11.1.1 Polyurethanes and thermoplastic polyurethanes

Polyurethanes, first discovered by Otto Bayer in 1937, encompass the series of polymers whose molecular backbone contains a significant number of urethane linkages, regardless of the content of the rest of the macromolecule.¹ The urethane linkage is formed via the reaction between an isocyanate group and a hydroxyl group. Initial studies into these polymers focused on forming linear polyurethanes by reacting diisocyanates with diols, but it was very quickly realized that a multitude of polymers with wide-ranging properties could be produced.² Early work focused on polyester-based polyols; however, the enhanced hydrolytic stability and immense versatility afforded by polyether-based polyols saw them become a preferred precursor in polyurethane synthesis.² These days, the vast selection of polyols, isocyanates, and chain extenders allows polyurethanes to be varied from soft thermoplastic elastomers to adhesives, coatings, flexible foams, and rigid thermosets. This chapter will focus solely on thermoplastic polyurethane (TPU) elastomers as host polymers for polymer nanocomposite systems. These TPUs are often called ‘segmented polyurethane block copolymers,’ and form a subset of a large and commercially important family of thermoplastic elastomers (TPEs) comprising styrene block copolymers (SBCs), thermoplastic olefins (TPOs), thermoplastic vulcanizates (TPVs), and copolyester elastomers (COPEs). TPEs combine the flexibility and resilience of rubbers with the processability of plastics. Global demand for TPEs is forecast to increase 6.3% pa through 2011 to 3.7 million tonnes. TPU materials currently comprise approximately 15% of the volume of TPE polymers sold annually.³ They are at the premium end of this class of materials and are known for their high strength, durability, and tremendous versatility in terms of tailored formulations.

Some common applications for TPUs include automotive interiors, footwear, flexible hose and tubing, cellphone buttons, closures, seals and o-rings, adhesives, cable jacketing, sport and leisure items, textiles and textile coatings, implantable medical device components, mining and mineral processing equipment, laminates for impact glazing, photovoltaic cell encapsulation, and wastewater treatment equipment, just to name a few.

The clever incorporation of functional nanofillers of various types into TPU host polymers can extend typical TPU property profiles, improving mechanical and tribological performance,⁴’⁵ dimensional stability at higher operating temperatures,⁶ gas barrier and flame retardancy performance,^7,8 electrical properties,^6,9 and biological properties.^10–12 This chapter reviews some of the exciting progress being made in these areas.

11.1.2 TPU chemistry, morphology, and properties

TPUs are linear, block copolymers of alternating hard and soft segments. The soft segments (SSs) are composed of long-chain diols (most typically polyester, polyether, or polycaprolactone-based) with a molecular weight of 1000–4000 g/mol, whilst the hard segments (HSs) commonly consist of alternating diisocyanates and short-chain extender sequences.¹ Thermodynamic incompatibility between the segments results in phase separation, and subsequent organization into hard and soft domains with a nanoscale texture, which gives TPUs their distinct mechanical properties and thermoplastic utility.^13–15

The flexible, amorphous soft domains primarily influence the elastic nature of the TPU, but still have some influence on the hardness, tear strength, and elastic modulus of the polymer. They are typically above their glass transition temperature (T_g) at room temperature and thus control TPU performance at low temperatures. In contrast, the rigid and typically semi-crystalline hard domains primarily influence elastic modulus, hardness, tear strength, and melt processability.¹ However, the hard domains also act as physical cross-links, imparting elastomeric properties to the soft domain phase.¹⁶ Thus, the mechanical properties of TPUs are strongly influenced by the complex microphase morphology of the hard and soft domains. These are, in turn, dependent on such factors as the hard/soft segment composition ratios, the segmental solubility parameters and associated compatibility, molecular weight and polydispersity of the hard and soft segments, and the thermal and processing history of the TPU. ^17–23 The unique microphase morphology confers on TPUs a higher tensile strength and toughness when compared with most other elastomers,²² and the absence of covalent cross-linking allows TPUs to be both melt and solution processed. However, under continuous or cyclic loading the absence of chemical cross-linking gives way to a degree of plastic flow whereby the hard domains can restructure, which results in large hysteretic losses and poor creep resistance.²⁴’²⁵ The clever introduction of nanofillers into this complex nanoscale morphology, and the characterization of the level of interplay between the various system components, represents a fascinating challenge for current researchers. Similarly, the typically moderate hard domain T_g in these systems seriously limits the upper use temperatures and high temperature performance and chemical or dimensional stability in aggressive environments. Again this is believed to be an area where innovation in TPU nanocomposite systems can effectively address these limitations.

