Melting of Variable-Thickness Liquid-Crystal Thin Films: A Synchrotron X-Ray Study

Abstract

The results of a synchrotron x-ray study of the crystalline~jB to smectic-A melting transition in 4-w-pentylbenzenethio-4'-n-tetradecyloxybenzoate (14S5) are presented. Samples of two, three, five, and twelve molecular layers were investigated with the freestanding thin-film technique. A single, abrupt, hysteretic transition is observed in twolayer films with temperature-independent positional correlations above the transition. All thicker films melt by two first-order phase transitions; the character of the intermediate phase depends strongly on film thickness. PACS numbers: 64.70.Ew, 61.30.Eb, 68.60.+ qThe planar melting behavior which occurs within the layers of smectic liquid crystals provides an opportunity to study the evolution of the melting transition from two to three dimensions with freestanding thin-film techniques/ The thinnest films consisting of only two layers are the only experimentally viable example of substrate-free two-dimensional (2D) melting known. The present study has been motivated by the expectations (1) that melting in two-dimensional systems might involve qualitatively new physical mechanisms and (2) that the behavior versus thickness would be relevant to our understanding of 3D melting transitions in smectics in particular and in solids in general. Additional interest in the problem has been stimulated by a detailed theory of second-order 2D melting via a dislocation mechanism,^ by controversial molecular-dynamics calculations/ and by recent experimental studies demonstrating second-order melting in both incommensurate Xe and Ar monolayers adsorbed on a pyrolytic graphite substrate."^ Liquid-crystal thin films were prepared as previously described/ However, a new material, 4-n-pentylbenzenethio-4'-n-tetradecyloxybenzoate (TiS5), was used, which has two distinct advantages over N-(4-w-butyloxybenzylidene)-4-woctylaniline (40.8) used in our previous studies of crystalline-B thin films: (1) T4S5 is thermally stable and (2) it has a wide smectic-A range (66.5 to 86.6 °C). This last point is important because thin-film transitions occur some 10 °C above the respective bulk transition temperatures,^ and films cannot be made when the bulk material is above the smectic regime. Our experiments were carried out on the focused wiggler beam line VII-2 at the Stanford Synchrotron Radiation Laboratory. Two different resolution configurations were used. A high-resolution mode using two Si(lll) crystals in the monochromator and one in the analyzer had a longitudinal resolution component AQn =7xlO"'*A"^ [full width at half maximum (FWHM)] at Q ~ 1.4 A" \ For studying broad liquid scattering, we used pyrolytic graphite (002) monochromator and analyzer crystals, which relaxed the longitudinal resolution to AQii c^O.03 A"^ (FWHM).The x-ray scattering work presented here is closely related to studies of the mechanical shear properties described in a Letter by Bishop et aL^ We begin with results for the two-layer ( AT = 2) system. The mechanical measurements^ found three distinguishable features, but x-ra...

Hexatic ordering in liquid crystal films

“…In 1979, D. E. Moncton and R. Pindak (Moncton and Pindak 1979, Moncton et al 1982 proposed that, in the thin film limit, freely suspended films of thermotropic liquid crystal should provide an ideal substratefree system for studying two-dimensional melting. They began a series of studies on the melting of very thin films at the S,-+S, phase transition, using a synchrotron X-ray source.…”

Direct Layer-by-Layer Freezing Transitions in Free-Standing Smectic-A 14S5 Films

“…It can be further inferred from previous x-ray studies [11] and our additional optical-reflectivity (OR) results that the frozen Cry-B surfaces consist of a single outermost layer on either side of the films. For films thicker than four molecular layers, there are additional layer-by-layer Sm-A À Cry-B transitions upon further cooling, which are most easily seen in the OR results presented below.…”

“…Our most significant observation is that there is no evidence whatsoever of a hexatic phase in all our films of various thickness as they undergo layer-by-layer freezing. This represents an important definitive result on the existence of direct Sm-A -Cry-B layer-by-layer freezing without involving a hexatic phase, since single-crystal ED is a much more sensitive technique to discern hexatic behavior than the previous studies on 14S5 films using torsionaloscillator [10], x-ray intralayer powder diffraction [11], or video observation of birefringence changes [7]. In particular, we see no evidence of an intermediate phase with no positional order but with bondorientational order in the interior layer of the three-layer film that was suggested earlier [10,11].…”

“…This represents an important definitive result on the existence of direct Sm-A -Cry-B layer-by-layer freezing without involving a hexatic phase, since single-crystal ED is a much more sensitive technique to discern hexatic behavior than the previous studies on 14S5 films using torsionaloscillator [10], x-ray intralayer powder diffraction [11], or video observation of birefringence changes [7]. In particular, we see no evidence of an intermediate phase with no positional order but with bondorientational order in the interior layer of the three-layer film that was suggested earlier [10,11]. The free-standing two-layer smectic film is of particular interest because of its possible use as a prototype to test the defect-mediated theory of 2D melting [8].…”

“…Unfortunately, there have been conflicting reports on the surface-freezing behavior of this material. Mechanical and x-ray measurements on 14S5 free-standing films indicated that, upon cooling, the outermost layers of Sm-A films undergo a single freezing transition into the Cry-B phase, to be followed by the abrupt freezing of the entire interior at a lower temperature [10,11]. These studies also suggested that the three-layer film might be somewhat unique in that the interior layer appears to show a hexatic intermediate phase.…”