L. M. León

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∗).

Crystallization, structural relaxation and thermal degradation in Poly(L-lactide)/cellulose nanocrystal renewable nanocomposites.

In this work, crystallization, structural relaxation and thermal degradation kinetics of neat Poly(L-lactide) (PLLA) and its nanocomposites with cellulose nanocrystals (CNC) and CNC-grafted-PLLA (CNC-g-PLLA) have been studied. Although crystallinity degree of nanocomposites remains similar to that of neat homopolymer, results reveal an increase on the crystallization rate by 1.7-5 times boosted by CNC, which act as nucleating agents during the crystallization process. In addition, structural relaxation kinetics of PLLA chains has been drastically reduced by 53% and 27% with the addition of neat and grafted CNC, respectively. The thermal degradation activation energy (E) has been determined from thermogravimetric analysis in the light of Kissingers and Ozawa-Flynn-Wall theoretical models. Results reveal a reduction on the thermal stability when in presence of CNC-g-PLLA, while raw CNC slightly increases the thermal stability of PLLA. Fourier transform infrared spectroscopy and energy dispersive X-ray spectroscopy results confirm that the presence of residual catalyst in CNC-g-PLLA plays a pivotal role in the thermal degradation behavior of nanocomposites.

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.

PLLA-grafted cellulose nanocrystals: Role of the CNC content and grafting on the PLA bionanocomposite film properties.

Cellulose nanocrystals (CNC), extracted from microcrystalline cellulose by acid hydrolysis, were grafted by ring opening polymerization of L-Lactide initiated from the hydroxyl groups available at their surface and two different CNC:L-lactide ratios (20:80 and 5:95) were obtained. The resulting CNC-g-PLLA nanohybrids were incorporated in poly(lactic acid) (PLA) matrix by an optimized extrusion process at two different content (1 wt.% and 3 wt.%) and obtained bionanocomposite films were characterized by thermal, mechanical, optical and morphological properties. Thermal analysis showed CNC grafted with the higher ratio of lactide play a significant role as a nucleating agent. Moreover, they contribute to a significant increase in the crystallization rate of PLA, and the best efficiency was revealed with 3 wt.% of CNC-g-PLLA. This effect was confirmed by the increased in Youngs modulus, suggesting the CNC graft ratio and content contribute significantly to the good dispersion in the matrix, positively affecting the final bionanocomposite properties.

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.

Thermal scanning rheometric analysis of curing kinetic of an epoxy resin. I. An anhydride as curing agent

The curing reaction of the system diglycidyl ether of bisphenol A (DGEBA), an organic anhydride (HMTPA), as curing agent and a tertiary amine (DMP 30) as initiator has been studied by Thermal Scanning Rheometry (TSR) under isothermal conditions. The gel time, which is defined by several different criteria, has been found to be a good parameter to determine the activation energy of this curing process; on the other hand, the gel time depends on the concentration of the initiator. An empirical model has been used to predict the change in viscosity (η*) of the system with time until the gelation is reached; the first-order kinetics, the apparent kinetic constant (k′), and the activation energy before gelation have been determined. Furthermore, these results are reported together with the reaction mechanism proposed by another authors.

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.

Development of magnetoelectric CoFe2O4 /poly(vinylidene fluoride) microspheres

Magnetoelectric microspheres based on piezoelectric poly(vinylidene fluoride) (PVDF) and magnetostrictive CoFe2O4 (CFO), a novel morphology for polymer-based ME materials, have been developed by an electrospray process. The CFO nanoparticle content in the (3–7 μm diameter) microspheres reaches values up to 27 wt%, despite their concentration in the starting solution reaching values up to 70 wt%. Additionally, the inclusion of magnetostrictive nanoparticles into the polymer spheres has no relevant effect on the piezoelectric β-phase content (≈60%), crystallinity (40%) and the onset degradation temperature (460–465 °C) of the polymer matrix. The multiferroic microspheres show a maximum piezoelectric response |d33| ≈ 30 pC N−1, leading to a magnetoelectric response of Δ|d33| ≈ 5 pC N−1 obtained when a 220 mT DC magnetic field was applied. It is also shown that the interface between CFO nanoparticles and PVDF (from 0 to 55%) has a strong influence on the ME response of the microspheres. The simplicity and the scalability of the processing method suggest a large application potential of this novel magnetoelectric geometry in areas such as tissue engineering, sensors and actuators.

Increased functional properties and thermal stability of flexible cellulose nanocrystal/ZnO films

In this work we attempt to improve the functional properties and thermal stability of cellulose nanocrystal (CNC) films by means of eco-friendly materials and processes. Mechanically flexible films of closely packed CNCs with concentrations up to 5 wt.% of zinc oxide (ZnO) nanoparticles have been prepared by a simple, standard and environmentally friendly method using solely water. Results reveal that ultraviolet light is blocked by 98.5% at 1 wt.% ZnO while good transparency is maintained. A sharp hydrophobicity increase is observed with the addition of ZnO which would enhance the durability of films by decreasing the water diffusion through the material. The thermal degradation activation energy (E) presents an increase of 141%, denoting a high thermal stability of films, which would result beneficial for their potential application in the field of flexible electronics. Mechanical results demonstrate a high structural integrity of CNC/ZnO as a result of the occurring strong cellulosic inter- and intramolecular interactions within the closely packed CNC network. In overall, this work highlights the potential for environmentally friendly processing of sustainable nanostructured functional materials based on cellulose.