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The elastic modulus of isolated polytetrafluoroethylene filaments

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      Abstract

      We report vibrational Raman spectra of small extended perfluoro-n-alkanes (CnF2n+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, noninteracting polymer at low temperatures and serves as a benchmark for quantum chemical predictions.

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      Most cited references 28

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      The crystal structure of long-chain normal paraffin hydrocarbons. The “shape” of the

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        Structures of Molecules and Crystals of Fluoro-Carbons

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          Longitudinal Acoustical Vibrations of Finite Polymethylene Chains

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            Author and article information

            Affiliations
            [1]Institut für Physikalische Chemie, Universität Göttingen, Tammannstr. 6, 37077 Göttingen, Germany
            Author notes
            [*]Corresponding author's e-mail address: msuhm@123456gwdg.de
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            Journal
            SOR-MATSCI
            ScienceOpen Research
            ScienceOpen
            2199-1006
            15 September 2014
            : 0 (ID: bb398bf3-5d4e-4069-b8db-5dcf53481c8a)
            : 0
            : 1-6
            1779:XE
            10.14293/S2199-1006.1.SOR-MATSCI.AKA0J6.v2
            © 2014 Drawe et al.

            This work has been published open access under Creative Commons Attribution License CC BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Conditions, terms of use and publishing policy can be found at www.scienceopen.com.

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            Figures: 4, Tables: 2, References: 29, Pages: 15
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            Answers to Reviewer #2: Ewan Blanch Comment 1: The authors state "This adds a gas phase perspective to polymer science which can be exploited for rigorous comparison between theory and experiment", but the conformational dynamics of many polymers are influenced by intermolecular interactions. Can this approach be applied to other polyalkanes, or even other polymers, to characterise their elastic modulii? Answer: Yes, we think that the approach will be more widely applicable, beyond the current examples of polyethylene and polytetrafluoroethylene. We are currently starting to work on other examples. One requirement is that the stretched conformation is a global minimum or at least close to the global minimum for short oligomers. The second requirement is that these oligomers should have a significant vapor pressure before they thermally decompose. The third requirement is that the longitudinal acoustic mode is prominent enough to be separated from vibrations of other competing conformations. The fourth requirement is that intermolecular interaction does not change the intramolecular force constants too much. Otherwise, the resulting elastic modulus of a single strand may deviate significantly from that of the perfectly ordered bulk material. We now summarize this briefly in the revised version by extending one of our last statements: Single molecular filament characterization in terms of their longitudinal elasticity is now feasible by supersonic jet spectroscopy for fragments with a vapor pressure on the order of at least 10 μbar, if the necessary heating prior to the expansion is tolerated for some hours without decomposition, if the longitudinal acoustic vibration is sufficiently Raman active, and if the conformational diversity can be controlled by rapid cooling. To make the result useful for the bulk material, it requires furthermore sufficiently weak inter-strand interactions. Comment 2: How do the low frequency longitudinal bands compare with those from the corresponding bulk solid phase Raman spectra? Answer: The deviations are rather small. For example, for C18H38 the jet LAM-1 wavenumber is 6 cm-1 (~5%) smaller compared to the solid state value [1,2]. Similarly, LAM-1 wavenumbers of jet-isolated perfluoroalkanes with chain length 9, 10, and 12 are between 6 and 10 cm-1 (<9%) smaller than solid state LAM-1 wavenumbers [3]. We summarize this briefly in the revised version before the discussion: Like in the case of alkanes [9,12], condensed phase frequencies of the perfluoroalkane LAM modes [4] are typically a few percent higher than the present unperturbed gas phase values due to lamellar and other intermolecular effects. Comment 3: Can molecular dynamics simulations be applied to these PTFE systems to verify that the conformation of the predicted low energy structure is significant? Answer: Such molecular dynamics calculations would be very valuable and we encourage them - in the case of PTFE, the spectra are so unambiguous in terms of conformational uniformity at low temperature that one can use them as an experimental benchmark for such calculations, but other cases may be less clear (see above). Comment 4: How is sample decomposition (and therefore sample integrity) monitored? How often was photodecomposition by the 18W laser detected (if at all)? Answer: Decomposition products would be noticed in the spectra and were not found. Note that the interaction of the PTFE molecules with the focussed 18W laser is on the order of only 100 ns, due to supersonic carrier gas flow. Due to the absence of an electronic absorption band in the visible range, there will be no significant excitation. More critical is the storage of the compound in the thermal oven prior to the expansion, but again this is uncritical for perfluoroalkanes at less than 400 K. We mention this briefly in the revised version: The risk of sample decomposition by the laser is negligible due to the sub-microsecond residence time and lack of absorption bands in the visible. [1] Schaufele & Shimanouchi, J. Chem. Phys. 47, 3605–3610 (1967). [2] Takeuchi et al., Chem. Phys. Lett. 28, 449–453 (1974). [3] Rabolt & Fanconi, Polymer 18, 1258–1264 (1977).
            2014-10-27 10:13 UTC
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            Answers to Reviewer #1: Anshu Mathur 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. Answer: Indeed, this is potentially misleading and we now clarify in the conclusions: The resulting elastic modulus amounts to two-thirds of the corresponding polyethylene value [8, 9], which is mainly due to the larger cross-section of a single PTFE strand caused by the fluorine atoms. Because the elastic modulus refers to the force per area, more polyethylene than PTFE strands must be stretched per unit area. This results in a higher stiffness of polyethylene for geometrical rather than energetical reasons. Comment 2: Is there an effect of the temperature gradient on the observed LAMs and sampling precision from one sample to the next? Answer: No, the band centers are close to their low temperature limit and because our samples are molecular and monoconformational, there is no sample variation issue. We now add to the end of the Results section: Residual thermal shifts of the band positions compared to the 0 K value are expected to be of a similar size as calibration shifts, at most. And we extend the introductory sentence: This technique allows us to determine the elastic modulus for cold molecules free of interactions with a molecular environment and free of any sample variations characteristic for macroscopic samples. 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? Answer: Yes, we now add: ... they contribute to the lower friction by allowing for motion far below the melting transition [27]. This motion is probably facilitated by the propensity for stretched conformations in perfluoroalkanes (see Fig. 2) in contrast to the ease with which normal alkanes kink and form gauche angles [9] and also by the absence of the regular zig-zag pattern of stretched alkyl chain segments, which tends to lock in when chains glide past each other.
            2014-10-27 10:11 UTC
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