Effect of barium chloride doping on PVA microstructure - Springer Link

Appl. Phys. A 87, 797–805 (2007)

Applied Physics A

DOI: 10.1007/s00339-007-3923-y

Materials Science & Processing

r.f. bhajantri1 v. ravindrachary1,u a. harisha1 c. ranganathaiah2 g.n. kumaraswamy2

Effect of barium chloride doping on PVA microstructure: positron annihilation study 1 2

Department of Physics, Mangalore University, Mangalagangotri 574199, India Department of Physics, University of Mysore, Manasagangotri 570006, India

Received: 24 November 2006/Accepted: 22 January 2007 Published online: 27 March 2007 • © Springer-Verlag 2007

Microstructural studies on BaCl2 doped polyvinyl alcohol (PVA) polymer films were carried out using density, PAL and dielectric measurements at room temperature. The positron annihilation studies on these samples shows considerable effect on the PVA microstructure due to doping and is understood by invoking the chemical reaction between Ba2+ ions of BaCl2 with OH groups of PVA via intra/inter molecular hydrogen bonding, which forms the complex. This complex formation modifies the free volume content in the amorphous fraction, and results in an enhancement of the polymer crystallinity. At higher dopant concentrations, the number of such complexes increases, and ends up with the formation of dopant aggregates or agglomerates leading to certain phase separation into a polymer-rich phase and a dopant-rich phase. These phase separations are thought to be due to the existence of two or more crystalline phases within the polymer matrix. The XRD study also supports this enhancement of PVA crystallinity due to doping. The electrical studies on the doped PVA reflects that the complex formation due to doping affects the microstructure and hence the dielectric properties including the dc and ac conductivities of the polymer. All of these observed results were analyzed and understood based on the microstructural modification of PVA as a function of dopant concentrations. ABSTRACT

PACS 78.70.Bj;

1

73.61.Ph; 77.84.Jd

Introduction

Over recent years PVA polymers have attracted attention due to their rich variety of applications. A semicrystalline polymer consisting of crystalline and amorphous phases in the nanometer range, forms lamellar stacks within a superstructure. A crystalline polymer may be regarded as an amorphous matrix in which small crystallites are randomly distributed. However, it is more natural to treat a crystalline polymer as a certain sufficiently imperfect crystalline lattice in which voids are filled with amorphous matter. The role of amorphous regions may be played by the sites saturated with crystal defects like free volume holes, which are formed in u Fax: +918-24-2287367, E-mail: [email protected]

chain-folds. The general properties of a polymer depends on the volume fraction of the crystalline domains as well as on their size and structure. One way of tailoring the polymeric properties for a particular application is by doping. When a polymer is doped, the dopant can induce modifications in the molecular structure and hence the microstructural property of the polymer. In particular the transition metal salts and nanoparticle-doped polymers are considered to be a new class of organic materials due to their considerable modification on physical properties including microstructural, optical, electrical and thermal properties. These changes in physical properties, depends on the chemical nature of the dopant and the way in which they interact with the host polymer [1–3]. Polymers contain local free volumes; these are the cavities or holes of atomic and molecular dimensions that arise because of irregular molecular packing in the amorphous phase (static and preexisting holes) and molecular relaxation of the polymer chains and terminal ends (dynamic and transient holes). The presence of these holes lowers the density of the amorphous phase by about 10% compared with the density of the crystalline phase of the same polymeric material. Therefore the presence of these free volume holes, within a polymeric system affects the thermal, mechanical, and relaxation properties of that polymer. Generally many techniques have been used to study the free volume related properties of the polymer like photochromic, fluorescent spectroscopy, X-ray diffractions, and positron annihilation lifetime spectroscopy (PALS). Among these, the PALS method plays an important role in polymeric study, because it carries important information about the free volumes and their microstructures. Hence PALS is emerging as a unique, nondestructive, potential tool, and is now widely used as a chemical probe for directly determining the nanometer scale free volume holes, and their number density (concentration) fluctuations within a polymer [4–6]. In this technique, positrons from a radioactive source (22 Na ) are injected in to a condensed medium like polymers, they get thermalized rapidly by losing their energy and annihilate with the electrons of the medium. Annihilation usually takes place from different positron states viz., the free annihilation process, or from a localized state (trapped state) or form a bound state called positronium atom Ps (a hydrogen like positron–electron bound state) by picking up an electron

