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Polymer Crystallization (polymer + crystallization)

Distribution by Scientific Domains
Polymers and Materials Science77%

Selected Abstracts

Density Fluctuations during the Early Stages of Polymer Crystallization: An Overview

MACROMOLECULAR MATERIALS & ENGINEERING, Issue 4 2008
Jimmy Baert
Abstract The present work provides a critical review of polymer crystallization studies using SALS; experimental methods, analysis techniques, observations and their relations with respect to other techniques are discussed. Furthermore, the fact that nucleation might be accompanied by large scale density fluctuations has been investigated for the flow-induced crystallization of iPB. SALS was applied to measure density and orientation fluctuations, whereas complementary results were obtained from optical microscopy. The observations from both crystallization and melting experiments seem to indicate that the detected density fluctuations result from the presence of weakly anisotropic structures, rather than being an indication of densification before the onset of crystallization. [source]

Polymer Crystallization Influenced by Initial Orientation of Cylindrical Diblock Copolymers in Thin Films

MACROMOLECULAR RAPID COMMUNICATIONS, Issue 19 2008
Ping Yang
Abstract The effect of the initial states (disordered perpendicular cylinder structure vs. parallel cylinder structure) on the crystallization of polystyrene- block -poly(ethylene oxide) (PS- b -PEO) thin films during cyclohexane annealing was investigated. The cylindrical domains perpendicular or parallel to the surface were obtained by controlling the film thickness. During solvent annealing, for the film with the perpendicular cylinders, the ordering degree of cylinders was increased. The enthalpic increase is large enough for the forming of square-shaped crystals, and subsequently the square-shaped single crystals surrounded by the ordered hexagonally packed perpendicular cylinders evolved to the dendrite ones. For the film with the parallel cylinders, the parallel cylinders were translated to the perpendicular ones. The increased enthalpy was not large enough for the formation of square-shaped single crystals. Instead, the dendrite-like crystals started at the edge of terraces. [source]

Polymer Crystallization: A DSC Approach to Building the T-CR-T Diagram

MACROMOLECULAR THEORY AND SIMULATIONS, Issue 2-3 2008
Rosa Berlanga
Abstract T-CR-T diagrams are one of the most reliable ways to predict the thermal behavior of material solidification from the melt. A brief description of solidification accounting for continuous cooling is presented for fast-crystallizing polymers. An isoconversional method is applied to the kinetic analysis of non-isothermal processes. The solidification process of different polymer materials: high density polyethylene based samples and mixtures of PEG with a pharmaceutical drug were analyzed by means of DSC. Several diagrams were constructed and the best agreement between experimental data and the calculated T-CR-T curves corresponds to the use of the isoconversional method. [source]

The continuous cooling transformation (CCT) as a flexible tool to investigate polymer crystallization under processing conditions

ADVANCES IN POLYMER TECHNOLOGY, Issue 2 2009
V. Brucato
Abstract An experimental route for investigating polymer crystallization over a wide range of cooling rates (from 0.01 to 1000°C/s) and pressures (from 0.1 to 40 MPa) is illustrated, using a method that recalls the approach adopted in metallurgy for studying structure development in metals. Two types of experimental setup were used, namely an apparatus for fast cooling of thin films (100,200 ,m thick) at various cooling rates under atmospheric pressure and a device (based on a on-purpose modified injection molding machine) for quenching massive samples (about 1,2 cm3) under hydrostatic pressure fields. In both cases, ex situ characterization experiments were carried out to probe the resulting structure, using techniques such as density measurements and wide-angle x-ray diffraction (WAXD) patterns. The cooling mechanism and temperature distribution across the sample thickness were analyzed. Results show that the final structure is determined only by the imposed thermal history and pressure. Experimental results for isotactic polypropylene (iPP), poly(ethylene terephthalate) (PET), polyamide 6 (PA6), and syndiotactic polystyrene (sPS) are reported, showing the reliability of this experimental approach to assess not only quantitative information but also a qualitative description of the crystallization behavior of different classes of semicrystalline polymers. The present study gives an opportunity to evaluate how the combined effect of the cooling rate and pressure influences the crystallization kinetics for various classes of polymer of commercial interest. An increase in the cooling rate translates into a decrease in crystallinity and density, which both experience a sudden drop around the specific "crystallizability" (or "critical cooling rate") of the material examined. The exception is sPS where competition among the various crystalline modifications determines a minimum in the plot of density vs. cooling rate. As for the effect of pressure, iPP exhibits a "negative dependence" of crystallization kinetics upon pressure, with a decrease of density and degree of crystallinity with increasing pressure, owing to kinetic constraints. PA6 and PET, on the other hand, due to thermodynamic factors resulting in an increase in Tm with pressure, exhibits a "positive dependence" of crystallization kinetics upon pressure. Finally, recent original results concerning sPS have shown that the minimum in the density vs. cooling rate curve shifts toward larger cooling rates upon increasing pressure. © 2009 Wiley Periodicals, Inc. Adv Polym Techn 28:86,119, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20151 [source]

Numerical modeling of nonisothermal polymer crystallization kinetics: Flow and thermal effects

POLYMER ENGINEERING & SCIENCE, Issue 10 2010
Matthieu Zinet
A numerical model able to simulate polymer crystallization under nonisothermal flows is developed. It is based on the assumption that the trace of the extra-stress tensor, calculated according to a viscoelastic multimode Upper Convected Maxwell (UCM) model, is the driving force of the flow-induced extra nucleation. Two distinct sets of Schneider equations are used to describe the growth of thermally and flow induced nuclei. The model is then coupled with the momentum equations and the energy equation. As an application, a shear flow configuration between two plates (Couette flow) is simulated. The relative influence of the mechanical and thermal phenomena on the crystallization development as well as the final morphology distribution is then analyzed as a function of the shearing intensity and the cooling kinetics, in terms of nucleation density and crystallite mean sizes. POLYM. ENG. SCI., 50:2044,2059, 2010. © 2010 Society of Plastics Engineers [source]

Influence of initial mixing methods on melt-extruded single-walled carbon nanotube,polypropylene nanocomposites

POLYMER ENGINEERING & SCIENCE, Issue 9 2010
Vinod K. Radhakrishnan
We report the first direct comparison of melt-extruded polypropylene,single-walled carbon nanotube (PP/SWNT) nanocomposites prepared by three different initial mixing methods. The standard deviation of the G-band intensity obtained using Raman mapping was found to be the best measure of dispersion uniformity in the extruded composites, and dispersion uniformity was found to generally correlate with rheological and thermal properties. For all three initial mixing methods, both unmodified and sidewall-functionalized purified SWNTs were evaluated. Surprisingly, in all cases, dodecylated SWNTs prepared using the reductive alkylation method were less uniformly dispersed in the final composite than the unmodified SWNTs. The simplest process, dry blending, resulted in poor nanotube dispersion and only polymer crystallization was significantly affected by the presence of the nanotubes. A slightly more complex rotary evaporation process resulted in significantly more uniform dispersion and significant changes in rheological properties, polymer crystallization, and thermal stability. The most elaborate process tested, hot coagulation, enabled the most uniform dispersion and the greatest change in properties but also resulted in some polymer degradation. POLYM. ENG. SCI., 50:1831,1842, 2010. © 2010 Society of Plastics Engineers [source]

Advanced nucleating agents for polypropylene,

POLYMERS FOR ADVANCED TECHNOLOGIES, Issue 9 2007
Dima Libster
Abstract The present short review aims to summarize advanced nucleating agents for polypropylene (PP). Reviewing the relevant literature, we focused on powerful nucleators that are capable of significantly increasing the crystallization temperature of the polymer at very low working concentrations and also serving as clarifying agents. The nucleation mechanism and efficiency of these compounds are discussed in detail. The nucleating agents were divided into groups according to their tendency to induce monoclinic (,), hexagonal (,), or orthorhombic (,) PP cell geometries. The major , -nucleators and clarifiers are sorbitol-based compounds that speed-up the polymer crystallization due to gelation phenomena and induction of epitaxial crystallization by the metal salts of substituted aromatic heterocyclic phosphate. Among , -nucleators, N,N,-dicyclohexyl-2,6-naphthalene dicarboxamide was found to be very efficient and its nucleation ability was highly concentration dependent. In addition, it was shown that nucleation efficiency of a nucleator can be significantly increased by a new dispersion method comprising its solubilization in a microemulsion. Moreover, the nucleator (HPN-68) increased the , -modification present in the polymer. Copyright © 2007 John Wiley & Sons, Ltd. [source]