Session A47: Focus Session: Gelation and Glass Transition in Colloids and Soft Matter Systems I

Gelation kinetics of gelatin using particle tracking microrheology

Previous studies with gelatin have observed four distinct stages during the physical gelation process [Normand et al. Macromolecules, 2000, 33, 1063]. In this presentation we report measurements of microrheology in an effort to examine the time evolution of the gel on short length scales and time scales. By tracking latex particles in gelatin solution at different temperatures we can follow the microrheological changes and kinetics of the gelation process. Using the generalized Stokes-Einstein relation viscoelastic properties of these quasi-static gel states the evolution of the storage and loss moduli, G' and G'', are examined as functions of both time and temperature. The data show that both G' and G'' exhibit power law scaling versus frequency with the same exponent. The temperature and concentration dependence of the frequency at which the system crosses over from viscous to elastic behavior will be presented.   

Temperature Dependent Dynamics and Structure of Soft Colloids

We have investigated the effect of temperature on the structure and dynamics of a particular class of soft colloids created by densely grafting polymer chains to the nanoparticle surface. These materials are able to display fluid like properties even in the absence of any external solvent and are termed as self-suspended fluids. Temperature dependent rheology of these materials has displayed several interesting features including increased solid like response with increase in temperature. Tethered polymer chain in this particular system is cis 1-4 polyisoprene, which is a type-A dielectric and allows the effect of temperature change on the global dynamics of the tethered chain to be separately investigated with broadband dielectric spectroscopy. Furthermore, we have investigated the effect of temperature change on the nanoparticle structure and dynamics with the help of small angle X-ray scattering (SAXS) and X-ray photon correlation spectroscopy (XPCS). Based on our finding, we have provided an alternative description of the jamming phase diagram applicable to this system.   

Confinement-induced solidification of attractive colloidal suspensions

Using a model colloidal-polymer suspension, we show that confinement induces solidification in attractive colloidal suspensions via a fundamentally different route from that active in hard sphere colloidal suspensions. For a range of polymer concentrations, the suspensions undergo a phase transition from a colloidal fluid with clusters to a colloidal gel with increasing confinement while polymer and particle concentration are held constant. Surprisingly, the effects of confinement appear at much larger thicknesses in attractive colloidal suspensions than in hard sphere suspensions. We find that solidification in confined attractive suspensions is not driven by structuring of the colloids at the walls or by shear-induced migration or clustering. Instead, by analyzing the cluster size distributions in the fluid phase as a function of confinement, we find that the strength of the interparticle attraction increases as the samples are confined. We further demonstrate that this change in the strength of attraction leads to increasingly arrested particle dynamics as colloidal gels are confined.   

Gelation of self-assembed bile acid-PEG conjugates

The aggregation of macromolecules and low-molar-mass compounds into elongated self-assemblies such as wormlike micelles, fibers, or tubules increases the viscosity of the solutions and often leads to gelation due to network formation, even in organic solvents. Such one-dimensional nanostructures are promising candidates for drug delivery vehicles, packing materials for separation, templates for metal nanowires, biocides, and photo- or biocatalysis. An interesting group of compounds capable of this type of self-organization are bile acids, which are endogeneous steroids known to form gels at high concentrations and appropriate pH conditions. Grafting poly(ethylene oxide) on bile acids via anionic polymerization brings along thermoresponsiveness represented by lower critical solution temperature (LCST), while self-assembling occurs below another threshold temperature leading to a gelation at high concentrations, as shown by rheological experiments. The latter transition is assigned to the nanotube formation of pegylated bile acids, visualized by electron microscopy.   

The Role of Charge Interactions in Colloidal Gelation

We demonstrate the gelation of a novel system of oppositely charged colloidal particles. The particles are charged by grafting a polyelectrolyte brush from the surface, and suspended in a polar solvent with added monovalent salt. Confocal microscopy allows us to study in detail the three-dimensional structure and dynamics of these binary gels as we vary the particle volume fraction, interaction strength, and relative number ratio of the two particle species, and we find a transition between a gel and a fluid state with each of these parameters. We find that the mean contact number of particles in the gel decreases as we approach the gel line, in contrast to what has been reported in the literature for depletion gels.   

Anomalous Phase Transitions in Soft Colloid-Polymer Binary Mixtures

We have shown earlier [1] that these PGNPs resemble star polymers or spherical brushes in terms of their morphology in the melt. However, these particles show dynamics in melt which is quite different from other soft colloidal particles. Since most of the work on soft colloidal particles have been performed in solutions we have now explored the phase behavior of the PGNPs in good solvent using microscopic structural and dynamical measurements on binary mixtures of homopolymers and soft colloids consisting of polymer grafted nanoparticles. We observe anomalous structural and dynamical phase transitions of these binary mixtures, including appearance of spontaneous orientational alignment and logarithmic structural relaxations, as a function of added homopolymers of different molecular weights. Our experiments points to the possibility of exploiting the phase space in density and homopolymer size, of such hybrid systems, to create new materials with unique properties. Reference: 1. Sivasurender Chandran, Sarika C. K., A. K. Kandar, J. K. Basu, S. Narayanan, and A. Sandy, J. Chem. Phys. \textbf{135}, 134901 (2011).   

