Photo-Enforced Stratification

Polymerization techniques typically result in a homogenous final polymer.  If surface and/or substrate interface chemistries that differ from the homogenous composition are desired for applications such as coatings and adhesives, a multiple step processes is necessary to achieve the desired chemistries.  It could be possible to overcome these obstacles with photo-enforced stratification; a method to allow for the production of a polymeric thin film with different surface and/or substrate interface chemistries in a single reaction step.  Photo-enforced stratification is a method of photopolymerization designed to induce a compositional gradient in a polymer thin film upon co-photopolymerization in the presence of a light gradient by taking advantage of the temporal and spatial control that photopolymerization affords.  The light gradient is established in the film by having a sufficiently high concentration of a chromophore in the pre-polymer mixture such that illumination of the mixture results in attenuation of the light as a function of depth in the mixture.  The light gradient will establish a gradient in the reaction rate of polymerization.  A co-photopolymerization in the presence of a reaction gradient results in the monomer with the higher reactivity ratio to react faster than the other monomer in the high light intensity regions at the top of the film where the rate of reaction is the fastest.  As the monomer with the higher reactivity ratio is depleted at the surface of the film, more of that monomer will diffuse from the bulk to the surface of the film.  At the same time, a counter diffusion of the lower reactivity ratio monomer from the surface into the bulk will occur.  This results in the surface of the thin film being enriched in the monomer that has the highest reactivity ratio.