Local Chain Conformation Research Articles

Glass Transitions in Aqueous Solutions of Protein (Bovine Serum Albumin)

Measurements by adiabatic calorimetry of heat capacities and enthalpy relaxation rates of a 20% (w/w) aqueous solution of bovine serum albumin (BSA) by Kawai, Suzuki, and Oguni [Biophys. J. 2006, 90, 3732] have found several enthalpy relaxations at long times indicating different processes undergoing glass transitions. In a quenched sample, one enthalpy relaxation at around 110 K and another over a wide temperature range (120-190 K) were observed. In a sample annealed at 200-240 K after quenching, three separated enthalpy relaxations at 110, 135, and above 180 K were observed. Dynamics of processes probed by adiabatic calorimetric data are limited to long times on the order of 10(3) s. A fuller understanding of the processes can be gained by probing the dynamics over a wider time/frequency range. Toward this goal, we performed broadband dielectric measurements of BSA-water mixtures at various BSA concentrations over a wide frequency range of thirteen decades from 2 mHz to 1.8 GHz at temperatures from 80 to 270 K. Three relevant relaxation processes were detected. For relaxation times equal to 100 s, the three processes are centered approximately at 110, 135, and 200 K, in good agreement with those observed by adiabatic calorimetry. We have made the following interpretation of the molecular origins of the three processes. The fastest relaxation process having relaxation time of 100 or 1000 s at ca. 110 K is due to the secondary relaxation of uncrystallized water (UCW) in the hydration shell. The intermediate relaxation process with 100 s relaxation time at ca. 135 K is due to ice. The slowest relaxation process having relaxation time of 100 s at ca. 200 K is interpreted to originate from local chain conformation fluctuations of protein slaved by water. Experimental evidence supporting these interpretations include the change of temperature dependence of the relaxation time of the UCW at approximately T(gBSA) approximately = 200 K, the glass transition temperature of protein in the hydration shell, similar to that found for the secondary relaxation of water in a mixture of myoglobin in glycerol and water [Swenson et al. J. Phys.: Condens. Matter 2007, 19, 205109; Ngai et al. J. Phys. Chem. B 2008, 112, 3826]. The data all indicate in hydrated BSA or other proteins that the secondary relaxation of water and the conformation fluctuations of the protein in the hydration shell are inseparable or symbiotic processes.

Coarse-Grained Polymer Melts Based on Isolated Atomistic Chains: Simulation of Polystyrene of Different Tacticities

We present a molecular coarse-graining approach applied to polystyrene which obtains both the bonded and nonbonded interactions of the coarse-grained model from the sampling of isolated atomistic chains and pairs of oligomers. Atomistic melt properties are not used in the parametrization. We show that the coarse-grained polystyrene model not only predicts melt properties, including the melt packing and the density between 400 and 520 K, in satisfactory agreement with the atomistic model, but also reproduces the local chain conformations of atactic as well as stereoregular polystyrene. The model takes into account and reproduces correlations between neighboring bonded degrees of freedom and therefore reproduces the conformations of detailed atomistic chains in the melt on all length scales.

Comparison Between Coarse‐Graining Models for Polymer Systems: Two Mapping Schemes for Polystyrene

AbstractWe present a detailed study of a new, optimized coarse‐grained (CG) model of polystyrene (PS) and compare it with a recently published one (Harmandaris et al.,Macromolecules2006,39, 6708). By implementing a different mapping scheme, the new model, augmented with softer nonbonded interactions, better reproduces the local chain conformations and melt packing observed in atomistic simulations of atactic PS. Both models properly predict the bonded distributions and are capable of simulating different tacticities without needing sidegroups. Both CG models fit dynamic data from long atomistic simulations after determining the scale factor for the simulation time. Together with a rigorous back‐mapping procedure from the mesoscopic to atomistic description, this opens up a very feasible way for generating very long atomistic trajectories.magnified image

First Cumulant of the Dynamic Structure Factor for Flexible Polymers. Excluded-Volume Effects

