The chemical and structural changes within thin films of (3-glycidoxypropyl)trimethoxysilane (GPS) after exposure for various periods of time to air saturated with either D 2O or H 2O at 80°C were studied. The X-ray and neutron reflectivity (XR and NR), combined wuth attenuated total reflection infrared (ATR-IR) spectroscopy were used. The chemical degradation mechanism was identified by IR as hydrolysis of siloxane bonds. GPS films were prepared by dip-coating, which resulted in a greater and more variable thickness than for the spin-coated samples, for ATR-IR. The little changes in the reflectivity data was observed for films conditioned with D 2O at 80°C for one month.
Poly(N-isopropyl acrylamide) (PNIPAM) is perhaps the most well known member of the class of responsive polymers. Free PNIPAM chains have a lower critical solution temperature in water at {approx}31 C. This very sharp transition ({approx}5 C) is attributed to alterations in the hydrogen bonding interactions of the amide group. Grafted chains of PNIPAM have shown promise for creating responsive surfaces. Examples include controlling the adsorption of proteins or bacteria, regulating the flow of liquids in narrow filaments or mesoporous materials, control of enzymatic activity, and releasing the contents of liposomes. Conformational changes of the polymer are likely to play a role in some of these applications, in addition to changes in local interactions. In this work we investigated the T-dependent conformational changes of grafted PNIPAM chains in D2O using neutron reflection and AFM. The molecular weight (M) and surface density of the PNIPAM brushes were controlled using atom-transfer radical polymerization. We discovered a strong effect of surface density. At lower surface densities, in the range typically achieved with grafting-to methods, we observed very little conformational change. At higher surface densities, significant changes with T were observed. The results will be compared with numerical SCF calculations employing an effective (conc.-dependent) Flory-Huggins chi parameter extracted from the solution phase diagram. For the case of high M and high surface density, a non-monotonic change in profile shape with T was observed. This will be discussed in the context of vertical phase separation predicted for brushes of water-soluble polymers within two-state models.
The effect of the density and in-plane distribution of interfacial interactions on crack initiation in an epoxy-silicon joint was studied in nominally pure shear loading. Well-defined combinations of strong (specific) and weak (nonspecific) interactions were created using self-assembling monolayers. The in-plane distribution of strong and weak interactions was varied by employing two deposition methods: depositing mixtures of molecules with different terminal groups resulting in a nominally random distribution, and depositing methyl-terminated molecules in domains defined lithographically with the remaining area interacting through strong acid-base interactions. The two distributions lead to very different fracture behavior. For the case of the methyl-terminated domains (50 μm on a side) fabricated lithographically, the joint shear strength varies almost linearly with the area fraction of strongly interacting sites. From this we infer that cracks nucleate on or near the interface over nearly the entire range of bonded area fraction and do so at nearly the same value of local stress (load/bonded area). We postulate that the imposed heterogeneity in interfacial interactions results in heterogeneous stress and strain fields within the epoxy in close proximity to the interface. Simply, the bonded areas carry load while the methyl terminated domains carry negligible load. Stress is amplified adjacent to the well-bonded regions (and reduced adjacent to the poorly bonded regions), and this leads to crack initiation by plastic deformation and chain scission within the epoxy near the interface. For the case of mixed monolayers, the dependence is entirely different. At low areal density of strongly interacting sites, the joint shear strength is below the detection limit of our transducer for a significant range of mixed monolayer composition. With increasing density of strongly interacting sites, a sharp increase in joint shear strength occurs at a methyl terminated area fraction of roughly 0.90. We postulate that this coincides with the onset of yielding in the epoxy. For methyl-terminated area fractions less than 0.85, the joint shear strength becomes independent of the interfacial interactions. This indicates that fracture no longer initiates on the interface but away from the interface by a competing mechanism, likely plastic deformation and chain scission within the bulk epoxy. The data demonstrate that the in-plane distribution of interaction sites alone can affect the location of crack nucleation and the far-field stress required.
