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The three-phase arrangement of random copolymers of butene-1 with ethylene was investigated and compared with isotactic poly(butene-1) homopolymer (iPB-1). In all the analyzed compositions, isothermal crystallization leads to a three-phase structure, made of one crystal phase and two amorphous fractions that differ in mobility: the mobile amorphous fraction (MAF), made of the polymer chains that relax at the glass transition, and a rigid amorphous fraction (RAF) made of the amorphous segments coupled with the crystal phase. Copolymerization with ethylene leads to a drop in crystal fraction and to a sizable increase of both the RAF, and of the specific RAF, i.e. of the RAF normalized to crystallinity. Analysis of crystal growth rate allowed quantifying the fold surface free energy, which increases of about 50 to 100% in the copolymers, compared to iPB-1 homopolymer. In the butene-1/ethylene random copolymers, ethylene units are mostly excluded from the crystals and accumulate at the crystal/amorphous interphase, thus affecting the rigid amorphous area. The varied composition and higher mobility of the rigid amorphous fraction of the copolymers affects also the Form II to Form I transformation of poly(butene-1) crystals, which occurs with enhanced kinetics in the copolymers, compared to iPB-1 homopolymer.

The assessment and control of the crystallization behavior is an important issue in polymer processing. The rate at which crystallization takes place can influence the outcome of a production line and the quality of the polymer products. Depending on the type of processing (e.g., extrusion, injection-, or blow-molding), as well as the needed properties of the finished products, polymer crystallization speed can be tailored by varying material composition or formulation, as well as through the manipulation of process conditions. For instance, the addition of nucleating or clarifying agents can accelerate the crystallization. Similarly, the processing temperatures can be varied in order to properly alter the rate of crystallization. Variations in both material formulation and processing conditions induce corresponding modifications in material properties, by affecting material crystallinity, crystal thickness and morphology, crystal structure, etc., all of which drive bulk material properties. In order to determine which changes in processing or formulation are needed to tailor the crystallization conditions, it is necessary to assess the polymer crystallization behavior. Polymer crystallization can be monitored, quantified, and modeled using several calorimetric techniques, the most common including differential scanning calorimetry (DSC), temperature-modulated differential scanning calorimetry (TMDSC), and fast scanning chip calorimetry (FSC). Calorimetry can also be coupled with additional instrumentation/methods such as X-ray diffraction, rheometry, or microscopy, to follow details of structure development. Thus, complementary relationships of crystallization kinetics, structure, and morphology can be established for a given thermoplastic polymer, which allows optimization of material formulation and processing conditions in view of the desired properties. A detailed investigation of the crystallization process is also needed for a deeper understanding of polymer structure formation, since many specifics of polymer crystallization are still under investigation.

A detailed analysis of the effect of calcium carbonate nanoparticles on crystallization of isotactic polypropylene (iPP) is reported in this contribution. CaCO 3 nanoparticles with different crystal modifications (calcite and aragonite) and particle shape were added in small percentages to iPP. The nanoparticles were coated with two types of compatibilizer (either polypropylene- g -maleic anhydride copolymer, or fatty acids) to improve dispersion and adhesion with the polymer matrix. It was found that the type of coating agent used largely affects the nucleating ability of calcium carbonate towards formation of polypropylene crystals. CaCO 3 nanoparticles coated with maleated polypropylene can successfully promote nucleation of iPP crystals, whereas the addition of nanosized calcium carbonate coated with fatty acids delays crystallization of iPP, the effect being mainly ascribed to the physical state of the coating in the investigated temperature range for crystallization of iPP, as well as to possible dissolution by fatty acids of heterogeneities originally present in the polypropylene matrix.

The influence of calcium carbonate nanoparticles with different shapes (spherical and elongated) on the thermal properties and crystallization behavior of isotactic polypropylene was investigated. CaCO3 nanoparticles were covered by an appropriate coating agent to improve the interfacial adhesion between the filler and the polyolefin matrix. The nanocomposites were prepared by melt mixing and subsequent compression molding. A remarkable effect of CaCO3 on the thermal properties of iPP was observed. Moreover, the analysis of crystallization kinetics showed that CaCO3 nanopowder coated with PP-MA are efficient nucleating agents for iPP, and the overall crystallization rate results higher than plain iPP.

