Self-Healing Polymers E-Book

• Herausgeber: Wiley-VCH
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 754

Veröffentlichungsjahr: 2013

Beschreibung

Self-healing is a well-known phenomenon in nature: a broken bone merges after some time and if skin is damaged, the wound will stop bleeding and heals again. This concept can be mimicked in order to create polymeric materials with the ability to regenerate after they have suffered degradation or wear. Already realized applications are used in aerospace engineering, and current research in this fascinating field shows how different self-healing mechanisms proven successful by nature can be adapted to produce even more versatile materials. The book combines the knowledge of an international panel of experts in the field and provides the reader with chemical and physical concepts for self-healing polymers, including aspects of biomimetic processes of healing in nature. It shows how to design self-healing polymers and explains the dynamics in these systems. Different self-healing concepts such as encapsulated systems and supramolecular systems are detailed. Chapters on analysis and friction detection in self-healing polymers and on applications round off the book.

Leseprobe

Table of Contents

Related Titles

Title page

Copyright page

List of Contributors

Introduction

Part One: Design of Self-Healing Materials

1: Principles of Self-Healing Polymers

1.1 Introductory Remarks

1.2 General Concept for the Design and Classification of Self-Healing Materials

1.3 Physical Principles of Self-Healing

1.4 Chemical Principles of Self-Healing

1.5 Multiple versus One-Time Self-Healing

1.6 Resume and Outlook

Acknowledgments

2: Self-Healing in Plants as Bio-Inspiration for Self-Repairing Polymers

2.1 Self-Sealing and Self-Healing in Plants: A Short Overview

2.2 Selected Self-Sealing and Self-Healing Processes in Plants as Role Models for Bio-Inspired Materials with Self-Repairing Properties

2.3 Bio-Inspired Approaches for the Development of Self-Repairing Materials and Structures

2.4 Bio-Inspired Self-Healing Materials: Outlook

Acknowledgments

3: Modeling Self-Healing Processes in Polymers: From Nanogels to Nanoparticle-Filled Microcapsules

3.1 Introduction

3.2 Designing Self-Healing Dual Cross-Linked Nanogel Networks

3.3 Designing “Artificial Leukocytes” That Help Heal Damaged Surfaces via the Targeted Delivery of Nanoparticles to Cracks

3.4 Conclusions

Part Two: Polymer Dynamics

4: Structure and Dynamics of Polymer Chains

4.1 Foreword

4.2 Techniques

4.3 Structure

4.4 Dynamics

4.5 Application to Self-Healing

4.6 Conclusions and Outlook

5: Physical Chemistry of Cross-Linking Processes in Self-Healing Materials

5.1 Introduction

5.2 Thermodynamics of Gelation

5.3 Viscoelastic Properties of the Sol–Gel Transition

5.4 Phase Separation and Gelation

5.5 Conclusions

6: Thermally Remendable Polymers

6.1 Principles of Thermal Healing

6.2 Inorganic–Organic Systems

6.3 Efficiency, Assessment of Healing Performance

6.4 Conclusions

Acknowledgments

7: Photochemically Remendable Polymers

7.1 Background

7.2 Molecular Design

7.3 Reversible Photo-Crosslinking Behaviors

7.4 Evaluation of Photo-Remendability

7.5 Concluding Remarks

Acknowledgments

8: Mechanophores for Self-Healing Applications

8.1 Introduction

8.2 Mechanochemical Damage

8.3 Activation of Mechanophores

8.4 Mechanochemical Self-Healing Strategies

8.5 Conclusions and Outlook

9: Chemistry of Crosslinking Processes for Self-Healing Polymers

9.1 Introduction

9.2 Extrinsic Self-Healing Materials

9.3 Intrinsic Self-Healing Materials

9.4 Concluding Remarks and Future Outlook

10: Preparation of Nanocapsules and Core–Shell Nanofibers for Extrinsic Self-Healing Materials

10.1 Selected Preparation Methods for the Encapsulation of Self-Healing Agents

10.2 Mechanically Induced Self-Healing

10.3 Stimuli-Responsive Self-Healing Materials

10.4 Novel Approaches and Perspectives

Part Three: Supramolecular Systems

11: Self-Healing Polymers via Supramolecular, Hydrogen-Bonded Networks

11.1 Introduction

11.2 Dynamics of Hydrogen Bonds in Solution

11.3 Supramolecular Gels

11.4 Self-Healing Bulk Materials

11.5 Conclusions

Acknowledgment

12: Metal-Complex-Based Self-Healing Polymers

12.1 Stimuli-Responsive Metallopolymers

12.2 Self-Healing Metallopolymers

12.3 Summary and Outlook

Acknowledgments

13: Self-Healing Ionomers

13.1 Introduction

13.2 Basic Principles of Ionomers

13.3 Ionomers in Self-Healing Systems

13.4 Actual Developments and Future Trends in Ionomeric and Related Self-Healing Systems

Part Four: Analysis and Friction Detection in Self-Healing Polymers: Macroscopic, Microscopic and Nanoscopic Techniques

14: Methods to Monitor and Quantify (Self-) Healing in Polymers and Polymer Systems

14.1 Introduction

14.2 Visualization Techniques

14.3 Healing of Mechanical Properties

14.4 Healing of Functional Integrity

14.5 Summary

15: Self-Healing Epoxies and Their Composites

15.1 Introduction

15.2 Capsule-Based Healing System

15.3 Vascular-Based Healing Systems

15.4 Intrinsic Healing Systems

15.5 Conclusions

16: Self-Healing Coatings

16.1 Introduction into Self-Healing Coatings

16.2 Concept of Micro- and Nanocontainer-Based Self-Healing Coatings

16.3 Types of Nanocontainers

16.4 Characterization of Nanocontainer-Based Self-Healing Coatings

16.5 Conclusions and Current Trends

17: Application of Self-Healing Materials in Aerospace Engineering

17.1 General Considerations

17.2 Conclusions

Index

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Editor

Wolfgang H. Binder

MLU Halle-Wittenberg

Institut für Chemie

Von-Danckelmann-Platz 4

06120 Halle

Germany

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List of Contributors

Anna C. Balazs

University of Pittsburgh

Chemical Engineering Department

Pittsburgh, PA 15261

USA

Stijn Billiet

Ghent University

Department of Organic Chemistry

Polymer Chemistry Research Group

Krijgslaan 281 S4-bis

9000 Ghent

Belgium

Wolfgang H. Binder

Martin-Luther-Universität Halle-Wittenberg

Institut für Chemie

Von-Danckelmann-Platz 4

06120 Halle

Germany

and

MLU Halle Wittenberg

Faculty of Natural Sciences II

von Danckelmannplatz 4

06120 Halle (Saale)

Germany

Stefan Bode

Friedrich-Schiller-University Jena

Laboratory of Organic and Macromolecular Chemistry (IOMC)

Humboldtstr. 10

07743 Jena

Germany

and

Friedrich-Schiller-University Jena

Jena Center for Soft Matter (JCSM)

Philosophenweg 7

07743 Jena

Germany

Dimitriya Borisova

Max Planck Institute of Colloids and Interfaces

Am Mühlenberg 1

14476 Golm

Germany

Ranjita K. Bose

Delft University of Technology

Faculty of Aerospace Engineering, Novel Aerospace Materials

Kluyverweg 1

2629HS Delft

The Netherlands

Ana Rita Brás

Forschungszentrum Jülich

Jülich Centre for Neutron Science

JCNS-1

52425 Jülich

Germany

Diana Döhler

Martin-Luther-Universität Halle-Wittenberg

Institut für Chemie

Von-Danckelmann-Platz 4

06120 Halle

Germany

Anthony M. Coppola

University of Illinois at Urbana-Champaign

Beckman Institute for Advanced Science and Technology

405 N. Mathews Ave.

Urbana, IL 61801

USA

Daniel Crespy

Max Planck Institute for Polymer Research

Ackermannweg 10

55128 Mainz

Germany

Charles E. Diesendruck

University of Illinois at Urbana-Champaign

Department of Chemistry and Beckman Institute for Advanced Science and Technology

