Addition of nanoclays or other nanoparticles into various polymers to produce nanocomposites has been extensively utilized in an attempt to enhance the mechanical, physical, and thermal properties of polymers. While some interesting properties have been demonstrated, the resulting nanocomposites have yet to realize their full potential. Nanoparticles in general, and nanoclays in particular, with their nanometer size, high surface area, and the associated predominance of interfaces in the nanocomposites, can function as structure and morphology directors, for example stabilizing a metastable or conventionally inaccessible polymer phase, or introduce new energy dissipation mechanisms. Thus, what distinguishes nanoparticles from conventional micrometer-size rigid reinforcements is that their role might not be limited to only adding stiffness to the polymer, but also to directing morphology, as well as introducing new energy-dissipation mechanisms leading to enhanced toughness in the nanocomposites. Herein we demonstrate this potential by reporting a remarkable (order of magnitude) increase in toughness with a concurrent increase in stiffness in a poly(vinylidene fluoride) (PVDF) nanocomposite.The kinetics of crystallite growth and the details of crystallite morphology of semicrystalline polymers can be affected by the presence of layered silicates. [1,2] Although some changes in morphology have been described in polymer/nanoparticle hybrids, [3±7] near-total stabilization and control of a crystalline phase, coupled with dramatic enhancements in materials properties, has not yet been reported. PVDF is an important engineering plastic. It is used extensively in the pulp and paper industry due to its resistance to halogens and acids, in nuclear-waste processing for radiation-, and hot-acid applications, and in the chemical processing industry for chemical and high-temperature applications. It is also used in various device applications, due to its unique piezoelectric [8±10] and pyroelectric [11] properties. There are five known crystalline forms or polymorphs of PVDF: a, b, c, d, and e.[12] The a phase is the most common in melt crystallization, and remains the dominant crystalline form versus the b, and c phases. The c phase does not form except at high temperatures and pressures. Earlier reports have shown that the a phase (chain conformationÐtrans-gauche trans-gauche, tgis inactive with respect to piezo-and pyroelectric properties, while the b form (all trans) exhibits the most activity, and is thus the focus for electromechanical and electroacoustic transducer applications. Thus, the b form has great technological utility and there have been numerous attempts to stabilize this phase. For example, the b form of the PVDF has been obtained by careful crystallization from solution, [13] by melt crystallization at high pressure, by application of a strong electric field, [14] by molecular epitaxy, [15] and by preparing a carbon-coated, highly oriented ultrathin film.[16] Earlier reports have indicated the possibility...

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In the past decade, attention has been focused on using polymer nanocomposites to overcome the trade-offs encountered in traditional composite systems.[1±12] Notwithstanding an increase in stiffness, most reported nanocomposites exhibit lower toughness than the matrix polymers, although Cohen and co-workers have found that some particulate-filled polymer composites show an increase in toughness compared to the neat polymer.[13±15] They attributed the effect to the combined mechanisms of crack deflection and local plastic deformation of the polymer around the particles following debonding. Recent work in our group has led to the development of nanohybrid materials, more specifically poly(vinylidene fluoride) (PVDF) nanocomposites, that exhibit a simultaneous increase in stiffness and toughness. The new nanocomposites are almost an order of magnitude tougher than the pure polymer. Recent molecular-dynamics studies have suggested that the mobility of the nanoparticles in the polymer might be crucial for introducing new energy-dissipating mechanisms that lead to enhanced toughness in the nanocomposite.[16] We present experimental evidence that nanoparticle orientation and alignment under tensile stress is responsible for this energydissipation mechanism. This mechanism is applicable to both semicrystalline and amorphous systems and is typically absent from conventional polymer composites. The response measured below and above the polymer glass-transition temperature (T g ) indicates that mobility of the polymer matrix is a precondition for this mechanism to be effective. In other words, the increase in toughness scales with the increase in the mobility of the polymer chains, which in turn dictates the mobility of the nanoparticles. Our results show that, although the degree of improvement in toughness is system dependent, nonetheless an increase can be induced across a broad range of polymer systems and morphologies. Semicrystalline PVDF and amorphous atactic polystyrene (PS) nanocomposites containing 5 wt.-% of nanoparticles (nanoclay) were synthesized via melt extrusion. X-ray and transmission electron microscopy (TEM) analysis of both systems confirmed the presence of a homogenous nanometer-scale dispersion of multilayers of alternating polymer and nanoclay stacks. Both systems showed an intercalated nanostructure. Thermal analysis in the form of dynamic mechanical analysis and differential scanning calorimetry showed no evidence of a change in either the degree of crystallinity for the PVDF nanohybrid or of the T g for both the PVDF and PS nanocomposites. The nanohybrids were then subjected to tensile testing at a strain rate of 5 mm min ±1 at different temperatures to measure the change in the response of the nanohybrids compared with the pure polymers. Figure 1 summarizes the results. From Figure 1a, it can be seen that at 30 C (the T g of PVDF is ±40 C), there is an order-of-magnitude increase (~700 %) in the toughness of the nanohybrid as measured by

The structure and dynamics of polymer-grafted two-dimensional silicate layers in solution were investigated. The geometry of the individual silicate layers was examined by looking at both polarized and depolarized light scattering from dilute solutions, while higher-concentration systems were used to study the interaction and dynamics of polymer-grafted silicate layers in suspension. The form factor for an oblate ellipsoid was used to fit the polarized intensity profile, and values of a approximately 80 nm and b approximately 380 nm for the semi-axes were obtained. The 80 nm value compares reasonably with the dimensions of the polymer brushes grafted on the surface of the silicate layers. The modulus of the grafted silicate in solution, as determined by Brillouin scattering, is of the order of 10 GPa. The cooperative diffusion mechanism, typical of interacting polymer chains, is suppressed due to the high polymer osmotic pressure. The osmotic pressure is also responsible for the weak interpenetration of the densely grafted polymer chains on the surface of the silicate layers. The scattering data indicates that the polymer-grafted nanoparticles move via collective diffusion and experience significant decrease in mobility above their overlap concentration.