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    Review of 'The elastic modulus of isolated polytetrafluoroethylene filaments'

    The elastic modulus of isolated polytetrafluoroethylene filamentsCrossref
    Overall an interesting manuscript. Please address the following comments.
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    The elastic modulus of isolated polytetrafluoroethylene filaments

    We report vibrational Raman spectra of small extended perfluoro-n-alkanes (C{n}F{2n+2} with n=6, 8-10, 12-14) isolated in supersonic jet expansions and use wavenumbers of longitudinal acoustic vibrations to extrapolate the elastic modulus of cold, isolated polytetrafluoroethylene filaments. The derived value E = 209(10) GPa defines an upper limit for the elastic modulus of the perfectly crystalline, non-interacting polymer at low temperatures and serves as a benchmark for quantum chemical predictions.

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      Comment 1:
      From the Manuscript Text
      "The longitudinal stiffness of a polyme chain is an important mechanical property which is usually hidden in a macroscopic polymer sample due to conformational flexibility and a lack of regularity. Its extraction from Raman spectra of finite oligomers in the condensed phase may require elaborate corrections for intermolecular interactions [28]. We provide the first interaction-free spectroscopic determination for PTFE from monoconformational, collisionally cooled perfluoro-nalkanes in supersonic jets."

      "The resulting elastic modulus amounts to two-thirds of the corresponding polyethylene value [8, 9], which is mainly due to the lower chain packing density in PTFE.”

      In the conclusions, you state that the extraction of mechanical properties from Raman data requires elaborated corrections for intermolecular interactions and the interaction free spectroscopic method that you have developed yields elastic modulus of cold molecules free of interaction with the molecular environment. In the following sentence you state that the elastic modulus is a function of lower chain packing density which is contradictory to your statement about the measurement not being affected by intermolecular interactions. Please clarify.

      Comment 2: Is there an effect of the temperature gradient on the observed LAMs and sampling precision from one sample to the next?

      Comment 3: From the text “Apparently, the small twisting of the carbon chain and substitution of hydrogen by fluorine in PTFE does not influence the stiffness of a single chain significantly [3], whereas they contribute to the lower friction by allowing for motion far below the melting transition [27].”

      ‘It should be emphasized that polyethylene prefers a nonhelical all-trans conformation [26].”

      “PTFE assumes a helical structure [10] which is also confirmed for small perfluoro-n-alkanes in the gas phase [11].”

      Can a short discussion be included to translate this difference between polyethylene and PTFE and the properties that are exhibited by the two at a macro scale highlighting how the single polymer chain characteristics are exhibited in polyethylene and PTFE as a material?


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