Oriented crystallization of cross-linked polybutadiene rubber. 2. Small-angle and wide-angle x-ray scattering studies of crystal orientation (original) (raw)
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Morphological changes during oriented polymer crystallization
Polymer Engineering and Science, 1976
A theory of the stress-induced crystallization of polymeric networks is presented which takes into account 1) the free energy of fusion, 2) crystal surface energies and 3) entropic changes in the amorphous sections of crystallizing chains. It is assumed that the vector running from one end to the other of the crystallite is oriented in the direction of network extension, irrespective of crystal morphology, thus minimizing the free energy of crystallization. Assuming that the network assumes the crystal morphology which minimizes the free energy ofthe network at a given degree of crystallinity and that the crystallization proceeds along this lowest free energy path, it is predicted for simple network extension that growth of a perfectly oriented extended-chain crystal occurs initially, changing to a one-fold crystal oriented perpendicular to extension at low extension and to a two-fold crystal having nearly perfect orientation at high extension. The stress is predicted to decay initially and then to rise as the network chains switch from an extended-to a folded-chain morphology. Spatial factors which may trap chains in the extended-chain morphology or prematurely stopping the crystallization process can result in a mixed crystal morphology. At high extension, the structure is similar to that of the shish kebab.
Oriented crystallization of crosslinkedcis-1,4-polybutadiene rubber
Journal of Applied Polymer Science, 2007
Crosslinked cis-1,4-polybutadiene (PB) rubber is known to undergo crystallization during mechanical deformation. This article presents small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), and stress-relaxation studies of the structural development in crosslinked PB samples crystallized under various orientation regimes. The studies are concerned with isothermal crystallization at various temperatures (10 to À508C) and at constant elongation ratios (1-4.5) and hence with the effects of weak-to-intermediate and high orientations on oriented crystallization versus the behavior of unoriented specimens. The results for weak-to-intermediate orientations indicate a decrease in the long spacing during crystallization, which is accompanied by an almost constant lamellar thickness as well as a significant time lag between the development of crystallinity revealed by WAXS and SAXS intensity measurements. The SAXS integral intensity shows an overshoot; that is, the intensity reaches a maximum after some crystallization time and then decreases. Stress measured during crystallization with constant elongation ratios shows decay, which depends on the isothermal crystallization temperature and elongation ratio. Although the crystallization in the weak-to-intermediate orientations is interpreted on the basis of nucleation growth, the crystallization under high orientations is best interpreted on the basis of spinodal crystallization. The most pronounced feature of structural formation during crystallization under high orientations is an almost constant long period from nearly the beginning of the crystallization, even in a timescale in which other physical quantities such as the WAXS crystallinity, SAXS integral intensity, and stress still change with time, suggesting spinodal-like crystallization, orientation-induced instability of molten network chains and their ordering, which involves spinodal-decompositionlike segregation of noncrystallizable parts (crosslinked points) and crystallizable parts (linear chains between the crosslinks) of network chains, and subsequent crystallization of crystallizable chains and pinning of the ordering process induced by crystallization. To the best of our knowledge, this work reports for the first time spinodal-like crystallization on oriented crosslinked polymer melts.
Crystals and Crystallinity in Polymers
2013
Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. For general information on our other products and services or for technical support, please contact our Customer Care Department within the 2 Packing of Macromolecules in Polymer Crystals 88 2.1 General Principles, 88 2.2 The Principle of Density (Entropy)-Driven Phase Formation in Polymers, 92 CONTENTS vi CONTENTS 2.3 Symmetry Breaking, 96 2.4 Impact of Chain Folding on Crystal Structure Symmetry, 103 2.5 Frustrated Polymer Crystal Structures, 107 2.6 Chiral Crystallization of Polymers with Helical Chain Conformations, 110 2.7 Packing Effects on the Conformation of Polymer Chains in Crystals: The Case of Aliphatic Polyamides, 113 References, 118 3 Methods in Crystal Structure Determination from X-Ray Diffraction 123
Role of orientation in kinetics of nucleation and growth of crystals in polymers
Journal of Polymer Science Part B: Polymer Physics, 1989
A theoretical framework is provided for generalizing the inferences drawn from the results of earlier experimental studies of kinetics of crystallization in oriented poly(ethy1ene terephthalate). The framework is obtained by combining extensions of classical nucleation theories in polymers '. ' and a theory of crystal growth with anisotropic incorporation of segments into growing crystals.'.-' I t is shown that, while a very strong dependence of rate of primary nucleation on orientation does exist, there is a only a much weaker dependence of rate of crystal growth on the orientation of the crystallizing polymer. The theoretical formulation provided here would allow qualitative estimates for comparison with experiments.
