| With the increasing serious shortage of resources,natural polymers have attracted wide attention due to their unique properties,such as rich abundance,environmentally-friendly nature,tunable performances,and so on.In the natural polymers solution,the gel which has a stable three-dimensional network space structure is formed by the weak intermolecular interaction.It is a typical soft substance with high swelling ratio,biocompatibility and good physical and chemical properties.Therefore,it has been widely applied in agriculture,industry,food processing,biological medicine and so on,creating great economic and social value.Similar to temperature and composition constitute,pressure,as an important extreme condition,is also an independent parameter in scientific research.The study on the structure and properties of soft materials including natural polymer under high pressure has attracted the close attention of many researchers.Due to the thermodynamic equivalence,the effect of pressure on the gelation behavior could be considered similar to that of temperature.During the formation process of natural polymer aggregates,it is crucial to obtain the pressure-induced impacts on molecular interaction between natural polymer molecules,phase transformation and kinetic information.In addition,microstructure is also a deep factor that influences the property of gel material.However,until now,there are few reports on the effect of pressure on the self-aggregation of natural polymer solution,especially in situ studies on the phase transition process under high pressure.Viscosity is a significant parameter for sol-gel transition.In this paper,we designed two sets of devices for measuring the viscosity of liquid at high pressure.Furthermore,we studied the phase transitions of some representative natural polymer hydrogels by using these devices,for example,methylcellulose and agarose.The primary content and conclusion could be divided into several parts:1.Based on the relation between emission intensity of fluorescent molecular probe and its microviscosity,we designed a set of microviscosity measuring device.It can be used under the pressure from 0.1 to 500 MPa with temperature from-30 to120℃.The accuracy of pressure and temperature is 1 MPa and 0.1℃,respectively.The fluorescence spectrum of ethanol and methylcellulose aqueous solution with/without fluorescent probe were investigated by using this device.The influences of both pressure and temperature were analyzed in detail.In order to study gelation of natural polymer aqueous solution on higher pressure,we designed another high-pressure viscosity measurement device,which consist of diamond anvil cell(DAC),microscope and CCD.Based on the Stocks theory,the viscosity of aqueous solution was measured by utilizing a falling-ball technique.Currently,the home-made device can work in wide ranges of both pressure(~10GPa)and temperature(~400℃).The viscosity of glycerine was measured under different pressures and temperatures.The result was in good agreement with the literature data,exhibiting a desired outcome of the designed device.2.The heat-and pressure-induced gelation of methylcellulose(MC)aqueous solution was in situ studied for the first time based on dynamic viscoelastic and fluorescence measurements.It was indicated that the microviscosity showed a dramatic change around the phase transition point.Based on the experimental data,the T vs.P phase diagram of methylcellulose aqueous solution was established,and it indicated that the melting point was an increasing function of pressure.At the same time,the pressure-induced gelation of methyl cellulose aqueous solution was observed by microscope.It could be clearly observed that gel phase of the sample gradually disappeared,while an opposite scenario of formation for sol phase was shown at the same time.Finally,the fluorescence results during decompression showed that the phase transition of MC solution was reversible.3.Gelation of MC aqueous solution was investigated using a high-pressure viscosity measurement device for the first time.The results showed that sol-gel thermal transition of 1 wt% MC solution occurred at 53℃ under atmospheric pressure.Upon compression,the viscosity showed a dramatic change with the pressure up to 478 MPa at room temperature(22℃),and the gelation process was completed.Parabolic phase diagram of MC aqueous solution was established,and it showed that its melting point was increasing/decreasing functions of pressure at the onset and final stages,respectively.The mechanism of sol-gel transformation of MC aqueous solution was also discussed.It might be assumed that both hydrogen and hydrophobic bonds were involved with the gel formation of MC aqueous solution.For thermal induced phase transition,the intermolecular hydrophobic methyl groups play the leading role;while,for pressure induced phase transition,the contribution of hydrogen bonding is greater than that of hydrophobic bonds.4.Sol-gel transition behavior of agarose aqueous solution was investigated by using rheology and fluorescence lifetime measurement.During the heating process,the storage modulus G′ gradually decreased,then deviated abruptly at the temperature around 65℃,and finally decreased slowly again.Results of fluorescence measurement showed that the phase transition point was kept at 65℃,independent of agarose aqueous solution concentration.This result is in agreement with the experimental results of rheology measurement.Results of high pressure experiment indicated that the fluorescence lifetime of the probe in the agarose aqueous solution showed a dramatic change in the vicinity of the phase transition point.Then,T vs.P phase diagram of agarose aqueous solution was constructed,which showed that the melting point was an increasing function of pressure.Based on the phase diagram,the agarose gels were prepared by cooling under atmospheric pressure and 300 MPa,respectively.From the result of the recovered samples studied by optical rheometry,it was found that agarose gel prepared under high pressure had a higher elasticity and lower viscosity index,compared with the sample prepared at atmospheric pressure.It could be speculated that such kinds of properties might be attributed to the smaller pore size during gelation under high pressure.The method of pressure-induced gelation provides a new idea for the preparation of highly elastic gel. |
