Emulsifying Peerformance Of The Self-assemmblies Of Amphiphilic Copolymers

Self-assembly of amphiphilic block copolymers (BCPs) has attracted more and moreattentions of scientists in the polymer field. Many mile-stone results have been achieved in therelevant studies. To date, the research focus of macromolecular self-assembly graduallytransfers from the structures and morphologies of the self-assembly aggregates to theirfunctional design and application. However, BCPs with regular molecular structures are stillthe first choice as the building blocks for most self-assembly. The applications of BCPs arelimited to some extent due to the rigid conditions for synthesis and the complex requirementsin applications which are always unachievable for BCPs simultaneously. Compared withBCPs, the preparation of random copolymers (RCPs) is much easier, which makes it moreapplicable and popular in industrial applications. Our group has successfully appliedself-assembled micelles prepared from various RCPs as polymeric particulate emulsifiers, orinto the fabrications of functional coatings. Currently, the detailed structure of self-assembledmicelle based on amphiphilic RCPs is still unclear. What is the correlation between itsstructures and properties? Different with block copolymer micelles which have clearcore-shell boundary, the self-assembled micelles from amphiphilic RCPs do not possessregular hydrophobic/hydrophilic micro-phase due to their irregular chain sequence. Somehydrophobic segments of the RCPs also exist in the surface of the RCPs micelles. Will theyendow the self-assembled micelles with specific performance at the biphase interface? Herein,the first question in this dissertation―whether the structure of polymer micelles effect theporformance of the micelles at the oil/water interface‖was proposed？About one century ago, Ramsden and Pickering discovered that solid particles were ableto absorb at the oil/water interface and thus formed stable emulsions. Recently, with thedevelopment of nanotechnology, several new types of particulate emulsifiers are prepared,such as inorganic/organic hybrid nanoparticles, steric-stabilised polymeric latexes, microgels,and micelles, etc, which exhibit certain properties different with classical solid particulateemulsifiers, such as extremely high emulsifying efficiency or stimuli-responsiveness.Moreover, their performances cannot be explained by the present mechanism for solidparticulate emulsifiers. This type of particulate emulsifiers contain polymer component, so wecall them polymeric particulate emulsifiers. Does the intrinsic structure of polymericparticulate emulsifiers affect their surface property or emulsifying performance? Is there anyuniversal mechanism for these polymeric particulate emulsifiers? This is the second questionthis dissertation focused on.Considering these two questions, polymeric micelles with different structures weredesigned and used as prototype particles here to mimic the structures of some typicalpolymeric particulate emulsifiers. Their behaviors on the oil/water interface and emulsifyingperformance are studied, which helps to understand the functions of colloids at the complexfluid interface and also provids inspirations for the design and fabrication of novel highefficient and stimuli-responsive particulate emulsifiers.The route of this thesis is: firstly, with BCPs micelles as comparison, the structure of RCPs micelles was confirm, and their behavior on oil/water interface was compared with thatof BCPs micelles before using them as a model for polymeric particulate emulsifiers;secondly, polymeric micelles were prepared via self-assembly from the water-solublemacromolecule emulsifiers after chemical modification, and used as particulate emulsifiers;further, the self-assembled micelles were controlled into four typical structures byenvironmental stimulation. Role of the structural transition in their emulsifying performancewas studied; finally, photo-cross-linking was utilised to control the deformability of themicelles and the conformation of the polymer chains on the surface of micelles, and theoil/water interfacial performance of the micelles were studied in various swollen states. It isverified that the synergistic action of polymer chain and particle on the oil/water interface forfor polymeric particulate emulsifiers.The work in this dissertation includes the following parts:1．