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1.
C.J.C. Edwards R.W. Richards R.F.T. Stepto K. Dodgson J.S. Higgins J.A. Semlyen 《Polymer》1984,25(3):365-368
Particle scattering functions P(Q) (where Q represents the wave vector), have been calculated using a Monte Carlo method for cyclic and linear poly(dimethyl siloxanes) (PDMS) containing up to 100 skeletal bonds. A maximum is found in the Kratky plot at for cyclic PDMS (with root-mean-square radii of gyration ) and this is in satisfactory agreement with the analytical calculations of Casassa and of Burchard and Schmidt. In addition, other clearly-defined maxima are found at u≈5.0 for PDMS ring molecules with less than ≈40 skeletal atoms. These maxima are believed to be characteristic of small cyclic molecules as they are also predicted for small polymethylene rings. Comparisons are made with experimental small-angle neutron scattering (SANS) data for cyclic and linear PDMS. A single maximum in the experimental Kratky plot at u≈2.0 is found for PDMS ring molecules with an average of 550 skeletal atoms. The experimental data for cyclic PDMS are in better agreement with the Monte Carlo calculations of P(u) than with analytical predictions up to u≈2.0. 相似文献
2.
The equilibrium shapes of cyclic and linear poly(dimethyl siloxane) (PDMS) molecules have been investigated using Flory, Crescenzi and Mark's rotational isomeric state model, together with a Monte Carlo method described previously. The principal axes of the equivalent ellipsoids have been computed and the major change on ring formation was found to be a reduction in the longest axis of the ellipsoid. This result is in agreement with previous studies of other ring molecules. The shapes of the cyclics in the region w=20 are disc-like and this observation is related to the different bond angles at silicon and oxygen atoms. Bulk densities and refractive indices of PDMS rings are discussed in relation to the calculated molecular shapes. Histograms are used to illustrate the proportional frequency functions of the radii of gyration and their components sx, sy and sz. 相似文献
3.
The diffusion coefficients (D) of cyclic and linear poly(dimethylsiloxanes) (PDMS) have been measured in bromocyclohexane at 288 K and 301 K. Bromocyclohexane has previously been reported to be a θ-solvent for high molar mass linear PDMS at 301 K, but the hydrodynamic radii reported here apparently show the effects of molecular expansion at both temperatures. In addition, the hydrodynamic radii of both linear and cyclic PDMS are found to be insensitive to whether the solvent is toluene or bromocyclohexane. The ratio of friction coefficients for the ring (r) and linear (l) molecules of the same number of segments (x) is in good agreement with the theoretical value of in the impermeable limit and with the experimental value found previously in toluene solution. As x decreases the ratio tends to unity, illustrating the increasing importance of free-draining at low molar mass. 相似文献
4.
Thomas C. Amu 《Polymer》1982,23(12):1775-1779
Intrinsic viscosity measurements were carried out on five well characterized fractions of poly(ethylene oxide) in aqueous solutions at 24.9°, 34.9°, and 45.5°C. The Stockmayer-Fixman extrapolation was applied to the data: it yields the unperturbed dimensions K0 of the chain. The unperturbed root-mean-square end-to-end distance calculated for the polymer fractions in water indicate that the polymer molecules are expanded in this solvent as the temperature is raised. The temperature coefficient of unperturbed dimension, , calculated for poly(ethylene oxide) in water using the present data is about 100 times higher than the literature values of 0.23 (±0.02) × 10?3 K?1 and 0.2 (±0.2) × 10?3 K?1, respectively, obtained from force-temperature (‘thermoelastic’) measurements on elongated networks of the polymer in the amorphouse state and form viscosity measurements on this polymer in benzene. A value of θ=108.3°C was obtained from the temperature dependence of the interaction parameter B in the Stockmayer-Fixman equation. 相似文献
5.
Small-angle neutron scattering studies have been made of molten and crystalline polypropylene using samples containing small amounts of deuterated polypropylene in a protonated polypropylene matrix. The specimens were characterized by small- and wide-angle X-ray scattering to determine the d-spacing and the degree of crystallinity χ and by gel permeation chromatography to determine molecular weight, Mw, and molecular weight distribution. The degree of crystallinity was varied from 0.5 to 0.7, the d-spacing from 120 to 250 Å and the molecular weight from 34 000 to 1 540 000. Clustering was not observed. The radius of gyration of the tagged molecules was approximately proportional to and almost independent of d and χ. In the melt similar values were obtained which are, within experimental uncertainties, the same as in a θ-solution. For 〈s2〉wk2? 1 the scattering law approaches a k?2 dependence. The results are discussed with reference to the chain-folded model but a fit cannot be obtained over all molecular weights. A simple random coil model fits the neutron scattering data partly but this does not explain the origin of the d-spacing. 相似文献
6.
