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Surface characterization of air plasma treated poly vinylidene fluoride and poly methyl methacrylate films.

INTRODUCTION

In recent years, plasma surface modification of polymeric material has been intensively investigated. The interaction of plasma with the surface of polymer films changes their chemical and physical properties. It is the consequence of different processes such as oxidation, degradation, cross linking, and structural changes, which may occur in the thin surface layer. The efficiency of these processes depends on atmosphere, gas pressure and temperature, kind of polymer surface, power, and time of plasma action (1-14). Treatment of polymers by different types of plasma (microwave, radio frequency, corona discharge) is often used for modification of wettability, printability, adhesion, durability, scratch resistance, hardness, membrane permeability (2-4), and compatibility with living tissues (6), (12).

The interaction of low temperature air plasma (non equilibrium plasma) with polymer films causes mainly the change of surface polarity. It may be a result of the creation of C = O, OH, and COOH groups in the oxidation process. Electrons present in such plasma conditions are known to have kinetic energy sufficiently high for breaking up the covalent bonds and to initiate the chemical reactions (1).

Specific techniques are required for understanding the mechanism processes caused in polymers by plasma. One of them is the measurement of the contact angle, (wettability angle) of the liquid drop on polymer film and the calculation of its surface free energy ([[gamma].sub.S]) which can not be measured directly (1), (15), (16). The relation needed for ([[gamma].sub.S]) calculation is determined by using the Young's equation (1), (17).

[[gamma].sub.S] = [[gamma].sub.SL] + [[gamma].sub.L]cos[theta] (1)

Where [[gamma].sub.SL] is the polymer-liquid interfacial energy, [[gamma].sub.L] is the surface tension of liquid used. The contact angle is usually measured by the tangent at the three-phase interface (solid-liquid-vapor). It can be also applied for the estimation of interface forces between two different polymeric materials (18).

According to Owens-Wendt theory (1), (19), the total free surface energy ([[gamma].sub.S]) is the sum of its dispersive ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) and polar ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) components:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

This can be estimated using two or more liquids for [theta] measurements.

Fluoropolymers such as poly vinylidene fluoride (PVDF) are materials of large technological importance, and they are characterized by high chemical inertness, thermal stability, and electrical properties, for example, piezoelectricity, its chemical and weather resistance, its durability and biocompatibility. They also have very low surface tension, which causes negligible adhesion to other materials, especially metals.

In recent years, increasing interest has been devoted to the modification of PVDF surface characteristics to obtain improved adhesion. For this, different treatments were used: chemical etching (20), high energy ion radiation (21), X-ray and electron irradiation (22), and plasma etching (23-25), the last being considered to be one of the most efficient techniques for surface activation of polymers (26). It is well known that plasma environments contain neutral species, energetic ions, electrons, and photons. The interaction of these plasma particles with the polymers can cause chemical and physical modifications to their surfaces, whereas the bulk properties remain unchanged (27). These modifications produce more reactive surfaces and affect wetting properties, cross linking, and molecular weight.

Poly methyl methacrylate (PMMA) is one of the most convenient polymers for optical applications. It offers excellent optical properties, high molding precision, and low costs. A coating of the soft material is mainly intended to improve its mechanical durability. The low surface energy of PMMA combined with the thermal stress of evaporated inorganic thin films may have a negative effect on the adhesion of coatings, which is the main problem for coating PMMA. Low pressure plasma treatment is a convenient way to activate polymer surface before coating (26). Different radio frequency (RF) glow discharge plasma treatments that influence the surface energy of PMMA have been described in the literature (28-31). Obviously, the mechanism of PMMA surface modification and the effect on the adhesion properties depend on the variety of plasma parameters.

In this investigation, the surfaces of PVDF and PMMA films were modified in air plasma with an aim of improving its wettability, adhesion, and printability. For this purpose, PVDF and PMMA films were prepared and characterized using various methods such as electron spectroscopy for chemical analysis (ESCA), ATR-FTIR, and contact angle to find out the surface energy of plasma processed PVDF and PMMA films. The adhesion of polymer films before and after plasma treatment was investigated by T-peel test method. Similarly, the printability was measured for different time of treatment by cross test method. The mechanism of improvement in the aforementioned surface related properties of PVDF and PMMA films are discussed.

