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Effect of Final Monomer Deposition Steps on Molecular Layer-by-Layer Polyamide Surface Properties Marissa E. Tousley, Devin L. Shaffer, Jung-Hyun Lee, Chinedum O. Osuji, and Menachem Elimelech Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02746 • Publication Date (Web): 03 Oct 2016 Downloaded from http://pubs.acs.org on October 7, 2016
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Effect of Final Monomer Deposition Steps on Molecular Layer-by-Layer Polyamide Surface Properties
Marissa E. Tousley,†* Devin L. Shaffer,‡ Jung-Hyun Lee,§ Chinedum O. Osuji,‡ Menachem Elimelech‡
To be submitted to Langmuir
†
Department of Chemical Engineering, Rose-Hulman Institute of Technology, Terre Haute,
Indiana 47803, USA ‡
Department of Chemical and Environmental Engineering, Yale University, New Haven,
Connecticut 06511, USA §
Department of Chemical and Biological Engineering, Korea University, Seoul, South Korea
* Corresponding author: Marissa E. Tousley, Email:
[email protected] ACS Paragon Plus Environment
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Abstract A current challenge to desalination membrane technology is the inability to precisely control the properties of the polyamide selective layer due to the complexity of interfacial polymerization. In this study, we investigate the ability of molecular layer-by-layer (mLbL) assembly, an alternative polyamide fabrication technique, to create polyamide surfaces with tunable chemistry. We explore the influence of terminating monomer, monomer deposition time, monomer size, and the presence of underlying ionizable functional groups on mLbL-derived polyamide surface properties. AFM colloidal probe measurements, contact angle titrations, QCM cesium adsorption experiments, and XPS data show that polyamide films terminated with m-phenylene diamine (MPD) or trimesoyl chloride (TMC) for 20-30 seconds are chemically similar. Increasing terminating monomer deposition time or using a smaller, more reactive monomer results in more distinct colloidal-probe adhesive interactions, contact angle titration curves, negative charge densities, and near surface atomic compositions. By optimizing the final monomer deposition steps, both amine-rich and carboxyl-rich polyamide surfaces can be fabricated, which has implications for the application of mLbL assembly to membrane-based desalination.
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Introduction Solution-based molecular layer deposition has enabled the controlled growth of thin, organic networks designed for photovoltaics,1 organic light emitting diodes,2 cancer treatment,3 and membrane separations.4 Similar to polyelectrolyte layer-by-layer assembly, a series of spin or dip coating steps are used to fabricate thin films.5,
6
In solution-based
molecular layer deposition, however, monomer layers are covalently bound together,6, 7, 8, 9 rather than assembled through less-stable, ionic or hydrogen bonds.10, 11 Most relevant to this study, the solution-based sequential deposition of diamine and tri-acid chloride monomers yields covalently crosslinked polyamide films (Figure 1).6, 12, 13 This polyamide fabrication approach is commonly referred to as the molecular layer-by-layer (mLbL) assembly of polyamide. mLbL assembly of polyamide is of particular interest in membrane desalination, where state-of-the-art thin-film composite (TFC) membranes contain a polyamide selective layer.14 Traditionally, the polyamide selective layer is formed through interfacial polymerization (IP). During IP, diamine monomers partition from an aqueous phase into an organic phase containing tri-acid chloride monomers. When the monomers come in contact near the water-oil interface, they react to form a nascent crosslinked polyamide film.15 This film grows and changes structure as diamine monomers continue diffuse across the interface and through the film to undergo further reaction.14, 16, 17 Precise and methodical control of IP polyamide properties is difficult to achieve due to the complexity of the IP process, namely the diffusion-controlled nature and interconnected process parameters (e.g. monomer reactivity and solubility, solvent viscosity, reactant ratio).18, 3
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This limited property control creates the need for post-fabrication treatment of TFC
membranes, such as surface modification to reduce membrane fouling propensity and chemical treatment to tune permeability and selectivity.14,
20, 21, 22
Our fundamental
understanding of membrane transport phenomena and fouling is also hindered by limited property control. Typically, altering one polyamide property (e.g. network structure) results in simultaneous changes to other properties (e.g. thickness and surface roughness), making it challenging to systematically investigate the influence of isolated properties on membrane performance.23, 24, 25, 26
