Structure and dynamics of block copolymer based soft materials

Abstract

Block copolymers are made by joining two or more polymer components into a single molecule. Due to the incompatibility between the interconnected chains, block copolymers form microphase separated domains of long range order at length scales of 5 - 100 nm. These materials have been the subject of intense study for the past four decades and the rich structural and dynamic behavior of block copolymers are still being explored. In this thesis, the structure, dynamics and mechanical properties of block copolymers and block copolymer blends were investigated using small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), dynamic mechanical spectroscopy (DMS), differential scanning calorimetry (DSC), and tensile testing. A new equilibrium block copolymer phase, the Frank-Kasper σ-phase was discovered in poly(1,4-isoprene-b-DL-lactide) (IL) diblock and poly(styrene-b-1,4- isoprene-b-styrene-b-ethylene oxide) (SISO) tetrablock copolymer melts. The σ-phase has tetragonal symmetry (P42/mnm) and possesses 30 spheres per unit cell. This gigantic crystal, a dodecagonal quasicrystal approximant, structure has been reported primarily in two heavy metals, numerous metal alloys, and dendrimers. Identification of the σ-phase in block copolymers provides new evidence regarding the complex nature of packing spheres on an ordered lattice. The dynamics ordered sphere-forming block copolymers was studied using SAXS and rheological techniques. The process of ordering into a body-centered cubic (BCC) morphology from the disordered state and the order-to-order transition (ODT) from the (metastable) BCC to σ-phase were found to follow nucleation and growth mechanisms. The IL diblock copolymer phase diagram was investigated as a function of composition and temperature. IL diblock copolymers are strongly segregated due to a relatively large Flory-Huggins interaction parameter χ between polyisoprene and poly(DL-lactide). Fluctuation effects strongly influence the ODT due to the low IL molecular weights and this was evidenced by thermal signatures in DSC thermograms. The structure and mechanical properties of poly(DL-lactide-b-1,4-isoprene-b-DLlactide) (LIL) triblock copolymer thermoplastic elastomer and low molecular weight IL diblock copolymers, and blends of these materials were studied. While the linear response is relatively invariant to the molecular architecture and molecular weight, the extensional properties were dramatically influenced by the triblock content. Finally, path dependency of microstructures of poly(1,2-butadiene-b-ethylene oxide) (PB-PEO) non-ionic block copolymer surfactants in oil and water was examined. Due to an extremely low critical micelle concentration due to the high molecular weight of the PB-PEO block copolymer surfactant, highly path dependent and long-lasting metastable microstructures were generated. This result offers new opportunities for the preparation of target block copolymer microstructures.