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Jian Hong, De-xia Zhou, Hong-xing Hao, Min Zhao, Hong-tao Bian. Ultrafast Infrared Spectroscopic Study of Microscopic Structural Dynamics in pH Stimulus-Responsive Hydrogels†[J]. Chinese Journal of Chemical Physics , 2020, 33(5): 540-546. doi: 10.1063/1674-0068/cjcp2006096
Citation: Jian Hong, De-xia Zhou, Hong-xing Hao, Min Zhao, Hong-tao Bian. Ultrafast Infrared Spectroscopic Study of Microscopic Structural Dynamics in pH Stimulus-Responsive Hydrogels[J]. Chinese Journal of Chemical Physics , 2020, 33(5): 540-546. doi: 10.1063/1674-0068/cjcp2006096

Ultrafast Infrared Spectroscopic Study of Microscopic Structural Dynamics in pH Stimulus-Responsive Hydrogels

doi: 10.1063/1674-0068/cjcp2006096
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  • Author Bio:

    Hong-tao Bian, E-mail: htbian@snnu.edu.cn

  • Part of the special issue for "the Chinese Chemical Society's 16th National Chemical Dynamics Symposium".
  • Received Date: 2020-06-13
  • Accepted Date: 2020-07-13
  • Publish Date: 2020-10-27
  • Part of the special issue for "the Chinese Chemical Society's 16th National Chemical Dynamics Symposium".
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Ultrafast Infrared Spectroscopic Study of Microscopic Structural Dynamics in pH Stimulus-Responsive Hydrogels

doi: 10.1063/1674-0068/cjcp2006096
  • Author Bio:

Part of the special issue for "the Chinese Chemical Society's 16th National Chemical Dynamics Symposium".
Jian Hong, De-xia Zhou, Hong-xing Hao, Min Zhao, Hong-tao Bian. Ultrafast Infrared Spectroscopic Study of Microscopic Structural Dynamics in pH Stimulus-Responsive Hydrogels†[J]. Chinese Journal of Chemical Physics , 2020, 33(5): 540-546. doi: 10.1063/1674-0068/cjcp2006096
Citation: Jian Hong, De-xia Zhou, Hong-xing Hao, Min Zhao, Hong-tao Bian. Ultrafast Infrared Spectroscopic Study of Microscopic Structural Dynamics in pH Stimulus-Responsive Hydrogels[J]. Chinese Journal of Chemical Physics , 2020, 33(5): 540-546. doi: 10.1063/1674-0068/cjcp2006096
  • Hydrogel is formed by a three-dimensional cross-linked polymer network with a large amount of water as the dispersion medium [1, 2]. The specific hydrogen bonding interactions between the polymer networks and water molecules can cause the water molecules to be locked in the three dimensional networks, thus making the system lose fluidity and turn into a quasi-solid substance. Though the water content trapped in the hydrogels can be extremely high (up to 90%), the hydrogels can still show excellent mechanical properties and its composition is close to the biological tissues [3]. Furthermore, the hydrogels are sensitive to the environmental stimulus [4, 5], such as pH, temperature, ionic strength, light, electric field and magnetic field, etc. Therefore, the soft materials based on the hydrogels can have broad applications such as in chemical sensing, controlled drug release, bioelectronics and tissue engineering [3, 6-11].

    One of the fundamental properties of hydrogel is its capability to show the measureable change in volume in response to the environmental stimulus [12, 13]. Hydrogels can display the volume change by the swelling or phase transition [14]. The diffusion and uptake of water molecules across the hydrogel networks and the swelling capacity are mainly affected by the degree of the crosslinking and the interaction between water and the polymeric chains in the hydrogel [13]. Typically, hydrogels can have the ionic and neutral side groups attached to their backbone chains. For the pH-sensitive hydrogels, the hydrogel can be ionized depending on the pH of the solution and swells as a result of the electrostatic charge repulsion among the polymeric chains [13]. However, the mechanism of the swelling kinetics and water diffusion at the molecular level is still not clear yet. It is expected that the structure and dynamics of water molecules trapped in the three dimensional networks formed by the polymeric chains should be very different from that in the bulk solutions [15]. And the rearrangement of the hydrogen bond network of water molecules should occur at the ultrafast time scale [16]. The hydrogen bonding of water molecules, pore size and rotational motion of the polymeric chains in the hydrogels are believed as the key factors that control its swelling capacity and behavior. Understanding the structural dynamics of the polymer networks would be critical for the development of future hydrogels with desired functions.

