Dual Thermoresponsive Aggregation of Schizophrenic PDMAEMA-b-PSBMA Copolymer with an Unrepeatable pH Response and a Recycled CO2/N2 Response.

Abstract

A dual thermoresponsive block copolymer of poly[2-(dimethylamino)ethyl methacrylate]-block-poly(sulfobetaine methacrylate) (PDMAEMA-b-PSBMA) exhibited reversible schizophrenic aggregation behavior in water because of the upper critical solution temperature (UCST) of the PSBMA block and the lower critical solution temperature (LCST) of the PDMAEMA block. Both the UCST and LCST shifted to lower values with increasing DMAEMA/SBMA block ratios, which was ascribed to the hydrophobic/hydrophilic balance of both blocks. Because of the salt-sensitive PSBMA and pH-responsive PDMAEMA, the UCST and LCST values of PDMAEMA-b-PSBMA were codetermined by varying the salt concentrations and pH. Specifically, increasing the salt concentration resulted in a notable decrease in the UCST and a slight increase in the LCST due to the salt-induced screening of the electrostatic attractions of the PSBMA and salt-enhanced solubility of the PSBMA blocks, respectively. The LCST decreased with increasing pH because of the deprotonation of PDMAEMA, and the UCST first increased and then decreased with increasing pH. Besides, the copolymer with larger PDMAEMA content was more sensitive to pH. For the repetitive adjustment to thermoresponsive aggregation, repeated addition of acids and bases induced salt accumulation and diminished the switchability of pH, whereas repeated switching cycles were achieved by CO2/N2 bubbling without introducing salt enrichment. The difference in HCl/NaOH titration and CO2/N2 bubbling also existed in the switching cycles when PDMAEMA-b-PSBMA served as a stimulus-responsive emulsifier.