Chain scission and plastic deformation in the strained crystalline polymer

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Abstract
Depending on temperature and load rate an unoriented polymer solid under sufficiently large applied stress either deforms plastically (high temperature, low rate of loading) or undergoes brittle fracture (low temperature, high rate of loading). The fracture proceeds through the areas of minimum strength, i.e., along the boundaries between adjacent spherulites and between parallel lamellae. Very few chains, i.e., interlamella tie molecules and molecules bridging the crack if a crystal lamella is broken, are ruptured during this process. Hence the number of radicals per cm2 of new surface is small, about 1013/cm2.The plastic deformation gradually transforms the sample into the extremely well oriented fiber structure of much higher elastic modulus and strength but of smaller strain to break. The basic element of the fiber structure is the long and narrow microfibril formed by micronecking of the crystal lamella. The microfibril consists of fully oriented folded chain blocks connected by a great many tie molecules obtained by partial chain unfolding in the micronecks. Their number per amorphous layer increases with the draw ratio and so does the tensile strength of the microfibril.Under load the fiber structure breaks at very small strain. The fracture proceeds through the weakest elements of the structure, i.e., through the boundary between adjacent microfibrils and the amorphous layers between subsequent blocks of the microfibrils. With axial stress the fracture must cut at least one full cross section of the fiber and hence rupture about 1014 intrafibrillar tie molecules per cm2. ESR experiments show long before the final break a much higher number of radicals throughout the whole strained sample upto 1017/cm2. The number of broken chains depends on strain and not on stress. One imagines that at every applied strain one breaks the most strained tie molecules which as a consequence of stress concentration have to carry most of the load. The stress concentration caused by the special morphology of the microfibrils and tie molecules reduces the tensile strength of the polymer far below the theoreticall limit calculated for uniform stress field even if one considers that the number of chains passing through the fracture plane is only a fraction of the maximum number calculated with all macromolecules extended and perfectly aligned.
Funding Information
  • Camille and Henry Dreyfus Foundation

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