Effects of Water and Temperature on Conformational Order in Model

Jul 13, 2009 - Christopher J. Orendorff*, Dale L. Huber and Bruce C. Bunker ... Edin Nuhiji , Cynthia S. Wong , Alessandra Sutti , Tong Lin , Mark Kir...
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J. Phys. Chem. C 2009, 113, 13723–13731

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Effects of Water and Temperature on Conformational Order in Model Nylon Thin Films Christopher J. Orendorff,* Dale L. Huber, and Bruce C. Bunker Sandia National Laboratories, Albuquerque, New Mexico 87185 ReceiVed: February 12, 2009; ReVised Manuscript ReceiVed: May 6, 2009

Nylon 6,6 thin films have been examined by Raman spectroscopy to determine how chain conformation is influenced by environmental parameters such as exposure to water and temperature variations. The motivation for this work is to elucidate how interactions between water and the model polymers mediate polymer structures in applications such as the removal of salt from water in reverse osmosis membranes. Raman spectra show that model self-assembled monolayers containing Nylon 6,6 chains are semicrystalline under ambient conditions. The native chains adopt an unusual kinked and folded conformation related to that found in γ-Nylon 6,6. The regular chain deformations allow adjacent tethered chains to maximize hydrogen-bonding between neighboring amide groups under the constraints imposed by surface tethering. With increasing temperature, the films undergo a phase transition associated with the disruption of hydrogen bonds leading to structures containing more linear regions that are closer to the “all trans” case. Similar structural changes are observed on exposing the Nylon films to water. The salt content of the water does not appear to have a significant impact on the structure or phase transition in the Nylon 6,6. These results suggest that while inclusion of water has a profound effect on the polymer structure, either salt is excluded from the polymer or there is sufficient free volume within the films to accommodate hydrated ions without inducing further structural changes. Introduction Interactions between water and polymers dictate the performance of materials used in a wide range of technologies, including the biocompatibility of medical implant devices,1 the adsorption resistance of antifouling coatings,2 and charge transport in organic electronics.3,4 In all of these applications, it is clear that polymers exert a strong influence on the local structure of water, that water can exert a strong influence on the structure of the polymer, and that these structural interactions can have a profound influence on materials performance. For example, it has been demonstrated that polymers such as polyethylene oxide used to inhibit biofouling can nucleate a protective layer of ordered water that can be up to 5 nm thick and have a viscosity that is up to 107 times greater than that of bulk water.5 For thermally programmable polymers such as poly(n-isopropylacrylamide), it is known that the arrangement and “melting” of ordered water around the individual chains controls the inverse temperature swelling transition. Below the transition, the polymer is swollen, hydrophilic, and antifouling, while above the transition, the chains collapse to produce a more rigid, hydrophobic material with much lower occluded solution volume.6-8 We are interested in understanding water-polymer interactions for the specific application of polymers for use in reverse osmosis (RO) or nanofiltration (NF) systems. In reverse osmosis, pressure is applied to the salt-rich “feed” side of a nanoporous polymeric membrane, forcing pure water through the membrane while excluding salts and other dissolved substances. It is well known that interfacial interactions of the polyamide membranes typically used for RO membranes govern their desalination and remediation capabilities.9-12 However, the mechanisms by which the polymer separates pure water from salt water are not well understood. Chemical phenomena such as competitive hydration * To whom correspondence should be addressed. E-mail: corendo@ sandia.gov.

Figure 1. Schematic of surface-bound Nylon 6,6 (N3.0) on silica.

of the polymer relative to dissolved salts are clearly important, as are the physical structure and dynamics of the effective channels or “pores” through which the water must pass. In these systems, it is clear that water-polymer, polymer-polymer, and dissolved species-polymer interactions all contribute to chain configurations, which in turn influence both the stability and performance of the active polymeric phases. One of the difficulties associated with understanding and controlling water-polymer interactions is that most commercial membrane materials are multicomponent, amorphous materials that contain an ill-defined spectrum of domain structures and local ordering between chains. For fundamental studies, we have developed a methodology for creating model polymer films in which the structure is both uniform and well characterized. Our approach involves the controlled growth of oligomer films within self-assembled monolayers. Here, the spacing between oligomer chains is controlled at the monolayer base, and chains are grown to have exactly the same sequences of monomeric units, each of which is in registry with its neighbors. The structures of the as-prepared films can be determined with precision, and the structural changes associated with changes in environmental conditions can be monitored in detail. The specific model system we have selected as a starting point for understanding water-polymer interactions is Nylon 6,6 (Figure 1), which is a member of one of the most ubiquitous and widely studied polyamide systems. X-ray diffraction studies on crystalline bulk Nylon 6,6 indicate that the structure of this polymer is driven by hydrogen-bonding interactions between amide and carbonyl groups on adjacent chains.13-17 Crystalline Nylon 6,6 exists in three phases. In the R and β phases, hydrogen

