Poly(2,6-dimethyl-1,4-phenylene)oxide Research Articles

Transitions between Nanoporous crystalline α and β forms of poly(2,6-dimethyl-1,4-phenylene) oxide (PPO)

Nanoporous-crystalline (NC) α and β forms of Poly(2,6-dimethyl-1,4-phenylene) oxide (PPO), exhibit two different large helix conformations, both being suitable for intrahelical low-molecular-mass guest inclusion. α → β and β → α transitions can be achieved by sorption of liquid β-guests and α-guests (i.e., of guest inducing β and α forms by solution casting and by sorption in amorphous PPO), respectively. Both α → β and β → α transitions occur with complete loss of possibly present crystalline phase orientations (both axial and planar). Moreover, the reported data show that transitions between the two phases and the associated loss of crystalline phase orientation occur only for guests being good PPO solvents. These observations rule out the possibility of solid–solid transitions while can be rationalized by mechanisms involving a local dissolution of the NC phases. This local polymer dissolution is followed by host/guest co-crystallization with the helical conformation being different from the starting one and more suitable to host the used guest. These data fully confirm complete separation between the two NC phases while rule out the hypothesis of a continuum of NC phases between two limit ones.

All‐Atom Molecular Dynamics Simulations of Poly(2,6‐dimethyl‐1,4‐phenylene) Oxide: Validation of OPLS‐AA Force Field for Conformational Behaviour in Vacuum and in Carbon Tetrachloride

AbstractOPLS force field has been successfully validated for poly(2,6‐dimethyl‐1,4‐phenylene) oxide (PPO) in comparison with quantum chemical results and available experimental data. The formation of stable polymer solvent complexes, where solvent molecules are hosted in closed loops, has been detected in molecular dynamics (MD) simulations of OPLS all‐atom (OPLS‐AA) models of diluted solutions of PPO in CCl4. Estimations of free energy of solvation indicate an entropic stabilization of the polymer solvent complexes. A very high similarity of these structures with the intra‐helical channels observed in the structure of PPO co‐crystals suggests their possible role in the nucleation process inducing the crystallization of PPO from organic solvents.

Hydrogen bonded dimer of an alcohol with the derived carboxylic acid triggering their sorption by nanoporous-crystalline PPO films.

Films exhibiting nanoporous-crystalline (NC) phases of poly(2,6-dimethyl-1,4-phenylene) oxide (PPO), which are highly effective to absorb apolar organic guest molecules, are also able to absorb polar molecules (like alcohols and carboxylic acids) but only from concentrated organic solutions. NC PPO films, which do not absorb alcohols and carboxylic acids from diluted aqueous solutions, exhibits a huge uptake (even above 30wt%) of benzyl alcohol (BAL) and benzoic acid (BA), if BA is obtained by spontaneous room temperature oxidation of BAL in aqueous solution. This phenomenon is rationalized by an easy uptake, mainly by the PPO intrahelical crystalline empty channels, of a BAL/BA 1/1 hydrogen-bonded dimer. This huge uptake of BAL/BA dimer by NC PPO films, which is also fast for films exhibiting the orientation of the crystalline helices perpendicular to the film plane (c^ orientation), can be exploited for purification of water from BAL, when present in traces. High and fast sorption of a hydrogen bonded dimer and negligible sorption of the two separate compounds is possibly unprecedented for absorbent materials.

Sorption of CO2, CH4 and Their Mixtures in Amorphous Poly(2,6-dimethyl-1,4-phenylene)oxide (PPO).

