Synthesis and Characterization of Multiphase Block Copolymers: Influence of Functional Groups on Macromolecular Architecture
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Low molecular weight liquid polybutadienes (1000 â 2000 g/mol) consisting of 60 mol% 1,2-polybutadiene repeating units were synthesized via anionic telomerization and conventional anionic polymerization. Maintaining the initiation and reaction temperature less than 70 Â°C minimized chain transfer and enabled the telomerization to occur in a living fashion, which resulted in well-controlled molecular weights and narrow polydispersity indices. MALDI-TOF mass spectrometry confirmed that the liquid polybutadienes synthesized via anionic telomerization contained one benzyl end and one protonated end. Subsequently, 2-ureido-4[1H]-pyrimidone (UPy) quadruple hydrogen-bonding was introduced to telechelic poly(ethylene-co-propylene), and mechanical characterization of the composites with UPy-functionalized carbon nanotubes was performed. The composites enhanced the mechanical properties and the UPy-UPy association between the matrix polymer and carbon nanotubes prevented the decrease of an elongation at break. The matrix polymer was also reinforced without sacrificing the processability. Additionally, UPy groups were introduced to styrene-butadiene rubbers (SBRs). Introducing UPy groups to SBRs drastically changed the physical properties of these materials. Specifically, the SCMHB networks served as mechanically effective crosslinks, which raised Tg and enhanced the mechanical performance of the SBRs. Novel site-specific sulfonated graft copolymers, poly(methyl methacrylate)-g-(poly(sulfonic acid styrene)-b-poly(tert-butyl styrene)), poly(methyl methacrylate)-g-(poly(tert-butyl styrene)-b-poly(sulfonic acid styrene)), and the corresponding sodium sulfonate salts were successfully synthesized via living anionic polymerization, free radical graft copolymerization, and post-sulfonation strategies. The graft copolymers contained approximately 9 â 10 branches on average and 4 wt% of sulfonic acid or sodium sulfonate blocks adjacent to the backbone or at the branch terminus. The mobility of the sulfonated blocks located at the branch termini enabled the sulfonated blocks to more readily interact and form ionic aggregates. The glass transition temperatures (Tg) of the sulfonated graft copolymers with sulfonated blocks at the branch termini were higher than that of copolymers with sulfonated blocks adjacent to the backbone. More facile aggregation of sulfonated blocks at the branch termini resulted in the appearance of ionomer peaks in SAXS whereas ionomer peaks were not observed in sulfonated graft copolymers with sulfonated blocks adjacent to the backbone. In addition, similar analogues, novel site-specific sulfonated graft copolymers, poly(methyl methacrylate)-g-(poly(sulfonic acid styrene)-b-poly(ethylene-co-propylene)) (PMMA-g-SPS-b-PEP), poly(methyl methacrylate)-g-(poly(ethylene-co-propylene)-b-poly(sulfonic acid styrene)) (PMMA-g-PEP-b-SPS), and the corresponding sodium sulfonate salts were successfully synthesized. Estimated ï £N values predicted the phase separation of each block and differential scanning calorimetry (DSC) and dynamic mechanical analysis confirmed the phase separation of each block component of the graft copolymers. The aggregation of sulfonic acid or sodium sulfonate groups at the branch termini restricted the glass transition of the PEP block. This lack of the glass transition of the PEP block resulted in higher storage modulus than a sulfonated graft copolymer with sulfonated blocks adjacent to the backbone. The location of sulfonated blocks in both sulfonic acid and sodium sulfonate graft copolymers significantly affected the thermal, mechanical and morphological properties. Lastly, symmetric (16000 g/mol for each block) and asymmetric (14000 g/mol and 10000 g/mol for each block) poly(ethylene-co-propylene)-b-poly(dimehtylsiloxane) (PEP-b-PDMS) were synthesized using living anionic polymerization and subsequent hydrogenation. The onset of thermal degradation for the PEP-b-PDMS diblock copolymer was higher than 300 ÂºC and PEP-b-PDMS was more thermally stable than the precursor diblock copolymer, polyisoprene-b-PDMS. DSC analysis of PEP-b-PDMS provided Tg of PDMS -125 ÂºC, Tg of PEP -60 ÂºC, Tc of PDMS -90 ÂºC, and Tm of PDMS -46 and -38 ÂºC, respectively. Appearance of thermal transitions of each PEP and PDMS block revealed the formation of phase separation. Estimated Ï N also supported the phase separation.
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