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. 2018 Aug 30;3(8):10172-10179.
doi: 10.1021/acsomega.8b01308. eCollection 2018 Aug 31.

Soluble-Insoluble-Soluble Transitions of Thermoresponsive Cryptand-Containing Graft Copolymers

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Free PMC article

Soluble-Insoluble-Soluble Transitions of Thermoresponsive Cryptand-Containing Graft Copolymers

Ming Zhang et al. ACS Omega. .
Free PMC article

Abstract

Cryptand-containing alternative copolymers were first made from copolymerization of styrenic derivatives and maleic anhydride and then chemically modified in this work by grafting methoxy poly(ethylene glycol) (MPEG) onto the maleic functional groups. These graft copolymers show interesting multistep soluble-insoluble-soluble (S-I-S) transitions in acidic aqueous media at a cloud point (T cp) and a subsequent mixing temperature (T mix). Turbidity measurements and dynamic light scattering studies indicate that such complex transitions may be attributed to the entropic contribution associated with the dehydration and aggregation of the MPEG groups and then the enthalpic contribution associated with the hydrogen bonding between ethylene glycol and carboxylic acid groups. More importantly, the phase transition temperatures and insoluble temperature ranges are very sensitive to changes in subtle hydrophobic-hydrophilic balance of the copolymers, such as the variation of pH, the cryptand size, and the length of the MPEG graft. The understanding of the S-I-S transition in relation to the structure of the copolymers and the external conditions may be useful in the design of smart materials and sensors.

Conflict of interest statement

The authors declare no competing financial interest.

Figures

Scheme 1
Scheme 1. Tunable Hydrophobic–Hydrophilic Balance of Amphiphilic Cryptand-Containing Graft Copolymers
PEGm indicates PEG blocks on the cryptand, and MPEG indicates the grafted PEG chains on the maleic acid groups.
Figure 1
Figure 1
(A) Temperature dependence of the transmittance at 500 nm (heating–cooling cycle) and the hydrodynamic diameters (Dh) measured by dynamic light scattering (DLS) experiments of PDStPEG0MA10-g-MPEG350 at 0.5 mg/mL at pH 3.0. (B) Temperature dependence of the transmittance at 500 nm of PDStPEG0MA10-g-MPEG750 at various pH values. The Tcps are higher and less pronounced at higher pH, when the COOH groups become more deprotonated.
Figure 2
Figure 2
(A) Temperature dependence of the transmittance of PDStPEG0MA10-g-MPEG350 at different pH values. (B) Variation of Tcp and Tmix of PDStPEG0MA10-g-MPEG350 as a function of pH.
Figure 3
Figure 3
Temperature dependence of the transmittance (0.5 °C/min) and the hydrodynamic diameters (Dh) measured by DLS at pH 3.0. (A) PDStPEG1MA10-g-MPEG350, (B) PDStPEG2MA10-g-MPEG350, and (C) PDStPEG3MA10-g-MPEG350.
Figure 4
Figure 4
TEM images of the aggregates formed by aqueous solution of PDStPEG1MA10-g-MPEG350 at different temperatures: (A) 25 °C, (B) 50 °C, and (C) 80 °C.
Scheme 2
Scheme 2. Schematic Illustration of the Aggregation Process of PDStPEGmMA10-g-MPEG350

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