Article pubs.acs.org/JPCB
Molecular-Level Changes of Aqueous Poly(N‑isopropylacrylamide) in Phase Transition Iina Juurinen,*,† Szabolcs Galambosi,† Adina G. Anghelescu-Hakala,‡,§ Jaakko Koskelo,† Veijo Honkimak̈ i,∥ Keijo Ham ̈ al̈ aï nen,† Simo Huotari,† and Mikko Hakala*,† †
Department of Physics, University of Helsinki, P.O.B. 64, FI-00014, Helsinki, Finland VTT Technical Research Centre of Finland, Patruunantie 19, FI-05200, Rajamäki, Finland § Department of Chemistry, Laboratory of Polymer Chemistry, University of Helsinki, P.O.B. 55, FI-00014, Helsinki, Finland ∥ European Synchrotron Radiation Facility, F-38043, Grenoble Cedex 9, France ‡
ABSTRACT: We report a Compton scattering study on the molecular-level structural changes of aqueous poly(N-isopropylacrylamide) (PNIPAM) across the conformational phase transition. PNIPAM is a thermoresponsive polymer that changes its conformation in water from the hydrophilic coil state to the collapsed hydrophobic globule state at 32 °C. Combined with density functional theory calculations, the Compton scattering experiments detect two type of changes in the phase transition. The amount of hydrogen bonds is found to reduce, and an elongation of the internal covalent bond lengths is observed. The elongation of the bonds indicates that not only the hydrogen bonding changes but there are other processes, most likely related to hydrophobic interaction, that should be taken into account in the phase transition.
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INTRODUCTION Smart polymers are materials that respond to slight changes in their environment, such as temperature, pH, or magnetic field. Poly(N-isopropylacrylamide) (PNIPAM) is a thermoresponsive polymer, with a lower critical solution temperature at 32 °C in aqueous solution. At this temperature, it undergoes a change from a hydrophilic coil state to a collapsed hydrophobic globule state. The applications for such a thermoresponsive polymer are abundant, especially as the phase transition can be tailored to react to different external stimuli, or the transition temperature can be adjusted by proper modification of the polymer.1,2 For example, responsive or self-arranging polymers can be used in drug delivery3,4 or optical devices.5,6 Thus, the properties of PNIPAM have been widely studied.1,3,7−25 PNIPAM has both hydrophilic and hydrophobic groups. The amide part of the PNIPAM side chain has the ability to be hydrogen bonded to the solvating water in two different sites: the carbonyl group (CO) and the amine group (NH). In the fully hydrated state (T < 32 °C), the carbonyl group accepts two hydrogen bonds and the amine group donates one bond (see Figure 1).26,27 This enables PNIPAM to form intrachain hydrogen bonds between different parts of the polymer. The hydrophobicity in PNIPAM arises from the carbon backbone as well as the isopropyl group in the side chain. The phase transition of aqueous PNIPAM is proposed to be related to the loss of the hydrogen bonds between water and PNIPAM and on the other hand a creation of intrachain hydrogen bonds, with the hydrophobic parts of the polymer playing a role. However, it has been suggested that not all of the hydrogen bonds between water and PNIPAM are broken in the phase transition.8,19,21 For example, in a study using resonant © 2014 American Chemical Society
Figure 1. Schematic picture of PNIPAM polymer with water hydrogen bonded to the side chain. The rest of the water molecules surrounding the polymer are not shown. The gray atoms are carbon, red atoms oxygen, white hydrogen, and blue nitrogen.
