Phase Transition Mechanism Induced By Hydration Structure Of Dense Polymer Chains In Hydrogel

Cryopreservation is a technology that quick-freezes targets,such as biological materials(such as cells,tissues,and organs)at extremely low temperatures,stops metabolism,and prolongs the life cycle.Therefore,it has great application prospects in medicine,biology and other fields.The traditional cryopreservation method is to achieve cryopreservation of biological materials by adding salts,organic solvents and other substances to cells or tissues.However,the toxic effects carried by the additive itself can cause irreversible damage to cells,tissues,etc.Therefore,how to construct green,mild and efficient antifreeze materials is an important challenge facing the current cryopreservation technology.In extreme environments(extremely cold/heat),tardigrades can reduce the free water content in the body through their own volume contraction,and at the same time increase the proportion of bound water in the body to effectively inhibit the freezing of water in the body.This phenomenon provides a good research idea for the research and development of green,mild and efficient freezing of organisms at low temperature and antifreeze materials with good biocompatibility.Hydrogels are soft materials composed of a large number of water and hydrophilic three-dimensional(3D)polymer networks,which have excellent biocompatibility and are ideal candidates for the construction of cryopreservation biological tissues.The water molecules inside the hydrogel mainly exist in two forms: one is the free water molecules free in the gel network,referred to as free water;The other is water molecules bound to polymer chains,referred to as bound water.By constructing a pure hydrogel network model,regulating the type and quantity of hydrated structures of polymer segments,and exploring the regulation law and mechanism of free water and combined water ratio on the phase transition behavior of hydrogel in the hydrogel system,it is expected to realize the preparation of new green,mild and efficient antifreeze hydrogel materials.The specific research content is as follows:1.Based on the dense hydrogel network model,the behavior mechanism of polymer chain hydration structure induced by hydrogel phase transition was explored Taking acrylic monomer as an example,N,N-methylenebis-acrylamide was used as a crosslinking agent,and 2-hydroxy-2-methyl-1-[4-(2-hydroxyethoxy)phenyl]-1-propanone was used as a photoinitiator to prepare polyacrylic hydrogel.By adjusting the concentration of monomers,cross-linking agents and initiators,adjusting the ratio of bound water and free water in the hydrogel system,the phase transition process of materials was tested by differential scanning calorimetry.The test results show that the freezing point of polyacrylic hydrogel gradually decreases with the increase of acrylic monomer concentration,and the glass transition occurs when the monomer concentration reaches 7.5 mol/L,at this time,the antifreeze temperature of the hydrogel is-70 °C,and the glass transition temperature(Tg)of the gel gradually increases with the continuous increase of the monomer concentration.In addition,traditional hydrogel systems such as PAAm,PAMPS and PDMAA were also studied,and similar phase transition conversion processes were found.A series of experiments show that with the increase of the concentration of hydrogel monomer,the number of functional groups of polymer segments increases,the hydration capacity of polymer network is enhanced,free water gradually decreases,and bound water gradually increases,resulting in a gradual decrease in the freezing point of hydrogel.When the monomer concentration reaches a certain value,all the water inside the hydrogel will exist in the form of bound water,and the hydrogel will undergo glass transition.Studies have shown that PAMPS hydrogels can exhibit excellent frost resistance at 2 mol/L compared to other types of hydrogels,with a freezing point of-75°C.In summary,antifreeze hydrogel materials with excellent antifreeze performance and good biocompatibility were successfully prepared without the participation of solutes(salts,polar solvents,etc.).2.Preparation of low-temperature non-swelling antifreeze double-network hydrogel based on PAMPS-PAAm.Based on the fact that when actually applied in a low temperature environment,the water molecules in the environment will spontaneously penetrate into the hydrogel,resulting in an uncontrollable increase in the water content in the hydrogel and a decrease in the concentration of polymer chains,which damages the actual application effect of the material in the low temperature environment and greatly shortens the life.To solve this dilemma,we use PAMPS as a rigid first network and PAAm as a flexible second network.By regulating the monomer concentration and crosslinker content of the second network,controlling the swelling behavior of the hydrogel,and combining the phase transition transformation characteristics of the PAAm system in(1),a dual-network antifreeze hydrogel with excellent antifreeze performance(freezing point-35 °C)and excellent anti-swelling ability was obtained,which provided a new idea for the long-term continuous use of antifreeze hydrogel materials such as antifreeze hydrogel materials without the participation of solutes(salts,polar solvents,etc.)in a low temperature environment.In summary,by changing the hydration strength of the polymer segment,the ratio of free water and bound water in the hydrogel system can be effectively regulated,the freezing point of the hydrogel can be continuously reduced and eventually the glass transition phenomenon can occur,and the non-swelling antifreeze hydrogel strategy that can achieve long-term stable use at low temperature through the construction of the non-swelling dualnetwork hydrogel system provides a research basis and innovative ideas for the construction of high-efficiency antifreeze materials without the participation of solutes(salts,polar solvents,etc.).