Preserving Porosity of Mesoporous Metal-Organic Frameworks through the Introduction of Polymer Guests

High internal surface areas, an asset that is highly sought after in material design, has brought metal-organic frameworks (MOFs) to the forefront of materials research. In fact, a major focus in the field is on creating innovative ways to maximize MOF surface areas. Despite this, large-pore MOFs, particularly those with mesopores, continue to face problems with pore collapse upon activation. Herein, we demonstrate an easy method to inhibit this problem via the introduction of small quantities of polymer. For several mesoporous, isostructural MOFs, known as M-2(NDISA) (where M = Ni2+, Co2+, Mg2+, or Zn2+), the accessible surface areas are increased dramatically, from 5 to 50 times, as the polymer effectively pins the MOFs open. Postpolymerization, the high surface areas and crystallinity are now readily maintained after heating the materials to 150 degrees C under vacuum. These activation conditions, which could not previously be attained due to pore collapse, also provide accessibility to high densities of open metal coordination sites. Molecular simulations are used to provide insight into the origin of instability of the M-2(NDISA) series and to propose a potential mechanism for how the polymers immobilize the linkers, improving framework stability. Last, we demonstrate that the resulting MOF-polymer composites, referred to as M-2(NDISA)-PDA, offer a perfect platform for the appendage/immobilization of small nanocrystals inside rendering high-performance catalysts. After decorating one of the composites with Pd (average size: 2 nm) nanocrystals, the material shows outstanding catalytic activity for Suzuki-Miyaura cross-coupling reactions.

Self-Healing Fiber-Reinforced Composites with Tailored Supramolecular Matrices

Federica Sordo

Fiber Reinforced Composites (FRCs), due to their intrinsic heterogeneity, are generally affected by complex multi-scale failure mechanisms leading to the formation of matrix cracks that are difficult to detect and repair. FRCs with the ability to heal and recover at least part of their initial properties after damage could present a solution to extend their lifetime and reliability. A possible approach is to introduce external healing agents in the matrix of the composite, through capsules or micro-channels, but this is often difficult to achieve in a high volume fraction reinforcement composite. An alternative approach is to rely on intrinsic self-healing polymers as matrices, which alleviates the need for additional phases beyond matrix and reinforcement. The present work investigated this second approach, by exploring for the first time the feasibility of processing glass fabric reinforced composites based on supramolecular hybrid network self-healing matrices and analysing their resulting properties and transfer of the self-healing ability to the composite. Hybrid networks based on epoxy chemistry, combining covalent cross-links and cooperative reversible hydrogen bonds, were selected as benchmark materials. These systems were charac- terized, in particular in terms of processing parameters and self-healing ability, and showed to be a-priori compatible with FRCs manufacturing techniques and demonstrated higher healing efficiency by increasing the H-bonds content in the polymer network, from none to 50%. On the other hand, networks characterized by large number of physical cross-links exhibited limited solvent and creep resistance, hindering their potential application. These issues were addressed by modifying the average functionality of the monomers through the combination of bifunctional and tetrafunctional epoxy resins in the compositions while keeping the use of a catalyst, leading to better controlled networks, both in terms of kinetics and final structure, and to propose a range of materials with several levels of compromise between self-healing efficiency, mechanical strength, creep resistance, and damping properties. In particular, 50% to 100% self-healing recovery of tensile properties were observed after one day, depending on the tetraepoxide content. In parallel, since the Tg of benchmark supramolecular hybrid net- works was in the range from 11-16 âŠC, an aliphatic diepoxide was introduced in the synthetic procedure to propose materials with a Tg down to -10 âŠC, to reach applications closer to those of elastomers. In view of FRCs development, the ability of partially supramolecular self-healing polymers to bond to inorganic reinforcements and to heal interfacial failure was then investigated: two dedicated butt joint-like and tack-like test methods were developed. Adhesion strength of the polymers cured on glass substrates decreased with increasing H-bonds content in the polymer networks (from 0 to 50 %) as the mechanical strength of the polymer also decreased, while the failure mechanism shifted from adhesive to cohesive due to the possibility to form hydrogen bonds with the glass substrates. Interface restoration through healing of the polymer matrices however increased from none to a strength recovery up to 70 % after 1 h healing time for the 50% H-bond polymer...

