Physical properties of novel polypropylenes

Abstract

High molecular polypropylene (MW ~ 1,100,000 g/mol) was added to a matrix of polypropylene with an average molecular weight (MW ~ 250,000 g/mol). By blending both materials we tried to combine the respective advantages regarding stability and processability. The long chains from the high molecular polypropylene act as tie molecules between the crystalline regions in the polymers, providing direct covalent bonds. Theoretical calculations showed that the probability for tie molecules is about 4% in the matrix material and about 24% in the high molecular polypropylene. Conventional tensile tests at the prepared specimen showed the desired effects at elevated temperature. The evolution of micro-voids during stretching has been analyzed by small-angle x-ray scattering (SAXS) experiments. SAXS belongs to the indirect measuring methods. Only the absolute squared of the Fourier transform of the electron density distribution is recorded. In order to evaluate the scattering from micro-voids a model by Wilke has been adapted to our polymers. The model accounts for randomly distributed voids which are aligned along the stretching direction and show a log-normal size distribution. A fitting algorithm returns the parameters of the size distribution function. As most important result we found that the dimensions of the voids decrease with increasing fraction of long chains. As a consequence of these results void sizes in polymer materials can be determined by blending and subsequent stretching. Melt-spun fiber polymers also have a significant industrial relevance. We therefore prepared such fibers from the matrix material and one blend. In order to evaluate the scattering data a model developed by Wilke has been adapted to the samples. One of the distinct advantages of this model compared to other approaches is that the radius of the crystallites can be obtained. We found that in the blends we get significant orientation and increased lamella radii.