Olivier T. Picot

Humidity-Responsive Bilayer Actuators Based on a Liquid-Crystalline Polymer Network

A humidity-responsive bilayer actuator has been developed that consists of an oriented polyamide-6 substrate and a liquid-crystalline polymer coating. The oriented substrate acts as an alignment layer for the liquid crystal. The liquid-crystalline polymer consists of a supramolecular network having hydrogen-bonded entities that, after activation with an alkaline solution, exhibit deformation in response to a change in humidity. The bending behavior of the bilayer actuator was analyzed, showing a large response to a change in the humidity.

In Situ Exfoliation of Graphene in Epoxy Resins: A Facile Strategy to Efficient and Large Scale Graphene Nanocomposites

Any industrial application aiming at exploiting the exceptional properties of graphene in composites or coatings is currently limited by finding viable production methods for large volumes of good quality and high aspect ratio graphene, few layer graphene (FLG) or graphite nanoplatelets (GNP). Final properties of the resulting composites are inherently related to those of the initial graphitic nanoparticles, which typically depend on time-consuming, resource-demanding and/or low yield liquid exfoliation processes. In addition, efficient dispersion of these nanofillers in polymer matrices, and their interaction, is of paramount importance. Here we show that it is possible to produce graphene/epoxy nanocomposites in situ and with high conversion of graphite to FLG/GNP through the process of three-roll milling (TRM), without the need of any additives, solvents, compatibilisers or chemical treatments. This readily scalable production method allows for more than 5 wt % of natural graphite (NG) to be directly exfoliated into FLG/GNP and dispersed in an epoxy resin. The in situ exfoliated graphitic nanoplatelets, with average aspect ratios of 300-1000 and thicknesses of 5-17 nm, were demonstrated to conferee exceptional enhancements in mechanical and electrical properties to the epoxy resin. The above conclusions are discussed and interpreted in terms of simple analytical models.

New Approach toward Reflective Films and Fibers Using Cholesteric Liquid-Crystal Coatings

A new approach for the production of oriented films and fibers with angular-dependent reflective colors is presented. The process consists of spray coating a solution of cholesteric liquid-crystalline monomers onto a melt-processed and oriented polyamide-6 substrate followed by UV curing. Reflectivity measurements and optical microscopy show that a well-defined liquid-crystalline and planar alignment is obtained. It is further demonstrated that a reflection up to 80% is obtained by coating oriented films on both sides of the oriented substrate with a single-handedness cholesteric liquid-crystal coating. The high reflectivity is attributed to the close to half-wave retardation induced by the anisotropic polymer substrate. Also, polyamide-6 filaments are successfully coated and fibers are obtained with an angular-dependent color in a single dimension along the fiber direction, which originates from the planar cholesteric alignment on a curved surface.

A real time optical strain sensor based on a cholesteric liquid crystal network

A flexible, strain sensor for monitoring uniaxial deformations in oriented polymer films is demonstrated. The sensor consists of a crosslinked cholesteric liquid crystal layer which is spray-coated on a uniaxially oriented polyamide 6 substrate. When no deformation is applied, reflectivity measurements showed a well-defined reflection band. Upon uniaxial extension, the reflection band shifted towards lower wavelengths. A 40 nm shift was recorded for 13% of strain. This wavelength shift is attributed to a decrease in the cholesteric helixs pitch which is induced by the change in thickness of the CLC coating due to Poissons contraction. As a result the colour of the film changed from orange to green with increasing strain. The optical response was found to follow the mechanical behaviour of the polymer substrate. Cycling loading showed a time dependent optical response of the sensor fitting with the viscoplastic mechanical response of the substrate giving real time information on the deformation.