11.1.3 TPU nanocomposites versus traditional microcomposites

The polymer industry is continually researching to find new materials that offer increased performance at lower costs, often necessitating the introduction of fillers. There have been various attempts to improve the mechanical performance, creep resistance, and permanent set of TPUs either by varying the composition of the hard and soft segments^19,26 or by introducing macro- or micro-sized particulate fillers,^27,28 with the majority of these studies yielding suboptimal results. Generally microfillers significantly stiffen TPUs and also substantially reduce elongation to break. They can also be associated with major wear of polymer processing equipment and dies due to the high loading levels required (up to 40% by weight) and the highly abrasive nature of these fillers.

Nanofillers offer significant advantages over macro-sized or micro-sized fillers, including vastly superior reinforcement efficiency, a greater surface area to mass ratio, low percolation threshold, and often very high aspect ratios, and as such there has been significant research and investment into the production of polymeric nanocomposites. Recently, a range of nanoparticles has been used in TPU-based composites, with varying levels of success. The bulk of this work has involved the use of carbon nanotube or layered silicate-based nanofillers.

11.2.1 Mechanical influence of nanofillers on TPU stiffness and ultimate tensile strength

Figure 11.1 and 11.2 compare the mechanical properties of many of the best literature examples of TPU nanocomposites reinforced with layered silicate nanofillers or carbon nanotubes or nanofibres. We have chosen only the most impressive results from the academic and commercial literature. This is because we found that many studies, while presenting large percentage changes as a result of nanofiller reinforcement, did not benchmark particularly well to typical commercial TPU property ranges. While clearly there are many other material properties of great interest for TPU nanocomposites which will be discussed elsewhere in this chapter, these figures do present some very noteworthy trends.

First, Fig. 11.1 plots ultimate tensile strength versus elastic modulus. Even taking into account obvious differences in the particular TPU matrix materials, nanofillers and nanofiller loadings used in each study, the data clearly shows a more pronounced stiffening effect for carbon nanotubes and a more pronounced strengthening effect for layered silicates. The one exception is the very stiff nanocomposite prepared by solvent exchange at an unusually high loading (20 wt%) of unmodified synthetic hectorite.²⁹¹ The highest tensile strengths achieved (anything above 45 MPa is considered very good in the industry) were all layered silicate reinforced systems, with the strongest being prepared by reactive extrusion.³⁰

Figure 11.2 plots tensile strength against the more industrially acceptable parameter for TPUs of stress at 50% strain. The grouping of the layered silicate data in the top right quadrant tells us that nanofiller-TPU molecular interactions are more substantial and perhaps ‘cooperative’ during deformation in layered- silicate reinforced systems, and that carbon nanotube (CNT)-based nanofillers need further optimization for utility in TPU systems.

A TPU property profile which engenders strength and toughness while maintaining flexibility and resilience has been traditionally very difficult to achieve via the composite approach due to substantial stiffening, and moving to softer TPU matrix polymers is not possible because they tend to suffer from a lack of strength due to the limits of segmental demixing evident at low hard segment composition ratios. In addition, typically if a stiffer, harder TPU is required, a higher hard- segment composition ratio can readily be formulated. Therefore stiffening of this class of polymers without the accompaniment of other improved functional properties or performance benefits is not generally of commercial interest.

11.2.2 TPU nanocomposites employing nanofillers other than layered silicates

There are reports in the literature of TPU nanocomposites incorporating nanosilica in the form of either fumed silica,^31,32 to improve the mechanical or adhesive properties of TPUs, or in the form of polyhedral oligomeric silsesquioxane POSS components to improve either biostability¹⁰ or resistance to thrombosis.^10,11 Magnetite nanofillers have recently been dispersed and solution cast to produce TPU nanocomposite films with superparamagnetic properties/ and zinc oxide- TPU nanocomposites have recently been prepared and fabricated into electrospun web constructs, which demonstrated enhanced UV absorption and antimicrobial performance for potential use in protective clothing.¹²

