Wetting Properties of XLPE based Inorganic Hybrid Nanocomposites: Contact Angle Studies

Transcription

1 Chapter 8 Wetting Properties of XLPE based Inorganic Hybrid Nanocomposites: Contact Angle Studies Abstract: Wetting properties and surface energy characteristics of nanocomposites are studied using contact angle measurements of nanocomposites with water and dimethyl sulfoxide (DMSO) as solvents. Various wettability parameters such as total surface free energy, work of adhesion, interfacial free energy, spreading coefficient and Girifalco- Wetting (θ<90 0 ) Non wetting (θ>90 0 ) Good s interaction parameter are analysed. The surface properties of a particular polymeric system are controlled by its chemical Neat XLPE XLPE/10 wt% Al 2O 3 Nanocomposite structure, composition and morphology. This chapter is presented as (i) contact angle measurements of XLPE/Al 2 O 3 nanocomposites as a function of Al 2 O 3 filler concentration, (ii) contact angle measurements of hybrid nanoparticle based XLPE/Al 2 O 3 /clay ternary nanocomposites and (iii) contact angle measurements of nanocomposites as a function of different nanofillers of same concentration. The nanocomposites exhibit higher contact angles, i.e. more hydrophobic compared to neat XLPE (abstract graphic). Results of contact angle studies have been correlated with the morphology and microstructure development in each system of nanocomposites. The results of this chapter have been communicated to Industrial and Engineering Chemistry Research, ACS.

2 264 Chapter Introduction A great deal of research interest has been made on the study of the wetting phenomena of polymeric systems. Non-wettable solid surfaces are of great interest in the applications of microelectronics [1]. An appropriate experimental technique for quantifying the wetting properties and surface characteristics of solids is the measurement of contact angles of liquids on solid surfaces. The term wettability is used to describe the extent to which a liquid spreads on a solid surface. The interfacial properties between a liquid and a polymer component are characterized by the surface energies of each phase and the contact angle between them [2]. Using contact angle measurements, one can determine the surface free energy, interfacial free energy as well as polar and dispersion components of surface energy. In addition, information regarding hydrophobichydrophilic alterations, polar group orientations, and restructuring of the surface in long time contact with a liquid are obtained [3]. Also the surface properties of the composites largely contribute to the adhesion behaviour and wetting phenomena. The nanofillers contribute a great extent to this aspect. Contact angle measurements are often used as an empirical indicator of wettability and interfacial tension. If we consider the stable configuration of a liquid placed on a solid surface, the equilibrium shape conforms to the minimum total interfacial energy for all the phase boundaries present. Forces that control the wetting of a surface are shown in Figure 8.1. If the solid liquid interfacial energy is high, the liquid tends to form a ball having a small interfacial area. In contrast, if the solid-vapour interfacial energy is high, the liquid tends to spread out indefinitely to eliminate the interface [4]. During wetting the contact angle between a liquid and a solid is zero or close to

3 Wetting Properties of XLPE based Inorganic Hybrid Nanocomposites 265 zero that the liquid spreads over the solid easily. On a nonwetting surface, the contact angle is greater than 90 o, so that the liquid tends to ball up and run off the surface easily [1]. Fig. 1 (a) Fig. 1 (b) θ=180 0, drop formation Fig.1 (c) θ>90 0, non wetting Fig. 1 (d) θ=90 0 Fig. 1 (e) θ<90 0, wetting Fig. 1 (f) θ~0 0, nearly complete wetting Fig. 1 (g) θ=0 0, spreading of liquid on solid Fig. 8.1 (a-g) Forces that control the wetting of a surface.

4 266 Chapter 8 For polymer production, where particulates or fibers are used for reinforcement, colorant, flame retardancy or stability, understanding the wetting phenomena has considerable value in relation to the material performance. To lower the free energy of the system, the polymer chains must preferentially interact with the filler surface, where wettability plays a dominant role in successfully achieving the desired structure [5]. Chemical modification of the surface of the nanoparticles can be considered to reduce their surface energy, in order to improve their dispersibility/compatibility with non-polar media of polymeric matrix [6]. The variation of contact angle with respect to the filler loading will give insight into the behaviour of the nanoparticles on the surface of the membranes and relation to the hydrophilicity or hydrophobicity of the polymer matrix as the case may be [7]. The purpose of this study is to investigate contact angle measurements of XLPE/Al 2 O 3 nanocomposites as a function of Al 2 O 3 filler concentration, (ii) contact angle measurements of hybrid nanoparticle based XLPE/Al 2 O 3 /clay ternary nanocomposites, (iii) contact angle measurements of nanocomposites as a function of different nanofillers of same concentration. This study focuses on the effect of filler concentration and hybrid nanofillers on wetting characteristics, and the effect of different nanofillers of same concentration on wetting characteristics of XLPE matrix. The surface of XLPE containing nanofiller was found to be more hydrophobic than the pure XLPE resin, as suggested by static contact angle measurements. The improvement in hydrophobicity of nanocomposite s surface was attributed to the enrichment of the polymer surface with nanoparticles.

