Journal of Babylon University/Pure and Applied Sciences/ No.(7)/ Vol.(22): 2014
Effect Of Molecular Weight On The Dynamic Electro – Optic Properties Of Polysiloxane Liquid Crys tal Polymers Khalid Al.Ammar , Zahra'a Adel jawad Physics Department – college of Education for pure science–Babylon University E-mail : [email protected] Abstract : This study has focused on the electro- optic properties of a series of polymers with differing molecular weight based on the polysiloxane back bone. It is found that reducing the molecular weight lowers the phase transition temperature and eventually leads to a destabilisation of the nematic phase the effect of reducing the molecular weight of the polymer is to lower the observed threshold voltage. This may be related to an increase in the intrinsic elastic constant of the liquid crystal polymers it is found there is a strong coupling between the mesogenic side chain groups and the polymer chain and that the elasticity of the polymer chain plays a strong role in the electro –optic properties. The orientational order of the mesoginic units follows a similar temperature dependence for each of the differing polymer molecular weight, although the magnitude of the order parameter is much reduced as the molecular weight increases the variation in the polymers molecular weight leads to changes in the dynamic of the electro – optic response but these time scales are dominated by temperature and molecular weight effects. Keyword : polysiloxane , electro – optic properties الخلصة : هذه الدراسة ركزت على الخواص الكهرو بصرية لمجموعة أو سلسلة من البوليمرات التي تمتلك أوزان جزيئية مختلفة ومبنية على عمود فقري لبوليمر البولي سايلوكسين. وجدت ان الوزان الجزيئية المنخفضة تقلل من درجة حرارة انتقال الطور وفي النهاية تسبب عدم استقرار في الطور النيماتي، وتأثير انخفاض الوزن الجزيئي للبوليمر تقلل فولتية العتبة وهذا ومن الممكن أن يكون له علقة بزيادة ثابت المرونة للبوليمر. وجد إن هنالك ارتباط قوي بين الجانب الميزوجيني في البوليمر ولذلك فان مرونة البوليمر تلعب دور قوي في الخواص الكهرو بصرية. معامل الترتيب التوجيهي للوحدات الميزوجينية يعتمد على درجة الحرارة لكل الوزان الجزيئية المختلفة للبوليمر على الرغم من ان معامل الترتيب التوجيهي يقل بزيادة الوزن الجزيئي وان التباين في الوزان الجزيئية للبوليمرات يقود الى التغير الديناميكي في الستجابة الكهرو بصرية لكن الفترات الزمنية تتأثر بدرجة الحرارة والوزن الجزيئي. Introduction Material that exhibit liquid crystalline phases have been recognized for almost a century [RENTZER, 1888] yet it is only over the last two decades that such materials have evolved from the laboratory curiosity stage to become the basis of a new science. Liquid crystalline phases have recently been called the fourth state of matter [SAEVA, 1979]. However, such a definition belies the richness of structural properties that characterizes the many different liquid crystalline phases and leads to the immense interest in their behavior.A liquid crystalline phase is generally defined in terms of the long – range intermolecular order exhibited by its constituent molecules in a crystalline phase there are three degrees of long – range positional order. In a liquid crystalline phase at least one of their in on long – range positional order. There may, however, exist short – range rotation or orientational order between the molecules if they exhibited a shape an anisotropy, so that a true isotropic liquid phase is only exhibited when neither positional long – range nor orientational short – range order exist. Thus, if one or more of these degrees of positional or rotation order are manifest by the anisotropy molecules, then we have a useful definition of the crystalline phases [ HILSUM , 1983 ] As shape anisotropy is a prerequisites in this definition we might expect macroscopic properties such as refractive index, electric conductivity, viscosity and elasticity also to be anisotropic. Since liquid crystalline phases are also fluid, external fluids (magnetic, electrical, optical, mechanical and thermal) may be used to induce changes these properties. It
165 Journal of Babylon University/Pure and Applied Sciences/ No.(7)/ Vol.(22): 2014 was the interdependence of the optical and electrical properties and the quest for new electro – optic devices that led to the recent resurgence of interest in liquid crystalline phases. 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 6 cm2 area, and etched using HCl acid with zinc metal powder, as a catalyst, to give 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 [Saemgsuman , 2007] . 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 [Zhang, 2007]. 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 Geigy) the majority of the cell used in this work were prepared using " kapton " sheet of nominal thickness 0.025 mm, as spacers, to separate the electrodes [Mitchell, 2005]. 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 [Auriemma, 2007; Channuau, 2008; Wangsonb, 2008], fig. (1) shows the electro-optical cell construction.
