Influence of entanglements on glass transition of atactic polystyrene

Abstract

Freeze‐dried samples were prepared from dilute solutions of atactic polystyrene (a‐Ps) in benzene in a concentration range of 1 × 10−1 to 2 × 10−5 g/mL, and their glass transition temperatures (Tg) were determined by differential scanning calorimetry. It was found that below a critical concentration (C), the Tg of samples decreases linearly with decreasing logarithmic concentration of solutions. The freeze‐dried samples were annealed at a constant temperature for various times or at various temperatures for the same period of time. The Tg of samples prepared from solutions of concentration below C was observed to shift to higher temperature with increasing annealing temperature or annealing time, finally approaching that of the bulk sample. Fourier transform infrared (FTIR) spectra of the freeze‐dried samples were determined and were compared with the bulk sample. Significant changes in line width and absorption intensity were observed in the spectra of the freeze‐dried samples, indicating that the packing of segments is in a more dilatant state. During annealing, the packing of segments in the samples gradually approaches that of the bulk sample, diminishing the differences in FTIR spectra between the freeze‐dried sample and the bulk sample. The experimental observations are discussed, and it is assumed that intrachain and interchain entanglements may play an important role in glass transition. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2243–2251, 2005

Polystyrene freeze-dried from dilute solution: Tg depression and residual solvent effects

“…[10,11] It is found that no depression in T g occurs for polymer concentrations greater than a critical value (ca. 1!10 K3 g/ml for polystyrene) and that depressions increase as the polymer concentration decreases [6,[9][10][11]; for example, in the early work of Braun and Kovacs, no T g depression is reported for polystyrene freeze-dried from a 1% solution in benzene [15]. Although a depression in T g upon freeze-drying from dilute solution is the general result, an increase in T g upon freezedrying from dilute solution is observed [12,13] for polyacrylamide, which has strong hydrogen bonding.…”

“…The magnitude of the change in T g upon freeze-drying from dilute solution depends on the details of sample preparation and the sample itself [2][3][4][5][6][7][8][9]. For example, very large T g depressions, 40-64 K, are reported for polystyrene freeze-dried from dilute cyclohexane solutions [2,3], whereas depressions of between 2 and 15 K have been reported for various polystyrenes freezedried from dilute benzene solutions.…”

“…The cause of the change in T g from the bulk value upon freeze-drying, or other rapid drying techniques, from dilute solution has not been understood although many explanations have been proposed, such as the high surface to volume ratio or lower density [2,14], a change in specific structure or chain conformation during preparation [3,6], and stresses incurred due to freeze-drying [4]. A reduction of entanglements is also thought to result from freeze-drying from dilute solution [4,16,17] although there is contradictory evidence that significant chain interpenetration exists [18,19]; in any case, evidence suggests that reduction of entanglements is not the cause of the T g depression in either freeze-dried materials [4,5] or in thin films [20][21][22], in spite of recent works, which make claims to the contrary [6,8,13].…”

“…For example, very large T g depressions, 40-64 K, are reported for polystyrene freeze-dried from dilute cyclohexane solutions [2,3], whereas depressions of between 2 and 15 K have been reported for various polystyrenes freezedried from dilute benzene solutions. [4][5][6] In related work, rapid vaporization of solvent from dilute polymer solutions has also resulted in the depression of T g for various polymers. [10,11] It is found that no depression in T g occurs for polymer concentrations greater than a critical value (ca.…”

“…Materials confined at the nanoscale, such as in polymer thin films or confined in the nanopores of controlled pore or sol-gel glasses, show behavior that differs from the bulk. [1] Amorphous polymers freezedried from dilute solutions have also been reported to show changes in the glass transition temperature, T g [2][3][4][5][6][7][8][9][10][11][12][13][14]. It was originally speculated that depressions in T g in polymer thin films and in freeze-dried polymers from dilute solutions were related [4,5].…”