    Ultrafast IR spectroscopy has been demonstrated to be a powerful technique to measure the hydrogen bond dynamics in the bulk water [17, 18] as well as electrolyte aqueous solutions [19-22], confined systems [23] to understand the structure and dynamics of water molecules. However, due to that the vibrational lifetime of OH stretch in the water molecules is relatively short [17], only limited information can be obtained mainly due to the shorter time window of the measurable dynamic process. As an alternative method, the solutes with longer vibrational lifetime were typically utilized as a local vibrational probe to report the structural dynamics in the condensed phases [24-31]. Fayer and co-workers investigated the reorganization dynamics of both water and anionic solute in the polyacrylamide hydrogel by using the ultrafast two-dimensional infrared (2D IR) spectroscopy [15]. It was found that the hydrogen bond network reorganization dynamics reported by water and anionic probe are significantly slower than in bulk water. However, the studies on elucidating the structural dynamics in the hydrogels under different stimuli are still not reported yet.

    In this report, the pH stimulus-responsive hydrogels were chosen as a model system to investigate the structural dynamics in the hydrogels using the ultrafast infrared spectroscopic method. The poly(DMAEMA-co-AA) hydrogels were synthesized based on the copolymerization method and characterized by the macroscopic swelling measurements under different pH conditions. And the microscopic structural dynamics in the polymeric networks of hydrogels were studied using FTIR and ultrafast IR spectroscopies, where the SCN- anion was employed as a local vibrational probe. Vibrational relaxation dynamics and rotational anisotropy measurements firmly confirmed that the rotation of SCN- is strongly restricted due to the formation of the three dimensional polymeric networks in the hydrogels. Also the rotational time constants of SCN- in the swollen and collapsed states of hydrogels were determined and correlated well with the macroscopic swelling measurements.

  • The experimental setup for the ultrafast IR spectroscopic measurement has been described in our previous reports [32-34]. A picosecond (ps) amplifier and a femtosecond (fs) amplifier with output frequencies at 800 nm were independently operated and synchronized with the same seed pulse generated from a 78 MHz Ti-sapphire oscillator. The ps amplifier was used to pump an optical parametric amplifier (OPA) to produce ~1 ps Mid-IR pulses with a bandwidth ~18 cm-1. The fs amplifier was used to pump the fs OPA to produce ~140 fs Mid-IR pulses with a bandwidth ~200 cm-1. The ultrafast IR spectroscopic measurement was based on the pump-probe scheme, where the narrow-band ps IR pulse was used to selectively excite the vibrational modes in the sample and denoted as the pump beam. The broadband fs IR pulse was used as the probe beam and was frequency resolved by a spectrograph. Two IR polarizers were added into the probe beam path to selectively measure the parallel or perpendicular polarized signal relative to the pump beam. Vibrational lifetimes of the anionic probe were determined from the isotropic pump-probe signal based on the polarization selective measurement. Rotational time constants of the anionic probe in the hydrogels under different pH conditions were determined from the waiting time dependent anisotropy.

    The poly(DMAEMA-co-AA) hydrogels were synthesized in the aqueous solution following the method reported in previous studies [14, 35]. A certain amount of dimethylaminoethyl methacrylate (DMAEMA), and acrylic acid (AA) were weighed based on the calculation and mixed in a beaker. The NaSCN was also added and the concentration was controlled at 0.1 mol/kg. Then the prepared crosslinking agent $ N $, $ N' $-methylene bisacrylamide (BIS) and initiator (NH4)2S2O8 (APS) were added into the reaction system to initiate the reaction. The transparent and soft DMEMA/AA copolymer hydrogel was obtained after the reaction. The optimized reaction conditions were as follows: monomer ratio of DMAEMA to AA was 4:1, the monomer mass fraction was 30%, the amount of crosslinking agent was 1.4% of the total monomer mass, and the amount of initiator was 0.4% of the total monomer mass. The above chemicals were purchased from Aladdin with analytical grade and used as received. The hydrogel was placed in the deionized water for 7 days to remove the unreacted residual monomers and impurities. The hydrogels were molded to the disks (13 mm×8 mm) and placed in the oven under vacuum drying.

    To determine the dynamic swelling behavior, the dried hydrogel disks were placed in the phosphate citrate buffer solutions under different pH conditions (2.0, 5.0, 7.0, 9.0 and 12.0) at room temperature. Here D2O is used instead of H2O to make sure the SCN- anionic probe can have a longer vibrational lifetime in the ultrafast IR spectroscopic measurement. The swollen hydrogels were taken out from the buffer, blotted for the removal of surface water, and weighed at specified time intervals. The swelling of the hydrogel network can be expressed by the weight of swelling ratio (SR):

    where $ W_t $ is the weight of the swollen hydrogel at specific time $ t $ (hour) and $ W_0 $ is the weight of the initially dried hydrogel. The equilibrium swelling ratio (ESR) is obtained when the weight of the swollen hydrogel doesnot change with time.

    The Fourier transform infrared (FTIR) measurements were employed to characterize the polymerization of the hydrogels. The C = C spectral features originated from the monomers of AA and DMEMA disappeared after the copolymerization reaction. The FTIR measurements were performed on the Nicolet iS10 spectrometer, and all the samples were sandwiched in a home-made sample cell composed of two CaF2 windows separated by a Teflon spacer. The thickness of the spacer was controlled and can be adjusted depending upon the optical densities. All the measurements were carried out at room temperature (22±1 ℃).

    Figure Scheme 1.  The reaction route to prepare the poly(DMAEMA-co-AA) hydrogels.

  • To characterize the swelling behavior of the hydrogels, the pH dependent swelling experiments were performed to evaluate the swelling capacity of the poly(DMAEMA-co-AA) hydrogels with the pH ranging from 2.0 to 12.0 at room temperature, and the results are shown in FIG. 1(a). As shown in FIG. 1(a), the weight of the swollen hydrogels increases almost linearly with the duration time of swelling and reaches a plateau at approximately 40 h. Furthermore, the swelling ratios of the hydrogels are very sensitive to the pH, which indicates that the as-prepared hydrogels are pH stimulus-responsive. FIG. 1(b) shows the pH dependence of the equilibrium swelling ratio (ESR) for the poly(DMAEMA-co-AA) hydrogels. It is expected that the specific interaction between the carboxyl groups and the tertiary amine side groups in the polymer chains of the hydrogels should be the governing factors that affect the pH-dependent swelling behavior. When the pH is varied from 7.0 to 2.0, the tertiary amide side groups should be positively charged and the hydrogels are in the swollen state. This can be explained by the fact that the electrostatic repulsion between the charged sides groups in the chains of the hydrogel can lead to a high swelling capacity. At pH = 2.0, the poly(DMAEMA-co-AA) hydrogels shows the maximum swelling capacity which is around three times higher than that in the neutral condition. When the pH changes from 7.0 to 12.0, the carboxyl groups in the poly(DMAEMA-co-AA) hydrogels would be converted to the carboxylate anions with negative charges. And the repulsion interaction between the negatively charged groups would further cause the swelling but with a less degree. At the neutral condition of pH varying from 6 to 8, the attraction and repulsion between the carboxyl groups and the tertiary amine side groups would be balanced and showed the minimum swelling capacity. The p$ K_ \rm{a} $ of DMAEMA in the water solution also falls in this pH range which suggests that the poly(DMAEMA-co-AA) hydrogels show the collapsed state to some extent [36].

