Exfoliation by extensional flow deformation

Fornes et al. (2001) proposed that the layered-silicate exfoliation during an extrusion process was initiated by the break-up of taller silicate stacks into smaller stacks, following a layer-by-layer peeling mechanism of the top and bottom silicate stack platelets by the extrusion shear stress. They also reported that this mechanism was proportional to the polymer Mw (viscosity), in which higher effective shear stress associated with higher Mw would lead to a more exfoliated morphology.

Contrary to conventional shear deformation via an extrusion process, the elongational stress deformation associated to the spin speed in fibre extrusion can, in many instances, being inversely proportional to the Mw and MWD. This is because, although higher Mw polymers will produce higher spinline tension associated with the elongational viscosity, the molten filament is also more susceptible to spinline fracture and thus less spinnable. On the other hand, lower Mw polymers, providing they possess sufficient melt strength, are more spinnable at much higher take-up speeds and hence would experience greater elongational stress and chain alignment. A recent schema proposed to describe the exfoliation mechanism of layered-silicate under the influence of extensional flow stress deformation can be found in the literature (Lew, 2004).

Correlations drawn from the experimental results have led to the postulation that melt-spinning and hence extensional flow stress per se will not evoke exfoliation of layered-silicate in a pristine state. However, interestingly, melt-spinning is found to greatly promote exfoliation and nanodispersion of a precedently intercalated layered-silicate. This postulation is in part based on the XRD and HRTEM results and further corroborated by the following observations and contentions.

Firstly, the two peaks attributed to the third and fourth orders of basal spacing of the layered-silicate seen in the XRD spectra for the injection-moulded specimens and feedstock compounds were absent in the fibre spectra. Because the disappearance of the third and fourth order peaks could be an effect of exfoliation or because of low X-ray beam intensity, it is not possible to distinguish which of the two effects is more prevalent. However, the first and second order peaks of the hsv fibres manifested a more depressed peak shape and exhibited a lower peak intensity compared with the Isv fibres, but, in theory, the Isv fibre should exhibit a greater peak intensity because of the larger circumference and hence area exposed for beam diffraction. It is quite apparent, therefore, that the disappearance of the third and fourth order peaks, and the suppressed peak shape and intensity for the first and second order peaks, are mainly attributed to further exfoliation of the precedently intercalated layered-silicate through an extensional deformation effect during the melt-spinning process.

Secondly, because the XRD spectra of PPEX did not show any peak in the compounded feedstock form, its absence in the fibre spectra could not be taken as convincing evidence of layered-silicate exfoliation associated with the melt-spinning. However, given that the HRTEM images of PPEX fibre have manifested layered-silicates of thinner lateral dimension and more uniform dispersion than the injection-moulded PPEX, this would imply the occurrence of exfoliation during the melt-spinning process.

Finally, the larger lateral dimension of layered-silicates observed in the feedstock compounds and the injection-moulded samples compared with the corresponding melt-spun fibres would confirm that the extensional flow stress associated with melt-spinning had indeed extended the exfoliation of the layered-silicate. The above analyses would lead to the following proposal of the exfoliation mechanism of layered-silicate during the melt-spinning process and its schematic diagram is given in the literature (Lew, 2004).

• The coiled polymer chains intercalated in the layered-silicate disentangle, enabling the layered-silicates to begin to realign themselves in the flow direction. The polymer chains residing in the intercalated layered-silicate spacing will also be oriented in the flow direction.

• Drawdown of the melt stretches the polymer chains and promotes further penetration of the co-entangled polymer chains into the layered-silicate spacing.

• Under the very high elongational deformation rates, the silicate interlayers begin to shear apart and are subsequently delaminated by the polymer chains that reside in the silicate interlayers.

• The exfoliated silicate platelets improve the slippage of the polymer chains during drawdown from the capillary, enhance the melt stability and result in highly regular chain orientation in the spinline direction. Depending on the extent of exfoliation, the exfoliated silicate platelets preserve the chain alignment by suppressing molecular relaxation in the fibre traverse axis, leading to an overall improvement in the birefringence and mechanical performance of the fibre, as shown by the PPEX fibre.

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