EXPERIMENTAL

Materials

The PVDF used in this work was supplied in RG granules form by Kureha Chemical Industry, Co. (Japan) and PMMA supplied in RG granules form by IPCL, Baroda, India. Double-distilled water was used throughout the study. Glycerol, Formamide, Ethylene glycol, Dimethyl formamide, and Toluene of (AR grade) were obtained from Merck, Mumbai, India.

Film Preparation

The films were prepared by solution casting method. To prepare the PVDF and PMMA film, the polymers were dissolved at 60[degrees]C in the solution of dimethyl formamide and toluene, respectively. The solution was spread on glass petri dish for evaporation of solvent. On drying, the solutions stood at room temperature for 2 h, and thin transparent films of PVDF and PMMA having thickness of 40-50 [micro]m were obtained. The prepared films were stored in vacuum desiccators for further study. Before plasma treatment, the films were cleaned with acetone in an ultrasonic bath for 5 min and then dried.

Plasma Processing Unit

A typical glass bell jar type plasma reactor having a height of 30 cm and a diameter of 30 cm was used. The two electrodes were capacitively coupled to the RF source capable of giving power out put up to 100 W. Various ports were fitted on the base plate for gas and monomer inlet. Pirani gauge was fitted on to the top plate. To confine the glow discharge to the specific volume, the magnetron was mounted on the base plate. Because of magnetron, the plasma confined to a volume of 500 c[m.sup.3], and the maximum sample that can be uniformly treated in our plasma chamber is 10 cm X 10 cm. The working pressure was adjusted to 0.2 mbar and gas flow rate to 15 [cm.sub.3] (STP)/min. The detail of the plasma processing chamber has been reported in the literature (32).

% Weight Change of the Substrate

Many researchers have used thickness measurement or surface profiling as a technique for determining the extent of surface etching or grafting. In this study, on account of the use of magnetron, the etching phenomenon or deposition is assumed to be uniform over the processed surface. When the surface is treated in plasma with monomer, etching and deposition occurs simultaneously {competitive ablation and polymerization (CAP)}. Hence, thickness measurement may not be an accurate technique. We therefore thought it to be appropriate to measure the extent of treatment as a function of change of weight of the substrate. For this purpose, following method was used.

The substrate was weighed before subjecting it to plasma treatment, using a Mettler (Model-AE 240) single pan high precision balance with capacity of measuring weights up to 10 g accuracy. For each sample, five readings were taken and an average value was determined. Percentage deviation of weight was calculated and found to be 0.012%. After the treatment, the substrate was weighed immediately, and the percent change of weight was determined by the following formula:

% Weight change = {(W - [W.sub.0])/[W.sub.0]} x 100 (3)

Where [W.sub.0] is the initial weight of the substrate, and W is the weight of substrate after treatment. A positive change implies a gain in the weight, and a negative change implies weight loss in the substrate.

Using the sessile drop method (33), the contact angle (CA) of water (W), glycerol (G), formamide (F), and ethylene glycol (E.G), on the surfaces of air plasma treated PVDF and PMMA films, were measured using a contact angle meter with a projection microscope (Poland Model MP3 Nr 3905). The projection microscope is modified to get the image of drop directly on the screen in magnified form. Light coming from the lamp falls on the reflector kept at 45[degrees], which passes through magnifying and focusing lenses and falls on the reflector inside the microscope and an image is projected on the screen. The liquid drop (~2-3 [micro]1) was placed on to the polymer surface with the help of micro syringe. The image of liquid is directly projected on the screen. The screen has two calibrated axes mutually perpendicular to each other, which can be rotated.

Angle of contact can also be calculated by measuring height and base length of spherical drop. Figure 1 shows the sessile drop method for calculating angle of contact using height and base length.