Injectable Solid Peptide Hydrogel as Cell Carrier: Effects of Shear Flow on Hydrogel and Cell Payload

Peptides were designed to intramolecularly fold into $\beta $-hairpins once they are exposed to physiological conditions and then consequently self-assemble into a rigid hydrogel with a network structure of branched and entangled, 3nm-wide fibrils. These physical hydrogels can be injected as preformed solids, because they can shear-thin and consequently flow under an appropriate shear stress but immediately recover back into solids on removal of the stress with gel stiffness restoring over time. In this work, mechanisms of gel shear-thinning and immediate recovery were elucidated by investigating gel behavior during and after flow via mechanical and structural characterizations. Importantly, hydrogel flow behavior was studied in a capillary geometry that mimicked the actual situation of syringe injection. Hydrogel flow profiles were obtained via fluorescent particle tracking and the profile shape was found dependent on flow rate and gel stiffness. Hydrogel nanostructure was probed with small angle neutron and x-ray scattering. The results demonstrate that these hydrogels can be excellent candidates for tissue regeneration substrates and injectable therapeutic delivery vehicles.   

Shear-driven gelation of dilute colloidal suspensions

Shear-driven solidification of dilute colloidal suspensions has dramatic impact on their applications, ranging from industrial making of paints to artificial or natural microfluidic devices and is a prototype of far from equilibrium transitions. In a set of experiments on a dilute charge-stabilized colloidal suspension, we have monitored shear-induced aggregation in a fully controlled way and rationalized the effect of the shear stress from the initially liquid suspension to the final solid. By combining light scattering, rheology and microscopy images, we show that the suspension changes, under shear, into a suspension of non-Brownian aggregates whose packing fraction increases with the shearing time. Upon flow cessation, these aggregates can eventually form cohesive random packings where each inter-aggregate bond involve a large number of colloidal bonds. Such solidification mechanism is thus a hybrid between colloidal gelation and the packing-driven jamming of non-Brownian suspensions.   

Aggregation and network formation in aqueous methylcellulose near the sol-gel transition

Methylcellulose (MC) is a semi-flexible polymer which can be soluble in water at low temperatures, depending on the average number of methyoxyl groups on each repeat unit. Upon heating, soluble MCs pass through a lower critical solution temperature (LCST) and undergo thermoreversible gelation, which is well described by Winter-Chambon critical gelation theory. The relaxation exponent ($n$) exhibits a smooth variation with concentration, approaching $n = 1$ at low concentration and $n = 0.5$ high concentration. We selected a set of commercial MC for materials with similar degrees of substitution, but known for their significant variations in gelation temperature in water. MCs which gel at higher temperatures also exhibit a plateau in elastic modulus at low frequencies, which indicates two relevant length scales coexist just below the gel point. Scattering experiments (static, dynamic, and small angle x-ray and neutron) are compared to rheological measurements to reveal the MC chain structures and aggregation associated with phase separation and gelation and enable a mechanistic understanding of these phenomena.   

Effect of particle stiffness on glassy dynamics of dense colloidal liquids

``Fragile'' glassy materials show a non-Arrhenius dependence of relaxation time with temperature close to the glass transition and have been extensively studied for molecular glass formers as model ``hard-sphere'' colloidal suspensions, but we lack a complete understanding of ``strong'' glass formers which show an Arrhenius dependence on temperature approaching the glass transition. In this work, we investigate the glassy dynamics of microgels of varied particle stiffness in dense aqueous suspensions using confocal microscopy. Poly(N-isopropylacrylamide) (PNIPAM) microgel particles of variable stiffness in aqueous media are synthesized by precipitation polymerization varying the cross-linking density to resemble ``strong'' glass forming liquids owing to their directional elastic interparticle interactions at increased microgel volume fraction. The fragility effect on the glassy dynamics in dense colloidal suspension is investigated as we tune the behavior from ``soft-sphere'' to ``hard-sphere'' limits. We find that dynamic heterogeneity, specifically string-like motion, is more pronounced as stiffness increases.   

