摘要:

AbstractA nonlinear diffusion equation is used to study the spinodal decomposition of binary polymer mixtures. The structure of steady‐state solutions is analyzed. The free energy of the system for the solutions is shown to be a nonincreasing time function. The solution with the minimum free energy is the only stable solution and corresponds to complete phase separation in the system. A numerical analysis shows that transition to complete equilibrium takes place through a succession of fast concentration structure changes interspersed by periods in which the process is abruptly showed down. The free energy of the system is almost constant in the kinetically stable periods. All these facts indicate a relaxation process of an absolutely new type. Slow variables are introduced to describe the spinodal decomposition in macroscopic regions. These variables govern the spatial transformation of the forms and sizes of microphases. The perturbation theory is used to derive the equations for slow variables. The final relations of the simplest type give an exponential time dependence of the average size of a microphase. The problem of a contact of two pure components that are not compatible in the entire composition range is se

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280101

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractA study of the Seebeck coefficient has shown that doping of polyparaphenylene by ion implantation makes it possible to obtain an electronically doped semiconductor at low energy:n‐type with alkali metal ions andp‐type with halide ions. At the highest energies (E>100 keV) thep‐type conductivity is due to the creation of defects by irradiation. Generally the semiconductor obtained is degenerate with a Seebeck coefficient close to that obtained by chemical doping. Study of the mechanisms of conduction suggests plots of log σ = (T−l/n); the greaternis, the better is the agreement between the experimental curve and theory. Representation of the conductivity is proposed according to Mott's theories, which are applicable to amorphous semiconductors and involve several conduction processes in the temperature space. For the variable‐range‐hopping (VRH) mechanism at low temperature, two parameters, α−1(representing the spatial separation of hopping sites) andN(EF) (the density of states at the Fermi Leve

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280102

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractNormal mode analyses were done for the exact structures of model oligomers of PEO to investigate the longitudinal acoustic mode (LAM) in this helical polymer. It quickly became apparent that it is impossible to reproduce observed LAM frequencies without incorporating explicit interchain interactions. We have developed a force field that includes interchain interactions through an atom‐atom potential and have shown that this force field reproduces very well the observed LAM frequencies of helical‐chain oligomers of PEO. This result indicates that the LAM of helical chains is much more sensitive to intermolecular interactions than that of planar zigzag cha

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280103

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractNormal mode calculations were carried out on model folded‐chain molecules to determine the effect of a fold on the LAM of PEO. The presence of a chain fold results in a decrease in the LAM frequency and intensity, the change in frequency being very much smaller than the change in intensity. End interactions do not seem to have much effect on the LAM when a fold is present at the chain ends. The effective chain length for determining the LAM frequency was found to be that part of the chain experiencing interchain interaction

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280104

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe chain modulus of poly(ethylene oxide) is determined by the Raman longitudinal acoustic mode method with the aid of a normal coordinate analysis and found to be 1.3 × 1011dyn/cm2. Our normal mode calculations provide a clear basis for explaining the discrepancy between chain moduli determined from Raman and x‐ray experiments. They show that the uncorrected Raman modulus of helical polymers reflects the presence of interchain interactions and therefore should not be taken as the correct elastic modulus of a single cha

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280105

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe sub‐Tgrelaxations of bisphenol‐A–based thermosets cured with diaminodiphenyl methane and diaminodiphenyl sulfone have been studied by dielectric measurements over the frequency range 12 Hz to 200 kHz from their ungelled or “least” cured states to their fully cured states. Both thermosets show two relaxation processes, γ and β, as the temperature is increased toward theirTgs. In the ungelled states, the γ process is more prominent than the β process. As curing proceeds, the strength of the γ process decreases and reaches a limiting value, while that of the β process initially increases, reaches a maximum value, and then decreases. An increase in the chain iength and the number of crosslinks increases the number of ‐OH dipoles and/or degree of their motions in local regions of the network matrix. This is partly caused by the decreasing efficiency of segmental packing as the curing proceeds. The sub‐Tgrelaxations become increasingly more, separated from the α relaxation during curing. Physical aging causes a decrease in the strength of the β relaxation of the thermosets as a result of the collapse of loosely packed regions of low cross‐linking density, and this decrease competes against an increase caused by further crosslinking during

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280106

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractOwing to enthalpy relaxation, values of the glass transition temperature (Tg) for partially reacted polymers may depend on the thermal history of samples and the heating rate used for measurements. Use of theoretical relations betweenTgand the extent of reaction (x) of a thermoset must take this fact into account. The original DiBenedetto equation has been reevaluated as a convenient constitutive equation for expressingTgversusx. An extension of Couchman's approach for the expression of the compositional variation ofTgenabled us to derive the same functionality as given by the DiBenedetto equation. Thus, the DiBenedetto equation may be regarded as based on entropic considerations applied to a model of the thermosetting polymer consisting of a random mixture of a fully reacted network with the initial monomers in an amount which depends on the particular conversion level. These two equations have been applied with success to different diepoxy‐diamine copolymer

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280107

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractTime‐resolved light scattering studies were undertaken to elucidate the kinetics of phase separation in polycarbonate (PC)/polymethyl methacrylate (PMMA) blends. The 40:60 PC/PMMA blend undergoes thermally induced phase separation through spinodal decomposition. Temperature jump experiments were carried out from a single‐phase to a two‐phase temperature region. The general trend of spinodal decomposition in this blend system is nonlinear in character and obeys the power laws. The time evolution of scattering curves was analyzed in accordance with dynamical scaling laws for self‐similarity and the shape of scaled structure fu

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280108

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractMonomolecular particles of polystyrene (Mw/Mn= 1.04,Mw= 3.84 ? 106) formed by spreading of a dilute solution in benzene over a water surface were successfully accumulated onto a hydrophobic substrate by the horizontal lifting method. The accumulation was quantitative (up to 66 layers) to give a multilayer film. The substrates used were silicon single crystals, quartz coated with an iron(III) stearate monolayer, and poly(methylmethacrylate) plates. The film contained voids amounting to 20‐30 vol%. The surface structure observed by transmission electron microscopy clearly showed a multilayer, particle structure. These facts indicate that the molecules exist as monomolecular particles in the film. The film should be a suitable material to study properties of polymeric monomolecules in a very unusual state as compared with the ordinary soli

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280109

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY

摘要:

AbstractSteady‐state permeation rates for pure CO2and CH4and their binary mixtures through homogeneous dense cellulose triacetate membranes have been measured at three temperatures between 20 and 40°C and pressures up to 2.8 MPa. The pressure dependence of the mean permeability coefficient for CO2can be described by the total immobilization model in conjunction with a modified free‐volume model. No appreciable pressure dependence of the permeability coefficient for CH4is observed, while the permeability coefficients for CH4in binary mixture of CO2and CH4depend on applied gas pressure. The pressure dependences of the mean permeability coefficients for the components in the binary mixture are discussed in terms of the above mobility model. Membrane plasticization induced by CO2affects permeation by both g

ISSN:0887-6266    

DOI:10.1002/polb.1990.090280110

出版商:John Wiley&Sons, Inc.    

年代:1990

数据来源: WILEY