Temperature-memory polymer actuators

Abstract

Reading out the temperature-memory of polymers, which is their ability to remember the temperature where they were deformed recently, is thus far unavoidably linked to erasing this memory effect. Here temperature-memory polymer actuators (TMPAs) based on cross-linked copolymer networks exhibiting a broad melting temperature range (ΔT m ) are presented, which are capable of a long-term temperature-memory enabling more than 250 cyclic thermally controlled actuations with almost constant performance. The characteristic actuation temperatures T act s of TMPAs can be adjusted by a purely physical process, guiding a directed crystallization in a temperature range of up to 40°C by variation of the parameter T sep in a nearly linear correlation. The temperature T sep divides ΔT m into an upper T m range (T > T sep ) forming a reshapeable actuation geometry that determines the skeleton and a lower T m range (T < T sep ) that enables the temperature-controlled bidirectional actuation by crystallization-induced elongation and melting-induced contraction. The macroscopic bidirectional shape changes in TMPAs could be correlated with changes in the nanostructure of the crystallizable domains as a result of in situ Xray investigations. Potential applications of TMPAs include heat engines with adjustable rotation rate and active building facades with self-regulating sun protectors.reversible shape-memory polymer | active movement T he alignment and coupling of thermally controlled volume changes on the nanoscale has emerged as most important working principle to translate shape changes from the nanolevel to the macrolevel in polymers (1-4). In stimuli-responsive polymers capable of a free-standing shape-changing effect, this alignment is achieved during synthesis or processing by either application of external stress or the utilization of templates and fixed by covalent cross-links (5-12). Once synthesis is completed, the geometry of the shape change cannot be changed anymore (13-15) and the actuation temperature is fixed; this relies on thermal transitions with a defined temperature. Here we explored whether it is possible to implement a thermally controlled bidirectional actuation into free-standing polymers by purely physical manipulation enabling to adjust (repeatedly) the actuation temperature and (shape changing) geometry.Although programmable shape changes have been realized in shape-memory polymers (SMPs), this effect is generally a onetime, one-way effect in free-standing SMPs (16-18). In SMPs the switching domains, which can solidify by crystallization or vitrification, provide two functions: they determine the geometry of the shape change and cause the entropy elastic recovery. A reversible movement could be observed when polymers with crystallizable segments are held under an externally applied constant stress (3,8). Recently temperature-memory polymers (TMPs) enabled the programming of the switching temperature (19,20). Also this temperature-memory effect (TME) is limited to a onetime, one-way effect. The aim of th...

Shape-memory behavior of cross-linked semi-crystalline polymers and their blends

“…So, the thermally induced reversible bending of active asymmetric layered polymer composites-laminates reported by Basit et al [28] and Imai [29] occurs similarly to the well-known bimetallic strip effect in unconstrained as well as constrained state and under load [28,29]. Westbrook et al [30] described the reversible deformation of free-standing cross-linked polymer system in bending mode, while Behl and coworkers [31,32] as well as Wu et al. [33] reported on reversible deformation of freestanding cross-linked polymer systems in tensile mode caused by internal stresses stored at least in one semi-crystalline phase of the polymer system during its preparation.…”

“…Some current papers [28][29][30][31][32][33] containing results on two-way SME of layered polymer composites consider two other types of thermally induced two-way SME based on quite different physical principles as described above. So, the thermally induced reversible bending of active asymmetric layered polymer composites-laminates reported by Basit et al [28] and Imai [29] occurs similarly to the well-known bimetallic strip effect in unconstrained as well as constrained state and under load [28,29].…”

Biodegradable polyester-based shape memory polymers: Concepts of (supra)molecular architecturing

“…Recently, another strategy was proposed to produce 2W-SM polymers. Behl et al [68] suggested that creation of a special molecular conetwork (skeleton) containing reversible crystallizing/melting segments can overtake the job of external stress. Important prerequisites of this strategy are: presence of nanoscaled crystalline domains which are kept aligned by the skeleton network (thereby overtaking the role of external loading) and a broad melting range of the related crystallites.…”

An Overview of Novel Actuators for Soft Robotics

“…According to the same study, when immersed in water, a two-ply, coiled, silver-plated 180 µm diameter nylon fiber can be electro-thermally actuated at 5 Hz while producing 10% stroke and lifting 22 MPa load. In Reference [63], the focus was to program the SMPs for a sustainable and repeatable operation of bidirectional actuation. They have achieved at least 250 cyclic thermally controlled actuations with almost constant performance.…”