Polyolefin/layered silicate nanocomposites via surface initiated ring opening metathesis polymerization
This thesis reports about the synthesis of thermoplastic elastomers via polyolefin/layered silicate nanocomposites and fabrication of porous membranes using a triblock Poly(styrene-b-isoprene-b-dimethylsiloxane) (SID) block copolymer. The first project work started as second phase of a NSF CAREER grant to study the properties of nanocomposites synthesized using polyolefins like norbornenes, alkyl substituted norbornenes and cyclopentene to engineer high performance nanocomposites. The polymers will be surface initiated from clay using ring opening metathesis polymerization and subsequent hydrogenation forms thermoplastic elastomeric nanocomposites with synergistic properties from mechanical reinforcement offered by layered silicates, enhanced thermal properties due to changed chain conformations and effects of block copolymers to combine different polymeric properties.
First published article on this work reported the successful synthesis of diblock copolymer/layered silicate nanocomposite via surface initiated ring opening metathesis polymerization (SI-ROMP) of norbornene and cyclopentene from montmorillonite clay (MMT) surface. The surface of MMT clay was organo modified using norbornene-terminated alkyl ammonium surfactant and block copolymer brushes were polymerized from the clay surface and disordering and dispersing the clay platelets in polymer matrix. Non-clay counterparts were obtained via reverse ion exchange methods and characterized in parallel with corresponding nanocomposite. Thermal, mechanical and morphological characterization showed the property enhancement due to addition of clay and showed good exfoliation of MMT.
After successful demonstration of exfoliated nanocomposites using surface initiated polymerization technique, we focused on development of nanocomposites with ethylidene norbornene as first block and cyclopentene as second block; subsequent hydrogenation of first block results in a soft and elastomeric poly(ethyl norbornene) block and second block results in a perfectly linear semi crystalline poly(ethylene) block. We followed the same experimental procedure used in the first article with necessary changes accommodating the needs of new monomer to synthesize exfoliated nanocomposites. We were successful in synthesizing surface initiated hydrogenated nanocomposites with clay exfoliation, however subsequent property evaluation in conjunction with the non-clay analogs showed that addition of clay negatively effects the thermal properties of hydrogenated poly(ethylidene norbornene) block. This observation was contrary to our hypothesis that clay confines the polymer chain movement and results in enhancement of thermal properties.
The second project in my thesis work focused on the study of porous membranes fabricated from a class of triblock copolymers for carrying out separations. This project exploits the microstructures formed due to self-assembly of block copolymers to form percolating network of pores in membranes fabricated using block copolymer material. A Poly(styrene-b-isoprene-b-dimethylsiloxane) (SID) block copolymer was studied as a potential system. A SID triblock copolymer resulting in gyroid phase was synthesized, characterized and fabricated as a thin porous membrane after chemical etching of D block and forming a network of pores with I block as pore lining and S block as membrane backbone. This membrane was tested via flow through experiments to establish successful separation based on size.