There is also a growing body of work on TPU nanocomposites incorporating single-walled carbon nanotubes (SWNTs),³³ double-walled carbon nanotubes,³⁴ and multi-walled carbon nanotubes (MWNTs).⁸ The manipulation and incorporation of large numbers of individual CNTs into polymeric systems is a difficult task because the high molecular weights and strong intertube forces (both van der Waals and electrostatic) promote the formation of micron-sized bundles and ropes,i⁵ leading to phase separation and poor mechanical properties. As a result both surfactant and covalent functionalizations have been employed in an attempt to fragment and disperse these micron-sized inclusions.^33,36–40 The majority of these TPU-CNT nanocomposites have been formulated via solution processing techniques,⁸’³³’^41–49 although in situ polymerization⁴⁶’^50–53 and melt processing⁸ have been reported as well. Despite the superior mechanical performance of SWNTs, MWNTs are the most common variety of CNT studied and a wide variety of surface functionalizations has been investigated. Koerner and co-workers reported the first study into TPU-CNT nanocomposites, in which the researchers attempted to improve the stress-recovery characteristics of Morthane (a commercially available, shape memory TPU) by adding 1–5 vol.% MWNT via solution processing.⁴⁵ The researchers demonstrated that the nanocomposite could store and subsequently release up to 50% more recovery stress than the unmodified TPU. The nanocomposite also demonstrated the ability to recover stress, above that of the host TPU, when exposed to increased temperatures, infrared radiation, and electrical current. The authors theorized that the absorption of energy by MWNTs caused localized heating, which in turn melted the strain-induced TPU soft segment crystals, increasing stress recovery.

Sen and co-workers were the first to investigate the fabrication of TPU nanocomposites incorporating functionalized SWNTs³³ The solution-processed nanocomposites demonstrated significantly increased mechanical properties over the host TPU. Whilst the final material was mechanically inferior to several commercially available TPUs without nanoparticle reinforcement, the study was the first to demonstrate both the possibility of increased dispersion in a TPU-CNT nanocomposite and that TPU-CNT interactions can be enhanced by CNT functionalization.

As a result, the majority of research into TPU-CNT nanocomposites has focused on using functionalized CNTs; however, there have been several studies published describing the use of pristine CNTs.^8,43,48 Importantly, these investigations have demonstrated poor CNT dispersion and phase separation between the SWNTs and the host TPU and suggest that the future of TPU-CNT nanocomposites lies with modification of the CNT surface. A wide variety of functional groups have been attached to CNTs in an attempt to overcome these problems. CNT surface modifiers are typically either short hydrophilic groups like COOH,^47,49 NH₂,⁴⁶ and OH⁴¹ capable of significant hydrogen bonding with the host polymer, or long- chain hydrophobic groups designed to break apart CNT aggregates and increase dispersion.^33,54 Recently, significant results have been achieved by Deng and co-workers,^42,55 who demonstrated dramatic mechanical enhancements with very small loadings of MWNTs modified with novel functional groups. Several research groups have also attempted to use CNTs as cross-linking agents, covalently bonding individual CNTs to the TPU backbone. However, results have been mixed due to the fine stoichiometric control required to produce nanocomposites with the same molecular weights and hard/soft segment ratios as the control TPU.^50,52

Despite the abundance of work into TPU-CNT nanocomposites, the only published investigations of TPU-functionalized CNT nanocomposites formulated by melt processing techniques have been conducted by Chen and co-workers⁵⁶ and more recently Barick and Tripathy.⁵⁷ Chen et al. demonstrated that acid-treated MWNTs had dramatically increased mechanical properties with an eightfold increase in Young’s modulus and a 2.4-fold increase in tensile strength due to the strong interfacial interactions and good MWNT dispersion within the melt-drawn fibers. Barick and Tripathy reported substantial stiffening and toughening of a soft aliphatic grade of TPU by melt compounding vapor-grown carbon nanofibres (VGCNF), which had been purified, treated with acid and heat, into the host polymer. They also observed significant shifts in TPU thermal transitions and thermal stability, suggesting strong interfacial effects. Significantly, these results corroborate computational studies that suggest melt processing may induce a residual thermal radial stress (through differences in the coefficients of thermal expansion between the CNT and the host polymer), forcing the polymer into closer contact with the tube surface and thereby enhancing molecular interactions.⁵⁸

11.2.3 TPU layered silicate nanocomposites

The vast majority of work on TPU nanocomposites has involved the use of layered-silicate-based nanofillers. Therefore, the remainder of this chapter will focus upon this class of nanocomposites and their chemistry, processing, structure, performance and safety or nanotoxicology.

