Fischer–tropsch waxes. III. Indications of molecular order in molten wax

Although paraffins cannot form liquid crystals, evidence of the existence of some kind of molecular order at temperatures above their melting points can be found in the literature. When the infra-red spectra of certain Fischer–Tropsch waxes were recorded at different temperatures, it was noticed that the intensity of the 1303 cm^?1 band (A₁₃₀₃), which is due to methylene groups in amorphous regions, was abnormally low just above the melting points of the waxes. At higher temperatures the intensity rose to a maximum followed by the normal decline. This effect was shown most strikingly by super-hard wax, which has a high crystallinity and high average molecular weight. A dilatometric experiment with super-hard wax showed that the specific volume rises linearly from the melting point (111°) to ～ 125°. At higher temperatures, although the volume continues to rise linearly, the slope is reduced by 11%. The explanation presented for both phenomena is that bundles of molecules are present above the melting point. They are remnants from crystallites and disappear at ～ 125°. in the case of super-hard wax. Upon cooling, however, the ordering of molecules only starts taking place near the congealing point. This is borne out by the linear changes in A₁₃₀₃ and specific volume that have been observed.     

Fischer-tropsch waxes v. Study of branching and its effect on crystallinity using an improved infra-red method

A method for the determination of degree of branching, much faster than the one previously used, is described. A sample cell designed for fast introduction and removal of samples is used and the method for measuring band intensities has been adapted to computer use. It is shown how the degree of branching decreases from the first member of a series of extract waxes to the residue, because of the higher solubility of isoparaffins compared with that of n-paraffins. A linear relationship between crystallinity and degree of branching was found to exist for the series. Fischer-Tropsch ‘medium wax’ exhibits an abnormally low crystallinity. Possible causes for this are the fact that it has a much lower average molecular weight than the extract waxes, which were derived from ‘hard wax’, and that there can be differences in chain-length distribution. The extract waxes were found to contain 4–6% more amorphous material than isoparaffins, as determined by urea adduction. This difference is explained in terms of the tendency of the urea method to give too low values, the fact that some normal molecules can be present in amorphous regions and the uncertainty of the significance of infra-red crystallinity in absolute terms.     

Wax crosslinking with dicumyl peroxide

A Fischer—Tropsch paraffin wax of high molecular weight, congealing point 205°F (96·1°C), was crosslinked with dicumyl peroxide as initiator. At peroxide/wax molar ratios of less than 1·15 : 1, crosslinked waxes with molecular weights up to approximately double that of the starting material are obtained. These products are characterised by an increase in elasticity. At peroxide/wax ratios greater than 1·15 : 1 insoluble and infusible gels are formed. The efficiency and fate of the initiator and possible side reactions are discussed.     

Fischer-tropsch waxes. I. An infra-red method for the determination of crystallinity

Crystallinity is, ideally, the fraction or percentage of perfectly crystalline matter in a material which is partially crystalline. Because transition structures can exist, however, measured crystallinity will depend largely on the method used. The differences in the infra-red spectra of the same substance in the crystalline and amorphous states can be exploited for quantitative measurement. Methods based on bands that appear only in the ‘amorphous spectrum’, such as the method described here, have definite advantages over those based on ‘crystalline bands’. While testing the method it was found that the rate of cooling has only a small effect on the crystallinity of Fischer–Tropsch waxes. Surface effects play an important role, however. Thin slices cut from the central parts of thick discs were used for measurements. Before they were sampled, certain waxes had to be homogenised by melting. Results were reproducible within ± 1·5 to ±6 per cent for different waxes. Because of the dependence of crystallinity upon the method used, however, the value of this method lies in its use in comparative studies.     

The role of crystallinity and morphology in the preparation of ultra-highly oriented polyethylene

The influence of the degree of crystallinity and morphology on the drawing behavior of linear polyethylene (LPE) has been investigated on three polymers with molecular weights ranging from 67,000 to over 300,000. Samples of similar crystallinity, but different morphology, were obtained by subjecting the polymers to two distinct preparations: slow cooling from the melt in one case and rapid quenching followed by annealing at 120°C in the other. The resulting isotropic products, characterized by optical microscopy and density measurements were drawn at constant speed and the deformation process monitored by recording the draw ratio/draw time relationship and the stress/strain curves. The results indicate that crystallinity per se does not have a primary effect in determining the rate of local deformation except in the case of polymers of very low weight average molecular weight and narrow molecular weight distribution. A dominant role seems to be played by the broad features of the sample morphology as detected by optical microscopy. A comprehensive explanation of these results is based on the concept of a network structure whose nature is affected by the annealing treatment to an extent which depends on the degree of coupling between adjacent crystalline regions in the isotropic undrawn polymer.     

