Effect of hydrophobic alkyl chains on the plasticization properties of citrate: Experiments and MD simulation

In this work, the citrate plasticizers with different alkyl chain lengths, tributyl citrate (TBC), trihexyl citrate (THC), trioctyl citrate (TOC) and tridecyl citrate (TDC) were synthesized by esterification reaction. The effects of hydrophobic alkyl chains on plasticization properties of citrate were studied by using a combination of characterization techniques and molecular dynamics (MD) simulation. Tensile tests showed that the elasticity of citrate/PVC was basically negatively correlated with the length of citrate hydrophobic alkyl chain, while its strength was positively correlated. The citrate plasticizer migration stability test indicated that in deionized water and 50 % (v/v) ethanol solution, the mobility of TBC was the highest and that of TOC was the lowest. The MD simulation results found that the interaction between PVC and citrate mainly included hydrogen bonding (OH-Cl), van der Waal force (COO-Cl), and the mutual winding between the alkyl chain of citrate and the PVC chain. Among them, TBC, THC and TOC can be spontaneously dispersed in PVC resins, but TDC cannot, because its long hydrophobic alkyl chains make TDC more inclined to self-polymerize and form large hydrophobic aggregation zones. However, in the suitable range (number of carbon atoms ≤ 8), longer alkyl chains can be effective in improving the stability of citrate in resins. Thus, for citrates, alkyl chains of 8 carbons perform better. Our research is helpful to understand the effect of molecular structure on the plasticizer of citrate and propose an optimized citrate structure with higher performance.