摘要:

AbstractAlthough the rate of heterogeneous nucleation of crystallization in isotactic polystyrene, as studied by photomicroscopy, is markedly increased by addition of fine‐particle silica, the rate of subsequent radial growth of spherulites formed is diminished. The latter observation is rationalized on the basis of a modified Hoffman–Lauritzen treatment wherein the nucleant is depicted as a quasicrosslink which impedes the transport of polymer segme

ISSN:0098-1273    

DOI:10.1002/pol.1983.180210811

出版商:John Wiley&Sons, Inc.    

年代:1983

数据来源: WILEY

摘要:

AbstractThe autohesion (tack) and cohesion of a random styrene‐butadiene elastomer have been examined as a function of test temperature and speed using a T‐peel geometry. Both properties have been reduced to a single master curve by horizontally shifting the data with the same set of shift factors. The cohesive strength increases with increasing reduced test rateRaTand appears to approach a plateau at the highest rates. Tack also increases withRaTbut decreases abruptly at a critical rate and peeling the occurs in a stick‐slip fashion. Tack again increases at sufficiently high test rates. In the range of rates where tack is maximized, its value is essentially the same as its cohesive strength. Above or below this range, tack is substantially less than the elastomer's cohesive strength. Mechanisms are proposed to explain why relative tack (i.e., tack divided by cohesive strength) isnota simple measure of the extent of completion of a tack bond and may indeed be equal to one in spite of incomplete tack bond form

ISSN:0098-1273    

DOI:10.1002/pol.1983.180210812

出版商:John Wiley&Sons, Inc.    

年代:1983

数据来源: WILEY

摘要:

AbstractThe linear thermal expansivity of poly(ethylene terephthalate) extruded at 50 and 90°C to extrusion ratios λ of 1–4.8 has been measured between 120 and 300 K. With increasing λ, the expansivity along the extrusion direction (α∥) decreases sharply, while that in the transverse direction (α⊥) shows a slight increase. For λ<3, the large drop in α∥and the accompanying increase in the axial Young's modulusE∥can be ascribed to chain alignment in the crystalline regions and to an increase in number and tautness of intercrystalline tie chains. At higher λ, however, the crystalline orientation apparently becomes saturated, so that taut tie molecules are solely responsible for further changes in both α∥andE∥. On the other hand, α⊥is mainly determined by crystalline orientation for all λ, thus showing very little increase at large λ. For the highly oriented samples (λ ≥ 3), the Takayanagi model provides a reasonable descripti

ISSN:0098-1273    

DOI:10.1002/pol.1983.180210813

出版商:John Wiley&Sons, Inc.    

年代:1983

数据来源: WILEY

摘要:

AbstractThe small‐angle x‐ray scattering (SAXS) technique has been used to characterize the detailed microphase structure of two crosslinked segmented polyurethane elastomers. Both copolymers contain trifunctional polypropylene ether triols in the rubbery elastomeric block, but are synthesized with different hard segments: a symmetric 4,4′‐diphenylmethane diisocyanate (MDI) chain extended with butanediol (BD); and an 80/20 mixture of asymmetric 2,4‐toluene diisocyanate (TDI) and symmetric 2,6‐toluene diisocyanate (TDI) chain extended with ethylene glycol (EG). Calculations of SAXS invariants and determinations of deviations from Porod's law are used to examine the degree of phase segregation of the hard‐ and soft‐segment domains. Results show that the overall degree of phase separation is poorer in the asymmetric TDI/EG‐based copolymer than in the symmetric MDI/BD‐based copolymer. Determination of diffuse phase boundary thicknesses, however, reveals that the domain boundaries are sharper in the asymmetric TDI/EG system. The contrasting morphologies found in the two systems are interpreted in terms of differences in hard‐soft segment compatibility, diisocyanate symmetry, and diisocyanate length. Coupled with conformational considerations, this information is used to construct a new model for polyurethane hard‐segment microdomain structure. Important features of the model are that it takes into account the effects of hard‐segment sequence length distribution and allows for folding of the longer hard‐segment sequences back i

ISSN:0098-1273    

DOI:10.1002/pol.1983.180210814

出版商:John Wiley&Sons, Inc.    

年代:1983

数据来源: WILEY

摘要:

AbstractAn analysis is presented of the effects of external flow kinematics on the so‐called local flow in seeded, flow‐induced crystallization and orientation. The flow field around a growing crystal or nucleation seed is modelled by the Stokes flow equations past a prolate ellipsoid of high aspect ratio. Exact solutions for various flow kinematics, worked out elsewhere by the singularity method, are applied here to the analysis of local gradients. The results show that along the symmetry axis of the spheroid, the extensional gradients which result for various free‐stream velocity fields are primarily the result of the constant‐velocity free‐stream component. However, free‐stream, extensional flow can significantly enhance the region of such high gradients. Along the symmetry plane of the spheroid, primarily shearing gradients result, with small extensional gradients occurring when the free‐stream flow has extensional components. Results of chain extension and birefringence calculations are also presented

ISSN:0098-1273    

DOI:10.1002/pol.1983.180210815

出版商:John Wiley&Sons, Inc.    