Figure 1. Schematic depicting the mLbL polyamide film fabrication process, where diamine and tri-acid chloride monomers are sequentially deposited onto a substrate to form a crosslinked polyamide film. Chemical structures of the monomers used for the final deposition step are shown below the film fabrication depiction: trimesoyl chloride (TMC), m-phenylene diamine (MPD), and ethylene diamine (ED). 4
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The sequential and surface-site limited nature of mLbL assembly of polyamide, in contrast, allows for monomer-length-scale control over properties.6 This polyamide fabrication technique, therefore, is a useful tool for creating “model membranes” which can be used to gain fundamental insight on TFC membrane performance. mLbL assembly was recently employed to investigate the isolated effects of film thickness and network structure on membrane permeability and salt selectivity.5,
27
These effects were studied by
systematically controlling the number of monomer deposition steps and the monomers used for film fabrication. While mLbL assembly holds promise for enabling the development of high-performance, highly tunable TFC membranes, this fabrication approach is still in the incipient stages and is not fully understood. In this study, we explore another aspect of property control achievable through mLbL assembly of polyamide—surface chemistry. Specifically, we investigate the influence of the final monomer deposition step on mLbL polyamide surface properties. We report the surface characteristics of tri-acid chloride- and diamine-terminated mLbL polyamide films and demonstrate that complete reaction of available surface sites is not achieved during monomer deposition. Methods to further tailor mLbL polyamide to be more chemically distinct and possess nearly homogeneous surface functionality are also discussed. The ability to fabricate polyamide surfaces with homogeneous functionality has implications in fundamental transport studies of desalination membrane selective layers, fundamental studies of membrane fouling and scaling propensity, and the development of robust, molecular-scale antifouling coatings that do not compromise desalination membrane performance.
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Experimental Section Materials. Polyethyleneimine (PEI, Mw ~750,000, 50% wt% in water), poly(acrylic acid) (PAA, Mw ~ 100,000, 35 wt% in water), ethylene diamine (ED), trimesoyl chloride (TMC), citric acid, MES monohydrate, and HEPES buffer were purchased from Sigma-Aldrich and used as received. Acetone (ACS grade, Fisher Scientific), toluene (Macron Fine Chemicals),
m-phenylenediamine (MPD, Acros Organics), acetic acid (25 % v/v, Fisher Scientific), sodium bicarbonate (Fisher Scientific), sodium hydroxide (J.T. Baker), boric acid (Acros Organics), and cesium chloride (CsCl, 99.9%, Alfa Aesar) were also used as received. Polyacrylonitrile (PAN) support layers (PA50, 75,000 molecular weight cut-off) were purchased from Sepro Membranes (Oceanside, CA) and silicon wafers were purchased from University Wafer (Boston, MA). Ultrapure water was obtained from a Millipore Milli-Q system.
mLbL Polyamide Surface Preparation. Polyamide surfaces were prepared by mLbL assembly, using spin6, 12 and dip coating5, 27 protocols adapted from the literature. Dip coating enabled the fabrication of large area samples (desired for contact angle titration measurements). Spin coating enabled to fabrication of surfaces on specialized substrates (i.e. quartz-crystal microbalance sensors). Spin coated films were prepared by sequential deposition of TMC monomer solution (1 wt% in toluene), toluene rinse solution, MPD monomer solution (1 wt% in toluene), and acetone rinse solution on a silicon wafer or silicon dioxide coated quartz crystal microbalance sensor. These deposition steps were repeated for 20 (n = 40 monomer deposition steps) or 20.5 (n = 41) cycles for MPD- and TMC-terminated films, respectively. After monomer 6
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solution deposition, the substrate surface was in contact with the solution for 20 s, followed by a 20 s spin cycle at 3000 rpm to remove excess monomer solution. Similarly, rinse solutions were allowed to contact the substrate for 20 s after deposition, followed by a spin cycle at 4000 rpm for 15 s. Following mLbL deposition, films were annealed at 215 °C for 1 minute. Prior to mLbL polyamide deposition, substrates were cleaned for 10 minutes in a UV/ozone cleaner (Jelight Co., Irvine, CA). All of the differently terminated mLbL surfaces were prepared using the aforementioned protocol, with small adjustments to the final monomer deposition steps as explained in Table 1. In this table, nterm refers to the final or terminating monomer deposition step, nterm-1 refers to the monomer deposition step directly preceding the terminating monomer deposition step or the second to last monomer deposition step, nterm-2 refers to the third to last monomer deposition step, and nterm-3 refers to the fourth to last monomer deposition step.
