Structure, molecular orientation, and resultant mechanical properties in core/ sheath poly(lactic acid)/polypropylene composites
Graphical abstract
Introduction
Polymer composites are increasingly important for structural building [1], [2], [3], [4], electronic [5], [6], [7] and biomedical materials [8], [9]. An ideal composite is one that benefits from a net increase in the performance of the material derived from the desirable characteristics of the constituent polymers and interactions between them, with suppression of unfavorable traits of each constituent. The mechanical behavior of semicrystalline polymers is strongly dependent on crystallinity and microstructure, which are determined by the thermomechanical history. However, co-processing two (or more) polymers provides additional challenges in controlling and predicting the material behavior. To create a polymer–polymer composite, the materials must be compatible in terms of the conditions at which they can be processed as well as the conditions at which each exhibits its desired property (e.g., toughness, conductivity, optical), as well as strong consideration of interfacial compatibility.
With recent focus on environmental awareness, much consideration has been given to polymer composites that include all or some fraction of materials that can be produced from sustainable resources such as bamboo [10], cotton [11], hemp [12], jute [13], kenaf [14], linseed and caster oil [15] and poly(lactic acid) [13], [16], [17]. Polymers of lactic acid or lactide monomer (PLA), which can be produced from agricultural rather than petroleum raw materials, are particularly interesting for biomedical, packaging, and disposable applications. PLAs are biodegradable and biocompatible and may solve some solid waste disposal problems. Manufacturing PLA consumes up to 65% less fossil fuel and produces up to 80–90% less greenhouse gas emissions relative to petroleum-based polymers [18] and are currently in use for sutures and other medical devices and increasingly in food packaging in Japan, the US, and Europe [19]. Yet, despite high tensile strength and the potential environmental advantages (outlined above), PLA has found limited use due to its brittleness and low ductility [20], poor gas barrier properties [21] and hydrolysis at temperatures suitable for melt processing, dyeing, or laundering [22], [23], [24].
Unlike PLA, polypropylene (PP) is not biodegradable or biocompatible, but possesses superior mechanical strength, abrasion resistance, resistance to chemical or biological agents, and benefits from being a ubiquitous and inexpensive leader in numerous polymer applications including woven and nonwoven fibers, extrusion molding, and packaging. Excellent quality extrudates such as fibers, films, and molded parts may be produced from PP alone, though the lack of reactive sites and low surface energy has lead to a number of creative methods to increase PP functionality. Additives may impart antimicrobial activity [25], [26], [27]. Some recent post-spinning treatments to PP fibers such as plasma exposure [28], [29], chemical modification [28], [30], [31], [32], or metal or metal oxide deposition [33], [34] have been applied to increase the reactivity, conductivity, abrasion resistance, or wettability of the surface though possible drawbacks to these approaches include changes in morphology, loss of polymer mechanical strength, and increase in web permeability when additives are included [35], [36] and high processing temperatures, high cost, and/or low efficiency [28] for many of the post-spinning treatments.
In this work we focus on composite PLA – PP fibers produced by co-extrusion with the ultimate goal of improving the mechanical properties over that of PP-PLA blends, providing a better understanding the effect of co-extrusion on the morphology of each polymer, and ultimately insight into the design of other polymer composite structures. We use fibers as our system of study because of their preponderance in composite applications. In addition, the ability to examine the role of core versus sheath materials would provide insight in multilayered fibrous systems, which a study of a bilayered planar composite system would fail to achieve. Bicomponent fibers are widely used in nonwoven fiber materials where the sheath has a lower melting point to promote bonding [37], [38] while the higher Tm core component experiences less thermal exposure and morphological modifications during bonding. Beyond the expressed advantages of PP and PLA already discussed, both polymers melt and can be processed within the same range of temperatures (Tmelt = 155–165 °C, Tprocess = 200–230 °C), which is markedly lower than that for other common fiber forming polymers such as nylon-6 or poly(ethylene terephthalate). There are a number of reports of bicomponent spinning and bicomponent PP-PLA fibers have recently been commercialized [39], but to our knowledge, no reports in the open literature describe the melt spinning process for core/sheath bicomponent PLA with PP fibers nor the effect of spinning parameters on morphology of such fibers.1 While blending PLA with PP lead to decreased fiber strength relative to either polymer spun individually due to poor interfacial compatibility [40], [41], we believe that greatly reducing interfacial contact area in core-sheath configuration, where each polymer is continuous along the length of the fiber, will maintain the mechanics of the stronger component. Further, co-spinning a sheath of PLA on PP provides routes to surface-functionalize PP [36], [37], [38], [41], [42], [43] fibers for biological and filtration applications, while encapsulating PLA within a sheath of PP can provide for resistance to solvent or biological degradation.
Section snippets
Materials and fiber spinning
Isotactic polypropylene and poly(lactic acid) were provided by Sunoco Chemicals Polymers Division, Pittsburg, PA (product CP360H) and NatureWorks® LLC, Minnetonka, MN (product PLA polymer 6202D, 98% l-lactide), respectively. Polymer viscosities are listed in Table 1. Single and bicomponent fibers were spun at the Nonwovens Cooperative Research Center (NCRC) Partners’ Pilot Spunbond line located at North Carolina State University (NCSU) over a range of aspirator pressures.2
Fiber spinning
PLA/PP bicomponent fibers were spun with varying mass ratios of PLA:PP approximately equal to 10:90, 50:50, and 85:15. Fibers were fully solidified at the collector distance of 1.3 m. The focused ion beam induced secondary electron (ISE) images of the cross sections of representative bicomponent fibers are illustrated in Fig. 1. These micrographs are taken with a Ga+ FIB beam current of approximately 5 pA. The cross sections are viewed at ∼52° with respect to the cross section plane normal.
Conclusions
The effect of co-spinning poly(lactic acid) and polypropylene on the crystallization of each polymer and on the strength and molecular orientation of fibers was investigated. A modified method of estimating molecular orientation via the tensile strain of fibers was established and indicates that spinning PLA or PP as the sheath component may result in greater molecular orientation (strain shift) than for either polymer spun individually. Core-sheath fibers of PP and PLA were as strong as either
Acknowledgments
The authors gratefully acknowledge the Nonwovens Cooperative Research Center (NCRC) for financial support of this work. Thanks are also due to Dr. Behnam Pourdeyhimi for fruitful discussions.
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