6 research outputs found
Probing Gelation and Rheological Behavior of a Self-Assembled Molecular Gel
Molecular gels have
been investigated over the last few decades;
however, mechanical behavior of these self-assembled gels is not well
understood, particularly how these materials fail at large strain.
Here, we report the gelation and rheological behavior of a molecular
gel formed by self-assembly
of a low molecular weight gelator (LMWG), di-Fmoc-l-lysine,
in 1-propanol/water mixture. Gels were prepared by solvent-triggered
technique, and gelation was tracked using Fourier transform infrared
(FTIR) spectroscopy and shear rheology. FTIR spectroscopy captures
the formation of hydrogen bonding between the gelator molecules, and
the change in IR spectra during the gelation process correlates with
the gelation kinetics results captured by rheology. Self-assembly
of gelator molecules leads to a fiber-like structure, and these long
fibers topologically interact to form a gel-like material. Stretched-exponential
function can capture the stress-relaxation data. Stress-relaxation
time for these gels have been found to be long owing to long fiber
dimensions, and the stretching exponent value of 1/3 indicates polydispersity
in fiber dimensions. Cavitation rheology captures fracture-like behavior
of these gels, and critical energy release rate has been estimated
to be of the order 0.1 J/m<sup>2</sup>. Our results provide new understanding
of the rheological behavior of molecular gels and their structural
origin
Increasing Molecular Mass in Enzymatic Lactone Polymerizations
Using a model developed for the enzyme-catalyzed polymerization
and degradation of polyÂ(caprolactone), we illustrate a method and
the kinetic mechanisms necessary to improve molecular mass by manipulating
equilibrium reactions in the kinetic pathway. For these polymerization/degradation
reactions, a water/linear chain equilibrium controls the number of
chains in solution. Here, we control the equilibrium by adding water-trapping
molecular sieves in the batch polymerization reactions of ε-caprolactone.
While ring-opening rates were mostly unaffected, the molecular mass
shifted to higher molecular masses after complete conversion was reached,
and a good agreement between the experimental and modeling results
was found. These results provide a framework to improve the molecular
mass for enzyme-catalyzed ring-opening polymerization of lactone
Molecular Insights into Gelation of Di-Fmoc‑l‑Lysine in Organic Solvent–Water Mixtures
Despite
significant interest in molecular gels due to their intriguing
structure formation through self-assembly and their stimuli-responsive
behavior, our understanding of the gel formation mechanism of a low-molecular-weight
gelator (LMWG) is incomplete. Here, we report a combined experimental
and computational study on a LMWG, di-Fmoc-l-lysine, that
has two aromatic moieties and multiple hydrogen bond donors and acceptors.
Gelation in various organic solvent–water mixtures was obtained
through the solvent-triggered technique. We show that an approach
based on approximate cohesive energy density derived from density
functional theory (DFT) calculations can capture the experimental
solubility trend of LMWGs in different organic solvents. Furthermore,
DFT calculations indicate parallel and helical structures to be the
preferred structural motifs for gelator dimers. We believe that these
motifs can potentially lead to fiber formation as observed with microscopy.
Our work provides a relatively simple yet effective approach to quantify
interactions between solvents and complex gelators that can help rationalize
solubility and gelation behavior
Microstructure and Mechanical Properties of In Situ <i>Streptococcus mutans</i> Biofilms
Insight into live microbial biofilm
microstructure and mechanical
properties and their interactions with the underlying substrate can
lead to the development of new remedial strategies and/or materials.
Here we report mechanical properties of dental pathogenic <i>Streptococcus mutans</i> biofilms, grown on a polystyrene-coated
plate of a shear rheometer in physiologically relevant conditions,
precisely controlled in a custom built bioreactor. In situ measurements
demonstrated the importance of microstructure and composition of extracellular
polymeric substances on the biofilm modulus. The biofilms behave like
a weak gel with storage moduli higher than loss moduli. The simple
but robust experimental technique presented here can easily be extended
to other biofilm-material systems
Anisotropic Nanoparticles Contributing to Shear-Thickening Behavior of Fumed Silica Suspensions
Rheological characteristics
of a concentrated suspension can be
tuned using anisotropic particles having various shapes and sizes.
Here, the role of anisotropic nanoparticles, such as surface-functionalized
multiwall carbon nanotubes (MWNTs) and graphene oxide nanoplatelets
(GONPs), on the rheological behavior of fumed silica suspensions in
polyÂ(ethylene glycol) (PEG) is investigated. In these mixed-particle
suspensions, the concentrations of MWNTs and GONPs are much lower
than the fumed silica concentration. The suspensions are stable, and
hydrogen-bonded PEG solvation layers around the particles inhibit
their flocculation. Fumed silica suspensions over the concentration
range considered here display shear-thickening behavior. However,
for a larger concentration of MWNTs and with increasing aspect ratios,
the shear-thickening behavior diminishes. In contrast, a distinct
shear-thickening response has been observed for the GONP-containing
suspensions for similar mass fractions (MFs) of MWNTs. For these suspensions,
shear thickening is achieved at a lower solid MFs compared to the
suspensions consisting of only fumed silica. A significant weight
reduction of shear-thickening fluids that can be achieved by this
approach is beneficial for many applications. Our results provide
guiding principles for controlling the rheological behavior of mixed-particle
systems relevant in many fields
Effects of Poly(3-hexylthiophene) Molecular Weight and the Aging of Spinning Solution on the Electrospun Fiber Properties
The electrospinning technique is an attractive route
for processing
conjugated polymers in a significant quantity for large-scale applications.
However, the processing–structure–property relationship
of the electrospinning process for conjugated polymers is not well
understood. Here, we report the electrospinning of poly(3-hexylthiophene)
(P3HT) for three different molecular weights of P3HT: 31, 58, and
83 kDa. Chloroform was used as a solvent, and a high molecular weight
poly(ethylene oxide) (PEO) was utilized to facilitate the processing
of P3HT. Electrospinning was performed on the freshly prepared and
24 h aged spinning solutions. The aging of the spinning solution led
to the self-assembly of P3HT chains, particularly with dominant H-aggregation
for 83 kDa P3HT. The structure development and properties of the fibers
were investigated, including the single-fiber electrical conductivity
measured using a custom-built setup. Electrical conductivity has been
found to increase with increasing molecular weight, and as high as
a fivefold enhancement in single-fiber electrical conductivity was
obtained for the fibers from the aged solution compared to the fiber
from the freshly prepared solution. Despite a 25% PEO concentration
in the fibers, the maximum electrical conductivity of a single fiber
was found to be ≈2.7 × 10–5 S/cm, similar
to the pristine P3HT thin films. Our study provides an additional
understanding of P3HT structure development in electrospun fibers
as a function of polymer molecular weight and processing steps and
relates that to fiber properties