Detection of tethered biocide moiety segregation to silicone surface using sum frequency generation vibrational spectroscopy

Polymer surface properties are controlled by the molecular surface structures. Sum frequency generation (SFG) vibrational spectroscopy has been demonstrated to be a powerful technique to study polymer surface structures at the molecular level in different chemical environments. In this research, SFG has been used to study the surface segregation of biocide moieties derived from triclosan (TCS) and tetradecyldimethyl (3-trimethoxysilylpropyl) ammonium chloride (C-14 QAS) that have been covalently bound to a poly(dimethylsiloxane) (PDMS) matrix. PDMS materials are being developed as coatings to control biofouling. This SFG study indicated that TCS-moieties segregate to the surface when the bulk concentration of TCS-moieties exceeds 8.75% by weight. Surface segregation of C-14 QAS moieties was detected after 5% by weight incorporation into a PDMS matrix. SFG results were found to correlate well with antifouling activity, providing a molecular interpretation of such results. This research showed that SFG can aid in the development of coatings for controlling biofouling by elucidating the chemical structure of the coating surface.     

Molecular level structures of poly(n-alkyl methacrylate)s with different side chain lengths at the polymer/air and polymer/water interfaces

Sum frequency generation (SFG) vibrational spectroscopy has been successfully applied to study molecular structures of several poly(n-alkyl methacrylate)s (PAMAs) with different side chain lengths at the PAMA/air and PAMA/water interfaces. We have observed that the ester side chains from all PAMAs always dominate the interface, but the orientation information of the methyl end group on the side chains varies, depending on the length of the side chain. The contributions from methylene groups on the side chains have been evaluated, and the surface structures have been related to the surface tension of these polymers. Different water restructuring behaviors have been observed for different PAMAs. This phenomenon and its reversibility are strongly dependent on the glass transition temperature of each polymer, which is influenced by the side chain length. Detailed data fitting and analysis has been discussed.     

Measuring polymer surface ordering differences in air and water by sum frequency generation vibrational spectroscopy

Molecular structures of poly(n-butyl methacrylate) (PBMA) at the PBMA/air and PBMA/water interfaces have been studied by sum frequency generation (SFG) vibrational spectroscopy. PBMA surfaces in both air and water are dominated by the methyl groups of the ester side chains. The average orientation and orientation distribution of these methyl groups at the PBMA/air and PBMA/water interfaces are different, indicating that surface restructuring occurs when the PBMA sample contacts water. Analysis shows that the orientation distribution of side chain methyl groups on the PBMA surface is narrower in water than that in air, indicating that the PBMA surface can be more ordered in water. To our knowledge, this is the first time that quantitative comparisons between molecular surface structures of polymers in air and in water have been made. Two assumptions on the orientation distribution function, including a Gaussian distribution and a formula based on the maximum entropy approach, are used in the analysis. It has been found that the orientation angle distribution function deduced by the Gaussian distribution and the maximum entropy distribution are quite similar, showing that the Gaussian distribution is a good approximation for the angle distribution. The effect of experimental error on the deduced orientational distribution is also discussed.     

Effect of oxidation on the wettability of poly(dimethylsiloxane) surfaces

The wetting of amorphous poly(dimethylsiloxane) (PDMS) surfaces by water has been studied using molecular dynamics simulations. PDMS surfaces were generated by compressing a long PDMS chain between two elastic boundaries at atmospheric pressure. Oxidation of the PDMS surface, achieved in real systems by exposure to air plasma or corona discharge, was modeled by replacing methyl groups on the PDMS chain with hydroxyl groups. Three surfaces of varying degrees of oxidation were characterized by measuring the water contact angle and the roughness. The dependence of the microscopic contact angle on drop size was measured from time averaged density profiles. The macroscopic contact angle was measured directly using a cylindrical drop of infinite length with zero contact line curvature. The measured macroscopic contact angle ranged from approximately 125 degrees on the untreated surface to 75 degrees on the most oxidized surface studied. The line tension was found to increase with increasing degree of oxidation, from a negligible value on the untreated surface to approximately 5x10(-11) J m(-1) on the most heavily oxidized surface.     

