All-Organic, Stimuli-Responsive Polymer Composites with Electrospun Fiber Fillers

Abstract

Stimuli-responsive materials are desired for a wide range of applications. Here, we report the design and fabrication of all-organic, stimuli-responsive polymer composites using electrospun nanofibers as the filler. The incorporation of 4 wt % of filler into the polymer matrix increased the tensile storage modulus by 2 orders of magnitude. Upon exposure to water, the filler fibers plasticize and no longer provide mechanical reinforcement. The tensile storage modulus subsequently diminishes 2 orders of magnitude to the value of the neat matrix polymer. M aterials that can change their mechanical properties on command upon exposure to specific stimuli are desired for a wide variety of applications, including drug delivery, sensors, actuators, and shape-memory materials. 1−7 Many different strategies have been developed to impart stimuliresponsive properties into soft and hard materials upon exposure to a variety of stimuli. 8 One of the most interesting approaches to dynamic materials is using stimuli-responsive filler materials for polymer composites. Because the filler is responsible for the dynamic response, it can be blended with a wide variety of polymers to impart stimuli-responsive properties to materials that are otherwise mechanically static. 9,10 As an example, Rowan and colleagues demonstrated significant mechanical switching in a variety of polymers using cellulose nanowhiskers as filler for polymer nanocomposites inspired by the dermis of a sea cucumber. 11−14 Here, we report all-organic, stimuli-responsive polymer composites fabricated using an electrospun mat of poly(vinyl alcohol) (PVA) as the filler, which undergoes a 2 orders of magnitude change in the storage modulus upon exposure to water.Electrospinning uses electrostatic forces to produce continuous polymer nanofibers that have been used for a variety of applications from cell scaffolds to filtration membranes and electronic devices to drug delivery vehicles. 15,16 In electrospinning, fibers are generated by applying an electric field between a polymer solution and a grounded collector. When the electrostatic force overcomes the surface tension of the polymer solution, a stable jet or "Taylor cone" can be formed. As the jet travels toward the collector, it is constantly subjected to a stretching movement, producing nanofibers of tunable diameter. 17 Outside of the uses for the fibrous mat, nanofibers fabricated via electrospinning have also been used as the filler component in polymer nanocomposite materials. 18,19 The incorporation of electrospun nanofibers into a polymer matrix was found to increase the strength of the composite films compared to the corresponding neat polymers. More recently, stimuli-responsive polymer composites have been fabricated from electrospun mats. Luo and Mather have demonstrated shape memory and actuation properties of electrospun polymer composites using poly(ε-carprolactone) and carbon nanofibers, respectively, as filler materials. 20,21 To the best of the authors' knowledge, a controlled change in material modu...

Tunable Mechanics in Electrospun Composites via Hierarchical Organization

“…This minimal modulus enhancement was due to the relatively similar storage moduli of the filler and matrix components. Upon hydration and subsequent drying, all of the PVA/MMT.PVAc composites with 2.5, 7, and 12 wt % of MMT in PVA filler exhibited significantly restored moduli by 97, 96, and 91% to 1.8, 1.9, and 2.0 GPa, respectively (Figure 4a), similar to the two-way switchable behavior observed 19 for PVA.PVAc films without MMT. In contrast, the mechanical behavior of PVA/MMT.EO-EPI composites was quite different (Figure 4b).…”

“…18 Recently, we developed a series of responsive composite systems with electrospun fibers as the filler, exhibiting tunable matrix−filler interactions and responsive mechanics upon hydration. 19,20 In this investigation, by expanding the influence of scale, we have added an additional level of complexity via the incorporation of inorganic MMT into PVA electrospun nanofibers, which were embedded within a rubbery ethylene oxide-co-epichlorohydrin (EO-EPI) or poly(vinyl acetate) (PVAc) matrix. PVAc was introduced as a matrix polymer to enhance the stiffness contrast between hard and soft states and for its biocompatibility.…”

Stiffness‐Changing of Polymer Nanocomposites with Cellulose Nanocrystals and Polymeric Dispersant

“…The decoration of CNCs with specific binding motifs has also allowed extending the effect to other stimuli . Several nanomaterials have been used to create such adaptive nanocomposites, but CNCs remained the filler of choice due to their high strength (0.25–6 GPa) and stiffness (105–150 GPa), benign nature, and the renewability and low cost of the sources from which they are isolated . The properties of CNCs depend on the source from which they are extracted and also the isolation process .…”

Temperature scanning stress relaxation behavior of water responsive and mechanically adaptive elastomer nanocomposites

“…Inspired by nature, several efforts were made to mimic the properties of sea cucumber dermis . Most of the literature utilized the concept of formation/breakage of three dimensional filler network(cellulose nanocrystals, poly(vinyl alcohol) mats, chitin nanocrystals) within the polymer composite (epichlorohydrin, poly(vinyl acetate), carboxylated styrene butadiene rubber) upon the addition/removal of water, which in turn led to the mechanoadaptive characteristics …”