Processing dependence of surface morphology in condensation cured PDMS nanocomposites

A processing method using high speed mixing was developed for the generation of nanocomposites comprised of α,ω-dihydroxy-polydimethylsiloxane (PDMS) and untreated fumed silica nanoparticles (U-FSN). Conventional condensation cure with poly(diethoxysiloxane) (PDES) was employed so as to generate PDMS/FSN nanocomposites with increasing weight fractions of siliceous domain. This study focuses on the changes in surface morphology imaged by non-contact or tapping mode AFM (TM-AFM) as a function of increasing the initial concentration of PDES. The ratios (X) of Si-OEt from PDES to Si-OH from PDMS end groups are 4X, 14X, 28X, 35X, 45X and 60X. Compared to prior work, two important differences in the present investigation are (1) the use of a high shear mixer as a processing tool to facilitate nanoparticle dispersion and (2) spin coating instead of dip coating. Common to the present and prior work is the use of TM-AFM for investigating surface morphology as a function of siliceous phase precursor concentration. TM-AFM phase images show nanoparticles are “reporters” and reflect effects of composition and processing on surface morphology. Near-surface nanoparticles are clearly imaged up to 35X, “disappear” at 45X, then “reappear” at 60X. These results are different from those previously reported where “disappearance” was noted for 14X compositions. The differing results reveal that processing conditions have an important effect on surface morphology. A model is presented that accounts for the surface morphological observations and wetting behavior via sessile drop measurements.