1992 рік

An equilibrium thermodynamic analysis of composite particle morphology development within artificial latices is presented. The analysis emphasizes the role of the solvent upon phase compositions and interfacial tensions, and predictions of the favoured morphology are made as a function of the extent of solvent removal. Experimental observations of the morphology of poly(methyl methacrylate)/polystyrene composite particles agree well with the predictions and demonstrate the significant role that the surfactant can have upon the preferred particle structure. Consideration is given to the choice of solvent used to produce the artificial latex and it is predicted that the preferred particle morphology is unlikely to be dependent upon the type of solvent used in the process.

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Thermally responsive gels of poly(vinyl methyl ether) (PVME) were formed by the v-irradiation of solutions of linear PVME. Under precisely defined synthesis conditions, the shrinking rates of the gels could be increased to values well above the range observed for gels formed by copolymerization/crosslinking reactions. This enhancement in rate appears related to the development of a microporous structure which allows the convective expulsion of solvent from the network; this occurs more quickly than the diffusive motion of the network. The overall rate of a cyclical process using responsive gels can also be enhanced if one stage of the process, either swelling or shrinking, is reversed before equilibrium is achieved.

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Rheology and birefringence measurements showed conclusively that quasi-full chain extension could be achieved in stagnant extensional flow above some critical strain rate of the order of the reciprocal of the chain characteristic time. Degradation in this highly expanded state could be rationalized with a model of chain scission through frictional loading in which the fracture event is controlled by the force balance between polymer solvent viscous friction and the breaking strength of the bond under consideration. In contrast to the process of chain extension, which could be observed only under rather restrictive conditions of flow, polymer degradation is ubiquitous and is readily detected in situations as diverse as convergent flow, ultrasonic irradiation, solution crystallization, turbulence and flow across porous media. Although some extensional flow component was always present in these types of degradation, the residence time in the high strain-rate region is so short that only a limited degree of coil expansion could be expected. Degradation in transient elongational flow indicated a behaviour definitely distinct from the degradation of fully extended chains under stagnant conditions. In particular, the following experimental facts cannot be explained by the model of fully extended frictionally loaded chains : ~e -~ M- 1 (where ~ is the critical strain rate for chain fracture); ~f -~ ~s-0.25 (where ~ is the solvent viscosity); degradation yield is a fairly unique function of average fluid velocity or entrance pressure drop and is almost independent of differences in elongational strain rate introduced by nozzle geometry. These facts lead us to the conclusion that the fundamental parameter which governs the rate of degradation of partly deformed coils is not the magnitude of the local stress, but rather the deformation energy accumulated by the coil up to the moment of rupture

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The deformation behaviour of the poly(tetramethylene oxide) subchains of two sodium sulphonated model polyurethane ionomers is examined by small-angle neutron scattering (SANS). Surprisingly, the scattering patterns from both ionomers remained isotropic at low elongations. At higher elongations, visible anisotropy appeared in the SANS patterns, but the change in the radii of gyration in the directions parallel and perpendicular to the stretching direction was small. Three models of network deformation--the junction affine, the phantom network and the affine deformation model--failed to reproduce the experimental results. A two-stage deformation model comprising an initial stage of aggregate rearrangement and subchain relaxation at low elongations, followed by subchain stretching at higher elongations, is postulated to explain the observed results

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Glyoxal was used to crosslink hydroxypropyl cellulose (HPC) chains. The crosslinking was performed by introducing glyoxal into a water or ethanol solution of HPC at room temperature and allowing the solvent to evaporate under ambient conditions. The mechanical properties of the resultant films were estimated in relation to morphological properties, which were dependent upon the amount of glyoxal in solution. It turned out that a suitable amount of glyoxal causes a significant effect on crosslinking, while an excess amount of glyoxal causes drastic decreases in Young's modulus and tensile strength due to main-chain scission. The crosslinking of HPC molecules in solution could be estimated by dynamic light scattering. The diffusion coefficient and the Stokes radius were very sensitive to the degree of crosslinking. The former became lower and the latter became bigger on increasing the glyoxal content in solution. In contrast, an excess amount of glyoxal invoked the inverse relationship because of main-chain scission. These results indicate that the crosslinking effect in solution is sensitive to the morphology and mechanical properties of the resultant films.

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This paper reviews recent progress in the study of gelatin gels. For example, it is now established that gel junction zones do not have a large cross-sectional radius of gyration, and both the modulus and the absolute optical rotation appear to increase slowly, but without limit, even when plotted on a log time axis. Such data are not consistent with substantial side-by-side aggregation following the initial renaturation of the collagen-like triple helix. The rheological consequences of this are discussed. At very low concentrations, atypical effects are seen, indicating the existence of a transient weak network

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13C spin-lattice relaxation times and NOE factors were measured as a function of temperature in two magnetic fields for the protonated backbone carbons of poly(/~-hydroxybutyrate-co-/~-hydroxyvalerate) (PHB/V) copolymer in chloroform-d. Analysis of the relaxation data by employing the Dejean-Laupr&re- Monnerie model reveals the following motional characteristics of the copolymer chain: (1) the local dynamics of a comonomer unit, e.g. HB, is independent of the presence of a nearby HV unit and vice versa; (2) different local dynamics prevail not only between the various comonomers, but also within a given comonomer unit in the random copolymer chain; (3) chain segmental motion of the copolymer described by co-operative conformational transitions is similar to that observed for the PHB homopolymer in a previous study

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We present the scattering function for AB diblock copolymers, and their mixtures with homopolymers A and/or B, in the single-phase state. The calculation is a generalization of earlier random-phase approximation calculations to include differences in monomer volumes and statistical segment lengths, as well as homopolymer molecular-weight distributions and copolymer composition distributions. Their individual and combined effects on the scattering profiles l(q) and on the Flory interaction parameter determined from l(q) are discussed. Using poly (styrene-b-isoprene) as a specific example, we found that for pure block copolymer the asymmetry effects are negligible if the volume fraction of one of the blocks, )cA, is close to 0.5, but that they become significant as fA approaches zero or unity. For a blend, we found asymmetry effects over a wide range of system characteristics.

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The structure of starch in its native state and during gelatinization was studied using small-angle X-ray scattering (SAXS). The scattering data were then modelled by considering the theoretical scattering from a finite number of infinite lamellar planes of crystalline and amorphous material, embedded in a medium of specified electron density. The changes in the SAXS pattern during gelatinization could be modelled simply by changing the relative electron densities of the system, thereby observing directly the absorption of water and the disruption of crystallinity. This rigorous analysis of the SAXS data from starch has provided new insights into the structure of native starch and the mechanisms of gelatinization.

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Solution-state 13C nuclear magnetic resonance spectra are presented for the emeraldine base and leucoemeraldine base forms of polyaniline. The bulk of the leucoemeraldine base sample gives a simple two-line spectrum, which is readily assignable. The emeraldine base samples give spectra of much greater complexity, showing many more lines than there are carbons in the postulated chain repeat unit. This is rationalized in terms of slow interchange of the many conformers available to the emeraldine base form. A quantitative 'region assignment' is made. Gel permeation chromatography measurements show that interconversion between the two forms is achieved without either significant crosslinking or chain scission. The minor peaks in the spectrum of the leucoemeraldine base form due to chain defects (including chain ends) are compared with chemical shift values calculated for postulated structures, and some structures can be rejected on the basis of this.

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