Powder processing of ultra-high molecular weight polyethylene I. Powder characterization and compaction

Powders of ultra-high molecular weight polyethylene from three suppliers were characterized for density, crystallinity, particle size, particle size distribution and particle morphology. The powders had molecular weights in the range 2-5 million. Bulk powder behavior, compressibility, green strength and springback were evaluated and explained in terms of particle characteristics. The green densities of the powders were found to reach a plateau at pressures of about 100 MPa. These plateau density levels were found to depend upon powder, characteristics and to lie between 80 and 90 percent relative density. Green strength is shown to be a unique function of a densification parameter.     

Structure and properties of polypivalolactone

Pivalolactone (α,α-dimethyl-β-propiolactone) can be polymerized to linear polyesters with widely different molecular weights. The polymer has a high degree of crystallinity and a high crystalline melting point. Several other basic properties of the polymer have been determined, such as molecular weight, glass-transition temperature, rheological characteristics, etc., and its possible use in the fibre and plastics field has been extensively investigated. It was found that the polymer, when adequately stabilized, has a high thermal stability and shows hardly any discoloration upon processing.Ultimate products such as fibres and injection-moulded articles based on polypivalolactone have a high resistance to hydrolysis, heat and chemicals, and exhibit excellent weathering properties. A remarkable characteristic of the products is their high elastic recovery, in particular after annealing at high temperatures. Many of the observed properties can be explained by means of the molecular and crystalline structure of the polymer.     

Dampfdruckosmometrische Molekulargewichtsbestimmungen von hochschmelzenden,oxidierten Polyethylenwachsen

Molecular Weight Determination of High-melting, Oxidized Polyethylene Waxes by Vapour Pressure Osmometry Amongst the high-malting oxidized polyethylene waxes PE-Wax AC 392 of Allied Chemical Corp, holds an exceptional position as for its specification. Because of its crystallinity and the high melting point AC 392 can only be kept in solution at high temperatures. Several attempts to determine the molecular weight failed so far, because of these difficulties. The average number of the molecular weight (M?n) was determined for AC 392 and some similar waxes with a commercial vapour pressure osmometer at a working temperature of 125°C. The selection of the solvent is of importance; the high measuring temperature requires special experimental conditions.     

An anomaly in the necking behavior of polyethylene

Tensile creep measurements at constant load on nonoriented polyethylene have shown a marked transition at a certain stress level from a neck formation followed by instantaneous fracture to the formation of a neck which resists fracture for a considerable time. The transition, which shifts towards shorter time and higher nominal stress with increasing molecular weight, has been studied for 16 polyethylenes of different molecular weights, degrees of branching and crystalline structures. The marked. transition has only been observed for high density polyethylene of high molecular weight. Deformation measurements show a more distinct necking for the high density than for the medium density polyethylenes. This is consistent with current molecular deformation theories. A hypothesis for the transition is proposed based on the distinctness of the neck process in the high density polyethylene and the large difference in strength between the spherulitic structure and the fibrillar structure. The dependence of the transition on molecular weight is expected since the number of tic chains incrcrtses with increasing molecular weight.     

Surface segregation of polyethylene in low‐density polyethylene/ethylene–propylene–diene rubber blends: Aspects of component structure

