The coarsening and internal dynamics of a bulk foam are probed noninvasively by exploiting the strong multiple scattering of light that gives foams their familiar white color. By approximating the light propagation as a diffusion process, transmission measurements provide a direct probe of the average bubble size d. A second corroborating measure of d is obtained by analyzing temporal intensity fluctuations of the multiply scattered light within the framework of diffusing-wave spectroscopy. Both sets of measurements show the same behavior: At early times d is approximately a constant (20µm). After about twenty minutes the foam begins to coarsen and scaling behavior is observed such that the growth of d is a power-law in time, tz with z=0.45±0.05. This result is in near accord with the theoretical prediction, z=1/2, for foam in the limiting case of space-filling polyhedral bubbles. In addition, the change in packing conditions during coarsening gives rise to a nonequilibrium dynamical process which also exhibits temporal scaling: Neighboring bubbles undergo sudden structural rearrangements at a rate per unit volume which decays as t−Y with y=2.0±0.2. This value of y cannot be explained by the presence of only a single time-dependent length scale in the foam structure. Since these bubble rearrangement events serve to relax local stress, they must also play a role in the relaxation of externally imposed stress. Therefore, elucidation of their origin and scaling behavior will lead to an increased understanding of the rheology and stability of foams.

Emulsions are commonly made of macrosized droplets dispersed in a continuous solvent. These systems are subject to two types of instabilities. One is reversible: flocculation, the other is irreversible: coalescence. The reversible flocculation induced by excess surfactant is studied. It is shown that an attractive interaction arises from depletion of surfactant micelles and leads to a fluid-solid phase transition. The sensitivity of this phase transition to the oil droplet diameter leads to a fractionated crystallisation method for purifying in size highly polydisperse emulsions. These well characterized systems are used to study the stability of the film against coalescence. Two distinct regimes of coalesence are found. One is driven by the surfactant chemical potential and occurs in a dilute emulsion. The other is driven by the water chemical potential and takes place in a concentrated emulsion which ressembles a biliquid foam.

A totally unexpected phenomenon has been observed for the first time a few years ago when studying, by small angle neutron scattering, rubbery networks containing a small proportion of uncrosslinked labelled (deuterated) chains [1].When such a system is elongated, the scattered intensity strongly increases with the extension ratio λ for q vectors parallel to the stretching direction (and for low q values). Conversely, in the direction perpendicular to the elongation axis, the scattered intensity slightly decreases at low q for small extension ratios and then remains approximately constant when λ is further increased. The changes which are observed occur therefore in a direction opposite to the “conventional” one, i.e. that corresponding to chains effectively linked by at least two chemical junctions to the network and thus oriented on an average along the direction of stretching.

There are a variety of gels (highly swollen solids) that are of considerable interest to polymer scientists, materials scientists, and ceramists. One type consists of typical organic polymers such as polyethylene or polystyrene, in networks which are formed by means of physical cross links, such as crystallites or physical aggregates. Such gels are thermoreversible in that liquefaction occurs upon heating. Another type consists of chain-like structures permanently bonded into covalent networks. These permanently branched and cross-linked chains can be either organic (phenol-formaldehyde resins, epoxies, etc.), or inorganic [silica (SiO2), titania (TiO2), zirconia (ZrO2), etc.] Both the organic and inorganic covalent types have been used to prepare aerogels, and the inorganic ones are now much used to prepare high-tech ceramics by the new sol-gel route.

In the case of the thermoreversible, organic polymer gels, moduli can be measured as a function of concentration, temperature, and structural characteristics of the polymer (molecular weight, molecular weight distribution, and nature and degree of any chain branching). Such equilibrium results give information on the nature of the gels, including the influence of morphology, and the presence of dangling-chain irregularities. Measurements carried out as a function of time, for example, on polyethylene homopolymers and copolymers, can give information about their gelation kinetics.

In the case of the ceramic materials, the evolution of the shear modulus with time is very useful in establishing induction times, rates of gelation, and aging effects. Correlation of such information with results of scattering studies can give much insight into the nature of the sol-gel process.

We have investigated the question of heterogeneity in gels using Brillouin scattering, depolarized Rayleigh scattering and dynamic light scattering measurements on methylmethacrylate (MMA) gels crosslinked with varying amount of ethylene-dimethacrylate (EDMA). We find that the k dependence of the phase velocity changes on increasing the cross-link content. For higher concentrations of crosslink we observe maxima and minima in the k dependence of the phase velocity which e associate with spatial inhomogeneities in the gel, with the formation of regions of low and high cross-link density, respectively. This micro-phase separation occurs around crosslink content of 3-4 %. At this same concentration depolarized Rayleigh measurements show a marked decrease in the number of MMA side groups capable of rotating.

The property of elasticity and the effect of swelling of mechanically induced silica-siloxane mixtures are interpreted within a percolation framework. This approach is illustrated by varying the specific amount of adsorbed polymer upon the silica surface of aggregates.

