J.L. Gómez Ribelles

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

The aim of this work is to determine the relaxation times of the cooperative conformational rearrangements of the amorphous phase in semi-crystalline poly(ethylene terephthalate) (PET) and compare them with those calculated in amorphous PET. Samples of nearly amorphous polymer were prepared by quenching and samples with different crystallinity fractions were prepared from the amorphous one using cold crystallisation to different temperatures. The differential scanning calorimetry (DSC) thermograms measured on samples rapidly cooled from temperatures immediately above the glass transition show a single glass transition which is much broader in the case of high-crystallinity samples than in the amorphous or low-crystallinity PET. To clarify this behaviour, the samples were subjected to annealing at different temperatures and for different periods prior to the DSC measuring heating scan. The thermograms measured in samples with low crystallinity clearly show the existence of two amorphous phases with different conformational mobility, these are called Phases I and II. Phase I contains polymer chains with a mobility similar to that in the purely amorphous polymer, while Phase II shows a much more restricted mobility, probably corresponding to conformational changes within the intraspherulitic regions. The model simulation allows to determine the temperature dependence of Phase II relaxation times, which are independent from the crystallinity fraction in the sample and around two decades longer than those of the amorphous polymer at the same temperature.

Dielectric relaxation spectroscopy of polyethylene terephthalate (PET) films

The dielectric properties of biaxially stretched polyethylene terephthalate (PET) films of thickness and 68% degree of crystallinity were investigated by means of dielectric relaxation spectroscopy in the frequency range Hz and the temperature range C. Differencial scanning calorimetry (DSC), in the range C, was employed to investigate the thermal properties of the PET samples. Besides measuring the relaxation associated with the glass transition and the secondary relaxation, special attention has been paid to the investigation of DC-conductivity-related effects. They give rise to high dielectric permittivity values and dielectric losses at low frequencies and high temperatures. The results are analysed within the complex permittivity formalism and discussed in terms of interfacial Maxwell - Wagner - Sillars polarization, the peak, conductivity relaxation, space-charge polarization, electrode polarization and DC conductivity. DC conductivity values determined from frequency plots of the AC conductivity follow the Vogel - Tamman - Fulcher equation at temperatures higher than the glass transition temperature, indicating that the charge-carrier transport mechanism is governed by the motion of the polymeric chains. On the basis of the temperature dependence of the DC conductivity PET is classified as a fragile system.

Porous poly(2-hydroxyethyl acrylate) hydrogels

Abstract Porous hydrogels were prepared by copolymerisation of 2-hydroxyethyl acrylate and ethyleneglycol dimethacrylate (as crosslinking agent) in solution using water or ethanol as solvents. Macroscopic pores are formed due to the segregation of the solvent from the polymer network during the polymerisation process. In the dry state the polymer network had nearly the same density as the poly(2-hydroxyethyl acrylate) polymerised in bulk thus showing that the pores collapse during the drying process. When the dry samples were swollen in water the pores opened and the volume fraction of pores could be determined by weighing. The pore morphology was observed by scanning electron microscopy. The dependence of the pore size on the solvent used and on the monomer/solvent ratio in the polymerisation process is shown. The elastic modulus and loss tangent were measured as a function of temperature in the region of the main (or α) dynamic-mechanical relaxation process. These spectra were correlated with the morphology of the samples.

Hybrid structure in PCL-HAp scaffold resulting from biomimetic apatite growth

Polymer–ceramic composites are favourite candidates when aiming to replace bone tissue. We present here scaffolds made of polycaprolactone-hydroxyapatite (PCL-HAp) composites, and investigate in vitro mineralisation of the scaffolds in SBF after or without a nucleation treatment. In vitro bioactivity is enhanced by HAp incorporation as well as by nucleation treatment, as demonstrated by simulated body fluid (SBF) mineralization. Surprisingly, we obtained a hybrid interconnected organic-inorganic structure, as a result of micropore invasion by biomimetic apatite, which results in a mechanical strengthening of the material after two weeks of immersion in SBF×2. The presented scaffolds, due to their multiple qualities, are expected to be valuable supports for bone tissue engineering.

A phenomenological study of the structural relaxation of poly (methyl methacrylate)

Abstract The structural relaxation process of poly(methyl methacrylate) has been studied by differential scanning calorimetry. The sample was subjected to different thermal histories with isothermal stages at ageing temperatures of 100 and 80°C for different ageing times. The enthalpy increment suffered by the sample in the isothermal stage of the thermal history (enthalpy loss due to ageing) was calculated as a function of the ageing time and temperature. The specific heat curves measured were fitted to two phenomenological models.