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Applied Physics A – Materials Science & Processing

from the molecules. Four possible locations for the positron and Ps before the annihilation are: in the polymer bulk matrix, a defect such as a hole or free volume, on the surface, and in the vacuum. Once Ps is formed, it can be trapped in a pre-existing free volume hole in the amorphous region of the polymer during its lifetime. Usually, the positronium exists in two spin states: para-Positronium (p-Ps) in which particle spins are antiparallel and decays into two gamma quanta with a mean lifetime of 0.125 ns, and is not useful to characterize the free volume of polymers. The other Ps state is ortho-Positronium (o-Ps) in which the particle spins are parallel and decays into three photons with a lifetime of 140 ns in vacuum. In molecular solid like polymers its life time is reduced to few nano-seconds depending on the free volume properties of the polymer and annihilates with an electron of the medium possessing an opposite spin – a process called pick-off annihilation, in which two gamma quanta are emitted. It is a well-established view that the Ps are usually localized in the free volume holes and according to the free volume model, the o-Ps lifetime represents the probability of overlap of the o-Ps wave function with the wave function of the electron associated with the surrounding cavity wall (the positron annihilation rate λ is proportional to the overlap integral of positron and electron densities). As the size of the free volume site increases, the electron density seen by the o-Ps decreases and thus o-Ps lives longer resulting in a greater value of the o-Ps lifetime. The intensity of the o-Ps increases with an increase in population of free volume sites due to higher o-Ps formation and trapping rates. Hence the o-Ps lifetime study is the focus of the PAL technique in polymeric research [4–12]. Various research groups used positron annihilation lifetime spectroscopy and other complementary techniques to understand the microstructural properties of doped polymers in terms of its free volume content. Some of the studies are: thermally induced changes on doped PAN [6], effects of iodine doping on PVA [7], temperature dependence of CuCl2 doped PVA [8], microstructural and conductivity variation of HDPE upon doping [9], iodine sorption on polycarbonate [10], disperse red1 (DR1) and disperse orange3 (DO3), chromophores doping effects on PMMA [11], FeCl3 and CrCl3 doping effects on polyhydroxamine acid [12]. The main study on the electrical properties of a polymeric material is to obtain information on molecular motion and structural transitions. In a semicrystalline polymer, containing a mixture of amorphous and crystalline regions, the conductivity may be dominated by the properties of amorphous regions which gives rise to localized states. Since there are many localized states in a semicrystalline polymer, the release or excitation of carriers in these states dominates the conduction process. In the doped polymers, if the dopant molecules are present in low concentration, they will give rise to additional molecular sites for the trapping of charge carriers and such localized sites can be defined in molecular terms using the difference in ionization potential as an indication of trap depth. As the dopant concentration increases, the molecules start bridging the gap separating the two localized states and lowering the potential barrier between them thereby facilitating the transfer of charge carriers. This results in an enhancement of the conductivity of the polymeric system. Thus the conductiv-

ity of a polymer can be controlled by properly choosing the dopants, their shape and their relative concentrations [13, 14]. PVA is a semicrystalline polymer that has been studied intensively due to its interesting physical properties. The special properties of PVA arise from the role of OH groups and hydrogen bonding. PVA is normally a poor electrical conductor; it becomes conductive upon doping with some dopants. Conductivity in PVA arises due to the high physical interactions between polymer chains, via hydrogen bonding between the hydroxyl groups and the dopant, and is mainly dominated by the properties of the amorphous regions. In addition, the ionic mobility in the amorphous region changes by segmental motion of the PVA host and it changes with crystallization and structural order. Hence the ionic conductivity of a doped polymer is mainly localized into the amorphous phase. Here the dopant may reside within the amorphous regions forming a charge transfer complex (CTC), or it may exist in the form of molecular aggregates between the polymer chains of PVA. In this case the CTC thus created are supposed to create localized states of various depths, which will lead to the trapping sites distributed over a considerable wide energy range. Therefore from the basic point of view, when a polymer is doped with a dopant, it is important to understand its ability to form CTCs, the mechanism of charge production and its subsequent localization in surface and bulk traps. Several studies of the electrical and dielectric properties of multiple valance metal ions halogen doped PVA, showed a strong dependence of the donor-acceptor mechanism between the metal ion and the polymer matrix [3, 7, 15]. In view of this it is important to note that the dopant modifies the microstructure of a polymer and hence free volume related properties. Particularly, the doping process changes the molecular mobility and in turn affects the chemical structure, crystallinity, and the degree of cross-linking of a polymer. In this paper we have chosen a divalent metal salt BaCl2 and doped with different concentrations to the semicrystalline polymer PVA. The effect of BaCl2 doping on the microstructural properties and electrical properties were studied using positron annihilation lifetime spectroscopy, powder XRD, dielectric properties, and dc as well as ac conductivity techniques. 2 2.1

Experimental section Materials and sample preparation

The polymer PVA used in this work was obtained in powder form from M/s.s.d. fine-chemical Mumbai. It has an approximate molecular weight of 1,25,000 and its degree of saponification is 86% – 89%. The BaCl2 was procured from Glaxo Mumbai. The PVA films with different mass fractions of 0, 1, 5, 10, 20, and 25 (wt.%) of BaCl2 dopant were prepared by a solution casting method as given in our earlier communication [2]. The thickness of the films is in the range of 0.05– 0.2 mm. The densities of the films were measured using Archimedes’s principle, by considering the weight of the films in air and water. 2.2

Positron annihilation lifetime measurements

Positron lifetime spectra were recorded in pure as well as BaCl2 doped PVA using positron lifetime spectrometer

BHAJANTRI et al.

