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A novel method to enhance and control bubble nucleation in poly(l‐lactic acid) (PLLA) foaming, not relying on external nucleating agents, is presented. Amorphous PLLA is annealed at temperatures close to its glass transition, to allow local alignment of polymer chains in the form of nanosized aggregates, called homogeneous crystal nuclei (HCN). PLLA containing HCN is then foamed after sorption of CO2 at 120 °C and 10 MPa. Under the chosen experimental conditions, HCN can promote growth of PLLA spherulites that can be controlled by thermal‐pressure history. Foam morphology is strongly affected by the presence of HCN, which act as heterogeneous nucleation sites for bubbles. Resulting foams are characterized by a bimodal morphology, with bubbles of about 50–70 µm in diameter developed in the bulk, and much smaller cells of few µm, grown upon pre‐existing crystal aggregates. The method detailed here to attain PLLA foams with tailored morphology can be exploited also for other semicrystalline polymers. [ABSTRACT FROM AUTHOR]

Poly(l‐lactic acid) (PLLA) is a biodegradable and biocompatible thermoplastic polyester produced from renewable sources, widely used for biomedical devices, in food packaging and in agriculture. It is a semicrystalline polymer, and as such its properties are strongly affected by the developed semicrystalline morphology. As a function of the crystallization temperature, PLLA can form different crystal modifications, namely α′‐crystals below about 120 °C and α‐crystals at higher temperatures. The α′ modification is therefore of special importance as it may be the preferred polymorph developing at processing‐relevant conditions. It is a metastable modification which typically transforms into the more stable α‐crystals on annealing at elevated temperature. The structure, kinetics of formation and thermodynamics of α′‐ and α‐crystals of PLLA are reviewed in this contribution, together with the effect of α′‐/α‐crystal polymorphism on the properties of PLLA. © 2018 Society of Chemical Industry The structure, kinetics of formation and thermodynamics of the two main crystal modifications of PLLA are reviewed, together with the effect of crystal polymorphism on material properties. [ABSTRACT FROM AUTHOR]

The isotropic melt of poly(L-lactic acid) (PLLA) and random L/D-lactide copolymers with 2% and 4% D-isomer co-units in the chain has been crystallized at 90 °C, which leads to growth of α'-crystals. The maximum melting temperature of the α'-crystals is around 150 °C in case of the homopolymer, and decreases by about 10 and 15 K in the random copolymers containing 2% and 4% D-isomer co-units, respectively. Analysis of the stability of the α'-crystals requires suppression of formation of α-structure, which is achieved by fast heating using a fast scanning chip calorimeter. For the PLLA homopolymer, the critical heating rate above which the α'/α-transition is completely suppressed is around 30 K s-1, which significantly decreases in the copolymers due to the presence of D-isomer co-units. The slower kinetics of the melting-recrystallization process in such random L/D-lactide copolymers is explained by the required segregation of the chain defects. [ABSTRACT FROM AUTHOR]

Melting and reorganization of conformationally disordered crystals (α′-phase) of poly( l-lactic acid) (PLLA) are analyzed as a function of the rate of heating in a wide range between about 10−1 and 103 K s−1. It is found for the first time that the reorganization of conformationally disordered α′-crystals into stable α-crystals can be suppressed by fast heating. Heating of α′-crystals of PLLA at a constant rate, faster than 30 K s−1, leads to its complete melting between 150 and 160 °C and suppression of formation of α-crystals on continuation of heating. Non-isothermal reorganization of α′-crystals into α-crystals only occurs when heating at a rate slower than 30 K s−1. It is evidenced that isothermal reorganization of α′-crystals into α-crystals at 150-160 °C proceeds via melting followed by recrystallization rather than a solid-solid phase transition. [ABSTRACT FROM AUTHOR]

Abstract: The kinetics of crystal nucleation of poly(L-lactic acid) (PLLA) has been analyzed by fast scanning chip calorimetry in a wide temperature range between 313 and 383 K, that is, between temperatures about 30 K below and 40 K above the glass transition temperature. The relaxed melt was rapidly cooled to the analysis temperature and subsequently aged between 10−1 and 104 s, to permit formation of nuclei. The number of formed crystal nuclei has been probed by analysis of the crystallization rate at 393 K. The nucleation rate is maximal at 370–375 K and decreases monotonously with decreasing temperature in the analyzed temperature range. The observation of a monomodal dependence of the nucleation rate on temperature points to predominance of a single nucleation mechanism in the analyzed temperature range, regardless nucleation occurs in the glassy or devitrified amorphous phase. The collected data suggest that nuclei formation at ambient temperature requires a waiting time longer than about 108 s. The performed study is considered as a quantitative completion of nucleation-rate data available for PLLA only at temperatures higher than 360 K, suggesting that the nucleation mechanism is independent on temperature in the analyzed temperature range between 313 and 383 K. [Copyright &y& Elsevier]

