José Manuel Laza

Thermal scanning rheometer analysis of curing kinetic of an epoxy resin: 2. An amine as curing agent

Abstract The curing reaction of the system diglycidyl ether of bisphenol A (DGEBA) with different amine concentrations (TETA) has been studied by means of thermal scanning rheometer (TSR) and dynamic mechanical thermal analyser (DMTA). The aim of this work was focused on studying the effect of the amine concentration on the kinetic, the rheologic characteristics during the crosslinking process, and the dynamic-mechanical properties of the system. Through TSR measurements, the gel time was observed to vary with amine concentration and cure temperature when measurements were carried out under isothermal conditions. The gel time also was found to depend on the heating rate when the measurements were done varying the temperature. The apparent activation energy (Ea) of each system was calculated from two different methods; the gel time (tg) and the complex viscosity ( η ∗ ) measurements, respectively. The study of the dynamic-mechanical properties of the resins such as epoxy by means of the three-point bending mode in DMTA, gives information of their viscoelastic properties allowing the calculation of the glass transition temperature (Tg) and the apparent activation energy of the relaxation process (Ea∗).

Study of the curing process of a vinyl ester resin by means of TSR and DMTA

Abstract The curing reaction of a vinyl ester resin (VER), using methyl ethyl ketone peroxide (MEKP) as initiator and cobalt hexanoate (HxCo) as promoter has been studied by thermal scanning rheometry (TSR) and dynamic-mechanical thermal analysis (DMTA) under isothermal conditions. The gel time, which is defined by several criteria, has been utilized to determine the apparent activation energy (Ea) of the process. Furthermore, an empirical model has been used to predict the change of complex viscosity (η∗) with time, and assuming a first order kinetics, a new value for Ea is obtained independent of HxCo concentration. Finally, the vitrification time has been obtained from DMTA experiments.

Triple-shape memory effect of covalently crosslinked polyalkenamer based semicrystalline polymer blends

Triple-shape memory polymers are developed by blending and crosslinking two semicrystalline polymers (poly(cyclooctene), PCO, and polyethylene, PE) towards creating two pronounced segregated crystalline domains within a covalently crosslinked network. The key thermo-mechanical properties of a series of a polyalkenamer and a polyolefin based polymer blends are characterised using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). Furthermore, the versatile multi-shape memory functionality is demonstrated, and main shape memory response is evaluated by performing consecutive thermo-mechanical bending experiments based on a two-step programming process and subsequent progressive thermal recovery. The proposed approach, thanks to the excellent achieved shape memory properties, as well as the possibility of tailoring the thermo-mechanical response, is presented as a versatile method to increase the potential applications of these thermo-active materials by designing optimal compositions.

Unsaturated polyester resins cure : Kinetic, rheologic, and mechanical-dynamical analysis. I. Cure kinetics by DSC and TSR

The curing reaction of a polyester resin, using methyl ethyl ketone peroxide and cobalt octoate as promoter, has been studied by differential scanning calorimetry and thermal scanning rheometry under isothermal conditions. All kinetic parameters of the curing reaction, including the reaction order, activation energy, and the rate constant, were calculated and reported using different empirical relationships. The gel time, which is defined by several criteria, was used to determine the apparent activation energy of the process.

Magneto-active shape memory composites by incorporating ferromagnetic microparticles in a thermo-responsive polyalkenamer

Covalently crosslinked semi-crystalline polyalkenamer-based shape memory polymers (SMPs) were prepared and characterized. Thermal and thermo-mechanical properties of thermo-sensitive polymers manufactured by melt compounding were investigated, and shape memory features demonstrated. For remote activation of shape recovery properties, electromagnetic inductive heating of a series of iron-based ferromagnetic microparticles was evaluated for subsequent incorporation into a shape memory polymeric matrix. The inductive heating capacity of micro-sized iron-filled polyalkenamers with different volume fraction contents was optimized and a comparison of thermo-mechanical properties of filled and unfilled shape memory polymeric networks was performed. Electromagnetically triggered shape memory properties of easily formed composites were documented and shape memory recovery rates comparable to those obtained by conventional heating methods were demonstrated for further research and design of new types of applications.

