Small-angle neutron scattering from poly(vinyl alcohol)-borate gels

Phase transition and gelation in cellulose nanocrystal-based aqueous suspensions studied by SANS

HYPOTHESIS

Aqueous suspensions of cellulose nanocrystals (CNC) form a re-entrant liquid crystal (LC) phase with increasing salinity. Phase separation occurs in this LC state leading to a biphasic gel with a flow programmable structure that can be used to form anisotropic soft materials. We term this state a Liquid Crystal Hydroglass (LCH). Defining the mechanisms by which the LCH forms requires detailed structural analysis at the mesoscopic length scale.

EXPERIMENTS

By utilising Small Angle Neutron Scattering (SANS), we investigated the microstructure transitions in CNC suspensions, with a particular focus on the unique LC re-entrancy and gelation into the biphasic LCH.

FINDINGS

Scattering from LCH gels comprises contributions from a dispersed liquid state and static heterogeneity, characterised using a Lorentzian-Gaussian model of inhomogeneity. This conceptually supports a gelation mechanism (spinodal decomposition) in CNC suspensions towards a biphasic structure of the LCH. It also demonstrates that, with increasing salinity, the non-monotonic variation in effective volume fraction of CNC rods fundamentally causes the LC re-entrancy. This work provides the first experimental characterisation of the LC-re-entrancy and formation of an anisotropic LCH gel. The proposed mechanism can be extended to understanding the general behaviour of anisotropic colloids.

Polymeric Sol-Gel Transition with the Diverging Correlation Length Verified by Small-Angle X-ray Scattering

Sol-gel transitions of polymers are pivotal phenomena in material science, yet the critical phenomenon of structure during gelation has remained unclear. Here, we investigated the sol-gel transition of a fluorous polymer, poly(vinylidene fluoride-co-hexafluoropropylene), in a blend of two ionic liquids. This system features a quite high amount of cross-linker and binding sites with ion-dipole interactions between the cation and C-F dipoles, thereby facilitating easy exchange of the cross-links. Changing the mixing ratio of the two ionic liquids enabled tuning the ion-dipole interactions and inducing sol-gel transition. Notably, the correlation length and molar mass, obtained by small-angle X-ray scattering, diverged at the gelation point. Moreover, the derived critical exponents (ν = 0.85 ± 0.05) aligns remarkably well with the prediction from percolation theory (ν = 0.88). To our knowledge, this is the first report on the evident divergence during polymeric gelation by small-angle scattering and the verification of the critical exponents of the percolation theory.

Structural and Mechanical Properties of Konjac Glucomannan Gels and Influence of Freezing-Thawing Treatments on Them

Freezing has been widely used for long-term food preservation. However, freezing-thawing (FT) treatment usually influences the texture and structure of food gels such as konjac. For their texture control after FT treatment, it is important to clarify the structural change of food gels during the FT process. In this study, we investigated the aggregated structures of konjac glucomannan (GM) gels during the FT process using simultaneous synchrotron small-angle X-ray/wide-angle X-ray scattering (SAXS/WAXS) techniques. The FT treatment resulted in more crystallization of GM, and consequently, a large increase in compressive stress. In-situ SAXS/WAXS measurements revealed the following findings: on freezing, water molecules came out of the aggregated phase of GM and after the thawing, they came back into the aggregated phase, but the aggregated structure did not return to the one before the freezing; the gel network enhanced the inhomogeneity due to the growth of ice crystals during freezing. Furthermore, we examined the influence of additives such as polyvinyl (alcohol) (PVA) and antifreeze glycoprotein (AFGP) on the mechanical and structural properties of freeze-thawed GM gels. Although the addition of PVA and AFGP suppressed the crystallization of GM, it could not prevent the growth of ice crystals and the increase in the inhomogeneity of the gel network. As a result, the compressive stresses for freeze-thawed GM gels containing PVA or AFGP were significantly higher compared with those of GM gels without FT treatments, although they were lower than those of freeze-thawed GM gels. The findings of this study may be useful for not only the texture control of freeze-thawed foods but also the improvement of the mechanical performance of the biomaterials.

