Thermal Reshaping Research Articles

Reshapable bio-based thiol-ene vitrimers for nanoimprint lithography: Advanced covalent adaptability for tunable surface properties

Taking inspiration from natural surfaces known for their ability to adaptively remodel their shape, we fabricated stimuli-responsive microstructures by using UV-induced nanoimprint lithography. For this, a fully bio-based dynamic thiol-ene photopolymer was synthetized by the radical mediated addition of a trifunctional eugenol-based thiol (SH3E) crosslinker across an allylated linseed oil (ALELO). To catalyze the bond exchange reactions between the hydroxyl and ester groups within the network, a bio-based eugenol phosphate ester was introduced as a transesterification catalyst. Pure eugenol was further added as a reactive diluent to increase the number of hydroxyl groups and thus, accelerate the thermo-activated bond exchange reactions. Once cured by UV exposure, the dynamic photopolymer is thermally stable up to 250 °C and is able to relax 63% of the original stress within 62 min at 160 °C. Subsequently, films with micropillars, having an aspect ratio of 1:2.5 were prepared by using nanoimprint lithography. The macroscopic reflow capability of the dynamic network enabled a reorientation of the imprinted structures during a thermal reshaping step. The imprints were characterized by 2D/3D optical microscopy, μCT imaging and static water contact angle measurements. Based on the orientation of the micropillars, the water contact angle was varied between 118° and 95°, giving rise to a possible application in microfluidic devices.

Simulated end-of-life reuse of composites from marine applications using thermal reshaping of seawater-aged, glass fibre-reinforced acrylic materials

In this work, end-of-life reuse of continuous fibre-reinforced thermoplastic composites from marine applications has been simulated by thermally reshaping (reprocessing) seawater-aged materials. Specifically, resin-infused, V-shaped glass/acrylic composites were seawater-aged (60 °C; 170 h) and reprocessed in a heated hydraulic press (120 °C; 10 bar). Reprocessing promoted void collapse and increased short beam shear strengths. Unaged reprocessed materials retained 52 % and 57 % performance in flexural strength and modulus, (relative to virgin material), with corresponding retentions of 20 % and 62 % in the aged reprocessed materials, respectively. Ageing- and reprocessing-related flexural performance changes occurred due to mild matrix plasticisation and fibre misalignment, respectively.

Thermal reshaping as a route for reuse of end-of-life glass fibre-reinforced acrylic composites

Thermal reshaping has been employed to simulate the end-of-life reuse of liquid-resin-infused thermoplastic acrylic composite laminates, and the associated effects on matrix-dominated mechanical performance and microstructure have been studied. L-shaped laminates were infused at room temperature and subjected to 1 or 4 hot-press flattening cycles (25 min at 120 °C; 11 bar). Compared to the original references, up to 13% higher transverse flexural strengths were measured for the reprocessed laminates. Such a scheme may be readily implemented for high-value reuse without sacrificing fibre length scales, and with minimal cumulative mass loss over successive reheating cycles (10 cycles: 2% and 15 cycles: 2.6%). This study provides important insights to foster a greater understanding of the performance limits of hot-press reprocessing to inform the practical reuse and re-application of sustainable composites in a circular economy.

Ultrafast Laser-Induced Formation of Hollow Gold Nanorods and Their Optical Properties.

Laser irradiation has been shown to be an efficient means to modify structures and shapes of plasmonic nanoparticles for tuning their properties. Thermomechanical deformations of single-crystal and penta-twinned gold nanorods by femtosecond laser irradiations have been studied by classical molecular dynamics simulations. It is demonstrated that hollow gold nanorods could be formed by femtosecond laser irradiations under certain conditions of maximum temperatures in nanorods by laser heating and cooling rates due to the extrinsic solvent. For a given maximum temperature and cooling rate, a larger cavity is induced in the irradiated single-crystal nanorod. The results also indicate that at the same cooling rate a higher threshold of maximum temperature can be required for producing the cavity in the twinned nanorod. The optical spectra of the laser-irradiated gold nanorods are calculated, and the shifts in the surface plasmon resonance peak of the nanorods are illustrated due to the thermal reshaping and the plasmon hybridization mechanism. Moreover, we show the formation of the hollow gold nanorod possessing the surface plasmon resonance peak in the second near-infrared window and a relatively small aspect ratio (∼2.8), which is highly desirable and suitable for serving as agents in biomedical imaging and photothermal therapy applications.

