Supramolecular Gel Properties Of Dendron-Polyelectrolyte Complexes

As an important part of soft materials, gels, which possess both the appearance of liquid and the property of solid, have been applied in various fields owing to their unique properties. The types of gels are divided into polymer gels and low-mass-molecule (LMM) gels in accordance with gelator molecular weight. Polymer gels, which are constructed either by covalent cross-linking to get chemical gels or through physical changes contributed by non-covalent interactions between macromolecules to form physical gels, have been extensively investigated and widely applied. LMM gels, which are always supramolecular, are usually initiated by the self-assembly of LMM gelators through non-covalent interactions, such as hydrogen bond,π-πstacking, donor-accepter interactions, etc., to construct physical gels. The most interesting functionality is that they could be destroyed and rebuilt under special conditions. In recent years, LMM gels stand to attention for their unique assembly properties in supramolecular chemistry and potential applications in various fields. Both two kind gels have their own properties, and we are interested in developing a new kind of gels which contain both their properties. This dissertation will focus on this kind of gels, which are polymer-based supramolecular gels.Due to well-defined architectures and diverse functionalities dendrimers and dendrons have been widely applied in the study of supramolecular assembly field. It has been reported that some dendrons, with various decorations on their apexes or peripheries, could form gel-phase materials mainly in organic media. They can be called as dendritic gelators with a wedge shape. Most of them contain amide and/or urethane groups which provide multiple intermolecular hydrogen bond sites and peripheral alkyl chains which provide interactions with organic solvents. During gelation, the formation of multiple hydrogen bonds is the advantage and key factor for dendrons in constructing supramolecular aggregates, such as ribbons, which could braid into networks and finally create a gel. To promote the gel-formation ability of such dendritic gelators, it has been reported that the dumbbell-shaped dimers and diblock dendrimers formed by covalent or non-covalent connections of these dendrons with bifunctionalized motifs can enhance the gel-forming ability of dendritic gelators. Meanwhile, dendron-polymer gelators, which were prepared by linking dendron to a linear polymer, have been also studied. In one of our previous work we created the codendrimer gelators by covalently linking poly(urethane amide) (PUA) dendrons and poly(methallyl dichloride) (PMDC) dendrons together, and found that their gel-forming ability was expedited with increasing generation of both the dendrons because higher polarity and multiple hydrogen bonds can promote the aggregation ability of the gelator. At this point of view, we think about using dendronized polymer structure to promote gelation. We choose ionic supramolecular interaction as the linkage between dendron-branch and polymer backbone to form a kind polymer-supporting supramolecular gel, in which a positive macromolecular effect occurred.We have synthesized three kind dendronized polymers, in which ionic interactions is chosen as the linkage between Kim-type PUA-dendron (g2-PUA) and linear polyelectrolyte backbones. In the first part of our work, we chose poly(diallyldimethylammonium chloride) (PDADMAC) as backbone and synthesized g2-PUA-PDADMA. We found that both g2-PUA and the g2-PUA-PDADMA could form organogels in toluene. Interestingly, both the minimum gelation concentration and gelation time of the g2-PUA-PDADMA were greatly reduced. Our investigations showed that the intermolecular hydrogen bonding between adjacent dendrons creates in the gel-phase the similar supramolecular structures. Our further studies on the gelation kinetics indicated that the polyelectrolyte backbones played an important role in prearranging the attached dendritic gelators orderly and quickly into the supramolecular structures via a nucleation-elongation mechanism. Therefore, the gel-forming ability of the g2-PUA was enhanced by complexing with the polyelectrolyte. In this work this positive macromolecular effect was discussed.In the second part of our work, dendronized polymer were prepared via complexation of g2-PUA with three positively charged polyelectrolytes. We chose g2-PUA-PDADMA, g2-PUA-PAH and g2-PUA-QP4VP as research objects, whose backbone were PDADMA, poly(allylamine hydrochloride) (PAH) and quaternized poly(4-vinylpyridine) (QP4VP) respectively. All these complexes could form organogels in toluene, and their gel properties were studied through determination of their minimum gelation concentration and characterization of the networked supramolecular structures formed in their gel-phase using TEM and AFM. Our findings showed great differences in the gel-forming ability and supramolecular structure among these gelators. A simple simulation on the conformations of the three polyelectrolytes illustrated that the differences came originally from the polyelectrolyte conformations which produced different distribution and orientation of the dendritic gelators along the backbones. In case that the backbone could induce the dendritic gelators to form a pre-ordered structure before gelation, the dendronized polymer gelator would form nice supramolecular aggregates so show up an enhanced gel-forming ability. On the contrary, such the ability would be declined.In the third part of our work, we compared the gel abilities of g2-PUA、g2-PUA-PDADMA and g2-PUA-PAH in different low-polar organic solvents, such as toluene, cyclo-hexane and n-hexane. We found that g2-PUA could only gel in toluene, while g2-PUA-PDADMA could gel in all the three solvents and g2-PUA-PAH could gel in toluene and n-hexane. Through gel-phase morphology detection by TEM and AFM, we found that g2-PUA presented a membranous morphology in n-hexane, while in the same solvent dendronized polymer gels presented similar morphologies. SAXS and XRD results showed that alkyl chains in g2-PUA performed a better stacking structure in n-hexane. This phenomenon illustrated that the aggregation of g2-PUA in toluene preferred enlongational growth via hydrogen bonding, while in n-hexane it preferred lateral growth through alkyl chain stacking. The promotion of gel-forming ability in poor solvents of dendronized polymers was resulted from polyelectrolyte backbones preventing the tight aggregation of dendrons, which led to precipitation, and the linear structure gave better chance for dendrons forming intermolecular hydrongen bonds, which conduced to gel-forming. These results provide a new strategy for the design of advanced supramolecular structure materials.