Biofilms are the habitat of 95% of bacteria successfully protecting bacteria from many antibiotics. However, inhibiting biofilm formation is difficult in that it is a complex system involving the physical and chemical interaction of both substrate and bacteria. Focusing on the substrate surface and potential interactions with bacteria, we examined both physical and chemical properties of substrates coated with a series of phenyl acrylate monomer derivatives. Atomic force microscopy (AFM) showed smooth surfaces often approximating surgical grade steel. Induced biofilm growth of five separate bacteria on copolymer samples comprising varying concentrations of phenyl acrylate monomer derivatives evidenced differing degrees of biofilm resistance via optical microscopy. Using goniometric surface analyses, the van Oss-Chaudhury-Good equation was solved linear algebraically to determine the surface energy profile of each polymerized phenyl acrylate monomer derivative, two bacteria, and collagen. Based on the microscopy and surface energy profiles, a thermodynamic explanation for biofilm resistance is posited.
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Leeuwenhoek, one of the first microscopists, understood that the interpretation of features in a magnified image depends largely on the quality of the prepared sample. Today, one of the most utilized microscopes is the scanning electron microscope (SEM) due to its high throughput, ease of operation, and simple sample preparation. While there has been extensive research on sample coatings for SEM imaging (e.g., conductive metals, coating thickness, charge effects) [1], less attention has been given to artifacts introduced from the fracture and cutting of samples for high magnification imaging. In the field of gas separations, liquid nitrogen is commonly used to "freeze-fracture" polymer membranes to visualize internal microstructure. While this method allows for high throughput, it also imparts artificial morphology and topology that can easily be misinterpreted [2]. In this paper, SEM and atomic force microscopy (AFM) imaging techniques were utilized to showcase the importance of microtomy for gas separation membrane research.A variety of polymers and additives were selected to represent the numerous materials and common internal morphologies present in membranes for gas separation research. Polybenzimidazole (PBI) was purchased from PBI Performance Products, Inc. (M w ~30 KDa, 26 wt% in dimethylacetamide with 1.5% (w/w) LiCl). Matrimid 5218 ® was obtained from Ciba Specialty Chemicals (Huntsman Advance Materials Americas, Inc.). Polysulfone (PSf) was acquired from Solvay Chemicals, Inc. (Udel ® P-3500 LCD). Cellulose acetate (M w ~100 KDa) was purchased from Acros Organics (Fisher Scientific). Basolite ® Z1200 (ZIF-8) was purchased from Aldrich. The co-polyimide 6FDA-DAM:DABA (3:2) (6FDD) and colloidal ZIF-8 (cZIF-8) were synthesized according to published procedures [3][4]. Polymer, polymer blend, and mixed-matrix membranes (MMM) were fabricated and annealed using our custom protocol [3]. Cross-sections of membranes were prepared via freeze-fracture using liquid nitrogen and by microtoming. Membranes were submerged in liquid nitrogen for several seconds then pressed against the bottom of the low-form nitrogen dewar flask to fracture the membrane and reveal a cross-section. A Leica EM UC7 Ultramicrotome was used to cut the membrane samples with either 45° glass knives or a 45° diamond knife supplied by Diatome. The freeze-fractured and microtomed samples were imaged by SEM (Zeiss Supra 40) and AFM (Bruker Multimode 8 with "J" scanner in PFQNM) to compare their microstructures.The most common features observed in membrane cross-sections prepared by freeze-fracture are stretched or elongated deformations, craters, flakes, and web-like structures (Figure 1). In MMMs or polymer blend membranes, interfacial voids are typically amplified or introduced during freeze-fracture due to the disruptive force applied during the preparation. It is also difficult to determine the level of miscibility of blends and wetting of additives by the polymer matrix using samples prepared by freezefracture. An example of the difference ...
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