摘要:

AbstractA theory of nuclear magnetic resonance free induction decay (FID) in crosslinked poly‐dimethysiloxane networks is proposed. The anomalously small amplitude of the FID asymptotes at high temperatures is explained as due to crosslink motions. The Anderson‐Weiss theory is generalized to account for the non‐Gaussian distribution of local dipolar fields. © 1993 John Wiley&Son

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310301

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractAn algorithm is proposed for the construction of Miyazawa's equations with an arbitrary number of junction atoms in the repeat unit of the macromolecule. In these equations the helical parameters of an infinite polymer chain and the internal parameters of the chain, bond lengths, bond angles, and internal rotation angles, are interrelated. © 1993 John Wiley&Sons, Inc

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310302

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractA wide‐angle x‐ray diffraction (WAXD) study of the development of molecular orientation in the crystalline phase of ultra‐high–molecular weight polyethylene films prepared by the gelation–crystallization method is presented. WAXD scans of the undrawn films show that the lamellae are oriented in the plane of the films. Upon drawing at 130°C, the orientation of the molecular chains changes from the direction normal to the film surface (ND) to the elongation direction. The decrease of the 200/020 intensity ratio at low draw ration (λ<10) indicates that double orientation develops during the transformation from the lamellar to the fibrillar morphology, with thea‐axis oriented parallel to ND. The orientation distributions of the 110, 200, 020, and 002 planes of the orthorhombic unit cell of polyethylene were studied and characterized by the coefficients of a Legendre polynomial series. At a draw ratio of 4.5, the second‐order coefficient, 〈P2(cos χ〉, already gets close to its limiting value, but it is shown that higher order coefficients of the polynomial series can be used to describe the evolution of the orentation, even up to λ = 50. The coefficients relative to the molecular chain orientation, 〈Pn(cos χ)〉c, can be calculated from different crystalline reflections. Curve‐fitting calculations were made in order to improve the correlation between the results obtained from the orientation distribution of the 110, 020, and 002 planes. A Person VII function was found to give a better fit of the experimental curves than Gaussian or Lorentzian equations.

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310303

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe dielectric properties of 30% crystalline dry Nylon‐12 have been measured over the frequency range 10–105Hz and temperature range 300–450 K, and the effect of its annealing at 423 K investigated both by dielectric measurement and differential scanning calorimetry. Annealing causes its crystallization to α phase, which increase the dc conductivity and decreases the contribution to orientation polarization, but does not alter the shape of the relaxation spectrum. The orientation polarization in Nylon‐12 involves two processes, each of which occurs above the glass‐transition temperature of Nylon‐12, but only the spectra of the lowest temperature process could be clearly resolved. © 1993 John Wi

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310304

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractPreviously we have shown that the Lattice Fluid (LF) model can quantitatively predict, without adjustable parameters, gas solubilities for hydrocarbon and chlorinated hydrocarbon vapors in nonpolar polymers. For polar polymers, the model can also predict, with reasonable success, the solubilities of polar and aromatic vapors. However, the solubilities in polar/nonpolar combinations of gas and polymer are systematically overestimated. These are cases in which the geometric mean approximation for the interaction parameter is not expected to be valid. This paper demonstrates that with the addition of a simple empirical correlation for the interaction parameter based on Hansen's three‐dimensional solubility parameters, the LF model is then able to quantitatively predict solubilities in all types of gas/polymer systems (excluding strongly self‐associating systems, such as alcohols). No adjustable parameters are used; only the pure component equation‐of‐state and solubility parameters are required. © 1993 John Wiley&S

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310305

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractRecently heat capacitiesCpof poly(amino acid)s of all naturally occurring amino acids have been determined. In a second step the heat capacities of four copoly(amino acid) s are studied in this research. Poly(L‐lysine · HBr‐alanine), poly(L‐Lysine · HBr‐phenylalanine), poly(sodium‐L‐glutamate‐tyrosine), and poly(L‐proline‐glycine‐proline) heat capacities are measured by differential scanning calorimetry in the temperature range 230–390 K. This is followed by an analysis using approximate group vibrations and fitting theCpcontributions of the skeletal vibrations of the corresponding homopolymers to a two‐parameter Tarasov function. Good agreement is found between experiment and calculation. Predictions of heat capacities based on homopoly(amino acid)s are thus expected to be possible for all polypeptides, and enthalpies, entropies, and Gibbs functions for the s