Assuming the drop to be sphere, we can see from the Fig. 1 that the radius of the sphere can be calculated as:

[FIGURE 1 OMITTED]

R = [[[r.sup.2] + [h.sup.2]]/2h] (4)

where "r" is the half the base length of drop and "h" is the height of the drop. "R" is the radius of the sphere.

Since,

Sin[theta] = [r/R] (5)

[theta] = [Sin.sup.-1][2rh/[[r.sup.2] + [h.sup.2]]] (6)

By measuring "r" and "h," [theta] can be calculated from above equation (34).

The measurements were done quickly to avoid errors due to fast contamination of surfaces as well as evaporation of liquid droplets. Contact angles were determined immediately after the plasma treatment at 10 different places of the same sample, and an average value was determined. The error involved in the measurement of contact angle was [+ or -] 1.4[degrees]. The drop shape was solved numerically and fitted by means of mathematical functions. The surface energy was calculated from contact angle measurement using Fowke's approximation. The detailed calculation is given elsewhere (32).

To study the effect of plasma on adhesion, a standard T-peel test (ASTM D1876) was carried out using Instron Instrument (Model 1026) at a rate of 100 mm/min at room temperature. For the aforementioned study, a scotch tape of width 3 cm was stuck over a length of 4.0 cm on the PVDF and PMMA films, and care was taken to see that there were no air gaps or wrinkles and was kept under pressure of 1.0 kg for 10 min. T-peel test was carried out after fixing one end of the sample in one jaw and the scotch tape end with a piece of paper adhere to it in another jaw. T-peel strengths are reported as force of peel per centimeter of sample width (sample width was 3.0 cm). Ten samples were tested for one set and an average value was obtained. Printability was measured by cross test method (ASTM D 3359). Ink manufactured by Hindustan Inks and Resin was used. A set of 10 readings were taken and an average value was determined.

The ATR-FTIR spectra were recorded by using a Per-kin-Elmer Paragon 500 FTIR spectrometer. A KRS-5 crystal with an angle of incidence of 45[degrees] was used for recording the ATR spectra. ATR spectra were recorded immediately after plasma treatment. For every ATR spectrum 64 scans were taken with a resolution of 4 [cm.sup.-1].

Thermo VG scientific Multilab 2000 was used for ESCA recording and analysis.

Curve fitting: Lorentzian non linear curve fitting program of ORIGIN-6 software was used.

Principle of ESCA

Surface analysis by ESCA is accomplished by irradiating a sample with mono energetic soft X-rays and analyzing the energy of the ejected electrons. Monochromatic Mg K[alpha] (1253.6 eV) or Al K[alpha] (1486.7 eV) X-rays are usually employed as source of irradiation. When the sample is irradiated with such monochromatic X-rays, photo ionization of the inner core electrons will occur. The resulting photoelectron will have a kinetic energy given by

[E.sub.k] = hv - [E.sub.B] - [[PHI].sub.S] (7)

Here [E.sub.k] is the kinetic energy of emergent electron, hv is the energy of incident X-rays, [E.sub.B] is the binding energy of the core electron and [PHI]s is the spectrometer work function.

As hv and [PHI]s are known, [E.sub.k] and [E.sub.B], that is, binding energy of core electron can be measured. Because each element has a unique set of binding energies, ESCA can be used to identify and determine the concentration of the elements on the surfaces.

RESULTS AND DISCUSSION

Etching and Weight Loss Study

Air plasma treatments affect both the wettability and the elemental surface composition of all materials. The most obvious result of the plasma treatment was the improved wettability (35). The interaction of ions, electrons, and energetic species of neutral atoms causes rapid removal of low molecular contaminants such as additives processing aids and adsorbed species, which is also called as "plasma cleaning." After plasma cleaning, ablation of polymer chain starts. It was found in the literture (36) that the bombardment by energetic particles, neutrals vacuum, UV radiations, electrons, and ions results in etching of the surface. This is either due to the physical removal of molecules of fragments or the breaking up of bonds, chain scission, and degradation processes. The gases evolved in the reaction may be pumped out. This causes loss in the weight.