11.2.4 Layered silicates as nanofillers

The incorporation of layered silicates into polymer matrices has been performed for over 50 years. During recent years, polymer nanocomposites containing natural clays and synthetic layered silicates have been most widely investigated, since the starting clay materials have been readily available in various sizes and shapes, and because their intercalation chemistry and interactions with surfactants and polymers have been studied for several decades.⁵⁹ In particular, the development of instrumentation to characterize polymer nanocomposites at small length scales, such as scanning force, laser scanning fluorescence, and electron microscopes, have spurred researchers into probing the influence of particle size, shape, and surface chemistry on the properties of these new materials.

11.2.5 Fabrication techniques for TPU-clay nanocomposites

Typically smectite-type clays are usually used as fillers. Both natural clays (montmorillonite and hectorite) and synthetic clays (synthetic hectorites and fluoromicas) have been investigated, and TPU nanocomposite processing has seen many different approaches. The most common dispersion methods⁶⁰ are solvent casting,⁶¹ melt processing,⁵⁷ in situ polymerization,⁶² and reactive extrusion.^30,38

In solvent casting, a polymer is solubilized in an organic solvent, then the nanoclay is dispersed in the resulting solution, often assisted with high shear homogenization or ultrasonic energy. The solvent is then allowed to evaporate, leaving the nanocomposite behind, typically as a thin film. The solvent imparts the enhanced mobility the polymer needs in order to intercalate between the silicate layers, while thermodynamic compatibility combined with physical mixing gives rise to a dispersed system. There are limitations to solvent casting. The selected solvent must be able to completely dissolve the polymer and disperse the nanoclay. This approach can also lead to poor clay dispersion, as well as other problems such as high costs due to the large amounts of solvent required to achieve appreciable filler dispersion, technical phase separation problems, and health and safety problems.^60,63

In situ polymerization involves the dispersion of clay layers into the matrix by polymerization, mixing the silicate layers with the monomer, in conjunction with the polymerization initiator or the catalyst. This method, which was used by Usuki et al.⁶⁴ in their pioneering research, takes advantage of the relative ease with which a small monomer molecule can intercalate silicate layers compared with a much larger polymer molecule.

Melt processing refers to the process of single or twin screw extrusion, whereby polymer chains are melted to increase their mobility and then the nanoclays are dispersed into the melt to allow intercalation of the nanoclays. Twin screw extruders or Brabender mixers require much higher shear energy, so care is needed when processing thermally sensitive TPU materials, as aggressive melt processing inevitably leads to a loss in molecular weight and a reduction in properties and performance. The selection of thermally stable organoclay modifiers is paramount, and careful washing away of excess exchanged alkylammonium cations is also important to avoid unwanted TPU degradation in the extruder.⁶⁵ Melt processing of TPU nanocomposites can be easily scaled to manufacturing quantities, but care must be taken to avoid TPU degradation.⁶⁶

Reactive extrusion of TPU involves the in situ polymerization of TPU precursors (polyol, diisocyanate, and a chain extender) in a twin screw extruder. Nanofillers can be either side fed as dry powders or pre-dispersed into the polyol liquid precursor.^30,38 This method is used for chemical modification of existing polymers and the function of the extruder is as a continuous chemical reactor for polymerization.⁶⁷ This technique requires chemical reaction control besides conventional screw extruder parameter control (of materials conveying, melting, and mixing). Compared with conventional solution casting, reactive extrusion has the following advantages: no solvent is required, high-viscosity polymers can be used, and processing conditions, such as mixing time and temperature, are flexible.⁶⁷ Importantly, it avoids TPU reprocessing and thermal degradation, potentially avoids the handling of dry, inhalable nanopowder feeds, and effectively shifts the TPU nanocomposite processing route further up the TPU supply chain.³⁰ The majority of commercial TPU materials available today are manufactured using reactive extrusion.

