No abstract

Previous work has shown that focused ion beam (FIB) can be effectively utilized for the cross-sectional analysis of polymers such as core-shell solid micro-spheres [1] and hollow latex nanospheres [2]. While these have clearly demonstrated the precise location selection and milling control provided by the FIB technique, the samples studied consisted of only a single polymer. In this work, FIB is used to investigate bicomponent polymeric fiber systems by taking advantage of the component's differing physical properties. An approach for cross sectioning and thus revealing the morphology with respect to the polymeric components in a bicomponent polymeric fiber crosssection with the island-in-the-sea (I/S) structure is presented. The I/S fibers investigated were fabricated using the spunbonding process and are composed of bicomponent combinations of polypropylene (PP), polylactic acid (PLA), polyethylene terephthalate (PET), or nylon 6 (PA6).Bicomponent I/S fibers [3] were coated with approximately 30 nm of palladium-gold using a Denton Desk II sputter coater. An FEI Quanta 200 3D Dual Beam system was used for crosssectioning and imaging. The fibers were cross-sectioned by using a 30keV Ga + ion beam for both the mass removal and cross-section polishing steps of the process. Initial mass removal was carried out using a water injection system to chemically enhance material removal [4]. The injection of the water vaper into the vacuum chamber resulted in an increase of chamber pressure of about 1 order of magnitude (from about 5E-6 Torr to 5E-5 Torr). Prior to the mass removal step, a platinum strip was deposited onto a selected area across the width of the fiber to protect the surface of the fiber by preventing unwanted etching of the fiber surface. An approximately 20x20 µm 2 area with a depth equals to the fiber diameter (~10µm) was removed in the mass removal step by using a 20nA beam and a rectangle raster pattern. Beam currents of 5nA, 1nA and 100pA were subsequently used for cross-section polishing using a cleaning cross-section milling pattern to provide line-by-line material removal. Finally, a 30pA beam with a dwell time of 55µs and a 1024x768 pixels scan resolution was directed perpendicular to the cross section surface to provide a secondary electron (SE) image. An example of an I/S fiber cross-section is presented in Figure 1. Differential sputtering of the respective polymers gave rise to the necessary topographical contrast since sputtering occurred over the cross-section surface during ion induced secondary electron imaging. This differential sputtering combined with the contrast generated by ion-induced secondary electron microscopy provided the ability to differentiate the structure of the bicomponent fiber.Various spunbonding process parameters govern the final I/S fiber structure and properties. Previous studies on revealing the cross-section morphology utilizing oxygen plasma [5][6] and scanning electron microscopy (SEM) can also provide structural information. However, the sample preparation and ...

Previous work has shown that focused ion beam (FIB) nanomachining can be effectively utilized for the cross-sectional analysis of polymers such as core-shell solid microspheres and hollow latex nanospheres. While these studies have clearly demonstrated the precise location selection and nanomachining control provided by the FIB technique, the samples studied consisted of only a single polymer. In this work, FIB is used to investigate bicomponent polymeric fiber systems by taking advantage of the component's differing sputter rates that result from their differing physical properties. An approach for cross sectioning and thus revealing the cross-sectional morphology of the polymeric components in a bicomponent polymeric fiber with the island-in-the-sea (I/S) structure is presented. The two I/S fibers investigated were fabricated using the melt spinning process and are composed of bicomponent combinations of linear low density polyethylene (LLDPE) and nylon 6 (PA6) or polylactic acid (PLA) and an EastONE proprietary polymer. Topographical contrast as a result of differential sputtering and the high surface specificity and high signal-to-noise obtained using FIB-induced secondary electron imaging is shown to provide a useful approach for the rapid characterization of the cross-sectional morphology of bicomponent polymeric fibers without the necessity of staining or other sample preparation.