Table 1. Final Deposition Steps for Differently Terminated mLbL Polyamide Films Surface Termination
nterm-3
nterm-2
nterm-1
nterm
TMC* (20 min)
MPD*, standard**
TMC, standard
MPD, standard
TMC, 20 min
TMC (standard)
MPD, standard
TMC, standard
MPD, standard
TMC, standard
MPD (standard)
TMC, standard
MPD, standard
TMC, standard
MPD, standard
MPD (20 min)
TMC, standard
MPD, standard
TMC, standard
MPD, 20 min
MPD (2x20)
TMC, standard
MPD, 20 min
TMC, standard
MPD, 20 min
ED (enriched)
MPD, standard
TMC, standard
MPD, standard
ED, 20 min
*TMC is trimesoyl chloride, MPD is m-phenylene diamine, and ED is ethylene diamine. **Standard deposition times for spin and dip coated films are 20 and 30 seconds, respectively.
Dip coated mLbL polyamide films were formed on top of a hydrolyzed PAN support coated with a polyelectrolyte bilayer. PAN supports were hydrolyzed in a 2 M NaOH aqueous solution at 50 °C for 2 h and then rinsed in a regularly changed deionized water bath for 5 7
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days. A polyelectrolyte bilayer was deposited on the hydrolyzed PAN, first by dip coating in a PEI solution (0.1 wt% PEI in 0.5 M NaCl, pH 10.6) for 15 min, followed by two 2 min rinsing steps in de-ionized water, and then by dip coating in a PAA solution (0.1 wt% PAA in 0.5 M NaCl, pH 3.5) for 10 min, followed by two additional 2 min rinsing steps in de-ionized water. The purpose of this bilayer is to block pores on the underlying PAN support and increase the density of negative surface sites necessary for MPD attachment. The hydrolyzed PAN supports were air dried for 10 min, prior to performing solvent exchange in acetone (30 min) and toluene (1 h) to remove residual water from the support. After solvent exchange, mLbL polyamide was formed on top of the polyelectrolyte bilayer by sequential dip coating of the PAN support into the following monomer and rinse solutions for 30 s intervals: MPD (1 wt% in toluene), acetone, acetone, toluene, TMC (1 wt% in toluene), toluene, toluene. This deposition process was repeated for 14.5 cycles (n = 29) for MPD-terminated surfaces and 15 cycles (n = 30) for TMC-terminated surfaces. There were slight variations to the final monomer deposition steps, described in Table 1, depending on the specific mLbL surface termination. Following mLbL polyamide formation, films were annealed at 70 °C, dried in air for 1 h to allow for evaporation of residual solvent, stored in water for 2 h to ensure hydrolysis of unreacted acid chloride groups, and then stored dry until use.
X-ray Photoelectron Spectroscopy.