Raman spectroscopy, it was observed that only one hydrogen bond is lost of the two with the carbonyl and none is lost with the amine group.15 It is well-known that the temperature dependence of the globule size exhibits hysteretic behavior,24 which has been attributed to intrachain bonding.19,24 Indeed, for polymers with a reduced ability to form intrachain bonds, the hysteretic effect is reduced.12,16 Consistently, for polymers with an increased ability to form intrachain bonds, the hysteretic effect is in turn increased.11 Received: February 24, 2014 Revised: April 29, 2014 Published: April 30, 2014 5518
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was in a 2 mm diameter borosilicate glass capillary with 10 μm thick walls placed in a vacuum chamber with 25 μm thick Mylar windows. The incident X-ray energy was 86.72 keV, and the scattering angle was 163.3°. The scattered photons were detected with a 9-element germanium solid state detector with a momentum resolution of 0.64 atomic units (au). The temperature of the sample was varied between 300 and 308 K in cycles. The phase transition was confirmed with structural analysis by X-ray diffraction. The phase transition was also observed by a visual inspection of the sample. The measured spectra were corrected for background and the necessary energy-dependent corrections such as absorption, detection efficiency, and cross section. Finally, the spectra from all the analyzers were summed to obtain the total result. Within the impulse approximation,42 the experimentally measured scattering cross section from an isotropic system is proportional to the Compton profile43
Although getting a grasp on the hydrophobic interaction is more difficult, the phase transition is found to be affected by the hydrophobic parts of the PNIPAM polymer. In the phase transition, the amount of water near the hydrophobic parts reduces.7−9,20 The hydrophobic parts also affect the phase transition temperature and hysteresis. For example, when molecules resembling the side chain of PNIPAM were studied, a lack of the carbon backbone was found to increase the transition temperature.9,17 On the other hand, an extra methyl group in the side chain of the polymer has been found to increase the hysteresis as well as the transition temperature.14 In this work, we study the details of the proposed hydration scheme in the phase transition of aqueous PNIPAM with X-ray Compton scattering. In X-ray Compton scattering, inelastically scattered X-ray photons from the sample are observed. The spectrum of the scattered photons is related to the ground state momentum density of the electrons,28 and it is extremely sensitive to small changes in the intra- and intermolecular bond lengths.29−31 Therefore, Compton scattering has recently been used to characterize molecular systems under different conditions.29−41 In this work, the results of the Compton scattering experiments are supported by density functional theory (DFT) calculations from clusters obtained from molecular dynamics (MD) simulations.
J(q) =
1 2
∞
2π
∫|q| ∫0 ∫0
π
⟨n(p)⟩ sin θ dθ dϕ pdp
(1)
where ⟨n(p)⟩ is the time-averaged electron momentum density of the liquid and q a scalar momentum variable. J(q) is normalized so that its integral corresponds to the number of electrons. As the observable, we use the difference Compton profile
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METHODS PNIPAM was synthesized using reversible addition−fragmentation chain transfer (RAFT) polymerization with 4-cyanopentanoic acid dithiobenzoate (cpa-RAFT) (Aldrich) as the RAFT agent and 2,2′-azobis(isobutyronitrile) (AIBN) (Fluka) as the free radical indicator. N-Isopropylacrylamide (NIPAM, Polyscience Inc.) was recrystallized from benzene, and AIBN was recrystallized from methanol. 11.016 g of NIPAM monomer (1.77 M), 137.337 mg of cpa-RAFT agent (8.95 mM), and 5.87 mg of AIBN (0.65 mM) were dissolved in 55 mL of dioxane in a round-bottom flask equipped with a magnetic stirrer. The mixture was degassed by three freeze−thaw cycles, sealed under a vacuum, and polymerized in a thermostated oil bath at 60 °C for 48 h. The polymer was precipitated in diethyl ether, purified by repeated precipitations, and dried under a vacuum. The molecular weight (Mn) and polydispersity index (PDI) of PNIPAM were determined with a Waters liquid chromatography system equipped with a Waters 2414 Refractive Index Detector. Tetrahydrofuran (THF) was used as an eluent at a flow rate of 0.8 mL/min. Results were calibrated against polystyrene standards. The PNIPAM used in the experiment (see Figure 2) had a molecular weight of Mn = 7860 g/mol and a polydispersity of 1.34. It was dissolved in water to a concentration of 5 wt %. The experiment was performed at the ID15 beamline at European Synchrotron Radiation Facility (ESRF). The sample
ΔJ(q) = (J308K (q) − J300K (q))/J300K (0)
(2)
