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Impact of Helical Chain Shape in Sequence-Defined Polymers on Polypeptoid Block Copolymer Self-Assembly
Controlling the self-assembly
of block copolymers with variable
chain shape and stiffness is important for driving the self-assembly
of functional materials containing nonideal chains as well as for
developing materials with new mesostructures and unique thermodynamic
interactions. The polymer helix is a particularly important functional
motif. In the helical chain, the traditional scaling relationships
between local chain stiffness and space-filling properties are not
applicable; this in turn impacts the scaling relationships critical
for governing self-assembly. Polypeptoids, a class of sequence-defined
peptidomimetic polymers with controlled helical secondary structure,
were used to systematically investigate the impact of helical chain
shape on block copolymer self-assembly in a series of polyÂ(<i>n</i>-butyl acrylate)-<i>b</i>-polypeptoid block copolymers.
Small-angle X-ray scattering (SAXS) of the bulk materials shows that
block copolymers form hexagonally packed cylinder domains. By leveraging
sequence control, the polypeptoid block was controlled to form a helix
only at the part either adjacent to or distant from the block junction.
Differences in domain size from SAXS reveal that chain stretching
of the helix near the block junction is disfavored, while helical
segments at the center of cylindrical domains contribute to unfavorable
packing interactions, increasing domain size. Finally, temperature-dependent
SAXS shows that helix-containing diblock copolymers disorder at lower
temperatures than the equivalent unstructured diblock copolymers;
we attribute this to the smaller effective <i>N</i> of the
helical structure resulting in a larger entropic gain upon disordering.
These results emphasize how current descriptions of rod/coil interactions
and conformational asymmetry for coil polymers do not adequately address
the behavior of chain secondary structures, where the scalings of
space-filling and stiff–elastic properties relative to chain
stiffness deviate from those of typical coil, semiflexible, and rodlike
polymers
Electrochemical Effects in Thermoelectric Polymers
Conductive polymers such as PEDOT:PSS
hold great promise as flexible
thermoelectric devices. The thermoelectric power factor of PEDOT:PSS
is small relative to inorganic materials because the Seebeck coefficient
is small. Ion conducting materials have previously been demonstrated
to have very large Seebeck coefficients, and a major advantage of
polymers over inorganics is the high room temperature ionic conductivity.
Notably, PEDOT:PSS demonstrates a significant but short-term increase
in Seebeck coefficient which is attributed to a large ionic Seebeck
contribution. By controlling whether electrochemistry occurs at the
PEDOT:PSS/electrode interface, the duration of the ionic Seebeck enhancement
can be controlled, and a material can be designed with long-lived
ionic Seebeck enhancements