Effect of barium chloride doping on PVA microstructure: positron annihilation study

(PLT). The spectrometer consists of a fast-fast coincidence system with BaF2 scintillators coupled to photo multiplier tubes of type XP2020/Q with quartz window as detectors. The BaF2 scintillators were conical shaped to achieve a better time resolution. Two identical pieces of a stacked sample were placed on either side of a 17 µCi 22 Na positron source, deposited on a pure kapton foil of 0.0127 mm thickness. This source-sample sandwich was placed between the two detectors of the PLT to acquire lifetime spectra. A 180 ps resolution is achievable for this spectrometer with an 60 Co prompt spectrum. However, to get a higher count rate, the spectrometer was operated at 220 ps and all measurements were carried out at room temperature. Two to three lifetime spectra with more than a million counts under each spectrum were recorded within 1 – 2 h of duration. Consistently reproducible spectra were analyzed into three lifetime components with the help of a computer program PATFIT-88 [16] with proper source and background corrections. The source correction and resolution function terms were estimated from the lifetime of wellannealed aluminum using the program resolution function. In three-component analysis, both ‘fixed’ and ‘free’ analysis were carried out on the PALS spectra. Consistently better χ 2 values and standard deviations are obtained in ‘fixed analysis’ with 0.125 ns (p-Ps) and the variance of the fit is around 1.0. 2.3

X-ray diffraction measurements

From the scanned XRD graphs given in our previous paper [2], the degree of crystallinity X c (%) was estimated from the ratios of the areas under the crystalline peaks and the respective halos using the relation using Powder X software [17]. Xc =

Ac × 100% , Ac + Aa

(1)

where Ac and Aa are the area of crystalline and amorphous (halo) region, respectively. 2.4

Electrical measurements

I –V measurements are conducted using a Keithley 236 I –V programmable source measure unit, by the two probe method at room temperature. The polymer films were coated with a good quality silver paste on both sides to ensure a good electrical contact. These films were sandwiched between two silver electrodes and the measurements were carried out in resistance mode: by applying voltage in the range (0 – 100 V) in multiple steps of 2 V, the current is measured. The current–voltage graphs show the linear relationship i.e., ohmic ( IαV) behavior. The dc conductivity was calculated using the relation σdc =

d (S/cm) RA

(2)

where A = 2.55 cm2 is the area of the silver electrode, d is the thickness of the sample. R is the resistance of the samples, which is estimated by linear fit of the I –V graph.

2.5

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Dielectric measurements

The dielectric measurements were carried out at room temperature using a Keithley 3322 LCZ meter. By sandwiching the sample between two copper electrodes of 2.4 cm diameter, the capacitance C and the dielectric loss (tan δ) were recorded for different frequencies. Using the capacitance values C , the dielectric constant εr is calculated with the relation, εr =

Cd , ε0 A

(3)

where d is the thickness of the sample, A is the area of the electrode and ε0 = 8.85 × 10−12 F/m is the permittivity of free space. The ac conductivities were calculated using the relation, σac = 2π fε0 εr tan δ ,

(4)

where f is the frequency, εr dielectric constant and tan δ is the dielectric loss. 3 3.1

Results and discussion PALS studies

The acquired positron lifetime spectra were resolved into three-lifetime components. The first short-lived component τ1 with intensity I1 (τ1 = 0.123 ns, I1 = 8.48%– 10.77%) is attributed to self-annihilation of p-Ps and free annihilations of positrons in bulk samples. The intermediate lifetime component τ2 with intensity I2 (τ2 = 0.291 – 0.321 ns, I2 = 70.80%–77.28%) is considered to be the annihilation of trapped positrons at the defects present in the crystalline regions or trapped at the crystalline-amorphous interface regions. The third and longest lifetime τ3 with intensity I3 (τ3 = 1.554– 1.605 ns, I3 = 13.22%–18.76%) is attributed to the pick-off annihilation of the ortho-positronium (o-Ps) in the free volume sites present mainly in the amorphous regions of the polymer matrix. Following the treatment developed by Nakanishi et al. [18], a semi-empirical correlation between the free-volume hole radius and the o-Ps lifetime τ3 has been established. In this model, a positronium is assumed to be localize within a spherical potential well having an infinite potential barrier of radius R with an electron layer in the region R < r < ∆R. The relation between τ3 and the radius R of the free volume hole or cavity is given by 1 1 R 1 2πR −1 (τ3 ) = = 1− + sin (ns) , (5) λ3 2 R0 2π R0 where τ3 (o-Ps lifetime) and R (hole radius) are expressed in ns and Å respectively. R0 = R + ∆R where ∆R = 0.1657 nm is the fitted empirical electron layer thickness. Here the o-Ps lifetime in the electron layer of thickness ∆R is the spinaveraged Ps lifetime of 0.5 ns. The value of ∆R = 0.1657 nm was determined by fitting the above equation with experimental values of τ3 obtained for molecular materials with known hole sizes such as zeolites. Using ∆R, the free volume radius R has been estimated from (5) and the average size of the free

