Photon Factory Activity Report 2022 #40 (2023)

BL-6A/2020G520

Formation of Homocrystal with Remarkably High Melting Temperature in
PLLA/PDLA Blend Revealed by WAXD
Neimatallah MAHMOUD1, Hideaki TAKAGI2, Nobutaka SHIMIZU2, Noriyuki IGARASHI2, andShinichi SAKURAI1
1
Graduate School of Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan2
Photon Factory, Institute of Materials Structure Science,High Energy Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
1 Introduction
Poly(lactic acid) (PLA) is an environment friendly
material. It is biodegradable, non-toxic and highlytransparent. However, PLA mechanical and thermal
properties require significant enhancements. PLA exists in
two enantiomeric forms, L-lactic acid (PLLA) and D-lacticacid (PDLA). Each on their own can form homo
crystallites (HC). Moreover, these two enantiomers,
together, can form stereocomplex crystallites (SC). The
tightly packed SC has a melting temperature 50°C higher
than HC.
2 Experiment
The isothermal crystallization of PLLA/PDLA (50/50)
blend was examined, at different temperatures and fordifferent durations, by means of differential scanningcalorimetry (DSC), time-resolved wide-angle X-ray
diffraction (WAXD) and polarized optical microscope(POM) observation. WAXD measurements wereperformed at BL-10C beamline of photon factory (PF) of
high-energy research organization (KEK), Japan, with thewavelength of 0.10 nm and the size of the incident X-ray
beam being 182µm (H) × 346µm (V). The distance
between the specimen and the 2-dimensional detector(PILATUS3 2M, DECTRIS, Switzerland) was set at 0.25m. The exposure time was 30s.
3 Results and Discussion
Figure 1 shows the particular case of Tc = 170°C,
where the isothermal crystallization duration plays a majorrole in the formed crystallites and their meltingtemperatures. For the 1- and 5-min durations, noendothermic peaks appeared, indicating the absence ofcrystallites formation. However, for the durations between
10 and 65 min, two endothermic peaks appeared, bothascribed to the melting of SC. The HC melting peak
appeared after 100 min of isothermal crystallization with asignificantly high melting temperature of 184°C. Moreover,the HC melting temperature increased with the longer
isothermal crystallization durations, resulting in aremarkably high melting temperature of Tm (5 h) =187.3°C. To the best of our knowledge, such high Tm has
never been reported before.
The assignment of the melting peaks to HC and SC wasconfirmed by the time-resolved wide-angle X-ray
diffraction experiments performed after quenching thesample from 250°C to 170°C. As shown in the inset inFigure 2, the first peak of HC emerged 70 min after thetemperature stabilized at 170°C. While the second peakFig. 1 DSC curves during the subsequent heating to 250°C after theisothermal crystallization at 170°C of the PLLA/PDLA (50/50)
specimens.

Fig.2 Changes in the WAXD profiles along the isothermalcrystallization at Tc = 170 C upon quenching from 250 C for thePLLA/PDLA (50/50) blend specimen and a close-up showing theemergence of HC peaks

appeared after more than 2 h and 13 min. Furthermore, thePOM observations carried out at the same Tc indicate that
HC was formed within the already established SCspherulites, as the SC completely filled the observed spaceprior to the HC crystallization setting in.
As for the isothermal crystallization performed atdifferent temperatures, the DSC results at Tc = 110°C andTc = 150°C showed the appearance of HC (along with SC)
melting peaks after 1- and 5-min durations, respectively,
with its melting temperature increasing with the longer
crystallization durations. For Tc = 110°C, Tm of HC
increased from 173.5°C to 176°C after 1 min and 5 hdurations, respectively. As for Tc = 150°C, Tm of HC
increased from 176.3°C to 180.1°C after 5 min and 5 hdurations, respectively. However, for Tc = 180°C only SC
was formed after 5 h of isothermal crystallization.
* shin@kit.ac.jp