405 N. Mathews Ave.,

Urbana, IL 61801

USA

Filip E. Du Prez

Ghent University

Department of Organic Chemistry

Polymer Chemistry Research Group

Krijgslaan 281 S4-bis

9000 Ghent

Belgium

and

SIM

Technologiepark Zwijnaarde 904

9052 Ghent

Belgium

Tom Engel

Saarland University

Inorganic Solid-State Chemistry

Am Markt, Zeile 3

66125 Saarbrücken

Germany

Santiago J. Garcia

Delft University of Technology

Faculty of Aerospace Engineering, Novel Aerospace Materials,

Kluyverweg 1

2629HS Delft

The Netherlands

Ryan C. Gergely

University of Illinois at Urbana-Champaign

Beckman Institute for Advanced Science and Technology

405 N. Mathews Ave.

Urbana, IL 61801

USA

Liberata Guadagno

Università di Salerno

Dipartimento di Ingegneria Industriale

Via Ponte Don Melillo

84084 Fisciano (SA)

Italy

and

Università di Salerno

Nano-Mates – Research Centre for NANOMAterials and nanoTEchnology

Via Ponte don Melillo

84084 Fisciano (SA)

Italy

Martin D. Hager

Friedrich-Schiller-University Jena

Laboratory of Organic and Macromolecular Chemistry (IOMC)

Humboldtstr. 10

07743 Jena

Germany

and

Friedrich-Schiller-University Jena

Jena Center for Soft Matter (JCSM)

Philosophenweg 7

07743 Jena

Germany

Kevin R. Hart

University of Illinois at Urbana-Champaign

Beckman Institute for Advanced Science and Technology

405 N. Mathews Ave.

Urbana, IL 61801

USA

Florian Herbst

Martin-Luther-Universität Halle-Wittenberg

Institut für Chemie

Von-Danckelmann-Platz 4

06120 Halle

Germany

Xander K.D. Hillewaere

Ghent University

Department of Organic Chemistry

Polymer Chemistry Research Group

Krijgslaan 281 S4-bis

9000 Ghent

Belgium

and

SIM

Technologiepark Zwijnaarde 904

9052 Ghent

Belgium

Nico Hohlbein

Universität zu Köln

Institut für Physikalische Chemie, Department Chemie

Luxemburger Str. 116

50733 Köln

Germany

and

Fraunhofer Institut für Umwelt-,

Sicherheits- und Energietechnik UMSICHT

Osterfelder Str. 3

46047 Oberhausen

Germany

and

Institut für Zukunftorientierte Kompetenzentwicklung

Hochschule Bochum

Lennershofstr. 140

44801 Bochum

Germany

Henghua Jin

University of Illinois at Urbana-Champaign

Beckman Institute for Advanced Science and Technology

405 N. Mathews Ave.

Urbana, IL 61801

USA

Hans-Werner Kammer

Martin Luther University Halle-Wittenberg,

Department of Chemistry

06099 Halle (Saale)

Germany

Guido Kickelbick

Saarland University

Inorganic Solid-State Chemistry

Am Markt, Zeile 3

66125 Saarbrücken

Germany

German V. Kolmakov

New York City College of Technology

Physics Department

300 Jay Street

Brooklyn, NY 11201

USA

Joerg Kressler

Martin Luther University Halle-Wittenberg

Department of Chemistry

06099 Halle (Saale)

Germany

Ugo Lafont

Delft University of Technology

Faculty of Aerospace Engineering, Novel Aerospace Materials

Kluyverweg 1

2629HS Delft

The Netherlands

Jun Ling

Sun Yat-sen (Zhongshan) University

Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education

DSAPM Lab

School of Chemistry and Chemical Engineering

Guangzhou 510275

China

Pasquale Longo

Università di Salerno

Nano-Mates – Research Centre for NANOMAterials and nanoTEchnology

Via Ponte don Melillo

84084 Fisciano (SA)

Italy

and

Università di Salerno

Dipartimento di Chimica e Biologia

Via Ponte Don Melillo

84084 Fisciano (Salerno)

Italy

Philipp Michael

Martin-Luther-Universität Halle-Wittenberg

Institut für Chemie

Von-Danckelmann-Platz 4

06120 Halle

Germany

Helmuth Möhwald

Max Planck Institute of Colloids and Interfaces

Am Mühlenberg 1

14476 Golm

Germany

Jeffrey S. Moore

University of Illinois at Urbana-Champaign

Department of Chemistry and Beckman Institute

405 N. Mathews Ave.

Urbana, IL 61801

USA

and

University of Illinois at Urbana-Champaign

Beckman Institute for Advanced Science and Technology

405 N. Mathews Ave.

Urbana, IL 61801

USA

Rolf Mülhaupt

Freiburg Materials Research Center (FMF) and Freiburg Center for Interactive Materials and Bio-Inspired Technologies (FIT)

Stefan-Meier-Straße 21

79104 Freiburg

Germany

and

University of Freiburg

Institute for Macromolecular Chemistry

Stefan-Meier-Str. 31

79104 Freiburg

Germany

Carlo Naddeo

Università di Salerno

Dipartimento di Ingegneria Industriale

Via Ponte Don Melillo

84084 Fisciano (SA)

Italy

Anke Nellesen

Universität zu Köln

Institut für Physikalische Chemie, Department Chemie

Luxemburger Str. 116

50939 Köln

Germany

and

Fraunhofer Institut für Umwelt-

Sicherheits- und Energietechnik UMSICHT

Osterfelder Str. 3

46047 Oberhausen

Germany

and

Institut für Zukunftorientierte Kompetenzentwicklung

Lennershofstr. 140

44801 Bochum

Germany

Wim Pyckhout-Hintzen

Forschungszentrum Jülich

Jülich Centre for Neutron Science

JCNS-1

52425 Jülich

Germany

Marialuigia Raimondo

Università di Salerno

Dipartimento di Ingegneria Industriale

Via Ponte Don Melillo

84084 Fisciano (SA)

Italy

Dieter Richter

Forschungszentrum Jülich

Jülich Centre for Neutron Science

JCNS-1

52425 Jülich

Germany

Min Zhi Rong

Sun Yat-sen (Zhongshan) University

Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education

DSAPM Lab

School of Chemistry and Chemical Engineering

Guangzhou 510275

China

Isaac G. Salib

University of Pittsburgh

Chemical Engineering Department

Pittsburgh, PA 15261

USA

Benedict Sandmann

Friedrich-Schiller-University Jena

Laboratory of Organic and Macromolecular Chemistry (IOMC)

Humboldtstr. 10

07743 Jena

Germany

and

Friedrich-Schiller-University Jena

Jena Center for Soft Matter (JCSM)

Philosophenweg 7

07743 Jena

Germany

Annette M. Schmidt

Universität zu Köln

Institut für Physikalische Chemie, Department Chemie

Luxemburger Str. 116

50733 Köln

Germany

and

Fraunhofer Institut für Umwelt-

Sicherheits- und Energietechnik UMSICHT

Osterfelder Str. 3

46047 Oberhausen

Germany

and

Institut für Zukunftorientierte Kompetenzentwicklung

Hochschule Bochum

Lennershofstr. 140

44801 Bochum

Germany

Ulrich S. Schubert

Friedrich-Schiller-University Jena,

Laboratory of Organic and Macromolecular Chemistry (IOMC)

Humboldtstr. 10

07743 Jena

Germany

and

Friedrich-Schiller-University Jena

Jena Center for Soft Matter (JCSM)