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volume holes, Vf (in Å3 ), for spherical cavities was calculated using the relation [18]. Vf =

4πR3 . 3

(6)

Thus the o-Ps lifetime is sensitive to the molecular environment and in particular to the size of the free-volume region in which it is localized. The intensity I3 of o-Ps in the polymers have been influenced by many factors, such as temperature, positron irradiation, doping, electric field, and polar group. The variation of o-Ps lifetime τ3 and the free volume size Vf with different concentrations of BaCl2 filled PVA is shown in Fig. 1 and I3 in Fig. 2. The effect of various additives on Ps formation has been extensively studied for molecular liquids. One of the most important findings is that the addition of an electron acceptor, such as nitrobenzene derivatives and chlorinated hydrocarbons, causes a marked reduction in the Ps intensity (inhibition of Ps formation). Interestingly, certain electron acceptors were found to increase the Ps yield (enhancement of Ps formation). Addition of these compounds to a solution containing a Ps inhibitor was found to recover the Ps yield reduced by the inhibitor (anti-inhibition of Ps formation). These phenomena of inhibition, enhancement and anti-inhibition of Ps formation have been successfully explained on the basis of the spur reaction model [19–22].

In molecular solids a decrease in the o-Ps pick-off component intensity I3 are thought to be due to a decrease in the number of o-Ps annihilation sites (decrease in relative free volume concentration) or a decrease in the probability of Ps formation (o-Ps inhibition). Here the decrease in I3 due to o-Ps inhibition (introduction of a competing mechanism to Ps formation) is observed due to the addition of salts, which can cause the increasing dipole character of the polymer molecules and increase localization of negative charge. It is thought that the formation of o-Ps is chemically inhibited in favour of positron capture caused by the presence of anions. The decrease in I3 due to chemical inhibition is approximately exponential with the anion concentration approaching some saturation value. In the present work it is possible that Ba2+ or Cl− ions provide a competing mechanism of Ps formation. Accordingly the Cl atom in the BaCl2 molecule is considered to be a chemical quencher for Ps and is expected to cause inhibition of Ps formation but no quenching or conversion of already formed Ps. In this case of inhibition, the o-Ps intensity can be described by I3 /I3 (C = 0) = 1/[1 + (KC)β ] where C is the concentration of the inhibitor and K and β are constants and τ3 are not given by equation (5); τ3 remains unaffected. But from the Fig. 1 it is clear that the quenching effect does not appear: o-Ps lifetime τ3 decreases up to 10 wt. % and later it increases slightly and I3 also changes. If Ps inhibition were occurring, a concomitant decrease in the intensity I1 of the para-Positronium (p-Ps) component would be expected and the o-Ps lifetime and I3 should decrease continuously in the whole range of doping as observed by various groups on polymer with different quenching agents [8, 11, 23]. In the present

Variation of (a) o-Ps lifetime τ3 and (b) the free volume size Vf of PVA doped with different BaCl2 concentrations FIGURE 1

Variation of o-Ps intensity I3 and relative fractional free volume as a function of BaCl2 concentration in PVA FIGURE 2

Variation of (a) I1 , (b) I2 , and (c) τ2 as a function of BaCl2 concentration in PVA FIGURE 3

BHAJANTRI et al.


