Guy Van Assche

Phase diagram of P3HT/PCBM blends and its implication for the stability of morphology.

In this work, the phase diagram of poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blends is measured by means of standard and modulated temperature differential scanning calorimetry. Blends were made by solvent-casting from chlorobenzene, as blends cast from toluene or 1,2-dichlorobenzene prove to retain effects of phase segregation during casting, hindering the determination of the phase diagram. The film morphology of P3HT/PCBM blends cast from chlorobenzene results from a dual crystallization behavior, in which the crystallization of each component is hindered by the other component. A single glass transition is observed for all compositions. The glass transition temperature (Tg) increases with increasing concentration of PCBM: from 12.1 degrees C for pure P3HT to 131.2 degrees C for pure PCBM. The observed Tg defines the operating window for the thermal annealing and explains the long-term instability of both the morphology and the photovoltaic performance of the P3HT/PCBM solar cells.

Measurements of Thermal Properties of Carbon/Epoxy and Glass/Epoxy using Modulated Temperature Differential Scanning Calorimetry

Major potential of composite materials relies in the nonlinear behavior triggered by their inhomogeneous nature. Particularly in heat diffusion, composite materials present a high variation of thermal properties as a function of temperature. Therefore, the spectrum of a propagating thermal wave can contain higher harmonics of the excitation frequency. The amplitude of these harmonics depends on the range of temperatures developed inside the material. This study is focused on the mathematical formulation of the relationship between thermal properties and temperature. To this end, the heat capacity and the thermal conductivity of Carbon/Epoxy and Glass/Epoxy cross-ply laminates were determined in a temperature range of interest for the aircraft industry using an ASTM method based on Modulated Temperature Differential Scanning Calorimetry. The results are indispensable toward a nonlinear treatment of heat diffusion phenomena and the respective exploitation for nondestructive testing.

Toward bulk heterojunction polymer solar cells with thermally stable active layer morphology

Abstract. When state-of-the-art bulk heterojunction organic solar cells with ideal morphology are exposed to prolonged storage or operation at elevated temperatures, a thermally induced disruption of the active layer blend can occur, in the form of a separation of donor and acceptor domains, leading to diminished photovoltaic performance. Toward the long-term use of organic solar cells in real-life conditions, an important challenge is, therefore, the development of devices with a thermally stable active layer morphology. Several routes are being explored, ranging from the use of high glass transition temperature, cross-linkable and/or side-chain functionalized donor and acceptor materials, to light-induced dimerization of the fullerene acceptor. A better fundamental understanding of the nature and underlying mechanisms of the phase separation and stabilization effects has been obtained through a variety of analytical, thermal analysis, and electro-optical techniques. Accelerated aging systems have been used to study the degradation kinetics of bulk heterojunction solar cells in situ at various temperatures to obtain aging models predicting solar cell lifetime. The following contribution gives an overview of the current insights regarding the intrinsic thermally induced aging effects and the proposed solutions, illustrated by examples of our own research groups.

Self-healing soft pneumatic robots

Soft robots manufactured out of self-healing polymers have the ability to repair macroscopic damage. Inspired by the compliance found in many organisms, soft robots are made almost entirely out of flexible, soft material, making them suitable for applications in uncertain, dynamic task environments, including safe human-robot interactions. Their intrinsic compliance absorbs shocks and protects them against mechanical impacts. However, the soft materials used for their construction are highly susceptible to damage, such as cuts and perforations caused by sharp objects present in the uncontrolled and unpredictable environments they operate in. In this research, we propose to construct soft robotics entirely out of self-healing elastomers. On the basis of healing capacities found in nature, these polymers are given the ability to heal microscopic and macroscopic damage. Diels-Alder polymers, being thermoreversible covalent networks, were used to develop three applications of self-healing soft pneumatic actuators (a soft gripper, a soft hand, and artificial muscles). Soft pneumatic actuators commonly experience perforations and leaks due to excessive pressures or wear during operation. All three prototypes were designed using finite element modeling and mechanically characterized. The manufacturing method of the actuators exploits the self-healing behavior of the materials, which can be recycled. Realistic macroscopic damage could be healed entirely using a mild heat treatment. At the location of the scar, no weak spots were created, and the full performance of the actuators was nearly completely recovered after healing.

Roles of in situ surface modification in controlling the growth and crystallization of CaCO3 nanoparticles, and their dispersion in polymeric materials

The in situ surface modification of inorganic nanoparticles (NPs) and its influence on the size, morphology, and particle surface properties is increasingly receiving attention. Control of the size and morphology and perfect dispersion of inorganic NPs in polymer matrices fabricates soft materials with unique optical, electrical, magnetic, gas barrier, self-healing, and thermal and mechanical properties. This study explores the strategy of the in situ modification of inorganic NPs (CaCO3) with cationic and anionic surfactants and the role of in situ modification on the dispersion of these NPs in thermoplastic polymers (poly ε-caprolactone, PCL). The surfactants having an appropriate polar head with a high charge density bind onto the crystal’s nuclei, protect them against extensive aggregation, and consequently control the size, morphology, and surface properties of the produced NPs. This permits formulation of hybrid materials with enhanced thermal stability and tensile modulus and with a marked increase of the crystallization rate.

