Order Parameter and Elastic Constants Studies of Liquid Crystals Khalid Al.Ammar1, Naheda Humood Al.Garah2 1Physics Department – college of Education for pure science–Babylon University 2Physics Department – college of Dentistry –Babylon University Abstract This study focused on the smectogenic polysiloxane, PG296, and the nematogenic, PG375, Where noted the lower order parameter at higher molecular weight polymers, they do support the overall trend of an increased nematic range with increasing polymer concentration. The reduced solvent power of 5CB when used with the polymer PG375 might be associated with the lack of side groups of similar cyanobiphenyl structure in the polymer but could also be related to the strong dipolar anti-parallel associations that might be expected to form between the cyano terminated polymer side groups, thus locking in the backbone configuration. The elastic constants and S2 as a function of polymer concentration, where the constants decreases with increasing concentration of the solution. Keyword: liquid crystals, polysiloxane
RESEARCH ARTICLE OPEN ACCESS
I. Introduction


The chemical structures of polysiloxane the smectogenic polysiloxane used in these studies, PG296, and the nematogenic, PG375, are presented for side-by-side comparison in figure 4. Both materials have a 52-unit polysiloxane backbone, flexibly coupled to the rigid aromatic groups by an aliphatic chain that contains four or six carbon atoms. The side group elements in the homopolymer PG375 possess many of the features of the side group elements of the copolymer PG296. These similarities in chemical structure were selected to minimise the number of variables that might have been expected to influence the properties of the solutions formed as increasing concentrations of the polymer were added to the pentyl cyanobiphenyl, 5CB. The remaining differences in polymer structure, which were expected to be highlighted by experimental investigation of the solutions, were the similarity between PG296 biphenylsubstituentR(see figure4) and 5CB, expected to aid the solubility of the polymer in this solvent. The non-polar group R -, copolymerized with R in PG296, expectedto influence the anti-parallel ordering of the nematogenic elements and the ratio of bend to splay elastic constants [1,2]. The ester linkage in the homopolymer PG375 side group which, together with the methyl substituted benzene ring decreases the length to width ratio of the group. This was expected to more noticeably affect the order parameter of the solutions in the higher concentration of benzoate ester associated with the homopolymer solutions.
II. Experimental

Blazers tin oxide coated glass slides were used for all electro – optic cells constructed in this work. This was easily cut, and was sufficiently flat to allow the fabrication of cells with parallel plate separation to within a few seconds of arc. The glass was cut into plate of approximately 6cm2 area, and etched using HCL acid with zinc metal powder, as a catalyst, to give a 2 cm square electrode surface. Each glass plate was then washed by hand, in soup and water and cleaned in an ultrasonic bath for 30 minutes at 60 oC. One technique was used to achieve uniform planer alignment of the liquid crystal director. The cell electrodes were coated with a thin layer of polyimide precursor [consisting of a 5% solution of Rodehftal 322 (Rh one Poulenc chemical Ltd.) in dimethylformaimed], using a spin – coater running at 4,500 r. p.m these coated slides were heated in an oven for 30 minutes at 80oC they were then rubbed at room temperature in a single direction with a cloth using controlled repeatable procedure, heated again for 30 minutes at 130oC [3].
As a consequence cell construction involved the following procedure a small portion of the selected polymer sample was carefully applied to one of the treated glass electrodes this was then heated for some time (typically 10-15 minutes) above the clearing point of the polymer in order to allow trapped air to escape [4]. The second glass electrode was then mounted above the first electrode, and the complete assembly inserted into a homemade clamping frame. The electrodes were fixed permanently using (Araldite Rapid epoxy resin ciba Geiger) the majority of the cell used in this work were prepared using "kapton" sheet of nominal thickness 0.025 mm, Khalid Al.Ammar Int. Journal of Engineering Research and Application www.ijera.com ISSN : 2248-9622, Vol. 5, Issue 2, ( Part -1) February 2015, pp.30-39 www.ijera.com 31 | P a g e

as spacers, to separate the electrodes [5]. The thickness of the cells were measured using micro meter techniques, both gave similar results, with typical electrode separations in the range 0.0026 – 0.030 mm the final step in the preparation of the electro – optic cell was the connection of the leads which carried the electric field from the power supply [6, 7, 8] fig (1) shows the electro-optical cell construction.
Figure (1) Electro – optic cell

The most important three devices were used in this work for electro – optic measurement are: 1. Mk 1000 2. Hcs 402 3. ALCT The Mk 1000 series temperature controller offers precision, accuracy, and stability for temperature measurement and control. When coupled with heating / cooling equipment from instc, the Mk 1000 can provide temperature control that is accyrate to 0.001oC. Two operation modes, keypad operation using the front panel of the controller, or software control though pc as well as. Adjustable ramp (rate of heating / cooling) to user set temperature point. Programmable operation command set. Precisely controls temperature to 0.001oC option save temperature data to the computer. RTD thermistor or thermocouple, LC cell holders for many types of LC cells.
Place the mk1000 controller on a level clean surface with sample ventilation if the mk 1000 controller is equipped with a cooling fan. Make sure the vents on the sides are unobstructed the rear parallel of a standard mk 1000. All connections to the mk 1000 use different types of ports eliminate the possibility of an incorrect connection a minimum configuration uses only the sensor / heater port for connecting the hot stage or hot chuck cable to, and the line power input port for connection the power cable to additional connection for communications and cooling accessories are available depending on the feature purchased do hot connect power cable and turn on the mk 1000 until all other connections have been made. Temperature control system, which includes MK 1000 controller, nitrogen container nitrogen pump (LN2 - p), and hot – cooling stage. Large viewing Aperture Dual pane window for better thermal isolation Khalid Al.Ammar Int. Journal of Engineering Research and Application www.ijera.com ISSN : 2248-9622, Vol. 5, Issue 2, ( Part -1) February 2015, pp.30-39 www.ijera.com 32 | P a g e