Local Chain Conformation Research Articles

Quantification of conformational disorder in glassy polycarbonate by two-dimensional nuclear magnetic resonance spectroscopy

We have investigated the localchain conformation in amorphous bisphenol-A polycarbonate at the carbonate–phenylene fragment by a solid-state nuclear magnetic resonance study. Chemical-shielding anisotropy (CSA) powder patterns have been analysed, and their sensitivity to local conformation is discussed. CSA–dipole tensor correlation experiments on isotopically enriched samples were analysed in terms of dihedral-angle distributions. The comparison with atomistic simulations allowed for a critical test of the glass model and provided information for the improvement and refinement of the model.

Significance of bound water to local chain conformations in protein crystals.

We examine how the polypeptide chain in protein crystal structures exploits the multivalent hydrogen-bonding potential of bound water molecules. This shows that multiple interactions with a single water molecule tend to occur locally along the chain. A distinctive internal-coordinate representation of the local water-binding segments reveals several consensus conformations. The fractional water occupancy of each was found by comparison of the total number of conformations in the database regardless of the presence or absence of bound water. The water molecule appears particularly frequently in type II beta-turn geometries and an N-terminal helix feature. This work constitutes a first step into assessing not only the generality but also the significance of specific water binding in globular proteins.

Second Virial Coefficient of Isotactic Oligo- and Poly(methyl methacrylate)s. Effects of Chain Stiffness and Chain Ends

The second virial coefficient A 2 was determined for isotactic poly(methyl methacrylate) (i-PMMA) over a wide range of weight-average molecular weight M w from 7.89 x 10 2 to 1.93 x 10 6 in acetone at 25.0 °C, for M w ≥ 1.55 x 10 6 in chloroform at 25.0 °C and in nitroethane at 30.0 °C, and for M w = 6.88 x 10 5 in acetonitrile at 35.0, 45.0, and 55.0 °C. (Some of the data had already been obtained in the previous work.) It is shown that the observed dependence of A 2 on M w in the oligomer region may be quantitatively explained by the Yamakawa theory that takes account of the effect of chain ends. The values of the effective excess binary-cluster integrals β 1 and β 2 associated with the chain end beads are then found to be 66 and 360 A 3 , respectively, in acetone at 25.0 °C by taking the repeat unit as a single bead. The analysis shows that the effect of chain ends remains even for relatively large M w in the good solvent as in the cases of atactic polystyrene (a-PS) and atactic poly(methyl methacrylate) (a-PMMA). The results for the true interpenetration function Ψ in A 2 without the effect of chain ends indicate that the two-parameter theory breaks down completely, as found previously for a-PS and a-PMMA ; the observed Ψ as a function of the cubed gyration-radius expansion factor α s 3 depends separately on M w and on the reduced excluded-volume strength λB. It is found that the values of Ψ for i-PMMA are appreciably smaller than thsoe for a-PMMA in the same solvent, i.e., for the same value of the binary-cluster integral β, while the former values almost coincide with the latter for the same λB. The Yamakawa theory that takes account of the effects of chain stiffness and local chain conformation on the basis of the helical wormlike chain may explain satisfactorily the observed behavior of Ψ and also the remarkable difference in it between i-PMMA and a-PMMA, the effects appearing in Ψ through both λB and the mean-square radius of gyration (S 2 ).

A verification by FTIR spectroscopy of the effect of some local chain conformations on the specific molecular interactions of solvents, esters, and polyesters with PVC

