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Tuning The Tensile Strength Of Cellulose Through Vapor Phase Metallization Keith E. Gregorczyk, David F Pickup, Miren Garcia Sanz, Itxasne Azpitarte Irakulis, Celia Rogero, and Mato Knez Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm503724c • Publication Date (Web): 04 Dec 2014 Downloaded from http://pubs.acs.org on December 15, 2014

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Chemistry of Materials

Tuning The Tensile Strength Of Cellulose Through Vapor Phase Metalation Keith E. Gregorczyk1, David F. Pickup, Miren Garcia Sanz1, Itxasne Azpitarte Irakulis1, Celia Rogero2,3, and Mato Knez,1,4 1

CIC nanoGUNE Consolider, Tolosa Hiribidea, 76, E-20018 Donostia-San Sebastian, Spain 2

Centro de Física de Materials, Paseo Manuel de Lardizabal, 5, 20018 Donostia-San Sebastian, Spain

3

Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal, 4, 20018 Donostia-San Sebastian, Spain 4

Ikerbasque, Basque Foundation for Science, 48011, Bilbao, Spain

Corresponding author: [email protected] Abstract The infiltration of transition metals into biopolymers by means of vapor phase processes has been shown to unusually change the bulk mechanical properties of those materials. Here for the first time a novel, single precursor infiltration process was applied to cellulose. The mechanical properties, as measured through uni-axial tensile testing, showed improvement as a function of the total number of infiltration cycles as well as the precursor used. For cellulose infiltrated with diethyl zinc with only four infiltration cycles the ultimate tensile strength was seen to nearly double from ~160 MPa to ~260 MPa. A significant increase is also seen in the elastic modulus with values increasing ~2.5X, from ~1.8 GPa to ~4.5 GPa. In contrast, cellulose infiltrated with trimethyl aluminum showed very little improvement in the mechanical properties. By choosing the appropriate 1 ACS Paragon Plus Environment


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precursor and/or number of cycles the mechanical properties become tunable. The chemical changes in the cellulose structure were measured with Raman spectroscopy and a novel semi in-situ x-ray photoelectron spectroscopy experiment. The results of both spectroscopic techniques were used to propose a reaction scheme.

1. Introduction Vapor phase infiltration techniques are modified versions of atomic layer deposition (ALD) that take advantage of the mobility of a vaporized precursor to diffuse into and react with synthetic and/or natural polymers. Unlike ALD, however, these techniques do not follow the same self-limiting surface reaction rules typically associated with ALD, but rather rely on diffusion of the precursors into subsurface areas and their reaction with buried functionalities.1 Despite this, the infiltration methodology relies on the same technical principles, i.e. the sequential supply of vaporized precursors in a solvent-free environment and has been found to be uniquely suited for subsurface reactions with polymers. Sub-surface reactions due to precursor infiltration were originally witnessed as a side effect of standard ALD processes on soft synthetic materials.2 The first intentional use of the technique was done with standard paired precursors on spider silk and showed significant changes in the mechanical properties due to chemically induced changes in the protein structure and cross-linking of the proteins with metal ions. The process was named multiple pulsed vapor phase infiltration (MPI).3 Subsequently several variations of infiltration were published with similar precursor pulsing strategies, each with a slightly different name such as, sequential vapor infiltration (SVI)4 and sequential infiltration synthesis (SIS).5, 6 All of these processes are inherently the same; using vapor phase

