10 research outputs found
Effects of Annealing Solvents on the Morphology of Block Copolymer-Based Supramolecular Thin Films
We report a strategy that combines supramolecular assembly
and
solvent annealing to manipulate the microdomain orientation in block
copolymer thin films. In supramolecular thin films formed by polystyrene-<i>b</i>-polyÂ(4-vinylpyridine) (PS-<i>b</i>-P4VP) with
3-pentadecylphenol (PDP) hydrogen-bonded onto P4VP blocks, where PS
blocks phase-separate into cylindrical microdomains, we found that
the orientation of PS cylinders can be controlled by using different
types of solvents to anneal the films. As films are annealed under
the vapors of solvents bearing no oxygen atoms, such as chloroform,
PS cylinders are perpendicular to the surface, while under those of
solvents bearing oxygen, such as THF, PS cylinders are parallel to
the surface. Furthermore, the orientation is switchable upon the alternate
use of different types of solvents. The <sup>1</sup>H NMR measurements
showed that the strengths of hydrogen bonds between PDP and P4VP are
greatly weakened in oxygen-bearing solvents due to the competition
of the highly electronegative oxygen atoms. We thus suggest that the
binding capability of PDP onto P4VP is the key to determine whether
the supramolecules can be assembled in the vapors of different types
of solvents, which in turn, regulates the orientation of PS cylinders
Biological Hydrogels Formed by Swollen Multilamellar Liposomes
The
self-assembly of lecithin-bile salt mixtures in solutions has
long been an important research topic, not only because they are both
biosurfactants closely relevant to physiological functions but also
for the potential biomedical applications. In this paper, we report
an unusual biological hydrogel formed by mixing bile salts and lecithin
at low bile salt/lecithin molar ratios (<i>B</i><sub>0</sub>) in water. The gel can be prepared at a total lipid concentration
as low as âŒ15 wt %, and the solidlike property of the solutions
was confirmed by dynamic rheological measurements. We used cryo-TEM
and SAXS/SANS techniques to probe the self-assembled structure and
clearly evidence that the gel is made up of jammed swollen multilamellar
vesicles (liposomes), instead of typical fibrous networks found in
conventional gels. A mechanism-based on the strong repulsion between
bilayers due to the incorporation of negatively charged bile salts
is proposed to explain the swelling of the liposomes. In addition
to gel, a series of phases, including viscoelastic, gel-like, and
low-viscosity fluids, can be created by increasing <i>B</i><sub>0</sub>. Such a variety of phase behaviors are caused by the
transformation of bilayers into cylindrical and spheroidal micelles
upon the change of the effective molecular geometry with <i>B</i><sub>0</sub>
Nonvolatile Organic Field-Effect Transistors Memory Devices Using Supramolecular Block Copolymer/Functional Small Molecule Nanocomposite Electret
Organic
field-effect transistors (OFETs) memory devices based on hybrid nanocomposite
electret were fabricated by cooperative supramolecular polystyrene-<i>block</i>-polyÂ(4-vinylpyridine) (PS-<i>b</i>-P4VP)
with two different block compositions (asymmetric L1 and symmetric
L2) that contain hydroxyl-functionalized ferrocene small molecules
(FMs). Because of the selective hydrogen interaction between the hydroxyl
groups of FM and pyridine groups in P4VP block, the small FMs can
preferentially disperse in the P4VP nanodomain, which can be used
as nanostructured charge-trapping nanocomposite electret (L1-FMX and
L2-FMX) under solvent-annealing process. The charge-storage functionalities
can be easily tailored by morphologies of the hybrid nanocomposite
thin film and spatial distribution of the FM molecules in which the
relative molecular mass of block copolymers and the FM loading ratio
can further control both of them. These block copolymer nanocomposite
thin film electrets with charge-controlling guest FM for OFETs memory
devices exhibit significant features including the ternary bits storage,
high-density trapping sites, charge-carrier trapping of both polarities
(ambipolar trapping), and solution processing that can make important
progress for future advanced storage and memory technology
Mixtures of Lecithin and Bile Salt Can Form Highly Viscous Wormlike Micellar Solutions in Water
The self-assembly of biological surfactants
in water is an important
topic for study because of its relevance to physiological processes.
Two common types of biosurfactants are lecithin (phosphatidylcholine)
and bile salts, which are both present in bile and involved in digestion.
