Cellulose acetate butyrate has been studied in regard to its hydrodynamic properties in several solvents. The polymer was fractionated by precipitation, and the molecular weight distribution of the polymer fractions was determined by gel permeation chromatography. The number-average molecular weight was estimated by osmometry. The Mark–Houwink–Kuhn–Sakurada relations between viscosity and molecular weights were established, and the unperturbed dimensions of the polymer chains were evaluated. In light of these data, current theories of polymer solutions are discussed. A new method of estimating unperturbed dimensions is proposed for semiflexible linear polymers. 相似文献
The crystallization kinetics and morphology of poly(tetramethyl-p-silphenylene)siloxane spherulites have been investigated over a temperature range of 25–130°C. The effect of molecular weight on the spherulitic growth rates, ranging from the monomer to molecular weights about 106, is discussed in terms of conventional rate theory. Surface free energies of crystal growth are computed on the basis of a spherulitic model in which the polymer chains are presumed to be incorporated within the lamellar crystallites which are comprised in the spherulites. Mention is made of the change in mechanical properties with molecular weight. 相似文献
The polymerization of methyl methacrylate (MMA) was carried out in water by use of various concentrations of sodium bisulfite as initiator in the presence and absence of colorless and colored soda lime glass at 40°C. The reaction products were filtered, washed with water, methanol and finally dried at 50°C. The glass–polymer combination was subjected to Soxhlet extraction with benzene. The molecular weights of the soluble polymers were determined viscometrically. In the absence of glass, the conversion was lower but the average molecular weights were higher than in presence of soda lime glass. The effect of grain size of the glass on conversion and molecular weights was such that larger grain size of glass resulted in lower conversion and higher average molecular weights. 相似文献
Some physical properties of the block copolymers with PVC, described in Parts I and II, are discussed. The effect of chemical composition, molecular architecture, and film preparation conditions on their two-phase structure is shown. Especially in the case of high PVC content, the morphological aspect is largely influenced by the molecular architecture—pronged-fork polymer chains—and the nature of the solvent for casting. The incorporation of rubbery poly(alkyl acrylate) particles in the PVC matrix leads to an interesting stress–strain behavior. A relation between this behavior and the morphology is established. 相似文献
The cationic polymerization of p-chloro-α-methylstyrene was investigated for the effect of initiator and solvent on polymer tacticity, molecular weight, and molecular weight distribution. The products were generally crystalline polymers of 80–90% syndiotactic content with fairly high molecular weights and broad molecular weight distributions. Tacticities and molecular weight distributions followed similar patterns suggesting that the effect of reaction conditions on ionpair end-group structures was the dominant factor. 相似文献
The mechanism of carbon black reinforcement was studied by examining stress–strain properties of preswollen (i.e., cured in the swollen state then deswollen) SBR-1500 vulcanizates and the effect of heat treatment on such samples. It was found that the supercoiling of molecular chains leads to a purely viscous response at low elongations. On heating of preswollen samples, molecular rearrangement at the carbon black surface occurs, leading to a considerable increase in modulus. The activation energy of that process was determined by using the superposition principle. 相似文献
Model networks of poly(dimethylsiloxane) (PDMS) were prepared by tetrafunctionally endlinking hydroxyl-terminated chains of various molecular weights. Some networks were prepared from mixtures of chains so as to yield a bimodal distribution of network chain lengths and, in some cases, these networks were prepared in solution. The stress–strain behavior of these unimodal and bimodal networks was studied in pure shear, which was imposed by stretching a sheet of the network having a large ratio of width to length in the direction perpendicular to the width. The pure-shear moduli of both types of networks generally were found to depend markedly on strain. Stress–strain isotherms for unimodal networks prepared from chains of one or the other of two molecular weights were well interpreted using the constrained-junction model of Flory and Erman. The bimodal networks showed large increases in the pure-shear modulus at high strains which were similar to those reported for uniaxial extension and compression. Endlinking in solution decreases the modulus in general and its upturn in particular, presumably because of diminished chain-junction entangling. 相似文献
Further stress relaxation experiments, mostly at 50°C, are reported on mixtures of crosslinkable ethylene–propylene terpolymer with saturated ethylene–propylene copolymer (molecular weights 3.6 and 45 × 104) containing up to 50% by weight of copolymer, crosslinked by sulfur to leave the saturated copolymer unattached and free to reptate in the copolymer network. Stress relaxation was measured in small simple elongations (stretch ratio about 1.15) on samples which had been extracted to remove a large part of the unattached copolymer and dried. The relative increase in modulus at long times (104 sec) increased with the proportion extracted; at short times (1 sec), extraction of the lower molecular weight copolymer increased the modulus to about the same extent but extraction of the higher molecular weight copolymer affected it very little. The relaxation modulus of the copolymer extracted from sample 50H (50% copolymer of high molecular weight), obtained by difference, agreed with that for the total copolymer except for a small difference probably attributable to molecular weight selectivity in the extraction. Stress relaxation was measured on sample 50H at six higher elongations up to a stretch ratio of 3. The dependence of stress on time and strain was consistent with an analysis based on the following assumptions: (a) linear additivity of the network and unattached copolymer contributions, (b) strain–time factorization of the stress contributions from the individual components, (c) a strain dependence for the unattached component corresponding to the presence of a Mooney–Rivlin C2 term only, (d) a strain dependence for the network component which does not follow the Mooney–Rivlin equation but is dominated by a simple neo-Hookean term. 相似文献
Homogeneous trimethylsilylation of silicic acid in nonaqueous media was studied. The silylation process consisted of three steps: acidification of sodium silicate, solvent extraction. and trimethylation of silicic acid. Sodium silicate was first acidified with dilute sulfuric acid to form silicic acid: extraction followed with hydrophilic organic solvents. The Silicic acid obtained was finally allowed to react with trimethylchlorosilane to give trimethylsilylates with molecular weights of 1000–12,000. Optimum conditions for each step have been intensively sought to achieve trimethylsilylates in high yield. The trimethylsilylates exhibited excellent thermal stability and surface properties, such as water repellency and antifoaming properties, comparable to those of conventional polydimethylsiloxanes. 相似文献
Summary: The phenomenon of crazing in polymers has received considerable attention in the past as it is thought to play a pivotal role in determining the performance of polymers under load. One aspect of particular interest has been the interconnection between molecular structure, craze characteristics, and macromechanical properties. In the present study, three different grades of polystyrene (PS) with different molecular weights have been systematically investigated in situ with synchrotron radiation microfocus small‐angle X‐ray scattering (µSAXS). The results suggest that there are different mechanisms operating in PS samples with low and very high molecular weights, compared to those of medium‐to‐high molecular weight. Previously it was thought that, above the critical molecular weight of entanglement, the effect of molecular weight on PS's mechanical behaviour at room temperature was negligible.