AbstractThe molecular interactions of poly(vinylchloride) (PVC) with some solvents [cyclohexanone (CH), methyl ethyl ketone (MEK) and N‐methylpirrolidone (MP)], esters [dioctylphthalate (DOP) and butyl stearate (BuSt)], and polyesters [poly(ethylene adipate) (PEA) and poly(ε‐caprolactone) (PCL)] have been investigated by FTIR spectroscopy. In all cases the band of the carbonyl group is found to shift to lower frequencies, but significant differences between the solvent and the esters, whether polymeric or not, are evidenced. For PVC‐solvent systems, the shift proves to increase linearly as PVC/solvent ratio increases, what suggests that only a definite number of polymer sites is involved. From the slopes of the straight lines this effect of composition is shown to increase in the order MP < MEK < CH, i.e., as the basicity of the solvent decreases. In contrast, for the PVC‐esters or polyester blends, a nonlinear behavior consisting of two distinct interaction processes, is obtained. The increase of shift as PVC/ester ratio increases is faster in the first process for all PVC‐ester systems and it is particularly enhanced for BuSt and, to a lesser extent, for DOP. Instead, during the second process, that increase is of little significance for BuSt relative to DOP and PCL. These results account for the saturation of the polymer structures that are capable of interacting, at different rates depending on the type of ester. Besides, the whole number of those structures appears to be lower than in the case of solvents.The results are discussed on the ground of, on one side, the mechanism of nucleophilic substitution on PVC, in the same solutions and blends, which, as found previously, is of a stereospecific nature, and, on the other, the electron‐donor‐acceptor concept (EDA) and the hard‐soft‐acid‐base concept (HSAB) as applied to both the interacting agents (solvents and esters) and the isotactic GTGT and GTTG− triad conformations as well as the heterotactic GTTT. In the light of the resulting conclusions it is suggested that: (i) the linear behavior shown by the solvents obeys the solvent ability to ensure a dynamic equilibrium between the two possible conformations of ‐mmr‐ sequence, i.e., GTGTTT and GTTG−TT, through the preferential interaction with the little likely GTTG− conformation, the content of which happens so to be constant as long as there are ‐mmr‐ sequences in solution; (ii) the nonlinear behavior of PVC‐ester or polyester binary systems reveals a nonequilibrium situation and so the conformational change GTGTTT ⇒ GTTG−TT, which is highly hindered, will occur occasionally depending on the ester nature. This enables one to attribute the fast and the slow interaction processes to the permanent GTTG−TT conformations derived from the polymerization and to the same conformations formed as the result of the conformational changes, respectively.Strong support for the above novel finding that PVC … OC interaction is of a local conformational nature is given by two additional investigations. First, a similar study with a PVC sample prepared at −50°C, shows that the carbonyl band shifts of CH and PCL are appreciably lower than those of PVC prepared at 70°C. The same holds for the blendof PCL with the latter PVC sample after substitution reaction (0.6%) at −15°C in CH with sodium benzenethiolate (NaBT). Since the PVC obtained at −50°C and the 0.6% substituted polymer exhibit a lower content of both permanent GTTG−TT conformations ad ‐mmr‐ sequence, these results agree with expectatins and confirm the above suggestions. Secondly, the changes in the CCl stretchign frequencies of PVC with increasing amounts of solvent or ester, as extensively studied, clearly indicate the occurrence of the aforementioned conformational change, and so they are consistent with our proposals as to the actual conformational nature of PVC…OC interactions. © 1995 John Wiley & Sons, Inc.

Transport Coefficients of Isotactic Oligo- and Poly(methyl methacrylate)s in Dilute Solution

The intrinsic viscosity [η] was determined for 25 samples of isotactic oligo- and poly(methyl methacrylate)s (i-PMMA), each with the fraction of racemic diads f r ≃ 0.01, in the range of weight-average molecular weight M w from 3.58 x 10 2 (trimer) to 1.71 x 10 6 in acetonitrile at 28.0°C (Θ). The translational diffusion coefficient D was also determined from dynamic light scattering measurements for 12 of them in the range of M w from 6.58 x 10 2 (hexamer) to 9.46 x 10 5 under the same solvent condition. It is found that [η] is proportional to M w 1/2 for M w ≥ 5 x 10 4 and its deviation from this asymptotic behavior is small even for smaller M w , while D is inversely proportional to M w 1/2 except for M w ≤ 2 x 10 3 . Such apparent Gaussian behavior of [η] and D over a wide range of M w is the result expected from that previously obtained for the mean-square radius of gyration (S 2 ). From an analysis of these transport coefficients on the basis of the helical wormlike (HW) chain model, it is shown that the above M w dependences of [η] and D may be well explained by the HW theories with the values of the model parameters consistent with those previously determined from (S 2 ). A comparison is also made of the present results for [η] and D for i-PMMA with the previous ones for atactic (a-) PMMA with f r = 0.79. This leads to the confirmation of the previous conclusion derived from (S 2 ) concerning the f r dependence of the chain stiffness and local chain conformation of PMMA. That is, the i-PMMA chain is of weaker helical nature than the chain that retains rather large and clearly distinguishable helical portions in dilute solution.

The interplay between the chain stiffness and the local structure of fluorine containing polymers in the melt

A detailed study of the relationship between chain stiffness and the local structural arrangements in a series of molten Fluorine containing polymers is presented. Four polymers of the form -(CF2CFX)n- are considered with X = F, Cl, Br and D together with poly(ethylene), -(CH2CH2)n-. By comparing the experimental structure factor with those obtained from atomistic models quantitative structural parameters are obtained which describe the intrachain structure. The introduction of differing substituents into these Fluorine containing polymers has a marked impact on the nature of the local chain conformation. Using these models of the intrachain structure as a starting point, we have utilised reverse Monte Carlo techniques to derive information on the spatial and orientational correlations between these chains segments in the melt. Somewhat surprisingly the rather anisotropic chains segments of -(CF2CF2)n- show less orientational correlations between near neighbour segments than the rather flexible -(CH2CH2)n- system.

