Effects of Erucamide on Fiber “Softness”: Linking Single-Fiber Crystal Structure and Mechanical Properties

Erucamide is known to play a critical role in modifying polymer fiber surface chemistry and morphology. However, its effects on fiber crystallinity and mechanical properties remain to be understood. Here, synchrotron nanofocused X-ray Diffraction (nXRD) revealed a bimodal orientation of the constituent polymer chains aligned along the fiber axis and cross-section, respectively. Erucamide promoted crystallinity in the fiber, leading to larger and more numerous lamellae crystallites. The nXRD nanostructual characterization is complemented by single-fiber uniaxial tensile tests, which showed that erucamide significantly affected fiber mechanical properties, decreasing fiber tensile strength and stiffness but enhancing fiber toughness, fracture strain, and ductility. To correlate these single-fiber nXRD and mechanical test results, we propose that erucamide mediated slip at the interfaces between crystallites and amorphous domains during stress-induced single-fiber crystallization, also decreasing the stress arising from the shear displacement of microfibrils and deformation of the macromolecular network. Linking the single-fiber crystal structure with the single-fiber mechanical properties, these findings provide the direct evidence on a single-fiber level for the role of erucamide in enhancing fiber “softness”.


SI.01 Materials
Polymer fibres were donated by Procter & Gamble (Beijing).Samples were single fibres of polypropylene (PP), a homopolymer designed by ExxonMobil™ specifically for spunbonded nonwovens, with one sample also containing 1.5 wt% of erucamide (C 22 H 43 NO; Figure 1).The erucamide was introduced prior to fibre extrusion by mixing the pure polypropylene resin with masterbatch (composed of 10 wt% erucamide and 90 wt% polypropylene) in an 85:15 ratio, yielding 1.5 wt% erucamide per fibre.The fibres also contained 1 wt% titanium dioxide (TiO 2 ), added via premixing with the PP resin before fibre spinning.The fibres, denoted as PP and PP + ER for the erucamide-loaded fibre, thus differed only in the 1.5 wt% erucamide additive in the PP + ER fibres, with the same PP matrix and fibre extrusion process.For sample preparation, single fibres were carefully removed from fibre bundles with tweezers, avoiding or minimizing any fibre stretching and breakage.Erucamide (Sigma Aldrich, >85%) was recrystallised once from acetone (Sigma Aldrich, 99.5%) prior to being transferred to glass capillaries for XRD measurements.showing the meridional (or vertical) plane (q z ) and the equatorial (or horizontal) plane (q xy ) scattering vectors on the 2D detector plane.(C) 2D XRD pattern obtained for a fibre titled by the angles β and φ with respect to the incident beam, and the fibre c-axis can be found at the centre of the off-equatorial and off-meridional Bragg reflections (4.07 Å, [(041)/( 1 31)]) (indicated by the green arrows).The mirror symmetry about the c-axis allowed determination of fibre tilt φ.Fibre alignment and Fraser correction [1] were attained via measurement of the azimuthal angle φ, defined as the angle between the normal direction (scattering vector q z ) and the fibre axis (pink arrow).β is calculated as tan      , where λ is the X-ray wavelength, s r is the radius of Polanyi sphere of the reference 041/1 31 reflection, A four-quadrant fibre pattern mapped into reciprocal space with the missing information for the lower half copied from the upper half.

