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Abstract: For two variable chemical oscillators, a simple strategy is to combine autocatalysis with a term including a binomial Michaelian denominator (constant+concentration). This strategy is applied in this paper to the context of polymerization reactions. In order to achieve autocatalysis a kinetic scheme of the form is applied, where the dimeric form D is included. The Michaelian term can be obtained in different ways, for example: (i) a third compound Z flows through the system and eliminates the monomer, (ii) compound Z can be radical initiators, and (iii) compound Z, which eliminates monomer, can be a special conformation (adsorbent) of the polymer. Three mechanisms are developed using these Michaelian terms. After decreasing the number of variables to two, by assuming a non-significant variation of the others, and normalizing the kinetic equations with respect to a certain steady state, phase plane techniques are applied to prove the existence of a limit cycle in the three models. Both monomer concentration and propagator concentration in the three models (cases 2.1 and 2.2) or the concentration of the adsorbent polymers are taken as oscillating variables. [Copyright &y& Elsevier]

Abstract: A simple polymerization scheme has been studied introducing small modifications leading to a stable focus type steady state (with damped oscillations) or unstable focus type (which combined with a no return enclosure for phase trajectories will show cycle limit sustained oscillations). Two variables have been employed in this analysis: X∝ monomer, . Limit cycle oscillations requires the addition of autocatalysis with respect to the monomer, J+X→2X, and so does an “enzymatic” block assuming that . The combination of both collateral additions makes the steady state an unstable focus and allows a simple Poincaré–Bendixson proof for the existence of the limit cycle. [ABSTRACT FROM AUTHOR]

Highlights: [•] New gels based on PEG and PVA of different degrees of hydrolysis were synthesized. [•] PVA degree of hydrolysis and PEG content of the network control the swelling capacity. [•] The pH influences the degree of swelling and the rate of hydrolysis of the hydrogel. [•] The degree of hydration decreases with temperature because their entropy is negative. [Copyright &y& Elsevier]

Abstract: Networks of β-cyclodextrin have been prepared by reaction with acylated poly(ethylene glycol) with a molar mass of 600g/mol. Samples with different β-cyclodextrin/poly(ethylene glycol) ratios: 1/4, 1/6, 1/8 and 1/10 have been prepared. Both components are bonded by ester groups, resulting in a network that can be degraded by hydrolysis in basic and acidic media. The maximum stability of the hydrogels is reached at pH 4. The hydrogel percentage water content depends on β-cyclodextrin content ranging from 82 to 98, and the swelling data obtained for these hydrogels fit well with a second order kinetics. The sorption behavior of these hydrogels has been tested by employing 1-naphthol as model molecule. The sorption capacity is close to other cyclodextrin networks previously reported and depends on the hydrogel composition and the concentration of 1-naphthol. [Copyright &y& Elsevier]

The thermoreversible gelation of blends of polystyrene-block-poly(ethylene/butylene)-block-polystyrene (SEBS) and polystyrene-block-poly(ethylene/propylene) (SEP) copolymers in n -octane was studied. The solvent is selective for the polyolefine blocks of the copolymers. The influence of the composition of the hybrid gels on the sol–gel transition and on the mechanical properties of the gels was analyzed. The sol–gel transition temperature increased with the concentration of both type of copolymers and did not depend on the hybrid gel composition for SEBS2 proportions higher than 50% at a total copolymer concentration higher than 6 wt%. The mechanical properties of the different gels were examined through oscillatory shear and compressive stress relaxation measurements. The elastic storage modulus increased with the triblock copolymer concentration but kept almost constant with the diblock copolymer concentration for SEBS concentrations higher than 5.0%. The stress relaxation rate was not dependent on the concentration of triblock and diblock copolymers, but the hybrid gels show lower stress relaxation rates than the pure SEBS2 gels. In the hybrid SEBS/SEP gels the SEP chains impart stability to the micelles or nodes of the network whereas the SEBS chains are responsible for the bridges that keep the gel as one-phase system. [ABSTRACT FROM AUTHOR]

Chitosan nanogels functionalized with folic acid and crosslinked with PEG diacid or tartaric acid have been prepared by inverse microemulsion method for targeted delivery into cancer cells. Nanogels with varying degrees of crosslinking (4.5–10 mol% of crosslinker) have been obtained. The particle sizes determined by TEM are below 50 nm with narrow size distributions. Nanogels show a clear improvement in solubility compared with pure chitosan. Their swelling ability is pH sensitive, due to the protonation of the free amino groups when the pH is varied from 10 to 4. The nanogels have been loaded with 5-fluorouracil, which is retained at pHs greater than 7.4 and released at pH = 5.5. These nanoparticles are degraded by lysozyme at physiological pH in a period of 20 days and are easily labeled with fluorescein isothiocyanate. All these features make them promising materials for the controlled release of anticancer drugs. [ABSTRACT FROM AUTHOR]

Highlights: [•] Folate-chitosan nanogels were crosslinked with genipin in microemulsion medium. [•] Nanogels are water soluble at neutral and basic pHs. [•] Degradation rate was slightly higher at endosomic pH 5.5 than at physiological 7.4. [•] Nanogels displayed a pH-responsive release of 5-fluorouracil. [ABSTRACT FROM AUTHOR]

Abstract: Chitosan nanoparticles crosslinked with genipin were prepared by reverse microemulsion that allowed to obtain highly monodisperse (3–20nm by TEM) nanogels. The incorporation of genipin into chitosan was confirmed and quantitatively evaluated by UV–vis and 1H NMR. Loosely crosslinked chitosan networks showed higher water solubility at neutral pHs than pure chitosan. The hydrodynamic diameter of the genipin–chitosan nanogels ranged from 270 to 390nm and no remarkable differences were found when the crosslinking degree was varied. The hydrodynamic diameters of the nanoparticles increased slightly at acidic pH and the protonation of ionizable amino groups with the pH was confirmed by the zeta potential measurements. The biocompatible and biodegradable nature, as well as the colloidal and monodisperse particle size of the prepared nanogels, make them attractive candidates for a large variety of biomedical applications. [Copyright &y& Elsevier]

Abstract: Chitosan nanoparticles were obtained by crosslinking with two biocompatible dicarboxylic acids: polyethylene glycol dicarboxylic acid and tartaric acid. The water-in-oil (W/O) microemulsion method yielded particle sizes around 10–15 nm in the dried state (TEM) and 200–700 nm in the swollen state (QELS) from commercial chitosan. All the synthesized nanogels showed improved water solubility and most of them were stable at physiological pH. QELS studies revealed the influence of the hydrophilic character and flexibility of the crosslinker on the swelling behaviour of the nanogels. The nanoparticles showed a pH-sensitive volume transition that was consistent with the pKa of chitosan. The collected zeta potential data corroborated the electrostatic repulsion mechanism responsible for the pH-responsive behaviour. [Copyright &y& Elsevier]