Molecular packing of lysozyme,fibrinogen, and bovine serum albumin on hydrophilic and hydrophobic surfaces studied by infrared-visible sum frequency generation and fluorescence microscopy

Infrared-visible sum frequency generation (SFG) vibrational spectroscopy, in combination with fluorescence microscopy, was employed to investigate the surface structure of lysozyme, fibrinogen, and bovine serum albumin (BSA) adsorbed on hydrophilic silica and hydrophobic polystyrene as a function of protein concentration. Fluorescence microscopy shows that the relative amounts of protein adsorbed on hydrophilic and hydrophobic surfaces increase in proportion with the concentration of protein solutions. For a given bulk protein concentration, a larger amount of protein is adsorbed on hydrophobic polystyrene surfaces compared to hydrophilic silica surfaces. While lysozyme molecules adsorbed on silica surfaces yield relatively similar SFG spectra, regardless of the surface concentration, SFG spectra of fibrinogen and BSA adsorbed on silica surfaces exhibit concentration-dependent signal intensities and peak shapes. Quantitative SFG data analysis reveals that methyl groups in lysozyme adsorbed on hydrophilic surfaces show a concentration-independent orientation. However, methyl groups in BSA and fibrinogen become less tilted with respect to the surface normal with increasing protein concentration at the surface. On hydrophobic polystyrene surfaces, all proteins yield similar SFG spectra, which are different from those on hydrophilic surfaces. Although more protein molecules are present on hydrophobic surfaces, lower SFG signal intensity is observed, indicating that methyl groups in adsorbed proteins are more randomly oriented as compared to those on hydrophilic surfaces. SFG data also shows that the orientation and ordering of phenyl rings in the polystyrene surface is affected by protein adsorption, depending on the amount and type of proteins.     

Kinetics of ultraviolet and plasma surface modification of poly(dimethylsiloxane) probed by sum frequency vibrational spectroscopy

In numerous applications in microfluidics, cell growth, soft lithography, and molecular imprinting, the surface of poly(dimethylsiloxane) (PDMS) is modified from a hydrophobic methyl-terminated surface to a hydrophilic hydroxyl-terminated surface. In this study, we investigated molecular structural and orientational changes at the PDMS-air interface in response to three commonly used surface modification processes: exposure to long-wavelength ultraviolet light (UV), exposure to short-wavelength UV that generates ozone (UVO), and exposure to oxygen plasma (OP). The surfaces of two PDMS compositions (10:1 and 4:1 of base polymer/curing agent) were probed during modification, using monolayer-sensitive IR + visible sum frequency generation (SFG) vibrational spectroscopy, with two different polarization combinations. During PDMS surface modification, the peak intensities of CH3 side groups and CH2 cross-link groups decreased, while peak intensities of Si-OH groups increased. There was no significant change in the average orientation of the CH3 groups on the PDMS surface during modification. The concentration of CH3 groups on the surface decreased exponentially with time, for all three UV, UVO, and OP modification processes, with first order kinetics and time constants of approximately 160, 66, and 0.3 min, respectively. At steady state, residual CH3 groups were detected at the PDMS surface for UV and UVO treatments; however, there were negligible CH3 groups detected after OP modification.     

Chemical force titrations of amine- and sulfonic acid-modified poly(dimethylsiloxane)

Chemical force titrations-measurements of the adhesive interaction between a pair of suitably chemically modified atomic force microscopy (AFM) tip and sample surfaces as a function of pH-have been carried out for various combinations of silanol, amine, carboxylic acid, and sulfonic acid functional groups on both tip and sample. The primary surface material studied was poly(dimethylsiloxane) (PDMS). Surface modification was carried out using a plasma oxidation process to form silanol sites; further modification with amine or sulfonic acid sites was carried out by reaction of the silanol sites with the appropriate trialkoxysilane derivative. AFM tips were also modified using trialkoxysilane compounds. In the cases of tip/sample combinations with the same functional group on each, surface pK(1/2) values could be determined. In several "mixed" tip/sample combinations, a peak appeared in the titration curve midway between the surface pK(1/2) values of the tip and sample, consistent with an ionic H-bonding model for the interactions. The amine/sulfonic acid pair showed more complex behavior; the amine-terminated tip/sulfonic acid-terminated PDMS surface force titration curve consisted of two peaks centered at pH 4 and pH 8. Reversing the tip/sample pair resulted in the peak positions being shifted upward by 1.0 pH unit. The peak appearing at lower pH is assigned to electrostatic interactions between the two oppositely charged surfaces, whereas the higher pH peak is believed to arise due to ionic H-bonding interactions. AFM images show the effects on surface patterning of amine- and sulfonic acid-modified PDMS surfaces that have undergone two different oxidation methods (air plasma oxidation and Tesla coil oxidation). The surface morphologies of freshly prepared and 24 h aged air plasma oxidized PDMS are also discussed in this study.     

