Rw. Wang et al., Packing and thermal stability of polyoctadecylsiloxane compared with octadecylsilane monolayers, LANGMUIR, 16(15), 2000, pp. 6298-6305
FTIR, Raman spectroscopy, and thermogravimetric analysis (TGA) were used to
compare the structure and thermal properties of polyoctadecylsiloxane (POS
) and octadecyl trichlorosilane (OTS) monolayers. The octadecyl chains in P
OS had higher conformational and intermolecular order than the same chains
of OTS adsorbed as self-assembled monolayers (SAMs) on 106 nm hydrated sili
ca beads. The latter were identical to the structures observed for OTS SAMs
on superhydrated fumed silica that had primary particle sizes an order of
magnitude smaller than the 106 nm beads, indicating that curvature did not
affect the chain packing. The chains on POS were also more thermally stable
to conformational and intermolecular disorder than the OTS chains, and thi
s was attributed to increased free volume in the latter case. The differenc
es in structure were partially determined by steric restrictions that arise
because the Si-O-Si bond distance is less than the van der Weals radii of
the alkyl chains. In POS, the most likely structure is therefore one in whi
ch the octadecyl chains are pointing in the same direction from every other
Si atom, permitting good lateral chain packing. In the OTS SAMs, the octad
ecyl chains must all point in the same direction, away from the silica surf
ace. Therefore, linear chains would be excluded but small clusters of dimer
s or trimers could be accommodated. These restraints increase the nearest n
eighbor distance between alkyl chains on SAMs and do not permit them to be
as closely packed as for POS. This in turn contributes to the increased irr
eversible disordering of the chains with temperature.