Characterization Of Polylactic Acid And Its Structure In Situ Starch Graft

Copolymers of starch grafted with poly (lactic acid) were directly prepared by in situ reaction of cornstarch with lactic acid in aqueous media in the presence of stannous 2-ethyl hexanoate [Sn(Oct)₂]. The procedure of the graft reaction was elucidated, based on the HPLC analysis, as the ring-opening polymerization (ROP) from starch surface of the small amounts of lactide produced in situ in the reaction system. Analysis of ¹³C-NMR spectra of starch graft copolymers indicates that the ROP of lactide takes place through acyl-oxygen cleavage co-initiated by hydroxyl group at glucopyranan unit, yielding PLA grafts with a hydroxyl terminus.The resultant starch-g-PLA was characterized by FTIR, ¹H-NMR and ¹³C-NMR. The assignments of the resonance in ¹H - and ¹³C-NMR spectra of the copolymer were clarified. The FTIR and quantitative ¹³C-NMR analysis indicates that the degree of polymerization of starch graft copolymers could be controlled by changing the reaction durations.The detailed chemical microstructure of the resultant graft copolymer was clarified by one- and two-dimensional NMR spectroscopy. The positions of lactyl units at the glucopyranan ring as well as the corresponding distribution were determined. The DS values of the graft copolymer calculated based on ¹³C-NMR spectra of starch ring carbons are identical to that of carbonyl carbons in LA moiety. Analysis of quantitative ¹³C-NMR spectra of the carbons in starch ring as well as the carbonyl carbons in LA moiety demonstrates that the reactivity of hydroxyl groups at glucopyranan unit of starch decreases in the order of C-6>C-3>C-2. The average length of PLA grafts at individual starch ring carbons was also investigated.The thermal properties as well as the solubility of starch graft copolymers in organic solvents were investigated. The DSC result indicated that the glass transition temperature of starch graft copolymers decreased with the increase of the DS and DP values of starch graft copolymers.