Multiplexed two-photon excitation spectroscopy of single gold nanorods.

Plasmonic metallic nanoparticles are commonly used in (bio-)sensing applications because their localized surface plasmon resonance is highly sensitive to changes in the environment. Although optical detection of scattered light from single particles provides a straightforward means of detection, the two-photon luminescence (TPL) of single gold nanorods (GNRs) has the potential to increase the sensitivity due to the large anti-Stokes shift and the non-linear excitation mechanism. However, two-photon microscopy and spectroscopy are restricted in bandwidth and have been limited by the thermal stability of GNRs. Here, we used a scanning multi-focal microscope to simultaneously measure the two-photon excitation spectra of hundreds of individual GNRs with sub-nanometer accuracy. By keeping the excitation power under the melting threshold, we show that GNRs were stable in intensity and spectrum for more than 30min, demonstrating the absence of thermal reshaping. Spectra featured a signal-to-noise ratio of >10 and a plasmon peak width of typically 30nm. Changes in the refractive index of the medium of less than 0.04, corresponding to a change in surface plasmon resonance of 8nm, could be readily measured and over longer periods. We used this enhanced spectral sensitivity to measure the presence of neutravidin, exploring the potential of TPL spectroscopy of single GNRs for enhanced plasmonic sensing.

Shape Stability of Truncated Octahedral fcc Metal Nanocrystals.

Metallic nanocrystals (NCs) can be synthesized with tailored nonequilibrium shapes to enhance desired properties, e.g., octahedral fcc metal NCs optimize catalytic activity associated with {111} facets. However, maintenance of optimized properties requires stability against thermal reshaping. Thus, we analyze the reshaping of truncated fcc metal octahedra mediated by surface diffusion using a stochastic atomistic-level model with energetic input parameters for Pd. The model describes NC thermodynamics by an effective nearest-neighbor interaction and includes a realistic treatment of diffusive hopping for undercoordinated surface atoms. Kinetic Monte Carlo simulation reveals that the effective barrier, Eeff, for the initial stage of reshaping is strongly tied to the degree of truncation of the vertices in the synthesized initial octahedral shapes. This feature is elucidated via exact analytic determination of the energy variation along the optimal reshaping pathway at low-temperature (T), which involves transfer of atoms from truncated {100} vertex facets to form new layers on {111} side facets. Deviations from predictions of the low-T analysis due to entropic effects are more prominent for higher T and larger NC sizes.

Atomic Scale Mechanisms Underlying Thermal Reshaping of Anisotropic Gold Nanocrystals Revealed by in Situ Electron Microscopy

Understanding the shape transformation processes of anisotropic nanocrystals is an essential step toward fully exploiting their remarkable shape-dependent properties. Here, we employ environmental ...

Highly selective cis-1,4 copolymerization of dienes with polar 2-(3-methylidenepent-4-en-1-yl) pyridine: an approach for recyclable elastomers

We report the coordination copolymerization of a new polar diene-based monomer (MPEP), with isoprene or butadiene. After being quaternized, the resultant materials exhibit remarkable thermal reshaping and recycling properties.

Thermal Reshaping of Gold Microplates: Three Possible Routes and Their Transformation Mechanisms.

The thermal stability of Au micro/nanomaterials (AuMNLs) has always been a hot topic because their physicochemical properties, like surface plasmon resonance and catalytic activity, are closely related to their morphology or exposed crystal planes which are heat-sensitive. In this study, we made careful and systematic investigation of thermal deformation and reshaping of individual Au microplates (AuMPs) using atomic force microscopy. We found that AuMPs could transform into walled AuMPs (WAuMPs) and concave AuMPs (CAuMPs) at just 300 °C, which are thermodynamically and kinetically favorable products, respectively. A small fraction of AuMPs, named invariable AuMPs (IAuMPs), remained intact. However, both CAuMPs and IAuMPs can be converted to WAuMPs if the temperature is further increased. We also showed that melting of AuMPs begins from vertices and the boundaries between the top and side plane, rather than only side crystal planes as envisaged before. Finally, we performed a series of electrochemical studies showing that WAuMPs exhibited substantially higher electrocatalytic conversion of methanol at lower formal potential compared to intact AuMPs. This work shows that thermal reshaping of Au is far more complicated as was expected before. It also demonstrates how thermal reshaping can be used to improve electrocatalytic performance of Au and potentially other MNLs.

