Poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) is a high-performance commercial thermoplastic polymer that exhibits cocrystalline phases (CC) with many low molecular mass guest molecules as well as nanoporous crystalline (NC) phases. In this paper, preparation and characterization, mainly by WAXD and FTIR techniques, of PPO films exhibiting the CC phase with carvacrol (a relevant natural antimicrobial) are reported. The study shows that the relative intensities of two O–H out-of-plane deformation peaks (at 698 and 717 cm−1) of carvacrol are highly sensitive to its concentration in CC films. The reported data can be rationalized by assuming that 698 and 717 cm−1 peaks are due to isolated and hydrogen-bonded guest molecules, respectively, and by preferential inclusion of carvacrol in the cavities of the NC phase. Polarized FTIR spectra of axially stretched CC PPO/carvacrol films show that the guest peak at 717 cm−1 is not dichroic, while the guest peak at 698 cm−1 is markedly dichroic. This confirms that the O–H out-of-plane deformation peak at 698 cm−1 is due to isolated guest molecules, which are mainly enclosed in the cavities of the axially oriented CC phase.
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