Material Properties Of Films Research Articles

Thickness dependence of the mechanical properties of piezoelectric high-Q_m nanomechanical resonators made from aluminium nitride

Abstract Nanomechanical resonators with high quality factors (Qm) enable mechanics-based quantum technologies, in particular quantum sensing and quantum transduction. High-Qm nanomechanical resonators in the kHz to MHz frequency range can be realized in tensile-strained thin films that allow the use of dissipation dilution techniques to drastically increase Qm. In our work, we study the material properties of tensile-strained piezoelectric films made from aluminium nitride (AlN). We characterize crystalline AlN films with a thickness ranging from 45nm to 295nm, which are directly grown on Si(111) by metal-organic vapour-phase epitaxy. 
We report on the crystal quality and surface roughness, the piezoelectric response, and the residual and released stress of the AlN thin films. Importantly, we determine the intrinsic quality factor of the films at room temperature in high vacuum. We fabricate and characterize AlN nanomechanical resonators that exploit dissipation dilution to enhance the intrinsic quality factor by utilizing the tensile strain in the film. We find that AlN nanomechanical resonators below 200nm thickness exhibit the highest Qf-product, on the order of 1012Hz. We discuss possible strategies to optimize the material growth that should lead to devices that reach even higher Qf-products. This will pave the way for future advancements of optoelectromechanical quantum devices made from tensile-strained piezoelectric AlN.

Freshness monitoring of some food products using pH-responsive smart films containing cyanidin cation

In this study, we introduced an innovative method by incorporating 1-butyl-3-methylimidazolium chloride (BmimCl) as a versatile component and fine-tuning the mixture of chitosan, BmimCl, and cyanidin cation (Anth) to create pH-responsive smart films for food freshness. Owing to the dual functionality of BmimCl, it served as a plasticizer as well as enhanced the film's capacity to display a broader spectrum of visible color changes. The material properties of chitosan films containing BmimCl and Anth at varying concentrations were characterized using FTIR, XRD, mechanical test, TGA, UV–vis and pH sensitivity. The combined results indicate that the chitosan-based pH-responsive smart film possessed excellent mechanical properties, great thermal stability, high light transmission, and a broad pH sensitivity. pH-responsive films were used for real-time freshness monitoring of freshwater shrimps, sterilized milk, and hairtails. Results revealed that BmimCl disrupts the hydrogen bond network of chitosan molecules. The films exhibit a transition from green to red as the food turns from fresh to spoiled, effectively demonstrating their capability to monitor freshness.

Transport and material properties of doped BiSbX topological insulator films grown by physical vapor deposition

Abstract Topological Insulator (TI) BiSb is a promising material for spin-orbit torque (SOT) devices because of its high spin Hall angle, relatively higher electrical conductivity, and can be produced at room temperature by physical vapor deposition. In this work, we systematically investigate the effects of doping BiSb with several dopants. We found that doping with elemental dopants does not change the bulk conductivity, but doping with metal-nitride or metal-oxide molecular dopants can substantially increase or decrease BiSb’s bulk film conductivity with minor impacts on its TI properties. Furthermore, doping can significantly improve other TI film material properties such as increased melting point and film hardness, producing smaller grain sizes with improved interfacial roughness, and enhanced (012) film growth texture, all of which can contribute to enhanced atomic migration resistance in and out of the BiSb layer. Thus, our results open a path for using doped BiSb in integrated SOT devices

Importance of Primary and Secondary Hydrogen Bonding Interactions of Polyols on the Plasticization of Chitosan.

Molecular-level interactions between glucosamine and glycerol have shed light on the specific binding motif required for the plasticization of chitosan with glycerol via the gel theory mechanism. Here, we describe a spectroscopic study of the intermolecular interactions between the monomeric repeat unit of chitosan, glucosamine, and simplified 1,2- and 1,3-diol units of glycerol (i.e, 1,3-propanediol and ethylene glycol). The material properties of chitosan films containing these diols at varying concentrations were characterized using ATR-IR, DMA, TGA, and SEM. The combined results indicate that these diols plasticize chitosan via the lubricity theory mechanism, which differs from glycerol that plasticizes via the gel theory mechanism. At low concentrations, this difference in mechanism has a minimal impact on the material properties. However, at high concentrations of the diols, the necessity of a secondary hydrogen bonding interaction for the retention of chitosan plasticization is observed with a significant increase in the Young's modulus of the materials. The impact of hydrophobicity within the diols was also investigated in chitosan films using 1,2-propanediol and 2-methyl-1,3-propanediol. The combined analyses provide strong evidence that both primary and secondary interactions are responsible for determining the mechanism of chitosan plasticization.