11.2.6 TPU clay nanocomposite structure–property relations

The dispersion of clay nanolayers and the morphology of the nanocomposite depend on various factors, such as the nature of the components used (layered silicate, surface modifier, and polymer matrix) and method of preparation (mixing method, time, and temperature).^63,68,69 Generally, four main nanocomposite structures can be obtained, as illustrated in Fig. 11.3.⁶⁰

In an intercalated structure, the polymer chains are sandwiched in between the silicate layers, which display limited dispersion. On the other hand, exfoliated nanocomposites demonstrate separated, individual silicate layers, which are more or less uniformly dispersed in the polymer matrix. The ordered structure is lost and the uniform dispersion of the anisotropic nano-sized particles can lead to a large interfacial area between the constituents at extremely low loadings of the nanoparticles. This large interfacial area and the nanoscopic dimensions between constituents differentiate polymer nanocomposites from conventional composites and filled plastics. Hence, new mechanical, optical, and electrical properties can be developed, which may not occur in the macroscopic counterparts.⁶⁰ In fact, exfoliation of the platelets is the desirable objective of dispersion, because it achieves optimum enhancement of the properties. As well as these two well- defined structures, other intermediate organizations can exist presenting both intercalation and exfoliation.

Compared with the large body of research into polymer-clay nanocomposites, the number of studies conducted on TPU nanocomposites is relatively small. The use of TPU as the nanocomposite matrix presents some interesting challenges in understanding the nanoscale and microscale morphology, due to the pre-existing nanoscale morphology of the segmented TPU domain (the so-called soft and hard segments). TPU nanocomposites have been prepared predominantly through 3n situ polymerization,⁶²’^70–74 solvent casting,⁶’^75–77 melt processing,^78,79 and to a lesser extent via reactive extrusion.^80,81

Previous research shows that TPUs can be tailored to meet the specific property requirements by incorporating organically modified smectic clays. The first examples of TPU nanocomposites with greatly improved performance were reported by Wang and Pinnavaia.⁶² The nanocomposites were prepared via in situ polymerization and alkylammonium-exchanged montmorillonites were used as nanofillers. They showed that the tensile strength, modulus, and strain at break all increased by more than 100% at a loading of only 10 wt% organoclay.

Meng et al.⁸² employed different molecular weight polyether-based TPUs and organically modified montmorillonite (OMMT) in their investigation of the exfoliation of OMMT in a TPU matrix by melt processing. They concluded that optimized exfoliated nanocomposites can be obtained if there is adequate shear stress and appropriate molecular diffusion to accommodate interaction between the OMMT and the TPU.

Low permeability TPU nanocomposites were produced by Runt and co-workers⁶¹ using polyether-based TPU and octadecylammonium-modified montmorillonite (MMT) by solvent casting. The water vapor permeabilities of the nanocomposites were reduced by as much as a factor of five at the highest OMMT content (20 wt%) and the enhancement of modulus was achieved without loss of ductility.

Cai et al.⁸¹ produced TPU-MMT nanocomposites by reactive extrusion, which involved a one step direct polymerization-intercalation technique with a twin screw extruder. Morphological characterization of the nanocomposite indicated that there were delaminated nanocomposite structures with well-dispersed nanoclays in the TPU matrix. Improvements in thermal stability, flame retardancy, and tensile strength were achieved with the addition of up to 4 wt% MMT.

Other successful TPU nanocomposites have been reported in the following publications. Depending on the system, increases in modulus⁶²’⁷⁵’⁷⁷’⁸³’⁸⁴ tensile strength,^{70,74,75,78,83} elongation,^6,74 flame resistance and thermal stability,^70,72,83 barrier resistance,^70,75,83 tear strength,⁸⁵ abrasion resistance,⁷² fracture toughness,⁷² fatigue resistance,⁸⁶ and corrosion resistance⁸⁷ have been reported.

11.2.7 Hard and soft segment interactions with nanofillers

Knowledge in the TPU nanocomposite area has progressed recently with several research papers discussing the effects of nanofillers on the underlying complex TPU morphology, highlighting the importance of understanding the specific hard and soft segment interactions with the nanofillers in correlation with the resulting nanocomposite properties. Some of the literature in this field is summarized in Table 11.1.

The nanofiller in a nanocomposite should be distributed on a nanoscale level and therefore undergo substantial interactions with polymer chains and sequences. There is therefore tremendous potential ‘reinforcement efficiency’ due to the very high nanofiller aspect ratio and surface area, the local confinement of molecular motion,^88,89 and the tailoring of specific hard or soft segment interactions with the nanofiller surface.^34,90,91 However, due to the layered structure of silicate nanofillers, the organization of the resultant nanocomposite depends on the thermodynamic compatibility of the polymer and the layered silicate, the intercalation kinetics, and the processing conditions.^65,92 The types of structures encountered are shown in Fig. 11.3. Virgin layered silicate particles are naturally hydrophilic, and as such are largely immiscible with TPUs, which contain a significant hydrophobic component in the form of the soft segments. This natural repulsion between the untreated silicate and the TPU discourages the formation of an exfoliated system, which is required for an effective nanocomposite. Instead a phase-separated composite or microcomposite is formed, whose properties do not exceed those of traditional composites.