Philosophenweg 7

07743 Jena

Germany

Dmitry G. Shchukin

Max Planck Institute of Colloids and Interfaces

Am Mühlenberg 1

14476 Golm

Germany

Nancy R. Sottos

University of Illinois at Urbana-Champaign

Beckman Institute for Advanced Science and Technology

405 N. Mathews Ave.

Urbana, IL 61801

USA

Olga Speck

University of Freiburg

Plant Biomechanics Group and Botanic Garden

Schänzlestr. 1

79104 Freiburg

Germany

and

Competence Networks Biomimetics and BIOKON

Stefan-Meier-Straße 21

79104 Freiburg

Germany

Thomas Speck

University of Freiburg

Plant Biomechanics Group and Botanic Garden

Schänzlestr. 1

79104 Freiburg

Germany

and

Competence Networks Biomimetics and BIOKON

Schänzlestr. 1

79104 Freiburg

Germany

and

Freiburg Materials Research Center (FMF) and Freiburg Center for Interactive Materials and Bio-Inspired Technologies (FIT)

Stefan-Meier-Straße 21

79104 Freiburg

Germany

Max von Tapavicza

Universität zu Köln,

Institut für Physikalische Chemie, Department Chemie

Luxemburger Str. 116

50939 Köln

Germany

and

Fraunhofer Institut für Umwelt-

Sicherheits- und Energietechnik UMSICHT

Osterfelder Str. 3

46047 Oberhausen

Germany

and

Institut für Zukunftorientierte Kompetenzentwicklung

Hochschule Bochum

Lennershofstr. 140

44801 Bochum

Germany

Roberto F.A. Teixeira

Ghent University,

Department of Organic Chemistry

Polymer Chemistry Research Group

Krijgslaan 281 S4-bis

9000 Ghent

Belgium

and

SIM

Technologiepark Zwijnaarde 904

9052 Ghent

Belgium

Jesús M. Vega

Delft University of Technology

Faculty of Aerospace Engineering, Novel Aerospace Materials

Kluyverweg 1

2629HS Delft

The Netherlands

Scott R. White

University of Illinois at Urbana-Champaign

Beckman Institute for Advanced Science and Technology

405 N. Mathews Ave.

Urbana, IL 61801

USA

Andreas Wischnewski

Forschungszentrum Jülich

Jülich Centre for Neutron Science

JCNS-1

52425 Jülich

Germany

Ming Qiu Zhang

Sun Yat-sen (Zhongshan) University

Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education

DSAPM Lab

School of Chemistry and Chemical Engineering

Guangzhou 510275

China

Yi Zhao

Max Planck Institute for Polymer Research

Ackermannweg 10

55128 Mainz

Germany

Sybrand van der Zwaag

Delft University of Technology

Faculty of Aerospace Engineering, Novel Aerospace Materials

Kluyverweg 1

2629HS Delft

The Netherlands

Introduction

Wolfgang Binder

When the Romans invented concrete as a construction material more than 2000 years ago for erecting arches, water-pipes and monuments such as the Pantheon, it can be assumed that they did not know about any of the molecular mechanisms of self-healing. However, they surely knew and realized by observation of, for example, old Egyptian pyramids, that the construction of a many-century-lasting empire needs even longer lasting materials, remaining unchanged over many thousands of years, even into our modern times. As all modern materials, fabricated with a usually huge amount of intellectual and also hand- or machine-driven force, are subject to thermal or mechanical destruction as well as chemical degradation during their active lifetime, their use is therefore limited. Despite the inevitable fact that the renewing and destructive force has allowed new civilizations to emerge during the past (historical) times, a short look at nature makes the possibilities of repair and restoration of properties obvious – why would modern man not be able to achieve the same, similar, or even better?

As polymers and polymeric materials are “the” smart invention and technological driving force of the twentieth century, the quest for implementing self-healing-properties into polymers [1] is strong. Not only the practical demand of maximum usage-times of each fabricated thing, but also the everlasting limitation on natural resources and costs inevitably leads to the quest for generating self-repairing polymer materials. Similar to repair-mechanisms active in living nature, regeneration of material properties should be reachable without external action [2]. In such materials, stress of a certain magnitude (either chemical, physical, or thermal) induces a mechanical deformation in the polymer, which in turn activates a response [3] within the material, leading to “healing” of the generated (physical) damage.

Looking at even superficial injuries in mammal organisms shows that a vascular (e.g., bloodstream-supported) supply-system helps to restore and heal mechanical damage via the blood-clotting cascade and subsequent tissue regeneration. This very simple principle demonstrates that biomimicry might help in the design of self-healing polymers by applying similar capsule- or vascular-based logics. If one looks further, the principles of DNA-repair based on the radical scission of DNA-chains can induce a DNA-repair system, which in its complexity cannot be copied into simple bulk polymeric materials. Thus, an important aspect of self-healing is the presence of a structure which is able to dynamically respond to an external stimulus [3], enabling the restoration of the initial material properties. Due to their highly complex chain structure, polymers, in particular, are ideally suited to serve as molecules for dynamic and thus self-healing properties, given that they are coupled to fast and efficient crosslinking reactions [4].

In self-healing polymers, many complex issues of chemical and physical-principles are interlinked, only then providing the necessary understanding of the underlying processes (see Chapter 1 by Binder et al. on the principles of self-healing polymers). Only a material able to recognize the damage-event can be able to heal autonomously, thus repairing without external action, similar to what is known in nature (see Chapter 2 by Speck et al. on biomimicry related to plants in self-healing polymers). Thus an inherent “sensing” ability of the polymer is required (see Chapter 8 by Moore et al. on mechanochemistry), which then shows a dynamic response to induce a repair mechanism and thus healing, usually via crosslinking processes. As the complexity of the “clotting-cascade” in living organisms cannot be reached in technical polymers, network-formation is among the central aspects of the healing mechanism, often related to permanent chemical crosslinking processes (see Chapter 9 by Du Prez et al.), reversible (covalent) crosslinking (see Chapter 6 by Kickelbick et al.) and via photochemical crosslinking (see Chapter 7 by Zhang et al.), requiring knowledge on the physicochemical principles of network formation (see Chapter 5 by Kressler et al.). Spatial separation of highly reactive intermediates by encapsulation (see Chapter 10 by Crespy et al.) is thus important.

Chain dynamics and principles of polymer physics are an important aspect for the design of self-healing polymers (see Chapter 3 by Balazs et al. and Chapter 4 by Pyckhout-Hintzen et al.). In order to design materials with dynamic properties, a reversible bonding system is required to enable a self-healing material, including aspects of supramolecular polymer chemistry. Thus noncovalent bonds such as hydrogen bonds and π–π-stacking (see Chapter 11 of Binder et al.); metal–metal-complexes (see Chapter 12 by Schubert et al.) and ionomers (see Chapter 13 by Schmidt et al.) are crucial elements, demonstrating the possibility of multiple healing cycles in contrast to covalently linked networks.

Self-healing polymers are already used in industrial engineering, fabricating material parts with self-healing properties. Thus reliably testing and understanding the mechanical properties of self-healing polymers is a crucial aspect, including modern micro- and nanoscaled testing methods, besides the classical mechanical testing-methods (see Chapter 14 by van der Zwaag et al.). Chapters on the applications of epoxy resins as the largest class of self-healing polymers (see Chapter 15 by White et al.), on self-healing polymers in the aerospace industry (see Chapter 17 by Guadagno et al.) and the use of layer-by-layer deposition for self-healing anticorrosion-surfaces (see Chapter 16 by Möhwald et al.) provide insight into the technological features of this fascinating class of materials.

The self-healing polymers are there on the market, they have become reality in material science. All in all, despite the different aspects of many research activities and fields, spanning the range from molecular chemistry to polymer physics, as well as mechanical testing and industrial engineering aspects, there are still many open fields to discover. In the future, it will not be the versatility of chemists and physicists that will decide about self-healing polymers, but the markets and the needs of engineers, where self-healing polymers will be used – it is thus just a question of market and history that will decide if new emerging cultures of the future will find our current technical achievements after 2000 years or more still present in everyday life . . . self-healing principles surely will play a role in this.

