The experimentally observed effect of forming a dilute oriented web of polymer fibrils in a nematic solution is to reduce or remove the discontinuity in order parameter that occurs as the temperature is raised. At high temperatures, some nematic ordering persists as a consequence of the presence of oriented polymer. At low temperatures the effective nematic order parameter is reduced below that of the pure material. This is presumably because there are some components of the polymer network that are not favorable to ordering in the predominant direction, and these prevent a uniformly ordered system from forming.
A characteristic found in both the experiments and the theoretical prediction is the existence of a range of temperatures over which the order parameter varies linearly with temperature. We interpret this phenomenon as arising from the existence of separate domains of material in the nematic and isotropic phases, respectively. The effect of raising the temperature while in this regime is to reduce the size of the nematic domains and consequently to increase the size of the isotropic domains. While the system is in this regime there will be domain walls between the two phases. Outside the range of this phenomenon there will be a single phase in which there is a smaller spatial variation in order parameter. At present it is not clear whether there might be continuous phase transitions at the boundaries of the linear regime, and this will be the subject of further experimental and theoretical study.
This work was supported by the NSF ALCOM Science and Technology Center under Grant DMR89-20147.
Figure 1: Measured birefringence of the liquid crystal 5CB in a cell
containing an ordered network of
polymer fibrils is shown as a function of temperature for various
concentrations of polymer.
Figure 2: Numerical solutions of the theoretical equation for the average
order parameter of the
liquid crystal are shown as a function of temperature. The concentration of
polymer is represented by
the strength of the effective field, 
. Here 
is
given in umits of the
mean-field parameter 
, and 
.
