Influence of conformational change of chain unit on the intrinsic negative thermal expansion of polymers

Relaxation Model of the Relations between the Elastic Modulus and Thermal Expansivity of Thermosetting Polymers and FRPs

This research was completed in the development of studies devoted to relations between the elastic modulus (MoE) and thermal expansivity (CTe) of different materials. This study, based on experimental data, confirmed the models of the relations between MoE and CTe under normal and heating temperatures for thermosetting epoxy polymers and glass-fiber FRPs in two variants (unfilled and filled by mineral additives), after the usual glassing and prolonged thermal conditioning (thermo-relaxation). The experiment was based on dilatometric and elastic deformation testing. Two models of MoE/CTe were tested: Barker's model and our authors relaxation model (MoE = f(CTe)), which is based on previous modelling of the non-linearity of the physical properties of polymers' supramolecular structures. The result show that the models' constants depend on composition; Barker's model is applicable only to polymers with satisfying agreement degrees in the range 10-20%; our model is applicable to polymers and FRPs with satisfying agreement degrees in the range of 6-18%.

Diverse Thermal Expansion Behaviors in Ferromagnetic Cr1-δTe with NiAs-Type, Defective Structures

Negative thermal expansion (NTE) and zero thermal expansion (ZTE) properties are of great significance for the long-life stable operation of precision equipment. However, there are still existing challenges in finding new materials that exhibit NTE or ZTE over a wide temperature range. Here, we report negative, zero, and positive thermal expansion in NiAs-type, defective Cr_1-δTe, containing three compounds: hexagonal CrTe, monoclinic Cr₃Te₄, and trigonal Cr₅Te₈. CrTe shows the NTE behavior from 280 to 340 K with the volume coefficient of thermal expansion α_V = -27.6 × 10^-6 K^-1. Cr₃Te₄ shows the ZTE behavior over a wide temperature range of 180-320 K (α_V = 0.16 × 10^-6 K^-1). And Cr₅Te₈ holds the PTE behavior over the whole temperature range (α_V = 38.5 × 10^-6 K^-1). All of the samples show obvious anisotropic thermal expansion on heating. Combined with the magnetic measurements, it can be confirmed that the NTE and ZTE properties in ferromagnetic Cr_1-δTe originate from the magnetovolume effect (MVE). Such NiAs-type, defective compounds with similar compositions but different structures provide a new perspective for tuning the NTE properties of materials and searching for new materials with ZTE over a wide temperature range.

Directional, Low-Energy Driven Thermal Actuating Bilayer Enabled by Coordinated Submolecular Switching

The authors reveal a thermal actuating bilayer that undergoes reversible deformation in response to low-energy thermal stimuli, for example, a few degrees of temperature increase. It is made of an aligned carbon nanotube (CNT) sheet covalently connected to a polymer layer in which dibenzocycloocta-1,5-diene (DBCOD) actuating units are oriented parallel to CNTs. Upon exposure to low-energy thermal stimulation, coordinated submolecular-level conformational changes of DBCODs result in macroscopic thermal contraction. This unique thermal contraction offers distinct advantages. It's inherently fast, repeatable, low-energy driven, and medium independent. The covalent interface and reversible nature of the conformational change bestow this bilayer with excellent repeatability, up to at least 70 000 cycles. Unlike conventional CNT bilayer systems, this system can achieve high precision actuation readily and can be scaled down to nanoscale. A new platform made of poly(vinylidene fluoride) (PVDF) in tandem with the bilayer can harvest low-grade thermal energy and convert it into electricity. The platform produces 86 times greater energy than PVDF alone upon exposure to 6 °C thermal fluctuations above room temperature. This platform provides a pathway to low-grade thermal energy harvesting. It also enables low-energy driven thermal artificial robotics, ultrasensitive thermal sensors, and remote controlled near infrared (NIR) driven actuators.

All-carbocycle hydrocarbon thermosets with high thermal stability and robust mechanical strength for low-k interlayer dielectrics

Two kinds of hydrocarbon precursors were synthesized and cured at elevated temperatures to give cross-linked all-aliphatic/aromatic-ring polymers with a low dielectric constant for next-generation interlayer dielectrics.