Influence of the nematic order on the rheology and conformation of stretched comb-like liquid crystalline polymers

We have studied the rheology and the conformation of stretched comb-like li quid-crystalline polymers . Both the influence of the comb-like structure a nd the specific effect of the nematic interaction on the dynamics are inves tigated. For this purpose, two isomers of a comb-like polymetacrylate polym er, of well-defined molecular weights, were synthesized: one displays a nem atic phase over a wide range of temperature, the other one has only an isot ropic phase. Even with high degrees of polymerization N, between 40 and 100 0, the polymer chains studied were not entangled. The stress-strain curves during the stretching and relaxation processes show differences between the isotropic and nematic comb-like polymers. They suggest that, in the nemati c phase, the chain dynamics is more cooperative than for a usual linear pol ymer. Small-angle neutron scattering has been used in order to determine th e evolution of the chain conformation after stretching, as a function of th e duration of relaxation t(r). The conformation can be described with two p arameters only: lambda (p) the global deformation of the polymer chain, and p, the number of statistical units of locally relaxed sub-chains. For the comb-like polymer, the chain deformation is pseudo-affine: lambda (p), is a lways smaller than lambda (the deformation ratio of the whole sample). In t he isotropic phase, lambda (p) has a constant value, while p increases as t (r). This latter behavior is not that expected for non-entangled chains, in which p varies as t(r)(1/2) (Rouse model). In the nematic phase, lambda (p ) decreases as a stretched exponential function of t(r),, while p remains c onstant. The dynamics of the comb-like polymers is discussed in terms of li ving clusters from which junctions are produced by interactions between sid e chains. The nematic interaction increases the lifetime of these junctions and, strikingly the relaxation is the same at all scales of the whole poly mer chain.