Shape-memory materials based on thermoreversibly cross-linked poly-ε-caprolactones

Shape memory polymers (SMPs) are materials that are able to change their shape from a temporary shape to a permanent one by application of a stimulus such as heat or light. SMPs are usually chemically or physically crosslinked materials that exhibit an elastomeric behaviour above a glass or melting transition temperature. Poly(ε-caprolactone) (PCL) is one of the most widely studied polymers for the development of SMPs. PCL presents several advantages such as a melting transition temperature close to human body temperature, a high biocompatibility, is (bio)degradable and potentially biosourced. So, this polymer is highly relevant for both degradable packaging and also for biomedical devices such as resorbable suture wires or stents. However, after crosslinking, the material can not be reprocessed, preventing any reuse/recycling. The main purpose of this work is to provide a solution to this major drawback, which would then enable, for example, to reshape packaging films after use or to reprocess trimmings remaining after production.

Amongst current trends in the design of new polymer and composite materials, organic reactions that are able to create and reversibly disrupt chemical bonds upon an external stimulus (temperature, irradiation,…) are currently gaining more and more attention in macromolecular engineering and are used in various areas such as remendable materials, drug delivery systems, stimulus-degrading materials or recyclable materials. Amongst all the reversible links described in the literature, thermally (4+2) reversible cycloadditions present interesting properties such as the creation of robust bonds and well defined reversibility conditions. As an example, the application of furan/maleimide adducts as covalent link, which cycloreversion is largely favored in the range of temperature (90-120°C), is widely reported.

This contribution aims at reporting a new concept for the preparation of well defined and recyclable PCL based reversibly cross-linked SMPs by the formation of reversible carbon-carbon bonds.

For this purpose, commercially-available linear and multi-arm star shaped PCL precursors have been selected and selectively functionalized at their chain ends either by a diene (furan, anthracene) or a dienophile (maleimide). Typically, PCL-based shape memory materials have been prepared by mixing a stoichiometric amount of diene-bearing and maleimide-bearing PCLs in a twin-screw mini-extruder at a temperature which favors cycloreversion. The polymer blend is then cured at 65°C (just above PCL melting temperature), with the purpose to increase chains mobility and improve the formation of the adducts. Cross-linked PCLs were obtained, as evidenced by swelling experiments. The shape memory properties of the materials have been studied by cyclic tensile thermomechanical analysis. The influence of the architecture of the PCL precursors as well as the nature of the Diels-Alder moieties on the cross-linking rate and on the shape memory properties has been studied. Reversibility of the network formation in the case of furan, used as diene, has been assessed by rheology and by recycling experiment.