Ultrathin Cellulose Films of Tunable Nanostructured Morphology with a Hydrophobic Component

Abstract

In recent years, a wide range of different methods to implant nanosized patterns on surfaces have been developed. Spin coating immiscible binary polymer blends is a straightforward method to prepare micro- and nanostructures on thin films. This study utilizes binary blends to effortlessly prepare stable, surface-functionalized cellulose films. Blends of trimethylsilyl cellulose (TMSC) majority phase and polystyrene (PS) minority phase in toluene were spin coated into an ultrathin film, and TMSC was hydrolyzed to cellulose. The films were characterized and quantified using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), contact angle measurements, and quartz crystal microbalance (QCM-D). AFM revealed that horizontally phase-separated structures form during spin coating: after the hydrolyzation of TMSC to cellulose, PS protrudes from cellulose as distinct patches. The patches are disk-like structures with a circular radial cross-section and a height of ca. 5-20 nanometers. The smaller the amount of PS in the original spin coating solution, the smaller the PS island dimensions in the films. The results obtained from the XPS measurements support the AFM results. Water contact angle of the PS/cellulose films increases from 61 degrees to 71 degrees when the relative amount of hydrophobic PS is increased from 1:100 to 1:5. Thus by simply varying the ratio of the film components subtly tailored hydrophobic properties can be achieved. The swelling of the films due to exposure to water was studied by QCM-D. The swelling was not affected by the amount of PS in the blend, and at equivalent cellulose content the blends and pure cellulose films exhibited similar swelling characteristics. In addition, the QCM-D evaluation demonstrated that the films are stable over extended periods of time and are suitable for fundamental studies by QCM-D.