Polystyrene ring motion

We have chosen motion about the C2 axis to model ring motion in polystyrenes. We acknowledge that these motions are likely to be more complicated than just C2 rotations. 

Comment For polystyrene, a /J-like relaxation process was attributed to phenyl-ring flips, while the main chain does not participate, see Section 3.3. Hence, when the backbone motion of a selectively labeled compound is studied, polystyrene can be regarded as a type A glass former. 

Schaefer and coworkers, in another chapter in this text, used 1h - 13(j dipole-dipole "line shapes" obtained in a very clever way to investigate rotational motion of the aromatic rings in polystyrene. The method used a WAHUHA pulse sequence to decouple proton-proton dipolar interactions, cross polarization to enhance signal acquisition and an overall sampling technique synchronous with the sample rotation. The dipole-dipole interaction was mapped in rotational sideband spectra obtained from 16 "normal" CP/MAS spectra. The method, though somewhat involved, provided a measure of dipole-dipole line-shapes which can be interpreted in terms of side-chain rotation in the polymer. 

The amplitudes of ring- and main-chain motions of a variety of polystyrenes have been established from the 13C NMR magic-angle spinning sideband patterns of dipolar and chemical shift tensors. The frequencies of the same motions have been determined by TiCC) and TjpCC) experiments. The most prevalent motion in these polymers is restricted phenyl rotation with an average total displacement of about 40°. Both the amplitude and frequency of this motion vary from one substituted polystyrene to another, and from site to site within the same polystyrene. A simple theory correlates the observed ring dipolar patterns with s. 

An increase in side group bulkiness generally serves to raise Tg. Ring-substituted polystyrenes show this effect and, in the case of polyacenaphthalene, the motions of the chain are so severely restricted that the Tg occurs at 264 °C. Chain microstructure is clearly important in determining chain mobility. We have already discussed the case of random copolymers on the basis of the Gibbs-DiMarzio theory and have developed equation (5-19), which shows that the Tg of a random copolymer is intermediate between the Tg s of the corresponding homopolymers. Equation (5-19) is similar in form to the empirical expression of Wood7... 

The presence of dissolved CO2 molecules in a polymer results in the plasticization of the amorphous component of the matrix. In this respect CO2 mimics the effect of heat but with the important distinction that the Tg is depressed. The extent of the Tg depression is dependent on the wt% of CO2 in the matrix. As previously mentioned, one of the characteristics of plasticization is the enhancement of segmental motion, which has been observed spectroscopically for the ester groups of PMMA [20] and the phenyl rings of polystyrene [21]. The consequential increase in free volume of the matrix has been studied by methods such as laser dilatometry [22], in situ FTIR spectroscopy [20], high-pressure partition chromatography [23], and inverse gas chromatography [24]. 

The fields have advanced way beyond the simple determination of spectra and correlating particular bands with particular chemical groups. Today, specific motions are calculated. For an example, see Figure 2.1 (4). Here, two conformational displacements of polystyrene are shown—one near 550 cm" in the infrared spectrum, and one near 225 cm in the Raman spectrum. These motions illustrate a degree of coupling between the ring and backbone vibrations. 

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