Glass transition and structural relaxation in semi-crystalline poly(ethylene terephthalate): a DSC study

An experimental study and model development of poly(ethylene terephthalate) yarn morphology

In this study, macromolecular arrangements in fibers were investigated and a novel approach based on model development for partially oriented (POY) polyethylene terephthalate (PET) yarn morphology was proposed. Differential scanning calorimeter (DSC) analysis, Fourier transform infrared (FT-IR) analysis, boiling water shrinkage and tensile tests were performed through the study. DSC thermogram, FT-IR spectra, stress-strain curve, the change in the boiling water shrinkage properties and data from the literature were used to develop the model for POY PET fiber morphology. In order to make a comparison, textured PET yarn morphology was also investigated. The neck formations during tensile tests, cold crystallization on the DSC thermogram, peak formation on the FT-IR spectrum of the POY PET yarn after texturizing and shrinkage test at 971 and 845 cm-1 and shrinkage phenomenon have been explained by this model.

Relaxation dynamics of poly(vinylidene fluoride) studied by dynamical mechanical measurements and dielectric spectroscopy

The aim of this study is to analyze the mobility of polymer chains in semicrystalline poly(vinylidene fluoride) (PVDF). PVDF crystallizes from the melt in the α crystalline phase. The transformation from the α phase to the electroactive β phase can be induced by stretching at temperatures in the range between 80 and 140 °C. The spherulitic structure of the crystalline phase is deformed during stretching to form fibrils oriented in the direction of the strain. The amorphous phase confined among the crystalline lamellae is distorted as well and some degree of orientation of the polymer chains is expected. Dynamic-mechanical and dielectric spectroscopy measurements were performed in PVDF films stretched to strain ratios up to 5 at temperatures between 80 and 140 °C. Dynamic-mechanical measurements were conducted between -60 °C and melting and in this temperature range the relaxation spectra show the main relaxation of the amorphous phase (called β-relaxation) and at higher temperatures a relaxation related to crystallites motions (α (c)-relaxation). Although the mean relaxation times of the β-relaxation are nearly equal in PVDF before and after crystal phase transformation, a significant change of shape of the relaxation spectrum proves the effect of chain distortion due to crystal reorganization. In stretched PVDF the elastic modulus of the polymer in the direction of deformation is significantly higher than in the transversal one, as expected by chain and crystals fibril orientation. The recovery of the deformation when the sample is heated is related with the appearance of the α (c)-relaxation. Dielectric spectroscopy spectrum shows the main relaxation of the amorphous phase and a secondary process (γ-relaxation) at lower temperatures. Stretching produces significant changes in the relaxation processes, mainly in the strength and shape of the main relaxation β. The Havriliak-Negami function has been applied to analyze the dielectric response.

Phenomenological model for the confined dynamics in semicrystalline polymers: the multiple alpha relaxation in cold-crystallized poly(ethylene terephthalate)

The segmental relaxation in poly(ethylene terephthalate), crystallized from either an isotropic or a cold-drawn glass, is investigated by means of dielectric spectroscopy. It is shown that there exist two distinct alpha relaxation modes: a slow one, characterized by a rather wide frequency interval, and a faster, much narrower one. A simple phenomenological model is developed in order to analyze the polarization autocorrelation functions phi(t)'s associated with these relaxation modes. The model is based on the idea that the growth of crystalline domains causes a progressive confinement of the amorphous regions where, eventually, the observed alpha processes take place. The mechanism of confinement is accounted for by applying to the case of constrained density fluctuations, well known concepts introduced by Adam and Gibbs [J. Chem. Phys. 43, 139 (1965)] concerning the relaxation dynamics in liquids close to the glass transition. Randomness on confining conditions is then introduced, leading to the derivation of analytical expressions which are used afterwards to fit the asymptotic behavior of the phi's for long-time tails. It is found that the slow, broad alpha process takes place in regions where the confining effect of crystals is strong, whereas the amorphous domains relaxing via the fast mode are those where the confinement effect of crystals is weak. The analysis of the phi's by means of this model allows us to relate the fitting exponents to the dispersion in the free energy associated with structural rearrangement.

