surface energy characteristics -...
TRANSCRIPT
CHAPTER-6
SURFACE ENERGY CHARACTERISTICS Surface energy characteristics of the composites were measured using
contact angle measurements of the composites with water and methlene
iodide. The solid surface free energy was calculated from harmonic mean
equations. Various contact angle parameters such as total solid surface
free energy, work of adhesion, interfacial free energy and spreading
coefficient were analyzed. A computer programe was developed for
calculating the different parameters from the harmonic mean equation.
The work of adhesion and interfacial free energy, spreading coefficient,
and Girifalco-Good’s interaction parameter changed with composition.
The results of this chapter have been accepted for publication in International Journal of NanoSystems
166 Chapter VI
6.1 Introduction
The relationship between surface and interfacial energies determines to a
large extent the wetting behaviour of a liquid on a solid surface and the
phase morphology of mixtures of two or more phases. 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. 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.
Contact angle measurements of solids are a widely accepted
characterization technique for the surface energy characteristics such as
wettability and adhesion. Using this technique, one can determine the
surface free energy, interfacial free energy as well as polar and dispersion
components of surface energy. In addition, information regarding
hydrophobic-hydrophilic alterations [1-3], polar group orientations, and
restructuring of the surface in long time contact with a liquid are obtained.
Non wettable solid surface are of great interest in the applications of
microelectronics. Wettable solid surfaces are mostly used in applications
such as biocompatibility, printing, coating and oil recovery, membrane for
reverse osmosis, detergency, ore flotation, and retention of pesticides on
leaves. The surface and interfacial energies of polymers can be varied for
specific applications through their surface modifications. Controlled
wettability of the polymer surface has been accomplished through several
chemical modification techniques. These include oxidation, chemical
etching of the surface, grafting with a reactive substance, ultraviolet
Surface Energy Properties of Polystyrene and Composites 167
irradiation, plasma polymerization and treatment with surface active
compounds
In spite of several chemical modification techniques, reports on contact
angle characterization of physically modified polymers such as polymer
composites are quite large in literature. Composite preparation has been
considered as an important route to fulfill several technological
applications of polymeric materials. In contact angle studies it has been
shown that powder fillers decreases the surface free energy of
thermoplastic polymers [4-6].
In this chapter the results of contact angle characterization of the prepared
composites are reported.
6.1.1 Structure of Surfaces and Interfaces
Many scientifically fascinating and technically important processes
depend on the unique properties of molecules between phases. A surface
is defined as the boundary between two phases; it might be more accurate
to call it an interface, because two phases cannot exist without an
interface. The surface of a phase or the interface between phases is a
region of high energy relative to the bulk [7]
6.1.2 Surface Tension and Surface Energy
It is observed experimentally that a force is required to extend a liquid
surface. The surface tension or interfacial tension ‘γ’ is defined as the
reversible work wr, required to increase the surface of the liquid by a unit
area. It is common to refer ‘γ’ as the surface tension when discussing
fluid surface, and as surface free energy when discussing solid surface [8].
168 Chapter VI
Since surface tension is the force (per unit length) that opposes the
stretching of the liquid film, it is a measure of intermolecular forces.
dwr = γ dA
Fig 6.1 Surface layer between the phases
It is well established from the literature that contact angle measurements
can be used in the calculation of surface energy. In the case of pure
liquids and smooth, homogeneous, rigid and insoluble solid surfaces, the
contact angle is a thermodynamic parameter, which can be used to
calculate the interfacial tension by different techniques. Specification of
the conditions under which this surface work is done allows the surface
tension to be related to other thermodynamic properties of the system. In
two phase multi component system as in fig 6.1, in solid-liquid phase
boundaries, by the first and second law of thermodynamics, the variation
in the internal energy E or the Gibb’s free energy G of the system , is
expressed by the variables, P, V, T and entropy ‘S’ as (equ6.1 and 6.2) [9]
idE TdS PdV dA dnιγ µ= − + + ∑ (6.1)
idG SdT VdP dA dnιγ µ= − + + + ∑ (6.2)
where ‘µi’ is the number of moles and ‘dni’ is the excess moles of ‘i'
within the boundary.These relations apply to systems with a plane
interface separating the bulk phases.
Surface tension relative to internal energy and Gibb’s free energy as
Surface Energy Properties of Polystyrene and Composites 169
, , iP T n
GA
δγδ
=
= , , iS V n
EA
δγδ
=
(6.3)
The subscripts refer to the independent variables which have to remain
constant during the increase of surface area by a unit amount [10]
Thus the surface tension of a flat interface is the excess Gibb’s free
energy per unit area. At constant temperature and pressure a small change
in the surface free energy of the system is given by the total differential
with respect to area is taken as zero
S SL LS SL L
dG dG dGdG dA dA dAdA dA dA
= + + (6.4)
but
L S SLdA dA dA= − = where A constitute the solid phase, and
SS
GA
δ γδ
= and so on.
The coefficient L area
GA
δδ
−
gives the free energy change for the
spreading of a film of liquid over the substrate and is called the spreading
coefficient of liquid on solid. Thus
/l s s l sl cS Sγ γ γ= − − = (6.5)
Spreading coefficient is positive if spreading is accompanied by a
decrease in free energy that is spontaneous spreading.
6.1.3 Wetting The relationship between surface and interfacial energies determines to a
large extent the wetting behaviour of a liquid on a solid surface and the
phase morphology of mixtures of two or more phases [11]. Many
170 Chapter VI
important and interesting technical phenomena are related to the
spreading of liquids on solids or wetting. Fig6.2 (a) shows a drop of
liquid on solid surface and three interfacial tensions that govern the shape
of the liquid. (γ sv) is the interfacial tension between solid phase and
vapor phase, (γ sl), is the interfacial tension between solid and liquid
phase and γ lv is the surface tension of the liquid or the interfacial tension
between the liquid and the vapor phase.
The angle between the solid surface and the tangent to the liquid surface
at the contact point, the contact angle, may vary between 0 and 1800. The
angle specifies the conditions for minimum energy according to the
equation (6.6) [12]. The equation is a thermodynamic equilibrium
condition for an ideal solid-liquid system.
(γ lv) cosθ = (γ sv) - (γ sl) (6.6)
where (γ sv) , (γ sl), (γ lv) are the interfacial energies between the phases
actually present in the system at the time of measurement.
For simplicity of the above equations (γ sv), is now onwards taken as (γ s),
and (γ lv) as (γ l)
We may define θ= 900 as the boundary between non wetting (θ>900), and
wetting (θ< 900). Spreading is the condition in which the liquid completely
covers the solid surface (θ= 00) (Fig 6.2[a-g]). Relationship between the
surface energies determine the wetting behavior and spreading coefficient
‘Sc’ as equation 6.5.
For spreading to occur it is necessary that (Sc) be positive. A
corresponding necessary, condition for spreading is that the liquid-vapor
interface energy be less than solid vapor interfacial energy ((γ lv) < (γ sv)).
Surface Energy Properties of Polystyrene and Composites 171
In general, all the interface energies can be affected by changing
composition.
Fig6. 2 (a)
cos SV SL
LV
γ γθγ
−=
Fig 6.2 (b) θ=1800, drop formation
θ>900, non wetting θ=900
Fig 6.2 (c) Fig 6.2(d)
172 Chapter VI
θ<900, wetting θ~00, nearly complete wetting
Fig 6.2 (e) Fig 6.2 (f)
θ=00 , spreading of liquid on solid
Fig 6.2 (g)
Fig6. 2 (a-g) Forces that control the wetting of a surface.
6.1.4 Wettability Studies Theory and Calculations The contact angle is measured as the tangent angle formed between a liquid
drop and its supporting surface. The techniques for measuring contact angle
have been reviewed in detail by Neumann and Good [13]. The most
commonly used method involves directly measuring the contact angle for a
drop of liquid resting on a horizontal solid surface (a sessile drop). Computer
oriented contact angle goniometers employ a microscope objective to view
the angle directly and capture a digital image of the drop. The contact angle
and other important details are collected from the image.
The classic equation relating the surface energy of a solid in contact with
vapour (γsv ) or (γ s),
Surface Energy Properties of Polystyrene and Composites 173
liquid in contact with vapor (γlv ), or (γ l), interfacial free energy
between solid and liquid (γsl ) and the contact angle (θ ) due to Young is
given in equation 6.6 [14]. The extensive use of Young’s equation
reflects its general acceptance.
In equation (6.6), γ s and (γ sl) are not amendable to direct measurement.
A plot of (θ) against surface tension for a homologous series of liquids,
(γl) can be extrapolated to give a critical surface tension γc, at which
cosθ=1. γc, has taken as an approximate measure of surface free energy γs,
of the surface. However, a limitation of this consideration is that the
precise value of γc, depend on the particular series of liquids used to
determine it.
6.1.5 Semi empirical model.
A model that has proved to be very stimulating to research on contact
angle phenomena begins with the proposal by Girifalco and Good [15]. If
it is assumed that the two phases are mutually entirely immiscible and γs1
is related to γs and γ1 through geometric mean law relationship i.e.
γs1 is equivalent to (γ1 γs) ½ through the interaction parameter φ
Interfacial free energy should obey the equation
γs1 = γs + γ1 -2φ (γ1 γs) ½ (6.7)
The parameter φ is the interaction parameter.
In order to include other interactions such as dipolar or hydrogen bonding,
many semi empirical approaches have been tried, including adding terms
to the above equation or modifying the definition of ‘φ’. Perhaps the most
174 Chapter VI
well known of these approaches is come from Fowkes [16-18] suggesting
that the interaction across a water –hydrocarbon interface are dominated
by dispersion forces such that
γs1 = γs + γ1 -2φ (γ1d
γsd) ½ (6.8)
Where (γ1d
γsd) denote the surface tension due to the dispersion or Van
Der Waals component of the interfacial tension [19,20].
Later, Owens Wendt [21] and Kaelble [22] modified Fowkes approach
further by assuming the polar attraction component of forces which also
included in the surface forces. Wu [23, 24] has found a still better
agreement to obtain γs when he uses harmonic mean equations which
combines both the dispersion and polar interactions. Many researchers
have tested this model on several materials to find the surface tension
components [25, 26]
In order to verify Wu’s approach, by including the two component of
surface tension, two liquids of dissimilar polar types are to be selected
[27]. One polar liquid (water) and other non polar liquid (methylene
iodide) are selected for study
Wu’s harmonic mean equations are
(1 cos( )) wrωθ+ = 4d d p p
w s w sd d p p
w s w s
r r r rr r r r
+ + +
(6.9)
and
(1 cos( ))m mrθ+ = 4d d p p
m s m sd d p p
m s m s
r r r rr r r r
+ + +
(6.10)
Surface Energy Properties of Polystyrene and Composites 175
where the superscripts d and p stand for contributions due to dispersion
and polar forces, respectively. Data for water and methylene iodide were
taken from the literature (Water γw = 72.8 mJ/m2, γdw = 21.8 mJ/ m2,,
γpw = 51.0 mJ/ m2,, methylene iodide; γm = 50.8 mJ/m2, γd
m = 49.5mJ/m2,
γpm = 1.3 mJ/m2
. Dispersive and polar component of surface energy of the
composites γds and γp
s for different compositions of PS were determined
by solving equations (6.9) and (6.10) with the help of computer program
in C language (Appendix). According to Owens-Wendt theory, the total
solid surface free energy is represented as
γs = γds + γp
s (6.11)
The work of adhesion, WA, was calculated using the equation
WA = (1 + cos θ) γ1 (6.12)
Where γ1 is the surface tension of the liquid used for the contact angle
measurement.
The interfacial free energy, γs1 was calculated from Dupres equation [28]
γs1 = γs + γ1 - WA (6.13)
Girifalco – Goods interaction parameter,φ, between the polymer and the
liquid was determined using the equation given below [29]
1/ 2
(1 cos2( )
l
l s
rr r
θφ + )= (6.14)
The total surface energy γs and its dispersive and polar components are
shown in Table 6.1 & 6.2.
176 Chapter VI
6.2 Results and Discussion
The wetting behaviour of the composites with respect to water and
methylene iodide is analyzed which focuses on the effect of volume
percentage of the filler on wetting characteristics such as work of
adhesion, total surface free energy, interfacial free energy and spreading
coefficient. The hydrophobic nature of the composites is found to increase
with the addition of fillers.
6.2.1 Contact angle variations
The composites posses higher contact angle with water and methylene
iodide compared to neat PS (fig 6.3[a-d] &6.4[a-b]).
(a) (b )
(c) (d)
Fig 6.3 Representative figures of contact angle measurements of (a) BNN40 with methylene Iodide (b) PS with metylene Iodide (c) BNN40 with water (d) PS with water.
Surface Energy Properties of Polystyrene and Composites 177
0 10 20 30 4034
36
38
40
42
44
46
48
Methylene iodide Water
Volume of YBCO, %
Con
tact
ang
le (θ
) met
hyle
ne io
dide
84
86 Contact angle (θ) w
ater
(a)
0 10 20 30 40
34
36
38
40
42
44
46
48
50
52
54
methylene Iodide Water
Volume of BNN, %
Con
tact
ang
le(θ
) met
hyle
ne io
dide
84.0
84.2
84.4
84.6
84.8
Contact angle(θ) w
ater
(b)
Fig 6.4 (a) & (b) Variation of contact angle with water and M.I of the composites with (a) filler YBCO (b) filler BNN
178 Chapter VI
The increase in contact angle with water shows an increment in the
hydrophobic nature of the composites. Even though the water absorption
percentage of PS is 0.05, and the addition of fillers increases the
hydrophobic nature to a higher value. The increase in contact angle value
can also be compared with the increase in surface roughness of the polymer
surface [30].The roughness of the sample surfaces increases by the addition
of BNN and YBCO. The roughness increases highly in YBCO40 and a
rapid rise in its contact angle with water and methylene iodide. The
roughness of the composite surface was observed with SEM also.
6.2.2 Surface Free energy
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. Surface free energy based on contact
angle values of water and methylene iodide is reported in table 6.1&6.2.
It can be seen that the surface free energy decreases with filler and the
least value was observed in the case of BNN40. Hydrophobicity is
indicated by higher contact angle and smaller surface energy. Addition of
ceramic powders results a reduction in surface free energy [31]. Thus the
surface energy of the composites drops below 46.2564, which is the
surface energy of the neat PS at room temperature [32]. 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. Surface energy can be divided into polar and non polar
components. The dispersive (non polar) component of surface energy is
Surface Energy Properties of Polystyrene and Composites 179
found to decreases and polar component increases with filler content
whereas, no significant variation in total surface energy. YBCO composites
and BNN composites have almost equal surface free energy values. This
suggests that the natures of forces acting at the surface of these two
composites are approximately the same. The polar surface free energy of
the composites goes on increasing with the nature of filler. BNN, being a
ferroelectric material shows maximum value for polar component and that
is for its higher loading. Due to the polar nature of YBCO and BNN
functional groups formed on the surface of the polymer increases polar
surface energy. In the case of YBCO40, polar surface energy is less than
YBCO30 because of percolation of the filler. Improved contact angle has
been related to the decrease of surface free energy [33].
Table 6.1 Components of surface energy of BNN-PS
Name Contact Angle
(water0)
Contact angle
(meth.Iod0)
(Dispersion component) of surface energy
γsd (mj/m2)
(Polar component) of surface energy γs
p
(mj/m2)
( Surface free energy) (mj/m2)
γs=γsd + γs
p
PS 84 35 39.6814 6.575 46.2564
BNN10 84.6 50 32.89 7.585 40.475
BNN20 84.7 51 32.33 7.665 39.995
BNN30 84.72 52 31.8814 7.75 39.6314
BNN40 84.8 53 31.4652 7.805 39.2702
180 Chapter VI
Table 6.2 Components of surface energy of YBCO-PS
Name Contact Angle
(water 0)
Contact angle
(meth.Iod 0)
(Dispersion component) of
surface energy γsd
(mj/m2)
(Polar component) of surface energy γs
p
(mj/m2)
( Surface free energy)
(mj/m2) γs=γs
d + γsp
PS 84 35 39.6814 6.575 46.2564
YBCO10 84.1 45 35.2204 7.3556 42.576
YBCO20 84.15 46 34.759 7.3845 42.1435
YBCO30 84.18 46.5 34.5255 7.4165 41.942
YBCO40 86 47 34.6486 6.725 41.3736
6.2.3 Work of adhesion The work of adhesion, (WA) which is the work required to separate the
composite surface and the liquid drop, decreases with filler concentration.
This fact is clear from fig 6.5, which represents the variation of (WA) with
respect to the change in filler content .The values of work of adhesion
related to water and methylene Iodide are presented in the third column of
table 6.3, 6.4, 6.5 & 6.6. Work of adhesion decreased as the interfacial
bonding is decreased [34]. The possibility of polar non-polar interactions
across interface is a measure of adhesion between the test liquid and
polymer surface. For maximum adhesion the ratio of polar to non polar
component across the interface should be same. In the studied composites
the ratio between water composite systems should be same as that
between composite water system for perfect adhesion of water and
composite. This ratio is greater than one for water-composite systems and
less than one for composite water system. This ratio is less than one for
Surface Energy Properties of Polystyrene and Composites 181
M I –composite system and greater than one for composite-M.I system
.The trend is same but in opposite manner because of the inherent
property of water as polar liquid and M I as non polar liquid. The
magnitude of difference between work of adhesion of neat polymer and
that of composites are much higher for M.I than that of water. Generally
work of adhesion can be correlated to the filler matrix interaction. Thus
the effective dispersion of fillers into the matrix that have caused decrease
in work of adhesion, and that is due to the increment in the hydrophobic
nature of the composite.
0 10 20 30 40
78
80
82
84
86
88
90
92
94
Wor
k of
adh
esio
n (m
j/m2 )
Volume of filler %
YBCO-PS with water BNN-PS with water YBCO-PS with MI BNN-PS with MI
Fig 6.5 Variation of work of adhesion with different volume % of the
filler with two liquids
182 Chapter VI
Table 6.3 Properties of BNN-PS composites related to water
Name Contact Angle
(water0 )
Work of adhesion
(WA)=(1+cosθ)γ1
Interfacial energy γs1
= γs + γ1 - WA(mj/m2)
Spreading coefficient Sc = γs - γs1
-γ1(mj/m2)
Interaction parameter Φw
γ1 1/ 2l
(1+cos )2(r )sr
θ
PS 84 80.4648 38.5916 -65.1352 .6933
BNN10 84.6 79.650 33.624 -65.949 .7366
BNN20 84.7 79.5245 33.2705 -66.0755 .7368
BNN30 84.72 79.4992 32.9322 -66.1008 .7400
BNN 40 84.8 79.3980 32.6722 -66.202 .7424
Table 6.4 Properties of YBCO-PS composites related to water
Name Contact Angle (water)
Work of adhesion
(WA)=(1+cosθ)γ1
Interfacial energy γs1
= γs + γ1 - WA(mj/m2)
Spreading coefficient
Sc = γs - γs1 -
γ1(mj/m2)
Interaction parameter
Φw =γ1
1/ 2l
1(1+cos )2(r )sr
θ
PS 84 80.4096 38.5916 -65.1352 .6933
YBCO10 84.1 80.2832 35.093 -65.3168 .7210
YBCO20 84.2 80.2201 34.723 -65.3799 .7241
YBCO30 84.6 80.821 34.5599 -65.4179 .7255
YBCO40 86 77.8782 36.2954 -67.7218 .7095
Surface Energy Properties of Polystyrene and Composites 183
Table 6.5 Properties of BNN-PS composites related to methylene Iodide
Name Contact angle
(meth.Iod)
Work of adhesion
(WA)=(1+cosθ)γ1
Interfacial energy γs1
= γs + γ1 - WA(mj/m2)
Spreading coefficient
Sc = γs - γs1 -
γ1(mj/m2)
Interaction parameter
Φm =γ1
1/ 2l
1(1+cos )2(r )sr
θ
PS 35 92.4129 4.6435 -9.1871 0.9532
BNN10 50 83.4536 7.8214 -18.1464 0.9202
BNN20 51 82.7694 8.0256 -18.8306 0.9181
BNN30 52 82.0756 8.3558 -19.5244 0.9146
BNN40 53 81.3722 8.698 -20.2278 0.9109
Table 6.6 Properties of YBCO-PS composites related to methylene Iodide
Name Contact angle
(meth.Iod)
Work of adhesion
(WA)=(1+cosθ)γ1
Interfacial energy γs1
= γs + γ1 - WA(mj/m2)
Spreading coefficient
Sc = γs - γs1 -
γ1(mj/m2)
Interaction parameter
Φm =γ1
1/ 2l
1(1+cos )2(r )sr
θ
PS 35 92.4129 4.6435 -9.1871 0.9532
YBCO10 45 86.7210 6.649 -14.879 0.9291
YBCO20 46 86.0886 6.8549 -15.5114 0.9371
YBCO30 46.5 85.7684 7.9736 -16.8316 0.9290
YBCO40 47 85.4455 6.7281 -16.1545 0.9318
184 Chapter VI
6.2.4 Interfacial energy
The interfacial energy between the composites and the liquids are
reported in the fourth column of the tables 6.3, 6.4, 6.5&6.6. 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 as 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 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 [35]. The interfacial free energy γs1
between the composite surface and the test liquids, water and methylene
iodide behaves in opposite manner. With water the interfacial free energy
decreases markedly and with methylene iodide it increases compared to
neat PS. The polar sites present in the surface gave path for the increment
in interfacial energy with methylene iodide. The different kinds of
intermolecular forces such as dispersion, hydrogen bonding and polar
may not equally contribute to solid-solid, liquid-liquid and solid-liquid
interactions. For water composite system, the potential function of water
Surface Energy Properties of Polystyrene and Composites 185
contains important hydrogen bonding contributions that are absent in MI
composite system. The effects of polar-polar and polar non polar
interactions are not contributing in an additive manner with the two
composite systems (M.I-composite system, Water-composite systems).
There has been considerable theoretical development in the treatment of
interfacial tension and work of adhesion. As illustrated in fig 6.6(a),
schematic model for interfacial tension, ‘γsl’ may be regarded as the sum
of work to bring solid and liquid molecules to their respective solid-
vapour and liquid-vapour interface less the free energy of interaction
across the interface. This is determined through equ 6.13. Calculation of
this can be carried out if potential energy function for solid-liquid
interaction is known. Equation 6.14 is based on the geometric mean law
relation ship and the calculations were done according to the above
Relations.
(a)
Solid
Liquid
186 Chapter VI
0 10 20 30 40
5
10
15
20
25
30
35
40
Inte
rfaci
al e
nerg
y (m
j \ m
2 )
Volume of filler, %
BNN-PS with water YBCO-PS with water BNN-PS with M.I YBCO-PS with M.I
(b)
Fig 6.6 (a) Schematic model of interfacial energy (b) Variation of interfacial energy with different volume % of the filler with different liquids
6.2.5 Spreading Coefficient
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. This
means the existence of a finite contact angle. (i.e. θ >0). The spreading
coefficients of composites for water and methylene iodide are given in the
fifth column of tables6.3, 6.4, 6.5&6.6. By the fig.6.7, it can be noted that
the wetting due to methylene iodide and water decreases upon filler
addition. Comparing water and methylene iodide, the less negative value
is given by methylene Iodide which means that it is a better wetting agent
for the current composites. Thus wetting has decreased for water with the
addition of fillers indicating more hydrophobic nature of the composites
Surface Energy Properties of Polystyrene and Composites 187
and which in turn reduces the water absorption capacity of
Polystyrene[36,37]. The polar –polar interactions across the interface is a
measure of wetting. The ratio of polar to polar interactions in water-
composite system is much greater than polar to polar interactions between
composite water system. The ratio of polar to polar interactions in MI-
composite system is much less than polar to polar interactions between
composite MI system. The trend is same for both composite system but in
opposite manner. A less negative value for spreading coefficient is
experienced in between M.I-composite system.
0 10 20 30 40-70
-60
-50
-40
-30
-20
-10
BNN-PS with waterYBCO-PS with water BNN-PS withM.I YBCO-PS with M.I
spre
adin
g co
effic
ient
(mj/m
2 )
Vol ume of filler,%
Fig 6.7 Variation of spreading coefficient of the composites with
volume % of the filler
6.2.6 Interaction Parameter
Girifalco-Good’s interaction parameter (Φ) calculated using the equation 6.17
provides a good understanding of the degree of interaction between the test
liquid and the polymer surface as shown in fig 6.8. Empirically the value of (Φ)
188 Chapter VI
ranges from 0.5 to 1.15. In this type of calculation Skapski [38] 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 solid-solid, liquid-
liquid, and solid-liquid interactions. For water composite system, the potential
function for water contains important hydrogen bonding contribution that are
absent in M.I-composite interactions. Work of adhesion and spreading
coefficient are less in water-composite system and that is a measure of
interaction parameter. The values are given in the last column of table 6.3, 6.4,
6.5 & 6.6. A higher value indicates greater interaction. Φw and Φm are the
Girifalco Good’s interaction parameter due to water and methylene iodide
respectively. Thus based on Girifalco-Good’s interaction parameter, Φw and Φm
it can be suggested that the interaction parameter, Φw is less than Φm. [39-40].
0 10 20 30 40
0.68
0.70
0.72
0.74
0.76
0.78
0.80
0.82
0.84
0.86
0.88
0.90
0.92
0.94
0.96
Inte
ract
ion
para
met
er
Volume of filler,%
Water(YBCO-PS)Water(BNN-PS)M.I(YBCO-PS)M.I(BNN-PS)
Fig 6.8 Variation of Interaction parameter of the composites with
different volume % of the fillers and with different liquids
Surface Energy Properties of Polystyrene and Composites 189
6.2.7 Theoretical Modeling of Contact angle
Li and Neumann sought an equation of state of interfacial tensions of the
form ( , )sl l sfγ γ γ= .Based on a series of measurements of contact angles
on polymeric surfaces; they revised an older empirical law to produce a
numerically robust expression. The equation holds good for some solid
liquid systems. But some controversy has surrounded the equation [41-44]
1/ 2 22( ) exp 0.0001247( )sl l s l s l sγ γ γ γ γ γ γ= + − − − (6.15)
or when combined with the Young equation
1/ 2
2cos 1 2 exp 0.0001247( )sl s
l
γθ γ γγ
= − + − −
(6.16)
Table 6.7 Modeling (Contact angle) (Water)
Sample name Equ (6.16) (θ) Experimental(θ)
Poly 62.60 84
YBCO10 68.60 84.1
YBCO20 69.30 84.2
YBCO30 69.57 84.6
YBCO40 70.54 86
BNN10 71.99 84.6
BNN20 72.76 84.7
BNN30 73.35 84.72
BNN40 73.93 84.8
190 Chapter VI
Table 6.8 Modeling (Contact angle) ( Methylene Iodide)
Sample name Experimental By equation 6.16
Value of θ Value of θ
Poly 35 25.37
YBCO10 45 35.35
YBCO20 46 36.41
YBCO30 46.5 36.90
YBCO40 47 38.27
BNN10 50 40.39
BNN20 51 41.5
BNN30 52 42.33
BNN40 53 43.16
The experimental and theoretical values of contact angles are given in
Table 6.7&6.8. It can be seen that theoretical predictions do not match
with the experimental results. Some controversy has still surrounded the
equation of state for modeling. In theoretical and experimental
calculations the value of (θ), the contact angle is found to be increased
with filler concentrations
6.3 Conclusions
The contact angle with water and methylene Iodide got increased by the
addition of ceramic filler to polystyrene matrices. The nature of surface
forces appears to be the same in YBCO and BNN composites. The
hydrophobic nature of PS increases with composite formation. The
decrease in solid surface free energy of the composite is the result of
Surface Energy Properties of Polystyrene and Composites 191
accumulation of excess polar sites at the surfaces. γsd is decreased due to
the restricted chain mobility by the addition of fillers. The solid surface
free energy of the composites decreased and thereby decreased work of
adhesion by the addition of filler to PS. Methylene Iodide showed less
negative value for the spreading coefficient than water and it is a good
wetting agent than water. Finally the nature of the particle and particle
dispersion in the matrix affected the various parameters analyzed as each
one of them changed according to the filler loading.
Theoretical predictions of contact angle and the experimental results
showed the similar trend with progression in contact angle with filler
addition.
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