Methods
The nano-focused XRD experiment was performed at beamline I14 at Diamond Light Source (DLS), Didcot, UK, using an X-ray beam with an energy of 20 keV (λ = 0.619 Å), with an integration time of 30 seconds per frame.The beam size was 60 nm × 60 nm with an angular resolution of 10 millidegrees (dq = 0.02 nm -1 ) [2].The experimental setup is schematically shown in Figure S2A, along with the measurement details, in Section SI.01.A single fibre was scanned at four sections (S 1 -S 4 ) along 300 µm of the length, with 3 scans in each section 1 µm apart, with a 100 nm step size in S 1 and a 500 nm step size in S 2 -S 4 .The crystal structure along fibre width was investigated at three areas (A 1 -A 3 ) marked in orange in S 1 .A single fibre was fixed (vertically w.r.t. the incident X-ray beam) on the copper frame of a sample holder using Kapton (poly (4,4'-oxydiphenylene-pyromellitimide)) tape.All the measurements were performed under ambient conditions.The XRD scans were performed in the transmission mode and the intensity was detected with a 285 mm sample-to-detector distance using a 3module Excalibur detector [3].Calibration of the detector was performed using the cerium oxide (CeO 2 ) powder standard.
Optical microscopy followed by X-ray fluorescence (XRF) spatial mapping of TiO 2 particles distributed within the fibre matrix were applied to locate and align the fibre in the nanofocused beam.The fluorescence scan was attained at a scan size ~25-30 µm, exceeding that of fibre diameter (d f <15 µm), at a step size of 500 nm and an exposure time of 1 s.An example XRF scan is shown in Figure S2C, showing a uniform distribution of TiO 2 particles across the PP fibre.The apparent fibre diameter values obtained based on the XRF scans were consistent with those attained later via FDAS770 Laser Scanning Micrometre (Dia-stron).
For the nXRD measurement to probe the fibre crystal structure along its length and width, the vertically aligned fibre was scanned with the nano-focused beam, 3 times along the lines 1 µm apart (Figure S2B) at each of the four sections (S 1 -S 4 ) along 300 µm of the fibre length.
The scans in the first section (S 1 ) were attained with a 100 nm step size and in S 2 -S 4 with a 500 nm step size.Based on the initial scans, the fibre crystal structure was further analysed at three discrete positions in S 1 , denoted as A 1 -A 3 , each of 3 µm in length across the fibre width (Figure S2B).The XRD scan for the control erucamide powder sample was attained at microXAS beamline (X05LA), Swiss Light Source (SLS), Paul Scherrer Institute, Villigen, Switzerland, using an X-ray beam with an energy of 17.2 keV (λ = 0.721 Å), with an integration time of 4 s.The beam size was 1 µm × 1 µm and the angular resolution 10 millidegrees (dq = 0.02 nm -1 ).The diffraction patterns were recorded in the transmission mode using an 8-module Eiger detector (Dectris, Switzerland).Lanthanum hexaboride (LaB 6 ) powder standard was used to calibrate the detector geometry.
Raw XRD 2D images from DLS and SLS were processed using Dawn [4] and Datasqueeze 3.0 software [5], respectively, involving appropriate masking to remove the detector grid, intermodular gaps (123 pixels, vertical) and overexposed or dead pixels, as well as applying corrections for the diffraction intensity (i.e.polarization) and geometric effects associated with the fibre alignment (Figure S1) [1,6,7].The azimuthal integration over 1793 x 2069 pixels generated 1D diffraction curves plotted as intensity (arb.u.) vs momentum transfer q (nm -1 ), as well as azimuthal profiles plotted as intensity (arb.u.) vs azimuthal angle  (°).A baseline corrections was applied in Python using the asymmetric least squares smoothing method [8].
SI.02 XRD peak analysis; Fibre unit cell parameters; Degree of crystallinity XRD peak analysis was performed in IGOR Pro (WaveMetrics, Portland, USA) using the Multipeak Fit operation.The XRD profiles were considered composed of Gaussian or Lorentzian profiles and were fitted to pseudo-Voigt and Gaussian functions.The characteristic lattice spacing (d-spacing) could be ascertained according to , Equation S1: where q is a momentum transfer vector defined as:

Equation S2
and the crystallite size L a , in the direction of hkl plane (i.e.perpendicular to the hkl plane), could be obtained using Scherrer's equation [9][10][11][12][13][14][15][16][17]: where K is the Scherrer constant (i.e. a shape factor) of order unity depending on the actual shape of the crystalline (here K = 1).The number of vertically aligned polymer chains ordered in the direction of the planes responsible for the formation of hkl Bragg reflection was estimated as: m = .