X-ray photoelectron spectroscopy (XPS)
measurements were performed using a Kratos AXIS Ultra DLD Spectrometer (Kratos Analytical, Manchester, UK), with a monochromated Al Kα source operating at 1486.6 eV and 140 W. The base pressure of the sample analysis chamber was ca. 2.0 × 10-9 Pa, and spectra were collected in hybrid mode using electrostatic and magnetic lenses from a nominal 8
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spot size of 300 µm × 700 µm. Near-surface atomic composition was determined from survey scans at a 15 degree angle from the plane of spin coated polyamide films over a binding energy range of 0-1200 eV, pass energy of 160 eV, step size of 0.5 eV, and dwell time of 0.1 s. XPS data analysis was performed using the CasaXPS software package, using a Shirley-type background. The
National
Institutes
of
Standards
and
Technology
(NIST)
Electron
Effective-Attenuation-Length Database, Standard Reference Database 82, was used to estimate measurement mean escape depth (MED) and information depth. Estimations were made assuming fully crosslinked polyamide with a stoichiometric ratio of 75% carbon, 10% nitrogen, and 15% oxygen, and a density of 1.3 g/cm3. The MED, defined as the average depth normal to the surface from which radiation escapes, was estimated for carbon (0.80 nm), oxygen (0.67 nm), and nitrogen (0.74 nm). These three MED values were averaged, yielding an estimated MED of 0.74 nm for the polyamide films.28 The information depth, or the maximum depth normal to the surface from which 95% of detected photoelectrons originate, is approximately three times the MED, or approximately 2.4 nm for the measured films.29
Atomic Force Microscopy Roughness and Adhesion Force Measurements. Atomic force microscopy (AFM) surface imaging was performed using a Dimension FastScan AFM (Bruker, Santa Barbara, CA). The surface roughness of spin and dip coated mLbL samples was determined from 1 µm × 1 µm AFM images collected under ambient conditions. Reported roughness values were determined by averaging measurements from two separately prepared samples (three spots per sample) for each surface type. 9
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Adhesion force measurements were performed in a 1 mM NaCl solution at pH 7 using a Dimension Icon AFM (Bruker, Santa Barbara, CA). A tip-less silicon nitride cantilever (Bruker NP-O10) functionalized with a 4 µm carboxyl-modified latex particle (CML, carboxyl content 19.5 µeq/g, Life Technologies, Eugene, OR) was used to measure adhesive interactions with mLbL surfaces (dip coated).30 The particle was attached to the cantilever using UV-curable adhesive (Norland Optical Adhesive, Norland Products, Cranbury, NJ), which was cured in a UV/ozone cleaner (BioForce Nanosciences, Ames, IA) for 20 min. Prior to each experiment, the cantilever deflection sensitivity and spring constant were determined using the thermal noise method.31 Force versus separation curves were then collected using a 0.5 Hz ramp rate, 1.5 µm ramp size, and a trigger force of 10 nN. The Peak Analysis function of Nanoscope Analysis v1.5 (Bruker) was used to determine the maximum pull-off force of each force-separation curve. In every experiment, adhesive interactions were measured in 3 spots per surface type. The statistical difference of the average adhesion forces of the differently terminated mLbL polyamide surfaces (measured in the same experiment) were compared using a two sample t-test assuming unequal variances.
Quartz Crystal Microbalance-Based Surface Charge Measurements.
The negative
surface charge densities of the mLbL polyamide films were estimated using a quartz crystal microbalance (QCM) technique (QCM E-4 model, Q-Sense Inc., Biolin Scientific, Linthicum Heights, MD).32
Spin coated mLbL polyamide films were formed on silicon dioxide coated
quartz QCM sensors (Q-Sense Inc.) with 20 deposition cycles and 20.5 cycles for MPD- and TMC-terminated polyamide, respectively. Changes in the fundamental frequencies of the coated QCM sensors were measured over time as the sensors were exposed to a solution 10
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containing cesium ion probes that ionically associated with negatively charged sites in the polyamide films. The polyamide coated QCM sensors were subjected to 5-6 cycles of exposure to a 1 mM CsCl solution at pH 10 for 1 h followed by rinsing the sensors in ultrapure water at pH 5.3 for 45 minutes. Using the Sauerbrey Equation, the changes in frequency at the third overtone of the fundamental frequency were used to calculate changes in mass of the films due to cesium ion association with negatively charged sites in the polyamide films.32 The surface densities of negatively charged sites on the polyamide films were estimated from the cesium ion mass accumulations, knowing the atomic weight of cesium (132.9 g mol-1) and assuming a single cesium ion ionically associated with a single negatively charged site. The mean value of negative surface charge densities calculated for CsCl exposure cycles 4-6 for each polyamide film sample was taken as the representative negative surface charge density.
Four or five samples of each differently terminated
polyamide film were tested, and statistical differences between surface charge density of the polyamide film samples were determined by two-sided t-tests assuming unequal variances.