where J308K(q) and J300K(q) are the Compton profiles of aqueous PNIPAM at 308 and 300 K, respectively. We examined the experimental results by comparing them to computational difference Compton profiles obtained from MD simulation snapshots. The Compton profiles were calculated with DFT using the ERKALE code,44−46 with Becke 8847 and Lee, Yang, and Parr48,49 exchange and correlation functionals. The employed pcemd-3 basis sets are optimized for electron momentum density calculations.50 A set of molecular conformations used in the DFT calculations was obtained with classical MD simulations. In the MD simulations, only Nisopropyl-2-methyl-propanamide (NIPMPA) molecules (essentially the side chain of PNIPAM) were simulated in water, because only a small fraction of the polymer can be calculated with DFT. Thus, as the main point of interest lays in the interaction between water and the side chain of PNIPAM, simulating only the NIPMPA molecules in water is sufficient to reproduce clusters needed in the subsequent calculations. The experimental concentration (of 5 wt %) of PNIPAM polymer with approximately 70 side chains each corresponds to a molar concentration of 0.014% PNIPAM. This corresponds roughly to 1% NIPMPA in water, which was simulated in a periodic cubic box containing 25 NIPMPA molecules and 2500 water molecules. The MD simulations were performed with the Gromacs software.51 The four-point transferable intermolecular potential (TIP4P)52 was used as the force field for water and optimized potentials for liquid simulations (all atoms) (OPLS-AA)53 for NIPMPA. The cut-offs for the potentials in the simulation were 2 nm. The duration of the simulation was 4 ns. Prior to that, there was an equilibration simulation of 6 ns to ensure that the starting conformation does not have an effect on the results. For the simulation, the leapfrog integrator was used with a time step of 1 fs, the velocity-rescale thermostat was used with a reference temperature of 300 K and coupling constant of 0.1 ps,
Figure 2. Structure of synthesized cpa-PNIPAM (molecular weight 7860 g/mol, polydispersity index 1.34). 5519
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and the Parrinello−Rahman method was used with a reference pressure of 1 bar and a coupling constant of 0.5 ps. To define whether the molecules were hydrogen bonded, we used the criteria where the distance between the donor and the acceptor is less than 3.5 Å and the acceptor−donor−hydrogen angle is less than 30°. With the DFT calculations, we studied the effects of hydrogen bonding, as well as those of internal bond lengths. The hydrogen bonds were studied with clusters with 1 NIPMPA and 17 (closest) water molecules, with three different hydrogen-bonding topologies: the CO site with two hydrogen bonds and the NH site with one bond, the C O site with one hydrogen bond and the NH site with one bond, and the CO site with two hydrogen bonds and the NH site without bonds. For the CO and NH sites separately these topologies yield difference Compton profiles ΔJ(q) = (J2HB (q) − J3HB (q))/J3HB (0)
(3)
Figure 3. Difference Compton profile of aqueous PNIPAM, where the hydrophobic globule state (308 K) is compared to the hydrophilic coil state (300 K). The vertical lines denote the statistical error bars. The inset shows individual Compton profiles for aqueous PNIPAM at 300 and 308 K. The Compton profiles are offset for clarity.
where J2HB(q) is the Compton profile of clusters with two hydrogen bonds between water and PNIPAM and J3HB(q) with three hydrogen bonds. Additionally, the intrachain bonding was studied with clusters with 2 NIPMPA and 12 water molecules, where the NIPMPA molecules were either hydrogen bonded or not hydrogen bonded to each other. There were no other structural restrictions in the clusters apart from the abovementioned. All clusters were randomly selected from the MD simulations, thus containing variations in the structure. In all cases, Compton profiles from at least 100 clusters were computed to obtain a statistically averaged result. The variation in the internal bond lengths was studied with the above-mentioned clusters so that a subset of clusters were selected according to having a longer than average bond length at a certain covalent bond. The Compton profiles from the subset were averaged (Jsubset(q)) and compared to the Compton profile of the whole set of clusters (Javerage(q)) with the same hydrogen bonding topology (for example, both CO and NH sites having one hydrogen bond), resulting in a difference Compton profile ΔJ(q) = (Jsubset (q) − Javerage (q))/Javerage (0)
Figure 4. Computational effect of forming and breaking of hydrogen bonds on the Compton profile of aqueous PNIPAM.
As was done for the analysis of hydrogen-bond-induced changes, the results concerning the internal bond length variations were obtained by averaging Compton profiles of at least 100 individual clusters.
