Surface crystallization in normal-alkanes and alcohols

A new, rare surface freezing, phenomenon is observed in molten normal-alkanes and their derivatives (alcohols, thiols, etc.). X-ray and surface tension measurements show the formation of a crystalline monolayer on the surface of the liquid alkane at temperatures up to 3 C above the bulk solidification temperature, T{sub f}. For alcohols, a single bilayer is formed. In both cases, the molecules in the layer are hexagonally packed and oriented normal to the surface for short chain lengths, and tilted for long ones. In both cases the single layer persists down to T{sub f}. In terms of wetting theory, this constitutes a very limited partial wetting of the liquid surface by the crystalline layer. The new surface phase is obtained only for chain lengths 14 < n {le} 50 in alkanes, and 16 < n < 30 in alcohols. The measurements are satisfactorily accounted for within a simple theory based on surface energy considerations

A model for a generalized liquid crystal phase sequence

A simple model is presented which is shown to qualitatively explain the structures of the generalized liquid crystal phase sequence : Crystalline-G/Smectic-F → Crystalline-J/Smectic-I → Crystalline-B → Smectic-C → Smectic-A. The model assumes an effectively long range potential which keeps the layers flat, and a partially frustrated nearest neighbour interaction which induces molecular tilt. In addition to determining the order of the phases, the model also explains the observed effects of a free surface, on the phase diagram.On présente un modèle simple qui explique qualitativement les structures de la séquence généralisée de phases cristal-liquide : cristal G/smectique F → cristal J/smectique I → cristal B → smectique C → smectique A. Le modèle suppose un potentiel effectif à longue distance qui impose des couches plates et une interaction partiellement frustrée entre proches voisins qui induit une inclinaison des molécules. Le modèle explique également l'effet d'une surface libre sur le diagramme de phases

Undulating membrane structure under mixed extensional-shear flow

We report on studies using a new X-ray extensional flow cell to examine, for the first time, the structure of undulating lamellar lyotropic liquid crystal systems under extensional flow. The extensional component of the flow profile produced within this cell causes the lamellae to orient. We find that, although the intermembrane spacing does not change at low flow rates, it suddenly decreases after a critical flow rate has been attained due to the stretching and straightening of the lamellae. The effects of the shear component of flow on this oriented system have been examined in the context of a theoretical model developed by Ramaswamy

Role of Metastable Phases in Polymer Crystallization; Early Stages of Crystal Growth

In this chapter we have addressed issues concerning crystallization of flexible polymers from melt. We have described several examples where crystallization proceeds via a transient phase before a thermodynamically stable phase intervenes. With several examples it has been shown that transient state of a phase strongly depends on the crystal size. A phase that is thermodynamically stable during the initial stages of crystallization becomes metastable with crystal growth. Taking linear polyethylene as an example, it is shown that crystal size can be controlled during polymerization, thus the phase behavior. These findings have implications in polymer processing like sintering of intractable ultra high molecular weight polyethylene