Improving the Frost Resistance Performance of Hydrophilic Coatings by Modulating the Formation of Zwitterionic Polymers

In the past few years, less attention has been paid to hydrophilic materials in frost resistance studies due to the fact that the hydrated layer may not be able to resist frost formation under cold conditions. Zwitterionic polymers can tightly interact with water molecules through hydrogen bonding and electrostatic interactions, forming a hydrated layer that is stable at low temperatures. In this work, polyethyleneimine (PEI) and 1,3-propanesultone (PS) constitute the basis of a zwitterionic polymer, on which different catechol derivatives such as protocatechualdehyde (PA), protocatechuic acid (PCA), and 4-(chloroacetyl)­catechol (CAC) are grafted. These coatings exhibit hydrophilicity with a water contact angle below 10°, where mussel mimetic catechol derivatives are used as anchor points to deposit the coating on the substrate. In addition, the relationship between the molecular structure and frost resistance capability was investigated by changing the type of catechol derivative, as well as revealing the frost resistance mechanism of the hydrophilic coating. In particular, the PEI-CAC-PS coating has strong hydrogen bonding due to the formation of a five-membered ring with amino and carbonyl groups attached to the same carbon, which, together with the electrostatic interaction, gives the coating a high content of nonfreezable water and thus excellent frost resistance. In addition to frost resistance properties, the hydrophilic zwitterionic polymer coating is multifunctional with antifogging and self-cleaning capabilities.