Helical Chain Conformation Research Articles

Morphology and Structure of Poly(di-n-butylsilane) Single Crystals Prepared by Controlling Kinetic Process of Solvent Evaporation

The silicon backbone conformation in poly(di-n-butylsilane) (PDBS) has been shown to be a 7/3 helix at ambient conditions, which is in marked contrast to the near-planar conformation of its homologous polymers with side chain lengths of one to three or six to eight carbon atoms. In this work, both the 7/3 helical and near-planar chain conformations are achieved by controlling the solvent evaporation rate around room temperature. The chain conformation and crystal structure obtained in this method have been correlated to the crystal morphology by wide-angle X-ray diffraction, transmission electron microscopy, electron diffraction, optical microscopy, atomic force microscopy, and UV absorption spectrum. The lath-shaped single crystals obtained at 12 °C correspond to an orthorhombic form with near-planar chain conformation whereas the lozenge-shaped single crystals obtained at 30 °C (in coexistence with the lath-shaped crystals) are orthohexagonal with a 7/3 helix.

Chain conformations of syndiotactic poly(m-methylstyrene) in the crystalline state

A conformational energy analysis of the isolated chain of syndiotactic poly(m-methylstyrene) under the constraint of a crystalline field is reported. Two different minimum energy conformations having similar energy have been found; the trans-planar conformation with tcm symmetry and the two-fold helical conformation with s(2/1)2 symmetry, according with the observed polymorphic behavior of this polymer. The calculated chain axes are in agreement with the experimental axes of 5.1 and 7.9Å found for the different polymorphic forms of syndiotactic poly(m-methylstyrene). However, only a metastable disordered modification (form III) having chains in trans-planar conformation has been described. This indicates that, even though the trans-planar conformation is, in the isolated chain as stable as the helical conformation, the packing of the chains in helical conformation is probably more efficient than that of the trans-planar chains.

Synthesis and Hierarchical Structures of Amphiphilic Polyphenylacetylenes Carrying l-Valine Pendants

In this work, we synthesized amino acid-containing poly(phenylacetylene)s and demonstrated the possibility of tuning their hierarchical structures by internal and external perturbations. A valine-acetylene adduct, 4-ethynylbenzoyl-l-valine methyl ester (5), was readily polymerized by [Rh(nbd)Cl]2, and the resultant “polyester” (1) was selectively hydrolyzed by KOH to its “polyacid” congener, poly(4-ethynylbenzoyl-l-valine) (2). (Supra)molecular structures of the polymers were characterized by NMR, IR, UV, CD, and AFM techniques. The macromolecules took helical chain conformations in solutions. Their Cotton effects varied to different extents, when the polymer, solvent, and pH changed respectively from 1 to 2, from methanol to THF, and from neutral to basic. Upon evaporation of their methanol solutions, 1 and 2 self-assembled into micellar spheres and helical cables, respectively. Changing the solvent of 1 to THF changed its folding structure to helical cables, while increasing the pH of the methanol solution of 2 led to the formation of random threads. The denaturation from helical cables to random threads is probably caused by the cleavage of interstrand hydrogen bonds by base-mediated ionization of carboxyl groups of the valine pendants.

Synthesis of poly(1-β-naphthyl-2-phenylacetylene) membranes through desilylation and their properties

The polymerization of 1-β-naphthyl-2-[( p-trimethylsilyl)phenyl]acetylene ( 8a) with TaCl 5– n-Bu 4Sn in cyclohexane provided a high molecular weight polymer ( 9a) ( M w=3.4×10 6). The corresponding monomers having p-dimethyl- t-butylsilyl and p-dimethyl(10-pinanyl)silyl groups in place of p-trimethylsilyl group in 8a also polymerized in a similar way to give high molecular weight polymers ( 9b, 9c, respectively; M w>1×10 6). All these polymers were soluble in many common solvents such as toluene and chloroform, and provided free-standing membranes by casting from toluene solution. The oxygen permeability coefficients ( P O 2 ) of 9a at 25 °C was as high as 3500 barrers. The membrane of poly(1-β-naphthyl-2-phenylacetylene) ( 10a) was prepared by desilylation of the membrane of 9a with trifluoroacetic acid. Polymer 10a was insoluble in any solvents, and showed high thermal stability (the onset temperature of weight loss in air ∼470 °C). The P O 2 value of 10a reached 4300 barrers. Not only the membrane of 9c but also its desilylation product 10c exhibited large optical rotations ([ α] D=+2924 and +9800°, respectively) and strong CD signals. This indicates that the membrane of 10c maintains the helical main chain conformation of 9c with a large excess one-handed helix sense.

