Shear-Induced Reactive Gelation

In this work, we describe a method for the production of porous polymer materials in the form of particles characterized by narrow pore size distribution using the principle of shear-induced reactive gelation. Poly­(styrene-<i>co</i>-divinylbenzene) primary particles with diameter ranging from 80 to 200 nm are used as building blocks, which are assembled into fractal-like clusters when exposed to high shear rates generated in a microchannel. It was found that independent of the primary particle size, it is possible to modulate the internal structure of formed fractal-like aggregates having fractal dimension ranging from 2.4 to 2.7 by varying the residence time in the microchannel. Thermally induced postpolymerization was used to increase the mechanical resilience of such formed clusters. Primary particle interpenetration was observed by SEM and confirmed by light scattering resulting in an increase of fractal dimension. Nitrogen sorption measurements and mercury porosimetry confirmed formation of a porous material with surface area ranging from 20 to 40 m²/g characterized by porosity of 70% and narrow pore size distribution with an average diameter around 700 nm without the presence of any micropores. The strong perfusive character of the synthesized material was confirmed by the existence of a plateau of the height equivalent to a theoretical plate measured at high reduced velocities using a chromatographic column packed with the synthesized microclusters

Kinetics and Cluster Morphology Evolution of Shear-Driven Aggregation of Well-Stabilized Colloids

We investigate the shear-driven aggregation of polystyrene colloids that are stabilized by both fixed and surfactant charges, using a microchannel device, in various particle volume fractions. The objective is to understand how the primary particles evolve to clusters with shearing time, how the cluster morphology develops along the aggregation with the effect of breakage and restructuring, and whether non-Derjaguin–Landau–Verwey–Overbeek (DLVO) interactions are present, affecting the kinetics. The time evolution of the primary particle conversion to big clusters is characterized by an induction time, followed by an explosive increase when the cluster size reaches a certain critical value, which confirms the self-acceleration kinetics developed in the literature. The size of the critical clusters has been quantified for the first time, and its scaling with the shear rate follows the literature prediction well. Moreover, analysis of the shear-driven kinetics confirms the presence of substantial non-DLVO interactions in the given system

Effect of Dispersed Polymeric Nanoparticles on the Bulk Polymerization of Methyl Methacrylate

We proposed a methodology to investigate the effect of dispersed nanoparticles (NPs) on bulk polymerization of methyl methacrylate (MMA), based on DSC experiments and modeling of the bulk polymerization kinetics. As examples, we have applied it to polymeric NPs, polytetra­fluoroethylene (PTFE) and polystyrene (PS), and compared the results with those from linear PMMA and cross-linked PMMA (cPMMA). The presence of dissolved linear PMMA enhances the MMA bulk polymerization kinetics, as if the system was prepolymerized at a conversion equal to the dissolved amount of the linear PMMA. The dispersed cPMMA also enhances the MMA polymerization kinetics, but the enhancement decreases as the cross-linker in the cPMMA increases. The PTFE NPs behave like (inert) dead volume, while the PS NPs do enhance the MMA polymerization kinetics due to their slight swelling in MMA. Therefore, we can conclude that dispersed nonswellable polymeric NPs behave like inert dead volume, and swellable polymeric NPs enhance the MMA bulk polymerization kinetics and the enhancement extent increases as the swelling extent increases. The CryoSEM image of the bulk PMMA shows grainlike microstructure when the NPs are present

Equilibrium Theory Based Design Space for the Multicolumn Countercurrent Solvent Gradient Purification Process