The ability to examine the topographical and morphological properties of polymeric material can provide significant insight into their functional properties. Focused ion beam (FIB) nanomachining and ion induced secondary electron imaging (ISE) is gaining acceptance as the method of choice to accomplish this [1]. The development of optimized nanomachining methodologies is essential to the continued expansion of FIB technology to the myriad of polymeric materials of technological importance. Recently, a FIB in situ technique was developed for revealing and imaging the cross sectional morphology of the polymeric components in a bicomponent polymeric fiber with the island-in-the-sea (I/S) structure [2]. This technique exploits the topographical contrast generated as a result of differential sputtering. When combined with the high surface specificity and high signal-to-noise ratio obtained using Ga + ISE imaging, the capability of FIB to provide a useful approach for efficient characterization of the cross-sectional morphology of bicomponent polymeric fibers without the necessity of staining or other sample preparation required for previously employed imaging techniques was demonstrated.While the utility of FIB nanomachining and ISE imaging was demonstrated for two specific bicomponent polymer systems, harnessing the full potential of nanomachining and differential sputtering to reveal morphology hinges on the ability to optimize FIB sputtering conditions. Sputtering of polymers is likely a complex process dependent on the molecular structure, thermal sensitivity, and chemical reactivity of the polymers which differs significantly from sputtering of materials such as semiconductors, ceramics and metals [3]. In polymers, material removal rate will be strongly affected by the stability of the resulting radicals and ionic fragments produced as the Ga + looses energy in the polymer(s). The generation of radicals and other factors resulting in the destruction of the polymer can also be affected by sputtering conditions such as Ga + dose rate, beam overlap and other factors. To gain a better understanding of the process of the nanomachining of polymers, dose rate, beam overlap and other factors affecting the material removal rates of selected technologically important polymers were investigated.Five technologically important thermoplastics were investigated: linear low density polyethylene (LLDPE), polypropylene (PP), polylactic acid (PLA), polyethylene terephthalate (PET), and nylon 6 (PA6). A FEI Quanta 200 3D DualBeam FIB system (FEI Company, USA) with a 30kV Ga + beam was used for all sputtering experiments. Polymers used for sputter rate determination were in the form of beads from the suppliers. A hand microtome was used to obtain flat areas on the polymer beads. To determine the sputter rate of each polymer, 5µm x 5µm craters were sputtered into the respective polymer beads (see example in Figure 1) under various experimental conditions. Craters were sputtered by scanning the Ga + beam in a serpentine pattern at norm...

The development of optimized focused ion beam (FIB) nanomachining methodologies is essential to the advancement of FIB for characterization polymeric materials. The ability to examine the topographical and morphological properties of polymeric materials can provide significant insight into their functional properties. Recently, a FIB in situ technique was developed for revealing and imaging the cross sectional morphology of the polymeric components in a bicomponent polymeric fiber with the island-in-the-sea (I/S) structure [1]. This technique exploits the topographical contrast generated as a result of differential sputtering. When combined with the high surface specificity and high signal-to-noise ratio obtained using Ga + FIB induced secondary electron (ISE) imaging, the capability of FIB technology to provide a useful approach for efficient characterization of the cross-sectional morphology of bicomponent polymeric fibers was demonstrated without any sample pretreatment. While the utility of FIB nanomachining and ISE imaging was demonstrated for two specific bicomponent polymer systems, harnessing the full potential of nanomachining and differential sputtering to reveal morphology hinges on the ability to optimize FIB sputtering conditions for different polymeric systems. Sputtering of polymers is likely a complex process dependent on both the polymer's properties and FIB beam parameters [2,3]. Of all the polymer properties, crystallinity influences many of the polymer mechanical properties such as hardness. To gain a better understanding of the process of the nanomachining of polymers, the fundamental relationships between material removal rate and dose for polymers having different cystallinities were investigated.Two technologically important polyolefins, linear low density polyethylene (LLDPE; Dow Chemical Company, USA) and isotactic polypropylene (PP; Product CP360H, Sunoco Chemicals Polymers Division, USA), were investigated in this study. Polymer beads of LLDPE and PP were melted at 140 o C and 180 o C, respectively, in an oven (Model 280A, Fisher Scientific, USA) under argon gas flow. Polymers beads were melted between two pieces of Si to form the polymer into a film having a smooth surface. The polymer melt films were then quenched in a commercial freezer to -15 o C. Subsequent thermal annealing of each polymer was carried out at 115 o C and 125 o C for LLDPE and PP, respectively, for different time intervals to produce different levels of crystallinity. The crystallinity of the annealed polymer samples was measured using a SmartLab x-ray diffractometer (XRD) and quantified with the SmartLab Guidance software (Rigaku Americas, USA). A Quanta 200 3D DualBeam FIB/SEM system (FEI Company, USA) with a 30kV Ga + beam was used for all sputtering experiments. To determine the material removal rate for each polymer sample, 5µm x 5µm and 10µm x 10µm craters were sputtered into the respective polymer films under various experimental conditions. Craters were sputtered by scanning the Ga + beam in a serpentine p...