Study On The Anisotropic Interfacial Properties,morphology Evolution And Rheological Behavior Of Liquidcrystal/flexible Polymer Immiscible Blend

The properties of a polymer blend are closely related to the processing conditions and its morphology. The investigations on the evolution of the morphology of the polymer blend and its rheology play an important role in the application of the polymer blend. Liquid crystal (LC) dispersed in an amorphous liquid matrix is of great interest in a variety of applications. One of the essential steps in mixing process is the deformation and retraction of dispersed droplets in an immiscible matrix. In order to better control the properties of such materials, it is crucial to understand the dynamics of morphology evolution in such systems. These kinds of system show many different behaviors from the flexible polymer materials due to the easy orientation and anisotropic properties of LC.When the dispersed droplet becomes an anisotropic nematic liquid crystal, parameters such as the orientation of director, tumbling parameter of director, and the interfacial anchoring energy are going to affect the dynamics of the shape evolution of droplet and the interfaical properties. However, less extensive works have been done to understand the effect of the director orientation and anchoring on the morphology evolution and the anisortropic properties in flow field in a polymer-liquid-crystal system. A sysmatic theoretical and experimental study on the LC dispersed polymer blends can help us to better understand the processing and control the final porperties of products.(1) We have measured the apparent interfacial tension between a low molar mass liquid crystal and an isotropic PDMS matrix by using the deformed droplet retraction method. The apparent interfacial tension is expected to reflect the change of microstructure of LC droplet under a small transient shear flow. The apparent interfacial tension shows an increase with the initial deformation of LC droplets. It may be attribute to the larger initial deformation can produce higher order parameter of director in the LC droplet. In addition, the apparent interfacial tension may be also dependent on the droplet size that reflects the effect of interfacial anchoring. Within the range of droplet sizes in our experiments, both 5CB and 8CB exhibit the trend that the apparent interfacial tension increased with radius of droplets. It may be account for that the interfacial anchoring of director has a greater effect on the director orientation for smaller droplets, while less effect on the director orientation for larger droplets. Moreover, the apparent interfacial tension shows an increased function of temperature below the nematic-isotropic transition, and a decreased function of temperature above the N-I transition for both 5CB and 8CB.(2) We have studied the interfacial properties between a liquid crystal and a flexible polymer by deformed droplet retraction method under an external DC electric field. The apparent interfacial tension is expected to reflect the change of microstructure of LC droplet under transient shear flow and electric field. The LC droplets were applied a DC electric field to create a controlled orientation. In order to obtain different orientation in LC, a shear strain was applied parallel and perpendicular to the electric field, respectively. The behaviors of LC microstructure reorientation during droplet retraction were discussed. The linear fit of retraction data by using YB large deformation model show that the curves of LC 5CB bend downward for parallel configuration and bend upward slightly for parallel configuration in the late stage of droplet retraction. It may be account for that the electric field and the internal flow of droplet affect the director orientation for 5CB droplets due to its flow-aligning properties. In addition, the initial apparent interfacial tension decreases for perpendicular configuration and slightly increases for parallel configuration with the DC electric strength, which can be explained that the stronger electric field induce a higher order parameter in droplet.(3)We investigated deformation and retraction of a single low molar mass liquid crystal 5CB and 8CB droplet dispersed in PDMS matrix under 2-D linear flow by a computer-controlled four-roll mill, which equipped with an optical microscope and a digital camera. The deformation parameter and orientation angle during deformation versus capillary number was obtained and compared with calculations of using MM model and YZ-LC model. The results showed that the deformation and rotation of LC droplet is more difficult than Newtonian droplets, possibly because the resistance of the nematic elastic energy induced by the nematic mesogens deformation and orientation under flow field. The calculations of YZ-LC model are found to be better for describing the LC droplet deformation than MM model since the former integrate the coupling effects between the LC droplet shape and the director, the LC droplet shape and the conformation of polymer chain. We have also observed the evolution of orientation angle of dispersed liquid crystal droplet during retraction. The results showed that the orientation angle of droplet of liquid crystal changed with time after transient flow during retraction, whereas after application of steady flow, the angle did not change basically. The change of orientation angle was attributed to the recovery of directors in the bulk of LC droplet, which induced the rotation of droplet shape. A theoretical model proposed by Yu was found to be able to predict the evolution of orientation angle of LC droplet during retraction both for transient flow and for steady flow. The apparent interfacial tension between 8CB and PDMS below smectic-nematic transition can also be obtained, which is difficult to measure by the pendant drop method.(4)We investigated the electrorheological (ER) effect of the immiscible blend of liquid crystal and flexible polymer. The nematic dispersed 5CB and 8CB in PDMS showed a positive ER effect, at the same time the electric field induced morphology change influenced the viscoelasticity of the blend. It was considered that besides the LC droplets orientated alignment along electric field, there also has the droplets coalescence of elongate LC along the field between the electrodes.The smectic dispersed 8CB in PDMS has a gel-like structure and showed very different ER effects. The shear viscosity shows shear thickening under smaller shear rate, and shear thinning under larger shear rate, which is attributed to the yield of gel-like structure. With the increasing voltage, the shear viscosity of the blend increased apparently and the LC orientation has a saturation value in our experiments. A transition from positive ER effect to negative ER effect would happen when the flow field over a critical stress and the stress is larger than the yield stress of gel-like structure. The yield stress and the transition from positive ER effect to negative indicate the change of gel-like structure of smectic 8CB/PDMS.