Estimation of crystallinity of isotactic polypropylene using Raman spectroscopy

A method to estimate the degree of crystallinity in isotactic polypropylene has been developed. The method is based on integrated intensities of the Raman bands at 808 and 841 cm −1. From the observation of correlation splitting, Raman bands related to different conformational states were identified. This analysis indicates the existence of three different conformational states. The 808 cm −1 band was assigned to helical chains within crystals. The 840 cm −1 band was shown to be composed of a band at 840 cm −1, assigned to shorter chains in helical conformation, and a broader band at 830 cm −1 assigned to chains in non-helical conformation. In order to establish a quantitative relation between Raman scattering intensity and crystallinity samples subjected to different cooling rates and crystallisation temperatures were analysed. These results correlate well with those of differential scanning calorimetry.

Structural changes during annealing of the crystalline helical form of syndiotactic polypropylene

A sample of syndiotactic polypropylene was quenched from the melt at 0°C and kept at this temperature for 1 min. Upon heating to room temperature, it shows the disordered crystalline form I, with the chains in helical conformation. In addition, it also contains a fraction of trans-planar mesophase, formed during the short time at 0°C. It was annealed at increasing temperatures, up to 120°C, and the structural changes analyzed by x-rays and FTIR measurements. Annealing up to 90°C results in a progressive decrease in the trans-planar mesophase, and also the melting of the more disordered and smaller helical crystals. In this range, the structural parameters such as crystallinity and crystal perfection smoothly increase, whereas a steeper increase is observed after 90°C when the disordered phases have disappeared. Afterwards, the formation of the more ordered form I, characterized by a peak at 18.8° of 2θ in the x-ray diffractogram, becomes possible. The content of the mesophase was determined by measurements of sorption of a vapor at low activity; it was 30% in the initial sample. This value decreased up to the annealing temperature of 90°C, where the value of the crystallinity was found coincident with the fraction of impermeable phase derived by sorption. In addition, transport properties confirm that the disappearance of the mesophase allows the formation of the ordered crystalline form I.

Mechanical and thermal induced chain conformational transformations in syndiotactic polypropylene (sPP)

Stretching a stacked sPP lamellar morphology at room temperature leads to a mechanical induced transformation from the (t2g2)2 (helical) into the (tttt) (zigzag) chain conformation of the polymer. The so prepared samples exhibit after annealing above 80°C a thermal induced retransformation into the cell I and cell III crystal structure of the helical chain conformation. The mechanical induced chain conformational transformation as well as the thermal induced retransformation was studied by means of transmission electron microscopy and electron diffraction.

Polymorphism of Oriented Syndiotactic Polypropylene

Two syndiotactic polypropylene (sPP) samples were obtained by quenching the melt in a bath at 0 °C, for 1 min (sample 1A) and 3 days (sample 1B). In these conditions sample 1A crystallized in the disordered form I, having the chains in helical conformation, whereas sample 1B formed a mesophase with the chains in trans-planar conformation, as evident by X-rays and FTIR analysis. Samples 1A and 1B were drawn at room temperature, obtaining fibers with a draw ratio between 4 and 7, which were analyzed under tension. The fibers, with a draw ratio 4 and 5, obtained from sample 1A, show a well oriented helical form and a fraction of oriented trans-planar form. When the draw ratio reaches the value of 6 and 7, the crystalline form III with chains in trans-planar conformation is obtained. At variance, the fibers obtained from sample 1B show a progressive orientation of the trans-planar mesophase, and the appearance of form III at draw ratio of 6 and 7. Therefore, the fibers drawn at λ = 6 and 7, coming from the tw...

