Spatial Structure Investigation of Porous Shell Layer Formed by

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Spatial structure investigation of porous shell layer formed by swelling of PA66 fibres in CaCl2/H2O/EtOH mixtures Barbara Rietzler, Thomas Bechtold, and Tung Pham Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03741 • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019

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Spatial structure investigation of porous shell layer formed

by

swelling

of

PA66

fibres

in

CaCl2/H2O/EtOH mixtures Barbara Rietzler, Thomas Bechtold, Tung Pham * Research Institute of Textile Chemistry and Textile Physics, University of Innsbruck, Hoechsterstrasse 73, 6850 Dornbirn, Austria *Corresponding author: [email protected]

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Abstract: This is a continuation of work on interactions between polyamide 66 (PA66) fibres and CaCl2/H2O/EtOH mixtures. It was observed that the mixtures dissolved the fibres, but with or without an intermediate stage of visibly evident swelling depending on the mixture composition. The interaction proceeds via Lewis acid-base complexation between the polymer carbonyl groups and Ca2+ ions and can be interrupted by rinsing the fibres with water. Swollen fibres retained their expanded diameters even after rinsing and exhibited a highly rough surface and increased water retention. The observed effects suggest such mixtures may be used to increase surface roughness of PA66 fibres for increasing interfacial adhesion in composites applications.

In this publication, we report the results of further investigations into the spatial structure of crosssections of swollen fibres. Using atomic force microscopy coupled with infrared spectroscopy on the length scale of hundred nanometres (nanoIR-AFM), we could show, for the first time, the PA66 core-shell structure, where the shell thickness increases with treatment extent and exhibits a highly porous structure. Thus, the surface roughness observed previously is not limited only to the surface but extends towards the fibre core. The examination also showed no evidence of Ca2+ complexation in the fibre cores, which confirms a near-complete removal of the ions. Additional measurements of the crystallinity with differential scanning calorimetry (DSC) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) showed that the shell exhibits lower crystallinity than the core.

Keywords nanoIR-AFM, composites, interfacial adhesion, polyamide 66, porous structure, shell/core, surface modification


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Introduction One of the most widespread synthetic polymers is polyamide and especially polyamide 66 (PA66). In 2015, more than 7 million tons PA66 were produced worldwide.1 It is used in the automotive segment, engineering plastics and textiles largely due to its excellent mechanical properties, such as high tensile strength, high abrasion resistance and also its chemical stability. However, the polymer also exhibits a low surface energy, which impedes its use in applications requiring interfacial adhesion, for instance in textile composites.

Various research groups have employed plasma treatments to etch polymer surfaces or to add functional groups to increase the surface energy.2-6 For instance, Schaefer et al.6 used atmospheric plasma treatments to improve the bonding ability between polyamide 6 and polyurethane in composites. Another approach is the introduction of new functionalities on the surface.7-10 For instance, Choi et al. chemically grafted nanodiamond into PA66 to enhance interfacial adhesion and mechanical properties.11 A further option is the use of solvents to roughen polymer surfaces. For instance, Krishna Prasad et al.12 used formic acid to improve adhesion in PA66-rubber composites.

The interaction between Ca2+ and polyamide was perhaps first reported by Wyzgoski et al. in 1987 13, who observed that aqueous CaCl2 solutions caused stress cracking of the polymer and that the salt is partially soluble in the polymer. Further investigations showed that Ca2+ absorption by PA66 increases with time, but does not degrade the polymer.14 The interaction proceeds through Lewis-acid base complexation between the CaCl2 and the carbonyl groups (C=O) in polyamides, and a similar mode of interaction is also observed with GaCl3 and AlCl3.15-17 The Ca2+ 3 ACS Paragon Plus Environment

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complexation with polyamide suppresses hydrogen bond formation between the polymer chains, and this effect has been employed to help increase the draw ratio in fibre spinning, by adding the salt to polymer melts and solutions.18-20 Moreover, the decomplexation process of PA6/CaCl2 in different non-solvents was investigated by Liu et al. and it was found that depending on the nonsolvent, different crystalline forms are obtained.21 The complexation interactions have also been tested on aromatic polyamides, e.g. by Li et al., who used boiling ethanolic CaCl2 solutions to improve the interfacial adhesion of Kevlar in fibre composites.22