Fig. (1): Electro – optic cell
166 Journal of Babylon University/Pure and Applied Sciences/ No.(7)/ Vol.(22): 2014
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 which 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. Temperature control system, which includes MK 1000 controller, nitrogen container nitrogen pump ( LN2 - p), and hot – cooling stage. It features, Large viewing Aperture. Dual pane window for better thermal isolation. Integrated Aperture window defrost system. Gas purge sample chamber. Inner lid for improved sample temperature uniformity. Vertical and horizontal mounting. Optional precision X –Y micropositionar for sample positioning. Application software, wintemp, allows remote control from host computer . ALCT Liquid Crystal measuring subsystem, which includes ALCT- EO1 (referred as ALCT after), test cell holder, photo detector head, and connecting cables. Using this system with well-prepared LC test cell and proper method, user can measure: Liquid crystal mixture physical parameters : - Dielectric constants ||,, Δ. - Elastic constants K11 and K 33. - Threshold Voltage Vth. - Polarization current Ip.
- Viscosity , 1 . Optical performance of LCD devices - Voltage – transmittance curve. - Switching speed, rise, falling time. Application software, WinLC, provides user an integrated tools to configure measurement setup, data collection, analysis and visualization. Optical test bench subsystem, which includes white LCD light source, polarizer, rotatable hot – cooling stage holder analyzer, and photo detector holder. this test bench allows user to:- Arrange polarizer and analyzer perpendicular and parallel to each other. Test cell in side of the hot-cooling stage can be rotated in full 360 o range. Light source , polarizer and analyzer are installed in sealed dark sections to prevent the contamination of optical components. Light sealable working chamber shields a way the room lighting.
167 Journal of Babylon University/Pure and Applied Sciences/ No.(7)/ Vol.(22): 2014
Fig. (2): Picture of a device with optical test bench subsystem.
Fig. (3): Picture of a device.
Materials : Chemical structures CH3 CH3
CH3 Si O Si CH3 ( CH2 )6 OR O OR = C O CN CH3
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Table (1): Molecular weight and phase transition data polysiloxane o o polymer Mw Dp Tg C TNI C 1 1.7×106 320 48 130 2 8.1×105 102 40 125 3 7.2×104 15 26 104 4 4.5×103 7 16 77 5 3.1×103 4 4 61 Results Dynamic electro–optic properties Using the system and the method described in work, we have implemented a cyclic experiment in which we use the values of on to determine when a steady state is reach. As mentioned in work the approach allows both on and off to be evaluated. in order to achieve complete switching we typically applied 147 to 240 volts(peak to peak) at a frequency of 500 Hz. The variation of the transmitted light intensity as a function of the applied voltage and the required voltage for each material in this work for complete switching. By reducing the temperature TNI the saturation states showed in these figures shifted, the required voltage for complete switching was increased in this case. The same behavior was observed for the materials in work although the materials in this work show larger differences between the required voltages for each material compared with the materials in work. Reproducible switching effect were. Observed at temperatures close to the nematic- isotropic transition temperature where the viscosity of the polymer is relatively low and measurement were made in the range of TNI to TNI -4 for materials no.1 and 2 and no response to the applied field had been observed for these materials in the smectic phase but measurements were made for the materials no.3.4 and 5 in the range of TNI to. TNI -5 at the same time reproducible switching effects were observed in the smectic phase fig.(6) no.3 show on as a function of the time for respectively , each set of experiment were made at constant temperature. These curves show similar characteristic features to the series of polysiloxan containing liquid crystal polymers in work although there are some differences in the time scale initially the switch on time allowed for complete relaxation to the predefined surface alignment state as the time period for which the field is removed off, increases, the following on increases until aquasi- equilibrium is reached. From such curves we can obtain values for on and off which represent switching between equilibrium conditions repetition of the cyclic experiment at different but fixed temperatures allows the temperature variation of on and off to be mapped out.
