Increased Hydrogel Swelling Induced by Absorption of Small Molecules

May 9, 2016 - The water and small molecule uptake behavior of amphiphilic diacrylate terminated poly(dimethylsiloxane) (PDMSDA)/poly(ethylene glycol d...
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Increased Hydrogel Swelling Induced by Absorption of Small Molecules Changwoo Nam, Tawanda J. Zimudzi, Geoffrey M. Geise, and Michael A. Hickner ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02069 • Publication Date (Web): 09 May 2016 Downloaded from http://pubs.acs.org on May 24, 2016

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Increased Hydrogel Swelling Induced by Absorption

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of Small Molecules

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Changwoo Nam1, Tawanda J. Zimudzi2, Geoffrey M. Geise3, Michael A. Hickner1* 1

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Department of Materials Science and Engineering, The Pennsylvania State University,

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University Park, Pennsylvania, 16802, United States 2

Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania,

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16802, United States 3

Department of Chemical Engineering, The University of Virginia,

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Charlottesville, Virginia, 22904, United States

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*Corresponding author: [email protected]

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ABSTRACT: Water and small molecule uptake behavior of amphiphilic diacrylate terminated

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poly(dimethylsiloxane) (PDMSDA)/poly(ethylene glycol diacrylate) (PEGDA) cross-linked

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hydrogels were studied using attenuated total reflectance-Fourier transform infrared (ATR-FTIR)

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spectroscopy. These hydrogel networks absorbed more water as the PEGDA content of the

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network increased. In contrast to typical osmotic deswelling behavior that occurs when liquid

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water equilibrated hydrogels are immersed in small molecule solutions with water activities less

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than unity, water-swollen gels immersed in 2-acrylamido-2-methylpropane sulfonic acid

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(AMPS-H) solutions rapidly regained their water content within 4 min following an initial

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deswelling response. In-situ ATR-FTIR analysis of the hydrogel film swelling indicated that

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small molecule absorption into the gel played an important role in inducing gel reswelling in low

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water activity solutions. This aspect of polymer gel water uptake and interaction with small

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molecules is important for optimizing hydrogel coatings and hydrophilic polymer applications

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where there is an interaction between the internal chemical structure of the gel and electrolytes or

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other molecules in solution.

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KEYWORDS: hydrogel, water uptake, ATR-FTIR spectroscopy, amphiphilic, sulfonic acid

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INTRODUCTION

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Water swelling of cross-linked hydrogel polymers can be described by Flory–Rehner

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theory1,2 which accounts for the balance between the free energy of hydration and the elastic free

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energy of the polymer matrix in a submerged cross-linked polymer gel.3 Additionally, increasing

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the number of ionic groups in gels is known to increase the degree of water swelling because the

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ionic groups provide an additional driving force for swelling through their large hydration

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energy.4 When equilibrated in aqueous solution, hydrogel water uptake is often sensitive to

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solute concentration because the presence of solutes depresses the thermodynamic activity of

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water which is linked to polymer swelling (Scheme 1).5-7 For example, a cross-linked PEGDA-

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based hydrogel uptake 1.02 g (water) per g (dry polymer) in pure water (aw = 1), but the same

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polymer absorbs 0.83 (water) g/g (dry polymer) in 1 M NaCl (aw = 0.967).8,9

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Scheme 1. Hydrogel swelling where polymer chains are represented as hydrophilic (blue line) or

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hydrophobic (red line) chains (a) hydrogel film on the substrate before exposure to water (b)

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swollen hydrogel film after exposure to water (c) hydrogel deswelling after introduction of

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organic and inorganic solutes (blue circle) (d) induced water swelling after introduction of ionic

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group-containing solutes (green circle).

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The PEGDA-based hydrogel interface with aqueous solution is hydrophilic, and high

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concentration of water molecules at the gel-aqueous interface has been suggested to play a role

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in preventing fouling.10-15 Several researchers have provided evidence of the water-induced

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repulsion between proteins and hydrogel films of varying composition.16-18 These studies have

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indicated that water molecules at the film interface prevent fouling. Magin et al.19 performed

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attachment studies with three fouling organisms on functionalized, cross-linked PEGDMA

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hydrogels, which showed improvements in antifouling and fouling release with the more

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hydrophilic surfaces.

