Coarsening of the phase structure in immiscible polymer blends. Coalescence or ostwald ripening?

Direct observation of degassing during decompression of basaltic magma

Transitions in eruptive style during volcanic eruptions strongly depend on how easily gas and magma decouple during ascent. Stronger gas-melt coupling favors highly explosive eruptions, whereas weaker coupling promotes lava fountaining and lava flows. The mechanisms producing these transitions are still poorly understood because of a lack of direct observations of bubble dynamics under natural magmatic conditions. Here, we combine x-ray radiography with a novel high-pressure/high-temperature apparatus to observe and quantify in real-time bubble growth and coalescence in basaltic magmas from 100 megapascals to surface. For low-viscosity magmas, bubbles coalesce and recover a spherical shape within 3 seconds, implying that, for lava fountaining activity, gas and melt remain coupled during the ascent up to the last hundred meters of the conduit. For higher-viscosity magmas, recovery times become longer, promoting connected bubble pathways. This apparatus opens frontiers in unraveling magmatic/volcanic processes, leading to improved hazard assessment and risk mitigation.

Partial to complete wetting transitions in immiscible ternary blends with PLA: the influence of interfacial confinement

In this study it is shown that the three different intermediate phases in melt blended ternary PLA/PHBV/PBS, PLA/PBAT/PE and PLA/PE/PBAT systems all demonstrate partial wetting, but have very different wetting behaviors as a function of composition and annealing. The interfacial tension of the various components, their spreading coefficients and the contact angles of the confined partially wet droplets at the interface are examined in detail. A wetting transition from partially wet droplets to a complete layer at the interface is observed for both PHBV and PBAT by increasing the concentration and also by annealing. In contrast, in PLA/PE/PBAT, the partially wet droplets of PE at the interface of PLA/PBAT coalesce and grow in size, but remain partially wet even at a high PE concentration of 20% and after 30 min of quiescent annealing. The dewetting speed of the intermediate phase is found to be the principal factor controlling these wetting transitions. This work shows the significant potential for controlled wetting and structuring in ternary polymer systems.

Suppressing phase coarsening in immiscible polymer blends using nano-silica particles located at the interface

Coalescence suppressing effect of nanoparticles at the interface of polymer blends.

Quantitative biorelevant profiling of material microstructure within 3D porous scaffolds via multiphoton fluorescence microscopy

This study presents a novel approach, based on fluorescence multiphoton microscopy (MPM), to image and quantitatively characterize the microstructure and cell-substrate interactions within microporous scaffold substrates fabricated from synthetic biodegradable polymers. Using fluorescently dyed scaffolds fabricated from poly(DTE carbonate)/poly(DTO carbonate) blends of varying porosity and complementary green fluorescent protein-engineered fibroblasts, we reconstructed the three-dimensional distribution of the microporous and macroporous regions in 3D scaffolds, as well as cellular morphological patterns. The porosity, pore size and distribution, strut size, pore interconnectivity, and orientation of both macroscale and microscale pores of 3D scaffolds were effectively quantified and validated using complementary imaging techniques. Compared to other scaffold characterizing techniques such as confocal imaging and scanning electron microscopy (SEM), MPM enables the acquisition of images from scaffold thicknesses greater than a hundred microns with high signal-to-noise ratio, reduced bulk photobleaching, and the elimination of the need for deconvolution. In our study, the morphology and cytoskeletal organization of cells within the scaffold interior could be tracked with high resolution within the limits of penetration of MPM. Thus, MPM affords a promising integrated platform for imaging cell-material interactions within the interior of polymeric biomaterials.