10.1021/jp901309y CCC: $40.75  2009 American Chemical Society Published on Web 07/13/2009

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bonds connect chains in a regular zigzag pattern without intersheet hydrogen bonding, while in the γ phase, the aliphatic chains are distorted to produce pseudohexagonal pleated sheets. Due to the regular periodic spacing of hydrogen bonds and dense packing of the aliphatic chains, the crystalline to amorphous phase transition in R-Nylon 6,6 occurs at high temperatures (250 °C). In other polyamides, including both even-even (such as Nylon 2,6 and Nylon 2,8) and odd-odd (e.g., Nylon 5,5) numbered nylons, the presence of more irregular or offset hydrogen-bonding configurations results in lower transition temperatures.18,19 We are interested in understanding how environmental parameters including temperature and exposure to water influence the hydrogen-bonding interactions between Nylon chains and how such changes influence chain configurations. In this study, we have monitored the structural responses of Nylon 6,6 using Raman spectroscopy. Raman spectroscopy is a powerful tool for obtaining detailed structural information for various phases of polyamides. Raman spectroscopy has been used to study the R-to-γ transition in Nylon 6, as well as conformation stresses and phase transitions in electrospun Nylon 6 fibers.20,21 Peak assignments and intensity changes observed in these previous Raman studies have been invaluable in helping us interpret the structural changes that we observe in our model Nylon 6,6 films. As will be shown below, both temperature changes and water content induce dramatic changes in chain conformations. The implications of such changes on waterpolymer interactions that are relevant to water treatment technologies are discussed. The focus in this paper is on how water influences the structure of the surface-bound oligomer. In a companion paper, we focus on how the oligomer surface influences the structure and properties of the water.22 Although the total interplay between water and oligomer structures is required to develop comprehensive models for phenomena such as the separation of salts from water in reverse osmosis membranes, the implications of the observed changes in polymer structures on water treatment will be discussed. Experimental Section Materials. Adipoyl chloride, 1,6-hexamethylene diamine, 3-aminopropyltrimethoxysilane (APTES), bulk Nylon 6,6, and anhydrous toluene were purchased from Aldrich. Ethanol (200 proof) was purchased from Aaper. Sodium chloride was purchased from Fisher Scientific. All chemicals were used as received. Kromasil silica (Krom-100-10, 309 m2/g) was obtained from Akzo Nobel. Methods. Surface-grafted Nylon 6,6 films were prepared on silica particulate substrates. Briefly, 3 g of silica was added to a clean, dry round-bottom flask containing 2% [v/v] 3-aminopropyltimethoxysilane (APTES) in anhydrous toluene and stirred under N2 for 4 h at 25 °C. The resulting APTES-modified silica was filtered and washed with toluene and ethanol. Nylon 6,6 films were prepared using a stepwise growth procedure in alternating diamine and diacid solutions. APTES-modified silica was added to 10 mL 0.1 M adipoyl chloride in dry toluene and stirred under N2 for 45 min. NMR measurements indicate that the APTES surface coverage is 78% of the theoretical maximum (see Supporting Information). Particles were vacuum filtered, washed with toluene, and added to 10 mL 0.1 M 1,6hexamethylene diamine solution in dry toluene and stirred under N2 for 30 min. The alternating filtration and diacid/diamine reactions were repeated to increase the Nylon 6,6 chain growth. In total, three adipoyl chloride and two 1,6-hexamethylene diamine steps were used to prepare the samples used in all

