Discrete Block Copolymers:Synthesis And Self-assembly Behaviors

Molecular weight is one of the main structural parameters that determine the structures and properties of polymers.Compared with biological macromolecules with uniform size,synthetic polymers always suffer from heterogeneity in molecular weight,that is,polydispersity.It has been proved that dispersity has an important effect on the mechanical properties,rheology,crystallization and self-assembly behavior.For example,in terms of the self-assembly of block copolymers,the introduction of dispersity could incur significant change in the morphology,domain spacing and stability of the ordered structures.With the development of synthesis technology,researchers have been able to regulate molecular weight distribution to a certain extent.However,discrete block copolymers with uniform chain length still remain as a big challenge.Experimental explorations always employ polymers with certain dispersity as model system.In contrast,block copolymers featuring perfectly uniform chain length are the research objects of related theoretical studies,which inevitably make theories divorced from experiments.In order to resolve the differences,some researchers turn to strategies for regulating dispersity to probe its effect on self-assembly.Unfortunately,they suffer from poor control over dispersity or incomplete description of dispersity,resulting in unconvincing conclusions.Based on above all,we designed and synthesized a series of discrete block copolymers with uniform chain length and defined molecular weight,hoping to reveal the intrinsic structure information by cautiously investigating the bulk self-assembly behaviors.On this basis,molecular weight distribution could be accurately controlled by reconstruction strategy,thus,providing an ideal platform for quantitative study the critical importance of molecular weight distribution.Concrete studying of this paper can be divided into four parts(1)Combined with iterative growth strategy and efficient chemical reactions,a series of discrete oligo lactic acid(oLA)and oligo dimethylsiloxane(oDMS)with different molecular weight were accurately synthesized.The iterative divergent/convergent approach and orthogonal protection strategy were applied to synthesize oLA,while chain extension was realized with esterification reaction.An iterative linear growth strategy,together with the reaction of silicon hydroxyl group and silicon chloride bond were employed to synthesize oDMS with polymerization degree 13 or 17.The discrete nature and absolute molecular weight can be unambiguously confirmed by a combination of techniques,including proton nuclear magnetic resonance(1H NMR),size exclusive chromatography(SEC)and matrix assisted laser desorption/ionization time-of-flight mass spectrometry(MALDI-ToF MS)(2)A library of AB-type discrete block copolymers with defined chemical structures were modularly synthesized by adopting discrete oDMS and oLA as building blocks.Detailed data on thermal properties and self-assembly behaviors could be obtained by differential scanning calorimetry(DSC)and small angle X-ray scattering(SAXS).Results shown that with the increase of oLA repeat units,transition temperatures(glass transition temperature and order disorder transition temperature)increase and morphologies change from disordered state to highly ordered lamellar structures with a positive correlation between the domain spacing and molecular weight(3)According to Gaussian distribution and Skew-normal distribution,samples with symmetrical and asymmetrical distributions of oLA block were constructed by blending AB-type discrete block copolymers,aiming at studying the effects of distribution width and shape,respectively.As for the symmetrical distribution,it is found that domain spacing increase continuously as the distribution width increases.Interestingly,the initial deviation from uniformity to inhomogeneity(i.e.,from D=1.000 to D=1.001),domain spacing increased by 3%despite the increment of D being merely 0.001.After that,the domain spacing just increases much slowly with the increase of D,which is never detected in the previous researches.On the other hand,the distribution shape could also have an important impact on the self-assembly behaviors:samples with negative skew distribution exhibit ordered structures with increasing domain spacing and stability than their positive counterparts(4)A series of ABx-type(x=2,4)hetero-arm star copolymers with uniform chain length were modularly prepared based on discrete oDMS and oLA.The precise chemical structure eliminates all the molecular defects associated with molar weight,dispersity,and compositional ratio.Studies on the bulk self-assembly behaviors reveal that,besides conventional structures(lamellae,double gyroid,cylinder),nonclassical spherical phases(Frank-Kasper A15,? and DQC phase)could be obtained due to increasing interface curvature caused by architecture asymmetry.A phase portrait with unprecedented accuracy was mapped out(up to one monomer resolution).It shows the phase region of various phases,phase transition order,and the effect of topological asymmetry,unraveling intriguing details of phase behaviors which have long been compromised by inherent molecular weight distribution.