In order to prepare effective nanocomposites, the hydrated cations in the interlayer spacing are replaced with organic cations to render the surface more organophilic and improve wetting by the polymer matrix^60,93 Furthermore, this substitution allows swelling of the gallery between the silicate layers, which in turn allows the polymer to enter the galleries during the formation of the nanocomposite. An unswollen silicate has an interlayer spacing of the order of 1 nm, which is smaller than the host polymer’s radius of gyration.^94,95 The control and enhancement of both the polymer-silicate interaction and the swelling of the gallery layers are thus critical factors in the production of any polymeric nanocomposite, particularly so for TPUs, which generally possess both hydrophilic (hard segments) and hydrophobic (soft segment) components, both of which are of a length scale much smaller than even the smallest synthetic clay-based nanofillers (~ 25 nm).

In 1998, Wang and Pinnavaia⁶² reported the first examples of TPU-layered silicate nanocomposites. The nanocomposites were prepared via in situ polymerization using montmorillonite (ion exchanged with alkylammonium salts) as the nanofiller. X-ray diffraction (XRD) patterns indicated that intragallery polymerization contributed to the dispersion of the organosilicates, and improvements in tensile strength, elongation, and modulus were observed.

The commercially available Cloisite® series of organosilicates have been used in a large number of TPU nanocomposite studies. Earlier studies employed solvent casting^54,61,96,97 to assist in overcoming some of the abovementioned thermodynamic barriers, and also to avoid complications due to thermal degradation of the alkylammonium modifiers.^98,99 Interestingly, the more hydrophobic Cloisite 15A generally did not disperse well, although increases in tensile and barrier properties^96,97 were reported with this nanofiller. Finnigan et al. reported that the more hydrophilic Cloisite 30B produced nanocomposites with relatively better dispersion, and very large associated increases in stiffness, tensile hysteresis, and permanent set.⁷⁷ This increase in hysteresis in nanocomposites has not been well documented in the literature, but is an important observation because many dynamic applications of TPUs require that resilience is maintained. This study was also the first to directly compare identical TPU nanocomposite formulations prepared via both solution and melt processing. However, the very small lab scale conical twin screw extruder employed caused extreme thermal degradation and loss of ultimate properties.

Due to the more commercially scalable nature of melt compounding, several workers have investigated TPU-Cloisite® nanocomposites prepared via melt compounding on a larger scale.^7,57,100,101 The relatively hydrophilic Cloisite® 30B and Cloisite® 10A generally disperse and delaminate well in TPUs.^7,54,57 This is due to their respective polar hydroxyl and benzyl functionalities, which promote thermodynamically favorable enthalpic driving forces, such as H bonding, for intercalation.^54,34,103 Cloisite® 25A, in the middle of the hydrophilicity/ hydrophobicity range, has also resulted in good dispersion and property enhancement.^75,83 Dan et al. achieved the best property improvements with Cloisite® 30B, and also found that higher tensile properties could be achieved in an ester-based TPU with respect to an ether-based TPU, despite better dispersion in the latter.¹⁰² They also reported the influence of some nanofillers on hard/soft segment demixing and morphology, as well as a decrease in observed properties after a second melt processing step, implying thermal degradation.