Equation S5
The unit cell volume (V) was calculated according to Equation S6 and crystalline density (ρ) , Equation S7: where Z is the total number of monomers per unit cell (for polypropylene Z = 12), M w the molecular weight, and N A the Avogadro number [18,19].
The degree of crystallinity was calculated as: where A c and A a are integrated intensities under fitted crystalline part (i.e.Bragg peaks) and amorphous halo, respectively (cf. Figure 2 in main text).

SI.03 Measurement of the tensile strength and Young's modulus of a single fibre
The tensile strength measurements were conducted using an LEX820 Linear Extensometer with a Laser Diffraction System (Dia-stron).To ensure that the fibre axis was aligned coaxially with the line of action of the testing machine, the single fibre was mounted within the grooves on a pair of plastic tabs.Single fibres were secured with a drop of glue (Loctite Super Glue for all plastics, Rapid Electronics Ltd) placed on each side of the sample holder and dried overnight.Ten fibres each for PP and PP + ER were measured.Prior to each measurement, the fibre diameter was attained with FDAS770 Laser Scanning Micrometre (Dia-stron) (Figure S3A).The tensile strength measurements were run at the pulling speed of 0.1 mm s -1 , with a gauge force of 0.005 N and a gauge length of 12 mm (Figure S3B) corrected via pre-tensioning of the fibres [20].The engineering tensile strength, , calculated as: where F is the load at the failure and d is the fibre diameter. was plotted as a functionn of the engineering strain, ɛ: where L 0 and L are the original and final lengths of the fibre, respectively.
To account for the changes in the fibre cross-section and length upon elongation, the true stress  and strain ɛ were calculated according to Equation S11 and Equation S12, respectively: The Young's modulus E was determined from the linear region of the engineering tensile stress vs engineering tensile strain curve according to Equation S13,

Equation S13
and toughness, T, was determined as the area under the stress-strain curve according to Equation S14:

Equation S14
Figure S3 Experimental set-up for (A) the fibre diameter measurement (the red dashed arrow indicates the laser beam direction) prior to (B) the tensile strength measurement.

SI.04 nXRD results on sing-fibre crystal structure
Table S1 Fitted crystal structural parameters for polypropylene (PP) and polypropylene fibre with erucamide (PP + ER): average peak position q, crystal planes (hkl), and d-spacing (Å).Two specimens per fibre type were studied.The errors for the d-spacing were calculated via the error propagation method based on the uncertainty of the peak position, also considering the angular resolution of the instrument, which is smaller than 0.1 Å.The errors calculated as the standard deviation between the 4 sections of two fibres per sample batch yielded less than 0.06 nm -1 for the peak position q.   Figure S5 shows the representative diffractograms obtained for the the replicates of polypropylene fibre (PP, Fibre 2) and polypropylene fibre with erucamide (PP + ER, Fibre 2).
The 2D diffraction patern for PP + ER closely resembled that of PP and for both fibres all Bragg reflections were assigned solely to isotactic α-polypropylene.Furthermore, the position of reflections corresponding to polypropylene was unaffected by the presence of the erucamide, which is in agreement with our previous findings for bundles of fibres [21].

SI.06 Crystal orientation with respect to fibre axis
Figure S6 shows the azimuthal intensity distribution of the equatorial and near-meridional contributions to (110) reflection (q band of 9-11 nm -1 ) obtained for fibre replicated PP (Fibre 2) and PP + ER (Fibre 2).