Electromagnetically Enhanced Atomic Layer Deposition for Modification of Thin Film Properties through in Situ Substrate Biasing

Atomic layer deposition (ALD) is broadly used for thin, conformal film growth in microelectronics, photovoltaics, and other industries. Enhanced film properties are required for advancement of device performance. Use of electrical current in situ during deposition to drive material properties of films produced by ALD toward a desired state (such as higher conductivity) will increase the utility of thin film materials for device applications. In an ALD reactor bias is applied directly to substrates during ALD precursor pulses. Higher conductivity and growth rates were observed for samples grown with ALD enhanced by in situ substrate biasing. Thicknesses were determined via spectroscopic ellipsometry, uniformity was investigated by atomic force microscopy, and crystallinity by x-ray diffraction. These systems are computationally investigated to probe the mechanism by which the ALD processes are enhanced.

Production of film compositions modified with carboxyl- and hydroxyl-containing compounds based on polyacrylamide

The effect of plasticizers on the physical and mechanical properties of film materials based on polyacrylamide has been studied. It has been established that the introduction of sorbitol, trimethylolpropane and PEG 400 into the composition reduces the tensile strength of film materials. It is shown that the use of plasticizers affects the relative elongation at break and film adhesion.

Wet Chemical Synthesis of AlxGa1−xAs Nanostructures: Investigation of Properties and Growth Mechanisms

This study focuses on the wet chemical synthesis of AlxGa1−xAs nanostructures, highlighting how different deposition conditions affect the film morphology and material properties. Electrochemical etching was used to texture GaAs substrates, enhancing mechanical adhesion and chemical bonding. Various deposition regimes, including voltage switching, gradual voltage increase, and pulsed voltage, were applied to explore their impact on the film growth mechanisms. SEM analysis revealed distinct morphologies, EDX confirmed variations in aluminum content, Raman spectroscopy detected structural disorders, and XRD analysis demonstrated peak position shifts. The findings emphasize the versatility and cost-effectiveness of wet electrochemical methods for fabricating high-quality AlxGa1−xAs films with tailored properties, showing potential for optoelectronic devices, high-efficiency solar cells, and other advanced semiconductor applications.

Incorporation of ground mustard seeds in an active packaging film: structural properties and controlled release of allyl isothiocyanate

SummaryGround mustard seeds can be incorporated into a packaging film material to have a controlled release of allyl isothiocyanates (AITC). However, the properties of film material loaded with ground mustard seeds need to be evaluated, since they might influence the release of AITC from the film materials. This study aimed to assess properties, including surface morphology, spectral analysis, crystallinity, the moisture‐related, thermal and mechanical properties, of the film materials incorporated with the ground seeds and the release of antimicrobial compounds. Furthermore, the effects of relative humidity on AITC release in the headspace were investigated. The results showed that incorporating a larger quantity of ground mustard seeds resulted in the quicker release of AITC due to direct exposure to moisture, as indicated by higher concentrations observed in the headspace. Meanwhile, a smaller quantity of ground seeds added to the film maintained the thermal properties of the materials until 100 °C, resulting in significantly better moisture‐related properties and higher mechanical strength in terms of tensile strength, elastic modulus, and strain at break. However, the smallest ratio (1:5) showed no release of AITC into the packaging headspace at 75% and 100% RH. Furthermore, the relative humidity led to the release of more AITC in the headspace, which is potentially applied for high‐moisture food products. Based on the influence of relative humidity on AITC concentrations, the mechanism of AITC release from the film materials is proposed, describing the AITC diffusion from ground mustard seeds, film, and headspace.

Two-step modification strategy enhances carbon fiber/old corrugated container fiber flexible conductive film

Carbon fiber (CF) possesses inherent properties of low density, high strength, and environmental stability. Its high aspect ratio and knittability enable the formation of flexible conductive networks. Old corrugated container (OCC) is rich in cellulose and hemicellulose, rendering them flexible and exhibiting a low coefficient of thermal expansion. In this paper, CF/OCC/PVA flexible conductive films were prepared using polyvinyl alcohol (PVA) as the matrix, CF as the conductive filler and OCC as the flexible support material. Firstly, the coupling agent was grafted on the surface of CF to enhance their surface activity, improve the dispersion of CF in solution and the chemical bonding of CF to the matrix. Subsequently, sizing nanoparticles were applied to increase the roughness of the CF surface. This served to increase the volume of the conductive network and enhance the mechanical interlocking between CF and PVA. The results show that the two-step combined modification strategy have a significant positive impact on the interfacial chemical activity, contact angle, and roughness of CF. Moreover, it leads to notable improvements in various properties of the conductive film materials. This finding is relevant for the preparation of CF reinforced other matrix materials as well as for expanding the special properties and application value of CF conductive thin film materials.

