Ingo Bellin

Polymeric triple-shape materials

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.

Dual-shape properties of triple-shape polymer networks with crystallizable network segments and grafted side chains

Triple-shape materials have recently been introduced as a promising class of active polymers, that can change on demand from a first shape (A) to a second shape (B) and from there to a third shape (C). Here, the dual-shape capability of such a triple-shape polymer network system having two distinct melting transitions is investigated by cyclic, thermomechanical experiments. These multiphase polymer networks are synthesized by photopolymerization from poly(ethylene glycol) monomethyl ether monomethacrylate and poly(e-caprolactone) dimethacrylate as crosslinker. While their permanent shape is determined by the chemical crosslinks formed during network preparation, the two different crystallizable phases can be used to fix other temporary shapes. When programmed appropriately in a two-step thermomechanical process these materials can exhibit a triple-shape effect. Here, one-step programming methods for dual-shape effects are applied to these polymer networks under variation of process parameters, especially thermal conditions. In this way, crystalline phases formed by both segments can be used either individually or simultaneously to fix a temporary second shape. The permanent shape can be recovered by heating, exceeding a specific switching temperature. This switching temperature correlates with the melting transition of the related domain, if one domain is used for fixation. If both domains are used, the switching temperature correlates with the higher melting temperature.