This article contains a detailed analysis of the melting process of poly(butylene terephthalate) performed by means of temperature-modulated differential scanning calorimetry at various temperature-modulation frequencies. The modulation period largely affects the apparent reversing specific heat capacity (cp), but not the total cp. An interpretation based on the analysis of the latent heat involved in the process is proposed. In addition, a detailed examination of the modulated heat-flow-rate curves is presented. The finding is that the melting process of poly(butylene terephthalate) is the sum of simultaneous events of the melting of small and defective crystals and their immediate recrystallization into more stable structures. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2191–2201, 2004

This article provides a thorough analysis of the crystallization process of poly(butylene terephthalate) (PBT). Isothermal crystal growth rates have been experimentally measured between 459 and 491 K, and analyzed according to the Hoffman and Lauritzen theory. A II-III regime transition occurs at 484 K, accompanied by a change in crystal morphology. By means of a non-linear fitting procedure, precise values of the nucleation constant K g were obtained. The lateral surface free energy a was calculated and, from this, the a parameter of the Thomas-Staveley expression was obtained.

This paper provides an analysis of contributions to the apparent, reversing heat capacity when measured by temperature-modulated differential scanning analysis (TMDSC) with an underlying heating rate in the temperature range where irreversible transitions with latent heats occur. To deconvolute the data of a TMDSC scan into a total and reversing part, it is common practice to use the sliding averages and the first harmonics of the Fourier series of temperature and heat-flow rate. Under certain conditions, this procedure produces erroneous reversing contributions which are detailed by experiment and simulation. Unless the response to the temperature modulation is linear, the total heat-flow rate is stationary, and the transition is truly reversible and occurs only once during the temperature scan, one cannot expect a true deconvolution of total and reversible effects. In the presence of multiple, irreversible transitions within a modulation period, however, each process involving latent heat can increase the modulation amplitude, as demonstrated by computer-simulation of polymer melting. As a result, the multiple transitions may give erroneously high latent heats when integrating the apparent reversing heat capacity with respect to temperature.

Nanocomposites composed of poly(ethylene terephthalate) (PET) filled with calcium carbonate particles of nanometer scale were prepared by polymerizing the polyester in the presence of the nanosized fillers. Besides plain calcium carbonate, carbonate nanoparticles coated with stearic acid were also used, in order to improve the compatibility between the polymeric matrix and nanofillers. Morphological analysis evidenced a good dispersion of both the nanopowders into the PET matrix, especially in the case of coated calcium carbonate. The strong interfacial adhesion between the two phases is also responsible for the increase of the glass transition and melting temperatures in the nanocomposites compared to plain PET. Finally, non-isothermal crystallization studies revealed that the coated CaCO3 is a good nucleating agent for PET. Analysis of non-isothermal crystallization data with the Ozawa theory was successful for plain PET and PET/un-CaCO3, but this method failed to describe the dynamic solidification of the PET/c-CaCO3 nanocomposite.

Chemical and eating quality of citrus fruit changes slowly after harvest, and quality alteration is mainly due to shrinkage, loss of firmness, excessive weight loss and decay rather than a reduction of nutritional value and taste features. Film wrapping may be a suitable means to reduce transpiration and preserve market quality provided film permeability to gases does not lead to: 1) a reduction of in-package O2 partial pressure at a point that would induce anaerobic respiration; 2) an increase of CO2 concentration to toxic levels. This experiment was carried out to study quality changes of 'Sanguinello' oranges treated or not treated with 500 mg/L imazalil (IMZ) and wrapped with continuous, macro- or micro-perforated polyolefinic films. Wrapped and no-wrapped fruit were stored at 20 degrees C and 60% RH for 20 or 30 days. In-package gas composition of the macro-perforated film showed no significant difference compared to air composition, while in-package partial pressure of CO2 and O2 ranged between 4 (continuous film) and 9.8 kPa (micro-perforated films), and 14.8 (continuous film) and 5 kPa (micro-perforated films), respectively. After 30 days of storage weight loss in fruit wrapped with the macro-perforated film was (4.3%) slightly lowerthan un-packed fruit (5%), while in all other packages weight loss never exceeded 0.7%.Quality changes were quite stable over storage in all treatments, although slight but significantly lower levels of total soluble solids and ascorbic acid were detected in micro-perforated films with the lowest degree of perforation. However, the sensory analysis denoted a remarkable decrease of firmness in un-wrapped or wrapped fruit with macro-perforated film, while a moderate build-up of off-flavour, which reduced the eating quality, developed in micro-perforated films. Decay ranged between 6 and 12% in not treated fruit, with the lowest incidence detected in un-wrapped fruit, whereas differences among the different films were not significant. No decay was detected in fruit treated with IMZ. 'Sanguinello' oranges can be stored under retail conditions for a month by the only means of film wrapping without important changes in chemical, eating and marketing quality provided fruit are treated with an effective fungicide to prevent decay and that in-package gas composition is not markedly changed with respect to air.

