Molecular Theory of Strength
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MOLECULAR THEORY OF STRENGTH
HENNING H. KAUSCH Polymer Laboratory, Swiss Federal Inst. of Technology, 32, chemin de Bellerive CH-1007 Lausanne ABSTRACT Polymer strength depends on the nature of the constituting elements and their mechanical, thermal, and otherwise physical, but also chemical interaction. Some of these mechanisms are discussed in this paper; special attention is given to disentanglement and chain scission. INTRODUCTION As opposed to the continuum mechanical theories, "molecular" theories of fracture recognize the presence of discrete particles or elements forming the material body. These theories intend to relate breakage, displacement and reformation of these elements with deformation, defect development and fracture of the structured material [1]. Some of these theories have in common the assumption that macroscopic failure is a rate process, that the basic fracture events are controlled by thermally activated breakages of secondary and/or primary bonds, and that the accumulation of these events leads to crack formation and/or breakdown of the loaded sample. These basic events are sometimes defined in a general manner ("damage") and not experimentally related to particular morphological changes or to the existence of (micro)-cracks. As opposed to these "morphological" theories, others are based on the physical evidence of molecular damage obtained from spectroscopic and/or different scattering methods. Strength then can be defined as that stress which activates one (or
more) of the above mechanisms at such a rate so that failure of a specimen occurs within the specified time-scale. Two important polymer characteristics must be taken into account :molecular anisotropy and sample heterogeneity. On a molecular level (1-10 nm) all polymers are strongly anisotropic and two basically different failure phenomena can occur : chain slippage and chain scission. The stress level at which local failure occurs varies tremendously from a few MPa (for the slip of adjacent segments in an elastomer) to 10-20 GPa (for stress-induced chain scission). The presence of crystal lamellae (5-50 nm), elastomeric or mineral fillers and defects (0.2-20 pmu)within a polymer material causes stress and strain concentration, influences the rate of straining, the mode of crack initiation and propagation and, consequently, the strength (failure load) and toughness (energy consumption during failure) of the material. The strength of a sample will be the higher, the more homogeneously in space and time, stresses can be transferred onto the molecular chains. In all molecular theories of strength, a polymer is modelled as an ensemble of elements interacting with each other and/or with the environment. The interaction is first of all mechanical (the applied load causes plastic deformation, craze initiation, flow, or chain scission),
Mat. Res. Soc. Symp- Proc. Vol. 79.
1987 Materials Research Society
380
but also chemical (oxidation, radical reactions), thermal (local concentration of dissipated energy) or otherwise physical (e.g
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