The first cumulant Ω(k) of the dynamic structure factor as a function of the magnitude k of the scattering vector was determined from dynamic light scattering (DLS) measurements for an atactic polystyrene (a-PS) sample with the weight-average molecular weight Mw=8.04×106 in toluene at 15.0 °C, an atactic poly(methyl methacrylate) (a-PMMA) sample with Mw=1.31×107 in acetone at 25.0 °C, an atactic poly(α-methylstyrene) (a-PαMS) sample with Mw=5.46×106 in toluene at 25.0 °C, and a polyisobutylene (PIB) sample with Mw=6.63×106 in n-heptane at 25.0 °C. It was also determined for the a-PS and a-PαMS samples in cyclohexane in the vicinity of the respective Θ temperatures (34.5 and 30.5 °C). The translational diffusion coefficient D was also determined from DLS measurements for the a-PS, a-PMMA, a-PαMS, and PIB samples in the same solvent conditions mentioned above. From a comparison of the present data with the previous ones for all the samples in the respective Θ temperatures, it is found that the observed dimensionless coefficient C of the k4-term in the expansion of Ω(k) does not almost depend on excluded volume, as predicted by the Tanaka–Stockmayer first-order perturbation theory of C, being in good agreement with the theoretical value for the Gaussian chain with nonpreaveraged hydrodynamic interaction. Further, it is found that the excluded-volume effect is very small on the quantity [Ω(k)/Dk2−1]/<S2>k2 over the whole range of k2 examined, where <S2> is the mean-square radius of gyration. Their first-order perturbation theory of the height of the plateau in the k3-region, however, cannot explain the behavior of the present experimental data. The disagreement between theory and experiment may be regarded as arising from the fact that the theory based on the Gaussian chain model cannot take proper account of effects of chain stiffness and local chain conformation.

Influence of Stereoregularity of the Polymer Chain on Interactions with Surfactants: Binding of Cetylpyridinium Chloride by Isotactic and Atactic Poly(methacrylic acid)

Association of a cationic surfactant cetylpyridinium chloride, CPC, with isotactic and atactic poly(methacrylic acid), i-PMA and a-PMA, respectively, in aqueous 0.01 M NaCl solutions was studied by pH and fluorescence measurements in conjunction with potentiometric studies using a surfactant-sensitive membrane electrode. pH measurements have demonstrated that the presence of an oppositely charged surfactant increases ionization of carboxyl groups on PMA at low degrees of neutralization. The increase is more pronounced in the case of i-PMA. The isotactic form of PMA is not soluble in water at zero degrees of neutralization but can be rendered soluble by the addition of CPC at the surfactant to a polyion molar ratio of around 0.4. In the solubilized complex, the positive charge of the CPC molecule is facing the polar solvent, whereas surfactant tails are oriented toward the i-PMA compact coil. Binding isotherms and cooperativity parameters show that chain tacticity has an important influence on the interaction of cetylpyridinium cation with polymethacrylate anion. At the onset of cooperative binding, the association is stronger with i-PMA than with the atactic form, as demonstrated by lower CAC values and higher values of the cooperativity parameters. In contrast, more surfactant is bound by a-PMA in the region where polyion becomes saturated with surfactant ions. Results are interpreted by taking into account local chain conformations as obtained from quantum mechanical semiempirical molecular orbital calculations. Greater hydrophobicity and possibly higher charge density of i-PMA on one hand and more flexibility of the a-PMA chain on the other are held responsible for these observations.

First Cumulant of the Dynamic Structure Factor for Polymers in Θ Solvents. Effects of Chain Stiffness and Local Chain Conformation

The first cumulant Ω(k) of the dynamic structure factor as a function of the magnitude k of the scattering vector was determined from dynamic light scattering (DLS) measurements for two atactic poly(α-methylstyrene) (a-PαMS) samples with weight-average molecular weights Mw=3.22×106 and 5.46×106 in cyclohexane at 30.5 °C (Θ) and for one polyisobutylene (PIB) sample with Mw=6.63×106 in isoamyl isovalerate at 25.0 °C (Θ). The translational diffusion coefficient was also determined from DLS measurements for the a-PαMS sample with the larger Mw and for the PIB sample in the same solvent conditions. The mean-square radius of gyration was determined from static light scattering measurements for the PIB sample. From a comparison of the present data for a-PαMS and PIB along with the previous ones for atactic polystyrene and poly(n-hexyl isocyanate) with the helical wormlike chain theory, it may be concluded that the universality predicted by the Gaussian chain theory for the plot of η0Ω(k)/kBTk3 against 1/2k cannot be realized for flexible polymers even with very high Mw (\\simeq106–107), and that the height of the plateau in the k3-region depends on chain stiffness and local chain conformation, where η0 is the solvent viscosity, kB the Boltzmann constant, and T the absolute temperature.

