Abstract
Films exhibiting co-crystalline (CC) phases between a polymer host and low-molecular-mass guest molecules are relevant for many applications. As is usual for semi-crystalline polymers, axially oriented films can give relevant information on the crystalline structure, both by Wide Angle X-ray diffraction fiber patterns and by polarized Fourier-transform infrared spectroscopy. Axially oriented CC phases of poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) with 1,3,5-trimethylbenzene (mesitylene) can be simply obtained by the stretching of CC PPO films. In fact, due to the plasticization effect of this highly boiling guest, PPO orientation can occur in a stretching temperature range (170–175 °C) nearly 50 °C lower than that generally needed for PPO films (220–230 °C). This low stretching temperature range allows avoidance of polymer oxidation, as well as formation of the mesomorphic dense γ PPO phase. Axially oriented CC phases of PPO with toluene, i.e., with a more volatile guest, can be instead obtained by the stretching (in the same low temperature range: 170–175 °C) of CC PPO blend films with polystyrene.
Highlights
Two well separated families of CC forms of PPO have been described: (i) highly ordered, only obtained with a few guest molecules (α-pinene, decalin, tetralin), which become amorphous after guest removal [46,47,48,49]; (ii) poorly ordered with highly extended polymer conformations, observed for many guests, which lead to NC phases by guest removal [13,14,15,16,17,18,19]
The experiments reported in this paper refer to the stretching of CC PPO phases with toluene and mesitylene, i.e., of two CC phases of family (ii) with α form
An axially oriented NC α form blend film (obtained by stretching at 190 ◦ C up to a draw ratio of 3 for a PPO/atactic polystyrene (aPS) (70/30; wt/wt) blend film, exhibiting a wide-angle X-ray diffraction (WAXD) pattern like that shown in Figure 2b,b’ of [43]), was immersed in a toluene 50 ppm aqueous solution for different times
Summary
Poly(2,6-dimethyl-1,4-phenylene) oxide [1,2,3,4], often abbreviated as PPO, is the only polymer beside syndiotactic polystyrene (s-PS) [5,6,7,8,9,10,11,12] that exhibits co-crystalline (CC) phases with low-molecular-mass guest molecules and corresponding nanoporous-crystalline (NC) phases, as derived by guest removal [13,14,15,16,17,18,19].As is well described in the literature, NC polymer samples can be useful for the removal of organic pollutants from water and air [20,21,22], in molecular sensors [18,23], in gas separation procedures [14,24,25], as support for catalysts [26,27] or even for chemical stabilization of unstable guest molecules [28].CC polymer samples (mainly films) have been proposed for antimicrobial [29], optical [30,31,32], magnetic [33] and dielectric [34,35] applications. Poly(2,6-dimethyl-1,4-phenylene) oxide [1,2,3,4], often abbreviated as PPO, is the only polymer beside syndiotactic polystyrene (s-PS) [5,6,7,8,9,10,11,12] that exhibits co-crystalline (CC) phases with low-molecular-mass guest molecules and corresponding nanoporous-crystalline (NC) phases, as derived by guest removal [13,14,15,16,17,18,19]. As is well established for s-PS, polarized FTIR spectra of axially stretched films can be powerful tools to study the relative orientations of polymer chain axes and guest molecules in CC phases by evaluating the amount and sign of linear dichroism of guest peaks [28,33,38,39,40,41]. The dichroism of the FTIR peaks of guest molecules absorbed in oriented amorphous phases is instead negligible for both
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