Effect of some tacticity‐depending local chain conformations on the behaviour of poly(vinyl chloride). Changes of glass transition temperature through stereoselective substitution in solution

Effect of some tacticity‐depending local chain conformations on the behaviour of poly(vinyl chloride). Changes of glass transition temperature through stereoselective substitution in solution

On a novel interpretation of PVC antiplasticization based on some local chain conformations

The β transition of a poly(vinyl chloride) after either nucleophilic substitution to various extents or plasticization with distinct amounts of diocytl phthalate has been measured by Dynamical Mechanical Thermal Analysis. Both sets of experiments are shown to result in similar loss of β transition. The results are argued in the light of prior work on the substitution mechanism determining role of the mmr chain segments at the end of isotactic sequences. They strongly suggest the mmr structures to interact specifically with plasticizer thereby becoming stiffer (antiplasticization).

Excluded-volume effects on the mean-square radius of gyration and intrinsic viscosity of oligo- and poly(methyl methacrylates)

The mean-square radius of gyration and intrinsic viscosity were determined for isotactic poly(methyl methacrylate) (i-PMMA) with the fraction of racemic diads f r ≃ 0.01 in acetone at 25.0 °C and in chloroform at 25.0°C in the range of weight-average molecular weight M w from 6.58 x 10 2 to 1.93 x 10 6 . The results for the gyration- and viscosity-radius expansion factors αs and α η for i-PMMA along with those previously obtained for atactic poly(methyl methacrylate) (a-PMMA) with f r = 0.79 in the same solvents are found to become functions only of the scaled excluded-volume parameter z defined in the Yamakawa-Stockmayer-Shimada theory on the basis of the helical wormlike chain. Here, α η for i-PMMA has been calculated as before by taking account of the dependence on solvent of the Flory-Fox factor Φ o in the unperturbed state. Thus the present results along with the previous ones for atactic polystyrene (a-PS), polyisobutylene, and a-PMMA lead to the conclusion that the quasi-two-parameter scheme is valid for αs and α η for a variety of polymer-solvent systems irrespective of the differences in chain stiffness, local chain conformation, and solvent condition. This also indicates that there is no draining effect on α η at least for these systems. It is found that the effects of chain stiffness on αs and α η remain appreciable even for M w > 10 6 for i-PMMA as well as for a-PS and a-PMMA. The effects are smaller for i-PMMA than for a-PMMA both in acetone and in chloroform, reflecting the fact that the former chain is less stiff than the latter. It is also found that the values of the binary-cluster integral β between beads (segments) for the two PMMAs in the same solvent are almost identical with each other, indicating that β is independent of the stereochemical structure of the polymer chain.

Poly(N‐n‐butylitaconimide). Preparation and characterization

AbstractPoly(N‐n‐butylitaconimide) was prepared by radical polymerization in benzene and in bulk at 60°C and was subsequently fractionated at 30°C with benzene and methanol as solvent and nonsolvent, respectively. Relationships between molecular weight and intrinsic viscosity (Mark‐Houwink‐Sakurada equations) in tetrahydrofuran, benzene, and toluene at 30°C are established. From the Burchard‐Stockmayer‐Fixman plot, the characteristic ratio of this polymer is determined, and local chain conformation is discussed in relation to the termination process in radical polymerization. © 1993 John Wiley & Sons, Inc.

Stochastic dynamics simulations of polymethylene melts confined between solid surfaces

Stochastic dynamics simulations of polymethylene chains (CnH2n+2 for n=13, 28, 60) represented by united atom models have been carried out in order to investigate both the equilibrium and dynamic properties of polymer melts confined between flat solid surfaces. The packing manner of monomer segments, segment orientation and local conformations of chains are found to be independent of chain length, and agree very well with the results of the previous Monte Carlo simulations of C13H28 melts. The chains are extremely flattened close to the surface and many molecules assume essentially two-dimensional train configurations even in the case of C60H122 melts. However, the local chain conformations are only slightly affected by the surfaces. The apparent self diffusivities are found to be independent of the distance from the surface as the result of a reduced mobility normal to the surface and an enhanced mobility parallel to the surface. The absolute magnitudes of self-diffusion constants are larger than experimental values, most likely due to the application of united atom model.

