The XRD patterns of polypropylene fibres produced in this work show regions of high count volumes at positions across the spectrum.
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Diffraction Angle (2 Theta) 180 °C Rotational speed 5 10 15 20 25 30 35 40 45 50 Norm alised Inte nsity
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Rotational speed 16.45 (° 2)
Figure 3.17: XRD spectra of PP centrifugal spun at 180 °C and at 210 °C with spinning speeds ranging from 11,000 rpm (black) to 16,000 (navy). Graphs are normalised with respect to temperature. The shoulder that occurs ~ 16.45° 2θ at higher temperatures is labelled for clarity.
This indicates that X-ray superposition occurs at specific angles of beam incidence and from that it can be argued that the polypropylene fibres contains long range order in some aspects. The patterns produced from typical centrifugal spun MF650Y polypropylene fibres, Figure 3.17, have been normalised as the raw data demonstrated differences in absolute intensity measured as a result of the sample preparation rather than molecular structure. The XRD patterns are normalised to make the height of the highest peak the same for all samples. The normalised curves presented show that the peak locations and shape were largely consistent for all the fibres regardless of processing temperature. The only difference observed in the traces was a slight increase in a shoulder height at 16.45° 2θ as the processing temperature is increased, this has been labelled for clarity in Figure 3.17. In terms of processing speed no significant changes in the XRD pattern was observed as the spinning speed was increased from 11,000 to 16,000 rpm. The peak heights observed in these diffraction patterns have a similar shape and the relative peak height remained consistent. Despite the slight shoulder observed at higher temperatures it can be surmised that the diffraction patterns produced by the polypropylene fibres were largely similar regardless of the processing conditions.
The inspection of the pattern shape and location of the features yields information regarding the order of the system. The XRD traces collected from fibres contain a broad hump which occurs between 10 and 30°2θ. This feature is highlighted in Figure 3.18 and occurs where an increase in intensity above the background baseline can be observed. This broad increase in diffraction intensity is caused by the scattering of the X-rays by the amorphous parts of the PP. Discretely separate from the amorphous halo is the presence of more defined peaks that have intensity maxima at 14, 21.5 and 42° 2θ. These peaks are created by regions of order within the PP fibres.
Figure 3.18: XRD trace of polypropylene fibres produced by melt centrifugal spinning at 180 °C and 13,000 rpm. The background count baseline and amorphous halo region are explicitly shown.
This two phase structure is expected as most high polymers are actually semi-crystalline: comprising long range molecular order amongst areas of disorder. In long chain polymers the two phases are considered to be intrinsically linked and through defects in the crystal and disordered chain alignment, lack definite boundaries. There are several ways of describing and modelling the phase separation of semi-crystalline materials
The XRD peaks of the PP fibres shown above in Figure 3.17 are not sharp enough to be linked to the classic crystal structure of PP. By comparing the XRD trace of as-spun PP fibres to the diffraction pattern of MF650Y PP that had been pressed into a film and cooled slowly there is a clear difference in the shape and form of the trace from the two forms of the same material. The XRD traces of pressed film and fibres is shown in Figure 3.19.
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Figure 3.19: Normalised XRD traces showing MF650Y slow cooled PP film compared to the traces of the centrifugal spun fibres.
The location and shape of the peaks is the major difference between them: film PP has primary peaks at 12, 17, 18.5° 2θ with a doublet at 21.5 and 22° 2θ that are not seen in the fibre traces. The peaks are also much sharper than those observed for the fibres. These peaks are consistent with those observed elsewhere for conventional semi-crystalline polypropylene. However, the crystal structure of polymers can vary depending on how the chains conform. Crystallisation of PP is known to be complex with the possibility of forming three crystalline forms: α-monoclinic, β-pseudo-hexagonal and γ-orthorhombic. Van der Meer (2003) models each isotactic PP helix as a triangle and draws the three crystal forms with their respective unit cells, Figure 3.21. As-received and slow cooled polypropylene is usually found in the α-monoclinic, the structure and dimensions of which are shown in Figure 3.22 (Cho et al., 2010). This means that the X-ray diffraction patterns observed in the fibrous polypropylene produced in this study are a result of a different fine structure being present and this means that the fibrous polypropylene produced through melt centrifugal spinning is not crystallising in the same manner as conventional polypropylene. This is not the first time such XRD patterns have been observed: Raghavan et al. (2013) also observed a similar XRD trace in melt blown polypropylene and concluded that this was due to crystal deformation and faults.