The dimensions of both cyclic and linear poly(dimethyl siloxanes) in dilute solution in benzene-d6 have been measured by small-angle neutron scattering. The mean-square radii of gyration of the linear polymers are consistent with values predicted from published data, including experimental molar cyclization equilibrium constants. The average dimensions of the cyclic poly(dimethyl siloxanes) in fractions containing z-average numbers of bonds in the range , were found to be considerably smaller than those of the corresponding linear polymers. The neutron scattering results give a value for the ratio of the z-average radii of gyration for linear and ring poly(dimethyl siloxanes) (containing the same number of monomer units) . This ratio may be compared with the value of 2.0 predicted theoretically for ‘flexible’ high molecular weight linear and cyclic polymers, unperturbed by excluded volume effects. 相似文献
7.
The synthesis and characterization of methacrylate-ended macromers ( 500 to 10 000) and their copolymerization with styrene (M2) is described. The experimental errors in the values of the reactivity ratios r1 render them meaningless. Values of r2 can be determined with more precision and increase from 1.06 to 1.55 as the molecular weight of the macromer increases. This behaviour is due to steric effects, not diffusion-controlled propagation. It is shown that the assumptions that and are only valid for macromers of . 相似文献
8.
High resolution neutron scattering experiments have been used to observe the diffusive motion of low molecular weight linear and cyclic poly(dimethyl siloxane) molecules in dilute solution in deuterated benzene. Diffusion coefficients (D) and hydrodynamic radii (RH) have been compared with values obtained by light scattering for higher molecular weight samples and with radii of gyration (Rg) obtained by small-angle neutron scattering. While the ratio is close to the predicted value of 0.85, the ratio falls below the theoretical value for both ring and chain molecules. The scattering curves show effects arising from both centre of mass diffusion and internal molecular motion, and the observed inverse correlation times are compared with calculated behaviour as a function of scattering vector, Q. 相似文献
9.
Extremely high molecular weight polystyrenes with a in the range 10.8 × 106 to 2.2 × 107 were prepared by emulsion polymerization initiated with a heterogeneous initiator at 30°C, which has a ‘living character’. Samples of polystyrene were characterized by light scattering and viscometry in toluene and benzene at 25°C, and in θ-solvent cyclohexane at 34.8°C. Also determined were the relationships of mean-square radius of gyration 〈s2〉 (m2) and the second virial coefficient A2 (m3 mol kg?2) on the molecular weight, which for toluene and benzene are described in equations: Toluene (25°C) ; ; Benzene (25°C) ; . The parameters in the Mark-Houwink-Sakurada equation were established, for extremely high molecular weight polystyrene in toluene and in benzene, at 25°C into the form giving for ; . The mentioned relations, as well as the obtained values of Flory parameter ?0 and of ratio were compared with solution properties of high molecular weight polystyrene with narrow molecular weight distribution prepared by anionic polymerization by Fukuda et al. 相似文献
10.
The technique of small-angle neutron scattering (SANS) has been used to study the chain configuration in pressure crystallized polyethylene. Two narrow molecular weight fractions of deuterated molecules (PED) of Mw 23 000 and 54 000 were solution blended with a protonated matrix polymer of Mw 81 500. Although pressure crystallization was shown to have produced a clustering of the PED molecules, the radii of gyration were, nevertheless, shown to be consistent with a model in which the PED molecules possessed rod-like configurations. The predicted rod lengths were in close agreement with the molecular stem lengths of the PEH matrix polymer, which were independently determined by nitric acid etching. Furthermore, a doubling of the PED molecular weight produced no change in the value of . This was interpreted in terms of a chain folding mechanism in which a molecule is bounded by the surfaces of a lamellar block and is therefore unable to increase its' rod length. 相似文献
11.
Thirteen fractions of poly(phenyl acrylate) have been prepared with weight-average molecular weight ranging from 0.158 × 106 to 2.57 × 106 g mol?1. The temperature coefficient of the unperturbed dimensions and the glass transition temperature were found to be ?1.8 × 10?3 deg?1 and 55.6°C respectively. Good accord was obtained among different methods for establishing θ-conditions of 11.5°C in ethyl lactate. From viscometry, osmometry and light scattering under θ-conditions, as well as in a good solvent, the unperturbed dimensions were determined via several procedures yielding a value of . This corresponds to a steric factor υ = 2.37 (±0.08) and a characteristic ratio C∞ = 11.3 (±0.8). The polymer chain is thus more rigid than poly(methyl acrylate), but less rigid than poly(phenyl methacrylate). With respect to its Tg and flexibility, poly(phenyl acrylate) bears a strong similarity to poly(benzyl methacrylate). 相似文献
12.