Role of the chain sequence of amphiphilic copolymers plays in the emulsifyingperformace of amphiphilic copolymeric micelles.Random copolymers P(AA-r-St) and block copolymers (BCPs) PAA-b-PSt with similarcomposition of hydrophobic/hydrophilic units in different chain sequences were synthesized,and self-assembled into spherical micelles in selective solvent DMF/H2O. DPD simulationwas used to illustate the inner structure of the as prepared micelles, and the results werecorrelated well with the results obtained from SEM, TEM, XPS, conductivity and pH titration,DLS, etc, which indicated that: PAA-b-PSt micelles had core-shell structure with PAA blocksformed shell and had a pKa of ca.6.2undependent on polymer composition. Only PAA chainsswelled with pH increasing, while hydropholic PSt core was stable. As a comparison, therewere hydrophilic microphase zones hybrid into the micelle’s hydrophobic core, while thehydrophobic units also existed in the micellar hydrophilic surface. With higher hydrophobiccomposition in RCPs, the hydrophobic microphases in micellar surface became larger, whichresulted in a more stable micelle structure. The pKa of RCPs micelles decreased as theincrease of PAA mol%content of polymers, and was obviously higher than the pKa of BCPsmicelles. As the pH of micellar solution increasing, the chains in the surface of RCPs micelleswere firstly deprotonated and swollen. The micellar structure then swelled with pH larger thanits pKa. With further increase of pH level, partially extremely swollen micelles woulddissemble into free polymer chains. Finally, the block copolymer B48and the randomcopolymer R49, with the ratio of hydrophobic and hydrophilic composition nearly equal to1:1, self-assembled into micelles. As prepared micelles were used as particulate emulsifiers tostudy their performance in oil/water interface. The resultes illustrated that: stable W/OPickering emulsion stabilised by B48micelles can only be obtained when the protonationdegree of PAA chain in B48micelles was less than4%(pH≤5); same condition (pH≤5,lower protonation) also worked for R49micelles to stabilize stable W/O Pickering emulsion.When7≤pH <pKa, with the protonation degree of polymer chain on the surface of R49micelles increased, the stable O/W Pickering emulsions were obtained, meanwhile, due to theswollen polymer chains on micelle surface, the emulsifying efficiency of the micelleincreased with significant decrease in the mean size of the emulsion drop. The micellesbecame extremely swollen when pH> pKa, and thus formed O/W emulsion which was actually stabilized by amphiphilic copolymer chain. Additionally, the Pickering emulsionstabilized by R49micelles exhibited a fascinating phenomenon, which was capable ofreversible pH-induced phase inversion, due to performance of the amphiphilic polymer chainson the surface of the R49micelles in response to pH change. The obtained results preliminaryconfirms that the micellar structure controlled by chain sequence plays an important role inthe micelles’ behavior in oil/water interface. RCPs micelles compared with BCPs micelleshave better liquid interfacial performance and are more tailorable and controllable, and thuscan be used as a model for the further study of the polymeric particulate emulsifiers.2. Self-assembly and Emulsification of Poly{[styrene-alt-maleic acid]-co-[styrene-alt-(N-3,4-dihydroxyphenylethyl-maleamic acid)]}This chapter focus on exploring the effect of chemistry on the self-assembly andemulsifying performance of polymers. A new type of amphiphilic random copolymer,poly{(styrene-alt-maleic acid)-co-[styrene-alt-(N-3,4-dihydroxyphenylethyl-maleamic acid)]}(SMA–Dopa), was designed and synthesized. SMA–Dopa possess dopamine moieties as sidegroups and alternating poly(styrene-alt-maleic acid)(HSMA) as main chain. HSMA is a typeof conventional macromolecular emulsifiers and water-soluble. Dopamine moiety facilitatedthe self-assembly of the SMA–Dopa in selective-solvent into stable moderately swollenmicelles, and increased the adsorption of the SMA–Dopa at the oil/water interface. TheSMA–Dopa micelles can maintain their structure at the oil/water interface with betteremulsification performance than HSMA. pH and salt concentration could change the micellestructure of SMA-Dopa52in water, then affect its structure at the oil/water interface, and thusaffect its final emulsifying performance. The emulsifying characteristics demonstrated thatself-assembled SMA–Dopa52micelles with moderately swollen structure (at2<pH<6)combine the advantages of the solid particulate emulsifiers and macromolecular emulsifiers,possessing excellent emulsifying efficiency and good emulsion stability. Moreover, theemulsifying performance of the SMA–Dopa52micelles could be enhanced by the addition ofsalt. Summarized from this chapter, starting from macromolecule emulsifier, the polymerchains can be self-assembled into micelles after chemical functionalization, which provides anew route to prepare high efficient particulate emulsifiers.3. Dual-Responsive Poly(styrene-alt-maleic