A procedure for designing the operation parameter space for the twin-column multicolumn countercurrent solvent gradient purification (MCSGP) process for the purification of therapeutic proteins is derived. This is based on the equilibrium theory, which assumes instantaneous equilibrium conditions. As the MCSGP process allows protein separation with a linear modifier gradient, all equations are derived in terms of the covered distance as a function of the modifier concentration in ion-exchange chromatography. All constraints, which need to be fulfilled in order to obtain a stable process with maximum yield and purity, are described as a function of the different process parameters. For operation parameters within the parameter space where all constraints are fulfilled, a stable process is predicted. Additionally, on the boundary of this region, the optimal operation point in terms of buffer consumption and productivity can be found. Besides, the presented design space can help to analyze the impact of different process parameters on performance and stability and therefore to establish conditions for a robust operation of the process

Population Balance Modeling of Antibodies Aggregation Kinetics

The aggregates morphology and the aggregation kinetics of a model monoclonal antibody under acidic conditions have been investigated. Growth occurs via irreversible cluster–cluster coagulation forming compact, fractal aggregates with fractal dimension of 2.6. We measured the time evolution of the average radius of gyration, ⟨<i>R</i>_<i>g</i>⟩, and the average hydrodynamic radius, ⟨<i>R</i>_<i>h</i>⟩, by in situ light scattering, and simulated the aggregation kinetics by a modified Smoluchowski‘s population balance equations. The analysis indicates that aggregation does not occur under diffusive control, and allows quantification of effective intermolecular interactions, expressed in terms of the Fuchs stability ratio (<i>W</i>). In particular, by introducing a dimensionless time weighed on <i>W</i>, the time evolutions of ⟨<i>R</i>_<i>h</i>⟩ measured under various operating conditions (temperature, pH, type and concentration of salt) collapse on a single master curve. The analysis applies also to data reported in the literature when growth by cluster–cluster coagulation dominates, showing a certain level of generality in the antibodies aggregation behavior. The quantification of the stability ratio gives important physical insights into the process, including the Arrhenius dependence of the aggregation rate constant and the relationship between monomer–monomer and cluster–cluster interactions. Particularly, it is found that the reactivity of non-native monomers is larger than that of non-native aggregates, likely due to the reduction of the number of available hydrophobic patches during aggregation

Fragmentation of Amyloid Fibrils Occurs in Preferential Positions Depending on the Environmental Conditions

Understanding the mechanism of amyloid fibril breakage is of fundamental importance in various research fields including biomedicine and bionanotechnology. The aim of this work is to clarify the impact of temperature and agitation speed on the fibril breakage rate constant, which depends both on the fibril length as well as on the position of fragmentation along the fibril longitudinal axis. In particular, we intend to discriminate between three fibril fragmentation mechanisms: erosion (i.e., breakage occurs preferentially at the ends of the fibril), random (i.e., breakage occurs with the same likelihood at any position), or central (i.e., breakage occurs preferentially at the center of the fibril). To do so, we compare the time evolution of the fibril length distribution followed with atomic force microscopy with simulations from a kinetic model based on population balance equations (PBE). In this frame, we investigate the breakage mechanism of insulin fibrils, which turns out to be affected by the operative conditions employed. Moreover, we compare our findings with literature data obtained with β-lactoglobulin and β2-microglobulin. It is observed that high temperature drives the breakage toward an erosion mechanism, while a high agitation rate rather induces a central breakage

Thermoresponsive Stability of Colloids in Butyl Acetate/Ethanol Binary Solvent Realized by Grafting Linear Acrylate Copolymers

We have developed a new class of thermoresponsive colloids that can exhibit a sharp reversible transition between dispersion and aggregation in binary BuAc/EtOH solvents based on the UCST (upper critical solution temperature)-type phase separation. This is realized by grafting linear PMMA-BA (random) copolymer onto the colloidal particles. We have selected TiO₂/PS hybrid spheres (HSs) as a model system to demonstrate our general design concept. By grafting the linear PMMA-BA copolymer onto the HS surface, with the molecular weight from 30 to 40 kDa, we found that the thermoresponsive transition between dispersion and aggregation is fast, sharp, and reversible. At high mass fractions of the HSs, we have even observed a sharp transition between dispersion and gelation (or phase separation). The transition temperature can be tuned by varying the binary solvent composition, BuAc/EtOH, and the molecular weight of the grafted linear copolymer in the range from 5 to 55 °C. One of the most important features of this work is that the thermoresponsive materials used in organic solvents are initially synthesized in water with widely applied conventional (instead of research-based) techniques, thus being well suited for industrial production. In addition, the proposed approach is rather general and applicable to realizing the thermoresponsive transition for various types of colloids and nanoparticles

