Extensive Development of Precursory Helical Pairs Prior to Formation of Stereocomplex Crystals in Racemic Polylactide Melt Mixture

Abstract

Melt crystallization of racemic polylactide (equimolar PLLA/PDLA) blend upon slow cooling (1 °C/min from 270 °C) was studied via a combination of wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), and Fourier-transform infrared spectroscopy (FTIR). Results indicated extensive development of racemic (32/31) helical pairs below 220 °C, followed by emergence of a broad mesomorphic peak in the WAXS profile below 190 °C; the intensity of this mesophase peak started to decrease at 150 °C, with concomitant emergence of WAXS- or DSC-discernible formation of stereocomplex (βc) crystals. Isothermal measurements at 200 vs 170 °C revealed the presence of low vs high populations of helical pairs; βc crystals were observed to develop only at 170 °C but not at 200 °C, indicating the need for adequate population of racemic helical pairs for formation of their mesomorphic clusters in the melt matrix as precursors of βc nuclei. The clear change in the melt structure well before the formation of incipient βc crystals reflects strong driving force under large supercooling toward transformation, but the transformation process is kinetically suppressed: only after extensive development of racemic helices and emergence of mesomorphic clusters in the melt matrix may nucleation occur. These observations suggest that the nucleation process proceeds in elementary units of preformed helical pairs in the melt matrix, with an intermediate stage of clustered helical pairs before incipience of βc crystals.

Spontaneous formation of polylactide stereocomplex microspheres containing metal ions

“…Also IR analysis of the stereocomplexation products (the FTIR spectrum of the stereocomplex obtained from PLA‐(COOH) 2 can be seen in Fig. S12) indicates the presence of the β helix formed by associated PLLA and PDLA chains (a band at 909 cm –1 derived from skeletal vibrations instead of a band characteristic for the α helix at 921 cm –1 typical for isotactic PLA).…”

“…The intensity of the signal corresponding to the carboxylate anion increases with increasing ratio of CaO to carboxyl groups up to 1:1 and then remains constant. It is worth mentioning that the signals in the 970–850 cm –1 region (skeletal stretching and a CH 3 rocking) were changed when passing from one stereoisomer α helix to the β helix characteristic for the stereocomplex …”

Hydrogen bonding assists stereocomplexation in poly(l‐lactic acid)/poly(d‐lactic acid) racemic blends

“…In contrast to cooling at 3000 K/s (dotted line), cooling the melt of the PLLA/PDLA racemic blends at 2.5 K/s resulted in an amorphous phase with apparent hydrogen bonding. Namely, the methyl CH stretching (asymmetric at 2990 cm −1 , symmetric at 2940 cm −1 ) and C α H stretching band at 2880 cm −1 are all slightly shifted to lower frequencies . In addition, when compared to cooling the melt of the PLLA/PDLA racemic blends at 3000 K/s, the broad CO stretching band at ~1750 cm −1 became narrower when cooling at 2.5 K/s.…”

“…For instance, Zhang et al investigated CH 3 ···OC hydrogen bonding during PLLA/PDLA stereocomplexation, and determined that the CH 3 ···OC interaction occurs in the induction period, even prior to structural adjustment of the COC backbone during stereocomplexation . Comparing the wide‐angle X‐ray scattering and Fourier‐transform infrared (FTIR) spectroscopy results of racemic blends upon cooling from 270 °C at 1 °C/min, Yang et al observed a development of hydrogen bonding between PLLA and PDLA chains well before formation of mesomorphic clusters, which eventually transformed into SC . According to Henmi et al, after rapid cooling from the melt to temperatures above 130 °C, density fluctuations larger than the long spacing periods grew even before nucleation.…”

Injection molding and characterization of polylactide stereocomplex

“…However, stereocomplex PLA, which has a melting temperature ranging from 200 to 245 C, could be injection molded at 200 and 230 C, which would result in a slower crystallization rate than at 180 C [14]. As described by Yang et al [15], the process of creating stereocomplex PLA begins with the formation of paired helices which serve as nucleation sites. Above 200 C, the development of paired helices remains inactivate, but at 170 C, significant development of paired helices occurs.…”