Despite of having good ionic conductivity and high mechanical strength, one of the most important issues related to the applications of polymers is the lack of biodegradability for the polymers. As report in Huang et al., keeping conductive polymers such as polyaniline in the body for a long time may induce enduring inflammation and require surgical removal (Huang, et al., 2007). Most of the research studies the poly-Æ-caprolactone (PCL) because its noticeable natural degradation and mechanical strength as it only able dissolve in organic solvent. Other than that, PCL believe to have better ionic and electrical conductivity compare with other biodegradable polymer. PCL was picked as host material for its noticeable natural degradation and mechanical strength as it only able to dissolve in organic solvent. (B.C. Ng et al., 2011). One of the primary concerns that discourage research in polymer electrolyte is believed to be due to the environmental impact it would bring if polymer electrolytes are used in large amounts. To address such concern, it is an important and very challenging effort to introduce a biodegradable polymer with the aim to reduce the environmental impact. So, the use of polylactide (PLA) as the biodegrdable polymer for its proved biocompatibility and biodegradability, and its reasonably good solubility and have same good conductivity properties if compared to polyaniline (Huang, et al., 2007).
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Various types of analytical and evaluative methods are available in the polymer field. Several instruments were used to characterize the SPE. These instruments include impedance spectrometer, X-ray diffractometer, scanning electron micrometer. Impedance spectroscopy was employed to study the ionic conductivity, electrical and dielectric properties of the polymer electrolytes. However, the structural and thermal analyses are of great interest. The structural characterizations of polymer blend electrolytes were also investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM).
X-ray diffraction (XRD)
The amorphous degree of polymer electrolytes was investigated using XRD. The XRD patterns were recorded on a Siemens D 5000 diffractometer with
Cu-KÎ± radiation (Î»=1.54060 Å), over the range of 2Î¸=5-80Â° at ambient temperature. Debye-Scherrer equation is used to determine the coherence length as shown as below:
where Î» is X-ray wavelength; Î¸b is glancing angle of the peak; Î”2Î¸bis full width at half maximum (FWHM). The peak at 2Î¸â‰ˆ18Â° was chosen to determine the coherence length for the polymer electrolytes.
Study of XRD
X-ray diffraction (XRD) is a versatile, non-destructive technique that reveals detailed information about the chemical composition and crystallographic structure of natural and manufactured materials. The use of XRD is to measure the average spacing between layers or rows of atoms, to determine the orientation of a single crystal or grain, to find the crystal structure of an unknown material and to measure the size, shape and internal stress of small crystalline regions. XRD is a powerful and rapid technique for identification of an unknown mineral. In most cases, it provides an unambiguous mineral determination and minimal sample preparation is required (Bruce King, 2005). XRD are widely available and its data interpretation is relatively straight forward. The intensity of diffracted X-rays is continuously recorded as the sample and detector rotate through their respective angles. A peak in intensity occurs when the mineral contains lattice planes with d-spacing appropriate to diffract X-rays at that value of Î¸. Although each peak consists of two separate reflections (KÎ±1 and KÎ±2), at small values of 2Î¸ the peak locations overlap with KÎ±2 appearing as a hump on the side of KÎ±1. Greater separation occurs at higher values of Î¸. Typically these combined peaks are treated as one. The 2Î» position of the diffraction peak is typically measured as the center of the peak at 80% peak height. Bragg's law was used to explain the interference pattern of X-rays scattered by crystals (Bruce King, 2005).
Bragg's law is executed in powder diffraction technique. In this approach, XRD behaves like "reflection" from the planes of atoms within crystal and that only at specific orientations of the crystal with respect to the source and detector are X-rays "reflected" from the planes. Figure 3.1 illustrates the Bragg condition for the reflection of X-rays by a crystal. Two X-rays beams, Ray 1 and Ray 2, are reflected from adjacent plans, where the spacing between the atomic planes occurs over the distance, d. Ray 1 reflects off the upper atomic plane at an angle of Î¸ which equal to its incident angle. Similarly, Ray 2 reflects off the lower atomic plane at the same angle Î¸. However, it has to travel the extra distance Az or Cz as compared to Ray 1. For the reflected beams to emerge as a single beam of reasonable intensity, they must reinforce, or arrive in phase with one another. This phenomenon is known as constructive interference. For constructive interference to take place, the path lengths of the interfering beams must differ by an integral number of wavelengths (nÎ»). The perpendicular distance between pairs of adjacent planes (d) and the angle of incidence, or Bragg angle (Î¸) are related to the distance AB by
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AB = BC = d sin Î¸ (3.)
