Polymeric triple-shape materials

Abstract

Shape-memory polymers represent a promising class of materials that can move from one shape to another in response to a stimulus such as heat. Thus far, these systems are dual-shape materials. Here, we report a triple-shape polymer able to change from a first shape (A) to a second shape (B) and from there to a third shape (C). Shapes B and C are recalled by subsequent temperature increases. Whereas shapes A and B are fixed by physical cross-links, shape C is defined by covalent cross-links established during network formation. The triple-shape effect is a general concept that requires the application of a two-step programming process to suitable polymers and can be realized for various polymer networks whose molecular structure allows formation of at least two separated domains providing pronounced physical cross-links. These domains can act as the switches, which are used in the two-step programming process for temporarily fixing shapes A and B. It is demonstrated that different combinations of shapes A and B for a polymer network in a given shape C can be obtained by adjusting specific parameters of the programming process. Dual-shape materials have already found various applications. However, as later discussed and illustrated by two examples, the ability to induce two shape changes that are not limited to be unidirectional rather than one could potentially offer unique opportunities, such as in medical devices or fasteners.

Fig. 1.

Polymer network architecture. (a) MACL network. (b) CLEG network. Color coding is as follows: green, PCHMA segments; red, PCL segments; blue, PEG side chains; gray, cross-links.

Fig. 2.

Thermograms for the second heating run of DSC at a heating rate of 1 K·min⁻¹. Thermograms: I, homonetwork from PCLDMA, CL(100); II, CL(60)EG; III, CL(30)EG; IV, homopolymer from PEG monomethylether-monomethacrylate, graft-EG.

Fig. 3.

Cyclic, thermomechanical experiments. (a) MACL (45) (fifth cycle) as a function of time. The solid line indicates strain; the dashed line indicates temperature. The variables are explained in Materials and Methods. (b) εT diagram showing the recovery of shapes B and C in cyclic, thermomechanical experiments (third cycle) for CL(40)EG for different combinations of ε_B⁰ and ε_A⁰: solid line, ε_B⁰ = 50% and ε_A⁰ = 100%; dashed line, ε_B⁰ = 30% and ε_A⁰ = 100%; dotted line, ε_B⁰ = 50% and ε_A⁰ = 120%.

Fig. 4.

Cyclic, thermomechanical experiments. (a) Recovery curves (third cycle) for CL(70)EG (solid line) and CL(100) (dashed line) after application of a triple-shape programming process with ε_B⁰ = 50% and ε_A⁰ = 100%. (b) Strain (black) and temperature (red) as a function of time for CL(100) illustrating the effect of cold drawing of PCL segments on the recovery properties.

Fig. 5.

Series of photographs illustrating the triple-shape effect. Two different demonstration objects prepared from CL(50)EG: tube (a); fastener consisting of a plate with anchors (b). The picture series show the recovery of shapes B and C by subsequent heating to 40°C and 60°C, beginning from shape A, which was obtained as a result of the two-step programming process.