Structure and thermomechanical properties of superlattice-stabilized TiNbTaZrSiAlN high-entropy coating

Stabilizing the phase structure is the prerequisite for obtaining excellent properties of film materials. Here, we construct a multi-layer superlattice structure including high-entropy sublayer to further tailor the structure and properties of Al-containing nitride coating. A fully cubic structure of Ti0.11Nb0.14Ta0.13Zr0.14Si0.09Al0.39N/Ti0.55Al0.45N (HEN_M) multilayer is realized by stabilizing the cubic/wurtzite-mixed Ti0.11Nb0.14Ta0.13Zr0.14Si0.09Al0.39N (HEN) sublayers with the cubic Ti0.55Al0.45N template layers. The coherent interface strain increases the hardness to 37.5 ± 1.1 GPa for HEN_M in comparison with the corresponding monolithic coatings with 28.2 ± 0.9 GPa for Ti0.55Al0.45N and 22.2 ± 0.7 GPa for HEN. Simultaneously, the HEN_M multilayer shows the optimal Vickers indentation toughness. All coatings exhibit age-hardening characteristics stemming from the spinodal decomposition. Compared to Ti0.55Al0.45N reaching its peak hardness of ∼32.9 ± 0.9 GPa at 900°C, the sluggish diffusion effect promotes HEN and HEN_M maintaining the maximum hardness of ∼25.6 ± 0.8 GPa and ∼39.6 ± 0.9 GPa upon annealing to 1100 and 1000°C, respectively. In addition, exposed to synthetic air at 850°C for 10 h, Ti0.55Al0.45N coating has been completely oxidized, whereas the HEN and HEN_M coatings only experience partial oxidation with oxide scales of ∼1.52 and 0.78 μm.

Chemically tailored graphite oxide nanoparticles for improving material properties of canola protein-based films

Canola protein obtained from canola meal, a byproduct of the canola industry, is an economical biopolymer with promising film-forming properties. It has significant potential for use as a food packaging material, though it possesses some functional limitations that need improvement. Incorporating nanomaterials is an option to enhance functional properties. This study aims to produce canola protein films by integrating GO exfoliated at several oxidation times and weight ratios to optimize mechanical, thermal, and barrier properties. Oxidation alters the C/O ratio and adds functional groups that bond with the amino/carboxyl groups of protein, enhancing the film properties. Significant improvement was obtained in GO at 60 and 120 min oxidation time and 3% addition level. Tensile strength and elastic modulus increased 200% and 481.72%, respectively, compared to control. Control films showed a 37.57 × 10−3 cm3m/m2/day/Pa oxygen permeability, and it was significantly reduced to 5.65 × 10−3 cm3m/m2/day/Pa representing a 665% reduction.

Comparison of three titanium-precursors for atomic-layer-deposited TiO2 for passivating contacts on silicon

Atomic layer-deposited (ALD) TiO2 thin films on silicon were deposited using titanium tetrachloride (TiCl4), titanium tetraisopropoxide (TTIP), and tetrakis(dimethylamino)titanium (TDMAT) together with water vapor as the oxidant at temperatures ranging between 75 and 250 °C. The Si surface passivation quality of as-deposited and isothermally annealed samples was compared using photoconductance lifetime measurements in order to calculate their effective surface recombination velocities Seff. A low Seff of 3.9 cm/s (J0s=24fA/cm2) is achieved for as-deposited TiCl4-TiO2 at 75 °C when a chemically grown (i.e., from RCA cleaning) SiOx interface layer is present. Depositing TTIP-TiO2 at 200 °C on a chemically grown SiOx interface layer yields equivalent Seff values; however, in this case, TTIP-TiO2 requires a 5–15 min postdeposition forming gas anneal at 250 °C. In contrast, TDMAT-TiO2 was not found to provide a similar level of passivation with/without a chemically grown SiOx interface layer and postdeposition anneal. Modeling of the effective lifetime curves was used to determine the magnitude of the effective charge densities Qf in the TiO2 films. In all cases, Qf was found to be of the order of ∼1011 q cm−2, meaning field-effect passivation arising from ALD TiO2 is relatively weak. By comparing the material properties of the various TiO2 films using ellipsometry, photothermal deflection spectroscopy, Raman spectroscopy, elastic recoil detection analysis, x-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy, we find experimental support for the role of Cl (in conjunction with hydrogen) playing a beneficial role in passivating dangling bond defects at the Si surface. It is concluded that low deposition temperature TiCl4 processes are advantageous, by providing the lowest Seff without any postanneal and a comparatively high growth per cycle (GPC).

