Intrinsic thermodynamic properties (e.g., transition temperature and enthalpy) of kappa- and iota-sequences in these copolymers are close to those of their parent homopolymers. With respect to the ordering capability, the kappa/iota-hybrid carrageenans seem to behave as random block copolymers with length sequence distributions truncated from the side of the small lengths. High-sensitivity DSC showed that the total transition enthalpy of the kappa/iota-hybrids goes through a minimum at 60 mol-% kappa-units, whereas for the mixture of kappa- and iota-carrageenan, the total transition enthalpy is a linear function of the composition. The conformational order-disorder transition of the kappa/iota-hybrids was studied using optical rotation and high-sensitivity differential scanning calorimetry. This allows one to control the gel strength independent of the gelation or melting temperature. On the other hand, the gelation temperature of the kappa-rich kappa/iota-hybrids is independent of their composition. The gel strength (storage modulus, G') of the kappa/iota-hybrids decreases with decreasing kappa-content. Their rheological properties were determined by small deformation oscillatory rheology. The kappa/iota-ratio of these kappa/iota-hybrids was determined by NMR spectroscopy. To this end, a series of kappa/iota-hybrid carrageenans ranging from almost homopolymeric kappa-carrageenan (98 mol-% kappa-units) to almost homopolymeric-carrageenan (99 mol-% iota-units) have been extracted from selected species of marine red algae (Rhodophyta). This paper describes the effect of the kappa/iota-ratio on the physical properties of kappa/iota-hybrid carrageenans (synonyms: kappa-2, kappa-2, weak kappa, weak gelling kappa).
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