References

1 (a) White, S.R., and Blaiszik, B.J. (2012) Selbstheilende Materialien. Spektrum Wissenschaft, 3, 82–90; (b) Fischer, H. (2010) Self-repairing material systems – a dream or a reality? Nat. Sci., 2 (8), 873–901; (c) Davis, D.A., Hamilton, A., Yang, J., Cremar, L.D., Van Gough, D., Potisek, S.L., Ong, M.T., Braun, P.V., Martinez, T.J., White, S.R., Moore, J.S., and Sottos, N.R. (2009) Force-induced activation of covalent bonds in mechanoresponsive polymeric materials. Nature, 459 (7243), 68–72; (d) White, S.R., Sottos, N.R., Geubelle, P.H., Moore, J.S., Kessler, M.R., Sriram, S.R., Brown, E.N., and Viswanathan, S. (2001) Autonomic healing of polymer composites. Nature, 409, 794–817; (e) Cordier, P., Tournilhac, F., Soulie-Ziakovic, C., and Leibler, L. (2008) Self-healing and thermoreversible rubber from supramolecular assembly. Nature, 451, 977–980; (f) Herbst, F., Dohler, D., Michael, P., and Binder, W.H. (2013) Self-healing polymers via supramolecular forces. Macromol. Rapid Commun., 34 (3), 203–220.

2 (a) Caruso, M.M., Davis, D.A., Shen, Q., Odom, S.A., Sottos, N.R., White, S.R., and Moore, J.S. (2009) Mechanically-induced chemical changes in polymeric materials. Chem. Rev., 109 (11), 5755–5798; (b) Murphy, E.B., and Wudl, F. (2010) The world of smart healable materials. Prog. Polym. Sci., 35 (1–2), 223–251.

3 (a) Xia, F., and Jiang, L. (2008) Bio-inspired, smart, multiscale interfacial materials. Adv. Mater., 20 (15), 2842–2858; (b) Youngblood, J.P., and Sottos, N.R. (2008) Bioinspired materials for self-cleaning and self-healing. MRS Bull., 33, 732–741; (c) Binder, W.H., Pulamagatta, B., Schunack, M., and Herbst, F. (2012) Biomimetic polymers, in Bioinspiration and Biomimicry in Chemistry (ed. G. Swiegers), John Wiley and Sons, Hoboken, NJ, pp. 323–366. ISBN 0470566671 and 9780470566671.

4 (a) Gragert, M., Schunack, M., and Binder, W.H. (2011) Azide/alkyne-“click”-reactions of encapsulated reagents: toward self-healing materials. Macromol. Rapid Commun., 32 (5), 419–425; (b) Binder, W.H., and Herbst, F. (2011) Click chemistry in polymer science, in McGraw-Hill Yearbook of Science & Technology (ed. D. Blumel), McGraw-Hill, New York, pp. 46–49; (c) Binder, W.H., and Sachsenhofer, R. (2008) “Click”-chemistry in polymer and material science: an update. Macromol. Rapid Commun., 29 (12–13), 952–981.

Part One

Design of Self-Healing Materials

1

Principles of Self-Healing Polymers*)

Diana Döhler1), Philipp Michael1), and Wolfgang Binder

1.1 Introductory Remarks

All matter is subject to thermal or mechanical destruction as well as chemical degradation during its active lifetime – thus restricting the use of each crafted piece of matter which has been fabricated with a usually huge amount of intellectual and also hand- or machine-driven force. When the Romans invented concrete as a construction material more than 2000 years ago for erecting arches, water-pipes and building of monuments, such as the Pantheon, it might be assumed that they did not know about any of the molecular mechanisms of self-healing in this material. However, they surely knew and realized by observation of, for example, old Egyptian pyramids, that the construction of an empire to last for many centuries needs even longer lasting materials. Indeed, a useful construction material does not only need to be strong and stiff, it also needs to be flexible in shape and application, thus self-hardening concrete definitely is advantageous over handcrafted stone. The self-healing properties of concrete have provided us with testimony of this technically advanced (Roman) culture, whose achievements can be seen even in our modern times.

Polymers and polymeric materials are “the” smart invention and technological driving force of the twentieth century, hence the quest for self-healing or self-repairing polymers [1, 2] is strong. Not only the practical demand for maximum usage-times of each fabricated thing, but also the everlasting limitation of natural resources and costs leads to the search for self-repairing polymeric materials needing no direct human action for repair. Therefore, as new polymers and polymeric materials are designed, the quest for materials with self-healing pro­perties (i.e., those which can regenerate similarly to living matter, especially after mechanical deformation and crack-formation) is increasing, culminating in the need for self-healing polymers after mechano-deformation [3–10]. In such materials, stress of a certain magnitude (either chemical, physical, or thermal) induces a mechanical deformation in the polymer, which in turn activates a response within the material, leading to “healing” of the generated (physical) damage. Despite the inevitable fact that the destructive and renewing force has allowed new civilizations to emerge during the past (historical) times, a short look at nature makes the possibilities of repair and restoration of properties obvious – why would man not be able to achieve the same, similar, or even better? Naturally, mankind is taking steps to increase the lifetimes of all materials, in particular those of polymeric materials and composites, thus reducing the need for repair and replacement of such materials. Looking at superficial injuries in mammal organisms we see that a vascular (e.g., bloodstream-supported) supply-system helps to restore and heal mechanical damage via the blood-clotting cascade and subsequent tissue regeneration. This very simple principle demonstrates that biomimicry might help in the design of self-healing polymers by applying similar capsule- or vascular-based logics (see also Chapter 2). Looking further, principles of DNA-repair based on the radical scission of DNA-chains can induce a DNA-repair cascade, which in its complexity cannot be copied in simple bulk polymeric materials, but shows that a dynamic system is required to enable a self-healing material. Thus, an important aspect of self-healing is the presence of a structure which is able to respond dynamically to an external stimulus, enabling the restoration of the initial material properties. Due to their highly complex chain structure, polymers are ideally suited to serve as molecules for dynamic and thus self-healing properties.

1.2 General Concept for the Design and Classification of Self-Healing Materials

A polymer displaying self-healing properties needs the ability to transform physical energy into a chemical and/or physical response able to heal the damage – a process which normally is not present in “conventional – non-self-healing” polymers. Thus, the polymer needs to “sense” the damaging force, transforming it autonomously (without further external stimulus) into a healing-event, ideally at the damaged site. The possible mechanistic designs of self-healing polymers are depicted in Figure 1.1. A self-healing polymer, therefore, is supposed to heal damage (see Figure 1.1a, imposed by shear-force or another rupturing event) by either physical processes alone (see Figure 1.1b) or via a combination of chemical and physical processes (see Figure 1.1c). The design of self-healing polymers, therefore, is a multidisciplinary process, requiring knowledge of their structure, their individual dynamics, as well as a deep knowledge of chemical processes. Thus, the design of self-healing polymers needs a thorough understanding of the polymer’s individual chain-dynamics (see also Chapters 3 and 4), and not only the dynamics of whole chains or molecules within the polymeric material, but also the dynamics of each segment interacting with a specific part of the new interface or other polymeric/monomeric molecules.

Figure 1.1 Design of self-healing polymers. (a) Shear-force or rupture is transformed into a crack, sensed by the polymer matrix. (b) Newly generated interfaces lead to concentration gradients, which allow enhanced diffusional processes (f), resulting in welding, swelling, patching, nanoparticle enrichment or simple molecular interdiffusion (physical entanglement) of polymer chains (d) and (e). (c) Molecular movements lead to contact between functional groups and thus to a chemical healing process, resulting in crack closure by a newly formed network (g) Healing induced by a crosslinking reaction, either via supramolecular forces (h), covalent chemistry (i), mechanochemical (j) or “switchable” reactions (k). After a second rupture event at the same area the number of healing-cycles can vary, implying either a one-time healing process (l) or multiple healing (m) and (n).

