9 research outputs found
Interplay between Liquid Crystalline Order and Microphase Segregation on the Self-Assembly of Side-Chain Liquid Crystalline Brush Block Copolymers
Herein we investigate the influence
of competing self-organizing
phenomena on the hierarchical self-assembly of liquid crystalline
brush block copolymers (LCBBCs). A library of LCBBCs are synthesized
using ring-opening metathesis polymerization (ROMP) of norbornene
side-chain functionalized monomers comprising (1) cholesteryl mesogen
with nine methylene spacer and (2) semicrystalline polyÂ(ethylene glycol)
(PEG). The self-assembly of LCBBCs with variations in LC block content
(7–80 wt %) are investigated in their melt state. All LCBBCs
show two distinct thermal transitions corresponding to PEG semicrystalline
phase and LC mesophases. Interestingly, the LCBBCs display a multilevel
hierarchical structure evidenced by the results from X-ray scattering
and transmission electron microscopy (TEM): (1) smectic A (SmA) mesophases
(<i>d</i> = 3–7 nm) by the assembly of cholesteryl
side chains and (2) microphase segregation into lamellar or cylinder
(<i>d</i> = 40–75 nm) resulting from the incompatibility
between LC moieties and PEG side chain. Surprisingly, the presence
of microphase-segregated domains in LCBBCs prevents the formation
of cholesteric mesophase in sharp contrast to side-chain liquid crystalline
homopolymer (SCLCP) bearing the same mesogen and the flexible spacer.
This could be attributed to very high surface to volume ratio at intermaterial
dividing surface (IMDS) in LCBBCs, by which only LC layers (i.e.,
SmA mesophase) are favored to form at the IMDS. On the fundamental
side, these LCBBCs are an interesting scaffold to explore the impact
of interactions between LC order and microphase segregation of side-chain
polymeric brushes on the self-assembly of LCBBCs. Moreover, these
new LCBBC scaffolds will serve as a tool box for rational design of
hierarchically organized functional materials for stimuli responsive
applications
Surface Aligned Main-Chain Liquid Crystalline Elastomers: Tailored Properties by the Choice of Amine Chain Extenders
A promising way to induce shape transformation
in soft materials
is via spatial variation in the orientation of the alignment of liquid
crystalline elastomers (LCEs). Here, we improve the nascent thermomechanial
shape transformation in main-chain LCEs prepared via aza-Michael addition
reactions. Specifically, increasing the alkyl length in the <i>n</i>-alkylamine chain extender effectively reduces the actuation
temperature by destabilizing the nematic phase as well as reduces
the glass transition temperature (<i>T</i><sub>g</sub>)
by increasing the free volume. In addition, incorporating a hydroxyl
end-group in the amine chain extender (i.e., <i>n</i>-alkanolamine)
increases the actuation strain and improves the film quality by preventing
side-chain aggregates of <i>n</i>-alkylamine-functionalized
LCEs. Interestingly, uniaxially aligned <i>n</i>-alkanolamine-functionalized
LCEs exhibit an unprecedentedly large elongation and an enhanced toughness
even along the loading direction likely due to hydrogen bonding between
chains. Thus, our study highlights that the choice of amine chain
extender during LCEs synthesis can be an efficient strategy to tailor
the properties as well as to provide a new functionality in the LCEs
which may expand their range of applications in shape morphing devices,
smart coatings, and dynamic substrates
Tunable Encapsulation Structure of Block Copolymer Coated Single-Walled Carbon Nanotubes in Aqueous Solution
Nanosized and shape-tunable molecular
building blocks can provide
great opportunities for the fabrication of precisely controlled nanostructures.
In this work, we have fabricated a molecular building block of single-walled
carbon nanotubes (SWNTs) coated by PPO–PEO–PPO block
copolymers whose encapsulation structure can be controlled via temperature
or addition of small molecules. The structure and optical properties
of SWNT block copolymers have been investigated by small-angle neutron
scattering (SANS), ultraviolet–visible (UV–vis) spectroscopy,
atomic force microscopy (AFM), and molecular dynamics (MD) simulation.