    Figure 1.  (a) Swelling behavior of the poly(DMAEMA-co-AA) hydrogels with time under different pH conditions. (b) pH dependence of the equilibrium swelling ratio (ESR) for the poly(DMAEMA-co-AA) hydrogels

    The swelling behavior observed in FIG. 1 can be further analyzed to determine the water diffusion process in the poly(DMAEMA-co-AA) hydrogels. The diffusion of the water molecules in the hydrogels can be described with the following equation [37]:

    where $ K $ is the swelling constant and $ n $ is the swelling exponent. These values can be determined from the slopes and the intercept of the plots between ln(SR) and ln($ t $). These plots and the fitting results are shown in FIG. 2 and Table Ⅰ.

    Figure 2.  Swelling kinetic in the poly(DMAEMA-co-AA) hydrogels under different pH conditions. The solid lines are the fitting curves with the expression of ln(SR) = ln$ K $+$ n $ln$ t $

    Table Ⅰ.  Swelling parameters of the poly(DMAEMA-co-AA) hydrogels under different pH conditions

    From Table Ⅰ, the values of $ n $ were determined to be 1.0 considering the experimental uncertainties and did not show the pH dependence. The swelling kinetics analysis indicated that the water diffusion in the hydrogels is the case Ⅱ diffusion [37], where the diffusion rate of water molecules is higher than the relaxation rate of the polymer chains in the hydrogels. Thus, the relaxation rate of the polymer chains in the poly(DMAEMA-co-AA) hydrogels should be the key factor that controls the swelling kinetics.

    From the above discussion, the synthesized poly(DMAEMA-co-AA) hydrogels did show the swelling capability and the pH stimulus-responsive behavior. However, the swelling mechanism in the hydrogels can only be speculated based on the macroscopic measurements. And the molecular level understanding of the structure and dynamics of the water molecules trapped by the polymeric chains in the hydrogels is still lacking. Here the SCN- anionic probe was employed as the local vibrational probe to report the structural dynamics in the hydrogels. It would be expected that the vibrational relaxation dynamics of the SCN- would provide further insights into the structure of the three dimensional networks formed in the pH responsive hydrogels.

  • FIG. 3 shows the pH dependence of the CN stretching of the SCN- anionic probe in the poly(DMAEMA-co-AA) hydrogels. It is found that the CN stretching frequency of the SCN- anionic probe in the hydrogels is slightly changed from 2064 cm-1 to 2060 cm-1 with the pH ranging from 7.0 to 2.0. However, the bandwidth of the CN stretching peak remains unaffected, which is determined to be 37 cm-1 by using a pseudo-Voigt function. Previous studies showed that the CN stretching frequency of SCN- anion is positioned at 2064 cm-1 in the neat water solution [39-41]. And the frequency and bandwidth of the CN stretching mode are sensitive to the local environments and the hydrogen bond interaction of the solvents [42, 43]. It is generally true that the CN stretching frequency is redshifted to lower frequencies when the SCN- anionic probe interacts with the charged interfaces [41]. As the pH vary from 7.0 to 2.0, the poly(DMAEMA-co-AA) hydrogels are in the swollen state, and the tertiary amide side groups are positively charged. It would be expected that the SCN- anionic probe would be located in the vicinity of the positively charged side groups of the hydrogels. And the water molecules in the surrounding environments would have a much weaker hydrogen bond interaction than that in the neutral condition. On the other hand, when the pH is varied from 7.0 to 12.0, the carboxyl groups in the poly(DMAEMA-co-AA) hydrogels would be converted to the carboxylate anions with negative charges. It is expected that the SCN- anionic probe should be located away from the charged polymer chains and interacts with the water molecules trapped by three dimensional polymer networks in the hydrogels. And the CN stretching frequency of SCN- is slightly shifted to 2062 cm-1. The vibrational relaxation dynamics of SCN- presented in the following would provide further information regarding the structural dynamics in the hydrogels under different pH conditions.

    Figure 3.  Normalized FTIR spectra of CN stretching in the SCN- anionic probe dissolved in the poly(DMAEMA-co-AA) hydrogels with different pH. The solid lines are the fitting curves using a pseudo-Voigt function [25, 38]. The background signal of poly(DMAEMA-co-AA) without the addition of SCN- has been subtracted

    FIG. 4(a) shows the pH dependence of the vibrational relaxation dynamics of the SCN- anionic probe in the poly(DMAEMA-co-AA) hydrogels. Similar to our previous studies [34, 40], the vibrational relaxation dynamics of the CN stretching in SCN- can be described by a bi-exponential function. The fast component is related to the intramolecular vibrational energy transfer within the SCN-, and the slow component is mainly contributed by the vibrational population decay of SCN-. The time constants of the fast and slow components are shown in FIG. 4(b). The time constant of the fast component is determined to be 1.2±0.2 ps and remains unchanged with the pH varying from 2.0 to 12.0. However, the slow component displays pH dependence. At the lower pH, the vibrational lifetime of SCN- shows a much faster decay rate than that in the neat water solution. The vibrational population decay of SCN- is determined to be 14±1 ps at the pH of 2. This can be explained by the fact that the SCN- is located in the vicinity of the positively charged side groups of tertiary amide in the hydrogels, where their specific interactions can facilitate the vibrational energy transfer of SCN-. When the pH of the hydrogels is higher than 7.0, the vibrational population decay of SCN- anionic probe is slowed down slightly. At a pH of 12.0, the vibrational lifetime of SCN- is determined to be 20±1 ps. The vibrational relaxation dynamics firmly confirms that the SCN- anionic probe should closely interact with the side groups of the polymer chain at the lower pH. When the pH of the hydrogel is 7.0 or higher, the SCN- anions should be confined in the pore formed by the polymer chain and solvated by the water molecules. Therefore, the structural dynamics in the hydrogels can be revealed from the viewpoint of the SCN- anionic probe.

    Figure 4.  (a) Vibrational relaxation for the CN stretching of the SCN- anionic probe in the poly(DMAEMA-co-AA) hydrogels under different pH conditions. The solid lines are the fitting curves using a biexponential decay function. (b) pH dependent vibrational lifetime constants of SCN- probe in the poly(DMAEMA-co-AA) hydrogels

    FIG. 5(a) shows the rotational anisotropy decay of the SCN- anionic probe in the hydrogels under different pH conditions. It is clear that the rotation of SCN- canot decay to zero, especially at pH of 7.0. Similar to previous studies [25, 34, 40], the rotational anisotropy decay of SCN- can be described by a biexponential decay function with the following expression:

    Figure 5.  (a) Rotational anisotropy decay of SCN- anionic probe in the poly(DMAEMA-co-AA) hydrogels under different pH conditions. The solid lines are the fitting curves using a biexponential decay function. (b) pH dependence of the rotational time constants of the SCN- probe in the poly(DMAEMA-co-AA) hydrogels.

    where $ \tau_1 $ and $ \tau_2 $ are the rotational time constants, which can be determined by fitting the curves shown in FIG. 5(a) using Eq.(3).

    The pH dependence of rotational time constants of SCN- is listed in Table Ⅱ and plotted in FIG. 5(b). The fast component is mainly explained by the wobbling motion within a restricted angle in a cone caused by the intermolecular interaction [23, 44]. The rotational time constant of the fast components is determined to be 3.2±0.2 ps on average and does not show the pH dependence. Previous studies showed that the rotational time constant of SCN- in the aqueous solution was determined to be 3.5±0.2 ps [39, 40], which is consistent with the rotational time constants of the fast component observed here. On the other hand, the slow component is mainly contributed by the molecular rotation in the investigated systems. And the rotational time constant of the slow component is strongly affected by the pH in the hydrogels. At pH of 7.0, the rotational time constant of SCN- was determined to be 13±1 ps. However, the rotational time constants of SCN- in the hydrogels are slowed down to 21±2 ps and 19±2 ps at pH of 2.0 and 12.0, which indicates that the rotational dynamics of the SCN- is strongly affected by the crosslinked polymer chains. Fayer and coworkers investigated the water dynamics in polyacrylamide hydrogels using SeCN- as the anionic probe [15]. They showed that SeCN- displayed two component dynamics, where the fast component was assigned to the anions solvated in the confined water pools in the hydrogels. The slower component was affected by the mass concentration of the polymer and was assigned to the interfacial anion strongly interacting with the polymer fibers. Our results are consistent with the study reported by Fayer and coworker, and the rotational time constant of the slow component of the SCN- anionic probe displays the pH dependence, which is correlated with the swelling behavior under different pH conditions.

    Table Ⅱ.  Rotational time constants of the SCN- anion in the poly(DMAEMA-co-AA) hydrogels under different pH conditions

    By comparing FIG. 1(b) and FIG. 5(b), it is interesting to see that the rotational dynamics of SCN- anionic probe can closely correlate with the swelling behavior in the hydrogels. When the pH is at the neutral condition of 7.0, the nonzero term of $ B_0 $ in the rotational anisotropy decay of SCN- is determined to be 0.325±0.003. This clearly indicated that the rotation of the SCN- anionic probe is restricted and confined in the pore formed by the cross-linked networks in the hydrogels. Also the hydrogel shows the minimum swelling capacity due to the balanced electrostatic repulsion and attraction between the carboxyl groups and the amine side groups. However, when the amine side groups are positively charged, the pore size in the hydrogels would be extended much larger which can accommodate higher capacity of water molecule. Also the SCN- would be located in the vicinity of the charged surface of the polymer chains. And the rotation of SCN- would be strongly affected at the charged interface, but can rotate with more degrees of freedom compared to the collapsed state. Here it should be noted that the rotational time constants of SCN- are sensitive to the nonzero term of $ B_0 $ [45]. If the nonzero term of $ B_0 $ is fixed to zero, the rotational time constants of SCN- would become much bigger. However, the physical picture of this assumption is not consistent with the swelling measurements and this situation is not discussed here. All these results firmly confirmed that the three dimensional networks formed by the polymeric chains in the hydrogels can be unraveled by the structural dynamics of the SCN- anionic probe. Most importantly, the microscopic rotational dynamics of SCN- correlates well with the pH-dependent swelling behavior observed in the macroscopic experiment, which can provide further understanding of the structural dynamics in the hydrogels at the molecule level. To the best of our knowledge, herein, the structural dynamics in the pH stimulus-responsive hydrogels is reported for the first time by using the ultrafast IR spectroscopy.

  • In summary, the poly(DMAEMA-co-AA) hydrogels with pH stimulus-responsive behavior were synthesized and characterized by combining the macroscopic swelling characterization and the ultrafast IR spectroscopic measurements under different pH conditions. The SCN- anionic probe was employed as a local reporter to reflect the structural dynamics in the hydrogels. Rotational anisotropy measurements reveal that the rotation of SCN- anion is strongly confined by the three dimensional networks formed by the polymeric chains in the hydrogels. The rotational time constant of the SCN- is determined to be 21±2 ps when the hydrogels are in the swollen state at the pH of 2.0, which is related to the electrostatic interaction of side groups in the hydrogels. While in the collapsed state, the rotation of SCN- is strongly restricted and cannot decay to zero. The pH-dependent rotational dynamics of SCN- is well correlated with the macroscopic pH-dependent swelling measurement. The results presented in this study are expected to provide a molecular-level understanding of the structural dynamics of the cross-linked polymeric network under the external stimulus.

  • This work was supported by the National Natural Science Foundation of China (No.21873062), the Fundamental Research Funds for the Central Universities (GK202001009), the Natural Science Basis Research Plan in Shaanxi Province of China (No.2020JM-295), the 111 Project (B14041) and Program for Changjiang Scholars and the Innovative Research Team in University (IRT-14R33). We also thank Dr. Somnath Mukherjee from Shaanxi Normal University for his advice on the editing of the manuscript.

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