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chemistry to modify subsurface regions of soft materials and will be referred to generally as infiltration. For the technique to be successful standard ALD precursors are given significantly longer exposure times, as compared to a standard ALD process, so as to allow for diffusion into the material, chemical modification of the substrate, and the creation of a hybrid organic/inorganic material or a hybrid area in a material. Infiltration has also been demonstrated on several synthetic polymer systems including polypropylene (PP), polyvinyl alcohol (PVA), polyamide-6 (PA6), various polyesters (PBT, PET, PLA), polycarbonate (PC), polymethyl methacrylate (PMMA), polytetraflouroethylene (PTFE), and polyethylene oxide (PEO). 5-10 Infiltration investigations with biopolymers have been somewhat more limited and include the previously mentioned spider silk3, 11 and egg collagen.11, 12 Vapor phase interactions with cellulose have only been studied via standard ALD processes. TiO2 replicas (where the cellulose substrate is thermally decomposed in a post annealing step) and composite TiO2/cellulose structures (no thermal decomposition) were fabricated. The resulting structures were shown to be photocatalytic by monitoring the formation of Ag nanoparticles through photo-reduction of Ag(I) ions.13 Additional studies with replicated cellulose were conducted using Ir decorated Al2O3/cellulose and TiO2/cellulose, in these cases the photocatalytic activity was monitored through methylene blue degradation.14 Similar work was done using nanocellulose to create core/shell aero gels and nanotubes.15 Transitions of the wetting properties of cotton fibers were studied as a function of ALD cycle numbers. It was shown that the fibers become hydrophobic after only a few cycles of a TMA/H2O; the authors further noted qualitatively similar results with other fiber



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systems modified with TiO2, TiN, and ZnO though no data were presented.9, 16 The conductivity of ZnO coated cotton and polypropylene were also studied.17 Finally, a high temperature, as compared to the results reported here, chemical vapor deposition study was conducted at 190°C using nanocellulose aerogels coated with TiO2.18 The TiO2 coated nanocellulose aerogels showed photoswitchable superabsorbency. Cellulose is a polysaccharide composed of individual d-glucose units and is of particular interest for a variety of reasons. It is the most abundant biopolymer on earth and exists in several different polymorphs cellulose I (mainly from plants), cellulose II (bacterial algae), cellulose III, and cellulose IV. Cellulose I, the primary source for cotton and paper, has two different crystalline phases Iα, a triclinic crystal structure, and Iβ with a monoclinic crystal structure. Furthermore cellulose also has a hierarchical structure of fibers, microfibrillated fibers, and microfibrils each containing both crystalline and amorphous regions and are largely bound together through hydrogen bonds.19 Given that cellulose is a virtually unlimited resource, adaptations of cellulose for a particular technology or application is very appealing. Besides the ecological aspects, using cellulose may play an important future role, as cellulose is CO2 neutral. Based on such considerations, a variety of potential applications in a large number of areas are currently being pursued including fiber based reinforcement,20 advanced water filtration,21 energy storage,22 and printed electronics.23 However, the strategy presented here may allow significantly more functional or even multifunctional hybrid cellulose materials. The earlier infiltration strategies applied standard precursor pairs as used for conventional ALD processes, i.e. TMA/H2O, DEZ/H2O, and/or TTIP/NH3. Here we present a simplified process design, the first infiltration study of cellulose and the first


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infiltration study using single precursors. This simplified strategy was chosen to specifically target and consume the –OH bonds that naturally create so-called ‘water bridges’ binding cellulose into its hierarchical structure. Furthermore this strategy allows better control over the by-products of the reaction, for example, the supply of a counter precursor like H2O or similar will produce novel functionalities within the substrate that were not present before. With this scope, the reaction pathways will differ in the single precursor vs. paired precursor strategies. With this new strategy in mind the mechanical properties were measured and shown to be dependent not only on the total amount of precursor the cellulose has been exposed to but also on which precursor is used for the process, introducing a certain level of tunability. Chemical changes were monitored through standard ex-situ Raman spectroscopy as well as through a novel procedure developed to allow semi in-situ XPS analysis of the initial stages of the reaction with cellulose samples while avoiding contamination due to adventitious carbon.

2. Results and Discussion

2.1. Uni-axial Tensile Testing As mentioned in the introduction, previous studies with spider silk3, 11 and egg collagen,11, 12 have shown that small modifications to the binding structure through infiltration can lead to dramatic changes in the bulk mechanical properties. Therefore the mechanical properties of cellulose were monitored as a function of infiltration cycles of either trimethyl aluminum (TMA) or diethyl zinc (DEZ). Figure 1(a-c) shows the cellulose fibers aligned on the jig, an optical image of the fiber after alignment, and the jig attached to the measurement system. After infiltration fibers are individually aligned 5 ACS Paragon Plus Environment


