Hydrodynamic constants and frictional properties of polystyrene in dilute solution

Experimental data from hydrodynamic measurements on eleven standard polystyrene samples in a poor and a good solvent (cyclohexane at theta-temperature 34,5°C, toluene at 25°C) are analyzed in terms of the two-parameter theory of the hydrodynamic properties of random coils. The discussion is focussed on the expansion factors of hydrodynamic radii α_f and α_μ, evaluated from the translation friction coefficients and the intrinsic viscosity, and on the hydrodynamic-interaction parameters P and Φ. The expansion factors α_f and α_η are shown to assume nearly equal values (at 1,15 ≤α_η ≤ 1,5). That implies that the dependences on coil expansion must be very similar for P and Φ^1/3. The value of the P parameter in the unperturbed state (i.e. P₀) is shown to be close to that recently calculated by Zimm and to be significantly higher than the results of previous theoretical calculations.     

Deuteron NMR relaxation and molecular motion of polystyrene in solution

Deuteron relaxation times T₁ of polystyrenes deuterated at the backbone (PS-d₃) and the phenyl ring (PS-d₅), respectively, have been measured in solutions of benzene and diethyl malonate as a function of concentration and temperature. We conclude that the motion of the phenyl ring is faster than that of the backbone, the difference being smallest at high temperatures around 180°C. The temperatures dependence is discussed in relation to the activation energies in polystyrene.     

Continuous fractionation and solution properties of PVC, 6. Pressure dependence of the viscosity — influence of molecular weight and composition

Viscosities were measured as a function of pressure and temperature with solutions of PVC 75 000 in cyclohexanone (CHO) and polymer contents ranging from 0,6 to 12 wt.-%, by means of a Searle-type (≥3 wt.-%) and a rolling-ball viscometer (<3 wt.-%). Furthermore, the influence of molecular weight was determined with solutions of 8 wt.-% of PVC 20 000, PVC 37 000 and PVC 100 000. (The numbers in the codes of the PVC specimens are their approximate molecular weights.) For all concentrations and molecular weights, the viscosity increases in a more or less exponential manner with increasing pressure. The ratio f₁₀₀₀ of the viscosity of the solution at 1 000 and 1 bar can be varied by the change of the polymer content from 2,5 (the value of the pure solvent, index s) to 3,5 (12 wt.-% PVC 75 000) at t = 25°C and from 2,23 to 2,94 at t = 80°C. An increase of the molecular weight of the polymer raises f₁₀₀₀ in a similar manner as the polymer concentration. Using the reduced variables V^≠/V^≠ (ratio of the volumes of activation of the solution and the pure solvent) and c? (product of the polymer concentration and the intrinsic viscosity), all results obtained by variation of T, c and M_w can be represented by a master curve. This means that it is possible to calculate the pressure dependence of a given polymer solution of arbitrary polymer concentration from a mere measurement of the intrinsic viscosity at normal pressure. Criteria are presented which allow a forecast concerning the occurrence of minima in the concentration dependence of the energy of activation of the viscous flow E^≠ and V^≠.     

Dilute solution properties and unperturbed dimensions of poly(di-ni-alkyl itaconate)s

The diesters of itaconic acid (methylenesuccinic acid) with higher C₁₂ – C₂₀ unbranched, saturated, aliphatic alcohols were prepared, purified, and polymerized in bulk or in solution with 2,2′-azoisobutyronitrile as initiator. The transparent to waxy polymers obtained were fractionated and all fractions characterized by viscosity and light scattering measurements. The Kuhn-Mark-Houwink-Sakurada relations were established and then extrapolated to thetaconditions, in order to calculate the unperturbed dimensions. High values of the steric factor σ, and the characteristic ratio C_∞ were obtained. The results correlate well with those obtained previously for poly(di-n-alkyl itaconate)s from the corresponding lower C₁ – C₁₁ alcohols.     

Intrinsic viscosity versus molecular weight relationship of polystyrene in cis-decahydronaphthalene

Intrinsic viscosities [η] and mean-square radii of gyration 〈R〉 of a polymer homologous series of commercially available narrow-molecular-weight-distribution polystyrene (PS) samples in a molecular-weight range of 800 up to 24 · 10⁶ were determined in cis-decahydronaphthalene at 25°C by light-scattering and viscosity measurements. These dilute solution properties were compared with those for PS in the good solvent toluene, in the constitutional isomer trans-decahydronaphthalene both at 25°C and in cyclohexane at 34,5°C as a theta-system.     

