Static wetting behaviour of diblock copolymers

D. Ausserre, V. A. Raghunathan, M. Maaloum
1993 Journal de Physique II  
~ Thin liquid films of ordered dihlock copolymers deposited on a solid substrate form a multilayer stacking parallel to the solid surface. A multilayer with a finite extend can he stable, metastable, or unstable, depending on thc relative values of the surface ener&ics of the various interfaces. The spreading parameter and chemical potential of a n-layer are derived, and used for classifying all possible situations. It is shown that only monoand hilayers can be siahle, and that non-wetting
more » ... at non-wetting multilayers are subjected to a long-time piling up instability, leading in practice to the formation of characteristic riggourat-like Structures. A diblock copolymer is made of two polymer chains with different chemical compositions A and B linked together end to end. Depending on the temperature, these materials are found in two different states. For T > To" A and I3 are mixed and form a homogeneous melt. On the other hand, for T < Too, A and B species phase-separate and form miccrodomains with one dimension at least comparable to the chain sire. To, is called the microphase separation temperature or the order-disorder transition temperature [ I ] . In the case of symmetric diblock copolymers, which have almost equal volume fractions of A and B, the microphase structure is lamellar. In thin films of these materials deposited on a solid substrate, these layers orient parallel to the substrate. In the present paper, we examine the wetting properties of these copolymer melts. The wetting behaviour of a simple liquid A on a solid surface is controled by the spreading parameter [Z], defined by where y s v , yLs and ypv are the surface tensions associated with the solid-vapour, liquid-solid and liquid-vapour interfaces, respectively. If S,\ is positive, the liquid spreads on the solid surface. Otherwise it does not wet the surface and forms a drop with a well-defined contact angle 8 , given by the Young relation (2) A Yfv cos H" = YS" -YLS . Equations (1) and (2) hold good for any homogeneous liquid including polymer melts 1486 JOURNAL DE PHYSIQUE I1 N' 10
doi:10.1051/jp2:1993215 fatcat:m45btxac3rg23e2o6tjw3xyt6u