Macroporous poly(l-lactide) of controlled pore size derived from the annealing of co-continuous polystyrene/poly(l-lactide) blends

A detailed study on the static annealing of co-continuous polystyrene/poly(L-lactide) (PLLA) blends is presented. The effects of temperature, time at temperature, viscosity of the phases and interfacial modification on the coarsening of the blend are discussed. In this paper, polystyrene and PLLA are blended at compositions of 50/50 and 60/40 to form co-continuous morphologies. These co-continuous morphologies are coarsened under quiescent annealing conditions, and the subsequent removal of the polystyrene phase leaves a macroporous PLLA structure. The microstructure is analyzed using three different techniques: the BET nitrogen adsorption technique, mercury intrusion porosimetry and SEM combined with image analysis. It is shown that static annealing can be used to generate a series of co-continuous networks with controlled pore sizes ranging from 1 to hundreds of microns. A non-linear pore size growth rate is observed for these systems due to the degradation of PLLA and this study indicates that controlled degradation can be used as an additional tool for morphology control. Compatibilized polystyrene/PLLA blends demonstrate significantly reduced coarsening effects due to the reduction of interfacial tension. The coarsening rate of the co-continuous structure was examined in terms of the pore size, R and this growth rate is discussed in terms of a previously proposed coarsening mechanism. This approach is a route towards the preparation of a macroporous PLLA structure with pore sizes in the range required for scaffolds for tissue regeneration.

Probing the mechanism of aqueous two-phase system formation for alpha-elastin on-chip

The kinetics of formation of the aqueous two-phase system (ATPS) for alpha-elastin was studied by dark field microscopy in an on-chip linear temperature gradient. Scattering intensities of protein solutions were recorded as a function of temperature and time, simultaneously at several concentrations. It was found that the formation rate of the ATPS could be fit as a first-order process and that the apparent rate constant increased with protein concentration. The activation energy for the process was 9.5 +/- 0.5 kcal/mol, and this result was consistent with a coalescence mechanism. Experiments were also conducted with varying concentrations of sodium dodecyl sulfate, which shut off the coalescence mechanism forcing ATPS formation to proceed through Ostwald ripening. When this was done, the activation energy increased to 33 +/- 2 kcal/mol and the kinetics became consistent with a second-order process.

Study of beta-lactoglobulin/acacia gum complex coacervation by diffusing-wave spectroscopy and confocal scanning laser microscopy

Complex coacervation has been investigated on mixtures of beta-lactoglobulin (beta-lg) and acacia gum (AG) at pH 4.2 where these two macromolecules interact electrostatically. Changes in beta-lg/AG complex coacervation induced by the presence of beta-lg aggregates were considered. The nature and structure of particles resulting from complex coacervation were determined by using confocal scanning laser microscopy (CSLM). CSLM revealed fundamental differences in the structure of each of the studied dispersions (at 1 wt.% total concentration). Spherical vesicular coacervates and precipitates (based on beta-lg aggregates) were the hallmark of BLG/AG dispersions (beta-lg dispersion containing insoluble aggregates). Only coacervates were visible in AF-BLG/AG dispersions (beta-lg dispersion free of insoluble aggregates). The latter were characterised by the presence of large foam-like coacervates induced by partial coalescence of single coacervates, especially at the 2:1 protein to polysaccharide (Pr:Ps) ratio. Diffusing wave spectroscopy (DWS) was used to study the stability of dispersions as a function of time. Depending on the Pr:Ps ratio and the presence of beta-lg aggregates, the intensity correlation function (g(2)(t)) shifted to lower correlation times rapidly after mixing of both macromolecules. This revealed the formation of a large number of small particles, characterised by faster Brownian motion. At 1 and 5 wt.% total concentration, the 8:1 Pr:Ps ratio exhibited a rapid decrease of the backscattered intensity in time, both for BLG/AG and AF-BLG/AG mixtures, revealing rapid sedimentation/coalescence of particles. This precluded the achievement of a stable correlation function. For the 2:1 Pr:Ps ratio, mixtures exhibited both coalescence and sedimentation phenomena as confirmed by shifts in the g(2)(t) towards larger correlation times and the decrease of the initial value of g(2)(t) with time. Mixtures obtained for the 1:1 Pr:Ps ratio were characterised by small variations in the DWS signal, emphasising the stability of produced particles. The increase of the total biopolymer concentration reduced the effect of both Pr:Ps ratio and presence of protein aggregates. From CSLM and DWS observations, possible differences in the complex coacervation mechanism in both types of mixtures were highlighted. The use of protein aggregates to control complex coacervation was underlined.