Self-Assembly And Application Of Amphiphilic Random-Alternating Copolymers

Due to the significance of the great range of potential applications, there has been great progress in the past decade in the research of self-assembly of amphiphilic copolymers. During the past few years, our group has focused on the research of non-covalently connected micelles(NCCM) formed by the non-covalent interaction between non-block copolymers, and has had many successes. Up to now, the hot topic of macromolecular self-assembly has shifted into the design and functionalization of functional assembly units as well as their application in drug delivery, encapsulation, and medical imaging.As yet, most self-assemblies are based on amphiphilic block copolymers, owning to their well-defined architectures and narrow polydispersities. However, the synthesis of block copolymers is relatively inconvenient, which limits their large-scale applications. Compared with block copolymer, random copolymers are abundant in sources, easy to obtain and are of more potential value in large-scale applications. Although random copolymers have,relatively ill-defined structures compared to block copolymers, there have been some publications of random copolymer self-assembly into stable and ordered aggregations. One main aspect of this dissertation is whether or not the random copolymer micelle could be endowed with responsiveness via molecular design and functionalization and whether it would be possible to finally turn the random copolymer micelle into a practical application. According to the structure of random copolymers, we speculate that the hydrophilic/hydrophobic micro-domains of the assembly should be of disordered distribution with enriched hydrophilic microdomain at the surface of micelle and should be enriched hydrophobic micro-domain inside the micelle, which makes them amphiphilic.As early as a century ago, Pickering has already discovered that the solid colloid particles possessed surface activity and could stabilize emulsions. However it is not until the development of nanotechnology in the 1990s,had the particle emulsifiers become the focus of attention and scored great usage in the field of inorganic and organic/inorganic hybrid particle emulsifiers. With both hydrophilic and hydrophobic micro-domains on its surface, particle emulsifiers have contacted angles normally in the range of 60°120°and can lie stably at the oil/water interface. We would like to consider and focus on whether the amphiphilic assembly micelle could also be used as a particle emulsifier. In this case, we proposed that we could obtain polymeric soft emulsifier by taking advantage of the surface activity of random copolymer micelle and systematically studied its emulsification as well as the application, which is the another point of research focus of this thesis.Based on those two areas of research foci mentioned above, we have designed and synthesized a series of simple and readily available amphiphilic non-block random alternating copolymers, in which responsive units have been incorporated to acquire stimuli-responsive property. Macromolecular colloid particles with structural stability and surface activity were obtained from the self-assembly of these randomly alternating copolymers as assembly units, and their function as particle emulsifiers was also studied and therefore we developed a new strategy to prepare polymeric soft particle emulsifier for application.The main framework and feature of this thesis are:(1) stable surface-active colloid particles were prepared from the self-assembly of functional alternating copolymers with determined structure and shortest hydrophilic/hydrophobic chain segments to confirm that they possess good emulsification property and form a novel kind of soft particle emulsifier; (2) surface-active colloid particles were prepared from the self-assembly and photo-crosslinking of randomly alternating amphiphilic photo-crosslinkable copolymers with partly disordered chain structure; (3) multi-sensitive surface active colloid particles with good emulsification properties were prepared from the self-assembly of completely disordered random amphiphilic copolymers with multi-sensitive units in selective solvents; (4) finally combining with biological macromolecules, the emulsification property of soy protein isolate(SPI) and its graft copolymers with random structures were examined, which confirmed the improvement of emulsification property after the graft modification of SPI. The results showed that the colloid particles of non-block amphiphilic copolymer with short hydrophilic/hydrophobic chain segments did not demonstrate the typical core-shell structure, whereas hydrophobic micro-domains still existed on the surface of the colloid particles, imparting surface activity to them. Therefore these results showed that the present methodology of preparing surface active colloid particles from the self-assembly of amphiphilic random-alternating copolymer is general and has a wide range of applications. The main contents