The structure of the hydrated block copolymer layer surrounding SWNT
can be controlled reversibly by varying temperature as well as by
irreversibly adding 5-methylsalicylic acid (5MS). Increasing hydrophobicity
of the polymers with temperature and strong tendency of 5MS to interact
with both block copolymers and π orbitals of the SWNTs are likely
to be responsible for the significant structural change of the block
copolymer encapsulation layer, from loose corona shell to tightly
encapsulating compact shell. Our result shows an efficient and simple
way to fabricate and manipulate carbon-based nano building blocks
in aqueous systems with tunable structure
Hierarchically Self-Assembled Photonic Materials from Liquid Crystalline Random Brush Copolymers
Here we report a general methodology
to attain novel hierarchical
nanostructures using new polymer scaffolds that self-assemble to form
cholesteric 1D photonic mesophases existing in conjunction with microphase
segregated domains. To achieve this, a series of liquid-crystalline
random brush copolymers (LCRBC) consisting of cholesteryl liquid crystalline
(LC) mesogen and brushlike PEG as side chain functionality are synthesized.
At room temperature, all LCRBCs exhibits microphase segregation of
PEG side chains on length scale of 10–15 nm, whereas LC domain
forms smectic mesophase (3–7 nm LC layers). Interestingly,
upon heating a cholesteric mesophase is exclusively observed for copolymer
containing 78 and 85 wt % of LC content (LCRBC78 and LCRBC85, respectively)
existing along with microphase segregated PEG domains. Moreover, the
phase behavior of these copolymers studied by temperature-controlled
small-angle X-ray scattering (SAXS) suggests the order–disorder
transition for the microphase segregated structure coincides with
the cholesteric–isotropic transition. Remarkably, LCRBC78 and
LCRBC85 quenched from cholesteric mesophase exhibits nanoscale hierarchical
order consisting of (1) smectic LC ordering with 3–7 nm periodicity,
(2) microphase segregation of PEG side chain on 10–12 nm length
scale, and (3) periodicities from helical mesophase (cholesteric phase)
on optical length scales of 150–200 nm. Thus, by exploiting
LCRBC molecular architecture and composition, hierarchical nanostructure
can be obtained and preserved which allows for the creation of unique
1D-photonic materials
Atomistic Structure of Bottlebrush Polymers: Simulations and Neutron Scattering Studies
We
have used small angle neutron scattering (SANS) measurement
and atomistic molecular dynamics (MD) simulations to investigate the
conformation of bottlebrush polymers with polyÂ(norbornene) (PNB) backbone
and different sizes of polyÂ(lactide) (PLA) side chains (PNB<sub>25</sub>-<i>g</i>-PLA<sub>5</sub>, PNB<sub>25</sub>-<i>g</i>-PLA<sub>10</sub>, and PNB<sub>25</sub>-<i>g</i>-PLA<sub>19</sub>). At early stage of simulations, stretched side chains with
visible spatial-correlations of about 30 Ă… were observed. The
experimentally measured SANS data, on the other hand, does not exhibit
any correlation peaks in the corresponding length scale indicating
a compact form rather than a stretched-hairy polymer conformation.