In this investigation, we found that treatment under air plasma results in loss of weight, which increases with treatment time and is depicted in Fig. 2. The percentage weight loss increases steadily with time and it may be due to etching reaction caused by plasma action (37). The etching process is predominant on the amorphous regions of the surface than on crystalline regions (38). Therefore, it is possible that the initial rates of etching are more rapid. Once all the etchable amorphous materials on the surface have been removed, the remaining crystalline and tightly bound amorphous material can not be removed easily, causing decline in the etching rate.

[FIGURE 2 OMITTED]

Contact Angle Measurements

Contact angle is a measure of the wettability and surface roughening of polymer surfaces. Extent of surface modification achieved due to plasma treatment can be evaluated using contact angle measurements. Before air plasma treatment, the surfaces of the materials PVDF and PMMA had equilibrium water contact angles of 88.23[degrees] and 77.64[degrees], respectively and for other liquids such as glycerol, formamide and ethylene glycol, the values of contact angle are less when compare with water. However, the interaction of water with PVDF is greater than that with PMMA. The changes of contact angle on the plasma treated PVDF and PMMA film surfaces with respect to four different liquids are shown in Tables 1 and 2, respectively. It is observed that all the samples show decrease in water contact angle values as the treatment time increases, which is shown in Fig. 2. From the microscopic point of view, the surface hydrophilization is to maximize hydration and hydrogen bonding interactions. Hydroxyl, carbonyl, carboxyl, and carboxylate groups contain lone pairs (unshared) of electrons and asymmetric charge distributions. All kinds of oxygen, nitrogen, or sulfur containing organic functional groups can interact more effectively than carbon-based repeating units (33).
TABLE 1. Values of contact angle, polar
([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) and
disperse ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII])
components of solid surface free energy, and S.E. ([[gamma].sub.s])
of air plasma treated PVDF film for various durations of time.

 Ethylene Polar
Treatment Water Glycerol Formamide Glycol Components
Time (min) C.A (W) C.A (G) C.A (F) C.A ([MATHEMATICAL
 (E.G) EXPRESSION NOT
 REPRODUCIBLE IN
 ASCII])

 0 88.23 79.95 67.33 59.42 4.53
 5 76.13 64.03 55.40 52.93 8.92
10 68.12 57.48 51.38 48.47 14.71
15 62.47 49.60 47.94 45.32 19.35
20 55.72 45.72 43.67 40.64 25.96

Treatment Dosperse Components (S.E) (mJ/[m.sup.2])
Time (min) ([MATHEMATICAL ([MATHEMATICAL
 EXPRESSION NOT EXPRESSION NOT
 REPRODUCIBLE IN REPRODUCIBLE IN
 ASCII]) ASCII])


 0 23.03 27.56
 5 25.11 34.03
10 21.87 36.58
15 20.66 40.01
20 17.97 43.93

TABLE 2. Values of contact angle, polar
([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) and
disperse ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII])
components of solid surface free energy, and S.E.
([[gamma].sub.s]) of air plasma treated PMMA film for
various durations of time.

Treatment Water Glycerol Formamide Ethylene Polar
Time (min) C.A. C.A. (G) C.A. (F) Glycol components
 (W) C.A. ([MATHEMATICAL
 (E.G) EXPRESSION NOT
 REPRODUCIBLE IN
 ASCII])

 0 77.64 70.22 61.55 50.98 10.55
 5 72.33 67.84 56.32 45.35 14.07
10 67.55 61.29 51.23 41.22 16.38
15 64.31 57.14 48.19 38.73 18.34
20 61.31 54.21 44.26 35.68 19.99

Treatment Disperse components (S.E) (mJ/[m.sup.2])
Time (min) ([MATHEMATICAL ([MATHEMATICAL
 EXPRESSION NOT EXPRESSION NOT
 REPRODUCIBLE IN REPRODUCIBLE IN
 ASCII]) ASCII])

 0 20.23 30.78
 5 19.34 33.41
10 20.52 36.90
15 20.76 39.10
20 21.24 41.23


Contact angle measurement is probably the most common method of solid surface tension measurement. Contact angle data, especially in the case of polymeric materials, can be obtained with simple techniques but the interpretation of data is not always straight forward, and the correct use of data requires knowledge of the thermodynamic status of the observed angle (34).