Formation of relief structures on fibres by photo-embossing

Relief structures are created on the surface of fibres to generate new effects in these fibres. More specifically, photo-embossing is used to generate the relief structures and to generate a non-contact process without etching procedures which could eventually be applied in high-speed and continuous spinning lines. Typically, a photopolymer mixture consisting of a polymeric binder, a monomer and a photo-initiator is directly spun into a monofilament fibre. The rheological properties of the photopolymer mixture are measured, and it is shown that the photopolymer mixture is visco-elastic at elevated temperatures. Fibres are prepared from the photopolymer mixture, and surface relief structures are generated perpendicular to the fibre axis by photo-embossing using a mask exposure. Optical microscopy (OM) and scanning electron microscopy (SEM) are employed to study the surface morphology of the fibres. It is shown that the height of the surface relief structures is strongly dependent on the pitch of the line mask and on the diameter of the fibres. At optimized conditions, patterned fibres are produced with well-defined surface relief structures. The fibres are rather brittle which is related to the crosslinked chemical network in the fibres.

Synergistic effects of filler size on thermal annealing-induced percolation in polylactic acid (PLA)/graphite nanoplatelet (GNP) nanocomposites

Abstract Two graphene-based nanofillers of different sizes were melt mixed with polylactide acid (PLA). Composite films based on graphite nanoplatelets (GNPs) with the largest lateral size showed superior electrical conductivity and a lower percolation threshold after melt-compounding and hot-pressing at room temperature. However, upon annealing in the melt, composites based on GNPs with the smallest lateral size displayed significantly improved electrical conductivity and a decrease in percolation threshold as a result of dynamic percolation while composites based on large GNPs showed hardly any change in conductivity and percolation threshold. Hybrid filler systems based on 5 wt% GNP with variable small/large GNP filler ratios displayed synergistic effects in the formation of a conductive network during thermal annealing, and an optimum filler ratio of 50/50 was found to achieve the highest conductivity after annealing. This annealing-induced increase in particle connectivity for composites based on 5 wt.% hybrid small/large GNPs was also reflected in rheological measurements by the manifestation of a plateau in the storage modulus at low frequencies after annealing while such phenomenon was not observed for nanocomposites solely based on either small or large GNPs at similar loadings.

Surface structuring of bi-component fibres with photoembossing

A novel microstructuring technique, photoembossing, is used to create relief structures on the surface of fibres to generate new functionalities, such as diffractive optical effects for fashion design. A typical photopolymer compound, which consists of a polymeric binder, a multifunctional monomer and a photoinitiator is coated on the surface of a conventional synthetic core fibre (PET). Photoembossing is performed via a non-contact exposure to an interference pattern to obtain surface-relief gratings with the grating vector along the fibre axis. The monofilament fibres with grating structures perpendicular to the fibre axis were produced with a period of 1 and 8 μm and a typical height of 60–110 nm and 900–1300 nm, respectively. In accordance with the grating equation, it is observed that the micro-structured fibres with a pitch of 1 μm exhibit a strong angular dispersion and this in contrast to fibres with a pitch of 8 μm. Separated diffracted colours are observed predominantly in the first case (red, green and blue) by varying the viewing angle.

Breaking the Nanoparticle Loading–Dispersion Dichotomy in Polymer Nanocomposites with the Art of Croissant-Making

The intrinsic properties of nanomaterials offer promise for technological revolutions in many fields, including transportation, soft robotics, and energy. Unfortunately, the exploitation of such properties in polymer nanocomposites is extremely challenging due to the lack of viable dispersion routes when the filler content is high. We usually face a dichotomy between the degree of nanofiller loading and the degree of dispersion (and, thus, performance) because dispersion quality decreases with loading. Here, we demonstrate a potentially scalable pressing-and-folding method (P & F), inspired by the art of croissant-making, to efficiently disperse ultrahigh loadings of nanofillers in polymer matrices. A desired nanofiller dispersion can be achieved simply by selecting a sufficient number of P & F cycles. Because of the fine microstructural control enabled by P & F, mechanical reinforcements close to the theoretical maximum and independent of nanofiller loading (up to 74 vol %) were obtained. We propose a universal model for the P & F dispersion process that is parametrized on an experimentally quantifiable “D factor”. The model represents a general guideline for the optimization of nanocomposites with enhanced functionalities including sensing, heat management, and energy storage.