Differences between Ziegler–Natta and Single‐Site Linear Low‐Density Polyethylenes as Characterized by Inverse Gas Chromatography

AbstractSummary: Zero‐pressure weight‐fraction‐based Henry's constants $(H_{{\rm 12}}^0 )$ of two alkanes, two alkenes, and benzene in Ziegler–Natta (ZN) and single‐site (ss) linear low‐density polyethylenes (LLDPE) with different average branch contents, molecular‐weight averages ($\overline M _{\rm n}$ and $\overline M _{\rm w}$), and polydispersity indices (PDI) were measured in the temperature range of 170 to 230 °C using the technique of inverse gas chromatography. For both types of LLDPEs, the measured $H_{{\rm 12}}^0$ was insensitive to molecular weight and PDI. However, the branch content dependence differed significantly. In particular, ZN‐LLDPE exhibited a strong branch content dependence on $H_{{\rm 12}}^0$ while ss‐LLDPE did not. In fact, depending on the temperature and solvent used, $H_{{\rm 12}}^0$ of ZN‐LLDPE could drop as much as 21% with an increase in an average branch content from 13 to 35 branches per 1 000 backbone carbons, while the differences of $H_{{\rm 12}}^0$ between ss‐LLDPEs with comparable differences in branch content were well within experimental uncertainties (5%). This observation is also supported by the fact that comparable ZN‐ and ss‐LLDPEs exhibited larger differences in $H_{{\rm 12}}^0$ at higher average branch contents. The above findings strongly suggested that the liquid morphologies of ZN‐ and ss‐LLDPEs differ considerably. In particular, the local chain conformation and free volume characteristics (i.e., the size distribution and/or spatial distribution of the free volume holes) of them are rather different. Such differences become more pronounced at higher temperatures.The weight‐fraction‐based Henry's constant of n‐hexane and hex‐1‐ene in Ziegler–Natta (ZN) and single‐site (ss) linear low‐density polyethylenes (LLDPE) with different average branch contents.imageThe weight‐fraction‐based Henry's constant of n‐hexane and hex‐1‐ene in Ziegler–Natta (ZN) and single‐site (ss) linear low‐density polyethylenes (LLDPE) with different average branch contents.

Oligo- and Polyacrylonitrile in Dilute Solution Excluded Volume Effect

The article is concerned with the conformational characteristics of polyacrylonitrile in dilute solutions of dimethylformamide at 25°C. The excluded volume effects on the intrinsic viscosity of oligo- and polyacrylonitrile were quantitatively explained by the Yamakawa theory that takes into account the effects of chain ends and chain stiffness. The quasi-two-parameter scheme is valid for different polymer-solvent systems irrespective of the larger differences in chain stiffness, local chain conformation, and solvent condition.

On the scratch deformation of micrometric wollastonite reinforced polypropylene composites

Scratch deformation characteristics of neat and wollastonite-containing polypropylenes under identical test conditions are examined by electron microscopy and atomic force microscopy techniques. The study indicates that the severity of plastic deformation during scratch deformation in filled polypropylenes is a strong function of the debonding/detachment of wollastonite mineral particles from the polypropylene matrix. Scratch resistance is evaluated in terms of scratch hardness, scratch depth, average scratch roughness, thickness and density of the scratch tracks. Atomic force microscopy suggests the presence of a localized region surrounding the reinforcement particle that is characterized by enhanced crystal nucleation in which the local chain conformation and kinetics are likely to be different from regions that are a significant distance away from the mineral particle.