Dissolution of cellulose into supercritical water and its dissolving state followed by structure formation from the solution system

Cellulose was treated with supercritical water at 668 K and 25 MPa for 0.04 s in this study. The cellulose/water system was transparent at room temperature for a while after supercritical water treatment before a precipitate gradually appeared over several hours. The precipitation process was monitored by synchrotron X-ray scattering. The scattering functions of fractal systems and flat-like structures were utilized to explain the experimentally observed small-angle scattering profiles. Immediately after supercritical water treatment, the cellulose appeared to dissolve with a fractal dimension D of approximately 1, indicating that the cellulose molecules were rigid, followed by aggregation into a 5-nm-thick flat-like structure. The flat-like structure was determined to be similar to the molecular sheets observed during the early stages of precipitation in the cellulose/aqueous sodium hydroxide and cellulose/aqueous lithium hydroxide/urea systems. Resultant regenerated cellulose had high crystallinity, large crystal size, and a low degree of polymerization.

Robust and Highly Stretchable Chitosan Nanofiber/Alumina-Coated Silica/Carboxylated Poly (Vinyl Alcohol)/Borax Composite Hydrogels Constructed by Multiple Crosslinking

We investigated the mechanical and structural properties of composite hydrogels composed of chitosan nanofiber (ChsNF), positively charged alumina-coated silica (ac-SiO2) nanoparticles, carboxylated poly (vinyl alcohol) (cPVA), and borax. ChsNF/cPVA/borax hydrogels without ac-SiO2 exhibited high Young's modulus but poor elongation, whereas cPVA/ac-SiO2/borax hydrogels without ChsNF had moderate Young's modulus but high elongation. ChsNF/ac-SiO2/cPVA/borax hydrogels using both ChsNF and ac-SiO2 as reinforcement agents exhibited high extensibility (930%) and high Young's modulus beyond 1 MPa at a high ac-SiO2 concentration. The network was formed by multiple crosslinking such as the complexation between borate and cPVA, the ionic complexation between ac-SiO2 and cPVA, and the hydrogen bond between ChsNF and cPVA. Structural analysis by synchrotron small-angle X-ray scattering revealed that the nanostructural inhomogeneity in ChsNF/ac-SiO2/cPVA/borax hydrogel was suppressed compared to those of the ChsNF/cPVA/borax and cPVA/ac-SiO2/borax hydrogels.

Double dynamic hydrogels formed by wormlike surfactant micelles and cross-linked polymer

HYPOTHESIS

Interpenetrating networks consisting of a polymer network with dynamic cross-links and a supramolecular network allow obtaining hydrogels with significantly enhanced mechanical properties.

EXPERIMENTS

Binary hydrogels composed of a dynamically cross-linked poly(vinyl alcohol) (PVA) network and a transient network of entangled highly charged mixed wormlike micelles (WLMs) of surfactants (potassium oleate and n-octyltrimethylammonium bromide) were prepared and studied by rheometry, SANS, USANS, cryo-TEM, and NMR spectroscopy.

FINDINGS

Binary hydrogels show significantly enhanced rheological properties (a 3400-fold higher viscosity and 27-fold higher plateau modulus) as compared to their components taken separately. This is due to the microphase separation leading to local concentrating of PVA and WLMs providing larger number of polymer-polymer contacts for cross-linking and longer WLMs with more entanglements. Such materials are very promising for the application in many areas, ranging from enhanced oil recovery to biomedical uses.

