Miscibility And Orientation Of Solution-Mixed PTFS/PMMA Blends For Optical Film Application

Liquid crystal display（LCD）technology has been widely used in many occasions because of its low power consumption and high display quality.However,LCD technology in the early stage has many shortcomings such as the narrow viewing angle and low image contrast.One commercial solution is sticking one or more layers of optical compensation film to the surface of polarizer to compensate the phase retardation at oblique angle.Optical compensation film is a kind of film with optical anisotropy similar to uniaxial or biaxial crystal,which has different phase compensation function according to the refractive index difference among the three dimensions.Cellulose triacetate is usually used as the substrate film,and its out-plane birefringence can be changed by adding retardation material while the in-plane birefringence can be changed by uniaxial stretching procedure.However,cellulose triacetate material still has many defects.For example,it is hard to stretch to a high draw ratio,and it has strong hygroscopicity and poor stability.Moreover,the related raw materials and processing technology are monopolized by foreign enterprises.Methacrylate polymer has been considered as the next generation of compensation film material,but its low toughness and poor heat resistance limit its application in compensation film.Blending modification is a low-cost but effective method to improve the property of methacrylate polymer,which is of great practical significance for the molecular design of new optical compensation film materials and the development of domestic LCD industry.In this paper,polymethyl methacrylate（PMMA）was used as the base material,blending with polytrifluorostyrene（PTFS）at different mass ratios to prepare the blend films with high transparency and uniform flatness.We had investigated many properties of the blends,such as thermal properties,surface properties,mechanical properties and optical properties.And then,the compatibility of the blend system and the source of interaction force were investigated by differential scanning calorimetry（DSC）,dynamic thermo-mechanical analysis（DMA）and solid state nuclear magnetic resonance（SSNMR）.Finally,we combined the wide-angle X-ray diffraction（WAXS）data with polarized FTIR technology to analyze the orientation structure of the blend film under different draw ratio and the origin of optical anisotropy in PTFS/PMMA blend system.The main research contents and results are as follows:（1）Preparation and characterization of PTFS/PMMA blend film.Using THF/MEK（1:1）as cosolvent to prepare uniform and transparent film by blade coating.The results of characterization show that the addition of PTFS enhanced comprehensive performance of PMMA.The TGA and DSC data show that PTFS inhibits the decomposition of weak H-H bond in PMMA at high temperature while the glass transition temperature increases with the content of PTFS,which indicates that the heat resistance of PMMA is enhanced.The stress-strain curves of the films with different component ratios show that PTFS enhances the strength and ductility of PMMA.The results of optical characterization show that the transmittance of the blend film is more than 92%in the visible light.The out-plane retardation of the film increases with the content of PTFS,and the in-plane retardation can be effectively improved by uniaxial stretching under high temperature.It indicates that retardation of the blend film can be adjusted from the formulation and stretching process,which is easy to meet with the application requirements of optical film.However,the contact angle experiment shows that the surface energy of the blends decreases with the content of PTFS which may lead to the deterioration of adhesion property with PVA layer.Therefore,the component proportion of PTFS can’t be too high.（2）The compatibility of PTFS/PMMA blend system was studied.The DSC and DMA data were used to determine the glass transition behavior of the blends.A single glass transition temperature can indicate that the system could be compatible at any ratio.The 2D ¹H SQ-SQ NOESY,2D ¹H-¹³C HETCOR and 2D ¹⁹F-¹³C HETCOR spectra were determined by SSNMR.The results of HETCOR spectra show that the system has reached the level of molecular blending.The H atom on the benzene ring of the PTFS side chain is closer to the H atom on the methoxy group of the PMMA side chain,and the F atom on the main chain of the PTFS has no contact with the PMMA side and main chain.Combined with FTIR data,it can be inferred that the compatibility driving force of the blend system comes from the interaction between the hydrogen atom on the benzene ring of the PTFS side chain and the carbonyl group of the PMMA side chain.The F atom on the main chain enhances the positive charge of the hydrogen nucleus of benzene ring,thus makes the system from phase separation to complete compatibility.（3）The orientation structure of PTFS/PMMA blend system was studied.The dominant conformation of benzene ring after stretching was determined by polarized FTIR.The results show that the transition moment vector of C-H in-plane bending vibration on benzene ring is perpendicular to the long axis of molecule,while the transition moment vector of out of plane bending vibration is parallel to the long axis of molecule.It can be speculated that the plane of benzene ring is perpendicular to the direction of molecular chain.Based on the birefringence data and WAXS data,the orientation structure of the blend films at different stretching ratios was studied.The results show that the orientation degree of the PTFS main chain increases with the draw ratio,and the specific orientation of benzene ring provides the difference of refractive index in different directions.However,the orientation degree of the PMMA main chain changes little with the draw ratio.It can be speculated that the molecular chain relaxes during the process of recovering to room temperature after stretching at high temperature.