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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]

The effect of the chain length/molar mass of poly ( l -lactic acid) (PLLA) on the formation of homogeneous crystal nuclei below and above the glass transition temperature ( T g ) has been analyzed by fast scanning chip calorimetry, employing Tammann’s nuclei development method. It has been found that the kinetics of crystal nucleation below T g is independent on the chain length within the investigated range of molar masses from about 60 to 600 kDa, similar as the kinetics of enthalpy relaxation. In contrast, at temperatures well above T g there is detected that crystal nucleation is slower in samples of higher molar mass. This observation is confirmed by non-isothermal nucleation experiments which showed that suppression of nucleation occurs on cooling at rates of 200, 100, and 30 K/s in samples with molar masses of around 60, 100, and 600 kDa, respectively. The different effect of molar mass on crystal nucleation in the glass and well above T g is discussed in terms of the length-scale of required chain-mobility and of the critical size of nuclei. [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]

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 s − 1 . 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 s − 1 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. [ABSTRACT FROM AUTHOR]

The structural evolution of poly(butylene succinate) (PBS) crystals after crystallization at 100 °C has been investigated by synchrotron wide-angle and small-angle X-ray scattering (WAXS and SAXS) analysis. Isothermal crystallization of PBS leads to a single lamellar population. Temperature-dependent WAXS/SAXS analysis has proven that an additional crystal population, characterized by thicker lamellae, develops upon heating, resulting in a dual lamellar population. Deconvolution of the Lorentz-corrected double SAXS profiles has allowed deriving the temperature dependence of the lamellar thickness for the two different crystal populations. The structural information derived by WAXS/SAXS has allowed elucidating the multiple melting behavior of PBS, so that the separate endotherms of the overall melting behavior could be associated with specific thermal events. The first melting peak has been connected to the fusion of original crystals, only minimally reorganized upon heating, whereas the second melting peak has been ascribed to the overlapping of two different melting processes, one linked to the fusion of original crystals thickened at high temperatures, most likely in the solid state, and the other one to the fusion of markedly thicker crystals, developed via melting/recrystallization mechanism. [Display omitted] • PBS multiple melting involves emergence of an additional population of thicker crystals. • Original crystals slightly thicken upon heating, likely in the solid state. • Thicker additional crystals develop via melting/recrystallization. • At high temperature, the two different crystal populations coexist. [ABSTRACT FROM AUTHOR]

Poly ( l -lactic acid) (PLLA) and its random copolymers with small amount of d -isomer co-units but identical mass-average molar mass of 120 kDa were analyzed regarding the non-isothermal and isothermal crystallization and nucleation behaviors. First, it was confirmed that the gross crystallization rate and crystal growth rate decrease with increasing d -isomer content in the chains. In extension to earlier work, the isothermal analysis of the crystallization rate was completed by determination of critical cooling rates to suppress ordering of the melt. While in case of the PLLA homopolymer the melt needs to be cooled faster than 0.5 K s −1 in order to suppress crystallization, the critical cooling rate is lowered by about one order of magnitude to 0.05 K s −1 in case of a copolymer with 4% d -isomer co-units. However, even if the melt is cooled faster than the critical cooling rate to suppress crystallization, nuclei formation still occurs in a specific cooling-rate range depending on the d -isomer content, ceasing only on cooling about two orders of magnitude faster than the critical cooling rate to suppress crystallization. The observation of slower nuclei formation at non-isothermal conditions in PLLA containing d -isomer co-units is confirmed by isothermal nucleation experiments. [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 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]

Highlights: [•] PLLA and PBT are not miscible. [•] Interactions between PLLA and PBT functional groups are established upon melt mixing. [•] Morphology of the blends depends on composition. [•] When PBT content is ⩽20%, PBT particles are finely dispersed within the PLLA matrix. [•] The PLLA/PBT 60/40 blend presents a co-continuous multi-level morphology. [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: This paper contains new views about the amorphous and partially ordered phases of the three polymers listed in the title. The discussion is based on information on structure, thermodynamic stability, and large-amplitude molecular motion. Polyethylene is the basic backbone of all alkene polymers, and the other two are the first members of the vinyl polymers which have stereospecifically placed alkyl side chains. Their multiphase structures consist of metastable crystals, mesophases, and surrounding rigid and mobile amorphous fractions. All these phases have sizes ranging from micrometer dimensions down to nanometers. Besides the phase structures, information about the molecular coupling between the phases must be considered. Depending on temperature, the polymer phases can vary from solid (rigid) to liquid (mobile). New knowledge is also gained by cross-comparison of the title polymers. The experimental information was gained from (a) various forms of slow, fast, and temperature-modulated thermal analysis to identify equilibrium and non-equilibrium states, (b) measurement of structure and morphology at various length scales, and (c) tracing of the large-amplitude molecular motion, the kinetics of order/disorder changes, and the liquid/solid transitions (glass transitions). It is shown that much more needs to be known about the various phases and their coupling to characterize a given polymer and to fine-tune its properties for a given application. [ABSTRACT FROM AUTHOR]

