Abstract
A series of azopyridine-terminated poly(N-isopropylacrylamide)s (PNIPAM) (C12-PN-AzPy) (∼5000 < Mw < 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 < Rh < 10.9 nm) that were characterized by light scattering (LS), 1H 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 τ = 2.3 ms at pH 3.0. It slows down slightly for nanoparticles in neutral water (τ = 0.96 s, pH 7.0), and it is very slow for AzPy-PNIPAM particles in alkaline medium (τ > 3600 s, pH 10). The pH-dependent dynamics of the cis-to-trans dark relaxation, supported by Fourier transform infrared spectroscopy, 1H 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.
Highlights
Among the various stimuli for responsive polymer-based devices, light possesses several advantages: it is directional, tunable in terms of energy, and it can be turned on and off rapidly.[1−3] Azobenzene, which undergoes reversible trans-tocis photoisomerization, is commonly used for such applications
The polymers were prepared by reversible addition− fragmentation chain-transfer (RAFT) polymerization of NIPAM in dioxane using the chain-transfer agents CTA-AzPy or CTA-Azo, the latter CTA leading to azobenzene-modified PNIPAM used in control experiments described below (Scheme 1)
It features a triplet at 0.88 ppm, ascribed to the resonance of the terminal methyl protons of the dodecyl chain and signals in the aromatic spectral region ascribed to the azopyridine protons: signals a and b are attributed to the protons of the pyridine ring while the signals c and d correspond to the protons of the aromatic ring connected to the polymer by an ether bond
Summary
Among the various stimuli for responsive polymer-based devices, light possesses several advantages: it is directional, tunable in terms of energy, and it can be turned on and off rapidly.[1−3] Azobenzene, which undergoes reversible trans-tocis photoisomerization, is commonly used for such applications. Synthetic Pathways to α-Azobenzene-, α-Azopyridine-, and α-Ethyl-azopyridinium-ω-n-dodecyl-PNIPAMs assemblies, such as liquid crystals,[9−11] metal−organic frameworks,[12,13] fibers,[14] films,[15,16] and gels.[17−19] It turns out that binding of the AzPy nitrogen to common H-bond donors, such as phenols, accelerates the cis-to-trans thermal relaxation rate of neutral AzPy because of the redistribution of the AzPy πelectron imposed by the H-bond formation This effect was exploited recently by Gelebart et al who succeeded in generating continuous, macroscopic mechanical waves by continuous irradiation of films containing AzPy H-bonded to benzoic acid moieties.[16]. This result is of interest in the context of polymer self-assembly and from the practical view point as an entry to fast responsive light-driven systems
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