Molecular mobility and hydration properties of segmented polyurethanes with varying structure of soft‐ and hard‐chain segments

The molecular mobility and hydration properties of model segmented polyurethanes from either poly(propylene glycol) (PPG) or poly(butylene adipate) (PBAD), both of molecular weight 2000 (soft segments), and three different diisocyanates (all-trans 4,4′-dicyclohexylmethane diisocyanate, 100% t,t HMDI; HMDI with 20% of trans isomers, 20% t,t HMDI; and 4,4′-diphenylmethane diisocyanate, MDI) (hard segments) were investigated using differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC) measurements, ac dielectric relaxation spectroscopy (ac DRS), equilibrium water-sorption isotherms (ESI), and dynamic water-sorption isotherms (DSI). No effects of the structure and of the amount of the soft segments on the overall degree of microphase separation (DMS) into microphases rich in soft and hard segments, respectively, were observed. On the contrary, DMS depends on the composition of the diisocyanates used and systematically increases in the order MDI, 20% t,t HMDI, 100% t,t HMDI as indicated by DSC, TSDC, and ac DRS. The PPG-based polyurethanes are characterized by larger values of water content at saturation, h, and smaller values of the diffusion coefficient of water, D. h increases with temperature, indicating that the sorption process is endothermic.

Polymer-water interactions in poly(hydroxyethyl acrylate) hydrogels studied by dielectric, calorimetric and sorption isotherm measurements

Abstract Poly(hydroxyethyl acrylate) hydrogels with water contents up to 0.96 (g water/g dry material) are studied with equilibrium and dynamic water sorption isotherm measurements, differential scanning calorimetry, thermally stimulated depolarization current measurements and broadband a.c. dielectric relaxation spectroscopy. Our main interest is focused on comparing results on the state of water in the hydrogel obtained with the different techniques on the same sample and on discussing in a quantitative way the relations among the different descriptions imposed by the different techniques. The results show that for water contents lower than about 0.20-0.30 the hydrogel is a homogeneous system in the temperature range down to −100 °C. At higher water contents phase separation occurs. Several critical water contents have been obtained by means of the different techniques used.

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 (αc-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 αc-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.

The length of cooperativity at the glass transition in poly(vinyl acetate) from the modeling of the structural relaxation process

Abstract The structural relaxation process of poly(vinyl acetate) has been studied by differential scanning calorimetry. The sample was subjected to different thermal treatments including an isothermal annealing at temperature Ta for a time ta. The heat capacity cp(T) was measured during a heating scan. Thus, the experimental results consist of a series of cp(T) curves determined after thermal histories with different values of Ta and ta. The experimental results were compared with the prediction of a multiparameter phenomenological model described in references [15,16]. This model follows the evolution of the configurational entropy of the sample during the whole thermal history. The parameters of the model were calculated by simultaneous fits to five cp(T) curves, corresponding to different thermal histories (the parameters of the model have the character of material parameters, independent of the thermal history). This allows the determination of the temperature dependence of the relaxation times and the β parameter of the Kohlrausch-Williams-Watts equation. From these parameters and an estimate of the internal rotational barrier obtained by molecular mechanics calculations, the size of the smallest cooperative rearranging region in the Adam-Gibbs theory was calculated. This result is compared with the length of cooperativity calculated from the temperature fluctuation theory proposed by Donth.

Electrosprayed poly(vinylidene fluoride) microparticles for tissue engineering applications

Poly(vinylidene fluoride) (PVDF) microparticles have been produced by electrospraying as a suitable substrate for tissue engineering applications. The influence of the polymer solution concentration and processing parameters, such as electric field, flow rate and inner needle diameter, on microparticle size and distribution has been studied. Polymer concentration is the most influential parameter on PVDF microparticle formation. Higher concentrations promote the formation of fibers while dilute or semi dilute concentrations favor the formation of PVDF microparticles with average diameters ranging between 0.81 ± 0.34 and 5.55 ± 2.34 μm. Once the formation of microparticles is achieved, no significant differences were found with the variation of other electrospray processing parameters. The electroactive β-phase content, between 63 and 74%, and the crystalline phase content, between 45 and 55%, are mainly independent of the processing parameters. Finally, MC-3T3-E1 cell adhesion on the PVDF microparticles is assessed, indicating their potential use for biomedical applications.