摘要:

AbstractThe previous description1 of highly oriented films of poly(p‐phenylene vinylene) is augmented by close packing energy calculations and quantitative analysis of layer line intensity profiles observed via electron diffraction. The chain axial translational disorder is of the paracrystalline type: partial registration of chains occurs within the (100) layers while the layers themselves are randomly shifted from one another along the chain directio

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270301

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractTwo techniques are described for measuring the shear viscosities of thin liquid films confined between two surfaces. Both techniques employ the surface forces apparatus, which has already been used extensively to measure the static interactions between surfaces in liquids. With either of the new dynamic techniques, shear viscosities of the confined liquids can be measured as a function of film thickness with a precision of about 0.1 nm (1 Å). The techniques complement each other: one is used to best advantage at high shear rates, the other at low shear rates. Results are presented for measurements made on low‐molecular‐weight polymer melts of polybutadienes. At low shear rates, these results provide detailed information on the relation between polymer molecular weight, the conformation of polymer molecules at the surfaces, the intermolecular forces between the surfaces, and the location of the shear plane (plane of no‐slip). At high shear rates, the results suggest the gradual evolution of non‐Newtonian behavior in submicron liqu

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270302

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractWe have used NMRT2relaxation and pulsed‐gradient spin‐echo diffusion techniques to study properties of Arco R45M hydroxyl‐terminated polybutadiene, either unfilled or containing 65 wt.% filler particles (SiO2, NaCl, Al) and cured with isophorone diisocyanate (IPDI), as functions of IPDI content. A shortT2relaxation component arises from the network (gel) whose amount is greatest (up to 92%) near NCO/OH stoichiometry. Gel fraction and stoichiometry concentration both are affected slightly by filler surface reactivity but principally by filler particle size. The diffusion rate of the nonnetwork (sol) molecules has a range of 1–2 orders of magnitude. This range is narrowest near stoichiometry for the smallest filler (SiO2), i.e., the situation in which the sol molecules are least mobile. Branching theory and the hypothesis of a layer of reduced mobility in a wide vicinity of the filler particles provides semiquantitative explanations of these obser

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270303

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractThree series of amorphous semicrystalline poly(styrene‐b‐ε‐caprolactone)s have been synthesized with polystyrene blocks of 6000 (series A), 40000 (series B), and 70000 (series C) molecular weight, respectively. In these materials, the polymer miscibility evolves from a situation where a diffuse interphase involves the major part of the volume of the copolymer (series A) to a sharp phase separation as observed for copolymers with the longest PS block (series C). The crystallization of PCL blocks is mainly governed by the phase morphology. In copolymers of series A, the crystallization rate of PCL blocks is slowed down the more as the miscibility with PS increases, and ultimately the degree of crystallinityXcdecreases significantly. When phase separation is sharp,Xcchanges dramatically at the phase inversion and decreases when PS forms the continuous phase. At the inversionXcdepends on the mean size of the PCL microdomains as compared with the thickness of the crystalline lamellae. The periodicity of the phase morphology as observed by TEM is influenced by the solvent used in casting films, whereas monolamellar monocrystals can be obtained by a self‐seeding t

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270304

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractThree series of polyurethanes, based on three polyols, diphenylmethane diisocyanate (MDI), and three chain extenders were synthesized. Polypropylene glycol (PPG) soft‐segment length (MW 1000, 2000, and 3000), soft‐segment concentration (30%, 50%, and 70%), and the type of chain extender (ethylene glycol, butane diol, and hexane diol) were varied and their effect on the amount of phase separation studied. Methods for assessing phase separation quantitatively, based on shifts of the glass transition temperatureTgand the enthalpy jump at the glass transition were tested. It was shown that they give incorrect results, especially with PPG 1000 as the soft segment. Dependence of the soft segmentTgon the polyol length was explained by the “network effect.” True phase mixing was found only with PPG 1000 series at low soft‐segment concentration, whereas, no clear indication of the phase mixing with PPG 2000 and PPG 3000 based polyurethanes was