Orendorff et al. experiments. The result is a three-repeat unit, acid-terminated Nylon 6,6 oligomer-modified silica substrate, referred to as N3.0; the chemical structure of which is shown in Figure 1. Ellipsometry measurements confirm the stepwise growth of the films, while elemental analyses show that the chain density is 50% of the theoretical maximum assuming closely packed all-trans chains (Supporting Information). Instrumentation. Samples for Raman spectroscopy were prepared by adding 50 mg of polyamide-modified silica to a quartz NMR tube (Wilmad Glass). Temperature control was achieved using a copper block sample holder fitted with either a resistive heater or a well for ice and the temperature was monitored with a K-type thermocouple. At each temperature, samples were equilibrated for 30 min prior to spectral acquisition. Raman spectra were collected using 25 mW of 785 radiation from diode laser in a commercial Thermo-Nicolet Almega Dispersion Raman spectrometer. Slit settings were 100 µm for all experiments and a spectral bandpass of 5.4 cm-1. Spectral acquisition times range from 2-5 min and are provided in the figure captions. A minimum of three spectra were acquired for each sample condition. Postacquisition, each Raman spectrum was background subtracted using a linear fit baseline. No other spectral processing or smoothing was done to these spectra. The total carbon content of the polyamide-modified silica was determined to be 13.2 wt % using a Perkin-Elmer 2400 Series II CHNS elemental analyzer (Desert Analytics, Tucson, AZ). Water dosing experiments were performed by adding a known volume of deionized water to 50 mg of N3.0 modified silica and sealing the dosed samples in NMR tubes. Given the Nylon 6,6 content of the modified silica of 13.2 wt %, the solution volumes added of 2.5, 5.0, and 50 µL correspond to water dosing levels of 16, 33, and 330 water molecules per chain (or 32, 49, and 90 wt % water assuming that all water is confined within the Nylon film), respectively. Tethered Nylon 6,6 samples were also immersed in 10-4 and 0.1 M NaCl solutions to study the role of dissolved salts on chain behaviors. Molecular Pictures. Pictures of the solvent-stationary-phase interface were constructed using ChemDraw 3D (CambridgeSoft). Single or two adjacent surface-bound Nylon 6,6 chains were arranged perpendicular to the silica surface. Given that these oligomer chains are grafted using a trifunctional precursor (aminopropyltrimethoxysilane), the two adjacent Nylon 6,6 chains are spaced by the average Si-O-Si spacing of 3.2 Å.23 There are other physically correct chain spacings that could be used based on the surface coverage; these pictures are meant to represent one possible scenario. First, Nylon chains are drawn in the all-trans conformation, fully extended from the silica surface. Gauche conformers and rotational disorder are manually added to the structure according to the experimental Raman data that show the presence and relative numbers of end-gauche, methylene gauche, and amide gauche conformers. Once the appropriate numbers of these conformers are added to the structure (given by the spectral data), the picture is energyminimized (see below). Solutions converging on chemically unrealistic configurations were discarded. Molecular models were energy-minimized using the modified MM2 force field computation provided by ChemDraw 3D. This computation employs numerous chemical parameters, including bond stretch energy, angle bend energy, torsion and nonbonding constraints, π-system calculations, charge and dipole terms, and cutoff parameters for van der Waals and electrostatic interactions. During the energy-minimization computation, the silicon atoms and the two proximal methylene groups on each Nylon chain were fixed in space, simulating surface-confined oligomer

Conformational Order in Model Nylon Thin Films

J. Phys. Chem. C, Vol. 113, No. 31, 2009 13725 TABLE 1: Peak Frequencies (cm-1) and Assignments for Bulk Nylon 6,6 and N3.0 peak frequency (cm-1) bulk Nylon 6,6

N3.0 at 23 °C

N3.0 at 90 °C

2933sh 2919 2903 2873 1635 1480 1444 1428 1298 1131

2930 2918 2903 2860 1650

2934sh 2924 2904sh 2865 1641

1444 1410 1302

1446

1082 1060 1042

1084sh 1060 1047

1063 1046

Figure 2. Raman spectra of (a) bulk Nylon 6,6 and (b) surface-tethered N3.0. Acquisition times are (a) 2 and (b) 5 min.

chains. It is important to note that although the models presented here are energy-minimized by the modified MM2 computation, the images are intended to aid in visualizing chain conformations rather than in providing a unique and unequivocal solution to how the chains might be oriented on the surface. Results and Discussion Raman Spectral Indicators of Conformational Order. Raman spectroscopy has proven to be a useful tool for determining structure and order of bulk aliphatic organic materials and organic thin films including lipid bilayers and selfassembled monolayers.24-28 Perhaps the most relevant body of recent work is focused on using Raman spectroscopy to determine the rotational and conformational order of alkylsilanemodified silica stationary phases for reversed-phase liquid chromatography (RPLC); materials that are closely related to our aliphatic Nylon 6,6-modified silicas.29-32 These reports have identified several spectral indicators of rotational and conformational order. Examples include the asymmetric and symmetric methylene stretching frequencies, νa(CH2) and νs(CH2), which are an indicator of alkyl chain-chain coupling. The intensity ratio of the νa(CH2) to the νs(CH2), is an empirical measure of rotational disorder, while the intensity ratio of the gauche carbon-carbon stretch, ν(C-C)G, to the trans carbon-carbon stretch, ν(C-C)T, is a direct measure of gauche conformers in the material. The other vibrational modes that provide structural information for the specific case of Nylon are provided below. Conformational Order in Bulk and Surface-Confined Nylon 6,6. Raman spectra of a commercial bulk Nylon 6,6 polymer and surface-bound N3.0 samples are shown in Figure 2. The corresponding peak frequency assignments are given in Table 1. While the amide I (1635 cm-1) and amide II (1444 cm-1) vibrational modes for polyamides are commonly used for simple material identification, the entire vibrational spectrum for polyamides contains a plethora of information regarding local structural and chemical environments. For bulk Nylon 6,6 in the ν(C-C) region, the ν(C-C)T,HB (1046 cm-1) and ν(C-C)T (1063 cm-1) modes correspond to trans conformers within the aliphatic chains. The ν(C-C)T,HB also indicates intermolecular (i.e., chain-chain) hydrogen-bonding, such as that found in the crystalline phases of bulk Nylon.33-35 The absence of ν(C-C)G