3 2.5
y
t 2 i s n
e 1.5 t n I 1 0.5 0 0 50 100 150 200 250 300 voltage (V)
Figure4: The relation between voltage and intensity for Polymer1.
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2.5
2
y t
i 1.5 s n e t 1 n I 0.5 0 0 50 100 150 200 250 300
voltage (V)
Figure5: The relation between voltage and intensity for Polymer 2.
2.5
2
y t
i 1.5 s n e t
n 1 I
0.5
0 0 50 100 150 200 250
voltage (V) Figure6: The relation between voltage and intensity for Polymer 3.
2.5
2
y t i 1.5 s n e t
n 1 I
0.5
0 0 20 40 60 80 100 120 140 160 180 200 voltage (V)
Figure7: The relation between voltage and intensity for Polymer 4.
3
2.5
2
y t i s
n 1.5 e t n I 1
0.5
0 0 50 100 150 200 voltage (V)
Figure8: The relation between voltage and intensity for Polymer 5.
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30 127oC 25
)
c 20
e o s 129 C
( 15 n o 10 5 0 0 500 1000 1500 2000 Time left off (min)
Figure9: The relation between on and time left off for polymer 1.
25 o
) 20 119 C
c o
e 124 C
s 15 ( n
o 10 5 0 0 500 1000 1500 2000 Time left off (min)
Figure10: The relation between on and time left off for polymer 2.
40 100oC ) 30 o c 101 C
e o s 103 C ( 20 n o
10 0 0 500 1000 1500 2000 Time left off (min)
Figure11: The relation between on and time left off for polymer 3.
10 71oC
) 8 c e
s 6 o ( 75 C n
o 4
2 0 0 500 1000 1500 2000 Time left off (min)
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Figure 12: The relation between on and time left off for polymer 4.
6 5
)
c 4 o e 58 C s 57 C (
3
n o
o 60 C
2 1 0 0 500 1000 1500 2000 2500 Time left off (min)
Figure13: The relation between on and time left off for polymer 5. Results: Order parameter: Measurements were made for each sample over a range of temperatures to include both nematic and smeatic phases the resultant order of the order parameter with temperature for each particular sample shows the classical shape predicted by the Maier and saupe theory [De Jeuw,1980]. The variation of the polymer molecular weight leads to distinctive trends in the order parameter versus temperature plots however each individual curve has the same basic form fig (14) it is clear that the materials with the highest molecular weight exhibit the lowest order parameter and this effect has also been observed by boeffel et al [Boeffel ch,1982] for same siloxaen based side chain polymers although it might appear that the lower order parameter at higher molecular weights reflect an incomplete mono domain formation due evidence to suggest that the measurements were anything other than true steady state values. The values and temperature dependence of the order parameter observed for polymer ( 2)closely follow those obtained for an siloxan based material with the same mesogenic unit and the same degree of polymerisation. The sample of material no.1 in this study had been held of fixed temperature for 60 days and there was no change observed on the value of order. This is to confirm that no kinetic phenomena has influence on the order parameter measurements. 0.8
0.7 ) S (
r
e 0.6 t e m
a 0.5 r a p
r
e 0.4 d r O 0.3
0.2 20 40 60 80 100 120 140 Tempereture (oC) 1 2 3 4 5 Figure14: The variation of the measured order parameter (S) obtained through measurement of the infra- red dichroism as a function of temperature.
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70 60 50 40 30 2 h t 20 V 10 0 0 0.2 0.4 0.6 0.8 Order parameter (s)
1 2 3 4 5 Figure15: Plots showing square of the threshold voltages against the order parameter S. Discussion : Variation of the degree of polymerization in this series of polysiloxan based side chain liquid crystal polymers has impact upon the phase behavior, the order parameter and upon the electro-optic properties. it is found that decreasing the molecular weight lower, the phase transition temperature for high molecular weight polymers (material no.1,2 and 3) a smectic phase is observed with very narrow nematic range however at low molecular weight (material no.4 and 5) only the nematic phase is observed. complementary to the reduction of the phase transition temperature with decreasing the molecular weight of the polymers there is a market lowering of the glass transition temperature of value of the glass transition is found [Boeffel, 1982] to increase as the molecular weight of the polymer is increased [Haasew, 1989]. Changing the molecular weight has a marked impact upon the switching times and the threshold voltage comparison of those results for off as a function of temperature with those measured using conventional optical relaxation techniques shows a significant difference which increases as the temperature is lowered, a similar trend was observed for a series of siloxane based side chain liquid crystal polymers figure shows that reorientation in an electric field results in electrical energy coupling to the mesogenic units. Since the polymer chain is coupled to the mesogenic side chain [ Kellerp,1985] then this well also under go motion. However, the response of the mesogenic units and that of the polymer need not be on the same time scale. The polymer chain may take much longer to reach equilibrium and because of the week nature of the coupling this need not be evidenced by the orientation of the side chains on removal of the electric field, the side chains can relax independently of the polymer chain and this occur slightly a head of the polymer chain relaxation. This slower relaxation of the polymer chain is only detected since if influences the subsequent response to the electric field [Finkelmann H, 1979]. The range of homo polymers behaved in a similar manner to many other reported liquid crystal polymers [Moussa F, 1987] for example the switch on time decreased as the temperature was increased above the glass transition, such effects are related to the reduction polymer viscosity. In this study it is found that the higher molecular weight has the lower switch off times, this is attributed to the contribution from limited segment lengths of the polymer back bone in work of this these is a suggestion has been made that the differences in the molecular weight of the polymers.
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The results of the orientational order parameter in this work confirms the expectation of work. References: Auriemma, F., de rosa, C., Esposito, S. and Mitcell, G.R. (2007): "polymer ph ac superelasticity in simicrystalline polymers" Angewandtle chemie international Edition, 46(23), pp.4325-4328. Bboeffel, Ch., Hisgaen, B., Pschorn, U., Ringsdorf, H. and Spiess, H., Chem, 23, 388, (1982). Channuau, W., Siripit ayananon, J., Molloy, R. and Mitchell, G.R. (2008): polymer, 49 (20), pp.4433-4445. De jeu, W. (1980): "physical properties of Liquid Crystalline Material" Gordon, Breach. Finkelmann, H., Naegel, D. and Ringsdorf, H., Markromol. (1979): Chem., 180, 803. Haase, W.,(1989):"Side chin Liquid Crystal polymers" ed. McArdle C. Blackie chapter 11. Hilsum, C., and Raynes, E.P. Eds , Phil trans R.soc, London,(1983), leon and Glasgow. Keller, P., Carvalho, B., Cotton, J.P., Lambert, M., Moussa, F. and Pepy G.J. (1985): Physique 46, L1065. Mitchell, G.R., Saengsuwan, S. and Bualek-limcharoen, S. (2005): "Evaluation of ereferred orientation in multi component polymer systems using x-ray scattering perocedures", proyress in colloid & polymer science, 130, pp.149-159. Mohan, S.D., Mitchell, G.R and Davis, F.J (2011): chain extension in electrospun polystyrene fibres: a SANS study. Soft matter, 7, pp.4397-4404. issn 1744-683x doi: 10.1039/c0sm01442g . Moussa, F., Cotton, J.P., Hardouin, F., Keller, P., Lambert, M., and Pepy G. J. (1987): Physique 48, 1079. Reintzer, F., Monatsh chem., (1888), 9, 421. Saeva , F.D.E.d , liquid crystals : the forth state of matter ,(1979), Marcel Dekker new York. Saengsuman, S., Bualek – limcharoen, S., Mitchell, G.R and Olley, R.H (2007): polymer, 44, pp.3407-3415. Wangsonb, S., Davis, F.J., Mitchell, G.R. And Olley, R.H (2008): Macromolecular rapid communications, 29(23), pp.1861-1865. Zhang, X.T, Song, W.H., Harris, P.J. and Mitchell, G.R. (2007):" Electrodeposition of chiral polymer carbon nanotube composite films", (hemphys chem , p112), pp.1766-1769.
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