Jean-Marc Haudin

Growth transition and morphology change in polypropylene

The growth rate of isotactic polypropylene is deduced from microscopic observations during isothermal crystallizations. A change in the growth regime is observed at 138 ‡C and interpreted as a Regime III → Regime II transition, according to Hoffmans kinetic theory of polymer crystallization. A Regime II → Regime I transition is also theoretically predicted at 155 ‡C, i. e. at a temperature outside the investigated temperature range. The Regime III → Regime II transition is related to the positive to negative change in the spherulite birefringence, which is generally attributed to a change in the organization of crystalline lamellae: quadritic arrays of intercrossing lamellae atTc < 138 ‡C (Regime III) and preferentially radiating lamellae atTc > 138 ‡C (Regime II). It is suggested that such a morphological change could be interpreted using the concept of “non-adjacent re-entry” introduced in Hoffmans kinetic theory. This interpretation could also explain the interspherulitic ruptures observed in negative spherulites.

Experimental study of shear-induced crystallization of an impact polypropylene copolymer

The crystallization kinetics of polypropylene was observed during shear and after shear experiments under isothermal condition. The crystallizations were performed in a plate-plate and a fiber pull-out device. The nucleation density, the crystalline growth and the overall kinetics were measured and compared with data obtained in a similar way but during static experiments. The morphologies are spherulitic and formed from nuclei which seem to be randomly distributed. α-phase spherulites are always observed but with a nucleation density and a growth rate which depend on shearrate. The nucleation density is strongly enhanced by shear and acts as the main factor on the overall kinetics. The overall kinetics can be analyzed with a two-step Avrami model, where an Avrami exponentn1 with a very high value is always observed first after shear and a more usual parametern2 for the subsequent crystallization period. This high value ofn1 seems to be related to the strong enhancement of nucleation density. The growth rate increases with the shear-rate, but the basic growth mechanisms do not seem to be modified. For crystallizations after shear the growth rate decreases with a long-time delay after shear but not down to the static value. The effect is characteristic of a partial relaxation of chain orientation after shear but with a very unusual time constant.

Strain-induced crystallisation in bulk amorphous PET under uni-axial loading

Strain induced crystallisation of poly(ethylene terephthalate) (PET) designed for stretch-blow moulding is studied combining well-controlled tensile tests, different quenching protocols and X-ray diffraction technique. As well known [Polymer, 33 (1992) 3182; Polymer, 33 (1992) 3189], crystallisation begins when a minimum strain has been reached. However, crystallisation is a progressive phenomenon that could involve intermediate stages or phases as suggested by some recent papers [Macromolecules, 31 (1998) 7562; Polymer, 41 (2000) 1217]. In this study, first evidence for the crystallisation is the appearance, parallel to the tensile direction, of zones having the crystal lateral order. The spreading out of the diffraction dots suggests that these zones are small in comparison to usual lamella thickness or that the order is imperfect. Longitudinal order appears later, progressively leading to fibre-texture. At low strain, before this texture is totally developed, cooling influences the final microstructure superposing steps in which crystallisation and relaxation compete illustrating the relative importance of cooling protocol when strain induced crystallisation of PET is studied. Nevertheless, strain hardening can be observed before the crystalline microstructure is totally developed, emphasising the fact that strain hardening is mainly controlled by first stages of crystallisation and that actual crystallisation occur during a following relaxation step.

Shear-induced crystallization of polypropylene: Influence of molecular weight

Three series of isotactic polypropylene characterized by different molecular weights and the same isotactic index have been studied during crystallization under static and shearing conditions. The shear is induced by the displacement of a glass fiber in the molten polymer. The monoclinic α-phase is here formed under shear with a columnar organization at the surface of the glass fiber, and does not appear under static condition. The growth-rate, constant during the shear-induced crystallization experiment, is compared with the result obtained from static crystallization. An important increase of the growth-rate due to the shear flow is observed. This increase depends on the molecular structure. The average molecular weights ―Mw and ―Mz seem to be the most important molecular parameters, for which an excellent correlation is obtained. The increase of these parameters ―Mw and ―Mz leads to a significant enhancement of the growth-rate, which can be multiplied by a factor of 10 in the present conditions.