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onto jigs using high tensile strength epoxy they are loaded into the mechanical tester. The final step is to cut the jig, see the black arrows in Figure 1c, allowing uni-axial tensile measurement of the individual fiber. The results of these experiments are shown in Figures 2a and 2b where changes in the ultimate tensile strength (UTS) and elastic modulus (EM) were witnessed. Untreated fibers were measured to have an UTS of ~160 MPa and an EM of ~1.75 GPa. TMA infiltrated cellulose shows a slight increase in both the UTS and EM at around 4 infiltration cycles with values of ~175 MPa and ~2.5 GPa respectively, after which a slight decrease back to the control value (within error) at 10 cycles of infiltration, followed by a steady increase to 50 cycles where both DEZ and TMA have, within error, the same value of ~225 MPa. A similar, though slightly more pronounced, trend is shown for the EM where after an increase to ~2.5 GPa at 4 cycles followed by a decrease to the control value after around 20 cycles and finally a slight increase back to ~2.5GPa at 50 cycles. A considerably larger increase is seen for cellulose infiltrated with DEZ. At 4 cycles the UTS nearly doubles to ~260 MPa and the EM shows an even higher increase, ~2.5x, with a value of ~4.5 GPa. After this initial peak both the UTS and EM drop back to values close to the control value, similar to the results with TMA. This improvement of tensile strength of cellulose mimics that of several other biopolymers where small amounts of Zn and Mn embedded in the mandibles, jaws, stingers, and carapaces of insects and marine animals have shown similar effects on their mechanical performance.24-28 2.2 Raman Spectroscopy


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To understand the changes occurring during these reactions Raman spectroscopy was conducted as a function of the number of infiltration cycles for each of the precursors. Cellulose, like most bioorganic polymers, has several highly active band regions with a variety of overlapping band structures that can make tracking and identifying individual peaks difficult.29 To mitigate this difficulty two regions were focused on; the spectral region between 550 cm-1-400 cm-1 where Al-O, Zn-O, and the skeletal bending modes of cellulose,29-31 in particular the δ(CCC), δ(COC), δ(OCC), and δ(OCO) are present, and the region between 3000 cm-1-2800 cm-1 where bands associated with the stretching modes of -CHx are present.32, 33 Figures 3a through 3c show the evolution of the Raman spectra as a function of infiltration cycles for both TMA and DEZ. In Figure 3a and 3c, a main peak is witnessed centered around 2894 cm-1 as well as several shoulders between 3000 cm-1 and 2900 cm-1 with two pronounced peaks at 2963 cm-1 and 2944 cm-1. The most prominent difference between cellulose infiltrated with TMA and infiltrated with DEZ is these two peaks, which steadily increase as a function of TMA infiltration and where no change is seen as a function of DEZ infiltration. This increase could be related to a variety of different possibilities such as partially unreacted TMA where some number of the –CH3 groups have not reacted. It also lends evidence to the possibility of TMA breaking open the glucose rings or severing the bonds between the individual glucose rings in the cellulose backbone ultimately weakening the overall structure as witnessed in the later cycle numbers in the mechanical data. In the region between 550 cm-1 and 400 cm-1, Figure 3b and 3d, changes in the relative ratios between peaks at 455 cm-1 and 434 cm-1 are witnessed for both the DEZ



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and TMA infiltrated samples. As mentioned, the peaks in this region are associated with Al-O, Zn-O, or cellulose skeletal bending modes. In the case of TMA infiltrated cellulose (Figure 2b) a steady increase in the intensity of the peak at 455 cm-1 is seen and may be assigned to the increasing number of Al-O bonds.34 This increase is also accompanied by a decrease in the peak at 434 cm-1 and a partial flattening of the peak at 517 cm-1. The change in ratio of these two peaks with respect to each other almost certainly indicates a more complicated set of changes related to the rearrangement of the skeletal bonds of the cellulose during the reaction. The complicated overlap of peaks associated with Zn-O and skeletal bending modes is also present for the DEZ infiltrated samples as shown in Figure 3d. The steady increase in the peaks at 434 cm-1 and 517 cm-1 can potentially be assigned to an increase in the presence of Zn-O bonds but, as for TMA infiltrated cellulose, it likely indicates a more complicated set of structural changes occurring with the cellulose backbone structure. 2.3 Semi In-situ X-ray Photoelectron Spectroscopy When compared to a process with two or more precursors, this single precursor process will introduce different chemical reactivity with the cellulose. Thus, it is difficult to derive the ongoing chemistry through direct comparison between the two process types. In fact, using a single precursor may make the chemistry much more complicated as the surface chemistry will not be constant as in a typical ALD process or an infiltration process that uses two precursors as was published previously.3, 11 To get a better understanding of the complicated changes seen in the Raman data and to probe the very initial stages of the reaction chemistry, where the changes in the mechanical properties


