Crystallization, Melting and Morphology of Syndiotactic/Atactic Polystyrene Blends: Molecular Weight Effects of Atactic Polystyrene

Abstract

Summary: Molecular weight (MW) effects of atactic polystyrene (aPS) on the morphologies and crystallization kinetics of syndiotactic polystyrene (sPS) blends have been investigated, and miscibility was confirmed for all the sPS/aPS blends. Addition of aPS(M) or aPS(L) leads to a significant increase in the entropy of mixing and gives rise to a pronounced decrease in the equilibrium melting temperature of the blends. On heating from the glassy state, the crystallization peak temperature of the sPS/aPS(H) blend increased with increasing aPS content due to a dilution effect, whereas those of sPS/aPS(M) and sPS/aPS(L) decreased mainly due to enhanced chain mobility. On cooling from the melt state, the crystallization peak temperature decreased with increasing aPS content regardless of the aPS MW. A higher activation energy is required for the sPS component transport from the miscible melt to the growth front in the sPS/aPS(L), suggesting that extra energy is required for the demixing process. Interspherulitic segregation of aPS diluents was evident in the sPS/aPS(L) blends, while interfibrillar segregation morphology was obtained in the sPS/aPS(H) as well as the sPS/aPS(U) blends.

Crystallization and Morphological Features of Syndiotactic/Atactic Polystyrene Blends at High Temperatures

“…If the lamellar thickening rate (l c /l à g ) is further assumed intact after aPS addition, the lamellar thickness developed in the blend is proportional to 1/DT, suggesting that l c should be increased with increasing aPS-L (or aPS-M) content due to the melting point depression. [23] Moreover, if l 1 were selected to be l c , the amorphous layers were thickened with increasing aPS-L content, giving rise to the interlamellar segregation morphology, which is inconsistent with TEM observations (discussed later). Based on the above arguments, l 2 is assigned to be l c ( Figure 10) and its composition dependence is shown in Figure 9.…”

“…According to the Flory-Huggins theory, T 0 m of the sPS/aPS-U and sPS/aPS-H blends remains substantially unchanged for different compositions, whereas the addition of low-M w aPS components leads to a pronounced increase in the entropy of mixing in the sPS/aPS-M and sPS/aPS-L blends and gives rise to a considerable decrease in T 0 m . [23] For instance, a depression of 4 and 12 8C in T m 0 has been reported respectively for the sPS/aPS-M and sPS/aPS-L blends containing 50% aPS. Here an intriguing question arises, regarding whether the lower T m obtained in the blends is exclusively associated with T m 0 depression or whether it is due to lamellar thickness variation as well.…”

“…[38] Due to this entropic driving force, aPS-U chains are unable to be accommodated in the amorphous layer, and IL segregation has never been observed for the sPS/aPS-U blend as it crystallizes at high temperatures (%245-265 8C). [23] In contrast, the IL segregation mode has been observed with a similar d value for the sPS/aPS-U blends cold crystallized at low temperatures near the glass transition (%100 8C). [39] In fact, lamellae with short lateral dimensions were found in the cold crystallized sPS [39] in contrast with those developed in the melt crystallized sPS where long and parallel lamellae were detected.…”

“…Using SEM and TEM, IF segregation was readily observed for the sPS/aPS-U and sPS/aPS-H blends crystallized at T c from 245 to 265 8C. [11,23] The dimensions of the segregated domains became larger at a higher T c . Moreover, interfibrillar aPS-M segregation was also dominant for the sPS/aPS-M blends containing less than 36% aPS-M. [23] For blends having a higher aPS-M content, however, the aPS-M component was expelled a large distance during sPS crystallization, and was confined mainly between sPS spherulites, leading to an IS segregation morphology.…”

“…[11,23] The dimensions of the segregated domains became larger at a higher T c . Moreover, interfibrillar aPS-M segregation was also dominant for the sPS/aPS-M blends containing less than 36% aPS-M. [23] For blends having a higher aPS-M content, however, the aPS-M component was expelled a large distance during sPS crystallization, and was confined mainly between sPS spherulites, leading to an IS segregation morphology. [11] On the other hand, IS segregation of aPS-L upon sPS crystallization was always observed for all the sPS/aPS-L blends studied.…”