Relaxation in Partially Crystalline Systems

The relaxation behavior of partially crystalline systems is complex and different from amorphous polymers. Observations give the general impression that, in comparison to amorphous systems, partially crystalline samples are much less uniform in behavior. Many of the systems exhibit peculiarities and these can dominate the viscoelastic properties. This is not the place to explore this large field in the necessary depth, which would mean we would have to discuss separately the mechanical behavior of polyethylene, poly(ethylene terephtha-late), polypropylene, it-polystyrene, poly(tetrafluoroethylene) etc. What can be done for illustration is to pick out one instructive example and we select polyethylene. 

The change in behavior from the highly crystalline LPE to samples with lower crystallinity like the LDPE occurs continuously, as is demonstrated by the measurement shown in Fig. 5.28. Here chlorination was employed for a controlled reduction of the crystallinity. We observe that decreasing the 

The use of the Greek letters in agreement with the convention to choose the symbols a, / , 7 etc. for a designation of the processes in the sequence they show up from high temperatures downwards, produces some confusion in the meaning of the term a-process . While for amorphous polymers, it is identical with the glass-rubber transition, this is not the case for polyethylene. Here, if the glass-rubber transition occurs at all, it is denoted as the -process . 

How can these different facts be reconciled and brought together in one common picture The answer is The a-process in polyethylene has a composite nature. The mechanical relaxation indeed originates firom an additional shear of the amorphous regions. The prerequisite for this shear, however, is a chain transport through the crystallites. By this intracrystalline motion the 

Under static conditions the line shape of a NMR absorption spectrum reflects directly the orientational distribution of the CH-bonds in the sample. Molecular dynamics modifies the spectral shape in a well-defined way. Only the reorientational motion of individual CH-bonds is probed by the experiment which leads generally to a narrowing of the line. A simple situation 

The relaxation behavior of amorphous polymers was dominated by two processes, the glass-rubber transition and the terminal flow region, which are both characterized by a temperature dependence given by the WLF equation. For polyethylene, one cannot expect a flow transition because flow is suppressed by the crystallites in the sample. The fact is that for linear polyethylene, i.e., polyethylene with high crystallinity, there is no WLF-controlled process at all. The numerous measurements in the literature provide clear evidence that the two processes observed in linear polyethylene, a and 7, are both based on activated mechanisms obeying the Arrhenius law. The process 

What is the origin of the a-process Different observations must be included in the considerations. First, remember the results of the NMR experiment presented in Sect. 5.4.2. Here, a longitudinal chain transport through the crystallites was clearly indicated. The chain motion is apparently accom- 

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