Antifreeze Hydrogels from Amphiphilic Statistical Copolymers

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Antifreeze Hydrogels from Amphiphilic Statistical Copolymers Chao Wang, Clinton G. Wiener, Pablo I. Sepulveda-Medina, Changhuai Ye, David S Simmons, Ruipeng Li, Masafumi Fukuto, R. A. Weiss, and Bryan D. Vogt Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03650 • Publication Date (Web): 13 Dec 2018 Downloaded from http://pubs.acs.org on December 17, 2018

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Chemistry of Materials

Antifreeze Hydrogels from Amphiphilic Statistical Copolymers Chao Wang, Clinton G. Wiener, Pablo I. Sepulveda-Medina, Changhuai Ye, David S. 1

1,†

1

1

Simmons, Ruipeng Li, Masafumi Fukuto, R. A. Weiss *, Bryan D. Vogt * 2

1

2

3

3

1,

1,

Department of Polymer Engineering, University of Akron, Akron, OH 44325

Department of Chemical and Biomedical Engineering, University of South Florida, Tampa, Florida, 33612

3

National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, New York 11973

ABSTRACT: Prevention of ice formation is a critical issue for many applications, but routes to overcome the large thermodynamic driving force for crystallization of water at significant supercooling are limited. Here, we demonstrate that supramolecular hydrogels formed from statistical copolymers of 2-hydroxyethyl acrylate (HEA) and 2-(N-ethylperfluorooctane sulfonamido)ethyl methacrylate (FOSM) exhibit a degree of ice formation suppression unprecedented in a synthetic material. The mechanisms of ice prevention by these hydrogels mimic two methods used by nature: 1) hydrogen bonding of water to highly hydrophilic macromolecular chains and 2) nano-confinement of water between hydrophobic moieties. From systematic variation in the copolymer composition to control the nanoscale (97%, but Cs drops sp

precipitously to 95% nf-water is consistent with the minimum water cluster size to begin to spectroscopically observe the signatures for the onset of water crystallization.

39

CONCLUSIONS This work demonstrates a novel approach to prevent nearly all of the water from freezing in a hydrogel to temperatures as low as 128 K with the efficacy coupled to the nanostructure (composition) of the hydrogel. The amount of non-frozen-water reported for these hydrogels can be much greater than has been previously reported for water in solid materials. The design of the hydrogel mimics two methods by which nature prevents water from freezing: 1) highly hydrophilic macromolecule segments that also include hydrogen bonding and 2) confinement of the water by hydrophobic nanodomains. When the number of water molecules within the average volume constrained between the hydrophobic nanodomains is less than 300, greater than 95 % of the water within the hydrogels does not freeze. This volume is consistent with the critical size to observe the onset of water crystallization in water clusters. One caveat with these materials is that the water 39

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content of the materials is limited to less than 50 wt% to be effective in suppressing crystallization, which will limit these materials to applications where high concentrations of polymer are acceptable. Nonetheless, this design of nanostructured hydrophobically-modified hydrogels may be useful for developing new antifreeze materials based on alternative chemistries that can efficiently prevent ice formation in technologies where freezing water has detrimental effects.

METHODS Materials. 2-(N-ethylperfluorooctane-sulfonamido)ethyl methacrylate (FOSM, ≥85.0%, BOC Sciences) was recrystallized from methanol. Hydroxyethyl acrylate (HEA, ≥ 96.0 %, Scientific Polymer Products, Inc.) was passed through a removal column for hydroquinones to remove the inhibitor(s).

2,2′-Azobis(2-methylpropionitrile)

(AIBN,

≥

98.0%,

Sigma-Aldrich)

was

recrystallized from methanol. 1,4-dioxane (≥99.0%), N,N’-Methylenebis(acrylamide) (MBAA) and Irgacure 2959 (2-hydroxy-40-(2-hyroxyethoxy)-2-methylpropiophenone) were obtained from Sigma-Aldrich and used as received. Deuterated chloroform (CDCl , 99.8% D) and deuterated 3

acetone (acetone-d6, 99.9% D) was obtained from Cambridge Isotope Laboratories, Inc., and used as received. HEA/FOSM copolymers were synthesized using free-radical solution polymerization at 333 K as described in the Supporting Information. The composition of the copolymers was determined by H NMR (Varian Mercury-300 NMR). The formulations and properties of the HFx hydrogels 1

