Clara Silvestre

Non-isothermal crystallization of polymers

Abstract 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.

Morphology, crystallization and melting behaviour of films of isotactic polypropylene blended with ethylene-propylene copolymers and polyisobutylene

Abstract The crystallization, the morphology and the thermal behaviour of thin films of isotactic polypropylene (iPP) blended with elastomers such as random ethylene-propylene copolymers (EPM) with different ethylene content and polyisobutylene (PiB) were investigated by means of optical microscopy, differential scanning calorimetry and wide angle X-ray diffractometry. During crystallization EPM copolymers are ejected on the surface of the film forming droplet-like domains. A different morphology is observed in iPP/PiB blends. For these mixtures the elastomers separate from the iPP phase forming spherical domains that are incorporated in the iPP intraspherulitic regions. Both EPM and PiB elastomers act as nucleant agents for iPP spherulites. This nucleation efficiency is strongly dependent on the chemical structure and molecular mass of the elastomers. The addition of EPM causes an elevation of the observed and equilibrium melting temperature of iPP. This unusual effect may be accounted for by assuming that the elastomers are able to extract selectively the more defective molecules of iPP. The depression of the growth rate of spherulites and the observed and equilibrium melting temperature of iPP, noted in iPP/PiB blends, suggests that these two polymers have a certain degree of compatibility in the melt.

Poly(ethylene oxide)/poly(methyl methacrylate) blends: Influence of tacticity of poly(methyl methacrylate) on blend structure and miscibility

Abstract The influence of different configurations of poly(methyl methacrylate) on the miscibility and superstructure of poly(ethylene oxide)/poly(methyl methacrylate) (PEO/PMMA) blends was examined using small-angle X-ray scattering and differential scanning calorimetry. The blends prepared by solution casting were isothermally crystallized at 48°C. The miscibility, the melting behaviour, the glass transition temperature and the structural parameters of the blends were strongly dependent on the tacticity and blend composition. The small-angle X-ray intensity profiles were analysed using a recently developed methodology. For the poly(ethylene oxide)/atactic poly(methyl methacrylate) (PEO/APMMA) and poly(ethylene oxide)/syndiotactic poly(methyl methacrylate) (PEO/SPMMA) blends, the long period and the amorphous and transition region thicknesses increased with increase of PMMA content, whereas for the poly(ethylene oxide)/isotactic poly(methyl methacrylate) (PEO/IPMMA) blends they are independent of composition. The structural properties of the blends were attributed to the presence of non-crystallizable material in the interlamellar or interfibrillar regions, depending on PMMA tacticity. From the glass transition and melting temperatures, it has been supposed that one homogeneous amorphous phase is present in the case of PEO/APMMA and PEO/SPMMA blends and that the PEO/IPMMA amorphous system is phase-separated. The free-volume contribution to the energy of mixing for the various tactic PMMAs is hypothesized to be responsible for the difference in mixing behaviour.

Crystallization, morphology, structure and thermal behaviour of nylon-6/rubber blends

Abstract An experimental study was carried out in order to investigate the morphological, kinetic, structural and thermodynamic properties of nylon-6/rubber (namely ethylene-propylene copolymer (EPM) and ethylene-propylene copolymer functionalized by inserting along its backbone succinic anhydride groups (EPM-g-SA)) blends. The morphology and the overall kinetics of crystallization of the blends strongly depend on the type of copolymer added to nylon and on the blend composition. The EPM-g-SA acts as a nucleating agent for the Ny spherulites and at the same time causes a drastic depression of the overall kinetic rate constant. This decrease is related to the increase of the melt viscosity observed in Ny/EPM-g-SA blends. The crystalline lamella thickness of the Ny phase in the blends is lower than that of pure Ny crystallized at the same T c suggesting that the presence in the melt of an elastomeric phase disturbs the growth of the Ny crystals. The rubber does not influence the thermal behaviour of the nylon. The results found lead to the conclusion that in the melt nylon-6 is incompatible with both EPM and EPM-g-SA copolymers.

Study of the non-isothermal crystallization of poly(ethylene oxide)/poly(methyl methacrylate) blends

Abstract Crystallization during cooling of a poly(ethylene oxide)/poly(methyl methacrylate) (PEO/PMMA) blend has been studied using differential scanning calorimetry. Five different cooling rates Vr have been used (namely 10, 5, 2.5, 1.25 and 0.625°C min−1). The presence of PMMA for a given Vr reduces the overall PEO crystallization rate. This effect can be ascribed to reduction of the mobility of the crystallizable chains due to the presence of the amorphous component. It was found that in quasi-static conditions at lower Vr, when nucleation and growth processes are determined by a thermal mechanism alone, the experimental data for the pure PEO and the PEO/PMMA 90 10 and 80 20 blends agree quite well with the theoretical results obtained using the zero-order approximation of Ziabickis theory. At higher Vr, in the case of the blends, athermal nucleation cannot be neglected, and then the same approximation does not fit the experimental results. The experimental data analysed showed no agreement with Ozawas theoretical predictions.

