Study On The Regulatory Mechanism Of Optical Properties Of Cellulose Triacetate Film By Synchrotron Radiation And Birefringence

Cellulose triacetate(CTA)is one of the polymer optical materials with the highest transparency,which is widely used in display panels as optical film,with the market share of CTA optical film accounting for more than 50%.However,domestic production of CTA optical thin films is difficult due to complex processing parameters and high technical thresholds.Only a few Japanese companies have mastered the mature processing technology of CTA optical film.The forming process of CTA optical films has experienced complex physical changes from raw material to film and oriented film,and the optical properties are determined by multi-scale structures such as the chemical structure of segments,segment orientation at sub-nanoscale,orientation structure of crystalline and amorphous regions at the nanoscale,and phase separation at the micron scale,which is a complex process involving multiple processing steps,multiple processing parameters,and multi-scale structures.Therefore,it is an enormous challenge to accurately characterize the contribution of different CTA structures to the birefringence during CTA processing.While an in-depth understanding of the relationship between multi-scale structural evolution mechanisms and optical properties is crucial in the fabrication of CTA optical thin films.In this thesis,based on the research methods of synchrotron radiation X-ray scattering(WAXS/SAXS/USAXS),optical birefringence,and polarized infrared spectroscopy,we systematically study the multi-scale structural evolution of a CTA film under a uniaxial tensile field.It is clear that the birefringence of a CTA film is determined by the multiscale structure of the chain chemical structure,the orientation of the pendant group,the orientation of the chain segments,the orientation of the main chain,the structure and morphology of the additive,the condensate structure,and the phase separation.The understanding of the birefringence regulation mechanism in optical thin films has been deepened by relating the microscopic multiscale structure to optical properties,which provides theoretical support for the optimization of processing parameters in the process of raw material selection,and modification,solution constitution,solution casting process,drying process,and post-stretching,etc.,during the development of high-performance CTA optical thin films.Finally,the details of the study are as follows:(1)Optical birefringence,polarized FTIR,and SAXS have been used to study the nematic interaction(NI)between CTA chains and rigid rod-shaped 5CB molecules,and the role of NI in the regulation of orientation birefringence(ΔnO).ΔnO increases with the increase of molecular mass fraction(ω5CB)and stretch strain(ε).The CTA film has negative intrinsic birefringence,while the 5CB molecule has positive intrinsic birefringence.The specific values of intrinsic birefringence of CTA and 5CB are quantitatively calculated,and the NI strength between the CTA chains and 5CB molecules are ξCTA,5CB≈0.43.Most importantly,the ΔnO programs of pure CTA films and CTA/5CB blend films in ε-ω5CB 2D space(Ts=170 and 200℃)are constructed.(2)The structural evolution of three CA-based films(CDA,CTA,and PCTA)was studied by in situ synchrotron radiation WAXS.It is found that the crystallinity(χc)and crystal size(L(110))of CDA films remain unchanged during heating and stretching.In contrast,the heating treatment of CTA and P-CTA films from room temperature can improve the perfection degree of existing crystals and also significantly promote the formation of new crystals,and the χc and L(110)of CTA and P-CTA films do not change much during the process of stretching to fracture.However,the f(110)of CDA,CTA,and P-CTA films increased with the increase of tensile strain.At the same time,the intrinsic birefringence of the crystalline and amorphous phases of the CTA film is significantly different due to the different distribution of chain conformation in the two phases,and the intrinsic birefringence of the crystalline and amorphous phases of the CTA film is quantitatively determined.(3)Using optical birefringence and USAXS techniques,we investigated the mixing dynamics of the two additives and the CTA and revealed the underlying mechanism of RWD optical film fabrication.This work found that CTA/DOS blend film can thermally induced phase separation at high temperatures,and has a lower critical solution temperature(LCST).The polymer matrix and DOS aggregates of the blend film are oriented by stretching,and after the DOS aggregates are removed by solvent extraction,oriented porous layers are formed within the polymer matrix.It is further found that under the special DOS mass fraction(ωDOS<10%)and unique strain range(0<ε<ε*,critical strain ε*),the coupling between the orientation porous structure in the film and the polymer matrix will contribute to the ΔnF,which is combined with the ΔnO of the CTA matrix,and the final ΔnT of the CTA film shows RWD behavior.(4)The molecular orientation of the 5CB in the thickness direction was controlled by a high-voltage electric field during the volatilization process of the CTA/5CB solution.As the electric field strength increases,the in-plane birefringence of the film will not be affected,while the out-of-plane birefringence of the film will be reduced.However,due to experimental limitations on the electric field strength,the CTA/5CB blend film cannot be prepared as a+C-type optical film.