摘要:

AbstractThree approaches towards a fracture theory are discussed, namely the statistical, the continuum‐mechanical, and the micro‐morphological approach. The latter recognizes that (amorphous or semicrystalline) high‐polymer solids are composed of chain molecules which show a close range order and/or a superstructure. Information from morphological and spectroscopic investigations is utilized to characterize the different elementary molecular processes (slip of chains, segment rotation, chain scission, crystal deformation) which may lead to fracture initiation, crack growth, or yielding.The statistical variability of the elementary processes and the correlation of these processes with each other determine the statistical variability of the cumulative process,e.g., time to failure or fracture strength. Thus, a relation between the results of the micro‐morphological approach and the terms of the statistical approach (underlying distribution) is obtained. A mathematical study of the effect of these processes (network degradation, stress relaxation) on the expectation values of strength or time to failure is carried out for networks of different degrees of orientation.The results obtained for one‐phase networks are applied to critically stressed regions of heterogeneous polymers (amorphous regions in oriented fibers, plastically deformed zone at tip of growing crack). Thus, a correlation between continuum‐mechanical terms (stress intensity factor, surface energy) and molecular terms (network properties, activation energies) is attempted. A failure criterion

ISSN:0449-2994    

DOI:10.1002/polc.5070320105

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1971

数据来源: WILEY

摘要:

AbstractCrystalline polymers consist of regions of varying order ranging from perfectly crystalline to random and rubber‐like assemblies of polymer chains. Such structures may thus be represented in terms of statistical parameters characterizing the size and shape of ordered regions, degree of order and orientation of these regions. These parameters may be characterized by such techniques as x‐ray and light scattering. A study of the deformation of such polymers requires the specification of the change in these descriptive parameters with time, temperature, and morphology. The application of a number of optical techniques to this determination is discussed and typical results are presented. A deformation mechanism for crystalline polymers is proposed which is consistent with these observati

ISSN:0449-2994    

DOI:10.1002/polc.5070320106

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1971

数据来源: WILEY

摘要:

AbstractSemicrystalline, as well as amorphous, polymers frequently develop some measure of molecular mobility at temperatures well below their melting temperature, Tm, or their glass transition temperature, Tg. In these polymers, one or more damping or internal friction peaks are usually found to occur in the low temperature region from 4°K to 300°K. The magnitude, and temperature position, of these maxima are dependent upon the structure of the polymer, its mode of preparation and past history, and the local environment of the moving units. For purposes of discussion, the observed relaxation processes may be classified, on the basis of their temperature location at low measuring frequencies (∼1 Hz) as δ‐processes, those occurring below 80°K; γ‐processes, those occurring in the temperature range from 80°K to about 240°K; and β‐processes, those occurring from about 200°K to above 300°K. Various examples will be given of polymers that show significant δ, γ, and β‐processes and the influence of structure, structural substitution, crystallinity and low molecular weight diluents will be illustrated.The extent to which relaxation behavior can be correlated to macroscopic ductility and fracture toughness is discussed. It is noted that the lowest temperature δ‐relaxations arise primarily from reorientational and/or wagging motions of pendant methyl, ethyl, and phenyl side chains and, as such, have little bearing on macroscopic impact strength. The intermediate temperature γ‐processes arise from reorientational motions, of short side chain sequences or from local motion of main chain sequences. Some degree of main chain mobility. appears to be an essential requisite if ductility and toughness are to be realized. The higher temperature β‐processes in the 200°K t o 300°K range arise from motion of polar side groups, from motion of short chain sequences involving water complexes, or from large‐scale motions associated with the Tgof the amorphous regions. In the latter instance, the β‐process i s accompanied by a large increase in ductility and impact strength and this feature is utilized in polyblends and copolymers to raise the fracture t

ISSN:0449-2994    

DOI:10.1002/polc.5070320107

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1971

数据来源: WILEY

摘要:

AbstractIn the first part the microstructure of polymer melts and semicrystalline bulk material is treated on the basis of the bundle model and with molecular parameters. The meander‐shaped arrangement of the bundles in the amorphous phase can be founded thermo‐dynamically and leads to a radius of the meander of 50 Å. Based on this model a molecular theory of the relaxation processes (α, β, γ) is given and experimentally proved for linear polyethylene. In the last part both limiting cases of the deformation behavior are quantitatively derived in a molecular picture: the ideal plasticity illustrated by the yield stress of linear polyethylene and the ideal para‐elasticity illustrated by the stress‐strain curve of the drawn poly

ISSN:0449-2994    

DOI:10.1002/polc.5070320108

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1971

数据来源: WILEY

摘要:

AbstractIn order to study the relationship between structure and molecular motion, drawn polymers are more suitable than isotropic samples since their morphology can be characterized more easily by quantitative methods such as small angle X‐ray scattering (SAXS) and electron microscopy (EM). The effects of changes in morphological structure on molecular motion are demonstrated by comparing the results of the SAXS and EM measurements with the dynamic mechanical behavior of drawn polyethylene. In addition the NMR broad line spectra of these samples were analyzed and the influence of annealing conditions and external stress was investigate

ISSN:0449-2994    

DOI:10.1002/polc.5070320109

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1971

数据来源: WILEY

摘要:

AbstractStarting from a review of recent research on yield behavior, the author emphasizes the need to extend classical yield criteria in two ways. First, for iso‐tropic polymers, inclusion of a term allowing for the hydrostatic component of stress. Second, for oriented polymers, allowance for differences of behavior in tension and compression. The causes for these differences and progress in defining yield criteria by classical plasticity and by regarding yield behavior as an extension of nonlinear viscoelastic behavior are discusse

ISSN:0449-2994    

DOI:10.1002/polc.5070320110

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1971

数据来源: WILEY

摘要:

AbstractA variety of annealing treatments at elevated temperatures have been characterized by a specific infrared absorption assigned to the folded chain segments in polyethylene terephthalate fibers. The higher the annealing temperature, the more regular is the chain folding introduced into the fiber. When a fiber is free to contract, it will have a much larger amount of refolding than one which is held at constant length. Stretching is able to prevent refolding at low temperatures but in these experiments stretching was not able to prevent refolding at the highest temperature.By comparing the above infrared results with small angle X‐ray diffraction and broadline nuclear magnetic resonance techniques, it is now possible to give validity to the interpretation that chain folds are major contributors to the long period.The external tension applied to a fiber during an annealing treatment will determine the internal shrinkage tension caused by the molecules during subsequent reheating. If chain refolding is allowed to occur during annealing, the potential for subsequent shrinkage and the amount of internal shrinkage tension are lowered, and higher temperatures are required to activate additional shrinkage. It thus appears that a major contributor to shrinkage and shrinkage tension in an oriented fiber is the refolding of molecules during thermal activatio

ISSN:0449-2994    

DOI:10.1002/polc.5070320111

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1971

数据来源: WILEY

摘要:

AbstractApplication of a homogeneous stress or deformation field to a polymeric solid does not produce locally homogeneous response. Rather, the local material response frequently displays major variations in degree. For example, the phenomenon of craze and crack formation in glassy polymers and the formation of microvoids in stretched elastomers show clearly that the deformation field is highly inhomogeneous.In elastomers and glassy high polymers one may anticipate such inhomogeneous response arising from the macromolecular structure of the system. For example, consideration of nonuniform cross‐link density in an elastomer suggests that a considerable variation in chain extensions would exist throughout the sample. The ultimate properties of such a system would be expected to depend markedly on the more highly deformed components of the system. However, the description of the macroscopic sample in terms of molecular response would entail simultaneous treatment of both linear and nonlinear (Gaussian and non‐Gaussian) chain statistics.In order to describe the macroscopic, mechanical response of a polymeric network in terms of the behavior of its constituent elements, the macromolecules, it is necessary to develop a formalism for averaging both linear and nonlinear behavior. This paper is concerned with such a scheme. A minimum free energy approach to the network problem is presented which allows prediction of the molecular deformations without invoking the tenuous assumption that the molecular deformation field is a continuum deformation field.The analysis involves the formulation of integral equations describing the continuum stress and deformation tensors and the macroscopic work function, in terms of appropriate averages of the deformations experienced by the elements of the network. The elastic, but not necessarily linear, response of the network. The elastic, but not necessarily linear, response of the network is characterized by a state of minimum free energy. The elemental deformations are determined such that they minimize the macroscopic work function subject to the constraints imposed on this minimum by the macroscopic network deformations. The minimizing functions, or extremals, are obtained using the calculus of variations.This approach to the network problem provides a self‐consistent formalism for comparing various elemental behaviors and the resultant network response. The elemental deformations are functionals of the elemental potential (or strain energy) function. Thus, a different family of extremals is obtained for a particular non‐Gaussian chain than for a Gaussian chain. In this way, network stress‐strain behavior for a multitude of chain statistics can be examined using a single, self‐consistent scheme. The procedure can be applied to chains exhibiting both internal energy and entropy changes upon extension.The original derivation of this theory has been applied to a two‐dimensional, anisotropic network of Hookean and non‐Hookean fibers and is given elsewhere [1]. The three‐dimensional treatment of the elastomer network for both Gaussian and non‐Gaussian chains is developed fully elsewhere [2]and constitutes a complete presentation of the paper given

ISSN:0449-2994    

DOI:10.1002/polc.5070320112

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1971

数据来源: WILEY

摘要:

AbstractCriteria for failure in multiaxial states of stress have associated with them geometrical representations as surfaces in principal stress space. Any combination of stresses is safe if the corresponding point in principal stress space falls within the failure surface. The material will fail if the point lies on or outside of the surface. The geometry of some simple surfaces and means for determining them on isotropic bodies are discussed, and the scanty information on failure surfaces for polymeric materials is reviewed.

ISSN:0449-2994    

DOI:10.1002/polc.5070320113

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1971

数据来源: WILEY

摘要:

AbstractMacroscopic fracture is preceded by localized deformation and rupture processes that depend on the state of combined stress (i.e., on the stress state) and other test conditions as well as on polymer structure and morphology. To illustrate the effect of stress state on the mode of fracture initiation and crack growth, data obtained on elastomers in triaxial tension are reviewed along with those for an unfilled styrenebutadiene rubber vulcanizate in simple tension, equal biaxial tension, and under the biaxial tensile conditions that give a pure shear deformation. The behavior of plastics, including the dependence of the strength of a notched specimen on its thickness, is briefly considered.

ISSN:0449-2994    

DOI:10.1002/polc.5070320114

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1971

数据来源: WILEY