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310306

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractA computer‐controlled chemiluminescence (CL) instrument incorporating a differential scanning calorimeter was used to simultaneously acquire photochemical and enthalpic data for poly(N‐vinyl‐2‐pyrrolidone) (PVP). Samples were subjected to a linear temperature ramp under nitrogen and their luminescence response recorded. The resultant dynamic CL peak is attributable to the decomposition of hydroperoxide groups and the subsequent mutual termination of secondary polymer peroxyl radicals. It is shown that dynamic CL can be used to characterize the level of oxidation in PVP, which in commercial samples, may be partly related to the level of residual polymerization inititor and to the drying process. The temperature at which maximum CL emission occurs correlates with the glass transition temperature (Tg) of the polymer and increases with increasing molecular weight. A marked increase in theTgof PVP occurs after it is aged in air for 24 h at 120°C. This is due to the loss of adsorbed moisture from the polymer which was confirmed by thermogravimetric analysis. Oxidation profiles of PVP were obtained by plotting the integrated CL peak area as a function of aging time. The profiles are compared with data obtained from isothermal CL and viscosity measurements. Gas perturbation experiments suggest that when drying PVP under nitrogen at elevated temperatures significant populations of longlived macroalkyl radicals are formed which can peroxidize the polymer on exposure to air. © 1993 John Wiley&

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310307

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractChanges in the dielectric permittivity ε′ and loss epsiv;″ during the curing of DGEBA catalyzed by 10 mole % dimethylbenzylamine have been studied from sol to gel to glass formation regions at different temperatures from 323 to 390 K. The ε′ monotonically decreases with time of cure, and ε″ initially decreases by several orders of magnitude and then increases to reach a peak value before finally decreasing to a low value characteristic of the glassy state. The features shift to shorter times and the peak vanishes as the curing temperature is increased. The decrease of ε″ at the initial stage of cure has been analyzed in terms of dc conductivity σ0, which follows a power law, σ0∝︁ (tg–t)x, as well as a new singularity equation, σ0∝︁ exp[–B/(t0–t)] wheretg, x,B, andt0are empirical constants that vary with the curing temperature;tgis close to the time for gelation; andt0≥ time for vitrification. The dielectric properties of the thermoset formed after different periods of cure have been studied from 77 to 325 K. Similar studies of the thermosets formed at different temperatures have been made. Increase in the curing period decreases the heights of both the γ‐and α‐relaxation peaks and increases their separation, while a β‐relaxation peak emerges. Isothermal curing at high temperatures decreases the height of the γ peak to a vanishingly small value and increases that of the β peak from a vanishingly small value. In both the uncured and fully cured states, there is only one sub‐Tgrelaxation process named γ for the uncured and β for the cured state. These results are discussed in terms of our general physical concepts of local mode motions in a

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310308

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe solubility of carbon dioxide, methane, and propane in poly(dimethyl silmethylene) [(CH3)2SiCH2]xand poly(tetramethyl silhexylene siloxane) [(CH3)2Si (CH2)6Si (CH3)2O]xwas measured in the temperature range from 10.0 to 55.0°C and at elevated pressures. The present results are compared with similar measurements made with other silicone polymers. At a given temperature and pressure, the solubility of the above three gases is highest in poly(dimethyl siloxane) (Me2SiO)x. The gas solubility is decreased by either backbone‐chain or side‐chain substitutions of functional groups in (Me2SiO)xwhich increase the stiffness of the polymer chains and decrease the specific or fractional free volume of the polymers. It is conjectured that a decrease in the free volume of silicone polymers has a greater effect in decreasing the gas solubility than differences in gas/polymer interactions [with the exception of specific interactions (e.g., between CO2and polar groups in the polymer)]. © 1993 John Wiley&Sons

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310309

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe glass transition (Tg) of thin polystyrene films (ca. 3000 A˚) cast on silicon wafers was determined by a new technique. An ellipsometer was used to determine the refractive index and thickness of the polystyrene films.Tgwas determined by measuring the temperature dependence of the refractive index. The change in thickness with temperature was used to calculate the linear and bulk thermal expansion coefficients of the material. A significant shift inTg, possibly due to strains induced in the cooled films, was observed between heating and cooling for polystyrene films. © 1993 John Wiley&Sons, In

ISSN:0887-6266    

DOI:10.1002/polb.1993.090310310

出版商:John Wiley&Sons, Inc.    

年代:1993

数据来源: WILEY