PHD CANDIDATE, NEW JERSEY INSTITUTE OF TECHNOLOGY
Probing Nanoscale Hydrophobicity and Chemical Distribution of Surface Modified PES Membranes.
Chemical modifications bring unique properties into polymeric membranes that may have enhanced filtration or separation efficiencies, antifouling, antimicrobial activity and selectivity. However, there is a lack of nanoscale characterization of chemical additive distribution and impacts of chemical modifiers or additives on membrane surface properties, especially those at nanoscale. In this study, a series of industrially relevant polyethersulfone (PES) membranes modified with poly (ethylene glycol) (PEG) and polyvinylpyrrolidone (PVP) were analysed systematically. Particularly, hydrophobicity and chemical distribution were scrutinized by atomic force microscopy (AFM) and AFM coupled with infrared analysis capability (AFM-IR) for the first time that successfully resolved nanoscale structural and chemical properties of the chemically modified PES membranes. Our results indicated the heterogeneous spatial distribution of PVP and PEG based on their characteristics IR bands and the resulting hydrophobicity distribution on modified membrane surfaces at nanoscale. Particularly, we established a linear correlation (R2=0.994) between the measured adhesion force and water contact angles, which enabled the examination of local surface hydrophobicity. The PES membranes became more hydrophilic with the increasing blend of PVP and PEG. With AFM-IR, trace amounts (1-4%) of PVP could be identified sensitively on PES membranes based on their unique characteristic IR bands, which were not achieved by FTIR or IR mapping. Overall, these novel characterization approaches hold paramount importance for the design and quality control of polymer membrane modification and manufacturing.
Abstract: Atomic force microscopy (AFM) has evolved to be one of the most powerful tools for the characterization of material surfaces especially at the nanoscale. Recent development of AFM has incorporated a suite of analytical techniques including surface-enhanced Raman scattering (SERS) technique and infrared (IR) spectroscopy to further reveal chemical composition and map the chemical distribution. This incorporation not only elevates the functionality of AFM but also increases the resolution limitation of conventional IR and Raman spectroscopy. Despite the rapid development of such hybrid AFM techniques, many unique features, principles, applications, potential pitfalls or artifacts are not well known to the community. This review systematically summarizes the recent relevant literature on hybrid AFM principles and applications. It focuses specially on AFM-IR and AFM-Raman techniques. Various applications in different research fields are critically reviewed and discussed, highlighting the potentials of these hybrid AFM techniques. Here, the major drawbacks and limitations of these two hybrid AFM techniques are presented. The intentions of this article are to shed new light on the future research and achieve improvements in stability and reliability of the measurements.
Pub.: 25 Jan '17, Pinned: 19 Jun '17