Chemical Engineering

We study the interfacial partitioning of poly(styrene-b-methyl methacrylate) (SM) diblock copolymers and nanogel particles (NGPs) comprising poly(methyl methacrylate arms) at interfaces between thin planar films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) homopolymers.

When copolymers with constant PS and varying PMMA block lengths are incorporated into the top PS layer, the resulting dewetting kinetics of the top PS film decrease by reducing the length of the PMMA block and increasing the molecular weight of the host PS homopolymer. Similar behavior is observed when the SM copolymers are added to the bottom PMMA homopolymer. The dewetting kinetics of systems with equimass mixtures of asymmetric copolymers added in the PS homopolymer lie between those of the individual copolymers. Systems incorporating SM copolymers possessing short PS blocks and long PMMA blocks, exhibit dewetting rates that are higher than those of the copolymer-free PS/PMMA bilayer. This behavior is attributed to the segregation of SM aggregates at the PS/PMMA interface, which destabilizes the film.

NGPs are also capable of compatibilizing the PS/PMMA interfaces but to a lesser extent than the SM copolymers. In contrast to the SM copolymers, however, the NGPs can segregate from PMMA homopolymers due to autophobicity. When in contact with a free surface, the particles migrate to the PMMA surface but remain inside the PMMA. When a thin film of PS is placed on top of a PMMA/NGP film, the NGPs segregate to and thus roughen the PMMA/PS interface. We attribute this behavior to a change in surface vs. interfacial energetics. Specifically, while the high surface energy of the native PMMA film keeps the particles inside the bulk PMMA, placing a thin PS layer on top of the PMMA/NGP film decreases the PMMA/PS interfacial tension by about an order of magnitude, which consequently permits segregation of the NGPs to the PMMA/PS interface. We further demonstrate the possibility of patterning the segregated NGPs by contacting a corrugated poly(dimethylsiloxane) (PDMS) layer to PMMA/NGP films. Regions of the PMMA/NGP film touching the PDMS layer exhibit NGP segregation, while non-contacted regions appear featureless.