Kinetics of Induced Crystallization of the LC1-xSilx System - The

Department of Physics, Worcester Polytechnic Institute, Worcester, Massachusetts 01609. J. Phys. Chem. B , 2007, 111 (8), pp 1916–1922. DOI: 10.1021...
0 downloads 0 Views 176KB Size
Download PDF
1916

J. Phys. Chem. B 2007, 111, 1916-1922

ARTICLES Kinetics of Induced Crystallization of the LC1-xSilx System Dipti Sharma* and Germano Iannacchione Department of Physics, Worcester Polytechnic Institute, Worcester, Massachusetts 01609 ReceiVed: NoVember 21, 2006; In Final Form: January 9, 2007

This study explores the kinetics of a new feature, called “induced crystallization (IC)”, observed in an Aerosil dispersed octylcyanobiphenyl (8CB) liquid crystal system. Heating rate dependent experiments were performed using modulation differential scanning calorimetry (MDSC) at various heating ramp rates. In the presence of Aerosil nanoparticles, a well-defined exothermic peak was found as an additional feature on the heating scan before the melting transition, which was absent in the bulk 8CB; hence, we like to call it an “IC” as it is induced by Aerosil nanoparticles in the system. The system LC1-xSilx was prepared by mixing Aerosil nanoparticles in the bulk 8CB by the solvent dispersion method (SDM) where LC represents bulk 8CB and Sil represents Aerosil nanoparticles with x as the Aerosil fraction. The concentration of the Aerosil nanoparticles (x) varied from 0 to 0.2 g/cm3 in the bulk 8CB. The IC transition peak showed a temperature shift and change in the shape and size in the presence of Aerosil nanoparticles. In addition, this transition shifted significantly with different heating ramp rates following an Arrhenius behavior showing activated kinetics. The presence of Aerosil nanoparticles caused a significant increase in the enthalpy and decrease in the activation energy for the IC transition as the density of Aerosil nanoparticles increases and showed a saturation for the highest density of Aerosil nanoparticles. This behavior can be explained in terms of molecular disorder and surface molecular interaction induced by adding Aerosil nanoparticles into the bulk of 8CB liquid crystal.

I. Introduction Liquid crystals (LC) are a particularly attractive system for the study of phase transitions into partially ordered phases. This makes them especially interesting for the study of the effects of quenched random disorder (QRD), which are typically introduced by the random fixed dispersion of solid surfaces. The effect of quenched random disorder on phase transitions as well as on melting or crystallization is an important area of study that continues to attract a great deal of research. Disorder is ubiquitous (ideal pure transitions being the exception rather than the rule in nature!), and the effect on phase transitions can be profound. Phase transitions are modified depending on the aspect of the system affected by the disorder and on the dimensionality. In LC + Aerosil systems, the quenched random disorder is created by a dispersed gel of Aerosil nanoparticles and is varied by changing the density of Aerosils in the dispersion. A convenient measure of the introduced disorder is the grams of silica per cubic centimeter of liquid crystal, denoted as the conjugate silica density Fs, which is directly related to the surface area of solids as well as to the mean distance between solid surfaces. The Aerosils used are silica spheres that can hydrogen-bond together to form a fractal-like random gel. Studies have previously been carried out by various groups on liquid crystals in an aerogel medium to study phase transitions.1,2 Aerogels are self-supporting structures, and this places a lower limit on the disorder strength that can be probed. By contrast, the Aerosil gel provides a weaker and more easily controlled perturbation and thus opens up a physically interesting regime. * Corresponding author.

Aerosil dispersed liquid crystal, octylcyanobiphenyl (8CB), has been studied by several authors to understand smectic-A to nematic (SmA-N) and nematic to isotropic (N-I) transitions.1,3-8 No literature has been found on a heating rate dependent study of the crystallization of the Aerosil dispersed liquid crystal 8CB system using Arrhenius theory. They are open to be investigated and to understand the role of Aerosil dispersion in the bulk liquid crystals. Therefore, we have undertaken a detailed heating rate dependent calorimetric study of an Aerosil dispersed bulk 8CB liquid crystal system using modulation differential scanning calorimetry (MDSC) to show the kinetics of crystallization using Arrhenius theory and also to understand the role of Aerosil dispersion in terms of crystallization. MDSC is a good tool to understand the rate kinetics of various materials and has been used by various authors for decades.9-12 In this work, we study the effect of quenched random disorder due to a dispersed thixotropic Aerosil gel on crystallization from the solid state to the crystalline state (S-K) as a function of heating rate in the Aerosil dispersed liquid crystal system LC1-xSilx using Arrhenius theory. The calorimetric results for melting, SmA-N, and N-I transitions of this liquid crystal system LC1-xSilx have been reported by us using Arrhenius theory in our earlier publications.13,14 Now we present the kinetics of a new feature, “induced crystallization (IC)”, observed on the heating scan before melting in the system. This study focuses particulary on “induced crystallization”. Heating scans were performed at four different heating ramp rates using MDSC to understand the rate kinetics of the induced transition of the system. Four different densities of the Aerosil nanopar-

10.1021/jp067736o CCC: $37.00 © 2007 American Chemical Society Published on Web 02/07/2007

Induced Crystallization of the LC1-xSilx System

Figure 1. Schematic of an Aerosil gel. The gel forms long, necklacelike chains that interconnect randomly and percolate to form a fractal gel at densities above ∼0.02 g/cm3. The polar LC molecules have a more-or-less homeotropic alignment at the silica surface. The zoomed region shows a cartoon of a distorted smectic-A phase by the presence of the Aerosil gel.

ticles in 8CB were studied to explore the effect of Aerosil nanoparticles in bulk 8CB. Samples and calorimetry are described in section II. The results are shown in section III, with discussion and conclusions drawn in section IV. II. Samples and Calorimetry II.A. Sample Preparation. The system LC1-xSilx represents an Aerosil dispersed liquid crystal 8CB where LC, Sil, and x represent the liquid crystal 8CB, Aerosil nanoparticles, and the Aerosil fraction, respectively. The system was prepared by mixing Aerosil nanoparticles in the bulk 8CB by the solvent dispersion method (SDM) described in our earlier publication.14-16 The specific surface area of type-300 Aerosil nanoparticle is 300 m2 g-1,17 and the diameter of the Aerosil nanoparticles is roughly 7 nm, whereas the length of the 8CB molecules is 2 nm and the width is 0.5 nm. The molecular weights of bulk 8CB and Aerosil nanoparticles (SiO2) are Mw ) 291.44 g mol-1 and Mw ) 60.08 g mol-1, respectively. The concentration of the Aerosil nanoparticles (x) varied from 0 to 0.2 g/cm3 in the bulk 8CB. The 8CB liquid crystal used in this work is a well-studied prototypical rodlike molecule, with a rigid biphenyl core at one end attached to an aliphatic tail and the other end attached to a polar cyano group. The pure material undergoes a weak firstorder isotropic to nematic transition at ToIN ) 313.98 K, undergoes a continuous nematic to smectic-A transition at ToNA ) 306.97 K, and crystallizes below 290 K,18 as reported using an ac calorimetry technique at a very slow ramp rate of 50 mK/ h. The Aerosil consists of SiO2 (silica) spheres coated with (-OH) hydroxyl groups exposed on the surface. The hydroxy groups on the surface enable the spheres to hydrogen-bond and form a thixotropic,19 fractal gel in an organic medium such as 8CB. A schematic of the Aerosil gel and LC molecules, drawn approximately to scale, is shown in Figure 1. The gel can be thought of as randomly crossing long silica chains with a very high pore volume fraction and no preferred orientation, as was shown by light scattering20 and small-angle X-ray scattering (SAXS) studies.5 However, these SAXS studies showed that the basic Aerosil unit consists of a few of these spheres fused together during the manufacturing process.5 The specific surface area of the type-300 Aerosil is a ≈ 300 m2 g-1 as determined by a Brunauer-Emmett-Teller (BET) adsorption isotherm as specified by the manufacturer. The gelation threshold for the Aerosil used in this experiment occurs approximately at a silica density of F ≈ 0.015. Because of the surface hydroxyl groups, the polar 8CB molecules should anchor homeotropically at the

J. Phys. Chem. B, Vol. 111, No. 8, 2007 1917 silica surface. However, due to the short radius of curvature and expected undulations of the silica surface, the actual surface orientation is most likely tilted. The hydrophilic nature of the Aerosils allows the silica particles to weakly hydrogen-bond to each other and form a gel in an organic solvent. However the basic free-floating Aerosil unit typically consists of several of these spheres fused together during the manufacturing process.5 Each 8CB and Sil sample was created by mixing appropriate quantities of liquid crystal and Aerosil together and then dissolving the resulting mixture in spectroscopic grade (low water content) acetone. The resulting solution was then dispersed using an ultrasonic bath for about an hour. As the acetone evaporates from the mixture, a fractal-like gel forms through diffusion-limited aggregation. Crystallization of the LC host can severely disrupt the gel structure, and so care was taken to prevent any formation of the solid phase of the liquid crystal during the experiments. The samples were subsequently dried under vacuum for more than 2 h at elevated temperature. This preparation method has been shown to produce uniform and reproducible dispersions.5 The silica density dependence of the induced crystallization was studied in four dispersion samples of the system LC1-xSilx. The density of the Aerosil nanoparticles in the bulk of 8CB was varied in four steps from 0 to 0.2 g/cm3. The concentration x was 0, 0.05, 0.10, 0.15, and 0.20 g/cm3 (grams of silica per cm3 of total volume) in the system LC1-xSilx where when x ) 0, it represents the bulk 8CB. These samples were degassed for about 1 h under a vacuum unit at room temperature (293 K) and then used in calorimetry. In addition, a pure Aerosil gel sample of unknown density was also studied. All results were compared with the results of bulk 8CB, which we reported recently.13,14 II.B. Calorimetry. To study the kinetics of induced crystallization of the Aerosil dispersed liquid crystal system LC1-xSilx, a model MDSC 2920 (TA Instruments) was used for modulation differential scanning calorimetry (MDSC). Four different densities of Aerosil nanoparticles were studied in bulk 8CB. First the samples were quenched at 243 K and then kept isothermal for 15 min. The sample (5 mg of the lowest density x ) 0.05 g/cm3) was heated from 243 to 333 K at a 20 K/min ramp rate and then cooled to 243 K at the same ramp rate. The respective heat flow of the sample was recorded along with the temperature change during the heating scans. The MDSC thermograms showed an additional exothermic peak before the melting transition in the system. We like to call this additional feature “induced crystallization”. Three endothermic peaks were also observed on the heating scans at the melting, SmA-N, and N-I transitions, which were described in our earlier publications.13,14 Similar measurements were made at heating scan rates of 10, 5, and 1 K/min with the same sample. Experimental and environmental conditions were kept identical for all runs so that a comparison of the phase transition parameters could be made to understand the effect of different heating rates on the phase transitions of the sample. The above steps were repeated for other densities of Aerosil nanoparticles (x ) 0.10, 0.15, and 0.20 g/cm3) using the same procedure. The results of bulk 8CB and bulk Aerosil nanoparticles are reported in our latest publications.13,14 The transition temperature of the “IC” peaks are reported at the maximum height of the peaks instead of the onset temperature because the temperature at the peak maxima reflects the maximum change in the enthalpy. III. Results of Induced Crystallization III.A. Heating and Cooling. Figure 2 shows the results of the heating and cooling scans of the Aerosil dispersed system

1918 J. Phys. Chem. B, Vol. 111, No. 8, 2007

Figure 2. Heat flow (W/g) versus temperature (K) plot at 10 K/min heating ramp rate for density x ) 0.05 g/cm3 in the LC1-xSilx system. The regions S, K, A, N, and I represent the solid, crystalline, smecticA, nematic, and isotropic state of the Aerosil dispersed system LC1-xSilx, respectively. Forward and backward arrows show the heating and cooling scans, respectively, whereas the upward arrow shows the new feature present in the heating scan called “induced crystallization”.

LC0.95Sil0.05 at 10 K/min, spanning the isotropic and crystal phases for the lowest density of Aerosil nanoparticles, 0.05 g/cm3. The forward and backward arrows show the heating and cooling scans, respectively. On heating, as the temperature increases from 243 to 333 K, the Aerosil dispersed system shows an exothermic peak (indicated by the upward arrow) at 275.2 K ()2.1 °C) as it moves from its solid state (S) to crystalline state (K). It is an additional and interesting peak observed in the presence of Aerosil nanoparticles in the system before melting. We like to call this exothermic peak “induced crystallization” as it is induced by Aerosil nanoparticles in the bulk of 8CB. As the heating scan continues further, three endothermic peaks were also observed at melting (K-A at 294.7 K), smectic-A to nematic (SmA-N transition at 305.3 K), and nematic to isotropic (N-I transition at 312.7 K), respectively. These endothermic peaks were discussed in our earlier publication.14 Here our study is focused on “IC” only. On the cooling scan, three exothermic peaks were observed at the I-N, N-SmA, and SmA-K transitions. No peak was observed corresponding to the S-K transition on the cooling scan. It seems that the S-K and K-A peaks were merged and became a single peak in the form of the SmA-K transition at 265.5 K. The bulk 8CB shows the absence of the “IC” peak on the heating scan, whereas on the cooling scan, the SmA-K (crystallization) transition shows a little shift of 1.1 K toward lower temperature in the presence of Aerosil nanoparticles when compared with the bulk 8CB results as shown in Figure 3. These results indicate that the presence of Aerosil nanoparticles induces a disorder because of quenching and produces a new exothermic peak before melting on the heating scan (called “IC”), which combines with the melting peak on the cooling scan and changes the thermodynamics of the system. III.B. Effect of Rate. A rate dependent study of the system LC1-xSilx for all densities of x was also performed at different heating ramp rates of 20, 10, 5, and 1 K/min. Significant shifts in the “IC” peak temperature were observed, and it moves toward lower temperature for all densities of Aerosil nanoparticles as the ramp rate decreases. Figure 4 shows the thermogram of the system LC0.95Sil0.05 as various heating ramp rates varied from 20 to 1 K/min. As the ramp rate decreases, “IC” or the S-K transition shifts toward lower temperature and the peak becomes smaller and a little broader. The rate of its shifting is higher than the rate of the melting transition (K-SmA), as

Sharma and Iannacchione

Figure 3. Heat flow (W/g) versus temperature (K) plot at 10 K/min heating ramp rate for densities x ) 0.05 and 0.00 g/cm3 in the LC1-xSilx system showing the effect of Aerosil nanoparticles in 8CB.

Figure 4. Rate effect on one density x ) 0.05 g/cm3 of the Aerosil dispersed system plotted as heat flow (W/g) versus temperature (K) for the heating scan for different ramp rates from 20 to 1 K/min.

Figure 5. The excess of specific heat capacity (J/(g K)) versus temperature (K) plot for the S-K transition with varying ramp rates from 20 to 1 K/min for density 0.05 g/cm3.

shown in Figure 4. To see the clear shifts in the “IC” transition, the excess of the specific heat capacity was plotted after subtracting the linear background. The excess specific heat capacity for the system was obtained by subtracting from the specific heat, Cp, a linear background,

∆Cp ) Cp - Cp(background)

(1)

where Cp(background) is the baseline and Cp is the specific heat capacity of the sample. Figure 5 shows the excess of the specific heat capacity of the S-K transition at various ramp rates. It is clear that as the ramp rate decreases, the peak becomes broader

Induced Crystallization of the LC1-xSilx System

J. Phys. Chem. B, Vol. 111, No. 8, 2007 1919 TABLE 1: Kinetics of Induced Crystallization “IC” (for the S-K Transition) in the System systema LC0.95Sil0.05 LC0.90Sil0.10 LC0.85Sil0.15 LC0.80Sil0.20

% F(x)b ∆HGrowthc crystald Tc - Tme ∆HS-Kf ∆ES-Kg 0.05 0.10 0.15 0.20

13.53 14.05 14.48 14.58

92.82 93.52 94.04 93.60

20.4 21.6 22.4 17.7

3.79 3.94 4.01 4.09

0.34 0.26 0.21 0.19

a System varied with density. b Density of Aerosil nanoparticles, F (in g/cm3). c Crystal growth for induced crystallization for the S-K transition. d Percent crystallinity at Tc. e Induced crystallization range (in K). f Enthalpy for the S-K transition (in kJ mol-1). g Activation energy for the S-K transition (in kJ mol-1).

Figure 6. Arrhenius plot: ln β (K/min) versus 1/T (K) for the S-K transition for all densities. The upper plot shows lower densities, and the lower plot shows higher densities of Aerosil. Smooth lines are the fits to the data points. Errors in TS-K = (0.01 K.

shifting toward lower temperature and follows an Arrhenius behavior. The height range of the “IC” peak is found to be ∼2 J/(g K). Using Arrhenius theory,21-24 the effective heating rate can be given by

( (-∆E RT ))

β ) βo exp

Figure 7. Enthalpy (J/g) versus ramp rate (K) for the S-K transition varying the ramp rates from 20 to 1 K/min.

(2)

where β is the effective heating rate in K/min, βo is a constant in K/min, ∆E is the activation energy in J/mol, R is the universal gas constant in J/(mol K), and T is the absolute temperature in kelvin. This equation can also be shown as

ln β ) ln βo -

(∆E RT )

(3)

where ∆E is determined from the slope of the graph, which is plotted between ln β and 1/T. Rate effects for “IC” were also found for other densities of Aerosil nanoparticles (x varies from 0.10 to 0.20) in the system LC1-xSilx. For simplicity, these plots are not shown here but these results are considered in the data analysis. All transitions follow an Arrhenius behavior with different heating ramp rates and show rate dependent kinetics. According to eq 3, when the transitions were plotted as the ln of the heating rate versus 1/T for all densities, all densities of Aerosil show different activation energies. Figure 6 shows the Arrhenius plot for the S-K transition where the upper section shows lower densities of Aerosil nanoparticles and the lower section shows higher densities. The activation energy for the S-K transition can be calculated from the slope of the lines for all densities using eq 3. The variation of the activation energy with different densities of Aerosil nanoparticles is shown in Table 1 for the “IC” transition. The activation energy decreases as the Aerosil density increases and becomes saturated for the highest density of Aerosil nanoparticles (0.20 g/cm3) in the system. The rate dependence of the associated enthalpy of the “IC” transition suggests that it is a fairly stable induced phase as shown in Figure 7. The exothermic character and relatively large

Figure 8. Density dependent heating scan at 5 K/min ramp rate showing the heat flow (W/g) versus temperature (K) plot for all densities where the black symbol shows the result of bulk 8CB.

enthalpy imply that significant phase ordering occurs in the temperature range just below melting. III.C. Effect of Density. Figure 8 shows the density effect on the “IC” and melting transitions of the system LC1-xSilx for densities varied from 0 to 0.20 at a 5 K/min heating ramp rate. It is clear that as the density increases, all transitions show a temperature shift in a random order with a change in the shape and size of the peak. The “IC” peak becomes a little smaller but broader as the Aerosil density increases. To see a clear density effect on “IC”, the excess of the specific heat capacity versus temperature was plotted after subtracting a baseline as shown in Figure 9. It is clear that as the density increases, the peak becomes a little broader and its area increases. The crystallization increases or the crystal grows as the Aerosil nanoparticle increases from 0.05 to 0.15 g/cm3 in the system. As the Aerosil density further increases from 0.15 to 0.20 g/cm3, the crystal becomes saturated as shown in Figure 10. The data details of crystal growth for all densities are shown in Table 1.

1920 J. Phys. Chem. B, Vol. 111, No. 8, 2007

Sharma and Iannacchione III.D. Percent Crystallinity. To know how much of the system is crystalline, latent heat for the “IC” and melting transitions were calculated and were used to calculate the percent crystallinity of the system.

∆H′ ) ∆Hm - ∆Hc ∆(% crystallinity) )

Figure 9. Blowup of the S-K transition for all densities at a 5 K/min heating ramp rate plotted as the excess of specific heat capacity (J/(g K)) versus temperature (K).

∆H′ × 100 ∆H/mmtotal

(4) (5)

where ∆Hm and ∆Hc are the latent heat of melting and “IC” transitions, respectively, and ∆H/m is the specific heat of melting and mtotal is the total mass of the sample used. The percent crystallinity of the system with Aerosil density is given in Table 1. It is clear from the table that as the Aerosil density increases in the system, the percent crystallinity increases for lower densities but becomes saturated for the highest density (0.20 g/cm3) of Aerosils. IV. Discussion and Conclusions

Figure 10. Crystal growth versus Aerosil density plot.

Figure 11. Upper section shows the induced crystallization range (K) and the lower section shows the position of the crystallization peak for the S-K transition versus the density plot.

The change in the S-K range and the peak position for “IC” versus Aerosil density was plotted in Figure 11. It is clear that as the Aerosil density increases in the system, the “IC” range increases up to 0.15 g/cm3 density of Aerosil but decreases as the density increases further. The “IC” peak shifts toward lower temperature as the density increases but moves toward higher temperature as the density becomes higher. This can be explained in terms of the surface interaction between the Aerosil nanoparticles and the 8CB molecules, as we discussed in our earlier publication.14

IV.A. Kinetics of Induced Crystallization. Because of quench random disorder (QRD), the presence of Aerosil nanoparticles in bulk 8CB induces a crystallization in the system below the melting transition on the heating scan and changes the thermodynamics of the system. As Aerosil nanoparticles are added in the system, they bring a molecular disorder and induce the surface interaction between Aerosil nanoparticles and liquid crystal molecules. As the density of Aerosil increases (from 0.00 to 0.15 g/cm3), mutual interaction between Aerosil nanoparticles (size 7 nm) and a liquid crystal molecule (length 2 nm and width 0.5 nm) increases, but as the density of Aerosil increases further (from 0.15 to 0.20 g/cm3), the mutual interaction decreases and self-interaction between Aerosil nanoparticles increases and becomes maximum for the highest density of Aerosil (0.20 g/cm3) and shows a saturation. The evidence of surface interaction can be given on the basis of infrared (IR) results, as described in our earlier publication.14 Because of QRD, the presence of molecular disorder between liquid crystal molecules and Aerosil nanoparticles brings an induced crystallization below melting and shows activated kinetics of “IC” that can be explained using the rate dependent Arrhenius theory. Using this theory, the activation energy of the transitions can be calculated by the slope obtained from the graph plotted between ln β versus 1/T as shown in Figure 6. This energy represents the ordered-disordered molecular motion and rearrangement of the system LC1-xSilx near the transition temperature.21-24 The change in activation energy of the transition shows a relationship with its respective enthalpy and can be compared together with density variation (as explained in our earlier publication).13,14 Figure 12 shows a comparative plot between enthalpy and activation energy of the system for all densities of the S-K transition. The data are plotted as the change in excess of enthalpy versus density on the upper section and as the change in excess of activation energy versus density on the lower section, where ∆H and ∆E are enthalpies and activation energies of the system for different densities. This plot shows that as the density of Aerosil nanoparticles increases in the system, enthalpy for the S-K transition or “IC” increases and becomes saturated for the highest density of Aerosils (0.20 g/cm3) and its respective activation energy decreases in the same pattern. Because of quench random disorder (QRD), the presence of Aerosil nanoparticles in bulk 8CB induces a random disorder that causes the system to release more energy during “IC” before

Induced Crystallization of the LC1-xSilx System

Figure 12. Enthalpy (kJ/mol) and activation energy (kJ/mol) versus Aerosil density plot where the upper section shows enthalpy and the lower activation energy. Errors in the activation energy are found to be (0.01 kJ/mol.

Figure 13. Percent crystallinity and crystal growth versus Aerosil density plot where the upper section shows percent crystallinity and the lower growth.

going to the melting transition, and hence, the system undergoes an increase in enthalpy as the density of the Aerosil nanoparticles increases (from 0.00 to 0.15 g/cm3). Since the system releases more energy during “IC”, it needs less activation energy and activation energy decreases in the same pattern. For further increase of the density of Aerosils (from 0.15 to 0.20 g/cm3), the self-interaction between Aerosil nanoparticles increases more than the mutual interaction and the enthalpy of “IC” shows a decrease and becomes saturated and hence activation energy shows an increase in the same way. Figure 13 shows a comparative plot between the percent crystallinity and crystal growth for the “IC” of the system. As the density increases in the system, the percent crystallinity of the system increases for lower densities (up to 0.15 g/cm3) and then it shows a decrease for a further increase of density (from 0.15 to 0.20 g/cm3) of the Aerosils. On the other hand, the crystal grows as the density of the Aerosil increases and becomes bigger for lower densities (0.15 g/cm3) and then it becomes saturated for the highest density (0.20 g/cm3). This can also be explained in terms of molecular interactions between the Aerosil nano-

J. Phys. Chem. B, Vol. 111, No. 8, 2007 1921 particles and the liquid crystal molecules, as explained above. The major change observed in quantities is after 0.15 g/cm3 density of Aerosils, which indicates a switching from mutual interaction to self-interaction between Aerosil nanoparticles, where self-interaction becomes maximum and shows a saturation in the system for the highest density of Aerosils, 0.20 g/cm3. Data details for all densities of Aerosil dispersed in the LC1-xSilx system, i.e., crystal growth, percent crystallinity, “IC” range, enthalpy, and activation energy are given in Table 1. IV.B. Conclusions. To understand the kinetics of “induced crystallization” in the Aerosil dispersed LC1-xSilx system, the thermodynamic measurements of the Aerosil dispersed bulk 8CB system were performed using MDSC. The heating scan shows an additional exothermic peak before melting for the Aerosil dispersed liquid crystal system. As the density of the Aerosil nanoparticles increases, the peak found on the “IC” transition shows a temperature shift following an Arrhenius behavior and shows activated kinetics. The “IC” peak shifts toward lower temperatures as the ramp rate decreases and provides the energy dynamics of “IC” for the LC1-xSilx system. The enthalpy of “IC” increases and the crystal grows as the Aerosil nanoparticles increase in the system and become saturated at the highest density (0.20 g/cm3) of the system. This can be explained in terms of molecular disorder created between the Aerosil nanoparticles and the liquid crystal molecules. The change in patterns of enthalpy and activation energy depends on the interaction between Aerosil nanoparticles and 8CB molecules. As Aerosil nanoparticles are added into bulk 8CB, a surface interaction takes place between molecules where Aerosil nanoparticles are found to be coated with 8CB molecules and interact with each other under a weak hydrogen bond interaction. As the density of Aerosil nanoparticles increases in the system, the mutual interaction between Aerosil nanoparticles and liquid crystal molecules increases and shows an increase in quantities, but as the density of Aerosil nanoparticles increases further from 0.15 g/cm3, the mutual interaction decreases and self-interaction between Aerosil nanoparticles increases and brings a saturation into the system. This change in interaction brings a significant increase in enthalpy, crystal growth, percent crystallinity, and “IC” with a range up to 0.15 g/cm3 density of Aerosils, but for a further increase of Aerosil density the system shows a saturation. The enthalpy of the “IC” increases as the Aerosil density increases and its respective activation energy decreases (up to 0.15 g/cm3). As the density further increases (from 0.15 to 0.20 g/cm3), the self-interaction between Aerosil nanoparticles increases due to formation of Si-O-Si bonds and increases the disorder in the system. This indicates a decrease in the enthalpy and an increase in the activation energy for the “IC” transition for the highest density, 0.20 g/cm3, of Aerosil nanoparticles in the system. Acknowledgment. This work was supported by the NSFCAREER Award DMR-0092786. References and Notes (1) Bellini, T.; Radzihovsky, L.; Toner, J.; Clark, N. A. Science 2001, 294, 1074. (2) Roshi, A.; Iannacchione, G. S.; Clegg, P. S.; Birgeneau, R. J. Phys. ReV. E 2004, 69, 031703. (3) Retsch, C.; McNulty, I.; Iannacchione, G. S. Phys. ReV. E. 2002, 65, 032701. (4) Jin, T.; Finotello, D. Phys. ReV. Lett. 2001, 86, 818. (5) Iannacchione, G. S.; Garland, C. W.; Mang, J. T.; Rieker, T. P. Phys. ReV. E. 1998, 58, 5966. (6) Sharma, D.; Iannacchione, G. S. J. Chem. Phys., in press. (7) Barjami, S.; Roshi, A.; Sharma, D.; Iannacchione, G. S. ReV. Sci. Instrum., submitted.

1922 J. Phys. Chem. B, Vol. 111, No. 8, 2007 (8) Sharma, D.; Mandal, A.; Arguello, J.; Iannacchione, G. S. Biophys. J., to be submitted. (9) Sharma, D.; Shukla, R.; Singh, A.; Nagpal, A.; Kumar, A. AdV. Mater. Opt. Electron. 2000, 10, 251. (10) Ernst, C. R.; Schneider, G. M.; Rflinger, A. W.; Weissflog, W. Ber. Bunsen-Ges. 1998, 102, 1870. (11) Donth, E.; Korus, J.; Hempel, E.; Beiner, M. Thermochim. Acta 1997, 304-305, 239. (12) Sharma, D.; Shukla, R.; Kumar, A. Thin Solid Films 1999, 357, 214. (13) Sharma, D.; MacDonald, J. C.; Iannacchione, G. S. J. Phys. Chem. B 2006, 110, 16679-16684. (14) Sharma, D.; MacDonald, J. C.; Iannacchione, G. S. J. Phys. Chem. B 2006, 110, 26160-26169. (15) Clegg, P. S.; Stock, C.; Birgeneau, R. J.; Garland, C. W.; Roshi, A.; Iannacchione, G. S. Phys. ReV. E 2003, 67, 021703.

Sharma and Iannacchione (16) Clegg, P. S.; Birgeneau, R. J.; Park, S.; Garland, C. W.; Iannacchione, G. S.; Neubert, M. E. Phys. ReV. E 2003, 68, 031706. (17) Degussa Corp., Silica Division, 65 Challenger Road, Ridgefield Park, NJ 07660. Technical data is given in the Degussa booklet Aerosila. (18) Thoen, J.; Marynissen, H.; Dael, W. V. Phys. ReV. A 1982, 26, 2886. (19) A property of certain gels is to become fluid when mechanically disturbed (as by shaking or stirring) and then to reset after a period of time. (20) Bellini, T.; Buscaglia, M.; Chiccoli, C.; Mantegazza, F.; Pasini, P.; Zannoni, C. Phys. ReV. Lett. 2000, 85, 1008. (21) Vogel, H. Phys. Z. 1921, 22, 645. (22) Fulcher, G. S. J. Am. Ceram. Soc. 1926, 6, 339. (23) Maximean, D.; Rosu, C.; Yamamoto, T.; Yokoyama, H. Mol. Cryst. Liq. Cryst. 2004, 417, 215. (24) Blum, F.; Padmanabhan, A.; Mohebbi, R. Langmuir 1985, 1, 127.