Preparation And Properties Of Cassava Dialdehyde Starch And Their Hydrogels

Dialdehyde starch is a modified starch which is prepared by the oxidation of native starch with sodium periodate.It has great value in biomedicine and packing areas for its low toxicity,good biodegradability and cross-linking ability.Cassava is a kind of non-food energy crops in China.Abundant research has been done on the preparation of dialdehyde starch from cassava starch,however there are few studies on the structure and properties of cassava dialdehyde starch,especially for the high aldehyde content cassava dialdehyde starch.In this paper,dialdehyde starch with different aldehyde content was prepared by using NaIO₄ and cassava starch.Relationships between the content of the aldehyde group and molecular weight distribution,crystallinity,gelatinization and solubility of dialdehyde starch was studied.Hydrogels with self-healing property and pH sensitivity were prepared by using dialdehyde cassava starch and bis?benzenamine?functionalized polyethylene glycol.The optimum conditions for the reaction of NaIO₄ and cassava starch were 40?,2 h and pH 2-4.Dialdehyde starch?DAS5-DAS85?with aldehyde content of 5%to 85%was prepared by using different dosages of NaIO₄.The weight-average molecular weight?M_w?of cassava starch was more than 10 million,and M_w of DAS5,DAS10 and DAS19 are 9300,4300 and2960,respectively.With further increasing aldehyde contents,M_w of dialdehyde starch didn't change evidently.The oxidation reaction occurred both in the amorphous and crystalline regions and destroyed the crystalline regions gradually.When the dialdehyde content was more than 39%,starch granules became amorphous.The surfaces of dialdehyde starch granules was rough,and the whole granules collapsed.Gelatinization temperature increased,and the gelatinization enthalpy decreased with increasing dialdehyde contents.Precipitation formed in DAS85 solution with 10%mass concentration after 6 h and occurred in DAS53 solution after12 h at 25?.DAS39 solution remained transparent after 48 h at 25?.DAS85 solution was prone to precipitate when the mass concentration was higher than 5%,and it kept clear and transparent when the mass concentration was below 3%.Three kinds of polyethylene glycol bis?p-aminophenyl?ethers?PEGBA-2K,PEGBA-6K,PEGBA-20K?were prepared by etherification of polyethylene glycol 2000,6000 and 20000with p-nitrochlorobenzene followed by the reduction reaction using Pd/C and HCOONH₄.The mixtures of DAS85 and PEGBA solution gelled spontaneously.When the total mass concentration of PEGBA and DAS85 was 10%,three hydrogels formed by DAS85 and PEGBA-2K,PEGBA-6K,PEGBA-20K showed highest strength when the molar ratio of aldehyde groups to amino groups were 3:1,7:1,10:1,respectively.The storage modulus of the three hydrogels were 19 kPa,26.4 kPa,and 6 kPa.Hydrogels formed by DAS85 and PEGBA had a self-healing ability.The compression fracture-healing experiment showed that when the total mass concentration of DAS85 and PEGBA-6K was 5%and the molar ratios of aldehyde groups to amino groups were 3:1,5:1,7:1,10:1,DAS85-PEGBA-6K hydrogels had rupture strength of 40 kPa,170 kPa,226 kPa,64 kPa,respectively.The compression strength of hydrogels was about 5 kPa after being cut from the middle and healing for 5 minutes.After healing for 12 h at 25?,the four hydrogels retained 45%,16%,12%,and 32%of their initial rupture strength,respectively.After healing at 37? for 12 h,the fracture strength recovery rates of hydrogels were 91%,22%,12%,and 33%.Hydrogels formed by DAS85 and PEGBA were stable in solutions of pH 3 to 9,and a reversible transformation of"gel-sol"appeared under neutral and strong acidic conditions.Scanning electron microscopy showed that hydrogels formed by DAS85 and PEGBA showed a porous structure after freeze-dried.