Revealing the glass transition in shape memory polymers using Brillouin spectroscopy

Mechano-Chemical Properties of Electron Beam Irradiated Polyetheretherketone

In this study, the mechano-chemical properties of aromatic polymer polyetheretherketone (PEEK) samples, irradiated by high energy electrons at 200 and 400 kGy doses, were investigated by Nanoindentation, Brillouin light scattering spectroscopy and Fourier-transform infrared spectroscopy (FTIR). Irradiating electrons penetrated down to a 5 mm depth inside the polymer, as shown numerically by the monte CArlo SImulation of electroN trajectory in sOlids (CASINO) method. The irradiation of PEEK samples at 200 kGy caused the enhancement of surface roughness by almost threefold. However, an increase in the irradiation dose to 400 kGy led to a decrease in the surface roughness of the sample. Most likely, this was due to the processes of erosion and melting of the sample surface induced by high dosage irradiation. It was found that electron irradiation led to a decrease of the elastic constant C₁₁, as well as a slight decrease in the sample’s hardness, while the Young’s elastic modulus decrease was more noticeable. An intrinsic bulk property of PEEK is less radiation resistance than at its surface. The proportionality constant of Young’s modulus to indentation hardness for the pristine and irradiated samples were 0.039 and 0.038, respectively. In addition, a quasi-linear relationship between hardness and Young’s modulus was observed. The degradation of the polymer’s mechanical properties was attributed to electron irradiation-induced processes involving scission of macromolecular chains.

Shape Memory Poly(β-hydroxythioether) Foams for Oil Remediation in Aquatic Environments

Shape memory poly(β-hydroxythioether) foams were produced using organobase catalyzed reactions between epoxide and thiol monomers, allowing for the rapid formation of porous media within approximately 5 min, confirmed using both rheology and physical foam blowing. The porous materials possess ultralow densities (0.022 g × cm^-3) and gel fractions of approximately 93%. Thermomechanical characterizations of the materials revealed glass transition temperatures tunable from approximately 50 to 100 °C, elastic moduli of approximately 2 kPa, and complete strain recovery upon heating of the sample above its glass transition temperature. The foams were characterized for their ability to take up oil from an aqueous multilayered ideal environment, revealing more than 2000% mass of oil (relative to the foam mass) could be collected. Importantly, while post-fabrication functionalization was possible with isocyanate chemistry followed by addition of hexadecanethiol or 3,3-bis(hexadecylthio)propan-1-ol, the oil collection efficiency of the system was not significantly enhanced, indicating that these materials, as porous media, possess unique attributes that make them appealing for environmental remediation without the need for costly modifications or manipulations.

Flash Brillouin Scattering: A Confocal Technique for Measuring Glass Transitions at High Scan Rates

Glass transition temperatures T _g are most commonly measured by differential scanning calorimetry, a method that has been extended to the flash scanning calorimetry (FSC) regime by reducing sample volumes. However, significant manual preparation effort can render FSC impractical for, e.g., local probing of spatially heterogeneous specimens. Another strategy can be to select a small volume by focusing down a laser beam, where Brillouin Light Scattering (BLS) is a proven method for confocal T _g measurement. Here, we introduce Flash Brillouin Scattering, which extends BLS to fast scan rates, achieved by periodically heating the probed region with an infrared laser. For comparison with conventional BLS, we first characterize T _g of pure glycerol, and show how rapid quenching produces a less packed glass with downshifted sound velocity. We then turn toward its aqueous solutions, which crystallize too fast for a nonflash approach, and demonstrate scan rates in excess of 10⁵ K/s. These results are of interest not only because glycerol is a model system for hydrogen-bonded glass formers, but also because of its applications as a cryoprotectant for frozen biological samples. Light scattering studies of the latter, currently limited to cryo-Raman spectroscopy, are likely to be complemented by the technique introduced here.

Thermoresponsive Shape Memory Fibers for Compression Garments

Their highly deformable properties make shape memory polymers (SMP) a promising component for the development of new compression garments. The shape memory effect (SME) can be observed when two polymers are combined. In here, polycaprolactone (PCL) and thermoplastic polyurethane (TPU) were melt spun in different arrangement types (blend, core-sheath, and island-in-sea), whereas the best SME was observed for the blend type. In order to trigger the SME, this yarn was stimulated at a temperature of 50 °C. It showed a strain fixation of 62%, a strain recovery of 99%, and a recovery stress of 2.7 MPa.

The Difference in Molecular Orientation and Interphase Structure of SiO2/Shape Memory Polyurethane in Original, Programmed and Recovered States during Shape Memory Process