5 Wetting Properties of XLPE based Inorganic Hybrid Nanocomposites Theory and Calculations The basic equation relating the surface free energy of a solid (γ s ) and that of the liquid in contact with the solid (γ l ), the interfacial free energy between the solid and the liquid (γ sl ), and the contact angle (θ) is due to Young s equation [8]. It is given by Equation (8.1): (γ lv ) cosθ = (γ sv ) - (γ sl ).8.1 In Equation (8.1), γ s and γ sl are not amenable to direct measurement. Plot of cosθ against the surface tension for a homologous series of liquids, γ l, can be extrapolated to give a critical surface tension, γ c, at which cosθ = 1 [9]. The γ c has been taken as an approximate measure of the surface energy, γ s, of the solid. However, a limitation of this consideration is that the precise value of γ c depends on the particular series of liquids used to determine it. A more appropriate method has been presented by Fowkes considering solid dispersion forces using a geometric mean equation [10]. Later, Owens and Wendt [11] and Kaelble [12] modified Fowkes equation further by also assuming the polar attraction forces which also included the hydrogen bonding forces. Wu [13] has found a still better agreement to obtain γ s when he used a harmonic mean equation which combines both the dispersion and the polar forces. Wu s approach has been quite satisfactorily verified by several authors [14 18]. In order to verify Wu s approach, two liquids of dissimilar polarity are selected. Water and DMSO have been selected for obtaining γ s of polymers [19]. Wu s harmonic mean equations for water and DMSO are written by Equation (8.2):

6 268 Chapter 8 Wu s harmonic mean equations are (1 + cos( θ ω )) r w = and 4[(r w d r s d /r w d +r s d ) +( r w p r s p /r w p )] (1+ cos( θ d ))r d = 4[(r d d r d s /r d d +r d s ) +( r p d r p s /r p d )] Where the superscripts d and p stand for contributions due to dispersion and polar forces, respectively. Data for water and DMSO were taken from the literature (water γ w = 72.8, γ d w= 21.8 mj/ m 2 ; γ p w= 51.0 mj/ m 2, DMSO ; γ d = 44, γ d d= 36 m/m 2, γ p d= 8 ). Dispersive and polar component of surface energy of the composites γ d s and γ p s for different compositions of XLPE were determined by solving equations (8.2) and (8.3). (i) According to Owens-Wendt theory, the total solid surface free energy is represented as γ s = γ d s + γ p s Surface energy is the energy associated with the interface between two phases. If the solid-vapour interfacial energy is low, the tendency for spreading to eliminate the interface will be less. (ii) The work of adhesion, (W A ) is the work required to separate the composite surface and the liquid drop. The work of adhesion, W A, was calculated using the equation. W A = (1 + cosθ) Where γ 1 is the surface tension of the liquid used for the contact angle measurement.

7 Wetting Properties of XLPE based Inorganic Hybrid Nanocomposites 269 (iii) The interfacial energy is defined as the energy necessary to form a unit area of the new interface in the system. The interfacial free energy, γ s1 was calculated from Dupres equation [12]. γ s1 = γ s + γ 1 - W A 8.6 (iv) The spreading coefficient Sc implies that a liquid will spontaneously wet and spread on the solid surface if the value is positive whereas, a negative value of Sc implies the lack of spontaneous wetting. Sc = γ s -γ sl -γ l..8.7 (v) Girifalco-Good s interaction parameter provides a good understanding of the degree of interaction between the test liquid and the polymer surface. Girifalco Goods interaction parameter, φ, between the polymer and the liquid was determined using the equation given below [13]. rl (1 + cosθ ) φ = / 2 2( r r ) l s 8A: Contact Angle Studies of XLPE/Al 2 O 3 Nanocomposites 8.3 Results and Discussion Contact Angle The wetting behaviour of the composites with respect to water and DMSO is analyzed, which focuses on the effect of the filler concentration on wetting characteristics such as work of adhesion, total surface free energy, interfacial free energy, spreading coefficient and Girifalco-Good s interaction parameter. XLPE shows non wetting behaviour by the addition of Al 2 O 3 nanofiller and the increase in contact angle value is in perfect

