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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 influence of both temperature and time on the solid–solid transformation from tetragonal Form II mesophase to trigonal Form I crystals in isotactic poly(butene-1) and its random copolymers with ethylene has been evaluated. The polymers were isothermally crystallized and then annealed at temperatures ranging from −30 to +30 °C for various times, up to 24 h. Incorporation of ethylene co-units into isotactic poly(butene-1) not only leads to a faster solid–solid transformation rate, as already discussed in the literature, but it also affects the temperature of maximum transformation rate. For all the analyzed compositions, at the initial stage of the process, a maximum rate is observed upon annealing around −20 °C, whereas when the transition proceeds to a later stage, it becomes much faster around +20 °C. The occurrence of two maxima in the transformation-temperature profile has been correlated with a nucleation and growth mechanism for the Form II to Form I transition. The influence of ethylene co-units on the transformation rate has been rationalized taking into account the varied composition of the rigid amorphous portions at the lamellar basal planes of the crystals, which confirms the hypothesis of a connection between crystal lamellae rearrangements and the amorphous-crystal boundaries. [ABSTRACT FROM AUTHOR]

The effect of different amounts of ethylene co-units in the butene-1 chain, on the fold-surface structure of crystals of isotactic polybutene-1, has been probed by analysis of the rigid amorphous fraction (RAF). The exclusion of ethylene co-units from crystallization in random butene-1/ethylene copolymers and their accumulation at the crystal basal planes leads to a distinct increase of the RAF with increasing concentration of co-units. A specific RAF was determined by normalization of the RAF to the crystal fraction. While in the butene-1 homopolymer a specific RAF of 20–30% is detected, it increases to more than 100% in copolymers with 5–10 mol% of ethylene co-units, being in accordance with the previously observed increase of the free energy of the crystal fold-surface due to copolymerization. It has also been shown that the specific RAF increases with decreasing temperature of crystallization, due to formation of a fold-surface of lower perfection, containing an increased number of chain segments traversing the crystalline-amorphous interface. [ABSTRACT FROM AUTHOR]

Abstract: The manuscript details an investigation on the kinetics of vitrification of the rigid amorphous fraction in poly[(R)-3-hydroxybutyrate] (PHB) in dependence of thermal history. PHB was cooled from the melt at various rates, then the combined evolution of crystal and amorphous fractions was monitored during storage at 25°C. The permanence at room temperature induces progressive growth of PHB crystals and concurrent vitrification of rigid amorphous fraction. During the initial stages of the crystal growth, the amount of rigid amorphous fraction that vitrifies depends on the previous thermal history. After prolonged storage at 25°C, the increase of the crystal fraction corresponds to a reduction of both mobile amorphous and rigid amorphous content. The decreased coupling between the amorphous and crystal segments, reported in the literature for a few other semicrystalline polymers upon annealing at high temperatures, is shown to occur also in PHB, but during crystallization at low temperature. This finding provides one step forward a thorough understanding of the relation between crystal formation and reduced mobility of the coupled amorphous portions. [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: A thorough analysis of the melting behavior of cis-1,4-polybutadiene (cis-PBD) is detailed in this contribution. Isothermal crystallization at −26°C, followed by cooling, provides a three-phase structure composed of a mobile amorphous fraction equal to 0.413, a crystallinity of 0.277, and a rigid amorphous fraction of 0.310. Similar to many other polymers, cis-PBD displays multiple melting after isothermal crystallization, and up to three main endotherms can be evidenced by calorimetry, in dependence of the scanning rate. The results of conventional and temperature-modulated calorimetry analyses presented in this contribution suggest a link between multiple melting and devitrification of the rigid amorphous fraction in cis-PBD. The small endotherm located a few degrees above the crystallization temperature appears to be caused by concurrent partial mobilization of both the crystal and the rigid amorphous portions. Additional partial mobilization of rigid amorphous segments seems to take place at around −11°C, and it is only above this temperature that large reorganization of the crystal phase becomes possible, allowing partial melting and recrystallization/annealing/crystal perfection. [Copyright &y& Elsevier]