High quality glasses may be prepared from liquid precursors, via sol - geL Starting materials and variable control determine the final characteristics of silica monoliths for optical fiber preforms, prepared via sol - gel, as we have found in previous studies. The rate of hydrolysis. and condensation reactions, as wcU as the time required for densification of a gel, is dependent on water/alkoxide ratio, pH and the presence of drying control chemical additives. The physical and optical properties of the preforms are also determined by the specific conditions of the sol - gel processing.

In this study we present an additional analysis of pH effects on the kdnetics, before and during the material syneresis. The effect of CO2 as a contaminant, is also considered. Samples are prepared from hydrolysis of tetrametoxysane in methanol - water mixtures, in the presence of a - picoline. At 20°C, the initial values of pH range between 8 and 6 and show an exponential decrease as the hydrolytic polymensation proceeds, tending to acidic or more acidic values. These steps also are followed by TEM and SEM and are correlated with the sort of aggregates formed in the material along the percolation process. Optical properties of the preforms are interpreted in terms of the clusters formed from those aggregates.

Details on instrumental monitoring of [H+] along the alcogelation process are discussed.

We have employed a variety of scattering techniques to study the structure of diblock copolymer aggregates in solution The system of interest is an anionically prepared poly(styrene-isoprene) diblock copolymer (PS-PI) with well defined molecular weight and composition. When the diblock copolymer is added to decane, which dissolves the polyisoprene but not the polystyrene, we observe that polymeric micelles form at concentrations above a critical micellar concentration. The aggregation number is determined by light scattering and neutron scattering to be roughly 120. The micellar aggregate consists of a spherical polystyrene core with a mean radius of 105 A surrounded by a PI corona. The size distribution is quite narrow (<10%). The PS core is swollen with the solvent by roughly a factor of 2 in volume. By combining dynamic light scattering and SANS, we determined the thickness of the corona to be 50 A, which is 70% larger than the radius of gyration of a free PI chain of the same molecular weight in a good solvent. Furthermore, SANS measurements suggest that the aggregates form an ordered phase at the nominal cross-over concentration as it is expected for a highly functionalized star-branched macromolecules in solution.

Binary blends of diblock copolymer (AB) and homopolymer (hA) self assemble upon solvent evaporation into a great variety of microphase separated morphologies. The ordered lamellar, bicontinuous double diamond, cylindrical and spherical morphologies were observed by TEM and SAXS in our studies, as well as a range of micellar morphologies.

The mean curvature (H) and the area per copolymer junction (σj), which characterize the intermaterial dividing surface, increased with increasing homopolymer concentration in the blend and/or with decreasing homopolymer molecular weight. These trends were generally obeyed both between and within ordered morphology types. The increase in H and σj was related to an increased degree of mixing between the homopolymer and the block of the copolymer.

Two types of isothermal morphology diagrams were constructed to consolidate the extensive morphological data and to illustrate the general morphological transitions in AB/hA blends. The constant molecular weight morphology diagrams illustrated the interdependence of the copolymer composition and the homopolymer concentration. The constant copolymer composition diagrams emphasized the importance of the relative homopolymer molecular weight and the overall blend composition.

We present a theoretical study of a melt of diblock copolymers consisting of a rigid rod and a flexible tail. It is shown that in addition to the lamellax phases previously discussed, there also exist phases of “hockey puck” micelles, where the rods are packed axially into cylinders. These phases occupy most of the phase diagram previously thought to consist of monolayer lamellae. It is argued that spherical micelles probably do not exist.

The microphase segregation properties of diblock copolymers composed of a neutral polymer joined to a polyelectrolyte are described. For weakly charged diblocks under melt conditions, we have constructed a theory of weak concentration fluctuations and have used it to study the spinodal instabiity to microphase separation that is driven by incompatibility of the polyion and neutral blocks. As in polyelectrolyte melts, the charging has a strong compatibilizing effect because of the large counterion entropy cost of phase separation. The electrostatic interactions also introduce a new length scale, the Debye screening length, which competes with the free chain radius to determine the microphase wavelength. A scaling theory of the strongly-segregated phase indicates that the salt-free microphases cannot have sharp interfaces beyond a threshold number of charges per chain.

Atomic Force Microscopy has been used to study the early stage evolution of the free surface of annealed symmetric poly(styrene-b-n-butylmethacrylate) diblock copolymer thin films. As the lamellar ordering propagates through the film thickness, Islands or holes are formed on the free surface. It Is shown that, depending on the Initial film thickness, I.e. on the fraction of the film surface occupied by the islands (or holes) In the ordered state, the existence or non-existence of spatial correlations characterizes the ordering kinetics of both islands and holes. However, the limit between these two regimes is not the same in the two cases : in the case of holes, spatial correlations occur for a higher value of the surface coverage than In the case of islands.

We report investigations of instabilities of thin polystyrene films (< 100 nm). In the liquid state films break up by creating cylindrical holes. The average distance between holes is approx. proportional to the film thickness squared. This dependence compares well with the prediction of a recent theory on spinodal decomposition of thin liquid films. The subsequent dewetting process (=growth of holes) depends strongly on the composition of the substrate surface whereas the break up does not seem to be influenced by this surface. The importance of long and short range interactions in this context is discussed.

