A. Ávila, Department of Electrical and Electronic Engineering, Universidad de los Andes, Bogotá, carrera 1E No 19A 40, Colombia. Tel: +57 1 3394949 ext: 2828; fax: +571 332 4316; e-mail: *email@example.com
The spatial distribution of electrical charges along the longitudinal axes of a polypropylene electret fiber was determined using Electrostatic Force Microscopy (EFM). EFM mapping on highly curved surfaces, such as those of polymeric fibers, is a challenging endeavour and most work reported in the scientific literature has been limited to single line-scan analysis or flat specimens. Charged polymeric fibers, electrets, are extensively used in high performance filtration applications and methods to determine the amount and magnitude of the charges on these fibers remain elusive. Electrical charge maps of individual fibers were obtained by biasing the tip to –10 V and maintaining a constant tip-sample distance of 100 nm. Spatially dependant EFM phase and magnitude gradients were determined and the developed technique may provide a unique understanding into the heterogeneous charge distribution on electrets fibers. Direct mapping of the charge distribution in electrets fibers can offer new insights in the development of antistatic additives, new means to facilitate electrostatic self-assembly of nano-moieties on the surface of fibrous materials and a quantitative metrics capable of determining discharge dynamics and predicting the shelf-life of filtration media.
Electrostatic charges are commonly injected to polymeric fibers in order to improve the use of these materials in electrostatic filtration applications (Ferreira & Figueiredo, 1992; Kim et al., 2006). Polymeric materials containing electrostatic charges are called electrets and have been widely used as air filters and respiratory masks. In electrets, the presence of embedded electrostatic charges improves aerosol capture efficiencies and lowers pressure drop across the filter as particles are not only captured via impaction and interception but also via electrostatic attraction mechanisms (Kim et al., 2007). Electrostatic charges are injected into polymeric fibers usually via corona and/or tribo-charging mechanisms. However, assessing the charge retention abilities of polymer electrets as well as determining the magnitude and exact location of these charges has been an enduring challenge.
Charge and charge spatial distribution have been previously measured in dielectric solids using thermal and acoustic techniques (Fleming, 2005). Thermal techniques included the pulse method, laser-intensity modulation-method, thermal steps and the thermo-elastically generated methods. Acoustic techniques include pressure wave-propagation, laser induced-pressure-pulse, nonstructured acoustic pulse, laser-generated acoustic pulse, pulsed-electroacoustic methods and the step-electro-acoustic technique (Ahmed & Srinivas, 1997). In thermal techniques, the maximum resolution is about 2 μm, whereas in acoustic techniques, the minimum thickness of the sample is about ten times the spatial resolution, which in most cases is on the order of tenths of micrometres (Fleming, 1999). Other charge measurement techniques include photoconductivity and field probe spectroscopy (Ahmed & Srinivas, 1997). The previously mentioned techniques have micrometre order resolution and/or use electrode pairs to carry out the measurements. Current techniques appear limited in resolution and require unique sample preparation procedures making them inappropriate to measure electrical charges in nanoscale systems. Polypropylene fibers commonly used in high performance filtration applications, such as the one used in this study, have diameters between 1 and 3 μm so probing their charge distribution requires the use of a technique not only capable of nanoscale resolution but also capable of long range scanning. Studies of localized charges on polymers samples such as PC (poly carbonate) and PMMA (poly methyl methacrylate) films have been reported since 1990s (Saurenbach & Terris, 1990; Terris et al., 1992). These measurements have relayed on the dual capacity of the tip to act as a voltage electrode when in contact with the samples as well as a probe of the charge injected into the material.
We hereby report on the use of EFM for the examination of charged cylindrical polymeric fibers by using a noncontact dual pass methodology. The presence of a curvature is an additional challenge to the EFM measurements as well as to the sample preparation procedures. The aim of this paper is to probe the charge distribution on the fibers and avoid modifying it by tip-sample contact charging.
Three main processes are used for charging electrets including corona charging, triboelectric charging and charging by induction (Brown, 1981). Corona-charged polypropylene fibers are the focus of this report. The charged ions from the corona treatment are embedded onto the surface and in the bulk of the polymeric specimen.
Current interest on high performance filtering applications has motivated studies to understand the retention of charge in the fibers. Kim et al. determined the presence and polarity of charges in a single polypropylene fiber and provided quantitative evidence of charge leakage in the fibers after immersion in isopropyl alcohol. However, their reported measurements were obtained only across a single line scan on the surface (Kim et al., 2006). In clear contrast, this work reports on phase and amplitude differences registered on sections across the longitudinal axis of the fiber covering up to 83% of the fiber's upper hemisphere.
The transformation of phase and height images into quantitative charge estimations remains a challenge not only for PP fibers but also for curved nanostructures. Up to date quantitative interpretation of charge density on fibers have been focused on geometrical planar approaches and only few have explored the exact influence of tip-sample geometries (Gomez et al., 2010). Establishing Semi-empirical and analytical models that correlate EFM measurements with the properties of the fiber could also provide a new insight into the electric forces acting on the tip and therefore and be translated into metrics to predict the shelf-life and optimal operating conditions of fibrous materials.