Electroosmotic flow in a poly(dimethylsiloxane) channel does not depend on percent curing agent

Poly(dimethylsiloxane) (PDMS) microfluidic devices were prepared from different ratios of "curing agent" (which contains silicon hydride groups) to "base" (which contains vinyl-terminated noncross-linked PDMS), to determine the effect of this ratio on electroosmotic flow (EOF). In fabricating devices for this purpose, a novel method for permanently enclosing PDMS channels was developed. As a supplement to the microfluidic method, the inner walls of capillaries were coated with PDMS formed from varying ratios of curing agent to base. EOF was found to be constant for PDMS formed with each ratio, which implies that the negative surface charges do not arise from chemical species present only in the base or the curing agent.     

Chain conformation of poly(dimethyl siloxane) at the air/water interface by sum frequency generation

This is to report a study of chain conformation of poly(dimethylsiloxane) (PDMS) in spread monolayers at the air/water interface (A/W) with the aid of vibrational sum frequency spectroscopy (VSFS). We find that methyl groups of PDMS chains at the interface are completely disordered in the dilute regime of the surface density. At higher surface densities, however, the two methyl groups on the repeating unit point into the air asymmetrically; one points more normal to the interface, whereas the other lies more parallel to the interface. In the first collapsed regime, where the surface pressure of the PDMS monolayer reaches a plateau value of 8.7 mN/m, the signal intensity at 2915 cm (-1), assigned to the symmetric vibrational frequency of the methyl groups, is found independent of the surface density. On the basis of this finding, we propose that PDMS chains, in the first collapse regime at the A/W, form asymmetric layers. Thus, our proposal lends support to earlier works by Langevin's group to refute a widely speculated helix model that was based on energy minimization in the crystalline state of PDMS. In short, the energy consideration in the bulk crystalline state does not provide meaningful guidance as to the chain conformation of the monolayer at the A/W.     

Spatially resolved vibrational energy transfer in molecular monolayers

We have shown that it is possible to input heat to one location of a molecule and simultaneously measure its arrival in real time at two other locations, using an ultrafast flash-thermal conductance technique. A femtosecond laser pulse heats an Au layer to approximately 800 degrees C, while vibrational sum-frequency generation spectroscopy (SFG) monitors heat flow into self-assembled monolayers (SAMs) of organic thiolates. Heat flow into the SAM creates thermally induced disorder, which decreases the coherent SFG signal from the CH-stretching transitions. Recent improvements in the technique are described, including the use of nonresonant background-suppressed SFG. The improved apparatus was characterized using alkanethiolate and benzenethiolate SAMs. In the asymmetric 2-methyl benzenethiolate SAM, SFG can simultaneously monitor CH-stretching transitions of both phenyl and methyl groups. The phenyl response to flash-heating occurs at least as fast as the 1 ps time for the Au surface to heat. The methyl response has a faster portion similar to the phenyl response and a slower portion characterized by an 8 ps time constant. The faster portions are attributed to disordering of the methyl-substituted phenyl rings due to thermal excitation of the Au-S adbonds. The slower portion, seen only in the methyl SFG signal, is attributed to heat flow from the metal surface into the phenyl rings and then to the methyl groups.     

