Dual Thermo- and Photo-Responsive Micelles Based on Azobenzene-Containing Random Copolymer

Abstract

Amphiphilic random copolymer poly(methacrylamido-azobenzene)-ran-poly(2-hydroxyethylacrylate) (PMAAAB-ran-PHEA) was synthesized via hydrolysis of poly(methacrylamido-azobenzene)-ran-poly[2-((2'-tetrahydropyranyl)oxy)ethylacrylate] (PMAAAB-ran-P(THP-HEA)), which was prepared by conventional radical polymerization. PMAAAB-ran-PHEA micelles were then prepared via dialysis method against water with DMF as solvent. The structure, morphology, size, and low critical solution temperature (LCST) of PMAAAB-ran-PHEA and its micelles were determined by ¹H-NMR, GPC, TEM, and DLS. The thermo- and photo-responsive behaviors of the resulting polymer micelles were investigated with Nile red as a fluorescence probe. The results showed that PMAAAB-ran-PHEA micelles were porous or bowl-shaped and its size was 135-150 nm, and its LCST was 55 °C when F_MAAAB of the random copolymer was 0.5351; the hydrophobicity of the micellar core was changed reversibly under the irradiation of UV light and visible light without release of Nile red or disruption of micelles; the size and solubilization capacity of the micelles were dependent on temperature, and Nile red would migrate for many times between the water phase and the micelles, and finally increasingly accumulated during the repeated heating and cooling processes.

Keywords: 2-hydroxyethylacrylate; Nile red; amphiphilic random copolymer; methacrylamido-azobenzene; micelles; photo-/thermo-responsiveness.

Scheme 1

Syntheses of PMAAAB-ran-P(THP-HEA) and PMAAAB-ran-PHEA.

Figure 1

GPC trace of PMAAAB-ran-P(THP-HEA) (F_MAAAB = 0.5351).

Figure 2

¹H-NMR spectrum of PMAAAB-ran-P(THP-HEA) (F_MAAAB = 0.5351).

Figure 3

¹H-NMR spectra of PMAAAB-ran-P(THP-HEA) copolymers.

Figure 4

¹H-NMR spectra of PMAAAB-ran-P(THP-HEA) (B) and PMAAAB-ran-PHEA (A).

Figure 5

Size distribution of polymericmicelles (S1 to S5 samples, 0.02 gL⁻¹).

Figure 6

TEM images of polymeric micelles S3, S4 and S5 (scale bar: 200 nm, 0.02 gL⁻¹).

Figure 7

Temperature-dependence of the R_h of polymeric micelles S3.

Figure 8

TEM images of Nile red-unloaded (S3) and Nile red-loaded (NR-S3) micelles (scale bar: 200 nm).

Figure 9

UV-vis spectra (a) and fluorescence spectra (b) of S3 and NR-S3 micellar solution.

Figure 10

Fluorescence spectra of NR-S3 micellar solution under the irradiation of UV light (365 nm, 200 μWcm⁻²) for 90 min (a) and the irradiation ofvisible light for 80 min (b).

Figure 11

Fluorescence spectra of NR-S3 micellar solution under alternating UV and visible light irradiation to the photostationary state four times (a) and evolution of their maximum fluorescence intensity (b).

Figure 12

Fluorescence spectra of NR-S3 micellar solution at the heating process from 25 to 60 °C (a) and the subsequent cooling process from 60 to 25 °C (b).

Figure 13

Fluorescence spectra (a) and maximum value ((b), at 645 nm) of NR-S3 micellar solution at 25 °C through the repeated heating and cooling processes for four times.