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similarly shaped difference Compton profiles. On the other hand, forming bonds yields difference Compton profiles that are roughly the same shape but of opposite sign. The difference Compton profile for forming a hydrogen bond between two NIPMPA molecules (PNIPAM side chains) is also presented in Figure 4. The effect of changes in the hydrogen bonding of water is similar to these cases (see, for example, refs 29, 30, and 40); i.e., breaking or weakening of the hydrogen bond yields a difference Compton profile characterized by ΔJ(0) > 0, whereas forming or strengthening of the bond yields profiles with ΔJ(0) < 0. While we cannot quantify the change in the number of hydrogen bonds, the experimental difference Compton profile is consistent with a loss of hydrogen bonds after a comparison of Figures 3 and 4. However, compared to the present experimental result, the computational difference Compton profiles of Figure 4 for both creating and breaking a hydrogen bond approach zero at lower q values (at q ≈ 2 au) than the experimentally observed profile. Thus, a good agreement with experiment cannot be reached by only
RESULTS AND DISCUSSION The experimental Compton profiles and their difference (eq 2) are shown in Figure 3. As discussed earlier, the Compton profiles are the one-dimensional projection of the electron momentum density. The difference Compton profile deviates from zero, implying variation in the electron momentum densities of aqueous PNIPAM at 300 and 308 K. This indicates that there are differences in the molecular structures. As the phase transition has been previously attributed to, for example, the decrease in the number of water hydrating PNIPAM and formation of intrachain hydrogen bonds, we will next take a closer look computationally at a few of the interactions and their effect on the Compton profiles. First, we consider the effect of hydrogen bond breaking and formation on the difference Compton profile. In Figure 4, the results are shown for different bonding topologies. Breaking a bond at either the carbonyl or amine site of PNIPAM produces 5520
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molecules are hydrogen bonded, the intramolecular bond lengths are elongated, and when the hydrogen bond breaks, the bond lengths shorten.54 In more complex molecules, the changes are less straightforward. For example, in PNIPAM, the OC and NH lengths are known to shorten as the hydrogen bond (at the site in question) is broken, but simultaneously the (O)CN bond length increases.26 In terms of changes in the Compton profile, this may cause cancellation effects. As is previously suggested, the phase transition of PNIPAM consists of a few stages. At first, hydrogen bonds between PNIPAM and water (at the CO site) begin to break. According to our calculations (Figure 4), this is in agreement with the experimental difference Compton profile. Subsequently, intrachain hydrogen bonds start to form. This effect, as modeled in the calculations, brings an effect opposite to the observed difference Compton profile. However, in order to form intrachain hydrogen bonds, at least the hydrogen bond between water and the NH group must be broken, since the NH group can contribute to only one hydrogen bond. Thus, it is possible that the two latter effects cancel out. This leads to an observation of hydrogen bond breaking resulting in a difference Compton profile with ΔJ(0) > 0. However, still some contribution is needed from stretching of the internal bonds. This stretching must overcome the relaxation in the bond lengths caused by the broken hydrogen bonds. As the stretching in all covalent bonds yields rather similar difference Compton profiles, we cannot pinpoint where the elongation takes place. The observed elongation in the (O)CN bond26 is unlikely to explain the experimental result on its own, because it is related to the bond length contractions at the OC or NH bonds. However, the effects of the hydrophobic interaction on the internal structures of the polymer and water are far less known. The amount of water molecules around PNIPAM reduces at the phase transition,7−9,20 which can affect the internal conformation in PNIPAM, as the hydrophobic groups of the polymer no longer interact with water but with other hydrophobic groups. Additionally, the amount of water molecules in the bulk portion is increased. If this transition increases the strength of the hydrogen bonds between water molecules, it would simultaneously elongate their internal bond lengths. The hydrophobic interaction, which is considered to play a role in the phase transition,7−9,13,14,17,20 may create a unique signature to the Compton profile besides those observed via the changes in the hydrogen bonding and the elongation of the bond lengths. However, to gain detailed knowledge on such features alone is not straightforward, and the interpretation would require calculations computationally so heavy that they are beyond the scope of this article. Additionally, there are other processes that may influence the Compton profiles. One of these is the resonance structures in PNIPAM.27 The DFT calculations should reproduce these structures provided that the bond length relaxation related to the change in the electronic structure is accurate. However, as the coordinates are obtained with classical MD simulations, the structures might differ from those required for the correct representation of the resonance structures, thus preventing seeing the effect fully in the calculations. In conclusion, the Compton scattering experiment suggests that in the phase transition of aqueous PNIPAM both the internal bond lengths and the number of hydrogen bonds change. More specifically, the results show an increased amount
considering hydrogen bond effects and other contributions must be searched for. The effect of the covalent bond lengths on the Compton profiles needs to be studied separately from the hydrogen bonds, since in the MD simulations used in this study the bond lengths of the NIPMPA molecules fluctuate approximately 1− 2% around the average bond length regardless of whether the molecule is hydrogen bonded or not. Moreover, the water molecules have fixed bond lengths. The covalent bond elongation or contraction related to forming or breaking of the hydrogen bond54 is so small in the MD simulation for NIPMPA that it is hidden under the general fluctuation of the bond lengths and the effect is not seen in the DFT calculations. Therefore, we use the approach outlined in the Methods section. The computed results for PNIPAM having increased bond lengths is shown in Figure 5, where the bond lengths of the
Figure 5. Computational effect of increasing bond lengths of different intramolecular bonds on the Compton profile of aqueous PNIPAM.
