l-Methionine sulfoxide and l-alanine copolymers for ice control†

Abstract

Amphiphilic copolymers are reported to show excellent ice control activity for their special interaction with ice and water. In this work, amphiphilic L-methionine sulfoxide and L-alanine copolymers (MSACPs) with different block ratios (PMet^o₄₈-b-PAla_y), different molecular weights and random (P(Met^o₄₈-co-Ala₂₀)) structures are prepared, respectively, to study the structure and ice control activity relationship. The ice control activity is well studied using ice nucleation, dynamic ice shaping (DIS) and ice recrystallization inhibition (IRI). The MSACPs are self-assembled into micelles of 11 to 80 nm in aqueous solution, which promotes the heterogeneous ice nucleation. Among block copolymers with different block ratios (PMet^o₄₈-b-PAla_y) and molecular weights, PMet^o₄₈-b-PAla₂₀ with an appropriate block ratio and molecular weight shows the highest nucleation temperature (−18.2 °C), and excellent DIS and best IRI (the mean largest grain size is 34%) activities. And, for the block copolymers with a similar block ratio to that of PMet^o₄₈-b-PAla₂₀, the DIS and IRI are closely related to the molecular weight as the ice control property is enhanced at high molecular weight. Compared with PMet^o₄₈-b-PAla₂₀, the corresponding random copolymer, P(Met^o₄₈-co-Ala₂₀), shows weaker ice control activity. Meanwhile, low-field nuclear magnetic resonance results show that the polymer–water interaction increases with hydrophobic block ratio, but has less effect on the molecular weight. The ice control activity is probably related to the balance between the polymer–water interaction and the polymer–ice interaction, and is optimized by both strong interactions.