8 270 Chapter 8 correlation with increase in filler concentration. The hydrophobic nature of the composites is found to increase with the addition of fillers. a b c d Figure 8.2: Contact angle measurements of (a) neat XLPE (b) XLPE/2 wt% Al 2 O 3 nanocomposite (A2) (c) XLPE/5 wt% Al 2 O 3 nanocomposite (A5) (d) XLPE/10 wt% Al 2 O 3 nanocomposite (A10) with water. Figure 8.2 presents the representative figures of contact angle measurements of XLPE/Al 2 O 3 nanocomposites with water. The increase in hydrophobicity is very well understood from the images. With DMSO also the same trend is shown by the XLPE/Al 2 O 3 nanocomposites. All parameters of contact angle studies are summarized in Table 8.1 & 8.2. Variation in contact angle with water and DMSO is given in Figure 8.3.

9 Wetting Properties of XLPE based Inorganic Hybrid Nanocomposites 271 Table 8.1: Wetting properties of XLPE/Al 2 O 3 nanocomposites of composite-water system. Sample Contact angle ( o ) Work of adhesion W A =(1+cosθ)γ l Interfacial energy (γ sl = γ s + γ l - W A ) Spreading coefficient (Sc = γ s - γ sl - γ l ) Interaction parameter Φ=[(1+cosθ)γ l ]/2 (γ s γ l ) 1/2 X A A A Table 8.2: Wetting properties of XLPE/Al 2 O 3 nanocomposites of composite-dmso system. Sample Contact angle ( o ) Work of adhesion W A = (1 +cosθ)γ l Interfacial energy (γ sl = γ s + γ l - W A ) Spreading coefficient (Sc = γ s - γ sl - γ l ) Interaction parameter Φ=[(1+cosθ)γ l ]/2 (γ s γ l ) 1/2 X A A A

10 272 Chapter Water DMSO 80 Theta ( o ) wt% of nano Al 2 O 3 Figure 8.3: Variation of contact angle of XLPE/Al 2 O 3 nanocomposites with water and DMSO as a function of filler concentration. The selected solvents, water is highly polar and DMSO is having less polarity. The nature of solvents affects the contact angle measurements, i.e. due to the polar-non polar nature of solvents and the polymer samples. XLPE/Al 2 O 3 nanocomposites show increase in contact angles with increase in nanofiller concentration with both solvents water and DMSO. This is an indication of the repulsion of non polar surface towards a polar liquid. By the addition of Al 2 O 3 nanofillers, polymer surface becomes more non polar, i.e. hydrophobic and the contact angle increase accordingly. With water, the contact angle of XLPE (85 o ) increases to 102 o for XLPE/Al 2 O 3 nanocomposite, i.e. it becomes a non wetting surface. The nanofillers are surface treated and the organic tail of surface treatment, interact well with the polymer chain and increase the homogeneity of the system. The intrinsic properties of nanofillers and its effective dispersion answer to the non wettable surface of nanocomposites. In nanocomposites, the high

11 Wetting Properties of XLPE based Inorganic Hybrid Nanocomposites 273 packing density of the macromolecules on the supermolecular level (with marked ordering even in quasi-amorphous regions) create steric and kinetic hindrances for penetration of solvent molecules, which may result in high degree of hydrophobicity [7] Surface Free Energy Surface free energy characteristics of XLPE/Al 2 O 3 nanocomposites are given in Figure 8.4. The total surface free energy is showing a decreasing trend with filler concentration. According to the principle of wetting process, if the solid-vapour interfacial energy is low, the tendency for spreading to eliminate the interface will be less. In that case the system exhibits more hydrophobic nature. Lowering the free energy of the system, the polymer chain must preferentially interact with the filler surface, where wettability plays a dominant role in successfully achieving the desired structure. Surface energy is the energy associated with the interface between two phases. The solid surface is rich in hydrocarbon molecule. The forces that hold between hydrocarbons together are much weaker than the force that acts between water molecules and consequently water on a hydrocarbon surface remain in non wetting foam. Herein, the improved dispersibility of the surface treated nanoparticle is mainly attributed to the substitution of possible hydroxyls of the nanoparticles, which has a dual positive effect: decreasing the surface free energy of the nanoparticles and preventing the formation of hydrogen bonds between the nanoparticles during the preparation of the nanocomposites [20]. Composites with silane modified nanoparticles, however, show significantly enhanced contact angles, indicating that the hydrophobicity of the nanoparticle was significantly enhanced, i.e., the surface free energy of the nanoparticle was reduced and by which it affects the total