Electrostatic Force Microscopy (EFM) was used to measure the spatial distribution of charges in polypropylene electret fibers commonly used in filtration media. The work described in this manuscript used the same specimens as those previously used by Kim et al. (2006, 2010) with diameters between 1.8 μm to 2.2 μm. EFM experiments were performed at ambient conditions using a MFP3D-Bio Atomic Force Microscope (Asylum Research, Santa Barbara, CA, USA). In these experiments, the topographical profile of the sample and the second (nap) scan were obtained using AC mode. These experiments used AC240TM silicon cantilever tips with a platinum coating purchased from Olympus (Olympus Corporation America Inc., USA) and a probe resistance of 350 μs, which was sufficiently low for performing high electro-potential resolution imaging (Olympus Corporation). The cantilevers had a regular pyramidal shape with front, back and side angles of 0°, 35± 1° and 15°± 1°, respectively. The radius of curvature of the tip was 28 ± 10 nm and the free amplitude was 80.92 nm. According to the manufacturer's specification, the spring constant of the cantilever was in the range 0.5 to 4.4 N m-1, with nominal value of 2 N m-1. The resonance frequency of the cantilever was measured to be 72 ± 2 kHz. Tip-sample separation was kept at 100 nm and the tip bias voltage applied was –10v for all experiments.
Electret fiber samples were separated from yarns using plastic tweezers to avoid charge removal during the preparation steps. Small yarns were sequentially obtained by pulling apart the fiber until a couple fibers were visualized under a 100× optical stereoscope. The fibers were then placed over a pair of DI water droplets on a glass slide as illustrated in Figure 1(a). As the water droplets slowly evaporated, the fibers attached to the substrate. Figure 1(b) shows a photograph of the cantilever on top of a single fiber.
EFM experiments were carried out in AC tapping mode. EFM is a two-scan technique that can detect a difference between attractive and repulsive forces. During the first scan, the fiber's topography was captured while in the second scan, conducted at a fixed distance of 100 nm from the fiber surface, hence limiting the forces measured to long-range interactions. The force measured (F) depends of two components: the attraction or repulsion between the biased tip and the underlying electrode (the metal sample holder disc) and Columbic forces (between the biased tip and the localized charged on the sample surface).
Figure 2(a) shows a schematic diagram of the two-scan methodology followed. The second scan is a lifted profile of the topography obtained in the first scan, yet with a voltage bias applied to the tip. Figure 2(b) illustrates the dependence of the scan profile on the tip shape. When using an AC240TM cantilever, the 15o side angle of the tip shape renders a maximum 150o subtended arc over the fiber, which corresponds to less than half of the upper cylindrical surface.
Figure 3(a) shows a 2 × 2 μm2 topography of an electret fiber. Three different cross sections are taken at three different sections along the fiber as indicated in Figure 3(b). The three different horizontal line scans were labelled as start, middle and end. In the electrostatic phase image (blue traces and right axes on Fig. 3b), one can observe that although the three sections present a similar topographical shape, there is variation of 0.5–1.5° in the phase shift signals. A higher contrast can be noted on the amplitude plots where variations of 1 to 2 nanometres on the tip oscillation can be discerned.
Figures 4 and 5 illustrate electrostatic force measurements overlapping with the topography of the fiber specimens. Colour lines along the axis of the fibers specify the positions where amplitude and phase signals were registered. The lines also indicate the scanning direction. To reveal the fiber's heterogeneous surface charge distribution, three different sections of a single fiber were measured. For each section of the fiber, the figure presents the phase shift, amplitude contrast images and the corresponding raw data. A contrast image is defined here as a 3D image of the fiber that contains information of the 3D topography and the electrostatic cantilever response. In these images surface charge distribution is easily visualized.
In Figures 4 (a)–(c), the phase shift in the fiber is presented with the glass slide phase (nonconductive substrate) as a reference. A negative change in the phase represents an attractive interaction between the fiber's surface charge and the negatively biased tip (notice the variations from 0° to –2°). No charge variations were detected in the glass slide.
The contrast phase images demonstrate the heterogeneity of the charge distribution along the 6 μm length of a single fiber. These images allow the identification of specific areas characterized for the absence or presence of charge, notice the contrast between blue, green and white domains on the images. Since these domains are observed at the edges and in the middle of the fiber, the data provides direct evidence of the nonuniformity of the corona charging method. The contrasting images provide evidence that the curvature of the fiber poses a challenge for the desired uniform charge injection.
The detection of several domains at the centre of the fiber could mean that the surface roughness may act as charge trap and/or that the areas more exposed to the discharge electrode cause a higher charge injection. Even though having larger surface areas for enhancing the interaction between particles and fiber surface is considered as a promising result in the potential applications of electrect fibers in filters, the larger curvature that is created poses a significant obstacle, as it could represent higher spatially inhomogeneous charge distribution that would certainly compromise the filters’ efficiency.
The images in Figures 4 and 5 illustrate the heterogeneity of the corona-discharge process. EFM phase measurements are more sensitive to the charge distribution than amplitude measurements. The phase measurement contains information about the direction of the force, while the amplitude difference is only an indication of the presence of charge, regardless of its polarity. These measurements can be useful in optimising the manufacturing process of electrets (fiber's roughness charge injection efficiency and surface coverage) as well as in determining parameters such as shelf-life of the filtration media.
A methodology to map surface charge distribution in highly curved surfaces was described and validated. The maps obtained via EFM illustrate a heterogeneous surface charge distribution, as a function of position, with larger variations in the longitudinal direction of the fiber.
Surface charge distribution determination in nanostructures is a critical factor that can define characteristics of fiber-based products such as its wetability, adhesion, detachment selectivity of surrounded impurities and evenness of colour. The technique introduced in this paper can be adapted for assessing charge degradation dynamics and estimating shelf-life of electret based products.
The authors acknowledge the support provided by the office of the Vice-President for Research that coordinates the activities of the Microscopy center of the Universidad de los Andes.