Crystallization Behavior Of Polymers Research Articles

Phase separation and polymer crystallization in a poly(4‐methyl‐1‐pentene)–dioctylsebacate–dimethylphthalate system via thermally induced phase separation

AbstractThe effects of the polymer concentration and quenching temperature on the phase separation, the membrane morphology and polymer crystallization behavior in a poly(4‐methyl‐1‐pentene) (TPX)‐dioctylsebacate (DOS)‐dimethylphthalate (DMP) system via thermally induced phase separation were studied with a pseudobinary phase diagram, with the weight ratio of DOS:DMP = 1:1. SEM was used to observe the membrane morphology and structure, whereas the TPX crystallization behavior was studied with DSC and WAXD. Liquid‐liquid phase separation occurred, although quenching under the crystallization temperature. As the quenching temperature decreased, the pore size decreased, with better connected pore structure formed. The membranes quenched at 333 and 363 K showed good cellular structures, with an average pore size of about 2.3μm, whereas the pores of the membranes quenched at 393 and 423 K were not well formed, with some lamellar crystals on the inner side. The diluent assisted the mobility of the polymer chain, which improved the polymer crystallization. Dual‐melting‐peak behavior occurred for all the samples studied here. As the quenching temperature increased, the first peak of the melting trace moved to a higher temperature, whereas the second one stayed almost the same. The flexibility of the TPX main chain was restricted by the side groups, which allowed liquid‐liquid phase separation to occur first when quenched below the equilibrium crystallization temperature. This allowed primary and secondary crystallization, which was responsible for the dual‐melting‐peak behavior. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 153–161, 2007

Crystallization behavior of single or pauci chain aggregates of isotactic polystyrene

Single and pauci chain aggregates of isotactic polystyrene (i-PS) were prepared by the freeze-drying process from dilute solutions with the concentration from 1×10−3 to 2×10−5 g/mL. It was found by DSC measurements that the melting point of samples gradually shifted to lower temperatures with the decrease of the solution concentration used for sample preparation. As a result, the lamella thickness of bulk samples and the samples prepared by the freeze-drying process from a solution of 2×10−5 g/mL was 19.3 and 12.6 nm, respectively. At 468.3 K the half crystallization time (t1/2) of samples freeze-dried from a solution of 1×10−4 g/mL was about 36 s, which was merely one tenth of that of the bulk sample. In addition, the growth rate of spherulite (dr/dt) of samples prepared from a solution of 2×10−5 g/mL was faster than that of the bulk sample annealed at 478.3 K. All these results should be attributed to the fewer entanglements in samples prepared by freeze-drying process from dilute solutions, and presented clear evidence for the influence of chain entanglements on the crystallization behavior of polymers.

Effect of nm-Thin Inorganic Layered Fillers on the Crystallization of Polymer Nanocomposites

ABSTRACTThe crystallization behavior of polymers in nanocomposites with inorganic fillers (montmorillonite layered silicate, MMT) is reviewed. Various different polymers are comparatively discussed [poly(vinyl alcohol) (PVA), polypropylene (PP), syndiotactic-polystyrene (sPS), and poly(ethylene oxide) (PEO)] representing three types of filler/matrix interactions: strong specific interactions (PVA/MMT), weak/negligible interactions (sPS and PP/o-MMT), and “unfavorable” (PEO/MMT). In the case of PVA/MMT, crystallization of PVA is strongly promoted by MMT, also stabilizing a new crystal form not found in bulk PVA. For sPS and PP/o-MMT, crystallization is only moderately affected, exhibiting traces of simple heterogeneous nucleation and mostly bulk-like crystal structures, with very small traces on non-bulk crystals. For PEO, crystallization is impeded near the MMT surfaces, due to coordination of the surface cations to the PEO. In all cases smaller spherulite sizes develop when filler is added, independent of the size of the bulk polymer spherulites, whereas the crystallization temperature changes reflect the strength of the polymer/surface interactions.

Effect of the rate of deformation on the crystallization behavior of polymers

Deformation has a significant influence on the crystallization process in a number of polymers. In this paper, the response of a recently developed model for crystallizing polymers is investigated when subject to uni-, bi-axial and constant width extensions for a range of strain rates. Both the loading and unloading behavior are examined for these deformations. The particular model studied here was developed to capture the effect of strain induced crystallization in polymers and has been applied to model crystallization in polyethylene terephthalate at temperatures just above its glass transition temperature. The model has been formulated using the notion of multiple natural configurations within a full thermodynamic framework. The connection between micro-structural changes taking place in the polymer and the form of the model are elucidated. The interplay between the relaxation processes, the rate of deformation and their combined effect on crystallization is illustrated. The results show an earlier onset of crystallization for high strain rates due to stretching of the polymer network. At low strain rates however, crystallization is not observed as the polymer network is able to relax during the deformation. A sharp upturn in the stress is observed after the onset of crystallization due to the formation of a rigid crystalline phase. The unloading curves clearly show a hysteric behavior with the amount of dissipation increasing for increasing values of strain rate. These results compare favorably with experimental observations available in literature.