Physical properties of xanthan, galactomannan and their mixtures in aqueous solutions

AbstractSome new information on the conformation of xanthan in aqueous solutions is given. A single helical chain conformation characterizes xanthan in the native state. When heated over the critical temperature for conformational change ( helixŕ coil), the xanthan is denatured and renatured when it is cooled down in a localy double helical structure. A galactomannan was also characterized and its persistence length was obtained (Lp ≈︁ 90A°). Then mixtures of the galactomannan and xanthan were investigated to propose a mechanism for the specific gelation. From the results of microcalorimetry and circular dichroism, it is concluded that a complex is formed between one disordered xanthan chain and one galactomannan chain and that an ordered conformation is stabilized at temperatures lower than 25°C when the galactomannan has a M/G ratio of ≈︁ 3. This temperature corresponds to the sol‐gel transition. This is the first time that a structure of the crosslink points is demonstrated.

Molecular modelling of helical and extended-chain polyhydroxybutyrates and polytetramethylene succinate

Molecular modelling (MM) has been employed to investigate the crystalline chain conformations of a series of related aliphatic biodegradable polyesters. The bacterial (isotactic) poly (3-hydroxybutyrate) helical conformation of 2.98 Å advance/repeat unit can be cold-drawn to a nearly extended chain conformation possessing an advance of 4.6 Å/repeat unit versus 4.8 Å for complete extension. Syndiolactic PHB has an extended helical chain conformation of 3.85 Å per repeat on account of steric interference between substituents of contiguous R and S units. Molecular modelling techniques have confirmed this result and suggest that up to 25% of S configured units spread randomly or in a stereoblock fashion are isomorphous with the R repeats of isotactic helix segments. From X-ray fibre diffraction data, a low energy crystalline chain conformation for elastomeric poly(4-hydroxybutyrate), P4HB, with a pitch of 11.9 Å per 2 repeat units has been proposed. A synthetic biodegradable polyester, poly(tetramethylene succinate) or Bionolle®, undergoes a reversible strain-induced crystal modification in which the fibre identity period increases from 10.91 Å to 11.97 Å. Plausible conformers are proposed respecting these observed pitches. Modelled P4HB and Bionolle® chains in the ‘all trans’ planar zigzag form correspond to a pitch of 12.4 Å, which overestimates the observed fibre repeat values. Analysis of the crystalline structure of synthetic and bacterial polyesters gives potential insight into their biodegradability, which is controlled by specific enzyme binding characteristics.

Polymorphism of syndiotactic polystyrene. Morphology of the solvent-induced crystalline forms

Abstract The morphology of the delta and gamma forms of syndiotactic polystyrene (sPS) was investigated using scanning electron microscopy. The crystalline forms, delta and gamma, characterized by the chains in helical conformation, were obtained by solvent-induced crystallization from the amorphous glassy polymer. Dichloromethane and acetone were used to obtain the delta and the gamma form, respectively. The last form was obtained by thermal treatments of the delta form, too. The surface of the solvent-crystallized samples shows a texture characterized by oriented elements and cavities, whereas the internal part is microspherulitic in the case of the delta form and is more complex in the gamma form. At variance, the gamma form obtained by annealing the delta form shows a much more regular microspherulitic texture.

Tubular Architectures from Polymers with Tapered Side Groups. Assembly of Side Groupsviaa Rigid Helical Chain Conformation and Flexible Helical Chain Conformation InducedviaAssembly of Side Groups

Syntheses of N-[3,4,5-tris(n-dodecan-1-yloxy)phenyl]maleimide (6) and exo,exo-5,6-bis[[[3,4,5-tris(n-dodecan-1-yloxy)benzoyl]oxy]methyl]-7-oxabicyclo[2.2.1]hept-2-ene (16) and their polymerization by radical, anionic and ring opening metathesis polymerizations, respectively, to yield the corresponding polymers 7 and 17 are described. Regardless of the polymerization method by which it was synthesized, 7 produced a hexagonal columnar (Φh) liquid crystalline phase generated from the tubular architecture of the rigid 7/2 helical chain conformation surrounded by its tapered side groups. 17 also produces a Φh phase. However, in this case, the assembly of tapered side groups of 17 into a tubular architecture induced a 3/1 helical chain conformation of its own flexible backbone. These two experiments support the model in which polymers containing tapered side groups form tubular supramolecular architectures by inducing a flexible helical chain conformation during the self-assembly of their own tapered side group...