Electro-optical properties and molecular flexibility of polyelectrolytes in dilute solutions

Measurements of the electric birefringence of aqueous polyelectrolyte solutions in the very dilute regime ( ≈ 10^?5 g · mL^?1) reveal that the macroions are fully extended. However, this rod-like conformation is easily altered towards a more coiled conformation when a strong static electric field is applied, particularly in the case of very high molar-mass polymers. With the application of rectangular dc pulses, as ordinarily used in Kerr-effect measurements, the change of molecular shape interferes with the transient rise and decay of the birefringence signal, because the rotational mobility as well as the optical anisotropy of the macroions become time-dependent. As a result, the mean relaxation time for the rise curve is considerably smaller than that of the field-free decay, and the steady-state birefringence is much lower than expected for the unperturbed macroions. The change of the molecular shape can be avoided if the orientation of the macroions is induced by high-frequency ac pulses. This experimental technique is therefore suitable to obtain the unperturbed shape and size of the macroions. A molecular interpretation for the dependence of the birefringence function on the pattern of the electric field is proposed.     

Conformational transition of block copolymers in dilute solution and their morphology in the solid state

Block copolymers poly(methyl methacrylate)-block-polystyrene-block-poly(methyl methacrylate) and polystyrene-block-poly(tert-butyl methacrylate) were prepared by anionic polymerization and were studied in dilute solution as well as in the bulk state. The viscometric study revealed that the block copolymers exhibit a conformational “transition” occurring in a narrow temperature range. Films were obtained with the same copolymers by casting at different temperatures, chosen below and above the “transition”. From the investigation of these films by differential scanning calorimetry, using the entropy relaxation method, and by dynamic mechanical measurements, it was shown that the block copolymers exhibit a rather segregated structure if they are obtained below the “transition” temperature, and that they adopt a quasi-Gaussian conformation (implying a better miscibility of the two kinds of blocks) if the solvent is evaporated above the “transition” temperature.     

Change in molecular weight distribution accompanying thermal degradation of isotactic polystyrene

The molecular weight distributions of isotactic polystyrene samples subjected to various degrees of thermal degradation were experimentally determined and compared with those derived theoretically for randoms scission of the polymer chains. Agreement between the theoretical and the experimental results was fairly good. It was found that the molecular weight distributions of the original and the thermally degraded isotactic polystyrene can be represented by the logarithmic normal distribution function. The degree of degradation is nearly linear with time. The scission of the main chains seems to occur at random up to 330°C. The activation energy for the thermal degradation of isotactic polystyrene at 290 to 330°C. was 42 kcal. /mole. The results of thermal degradation of the isotactic polystyrene were compared with those of the atactic polystyrene.     

The molecular weight dependence on the photoconductivity of poly(N-vinylcarbazole)

Poly(N-vinylcarbazole), (PNVC), was prepared, fractionated by gel permeation chromatography, and then characterized by viscometry and vapour pressure osmometry. The fractionated PNVC species with relatively narrow molecular weight distributions were successfully used to measure both their electrical dark-conductivity and photoconductivity using a surface type cell in high vacuum (ca. 10^?7 mm Hg) at room temperature. A molecular weight dependent photoconductivity was found for the fractionated PNVCs with weight average molecular weights in the range of 1,2·10³ to 2,4·10⁵. This observation is in contradiction to Epping's results who has found a molecular weight independent photoconductivity in the molecular weight range of 3·10⁵ to 7·10⁶. Our molecular weight dependence may be well understood in terms of the interrupted overlap of the π-electrons of adjacent carbazolyl groups at the terminal parts of the polymer chains, this effect being all the more stronger the smaller the molecular weight is.     

On the dilute solution properties of styrene/methyl methacrylate copolymers

The solution properties of unfractionated styrene/methyl methacrylate copolymers were investigated over the entire range of copolymer composition. The Mark-Houwink-Sakurada constants K_η, and a_η of the copolymers, as well as the constants K_Θ and B′ from the Stockmayer-Fixman procedure, related to the unperturbed chain dimensions and to the excluded volume, respectively, seem simply related to those of the parent homopolymers through some composition variables. In most cases, however, equations must be employed which have to be fitted to the experimental data, in order to evaluate parameters which are not otherwise predictable.     

Molecular weight dependence of magnetic circular dichroism for polystyrenes in solution

The molecular weight dependence of the magnetic circular dichroism (MCD) for polystyrenes (PS) has been studied in various solvents by using the normalized dipole-strength D^*. The normalized MCD intensity B^* is not so much affected by the molecular weight as the normalized dipole strength D+. The ratio of the Faraday parameter to the dipole strength, B^*/D^*, decreases as the molecular weight increases to 4,0.10³ and is constant above it. The increase of the normalized D^* with the increase of molecular weight is interpreted in terms of the closer approach of chromophores. These results indicate that MCD for polystyrenes is influenced by the interaction of chromophores in a polymer chain: the decrease of B^*/D^* comes from the increase of the interaction of chromophores. This tendency is enhanced by solvent effects on MCD.     