The adsorption of myoglobin to Langmuir monolayers of a metal-chelating lipid in crystalline phase was studied using neutron and X-ray reflectivity (NR and XR) and grazing incidence X-ray diffraction (GIXD). In this system, adsorption is due to the interaction between chelated divalent copper or nickel ions and the histidine moieties at the outer surface of the protein. The binding interaction of histidine with the Ni-IDA complex is known to be much weaker than that with Cu-IDA. Adsorption was examined under conditions of constant surface area with an initial pressure of 40 mN/m. After {approx}12 h little further change in reflectivity was detected, although the surface pressure continued to slowly increase. For chelated Cu{sup 2+} ions, the adsorbed layer structure in the final state was examined for bulk myoglobin concentrations of 0.10 and 10 {micro}M. For the case of 10 {micro}M, the final layer thickness was 43 {angstrom}. This corresponds well to the two thicker dimensions of myoglobin in the native state (44 {angstrom} x 44 {angstrom} x 25 {angstrom}) and so is consistent with an end-on orientation for this disk-shaped protein at high packing density. However, the final average volume fraction of amino acid segments in the layer was 0.55, which is substantially greater than the value of 0.44 calculated for a completed monolayer from the crystal structure. This suggests an alternative interpretation based on denaturation. GIXD was used to follow the effect of protein binding on the crystalline packing of the lipids and to check for crystallinity within the layer of adsorbed myoglobin. Despite the strong adsorption of myoglobin, very little change was observed in the structure of the DSIDA film. There was no direct evidence in the XR or GIXD for peptide insertion into the lipid tail region. Also, no evidence for in-plane crystallinity within the adsorbed layer of myoglobin was observed. For 0.1 {micro}M bulk myoglobin concentration, the average segment volume fraction was only 0.13 and the layer thickness was {le}25 {angstrom}. Adsorption of myoglobin to DSIDA-loaded with Ni{sup 2+} was examined at bulk concentrations of 10 and 50 {micro}M. At 10 {micro}M myoglobin, the adsorbed amount was comparable to that obtained for adsorption to Cu{sup 2+}-loaded DSIDA monolayers at 0.1 {micro}M. But interestingly, the adsorbed layer thickness was 38 {angstrom}, substantially greater than that obtained at low coverage with Cu-IDA. This indicates that either there are different preferred orientations for isolated myoglobin molecules adsorbed to Cu-IDA and Ni-IDA monolayer films or else myoglobin denatures to a different extent in the two cases. Either interpretation can be explained by the very different binding energies for individual interactions in the two cases. At 50 {micro}M myoglobin, the thickness and segement volume fraction in the adsorbed layer for Ni-IDA were comparable to the values obtained with Cu-IDA at 10 {micro}M myoglobin.
Silane adhesion promoters are commonly used to improve the adhesion, durability, and corrosion resistance of polymer-oxide interfaces. The current study investigates a model interface consisting of the natural oxide of 100 Si and an epoxy cured from diglycidyl ether of bisphenol A (DGEBA) and triethylenetetraamine (TETA). The thickness of (3-glycidoxypropyl)trimethoxysilane (GPS) films placed between the two materials provided the structural variable. Five surface treatments were investigated: a bare interface, a rough monolayer film, a smooth monolayer film, a 5 nm thick film, and a 10 nm thick film. Previous neutron reflection experiments revealed large extension ratios (>2) when the 5 and 10 nm thick GPS films were exposed to deuterated nitrobenzene vapor. Despite the larger extension ratio for the 5 nm thick film, the epoxy/Si fracture energy (G{sub c}) was equal to that of the 10 nm thick film under ambient conditions. Even the smooth monolayer exhibited the same G{sub c}. Only when the monolayer included a significant number of agglomerates did the G{sub c} drop to levels closer to that of the bare interface. When immersed in water at room temperature for 1 week, the threshold energy release rate (G{sub th}) was nearly equal to G{sub c} for the smooth monolayer, 5 nm thick film, and 10 nm thick film. While the G{sub th} for all three films decreased with increasing water temperature, the G{sub th} of the smooth monolayer decreased more rapidly. The bare interface was similarly sensitive to temperature; however, the G{sub th} of the rough monolayer did not change significantly as the temperature was raised. Despite the influence of pH on hydrolysis, the G{sub th} was insensitive to the pH of the water for all surface treatments.