of this thesis are as follows:1. Synthesis, modification and self-assembly of the amphiphilic alternating copolymer P(St-alt-MAn)Using toluene as solvent, an alternating copolymer P(St-alt-MAn) was prepared by free radical polymerzation which then underwent ammonolysis with dopamine to obtain an amphiphilic alternating copolymer P(St-alt-MA-Dopa). In selective solvent, both P(St-alt-MAn) and P(St-alt-MA-Dopa) assembled into stable spherical colloid particles. The P(St-alt-MA-Dopa) colloid particles showed the pH-sensitivity and ion responsitivity as well as surface activity. In addition, the P(St-alt-MA-Dopa) colloid particle was a novel polymeric soft particle emulsifier:with P(St-alt-MA-Dopa) solution as water phase and toluene as oil phase, stable Pickering oil-in-water emulsions were produced. The absolute zeta potential value of the colloid particle increased with the increase of pH and decreased with the increase of salt concentrations; the size of the colloid particles increased with the increase of pH. The hydrophobility of the P(St-alt-MAn-Dopa) colloid particle was determined by fluorescence technique and the result showed that the increasing pH led to the increase of hydrophilicity, whereas the ionic strength had little effect on its hydrophobility. The P(St-alt-MA-Dopa) colloid particle exhibited good emulsifying property within a wide range of pH and very low concentration (0.5 mg/mL) and the pH and salt concentration had some influence on its emulsification property. The above results preliminarily attest our proposed conjectures:functional and responsive colloid particles can also be prepared from the assembly of random-alternating copolymers; functional alternating amphiphilic copolymer with the shortest hydrophilic/hydrophobic chain segments can self-assemble into a surface active colloid particle with emulsification property, which makes it a novel soft particle emulsifier.2. The synthesis and self-assembly of amphiphilic random alternating PSMVM copolymersA new amphiphilic random alternating-like copolymer PSMVM P(St-alt-MAn)-co-(VM-alt-MAn) was prepared through free radical copolymerization with St, coumarin-containing styrene monomer (VM) and MAn as monomers. The copolymer self-assembled into a stable amphiphilic spherical colloid particle (50-100 nm diameter) in selective solvent, which has photo, pH and ionic strength sensitivity. The transmission electron microscopy (TEM) showed that the PSMVM colloid particle exhibited dispersion microphase separation structure which was possibly related to the structure of PSMVM. The UV irradiation led to the cross-linking of coumarin group inside PSMVM particle and the TEM showed that the colloid particle had a tendency of collapse deformation and smaller size with the increasing irradiation time and the over-irradiation led to aggregation of the colloid particles. Compared with the uncrosslinked colloid particles (UCL-PSMVM), the crosslinked colloid particles (CL-PSMVM) showed better solvent resistance. The XPS results confirmed that there were relatively enriched oxygen at the surface of the aggregate and it may be caused by the accumulation of hydrophilic carboxyl acid group on the surface of particles which stabilized the colloid particles. The carboxyl group of PSMVM imparted pH sensitivity to PSMVM colloid particle:the zeta potential became more negative and the size increased with increasing pH value. The addition of NaCl could shield the surface charge of the PSMVM colloid particle and thus led to a decrease in absolute zeta potential value and an increase in hydrophobility and size of the colloid particle. In this part, the immobilization of the PSMVM colloid particles was achieved by crosslinking and could be further inferred that it would have practical significance in the preparation and application of polymeric soft particle emulsifier.3.Emulsification behavior of amphiphilic random alternating PSMVM copolymer colloid particleThe focus of this section was soft particle emulsifier from self-assembling random alternating copolymer. The effect of environment on the emulsification behavior of PSMVM colloid particle was studied and the entrapment of oil substance as well as its absorption behavior on the oil/water interface was also explored. The results showed that PSMVM particle could serve as soft particle to stabilize oil phase such as toluene at very low concentration, resulting in stable oil-in-water Pickering emulsion. The colloid particle had good emulsification property in a wide pH range and the size of the emulsion droplet decreased with increasing pH, however the stability of emulsion declined leading to demulsification when pH value exceeded 11. After UV crosslinking, the obtained crosslinked PSMVM colloid particle showed improved emulsification property and demulsification was not observed in the pH range of 2-11. We also observed that the addition of salt improved the emulsification property. Using the simple emulsion and then solidification by the polymerization of oil phase, the PSMVM colloid particles