Hexapeptide Boc-(d-Ala-ΔZPhe-l-Ala)2-OCH3 [Boc = tert-butyloxycarbonyl; ΔZPhe = (Z)-β-phenyldehydroalanine] in the solid state exhibited a novel looping backbone consisting of three partially overlapped β-turns supported by intramolecular (i+3) → i hydrogen bonds. Here the looping backbone also included a β-bend ribbon structure.
It has been theoretically predicted that spherical particles added to the smectic phase formed by rod-like particles segregate between the smetic layers and significantly enhance the stability of the smectic phase due to an entropic effect based on the steric repulsion between the particles, which is referred to as the depletion effect. In this study, we describe the first experimental verification of the theoretical prediction, in which the binary mixtures of the rod-like polymer, poly[n-decyl-(S)-2-methylbutylsilane] (PDMS), with narrow molecular weight distributions, and a spherical branched alkane, squalane, were investigated by synchrotron radiation small-and wide-angle X-ray scattering (SR-SAXS and SR-WAXS) and atomic force microscopy (AFM) observations and found to reproduce the predicted segregation between the smectic layers.
The segregation of spherical molecules (squalane) between the smectic layers of rod-like polymers (polysilanes) with narrow molecular weight distributions were investigated by synchrotron radiation small-angle X-ray scattering (SR-SAXS), atomic force microscopy (AFM) observations, and molecular dynamics simulations to elucidate the effect of the polymer side chain length on the segregation. It has been theoretically predicted that the smectic phase of the rod-like particles will be stabilized by inserting the spherical particles into the interstitial region between the smectic layers when the diameter of the spherical particles is smaller than that of the rod-like particles whose length is sufficiently long. We found that the segregation of squalane was unaffected by the molecular weight (Mw) of the polysilane in the range of 9,200-44,100 g/mol, and the diameter of the polysilane showed the optimal size of 5.64 nm for the segregation of squalane whose diameter is 6.57 nm although the origin of these inconsistencies between theory and experiment is currently not clear.
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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