Light and Temperature as Dual Stimuli Lead to Self-Assembly of Hyperbranched Azobenzene-Terminated Poly(N-isopropylacrylamide)

Abstract

Abstract:Hyperbranched poly(N-isopropylacrylamide)s (HBPNIPAMs) end-capped with different azobenzene chromophores (HBPNIPAM-Azo-OC 3 H 7 , HBPNIPAM-Azo-OCH 3 , HBPNIPAM-Azo, and HBPNIPAM-Azo-COOH) were successfully synthesized by atom transfer radical polymerization (ATRP) of N-isopropylacrylamide using different azobenzene-functional initiators. All HBPNIPAMs showed a similar highly branched structure, similar content of azobenzene chromophores, and similar absolute weight/average molecular weight. The different azobenzene structures at the end of the HBPNIPAMs exhibited reversible trans-cis-trans isomerization behavior under alternating UV and Vis irradiation, which lowered the critical solution temperature (LCST) due to different self-assembling behaviors. The spherical aggregates of HBPNIPAM-Azo-OC 3 H 7 and HBPNIPAM-Azo-OCH 3 containing hydrophobic para substituents either changed to bigger nanorods or increased in number, leading to a change in LCST of´2.0 and´1.0˝C, respectively, after UV irradiation. However, the unimolecular aggregates of HBPNIPAM-Azo were unchanged, while the unstable multimolecular particles of HBPNIPAM-Azo-COOH end-capped with strongly polar carboxyl groups partly dissociated to form a greater number of unimolecular aggregates and led to an LCST increase of 1.0˝C.

The Dependence of the Cloud Point, Clearing Point, and Hysteresis of Poly(N-isopropylacrylamide) on Experimental Conditions: The Need for Standardization of Thermoresponsive Transition Determinations

“…Thermally responsive polymers with either lower critical solution temperature (LCST) or upper critical solution temperature (UCST), due to coil‐to‐globule or globule‐to‐coil transitions, respectively, belong to a unique class of smart materials with broad application possibilities ranging from nanotechnologies to biomaterials, tissue engineering scaffolds, intelligent drug release assemblies, sensors, self‐healing structures, responsive hybrid materials, etc. (see, e.g., refs . and references therein).…”

“…A detailed literature evaluation shows that the CSTs, usually but incorrectly claimed as LCSTs, for aqueous solutions of PNIPAAm were reported even as low as 24 °C and as high as around 60–70 °C . As shown by a variety of typical examples in Table S1 (Supporting Information), the thermal transition temperatures of PNIPAAm homopolymers lie between these data, and even a quick look at this dataset induces significant uncertainties due to the reliability of the reported so‐called LCST values of PNIPAAm in the literature, and other thermally responsive polymers as well, on the one hand . On the other hand, it can also be seen in Table S1 (Supporting Information) that a vari...…”

“…Undoubtedly, poly( N ‐isopropylacrylamide) (PNIPAAm) has been the most investigated temperature‐responsive polymer since the first description of its LCST‐type behavior . This is mainly due to its biocompatibility and LCST near to the body temperature in the range of ≈32 °C as usually considered (see, e.g., refs . and references therein).…”

Synthesis and Self-Assembly of Multistimulus-Responsive Azobenzene-Containing Diblock Copolymer through RAFT Polymerization

“…It also means that the observed fluorescence enhancement upon UV irradiation (i.e., Figure 6) can be related to changes in sizes and types of morphological aggregates. cores of regular E-isomer with an ordered array transformed to the Z-isomer, which packed into the loose cores as the unordered array [54].…”

“…After UV irradiation for 10 min, there was an obvious increase in the average diameter to 405.4 nm. Moreover, the D h of the poly(NIPAM-b-Azo) after UV irradiation increased from 170-420 to 250-650 nm, indicating that the hydrophobic E-isomer of the azobenzene transformed into the strong polar Z-isomer under UV irradiation, and the tight cores of regular E-isomer with an ordered array transformed to the Z-isomer, which packed into the loose cores as the unordered array [54]. cores of regular E-isomer with an ordered array transformed to the Z-isomer, which packed into the loose cores as the unordered array [54].…”