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Stabilizing the Ordered Bicontinuous Double Diamond Structure of Diblock Copolymer by Configurational Regularity
journal contribution
posted on 2018-05-18, 18:22 authored by Chih-Hsuan Lin, Takeshi Higuchi, Hsin-Lung Chen, Jing-Cherng Tsai, Hiroshi Jinnai, Takeji HashimotoWe
investigate the formation of the ordered bicontinuous structures
in a stereoregular diblock copolymer, isotactic polypropylene-block-polystyrene (iPP-b-PS), in which
the minority PP block possessed isotactic configuration. This diblock
displayed the conventional ordered bicontinuous double gyroid (OBDG)
morphology upon heating above the crystal melting point of the iPP
block from the as-cast state. The OBDG phase remained stable in the
heating process up to the order–disorder transition. In the
subsequent cooling process from the nearly disordered state, the OBDG
phase first developed, but when the temperature was sufficiently low,
an order–order transition from OBDG to the ordered bicontinuous
double diamond (OBDD) phase occurred, and OBDD eventually became the
dominant structure. The results attested that OBDD and OBDG represented
the thermodynamically stable structure at the lower and the higher
temperature, respectively, and the OBDG morphology formed in the as-cast
state was metastable. The present finding along with that of the syndiotactic
polypropylene-block-polystyrene (sPP-b-PS) reported previously consolidated the role of configurational
regularity in stabilizing the otherwise unstable OBDD phase for diblock
copolymers. The stability of the OBDD structure was attributed to
the cooperative effect of the relatively high polydispersity and the
helical segment formation of the stereoregular minority block, as
the conformational entropy loss arising from the packing frustration
of the minority block in the network domain was effectively compensated
by the release of enthalpy via the formation of helical segments and
their associations.