Thermal Healing, Reshaping and Ecofriendly Recycling of Epoxy Resin Crosslinked with Schiff Base of Vanillin and Hexane-1,6-Diamine.

A bio-derived dihydroxylimine hardener, Van2HMDA, for the curing of epoxy resin was prepared from vanillin (Van) and hexamethylene-1,6-diamine (HMDA) by Schiff base formation. The epoxy resin of diglycidyl ether of bisphenol A was cured with Van2HMDA in the presence of the catalyst, 2-ethyl-4-methylimidazole (EMI). The crosslinked epoxy resin showed thermal-healing properties at elevated temperatures. Moreover, the crosslinked epoxy resin can be reshaped by heating via imine metathesis of the hardener units. The imine metathesis of Van2HMDA was confirmed experimentally. Stress-relaxation properties of the epoxy resin crosslinked with Van2HMDA were investigated, and the activation energy obtained from Arrhenius plots of the relaxation times was 44 kJ/mol. The imine bonds in the epoxy polymer matrix did not undergo hydrolysis after immersing in water at room temperature for one week. However, in the presence of acid, the crosslinked polymer was easily decomposed due to the hydrolysis of imine bonds. The hydrolysis of imine bonds was used for the ecofriendly recycling of crosslinked polymer. It is inferred that thermal-healing, reshaping, and reprocessing properties can be implemented in the various crosslinked epoxy resins with the bio-derived dihydroxylimine hardener, albeit the recycled epoxy resin is of inevitably lower quality than the original material.

Thermal Reshaping Dynamics of Gold Nanorods: Influence of Size, Shape, and Local Environment.

The thermal reshaping of gold nanorods in a polymer matrix is an important phenomenon for many potential applications. However, a fundamental understanding of the various mechanisms that govern the nanorod reshaping dynamics is still lacking. Here, we provide evidence for a phenomenological model of the gold nanorod shape transformation based on the measurements and detailed analysis of the time-resolved thermal reshaping for a variety of gold nanorods having different geometries (aspect ratio, volume, diameter) in a cross-linked epoxy matrix at application relevant temperatures (120-220 °C). Our analysis suggests that (a) the nanorod reshaping dynamics consist of two temporal regimes that are governed by different phenomena and (b) the ultimate amount of reshaping at a given temperature depends strongly on the initial particle geometry and the mechanical stiffness of its surroundings. At short times, the shape transformation is dominated by a curvature-induced surface diffusion process, in which the activation energy for diffusion depends on curvature. At long times, however, the surrounding environment plays a key role in slowing the diffusion and stabilizing the nanorod shape. We show that the long-time behavior can be well described using a modified surface diffusion model that takes into account the slowing of atomic diffusivity as a result of external forces arising from mechanical constraints. The ability to tune both the final shape and the reshaping dynamics in nanocomposites opens up new possibilities in tailoring the optical properties of these materials.

Morphological control of gold nanorods via thermally driven bi-surfactant growth and application for detection of heavy metal ions

We report a modified synthesis route of colloidal gold nanorods (AuNRs) by combining the thermal re-shaping treatment and bi-surfactant modification using hexadecyltrimethylammonium bromide (CTAB) and sodium oleate (NaOL). Aspect ratios down to 1.3 ± 0.1 can be achieved in addition to good monodispersity, uniformity, and chemical stability of the materials. Furthermore, without needing post-treatment, metal ions directly interact with the AuNRs efficiently, allowing rapid and sensitive colorimetric detection of heavy metal ions such as Pb2+ and Cu2+ with a low concentration down to 2.5 μM. The detection performance in terms of selectivity, sensitivity and stability is systematically evaluated. The AuNRs with tunable aspect ratios as well as chemical stability have potential in surface-plasmon-based applications such as biochemical sensing, biochemical imaging, medical diagnostics, and cancer therapy.