Poly(N-isopropylacrylamide) (PNIPAM) exhibits a lower critical solution temperature (LCST) of {approx}30 C in water that is attributed to alterations in the hydrogen-bonding interactions of the amide group. PNIPAM in various forms has been explored for a variety of applications including controlled drug delivery, solute separation, tissue culture substrates, and controlling the adsorption of proteins, blood cells, and bacteria. Grafting PNIPAM onto surfaces is a promising strategy for creating responsive surfaces, since the physical properties of PNIPAM are readily controlled by changing the temperature. Considerable effort has been devoted to studying variations in chain conformations with temperature (T) in PNIPAM-based materials. Kubota et al. studied conformational changes of PNIPAM free chains with temperature for molecular weights ranging from 1.63 x 10{sup 6} to 2.52 x 10{sup 7} g/mol (M{sub w}/M{sub n} > 1.3) in water using laser light scattering. They reported a decrease in the radius of gyration (R{sub g}) as the solution temperature increased above the LCST. The magnitude of the effect was more pronounced with increasing molecular weight, ranging up to a factor of two for the highest molecular weight sample. In a similar study, Wu et al. observed a decrease in R{sub g} of a factor of seven for a high molecular weight PNIPAM sample with very low polydispersity (M{sub w} = 1.3 x 10{sup 7} g/mol, M{sub w}/M{sub n} < 1.05). Regarding grafted PNIPAM chains, Kidoaki et al. recently employed an iniferter-based graft polymerization method to generate a dense, high molecular weight brush and reported changes in the thickness measured by AFM. The thickness of the grafted layer was obtained from AFM images of the boundary between grafted and nongrafted (ablated by laser light) regions. They found that the swollen film thickness decreased by a factor of {approx}2 with increasing temperature from 25 to 40 C for samples with a range of dry film thickness from 250 to 1500 {angstrom}. More recently, Balamurugan et al. used surface plasmon resonance (SPR) to probe conformational changes in a PNIPAM brush grafted onto a gold layer by atom transfer radical polymerization (ATRP). For a sample with a dry film thickness of 517 {angstrom}, the SPR measurements indicated a significant contraction (extension of the layer with increasing/decreasing) temperature through the transition. Quantification of the change in profile characteristics was not reported, but it was noted that the change in the SPR signal occurred over a much broader range of temperature (15-35 C) than is typical of the transition for free chains in bulk solution. No systematic study of detailed PNIPAM chain conformations has yet been reported as a function of the two critical brush parameters, the surface density and molecular weight. A recent theoretical analysis by Baulin and Halperin has identified the surface density as a critical parameter demarcating different regimes of behavior. This arises from the concentration dependence of the Flory {chi} parameter as obtained from a recent phase behavior study of free chains in solution. Little attention has been paid to the surface density in previous experimental studies of grafted PNIPAM chains. We have begun a systematic study of the temperature-dependent conformational changes of PNIPAM grafted chains in water as a function of surface density and molecular weight using neutron reflection (NR). In previous work, we investigated the conformational changes of PNIPAM chains tethered to silicon oxide using two methods. The first was the 'grafting from' method in which N-isopropylacrylamide monomers were polymerized from the silicon surface with a chain transfer, free-radical technique. In the second method, preformed PNIPAM chains with carboxylic acid end groups associated with terminal hydroxyl groups of a mixed self-assembling monolayer. Detailed concentration profiles of the PNIPAM brushes were determined in D{sub 2}O as a function of temperature and also in d-acetone at room temperature. Profiles were obtained in the two solvents in order to investigate the role of the solvent in mediating interactions. The profiles in D{sub 2}O were bilayers, composed of a very thin layer with higher concentration at the surface and a low concentration layer extending well into the subphase. The very thin, higher concentration surface layer was attributed to attractive segment-surface interactions. The profiles in acetone were smoothly decaying single-layer profiles. The low segment concentration at the surface in acetone indicated that the surface density of these brushes was rather low. The dry film thicknesses were less than 40 {angstrom}, much lower than in the study of Kidoaki et al. On the basis of the molecular weights and dry film thicknesses, the surface density ({sigma}, chains/{angstrom}{sup 2}) ranged from 1 x 10{sup -4} to 2 x 10{sup -4} for those samples.