Synthesis of Macroporous Polymer Particles Using Reactive Gelation under Shear

By combining elements from colloidal and polymer reaction engineering a new approach toward macroporous, mechanically robust polymer particles is presented, which does not require any porogenic additives. Specifically, aggregation and breakage in turbulent conditions of aggregates originating from fully destabilized primary latex particles is applied to produce compact, micrometer-sized clusters. Post-polymerization of monomer introduced initially to swell the primary particles is imparting mechanical rigidity and permanence to the internal structure. The resulting microclusters exhibit an internal porosity on the order of 70% and relatively broad pore size distribution, with exceptionally large pores, ranging from about 50 nm to 10 μm in diameter. These particulate microclusters, produced via reactive gelation under shear, are fractal objects with fractal dimension around 2.7, as opposed to the more open fractal structure of a monolith produced via stagnant reactive gelation, with fractal dimension of 1.9. Such macroporous particles are thought to be useful in applications requiring pores on the micrometer scale, e.g., in the chromatography of biomolecules or for packing beds perfusive to convective flow

Contribution of Electrostatics in the Fibril Stability of a Model Ionic-Complementary Peptide

In this work we quantified the role of electrostatic interactions in the self-assembly of a model amphiphilic peptide (RADA 16-I) into fibrillar structures by a combination of size exclusion chromatography and molecular simulations. For the peptide under investigation, it is found that a net charge of +0.75 represents the ideal condition to promote the formation of regular amyloid fibrils. Lower net charges favor the formation of amorphous precipitates, while larger net charges destabilize the fibrillar aggregates and promote a reversible dissociation of monomers from the ends of the fibrils. By quantifying the dependence of the equilibrium constant of this reversible reaction on the pH value and the peptide net charge, we show that electrostatic interactions contribute largely to the free energy of fibril formation. The addition of both salt and a charged destabilizer (guanidinium hydrochloride) at moderate concentration (0.3–1 M) shifts the monomer-fibril equilibrium toward the fibrillar state. Whereas the first effect can be explained by charge screening of electrostatic repulsion only, the promotion of fibril formation in the presence of guanidinium hydrochloride is also attributed to modifications of the peptide conformation. The results of this work indicate that the global peptide net charge is a key property that correlates well with the fibril stability, although the peptide conformation and the surface charge distribution also contribute to the aggregation propensity

Role of Cosolutes in the Aggregation Kinetics of Monoclonal Antibodies

We propose a general strategy based on kinetic analysis to investigate how cosolutes affect the aggregation behavior of therapeutic proteins. We apply this approach to study the impact of NaCl and sorbitol on the aggregation kinetics of two monoclonal antibodies, an IgG1 and an IgG2. By using a combination of size exclusion chromatography and light scattering techniques, we study the impact of the cosolutes on the monomer depletion, as well as on the formation of dimers, trimers, and larger aggregates. We analyze these macroscopic effects in the frame of a kinetic model based on Smoluchowski’s population balance equations modified to account for nucleation events. By comparing experimental data with model simulations, we discriminate the effect of cosolutes on the elementary steps which contribute to the global aggregation process. In the case of the IgG1, it is found that NaCl accelerates the kinetics of aggregation by promoting specifically aggregation events, while sorbitol delays the kinetics of aggregation by specifically inhibiting protein unfolding. In the case of the IgG2, whose monomer depletion kinetics is limited by dimer formation, NaCl and sorbitol are found respectively to accelerate and inhibit conformational changes and aggregation events to the same extent