Relationship between the coil‐globule transition of an aqueous poly(N‐isopropylacrylamide) solution and structural changes in local conformations of the polymer

AbstractStructural changes in the local conformation of poly(N‐isopropylacrylamide) (PNiPA) during the thermally and solvent‐induced coil‐globule transitions in an aqueous solution were studied by using attenuated total reflection / infrared (ATR/IR) spectroscopy combined with density functional theory (DFT) calculation. DFT calculation makes it possible to connect the spectral changes observed during the transitions with the structural changes of the local conformation of polymer chains. The results suggest that some of the intramolecular C=O···H‐N hydrogen bonds of amide groups are broken, and the changes in local conformations occur during the coil‐globule transitions of PNiPA. In this paper, an overview of our recent studies on the coil‐globule transitions of PNiPA is given for introducing a new idea that may explain the stimulus sensitivities of PNiPA in solutions; the solubility of segments concerning with the local conformation of repeating monomer units is changed by an external perturbation, and then the polymer system shows the coil‐globule transition.

Mean-Square Optical Anisotropy of Oligo- and Poly(α-methylstyrene)s in Dilute Solution

The mean-square optical anisotropy 〈Γ2〉 was determined from anisotropic light scattering measurements for 10 samples of atactic oligo- and poly(α-methylstyrene)s (a-PαMS) with the fraction of racemic diads fr = 0.72 in the range of weight-average degree of polymerization xw from 2 to 67.1 in cyclohexane at 30.5 °C (ϑ). A comparison is made of the present data with the helical wormlike (HW) chain theory with the values of the model parameters previously determined from the mean-square radius of gyration and with those of the local polarizability tensor α0 per repeat unit properly assigned on the basis of the bond and group polarizabilities. The HW theory value of 〈Γ2〉/xw in the limit of xw → ∞ is found to be somewhat larger than the experimental one. It is then shown that if α0 is replaced by 0.90α0, the HW theory may reproduce quantitatively the experimental results for all the samples except those with xw ≤ 4 for which effects of chain ends become appreciable. A comparison is also made of the present results for a-PαMS with previous ones for atactic polystyrene (fr = 0.59), atactic poly(methyl methacrylate) (a-PMMA) (fr = 0.79), isotactic PMMA (fr ≃ 0.01), and poly(n-hexyl isocyanate), and the behavior of 〈Γ2〉 is shown to depend remarkably on chain stiffness and local chain conformation.

Viscosity- and Hydrodynamic-Radius Expansion Factors of Oligo- and Poly(α-methylstyrene)s in Dilute Solution

The intrinsic viscosity [η] was determined for 23 samples of atactic oligo- and poly(α-methylstyrene)s (a-PαMS), each with the fraction of racemic diads fr = 0.72, in the range of weight-average molecular weight Mw from 4.12 × 102 to 5.46 × 106 in three good solvents, toluene, 4-tert-butyltoluene, and n-butyl chloride, at 25.0 °C. The translational diffusion coefficient D was also determined from dynamic light-scattering measurements for 17 a-PαMS samples in the range of Mw from 1.04 × 103 to 3.22 × 106 under the same solvent conditions. It was found that the values of [η] in toluene and n-butyl chloride are appreciably smaller than the previous ones of [η]ϑ in cyclohexane at 30.5 °C (ϑ) in the oligomer region in which the intramolecular excluded-volume effect may be ignored, while those of [η] in 4-tert-butyltoluene are only slightly larger than the latter there because of the dependence on solvent of the hydrodynamic chain (bead) diameter. The disagreement in toluene and n-butyl chloride may be regarded as arising from the so-called specific interaction between polymer and solvent molecules, and therefore the viscosity-radius expansion factor αη in these solvents was calculated after removing its contribution. On the other hand, the values of the hydrodynamic radius RH defined from D in the three good solvents were found to agree well with the previous ones of RH,ϑ in cyclohexane at ϑ in the oligomer region, and therefore the hydrodynamic-radius expansion factor αH could be calculated straightforwardly. It was then found that both plots of αη and αH against the scaled excluded-volume parameter z̃ for a-PαMS in the three good solvents along with those for atactic polystyrene and atactic and isotactic poly(methyl methacrylate)s previously studied form their respective single-composite curves, confirming the validity of the quasi-two-parameter scheme that all expansion factors are functions only of z̃ irrespective of the differences in polymer species (chain stiffness and local chain conformation) and solvent condition.

