Increased Hydrogel Swelling Induced by Absorption of Small Molecules

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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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ACS Applied Materials & Interfaces

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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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27

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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


polydimethylsiloxane

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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).

191 192

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

239

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

247

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

248

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

249

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

250

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

251

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

252

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

254

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

259

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

261

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

262

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

264

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

267

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

268

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

269

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

270

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

272

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

273

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

274

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

275

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

276

6b).

277

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

278

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

279

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

280

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

281

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

282

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

283

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

284

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

285

group.


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Page 18 of 22

286 287

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

288

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

289

1 M AMPS-Na, respectively.

290 291

CONCLUSIONS

292

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

293

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

294

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

295

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

296

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

297

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

298

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

299

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

300

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

301

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

302

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


18

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303

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

304

intrusion into hydrophilic antifouling coatings.

305 306

ASSOCIATED CONTENT

307

Supporting Information

308

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

309

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

310

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

311

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

312

content during water swelling (Figure S3).

313

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

314

AUTHOR INFORMATION

315

Corresponding Author

316

[email protected]

317

ACKNOWLEDGEMENTS

318

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

319

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

320

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

321

Fellowship.

322

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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Page 20 of 22

323

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.

325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364

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Increased Hydrogel Swelling Induced by Absorption of Small Molecules

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