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assignmenta,b,c,d νa(CH2)/R-NH, β-NH, β-CO νa(CH2)/R-CO νa(CH2)/β-CO, γ-NH νs(CH2) ν(CdO), amide I δ(C-N-H) δ(CH2), δ(N-H), amide II δ(CH2) τ(CH2) ν(C-C)T ν(C-C)G ν(C-C)T ν(C-C)T,HB

a ν ) stretch; δ ) bend and/or scissor; τ ) twist. b R, β, and γ are methylene positions relative to NH and CO functional groups. c Assignments were made from refs 33-36. d G ) gauche; T ) trans; T,HB ) trans/hydrogen-bonded described in text.

(1080 cm-1) for bulk Nylon 6,6 suggests that the Nylon chains are in the all-trans configuration. The bulk Nylon 6,6 spectrum also has a pronounced methylene deformation mode, designated δ(CH2), at 1428 cm-1. In the literature for bulk polyethylene (PE), this mode has been referred to as δ(CH2)ORTHO and the relative intensity of that mode is a direct measure of the polymer chain-chain coupling in the crystalline orthorhombic phase.25 For Nylons, the relative intensity of the 1428 cm-1 δ(CH2) can also be used as a measure of chain-chain coupling analogous to that for the PE case. There is also a δ(C-N-H) bending mode observed at 1480 cm-1 for bulk Nylon 6,6, which indicates a trans amide group.21,36 In summary, the Raman spectra for bulk Nylon 6,6 confirm literature results showing that while this material is amorphous, most chains in the materials are in the all-trans configuration and are hydrogen bonded to each other in a zigzag pattern analogous to that found in the crystalline R-phase. For alkane-containing materials, the spectral indicator I[νa(CH2)]/ I[νs(CH2)] is used extensively as a sensitive empirical marker of rotational and conformational order.25,30,31 However, in the case of polyamides, the complexity of the ν(CH2) envelope is more complex and difficult to interpret.37 For this reason, our analysis of quantitative structural changes in Nylon 6,6 will focus on the ν(C-C) and δ(C-H) spectral regions. The Raman spectrum for surface-confined Nylon is dramatically different from that of bulk Nylon, indicating that chain conformations are not anything like those found in the crystalline R-phase. First, N3.0 has an appreciable ν(C-C)G mode (1082 cm-1), providing clear evidence that gauche conformations are present. The appearance of gauche defects is accompanied by a disappearance of the ν(C-C)T mode, indicating disruptions in the all-trans chain configurations found in the R-phase. In addition, the relative intensity of the δ(C-H-N) bending mode at 1480 cm-1 decreases to a weak shoulder. This suggests that the amide groups are primarily in the gauche configuration for the surface-confined Nylon. However, although gauche configurations are prevalent, the Raman spectra indicate that adjacent chains may be coupled to each other via hydrogen bonds even more strongly than in bulk Nylon. Stronger chain coupling for the N3.0 sample relative to bulk Nylon 6,6 is indicated by the increased relative intensity of the δ(CH2) mode at 1410 cm-1and ν(C-C)T,HB mode at 1040 cm-1, along with

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Figure 4. Raman spectra of N3.0 at (a) 23, (b) 60, and (c) 90 °C. Acquisition times are 5 min for all spectra. Figure 3. Molecular picture of surface-confined Nylon 6,6 at ambient temperature for (a) a single Nylon chain and (b) a 90° rotated view of two adjacent Nylon chains. Arrows highlight hydrogen-bonded amide groups. Models are energy minimized using the MM2 calculation. Structures are colored by atom: carbon atoms are pink, oxygen atoms are red, nitrogen atoms are blue, silicon atoms are purple, and hydrogen atoms are pale blue.

the peak frequencies of the νa(CH2) and νs(CH2) modes at 2918 and 2860 cm-1; all of which are fingerprints for coupled chains.21,25 We have developed molecular pictures for the configuration of the tethered Nylon 6,6 chains that are consistent with both the Raman spectra and known constraints imposed by the surface (Figure 3). Parameters that need to be considered include (1) maximizing the number and strength of hydrogen bonds between adjacent chains to minimize the energy of the total chain assembly, (2) adapting to the constraint of having all chains in exact registry due to attachment and growth on the substrate surface, and (3) filling the space that is available between individual chains, as determined by the lateral chain packing density. In bulk R- and β-Nylon 6,6, the first constraint is satisfied when adjacent chains are offset by one carbon to form a zigzag array of all-trans chains.13,14 However, when the chains are tethered on one end, the amide and carbonyl groups cannot be put into registry by sliding chains relative to each other to form a zigzag array. Here, maximization of hydrogen bonding requires either tilting of the chains or the formation of guache defects. In a totally dense monolayer, complete hydrogen bonding can be achieved by tilting the chains by roughly 30° to the surface normal (analogous to the chain offset observed in R-Nylon 6,6).33-35 However, elemental analyses indicate that we do not have fully dense films. The weight percentage of carbon on our silica particles (having a surface area of 309 m2/g) is 13.2%, which corresponds to a calculated chain density of 0.65/nm2.38 The surface area occupied per chain is roughly 1.5 nm2, which is about half the theoretical maximum chain density.38 At this chain density, the lateral chain spacing is such

