Comments on “The application of the reptation hypothesis to polyelectrolyte biomaterials”

Abstract

A recent review paper by Nicholson [1] criticizes the application of the reptation chain pull-out model of fracture to glass polyalkenoate and zinc polycarboxylate cements, and specifically two papers by Hill and co-workers [2, 3]. There are a number of points worthy of comment. First, neither of these two papers set out specifically to test the validity of the reptation chain pull-out model to fracture, but this model provided a very useful framework with which to analyse the toughness data. There are, of course, many other models of polymer dynamics, some of which take into account that the tube may not remain intact and that there may be co-operative motion. However, none of these models have been so widely applied or so successful as the reptation model and, to the author's knowledge, none of these models have been applied to polymer fracture. The two major criticisms of the reptation chain pull-out model of fracture are that it assumes that a chain only crosses the fracture plane once and that a single polymer chain is not sufficiently long to bridge the crack tip, which typically has dimensions of the order of 1/xm in most glassy polymers. Neither of these two very valid criticisms is included in the recent review paper. Secondly, the divergence of the toughness data for glass polyalkenoate and zinc polycarboxylate cements from that predicted by the reptation chain pull-out model is admittedly large, but it is worth pointing out that, before these two studies, these polymeric cements were largely regarded as cementitious materials and the matrix polymer chains were viewed as being rigidly cross-linked. Thus, there should have been no influence of the poly(acrylic acid) chain length on toughness. Indeed, the only study to treat these cements as polymer composites modelled them as thermoset materials [4]. Clearly the marked dependence of the toughness on the poly(acrylic acid) chain length indicates a thermoplastic character to these cements, and this was supported by a dynamic mechanical and dielectric study showing the presence of sharp loss peaks in these materials [5]. Nicholson states that there are more-advanced models of polymer dynamics than the reptation theory which account for the dynamic coupling of chains. However, these models would be expected to lead to a prediction that the toughness (G~) will be even more dependent on the chain length or degree of polymerization (N) than predicted by the