Chavarria and Paul¹⁰⁰ further probed the effect of organoclay structure by modifying a series of montmorillonite-based nanofillers with various alkylammonium surfactants, and studying the resulting structure–property relations for a high-hardness ether-based versus a medium-hardness ester-based host TPU. They concluded that for both host TPU systems: (a) one long alkyl tail on the ammonium ion rather than two, (b) hydroxyl ethyl groups on the amine rather than methyl groups, and (c) a longer alkyl tail as opposed to a shorter one, all lead to improved clay dispersion and TPU nanocomposite stiffness. However, although the harder ether-based (and presumably higher melt viscosity) TPU host polymer had better nanofiller dispersion, the percentage increase in properties remained higher in the softer TPU. Meng et al.⁸² also showed that the degree of dispersion of the nanofiller (Cloisite 30B) was improved by processing the TPU host so that it had a higher molecular weight and higher viscosity due to improved shear. They also showed that an optimum Brabender melt processing time was required to achieve the highest combination of nanofiller dispersion and tensile stress at 500% strain, due to excessive thermal degradation at longer processing times. Ma et al. organically modified a less well-known class of layered silicate, rectorite, and performed melt compounding studies on a polyester-based TPU and reported substantial increases in both tensile strength and tear strength.¹⁰¹ Most interestingly, the strongest formulations did not necessarily provide the best tear strength, suggesting that toughening mechanisms in these systems are complex and that non-delaminated clay tactoids may play more of a role in toughening than discrete organoclay platelets.

Whilst much of the research has focused on ionic substitution of bulky organic surfactants, several attempts at covalent modification of the silicate surface have been attempted.⁷⁴’¹⁰⁴ An initial report from Tien and Wei⁷⁴ utilized organosilicates with one to three hydroxyl groups per surfactant molecule, which were capable of participating in the polymerization process and promoting delamination from within the layers. Since then, other researchers have used this approach with Cloisite® 30B.¹⁰⁴ The tethering of TPU chains to the organosilicate surface has also been used to achieve a strong interfacial bond. However, it is difficult to reach the full potential for property improvements via this route, because the isocyanate groups can also react with bound water in the silicate interlayer, and with hydroxyl groups on the silicate edges, which, if not correctly accounted for, can lead to reduced molecular weight and matrix cross-link density.¹⁰⁵ The size of the layered silicate has been shown to influence the resulting nanocomposite morphology. Due to the relative scale of the hard domains compared with the most common silicates employed, few groups have investigated this area. However, research into smaller silicates has shown tensile strength can be significantly enhanced without significantly altering Young’s modulus or resilience.⁷⁸’¹⁰⁶ In particular, the number of polymer-filler interactions and entanglements increases as the platelet size decreases, because the smaller platelets have a larger surface area and a smaller interparticle distance. This can have a significant effect on the mechanical properties, particularly at high strains where smaller platelets can more readily rotate without overcrowding, and therefore can more readily interact with the matrix by communicating in shear. Finnigan et al⁷⁸ clearly demonstrated this by performing in situ strained synchrotron small-angle X-ray scattering measurements on a series of solution-cast TPU nanocomposites incorporating organically modified fluoromica nanofillers of controlled aspect ratio, which had been prepared via high energy milling. In a follow-up study they showed that superior resistance to stress relaxation could also be achieved by the incorporation of low aspect ratio organoclays. ¹⁰⁶ They explained that, as the particle size and aspect ratio of the filler increase, the TPU chains in the interfacial region are more restrained and experience greater localized stresses, which result in an increase in strain-induced slippage at the polymer-filler interface. This technology has been patented,¹⁰⁸ and a startup company, TenasiTech Pty Ltd, formed to commercialize the technology. TenasiTech has now substantially scaled up TPU nanocomposite production via reactive extrusion.³⁰ Typical performance gains for a common polyether-based aromatic TPU reinforced with a 2(w/w)% fluoromica-based nanofiller are shown in Table 11.2.

Notably, the property improvements reported are evidence that well-engineered nanofiller systems can not only avoid stiffening and hardening effects; they can also have a positive influence on critical viscoelastic properties such as creep and compressive set, in addition to ultimate tensile strength and tear strength.

Using a Monte Carlo simulation, Pandey et al.¹⁰⁹ predicted that smaller clay platelets should be more easily delaminated and dispersed in a compatible solventhost polymer than high aspect ratio clays. Furthermore, computational models by Balazs and co-workers on block copolymers indicated that a change in microphase texture and properties would be expected since nanofiller size scales with the phase domain size of the block copolymer.¹¹⁰ McKinley and co-workers^29,91 incorporated unmodified, low aspect ratio hectorite (Laponite) into a number of TPUs at up to 20 wt% using a novel solvent-exchange technique. They reported remarkable mechanical properties, heat distortion temperature, and morphological changes in fully delaminated nanocomposite systems. Some degree of ‘tuned’ segmental association of the Laponite® was demonstrated, depending on the solubility parameter of the chosen soft segment.⁹¹

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