Creating patterned poly(dimethylsiloxane) surfaces with amoxicillin and poly(ethylene glycol)

This paper reports a simple microwave plasma patterning of poly(dimethylsiloxane) (PDMS) surfaces, which is accomplished by allowing selective surface areas to microwave plasma exposure in the presence of gaseous monomer. When maleic anhydride is used for microwave plasma reaction in the presence of physical barrier on the PDMS substrate, the resulting patterned surfaces with chemically bonded maleic anhydride and carboxylic acid groups are generated. In this particular study we attached amoxicillin via ammonolysis under weak base conditions in the presence of a catalyst as well as poly(ethyleneglycol) (PEG). A combination of internal reflection IR imaging (IRIRI) and atomic force microscopy (AFM) revealed that amoxicillin and PEG can be readily reacted on the microwave plasma patterned PDMS surfaces. Surface areas directly exposed to microwave plasmons exhibit the highest reactivity due to higher content of functional groups. These studies also show that molecular weight of PEG has also significant effect on kinetics of surface reactions.     

Changes in silicon elastomeric surface properties under stretching induced by three surface treatments

Poly(dimethylsiloxane) (PDMS) substrates are used in many applications where the substrates need to be elongated and various treatments are used to regulate their surface properties. In this article, we compare the effect of three of such treatments, namely, UV irradiation, water plasma, and plasma polymerization, both from a molecular and from a macroscopic point of view. We focus our attention in particular on the behavior of the treated surfaces under mechanical stretching. UV irradiation induces the substitution of methyl groups by hydroxyl and acid groups, water plasma leads to a silicate-like layer, and plasma polymerization causes the formation of an organic thin film with a major content of anhydride and acid groups. Stretching induces cracks on the surface both for silicate-like layers and for plasma polymer thin coatings. This is not the case for the UV irradiated PDMS substrates. We then analyzed the chemical composition of these cracks. In the case of water plasma, the cracks reveal native PDMS. In the case of plasma polymerization, the cracks reveal modified PDMS. The contact angles of plasma polymer and UV treated surfaces vary only very slightly under stretching, whereas large variations are observed for water plasma treatments. The small variation in the contact angle values observed on the plasma polymer thin film under stretching even when cracks appear on the surface are explained by the specific chemistry of the PDMS in the cracks. We find that it is very different from native PDMS and that its structure is somewhere between Si(O2) and Si(O3). This is, to our knowledge, the first study where different surface treatments of PDMS are compared for films under stretching.     

Infrared-visible sum frequency generation spectroscopic study of molecular orientation at polystyrene/comb-polymer interfaces

Surface-sensitive infrared-visible sum frequency generation spectroscopy (SFG) in total internal reflection geometry has been used to study the structure of poly(vinyl n-octadecyl carbamate-co-vinyl acetate) (PVNODC) or poly(octadecyl acrylate) (PA-18) in contact with a deuterated or hydrogenated polystyrene (dPS or hPS) layer. SFG spectra from the PVNODC (or PA-18)/hPS interface show methyl and methylene peaks corresponding to PVNODC (or PA-18) and phenyl peaks corresponding to the PS. Analysis suggests that the methyl groups are tilted at angles less than 30 degrees with respect to the surface normal. The presence of a strong methylene peak suggests the PVNODC alkyl side chains contain more gauche defects at the PS/PVNODC interface in comparison to PVNODC (or PA-18)/air interfaces. On heating, the SFG intensity from the PS/PA-18 interface drops sharply near the bulk melting temperature (T(m)) of PA-18. Interestingly, a similar drop in SFG signal is also observed for the PS phenyl groups. This demonstrates the ability of the phenyl group at the PS/PA-18 interface to rearrange itself upon the solid-to-liquid transition of the PA-18 alkyl side chain, at a temperature well below the bulk PS glass transition temperature. For PS/PVNODC interfaces, the drop in SFG intensity is gradual and in agreement with the three broad transitions of PVNODC observed in the bulk.     