In a previous study, we reported on the interactions of CaCl2/H2O/EtOH mixtures with semicrystalline PA66 fibres.23 Depending on the composition of the solvent, dissolution or swelling were observed during the treatment. Solvents with a higher amount of ethanol and less water caused dissolution of the fibres without visibly evident swelling, whereas solvents with higher amounts of water and less ethanol caused swelling of the PA66 fibres prior to dissolution. Therefore, the dissolution rate is controlled by the composition of CaCl2/H2O/EtOH. In Figure 1, photomicrographs of the fibre in solvent (A) and after rinsing with water (B) are shown. The fiber in Figure 1 A, still is in the solvent and the swelling process is continuing, the diameter has increased to 25 µm. On the contrary, in Figure 1 B, the swelling process has been stopped by washing the fiber with deionized water. Although the swelling is stopped, the fibre diameter is not collapsing, as it still has an increased diameter of 25 µm. Furthermore, the investigation of the surface with 3D laser scanning microscopy showed an increased surface roughness.23 A significant increase in water retention has also been reported in our previous work.23 As the swollen structure of the fibre does not collapse after solvent removal, it was of interest to further investigate the


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structure of the swollen layer, which shows an increased surface roughness on the longitudinal direction.

Figure 1. Light transmission microscope images; A – Swollen PA66 fibre in solvent; B – Swollen and washed fibre In this paper, we report on examination of the fibres using a special technique, the atomic force microscopy coupled with infrared spectroscopy on the length scale of hundred nanometres (AFMIR). This technique allows for a combined analysis of the surface topography and of any variations in the chemical structure, with a high degree of resolution enabling the investigation of both structure and chemical composition. The technique was used on cross-sections of treated fibres to examine structural details of the shell layer, and to investigate if any traces of Ca2+ complexation remained in fibres after de-complexing with water. The crystallinity changes in the fibre after treatment were also investigated with DSC and ATR-FTIR measurements.

Experimental


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PA66 filaments were provided by Schoeller GmbH & CoKG, Hard, Austria and used as received. The filament diameters were approximately 15.7 µm ± 0.4 µm. The treatment solutions were prepared with calcium chloride dihydrate (analytical grade) and ethanol (99.9% for analysis) purchased from Sigma Aldrich (Buchs SG, Switzerland) and Oesterreichische Agrar-Alkohol Handeslges.m.b.H (Spillern, Austria) respectively, and deionised water.

The PA66 filaments were immersed in a solution of composition 12.50 mol% CaCl2, 68.75 mol% H2O, and 18.75 mol% EtOH at ambient temperature. This solution was found to cause swelling of the PA66 filaments.23 The treatment duration was varied over a range of time periods to vary swelling extents. At the end of each treatment, the filaments were rinsed with deionised water to remove the solvent and dried under ambient conditions before further experiments.

Analytical Methods Atomic force microscopy allows for the mapping of surface morphologies.24 A cantilever tip is brought into contact with the sample surface and a vertical force is applied. The cantilever stays in contact with the surface during the whole measurement and scans the sample line by line. The measurement of the deflection of the tip by an optical detector yields a topographical image of the sample surface. Because of the sharpness of the tip, a lateral resolution of less than one nanometre can be achieved.25 The AFM-IR instrument combines topographical analysis with a spectromicroscopic

technique

based

on

the

detection

of

photo

thermal

induced

resonance (PTIR).26-28 The sample surface is irradiated with infrared laser pulses. When a wavelength is in resonance with an absorption band of the molecular structure of the sample, the light is absorbed and therefore the sample is heated. This leads to thermal expansion of the sample 6 ACS Paragon Plus Environment

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and introduces a vibration on the cantilever. With a visible laser, focused on the cantilever, the deflection is measured with a four-quadrant detector. The amplitude of this signal is then proportional to the absorption.29 The signal is Fourier-transformed and recorded.27 During the measurement, the AFM tip is operated in contact mode. Specimens of treated and untreated fibres were embedded in an epoxy resin (Technovit® 7100, Kulzer GmbH, Germany) and cured at ambient temperature overnight. An ultra cryomicrotome (EM UC7, Leica Microsystems GmbH, Germany) equipped with a diamond knife (cryo 45°, DiATOME AG, Switzerland) was used to obtain 500-1000 nm thick cross-sections from the resin embedded specimens at -40°C. The cross sections were then placed on IR inactive ZnS windows and loaded into the nanoIR2™ instrument (Anasys Instruments, Inc., USA) for AFM-IR and topography measurements. Both measurements were performed in contact mode using probes with a resonance frequency of 13 kHz ± 4 kHz and a spring constant of 0.07-0.4 N/m, and the data was analysed with the on-board software. For the collection of IR spectra, the frequency range in the FFT was defined by a centre frequency of 180 kHz and a width of 70 kHz. In the nanoIR experiments, 10 ns infrared laser pulses with a rate of 1 kHz are generated. The IR spectra were recorded over a range from 900–1900 cm-1 and 2700–3600 cm-1. A co-average of 32 scans was used, which means that 32 scans per data point were collected and averaged.

