Polymeric nanostructures are important in integrated circuit fabrication as well as in a series of emerging applications ranging from sensors to biotechnology. The interest in achieving sub-100 nm features in integrated circuits raises a number of issues related to the mechanical integrity and thermophysical properties of such systems. While there has been extensive study of the effect of confinement on the thermophysical properties of polymeric films and nanocomposites, there has as yet been no study of the glass transition temperature (Tg) of 1-D polymeric nanostructures. Here we provide results of the first study of Tg in such nanostructures using a novel fluorescence method to determine Tg values in polymeric nanostructures.

Nanolines of varying thickness (h) and line width (LW) were made by E-beam lithography of thin and ultrathin poly(methyl methacrylate) (PMMA) films supported on silica slides. There are attractive polymer-substrate interactions (hydrogen bonds) in this system, resulting in an increase in Tg with decreasing thickness in ultrathin PMMA films supported on silica. However, in the nanoline geometry, of relevance for the semiconductor industry, there is a substantial increase in the free surface (polymer-air interface) area as compared to the thin films from which they were made. Previous studies of freely standing PMMA films (without a substrate) have shown that the Tg decreases with decreasing thickness, which is caused by the free surface regions being of higher mobility and lower Tg than those in bulk PMMA. We find that PMMA nanolines with h = 18 nm and LW = 50 nm yield Tg reductions of 15 K relative to that of an 18-nm-thick PMMA film and 5 K relative to that of bulk PMMA. This indicates that knowledge of how nanoconfinement modifies Tg in supported polymer films cannot be used without modification in predicting the Tg behavior of nanopatterned polymer on a substrate. Instead, the detailed geometry involved in the nanopattern must be explicitly considered in any experimental studies or simulations of such nanostructures.