Instituto de Investigaciones en Ciencia y Tecnología de Materiales

Vitrimers are a new class of polymeric materials with very attractive properties, since they can be reworked to any shape while being at the same time permanently cross‐linked. As an alternative to the use of transesterification chemistry, we explore catalyst‐free transamination of vinylogous urethanes as an exchange reaction for vitrimers. First, a kinetic study on model compounds reveals the occurrence of transamination of vinylogous urethanes in a good temperature window without side reactions. Next, poly(vinylogous urethane) networks with a storage modulus of ≈2.4 GPa and a glass transition temperature above 80 °C are prepared by bulk polymerization of cyclohexane dimethanol bisacetoacetate, m ‐xylylene diamine, and tris(2‐aminoethyl)amine. The vitrimer nature of these networks is examined by solubility, stress‐relaxation, and creep experiments. Relaxation times as short as 85 s at 170 °C are observed without making use of any catalyst. In addition, the networks are recyclable up to four times by consecutive grinding/compression molding cycles without significant mechanical or chemical degradation.

Polyhydroxyalkanoates (PHAs) are gaining increasing attention in the biodegradable polymer market due to their promising properties such as high biodegradability in different environments, not just in composting plants, and processing versatility. Indeed among biopolymers, these biogenic polyesters represent a potential sustainable replacement for fossil fuel-based thermoplastics. Most commercially available PHAs are obtained with pure microbial cultures grown on renewable feedstocks (i.e. glucose) under sterile conditions but recent research studies focus on the use of wastes as growth media. PHA can be extracted from the bacteria cell and then formulated and processed by extrusion for production of rigid and flexible plastic suitable not just for the most assessed medical applications but also considered for applications including packaging, moulded goods, paper coatings, non-woven fabrics, adhesives, films and performance additives. The present paper reviews the different classes of PHAs, their main properties, processing aspects, commercially available ones, as well as limitations and related improvements being researched, with specific focus on potential applications of PHAs in packaging.

Provides comprehensive coverage of the most recent developments in the theory of non-Archimedean pseudo-differential equations and its application to stochastics and mathematical physics--offering current methods of construction for stochastic processes in the field of p-adic numbers and related structures. Develops a new theory for parabolic equat

Abstract Owing to enthalpy relaxation, values of the glass transition temperature ( T g ) for partially reacted polymers may depend on the thermal history of samples and the heating rate used for measurements. Use of theoretical relations between T g and the extent of reaction ( x ) of a thermoset must take this fact into account. The original DiBenedetto equation has been reevaluated as a convenient constitutive equation for expressing T g versus x . An extension of Couchman's approach for the expression of the compositional variation of T g enabled us to derive the same functionality as given by the DiBenedetto equation. Thus, the DiBenedetto equation may be regarded as based on entropic considerations applied to a model of the thermosetting polymer consisting of a random mixture of a fully reacted network with the initial monomers in an amount which depends on the particular conversion level. These two equations have been applied with success to different diepoxy‐diamine copolymers.

Epoxidised soybean oil (ESO) was cross-linked with an aqueous citric acid (CA) solution without the addition of any other catalyst or solvent. Completely bio-based polymer networks were generated. The initial system was an emulsion, but it became a homogeneous and transparent polymer network by reaction. The ability of the final materials to self-heal without adding extrinsic catalysts was assessed by stress relaxation and lap-shear tests. This was achieved by molecular rearrangements produced by thermally activated transesterification reactions of β-hydroxyester groups generated in the polymerization reaction.

Polyheterocycles are one of the most desired synthetic targets due to their numerous and valuable applications in various fields. Multicomponent reactions (MCRs) are highly convergent one-pot processes, in which three or more reagents are combined sequentially to construct complex products, with almost all the atoms coming from the starting reagents. In this context, the syntheses of 'heterocycles' via MCR-based processes have been reviewed a number of times. However, there is not a single review (recent or otherwise) covering the synthesis of 'polyheterocycles' via a direct MCR or via a one-pot process involving MCRs coupled to further cyclizations (via ionic, metal-catalyzed, pericyclic, or free-radical-mediated cyclizations). This issue is consequently the main topic of the present review, which considers work from the last decade. The work is categorized according to the key processes involved in the syntheses of polyheterocycles, aiming to give readers an easy understanding of this MCR-based chemistry and to provide insights for further investigations. The reaction mechanisms providing novel elements to these MCR-based methods for the synthesis of polyheterocycles are also discussed.

