Kinetics of UV Irradiation Induced Chain Scission
and Cross-Linking of Coumarin-Containing Polyester Ultrathin Films
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Abstract
Photoresponsive thin films are commonly
encountered as high performance
coatings as well as critical component, photoresists, for microelectronics
manufacture. Despite intensive investigations into the dynamics of
thin glassy polymer films, studies involving reactions of thin films
have typically been limited by difficulties in decoupling segregation
of reacting components or catalysts due to the interfaces. Here, thin
films of coumarin polyesters overcome this limitation where the polyester
undergoes predominately cross-linking upon irradiation at 350 nm,
while chain scission occurs with exposure to 254 nm light. Spectroscopic
ellipsometry is utilized to track these reactions as a function of
exposure time to elucidate the associated reaction kinetics for films
as thin as 15 nm. The cross-linking appears to follow a second order
kinetic rate law, while oxidation of the coumarin that accompanies
the chain scission and enables this reaction to be tracked spectroscopically
appears to be a first order reaction in coumarin concentration. Because
of the asymmetry in the coumarin diol monomer and the associated differences
in local structure that result during formation of the polyester,
two populations of coumarin are required to fit the reaction kinetics;
10β20% of the coumarin is significantly more reactive, but
these groups appear to undergo chain scission/oxidation at both wavelengths.
These reaction rate constants are nearly independent (within 1 order
of magnitude) of film thickness down to 15 nm. There is maximum rate
at a finite thickness for the 254 nm exposure, which we attribute
to constructive interference of the UV radiation within the polymer
film, rather than typical confinement effects; no thickness dependence
in reaction rates is observed for the 350 nm exposure. The utilization
of a single polymer with two distinct reactions enables unambiguous
investigation of thickness effects on reactions