In order to further understand the shape memory mechanism of a silicon dioxide/shape memory polyurethane (SiO₂/SMPU) composite, the thermodynamic properties and shape memory behaviors of prepared SiO₂/SMPU were characterized. Dynamic changes in the molecular orientation and interphase structures of SiO₂/SMPU during a shape memory cycle were then discussed according to the small angle X-ray scattering theory, Guinier’s law, Porod approximation, and fractal dimension theorem. In this paper, a dynamic mechanical analyzer (DMA) helped to determine the glass transition start temperature (T_g) by taking the onset point of the sigmoidal change in the storage modulus, while transition temperature (T_trans) was defined by the peak of tan δ, then the test and the calculated results indicated that the T_g of SiO₂/SMPU was 50.4 °C, and the T_trans of SiO₂/SMPU was 72.18 °C. SiO₂/SMPU showed good shape memory performance. The programmed SiO₂/SMPU showed quite obvious microphase separation and molecular orientation. Large-size sheets and long-period structures were formed in the programmed SiO₂/SMPU, which increases the electron density difference. Furthermore, some hard segments had been rearranged, and their gyration radii decreased. In addition, several defects formed at the interfaces of SiO₂/SMPU, which caused the generation of space charges, thus leading to local electron density fluctuations. The blurred interphase structure and the intermediate layer formed in the programmed SiO₂/SMPU and there was evident crystal damage and chemical bond breakage in the recovered SiO₂/SMPU. Finally, the original and recovered SiO₂/SMPU samples belong to the surface fractal system, but the programmed sample belongs to the mass fractal and reforms two-phase structures. This study provides an insight into the shape memory mechanism of the SiO₂/SMPU composite.

Selective Reactivity of Myrcene for Vat Photopolymerization 3D Printing and Postfabrication Surface Modification

Biosourced materials are gaining interest industrially, but there are still limitations on the library of available materials suitable for advanced manufacturing, especially using photopolymerization-based processing techniques. Terpenes, such as myrcene, are naturally produced materials possessing structural features, specifically alkenes, that avail themselves for such techniques. Free-radical and anionic polymerization techniques were used to explore molecular architecture, such as branching, as well as molecular weight and dispersity on physical properties prior to the production of 3D printing photopolymer resins. The polymyrcene resins were printed into dogbones and mold templates for soft materials. Model reactions with monofunctional thiols were used to demonstrate the potential for postpolymerization and fabrication functionalization, accompanying a physical demonstration where the surface hydrophobicity of polymyrcene could be tuned from superhydrophobic when using an alkyl chain monothiol (greater than 100° water contact angle) to a hydrophilic surface displaying a water contact angle of less than 45° compared with that of the unmodified surface (∼60°). Tunable bulk and surface properties are a unique feature for 3D printing materials and demonstrate the potential of polymyrcene and other biosourced photopolymers to a wide range of research applications.

A Brief Review of the Shape Memory Phenomena in Polymers and Their Typical Sensor Applications

In this brief review, an introduction of the underlying mechanisms for the shape memory effect (SME) and various shape memory phenomena in polymers is presented first. After that, a summary of typical applications in sensors based on either heating or wetting activated shape recovery using largely commercial engineering polymers, which are programmed by means of in-plane pre-deformation (load applied in the length/width direction) or out-of-plane pre-deformation (load applied in the thickness direction), is presented. As demonstrated by a number of examples, many low-cost engineering polymers are well suited to, for instance, anti-counterfeit and over-heating/wetting monitoring applications via visual sensation and/or tactual sensation, and many existing technologies and products (e.g., holography, 3D printing, nano-imprinting, electro-spinning, lenticular lens, Fresnel lens, QR/bar code, Moiré pattern, FRID, structural coloring, etc.) can be integrated with the shape memory feature.

Nonlinear Brillouin spectroscopy: what makes it a better tool for biological viscoelastic measurements

Brillouin spectroscopy is an emerging tool in biomedical imaging and sensing. It is capable of assessing the high-frequency viscoelastic longitudinal modulus with microscopic spatial resolution. Nonlinear Brillouin spectroscopy based on impulsive stimulated Brillouin scattering offers a number of significant advantages over conventional spontaneous and stimulated Brillouin scattering. In this report, we evaluate the accuracy of Brillouin shift measurements in spontaneous and nonlinear Brillouin microscopy by calculating the Allan variance for both CW excited spontaneous Brillouin measurements and nonlinear Brillouin scattering measurements made with both nanosecond and picosecond pulse excitation. We find that impulsive stimulated Brillouin spectroscopy is superior to spontaneous Brillouin spectroscopy in terms of the accuracy of such measurements and demonstrate its application for assessing tiny changes in Brillouin frequency shifts associated with low concentrations of biologically relevant solutions.

Highly Cross-Linked Shape Memory Polymers with Tunable Oxidative and Hydrolytic Degradation Rates and Selected Products Based on Succinic Acid

Minimally invasive medical devices are of great interest, with shape memory polymers (SMPs) representing one such possibility for producing these devices. Previous work with low density, highly porous SMPs has demonstrated oxidative degradation, while attempts to incorporate hydrolytic degradation have resulted in rapidly decreasing glass transition temperature (T _g ), ultimately preventing strain fixity of the materials at clinically relevant temperatures. Through esterification of the amino alcohol triethanolamine, an alcohol containing network was synthesized and incorporated into SMPs. These ester networks were used to control the bulk morphology of the SMP, with the T _g remaining above 37 °C when 50% of the alcohol was contributed by the ester network. This methodology also yielded SMPs that could degrade through both hydrolysis and oxidation; by oxidation, the SMPs degrade at a similar rate as the control materials (0.2%/day mass) for the first 30 days, at which point the rate changes to 3.5%/day until the samples become too fragile to examine at 80 days. By comparison, control materials have lost approximately 30% of mass by 140 days, at a constant rate of degradation, demonstrating that the ester SMPs are a promising material system for producing more rapidly degradable, soft, porous biomaterials.