Tunable Multiple-Shape Memory Polyethylene Blends

Heating Rate Sensitive Polyethylene Terephthalate

Smart materials react to external triggers by changing size, color, mechanical properties, or permeability. The next generation of smart materials will be able to not only recognize and react to external triggers but also to their dynamics. The only existing example of such a material is heating rate-sensitive polymorphous cross-linked syndiotactic polypropylene. This study presents a new principle of a heating rate-sensitive material on the example of cross-linked and fully amorphous quenchable semi-crystalline polyethylene terephthalate (x-PET). The x-PET is stretched to high elongation above its melting temperature and constrained quenched to a fully amorphous state. Then the polymer is heated to 120-170 °C with different heating rates. Due to its heating-rate sensitivity, x-PET shrinks to different stabilized lengths dependent on the heating rate. The new length can be used to read out the heating rate and to specifically answer to this by mechanically switching a process. Detailed analytics of this process reveal that amorphous stretched x-PET is starting the retraction above Tg and simultaneously stopping it by crystallization. The different rates of these processes result in the heating rate sensitivity of x-PET.

Thermo-Responsive Shape Memory Behavior of Methyl Vinyl Silicone Rubber/Olefin Block Copolymer Blends via Co-Crosslinking

Shape memory polymers (SMPs) are developed by blending and cross-linking polymers which include crystalline domains and cross-linked networks. In this paper, we describe the morphology, thermal and shape memory behavior of methyl vinyl silicone rubber (MVMQ)/olefin block copolymer (OBC) blends prepared by a melt-blending and chemical cross-linking method. MVMQ without crystalline domains could not hold its temporary shape. After introducing the OBC, the obtained blends exhibited excellent dual shape memory properties. The cross-linking networks of MVMQ acted as reversible domains, while crystalline regions of OBC worked as fixed domains. When the blending ratio of MVMQ/OBC was 50/ 50, the blend had both a high shape fixity ratio and shape recovery ratio.

A Review of Shape Memory Polymers and Composites: Mechanisms, Materials, and Applications

Over the past decades, interest in shape memory polymers (SMPs) has persisted, and immense efforts have been dedicated to developing SMPs and their multifunctional composites. As a class of stimuli-responsive polymers, SMPs can return to their initial shape from a programmed temporary shape under external stimuli, such as light, heat, magnetism, and electricity. The introduction of functional materials and nanostructures results in shape memory polymer composites (SMPCs) with large recoverable deformation, enhanced mechanical properties, and controllable remote actuation. Because of these unique features, SMPCs have a broad application prospect in many fields covering aerospace engineering, biomedical devices, flexible electronics, soft robotics, shape memory arrays, and 4D printing. Herein, a comprehensive analysis of the shape recovery mechanisms, multifunctionality, applications, and recent advances in SMPs and SMPCs is presented. Specifically, the combination of functional, reversible, multiple, and controllable shape recovery processes is discussed. Further, established products from such materials are highlighted. Finally, potential directions for the future advancement of SMPs are proposed.

Polyethylenes functionalized with ureidopyrimidone: synthesis, thermomechanical properties and shape memory behavior

In this contribution, we reported an approach to functionalize polyethylene with quadruple hydrogen bonds.

Shape Memory Behavior of Natural Eucommia ulmoides Gum and Low-Density Polyethylene Blends with Two Response Temperatures

A series of shape memory blends of natural Eucommia ulmoides gum (EUG) and low-density polyethylene (LDPE) with a bicontinuous cross-linked structure were prepared by a physical blending method, which could be used in the field of thermal response with two different temperatures. We report the shape memory properties of these blended materials with two response temperatures for the first time. The mechanical, curing, thermal and shape memory properties of the blends were studied in this manuscript. Schematic diagrams are proposed to illustrate the dual shape memory behaviors of the EUG/LDPE blends. Our study focused on observing the relationship between the shape memory behavior and the microscopic crystalline phase states in the blends. In the blends, both the cross-linked network and the LDPE crystalline regions could act as fixed domains, while the crystalline regions of LDPE or EUG could act as the reversible domain. The shape memory properties were mainly determined by the components of the fixed and reversible domains. We focused on the shape memory behavior of blends at 60 °C and 130 °C in this manuscript. The results showed that when the peroxide dicumyl peroxide (DCP) dosage was 1.0 phr, the blends exhibited acceptable shape behavior at 60 °C (R_1f = 74.8%, R_1r = 63.3%). At the same time, when DCP dosage was 0.4 phr, the shape memory behavior of the blends at 130 °C was good and much better than that at 60 °C (R_2f = 91.1%, R_2r = 89.4%).

Stereocomplexed and Homochiral Polyurethane Elastomers with Tunable Crystallizability and Multishape Memory Effects

Design of the polymer networks with tunable mechanical properties and multishape memory effects (multi-SMEs) is highly desired in the engineering applications. Herein, we report on the stereocomplexed and homochiral polyurethane (PU) elastomers with tunable multi-SMEs by cross-linking the triblock prepolymers bearing the poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) enantiomeric segments. The homochiral PU is nearly amorphous, yet the stereocomplexed PU becomes highly crystalline due to the stereocomplexation of enantiomeric segments. Moreover, the two distinct thermal (glass, melting) transitions of PLLA (or PDLA) segments in PUs are integrated to realize the thermally induced triple- and quadruple-SMEs. Control over the enantiomeric segmental ratios allows the feasible manipulation of crystallizability, mechanical and thermal properties, and multi-SMEs of PUs.

