Relaxation based modeling of tunable shape recovery kinetics observed under isothermal conditions for amorphous shape-memory polymers

Ultra-Tough Waterborne Polyurethane-Based Graft-Copolymerized Piezoresistive Composite Designed for Rehabilitation Training Monitoring Pressure Sensors

Effective training is crucial for patients who need rehabilitation for achieving optimal recovery and reducing complications. Herein, a wireless rehabilitation training monitoring band with a highly sensitive pressure sensor is proposed and designed. It utilizes polyaniline@waterborne polyurethane (PANI@WPU) as a piezoresistive composite material, which is prepared via the in situ grafting polymerization of PANI on the WPU surface. WPU is designed and synthesized with tunable glass transition temperatures ranging from -60 to 0 °C. Dipentaerythritol (Di-PE) and ureidopyrimidinone (UPy) groups are introduced, endowing the material with good tensile strength (14.2 MPa), toughness (62 MJ-1 m-3 ), and great elasticity (low permanent deformation: 2%). Di-PE and UPy enhance the mechanical properties of WPU by increasing the cross-linking density and crystallinity. Combining the toughness of WPU and the high-density microstructure derived by hot embossing technology, the pressure sensor exhibits high sensitivity (168.1 kPa-1 ), fast response time (32 ms), and excellent stability (10 000 cycles with 3.5% decay). In addition, the rehabilitation training monitoring band is equipped with a wireless Bluetooth module, which can be easily applied to monitor the rehabilitation training effect of patients using an applet. Therefore, this work has the potential to significantly broaden the application of WPU-based pressure sensors for rehabilitation monitoring.

Stress Relaxation Behavior of Poly(Methyl Methacrylate)/Graphene Composites: Ultraviolet Irradiation

The graphene/poly (methyl methacrylate) (PMMA) composites are a promising candidate for electronic, optoelectrical, and environmental applications. Understanding the mechanical degradation of PMMA-based materials is of practical importance in improving the reliability and lifespan of the associated structures and systems. In this study, we investigate the effects of functionalized graphene (FG) and UV irradiation on the stress-relaxation of PMMA. Uniaxial tensile and stress -relaxation tests are performed to evaluate the mechanical properties of the composites. The mechanical strength and elongation at the break increase with the graphene concentration but decrease with the increase of the irradiation dose. Raman spectroscopy and intrinsic viscosity measurement are applied to examine the root cause of the degradation in the composites. UV irradiation leads to polymer chain scission and loss of molecular weight. The Kelvin representation of the standard linear solid model (SLSM) is used to describe the stress-relaxation curves of the composites. The value of the elastic modulus in the Kelvin element decreases with the increase in temperature. The viscosity follows the Arrhenius equation. The activation energy of viscosity increases with the increasing FGs concentration because the FGs hinder the chain motion of PMMA. However, UV irradiation makes chain scission of PMMA/FGs composite so that the polymer chain moves more easily and the activation energy of stress relaxation lowers. The steady-state stress follows the van 't Hoff equation that stress relaxation is an exothermal deformation process. Although Maxwell's representation of SLSM is mathematically identical to the Kelvin representation of SLSM, the former cannot interpret the stress-relaxation behavior of PMMA/FGs composite, which is against the concept of Young's modulus as a decreasing temperature function.

Effect of sorbitan monopalmitate on the polymorphic transitions and physicochemical properties of mango butter

The present study reports the effect of sorbitan monopalmitate (SM) as a crystallization modifier on the physicochemical properties of mango butter (MB). The concentration of SM was varied in the range of 1 and 5 wt%. The addition of SM promoted the aggregation of globular MB crystals. The FTIR patterns did not show any significant changes when SM was added. XRD and DSC analyses confirmed the crystallization of MB crystals in stable β' and β (V) polymorphic states. However, SM also introduced imperfections in the crystal lattices of MB. Among all formulations, M2 (SM; 1% w/w) possessed a mechanically stable network structure. The crystallization rate of MB was tailored by SM in a concentration-dependent manner. The solid content was highest in M4 (SM; 5% w/w) at 10 °C and 30 °C among all the oleogels. In gist, SM in manageable quantities can be utilized for preparing custom-tailored MB-based products.

Tunable Diffractive Optical Elements Based on Shape-Memory Polymers Fabricated via Hot Embossing

We introduce actively tunable diffractive optical elements fabricated from shape-memory polymers (SMPs). By utilizing the shape-memory effect of the polymer, at least one crucial attribute of the diffractive optical element (DOE) is tunable and adjustable subsequent to the completed fabrication process. A thermoplastic, transparent, thermoresponsive polyurethane SMP was structured with diverse diffractive microstructures via hot embossing. The tunability was enabled by programming a second, temporary shape into the diffractive optical element by mechanical deformation, either by stretching or a second embossing cycle at low temperatures. Upon exposure to the stimulus heat, the structures change continuously and controllable in a predefined way. We establish the novel concept of shape-memory diffractive optical elements by illustrating their capabilities, with regard to tunability, by displaying the morphing diffractive pattern of a height tunable and a period tunable structure, respectively. A sample where an arbitrary structure is transformed to a second, disparate one is illustrated as well. To prove the applicability of our tunable shape-memory diffractive optical elements, we verified their long-term stability and demonstrated the precise adjustability with a detailed analysis of the recovery dynamics, in terms of temperature dependence and spatially resolved, time-dependent recovery.

Modeling Based Characterization of Thermorheological Properties of Polyurethane ESTANE™

Shape-Memory Polymers (SMPs) have the ability to be deformed and memorize this deformation until an external activation stimulus (e.g., heat) is applied. Therefore, they have attracted great interest in many areas, especially for applications where reconfigurable structures are required (e.g., Shape-Memory (SM) stents or micro air vehicles). Nevertheless, prior to technical application, the effective thermomechanical behavior of SMPs must be thoroughly understood. In the current contribution, an assessment of thermorheological properties of the commercially available polyurethane system ESTANE is presented. Thermorheological properties were investigated using Dynamic Mechanical Thermal Analysis (DMTA) and complementary uniaxial stress relaxation experiments. Upon material parameter optimization, a finite viscoelastic and incompressible material model was used to model experimentally observed viscoelastic properties.

AO-resistant shape memory polyimide/silica composites with excellent thermal stability and mechanical properties

Shape memory polyimide/silica composite films show AO-resistant performance, good thermal stability and mechanical properties.

Memory-effects of magnetic nanocomposites

The thermally induced shape memory effect (SME) is the capability of a material to fix a temporary (deformed) shape and recover a 'memorized' permanent shape in response to heat. SMEs in polymers have enabled a variety of applications including deployable space structures, biomedical devices, adaptive optical devices, smart dry adhesives and fasteners. By the incorporation of magnetic nanoparticles (mNP) into shape-memory polymer (SMP), a magnetically controlled SME has been realized. Magnetic actuation of nanocomposites enables remotely controlled devices based on SMP, which might be useful in medical technology, e.g. remotely controlled catheters or drug delivery systems. Here, an overview of the recent advances in the field of magnetic actuation of SMP is presented. Special emphasis is given on the magnetically controlled recovery of SMP with one switching temperature T(sw) (dual-shape effect) or with two T(sw)s (triple-shape effect). The use of magnetic field to change the apparent switching temperature (T(sw,app)) of the dual or triple-shape nanocomposites is described. Finally, the capability of magnetic nanocomposites to remember the magnetic field strength (H) initially used to deform the sample (magnetic-memory effect) is addressed. The distinguished advantages of magnetic heating over conventional heating methods make these multifunctional nanocomposites attractive candidates for in vivo applications.