Peter Gin

Introduction of molecular scale porosity into semicrystalline polymer thin films using supercritical carbon dioxide

We report supercritical carbon dioxide (scCO2) technology used for forming a large degree of molecular scale porosity in semicrystalline polymer thin films. The following three steps were integrated: (i) pre-exposure to an organic solvent which melted crystalline structures but did not cause a decrease in thickness, (ii) scCO2 exposure under the unique conditions where the anomalous absorption of CO2 occurred, and (iii) subsequent quick evaporation of CO2 to preserve the swollen structures. This unified process resulted in homogenous low-density polyphenylene vinylene films (a 15% reduction in density) with the sustained structure for at least 6 months at room temperature.

Directed self-assembly of nanoparticles at the polymer surface by highly compressible supercritical carbon dioxide

We report a versatile route for self-assembly of polymer-soluble nanoparticles at the polymer surface using highly compressible supercritical carbon dioxide (scCO2). Polystyrene and poly(methyl methacrylate)-based nanocomposite thin films with functionalized polyhedral oligomeric silsesquioxane and phenyl C61 butyric acid methyl ester nanoparticles were prepared on Si substrates and exposed to scCO2 at different pressures under the isothermal condition of 36 °C. The resultant structures could be then preserved by the vitrification process of the glassy polymersvia quick pressure quench to atmospheric pressure and subsequently characterized by using various surface sensitive experimental techniques in air. We found that the surface segregation of these nanoparticles is induced in the close vicinity of P = 8.2 MPa where the excess absorption of the fluid into the polymers maximizes. However, when the film thickness becomes less than about 4Rg thick (where Rg is the radius of polymer gyration), the uniform dispersion of the nanoparticles is favorable instead even at the same CO2 conditions. We clarify that the phase transition is correlated with the emergence of a concentration gradient of the fluid at the polymer/CO2 interface and is a general phenomenon for different polymer–nanoparticle interactions.

Effect of supercritical carbon dioxide on molecular aggregation states of side chains of semicrystalline poly{2-(perfluorooctyl)ethyl acrylate} brush thin films

We report a carbon dioxide-based approach to induce highly ordered molecular aggregation states of perfluoroalkyl (Rf) chains of densely-grafted poly{2-(perfluorooctyl)ethyl acrylate} (poly(FA-C8)) brush in place of conventional thermal annealing. Poly(FA-C8) brush films of 40 nm thickness were prepared by surface-initiated atom transfer radical polymerization. In-situ neutron reflectivity measurements for the poly(FA-C8) brush films under the isothermal condition of T = 309 K, which is below the bulk melting temperature (Tm = 348 K), elucidated large expansion of polymer chains due to sorption of CO2 molecules. Comparison of the swelling behavior with an amorphous poly{2-(perfluorobutyl)ethyl acrylate} brush thin film clarified that the sorption of CO2 molecules results in the melting of the semicrystalline poly(FA-C8) brush at P > 4.1 MPa. In addition, by using out-of-plane grazing incidence wide-angle X-ray diffraction, it was found that subsequent slow quench from P > 4.1 MPa induces rearrangement of the rigid rod-like Rf groups, forming highly ordered molecular aggregation structures similar to those via a conventional thermal process. The appropriate CO2 process conditions for the effective induction of the highly ordered structures of the rigid Rf groups are discussed in detail.

Surface Segregation of Nanoparticles Driven by Supercritical Carbon Dioxide

Surface segregation (i.e., preferential segregation of one component to the surface in multicomponent systems) is common to all material classes and is typically driven by a reduction in surface energy which compensates for the entropy loss and/or energy gain associated with the demixing of the components. However, the conventional surface segregation for polymeric systems requires high temperatures, typically close to 200°C, and long annealing time, in order to ensure enough polymer mobility. Here we show a low-temperature method to preferentially migrate organoclay nanoparticles to the polymer surface using supercritical carbon dioxide.