Synthesis and electrical properties of functional block copolymer/inorganic nanocomposite materials

Hongying Chen

Abstract

Sulfonated poly[styrene-b-ethylene-co-butylene-b-styrene] (sSEBS) block copolymers/inorganic nanocomposite materials were synthesized via in situ formation of inorganic fillers and characterized particularly for their dielectric properties and proton conductivities. In preparation of sSEBS/SrTiO3 nanocomposites, titanium (IV) isopropoxide [Ti(OPri )4 ] complex was diffused into sSEBS film, followed by subsequent hydrolysis of [Ti(OPr i )4 ], diffusion of strontium cations in sSEBS domains, and in situ formation of crystalline SrTiO3 . sSEBS with sulfonation degree of 38.1% and 65.0% were employed, and relevant sSEBS/SrTiO 3 composites contain SrTiO3 of 10-15 wt%. Elemental composition characterization with ESEM/EDX indicated uniform distribution of Sr and Ti. TEM images revealed clusters of SrTiO3 rods were selectively formed in hydrophilic domains in sSEBS with nanophase separation in lamellar morphology. TEM/SAED of sSEBS/SrTiO3 confirmed crystalline SrTiO3 structure inside composite film. The dielectric enhancement and shift with frequency showed potential of these materials for energy storage and conversion devices. sSEBS/silicate composites as model nanocomposite proton exchange membranes were prepared via in situ sol-gel reactions of tetraethylorthosilicate (TEOS) in sSEBS solutions that were solution cast into films. These hybrid membranes exhibited nanophase separated morphology with the particles mainly dispersed in the hydrophilic sulfonated block. The number of water molecules per sulfonic acid group increased with silicate content. Some sSEBS/silicate membranes exhibited lower methanol permeability than Nafion ® 117 while others showed higher methanol permeability. Methanol permeability increased with introduction of silicate which was attributed to the broadening of hydrophilic domains by silicate insertion. Besides mechanical improvement, proton conductivity increase in membranes containing around 10wt% silicate is discussed in terms of the morphological change and synergetic effect by silicate particles. Macromolecular dynamics of sSEBSs were investigated using broadband dielectric spectroscopy (BDS). Two main relaxations corresponding to the glass transitions in the EB and S block phases were identified, and their temperature dependences were VFT - like. Tg for the S block phase shifted to higher temperature due to restrictions on chain mobility caused by hydrogen bonded SO3 H groups. While the EB block phase T g appeared to remain constant with degree of sulfonation in DMA experiments, it shifted somewhat upward in BDS spectra. The fragilities of the EB and S block domains in sSEBS decreased after sulfonation. The temperature dependence of the dc conduction contribution to sSEBS loss spectra also followed VFT-like behavior, and S block segmental relaxation time correlated well with conductivity according to the fractional Debye-Stokes-Einstein equation.