Core-shell nano-latex blending method to prepare multi-shape memory polymers

Multi-shape memory polymers were prepared by blending a series of styrene (St)-block-(styrene-random-meth acrylate (MA))-block-styrene triblock copolymer nano-latexes. These latexes synthesized using a reversible addition-fragmentation chain transfer (RAFT) emulsion polymerization method have a core-shell structure. The cores are formed by the middle poly(St-random-MA) blocks. They act as transition phases with variable transition temperatures via adjusting the St/MA ratio. When the latexes are blended with an identical PSt shell but different poly(St-random-MA) cores, the shells play a role in preventing the aggregation of these poly(St-random-MA) cores forming a crosslinked network after hot-press treatment. Therefore a polymer with well-distributed multiple nanophases is achieved, which shows a quadruple-shape memory behavior. Furthermore, the shape memory and recovery performance at a certain temperature can be improved on purpose by increasing the mass ratio of the corresponding transition phases, which can be realized via simply varying the blending ratio of different latexes. An optimized multi-shape memory polymer with the shape memory and recovery ratio higher than 80% at all the transition temperatures is achieved.

Programming of One- and Two-Step Stress Recovery in a Poly(ester urethane)

This work demonstrates that phase-segregated poly(ester urethane) (PEU) with switching segments of crystallizable poly(1,4-butylene adipate) (PBA) can be programmed to generate two separate stress recovery events upon heating under constant strain conditions. For programming, two elongations are applied at different temperatures, followed by unloading and cooling. During the adjacent heating, two-step stress recovery is triggered. The results indicate that the magnitude of the stress recovery signals corresponds to the recovery of the two deformation stresses in reverse order. As demonstrated by further experiments, twofold stress recovery can be detected as long as the elongation at higher temperature exceeds the strain level of the deformation at lower temperature. Another finding includes that varying the lower deformation temperature enables a control over the stress recovery temperature and thus the implementation of so-called "temperature-memory effects". Moreover, exerting only one elongation during programming enables a heating-initiated one-step stress recovery close to the deformation temperature. Based on these findings, such polymers may offer new technological opportunities in the fields of active assembly when used as fastening elements and in functional clothing when utilized for compression stockings.

Multi-shape memory effect of polyimides with extremely high strain

Polyimides exhibited high strain and multi-shape memory properties with the synergistic effects of physical and chemical crosslinks.

Heating Rate Sensitive Multi-Shape Memory Polypropylene: A Predictive Material

Here we report on a novel type of smart material that is capable of specifically responding to the changing rate of an environmental signal. This is shown on the example of lightly cross-linked syndiotactic polypropylene that reacts to a temperature increase by adapting its shape change according to the applied heating rate. In general, a material with such properties can be used to predict a system failure when used in a defined environment and is therefore called "predictive material".

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.

Shape-Memory PVDF Exhibiting Switchable Piezoelectricity

In this study, a material is designed which combines the properties of shape-memory and electroactive polymers. This is achieved by covalent cross-linking of polyvinylidene fluoride. The resulting polymer network exhibits excellent shape-memory properties with a storable strain of 200%, and fixity as well as recovery values of 100%. Programming upon rolling induces the transformation from the nonelectroactive α-phase to the piezoelectric β-phase. The highest β-phase content is found to be 83% for a programming strain of 200% affording a d33 value of -30 pm V(-1). This is in good accordance with literature known values for piezoelectric properties. Thermal triggering this material does not only result in a shape change but also renders the material nonelectroactive.

Programming of shape memory natural rubber for near-discrete shape transitions

Typical shape memory polymers are hot-programmed and show a shape transition over a broad temperature range of 10 K and more. Cold-programmed shape memory natural rubber (SMNR) recovers more than 80% of its original shape within 1 K. The trigger point can be increased upon aging the stretched SMNR over several weeks without losing the narrow trigger range. This process can be accelerated by treatment of the stretched SMNR with nonaffine solvent vapors. Affine solvent vapors of low concentrations afford higher trigger points than that achieved by aging. This way, even higher cross-linked natural rubber can be cold-programmed.

Dual-shape memory effect in radiation crosslinked thermoplastic blends: fabrication, optimization and mechanisms

Recently, as an important class of mechanically active smart materials, thermoplastic dual-shape memory polymers (SMPs) have attracted notable attention and can be fabricated in many different manufacturing techniques.

Environmental memory of polymer networks under stress

Generally reversible stimuli-responsive materials do not memorize the stimulus. In this study we describe an example in which stretched and constrained semi-crystalline polymer networks respond to solvent gases with stress and simultaneously memorize the concentration and the chemical nature of the solvent itself in their microstructure. This written solvent signature can even be deleted by temperature.