The attenuated total reflectance Fourier transform infrared spectra (ATR-FTIR) were recorded on a Bruker Vector 22 instrument (Karlsruhe, Germany). The ATR unit was equipped with a diamond crystal. A total of 32 scans were collected for each spectrum from 500 cm-1 to 4000 cm-1 at a resolution of 4 cm-1. The intensity measurements for crystallinity index calculation was done with Origin® Pro 2017G, and the results reported are the average of five determinations. To 7 ACS Paragon Plus Environment

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calculate the crystallinity index, absorption intensities at 1640 cm-1 and 1200 cm-1 were used (Equation 1). 𝐴𝑏𝑠1200 𝑐𝑚 ―1

𝐼𝑐𝑟𝑦𝑠𝑡 = 𝐴𝑏𝑠

1640 𝑐𝑚 ―1

Equation 1

The melting enthalpies of specimens were measured on a differential scanning calorimeter (DSC 3, Mettler Toledo, USA) under nitrogen flow (50 ml/min) in the temperature range of 20– 280°C with heating and cooling rates from 10 to 30°C/min, on samples in 40 µl aluminium crucibles sealed with pierced lids. The data was analysed with the on-board software and the reported results are the average of ten replicate determinations.

Results and Discussion

Figure 2. Cross-sections of a) untreated PA66 fibres; b) 10 min treated swollen PA66 fibres Photomicrographs, with light transmission microscopy at 50x magnification, of cross-sections from untreated fibres, and fibres treated for 10 min with the solvent, are shown in Figure 2. The untreated fibres show a smooth surface at the cross-section edges, whereas the swollen fibres show 8 ACS Paragon Plus Environment

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a rough shell layer surrounding a smooth core. The outer shell layer appears uneven and very porous. In the previous work, the longitudinal fibre surface was investigated with 3D confocal laser scanning microscopy to determine surface roughness. The treated fibres exhibited much greater surface roughness compared to the untreated fibre. Nonetheless, no further detail on the shell structure could be discerned. The preparation of cross-sections enables a deeper insight in the shell structure. An additional AFM picture of the longitudinal fibre surface can be found in the Supporting Information (see Figure 3, Supporting Information). As a greater magnification and resolution was not possible with the light transmission microscope than what is shown in Figure 2, the cross-sections were investigated with AFM. The AFM topographical images from crosssections of untreated fibres and fibres treated for 3 min, 5 min, 10 min and 15 min are shown in Figure 3.


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Figure 3. Topographical images of A) untreated; B) 3 min treated; C) 5 min treated, D) 10 min treated, and E) 15 min treated PA66 fibres. In part A, the symbol “*” marks a gap between the fiber and embedding resin, and in Part C, the symbol “**” marks a contaminant on the cross-section.


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As in Figure 2, the surface of the untreated fibre in Figure 3A also shows a smooth surface. The interface between the PA66 fibre and the epoxy is very sharp, but the adhesion between fibre and the epoxy was weak as is evident in Figure 3A. The dark section (marked with *) is a gap between the fibre and the embedding material. Even after 3 min of treatment with the solvent, a swollen shell can be observed (Figure 3B). The thickness of the shell layer is approximately 1 µm. In Figure 3C, 5 min treated PA66 fibres are shown and the shell layer thickness has increased to around 3 µm. Up to a swelling time of 10 min, single fibres with a circular shape and a swollen outer layer are found. The increase of the outer diameter with time found in the cross-sections agree with the diameters found under the light transmission microscope reported in the previous work. After 15 min (Figure 3E), the shell layers from different fibres appear fused with each other, and no single fibres are observed. Some remaining cores with their still quite circular shape are seen embedded in a porous network of swollen and reprecipitated PA66. These cores still exhibit a diameter of approximately 7-8 µm. The AFM pictures reveal a uniform thickness of the shell, which suggests that there was a uniform degree of solvent permeation into the fiber, from the periphery towards the center. Therefore, the fibres retain its circular shape even after a treatment time of 10 min at a high swelling degree. This can be explained by the dissolution mechanism of polymers based on diffusion. This has already been reported earlier for polymer films.30 The AFM images confirm that the solvent diffuses into the polymer and causes swelling of the polymer. The interface between the unaffected PA66 core and complexed PA66 shell is moving inwards as described in the model for the dissolution of polymers.31 Besides, it can be confirmed that the formed shell layer has a very porous structure. In other words, the obtained AFM images reveal that the swollen PA66 fibres do not only have a higher roughness on the longitudinal surface, but the whole shell layer has a very porous 11 ACS Paragon Plus Environment

