| Poly(L-lactic acid) (PLLA) is a biodegradable polymer produced from renewable biobased resources. Due to its biodegradability, biocompatibility and good mechanical properties, it has tremendous market potential in biomaterials, fibers, disposable commodities and package materials. So, PLLA has attracted many researchers'and companies'attention in biogegradable area. How to obtain high molecular weight PLLA via an efficient and low-cost way, and how to introduce branching into its chain to improve its mechanical properties, especially melt strength, are key points in its synthesis and modification. In this article, crystalline dicarboxylated PLLA prepolymer was first synthesized and then reacted with diglycidylester as a chain extender. Via the so-called melt polycondensation-chain extension method, high molecular weight PLLAs with branched long chains were prepared.Firstly, dicarboxylated PLLAs with number average molecular weight (Mi) of 1000-20000 g.mol-1 were synthesized via melt polycondensation of L-lactic acid in the presence of a small amount of succinic anhydride (SAD), using tin(Ⅱ) chloride and toluene-4-sulfonic acid as a binary catalyst (SnCI2/TSA). The terminal COOH percentage reaches over 98% and the molecular weight can be controlled by molar ratio of LLA to SAD. Thermal transition behaviors of the prepolymer depends on its molecular weight. It crystallizes slowly at Mn≤2000, but quickly at Mn≥4000. The crystallinity increases from 27% to 40% when the Mn grows from 4000 to 10000.Secondly, dicarboxylated PLLAs were chain extended with diglycidylester and the chain structures of the chain extended products were characterized by three-detector GPC. The effects of Mn of prepolyme, r epoxy/carboxyl molar ratio, reaction temperature and pressure on the chain extenstion reaction and on the chain structures of chain extended products were investigated. The reaction between the epoxy groups in diglycidylester and the carboxyl groups in prepolymer took place rapidly during chain extension, so the molecular weight of chain extended PLLA grew rapidly to high value in short time. Meanwhile, side reations between carboxyl/epoxy and side hydroxyl formed during chain extension ocuured too, leading to the formation of branched chains, together with broadening of molecular weight distribution and even with gellation. The Mn of prepolymer, epoxy/carboxyl molar ratio and reaction temperature exhibited significant effects on the chain extenstion reaction and on the chain structures of chain extended PLLAs. Chain-extending prepolymer with propriate molecular weight (ca. Mn 6000) at higher epoxy/carboxyl ratio and higher temperature was beneficial to the growth of the molecular weight. Lower Mn of prepolymer, epoxy/carboxyl molar ratio other than 1/1 and higher reaction temperature led to enhanced branching, broader MWD and even gellation. Reaction under vacuum also enhanced the branching to certain extent.The crystallization of the chain extended PLLAs was further studied. The chain extended PLLAs were crystallizable but their crystallizability was decresed with comparison to their prepolymers because of introduction of chain extender moiety and formation of branching. The higher extent of branching, the less the crystallizability. So chain extended PLLAs prepared from prepolymer with higher Mn has better crystallizability.. For comparison, linear chain extended PLLAs were prepared using 1,3-phenyl-Bis(2-oxazoline) as chain extender. It has better crystallizability than the branched counterpart, but is still interior to the prepolymer.In the end, when dicarboxylated PLLA prepolymers with Mn of 6000-10000 were chain extended with diglycidylester at optimized conditions, i.e., at epoxy/COOH ratio of 1 and 160℃for 30 min, branchd PLLAs with Mw of 90, 000-215,000, PDI of 4.88-9.18, branching factor of 0.27-0.79 and crystallinity of 3.7-10.7% were successfully synthesized. After sufficient isothermal cold crystallization, the crystallinity can reach 20-30%. |