The difference in path length between the two beams is given by:
Difference in path length = AB + BC (3.)
Thus, difference in path length = d sin Î¸ (3.)
This must be equal to an integral number, n, of wavelengths. If the wavelength of the X-rays is Î», then
nÎ»= 2d sin Î¸ (3.)
This is known as the Bragg equation. At angles of incidence other than Bragg angle, the reflected beams are out of phase and destructive interference or cancellation occurs
Figure 3.: Derivation of Bragg's law
Scanning Electron Microscopy (SEM)
Using Leica' s SEM (model S440) at 10kV, the morphology of polymer samples at room temperature was studied. Insulator such as pure PLA was coated with a thin layer of gold to prevent electrostatic charging.
Study of SEM
A scanning electron microscope (SEM) is a type of electron microscope that images a sample by scanning it with a beam of electrons in a raster scan pattern. The electrons interact with the atoms that make up the sample producing signals that contain information about the sample's surface topography, composition, and other properties such as electrical conductivity. In this technique, an electron beam is produced by heating the tungsten filament and then focused by magnetic fields in a high vacuum. The vacuum prevents the interaction of the beam with any inessential particles in the atmosphere. The electrons from this finely focused beam are scanned across the surface of a sample in a series of lines and frames called a raster. At any given moment, the specimen is then bombarded with electrons over a very small area. These electrons may be elastically reflected by the surface of the sample with no loss of energy (backscattered electrons), they may be absorbed and emitted secondary electrons of low energy, they may be absorbed and give rise to the emission of visible light, and they may give rise to electric currents within the specimen. All these effects can be detected and hence given a map of the surface topography of samples (Smart & Moore, 2005).
The ionic conductivities of the samples were determined, by using HIOKI 3532-50 LCR HiTESTER, over the frequency range of 50 Hz to 1 MHz. The samples were sandwiched between stainless steel blocking electrodes and sealed in a glove-box, and then be placed in a temperature-controlled oven at vacuum (<10âˆ’2 Torr) for 2 hour to assure the membrane is up to test temperature before removing outside the oven.
Ambient Temperature-Ionic Conductivity and Temperature Dependence-Ionic Conductivity Studies
The bulk ionic conductivity of polymer electrolytes is determined by using the equation below.
where ð“ is the thickness (cm); Rbis bulk resistance ( Î©) and A is the known surface area (cm2) of polymer electrolytes. The semicircle fitting was accomplished to obtain Rb value. Rbof the thin electrolytes film was calculated from extrapolation of the semicircular region on Z real axis ( Z'), as shown in Appendix A. Besides,
Z' and Z imaginary ( Z'') axis must be in equal scale because the radius of a circle must be the same.
Frequency Dependence-Ionic Conductivity Studies
In this study, the conductivity was expressed as below:
where G is the conductance (S) which obtained from impedance spectrometer by choosing this parameter.
Study of Impedance Spectroscopy
Impedance spectroscopy (IS) is a powerful analytical tool to characterize the electrical properties of materials and their interfaces with electronically conducting electrodes. It is also widely been used to investigate the dynamics of bound or mobile charge in the bulk or interfacial regions of any kind of solid or liquid material: ionic, semiconducting, mixed electronic-ionic and even insulator (dielectric). This spectroscopy is not only well known in determination of ionic conductivity for solid electrolyte; it also extended for other solid dielectrics (Barsoukov & Macdonald, 2005). It is an extensive method because it involves a simple electrical measurement that can readily be automated. The impedance diagram can be much more informative because it is correlated with complex materials variables, ranging from mass transport, rates of chemical reactions, corrosion and dielectric behavior, to defects of crystalline portion, microstructure, and compositional influences on the conductance of solids. The microscopic parameters such as mobility of charge carriers, concentrations and rates of electron transfer reaction can also be predicted through the characterization in the impedance response. In addition, it has been used to investigate the membrane behaviour in living cells as it can estimate the aspects of the performance of chemical sensors and fuel cells. It also serves as an empirical quality control procedure via the interpretation of fundamental electrochemical and electronic processes. The true grain resistance is obtained as it decouples the grain and grain boundary effects (Barsoukov & Macdonald, 2005).
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