The diffusion coefficients of cyclic and linear oligomeric and polymeric dimethyl siloxanes, containing number-average numbers of skeletal bonds in the range , have been measured in toluene solution at 298K. Impermeable diffusion behaviour was observed for all the siloxanes studied, in agreement with previous findings for ethylene oxide and hexamethylene oxide oligomers and polymers. The ratio of the friction coefficients for the ring (r) and linear (l) dimethyl siloxanes was found to be (within the limits of experimental error) over the whole range of molecular weights studied. Values of the expansion factor αf for the linear poly(dimethyl siloxanes) (obtained from the diffusion measurements) were found to be approximately equal to the corresponding values for αη (found previously by viscometric studies) for small values of the excluded volume parameter z; and αf was larger than αη for large z, as predicted by first-order perturbation theory. Mean-square radii of gyration 〈s2G〉 were calculated from the diffusion data for both cyclic and linear poly(dimethyl siloxanes) assuming Gaussian statistics, and found to be in good agreement with the corresponding values obtained by neutron scattering. However, the values of 〈s2G,l〉 for the short chain poly(dimethyl siloxanes) did not agree with the corresponding values of 〈s2O,l〉, calculated using the rotational isomeric state model of Flory, Crescenzi and Mark. This discrepancy is thought to arise from deviations from Gaussian behaviour, and considerably better agreement between experiment and theory was achieved by using appropriate values of a function ψl(x), which relates radii of gyration and impermeable hydrodynamic diffusion radii. 相似文献
13.
The applicability of the Gibbs—DiMarzio (G—DM) theory of the glass transition (Tg) is quantitatively evaluated for PS, PVC, PαMS and PMMA. The analysis was conducted under the assumption that both the inter-/intramolecular energy ratio (r) and the effective chain segment density (n) remain constant while the fractional free volume at Tg(V0) varies as a function of the reciprocal degree of polymerization (). Based upon reduced parametric plots of versus, the results showed that the G-DM equations were satisfactory for PS and PVC but unsuccessful in the cases of PαMS and PMMA. For the former cases the analysis indicated that when 0.015 ? V0 ? 0.045 optimum agreement occurred at n=1.80, r=10.5 and n=1.36, r=0.95, respectively. Although potential n, r values were obtained for PαMS when the allowable V0 range was expanded to 0.010–0.050, none of these combinations satisfied all of the analytical requirements. No agreement for the PMMA data sets could be obtained even when this less stringent V0 criterion was adopted. Attempts to improve this situation by incorporating ‘beads’ and ‘flexes’ into the statistical mechanical equations are also considered. 相似文献
14.
Transport in ternary polymer1, polymer2, solvent systems has been investigated using an n.m.r. spin-echo technique. The dependence of the self-diffusion coefficient of poly(ethylene oxide) polymers on the concentration and molecular size of dextran in aqueous solution has been measured. Monodisperse poly(ethylene oxide) fractions (, 2.8·105 and 1.2·106) and dextrans (, 1·105 and 5·105) have been employed over a range of concentration up to the miscibility limit in each system. It is found that when the molecular size of the diffusant is commensurate with or exceeds that of the matrix polymer, a relationship of the form: is applicable, where C[η] refers to the dextran component and is considered to describe the extent of coil overlap in concentrated solution. () is independent of the molecular size of the poly(ethylene oxide), at least in the range studied (Mw<300 000). 相似文献
15.
Qicong Ying Benjamin Chu Renyuan Qian Jingsheng Bao Jiyu Zhang Chaochou Xu 《Polymer》1985,26(9):1401-1407
Laser light scattering including angular dependence of total integrated scattered intensity and of the spectral distribution has been used to characterize five samples of poly(1,4-phenylene terephthalamide), PPTA (commercially known as Kevlar), of different molecular weights in 96% sulphuric acid and 0.1 NK2SO4. The data are supplemented by intrinsic viscosity measurements used to detect the possible effects of association, by differential refractometry providing a measure of the refractive index increments in mixed solvents (H2O, H2SO4 and K2SO4) and by spectrophotometry for the extinction coefficient needed in the correction of attenuation in light scattering studies. The results show 〈D〉Z = 2.11 × 10?5 in reasonable agreement with an average of many of the published intrinsic viscosity data obeying [η] = 1.09 × 10?3 Mw1.25 ml g?1 and w expressed in g mol?1. 相似文献
16.
