Actuation Mechanisms of a Semicrystalline Elastomer-Based Polymer Artificial Muscle with High Actuation Strain

Abstract

Semicrystalline elastomer-based artificial muscles (sePAMs) are of high actuation strain generally above 30% and are promising for applications of soft robots, flexible electronics, etc. However, a comprehensive and in-depth understanding of the actuation behavior and mechanisms of the sePAMs is lacking. This obstructs the future design and development of the sePAMs. In this paper, we reported a sePAM with a high actuation strain above 40% under 1.0 MPa. Furthermore, we presented an in-depth analysis of the underlying molecular and thermodynamic mechanisms related to this high actuation strain. It is found that both the entropy-elastic actuation and the crystallization-induced elongation (CIE)/melting-induced contraction (MIC) play important roles in controlling the actuation behavior. More importantly, it is revealed that the primary and direct cause for the CIE is the further elongation of the amorphous section in the stretched network during oriented crystallization. The oriented crystallization induces an entropy increase of the amorphous section, causing the stress of the network to be partly relaxed and the network to be further stretched by the external stress. This entropy increase was confirmed by theoretical calculation. The theory also predicts that an oriented crystallization of about 10% can induce a large elongation of the network with a strain increment of about 80%, which agrees well with the experiments reported in this paper as well as in the literature. This work greatly advances our understanding of the actuation behavior and mechanisms of the sePAMs, opening up new strategies for the future design and development of high-performance polymer actuators.