Membrane‐based mechanical characterization of screen‐printed inks: Deflection analysis of ink layers on polyimide membranes

AbstractMeasurements of Young's modulus and residual stresses of screen‐printed ink layers using a bulge test on coated polyimide‐based membranes are proposed in this work. The applied bulge test monitors the deflection of membranes under pressure with interferometry. The obtained Young's modulus ranges from 6 to 8 GPa for a carbon blend‐based ink and is around 12 GPa for a silver nanoparticle ink. These values are compared with standard nanoindentation and show good agreement. Besides, the residual stresses range from −4 to 8 MPa for the carbon blend‐based ink, while the silver ink is measured around −10 MPa. The use of the membrane‐based method underlines the influence of exact deposition and curing conditions on the ink film material properties. The impact of the substrate on the ink layer properties, such as the thickness and its uniformity, is discussed, especially with regard to the heat treatment of the membrane.

Preparation and properties of transparent, waterproof, and antibacterial composite film materials

AbstractTransparent polymer materials, such as plastics, have become widely used in the packaging industry due to their excellent properties. However, their nondegradability has led to a significant environmental issue known as “white pollution.” To address this issue, a transparent antibacterial film material was prepared using carboxymethyl cellulose and carboxymethyl chitosan as raw materials through an acid‐assisted freeze–thaw strategy. The material exhibits optical transmittance exceeding 90% and a tensile strength of up to 124.1 MPa. Meanwhile, this material demonstrated excellent waterproof performance as well as antibacterial activity to Staphylococcus aureus and Escherichia coli. Therefore, this film material holds promising potential for development in antibacterial food packaging.

Influence of Active-to-Passive Ratio on the Deformation in Circular Dielectric Elastomer Actuators.

To further improve the performance of dielectric elastomer actuaotrs (DEAs), the development of novel elastomers with enhanced electro-mechanical properties is focal for the advancement of the technology. Hence, reliable techniques to assess their electro-mechanical performance are necessary. Characterization of the actuator materials is often achieved by fabricating circular DEAs with the pre-stretch of the membrane fixed by a stiff frame. Because of this set-up, the electrode size relative to the carrier frame's dimension has an impact on actuator strain and displacement. To allow for comparable results across different studies, the influence of this effect needs to be quantified and taken into account. This paper presents an in-depth study of the active-to-passive ratio by proposing two simplified analytical models for circular DEA and comparing them. The first model is taking the hyperelastic material properties of the dielectric film into account while the second model is a linear elastic lumped parameter model based on the electro-mechanical analogy. Both models lie in good agreement and show a significant linear impact of the radial active-to-passive ratio on the electro-active strain and a resulting maximum of displacement around 50% radial coverage ratio. These findings are validated by experiments with actuators fabricated using silicone membranes. It is shown that the electrode size is not only an important parameter in the experimental design, but in some cases of higher significance for the accuracy of analytical models than the hyperelastic properties of the material. Furthermore, it could be shown that a radial coverage ratio of around 50% is desirable when measuring displacement as it maximizes the displacement and lowers the impact of deviations in electrode sizes due to fabrication errors.

Bending of polymer films: a method for obtaining a compressive modulus of thin films.

