Recognition‐Mediated Hydrogel Swelling Controlled by Interaction with a Negative Thermoresponsive LCST Polymer

Abstract Most polymeric thermoresponsive hydrogels contract upon heating beyond the lower critical solution temperature (LCST) of the polymers used. Herein, we report a supramolecular hydrogel system that shows the opposite temperature dependence. When the non‐thermosesponsive hydrogel NaphtGel, containing dialkoxynaphthalene guest molecules, becomes complexed with the tetra cationic macrocyclic host CBPQT4+, swelling occurred as a result of host–guest complex formation leading to charge repulsion between the host units, as well as an osmotic contribution of chloride counter‐ions embedded in the network. The immersion of NaphtGel in a solution of poly(N‐isopropylacrylamide) with tetrathiafulvalene (TTF) end groups complexed with CBPQT4+ induced positive thermoresponsive behaviour. The LCST‐induced dethreading of the polymer‐based pseudorotaxane upon heating led to transfer of the CBPQT4+ host and a concomitant swelling of NaphtGel. Subsequent cooling led to reformation of the TTF‐based host–guest complexes in solution and contraction of the hydrogel.


S4
Isothermal titration calorimetry (ITC) experiments were performed using a nano-ITC titration calorimeter from TA Instruments with a standard sample cell volume of 1 mL at 8 °C.
A 250 μL injection syringe was used with stirring at 400 rpm. TTF-PNIPAM (0.22 mM) was dissolved in deionized water and the solutions were degassed gently under vacuum before use.
Control experiments with identical injections of CBPQT 4+ into deionized water alone were used to correct titration data. ITC data collected during the analysis were fitted to an independent sites model where one independent site binds one ligand. The fit and the determination of thermodynamic parameters (Ka, n, H, S, G) were achieved by using the TA Instrument ITC analysis software (NanoAnalyze).

Turbidity measurements were performed on a Cary 50 UV-Visible spectrophotometer with
Peltier temperature control at a wavelength of 540 nm. The samples were first cooled to a suitable temperature to fully dissolve the polymer (0,5 mM) in deionized water, after which the sample was placed in the instrument and cooled to 5 o C. In the case of supramolecular hostguest complexes, 1 equivalent of CBPQT 4+ was added to polymer solution. The transmittance was measured during at least two controlled cooling/heating cycles with a cooling/heating rate of 1 °C min -1 from 5 o C to 40 o C with hold steps of 5 min at the extreme temperatures while stirring. TCP is given as the temperature when the transmittance drops to 50% during the second heating ramp.

Synthesis and characterization of NaphtGel and NaphtGel0
Synthesis of NaphtGel N,N-Dimethylacrylamide (1 g, 10 mmol), AIBN (8.6 mg, 0.052 mmol), N,N'methylenebisacrylamide (0.0128 g , 0.083 mmol) and NaphtAm 1 (0.122 g, 0.3 mmol) were stirred in DMF (4.5 g) in a sealed flask. The mixture was deoxygenated by nitrogen bubbling for 20 min in an ice/water and the solution was transferred between two glass plates (7 x 3 cm) placed into a sealed flask under nitrogen. The reactor was heated for 24 h at 70 °C in an oven.
Then, NaphtGel was removed from the glass plates at room temperature and washed with acetone and dried successively at room temperature for 24 h and at 60 °C for 12 h. The dried gel was put in water and left under stirring for 5 days to swell at equilibrium. NaphtGel0 was prepared using the same procedure without NaphtAm. The molar percentage of NaphtAm units (mol% NaphtAm) in the copolymer was calculated with the following equation: The mol% of NaphtAm for NaphtGel was estimated as 3 % (± 0.6).

Swelling and UV-Vis studies
Hydrogel samples were swelled in deionized water at 25°C. The swelling ratios of the samples were determined gravimetrically at different times to follow the swelling kinetics using the following equation: The required amount of CBPQT 4+ , 4Clto complex all naphthalene units within NaphtGel was calculated on the basis of the mol% of NaphtAm calculated by 1 H NMR spectroscopy.

Effect of crosslinker on the swelling ratio of NaphtGel
The amount of crosslinker in NaphtGel was optimized in order to obtain hydrogels with maximum swelling ratio. The results obtained by variation of the amount of MBA crosslinker are summarized in Table S1 revealing that 0.8 mol% crosslinker is optimal for this work as it is just sufficient for crosslinking while facilitating large chain motion and mobility as required for swelling.

Effect of [NaCl] on the swelling ratio of NaphtGel0, NaphtGel and NaphtGel/CBPQT 4+
After having reached their equilibrium swellings in deionized water, hydrogels pieces were immersed in different [NaCl] (from 0 to 357 g/L; 0 to 6.1 M) for 3 days and their new swelling ratios were estimated.

Absorbance (AU)
Heating-cooling cycle absorbance at 800 nm absorbance at 520 nm heating cooling