Crystallization and melting of poly(lactic acid)

This contributions summarizes the latest 15 years of research on crystallization and melting of poly(lactic acid), a biodegradable and biobased polyester of growing industrial interest. PLA is eco-friendly, since, apart from being derived from renewable resources like corn, wheat, or rice, it is recyclable and compostable [1, 2]. PLA is biocompatible, as it has been approved by Food and Drug Administration (FDA) for direct contact with biological fluids [3], and has better thermal processability compared to other biopolymers like poly(hydroxyalkanoates), poly(ethylene glycol), or poly(?-caprolactone) [4]. Moreover, PLA requires 25-55% less energy to be produced than petroleum-based polymers, and estimations show that this can be further reduced to less than 10% [5]. Unfortunately PLA has also a few drawbacks, which limit its use in certain applications. Limitations include poor mechanical properties and a low crystallization rate. The latter causes difficulties to processing of end-use articles. Kinetics of crystal nucleation of PLA is maximal at 90-100 °C, as probed by fast scanning calorimetry and optical microscopy [6-7]. Overall crystallization kinetics of PLA was determined in a wide temperature range, from just above the glass transition temperature to close the melting point [8]. The most peculiar behaviour is a discontinuity in crystal growth rate, around 110-120 °C, first highlighted in Ref. [9], which is caused by crystal polymorphism: crystallization at temperatures higher than about 120°C leads to formation of orthorhombic a- crystals, which is replaced by growth of pseudohexagonal a'-crystals at temperatures lower than about 120°C [10- 12]. Crystallization kinetics of poly(lactic acid) is largely affected by chain parameters. PLA is produced by polymerization of lactic acid, which has two optically active forms called L-lactic acid and D-lactic acid. Commercial PLA grades are usually produced from L-rich mixtures and typically comprise a minimum of 1-2 % Dlactic acid units [11-12]. The content of L- and D-lactic acid affects both the crystallization rate and the formation of a- and a'- crystals of PLA, with a decrease of the overall crystallization rate of PLA, as well as of the rate of spherulite growth of both the crystal modifications, when D-lactic acid content is increased [13]. The latter also causes a shift of the temperature range of the formation of the two crystal modifications to lower temperatures. Moreover, PLA chain length also affects the growth rate of the a- and a'- spherulites: independently of D-lactic acid content, an increase of molecular mass leads to a lower maximum crystallization rate, but does not affect the a/a'- crystal polymorphism of PLA upon melt crystallization [14] Crystallization of PLA into a'- or a- modification has a strong influence on material properties, including mechanical and barrier properties, since replacement of a'- crystals by a -crystals leads to quantitative change of properties like Young's modulus, elongation at break, and water vapor transmission rate [15-16]. Also the thermal properties of PLA are highly dependent on crystal polymorphism, as a-crystals have higher melting and enthalpy then a'-crystals [17]. The temperature dependence of melting enthalpy of PLA was recently quantified. At the respective melting temperatures of 150 °C and 180 °C, the equilibrium melting enthalpy values of the a'- and a- forms are 107 and 143 J g-1 [18]. At parity of temperature, the enthalpy of melting of a'-crystals is about 25 J g-1 lower than that of the a-form, which is linked to the presence of conformational defects in the disordered a'- modification [18]. During heating, the metastable condis a-phase reorganize into stable a-crystals via partial melting and recrystalliza