The analysis of the cp melting curve of poly(ethylene terephthalate) (PET) after non-isothermal crystallization, according to a three-phase enthalpy-based procedure, has allowed to monitor the correct crystalline and rigid amorphous fractions evolution during fusion at low heating rate and to identify the temperature of complete mobilization of the rigid amorphous fraction, located in the proximity of 215 °C. This limit temperature for the disappearance of the rigid amorphous fraction in PET corresponds to the transition between the triple and the double melting behaviour displayed by PET after isothermal crystallization. The multiple melting behaviour of PET is usually attributed to the fusion of original crystals recrystallized during the heating at conventional scanning rate, and to the presence of dual lamellar stacks. In the present study, PET was isothermally crystallized at various temperatures (Tc) ranging from 185 °C to 225 °C, and the double and triple melting profile that is observed at conventional DSC heating rates, after isothermal crystallization at Tc higher and lower than 215 °C respectively, has been put in relation with the absence and the presence of RAF around the original crystal lamellae. All the experimental evidences here reported indicate that the critical temperature for the formation and the disappearance of the rigid amorphous fraction in the PET sample analyzed in the present study at conventional DSC scanning rates, is around 215 °C. [ABSTRACT FROM AUTHOR]

The origin of "annealing peaks" in semicrystalline polymers, obtained with aging thermal protocols at temperatures between the glass transition and the melting temperature, and revealed by calorimetry as small endotherms preceding the main melting peak, has been recently debated in the literature. On the one side it is claimed that these small endotherms are related to the melting of defective/thinner crystals, on the other side it is proposed that they can be associated to the enthalpy relaxation of the rigid amorphous fraction (RAF), i.e., that part of the amorphous region directly coupled to the crystalline stems. With this work we aim to contribute to the debate, by exploiting the different thermal properties of the RAF coupled with different crystal modifications of polymorphic poly (butene-1) (PB-1). In this polymer the RAF mobilizes at largely different temperatures when it is coupled with crystals of trigonal Form I, or tetragonal Form II. By exploiting this peculiarity, we performed isochronous aging experiments with differential scanning calorimetry (DSC) on both crystalline phases, in a wide temperature range between the glass transition of the mobile (bulk) amorphous fraction and the onset of crystal melting. An endothermic peak above the aging temperature was typically observed. The trend of the enthalpy of this annealing peak with temperature can be described by a bell-shaped curve, approaching zero recovered enthalpy at temperatures of 100–110 °C, and 40–50 °C for Form I and Form II, respectively. These temperatures coincide with previous literature report about mobilization of PB-1 RAF and are thus identified as the upper limit of the RAF glass transition for the two polymorphs. Isothermal aging experiments as a function of time suggest that the enthalpy recovery of Form I RAF show the typical signature of glassy dynamics, similarly to what occurs for Form II RAF after short aging times. At longer times, the superposition of the polymorphic phase transition and RAF aging changes the kinetics of the recovered enthalpy for the latter polymorph, possibly due to the increased stress at the crystal-amorphous interface and creation of additional RAF in parallel with Form II to Form I transformation. Overall, our results demonstrate that for PB-1, at least within the investigated temperature range, the annealing peaks can be related to the RAF strive to attain thermodynamic equilibrium in its glassy state. Recovered enthalpy as a function of aging temperatures for PB-1 containing Form II or Form I crystals after annealing under certain temperature conditions. [Display omitted] • The endothermic peak appearing 10 °C above the aging temperature in PB-1 is related to structural recovery of the RAF. • The evolution of rigid amorphous fraction of PB-1 coupled with different crystal modifications is determined. • The glass transition of PB-1 RAF with Forms I and II crystals is approximately 100-110 °C and 40-50 °C, respectively. • The concomitant transformation of Form II to Form I during aging influences RAF glassy dynamics. [ABSTRACT FROM AUTHOR]