摘要:

The side-chain molecular motion of the α helix, ω helix, and β form of copoly(butyl-L-aspartate-benzyl-L-aspartate) with degrees of butylation 0%, 25%, and 43% was investigated by means of dielectric relaxation. Copoly(butyl-L-aspartate-benzyl-L-aspartate) with butylation 0% and 25% has a left-handed α helix, and, on the contrary, that of the 43% copolymer is a right-handed α helix. These two a helices transformed into the left-handed ω helix, with stacking between the benzyl groups, or between benzyl and butyl groups, or between butyl groups when heated to 140°C. The dielectric constant decreased at the α-ω transition temperature, irrespective of frequency. In particular, a remarkable decrease of dielectric constant was observed to accompany the reversal in helix sense. The ω-β transformation was observed at about 180°C by further heating, accompanied by an abrupt increase in dielectric constant. The side-chain dispersion temperature was in the following order: T(ω) ≲ T(β) < T(α). Relatively free rotation of the side chain for the ω helix is possible, although the side-chain molecular motion, the side-chain dispersion, is restricted by the stacking of the side chain in the ω helix. The side-chain molecular motion of the α helix, rotation of the benzene ring, is hindered by the presence of δ O and ϵ CH2atoms of the residue above and below the benzene ring.

ISSN:0022-2348    

DOI:10.1080/00222348308245746

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

PMMA doped with fluorescent dyes presents important modifications of thermally stimulated current spectra at high temperature. Glass and liquid-liquid transitions are no longer resolved and only one intense peak appears at 127°C. This peak cannot be resolved by the thermal sampling method. It has an Arrhenius behavior, and the plot of τ(1/T) shows the existence of a compensation phenomenon with a characteristic temperature which does not depend on the dye concentration. The modification is induced even if the dye concentration is as weak as some 10s of ppm. Thus it cannot be considered as a plasticization effect.

ISSN:0022-2348    

DOI:10.1080/00222348308245747

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

Thermally stimulated currents (TSC) have been measured in several samples of poly(vinyl chloride) differing in tacticity and molecular weight as a result of polymerizing them at different temperatures. This has allowed us to characterize the relaxation behavior of PVC. No dielectric relaxation can be observed by this experimental technique at temperatures between liquid helium and liquid nitrogen. The β relaxation is observed around 173°K, with similar parameters in all samples studied. Around the glass transition the relaxation times isolated in the α peak follow a compensation law. Molecular weight and tacticity have a strong influence on the temperature of the maximum and the intensity of this relaxation, respectively.

ISSN:0022-2348    

DOI:10.1080/00222348308245748

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

The transient morphological development of blends of sytrene-butadiene rubber (SBR) and cis-polybutadiene (BR) is investigated as a function of the viscosity of the individual rubbers and also the energy input (using a power integrator) during the mixing process. The gum rubber blends consist of SBR:BR = 80:20 and 20:80 by weight ratio. The ebonite technique is used to prepare specimens for electron microscopic (EM) morphological study. The EM results reveal that these 80:20 and 20:80 weight ratio blends always consist of two phases. The minor component always forms the dispersed phase throughout the major component matrix. The viscosity of each blend, as well as the energy input during the mixing operation, strongly governs the resultant blend morphology. In the initial mixing stages, the domain sizes of the dispersed phase vary over a broad range. As more energy is introduced to the system, the range of domain

ISSN:0022-2348    

DOI:10.1080/00222348308245749

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

Diffusion and sorption of cyclohexane vapor in styrenebutadiene copolymers were used to study the morphology of an SBS triblock copolymer (29.3 wt% styrene) cast from different solvents. The weight gains or losses of this SBS and several SBR (5, 23, 45, and 85 wt% S content) samples were continuously monitored at 40°C and at penetrant pressures of 30, 60, 90, and 120 Torr. Significant differences exist in the equilibrium sorption among the different samples. Possible reasons for this observation are explored. The one corroborated by dynamic mechanical measurements attributes the differences to varying interphase volume fractions. The interphase region separating the PB and PS phases in SBS was assumed to be a continuous profile of equivalent SBR with different S contents. Experimental results, coupled with simple calculations, indicated that the overall interphase volume fractions in the SBS are 0.28 for the MEK/THF-cast (1:9 volume ratio of MEK and THF) sample, 0.15 for the toluene-cast sample, and 0.06 for the cyclohexane-cast sample. Important observations consistent with these results are: (1) the existence of a relative minimum (at around 20–30 wt% S content) in the plot of the equilibrium sorption of cyclohexane vs S content in rubbery SBR; and (2) a relative maximum in the plot of the mutual diffusion coefficient of cyclohexane in SBR vs the concentration of cyclohexane in SBR.