Similar to biochemical healing processes, the initial damage (see Figure 1.1a) generates a free (usually fresh) interface (shown as a crack), which in turn can act as a site for molecular processes, such as swelling, patching or simple molecular diffusion, which can induce a welding process (see Figure 1.1d), subsequently leading to a closing of the crack and thus a “self-healing” process [11]. Nanoparticles (see Figure 1.1e), small, or even large molecules can diffuse to the interface, thus leading to changes in the local concentration, and also inducing changes in the individual local mobility of the molecules. This can in turn lead to the healing of the crack (see Figure 1.1f).

Chemical healing processes (see Figure 1.1c) always need a combination of physical and chemical healing principles, as a chemical reaction can only take place when contact between the reactants has been achieved. In general, after diffusion and reaction of the reactants, the crack is filled by a newly formed network (see Figure 1.1g [see also Chapter 5]), which results from the crosslinking reaction of individual polymer chains, either via purely physical (“supramolecular”) forces [9, 12–14], or by action of truly chemical forces resulting in partially reversible [15–17] or stable [18, 19] covalent bonds (see Figure 1.1h and i). Purely supramolecular interactions [9, 20–22] (see also Chapters 11–13), well known from molecular self-assembly, can reform, thus generating a network with dynamic properties by itself [23]; covalent chemistry (see Chapters 6–9, 15, and 17) is able to form a new network by a plethora of chemical crosslinks, often well known and well optimized by technical processes of resin-chemistry (“thermosets”). In particular Diels–Alder (DA) reactions [17, 24–28], epoxide chemistry [29–41], “click-based” chemistry [18, 42–54], isocyanate chemistry [55], olefin metathesis [19, 56–61], and thiol chemistry [62, 63] have gained significance in this respect. Choice of the chemical reactions to be used usually takes into account the efficiency (“free energy”) of the reactants as a major selection tool, besides the stability and selective incorporation of the respective functional groups into the final material, for example, via encapsulation strategies (see also Chapters 10 and 16).

Furthermore, so called mechanochemical reactions (see Figure 1.1j) (see also Chapter 8) can transform the physical energy of damage directly via a specially designed chemical group (“mechanophore”) [4, 64, 65] into an activated chemical state, which in turn allows self-healing. These specially designed reactions are intrinsically coupled to the existing polymeric chains, as the attached polymeric chains act as a “handle”, which by definition allows the conversion of applied mechanical energy into the actual chemical reaction [66]. Especially, ring-opening reactions [64, 65, 67–73] and carbene-based catalyst activation [74–78] have become prominent for realizing the concept of self-healing polymers.

Moreover, an inherent “switch” [79–82] (see Figure 1.1k) (see also Chapter 7) such as light or an electrochemical stimulus can be flipped, triggering a reversible network formation within the polymer and thus a self-healing response.

Additionally, the mentioned self-healing approaches can be divided into intrinsic and extrinsic concepts [7] (see Figure 1.1), where intrinsic self-healing polymers utilize an inherent material ability to self-heal, triggered either by a damage event or in combination with an external stimulus. In contrast, for extrinsic self-healing concepts the healing agents have to be pre-embedded into a (polymeric) matrix enabling their release during a rupture event and thus self-healing.

All these approaches display different features with respect to external conditions under which self-healing takes place (such as the required stress for activation, the temperature of healing as well as other external constraints imposed by the mechanism of the healing concept) the number of healing-cycles thus implying either a once-a-time-healing after one stress-event (see Figure 1.1g and k), or the possibility to repeatedly heal damage at the same position of the material (see Figure 1.1l and m) as well as the timescale on which the self-healing process is taking place. Hence, a large body of work has been dedicated to optimize the conditions of healing (temperature, additives, and optimization of catalysts) as well as the technical realization of the concept, thus being able to fabricate and produce a technically useful polymer at reasonable costs within a technical process (see Chapter 14).

The following sections will provide an overview of the chemical principles of the underlying concepts to fabricate self-healing polymers. The various possibilities of chemical and physicochemical approaches have, therefore, triggered a number of strategies which will be discussed in separate chapters as their chemical and physical principles are totally different.

1.3 Physical Principles of Self-Healing

In general, all self-healing concepts have the aim to generate crosslinked networks, either by covalent or supramolecular chemistry, or by purely physical crosslinking via chain entanglements. Self-healing principles based on physical interactions constituted one of the first historically observed self-healing behaviors of manmade plastics and will be discussed in the following. Chemical self-healing concepts will be presented in Chapter 9. Physical principles implementable in self-healing approaches are based on molecular diffusion, induced either by Brownian motion, chain segment motion or entropy driven movement of molecules or particles with or without external stimulus. Several self-healing mechanisms, such as molecular interdiffusion, welding, swelling, or self-healing via nanoparticles can be assigned to this category. Additionally phase-separation phenomena in polymers can be an enhancing factor for self-healing [83–95] (e.g., see Chapter 13).

In order to understand the phenomena of self-healing based on physical interactions it is necessary to get fundamental knowledge of molecular mechanisms. The elementary steps of all physical self-healing principles are interdiffusion and entanglement of polymer chains [11, 96]. Both properties depend on intermolecular forces, which are closely linked to the chemical nature of the polymer and the length of the molecules, including the dependence on the average molecular weight. However, both factors often oppose each other. Accordingly, short chains enable fast molecular interdiffusion, while long chains generate materials with the ability for high strength recovery at the interface [11].

One of the best known and well understood self-healing approaches based on physical methods is the so-called molecular interdiffusion, which can be divided into thermoplastic and thermoset self-healing occurring above and also, at least theoretically, below the glass transition temperature (Tg). It has been shown that the polymer/polymer interface gradually vanishes and the mechanical strength at the initial interface increases by bringing two pieces of identical or even compatible polymers into contact. Thus, the polymer matrix is actually healed just due to molecular diffusion along the polymer/polymer interface [3, 11].

In order to understand this phenomenon, Wool and O’Connor [97] developed, in 1981, a five-stage mechanism to unscramble the complexity of strength recovery at ruptured polymer/polymer interfaces, and provide an explanation for the functioning principle of many self-healing concepts (see Figure 1.2), being strongly related to molecular interdiffusion at or above the glass transition (Tg). At this temperature polymer segments are mobile enough to enable an efficient self-healing process. This relatively simple model is, in its basic steps, applicable as a universal mechanism for nearly all self-healing concepts [98].

Figure 1.2 Stages of self-healing mechanism. (a) Surface rearrangement, (b) surface approach, (c) wetting, (d) diffusion and (e) randomization.

The principle stages of healing according to the mechanism of Wool and O’Connor [97] are illustrated in Figure 1.2 including the steps of surface rearrangement (a), surface approach (b), wetting (c), diffusion (d) and randomization (e). Surface rearrangement and surface approach are the first steps after a damage event has occurred. These steps are the most critical ones as healing can only take place if the ruptured interfaces can come into contact with each other. The surface rearrangement (a) influences the rate of crack healing significantly due to the discontinuity of the topography and the roughness of the created crack surface, which may change with time, temperature and the applied pressure [11, 97, 98]. Higher surface roughness leads to a higher contact area and, therefore, to higher rates of diffusion, and thus expectedly higher rates of self-healing. The surface approach (b) under controlled laboratory conditions appears to be the most trivial step. However, in practice, this is one of the most crucial steps regarding surfaces pulled apart by the damage event which might prevent the contact of the surface layers and thus terminate the self-healing process [98]. Moreover, the surface approach stage determines the mode of healing, for example, healing in point or line mode [97]. Before the healing process can start, the wetting of the cracked surfaces by each other or by healing agents has to be ensured. This is mostly achieved by ensuring sufficiently high chain mobility of the initial material, and also by increasing the temperature or adding solvents. The wetting stage (c) enables diffusion (d) which results in the entanglement of polymer chains and, therefore, in the recovery of the mechanical properties of the healed material. Diffusion is a fractal random walk of polymer chains near to the surface which first results in the entanglement of the mobile chains and then in interpenetration into the unruptured matrix material. The diffusion stage is the most important step for restoring the mechanical properties during which the majority of these pro­perties are recovered or healed [97–99]. During the randomization stage (e) the complete loss of initial crack interfaces can be observed.

