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Low-molecular-weight poly(ethylene oxide) (PEO) was grafted onto high-surface-area (A380) fumed silica (SiO2) by the use of a silane coupling agent, (CH3O)3−Si−(CH2)3−(OCH2CH2) x −OCH3, with x = 6−9, corresponding to an average molecular weight for the PEO segment of 375 g/mol, PEO(375)−silane. The PEO-grafted silica was characterized by thermogravimetric analysis and differential scanning calorimetry. In the case of PEO(375)−SiO2, the maximum grafting ratio, σ ∼ 2 molecules/nm2 (approximately 33 wt %), corresponded to less coverage than that expected for a monolayer of crystalline upright chains. No crystallization was observed, and the glass transition temperature, T g, increased from −90 °C for the unattached PEO(375)−silane to −54 °C for the attached PEO(375)−SiO2 chains. In the case of PEO(5000)−SiO2, a similar maximum weight percent coverage (∼38%), corresponding to σ ∼ 0.2 molecules/nm2, was obtained. At the highest grafting ratio, the calculated unperturbed chain dimensions in solution were approximately the same as those of the area occupied by the grafted PEO(5000) chains. A melt endotherm, with a melt temperature, T m, and enthalpy, ΔH m, that were decreased with respect to the neat PEO(5000)−silane, but no Tg, was observed. With decreasing σ, as the surface area occupied by the grafted chains increased with respect to the calculated chain dimensions in solution, ΔH m, T m, and the crystallization temperature, T c, decreased. The results suggest that the interaction of ethylene oxide segments with surface silanols inhibits the chain mobility necessary for crystallization.

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Poly(ethylene oxide) (PEO) of 4600 molar mass (PEO‐4600) was crystallized from methanol in the presence of hydrophilic fumed silicas (A380, A200, and OX50) with nominal surface areas of 380, 200, and 50 m2/g and a hydrophobic fumed silica (R812s) modified with methyl groups. The composites were characterized by thermogravimetric analysis and differential scanning calorimetry. The inhibition of crystallization and the tendency for chain reorganization after melting were in the order of A380 > A200 > OX50 > R812s, respectively, that is, both were least for the hydrophobic silica and increased with increasing specific surface area for the hydrophilic silica. The interaction of PEO with the silica increased in the melt state as compared with the solution‐cast samples, resulting in enhanced suppression of crystallization. The following took place at a high silica content: (1) crystallization occurred at crystallization temperatures [Tc < Tc (bulk)], suggesting that the silica inhibited crystallization; (2) crystallites with melt temperatures [Tm < Tm (bulk)] were observed, indictive of smaller and/or less perfect crystals; and (3) melt entropies [ΔSm (surface) < ΔSm (bulk)] suggested that the interaction of surface silanols, SisOH, with PEO decreased both the melt entropy and crystallite size/perfection. Crystallinity was observed in solution‐cast composites when there were greater than ∼0.03 PEO molecules/nm2 for native and ∼0.01 PEO molecules/nm2 for methylated fumed silica, similar to reported plateau equilibrium adsorption values from methanol. These results were consistent with a model in which PEO interacted more strongly with native fumed silica as compared with hydrophobically modified silica because of hydrogen bonding of the ether oxygens of PEO with the acidic silanols, preventing chain mobility and crystallization. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1978–1993, 2003

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