Reducing the Gap between the Activation Energy Measured in the Liquid and the Glassy States by Adding a Plasticizer to Polylactide

Abstract

The kinetic fragility of a glass-forming liquid is an important parameter to describe its molecular mobility. In most polymers, the kinetic fragility index obtained from the glassy state by thermally stimulated depolarization current is lower than the one determined in the liquid-like state by dielectric relaxation spectroscopy, as shown in this work for neat polylactide (PLA). When PLA is plasticized to different extents, the fragility calculated in the liquid-like state progressively decreases, until approaching the value of fragility calculated from the glass, which on the other hand remains constant with plasticization. Using the cooperative rearranging region (CRR) concept, it is shown that the decrease of the fragility in the liquid-like state is concomitant with a decrease of the cooperativity length. By splitting the fragility calculated in the liquid, in two contributions: volume and energetic, respectively, dependent and independent on cooperativity, we observed that the slope of the fragility plot in the glass is equivalent to the energetic contribution of the fragility in the liquid. It is then deduced that the difference between the slopes of the relaxation time dependence calculated in both glass and liquid is an indicator of the cooperative character of the segmental relaxation when transiting from liquid to glass. As the main structural consequence of plasticization lies in the decrease of interchain weak bonds, it is assumed that these bonds drive the size of the CRR. In contrast, the dynamics in the glass are independent on plasticization structural effects.

Amorphous rigidification and cooperativity drop in semi−crystalline plasticized polylactide

“…The CRR concept suggests that the relaxation time and the activation energy brutally increase during cooling towards the glass transition because a higher number of structural units must be mobilized to achieve the relaxation process. It was observed [24,25] by modulated temperature differential scanning calorimetry (MTÀ DSC) and dielectric relaxation spectroscopy (DRS) that the ATBC molecules decrease the kinetic fragility index, and that this result coincides with the decrease of the size of CRR [25]. Recently [26], we have shown that the variations of the CRR size in plasticized PLA are consistent with the variations of the free volume obtained experimentally from positron annihilation lifetime spectroscopy (PALS).…”

“…x % ATBC was added to PLA, x being equal to 0 (neat PLA), 2.5, 5, 10 and 15% of PLA initial weight (samples named PLA_x). The procedure for blends processing was similar to those reported in previous studies [24,25]. PLA and ATBC were both dried at 80 � C under vacuum for 12 h, and then blended in the internal mixer (Haake Rheocord 9000®) at 160 � C and 60 rpm for 15 min.…”

“…The intrinsic effect of plasticization on the MTÀ DSC response of amorphous PLA was investigated in previous works [21,[23][24][25][26] and is briefly summarized in Supporting Information (Fig. S3).…”

“…Plasticization of PLA can be explained by several theories based on physical observations [22]. In previous works on fully amorphous PLAs [21,[23][24][25][26], we investigated the influence of ATBC on the glass transition dynamics by thermal analysis techniques. Polymer glassÀ formers usually exhibit a deviation from the ArrheniusÀ type temperature dependence of relaxation time, when approaching glass transition on cooling, this deviation being described by fragility index [27].…”

Antiplasticization of Polymer Materials: Structural Aspects and Effects on Mechanical and Diffusion-Controlled Properties

“…In any case, it should be borne in mind that the observations and measurements were made at room temperature and that the threshold conditions for the antiplasticization/plasticization transition are temperature dependant (as indicated earlier). A similar argument for plasticizers used for toughening of polylactic acid (PLA) [71]. The authors did not detect antiplasticization behaviour from studies of the relaxation spectra of mixtures of PLA and acetyl tributylcitrate (ATBC) even at concentrations as low as 2.5%.…”