Rheology of polypropylene in the solid state

The tensile behaviour of a commercial grade of isotactic polypropylene was tested in a temperature range between 20 and 150 °C with a video-controlled testing system which is capable of imposing a constant true strain-rate within the neck automatically. The results are displayed in the form of effective stress-strain curves and modelled by a constitutive equation in a multiplicative form. It is thus shown that, for each temperature, the plastic response can be described up to very large strains (ɛ ≃ 2.0) by a set of four parameters. The assumptions introduced in this modelling are critically discussed in order to check the validity of the simplifying hypotheses (strain homogeneity, isochoric deformation, etc.). The constitutive equation thus obtained was utilized in a finite difference code in order to predict the development of stretching instabilities of polypropylene. The simulation gives access to the engineering stress-strain response of the stretched test piece and to the detailed kinetics of the incipient neck. It is found that the severity of the instabilities is less at room temperature than near the melting point because of the decrease of the strain-hardening and of the strain-rate sensitivity with temperature.

Thermal dependence of nucleation and growth rate in polypropylene by non isothermal calorimetry

Crystallization of polypropylene at a constant cooling rate is studied by differential scanning calorimetry. The results are interpreted using Ozawas theory, which allows the determination of the type of nucleation. In the studied polypropylene, a transition between a sporadic in time and an instantaneous nucleation is found at about 122 °C. Furthermore, an extension of Ozawas theory is proposed to predict the thermal dependence of the spherulite growth rate. The results are consistent with those obtained by direct microscopic observation.

Transcrystallinity effects in thin polymer films. Experimental and theoretical approach

An experimental and theoretical investigation of the overall crystallization kinetics of thin polymer films containing transcrystalline regions on their surfaces is presented. DSC experiments on polyamide 6-6 show that typical features of crystallization curves can be associated with the occurrence of transcrystallinity. In order to interpret these experimental results, a theoretical model is built within the frame of overall crystallization kinetics theories. It makes it possible to correlate the thickness of transcrystalline zone both to the crystallization temperature, the shape of the peak and the density of nuclei within the polymer. Computer simulation is also used to describe the different steps of structure development within a thin film containing transcrystalline regions.

In situ observation of the spherulite deformation in polybutene-1 (Modification I)

Stress-strain behaviour under uniaxial tension was determined for polybutene-1 specimens with various spherulite sizes and degrees of crystallinity. Samples with large spherulites and a high crystallinity level exhibited a very specific behaviour, characterized by a higher elastic modulus, a remarkably homogeneous deformation (absence of necking), a stress whitening phenomenon and a brittle intraspherulitic rupture. The deformation mechanisms have been analysed by means of in situ microscopic observations. It was shown that interlamellar separation, which occurred first in the equatorial regions perpendicular to the tensile axis, is mainly responsible for the observed phenomena.

Isothermal and Anisothermal Models for Cast Film Extrusion

Abstract In cast film extrusion, the polymer is extruded through a slit die, stretched in air and then cooled on a chill roll. Isothermal and non-isothermal models have been developed to predict the polymer melt behavior between the die exit and the chill roll. The mechanical resolution is based on the assumption of a uniform thickness in the film cross section. It takes gravity and inertia forces into account and is performed in two steps: the equilibrium equation for a small film element is first written in a curvilinear coordinate system and the stress tensor is then calculated in a local Cartesian system. The temperature field in the film thickness is determined using local heat transfer coefficients, whose calculation is based on a precise description of the radiation and convection phenomena. This local description requires the measurement of air temperature close to and far from the film. The coupling between mechanical and thermal equations is achieved by introducing the temperature dependence of the viscosity. An iterative scheme is used to adjust the stretching force, the length of the film and the contact point on the roll. The theoretical predictions (film profile, velocity, width, temperature) are compared with experimental measurements performed on a pilot line. The agreement is always good, except for the width, whose computed linear variation is in contradiction with the experimental observations.

Influence of specimen thickness on isothermal crystallization kinetics. A theoretical analysis

In the DSC technique, isothermal crystallization experiments are usually performed on thin flat specimens, but their interpretation generally uses theories developed for an unbounded volume. In this paper, isothermal crystallization of spherical entities in the volume limited by two parallel infinite planes is considered. Our model, derived from Avramis theory, gives an analytical expression for the transformed volume fraction as a function of time. It is shown that the influence of thickness becomes important when thickness becomes of the order of or smaller than the average spherulite radius. The main effects of a decreasing thickness are a slower crystallization kinetics and a decrease in the Avrami exponent. These results can be used to interpret experimental data obtained in isothermal polymer crystallization.