As previously mentioned, it is possible to observe self-healing via molecular interdiffusion, not only above but also below the Tg, despite being a contradiction of the conventional knowledge of polymer movement suggesting no motion of polymer segments below their Tg. Nevertheless, wetting and diffusion can be observed at a certain healing temperature significantly below the Tg [96, 100–102]. Wool et al. [100, 102] observed crack healing processes in glassy polymers and related this to the interdiffusion of molecular chains and the subsequent re-formation of molecular entanglements. It is claimed in literature, and highly controversially discussed in the scientific community, that this self-healing ability is caused by a potential reduction of the Tg at polymer surface layers compared to the Tg in the bulk [11, 96, 101, 103–105].

Four possible explanations for this behavior [106–115] are illustrated in Figure 1.3: (a) The accumulation of polymer end groups at the outer surface due to their higher space requirement compared to the corresponding chain segments leads to a lower density of chain segments at the inner surface, causing a higher chain mobility and thus a lower Tg. (b) The confinements for polymers with high molecular weight force them to alter their chain conformation (induced by a break in symmetry at the polymer surface – flattened chain conformation), resulting in decreasing interchain entanglements and a reduction in the chain segment density. (c) The collective motions along the chain (loop motions) dominating in thin film scenarios require a weaker free volume compared to the standard motion, leading to a decrease in the effective chain segment density and, therefore, to a reduction in the Tg at the interface. (d) The accumulation of low molecular weight polymers at the surface layer can cause, according to the Flory–Fox equation, a reduction in Tg. This effect can be explained by their lower surface tension and the reduction in free energy by a smaller change in conformational entropy. Following these assumptions, the final reduction in Tg at the surface layer will enhance the mobility of the chain segments and thus enable self-healing according to the basic steps of Wool and O’Connor [97–99].

Figure 1.3 Reduction of Tg in polymer surface layers. The accumulation of polymer end groups at the outer surface (a), confinements for polymer chains due to high molecular weight (b), and collective motions along the chain (c) leading to a decrease in the effective chain segment density in all three cases (a)–(c) and therefore to a reduction in the Tg at the interface. The enrichment of low molecular weight polymers at the surface (d) causes a reduction in Tg according to the Flory–Fox equation.

A further self-healing concept based on physical interactions is welding. Beside the mentioned molecular interdiffusion it is one of the traditional self-healing methods. Welding relies on damage healing by forming chain entanglements between two contacting polymer surfaces in order to restore the original mechanical properties of the ruptured area [3]. Therefore, welding is, in a strict interpretation, a superordinated category of molecular interdiffusion. Nevertheless, it is often referred to separately due to its various manifestations. The welding process consists of the same healing stages mentioned above: surface rearrangement, surface approach, wetting, diffusion and final randomization (see Figure 1.2). However, the rate of rearrangement and reorganization is influenced by various factors, such as the welding temperature, surface roughness, remaining chemical bonds between the surfaces, and the presence or absence of solvents [3]. In the case of covering or replacing damaged material by new externally added material the approach is also known as patching. If some solvents are involved to increase the mobility of the polymer chains the healing process is called swelling.

A completely different approach is the nanoparticle-based self-healing concept (see also Chapter 3) [3, 98, 116], inasmuch as it does not involve separation and rejoining of polymer chains like the previously discussed methods. This concept is based on the migration of nanoparticles to the damaged area. After cooling below a certain temperature a solid phase is formed recovering the mechanical strength and thus healing the damage. This method assumes a sufficiently high mobility of the nanoparticles in the polymer matrix. In order to enable this accumulation of particles at the crack surface the healing occurs commonly above Tg. Moreover, the modification of the nanoparticle surface is significant to ensure a sufficiently high driving force based on entropic and enthalpic interactions between the particles and the polymer matrix.

1.4 Chemical Principles of Self-Healing

All self-healing methods have the aim to generate crosslinks in networks, either by physical interactions, as discussed previously, or by chemical reactions of various kinds of functional groups, which will now be discussed in detail.

Chemical self-healing principles (see Chapters 2, 6–9, 11–13, 15 and 17) can be classified into two main categories, based either on covalent (see Figure 1.4a) or on supramolecular network formation (see Figure 1.4b). In the case of covalent network formation chemical bonds between functional groups are generated and thus a permanent, but sometimes even reversible, network is established. In contrast, supramolecular networks are commonly reversible associates of polymers linked via supramolecular interactions and show a higher dynamic behavior. Moreover, a subdivision into inherent “switchable” polymer systems (see Figure 1.4c) and concepts using mechanochemical activation of molecules by direct effect of a mechanical force (see Figure 1.4d) can be made. Concepts assigned to these two categories could also be classified into covalent or supramolecular network formation. The separations are, thereby, often blurred, enabling the assignment of several self-healing methods into more than one category.

Figure 1.4 Different modes of chemical interactions resulting in crosslinked network formation. (a) Covalent network formation, (b) supramolecular network formation, (c) “switchable” network formation and (d) mechanochemical network formation.

1.4.1 Covalent Network Formation

A great challenge for chemists and material scientists involved in the evolution of self-healing polymers was the development of concepts that, in a fast and preferably simple way, form highyl crosslinked networks. Therefore, they tuned the properties of the self-healing agents so that damage healing could occur under ambient conditions, like low temperatures, or humid and oxygen-containing environments. Furthermore, a large diversity of protection methods, such as encapsulation or in situ activation of catalysts, was developed. Moreover, some scientists explored various catalytic self-healing methods to accomplish a fast network formation at ambient temperatures, while others pursue higher temperature methods. Another challenging task was to guarantee a healing response only in direct response to a damage event. Therefore, reversible reactions shiftable to a “broken” stage and cured subsequently by re-shifting to the “healed” stage were used. Furthermore, different protection methods, preventing not only disturbing external influences but also undesired premature crosslinking reactions, found application. Accordingly, a large diversity of self-healing concepts has been developed up today.

In self-healing concepts based on covalently crosslinked networks, covalently linked network points are generated in a chemical reaction (see also Chapters 6–9 and 15). They exist in a large diversity and can be subdivided into reversible andirreversible reactions. Reversible methods, like DA/retro-DA reactions (DA/rDA) [17, 24–28, 117–123] or polycondensations [124, 125] provide the opportunity for multiple healing cycles, while irreversible methods, like the microcapsule-based ROMP concept [19, 59–61, 126], epoxides [127–130], or various click approaches [18, 42, 43, 48, 51], cannot heal a once ruptured area a second time. For more information see also Section 1.5.

1.4.1.1 Irreversible Covalent Network Formation Concepts

One of the most prominent self-healing methods based on irreversible covalent network formation was developed by White et al. [19] in 2001, who investigated the microencapsulation of dicyclopentadiene and subsequent embedding into an epoxy-matrix containing a Grubbs catalyst (see Figure 1.5a; Table 1.1a) (see also Chapters 9, 10, and 15). A damage event rips the microcapsules and releases the dicyclopentadiene monomer into the crack plane, where it comes into contact with a ruthenium-based Grubbs catalyst and triggers a ring opening metathesis polymerization (ROMP) [19, 56–58]. The subsequent generation of a dense network results in the recovery of the material properties, whereby self-healing efficiencies up to 99% can be achieved [126]. Further studies have resulted in the development of alternative monomers suitable for these ROMP systems, such as endo- [19] and exo-dicyclopentadiene [59], endo-1,2-dihydrodicyclopentadiene [59], norbornene [59], 5-ethylidene-2-norbornene [60], 5-(chloromethyl) norbornene [61], 5-(bromomethyl) norbornene [61] and norbornene carboxylic acid ethyl ester [61] (see Table 1.1b). Important, therefore, is a microencapsulation approach that can be applied now to nearly all kinds of polymers, such as thermosets, thermoplasts, and elastomers [19, 37, 41, 133–139]. The advantage of applied encapsulation is the possibility to spatially separate the reactive compounds from each other, from the matrix and, therefore, from the environment, thus increasing the longtime stability of the healing system (see Chapter 9). However, the original material properties can be affected negatively by embedding microcapsules. Moreover, the high cost of the Grubbs catalyst may limit a wider technical implementation.