1991
Works on strictly uniform ultra-long n-alkanes enabled the exploration of the onset of chain folding with increasing molecular length. It was established that folding sets in beyond a certain chain length, more specifically dependant on crystallization temperature (T,), starting in an initially irregular chain deposition, reorganizing subsequently into a strictly quantized conformation of integral fractional fold lengths through either thickening or thinning of the crystal. In the final reorganized stage the folds are regular and sharp. The isothermal crystallization rates for extended chain crystals were found to go through a maximum followed by a minimum with decreasing T, where chain folding takes over. This remarkable rate inversion, observed for c 2 4 6 H494 and c198 H398, so far, occurs both for solution and melt crystallization and could be verified for both primary nucleation and crystal growth. We interpret it in terms of a "self poisoning" phenomenon, where chain depositions, which occur transiently in the "wrong" conformation, are blocking the nucleation and growth of the crystal, a phenomenon also reflected in the reversal of the temperature dependence of isothermal refolding on crystallization from solution. Rate reversals of all these kinds promise to be basic to our understanding and are giving rise to recent alternative explanations from elsewhere which are being quoted and discussed.
Crystallography Reports, 2007
A modified system of Mansfield equations with different step propagation rates to the right and left was applied to describe the shape of solution-grown single crystals of long-chain alkanes with asymmetrically curved faces {110}. Solution of this system of equations and simulation of the shape of single crystals of polyethylene, α-polyvinylidene fluoride, and polyethylene oxide shows good agreement with the experimental data. Comparison of the simulated shapes and experimental micrographs makes it possible to determine the ratio of the intensity of secondary nucleation to the average propagation rate of the layer as well as the ratio of the rates of lateral steps to the right and left and to calculate the absolute values of the main crystallization parameters when the growth rates of single crystals in certain directions are known. It is shown that the asymmetry of the step propagation rate to the right and left is significant only for crystallization at very small supercoolings.
Crystallization and ordering processes in polyhydrocarbons with chemically irregular chains
Journal of Polymer Science Part C: Polymer Symposia, 1967
An x-ray, electroii micrusc.ope, and mecliauical properties study was carried out o i l polyethylerie, isotactic polypropylene, and trans-1,4-polyisopretie (gutla-perch) with differelit degrees of chemical irregularity made by halogeriat.iori reactioii of these regular polymers. Thc gradual distortion of chemical regularity causes a change in crystalline laltice parameters, degeneration of crystalline spherulitic structure, and simplification of siipernioleciilar st,ructures of t,he polymers investigated. The introduction of chemical irregrilarity is analogoiis to supercooliiig of a crystalliiie polymer wilh suitable cliaiige of mechanical propert>ies aiid morphological forms of structures. In the case of guttapercha, ehloritiation i i i films also favors the ~1 to 8 transition within the crystalhe state. I t has been found that stretching of vulcanized samples of chlorogutta-percha at temperatures above its melting point proceeds with an essential change iii internal energy and increase of entropy. A suppositioii has been made about the structure peculiarities of chlorogutta-percha which permit the appearance of such an effect. A total piclure of chaiige of different parameters and properties is giveii from the point of view of gradual transformatioii of t,he stereoregular polymer to an irregular amorphous one.
Orientation prior to crystallisation during drawing of poly(ethylene terephthalate
Polymer, 2000
Wide angle X-ray scattering data have been recorded during the drawing of poly(ethylene terephthalate) (PET) using a wide range of draw rates (0.05-12 s Ϫ1 ), temperatures (90-120ЊC) and draw ratios. The data were analysed to follow the development of molecular orientation and the onset of crystallisation. The molecular orientation prior to crystallisation has been characterised in terms of the orientation order parameter ͗P 2 cos u͘: The rate of increase of ͗P 2 cos u͘ with draw ratio decreases with both increasing temperature and decreasing draw rate. A superposition of all the data to a common reference temperature of 90ЊC was obtained using a WLF shift factor to provide a master curve showing the dependence of the development of ͗P 2 cos u͘ on draw rate. A comparison of the known chain relaxation motions of PET with the observed relation between draw rate and the onset of crystallisation provides an explanation of a previous discrepancy in the literature concerning the point of onset of crystallisation. For draw rates faster than the rate of the chain retraction motion, the onset of crystallisation is delayed until the end of the deformation process. For draw rates slower than the chain retraction motion, there is evidence of the onset of crystallisation occurring before the end of the deformation process. ᭧