A thermogravitational cell is used to measure Soret coefficients (s) for dilute binary aqueous solutions of ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, and polyethylene glycol (PEG) fractions with average molecular weights from 200 to 20,000 g-mol–1. The cell design allows the top and bottom halves of the solution column to be withdrawn and injected into a high-precision HPLC differential refractometer detector for analysis. Previously reported mutual diffusion coefficients D and the measured Soret coefficients are used to calculate thermal diffusion coefficients DT. s and D vary with the PEG molecular weight M as M+0.53 and M–0.52, respectively; hence, DT = sD is essentially independent of M. The segmental model of polymer thermal diffusion predicts DT = DsegUS/RT2, where Dseg is the segment diffusion coefficient, US the solvent activation energy for viscous flow, R the gas constant, and T the temperature. The predicted DT values, although independent of M, are too large by a factor of five. Additional tests of the segmental model are provided using literature data for polystyrene + toluene, n-alkane + CCl4, and n-alkane + CHCl3 solutions. Agreement with experiment is not obtained. In particular, the measured DT values for the alkane solutions are negative. 相似文献
The application of Boltzmann statistics to a complete distribution of molecular conformation energies of simplified homo‐ and copolymer models gives meaningful information about temperatures at which phase transitions take place in the bulk. We have calculated in the conformation statistical distribution (CSD) approximation Helmholtz free energy variation versus temperature δF = δU–TδS, where U and S are, respectively, the internal molecular energy and the Gibbs statistical entropy of the considered polymeric model. The deepest minima correspond to glass‐transition temperature (Tg) and melting temperature (Tm) of modelled polymers, while the remaining peaks are related to some other transitions, the existence of which is also experimentally proven. The adopted method is able to give Tg and Tm as a function of the molecular weight of polymers. Some indications can also be achieved about the instability of polymers. The same procedure has been applied to copolymers and blends and has given acceptable results for Tg and Tm as functions of the material microstructure and composition. Other thermal and mechanical properties, such as moduli, mobilities, chemical resistance to oxidation, physical tendency to miscibility, have been directly or indirectly estimated. 相似文献
Phase behavior of blends of a liquid-crystalline (LC) polymer with a non-LC polymer and of a series of copolymers containing mesogenic and nonmesogenic units was studied by thermal, optical, and dynamic mechanical methods. The polymers composing the blends and the copolymers had the same constituent monomers. The blends exhibited phase separation over the whole range of compositions studied as observed by DSC and dynamic mechanical analysis. Two glass transition temperatures (Tg) corresponding to the two components and independence of melting (Tm) and isotropization temperatures (Ti) to changes in composition were observed for the blends. The copolymers did not show phase separation over most of the composition range studied. Only one Tg corresponding to that of the major component could be detected for the copolymers, and the Tg was found to increase with an increase in the amount of nonmesogenic monomer in the copolymers. The difference in phase behavior was explained on the basis of the chemical environment of the constituent units in the blends and in copolymers. Phase inversion in the blends was observed by microscopy when the blends contained 60 mol% or more of the non-LC polymer. 相似文献
The thermal diffusion coefficient DT has been obtained for 17 polymer-solvent combinations, each of them spanning a range of polymer molecular weights, using thermal field-flow fractionation. The polymers examined include polystyrene, poly(alpha-methyl)styrene, polymethylmethacrylate, and polysioprene. The solvents include benzene, toluene, ethylbenzene, tetrahydrofuran, methylethylketone, ethylacetate, and cyclohexane. Although DT was confirmed as essentially independent of polymer molecular weight, it was found to vary substantially with the chemical composition of polymer and solvent. The results were used to evaluate several thermal diffusion theories; the agreement with theory was generally found to be unsatisfactory. Attempts were then made to correlate the measured thermal diffusion coefficients with various physicochemical parameters of the polymers and solvent. A good correlation was found in which DT increases with the thermal conductivity difference of the polymer and solvent and varies inversely with the activation energy of viscous flow of the solvent. 相似文献
Thermal field‐flow fractionation (ThFFF) is an interesting alternative to column‐based fractionation being able to address different molecular parameters including size and composition. Until today it has not been shown to be able to fractionate polymers of similar molar masses and chemical compositions by molecular topology. The present study demonstrates that poly(butyl methacrylates) with identical molar masses can be fractionated by ThFFF according to the topology of the butyl group. The influence of the solvent polarity on the thermal diffusion behavior of these polymers is presented and it is shown to have a significant influence on the fractionation of poly(n‐butyl methacrylate) and poly(t‐butyl methacrylate). Fractionation improves with increasing solvent polarity and solvent polarity may have a greater influence on fractionation than solvent viscosity. It is found that the thermal diffusion coefficient, DT, as well as the hydrodynamic diameter, Dh, exhibit increasing trends with increasing solvent polarity. The solvent quality has a significant influence on the fractionation. It is found that cyclohexane, being a theta solvent for poly(t‐butyl methacrylate) but not for poly(n‐butyl methacrylate), significantly improves the fractionation of the samples by decreasing the diffusion rate of the former but not the latter.
