11 research outputs found
Anisotropic selfâassembly and gelation in aqueous methylcelluloseâtheory and modeling
Recent experimental studies demonstrated that the aqueous methylcellulose (MC) polymer chains in water can form nanoscale fibrils (diameter âŒ14 nm, persistence length âŒ60 nm), and those fibrils can organize into networks at higher temperatures and/or concentrations, forming the commonly observed gel. Here we propose that the fibrils are oneâdimensional selfâassemblies of stacked, fused polymer rings that are formed at elevated temperatures due to the changing nature of the MCâwater hydrogen bonding. This mechanism is analogous to the coilâhelix transition in polypeptides, although it is not clear whether the MC fibrils possess chirality. We perform coarseâgrained molecular simulations of MC chain structure at temperatures both above and below the hypothesized coilâtoâring transition, with CG forcefield tuned by atomistic molecular dynamics simulations, and observe the expected conformational change. We then develop a statistical mechanical theory to predict the fibril selfâassembly, gelation and rheology as function of temperature and concentration. The findings are in reasonable agreement with experimental data and could be generalized to other carbohydrate polymers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1624â1636Phase behavior of aqueous methylcellulose (MC) is studied using coarseâgrained molecular dynamics (CGâMD) and statistical mechanical theories. At low temperatures, MC chains have random coil conformation and are uniformly dissolved in water. At elevated temperatures, conformational transition from coil to ring is predicted. The rings, then, selfâassemble into fibrils and, ultimately, network of fibrils. The predicted structural and rheological characteristics of fibrillar gels are found to be in reasonable agreement with experiments.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/122438/1/polb24065.pd
Linear and Nonlinear Rheological Behavior of Fibrillar Methylcellulose Hydrogels
Cryogenic transmission electron microscopy
and small-angle neutron
scattering recently have revealed that the well-known thermoreversible
gelation of methylcellulose (MC) in water is due to the formation
of fibrils, with a diameter of 15 ± 2 nm. Here we report that
both the linear and nonlinear viscoelastic response of MC solutions
and gels can be described by a filament-based mechanical model. In
particular, large-amplitude oscillatory shear experiments show that
aqueous MC materials transition from shear thinning to shear thickening
behavior at the gelation temperature. The critical stress at which
MC gels depart from the linear viscoelastic regime and begin to stiffen
is well predicted from the filament model over a concentration range
of 0.18â2.0 wt %. These predictions are based on fibril densities
and persistence lengths obtained experimentally from neutron scattering,
combined with cross-link spacings inferred from the gel modulus via
the same model
Structure and Properties of Aqueous Methylcellulose Gels by Small-Angle Neutron Scattering
Cold, semidilute, aqueous solutions of methylcellulose
(MC) are
known to undergo thermoreversible gelation when warmed. This study
focuses on two MC materials with much different gelation performance
(gel temperature and hot gel modulus) even though they have similar
metrics of their coarse-grained chemical structure (degree-of-methylether
substitution and molecular weight distribution). Small-angle neutron
scattering (SANS) experiments were conducted to probe the structure
of the aqueous MC materials at pre- and postgel temperatures. One
material (MC1, higher gel temperature) exhibited a single <i>almost</i> temperature-insensitive gel characteristic length
scale (ζ<sub>c</sub> = 1090 ± 50 Ă
) at postgelation
temperatures. This length scale is thought to be the gel blob size
between network junctions. It also coincides with the length scale
between entanglement sites measured with rheology studies at pregel
temperatures. The other material (MC2, lower gel temperature) exhibited
two distinct length scales at all temperatures. The larger length
scale decreased as temperature increased. Its value (ζ<sub>c1</sub> = 1046 ± 19 Ă
) at the lowest pregel temperature was indistinguishable
from that measured for MC1, and reached a limiting value (ζ<sub>c1</sub> = 450 ± 19 Ă
) at high temperature. The smaller
length scale (ζ<sub>c2</sub> = 120 to 240 Ă
) increased
slightly as temperature increased, but remained on the order of the
chain persistence length (130 Ă
) measured at pregel temperatures.
The smaller blob size (ζ<sub>c1</sub>) of MC2 suggests a higher
bond energy or a stiffer connectivity between network junctions. Moreover,
the number density of these blobs, at the same reduced temperature
with respect to the gel temperature, is orders of magnitude higher
for the MC2 gels. Presumably, the smaller gel length scale and higher
number density lead to higher hot gel modulus for the low gel temperature
material
Gelation, Phase Separation, and Fibril Formation in Aqueous Hydroxypropylmethylcellulose Solutions
The
thermoresponsive behavior of a hydroxyÂpropylÂmethylÂcellulose
(HPMC) sample in aqueous solutions has been studied by a powerful
combination of characterization tools, including rheology, turbidimetry,
cryogenic transmission electron microscopy (cryoTEM), light scattering,
small-angle neutron scattering (SANS), and small-angle X-ray scattering
(SAXS). Consistent with prior literature, solutions with concentrations
ranging from 0.3 to 3 wt % exhibit a sharp drop in the dynamic viscoelastic
moduli <i>G</i>âČ and <i>G</i>âł upon
heating near 57 °C. The drop in moduli is accompanied by an abrupt
increase in turbidity. All the evidence is consistent with this corresponding
to liquidâliquid phase separation, leading to polymer-rich
droplets in a polymer-depleted matrix. Upon further heating, the moduli
increase, and <i>G</i>âČ exceeds <i>G</i>âł, corresponding to gelation. CryoTEM in dilute solutions
reveals that HPMC forms fibrils at the same temperature range where
the moduli increase. SANS and SAXS confirm the appearance of fibrils
over a range of concentration, and that their average diameter is
ca. 18 nm; thus gelation is attributable to formation of a sample-spanning
network of fibrils. These results are compared in detail with the
closely related and well-studied methylcellulose (MC). The HPMC fibrils
are generally shorter, more flexible, and contain more water than
with MC, and the resulting gel at high temperatures has a much lower
modulus. In addition to the differences in fibril structure, the key
distinction between HPMC and MC is that the former undergoes liquidâliquid
phase separation prior to forming fibrils and associated gelation,
whereas the latter forms fibrils first. These results and their interpretation
are compared with the prior literature, in light of the relatively
recent discovery of the propensity of MC and HPMC to self-assemble
into fibrils on heating