Abstract: The influence of crystal polymorphism on the thermal properties of poly(l-lactic acid) (PLLA) is discussed in this contribution. Crystallization of PLLA at high temperatures yields the stable α form, whereas at low temperatures the metastable α′ modification develops, which is characterized by slightly larger lattice dimensions compared to the α counterpart, and by some degree of conformational disorder. Quantitative analysis with conventional and temperature-modulated calorimetry revealed a three-phase structure of PLLA composed of a crystal phase and two amorphous fractions with different mobility, for all the analyzed thermal histories. A higher coupling of the amorphous chain segments with the crystal phase was found in the presence of α crystals, probably due to the slightly larger lattice dimensions and the looser arrangements of PLLA chains in the α′ structure. Some peculiarities in the thermal behavior were rationalized, like an unusual frequency-dependence of the reversing apparent heat capacity upon the solid–solid transition from the α′ to the α crystals. Devitrification of the rigid amorphous segments seems also to be differently affected by the coupled crystal structure for the two analyzed crystal modifications of PLLA. [Copyright &y& Elsevier]

Abstract: This article contains a detailed analysis of the crystallization behavior of poly(l-lactic acid) (PLLA). Crystallization rates of PLLA have been measured in a wide temperature range, using both isothermal and non-isothermal methods. The combined usage of multiple thermal treatments allowed to obtain information on crystallization kinetics of PLLA at temperatures almost ranging from glass transition to melting point. Crystallization rate of PLLA is very high at temperatures between 100 and 118°C, showing a clear deviation from the usual bell-shaped curve. This discontinuity has been ascribed to a sudden acceleration in spherulite growth, and is not associated to morphological changes in the appearance of PLLA spherulites. Experimental data of spherulite growth rates of PLLA have been analyzed with Hoffman–Lauritzen method. Applicability and limitations of this theoretical treatment have been discussed. [Copyright &y& Elsevier]

A method recently proposed for measuring spherulite growth rates (G) using temperature programs, tailored for each polymer, is reviewed. This method, compared to the conventional isothermal procedure, permits to expand the temperature range where spherulite growth rates can be measured. Examples of application of this method are reported, in particular, the spherulite growth rates of isotactic polypropylene (iPP) and poly(l-lactic acid) (PLLA) are analyzed. For iPP, growth rates were obtained from 112 to 148 °C using different cooling rates and a self-nucleation procedure. For PLLA, measurements in both isothermal and non-isothermal conditions allowed to overcome the difficulties due to the very high nucleation density that prevent determination of growth rates at low crystallization temperatures. For this polymer the entire curve of G vs. T was obtained. [Copyright &y& Elsevier]

This work details the first example of enhancement of crystallization rate of poly (l -lactic acid) (PLLA) by introducing optically pure short chain branches. A biobased PLLA copolymer was prepared by initial radical functionalization of commercial PLLA with itaconic anhydride (IAH) in a Brabender mixer, then by reaction with a tailor-made hydroxyl-terminated, optically pure PLLA with molar mass M n = 4 kDa, the latter prepared via ring opening polymerization. Gel permeation chromatography and infrared spectroscopy proved the efficiency of grafting reaction, with the amount of grafted IAH quantified via UV–Vis. The synthesized graft copolymer displays faster crystallization rate with respect not only to the commercial grade, but also to a binary blend with the same nominal composition. The role played by the short branches in favoring both crystal nucleation and growth was discussed in terms of molecular nucleation. The results detailed in this manuscript demonstrate that synthesis of a graft copolymer with optically pure short branches is an efficient way to improve the poor crystallization kinetics of PLLA, which is one of the major drawbacks of this polymer. [Display omitted] • Optically pure short branches were grafted on poorly stereoregular commercial PLLA. • Optically pure short chains crystallize faster and facilitate crystal growth of PLLA. • Crystallization kinetics of PLLA can be enhanced by tailored chain structure. [ABSTRACT FROM AUTHOR]