Isotactic poly(3-hydroxybutyrate) (P3HB) is an aliphatic polyester obtained by bacterial fermentation. This truly biodegradable polymer has been widely investigated, mainly with the aim to replace conventional plastics, which cause environmental pollution. To improve its properties, extensive studies have been conducted to modify it properly by copolymerization or blending with other polymers. Macroscopic properties of polymers, particularly of polymer blends, are strongly affected by their microstructure, especially by the allocation of different phases and their level of dispersion and adhesion. In nonreactive blends, the adhesion level is mainly determined by the similarity between the chemical structure of the components, and the arrangement of the phases is remarkably controlled by the crystallization conditions. The superstructure of the crystalline phases accounts for the mechanical resistance of the blend, but the composition and the distribution of the amorphous phases also play an important role...

The miscibility, crystallization, vitrification, and melting behavior of multiblock copolymers consisting of alternating components of oligo[imino(1-oxododecamethylene)] and oligo(oxytetramethylene) were investigated in their dependence of composition and molecular mass of the blocks. In all compositions studied, the copolymers present two separate glass transitions at temperatures not far from those of the corresponding homopolymers. The immiscible components are linked by chemical bonds to nanophase-separated layers. In such a situation, the structure and mobility of each phase affect the other, causing small shifts of the glass-transition temperatures. The enthalpy-based crystallinity was calculated separately for each component, using the known information on heat capacities and latent heats. The crystallinity is influenced by the block lengths. The oligoamide segments crystallize from an isotropic melt with a relatively high crystallinity, whereas the oligoether blocks solidify in the presence of cr ystals and glass of the second component in the adjacent solid nanophases, which greatly reduces their overall long-range chain mobility, and as a consequence, their crystallinity is rather small.

The heat capacity of poly[carbonyl(ethylene-co-propylene)] with 95 mol % C2H4CO (Carilon EP®) was measured with standard differential scanning calorimetry (DSC) and temperature-modulated DSC (TMDSC). The integral functions of enthalpy, entropy, and free enthalpy were derived. With quasi-isothermal TMDSC, the apparent reversing heat capacity was determined from 220 to 570 K, including the glass- and melting-transition regions. The vibrational heat capacity of the solid and the heat capacity of the liquid served as baselines for the quantitative analysis. A small amount of apparent reversing latent heat was found in the melting range, just as for other polymers similarly analyzed. With an analysis of the heat-flow rates in the time domain, information was collected about latent heat contributions due to annealing, melting, and crystallization. The latent heat decreased with time to an even smaller but truly reversible latent heat contribution. The main melting was fully irreversible. All contributions are discussed in the framework of a suggested scheme of six physical contributions to the apparent heat capacity. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1565–1577, 2001

Heat capacities in the solid state of four globular proteins (bovine β-lactoglobulin, chicken lysozyme, ovalbumine, and horse myoglobin) and of the poly(amino acid) poly(L-tryptophan) have been determined using the Advanced THermal Analysis System (ATHAS). The experimental measurements were performed with adiabatic and differential scanning calorimetry over wide temperature ranges. The heat capacities were linked to an approximate vibrational spectrum by making use of known group vibrations and of a set of parameters, Θ1 and Θ3, of the Tarasov function for the skeletal vibrations. Good agreement was found between experiments and calculations with root mean square errors mostly within ±3%. The experimental data were analyzed also with an empirical addition scheme using the known data for poly(amino acid)s measured earlier. Based on this study, vibrational heat capacities can now be predicted for all proteins with an accuracy comparable to common experiments. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2093–2102, 1999

