Capillary-Channeled Polymer (C-CP) Fibers: A Novel Platform for Liquid-Phase Separations

In spite of its relative maturity as a group of analytical methods, the field of liquid chromatography continues to evolve with regard to the development of new stationary phases that may provide certain advantages over existing materials. Stationary phases in the form of inorganic or polymeric fibers have been a curiosity of separation scientists for many decades in the hopes of providing highly directional mobile-phase flow with minimal resistance.1–5

In principle, fibers can be extruded from silica-based melts so that subsequent derivatization chemistries can be performed on the surface. Likewise, polymer blends having specific functionality can be employed as well. While the chemical aspects of using micron-diameter fibers have been realized in terms of performing reversed-phase, ion-exchange, and capillary electrochromatography separations, the performance-to-cost ratios have never been such that there has been serious commercial interest in developing fibrous stationary phases. In particular, poor repeatability and control of the packing process, unexpectedly high flow resistance, and relatively low specific surface areas are the shortcomings typically cited.

A novel polymer fiber format is being developed in the authors’ laboratory in an effort to take advantage of the expected benefits of fibrous stationary phases.6,7 Capillary-channeled polymer (C-CP) fibers possess a very unique geometry (see Figure 1a) that includes eight channels that extend the entire fiber length (which can be miles on a spool). The fibers are nominally an oblong shape with diameters ranging from 35 to 50 μm, with the individual channels ranging in size from 5 to 20 μm. This class of materials was developed for commercial applications wherein spontaneous fluid movement (wicking) is desired (e.g., disposable diapers, sanitary napkins and tampons, and sportswear).8 For example, 1 g of polyester C-CP fiber can wick 100 mL of water in 1 hr.

Figure 1 - Micrograph of C-CP fibers. a) Scanning electron micrograph of a polypropylene fiber.6 (Reprinted with permission from Elsevier.) b) Optical micrograph of a polyester fiber bundle.

These fibers have another very important attribute. Depending on drawing conditions, C-CP fibers have surface areas that are 2.4–3.0 times those of conventional circular cross-section fibers. The combination of enhanced fluid mobility and increased surface area is critical to the potential use of C-CP fibers as chromatographic stationary phases. At this point in their development as chromatographic stationary phases, a few definitive qualities have been identified and are listed in Table 1. This paper presents some basic properties and examples of the use of the C-CP fiber columns for chromatographic separations as well as projected areas of practical application.

Experimental

To date, three base polymer types have been employed for chromatographic separations: polypropylene (PP), polyethylene terephthalate (polyester, PET), and nylon. As seen in the structures below, 

solute–surfaces interactions can take place based on the polymer functionality. As PP presents a totally aliphatic surface, PET provides aromatic and carbonyl functionality, and nylon includes amine and carbonyl groups on an alkyl backbone. Atomic force microscopy (AFM) measurements performed with native and carboxylated polystyrene beads attached to the AFM tips revealed the strongly hydrophobic nature of the PP and PET fiber surfaces (nylon was not available at the time).

The types of polymers that can be formed into the C-CP structures are quite diverse. The primary qualification here is that the polymer solution must be amenable to a melt-spinning process. In order for a polymer to be melt-spinnable, the melt must be sufficiently stable over a broad temperature range. In a semicrystalline polymer such as PET, the processing range extends from the melting transition (Tm) up to the degradation temperature. For glassy polymers such as polysulfone that cannot crystallize, the processing range will extend from above the glass transition temperature (Tg) to the thermal stability limit. Examples of common classes of polymers that fit into this general category include polyamides, polyesters, polyesteramides, polyolefins, polyetherketones, polyacrylates, polysulfones, polycarbonates, and polyurethanes. Finally, many copolymer combinations are available that can be melt-spun into C-CP fibers.

In addition to the surface chemistry affected by the base polymer, there is a wealth of fiber postprocessing that can be employed to generate more specific surface characteristics. For example, PET fibers were treated in caustic solution to create a surface that is anionic in character by oxidizing the carbonyl groups to carboxylate functionalities. In chromatographic terms, a reversed-phase surface has been converted to a cation-exchange surface. It must be stressed here that the chemical change is not achieved by coordinating a functional group to the surface as in the case of derivatized silica, for example. As such, a more robust surface is created. The polymer base materials also add a great level of chemical resilience to pH extremes not seen in silica-based supports where silica dissolution (typically at pH >7) and other forms of surface degradation/hydrolysis (pH <2) are limitations. 9 pH stability is the major driving force in the use of polymer beads as supports for biomolecule separations.10 In addition to the choice of polymer base to achieve specific separation chemistries, there is some flexibility in the physical aspects of the C-CP fibers. For example, the nominal diameter of the fibers, and thus the channel diameters, may be altered through the use of different fiber drawing conditions. There are also some alternative methods that may be employed to add porosity (i.e., surface area) to the fibers while still maintaining the basic capillary structure.

The general process of column packing is depicted in Figure 2. Columns were prepared using stainless steel tubing with standard commercial HPLC dimensions (inner diameter and length). Additionally, capillary formats were created in polymeric tubing and glass capillaries. Columns are prepared by winding the fibers onto a spool mounted on an axle assembly, fitted with a handle and an analog counter. When the desired amount of fiber (i.e., the number of fibers in the column) is obtained, the fibers are removed from the spool as a loop. A monofilament is then passed through the fiber loop and fed through the column and the fiber bundle pulled through, leaving a short portion of the fiber sticking out of each end of the column. Depending on the polymer material, there is a potential for fiber shrinkage in different organic mobile phases. Interestingly, this shrinkage only occurs along the main fiber axis, and not in the radial direction. Therefore, columns are soaked in acetonitrile for 30 min to remove any organic contaminants and to preshrink the fibers. The fibers are then cut flush with the column ends and standard HPLC endfittings are employed to mount the columns, just as for commercial packed columns. The fiber packing density is such that frits are not required to retain the stationary phase. In fact, standard frits can be the major source of backpressure when used on C-CP fiber columns.

Figure 2 - Diagrammatic representation of the C-CP fiber column packing procedure.

Figure 1b is an optical micrograph of a fiber bundle showing the packing arrangement likely present within the chromatographic columns. Clearly seen is the interdigitating of the fiber “fingers” as opposed to the perhaps expected crushing of the capillary walls. The primary limitation on the number of fibers that can be pulled through a column is the friction between the outermost fibers and the inner wall of the tubing. At the extreme, either the mechanical force is too great to pull the fibers through, or the monofilament (typically 50-lb test line) used to pull the fibers breaks. In a comparison of flow characteristics with commercial 5-μm porous bead-packed columns of the same dimensions (4.6 mm i.d. × 250 mm long), C-CP fiber columns operated over a flow range of 1–9 mL/min with an upper backpressure of <2000 psi, while the particle-packed columns reached the maximum system pressure of 4000 psi at 4 mL/min.11,12 The overall interstitial fraction values are somewhat better than expected for porous silica columns, with the specific permeability values being quite comparable. To be fair, these favorable hydrodynamic characteristics are achieved for a stationary phase of fairly low specific surface area (~3 m2/g) versus porous beads (>100 m2/g). As such, the efficient flow characteristics of the C-CP fiber columns must be used to advantage in columns of greater length to achieve high plate numbers in isocratic separations.

Comments