Internal motions in high polymers

One useful approach to the description of internal motion is to follow time dependence of the ensemble-average mean-squared displacement, Z, along the laboratory frame gradient axis as tl PGSE pulse separation time, A, is varied. The characteristic behaviour of as respectively dependent on 1/4 1/2 various time regimes represents a signature for 

---

Heijboer [28] has reported the dynamic mechanical properties of poly(nethacrylate)s with different size of the saturated ring as side chain. The y relaxation in these polymers is attributed to a conformational transition in the saturated ring. In the case of poly(cyclohexyl methacrylate), the transition is between the two chair conformations in the cyclohexyl ring. However, this type of internal motion in hindered by rather high intramolecular barriers, which can reach about 11 kcal mol-1. 

N. Nemoto, Y Makita, Y. Tsunashima, and M. Kurata. Dynamic light scattering studies of polymer solutions. 3. Translational diffusion and internal motion of high molecular weight polystyrenes in benzene at infinite dilution. Macromolecules, 17 (1984), 425 30. 

Attempts have been made to identify primitive motions from measurements of mechanical and dielectric relaxation (89) and to model the short time end of the relaxation spectrum (90). Methods have been developed recently for calculating the complete dynamical behavior of chains with idealized local structure (91,92). An apparent internal chain viscosity has been observed at high frequencies in dilute polymer solutions which is proportional to solvent viscosity (93) and which presumably appears when the external driving frequency is comparable to the frequency of the primitive rotations (94,95). The beginnings of an analysis of dynamics in the rotational isomeric model have been made (96). However, no general solution applicable for all frequency ranges has been found for chains with realistic local structure. 

If an amorphoiis polymer is cooled it will usually attempt to crystallize, but because of the high internal viscosity of the medium it is often precluded from packing into its lowest energy conformation. At 0 K, the lack of thermal excitation prevents the occurrence of most photochemical reactions. As the temperature is increased, the specific volume of the polymer will also increase as a result of forming "free volume", that is, space vdiich is not occupied by hard-shell dimensions of the atoms comprising the polymeric structure. The amount of free volume will depend to a certain extent on the previous thermal history. As free volume increases along with thermal excitation, various kinds of molecular motions will be observed in the polymer vdiich can be detected by I ysical measurements. 

We shall show by a very simple example how these ideas can be transformed into mathematical relations, for it will appear later that many an important property of high polymeric substances is connected directly with the internal motions of large, chain or reticulate molecules. E. Wohlisch in 1927 advanced the idea that the tendency of rubber and muscles to contract is traceable to the thermal motion of molecules and K. H. Meyer showed in 1932 that it is not a question of the motions of entire molecules, but of links in the principal chains which cause the contraction of the extended chain by their thermal motions. The quantitative demonstration of this idea was furnished by H. Mark and his co-workers and independently by W. Kuhn the experimental proof occupies an important position in the field of high polymers. 

Polymers in solution phases have a high degree of freedom for translational and internal motion. They change their conformations randomly by Brownian movements. The purpose of this section is to see how these molecular characteristics of polymers lead to the macroscopic properties of the polymer solutions. 

---