A Study On The Morphology And Rheology Of Polymer/liquid Crystal Blends

The properties of polymer blends are closely related to the morphology, which is formed during processing and affected by the conditions including flow field and thermal history. It is crucial to understand the relations between the morphology and rheological properties of polymer blends. Liquid crystal(LC) owns some unique properties. The orientation of director and anisotropic interfacial properties make the morphology evolution and rheology of the liquid crystal/flexible polymer blends different from those of flexible polymer blends.In this thesis, the following aspects about the morphology and rheology properties of both isotropic polymer blends and liquid crystal/polymer blends are studied.(1) Shape evolution of liquid crystal droplets immersed in flexible polymer is different from flexible polymer droplets. The relaxation of a liquid crystal droplet after transient flow was studied and it was found that the liquid crystal droplet will rotate during the shape recovery, which has never been observed for an isotropic droplet. Different factors that influence the orientation angle of a single liquid crystal droplet during retraction progress were studied, including the temperature, the dimension of the droplets, the time of shear flow, the strain rate, the flow type, and the properties of liquid crystal molecules. The rotation of a liquid crystal droplet during shape recovery is mainly ascribed to the interfacial rotational torque from the interfacial anchoring energy. The dimension of the torque is related to the angle between the interfacial normal direction and the liquid crystal molecular direction.(2) It was discovered that a gel-like blend system was formed by mixing a small amount of 8CB(4-octyl-4-biphenylcarbonitrile) in smectic state with flexible polymer PDMS(Polydimethylsiloxane). The gel-like blend system is stable in both smectic and nematic states, but will be destroyed in isotropic state. The morphology of the gel-like blends was proved to be high concentration emulsion with 8CB as the continuous phase. The morphology is related to the concentration of 8CB, and both the dimension of the dispersed phase and the thickness of 8CB films decrease with the increase of PDMS concentration(decrease of 8CB concentration). The plateau value of storage modulus(’pG) increases with the decrease of 8CB concentration. ’pG is proportional to the apparent yield stress of 8CB films, which is determined by the thickness of 8CB films. The stableness of the gel-like blends is related to the elasticity of 8CB films, and is ascribed to the yield stress of 8CB films. A new mechanism of phase inversion is suggested based on alternating large amplitude oscillatory shear and small amplitude oscillatory shear experiments. The formation of the gel-like blends was the result of the erosion breakup the 8CB droplets and the subsequent local phase inversion.(3) We investigated the yield stress and thixotropic properties of 8CB/PDMS gel-like blend for the first time by different manners. It was discovered the dimension of the apparent yield stress is related to the experiment protocol, and it decreases with the increase of 8CB concentration. The relation between the yield stress and ’pG illustrates that the yield stress mainly comes from the elasticity of 8CB network. The area of thixotropic loops of 8CB/PDMS gel-like blends is related to the experimental time. 8CB/PDMS gel-like blends behave like a thixotropic yield stress fluid at lower 8CB concentration(10%-15%), while behave like simple yield stress fluid at higher 8CB concentration(20%-30%).(4) We have investigated the dimension of the dispersed phase and droplet size distribution of flexible polymer blends by different manners including optical microscopic experiments, fitting the linear viscoelasticity by Palierne model and fittingthe nonlinear viscoelasticity by LAOS-YB model. The possibility of the establishment of droplet size distribution by large amplitude oscillatory shear(LAOS) experiments was firstly suggested theoretically based on the YB model. Palierne model shows better results for dimension of the dispersed phase, while LAOS-YB model show some derivations for both the dimension of the dispersed phase anddroplet size distribution from optical microscopic experiments. It is mainly caused by two reasons. On the one hand, the model is derived on the assumption that the stain amplitude is in the weak nonlinear range, while the LAOS experiments are performed under larger stain amplitude. On the other hand, the model just assumed Newtonian fluids, while the nonlinear contribution of the components has significant effect on the nonlinearity of the blends. It still needs further research for the establishment of droplet size distribution from the nonlinearity of polymer blends.