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were seen, a novel set of semi in-situ XPS experiments were conducted. A major complicating factor when conducting XPS studies on carbon materials, especially when changes in the oxidation state of carbon species are of particular interest, is contamination due to atmospheric carbon, referred to as adventitious carbon. In order to keep the adventitious carbon constant before and after the reaction compressed cellulose powder pills were reacted with either DEZ or TMA in vacuum conditions in the XPS system. Each pill was scanned before and after the reaction and were never re-exposed to atmosphere, thus preventing contamination and allowing detailed measurement of the reaction process. The C1s and O1s core level spectra prior to exposure to TMA are shown Figure 4a and 4b. In the deconvolved C1s spectra four distinct peaks are present; C=O at 289.0 eV, O-C-O at 288.0 eV, C-O at 286.6 eV, and the aforementioned adventitious carbon peak at 285.0 eV. The O:C ratio was found to be 0.74 which indicates a relatively pure cellulose.35, 36 After the semi in-situ reaction a noticeable change is seen in the shape of the C1s core level shape, with new components appearing at lower binding energies (Figure 4c), shown in green and purple. The green component, at 285.1 eV, is assigned to the C of cellulose C-O group where Al has bound to the O causing a shift in the C signal to lower binding energy.37 The purple component at 283.6 eV, is consistent with an Al-C species from one of a number of possible reacted molecular forms of TMA such as; C-OAl-(CH3)x, C-Al-(CH3)x, C-Al-O, and/or C-Al-C.38 The bulk component to this new contribution are most likely due to partially reacted TMA species associated with dangling or unreacted methyl groups. Another important change in the spectrum is the



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increased intensity of the peak associated with C=O at 289.0 eV which has been related to the opening of the ring structure in a variety of polysaccharide materials.37, 39 Examination of the O1s core level spectrum showed similar changes related to the reaction with TMA. The cellulose reference sample, Figure 4b, was as expected and agreed well with the literature.35, 36 Similar to the C1s spectra, after reaction with TMA, a new lower binding energy component at 531.7 eV was found, consistent with the formation of C-O-Al and in agreement with that identified in the C1s core level spectra.38 Directly comparing the contribution of this to the same structure in the C1s data shows that the two peaks have a ratio of 1:1 and, despite the 0.1 eV difference between the C-OAl and adventitious C, and are therefore consistent. Further comparison between the C1s and O1s core level spectra show a change in the O:C ratio from 0.74 to 0.64. Typically this change is attributed to further contamination (increase of adventitious carbon), but as the reaction was carried out insitu this is considered unlikely and therefore attributed to unreacted methyl groups. Additionally the ratio of the cellulose C-O:O-C-O peaks has decreased from 4.7 in the reference sample to 3.4 after the reaction. This ratio is important as it helps to indicate differences in the complicated cellulose structure as has been shown with wood fibers.35, 36

C-O bonds are those which are more likely to be related to the bridging –OH bonds,

changes in the –OH binding structure would appear here in the XPS spectrum. O-C-O bonds are more likely to be related to the backbone structure of the cellulose. The change in the ratio seen here indicates that the TMA reacts with C-O more likely to be associated with the bridging –OH bonds, and the O-C-O carbon is at least initially less affected.


