Crystallization of bisphenol‐A polycarbonate induced by organic salts: Chemical modification of the polymer. III. Reaction mechanism

Abstract

In the presence of a sodium arylcarboxylate or arylphenoxide, bisphenol‐A polycarbonate (PC) undergoes complex chemical modifications at high temperatures. The reaction mechanism is similar to the one previously established for model systems. Initially, the salt reacts with the carbonate groups of the polymer. This lowers the number‐average molecular weight and produces ionic chain ends of the phenoxide type. A fast transesterification reaction is then induced by a continuous exchange between the phenoxide and the carbonate groups, affecting the molecular distribution until an equilibrium is attained. In the presence of CO2, the phenoxide‐terminated PC undergoes further chemical modifications (formation of phenyl salicylate and phenyl phenoxybenzoate groups) leading to progressive crosslinking of the polymer.

Melt Crystallization of Bisphenol A Polycarbonate in Blends of Polycarbonate with Zinc Salts of Sulfonated Polystyrene Ionomers

“…The effect of sodium benzoate (SB), on the degradation of PC is also shown in Figure 7; a substantial molecular weight decrease, ∼30%, occurred during a 7 min preheat at 250 °C, and the degradation during annealing at 190 °C was also significant. These data are consistent with the results reported by Legras and co-workers [7][8][9][10][11][12][13] in which the M w of PC decreased by more than 40% after mixing PC and 1% SOCB for only 2 min at 270 °C. After annealing the PC/SB mixture for 5-6 h at 190 °C, part of the sample was no longer soluble, indicating that the PC cross-linked, which is also consistent with the effect of SOCB on PC reported by the Legras group.…”

“…Although alkali metal salts of aromatic carboxylic acids, such as SOCB and SB, induced rapid melt crystallization of PC, once the polymer was melted at elevated temperature, the crystallization process was not repeatable. [7][8][9][10][11][12][13] That result is in stark contrast to the crystallization/melting behavior of the PC-ZnSPS blends where the PC could be repeatedly crystallized and melted, which supports the conclusion that the crystallization of the PC in the PC-ZnSPS blends was not due to the same "chemical nucleation" mechanism as described by Legras and co-workers [7][8][9][10][11][12][13] for the PC/ arylcarboxylate salt mixtures.…”

“…For that composition, the sulfonation levels used in this study, ca. 4-13 mol %, correspond to about 0.1-0.2 wt % zinc sulfonate, which is comparable to the 0.1-1 wt % SOCB and SOCP (see Introduction) used by Legras and co-workers [7][8][9][10][11][12][13] in their work on chemical nucleation of PC.…”

“…Although pure PC is relatively stable at this temperature, the thermal stability of PC can be significantly influenced by the presence of a second componentsas was discussed earlier in this paper for blends of PC with alkali metal salts of aryl carboxylates. [7][8][9][10][11][12][13] Degradation of PC lowers the molecular weight, which also accelerates melt crystallization. Therefore, with regard to the SPS induction of PC crystallinity, it is important to ascertain whether PC degradation is the origin of that phenomenon.…”

“…The authors called the nucleation of PC by SOCB and SOCP "chemical nucleation" to distinguish it from conventional heterogeneous nucleation where no chemical reaction occurs. [7][8][9][10][11][12][13] Lightly sulfonated polystyrene ionomers (SPS) are miscible with PC for a range of sulfonation level that depends on the cation used, and the blends exhibit upper critical solution temperature (UCST) phase behavior. [14][15][16] Weiss and co-workers 16,17 reported that the PC in PC/SPS blends crystallized when the blend was cooled from the melt, but the phenomenon was not well characterized in those papers.…”