Healable Polymeric Materials

BARNABY W. GREENLAND AND WAYNE HAYES

Department of Chemistry, The University of Reading, Whiteknights, Reading, RG6 6AD, UK

1.1 Introduction

Progress in technology which serves to improve living standards and increase life expectancy is frequently linked to the materials that we humans have learnt to master and manipulate. The connection between the basic fabrics that tools can be made from and the progression in human development has become so intertwined that these advances have come to define specific eras: the stone age, bronze age and iron age. For the past 40 years or so, the role that silicon has played in advancing man's ability to address significant challenges (perhaps, most notably the lunar landings from the 1960's and 1970's) via computer technologies cannot be underestimated. However, there is also a growing consensus that the current period may come to be known at the Plastic Age. From the seminal discoveries of Staudinger and Carothers in polymer science during the 1930's, carbon and inorganic-based polymeric products have proliferated through the modern world, finding applications in all areas from inexpensive disposable packing materials to life enhancing hip replacements and life saving body armour.

In 2010, sales of raw polymeric materials topped &8364;117Bn in the European Union, for the first time equalling the value of petrochemicals sold in the region. With demand for polymeric products growing even whilst the cost of the crude oil rises, there is a clear need to move away from the culture of disposable products that society has become accustomed to. This can be achieved one of several ways: producing more polymers from renewable feedstocks; increasing the proportion of recycled polymers in circulation and increasing the lifespan of the polymeric products. It is in the latter two potential solutions that healable materials have most to offer. As shall be discussed in Chapters 3 and 4, the reversible nature of both covalent and supramolecular bonds is frequently exploited in producing healable materials that lend themselves to efficient recyclable materials. Producing materials that can heal either small cracks or major fractures will have a significant impact on the longevity of a host of polymeric products, from sunglasses to aeroplanes.

The diverse nature of applications for polymeric materials has occurred because of the chemist's increasing ability to design and synthesise new monomers and polymeric architectures (for example: multi-block co-polymers, branched or network materials), delivering products with useful functionality and physical properties. As synthetic techniques have progressed, the materials scientist's 'toolset' for characterising the materials at the micrometre, nanometre and angstrom scales has also improved, so the ability to successfully predict and measure the properties of new polymeric materials improves year-on-year. This structure–property interplay serves to increase the speed at which innovative materials with step-changing properties can be conceived, produced and brought to the market, further enhancing the modern world.

In the formative years of the field of polymer science, in the late 1930's and early 1940's, the primary driving force of the research carried out was to produce new materials whose properties (i.e. strength and thermal stability) were suitable for producing inexpensive items that could be mass produced, i.e. polystyrene cups, PET water bottles, strong fibres for rope and vulcanised rubber for car tyres. Recently, however, research has focused on producing high value items whose properties dictate the requirement for new polymers in order to fulfill an ever expanding number of roles, for example: shape memory materials, photo-conducting or luminescence devices and composite polymers that exhibit strength surpassing that of the strongest metal alloys. By their very nature, such polymeric materials frequently necessitate a multidisciplinary approach to their study, requiring close collaboration between the synthetic chemist — who can generate and manipulate the materials that differ at the atomistic level — and the engineer, who builds the device to test the product (i.e. a working solar cell or printed circuit board). Optimisation of these complex systems necessitates multiple iterations, taking many man years of effort before finally settling upon a suitable balance between the time, manpower and synthetic and fabrication costs required to attain the desired design criteria.

This interdisciplinary work ethos is clearly applicable to the field of healable polymer research. The most basic definition of a healable material requires a polymer with a given strength to be damaged, reducing its physical properties, and then, at a later point, to have regained some of the lost strength. Any research plan will require at least:

i) the design, synthesis and characterisation of new polymers;

ii) a method to fabricate a sample suitable for mechanical testing;

iii) a mechanical test to assess the pristine sample and healing nature of the material and

iv) an iterative development cycle whereby data from the mechanical testing will feed back into the production of the next generation of materials.

In order to complete the iterative development loop successfully, from a chemist's perspective, it is important that whilst the chemistry of the new material is fully understood, it is also imperative to have an understanding both of the mechanical tests required to demonstrate healing and of the terminology and conventions used by engineers. After summarising the potential benefits of producing commercially viable healable polymers, this chapter will define the requirements and different categories of healable materials, before providing an overview, aimed at the practising chemist, concerning the techniques currently employed to study the healablity of these fascinating new materials.

1.2 Healable Polymers – Potential Applications

While producing healable materials is an intuitively rewarding challenge, it is worth considering why developing healable materials may be beneficial in differing circumstances. As noted previously, polymeric materials are now used across a wide range of applications and fulfill a multitude of roles. In many cases, the polymer itself may play little part in adding to the strength of the product but may simply provide an attractive finish, such as the outer surface coating on a modern car. In these circumstances, small scratches caused by wear and tear are unlikely to alter the safety of the passengers. However, coatings that can regain their original luster after damage are of considerable commercial importance both...