Search

The study of guest–guest interactions in cocrystalline polymer samples is in general complicated by their possible occurrence in both amorphous and crystalline phases. In this paper we show that polarized FTIR spectra of axially oriented films allow discrimination between spatially ordered and disordered guest–guest hydrogen bonds occurring in crystalline and amorphous polymer phases, respectively. In particular, the possible formation of hydrogen bonds between alkyl-esters of para-hydroxybenzoic acid (parabens, relevant antimicrobial molecules), when absorbed in s-PS films exhibiting nanoporous-crystalline monoclinic δ and orthorhombic e phases, has been studied. X-ray diffraction patterns show the formation, for high guest uptake, of triclinic δ and orthorhombic e cocrystalline forms. Infrared linear dichroism measurements show that for δ form films the parabens are present as isolated guests of the cavities of the crystalline form, while guest–guest hydrogen bonds between parabens are observed only in ...

An efficient method to align low-molecular-mass molecules by enclosing them as isolated guests of the nanoporous host δ phase of syndiotactic polystyrene oriented films, is presented. This new method is compared with the classic method consisting in the absorption of low- molecular-mass molecules as a solute of amorphous phases of highly oriented semicrystalline polymeric films. It is shown that this new molecular alignment method provides a powerful tool to study in detail several features (spectroscopic properties, molecular motions) of relevant molecules, by separating with unprecedented selectivity their anisotropic spectral responses. The reported results refer to infrared and solid-state- 2 H-NMR spectra.

Disordered crystalline modifications formed by s(2/1)2 helices of syndiotactic polystyrene (s-PS) can be prepared by the removal of bulky guest molecules from intercalate as well as from triclinic δ clathrate forms. The X-ray diffraction pattern of the disordered crystalline modification is characterized from 2θCuKα

The possibility to control guest transport in polymer films, by selecting between available nanoporous-crystalline phases and their uniplanar orientations, is explored. The study is conducted on syndiotactic polystyrene films, presenting a nanoporous-crystalline phase ( δ or e) or the dense γ phase and exhibiting uniaxial orientation or each of the three known uniplanar orientations. In particular, the different films are compared as for the uptake of a relevant pollutant (1,2-dichloroethane) from dilute aqueous solutions. High guest solubility is observed only for films exhibiting the nanoporous-crystalline phases while the diffusivity is minimum for δ form films with a // c // orientation and maximum for e form films with c ⊥ uniplanar orientation. These results can be easily rationalized on the basis of crystal structure considerations.

Syndiotactic polystyrene δ form films, exhibiting a//c// and a⟂c// uniplanar orientations and subjected to annealing procedures in the temperature range of 60–130 °C, have been thoroughly characterized by X-ray diffraction. The reported results indicate that the gradual irreversible transformation from the nanoporous δ phase toward the dense γ phase occurs with a continuous transformation of the 010 and reflections of the δ form into the 020 and 200 reflections of the γ form, respectively. This analysis allows ruling out of the occurrence of a new reflection typical of an intermediate phase, as suggested by X-ray diffraction measurements on unoriented or axially oriented samples.

Fourier Transform Infrared spectroscopy (FTIR) and Wide-Angle X-Ray Diffraction (WAXD) measurements have clearly established the occurrence of a dual sorption ability of sulfonated syndiotactic polystyrene samples, which exhibit the nanoporous δ crystalline phase. In fact, large uptake (up to 20–30 wt%) of ionic liquid (IL; e.g. 1-ethyl-3-methylimidazolium dicyanamide) occurs only in the hydrophilic amorphous sulfonated phases and does not disturb the hydrophobic nanoporous crystalline δ phase. On the other hand, a large uptake of organic guests (e.g. naphthalene) occurs prevailingly in the nanoporous hydrophobic crystalline phase, independently of the presence of the IL in the amorphous phase, eventually leading to the formation of syndiotactic polystyrene co-crystalline phases. The thermal stability of IL can be largely increased by their inclusion in the amorphous phase of sulfonated syndiotactic polystyrene films. Copyright © 2012 John Wiley & Sons, Ltd.