acid)-graft-Poly(N-isopropyl acrylamide)Micelles as Switchable EmulsifiersTo explore the relationship between the micellar structure and their emulsifyingperformance, pH-and temperature-responsive self-assembled micelles were prepared andused as emulsifiers, based on a novel grafted polymer poly(styrene-alt-maleicacid)-g-poly(N-isopropyl acrylamide)(PSMA-g-PNIPAm). Structure of PSMA-g-PNIPAmmicelles varies in response to pH and temperature changes and can be classified into fourtypical states, including shrunken, moderately swollen, extremely swollen, and inverted state,confirmed by a combination of electrophoresis, DLS, TEM, and1H NMR. The structuralvariation plays a key role in the emulsifying performance of PSMA-g-PNIPAm micelles,according to the emulsifying characteristics of the four typical PSMA-g-PNIPAm micelles aswell as the micelles’ morphologies on the surface of oil droplets observed by SEM. Theemulsion stabilised by micelles with moderately swollen structure is especially stable compared with either the shrunken micelles or the extremely swollen micelles, because themoderately swollen micelles combine the advantages of particulate emulsifiers and polymericsurfactants.4. The self-assembly and emulsification of the photo-cross-linkable copolymerPoly(7-(4-vinylbenzyloxyl)-4-methylcoumarin-r-acrylic acid) P(VM-r-AA)In this chapter, The introduction of photosensitive hydrophobic coumarin (VM) in PAAmacromolecular chain, a type of photo-cross-linkable amphiphilic random copolymersP(VM-r-AA) are synthesized via free radical polymerization. they can be self-assembled intospherical micelles in selective solvent DMF/H2O, and used as particulate emulsifiers. Roles ofthe swelling of the polymer chains of the micelles and the deformation of the micellarstructures in their behavior at the oil/water interface are investigated from three aspects.(a)Chemical composites: The size and deformation capability of micelles depend on thechemical composite of the polymers. With the molar content of hydrophobic units PVM mol%decrease, the micelles become smaller and more easy to deform, resulting in stable adsorptionat the liquid interface. And the stability of the Pickering emulsion by micelles is inverselyproportional to the PVM mol%. The PVMAA12micelles with PVM mol%of12%are chosedas candidate model emulsifiers.(b) Photo-cross-linking: With the photo-cross-linking betweenthe polymer chains, the micelles shrink and the deformation capability of the micellesdecrease, resulting in that the liquid interfacial area occupied by per shrunk micelle decrease,i.e. the emulsifying efficiency of the shrunk micelles decrease.(c) pH: When pH<8, theuncross-linked micelles, i.e. shringkage degree by photo-cross-linking (SDC) equals to zero,favor to deform at oil/water interface because of the swelling behavior of the micelles inresponse to the increase of pH, bringing the higher emulsifying efficiency and excellentstability of the emulsion. However, extremely swelling of the micelles will sacrifice thestability of the emulsion, special when pH>8. As a comparsion, micelles with SDC of95%can swell with pH increase, but they cannot deform at the oil/water interface because of theover cross-linking. They possess lower emulsifying efficiency compared with theuncross-linked micelles. When pH>8, highly cross-linked micelles cannot stably adsorb atthe oil/water interface, when pH=10, there is even no emulsion formed. However, theemulsion stabilised by highly cross-linked micelles shows pH-responsive performance.In summary, this thesis inversitigates the role of the micellar structure on the emulsifyingperformance and the oil/water interfacial behavior, with the structrual control throughpolymer chain sequence, chemical modification, and enviromental stimuli (ionic strength, pH,temperature, and UV irradiation). These research provide answers to the two main questions.(1) the polymeric micellar structures and chemistry play an important role in the emulsifyingperformance. Random copolymeric micelles shows excellent controllability over the oil/waterinteracial behavior.(2) The polymeric micelles, prepared via selective solvent method, couldbe used as a model of the polymer based particulate emulsifiers, which bridge the polymeremulsifiers and solid particulate emulsifiers. The underlying principle of the modelemulsifiers likes to be: The conformational change of the swelling amphiphilic polymerchains on the surface of micelles benefits both the stable adsorption at the oil/water interfaceand the deformation of the micelles. The deformed micelles occupy larger oil/water interfacial area, resulting in larger loss of Gibbs free energy. Hence, they exhibits higher emulsificationefficiency and extremely stability of the emulsion.