pH-Dependent Morphology and Photoresponse of Azopyridine-Terminated Poly(N-isopropylacrylamide) Nanoparticles in Water

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Ren , H , Qiu , X-P , Shi , Y , Yang , P & Winnik , F M 2019 , ' pH-Dependent Morphology and Photoresponse of Azopyridine-Terminated Poly(N-isopropylacrylamide) Nanoparticles in Water ' , Macromolecules , vol. 52 , no. 8 , pp. 2939-2948 . https://doi.org/10.1021/acs.macromol.9b00193

Title: pH-Dependent Morphology and Photoresponse of Azopyridine-Terminated Poly(N-isopropylacrylamide) Nanoparticles in Water
Author: Ren, Hao; Qiu, Xing-Ping; Shi, Yan; Yang, Peng; Winnik, Francoise M.
Contributor: University of Helsinki, Polymers
Date: 2019-04-23
Language: eng
Number of pages: 10
Belongs to series: Macromolecules
ISSN: 0024-9297
URI: http://hdl.handle.net/10138/314247
Abstract: A series of azopyridine-terminated poly(N-isopropylacrylamide)s (PNIPAM) (C12-PN-AzPy) (similar to 5000 <M-w <20 000 g mol(-1), polydispersity index 1.25 or less) were prepared by reversible addition fragmentation chain-transfer polymerization of NIPAM in the presence of a chain-transfer agent that contains an AzPy group and an n-dodecyl chain. In cold water, the polymers form nanoparticles (5.9 nm <R-h <10.9 nm) that were characterized by light scattering (LS), H-1 NMR diffusion experiments, and high-resolution transmission electron microscopy. We monitored the pH-dependent photoisomerization of C12-PN-AzPy nanoparticles by steady-state and time-resolved UV-vis absorption spectroscopy. Azopyridine is known to undergo a very fast cis-to-trans thermal relaxation when the azopyridine nitrogen is quaternized or bound to a hydrogen bond donor. The cis-to-trans thermal relaxation of the AzPy chromophore in an acidic nanoparticle suspension is very fast with a half-life tau = 2.3 ms at pH 3.0. It slows down slightly for nanoparticles in neutral water (tau = 0.96 s, pH 7.0), and it is very slow for AzPy-PNIPAM particles in alkaline medium (tau > 3600 s, pH 10). The pH-dependent dynamics of the cis-to-trans dark relaxation, supported by Fourier transform infrared spectroscopy, H-1 NMR spectroscopy, and LS analysis, suggest that in acidic medium, the nanoparticles consist of a core of assembled C12 chains surrounded by a shell of hydrated PNIPAM chains with the AzPy(+) end groups preferentially located near the particle/water interface. In neutral medium, the shell surrounding the core contains AzPy groups H-bonded to the amide hydrogen of the PNIPAM chain repeat units. At pH 10.0, the amide hydrogen binds preferentially to the hydroxide anions. The AzPy groups reside preferentially in the vicinity of the C12 core of the nanoparticles. The morphology of the nanoparticles results from the competition between the segregation of the hydrophobic and hydrophilic components and weak attractive interactions, such as H-bonds between the AzPy groups and the amide hydrogen of the PNIPAM repeat units.
Subject: SUPRAMOLECULAR POLYMERIC MATERIALS
HYDROGEN-BOND
LIGHT-SCATTERING
BLOCK-COPOLYMER
AMIDE HYDROGEN
CHAIN
VESICLES
BEHAVIOR
FABRICATION
ACIDS
116 Chemical sciences
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