(705f) 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.