M.A. Corcuera

Cellulose nanocrystals/polyurethane nanocomposites. Study from the viewpoint of microphase separated structure

Cellulose nanocrystals (CNC) successfully obtained from microcrystalline cellulose (MCC) were dispersed in a thermoplastic polyurethane as matrix. Nanocomposites containing 1.5, 5, 10 and 30 wt% CNC were prepared by solvent casting procedure and properties of the resulting films were evaluated from the viewpoint of polyurethane microphase separated structure, soft and hard domains. CNC were effectively dispersed in the segmented thermoplastic elastomeric polyurethane (STPUE) matrix due to the favorable matrix-nanocrystals interactions through hydrogen bonding. Cellulose nanocrystals interacted with both soft and hard segments, enhancing stiffness and stability versus temperature of the nanocomposites. Thermal and mechanical properties of STPUE/CNC nanocomposites have been associated to the generated morphologies investigated by AFM images.

Influence of cure schedule and stoichiometry on the dynamic mechanical behaviour of tetrafunctional epoxy resins cured with anhydrides

Abstract Epoxy networks based on N,N,N′,N′ -tetraglycidyl-4,4′-diamino diphenylmethane (TGDDM) prepolymer were prepared with cis -1,2,3,6-tetrahydrophthalic anhydride (THPA) curing agent at anhydride/epoxy group ratios varying from 0.3 to 1.0. For post-cured mixtures, dynamic mechanical tests show that the glass transition temperature reaches the maximum value at stoichiometric ratios between 0.8 and 0.9. This behaviour has been related to the crosslink density of the formed networks, and also to etherification reactions occurring during cure which lower the amount of anhydride needed in order to complete the curing process. The study of cure cycle variations on the viscoelastic properties showed that for epoxy/anhydride mixtures high post-cure temperatures could be needed to reduce the amount of unreacted epoxy groups after curing. Fourier transform infra-red spectroscopy has been used to analyse the residual epoxy groups and also to study the influence of the different cure reactions on the physical properties of these networks.

Design of morphology in PMMA‐modified epoxy resins by control of curing conditions. I. Phase behavior

Rheokinetic and phase separation behavior of diglycidylether of bisphenol-A–4,4′-diaminodiphenyl methane epoxy mixtures, modified with a constant amount (15 wt %) of poly(methyl methacrylate) (PMMA), have been investigated. Stoichiometric epoxy/amine mixtures precured at 80°C several times presented various levels of miscibility. Differential scanning calorimetry (DSC) and dynamic mechanic thermal analysis were used for rheokinetic studies of curing and also for testing the thermal behavior of the fully cured mixtures. Phase separation, through curing, was simultaneously studied by transmission optical microscopy and DSC, showing an excellent correlation between the results obtained with both techniques.

Reaction kinetics of tolyl isocyanate with polyhexamethylene-pentamethylene carbonate diol

In the present work, the kinetics of the reaction of p-tolyl isocyanate with a polycarbonate diol has been studied, as a model for similar urethane systems. Liquid chromatography has been used to monitor the reaction, which allows direct measurement of the reaction of isocyanate group. The results obtained have enabled us to propose a reaction path and an autocatalyzed third-order kinetic model. Activation energy values have been determined from measurements taken at different isothermal temperatures.

In situ polymerization and characterization of elastomeric polyurethane-cellulose nanocrystal nanocomposites. Cell response evaluation

Polyurethane/Cellulose nanocrystal (CNC) nanocomposites have been prepared by means of in situ polymerization using CNCs as precursors of polyurethane chains. Thermal, mechanical and morphological characterization has been analyzed to study the effect of CNC on the micro/nanostructure, which consisted of individualized nanocellulose crystallites covalently bonded to hard and soft segments of polyurethane. The incorporation of low CNC content led to a tough material whereas higher amount of CNC provoked an increase in soft and hard segments crystallization phenomenon. Moreover, from the viewpoint of polyurethane and polyurethane nanocomposites applications focused on biomedical devices, biocompatibility studies can be considered necessary to evaluate the influence of CNC on the biological behaviour. SEM micrographs obtained from cells seeded on top of the materials showed that L-929 fibroblasts massively colonized the materials surface giving rise to good substrates for cell adhesion and proliferation and useful as potential materials for biomedical applications.

