Polymer Chirality\nModulates Phase Transitions at Liquid–Liquid\nInterfaces

In the poly(lactic acid) family, poly(l-lactic\nacid) (PLLA),\npoly(d-lactic acid) (PDLA), and mesomeric poly(d,l-lactic acid) (mPLA) have the same chemical constituents\nbut distinct configurations. It is well-known that PLLA and PDLA could\ncrystallize to homocrystals individually (HC<sub>L</sub> and\nHC<sub>D</sub>) or form stereocomplex crystals (SC) together.\nBut it remains a debated question whether poly(lactic acid) mixtures\nwith different chiralities can phase separate in melt or in solutions.\nHere, we study the phase transitions of PLLA/mPLA and PLLA/PDLA in\ndichloromethane (DCM) solutions at double emulsion interfaces. We\nfind that crystallization is dominant in the low-molecular-weight\n(<i>M</i><sub>W</sub>) mixtures, while liquid–liquid\nphase separation indeed occurs ahead of crystallization in the high-<i>M</i><sub>W</sub> mixtures. The PLLA/mPLA double emulsion droplets\ntransform into a spindle-like shape when the homocrystallization of\nPLLA is dominant in the low-<i>M</i><sub>W</sub> region,\nwhile eyeball-like microcapsules form when phase separation is dominant\nin the high-<i>M</i><sub>W</sub> region. The PLLA/PDLA double\nemulsion droplets are solidified to spherical microcapsules with dense\nstereocomplex crystallites at low <i>M</i><sub>W</sub>s,\nwhile the multiphase patchy microcapsules form by sequential liquid–liquid\nphase separation, stereocomplex crystallization, and homocrystallizations\nat high <i>M</i><sub>W</sub>s. We quantify a very weak repulsive\nenergy of 0.0050<i>kT</i> (<i>kT</i> is the thermal\nenergy at room temperature) between the l- and d-segments and depict the complete phase diagrams to provide insight\ninto the multiple phase transitions. This study quantifies the interaction\nof poly(lactic acid) mixtures and provides a design principle for\ntaking advantage of chiral polymers to modulate the microcapsule structures\nand properties via phase transitions at the liquid–liquid interface.