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 m(2)/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 [T-c < T-c (bulk)], suggesting that the silica inhibited crystallization; (2) crystallites with melt temperatures [T-m < T-m (bulk)] were observed, indictive of smaller and/or less perfect crystals; and (3) melt entropies [DeltaS(m) (surface) < DeltaS(m) (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 similar to0.03 PEO molecules/nm(2) for native and similar to0.01 PEO molecules/nm(2) 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. (C) 2003 Wiley Periodicals, Inc.