Formation Of Extended Chain Crystals Research Articles

Optimum processing conditions for the maximum crystallization rate of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)

The effect of thermal and shear histories on the crystallization rate of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) was studied. As with other crystalline polymers, the shear history greatly affected the crystallization rate when the shear rate was beyond a critical value, i.e., the inverse of the Rouse relaxation time. Even after the formation of extended chain crystals, spherulite texture was clearly discernable. It grew from certain points on the extended chain crystals. Consequently, a row of spherulites appeared along the flow direction. The resin temperature in the molten state was also significant. When the sample was heated to 170 °C, which is beyond the main melting peak in the differential scanning calorimetry curve, unmolten crystals did not affect the linear viscoelastic properties. They acted as effective nucleating agents for the rest of the polymer during cooling. Therefore, the shear history hardly affected the crystallization rate and the number of spherulites.

Thermal Analysis of PE Extended Chain Crystal of PE Extended Chain Crystal from Gel Spinning

Abstract It has been recognized that ultra-high strength and high modulus ultra-highly drawn Polyethylene (PE) fiber is produced from the formation of extended chain crystal. Nevertheless, whether extended chain crystal exists in gel spinning ultra-highly drawn PE fiber is still an unsettled question. In this work, decalin was adopted as a solvent for PE gel spinning. By means of thermal analysis, X-ray diffraction methods etc, the existence of extended chain crystal was explored. Reasons for confusion in previous studies of this problem were discussed. In PE fiber from 3 wt-°/o dope, extended chain crystal starts to form during draw ratios over 20. When draw ratio equals 42, the content of extended chain crystal roughly equals that of folded chain crystal. The melting point of extended chain crystal is 6°C higher than that of folded chain crystal. They are 145.5° C and 150° C to 152.5° C respectively. The latter would slightly rise with arise of draw ratio. Multi-crystallization behavior of annealed PE fiber was determined and discussed. It was also confirmed that there is a linear logarithm relationship between thermal shrinkage (internal stresses) and fiber moduli.

Non-isothermal melt crystallization of poly(tetramethylene succinate) under high pressure: Characterization and kinetics

The non-isothermal melt crystallization behavior (characterization and kinetics) of poly(tetramethylene succinate) (PTMS) under high pressure was investigated using high pressure differential thermal analysis (HP-DTA). Wide angle X-ray diffraction (WAXD), small angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), polarized optical microscopy (POM), and density measurement were used to study the crystal structure, the melting temperature and the morphology of the samples crystallized under various pressures. No new crystal structure or formation of extended chain crystals were found in the pressure crystallized samples. Optical micrographs revealed nucleation density decreased and spherulite morphology changed (presence of banding pattern) above 300MPa. DTA crystallization curves showed a single exothermic peak at all pressures which shifted to higher temperatures with increasing pressure and became broad above 300MPa. The kinetics was studied using the Ozawa model. A change in the nucleation pattern was predicted from the Ozawa exponent, instantaneous at low pressures and sporadic above 300MPa. From kinetic and morphological results it was found that a critical pressure region existed around 300MPa.

Accessing the hidden lamellar nanostructure of semi-crystalline nascent polymers by small-angle X-ray scattering contrast variation

Most melt-crystallized polymers present a lamellar nanostructure of alternating crystalline and amorphous lamellae which is coherent enough to display a broad interference peak in small-angle X-ray scattering experiments (SAXS). Nascent semi-crystalline polymers, on the other hand, though highly crystalline, hardly show an interference peak. This has long been attributed either to the formation of extended chain crystals or to a highly incoherent lamellar stacking. Here it is shown that a coherent lamellar order is shaded by a large scattering contribution from the air/grain interface. This is revealed by a SAXS contrast variation technique that suppresses the air/grain interface scattering and leaves only the scattering contribution from the internal lamellae.

Effect of cross‐link density on the high pressure crystallization of UHMWPE

Ultrahigh molecular weight polyethylene (UHMWPE) is a bearing surface material for total joint implants. It is radiation cross-linked for high wear resistance and is melted or treated with vitamin E for oxidative stability. We investigated high pressure crystallization (HPC) of irradiated UHMWPE as an alternative method to improve the mechanical strength while stabilizing the residual free radicals from radiation cross-linking. HPC of uncross-linked UHMWPE has resulted in the formation of extended chain crystals and increased crystallinity, leading to improved strength. We hypothesized that increased cross-link density would hinder crystallization during HPC due to decreased chain mobility. Therefore, we investigated the crystalline structure and tensile mechanical properties of high pressure crystallized 25-, 65- and 100-kGy irradiated UHMWPE. We also determined free radical content and wear. The strength of 25- and 65-kGy irradiated UHMWPEs was improved by HPC with increased crystallinity and crystal size. 100-kGy irradiated UHMWPE did not show improved strength, supporting our hypothesis that decreased chain mobility would hinder crystal formation and strength improvement. None of the HPC irradiated UHMWPEs contained detectable free radicals and their wear properties were maintained, suggesting oxidative and mechanical stability in the long term. Therefore, HPC can be used effectively for imparting oxidative stability while strength improvement can be achieved for irradiated UHMWPE with low to moderate cross-link density.