The adsorption of myoglobin to Langmuir monolayers of a metal-chelating lipid in crystalline phase was studied using neutron and X-ray reflectivity (NR and XR) and grazing incidence X-ray diffraction (GIXD). In this system, adsorption is due to the interaction between chelated divalent copper or nickel ions and the histidine moieties at the outer surface of the protein. The binding interaction of histidine with the Ni-IDA complex is known to be much weaker than that with Cu-IDA. Adsorption was examined under conditions of constant surface area with an initial pressure of 40 mN/m. After {approx}12 h little further change in reflectivity was detected, although the surface pressure continued to slowly increase. For chelated Cu{sup 2+} ions, the adsorbed layer structure in the final state was examined for bulk myoglobin concentrations of 0.10 and 10 {micro}M. For the case of 10 {micro}M, the final layer thickness was {approx}43 {angstrom}. This corresponds well to the two thicker dimensions of myoglobin in the native state (44 {angstrom} x 44 {angstrom} x 25 {angstrom}) and so is consistent with an end-on orientation for this disk-shaped protein at high packing density. However, the final average volume fraction of amino acid segments in the layer was 0.55, which is substantially greater than the value of 0.44 calculated for a completed monolayer from the crystal structure. This suggests an alternative interpretation based on denaturation. GIXD was used to follow the effect of protein binding on the crystalline packing of the lipids and to check for crystallinity within the layer of adsorbed myoglobin. Despite the strong adsorption of myoglobin, very little change was observed in the structure of the DSIDA film. There was no direct evidence in the XR or GIXD for peptide insertion into the lipid tail region. Also, no evidence for in-plane crystallinity within the adsorbed layer of myoglobin was observed. For 0.1 {micro}M bulk myoglobin concentration, the average segment volume fraction was only 0.13 and the layer thickness was {le} 25 {angstrom}. Adsorption of myoglobin to DSIDA-loaded with Ni{sup 2+} was examined at bulk concentrations of 10 and 50 {micro}M. At 10 {micro}M myoglobin, the adsorbed amount was comparable to that obtained for adsorption to Cu{sup 2+}-loaded DSIDA monolayers at 0.1 {micro}M. But interestingly, the adsorbed layer thickness was 38 {angstrom}, substantially greater than that obtained at low coverage with Cu-IDA. This indicates that either there are different preferred orientations for isolated myoglobin molecules adsorbed to Cu-IDA and Ni-IDA monolayer films or else myoglobin denatures to a different extent in the two cases. Either interpretation can be explained by the very different binding energies for individual interactions in the two cases. At 50 {micro}M myoglobin, the thickness and segement volume fraction in the adsorbed layer for Ni-IDA were comparable to the values obtained with Cu-IDA at 10 {micro}M myoglobin.
Tethered films of poly n-isopropylacrylamide (PNIPAM) films have been developed as materials that can be used to switch the chemistry of a surface in response to thermal activation. In water, PNIPAM exhibits a thermally-activated phase transition that is accompanied by significant changes in polymer volume, water contact angle, and protein adsorption characteristics. New synthesis routes have been developed to prepare PNIPAM films via in-situ polymerization on self-assembled monolayers. Swelling transitions in tethered films have been characterized using a wide range of techniques including surface plasmon resonance, attenuated total reflectance infrared spectroscopy, interfacial force microscopy, neutron reflectivity, and theoretical modeling. PNIPAM films have been deployed in integrated microfluidic systems. Switchable PNIPAM films have been investigated for a range of fluidic applications including fluid pumping via surface energy switching and switchable protein traps for pre-concentrating and separating proteins on microfluidic chips.
