Inspired by nature, stimuli-responsive systems were established and extensively developed in the past two to three decades. Among them, ever increasing attention has been paid to the field of responsive polymers due to their adjustable molecular structure and polymorphism of morphologies. The recent advances in polymer chemistry gave an impetus to the design of multi-responsive polymeric materials that recognize independently or synergistically more than one stimulus exhibiting collective responses. Based on this principle, the main goal of this research is the synthesis and study of the responsive behavior of triple stimuli-responsive hybrid Janus and micellar nanoparticles. The ability of these materials to alter their physico-chemical properties in response to multiple changes in their environmental conditions renders them attractive candidates in a diverse range of applications.
Hybrid Janus nanoparticles represent a new class of hybrid materials with an inorganic core and asymmetric grafting of polymer brushes from their surface. The high demand for such particles contradicts their small-scale production methods. In response to that, this work takes advantage of the large surface area provided by spherical polymer latex particles to immobilize silica nanoparticles at the latex-solvent interphase and thus provide shielding to one hemisphere of the colloidal silica nanoparticles embedded in the latex particles, whereas the exposed silica surface can be chemically modified as required. Here, the exposed surface of the silica nanoparticles was functionalized with atom transfer radical polymerization (ATRP) initiating sites. These asymmetric functionalized nanoparticles were used for the growth of a hydrophobic polymer poly(methyl methacrylate) (PMMA); a hydrophobic polymer, poly(tert-butyl acrylate) (Pt-BA) that can be hydrolyzed to form an anionic and pH-responsive derivative poly(acrylic acid) (PAA); and a hydrophilic, cationic and pH- and temperature-responsive polymer, poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA). For comparison, the fully-coated nanoparticle analogues were also synthesized employing the same polymerization conditions. The successful grafting of the polymers from the surface of the silica nanoparticles was verified by TGA, while high molecular weight polymers of narrow molecular weight distributions were measured by GPC, verifying the control of the surface-initiated polymerization reactions. Observation by FESEM provided insight
Abstract
Page ii
on the topology of the hybrid Janus nanoparticles, suggesting the formation of acorn-like nanoparticles. The aqueous solution behavior of the Janus and fully-coated PDMAEMA and PAA nanoparticles were investigated by DLS, potentiometric titrations and zeta potential measurements verifying the responsive behavior of the nanoparticles. Additionally, well-defined amphiphilic hybrid Janus nanoparticles comprising an inorganic silica core and a shell consisting of compartmentalized PAA and PDMAEMA chains were synthesized via a multi-step ATRP surface-initiated polymerization process. The successful grafting of the polymer brushes on the opposite hemispheres of the nanoparticles was evidenced by TGA, whereas high molecular weight and narrow molecular weight distributions were measured for both polymers. The ampholytic hybrid Janus nanoparticles exhibited a pH-responsive behavior in aqueous solution due to the presence of both ionizable, DMAEMA and AA, groups on the nanoparticles‟ surface. DLS studies showed a variation of the hydrodynamic diameter of the polyamholytic hybrid nanoparticles as a function of solution pH. At the extreme pH values the size of the nanoparticles reached a maximum, while near the isoelectric point the nanoparticles‟ size collapsed.
In the second part of this work, hybrid Janus nanoparticles that respond to changes 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) were copolymerized from the surface of Janus initiator nanoparticles by surface-initiated ATRP. 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 was investigated in water by UV/Vis and DLS studies, verifying the triple-responsive behavior of the nanoparticles.
Finally, multi-responsive block copolymers were synthesized by the sequential ATRP of DMAEMA followed by the polymerization of the in-house synthesized monomer SPMA. Two block copolymers were synthesized bearing 3 and 14 mole % SPMA, respectively. The PDMAEMA-b-PSPMA block copolymers can self-assemble into well-defined spherical micelles, comprising a hydrophobic PSPMA core and a hydrophilic PDMAEMA shell, in aqueous solution. The responsive behavior of the micelles when applying three different stimuli (i.e. light, pH and
Abstract
Page iii
temperature) was verified, while their capability to encapsulate a model compound and release it in response to UV light irradiation was also investigated.
(EN)