Characterization of cellulose ethers by 13C NMR, 1. Studies of high molecular weight polymers in solution and in the solid state

Carbon-13 NMR was used to investigate the structure of a cellulose derivative, ethyl-O-(hydroxyethyl)cellulose (EHEC). Analysis of high molecular weight samples in solution showed that the line widths of the main chain carbon resonances are very sensitive to the concentration of the solution. Above a certain concentration, excessive broadening occurs due to gel formation, which makes a quantitative evaluation of the degree of substitution difficult. If the concentration is decreased below the critical value for gel formation, the use of NMR parameters suitable for quantitative measurements (no NOE and long pulse intervals) leads to excessively long accumulation times. Spectra of solid samples obtained by cross-polarization magic angle spinning (CP-MAS) NMR exhibit better defined resonances of the main-chain carbons than do the spectra of solutions with concentrations above the critical value for gel formation. Variable contact time experiments with a specific sample indicated that a suitable contact time should be 400 μs. The quantitative evaluation of the total degree of substitution by CP-MAS NMR yielded values in agreement with those obtained by GC-analysis.     

多糖分子质量与配基连接臂长度对改性普鲁兰药物载体的性质影响

目的 研究普鲁兰分子质量和胆固醇配基连接臂长度对胆固醇改性普鲁兰自组装、载药及体外释放等性质的影响.方法 将胆固醇通过2种长度的连接臂共价修饰到2种分子质量的普鲁兰多糖上,合成不同取代度的胆固醇改性普鲁兰(共8种),使其在水中组装成纳米粒,并考察多糖分子质量及连接臂长度对纳米粒形态、粒径的影响.同时,以阿霉素、米托蒽醌为模型药物,考察其在改性普鲁兰多糖纳米粒中的包封及释放行为特征.结果 所有改性普鲁兰多糖均可形成纳米粒,包封药物后可形成载药纳米粒;多糖分子质量和连接臂长度对其性质具有一定的影响.结论 载药前,分子质量较大、连接臂较短的改性多糖具有更好的稳定性;载药后,其对药物的包封、粒径、体外释放行为的影响规律与药物种类相关.     

Dilute solution properties of high molecular weight poly(isobutyl methacrylate). III. Effect of centrifuging on various parameters

Three high molecular weight samples of poly(isobutyl methacrylate(designated as 937, SW 63/0082, and SW 62/0298 were obtained from Edgewood Arsenal, Maryland. Sample 937 has been fractionated and described^l,2) earlier. Initial investigations have shown that the solution viscosities were shear dependent. The three samples were subjected to centrifugation at constant velocity for various times. The intrinsic viscosity and molecular weight (M?_w) dropped slightly for sample 937, but dropped drastically for the other two samples. The number average molecular weight (M?_n) remained constant for each sample under these conditions. In this presentation we have also shown the necessity for taking measurements at low angles daring light scattering measurements in order to obtain the correct molecular weight (M?_w) for these high molecular weight species.     

Estimation of the 0th-order diffusion constant of polydisperse polymer in dilute solution and its application to the “diffusion constant versus molecular weight relationship”

A useful method for determining the “diffusion constant vs. molecular weight relationship” for monodisperse polymers is proposed using polydisperse polymers. According to this method, the diffusion constant vs. molecular weight relationship can be established if the 0th-order diffusion constants are plotted against the 0th-order molecular weights. The method was tested experimentally by using polydisperse polystyrenes in ethyl methyl ketone at 25°C and in cyclohexane at 35°C (theta-state). It was ascertained that the diffusion constant vs. molecular weight relationship was well established without any effect of molecular weight distribution even if the polymers were polydisperse.     

Investigations on conformations of block and of random copolymers in dilute solution

Investigations on the viscosity behavior of well characterized block and random copolymers were carried out in a number of solvents. Account was taken of the expansion of the corresponding individual homopolymers in each of the chosen solvents. It was shown thus that for block copolymers heterocontact interactions do not affect the dimensions of the macromolecular coils. This confirms the “segregated” conformations of block-copolymers, characterized by a very low number of intramolecular heterocontacts. On the contrary, for random copolymers the χ_AB term does not only affect the expansion but also the unperturbed dimensions of the molecule. The heterocontact interactions are, as expected, strongly solvent dependent, and a relation was established between the expansion increment and the critical demixtion volume of the corresponding homopolymers.     

Second virial coefficient revisited, 3. Viscosity of dilute polymer solutions: Molecular weight dependence of the Huggins coefficient

The variation of the Huggins coefficient K_H with the relative molecular mass M of the polymers was measured for solutions of polystyrene and of polyisobutylene and found to be most pronounced in the case of thermodynamically good solvents but vanishing at the theta-temperature, where the individual curves k_H (T; M) intersect with each other. The experimental results are interpreted as a consequence of the rheological inequality of inter- and intra-molecular contacts between polymer segments. A model is presented according to which k_H should be a linear function of M?^(a?0,5), where a is the exponent of the intrinsic viscosity-relative molecular mass relationship (Kuhn-Mark-Houwink). The evaluation of the present data, together with information from the literature, yields the individual effects from inter- and intra-molecular intersegmental contacts, their interdependence, and their variation with the thermodynamic quality of the solvent. In thermodynamically poor solvents, were the polymer coils are nondraining, these effects are found to differ extremely; however, they approach each other as the goodness of the solvent improves and the coils become draining; their identity can be extrapolated for a = 1.