Yongmoon Kwon

Investigation of the Effect of Reaction Conditions on the Synthesis of Multiarm-star Polyisobutylene--polystyrene Block Copolymers

Abstract New multi-arm star block copolymers comprising of rubbery polyisobutylene (PIB) midsegment and glassy polystyrene (PS) end blocks have been synthesized by carbocationic polymerization using a new multifunctional initiator, hexaepoxy squalene (HES), with TiCl4 coinitiator, di-t-butylpyridine (DtBP) as a proton trap and N,N-dimethylacetamide (DMA) as an electron pair donor in methylcyclohexane (MeCHx)/methyl chloride (MeCl) solvent mixtures at −80 °C. It was found that reaction conditions, such as solvent composition, HES/isobutylene (IB) ratio and TiCl4 concentration, have profound influence on initiator efficiency and functionality. Living conditions were achieved in the presence of DMA in MeCHx/MeCl 60/40 v/v, while in the absence of DMA, the Mn–conversion plot showed a considerable intercept. Depending on the reaction conditions, the PIB midblocks had 3–10 arms. Reaction rates increased with increasing solvent polarity and TiCl4 concentration. Living narrow molecular weight distribution PIBs (Mw/Mn=1.1–1.2) were reacted with styrene (St) solution containing DtBP and DMA to yield multiarm-star PIB–PS block copolymers. Blocking was evidenced by SEC analysis and copolymers with 8.9–28.6 wt.% PS, Mn∼164,000–609,000 g/mol and Mw/Mn=1.32–1.88 were successfully synthesized.

Study of the Surface Morphology of Polyisobutylene-based Block Copolymers by Atomic Force Microscopy

This paper reports the investigation of the nanostructured surface morphology of linear polystyrene-block-polyisobutylene-block-polystyrene (SIBS) triblock copolymers and novel arborescent SIBS block copolymers by Atomic Force Microscopy (AFM) in the tapping mode. Thin films spin coated from toluene onto silicon wafers were studied. The nanostructured morphology of the block copolymers varied with the hard polystyrene (PS) and soft polyisobutylene (PIB) segment composition, ranging from spherical to lamellar nanometer-sized discreet PS phases dispersed in a continuous PIB matrix. Annealing the samples resulted in well developed/ordered structures. The arborescent blocks had irregularly distributed PS phases in the PIB matrix. Annealing had a dramatic effect on the morphology which still remained irregular. Three-dimensional AFM image and section analysis indicated the presence of a height difference between PIB (high-lying plateaus or hills) and PS (low-lying plateaus or valleys) in the block copolymers, which became more prominent during annealing. It is theorized that the rubbery PIB chains are able to relax, thereby protruding from the surface, anchored by the physically crosslinked PS phases.