Structural changes induced by thermal treatments on emptied and filled clathrates of syndiotactic polystyrene

The solvent molecules included in the clathrates (δ-form) of syndiotactic polystyrene (s-PS) are removed by suitable extraction procedures. An emptied δ-form is produced which is different from the known γ-form obtained by heating. The structural changes induced by thermal treatments on emptied and filled clathrates of s-PS, have been followed by thermogravimetry, X-ray diffraction analysis, differential scanning calorimetry and Fourier-transform infrared spectroscopy. It is shown that, under suitable conditions, the emptied clathrates can be transformed into a mesomorphic form, which retains the chains in the helical conformation. This “helical” mesomorphic form, different from the previously known “planar” mesomorphic form, crystallizes by thermal treatments into the “helical” crystalline γ-form.     

Phase transitions in Nylons 8, 10 and 12 crystallized from solutions

The peculiar structural modifications of Nylons 8, 10 and 12 prepared by crystallization from solutions and reported as α*, were studied by differential scanning calorimetry (DSC) and thermomicroscopy, supported also by some X-ray diffraction (XRD) measurements. The results indicate that the phase transition shown by the α*-forms of the Nylon 8–12 series exhibits the characteristics of a melting immediately followed by crystallization into γ-form, rather than of a α* → γ solid-solid transition. When the γ-crystallization is concluded, the melting of this form takes place. DSC and XRD data demonstrate that the α* → γ transition is irreversible. The changes in the DSC traces of the Nylon 8 – 12 series were interpreted as indicative of a gradual stability increase of the α-form in comparison with the corresponding α*-form, depending on the increase of the number of methylene groups in the repeating unit.     

Studies on the β-form of isotactic polypropylene, 1. Characterization of the β-form and study of the β-α transition during heating by wide angle X-ray diffraction

The conditions for the formation of the β-form of isotactic polypropylene in the presence of a β-nucleator and the β-α transition during heating were studied by wide angle X-ray diffraction. A conditional crystallinity of the β-form, X′_β, and a structural disorder parameter, S, deduced from WAXD data were defined for the characterization of the β-form in addition to the K value. The results show that X′_β and S obviously depend on the content of β-nucleator, and that the β-α transition during heating essentially consists in the melting of the β-form and the successive recrystallization to the α-form.     

New synthesis of a highly active δ-MgCl2 for MgCl2/TiCl4/AlEt3 catalytic systems

A good supporting material for Ziegler-Natta type catalysts was obtained by reaction between powdered metallic magnesium and 1-chloro-n-C_mH_2m+1 (m = 3–9). This reaction affords a highly disordered form of MgCl₂ which was characterized by FT-IR spectroscopy and powder X-ray diffraction (XRD) analysis. This MgCl₂ form shows a crystallographic disorder much higher than that exhibited by the products obtained following the commercial activation procedures of α-MgCl₂, either mechanical or chemical. As shown by the XRD spectra, the synthetized MgCl₂ is characterized by the typical structure of the δ-form. Therefore, this MgCl₂ form, in view of its highly disordered structure, appears as a promising material for the preparation of active supported Ziegler-Natta type catalysts. Thus, by titanation of the obtained δ-MgCl₂ we have prepared some supported catalysts which have been tested in the slurry propene polymerizations, showing high activity (136000–138000 gPP/gTi) and good stereoselectivity (82–88% I.I.).     

On the conformation and the crystalline structure of an ethylene-chlorotrifluoroethylene alternating copolymer

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 Å) and a nearly complete translational and rotational disorder along the chain axis.     

SAXS/WAXS study of the annealing process in quenched samples of isotactic poly(propylene)

The structural rearrangement in samples of quenched isotactic poly(propylene) (iPP) submitted to different annealing treatments has been studied using simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS) at the synchrotron radiation source of DESY, Hamburg. From a quantitative analysis of the WAXS profiles the values of the α-monoclinic, mesomorphic, and amorphous mass fractions coexisting in the material were determined. It is demonstrated that the SAXS patterns were characterized by two different long-period values that are attributed to α- and mesomorphic periodicity, respectively. The related α- and mesomorphic volume phase fractions, calculated from the analysis of the SAXS data, are compared with the corresponding WAXS results. The rearrangement of the initial structure involves i) the thickening of the already existing α-phase lamellae at the expenses of the amorphous regions and ii) the structural rearrangement of the mesomorphic phase leading to its transformation into the α-monoclinic one.     

Crystallization of the γ-form of isotactic poly(propylene), 2. Synchrotron radiation measurements

Synchrotron radiation measurements were employed in recording wide- and small-angle X-ray diffraction patterns during the crystallization of isotactic poly(propylene) in γ-form. In particular, crystallization phenomena of this form are reported for a fraction of a random propylene-ethylene copolymer (4 wt.-% of ethylene) previously investigated by traditional diffraction methods. On slow cooling from the melt, the main decrease in long period L occurs between 88 and 76°C where the degree of crystallinity is almost constant. This may be explained by a flattening of lamellae.     

Studies on the β-form of isotactic polypropylene, 2. The melting behavior of predominantly β-form samples

Effects of crystallization conditions, heating rate, and isothermal annealing on the melting behavior of isotactic polypropylene samples with predominantly β-form were studied by differential scanning calorimetry. The results show that the β-form of these samples has a higher thermal stability and a lower recrystallization ability to the α-form after melting than the β-form of other samples.     

Synthesis and thermal properties of liquid crystalline side chain polymers with pendant cellobiose residues

Two series of thermotropic polymethacrylate (PM) samples with pendant cellobiose residues and an alkyl spacer (number of carbon atoms n = 4 and 10) were synthesized, and their mesomorphic properties were investigated to elucidate the function of acylated cellobiose moieties as discotic mesogens by differential scanning calorimetry (DSC), polarization microscopy and X-ray diffraction. The PM-4 samples with a short spacer (n = 4) showed two kinds of mesophases in a wide temperature region up to the degradation temperature of the sample, i. e., about 230°C, whereas the PM-10 samples with a long spacer (n = 10) showed a single mesophase between 45 and 135°C. X-Ray diffraction data suggested that both the mesophases formed by PM-4 and PM-10 belong to a kind of discotic columnar phases in which the side chain mesogens form the discotic columns around the polymer backbone.     

Synthesis and characterization of novel side-chain liquid crystalline polycarbonates, 2. Polycondensation of mesogenic diols and diphosgene

A series of new side-chain liquid crystalline polymers was prepared with a polycarbonate backbone, bearing (E)-4′-nitrostilbene mesogenic groups, connected to the backbone by spacers of different length. The polymers were synthesized in 1,4-dioxane by polycondensation of diphosgene and diol monomers with the general structure 2-[ω-(4′-nitrostilben-4-ylcxy)alkyloxy]-1,3-propanediol in the presence of pyridine as a proton trap and catalyst. The mesomorphic properties of all monomers, polymers and intermediates were studied by polarized optical microscopy and differential scanning calorimetry (DSC). The polymers were also studied by X-ray diffraction of non-oriented and oriented samples (fibres). All monomers were liquid crystalline with nematic and/or smectic phases. For the polymers a clear relation was found between spacer length and glass transition temperature (T_g), which decreases with increasing spacer length and between spacer length and clearing temperature, which increases with increasing spacer length. X-ray diffraction measurements indicated the presence of a smectic A double layer structure (S_Ad) for all polymers at room temperature, with an antiparallel of overlapping mesogenic groups. A distinct difference in the orientation of the mesogens was found when fibres were drawn from the smectic phase or from the nematic or isotropic phase. In the first case the mesogenic groups were oriented perpendicular to the fibre axis and in the second case the mesogenic groups were oriented parallel to the fibre axis. By gel-permeation chromatography (GPC) measurements of heated samples and by thermogravimetric analysis (TGA) and DSC the polymers were found to be thermally stable up to temperatures well above their clearing point.     

Novel structural and thermotropic behavior of poly(diphenylphosphazene)

Poly(diphenylphosphazene) (PDPhP) was synthesized by a reaction of 2,2,2-trifluoroethyl diphenylphosphinite with trimethylsilyl azide. Characteristics of the polymer exhibit a marked molecular weight dependency. DSC measurements showed that the PDPhP undergoes a mesophase T(1) type transition in the range 180 ～ 190°C and another at 240 ～ 255°C depending upon the molecular weights of the polymers. Upon heating and cooling through T(1) the higher molecular weight PDPhP transforms to a 3D orthorhombic (γ-form) with a = 1,99 nm, b = 1,05 nm and c = 0,995 nm from its original form (α-form) in which a = 1,01 nm, b = 1,01 nm and γ = 101° after passing through the disordered mesophase (δ-form). Another 3D form (β-form) with c = 1,25 nm is formed from the lower molecular weight PDPhP after heat treatment of the 2D phase (δ-form) that is obtained by quenching the specimens into ice-water (4°C) from above T(1). The conditions for the formation of the β-form depend strongly on the polymer molecular weight and its history. Lath-shaped crystals were grown from dilute dichlorobenzene solution by adding p-xylene (a non-solvent). Electron diffraction patterns of solution grown PDPhP specimens exhibit unusual hexatic triple reflections associated with the mesophase. (100) twinning due to the thermal history of the specimens has been established. A complex (multistep) heat treatment process was required to elucidate the transformation behavior in PDPhP.     

The crystal structure of polycapryllactam (nylon 8) and of polycapriclactam (nylon 10)

From X-ray diffraction patterns of uniaxially oriented fibers, the structures of Nylon 8 and Nylon 10 were determined. They crystallize in the monoclinic system (pseudohexagonal), space group P2₁/b, with four monomeric units in the unit cell. The crystallographic parameters of Nylon 8 are: a=4,77; b=9,54; c=21,9 Å; γ=120°, and those of Nylon 10: a=4,78; b=9,56; c=26,9 Å; γ=120° (c fiber axis). Both structures show a γ-type molecular packing, characterized by a shortening of the fiber repeat distances and by a twisting of the amide groups. Therefore, pleated sheets of parallel chains joined by hydrogen bonds are formed and the direction of the chains in adjacent sheets is alternately “up and down”. These structures result very similar to those of Nylon 6 and Nylon 12 in γ-form, demonstrating once more that, in this series, by increasing the number of methylene groups, the usual crystal structure becomes the γ-form.     

Crystallization of the γ-form of isotactic poly(propylene)

The pure γ-form of isotactic poly(propylene) can be obtained by melt-crystallization starting from a fraction of a random ethylene-propene copolymer (ethylene content 4 wt.-%) soluble in p-xylene at 28°C. Some structural models previously reported are tested by comparing calculated wide-angle X-ray diffraction patterns with the experimental ones. A method for the quantitative determination of the γ-form crystallinity by X-ray line profiles is proposed. The original copolymer and its fractions are characterized by differential scanning calorimetry and ¹³C NMR techniques.     

Determination of the elastic modulus of polyamide crystals along the chain axis by x-ray diffraction, 1. The α-form of nylon-6

Studies were carried out to determine the crystalline elastic modulus E₁ along the chain axis for the α-form of nylon-6 [poly(ε-caprolactam)] by X-ray diffraction. To obtain an inherent E₁ value of this polymer the problem was solved that different orders of meridional reflections give different E₁ values. Thus the disagreement was found to be due to the increase of the crystallite size by the applied stress. When corrected for this effect, the values from the 040 and 0.14.0 reflections agreed well with each other. The resulting modulus was 183 GN/m² at 23°C, which was much lower than a theoretically calculated value of 262,8 GN/m² for the fully extended structure model by Manley and Martin. This seems to be due to the slight shortening at the amide group of the molecular chain because it was confirmed from the intensity changes of the 040 and 0.14.0 reflections of this polymer by the applied stress that the amide part is more extensible than the polymethylene section. Judging from similar considerations the previously reported E₁ values for nylon-66 and nylon-6,10 are valid.     

Polymorphism of poly[bis(p-methoxyphenoxy)phosphazene]

Well characterized poly[bis(p-methoxyphenoxy)phosphazene] (PB(4-MeO)PP) has been investigated employing X-ray diffraction and differential scanning calorimetry (DSC). The DSC heating scan of the original PB(4-MeO)PP specimen exhibits one exotherm at 110°C and two endotherms at 106°C and 126°C, respectively. Only a single endotherm at 131°C appears on the second heating. X-ray diffraction measurements at elevated temperature reveal that PB(4-MeO)PP crystal is monoclinic (β-form) with unit cell dimensions a = 2,47 nm, b = 1,98 nm, c = 0,983 nm and y = 102° in the range of 106–126°C. Initially PB(4-MeO)PP specimens are observed to be orthorhombic (α-form) with a = 2,39 nm, b = 1,79 nm and c = 0,983 nm. The crystal transforms at 106°C and then changes into the β-form at 110°C according to DSC measurements. At 126°C the β-form crystal changes into a mesophase with an intermolecular distance of 1,44 nm at 150°C. The 3-dimensional orthorhombic γ-form with a = 2,21 nm, b = 2,01 nm and c = 0,963 nm is found at room temperature after specimens are cooled through T(1) = 126°C from the mesophase.     

Synthesis,structure and crystal morphology of nylon 16

The new polyamide nylon 16 was produced by polycondensation of 16-aminohexadecanoic acid in bulk at 210°C. The monomer was obtained by ring-opening hydrolysis of 2-azacycloheptadecanone which was prepared from commercial 8-cyclohexadecen-1-one through a four-step synthesis route. The polymer has a molecular weight of 13 000 and melts at 164°C. Fiber X-ray diffraction and single crystal electron diffraction measurements revealed that nylon 16 crystallizes in a monoclinic lattice with the parameters a = 0.483 nm, b = 0.935 nm, c = 4.16 nm (chain axis), γ = 121° containing two chains in the unit cell. The packing for the chains was found to be similar to that described for the γ-form characteristic of nylons, whereas a crystal phase in the α-form was not observed. Lamellar crystals of nylon 16 grown in 2-octanol were examined by electron microscopy, and their structure and morphology is discussed in relation to the crystal model conventionally accepted for the γ-form of nylons.     

Study of the long-period changes in samples of isotactic poly(propylene) obtained by quenching from the melt and subsequent annealing at different temperatures

The structural modifications induced in samples of isotactic poly(propylene) obtained by quenching from the melt at 100°C/s and subsequently annealed at 40, 60 and 80°C for different annealing times have been studied using simultaneous wide-angle and small-angle X-ray scattering at the synchrotron radiation source of DESY. The occurrance of two different long-period values is demonstrated. These values are related to the mesomorphic phase, existing in the starting quenched material, and to the α-monoclinic one, which settles during the annealing process, respectively.     

The X-ray determination of the amounts of the phases in samples of isotactic poly(propylene) quenched from the melt at different cooling rates

A new procedure for the determination of the amounts of phases in samples of isotactic poly(propylene) quenched at different cooling rates from the same melted polymer is described. According to the procedure, all the patterns corresponding to the same quenching series are simultaneously analyzed for the amounts of phases so that reliable phase fractions relative to the different samples of the series are achieved. The analysis of the results points out that for increasing cooling rates above 30°C/s a remarkable increase of the mesomorphic phase settles, mainly at the expense of the α-monoclinic one.     

Temperature dependence of the polymorphous phases of nylon 6

The α, γ, γ^* polymorphous forms and the physical mixtures of the α and γ-like forms of Nylon 6 were examined from room temperature up to the melting point by X-ray diffraction techniques. The α- and γ^*-forms evinced a more evident structural change as a function of temperature than the γ-form. The study carried out on the polymorphous forms of Nylon 6 at temperatures higher than room temperature furnished new insights in comparison to usual annealing. The level of structural order and the specific aspects of the structure were found to be largely determinant for the equilibrium state in the polymorphous forms of Nylon 6 from room temperature up to the melting point.     

Crystal structure and mesophase morphology of MBPE polyethers containing odd-number methylene spacers

A series of polyethers was synthesized from 4-[2-(4-hydroxyphenyl)ethyl]-3-methylphenol and α, ω-dibromoalkenes, MBPE-n. The polymers with odd-numbered methylene spacers show a monotropic mesophase behavior during cooling from the isotropic melt before crystallization. The crystal structures of MBPE (n = odd) polyethers have been investigated through wide angle X-ray diffraction fiber patterns. Two crystallographic periods along the c-axis are found, one of which corresponds to the length of methylene spacers, and the other is close to an integral number times the length of the repeating unit. Polarized light microscopy observations of the uniaxially oriented samples obtained through mechanical shear show a banded texture, which is a typical morphology found in liquid crystal polymers. Transmission electron microscopy experiments provide morphological evidence to identify that this mesophase is a nematic liquid crystal state based on the disclination defect patterns defined by Frank. A relationship between splay elastic constant k₁₁ and bend elastic constant k₃₃ is discussed through lamellar decoration of disclination defects.