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structure. The shell seems to consist of small channels and pores. It is believed that these pores are formed when the fibres are washed with deionised water and the Ca2+ ions removed. The swollen PA66 goes back to a decomplexed and unswollen state, but the shell does not collapse, and free spaces filled with water are formed. These free spaces are what we see afterwards in the shell layer. This finding can explain previously reported properties like the high water retention of the treated fibre. After showing the high roughness of the longitudinal surface of the fibre in our previous work, we can now show the porous structure of the inside of the shell layer, which was not possible before.

Figure 4. IR Spectra of untreated PA66 fibre and epoxy


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Figure 4 shows IR spectra from the AFM-IR measurements of an untreated PA66 fibre and the epoxy used for embedding. On the cross-section of the fibre, the typical bands for PA66 are found. The band around 3300 cm-1, the Amide A band is assigned to N-H stretching. Around 1640 cm-1 the Amide I band is found, which are due to C=O stretching vibrations. The spectrum labelled B in Figure 4 shows the absorption spectrum of the epoxy resin. Prominent absorption bands are found at around 1722 cm-1, which is attributed to C=O stretching vibration in epoxy; at around 1250 cm-1, assigned to C-C stretching vibrations; and at 1160 cm-1, due to C-O stretching vibrations. The complexation of Ca2+ ions with the amide carbonyl groups leads to a red shift of the N-H band at 3300 cm-1 to lower frequencies (see Figure S2, Supporting Information). This is in accordance to the work reported by Sun et al.32 Such shifts were not observed at any of the marked locations shown in Figure 5, which is from a cross-section of a fibre specimen treated for 10 min. This indicates that after washing, the complex is severed and the hydrogen bonds between the PA66 chains are reformed. With this method, it is confirmed that the swollen shell layer is consisting of reformed PA66, and no complex between Ca2+ and the carbonyl group is present anymore.


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Figure 5. AFM-IR spectra of 10 min treated PA66 fibre. Several spectra are recorded with varying amount of epoxy depending on the examination site. 14 ACS Paragon Plus Environment

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Another finding was made when the spectra in the shell layer were recorded. The spectra in the core of the swollen fibre looked the same as the spectra taken on the cross-section of untreated PA66 fibres (compare the spectrum labelled A in Figure 4 with those labelled A and B in Figure 5). In spectra recorded in the shell layer, an additional peak at 1722 cm-1 was observed which is attributed to the epoxy resin (compare spectra labelled B in Figure 4 with those labelled C-F in Figure 5). This leads to the conclusion that the epoxy resin permeated through the entire shell layer during the curing period. This discovery suggests that also water and other substances can permeate through the whole shell layer and is consistent with the higher water retention previously observed in treated fibres.

Figure 6. I) DSC measurements of PA66 fibres; A–untreated PA66; B–3 min treated; C–5 min treated; D–10 min treated; E–15 min treated. The thermograms shown are representative examples from multiple measurements; II) Enthalpy of fusion of untreated and swollen PA66 fibres vs. time; the red line shows the average of all measurements 73.3 J/g.


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The results of the DSC measurements are shown in Figure 6-I. The endothermic peaks at about 100°C is due to evaporative moisture loss, and the peak at about 260°C are due to melting of the crystalline domains in the polymer. The degree of polymer crystallinity may be estimated from the melting enthalpies at 260°C after taking into account the moisture content of the polymer, and possible recrystallizations that occur in the temperature range between the glass transition and the melting.33 We estimated the moisture amounts in polymer from the endothermic peaks at about 100°C assuming the enthalpy of moisture evaporation did not differ from that of pure water. However, there was no evidence of exothermic recrystallization peaks before melting, and thus this possibility was ignored. In previous work (see Figure S1, Supporting Information), we measured the melting enthalpy of a sample that was reprecipitated after complete dissolution, and obtained a value of 61.3 ± 3.4 J/g, which is significantly lower than that of an untreated fibre (75.9 ± 4.9 J/g). However, as seen in Figure 6-II, the treated fibres from this work did not show significantly different melting enthalpies as compared to the untreated fibre. The melting enthalpy for 100% crystalline PA66 lies between 185-235 J/g, as reported in literature.34 From this, we estimate the degree of crystallinity in the reprecipitated polymer (previous work) to be ca. 29%, and of the untreated and treated fibres in this work to be about 36%. Therefore, we find a significant difference in crystallinity between the completely dissolved and reprecipitated polymer and the untreated fibre but no difference between the treated and the untreated fibre. The difference between the average crystallinities of the shell layer and the core is not big enough to cause a significant difference in the overall crystallinity. Moreover, the variance of the values is high and detection of differences using DSC is difficult.


