Scientific publications

The statistical anionic copolymerization of isoprene (I) and styrene (S) is commonly used to synthesize tapered block copolymers, enabling control of the phase behavior by adjusting the order–disorder transition temperature, TODT. Alkyllithium initiation in hydrocarbons is known to afford tapered block copolymers of I and S in one step. The effect of tetrahydrofuran (THF) on the copolymerization kinetics and the resulting copolymers was systematically investigated by increasing the [THF]/[Li] ratio from 0 to 2500 (0 to 29%vol THF). For this purpose, in situ near-infrared (NIR) spectroscopy was employed as a versatile and fast method to track the highly accelerated consumption of the individual monomers. Changes in the I/S comonomer sequence and in the polyisoprene (PI) regioisomers, caused by variation of the THF concentration, were independently determined via NMR and in situ NIR spectroscopy. Reactivity ratios were determined as a function of the [THF]/[Li] ratio. They revealed a gradual reversal from rI ≫ rS over rI ≈ rS to rI ≪ rS. Corresponding changes in the copolymer composition profile up to a complete inversion are evident in thermal properties and morphologies. Although all copolymers possess the same comonomer composition (50%mol = 57%vol polystyrene (PS) units), small-angle X-ray scattering and transmission electron microscopy give evidence of a wide variation in bulk morphologies depending on the gradient profile. Overall, the phase diagram is symmetric, and the succession of phases bears certain similarities to the PI-b-PS case. This is discussed in terms of the increasing incompatibility of PS with 3,4-PI and the more symmetric polymer conformational parameter. The degree of segregation, as well as the nanodomain structure, was found to control the mechanical properties, showing a remarkably different viscoelastic response leading to either hard/brittle or ductile/soft materials. The accessibility of tailored gradient profiles, as well as their in-depth understanding by simply using THF as a microstructural modifier, opens a variety of possible applications. As an example, the synthesis of a PI-selective hydrogenated tapered triblock, possessing a THF-modified, phase-compatibilizing tapered block incorporated in the well-established SIS block architecture, is presented.

Abstract Switchable underwater adhesion can be useful for numerous applications, but is extremely challenging due to the presence of water at the contact interface. Here, deformable cupped microstructures (diameter typically 100 µm, rim thickness 5 µm) are reported that can switch between high (≈1 MPa) and low (<0.2 MPa) adhesion strength by adjusting the retraction velocity from 100 to 0.1 µm s–1. The velocity at which the switch occurs is determined by specific design parameters of the cupped microstructure, such as the cup width and angle. The results are compared with theoretical estimates of water penetration into the contact zone and expansion of the cup during retraction. This work paves the way for controlling wet adhesion on demand and may inspire further applications in smart adhesives.

Thiol-maleimide and thiol-vinylsulfone cross-linked hydrogels are widely used systems in 3D culture models, in spite of presenting uncomfortable reaction kinetics for cell encapsulation: too fast (seconds for thiol-maleimide) or too slow (minutes-hours for thiol-vinylsulfone). Here, we introduce the thiol-methylsulfone reaction as alternative cross-linking chemistry for cell encapsulation, particularized for PEG-hydrogels. The thiol-methylsulfone reaction occurs at high conversion and at intermediate reaction speed (seconds-minutes) under physiological pH range. These properties allow easy mixing of hydrogel precursors and cells to render homogeneous cell-laden gels at comfortable experimental time scales. The resulting hydrogels are cytocompatible and show comparable hydrolytic stability to thiol-vinylsulfone gels. They allow direct bioconjugation of thiol-derivatized ligands and tunable degradation kinetics by cross-linking with degradable peptide sequences. 3D cell culture of two cell types, fibroblasts and human umbilical vein endothelial cells (HUVECs), is demonstrated.

Hydroxyapatite coatings need similarly shaped splats as building blocks and then a homogeneous microstructure to unravel the structural and chemical hierarchy for more refined improvements to implant surfaces. Coatings were thermally sprayed with differently sized powders (20–40, 40–63 and 63–80 µm) to produce flattened homogeneous splats. The surface was characterized for splat shape by profilometry and Atomic force microscopy (AFM), crystal size by AFM, crystal orientation by X-ray diffraction (XRD) and structural variations by XRD. Chemical composition was assessed by phase analysis, but variations in chemistry were detected by XRD and Raman spectroscopy. The resulting surface electrical potential was measured by Kelvin probe AFM. Five levels of structural hierarchy were suggested: the coating, the splat, oriented crystals, alternate layers of oxyapatite and hydroxyapatite (HAp) and the suggested anion orientation. Chemical hierarchy was present over a lower range of order for smaller splats. Coatings made from smaller splats exhibited a greater electrical potential, inferred to arise from oxyapatite, and supplemented by ordered OH− ions in a rehydroxylated surface layer. A model has been proposed to show the influence of structural hierarchy on the electrical surface potential. Structural hierarchy is proposed as a means to further refine the properties of implant surfaces.