Optically Active Polysilylenes: State-of-the-Art Chiroptical Polymers

Controlled synthesis, chiroptical characterization, and manipulation of artificial helical polymers are challenging issues in modern polymer stereochemistry. Although many artificial polymers adopting a preferential screw-sense helical structure have been investigated, optically active polysilylenes bearing chiral side chains may be among the most suitable to elucidate the inherent nature of the helical structure, since these polymers offer powerful spectroscopic probes as a result of their ideal chromophoric and fluorophoric main chain properties around 300–330 nm. The present paper will review comprehensively the helix-property-functionality relationship between side chain structure, global and local main chain conformation, (chir)optical properties, electronic properties, several helical cooperative phenomena, the effects of temperature and solvent polarity, and molecular imaging. This knowledge and understanding of the nature of the polysilylene helix might constitute a bridge between artificial polymers and biopolymers and will assist in designing and controlling new types of helical polymers directed to diverse screw-sense-related properties and applications in the future.

Adsorption of polymer/solvent complexes on silica surfaces

In this work local chain conformation, polymer/solvent interactions and particle size of different PMMA/solvent or PMMA/surfactant/solvent systems were investigated by dynamic light scattering, infrared spectroscopy and capillary viscosimetry. Aggregation can be forced, in the chlorinated solvents, through mixing with a small amount of PEG non-ionic surfactants. Nano-gel particles can also be formed by stereocomplexation of i- and s-PMMA in certain solvents. The adsorption of stereoregular PMMA on silica was investigated by 1H NMR. The strong relative lowering of the peak ascribed to isotactic sequences has provided evidence of a stereospecific adsorption of the isotactic segments. The conformation and the secondary structure induced by the PMMA stereoregularity may, therefore, be considered as a significant factor in the adsorption process. According to the miscibility of the components, PMMA/PEG mixtures are shown to increase the amount of polymer adsorbed and also to modify the stereoselectivity of the adsorption. Aggregative adsorption is assumed to deeply influence the organisation of the polymer at the silica/solvent interface. Finally, adsorption of aggregates of stereocomplexes can be used to pattern surfaces in thin films.

The application of reverse Monte Carlo modelling to a polymeric melt

The reverse Monte Carlo (RMC) modelling technique has been applied to the polymeric melt poly(propylene oxide) (PPO). The modelling was based on neutron diffraction experiments on both hydrogenous and deuterated samples. Inter-atomic distances and calculated bond angle distributions for the RMC produced model were found to be in good agreement with reported experimental results, showing the ability of the RMC method to produce realistic structural models of amorphous polymers on the molecular length scale. The strengths and limitations of the RMC models in studies of the intermediate and long range order in polymers are investigated with emphasis on the information content and information quality. Particularly, we find by studying the RMC model of PPO that the strong first diffraction peak at about 1.45 Å −1 is almost entirely due to weak inter-chain correlations. The local chain conformation was investigated by calculating the partial atomic pair correlation functions for atoms belonging to monomers close in sequence. The results show that the most probable conformation is a “stretched” trans conformation, where two consecutive methyl groups are pointing in almost opposite directions. Although the dihedral and bond angle distributions found in the model are probably somewhat broader than in reality, they show unambiguously interpretable features which support the findings above.

Hydration and Conformation of Temperature-Dependent Micellization of PEO−PPO−PEO Block Copolymers in Aqueous Solutions by FT-Raman

The micellization of four triblock copolymers of the poly(ethylene oxide)x−poly(propylene oxide)y−poly(ethylene oxide)x type (Pluronic P103, P104, P105, and F88) in aqueous solutions versus temperature is followed by Fouier transform-(FT-) Raman spectroscopy. The frequencies and relative intensities of C−H stretching bands in FT-Raman spectra are sensitive to the local polarity and conformation of block copolymer chains, and their variations with temperature are indicators of micellization. Therefore, the critical micellization temperatures of these copolymers of 10% concentration in aqueous solutions are determined. The deconvolution method is used to resolve the overlapping bands in the C−H stretching region, and the deconvoluted spectra provide information about the structural and microenvironmental changes of ethylene oxide (EO) and propylene oxide (PO) blocks, respectively. The hydration of the EO chains in the corona is found to diminish with increasing temperature, and the conformation changes to a...