(where x denotes the FOSM concentration (mol%) of the dry copolymer rounded to the nearest integer value) are summarized in Table S2. Chemically crosslinked HEA (XLHEA) hydrogels were synthesized as control hydrogels using photo-initiated free-radical polymerization of HEA and MBAA (crosslinker) in Milli-Q water at 295 K. The reaction solutions were purged with dry


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nitrogen for 20 min and then irradiated with a 365 nm wavelength UV lamp (USND-3601, 36 W) for 5 min. The formulations for the reactions and the properties of the XLHEA hydrogels are given in Table S1. The sample notation is XLHEA-y where y is the crosslink density, n , rounded to the ee

nearest integer value. The crosslink density of each hydrogel was calculated using the theory of rubber elasticity as described in the SI. Sample preparation: Dry HFx copolymers were compression molded into films with a 35 ton vacuum molding machine (Technical Machine Products) at 423 K and a vacuum of ~100 kPa. The hydrogels were prepared by swelling the copolymer films with Type 1 ultrapure water (Milli-Q, Millipore) for at least 14 days at room temperature. The mass of the hydrogel was monitored every 24 h using a Mettler Toledo XS104 Excellence XS Analytical Balance to ensure that the hydrogel reached equilibrium. When the mass changed by < 3 % over 48 h, we defined the hydrogel as being at equilibrium. The equilibrium swelling ratios (S ≡ mass hydrogel/mass polymer) at room temperature are listed in Table S2. Characterization: The thermal behavior of the HFx hydrogels upon cooling and heating was measured by differential scanning calorimetry (DSC, Perkin Elmer DSC 8500) using 2.00-6.00 mg surface-blotted film samples crimped into aluminum pans at 2K/min. Figure S3 illustrates a representative isotherm. The amount of ice that formed or melted was calculated by dividing the total energy of crystallization or melting from the cooling and thermograms by the specific enthalpy of fusion of hexagonal ice (I ), 334 J/g. The percentage (wt%) of freezable water, C , was h

f

calculated as the mass of ice formed divided by the total mass of water in the hydrogel. The percentage (wt%) of supercooled (non-freezing) water, C was C = 1 – C . s

s

f

Broad-band dielectric spectroscopy measurement: The HF21 copolymer was dissolved in chloroform to make a 10 wt% solution. The HF21 film was prepared by solution casting on one


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side of two cylindrical brass plates and dried in the oven at 383 K under vacuum for over 12 h. Then the two films on the brass plates were compressed face to face into one film using clamps at 433 K. The brass plates with the HF21 film were swollen until equilibrium in deionized water, then put in between the electrodes of the cryostat (129610A He/LN Cryostat System, Solartron 2

Analytical) for dielectric spectroscopy measurement. The dielectric measurement was conducted under isothermal conditions using Solartron Analytical 1260 Impedance/Gain-Phase Analyzer and 1296 Dielectric Interfaces (frequency range from 0.1 Hz to 10 Hz). The measured temperature 6

range was from 128 K to 298 K at 5 K intervals. The dielectric spectroscopy data was fit and analyzed using Grafity software (Roland, Naval Research Laboratory). The dielectric spectroscopy data at temperatures from 128 K to 193 K were fit by the sum of the three relaxation processes as described by Havriliak-Negami equation and dc conductivity contribution as shown in Equation (1)

72

e* = e¥ + å k

De k s + ak bk iwe 0 (1 + (iwt k ) )

(1)

where ε* is the complex dielectric constant, ε∞ is the limiting high-frequency dielectric constant, Δε is the relaxation strength, ω = 2πf is the angular frequency, τ is the relaxation time, a (0 < a ≤1) and b (0 < b ≤1) are the shape parameters which describe the symmetric and the asymmetric broadening of the curve, σ is the dc conductivity. The dielectric spectroscopy data at the temperature from 198 K to 298 K was fit by using derivative formalism as shown in Equation (2) to eliminate the interference of the strong electrode polarization and dc conductivity to the water relaxation processes:

e der (w ) = -

2 ¶e '(w ) p ¶ log w

(2)