SI.07 The off-equatorial and off-meridional reflections
The micro-focused X-ray Diffraction data shown in Figure S7

SI.08 Scherrer analysis
The crystallite domain size, L a , and the number of vertically aligned polymer chains, m, for PP and PP + ER obtained from different Bragg peaks are shown in Figure S8.The fibres with erucamide exhibited a larger crystallite domain size.The separation of the meridional and equatorial contributions to the (110) Bragg reflection allowed the assessment of the effect of erucamide on the structural order within the crystallites of bimodal orientation.The results obtained from the Scherrer analysis show that erucamide promoted the crystal growth in the direction corresponding to meridian, with an average parent lamellae crystal size of 243 ± 8 negligible effect on the size of the daughter lamellae, with a crystallite size of 104 ± 5 Å for PP + ER and 101 ± 12 Å for PP, respectively (Figure S8, Table S3).However, it promoted the growth of the daughter lamellae, with the average relative content of the daughter to the parent lamellae across four studied sections (S 1 -S 4 ) along the fibre length valuing at 16 ± 1% (daughter) to 84 ± 1% (parent) for PP + ER and 11 ± 1% (daughter) to 89 ± 1% (parent) for PP (Figure S8, Table S3).Table S3 Relative content of the parent and daughter lamellae based on the relative peak intensity of the equatorial and meridional contributions to the (110) Bragg reflection, for the PP fibre and the PP fibre with 1.5 % erucamide (PP + ER) at four sections (S 1 -S 4 ) along the fibre length.

SI.09 Crystallite size and degree of crystallinity along fibre length
The degree of crystallinity, D c (Equation S8), in the single fibre was calculated based on the Bragg reflections at the q range of 9.9 -17.8 nm -1 (cf. Figure 4, main text).Table S4 lists the degree of crystallinity (D c ) calculated for polypropylene (PP) and polypropylene with erucamide (PP + ER) for two samples per fibre type, across four sections (S1-S4) along the fibre length (cf. Figure 4B in the main text).
Table S4 Degree of crystallinity (D c ) calculated for polypropylene (PP) and polypropylene with erucamide (PP + ER) for two samples per fibre type, across four sections (S 1 -S 4 ) along the fibre length.The average values were calculated as the weighted arithmetic mean, to account for the larger number of points scanned for section S 1 (step size of 100 nm) than for sections S 2 -S 4 (step size of 500 nm).The errors for D c were calculated via the error propagation method S20 based on the uncertainties in the integrated intensities under the fitted Bragg peaks (A c ) and the amorphous halo (A a ).The errors for the average D c were calculated as weighted standard deviations from all the sections studied.Figure S10 shows the experimental XRD curves obtained for replicates (Fibre 2) of PP and PP + ER fibre at four studied sections (S 1 -S 4 ) along fibre length (Fibre 2). Figure S11 shows the representative radial intensity distribution of (110) Bragg reflection obtained for polypropylene fibre (PP, Fibre 2) and polypropylene fibre with erucamide (PP + ER, Fibre 2).

Section
Fitting of the data allowed to estimate the crystallite size and the degree of crystallinity.The fitting results are listed in main manuscript.

SI.12. Mechanical properties
Figure S1 (A) Schematic representation of rotational angles of a single fibre with respect to

Figure
Figure S2 (A) Nano-focused XRD (nXRD) experimental setup at Diamond Light Source Figure S2) along the fibre length: the (minimum) crystallite domain size (L a ) in the direction perpendicular to the (hkl) plane, and the number of vertically aligned polymer chains (m) oriented parallel to the plane (hkl) for the reflection.The fitted structural parameters corresponding to the equatorial (parent lamellae) and meridional (daughter lamellae) contributions to (110) reflection are denoted as 110 eq. and 110 m., respectively.The errors for L a are calculated via the error propagation using the uncertainties in FWHM (Δq).The errors for m calculated as a combination of the uncertainties in d and L a are all smaller than 1.