Due to the advent of various foldable electric devices, it is becoming increasingly important to understand the bending properties of film materials. Bending of isotropic materials may be trivial if elastic deformation is small within a range of linear elasticity, which is often the case for bending. However, bending of polymer films, often used in recent foldable devices, may not be the case. Polymer films are frequently fabricated with stretching, which induces anisotropic orientation of molecular chains. In addition, there are many studies on bimodulus materials, which suggest the importance of the difference in tensile and compressive elastic moduli, i.e., the importance of elastic asymmetry, considering that bending involves compression and extension. In this study, we extended the standard linear elastic theory to include elastic anisotropy and elastic asymmetry and developed a method for obtaining compressive moduli of films, which cannot be obtained by a simple compression test because thin films under compression buckle at small strains. Our method is based on a bending test combined with a uniaxial tension test, which allows the measurement of Poisson's ratio in addition to Young's modulus, which are both anisotropic and asymmetric. To test our theory and method, we further performed experiments on biaxially stretched poly(ethylene terephthalate) (PET) films. As a result, we found non-negligible anisotropy in Poisson's ratio and non-negligible asymmetry (bimodulus) in tensile and compressive moduli. We further justify our framework by demonstrating a clear data collapse to show agreement between experiment and theory, clarifying limitations. Our results suggest the importance of elastic anisotropy and elastic asymmetry in bending of industrial films and give fundamental knowledge on this subject, which would be useful for applications. Such applications include the control of the position of the neutral plane and precise measurements of elastic moduli and Poisson's ratio, which are crucial, e.g., for the development of tough flexible electric devices and for the structural designs using compliant mechanisms.

Plasma-Enhanced Atomic Layer Deposition of Lithium Nickel Oxide Thin Film Model Systems

The main bottleneck in the development of the next generation of Li-ion batteries with higher energy densities, as demanded for long-range electrical mobility, is the lack of understanding and control of the reactions at the electrode-electrolyte interfaces. Reactions at the cathode side cause rapid capacity loss during cycling and represent a major issue for batteries with high-voltage cathodes, which are crucial in reaching the required energy densities. Promising candidates for use in next generation of batteries are Ni-rich Li(Ni1-x-yMnxCoy)O2 (NMC) cathodes, which show high capacity, high potential, and contribute to address the geopolitical issue of decreasing the Co content in the cathode. Ni-rich NMCs exhibit similar electrochemical properties as pure LiNiO2 and therefore LiNiO2 thin films can be used as model systems to gain fundamental understanding of the interfacial processes that hinder the large-scale implementation of Ni-rich NMCs. Atomic layer deposition (ALD) is a suitable technique to fabricate these model systems, as it gives precise control over film thickness, uniformity, chemical composition, and crystal orientation.Previous work on LiNiO2 by ALD is not extensively reported in literature. Moreover, films grown by a supercycle process, in which two ALD subcycles are alternated, suffered from a low Ni at.% and a large C content because of precursor ligand residues, and films fabricated using a complex multilayered approach showed severe Si contamination from the Li precursor [1]. In this contribution we report on ALD lithium nickel oxide (LNO) thin films with stoichiometries close to LiNiO2 and low levels of contaminants.The ALD process is based on lithium hexamethyldisilazide (LiHMDS) and nickel bis(N,N’-di-tert-butylacetamidinate) (Ni(tBu-MeAMD)2) as precursors and O2 plasma as co-reactant. 1 ALD cycle of Li2O is combined with x cycles of NiO using a supercycle approach. In this contribution we explore the relation between the Ni/Li cycle ratio x and material properties of the LNO films.The growth of the LNO films is monitored by means of in-situ spectroscopic ellipsometry. The ellipsometric data of films with a high Ni/Li cycle ratio are fitted with a general oscillator model used for NiO films, while a Cauchy model was adopted in case of a low Ni/Li cycle ratio and Li2O films. The growth per supercycle (GPSC) of LNO shows an intermixing effect, as it does not follow the linear trend calculated from the growth per cycle (GPC) values of the individual Li2O and NiO ALD processes, which are 1.0 Å/cycle and 0.34 Å/cycle respectively.XPS surface measurements show that the Ni/Li atomic ratio is tuned from 0 to 10 as function of the Ni/Li cycle ratio x. Low levels of Si and N contamination, below 4% and 2% respectively, are observed. A shift of the main peak in the Ni 2p3/2 spectrum to higher binding energies indicates a shift in oxidation state from Ni2+ to Ni3+ as decreases [2]. According to surface measurements, the LNO film with x=5 shows a stoichiometry close to LiNiO2, but depth profile measurements indicate that the Ni at.% is significantly higher in the bulk than on the surface of the films, most certainly attributed to the formation of a Li2CO3 surface layer upon air exposure. Li2CO3 is also present in the bulk of the LNO films, but the bulk C at.% stays below 4%.These results show that we can use ALD to fabricate a high-voltage cathode model system, which we plan to characterize in-situ and post-mortem in combination with both liquid and solid electrolytes to study the cathode-electrolyte interface.[1] Maximov et al., Energies, 13 (2020) 2345[2] Fu et al., Applied Surface Science 441 (2018) 1048 This work is part of the ‘BatteryNL – Next Generation Batteries based on Understanding Materials Interfaces’ project (with project number NWA.1389.20.089) of the NWA research programme ‘Research on Routes by Consortia (ORC)’ funded by the Dutch Research Council (NWO). Figure:(a) The GPSC of LNO ALD process shows an intermixing effect when comparing the experimental results with the expected GPSC based on the two processes (b) Ni 2p3/2 XPS spectrum for different Ni/Li cycle ratios x. A shift to higher binding energies is observed with decreasing x, indicating a shift from Ni2+ to Ni3+. (c) XPS surface measurements of LNO films show low levels of contaminants and a Ni/Li atomic ratio varying with the Ni/Li cycle ratio. (d) Depth profile of LNO with x=5 shows an increased Ni at.% and a low C at.% in the bulk of the film. Figure 1

