materials Review Shape Memory Materials from Rubbers Arunima Reghunadhan 1 , Keloth Paduvilan Jibin 2 , Abitha Vayyaprontavida Kaliyathan 2 , Prajitha Velayudhan 2 , Michal Strankowski 3, * and Sabu Thomas 2,4,5   Citation: Reghunadhan, A.; Jibin, K.P.; Kaliyathan, A.V.; Velayudhan, P.; Strankowski, M.; Thomas, S. Shape Memory Materials from Rubbers. Materials 2021, 14, 7216. https:// doi.org/10.3390/ma14237216 Academic Editor: Gabriele Milani Received: 23 September 2021 Accepted: 14 November 2021 Published: 26 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Postgraduate Department of Chemistry, Milad-E-Sherif Memorial College, Kayamkulam, Alappuzha 690502, India; arunimarenjith02@gmail.com 2 School of Chemical Sciences, Mahatma Gandhi University, Kottayam 686560, India; jibinkp999@gmail.com (K.P.J.);abithavk@gmail.com(A.V.K.); prajipravi.11@gmail.com (P.V.); sabuthomas@mgu.ac.in (S.T.) 3 Polymer Technology Department, Chemical Faculty, Gda´ nsk University of Technology, 80-233 Gda ´ nsk, Poland 4 School of Energy Materials, Mahatma Gandhi University, Kottayam 686560, India 5 International and Interuniversity Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam 686560, India * Correspondence: micstran@pg.edu.pl; Tel.: +48-58-3472434 Abstract: Smart materials are much discussed in the current research scenario. The shape memory effect is one of the most fascinating occurrences in smart materials, both in terms of the phenomenon and its applications. Many metal alloys and polymers exhibit the shape memory effect (SME). Shape memory properties of elastomers, such as rubbers, polyurethanes, and other elastomers, are discussed in depth in this paper. The theory, factors impacting, and key uses of SME elastomers are all covered in this article. SME has been observed in a variety of elastomers and composites. Shape fixity and recovery rate are normally analysed through thermomechanical cycle studies to understand the effectiveness of SMEs. Polymer properties such as chain length, and the inclusion of fillers, such as clays, nanoparticles, and second phase polymers, will have a direct influence on the shape memory effect. The article discusses these aspects in a simple and concise manner. Keywords: shape memory; elastomer; fixity; recovery; applications 1. Introduction to Rubbers and Properties The first material, known as caoutchouc, is obtained from the weeping tree. This is polyisoprene recovered from the sap of Hevea Brasiliensis and is known as natural rubber (NR) in comparing with synthetically produced rubbers. In the course of developing synthetic analogues of NR, similar compounds were found, which can also be cross-linked with sulphur. However, macromolecular compounds, as they have unsaturation, can be cross-linked with sulphur. This unsaturation normally comes from (partly or totally) diene monomers, for example, polyisoprene (synthetic), polybutadiene, styrene–butadiene, or acrylonitrile–butadiene copolymers [1–4]. Over the years, the importance of rubber to modern life has constantly increased. About one-third of the total global rubber usage is natural rubber (NR); the remaining two-thirds of required rubber is produced synthetically by a great number of industrial countries, well distributed throughout the world. More than half of the world’s production of natural and synthetic rubber is used in tyres and the remainder is for a great variety of industrial and consumer products [5,6]. The most important property of rubbers is elastic behaviour after deformation (either in compression or tension). It is possible to stretch a rubber sample ten times its original length and after removal of the tension, the given sample will return to its original shape and length. In addition, elastomers have many other useful properties under static and dynamic conditions, such as abrasion resistance, impermeability to air and water, and re- sistance to swelling in oils/solvents, etc. These properties are exhibited at higher, ambient, Materials 2021, 14, 7216. https://doi.org/10.3390/ma14237216 https://www.mdpi.com/journal/materials