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While several studies have considered antifouling properties at the hydrogel-water interface20

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(i.e., surface fouling), few reports have investigated the possibility of small molecule absorption

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within the polymer network and the resulting influence on network properties.21,22 Since water

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swelling of the substrate can be linked to fouling; it is important to understand how water

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swelling is influenced by molecular absorption to optimize the design of antifouling coatings.23,24

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Numerous techniques can be used to probe water swelling in polymer films including

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gravimetric swelling measurements, fluorescence spectroscopy, and ATR-FTIR spectroscopy.25-

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swelling processes. While ATR-FTIR is considered a surface technique, it is controllable, finite

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depth of penetration (typically on the order of 0.65 to 1.73 µm for a wavelength 1000 cm-1)28 can

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be used to analyze the film environment below the polymer-solution interface of the sample.29-33

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For example, Hallinan, et al.34 demonstrated measurement of multicomponent sorption and

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diffusion of small molecules in NAFION® on a molecular scale in real time using ATR-FTIR.

Of these techniques, ATR-FTIR can be used to probe intermolecular interactions during

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For this study, a series of crosslinked PDMSDA-PEGDA hydrogel films (Scheme 2a) were

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cured under UV irradiation. The swelling behavior of the crosslinked PDMSDA-PEGDA

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hydrogel films, when exposed to different solutions, were investigated to evaluate swelling of

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hydrogels. We found that the swelling of hydrogel exhibit is increasing swelling ratio during

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swelling in AMPS-H solution. Induced swelling in hydrogels is closely associated with the

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sulfonic acid and amide group in the AMPS-H solution.

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Scheme 2. (a) Chemical structures of the PEGDA and PDMSDA polymers along with the

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corresponding schematics illustrating the idealized schematics of PEGDA (blue) and PDMSDA

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(red) polymer chains in the gel (b) reactor chamber with the hydrogel coating ZnSe crystal

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substrate and (c) ATR flow cell.

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EXPERIMENTAL SECTION

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Materials.

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(PDMSDA) (Mn = 600~1000 g/mol, Gelest Inc., Morrisville, PA), diacrylate terminated

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poly(ethylene glycol) (PEGDA) (Mn = 700 g/mol, Sigma-Aldrich, St. Lois, MO) and a radical

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photoinitiator (Irgacure 2959®, Sigma-Aldrich, St. Lois, MO) were used as received. Sodium

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chloride (NaCl), sulfuric acid (H2SO4), 2-acrylamido-2-methylpropane sulfonic acid (AMPS-H),

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2-acrylamido-2-methylpropane sulfonic acid sodium salt (AMPS-Na), 1, 3-propanesulfonic acid

(3-Acryloxy-2-hydroxypropoxy

propyl)

terminated

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(PSA) and solvents were also obtained from Sigma-Aldrich (St. Louis, MO) and used as

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received.

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Synthesis of PDMS/PEGDA Hydrogel. PEGDA-based hydrogels were prepared using a

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procedure developed by Hou et al. and modified with PDMSDA to acquire a hydrogel thin film

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with enhanced mechanical properties.35 Hydrogel films were prepared by first coating a viscous

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polymer mixture on a ZnSe crystal (New Era Enterprises, Inc., NJ, and 50 mm x 20 mm x 10

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mm) that was rinsed with methanol before use. The coating was performed using a flow coater36

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where 3 µL of prepolymer mixture were pipetted onto the crystal, and a glass blade was used to

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spread the prepolymer mixture at 7 mm/s. The mixture was cured under UV irradiation (325 nm,

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9 mW/cm2) for 6 min under argon at 25 °C (room temperature) (Scheme 2b) to form films

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approximately 2 µm thick as measured by a Tencor P-1 long scan profilometer (KLA-Tencor,

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CA). The hydrogel films are denoted as “PDMSDAXX-PEGDAYY” where ‘XX’ indicates the

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mass fraction of PDMSDA and ‘YY’ indicates the mass fraction of PEGDA (Table 1).

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Table 1. Compositions of prepolymer mixtures used to prepare hydrogel films.

