Mechanicochemical conversion and block copolymerization by freezing of starch solutions

Abstract

AbstractDuring the freezing (cryolysis) of aqueous solutions or suspensions of high molecular compounds degradation of the macromolecules is observed due to the pressures obtaining in the phase transition and in the growth of ice crystals. As a result of the cryolysis active degradation products are formed which are susceptible to further chemical conversion. Products of the cryolytic degradation of starch formed during single and repeated freezing of its solutions (from t = −4° to −195°C.) with subsequent thawing (to −3 to −4°C.) are characterized by enhanced reducing properties, increased iodine numbers, decreased ηsp values, the appearance of a water‐soluble fraction, a shift in maximum absorption of the iodine complex to shorter wave lengths (from 580–600 mμ to 540–560 mμ) and a twofold increase in hydrolysis rate under the action of mineral acids. This is explained by degradation of the macromolecules and decrease in degree of polydispersity during the repeated freezing and thawing of the solutions or of the moist starch. Prolonged storage of the thawed starch specimens in argon and in air at +20°C., as well as short heat treatment at +37–75°C. (5–30 min., respectively) leads to decrease in the reducing capacity, increase in ηsp and to a shift in the absorption maximum of the starch‐iodine complex in the longer wave length direction. On adding free radical acceptors to the starch solution no changes in the characteristics are observed. This gives grounds to the assumption that the relations are due to the chemical conversions of macroradicals. Heating the cryolytic degradation products for longer periods in air causes intensification of the hydrolysis. Such hydrolysis does not take place in an atmosphere of the inert gas. Heating under these conditions solutions of starch that had not been subjected to freezing does not cause any changes in the product. The increased reactivity of starch cryolysis products in air is evidently the result of the formation of labile peroxide bonds in the starch macromolecule. The degradation of the initial soluble starch by dilute (0.15–0.25%) solutions of its cryolyzate has been experimentally established. In order to elucidate the effect of mechanical rupture of the chains on enzymatic activity experiments were carried out on determination of the activity of the cryolytic degradation products of amylase and pepsin. It has been found that under suitable conditions of freezing and thawing sharp changes in the enzymatic activity during storage are brought about. In contrast with the enzymes not subjected to this action the activity of the cryolyzed enzymes increases and in a number of cases exceeds that of the original preparations. The radical concept of degradation of starch and other polymers gave grounds for carrying out the block copolymerization of mixtures of polymers in solutions or in the presence of moisture by the method of repeated freezing and thawing in an inert gas atmosphere. The formation of block copolymers has been demonstrated on the example of the freezing of finely dispersed polystyrene emulsions as well as styrene monomer in starch solution with varying ratios of the components. A study of the resultant products showed that under the chosen conditions block copolymers of nonpolar polystyrene blocks and polar starch blocks are formed.