Target Morphologies in Polymer Blends
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Target Morphologies in Polymer Blends Nigel Clarke and Ian Henderson Department of Chemistry, University of Durham, Durham, DH1 3LE, UK ABSTRACT We model a novel process for obtaining controlled morphologies in polymer blends. Particles of one type of polymer are allowed to dissolve in a matrix of a dissimilar polymer. Prior to complete dissolution the blend is quenched into the two phase region, such that phase separation takes place. The combination of the incomplete dissolution and the wavelength selection process associated with phase separation results in particles that during the ’intermediate’ stages have a core that is significantly rich in the matrix material. The concept is extended to consider the effect of phase separation on an inhomogeneous surface chemically patterned with regions which are more attractive to one component of the blend. INTRODUCTION There has recently been a great interest in targeted morphologies in polymer blends, block copolymers and polymer films. The ability to produce materials with a specified morphology is highly desirable since the final mesoscale structure determines many of the properties of the material. Polymer blends with co-continuous and droplet structures, and block copolymers that exhibit a wide variety of equilibrium and non-equilibrium morphologies, have long been exploited for their enhanced mechanical properties. More recently the use of twophase blends and copolymers has attracted attention for photonic[1], and other electronic applications[2, 3]. Block copolymers have particular appeal since the morphology can be controlled by varying the ratio of the block lengths and/or the architecture. Naturally, the lengthscale associated with the microstructure is dictated by the size of the polymer blocks. Although not impossible, it is non-trivial and costly to produce copolymers with lengths commensurate with the wavelength of light, a requirement if such polymers are to be used in photonic applications. Blends, on the other hand, can phase separate with a wide range of length-scales ranging from tens of nanometers to hundreds of microns. Phase separation occurs when a polymer blend is ’quenched’ from the one-phase region to the two-phase region of its phase diagram. The dynamics of phase separation, and hence the transient structures, depend on whether the blend is quenched into the metastable or the unstable state. In the former case, phase separation proceeds by nucleation and growth, resulting in droplet-like structures. In the latter, the blend phase separates spontaneously into a cocontinuous structure with a preferred length-scale dominating, a process known as spinodal decomposition. A more controlled approach to phase separation in blends was first explored by studying surface-directed spinodal decomposition[4]. Following on from this work, a considerable effort has been devoted to the study of phase separation in confined geometries[5]. Of particular potential is the use of patterned substrates to influence blend morphology and to produce anisotropi
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