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A similar process was followed with DEZ with the results shown in Figure 5a through 5d. As with the TMA reacted sample a reference measurement was carried out before and after in-situ reaction and used a similar fitting interpretation with four deconvolved components in the C1s core level spectra and a single peak in the O1s core level, as seen in Figure 5a and 5b respectively. After reaction with DEZ the C1s signal broadens with new components appearing at lower binding energy. The green component, at 285.1eV, is assigned to the C of cellulose C-O groups where Zn has bound to an O causing a shift in the C core level contribution to lower binding energies.40 The purple component at 283.6 eV is considerably smaller than the associated component after the TMA reaction, and it is similarly identified as Zn-C bonds. Changes were also observed in the O1s region after reaction with the DEZ as seen in Figure 5d. A new component at a lower binding energy of 531.3 eV was witnessed and assigned to C-O-Zn species.40 As with the in-situ TMA the ratio between the C-O-Zn peak in the C1s spectrum and O1s spectrum were found to be 1:1 and therefore consistent. As with the TMA reacted sample an increase in the C=O component is also seen. Again a decrease in the ratio of C-O:O-C-O from 4.4 in the reference sample to 3.45 after reaction is observed, indicating that the Zn reacts similarly to the TMA with the C-O groups of the cellulose being the preferred target and the O-C-O carbon being less affected. Comparing the change in ratio due to reaction with TMA and reaction with DEZ shows a smaller change when using DEZ. In the case of TMA the ratio changes from 4.7 to 3.4 a difference of 1.3, while in the case of DEZ the ratio changes from 4.4 to 3.45, a difference of 0.95. Given that the TMA and the DEZ where allowed the same reaction time it would appear that the TMA reacts more aggressively with cellulose than the DEZ.



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This result agrees well with the both the Raman data and the drastic changes seen in the mechanical data, as-well-as recently reported reaction schemes with the synthetic polymer poly(p-phenylene terephthalamide), i.e. Kevlar.41 2.4 Potential Reaction Schemes Considering the data presented in Figures 1-4 it is possible to suggest potential reaction schemes. Figure 6a through 6d shows the proposed reaction scheme for cellulose and TMA. The initial structure of cellulose is seen in Scheme 1a where –OH groups cross-link the individual molecular chains as has been described previously in the literature.19 Very little improvement is seen in the mechanical data for cellulose reacted with TMA as can be seen in Figure 2a and 2b. It is therefore safe to assume that the TMA does not significantly cross-link the individual chains of cellulose in a way that leads to improved mechanical properties. An increase in carbon species, most likely associated with –CHx are evidenced from the Raman data, Figure 3a, and the XPS, Figure 4a through 4d. Furthermore the more aggressive reactions witnessed from the change in the ratio of C-O:O-C-O, the increase in the C=O (indicative of glucose ring opening), and the higher Al-C peak all support the more aggressive nature of TMA. It is also well known that TMA is mainly a dimer in both the vapor and liquid phase.42 Therefore it is expected that the TMA will quickly consume all –OH groups that are not blocked because of steric hindrance while simultaneously attacking ring structure, as illustrated in Figure 6b. Continued exposure may lead to small amounts of crosslinking of the cellulose chains, thus explaining the small increase in the mechanical data at early cycle numbers, while simultaneously breaking open the individual glucose rings, red arrows in Figure 6b


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through 6d. After some total amount of TMA has been supplied to react with the cellulose continued cross-linking and degradation of the backbone must reach a balance. Similarly a potential reaction scheme for DEZ can be constructed and is shown in Figure 7a through 7d. Unlike TMA, DEZ exists as a monomer and is known to be a less aggressive Lewis acid as compared to TMA. The dramatic increase in the mechanical properties is almost certainly due to crosslinking of the backbone structure. From both the Raman, Figure 3b and 3d, and XPS data, Figure 5a through 5c, it can be seen that the DEZ reaction does not add new carbon to the system, i.e. very little increase in Raman peak intensities associated with –CHx and a smaller XPS peak associated with Zn-C bonds. The smaller DEZ molecules may have easier access to the –OH groups and more readily consume them before attacking the ring structure. However, despite the initial increase in mechanical properties after enough DEZ is supplied, the cellulose backbone is most likely attacked, see red arrows in Figure 7c and 7d, and the results become comparable to TMA, see Figure 2a and 2b at 20 cycles of exposure. While these Figures certainly do not capture the complexity of these reactions, which most likely happen simultaneously through a variety of routes, it does help to illustrate an over-all result. It may also help to explain the initial increase in mechanical properties, which is more pronounced in the case of DEZ, followed by a subsequent decrease. It is possible that this decrease is related to either the breaking open of the individual glucose rings, cleaving of the backbone, and/or steric hindrance related reaction limitations. In terms of UTS and in the case of TMA this degradation happens quickly with the lowest point found at ~10 cycles, Figure 2a. In the less aggressive DEZ case the same point is not found until 20 cycles. It could therefore be concluded that it