that tilting an array of all-trans chains does not put the amide and carbonyl groups into hydrogen-bonding contact (R-Nylon 6,6-like phase). However, at half the maximum chain density there is plenty of free volume to allow the chains to bend or coil in such a way that hydrogen bonding is maximized by creating gauche methylene and amide conformers. The configuration shown in Figure 3 (arrived at via energy minimization using the MM2 (Molecular Modeling 2) calculations) represents a molecular picture that is actually quite similar to the pleated sheet structure found in γ-Nylon 6,6.21 In this configuration, all amide and carbonyl groups are hydrogen bonded, gauche defects are present in both methylene and amide positions, and there is strong chain-chain coupling both among and between chains as indicated by the Raman spectra. The chain configurations shown represent one of many arrangements that are consistent with the Raman spectra. We recognize that other bent or twisted configurations are also possible (including the formation of helical chains). We also recognize that our highsurface-area substrates probably do not exhibit a local order that is perfectly planar as shown. However, almost all arrangements that involve surface-constrained chains and that are consistent with the Raman spectral data showing highly coupled, hydrogenbonded Nylon chains with methylene and amide gauche conformers generate structures in which the amide and carbonyl groups form discrete planes of hydrogen bonds that are parallel to the substrate surface. With this knowledge of the baseline structure of surface-tethered Nylon 6,6, we are now in a position to evaluate how this structure changes in response to either temperature changes or exposure to water. Conformational Order in Surface-Bound Nylon 6,6 As a Function of Temperature. Raman spectra of tethered Nylon 6,6 at 23, 60, and 90 °C are shown in Figure 4. The corresponding peak frequency assignments are given in Table 1. Qualitatively, there are appreciable spectral changes for N3.0 as a function of temperature. As temperature increases, the intensity of the ν(C-C)T,HB and ν(C-C)G modes decrease relative to the ν(C-C)T mode, the τ(CH2) peak frequency shifts

Conformational Order in Model Nylon Thin Films to higher frequency and the intensity ratio of the δ(CH2) mode (1410 cm-1) to the δ(CH2)+amide II mode (1444 cm-1) decreases. In the ν(C-H) region, the most intense νa(CH2) mode (2918 cm-1) shifts to higher frequency and the relative intensities of all νa(CH2) modes (2903, 2918, and 2930 cm-1) appear to decrease and become a part of a broad νa(CH2) envelope at elevated temperature. These temperature-dependent conformational order changes in tethered Nylon 6,6 are highlighted in Figure 5 with plots of I[ν(C-C)T,HB]/I[ν(C-C)T], I[ν(C-C)G]/I[ν(C-C)T], I[δ(CH2)]/ I[δ(CH2)+amide II] verses temperature. As N3.0 undergoes a temperature-induced phase transition, chain-chain hydrogen bonding is disrupted, shown by a decrease in ν(C-C)T,HB intensity with temperature as the value of I[ν(C-C)T,HB]/ I[ν(C-C)T] decreases from ∼1.5 at 0 °C to ∼1.05 at 90 °C (Figure 5a). This observation of thermally induced hydrogenbond breaking is consistent with those made previously for bulk Nylon 6,6.33 This also suggests that the N3.0 exhibits less crystalline-like character as chains decouple at elevated temperature. There are several spectral indicators of diminishing chainchain coupling in N3.0 with temperature; one of which is shown explicitly in Figure 5b in the plot of I[δ(CH2)]/I[δ(CH2)+amide II] versus temperature. The intensity of the δ(CH2) mode decreases relative to the intensity of the δ(CH2)+amide II mode from a value of ∼0.6 to ∼0.2. This is consistent with previous observations made for the solid-liquid phase transition of polyethylene, where the intensity ratio of the δ(CH2)ORTHO (1420 cm-1) to the δ(CH2) (1440 cm-1) decreases during the melting transition.25 This indicates that the Nylon chains in N3.0 are more decoupled at higher temperature and on average must have greater free volume between adjacent chains than at ambient temperature.25 Other indications of decreased chain coupling with increasing temperature include the shifts in the νa(CH2) and νs(CH2) modes (2918 and 2860 cm-1) to higher frequency (2924 and 2965 cm-1) and the subtle decrease in the relative intensity of the amide I mode (1650 cm-1). The polarizability, R, of an amide carbonyl bond is greater in the hydrogen-bonded state than for an isolated carbonyl and, therefore, the relative intensity should follow the same trend (IR proportional to R2). Then, the relative intensity of the amide I mode is expected to decrease as interchain hydrogen-bonding is disrupted and aliphatic chains decouple. While the observation of amide I mode sensitivity to chemical environment has been made previously, no detailed explanation has been provided.21 The most interesting observation regarding the melting transition in tethered Nylon 6,6 is the decrease in gauche conformers with increasing temperature (Figure 5c). The value of I[ν(C-C)G/ν(C-C)T] decreases from 0.95 at 0 °C to 0.4 at 90 °C. The melting of bulk aliphatic materials generally results in a higher degree of disorder, which is reflected in an increase rather than a decrease in the number of gauche conformers.24,25,39 However, in our tethered Nylon 6,6 films, the “crystalline” room temperature phase already contains a large population of gauche defects. These defects are a direct result of the surface constraints that require serious chain deformations to accommodate the maximum number of interchain hydrogen bonds and for each chain to fill the space that is available between the chains. It appears that thermal stimulation allows the chains to “stand up” and move toward an all-trans configuration. The straightening out of the chains would largely eliminate interchain-chain interactions, which is consistent with the dramatic decrease in the δ(CH2) band at 1410 cm-1. With molecular models, the standing up of the chains and weakening of chain-chain