Craze evolution as a function of strain in PS. 相似文献
A study has been carried out of the differences in mechanical properties of oriented fibers of poly(ethylene terephthalate) (2GT), poly(trimethylene terephthalate) (3GT), and poly(tetramethylene terephthalate) (4GT). The properties studied include the tensile stress–strain behavior, the recovery from strain, shrinkage at 100°C and the glass-transition temperatures. The stress–strain curves of the three materials differ markedly. 2GT shows a monotonic increase in stress with increasing strain up to failure, which occurs at ~20% strain, and the oriented fibers possess a comparatively high initial modulus. 3GT shows a much lower initial modulus and there is an inflection in the stress–strain curve at about 5% strain. The stress–strain curve of 4GT shows a number of distinct features. Although the initial modulus of 4GT is similar to that of 3GT, the stress–strain curve shows a pronounced plateau in the region between 4% and 12% strain. At higher strains the stresses rise rapidly before failure. These features of the stress–strain curves in the three polymers can be related to previous studies where the x-ray diffraction spectrum and the Raman spectrum have been examined for fibers under stress. The ranking of both the recovery and shrinkage behavior of these materials is in the order 3GT > 4GT > 2GT. These results can also be understood in terms of the results of the previous structural studies, and it is concluded that the molecular conformations in both the crystalline and noncrystalline regions play a key role in determining the mechanical behavior. 相似文献
Twu, C.H., 1984. An internally consistent correlation for predicting the critical properties and molecular weights of petroleum and coal-tar liquids. Fluid Phase Equilibria, 16: 137–150.The objective of this work was to use normal boiling points and specific gravities to develop simple but reliable and accurate methods for predicting the critical properties and molecular weights of petroleum and coal-tar liquids. The normal boiling points of the systems investigated range up to 1778 R and the specific gravities up to 1.436. This virtually covers the entire range of practical interest. The predictions of critical properties based on the above data show significant improvement over published correlations. 相似文献
Lithiated polydienes were readily prepared by direct metalation of the diene polymers with sec-butyllithium and tetramethylethylenediamine in cyclohexane at room temperature. Reaction of the polylithiodienes with styrene or a-methylstyrene formed graft copolymers. Poly-1, 4-butadiene, poly-1, 2-butadiene, cis-poly-1, 4-butadiene, and polyisoprene were the substrates lithiated. The extent of metalation was much greater than previously reported metalations with n-butyllithium and terramethylethylenediamine. The grafting efficiencies, determined by acetone extraction and gel permeation chromatography, were greater than 95%. The physical properties of the graft copolymers are compared as a function of molecular weight, graft site level, and composition. At certain molecular weights, graft site level, and compositions, elastomers are formed without vulcanization. Their properties are comparable to SBS rubber and offer higher melt flow as an advantage. 相似文献
The copolymerization of acrylamide (AM) with sodium-3-acrylamido-3-methylbutanoate (NaAMB) has been studied. The value of r1r2 has been determined to be 0.56 for the AM–NaAMB pair. The molecular weights of the copolymers were relatively unaffected by monomer feed ratios. The copolymer microstructures, including run numbers and sequence distributions, were calculated from the reactivity ratios. The solution properties of the AM–NaAMB copolymers, as well as the NaAMB homopolymer, will be reported in a subsequent paper. 相似文献
Butadiene, isoprene, and chloroprene have been polymerized in dioxane under free-radical initiation to give liquid prepolymers containing terminal carboxyl groups. 4,4′-Azobis(4-cyanovaleric acid) was employed as the initiator. The prepolymers were obtained with molecular weights of 12,000 to 2000 and with functionalities usually greater than 2.0. Experimental parameters were studied to determine their effect on prepolymer yields and properties. The general preparative procedure and prepolymer characterization are described. 相似文献