Free radical elimination in irradiated UHMWPE through crystal mobility in phase transition to the hexagonal phase

Ultra-high molecular weight polyethylene (UHMWPE) is radiation cross-linked to decrease wear in total joint applications. Irradiation decreases the strength of UHMWPE and introduces residual free radicals, which can cause oxidation in the long-term. We advanced a method eliminating the free radicals without a reduction in strength. UHMWPE exhibits a hexagonal phase at high pressure and temperature, where chain mobility in the crystalline phase is increased, leading to the formation of extended chain crystals. We hypothesized that the increased chain mobility during transformation from the orthorhombic to hexagonal phase could be used to eliminate the residual free radicals in irradiated UHMWPE. We eliminated the free radicals in 25-, 65- and 100-kGy irradiated UHMWPEs and these materials did not show oxidation after accelerated aging. The ultimate tensile strength and work-to-failure of 25 and 65-kGy irradiated UHMWPEs were improved significantly while that of 100-kGy irradiated UHMWPE was lower compared to irradiated UHMWPE melted at ambient pressure.

Analysis of the effect of pressure on compression moulding of UHMWPE.

Ultra-high molecular weight polyethylene (UHMWPE) powder is effectively processed by compression moulding due to its very high melt viscosity. Compression moulding involves application of temperature and pressure as a function of time. The pressure applied during processing has a significant influence on the part properties. The effect of pressure applied during compression moulding was studied by moulding parts at different pressures. Increase in the applied pressure causes increase in the melting and recrystallization temperatures. An increase in the pressure applied at the melt temperature ( approximately 140 degrees C) from 7.8 MPa to 15.6 MPa caused the crystallinity to increase from 54% to 61%, the stiffness of the moulded part to increase from 257 MPa to 435 MPa and oxidative index to increase from 0.055 to 0.059. Further increase in the pressure applied at the melt to 23 MPa caused the crystallinity to fall to 49%, the modulus to reduce significantly to 302 MPa and the oxidative index to change to 0.063. Increase in the pressure applied at the recrystallization temperature ( approximately 91 degrees C) from 38 MPa to 78 MPa increased the crystallinity from 54% to 65%, increased the modulus from 257 MPa to 279 MPa and increased the oxidative index from 0.055 to 0.065. Further increase in the applied pressure to 97 MPa, caused the crystallinity to drop slightly to 61% the modulus to reduce to 269 MPa, and the oxidative index to reduce to 0.057. The experiments showed that for obtaining maximum crystallinity and stiffness, the applied pressure should be within a narrow range. The highest recrystallization pressure (97 MPa) indicated the formation of extended-chain crystals in addition to the chain-folded crystals. The change in pressure applied at the melt temperature had a significantly greater effect on Young's modulus, as compared to change in pressure applied at the recrystallization temperature. Fourier transform-infrared spectroscopy analysis of the samples moulded at different pressures revealed that the increase in crystallinity and stiffness was accompanied by increase in oxidation within the part. By filling the die in a nitrogen atmosphere instead of air, the oxidation level in the moulded parts was reduced by almost 60%, without adversely affecting the crystallinity and the modulus.

Thermal Analysis of PE Extended Chain Crystal of PE Extended Chain Crystal from Gel Spinning

Abstract It has been recognized that ultra-high strength and high modulus ultra-highly drawn Polyethylene (PE) fiber is produced from the formation of extended chain crystal. Nevertheless, whether extended chain crystal exists in gel spinning ultra-highly drawn PE fiber is still an unsettled question. In this work, decalin was adopted as a solvent for PE gel spinning. By means of thermal analysis, X-ray diffraction methods etc, the existence of extended chain crystal was explored. Reasons for confusion in previous studies of this problem were discussed. In PE fiber from 3 wt-% dope, extended chain crystal starts to form during draw ratios over 20. When draw ratio equals 42, the content of extended chain crystal roughly equals that of folded chain crystal. The melting point of extended chain crystal is 6° C higher than that of folded chain crystal. They are 145.5° C and 150° C to 152.5° C respectively. The latter would slightly rise with arise of draw ratio. Multi-crystallization behavior of annealed PE fi...

The influence of the extraction process and spinning conditions on morphology and ultimate properties of gel-spun polyethylene fibres

The morphology of gel-spun polyethylene fibres prior to hot-drawing, depends on the spinning conditions and the extraction process. Paraffin oil containing fibres spun at relatively low spin temperatures and high take-up speeds show a c-axis orientation parallel to the fibre axis due to shish-kebab formation in the spinline. The extraction of the paraffin oil is accompanied by stresses leading to morphological changes. For fibres spun at low take-up speeds and/or high spin temperatures this leads to a craze-like disturbance and a preferential c-axis orientation perpendicular to the fibre axis. Fibres with this type of orientation exhibit an equatorial SAXS maximum corresponding to a periodicity of 12 nm ascribed to regular packing of 12 nm thick lamella oriented with the c-axis perpendicular to the fibre axis. The formation of extended chain crystals in the case of low spin temperatures and high take-up speeds is accompanied by the introduction of flaws, which have a deteriorating effect on the ultimate properties of the fibres.

