Small-angle X-ray scattering and
atomic force microscopy were used
to observe smectic–smectic phase segregation in binary mixtures
of rigid-rod-like helical poly[n-decyl-(S)-2-methylbutylsilane] (PDMS) polymers with molecular-weight ratios
of 4.82 and narrow molecular-weight distributions. Phase segregation
is attributed to entropic effects as the homopolymers in the binary
mixture differ only in molecular weight. Entropy-driven segregation
in smectic phases was theoretically predicted in mixtures of rodlike
particles with different lengths under high pressure. Binary mixtures
of long and short PDMS with broader molecular-weight distributions,
which do not form smectic phases, showed no such segregation, verifying
that the driving force for segregation is the entropy gained through
smectic-phase formation. The binary mixture of a long PDMS with a
narrow molecular-weight distribution and a short PDMS with a broad
molecular-weight distribution showed segregation of each component,
indicating that the entropy gain of short-polymer-only smectic phase
formation surpasses the loss of mixing entropy.
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