Traditional studies of crystallization kinetics are often limited to idealized conditions, in which the parameters of state (temperature, pressure, etc.) are constant. In real situations, however, the external conditions change continuously, which makes the kinetics of crystallization dependent on instantaneous conditions, as well as on rate of change. This article provides an overview of the current state-of-the-art of non-isothermal crystallization of polymers during the cooling from the melt. The majority of the proposed theoretical formulations that predict non-isothermal crystallization kinetics, concern bulk crystallization and are based on modifications of the Avrami equation. The Ziabicki, Nakamura and Ozawa models are examined here in some detail together with treatments from other authors. The basic hypotheses of the various models, as well as their relative drawbacks, are underlined. Alternative empirical approaches to calculate the main parameters of non-isothermal crystallization and to compare the crystallization rate of different polymeric systems are also discussed. This article reviews the data concerning non-isothermal crystallization processes for different classes of polymers. Major attention is directed towards the dynamic crystallization of polyolefins, a class of materials of large industrial interest. Other results for polyoxyolefins, polyesters, polyamides and polyketones are also examined.

The influence of two natural terpene resins on the morphology, phase structure and isothermal crystallization process of iPP was investigated by optical (OM) and electron microscopy (SEM) and differential scanning calorimetry (DSC). It was found that in dependence on temperature, composition and chemical nature of the resin, one, two or three phases can be present. The isothermal spherulite growth rate and overall crystallization rate of the blends are depressed compared to plain iPP. This depression was attributed to the increase of the energies relative to the transport of macromolecules in the melt and to the formation of nuclei of critical size, following the addition of resin. The presence of resin seems to lead to the formation of more regular folding surfaces.

This article discusses the influence of two natural terpene resins (NTR), poly(α-pinene) (PαP A115) and poly(d-limonene) (PL C115), on morphology, miscibility, thermal, and dynamic-mechanical properties of their blends with isotactic polypropylene (iPP). The NTR have interesting physical and chemical properties, and they are approved for food contact application. From the results of differential scanning calorimetry and dynamic-mechanical thermal analysis it was deduced that both the resins were completely miscible with the amorphous iPP up to the composition investigated here (70/30 wt %). Scanning electron microscopy (SEM) analysis instead showed that the 70/30 iPP/PαP A115 blend and 80/20 and 70/30 iPP/PL C115 blends contained very small domains homogeneously distributed into the matrix. It is hypothesized that the domains are likely formed by the terpene-rich phase, and the matrix by the iPP-rich phase (besides the crystallized iPP phase). The iPP-rich phase and the NTR-rich phase would have the glass transition temperatures so close that they cannot be resolved by DSC and DMTA. Finally, for the iPP/PαP A115 system an upper critical solution temperature (UCST) is proposed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 867–878, 1999

The response of temperature-modulated differential scanning calorimetry (TMDSC) to irreversible crystallization of linear polymers was investigated by model calculations and compared to a number of measurements. Four different exotherms were added to a typical modulated, reversible heat-flow rate in order to simulate irreversible crystallization. It was found that the reversing heat-flow rate of the TMDSC in response to such irreversible crystallization exotherms is strongly affected by tbe shape of the transition and the phase-angle where the exotherm occurs. A comparison with the experimental data gave valuable insight into the transitions, as well as the nature of the TMDSC response which is usually limited to an analysis of the first harmonic term of the Fourier series that describes the heat-flow rate.

The article discusses the influence of the oligomeric resin, hydrogenated oligo(cyclopentadiene) (HOCP), on the morphology and properties of its blends with isotactic poly( 1-butene) (PB-1). PB-1 and HOCP are found to be partially miscible in the melt state. Solidified PB-1/HOCP blends contain three phases: (1) a crystalline phase formed by PB-1 crystals; (2) an amorphous PB-1-rich phase; and (3) an amorphous HOCP-rich phase. The optical micrographs of the solidified blends show a morphology constituted by microspherulites and domains of the HOCP-rich phase homogeneously distributed in the intraspherulitic region. DSC and DMTA results show two glass transition temperatures (T g ), different from the T g values of the plain components. The lower T g is attributed to the PB-1-rich phase, and the higher T g , to the HOCP-rich phase. The tensile properties were investigated at 25 and 80°C. At 25°C, the PB-1-rich phase is rubbery and the HOCP-rich phase is glassy, so the addition of HOCP to PB-1 arouses a noteworthy hardening of the samples and this brings an increase of the Young's modulus, E' (although the blend crystallinity lessens), and decreases of stresses at yielding point (σ y ) and at rupture (σ r ). The 90/10 and 80/20 blends show high values of elongation at rupture (e r ). At 80°C, the blends show decreases of E' and σ r values with the HOCP content. These decreases are attributed to the rubbery state of the phases and reduction of the blend's crystallinity. At 80°C, all the blends show a high value of e r . This phenomenon is attributed to the fine-size domain dispersion of the phases and to sufficient densities of tie molecules and entanglements. Finally, the partial miscibility behavior proposed in this article is compared with the miscibility hypothesis reported elsewhere.