NIPMPA molecules were elongated in the hydrogen bond topology where both carbonyl and amine groups are hydrogen bonded to one water molecule. The difference Compton profiles for elongations in different bonds are similar in shape, and the features reach larger q values than in the case of hydrogen bond related features. The differences in the intensities can be partly explained by the average elongation depending on the bond, varying between 0.6 and 2% of the average bond length. For water molecules, the effect of intramolecular bond length elongation is similar (see, for example, refs 30 and 40). By comparing the experiment to the theoretical results, we can conclude that the experimental result is caused by multiple effects, because all the computational difference Compton profile magnitudes for individual changes are smaller than the change observed experimentally. In fact, the concentrations of the calculated clusters are higher than the experimental concentration, which makes the magnitudes in the computations higher than they would be in the experimental concentration. This further assures that multiple events take place in the phase transition. The experimental result suggests that overall in the system hydrogen bonds break, but the covalent bond lengths increase. This is an interesting result, since often hydrogen bonds are conversely related to intramolecular bond lengths: When two 5521
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(11) Berber, M. R.; Mori, H.; Hafez, I. H.; Minagawa, K.; Tanaka, M.; Niidome, T.; Katayama, Y.; Maruyama, A.; Hirano, T.; Maeda, Y.; Mori, T. Unusually Large Hysteresis of Temperature-Responsive Poly(N-ethyl-2-propionamidoacrylamide) Studied by Microcalorimetry and FT-IR. J. Phys. Chem. B 2010, 114, 7784−7790. (12) Lu, Y.; Zhou, K.; Ding, Y.; Zhang, G.; Wu, C. Origin of Hysteresis Observed in Association and Dissociation of Polymer Chains in Water. Phys. Chem. Chem. Phys. 2010, 12, 3188−3194. (13) Pamies, R.; Zhu, K.; Kjôniksen, A.-L.; Nyström, B. Thermal Response of Low Molecular Weight Poly-(N-isopropylacrylamide) Polymers in Aqueous Solution. Polym. Bull. 2009, 62, 487−502. (14) Dybal, J.; Trchová, M.; Schmidt, P. The Role of Water in Structural Changes of Poly(N-isopropylacrylamide) and Poly(Nisopropylmethacrylamide) Studied by FTIR, Raman Spectroscopy and Quantum Chemical Calculations. Vib. Spectrosc. 2009, 51, 44−51. (15) Ahmedd, Z.; Gooding, E. A.; Pimenov, K. V.; Wang, L.; Asher, S. A. UV Resonance Raman Determination of Molecular Mechanism of Poly(N-isopropylacrylamide) Volume Phase Transition. J. Phys. Chem. B 2009, 113, 4248−4256. (16) Zhou, K.; Lu, Y.; Li, J.; Shen, L.; annd Z. Xie, G. Z.; Wu, C. The Coil-to-Globule-to-Coil Transition of Linear Polymer Chains in Dilute Aqueous Solutions: Effect of the Intrachain Hydrogen Bonding. Macromolecules 2008, 41, 8927−8931. (17) Ono, Y.; Shikata, T. Contrary Hydration Behavior of NIsopropylacrylamide to Its Polymer, P(NIPAm), with a Lower Critical Solution Temperature. J. Phys. Chem. B 2007, 111, 1511−1513. (18) Ono, Y.; Shikata, T. Hydration and Dynamic Behavior of Poly(N-isopropylacrylamide)s in Aqueous Solution: A Sharp Phase Transition at the Lower Critical Solution Temperature. J. Am. Chem. Soc. 