Marina Tsianou

Nanoparticle surface modification by amphiphilic polymers in aqueous media: Role of polar organic solvents

We investigate the role of three polar organic solvents (dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), and glycerol) on the interfacial behavior of Pluronic P105 poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers on protonated silica nanoparticles in an aqueous dispersion. The polymer adsorption and self-assembly have been assessed from critical surface micelle concentration (csmc, measured by pyrene fluorescence spectroscopy) and adsorbed layer thickness (measured by capillary viscometry) data. Above its csmc, PEO-PPO-PEO block copolymers form hydrophobic domains on the nanoparticle surface. Below a critical concentration in water (known as critical displacer concentration, cdc), organic solvents act as displacers (molecules that can displace adsorbed polymer from a solid surface). The critical displacer concentration is obtained from the csmc and the polymer adsorbed layer thickness data. The cdc is found to be dependent on both the amount of nanoparticles present in the system as well as the nature of the displacer. Below the cdc, the csmc increases and the adsorbed polymer layer thickness decreases with increasing organic solvent concentration. Interfacial free energy calculations suggest that DMF, DMSO, and glycerol can adsorb onto the silica particles by displacing adsorbed PEO. These calculations are consistent with the experimental results in that, as a displacer, glycerol is the most effective and DMF is the least effective. Above the cdc, the influence of glycerol or DMSO on csmc is opposite to that of DMF which is attributed to the cosolvent effect.

Cyclodextrins and Surfactants in Aqueous Solution above the Critical Micelle Concentration: Where Are the Cyclodextrins Located?

Cyclodextrins (CDs) are known to bind surfactant molecules below the surfactant critical micelle concentration (CMC); however, interactions of CDs with surfactant micelles (above the CMC) are not well understood. In particular, direct structural evidence of the location of CDs in the different subphases found in micellar solutions is lacking. We have utilized small-angle neutron scattering (SANS) with contrast matching to probe the localization of α-cyclodextrin (α-CD) and 2-hydroxypropyl-β-cyclodextrin (HPβ-CD) in sodium dodecyl sulfate (SDS) micelles in aqueous (D2O) solutions. SANS data from solutions containing either hydrogenated or deuterated surfactants were analyzed by considering three different scenarios pertaining to the localization of cyclodextrin, either all in solution or some in the micelle shell or some in the micelle core, and were simultaneously fitted using the core-shell prolate ellipsoid form factor and the Hansen-Hayter-based structure factor. The scenario that considered a fraction of CD to localize in the micelle core well described the SANS data from both hydrogenated and deuterated SDS-CD-D2O solutions, while the other two scenarios did not. Among the various structural and interaction parameters obtained from this analysis, it emerged that the micelle core consisted of up to ∼10% HPβ-CD or ∼16% α-CD with respect to the total number of molecules (surfactants and CDs) present in the micelle at 25 mM SDS, and up to 14% HPβ-CD or 28% α-CD at 50 mM SDS. This is the first study that provides direct evidence on the location of cyclodextrin in the core of surfactant micelles. An improved understanding of CD interactions with surfactants and lipids would enable better strategies for drug encapsulation and delivery with CDs.

Assessment of solvents for cellulose dissolution

A necessary step in the processing of biomass is the pretreatment and dissolution of cellulose. A good solvent for cellulose involves high diffusivity, aggressiveness in decrystallization, and capability of disassociating the cellulose chains. However, it is not clear which of these factors and under what conditions should be improved in order to obtain a more effective solvent. To this end, a newly-developed phenomenological model has been applied to assess the controlling mechanism of cellulose dissolution. Among the findings, the cellulose fibers remain crystalline almost to the end of the dissolution process for decrystallization-controlled kinetics. In such solvents, decreasing the fiber crystallinity, e.g., via pretreatment, would result in a considerable increase in the dissolution rate. Such insights improve the understanding of cellulose dissolution and facilitate the selection of more efficient solvents and processing conditions for biomass. Specific examples of solvents are provided where dissolution is limited due to decrystallization or disentanglement.