Abstract Space cooling and heating currently result in huge amounts of energy consumption and various environmental problems. Herein, a switching strategy is described for efficient energy-saving cooling and heating based on the dynamic cavitation of silicone coatings that can be reversibly and continuously tuned from a highly porous state to a transparent solid. In the porous state, the coatings can achieve efficient solar reflection (93%) and long-wave infrared emission (94%) to induce a subambient temperature drop of about 5 °C in hot weather (≈35 °C). In the transparent solid state, the coatings allow active sunlight permeation (95%) to induce solar heating to raise the ambient temperature from 10 to 28 °C in cold weather. The coatings are made from commercially available, cheap materials via a facile, environmentally friendly method, and are durable, reversible, and patternable. They can be applied immediately to various existed objects including rigid substrates.

Most everyday surfaces are randomly rough and self-similar on sufficiently small scales. We investigated the tactile perception of randomly rough surfaces using 3D-printed samples, where the topographic structure and the statistical properties of scale-dependent roughness were varied independently. We found that the tactile perception of similarity between surfaces was dominated by the statistical micro-scale roughness rather than by their topographic resemblance. Participants were able to notice differences in the Hurst roughness exponent of 0.2, or a difference in surface curvature of 0.8 $$\hbox {mm}^{-1}$$mm-1for surfaces with curvatures between 1 and 3 $$\hbox {mm}^{-1}$$mm-1. In contrast, visual perception of similarity between color-coded images of the surface height was dominated by their topographic resemblance. We conclude that vibration cues from roughness at the length scale of the finger ridge distance distract the participants from including the topography into the judgement of similarity. The interaction between surface asperities and fingertip skin led to higher friction for higher micro-scale roughness. Individual friction data allowed us to construct a psychometric curve which relates similarity decisions to differences in friction. Participants noticed differences in the friction coefficient as small as 0.035 for samples with friction coefficients between 0.34 and 0.45.

One of the contentious issues associated with the high-cycle fatigue of Nitinol, a nominally equiatomic alloy of nickel and titanium, is the claim that increasing the applied mean strain can increase, or at least have no negative impact, on the fatigue lifetime, in conflict with reported behavior for the vast majority of other metallic materials. To investigate this in further detail, cyclic fatigue tests in bending were carried out on electropolished medical grade Nitinol at 37 °C for lives of up to 400 million cycles of strain involving various levels of the mean strain. A constant life model was developed through statistical analysis of the fatigue data, with 90% reliability at a confidence level of 95% on the effective fatigue strain. Our results show that the constant life diagram, a plot of strain amplitude versus mean strain, is monotonic yet nonlinear for lives of 400 million cycles of fatigue loading. Specifically, we find that in contradiction to the aforementioned claim, the strain amplitude limit at zero mean strain is 0.55% to achieve a 400 million cycle lifetime, at 90% reliability with 95% confidence; however, to achieve the same lifetime, reliability and confidence level in the presence of a 3% or more mean strain, the required strain amplitude limit is decreased by over a factor of three to 0.16%. Moreover, for mean strains from 3% to 7%, the strain amplitude limit that allows a 400 million cycle lifetime, at 90% reliability with 95% confidence, is ~ 0.16%, and essentially independent of mean strain. We conclude that the debatable claim that an increase in the applied mean strain can increase the fatigue life of Nitinol components is not supported by the current data.

A new series of amphiphilic block copolymers has been prepared by ring opening polymerization (ROP) of cyclic carbonates using PEG as initiator. The light responsive unit [7–(diethylaminocoumarin)–4–yl]methyl ester has been introduced by a modular and versatile supramolecular approach, while a reference covalent copolymer has been synthesized for the shake of comparison. Synthesized copolymers showed monomodal narrow distributions and were able to self-assemble into spherical micelles when dispersed in water. UV irradiation allowed us the modification of the self-assemblies morphology, as proved by means of fluorescence spectroscopy, dynamic light scattering (DLS) and transmission electron microscopy (TEM). Both the supramolecular and covalent functionalized block copolymers were able to encapsulate small fluorescent probes as payload and to release them upon UV and NIR irradiation.

Optical polymers cover only a rather narrow range of optical properties. This is a limiting factor for the design of polymer-based optical systems such as smartphone cameras. Moreover, it also poses a problem for femtosecond two-photon lithography, which is a state-of-the-art technology to 3D print high-quality optics from photopolymers. To overcome the limitations of conventional polymers, we introduce nano-inks based on the commonly used photopolymers IP-DIP and IP-S as polymer matrix and zirconium dioxide (ZrO2) nanoparticles. We show that the refractive index and dispersion of these nano-inks can be purposefully tailored by varying the constituent materials and the volume fraction of the nanoparticles. Furthermore, we demonstrate the suitability of our nano-inks for optical applications by 3D printing single micro-lenses and a multi-material achromatic Fraunhofer doublet. Our findings confirm that nanocomposites expand the range of optical properties that are accessible for polymer-based systems and allow for the design of tailored optical materials.