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 X-ray investigations. Potential applications of TMPAs include heat engines with adjustable rotation rate and active building facades with self-regulating sun protectors.

Keywords: active movement; reversible shape-memory polymer.

Fig. 1.

Working principle of the programmable temperature-memory polymer actuator. (A) Programming: Amorphous sample is deformed at T_prog (Right); ● chemical cross-links. At T_sep (Center) appearance is determined by the directed crystallization of the internal skeleton-forming domains (red). Actuation: reversible shape changes are realized in the polymer by crystallization/melting of oriented polyethylene segments in the actuation domains (green) between T_low and T_sep (Left). (B) Thermal and thermomechanical investigations of cPEVA. DSC apparatus specific transient signal oscillation at the beginning and at the end of temperature ramps not shown. (Top) The T_sep divides crystallizable domains in geometry determining and actuation domains. (Middle) Thermogram of first actuation cycle. A melting peak with a peak maximum, which is significantly lower compared with the peak maximum obtained when the sample was completely molten (Top), can be observed. (Bottom) Elongation as function of temperature, first reversible actuation cycle for T_prog = 90 °C, ε_ssp = 150%, T_low = 25 °C, T_sep = 75 °C (shapes A, B, black line). Shape A is obtained as ε_A at T_sep. Cooling to T_low results in shape B corresponding to ε_B. Heating to T_sep recovers ε_A again. (C) Photo series illustrating the temperature-memory actuation capability of a cPEVA ribbon (80 × 20 × 0.9 mm), which was inked at its edges with blue color to enhance contrast. A concertina appearance was created by folding at T_prog = 90 °C, cooling to T_low = 25 °C, and heating to T_sep, which was varied. The concertina shifted reversibly between an expanded concertina (shape A) and a contracted concertina (shape B).

Fig. 4.

Demonstration of the programmable actuation capability of cPEVA. Parameters of experiment: T_prog = 90 °C, T_sep = 75 °C, T_low = 25 °C. (A) (Left) Schematic illustration of the related shape change in a cPEVA based demonstrator providing programmable window shades. (Right) After programming the window shades to close upon heating and open when cooled. (B) Heat engine driven by a concertina-shaped cPEVA drive element, which moves an attached toothed rack forward when heated to T_sep and back when cooled to T_low, whereby its contact pressure to the tooth wheel is controlled by a second cPEVA concertina. Upon cooling to T_low, this concertina contracts resulting in a lower pressure on the rack enabling the driving element to contract as well. The number of folds in the driving element determines the distance of the forward motion. In this way the rotation speed of the tooth wheel can be adjusted by the programming of the driving element. The numbers indicate the cycle number of the actuation cycles.

Fig. 2.

Quantification of the shape-shifting capability of cPEVA in cyclic, thermomechanical tensile tests. Parameters of experiments: T_prog = 90 °C, T_low = 25 °C. (A) Influence of the vinyl acetate (VA) content on the relative reversible elongation ε′_rev as function of time. A higher VA content broadens the range in which T_sep can be varied. (Upper) cPEVA10d20, (Lower) cPEVA20d20. (B) Correlation between T_sep and the actuation temperatures upon cooling T_act(A→B) and heating T_act(B→A) in actuation cycles. [T_act(A→B): filled squares, T_act(B→A): open circles]. (Left) cPEVA10d20, (Right) cPEVA20d20. (C) Long-term study of actuation cycles of cPEVA20d20 applying T_sep = 75 °C with 120 cycles with ε_ssp = 100% and 130 cycles with ε_ssp = 150%, ε versus cycle number plot, Insets show bidirectional actuation at after various cycle numbers. The variation of temperature is shown only in lower insets to enhance readability (ε: black line, T: red line).

Fig. 3.

Structural changes occurring during the bidirectional actuation of a programmed cPEVA20d20 ribbon. (A and B) Changes of the scattering pattern determined by 2D WAXS (A) and 2D SAXS (B) recorded for shapes A and B as well as for shapes A′ and B′ after (re)programming and in subsequent reversible actuation cycles. (T_sep = 75 °C, T_low = 25 °C, upper series ε_ssp = 150%, and lower series ε_ssp = 100%). Numbers indicate steps during experiment. (C) Changes of long periods schematically shown for cPEVA during bidirectional actuation.