Structure–melt viscosity relationship of discrete sequence-specific linear oligo(dimethylsiloxane-co-diphenylsiloxane)s

Abstract

Polysiloxanes are widely used in various household, industrial, and biomedical products; however, the effects of their composition and sequence on the melt viscosity have not been investigated in detail. A series of discrete linear oligosiloxanes with precisely controlled compositions and sequences was prepared in this study, and the relationship between their molecular structures and melt viscosities was examined. The oligomers possessed linear structures composed of dimethylsiloxy (D) and diphenylsiloxy (P) units with trimethylsiloxy (M) ends. P-repeat oligomers contained consecutive P blocks including exact numbers of P units. The prepared oligomers were employed to explore the specific effects of block structures. The 26-meric sequence isomers exhibited identical M/D/P compositions and four sequence-repeating blocks: (PD)₁₂, (PPDD)₆, (PPPDDD)₄, and (PPPPDDDD)₃. This facile synthetic methodology enabled the preparation of well-defined oligomers on the gram scale and estimation of their melt viscosities through actual measurements. The oligodimethylsiloxanes were also compared using previously reported viscosity values. Each oligosiloxane was uniform in terms of the M/D/P composition, sequence, and molecular weight. These oligosiloxanes were distinguished by their viscosity–molecular weight relationships; a steep enhancement in viscosity with an increasing number of consecutively repeated P units and a distinct influence of block sequence patterns on the viscosity of the 26-mers were observed. To gain molecular insights, the oligosiloxane viscosities were analyzed using the Rouse model and its modifications, and the effects of structural factors on the melt viscosity of the oligosiloxanes were discussed. The findings of this work demonstrate that appropriate Rouse model variants can effectively explain how the composition and sequence influence the melt viscosity of oligosiloxanes.