Previous studies on lecithinâbile salt mixtures have reported
the formation of short, rodlike micelles. Here, we show that lecithinâbile
salt micelles can be further induced to grow into long, flexible wormlike
structures. The formation of long worms and their resultant entanglement
into transient networks is reflected in the rheology: the fluids become
viscoelastic and exhibit Maxwellian behavior, and their zero-shear
viscosity can be up to a 1000-fold higher than that of water. The
presence of worms is further confirmed by data from small-angle neutron
and X-ray scattering and from cryo-transmission electron microscopy
(cryo-TEM). We find that micellar growth peaks at a specific molar
ratio (near equimolar) of bile salt:lecithin, which suggests a strong
binding interaction between the two species. In addition, micellar
growth also requires a sufficient concentration of background electrolyte
such as NaCl or sodium citrate that serves to screen the electrostatic
repulsion of the amphiphiles and to âsalt outâ the amphiphiles.
We postulate a mechanism based on changes in the molecular geometry
caused by bile salts and electrolytes to explain the micellar growth
Molecular Interactions between Lecithin and Bile Salts/Acids in Oils and Their Effects on Reverse Micellization
It has been known that the addition
of bile salts to lecithin organosols
induces the formation of reverse wormlike micelles and that the worms
are similar to long polymer chains that entangle each other to form
viscoelastic solutions. In this study, we further investigated the
effects of different bile salts and bile acids on the growth of lecithin
reverse worms in cyclohexane and <i>n</i>-decane. We utilized
rheological and small-angle scattering techniques to analyze the properties
and structures of the reverse micelles. All of the bile salts can
transform the originally spherical lecithin reverse micelles into
wormlike micelles and their rheological behaviors can be described
by the single-relaxation-time Maxwell model. However, their efficiencies
to induce the worms are different. In contrast, before phase separation,
bile acids can induce only short cylindrical micelles that are not
long enough to impart viscoelasticity. We used Fourier transform infrared
spectroscopy to investigate the interactions between lecithin and
bile salts/acids and found that different bile salts/acids employ
different functional groups to form hydrogen bonds with lecithin.
Such effects determine the relative positions of the bile salts/acids
in the headgroups of lecithin, thus resulting in varying efficiencies
to alter the effective critical packing parameter for the formation
of wormlike micelles. This work highlights the importance of intermolecular
interactions in molecular self-assembly
Phase Behavior and Structure of Supramolecules Formed by Poly(4-vinylpyridine) and Fanlike Benzoic Acid Derivative with Long Hydrophobic Tails
We
investigated the self-assembly behaviors of supramolecules formed
by polyÂ(4-vinylpyridine) (P4VP) and a fanlike small molecule, 3,4,5-trisÂ(hexadecyloxy)Âbenzoic
acid (THBA), via hydrogen bonding interactions. Different from other
commonly studied small molecules, THBA bears three particularly long
hydrophobic tails and tends to form stable crystals by itself, which
gives rise to unusual phase behaviors and structures. We used FTIR
to investigate the degree of complexation and find that it is not
monotonically increased to a plateau with increasing THBA but reaches
a maximum and then decreased. This is attributed to the crystallization
of excess THBA that induces a debonding of the originally associated
THBA. The crystallization-induced dissociation and macrophase separation
are further evidenced by DSC and X-ray scattering analyses. Below
the orderâdisorder transition temperature, a coexistence of
lamellae and hexagonally packed cylinders is found in melted P4VPÂ(THBA)
supramolecules. After the associated THBA crystallizes, the complexes
exclusively form hexagonally packed cylinders due to the bulky tails
of THBA, in the absence of the lamellar structure generally seen in
other polymer-based supramolecules
Synthesis, Morphology, and Sensory Applications of Multifunctional RodâCoilâCoil Triblock Copolymers and Their Electrospun Nanofibers
We report the synthesis, morphology, and applications
of conjugated
rodâcoilâcoil triblock copolymers, polyfluorene<i>-block-</i>polyÂ(<i>N</i>-isopropylacrylamide)<i>-block</i>-polyÂ(N-methylolacrylamide) (<b>PF</b><b>-</b><i><b>b</b></i><b>-</b><b>PNIPAAm</b><i><b>-b-</b></i><b>PNMA</b>), prepared by
atom transfer radical polymerization first and followed by click coupling
reaction. The blocks of PF, PNIPAAm, and PNMA were designed for fluorescent
probing, hydrophilic thermo-responsive and chemically cross-linking,
respectively. In the following, the electrospun (ES) nanofibers of
PF-<i>b</i>-PNIPAAm-<i>b</i>-PNMA were prepared
in pure water using a single-capillary spinneret. The SAXS and TEM