Polymer surface science

Molecular level studies of the structure and mechanical properties of polymer surfaces have been carried out by sum frequency generation (SFG) surface vibrational spectroscopy and atomic force microscopy (AFM). The surfaces of different grades of polyethylene and polypropylene have been characterized-including during the glass transition and when mechanically stretched. Copolymers that have hard and soft segments with different glass transition temperatures show phase separation, an effect of hydrogen bonding between the hard and soft segments, that influences their adhesive and friction properties. AFM and SFG show that low surface energy additives migrate to the surface and alter the surface mechanical properties. Polymers, where the chemical nature of the end groups is different from the backbone, show surface segregation of the hydrophobic part of the chain in air and the hydrophilic part in water. Likewise, in miscible polymer blends, surface segregation of the more hydrophobic component in air and the more hydrophilic component in water is observed. This area of surface science requires increased attention because of the predominance of polymers as structural materials and as biomaterials.     

A simple approach to micropatterning and surface modification of poly(dimethylsiloxane)

Ozone treatment is an efficient economical, alternative method for surface activation of poly(dimethylsiloxane) (PDMS). This is illustrated by the derivatization of a PDMS surface with (3-aminopropyl)triethoxysilane (APTES). The apparent surface concentration of amino groups was found to be ca. 10(-8) mol/cm2 using UV/visible spectroscopy of the product from the reaction of the amino groups and fluorescamine. Potential application for micropatterning of biologically active interfaces was demonstrated by the covalent immobilization of oligonucleotides. A simple process for photolithographic patterning on PDMS surfaces has been developed.     

Surface structural characterization of protein- and polymer-modified polystyrene microspheres by infrared-visible sum frequency generation vibrational spectroscopy and scanning force microscopy

Structural investigations of bare and surface-modified polystyrene microspheres (beads) have been carried out by infrared-visible sum frequency generation (SFG) vibrational spectroscopy and scanning force microscopy (SFM). Bead surfaces have been modified by either the covalent linking of immunoglobulin G (IgG) and bovine serum albumin (BSA) or the nonspecific adsorption of a Pluronic surfactant. After surface modification with protein, SFG signals in the aliphatic CH-stretch region are detected at both the buffer/bead and air/bead interfaces, indicating that some amino acid residues in proteins adopt preferred orientations. SFG results indicate that the hydrophobic poly(propylene glycol) moieties in the Pluronic order when adsorbed onto the bead, at both the buffer/bead and air/bead interfaces, whereas hydrophilic poly(ethylene glycol) groups align to a lesser extent. SFG spectra also show that the phenyl rings of bare polystyrene beads in contact with air or buffer are ordered, with a dipole component directed along the surface normal, but become less ordered after the adsorption of either proteins or the polymer. Molecular orientation and ordering at the bead surface affect its hydrophobicity and aggregation behavior. SFM results reveal the formation of nonuniform islands when bare beads with more hydrophobic character are spun-cast onto a silica substrate. In the presence of adsorbed protein, a hexagonal packing of beads, with some defects, is observed, depending on the bulk pH and the type of attached protein. Adsorbed Pluronic causes the beads to aggregate in a disordered fashion, as compared to the behavior of bare and protein-modified beads.     

Fouling-release coatings prepared from alpha,omega-dihydroxypoly(dimethylsiloxane) cross-linked with (heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethoxysilane

Surface properties of pristine and water-aged polymeric films made of alpha,omega-dihydroxypoly(dimethylsiloxane) (PDMS) cross-linked with (heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethoxysilane (FTEOS17) or tetraethoxysilane (TEOS) were investigated. The FTEOS17-cured coatings showed stable advancing and receding contact angles over a period of 3 months of water exposure, compared to a 70 degrees decrease in receding contact angle for the TEOS-cured coatings. After immersion in water, hydroxyl groups were detected on the TEOS-cured coatings with attenuated total reflection infrared spectroscopy (ATR-FT/IR). Tapping-mode atomic force microscopy (TM-AFM) on pristine FTEOS17-cured coatings showed surfaces topologies ranging from smooth and featureless to topologically complex, depending on FTEOS17 concentration. The fluorinated coatings showed a stable surface morphology after water immersion, which we believe is due to the formation of a fluorinated siliceous phase that prevented the surface reconstruction, water penetration, and hydrolysis. The smooth pristine TEOS-cured coatings showed an increased roughness with cracks and erosion pits present on the surface after water immersion.     

