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...
SYNOPSISThe cure behavior of diglycidyl ether of bisphenol A (DGEBA) type of epoxy resins with three aromatic diamines, 4,4'-diaminodiphenyl methane (DDM ) , 4,4'-diaminodiphenyl sulfone (44DDS), and 3,3'-diaminodiphenyl sulfone (33DDS) was studied by torsional braid analysis. For each curing agent the stoichiometry of the resin mixtures was varied from a two to one excess of amino hydrogens per epoxy group to a two to one excess of epoxy groups per amino hydrogen. Isothermal cures of the resin mixtures were carried out from 70 to 210°C (range depending on epoxy-amine mixture), followed by a temperature scan to determine the glass transition temperature ( T,) . The times to the isothermal liquidto-rubber transition were shortest for the DDM mixtures and longest for the 44DDS mixtures. The liquid-to-rubber transition times were also shortest for the amine excess mixtures when stoichiometry was varied. A relatively rapid reaction to the liquid-to-rubber transition was observed for the epoxy excess mixtures, followed by an exceedingly slow reaction process a t cure temperatures well above the T,. This slow process was only observed for epoxy excess mixtures and eventually led to significant increases in T, . Using time-temperature shifts of the glass transition temperature vs. logarithm of time, activation energies approximately 50% higher were derived for this process compared to those derived from the liquid-to-rubber transition. The rate of this reaction was virtually independent of curing agent and was attributed to etherification taking place in the epoxy excess mixtures. 0
Curing of fiber-reinforced thermoset polymer composites requires an elevated temperature to accelerate the crosslinking reaction and also hydrostatic pressure to consolidate the part and suppress the formation of voids. These processing conditions can be provided by autoclaves of appropriate size, but these are expensive and sometimes difficult to schedule. Ultrasonic debulking followed by oven cure is an attractive alternative to autoclave cure. In this technique a movable "horn" driven at ultrasonic frequency is applied to the surface of the uncured part. This generates pressure and at the same time produces heating by viscoelastic dissipation. The part can be debulked to net shape and staged through the action of the ultrasound. There are a large enough number of experimental parameters in ultrasonic debulking and staging to make purely empirical process optimization difficult, and this paper outlines numerical simulation methods useful in understanding and developing the process.
The cure of an epoxy‐anhydride resin system used in pultrusion was characterized to develop an understanding of the cure behavior and to determine potential process controls parameters. Isothermal cures of neat resin formulations were monitored by differential scanning calorimetry (DSC), torsional braid analysis (TBA), and microdielectrometry (MDE). The processing conditions define a time/ temperature region in which monitoring would be applicable. Both DSC and MDE were found to yield useful information in this region, however, the events typically monitored by TBA either did not occur or occurred too quickly to be monitored. Significant ionic conductivity was observed in the fully cured resins at temperatures above the glass transition temperature and could possibly be used as a control parameter. This study revealed an apparent change in reaction mechanism with increasing cure temperature. DSC showed a change in activation energy with extent of reaction and a decrease in heat of reaction at the higher isothermal cure temperatures. The formation of a different network structure was indicated by a decreasing glass transition temperature of the cured resin with increasing cure temperature by both DSC and TBA.
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