Aspects of the molecular weight and its distribution, the branching of low‐density polyethylene (LDPE), and the molecular composition of the ethylene–propylene–diene rubber (EPDM) matrix are presented in this article in terms of their influence on the surface segregation of polyethylene (PE) in elastomer/plastomer blends. All of the PEs studied, despite different weight‐average molecular weights and degrees of branching, segregated to the surface of the LDPE/EPDM blends. Atomic force microscopy pictures demonstrated defective crystalline structures on the surface of the blends, which together with a decrease in the degrees of their bulk crystallinity and a simultaneous increase in their melting temperatures, pointed to a low molecular weight and a defective fraction of PE taking part in the surface segregation. The extent of segregation depended on the molecular structure of the EPDM matrix, which determined the miscibility of the components on a segmental level. The higher the ethylene monomer content in EPDM was, the lower was the PE content in the surface layer of the blends. The composition and structure of the surface layer was responsible for its lower hardness in comparison with the bulk of the blends studied. The surface gradient of the mechanical properties depended on the physicochemical characteristics of the components and the blend composition, which created the possibility of tailoring the LDPE/EPDM blends to dedicated applications. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 625–633, 2006     

Effect of the diisocyanate structure and the molecular weight of diols on bio‐based polyurethanes

Bio‐based polyurethanes (PU) containing poly(ε‐caprolactone) diol (PCL) and hydroxyl telechelic natural rubber (HTNR) were synthesized. The effect of the diisocyanate structure and the molecular weights of diols on the mechanical properties of PU were investigated. Three different molecular structures of diisocyanate were employed: an aliphatic diisocyanate (hexamethylene diisocyanate, HDI), an aromatic diisocyanate (toluene‐2,4‐diisocyanate, TDI) and a cycloalkane diisocyanate (isophorone diisocyanate, IPDI). Two molecular weights of each diol were selected. When HDI was employed, a crystalline PU was generated while asymmetrical structures of TDI and IPDI provided an amorphous PU. The presence of crystalline domains was responsible of a change in tensile behavior and physical properties. PU containing TDI and IPDI showed a rubber‐like behavior: low Young's modulus and high elongation at break. The crystalline domains in PU containing HDI acted as physical crosslinks, enhancing the Young's modulus and reducing the elongation at break, and they are responsible of the plastic yielding. The crystallinity increased the tear strength, the hardness and the thermal stability of PU. There was no significant difference between the TDI and IPDI on the mechanical properties and the physical characteristics. Higher molecular weight of PCL diol changed tensile behavior from the rubber‐like materials to the plastic yielding. Thermal and dynamic mechanical properties were determined by using DSC, TGA and DMTA. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

充油SEBS改性PP性能研究

用DSC与TG研究了不同的充油比及不同苯乙烯-乙烯/丁烯-苯乙烯(SEBS)摩尔质量时充油SEBS/聚丙烯(PP)共混体系的结晶性能与热稳定性;同时考察共混体系的力学性能和流变性能。结果表明:随着充油比(即油弹比,m油/mSEBS)的增大,熔体质量流动速率(MFR)显著增加,拉伸屈服强度、熔点、结晶度和硬度显著下降。SEBS的摩尔质量影响其对油的吸附能力,摩尔质量越大,吸油能力越好。随着SEBS摩尔质量的增大,材料的熔点、结晶度、拉伸屈服强度增大,硬度变化不明显。     

Ternary polymer blends based on poly(lactic acid): Effect of stereo‐regularity and molecular weight

The effects of stereo‐regularity and molecular weight of poly(lactic acid) (PLA) on ternary polymer blends was analyzed using optical clarity as the primary screening method. This enabled the ready identification of phase boundaries of optically clear and apparently miscible regions. Solvent‐mediated blends of amorphous poly(dl ‐lactide) (PDLLA) and semi‐crystalline poly(l ‐lactide) (PLLA) with various molecular weights from high to low, along with polycaprolactone (PCL) and cellulose acetate butyrate (CAB) were used in this study. The nature and extent of crystallinity of the blends was examined by X‐ray diffraction, which, in conjunction with differential scanning calorimetry, scanning electron microscopy, and Fourier transform infrared spectroscopy, provided information about the competition between polymer crystallization (self‐aggregating behavior) and intermixing of the macromolecules. Thus, allowing the primary physical cause of transparency loss to be identified. The results of the ternary blends optical clarity showed the position of the phase boundaries in PLLA/PCL/CAB and PDLLA/PCL/CAB blends are significantly affected by the stereo‐regularity and molecular weight of PLA. The PDLLA (amorphous) blend shows comparable regions of phase separation with high molecular weight and semi‐crystalline PLLA blends even though the molecular weight is much lower. The blends of the shorter chain PLLA1 tend to show more crystalline regions. The optical transparency, miscibility, and crystallinity of the blends are not only affected by the stereo‐regularity and molecular weight of PLA but also the crystallizable PCL, especially at high loading. These findings give useful information to the film‐packaging sector where good optical clarity is a critical performance requirement. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41780.     