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]

Abstract: A detailed analysis of the three-phase structure of isotactic poly(butene) was conducted by conventional and temperature-modulated calorimetry. The development of the crystalline, mobile amorphous, and rigid amorphous fractions was analyzed as a function of thermal history, upon isothermal and non-isothermal crystallization. It was found that, under the chosen experimental conditions, the amount of rigid amorphous phase (w RA) in PB-1 ranges from w RA =0.14 to 0.23, with higher values formed when the polymer is crystallized at low temperatures or at high cooling rates from the melt. Comparison of total and frequency-dependent reversing heat capacity curves suggested that the rigid amorphous phase of isotactic poly(1-butene) vitrifies after completion of the crystallization process and that its full mobilization takes place at around 50°C. The exact temperature of complete devitrification is slightly affected by the thermal history of the material. An effort to link the mechanical properties of PB-1 to the three-phase structure was attempted, and a correlation of Young''s modulus with the solid fraction at the temperature of analysis, composed of crystalline and rigid amorphous phases, was proposed. [Copyright &y& Elsevier]

Abstract: The isothermal crystallization kinetics and melting behavior of poly(butylene terephthalate) (PBT) in binary blends with poly(ε-caprolactone) (PCL) was investigated as a function of PCL molecular mass by differential scanning calorimetry and optical microscopy. The components are miscible in the melt when oligomeric PCL (M w =1250) is blended with PBT, whereas only partial miscibility was found in mixtures with higher molecular mass (M w =10,000 and 50,000). The equilibrium melting point of PBT in the homopolymer and in blends with PCL was determined through a non-linear extrapolation of the T m = f(T c) curve. The PBT spherulitic growth rate and bulk crystallization rate were found to increase with respect to plain PBT in blends with PCL1250 and PCL10000, whereas addition of PCL50000 causes a reduction of PBT solidification rate. The crystallization induction times were determined by differential scanning calorimetry for all the mixtures through a blank subtraction procedure that allows precise estimation of the crystallization kinetics of fast crystallizing polymers. The results have been discussed on the basis of the Hoffman–Lauritzen crystallization theory and considerations on both the transport of chains towards the crystalline growth front and the energy barrier for the formation of critical nuclei in miscible and partially miscible PBT/PCL mixtures are widely debated. [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]

This contribution provides an overview of the current state of art of spherulite growth rate in binary polymer blends. A detailed analysis of the various factors that contribute to increase or decrease the growth rate of spherulites is reported and supported by several examples. The reported results are discussed mainly with regard to miscibility of the components. Below the melting point, blends containing crystallizable polymers are heterogeneous systems, as the components separate from each other during solidification. However, miscibility within the remaining amorphous phase may still be possible and the homogeneity or heterogeneity of the amorphous phase affects the crystallization process. [Copyright &y& Elsevier]

Literature values of the enthalpy of melting (Δ h m °) of 100% crystalline poly( l -lactic acid) (PLLA) vary in a wide range (82–203 J g –1 ). In most cases, such values were determined without taking into account the presence of α′- or α-crystal modifications, or the influence of d -lactic acid units in the analyzed poly(lactic acid). The bulk enthalpy of melting was determined for the stable orthorhombic α form in only few studies, and thermodynamic properties of the metastable α′-modification are not available yet. In the present work, the temperature dependence of Δ h m ° for both the α′- and α-crystal polymorphs of PLLA are determined, by combining differential scanning calorimetry and X-ray diffraction. It is proven that the enthalpies of melting of α′- and α-crystals of 100% crystalline PLLA are different. At the respective melting temperatures of 150 °C and 180 °C, the Δ h m ° values of the α′- and α-forms are 107 and 143 J g −1 . At identical temperature, the enthalpy of melting of α′-crystals is approximately 25 J g −1 lower than that of the α-form. This observation is rationalized taking into account the presence of conformational defects in the disordered α′-modification. [ABSTRACT FROM AUTHOR]

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]

