Molecular hydrogels from bolaform amino acid derivatives: a structure-properties study based on the thermodynamics of gel solubilization

Insight is provided into the aggregation thermodynamics associated to hydrogel formation by molecular gelators derived from L-valine and L-isoleucine. Solubility data from NMR measurements are used to extract thermodynamic parameters for the aggregation in water. It is concluded that at room temperature and up to 55?°C, these systems form self-assembled fibrillar networks in water with quite low or zero enthalpic component, whereas the entropy of the aggregation is favorable. These results are explained by considering that the hydrophobic effect is dominant in the self-assembly. However, studies by NMR and IR spectroscopy reveal that intermolecular hydrogen bonding is also a key issue in the aggregation process of these molecules in water. The low enthalpy values measured for the self-assembly process are ascribed to the result of a compensation of the favorable intermolecular hydrogen-bond formation and the unfavorable enthalpy component of the hydrophobic effect. Additionally, it is shown that by using the hydrophobic character as a design parameter, enthalpy-controlled hydrogel formation, as opposed to entropy-controlled hydrogel formation, can be achieved in water if the gelator is polar enough. It is noteworthy that these two types of hydrogels, enthalpy-versus entropy-driven hydrogels, present quite different response to temperature changes in properties such as the minimum gelator concentration (mgc) or the rheological moduli. Finally, the presence of a polymorphic transition in a hydrogel upon heating above 70?°C is reported and ascribed to the weakening of the hydrophobic effect upon heating. The new soft polymorphic materials present dramatically different solubility and rheological properties. Altogether these results are aimed to contribute to the rational design of molecular hydrogelators, which could be used for the tailored preparation of this type of soft materials. The reported results could also provide ground for the rationale of different self-assembly processes in aqueous media.     

Strategy to establish the number of replicates of a medical laboratory test

The allowable error of a test in medical laboratories can be defined by the customer demands based upon the use of the test, or by the appropriate biological variation. After correction for systematic bias, the actual analytical variation is compared to the stated allowable error. On top of that, proper quality control rules should be implemented regarding the number of control materials and the desired analytical quality assurance as well as assessment. A strategy is outlined that balances all these factors, taking advantage of the possibility to diminish the actual analytical variation through the use of replicate tests.     

Morphology and modulus–temperature behavior of semicrystalline poly(ethylene terephthalate) (PET)

The influence of the thermal history on the morphology and mechanical behavior of PET was studied. The degree of crystallinity (density measurements) and the morphological structure (electron microscopy and small-angle x-ray diffraction) depend on the crystallization temperature. The viscoelastic parameters obtained from the modulus–temperature curves are mainly determined by the morphology of the samples. The glass-transition temperature, T_i, is a function of the crystallinity and the crystallization temperature. It is maximum for a crystallinity between 0.34 and 0.39 for a sample crystallized isothermally between 120 and 150°C. This dependence on crystallization conditions is ascribed to the conformation of the amorphous chain segments between the crystalline lamellae as well as the concentration and the molecular weight of the polymer material rejected during isothermal crystallization. Both factors are supposed to be temperature-dependent. The value of the rubbery modulus is a function of both the volume concentration of the crystalline lamellae and the structure of the interlamellar amorphous regions (chain folds, tie molecules, chain ends, and segregated low molecular weight material). Annealing above the crystallization temperature of isothermally crystallized samples has a marked influence on their morphology and mechanical behavior. The morphological structure and the viscoelastic properties of annealed PET samples are completely different from those obtained with samples isothermally crystallized at the same temperature.     

New developments in photochromic polymers

New developments on photochromic polymers have been obtained with systems containing spirobenzopyran derivatives. The importance of the chemical incorporation of the photochrome within the polymer backbone is stressed, and the influence of matrix orientation on the rate of fading is shown. Well known fatigue phenomena are illustrated on the basis of the changes of photostationary optical density on repeating the irradiation–dark recovery cycle. The photomechanical properties and characteristics of rubbery polyethylacrylates crosslinked with the photochrome bismethacrylate are considered in detail. Another principle of photochromism based on photodissociation phenomena is also discussed in the case of the photodissociation and photorecombination of anthracene derivatives and benzacridizinium tosylate, as well as in the case of the photodissociation and thermal recombination of benzpinacol groups incorporated in the polymer main chain. In these experiments in the solid state, it can be shown that the recombination involves always two successive steps: a rapid step corresponding to cage recombination and a second slow step related to diffusion and segment mobility (T_g) of the polymer.     

Dynamics of Multiple Degree Ginzburg-Landau Vortices

For the two dimensional complex parabolic Ginzburg-Landau equation we prove that, asymptotically, vortices evolve according to a simple ordinary differential equation, which is a gradient flow of the Kirchhoff-Onsager functional. This convergence holds except for a finite number of times, corresponding to vortex collisions and splittings, which we describe carefully. The only assumption is a natural energy bound on the initial data. This paper, together with [3,4], is companion to [2] where the higher dimensional case is considered.     

Tetrachlorobisphenol-A adipate polymers. I. Crystallization of poly(tetrachlorobisphenol-A adipate)

The properties of melt-crystallized poly(tetrachlorobisphenol-A adipate) were studied by using a differential scanning calorimeter. The dependence of melting point and the degree of crystallinity are reported as a function of the crystallization conditions. The heat of fusion is equal to 8.1 kcal/mole, while the equilibrium melting point, as determined by extrapolation, is 283°C. The polymer crystallized from the melt has a maximum degree of crystallinity of 0.53.     

Photochromism of spirofluorenylindolizines

The decoloration kinetics of three spirofluorenylindolizines have been followed in solution and in polymer matrix. They are the 5′-methyl-spiro[9H-fluorene-9.1′(1′H)-indolizine]-2′-3′-bis-methoxycarbonyl ( I ), its methacryloyl oxymethyl ( II ) derivative, and the copolymers of ( II ) with ethyl acrylate. In solution the reactions follow first-order kinetics and show pronounced negative solvatochromism. In solid polymer film, the incorporation of the photochrome as side group of polyethylacrylate affects strongly the decoloration below T_g of the film. Above T_g the decoloration was interpreted on the basis of the WLF equation.