| Coumarin and its derivates are one type of photoreactive chromophores which can undergo reversible dimerization and cleavage reactions at different wavelengths of light. By incorporating coumarin moieties into polymers and block copolymers (BCPs), the resulting materials become photoresponsive. They have attracted much attention due to their potential applications as shape-memory materials, photo-sensitizers for solar cells and drug delivery carriers. The research work presented in this thesis aimed at developing novel coumarin-containing polymers and BCPs, and investigating their potential applications as polymeric nanoparticles and films. To achieve this goal, we have designed, synthesized and characterized a number of such photoresponsive polymers and BCPs. We have studied their properties in solution and solid state in the form of several photocontrollable materials such as micelles, vesicles, nanogels, microgels, films and single chain nanoparticles (SCNPs).;In the second project, we have investigated the influence of the position of coumarin groups on the photoinduced volume change of nanogel particles. By synthesizing various DHBCPs with which the dimerization of coumarin (cross-linking) can occur either in the core, the shell or both the core and shell, our studies found that more significant volume expansion and contraction could be achieved with both core- and shell-linked nanogel particles. Moreover, at a relatively low dimerization degree (10∼40%), such doubly cross-linked nanogel displays an unusual thermal phase transition characterized by an upper critical solution temperature (UCST). This transition has been attributed to a reversible aggregation of nanogel particles.;In the third project, by synthesizing a new DHBCP, we have applied the same self-assembly strategy to first obtain giant vesicles (∼1.5 mum in diameter) with the photodimerization of coumarin, then a microgel (large, micrometer-sized particles of a hydrogel). The "soft" cross-linking of the vesicle corona with a small amount of coumarin groups (5 mol%) is capable of both retaining the vesicle structure and allowing the swelling of vesicle membrane at T<LCST. We have obtained particles that could display a large, reversible and fast volume transition (∼700%) induced by a temperature switching across the LCST. Under lambda<260 nm UV irradiation, the de-cross-linking reaction leads to disintegration of the particles.;In the fourth project, we have developed a method, based on the intramolecular dimerization of coumarin, to prepare SCNPs. With a very dilute solution of a coumarin-containing polymer with high molecular weight, the dimerization could dominantly occur intramolecularly, that is, between coumarin groups on the same polymer chain. This causes coil-to-globule transition leading to the collapse of single polymer chains and formation of small SCNPs (∼20 mn in diameter). The hydrodynamic diameter of these particles was tunable by controlling the photo-cross-linking density. Moreover, we showed that SCNPs could be used as nano-reactors for in situ synthesis of gold nanoparticles (AuNPs), the kinetics of which could be controlled by the dimerization degree.;In the first project, we demonstrated a general strategy to prepare photoresponsive nanogels by combining the self-assembly of double-hydrophilic BCPs (DHBCPs) and the photoreaction of coumarin. These water-soluble DHBCPs have a thereto-responsive block that has a lower critical solution temperature (LCST) and is functionalized with coumarin. In aqueous solution, when the temperature is raised to above the LCST, the DHBCP forms micelles that can be cross-linked through the photodimerization of coumarin groups at lambda>310 nm. Upon subsequent cooling of the solution to T<LCST, nanogels (nanoparticles of a hydrogel) is thus obtained. Under lambda<260 nm UV irradiation, the photocleavage reaction of coumarin dimers can be used to reduce the cross-linking density, resulting in swelling of nanogel particles in water. By using two wavelengths, the. reversible photo-cross-linking and de-cross-linking allows for a reversible volume change of nanogel particles. We also investigated the use of such photoresponsive nanogels for controlled release of model compounds.;The last project dealt with photo-deformable films of a supramolecular polymer of poly(4-vinylpyridine) hydrogen-bonded with coumarin side groups. We find that films of this polymer could be bent when exposed to UV light at lambda>310 nm. Our studies have attributed this phenomenon to an imbalanced surface stress arising from anisotropic dimerization of coumarin groups. As one side of the film is exposed to UV light, the photoreaction occurs only in a region close to the surface. Since polymer density is related to the cross-linking degree, this may create a difference in tension between the two surfaces of the film and result in the observed bending. The demonstration of large-scale photoinduced deformation of an amorphous polymer opens the way for further development of photodeformable polymers. |
