Rotation side-group

An example of a secondary transition is the side group rotation -CO-O-CH3 with PMMA, which is set free at about 25 °C on a time scale round 1 sec. PVC shows a weak transition at -30 °C as a result of freedom of rotation for the Cl atoms. A very elegant illustration is given by polycyclohexylmethacrylate in comparison with polyphenylmethacrylate (Heijboer). The polymers are nearly identical the difference is that the first has a saturated ring as a side group, and the second an unsaturated one ... 

A dipole moment of type (c) in a flexible side chain, e.g. as in poly(methyl methacrylate), imposes a dipole moment which can be resolved into a rigid, perpendicular component and a rotatable component. One might have supposed that two separate relaxation processes might then have occurred, but in practice only one, high-frequency, relaxation is observed and one concludes that in solution the side-group rotation and segmental motion in the main chain are combined in a single, fast process (North, 1972). 

Horle and Mita (5) measured the phosphorescence decay of benzophenone in PMMA over a wide temperature range (80 to 433K). Non-exponential decays were observed for temperatures between T (onset of ester side group rotation of the matrix polymer) and T (glass transition temperature), and the decay profile was ... 

Typical decay curves of benzophenone (BP) phosphorescence (analyzed at 450 nm) at various temperatures in PMMA excited by a 10-ns nitrogen laser pulse at 337 nm are shown in Fig. 1 (6). The phosphorescence intensity, l(t), decreases as a single exponential below the temperature corresponding to the ester side-group rotation (Tq = -3O C for PMMA). Deviations from a single-exponential decay are observed for T>Tg, which increase with increasing temperature, but the deviation becomes less marked above the glass transition temperature, T, of the matrix polymer and disappears at 15O C. 

A second molecular motion that might be able to take place at low temperatures is rotation of, or inside, the substituent side group on the chain.Very often, too, the libration and side group rotation are coupled, so that both occur at the same temperatures and require the same times. 

Side group rotations become important in polymers with ester side chains, of which there are very many. Of technological significance is the low temperature transition in poly(methyl methacrylate). Another interesting conformation change is the boat-chair switch in cyclohexyl groups (Figure 5.5). 

Evidently there are some cases where the thermal motion of a side chain has a significant effect on that of the main chain. An example of this is polymethyl methacrylate. in which the ester side-group rotation is rather strongly coupled to the thermal motion of the main chain. By liberation of the rotation of tte ester side group, the motion of the main chain is also affected (see Ref. 8, p. 122). 

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