Molecular recognition directed self‐assembly of tubular supramolecular architectures from building blocks containing monodendrons as exo‐receptors and crown‐ or pseudo‐crown‐ethers as endo‐receptors

AbstractThe principles of self‐assembly of tapered exo‐receptors constructed from monodendrons containing crown‐ether, oligooxyethylenic groups and polymer backbones as endo‐receptors into tubular supramolecular architectures are described with selected examples. In the case of polymers with flexible backbones it is suggested that a helical chain conformation is induced during the assembly of its own tapered side groups. This mechanism is supported by the enhanced self‐assembling ability of tapered side groups attached to a backbone with rigid helical conformation.

Conformational study of linear alternating and mixed D- and L-proline oligomers using electronic and vibrational CD and Fourier transform IR.

Vibrational CD (VCD) spectra of a series of blocked linear, alternating D- and L-proline containing oligopeptides, dissolved in D2O and in CDCl3, are reported. For the Boc-LDL-Pro3 to Boc-DLDLDLDL-Pro8 oligomers, the VCD spectra in the amide I band is a positive couplet, opposite in sense to that obtained for (L-Pro)n oligomers. While this admits the possibility of their favoring a right-handed helical chain conformation, the amide I ir spectra for these DL oligomers in D2O indicate a mixed, apparently alternate, cis-trans conformation that prevents a simple conclusion. Their VCD in D2O evidence no narrowing and has a progressive loss in intensity (measured as delta A/A) with an increase in chain length. In CDCl3 a similar pattern of positive VCD couplets decreasing in intensity with length was seen, but the ir spectra are narrower. Their electronic CD (ECD), in the uv, also indicates a loss in intensity with increasing length. Oligomers with odd or even numbers of Pro residues have different ECD patterns, indicating that those spectra are strongly influenced by local contributions arising in the N-terminal groups. The VCD arises from dipolar and vibrational coupling of the amides in the helical structure. All the spectra are consistent with the chiral end groups leading to formation of an excess of one helical handedness. With an increase in length, the influence of this selectiveness is less and the overall CD measured decreases.

Ionic conductivity and organisation of macromolecular polyether–alkali-metal salt complexes

The ionic association, ionic conductivity and macromolecular organisation of alkali-metal salt complexes with poly(ethylene oxide)(PEO) and related polymers are described. The amorphous phase is the principal pathway for ionic mobility in PEO and two regimes of salt concentration, polymer-rich and salt-rich, may be identified. Most attention so far has been paid to the former, where a better understanding of the state of aggregation of the salt and the mechanism of ion transport are current aims of research. The salt–PEO association gives rise to helical chain conformations in which the cations are enclosed and the anions stack alongside. Extensive macromolecular organisation into domain or fibrous textures may occur in particular systems and the stacking of radical anions such as tetracyanoquinodimethanide form electronically conducting systems. Premelting transformations involving ionic motion in the organised phase give rise to liquid-crystalline behaviour in systems with aromatic anions. The organisation and ionic conductivity of novel smectic liquid-crystalline polymer electrolytes are described.

Phase organization of solvent-crystallized syndiotactic polystyrene films

Abstract Sorption and diffusion of dichloromethane vapor in the helical forms—that is, δ and γ—of syndiotactic polystyrene were investigated. The thermal transition from the δ to the γ form occurs through an intermediate mesomorphic form, also characterized by chains in helical conformation but lacking true crystalline order. The mesomorphic phase was found impermeable to the vapor at low activity, becoming permeable at higher activities. The presence of a mesomorphic phase, along with the crystalline and the amorphous forms, was suggested in the crystallized samples, too, and its fraction increases greatly going from the δ to the γ form.

