Effect of molecular architecture on the crystalline structure and stiffness of iPP homopolymers: Modeling based on annealing experiments

Abstract

Five PP homopolymers were selected and their molecular structure was thoroughly characterized to determine the effect of molecular architecture on their annealing behavior and on the ultimate stiffness achieved by heat treatment. Molecular mass and its distribution were characterized by rheological measurements, while chain regularity was determined by calorimetry, by the stepwise isothermal segregation technique (SIST). The samples were annealed in two different ways. Tensile bars were treated in an oven at 165 °C for increasing times to determine changes in stiffness. Various defects developed during the annealing of tensile specimens that did not allow the reliable determination of modulus by direct measurement. On the other hand, the second approach, the annealing of small samples in a DSC cell clearly showed the changes occurring in crystalline structure and also the effect of nucleation and molecular architecture on them. The large molecular weight fraction used to facilitate nucleation hinders crystal perfection, while the presence of a heterogeneous nucleating agent increases overall crystallinity, but does not influence recrystallization during annealing. Melting traces were transformed into lamella thickness distributions, from which average lamella thickness was derived. Lamella thickness and crystallinity, the independent variables of the empirical equation used for the calculation of modulus, were extrapolated to infinite annealing time to predict maximum stiffness. The Full paper submitted for publication in Journal of Applied Polymer Science 2 value obtained, 3.5 GPa, is very far from the theoretically predicted 40 GPa of oriented crystals, which cannot be achieved under practical conditions.

Direct correlation between modulus and the crystalline structure in isotactic polypropylene

“…These simulated results indicate that the desired mechanical properties can be achieved if crystallinity is around 75% and lamella thickness is larger than 30 nm. This is in good agreement with our earlier results that 3 GPa of modulus can be reached by carefully designed annealing experiments [38].…”

“…Based on the earlier results [7,38] and our experimental data it can be concluded that stiffness depends predominantly on crystallinity and lamella thickness. Consequently, the following expression should be solved (Equation (9)):…”

“…Although, a linear function can be fitted to the plots from X ( 0.5 to 0.7, which covers the range of iPP grades in the practice. That is why the linear correlation developed by Pukánszky and coworkers [7,38] provides accurate results. The shape of the curve at lower lamella thickness and crystallinity can be speculated on (marked with dashed line in Figure 11), but the shape of the correlation at large lamella thickness and crystallinity is completely unknown.…”

Chain regularity of isotactic polypropylene determined by different thermal fractionation methods

“…Despite that, the crystallization during SSA-DSC is influenced by the presence of nucleating agents and kinetics especially in the samples of NPP2 and NPP3, which contain both UHMW fraction and nucleating agent. The presence of UHMW fraction hinders the crystallization [29] at high temperature resulting in apparently smaller chain regularity in these samples. Moreover, we have to emphasize that the evaluation of NMR results is much more complicated than that of SSA-DSC traces.…”

Decreased oxygen permeability of EVOH through molecular interactions

“…The modulus of crystalline polymers is determined mainly by crystallinity and the thickness of the lamella grown during crystallization [65]; lamella thickness increases with increasing crystallization temperature, thus also with nucleation [66,67]. The heat of fusion proportional to crystallinity is plotted against additive content in Figure 5.…”