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Structure-Property Relationships in Polyurethane-Carbon Particle Nanocomposites
[Thesis]. Manchester, UK: The University of Manchester; 2013.
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In this research work, the relationships between structure and properties in micro-composites and nano-composites of polyurethane (PU) and conductive carbon particles have been studied. PU is a class of block copolymers containing the urethane linkage (-NHCO-O-) within its structure. Most PU block copolymers consist of alternating ‘soft’ and ‘hard’ segments. The hard segment used in this study was based on 4,4’-methylenebisphenylisocyanate (MDI) and 2-methyl 1,3 propanediol (MP-Diol) which produced a stiff aromatic polyurethane. Two soft segments; poly(tetrahydofuran) (PTHF) and poly(propylene oxide) based polyol end-capped with ethylene oxide (PPO-EO) were used to study the effects of soft segment structure on PU properties. DMTA, DSC and modulated-DSC indicated that PU-PTHF had higher microphase separation due to greater immiscibility between PTHF and the MDI/MP-Diol hard segments. In order to improve the electrical and mechanical properties of PU, conductive carbon particles were incorporated. The critical factor was the dispersion of these conductive fillers in the PU matrix to obtain optimum properties. The first carbon filler studied was carbon black (CB). PU composites prepared by the adding of MP-Diol plus ultrasonication (MU) gave the best dispersion of CB aggregates resulting in higher thermal decomposition temperature and good conductivity. However, the mechanical toughness was reduced. In subsequent studies, PU composites incorporating three different treated multiwalled carbon nanotubes (MWCNT) were investigated. MWCNT were disentangled and shortened by ultrasonication and acid cutting treatments. The ultrasonicated MWCNT (MWCNT_U) had longer length than the acid-cut MWCNT (MWCNT_AC). Ultrasonication was the best technique for dispersing MWCNT since the storage modulus was increased by ~200% at low MWCNT_U loading and the toughness remained the same as unfilled PU. PU/MWCNT_AC nanocomposites at 1 – 3 wt% of MWCNT_AC exhibited similar electrical conductivities to unfilled PU at an order of 10-8 S/cm, implying that the acid cutting treatment might disturb the inherent conductivity in MWCNT. The conductive percolation thresholds of composites were determined following the percolation theory. It was found that the percolation thresholds for MWCNT-filled composites were significantly lower than that of CB-filled composites. The lowest percolation threshold was observed in MWCNT_U-filled composite at 0.31 wt%.