Amorphous rigidification and cooperativity drop in semi−crystalline plasticized polylactide
Graphical abstract
Introduction
The thermal resistance of polylactide (PLA) can be improved by processing semi−crystalline polylactide (sc−PLA) [[1], [2], [3]], but it often requires adding plasticizer in the formulation to mitigate its brittleness [[2], [3], [4], [5], [6], [7], [8]]. Recent works evaluated capabilities of biodegradable and biobased molecules as plasticizers [[9], [10], [11], [12], [13], [14], [15]]. These molecules are often compared to citrate esters [[16], [17], [18], [19], [20], [21]], which possess good miscibility with PLA [[16], [17], [18], [19]]. Acetyl tributyl citrate (ATBC) has shown good thermal stability during repeated heating/cooling cycles around the glass transition [21]. Despite the high applicative importance of polymer/diluent systems, reports on the physical consequences of plasticizers on the local microstructure on semicrystalline (sc) polymers are rare.
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 glass−formers usually exhibit a deviation from the Arrhenius−type temperature dependence of relaxation time, when approaching glass transition on cooling, this deviation being described by fragility index [27]. Its molecular representation can be provided within cooperative rearranging regions (CRR) concept as proposed by Adam and Gibbs [28]. 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 (MT−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).
Establishing the mechanism of the influence of plasticizer on the amorphous fraction dynamics of sc−PLA is challenging due to the interplay existing between the processing conditions, the obtained microstructure, and the resulting properties of the amorphous phase. But these microstructure features have a significant impact on transport [29] and mechanical [30] properties. Therefore, new results regarding these characteristics will be of interest for enlarging the use properties of PLA.
When PLA undergoes thermal crystallization, the amorphous region is progressively reduced, leading to constrained chains mobility. Due to the incomplete decoupling between the crystalline and amorphous phases, PLA like many polymers is described, by a three phase model, which considers crystalline phase, mobile amorphous fraction (MAF) and rigid amorphous fraction (RAF) [[31], [32], [33], [34], [35]]. The RAF behaves as an interphase with nanometric dimensions [36]. Contrary to MAF, the RAF does not relax at glass transition and devitrifies in a temperature domain between the glass transition and the fusion [37,38]. RAF can be easily formed in the imperfect crystallization conditions [37], and it is detrimental to PLA barrier properties [39].
In neat PLA, the constraining effect of crystals and RAF on the MAF mobility has been investigated in several studies [[40], [41], [42], [43]]. The increase of the fragility [40,41] and the decrease of cooperativity [42,43] have been reported. Besides, two distinct thermal signatures at the glass transition have been observed for the samples with intermediate crystallinity [[42], [43], [44], [45]]. It has been assumed that the low temperature process is the signature of relaxation in the amorphous matrix, which does not undergo geometrical restrictions. On the other hand, the high temperature process has been attributed to the signature of relaxation in the MAF confined into the spherulites with constrained dynamics. Righetti et al. [46] clearly identified the signatures of both constrained and unconstrained MAFs by MT−DSC and dielectric relaxation spectroscopy (DRS) when crystallizing at 85 °C. In contrast, after crystallizing at 145 °C the MAF remains practically unconstrained [46].
In this work we provide new results regarding the respective roles of crystallization and plasticizing molecules on the amorphous phase behavior in PLA. The temperature regime slightly above the glass transition region has been chosen as crystallization conditions to impact the MAF mobility and to promote RAF formation. MT−DSC has been used to calculate the content of MAF and RAF in plasticized sc−PLA for various crystallization times. The MAF dynamics have been investigated using Cooperative Rearranging Region (CRR) concept. The free volume variations with both plasticization and crystallization have been investigated with temperature−dependent PALS.
Section snippets
Materials
PLA grade was 4042D with a D content of 4% as provided by NatureWorks®. ATBC (CAS Number 77–90−7) was purchased from Sigma Aldrich® (France) and used as plasticizer. Hildebrand solubility parameters δ are close between both constituents: δ PLA = [21.9 (MJ m−3)0.5] and δ ATBC = [18.4 (MJ m−3)0.5] [21]. 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
Impact of the annealing on the amorphous fractions characteristics
The intrinsic effect of plasticization on the MT−DSC response of amorphous PLA was investigated in previous works [21,[23], [24], [25], [26]] and is briefly summarized in Supporting Information (Fig. S3). Fig. 1a shows MT−DSC average heat flow curves for amorphous and sc−PLA_2.5 samples after the annealing at Tg + 20 °C for tc = 1–1000 min. The heat capacity step gradually decreases with tc and seems to disappear for the longest annealing time. The cold crystallization peak progressively shifts
Conclusion
The amorphous phase characteristics of plasticized sc−PLA investigated through the three−phase model, the cooperativity concept, and the quantification of free volume exhibit significant changes in comparison to neat sc−PLA. Our results show that the glass transition temperature of plasticized PLA decreases with crystallization, as the probable consequence of the enrichment of amorphous phase by plasticizer expulsed out of the crystalline phase. Besides, a complex interplay between parameters
CRediT authorship contribution statement
N. Varol: Investigation. N. Delpouve: Writing - original draft, Writing - review & editing, Conceptualization, Methodology, Validation. S. Araujo: Formal analysis. S. Domenek: Supervision. A. Guinault: Investigation. R. Golovchak: Investigation, Formal analysis, Supervision. A. Ingram: Investigation. L. Delbreilh: Validation, Methodology, Supervision. E. Dargent: Project administration, Conceptualization, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledegements
RG acknowledges U.S. National Science Foundation (Grant No. DMR-1725188) for the acquisition of PAL spectrometer.
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