High-resolution three-dimensional structure of horse heart cytochrome c

The 1.94A˚resolution three-dimensional structure of oxidized horse heart cytochrome c has been elucidated and refined to a final R-factor of 0.17. This has allowed for a detailed assessment of the structural features of this protein, including the presence of secondary structure, hydrogen-bonding patterns and heme geometry. A comprehensive analysis of the structural differences between horse heart cytochrome c and those other eukaryotic cytochromes c for which high-resolution structures are available (yeast iso-1, tuna, rice) has also been completed. Significant conformational differences between these proteins occur in three regions and primarily involve residues 22 to 27, 41 to 43 and 56 to 57. The first of these variable regions is part of a surface β-loop, whilst the latter two are located together adjacent to the heme group. This study also demonstrates that, in horse cytochrome c, the side-chain of Phe82 is positioned in a co-planar fashion next to the heme in a conformation comparable to that found in other cytochromes c. The positioning of this residue does not therefore appear to be oxidation-state-dependent. In total, five water molecules occupy conserved positions in the structures of horse heart, yeast iso-1, tuna and rice cytochromes c. Three of these are on the surface of the protein, serving to stabilize local polypeptide chain conformations. The remaining two are internally located. One of these mediates a charged interaction between the invariant residue Arg38 and a nearby heme propionate. The other is more centrally buried near the heme iron atom and is hydrogen bonded to the conserved residues Asn52, Tyr67 and Thr78. It is shown that this latter water molecule shifts in a consistent manner upon change in oxidation state if cytochrome c structures from various sources are compared. The conservation of this structural feature and its close proximity to the heme iron atom strongly implicate this internal water molecule as having a functional role in the mechanism of action of cytochrome c.

A Monte Carlo study of the excluded‐volume effect on the amylosic chain conformation

AbstractA Monte Carlo procedure was used to determine the effect of excluded volume on the conformational dimensions of amylosic chains. The excluded volume was introduced into the model by assuming that hard spheres, which cannot overlap each other, exist at the center of mass of each glucose unit in the chain sequence. Monte Carlo chains, which were generated to be distributed consistent with the potential energy of nonbonded nearest‐neighbor interactions, underwent self‐intersections, and the attrition rate in the generation of self‐avoiding chains was found to obey an exponential decay law with increasing chain length x. Thus, in order to generate effectively a large number of self‐avoiding chains with long sequences, we used the Wall–Erpenbeck s‐p method of chain enrichment [F. T. Wall and J. J. Erpenbeck (1959) J. Chem. Phys. 30, 634–637]. By examination of the radial distribution of the end‐to‐end distance and the chain‐length dependence of the quantity 〈r2〉/xl2 (〈r2〉 is the mean square end‐to‐end distance and l is the virtual bond length), it was found that unperturbed amylosic chains change in overall conformation from a non‐Gaussian chain having a helical character to Gaussian as x is increased, whereas perturbed chains do not show Gaussian behavior even at x = 500. For the perturbed chains, 〈r2〉 can be expressed by the equation 〈r2〉 = axb in the range of 100 ≤ x ≤ 500, where a and b > 1 are constants. From comparisons of the persistence vectors and perspective drawings of representative unperturbed and perturbed chains, we felt the local conformation of the amylosic chains, i.e., the local helical character, is also affected by the long‐range excluded‐volume interaction.

The influence of solvent on the static properties of polymer chains in solution

The conformational properties of flexible polymer chains in dilute and concentrated solutions have been studied by computer simulation using the molecular dynamics technique. The polymer itself is represented by a freely linked chain consisting of 16 beads and the low-molecular solvent by the same particles as the chain beads. Equilibrium properties of the chain such as the end-to-end distance, the radius of gyration and the average bond angle are computed. All these quantities are compared with those obtained for an isolated chain without solvent particles. We conclude that within the accuracy of our calculation the behaviour of the chain immersed in a monomeric solvent is mainly determined by intrachain interactions rather than by solvent effects, i.e. the solvent has almost no effect on the static structure of the chain. Studies of multiple chain systems at a density of 0·8 demonstrate that both the local chain conformation and chain dimensions are very similar to those expected for an ideal gaussian chain. The influence of attractive forces on the static properties is also investigated and a qualitative comparison of present results with those of other authors is made.