Evidence for the absence of enzymatic reactions in the glassy state. A case study of xanthophyll cycle pigments in the desiccation-tolerant moss Syntrichia ruralis

Desiccation-tolerant plants are able to withstand dehydration and resume normal metabolic functions upon rehydration. These plants can be dehydrated until their cytoplasm enters a ‘glassy state’ in which molecular mobility is severely reduced. In desiccation-tolerant seeds, longevity can be enhanced by drying and lowering storage temperature. In these conditions, they still deteriorate slowly, but it is not known if deteriorative processes include enzyme activity. The storage stability of photosynthetic organisms is less studied, and no reports are available on the glassy state in photosynthetic tissues. Here, the desiccation-tolerant moss Syntrichia ruralis was dehydrated at either 75% or <5% relative humidity, resulting in slow (SD) or rapid desiccation (RD), respectively, and different residual water content of the desiccated tissues. The molecular mobility within dry mosses was assessed through dynamic mechanical thermal analysis, showing that at room temperature only rapidly desiccated samples entered the glassy state, whereas slowly desiccated samples were in a ‘rubbery’ state. Violaxanthin cycle activity, accumulation of plastoglobules, and reorganization of thylakoids were observed upon SD, but not upon RD. Violaxanthin cycle activity critically depends on the activity of violaxanthin de-epoxidase (VDE). Hence, it is proposed that enzymatic activity occurred in the rubbery state (after SD), and that in the glassy state (after RD) no VDE activity was possible. Furthermore, evidence is provided that zeaxanthin has some role in recovery apparently independent of its role in non-photochemical quenching of chlorophyll fluorescence.

Unsaturated polyester resins cure: Kinetic, rheologic, and mechanical dynamical analysis. II. The glass transition in the mechanical dynamical spectrum of polyester networks

Unsaturated polyester networks with various structures built from an orthophtalic polyester, with methyl ethyl ketone peroxide as an initiator and cobalt octoate as a promoter, were studied with dynamic mechanical thermal analysis from -50 to 200 °C to characterize changes in the mechanical properties as a function of the temperature. From these measurements, the glass-transition temperatures of the different networks were determined, their dependence on conversion being fitted to an equation related to the Couchman and DiBenedetto equations. Finally, the different transitions were analyzed as a function of the cure conditions.

Study of the chain microstructure effects on the resulting thermal properties of poly(L-lactide)/poly(N-isopropylacrylamide) biomedical materials.

The development of thermally-sensitive poly(N-isopropylacrylamide) (PNIPAAm) and biocompatible/biodegradable poly(L-lactide) (PLLA) blends offers us an efficient strategy in order to obtain materials with improved functional properties to be used in the emerging field of biomedicine. In this sense, thermal properties of PLLA and PNIPAAm have been investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and wide angle X-ray diffraction (WAXD) were conducted to shed more light on the obtained results. For a better understanding of PLLA/PNIPAAm system, both low and high molecular weight PLLA and PNIPAAm have been synthesized by ring opening polymerization and aqueous redox polymerization respectively. Obtained results are interpreted from the viewpoint of chain microstructure of each homopolymer and the ratio between two constituent materials. DSC, SEM and WAXD results show a phase separation over the entire composition range irrespectively of the molecular weight of both homopolymers. Additionally, it was found a nucleating agent behavior of low molecular weight PNIPAAm, while high molecular weight PNIPAAm hinders the crystallization of PLLA. FTIR results suggest that the strong autoassociation present in PNIPAAm plays a key role impairing the miscibility of the whole system. Thermogravimetric analysis reveals that thermodegradation process of PLLA could be continuously delayed with the addition of PNIPAAm due to the increased thermal stability of N-isopropylacrylamide in regard to L-lactide sequences.

Shape memory composites based on glass-fibre-reinforced poly(ethylene)-like polymers

The mechanical response of a series of semicrystalline shape memory polymers was considerably enhanced by incorporating short glass fibres without modifying the thermo-responsive actuation based on balanced crystallinity and elasticity. The effect of different fractions of inorganic reinforcement on thermo-mechanical properties was evaluated using different instrument techniques such as differential scanning calorimetry (DSC), thermogravimetry (TGA), dynamic mechanical thermal analysis (DMTA) and three-point flexural tests. Moreover, we studied the inorganic reinforcement influence on the shape memory actuation capabilities by thermo-mechanical bending cycle experiments. As demonstrated, the manufactured polymer composites showed excellent shape memory capacities, similar to neat active polymer matrices, but with outstanding improvements in static and recovering mechanical performance.

Construction of antibacterial poly(ethylene terephthalate) films via layer by layer assembly of chitosan and hyaluronic acid

Polyelectrolytic multilayers (PEMs) with enhanced antibacterial properties were built up onto commercial poly(ethylene terephthalate) (PET) films based on the layer by layer assembling of bacterial contact killing chitosan and bacterial repelling highly hydrated hyaluronic acid. The optimization of the aminolysis modification reaction of PET was carried out by the study of the mechanical properties and the surface characterization of the modified polymers. The layer by layer assembly was successfully monitored by TEM microscopy, surface zeta-potential, contact angle measurements and, after labeling with fluorescein isothiocyanate (FTIC) by absorption spectroscopy and confocal fluorescent microscopy. Beside, the stability of the PEMs was studied at physiological conditions in absence and in the presence of lysozyme and hyaluronidase enzymes. Antibacterial properties of the obtained PEMs against Escherichia coli were compared with original commercial PET.