Study On Crystallization-driven Self-assembly Of PE-based Block Copolymers In Soulution

The self-assembly of block copolymer(BCP)has attracted considerable attention for decades because it can yield ordered structures with various kinds of morphologies.These aggregates provide potential or practical applications in many fields.The"epitaxil growth" nature of crystals offers crystallization-driven self-assembly of semi-crystalline BCPs with the characteristics of well regulation and design of micellar structures.In this dissertation,the self-assembly behaviors of polyethylene-block-poly(tert-butylacrylate)(PE-b-PtBA)and polyethylene-block-polyacrylic acid(PE-b-PAA)BCPs in selective solvents were studied.In the first part,the crystallization-driven one-dimensional self-assembly of PE-b-PtBA in DMF was presented.It is found that all three PE-b-PftBA BCPs used in this work can form crystalline cylindrical micelles in DMF.When the BCP solution is cooled to crystallization temperature(Tc)from 130 ?,the seed micelles may be produced via two competitive processes in the initial period:stepwise micellization/crystallization and simultaneous crystallization/micellization.Subsequently,the seed micelles can undergo growth driven by the epitaxial crystallization of the unimers.The lengths of both the seed micelles and the grown micelles are longer for the BCP with a longer PtBA block at a higher Tc.Quasi-living growth of the PE-b-PtBA crystalline cylindrical micelles is achieved at a higher Tc.A longer PtBA block evidently retards the attachment of unimers to the crystalline micelles,leading to a slower growth rate.Then crystallization-driven co-assembly of micrometric polymer single crystals and nanometric block copolymer micelles was achieved.These hybrid single crystals are first formed by cocrystallization of PE52 homopolymer and PE100-b-PftBA48 BCP in DMF or DMF/o-xylene mixed solvent.The morphology of the obtained hybrid single crystals can be regulated via changing the crystallization temperature,solvent composition and mass ratio of BCP/homopolymer.Due to the difference in crystallization rate,the distribution of PE-b-PtBABCP in the hybrid single crystals may be inhomogeneous,leading to a concave gradient surface structure.Atom force microscope(AFM)measurements reveal that the hybrid single crystals have a double-layer structure,in which PE homopolymer chains adopt extended conformation and the PE blocks in PE-b-PtBA are once-folded.After the PE homopolymer is consumed,cylindrical micelles of PE-b-PtBA can further epitaxially grow on the lateral surface of the hybrid single crystals and "ciliate paramecium-like" co-assemblies are yielded.The single crystal/micelles co-assemblies can be prepared either by one-step method or by two-step method.This work provided a simple route to construct hierarchical assemblies composed of objects with different size levels by using crystallization as the key driving force.Finally,the disassembly of PE-b-PAA crystalline micelles in aqueous solution was studied.Cylindrical micelles of PE-b-PAA are first fabricated via crystallization-driven self-assembly(CDSA)of PE-b-PAA in DMF/o-xylene(1/1 v/v)mixed solvent followed by dialysis against DMF and water.The effects of kind and amount of amines on the morphology of cylindrical micelles in aqueous solution were investigated with TEM and DLS.Long crystalline cylindrical micelles(150-400 nm)would be fractured into short stub-like micelles and spherical micelles(20-50 nm)by adding monoamines like lysine,diethylamine and triethylamine.Fourier transform infrared(FT-IR)characterization showed that the effect of monoamines on the morphology of cylindrical PE-b-PAA micelles can be ascribed to the electrostatic interaction between amine and PAA and the resultant steric repulsion among PAA corona,which is largely related to the amino functionality and molecular size(weight)of amines.Further calculation of free energy proves the thermodynamic accessibility of fragmentation in crystalline PE core by utilizing the electrostatic interaction and steric repulsion within ionized PAA coronas.