年代:1983

数据来源: WILEY

摘要:

AbstractExperimental values of the dipole moment ratio 〈μ2〉/nm2and its temperature coefficientdln 〈μ2〉2/dT, where 〈μ2〉 is the mean‐square dipole moment of a chain withnskeletal bonds andm2is the mean square of the skeletal bond dipole moments, are reported for well‐characterized random copolymers of 3,3‐dimethyloxetane and tetrahydrofuran. The results are interpreted in terms of the rotational isomeric state theory in a manner consistent with that developed for the parent homopolymers. The theory gives a good account of the experimental results corresponding to copolymers in which the mole fraction of tetrahydrofuran lies in the range 0.11–0.89. It is found that whenever the copolymerization obeys Bernouillan statistics, the dipole moments are quite insensitive to the comonomer distribution. The theoretical analysis suggests, however, that the value of the dipole moment ratio of alternating copolymers of 3,3‐dimethyloxetane and tetrehydrofuran should be near that of the parent homop

ISSN:0098-1273    

DOI:10.1002/pol.1983.180210816

出版商:John Wiley&Sons, Inc.    

年代:1983

数据来源: WILEY

摘要:

AbstractLow‐frequency Raman spectra are reported for monodisperse methylene‐oxyethylene‐methylene triblock oligomers. Assignments are made to the longitudinal acoustical modes (LAMs) of the oxyethylene block and the methylene block. The results for LAM‐1 can be modeled by the vibrations of rods with perturbing forces and masses. The results for LAM‐3 are not so readil

ISSN:0098-1273    

DOI:10.1002/pol.1983.180210817

出版商:John Wiley&Sons, Inc.    

年代:1983

数据来源: WILEY

摘要:

AbstractA series of experiments has been carried out investigating several features of the surface growth method for observing longitudinal growth of polyethylene fibers in Couette geometry. Attempts to obtain limiting steady‐state takeup rates using a Teflon rotor were hampered by fiber breakage; however, maximum growth rates before breakage were found to be considerably higher than those observed in previous studies. Growth rates were also obtained using a static method, and for the Teflon rotor indicated above a critical concentration a linear growth rate equal to the stirrer velocity with rates essentially independent of temperature. With a silanized glass rotor, the same method gave much lower growth rates at comparable stirrer speeds and temperatures and showed a temperature dependence suggestive of a nucleation‐controlled mechanism. The implications of these results for other studies of the mechanisms of growth by the surface method are also discus

ISSN:0098-1273    

DOI:10.1002/pol.1983.180210818

出版商:John Wiley&Sons, Inc.    

年代:1983

数据来源: WILEY

摘要:

AbstractSorption of iodine by bulk polyacetylene was studied under various I2gas pressures at 25°C. The sorption dynamics show that the penetration of iodine into PA is not Fickian and the diffusion coefficient increases with time of sorption. A discontinuous increase in the sorption isotherm is observed atP/P0= 0.25 (Pis the pressure of the I2gas andP0is the saturation value at 25°C). It is due to iodine penetration into PA crystals, as evidenced by x‐ray analysis. The distribution of iodine within crystals is apparently inhomogeneous: some unit cells are changed into “iodine‐PA” cells, while others remain unchanged. The electrical conductivity depends not only on the amount of iodine but also on the I2gas pressure under which sorption is carried out. At given iodine content, the conductivity of a sample doped under higher I2pressure is greater than that of a sample doped under lower

ISSN:0098-1273    

DOI:10.1002/pol.1983.180210819

出版商:John Wiley&Sons, Inc.    

年代:1983

数据来源: WILEY

摘要:

AbstractUsing recently developed quantitative FTIR methods, thegaucheandtransconformer compositions have been measured for polyethylene terephthalate (PET) samples exposed to benzene and acetone. The kinetics of the primary and secondary isomerization stages of the solvent‐induced crystallization process have been studied and compared with the thermally crystallized system. The Arrhenius activation energies of isomerization in the primary stage in which Fickian diffusion behavior is followed for the benzene‐ and acetone‐induced PET systems are 8.2 ± 0.4 and 7.7 ± 0.4 kcal/mol, respe

ISSN:0098-1273    

DOI:10.1002/pol.1983.180210820

出版商:John Wiley&Sons, Inc.    

年代:1983

数据来源: WILEY