Figure S4 Fibre 1
Figure S4 Fibre 1 XRD diffractograms of (A) a polypropylene single fibre (PP) and (B) a

Figure S6
Figure S6Representative azimuthal intensity distributions of equatorial (φ = 180° ± 30°) and were attained at microXAS beamline (X05LA), Swiss Light Source (SLS), Paul Scherrer Institute, Villigen, Switzerland.The fibre samples were irridatied with micro-focused beam of 1 x 1 µm in size, with the beam energy of E = 17.2 keV (λ = 0.721 Å).Vertically aligned single PP and PP + ER fibres were investigated along 300 µm in fibre length, while the sample was rotated around its axis.The application of 8-module detector, with the intermodular gaps along fibre meridian and equator excluded the important contributions corresponding to the (110), (040) and (130) crystal planes compromising the overall crystal structure profiles.Nevertheless, since the offequatorial and off-meridional reflections consistent with periodic distance yielding at d = 4.21 Å and d = 4.08 Å (d = 4.07 Å for PP + ER), that corresponds to (111) and (041/131)crystal planes respectively, were unsusceptible to the alignment of detector modules, we were able to confirm that the appearance of the Bragg peaks was different for PP and PP + ER samples as a result of stronger scattering from the crystalline fraction of semicrystalline polymer from the fibre with erucamide masterbatch, suggesting erucamide facilitating a higher crystalline order.

Figure S7
Figure S7Example XRD patterns of (A) polypropylene fibre (PP) and (B) polypropylene fibre

Figure S8
Figure S8 The crystallite domain size (L a ) and the number of vertically aligned polymer chains

Figure
Figure S9 shows the experimental XRD curves obtained for Fibre 1 of PP and PP + ER fibre at

Figure S9
Figure S9 Experimental XRD curves obtained at four sections (S 1 -S 4 ) along the fibre length for

Figure S10
Figure S10 Experimental XRD curves obtained at four sections (S 1 -S 4 ) along fibre length for

Figure S11
Figure S11Representative radial intensity distributions of equatorial and near-meridional

Figure
Figure S12 shows the experimental XRD curves obtained for replicates (Fibre 2) of PP and PP

Figure S12
Figure S12Experimental XRD curves along with the decomposed Gaussian and Voigt fits for

Figure S13
Figure S13 Experimental XRD curves obtained at three sections (A 1 -A 3 ) across fibre width for

Figure S14
Figure S14 Radial intensity distributions of equatorial and near-meridional (110) reflections

Figure S15
Figure S15The DSC curves obtained for PP and two PP + ER samples (2 repeats each).The curves correspond to second heating cycle, after removing the thermal history and a re-crystallization process under the same controlled rate.

Figure S16
Figure S16Comparison between true and engineering stress vs strain ( vs ) curves for both

Table S6
Fitted structural parameters for polypropylene fibre (PP) and polypropylene fibre with erucamide (PP + ER) measured at three sections (A 1 -A 3 ) across the fibre width: the crystallite size (L a ) and the number of chains (m) ordered in the direction normal to the direction of the plane (hkl) giving rise to the reflection.The fitted structural parameters corresponding to the equatorial (parent lamellae) and the meridional (daughter lamellae) contributions to (110) reflection are denoted as 110 eq. and 110 m., respectively.The errors for L a were calculated via error propagation using the uncertainties for FWHM (Δq).The errors for m calculated as a combination of uncertainties in d and L a are all smaller than 1.

Table S7
Relative content of the parent and daughter lamellae based on the relative peak intensity of the equatorial and meridional contributions to the (110) Bragg reflection, for PP fibre and PP fibre with 1.5 wt% erucamide (PP + ER) at three sections (A 1 -A 3 ) across the fibre width.

Table S8
Mechanical parameters obtained for ten specimens per sample for PP and PP + ER fibres: fibre diameter (d f ), engineering tensile strength (σ), engineering elongation at break (ɛ), true tensile strength (σ t ), true elongation at break (ɛ t ), Young's modulus (E), yield strength (σ y ), and toughness (T).The errors for the average values were calculated as a standard deviation from 10 fibres per sample.