Effect of solubility of a hydrazide compound on the crystallization behavior of poly(l-lactide)

Abstract

Tetramethylenedicarboxylic di-(2-hydroxybenzohydrazide) (TMBH) has been proved to be an effective nucleating agent for poly (L-lactide) (PLLA). The correlation between the solubility of TMBH in a PLLA melt during the annealing treatment and its nucleating efficiency has been systematically investigated. The phase diagrams of the binary system consisting of PLLA and TMBH were constructed by differential scanning calorimetry and polarized optical microscopy. Depending on the annealing temperature (T_f) and TMBH concentration (C_TMBH), TMBH may remain crystallites, or partially/completely dissolve into a PLLA melt, which greatly affected the crystallization kinetics and crystalline morphology of PLLA. When T_f was relatively lower and/or TMBH concentration was higher, TMBH crystallites remained solid in the PLLA melt after annealing treatment (e.g. T_f@180 °C and C_TMBH@1 wt%), which could effectively enhance the crystallization rate and nucleation density of PLLA. The solubility of TMBH in the PLLA melt would be increased gradually with the increase of T_f and/or the decrease of C_TMBH, e.g. TMBH would be partially dissolved into the PLLA melt for TMBH-1 at 200 °C. Furthermore, TMBH would be completely dissolved into the PLLA melt as T_f was raised to 230 °C with C_TMBH ranging from 0.1 to 1 wt%. In this case, the soluble TMBH molecules would assemble into rod-like structures with lower specific area during the cooling process, which decreased the nucleating efficiency of TMBH and produced rod-like PLLA crystallites.