J. M. Meseguer Dueñas

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.

Structural relaxation of glass‐forming polymers based on an equation for configurational entropy, 4. Structural relaxation in styrene‐acrylonitrile copolymer

The structural relaxation process in styrene-acrylonitrile copolymer has been characterized by means of differential scanning calorimetry (DSC) experiments. The results in the form of heat capacity, cp(T), curves are analyzed using a model for the evolution of the configurational entropy during the process recently proposed by the authors.11,12 The model simulation allows one to determine the enthalpy (or entropy) structural relaxation times and the β parameter of the Kohlrausch-Williams-Watts equation characterizing the width of the distribution of relaxation times. This material parameters are compared with their analogues determined from the dielectric and dynamic-mechanical relaxation processes.

Forced compatibility in poly(methyl acrylate)/poly(methyl methacrylate) sequential interpenetrating polymer networks

The aim of this work is to study the miscibility of poly(methyl acrylate)/poly(methyl methacrylate), (PMA/PMMA), sequential interpenetrating networks, (IPNs), as a function of the crosslink density using dielectric and dynamic-mechanical techniques. The PMA/PMMA system is immiscible and so, for low crosslink densities, phase separation appears, as detected by the occurrence of two clearly differentiated main dynamic-mechanical relaxation processes corresponding to the two components. If crosslink density is high enough, a homogeneous IPN can be obtained, achieving a forced compatibilization of both networks. The IPN crosslinked with 10% ethyleneglycol dimethacrylate shows a single main dynamic-mechanical relaxation process. Only the α main relaxation process appears in the PMA networks within the temperature range (−60 to 200°C) of the experiments conducted in this work. The dielectric relaxation spectrum of PMMA networks shows the secondary β relaxation followed by a small α relaxation partially overlapped with it. In the IPNs, both the main relaxation processes tend to merge into a single one and the dielectric spectrum shows a single peak that mainly corresponds to the secondary relaxation of the PMMA.

Structural relaxation of glass-forming polymers based on an equation for configurational entropy: 3. On the states attained at infinite time in the structural relaxation process. Results on poly(ether imide)

The aim of this paper is to discuss the limit state attained at infinite time in the structural relaxation process. This state usually is identified with the equilibrium state extrapolated from the experimental data obtained at temperatures above the glass transition. The analysis is conducted with the help of a phenomenological model with fitting parameters, based on an equation for the evolution of the configurational entropy during the process. The model avoids the use of the fictive temperature, which makes it easier to introduce a different hypothesis on the limit states of the process.

Dielectric relaxations in poly(hydroxyethyl acrylate): influence of the absorbed water

Abstract Poly(hydroxyethyl acrylate) presents two relaxation zones, labelled γ and α, when it is completely dry. The temperature of the maximum of the γ relaxation, as well as its apparent activation energy, are somewhat higher than in poly(hydroxyethyl methacrylate), a fact that could be explained by higher intermolecular interactions in the series of polyacrylates than in the series of polymethacrylates. The absorption of even slight traces of water causes a new relaxation to appear, the intensity of which increases with the content of water, while at the same time the intensity of the γ relaxation decreases. This fact suggests the formation of an association of the water molecules with the side groups of the polymer. The characterization of the α relaxation is difficult because of the high d.c. conductivity component of the permittivity. Its temperature suggests the presence of hydrogen bonds which render the main chains rigid.

Dielectric relaxation spectrum of poly (ε-caprolactone) networks hydrophilized by copolymerization with 2-hydroxyethyl acrylate

Abstract.The dielectric relaxation spectrum of polycaprolactone (PCL) networks hydrophilized with different amounts of 2-hydroxyethyl acrylate (HEA) is investigated. PCL is a semicrystalline polyester with a complex relaxation spectrum that includes the main α relaxation and two secondary modes (β, γ) at lower temperatures. The overlapping of the different relaxational modes was split by using several Havriliak-Negami functions. Crosslinking the material modifies the dynamics of the main relaxation process as reflected by the parameters that characterize the Vogel behavior of the process and the dynamic fragility. The incorporation of HEA units in the network results in a material with microphase separation: two α processes are detected, the one corresponding to the PCL chains and the new one associated to nanometric regions that contain different amount of both comonomers. The incorporation of the HEA units in the system involves the presence of a new βsw relaxation due to the link of two side chains by water molecules through hydrogen bonding.

