Glass Transition Temperatures in Poly(cyclohexyl methacrylate) Thin Films Prepared By Initiated Chemical Vapor Deposition

The glass transition temperature (T_g) is a critical physical parameter in amorphous polymers, defining the thermal transition of the material from the glassy to rubbery state. Consideration of T_g is important in designing polymers for practical applications. A large body of previous research has shown that the T_g is strongly altered in ultrathin films (<100 nm thick) due to nanoconfinement effects.¹ The T_gs in these films can be 10-30 °C lower than their bulk counterparts. The film fabrication approach and processing history has also been shown to influence T_gs of films. Understanding how to systematically reduce T_gs though film processing would prove beneficial in many applications, including solid polymer electrolyte layers for lithium ion batteries and gas separation layers in thin film composite membranes.

This research project examines how T_gs are altered in films prepared by initiated chemical vapor deposition (iCVD). In iCVD, monomer and radical species in the gas phase adsorb to a substrate and react through a free radical polymerization mechanism to yield thin films with thicknesses that can be controlled from 10nm to several micrometers. As iCVD integrates film synthesis and processing into a single step, the T_g is expected to deviate strongly from films prepared by conventional means (e.g. spin coating, solution casting, etc.) This research focuses on the polymer poly(cyclohexyl methacrylate) (PCHMA), which has previously been used as a sacrificial material in air-gap fabrication of microelectromechanical systems (MEMs). iCVD PCHMA has been thoroughly characterized using differential scanning calorimetry (DSC) to measure the T_g; thermal gravitational analysis (TGA) to measure the polymer’s thermal stability; Fourier transform infrared spectroscopy (FTIR) for composition analysis; and size exclusion chromatography (GPC) to measure molar mass distributions.

Preliminary results indicate that the iCVD PCHMA films (weight-average molecular weight around 20-25,000 and thicknesses between 800nm-2µm) have a lower T_g at 65-68°C in comparison to standard samples of PCHMA (5 mg powder, hydraulic pressed, and 1 µm thick spun cast films) with a T_gbetween 90-120°C. However, DSC measurements on thinner films (thicknesses less than 1 μm) are still underway as the heat flux resolution of the DSC measurement is inadequate for the miniscule film masses associated with thinner samples.

Research is ongoing to determine if T_gs in ultrathin films (on the order of 50-100nm) are even lower than what is observed with the thicker PCHMA films. In order to be able to measure the T_g of such thin films, the next step of this research is to build an environmental chamber that integrates active temperature control with in situ specular reflection measurements. As T_g is a second-order phase transition, the coefficient of thermal expansion undergoes a marked change in slope at the T_g. Measuring thickness as a function of temperature will provide a means to measure the T_gof thin PCHMA films prepared by iCVD.

¹ Ediger, M. D., & Forrest, J. a. (2013). Dynamics near Free Surfaces and the Glass Transition in Thin Polymer Films: A View to the Future. Macromolecules, 131127135402009. http://doi.org/10.1021/ma4017696