Exothermal Process in Miscible Polylactide/Poly(vinyl phenol) Blends: Mixing Enthalpy or Chemical Reaction?

Abstract

Summary: In a previous paper (Macromolecules, 2005, 38, 9221), the enthalpy of mixing in poly(DL‐lactide) /poly(vinyl phenol) blends was directly measured by DSC. The first DSC scan for solution/precipitation blends showed phase separation, but miscibility was observed in the second DSC scan. Hence, miscibility was achieved after thermal treatment, an unusual behavior in polymer blend current research. However, the exothermal event observed during the first heating scan could also be the result of a chemical reaction. In this work, a new research study conducted to elucidate the nature of the exothermal heat observed in PDLA/PVPh during the first DSC heating scan. Since the single‐phase PDLLA/PVPh blend obtained after thermal treatment can be redissolved and reprecipitated, results obtained in the consecutive DSC scan prove that the process is completely reversible. Furthermore, GPC and 13C NMR results provide evidence that there is no change in the chemical structure of the studied polymers before and after the thermal treatment, which evidences the absence of transesterification reactions. Therefore, it can be concluded that the exothermal heat is the result of a mixing process, and miscibility is a consequence of specific interactions.New structure expected for transesterfication reactions.magnified imageNew structure expected for transesterfication reactions.

Compatibilization through Specific Interactions and Dynamic Fragility in Poly(D,L‐lactide)/Polystyrene Blends

“…The analysis by gel permeation chromatography (GPC) of samples subjected to the first heating scan confirmed that the molecular weight distribution of PDLLA (M w ¼ 1.1 Â 10 5 ) does not change during the mild thermal treatment; in addition transesterification or other possible degradation reactions did not occur, because no new bonds were observed by 13 C NMR. [18] An exotherm located at relatively low temperatures (about 140 8C) could also be observed during the first heating scan of the blends obtained by solution/precipitation. This low temperature should be safe for both PHS and PDLLA and can be hardly related to any degradation process.…”

“…Hence a single T g was observed in the following scan, leading to miscibility. [18] The PLLA/ PSHS16 system studied here behaves similarly, yet there is no enthalpy of mixing appearing since fewer specific interactions than in PDLLA/PHS blends are taking place. In addition, the glass transitions observed for the blends with intermediate compositions are broader than those observed for the blends with one dominating component.…”

“…These results agree with the reported immiscibility between PLLA and PS. [2] Taking into account that polylactide/PHS blends were reported miscible when prepared by solution/ precipitation method, [15,16,18] we have been interested in blending PDLLA with different PSHS copolymers to determine the minimum amount of HS necessary in PS chains to achieve miscibility. Figure 2 shows the second DSC scan for PDLLA/PSHS 50:50 blends obtained by solution-precipitation.…”

“…This result suggests certain compositional heterogeneity and can be attributed to the expected domain sizes in the initial samples obtained by solution/precipitation (larger domain sizes can be expected for samples in the middle of the compositional range). It must be noted that in the PDLLA/PHS blend studied in a recent paper, [18] broad transitions were not observed, probably because in that system the larger number of specific interactions provides a larger driving force for miscibility. In summary PSHS16 when blended with PDLLA establishes the minimum interassociation extent required for miscibility, showing no mixing enthalpy.…”

“…As obtained mixtures were found phase-separated at the molecular level as confirmed in the initial DSC scan, carried out in specifically mild thermal conditions (heating rate 20 8C Á min À1 , final temperature 200 8C) to avoid any polymer degradation reaction (the occurrence of these undesired side reactions was also discarded using DSC and NMR). [18] In the consecutive scan (recorded immediately after completion of the first one) a single T g was observed, suggesting that miscibility was achieved during the first heating scan. The mixing process observed during the first heating scan cannot be attributed to degradation reactions.…”