The influence of molecular weight and chemical structure of soft segment in reaction kinetics with tolyl isocyanate

Abstract The influence of molecular weight and chemical structure of several polycarbonate diols on the kinetics of condensation reaction with p-tolyl isocyanate has been studied. Size exclusion chromatography has been used to monitor the reaction. The condensation reaction kinetics is adequately described by an autocatalysed third order rate equation. The values obtained for rate constants, using a Runge–Kutta mathematical model, suggest an association phenomena by hydrogen bonding implying hydroxyl groups but also urethane groups. In bulk and in stoichiometric conditions, the association phenomena observed increases proportionally, on one hand to the decrease of molecular weight of macrodiol, and on the other hand to the tendency to form intramolecular hydrogen bonds.

Diels-Alder "click" chemistry for the cross-linking of furfuryl-gelatin-polyetheramine hydrogels†

This paper deals with the design of biopolymeric hydrogels after the chemical cross-linking of gelatin through the Diels–Alder (DA) reaction. Furan modified gelatin (Gel-FGE) was prepared by the reaction of furfuryl glycidyl ether with the free amino groups present in the gelatin. A Jeffamine®-based bismaleimide was employed as a cross-linking agent and the influence of the amount of cross-linker used on the final hydrogel properties was studied. The DA and retro-DA reactions were followed by ultraviolet spectroscopy and the final properties of the hydrogel assessed. Scanning Electron Microscopy was used to analyze the structure of the final material and rheology studies confirmed the formation of a chemically cross-linked network. The swelling behavior in response to external stimuli such as pH and salt concentration was also studied. By virtue of the DA “click” reaction, biopolymer-based cross-linked hydrogels, with promising properties for biomedical applications, were obtained in a simple one step procedure free of catalysts, additives or coupling agents.

Effect of maleimide-functionalized gold nanoparticles on hybrid biohydrogels properties

The role of well-dispersed gold nanoparticles as cross-linking agents in nanocomposite hydrogel formation was studied. Maleimide-coated gold nanoparticles were synthesized and used for Diels–Alder cycloaddition with furan modified gelatin. Hydrogel formation was aided by additional amide coupling of the modified gelatin with chondroitin sulfate. The cross-linking ability of the functionalized nanoparticles was evaluated and the final hydrogel properties were compared to those of a hybrid hydrogel containing inert-linker coated gold nanoparticles. The storage modulus of the nanoparticle cross-linked nanocomposites was 2.5–3 times compared to the controls. The presence of nanoparticles also affected the swelling properties, resulting in lower swelling ratios due to the formation of the more cross-linked structures. Conducted drug delivery experiments and the study of the light irradiation on the drug release behavior revealed promising features for the applications of nanocomposite polymer for drug delivery.

The role of reactive silicates on the structure/property relationships and cell response evaluation in polyurethane nanocomposites

Precursors of polyurethane chains have been reacted by means of in situ polymerization with organically modified montmorillonite clay to obtain polyurethane nanocomposites containing from 1 to 4 wt % of nanoreinforcement. The effective final dispersion of inorganic component at nanometric scale was investigated by X-ray diffraction, atomic force microscopy, and transmission electron microscopy. In addition, the effect of the nanoreinforcement incorporation on thermal and mechanical behavior of polyurethane nanocomposites was evaluated. Nanocomposites showed similar mechanical properties to polyurethanes containing high-hard segment contents with higher tensile modulus and a decrease in elastomeric properties of polyurethane materials. Finally, biocompatibility studies using L-929 fibroblast have been carried out to examine in vitro cell response and cytotoxicity of the matrix and their nanocomposite materials. Results suggested that the organic modifier in the clay is unsuitable for biomedical devices in spite of the fact that the matrix is a good candidate for cell adhesion and proliferation.

Designing hydrogel nanocomposites using TiO2 as clickable cross-linkers

Titanium dioxide (TiO2) nanoparticles with clickable functional groups were prepared to allow for the Diels–Alder “click” reaction with a furan-modified pigskin gelatin. Bifunctional dopamine-maleimide linker was employed for TiO2 functionalization with maleimide group. The obtained nanoparticles were characterized using TEM, Zeta potential and FTIR spectroscopy. Functional nanoparticles were subsequently used, together with chondroitin sulphate, as cross-linkers for gelatin hydrogels. Hydrogel controls with bare TiO2 and without nanoparticles were prepared for comparison. The swelling and rheological properties of the nanocomposite hydrogels confirmed the formation of the covalently linked heterogeneous networks. An increase in the storage moduli values was recorded when using maleimide-coated nanoparticles. At the same time, the swelling of the network was significantly reduced indicating the formation of more cross-linked networks. The participation of the surface attached maleimide functional groups through the Diels–Alder cycloaddition was thus confirmed. In addition, hydrogels responded to electrostatic forces as observed by electrostatic force microscopy.