Crystallization of polypeptides during polymerization

In order to pursue the possibilities of formation of extended chain crystals in the course of polymerization, kinetics of the heterogeneous polymerization of L- and DL-alanine N-carboxy anhydride (NCA) initiated by butylamine in acetonitrile and the morphologies of the resultant polymers were studied. The fibers formed in the beginning of polymerization thickened as the polymerization proceeded. The growing chain molecules, lost solubility in the system when they attained a degree of polymerization of 3 to 4 and crystallized in the form of ribbon-like fibers in which molecules with the β-conformation were aligned perpendicularly to the fiber axis. As the growing chains attained a critical length by further polymerization, they began to take the α-helical conformation. The crystals grew successively thereafter, taking the α-helical conformation through addition of the monomer molecules onto the reactive chain ends. Possible mechanisms are proposed for the nucleation and the subsequent growth of the crystals in the course of polymerization. The polymerization of NCA's of glycine and β-alanine has also been studied. The polymer precipitated in the form of spherulites consisting of ribbon-like crystalline units. The molecules in the ribbon-like crystals were arranged antiparallel, forming a hydrogen-bonded sheet. Homopolymerizations of other NCA's and copolymerizations were also examined in the same heterogeneous system.

Isolation of extended-chain crystals from high-pressure crystallized polyethylene by selective dissolution

Abstract In recent years the formation of extended-chain crystals has received increasing attention, and a number of mechanisms for it have been proposed [1–5]. These mechanisms are classified into two groups, one being the thickening of folded-chain crystals [1, 2] and the other being the direct formation of extended-chain crystals from the melt [3–5]. In order to clarify the mechanism, it is necessary to study the microstructure of the isolated extended-chain crystals in detail. In this paper the isolation of the extended-chain crystals from high-pressure crystallized polyethylene by selective dissolution of folded-chain crystals is described.

Hydrodynamically induced crystallization of polymers from solution

This paper primarily describes the large diversity in morphology of fibrillar polyethylene crystals precipitated from stirred solutions at elevated temperatures. Shish-kebabs crystallized at 112°C from stirred xylene solutions of linear polyethylene exhibit lamellar overgrowths having thicknesses of the order of 150 A from which it was concluded i. a. that at crystallization temperatures above 100°C only backbones are formed and that the surrounding polymer molecules deposit epitaxially on the substrate at lower temperatures. The variation in the appearance of the overgrowth and the inhomogeneity of the samples can be traced back to irregular stacking and to differences in washability of the fibrillar crystals, these differences being largely due to the non-uniform flow pattern along the various stirrer parts. Unusual lamellar shapes, interconnections, sheets between lamellae, the “veil” on the Shish-kebab noted byKeller andMachin, may well be explained byKeith, Padden andVadimskys mechanism of crystallization-induced orientation of macromolecules between the lamellae and the consequent formation of extendedchain crystals. The lamellar overgrowth appears to be arranged along the backbone according to a logarithmic normal distribution, and the average diameter of the lamellar crystals increases with the average spacing, both being approximately equal in size. The fibrillar crystals end in smooth tapered tails having lengths varying between 5 and 8 μ. These observed fibre ends agree best with the “unroll” model proposed for fibrillar crystal growth in flow fields if on the average each molecule introduces one chain fold and the chain ends are randomly assimilated by the crystal lattice. Linear polyethylene below molecular weight of 50×103 and low density polyethylene did not crystallize in fibrillar habit. whereas copolymers of ethylene and slight amounts of propylene and butylene proved capable of forming fibrillar crystals and isotactic polypropylene yielded smooth and overgrown fibres.

Molecular weight dependence of isothermal long period growth of polyethylene single crystals

The long period growth L of polyethylene single crystals during annealing requires an increase in length of all straight sections of every single macromolecule between the fold planes. The material needed has to be transported from the free ends toward the centre of the macromolecule. In order to explain the temperature dependence of long period growth rate, the transport has to proceed by collective jumps of all monomers of one straight chain section between the fold containing surfaces. The number of such jumps needed for a given thickness increase in unidirectional motion or random diffusion is proportional to ( M L ) 2 and ( M L ) 3 respectively. The growth rate hence turns out to be strongly dependent on molecular weight M. In the most probable case of random diffusion the L versus log t curves are shifted by 3 log M to higher times. The growth rate extremely rapidly increases when the extended chain length is only two, three or even four times larger than the long period L. The transformation of the folded into extended chain crystal in such a case is achieved in a time which roughly equals the time needed for isothermal single crystal growth to L ext. 4 . This time, of course, is shorter the lower the molecular weight and the higher the temperature. Such a mechanism may eventually explain the formation of extended chain crystals observed in fractionated samples of low molecular weight isothermally crystallized from melt close to 130°C and in whole Marlex samples crystallized from melt under high pressure at 230°C.