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The crystal structure of the δ clathrate form of syndiotactic polystyrene (s-PS) containing CHCl 3 , a molecule having a pivotal role in respect to the co-crystalline phases formation of this polymer, has been determined through X-ray diffraction data and molecular mechanics calculations. Analogously to all the other δ clathrate forms of s-PS, this structure presents a monoclinic unit cell (cell constants a = 1.77 nm, b = 1.32 nm, c = 0.78 nm and γ = 121.5°) in which the s(2/1)2 polymer helices and guest molecules are packed according the space group P 2 1 / a . At variance with all the other δ clathrate forms of s-PS whose crystal structure has been reported in the literature, probably due to the not planar shape of the chloroform guest molecule, in this structure guest molecules occupy each centrosymmetric cavity in a very low efficient way, giving rise to a disorder in the positioning of the guest molecules along the b + a / 2 direction of the unit cell. A comparison with the e type clathrate with the same guest, for which some preliminary results have been reported too, is also presented.

The detailed description of the crystalline structure of a channel type e clathrate cocrystal of syndiotactic polystyrene (s-PS) is presented here for the first time. The proposed structure is base...

The self-assembling of fluorescent guest molecules, with the δ nanoporous host crystalline phase of syndiotactic polystyrene (s-PS) films, reduces chromophore diffusivity. Moreover, the chromophore...

The crystal structure of the clathrate form of syndiotactic poly(p-methylstyrene) containing benzyl chloride is presented. This structure presents a monoclinic unit cell (cell constants a = 26.2 A, b = 11.8 A, c = 7.9 A and γ = 116°) in which the s(2/1)2 polymer helices and guest molecules are packed according the space group P21/a. This molecular complex represents, to our knowledge, the first case of polymeric clathrate form in which in the same crystal are present two kinds of cavities having different shape and dimensions: α class cavities (containing two guest molecules) and β class cavities (containing one guest molecule). The contemporaneous presence of these two types of cavities is originated by a disorder in the arrangement of right- and left-handed helices in the crystals related to the particular staggered layout of the layers of chains containing the cavities in which helices of a given chirality can be randomly substituted by enantiomorphous ones.

The crystalline structure of form I of syndiotactic poly(para-methylstyrene) is described. In this form, the polymer chains assume a s(2/1)2 helical conformation and are packed according to the space group P2 1 /a in a unit cell with a = 24.5 A, b = 12.4 A, c = 8.1 A, and y = 143.5°. The proposed model may be described by a stacking of layers of alternating right- and left-handed helical chains efficiently packed in an "interdigital structure" in which phenyl rings belonging to one chain interlock with the phenyl rings of the two adjacent chains of opposite chirality. A disorder in the correct succession of the layers is suggested. A possible similarity between this form and the nanoporous 6 form of syndiotactic polystyrene, suggested by the fact they are both obtainable by guest removal from a class clathrates, is analyzed.

The crystal structures of two molecular complex phases of syndiotactic polystyrene (s-PS) with 1,3,5-trimethyl-benzene (TMB) and 1,4-dimethyl-naphthalene (DMN) have been described. These structures present a monoclinic unit cell in which the s(2/1)2 polymer helices and guest molecules are packed according the space group P21/a and unit cell constants: a=17.3 A, b=15.4 A, c=7.8 A and γ=95.7° for s-PS/TMB and a=17.4 A, b=17.2 A, c=7.8 A and γ=116.4° for s-PS/DMN. Both structures can be described as intercalates, since they present ac layers of polymer helices alternated to layers of contiguous guest molecules and a guest/monomeric-unit molar ratio of 1/2, as recently observed only for s-PS/norbornadiene molecular complex. On the basis of a comparison between crystalline structures and X-ray diffraction data of several s-PS molecular complexes, a simple criterion to anticipate their clathrate or intercalate nature has been suggested.

The crystal structure of the clathrate form of syndiotactic polystyrene containing ortho-dichlorobenze-ne has been determined, and compared with those of the other crystalline clathrate structures of s-Ps already known. This guest molecule, characterized by a quite different shape and volume from the others already described, gives rise to the only example of clathrate form of s-PS presenting a statistical orthorhombic symmetry (cell constants a = 17.5 A, b = 22.9 A, c = 7.8 A and space group Ccmb). This symmetry is generated by the random coexistence in the crystals of domains, with the same monocliulc P21/a symmetry found for the other clathrate forms, that can be realized by two different but energetically equivalent, packings of the chains. 0 The unit cell constants are a = 17.5 A, b = 14.4 A, c = 7.8 A and γ = 127.4°. The shape of the guest molecule produces a significant modification of the cavities (if compared to δ form) realized mainly through a shift of the ac layers of chains delimiting the cavities along the a axis direction that turns out in the highest y value found for these kinds of structures.

The crystal structure of the clathrate form of syndiotactic polystyrene containing different amounts of CS 2 has been analyzed, and compared with that of the clathrate containing I 2 . These two guest molecules, which are characterized by a similar shape and volume, give rise to the only two known examples of clathrate forms containing up to two molecules per cavity. The two structures are isomorphous both in the case of complete and of partial occupation of the cavities. In the second case, CS 2 and I 2 molecules when alone in the cavity occupy almost identical positions at its center. Some of the literature on the crystal structure of these two clathrates is ambiguous and this is discussed. A comparison with the CS 2 clathrates of syndiotactic poly(p-methylstyrene) and poly(m-methylstyrene) is also made.

The crystal structures of the clathrate forms of syndiotactic polystyrene (s-PS), poly (p-methylstyrene) (s-PPMS) and poly (m-methylstyrene) (s-PMMS) containing guest molecules having widely different steric hindrance are compared in detail. Common features and differences concerning the packing of the chains, the shape and the dimensions of the cavities and the stability of the forms deprived of the guest molecules are pointed out. A new clathrate form of s-PPMS containing CS 2 is also described.

Models for the crystal structure of cis-1,4-poly(2,3-dimethyl-1,3-butadiene) are elucidated by the joint use of molecular mechanics and WAXS. The chain conformation can be predicted by using different sets of potential functions. A model of crystal structure calculated by molecular mechanics has been refined in order to find the best accordance with the experimental X-ray powder spectra. Another model is proposed to explain a few minor discrepancies between the calculated and the experimental WAXS powder spectra.

The crystal structure of the clathrate form of syndiotactic poly(m-methylstyrene) containing carbon disulfide is presented. The structure is characterized by polymer chains in s(2/1)2 helical conformation and CS2 molecules packed in an orthorhombic unit cell with axes a = 17.8 A, b = 13.1 A, and c = 7.8 A, according to the space group Pcaa. The calculated crystalline density is 1.14 g/cm3 for two polymer chains (eight monomer units) and four carbon disulfide molecules included in the unit cell. As in the packing model proposed for the crystal structures of the β class of the syndiotactic poly(p-methylstyrene) clathrates, the guest molecules occupy cavities delimited by isomorphous helices along the b axis and are related by the crystallographic 2-fold screw axis of the unit cell. The packing of the helices along the a axis, instead, is similar to the close packing realized by the chains only in the clathrates and in the δ form of syndiotactic polystyrene.

The crystal structure of the β class clathrate of syndiotactic poly(p-methylstyrene) containing benzene is presented and compared to that of the clathrate containing tetrahydrofuran, also belonging to the β class. The structure is characterized by polymer chains, in s(2/1)2 helical conformation, and benzene molecules packed in an orthorhombic unit cell with axes a = 19.5 A, b = 13.3 A, and c = 7.7 A, according to the space group C2221. The calculated crystalline density is 1.05 g/cm3 for two polymer chains (eight monomer units) and four benzene molecules included in the unit cell. Our analysis affords an insight into the structural features typical of the β class clathrates. As in the packing model proposed for the crystal structure of the s-PPMS clathrate containing tetrahydrofuran, the guest molecules occupy cavities delimited by isomorphic helices along the b axis and are related by the crystallographic 2-fold screw axis of the unit cell. The packing of the helices along the a axis is, instead, complet...

The crystal structure of the clathrate form of syndiotactic poly(p-methylstyrene) (s-PPMS) containing o-dichlorobenzene (o-DCB) is presented. The structure is characterized by polymer chains in s(2/1)2 helical conformation and o-dichlorobenzene molecules packed in a monoclinic unit cell with axes a = 23.4 Angstrom, b = 11.8 Angstrom, c = 7.7 Angstrom, and gamma = 115 degrees, according to the space group P2(1)/alpha. The calculated crystalline density is 1.07 g/cm(3) for two polymer chains (eight monomer units) and two o-DCB molecules included in the unit cell. The o-DCB molecules occupy cavities delimited by the phenyl rings of two enantiomorphic polymer chains. A disorder in the positioning of the o-DCB molecules inside the cavity is present. A comparison with the crystal structure of the clathrate form of s-PPMS containing tetrahydrofuran is presented. Remarkable differences were found, confirming the previous hypothesis that two different types of crystal structures (alpha and beta class) are presented by the clathrate forms of s-PPMS depending on the guest molecule.

Possible routes for obtaining oriented fibres of the various known crystalline forms of syndiotactic poly(p-methylstyrene) are described. These routes include solvent diffusion and/or annealing on mesomorphic fibres. The X-ray fibre diffraction patterns show that form I, form II and all the clathrate forms are characterized by chains with helical s(2/1)2 conformation and repeating distance of approximate to 7.8 Angstrom, while the mesomorphic form (form IV), form III as well as a new crystalline form (form V), are characterized by chains with trans planar conformation and repeating distance of approximate to 5.1 Angstrom. Copyright (C) 1996 Elsevier Science Ltd.

Yarns of different inherent viscosities, in the range 0.6-1.1 dL/g, spun in industrial plants, and drawn at room temperature to obtain mesomorphic samples, have been characterized. The evolution from the mesomorphic form toward the triclinic crystalline form has also been studied by combined differential scanning calorimetry (DSC), dynamic mechanical analyses (DMA), and accurate wide angle X-ray diffraction experiments. The DMA analysis of the mesomorphic samples allows better resolution of the glass transition and crystallization phenomena, which are superimposed in the DSC scans. The degree of molecular orientation in the mesomorphic samples, and the temperature of crystallization from the mesomorphic form (60-80-degrees-C), are essentially independent of the polymer molar mass. (C) 1994 John Wiley & Sons, Inc.

The δ -clathrate forms of syndiotactic polystyrene (s-PS) containing a homologous series of guests characterized by an increasing molecular volume ( n -alkanes ranging from n -pentane to n -dodecane) have been investigated. The present analysis allows concluding that the volume of the guest plays a crucial role in the choice of the type of δ -clathrate that can be obtained. As a matter of fact, longer n -alkanes, such as n -octane, n -nonane or n -decane, turned out to give rise triclinic δ -clathrates presenting a guest/monomeric-unit ratio of 1/8, while δ -clathrates containing shorter alkanes, like n -hexane or n -heptane, instead, present a monoclinic or a triclinic structure with a guest/monomeric-unit ratio of 1/4. These results confirm the validity of a simple criterion to predict the type of s-PS δ -clathrate with a certain guest on the basis of the experimental d 010 , as described in Macromolecules 2010, 43 , 8549–8558. Moreover, our analysis confirms previous solid state 13 C NMR and Raman spectroscopy data, indicating that shorter n -alkanes molecules, like n -hexane, are hosted in the cavities with an all- trans conformation while longer alkanes, like n -nonane, present bent conformations. The effect of n -alkanes molecules on the crystallization of s-PS amorphous samples has been investigated too.

The nucleating activity of antimony catalyst remnants on the melt crystallization of poly(ethylene terephthalate) (PET) is proved through comparisons, for samples with Sb concentrations in the range 180-1850 ppm, of the crystallization behaviors before and after a complete extraction of the catalyst remnants. These comparisons are performed by analysis of the crystallization peaks of differential scanning calorimetry (DSC), by the study of the isothermal crystallization kinetics from X-ray diffraction experiments, and by optical microscopy observations of the size of the spherulites. The dependence of the crystallization temperature (Tc, as evaluated by the position of the DSC peak) on the Sb concentration, reported in the literature, is shown to be largely overestimated. In particular, it is found that the Tc of commercial samples are essentially unaffected by the presence of the standard concentrations of Sb catalyst remnants. © 1993 John Wiley & Sons, Inc.

In this contribution, the X-ray diffraction (XRD) pattern of a highly oriented sample of syndiotactic polystyrene in the mesomorphic form, collected by an automatic diffractometer, is presented, thus providing quantitative and more complete information relative to the diffracted intensity. The structural changes induced by annealing procedures in mesomorphic samples are shown through the analysis of the XRD patterns: both unoriented and oriented mesomorphic samples are transformed gradually into α-form crystals. On the basis of these evidences and of preliminary comparisons between the calculated Fourier transform of simplified models and the experimental diffraction intensity, it is suggested that the local organization in triplets of trans-planar chains, typical of the different modifications of the α-form, would be largely present also in the disordered chain agglomerates of the mesomorphic form.

The X-ray diffraction patterns at different temperatures of alternating ethylene-tetrafluoroethylene (ETFE) and of alternating ethylene-chlorotrifluoroethylene (ECTFE) copolymer samples are compared. For ECTFE, a reversible variation with temperature of the half-width of the diffraction peaks, analogous to but much smaller than that one which corresponds to the topotactic crystal-to-crystal transition of ETFE, was found. A comparison of the dynamic mechanical spectra of the two copolymers seems to confirm the different nature of the two high-temperature relaxation peaks. In particular, the so called α and β relaxations of ECTFE occur in the amorphous phase while the so called α and α′ relaxations of ETFE occur in the amorphous and crystalline phase, respectively.

The conformation and the packing of poly(ethylene terephthalate) (PET) chains in the mesomorphic form have been investigated by accurate X-ray diffraction measurements, conformational energy analysis, and calculations of Fourier transforms of models. The X-ray diffraction pattern presented here is richer than those previously reported. In particular, besides the known meridional peaks at zeta = 1/c and 3/c, corresponding to a periodicity of 10.3 angstrom, a stronger meridional peak at zeta = 5/c has been shown. The reported Fourier transform calculations of isolated-chain models indicate that the experimental diffraction pattern of the mesomorphic form of PET can be qualitatively accounted for by extended linear chains obtained by random sequences of monomeric units in different minimum energy conformations. These random sequences of monomeric units generate chains in which any long-range correlation between the planes of the phenylene rings, along the chain, is absent.

The conformation and the packing of the chains of the alternating ethylene-chlorotrifluoro-ethylene copolymer (ECTFE), in the crystalline phase, are investigated by X-ray diffraction analysis, by calculations of the conformational energy and by calculations of the Fourier-transforms of models. These analyses suggest that the crystalline form of ECTFE is a mesomorphic form, with a pseudo-hexagonal packing of the axes of nearly trans-planar chains (interchain distances of 5,69 A) and a nearly complete translational and rotational disorder along the chain axis.

The crystallization conditions for the preparation of different modifications of the orthorhombic form (beta-form) of syndiotactic polystyrene (s-PS) are described. Crystallizing from the melt produces the beta' or alpha-form depending on the cooling rate. Crystallization by casting a film from omicron-dichlorobenze solutions produces clathrate structures (delta-form) at low temperatures (T < 130-degrees-C) and ordered beta" modifications at higher temperatures. In particular, modification very close to the beta" limiting one are obtained in the temperature range 130-140-degrees-C. Recrystallization of the beta-form has been detected by differential scanning calorimetry and studied by X-ray diffraction analysis of the annealed samples. These studies indicate that the recrystallization of the beta' form in the melting region does not involve a beta' to beta" transition, but only morphological changes. Models for the chain packing of the limiting modifications of the beta-form are suggested, by quantitative comparisons of the X-ray diffraction intensities and calculated structure factors.

The crystal structures of δ-clathrates of syndiotactic polystyrene (s-PS) with highly polar guest molecules (nitrobenzene, NB and p-nitroaniline, NA) have been defined, on the basis of X-ray diffraction and FTIR linear dichroism analyses. The s-PS/NB co-crystal presents a structure very similar to those of the δ-clathrates already described in the literature, i.e. monoclinic and with the guest molecular plane nearly perpendicular to the host helical axes. The s-PS/NA co-crystal presents a definitely different structure, i.e. triclinic and with guest molecular planes inclined with respect to the host helical axes. The definition of the co-crystalline structures has allowed to establish that the a⊥c// uniplanar orientation of the co-crystalline phase leads the guest dipoles roughly perpendicular to the film plane, i.e. to a dipole orientation suitable for poling processes. Films presenting a high degree of a⊥c// uniplanar orientation of the s-PS/NA co-crystalline phase have been obtained and characterized by X-ray diffraction measurements.

Models for chain packing for ordered and disordered modifications of the α-form of syndiotactic polystyrene are suggested, through quantitative comparisons between X-ray diffraction intensities and calculated structure factors. According to the reported analysis, zig-zag planar chains are packed in the hexagonal unit cell proposed by Greis et al., but with different orientation and relative height of the phenyl groups. In particular, the phenyl groups in both ordered and disordered modifications are packed according to rhombohedral symmetry. The disorder in the α′ modification corresponds to the statistical occurrence of two different, nearly isosteric, orientations of triplets of chains at well defined locations of a threedimensional lattice, which generate a rhombohedral symmetry for the whole unit cell.

The degree of flexibility of the mesogenic unit -PhOCH 2 PhCH 2 OPh- was studied by a geometrical and energetical analysis. The energetical analysis was effected by the calculation of maps of the conformational energy. The results indicate that, at the temperature at which LC behavior is observed for the polymers in which these units are inserted (460K), a large fraction of non-extended conformations (at least ≃50%) is foreseen for the isolated mesogenic unit.

The complex polymorphic behavior of syndiotactic polystyrene (s-PS) in melt-crystallized samples is altered by blending with poly (2,6-dimethyl-1,4-diphenylene oxide) (PPO). In particular, to render the beta form in these blends, starting with samples containing the alpha or gamma forms, requires much lower temperatures and shorter melting times than for pure s-PS. On the basis of the results, it is suggested that this phenomenon is due to more rapid loss of the memory of the alpha form, for the same temperature and time, in the melt in the presence of PPO molecules.

The crystal and molecular structure of poly(aryl ether ether diketone) has been determined through wide-angle X-ray diffraction methods and conformational analysis. The unit cell, corresponding to a statistical structure, is orthorhombic, with cell dimensions a = 9.96 ± 0.10 A , b = 4.78 ± 0.05 A and c = 10.7 ± 0.10 A . The space group is Pbcn . The statistical subperiodicity along the chain axis c = 10.7 A is slightly larger than in the case of poly(aryl ether ketone) and poly(aryl ether ether ketone), which also have statistical structures. In addition, in the fibre spectra, weak haloes in layers are observed, which correspond to a periodicity three times larger, in agreement with the chemical constitution. While the phenylenic groups take approximately ordered positions in the crystals, this is not so for the constitutional units COCO and O of neighbouring chains.

The paper is intended to summarize the studies devoted to the elucidation of the different crystal structures and crystal morphologies and of the different conditions favouring one or the other of the polymorphs of isotactic polypropylene. This outline takes into account 103 contributions produced in the course of about thirty-five years and is completed with 22 figures.

A cheap, commercial and easy-to-process hydrocarbon polymer (syndiotactic polystyrene, s-PS) can be crystallized in two different nanoporous phases (delta and epsilon) that show large uptake of several guest molecules and hence are suitable for applications in chemical separations and air/water purification as well as in sensorics. The recently discovered epsilon phase of s-PS is characterized by channel-shaped cavities crossing the unit cells along the c axis, rather than by isolated cavities as observed for the already known delta phase. Guest sorption from the delta and epsilon nanoporous phases eventually leads to two different classes of polymer co-crystals, where planar guest molecules present their planes roughly perpendicular and parallel to the polymer chain axes. The epsilon phase channels allow co-crystal formation with guest molecules much longer than the chain axis periodicity and, due to possible reactions between guest molecules, open the possibility to achieve new hybrid 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.

In the present paper we report on the clathrate structures of syndiotactic poly( p -methylstyrene) (s-PPMS) containing chlorobenzene (cell constants: a = 23.5 A, b = 12.0 A, c = 7.9 A and γ = 112.4°; space group: P 2 1 / a ) and toluene (cell constants: a = 19.5 A, b = 13.5 A, c = 7.9 A and γ = 90°; space group: P 2 1 ). Despite the extreme similarity between the shape and volume of these two molecules, they give rise to completely different clathrate structures, the first belonging to α class, the second belonging to β class. Moreover the clathrate form containing chlorobenzene represent the first case in which a α class clathrate form of s-PPMS contains two guest molecules in each cavity while the crystal structure here proposed for the s-PPMS/toluene clathrate represents a new example of chiral crystalline phase in which the polymer helices assume all the same chirality in the lattice. These results underline the fact that the choice of a particular structural organization in the process of clathrates’ formation of s-PPMS is not easily referable only to steric effects but seems to be dependant even on the chemical structure of the guest molecules. A comparison with syndiotactic polystyrene is also done.

A new crystalline clathrate class of syndiotactic poly(p-methylstyrene) has been found through thermal and WAXS analyses. In these clathrate forms, the polymer chains assume a T6G2T2G2 conformation with a repetition period of 11.7 ± 0.1 A and t2 symmetry. The new conformation, supported by molecular mechanics calculations, presents a cavity, favorable to the formation of clathrate structure, only on one side of the polymer chain. This cavity is bigger than those presented by this polymer with a helical s(2/1)2 chain conformation found for all the other clathrates. After annealing, the clathrates of this new class are transformed in form II. For this new class of clathrates the term γ is proposed.

The crystal structure of the molecular complex of syndiotactic polystyrene (s-PS) with bicyclo-[2.2.1]hepta-2,5-diene (norbornadiene, NB) is presented. In analogy to the already described s-PS molecular complexes, this structure presents a monoclinic unit cell (cell constants a = 17.5 Å, b = 14.5 Å, c= 7.8 Å, and gamma= 107.8°) in which the s(2/1)2 polymer helices and guest molecules are packed according to the space group P21/a. However, the structure of the s-PS/NB complex presents layers of contiguous guest molecules alternated with layers of enantiomorphous polymer helices and hence can be described as an intercalate phase. On the other hand, all the known structures of other s-PS molecular complexes present isolated molecules (or a couple of small molecules) into the cavities typical of the nanoporous s-PS ä form and hence can be described as clathrate phases. The molar ratio between the monomeric unit and the guest is 2/1 for the s-PS/NB intercalate phase, while it is generally 4/1 for the s-PS clathrate phases.

The crystal structures of clathrate forms of syndiotactic poly(m-methylstyrene) containing guest molecules having different steric hindrance (CS 2 , benzene and orto-dichlorobenzene) are presented. The structures are all characterized by polymer chains in s(2/1)2 helical conformation and guest molecules packed in an orthorhombic unit cell according to the space group Pcaa. All the presented clathrates belongs to β class indipendently from the dimensions of the guest molecule. In this aspect they differ both from clathrate forms of syndiotactic polystyrene, all belonging to a class, and from clathrate forms of syndiotactic polyp-methylstyrene) that belong to a or β class according to the steric hindrance of the guest molecule.

The synthesis and a preliminary structural characterization of syndiotactic poly(m-methylstyrene) are reported. A complex polymorphic behavior has been revealed. Three different polymorphic forms and two clathrate structures, characterized by the inclusion of guest molecules in the crystalline lattice, have been found. The conditions for the obtaining of the various polymorphic forms, in unoriented and. oriented fiber samples, are described. Form I, obtained by crystallization from the melt or from the amorphous phase, is characterized by chains in the s(2/1)2 helical conformation. The same helical conformation has been found in the clathrate structures containing molecules of o-dichlorobenzene or benzene. Form II can be obtained by crystallization from solutions of solvents which do not form clathrate structure or by removing the guest molecules from the clathrate forms. Form III, obtained in oriented fibers by stretching amorphous films, is a mesomorphic modification characterized by chains in the trans-planar conformation with a high degree of disorder in the lateral packing of the chains. Annealing treatments or solvent-induced crystallization of the oriented mesomorphic form allow the obtaining of oriented fibers in some of the polymorphic forms. The differences and analogies with the polymorphic behavior of syndiotactic poystyrene and poy(p-methylstyrene) are outlined.

The influence of the shape and size of guest molecules on the structural organization of the a and β class clathrates of syndiotactic poly(p-methylstyrene) is described, through the analysis of the packing model proposed for the crystal structures of the clathrate forms containing o-dichlorobenzene and tetrahydrofuran. Preliminary data of the crystal structure of the s-PPMS clathrate containing benzene and a comparison with the crystal structure of the clathrate forms of syndiotactic polystyrene are also presented.

Novel data about the crystal morphology and the electron diffraction of syndiotactic poly(p-methylstyrene) (s-PPMS) in form I and in the clathrate forms including o-dichlorobenzene and tetrahydrofuran are presented. Preparative routes of s-PPMS samples in the various polymorphs suitable for electron microscopy and diffraction have been devised. This investigation provides data, unavailable in the past literature, helpful to get direct structural information and to better understand the ways of obtaining and stabilizing the various polymorphs. (C) 2000 Elsevier Science Ltd. All rights reserved.

The crystal structure of the clathrate δ form of syndiotactic polystyrene containing 1,2-dichloroethane is presented. Polymer chains in s(2/1)2 helical conformation and 1,2-dichloroethane molecules are packed in the monoclinic unit cell with axes a=17.11 A, b=12.17 A, c=7.70 A and γ=120° according to the space group P21/a. The crystalline density of the clathrate δ form is 1.23 g cm−3 with two chains (eight monomer units) and two 1,2-dichloroethane molecules inside the unit cell. Molecules of 1,2-dichloroethane occupy isolated cavities between the benzene rings of adjacent polymer chains and are essentially only in the trans conformation. The location and the conformation of the 1,2-dichloroethane molecule are such that both chlorine atoms are sandwiched between two benzene rings of the same polymer chain.

The crystal structure of the clathrate form of syndiotactic poly(p-methylstyrene) containing tetrahydrofuran is presented. Polymer chains in the s(2/1)2 helical conformation and molecules of tetrahydrofuran are packed in the monoclinic unit cell with axes a = 18.8 Angstrom b =12.7 Angstrom, and c = 7.7 Angstrom, and with gamma = 100 degrees according to the space group P2(1)/a. Two polymer chains (eight monomer units) and four molecules of tetrahydrofuran are included in the unit cell. The calculated crystalline density is 1.13 g/cm(3). Tetrahydrofuran molecules occupy cavities delimited by the benzene rings of adjacent polymer chains along the b axis; each cavity is occupied by two molecules of tetrahydrofuran related by the 2-fold screw axis. A comparison with the crystal structures of the clathrate forms of syndiotactic polystyrene is presented.

A possible structural model for the mesomorphic form of syndiotactic poly(p-methylstyrene) is discussed. The structural analysis is performed through the comparison between the experimental X-ray diffraction intensities and the calculated intensities for model structures constituted by ordered and disordered aggregates of chains. This analysis provides support for the hypothesis that the mesomorphic form of sPPMS may be described in terms of small aggregates of trans-planar chains having a packing mode very close to that one of form III, in which disorder in the relative arrangement of neighbouring chains is present to some extent. (C) 1998 Elsevier Science Ltd. All rights reserved.

A classification for clathrate structures of syndiotactic poly(p-methylstyrene) in two classes (α and β), on the basis of X-ray diffraction analyses as well as of the different observed behavior with respect to the thermal and acetone treatment, is presented. A structural correlation between α and β class clathrates and forms I and II, respectively, is evidenced. It is also suggested that form I and form II could be the “emptied forms” of the α and β class clathrates, respectively. Similar steric hindrance of the molecules inducing clathrate structures, within each class has been observed.

Some properties and structural aspects of fibers obtained by spinning, in a wide range of take-up speed (2,800–4,400 m/min), of commercial samples of PET, produced by dimethyl terephthalate (DMT) and the terephthalic acid (TPA) processes, are compared. For a same take-up speed, the considered fibers from TPA are spun at lower pack pressures and always show higher tenacity and lower ductility than fibers from DMT. X-ray diffraction, density, and birefringence measurements indicate that, for the fibers from the DMT process, an earlier crystallization occurs which prevents the progress of the orientation of the amorphous phase. The lower molecular orientation in the amorphous phase of the fibers from DMT, in turn, accounts for their lower tenacity. The earlier crystallization of the fibers from DMT would be related to their slightly higher melting temperatures and melt viscosities, which could be due to the lower amount of constitutional defects (diethylene glycol content) in the considered PET samples from DMT compared with those from TPA. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 889–896, 1997

The structure of poly (epsilon-caprolactam) (nylon 6) in the beta mesomorphic form is here examined. The comparison of the diffraction intensity, calculated on modeled structures, with the experimental profiles, collected by us through an automatic diffractometer, is presented. This analysis has put into evidence the following limiting structural features. (a) The beta form of nylon 6 is made of small mesomorphic aggregates of chains (where the matter scatters coherently) with axes arranged in a hexagonal lattice (a = b = 0.48 nm; gamma = 120 degrees). (b) The chains have disordered conformations (and do not have a definite chirality, as it is the case for the 2(1) helices in the alpha and gamma forms) with the -CH2- chains close to nearly all-trans (antiperiplanar) and the two dihedral angles adjacent to the amide bond, +/-120 degrees) to antiperiplanar (180 degrees). This notwithstanding, the chains are straight and extended. As a result, the mean chain periodicity is close to 0.835 nm. (c) The H-bonds are formed along lines in the [100], [010], and [1(1) over bar0$] directions; they force neighboring chains within the small mesomorphic aggregates to adjust their conformation in such a way that nearly 100% of hydrogen bonds are always formed, in agreement with the IR data. In the case of the alpha and gamma more ordered forms of nylon 6, such lines are all in a unique direction, leading to hydrogen-bonded sheets (parallel to the chain axis) of enantiomorphous anticlined, alternately up and down chains (in the alpha form) or of isomorphous isoclined chains, (in the gamma form), (d) As a consequence, since the amide groups lie all at nearly the same height along z, in the beta form of nylon 6 the lines of hydrogen bonds lie in layers perpendicular to the chain asis and have the same direction within each layer. However, consecutive layers along z may have the lines of H-bonds which are not parallel (e.g., occasionally rotated by +120 or -120 degrees instead of the ''normal'' 180 degrees). (e) In the beta form of nylon 6, disorder arises also from the random substitution of up and down chains in the lattice positions.

Accurate wide angle X-ray diffraction measurements for a highly oriented polyacrylonitrile (PAN) fiber suggest the presence, in the pseudohexagonal form of PAN, of sequences with an average periodicity close to 2.4 Angstrom. This is in qualitative agreement with the mean periodicity anticipated by the knowledge of lateral spacing of the pseudohexagonal cell and of the experimental density value. A conformational analysis has allowed location of minimum energy extended conformations for stereoregular and stereoirregular model chains of PAN. The Fourier transforms of these minimum energy extended model chains have been calculated and compared to the experimental X-ray diffraction data. This comparison indicates that the paracrystalline pseudohexagonal phase of PAN is formed by elongated chain stretches, with atactic configuration. The backbone dihedral angles of these atactic stretches in the minimum energy conformation assume values close to 180 degrees, corresponding to r diads, but may present also large deviations from 180 degrees, corresponding to m dads. Where these deviations occur, the length of the projection along the chain axis of the vector connecting the centers of consecutive monomeric units is shorter than 2.5 Angstrom and close to 2.3 Angstrom, giving a mean chain periodicity in a good agreement with the experimental value.

The influence of constitutional defects (those resulting from variations of the comonomer molar ratio from 50/50 and those due to the introduction of termonomer units) and thermal history on the polymorphic behaviour of ethylene-tetrafluoroethylene (ETFE) copolymer is investigated. At low temperatures, while well developed orthorhombic structures are obtained for copolymers with a C2F4 content in the range 50–70 mol%, a content of the perfluoropropylperfluorovinylether termonomer of 1.8 mol% is already sufficient to favour formation of the so-called pseudo-hexagonal mesophase. At high temperatures, the degree of crystallinity as well as the correlation length of the hexagonal domains remain essentially unaltered by an increase of the C2F4 molar content from 50 to 80%, while both are strongly reduced by low concentrations (1–3 mol%) of the termonomer units (substantially excluded from the crystalline phase). This is expected to be a determinant factor for the improved ultimate properties typical of commercial terpolymers of ETFE.

The literature methods for the determination of the mean of the crystallite orientation distribution for the c axis, that is of the orientation coefficient f(c), for poly(ethylene terephthalate) (PET), based on the azimuthal scan of the (($) over bar 105) reflection, are reviewed. These methods appear unsuitable for samples presenting the ''tilted orientation''; that is, the molecular chain axis inclined by some degrees with respect to the fiber axis, as frequently occurs for PET fibers. A new method for the determination of f(c) for PET, also based on the azimuthal scan of the (($) over bar 105) reflection (which can be applied also to samples with ''tilted orientation''), is proposed. This method implies as a first step the determination of the tilt angle, for which the complete fiber pattern is required. A possible simplifying assumption, which allows use of the sole azimuthal (($) over bar 105) profile and makes the method also applicable to poorly oriented samples (for which the determination of the tilt angle is not easy), is also discussed. (C) 1995 John Wiley and Sons, Inc.

Conformational and packing energy calculations have been performed on the α modification of syndiotactic polystyrene. The conformational energy has been optimized as a function of the internal parameters of the chain. The packing energy has been calculated considering at first the best packing of chains in triplets and then the best packing of triplets in the space groups R 3 and P 3 . The comparison of the results of the energy calculations with the X-ray experimental data corroborates a rhombohedral mode of packing of the triplets, corresponding to the space group R 3 .