Study On Preparation Of Porous Polymer Monoliths Via Emulsion Template Polymerization Method

When the external phase of high internal phase emulsion (HIPE) is polymerized, a porous polymer monolith with interconnected pore structure can be achieved with the internal phase serving as the template. The monolith prepared by HIPE template polymerization method is called PolyHIPE. Benefiting from the high porosity, uniform pore structure and precise controlling on the porous stucture, these monolithic materials have shown great potentials in seperation, catalyst and tissue engineering etc. Nevertheless, PolyHIPEs always possess poor mechanical properties, brittle and chalky. They normaly have implex and hydrophobic surface which requires further functionalization. This thesis is to develop the innovative study to solve the problems above.Here, the typical water-in-oil (W/O) HIPE containing the external phase of styrene (St) / divinylbenzene (DVB) and internal phase of water was chosen. Several novel methods were introduced to improve the mechanical property and surafce functionality of PolyHIPEs, such as RAFT polymerization, miniemulsion and interfacial polymerization etc. The following aspects were covered.(1) A series of PolyHIPEs with open cellular structure were prepared via HIPE RAFT polymerization by using benzyl dithioisobutyrate (BDIB), 4-cyano-4-(dodecylsulfanylthiocarbonyl)sulfanyl pentanoic acid (CDSPA), 2-{[(dodecylsulfanyl)-carbonothioyl] sulfanyl} isobutyronitrile (DSCSIB) and 2-{[(dodecylsulfanyl)-carbonothioyl]sulfanyl}propanoic acid (DCSPA) as the polymerization control reagent, respectively. Compared with conventional HIPE polymerization, the porous interconnectivity was enhanced as well as the mechanical properties. Especially when DCSPA was used as the RAFT agent, the crush strength and compressive modulus reached 1.5 MP and 45 MPa, which was 2 and 2.6 times increase, respectively. Besides, PolyHIPEs prepared by HIPE RAFT polymerization remained intergrity during compression which indicated the chalkiness problem was well solved. This advantageous performance was attributed to the different polymerization process. In the conventional HIPE crosslinking polymerization, a small number of macromolecules with pendant double bonds were generated at the very beginning. These high molecular weight chains intended to form microgels further through intramolecular crosslinking. Eventually, the microgels connected with each other and formed a complete polymer network. These microgels owing certain strength can restrain the volume shrinkge induced by the convertion from monomers to polymers which resulted in the formation of amounts of nanopores. And these nanopores existed as the defects which leaded to poor mechanical performance of PolyHIPEs. In the HIPE RAFT crosslinking polymerization, however, amounts of oligomer radicals were generated at the beginning. During the entire polymerization, microgels were avoided and intermolecular crosslinking occurred until a homogeneous polymer network was developed, which contributed to superior mechanical properties.(2) W/O miniemulsions containing 40～60% internal phase were prepared via ultrasound treatment. The porous monolith with average void size of 300 nm was then synthesized by polymerizing the organic phase of miniemulsion, which was called Poly(miniMIPE). It was identified that using miniemlsion template polymerization method the open-celluar structure could be achieved even with a dispersed phase volume fraction down to 40%, which relieves the traditional restrict that internal phase fraction of more than 74% was necessary to form an interconnected void-window structure. The mechanical strength of Poly(miniMIPE) was enhanced remarkably, where crush strength and compressive modulus were increased to 23.5 MPa and 410 MPa, respectively. The porous structure was uniform. The average size of voids and windows were decreased down to 300 nm and 85 nm respectively. It was inferred that the distance between adjacent droplets was reduced greatly by introducing miniemulsion template, which was crucial for the formation of interconnected windows.(3) Three different methods of using flexible crosslinker, copolymerizing soft monomer and introducing elastic oligomer were employed to decrease the brittleness of polystyrene-based PolyHIPEs. It was identified that copolymerizing 2-Ethylhexyl acrylate (EHA) as soft monomer has little influence on the void-window microstructure. And the brittleness of porous monoliths was decreased effectively on the assurance of certain mechanical strength.(4) Porous polymeric monoliths with carboxyl groups enriched on the surface of voids were prepared via using acrylic acid (AA) as water soluble monomer or introducing macroRAFT agent containing poly-(AA) chains as reactive surfactant, respectively. Interfacial polymerization was realized by using water soluble initiator (KPS) and monomer (AA) in W/O HIPE. Amount of carboxyl groups on the surface of voids was increased with the increase of AA content, while the interconnectivity of monolithic materials was decreased. Moreover, crush strength and compressive modulus were increased greatly with the introduction of AA segments.