Isothermal and non-isothermal crystallization kinetics of poly(L-lactic acid)/poly(butylene terephthalate) (PLLA/PBT) blends containing PLLA as major component is detailed in this contribution. PLLA and PBT are not miscible, but compatibility of the polymer pair is ensured by interactions between the functional groups of the two polyesters, established upon melt mixing. Crystal polymorphism of the two polyesters is not influenced by blending, as probed by wide-angle X-ray analysis. The addition of PLLA does not affect the temperature range of crystallization kinetics of PBT, nor the crystallinity level attained when the blends are cooled from the melt at constant rate. Conversely, PBT favors crystallization of the biodegradable polyester. The addition of PBT results in an anticipated onset of crystallization of PLLA during cooling at a fixed rate, with a sizeable enhancement of the crystal fraction. Isothermal crystallization analysis confirmed the faster crystallization rate of PLLA in the presence of PBT.

Hydrogels based on N-isopropylacrylamide and sodium acrylate as ionic comonomer were synthesized by free r a d i c a l polymerization in wa t e r using N ,N ? -methylenebisacrylamide as crosslinker and ammonium persulfate as initiator. The glass transition of dried copolymers poly(N-isopropylacrylamide) (PNIPA) and poly(sodium acrylate) (SA) gels and demixing/mixing transition of PNIPA-SA hydrogels swollen with increasing amounts of water were studied using conventional differential scanning calorimetry. In the crosslinked polymers, the glass transition linearly increases, and the transition range becomes broader, with increasing crosslinker content. Increasing content of ionic comonomer also produces an increase of glass transition temperature, which moves to higher temperatures with higher sodium acrylate fraction. The influence of chemical structure of PNIPA-SA hydrogels on the lower critical solution temperature (LCST) of PNIPA-SA/water mixtures during heating and cooling was quantified as function of the content of the crosslinker and the ionic comonomer, as well as water content of the hydrogel in the range from 95 to 70 wt%. At parity of water content, the LCST occurs at higher temperatures for gels containing higher amounts of sodium acrylate. Simi larly, the int roduction of N ,N ? -methylenebisacrylamide causes an increase of the LCST, which grows with increasing of crosslinking degree of the hydrogel.

The influence of mobility of the amorphous chains on crystallization of the naturally occurring polyester poly(3-hydroxybutyrate) (PHB) was investigated by conventional and temperature-modulated calorimetry. Thermal analysis of initially amorphous PHB shows two distinct exotherms, indicating that cold crystallization of PHB proceeds via a two-stage process. The experimental data were interpreted taking into account the role of vitrification/devitrification of the rigid amorphous fraction (RAF) on crystallization kinetics of PHB. The rigid amorphous structure, which is established simultaneously with the crystals growth during the first stage of cold crystallization, slows down crystallization before completion. The coupling between the crystalline and amorphous segments hinders further crystallization by the creation of an immobilized amorphous layer that surrounds the crystals and leads to metastability of the system before completion of ordering. Further increase of the temperature results in mobilization of the RAF. Once the amorphous layer coupled with the crystals has gained sufficient mobility, chain rearrangements needed to accommodate the PHB segments into ordered structures can take place, and crystallization can proceed further. Moreover, the role of the three-phase structure on embrrittlement of PHB upon storage is discussed. Soon after molding, PHB is ductile, but when the molded product is stored at room temperature, a detrimental ageing process makes the material very brittle. The change in mechanical behavior upon storage is caused by a variation of the three-phase structure, including progressive crystallization and vitrification of the rigid amorphous fraction.

In this contribution we report the preparation and the luminescence property of Eu(TTA)3phen complex doped toluene gels. Gels were prepared by using either a low molecular weight gelator, 12-hydroxystearic acid (HSA), or a macromolecular gelator, syndiotactic polymethylmethacrylate (s-PMMA). The gelation properties and their reversible behavior from solid-like to liquid systems have been investigated. In addition, photophysical investigations, as well as morphology, thermal properties and ageing behavior of the gels were analyzed as a function of composition of the gels.

