Nano-Engineered Polymer-Steel Hybrids : Chemical and Physical Compatibilization

The exploitation of the toughness of steel in steel-polymer hybrids in structural applications is limited due to the huge difference in stiffness of the reinforcement (200 GPa for steel) compared to the polymer matrix (between 1 and 3 GPa only). This stiffness mismatch leadsto stress concentrations at the steel-polymer interface, which give rise to either early interface fracture or plastic yielding. The guiding hypothesis within this PhD research, which focuses on the optimization of polymer-steel hybrids, is that it is necessary to decrease the polymer-steel stiffness mismatch and that the steel-polymer adhesion as well as the polymer toughness and/or yield stress in the interphasial region need to be improved.nrpages: 300status: publishe

Linking the morphology of a high hard segment content polyurethane to its thermal behaviour and mechanical properties

Understanding and controlling the morphology of thermoplastic polyurethane (TPU) is crucial, as it is closely linked with its thermal and mechanical properties. The morphology of a TPU with a high hard segment content was investigated. When hard segment crystallisation was avoided by fast cooling, a crystallisation-induced phase separation occurred upon reheating. At higher temperatures, a second polymorph was additionally created. Cooling slowly from the melt directly induced the formation of both polymorphic forms. The complex thermal behaviour could hence be explained by the (cold) crystallization and melting of two polymorphs. The disordered two-phase nanomorphology, revealed by AFM at room temperature after cooling slowly, was validated for both fast and slowly cooled samples at higher temperatures by fitting model SAXS patterns to time resolved synchrotron SAXS data. Annealing fast cooled samples at high temperature induced some ordered, lamellar stacks in addition. Finally, the morphology was linked to the evolution of storage modulus with increasing temperature.publisher: Elsevier articletitle: Linking the morphology of a high hard segment content polyurethane to its thermal behaviour and mechanical properties journaltitle: Polymer articlelink: http://dx.doi.org/10.1016/j.polymer.2015.11.007 content_type: article copyright: Copyright © 2015 Elsevier Ltd. All rights reserved.status: publishe

Ductile steel fiber/epoxy composites with modified adhesion

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Highly efficient hyperbranched CNT surfactants : influence of molar mass and functionalization

End-group-functionalized hyperbranched polymers were synthesized to act as a carbon nanotube (CNT) surfactant in aqueous solutions. Variation of the percentage of triphenylmethyl (trityl) functionalization and of the molar mass of the hyperbranched polyglycerol (PG) core resulted in the highest measured surfactant efficiency for a 5000 g/mol PG with 5.6% of the available hydroxyl end-groups replaced by trityl functions, as shown by UV-vis measurements. Semiempirical model calculations suggest an even higher efficiency for PG5000 with 2.5% functionalization and maximal molecule specific efficiency in general at low degrees of functionalization. Addition of trityl groups increases the surfactant-nanotube interactions in comparison to unfunctionalized PG because of pi-pi stacking interactions. However, at higher functionalization degrees mutual interactions between trityl groups come into play, decreasing the surfactant efficiency, while lack of water solubility becomes an issue at very high functionalization degrees. Low molar mass surfactants are less efficient compared to higher molar mass species most likely because the higher bulkiness of the latter allows for a better CNT separation and stabilization. The most efficient surfactant studied allowed dispersing 2.85 mg of CNT in 20 mL with as little as 1 mg of surfactant. These dispersions, remaining stable for at least 2 months, were mainly composed of individual CNTs as revealed by electron microscopy

Tensile behaviour of stainless steel fibre/epoxy composites with modified adhesion

In this study we investigate the tensile behaviour of unidirectional and cross-ply composites reinforced with ductile stainless steel fibres and modified adhesion to the epoxy matrix. Results show that annealed stainless steel fibres have a potential in designing tough polymer composites for structural applications. The stiffness of the UD composites made from these fibres is 77GPa combined with the strain-to-failure between 15% and 18% depending on the level of adhesion. Silane treatments were used to modify the adhesion. By treating the stainless steel fibres with different silane coupling agents, an increase of 50% in the transverse 3-point-bending strength was realised. Increasing the adhesion by 50% leads to a higher tensile strength and strain-to-failure in both UD and cross-ply laminates and a higher in-situ strength of the 90_ plies. It also delays formation of matrix cracks and hinders growth of debonding.publisher: Elsevier articletitle: Tensile behaviour of stainless steel fibre/epoxy composites with modified adhesion journaltitle: Composites Part A: Applied Science and Manufacturing articlelink: http://dx.doi.org/10.1016/j.compositesa.2014.11.022 content_type: article copyright: Copyright © 2014 Elsevier Ltd. All rights reserved.status: publishe

Improving the properties of polymer-steel composites by using silane coupling agents

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Optimisation of wet chemical silane deposition to improve the interfacial strength of stainless steel/epoxy

© 2014 Elsevier B.V. All rights reserved. The evaluation of various wet chemical deposition conditions of γ-aminopropyltriethoxysilane (APS) on stainless steel resulted in stainless steel/epoxy hybrids with improved interfacial strength. Nuclear magnetic resonance spectroscopy (NMR) revealed the working window of the silane solution used, while scanning electron microscopy (SEM) and spectroscopic ellipsometry (SE) served at characterising the final APS film structural properties. With pull-off testing the interfacial strength of surface treated steel plates in contact with an epoxy resin was determined. Fracture surface morphological features allowed identifying the failure mode. Optimisation of the different silane deposition conditions led to a doubling of the interfacial strength compared to non-treated samples. The fracture strength value of more than 60 MPa by far exceeds values currently found in literature and approaches the epoxy fracture strength by which the original adhesive failure mode converts into a more cohesive failure mode.publisher: Elsevier articletitle: Optimisation of wet chemical silane deposition to improve the interfacial strength of stainless steel/epoxy journaltitle: Applied Surface Science articlelink: http://dx.doi.org/10.1016/j.apsusc.2014.10.075 content_type: article copyright: Copyright © 2014 Elsevier B.V. All rights reserved.status: publishe

Highly Efficient Hyperbranched CNT Surfactants: Influence of Molar Mass and Functionalization

End-group-functionalized hyperbranched polymers were synthesized to act as a carbon nanotube (CNT) surfactant in aqueous solutions. Variation of the percentage of triphenylmethyl (trityl) functionalization and of the molar mass of the hyperbranched polyglycerol (PG) core resulted in the highest measured surfactant efficiency for a 5000 g/mol PG with 5.6% of the available hydroxyl end-groups replaced by trityl functions, as shown by UV–vis measurements. Semiempirical model calculations suggest an even higher efficiency for PG5000 with 2.5% functionalization and maximal molecule specific efficiency in general at low degrees of functionalization. Addition of trityl groups increases the surfactant–nanotube interactions in comparison to unfunctionalized PG because of π–π stacking interactions. However, at higher functionalization degrees mutual interactions between trityl groups come into play, decreasing the surfactant efficiency, while lack of water solubility becomes an issue at very high functionalization degrees. Low molar mass surfactants are less efficient compared to higher molar mass species most likely because the higher bulkiness of the latter allows for a better CNT separation and stabilization. The most efficient surfactant studied allowed dispersing 2.85 mg of CNT in 20 mL with as little as 1 mg of surfactant. These dispersions, remaining stable for at least 2 months, were mainly composed of individual CNTs as revealed by electron microscopy