Abstract The curing reaction of a commercial bisphenol A diglycidyl ether (BADGE) with ethylenediamine (EDA) was studied by differential scanning calorimetry. Different kinetic expressions were found with isothermal (low temperature range) and dynamic (high temperature range) runs. Two competitive mechanisms are shown to be present: an autocatalytic one (activation energy E = 14 kcal/mol) and a noncatalytic path characterized by a second‐order reaction with E = 24.5 kcal/mol. At low temperatures both mechanisms took place simultaneously, showing a significant decrease in the reaction rate after the gel point. At high temperatures only the noncatalytic reaction was present, without showing a noticeable rate decrease in the rubber region. Also, a third‐order dependence of the glass transition temperature on reaction extent is shown.

Contemporary researchers have specified that natural flax fiber is comparable with synthetic fibers due to its unique physical and mechanical characteristics which have been recognized for decades. Flax fiber-reinforced composites have the potential for wide usage in sport and maritime industries, and as automotive accessories. In addition, this composite is in the development stages for future applications in the aeronautical industry. However, designing the flax composite parts is a challenging task due to the great variability in fiber properties. This is caused by many factors, including the plant origin and growth conditions, plant age, location in the stem, fibers extraction method, and the fact that there is often a non-uniform cross section of the fibers. Furthermore, the water and moisture absorption tendency of the flax fibers and their composites and the consequent detrimental effects on their mechanical performance are also major drawbacks. Fibers may soften and swell with absorbed water molecules, which could affect the performance of this bio-composite. Flax fibers’ moisture absorption propensity may lead to a deterioration of the fiber–matrix interface, weakening the interfacial strength and ultimately degrading the quality of the composite. This review represents a brief summary of the main findings of research into flax fiber reinforced composites, focusing on the challenges of its water and moisture absorption behavior on their performance.

Furan-based thermoset polyurethanes have been prepared in a one-pot fashion with the ability to self-mend under mild temperature conditions, by making use of a Diels–Alder shape-memory assisted self-healing (DASMASH) approach. For this, thermoreversible covalent bonds, obtained by Diels–Alder chemistry, are introduced as cross-linkers into a polycaprolactone (PCL) containing polyurethane material. It is demonstrated that, after introduction of a crack into the PU-thermoset, Diels–Alder bonds preferentially break, regenerating free furan/maleimide functional groups, while the shape memory effect favors the crack closure at temperatures above the melting point of PCL, simultaneously resulting in a reformation of the reversible cross-links. The reversibility and shape memory ability of the materials were optimized and studied by FTIR, 1H NMR and tensile measurements. Different compositions were used to properly understand the role and influence of each component. The polyurethane materials healed at 50 °C after mechanical damage induced by either the application of a large tensile deformation or by performing controlled macro/micro scratches with a depth sensing indenter. Online FT-IR monitoring provided a kinetic description of the system reversibility for numerous cycles. Furthermore, mechanical recovery with complete disappearance of the microscratches was accomplished after multiple cycles of large tensile deformation. The results were not only confirmed by an optical inspection and scanning electron microscopy, but also with confocal microscopic mapping, by comparison of the cross-section profiles of the microscratches before and after healing.

Abstract Nanocomposites of cassava starch reinforced with waxy starch nanocrystals were prepared. They showed a 380% increase of the rubbery storage modulus (at 50 °C) and a 40% decrease in the water vapor permeability. X‐ray spectra show that the composite was more amorphous than the neat matrix, which was attributed to higher equilibrium water content in the composites. TGA confirmed this result and its thermal derivative suggested the formation of hydrogen bonding between glycerol and the nanocrystals. The reinforcing effect of starch nanocrystals was attributed to strong filler/matrix interactions due to the hydrogen bonding. The decrease of the permeability suggests that the nanocrystals were well dispersed, with few filler/filler interactions. magnified image

The last two decades have witnessed an exponential growth in the interest for using bio-derived products, which has been driven by the need for replacing petroleum based materials reducing the fuel consumption and, equally important, for producing materials with lower environmental impact. Vegetable oils constitute a rich source for many different polymers and polymer precursors and they are being considered for the production of “greener” composites. The wide range of possible combinations of vegetable oils, chemical modifications, polymerization routes, nature of the fillers and fibers used as reinforcement materials allows tailoring the composite properties to fit the requirements of structural or functional materials. Thus, a wide range of macro, micro and nanosized particles and fibers have been proposed as reinforcements/fillers, including organic and inorganic ones, natural or synthetic, in order to give adequate answers to specific requirements. Although, the role of oil-based products may seem modest in some cases (partial replacement of synthetic materials), there is a clear trend to increase the percentage of “green”-based raw materials in the formulations of commodities as well as specialty polymers/composites for high added value applications. Examples of different types of reinforced thermoset and elastomeric bio-composites are presented in this short review.

