Our latest work on biodegradable and biobased block copolymers prepared by reactive extrusion, which was performed in collaboration with AIMPLAS was just published in Soft Matter. In this paper, with the title "Direct and Indirect Effects on Molecular Mobility in Renewable Polylactide-Poly(propylene adipate) Block Copolymers as Studied by Dielectric Spectroscopy and Calorimetry", is the next step of our ongoing research on biodegradable and biobased copolymers that can be synthesized with reactive extrusion, a fast and economical alternative to the traditional polymerization procedure. In fact, with reactive extrusion, PLA and its copolymers can by synthesized in under 20 min.
Abstract
In this work, we study a series of sustainable block copolymers based on polylactide, PLA, and poly(propylene adipate), PPAd, both polymers being prepared from renewable resources. Envisaging wide range of future applications in the frame of green and circular economy, e.g., packaging materials replacing conventional petrochemicals, the employment of PPAd aims at lowering of the glass transition and melting temperatures of PLA and, finally, facilitating the enzymatic degradation and compostability. The copolymers have been synthesized via ring opening polymerization of lactide in the presence of propylene adipate oligomers (5, 15 and 25 %). The direct effects on molecular mobility by the structure/composition are assessed in the amorphous state employing broadband dielectric spectroscopy (BDS) and calorimetry. BDS allowed the recording of local PLA and PPAd dynamics at all cases. The effects on local relaxations suggest favoring of interchain interactions, both PLA-PPAd and PPAd-PPAd. Regarding the more important segmental dynamics, the presence of PPAd leads to faster polymer chains diffusion, as monitored by the significant lowering of the dielectric and calorimetric glass transition temperature, Tg. This suggests the plasticizing role of PPAd on PLA (majority) in combination to the lowering of the average molar mass, Mn, in the copolymers from ~75 to ~30 kg/mol, which is an actual scope for these materials’ synthesis. Interestingly, a strong suppression in fragility (chains cooperativity) is additionally recorded. In contrast to calorimetry and due to high resolving power of BDS, for the higher PPAd fraction, the weak segmental relaxation of PPAd was additionally recorded. Overall, the recordings suggest strong increase in free volume and two individual dynamics states, one for 0 and 5 % PPAd and another for 15 and 25 % PPAd. Within the latter, we gained indications for partial phase nano-separation of PPAd. Regarding indirect effects, these were followed via crystallization. Independently from the method of crystallization, namely, melt- or cold-, the presence of PPAd leaded to systematic lowering of crystallization and melting temperatures and enthalpies. The effects reflect the decrease of crystal nuclei, which is confirmed by optical microscopy as in the copolymers fewer, however, larger crystals are formed.
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