results suggested the lamellar structure of the <b>PF</b><b>-</b><i><b>b</b></i><b>-</b><b>PNIPAAm</b><b>-</b><i><b>b</b></i><b>-</b><b>PNMA</b> along the fiber axis. These obtained nanofibers showed
outstanding wettability and dimension stability in the aqueous solution,
and resulted in a reversible on/off transition on photoluminescence
as the temperatures varied. Furthermore, the high surface/volume ratio
of the ES nanofibers efficiently enhanced the temperature-sensitivity
and responsive speed compared to those of the drop-cast film. The
results indicated that the ES nanofibers of the conjugated rodâcoil
block copolymers would have potential applications for multifunctional
sensory devices
Tunable Thermoelectric Performance of the Nanocomposites Formed by Diketopyrrolopyrrole/Isoindigo-Based DonorâAcceptor Random Conjugated Copolymers and Carbon Nanotubes
This
paper presents the development of thermoelectric properties
in nanocomposites comprising donorâacceptor random conjugated
copolymers and single-walled carbon nanotubes (SWCNTs). The composition
of the conjugated polymers, specifically the ratio of diketopyrrolopyrrole
(DPP) to isoindigo (IID), is manipulated to design a series of random
conjugated copolymers (DPP0, DPP5, DPP10, DPP30, DPP50, DPP90, DPP95,
and DPP100). The objective is to improve the dispersion of SWCNTs
into smaller bundles, leading to enhanced thermoelectric properties
of the polymer/SWCNT nanocomposite. This dispersion strategy promotes
an interconnected conducting network, which plays a critical role
in optimizing the thermoelectric performance. Accordingly, the effects
of morphologies on the thermoelectric properties of the nanocomposites
are systematically investigated. The DPP95/SWCNT nanocomposite exhibits
the strongest interaction, resulting in the highest power factor (PF)
of 711.1 ÎŒW mâ1 Kâ2, derived
from the high electrical conductivity of 1690 S cmâ1 and Seebeck coefficient of 64.8 ÎŒV Kâ1.
The prototype flexible thermoelectric generators assembled with a
DPP95/SWCNT film achieve a maximum power output of 20.4 ÎŒW mâ2 at a temperature difference of 29.3 K. These findings
highlight the potential of manipulating the composition of random
conjugated copolymers and incorporating SWCNTs to efficiently harvest
low-grade waste heat in wearable thermoelectric devices
Using a Single Electrospun Polymer Nanofiber to Enhance Carrier Mobility in Organic Field-Effect Transistors toward Nonvolatile Memory
In
this work, a single electrospun polymer nanofiber was employed
as an additional dielectric in organic field-effect transistors where
the active channel was a layer of pentacene. A high field-effect mobility
(>1.50 cm<sup>2</sup>/(V·s)) and a high ON/OFF current ratio
(>10<sup>6</sup>) could be achieved by the use of such a nanofiber.
Probing by electron microscopy, atomic force microscopy, and scattering
techniques, we found that the geometry of the fiber is key to induce
a pentacene morphology with large and oriented grains that facilitates
the charge transport in pentacene layer along the fiber. The feasibility
of nonvolatile memory based on this new type of transistor has been
explored and the devices showed a fairly high memory window and reliable
memory characteristics. In addition to pure polymers, the effects
of composite nanofibers with dispersed [6,6]-phenyl-C<sub>61</sub>-butyric acid methyl ester were also investigated, and the electrical
properties and memory characteristics of the transistors were found
to be further improved. This study highlights the importance of dielectric
geometry to pentacene morphology that is decisive for the performances
of organic field-effect transistors
Effects of Alkali Cations and Halide Anions on the Self-Assembly of Phosphatidylcholine in Oils
The
interactions between ions and phospholipids are closely associated
with the structures and functions of cell membrane. Instead of conventional
aqueous systems, we systematically investigated the effects of inorganic
ions on the self-assembly of lecithin, a zwitterionic phosphatidylcholine,
in cyclohexane. Previous studies have shown that addition of inorganic
salts with specific divalent and trivalent cations can transform lecithin
organosols into organogels. In this study, we focused on the effect
of monovalent alkali halides. Fourier transform infrared spectroscopy
was used to demonstrate that the binding strength of the alkali cations
with the phosphate of lecithin is in the order Li<sup>+</sup> >
Na<sup>+</sup> > K<sup>+</sup>. More importantly, the cationâphosphate
interaction is affected by the paired halide anions, and the effect
follows the series I<sup>â</sup> > Br<sup>â</sup> > Cl<sup>â</sup>. The salts of stronger interactions with
lecithin, including LiCl, LiBr, LiI, and NaI, were found to induce
cylindrical micelles sufficiently long to form organogels, while others
remain organosols. A mechanism based on the charge density of ions
and the enthalpy change of the ion exchange between alkali halides
and lecithin headgroup is provided to explain the contrasting interactions
and the effectiveness of the salts to induce organogelation