3D characterization of heat-induced morphological changes of Au nanostars by fast in situ electron tomography.

A thorough understanding of the thermal stability and potential reshaping of anisotropic gold nanostars is required for various potential applications. Combination of a tomographic heating holder with fast tilt series acquisition has been used to monitor temperature-induced morphological changes of Au nanostars. The outcome of our 3D investigations can be used as an input for boundary element method simulations, enabling us to investigate the influence of reshaping on the nanostars' plasmonic properties. Our work leads to a better understanding of the mechanism behind thermal reshaping. In addition, the approach presented here is generic and can hence be applied to a wide variety of nanoparticles made of different materials and with arbitrary morphology.

Solvent-free optical recording of structural colours on pre-imprinted photocrosslinkable nanostructures

Optical digital information storage media, despite their ever-increasing storage capacity and data transfer rate, are vulnerable to the potential risk of turning inaccessible. For this reason, long-term eye-readable full-colour optical archival storage is in high demand for preserving valuable information from cultural, intellectual, and scholarly resources. However, the concurrent requirements in recording colours inexpensively and precisely, and preserving colours for the very long term (for at least 100 years), have not yet been met by existing storage techniques. Structural colours hold the promise to overcome such challenges. However, there is still the lack of an inexpensive, rapid, reliable, and solvent-free optical patterning technique for recording structural colours. In this paper, we introduce an enabling technique based on optical and thermal patterning of nanoimprinted SU-8 nanocone arrays. Using photocrosslinking and thermoplastic flow of SU-8, diffractive structural colours of nanocone arrays are recorded using ultra-violet (UV) exposure followed by the thermal development and reshaping of nanocones. Different thermal treatment procedures in reshaping nanocones are investigated and compared, and two-step progressive baking is found to allow the controllable reshaping of nanocones. The height of the nanocones and brightness of diffractive colours are modulated by varying the UV exposure dose to enable grey-scale patterning. An example of recorded full-colour image through half-tone patterning is also demonstrated. The presented technique requires only low-power continuous-wave UV light and is very promising to be adopted for professional and consumer archival storage applications.

Retardation of shape change of Au nanorods using photo‐cross‐linkable ligands

ABSTRACTThe thermal reshaping of gold nanorods has been slowed by grafting a diblock copolymer [P(S‐b‐S‐N3)] containing an outer polystyrene (PS) brush and a short, inner photo‐cross‐linkable PS‐azide block. The P(S‐b‐S‐N3)‐Au NRs were dispersed in a PS thin film and reshaping was investigated using scanning electron microscopy and UV–Vis spectroscopy. For P(S‐b‐S‐N3)‐Au NRs in PS, the longitudinal surface plasmon resonance decreased from about 880 toward 750 nm upon annealing at 100 °C, 150 °C, and 200 °C. This blue shift increased in strength as temperature increased. However, this reshaping of P(S‐b‐S‐N3)‐Au NRs was slower than that of Au NRs grafted with a poly(ethylene glycol) brush that was dispersed in poly(methyl methacrylate). By slowing down reshaping at elevated temperature, polymer thin film devices that heat during use (e.g., polarization dependent filters) can exhibit a longer lifetime. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 301–307

Improved Alignment of Gold Nanorods Embedded in Polymer Films

The multiple applications of gold nanorods (AuNRs) are most effective if the longitudinal surface plasmon resonance (SPR) can be simultaneously excited in all the involved AuNRs. This can be achieved in well aligned ensembles of AuNRs, obtained by stretching AuNR-polymer composite films, as a possible method. One of the difficulties encountered in applying this method, is the thermal reshaping of the rods, which would change their optical properties. To mitigate this risk, while increasing the plastic deformability of the film, a polymer of high molecular weight mixed with a small amount of plasticizer was chosen as the composite matrix. The resulting composite films could be stretched up to seven times their initial length. To quantify the alignment of the rods, the average orientation angle was calculated based on scanning electron micrographs. An orientational order parameter of 0.92 was found, as compared to 1, which is the parameter value corresponding to perfect alignment. The anisotropy of the composite film was tested in linearly polarized light.