ATR-FTIR Analysis of Plasma-Treated PVDF and PMMA Films

To understand the extent and type of chemical modification on the surface due to plasma treatment, ATR-FTIR spectra of treated films were recorded. Plasma modifies surface of polymeric films hundred of angstrom unit deep and not the bulk (39). The plasma initiated both chemical modification process and degradation reaction on every polymeric material. There are various active species such as highly excited electron, ion, and radical species in the plasma (40). Hence, an ATR-FTIR spectrum is an appropriate technique.

It is well known that treatment of polymer films in gaseous plasma incorporates hydrophilic functionality, for example, treatment in nitrogen plasma incorporates nitro and primary amino groups at the surface (41), (42), treatment in oxygen plasma incorporates hydroxyl, carbonyl, carboxylic acid functional group (43). Air plasma treatment gives the combined effect of oxygen and nitrogen plasma. In general, the major observations of the membrane treated with air plasma are formation of crosslink and densification. The degree of membrane cross linking may be controlled by varying the plasma treatment time or changing the power of plasma. To see the changes in the chemical structure, FTIR spectroscopy was employed.

PVDF being a simple structure (-[CH.sub.2]-[CF.sub.2]-) has [CF.sub.2] stretching and bending and [CH.sub.2] rocking vibrations. There are large difference in bonding energy between C-F and C-H bond in the fluoropolymers such as PVDF. If the chemical structures of the polymers modify only the C-F component by plasma, it means defluorination (C-F bond scission) reaction occurring on the surface. On the other hand, if only a C-H component was modified by plasma, it means dehydrogenation (C-H bond scission) reaction occurring on the surface. If this plasma treatment can abstract some atoms in fluoropolymer surfaces selectively, that may introduce other desired functional groups such as carbonyl, carboxyl, hydroxyl, etc. using this technique introduced by plasma treatment, various polymeric materials properties can be controlled, for example, adhesion, wettability, and biocompatibility of polymers.

When PVDF is treated in air plasma, the following changes take place, which is shown in Fig. 3. The band at 3021 [cm.sup.-1] confirms the presence of stretching vibration of [CH.sub.2] group. The intensity of this band is increased as treatment time is increased The band at 1383 [cm.sup.-1] is corresponds to the bending vibration of C-F group. Bands at 510-530 [cm.sup.-1] condirm the presence of [CF.sub.2] group and band at 845 [cm.sup.-1] corresponds to rocking vibration of [CH.sub.2] group. A band at 872 [cm.sup.-1] confirms the presence of stretching vibration of [CF.sub.2] group.

[FIGURE 3 OMITTED]

When PMMA film is treated in air plasma, the following changes take place which are shown in Fig. 4. No peak shifts were observed implying no oxidation or reduction change in the (C=O) or (C-O) chemical environment (29). Also observed was an increase in the (C-H) stretching (aliphatic peak at 2950 [cm.sup.-1]) band with increase in the treatment time. The band at 1700 [cm.sup.-1] confirms the presence of C=C bond of PMMA. The intensity of this band is increased as the treatment time increased. The band in the region 750-755 [cm.sup.-1] corresponds to the [CH.sub.2] group of PMMA. The intensity of the band at 750 [cm.sup.-1] is increased as treatment time increased. The cross linking of the film was characterized by well defined peak at 1719 [cm.sup.-1]. As the plasma treatment time increased, the adsorption at 1719 [cm.sup.-1] continously decreased. This result may be attributed to the occurrence of cross linking through the plasma treatment (44). This type of analysis can project most predominant chemical changes occurring on the polymeric surfaces.

