Search

Poly(ethylene terephthalate) (PET) is a thermoplastic polyester with excellent thermal and mechanical properties, widely used in a variety of industrial fields. It is a semicrystalline polymer, and most of the industrial success of PET derives from its easily tunable crystallization kinetics, which allow users to produce the polymer with a high crystal fraction for applications that demand high thermomechanical resistance and barrier properties, or a fully amorphous polymer when high transparency of the product is needed. The main properties of the polymer are presented and discussed in this contribution, together with the literature data on the crystal structure and morphology of PET. This is followed by an in-depth analysis of its crystallization kinetics, including both primary crystal nucleation and crystal growth, as well as secondary crystallization. The effect of molar mass, catalyst residues, chain composition, and thermo-mechanical treatments on the crystallization kinetics, structure, and morphology of PET are also reviewed in this contribution. [ABSTRACT FROM AUTHOR]

Shear-induced formation of crystal nuclei in polyamide 11 (PA 11) was studied using a conventional parallel-plate rheometer. Crystallization of PA 11 after shearing the melt at different rates for 60 s was followed by the evolution of the complex viscosity. The sheared samples showed in an optical microscope a gradient structure along the radius, due to the increasing shear rate from the center to the edge. The critical shear rate for shear-induced formation of nuclei was identified at the position where a distinct change of the semicrystalline superstructure is observed, being at around 1 to 2 s−1. Below this threshold, a space-filled spherulitic superstructure developed as in quiescent-melt crystallization. Above this value, after shearing at rates between 1 and 5 s−1, an increased number of point-like nuclei was detected, connected with formation of randomly oriented crystals. Shearing the melt at even higher rates led to a further increase of the nuclei number and growth of crystals oriented such that the chain axis is in parallel to the direction of flow. In addition, optical microscopy confirmed formation of long fibrillar structures after shearing at such condition. The critical specific work of flow of PA 11 was calculated to allow a comparison with that of polyamide 66 (PA 66). This comparison showed that in the case of PA 11 more work for shear-induced formation of nuclei is needed than in the case of PA 66, discussed in terms of the chemical structure of the repeat unit in the chains. [ABSTRACT FROM AUTHOR]

Poly[(R)-3-hydroxybutyrate] (PHB) is the first produced and most studied polyhydroxyalkanoate (PHA), a class of polymers that can be synthesized by a variety of bacteria through fermentation. PHB, and with it most PHAs, is produced in vivo, contrarily to all other types of bio-based and biodegradable polymers that are attained by chemical reactions, like polycondensation or ring-opening polymerization. The in vivo polymerization leads to a number of special features, like the absence of catalyst residues and other impurities typically present in synthetic polymers, which can promote crystal nucleation. Moreover, PHB has a perfectly isotactic structure with all chiral carbon atoms in (R)-configuration. These peculiarities make PHB a model compound for the study of polymer crystallization; hence its crystallization behavior has received considerable attention over the years. The main properties of the polymer are presented and discussed, together with literature data on crystal structure and morphology of PHB. This is followed by in-depth analysis of the crystallization kinetics, including both the primary crystal nucleation and crystal growth. Secondary crystallization is also discussed, as it largely affects timedependent properties of molded PHB products, which progressively becomes brittle upon storage: the glass transition of PHB is around 0°C; hence at room temperature PHB amorphous chains have sufficient mobility to continue to crystallize, which in turn implies also vitrification of part of the amorphous chains coupled with the crystals, the rigid amorphous fraction (RAF). The influence of the RAF on crystallization of PHB is also discussed, since its vitrification during crystallization can hinder or even stop further crystal growth. The analysis of the crystallization behavior of PHB is completed by a thorough discussion of melting of PHB crystals, which also presents a number of notable features. [ABSTRACT FROM AUTHOR]

The crystallization kinetics of poly( l/d -lactic acid) (PLLA) with a molar mass of around 120 kDa and containing 4% of d -isomers is sizably improved by blending with low amount (<10 m%) of a PLLA of high stereoregularity, made of pure l -isomer, and molar mass of 4 kDa. Blends of the two PLLA’s are prepared by solution mixing. The blends are homogeneous in the melt and display a single, composition-dependent glass transition. Increasing concentration of the highly stereo-regular low-molar mass component in the blends results in an earlier onset of crystallization and a higher crystal growth rate. The blend composition affects the crystal polymorphism of PLLA, as increasing content of the highly stereoregular and low molar mass polymer favors growth of α-crystals at lower temperatures than typically observed in unmodified PLLA. It is suggested that the short PLLA molecules of the low-molar mass component start to crystallize at higher temperature than the long molecules of the high-molar mass component, acting as crystal nuclei for the subsequent crystal growth that involves both species. [ABSTRACT FROM AUTHOR]

