Langmuir 1998, 14, 7065-7071
7065
Autophobicity and Layering Behavior of Thin Liquid-Crystalline Polymer Films M. W. J. van der Wielen,* M. A. Cohen Stuart, and G. J. Fleer Laboratory for Physical Chemistry and Colloid Science, Wageningen Agricultural University, Dreijenplein 6, 6703 HB Wageningen, The Netherlands Received June 18, 1998. In Final Form: September 25, 1998 The stability against breaking-up of thin spin-coated films of liquid-crystalline polymers depends on the film thickness and annealing temperature. This study concerns side-chain liquid-crystalline polymers, based on alternating copolymers of maleic anhydride and mesogenic alkenes. The mesogenic group is a methoxybiphenyl. The as-prepared films are homogeneous and remain stable below the glass transition temperature. Upon annealing above this temperature, the films start to dewet. At the final stage of dewetting only droplets remain on top of a rather stable bilayer, which itself does not participate in the dewetting. This indicates autophobic behavior. The bilayer is even present above the isotropization temperature. In the mesophase we have a layered film, and dewetting may occur over several ordered layers. In all cases the dewetting is not linear in time and polymer slippage seems to take place on top of the stable bilayer. In the case of polymer slippage, a t2/3 dependence is expected for the growth rate. That is indeed found above the isotropization temperature. In the mesophase the dewetting differs from the “normal” slippage behavior and a weaker time dependence is observed. Around the isotropization temperature there is a strong increase in the (initial) dewetting velocity of over more than 2 orders of magnitude, due to the sudden drop in viscosity.
1. Introduction The adhesion between a polymer and a solid substrate is of great importance in many practical applications, such as coatings and optical and electrical devices. In these applications the polymer film should be stable with respect to morphology and thickness. When the polymer film is thicker than a few micrometers, gravity affects the film stability. Thinner polymer films (
