Elsevier

Polymer

Volume 114, 7 April 2017, Pages 144-148
Polymer

Effect of molar mass on the α′/α-transition in poly (l-lactic acid)

https://doi.org/10.1016/j.polymer.2017.02.063Get rights and content

Highlights

  • The molar mass affects the kinetics of the α′/α-crystal transition in PLLA.

  • The rate of the α′/α-crystal transition of PLLA decreases with increasing molar mass.

  • The kinetics of the α′/α-crystal transition of PLLA was analyzed by FSC.

Abstract

The effect of molar mass on the reorganization behavior of α′-crystals of poly (l-lactic acid) (PLLA) has been investigated by fast scanning chip calorimetry. The α′-crystals of PLLA were formed by isothermal crystallization of the relaxed melt at 90 °C or lower temperatures, and were then heated at different rates between 2 and 200 K s1. Slow heating permits for all PLLA samples of different molar mass between 2 and 576 kDa reorganization of α′- into α-crystals. It was found that the α′/α-transition is suppressed if heating occurs faster than 10 K s1 in case of the sample with a molar mass of 576 kDa, while the critical heating rate to suppress the formation of α-crystals is increased to 30 and 100 K/s in PLLA with molar masses of around 100 kDa and 2 kDa, respectively. The faster kinetics of the α′/α-transition in case of shorter macromolecules may be explained by faster melting of smaller α′-crystals, faster growth of α-crystals from the non-isotropic melt containing remnants/self-seed from molten α′-crystals, and/or a higher number of such α′-crystal remnants/self-seed.

Introduction

Poly(l-lactic acid) (PLLA) is a biodegradable and bio-based aliphatic polyester derived from renewable sources such as corn sugar, potato, and sugar cane. It is compostable and therefore considered a promising alternative to reduce the municipal solid waste disposal problem by offering additional end-of-life scenarios. Moreover, PLLA has similar optical, mechanical, thermal, and barrier properties as commercially available commodity polymers, therefore rapidly replacing fossil-based polymers for numerous applications. Commercial use of PLLA was initially limited to medical applications due to its high cost and low availability; however, new synthetic routes allow cost-efficient production of high-molar mass PLLA, with the potential to become a commodity. Both, variation of the molar mass as well as the concentration of stereodefects can effectively be used to tailor the rheological properties, structure formation on solidification the melt, and ultimate material properties [1], [2], [3], [4], [5], [6], [7]. The present study focuses on the analysis of the effect of the molar mass on the stability and the reorganization behavior of α′-crystals which typically form on fast cooling of the melt, at temperatures below about 110 °C.

PLLA is polymorphic, that is, depending on crystallization conditions different crystal structures may develop [8]. Crystallization of the melt at temperatures higher than about 120 °C leads to formation of orthorhombic α-crystals in which the molecular segments adopt a 103 helix conformation [9], [10], [11], [12]. At lower crystallization temperatures growth of pseudohexagonal α′-crystals is favored, with the molecule segments showing the same helical structure as in α-crystals, but exhibiting conformational disorder [13]. As such the α′- modification is classified as a mesophase, being a crystal with conformational disorder leading to lower packing density, slightly increased lattice spacings, and lower specific enthalpy of melting compared to the α-phase [14], [15], [16], [17], [18]. Both the α- and α′-crystal modifications may develop during industrial processing, and knowledge about the prevailing crystal form is of importance due to the large impact on ultimate properties, including thermal, barrier and mechanical properties [19], [20], [21].

