Conformational Energies of Perfluoroalkanes. II. Dipole Moments of H(CF<sub>2</sub>)<sub>n</sub>H

Bates, T. W.; Stockmayer, W. H.

doi:10.1021/ma60001a003

Cited by 53 publications

(27 citation statements)

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“…The backbone helices must be quite stiff; indeed, the persistence length of PTFE near ambient temperature is 2-5 nm. [41,[52][53][54] This backbone stiffness can be attributed at least in part to the steric crowding of fluorine atoms, which are larger than the hydrogen atoms in polyethylene. The two energy minima near the trans-trans conforma- tion, which produce the left-or right-handed PTFE helices, are much deeper than the corresponding single minimum in polyethylene; gauche conformations, which produce kinks in the chain, have a probability of 20% or less in PTFE at 300 K. [53,54] Implications of Chain Stiffness for Models of Nafion…”

Section: Implications Of Dynamics For Nafion Chain Conformationmentioning

confidence: 99%

Backbone Dynamics of the Nafion Ionomer Studied by ¹⁹F‐¹³C Solid‐State NMR

Chen

Schmidt‐Rohr

2007

Macro Chemistry & Physics

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The chain dynamics of a perfluorinated ionomer, Nafion®, have been studied by 19F and 19F‐13C solid‐state NMR at 295 K. The backbone of Nafion is essentially poly(tetrafluoroethylene) (PTFE), which was investigated for reference. Fast uniaxial rotations of the helical backbone were confirmed in PTFE and detected similarly in Nafion, though with a distribution of amplitudes. The rotations produce motionally averaged 19F‐13C dipolar couplings and chemical shift anisotropies (CSAs) that are linearly correlated. Additional narrowing of the CSAs indicated that the backbone axis in hydrated Nafion moves with an amplitude >15°. Motional amplitudes of various backbone and side‐branch sites were inferred from motionally averaged 19F CSA parameters measured with CSA recoupling. They increase with the distance from the branch point, e.g., to >25° in the center of the side branch. Implications for the chain and supramolecular structure of Nafion are discussed.magnified image

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Section: Implications Of Dynamics For Nafion Chain Conformationmentioning

confidence: 99%

Backbone Dynamics of the Nafion Ionomer Studied by ¹⁹F‐¹³C Solid‐State NMR

Chen

Schmidt‐Rohr

2007

Macro Chemistry & Physics

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“…Each subsequent bond must be similarly placed. For PTFE, both 3-state and 4-state statistical weight matrices have previously been studied, where the 4-state matrices have been shown to be somewhat preferred [30]. For the 4-state case the possible out-of-plane angles are +15 o , +120 o , −120 o , and −15 o (or trans+, gauche+, gauche-,trans-).…”

Section: Simulation Model For Estimating Chain Lengthmentioning

confidence: 99%

“…where σ = 0.2, σ ′ = 2.0, ω = 0.2, β = 0.5 at 25 o C [30], and the rows and columns are indexed in the order +15 o , +120 o , −120 o , and −15 o . After these matrices have been normalized [31], the orientation of the second bond is dictated by U 2 , the third by U 3 , the last by U N , and all others by U k , with two exceptions.…”

Section: Simulation Model For Estimating Chain Lengthmentioning

confidence: 99%

“…A later preprint [29] suggests that efforts are underway to account for the load carrying capacity of the inclusions as well, and subsequently the methodology may be used for stiffness predictions. To date this method has not been used for stiffness predictions for reasons largely related to the limitations introduced by way of the simplifying assumptions (i.e., the commonly applied 3-state models of chain bond angles are unable to capture helical chain coiling [30]; uncompensated free-end effects, accurate identification of polymer molecular weight, etc. [22]).…”

