Lamellar orientation at the surface of isotactic polystyrene thin films analyzed by sum frequency generation spectroscopy.

Analyzing the orientation of polymeric crystalline lamella at the surface of thin films can be challenging. Even though atomic force microscopy (AFM) is often sufficient for this analysis, there are cases when imaging is not sufficient to confidently determine lamellar orientation. Here, we used sum frequency generation (SFG) spectroscopy to analyze the lamellar orientation at the surface of semi-crystalline isotactic polystyrene (iPS) thin films. The SFG orientation analysis indicated that the iPS chains are oriented perpendicular to the substrate (flat-on lamellar orientation), which was confirmed by AFM. By analyzing the evolution of the SFG spectral features with the progress of crystallization, we demonstrated that the ratios of the SFG intensities of the phenyl ring resonances are a good indication of the surface crystallinity. Furthermore, we explored the challenges associated with SFG measurements of heterogeneous surfaces, which is commonly present in many semi-crystalline polymeric films. To the best of our knowledge, this is the first time that the surface lamellar orientation of semi-crystalline polymeric thin films was determined by SFG. Also, this work pioneers in reporting the surface conformation of semi-crystalline and amorphous iPS thin films by SFG and in linking the SFG intensity ratios to the progress of the crystallization and the surface crystallinity. This study demonstrates the potential applicability of SFG spectroscopy in the conformational analysis of polymeric crystalline structures at interfaces and opens the way to the investigation of more complex polymeric structures and crystalline arrangements, especially for the case of buried interfaces, where AFM imaging is not an option.

Photocuring Graphene Oxide Liquid Crystals for High-Strength Structural Materials.

Graphene is the strongest known material. However, the challenge of translating that strength from the microscale to the more useful macroscale remains unmet. Preparing solid structures from self-assembled graphene oxide liquid crystals has allowed the creation of paper and fibers with excellent mechanical properties. Conventionally, vacuum filtration, wet spinning, and freeze-drying are used to prepare such structures from graphene oxide liquid crystals. Here, we introduce photocuring as an additional option to create solid structures of self-assembled graphene oxide liquid crystals that allows for thicker samples and other shapes to be realized. The photocured graphene oxide paper prepared here exhibited mechanical properties comparable to those of benchmark samples prepared by vacuum filtration.

Influence of Polymer Molecular Weight on Chain Conformation at the Polystyrene/Silver Interface.

The dependence between the conformation of polystyrene (PS) and its molecular weight (Mw) in the vicinity of a metal interface was investigated by sum frequency generation (SFG) spectroscopy. Tilt angles θ ≥ 50° (the angle between the C2 axis of the pendant phenyl ring and the surface normal) were observed for all samples because of the interaction between the aromatic rings and the metal surface. Furthermore, it was found that θ decreases with increasing Mw for PS samples ranging from 20 × 103 g/mol to 400 × 103 g/mol. The intensity of the backbone SFG signal was higher for high Mw PS, compared to low Mw PS, indicating a greater number of backbone interactions with the silver substrate surface for the high Mw sample. These structural differences are driven by different entropic and enthalpic contributions to the free energy of adsorption for different polymer molecular weights. Differences in the polymer free volume and in the relative amount of chain ends with higher mobility may also influence the chain conformation. These results suggest that important interfacial properties of polymeric thin films, such as adhesion and wettability, could be tailored by modifying the polymer Mw to achieve the desired interfacial conformation.

Metal Oxide Quantum Dots Embedded in Silica Matrices Made by Flame Spray Pyrolysis.

Quantum dots have unique size-dependent properties and promising applications. However, their use in many applications remains hindered by mechanical, thermal, and chemical instability and the lack of viable quantum dot mass-production processes. Embedding quantum dots in matrices such as silica counteracts the instability challenges in some applications while preserving their unique properties and applicability. Here, we synthesize quantum dots of four different metal oxides embedded in a silica matrix in a one-step mass-production process using flame spray pyrolysis.

Morphological examination of highly porous polylactic acid/Bioglass® scaffolds produced via nonsolvent induced phase separation.

In this study, we produce highly porous (up to ∼91%) composite scaffolds of polylactic acid (PLA) containing 2 wt % sol-gel-derived 45S5 Bioglass® particles via nonsolvent induced phase separation at -23°C with no sacrificial phases involved. Before the incorporation of the bioglass with PLA, the particles are surface modified with a silane coupling agent which effectively diminishes agglomeration between them leading to a better dispersion of bioactive particles throughout the scaffold. Interestingly, the incorporation route (via solvent dichloromethane or nonsolvent hexane) of the surface modified particles in the foaming process has the greatest impact on porosity, crystallinity, and morphology of the scaffolds. The composite scaffolds with a morphology consisting of both mesopores and large macropores, which is potentially beneficial for bone regeneration applications, are examined further. SEM images show that the surface modified bioglass particles take-up a unique configuration within the mesoporous structure of these scaffolds ensuring that the particles are well interlocked but not completely covered by PLA such that they can be in contact with physiological fluids. The results of preliminary in vitro tests confirm that this PLA/bioglass configuration promotes the interaction of the bioactive phase with physiological fluids. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 2433-2442, 2017.

Synthesis of 45S5 Bioglass® via a straightforward organic, nitrate-free sol-gel process.

More than four decades after the discovery of 45S5 Bioglass® as the first bioactive material, this composition is still one of the most promising materials in the tissue engineering field. Sol-gel-derived bioactive glasses generally possess improved properties over other bioactive glasses, because of their highly porous microstructure and unique surface chemistry which accelerate hydroxyapatite formation. In the current study, a new combination of precursors with lactic acid as the hydrolysis catalyst have been employed to design an organic, nitrate-free sol-gel procedure for synthesizing of 45S5 Bioglass®. This straightforward route is able to produce fully amorphous submicron particles of this glass with an appropriately high specific surface area on the order of ten times higher than that of the melt-derived glasses. These characteristics are expected to lead to rapid hydroxyapatite formation and consequently more efficient bone bonding.

Space-filling trialkoxysilane: synthesis and self-assembly into low-density monolayers.

A novel space-filling trialkoxysilane derivative was synthesized using a two-step strategy from commercially available starting materials to produce the precursor for the formation of low-density self-assembled monolayers. Self-assembled monolayers of the synthesized compound were prepared on three different substrates (Si/SiO(2), glass and ITO) and were characterized using contact angle, ellipsometry and sum-frequency generation spectroscopy. Removal of the space-filling protecting group, (2-chlorophenyl)diphenyl methanol, yields a carboxy-terminated surface. Correspondingly, the contact angle and film thickness decrease and the SFG spectra clearly indicate an increase in gauche defect concentration characteristic of a low-density disordered monolayer.

Influence of Fluid Cell Design on the Frequency Response of AFM Microcantilevers in Liquid Media.

A study of the frequency response of AFM microcantilevers in liquid media contained in a commercial fluid cell is presented. Such systems exhibit complicated dynamics which are often not well described by available theories. Their dynamic behavior has a direct effect on the use of the AFM in dynamic mode while imaging in liquid or while extracting the rheological properties of the fluid. We explore the issues related to the design of the cantilever holder/fluid cell and propose an approach for evaluating, minimizing and recognizing the ultimate limitations of commercial cantilever holders. A technique for estimating the frequency response spectrum of the fluid cell itself from experimental data is presented. This spectrum can then be used to evaluate whether or not the fluid cell is suited for the desired purpose.