The α′-form of PLLA is only metastable at the temperature of its formation and below; on heating it transforms irreversibly into the stable α-form or into liquid phase [15], [16], [17], [22], [23], depending on the heating rate [24], [25], [26]. The α′/α-crystal transformation has extensively been studied by temperature-resolved X-ray diffraction (XRD), infrared spectroscopy and by differential scanning calorimetry (DSC). DSC heating scans of PLLA containing α′-crystals typically display first a small endothermic peak at around 150 °C, which is contiguously followed by a small exothermic event, and final endothermic melting around 170 °C. XRD data show that on slow heating α′-crystals transform first to α-phase before melting of the latter [15], [16], [17], [22], [23]. Controversy exists in the literature about the mechanism of the α′/α-crystal transformation. There is suggested that the transformation occurs via a solid-solid phase transition [16], [22] or by melting and melt-recrystallization [24], [25], [26], [27], [28]. The solid-solid phase transformation mechanism was suggested accounting for (i) the faster development of α-crystals upon transformation from the α′-crystals, compared to crystallization of the non-nucleated melt [16], (ii) not detected melting in temperature-resolved XRD experiments [13], [16], and (iii) preservation of high degree of chain orientation during the phase transition in X-ray fiber analyses [13], [22]. Instead, melting of α′-crystals and formation of the α-phase from the melt was proven by temperature-modulated DSC [27], [28], and by fast scanning chip calorimetry (FSC) [24], [25], [26]. FSC showed that in case of a PLLA homopolymer with a molar mass of about 100 kDa, α′-crystals can completely be melted without prior transformation to α-crystals when heating occurs faster than 30 K s-1 [24], [25]. Recently the initial study of the PLLA homopolymer was expanded toward the analysis of the effect of presence of low amount of d-isomer co-units at random position in the chain on the stability and the reorganization behavior of α′-crystals [26]. It was found that the critical heating rate needed to avoid formation of α-crystals is decreased to 20 and 5 K s-1 in PLLA copolymers containing 2 and 4% d-lactic acid co-units in the chain, respectively, which has been interpreted with required segregation of d-isomer co-unit-chain defects and their exclusion from crystallization, slowing down the kinetics of melt-recrystallization. The melting temperature/maximum stability of α′-crystals is also affected by the d-isomer content in the chain, as it decreases from about 150 °C in the PLLA homopolymer to 140 and 135 °C in the copolymers containing 2 and 4% d-isomer co-units, respectively [26]. These studies are now expanded, to take into account the effect of molar mass on the maximum stability and the reorganization behavior of the α′-form of PLLA.

Section snippets

Experimental

PLLA homopolymers were purchased from Polysciences Inc. (USA). There was available a grade with a viscosity-molar mass average of 2 kDa, and three samples with mass-average molar masses of 60, 143, and 576 kDa and polydispersities of 1.86, 1.37, and 1.46, respectively. Note that except for the sample with a molar mass of 2 kDa, the molar mass was determined by GPC after their delivery. The polymers were obtained in form of small chips from which samples were directly prepared for

Results and discussion

Fig. 2 shows three sets of FSC heating scans in the temperature range of melting and reorganization of α′-crystals of PLLA with a molar mass of 2 kDa. The various sets of curves were obtained after α′-crystal formation at 90 °C (bottom), 80 °C (center), and 70 °C (top), with the individual scans recorded at different rates between 5 K s−1 (black curves) and 100 K s−1 (red curves). Slow heating at 5 K s−1 reveals for all samples which were crystallized at different temperature two distinct

Summary

Crystallization of PLLA at high supercooling of the melt leads to formation of conformationally disordered crystals (α′-crystals). These crystals are metastable at the temperature of their formation and at lower temperatures. Heating of α′-crystals first leads to their stabilization [25] and then, at temperatures above their maximum stability limit, that is, at temperatures higher than the zero-entropy production melting temperature [34], to their melting. If the heating rate is sufficiently

Acknowledgments

Financial support by the Deutsche Forschungsgemeinschaft (Grant AN 212/20) is greatly acknowledged. MLDL is thankful for a short term research grant by the Martin Luther University Halle-Wittenberg.

References (34)

  • M. Pyda et al.

    Heat capacity of poly(lactic acid)

    J. Chem. Thermodyn.

    (2004)
  • J. Pretula, S. Slomkowski, S. Penczek, Advanced Drug Delivery Reviews, Polylactides – Methods of Synthesis and...
  • J. Ren

    Biodegradable Poly(lactic acid): Synthesis, Modification, Processing and Applications

    (2011)
  • A.J. Müller et al.

    Crystallization of PLA-based materials

  • J.J. Cooper-White et al.

    Rheological properties of poly(lactides). Effect of molecular weight and temperature on the viscoelasticity of poly(l-lactic acid)

    J. Polym. Sci. Polym. Phys.

    (1999)
  • P. De Santis et al.

    Molecular conformation of poly(S-lactic acid)

    Biopolymers

    (1968)
  • W. Hoogsteen et al.

    Crystal structure, conformation, and morphology of solution-spun poly(l-lactide) fibers

    Macromolecules

    (1990)
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