Structure development and mechanical performance of polypropylene

Abstract

Polymers are known for their ease of processability via automated mass production technologies. The most important process is injection molding that, due to its freedom in material choice and product design, allows producing a wide variety of thermoplastic products. Mechanical failure of these products, either upon impact or after prolonged exposure to load, limits their ultimate useful lifetime. To predict and control lifetime, understanding of the route from production to failure, i.e. the processing-structure-property relation, is necessary. This is a complex issue; especially in the case of semi-crystalline polymers. These are heterogeneous systems comprised of amorphous and crystalline fractions, of which the latter can be highly anisotropic with size and orientation that are strongly dependent on the precise processing conditions. As a consequence, these structural features in the microstructure, and the associated mechanical properties, generally exhibit distributions containing different orientations throughout a single processed product. Understanding polymer solidification under realistic processing conditions is a prerequisite to predict final polymer properties, since only a complete characterization of the morphology distribution within a product can lead to a meaningful and interpretable mechanical characterization. In this thesis we study the relation between processing conditions, morphology and mechanical performance of a semi-crystalline polymer, isotactic polypropylene. Key issue is the accurate control over all relevant processing parameters. Therefore, different experimental techniques are used to obtain samples at different high cooling rates, at elevated pressures, and high shear rates. A custom designed dilatometer (PVT- ?T -?? -apparatus) proves to represent the most important and useful technique. First, a predictive, quantitative model is presented for the crystallization kinetics of the multiple crystal structures of polypropylene, under quiescent conditions. The approach is based on the nucleation rate and the individual growth rate of spherulites of each type of polymorphism (a-, s-, ?- and mesomorphic phase), during non-isothermal, isobaric solidification. Using Schneider’s rate equations, the degree of crystallinity and distribution of crystal structures and lamellar thickness is predicted. Next, the effect of flow is introduced. Flow strongly influences the kinetics of the crystallization process, especially that of nucleation. Three regimes are observed in the experiments; quiescent crystallization, flow enhanced point nucleation and flow-induced creation of oriented structures. To assess the structure development under flow, a molecular-based rheology model is used. Combining the models derived for quiescent and for flow-induced crystallization, yields the tool that is capable of predicting the volume distributions of both isotropic and oriented structures, under realistic processing conditions. The kinetics of mechanical deformations strongly depend on the anisotropy in the crystalline morphology, thus the local orientation. To study this, uniaxially oriented tapes with a well defined, and high, degree of anisotropy are used as well as injection molded rectangular plates. Yield and failure are described using an anisotropic viscoplastic model, applying a viscoplastic flow rule. It uses the equivalent stress in Hill’s anisotropic yield criterion, and combines the Eyring flow theory with a critical equivalent strain. Factorization is used and the model is capable to quantitatively predict the rate, the angle and the draw ratio dependence of the yield stress, as well as the time-tofailure in various off-axis tensile loading conditions. To use the model, also for other polymers, characterization of only the isotropic state is sufficient. Therefore, the influence of the cooling rate on the deformation kinetics is studied in-depth on isotropic systems. Different cooling rates induce different crystal phases, both the stable a-phase and the mesomorphic phase, while also the degree of crystallinity and lamellar thickness are influenced. The deformation kinetics prove to be the same for the different microstructures, which means that the activation volume and energy are independent of the thermodynamic state. Differences in thermal history are, consequently, solely captured by two rate constants which are a function of the microstructure.