Development of novel melt-spun nanocomposite fibers
Polymer nanocomposites are derived by incorporating nanoparticles into a bulk polymer. The resulting composite materials often have exceptional physical properties compared to the bulk material alone.1, 2 Recent interest for novel properties in functional textiles has spurred research dedicated to the conversion of polymer nanocomposite systems into fibers using the efficient and versatile melt-spinning process. However, industrial implementation of synthetic nanocomposite fibers has been impeded by serious issues during melt-spinning. For example, increased melt viscosity, agglomeration of the nanoparticles, blockage, and frequent breaking of the fibers during winding have prevented their industrialization. Inclusion of layered silicates, cage-structured silicone, and carbon nanotubes into host polymers has found varying degrees of success to improve fiber properties, such as tensile strength, dyeing ability, fire performance, shielding, and electrical conductivity.3–5 Similarly, specific interactions between the components in ternary polymer nanocomposites and unique synergistic effects in properties have been demonstrated with various classes of nanoparticles.6–8 Here, we use a semi-industrial process to melt-spin a number of complex multi-phase nanocomposite systems into fibers.
Melt-spinning is the process by which a molten polymer flows through the multiple orifices of a spinneret before being rapidly cooled and drawn into fibers. The drawing process induces alignment of the molecules along the fiber axis (i.e., molecular orientation) and crystallization, which in turn increases the modulus and tenacity of the fibers. After air cooling and crystallization of the polymer, the multifilament bundle is passed through a godet roller system consisting of several cold and heated rolls (see Figure 1). In sequence, the bundle is passed through two cold rolls (R2), two induction-heated rolls (R3), and two final cold rolls. The fibers are then wound onto a bobbin. The speeds achieved by the extruder can be varied, and the draw ratio—defined here as the velocity of R3:R2—affects the properties of the resultant fibers.
We began by developing a number of binary and ternary nanocomposite systems based on nylon 6 using a melt-compounder. Specifically, we compounded nylon 6 with a nano-structured cage silicon (polyhedral oligomeric silsesquioxanes, POSS), a layered-type silicate (montmorillonite, MMT), and a combination of the two nanoparticles (see Figure 2). We evaluated the ability of these systems to be melt-spun into fibers at various draw ratios (between 1:2 and 1:3). We successfully produced a range of fully orientated fibers of nylon-6 nanocomposites.9 From a manufacturing perspective, we found that most of our nanocomposite fibers processed well, despite observing some processing difficulties—such as breakage—at high draw ratios. This is likely an indication of weakened polar bonds at the nylon-6/nanoparticle interface, a result of high stresses in the drawing process. Alternatively, the presence of agglomerates in the fiber may act as stress concentration sites to induce breakage.
Having fabricated a range of nanocomposites, we interpreted the changes in tensile properties caused by cold drawing from crystallinity measurements, changes in polymorphic crystal forms, and molecular orientation of the nylon 6. We found the tensile properties of the fibers to be skewed (see Figure 3). However, some systems offered substantial improvements to the bulk tensile modulus (up to 80%) at the lower draw ratios (for example, 1:2 and 1:2.5) without compromising the extensibility of the fibers. At higher draw ratios (1:3), the increases in modulus were not as pronounced and were counter-balanced by some losses in elongation at break.
We next evaluated the changes in molecular orientation of our nylon-6 nanocomposites by measuring the optical birefringence of the fibers. (Optical birefringence is a manifestation of the existence of orientation-dependent differences in refractive index, found in molecularly ordered materials such as synthetic fibers.) We measured the birefringence of our fibers—using a Michel-Levy chart10—by placing them between cross-polarizers in an optical microscope. We determined the interference color resulting from the retardation between ordinary and extraordinary waves at angles ±45° (where maximum brightness was observed) and used a retardation plate in order to shift the interference color of one full wave (±530nm). We found that, for example, our nylon-6/MMT nanocomposite fibers (draw ratio 1:2) displayed uniformity of pink interference color in the mid-section of the fiber, indicating uniform molecular orientation (see Figure 4). Additionally, the change in color across the diameter of the fibers was evident as the thickness (and light path through the fibers) varied.
We also examined the development of α- and γ-polymorphic crystalline forms of nylon 6 upon drawing from the x-ray diffraction patterns of the fibers (see Figure 5). Although the nucleation activity of the particles was noted from an increase in crystallization temperature, the nanocomposite fibers were less crystalline. This suggests that the particles have hindered the nylon-6 chain mobility and delayed crystal growth during the second stage of the crystallization process (i.e., after spherulite-impingement).
Although tensile properties of fibers are generally governed by changes in molecular orientation and polymorphic crystal structures of the host polymer, it has been proposed that they are critically affected by the chemical compatibility between the various components in multi-phase systems.11 We found that limited chemical compatibility—or a weak polymer/nanoparticle interface—in some cases had an overpowering effect over the influence of both molecular orientation and polymorphism, leading to fibers that prematurely broke and underperformed in tensile tests.9
In summary, we provided insight into the influence of the relevant parameters for melt-spinning of complex multiphase nanocomposite systems. In some cases, the loss in properties highlighted some issues of compatibility. However, it is possible to improve the properties of melt-spun nylon-6 fibers using nano-sized particles. Similar to polymer blends, the optimization of the physical properties of these nanocomposite fibers requires an understanding and control of morphology, of both the interface and interphase. In the future, we will optimize the physical properties—such as fire retardancy and heat stability—of commonly used polymeric fibers using a range of nanoparticles for various technical applications.
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- Michel-Levy chart for optical birefringence measurement. http://www.olympusmicro.com/primer/techniques/polarized/michel.html
- L. A. Utracki, Fibers from polymeric nanocomposites, Indian J. Fibre Text. Res. 31, pp. 15-28, 2006.