A detailed single chain model for molten poly(tetrafluoroethylene) from a novel structure refinement technique on neutron scattering data

We present a new methodology that couples neutron diffraction experiments over a wide Q range with single chain modelling in order to explore, in a quantitative manner, the intrachain organization of non-crystalline polymers. The technique is based on the assignment of parameters describing the chemical, geometric and conformational characteristics of the polymeric chain, and on the variation of these parameters to minimize the difference between the predicted and experimental diffraction patterns. The method is successfully applied to the study of molten poly(tetrafluoroethylene) at two different temperatures, and provides unambiguous information on the configuration of the chain and its degree of flexibility. From analysis of the experimental data a model is derived with CC and CF bond lengths of 1.58 and 1.36 Å, respectively, a backbone valence angle of 110° and a torsional angle distribution which is characterized by four isometric states, namely a split trans state at ± 18°, giving rise to a helical chain conformation, and two gauche states at ± 112°. The probability of trans conformers is 0.86 at T = 350°C, which decreases slightly to 0.84 at T = 400°C. Correspondingly, the chain segments are characterized by long all- trans sequences with random changes in sign, rather anisotropic in nature, which give rise to a rather stiff chain. We compare the results of this quantitative analysis of the experimental scattering data with the theoretical predictions of both force fields and molecular orbital conformation energy calculations.

Hydrogen bond strength and β‐sheet propensities: The role of a side chain blocking effect

Amino acid side chains can enhance peptide group hydrogen bond strength in protein structures by obstructing the competing hydrogen bond to solvent in the unfolded state. Available data indicate that the steric blocking effect contributes an average of 0.5 kJ per residue to protein hydrogen bond strength and accounts for the intrinsic beta-sheet propensities of the amino acids. In available data for helical models, the contribution to alpha-helix propensities is obscured especially by large context-dependent effects. These issues are all related by a common side chain-dependent steric clash which disfavors peptide to water H-bond formation, peptide to catalyst complexation in hydrogen exchange reactions (Bai et al., Proteins 17:75-86, 1993), and peptide to peptide H-bonding in the helical main chain conformation (Creamer and Rose, Proc. Natl. Acad. Sci. U.S.A. 89:5937-5941, 1992) but not in beta-strands.

A pseudo threefold helical structure found in silk Langmuir-Blodgett films by electron diffraction

The high performance tensile properties of silkworm (Bombyx mori) silk fiber has resulted in a long standing interest in the semicrystalline morphology of this material. The properties of silk fiber depend not only on the chemical composition (primary protein structure) but also on the fiber spinning conditions present in the silk gland which induce the formation of a (β-sheet based crystalline morphology (secondary protein structure). Knowledge of the silk structure is essential for understanding how the natural spinning processes results in such excellent material properties, but surprisingly few experimental results are available concerning the detailed structures of silk proteins. Two β-sheet based silk fibroin crystalline structures (e.g. silk I and silk II) have been studied by many authors, but the silk I structure remains largely uncharacterized. Here we report results from thin silk films prepared by the Langmuir-Blodgett (LB) technique which display a new silk fibroin structure with a threefold helical chain conformation.

Structural analysis of random propylene-ethylene copolymers

Abstract Structural properties of random propylene ethylene copolymers with low ethylene contents between 0 % and 11 mol % were investigated using mainly X-ray techniques. One of the essential issues of this paper is that a cocrystallization of the comonomer units does not take place. This is based on the following observations: The long period and the thickness of the crystalline lamellae decrease with increasing ethylene content. The electron density difference between crystalline and amorphous phase, however, as determined with the invariant, does not show any changes. The heat of fusion, normalized to the degree of crystallinity, does not decrease. This leads to the conclusion that the observed changes are not due to an incorporation of the ethylene units as defects, but to a reduced extension of the crystalline regions, now confined by the ethylene units which interrupt the helical chain conformation. The γ-modification, detectable by means of wide-angle X-ray diffractometry from an ethylene content of 5 mol % on, partially converted into the α-modification on annealing.