Sample

Initiator (wt %)

PDMSDA (wt %)

PEGDA (wt %)

PDMSDA100-PEGDA0

1

100

0

PDMSDA80-PEGDA20

1

80

20

PDMSDA70-PEGDA30

1

70

30

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ATR-FTIR Spectroscopy. An ATR-FTIR flow cell was used to conduct the swelling and

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intrusion experiments as illustrated in Scheme 2c. The flow cell consisted of a hydrogel-coated

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ZnSe crystal, peristaltic pump, solution reservoir, and HorizonTM multiple reflection ATR

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accessory (Harrick, NY). The experimental setup formed a rectangular channel with dimensions

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measuring 5 cm long, 2 cm wide and 0.2 cm high while the solution flow rate was maintained at

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5 mL/min (the Reynolds number was approximately 250). The ATR-FTIR spectra were collected

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using a Bruker Vertex 70 FTIR spectrometer (Bruker, Billerica, MA) equipped with a liquid

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nitrogen cooled mercury cadmium telluride detector and a CO2-free dry air purge. The incidence

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angle was fixed at 45°, yielding 12 internal reflections across the element. Spectra at 25 °C were

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collected once a min after introducing solutions into the flow cell. The penetration depth of the

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evanescent wave ranged from 0.46 µm at 3600 cm-1 to 1.66 µm at 1000 cm-1. All spectra were

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processed using Bruker OPUS 6.5 software.

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Swelling Studies. The hydrogel coated ZnSe crystal was equilibrated in deionized water (DI

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water) under flow conditions (5 mL/min) for 60 min. ATR-FTIR spectra were acquired at 1 min

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intervals using a rapid scan mode during hydration. Then, swelling in the presence of various

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solutions was measured by exposing fresh films to solutions containing the small molecule of

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interest. The water-equilibrated hydrogel coated crystals were exposed to the solution at a flow

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rate of 5 mL/min. After small molecule exposure, reswelling of the samples in pure water was

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performed by switching the flowing solution back to DI water to investigate the hydrogel water

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absorption properties after exposure to the small molecule solutions.

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RESULTS AND DISCUSSION

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Structure of PDMSDA-PEGDA Hydrogels. FTIR spectra (Figure 1) of the hydrogel coated

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crystal confirmed that the PDMSDA-PEGDA hydrogels were successfully formed on the crystal.

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The film spectra have an O–H peak at around 3580–3400 cm−1 due to OH group of PEGDA. The

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band at 1091-1021 cm-1 is consistent with a vs(Si-O-Si), which would be expected from the

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siloxane backbone of PDMSDA. Also, the δ(Si-CH3) bending peak was observed at 1261 cm-1.

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The v(C=O) band at 1730 cm-1 was observed and assigned to the CO of the acrylate group. Table

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2 lists the assignment of the individual bands where v denotes a vibrational stretch, vs

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symmetrical stretch, vas the asymmetric stretch, and δ represents a bending mode.

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Table 2. FTIR spectroscopy adsorption bands of PDMS/PEG hydrogel coatings. Frequency (cm-1)

Peak assignment39-41

3400-3580

bulk v(OH)

3350-3250

bound v(OH)

2960-2920

vs(CH)

1690-1760

vs(CO)

1412-1444

vas(Si-CH3)

1261

δ(Si- CH3)

1091-1021

vs(Si-O)

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To examine the influence of small molecules absorbed into the hydrogel network on the

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swelling properties of the hydrogel, we chose a sample thickness of 2 µm which is greater than

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ATR penetration depth. For these experiments, a ZnSe crystal with refractive index n = 2.4 was

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employed with the PDMSDA-PEGDA hydrogel film having n ≈ 1.4. The ATR depth of

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penetration for this system is 1.52 µm at 1091 cm-1, determined from Harrick’s equation.42 So,

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for films with thickness less than the depth of penetration, the spectrum of the sample includes

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contributions from a bulk solution that lies above the film. On the other hand, for films thicker

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than 1.52 µm, the only solution within the film and the film itself appears in the ATR spectrum

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resulting in strong absorbance for the peak at 1091 cm-1 (Figure 1a).

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Figure 1. ATR-FTIR spectra of PDMSDA70-PEGDA30 (0.5 µm and 2 µm) dry film (a) 1091-

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1021 cm-1 (Si-O-Si stretching), (b) 1261 cm-1 (CH3 symmetric deformation of Si-CH3), (c) 1412

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cm-1, 1444 cm-1 (CH3 asymmetric stretch of Si-CH3), (d) 1690-1760 cm-1 C=O stretching, (e)

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2963 cm-1 vs(CH) of CH3.