Cellulose dissolution: insights on the contributions of solvent-induced decrystallization and chain disentanglement

The dissolution of cellulose is a critical step for the efficient utilization of this renewable resource as a starting material for the synthesis of high value-added functional polymers and chemicals and also for biofuel production. The recalcitrance of semicrystalline cellulose microfibrils presents a major barrier to cellulose dissolution. Despite research efforts, important aspects of cellulose dissolution such as solvent-induced decrystallization and chain disentanglement are not well-understood. Here we address these fundamental issues with the practical goal of gaining insights into the swelling and dissolution of cellulose that cannot be obtained from macroscopic experimental data. To this end, we have used a newly-developed phenomenological model that captures the phenomena governing the dissolution of semicrystalline polymers as well as the thermodynamics and kinetics of dissolution. This model fits well experimental data for swelling and dissolution of cotton fibers in the ionic liquid [bmim]Cl, and allows the quantification of two important aspects, i.e., solvent effectiveness in cellulose (1) decrystallization and (2) chain disentanglement, the balance of which controls the mechanism and kinetics of cellulose dissolution. The activation parameters of cellulose decrystallization, estimated using the obtained decrystallization constant values, reveal that the decrystallization of cellulose in [bmim]Cl is associated with positive enthalpy and entropy and it is also very sensitive to temperature. When the solvent effectiveness in the disruption of cellulose crystals is relatively lower than its ability to disentangle the chains, the kinetics of dissolution are controlled by decrystallization. Furthermore, conditions that facilitate cellulose chain disentanglement, in addition to increasing the rate of dissolution, can result in faster decrystallization. The solvent effectiveness in chain disentanglement is the only factor that determines the decrease of the cellulose fiber radius. In cases where the fiber dissolution rate is lower than the decrystallization rate, the dissolution of cellulose is mostly controlled by the solvent ability to disentangle the chains. The insights obtained from this study improve the understanding of cellulose–solvent interactions underlying decrystallization and disentanglement and their contributions in controlling the kinetics of cellulose swelling and dissolution.

Polyelectrolyte nanocages via crystallized miniemulsion droplets

Polyelectrolyte nanocages were synthesized by interfacial cross-linking of monolayers of vinyl-functionalized surfactant molecules adsorbed by crystallized miniemulsion droplets. The monolayer-thick shell of these nanocages was confirmed by AFM analysis.

From polyelectrolyte complexes to polyelectrolyte multilayers: Electrostatic assembly, nanostructure, dynamics, and functional properties

Polyelectrolyte complexes (PECs) are three-dimensional macromolecular structures formed by association of oppositely charged polyelectrolytes in solution. Polyelectrolyte multilayers (PEMs) can be considered a special case of PECs prepared by layer-by-layer (LbL) assembly that involves sequential deposition of molecular-thick polyelectrolyte layers with nanoscale control over the size, shape, composition and internal organization. Although many functional PEMs with novel physical and chemical characteristics have been developed, the current practical applications of PEMs are limited to those that require only a few bilayers and are relatively easy to prepare. The viability of such engineered materials can be realized only after overcoming the scientific and engineering challenges of understanding the kinetics and transport phenomena involved in the multilayer growth and the factors governing their final structure, composition, and response to external stimuli. There is a great need to model PEMs and to connect PEM behavior with the characteristics of the PEC counterparts to allow for prediction of performance and better design of multilayered materials. This review focuses on the relationship between PEMs and PECs. The constitutive interactions, the thermodynamics and kinetics of polyelectrolyte complexation and PEM formation, PEC phase behavior, PEM growth, the internal structure and stability in PEMs and PECs, and their response to external stimuli are presented. Knowledge of such interactions and behavior can guide rapid fabrication of PEMs and can aid their applications as nanocomposites, coatings, nano-sized reactors, capsules, drug delivery systems, and in electrochemical and sensing devices. The challenges and opportunities in future research directions are also discussed.

Competitive Adsorption Between PEO-Containing Block Copolymers and Homopolymers at Silica

The ability to manipulate polymer adsorption is useful for applications involving colloidal stabilization, for example, paints, cosmetics, lubricants, and mineral and waste-water treatment. We have an ongoing interest on the use of organic molecules for modulating the aqueous solution and adsorption properties of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) block copolymers. In the present study, the influence of low molecular weight PEO homopolymer on the adsorption of a representative PEO–PPO–PEO block copolymer (Pluronic P105: EO37-PO56-EO37) at the surface of protonated silica nanoparticles dispersed in water is investigated. Pluronic P105 forms hydrophobic domains on the surface of protonated silica at a critical surface micelle concentration, csmc, of 0.02 wt% in the presence of 0.1 wt% silica nanoparticles in water, well below the cmc of Pluronic P105 in water (0.6 wt%). Dye solubilization experiments reveal an increase in the PEO–PPO–PEO block copolymer csmc with increasing amounts of added PEO homopolymer. The resulting critical displacer concentration for PEO homopolymer of molecular weights 200 and 600 Da was measured to be 0.1 wt% and 0.07 wt%, respectively, in the presence of 0.1 wt% silica nanoparticles. Capillary viscometry measurements indicate a decrease in the adsorbed layer thickness at the protonated silica surface with increasing PEO homopolymer concentration. The data presented herein are consistent with a physical model which considers “patches” of PEO–PPO–PEO block copolymer and PEO homopolymer adsorbed at the silica surface.