The dilute solution properties of linear, 18-arm, and 270-arm star polybutadienes have been studied in a theta solvent and in a good solvent. Values of the radius of gyration RG, the second virial coefficient A2, the intrinsic viscosity [η], and the diffusion coefficient D0 have been measured for each polymer. The ratios RT/RG, RV/RG, and RH/RG for each type of polymer are used to compare the four dilute solution properties. RT is termed the “thermodynamic radius.” It is the radius of the hard sphere with the same excluded volume as the polymer coil. RT is calculated from A2 by RT = (3A2M2/16ηNA)1/3. RV and RH are equivalent hard spheres defined for the intrinsic viscosity and translational diffusion coefficient, respectively. RT/RG, RV/RG, and RH/RG increase from about 0.7 for linear polymer coils as the number of arms in the star increases. Values of the ratios for the 18-arm stars are less than the value for the hard-sphere, but the values of the ratios of the 270-arm stars are equal to the hard-sphere limit within experimental error. 相似文献
The effect of butyl acrylate (BA), divinyl benzene (DVB) and vinyltrimethoxysilane (TMVS) on the thermal properties of poly(methyl methacrylate-co-butyl acrylate-co-acrylic acid) was investigated. Glass transition temperature (Tg), melting temperature (Tm) and specific heat capacity of the copolymers were investigated using Differential Scanning Calorimetry. Thermal stability of the copolymers which is associated with the degradation temperature (Td) was studied by Thermogravimetric Analysis. Polyacrylates with Tg ranges between -19°Cand 19°C were obtained. With the incorporation of >7 wt% of DVB, the Tg of the copolymer increases from about ?17°C to ?10°C even though they have not undergone UV irradiation. Gel content results prove that crosslinking has occurred in the copolymers. With increasing amount of TMVS from 0 wt% to 7 wt%, the Tm of the copolymers prepared at acidic pH is about 40-60°C higher than that at the alkaline pH. However, the addition of TMVS gives no significant effect to the Tg and Td of the copolymer films. The thermal stability of the copolymer has improved with increasing amount of BA and DVB, with DVB being more effective. The highest Td of 425°C with 8% of DVB has been obtained. Consequently, a polyacrylate copolymer with a Tg of about ?13°C, a Tm of 170 °C and a Td of about 424°C has been successfully synthesized. Hence, the soft polyacrylate with its relatively high Tm and Td could serve as a superb material especially to be applied in the areas that require high melting temperature and good thermal stability. 相似文献
Type II diffusion into uniform spheres (radius R) and sheets (thickness 2l) is calculated under the assumption that the glass-gel boundary proceeds at a constant velocity v from the surface towards the interior of the sample, that the diffusion coefficient Dg in the glass is constant and that the diffusion coefficient Dr of the rubbery gel is so much higher than vR or vl that practically no sorbate gradient is needed for the transport through the gel of the sorbate. The diffusion process is completed when this boundary reaches the center of the sample. The concentration profile of the sorbate in the glassy matrix in front of the boundary varies with time and velocity v. It does not, however, influence the boundary propagation velocity. Hence the often observed increase of the rate of the weight gain just at the end of the diffusion process is not considered at all. The relative weight gain of the sample W(t)/W∞ as a function of time is the only quantity usually measured. From the ordinate intercept A and the initial slope B of the plot of W(t)/t1/2W∞ vs. t1/2, one can calculate the characteristic transport properties, i.e., the diffusion coefficient Dg of the glass and the velocity v of the glass–gel boundary. 相似文献