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Scattering: X-ray scattering (small-angle, SAXS, and wide-angle, WAXS) measurements were carried out simultaneously using beamline 11-BM CS with 13.5 keV X-rays (l = 0.0918 nm) at the National Synchrotron Light Source II (NSLS II) at Brookhaven National Laboratory in Upton, NY. The sample-to-detector distance was 2.02 m for SAXS and 0.231 m for WAXS. Each sample was measured for 10 s with 10 s intervals between measurements. A Dectris Pilatus 300K CCD detector (pixel size = 172 µm × 172 µm) and a Photonic Science ImageStar 135 mm CCD detector (pixel size = 101.7 µm × 101.7 µm) were used for SAXS and WAXS, respectively. Details on the sample preparation for x-ray measurements are included in the SI. Small angle neutron scattering (SANS) measurements were performed using the NGB 30 m SANS beamline at the NIST Center for Neutron Research (NCNR) in Gaithersburg, MD using a titanium liquid cell, a neutron wavelength of l = 0.6 nm with a wavelength spread Δl/l of 14% 73

and a beam diameter of 1.91 cm. Excess solvent was added to cover the hydrogel sample to ensure that the sample remained fully hydrated during the SANS measurements. Three sample to detector distances: 1.33 m (with 7 neutron guides), 4.00 m (with 5 neutron guides), and 13.2 m (with 1 neutron guide) were used, which covered scattering vector (q) ranges of 0.300 – 4.71 nm , 0.0854 -1

– 0.830 nm and 0.0343 – 0.232 nm , respectively. Hydrogels for SANS experiments were prepared -1

-1

by swelling the copolymers with either D O or a (23.5/76.5 v/v) mixture of D O/H O. Samples 2

2

2

(~2.0 cm diameter circular disks) were cut from hydrogel films. The SANS data are shown in Figure S7. Details of the data analysis are included in the SI.

ASSOCIATED CONTENT


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Supporting Information. Experimental section for synthesis of copolymer, determination of copolymer composition, calculation of crosslink density, SAXS/WAXS characterization and SANS analysis; H NMR spectra of HEA/FOSM copolymers; Compositions, swelling ratios, 1

plateau moduli and crosslink densities of HEA/FOSM and XLHEA hydrogels; DSC thermograms for HEA/FOSM hydrogels; Temperature-dependent dielectric response function for the HF21 hydrogel; Temperature-dependent X-ray diffraction patterns for HEA/FOSM and XLHEA hydrogels; SANS patterns of HEA/FOSM hydrogels and fitted curves for the patterns; Interdomain spacings of HEA/FOSM hydrogels after a F/T cycle; SAXS and WAXS measurements of supercooled HF18 hydrogel during uniaxial deformation; Crosslink densities and optical photographs of HEA/FOSM and XLHEA hydrogels before and after a F/T cycle.

AUTHOR INFORMATION Corresponding Author *[email protected] (B.D.V.) or [email protected] (R.A.W.) Present Addresses 3M Center St., St. Paul, MN, 55144

†

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT


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This work was supported by a grant (CBET-1606685) from the Chemical, Bioengineering, Environmental, and Transport Systems Division of the Directorate for Engineering of the National Science Foundation. The research used the Complex Materials Scattering (CMS/11-BM) beamline, operated by the National Synchrotron Light Source II and the Center for Functional Nanomaterials, which are U.S. Department of Energy (DOE) Office of Science User Facilities operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. Access to the NGB 30m SANS was provided by the Center for High Resolution Neutron Scattering, a partnership between the National Institute of Standards and Technology and the National Science Foundation under Agreement No. DMR-1508249. We acknowledge the support of the National Institute of Standards and Technology, U.S. Department of Commerce, in providing the neutron research facilities used in this work. We thank Z. Cheng, Y. Xia, Z. Zhao, F. Peng and G. Deng for their help with measurements.

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Antifreeze Hydrogels from Amphiphilic Statistical Copolymers

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