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Effect of PEGDA Content on Swelling Behavior. The swelling behavior of the PDMSDA-

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PEGDA hydrogels of varying PEGDA content was studied as a function of time using ATR-

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FTIR. The v(OH) bands and fingerprint region of PDMSDA-PEGDA hydrogels during DI water

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swelling provide information on changes within the film at 1, 5, 10, and 60 min, respectively

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Analysis of changes in the OH region of an FTIR spectrum is an established method of studying

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the structure and interaction of water with polymer networks or surfaces.41 The v(OH) peak is

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centered at ~3400 cm-1 in liquid water and is highly sensitive to changes in hydrogen

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bonding.44,45 The v(OH) peak has been shown to comprise of three main components that depend

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on the hydrogen bonding of the water molecules. First, a peak at ∼3640 cm-1 corresponds to

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“free” OH when no hydrogen bonding is experienced.46 Second, when an average of two to three

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hydrogen bonds is experienced as in “liquid” water the peak is observed at ∼3400 cm-1. Third,

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when each water molecule forms four hydrogen bonds with its nearest neighbors in a tetrahedral

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arrangement as in ice, the OH stretching vibration is located at ∼3220 cm-1.47 In general when

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the average number or strength of hydrogen bonds experienced by each water molecule increases

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the v(OH) frequency decreases (red-shifts) from the “liquid-like” (bulk) water peak position

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towards the “ice like” (bound) water peak position.

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Upon exposure of the film to AMPS-H solution we observed this progressive red-shift from a

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peak maximum at 3374 cm-1 at the beginning of the swelling experiment to 3250 cm-1 at the end

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of the experiment which is indicative of an increase in hydrogen bonding and hence interaction

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of water molecules with the polymer network as demonstrated for the PDMSDA70-PEGDA30 in

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Figure 2. This shift cannot be explained as simply being the interaction of water with AMPS-H

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molecules in solution as the peak position of v(OH) in a 1 M solution of AMPS-H is 3347 cm-1 ,

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which is close to that of liquid like water.

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This shift was not observed upon exposure to NaCl solution which indicates that the

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introduction of sulfonic acid groups alters the hydrogen bonding environment in the system and

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facilitates stronger water interaction with the polymer network.

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Figure 2. Spectra of the v(OH) region of the PDMSDA70-PEGDA30 cross-linked hydrogel

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swelled in water after exposure to 1 M AMPS-H and 1 M AMPS-H on the ZnSe.

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Figure 3. (a) ATR-FTIR spectra of OH stretching in PDMSDAXX-PEGDAYY hydrogel films

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after exposure to water flow for 1, 5, 10, 60 min, respectively and (b) the hydrogel film FTIR

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spectra after exposure to DI water (referenced to dry film spectra).

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Also, for all of the samples studied, an increase in PEGDA content of cross-linked hydrogel

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from 0 to 30 wt % increased the water swelling (Figure 3 and Figure S3). For a 60 min period of

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equilibration of the sample with liquid water, the maximum absorbance at 3350 cm-1 increased

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from 1.04 for the PDMSDA100-PEGDA0 sample to 1.22 for the PDMSDA70-PEGDA30

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material (Figure 3a). This increase in uptake is due to the PEGDA content of hydrogels

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increasing the hydrophilicity of the gels. The acrylate crosslink junctions render the 100 wt %

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PDMSDA gel moderately hydrophilic. Increasing the PEO content of the sample lead to

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increased water binding inside the hydrogel network causing the v(OH) band to shift to lower

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frequencies.

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The cross-linked PDMSDA-PEGDA hydrogel has a number of main absorption bands in the

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lower-frequency fingerprint region between 900 and 1500 cm-1.48 Figure 3b shows ATR-FTIR

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spectra taken from several hydrogel films during water swelling. There was a noticeable decrease

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in absorbance in the fingerprint region between 1000 and 1300 cm-1 mostly due to vs(Si-O) at

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1091 cm-1, partly in combination with the δ(Si-CH3) at 1261 cm-1. These changes are due to the

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expansion of the hydrogel film resulting in less polymer in the ATR-FTIR penetration depth as

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the polymer swells. The negative absorbance of the fingerprint bands of the films demonstrates

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that the water swelling is related to the change in film thickness and a decrease in polymer

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concentration at the crystal interface.