As the simulation continued, the spatial correlations between side
chains disappeared after about 40 ns of chain relaxation, and the
scattering intensity calculated for the simulated structure becomes
reasonably close to the measured one. Statistical approach is used
to overcome the time scale limitation and search for optimal conformation
space, which also provides a good agreement with the experimental
data. Further coarse-grained simulation results suggest that the side
chain conformation strongly depends on the solubility competition
among side chain, backbone, and solvent. Significant changes of backbone
dynamics due to the side chain encapsulation have been revealed and
discussed
Molecular Design of Liquid Crystalline Brush-Like Block Copolymers for Magnetic Field Directed Self-Assembly: A Platform for Functional Materials
We report on the development of a
liquid crystalline block copolymer
with brush-type architecture as a platform for creating functional
materials by magnetic-field-directed self-assembly. Ring-opening metathesis
of <i>n</i>-alkyloxy cyanobiphenyl and polylactide (PLA)
functionalized norbornene monomers provides efficient polymerization
yielding low polydispersity block copolymers. The mesogenic species,
spacer length, monomer functionality, brush-chain length, and overall
molecular weight were chosen and optimized to produce hexagonally
packed cylindrical PLA domains which self-assemble and align parallel
to an applied magnetic field. The PLA domains can be selectively removed
by hydrolytic degradation resulting in the production of nanoporous
films. The polymers described here provide a versatile platform for
scalable fabrication of aligned nanoporous materials and other functional
materials based on such templates
Azobenzene Molecular Machine: Light-Induced Wringing Gel Fabricated from Asymmetric Macrogelator
To develop light-triggered
wringing gels, an asymmetric macrogelator
(1AZ3BP) was newly synthesized by the chemically bridging a photoisomerizable
azobenzene (1AZ) molecular machine and a biphenyl-based (3BP) dendron
with a 1,4-phenylenediformamide connector. 1AZ3BP was self-assembled
into a layered superstructure in the bulk state, but 1AZ3BP formed
a three-dimensional (3D) network organogel in solution. Upon irradiating
UV light onto the 3D network organogel, the solvent of the organogel
was squeezed and the 3D network was converted to the layered morphology.
It was realized that the metastable 3D network organogels were fabricated
mainly due to the nanophase separation in solution. UV isomerization
of 1AZ3BP provided sufficient molecular mobility to form strong hydrogen
bonds for the construction of the stable layered superstructure. The
light-triggered wringing gels can be smartly applied in remote-controlled
generators, liquid storages, and sensors
Structural Evolution of Polylactide Molecular Bottlebrushes: Kinetics Study by Size Exclusion Chromatography, Small Angle Neutron Scattering, and Simulations
Structural evolution from polyÂ(lactide)
(PLA) macromonomer to resultant
PLA molecular bottlebrush during ring opening metathesis polymerization
(ROMP) was investigated for the first time by combining size exclusion
chromatography (SEC), small-angle neutron scattering (SANS), and coarse-grained
molecular dynamics (CG-MD) simulations. Multiple aliquots were collected
at various reaction times during ROMP and subsequently analyzed by
SEC and SANS. These complementary techniques enable the understanding
of systematic changes in conversion, molecular weight and dispersity
as well as structural details of PLA molecular bottlebrushes. CG-MD
simulation not only predicts the experimental observations, but it
also provides further insight into the analysis and interpretation
of data obtained in SEC and SANS experiments. We find that PLA molecular
bottlebrushes undergo three conformational transitions with increasing
conversion (i.e., increasing the backbone length): (1) from an elongated
to a globular shape due to longer side chain at low conversion, (2)
from a globular to an elongated shape at intermediate conversion caused
by excluded volume of PLA side chain, and (3) the saturation of contour
length at high conversion due to chain transfer reactions
Poly(3-hexylthiophene) Molecular Bottlebrushes via Ring-Opening Metathesis Polymerization: Macromolecular Architecture Enhanced Aggregation
We report a facile synthetic strategy
based on a grafting through
approach to prepare well-defined molecular bottlebrushes composed
of regioregular polyÂ(3-hexylthiophene) (<i>rr</i>-P3HT)
as the conjugated polymeric side chain. To this end, the <i>exo</i>-norbornenyl-functionalized P3HT macromonomer was synthesized by
Kumada catalyst transfer polycondensation (KCTP) followed by postpolymerization
modifications, and the resulting conjugated macromonomer was successfully
polymerized by ring-opening metathesis polymerization (ROMP) in a
controlled manner. The P3HT molecular bottlebrushes display an unprecedented
strong physical aggregation upon drying during recovery, as verified
by several analyses of the solution and solid states. This remarkably
strong aggregation behavior is attributed to a significant enhancement
in the number of π–π
interactions between grafted P3HT side chains, brought about due to
the bottlebrush architecture. This behavior is qualitatively supported
by coarse-grained molecular dynamics simulations