Two different kinds of uniplanar–axial orientations, exhibiting the polymer chain axis parallel to the main draw direction (cax)and the ac plane parallel or perpendicular with respect to the film p...

Inclusion of molecules in the crystalline phases of s-PS induces on the guest long range order and strongly reduces the guest diffusivity and mobility with respect to a random dispersion at the molecular level in polymeric amorphous phases. Moreover, when included in highly oriented s-PS films, the degree of guest orientation is comparable to the host orientation and detailed information on directional properties within molecules can be unambiguously achieved. In this paper, the behavior of dichloromethane in the δ-clathrate phase of syndiotactic polystyrene is studied by combined FTIR and solid state 2H NMR techniques, defining the molecular orientation in the crystalline phase and the constrained reorientational dynamics of the guest molecules in the cavities.

Polymer films exhibiting large negative birefringence, suitable for compensating residual positive birefringence of liquid crystal displays, have been achieved for syndiotactic polystyrene. Procedures involving axial stretching at different draw ratios of unoriented films, followed by different crystallization routes, have been compared. The control of the polymer polymorphic behavior, i.e., of the formation of crystalline and co-crystalline (with low-molecular-mass guest molecules) phases, allows to control the negative birefringence up to Δn = n ⊥ - n ∥ = 0.17.

Guest-induced transitions from α and γ phases of syndiotactic polystyrene (s-PS) toward cocrystalline and nanoporous phases, and related guest-induced orientation phenomena, have been investigated by X-ray diffraction. Guests, being able to induce cocrystal formation in amorphous s-PS samples, are able to induce room-temperature cocrystal formation from γ and α form films, only if their vapor pressure is higher than 20 and 60 Torr, respectively. Guest-induced recrystallizations of unoriented γ form films generally lead to unoriented cocrystalline and δ form films, while chloroform-induced recrystallization leads to cocrystalline and e form films with perpendicular orientation of chain axes. On the other hand, guest-induced recrystallizations of unoriented α form films generally lead to oriented cocrystalline and derived δ form films. In particular, recrystallizations of unoriented α form films by room-temperature sorption of trichloroethylene and CHCl3 lead to films with (002) uniplanar and (210) uniplan...

The possible formation of molecular−complex phases of s-PS with styrene dimers has been investigated by their sorption in amorphous and nanoporous δ-form films. X-ray diffraction, 13C solid state NMR, and FTIR analyses have shown that only 1,3-diphenylbutane (dpb), but not 1,4-diphenylbutane, forms a highly stable molecular−complex with s-PS. Preliminary structural considerations indicate that this s-PS/dpb molecular−complex phase presents an intercalate structure, with host-monomer-unit/guest stoichiometric ratio of 4/1 and with a guest conformation with anti C2−C3 bond. Guest conformational behavior has allowed to discriminate, for the first time by a NMR technique, between guest molecules located in crystalline and amorphous s-PS phases. Presently dpb is the s-PS guest with maximum volume (≈0.36 nm3) and maximum number of non-hydrogen atoms (n = 16).

By quantitative evaluation of the spectral changes associated with guest removal from clathrate films as well as by infrared dichroism measurements on uniaxially oriented films, three infrared absorbance peaks (at 1379, 1154 and 841 cm −1 ) prevailingly associated with the amorphous phase have been located for syndiotactic polystyrene (s-PS) samples with helical crystalline phases. A detailed study relative to the infrared peaks in the wavenumber range 870–820 cm −1 , corresponding to phenyl C–H out-of-plane bending, has shown that the 841 cm −1 peak is fully associated with conformationally disordered sequences only for samples including trans-planar crystalline phases (α and β) but not for samples including helical crystalline phases (γ, δ and clathrates). This result has been rationalized on the basis of different intermolecular interactions between phenyl rings, occurring for trans-planar and helical s-PS crystalline phases. Moreover, a preparation procedure leading to fully unoriented films has been described for helical semicrystalline films, which generally present different kinds of uniplanar orientation. FTIR spectra of these fully unoriented semicrystalline films, by subtracting out the amorphous phase contribution with a procedure based on the 1379 cm −1 peak, have allowed a straightforward isolation of the spectra of the helical s-PS crystalline phases and the quantitative determination of the amorphous content.

The crystallization of amorphous syndiotactic polystyrene (s-PS) films when induced by bulky solvents, whose molecules are too big to be enclosed as guest of s-PS clathrate phases, generally leads to the formation of γ-phase. This kind of solvent induced crystallization in amorphous films generates a preferential (200) uniplanar orientation of the γ-crystalline phase. Moreover, the presence of highly boiling solvents in the amorphous phase of these γ-form samples can induce a γ→β phase transition, as a consequence of thermal annealing procedures at atmospheric pressure. A comparison with literature data suggests that the formation of the thermodynamically stable β-phase is generally favored with respect to the formation of the kinetically favored α-phase, by any s-PS dilution.

2H NMR solid-state spectra of aromatic molecules (benzene, toluene, chlorobenzene, naphthalene) included in clathrate phases of syndiotactic polystyrene (s-PS) or absorbed in the amorphous phase of s-PS, have been recorded and analyzed for a broad range of temperature (230-360 K). The dynamic processes accessible to aromatic molecules absorbed in s-PS clathrate phases are essentially temperature independent for the whole temperature range under investigation. Moreover, the molecular re-orientations reflected in the NMR line-shapes are similar to those accessible to the same molecules when absorbed in polystyrene amorphous phase at temperatures substantially lower than the polymer glass transition temperature. A simulation procedure for 2 H NMR line-shape analysis has been used to establish models for the dynamic processes of the different aromatic guest molecules.

Syndiotactic polystyrene films uniaxially stretched at different draw ratios, and including different crystalline (δ and γ) phases as well as clathrate phases with 1,2-dichloropropane, 1,2-dichloroethane, and 1-chloropropane, have been studied by combined infrared linear dichroism and X-ray diffraction measurements. Information relative to the orientation and mobility of trans and gauche conformers of these chlorinated guests in their clathrate structures with syndiotactic polystyrene has been achieved by comparison between experimentally evaluated and calculated vibrational transition moment directions of conformationally sensitive infrared peaks of the guest molecules.

The kind and degree of crystalline phase orientation for clathrate s-PS films, obtained by solution-casting procedures, and for semicrystalline films, obtained by suitable treatments on these cast films, are analyzed. Clathrate as well as nanoporous form films can present high degrees of uniplanar orientation, corresponding to the tendency of the ac crystallographic planes, to be parallel to the film plane. The driving force for this kind of orientation is the tendency of rows of parallel s(2/1)2 helices with minimum interchain and maximum interplanar distances to be parallel to the film plane. A high degree of uniplanar orientation is also maintained after annealing procedures leading to gamma form, that is the other crystalline form which presents the same helical conformation. A tentative indexing of the gamma form reflections, based on an orthorhombic unit cell, again indicates the presence of parallelism of ac planes with respect to the film surface. A substantial planar orientation, corresponding to the placement of the chain axes parallel to the film plane can also be maintained after annealing procedures at higher temperatures (220-230 °C) leading to alfa and beta crystalline forms, where trans-planar rather than helical chains are included.

A monoclinic δ-clathrate form of syndiotactic polystyrene (s-PS) with carvacrol (a relevant natural phenolic antimicrobial) has been prepared and characterized by X-ray diffraction. Very informative are Fourier transform infrared spectra, in particular their OH stretching region that shows a narrow peak and a broad band, corresponding to carvacrol molecules being isolated guest of the co-crystalline phase or dissolved in the amorphous phase, respectively. Analogous spectral features allow discriminating, for many different s-PS guests, between molecules being in crystalline or in amorphous phases. s-PS co-crystalline films with carvacrol molecules being prevailingly (more than 90%) guest of the co-crystalline phase have been prepared, even for high carvacrol content (up to 10–11 wt %). The location of most antimicrobial molecules in the crystalline phase assures a decrease of desorption diffusivity of two to three orders and hence long-term antimicrobial release. © 2014 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014, 52, 657–665

Three different co-crystalline phases of syndiotactic polystyrene (s-PS) with azobenzene have been prepared and structurally characterized. The triclinic δ -clathrate phase is characterized by a styrene-monomeric-unit/guest ratio (SU/G) equal to 8/1 while the δ -intercalate phase is characterized by SU/lG = 4/1. The orthorhombic e -clathrate phase allows stacking of a variable number of guest molecules per monomer unit with SU/G ≥ 9/1. Isomerization reactions of the azobenzene guest molecules (both photo-induced trans → cis and spontaneous cis → trans) have been studied by UV–Visible and FTIR measurements. Repeated isomerization cycles produce progressive desorption from the polymer film of azobenzene molecules being simply dissolved in s-PS amorphous phases, while poorly affect the content of azobenzene molecules being guest of s-PS co-crystalline phases. Polarized FTIR spectra of axially oriented films indicate that isomerizations occur without expulsion of azobenzene guest molecules from the polymer co-crystalline phases. The whole set of data indicate that films exhibiting co-crystalline phases of azobenzene with s-PS are suitable for producing self-erasing photo-isomerization patterns with molecular size marks.

The interaction of some volatile organic compounds, namely, 1,2-dichloroethane, 1,2-dibromoethane, and 1,1,2,2-tetrachloroethane, included in the δ crystalline phase of syndiotactic polystyrene (sPS) has been studied in terms of conformation, orientation, and dynamical behavior. By combination of X-ray diffraction (XRD), Fourier-transform infrared (FTIR), and solid-state (2)H NMR analyses, it has been shown that despite the differences in guest molecular properties (mass, boiling temperature, and volume), stable sPS/guest δ-clathrate cocrystals are formed since the nanoporous δ crystalline form has a flexible structure able to adapt itself to the guest molecule. As a consequence of inclusion, it has been shown that the guest diffusivity is strongly reduced and the dynamical processes are constrained, particularly when these guests are in trans conformation. This suggests the nanoporous sPS δ form to be an efficient tool for water and air purification through volatile organic compound absorption.

Extensive crystallization studies have been conducted on syndiotactic polystyrene in the presence of the nitroxide radical compound 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO). Depending on the selected crystallization conditions, three different co-crystalline phases are obtained: beside the known intercalate structure (guest layers intercalated with layers of alternated enantiomorphous helices), a δ-clathrate structure (isolated guests imprisoned into cavities formed between the same kinds of layers), and a e-clathrate structure (guests confined into channels). All three s-PS/TEMPO co-crystalline phases exhibit a long-term stability for temperatures below 60 °C. The control of the polymorphism of the co-crystals, associated with the control of film morphology (axial and/or uniplanar orientations) allows a control of concentration and orientation of the radical guest molecules, which can be helpful for the preparation of new organic magnetic materials.

The full vibrational spectra of the gamma, delta, and epsilon crystalline phases of syndiotactic polystyrene (sPS), i.e., phases presenting the s(2/1)2 helical conformation, have been experimentally determined and compared with that calculated at the B3LYP/6-31G(d,p) level of theory for an infinite helix. The assignment of the different modes was highly facilitated and validated by the experimental evaluation of the direction of the transition moment vector of most IR peaks, which was made possible for the first time by measurements on sPS films with different uniplanar orientations of the crystalline phase. The normal vibration analysis of most representative modes of the periodic model allowed for a general description of each one to be obtained, which was further confirmed by the direct inspection of mode animations.

The uniplanar orientations that can be achieved for the e phase of syndiotactic polystyrene, i.e., the first polymeric crystalline phase exhibiting channel-shaped porosity, have been explored by wide-angle X-ray diffraction. The formation of three different uniplanar orientations, presenting the three unit-cell axes perpendicular to the film plane, can be controlled by suitable processes, which are based on combined solvent-induced crystallizations and on thermal treatments. This allows gaining control of three different orientations of the crystalline nanochannels with respect to the film surface and hence control of diffusivity as well as orientation of low-molecular-mass guest molecules, also at the macroscopic scale.

Syndiotactic poly( p -fluoro-styrene) (s-PPFS) has been prepared with a polymerization procedure which allows reaching high average molecular masses and satisfactory yields. The polymorphic behavior of the polymer has been mainly studied by X-ray diffraction, calorimetric and infrared analyses. The main crystalline phase of s-PPFS, obtained by melt processing or cold-crystallization, exhibits trans -planar chains, is orthorhombic ( a = 9.5 A, b = 28.7 A, c = 5.1 A) and melts at nearly 320 °C. The X-ray analysis shows a strict analogy of this orthorhombic phase with the β phase of s-PS, also as for the occurrence of two limit ordered (β″) and disordered (β′) modifications, which differ for the intensity of reflections characterized by h + k = 2 n + 1. A metastable crystalline phase, also exhibiting trans -planar chains, has been observed for as-polymerized samples as well as for amorphous samples crystallized by sorption of toluene or 1,4-difluoro-benzene. Mainly on the basis of solvent sorption and desorption experiments, it is suggested that this metastable phase is a co-crystalline phase with the low-molecular-mass guest molecules.

Nanoporous crystalline phases of syndiotactic polystyrene can act as host for organic radicals allowing to produce 3D ordered structure of magnetic molecules, as probed by angle-dependent EPR, demonstrating the feasibility of cost-effective 3D nanostructured magnetic materials.

Thorough analyses of X-ray diffraction patterns of syndiotactic polystyrene (s-PS) films, exhibiting different crystalline and co-crystalline phases, and related evaluations of degrees of orientation have allowed to give a more accurate as well as simpler description of the three known uniplanar orientations. This new description involves the three simplest orientations of the high density ac layers (i.e. of the layers of close-packed alternated enantiomorphous s-PS helices) with respect to the film plane. On this basis it is proposed that the three uniplanar orientations should be named a∥c∥, a∥c⊥ and a⊥c∥, i.e., by a nomenclature which also presents the advantage to be independent of the helical crystalline or co-crystalline s-PS phase.

Robust syndiotactic polystyrene (sPS) films exhibiting highly stable co-crystalline phases can be obtained with a large variety of low-molecular-mass chromophore guest molecules. In this report different aspects relative to the structure and the optical properties of these films are described. In particular, possible applications of these films as fluorescent materials, as optical memories (based on the co-crystallization of photoreactive guest molecules), as non-linear optical materials (with polar guests) and as chiro-optical memories (based on temporary co-crystals with chiral guest molecules) are presented.

The full vibrational spectra of alpha and beta crystalline phases of syndiotactic polystyrene (sPS), that is, phases presenting the trans-planar conformation, have been experimentally determined and compared with that calculated at the B3LYP/6-31G(d,p) level of theory for an infinite trans-planar chain. The normal vibrational analysis of most representative modes of the periodic model allowed us to give a general description of each one, which was further confirmed by the direct inspection of mode animations. An assignment of the different modes was performed in terms of frequency, relative intensity, and direction of the transition-moment vector of the observed IR peaks as well as Raman vibrational frequencies.

The orientation of the crystalline δ nanoporous phase in syndiotactic polystyrene films, obtained by different procedures, have been characterized. For both solution cast and biaxially stretched films a high degree of uniplanar orientation, corresponding to the tendency of the ac crystallographic planes, to be parallel to the film plane has been observed and rationalized. According to molecular dynamics simulations of diffusion of small molecules into the δ nanoporous phase, this uniplanar orientation would minimize the molecular diffusivity through the nanoporous crystalline phase.

The uptake and diffusivity of ethylene in amorphous and semicrystalline s-PS (syndiotactic polystyrene) films presenting different crystalline phases and different crystalline-phase orientations have been studied by gravimetric and Fourier transform infrared measurements. High solubility and low diffusivity of ethylene have been observed for the nanoporous host δ phase of s-PS, due to the formation of a co-crystalline phase, where ethylene molecules are oriented nearly perpendicular to the polymer helices. In addition, ethylene diffusivity can be minimized by suitable (002) uniplanar orientation of the host crystalline phase, i.e. by placing the crystalline polymer helices perpendicular to the film surface. Ethylene-desorption measurements conducted at different temperatures have allowed us to evaluate activation energies relative to ethylene diffusion. The reported results suggest that this cheap, robust and easy-to-process polymeric material is suitable for (also for repeated use) food packaging requiring ethylene removal.