Lidong Feng

Synergistic effect of PLA–PBAT–PLA tri-block copolymers with two molecular weights as compatibilizers on the mechanical and rheological properties of PLA/PBAT blends

Two types of polylactide–poly(butylene adipate-co-terephthalate)–polylactide (PLA–PBAT–PLA) tri-block copolymers with different molecular weights (CP1 and CP2) were synthesized as compatibilizers for PLA/PBAT blends. Synergistic effects of CP1 and CP2 on the mechanical and rheological properties of the blends have been studied in detail. The addition of small amounts of CP1 and CP2 remarkably increased the elongation at the break point. 0.5 and 0.5 wt% of CP1 and CP2 led to an increase of elongation by over eightfold. Thermal, morphological and rheological analyses showed that addition of CP1 and CP2 increased the miscibility and interfacial bonding strength between PLA and PBAT, in addition to decreasing the melt viscosity. It was thought that the low-molecular-weight compatibilizer CP1 with high mobility would have a positive effect during the transportation of the high-molecular-weight CP2 from the matrix to the interface. In addition, CP2 played a key role in improving the interaction at the interface.

Effects of molecular weight on the crystallization and melting behaviors of poly(L-lactide)

In this study, a series of monodispersed poly(L-lactide) (PLLA) were synthesized by the ring-opening polymerization with Schiff base aluminum catalyst, and the effects of the number-average molecular weight (Mn) on the crystallization and melting behaviors of PLLA were investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The total crystallization rate of PLLA was Mn-dependent, which reached the maximum value for PLLA with Mn of 18.6 kg/mol. In addition, when Mn of PLLA was 18.6 kg/mol, the melting enthalpy (ΔHm) showed a maximum value (87.1 J/g), which was the highest reported value till now. The critical temperature for change of crystal formation from δ- to α-form crystals increased in the isothermal crystallization process with Mn increasing. In the reheating procedure, high-Mn PLLA demonstrated a small exothermal peak prior to the dominant melting peak, corresponding to crystal transition from δ- to α-form, but low-Mn PLLA didn’t show the peak of crystal transition. These different crystallization and melting behaviors were attributed to the different chain mobility of PLLA with different Mn.

Toughening modification of PLLA by combination of copolymerization and in situ reactive blending

The inherent brittleness and low melt strength of poly(L-lactide) (PLLA) are major bottlenecks for its large scale commercial applications. In this work, high toughness and high melt strength PLLA blends were prepared by reactive blending of PLLA, PLLA-b-poly(butylene adipate-co-terephthalate)-b-PLLA (PLLA-b-PBAT-b-PLLA) in the presence of (PLLA-block-poly(glycidyl methacrylates))3 (PLLA-b-PGMA)3. Among them, PLLA-b-PBAT-b-PLLA was synthesized by the combination of melt condensation and ring-opening polymerization (ROP), and (PLLA-b-PGMA)3 was synthesized by the combination of ROP and atom transfer radical polymerization (ATRP). The structure was confirmed by NMR spectra, FTIR spectra and GPC. Mechanical testing demonstrated that the blends containing PLLA, PLLA-b-PBAT-b-PLLA, and (PLLA-b-PGMA)3 exhibited higher elongation at break compared to the neat PLLA, and did not significantly lose tensile strength. The higher viscosity and storage modulus of PLLA blends indicated the production of the longer chains or even long chains branching. The strain hardening behaviour was observed obviously with increased elongational viscosity. The imperfect crystallization of PLLA/PLLA-b-PBAT-b-PLLA/(PLLA-b-PGMA)3 blends was demonstrated by the lowered melt point of PLLA. Scanning electron micrographs showed that the PLLA-b-PBAT-b-PLLA was dispersed well in the PLLA, and the interface adhesion was further increased after addition of (PLLA-b-PGMA)3. Moreover, optimization of parameters such as the PLLA-b-PBAT-b-PLLA concentration, and (PLLA-b-PGMA)3 content revealed that blends containing 30 wt% PLLA-b-PBAT-b-PLLA and 2 wt% (PLLA-b-PGMA)3 were optimal in terms of comprehensive properties.

Thermal, morphological, mechanical and aging properties of polylactide blends with poly(ether urethane) based on chain-extension reaction of poly(ethylene glycol) using diisocyanate

Poly(ether urethane)s (PEU), including PEUI15 and PEUH15, were prepared through chain-extension reaction of poly(ethylene glycol) (PEG-1500) using diisocyanate as a chain extender, including isophorone diisocyanate (IPDI) and hexamethylene diisocyanate (HDI). These PEUs were used to toughen polylactide (PLA) by physical and reactive blending. Thermal, morphological, mechanical and aging properties of the blends were investigated in detail. These PEUs were partially compatible with PLA. The elongation at break of the reactive blends in the presence of triphenyl phosphate (TPP) for PLA with PEUH15 or PEUI15 was much higher than that of the physical blends. The aging test was carried out at -20 °C for 50 h in order to accelerate the crystallization of PEUs. The PEUs in the PLA/PEU blends produced crystallization and formed new phase separation with PLA, resulting in the declined toughness of blends. Fortunately, under the aging condition, although PEUH15 in blends could also form crystallization, the reactive blend of PLA/PEUH15/TPP(80/20/2) had higher toughness than the other blends. The elongation at break of PLA/PEUH15/TPP(80/20/2) dropped to 287% for the aging blend from 350% for the original blend. The tensile strength and modulus of PLA/PEUH15/TPP blend did not change obviously because of the crystallization of PEUH15.

Determination of D-lactide content in lactide stereoisomeric mixture using gas chromatography-polarimetry

An analytical method has been proposed to quantify the D-lactide content in a lactide stereoisomeric mixture using combined gas chromatography and polarimetry (GC- polarimetry). As for a lactide stereoisomeric mixture, meso-lactide can be determined quantitatively using GC, but D- and L-lactides cannot be separated by the given GC system. The composition of a lactide stereoisomeric mixture is directly relative to its specific optical rotation. The specific optical rotations of neat L-lactide were obtained in different solutions, which were -266.3° and -298.8° in dichloromethane (DCM) and toluene solutions at 20°C, respectively. Therefore, for a lactide sample, the D-lactide content could be calculated based on the meso-lactide content obtained from GC and the specific optical rotations of the sample and neat L-lactide obtained from polarimetry. The effects of impurities and temperature on the test results were investigated, respectively. When the total content of impurities was not more than 1.0%, the absolute error for determining D-lactide content was less than 0.10% in DCM and toluene solutions. When the D-lactide content was calculated according to the specific optical rotation of neat L-lactide at 20°C, the absolute error caused by the variation in temperature of 20±15°C was not more than 0.2 and 0.7% in DCM and toluene solutions, respectively, and thus usually could be ignored in a DCM solution. When toluene was used as a solvent for the determination of D-lactide content, a temperature correction for specific optical rotations could be introduced and would ensure the accuracy of results. This method is applicable to the determination of D-lactide content in lactide stereoisomeric mixtures. The standard deviation (STDEV) of the measurements is less than 0.5%, indicating that the precision is suitable for this method.

An Analytical Method for Determining Residual Lactide in Polylactide by Gas Chromatography

An analytical method to determine the residual lactide in polylactide (PLA) was proposed using an internal standard method of gas chromatography (GC). PLA samples and diphenyl ether (DPE) as an internal standard were dissolved in dichloromethane, then PLA was precipitated in anhydrous alcohol. The residual lactide and DPE were extracted to alcohol for GC analysis. At room temperature, lactide could react with alcohol and change into ethyl lactoyl lactate (ELL), but the relative response factor of lactide versus DPE could be obtained through a numerical analysis method. Therefore, the residual lactide content could be quantitatively calculated in PLA. The relative standard deviation (RSD) of the measurements is not more than 7.0%, indicating that the method is suitably precise.