Rheology of oligomer melts in the nematic and isotropic states

Oligomers prepared by chain extension of liquid crystalline monomers are thermotropic. The alignment of liquid crystalline oligomers to shear flow via direct ink write printing is an increasingly popular approach to prepare aligned and 3-D printed liquid crystalline elastomers (LCEs). Here, we are concerned with the contribution of order and thermal history on the rheological properties of liquid crystalline. When the oligomers begin in a polydomain nematic state, the transition to an aligned nematic state occurs gradually over a wide range of shear rates. Conversely, when the oligomers begin in an isotropic state they behave as a Newtonian fluid until a critical shear rate is reached, at which point they align in a critical manner. It is shown that by either decreasing liquid crystalline content or increasing temperature, the viscosity of the oligomer melt decreases while this critical shear rate increases. In addition, the normal stress of oligomers is positive over all shear rates but decreases significantly in magnitude with increasing temperature. By combining the analysis of both temperature and liquid crystalline content, it is demonstrated that the temperature relative to the nematic-isotropic transition temperature is key to the oligomers' unique flow behaviors. Parallel plate experiments highlight the different rheological properties of liquid crystalline oligomers in the nematic and isotropic states.

Automated summary

The alignment and thermal history play important roles in the rheological properties of liquid crystalline oligomers. The transition to an aligned nematic state occurs gradually when oligomers start in a polydomain nematic state, while they behave as a Newtonian fluid until a critical shear rate is reached if they start in an isotropic state. The viscosity of the oligomer decreases and the critical shear rate increases with the decrease of liquid crystalline content or the increase of temperature. The normal stress of oligomers decreases significantly with increasing temperature.