The properties of polymers are not solely determined by their chemical structure but also by the processing step, which determines the orientation of the molecules in the final products. Nowadays, the majority of polymers are processed via the melt. Generally, the mechanical properties of polymers increase with molar mass. However, the melt-viscosity also significantly increases with molar mass. The (zero-shear) viscosity ??0 of polymer melts scales with the weight average molar mass Mw to the power 3.4, when Mw is above a certain threshold value. Consequently, processing polymers is often a compromise between properties and processibility with an optimum molar mass. In addition to chemistry and processing, the final product properties can be modified by additives and/or (nano)fillers. It has been reported that the melt viscosity of polymers can be reduced considerably, with the addition of small amounts of nanoparticles. The technological consequences of the viscosity reduction can be enormous. However, this viscosity reduction cannot be predicted at this moment, and the underlying mechanisms are not yet well understood. The objective of this thesis is to understand the mechanisms that lead to the improvement in processibility of semi-crystalline and amorphous polymers, and thus be able to control the processibility and property balance. In the present study, isotactic poly(propylene) (iPP)/silica and poly(carbonate) (PC)/silica nanocomposites were investigated. In the part on iPP/silica nanocomposites, different preparation methods were investigated, i.e., in-situ silica particle synthesis, melt compounding and solution processing. The rheology and crystallization behavior of such prepared iPP/silica nanocomposites were studied. Considering that melt compounding is one of the most used methods to prepare the polymeric products, melt compounding was applied to prepare iPP/silica nanocomposites. The addition of silica nanoparticles, with diameter ~20 nm, induced viscosity reduction. The viscosity reduction depended on the particle size and distribution. With decreasing silica particle size from ~20 nm to ~10 nm, a lower viscosity reduction was observed. Compared to in-situ silica particle synthesis method and melt compounding, better silica dispersions were obtained via solution methods, in which three different drying procedures were used. These three different drying procedures, i.e., gradually slow evaporation, precipitation and vapor rotation, produced comparable states of dispersion and demonstrated a similar viscosity reduction. In brief, the viscosity reduction can be obtained via different methods with selecting suitable particle size and distribution. After the viscosity reaches its minimum value, an increase in viscosity was observed with the further addition of silica nanoparticles. The viscosity reduction was explained by the selective adsorption of the high molar mass chains to the silica nanoparticles’ surfaces, while the low molar mass chains were in the polymer matrix. In addition to the viscoelastic behavior, the addition of silica nanoparticles influences the crystallization behavior of the iPP/silica nanocomposites. The isothermal crystallization kinetics of the iPP/silica nanocomposites were studied using the Avrami analysis. A two-stage crystallization process was observed: the primary stage characterized by nucleation and spherulitic growth and the secondary stage characterized by crystal perfection. The addition of silica increased the crystallization rate first, followed by the crystallization rate decreased. The highest crystallization rate occurred to the sample with the lowest viscosity. The addition of silica also increased the crystallization temperature and slightly increased the crystallinity, while the melt temperature remained constant. The flow-induced crystallization behavior was studied by using in-situ small-angle X-ray scattering (SAXS). It is well known that flow-induced crystallization is governed by the high molar mass fraction of the molar mass distribution. The low molar mass matrix, which experiences lower shear stress due to its low viscosity, results in less orientation under flow. Therefore, uniform and isotropic structures are obtained. The SAXS study showed that the orientation was minimal for the nanocomposites with the lowest viscosity. In the part of PC/silica nanocomposites, the addition of ~0.8 vol% silica (10-15 nm) induced ~26% viscosity reductions, after considering the effects of molar mass and glass transition temperature Tg. The effect of particle size and geometry on the viscoelastic behavior of PC/silica nanocomposites was also studied. Three different mechanisms were used in literature to explain the decrease in viscosity of nanocomposites, i.e., ball-bearing effect, free volume and selective adsorption, while none of them can explain the phenomena we observed in the PC/silica nanocomposites. We proposed that the viscosity reduction of the PC/silica nanocomposites can be attributed to the variations in the entanglement density. This explanation was confirmed from calculation and modeling results. The addition of silica nanoparticles increased the molar mass between entanglements. In addition, the effect of the affinity between PC and silica surface was also studied via adding brominated PC (PC_Br) to PC/silica nanocomposites. The viscosity reduction of the system was related to the weight ratio of PC and PC_Br. The largest viscosity reduction percentage ~49% was observed in the (PC/PC_Br)(50/50)/silica system. The mechanical properties of the PC/silica nanocmposites and the effect of annealing were explored as a function of the silica concentration. The results from uniaxial compression tests demonstrated that the addition of silica nanoparticles slightly increased the modulus and the yield stress of the PC/silica nanocomposites. The increases in the modulus and the yield stress are due to both the reinforcement of the silica nanoparticles and the interaction between PC and silica nanoparticles. With increasing annealing time and silica content, an increase in the yield stress was observed, which as confirmed from uniaxial compression and uniaxial tensile tests. Negligible changes in the softening and the strain hardening modulus were observed with increasing silica content as displayed by the true strain-stress curves. In summary, the viscosity of the both studied semi-crystalline polymer (iPP) and amorphous polymer (PC) can be reduced by the addition of silica nanoparticles. The viscosity reduction depends on the size (distribution) and the geometry of the nanoparticles. This viscosity variation does not influence the mechanical properties of the studied polymers. Different preparation methods can be applied to obtain the rheology and property balance with choosing suitable nanoparticle size and concentration.
|Qualification||Doctor of Philosophy|
|Award date||21 Sep 2010|
|Place of Publication||Eindhoven|
|Publication status||Published - 2010|