Polystyrene, local free volume

J. G. Victor and J. M. Torkelson, Photochromic and fluorescent probe studies in glassy polymer matrices. 3. Effects of physical aging and molar weight on the size distribution of local free volume in polystyrene, Macromolecules 21, 3490-3497 (1988). 

Some secondary relaxations of the components in thermoplastic AIPNs have been investigated by thermally stimulated depolarization current (TSDC) techniques and thermally stimulated conductivity (TSC) measurements [10,11]. It was found that upon addition of S-co-AA to CPU, the secondary and 3 CPU peaks (at ca. -140 °C and ca. -100 °C, respectively) shift slightly to lower temperatures, i.e., the corresponding relaxations become faster, these shifts being more pronounced at low S-co-AA contents. The shifts can be related to physical interactions between the IPN components and to their partial miscibility. Rizos et al. [15] have shown that as a result of such interactions, changes in the local free volume may occur, affecting the secondary relaxation times. The same changes in the [3 relaxation of PU have been found in polyurethane/polystyrene IPNs by Pandit and Nadkarni [16]. 

Fig. 5.1 Correlation between the normalized local free volume, and the shift in Tg for 20 nm thin films of polystyrene of different molecular weight (here indicated in kg/mol) 97 (blue circles), 160 (red diamond), 640 (green star), 932 (black triangle). In the panel, shift in Tg as a funetion of the thickness of the irreversibly adsorbed layer. Reproduced with permission from Ref. [7]...

The fluorescence intensity of quinoline derivatives has been found to increase dramatically with an increase in the molecular weight of the host polystyrene. " This is attributed to a decrease in the free volume in the polymer matrix restricting molecular rotation/motion of the fluorophore. Similar effects have been observed for juliodinemalononitrUe in different stereo-regular poly(methyl methacrylates), and temperature effects on the luminescence properties of indole and coumaric acid derivatives in different polymer matrices showed abrupt changes in emission intensity at temperatures which correspond to the onset of local relaxation processes in the polymer. ... 

A similar model can also be constructed for the motion of polymer chains, the main difference being that more than one hole will now be required to be in the same locality, as cooperative motions of a host of consecutive chain atoms are required for the movement of polymer chain segments. Thus, for a polymer chain segment to move from one position to an adjacent site, a critical void volume must first exist before the segment can jump. That empty spaces exist can be inferred from the fact that when a sample of polystyrene is dissolved in benzene there is a contraction in the total volume. This indicates that the polymer can occupy less volume when surrounded by benzene molecules and there must have been unused space in the polymer matrix to allow this increase in packing eflSciency to occur. On this basis, the observed specific volume of a sample, v, can be described as a sum of the volume actually occupied by the polymer molecules, iiq, and the free volume, Vf, in the system [see Fig. 2.27(a)],... 

The surface of a polymeric specimen may behave differently fi-om the bulk in its glass transition behavior. For example (82), the surface Tg of a monodis-perse polystyrene film was observed to be lower than the bulk Tg. This result was interpreted in terms of an increase in the free volume near the surface region, being induced by the preferential surface localization of chain end groups. 

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