Morphology Regulation And Formation Mechanism Of Functional Polymeric Particles In Soap-Free Emulsion System

Colloidal particles prepared by soap-free emulsion polymerization have been found widely applications in the fields of medical treatment,medicines,biochemistry,electronics,chemical engineering,energy and environment owing to their clean surfaces and outstanding monodispersity.As the increasing demand of applications,it is eager to design and produce polymeric particles with complex microstructures,various morphologies,tunable size and surface properties by chosing functional monomers based on specific requirements.However,due to the lack of systematical illumination of the formation mechanism of polymeric colloids with complex morphology by the copolymerization of comonomers and the guide of corresponding fundmental theory,it is difficult to prepare functional monodisperse particles with complex morphology by one-step soap-free emulsion polymerization.And polymeric particles with controllable size,monodispersity and morphology could hardly be produced for the poor controllability of the molecular weight of polymer and the process of nucleation.Meanwhile,colloidal particles prepared in soap-free emulsion system by the integration of control/“living” polymerization mainly ATRP,NMP and RAFT polymerization possess tedious processes for preparation and purification.Based on the above problems in producing particles by soap-free emulsion polymerization,this dissertation focuses on the morphology regulation and formation mechanism of polymeric particles with controllable phase-separated structures and tunable size and molecular weight by one-step soap-free emulsion polymerization,and the main contents of this dissertation include the following aspects:Using styrene(S)and glycidyl methacrylate(GMA)as comonomers,divinylbenzene(DVB)as crosslinker and potassium persulfate(KPS)as initiator,phase-separated P(S-GMA)particles were synthesized by one-step soap-free emulsion polymerization.The effects of mass ratio of monomers and adding amount of crosslinker on the morphologies of P(S-GMA)particles were discussed.Particles with various morphologies including homogeneous structure with smooth surfaces,raspberry-like structures and irregular spheres can be produced by regulating the mass ratio of S in the total monomers added.The formation mechanism of phase-separated P(S-GMA)particles by one-step soap-free emulsion polymerization were as follows,GMA was more likely to polymerize than S because of its higher reactivity and larger solubility in water,then GMA-enriched copolymers were produced in the early stage of the polymerization,and the S-enriched copolymers produced in the later stage of the polymerization were incompatible with previous GMA-enriched core particles and migrated from the interior part of the particles to the surface,forming raspberry-like particles.Particles with large nodules were obtained when the amount of crosslinker was relative low,for the phase separation process in the presence of crosslinker was much earlier than that without crosslinker.Particles with small coronas on the surface were produced when the adding amount of DVB was further increased since the time of phase separation was delayed.Phase-separated P(S-AA)particles with various surface roughnesses and morphologies including flower-like,raspberry-like structures and homogeneous spherical particles with smooth surfaces could also be produced by regulating the monomer ratio of S and acrylic acid(AA).When the mole ratio of DVB was increased,the surface roughness of the obtained raspberry-like particles was also increased.The formation mechanism of raspberry-like P(S-AA)particles was slightly different from that of raspberry-like P(S-GMA)particles.The S-enriched copolymers produced in the later stage of the polymerization were incompatible with AA-enriched copolymers obtained in the early stage of the polymerization and migrated towards the interior of the AA-enriched particles,forming raspberry-like particles.Coatings were assembled using raspberry-like P(S-AA)particles by vertical deposition method,and the coatings assembled at different temperatures showed different wettabilities.The coatings assembled at room temperature were hydrophilic,the water contact angles of the coatings increased as the increase of assembly temperature,and superhydruphobic coatings with water contact angle of 154.2° could be produced.However,the water contact angles dereased when the assembly temperature was above the glass transition temperature(Tg)of the polymers.In the process of assembly of P(S-AA)particles with increasing temperature below the Tg of polymers,the migration of hydrophobic components in the particles towards the surface of the particles driven by thermodynamics dereased the surface enery of the coatings,increasing the hydrophobicity.While in the process of assembly of P(S-AA)particles at temperature above the Tg of polymers,the surface roughness was decreased for the softening and fusing of the particles,decreasing the hydrophobicity.A series of “living” and monodisperse PS particles with tunable size ranged from 89 nm to 307 nm and tunable number average molecular weight ranged from 1180 g/mol to 11760 g/mol were produced using S as monomer,1,1-diphenylethlene(DPE)as control agent in soap-free emulsion polymerization by regulating the adding amount of S and DPE.By quantitatively researching the variation of mean diameter,zeta potential and number of particles in the process of the polymerization,two reasons were found to be responsible for producing particles with smaller size in the presence of DPE.One was that DPE reduced the monomer conversion.And the other was that primary particles produced in the presence of DPE were much stabler,and the coalescence between particles was reduced,resulting in increasing number of particles.These “living” colloids could be used as seeds to produce particles with various morphologies by initiating other monomers.The PGMA particles produced in the early stage of the polymerization “disappeared” and formed transparent polymer “suspensions” in the soap-free emulsion polymerization of GMA using DPE as control agent.Regulating the adding amount of GMA and DPE within a certain range,transparent polymer “suspensions” were produced,and the morphology of PGMA in TEM was thin film rather than colloids.The formation of the transparent PGMA “suspensions” may be related to the following factors:(1)The PGMA particles were swelled by large amount of water since that epoxy groups in PGMA were catalyzed to react with water to produce large amount of hydroxyl groups in the polymerization process;(2)The hydroxyl groups in the polymer could further react with epoxy to produce flexible aliphatic ether,which endowed the particles with flexible and proteiform;(3)Much shorter main polymer chain produced in the presence of DPE formed tiny hydrophobic domains surrounded by large amount of hydrophilic pendant groups.The synergistic effect of the above three factors resulted in the “disappearance” of PGMA colloids produced in the early stage of the polymerization.Various phase-separated structures including porous and raspberry-like P(S-GMA)paricles,and hollow and raspberry-like P(S-AA)particles were produced using DPE as control agent,S and GMA,and S and AA as comonomers in DPE controlled one-step soap-free emulsion polymerization by regulating the monomer ratio and crosslinking degree.By regulating the mass ratio of S and GMA,the obtained P(S-GMA)turned from transparent polymer “suspensions” to porous structures and then to irregular particles.The P(S-GMA)particles turned from folwer-like structures to raspberry-like structures and then to irregular structures with smooth surface as the increase of crosslinking degree.By adjusting the mole ratio of S and AA,the obtained P(S-AA)particles turned from transparent polymer solution to hollow structures and then to homogeneous particles with smooth surfaces.The P(S-AA)particles turned from irregular structures with smooth surface to raspberry-like structures as the increase of crosslinking degree.The formation mechanism of porous P(S-GMA)and hollow P(S-AA)particles was that the unpolymerized S resulted from the low conversion of monomers in the presence of DPE aggregated to from nano-sized monomer droplets,these droplets were incompatible with GMA-enriched particles and migrated towards the external of the particles,and the droplets were also incompatible with AA-enriched particles and migrated towards the internal of the particles,porous and hollow particles were produced when the unpolymerized nano-sized S droplets performed as in situ porogen were washed away.The drug loading and release behavior of porous P(S-GMA)and hollow P(S-AA)particles showed that the release rates of DOX were very slow in the weakly basic physiological environment and became very quick in weakly acidic environment.These colloids may be found widely applications in controlled drug delivery owing to their smaller sizes and p H responsive behavior.