Study On Phase Structure Control For Co-Continuous Polymer Blend And Its Application

Polymer blending has been considered as an attractive and inexpensive way to produce special high performance materials. As most polymers are incompatible, the blend phase structure derived from mixing plays a key role in determining the final performance of resultant products. In order to achieve desired material property,researchers have tried many versatile methods to control the morphology including adjusting the components or concentration of polymer components, modifying processing parameters and etc.The phase morphology of blending, such as particle-matrix structure, fiber-matrix structure, lamella structure and co-continuous structure, gains extensive attention from researchers. Among various structures, the co-continuous structure is of particular interest due to the unique inter connected phase throughout the whole material. Due to the presence of interfacial tension, such co-continuous structures start to coarsen when heated to a temperature higher than the melting/softening temperature of both phases. At present,there are still many controversies over the dominant form during structure evolution. The main forms include: hydrodynamic flow with the capillary zone, retraction at the top of the cylindrical phase, and integration of two nearby phases. In addition, there are also a large amount of literatures concentrating on how to cease the coarsening process and preserve the fine co-continuous morphology within the as-mixed blend. Up to present, the most widely used solution is the using of copolymer which acts as compatibilizing agent.This method could improve the adhesion between the two phases and slow down the rate of phase separation to a certain extent, but not completely eliminate this phenomenon.More critically, the introduction of such additives could not effectively enhance the thermal-mechanical stability of the co-continuous structure. Also, it has been found that we could obtain porous material out of the co-continuous by extracting one of the two phases. However, the more detail study on the development of its potential application is relatively rare.To address these challenges, a systematic study focusing on the internal structure of co-continuous polymer blend was conducted. The main contributions are as follows:1) In order to understand the phase separation process and figure out the dominant structure evolution form, we established two-dimensional and three-dimensional model based on analysis of the phase separation process. The kinetics of the phase separation was obtained through the simulation, utilizing the inverse of interfacial length per unit area or inverse of interfacial area per unit volume as the characteristic length respectively. It was found that the results based on simulation were consistent with experimental reports.At the beginning stage of heat treatment, rate of phase coarsening was fast; coarsening rate decreased significantly after a certain time; and at the later stage, the phase growth rate obtained based on three-dimensional results was relative higher. This indicates that our simulation assumption captured the main features of the phase separation and the retraction at the top of the cylindrical phase was a key factor for the phase separation process. In addition, we introduced the Cross-WLF equation model into the system to study the phase separation under different thermal boundary conditions. Simulation results were consistent with the experimental results, which verified the reliability of such model. Due to the temperature gradient field, the polymer viscosity gradually changed along the direction perpendicular to the isotherm lines, thus resultant final size of phase structure correspondingly reduced from high temperature to low temperature.2) Considering the critical influence of phase structure resulted from the processing on the material property, detailed study on relationship between the stress and temperature field and internal structure could give us some meaningful guidance on fabrication.Specifically, a co-continuous polymer blend was hot embossed against a micropatterned mold and then annealed inside the mold to obtain the desired internal morphology. It was found that embossing at a low temperature below the polymer flow temperature led to a significantly deformed internal structure. This orientation effect was largely suppressed by hot embossing the blend above the flow temperature, especially when an in-mold annealing stage was applied after 'embossing. In mold annealing was found to be a versatile technique for controlling the internal structure of the embossed pattern; not only the overall phase size but also its gradient and distribution can be controlled by varying the boundary conditions during in mold annealing. This in-mold annealing process is superior to pre-embossing annealing, since the latter in combination with hot embossing led to the formation of a closed pore surface and undesired structural deformation.In the extrusion process with large deformation, continuous phase structure for blend with components of similar viscosity was more prone to conformal changes. On the contrary, for the co-continuous phase with high viscosity ratio, such as PTFE/lubricant additive, the stress transfer mainly through the phase of high viscosity and then deformation was often not simultaneous. This led to the fiberization of PTFE phase.Another major difference for these two types of the blends during the extrusion was the structural uniformity in the radial direction. The flow for PLA/PS polymer melt in capillary was similar to parabolic curve with minimum shear rate at the capillary axis and high shear rate near the wall of the capillary, which led to the inhomogeneous structure within the extrudate. The flow for PTFE/lubricant system in the capillary was in the form of piston movement, thus there's no obvious shear rate difference along the radial direction. The internal structure of obtained extrudate was uniform.3) Phase structure and corresponding rheological behavior of PLA/PS co-continuous polymer blend modified with nano-particles of different dimension, including two-dimensional clay, one-dimensional carbon nanotube, or zero-dimensional nano diamond and agglomerated clay, were studied and compared. It was found the two-dimension nano-clay could work as stabilizer and quenched the interface of the blend completely. Blending system containing nano-clay exhibited excellent stable morphology during high temperature treatment. This was mainly because both PLA and PS molecular chains from two phases intercalated into layered structure of the clay at the interface,which was verified by TEM and X-ray diffraction. To some extent, the nano-clay played the role of nail at the interface.Further strain relaxation tests showed that blends containing two-dimensional nano-clay exhibited higher elastic modulus by comparing with other blends and it relaxed much slower under the same strain. In addition, PLA/PS/Clay blend showed enhanced thermo-mechanical stability. Blends with nano clay didn't present apparent phase separation after went through extrusion process. Also the orientation degree of the blend stabilized by nano-clay was significantly smaller than the pure blend the along the axial direction.4) A novel enhanced dual-percolation technique was developed to create ultra conductive porous nanocomposite at a record low carbon black concentration. The percolation threshold of obtained material was about 1wt% which decreased nearly 5 times compared with that of as-mixed composite. Moreover, a significant enhancement of conductivity was achieved for such porous composite. Comparing with the conventional carbon composite, electrical conductivity of resultant porous composite increased nearly nine orders of magnitude with Carbon black CB loading ranged from lwt% to 5wt%. In addition, the obtained material integrates interconnected micro pores of the substrate and nano pores derived from deposited CB. A phenomenological theory for this unusual behavior was proposed on the basis of the evidenced multi-scale porosity of the resultant material.Carbon black (CB) and/or carbon nanotube (CNT) loaded porous PP substrate with enhanced conductivity was facilely fabricated via this facile and novel method. The SEM result indicated that the agglomerated carbon additives formed continuous phase on the inner wall of the interconnected micro channel. Nitrogen gas adsorption test was carried out to characterize the nano pores resulted from these nano particles. In addition, the electrochemical performance of the material was evaluated by cyclic voltammetry and galvanostatic charge-discharge cycling. PP/CNT exhibited relatively high specific surface area, and high pore volume with narrower pore size distribution. However,PP/CB/CNT demonstrated lower equivalent series resistance (ESR) and higher charge capacity under the same testing conditions, which may be attributed to the synergistic effect of zero dimensional CB particle and CNT with large L/D ratio. Experimental results demonstrate the effectiveness of this novel method to construct conductive material with hierarchical porous structure which integrates co-continuous porous structure with nano pores derived from deposited carbon additives.