The adsorption of the strong polyelectrolyte poly(styrenesulfonate) (PSS) to the air surface of dilute aqueous solutions was investigated as a function of molecular weight and salt concentration. Detailed segment profiles of the deuterated polymer were determined by neutron reflection. Surface activity was also examined through surface tension measurements. In general, the segment profiles are composed of a thin layer (10-20 angstrom thick) of high concentration at the air surface, followed by a distinct second layer of much lower segment concentration that extends to larger depths into the liquid. The high segment density at the air surface is due to a strong surface attraction, arising from the low surface energy of the PSS backbone relative to the surface tension of water. At low salt concentration, the profiles tend toward a single dense layer, suggesting that the chains lie nearly flat at the interface in that limit. The adsorbed amount increases with salt concentration, with a stronger dependence for higher molecular weight chains. The adsorbed amounts at the air/water interface are higher than reported previously for PSS adsorbed onto neutral solid surfaces, consistent with the fact that the air-liquid interface provides a stronger surface attraction. While the trends of adsorbed amount with salt concentration and molecular weight are in good agreement with numerical self-consistent-field lattice calculations, the measured bilayer profiles are rather different from the smoothly decaying theoretical profiles. The surface tensions of the PSS solutions are significantly lowered relative to those of pure salt solutions. Combining the reflectivity and surface tension measurements, an approximately linear relationship is established between the surface pressure and the PSS adsorbed amount.
The relationships between fundamental interfacial interactions, energy dissipation mechanisms, and fracture stress or fracture toughness in a glassy thermoset/inorganic solid joint are not well understood. This subject is addressed with a model system involving an epoxy adhesive on a polished silicon wafer containing its native oxide. The proportions of physical and chemical interactions at the interface, and the in-plane distribution, are varied using self-assembling monolayers of octadecyltrichlorosilane (ODTS). The epoxy interacts strongly with the bare silicon oxide surface, but forms only a very weak interface with the methylated tails of the ODTS monolayer. The fracture stress is examined as a function of ODTS coverage in the napkin-ring (pure shear) loading geometry. The relationship between fracture stress and ODTS coverage is catastrophic, with a large change in fracture stress occurring over a narrow range of ODTS coverage. This transition in fracture stress does not correspond to a wetting transition of the epoxy. Rather, the transition in fracture stress corresponds to the onset of deformation in the epoxy, or the transition from brittle to ductile fracture. The authors postulate that the transition in fracture stress occurs when the local stress that the interface can support becomes comparable to the yield stress of the epoxy. The fracture results are independent of whether the ODTS deposition occurs by island growth (T{sub dep} = 10 C) or by homogeneous growth (T{sub dep} = 24 C).
The relationship between the nature and spatial distribution of fundamental interfacial interactions and fracture stress/fracture toughness of a glassy adhesive-inorganic solid joint is not understood. This relationship is important from the standpoint of designing interfacial chemistry sufficient to provide the level of mechanical strength required for a particular application. In addition, it is also important for understanding the effects of surface contamination. Different types of contamination, or different levels of contamination, likely impact joint strength in different ways. Furthermore, the relationship is also important from the standpoint of aging. If interfacial chemical bonds scission over time due to the presence of a contaminant such as water, or exposure to UV, etc, the relationship between joint strength/fracture toughness and interface strength is important for predicting reliability with time. A fundamental understanding of the relationship between joint strength and fundamental interfacial interactions will give insight into these issues.
The focus of this work is the structure within highly crosslinked, two component epoxy films. The authors examine variations in crosslink density within thin epoxy films on silicon substrates by solvent swelling. The method is based on the fact that the equilibrium volume fraction of a swelling solvent is strongly dependent upon the local crosslink density. The authors examine the volume fraction profile of the good solvent nitrobenzene through the epoxy films by neutron reflection. Isotopic substitution is used to provide contrast between the epoxy matrix and the swelling solvent.