Physics (Internal)

Aggregation States and Dynamics of Polymers at Non-solvent Interfaces

This event is part of the Biophysics/Condensed Matter Seminar Series.

Abstract: Interfaces of polymers with “nonsolvents” play an important role in their functional properties such as wettability, friction with lubricants, cell adhesion, biocompability, etc. To design and construct highly functionalized polymers for applications that exploit these characteristics, aggregation states of the polymers at the liquid interfaces must be understood as the first benchmark. However, this is experimentally difficult because such interfaces are buried. In this study, we use neutron reflectivity (NR) and sum-frequency generation (SFG) spectroscopy to study, respectively, the density profile and local conformation of poly(methyl methacrylate) (PMMA) at the interface with air or gaseous nitrogen (N2) and at water interfaces.

Specular neutron reflectivity (NR) was applied to a perdeuterated poly(methyl methacrylate) (dPMMA) film spin-coated on a substrate. The interface of dPMMA with water was diffuse in comparison with the pristine interface with air. Interestingly, the dPMMA film was composed of a swollen layer and the interior region, which also contained water, in addition to the diffused layer. To conserve mass, the swelling of the film by water is accompanied by an increase in the film thickness. The change in the film thickness estimated by NR was in excellent accord with the result of direct observation using atomic force microscopy (AFM). The modulus of dPMMA in the vicinity of the water interface was also examined on the basis of force-distance curves measured by AFM. The modulus decreased closer to the outermost region of the film. The extent to which the modulus decreased in the interfacial region was consistent with the amount of water sorbed into the film.

The local conformation of PMMA chains at the N2 and water interfaces was studied by infrared (IR)-visible SFG spectroscopy. Although SFG spectra in the C-H region for PMMA at the N2 interface have been hitherto reported, the peak assignments are not in accord with one another. Thus, we first made the accurate assignments of SFG peaks using films, which had been well annealed at a temperature above the glass transition temperature for a long time, of three different deuterated PMMAs as well as normal protonated PMMA. At the N2 interface, hydrophobic functional groups such as  methyl, ester methyl and methylene groups were present. While the  methyl group was oriented along the direction normal to the interface, ester methyl and methylene groups were oriented parallel to the interface. Quantitative discussion concerning the orientation of the functional groups of PMMA at the N2 interface was aided by a model calculation. Once the PMMA film was contacted with water, the carbonyl groups of the PMMA side chains were oriented to the water phase to form hydrogen bonds with water molecules, resulting in the migration of ester methyl into the internal region of the film. Concurrently, the methylene groups became randomly oriented at the water interface and/or in part migrated into the internal region. Interestingly, the  methyl groups still existed at the water interface oriented along the parallel direction. The outermost region of PMMA in water can be consisted of hydrophilic and hydrophobic domains with sub-nanometer scale. Water molecules H-bond to themselves near the hydrophobic domains, leading to the formation of an ice-like structure of water molecules. On the other hand, water molecules adjacent to the hydrophilic domains H-bond with carbonyl groups.