Properties of thin films of isotactic polypropylene blended with polyisobutylene and ethylene-propylene-diene terpolymer rubbers

An experimental study was carried out to investigate the kinetic, morphological and thermodynamic properties of thin films of isotactic polypropylene (iPP) blended with several elastomers such as ethylene-propylene-diene terpolymer (EPDM) and three samples of polyisobutylene (PIB) with different molecular masses. The addition of the rubber to iPP causes drastic modifications in the morphology, nucleation density, spherulite growth rate and thermal behaviour of iPP. Such modifications depend strongly on the chemical and molecular mass of the added elastomer and on the composition of the blend. All the elastomers studied seem to act as nucleating agents for the iPP spherulites. The addition of PIB to iPP results in a reduction of the spherulite growth rate G, whereas the addition of EPDM does not seem to have a great influence. For the iPP/PIBHM iPP/PIBMM and iPP/EPDM blends a depression of the equilibrium melting temperature Tm, with respect to that of pure iPP, is observed. This depression is increased for the blend containing 20% rubber. This effect is probably related to phenomena of partial miscibility in the melt and to the coexistence of processes such as molecular fractionation and preferential dissolution of the more defective molecules.

Syndiotactic polystyrene: crystallization and melting behaviour

Abstract The results of an investigation into the isothermal crystallization and thermal behaviour of syndiotactic polystyrene (sPS) are reported. The study was carried out using differential scanning calorimetry, optical microscopy and dilatometry. From the melt, sPS crystallizes according to a spherulite morphology. The spherulite growth rate and overall crystallization rate depend on crystallization temperature and sample preparation. During the crystallization process, segregation of impurities and non-crystallizable molecules at the spherulite growth front was observed. The nucleation rate, probably very fast, hindered the determination of crystallization kinetic parameters for some crystallization temperatures. Comparing the melt crystallization process of sPS with that of isotactic polystyrene (iPS) for the same value of undercooling, sPS crystallizes more rapidly than iPS, indicating the influence of the tacticity of the materials on the crystallization process. The equilibrium melting temperature T°m was determined and found to be 275°C.

Isotactic polypropylene/polyisobutylene blends: morphology-structure-mechanical properties relationships

Morphological observations by optical and scanning electron microscopy, wide (WAXS) and small (SAXS) angle X-ray scattering, differential scanning calorimetry (DSC) and mechanical tests have been performed on sheet specimens of isotactic polypropylene (iPP)/polyisobutylene (PIB) blends obtained under different crystallization conditions. Two kinds of morphologies have been observed, particularly at high crystallization temperatureTc, on thin sections of the same sheets: a spherulitic one in the centre and a row-like structure on the edges. The size of the spherulites, as well as the thickness of the row-like regions, decreases with diminishingTc, and seemsto be independent of the amount of rubber. The adhesion among the spherulites and between the spherulites and the row-like regions seems to become poorer with higherTc. The rubber particles seem to be evenly dispersed into the iPP matrix for samples quenched at low temperatures, whereas for samples isothermally crystallized (at highTc) their concentration seems to be slightly higher at the border of the spherulites than in the centre. The overall crystallinity measured by DSC and by WAXS is an increasing function ofTc and decreases with increasing amount of PIB. Theβ index of iPP phase, quite low indeed (max ≃ 3%), drops with loweringTc and with enhancing PIB percentage. The long spacingL for a given quenching temperatureTq is independent of PIB content, whereas for isothermally crystallized samples at low undercooling varies differently according toTc. The lamellar thicknessLc is always a decreasing function of rubber content. Stress-strain analysis shows a more and more brittle behaviour both with increasingTc (beyondTc=122° C all the specimens are very brittle irrespective of PIB amount) and PIB amount in accordance with the morphological observations. Some tentative hypotheses have been made to explain the observed behaviour.

Syndiotactic polystyrene-based blends: crystallization and phase structure

The crystallization from the melt, the morphology and the miscibility of Syndiotactic Polystyr-ene/Poly (vinylmethylether) (SPS/PVME) blends and SPS-/Poly (2, 6-dimethyl-l, 4-diphenyleneoxide) (SPS/PPO) blends has been analyzed by differential scanning calorimetry and optical microscopy.

Isotactic polypropylene/hydrogenated oligo(cyclopentadiene) blends: phase diagram and dynamic-mechanical behaviour of extruded isotropic films

Abstract The phase structure and the dynamic-mechanical properties of isotropic films of isotactic polypropylene iPP)/hydrogenated oligocyclopentadiene (HOCP) blends are reported. It is found that the phase diagram presents both the lower critical solution temperature and the upper critical solution temperature. The dynamic-mechanical tests (performed at a frequency of 1 Hz and at a heating rate of 4°C min−1) show that the addition of HOCP > 20 wt% causes an increase of the storage modulus at low temperatures; the curves of storage modulus and loss modulus of the isotropic films containing HOCP > 20 wt% show the transition of the smectic form of iPP to the crystalline α form.