Induced birefringence of rubbed and stretched polyvinyl alcohol foils as alignment layers for nematic molecules

Abstract

Some optical properties of poly (vinyl alcohol) (PVA) films are investigated prior and after the rubbing and stretching of the samples. Birefringence of the prepared foils is enhanced as their stretching degree and rubbing become larger. These processing factors are also affecting the values of the ordinary and extraordinary refractive indices. The induced anisotropy in PVA foils is evidenced in the morphological characteristics observed by optical microscopy. The spreading and adhesion behaviors of a nematic liquid crystal on PVA foils are analyzed by means of contact angle measurements. Preliminary testing of the nematic on the PVA films revealed significant variations in transmitted light intensity during sample rotation under crossed polarizers. The high contrast between dark and bright patterns is indicative of uniform and homogenous alignment of nematic on the rubbed and stretched PVA foils. KEYWORDS polyvinyl alcohol, birefringence, rubbed, stretching, alignment layer 1 | INTRODUCTION Among the applications of the most known hydrosoluble synthetic polymers, polyvinyl alcohol (PVA) has been used in display industry. 1 Liquid crystal display (LCD) devices are optoelectrical systems, which are mainly working based on the light-modulating properties of nematic liquid crystals (NLCs). 2 The cell contains a liquid crystal (LC) layer placed between two alignment layers (ALs) casted on indium tin oxide (ITO)-covered glass substrates. 3 The cell also contains polarizing plates, electrodes, retardation films, color filters, backlight unit, etc. In the fabrication technology of the LCDs, PVA films are used to manufacture polarizers, 4 retarders, 5,6 optical filters, 7,8 and ALs. 9-12 The ALis a polymer layer coated on the glass plates, which through its textured surface is able to align the NLC molecules. In the absence of AL layers, the NLC molecules would be disposed to orient themselves randomly. 13 The capacity to control the alignment of the NLC on AL surfaces is essential for the performance of LCDs since it is known that weak anchoring of the NLC can significantly reduce the operating voltages and can improve the steepness of the electro-optic response of device. 14 Because NLC molecules follow the alignment imposed by the surface pattern of AL, 1 various techniques were developed to control the NLC orientation through adaptation of the AL surface characteristics. Among these methods, the most reliable ones are stretching, 11 soft lithography, 15 photolithography, 16 nanoimprint lithography, 17 and rubbing. 18 The alignment of NLCs on a surface is affected by the polymer chemical structure and conformation 10,19,20 rubbing strength, 21 type of rubbing cloth, 18,22,23 film thickness, 9 surface roughness anisotropy, 24 and surface wetting characteristics. 25 Currently the display market still uses the rubbing procedure to prepare ALs. The process is quite simple and involves a close contact between the polymer film and the velvet covered rotating cylinder. 26 The pattern formed on the surface of the rubbed polym...

Effect of mechanical treatments on orientation behavior and spectral properties of azoderivative dyes incorporated in poly(vinyl alcohol) films

“…[ 31, 32 ] Other important contributions on this subject were made by Dorohoi and collaborators. [ 33‐41 ] The authors developed a method for evaluation of the birefringence and its dispersion, based on the light polarization state of the stretched polymer films, and analyzed the dichroism of the pure and dye‐containing polymers. Their studies showed that transparent polymer films represent adequate media for the orientation of certain spectrally active molecules, and their dichroism allows establishing the direction of the electronic transition.…”

“…Mechanically drawn PVOH films are usually uniax materials, [ 35 ] while their anisotropy increases during the stretching process, which is performed under mild heating. [ 38, 42 ]…”

Nanofluid Based on Carbon Dots Functionalized with Ionic Liquids for Energy Applications

“…Figure 8 shows that although the initial fluorescence intensity of all membranes was different, upon 180 min of production, they all exhibited a similar value. This fluorescence quenching, previously described by Nechifor et al [34], is due to the chemical interaction between the entrapped species (nanofluids and [Tmi][Trif]) and the crosslinker (GA), which results in an increase in the non-radiative decay mechanism.…”

“…The entrapment of Cdots within PVA ( Figure 7) was previously accomplished using Cdots with different functionalization, which resulted in refractive index tuning [30], shape-memory effects [31,32], and charge-trapping composites [33]. Indeed, and even though crosslinking of PVA is necessary to increase water resistance, this process has a substantial effect on the optical properties of the entrapped material [34]. (Figure 8).…”

“…The PVA ability to entrap molecules has been extensively studied [34], and it has been determined that due to the random orientation of the different macromolecular chains, a non-rubbery and non-stretched polymer membrane is usually characterized by an intrinsic optical anisotropy [35,36]. Figure 9 shows non-treated visual images of the produced membranes (Figure 9a), the fluorescence image that is similar in all PVA membranes with entrapped species (Figure 9b), and the POM (under crossed polarizers) images of all membranes (Figure 9c-e).…”