PBTTT-C16 sol-gel transition by rod associations and networking

A low-molecular-weight poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (designated as Lw-pBTTT-C₁₆) in a fair solvent (chlorobenzene, CB) displays peculiar structural, mechanical, and electronic features during sol-gel transition. Using comprehensive (multiscale) dynamic/static analysis schemes, the Lw-pBTTT-C₁₆/CB solution (10 mg mL^-1) is shown to capitalize on rod associations and networking to form a gel, in stark contrast with its high-molecular-weight companion previously reported to form gels through hierarchical colloidal bridging. The present study reveals, however, that the molecular weight of pBTTT-C₁₆ has a subtle impact on the gelation behaviors through the rarely recognized, contrasting supramolecular conformations (rod-like vs. wormlike) of the aggregate clusters fostered in the pristine solution. The ac conductivity nearly doubles as a result of improved (mesoscale) packing of cylindrical aggregates near the gel state as well as enhanced backbone rigidity of the constituting chains. Other distinguishing features include: (1) there is no real crossover of the dynamic moduli (G' and G'') upon increasing the temperature from gel (T = 15 °C) to solution (T = 80 °C) states. (2) The gel is about a hundredfold softer in dynamic modulus, yet more resilient with a fivefold increase in the yield strain. Both viscoelastic features are expected to greatly benefit the gel processability. (3) The coexistent microgels and cylinder (aggregate) bundles form a peculiar gel network that has not been reported previously with polymer or colloidal gels. The overall findings provide new mechanistic insight into the phenomenological effects of molecular weight for the pBTTT-C_n series in solution, sol, gel, and thin film.

High strength of hybrid double-network hydrogels imparted by inter-network ionic bonds

Interaction between networks has been proven to be of importance for mechanical property enhancement of double-network (DN) hydrogels.

Investigating the effect of supramolecular gel phase crystallization on gel nucleation

Supramolecular gel phase crystallization offers a new strategy for drug polymorph screening and discovery. In this method, the crystallization outcome depends on the interaction between solute and gel fibre. While supramolecular gels have shown success in producing new polymorphs and crystals with novel morphologies, role of the gel and nature of gel-solute interaction remains largely unexplored. The present study aims to provide a comprehensive picture of the structural evolution of a supramolecular gel produced from a bis(urea) based gelator (G) in the presence of a polymorphic drug carbamazepine (CBZ). The structural aspects of the gel have been assessed by single crystal X-ray analysis, X-ray powder diffraction (XRPD) and solid state NMR spectroscopy. Small Angle Neutron Scattering (SANS) has been used to follow the changes in gel structure in the presence of CBZ. Visual evidence from morphological study and structural evolution observed at a macroscopic level from rheological measurements, shows good agreement with the SANS results. The concentration of the gelator and the relative proportion of G to CBZ were found to be crucial factors in determining the competitive nucleation events involving gelation and crystallization. At a critical G to CBZ ratio the effect of CBZ on gel structure was maximum and fiber bundling in the gel was found to be critically affected. This study offers important information about how the interplay of gelator assembly and gel-solute interactions can fine-tune the nucleation events in a supramolecular gel phase crystallization.

Solvent transport through hard–soft segmented polymer nanocomposites

This article illustrates the non-barrier influence of filler on solvent transport through phase segmented block co-polymer nanocomposite.

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 almost temperature-insensitive gel characteristic length scale (ζ(c) = 1090 ± 50 Å) 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(c1) = 1046 ± 19 Å) at the lowest pregel temperature was indistinguishable from that measured for MC1, and reached a limiting value (ζ(c1) = 450 ± 19 Å) at high temperature. The smaller length scale (ζ(c2) = 120 to 240 Å) increased slightly as temperature increased, but remained on the order of the chain persistence length (130 Å) measured at pregel temperatures. The smaller blob size (ζ(c1)) 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.

Cosolvent gel-like materials from partially hydrolyzed poly(vinyl acetate)s and borax

A gel-like, high-viscosity polymeric dispersion (HVPD) based on cross-linked borate, partially hydrolyzed poly(vinyl acetate) (xPVAc, where x is the percent hydrolysis) is described. Unlike hydro-HVPDs prepared from poly(vinyl alcohol) (PVA) and borate, the liquid portion of these materials can be composed of up to 75% of an organic cosolvent because of the influence of residual acetate groups on the polymer backbone. The effects of the degree of hydrolysis, molecular weight, polymer and cross-linker concentrations, and type and amount of organic cosolvent on the rheological and structural properties of the materials are investigated. The stability of the systems is explored through rheological and melting-range studies. (11)B NMR and small-angle neutron scattering (SANS) are used to probe the structure of the dispersions. The addition of an organic liquid to the xPVAc-borate HVPDs results in a drastic increase in the number of cross-linked borate species as well as the agglomeration of the polymer into bundles. These effects result in an increase in the relaxation time and thermal stability of the networks. The ability to make xPVAc-borate HVPDs with very large amounts of and rather different organic liquids, with very different rheological properties that can be controlled easily, opens new possibilities for applications of PVAc-based dispersions.

