Nanostructuration in Thin Epoxy−Amine Films Inducing Controlled Specific Phase Etherification: Effect on the Glass Transition Temperatures

Abstract

Inordertopreparecuredthin filmsthicknessesin the range of 90 300 nm, epoxy amine mixtures of di fferent concentrations in toluene are spin-cast onto oxidized silicon substrates. The glass transition temperature of the cured thin films is measured by microthermal analysis, revealing the existence of two distinct glass transitions temperatures for all the samples due to amine segregation at air/ film interface. These transitions are ascribed to two layers. The upper layer properties of the film due to the air/polymer interface are independent of the film thickness. This layer is around 30 nm thick, and its glass transition temperature is about 97 C and matches to a constant amine/epoxy composition at the air/ film interphase.Consecutively,the filmthicknessreductioninducesanincreaseoftheepoxyexcessinthesublayer,promotingsideepoxy reactions revealed by aliphatic ether bonds formation. Thus, signi ficant increase of the sublayer Tg, in comparison to the Tg of the bulk sample with an equivalent amine/epoxy ratio at the same amine conversion rate, is mainly due to these new ethers bonds for film thickness less than 160 nm. These bonds, surface catalyzed, are created at low temperature by the consumption of the epoxy excess. Etheri fication enhancement is finely controlled by the amine epoxy o ffstoichiometry in the sublayer tuned by its thickness and mainly results in glass transition temperature increase at lightly amine conversion. At complete epoxy and high amine conversion reached by postcuring, etheri fication in the sublayer leads to a constant amount whatever the thickness of the sample. The glass transition temperature of the sublayer postcured is around 180 C, equivalent to the bulk sample and does not bring out con finement e ffects.