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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.

A detailed investigation of the low-temperature endotherm of poly(ethylene terephthalate) (PET) performed by temperature-modulated differential scanning calorimetry is presented. The origin of the small endotherm, generally observed a few degrees above the crystallization temperature in PET and in many other polymers, is a widely discussed matter. The most frequent interpretation considers it the result of partial fusion with superposition of a recrystallization process even if it has also been proposed that it can originate from enthalpic recovery connected to mobilization of the rigid amorphous fraction. In an attempt to resolve the question, a new method for the interpretation of the modulated heat-flow-rate curve resulting from a temperature modulation program is proposed. The procedure consists of the analysis of the initial points of the steady-state heatflow-rate signals in the heating and cooling semiperiods with the temperature modulation being performed with a sawtooth profile. The study conducted in parallel on the reversing specific heat capacity and the heatflow-rate curves, observed on heating after isothermal crystallization at various temperatures, showed that multiple processes, involving both the crystalline and the rigid amorphous fraction, overlap in the temperature range in which the low-temperature endotherm is observed. The origin of the endotherm under investigation is therefore connected with both partial fusion of the crystalline portions and enthalpy recovery subsequent to structural relaxation of the rigid amorphous fraction. An estimation of the relative percentages of the two different processes is presented and discussed.

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 Kg were obtained. The lateral surface free energy s was calculated and, from this, the á parameter of the Thomas-Staveley expression was obtained.

Polymer blends have been widely studied for the property improvement of commercial polymers. Most polymer blends, however, present coarse and unstable phase morphologies and poor adhesion at the interface, which is due to their immiscibility. Such problems have often been overcome with the addition of a proper compatibilizer that is able to modify the interfacial condition. In this way, it is possible to control the morphology of the material and the interphase adhesion in an appropriate manner to achieve an effective stress transfer within the blend. Compatibilization criteria and procedures, either by adding a third component (compatibilizer) or by inducing in situ chemical reactions between blend components, are reviewed. Some theoretical aspects of compatibilization processes have also been considered, giving examples of experimental consistency. The authors also report on the different compatibilizers used for several pairs of immiscible polymers with indications of their action to promote homogenization between phases.