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Effect of Ion and Small Molecule Diffusion into the Hydrogel. A typical set of FTIR spectra

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obtained during a swelling experiment for the PDMSDA70-PEGDA30 cross-linked hydrogel

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film is shown in Figure 4. These spectra were recorded during hydrogel exposure to 1 M aqueous

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NaCl solution (Figure 4a) and 1 M AMPS-H (Figure 4b) as a function of time.

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Figure 4. The spectra of the PDMSDA70-PEGDA30 cross-linked hydrogel referenced to the

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spectra of hydrogel film swelled in water as a function of time after exposure to (a) 1 M NaCl

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and (b) 1 M AMPS-H.

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As shown in these figures, the absorbance of v(OH) band at 3350 cm-1 decreased from -0.33 to

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-0.80 as the exposure time increased from 0 to 60 min (Figure 4a), indicating that exposure to 1

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M aqueous NaCl solution effectively decreased the water content of the hydrogels, which is

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consistent with the process of osmotic deswelling. Deswelling occurs because the activity of

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water is reduced by the presence of solutes in solution.

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The introduction of 1 M AMPS-H aqueous electrolyte in contact with a water-hydrated gel

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resulted in an initial deswelling event for the first 4 min of contact, similar to that observed for 1

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M NaCl solution, but surprisingly, reswelling occurred after the initial deswelling event. After 60

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min of equilibration in 1 M AMPS-H, the v(OH) reached -0.28 after 60 min (Figure 4b). This

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result is different from that typically observed when the hydrogels are exposed to ionic solutions

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or small molecules (i.e., osmotic deswelling). After exposure to AMPS-H solution, the bands at

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1550 and 1210 cm-1, which can be attributed to the amide and sulfonic acid group of AMPS-H,

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increased indicating that AMPS-H penetrated the hydrogel network to cause the reswelling.

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To understand the origin of this unusual reswelling behavior, the intensity of the v(OH)

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absorbance during the water and water-electrolyte cycles was measured, after exposure to

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various solutions as a function of time (Figure 5 and Figure S1). The plots report v(OH)

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absorbance at 3350 cm-1 as a function of time for experiments where the hydrogel was exposed

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to 1 M AMPS-H, 1 M H2SO4, 0.1 M AMPS-Na, 1 M NaCl, 0.1 M PSA and 0.1M AMPS-H.

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First, the O-H absorbance decreased with the introduction of small molecules (i.e., osmotic

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deswelling) in the first 4 min after the solution was changed from DI water to electrolyte

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solution. Then, the absorbance data shows a slow reswelling process occurring in the hydrogels

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exposed to 0.1 and 1 M AMPS-H, although this reswelling process does not appear to occur in

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films exposed to other molecules. This result suggests that sulfonic acid groups facilitate the

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reswelling process. The hydrogel exposed to 0.1 M PSA also reswelled slightly. However, the

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swelling magnitude is smaller, and the kinetics were slower in the hydrogels exposed to 0.1 M

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PSA compared to those exposed to 0.1 M AMPS-H.

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It appears that a sulfonic acid functionality is necessary for reswelling while Na+ ions do not

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induce reswelling. After exposure to AMPS-Na solution, the water absorbance sharply decreased

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and remained constant -0.25 (Figure 5b). After changing the electrolyte to DI water, the water

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absorbance increased to 0.9 (Figure 5c) and displayed swelling similar to the initial swelling

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event observed in Figure 4a for the NaCl solution. The osmotic deswelling of AMPS-Na (Figure

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5b) is caused by a reduced water activity of the electrolyte solution compared to pure water. The

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activity gradient that develops when a water-swollen gel is equilibrated with electrolyte causes

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water to flow out of the gel, and a deswelling response was observed. After changing the

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electrolyte to DI water, the water absorbance indicated reswelling due to the removal of Na+, and

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restoring the activity of water in the film to the initial state.

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Figure 5. Variation of OH absorbance of the hydrogel when exposed to changing solutions. In

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regions (a) and (c), DI was introduced; in the region (b) 1 M AMPS-H, 1 M PSA and 1M

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AMPS-Na were introduced.

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Upon exposure to PSA, the water absorbance initially showed a decrease to -0.85 and then a

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slight increase to -0.75 (Figure 5b). After changing the electrolyte to DI water, the water

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absorbance increased to 1.4 (Figure 5c) and displayed a pure water reswelling greater than the

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initial swelling event observed in Figure 5a. The sulfonic group in PSA interacted with the

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hydrogel and increased the hydrophilicity of the network resulting in more water uptake during

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reswelling (Figure 5c).

