摘要:

AbstractTwo somewhat separate issues have been delat with: the diffusion coefficients of various poly(ethylene oxide) (PEO) fractions at different temperatures; and morphological observations of spherulitic textures of a PEO mixture containing low‐ and high‐molecular mass components at different supercooling and cooling rates. We found that simultaneous knowledge of diffusion coefficient of PEO fractions and crystal growth rate of the mixture leads to a better description of the crystal growth. As observed, the spherulitic texture of the PEO mixture comprises three texture levels: the spherulite as a whole; the crystalgrowth unit comprised of different number of lamellae in a stack; and the individual lamellar crystal. Under certain crystallization conditions, the thickness of the lamellar stack is on the same order of magnitude as the parameter δ proposed by Keith and Pa

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290501

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe nature of the crystal transition of the α‐helical forms of poly (γ‐n‐alkyl glutamate)s (alkyl = ethyl, propyl, and butyl) is described. The transition is thermally reversible, and its temperatureT2is much higher than the glasslike transition temperatureT1associated with the side‐chain motion. The main chains undergo large‐scale motion (librational about the chain axis and translational along the axis) aboveT3≈ 200°C. The structure observed belowT2is anomalously disordered compared with that observed betweenT2andT3. The crystal structure emerging aboveT2is analyzed for a typical sample of poly(γ‐n‐propylL‐glutamate). The trigonal unit cell contains three α‐helices so that each helix is surrounded by other helices in the same fashion, but the helices are not interrelated by a crystallographic symmetry element. The side chains suffer no particular change atT2. The main‐chain motion gives rise to theT2transition by inducing attractive forces between in

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290502

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractAn integral constitutive equation which includes strain‐coupling effects is proposed. The constitutive equation is more general than the K‐BKZ equation, and it is shown that this equation can explain various important aspects of double‐step stress‐relaxati

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290503

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThis article reports the use of classical light scattering to study cellulose in the NH3/NH4SCN solvent system. Three solvent compositions were used, 27.01 73.0,25.5/ 74.5, and 24.51 75.5 weight ammonia/weight ammonium thiocyanate. The coefficient, (dn/dc)υ, was determined by back calculating using the molecular weight determined by solution viscometry in the solvent system cupriethylenediamine and the classical light‐scattering results. Second virial coefficients were found to be similar to those values measured for cellulose in the FeTNa and LiCl/DMAC solvent systems. The characteristic ratios were found to vary with solvent composition with the highest values being at a composition of 25.5/74.5 weight ammonia/ weight ammonium thiocyanate. Persistence lengths were also found to vary with solvent composition with the highest value being 264 × 10−8cm at solvent composition 25.5

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290504

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThis is the second part of a two–part study of the NH3NH4SCN cellulose solvent system. Quasielastic light scattering was used to determine the diffusion coefficients of cellulose in solution and the effective hydrodynamic radius of the dissolved molecules. Additionally, the system was studied using light microscopy to determine the minimum critical volume fraction or liquid crystal formation. Very little change was found in the diffusion coefficients with change in cellulose concentration indicating little interaction between the chains in solution. Values of 7.69 and 2.66 × 108cm2/s were measured for samples having a degree of polymerization of 153 and 969. The value of the coefficient relating the hydrodynamic volume to the radius of gyration was found to be in the range of 0.33 to 0.53, indicating an extended coil conformation according to the Kirkwood‐Riseman theory. The minimum critical volume fractions necessary for liquid crystal formation, υ2′ were 0.039, 0.038, and 0.048 for the three solvent compositions studied. The values calculated for υ2′ based on the measured persistence lengths were much larger than the predicted values, indicating strong deviation from theory or possible aggregation in

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290505

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe accessibility of starch in polyethylene starch blends was investigated by computer simulation, percolation theory, and acid hydrolysis experiments. The object of this work was to model the bilateral invasion of microbes in polyethylene‐starch blends as a function of starch concentration (p), and thickness of the material. It was found that computer simulations in three dimensions were in agreement with both percolation theory and the acid digestion experiments. In computer simulation the accessibility is highly dependent on the percolation threshold concentration (pc), which is 31.17%. Similarly, the accessibility of starch is highly dependent on an apparent percolation threshold near 30% by volume or approximately 40% by weight of starch. Atppcconnected pathways existing throughout the bulk of the material facilitate large amounts of starch extraction. The sharpness of the transition atpcincreases with the ratio of sample thickness to starch particle size. The results of this work have application to conduction and reacting systems where one component is dispersed in a matrix of the othe

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290506

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractPolyurethanes based on methylenebis(p‐phenyl isocyanate),bis(hydroxymethyl) propanoic acid, and poly (tetramethylene oxide) were synthesized, and their morphology and physical properties investigated. The acid polyurethanes exhibited very poor phase separation and mechanical properties, but upon neutralization with alkali metals phase separation improved dramatically and the materials resembled tough elastomers. The choice of cation played a much less important role than hard segment content in determining morphology and properties. Small‐angle x‐ray scattering patterns of these ionic polyrethanes exhibit two peaks, one characteristic of scattering between the hard and soft domains and the other reflecting scattering from aggregates of ionic groups residing within the hard domains. While ionic aggregation is the primary driving force for phase separation in these materials, their morphology more closely resembles that for nonionic polyurethanes than that for conventional ionomers, because the ionic groups exist in blocks composed primarily of rigid diisocyanate res

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290507

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractChain stiffness is often difficult to distinguish from molecular polydisperity. Both effects cause a downturn of the angular dependence at largeq2(q= (4π/λ)sin θ/2) in a Zimm plot. A quick estimation of polydisperity becomes possible from a bending rod (BR) plot in which lim (c→ 0)qRθ/Kcis plotted againstq(〈S2〉z)1/2=u. Flexible and semiflexible chains show a maximum whose position is shifted fromumax= 1.41 for monodisperse chains towards larger values as polydispersity is increased, while simultaneously, the maximum height is lowered. Stiff chains display a constant plateau at largeq, its value is πMLwhereMLis the linear mass density. Using Koyama's theory, the number of Kuhn segments can be determined from the ratio of the maximum height to the plateau height, if the polydispersity indexz= (Mw/Mn− 1)−1is known. Thus, if the weight‐average molecular weightMw, is known, the contour lengthLw, the number of Kuhn segments (Nk)w, the Kuhn segment lengthlkand the polydispersity of the stiff chains can be determined. The influence of excluded volume is shown to have no effect on this set of data. The reliability of this set can be cross‐checked with the mean‐square radius of gyration 〈s2〉zwhich can be calculated from the Benoit‐Doty equation for polydisperse chains. Rigid and slightly bending rods exhibit no maximum in the BR plot, and the effect of polydispersity can no longer be distinguished from a slight flexibility if only static scatter

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290508

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractFive different analytical schemes for examining isothermal and nonisothermal degradation of polymers were reviewed and found inadequate for describing multistage decomposition. The different schemes were experimentally tested using thermogravimetric analysis data for an ethylene‐vinyl acetate (EVA) copolymer, which exhibited a well‐behaved two‐step decomposition process in a nitrogen environment. Based on these experimental and analytical findings, a generalized methodology was developed capable of describing the exhibited complex decomposition behavior of polymers by combining two or more kinetic mechanisms in a series or parallel arrangement. This composite of combinational methodology thus extends established analytical schemes for describing complex decomposition of polymers in a rational manner, incorporating both experimental and theoretical considera

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290509

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe phase behavior of poly (vinyl alcohol) dissolved in ethylene glycol and propylene glycol was studied. On cooling a solution in the low concentration range, solidification results from a combination of a liquid‐liquid demixing and a crystallization or liquid‐solid demixing. This conclusion is based on the results obtained from optical and calorimetric observations and is further supported by the influence of the molecular weight and the solvent quality. It also explains the melting point‐crystallization relationship observed after isothermal crystallization of the polymer in presence of the solvent. The mechanism proposed for the thermoreversible gelation of these solution is conf

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290510

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY