Conference-CIF ’ 15-Nano-Segregation of Squalane between Smectic Layers of Rigid-Rod Polysilane

It has been theoretically predicted that spherical particles added to the smectic phase formed by rod-like particles segregate between the smetic layers and significantly enhance the stability of the smectic phase due to an entropic effect based on the steric repulsion between the particles, which is referred to as the depletion effect. In this study, we describe the first experimental verification of the theoretical prediction, in which the binary mixtures of the rod-like polymer, poly[n-decyl-(S)-2-methylbutylsilane] (PDMS), with narrow molecular weight distributions, and a spherical branched alkane, squalane, were investigated by synchrotron radiation smalland wide-angle X-ray scattering (SR-SAXS and SR-WAXS) and atomic force microscopy (AFM) observations and found to reproduce the predicted segregation between the smectic layers. [DOI: 10.1380/ejssnt.2015.121]


INTRODUCTION
Liquid crystal (LC) phases formed by rod-like particles with monodispersed lengths have been extensively studied using both in theoretical and computational methods.During the initial stage of these studies, the numerical experiments and computer simulations revealed that the most common LC sequence of the nematic-smecticcolumnar phases could be reproduced in the hard-rod particle model systems [1,2].These theoretical predictions agreed well with the thermotropic LC behavior of rigidrod helical polysilanes with narrow molecular weight distributions [3][4][5][6][7][8][9].
These theoretical approaches have been recently extended to binary mixtures of the hard-rod-like and spherical particles, showing the segregation of spheres between the smectic layers of rods due to an entropic effect driven by steric repulsion between the rod-like and sphere-like particles which is referred to as the depletion effect, schematically illustrated in Fig. 1 [10,11].
In this study, we present an experimental finding of the predicted nano-segregation based on synchrotron radiation small-and wide-angle X-ray scattering measurements (SR-SAXS and SR-WAXS) and atomic force microscopy (AFM) observations, in the thermotropic LC systems of the binary mixtures of poly[n-decyl-(S)-2methylbutylsilane] (PDMS) with narrow molecular weight distributions and a sphere-like saturated branched alkane, squalane, with a diameter comparable to that of the PDMS (Fig. 1).

II. EXPERIMENTAL
PDMS (M w =28,900 M w /M n =1.26 for SR-SAXS and M w =91,700 M w /M n =1.23 for AFM) was synthe-sized with the dichlorosilane monomer bearing (S)-2methylbutyl and n-decyl groups and fractionated into the samples with narrow molecular weight distributions according to a previously reported method.[12] The molecular weights of PDMS were selected so that the smectic layer reflection for the SR-SAXS and smectic banded pattern for AFM can be easily observed.Squalane was purchased from Tokyo Kasei (TCI, Tokyo, Japan) and used without further purifications as its purity has been found sufficiently high.PDMS and squalane were dissolved in chloroform to provide the binary mixtures with a designated mixing ratio.The solutions were gradually evaporated to dryness for the X-ray scattering measurements, or spin-cast on glass plates and annealed under saturated chloroform vapors for 12 h for the AFM observations.
The molecular weights of PDMS were determined by size exclusion chromatography (SEC) using a Shimadzu SCL-10Avp liquid chromatograph system controller equipped with UV-visible (Shimadzu SPD-10Avp) and RI (Shimadzu RID-10A) detectors, and a Shodex GPC K-805L column (Showa Denko, Tokyo, Japan).Chloroform was used as the eluent at the flow rate of 1.0 mL/min.The molecular weight calibration curve was obtained with standard polystyrenes (Showa Denko).
The AFM measurements were performed using a JSPM5200 scanning probe microscope (JEOL, Tokyo, Japan) in the AC-AFM mode at ambient temperature (ca. 25 • C) under an ambient atmosphere.Imaging was conducted with standard silicon cantilevers (µ-mash HQ:NSC36/AIBS, Nanoworld AG, Neuchâtel, Switzerland) with a typical resonance frequency of 65 kHz.
The SR-SAXS and SR-WAXS measurements were performed at the Institute of Materials Structure Science, Tsukuba, Japan (Photon Factory), with small-and wideangle X-ray equipment installed on the beam line, BL10C, with the approval of the Photon Factory Program Advisory Committee (No. 2013G724).The wavelength of the incident X-ray beam was 0.1488 nm.X-ray photographs were taken using a PILATUS 2M (Dectris, Switzerland) positioned 1064.6 mm from the sample's position.The samples were kept in glass capillary tubes at ambient temperature (ca. 25 • C) during the X-ray exposures for 60 s.The SAXS profiles were finally obtained as a function of the wave vector (q = (4π/λ) sin θ) by using silver behenate as a low-angle calibration standard.

III. RESULTS AND DISCUSSIONS
Figure 2 shows the SR-SAXS (A) and SR-WAXS (B) profiles observed for the binary mixtures of PDMS (M w =28,900 M w /M n =1.26) and squalane with various mixing ratios (weight of squalane/weight of binary mix- ture).The layer reflections of the smectic phase in SR-SAXS profiles shift toward smaller angles along with slightly sharpening of their peaks when increasing the mixing ratio of the squalane, although the layer reflection is generally weakened and broadened because smectic ordering is destabilized when the LC system is diluted with solvent.While at the same time, several reflections in SR-WAXS profiles, which were assigned to monoclinic molecular packing with a two-dimensional lattice of a = 18.6 Å, b = 24.7 Å, and γ = 89.9• , stay the same, indicating no squalane penetrating between the PDMS molecules.
In Fig. 3, the observed layer spacings for the binary mixtures are plotted versus the mixing ratio.The blue line (line A) is the theoretical curve when it is assumed that squalane is completely segregated in the interstitial region of the smectic layers, which is given by Eq. ( 1), and the green line (line B) is calculated on the assumption that the smectic phase is uniformly diluted with squalane, which is given by Eq. ( 2), where d max and d min are the layer spacings of the binary mixture calculated on the assumption of complete segregation and simple dilution, respectively, d 0 is the layer spacing of the polysilane, r is the mixing ratio, and ρ polysilane and ρ squalane denote the densities of the polysilane (0.91 g/cm 3 ) and squalane (0.88 g/cm 3 ), respectively.The observed layer spacings are between line A and B at the lower mixing ratios and starts to become closer to line B at the higher mixing ratio around 0.3, indicating the segregation of the squalane between the smectic layers and the partial separation from the smectic phase, although it has been predicted that the spheres up to 89 mol% of the binary mixture go between the smectic layers of the rods and stabilize the smectic phase [11].This might be at least partly due to the fact that the orientational freedom in real systems reduces the smectic phase stability enhacement which is induced by depletion effect because most of the theoretical studies have been performed using the frozen-orientation approximation [10].This characteristic segregation was confirmed by the AFM observations.Figure 4 shows AFM images, corresponding Fourier transforms, and cross-section profiles observed on the film of the binary mixture of PDMS (M w =91,700 M w /M n =1.23) and squalane with designated mixing ratios.All the AFM images show fairly clear banded textures characteristic of the smectic layers.Their repeating distance constantly increased with the increasing mixing ratio of squalane in good quantitative agreement with the SR-SAXS measurements.

IV. CONCLUSIONS
In conclusion, we have demonstrated the experimental verification of the theoretically predicted segregation of spheres between the smectic layers of rods in the binary mixtures of rod-like polysilane (PDMS) and a spherical branched alkane, squalane, although the segregation was not perfect.Further studies of the binary systems with various diameters of the spheres are needed because it has also been predicted that the depletion force is susceptible to the particle size.Work along this line is currently in progress.

FIG. 3 .
FIG. 3. Dependence of the observed layer spacing on the mixing ratio.Lines A (blue line) and B (green line) are calculated based on the assumption of complete segregation and simple dilution, respectively.
FIG. 4. AFM images observed on the film surface of the binary mixture of PDMS and squalane at the mixing ratios of (A) 0, (B) 0.1, (C) 0.2, (D) 0.3, (E) 0.4, corresponding Fourier transforms, and cross-section profiles taken along the red lines in the AFM images.Scale = 2 × 2 µm.