12 274 Chapter 8 surface energy of the entire composite system. The surface energy of the composites drops below 36.72, which is the surface energy of the neat XLPE at room temperature. This depression which occurs over the whole range of the concentration must result from a decrease in free energy due to the mixing of two components at the interface [21]. 38 Surface free energy (mj/m 2 ) wt% of nano Al 2 O 3 Figure 8.4: Surface energy plot of XLPE/Al 2 O 3 nanocomposites as a function of filler concentration Work of Adhesion Figure 8.5 presents the work of adhesion as a function of nanofiller concentration. The work of adhesion, (W A ) which is the work required to separate the composite surface and the liquid drop, decreases with filler concentration. Work of adhesion shows a decreasing tendency as the interfacial bonding is decreased [22]. The possibility of polar/non-polar interactions across interface is a measure of adhesion between the test liquid and polymer surface. It is clear from the graph that, for the present system, nanofiller addition increases the non polar behaviour of the surface and by which it shows a decreasing tendency of work of adhesion

13 Wetting Properties of XLPE based Inorganic Hybrid Nanocomposites 275 towards polar solvents. Generally work of adhesion can be correlated to the filler matrix interaction. The effective dispersion of fillers into the polymeric matrix that have caused decrease in work of adhesion, and that is due to the increment in the hydrophobic nature of the composite [2]. 80 Water DMSO Work of adhesion ( ) wt% of nano Al 2 O 3 Figure 8.5: Work of adhesion of XLPE/Al 2 O 3 nanocomposites with water and DMSO as a function of filler concentration Interfacial Energy Figure 8.6 shows the interfacial energy vs. filler concentration graph. For both the test liquids, nanocomposites exhibit increasing trend of interfacial energy with respect to the alumina filler content. The interface between phases is a region of high energy relative to the bulk. In order to maintain the lowest total energy for the system, the configuration of the surface adapts itself to minimize its excess energy. The dipoles orient themselves in such a way to give minimum surface energy. Ions can fit on the surface with relatively low energy, only if they are highly polarizable ions, such that the electron shells can be distorted to minimize the energy increase produced by the surface configuration. Consequently highly

14 276 Chapter 8 polarizable ions tend to form the major fraction of the surface layer. The interfacial energy is defined as the energy necessary to form a unit area of the new interface in the system. This is always less than the sum of the separate surface energies of the two phases, since there is always some energy of attraction between the phases. It may be any value less than this sum, depending on the mutual attraction of the two phases. In general, interfacial energy between immiscible phases is low compared with the sum of their surface energies [23]. The interfacial free energy γ s1 between the composite surface and the test liquids, water and DMSO behaves in the same manner. With water and DMSO the interfacial free energy increases markedly compared to neat XLPE. The different kinds of intermolecular forces such as dispersion, hydrogen bonding and polar interaction may not equally contribute to solid-solid, liquid-liquid and solid-liquid interactions. For water composite system, the potential function of water contains important hydrogen bonding contributions that are absent in DMSO composite system. The effects of polar-nonpolar interactions responsible for the increment in interfacial energy of the system. As illustrated in Figure 8.7, schematic model for interfacial tension, γ sl may be regarded as the sum of work to bring solid and liquid molecules to their respective solid-vapour and liquid-vapour interface. This is determined through equation 8.6. Calculation of this can be carried out if potential energy function for solid-liquid interaction is known.