Three samples of natural rubber were crosslinked in n-decane solution. After the solvent had been removed, a thermodynamic investigation was made of the elastic behaviour of the samples in simple extension. Thermoelastic measurements at constant pressure and length were carried out on the first two samples. The third sample was subjected to a thermomechanical heat of extension study using a Calvet microcalorimeter. The experiments enabled the temperature coefficient of the mean-square unperturbed dimensions, 〈r20〉, to be derived. From the thermoelastic measurements average values for dln of (0.44 ± 0.08) × 10?3deg?1 and (0.38 ± 0.07) × 10?3deg?1 were obtained, whilst the thermomechanical measurements gave a value of (0.54 ± 0.04) × 10?3deg?1. 相似文献
17.
The isothermal crystallization of poly(ethylene-terephthalate) (PETP) fractions, from the melt, was investigated using differential scanning calorimetry (d.s.c.). The molecular weight range of the fractions was from 5300–11750. Crystallization temperatures were from 498–513 K. The dependence of molecular weight and undercooling on several crystallization parameters has been observed. Either maxima or minima appear at a molecular weight of about 9000, depending on the crystallization temperature. The activation energy values point to the possibility of different mechanisms of crystallization according to the chain length. A folded chain process for the higher chains and an extended chain mechanism for the lower chains. The values of the Avrami equation exponent n vary from 2–4 depending on the crystallization temperature; non-integer values are indicative of heterogeneous nucleation. The rate constant K depends on Tc and , showing maxima related to the Tc used. The plot of log K either vs. (ΔT)?1 and (ΔT)?2 or and is linear in every case. 相似文献
18.
This article reports the synthesis and free radical polymerization of ortho-vinylbenzophenone. The glass transition temperature Tg of the homopolymer is 136°C. The products synthesized appeared to be atactic and amorphous. The Mark-Houwink constants for poly (o-vinylbenzophenone) in tetrahydrofuran are K = 4.2 × 10?2 cm3 g?1 and a = 0.765. The pre-exponential constant under theta conditions, Kθ, is estimated to be 5.93 × 10?2 cm3 g?1. The ratio of unperturbed dimensions of the actual polymer and free rotating analogue chain is 3.93, which is almost double that of polystyrene. The Flory-Huggins interaction parameter for poly is 0.48 at room temperature. The ratio at 60°C is . In free radical copolymerizations with styrene at 70°C, r1 (o-vinylbenzophenone) = 1.216, r2 = 0.751. This copolymerizations is virtually random. 相似文献
19.
J.E.L. Roovers 《Polymer》1975,16(11):827-832
A new method for the synthesis of comb shaped polystyrenes of predetermined structure is described. Silicon-chlorine bonds are introduced into the backbone polystyrene by reaction of SiMe2Cl2 with hydrolysed styrene/vinyl acetate copolymers and coupled with polystyryl-lithium in benzene. From a common backbone polymer a series of comb polymers are prepared that have a constant number of branches but vary in branch length. The of the whole comb polymers is about 1.3. The comb polymers with high branch density show θ (A2) temperatures lower than that for linear polystyrene. The radius of gyration at θ (A2) [〈S2〉θ (A2)] is always larger than calculated from random flight statistics. For comb polymers with 20–30 branches increases with λ?0.46 where λ is the fraction of polymer in the backbone. The intrinsic viscosities of the comb polystyrenes at θ (A2) are equal to that of the parent backbone polymer when λ > 0.25 and increase only little when λ becomes equal to 0.1. Similar behaviour is found in toluene. Intrinsic viscosities in cyclohexane at 35°C show a complex pattern because of the θ-temperature variation. 相似文献
20.
Several polystyrene-poly(ethylene oxide)-polystyrene (PS-PEO-PS) and poly(methyl methacrylate)-polystyrene-poly(methyl methacrylate) (PMMA-PS-PMMA) block copolymers, synthesized by free-radical polymerization, were studied in various solvents by using a light-scattering technique. The copolymers, which have different lengths of central blocks, had molecular weights within the range 3.0 × 104 to 1.6 × 106. It was found that almost all of them were confirmed as block copolymers from the variation of the product with , although they were rather heterogeneous. The copolymer compositions determined either from the additivity of the refractive index increments of its constituent parts and the copolymer or from ultra-violet analysis were in excellent agreement with each other. 相似文献