Bulk Phase Structures And Solution Self-Assembly Behaviors Of Amphiphilic Azobenzene-Containing Liquid Crystalline Triblock Copolymers

Amphiphilic block copolymers have attracted great attention since they possess powerful self-assembling capability in solution and the resulting nano-or microsized aggregates,such as spherical micelles,vesicles and nanotubes,have potential applications in various fields such as drug delivery,nanoreactors and nanotemplates.Liquid crystalline block copolymers(LCBCPs)combining the molecular level LC order(around 1?10 nm)with long range ordered microphase-separated domain morphologies(around 10?100 nm)provide such a platform for studying the influence of morphology of block copolymers on the phase behavior of the LC segments and elucidating the orientation of mesogens with respect to the morphologies and mesophase transitions under confinement.The introduction of liquid crystalline hydrophobic blocks to amphiphilic block copolymers creates conditions for building stable self-assembled structures of varied morphologies and designing intelligent assembly systems capable of responsing to external stimuli,such as light,electricity and magnetism.Azobenzene-containing LCBCPs have been one of the frontier research focus due to the photoresponsive property of azobenzene mesogen and their potential applications in optical devices,photoresponsive nanomaterials and molecular-transport channels.Recently,a wide variety of amphiphilic block copolymers with liquid crystalline polymers(LCPs)as the hydrophobic block have been synthesized by different synthetic routes.Combining controlled/living free radical polymerization techniques,such as atom transfer radical polymerization(ATRP)and reversible addition-fragmentation chain transfer(RAFT)polymerization,with click chemistry is a highly effective method for preparing block copolymers with controlled molecular weight and narrow molecular weight distributionIn this thesis,with 2,6-dibromoheptanedioate as the initiator,by optimizing reaction conditions and adjusting reaction time to reduce the changes of end-bromide,homopolymers LCP-Br and MJLCP-Br have been well prepared through ATRP of the ten-methylene spacer monomer 8-AZO-10-macr or monomer 8-AZO-0-macr without a spacer.Then bromide ended homopolymer were modified with NaN3 to obtain azido-terminated LCP-N3 and MJLCP-N3.Further through click reaction with alkyne-functionalized mPEG,a series of well-defined azobenzene-containing LC triblock copolymers with narrow molecular weight distribution were obtained.The polymer products were fully characterized by 1H NMR,FT-IR and GPC.The thermal behavior and self-assembled structures of these copolymers in bulk state were investigated by a combination of DSC,POM and SAXS/WAXS.For LCP series,glass transition or melting of partially crystalline alkyl chains and transition from lamellar structure to isotropic phase were observed during the heating process for the homopolymers.In the mesophases,the homopolymers showed a long-range ordered lamellar structure and the layer spacing was in good agreement with the calculated molecular contour length in a fully extended conformation of the side-chain mesogens,suggesting a fully interdigitated smectic A structure(Smd).The copolymers showed similar lamellar structure to the homopolymers from SAXS analyses.Due to the minor fraction of PEG block,the PEG domains was supposed to be confined in the continuous main-chain layers of the microphase-separated lamellar structure of the azobenzene-containing LC triblock copolymers.During the cooling process,except for bLCP15-PEG5k with the highest fraction of PEG block exhibited partial PEG crystallization from a confined layer of phase-segregated main chains in the LCP matrix.While for all other copolymers,the crystallization of PEG was completely inhibited based on DSC and SAXS/WAXS analysis results.For MJLCP series,both homopolymer and copolymers showed no significant thermal effects during heating and cooling processes,probably due to a combination of high rigidity and steric crowding of the polymer backbone without a spacer.MJLCP homopolymers exhibited hexagonal columnar phase in the bulk phase and their copolymers displayed less organized hexagonal columnar phase with slightly increased lattice spacing dimensions.The self-assembly behavior of thus synthesized amphiphilic azobenzene-containing LC copolymers in tetrahydrofuran/water(THF/H2O)solution system has been systematically studied by DLS and TEM.The copolymers were first dissolved in THF,a common solvent for both PEG and LC block,to give a stock solution.Subsequently,deionized water was added dropwise into the copolymer solution to induce the self-assembly and formation of aggregation.The effects of the initial concentration of copolymers and the ratio of hydrophilic and hydrophobic blocks on the aggregation structure were studied.For LCP series,when the initial concentration was 0.5 wt%,colloidal spheres composed of ill-defined colloidal cores and hydrophilic corona of PEG blocks attaching on the surfaces were observed for all copolymers,reflecting the characteristics of large aggregates or clusters.In addition,copolymers with PEG2k block assembled into colloidal spheres with larger diameters than copolymers of the same series with PEG5k.With the initial concentration of 0.1 wt%,leaf-like aggregates were formed for all copolymers and nucleation-like phenomenon occurred for most of the aggregates to combine into impeller like structures.Very interestingly,the high-magnification TEM images clearly showed stripes along the leaves with a spacing in good agreement with the layer spacing of bulk phase structure of the LC copolymers obtained by SAXS,which represented the smectic properties within the organized core of the aggregates.When the initial concentration was 0.05 wt%,leaf-like aggregates similar to micellar solutions with an initial concentration of 0.1 wt%could also be observed,but impeller-like aggregation became rare.For MJLCP series,bMJLCP20-PEG2k exhibited only simple colloidal particle morphology.While for bMJLCP20-PEG5k,very interestingly,large numbers of aggregats arranging in a well-organized hexagonal arrays were observed with different initial concentrations,which reflected their bulk organized structures.Furthermore,potential photoresponsive property of these assemblied aggregates was preliminarily investigated.For aggregates assemblied by bLCP15-PEG2k with a initial concentration of 0.1 wt%,after UV light irradiation,partial breakdown of impeller-like aggregates was observed due to a bent shaped cis structure formation triggered by the trans-cis transtion of azobenzene mesogens,while which could not transform back into the original morphologies upon visible light illumination.