A polyhedral oligomeric silsesquioxane (POSS) containing one epoxy group and seven isobutyl groups per molecule was incorporated into an epoxy network following a two-stage process. In the first stage, POSS was reacted with an aromatic diamine, employing a 1:1 molar ratio of both reactants. The distribution of species at the end of reaction, determined by size exclusion chromatography (SEC), was close to the ideal one. In a second step, this precursor was reacted with the stoichiometric amount of an aromatic diepoxide to generate an organic−inorganic hybrid material containing 51.8 wt % POSS. A primary liquid−liquid phase separation process occurred at the time of adding the diepoxide to the POSS−diamine precursor. This led to a macrophase separation into epoxy-rich and POSS-rich regions, possibly derived from the incompatibility of the isobutyl groups attached to the POSS with the aromatic epoxy−amine network. A secondary phase separation occurred in the epoxy-rich phase in the course of polymerization, producing a dispersion of small POSS domains. Both modulated local thermal analysis (LTA) and differential scanning calorimetry (DSC) showed that most POSS-rich domains were amorphous. A small fraction of POSS crystals was also detected. A postcure cycle led to an increase in the glass transition temperature and the disappearance of crystallinity. A reference network was synthesized by replacing POSS by phenyl glycidyl ether (PGE) in equimolar amounts. The resulting network was homogeneous but exhibited a lower glass transition temperature than the POSS-modified network. As both networks had the same topology, the higher Tg observed for the POSS-modified epoxy may be associated with the hindering of polymer chain motions by their covalent bonding to POSS clusters. The most important concept arising from these results is that a phase separation process may take place when employing a POSS bearing organic groups that are not compatible with the epoxy network.

The room temperature, photoluminescent properties of manganese-doped zinc sulfide films deposited by spray pyrolysis are reported. These films were deposited on Pyrex glass substrates at atmospheric pressure using air as a carrier gas. All films were polycrystalline with a wurtzite (hexagonal) structure. The manganese doping was achieved by mixing MnCl3 with the starting solution to deposit ZnS. The photoluminescence spectra was measured at room temperature as a function of the different deposition parameters and the Mn concentration. Besides the characteristic light emission associated with Mn impurities in a ZnS matrix, a peak associated with the self-activated emission was also observed under certain deposition conditions (low substrate temperatures and/or long deposition times). The presence of chlorine impurities in the films is suggested to be associated with this emission. The Mn luminescence presents a quenching effect with the Mn concentration. This quenching effect is similar to the one reported on films deposited by other techniques. The light emission at this center has an activation energy of 0.71±0.05 eV with the deposition temperature. This energy is proposed to be related with the energy required by the Mn atoms to find a proper site during the growth process to form a Mn2+ center.

Abstract BACKGROUND: Shape memory polymers are capable of fixing a transient shape and of recovering their original dimensions by the application of an external stimulus. Their major drawback is their low stiffness compared to smart materials based on metals and ceramics. To overcome this disadvantage, nanocellulose was utilized as reinforcement. RESULTS: Composites were prepared by casting stable nanocellulose/segmented polyurethane suspensions. The heat of melting of the polyurethane soft segment phase increased on cellulose addition. Composites showed higher tensile modulus and strength than unfilled films (53% modulus increase at 1 wt% nanocellulose), with higher elongation at break. Creep deformation decreased as cellulose concentration increased (36% decrease in 60‐minute creep by addition of 1 wt% nanocellulose). The nanocomposites displayed shape memory properties equivalent to those of the neat polyurethane, with recoveries of the order of 95% (referred to second and further cycles). CONCLUSIONS: It is possible to markedly improve the rigidity of shape memory polymers by adding small amounts of well‐dispersed nanocellulose. However, this improvement did not have substantial effects on the material shape fixity or recovery. Shape memory behavior seems to continue to be controlled by the polymer properties. Copyright © 2007 Society of Chemical Industry

