Effect of Nanoparticle Mobility on Toughness of Polymer Nanocomposites

Abstract

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

Clay nanocomposites based on poly(vinylidene fluoride-co-hexafluoropropylene): Structure and properties

“…Specifically, we note the organization of the polymer matrix into fiber-like conformations and superstructures oriented parallel to the stretching direction. In rubbery matrices, the realignment of the clay particles themselves along the stretching direction effectively blocks crack propagation and deflection [41,42].…”

Crystal structure transformation and dielectric properties of polymer composites incorporating zinc oxide nanorods

“…When the content of ZnO increases further, the nanocomposites display not only a reduced peak intensity but also disappearance of some peaks and occurrence of a new reflection peak. It is well known that the incorporation of nanoparticles would result in the formation of β phase [6,19] . In our study, it was found that when the content of ZnO nanorods reach as high as 20 wt%, the new developed β phase can be easily seen, which is consistent with the study reported previously [2,7,20] .…”

“…Through the β phase is not a stable phase compared with the α phase, it still raises a lot of research interest, since it has an impressive effect on the piezoelectric and pyroelectric properties [5] . To date, numerous fillers, such as nanoclay [6,7] , carbon nanotubes [8] , and graphene sheets [9,10] have been used as an effective method for the forming of β phase PVDF. However, there are only several literatures about P(VDF-HFP) based composites.…”

Influence of nanoparticle size, loading, and shape on the mechanical properties of polymer nanocomposites

“…The dynamical arguments are based on the observation that a PNC, where NPs are more mobile than the polymer chains, achieves a better resistance against deformation via improved release of local tension. 17,18,33,34 The structural arguments relate the NPs ability to create temporary bonds between the chains, thus creating a NP-polymer network, to the mechanical properties of the PNC. 19,22,24,26,35,36 In a recent publication 28 we demonstrated that the mechanical properties of PNC loaded with small, spherical NPs are related to structural features, such as the filler loading, the surface area of the polymerfiller interface, and the polymer-filler network structure.…”