15 Wetting Properties of XLPE based Inorganic Hybrid Nanocomposites 277 Interfacial energy with water ( ) wt% of nano Al 2 O 3 Water DMSO Interfacial energy with DMSO ( ) Figure 8.6: Interfacial energy of XLPE/Al 2 O 3 nanocomposites with water and DMSO as a function of filler concentration. Liquid Solid Figure 8.7: Schematic model of interfacial energy Spreading Coefficient The spreading coefficient (S c ) indicates that a liquid will spontaneously wet and spread on the solid surface if the value is positive whereas, a negative

16 278 Chapter 8 value of (S c ) implies the lack of spontaneous wetting. This means the existence of a finite contact angle. (i.e. θ>0). The spreading coefficients of composites for water and DMSO are given in Figure 8.8. Wetting characteristics of both, water and DMSO decreases upon filler addition. The polar polar interactions across the interface are a measure of wetting. While comparing the test liquids, DMSO shows less negative values and this indicates that, DMSO is a better wetting agent compared to water for the present system of polymer composites. As the filler concentration increases the value of Sc shifted to higher negative values and this explains the enhanced hydrophobicity of nanocomposites with water. Spreading coefficient with water ( ) wt% of nano Al 2 O 3 Water DMSO Spreading coefficient with DMSO ( ) Figure 8.8: Spreading coefficient of XLPE/Al 2 O 3 nanocomposites with water and DMSO as a function of filler concentration Girifalco-Good s Interaction Parameter The Girifalco-Good s interaction parameter values are given in Figure 8.9. Girifalco-Good s interaction parameter (Φ) provides a good perceptive of the degree of interaction between the test liquid and the

17 Wetting Properties of XLPE based Inorganic Hybrid Nanocomposites 279 polymer surface. Empirically the value of (Φ) ranges from 0.5 to In this type of calculation Skapski [24] type of approach is used, all interactions except nearest neighbour interactions are neglected. The different kinds of nearest neighbour intermolecular forces (dispersion, polar, hydrogen bonding etc.) may not be equally contributed to solidsolid, liquid-liquid, and solid-liquid interactions. The higher the value of Girifalco-Good s interaction parameter indicates higher interaction. Highest value is observed for neat XLPE and it decreases upon filler addition and which is in direct correlation with the filler content for both water and DMSO. The interaction is higher for DMSO compared to water for neat XLPE and all concentrations of nanofiller Water DMSO 0.98 Interaction parameter with Water Interaction parameter with DMSO wt% of nano Al 2 O 3 Figure 8.9: Girifalco-Good s interaction parameter of XLPE/Al 2 O 3 nanocomposites with water and DMSO as a function of filler concentration Theoretical Modelling of Contact Angle Li and Neumann sought an equation of state of interfacial tensions of the form γ = f ( γ, γ ). Based on a series of measurements of contact angles sl l s on polymeric surfaces they revised an older empirical law to produce a

18 280 Chapter 8 numerically robust expression. The equation holds good for some solid liquid systems. But some controversy has surrounded the equation [25-29] γ = γ + γ 2( γ γ ) exp ( γ γ ) 8.9 1/ 2 2 sl l s l s l s Or when combined with the Young equation 1/ 2 γ s cosθ = 1+ 2 exp ( γ l γ s ) γ l Theoretical value of contact angle is estimated using equation Table 8.3: Comparison of experimental and theoretical values of contact angle. Sample Experimental value of contact angle ( o ) [water] Theoretical value of contact angle ( o ) [water] X A A A Sample Experimental value of contact angle ( o ) [DMSO] Theoretical value of contact angle ( o ) [DMSO] X A A A Experimental and theoretical values of contact angle are compared in Table 8.3. Experimental values are higher compared to theoretical values for both the test solvents of water and DMSO. The increasing trend of values of contact angle with filler concentration is same for experimental

19 Wetting Properties of XLPE based Inorganic Hybrid Nanocomposites 281 and theoretical values. For the water-composite system the variation is slightly higher for the neat XLPE. For the nanocomposites of 2 and 5 wt% filler concentration theoretical value is in almost perfect agreement with the experimental value. As the filler concentration goes up, at 10 wt% again theoretical value shows deviation from the experimental value. Up to 5 wt% nanofiller concentration, morphology of nanocomposite is uniform as the nanofillers are nicely dispersed in the matrix and these composites show good correlation between experimental and theoretical values. The DMSO-composite system also follows the trend of water-composite system. For this system 2 and 5 wt% filler concentration theoretical value is in perfect agreement with the experimental value. The unfilled and highly filled system deviates from the theoretical contact angle values. 8B: Contact Angle Studies of XLPE/Al 2 O 3 /Clay Binary & Ternary Hybrid Nanocomposites 8.4 Results and Discussion: Wetting Characteristics of Hybrid Composites Very recently, hybrid ternary systems, having two nano materials has found promising in super microstructure development by the synergism between the nano fillers, extending increased properties [30-31]. Hybridisation with more than one filler type in the same matrix provides another dimension to the potential versatility to the parent system [32]. The aim of this work is to provide some insight into the wetting properties of binary composites of XLPE/Al 2 O 3, XLPE/clay and their ternary hybrid nanocomposites of XLPE/Al 2 O 3 /clay, in which spherical Al 2 O 3 and clay platelets are used as fillers. Table 8.4 and 8.5 summarize the wetting properties of binary and ternary hybrid nanocomposites with water and DMSO and the surface energy values are plotted in Figure 8.10.

