Elucidation of Chiral Symmetry Breaking in a Racemic Polymer System with Terahertz Vibrational Spectroscopy and Crystal Orbital Density Functional Theory.

The conservation of chiral symmetry has been used as a fundamental rule to determine polymer packing conformations in racemic systems. We have illustrated, through the interplay of polarization terahertz (THz) spectroscopy and solid-state density functional theory, that the chiral symmetry is not conserved in a poly(lactic acid) stereocomplex (scPLA) system. Poly(l-lactic acid) (PLLA) displays a weaker violation of the 31 screw symmetry than poly(d-lactic acid) (PDLA) and possesses a stronger intramolecular vibrational energy, on average, in the low-frequency gamma-point phonon modes than does PDLA. Polarization THz spectroscopy adds a new dimension to polymer crystallography through which new phenomena are expected to be revealed.