[FIGURE 4 OMITTED]

ESCA Analysis of Plasma-Treated PVDF and PMMA Films

Atomic composition for the PVDF surfaces modifies with air plasma was estimated from relative intensities of Cls, Fls, and Ols spectra. Results of ESCA analysis for PVDF and PMMA surfaces treated with air plasma are summarized in Table 3. Figure 5a shows Cls spectra of untreated PVDF film. The untreated PVDF surface showed a simple two peaks at 291.0 eV (due to [CF.sub.2]) and 286.4 eV (due to [CH.sub.2]) components. The Cls spectrum for untreated PVDF is shifted toward higher energy side then etched PVDF. This is probably due to the presence of fluorine in high percentage on the untreated surface, which causes a much higher degree of surface charging during ESCA measurements (45).
TABLE 3. Percentage atomic concentration of various elements in PVDF
and PMMA films subject to plasma treatment.

Sample CIs Ols Fls NIs

PVDF Untreated 51.7 0.21 48.09 -
PVDF treated in air plasma for 5 min. 46.02 0.90 53.08 -
PVDF treated in air plasma for 20 min. 43.24 2.42 54.34 -
PMMA Untreated 64.58 33.12 - 2.3
PMMA treated in air plasma for 5 min. 63.41 33.52 - 3.07
PMMA treated ill air plasma for 20 min. 58.84 36.51 - 4.65


[FIGURE 5 OMITTED]

Figure 5b and c show Cls spectra of PVDF film treated in air plasma for 5 and 20 min. The four components appeared at 286.2, 286.6, 288.3, 289.3, and 291.4 eV which were assigned to-[CH.sub.2]-[CF.sub.2]-, CHF-[CH.sub.2]-CHF, and O-[CH.sub.2] groups; [CH.sub.2]-CHF-[CH.sub.2], [CH.sub.2]-CHF-CHF, and O-[CH.sub.2]-[CF.sub.2] groups; [CH.sub.2]-CHF-[CF.sub.2], CHF-CHF-CHF, and O-CHF-CHF groups and [CF.sub.2]-[CH.sub.2], respectively. The relative intensity of the peak at 286.6 eV was about 12.03% of the total intensity of Cls core level spectrum. The composition in Fig. 5 indicates that [CF.sub.2] carbons were modified into CHF and [CH.sub.2] carbons and O-C carbons (O-[CH.sub.2] and O-CHF) during plasma exposure (37). These changes are consistent with several ESCA studies of PVDF, which had undergone other treatments (46-48). From the Ols spectrum of untreated PVDF film, a very small amount of oxygen was evident, this probably being due to the polymer surface contamination that was not removed by the solvent. From the Ols spectra of PVDF film treated in air plasma for 5 and 20 min, a vivid Ols spectra that appeared at 533.5-533.7 eV can be seen. These spectra mean they produced C--O groups on the PVDF surfaces.

The Fls spectra of untreated PVDF film shows highly intense peak corresponding to the carbon of-[CH.sub.2] at the lower binding energy and to the carbon of-[CF.sub.2] at the higher binding energy (49) and after plasma treatment for 5 min and 20 min the intensity of this peak was decreased.

The Cls spectrum of untreated PMMA film reveals peaks at 289, 286.8, and 285 eV corresponding to O-C=0, C-O and C-C groups, respectively (50). After air plasma treatment for 5 and 20 min the peak at 286.8 which is assigned to C-O group is significantly decreased, whereas that at 289 eV is increased.

The Ols spectra of untreated and air plasma treated PMMA film for 5 and 20 min is shown in Fig. 6. The untreated PMMA film shows two peaks at 533.77 and 532.21 eV due to the presence of C-O and O-C=0 groups. After plasma treatment for 5 and 20 min., the Ols spectra reveals the presence of C-O and O-C-O groups at 532.19 and 533.53 eV, respectively which are very close to untreated PMMA. It is probably due to hydroxyl group bonded from the plasma on top of the modified polymer. These groups seem to determine the hydrophilic surface character established by contact angle measurements. From the Nls spectra of untreated and air plasma treated PMMA films for 5 min and 20 min, it was observed that the area of nitrogen peak at 399.8 eV increases with an increase in treatment time. The atomic concentration of Nls element has increased to 4.65% (Table 3).

