Tuning Viscoelasticity Of Polypropylene Foaming System Containing Entangled Network Structure

Foam contains porous structures perceived as a gas-solid composite. It has been widely used in industries of package, auto part, insulation, building and aerospace due to light weight, acoustic energy absorption and thermal insulation. Compared to general plastic foams like polyethylene and polystyrene foams, polypropylene (PP) foam possesses superior thermal and mechanical properties and attracts so much attention. However, defects of low expansion ratio, poor cell uniformity and narrow foaming temperature zone exist in linear polypropylene (L-PP) foaming, resulting from low melt strength and rapid decline in viscoelasticity after the melt of crystalline. So far, enhancing the melt strength is the most direct and effective way to improve the foamability of PP, and specific modification methods include long chain branching, controlled crosslinking/grafting and nonreactive blending of polymers. Nevertheless, they all have their own flaws that have to be overcomed:shear modification and high cost of long chain branched PP (LCB-PP), poor efficiency of crosslinking/grafting of PP, limited improvement of viscoelasticity by the blending of immiscible materials.In this dissertation, the network chains of LCB-PP were tuned by shear and annealing process, and the corresponding rheological, crystallization and foaming behaviors were investigated; fibrillar morphology of dispered phase was controlled by process in twin-screw extruder to induce strain hardening in L-PP; entangled network of immiscible polymer was designed and its percolation threshold was identified to study the effect of fibrillar network on the viscoelasticity, crystallization and foamability of L-PP. The detailed work is as follows:1. The effects of processing history and annealing procedure on the rheological properties of LCB-PP and L-PP, especially melt flow index, swelling ratio, storage modulus and complex viscosity, were studied intensively. It is found that melt viscoelasticity of LCB-PP was greatly influenced even by the short-time processing in an internal mixer due to molecular disentanglement and alignment of side chains along the backbone; while the rheological property of L-PP was nearly independent from shear processing. According to the changes of parameters regarded as measures of viscoelasticity for LCB-PP, the relationship between the magnitude of shear modification and total shear history was linear on a logarithmic scale. Moreover, the rheological properties of shear modified LCB-PP were recoverd by annealing to some extent. For vacuum-annealing, this recovery process could be accelerated at higher annealing temperature and longer annealing time. While for the specific high pressure CO2 atmosphere in foaming, this recovery process of viscoelasticity was much more effective, and more sufficienct recovery was brought by higher CO2 pressure. According to the result of molecular dynamics simulation, a larger free volume under high pressure CO2 made these aligned side chains of LCB-PP easier to recover to their initial states, resulting in more effective recovery process for high pressure CO2 saturation annealing. Besides, the peak temperature during the crystallization of LCB-PP was substantially improved after processing. Meanwile, the foamability of LCB-PP was deteriorated after shearing process, which manifested as larger cell size and lower cell density.2. The effect of dispersed phase morphology of poly(butylenes terephthalate) (PBT) on solid-state foaming behavior of L-PP was studied. PP/ nanofibrillar PBT composite was prepared by'hot drawing method', and the effect of PBT on the crystallization, rheological and solid-state foaming behaviors of PP were also investigated. The results show that the foaming performance of PP was significantly impacted by crystal morphology and rheological property. For pure PP, microcells appeared at the centers of spherulites where the melting started, followed by the foaming of radial lamellae, eventually leading to low cell density, poor cell uniformity and bi-modal cell structure. However, the spherulites of PP were refined by the added PBT phase, no matter spherical or fibrous phase. The cell density was increased and the cell uniformity was improved with the heterogeneous nucleation of PBT. The storage modulus (G') at low frequency and the melt elasticity of PP/PBT composite were distinctly enhanced due to entangled network structure of nanofibrillar PBT, thus preserving cell shapes at relatively high temperature.3. The effect of processing parameters on the morphology of in situ microfibrils in a twin-screw extruder was studied by an orthogonal design method, and a rheological model between morphology parameters of PBT fibers and extensional viscosity of PP/PBT composite was established. Orthogonal experiment results demonstrate that PBT content was the most important factor affecting the aspect ratio of fiber and fibrillation content, and the optimal parameter values of fibrillation process was 180?,200-300r/min, 30wt% and 4kg/h. The rheological tests of PP/PBT blends show that melt viscoelasticity and strain hardening increased with the enhancement of anisotropy of dispersed phase. Strain hardening factor and fibrillar morphology parameters (L/d,?) could be connected by the rheological model obtained by experiment data fitting, and this model could be used to predict the strain hardening in extensional flow. On the other hand, the rheological property was hardly affected by the isotropic or spherical PBT phase.4. In situ polymer-fibrillar blend of PP/PBT was prepared under the optimal parameters obtain from orthogonal design method, and the effect of fibrils on the rheological, crystallization and foaming behaviors of the composite was studied. The results indicate that both viscoelasticity and crystallization of PP were affected by PBT morphology. With the increase of PBT fibril content, the terminal slope of G' gradually approached to 0, the frequency dependence of tan? became slight, and the semicircle radius in Cole-Cole plot increased with an evident upturning at high viscosity. The critical fibril concentration of physical entanglement network was estimated to be 4wt% by rheological measurement. Therefore, a remarkable strain hardening was observed in the extensional flow of the PP/5wt% fibrillar PBT blend, while this phenomenon was hardly exist in PP/spherical PBT blend or pure PP. With the existence of PBT fibers, an earlier onset of crystallization of PP was observed. The foamability of PP was dramatically improved by the physical network of PBT fibers served as heterogeneous nuclei, which manifested as higher cell density, smaller cell size and broader foaming temperature zone.