A trifunctional aziridine linker, trimethylolpropane tris(2-methyl-1-aziridinepropionate) (TTMAP), was melt blended with linear poly(lactic acid) (PLA) to make (i) star-shaped PLA and (ii) long chain branched (LCB) PLA by utilizing both pyromellitic dianhydride (PMDA) and TTMAP. Mixing torque evolution during melt processing revealed high reactivity of TTMAP with the carboxyl end group on PLA and low reactivity of PMDA with the hydroxyl end group. Star-shaped PLA exhibited higher viscosity than linear PLA, but no strain hardening in extensional flow; while LCB PLA showed significant extensional hardening. The strain rate dependence of extensional hardening coefficient indicated that concentration of LCB molecules in LCB PLA is low and the possible topological chain structure of those molecules is H-shaped. Unlike current methods used to branch PLA, (e.g., free radical chemistry or the use of an epoxy functional oligomers), our branching strategy produced strain hardening with less increase in shear viscosity. The topological structures of LCB chains by free radical chemistry and epoxy functional oligomers are also discussed.
Bibliographical noteFunding Information:
This work was supported by the National Science Foundation under the Center for Sustainable Polymers (CHE-1413862).