Structure and Performance of Carboxylic Styrene Butadiene Rubber/Citric Acid Composite Films

Herein, we report rubber composite films which are fabricated via film formation of carboxylic styrene butadiene rubber (XSBR) latex and citric acid (CA) solution. CA is regenerated in the interstitial space between XSBR latex particles, forming various structures with increasing contents. Before the CA nanocrystals appeared, the XSBR/CA composite films exhibit healable characteristics because of the massive hydrogen bonds. When the CA content achieves 30 wt %, CA nanocrystals appear and are regularly arranged in the rubber matrix. Then, the CA nanocrystals serve not only as multifunctional cross-linkers via hydrogen bonding with XSBR but also as effective reinforcers to improve the mechanical properties of the composite films. As the CA content achieves 60 wt %, the CA nanocrystals grow to micron size, and the tensile strength of the XSBR/CA film is 7.83 MPa (about 4.1 times that of neat XSBR) and Young's modulus is 31.2 MPa (almost 60 times that of neat XSBR). Interestingly, although the regenerated CA slightly decreases the T-g of XSBR, the maximum shape-fixing temperature of XSBR/CA composite films increases to about 12-14 degrees C, which can program the shape memory behavior of the composite films. Moreover, because of the water-soluble nature of CA, the XSBR matrix can be recycled completely by soaking the films into water to remove the CA in a mild condition.