| Polymers with periodic ordered nanostructures are promising soft matter materials for a wide range of applications.For instance,such a class of nanomaterials have potential usages in emerging fields such as soft lithography and precision membrane separation.Self-assembly of block copolymers is one of the most effective approaches to ordered nanostructures in polymers.However,the feature size of the ordered structures derived from the self-assembly of block copolymers is typically above 5 nm,thereby limiting their applications in many areas.In contrast,lyotropic liquid crystal mesophases formed by small-molecule amphiphiles possess ordered morphologies similar to those of block copolymers,which however display periodicities below 5nm.Furthermore,by incorporating polymerizable groups into the amphiphilic molecules,one can readily transform the fluidic liquid crystal mesophases into mechanically robust polymers with the preservation of the original ordered structures.Therefore,polymeric materials derived from polymerizable lyotropic liquid crystals have been intensively studied for many functional applications.To realize the functionalization of polymerizable lyotropic liquid crystals,the first step is to explore the relationship between the molecular structure of polymerizable amphiphiles and the physicochemical properties of liquid crystals.The presence of reactive groups in the molecular architectures of polymerizable amphiphilic molecules significantly influences the corresponding liquid crystal morphologies.Previously,the influence of polymerizable groups on the assembled structures of liquid crystals has been considered detrimental to the ordered assemblies of lyotropic liquid crystals,which however,has not been systematically studied.The classical critical packing parameter theory qualitatively predicts the geometry of lyotropic liquid crystals by considering the volume ratio of amphiphilic hydrophobic tails to hydrated hydrophilic headgroups.However,by ignoring the chemical details,and it cannot precisely guide the design and synthesis of polymerizable amphiphiles.The majority of polymerizable liquid crystals have been formed by acrylate-bearing mesogens or amphiphiles due to the synthetic modularity and the high reactivity for free radical polymerization.Compared with the saturated methyl group or methylene group,the acryloyloxy or methacryloyl groups,while introduced into amphiphiles,bring additional polarity and steric hindrance.Therefore,the acryloxyl or methacryloxyl groups have been roughly considered to be equally detrimental to the ordered structure of liquid crystals.This argument is also supported by the consideration based on critical packing parameter theory,where an extraα-methyl group in the methacryloyloxy group contributes trivially to the volume of the hydrophobic chain.In view of the wide application of acrylate-based polymerizable amphiphiles,this thesis focused on the understanding of the relationship between molecular structure and lyotropic liquid crystal phase behavior,with the goal of the large-scale preparation of polymers with sub-5-nm ordered structure.The phase behavior of lyotropic liquid crystals with methacryloyloxy or acryloyloxy amphiphiles has been compared,which demonstrate that the α-methyl group plays a non-negligible role in the lyotropic liquid crystal behavior of acrylate-based amphiphiles.In this study,a series of polymerizable amphiphiles are designed and synthesized,and the corresponding temperature-composition phase diagrams of the lyotropic liquid crystals have been investigated.We have fixed the hydrophilic head group of each pair of amphiphile,but slightly varied the chemical structure of the hydrophobic tail chain.The only difference is the presence or absence of an α-methyl at the acryloyloxy group at the tip of the hydrophobic chain.We have found that the presence of the α-methyl group does not increase the steric hindrance effect,but enhances the packing of the hydrophobic chains of the amphiphile molecules.As a result,the amphiphile molecules display highly tunable interface curvatures,eventually a rich lyotropic mesomorphism.The main content of this thesis is as follows:1.Six pairs of polymerizable amphiphiles were synthesized,namely sulfobetaine zwitterionic amphiphile pairs of SB114MA/SB114 A and SB113MA/SB113A;sulfo-imidazolium zwitterionic amphiphiles of SIm113 MA and SIm113A;sulfo-pyridinium zwitterionic amphiphiles of SP103 MA and SP103A;sulfo-triazolium zwitterions ST113 MA and ST113A;quaternary ammonium Genimi amphiphile G6-MA and G6-A.Each pair of the amphiphilic molecules has the same head group and the carbon chain length between the polymerizable group and the head group.The difference is the use of methacryloyloxy or acryloxy at the end of the tail as the polymerizable group.2.The temperature-concentration phase diagram of the lyotropic liquid crystals formed by the each amphiphile and water have been precisely mapped by using polarizing microscopy and small-angle X-ray scattering.We have found that amphiphiles with an α-methyl in the methacryloyloxy group tend to have richer mesophase morphologies and broader phase windows.3.The packing of the hydrophobic tails of each amphiphile has been compared by measuring the air/water interfacial tension of each surfactant in aqueous solutions.We have found that the introduction of an α-methyl group increases the saturated density of amphiphile molecules at the air interface,which is clearly due to the promotion of hydrophobic stacking of tail chains.In addition,using dynamic light scattering to characterize the possibly presented micelles or vesicles inside dilute solutions of each amphiphile,it is found that amphiphiles with α-methyl groups are more likely to form micelles with higher interfacial curvature in aqueous solutions.This finding is consistent with the conclusion by tensiomentry measurements.4.Lyotropic liquid crystals have been formulated with appropriate amounts of a cross-linker and a photo-initiator,for the desired morphologies such as lamellae,cylinders,or gyroids,etc.The ordered structures with sub-5-nm feature size have been preserved into solid polymers by UV-induced photocuring.The high-fidelity preservation of the photo-cured liquid crystal structures has been confirmed using small-angle X-ray scattering and transmission electron microscopy.5.The effect of α-methyl on the mechanical properties of polymers has been studied.Tensile strength measurements by elongation measurements show that polymer films derived from amphiphilic monomers with the α-methyl group had higher tensile modulus and but lower elongation at break. |