work we observed that the I1 is approximately constant (10 ± 0.6%) with an increase in BaCl2 concentration (Fig. 3). Hence it is argued that the decrease in τ3 with increasing BaCl2 concentration is due to a reduction in the number of inter-and intrachain free volume cavities. This reduction may be due to the coordinating effects of Ba2+ as well as Cl− and the filling of cavities by the ions as well as complex formation. To analyze this further, density measurements have been performed on this system in order to evaluate the postulated reduction in free volume as opposed to chemical quenching or inhibition of o-Ps. The density data shown in Fig. 4 reflect an additive increase with BaCl2 concentration and do not give an indication of a change in packing of the polymer chains. The simple additive response can be contrasted with the non-linear trends of the PALS free volume parameters. Under proper conditions, the fractional free-volume or the number density of the free volume is estimated using f V = CVf I3 , where C is a structural constant, Vf and I3 are the free volume size, and o-Ps intensity and this quantity varies with salt concentration Fig. 2. Although the density data show additivity and give no information on the polymer/salt interaction, the PALS results show a variation in the relative fractional free volume of the polymer-salt complex with an increase of salt concentration, indicative of changes in the atomic scale free volume and polymer packing. Hence the PALS results support the postulated effect of BaCl2 on the polymer structure: a coordination causing conformational changes and increased segmental interaction including the formation of charge transfer complex via the cations and anions [12, 23]. We assume therefore that Ps quenching is not an important effect in our case and equation (5) can still be applied for the estimation of the free volume hole size in the doped PVA and the variation of τ3 and I3 in the whole dopant range can be attributed to a change in the free volume related to the microstructure within the polymer due to doping. In particular, a decrease of I3 with dopant concentration is attributed to both an increase in crystallinity as well as partial inhibition effects within the polymer. From the result of Zidan on the structural modifications of an amorphous polymer poly(vinyl acetate) due to the metal halides MgCl2 and MgBr2 filling, it is clear that the halide ions also changes the structural properties of PVAc via formation of complex. The chloride ion (Cl− ), being smaller than the bromide ion, has a greater ability to form a complex. This indicates that the structural modifications in the polymer could be done by both anion and cation [24, 25].

FIGURE 4

Variation of PVA density as a function of BaCl2 concentration

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In Fig. 1 it is observed that the o-Ps lifetime and free volume size decreases up to 10 wt. % dopant concentrations and then increases slightly on further increase of dopant concentrations. Correspondingly the variations of o-Ps intensity I3 and relative fractional free volume for different dopant concentrations are shown in Fig. 2. From this figure, we observed that, the fractional free volume varies from 0.00127 to 0.00193 and the intensity I3 decreases from 18.76% to13.22% up to 10 wt. % dopant concentration. After 10 wt. % of dopant concentration these two parameters remain almost constant. It is known that the o-Ps atoms tend to localize in the disordered parts (low-density parts) or amorphous regions and crystalline-amorphous interfaces of a semicrystalline polymer. From various studies it is clear that the possibility of molecular ordering (densification) reduces the free-volume spaces available for o-Ps and leads to a decrease in I3 . Nakanishi et al. [18] observed in poly(ether-ether ketone) that the o-Ps lifetime did not change with crystallinity, but the o-Ps yield decreases linearly with increasing crystallinity. Similarly Lind et al. [26] observed a decrease in I3 but very little change in τ3 with an increase of polypropylene crystallinity. Therefore, a correlation between the molecular ordering by density fluctuations due to doping or crystallinity of the sample and the o-Ps formation probability I3 is often observed. Hence in the present case the results are somewhat different and a decrease of τ3 and I3 in the initial doping level is attributed to local molecular orientation of amorphous chains due to doping, leading to density fluctuations. This molecular ordering and the decrease in both τ3 and I3 , are understood using FTIR results as reported in our earlier paper [2]. The polymer-salt composites are characterized by interaction of the salt with the polar group of the polymer, which gives rise to complex formation. This complex formation is mainly dominated by the cations (Ba2+ ) with the OH units in the polymer. That is, the cations (Ba2+ ) bound to several OH atoms in a polymer chain may induce a stiffening of the chain (intrachain effect) and bounding with other chains may act as temporary cross-links (interchain effect). PVA-Cl− also shows a similar effect, although reduced in magnitude, which may be attributed to inhibition of segmental motion by the anions, despite not being bound to the chain. These processes cause structural rearrangement within the polymeric matrix and result in local molecular orientation of amorphous chains due to doping, leading to density fluctuations, which increases the degree of crystallinity. At the initial stages of doping the complex molecules seem to diffuse into the amorphous regions and get filled into the free volume cavities as a result both free volume size as well as its number density decreases. This variation is reflected in o-Ps lifetime τ3 and the intensity I3 decrease up to 10 wt. %. The increase of salt concentration is connected to an increase in the number of charge carriers and a reduction in the free volume size and fractional free volume. From 10 wt. % dopant concentration onwards we found a small increase in τ3 , suggesting that no more free volumes of appropriate size exist to fill the dopant molecules. Here the dopant particles are distributed in a normal polymer volume and arranged into flat agglomerates. These agglomerates are distributed unevenly within the polymeric host. As the dopant concentration increases further, these interactions increase

