Photo-cross-linking: A powerful and versatile strategy to develop shape-memory polymers

Abstract

Shape-memory polymers (SMPs), as an important class of smart materials, have been attracting interests from both academia and industries. Over the past two decades, the fundamental principles of SMPs have been well established, facilitating research to design different types of SMPs at the molecular structure level. Meanwhile, in recent years, increasing attention has been paid to design SMPs with unique shape-memory behavior and manufacture SMP articles with sophisticated shape towards practical applications. Notably, photo-cross-linking strategy has actively established a powerful platform to develop SMPs from basic molecular structure design to advanced design and manufacturing. This review emphasizes the view that there is a close connection between photo-cross-linking strategy and SMPs. Following a brief introduction on the important general aspects of SMPs, progress in research on various photo-cross-linking approaches used for molecular structure design of SMPs is reviewed. To depict the significance of photo-cross-linking strategy for SMPs beyond conventional molecular structure design, achievements at unconventional aspects of advanced design and 4D printing are highlighted.

Introduction

Stimuli-responsive behavior is a very common phenomenon in nature, for both animals and plants, and this biological intelligence has inspired scientists and engineers in materials science for a long time [[1], [2], [3]]. Chameleon and Venus flytrap are two good candidates to depict natural stimuli-responsive behavior vividly. A chameleon can change the color of its skin in response to the variation of environment, weather and motion [4,5], and Venus flytrap is capable of opening and closing to capture preys [6,7]. Both have provided numerous ideas to develop emerging materials [[8], [9], [10], [11], [12], [13], [14], [15]]. In the wide world of stimuli-responsive materials inspired by nature, shape-memory materials (SMMs), featuring programmable shape change, are very popular and important. The concept of “shape memory” seems to be first mentioned by Vernon in 1941 [16], as revealed by previous reviews [17,18]. With the development of materials science and engineering, SMMs have been widely studied worldwide and now comprise three well-developed categories, shape-memory alloys (SMAs), shape-memory ceramics (SMCs) and shape-memory polymers (SMPs) [[19], [20], [21], [22]]. Compared to SMAs and SMCs, SMPs have attracted more interest in recent years due to their unique advantages, such as light weight, large recoverable strain and easy tailoring enabled by their inherent thermal and mechanical properties. In accordance with the essence of shape memory, SMPs are capable of recovering the original shape from a temporarily deformed shape in response to the relavent stimulus. Widely used heat-shrinkable cross-linked polyethylene (PE) is acknowledged as the first commercialized SMP product [23]. Here, we use the classical cross-linked PE to briefly introduce the general aspects and recent advances of SMPs.

Apart from heat-shrinkable materials in packaging industry, SMPs have also been proposed for many other applications, including biomedical application [[24], [25], [26]], structural component [[27], [28], [29]], aerospace [[30], [31], [32]], actuators and sensors [[33], [34], [35], [36], [37], [38], [39]]. After being deformed, cross-linked PE can show shape recovery when heated, so it is defined as thermo-responsive SMP (TSMP). Indeed, most polymers have the potential to be TSMPs. Now, through elaborate design of molecular structure, SMPs can respond to many stimuli, including light, magnetism, electricity, pH and even chemicals [[40], [41], [42], [43], [44]]. A typical shape-memory cycle of cross-lined PE and many other SMPs includes programming and recovery. If only one temporary shape is created in the programming process, SMPs can only present two shapes (an original shape and a temporary shape) in a completed shape-memory cycle, defined as dual shape-memory effect (SME). Actually, some SMPs with adaptable molecular structure can exhibit three or more shapes in a shape-memory cycle by step-by-step programming and recovery, accordingly, these phenomena should be triple and multiple SMEs [45,46]. In terms of the shape-shifting module, SME of cross-linked PE is not reversible as reprogramming is required between successive shape-memory cycles, and this is the classical one-way SME. Towards cyclic actuation, in contrast, reversible two-way SME (essentially shape changing effect that refers to mechanical two-way and materials two-way [47]) has been put forward and widely studied in recent years [48]. Finally, besides fundamental researches on the molecular structure design of SMPs, increasing efforts have been made to design SMPs with unique shape-memory behavior and manufacture SMPs with sophisticated shape towards real applications. Overall, SMPs have been well studied during the past decades at the aspects of fundamental researches and technical innovations. No matter how SMPs develop, from the standpoint of molecular structure, all SMPs basically consist of netpoints and molecular switches [49]. Netpoints determine the permanent shape and can be formed by chemical or physical cross-linking; the latter enables the fixation of a temporary shape and recovery.

Photo-cross-linking, as a facile and versatile strategy, has contributed considerably to the achievements of SMPs. As for the basic molecular structure design, an irreversible photo-cross-linking approach only constructs netpoints, while a reversible approach can provide both netpoints and molecular switches, depending on the specific circumstance of an SMP. In view of this, reversible photo-cross-linking approaches can be more versatile than irreversible alternatives. In addition, photo-cross-linking can enable spatial and temporal control, facilitating the on-demand treatment of SMPs toward unique shape-memory behavior. In this case, a photo-cross-linking strategy can bring fresh vigor to the design of advanced SMPs. When it comes to facile fabrication of SMPs with chemical netpoints, photo-cross-linking strategy can effectively work as an important part of the “Pre-shaping and Post-cross-linking” methodology. By taking accurate shaping into account, photo-cross-linking strategy can further work together with advanced manufacturing techniques, for example, three-dimensional (3D) printing, and thus significantly benefits the manufacturing of SMPs with sophisticated shape towards practical applications. As an update, 3D printing to form SMPs is now often described as 4D printing. In all, photo-cross-linking strategy has boomed the development of SMPs in three major aspects, from basic molecular structure design to advanced design and manufacturing. However, a comprehensive review to depict the power of photo-cross-linking strategy in developing SMPs is still lacking.

This review aims to introduce the recent advances in the development of SMPs where photo-cross-linking strategy has been significant, and to emphasize the great value of photo-cross-linking as a powerful and versatile strategy for developing SMPs in future. Generally, the layout of this review is as follows. First, several general aspects of SMPs are introduced, including molecular structure and mechanism, and classifications. As not all the general aspects of SMPs will be included, we strongly recommend readers to refer to several previously published reviews to get more basics of SMPs [17,[50], [51], [52], [53], [54], [55], [56], [57], [58]]. That is followed by a section focused on research progress in the development of SMPs by various photo-cross-linking reactions, both irreversible and reversible, and this section refers to the molecular structure design of SMPs. That is followed by a section on research progress in unconventionally using photo-cross-linking strategy for advanced design of SMPs and manufacturing SMP articles via 4D printing, followed by a brief outlook on the future and challenges of photo-cross-linking strategy in the SMP field. The review delivers a viewpoint that there is a close connection between photo-cross-linking strategy and SMPs. We believe it will be of great value for professional scientists and research in SMPs on molecular structure design, advanced design and manufacturing.