[FIGURE 6 OMITTED]

Surface Energy Measurements

Figure 7 shows the variation of surface energy and polar and disperse components of surface energy with time for air plasma treated PVDF and PMMA films. One can observe a steady increase in surface energy ([[gamma].sub.s]) of PVDF and PMMA with the plasma treatment time. Similar kind of trend was observed in the increase in polar components ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) of PVDF and PMMA. It is mainly due to the incorporation of polar groups such as CO, COO, OH etc (51). However, there is an increase in the dispersed components ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) of PVDF for 5 min of treatment time and then sudden decrease in its value for higher treatment time. In case of PMMA, the dispersed components ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) value decreases for 5 min. of plasma treatment and increases for higher treatment time. Hence, the increase in surface energy ([[gamma].sub.s]) is mainly due to incorporation of polar groups on to the PVDF and PMMA surfaces. The wettability and hence surface energy of PVDF and PMMA films is increased because of interaction between the hydrogen bond and dipoles in the vertical direction of interface (52). The values of contact angle, polar ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) and disperse ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) components of solid surface free energy, and S.E. ([[gamma].sub.s]) of air plasma treated PVDF and PMMA film for various durations of time is given in Table 1 and 2, respectively. The properties such as wettability, adhesion, printability etc. strongly depend on the surface energy.

[FIGURE 7 OMITTED]

Surface Roughness

To control wettability and adhesion of polymers numerous surface modification technique such as exposure to flames, chemical modification, corona discharge, and low pressure gas plasma are used. In many cases, the wettability is regulated by changes in the chemical composition of the surface, but it has longer been recognized that the surface roughness can be important for wettability. Wenzel (53) was the first to discover the influence of surface roughness on contact angle. He introduced the roughness factor "R" into the Young's equation because he argued that in case of solid surface, the interfacial tension [[gamma].sub.SV] and [[gamma].sub.SL] should not be referred to the geometric area, but to actual surface area, and thus

R = [True surface area/Geometric surface area] (8)

or

R([[gamma].sub.SV] - [[gamma].sub.SL]) = [[gamma].sub.LV]cos[[theta].sub.W] (9)

For the contact angle on a rough surface, Wenzel obtained:

R = [Cos[theta]/Cos[theta]*] (10)

where "R" is the ratio of the treated area to the untreated area. Based on the equation, it can be predicted that roughness should have a major effect on contact angle and hence wettability of surface.

In this study, the surface roughness was determined from contact angle measured with reference to water. From Fig. 8, it can be seen that surface roughness increases as treatment time is increased. The roughness was found to increase considerably with the plasma exposure time. The surface roughness can enhance the mechanical interlocking and this has relatively a strong influence on the adhesive properties (54).

[FIGURE 8 OMITTED]

Adhesion

Adhesion is the joining of two different materials whereas cohesion is the joining of different parts made by same materials (33). A good adhesion can be guaranteed if the adhesive adequately wets the substrate of the two materials. Therefore, adhesion and wettability are two related properties (55).

Polymers have wide ranging application in food packaging and decorative products and as, insulation for electric devices. For these applications, the adhesion of material deposited on to polymer substrates is of primary importance. Not all polymer surfaces posses the required physical and chemical properties for good adhesion. Plasma treatment is one means of modifying polymer surfaces to improve adhesion while maintaining the desirable properties of the bulk material (56).

Work of Adhesion

The contact angle data can be used to evaluate the work of adhesion between PVDF, PMMA, and the substrate. The equilibrium contact angle for a liquid drop on an ideally smooth homogeneous and nondeformable surface is related to the various interfacial tensions by Young's equation (1), (17).

[[gamma].sub.LV]cos[theta] = [[gamma].sub.SV] - [[gamma].sub.SL] (11)

where [[gamma].sub.LV] is the surface tension of the liquid in equilibrium with its saturated vapor; [[gamma].sub.SV] the surface tension of the solid in equilibrium with its saturated vapor; and [[gamma].sub.SL] the interfacial tension between the solid and the liquid.

