Elsevier

Polymer

Volume 194, 24 April 2020, 122373
Polymer

Amorphous rigidification and cooperativity drop in semi−crystalline plasticized polylactide

https://doi.org/10.1016/j.polymer.2020.122373Get rights and content

Highlights

  • Plasticized PLA has been crystallized 20 °C above its glass transition.

  • Crystallization induces a shift of the glass transition to lower temperatures.

  • Fastest amorphous rigidification in plasticized PLA.

  • High increase of free volume during non-isothermal crystallization.

  • Cooperativity drops with the amorphous rigidification.

Abstract

Plasticization of amorphous polylactide shifts the glass transition and extends its temperature range of crystallization to lower temperatures. In this work, we focus on how low−temperature crystallization impacts the mobility of the amorphous phase. Plasticizer accumulates in the amorphous phase because it is excluded from the growing crystal. The formation of rigid amorphous fraction is favored by the low crystallization temperature. It reaches values up to 50% in plasticized polylactide. The increase in the content of rigid amorphous fraction coincides with both the increase of free volume quantified by positron annihilation lifetime spectroscopy, and the decrease in the cooperativity length obtained from the temperature fluctuation approach. The drop of cooperativity is interpreted in terms of mobility gradient due to the amorphous rigidification.

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.

References (88)

  • M.C. Righetti et al.

    Crystalline, mobile amorphous and rigid amorphous fractions in poly(L-lactic acid) by TMDSC

    Thermochim. Acta

    (2011)
  • A. Magon et al.

    Study of crystalline and amorphous phases of biodegradable poly(lactic acid) by advanced thermal analysis

    Polymer

    (2009)
  • S. Fernandes Nassar et al.

    Multi-scale analysis of the impact of polylactide morphology on gas barrier properties

    Polymer

    (2017)
  • E. Zuza et al.

    Glass transition behavior and dynamic fragility in polylactides containing mobile and rigid amorphous fractions

    Polymer

    (2008)
  • N. Delpouve et al.

    Cooperative rearranging region size in semi–crystalline poly(L–lactic acid)

    Polymer

    (2008)
  • K. Liao et al.

    Effects of physical aging on glass transition behavior of poly(dl−lactide)

    Eur. Polym. J.

    (2002)
  • J. Kansy

    Microcomputer program for analysis of positron annihilation lifetime spectra

    Nucl. Instrum. Methods A

    (1996)
  • M. Eldrup et al.

    The temperature dependence of positron lifetimes in solid pivalic acid

    Chem. Phys.

    (1981)
  • R. Zaleski et al.

    Pick–off models in the studies of mesoporous silica MCM–41. Comparison of various methods of the PAL spectra analysis

    Radiat. Phys. Chem.

    (2007)
  • T. Goworek et al.

    On possible deviations of experimental PALS data from positronium pick-off model estimates

    Chem. Phys.

    (2002)
  • S. Weyer et al.

    Phase angle correction for TMDSC in the glass−transition region

    Thermochim. Acta

    (1997)
  • M. Pyda et al.

    Heat capacity of poly(lactic acid)

    J. Chem. Thermodyn.

    (2004)
  • M.C. Righetti et al.

    Enthalpy of melting of α'– and α–crystals of poly(L–lactic acid)

    Eur. Polym. J.

    (2015)
  • M.C. Righetti et al.

    Temperature dependence of the rigid amorphous fraction in poly(ethylene terephthalate)

    Eur. Polym. J.

    (2014)
  • Q. Ma et al.

    Relationship between the rigid amorphous phase and mesophase in electrospun fibers

    Polymer

    (2013)
  • Q. Ma et al.

    Constraints in semicrystalline polymers: using quasi–isothermal analysis to investigate the mechanisms of formation and loss of the rigid amorphous fraction

    Polymer

    (2011)
  • J. Lin et al.

    Oxygen solubility and specific volume of rigid amorphous fraction in semicrystalline poly(ethylene terephthalate)

    Polymer

    (2002)
  • Y. Furushima et al.

    The characteristic length of cooperative rearranging region for uniaxial drawn poly(ethylene terephthalate) films

    Polymer

    (2013)
  • X. Shi et al.

    Synergistic effects of nucleating agents and plasticizers on the crystallization Behavior of poly(lactic acid)

    Molecules

    (2015)
  • M.P. Arrieta et al.

    Combined effect of poly(hydroxybutyrate) and plasticizers on polylactic acid properties for film intended for food packaging

    J. Polym. Environ.

    (2014)
  • S. Jacobsen et al.

    Plasticizing polylactide—the effect of different plasticizers on the mechanical properties

    Polym. Eng. Sci.

    (1999)
  • M. Baiardo et al.

    Thermal and mechanical properties of plasticized poly(L–lactic acid)

    J. Appl. Polym. Sci.

    (2003)
  • A. Ruellan et al.

    Solubility factors as screening tools of biodegradable toughening agents of polylactide

    J. Appl. Polym. Sci.

    (2015)
  • R.N. Darie–Nita et al.

    Evaluation of some eco-friendly plasticizers for PLA films processing

    J. Appl. Polym. Sci.

    (2016)
  • B.W. Chieng et al.

    Epoxidized vegetable oils plasticized poly(lactic acid) biocomposites: mechanical, thermal and morphology properties

    Molecules

    (2014)
  • B. Brüster et al.

    Plasticization of polylactide with myrcene and limonene as bio-based plasticizers: conventional vs. reactive extrusion

    Polymers

    (2019)
  • X. Yang et al.

    Migration resistant glucose esters as bioplasticizers for polylactide

    J. Appl. Polym. Sci.

    (2015)
  • J.M. Ferri et al.

    The effect of maleinized linseed oil as biobased plasticizer in poly(lactic acid)‐based formulations

    Polym. Int.

    (2017)
  • L.V. Labrecque et al.

    Citrate esters as plasticizers for poly(lactic acid)

    J. Appl. Polym. Sci.

    (1997)
  • N. Ljungberg et al.

    The effects of plasticizers on the dynamic mechanical and thermal properties of poly(lactic acid)

    J. Appl. Polym. Sci.

    (2002)
  • I. Harte et al.

    The effect of citrate ester plasticizers on the thermal and mechanical properties of poly(DL–lactide)

    J. Appl. Polym. Sci.

    (2013)
  • S. Singh et al.

    Crystallization of triethyl‐citrate‐plasticized poly(lactic acid) induced by chitin nanocrystals

    J. Appl. Polym. Sci.

    (2019)
  • L. Dobircau et al.

    Molecular mobility and physical ageing of plasticized poly(lactide)

    Polym. Eng. Sci.

    (2015)
  • A. Ruellan et al.

    Plasticization of polylactide

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