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Figure 5. Plots of (a) I[ν(C-C)T,HB]/I[ν(C-C)T], (b) I[δ(CH2)]/ I[δ(CH2)+amide II], and (c) I[ν(C-C)G]/I[ν(C-C)T] as a function of temperature. Error bars represent one standard deviation.

interactions requires a significant reduction in the number of hydrogen bonds between the chains. However, the Raman spectra indicate that the chain relaxation does not completely eliminate interchain hydrogen bonding. In particular, no increase in intensity is observed in the δ(C-H-N) region, indicating that the amine bonds retain their gauche configuration even at

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Figure 6. Molecular picture of tethered Nylon 6,6 at 23 and 90 °C for (a) a single Nylon chain and (b) a 90° rotated view of two adjacent Nylon chains. Models are energy minimized using the MM2 calculation. Structures are colored by atom: carbon atoms are pink, oxygen atoms are red, nitrogen atoms are blue, and hydrogen atoms are pale blue.

high temperature. In addition, the ν(C-C)T,HB band is still detectible. It is probable that at high temperatures, the chain motions and configuration changes are quite dynamic, and that transitory hydrogen bonds are always present. (Note: The dynamic nature of the chains is confirmed via NMR results to be published in a companion paper.) In summary, the Raman spectra suggest that elevated temperatures disrupt hydrogenbond interactions between tethered nylon chains. For low chain densities, heating results in a “phase transition” between the pleated or helical structures required to facilitate hydrogen bonding to a high temperature structure containing more isolated and nearly all-trans chains. This conformational change is shown in a molecular picture of N3.0 in Figure 6. Water and Saltwater Interactions with Nylon 6,6 Thin Films. For applications involving the exposure of polymers to aqueous solutions, such as the purification of water via reverse osmosis, it is critical to understand how water influences polymer chain conformations and hydrogen-bonding interactions. Studies on bulk Nylon suggest that while Nylon does adsorb water, the extent of water uptake (10-15 wt %) and swelling (2.5 vol %) are relatively minor. In addition, neutron scattering experiments indicate that water uptake and swelling are confined to amorphous regions, where water uptake is estimated to be as high as 60 wt %. No water appears to penetrate the crystalline R-Nylon domains under ambient conditions. However, the Raman results presented above clearly show that the structure and physical properties (e.g., melting transition) for our tethered Nylon 6,6 films are dramatically different from those of bulk or R-Nylon. In terms of the response to water, the most important difference between our tethered films and bulk Nylon is that in the R-phase, all amine-tocarbonyl hydrogen bonds are encapsulated and protected by close-packed hydrophobic aliphatic domains that water cannot penetrate. In contrast, within our films, each chain is isolated from other chains except at hydrogen-bonded contact points, and all chain components are accessible to environmental species such as water. The consequences of this open access to water are reflected in the structural changes induced by aqueous exposures as described below. Another potential difference between the thin films and bulk Nylon is that sites on the underlying substrate are also available to interact with water. In our films, roughly 50% of the available substrate sites are

Figure 7. Raman spectra of N3.0 dosed with (a) 16, (b) 33, and (c) 330 water molecules per Nylon 6,6 chain. Acquisition times are 5 min for all spectra.