were immobilized on the surface of the solid oil latex, which indicated that PSMVM colloid particles formed a particle film at the oil/water interface to stabilize the oil droplet, confirming the emulsion mechanism. The PSMVM colloid particle emulsion had longterm stability. Furthermore, successful entrapment of oil dye by the PSMVM colloid particle as emulsifier has been achieved and stable emulsions have been obtained. The molecular weight of PSMVM also had effects on its emulsification property and for the colloid particle with low molecular weight, demulsification occurred at pH=10.4.Self-assembly and property of amphiphilic random copolymer P(DM-co-AA-co-CA)A completely random copolymer P(DM-co-AA-co-CA) were synthesized via free radical copolymerization of coumarin-containing acrylic acid monomer (CA), together with dimethylaminoethyl methacrylate(DM) and acrylic acid(AA). In the selective solvent the copolymer P(DM-co-AA-co-CA) self-assembled into multi-sensitive colloid particle with emulsification property. The results showed that the size, charge, LCST and emulsification of the colloid particle could be controlled through the tuning of the external stimuli such as light, pH, ionic strength and temperature. The PDM and PAA segment in P(DM-co-AA-co-CA) endowed it amphoteric polyelectrolyte characteristics and the charge of the colloid particle was positive in acid condition and negative in base condition. At its isoelectric point (pH=8), the P(DM-co-AA-co-CA) colloid particle was neutral and become the smallest, and it almost lost surface activity, as a consequence its emulsification property disappeared. However, the P(DM-co-AA-co-CA) colloid particle had good emulsification in both acid and base environment and a stable oil-in-water emulsion could be obtained. The PDM chain segment endows the colloid particle thermosensitivity:the addition of salt increased low critical solotion temperature (LCST) whereas the addition of base led to the decrease of the LCST. The ionic strength had a complex influence on its emulsification property. The UV absorption and fluorescence intensity of the micelle solution reached the maximum at pH 8.54. In addition, the photo-crosslinking improved the stability and emulsification property of the colloid particles.5.Synthesis and aqueous solution properties of soy protein isolate graft copolymers SPI-g-NH-PAMPSGraft copolymers based on soy protein isolate with amino-terminated poly(2-acrylanmido-2-methyl propane sulfonic acid) (SPI-g-NH-PAMPS) were prepared by a grafting reaction between the free carboxylic acid groups of soy protein isolate (SPI) and the amino groups of poly(2-acrylanmido-2-methyl propane sulfonic acid) (H2N-PAMPS) using 1-(3-(dimethylamino)propyl)-3-ethyl-carbodiimide hydrochloride/N-Hydroxy-succinimide as the condensing agents in a buffer solution. The results showed that the hydrophilicity of SPI was improved after the graft reaction, which was attributed to that the hydrophilic sulfonic groups of the grafted H2N-PAMPS chains change the surface charge of SPI. SPI belonged to globulin and its size was around 180nm, whereas SPI-g-NH-PAMPS aggregates presented core-shell structure with a light halo PAMPS grafting shell around SPI and the size increased to ca.200 nm. The graft polymer SPI-g-NH-PAMPS showed no isoelectric point and its hydrophilicity and emulsification properties were improved at the isoelectric point of SPI (pH 4.5).In conclusion, we have synthesized four kinds of macromolecular amphiphilic systems with different chain structures, namely alternating copolymer, random alternating copolymer, random copolymer and graft copolymer of SPI and studied their self-assembly and emulsification properties. The above results showed that we have resolved the two research foci we proposed:1) stable and surface active polymer colloid particle could be prepared from the self-assembly of simple and readily available amphiphilic random alternating copolymers in common selective solvent; 2) this colloid particle have good emulsification property and could serve as polymeric soft particle emulsifier to prepare Pickering emulsion and entrap oil substance. Hence, a new methodology in the exploration of preparing particle emulsifier from self-assembling random alternating copolymer for application has been developed in this thesis. In addition, compared to the commonly reported preparation methods of surface active colloid particles—emulsion polymerization, dispersion polymerization and surface modification, self-assembly can more easily produce colloid particles with different shapes and properties by designing and tuning the chemical composition and chain structure as well as self-assembling environment, which allowed us more easily to control the assembly and disassembly process, benefiting the research of the emulsion mechanism of colloid particles.Our work has expanded the horizons of research interests in polymer self-assembly micellization and has great significance to ultimately realize its commercial or industrial applications.