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Another approach was the determination of a crystallinity index through FTIR measurements. It is reported in several studies that the absorption band at 1640 cm-1 can be used as a reference band.35 The band at 1200 cm-1 is assigned to the crystalline phase in PA66. Therefore, the calculation of a ratio between the intensities of the peak at 1200 cm-1 and at 1640 cm-1 indicates the proportion of crystalline phase in the polymer. In a first attempt, spectra from the AFM-IR were evaluated. However, the indices obtained showed a very high variance even at the core. Since PA66 is a semi-crystalline polymer, it is possible to find amorphous phases not only in the shell but also in the core. The epoxy resin also showed absorbance at 1200 cm-1, and this interfered with crystallinity index determinations in the shell region. Hence, for the determination of the crystallinity index we used an ATR-FTIR instrument. Since the penetration depth of the ATR-FTIR is around 1 µm, it is possible to determine the crystallinity index of the shell layer. In Figure 8, the calculated indices vs. treatment time are shown. It can be clearly seen that the crystallinity index of the treated fibres is decreasing with time. The untreated fibre exhibits a crystallinity index of 0.33±0,01. After 5 min treatment, this index decreases to 0.27±0,004. This means that the crystallinity of the untreated fibre is higher than that of the treated fibres, which was expected. However, it is noticed that after 5 min treatment, the crystallinity index is not decreasing anymore. As soon, as there is no core PA66 detected anymore by the ATR, which is the case when the shell has a larger thickness than 1 µm, a plateau is observed in the observed crystallinity changes. Thus, the results indicate a relative decrease of crystallinity in the shell layer.


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Figure 7. Diagram of the crystallinity index vs. treatment time. Conclusion In conclusion, this study shows a detailed analysis of the cross-section of PA66 fibres. It is shown that by rinsing with water, all Ca2+ complexation is removed, and the polymer-solvent interactions are stopped. The result is a porous shell of PA66 enveloping an unaffected core, and there are no changes of the chemical structure throughout the fibre. In addition, it was determined that the treatments did not change the bulk crystallinity of fibres, but that in shell regions were lower.

The results show that the treatment effects are limited principally to the fibre surfaces and the inner regions are unchanged. The depth of effects, i.e. shell thicknesses in the treated fibres, can easily be controlled with the treatment variables, and the CaCl2/H2O/EtOH mixtures are likely very suitable for surface-limited modifications of PA66 fibres.

Furthermore, permeation of epoxy throughout the shell layer was observed. This finding is consistent with the high water retention exhibited by the treated fibres. Therefore, it can be concluded that other substances with similar viscosities would also be able to permeate the shell 18 ACS Paragon Plus Environment

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layer and thus the fibres may act as substrates for controlled release applications. Investigations of this additional aspect are also planned.

It is worth mentioning, that a completely dissolved PA66 in formic acid and subsequent solvent evaporation forms films with higher crystallinity. This was observed in our own measurements (see Figure S1, Supporting Information) and is also reported by Muellerleile et al.36 This is in contrast to films obtained from the solvent used in our work and perhaps indicates differences in solvent action or mechanism of precipitation. It will be further investigated in future work.

Acknowledgments The authors gratefully acknowledge financial support from the Austrian Federal Ministry of Transport, Innovation and Technology (BMVIT) to the Endowed Professorship Advanced Manufacturing FFG-846932. Supporting Information. Figure S1: Representative DSC curves of a PA66 after complete dissolution and reprecipitation from CaCl2/H2O/EtOH mixture and formic acid compared with untreated PA66 fibres. References (1)

Scheibitz, M.; Kaneko, R.; Spies, P., Polyamide 6 and 66 (PA6 and PA66) Demand for

Polyamides Is Determined by Asia and the Automotive Industry. Kunststoffe Int 2016.


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For Table of Contents use only


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