Order phenomena in polymer crystallization
Within the frame work of these thesis the influence of order phenomena in polymer melts on crystallization kinetics has been investigated by means of nuclear magnetic resonance (NMR) spectroscopy. In particular the so-called memory effect in syndiotactic polypropylene (s-PP) and the influence of entangled polymer chains (entanglements) on the crystal growth rate have been the key points of investigation. Qualitative differences in the degree of order of chain segments were measured using relaxation as well as static double quantum (DQ) experiments. Crystallisation kinetics in polymer melts has been followed with an improved NMR pulse sequence which is sensitive to differences in the mobility of polymer chains or chain segments. In comparison to X-ray measurements, in which structural changes can be monitored, with these experiments we were hoping to gain new insights in to the mechanism of polymer crystallisation. Besides the NMR experiments DSC measurements have been conducted. Crystallization Kinetics Experiments: An improved NMR pulse sequence (MSE-CPMG) which consists of a combination of a magic sandwich echo (MSE) and a modified Carr-Purcell sequence (CPMG), has been used. The MSE allows the measurement of the free induction decay (fid) without signal loss from fast decaying signal of immobile chain segments during the so called dead time. The decay of the slowly decaying amorphous regions was measured via the CPMG sequence. Three different fractions of the sample can be identified via a simple analysis of the acquired signal: A crystalline fraction, a mobile-amorphous fraction and amorphous fraction with a highly reduced mobility. A continuous repetition of the experiment during the course of isothermal crystallization allows the analysis of crystallization kinetics. Memory Effect in s-PP: The memory effect was reproduced in NMR crystallization kinetics experiments. Differences between NMR and other methods were explained by the influence of sample preparation. However, the analysis of the NMR kinetics data did not yield further information about the mechanism of polymer crystallization. In literature, the memory effect is often explained by the existence of a pre-ordered melt. Neither relaxation nor DQ experiment could show that this picture is justified. Instead of that it was shown that the memory effect might be related to thermo-oxidative modification of the outer shell of the granular PP pellets. Entropic Effects in the Crystallization of PDMS: The presence of entanglements accelerate the crystallization of PDMS as demonstrated in DSC and NMR experiments. All samples that crystallized from the entangled melt crystallized faster than the unentangled, low-molecular weight sample. Bimodal mixtures of high (Mn = 115000 g/mol) and low (Mn = 5000 g/mol) molecular weight PDMS did not show a continuous increase in their crystallization rates but a maximum at a mixture with 40 % short and 60 % long chains. The existence of a maximum indicates that the crystallization rate of PDMS is governed by both entropic effects and the viscosity. The idea of an antagonistic interplay between these two factors was affirmed by the implementation of piezo-rheological experiments.
Polymerkristallisation, Ordnungsphänomene, Doppelquanten-NMR, Kristallisationskinetik, Relaxationsexperimente, polymer crystallization, order phenomena, double quantum NMR, crystallization kinetics, relaxation experiments
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