Revisiting Non-Conventional Crystallinity-Induced Effects on Molecular Mobility in Sustainable Diblock Copolymers of Poly(propylene adipate) and Polylactide

Abstract

This work deals with molecular mobility in renewable block copolymers based on polylactide (PLA) and poly(propylene adipate) (PPAd). In particular, we assess non-trivial effects on the mobility arising from the implementation of crystallization. Differential scanning calorimetry, polarized light microscopy and broadband dielectric spectroscopy were employed in combination for this study. The materials were subjected to various thermal treatments aiming at the manipulation of crystallization, namely, fast and slow cooling, isothermal melt- and cold-crystallization. Subsequently, we evaluated the changes recorded in the overall thermal behavior, semicrystalline morphology and molecular mobility (segmental and local). The molecular dynamics map for neat PPAd is presented here for the first time. Unexpectedly, the glass transition temperature, Tg, in the amorphous state drops upon crystallization by 8–50 K. The drop becomes stronger with the increase in the PPAd fraction. Compared to the amorphous state, crystallization leads to significantly faster segmental dynamics with severely suppressed cooperativity. For the PLA/PPAd copolymers, the effects are systematically stronger in the cold- as compared to the melt-crystallization, whereas the opposite happens for neat PLA. The local βPLA relaxation of PLA was, interestingly, recorded to almost vanish upon crystallization. This suggests that the corresponding molecular groups (carbonyl) are strongly involved and immobilized within the semicrystalline regions. The overall results suggest the involvement of either spatial nanoconfinement imposed on the mobile chains within the inter-crystal amorphous areas and/or a crystallization-driven effect of nanophase separation. The latter phase separation seems to be at the origins of the significant discrepancy recorded between the calorimetric and dielectric recordings on Tg in the copolymers. Once again, compared to more conventional techniques such as calorimetry, dielectric spectroscopy was proved a powerful and quite sensitive tool in recording such effects as well as in providing indirect indications for the polymer chains’ topology.

Segmental mobility in sustainable copolymers based on poly(lactic acid) blocks built onto poly(butylene succinate) in situ

“…That is, the PLA chains mainly crystallize and, thus, become immobile, whereas the PBSu-rich segments are mobile and their corresponding weak glass transition is enhanced at the expense of a PLA . 58,65,70 This second scenario implies a crystallization-driven phase separation, 90,91 which is compatible with recent data using X-ray diffraction, 71 denoting that the PLA chains dominate crystallization in the copolymers.…”

“…4c), the maximum change in the T g coincided with the maximum increase in CF c . This observation is not usual, but has been noted in a recent work on PLA-based block copolymers with poly(propylene adipate), the latter being the low-T g component 65 (similar to PBSu in the present work). This effect can be interpreted via two realistic scenarios.…”

“…In general, good agreement between the two methods was recorded, but this is not always the case. 65,84 For neat PLA and copolymers with 1-10 wt% PBSu, the a relaxation points shown in Fig. 10a obeyed the VFTH model function (i.e., the characteristic for cooperative dynamics).…”

“…Among others, the mechanisms of crystallization (i.e., nucleation, chain entanglements, chain folding, spherulitic growth 19,59 ) are strongly connected with the structure and mobility of the polymer chain (length, fragility, diffusion). [60][61][62][63][64][65] An exceptionally useful method to study molecular mobility and dynamics in polymers is dielectric spectroscopy, 66,67 which can be combined with other methods for the study of structure-property relationships.…”

“…Recently, we determined this relationship in a series of renewable copolymers based on PLA 58,65,68 as well as other esters. 57,69 Furthermore, we showed that, for block copolymers, the effects on crystallization related to polymer mobility could be more easily tuned but were stronger when compared with the same polymers in the form of blends.…”