The influence of polysilane chemical structure on optical properties, rubbed film morphology and LC alignment

Abstract

Abstract. Polysilane films were prepared by the drop casting method and their optical and morphological properties have been analyzed in order to investigate their suitability as alignment layers for nematic molecules. The samples do not absorb the radiations in the visible domain, particularly those containing methylhydrosilyl units, and present a transmittance of about 90% starting from 390 to 1100 nm. The optical band-gap is higher than 3.26 eV for all polysilanes indicating a low probability of optical absorption processes in the visible range. The morphology of the pristine samples shows isotropically distributed granular formations. The polymer surface was oriented by rubbing with two types of velvet: one with short fibers and the other with long fibers. The latter generates higher surface anisotropy, as shown by the reduction of the surface texture direction index values. The presence of methylhydrosilyl units allows a denser packing of the polymer structure and thus finer surface periodicities, leading to better orientation of the nematic molecules on the polysilane surface.Keywords : material testing, polysilane, rubbing, morphology, LC alignment eXPRESS Polymer Letters Vol.9, No.5 (2015) 456-468 Available online at www.expresspolymlett.com DOI: 10.3144/expresspolymlett.2015.43 * Corresponding author, e-mail: irina_cosutchi@yahoo.com © BME-PT layer in the visible domain. The surface morphology of the rubbed alignment layer, the pretilt angle and surface energy of the alignment film are influenced by the conformation of the polymer molecular backbone [9]. Many studies are focused on preparing alignment layers by rubbing polyimides [10][11][12][13], poly(vinylidene fluoride) [14] or polyamides [15]. Since the absorption characteristics of alignment layer determines unwanted light-transmission modulation of the liquid crystal leading to ghost images on the LCD panel [16,17], the investigations are focused on transparent polymers with good surface alignment ability. Another class of transparent polymers is represented by polysilanes. They consist of chains exclusively made up of silicon atoms with significant delocalization of the sigma electrons along the backbone. This aspect leads to interesting electrical and optical properties, such as high quantum efficiency of charge generation, high charge-carrier mobility, efficient luminescence, and optical non-linearity [18]. Their photophysical properties are strongly influenced by the chemical structure of the polymer side groups and/or dimensionality of the skeletal framework [19,20]. To our knowledge there are no investigations on polysilane-based alignment layers patterned by rubbing. Previous papers [21][22][23] showed that polysilanes exhibit good ability of aligning molecules. The morphological features of these polymers are affected by UV exposure due to the scission of the main chain and oxidation processes [21]. Since UV-irradiated polysilane generated different orientation of nematic molecules from that induced non-irradiated polymer, pattern...

Insights on Light Dispersion in Semi‐Alicyclic Polyimide Alignment Layers to Reduce Optical Losses in Display Devices

“…The device has a rotating cylinder (12 mm diameter) coated with velvet composed of long acrylic fibers. Their dimensions (4000 µm length and 23 µm thickness) were determined from scanning electron microscopy measurements, as previously reported …”

“…Their dimensions (4000 µm length and 23 µm thickness) were determined from scanning electron microscopy measurements, as previously reported. [24] Characterization: Infrared Fourier transformed (FTIR) spectra were collected using a Bruker Vertex 70 spectrometer. The spectra show distinct peaks that confirm the expected PI structures.…”

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