Diffusion of poly (l-lactide) in activated metal organic frameworks; crystallization evidenced by ssNMR and subsequent shape memory effects

Abstract

Shape-memory polymers are a class of materials that show dual or multi-shape competence. Here a solvent casting method was used to prepare composite films from Cu-BTC crystals in poly (l-lactide) (PLLA) to induce crystallization leading to a marked shape memory effect (SME). Extensive physico-chemical characterizations including solid-state NMR indicated that Cu-BTC triggered the crystallization of PLLA for ensuring the entropy needed for the shape memory behavior. The diffusion of polymeric chains inside pores of activated Cu-BTCs ensured interactions which increases the crystallinity as both simulation and experimental results show. Moreover, the segregation of Cu-BTC crystallites to small copper oxide particles due to the activation increases the volume of the crystalline region observed by microscopic images. We compared the identical features of composite prepared in the same method in where small molecule was encapsulated inside Cu-BTC with PLLA/5 wt% Cu-BTC; we were able to show that having the sharp transition of elastic to rubbery state in the vicinity of cold crystallization temperature in PLLA/5 wt% Cu-BTC provides Cu-BTC crystallites to act as netpoints. Moreover, the reinforcement accompanied by increased toughness demonstrates the combination of improved mechanical properties as well as SME of polymers which rarely have attained.