The kinetics of the structural relaxation process in PHEMA-silica nanocomposites based on an equation for the configurational entropy

The enthalpy relaxation of polymer-silica nanocomposites prepared by simultaneous polymerization of poly(2-hydroxyethyl methacrylate) (PHEMA) and tetraethyloxysilane, TEOS, a silica precursor, is investigated. Both the glass transition temperature, Tg, and the temperature interval of the glass transition, DeltaTg , increase as the silica content in the sample does. Structural relaxation experiments show that the temperature interval in which conformational motions take place broadens as the silica content in the hybrid increases. A phenomenological model based on the evolution of the configurational entropy during the structural relaxation process, the SC model, has been used for determining the temperature dependence of the relaxation times during the process. The results show an increase of the fragility of the polymer as the silica content increases, a feature that can be related to the broadening of the distribution of relaxation times characterized by the beta parameter of the stretched exponential distribution. On another hand the silica content increase produces a significant change of the relaxation times in the glassy state.

Glass transition and the origin of poly(p-phenylene sulfide) secondary crystallization

The effect of low temperature cold-crystallization on quenched poly(p-phenylene sulfide) (PPS) amorphous phase behaviour was systematically investigated by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) over the entire range of the process, from its early stage to the end. For the first time a well resolved double glass transition of partially cold-crystallized samples was evidenced. A T(g) steady increase was observed during the primary crystallization process, due to the reduction of amorphous chain segmental mobility imposed by the growing rigid phase. A shift of the relaxation temperature of about 10 degrees C was recorded at the end of the primary crystallization process. As the secondary crystallization takes place a new glass transition appears at higher temperature. For longer annealing time the lower T(g) disappears while the intensity of the upper one increases. The upper temperature glass transition of semi-crystalline PPS is explained as a consequence of the PPS secondary cold-crystallization process. In the light of the thermal and dynamic mechanical results, an interpretation is given of step-wise double crystallization, evidenced in non-isothermal cold-crystallization experiments carried out at low heating rate and followed by means of FT-IR techniques.

Advanced thermal characterization of fractionated natural organic matter

This work focuses on an experimental investigation of the thermodynamic properties of natural organic matter (NOM), and whether fractions of NOM possess the same thermodynamic characteristics as the whole NOM from which they are derived. Advanced thermal characterization techniques were employed to quantify thermal expansion coefficients (alpha), constant-pressure specific heat capacities (C(p)), and thermal transition temperatures (T(t)) of several aquatic- and terrestrial-derived NOM. For the first time, glass transition behavior is reported for a series of NOM fractions derived from the same whole aquatic or terrestrial source, including humic acid-, fulvic acid-, and carbohydrate-based NOM, and a terrestrial humin. Thermal mechanical analysis (TMA), standard differential scanning calorimetry (DSC), and temperature-modulated differential scanning calorimetry (TMDSC) measurements revealed T(t) ranging from -87 degrees C for a terrestrial carbohydrate fraction to 62 degrees C for the humin fraction. The NOM generally followed a trend of increasing T(t) from carbohydrate to fulvic acid to humic acid to humin, and greater T(t) associated with terrestrial fractions relative to aquatic fractions, similar to that expected for macromolecules possessing greater rigidity and larger molecular weight. Many of the NOM samples also possessed evidence of multiple transitions, similar to beta and alpha transitions of synthetic macromolecules. The presence of multiple transitions in fractionated NOM, however, is not necessarily reflected in whole NOM, suggesting other potential influences in the thermal behavior of the whole NOM relative to fractionated NOM. Temperature-scanning X-ray diffraction studies of each NOM fraction confirmed the amorphous character of each sample through T(t).