Section: Introductionmentioning

confidence: 99%

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Application of Rotational Isomeric State Theory to Ionic Polymer Stiffness Predictions

et al. 2005

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Presently, Rotational Isomeric State (RIS) theory directly addresses polymer chain conformation as it relates to mechanical response trends. The primary goal of this work is to explore the adaptation of this methodology to the prediction of material stiffness. This multi-scale modeling approach relies on ionomer chain conformation and polymer morphology and thus has potential as both a predictive modeling tool and a synthesis guide. The Mark-Curro Monte Carlo methodology is applied to generate a statistically valid number of end-to-end chain lengths via RIS theory for four solvated Nafion cases. For each case, a probability density function for chain length is estimated using various statistical techniques, including the classically applied cubic spline approach. It is found that the stiffness prediction is sensitive to the fitting strategy. The significance of various fitting strategies, as they relate to the physical structure of the polymer, are explored so that a method suitable for stiffness prediction may be identified.

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“…4,8,12,14,15 Approximate energies of conformational defects have been estimated from IR measurements ͑1.1 kcal/mol͒, 16,17 and from measurements of the dipole moments of ␣,-substituted dihydroperfluoroalkanes ͑1.2 kcal/mol.͒ 18 For the smallest perfluorocarbon molecules, the gas phase properties have also been investigated experimentally. [19][20][21][22][23][24] It was, however, not possible to determine the exact structure or internal rotational angles for different conformational isomers for molecules with nу4.…”

Section: Introductionmentioning

confidence: 99%

The torsional potential of perfluoro n-alkanes: A density functional study

Röthlisberger

Laasonen

Klein

et al. 1996

The Journal of Chemical Physics

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The structural, vibrational, and conformational properties of small perfluoro n-alkanes C n F 2n ͑nр5͒ have been studied with different density functional models. Our calculations show that the relative conformational energies are severely underestimated within the local density approximation ͑LDA͒. The inclusion of gradient corrections ͑GC͒, on the other hand, leads to results in close agreement with experimental values, e.g., the barrier to internal rotation in C 2 F 6 is calculated to be 2.9 kcal/mol and 3.8 kcal/mol at the LDA and GC level, respectively, whereas corresponding experimental values range from 3.7-3.9 kcal/mol. A calculation of the torsional potential about the central C-C bond in C 4 F 10 gives two degenerate chiral minimum energy configurations ͑t ϩ and t Ϫ ͒ shifted away from the usual trans position at zero dihedral angle by ϳϮ12°. These two minima are separated by a barrier of ϳ0.3 kcal/mol. At least four local minima were determined on the torsional potential energy surface. Two enantiomeric gauche minima ͑g ϩ and g Ϫ ͒ at ϭϮ125°are ϳ1.0 kcal/mol higher in energy than the t configuration. Two further minima ͑gЈ ϩ and gЈ Ϫ ͒ with a relative energy of 1.9 kcal/mol are located at ϭϮ83°. The role of nonbonded interactions in determining the conformational energy landscape is discussed in some detail.

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Conformational Energies of Perfluoroalkanes. II. Dipole Moments of H(CF₂)_nH

Cited by 53 publications

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Backbone Dynamics of the Nafion Ionomer Studied by ¹⁹F‐¹³C Solid‐State NMR

Backbone Dynamics of the Nafion Ionomer Studied by ¹⁹F‐¹³C Solid‐State NMR

Application of Rotational Isomeric State Theory to Ionic Polymer Stiffness Predictions

The torsional potential of perfluoro n-alkanes: A density functional study

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Conformational Energies of Perfluoroalkanes. II. Dipole Moments of H(CF2)nH

Cited by 53 publications

References 0 publications

Backbone Dynamics of the Nafion Ionomer Studied by 19F‐13C Solid‐State NMR

Backbone Dynamics of the Nafion Ionomer Studied by 19F‐13C Solid‐State NMR

Application of Rotational Isomeric State Theory to Ionic Polymer Stiffness Predictions

The torsional potential of perfluoro n-alkanes: A density functional study

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Conformational Energies of Perfluoroalkanes. II. Dipole Moments of H(CF₂)_nH

Backbone Dynamics of the Nafion Ionomer Studied by ¹⁹F‐¹³C Solid‐State NMR

Backbone Dynamics of the Nafion Ionomer Studied by ¹⁹F‐¹³C Solid‐State NMR