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the formation of dopant aggregates or agglomerates leading to a certain phase separation into a polymer-rich phase and a dopant-rich phase. Here many of the BaCl2 particles fluctuate with each other, the flat agglomerates are transformed into two dimension conductive islands and three-dimension conductive networks [9]. This results in the existence of two or more crystalline phases within the polymer. In this case, the first crystalline phase may be the main phase of PVA and second phase characterized due to the PVA-Ba2+ complex, may be because the Ba2+ ion (ionic radius ∼ 1.35 Å) is too large to fit into the crystalline structure of PVA and the defects at the PVA + Ba2+ interface. Hence the observed crystallinity from I3 variation is significant up to 10 wt. % and at higher dopant concentrations the non-existence of the correlation between the crystallinity with I3 is understood. This deviation from linearity at higher concentration may also be due to the fact that the dopant BaCl2 not only alters the degree of crystallinity but also the structure of the amorphous regions in PVA matrix. This shows that the crystallites in the doped PVA films were more highly ordered than in the pure PVA, and this reflects microstructural variations within the doped PVA. The intermediate lifetime component τ2 and its intensity I2 , intensity I1 , as a function of dopant concentration shows an interesting change as shown in the Fig. 3. From the figure we observe an initial increase in I2 and a slight decrease in the value of τ2 which is attributed to the increase in the number of trapped positrons in the defected sites within the crystalline region. These defects are thought to be kink type, which arises due to doping within the crystalline matrix [27]. After 10 wt. % of doping, these defects are seems to be almost saturates. As the dopant concentration increases, the dopant molecules in the free volume exerts a pressure on the crystalline-amorphous boundary, and at the same time the dopant ions in the crystalline regions forms a complex. This results in an additional creation of crystalline defects but of smaller size, hence I2 should increase and τ2 should decrease. In this way, the crystalline-amorphous boundary is also affected by complex formation due to doping. 3.2

XRD Studies

The microstructural variations of PVA upon the incorporation of BaCl2 salt are investigated using wide angle X-ray diffractogram (WAXD) as reported in our previous communications [2]. WAXD result shows many multiple peaks with respect to the main peak, and indicates the enhancement of PVA crystallinity due to BaCl2 doping. Using these WAXD results, the degree of crystallinity with BaCl2 concentration has been estimated and the same is given in Fig. 5 and Table 1. From this it is evident that complex formation due to doping affects the molecular mobility and results in the formation of molecular ordering or an enhancement of crystallinity of the polymer as seen in Fig. 5. At higher doping concentrations the dopant forms aggregates or agglomerates leading to a certain phase separation into a polymer-rich phase and a dopant-rich phase or an increase in crystallinity that is reflected as a saturated crystallinity. This is consistent with the positron results constancy in I3 Fig. 2. In some cases it is known that the inter chain distance in the amorphous state calculated by WAXD patterns from

Variation of PVA Degree of crystallinity as a function of BaCl2 concentration FIGURE 5

the position of the maximum of the first halo dam is equated with the D value calculated from positron annihilation data, within the experimental error. Here the average distance between amorphous segments, D = 2R is the diameter of the nano pores, whose value can be estimated from the PALS results using equation (5). According to Bragg’s law the value of dam is given by [28] nλ . (7) 2 sin θ The estimated results of D and dam are shown in Fig. 6. Usually, as observed by many researchers, in most polymers there is no good correlation between these results (dam and D) of these methods. Similarly in our case (Table 1) we also observe about 9% difference between dam and D. This may be due to the fact that the PALS method is sensitive not only to dam =

BaCl2 doping level (wt.%) 0 1 5 10 20 25

Crystallinity X c (%)

dam (nm)

D (nm)

56 62 69 85 90 87

0.453293 0.454627 0.542108 0.532273 0.544891 0.542339

0.4924 0.4850 0.4832 0.4812 0.4812 0.4860

Crystallinity, inter-chain distance (dam ) from WXRD, and free volume diameter (D) from PALS in doped PVA TABLE 1

FIGURE 6 Variation of free volume diameter D measured by PALS and inter-chain distance dam measured by WXRD as a function of BaCl2 concentration in PVA

BHAJANTRI et al.


the static open volume holes but also to the holes living for short times, while the XRD measurements give the average distance between molecules. In both the cases, the measurements were made at room temperature, ie below the glass transition temperature of the material and the thermal motions of the amorphous segments are expected to be very limited. However the calculated values using these methods are almost similar and the variations are seems to be independent of dopant concentrations after 5 wt. %. 3.3

dc conductivity studies

The dc electrical conductivity of the samples with different BaCl2 concentrations is shown in the Fig. 7. From the figure it is clear that the conductivity of the PVA increases up to 10 wt. % of dopant concentration and suddenly jumps at 20 wt. %, then attains its maximum at 25 wt. %. Further increase of dopant concentration makes the value of conductivity decrease. These variations of conductivity are understood by invoking the formation of charge transfer complex, CTC, within the PVA due to doping. It is known that in semicrystalline polymers, the dopant forms CTC and is expected to reduce the barrier between the trapping sites. This reduction in the barrier provides a conducting path through the amorphous regions of the polymer matrix, which results in an enhancement of its conductivity [3, 29]. Such a phenomena is supposed to decrease the activation energy of the carrier and increases the mobility towards the electrode during polarization. The increase in current with the dopant concentration leads to the accumulation of more and more positive charges in front of the cathode, resulting in an increasing strength at the polymer-electrode interface. Accordingly, in the present case it is reasonable to assume that the dopant molecules fill the free volume holes (or amorphous phase) and occupy the interstitial positions between the polymer chains in amorphous phase, and links these chains to some kind of bonds by a charge-exchange process between the dopant molecules and OH groups of PVA. As observed from FTIR and PALS results this interaction results in the formation of PVA-Ba2+ complex, which hinders the mobility of the polymeric chain in the amorphous fraction. This complex formation increases with doping level as a result both charge carrier density and