Moreover, the work of adhesion [W.sub.adh] is given by the following equation (57).

[W.sub.adh] = [[gamma].sub.SV] + [[gamma].sub.LV] - [[gamma].sub.SL] (12)

Combining Eqs. 8 and 9

[W.sub.adh] = [[gamma].sub.LV](1 + cos[theta]) (13)

Work of adhesion was calculated from contact angle with reference to water and depicted in Fig. 8. It clearly shows that there is a pronounced rise in the work of adhesion over a short period of plasma treatment and then steadily increased with plasma treatment time.

T-Peel Strength

Treatment of polymer films in plasma environment incorporates hydrophilic groups such as hydroxyl, peroxyl, carbonyl, amine, amide etc. These functional groups contribute for the increase in wettability and as a result adhesive layer spread on the surface more easily. Moreover, when these functionalities come in contact with the material a weak bond is formed due to vander Waal's forces. This force of attraction between plasma treated polymer surface and adhesive material contributes for the observed increase in bonding strength. To understand the effect of plasma treatment on bonding strength of PVDF and PMMA films, treated films were subjected to standard T-peel test. The variation in the peel strength for samples treated in air plasma is shown graphically in Fig. 9. It may be seen that the peel strength increases with time of treatment in plasma right up to 20 min. of treatment time. This is caused on account of surface roughness and an increase in surface energy due to plasma treatment.

[FIGURE 9 OMITTED]

Printability

It is well known that before printing is made, polymers are subjected to corona treatment (58).This treatment requires very high power and processing cost is also high. In this study, we have modified the surface of PVDF and PMMA films using cold plasma to have a good printability and good adhesion to ink. The printability was measured using cross test method. This method offers a simple way to measure the degree of adhesion of the ink (coating) on a substrate.

The improvement in printability of air plasma treated PVDF and PMMA film is shown in Fig. 10. Improvement in ink adhesion is generally observed after plasma treatment (51). Such modifications are basically observed because of possible improvement in wettability due to incorporation of polar groups on to the surface and phenomenon of plasma etching in turn increases an effective area for contact for spreading of ink material. Both the processes contribute for improvement in ink adhesion. Anchoring of ink takes place at the rough surface, causing better adhesion.

[FIGURE 10 OMITTED]

CONCLUSIONS

The interaction of air plasma with polymer films causes mainly the change of surface polarity. The contact angle ([theta]) decreases with increase in the plasma treatment time. Air plasma treatment incorporates polar functional groups on to the surface of the polymeric films causing a rise in solid surface free energy ([[gamma].sub.s]) including its polar components ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) with small changes in the dispersed ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) components values. It means that the amount of functional groups on polymer surface increases significantly after plasma treatment. The surfaces of PVDF and PMMA were modified effectively by air plasma. It was also proved that side groups in the main chain of polymers restrain the processes caused by plasma action. The plasma processes generates wide range of reactive species in the treated system, which undergo consecutive chemical reactions creating thus several oxygen based functionalities at the interface. Simultaneously, the vigorous increase of the surface roughness was observed as a result of the successful plasma etching. Mechanical interlocking due to surface roughness and chemical interaction as observed by ATR-FTIR and ESCA studies.

ACKNOWLEDGMENTS

The authors express sincere gratitude to Dr. R.R. Desh-mukh, from Department of Physics, University Institute of Chemical Technology, University of Mumbai, for his insightful comments and fruitful discussions. The authors gratefully acknowledge the inputs from Mr. Akshath R. Shetty and his technical assistance during this research work.

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S.M. Pawde, Kalim Deshmukh

Department of Physics, University Institute of Chemical Technology, University of Mumbai

Department of Physics, University Institute of Chemical Technology, University of Mumbai,

Matunga, Mumbai 400019, India

Correspondence to: Kalim Deshmukh; e-mail: kalim_deshmukh@yahoo.co.in

DOI 10.1002/pen.21319 Published online in Wiley InterScience (www.interscience.wiley.com).
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