Morphological responses in thermally conditioned linear low density polyethylene

An octene-modified linear low density polyethylene has been used to examine the mechanisms involved during thermal annealing. Annealing temperatures ranged from 60 to 100°C. Annealing results in crystallinity increments and these respond to two concurrent effects. One involves the segregation from crystalline regions of low molecular weight moieties in the polymer's molecular weight distribution; the other is lamellar thickening, leading to the formation of more highly perfected crystalline domains. In the present polymer, the two effects were found to be in balance at annealing temperatures near 80°C leading to the optimum distribution of crystalline regions in the amorphous portions of the polymer. The effect of thermal conditioning on mechanical properties of the polymer was illustrated in terms of the initial modulus and the polymer's yield strength. The twin mechanisms of molecular fractionation and lamellar thickening were found to influence both of the mechanical property parameters.     

A comparison of six solvents for the extraction of jojoba seed

Summary Data are presented which show the effects of different solvents on the yield and properties of liquid wax fromSimondsia chinensis (jojoba) and on the characteristics of the hydrogenated waxes obtained from the liquid waxes. Three reagent grade solvents, carbon tetrachloride, benzene, and isopropyl alcohol, and three commercial grade solvents, heptane, hexane, and tetrachloroethylene, were evaluated as extractants for the liquid wax from jojoba. Soxhlet-type of extractions were carried out under conditions in which the solvent was the only significant variable. Four of the solvents extracted essentially the same amount of material from the seed while isopropyl alcohol extracted significantly more material and tetrachloroethylene significantly less. Obviously the difficulties involved in separating the solids recovered from the isopropyl alcohol extraction preclude its use as the extracting solvent for jojoba wax. The density of the liquid waxes varies from 0.8631 to 0.8648; the waxes from the tetrachloroethylene and hexane extractions had the lowest value and the wax from isopropyl alcohol the highest. In each case, regardless of the solvent used, a precipitate developed in the liquid wax after it had been desolventized and stored for 7–10 days. Hydrogenation of clear fractions and precipitate containing fractions of these liquid waxes showed that the precipitate had no apparent effect upon the melting point or hardness of the resulting solid wax. Some of the liquid waxes required a longer hydrogenation time to attain an iodine value of about 1. At this iodine value all of the solid waxes had melting points between 66 and 68°C. Hardness values of all the solid waxes as measured by the Trionic hardness gauge were 90. One of the laboratories of the Southern Utilization Research and Development Division, Agricultural Research Service, U. S. Department of Agriculture.     

Copolymerization of ethylene with vinyl acetate. I. Effect of polymerization temperature on degree of branching

The degree of branching of a series of ethylene–vinyl acetate copolymers was found to be strongly dependent upon polymerization temperature. The copolymers were prepared by free-radical polymerization and had low molecular weights and molar ratios of ethylene: vinyl acetate greater than 3:1. Nuclear magnetic resonance and infrared studies showed that copolymers prepared at 150°C were highly branched and had little crystallinity. Branches were mainly alkyl groups on the polymethylene backbone segments. There was no evidence of δ-acetoxyalkyl branches. Long branches originating by intermolecular H abstraction from the acetylmethyl groups were also expected but could not be detected. These results were consistent with an intramolecular “backbiting” mechanism similar to that found in ethylene homopolymerizations. There was little or no participation by the vinyl acetate moiety in the branching scheme. Copolymers prepared at about 90°C had very few long or short branches and were more crystalline. Copolymers prepared between these temperatures had intermediate degrees of branching and crystallinity.     