20 282 Chapter 8 Table 8.4: Wetting properties of hybrid nanocomposites of compositewater system. Sample Contact angle ( o ) Work of adhesion W A = (1 + cosθ )γ l Interfacial energy (γ sl = γ s + γ l - W A ) Spreading coefficient (Sc = γ s - γ sl - γ l ) Interaction parameter Φ=[(1+cosθ)γ l ]/2(γ s γ l ) 1/2 X A C A1C A2C Table 8.5: Wetting properties of XLPE/Al 2 O 3 nanocomposites of composite-dmso system. Sample Contact angle ( o ) Work of adhesion W A = (1 + cosθ)γ l Interfacial energy (γ sl = γ s + γ l - W A ) Spreading coefficient (Sc = γ s - γ sl - γ l ) Interaction parameter Φ = [(1 + cosθ)γ l ]/ 2(γ s γ l ) 1/2 X A C A1C A2C

21 Wetting Properties of XLPE based Inorganic Hybrid Nanocomposites Surface free energy ( ) X A C A1C A2C Figure 8.10: Surface energy plot of hybrid nanocomposites as a function of filler concentration. From the wetting properties and surface energy values, it is understandable that, compared to ternary hybrid composites XLPE/Al 2 O 3 binary composite exhibit better non wetting characteristics. During hybridisation of nanofillers wetting properties decrease, but the values stay in between the XLPE/Al 2 O 3 and XLPE/clay binary combinations. All nanocomposites have enhanced non wetting property compared to neat XLPE. The surface treatment on Al 2 O 3 has a major role in decreasing the surface free energy of the nanoparticles and thus the entire system. Also it provides an efficient dispersion by acting as a link between non polar polymer and polar nanoparticle [33-34]. As the surface of nanoparticle is totally covered, it is highly hydrophobic. The discontinuity introduced by the addition of clay affect the ternary hybrid system and this negative effect is decreased as the Al 2 O 3 content increases.

22 284 Chapter 8 8C: Contact Angle Studies of Nanocomposites with Different Nanofillers of Same Concentration 8.5 Results and Discussion: Wetting Characteristics of XLPE with Different Fillers In the continuous phase of XLPE system, different dispersing minor phases have variations in properties according to their intrinsic properties, level of dispersion and extent of filler/polymer interactions. In this piece of work, the wetting characteristics of different spherical nanofillers of Al 2 O 3, SiO 2, TiO 2 and clay platelets of same concentration (5 wt%) is analysed. Table 8.6 and 8.7 summarize the wetting properties of different hybrid nanocomposites with water and DMSO. For the composite-water system, all nanocomposites exhibit improved hydrophobic nature compared to neat XLPE. While comparing spherical nanofillers and clay, 3D spherical nanofiller reinforced system is having higher contact angle and better hydrophobicity. Among different spherical nanofillers TiO 2 and Al 2 O 3 is showing higher values for contact angle. Three dimensional nanofillers are of higher efficiency as they have all the dimensions in nanoscale and having maximum surface area [35]. Due to the large surface area available, interaction between spherical nanofillers and polymer chains will be efficient compared to 1D and 2D nanofillers. Total surface energy of different nanocomposites is given in Figure Surface energy values are in direct correlation with the contact angle values of nanocomposites. XLPE/TiO 2 nanocomposite shows lowest surface energy followed by other nanocomposites and the highest for neat XLPE. Coming to the work of adhesion and spreading coefficient, lowest values for TiO 2 and Al 2 O 3 filled nanocomposites indicate the least interfacial adhesion. On comparing other parameters of interfacial energy and