the PVA conductivity increases. Since the motion of the carriers is impeded at the crystalline–amorphous interfaces and at higher dopant concentrations, the presence of the molecular aggregates, generally tend to preferentially diffuse into the amorphous regions of the polymer. The presence of aggregates in these regions causes a reduction of the crystalline– amorphous interface, which affects the conductive pathways through the amorphous regions. This process of reduction in the interstitial barrier increases the transition probability of electron hopping across the barrier [14]. Here the interfacial barrier also expected to decrease at higher level of doping because of the presence of dopant at the interfaces itself and the same is reflected with the sudden jump in the conductivity after 20 wt. % dopant concentration. 3.4

Variation of PVA dc conductivity with BaCl2 concentration

Dielectric studies

The variation of dielectric constant with dopant concentrations as well as frequency is shown in Fig. 8. This figure shows a high value of dielectric constant at low frequency, and it decreases with increase of frequency. Generally the dielectric properties of a polar polymer will depend on whether the dipoles are attached to the main chain or not and the dipole polarization will depend on segmental mobility, which is low at temperatures below the glass transition temperature. In view of this, the observed variation of dielectric properties are understood by invoking the Maxwell–Wagner– Sillars (MWS) effect of polarization [13]. Accordingly the enhancement in the dielectric properties is attributed to the interfacial polarization: a phenomenon that appears in heterogeneous media consisting of phases with different dielectric permittivity and conductivity and is mainly due to the accumulation of charges at the interfaces. In the present case like any other polar material, PVA is a polar polymer where each dipole can act as an electron trap and hence the observed high value of the dielectric constant is understood. The presence of BaCl2 complex in the PVA matrix acts as charge clusters and these clusters increases with dopant concentrations, which enhances the average polarization, where the mobile ions and electrons may be responsible for enhancement of the dielectric constant. After 20 wt. % concentration of BaCl2 the increase in the dielectric constant reflects the α-relaxation (space charge relaxation). The process connected with the

Variation of dielectric constant with BaCl2 concentration in PVA for different frequencies FIGURE 8

FIGURE 7

803

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segmental motion in the polymer may increase the degree of crystallinity within the system, which is in agreement with the positron results i.e., a decrease in the amount of amorphous part [30]. The decrease in dielectric constant with increase in frequency for all dopant concentrations can be appropriately explained on the basis of charge carriers being blocked at the electrodes. This may be attributed to the tendency of dipoles in a macromolecule to orient themselves in the direction of the applied field in the low frequency range. However, as the frequency of the applied field increases the dipoles will hardly be able to orient themselves in the direction of the applied field, and hence the value of the dielectric constant decreases at high frequency [15]. This result is also reflected in the variation of dielectric loss (tan δ) versus dopant concentration for different frequencies as shown in Fig. 9. From the figure we observed an increase in the loss factor with frequency and decreases for initial doping up to 5 wt. %, then increases up to 20 wt. %, afterwards it decreases for higher concentrations. These results can be understood by using the fact that, the increase of dielectric loss with frequency may be due to the resonant loss of dipoles. As the frequency increases the dipole polarization effect will tend to zero and the dielectric loss factor depends only on the electronic polarization. This may be due to the structural rearrangement of the molecules within the polymeric matrix. 3.5

ac conductivity

The frequency as well as BaCl2 dopant concentration dependence of PVA conductivity is shown in Fig. 10. This variation shows a maximum value of conductivity at 10 wt. % in 120 Hz graph, at 20 wt. % in 1 kHz, and 25 wt. % at 10 kHz, and 100 kHz indicates that the maximum value of ac conductivity shifts towards higher dopant concentration as the frequency increases. Generally it is known that the concentration of the dopant influences the polarization and hence the conduction process inside the composite system. In heterogeneous media, as seen in dielectric property dependence with dopant concentration, interfacial polarization occurs due to the accumulation of charges at the interfaces and the formation of large dipoles on metal particles or clusters. This interfacial relaxation affects the conductivity and permittivity of the constituents of