Crystallization of Polypropylene and Poly(Ethylene Terephthalate)

1. INTRODUCTION The prediction of polymer crystallization behavior at a reasonable semiquantitative level of accuracy is important in assessing processability and performance. For example, in both polypropylene (PP) [1–3] and poly(ethylene terephthalate) (PET), the interplay of the effects of rheology and crystallization kinetics under various types of fabrication conditions is crucial in determining the semicrystalline morphology. In turn, morphology affects the mechanical, optical, and gas barrier properties of the fabricated articles. On the other hand, since many important plastics fabrication processes are carried out with the polymer in semicrystalline state (below the crystalline melting temperature, but above the amorphous glass transition temperature), while others are carried out from the melt and above the recrystallization temperature, the crystallization kinetics and the morphology, in turn, also affect the processing of the polymer by these fabrication methods.

THE ROLE OF ELASTOMERS IN THE STUDY OF POLYMER CRYSTALLIZATION

Abstract Crystallization in polymers involves the development of three-dimensional order in the system. In this respect crystalline polymers resemble very closely the crystalline state of monomeric systems. In contrast to low molecular weight systems, however1 polymers never crystallize completely. The crystallinity level can vary from a few to 90% depending on the molecular constitution of the polymer chains and the external crystallization conditions. Consequently, a polycrystalline system having a complex morphology develops. When a polymer system crystallizes, either quiescently, or when held in a deformed state, there are major changes in properties from the typical rubber-like elasticity of noncrystalline (amorphous) polymers. These changes are particularly evident in many physical and mechanical properties. Therefore, it is important to understand the nature of the crystallization process and the structure and morphology that evolves. In addition to their intrinsic scientific interest these factors have great bearing on properties and the end-uses of polymeric substances. In this connection, elastomers, particularly natural rubber, have played a vital role that has led to our present understanding of the crystallization behavior of polymers, and the properties of the crystalline state. The significance of these pioneering contributions has not been generally recognized in today's study of the diverse number of crystalline polymers that constitute the present world of commerce.

Comparative crystallization behavior of polymers and aqueous solutions

This paper gives a short introduction to the modes of crystallization of polymers and shows, by a series of micrographs, that the resulting morphology is very similar to that of ice crystals growing from aqueous solutions. The similarity is explained by similar conditions of nucleation and growth, leading in both cases to a hindered and partial crystallization. It is shown that the resulting crystal patterns can be qualitatively explained by estimating the relative growth rates in different directions on a developing crystal face as a function of supercooling and concentration.

Kinetic and equilibrium phenomena in the system: Acetone vapor and polycarbonate film

AbstractAmorphous films of Lexan polycarbonate have been exposed to acetone vapor at controlled temperatures and partial pressures in order to study sorption kinetics and thermodynamics and polymer crystallization behavior. Sorption isotherms show a discontinuity is slope at or near the depressed glass transition, which itself was identified by torsion pendulum measurements. Crystallization abruptly begins to occur at partial pressures equal to or slightly above that of the solubility transition and is manifested by delayed desorption and whitening phenomena. In this process 20% crystallinity is usually developed, as measured by calorimetry which, however, produces a 40% drop in acetone solubility. Although the depressed glass temperature is near 0°C. in saturated atmospheres—a drop of 145°C.—the melting point is only depressed 60 or 70°C. Such disparity probably accounts for the enhanced polycarbonate crystallization rate in acetone over that in the dry bulk polymer above the normal Tg.

Dynamic Damping and Young's Modulus in the Range of Rubberlike Elasticity

Abstract After a brief interpretation of current theoretical concepts of the high elasticity of macromolecular substances, as they have been developed by a number of authors from the laws of thermodynamics, the most important experimental methods of dynamic vibration are pointed out. A newly developed apparatus, which permits the study of the plastic-elastic behavior of high polymers in the rubber-elastic field by means of forced vibrations under tensile stress is described. With a plentiful supply of experimental material, various factors were studied, including the crystallization behavior of polymers, the reinforcing effect of various fillers, polymerization influences, modification of Neoprene-W by accelerators, the elastic behavior of Butyl and Silicone rubber, the influence of certain activators on the reinforcing properties of alumina gel, and the action of softeners in polyvinyl chloride.