Poisoning by Purity: What Stops Stereocomplex Crystallization in Polylactide Racemate?

Formation of stereocomplex crystals (SC) is an effective way to improve the heat resistance and mechanical performance of poly(lactic acid) products. However, at all but the slowest cooling rates, SC crystallization of a high-molecular-weight poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) racemate stops at a high temperature or does not even start, leaving the remaining melt to crystallize into homochiral crystals (HC) or an SC–HC mixture on continuous cooling. To understand this intriguing phenomenon, we revisit the SC crystallization of both high- and low-molecular-weight PLLA/PDLA racemates. Based on differential scanning calorimetry (DSC), supplemented by optical microscopy and X-ray scattering, we concluded that what stops the growth of SC is the accumulation of the nearly pure enantiomer, either PDLA or PLLA, that is rejected from the SC ahead of its growth front. The excess enantiomer is a result of random compositional fluctuation present in the melt even if the average composition is 1:1. The situation is more favorable if the initial polymer is not fully molten or is brought up to just above the melting point where SC seeds remain, as proven by DSC and X-ray scattering. Moreover, we find that not only is SC growth poisoned by the locally pure enantiomer but also that at lower temperatures, the HC growth can be poisoned by the blend. This explains why SC growth, arrested at high temperatures, can resume at lower temperatures, along with the growth of HC. Furthermore, while some previous works attributed the incomplete SC crystallization to a problem of primary nucleation, we find that adding a specific SC-promoting nucleating agent does not help alleviate the problem of cessation of SC crystallization. This reinforces the conclusion that the main problem is in growth rather than in nucleation.

. Gel permeation chromatograms of LMW and HMW PLLA and PDLA.  Figure S2. To avoid degradation, the sample was firstly heated to 220 °C at a rate a 50 K/min (grey dashed curve) then continuously heated to 260 °C at a rate of 3 K/min (blue curve). The selected T s range is shaded yellow in the heating thermogram shown in Figure S2. The melting peak temperature of SC was calibrated by indium. Figure S2. DSC heating curve of LMW PLLA/PDLA racemate. For temperature lower than 200 °C, heating rate was 50 K/min (grey dashed curve); for 200 -260 °C, it was 3 K/min (blue curve).

Curve resolution of WAXS profiles of HMW PLLA/PDLA racemate
To evaluate the crystallinity of SC in HMW racemate at different temperature, the WAXS profiles were resolved into individual Bragg components and the amorphous scattering curve using Origin.
WAXS profile recorded at 220 °C is shown as an example in Figure S3. Figure S3. Peak-fitting of WAXS profile collected at 220 °C in heating.

Assignment of exotherm of SC and HC in HMW racemate with different T s
For T s ≤ 247 °C, enthalpies of higher and lower temperature exotherm are consistent with melting enthalpies of SC and HC melting peak, respectively. However, for T s ≥ 248 °C, the higher and lower temperature exotherms are partially overlapped with each other. It was assumed that the higher and lower temperature exotherm respectively corresponds to SC and HC ( Figure S4). To confirm this, the exotherm was decomposed by peak-fitting using modified Gaussian function S1,S2 and the enthalpies were compared with endotherms in second heating, as shown in Figure S5, higher and lower temperature exotherm corresponds to SC and HC, respectively. were obtained from DSC thermograms of HMW PLLA/PDLA racemate.

Assignment of exotherm of SC and HC in HMW racemate with cooling rate of 10 and 20 K/min from T s of 247 °C
For HMW PLLA/PDLA racemate, the crystallization peaks of SC and HC (α+α') are partially overlapped when the cooling rate is 10 and 20 °C/min. The crystallization enthalpies of SC and HC at these two cooling rates were obtained by peak fitting with modified Gaussian components, as shown in Figure S6. Figure S6. Resolution of HC and SC exotherms by peak fitting for HMW racemate with cooling rate of (a) 10 K/min and (b) 20 K/min.

Assignment of SC and HC crystallization in HMW PLLA/PDLA racemate + 1 wt% TMB-5 with T s ≥ 248 °C
The enthalpies of crystallization and melting of HMW PLLA/PDLA racemate + 1 wt% TMB-5 were measured. Analogous to HMW PLLA/PDLA racemate, the higher and lower crystallization exotherms corresponds to SC and HC (α+α'), respectively. Figure S7 shows the decomposition of exotherms for T s ≥ 248 °C. Enthalpies as a function of T s are shown in Figure S8.

Effect of TMB-5 on crystallization of HMW PLLA/PDLA racemate
The crystallization temperature of SC and HC in HMW PLLA/PDLA racemate + 1 wt% TMB-5 were compared with that in neat HMW PLLA/PDLA racemate. Figure S9 shows that crystallization temperature of SC and HC were both improved with addition of TMB -5 when sample was completely melted (T s ≥ 248). Figure S9. Comparison of crystallization temperature of SC and HC in HMW PLLA/PDLA racemate before and after added 1 wt% TMB-5. S11

POM observation of HMW PLLA/PDLA racemate after annealing at three selected T s
Crystalline morphologies of HMW PLLA/PDLA racemate annealed at T s of 236 °C, 246 °C and 250 °C, were studied by POM. No birefringent texture can be observed at the beginning of cooling for the selected three different T s , as shown in Figure S10. Figure S10. POM micrographs of HMW racemate collected after annealing at different T s for 2 min.