The method recently proposed in order to monitor simultaneously the development of crystalline, mobile amorphous and rigid amorphous fractions during cooling from the melt in semicrystalline polymers is here applied to isotactic polystyrene. The procedure, which allows the determination of the temperature evolution of the three nanophases, is the development of a previous enthalpy-based method founded on a two-phase description of semicrystalline polymers, with only one crystalline phase and one amorphous phase accounted for. The three-phase method that is here utilized proves to be able to correct the erroneous trend predicted by the two-phase method in the temperature region close to Tg. For isotactic polystyrene it was found that the rigid amorphous fraction starts to vitrify during the final stages of the non-isothermal crystallization process, with the full establishment of the rigid amorphous structure completed during the subsequent cooling to room temperature. The reported results confirm the existence of a link between vitrification of the rigid amorphous phase and secondary crystallization, and prove that in isotactic polystyrene only the rearrangements of the amorphous regions localized in proximity of the growing secondary lamellae are subjected to geometrical restrictions, with consequent formation of rigid amorphous phase.

A new method to simultaneously monitor the development of crystalline, mobile amorphous and rigid amorphous fractions during cooling from the melt in semicrystalline polymers is here presented. The method, which allows the determination of the temperature evolution of the three nanophases, is the development of a previous enthalpy-based procedure founded on a two-phase description of the semicrystalline polymers, with only one crystalline phase and one amorphous phase accounted for. The proposed method has been applied to determine the development of the nanophase structure in poly(ethylene terephthalate) (PET) upon cooling from the melt. In addition the temperature-dependent enthalpy of the rigid amorphous phase has been calculated. For the cooling rates investigated, it was found that the rigid amorphous fraction starts to vitrify when the crystallization process is almost finished and continues further on cooling to the glass transition of the bulk amorphous phase. A possible connection between secondary crystallization and initial vitrification of the rigid amorphous phase has been hypothesized for PET.

Quantitative temperature-modulated differential scanning calorimetry (TMDSC) and superfast thin-film chip calorimetry (SFCC) are applied to poly(butylene terephthalate)s (PBT) of different thermal histories. The data are compared with those of earlier measured heat capacities of semicrystalline PBT by adiabatic calorimetry and standard DSC. The solid and liquid heat capacities, which were linked to the vibrational and conformational molecular motion, serve as references for the quantitative analyses. Using TMDSC, the thermodynamic and kinetic responses are separated between glass and melting temperature. The changes in crystallinity are evaluated, along with the mobile–amorphous and rigid–amorphous fractions with glass transitions centered at 314 and 375 K. The SFCC showed a surprising bimodal change in crystallization rates with temperature, which stretches down to 300 K. The earlier reported thermal activity at about 248 K was followed by SFCC and TMDSC and could be shown to be an irreversible endotherm and is not caused by a glass transition and rigid–amorphous fraction, as assumed earlier. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1364–1377, 2006

A throughout investigation of crystallization and melting behavior of poly(L-lactic acid) is detailed in this contribution. Crystallization analyses, conducted in both isothermal and non-isothermal conditions, revealed the occurrence of a sudden acceleration in phase transition rate in the temperature range between 100 and 118 °C. This unusual increase, due to very high rates of spherulite growth, seems not related to morphological changes of PLLA spherulites, nor to unexpected variations in nucleation rate. DSC analyses disclosed multiple melting behavior of PLLA, depending on crystallization temperature. At low temperatures the very high crystallization rates lead to the achievement of low values of crystalline degree, with formation of small and/or defective crystals, which have a large tendency to reorganize into more stable structures during the heating scan that leads to complete fusion. The multiple melting process of PLLA was also analyzed at different heating rates.

This article contains a detailed analysis of the melting process of poly(butylene terephthalate) performed by means of temperature-modulated differential scanning calorimetry at various temperature-modulation frequencies. The modulation period largely affects the apparent reversing specific heat capacity (cp), but not the total cp. An interpretation based on the analysis of the latent heat involved in the process is proposed. In addition, a detailed examination of the modulated heat-flow-rate curves is presented. The finding is that the melting process of poly(butylene terephthalate) is the sum of simultaneous events of the melting of small and defective crystals and their immediate recrystallization into more stable structures.