JOURNAL OF MATERIALS SCIENCE 21 (1986) 3231-3236 Yield strength and anelastic limit of amorphous ductile polymers Part 1 Amorphous structure and deformation Z. H. STACHURSKI Department of Materials Engineering, Monash University, Melbourne 3168, Australia The microstructure of amorphous polyethylene below its glass transition temperature is described in some detail. The dimensions, shape and statistical mechanics of a polyethylene chain are already well understood. The packing of such molecular chains is less understood and it is considered here in terms of a CH2 pair distribution function. The pair distribution function is derived on the basis of (i) the variation of specific volume with temperature for completely amorphous and ideally crystalline polyethylene, and (ii) randomness of packing of the mole- cular chains. A scheme for the description of deformation of amorphous polymers is proposed. Points of constriction along the molecular chain are defined in terms of variation of cross- sectional area of the molecular tube. During deformation the points of constriction are convected with the body of the polymer. However, the deformation of the chain segment between the points of constriction is analysed in terms of kinematics of chain linkages. 1. Introduction The process of yield involves breaking and recon- stituting atomic bonds. In order to be able to say anything sensible about this process it is necessary to understand; (i) the amorphous microstructure of the material on a molecular level, and (ii) the existing hierarchy of the atomic bonds. A detailed knowledge of the amorphous micro- structure in polymers below the glass transition tem- perature is not generally available. Perhaps the best described microstructure of a glassy polymer is that of atactic polystyrene [1]. Some, albeit incomplete, knowledge about other amorphous polymers also exists [2, 3]. In this paper polyethylene is chosen as the model material of an amorphous polymer for the description of the yield process below the glass tran- sition temperature. Since a completely amorphous polyethylene is difficult to obtain experimentally below its glass transition, this choice requires some explanation and justification. For the purpose of the model of yield presented in this paper the most desirable feature in the polymer is the simplicity and uniformity of its molecular struc- ture. This is exemplified by linear polymers with the most basic repeat units such as polyethylene, poly- tetrafluoroethylene, amorphous sulphur, selenium, and others. Of these, polyethylene is the most thor- oughly studied polymer; i.e. the type and strength of the atomic interactions is well established [4-11]. The statistical conformations of the polyethylene chain have been worked out theoretically [12], and confir- med experimentally [13, 14]. The relevant physical properties, thermodynamic data and the glass tran- sition temperature are known [15, 16]. On the basis of all this information it is possible to describe and vis- ualize the amorphous structure of polyethylene below its glass transition temperature with a high degree of confidence. Thus point (i), as stated in the first para- graph, can be satisfied. As regards point (ii), Wunderlich [16] lists the energy associated with chemical bonds in polymers in the following general order: I. Covalent bonds (order of magnitude 420 kJ mol- of bonds). 2. The next biggest contribution comes from the change of potential energy on rotation (approximately 4 to 50 kJ mol- l). 3. The next contribution is from interactions due to hydrogen bonds or dipoles (usually between 4 and 40 kJ mol- l ). 4. Finally the van der Waals dispersion bonds (around 0.4 to 2 kJ mol l). Thus the first manifestations of mechanical yield in polyethylene (in which bonds of type 3 are absent) should occur at stress levels which are just sufficient to break the weakest type, i.e. van der Waals interchain bonds. However, it is important to point out that with increasing deformation strain-hardening due to entanglement may take place. This will increase the stress level, and consequently cause breaking of the next weakest bond, i.e. rotation around the covalent C-C bond [17]. Several molecular theories for yield in amorphous polymers have been proposed [18-22] and are reviewed in Part 2 [23]. In contrast to these theories the model presented in Part 2 of this publication places the res- ponsibility for the onset of yield in amorphous poly- ethylene on the breaking of the interchain van der Waals bonds [23]. There seem to be two general argu- ments supporting this view. Firstly the interchain bonds are the weakest of all 0022-2461/86 $03.00 + .12 9 1986 Chapman and Hall Ltd. 3231