Water Electrolysis Dominated Adhesion Of Borate Polymer Hydrogels On Solid Surfaces

Adhesive hydrogels have important applications in surface engineering,wound heal:ing,flexible electronic devices,and drug delivery.However,the water in the hydrogel will weaken the interfacial adhesion.To achieve high adhesion strength,the polymer chains in the hydrogel need to form a strong interface interaction with the solid surface.The catechol group can overcome the shielding of the water molecule layer and bind to the solid surface through covalent bonds,coordination bonds,hydrogen bonds and electrostatic adsorption.In recent years,the introduction of catechol groups into polymer networks has become an important approach for the design of adhesive hydrogels.The strong interfacial binding force of the catechol group can make the hydrogel adhere to various dry/wet solid surfaces.However,it is difficult to precisely control the adhesion strength,and hard to realize the reversible,cyclic adhesion/nonadhesive transition of hydrogels on various surfaces.This research focuses on the above key scientific issues.Based on the previous research on electrically controlled adhesive hydrogels of our group,a series of borate ester polymer hydrogels were prepared using catechol group-containing molecules with different chemical structures.The relationship between the molecular structure of the monomer and the physical and mechanical properties,adhesion properties,and electrically responsive properties of the hydrogel was discussed.Relying on the integrated design of the electrode and the hydrogel,the electrically controlled adhesion/nonadhesion evolution of the borate ester polymer hydrogels to conductive and non-conductive surfaces was realized.The main research contents are as follows:(1)Molecular building blocks containing catechol groups with different chemical structures:chlorogenic acid(CA),ellagic acid(EA),rutin hydrate(RH),tannic acid(TA),6,6’,7,7’-tetrahydroxy-4,4,4’,4’-tetramethyl-2,2’-spirobischroman(TTS),were applied to react with boric acid(BA)under alkaline conditions to prepare oligomer cross linkers CAB,EAB,RHB,TAB and TB which were terminated by tetrahydroxyboric acid anions.A series of borate ester polymer hydrogels denoted as CABVA,EABVA,RHBVA,TABVA and TBVA were prepared by the reaction between the above-mentioned crosslinkers and polyvinyl alcohol.Through infrared spectroscopy(FT-IR),proton nuclear magnetic resonance spectroscopy(1H NMR),and scanning electron microscope(SEM),the structure of the cross linkers and the chemical composition and microscopic morphology of.the hydrogel were characterized.(2)The influence of crosslinking density on the mechanical properties of hydrogels was studied.Properly increasing the content of catechol groups and the proportion of boric acid in the hydrogel crosslinker can increase the number of tetrahydroxyboric acid at the end of the crosslinkers,thereby increasing the crosslinking density of the hydrogel and improving the mechanical properties.The mechanical properties of CABVA-2,EABVA-2,RHBVA-2,TABVA-2,TBVA-2,which have the most excellent comprehensive mechanical properties,are comparatively studied.The elongation of the five hydrogels are 2491.0±8.5%,1838.7 ± 35.4%,1708.0 ± 13.9%,2458.7 ± 40.5%,2448.3 ± 45.5%,and the corresponding breaking strengths are 19.5 ± 0.4 kPa,53.9 ± 0.4 kPa,39.9 ± 1.6 kPa 27.0 ± 0.6 kPa,39.4 ± 2.5kPa.These five hydrogels also have excellent self-healing properties,which can achieve a strain self-healing efficiency of more than 90%and a stress self-repairing efficiency of more than 87%within 30s.This is the result of the combined effect of borate and hydrogen bonds in the hydrogel.(3)The electrical conductivity of the hydrogels of CABVA-2,EABVA-2,RHBVA-2,TABVA-2,and TBVA-2 with a molar ratio of catechol group molecule to boric acid of 1:2.5 can reach 4.97,5.0,4.90,4.96,5.18 mS/cm,respectively.Our previous work has proved that the pH value of the cathode and anode of the hydrogel can be adjusted by controlling the electrolysis reaction of the water in the hydrogel,thereby adjusting the adhesion performance of the hydrogel to conductive surfaces.According to this mechanism,we further broadened the system to five borate polymer hydrogels.Electrical stimulation for 2 s,five kinds of borate polymer hydrogels can generate adhesion to a variety of conductive substrates(stainless steel,copper plate,aluminum sheet carbon paper and ITO glass).By simply switching the direction of the electric field,the hydrogel can be reversibly dettaching from the conductive substrate.The adhesion strength of the hydrogel can be achieved by controlling the crosslinking density of borate bonds in the hydrogel.By appropriately increasing the content of catechol groups in the hydrogel and the proportion of boric acid in the crosslinking agent,the electronically controlled adhesion/nonadhesion performance of the hydrogel can be improved.(4)Through the design of a hydrogel-electrode integrated device,the in-situ water electrolysis reaction can program the adhesion/nonadhesion of the hydrogels to non-conductive surfaces.CABVA-2,EABVA-2,RHBVA-2,TABVA-2 and TBVA-2 have adhesion to a variety of non-conductive substrate surfaces,and the strength can reach 6.32 kPa,9.59 kPa,14.0 kPa,13.0 kPa,12.75 kPa;Adhesive strength changes up to 10,30,16,31,and 27 times during the transition of adhesion/no nadhesion.