We apply a scaling model to study adsorption of neutral-strongly charged diblock copolymer from concentrated salt solutions, where the range of electrostatic interactions is smaller than that of excluded volume correlations. Adsorbed layer structure is determined by an equilibrium between the solvated block stretching energy, surface-adsorbed block interactions, and the solution chemical potential. We find that adsorption from micellar solutions results in a lower surface density than adsorption from solutions below the CMC. In both cases the surface density increases, and the layer thickness decreases, with salt concentration.

We have used neutron reflectivity to measure the concentration profile of carboxylic acid-terminated polystyrene at the liquid/solid interface. Two isotopic systems were studied: (1) deuterated polystyrene (DPS) in cyclohexane and (2) polystyrene (PS) in deuterated cyclohexane. Although measured at 24°C (a poor solvent condition), the high grafting density of PS tethered chains causes the chains to weakly stretch to 3-5 times its unperturbed dimension. Although of similar molecular weight, the height of the DPS layer is about 50% higher than that of the PS. In both systems, the volume fraction of polymer near the wall is ∼ 0.60. For the DPS system, reflectivity profiles can be simulated using a parabolic profile with a depletion layer at the silicon-liquid interface. However, in the PS system, no depletion is observed. Here, the shape. of the PS concentration profile is parabolic with a rounded tail.

The presence of copolymers at an interface between two immiscible fluids is crucial to such processes as emulsion stabilization and microemulsion formation [1]. Recently Marques and Joanny [2] and Garel et al. [3] have studied the behavior of a random copolymer at a liquid-liquid interface. However, the arrangement or sequence distribution of the monomers in a copolymer can vary widely from random to blocky or purely alternating. In this paper, we use both analytic arguments and molecular dynamics simulations to determine how the sequence distribution affects the adsorption and conformation of a single macromolecule at the boundary between two immiscible fluids [4]. In particular, we derive an expression for the free energy of a copolymer at the interface and compare the resulting predictions with the outcome of the simulations. Our findings yield design criteria for fabricating polymers that display the desired interfacial properties.

The kinetics of adsorption of polyvinyl acetate at the solid-liquid interface has been studied to verify the correctness of a description in a paper [Peterson and Kwel, J.Phys.Chem. 65, 1330(1961)] : “the initial rate of adsorption of polyvinyl acetate was found to be rapid”. This is inconsistent with the widely accepted knowledge that polymer adsorption is a slow process. Polyvinyl acetate (Mw = 124,800) was adsorbed from benzene (0.001 to 0.05 mg ml−1) onto mica at 295.5 K. The adsorbed amount per unit area i.e. adsorbance has been determined as a function of incubation time using an ultramicrobalance [Mettler UM3]. The results obtained show that the adsorbance rises rapidly at the beginning of adsorption and then reaches an apparent plateau, where the adsorbance still increases at negligibly slow rate in comparison with the initial rate. The Peterson and Kwei's results have been confirmed to be correct. We regarded the plateau as an adsorption equilibrium and constructed adsorption isotherms, in which the Peterson and Kwei's results were incorporated. These isotherms are found to be less dependent on concentration in the dilute region concerned. This dependency is in agreement with the prediction of the Scheutjens and Fleer theory based on the loop-train-tail model.

Interfacial behavior has been one of the current focuses of polymer research. 1,2 The practical importance of the problem is directly related to coating, colloidal stability, etc. Several of the important issues on the kinetics of the polymers at the interface such as reversibility, equilibrium and polymer rearrangement at the surface have drawn particular interests recently.3,4

We investigate the end grafted polyelectrolyte brush in the poor solvent regime of the corresponding neutral polymer system. Using Poisson-Boltzmann theory for the electrostatics and Flory-Huggins-mean-field theory for the excluded volume and Van-der-Waals-like monomer interactions, we find the existence of a first order phase transition to a collapsed state for moderate to highly charged polyelectrolytes in the poor solvent regime. Irreversibilities in the disjoining pressure between planar grafted surfaces are predicted. For polyelectrolytes grafted to spherical and cylindrical surfaces with small radii of curvature, the phase transition is predicted to become second order in the infinite molecular weight limit. A phase diagram for the entire poor solvent regime is given.

We present the results of phase-separation experiments performed on the binary polymer blend polyisoprene-poly(ethylene-propylene). Elastic light scattering and optical microscopy were used to follow the phase-separation after quenches into the coexistence region of the phase diagram. For critical quenches, we observed the well known bicontinuous infinite cluster morphology normally associated with spinodal decomposition, but with two distinct growth modes. In the bulk, we observed L(t) - t1/3 consistent with diffusion driven dynamics, whereas near the sample walls, there obtained a novel fast growth with L(t) -t 3/2, inconsistent with either diffusion or interface driven dynamics. We attribute this large exponent to wetting effects, but still lack any theoretical understanding of the phenomenon.