Dielectric relaxation of poly(vinyl acetate)

Dielectric relaxation measurements of undiluted and diluted poly(vinyl acetate) PVAc have been made at temperatures 288<T<366 K over an extremely wide frequency range from 1 μHz to 150 MHz. The complex permittivity can be described quantitatively by the Havriliak–Negami equation ε*=ε∞+Δε[1+(jωτ0)β]−α, 0<α, β≤1. It has been found that the parameter α is given by a linear equation of dipole moment ratio g as α=1.09–0.91g. This observation suggests that α is connected closely with local chain conformations. On the other hand, β is nearly constant ∼0.87 above the glass transition temperature and is independent of effect of the diluent. This suggests that the constant value of β is an inherent characteristic of the relaxation spectrum of PVAc.

Radial Distribution Function Study for Ethylene–Propylene Random Copolymer by X-Ray Diffraction Method

The local chain conformation and packing in the amorphous solid of a polymer were determined for ethylene–propylene random copolymer (EPRC) by the analysis of the radial distribution function (RDF) obtained from X-ray diffraction measurement at room temperature. A new method was devised for removing the contribution of H atoms to the observed RDF in such a polyatomic system consisting of C and H atoms, and the C-C pair RDF for EPRC was derived. From the C-C pair RDF, the number of intrachain and interchain atoms were obtained. From this result, it was found that the amorphous bulk state and the isolated state in θ-solvent are nearly equal in the conformational probabilities for trans and gauche states in the EPRC chain.

ON THE COLD CRYSTALLIZATION OF POLY (ETHYLENE TEREPHTHALATE) FROM ITS GLASSY STATE

In this paper we studied the molecular mechanism of the cold crystallization upon heating quenched Poly(ethylene terephthalate) (PET). From morphological observations we found spherulites in the samples annealed at lower temperature although it was deduced in early papers that cold crystallization occurs in the form of fringed micellar structure. The cold crystallization kinetics of PET was also examined by the dilatometric method. The dependence of the Avrami exponent on the thermal history indicated that the interpretation of crystallization process in terms of the Avrami treatment is inadequate. A structural similarity was found for samples crystallized by the solvent induced crystallization and by the cold crystallization. The DSC exothermic peak owing to the cold crystallization disappeared in cold drawn PET in which the changes of local chain conformation in glassy state occurred by stress. From these facts we discussed the molecular mechanism of cold crystallization in quenched PET.

Use of the vibrational spectrum for the determination of the average local molecular structure of liquid alkanes by selective deuteration

It is shown that combined use of the infrared spectrum and selective deuteration provides a way to study the local average conformation of polymethylene chains. The method is applied to the case of liquid n-nonadecane with CD 2 groups selectively placed in positions 2 or 10.

Simulation of the brownian motion of macromolecular chains. I. Local motions and chain conformations

AbstractChains confined to a tetrahedral lattice can be considered as models for aliphatic polyethers and polyethylene. The deformations of such chains are analyzed by computer simulation using the Monte Carlo method. The elementary motions that are taken into consideration are three‐bond and four‐bond motions. The local mobility is analyzed in terms of conformational characteristics and compared with some NMR relaxation data.

Angular correlations in linear copolymers from the compositional dependence of their dipole moments. V. Solvent–polymer interactions

Mean square effective dipole moments μ2e per structural unit have been determined at 298 K from a set of poly(4-chlorostyrene, 4-methylstyrene) copolymers, P (4CS,4MS), in p-xylene solutions containing 10 mole % copolymer. Coefficients which are related to local chain conformations have been evaluated from an expansion of the dipole moment ratio R (x) =μ2e(x)/μ2e(1) into a series of linearly independent functions of the mole fraction x of 4 CS units in the copolymer. The coefficient of the quadratic term, F2=1.01±0.06, obtained from these solutions is compared with the value 1.36±0.09, reported previously for the same set of copolymers in the undiluted state at 428 K. Extrapolations based on smoothed temperature coefficients which were derived from measurements in the range 360 to 440 K, yield a value F2=1.15±0.04 for the undiluted copolymers at 298 K. Uncertainties are standard deviations. It is argued that the copolymer method is relatively immune to the usual difficulties of determining dipole moments of molecules in condensed matter because it is framed in terms of ratios of experimental values of effective dipole moments. It is therefore suggested that the observed difference in values of F2 at 298 K reflects a difference in the local chain conformations resulting from the difference in the ’’solvent’’–polymer interactions in the two cases. Approximate values of the Kirkwood correlation factor g=μ2e/μ21 of P4CS, where μ1 is the moment of an equivalent freely orienting 4CS unit, are calculated from our results and compared with values obtained by others. This comparison supports the finding of Burshtein and Stepanova that g of P4CS increases toward unity as the thermodynamic quality of the solvents increase. We suggest that the copolymer method may provide a reliable means of separating a local component from the long range (excluded volume) dimensional changes that occur with changes of solvents.