Abstract
In this research, poly(ε-caprolactone) (PCL) was melt-mixed with sepiolite nanoclays in a twin-screw extruder. In a subsequent step, the extruded films were drawn in the solid state to highly oriented nanocomposite films or tapes. A twin-screw extruder equipped with a Sultzer mixer for improved mixing in combination with a bench top drawing unit was used to prepare oriented nanocomposite tapes of different sepiolite loading and draw ratios. In order to study the influence of the solidification step on the drawability of the materials, different cooling procedures were applied prior to drawing. Optical microscopy images showed that slow or fast solidification using different chill rolls settings (open or closed) for the cast films resulted in different morphological conditions for subsequent drawing. The addition of sepiolite nanofillers led to nucleation and faster crystallization kinetics and oriented tapes which deformed by homogenous deformation rather than necking. The addition of sepiolite significantly improved the mechanical properties of both undrawn and drawn PCL tapes and Young’s modulus (1.5 GPa) and tensile strength (360 MPa) for composites based on 4 wt% sepiolite were among the highest ever reported for PCL nanocomposites. Interestingly, samples cooled with open chill rolls (slow crystallization) showed the highest modulus while solidification with closed rolls (fast crystallization) showed the highest tensile strength after drawing.
Highlights
In order to fully explore the reinforcement potential of anisotropic nanofillers, orientation in the direction of the load is preferred
This was shown by an increased elongation at break for samples containing sepiolite and confirmed in this study by the results presented in Figure 9, where it could be seen that, for samples produced with open chill rolls, the elongation at break was increased slightly with filler content from 590% for neat PCL to 630% for PCL + 4 wt% sepiolite
A twin-screw extruder equipped with a Sultzer mixer together with a bench top drawing unit was used for the extrusion and solid-state drawing of nanocomposite films
Summary
In order to fully explore the reinforcement potential of anisotropic nanofillers, orientation in the direction of the load is preferred. Films, and tapes are typically drawn in the solid state after the initial melt or solution-based processing step, which provides a possibility to effectively orient the polymer chains, and the anisotropic filler in the drawing direction. Upon solid-state drawing, the initial, often spherulitic, structure of the polymer is transformed into a fibrillar structure, and uniaxially strong fibers or tapes are obtained [1]. The attainable draw ratio and degree of orientation of the polymer chains are correlated to the structure of the polymer, and strongly dependent on the processing history. Compared to the relatively abundant literature available on the drawability and structure development upon drawing of different homopolymers, in particular polyolefins [2,3,4,5], nylons [6], and polyesters [7,8], there are relatively few articles on the drawing of polymer nanocomposites.
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