Crystallinities of poly(chlorotrifluoroethylene) and its copolymers by differential scanning calorimetry,X-ray diffraction,and density measurements

Crystallinities of films of copolymers of chlorotrifluoroethylene with 0 to 4% vinylidene fluoride, different molecular weights and thermal histories were calculated from differential scanning calorimetry (DSC), X-ray diffraction (XRD), and density measurements. The crystallinities for these films range between 16 to 58%, 11 to 71%, and 13 to 77% as determined by DSC, XRD, and density, respectively. Although the numerical value of the DSC, XRD, and density crystallinities obtained by each of the techniques are different, they correlate well with each other. The primary source of error in DSC is the uncertainty in drawing the baseline for separating the heat of fusion and heat of crystallization. In XRD, the ambiguities in the separation of the amorphous and crystalline peaks reduce the precision of the results. Calculation of crystallinity from density values requires that the distribution of the VF₂ between the amorphous and crystalline regions be known. In addition, the density technique is not sensitive to variations in the crystalline and amorphous densities.     

Highly active peroxides in the radical polymerization of ethylene under high pressure. Properties of the polymers

Using highly active peroxide initiators, a number of polyethylene samples were prepared at low temperatures of 70–140°C and a pressure of 1900 bar. The average molecular weight and the melt flow index were adjusted by adding propionaldehyde as modifier. The characteristic properties of the samples as regards the molecular weight distribution, the average molecular weight, the frequency of long chain branching, the crystallinity and the density were determined. As expected, the polymers are characterized by density values of more than 0.955 g/ml and crystallinity values of 65–80% which are characteristic of PE-HD. The molecular weight distribution, too, remains within narrow limits. On the other hand, rather surprisingly, a relatively high frequency of long chain branching typical of PE-LD is obtained. The investigation of the phase behaviour has shown that polymerization under high pressure at temperatures above 115–118°C takes place in the homogeneous range. The samples polymerized in the transition zone between the single and the two phase area exhibit maximum crystallinity and melt enthalpy. The frequency of long chain branching passed through a minimum.     

Influence of molecular weight on crystallization rate of oriented,glassy nylon 6

Differential scanning calorimetry and wide angle X-ray diffractometry were used to investigate the effects of molecular weight and molecular weight distribution on the crystallization kinetics of oriented, glassy nylon 6. The samples had number average molecular weights ranging from 10,000 to 42,000 and polydispersity indices ranging from 2.0 to 3.1. Noncrystalline films were prepared by quenching molten films between plattens chilled with liquid nitrogen. These films were drawn 4X and 5-1/3X, and the resultant uniaxial orientation was observed to enhance markedly the room temperature crystallization kinetics. Although macroscopic deformation can be assumed to be affine at the molecular level, it is hypothesized that wholechain molecular relaxation occurs at rates inversely proportional to the square of molecular weight, thereby creating a distribution of extension ratios which reflect the actual molecular weight distribution. Thus, the crystallization rate and the degree of crystallinity depend on the fraction of high molecular weight molecules present in the sample. Given two samples with the same molecular weight, the one with the broader distribution crystallizes more rapidly. Similarly, samples having the larger molecular weight crystallize to a greater extent when two samples have the same molecular weight distribution.     

Influence of branching on the thermal and crystallization behavior of bimodal polyethylenes synthesized with binary late‐transition‐metal catalyst combinations

Two reactor blends of linear and branched polyethylene resins with bimodal molecular weight distributions were synthesized in a one‐reactor polymerization process through the combination of 2,6‐bis[1‐(2,6‐dimethyphenylimino)pyridyl]cobalt dichloride ( 1 ) and 2,3‐bis(2,6‐diisopropylphenyl)butanediimine nickel dibromide ( 2 ) or 1,2‐bis(2,6‐diisopropylphenyl)cyclohexene diimine nickel dibromide ( 3 ) in the presence of modified methylaluminoxane. The linear correlation between the catalyst activity and concentration of the nickel compounds suggested that the catalysts performed independently of one another. The molecular weights, molecular weight distributions, and crystalline and phase structures of the blends were investigated with a combination of high‐temperature gel permeation chromatography, differential scanning calorimetry, wide‐angle X‐ray diffraction, and small‐angle X‐ray scattering techniques. The branching degree of the polyethylene produced with 3 was much higher than the branching degree of the sample produced with 2 , although their molecular weights were relatively close. In addition, the crystallization rate, melting temperature, degree of crystallinity, and crystallization temperature of more highly branched blends produced with 1 / 3 were lower. The long periods and thickness of the crystalline region were greatly influenced by the addition of highly branched polyethylene. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010