Fine-tuning longitudinal plasmon resonances of nanorods by thermal reshaping in aqueous media

Metallic nanoparticles that support surface plasmons are potential building units for future nanophotonic circuits, metamaterials, high-density optical data storage, etc. Many of these applications require the ability to ‘dial-up’ the desired plasmonic resonance modes and frequencies with high precision. Here, we demonstrate a thermal reshaping route that can be used to tailor longitudinal plasmon resonance energies of gold nanorods almost continuously from ∼800 to ∼560 nm. The longitudinal plasmon resonance wavelength exhibits an exponential decay function of the thermal annealing time at a given temperature. This correlates with the transmission electron microscopy characterization (TEM) which showed that the nanorod aspect ratio decreases exponentially with time, accompanying a gradual shape transformation from rod to sphere. The exponential decay half-time decreases with increasing annealing temperatures, with a value of 1.43 × 105 s at 50 °C down to 0.02 × 105 s at 100 °C. Our experimental results show that the shape transformation could be attributed to desorption of silver ions and side facet-binding Ag–Br–CTA ligands, which therefore promote the side growth leading to nanorod fattening. Compared to other synthetic methodologies to tune plasmonics, our thermal reshaping approach presents a straightforward paradigm for precisely tailoring plasmon resonance energy with a single parameter.

Thermal Reshaping of Gold Nanorods in Micellar Solution of Water/Glycerol Mixtures

Gold nanorods (Nds) with aspect ratios of 4, 3.5, and 2.8 were prepared by the electrochemical method. The nanorods were thermally studied in binary solvents of aqueous glycerol at different ratios (25%–75%). The results illustrated that the longitudinal surface plasmon resonance (SPL) is strongly dependent on the dielectric constant. The maximum absorption is red shifted with increasing the glycerol/water ratio. This was attributed to the decreasing value of the dielectric constant of the binary solvents. Moreover, by increasing the temperatures, the results showed relative instability of the gold nanorods. This attributed to the relative instability of the micelle capping the nanorods.

Tuning optical properties of gold nanorods in polymer films through thermal reshaping

The thermal reshaping of gold nanorods (NRs) in a poly(methyl methacrylate) (PMMA) nanocomposite film is investigated by UV-vis and TEM. To ensure dispersion, the NRs are modified with PEG brushes, and then dispersed in PMMA. Thermal annealing of the PMMA–NR film results in a blue shift of the longitudinal plasmon resonance, caused by a decrease in the length of the NR. The rate of the blue shift increases as temperature increases from 100 °C to 200 °C, and the longitudinal absorption peak approaches a constant value that scales linearly with temperature. We demonstrate a potential application by fabricating a device with a gradient in optical properties.

Hydrogen Storage in Pd Nanodisks Characterized with a Novel Nanoplasmonic Sensing Scheme

A novel nanoplasmonic sensing scheme is introduced based on remote real-time detection of induced electronic and shape/structural changes in a metal nanoparticle during the metal-hydride formation process. The localized surface plasmon resonance (LSPR) of the nanoparticle is utilized as signal transducer for optical readout. As a model system, hydrogen storage through metal-hydride formation is studied in Pd nanodisks. The experimentally obtained plasmonic response to hydrogen uptake yields pressure-LSPR-response isotherms. These isotherms are found to obey Sievert's law in the low-pressure range and exhibit a characteristic "plateau" at 18 Torr upon hydrogen charging and 7.5 Torr upon hydrogen discharging. An additional experiment also clearly shows the typical temperature dependence of the plateau pressure. Conversion of the LSPR signal to absolute hydrogen concentration, based on a proposed linear dependence of the LSPR response to hydrogen uptake, results in p-C isotherms in excellent agreement with those in the literature. This puts forward that the LSPR response is an extremely sensitive, remote, and real-time probe for "bulk" changes in a metal nanoparticle and can readily be used to study processes such as metal-hydride formation for hydrogen storage applications, alloying on the nanoscale, thermal reshaping, and so forth.