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 from an aesthetic viewpoint and due to improved corrosion resistance. Indeed healable coatings have been put into production in recent years by manufacturers such as Nippon Paint and Bayer.

Focusing on higher strength materials, one can envisage the benefit of healable gas and water pipes. In these instances, producing new materials that can autonomously fix fractures in situ, without the need to dig up the roads, would be a clear advantage to consumers and suppliers alike. There are also more high profile (although currently lower volume) applications associated with the high strength polymers and composites that are used in performance cars, military aircraft and the current generation of passenger aeroplanes. For example, over 20% of the airframe of the new Airbus A380 'Superjumbo' is polymer composite in nature. In these situations, where safety is an overriding design criterion, it might be expected that producing a polymer that can completely regain its strength after damage would be highly desirable. The ability of a material to self-heal in an autonomous manner is especially important in composite materials, where detecting damage within the interior of these structures is inherently difficult (and less well developed) than the identification of damage sites in conventional aircrafts made from metal–alloy airframes.

In all these situations, a number of restraining factors must be considered:

Cost: In the majority of cases, new technologies decrease in cost as they become more established and are produced on a greater scale. From the outset, healable materials could also offset a higher initial outlay by increasing longevity and lowering (or even serving to eliminate) routine maintenance costs, which are especially high for safety critical situations such as transport.

Performance compared to conventional materials: Introducing healabilty into a functional component must not compromise the strength or increase the density of the component. A healing polymer that is double the density of its conventional (non-healing) predecessor will not be suitable for many applications, for example transport, where the energy required to move the heavy healing system will be seen to outweigh any potential safety advantage. This is particularly important when considering the mode of healing at the molecular level. Conceptually, introducing latent, unpolymerised monomers and functionalities into a material that can form new bonds in order to heal a fracture is a pleasing potential solution, but one must consider how much stronger the material would have been if the latent functionalities were fully polymerised in the original formulation.

1.3 Categorisation of Healable Materials

As discussed, the simplest criterion of a healable material is that it is able to regain its strength after a damage event. This can be visualised by plotting strength of the polymer as a function of time (Figure 1.1). In this simplified example, the sample remains undamaged between time points A and B, indicated by the constant strength of the system. At time point B, the material is damaged, resulting in an instantaneous loss of strength (point C on the plot). Subsequently, the sample heals, and the strength of the material increases to match that of the pristine, undamaged sample (time point D). The rate of healing is indicated by the gradient of the slope between time points C and D. After healing, the strength remains constant for the remainder of the time.

A truly ideal healable material may have additional properties. For example, it may be able to heal on multiple occasions (Figure 1.2). These break/heal cycles may have several effects on the material. Figure 1.2A shows the strength versus time plot for an ideal sample that can regain its strength completely over three break/heal cycles. Figure 1.2B shows the situation encountered more frequently, whereby the sample only partially recovers its strength after each damage event. Thus, ultimately, the performance of sample will degrade, but, depending on the timescales involved, this may be adequate to fulfill the typical lifecycle of the product.

This simple treatment immediately raises several pertinent issues concerning healable material characterisation, some of which are highlighted:

i) How strong is the pristine material?

ii) How significantly can the material withstand damage yet still perform adequately with respect to its use?

iii) How quickly can it regain its strength?

iv) Under what conditions does the material heal?

v) How many break/heal cycles can the material undergo? and

vi) How easily can the site of damage in the polymer-based assembly be accessed?

The targets values for many of these questions will be application dependent. For example, the strength and number of healing cycles that a mobile phone case may be expected to pass through in a typical lifetime will be very different from that of a polymer component embedded deep within the wing of an aeroplane. However, a key universal consideration is what parameters should be measured to quantify healing in a polymeric system, and how they should be reported. The following two sections provide an overview of these parameters from a synthetic chemistry perspective, to provide an insight into some of the most common techniques and measurements reported for this rapidly evolving field of research.

1.4 Healing – Definitions

Although the initial design criterion for a healable material is the production of a component that can completely regain its physical properties after sustaining damage, there are many practical situations where this may not be possible or indeed necessary. In these situations, it is necessary to quantify the loss in performance of the healed material. This is most frequently expressed as the 'healing efficiency' (ηeff) of the material. The healing efficiency is a dimensionless parameter; it is the ratio of a specific mechanical property of the material before and after healing and is expressed as a percentage. It is given by Equation (1).

Healing Efficiency, ηeff = [Mechanical Value (healed)/Mechanical Value (pristine)] 𵿣 100 (1)

Thus materials with ηeff approaching 100% (note: it is possible to have ηeff >100%) have regained the strength of the pristine material, whereas a low value for ηeff indicates that the material has not regained significant strength. Healing efficiency is therefore an easy parameter to obtain and clearly indicates the success of a healing process, but the simplicity of this term can lead to misleading results, especially when only a single healing parameter is measured. In a typical break/heal experiment, a sample will be stretched to its breaking point and a variety of mechanical properties measured, for example: tensile modulus, elongation to break and modulus of toughness. After healing, the sample may, and indeed frequently does, exhibit a higher ηeff value for one of these parameters when compared the other two. Thus only by measuring and quoting ηefffor all of the appropriate parameters for a specific polymer can a true assessment of the nature of the healable material be made.

A further complication for a researcher new to this relatively young field is that there is not a standard definition of what actually constitutes a damaged material. In this respect, papers describe ηeff values that may have been calculated for the healing of a single microscopic crack in a bulk polymer, or, alternatively, after a material has been physically broken into multiple sections and the parts then separated and rejoined prior to obtaining the healing efficiency data. To this end, the term 'healing' serves to cover several repair scenarios and is evidently subject specific; readers of articles in this area must always bear this caveat in mind.

1.5 Measuring Healing

Testing procedures for healable polymers have been inspired by the well-established areas of materials chemistry and engineering and broadly fall into two categories:

i) mechanical load testing where a monotonic force is applied;

ii) rheological tests where an oscillating force is applied.

Both these experimental protocols have been used to assess the healing properties of new polymeric materials. The most frequently studied of these techniques are covered in the following sections and also in Chapter 2.

1.5.1 Cantilever Beam Tests to Determine Healing

Cantilever beam (CB) tests are suited to testing non-elastomeric (brittle) samples, generally where the glass transition temperature (Tg) of the material is significantly above the testing temperature, or the polymer is highly cross-linked.

Mechanical load testing requires fabrication of test samples with typical maximum dimensions of the order of 100 × 75 × 7 mm (typically requiring 50 g of material). The geometry of the sample depends on the precise experiment (see Figure 1.3), with the four most common forms being:

i) tapered double cantilever beam (TDCB);

ii) compact tension (CT);

iii) single edge notch bend (SENB) and

iv) single edge notch tension (SENT).

In each experiment, the sample is prepared by manually forming a pre-crack, typically by scoring with a razorblade. The distance between the tip of the precrack and the applied force is termed apristine (Figure 1.4). The sample is then subjected to a force which induces a crack of length apropagated. During this fracture event, the mechanical properties of the pristine material can be obtained. The load is removed and the sample is then left to heal. Whilst unloaded, the fracture faces will come into close proximity, facilitating healing; during this period, the crack reduces in length, resulting in a new distance between the fracture tip and the applied force (ahealed). The sample is then tested again and the healing efficiency calculated by comparison of the response of the pristine and healed samples.

The results from healing experiments are presented as a plot demonstrating how applied load varies as a function of displacement, which is generally only a few millimetres. Representative load versus displacement plots for a break/heal experiment from samples in the TDCB geometry are shown in Figure 1.5.