Dissolution of Cellulosic Fibers: Impact of Crystallinity and Fiber Diameter

With the aim of informing the selection of biomass pretreatment options and to assist in interpreting experimental results from different biomass/solvent combinations, this study addresses the impact of crystallinity and size on the kinetics of semicrystalline cellulose fiber swelling and dissolution. To this end, a newly developed phenomenological model is utilized that accounts for the role of decrystallization and disentanglement as two rate-determinant steps in the cellulose dissolution process. Although fibers with lower crystallinity swell more and faster, the degree of crystallinity does not affect the dissolution rate. Fibers of smaller diameter swell more and become amorphous faster. When decrystallization is important, the solubility of thinner fibers increases more with a reduction in the crystallinity compared to the diameter. However, when the dissolution is controlled by chain disentanglement, or in the case of dissolution of fibers having larger diameters, milling the fibers to reduce the particle size could increase the solubility.

Mono- and Di-valent Salts as Modifiers of PEO-PPO-PEO Block Copolymer Interactions with Silica Nanoparticles in Aqueous Dispersions

Colloidal stabilization of nanoparticle dispersions is central to applications including coatings, mineral extraction, and dispersion of oil spills in oceanic environments, which often involves oil-mineral-aggregates (OMAs). We have an ongoing interest in the modulation of amphiphile micellization and adsorption behavior in aqueous colloidal dispersions in the presence of various additives. Here we evaluate the effect of added salts CaCl2, MgCl2, and NaCl on the micellization and adsorption behavior of the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer Pluronic P105 (EO37PO56EO37). In 0.10 wt% silica nanoparticle (10.6 nm average diameter) dispersion, adsorbed block copolymer layer formation begins at a critical surface micelle concentration (csmc) of 0.02 wt%, well below the critical micellization concentration of Pluronic P105 in water. Dye solubilization experiments demonstrate an increase in the csmc upon addition of each salt. Each added salt reaches a level of maximum effectiveness in its ability to disfavor Pluronic P105 adsorption at the silica surface. These peak levels occur at concentrations of 0.005, 0.03, and 0.05 M for CaCl2, MgCl2, and NaCl, respectively, in the presence of 0.10 wt% silica nanoparticles. We explain these results in the context of an electrostatic displacer mechanism and discuss possible connections to OMA-dispersant formation. GRAPHICAL ABSTRACT

Product Design Applied to Formulated Products: A Course on Their Design and Development that Integrates Knowledge of Materials Chemistry, (Nano)Structure and Functional Properties

Product development is a multi-faceted role that a growing number of engineers are tasked with. This represents a significant shift in career paths for those employed in the chemical and materials engineering disciplines, who typically were concerned with bulk commodity manufacturing. This paradigm shift requires the undergraduate curriculum to be adapted to prepare students for these new responsibilities. The authors present here on a product design capstone course developed for chemical engineering seniors at the University at Buffalo (UB), The State University of New York (SUNY). The course encompasses the following themes: a general framework for product design and development (identify customer needs, convert needs to specifications, create ideas/concepts, select concept, formulate/test/manufacture product; and (nano)structure-property relations that guide the search for smart/tunable/functional materials for contemporary needs and challenges. These two main themes are enriched with case studies of successful products. Students put the course material into practice by working through formulated product design projects that are drawn from real-world problems. The authors begin by presenting the course Product Design Applied to Formulated Products: A Course on Their Design and Development that Integrates Knowledge of Materials Chemistry, (Nano) Structure and Functional Properties Andrew M. Bodratti University at Buffalo (UB), The State University of New York (SUNY), USA Zhiqi He University at Buffalo (UB), The State University of New York (SUNY), USA Marina Tsianou University at Buffalo (UB), The State University of New York (SUNY), USA Chong Cheng University at Buffalo (UB), The State University of New York (SUNY), USA