Sorption of pure CO2 and CH4 and CO2/CH4 binary gas mixtures in amorphous glassy Poly(2,6-dimethyl-1,4-phenylene) oxide (PPO) at 35 °C up to 1000 Torr was investigated. Sorption experiments were carried out using an approach that combines barometry with FTIR spectroscopy in the transmission mode to quantify the sorption of pure and mixed gases in polymers. The pressure range was chosen to prevent any variation of the glassy polymer density. The solubility within the polymer of the CO2 present in the gaseous binary mixtures was practically coincident with the solubility of pure gaseous CO2, up to a total pressure of the gaseous mixtures equal to 1000 Torr and for CO2 mole fractions of ~0.5 mol mol-1 and ~0.3 mol mol-1. The Non-Equilibrium Thermodynamics for Glassy Polymers (NET-GP) modelling approach has been applied to the Non-Random Hydrogen Bonding (NRHB) lattice fluid model to fit the solubility data of pure gases. We have assumed here that no specific interactions were occurring between the matrix and the absorbed gas. The same thermodynamic approach has been then used to predict the solubility of CO2/CH4 mixed gases in PPO, resulting in a deviation lower than 9.5% from the experimental results for CO2 solubility.

Transparent and flexible high-surface area nanoporous crystalline PPO films

Guest and processing conditions leading to high surface area (SA) nanoporous-crystalline (NC) poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) films are explored.

Segregation of Benzoic Acid in Polymer Crystalline Cavities.

Benzoic acid (BA) and its derivatives are very attractive because of their pharmacological properties, such as antioxidant, radical-regulating, antiviral, antitumor, anti-inflammatory, antimicrobial and antifungal. Syndiotactic polystyrene (sPS) and poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) films exhibiting co-crystalline phases with BA were prepared and characterized by WAXD, FTIR and polarized FTIR measurements. The FTIR measurements clearly showed that BA was present mainly as a dimer in the crystalline channels of the ε form of sPS as well as in the α form of PPO, as generally occurs not only in the solid state but also in organic dilute solutions. BA was instead present as isolated molecules in the crystalline cavities of the δ form of sPS. In fact, the FTIR spectra of BA guest molecules exhibited vibrational peaks close to those of BA in its vapor phase. Hence, the nanoporous-crystalline δ form of sPS not only avoids additive aggregation but also leads to the separation of dimeric molecules and the segregation of monomeric BA.

Polymorphism of poly(2,6-dimethyl-1,4-phenylene) oxide (PPO): Co-crystalline and nanoporous-crystalline phases

Relevant information on PPO crystallization, with particular emphasis on nanoporous crystalline (NC) and related co-crystalline (CC) forms, is reviewed. Preparation methods, structures, properties and possible applications of NC and CC samples are discussed. Reviewed results mainly refer to films (unoriented or with axial or planar orientations) but also to NC powders and NC aerogels. Particular attention is given to the impressive solubility and diffusivity of organic guests, which can be achieved for NC PPO samples. X-ray diffraction patterns and infrared linear dichroism, as collected for axially oriented NC films, indicate the presence of large diameter crystalline helices with intrahelical location of the guest molecules. The final section is mainly devoted to perspectives of basic and industrial research that are expected in the near future for CC and NC PPO samples. As for research studies, particularly suitable appear NC films with orientation of the crystalline empty channels perpendicular to the film plane ( c ⊥ orientation), mainly due to their fast guest sorption and good optical transparency.

Intrahelical empty channels of nanoporous-crystalline α and β forms of PPO

A linear dichroism study of vibrational peaks of amorphous and semi-crystalline axially stretched poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) films shows an unusual behaviour of co-crystalline and nanoporous-crystalline (α and β form) phases. In fact, signs of dichroism for all vibrational peaks of α and β phases are opposite with respect to those observed for amorphous phases. This behaviour indicates the occurrence in PPO α and β forms of large polymer helices, whose thermodynamic stability can be rationalized by placement of guest molecules in intrahelical channels. A large diameter helix, with chain periodicity of 1.06 nm, is compatible with layer-line position in WAXD patterns of axially stretched α form films. The occurrence for both α and β NC forms of PPO of intrahelical empty channels, can rationalize the very high guest diffusivity of semi-crystalline PPO films, which becomes particularly high when crystalline helical axes are preferentially perpendicular to the film plane.

Nanoporous-crystalline and amorphous films of PPO including off-on vapochromic fluorescent 7-hydroxy coumarin guests

This study examines the preparation of nanoporous-crystalline and amorphous films of poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) containing 7-hydroxy coumarin (i.e., umbelliferone) for the detection of chloroform vapours. Chloroform is selected as the volatile contaminant model being considered harmful and possibly carcinogenic after prolonged exposure. Umbelliferone is chosen as a fluorescent molecule being mostly unemissive in the solid-state and apolar media. α and β nanoporous-crystalline and amorphous PPO films presenting ≈40 μm thickness have been prepared by casting from carbon tetrachloride, benzene and chloroform solutions, respectively, whereas umbelliferone inclusions (1–2 wt%) have been realized in saturated methanol mixtures. Films show fluorescence maximum at about 430 nm with very low quantum yield (Φ F ) of around 1–2%, and particularly reduced in the α and β PPO films, thus suggesting stronger molecular interactions which might facilitate fluorescence self-quenching. It has demonstrated that the exposure to chloroform vapours under saturated conditions (10 5 ppm) turns rapidly on the fluorescence of the polymer films thanks to the solvation effect of the solvent and in agreement with the Φ F of 7% of umbelliferone in chloroform. Notably, a slower response was detected by using dichlorometane as VOC, possibly due to the weaker H-donor strength with respect to chloroform. It was worth noting, that the phenomenon results fastest for the α PPO films, i.e., in case of the lowest Φ F and quenched emission. Vapochromism appears therefore sensitive to the different fluorophore confinement, and the reported results may open new avenues for the rapid detection of VOC vapours. • Nanoporous-crystalline PPO films were prepared and characterized by WAXD and FTIR. • Umbelliferone inclusions in PPO films showed very low Φ F of around 1–2%. • Exposure to CHCl 3 vapours turned the fluorescence on. • Faster vapochromism for the α and β co-crystalline PPO films.

C‐perpendicular orientation in thin nanoporous‐crystalline poly(2,6‐dimethyl‐1,4‐phenylene)oxide films

AbstractNanoporous‐crystalline films of poly(2,6‐dimethyl‐1,4‐phenylene)oxide (PPO), exhibiting c‐perpendicular orientation (c⊥) orientation (i.e., the orientation of the crystalline chain axes being preferentially perpendicular to the film plane), show the advantage of faster guest diffusivity and higher transparency. However, by using the methods described in the literature, this perpendicular orientation cannot be achieved for low thickness films (≤20 μm). In this paper, we describe four different preparation methods, which are suitable to get high degree of crystallinity (Xc >40%) as well as high degree of c⊥ orientation (−0.5< fc <− 0.3) of CC and NC α forms, for film thickness even as low as 5 μm. All the described methods are based on reduction of guest sorption kinetics. The reported results also confirm our previous hypothesis that c⊥ orientation is due to formation of flat‐on crystalline lamellae, which in turn are due to nanoconfined co‐crystallization of PPO. An anomalous temperature dependence of c⊥ orientation for limonene‐induced co‐crystallization with PPO is described and rationalized.

Nanoporous-Crystalline Poly(2,6-dimethyl-1,4-phenylene)oxide Aerogels with Selectively Sulfonated Amorphous Phase for Fast VOC Sorption from Water.

This paper describes the preparation and characterization of poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) highly porous monolithic aerogels with a hydrophobic nanoporous–crystalline phase and a hydrophilic sulfonated amorphous phase. The sulfonated aerogels were obtained by the sulfonation of PPO physical gels, followed by the supercritical CO2 extraction of solvents. WAXD and FTIR analysis showed that the nanoporous–crystalline phase was preserved for a degree of sulfonation up to c.a. 35%, allowing a highly volatile organic compound (VOC) sorption capacity. The sulfonated PPO aerogels exhibited a high water sorption capacity, with a water uptake of up to 500 wt%, and faster VOC sorption kinetics from water with respect to unsulfonated aerogels.

Molecular Features Behind Formation of α or β Co-Crystalline and Nanoporous-Crystalline Phases of PPO.

Guest molecular features determining the formation of α and β phases of poly(2-6-dimethyl-1,4-phenylene) oxide (PPO) are explored by collecting literature data and adding many new film preparations, both by solution casting and by guest sorption in amorphous films. Independently of the considered preparation method, the α-form is favored by the hydrophobic and bulky guest molecules, while the hydrophilic and small guest molecules favor the β-form. Furthermore, molecular modeling studies indicate that the β-form inducer guests establish stronger dispersive interactions with the PPO units than the α-form inducer guests. Thus, the achievement of co-crystalline (and derived nanoporous crystalline) α- and β-forms would result from differences in energy gain due to the host–guest interactions established at the local scale.

Fast uptake of organic pollutants from dilute aqueous solutions by nanoporous-crystalline PPO films with c-perpendicular orientation

The extraordinary ability of poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) dense films, exhibiting a nanoporous-crystalline (NC) phase, to absorb traces of organic pollutants dissolved in water is shown. Equilibrium uptakes and sorption kinetics of perchloroethylene (PCE) are much higher and faster than for the corresponding amorphous films. For sorption kinetics, particularly relevant is a control of the planar orientation of the NC phase. For instance, for PCE sorption from 50 ppm aqueous solutions, a NC α form film with orientation of the crystalline polymer chains being preferentially perpendicular to the film plane (c⊥ orientation) exhibit a diffusivity nearly 14 times higher than for a NC film with crystalline polymer chains being preferentially parallel to the film plane (c// orientation) and nearly 88 times higher than for the starting amorphous PPO film. This phenomenon is rationalized in terms of crystalline channels being preferentially perpendicular to the film plane. Moreover, equilibrium uptakes for NC PPO films are much higher than for NC sPS films. For instance, for sorption experiments from PCE 50 ppm aqueous solutions, equilibrium uptakes for NC PPO and sPS films are nearly 15 wt% and 2 wt%, respectively. This phenomenon is rationalized by the high-free volume of both NC and amorphous phases of PPO.

Dependence on Film Thickness of Guest-Induced c Perpendicular Orientation in PPO Films.

For poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) films exhibiting nanoporous-crystalline (NC) phases, c⊥ orientation (i.e., crystalline polymer chain axes being preferentially perpendicular to the film plane) is obtained by crystallization of amorphous films, as induced by sorption of suitable low-molecular-mass guest molecules. The occurrence of c⊥ orientation is relevant for applications of NC PPO films because it markedly increases film transparency as well as guest diffusivity. Surprisingly, we show that the known crystallization procedures lead to c⊥ oriented thick (50–300 μm) films and to unoriented thin (≤20 μm) films. This absence of crystalline phase orientation for thin films is rationalized by fast guest sorption kinetics, which avoid co-crystallization in confined spaces and hence inhibit formation of flat-on lamellae. For thick films exhibiting c⊥ orientation, sigmoid kinetics of guest sorption and of thickening of PPO films are observed, with inflection points associated with guest-induced film plasticization. Corresponding crystallization kinetics are linear with time and show that co-crystal growth is poorly affected by film plasticization. An additional relevant result of this study is the linear relationship between WAXD crystallinity index and DSC melting enthalpy, which allows evaluation of melting enthalpy of the NC α form of PPO (ΔHmο = 42 ± 2 J/g).

Fast uptake of organic pollutants from dilute aqueous solutions by nanoporous-crystalline PPO films with c-perpendicular orientation

The extraordinary ability of poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) dense films, exhibiting a nanoporous-crystalline (NC) phase, to absorb traces of organic pollutants dissolved in water is shown. Equilibrium uptakes and sorption kinetics of perchloroethylene (PCE) are much higher and faster than for the corresponding amorphous films. For sorption kinetics, particularly relevant is a control of the planar orientation of the NC phase. For instance, for PCE sorption from 50 ppm aqueous solutions, a NC α form film with orientation of the crystalline polymer chains being preferentially perpendicular to the film plane (c⊥ orientation) exhibit a diffusivity nearly 14 times higher than for a NC film with crystalline polymer chains being preferentially parallel to the film plane (c// orientation) and nearly 88 times higher than for the starting amorphous PPO film. This phenomenon is rationalized in terms of crystalline channels being preferentially perpendicular to the film plane. Moreover, equilibrium uptakes for NC PPO films are much higher than for NC sPS films. For instance, for sorption experiments from PCE 50 ppm aqueous solutions, equilibrium uptakes for NC PPO and sPS films are nearly 15 wt% and 2 wt%, respectively. This phenomenon is rationalized by the high-free volume of both NC and amorphous phases of PPO.

Mechanisms determining different planar orientations in PPO films crystallized by guest sorption

Co-crystallization of amorphous poly(2,6-dimethyl-1,4-phenylene) oxide (PPO) films, as induced by sorption at different temperatures of many liquid guest molecules, is explored. Both for co-crystalline (CC) and corresponding nanoporous-crystalline (NC) films, kinds of and degree of planar orientation, which are relevant for their properties and applications, can be controlled by the guest sorption procedure. In particular, crystalline polymer chain orientation preferentially perpendicular to the film plane is favored by bulky guest molecules and by low guest sorption temperatures, i.e. by slow guest uptakes. Guest sorption kinetics and Scanning Electron Microscopy (SEM) support a simple mechanism that rationalizes the kinds of planar orientation of crystalline PPO chains. Slow guest uptakes imply polymer co-crystallization in spatial confinement, producing lamellae that are oriented flat-on to the film surface and hence polymer chain axes preferentially perpendicular to the film plane (c⊥ orientation). Fast guest uptakes imply polymer co-crystallization leading to fibrils that are parallel to the film surface and hence to polymer chain axes being preferentially parallel to the film plane (c// orientation).

High diffusivity dense films of a nanoporous-crystalline polymer

Nanoporous-crystalline (NC) films of poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) exhibit much higher guest uptakes with respect to amorphous PPO films as well as with respect to films based on different polymers. Particularly high guest diffusivities are observed for PPO films with orientation of the chain axes of their NC phases being preferentially perpendicular to the film plane (c⊥ orientation). Remarkably high diffusivities of c⊥ oriented NC PPO films are also observed for high film thickness (at least up to 50 μm). This phenomenon is due to the high free-volume of the amorphous PPO phase combined with crystalline channels being preferentially perpendicular to the film plane. Guest uptakes for NC PPO films, when expressed as mass/volume, are also much higher than for NC PPO powders and high-surface-area NC PPO aerogels, as well as much higher than for benchmark adsorbent polymers (Tenax). High guest uptakes and diffusivities of NC PPO films with c⊥ orientation are relevant for their possible applications as dense membranes for organic pollutant control, molecular separations and molecular sensors.

Planar Orientation and Transparency of Nanoporous-Crystalline Polymer Films

The dependence of polymer film transparency on different kinds of planar orientations of nanoporous-crystalline (NC) phases of syndiotactic polystyrene (sPS) and poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) is described. The two polymers exhibit opposite behavior: film transparency is improved for sPS and PPO by orientations of the chain axes of the NC phases being parallel (c∥) and perpendicular (c⊥) to the film plane, respectively. This behavior can be rationalized by the negative and positive birefringence of sPS and PPO chains, respectively. In fact, to maximize transparency, the refractive index of the NC phase along the perpendicular to the film plane has to be increased to come closer to that of the corresponding amorphous phase. This can be pursued by controlling the orientation of the NC phases and hence of the associated refractive index ellipsoids.

Control of Guest Thermal Release by Crystalline Host Orientation

Thermal release of guest molecules from poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) films, exhibiting host–guest co-crystalline (CC) phases with orientation of their chain axes being preferentially...

Axial Orientation of Co-Crystalline Phases of Poly(2,6-Dimethyl-1,4-Phenylene)Oxide Films

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.