2006, 128, 10030−10031. (19) Cheng, H.; Shen, L.; Wu, C. LLS and FTIR Studies on the Hysteresis in Association and Dissociation of Poly(N-isopropylacrylamide) Chains in Water. Macromolecules 2006, 39, 2325−2329. (20) Longhi, G.; Lebon, F.; Abbate, S.; Fornili, S. L. Molecular Dynamics Simulation of a Model Oligomer for Poly(N-isopropylamide) in Water. Chem. Phys. Lett. 2004, 386, 123−127. (21) Yang, H.; Cheng, R.; Wang, Z. A Quantitative Analyses of the Viscometric Data of the Coil-to-Globule and Globule-to-Coil Transition of Poly(N-isopropylacrylamide) in Water. Polymer 2003, 44, 7175−7180. (22) Maeda, Y.; Yamamoto, H.; Ikeda, I. Phase Separation of Aqueous Solutions of Poly(N-isopropylacrylamide) Investigated by Confocal Raman Microscopy. Macromolecules 2003, 36, 5055−5057. (23) Maeda, Y.; Higuchi, T.; Ikeda, I. FTIR Spectroscopic and Calorimetric Studies of the Phase Transitions of N-Isopropylacrylamide Copolymers in Water. Langmuir 2001, 17, 7535−7539. (24) Wu, C.; Wang, X. Globule-to-Coil Transition of a Single Homopolymer Chain in Solution. Phys. Rev. Lett. 1998, 80, 4092− 4094. (25) Shibayama, M.; Morimoto, M.; Nomura, S. Phase Separation Induced Mechanical Transition of Poly(N-isopropylacrylamide)/ Water Isochore Gels. Macromolecules 1994, 27, 5060−5066. (26) Guo, H.; Karplus, M. Ab Initio Studies of Hydrogen Bonding of N-Methylacetamide: Structure, Cooperativity, and Internal Rotational Barriers. J. Phys. Chem. 1992, 96, 7273−7287. (27) Torii, H.; Tatsumi, T.; Tasumi, M. Effects of Hydration on the Structure, Vibrational Wavenumbers, Vibrational Force Field and Resonance Raman Intensities of N-Methylacetamide. J. Raman Spectrosc. 1998, 29, 537−546. (28) Cooper, M. J., Mijnarends, P. E., Shiotani, N., Sakai, N., Bansil, A., Eds. X-ray Compton Scattering; Oxford University: Oxford, U.K., 2004. (29) Hakala, M.; Nygård, K.; Manninen, S.; Huotari, S.; Buslaps, T.; Nilsson, A.; Pettersson, L. G. M.; Hämäläinen, K. Correlation of Hydrogen Bond Lengths and Angles in Liquid Water Based on Compton Scattering. J. Chem. Phys. 2006, 125, 084504-1−084504-7. (30) Hakala, M.; Nygård, K.; Manninen, S.; Pettersson, L. G. M.; Hämäläinen, K. Intra- and Intermolecular Effects in the Compton Profile of Water. Phys. Rev. B 2006, 73, 035432-1−035432-9.
of broken hydrogen bonds as well as stretching of the covalent bond lengths either in the polymer or in the water molecules when the polymer changes from hydrophilic to hydrophobic phase. The observed increased amount of broken hydrogen bonds in the hydrophobic phase is in line with the generally proposed changes taking place in the phase transition. However, stretching of the internal bonds supports the interpretation of the importance of the hydrophobic interaction in the self-assembly of polymers.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: iina.juurinen@helsinki.fi. *E-mail: mikko.o.hakala@helsinki.fi. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge ESRF for provision of beam time. We thank Susi Lehtola for discussions. This work has been supported by the Academy of Finland (NGSMP, contract numbers 1259526, 1256211, and 1254065), the Research Funds of the University of Helsinki (projects 490064 and 490076), and Väisälä foundation.
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REFERENCES
(1) Maeda, Y.; Yamamoto, H.; Ikeda, I. Effects of Ionization on the Phase Behavior of Poly(N-isopropylacrylamide-co-acrylic acid) and Poly(N,N -diethylacrylamide-co-acrylic acid) in Water. Colloid Polym. Sci. 2004, 282, 1268−1273. (2) Islam, M. R.; Lu, Z.; Li, X.; Sarker, A. K.; Hu, L.; Choi, P.; Li, X.; Hakobyan, N.; Serpe, M. J. Responsive Polymers for Analytical Applications: A Review. Anal. Chim. Acta 2013, 789, 17−32. (3) Petrusic, S.; Jovancic, P.; Lewandowski, M.; Giraud, S.; Grujic, S.; Ostojic, S.; Bugarski, B.; Koncar, V. Properties and Drug Release Profile of Poly(N-isopropylacrylamide) Microgels Functionalized with Maleic Anhydride and Alginate. J. Mater. Sci. 2013, 48, 7935−7948. (4) Chung, J. E.; Yokoyama, M.; Yamato, M.; Aoyagi, T.; Sakurai, Y.; Okano, T. Thermo-Responsive Drug Delivery from Polymeric Micelles Constructed Using Block Copolymers of Poly(N-isopropylacrylamide) and Poly(butylmethacrylate). J. Controlled Release 1999, 62, 115−127. (5) Hu, L.; Sarker, A. K.; Islam, M. R.; Li, X.; Lu, Z.; Serpe, M. J. Poly(N-isopropylacrylamide) Microgel-Based Assemblies. J. Polym. Sci., A: Polym. Chem. 2013, 51, 3004−3020. (6) Chen, M.; Zhou, L.; Guan, Y.; Zhang, Y. Polymerized Microgel Colloidal Crystals: Photonic Hydrogels with Tunable Band Gaps and Fast Response Rates. Angew. Chem., Int. Ed. 2013, 52, 9961−9965. (7) Deshmukh, S. A.; Li, Z.; Kamath, G.; Suthar, K. J.; Mancini, S. K. R. S. S. D. C. Atomistic Insights into Solvation Dynamics and Conformational Transformation in Thermo-Sensitive and NonThermo-Sensitive Oligomers. Polymer 2013, 54, 210−222. (8) Deshmukh, S. A.; Sankaranarayanan, S. K. R. S.; Suthar, K.; Mancini, D. C. Role of Solvation Dynamics and Local Ordering of Water in Inducing Conformational Transitions in Poly(N-isopropylacrylamide) Oligomers Through the LCST. J. Phys. Chem. B 2012, 116, 2651−2663. (9) Lai, H.; Wu, P. A Infrared Spectroscopic Study on the Mechanism of Temperature-Induced Phase Transition of Concentrated Aqueous Solutions of Poly(N-isopropylacrylamide) and Nisopropylpropionamide. Polymer 2010, 51, 1404−1412. (10) Cui, S.; Pang, X.; Zhang, S.; Yu, Y.; Ma, H.; Zhang, X. Unexpected Temperature-Dependent Single Chain Mechanics of Poly(N-isopropylacrylamide) in Water. Langmuir 2012, 28, 5151− 5157. 5522
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(31) Bellin, C.; Barbiellini, B.; Klotz, S.; Buslaps, T.; Rousse, G.; Strässle, T.; Shukla, A. Oxygen Disorder in Ice Probed by X-ray Compton Scattering. Phys. Rev. B 2011, 83, 094117-1−094117-5. (32) Reiter, D. F.; Deb, A.; Sakurai, Y.; Itou, M.; Krishnan, V. G.; Paddison, S. J. Anomalous Ground State of the Electrons in Nanoconfined Water. Phys. Rev. Lett. 2013, 111, 036803-1−036803-5. (33) Lehmkühler, F.; Sakko, A.; Steinke, I.; Sternemaan, C.; Hakala, M.; Sahle, C. J.; Buslaps, T.; Simonelli, L.; Galambosi, S.; Paulus, M.; Pylkkänen, T.; Tolan, M.; Hämäläinen, K. Temperature-Induced Structural Changes of Tetrahydrofuran Clathrate and of the Liquid Water/Tetrahydrofuran Mixture. J. Phys. Chem. C 2011, 115, 21009− 21015. (34) Juurinen, I.; Nakahara, K.; Ando, N.; Nishiumi, T.; Seta, H.; Yoshida, N.; Morinaga, T.; Itou, M.; Ninomiya, T.; Sakurai, Y.; Salonen, E.; Nordlund, K.; Hämäläinen, K.; Hakala, M. Measurement of Two Solvation Regimes in Water-Ethanol Mixtures Using X-Ray Compton Scattering. Phys. Rev. Lett. 2011, 107, 