ISSN:0022-2348    

DOI:10.1080/00222348308245750

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

Structure studies on the segmented polyurethane-urea based on poly(tetramethylene glycol), 4,4'-diphenylmethane diisocyanate, and 4,4'-diaminodiphenylmethane have been made using wide- and small-angle x-ray diffraction. It was found that the hard segments have a well-defined crystal structure indexed by a monoclinic unit cell with dimensions a = 4.72 Å, b = 11.33 Å, c (chain axis) = 11.64 Å, and γ = 116.5°. Further, the hard segments have an anisotropic supermolecular structure, that is, a spherulitic crystalline texture. The microdomain structure depends on the molecular weight of the soft segments, with phase separation becoming distinct with an increase in the molecular weight of the soft segment. Based on the long period observed in small-angle x-ray patterns of heat-set specimens, the thickness of lamellae was estimated to be about 93 Å. This corresponds to an average hard segment sequence length of 8. On deformation, the hard segment orients preferably transverse to the stretch direction up to high elongations, and it shows a hysteresis of orientation. This phenomenon can be explained in terms of the anisotropic supermolecular structure. Further, deviation from ideal rubber elasticity observed in the stress-strain relationships of the segmented polyure-thane-urea elastomers might also be caused by the anisotropic structure.

ISSN:0022-2348    

DOI:10.1080/00222348308245751

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

The structure and morphology of vinylidene chloridemethyl acrylate copolymers were studied using small-angle and wide-angle x-ray scattering. Long period increased with comonomer concentration for compressionmolded films crystallized at the same temperature. An increase in the “a” dimension of the unit cell was noted for the “as polymerized” powder as comonomer content increased. Sequence distribution information obtained from the instantaneous copolymer composition equation agreed well with the observed powder data. No such trend was observed for melt-crystallized films, possibly because of a greater restriction on crystallization and a lower level of crystallinity within these samples.

ISSN:0022-2348    

DOI:10.1080/00222348308245752

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

The thermal behavior of random copolymers of vinylidene chloride-methyl acrylate (VDC-MA) was monitored with differential scanning calorimetry. The effects of composition, annealing, and heating rate on the copolymer melting process were studied. Increasing levels of comonomer decreased the crystalline melting point and percent crystallinity of the copolymers. A combination of information obtained from heating rate and small-angle x-ray scattering studies led to a model of the melting behavior of PVDC and its copolymers. Apparently a simultaneous melting/recrystallization phenomenon occurs in these materials during heating.

ISSN:0022-2348    

DOI:10.1080/00222348308245753

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

Studies on the structure and properties of segmented polyurethane-ureas having binary hard segment components have been made using infrared dichroism, small-angle x-ray diffraction, and mechanical testing. The results clarify the effect of the domain structure of the hard segments on the mechanical hysteresis behavior of the segmented polyurethane—urea elastomers. Experiments were carried out for film specimens prepared from poly(tetramethylene glycol), 4, 4'-diphenylmethane diisocyanate, and chain extenders such as ethylenediamine and hydrazine. The domain structures of the hard segments were controlled by blending segmented polyurethane-ureas having different chain extenders. The chain-extending reactions were conducted both simultaneously and in a stepwise manner during polymerization using two different chain extenders. The experimental results revealed that the mechanical hysteresis greatly decreased for the specimens of segmented polyurethane-ureas with binary component hard segments which were randomly copolymerized using the mixture of two different chain extenders. It is suggested that the decrease of hysteresis is related to the microdomain structure of segmented polyurethane-ureas composed of small fragments of lamellae. The small lateral size is caused by the restriction of crystal growth in the direction of hydrogen bond formation due to the heterogeneous hard segment components. This hysteresis phenomenon could be verified by the fact that the transverse orientation of the hard segments decreased in the case of the segmented polyurethane-ureas having randomly copolymerized hard segments. To impart the property of ideal rubber elasticity to segmented polyurethane-urea, it is deduced that it is important to control the hard segment domain structure so that it does not form higher-order structures such as lamellae of large lateral size.

ISSN:0022-2348    

DOI:10.1080/00222348308245754

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

Recurrence equations are proposed to calculate Cn, the total number of configurations, andxn, the mean x projection of the end-to-end separation, for regularly alternating copolymers confined to strips and capillaries. We suppose that the monomer units are incompatible in solution and investigate the asymptotic behavior ofxnaccording to the solubility of the constituent monomers.

ISSN:0022-2348    

DOI:10.1080/00222348308245755

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor