Thermostable physically crosslinked cryogel from carboxymethylated konjac glucomannan fabricated by freeze-thawing

Physically crosslinked cryogels of carboxymethylated konjac glucomannan (CMKGM) with different molecular weights (Mw, 1.57 × 105–7.33 × 105 Da) and degrees of substitution (DS, 0.19–1.63) were fabricated by using the freeze-thaw technique. The acidification of CMKGM solution at a sufficiently low pH of 1.0 is a prerequisite to produce a cryogel. Both the Mw and DS of CMKGM significantly affected the formation and mechanical strength of the cryogels. A high DS value cannot ensure the formation of the cryogel when the Mw of CMKGM is not sufficiently high. While the cryogel from CMKGM with the highest Mw of 7.33 × 105 Da and lowest DS of 0.19 exhibited the highest gel strength, cryogels could not be effectively formed for CMKGM with a DS beyond 1 and Mw below 2.24 × 105 Da. Furthermore, the gel strength could be enhanced by increasing polysaccharide concentration, freezing time, and the number of freeze-thaw cycles or thawing at a moderate temperature (25 °C). It was demonstrated that the intermolecular multiple hydrogen bonding was the major driving force in the formation of CMKGM cryogels with negligible, if any, participation of crystalline ordered structures. Detailed hydrogen bonding patterns in the gel network were proposed. In acidic cryogels, there existed three types of hydrogen bonds formed by –COOH and –OH groups, whereas in neutral cryogels, two kinds of hydrogen bonding among -COO- and –OH groups were involved. The intermolecular multiple hydrogen bonding endows the physical hydrogels with excellent thermostability.