23 Wetting Properties of XLPE based Inorganic Hybrid Nanocomposites 285 interaction parameter, and all other properties XLPE/Al 2 O 3 nanocomposite is found to be the best candidate having higher non wetting property. For the composite-dmso system, interaction and wetting characteristics of XLPE and a less polar liquid is analysed. Composite-DMSO system also follows the same trend of contact angle values of composite-water system compared to the neat XLPE. Spherical nanofiller reinforced composites exhibit better properties followed by clay reinforced system and then neat XLPE. Comparing all parameters, XLPE/TiO 2 nanocomposite shows better non wetting property. Polar-non polar interactions, polar-polar interactions, hydrogen bonding, and vander Waal s interactions contribute towards the polymer-test liquid interactions. The variations in these factors may affect different nanocomposites as its filler type varies. Proper dispersion of nanofiller in the XLPE matrix reduces the surface free energy and thus an increase in contact angle. All other properties are derived from the contact angle values and so it is an indirect expression of contact angle values with other factors of interaction. TEM images of different nanocomposites given in the first chapter confirm the dispersion level of nanofillers in the XLPE matrix and they are in complete correlation with the contact angle results.

24 286 Chapter 8 Table 8.6: Wetting properties of nanocomposites of different nanofillers (5 wt%) of composite water system. Sample Contact angle ( o ) Work of adhesion W A = (1 + cosθ)γ l Interfacial energy (γ sl = γ s + γ l - W A ) Spreading coefficient (Sc = γ s - γ sl - γ l ) Interaction parameter Φ=[(1+ cosθ)γ l ]/2(γ s γ l ) 1/2 X A S T C Table 8.7: Sample Wetting properties of nanocomposites of different nanofillers (5 wt%) of composite DMSO system. Contact angle ( o ) Work of adhesion W A =(1+cosθ)γ l Interfacial energy (γ sl =γ s + γ l - W A ) Spreading coefficient (Sc = γ s - γ sl - γ l ) Interaction parameter Φ=[(1+ cosθ)γ l ]/2(γ s γ l ) 1/2 X A S T C

25 Wetting Properties of XLPE based Inorganic Hybrid Nanocomposites Surface free energy ( ) X A S T C Figure 8.11: Surface energy plot of nanocomposites of different nanofillers (5 wt%). 8.6 Conclusions The results of the contact angle measurements can be summarized as follows. 1. For both the solvents water and DMSO, XLPE/Al 2 O 3 nanocomposites show increase in contact angle with increase in nanofiller concentration. 2. Theoretical predictions of contact angle and the experimental results showed similar trend with progression in contact angle values with filler addition. 3. From the wetting properties and surface energy values, it is understandable that, XLPE/Al 2 O 3 binary composite exhibits better non wetting characteristics compared to ternary hybrid nanocomposites and XLPE/clay nanocomposites.

26 288 Chapter 8 4. While comparing spherical nanofillers and clay, 3D spherical nanofiller reinforced system is having higher contact angle and better hydrophobicity. Among different spherical nanofillers, TiO 2 and Al 2 O 3 filled composites show higher values for contact angle. 5. Comparing other parameters of interfacial energy and interaction parameter, and all other properties XLPE/Al 2 O 3 nanocomposite is the best system having higher non wetting property. 8.7 References 1. Abraham, R.; Varughese, K. T.; Isac, J.; Thomas, S. Macromolecular Symposia, 2012, 315, Thomas, S. P.; Abraham, R.; Bandyopadhay, S. express Polymer Letters, 2008, 2, Hameed, N.; Thomas, S. P.; Abraham, R.; Thomas, S. express Polymer Letters, 2007, 1, Parvinzadeh, M.; Moradian, S.; Rashid, A.; Yazdanshenas, M. E. Polymer- Plastics Technology and Engineering, 2010, 49, Vladuta, C.; Voinea, M.; Purghel, E.; Duta, A.; Material Science and Engineering B, Dufresne, A. Macromolecules, 2010, 15, Thomas, S.; Thomas, S.; Sreekumar, P. A.; Bandyopadhyay, S. Journal of Polymer Research, 2011, 18, Young, T. Philosophical Transactions of the Royal Society of London, 1805, 95, Fox, H.; Zisman, H. W. Journal of Colloid Science, 1952, 7, Fowkes, F. M. The Journal of Physical Chemistry, 1963, 67,

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