Synthetic Design And Characterization Of Chitosan Aerogels

Chitosan aerogels own the characteristics of abundant raw material resources,low environmental load,and excellent thermal insulation property,but there are several problems such as low gel activity,lacking of three dimensional pore structure,difficult control of microstructure,huge shrinkage of aerogles mentioned before and after supercritical fluid drying,these drawbacks limit their application potential in the field of thermal insulation.To address these issues,a strategy to improve the sol solvent system of chitosan was proposed,and a kind of original ethanol/water binary solvent system was adopted as the solvent of chitosan so that to solve the problem of no gel formation or difficult to form gel in terms of chitosan sol in the condition of ultra-low substrate concentration,ultimately to prepare a type of chitosan aerogels with superior three dimensional pore structure,high specific surface area and strong adsorption capacity.On this basis,chitosan chains by selective oxidation reactions was introduced to together implement the formation course of chitosan gel network,so the chitosan aerogels with controllable morphological textures were systhesized.The construction of chitosan bybrid aerogels with network structure was achieved for the first time by the rigid crosslinker of OPA?o-phthalaldehyde?and by nanoscale-compositing linear polymer PVA,finally chitosan aerogels with moderate compression performance,excellent thermal insulation property and good thermal stability were prepared,hence the traditional problem for the uncontrollable shrinkage of chitosan aerogels before and after supercritical fluid drying was conquered perfectly.The main conclusions obtained are presented as follows:?1?The affecting evolutions of water and ethanol/water binary solvent system on chitosan sol-gel was studied,the mechanism for sol-gel formation driven by ethanol/water binary solvent system was also proposed.The optimum ratio of ethanol/water binary solvent system as solvent was 1.5 v/v.Besides,after experiencing the same gelling and aging processes,the w-CG5 series of chitosan sol with the concentration of 5 g/L in water solvent happened almost no any gel,while the ew-CG5series with the concentration of 5 g/L in ethanol/water binary solvent all formed gels,suggesting the remarkable effect for promoting the sol-gel process of chitosan in the solvent above,and it is an influential choice for the enhancement of chitosan gelling sheletons formation.The reasons for the mechanism concerning the gelling promotion via ethanol/water binary solvent were foucused on the difference of the miscibility between chitosan and formaldehyde in binary solvent system,then resulting in microscale active particles to enhance the activity of crosslinking reaction,eventually improving the sol-gel process of chitosan and greatly increasing the gel probability of chitosan sol.?2?To obtain chitosan aerogels with high surface area and uniform pore structure,the inflences of formaldehyde dosage on the morphological structure,specific surface area,pore size distribution and thermal stability of chitosan aerogels were investigated in the condition of ethanol/water binary solvent,and the mechanism of constructing chitosan aerogels was also revealed.With the increase of formaldehyde dosage,the textural structure of chitosan aerogels presented a loose structure at first and then a relative dense strucrure,the specific surface area decreased firstly and then increased finally decreaed,the average pore size increased firstly and then decreased,the pore size distribution was concentrated between 15-120 nm,and the upper limit temperature of thermal stability was focused upon 130-175°C range.When the concentration of formaldehyde was 6 wt.%,the specific surface area of chitosan aerogel was up to 973m²/g,which was higher than 78.53%of the highest value of chitosan aerogels for thermal insulation reported.The adsorption experiments showed that the original MO solution of 40 mg/L could be recovered as nearly colorless solution,which removal rate for MO was 99.51%.The key to construct chitosan aerogel network was that the crosslinking reactions between the aldehyde groups from formaldehyde and the carbonyl amine groups from chitosan happened to form chemical crosslinking bonds such as C-N-C and N=C.?3?By introducing chitosan chains endowed with aldehyde groups and/or carboxyl groups,chitosan aerogels with controllable morphological and texture were prepared.The synthesis mechanism of chitosan aerogels with specific morphological texture which were guided by chitosan derivatives obtained from selective oxidation treatments was proposed.The microstructure of chitosan aerogels prepared by introducing chitosan oxidized with SPD?sodium periodate?possessed nanoflake-like crosslinked network,and the morphological texture of nanofiber-like crosslinked network was obtained by introducing chitosan oxidized with APS?ammonium persulfate?and SPD.The reasons why chitosan aerogels had different morphological textures were listed below:1)When the SPD-oxidized chitosan was introduced,so there were ring-opening aldehydes in chitosan chains.With carbonyl ammonia reaction performing,the networking skeleton of chitosan could grow along two or even three directions at the same time,thus tending to form nanoflake-like units and finally forming nanoflake-like networking structures.2)When the APS-SPD-oxidized chitosan was introduced,owing to the presence of carboxylate ions in the formation of chitosan network structure,and the networking skeleton of chitosan aerogels was more favorable to form nanofiber-like structures by the driving force of electrostatic,hydrogen bonding and steric hindrance.?4?Under the condition of using rigid OPA as crosslinker,it is the first time to present linear polymer PVA to nanoscale-composite and strengthen the networking skeleton of chitosan aerogels,greatly restraining the shrinkage of aerogels mentioned before and after supercritical fluid drying.By means of physical entanglement,supramolecular and chemical crosslinking interaction,to successfully prepare chitosan hybrid aerogels with networking skeleton enhacement at a molecular level.Comparing with chitosan aerogels with the density higher than 80.4%for that of chitosan hybrid aerogels,the shrinkage inhibition effect of chitosan hybrid aerogels(0.097 g cm^-3)along the radial direction was still superior to that(0.175 g cm^-3)reported in the literature of38.2%.At the comparative density,the shrinkage of reported chitosan aerogels(0.103 g cm^-3)along radial direction was up to 38%,while that of chitosan hybrid aerogels was less than 20%,and the decline of shrinkage inhibition was alomost 50%.The OPA crosslinker and linear polymer PVA played important roles along axial and radial direction in restraining shrinkage,respectively.?5?The compressive propery,thermal insulation property and thermal stability of chitosan hybrid aerogels were tested.The mechanism for constructing networking structure of chitosan hybrid aerogels was proposed.The general trend of compressive stress-strain curves of chitosan hybrid aerogels along axial and radial directions compared to that of non-brittle porous polymer foams.Thermal conductivity of chitosan hybrid aerogels with the density of 0.097 g cm^-3 was 0.0271 W m^-1 K^-1,very comparative to 0.0262 W m^-1K^-1of stationary air,and the maximum use temperature of chitosan hybrid aerogels reached 225°C.In the condition of condensation reaction,hydrogen bonding,and physical chain entanglement between chitosan chains or already crosslinked chitosan chains and linear polymer PVA,the formation for double networking structure chains was the key to construct chitosan hybrid aerogels.