197401-1−197401-5. (35) Lehmkühler, F.; Sakko, A.; Sternemann, C.; Hakala, M.; Nygård, K.; Sahle, C. J.; Galambosi, S.; Steinke, I.; Tiemeyer, S.; Nyrow, A.; Buslaps, T.; Pontoni, D.; Tolan, M.; Hämäläinen, K. Anomalous Energetics in Tetrahydrofuran Clathrate Hydrate Revealed by X-ray Compton Scattering. J. Phys. Chem. Lett. 2010, 1, 2832−2836. (36) Hakala, M.; Nygård, K.; Vaara, J.; Itou, M.; Sakurai, Y.; Hämäläinen, K. Charge Localization in Alcohol Isomers Studied by Compton Scattering. J. Chem. Phys. 2009, 130, 034506-1−034506-8. (37) Nygård, K.; Hakala, M.; Manninen, S.; Itou, M.; Sakurai, Y.; Hämäläinen, K. Configurational Energetics in Ice Ih Probed by Compton Scattering. Phys. Rev. Lett. 2007, 99, 197401-1−197401-4. (38) Sit, P. H.-L.; Bellin, C.; Barbiellini, B.; Testemale, D.; Hazemann, J.-L.; Buslaps, T.; Marzari, N.; Shukla, A. Hydrogen Bonding and Coordination in Normal and Supercritical Water from Xray Inelastic Scattering. Phys. Rev. B 2007, 76, 245413-1−245413-7. (39) Nygård, K.; Hakala, M.; Pylkkänen, T.; Manninen, S.; Buslaps, T.; Itou, M.; Andrejczuk, A.; Sakurai, Y.; Odelius, M.; Hämäläinen, K. Isotope Quantum Effects in the Electron Momentum Density of Water. J. Chem. Phys. 2007, 126, 154508-1−154508-7. (40) Nygård, K.; Hakala, M.; Manninen, S.; Andrejczuk, A.; Itou, M.; Sakurai, Y.; Pettersson, L. G. M.; Hämäläinen, K. Compton Scattering Study of Water Versus Ice Ih: Intra- and Intermolecular Structure. Phys. Rev. E 2006, 74, 031503-1−031503-5. (41) Nygård, K.; Hakala, M.; Manninen, S.; Hämäläinen, K.; Itou, M.; Andrejczuk, A.; Sakurai, Y. Ion Hydration Studied by X-ray Compton Scattering. Phys. Rev. B 2006, 73, 024208-1−024208-9. (42) Eisenberger, P.; Platzman, P. M. Compton Scattering Of X-rays from Bound Electrons. Phys. Rev. A 1970, 2, 415−423. (43) Cooper, M. J. Compton Scattering and Electron Momentum Determination. Rep. Prog. Phys. 1985, 48, 415−481. (44) Lehtola, J.; Hakala, M.; Sakko, A.; Hämäläinen, K. ERKALE - A Flexible Program Package for X-ray Properties of Atoms and Molecules. J. Chem. Phys. 2012, 33, 1572−1585. (45) Lehtola, S. ERKALE - HF/DFT from Hel. 2012; http://erkale. googlecode.com. (46) Lehtola, J.; Hakala, M.; Vaara, J.; Hämäläinen, K. Calculation of Isotropic Compton Profiles with Gaussian Basis Sets. Phys. Chem. Chem. Phys. 2011, 13, 5630−5641. (47) Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A 1988, 38, 3098−3100. (48) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (49) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Results Obtained with the Correlation Energy Density Functionals of Becke and Lee, Yang and Parr. Chem. Phys. Lett. 1989, 157, 200−206. (50) Lehtola, S.; Manninen, P.; Hakala, M.; Hämäläinen, K. Contraction of Completeness-Optimized Basis Sets: Application to Ground-State Electron Momentum Densities. J. Chem. Phys. 2013, 138, 004109-1−004109-8.
(51) Berendsen, H. J. C.; van der Spoel, D.; van Drunen, R. GROMACS - A Message-Passing Parallel Molecular-Dynamics Implementation. Comput. Phys. Commun. 1995, 91, 43−56. (52) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. Comparison of Simple Potential Functions for Simulating Liquid Water. J. Chem. Phys. 1983, 79, 926−935. (53) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. Development and Testing of the OPLS All-Atom Force Field on Conformational Energetics and Properties of Organic Liquids. J. Am. Chem. Soc. 1996, 118, 11225−11236. (54) Goryainov, S. V. A Model of Phase Transitions in Double-Well Morse Potential: Application to Hydrogen Bond. Physica B 2012, 407, 4233−4237.
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