Thermo-adjustable mechanical properties of polymer, lipid-based complex fluids

Combined rheology (oscillatory and steady shear) and SAXS studies reveal details on the temperature-dependent, reversible mechanical properties of nonionic polymer, lipid-based complex fluids. Compositions prepared by introduction of the polymer as either a lipid conjugate or a triblock copolymer form an elastic gel as the temperature is increased to 18 C. The network is produced from PEO chain entanglement and physical crosslinks confined within the intervening aqueous layers of a multilamellar structured lyotropic mesophase. Although the complex fluids are weak gels, tuning of the gel strength can be achieved by temperature adjustment. The sol state formed at reduced temperature arises as a consequence of the well-solvated PEO chains adopting a non-interacting, conformational state. Complex fluids prepared with the triblock copolymers exhibit greater tunability in viscoelasticity than those containing the PEGylated-lipid conjugate because of the temperature-dependent water solubility of the central PPO block. The water solubility of PPO at reduced temperatures results in the polymer being expelled from the self-assembled amphiphilic bilayer, causing collapse of the swollen lamellar structure and loss of the PEO network. At elevated temperatures, the triblock reinserts into the bilayer producing an elastic gel. These studies identify macromolecular architectures for the facile preparation of dynamic soft materials possessing a range of mechanical properties that can be tuned by modest temperature control.   

Mechanics of networks of aliphatic fibers in aqueous surfactant media

We investigate the structural and rheological properties of aliphatic fibers dispersed in aqueous solutions of anionic surfactants, typically used in liquid detergents to provide yield stress. This system displays an onset to solid-like properties that depends on fiber concentration. In this contribution we will discuss how tuning the state of the surfactant background influences the fiber-fiber interactions and the mechanical properties of the gel.   

Elimination of branching in self- assembled beta hairpin peptide fibrils

Hydrophobic collapse of amphiphilic -hairpin peptides (e.g. MAX1 VKVKVKVKV$^{D}$PPTKVKVKVKV-NH$_{2})$ into fibrils and their hierarchical assembly into branched, hydrogel networks has been extensively studied. A physically crosslinked hydrogel network is formed due to fibrillar entanglement and branched defects in hydrophobic collapse during fibril formation. Alternating valine residues with side chains of the same size are responsible for the hydrophobic collapse of the molecule into a b-hairpin and fibril nanostructure with branching. In a new sequence LNK1 (LNK1 (Nal)K(Nal)KAKAKV$^{D}$PPTKAKAK(Nal)K(Nal)-NH$_{2})$ the non-beta turn valines were replaced with Napthylalanine and alanine amino acid residues, with hydrophobic side chains of larger and smaller volume, respectively, than valine. Thus, formation of a ``lock and key'' type structure was attempted in the hydrophobic core of the peptide fibrils that would eliminate fibril branching. The folding and network formation of LNK1 has been studied by Circular Dichroism spectroscopy (CD), Transmission Electron Microscopy (TEM) and Oscillatory Rheology. Preliminary rheological characterization suggests the elimination of branching in the fibrils and also a possibility that LNK1 networks, unlike MAX1, are just nanofibrillar suspensions rather than permanently physically crosslinked hydrogels.   

Bond orientational order in randomly-packed colloidal spheres

Systems of jammed particles are abundant, yet poorly understood. These systems are often naively assumed to be disordered, such that only short-range correlations are present and all spatial directions are equivalent. Yet, the mechanical stability of these materials implies that a network of mechanical forces percolates through the sample, which may give rise to long-range correlations and symmetry breaking. We directly measure, by confocal microscopy, the positions of hard colloids, which are sedimented by centrifugation, to form a jammed matter. We follow the centrifugation process in motion, measuring the density profile of our particles along the sample. Strikingly, while only short-range positional order exists in our system, both in the fluid and in the jammed state, the orientations of the bonds between the nearest neighbors are correlated in the jammed state, throughout the system. This breaks the rotational symmetry of the jammed state. Moreover, the rotational symmetry is correlated with the direction of gravity, suggesting that the mechanical network of forces plays an important role in our system. This breaking of rotational symmetry, observed in our system, must have an impact on a wide range of properties in other, more complex, randomly packed systems.   

Anharmonicity in Amino Acids

Two special dynamical transitions of universal character have been recently observed in macromolecules (lysozyme, myoglobin, bacteriorhodopsin, DNA, and RNA) at $T^{*}\sim 100 - 150$ K and $T_{D}\sim 180 - 220$ K. The underlying mechanisms governing these transitions have been subject of debate. In the present work it is reported a survey on the temperature dependence of structural, vibrational and thermodynamical properties of a nearly anhydrous amino acid (orthorhombic polymorph of the amino acids L-cysteine and L-proline at a hydration level of $3.5\%$). The temperature dependence of X-Ray diffraction, Raman spectroscopy, and specific heat were considered. The data were analyzed considering amino acid-amino acid, amino acid-water, and water-water phonon-phonon interactions, and molecular rotors activation. Our results indicated that the two referred temperatures define the triggering of very simple and specific events that govern all the interactions of the biomolecule: activation of CH$_{2}$ rigid rotors ($TT_{D}$).