Abstract: Polarized‐light optical microscopy (POM) is applied for investigation of homogeneous crystal nucleation in polymers, using the advantage of precise control of the nucleation pathway by application of fast scanning chip calorimetry (FSC). In the first part of this paper, homogeneous crystal nucleation in glassy poly ( l‐lactic acid) (PLLA) employing Tammann's two‐stage crystal nuclei‐development method is highlighted. PLLA samples of different nucleation history are prepared in an FSC, and then POM micrographs are studied regarding the effect of time and temperature of annealing the glass on the number of spherulites, which developed in the growth‐stage at higher temperature. The obtained images provide ultimate evidence about the validity of Tammann's approach for obtaining information about the kinetics of homogeneous crystal nucleation using calorimetry, when quantifying the number of nuclei by the enthalpy of crystallization during their growth to crystals in the development stage. In the second part of this study, information about the semicrystalline morphology of samples of poly(butylene terephthalate) and polyamide 66 crystallized at different supercooling using FSC is presented. POM analysis confirms the origin of the frequently observed bimodal temperature‐dependence of the crystallization rate as being caused by different mechanisms of crystal nucleation, resulting in qualitatively different structures. [ABSTRACT FROM AUTHOR]

Low-temperature crystallization of poly(butylene succinate) (PBS) was investigated by conventional and temperature-modulated differential scanning calorimetry (DSC and TMDSC). After completion of primary crystallization, cooling of semicrystalline PBS to temperatures near or even below the glass transition temperature ( T g ) leads to additional crystal growth. Such crystallization below T g is identified by detection of frequency- and time-dependent reversing heat-capacity in non-isothermal and quasi-isothermal TMDSC analyses, respectively. Moreover, crystallization in the glassy state causes a shift of T g to higher temperatures, thus affecting the properties of the amorphous phase. Low-temperature crystallization of amorphous PBS was recently reported in the literature and linked to completion of enthalpy relaxation of the PBS glass. In the present study, low-temperature crystallization has been shown to occur also in semicrystalline PBS, with low temperature crystallization probably involving constrained amorphous segments already coupled with the crystals. [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]

Bio-based composites made of poly(l-lactic acid) (PLLA) and pine wood were prepared by melt extrusion. The composites were compatibilized by impregnation of wood with γ-aminopropyltriethoxysilane (APE). Comparison with non-compatibilized formulation revealed that APE is an efficient compatibilizer for PLLA/wood composites. Pine wood particles dispersed within PLLA act as nucleating agents able to start the growth of PLLA crystals, resulting in a faster crystallization rate and increased crystal fraction. Moreover, the composites have a slightly lower thermal stability compared to PLLA, proportional to filler content, due to the lower thermal stability of wood. Molecular dynamics was investigated using the solid-state 1H NMR technique, which revealed restrictions in the mobility of polymer chains upon the addition of wood, as well as enhanced interfacial adhesion between the filler and matrix in the composites compatibilized with APE. The enhanced interfacial adhesion in silane-treated composites was also proved by scanning electron microscopy and resulted in slightly improved deformability and impact resistance of the composites. [ABSTRACT FROM AUTHOR]

The generation of long-chain branches (LCB) in biobased and biodegradable polylactide (PLA) by adding different amounts of a chain extender is studied. The rheological and calorimetric behavior have been used to determine the effect of LCB presence and their topology on PLA melt strength and crystallization behavior. Rheological modeling of linear and non-linear viscoelastic shear and extensional properties identified several possible branched structures. Moreover, remarkable differences were observed for the different topologies regarding the intrinsic non-linear parameters and the intra-cycle elastic and viscous non-linearities. Differential scanning calorimetry and polarized light optical microscopy measurements revealed a significant increase in the nucleation density and rate of PLA with increasing the amount of LCB, albeit they provoke a decrease in the growth rate due to a reduction in chain diffusion. Nevertheless, overall crystallization rate values revealed a predominant effect of nucleation over crystal growth. The introduction of LCB within the chains is highly beneficial as they increase nucleation, crystallinity, and elongational viscosity, thus improving the properties of biodegradable PLA. [ABSTRACT FROM AUTHOR]

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]