Gel thermoresponsiveness driven by switching of the charge-transfer interaction

Abstract

A novel gel LCST system consisting of a pyrene containing acrylate network polymer and external effectors is demonstrated. The LCST behaviour was conducted by switching of the CT interaction between the gel and effector, which was readily tuned by effector concentration or molecular structure of the effector.

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Introduction

Stimuli-responsive polymer gels, driven by environmental changes such as light, pH, temperature, and solvent composition, have attracted considerable interest in the field of smart functional materials with large volumetric change.¹ Among various stimuli, heat controllable polymer gels possess a potential advantage derived from their facile operability without variation of chemical components.² Thus, many researchers have focused on the applications of thermoresponsive polymer gels for smart materials such as drug delivery,³ gene therapy,⁴ thermosensitive chromatography,⁵ surface modifiers,⁶ and cell cultivation sheets.⁷ The basic structural feature of these thermoresponsive polymer gel lays on chemical crosslinking of linear polymer chain having thermoresponsiveness. For a typical example water, a solution of poly(N-isopropyl acrylamide) (PNIPAM) exhibits lower critical solution temperature (LCST) type phase transition around at 32 °C, which shows sharp phase separation from solution to precipitation above the critical solution temperature.⁸ Despite the convenience of thermoresponsive polymer gels at ambient temperature, the systematic molecular design of them are still unclear, especially in the media other than water.⁹

In this circumstance, we have recently reported LCST behaviour driven by charge-transfer (CT) interaction between electron-rich and poor aromatic compounds.¹⁰ Therein, poly(1-pyrenemethyl acrylate) (PPMA) bearing pyrene side groups as a π electron donor showed LCST behaviour in toluene when a π electron acceptor (so-called effector) was added. The effector interacts with the pyrenyl group in the polymer chain via formation of CT complex to increase solubility, and heating induces formation of aggregates through decomposition of the CT complex. This result prompted us to design a novel gel LCST system driven by CT interaction. Herein, we demonstrate a synthesis of chemically crosslinked polymer gel consisting of PPMA, and its LCST behaviour using π accepting effectors (Fig. 1).

Fig. 1 (a) Synthetic method for PPMAgel. (b) Chemical structures of effectors 2 and 3.

Results and discussion

A gel having CT interactive moiety was synthesized via ordinary free radical polymerization of 1-pyrenemethyl methacrylate (1)^10a initiated by 2,2′-azobisisobutyronitrile (AIBN) in a sealed capillary tube under 65 °C for 48 hours (see also ESI†). The obtained capillary gel (PPMAgel) was thoroughly washed and dried under vacuo. On Fourier transform infrared (FTIR) spectroscopy, disappearance of the peak assigned to stretching vibration of vinyl C=C (ν_C=C, 1615 cm⁻¹) and deformation vibration of alkenyl C–H (δ_C–H, 1406 cm⁻¹) was confirmed after the gel formation, and hypsochromic shift of stretching vibration of ester C=O (ν_C=O, 1716 cm⁻¹ → 1724 cm⁻¹) was also observed, indicative of completion of the polymerization (Fig. S1†). The dried gel with a length of ca. 3.5 mm were soaked and swollen in a wide range of organic solvents, such as hexane, toluene, chloroform, tetrahydrofuran (THF), acetone, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and methanol, so as to evaluate an equilibrium swelling degree (Q₁ = L_wet/L_dry, L; length of the longer side, Fig. S2†), implying compatibility of the solvent to the gel. In chloroform (Q₁ = 1.6) or THF (Q₁ = 1.5), PPMAgel was moderately swollen, whereas it collapsed in toluene, hexane, methanol, and DMSO, due to the poor compatibility of the polymer chain to these organic solvents.

To elucidate the influence of effectors, we firstly chose toluene, which has poor compatibility (Q₁ = 1.0) to the polymer chain. In other words, PPMAgel was expected to swell in a poor solvent only by the aid of effectors. Additionally, toluene exhibits high boiling point (T_b = 111 °C) and good solubility of acceptors. PPMAgel was firstly soaked in toluene for 6 hours, and then transferred into different concentrations of effector 2 and 3 at 25 °C. After the addition of the effectors, Q₂ (Q₂ = L_wet2/L_wet1, a swelling degree in the presence (L_wet2) or absence (L_wet1) of effectors 2 and 3) of PPMAgel gradually increased. For all gel samples, Q₂ increased with increment of effector concentration as shown in Fig. 2a, and swelling-saturation concentration of effector 2 (0.90 M) differed from that of 3 (0.25 M). Furthermore, the colour of PPMAgel was immediately changed after addition into the effector solution from light yellow to orange (2, Fig. 2b) or red (3), meaning a formation of CT complex. This observation simply shows that the improvement of swelling behaviour of PPMAgel after addition of effectors predominantly derived from disruption of strong π–π stacking between the polymer chains by CT complexation in PPMAgel and subsequent swelling. The saturation of swelling degree of PPMAgel in effector solution of toluene took ca. 4 days, probably due to slow diffusion of the effectors into the gel network, since the gel initially collapsed because of poor compatibility to toluene. At low concentration, Q₂ with 3 (0.25 M) was 2.4, while Q₂ with 2 (0.20 M) was 1.0 (Fig. 2a). This difference in CT complexation between PPMAgel and effectors can be explained from the association constant (K_a). Our previous study already showed CT complexation between PPMA and effectors derived from pyromellitic acid and mellitic acid, similar to 2 and 3, with K_a of 0.74 M⁻¹ and 4.68 M⁻¹, respectively.^10a Therefore, the use of effector 2 with a lower association constant required a larger acceptor concentration for swelling of PPMAgel than that of 3 with a higher association constant.

Fig. 2 (a) Q₂ of PPMAgel versus effector concentration in toluene, and (b) photographs representing the change of PPMAgel with effector 2 in toluene.

When swollen PPMAgel in an effector solution was heated, the gel showed shrinkage at high temperature and hypochromic colour change, i.e., volume phase transition with dissociation of CT complex. To shed light on the thermal transition behaviour of PPMAgel in the presence of effectors, PPMAgel were steadily heated to 80 °C at 5 °C intervals. As shown in Fig. 3a and 4a, the swelling degree of the gel upon temperature elevation. In the case of effector 2, the decrease of swelling degree occurred with above 0.75 M, while it was caused with above 0.25 M for effector 3. The difference in K_a between 2 and 3 is probably responsible for this result. Higher concentration of the effectors resulted in high swelling degree, although PPMAgel with high effector concentration cannot shrink even at high temperature, due to too much amount of effectors surrounding the gel. Having successfully shown LCST behaviour of PPMAgel, we subsequently investigated recyclability of the gel LCST system. As a result, PPMAgel and 2 (0.75 M) exhibited reversible volume transition at 25 °C (Q₂ = 1.3–1.5) and 80 °C (Q₂ = 1.8–2.0), and the gel and 3 (0.25 M) also showed it at 25 °C (Q₂ = 1.2–1.4) and 100 °C (Q₂ = 2.3–2.5), respectively (Fig. 3b and 4b). These results illustrate that the gel LCST system can be reproduced on repeating cycles of heating and cooling, thus suitable for a long time usage.

Fig. 3 (a) Q₂ of PPMAgel against temperature, with various effector 2 concentration in toluene, and (b) switching of Q₂ of PPMAgel between 25 °C and 80 °C and (c) photographs of PPMAgel along heating and cooling cycle under the existence of effector 2 (0.75 M) in toluene.

Fig. 4 (a) Q₂ of PPMAgel against temperature, with various effector 3 concentration in toluene, and (b) switching of Q₂ of PPMAgel between 25 °C and 80 °C, and (c) photographs of PPMAgel along heating and cooling cycle under the existence of effector 3 (0.25 M) in toluene.

To show evidence of cleaving of CT complexation with increase in temperature, UV-vis spectra of monomer 1 with acceptor 2 was measured with increased temperature (Fig. S3†). When CT takes place between the donor to acceptor, then it leads to rise on absorption spectrum attributed to CT band with λ_max at around 430 nm. Mixing 1 and 2 in toluene ([1] = 2 mM and [3] = 100 mM) brought about formation of a distinct yellow solution. Increasing temperature of the solution gradually from 25 °C to 100 °C decreases the absorbance at 428 nm, presumably due to the decrease of concentration to form the CT complex with increasing temperature, resulting in its individual component monomer 1 and effector 2. To obtain deeper insight, we assessed the association constants (K_a) between 1 and 2, and the thermodynamic parameters for the association (ΔH and ΔS) by Benesi–Hildebrand and van't Hoff plots, respectively (Fig. S4 and Table S1†), which founds ΔH = 16.4 kJ mol⁻¹ and ΔS = 47.5 J mol⁻¹ K⁻¹.

Conclusions

In summary, we have demonstrated novel gel LCST system consisting of pyrene containing acrylate gel and external effectors. The LCST behaviour was driven by CT interaction between the gel and effector, which was readily tuned by effector concentration as well as molecular structure of effector. To the best of our knowledge, this is the first example of LCST polymer gels using donor–acceptor interactions. This study show that, through proper selection and design of intermolecular interactions, gels exhibiting ambient LCST temperatures can be prepared. Therefore, other intermolecular interactions will be explored in the design of thermoresponsive polymer gels.

Acknowledgements

The project was supported by JSPS Grant-in-Aid for Scientific Research (B) (26288054) and JSPS Grant-in-Aid for Young Scientists (B) (23750117).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental detail, swelling degree in organic solvents, UV-vis absorption spectra of the CT complex and Benesi–Hildebrand plot. See DOI: 10.1039/c5ra18388j

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