We have successfully synthesized linear bifunctional
PNIPA polymers
with uniform molecular weight using living radical polymerization.
Our objective was to construct a polymer network with a consistent
and an ordered structure. To achieve this, we prepared a solution
with a concentration higher than the overlap concentration (C*) of
the linear PNIPA polymers with uniform molecular weight. Next, we
introduced a cross-linking agent to bond the polymer ends together.
Under our experimental conditions using this linear PNIPA polymer,
we prepared polymer networks at different concentrations: 1.2C*, 1.5C*,
and 2.0C*. Notably, the polymer network formed at 1.2C*, which is
slightly higher than the overlap concentration, exhibited a significantly
more ordered structure compared to those prepared at 1.5C* or 2.0C*.
This observation was substantiated by the analysis of the polymer
network degradation products through SEC measurements and the SAXS
results of the polymer network. In the case of PNIPA star-shaped polymers,
it has been found that a uniform network structure can be obtained
by preparing the polymer network at a concentration of about 2.0C*,
which is much higher than C*. The difference in excluded volume between
linear polymers and star polymers appears to play a pivotal role in
favoring the formation of a more ordered network structure when the
concentration reaches the overlap concentration threshold. In summary,
our study showcases the successful synthesis of linear bifunctional
PNIPA polymers with uniform molecular weight through living radical
polymerization. Moreover, we demonstrate that by carefully controlling
the concentration during polymer network synthesis, it is possible
to achieve a relatively ordered structure, especially when the concentration
slightly exceeds the overlap concentration threshold. The insights
gained from our findings contribute to a better understanding of polymer
network formation and have implications for the design and fabrication
of advanced materials.