Seed-Mediated Hot-Injection Synthesis of Tiny Ag Nanocrystals on Nanoscale Solid Supports and Reaction Mechanism

Controlling the size and shape of noble Ag nanocrystals (NCs) is of great interest because of their unique size- and shape-dependent properties, especially below 20 nm, and because of interesting applications in drug delivery, sensing, and catalysis. However, the high surface energy and tendency of these tiny NCs to aggregate deteriorates their unique properties and limits their applications. To avoid the aggregation of Ag NCs and improve their performance, we report a seed-mediated hot injection approach to synthesize highly dispersed tiny Ag NCs on a nanosized solid CaCO3 support. This simple, low-cost, and effective chemical approach allows for synthesizing highly uniform Ag NCs (∼10 nm) on the surface of presynthesized CaCO3 single NCs (∼52 nm) without any aggregation of the Ag NCs. Viscose fibers were coated with the Ag@CaCO3 composite nanoparticles (NPs) produced, as well as with ∼126 nm Ag NPs for reference. The Ag@CaCO3 composite NPs show excellent UV protection and antibacterial activity against Escherichia coli. In addition, they give a satin sheen gold to a dark gold color to the viscose fibers, while the Ag NPs (∼126 nm) result in a silver color. The proposed synthesis approach is highly versatile and applicable for many other noble metals, like Au or Pt.

Atomic force microscopy–based study of self-healing coatings based on reversible polymer network systems

A self-healing polymer system is created by incorporating reversible covalent bonds into an epoxy–amine-based network structure. The self-healing concept is based on the reversible Diels–Alder reaction between furan and maleimide functional groups. The thermal and mechanical properties of the reversible network structure are tailored in order to achieve good self-healing properties for the corrosion protection of metal surfaces. Atomic force microscopy is proposed as a technique to study the self-healing behavior of coatings. Local thermal analysis techniques are used to study the local thermomechanical behavior of the reversible network. Nanosized defects in the coatings are made by means of nanolithography. The actual self-healing behavior is studied by atomic force microscopy imaging before and after the heating steps. The healing capability of elastomeric and glassy model systems is compared.

Influence of temperature and UV intensity on photo-polymerization reaction studied by photo-DSC

Photo-DSC was used to investigate the cure kinetics of a photo-initiated resin. The exothermal photo-polymerization reactions were performed in isothermal mode. The irradiation of photo-initiated resin was studied under different conditions of temperature, UV lamp intensity, and reaction atmosphere (nitrogen and air). The results obtained by photo-DSC allowed us to determine kinetic data of the photo-polymerized reactions: the global activation energy and reaction enthalpy, and the conversion as a function of time and temperature. Modulated temperature DSC measurements were carried out to verify whether vitrification occurs during polymerization. The conversion at the top and bottom of irradiated samples was obtained by FT-IR spectroscopy before and after photo-polymerization. A non-homogenous photo-polymerization into the material was observed, probably because of the light absorptions effects within the uppermost layers.

Modulated Differential Scanning Calorimetry to Study Reacting Polymer Systems

Modulated (temperature) differential scanning calorimetry (MTDSC or MDSC) is used to study simultaneously the evolution of heat flow and heat capacity for the isothermal and non-isothermal reactions of polymer systems. An epoxy-anhydride thermosetting system is treated as an example. Vitrification and devitrtification steps are observed. The heat flow phase during isothermal and non-isothermal cure is always small, but its evolution contains information on relaxation phenomena, vitrification and devitrification, in the course of the chemical reaction. Modelling of the (heat flow related) chemical kinetics and the (heat capacity related) mobility restrictions contributes to a better understanding of the reaction mechanism and reaction kinetics up to a high degree of chemical conversion. The use of MTDSC as a tool for a quantitative construction of the vitrification curve in Temperature-Time-Transformation (TTT) and Continuous-Heating-Transformation (CHT) diagrams is illustrated.Modulated (temperature) differential scanning calorimetry (MTDSC or MDSC) is used to study simultaneously the evolution of heat flow and heat capacity for the isothermal and non-isothermal reactions of polymer systems. An epoxy-anhydride thermosetting system is treated as an example. Vitrification and devitrtification steps are observed.The heat flow phase during isothermal and non-isothermal cure is always small, but its evolution contains information on relaxation phenomena, vitrification and devitrification, in the course of the chemical reaction.Modelling of the (heat flow related) chemical kinetics and the (heat capacity related) mobility restrictions contributes to a better understanding of the reaction mechanism and reaction kinetics up to a high degree of chemical conversion. The use of MTDSC as a tool for a quantitative construction of the vitrification curve in Temperature-Time-Transformation (TTT) and Continuous-Heating-Transformation (CHT) diagrams is illustrated.

Evaluation of the Yasuda parameter for the atmospheric plasma deposition of allyl methacrylate

This work studies the influence of the proportional change in discharge power and the monomer feed on the morphology and the chemistry of atmospheric plasma deposited films. Atmospheric plasma coatings of allyl methacrylate were deposited using dielectric barrier discharge plasma under different conditions but always under the same ratio between the discharge power and monomer feed (W/FM). It is shown that a constant W/FM does not necessarily provide the same chemistry and the same morphology for atmospheric pressure plasma. This is explained by the higher discharge power of the plasma resulting in an increase of streamers which alter the distribution of energy among the plasma species. On the surface of the deposited coatings, globular-like features were observed, which are suggested to be formed in the volume of the discharge. The deposition rate is also influenced. providing thicker coatings, when high monomer feed/high power are used. Finally, infrared spectra showed a higher retention of the ester functionality at high power/high monomer feed.