Bicomponent fiber spinning involves co-extruding two polymers or polymer blends from the same spinneret into a filament that contains both. Possible benefits of co-spinning include the lower costs associated with single-step processes, the performance of fibers based on multiple components, and the suppression of an unfavorable rheological behavior (i.e., Newtonian) of one polymer by spinning with a small fraction of another more advantageous (i.e., Maxwell-fluid-like) polymer [Park et. al (1990)]. While polymer blends generally result in properties intermediate to the individual components and requires trading off one quality for another, bicomponent spinning has the potential to retain the individual component properties.1 However, bicomponent spinning can lead to fibers that split when subjected to mechanical force.2 Maintaining adhesion between the individual components retains the mechanical properties of fibers and can direct the morphology of molecules within fibers.
At the polymer-polymer interface, immiscible polymers exhibit reduced entanglements which can lead to rupture of the interface at temperatures below the Tm of the components. Above the Tm of both components, subjecting the material to external forces in the plane of the interface may result in “slip” of one polymer across the other, also due to reduced polymer chain entanglements. Analogous to surfactants in aqueous-oil emulsions, block copolymers may be used to lower the interfacial tension at polymer-polymer interfaces which can improve adhesion between the polymers and reduce slip during extrusion, a concept which is commonly referred to as “compatibilization”. Block copolymers make excellent surfactants for multiple-polymer systems and are judged to be effective if they reduce the domain size(s) of the individual phases in blends, reduce the interfacial tension of a polymer pairs, and/or increase the interfacial adhesion and mechanical properties of the composite.1,3-4 The concept of compatibilizing polymer-polymer interfaces that are not blended but rather contacted, such as those involved in core-sheath fibers or laminates, however, has largely been ignored. To our knowledge there have been no reports on the compatibilization of the core and sheath of bicomponent fibers by the addition of a block copolymer to one component.
We found that during large scale mechanical deformations, PP/PLA co-spun fibers split at the interface when PP was in the core and PLA the sheath.2 We explore the concept of compatibilization of two non-blended polymers by incorporating a triblock copolymer to influence the mechanical properties of melt spun bicomponent core/sheath fibers of PP and PLA. The morphology of the block copolymer in fibers is compared to that formed during melt mixing without extrusion and from solvent casting to elucidate the effect of elongational forces on block copolymer morphology. We discuss the application of steady shear rheological methods to evaluate the existence of slip at polymer-polymer interfaces and the efficacy of block copolymers in compatibilizing two polymer melts.
1R. Holsti-Miettinen and J. Seppälä. Polym. Eng. Sci. 32(13), 868 (1992).
2S. Arvidson, et al. ACS Appl. Mater. Interfaces (under review).
3T. Del Castillo-Castro et al. J. Appl. Polym. Sci., 119, 2895 (2011).
4D. Wang, et al. Polymer 52, 191 (2011).