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270305

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractStudies of lamellar shapes and profiles in linear polyethylene have often been implemented by microscopic examination of appropriately etched random slices through spherulitic specimens. However, since a spherulite crystallized from a bulk specimen is a three‐dimensional assembly of radially oriented lamellae with a twisting orientation (except for very highTcs), it may be difficult to draw appropriate conclusions from a random slice. A technique has been developed that allows one to prepare spherulitic surfaces such that their distance relative to the geometric center can be well characterized. A computer‐based graphical representation is also presented which suggests that the projection of lamellae on such surfaces is adequately represented by a radially oriented assembly of helicoidally twisted lamellae. Based on the agreement between the experimental results and the computer model, it is suggested that the C and S shapes previously observed by other researchers are not necessarily intrinsic to the lamellar profile but may arise due to geometrical effects as the lameliae project onto a surface at various angles. From these results it is also evident that in ringed spherulites lamellae undergo continuous twisting rather than successive misalignment of essentially untwisted segme

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270306

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractHard elastic monofilaments from polypropylene (PP) and its blends with polyethylene (PE) have been studied. Filaments with sharply different degrees of hard elasticity have been obtained by the same melt‐spinning process from three types of PP resin with almost the same isotacticity, molecular weight, and melt index and from blends of one of the PP resin with PE or high‐molecular weight PP (HMPP). With addition of a small amount of HDPE or HMPP to the parent PP resin, the hard elasticity of the filaments is increased significantly. Filaments with high hard elasticity can be prepared from PP resins of blends which show a relatively slow spherulitic growth rate. It is argued that the increased hard elasticity is due to row lamellae in PP fibers containing more adjacent folded chains and fewer tie molecules between lamellae and chain entanglements in the amorphous ph

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270307

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractA model for case II diffusion into polymers is presented. The addition of stress terms to the Fickian flux is used to produce the characteristics progressive front. The stress in turn obeys a concentration‐dependent evolution equation. The model equations are analyzed in the limit of small diffusivity for the problem of penetration into a semiinfinite medium. Provided that the coefficient functions obey two monotonicity conditions, the solvent concentration profile is shown to have a steep front that progresses into the medium. The formulas governing the progression of the front are developed. After the front decays away, the long time behavior of the solution is shown to be a similarity solution as in Fickian diffusion. Two techniques for approximating the solvent concentration and the front position are presented. The first approximation method is a series expansion; formulas are given for the initial speed and deceleration of the front. The second approximation method uses a portion of the long time similarity solution to represent the short time solution behind the fron

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270308

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractA dual‐mode transport treatment for glassy polymer‐gas systems is presented in which due account is taken of random‐walk topology. This permits elimination of the restrictive homogeneous random‐walk condition, which must hold if the existing simple dual‐mode transport model is to be considered rigorous. The results obtained from the new treatment indicate that the aforesaid simple model requires only minimal modification, as far as transport in the solution (Henry's law) mode is concerned. The simple model description of adsorption (Langmuir) mode transport, on the other hand, is shown to be subject to minor or major limitations and qualifications, according to whether the mobility of adsorbed molecules is, respectively, comparable to/lower than, or substantially higher than, that required by the homogeneous random‐walk condition. The present treatment permits reasonable semiquantitative interpretation, within the context of current dual‐mode sorption theory, of the transport behavior typically observed experimentally. The results indicate that experimental adsorption mode diffusion coefficients, as currently calculated, cannot be considered to be quantitative

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270309

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractThe structure and thermal behavior of freeze‐dried gels of radiation‐crosslinked high molecular weight poly(ethylene oxide) (PEO) were investigated by optical and electron microscopy, wide‐angle x‐ray scattering (WAXS), DTA, TGA, and thermomechanical analysis. The gels are highly porous with thin crystalline walls. Small spherulite and hedrite structures are observed on the walls. A model for gel formation in solution is suggested. A statistically homogenous chemical network is formed as a result of intrachain and interchain crosslinking. Simultaneous grafting of macromolecular fragments formed by chain scission also occurs.On increasing the irradiation dose from 1 to 15 Mrad, the degree of crystallinity determined by x‐ray diffraction and the total intensity of diffraction gradually decrease. The temperature and enthalpy of melting diminish steeply up to 5 Mrad, fall only slightly from 5 to 8 Mrad, and do not change from 8 to 15 Mrad. By comparing the x‐ray and DTA crystallinity values, this is shown to be due not only to reduced crystallinity at higher network density but also to Tree energy changes of entropic origin in crystalline and amorphous regions. Radiation chemical yields,G(‐units), for these dose ranges are 100, 38, and 0, respectively.Thermomechanical analysis was used to determine the elastic modulus of compression as a function of the dose absorbed, and the average molecular weight$ \bar M_{\rm c} $of network chains was estimated.$ \bar M_{\rm c} $decreases with doses up to 10 Mrad and does not change with furthe

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270310

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY