In nature, the hierarchical structure of biological nanocomposites is synergistically designed to achieve the desired mechanical properties and complex biological functions. The deformation behavior of these composites is governed by molecular mechanisms that involve the interaction between different levels of hierarchy.1 These bio-inspired design principles have begun to attract attention in the design of synthetic hybrid materials.2 Polymer nanocomposites provide the potential to combine the properties of individual components in a strategic way to achieve unique material properties. The unique interplay between filler dispersion, interfaces, and hierarchy play an important role in the rational design of polymer nanocomposites.
The precise control of the filler dispersion and the matrix-filler interactions are of great significance due to their huge potential in mediating the mechanical response of composites. The engineered matrix-filler interfaces determine the functionality and applicability of these composites. The understanding of these interactions at the molecular level opens up a new range of applications in energy, sensors, filters, and biotechnology.3 This contribution focuses on the use of responsive and active filler elements in polymer nanocomposites and highlights the role of dispersion strategies, interfacial control, and component interactions in responsive mechanics.
Dispersion Strategies Via Tailored Interactions
Critical to the mechanical reinforcement and responsive mechanics of polymer matrices is the uniform dispersion and tunable arrangement of responsive filler elements. The dispersed state of these nanofillers provides the network microstructure necessary for effective stress transfer and reinforcement to achieve mechanical enhancement and material switchability.4 Emerging strategies, such as utilizing active nanoscale fillers or tailoring dispersed state switching via surface functionalization,5 are being actively investigated in the design of new classes of functional polymer nanocomposites. It is the relationship between the dispersed, percolating filler morphology and mechanical response that has opened up new insight into novel and facile pathways for controlled and tailored dispersion.
Inspired by the sea cucumber dermis and motivated by the task of designing responsive materials for neural probes, a research team led by Rowan and Weder6 has developed a new class of responsive polymer nanocomposites utilizing high modulus, highly crystalline cellulose nanowhiskers (CNWs). In these adaptable nanocomposites, the formation and collapse of the percolating nanowhisker network can be selectively and reversibly modulated via response to various triggers, such as hydration or pH. These nanocomposite hybrids were comprised of a 1:1 ethylene oxide/epichlorohydrin copolymer (EO-EPI) matrix and CNWs extracted from sea tunicates. CNWs in the nanocomposite are in the “switched on” state forming a percolating network (Fig. 1), facilitated by strong hydrogen bonding interactions among the whiskers for efficient stress transfer and enhanced elastic modulus of the composite. The interactions among whiskers are “switched off” upon hydration due to competitive hydrogen bonding, leading to collapse of the percolating network and homogenous dispersion within the polymer matrix. This change in the dispersed state resulted in a significant reduction in the storage modulus. Utilizing this versatile strategy, functional nanocomposites were designed that varied the dispersed state of active filler to moderate the mechanical response to specific triggers.
In a recent investigation, Rowan and coworkers7 also engineered adaptable nanocomposites by controlling the surface chemistry of the filler, which tunes filler–filler interactions and the dispersed state upon switching. Surface functionalization of cellulose nanocrystals (CNCs) with either carboxylic acid (COOH) or amine (NH2) moieties was utilized to tune the attractive and repulsive forces between the CNC fillers to yield mechanical tunability modulated by pH [Fig. 2(A)]. At low pH, protonation of the amine functional groups occurred, producing an aqueous dispersion of CNCs due to electrostatic repulsion. However, at high pH, neutralization of amine-functionalized CNCs was achieved and hydrogen bonding-mediating attractive forces resulted in gelation. In contrast, aqueous dispersions driven by deprotonation were achieved at high pH for CNCs surface functionalized with COOH, while attractive interactions dominated at low pH conditions, which resulted in hydrogel formation [Fig. 2(B,C)]. These pH-dependent interactions derived from functionalized CNCs offer a unique handle to elegantly control the mechanical response and adaptability via dispersed state switching of percolating aqueous gels.
In an orthogonal strategy, small molecule self-assembly as a framework for the dispersed filler morphology in the design of mechanically enhanced materials has been recently explored by us7 and others.8 In this family of polymer nanocomposites, the associative nature of the small molecule filler ensures a dispersed, network structure within the polymer matrix, much like that observed in natural materials, such as tendon and bone. These self-organized small molecules may form fibrillar or sheet-like nanostructures through strong, directional interactions, offering a pathway towards mechanically robust materials influenced by the network architecture and filler dimensions (e.g., platelet vs. fiber reinforcement). We reported on the fabrication of a series of new bioinspired nanocomposites utilizing a cholesterol-derived diacetylene small molecule organogelator as the nanoscale filler in an elastomeric matrix.7 Using a facile process, the diacetylene-based small molecules self-organized into monodisperse one-dimensional nanofibers within the matrix [Fig. 3(A,B)], providing significant reinforcement compared to the non-fiber forming cholesterol control. At the highest filler loading, an almost two order of magnitude increase in the tensile storage modulus was achieved above the matrix glass transition temperature through this assembly driven approach to dispersion. Ultraviolet-activated polymerization of the diacetylene units was also observed as an in situ color change to the diacetylene red, nanostructured phase. Although this topochemical modification did not impact the linear region of the mechanical response, the role of polymerization on the non-linear region of the stress–strain response is currently under investigation.
Interfacial interactions also play a pivotal role in controlling the self-assembly of small molecules in a matrix material. A recent study by Cai et al.9 demonstrated the self-arrangement of high energy density 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (HNIW) in an inorganic host matrix that is comprised, of hexagonally oriented channels [Fig. 3(C)]. The silica-derived matrix contains a large number of surface hydroxyl groups, which act as an electron acceptor for HNIW, resulting in host–guest hydrogen bonding between the matrix and the HNIW filler. These specific interactions tailored the self-assembly process of HNIW within the nanoporous silica matrix to fabricate hierarchical nanocomposites. Although this investigation has been limited to structure and ordering of HNIW inside the nanochannels of the matrix, one can envision that these strategies for tunable assembly within nanocomposites architectures, particularly nanoporous channels, will drive new technology in transport-based applications, such as energetics and controlled released constructs.
Tailored Interfacial Adhesion Via Single Polymer Composites
Strong interfacial adhesion in polymer nanocomposites can be realized by utilizing matrix and filler elements of the same chemical composition. This new class of materials, single polymer composites (SPCs),10 inherently exhibits excellent matrix-filler adhesion, resulting in superior mechanical properties. A variety of SPC fabrication methods allow the incorporation of fillers of various phase states (i.e., amorphous, semicrystalline, or crystalline) as reinforcement fillers. For example, nanoscale fiber formation processes, such as electrospinning, have enabled the incorporation of high modulus, semicrystalline fillers into a low modulus, amorphous matrix of the same polymer material. Duhovic et al.11 have fabricated SPCs utilizing semicrystalline polyethylene terephthalate (PET) nanofibrils as fillers in an amorphous PET matrix to yield optically transparent composites with enhanced tensile strength and elastic modulus. These improvements in mechanics were attributed to the strong interfacial adhesion facilitating efficient stress transfer from the matrix to the filler. Another avenue in which SPCs are of extreme interest is in biomedical drug delivery systems, where utilizing a biodegradable polymer allows its dissolution in body fluids to be tailored by controlling the filler/matrix crystallinity. Li and Yao12 have utilized poly(lactic acid) (PLA) to tune the crystallinity of PLA filler/matrix of SPCs. By engineering the degree of interfacial adhesion via the SPC arrangement and tailoring the composite crystallinity, the design of material systems with tunable hydrolytic stability and water-responsive mechanics was achieved.
Understanding the molecular and structural deformation mechanics, particularly the role of the interfacial region, of SPCs is key to the design of materials with tunable properties. A research team led by Eichhorn and coworkers13 has utilized Raman spectroscopy to investigate the complex interactions and map the strain-induced stress transfer between a microcrystalline cellulose matrix and CNW fillers in all-cellulose nanocomposites. The composites with higher volume fractions of CNWs exhibited increased stress transfer due to strong adhesion at the interface, and interestingly, no increase in stress transfer was noticed with increasing tensile strain. The evolution of technologies like Raman spectroscopy has enabled the evaluation of nanodeformation mechanics at these tailored interfaces.
Percolating Networks Via Electrospinning
Electrospinning is another elegant pathway toward the formation of percolating, nanofiber mats for use as nanocomposite fillers. The inherently dispersed, network structure of fibers provides a larger interfacial area to be manipulated for a desired response to a specific trigger. In a recent work by our group,14 we have developed water-responsive, all-organic nanocomposites containing percolating nanofiber networks, which provide either diffused or discrete interfaces based on the chosen polymer matrix. The interface between non-woven nanofibers of poly(vinyl alcohol) (PVA) filler and a rubbery EO-EPI matrix exhibited diffuse and molecularly blended interfaces due to intense hydrogen bonding, resulting in partially restored storage modulus after drying (Fig. 4). In contrast, nanocomposites utilizing the same PVA filler and poly(vinyl acetate) matrix exhibited a sharp, but well-bonded interface, leading to two-way switchable mechanics. By controlling the degree of interaction between the fibrous electrospun mat filler and the matrix material, mechanical responsiveness can be selectively tuned. These responsive materials have potential applications as therapeutic delivery systems and sensor platforms.
The tunable mechanics of electrospun composites can also be realized by designing the matrix, filler or both components to be active or switchable. Mather and coworkers15 have utilized an active filler containing welded intersections of percolating, nanofiber networks to provide continuous conducting pathways in an inert polymer matrix to fabricate a series of thermo-responsive electrospun composites for applications in fuel cell membranes and shape memory actuation. These electrospun nanocomposites exhibited excellent proton conductivity at higher temperatures under a range of humidity conditions. Utilizing a matrix material and a filler, which are both active, enable the synergistic design of nanocomposites with tailored responsive mechanics. For example, Mather and coworkers have also developed an array of nanocomposites by incorporating electrically conducting carbon fiber percolating networks into an active epoxy-based shape memory polymer matrix.16 These nanocomposites exhibited both thermally and electrically activated shape memory behavior with higher electrical conductivities for fast activation of shape recovery.