occupied by Nylon chains, with the vast majority of the remaining sites being terminated by amine groups in the aminopropyl silane base layer (i.e., few unreacted silanols remain on the silica surface). These amine groups are available to hydrogen bond to water and could contribute to water retention. However, as the Nylon overlayer must be penetrated to access these amine groups, and as the number of potential hydrogen bonding sites is low relative to those present in the Nylon film, these substrate sites are being neglected in the discussion that follows. Raman spectra were obtained for tethered Nylon 6,6 films exposed to doses of deionized water corresponding to 16, 33, and 330 water molecules per Nylon 6,6 chain. At the highest water loading of 330 H2O/chain, the Raman spectra reveal that water disrupts interchain hydrogen bonding and stands up the tethered chains in much the same way as thermal stimulation (Figure 7c). The relative intensities of the ν(C-C)T,HB, ν(C-C)G, and δ(CH2) modes are all much lower than in the dry film, indicating that there is much less interchain hydrogen bonding, few gauche defects, and less chain-chain coupling, respectively. If the chains are standing up in a nearly all-trans configuration, the chains themselves only occupy 12% of the volume available. If all the remaining volume is occupied by water, there is room for on the order of 230 water molecules within the oligomer film, which means that an excess of water is present at the 330 H2O/chain dosing level. The occupation of all free volume by water and the disruption of interchain hydrogen bonds probably both contribute to the straightening of the Nylon chains into the all-trans configuration. However, as in the thermal case, it appears that the dominant contribution is the disruption of interchain hydrogen bonds. At the intermediate dosing of 33 H2O/chain, there is not nearly enough water to fill the void spaces between the chains, but there is sufficient water (3 molecules per each amine and carbonyl group) to associate with all chain sites that are available for hydrogen bonding. The Raman spectrum corresponding to

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Figure 9. Raman spectra of N3.0 in aqueous (a) 10-4 M and (b) 1 M NaCl. Acquisition times are 5 min for all spectra.

Figure 8. Plot of (a) I[ν(C-C)T,HB]/I[ν(C-C)T] and (b) I[ν(C-C)G]/ I[ν(C-C)T] as a function of number of dosed water molecules per Nylon chain. Error bars represent one standard deviation.

this dosing level is almost identical to that of the completely saturated (330 H2O/chain) case, indicating that the transition from gauche to all-trans chains occurs well before the void volume between the chains is filled. In fact, plots of Raman peak intensities vs water content (Figure 8) suggest that the midpoint of the phase transition occurs at a water content of around 15-16 H2O/chain, or 1-2 water molecules per each amine and carbonyl group. It appears that at low water doses, the water selectively binds to sites capable of hydrogen bonding and only hydrates the aliphatic domains once the hydrogen bonding sites are saturated (as might be expected). The Raman spectrum for tethered Nylon 6,6 at the lowest water dose examined (16 water molecules per chain) appears to have captured the onset of the swelling transition. Here, the decrease in the intensity of the ν(C-C)T,HB band is actually accompanied by an increase in the number of gauche defects, which is in contrast to the thermal results. We believe that this observation indicates that the onset of the “swelling transition” in tethered Nylon involves the insertion of water molecules into the hydrogen bonds between adjacent amine and carbonyl groups. The insertion of a water molecule into these linkages would weaken but not eliminate the hydrogen-bonding connections between chains and would produce steric crowding leading to the creation of more gauche defects. Eventually, sufficient water is present such that all amine and carbonyl groups can satisfy their hydrogen-bonding requirements by associating with col-

lections of uncorrelated water molecules rather than with a common water bridge, which allows the chains to separate. Once separated, the free volume between the chains is available to be filled with water, further stabilizing the upright, all-trans configuration. For RO membranes, it is important to understand how dissolved salts influence chain conformations and phase behavior. Raman spectra obtained for tethered Nylon 6,6 in aqueous NaCl solutions are shown in Figure 9. The spectra obtained in both 10-4 and 1 M NaCl solutions are identical to the spectrum for Nylon 6,6 saturated with deionized water. The addition of salt appears to have no influence on the conformation of the Nylon 6,6 chains. The chains stand up into an array of isolated all-trans chains whether salt is present or not. From a mechanistic perspective, these data could be consistent with two extreme scenarios, either that (1) salt is excluded from the Nylon 6,6 film, which only contains deionized water or (2) that hydrogen-bond disruption and chain hydration are largely independent of the presence of dissolved salts, which are free to penetrate the film in substantial concentrations. The Raman results alone cannot resolve the difference between the two extremes. However, it is likely that the salt/Nylon/water interactions will be dependent on chain spacings and configurations and that bulk Nylon and our tethered films will not behave the same. To establish a perspective of what could be going on in our films, a 1 M solution of NaCl contains roughly one Na+ and one Cl- per 50 water molecules. If this solution were to penetrate the film intact, there would be on the order of 5 Na+ and 5 Clper Nylon chain (i.e., the total ion concentration would be comparable to the number of amide and carbonyl groups in the chains). The expectation is that these ions would be hydrated, with a total hydration number of around 6 per ion pair (12% of the water in a 1 M aqueous NaCl solution is tied up in hydration shells). This means that there is more than enough water available to hydrate both the ions and the hydrogen-bonded sites simultaneously. The diameter of a hydrated Na+ ion is on the order of 0.95 nm, which means that there is sufficient room for hydrated ions between the chains (having a chain-chain separation distance of 1.2 nm) without introducing steric crowding. However, there is insufficient room for establishing independent hydration shells around both the chains and the ions simultaneously. Dehydration of the ions (or for that matter