However, it is known in the art that polypropylene, under the right conditions can form a paracrystalline structure known as a smectic phase (Natta and Corradini, 1960). The reasons for this and the exact structure of this phenomenon varies throughout the literature but a comprehensive hypothesis was supplied by Corradini et al. (1986). They concluded that the smectic phase observed was not due to microcrystals or crystal defects but caused by amorphic chain alignment in an arrangement more akin to the α-monoclinic unit cell than the β or γ-crystal forms. The cause of this structure is rapid quenching and elongation of the polymer chains during processing. This rapid quenching is observed in melt blowing but would also apply to centrifugal spinning. In the latter technique polymer exiting the spinneret is exposed to ambient air which leads to rapid cooling and solidification of the jet. The polymer solidifies so rapidly that the chain mobility is limited before crystallisation can occur.
The close approximation of the smectic phase to the α-monoclinic unit cell means that the fibre can be induced to crystallise through the application of heat. This additional thermal energy provides the mobility for the smectic phase to flip to the crystal form. This transition is observed via DSC through the low temperature exothermic peak. To evaluate this theory that temperature can induce a crystallisation below the melting point an annealing process was performed. Selected fibres were annealed for 1 hour at 120 °C and then cooled in a laboratory oven prior to XRD analysis. The different trace generated is shown in Figure 3.20.
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Figure 3.20: Annealed polypropylene fibres compared to untreated source fibre.
The annealed fibres exhibit primary peaks at 14.1, 16.8 and 18.6° 2θ which correlate to the peaks observed for slow cooled polypropylene film, Figure 3.17. In the position of the doublet observed in the film there is now single peak at 21.66° 2θ, proving that annealing of the fibres induces crystallisation. The process of annealing raises the fibres above the glass transition which increases chain mobility and allows the partially orientated fibres to form α-crystals.
The structure formed are known to be α-crystals as the crystal form and size can be calculated using Equation 2.6 and Equation 2.7 along with known information regarding polypropylene (de Villiers et al., 1998). The structure formed in the PP fibres is likely the monoclinic α-crystal but there are also alternative forms of polypropylene, Figure 3.21 (Van der Meer, 2003).
Figure 3.21 : Unit cell models of left: α-monoclinic, centre: β-pseudo-hexagonal and right: γ-orthorhombic. The grey triangles represent a right handed helix and the blank triangles a left handed helix (Van der Meer, 2003).
The polypropylene β-crystal has a triangular unit cell of dimensions a = b = 11.0 Å, c = 6.5 Å angle γ= 120° (Hirte, 1984). The β-crystals has been previously observed in fibres through the presence of peaks in DSC and XRD scans and are known to form in polypropylene melts experiencing high elongation and deformation of PP in conjunction with rapid solidification (Zhou, 2007). The conditions required for β-crystal formation are similar to those applied to the centrifugal spun fibres. The γ-crystal form is considered to be rare and usually requires a nucleating agent to be present in the melt (Van der Meer, 2003). However, the diffraction peaks typical of these structures are not observed in the XRD trace of either untreated or annealed fibres.
2.09 x 10 m-1 0 6 .6 5 x 1 0 m -10 chain axis dire ction
Figure 3.22: α-crystal form of PP in the 001 plane. The view is along the chain axis. Sketched from (Hirte, 1984).
The implication for processing is that in order to obtain semi-crystalline polypropylene a secondary heating process must occur to allow the chains to move from smectic to crystalline phases. This heating could occur within the centrifugal spinner, relying on either residual heat or auxiliary heating. Alternatively this could be done in a subsequent annealing process as performed in this study.
In summary, the XRD data provided here, combined with the DSC results indicate that centrifugally spun PP has a different fine structure compared to generic PP products. This new form is caused by the rapid quenching that occurs during melt centrifugal spinning. It is thought that rotational speed of the process plays only a marginal role in the quenching rate as the minimum speed required for spinning (11,000 rpm) provides a very high quenching rate and no trend was found to link the level of smectic phase to operating speed.