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takes DEZ roughly twice as long to begin destroying the cellulose structure as compared to TMA. Furthermore the chemical changes seen here may offer a secondary explanation to the abrupt wetting transitions seen at similar cycle numbers in coated cellulose (mentioned in the introduction).16 Parsons et al. explained this abrupt wetting transition with nanoscale roughness, however it may be possible, based on the work shown here, that the consumption of the cellulose related –OH groups may be contributing, if not the main cause. However, further experiments would need to be conducted to confirm this. The choice of using single precursors over standard paired precursors introduces the possibility of forced reactions that may not be present when these two precursors are paired with H2O. Here several differences arise such as the single precursor being forced to react with the functional sites of the substrate even if the gain in reaction enthalpy (∆HR) is lower than with H2O and continually changing surface sites, which would normally ‘reset’ by a second precursor. Additionally, the local heat evolution during the reaction of the precursor and counter precursor might further alter the chemistry of the substrate or even thermally destroy parts of the cellulose molecular chain. 3. Conclusions The modification of cellulose through single precursor infiltration shows that significant improvement in the mechanical properties can be gained. This improvement not only depends on which precursor is used but also on the total amount of the precursor exposed to the cellulose substrate, thus allowing a high degree of tunability. At higher numbers of cycles the reaction between TMA and DEZ with cellulose is indistinguishable. At lower cycles, however, a distinct difference between the two is witnessed. The initial cycles of DEZ infiltration lead to dramatic increases in the


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mechanical properties most likely due to a crosslinking of the backbone structure with Zn-O bonds. On the other hand initial cycles of TMA show very little improvement. This work may also open up the possibility of making truly multifunctional cellulose hybrid materials. By appropriately choosing the precursors and whether or not a second precursor is used it may be possible to introduce new functionalities combined with improved mechanical behavior. For example, stronger, hydrophobic, and electrically conductive cellulose would give the material a serious push towards new applications while largely maintaining CO2 neutrality.

4. Experimental Section

Cellulose was obtained from two sources. Cellulose powder from cotton linters was purchased from Sigma Aldrich and compressed into pills. Cotton fibers (99.99% pure) were individually removed from balls of cotton that were purchased from a local medical supply company. Before infiltration cotton fibers were baked at 70°C overnight16. Samples were in UHV before being exposed to the precursors in the semi in situ XPS experiments, largely removing any residual water. Vapor phase infiltration was done using a Cambridge Nantech (now Ultratech) Savannah atomic layer deposition tool. Cotton fibers were exposed to either diethyl zinc (Strem, 99.99%) or trimethyl aluminum (Strem, 99.9%). One cycle of the process was as follows; the precursors were pulsed into the reaction chamber for 0.05s and held in the reaction chamber for 30s, followed by a 30s purge step. The delivery and purge gas was N2 (99.99%). The reaction temperature was 100°C and the base pressure of the reactor was ~ 50mTorr. 15 ACS Paragon Plus Environment


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Uni-axial tensile tests were conducted using a CETR (now Bruker) universal material tester following ASTM standard C1557-03. Cotton fibers were aligned on to jigs using the aforementioned standard as a guide and had a gauge length of 6 mm. The diameter of the fibers was measured to be 18.6±0.2 µm with the error being reported as the standard deviation of the mean and is based on the measurement of 100 individual fibers. The ultimate tensile strength and elastic modulus are reported as the average of at least 10 fibers that broke in a manner consistent with the ASTM standard with the error reported as the standard deviation of the mean. XPS experiments were performed using a Phoibos photoelectron spectrometer equipped with an Al Kα X-ray source (12 mA, 8.33 kV) as the incident photon radiation. The overall resolution of the instrument is approximately 0.9 eV. At this resolution the line energy positions could be determined with an accuracy of ±0.2 eV. The pressure in the analyzing chamber was

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