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the chains) is possible, but would come at a significant energy cost. In many ion-exchange materials, that cost is overcome by interactions between the ions and fixed counterion charges in the exchange material. However, without dehydration, the only way to accommodate the ions would be to distort the chains via steric crowding. Such distortions would be clearly evident in the Raman spectra, and no such distortions are seen. Techniques other than Raman spectroscopy are needed to probe whether ions do in fact penetrate the Nylon 6,6 films and to determine the ordering of water that accompanies the net hydration of both the chains and the water. Techniques we have utilized to obtain such information include neutron scattering, sum frequency generation vibrational spectroscopy, and nanoscale measurements of water ordering using a scanning probe system called the interfacial force microscope. The influence of Nylon on the ordering of water and ion partitioning will be highlighted in a separate publication.22 However, the neutron scattering results on films exposed to 1 M CsF solutions suggest that the salt concentration within the Nylon film is less than 10% of that in the bulk solution. This indicates that the driving force for having ions penetrate the film is insufficient to overcome the energy penalty associated with dehydrating either the ions or the chains. The chain packing density in the films examined here is predicted to be right near the threshold of where competitive hydration effects should become significant (assuming hydration layers that are one water molecule thick). At lower chain densities, there should be no barrier to salt penetration, while at higher chain densities, even water may start to have a hard time penetrating the film (as in crystalline R-Nylon 6,6). Systematic studies in which chain packing densities are varied will be required to more clearly define the boundaries for water and salt penetration in such films.

hydrogen bonds are disrupted, the chains are isolated from one another and reconfigure themselves from bent and twisted structures containing gauche defects into a more linear, all-trans configuration. The Raman results obtained in this investigation clearly show that the physical and chemical properties of polymeric chains such as Nylon 6,6 are highly dependent on chain configurations. It is also clear that the chain configurations are highly dependent on anchoring and chain spacing geometries. For multicomponent materials, such as the complex polymers used for water treatment technologies, it is clear that the ultimate properties, including environmental stability, chain hydration, swelling, and even phenomena such as salt exclusion, will be dependent not only on the chemical sequences in the individual chains but also on how the chains are spaced and arranged relative to each other. We believe that studies involving the combined use of tethered chains and Raman spectroscopy represent a methodology that will eventually allow scientists and engineers to design membrane materials for applications such as water treatment based on a sound mechanistic understanding of structure-property relationships in polymers.

Conclusions

Supporting Information Available: Additional figures and surface coverage estimation from 29Si NMR. This material is available free of charge via the Internet at http:// pubs.acs.org.

Raman spectroscopy clearly shows that the structure of tethered Nylon 6,6 is dramatically different from that of bulk Nylon. The differences arise from two constraints imposed by our surface tethering geometry: (1) all chain ends are tethered, preventing them from sliding relative to each other in order to maximize interchain hydrogen bonding as in crystalline R- and β-Nylon 6,6, and (2) the lateral spacing between chains (1.2 nm) is too great to allow intimate and extensive chain-chain contact, in sharp contrast to the tightly packed chains in crystalline Nylon phases. The consequences of these two constraints are that chains in the tethered film assume bent or coiled geometries that maximize hydrogen bonding, allow chain-chain interactions, and fill the available free volume between the chains. The responses of the tethered Nylon to temperature changes and exposure to water are also in sharp contrast to what is seen in bulk materials and are a direct consequence of the structural differences induced by tethering. Crystalline phases such as R-Nylon 6,6, which contain tightly packed arrays of all-trans chains, exhibit high melting transition temperatures (as high as 250 °C) and are resistant to water penetration at room temperature. The tethered chains, which are open structures that are more loosely connected via hydrogen bonds, exhibit a low melting temperature (around 55 °C) and undergo a pronounced swelling transition at exposures of 1-2 water molecules per hydrogen bonding site (corresponding to 30 wt % water). For the tethered films, the observed phase transition, whether triggered by temperature or water exposure, involves the disruption of most interchain hydrogen bonds. Once the

Acknowledgment. The authors are grateful to research support from Sandia National Laboratories Laboratory Directed Research and Development program. The authors thank J. A. Timlin, D. R. Tallant, and R. L. Simpson for their technical assistance and L. M. G. Minear, C. A. Gresham, K. M. Alam, and L. E. Martin for allowing us to use their spectroscopy facilities. The authors also acknowledge Akzo Nobel for providing bulk silica materials. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.

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