Figure 1.5 Self-healing methods based on covalently crosslinked networks, utilizing (a) ROMP, (b) irreversible polycondensations, (c) epoxide chemistry, (d) CuAAC or (e) thiol-ene/thiol-yne click reactions, (f) Michael additions, (g) DA/rDA reactions, (h) thiol-based reactions, (i) radical reactions, or (j) reversible polycondensations as concept.

Table 1.1 Irreversible covalent network formation concepts for self-healing applications.

In order to overcome these shortcomings, in 2006 White et al. [131] studied a microcapsule-based self-healing systems using polycondensation reactions of technically available hydroxyl-terminated poly(dimethylsiloxane)s and poly­(diethoxysiloxane)s, efficiently catalyzed by cheaper organotin compounds (see Figure 1.5b; Table 1.1, entry 3). In order to prevent a premature reaction between the macromonomers, the tin compound was encapsulated in polyurethane microcapsules and embedded into the polymer matrix. In case of a rupture event the fluidity of the poly(dimethylsiloxane) enables flow into the crack, where a polycondensation reaction between the poly(dimethylsiloxane) and the poly(diethoxysiloxane), catalyzed by the tin compound, is triggered, forming a siloxane-crosslinked network within the damaged zone. Moreover, the healing system is applicable under aerobic and humid conditions due to the hydrophobic nature of both siloxane-based macromonomers and the stability of the released tin catalyst [131, 140]. This is of crucial importance for the practical realization of self-healing, and together with the lower cost of the catalyst was the prime motivation for developing such systems. However, it was not possible to achieve healing efficiencies of more than 24%, making a technical realization doubtful.

Another self-healing approach much closer to technical implementation utilized epoxides as matrix-embedded encapsulated healing agents [29–39] (see Figure 1.5c). Epoxides are highly strained three-membered rings which can undergo rapid coupling reactions with the second healing reagent, the so-called hardener, commonly substrates with activated hydrogen atoms, such as amines, alcohols, carboxylic acids, or mercaptans, as well as anhydrides and maleimides. In the course of the rupture event, the epoxide-containing capsules are cracked and release their content in order to react with the also embedded hardener via covalent bond formation. After treatment with these multifunctional and coreactive curing agents 100% conversion is usually reached, and insoluble thermosets are created composed of three-dimensional networks which fill the crack and restore the strength of the material. The crosslinking process can occur either at low temperatures (“cold curing”) or at elevated temperatures (“hot curing”), the latter being of minor interest for self-healing applications. Thus, primary and secondary aliphatic amines are most often used as hardeners because of their higher reactivity at low temperatures compared to other hardeners. Other important curing agents are acid anhydrides and formaldehyde resins which can react with the hydroxy groups of high molecular weight resins, resulting in further crosslinking reactions. While choosing the curing agent as well as the curing conditions the characteristics of the final resin, such as the crosslinking density and the morphology, can be freely tuned. Further, cationic curing can be induced photochemically [127–130]. Epoxy curing reveals some special advantages which make it to one of the most powerful self-healing concepts, especially for epoxy resin materials. While using epoxides as healing agents the same kind of material as the matrix is produced, thus ensuring a good adhesion between them and enabling full recovery of the initial material properties. Furthermore, together with the good healing efficiencies even at low temperatures, cold curing is the self-healing concept currently closest to commercial application. The sometimes lower longtime stability of the epoxy compounds can be partially compensated by the encapsulation, using specific wall materials or utilizing inherently more stable materials.

Recently, a self-healing system based on epoxy-amine resin formation [40] at ambient temperature was developed (see Table 1.1 entry 4). A modified aliphatic polyamine (EPIKURE 3274) and a diluted epoxy monomer (EPON 815C) were separately encapsulated and embedded into a thermoset epoxy matrix. The optimal mass ratio of amine to epoxy-containing capsules was determined to be 4 : 6 with an overall capsule content of 17.5% and an average healing efficiency up to 91%. However, a notable influence of the high capsule content within the polymer matrix on the material properties can be expected. A long term stability of the healing system of at least 6 months was shown. Another example of cold curing epoxy systems consists of the conventional epoxy bisphenol-A-diglycidyl ether (EPON 828) and the mercaptane hardener pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) acting as a two-compound healing system at room temperature [41] (see Table 1.1 entry 5). Both compounds were encapsulated into different types of microcapsules and incorporated into an epoxy resin matrix. An attractive healing efficiency of 104.5% with a low capsule content of 5% could be achieved after 24 h at 20 °C and also a sufficiently long term stability of the healing agents for at least 1 year could be demonstrated.

An approach of crosslinking concepts acting at ambient temperatures based on “click” reactions has been pursued by the groups of Binder [18, 42, 43] (see Figure 1.5d; Table 1.1 entry 6) and Kessler [48, 51], as well as by the thiol-ene/thiol-yne click concept [52–54] (see Figure 1.5e; Table 1.1 entry 7). However, they used a set of reactions called click chemistry [44–50], characterized by generating only one, mostly regiospecific pure product in high yields, working under simple reaction conditions and rapidly achieving complete conversion via a thermodynamic driving force greater than 20 kcal mol−1. This definition was given by Sharpless et al. [141] and thus primed the capability of this reaction type for self-healing applications. Binder et al. [18, 42, 43] (see Table 1.1 entry 6) have developed a click-based self-healing system using the copper(I) catalyzed alkyne-azide cycloaddition (CuAAC) reaction. They investigated a system assembling fluid trivalent azido-telechelic poly(isobutylene)s (PIB) and various low molecular weight multivalent alkynes [18, 42]. Both reactants were encapsulated in urea-formaldehyde capsules and embedded into a polymer matrix containing CuI(PPh3)3Br and TBTA as catalytic system. After the rupture event the healing agents came into contact with each other and the catalyst, triggering the cycloaddition reaction, forming a crosslinked network, and thus healing the damage. In the present case, the crosslinked network formation proceeded at ambient temperatures and resulted in nearly full recovery of the tensile modulus of 91% at 25 °C or an increased value of 107% at 60 °C. Moreover, the network formation of two multivalent fluid polymers [43] was investigated and optimized using fluid trivalent star-shaped azido- and alkyne-telechelic PIBs or multivalent side-chain functionalized copolymers of poly(acrylate), respectively, with CuI(PPh3)3Bras catalyst. Thus, higher network strand densities and faster crosslinking could be observed, even at room temperature. Furthermore, autocatalytic behavior of the crosslinking reaction has been proven, increasing further the efficiency of the CuAAC toward self-healing polymers. Kessler et al. [48, 51] (see Table 1.1 entry 6) followed a very similar approach also based on the CuAAC which is potentially applicable for self-healing, using two low molecular weight reactants (bisphenol-A-based bisazide and bisphenol-E or tetraethylene glycol-based diynes), and thus ensuring flow of the components into the crack. After triggering the CuAAC by the copper(I) catalyst the network filled the crack and restored the material properties. However, due to the bivalent monomers, linear polymers were generated which formed networks only via physical chain entanglements, while the previously described polymers [18, 42, 43] were able to form three-dimensional networks by covalent chemical bonds. Other interesting potential self-healing approaches based on click chemistry might be strain-promoted azide-alkyne cycloaddition reaction or photochemically triggered thiol-ene or thiol-yne click reactions (see Table 1.1 entry 7). Thereby, the absence of biotoxic copper salts enables application in living systems. Moreover, thiol-ene and thiol-yne click reactions attract the attention of scientists due to their ability of combining the aforementioned advantages of click chemistry and the potential of light-triggered reactions, enabling a spatially and temporally controlled self-healing process. Further photochemically triggered reactions with similar advantages, like [2+2] or [4+4] cycloadditions, are discussed in Section 1.4.4 [6, 17, 142–144].

