Phase-specific pore growth in ultrathin bicomponent films from cellulose- based polysaccharides†

The preparation of ultrathin (<100 nm) bicomponent films from hydrophobic polysaccharides with phase-specific pore growth was demonstrated and the underlying phenomena behind morphology formation were fundamentally investigated. The films were constructed, in a single-step process, by spin coating mixtures of trimethylsilyl cellulose (TMSC) and cellulose triacetate (CTA) from a common solvent. Atomic force microscopy (AFM) revealed a nanoand micron-scale phase separated structure, typical for interfacial polymer blends. Vertical phase separation had resulted in a continuous layer of TMSC next to the substrate with laterally phase separated CTA and TMSC on top. Furthermore, Xray photoelectron spectroscopy (XPS) and contact angle measurements indicated the presence of a thin overlayer of TMSC. In addition, increased relative humidity conditions during spin coating resulted in the formation of pores when the CTA weight percent in the blend was in the range from 17 to 83% (i.e., in TMSC/CTA blend ratios 5 : 1, 2 : 1, 1 : 2, and 1 : 5). Closer analysis of the morphology indicated that the pores resided exclusively in the CTA phase. Hypothetically, the formation of the observed peculiar morphologies was ascribed to various phenomena occurring upon spin coating: vertical and lateral polymer phase separation, dewetting under humid atmosphere, and layer inversion during dewetting. It is concluded that the obtained ultrathin polysaccharide films with tailored surface pores, morphology and wettability are expected to be useful in emerging nanotechnologies while having the advantage of an effortless manufacturing process.