Abstract The morphology of a system consisting of a bisphenol A diglycidylether (DGEBA) based epoxy, cured with a cycloaliphatic diamine (4,4′‐diamino‐3,3′‐dimethyldicyclohexylmethane, 3DCM), in the presence of an epoxy‐terminated butadience‐acrylonitrile random copolymer (ETBN), was studied as a function of the cure schedule and the initial rubber concentration. Scanning (SEM) and transmission (TEM) electron microscopy, differential scanning calorimetry (DSC) and dynamic mechanical analysis were used to characterize the generated morphology. SEM results were not affected by the type of mechanical test and strain rate. Trends observed for the particle size distribution, the volume fraction of dispersed phase, the concentration of dispersed phase particles and the composition of both phases as a function of polymerization temperature and rubber concentration, were discussed. A correlation between the viscosity at the cloud point and the average size of dispersed phase particles was found for different systems, independently of the cure temperature and the initial rubber amount.

. Understanding the role of different chemical functionalities, within the pores of MOFs, is the key for accomplishing superior captures of these toxic gases. Thus, the interactions of such functional groups (coordinatively unsaturated metal sites, μ-OH groups, defective sites and halogen groups) with these toxic molecules, not only determines the capture properties of MOFs, but also can provide a guideline for the desigh of new multi-functionalised MOF materials. Thus, this perspective aims to provide valuable information on the significant progress on this environmental-remediation field, which could inspire more investigators to provide more and novel research on such challenging task.

MIL-53(Al) can be prepared <italic>via</italic> reaction in continuous flow in only 5–6 minutes with a space time yield of 1300 kg m⁻³ d⁻¹. Extraction of free terephthalic acid from within the pores of MIL-53(Al) using supercritical ethanol has been developed.

A natural polyol was prepared from castor oil by alcoholysis with triethanolamine. The oil and the oil-based polyol were characterized by infrared spectroscopy and through the analytical determination of their functional groups, both techniques indicating that the hydroxyl content increased significantly after the alcoholysis reaction. The modified oil was subsequently used as the polyol component in the formulation of rigid polyurethane foams. Wood flour was chosen to be incorporated as filler in these materials. Physical, thermal, and mechanical properties of the neat and reinforced foams were measured, analyzed, and compared to a reference commercial system. The chemical reaction between wood flour and isocyanate strongly affected the composites’ response to thermo-gravimetric tests. Compression modulus and yield strength decreased as wood flour content increased. The effect of the foam density on the compression properties was also investigated.

Biocomposites were produced using a biodegradable material as matrix, and sisal fibers as reinforcement. The biodegradable material is a commercial product called MaterBi-Y, which is based on a cellulose derivatives and starch system. The characterization of these biocomposites was not done before and it is necessary in order to select a material instead of nonbiodegradable matrices. An alkaline treatment was performed in order to improve the mechanical properties of the fiber. The effect of the treatment on fiber properties and on impact, flexural, and tensile properties of composites were determined. Fiber content enhances the tensile properties of the biodegradable matrix. Water sorption studies were performed. The experimentally observed tensile properties (modulus and tensile strength) of short sisal fiber-reinforced cellulose derivatives/starch composites with different fiber loading are compared with the calculated values obtained from the existing theories of reinforcement.

Anatase nanoparticles were obtained through a modified sol–gel route from titanium isopropoxide modified with acetic acid in order to control hydrolysis and condensation reactions. The modification of Ti(OiPr)4 with acetic acid reduces the availability of groups that hydrolyze and condense easily through the formation of a stable complex whose structure was determined to be Ti(OCOCH3)(OiPr)2 by means of FTIR and 13C NMR. The presence of this complex was confirmed with FTIR in the early stages of the process. A doublet in 1542 and 1440 cm−1 stands for the asymmetric and symmetric stretching vibrations of the carboxylic group coordinated to Ti as a bidentate ligand. The gap of 102 cm−1 between these signals suggests that acetate acts preferentially as a bidentate rather than as a bridging ligand between two titanium atoms. The use of acetic acid as modifier allows the control of both the degree of condensation and oligomerization of the precursor and leads to the preferential crystallization of TiO2 in the anatase phase. A possible reaction pathway toward the formation of anatase is proposed on the basis of the intermediate species present in a 1:1 Ti(OiPr)4:CH3COOH molar system in which esterification reactions that introduce H2O into the reaction mixture were seen to be negligible. The Rietveld refinement and TEM analysis revealed that the powder is composed of isotropic anatase nanocrystallites.