An alternative self-healing concept uses the Michael addition of tetravalent thiols and bivalent maleimides (see Figure 1.5f; Table 1.1 entry 8) and is thus an elegant method for damage healing of epoxy resin materials due to the ability of crosslinking between the residual amino groups at the crack surface and the maleimide moieties [132]. Therefore, a strong dependence of the healing efficiency on the used matrix resin can be observed, for example, using EPON 828 as matrix material reveals an average healing efficiency of 121% after curing for 5 days at 25 °C.

1.4.1.2 Reversible Covalent Network Formation Concepts

Prime examples of the class of reversible covalent self-healing concepts are versatile DA cycloaddition reactions [17, 24–28, 117–123] (see Figure 1.5f) (see Chapter 6). DA reactions are amongst the most important reactions in organic chemistry due to their ability for C–C bond formation in high yields and good stereochemistry. Therefore, a conjugated diene and a dienophile, both commonly electrochemically activated by substituents, react with each other via a [4+2] cycloaddition. Moreover, many DA products are able to undergo a thermally induced [4+2] cycloreversion, the so-called rDA reaction which is the basis of the self-healing ability of polymeric DA/rDA systems. Due to the weaker bond strength between diene and dienophile of the DA adduct compared to all the other covalent bonds, this bond will break preferentially during the damage event and the rDA reaction will take place [25]. If the damaged sample is heated again, the diene and dienophile will reconnect and the crack plane is healed. Heating is commonly necessary to enhance the mobility of the reactive groups at the crack plane and, therefore, to accelerate the DA reaction [17, 25]. Nevertheless, nowadays some DA/rDA systems are known which are able to mend a crack even at ambient temperatures [123]. Advantageous self-healing principles based on DA/rDA reactions do not need additional ingredients like microcapsules or microvascular networks, and allow multiple runs of self-healing processes in the same area. Nevertheless, the mentioned high mending temperatures, as well as oxidative side-reactions restrict the application area of the DA reaction. In principle, two different polymer architectures for DA/rDA systems are known: first the polymer backbone can either be functionalized along itself [27, 28, 120] (see Table 1.2 entries 11 and 13) or in the side chains [26, 121] (see Table 1.2 entry 12), and secondly, multivalent, often star-shaped systems with DA functionalities as end groups [24, 25, 27, 148] (see Table 1.2 entry 10) can be applied. Chemically, the utilizable functionalities for DA/rDA systems can be classified into three main categories: furan-maleimide-based polymers [17, 24–26, 117] (see Table 1.2 entries 10 and 11), dicyclopentadiene-based polymers [17, 27] (see Table 1.2 entry 12), and anthracene-based polymers [17, 28, 118, 120, 148] (see Table 1.2 entry 13). Thus, Wudl et al. [24, 25] developed a self-healing concept using the thermally reversible DA/rDA reaction between star-shaped trivalent maleimides and tetravalent furans (see Table 1.2 entry 10), resulting in a highly dense network with thermoset properties via a polyaddition reaction. The damage event will break the DA-crosslinking points to form terminal maleimide and furan species, which in turn are able to react preferentially in a DA reaction, resulting in an average mending efficiency of about 50% at a temperature of 150 °C. Another thermally reversible DA method used modified poly(N-acetylethyleneimine)s bearing either maleimides or furans in the polymer side chain [26] (see Table 1.2 entry 11). Mixing of two suitable substituted polymers resulted in a dense network at a healing temperature of 80 °C. Further investigations of similar DA/rDA concepts consisting of polystyrene-bearing maleimides and several furan moieties indicate a limited thermal stability of furfuryl moieties with a therefore limited applicability for self-healing materials [121]. Furthermore, a single component self-healing polymer was developed using cyclic monomers containing dicyclopentadiene cores as reactive units [27] (see Table 1.2 entry 12), which were utilized simultaneously as diene and dienophile in the DA cycloaddition reaction. Accordingly, a polymer consisting of thermally reversible DA adducts with further reactive sides could be synthesized. A second DA reaction can be started, resulting in the trimerization and thus a highly crosslinked polymer, showing the ability for self-healing achieving an average healing efficiency of 46% upon heating to 120 °C. Moreover, a further single-component system potentially applicable for self-healing using DA/rDA reactions was developed applying anthracene-substituted acrylates simultaneously as diene and dienophile [28] (see Table 1.2 entry 13). The polymerization of this monomer results in a network with thermally reversible crosslinkers and represents thus a probable healing concept for thermoset materials. Beside the aforementioned systems, some other classical DA/rDA reactions using acyclic dienes and dienophiles [123], as well as some hetero-DA reactions [122] are potentially applicable for self-healing.

Table 1.2 Reversible covalent network formation concepts for self-healing applications.

Some other potential self-healing methods based on reversible bond formation are known, mostly disulfide bridges (SS) can be ruptured by reversible reduction into two thiol groups (SH) [62] (see Figure 1.5h). Thus, subsequent recrosslinking under re-formation of disulfide bridges via oxidative conditions enables the restoration of the initial material properties. For example, a redox-reversible hydrogel system based on thiol-modified poly(N-acetylethyleneimine) (see Table 1.2 entry 14) might use the interconversion between disulfide and thiol groups to heal the damaged area [145]. Another example can be a polystyrene-based block copolymer bridged by disulfides [62] (see Table 1.2 entry 15). The internal disulfide linkers could be cleaved under reducing conditions to thiols, separating the single blocks and thus releasing the accrued stress. Healing could occur after subsequent oxidation of the thiols re-forming the disulfide linkers.

Furthermore, another potential self-healing concept working in a similar, but slightly different way used the photoinduced reversible cleavage of allyl sulfide linkages in a polymer backbone under generation of thiyl radicals [146] (see Figure 1.5i; Table 1.2 entry 16) (RS·) which increased the chain mobility. Rearrangement of polymer chains in crosslinked rubber-like materials was thus assumed, enabling rapid stress release at ambient conditions without degradation of mechanical properties. Stress application without light irradiation resulted in a strained sample as expected, but under irradiation stress relaxation was observed due to homolytic photolysis via addition–fragmentation chain transfer reaction forming radicals by the allyl sulfide functionalities.

Other self-healing concepts based on thermally reversible cleavage of alkoxyamine bonds use a similar stress relaxation mechanism (see Table 1.2 entry 17). For example, a poly(alkoxyamine ester) was developed which is able to undergo a reversible radical exchange reaction at 60 °C by homolytic cleavage of the C–O bond [147]. Thus, these systems reduced the destructive stress within the polymeric material and are potentially suitable for self-healing applications.

Another, totally different approach for self-healing uses polycondensation reactions in order to restore the initial material properties (see Figure 1.5j). In contrast to the previously discussed reversible self-healing concepts which are, at least theoretically, infinitely repeatable, polycondensations are limited in the number of healing cycles due to the consumption of functional groups during the self-healing process. Nevertheless, they are commonly more than one-time healable and can be, therefore, attributed to the (partially) reversible concepts. However, the previously discussed example of White et al. [131] (irreversible covalent network formation concepts) based on a polycondensation reaction of two different poly­(siloxane)s with tin catalysis cannot be attributed to the reversible self-healing systems due to the necessity of encapsulation of the catalyst (see Table 1.1 entry 3). Therefore, multiple healing of a once ruptured and subsequently healed area is not possible. However, as an example, a polycarbonate-based self-healing polymer for thermoplastic materials was developed [124, 125] (see Table 1.2 entry 18) as a reversible covalent self-healing polymer, splitting the carbonate bond by thermolysis or hydrolysis. Thus, phenoxy groups with terminal hydroxy moieties are generated, which are able to react with a phenyl end of an uncleaved chain, using sodium carbonate as healing agent in order to accelerate the reaction by exchanging protons of the hydroxy group with sodium ions. This healing mechanism is limited to certain types of thermoplastic materials which enable the recombination of chain ends via condensation reactions. This and the elevated temperatures restrict the range of polymers and applications to which this technology can be applied.

1.4.2 Supramolecular Network Formation