Conformational Characteristics of Polymethylene Chains in Melts and in Various Phantom Chains from Explicit Atom Molecular Dynamics Simulations

Conformer populations, mean-square chain dimensions, and end-to-end distance distributions for polymethylene (PM) chains, H(CH2)nH with n = 13, 44, and 100, in melts and in various phantom chains have been studied using a parallel computational method of molecular (and Brownian) dynamics simulations with a well-calibrated explicit atom force field. The specific volume−temperature properties of the simulated melt systems are in excellent agreement with experiments. Conformational characteristics of PM chains in melts are identical to those of 1,5 phantom chains, characterized by short-range intramolecular interactions between the atoms separated by up to four skeletal bonds, which correspond to unperturbed chains in ϑ solvents according to recent implicit solvent model results of Sariban et al. and Destrée et al. Moreover, a Gaussian approximation of end-to-end distance distributions is found to be valid only for long PM chains with n greater than 100. According to our simulation results, the characteristic ratio C∞ for unperturbed polyethylene chains is found to be ca. 7.9 at 413 K. This value is larger than the generally accepted value, 6.8 (derived from intrinsic viscosity data for relatively polydisperse polyethylene samples), but is corroborated by recent small angle neutron scattering results (7.8 ± 0.4) for melt chains of nearly monodisperse samples. The fact that PM chains exhibit identical conformational characteristics in melts, 1,5 phantom chains, and ϑ solvents is attributed to the lack of any correlation between local chain conformations and (polymer−polymer or polymer−solvent) intermolecular attractions. Departures from such an ideal behavior seen for other polymers are also discussed.

Chain dimensions and scattering function of (1 → 3)- β- d-glucan simulated by the Monte Carlo method

A Monte Carlo method was used to evaluate the chain dimensions and scattering function of (1 → 3)- β- d-glucan in solution. Monte Carlo samples of the glucan chain were generated on the basis of a conformational energy map of the dimeric subunit, i.e. β-laminarabiose. The effect of the excluded volume was introduced into the model by placing hard spheres with a radius R ex at the center of mass of each glucose unit in the chain sequence. The results for the chain length dependence of the quantity 〈S 2〉 x (〈S 2〉 is the mean square radius of gyration and x is the degree of polymerization) and the radial distribution of the end-to-end distance have confirmed that the unperturbed (1 → 3)- β- d-glucan chain shows non-Gaussian-type behavior in the region of x < 200, above which the chain behaves in a Gaussian manner in its overall conformational character. However, local helical character was observed throughout the x range. In contrast with the unperturbed chain, the perturbed chain is characterized by non-Gaussian behavior over the chain length examined ( x ≤ 2000). The experimental 〈S 2〉 x vs. x data of the curdlan fractions in water-diluted cadoxen, evaluated by light scattering, were found to be simulated by perturbed chains with an excluded volume R ex = 1.0 A ̊ . The scattering function was calculated from the ensemble of generated (1 → 3)- β- d-glucan chains as a function of x ( x ≤ 40) in the q range up to 1.0 A ̊ −1 ( q is the scattering vector). The scattering function in the range 0.1 ≤ q ≤ 0.8 A ̊ −1 , where the scattering function is sensitive to the local chain conformation, reveals an appreciable oscillation. The calculated scattering function and 〈 S 2〉 for the Monte Carlo chains with x = 2–7 were in reasonable agreement with preliminary experimental results obtained by small-angle X-ray scattering for the corresponding laminara-oligosaccharides in water.

Dynamics of coarse-grained helical wormlike chains. II. Eigenvalue problems

The eigenvalue problem for the matrix representation of the diffusion operator appearing in the diffusion equation previously derived for the coarse-grained helical wormlike (HW) chain model is considered. Following the procedure already established in the study of the original diffusion equation for the discrete HW chain, the problem is solved in the subspace and block-diagonal approximations in the subspace L(1), where L is the ‘‘total angular momentum quantum number’’ and (1) indicates the one-body excitation basis set. The 1(1) eigenvalues λ1,kj thus obtained form three branches of the eigenvalue spectrum specified by the index j as before, and it is shown that λ1,k0 in the j=0 (lowest) branch at small wave number k is approximately proportional to the corresponding Rouse–Zimm eigenvalue in the Hearst version. The behavior of the eigenvalue spectra is also numerically examined taking atactic polystyrene with the fraction of racemic diads fr=0.59 and atactic poly(methyl methacrylate) with fr=0.79 as typical examples of flexible polymers. It is found that the ratio λ1,k0/λ1,10 at small k depends somewhat (appreciably for large k) on chain stiffness and local chain conformation.

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.