Surface mechanical properties of amorphous polymers using spontaneous particle embedment: Epoxy, poly(α-Methylstyrene), polystyrene of different architecture, and poly(vinyl acetate)

The assessment of surface mechanical properties of polymer has become a concern to the glassy material research community because of property deviation from macroscopic material reported by many studies. The assessment of the deviation is very crucial for microelectronics, paintings, coatings, and applications such as coatings in hard disc, CDs, DVDs and cameras and for paintings, adhesive and printing industries. The particle embedment technique developed by Teichroeb and Forrest is a novel method which can be used to investigate the surface softening or stiffening compared to macroscopic material by evaluation of modulus/compliance. Forrest’s group observed the enhanced surface dynamics for PS at temperature below glass transition temperature. Hutcheson and McKenna analyzed their data and found that the enhanced dynamics is still in the segmental region.
In the present work, we used the combination of particle embedment technique and elastic contact mechanics model, named the Johnson, Kendall and Roberts (JKR) model, to estimate the surface modulus of neat epoxy, epoxy/POSS composites and polystyrene films at room temperature. The work of adhesion between submicron particle and the polymer surface was used as the driving force for particle embedment. Atomic force microscopy (AFM) was used for particle size characterization and particle embedment depth estimation. The surface modulus value was found softer than the macroscopic glassy modulus implying the surface softening at room temperature. The findings were confirmed by experiments with particles of different surface energy or different work of adhesion with polymers such as silica and gold particles. The maximum stress values on all surfaces induced by the particle embedment were estimated to verify the expected response to be close to the linear regime. In our second study, we extend the technique to determine the surface compliance of poly(α-methylstyrene) (PαMS) for temperatures from room temperature to Tg + 21 °C for which existence of no mobile surface layer was reported by Paeng and Ediger. We used the Lee and Radok (LR) model for surface compliance estimation. Results were compared with macroscopic viscoelastic data for PαMS obtained from the literature and with new experimental data from the present study. We observed surface softening for the PαMS in the temperature range from room temperature to Tg − 21 °C. This was followed by a crossover to a surface mechanical response that was stiffer than the macroscopic material for temperatures above Tg − 21 °C to Tg + 21 °C. These novel findings were observed for both 199 and 60 nm diameter silica particles. In the next study, we explored the surface compliance of linear polystyrene (PS) and studied the architectural effects by comparing the difference among linear PS, three arm star PS and eight arm star PS for a temperature range from Tg – 10 °C to Tg + 10 °C. We found the similar behavior for both linear and star PS. All three PS showed the surface softening up to Tg, then surface stiffening appeared at several degrees above Tg and continued up to Tg + 10 °C. The difference between PαMS and all PS is that PαMS showed surface softening at lower temperature relative to Tg when compared with PS. For the last study, we examined the effect of relative humidity (RH) on time-dependent embedment behavior of silica particles on poly (vinyl acetate) (PVAc) surface.