摘要:

A number of techniques have been used to study structural organization and transitions in polyether- and polyester-based urethane elastomers. Scanning thermal methods reveal three characteristic transitions common to samples of varying composition, namely, the major glass transition and two higher transitions believed due to disruption of different types of hydrogen bonding. Other transitions are assigned to melting of ester and of urethane crystallites. Light-scattering measurements on turbid polyether-based samples indicate both isotropic and anisotropic contributions to scattered intensity, the latter reflecting structural ordering on a small scale. X-Ray diffraction studies of stress-induced crystallization in the polyetherbased elastomer show reflections attributed to crystallites of polyether segments. It is concluded that separation into a domain structure occurs in both types of elastomers, but to a higher degree in the polyether-based polymers than in the polyester-based polymers, possibly due to restrictions in the latter imposed by interaction of the ester and urethane groups.

ISSN:0022-2348    

DOI:10.1080/00222346808212458

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

An X-ray study has been applied to clarify the relationship between crystal structures and crystal orientations of tetraoxymethylene and polyoxymethylene obtained by solid- state polymerization of tetraoxymethylene. The polyoxymethylene crystal obtained by the solidstate polymerization of a tetraoxymethylene single crystal was found to be definitely oriented with respect to the original tetraoxymethylene single crystal without any aftertreatment; i.e., the polymer chain (the c-axis) is parallel to the b-axis of tetraoxymethylene and the a-axis of polyoxymethylene coincides to the c-axis of tetraoxymethylene.

ISSN:0022-2348    

DOI:10.1080/00222346808212459

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

Considerations based on the Maron theory show that for solutions of an amorphous polymer in a solvent the values of A obtained from heats of solution, heats of dilution, osmotic pressure, and vapor pressure should all be identical. Here λ = (aWaT)2, where μ is the Maron theory interaction parameter, T the temperature, and v2 the volume fraction. On the other hand, for solutions of a crystalline polymer in a solvent the A's obtained from heats of solution should always differ from those given by the other measurements. The difference is due to the heat of fusion involved in melting the crystalline portion of the polymer. A method is presented by means of which this heat of fusion can be found from the observed heats of solution, and this heat of fusion can either be used as a relative measure of crystallinity, or it can be converted to percent crystallinity when the heat of fusion of the completely crystalline polymer is known. The method described is very precise, and highly sensitive to low degrees of order.

ISSN:0022-2348    

DOI:10.1080/00222346808212460

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

The growth mechanism and crystalline texture of solution-grown crystals of poly(vinyl alcohol) have been studied by electron microscopy. From morphological data it is shown that single crystals of poly(vinyl alcohol) (PVA) have two growth faces, i.e., the (100) and (101) fold planes. It is suggested that the parallelogrammic single crystals grow from the center by molecular folds. The rate of addition on the (100) plane is about three times larger than that on the (101) plane. Twinning of the PVA crystals takes place at the (100), (001), (101), and (101) planes at the time of the nucleation or during the growth. The single crystal and twins of PVA are corrugated lamellae. Granulated structure is observed on the surface in a figure similar to the external form of the crystals. Dark-field micrographs show that the PVA single crystals possess a mosaic structure with the arrangement of the blocks deviating slightly from parallel alignment in the lamella. The reactivities of single crystals to various aldehydes indicate that the single crystals contain crystal lattice defects.

ISSN:0022-2348    

DOI:10.1080/00222346808212461

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

Experimental GPC calibration data for polystyrene, poly(methyl methacrylate), and poly(dimethyl siloxane) in chloroform is reported. These polymers have similar hydrodynamic volumes, since their unperturbed dimensions and extensions in chloroform, due to polymersolvent interaction, are very similar. As a result, the same molecular weight calibration curve is obtained for the three polymers. Recently published GPC data is examined on a plot of logarithm of unperturbed end-to-end distance against elution volume. This procedure gives a universal calibration for linear polymer molecules, providing the column combination, solvent, and temperature do not change. For the data of Meyerhoff, use of the unperturbed dimensions is superior to the plot of η suggested by Grubisic, Rempp, and Benoit. A possible explanation, based on a model for the polystyrene gel pores, is proposed for the use of the unperturbed dimensions.

