Fabrication of pseudo-pi vertical alignment mode liquid crystal devices with ultra-violet polymerisation and investigations of their electro-optical characteristics

Abstract

In this study, we applied the non-contact photo-polymerisation method in fabricating pseudo-polyimide (PI) liquid crystal (LC) devices with vertical alignment copolymer films, in the hope of solving the problems associated with traditional rubbing alignment processes. We mixed different photo-polymerisation-type acrylic monomers and negative dielectric anisotropy LC at proper weight percentages, and induced phase separation behaviour between the photo-polymerisation-type acrylic monomers and the LC molecules in the LC mixture solution through ultraviolet light irradiation, thereby forming copolymer films with vertical alignment effects. We further conducted preliminary characterisation of the alignment mechanism, photo-polymerisation, etc., of the LC molecules and investigated their interactions with alkyl acrylic monomers with different carbon chain lengths (the analogue of alkyl side chains in traditionally used vertical alignment-type PI) and electro-optical properties such as the transmittance, contrast ratio, and response time.

Improvement in device performance from a mixture of a liquid crystal and photosensitive acrylic prepolymer with the photoinduced vertical alignment method

“…In previous studies of photoinduced vertical alignment in LC devices, POM and scanning electron microscopy (SEM, JEOL, JSM-6390LV) were used to prove that the copolymer morphology after photoalignment is indeed a VACOF with a smooth surface, instead of a networked PDLC film (where the devices operated in the scattering mode) [40,41].…”

“…As detailed in [40] and [41], we mixed the alkyl long-carbon-chain acrylic prepolymer A C10 , the biphenol acrylic prepolymer B1 or B2, the photoinitiator C, and an NLC to obtain materials impacts with two different main chain structures, denoted as NLC/B1 and NLC/B2. We used organic polymers with a side chain structure similar to that of polyimide to simulate the traditional rubbing process typically involved in the fabrication, and the long carbon alkyl side chain of A C10 (alkyl carbon number 10, dodecyl prop-2-enoate, SeaEn Special Material Co) was supposed to affect the vertical alignment of LC molecules.…”

“…We employed a He-Ne laser (10 mW, λ = 632.8 nm) as the incident light source [40,41,86] and positioned the LC cell between crossed polarizers for measurements. After the application of an alternating saturation voltage (10 Hz, square wave), the signal was detected by a photodiode and displayed on an oscilloscope.…”

“…In our previous reports on noncontact photoinduced vertical alignment [40,41], we specifically highlighted the proposal of Vorflusev and Kumar from Kent State University in 1999 on multilayered phase separation composite film (PSCOF) technology [42]. Their seminal work has served as the theoretical basis of the present study of multicomponent mixtures of LCs and photosensitive prepolymers.…”

“…In 2005, by following the theoretical framework previously established, Kumar et al further varied process parameters, such as UV irradiation intensity, environment temperature and cell thickness and empirically demonstrated the effective control of these parameters to obtain polymer (PSCOF or PDLC) films with different morphologies [77]. Thus, in our previous studies [40,41], by building on the works of Kumar et al, we demonstrated the fabrication of vertical alignment copolymer film (VACOF) LC devices with double vertical-alignment-mode layers.…”