Characterization of thickness dependent morphology in nanoconfined PS/PLA polymer blended thin films

Poly(lactic acid) (PLA), a bio-derived, biodegradable polymer, has enhanced mechanical properties when blended with polystyrene (PS). This study investigates the nanoconfinement effects on film morphology in PS/PLA blends, examining film thickness and blend composition influences.

PS (280,000 g/mol) and PLA (111,000 g/mol) blends were prepared and diluted with chloroform solvent to achieve film thicknesses ~5nm to ~60nm and spuncast onto planar silicon substrates. Morphologies were characterized by atomic force microscopy (AFM), film thickness by ellipsometry, and surface composition by water contact angle model.

AFM revealed distinct thickness-dependent morphological changes. Below ~10nm, PLA crystallinity was suppressed by substrate-imposed confinement, yielding smoother surfaces and reduced domain sizes. 

At low PLA fractions (<50 wt%), decreasing film thickness induced  contact angles approaching pure PLA, suggesting preferential PLA surface segregation and that nanoconfinement restricts PS chain mobility over confined PLA. Conversely, high PLA wt% films (>75 wt%) had contact angles approaching pure PS, suggesting a thin PS surface layer.

Our findings demonstrate that nanoconfinement alters the domain morphology of PS/PLA thin films, allowing for tunable film functionality through thickness and composition changes. Future modulus measurements, XPS, and selective solvent leaching will quantify roughness and surface morphology changes and confirm surface composition.