The influence of block copolymers on silica-filled polyisoprene

In the present work, precipitated silica, liquid polyisoprene, and PDMS-PI block copolymer are mixed in different formulations. The dynamic Theological properties of the resulting uncured materials are measured and are used to assess filler-filler networking. In small-amplitude tests, composites containing PDMS-PI exhibit lower values of the dynamic storage modulus (G′) than those observed in control samples prepared without copolymer additives. The control samples remain stable during several months of storage, while samples containing low molecular weight block copolymers show evidence of an aging process. The Theological manifestations of this aging include a pronounced decrease in small-amplitude G′ with time, accompanied by weakening amplitude-dependence of both G′ and G″. These suggest a progressive reduction of filler-filler interactions, presumably caused by the development of a block copolymer layer at the filler-polymer interface. The make-up of that interfacial region is investigated by means of magic-angle spinning solid-state ¹H NMR experiments carried out on bound polymer fractions that survive extraction in a good solvent. Spin-spin relaxation times and the intensities of the relaxation modes associated with these characteristic times are used to identify the proportion of both PDMS and polyisoprene among the segments strongly immobilized by close contact with the silica surface and among the rest of the bound (un-extracted) interfacial layer. The changes in mechanical properties are correlated not with the overall composition of the bound polymer fraction, but with the increase in PDMS segments in close contact with the filler surface.