Surface structures and properties of polystyrene/poly(methyl methacrylate) blends and copolymers

Sum frequency generation (SFG) vibrational spectroscopy has been applied to study the molecular surface structures of polystyrene (PS)/poly(methyl methacrylate) (PMMA) blends and the copolymer between PS and PMMA (PS-co-PMMA) in air, supplemented by atomic force microscopy (AFM) and contact angle goniometer. Both the blend and the copolymer have equal weight amounts of the two components. SFG results show that both components, PS and PMMA, can segregate to the surface of the blend and the copolymer before annealing, although PMMA has a slightly higher surface tension. Upon annealing both SFG results and contact angle measurements indicate that the PS segregates to the surface of the PS/PMMA blend more but no change occurs on the PS-co-PMMA surface. AFM images show that the copolymer surface is flat but the 1:1 PS/PMMA blend has a rougher surface with island like domains present. The annealing effect on the blend surface morphology has also been investigated. We collected amide SFG signals from interfacial fibrinogen molecules at the copolymer or blend/protein solution interfaces as a function of time. Different time-dependent SFG signal changes have been observed, showing that different surfaces of the blend and the copolymer mediate fibrinogen adsorption behavior differently.     

CF4 plasma treatment of poly(dimethylsiloxane): effect of fillers and its application to high-aspect-ratio UV embossing

Surface modification of poly(dimethylsiloxane) (PDMS) was carried out via CF4 plasma treatment. The test PDMS used contains significant amounts of quartz and silica fillers, while the control material is the same PDMS with quartz removed by centrifugation. Fluorination accompanied with roughening was produced on both PDMS surfaces. With short plasma times (15 min or less), a macromolecular fluorocarbon layer was formed on the PDMS surfaces because of the dominant fluorination, leading to significant increase in F concentration, decrease of surface energy, and some roughening. With intermediate plasma times (15-30 min), dynamic balance between fluorination and ablation was achieved, leading to a plateau of the surface roughness, fluorine content, and [F-Si]/[F-C] ratio. At our longest investigated plasma time of 45 min, the plasma ablated the fluorinated covering layer on the PDMS surfaces, leading to significant increase in roughness and [F-Si]/[F-C] ratio and decrease of surface F concentration. The effect of additional quartz in the test PDMS on surface F concentration, [F-Si]/[F-C] ratio, and roughness was dramatic only when ablation was significant (i.e., 45 min). The obtained Teflon-like surface displays long-term stability as opposed to hydrophobic recovery of other plasma-treated PDMS surfaces to increase hydrophilicity. On the basis of the optimized plasma treatment time of 15 min, a microstructured PDMS mold was plasma treated and successfully used for multiple high-aspect-ratio (about 8) UV embossing of nonpolar polypropylene glycol diacrylate (PPGDA) resin.     

Deactivation by polysiloxane and phenyl containing disilazane: A 29SI CP-MAS NMR study after the formation of polysiloxane chains at the surface

A high degree of deactivation of glass and fused-silica capillary column walls is attainable by means of high temperature silylation (HTS) with or without a preceding leaching process. HTS with a phenyl containing disilazane, diphenyltetramethyldisilazane (DPTMDS), and polydimethylsiloxane (PDMS) are studied on Cab-O-Sil, a fumed silica, as a model substrate. Using ²⁹Si CP-MAS NMR, it was shown that no dimethylsiloxane chains were formed upon silylation with DPTMDS under different conditions of humidity and stoichiometry at 377°C. With DPTMDS deactivation it is possible that amino trisiloxy silane groups are formed, these groups add extra activity to the surface. Silylation with a PDMS, OV 101, at various temperatures between 300°–420°C did show that dimethylsiloxane chains were bonded at the surface. Using the ²⁹Si CP-MAS NMR technique with variable contact times to reveal siloxy group mobility, the degradation of dimethylsiloxane chains at the surface was studied. PDMS degradation at an optimal temperature gives a more effective diminuation of the silane activity caused by chemical reaction with the silanol groups and the effective screening of the remaining silanol groups with anchored polydimethylsiloxane chains and small cyclodimethylsiloxane ring structures at the surface.