Molecular motions in an amorphous

In the amorphous state, the distribution of polymer chains in the matrix is completely random, with none of the strictures imposed by the ordering encoimtered in the crystallites of partially crystalline polymers. This allows the onset of molecular motion in amorphous polymers to take place at temperatures below the melting point of such crystallites. Consequently, as the molecular motion in an amorphous polymer increases, the sample passes from a glass through a rubberhke state imtil finally it becomes molten. These transitions lead to changes in the physical properties and material application of a polymer, and it is important to examine physical changes wrought in an amorphous polymer as a result of variations in the molecular motion. 

Brown (24,29) proposed a model of homogeneous yielding based on the Mie (Lennard-Jones) interaction potential. According to Brown, there are three types of molecular motions in an amorphous polymer up to shear 5deld strain ... 

As indicated above, a relaxation is associated with molecular motions in which crytalline entities take part. However, the development of this process apparently requires the presence of an amorphous phase. Actually, as shown in Figure 12.32, the relaxation curves of polymethylenic waxes in which the crystallites are formed by totally extended chains (degree of crystallinity 100%) do not present a relaxation (41,42). Since neither totally crystalline nor totally amorphous polymers display a relaxation, one must conclude that this absorption is caused by molecular motions occurring in the crystalline-amorphous interphase. 

Techniques which are more specific to the various morphological states, especially the amorphous domain, are needed. NMR and ESR are two such molecular probes. By monitoring the mobilities of protons as a function of temperature, Bergmann has defined the onset of molecular motion in various polymers (14). The applicability of NMR as a measure of molecular motion in polymer solids has been the subject of several reviews 15,16,17). ESR monitors the rotational and translational properties of stable radicals, usually nitroxides, and relates their mobilities to polymeric transitions. As stated in several works (18,19), the radical s sensitivity to freedom of motion of the polymer chain is infiuenced by its size, shape, and polarity. The above probes are both high frequency in nature, 10 -10 Hz. Measurement at high frequency has decreased resolving power for the various transitions in contrast to low frequency or static experiments, such a dilatometry with an effective frequency of 10 Hz (20). 

The physical nature of an amorphous polymer is related to the extent of the molecular motion in the sample, which in turn is governed by the chain flexibility and the temperature of the system. Examination of the mechanical behavior shows that there are five distinguishable states in which a linear amorphous polymer can exist, and these are readily displayed if a parameter such as the elastic modulus is measured over a range of temperatures. 

The molecular motion of a segment at a chain end is more rapid than that in an inner chain site. The fact is clarified by the selective spin labeling in an amorphous polymer. The molecular weight dependencies of glass-rubber transitions at two kinds of sites, the chain end and inner chain sites give a free volume size of micro Brownian motion. 

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