Page 307 - Fiber Fracture

FRACTURE OF SYNTHETIC POLYMER FIBERS 289

INTRODUCTION

The achievement of high mechanical stiffness and strength from flexible and linear
commodity polymers has received extensive investigation over the last 20 years (Kinloch
and Young, 1983; Ward, 1983). Tensile drawing of polyethylene fibers to very high draw
ratios has allowed one to produce fibers with Young moduli above 100 GPa. In view of
the obvious commercial interest in these materials, it is of primary importance to have a
detailed knowledge of the factors controlling the tensile deformation and failure of solid
flexible polymers.
Several models have been proposed for describing the orientation and morphological
changes that occur during deformation of polymer fibers (Ward, 1983; Kausch, 1987).
All these approaches are, however, essentially phenomenological or semi-empirical
descriptions which provide no fundamental understanding of the phenomena occurring
at the molecular level. In the present paper, we wish to present a more comprehensive
approach which allows a unified description of polymer deformation and fracture
encompassing all the effects of molecular weight, molecular weight distribution, defects,
entanglement density, etc. . -
Before describing our approach, it is important to briefly describe how fibers are
being processed in industry. Immediately after polymerization, polymer chains are in
a random coil configuration and they can be compared to an agglomerate of ‘cooked
spaghetti’, see Fig. la. The mechanical properties of these systems are extremely poor
as any applied load is carried essentially by the weak attractive bonds between chains
with little contribution from the strong chain backbone.
For that reason, these agglomerates are further processed by drawing into a fiber form
wherein the polymer chains are now perfectly ordered and extended along the fiber axis,
see Fig. 1 b. In such a configuration, the strong covalent backbone chains play a crucial
role and tensile mechanical properties are optimized. Experiments clearly indicate that
the higher the draw applied to the macromolecular chains of Fig. la, the better their
orientation in Fig. 1 b and the higher the fiber tensile strength.
The present paper is organized as follows. We start by describing our molecular
model for the study of the factors controlling the drawability (Fig. 1) of flexible
polymer chains. Effects such as polymer molecular weight, density of entanglements
and temperature of drawing are explicitly taken into account. We then present our model
for the perfectly oriented fiber (Fig. lb) and its mode of fracture. Our approach allows
for both covalent and non-covalent bonds to break during deformation.

MODEL

Unoriented Fiber

Fig. 2 gives our model representation of the entangled solid polymer network prior
to deformation (Termonia and Smith, 1987, 1988). The dots denote the entanglement
loci. The dashed lines represent the weak attractive (Van der Waals) intermolecular
bonds connecting sections of either the same chain or, of different chains. Since the