No abstract

Hybrid materials of the metal−organic framework (MOF), chromium(III) terephthalate (MIL-101), and phosphotungstic acid (PTA) were synthesized in aqueous media in the absence of hydrofluoric acid. XRD analysis of the MIL101/PTA composites indicates the presence of ordered PTA assemblies residing in both the large cages and small pores of MIL-101, which suggests the formation of previously undocumented structures. The MIL101/PTA structure enables a PTA payload 1.5−2 times higher than previously achieved. The catalytic performance of the MIL101/PTA composites was assessed in the Baeyer condensation of benzaldehyde and 2-naphthol, in the three-component condensation of benzaldehyde, 2-naphthol, and acetamide, and in the epoxidation of caryophyllene by hydrogen peroxide. The catalytic efficiency was demonstrated by the high (over 80−90%) conversion of the reactants under microwave-assisted heating. In four consecutive reaction cycles, the catalyst recovery was in excess of 75%, whereas the product yields were maintained above 92%. The simplicity of preparation, exceptional stability, and reactivity of the novel composites indicate potential in utilization of these catalytic matrices in a multitude of catalytic reactions and engineering processes.

Drug delivery systems (DDS) capable of releasing an active molecule at the appropriate site and at a rate that adjusts in response to the progression of the disease or to certain functions ⁄ biorhythms of the organism are particularly appealing. Biocompatible materials sensitive to certain physiological variables or external physicochemical stimuli (intelligent materials) can be used for achieving this aim. Light-responsiveness is receiving increasing attention owing to the possibility of developing materials sensitive to innocuous electromagnetic radiation (mainly in the UV, visible and near-infrared range), which can be applied on demand at well delimited sites of the body. Some light-responsive DDS are of a single use (i.e. the light triggers an irreversible structural change that provokes the delivery of the entire dose) while others able to undergo reversible structural changes when cycles of light ⁄ dark are applied, behave as multiswitchable carriers (releasing the drug in a pulsatile manner). In this review, the mechanisms used to develop polymeric micelles, gels, liposomes and nanocomposites with light-sensitiveness are analyzed. Examples of the capability of some polymeric, lipidic and inorganic structures to regulate the release of small solutes and biomacromolecules are presented and the potential of lightsensitive carriers as functional components of intelligent DDS is discussed.

Gel microparticles composed of lightly cross-linked poly(acrylic acid) networks, onto which polyether chains (Pluronic F127) are grafted, are introduced. The hydrophobic poly(propylene oxide) chains aggregate within the microgel structure, and the resulting aggregates are capable of solubilizing hydrophobic drugs, such as taxol. At temperatures where the Pluronic chains are not aggregated, the microgels behave like networks without spatial heterogeneity. Upon formation of aggregates within hydrogels, their equilibrium swelling diminishes, and the swelling behavior indicates non-Gaussian chain distribution. The kinetics of gel swelling shows unusual temperature dependence of the effective diffusion coefficient, indicative of chain rearrangement within a certain temperature range. The microgels exhibit high ion-exchange capacity for cationic hydrophilic drugs. The potential for the newly obtained microgels to be used as drug carriers is discussed.

Rheological properties of thermoreversible hydrogels of poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide)-g-poly(acrylic acid) have been studied. The relaxation exponent ∆ ) 0.69 is found from the frequency-independent loss tangent at the gel point. The distance from the gelation threshold (ε) is varied by changing the concentration or temperature. Dependencies of the zero-shear viscosity (ηo) and equilibrium modulus (Go) scale as ηo ∼ ε s and Go ∼ ε t . The transient exponents s ) 1.26-1.28 and t ) 2.64-2.65 are obtained in the vicinity of the gelation threshold. The scaling relation between all exponents is in excellent agreement with the Rouse model and is consistent with the percolation theory. Transient rheological properties exhibit complex scaling behavior above the gel point depending on ε. Scaling above the gel point correlates with the polymer structure.

Oxygen-transfer enhancement has been observed in the presence of colloidal dispersions of magnetite (Fe3O4) nanoparticles coated with oleic acid and a polymerizable surfactant. These fluids improve gas−liquid oxygen mass transfer up to 6-fold (600%) at nanoparticle volume fractions below 1% in an agitated, sparged reactor and show remarkable stability in high-ionic strength media over a wide pH range. Through a combination of experiments using physical and chemical methods to characterize mass transfer, it is shown that (i) both the mass transfer coefficient (k L) and the gas−liquid interfacial area (a) are enhanced in the presence of nanoparticles, the latter accounting for a large fraction of the total enhancement (80% or more), (ii) the enhancement in k L measured by physical and chemical methods is similar and ranges from 20 to 60% approximately, (iii) the enhancement in k L levels off at a nanoparticle volume fraction of approximately 1% v/v, and (iv) the enhancement in k L a shows a strong temperature dependence. These results are relevant to a wide range of processes limited by the mass transfer of a solute between a gas phase and a liquid phase, such as fermentation, waste treatment, and hydrogenation reactions.

The study dealt with microgels that consist of loosely cross-linked poly(acrylic acid) onto which poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO−PPO−PEO) copolymers such as Pluronic F127 (average composition EO99PO67EO99) or L92 (EO8PO52EO8) segments are grafted. Microgels based on the more hydrophobic Pluronic L92 exhibited highly porous structure, while the microgels containing Pluronic F127 were generally larger and possessed smooth surfaces, a homogeneous structure, and lower ion-exchange capacity. A good correlation was observed between the microgel surface potential obtained from the potentiometric titration data and the independently measured electrophoretic ζ-potential. The differences in the microgel ion-exchange capacity account for the differences in the capacity of the microgels to absorb weakly basic anticancer drugs such as mitomycin, mitoxantrone, and doxorubicin. The uptake of hydrophobic drugs such as Taxol, estradiol, progesterone, and camptothecin was dictated by the content of PPO in the microgels, which determines their solubilizing capacity. The exclusion of proteins of varying molecular weight by the microgels reveals the effective pore size, which is below 7.5 nm in the F127-based microgels but is on the order of tens of nanometers in the L92-based microgels.

Poly(acrylic acid) (PAA) copolymers modified with block-copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) are inherently attractive for medicinal applications as their components are considered pharmaceutically safe. The laboratory-scale synthesis of poly(acrylic acid) bonded onto a PEO−PPO−PEO triblock (Pluronic) backbone via dispersion polymerization is reported. Initial optimization of the synthesis defines appropriate levels of initial loading of acrylic acid and Pluronic, with a mixture of 2,2‘-azobis(2,4-dimethylpentanenitrile) and lauroyl peroxide as an initiator system. The synthesis results in a copolymer with low residual monomer content and a very high degree of bonding between Pluronic and PAA. Diluted aqueous solutions of Pluronic-g-PAA exhibit rapid thermogelation. Rheological parameters of the copolymers urge on their ophthalmic applications.