Unusual liquid crystalline (LC) behavior and interesting
nanophase
separation behavior in the self-assemblies of a series of diblock
copolymers (BCPs), poly(dimethylsiloxane)-b-poly{2,5-bis[(4-butoxyphenyl)oxycarbonyl]styrene}
(PDMS-b-PBPCS), were observed. The synthesized PDMS-b-PBPCS BCPs with PBPCS blocks of different molecular weights
had volume fractions of PBPCS (f
PBPCS’s)
from 77% to 90%. At the molecular scale, the BCPs with f
PBPCS values of 77% and 83% were in amorphous isotropic
state in the entire temperature range studied, and thus, they were
in coil–coil BCPs. For the BCPs with f
PBPCS values of 87% and 90%, the PBPCS blocks displayed a thermodynamically
stable isotropic phase at lower temperatures and an LC phase at higher
temperatures. Therefore, they exhibited a transition from the coil–coil
to rod–coil BCPs with increasing temperatures. At the nanophase
structure scale, all of the BCPs were coil–coil type and self-assembled
into body-centered cubic (BCC) structures in the low temperature range.
Moreover, some of the nanophase-separated structures of these BCPs
could be thermoreversibly transformed by changing temperatures. For
the BCPs with f
PBPCS values between 77%
and 83%, they remained BCC nanophase-separated structures during heating
(up to 260 °C) as well as during subsequent cooling. For the
BCPs with f
PBPCS values of 87% and 90%,
when the temperature was higher than the isotropic-to-LC transition
temperatures of the PBPCS blocks, the new nanophase-separated hexagonally
packed cylinder (HEX) structures started to form. However, only the
nanophase-separated structure of the BCP with an f
PBPCS value of 90% could completely transform to the HEX
structure when the temperature reached 250 °C. As the temperature
was reduced, the nanophase-separated structures of these two samples
gradually reverted to the BCC structures below the LC-to-isotropic
transition temperatures of the PBPCS blocks.
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