ISSN:0022-2348    

DOI:10.1080/00222346808212462

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

Small-angle X-ray scattering is shown to be an effective technique, complementary to mechanical and thermal methods, for the study of structural ordering in polyurethane elastomers. Wide variations in intensity were found for different sample compositions and are interpreted in terms of the degree of hard segment aggregation. The results support previous conclusions regarding the organization in these samples based on analysis by scanning thermal methods. The polyether polyurethanes studied showed a higher degree of organization than the equivalent polyesters, and longer urethane segments enhance the extent of domain structure. Studies of the temperature dependence of scattering strengthen the previous assignment of a transition at about 150°C to the disordering of domain structure.

ISSN:0022-2348    

DOI:10.1080/00222346808212463

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

It was found experimentally that bilayered polyethylene single crystals with an orientational misfit angle larger than the critical angle, θ*, show moire pattern and only those with an angle smaller than θ* show the interlamella dislocation network. The intermediate pattern, which could neither be classified into the typical moire pattern nor the typical interlamella dislocation network, was found in the vicinity of θ*. Criteria for discrimination between the moire pattern and the interlamella dislocation network are discussed. The crystal with a misfit angle a little larger than θ* also comes to show the feature of the dislocation network during long storage of the crystal in the mother solution at the crystallization temperature. θ* is, therefore, a function of the time of storage. A mechanism is presented such that crystal lattices near the interfacial boundary are distorted to form the dislocation network by intermolecular force between the overlying crystals when the misfit angle is smaller than θ*.

ISSN:0022-2348    

DOI:10.1080/00222346808212464

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

Wool and other a-keratin fibers are known to consist mainly of aligned crystalline fibrils embedded in a less ordered matrix. The fibrils contain a large proportion of protein chains folded in the a-helical configuration. Under an external stress the fibril is capable of transforming to a second thermodynamic phase in which the protein chains are nearly fully extended into the pleated-sheet crystalline structure of β-keratin. This transformation involves local extensions in the fibril of the order of 100%. A constant stress, known as the equilibrium stress, is necessary to maintain the thermodynamic equilibrium between the two phases in the one fibril and this stress is independent of the relative quantities of each phase present. However, the concept is developed that a stress greater than the equilibrium stress is required to nucleate the extended β form from the folded a-form. In the composite fibril-matrix system undergoing extension, the matrix surrounding portions of the fibril which have transformed to the extended phase will be subjected to large complex strains. The resultant behavior of the system is studied by analyzing a simple mathematical model. With reasonable values of the parameters inserted, forward and reverse stress-strain curves have been obtained which compare qualitatively with the stressstrain behavior of wet wool fibers. The theory developed, by suitable adjustment of the parameters, can be applied to other composite systems consisting of aligned fibrils in a matrix.

ISSN:0022-2348    

DOI:10.1080/00222346808212465

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

The mechanical behavior of a system of parallel fibrils, capable of a phase change and embedded in a matrix, is analyzed by considering a simplified model. The model consists of a single infinitely long isolated fibril together with its associated matrix. Furthermore, it is assumed that slippage cannot occur at the fibril-matrix interface but that the matrix deforms in both tension and in shear. The phase change occurring in the fibrils gives rise to large discontinuous local strains in the fibril and is characterized by a “critical stress” and an “equilibrium stress.” Analysis of the model shows that distinct zones of the secondary phase will form consecutively along the fibril length when the system is extended. These zones are mobile and the stress fields in the matrix associated with each zone result in the mutual repulsion of adjacent zones. Stress-strain curves for both extension and recovery are calculated using linear and nonlinear forms of the system parameters. It is suggested that the model studied is relevant to the mechanical behavior of keratin fibers.

ISSN:0022-2348    

DOI:10.1080/00222346808212466

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

The Maron theory was used to deduce from mostly osmotic pressure and light-scattering data the interaction parameters as a function of concentration, temperature, and polymer molecular weight for solutions of polystyrene in seven different solvents. These interaction parameters were then utilized to calculate the activity coefficients of the solvent in more concentrated solutions for comparison with experimental values deduced from vapor pressure measurements. When order was absent in the polymers (all cases but two), the agreement between calculated and observed activity coefficients was found to be very good. In the two cases where order was present, the activity coefficients observed in the concentrated solutions were higher than those calculated. These deviations were employed to find the degree of polymer order both in solution and in the original polymers.

ISSN:0022-2348    

DOI:10.1080/00222346808212467

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor