Trans-planar Conformation Research Articles

Stress-induced structure transition of syndiotactic polypropylene via melt spinning

Syndiotactic polypropylene (sPP) fiber was prepared by melt spinning with the taken-up velocity of 200–700m/min, the conformation and crystallization of which were systematically investigated by a combination of Fourier transform infrared (FTIR) spectroscopy, wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The results indicated that sPP fibers consist of form I crystal with helical conformation at the spinning velocity of 200–300m/min, and the crystallinity and orientation are improved with the increase of spinning velocity in this range. As the spinning velocity exceeds 300m/min, sPP fibers contain mainly mesophase with trans-planar conformation and the content of form I decreases correspondingly. The crystallization behavior of sPP fiber with spinning velocity is different from that of most other crystalline polymers, i.e., the theory of orientation-induced crystallization is not well conformed to. For sPP, form I comprising of helical conformation is thermodynamically stable, though extensional stress can lead to transition from helical to trans-planar conformation, which is not favorable for the crystallization of form I.

Conformation Orientation of Syndiotactic Polypropylene Induced by Deformation at Different Temperatures

Two syndiotactic polypropylene (sPP) sample films (S0 and S90) with different initial structures were prepared in this paper by isothermal crystallization from the melt at 0 and 90 degrees C, respectively. The polymorphic transitions of the two samples induced by deformation at different temperatures (20, 40, and 60 degrees C) were investigated by rheo-optical FTIR spectroscopy. The results indicated that stretching leads to the transition from the helical to trans-planar conformation and improves the orientation of both helical and trans-planar conformations for the sPP films. With increasing stretching temperature, the conformational transitions for the two sPP samples are all suppressed, and the orientation behavior of the two samples appears completely different. The orientation degree of S0 decreases with increasing stretching temperature, while that of S90 increases. These results suggest that the stretching temperature and initial structure have great influence on the conformation transition and orientation behavior of the sPP sample. On the basis of the experimental results, the schematic illustration of the conformation transition and orientation behavior of sPP during stretching has been proposed.

Effect of 1,1,5-trihydroperfluoropentanol on the structure of unoriented poly(ɛ-caproamide) films

A change in the X-ray patterns of unoriented poly(ɛ-caproamide) films cast from poly(ɛ-caproamide) solutions in formic acid in the presence of 1,1,5-trihydroperfluoropentanol has been examined. It has been shown that the polyfluorinated alcohol increases the proportion of polymer molecules in the planar trans conformation owing to their transition from the gauche conformation constituting the amorphous phase and facilitates formation of hydrogen-bonded layers of macromolecules oriented perpendicular to the film plane.

N,N′-Diisopentyldithiooxamide

In the crystal structure of the centrosymmetric dithio­oxamide title compound, C12H24N2S2, one half-mol­ecule is present in the asymmetric unit and the entire molecule is generated by inversion. As is usual for secondary dithio­oxamides, the title compound shows the trans-planar conformation. Crystal packing is mainly supported by inter­molecular N—H⋯S and C—H⋯S inter­actions developing flat ribbon-like one-dimensional arrays. These are stacked vertically through π–π inter­actions of the core frames [regular mean plane distances of 3.659 (1) A], and the three-dimensional packing is completed by nonpolar inter­actions involving the alkyl chains.

Competitive influence of atactic polystyrene and supercritical carbon dioxide on the conformation of syndiotactic polystyrene

AbstractThe crystallization behavior of miscible syndiotactic polystyrene (sPS) and atactic polystyrene (aPS) blends with different sPS/aPS weight ratios was investigated in supercritical CO2 by using Fourier‐transform infrared spectroscopy, differential scanning calorimetry, and wide‐angle X‐ray diffraction. Supercritical CO2 and aPS exhibited different effects on the conformational change of sPS and competed with each other. Increasing the content of amorphous aPS in the blends made its effect on the conformational change of sPS gradually surpass that of supercritical CO2. Supercritical CO2 favored the formation of the helical conformation of sPS in lower temperature range and the all trans planar conformation in higher temperature range, instead of forming the latter one only in higher temperature range in ambient atmosphere. However, increasing aPS content in the blends pushed the range for forming the helical conformation to lower temperature and made the all trans planar conformation dominant in aPS/sPS 25/75 blend after treating in supercritical CO2 above 60 °C. The all trans planar zigzag conformation was more favorable than the helical conformation after mixing aPS in sPS in supercritical CO2. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1755–1764, 2007

Normal Vibrational Analysis of a trans-Planar Syndiotactic Polystyrene Chain

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.

Rotational barriers in monomeric CH 2[dbnd]CX–COOH and CH 2[dbnd]CX–CONH 2 (X is H or CH 3) and vibrational analysis of methacrylic acid and methacrylamide

The internal rotations in acrylic and methacrylic acids CH 2 CX–COOH and their amides CH 2 CX–CONH 2 (X is H or CH 3) were investigated by DFT-B3LYP calculations with 6-311+G** basis set. The potential energy curves were consistent with two minima that correspond to planar cis and trans conformation in the case of the acids (or cis and near- trans forms in the case of the amides). Acrylic acid and acrylamide were predicted to have the cis form as the low and predominant conformation of the molecules. In the case of the methacrylic acid and methacrylamide, the conformational relative stability was predicted to reverse as going from the acrylic to the metha compounds. The trans conformer in methacrylic acid or the near- trans in methacrylamide were predicted to be thermodynamically low energy structures of the molecules. The CCCO rotational barrier was calculated to vary from 4 to 6 kcal/mol in the four molecules. The OCOH and OCNH torsional barriers were calculated to be about 13 and 22 kcal/mol in the acids and the amides, respectively. The vibrational frequencies of methacrylic acid and methacrylamide were computed at the DFT-B3LYP/6-311+G** level and reliable vibrational assignments were made on the basis of normal coordinate analyses and comparison with experimental data of both molecules in their low energy conformations.

Preparation and characterization of functional disilaalkanes: Supramolecular motifs in the crystal structures of [formula omitted] [R = Me, R′ = Cl; R = R′ = Ph] mediated by C–H⋯Cl and/or C–H⋯π interactions

The hydrosilylation reaction between equimolar quantities of triphenylsilane and dichloromethyl/dichlorophenyl/trichlorovinylsilane in the presence of Karstedt’s catalyst (10 −5 mol Pt/mol of silane) affords the corresponding disilaalkanes, Ph 3 SiCH 2 CH 2 SiRR 2 ′ [R = Me, R′ = Cl ( 1); R = Ph, R′ = Cl ( 2); R = R′ = Cl ( 3)], along with minor amounts of Ph 3SiCH 2CH 2SiPh 3 ( 4) as a result of redistribution of the ligands at silicon (vinyl/H exchange). On the other hand, the analogous reaction between triphenylsilane and triethoxyvinylsilane proceeds without ligand exchange to yield the disilaalkane, Ph 3SiCH 2CH 2Si(OEt) 3 ( 5) as the sole product, which upon subsequent treatment with lithium aluminium hydride gives Ph 3SiCH 2CH 2SiH 3 ( 6). X-ray crystal structures of 1 (monoclinic, P2 1/ c) and 4 (triclinic, P 1 ¯ ) reveal a trans planar conformation of the Si–C–C–Si unit and propeller-shape stereochemical arrangement of the triphenylsilyl fragment. These molecules self assemble into 3-D supramolecular motifs via C–H⋯Cl and/or C–H⋯π interactions.

Synthesis, Characterization, and Crystalline Structure of Syndiotactic 1,2-Polypentadiene: The Trans Polymer

(E)-1,3-Pentadiene was polymerized with the CoCl2(PiPrPh2)2−MAO system, obtaining a crystalline polymer with a melting point of 168 °C. The polymer was characterized by IR, NMR (13C and 1H in solution and 13C in the solid-state), DSC, GPC, and X-ray diffraction, and it was found to have a trans-1,2 syndiotactic structure. The fiber periodicity of 5.14 ± 0.03 Å indicates a trans-planar main-chain conformation similar to the one found in 1,2-syndiotactic polybutadiene. Molecular mechanics calculations using the MM2 force field show that, for trans-1,2 syndiotactic polypentadiene (t-12 sPPD), the all-trans main-chain conformation is more stable with respect to the (T2G2) arrangement, consistent with structural data of most 1,2-syndiotactic polydienes but at variance with the cis isomer and with 1,2-poly(4-methyl-1,3-pentadiene). The crystalline structure of t-12 sPPD has been determined on the basis of a unit cell derived from fiber data and refined using the Rietveld method on unoriented patterns. Like both 1,2-syndiotactic polybutadiene and 3,4-syndiotactic polyisoprene, it adopts the high-symmetry orthorhombic space group Pbcm with the following refined lattice parameters: a = 6.59(1), b = 14.69(1), and c (chain axis) = 5.11(3) Å. The two chains in the unit cell also present, besides crystallographic intramolecular glide planes, intramolecular 2-fold axes and planes of symmetry, both orthogonal to the glide plane. The main-chain conformation is slightly distorted from planarity, while the side chains adopt the same idealized skew arrangement found in other trans-1,2 syndiotactic polydienes. The refinement, giving the quality of the available data, is satisfactory. The thermal parameters, the relatively loose packing, and some features of the refined geometry points to disorder in the structure that we were unable to detail further using simple crystallographic models.

Monitoring elasticity and orientation in syndiotactic polypropylene

The effect of temperature on the elasticity and structure of syndiotactic polypropylene (sPP) is investigated using a combination of WAXD and rheo-optical FTIR spectroscopy. sPP has a rich crystal structure, which leads to unique mechanical behavior. Beyond yield point, it exhibits elastic response associated with deformation-induced structure–structure transformation. The structure and orientation were measured both during and after uniaxial tensile stretching of films (up to 200%) as a function of temperature (25–70 °C). Our WAXD and rheo-FTIR results suggest that as the temperature increases, there is a reduction in the extent of helical to trans-planar conformational change upon stretching. When the strain is released, there is partial transformation of trans-planar conformation back to helical. The presence and orientation of the trans-planar conformation plays a key role on the elastic behavior of sPP beyond the yield point. Rheo-optical FTIR dichroism studies provide further insights into the elasticity in syndiotactic polypropylene. Different proportions of helical and trans-planar conformations orient to different extents. The helical conformation does not orient appreciably at higher temperature though they are present beyond the yield point. In contrast, the trans-planar chains show a significant increase in dichroism beyond the yield point, suggesting that there is a difference in mobility (orientation) of the helical and trans-planar chains. This further supports the importance of trans-planar chains on the elastic behavior.

Methods of preparation of novel composites of poly(ϵ‐caprolactone) and a modified Mg/Al hydrotalcite

Abstract12‐Hydroxydodecanoate (HD) anions were intercalated, via an ion‐exchange procedure, onto a Mg/Al hydrotalcite‐like compound with the formula [Mg0.65Al0.35(OH)2](NO3)0.35·0.56H2O. The obtained intercalate, characterized by chemical and thermal analyses, X‐ray powder diffraction, and Fourier transform infrared spectroscopy, had the formula [Mg0.65Al0.35(OH)2](NO3)0.08(HD)0.28·0.56H2O and an interlayer distance of 2.27 nm. Structural considerations indicated that the charge‐balancing HO(CH2)11COO− anions were accommodated in the interlayer region as a monofilm of partially interdigitated alkyl chains in a trans planar conformation and bearing the alcoholic group. The organically modified hydrotalcite was used to prepare novel composites based on poly(ϵ‐caprolactone) (PCL) with different procedures: (1) solvent casting, (2) ring‐opening polymerization of ϵ‐caprolactone, and (3) blending of precursors consisting of a PCL intercalated oligomer with a high‐molecular‐weight PCL. Microcomposites were obtained by the solvent casting of a mixture of a high‐molecular‐weight PCL and the modified hydrotalcite. The ring‐opening polymerization of ϵ‐caprolactone initiated by the OH groups of the alkyl chains intercalated in the hydrotalcite led to hybrid materials in which a low‐molecular‐weight PCL was in part intercalated into the modified hydrotalcite. Nanocomposites containing exfoliated hydrotalcite were obtained through the mixing, in different weight ratios, of hybrids consisting of PCL oligomers and modified hydrotalcite with a commercial high‐molecular‐weight PCL. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2281–2290, 2005

Vibrational spectra and molecular structure of 4,4′-azobis(pyridine N-oxide)

The vibrational and electron absorption spectra of 4,4′-azobis(pyridine N-oxide) have been studied with the help of density functional theory calculations to gain information on the molecular structure in solution. The molecules assume a trans-planar conformation in non-polar solvents, with a strong conjugation between the π electrons of the heteroaromatic rings and of the N N double bond. In polar solvents the molecules have still a center of symmetry, but the rings are not coplanar.

2,2'-Diaminodibenzyl: a rare case of crystallographically non-compliant molecular symmetry

The title compound, C14H16N2, adopts a trans-planar conformation. However, the mol­ecule, which possesses Ci point group symmetry, crystallizes in the non-centrosymmetric space group P212121. In the crystal structure, the mol­ecular symmetry is only approximately retained. The crystal packing is predominantly stabilized by N—H⋯N hydrogen bonds. Weak C—H⋯π inter­actions also contribute to the stability.

Conformational variation and crystalline phase transformation of low syndiotactic polypropylene films in stretched and stress-relaxed states

Low syndiotactic polypropylene (sPP; rrrr = 80%) films were isothermally crystallized at 0 °C (sample S0) and 90 °C (sample S90) for 65 h, respectively. Fourier transform infrared spectroscopy, differential scanning calorimetry, and wide-angle X-ray diffraction were used to characterize the structure transformation and orientation behavior of samples S0 and S90 at both stretched and stress-relaxed states. It was found that stretching (λ = 0–700%) induces the transformation of the chain conformation from helical to trans-planar form for both S0 and S90 films. The stretched S0 and S90 samples show well oriented trans-planar chains as well as partially retained helices. Simultaneously, crystalline phase transformation occurs during the stretching and relaxing processes of the investigated sPP samples, i.e., stable form I crystals can be transformed into metastable form III or mesophase by stretching samples, and vice versa. For stretched S0 sample, form III with trans-planar conformation, which generally exists in highly stretched sPP, cannot be observed, even at higher strains. For sample S90, however, stretching might induce the formation of both the form III crystals and mesophase with trans-planar chains; releasing the tension, form III again gets converted into trans-planar mesophase and form I crystals. In the stretched and stress-relaxed states of samples S0 and S90, the difference of the delicate orientation behavior and relative content of chain conformation and crystalline form can be attributed to the different heat-treating methods of the low syndiotacticity sPP. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2924–2936, 2005

Structural and Morphological Changes during UV Irradiation of the Crystalline Helical Form of Syndiotactic Polypropylene

Syndiotactic polypropylene films, in the most usual crystallographic modification (form I), were exposed to accelerated weathering in a UV device at 45 °C for increasing times. A different series of films characterized by the same structural organization were annealed at 45 °C to discern the thermal effects from those due to UV irradiation. The influence of UV irradiation and thermal treatments on the structural changes in terms of phase composition, crystallite sizes, and chain conformations was investigated. The analyzed samples were crystallized from the melt at 25 °C, obtaining the disordered crystalline form I, with chains in helical conformations, in large amounts, and a small fraction of mesophase with chains in trans-planar conformations. Diffractometric and thermal analysis have shown that UV radiation causes a strong increase in the crystallinity degree within the first 170 h of treatment. Such an increase mainly occurs by an increase in the coherent crystalline domain size in the direction perpendicular to the (200) planes. Spectroscopic data (FTIR) confirm that the increase in the crystallinity degree occurs not only by a strong reduction of the amorphous phase and a consistent increase of the conformational order of the chains in helical conformation but also by a reduction of the domains with chains in the trans-planar conformation. The trans-planar domains, very small and arranged in the amorphous matrix, are attached by UV radiation together with the amorphous phase. The observed phenomena are to be attributed to the effects of chain scission due to the photooxidation. Samples only annealed at 45 °C show only small effects of physical aging within the first 10 h of treatment.

Mechanically and thermally induced chain conformational transformations between helical form I and trans-planar form III in syndiotactic polypropylene using FT-IR and Raman spectroscopic techniques

The mechanically as well as the thermally induced chain conformational transformations in syndiotactic polypropylene (sPP) were monitored using both Raman and FT-IR spectroscopic techniques. Samples of syndiotactic polypropylene of helical crystalline conformations were elongated at different elongations up to 600%, while Raman and infrared spectra were simultaneously recorded. Quantitative analysis of these spectra using the ratio of helical to trans-planar content known as the ‘Transformation Index’ calculated from both Raman and infrared methods revealed that the relative proportion of the helical to the trans-planar conformation dramatically alters when the 100% elongation is reached, while any residual helical proportion has vanished above 500% elongation. Furthermore, we report for the first time an overall comprehensive Raman study of the helical to the trans-planar transformation of sPP during elongation up to 600% and consequent relaxation. In addition, a study of the thermal treatment of an sPP sample in the trans-planar conformation also took place. Furthermore, molecular orientation studies with the use of polarized infrared spectroscopy have been performed in order to monitor the changes in the orientation of the molecular chains during the application of stress. Both Raman and infrared experiments have demonstrated that when the tension is removed, a mixed stage of conformations is observed. Finally, the use of the above-mentioned techniques allows the monitoring of the thermally induced phase transformation from trans-planar configurations to helical in the temperature range between 60 and 90 °C. These spectroscopic findings are in agreement with differential scanning calorimetry thermal studies, which also confirm the thermally induced phase transformation.

Construct 2D nanostructure of co-adsorbed dipyridyl derivative by stabilization of hydrogen bond and long alkyl chains

The self-assembled structures of 4, 4′-dionoyl-2, 2′-dipyridyl and its co-adsorption with stearic acid on highly oriented pyrolytic graphite (HOPG) have been studied by the scanning tunneling microscopy (STM). As a result, 4,4′dionoyl-2, 2′-dipyridyl forms a planar trans conformation in two cases, and through the co-adsorption with stearic acid, 4, 4′-dionoyl-2,2′-dipyridyl and stearic acid form a well-ordered nanostructure.

Helix Distortion and Crystal Structure of the α-Form of Poly(l-lactide)

The crystal structure of the α-form of poly(l-lactide) was analyzed by the linked-atom least-squares refinements for the X-ray fiber diffraction data. The space group was orthorhombic P212121, and the unit cell contained the antiparallel two chains. The chain conformation was the 2-fold s(15*2/7) helix distorted periodically from the regular s(3*10/7) helix. The ester groups were deformed from the planar trans conformation. Potential energy calculations suggested that the periodic chain distortion resulted from the impartial stress distribution due to the interchain interactions.

Solvent induced polymorphism of quenched syndiotactic polypropylene in different liquids

The solvent induced crystallization phenomenon (SINC) was studied for syndiotactic polypropylene quenched from the melt at 0 °C and kept at this temperature a for long time. In these conditions a mesophase having the chains in trans-planar conformation was formed. The interaction polymer-solvent with liquids having different solubility parameters, derived by both the swelling and the weight uptake, considerably varies among the different liquids, showing a maximum corresponding to carbon tetrachloride (δ=8.6). A smaller maximum was found for chloroform (δ=9.3). These two maxima were attributed to interaction either with the amorphous phase or with the trans-planar mesophase.

Physical Gelation of Syndiotactic Polystyrene in the Presence of Large Molar Volume Solvents Induced by Volatile Guests of Clathrate Phases

Cooling of syndiotactic polystyrene (s-PS) solutions with bulky solvents, unsuitable as guests of s-PS clathrate phases, results in the formation of pastelike gels characterized by a trans-planar chain conformation and poor mechanical properties. Strong elastic gels with s(2/1)2 helical polymer chains forming a three-dimensional network can be, instead, obtained with the same bulky solvents by the temporary presence of a volatile solvent which is instead a suitable guest for a s-PS clathrate phase. After gel formation, the volatile solvent can be easily removed without breaking the gel structure, and the big size solvent molecules remain entrapped in the three-dimensional polymer network. The gel structures have been investigated by Fourier transform infrared spectroscopy, wide-angle X-ray diffraction, and rheological measurements.