Etude par analyse thermique differentielle de melanges diphasiques de polyethylene basse densite et de polystyrene atactique
Résumé
L'étude par analyse thermique différentielle de mélanges diphasiques de polyéthylène basse densité et de polystyrène montre que les conditions de réalisation de ces mélanges influent sur le comportement à la cristallisation. Un pic de cristallisation secondaire, qui peut être très important, apparaît à une température voisine de 71° pour des mélanges à faible teneur en polyéthylène. La forme générale des thermogrammes dépend du type de mélangeur utilisé et de l'histoire thermomécanique des échantillons. L'influence des agents d'interface est également notée.
Abstract
Differential thermal analysis of two-phase polymer blends (low density polyethylene/polystyrene) shows that the way of marking the blend has an effect on the crystallization behavior. A secondary crystallization peak (that may be important) appears around 71° for high PE content. The general shape of the thermograms depends on the way of blending and on the thermomechanical history. The influence of interface agents is also pointed out.
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Cited by (20)
Fractionated crystallization in semicrystalline polymers
2021, Progress in Polymer ScienceThe crystallization of heterogeneously nucleated bulk polymers typically occurs in a single exothermic process, within a narrow temperature range, i.e., a single exothermic peak is detected by differential scanning calorimetry when the material is cooled from the melt. However, when a bulk semicrystalline polymer is subdivided or dispersed into a multitude of totally (or partially) isolated microdomains (e.g., droplets or cylinders), in number comparable to that of commonly available nucleating heterogeneities, several separated crystallization events are typically observed, i.e., fractionated crystallization. This situation is often found for the minor crystallizable component in immiscible blends.
When the bulk polymer is dispersed into a number of microdomains that is several orders of magnitude higher than the available number of heterogeneities within it, most microdomains will be heterogeneity-free. In these clean microdomains the nucleation can occur by contact with the interfaces (i.e., surface nucleation) or by homogeneous nucleation inside the microdomain volume. These cases can be easily encountered in cylinders or spheres within strongly segregated block copolymers, or in infiltrated polymers within nanopores of alumina templates.
In this work, a comprehensive review of the known cases of fractionated crystallization is provided. The changes upon decreasing microdomain sizes from a dominant single heterogeneous nucleation, through fractionated crystallization, to surface or homogeneous nucleation are critically reviewed. Emphasis is placed on the common features of the phenomenon across the different systems, and thus on the general conclusions that can be drawn from the analysis of representative semicrystalline polymers. The origin of the fractionated crystallization effects and their dramatic consequences on the nucleation and crystallization kinetics of semicrystalline polymers are also discussed.
Confinement effects on the crystallization and SSA thermal fractionation of the PE block within PE-b-PS diblock copolymers
2006, European Polymer JournalWell defined polyethylene-b-polystyrene (PE-b-PS) diblock copolymers have been synthesized by anionic polymerization in a wide composition range, keeping the length and the microstructure of the PE block constant. These copolymers provide a model system to study the confined crystallization of PE. A fractionated crystallization behavior (i.e., multiple exotherms are observed upon cooling from the melt) was observed for the PE block within block copolymers with 26% and 11% PE (i.e., and diblock copolymers, where the subscripts indicate the composition in wt% and the superscript the molecular weight in kg/mol). For confined PE, annealing is always observed when the material is self-nucleated. In the case of , most of the PE spheres crystallize at very large supercoolings, a behavior previously reported in the literature and associated with homogeneous nucleation. However, in our case, the peak crystallization temperature (i.e., Tc = 46.6 °C) is much lower (16 °C) than that reported for similar size PE nano-droplets but in diblock copolymer whose second block is chemically different to PS. We therefore conclude that the nature of the interphase between the two neighboring blocks may be responsible for such low temperature nucleation, since this Tc is still quite high with respect to the vitrification temperature of PE (in comparison with other polymers whose homogeneous nucleation temperature has been found close to Tg), a fact that could indicate that the homogeneous nucleation temperature has not been reached because surface (or interfacial) effects can dominate. Thermal fractionation takes advantage of the distribution of methyl sequence length in hydrogenated polybutadiene in order to produce by successive thermal annealing a distribution of lamellar thickness within each PE block. Such a distribution of thermal fractions is affected by confinement and therefore these experiments demonstrate the influence of morphological restrictions on the crystallization of the PE block within the PE-b-PS diblock copolymers. As the PE content in the copolymer decreases, topological confinement effects limit the size of the lamellar crystals that can be formed within the reduced dimensions of the microdomains (MD). By the use of the Gibbs-Thomson equation and the thermal fractionation results, a distribution of crystalline lamellar thickness within each MD was obtained and the orientation of the chains within the MD was deduced.
