Sheng Xiang

The stereocomplex formation and phase separation of PLLA/PDLA blends with different optical purities and molecular weights

In this study, the poly(L-lactide)/poly(D-lactide) (PLLA/PDLA) blends with different optical purities of PLLA and various molecular weights of PDLA are prepared by solution mixing, and the stereocomplex formation and phase separation behaviors of these blends are investigated. Results reveal that optical purity and molecular weight do not vary the crystal structure of PLA stereocomplex (sc) and homochiral crystallites (hc). As the optical purity increasing in the blends, the melting temperature of sc (Tsc) and the content of sc (ΔHsc) increased, while the melting temperature of hc (Thm) hardly changes, although the content of hc (ΔHhm) decreased gradually. The Tsc and ΔHsc are also enhanced as the molecular weight of PDLA reduces, and the ΔHhm reduces rapidly even though the Thm does not vary apparently. With lower optical purities of PLLA and higher molecular weights of PDLA, three types of crystals form in the blends, i.e., PLA sc, PLLA hc and PDLA hc. As molecular weight decreases and optical purity enhances, the crystal phase decreases to two (sc and PDLA hc), and one (sc) finally. This investigation indicates that the phase separation behavior between PLLA and PDLA in the PLLA/PDLA blends not only depends on molecular weights, but also relies on the optical purities of polymers.

The morphology and spherulite growth of PLA stereocomplex in linear and branched PLLA/PDLA blends: effects of molecular weight and structure

Linear and three-armed poly(L-lactide)/poly(D-lactide) (PLLA/PDLA, 3PLLA/3PDLA and 3PLLA/PDLA) specimens were prepared through a solution mixing method, and the morphology and the spherulite growth of poly(lactide) stereocomplex crystallites (PLA sc) from the melt with various structures and molecular weights were investigated. Polarized optical microscopy images revealed that regular spherulites and a visible boundary developed in all of the PLLA/PDLA blends with relatively lower molecular weights (Mn 3PLLA/PDLA > 3PLLA/3PDLA).

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.

Synthesis of multi-arm poly(l-lactide) and its modification on linear polylactide

Multi-arm poly(l-lactide)s (PLLAs) were prepared by using alcohols with different numbers of hydroxyl groups as initiator and Sn(Oct)2 as catalyst. The structure and composition was confirmed by NMR and GPC analysis. A series of blends of PLLA/multi-arm PLLA were prepared by melt-mixing. Several techniques were applied to investigate the effects of multi-arm PLLA on the melt rheology, crystallization, thermal stability, morphology and mechanical properties of the linear PLLA. Multi-arm PLLA enhanced crystallization rate of PLLA as a nucleating agent. Rheological analysis showed that the viscosity of PLLA at low frequencies increased after addition of multi-arm PLLA. In addition, the tensile strength was increased with the increase of multi-arm PLLA and can achieve a maximum. FE-SEM measurements revealed that the surface of blends was homogenous, indicating good compatibility between PLLA and multi-arm PLLA. The results had an important guiding role in PLA modification, which provided an opportunity for generating new PLA blending systems with enhanced 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.