Variation of dielectric loss factor with BaCl2 concentration in PVA for different frequencies FIGURE 9

the composite material. It is also known that the solid consisting of phases with different conductivities influences the overall conductivity of the material, which increases with frequency. This is because at high frequencies localized charge carrier motion makes it possible to take maximum advantage of conducting regions, while at lower frequencies charge transport must extend over longer distances and is limited by the presence of isolated conducting regions. Accordingly the observed results are understood based on FTIR studies: in the semicrystalline polymer PVA contains OH groups and many inter and intra molecular interactions are possible via hydrogen bonding, which results in the formation of complexes within the polymer. This complex formation will cause structural variations including microcrystalline domains having different conductive properties within the polymeric network. Accordingly, the present increase in conductivity upon doping suggests that the polymer-dopant interaction will increase the charge carrier concentration or mobility or both within the microcrystalline domains of the PVA. From the point of view of the free volume, these dependencies are understood, i.e., the free volume allows segmental motion of polymer chains and facilitates ion conduction. In this doped polymer-salt composite films the variation in ion conductivity indicates the change in free volume. As seen in the positron lifetimes τ2 and τ3 dependencies on salt fraction, the ionic conductivity can also be related to the salt-fraction. Generally the crystalline regions do not allow easy passage of charge carriers, and hinder the conduction process. Although the amorphous regions and crystalline–amorphous boundary regions allow passage of charge carriers very easily, and helps the conduction process [31]. As seen in the positron data the oPs localizes in free volume in the amorphous region, and dig in there. The o-Ps lifetime (τ3 ) is affected by the variation in the dopant concentration in which an increase in conductivity is correlated to the Ps trapping in both the crystalline and amorphous regions. From equation (5), longer values of τ3 correspond to a larger free volume and hence higher conductivity. However, the positrons may be trapped in the crystalline region or in the crystalline–amorphous interface. If the ions of the dopant (complex) are trapped in the intercrystalline regions or in the bulk crystalline regions, they contribute very little to ionic conductivity. A reduction in the size of such in-

FIGURE 10 Variation of log (ac conductivity) with BaCl2 concentration in

PVA for different frequencies

BHAJANTRI et al.


tercrystallite traps would, therefore, help in ion conduction. Accordingly at higher concentrations, the complex formation and agglomerations of the dopant molecules within the free volume exert a pressure on the crystalline–amorphous boundary, at the same time the dopant ions in the crystalline regions also forms a complex. Hence the observed variation of free volume in the amorphous phase provides a continuous path for ion motion and helps in ion conduction within the doped polymer, which explains the variation of conductivity with dopant concentration. The observed frequency dependence reveals that the mechanism responsible for the ac conduction could be a hopping one. This indicates that the conduction is taking place via hopping of charge carriers between randomly distributed trapping centers. When the charge carriers are localized, there is no free motion of charge carriers and the conduction proceeds via the phonon-assisted hopping of charge carriers between localized sites. 4

Conclusions

Microstructural changes within semicrystalline polymer PVA films doped with different concentrations of BaCl2 were studied using density, PAL and dielectric measurements at room temperature. The observed results of PAL on doped PVA are explained by invoking the chemical interaction between Ba2+ ions of BaCl2 and the OH group of the PVA via intra/inter molecular hydrogen bonding which leads to complex formation. The variation of this complex due to doping, affects the amorphous fraction of the PVA, which increases the crystallinity of the sample. This study also suggests that at high dopant levels, the molecular interaction leads to the formation of dopant aggregates or agglomerates, and ends up with a separation of phases into a polymer-rich phase and a dopant-rich phase. These phase separations result in the existence of two or more crystalline phases within the polymer. In this case, the first crystalline phase may be the main phase of PVA, and second phase is characterized as being due to the PVA-Ba2+ complex. This study also indicates the existence of defect sites within the crystalline region. The variation of crystallinity (evaluated from powder XRD) study also indicates the enhancement of complex formation due to doping. The electrical studies on the doped PVA reflects that the doping forms a charge transfer complex (CTC) and affects the microstructure and hence the dc and ac conductivities of the polymer. These variations of conductivity are understood by invoking the formation of CTC within the PVA. The observed dielectric properties are understood by invoking interfacial polarization due to the doping. ACKNOWLEDGEMENTS R.F.B. and A.H. are grateful to UGC, Government of India for awarding of the FIP Fellowship. The authors thank also the Department of Science and Technology, Government of India

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for financial assistance (in the form of project order no. SP/S2/M-18/2000). The authors are thankful to The Research Coordinator, OSTC, Dr. S. Ganesh, Microtron Center, Mangalore University, Dr. G.K. Shivakumar, Department of Physics, National Institute of Technology, Karnataka, for extending the experimental facilities and to Dr. Cheng Dong, Institute of Physics, Chinese Academy of Sciences, Beijing, P.R. China for providing PowderX software to this work.

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