Graphical abstract Highlights • This is the first literature example of electrospun PLLA fibers containing DEET. • The biodegradable fibers are defect-free with uniform diameter of 1 µm. • It is possible to incorporate more than 50 wt% of DEET into electrospun PLLA. • Evaporation of DEET is delayed in the electrospun PLLA fibers. • PLLA/DEET fibers can be used as tools for fighting mosquito-caused diseases. Abstract The manuscript details the first literature example of fibers of poly(l -lactic acid) (PLLA) containing a mosquito repellent agent, N , N -diethyl-3-methylbenzamide (DEET). The fibers were prepared by electrospinning, are defect-free and have uniform diameter of the order of magnitude of 1 µm. Thermogravimetry and infrared spectroscopy showed that it is possible to incorporate more than 50 wt% of DEET, which is then slowly released to the environment. As such, the fibers can function as a drug delivery device. Further structural analyses disclosed that PLLA in DEET-containing fibers is semicrystalline, and contains α-crystals formed during electrospinning. Thermal, morphological and structural characterization revealed that the PLLA/DEET electrospun fibers have potential as novel material for controlled release of DEET, which, coupled with the ease of fabrication, detailed here, can provide innovative tools for fighting mosquito-caused diseases. [ABSTRACT FROM AUTHOR]

A mild CO 2 -treatment of a poly(l -lactic acid) PLLA disk with tailored geometry allowed to compare cold crystallization of PLLA containing α"-mesophase, with cold-crystallization of fully amorphous PLLA, both coexisting within a single sample. The use of PLLA disks with 0.5 mm thickness, low CO 2 pressure (3.0 MPa) and short treatment time (5 min) at 25 °C, led to only partial diffusion of the gas into the polymer. CO 2 -induced mesophase developed in surface-near regions of the disks, where CO 2 penetrated, whereas the core of the samples was not reached by the gas and remained amorphous. Cold-crystallization of CO 2 -treated PLLA was analyzed after full desorption of the gas, using different heating rates. Slow heating allowed to identify two cold-crystallization events: low-temperature cold-crystallization in the outer parts of the sample treated with CO 2 , promoted by prior formation of the α"-mesophase, and high-temperature cold-crystallization, occurring at similar temperatures as in case of non-CO 2 -treated amorphous PLLA. In both cases, cold-crystallization led to formation of conformationally disordered α′-crystals, as revealed by temperature-resolved wide-angle X-ray scattering. This study confirms that formation of α′-crystals is mainly controlled by the crystallization temperature, also when crystallization is promoted by the prior presence of a mesophase. [Display omitted] • Diffusion-controlled CO 2 -treatment allows inducing structural gradients in amorphous PLLA. • CO 2 -treated, surface-near parts develop α"-mesophase, while the core remains amorphous. • Presence of α"-mesophase enhances cold-crystallization compared to amorphous PLLA. • Growth of α′-crystals is mainly controlled by temperature. [ABSTRACT FROM AUTHOR]

This study details new polypropylene-based materials containing a siloxane-silsesquioxane resin and a sorbitol derivative. Molecular adducts originating from the synergistic interactions of siloxane-silsesquioxane resin (SiOPh) containing phenyl functionalities and 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol, Millad 3988 (DMDBS), strongly influence crystallization of isotactic polypropylene (iPP) as well as its morphology, transparency and mechanical properties. DMDBS is a commonly used clarifying agent for a variety of iPP grades. However, its disadvantage is that when added into iPP during extrusion, it allows only low draw ratios, because it increases crystallization temperature of iPP. Addition of SiOPh allows for control of the nucleation efficiency of the sorbitol derivative and adjust crystallization rate of iPP, to attain transparent formulations suitable for extrusion processes. iPP-based compounds were produced by co-rotating twin screw extrusion and further analyzed by differential scanning calorimetry, wide-angle X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy. Moreover, rheological, haze and static tensile measurements were conducted to determine the influence of composition on material properties. It was found that the best combination of properties is achieved when 1 wt% of SiOPh and 0.25 wt% DMDBS are added to iPP. This formulation can significantly change the crystallization behavior of iPP to be tailored for production of highly oriented and transparent products. [ABSTRACT FROM AUTHOR]

Structural, morphological and thermal properties of films based on poly(butylene succinate) (PBS) and inclusion complex (IC) of β-cyclodextrins (β-CD) and d -limonene were studied, in order to prepare novel bio-active food packaging materials. Formation of the inclusion complex was proved by infrared spectroscopy, nuclear magnetic resonance, and X-ray diffraction. The IC contains 7 wt% of d -limonene, as quantified by precipitation, as well as by 13 C NMR and by thermogravimetry. Inclusion within β-CD cavity stabilizes d -limonene against evaporation, which allows melt compounding with PBS. Infrared spectroscopy of compression molded composite films, evidenced both the d -limonene encapsulation inside β-CD cages, and the occurrence of interactions between hydroxyl groups of β-CD-lim complex and the carbonyl groups of PBS units. Optical micrographs showed a phase separated morphology, with homogeneous distribution of the IC within the PBS matrix, probably due to interactions between the polar groups of the composite components. DSC analysis highlighted the anti-nucleating action of β-CD-lim complex towards PBS, evidenced by a significant delay in crystallization kinetics of PBS. [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]

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]