In this paper the crystallization behavior of PA6, dispersed as droplets in various immiscible amorphous polymer matrices, is reported. PA6 was melt-mixed at various compositions with PS, (PPE/PS 50/50 wt/wt) and PPE using twin-screw extrusion. The phase morphologies of the obtained blends were analysed using SEM, etching experiments and image analysis. The crystallization behavior of PA6 was investigated by dynamic and isothermal DSC experiments. In case PA6 is dispersed as droplets, fractionated crystallization behavior occurs, characterized by several crystallization events at different, lowered crystallization temperatures. It is found to depend on the blend morphology (size of the droplets) and the thermal history of the samples (heterogeneous nucleation density). The PA6 droplet size distribution is shown to strongly influence the crystallization behavior of the droplets. Vitrification of the matrix appears to cause nucleation in the droplets at the interface. Decreasing the PA6 droplet size results in slower overall crystallization rates.
The semicrystalline structure and degree of crystallinity of fractionated crystallizing poly(methylene oxide)/(polystyrene/poly(2,6-dimethyl-1,4 phenylene ether) POM/(PS/PPE) blends have been investigated by DSC, SAXS and WAXD. The three techniques yielded highly correlated results.
The degree of crystallinity of the POM phase determined by DSC (Xc,DSC) decreases with decreasing POM content in the blends and this is accompanied by a shift from bulk to homogeneous crystallization.
The reduction in the measured degree of crystallinity determined by WAXD (Xc,WAXD) is even more pronounced and indicates, in absence of evidence for the formation of different polymorphs, that only small and imperfect crystals are formed during homogeneous crystallization in finely dispersed droplets. Analysis of the width of the WAXD reflections, which is also related to Xc,WAXD, yields a linear correlation between L1, a measure of the lateral dimensions of the crystallites, and the average dispersed particle diameter. The parameter L2, corresponding to the crystalline lamellar thickness, is non-linearly correlated with the degree of crystallinity, indicating that the decrease in Xc,WAXD is not solely due to the formation of thinner lamellae at higher degrees of undercooling. There is a simple relationship between the SAXS long period and the crystallization temperature, corresponding to the formation of thinner and less perfect crystalline lamellae during fractionated crystallization at higher degrees of undercooling.
As the lateral dimensions of the crystallites of finely dispersed crystallizing droplets is governed by their size, Xc,WAXD can be directly related to the particle diameter, since the fraction of small or imperfect crystallites will not be measured by WAXD.
The fractionated crystallization behavior of POM in immiscible POM/(PS/PPE) blends has been investigated by Differential Scanning Calorimetry (DSC) and correlated to the blend phase morphology. By varying the PS/PPE composition, homogeneous amorphous phases with different glass transition temperatures, varying between 100 (Tg,PS) and 215°C (Tg,PPE), and melt-viscosities were obtained, without altering the interfacial tension of the blend system. As such, a model blend system has been created which allows to investigate both the influence of the blend phase morphology and of the physical state of the amorphous PS/PPE matrix, on the crystallization behavior of the minor POM phase.
The difference between low viscosity/low viscosity and low viscosity/high viscosity blend systems with respect to the development of the phase morphology during melt-mixing is reflected in various aspects of the fractionated crystallization behavior. The onset composition of fractionated crystallization can be related to the center of the phase inversion region for all blend systems. Within the same blend type, the extent of homogeneous crystallization can be related to the blend phase morphology (i.e. the number of droplets per volume unit of the dispersed phase). However, comparing different blend types reveals that other factors, such as the physical state of the amorphous matrix phase, also play a role. Further, multiple crystallization peaks were observed and have been related to the width of the particle size distribution of the dispersed POM phase.
Étude de mélanges diphasiques de polymères par analyse thermique différentielle
1983, European Polymer JournalL'Analyse Thermique Différentielle (ATD) d'un polypropylène, d'un éthylène acétate de vinyle et des mélanges correspondants réalisés avec ou sans agent d'interface à l'aide de divers dispositifs permet l'étude en particulier des cristallisations des phases continues et des dispersions. L'ATD sous haute pression permet de préciser la nature des phases concernées par la cristallisation dans le domaine étudié (P ≤ 3000 bars).
Differential Thermal Analysis (DTA) has been performed on polypropylene and ethylene vinyl acetate samples and their blends, with or without interface agents, in order to study crystallization of dispersed and continuous phases. High-pressure DTA makes it possible to define the nature of the various crystalline phases for pressures up to 3000 bars.