Influence of the molecular mass on the segmental relaxation times of polystyrene determined by DSC

The segmental dynamics of narrow fractions (áMwñ/áMnñ ≈1.05) of polystyrene with molecular masses ranging from 4000 to 600000 has been characterised by DSC. The samples were subjected to different thermal histories previously to the recorded heating scans including cooling from equilibrium at different cooling rates and annealing at different temperatures for different times. The fragility parameter m=[∂logτ/∂ (Tg/T)]Tggwas determined from the dependence of the glass transition temperature on the cooling rate from an equilibrium state. The curve that represents the enthalpy loss during the isothermal annealing vs. the annealing temperature for a fixed annealing time (180 min) shows a peak which can be used to describe the temperature interval in which the conformational rearrangements takes place at a rate compatible with the annealing time. This peak shifts towards higher temperatures when the molecular mass increases due to the shift of the glass transition temperature but the plot vs.Tg/T is independent on the molecular mass. The b parameter of the stretched exponential and the temperature dependence of the relaxation time were calculated through modelling that confirms that the fragility of the polymer is independent of molecular mass.The segmental dynamics of narrow fractions (aMwn/aMnn ≈1.05) of polystyrene with molecular masses ranging from 4000 to 600000 has been characterised by DSC. The samples were subjected to different thermal histories previously to the recorded heating scans including cooling from equilibrium at different cooling rates and annealing at different temperatures for different times. The fragility parameter m=[∂logτ/∂ (Tg/T)]Tggwas determined from the dependence of the glass transition temperature on the cooling rate from an equilibrium state. The curve that represents the enthalpy loss during the isothermal annealing vs. the annealing temperature for a fixed annealing time (180 min) shows a peak which can be used to describe the temperature interval in which the conformational rearrangements takes place at a rate compatible with the annealing time. This peak shifts towards higher temperatures when the molecular mass increases due to the shift of the glass transition temperature but the plot vs. T g/T is independent on the molecular mass. The b parameter of the stretched exponential and the temperature dependence of the relaxation time were calculated through modelling that confirms that the fragility of the polymer is independent of molecular mass.

Glass transition in homogeneous and heterogeneous interpenetrating polymer networks and its relation to concentration fluctuations

Abstract The glass transition of sequential poly(methyl acrylate)–poly(methyl methacrylate) interpenetrated polymer networks (IPNs) has been studied by means of temperature modulated differential scanning calorimetry. The poly(methyl acrylate) network was polymerised first, swollen up to equilibrium in methyl methacrylate monomer which was then polymerised. Ethyleneglycol dimethacrylate was used as cross-linking agent in a proportion of 0.1% or 10% by weight. Both networks in the IPNs had the same cross-linking concentration. The loosely cross-linked IPN presents clearly two glass transitions as corresponds to a phase-separated system. On the contrary a single broad glass transition can be observed in the highly cross-linked network. A simple model is presented to determine at each temperature the fraction of material which has a conformational mobility characteristic of a liquid. The temperature dependence of this liquid fraction calculated for the highly cross-linked network is very similar to the experimental temperature modulated differential scanning calorimetry thermogram. This allows one to conclude that at least a part of the broadening of the glass transition temperature interval of the IPN with respect to the pure component networks is expected even in a completely homogeneous blend due to the probability that the local composition is different than the average. A great part of the material vitrifies at temperatures lower than the glass transition that would correspond to the average composition, what can be explained by the temperature dependence of the length of cooperativity.

Conformational mobility in a polymer with mesogenic side groups Dielectric and DSC studies

Abstract The relaxation times of the conformational rearrangements of the main-chain segments of a liquid crystalline side-chain polymer was determined from differential scanning calorimetry experiments in the temperature interval around and below its glass transition. Phenomenological models with fitting parameters were used to evaluate the temperature dependence of the relaxation times and the form parameter of the relaxation times distribution. These parameters were compared with its counterparts in the dielectric α relaxation process which appear in the temperature interval immediately above the calorimetric glass transition. For the temperature interval below the calorimetric glass transition the differential scanning calorimetry (DSC) results were compared with the dielectric results obtained by the thermally stimulated depolarisation technique.

Segmental dynamics in poly(methyl acrylate)–poly(methyl methacrylate) sequential interpenetrating polymer networks: structural relaxation experiments

The miscibility of poly(methyl acrylate)–poly(methyl methacrylate) sequential interpenetrating polymer networks (IPNs) has been studied by probing the conformational mobility of the component polymer chains. These IPNs exhibit the phenomenon of forced compatibilization. In a conventional heating differential scanning calorimetry (DSC) thermogram, the highly cross-linked IPN shows a single glass transition which covers a temperature interval of around 100° C; in contrast, loosely cross-linked IPNs show two glass transitions. The conformational mobility in these IPNs is studied by subjecting them to isothermal annealings at temperatures in the region of the glass transition and below it. The DSC scans measured after these treatments allow one to determine the temperature interval in which the sample is out of thermodynamic equilibrium but keeps enough conformational mobility to relax during the isothermal annealing in such a way that the enthalpy loss is measurable with the sensitivity of a conventional DSC. The results allow one to reach some conclusions about the compositional distribution of the IPN on the nanometre scale.