Synthesis and solution behavior of triple stimuli-responsive micellar and hybrid janus nanoparticles

Abstract

Inspired by nature, stimuli-responsive systems were established andextensively developed in the past two to three decades. Among them, ever increasingattention has been paid to the field of responsive polymers due to their adjustablemolecular structure and polymorphism of morphologies. The recent advances inpolymer chemistry gave an impetus to the design of multi-responsive polymericmaterials that recognize independently or synergistically more than one stimulusexhibiting collective responses. Based on this principle, the main goal of this researchis the synthesis and study of the responsive behavior of triple stimuli-responsivehybrid Janus and micellar nanoparticles. The ability of these materials to alter theirphysico-chemical properties in response to multiple changes in their environmentalconditions renders them attractive candidates in a diverse range of applications.Hybrid Janus nanoparticles represent a new class of hybrid materials with aninorganic core and asymmetric grafting of polymer brushes from their surface. Thehigh demand for such particles contradicts their small-scale production methods. Inresponse to that, this work takes advantage of the large surface area provided byspherical polymer latex particles to immobilize silica nanoparticles at the latexsolventinterphase and thus provide shielding to one hemisphere of the colloidal silicananoparticles embedded in the latex particles, whereas the exposed silica surface canbe chemically modified as required. Here, the exposed surface of the silicananoparticles was functionalized with atom transfer radical polymerization (ATRP)initiating sites. These asymmetric functionalized nanoparticles were used for thegrowth of a hydrophobic polymer poly(methyl methacrylate) (PMMA); ahydrophobic polymer, poly(tert-butyl acrylate) (Pt-BA) that can be hydrolyzed toform an anionic and pH-responsive derivative poly(acrylic acid) (PAA); and ahydrophilic, cationic and pH- and temperature-responsive polymer, poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA). For comparison, the fully-coatednanoparticle analogues were also synthesized employing the same polymerizationconditions. The successful grafting of the polymers from the surface of the silicananoparticles was verified by TGA, while high molecular weight polymers of narrowmolecular weight distributions were measured by GPC, verifying the control of thesurface-initiated polymerization reactions. Observation by FESEM provided insight on the topology of the hybrid Janus nanoparticles, suggesting the formation of acornlikenanoparticles. The aqueous solution behavior of the Janus and fully-coatedPDMAEMA and PAA nanoparticles were investigated by DLS, potentiometrictitrations and zeta potential measurements verifying the responsive behavior of thenanoparticles. Additionally, well-defined amphiphilic hybrid Janus nanoparticlescomprising an inorganic silica core and a shell consisting of compartmentalized PAAand PDMAEMA chains were synthesized via a multi-step ATRP surface-initiatedpolymerization process. The successful grafting of the polymer brushes on theopposite hemispheres of the nanoparticles was evidenced by TGA, whereas highmolecular weight and narrow molecular weight distributions were measured for bothpolymers. The ampholytic hybrid Janus nanoparticles exhibited a pH-responsivebehavior in aqueous solution due to the presence of both ionizable, DMAEMA andAA, groups on the nanoparticles‟ surface. DLS studies showed a variation of thehydrodynamic diameter of the polyamholytic hybrid nanoparticles as a function ofsolution pH. At the extreme pH values the size of the nanoparticles reached amaximum, while near the isoelectric point the nanoparticles‟ size collapsed.In the second part of this work, hybrid Janus nanoparticles that respond tochanges of the solution pH and temperature and to light irradiation were synthesized.For their synthesis, DMAEMA and the in-house synthesized monomer, 1',3',3'-trimethyl-6-methacryloyloxy-spiro(2H-1-benzopyran-2,2'-indoline) (SPMA) werecopolymerized from the surface of Janus initiator nanoparticles by surface-initiatedATRP. Two hybrid Janus nanoparticles were synthesized bearing 3 and 15 mole %SPMA, respectively. The pH- thermo- and light-responsive behavior of the SiO2-g-(PDMAEMA-co-PSPMA) hybrid Janus nanoparticles bearing 15 mole % SPMA wasinvestigated in water by UV/Vis and DLS studies, verifying the triple-responsivebehavior of the nanoparticles.Finally, multi-responsive block copolymers were synthesized by the sequentialATRP of DMAEMA followed by the polymerization of the in-house synthesizedmonomer SPMA. Two block copolymers were synthesized bearing 3 and 14 mole %SPMA, respectively. The PDMAEMA-b-PSPMA block copolymers can selfassembleinto well-defined spherical micelles, comprising a hydrophobic PSPMAcore and a hydrophilic PDMAEMA shell, in aqueous solution. The responsivebehavior of the micelles when applying three different stimuli (i.e. light, pH and temperature) was verified, while their capability to encapsulate a model compoundand release it in response to UV light irradiation was also investigated.