EPFL2016

The Effect Of Processing Conditions On The Morphology, Mechanical And Electrical Properties Of Batio3-Polymer Composites

Sara Dalle Vacche, Yves Leterrier, Jan-Anders Månson, Véronique Michaud

Ferroelectric ceramic – polymer composites have a unique mix of electrical and mechanical properties. Piezoelectric and pyroelectric activity, a wide range of dielectric constants and high breakdown strength are combined with mechanical flexibility, formability and low acoustic impedance. Furthermore their properties may be tailored by the judicious choice of the polymeric matrix and ceramic filler, of their volume fraction, and of the type of connectivity, making these materials very attractive for applications as embedded capacitors, sensors and actuators. Composites with 0-3 connectivity, i.e. a continuous polymer matrix filled with a ceramic phase discontinuous in all directions, are particularly interesting because of their relative ease of fabrication and processing. The functionality of these composite materials is mostly due to the dielectric, ferroelectric and piezoelectric properties of the ceramic phase, which therefore often needs to constitute more than 50% in volume. On the other hand the possibility of integrating these composites as embedded devices in host structures, which are often polymer based, or using them as efficient actuators depends greatly on their thermomechanical properties. Most of the studies on ferroelectric ceramic-polymer composites, however, focus on their electrical properties, and relatively few systematic studies on their thermal and mechanical behaviour have been published so far. Furthermore the ceramic volume fractions considered are often much lower than those used in most applications [1, 2], studies of highly filled composites mainly being limited to lead-based ferroelectric materials [3]. In the present study 0-3 BaTiO3 – polymer composites were prepared, containing up to 60% volume fraction of ceramic phase. BaTiO3 was chosen as the ceramic filler as it is a common piezoelectric material with high dielectric constant, having the additional benefit from an environmental point of view of being lead-free. For the matrix, a low viscosity epoxy resin and a polyvinylidene fluoride based polymer were chosen. The first is a thermoset resin commonly used for several applications, and the second is a thermoplastic semicrystalline polymer which displays piezoelectric activity. A common route for achieving very high loading of ceramic filler is the use of solvents to reduce the viscosity during processing. However, as the materials prepared via the solvent route were found to have poor mechanical properties, attributed to the presence of residual solvent, most of the effort focused on obtaining high ceramic volume fractions via a solvent free process. The thermomechanical, dielectric, ferroelectric and piezoelectric properties of the obtained composites were investigated, and discussed in the light of the properties of the basic components, the processing route and the resulting morphology. X-ray diffraction was carried out to determine the crystal structure of the BaTiO3 in the pure powder and in the composites, confirming that processing did not alter the crystallographic structure of the ceramic phase. Scanning electron microscope observation of fracture surfaces of the composites evidenced that a satisfactorily homogeneous dispersion of BaTiO3 in the matrix could be obtained, although aggregates with diameter of some micrometers and some porosity were detected. The thermal and mechanical behaviour of the composites was studied by differential scanning calorimetry and dynamic mechanical analysis. The glass transition temperature was found not to significantly depend on the amount of filler, while both the storage and loss moduli increased upon addition of BaTiO3. Furthermore, all the composites displayed a region of linear stress-strain behaviour smaller than the pure matrices. Finally, experimental results also evidenced the possibility to obtain low leakage current and high breakdown voltage.

2010

Approche expérimentale du comportement mécanique des polymères en sollicitation rapide

In the more recent applications of technical polymers, they are frequently submitted to high strain rates. Being viscoelastic by nature, their mechanical properties are rate dependent. It is therefore important to investigate their properties over a wide range of strain rates. The aims of this study are twofold: i) to propose an experimental technique applicable to a large domain of strain rates, included those covering impact tests, and ii) to apply this technique to the study of the rate dependence of the mechanical properties of some technical polymers. In the first section, the deformation mechanisms of polymers, and the way in which they are influenced by strain rate, are reviewed. Some basics of fracture mechanics are also briefly presented. A detailed study of our original approach to the high rates tests then follows. This technique is based on the reduction of dynamic effects which often affect the measurements at high loading rates. Rather than impact tests, our technique is a quasi-static tests at high rates. This results from the use of a damper as coupling element between the servohydraulic piston and the specimen, together with the specific design of load and displacement detectors. The transient acceleration, responsible for the dynamic effects is decreased, but the high loading rates are maintained. The second section is devoted to the investigation of the rate effect on the tensile and rupture properties of polyoxymethylene (POM) and impact modified polymethylmethacrylate (PMMA). It is shown that high molecular weight POM is less sensitive to the increase of the loading rates than for lower molecular weights. The physical structure, which depends on the processing conditions, plays an important role as well. Results show greater embrittlement by increasing the loading rate than by lowering the temperature. Finally, a molecular model of fracture is proposed. PMMA modified by different fractions and morphologies of the rubbery phase are submitted to tensile and fracture tests at room temperature. Several stages in the micromechanisms of deformation are identified by transmission electron microscopy. The progressive disappearance of some of these stages leads to ductile to brittle transitions in some of the materials studied. The velocity at which these transitions occur depends on the fraction and on the morphology of the modifying phase. It is also shown that a spherical morphology of the rubbery phase is not necessarily required to improve the toughness, but that this latter depends on the interactions between the phases.

EPFL1996