Contact Angle Titrations. Contact angles were measured with an optical tensiometer (OneAttension, Biolin Scientific, Paramus, NJ) using the sessile drop method. For each measurement, a 1 µL buffered water droplet of known pH was placed on a dried mLbL polyamide surface (dip coated) and photographed after 20 s. Image analysis (OneAttension software) was then performed to calculate the left and right contact angles between the mLbL surface and water droplet. Titration curves were prepared by measuring the contact angle of buffered aqueous solutions at pH 2-10. Measurements were performed in at least 5 random locations for each pH, where droplets were always placed in a pristine location (not 11
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previously used). Full sets of pH measurements were performed on a minimum of two separately prepared samples for each of the differently terminated mLbL polyamide surfaces. Aqueous solutions containing 1 M of the following buffers were used for measurements: sodium phosphate monobasic (pH 2), citric acid (pH 3 and 4), acetic acid (pH 5), MES (pH 6), HEPES (pH 7 and 8), boric acid (pH 9), sodium bicarbonate (pH 10). The surface ionization fraction, α, as a function of pH was calculated from buffered contact angle measurements, using:33, 34
=
cos − cos cos − cos
(1)
where θ is the contact angle, A is the limiting contact angle at low pH, and B is the limiting contact angle at high pH. This calculation assumes that only ionizable functional groups in direct contact with the water droplet influence the surface free energy and hence the measured contact angle.
Results and Discussion Characteristics of Standard mLbL Polyamide Surfaces. The root mean square roughness (RMS) of our standard mLbL polyamide surfaces was measured using AFM (Table SI) and is the same order of magnitude as the roughness of mLbL polyamide films previously described in the literature.5, 6 Surfaces prepared by dip coating have a higher roughness, 4.42 ± 0.52 nm, than those prepared by spin coating, 0.92 ± 0.25 nm. This difference in roughness is attributed to the underlying substrate. Dip coated films were prepared on top of polyelectrolyte coated, PAN ultrafiltration membranes, while spin coated films were prepared 12
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on silicon wafers. Regardless of film preparation technique, surface roughness is independent of terminating monomer. The near surface atomic composition of standard TMC and MPD-terminated films was measured by XPS (Table 2). The information depth, or maximum depth normal to surface where 95% of the detected photoelectrons originate, is estimated to be 2.4 nm for our measurement conditions, assuming fully crosslinked polyamide films.28, 29 TMC-terminated mLbL polyamide has a lower carbon to oxygen ratio (C/O, 4.82) and a higher oxygen to nitrogen ratio (O/N, 2.80) than MPD-terminated polyamide (5.07 C/O and 1.91 O/N). This trend is consistent with a previous report6 and can be explained by the chemical nature of the terminating monomers (Figure 1). MPD contains two amine groups, while TMC contains three acid chloride groups, which can hydrolyze to form carboxyl groups if not participating in another bond (i.e. amide bond). The oxygen content measured at the surface of an mLbL film terminated with TMC should therefore be higher than for one terminated with MPD, leading to a lower C/O ratio and a higher O/N ratio. C/O and O/N ratios for IP polyamide from the literature6 have also been included in Table 2. IP polyamide has a larger C/O and smaller O/N ratio than standard MPD polyamide, indicating that IP polyamide may have a lower surface carboxyl and a higher surface amine content than standard mLbL polyamide films.
Table 2. Atomic Ratio Compositions of mLbL Polyamide Surfaces Determined by X-ray Photoelectron Spectroscopy.
Surface Termination
C/O
O/N 13
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TMC (20 min)
3.40
18.9
TMC (standard)
4.82
2.80
MPD (standard)
5.07
1.91
MPD (20 min)
7.19
0.984
MPD (2x20)
7.13
0.955
ED (enriched)
8.77
0.804
IP Polyamide6
6.24
1.08
Standard TMC- and MPD-terminated surfaces also exhibit distinct adhesive interactions during solution-based AFM colloidal probe measurements (Figure 2). Upon retraction of a cantilever functionalized with a 4 µm carboxyl-modified latex particle (Figure S1), a greater adhesion force is measured for MPD-terminated polyamide than for TMC-terminated polyamide. The distribution of adhesion forces (represented by the maximum pull-off force divided by the particle radius, FMax/RP) from a representative experiment is shown in Figure 2. The mean adhesion forces for the MPD-terminated (-0.388 ± 0.189 mN/m) and TMC-terminated (-0.240 ± 0.158 mN/m) mLbL surfaces are statistically different (p