Exploring the electronic properties and oxygen vacancy formation in SrTiO3 under strain

SrTiO3 (STO) thin films are widely used as substrates in oxide-based devices, and although STO is one of the most studied materials, both experimentally and computationally in its bulk form, the potential for strain engineering of STO thin-films is largely ignored. In this work, we perform Density Functional Theory (DFT) calculations to investigate the tuning of the structural and electronic properties of STO thin films under uni- and biaxial strain. We find that for TiO2-terminated STO slab, the band gap changes due to strain are more significant than that of the SrO-terminated surface because the band gap is determined by the TiO2 layers closer to the surface. Furthermore, we also find that the formation energy of oxygen vacancies depends on the type of strain applied and the position of oxygen vacancies. These findings demonstrate the large and hitherto underexplored perspectives of using strain engineering to modulate the structural and electronic properties of perovskite film materials for a wide variety of energy and electronic applications.

Nanostructured SnS-Si hybrid photodetectors by pulsed laser processed nanocolloids

This research investigates the fabrication of SnS-Si hybrids for photodetection and photocatalysis using nanocolloids of tin sulfide (SnS), monocrystalline silicon (Sim) and polycrystalline silicon (Sip) produced by laser-assisted techniques (pulsed laser fragmentation as well as ablation in liquid). Characterization of the nanocolloids and the hybrid thin films obtained by techniques such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron Spectroscopy (XPS), Raman spectroscopy, UV–Visible spectroscopy and photoluminescence spectroscopy (PL) are presented in this work. Post-deposition annealing improved the material properties of the hybrid films and the films exhibited optical bandgaps between 1.51 and 1.68 eV, suitable for light detection. In the case of hybrid SnS-Si films, SnS-Sim showed two orders of current higher than the SnS-Sip films. The photocurrent dynamic curves (current vs time) and current-voltage (I–V) characteristics are investigated to verify the improved device performances. The photodetector parameters such as sensitivity, responsivity, detectivity are also reported. Compared with conventional SnS thin film devices, all the films (SnS and SnS-Si hybrids) in this work responded to illumination sources of wavelengths up to 1064 nm. This is one of the initial studies on hybrid devices with Si nanoparticles for photodetector applications. All the films showed ultra-short response/recovery times in the range of 400–800 ms and their photodetector parameters were recorded. The films demonstrated good stability and cyclic photocatalytic activity to visible light.

Mechanochromism and Strain-Induced Crystallization in Thiol-yne-Derived Stereoelastomers.

Most elastomers undergo strain-induced crystallization (SIC) under tension; as individual chains are held rigidly in a fixed position by an applied strain, their alignment along the strain field results in a shift from strain-hardening (SH) to SIC. A similar degree of stretching is associated with the tension necessary to accelerate mechanically coupled, covalent chemical responses of mechanophores in overstretched chains, raising the possibility of an interplay between the macroscopic response of SIC and the molecular response of mechanophore activation. Here, thiol-yne-derived stereoelastomers doped covalently with a dipropiolate-derivatized spiropyran (SP) mechanophore (0.25-0.38mol%) are reported. The material properties of SP-containing films are consistent with undoped controls, indicating that the SP is a reporter of the mechanical state of the polymer. Uniaxial tensile tests reveal correlations between mechanochromism and SIC, which are strain-rate-dependent. When mechanochromic films are stretched slowly to the point of mechanophore activation, the covalently tethered mechanophore remains trapped in a force-activated state, even after the applied stress is removed. Mechanophore reversion kinetics correlate with the applied strain rate, resulting in highly tunable decoloration rates. Because these polymers are not covalently crosslinked, they are recyclable by melt-pressing into new films, increasing their potential range of strain-sensing, morphology-sensing, and shape-memory applications.