Reversible stiffening transition in beta-hairpin hydrogels induced by ion complexation

We have previously shown that properly designed lysine and valine-rich peptides undergo a random coil to beta-hairpin transition followed by intermolecular self-assembly into a fibrillar hydrogel network only after the peptide solutions are heated above the intramolecular folding transition temperature. Here we report that these hydrogels also undergo a stiffening transition as they are cooled below a critical temperature only when boric acid is used to buffer the peptide solution. This stiffening transition is characterized by rheology, dynamic light scattering, and small angle neutron scattering. Rheological measurements show that the stiffening transition causes an increase in the hydrogel storage modulus (G') by as much as 1 order of magnitude and is completely reversible on subsequently raising the temperature. Although this reversible transition exhibits rheological properties that are similar to polyol/borax solutions, the underlying mechanism does not involve hydroxyl-borate complexation. The stiffening transition is mainly caused by the interactions between lysine and boric acid/borate anion and is not driven by the changes in the secondary structure of the beta-hairpin peptide. Addition of glucose to boric acid and peptide solution disrupts the stiffening transition due to competitive glucose-borate complexation.

A Gel Network Constituted by Rigid Schizophyllan Chains and Nonpermanent Cross-Links

This work reports a gel network formed by rigid schizophyllan (SPG) chains with Borax as a cross-linking agent. The formed cross-links are non-permanent and somewhat dynamic in nature because the cross-linking reaction is governed by a complexation equilibrium. Gelation processes are traced by dynamic viscoelastic measurements to examine the effects of Borax content, SPG concentration, temperature, salt concentration, salt type, and strain. The first-order kinetic model containing three parameters, t(0) (induction time), 1/tau(c) (gelation rate), and (saturated storage modulus), is successfully applied to describe the gelation of the SPG-Borax system. Gelation occurs faster at higher Borax content, higher SPG concentration, higher salt concentration, or lower temperature. Moreover the gelation is cation-type-specific. Storage modulus is a linear function of both Borax content and SPG concentration. The linear relationship between storage modulus and Borax content can be explained by a modified ideal rubber elasticity theory with a front factor alpha to take into account the presence of ineffective cross-links and the effect of SPG chain rigidity. On the other hand, the linear dependence of storage modulus on SPG concentration could be explained on the basis of chain-chain contacting behavior of extended SPG chains. Apparent activation energy and cross-linking enthalpy are calculated to be -74.5 and -32.4 kJ/mol for the present system. Strain sweep measurements manifest that the elasticity behavior of this gel starts to deviate from Gaussian-chain network at a small strain of 10%.

Small angle neutron scattering studies of structural characteristics of agarose gels

The 30 m small angle neutron scattering facility at the National Institutes of Standards and Technology has been used to examine neutron scattering from agarose gels formed in D2O. Differential scattering cross sections have been acquired over a continuous range of Q between 0.005 and 0.3 A-1. Subtle changes in gel structure are observed when pre-gelation agarose concentration is varied. Similarly, except when the gelling solution is rapidly cooled to a low temperature, the rate at which the gels are formed does not seem to have much effect. Clearer evidence of structural rearrangement is observed when the solvent quality is changed by the addition of dimethyl sulfoxide, or when the temperature of the gel is elevated above 70 degrees C. These data are consistent with a description of a randomly structured polymer network containing discrete self-similar, hydrogen-bonded, junctions normally of minimal thickness approximately 35-40 A.