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Following exposure to AMPS-H, the water absorbance initially decreased sharply to -0.75 and

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then steadily increased to -0.25 (Figure 5b). After changing the solution to DI water, the water

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absorbance increased to 1.4 (Figure 5c) and displayed reswelling greater than initial swelling

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event observed in Figure 5a. This data indicated that in AMPS-H and PSA the sulfonic acid

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group promoted swelling of the hydrogel films (Figure 4b) by increasing their hydrophilicity.

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To determine the structures and the presence of AMPS-H and AMPS-Na in the films after

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swelling in DI water, the hydrogel films were rinsed with DI water and dried under vacuum at

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room temperature for 1 h. Figure 6 shows data from films after being exposed to AMPS-H

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displaying the N-H vibration (1550 cm-1) which indicates the existence of AMPS-H residue in

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the hydrogel films (Figure 6a) while AMPS-Na was not present in the hydrogel network (Figure

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6b).

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The comparison of films exposed to AMPS-H and AMPS-Na demonstrated that AMPS-H did

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not change the chemical properties of the hydrogel network, such as chain scission, since the

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only change in the spectrum was the appearance of the amide peaks once the water was removed

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(Figure 6a and 6b). The observed amide signal is from AMPS-H, likely due to hydrogen bonding

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between the acrylate carbonyl of the network and the amide group of AMPS-H.49 The spectra

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confirm the presence of AMPS-H in the hydrogel network after rinsing with water. This

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retention of AMPS-H in the film suggests that anomalous swelling is due to water interacting

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with this trapped AMPS-H, which carries significant hydration due to the presence of the acidic

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group.

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Figure 6. ATR-FTIR spectra of the pure PDMSDA70-PEGDA30 hydrogel (dash) and vacuum

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dried (solid) PDMSDA70-PEGDA30 after water-electrolyte cycle with (a) 1 M AMPS-H and (b)

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1 M AMPS-Na, respectively.

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CONCLUSIONS

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ATR-FTIR spectroscopy was used to examine the changes in water absorption of PDMSDA-

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PEGDA hydrogels after water and small molecules in solution were introduced to the gel. It was

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found that gels exposed to AMPS-H and PSA solutions exhibited an increase in the water

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swelling as a function of time after an initial osmotic deswelling response and a high reswelling

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in liquid water. On the other hand, films exposed to AMPS-Na solution showed a typical

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osmotic deswelling response. This experimental data provides evidence that the introduction of

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sulfonic acid groups into the gel by absorption of AMPS-H and PSA results in an increase in

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water uptake while Na+-form salts do not show such a response. It was found that the amide

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group of AMPS-H was detected in the films that showed large reswelling responses. The finding

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suggests the existence of AMPS-H a plays a fundamental role in inducing water swelling in

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hydrogel films. The small molecule induced swelling observed in this study will provide useful

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information for future designs of hydrogel networks and prompt new studies of small molecule

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intrusion into hydrophilic antifouling coatings.

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ASSOCIATED CONTENT

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Supporting Information

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The swelling behavior of the PDMSDA70-PEGDA30 hydrogel film in different solutions,

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represented by the v OH (3350 cm-1) absorbance intensity (Figure S1). The swelling behavior of

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the PDMSDA100-PEGDA0 hydrogel film in AMPS-H solutions, represented by the v OH (3350

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cm-1) absorbance intensity (Figure S2). The systematic increase v(OH) absorbance with PEGDA

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content during water swelling (Figure S3).

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This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Author

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[email protected]

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ACKNOWLEDGEMENTS

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The authors acknowledge the support of the US Office of Naval Research through PECASE

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grant N00014-10-1-0875. C.N. thanks the Materials Science and Engineering Department at The

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Pennsylvania for financial support through the George W. Brindley/Jyung-oock Choe Graduate

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Fellowship.

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regarding flow coater and water binding. Finally, the authors thank Professor Thomas H. Epps,

The authors thank Douglas Kushner and Sarah Black for helpful discussion

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III, and the Epps Research Group in the Department of Chemical & Biomolecular Engineering at

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the University of Delaware for helping us construct and optimize our flow coater.

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