Surface-modification and characterization of H-titanate nanotube

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Plasmas and Polymers [papo] pp1013-papo-474404 October 20, 2003 16:19 Style file version June 28th, 2002

Plasmas and Polymers, Vol. 8, No. 4, December 2003 (C© 2003)

Surface Modification and Characterizationof Dichloromethane Plasma TreatedPolypropylene Film

D. J. Upadhyay1,3 and N. V. Bhat2

Received September 9, 2002; accepted June 10, 2003

Surface of polypropylene (PP) film was modified in plasma of dichloromethane(CH2Cl2). The nature of surface modifications and formation of cross-linked layerdue to plasma polymerization was studied by surface energy measurements and solu-bility test. Surface modification achieved by CH2Cl2 plasma was compared with thereported work on chloroform (CHCl3) and carbontetrachloride (CCl4) plasma modifi-cations. Modified surface characterized by ATR-FTIR technique indicated formation ofsaturated and unsaturated cross-linked product. On the basis of relative intensity changeof the specific bands, the site of attachment of chlorine on PP surface was investigated.Adhesive strength of modified film was measured by T-peel test method. Stability ofmodified surface was studied by measuring surface energy and peel strength after twomonths.

KEY WORDS: Plasma polymerization; polypropylene; surface energy; ATR-FTIR;T-peel strength.

1. INTRODUCTION

Polypropylene (PP) is an important engineering plastic due to its excellentmechanical properties and chemical stability. The hydrophobic nature of PP makesit excellent water repellent but the bondability and printability are poor.(1) YetPP is permeable to gases and on account of growing interest in the applicationof polymeric membranes in mixture separation processes, the permeation andpermselective properties of PP for liquid and vapour are being studied.(2) Thedevelopment in the microporous PP membrane has extended its application infiltration and separation processes.(3–6) If the surface of PP can be specifically

1University of Ulster, Coleraine, BT52 1SA, Northern Ireland, UK.2Bombay Textile Research Association (BTRA), LBS Marg, Ghatkopar, Mumbai-86, India.3To whom all correspondence should be addressed; e-mail: [email protected]

2371084-0184/03/1200-0237/0C© 2003 Plenum Publishing Corporation

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238 Upadhyay and Bhat

modified while retaining its bulk properties, the membrane of PP could be usedfor additional application and processes in aqueous environment. Further adhesionproperties could be improved for making multilayered films or laminates.

Surface modification of material is a growing research field having applica-bility in many areas. In order to improve adhesive properties of polymeric films,chemical etching, thermal/flame treatment, and acid/alkali swelling.(7–9) are somesurface modification techniques available, but the plasma glow discharge treatmentoccupies a special importance.(10–18) Plasma-induced grafting and treatment ingases such as N2, O2, and NH3 improves bonding strength of polymeric films.(19–23)

Being a surface specific technique, plasma modifies the surface without affectingthe bulk properties.

It has been reported that in plasma various processes such as interaction withenergized species, etching, surface functionalization, fragmentation, cross-linking,and plasma polymerization occur, depending upon gas/precursor used, and surfacemodification of polymer can be tailored by controlling processing parameters suchas pressure, power, and treatment time.(12,14)

Inagakiet al.(24,25)have studied the effect of surface chlorination of polypropy-lene using chloroform and carbontetrachloride plasma. Our recent study on 1,2-dichloroethane plasma processing of polypropylene revealed surface chlorinationand deposition of plasma-polymerized thin film with good bonding strength.(26)

The aim of this paper is to investigate the nature of surface modification achievedon PP film using CH2Cl2 plasma. This is done with a view to increase wettingand bonding strength of PP film. Further, the effectiveness of surface modificationwith CH2Cl2 is compared with other chlorinating plasmas.

2. EXPERIMENTAL

PP films of commercial variety were obtained from Reliance Industries Lim-ited, Bombay, India. The thickness of the film was 85µm of density 0.98 g/cm3.The PP film was of isotactic variety. The PP sample of dimension 9× 9 cm2 wascut for plasma processing. The films were washed in ultrasonic bath with ace-tone for 9 min and then dried in air. All chemicals used were of AR grade. Theplasma processing chamber consisted of a glass bell jar of diameter 30 cm andheight 30 cm. The schematic diagram of plasma chamber is shown in Fig. 1. Thetop and base plates have various ports. The inlets for the gas and monomer wereprovided on the base plate. The gas inlet was externally connected to mass flowcontroller (Unit Model URS-100). The top plate consists of a port on which PiraniVacuum Gage was mounted. The chamber evacuated using a two-stage RotaryPump (Model- STA-6P4M Hind High Vac Pvt. Ltd., India).

Two stainless steel parallel plates, inside the chamber, were capacitively cou-pled with RF source. The frequency of the power source was 13.56 MHz and coulddeliver a power up to 100 W. The power delivered between the two parallel plates

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Characterization of CH2 Cl2 Plasma Treated Polypropylene Film 239

Fig. 1. Schematic view of plasma processing chamber.

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was 20 W for CH2Cl2 plasma, whereas for CCl4 and CHCl3 plasmas it was 55 W.The exposure time for plasma processing was varied from 1 to 15 min. The cham-ber could be purged with nitrogen gas and evacuated to a pressure of 0.05 mbar.Alternately, vapors of organic solvent could be allowed to flow into the chamber.The working pressure was adjusted to 0.1 mbar, when the flow of the organic vaporwas 10 sccm. The chamber was slowly brought to atmospheric pressure by passingair or nitrogen gas. The samples were removed and preserved in vacuum desic-cator for further characterization. Weight change due to plasma modification wascalculated using METTLER AE 240 five-digit balance. The films were weighedbefore and after exposure to plasma, and an average of five readings was taken foreach sample. The error in the measurement was 0.012%. In order to understandthe nature of the deposited layer on PP surface, the modified films were subjectedto solubility test. A piece of PP film weighing from 0.5–1 g was soaked in hotp-xylene (100◦C) for 5 min, from the difference in weight extent of dissolutionwas calculated.

The surface energy was measured by measuring contact angle by sessile dropmethod. The liquids used for calculating surface energy are water, glycerol, andformamide of knownγ p (polar contribution) andγ d (disperse contribution). Asmall drop of double distilled liquid was placed on the polymeric surface witha microsyringe and observed through a microscope. The height (h) and radius(r ) of the spherical segment were measured and the angle was calculated by thefollowing equation.(27)

Contact angle(θ ) = sin−1

[2hr

r 2+ h2

].

At least 10 readings were taken at different places and an average value ofθ was determined. The surface energy of polymeric film was calculated usingFowkes approximation(28) using the equation[

1+ cosθ

2

]× γ`√

γ d`

= √γ ps ×

√γ

p`

γ d`

+√γ d

s

From the plot of LHS vs. RHS, values of slope and intercept can be obtained,which are used to calculate the surface energy of the modified film. The error inthe measurement of angle is±0.5◦. In the experimental setup, an error can alsoarise due to variation in the base pressure of the chamber. The observed pressurechanges are±0.5 mbar. The error in the time of measurement can be±1 s. On thebasis of these considerations, the cumulative error in the surface energy works outto be±1.8 mJ/m2.

The attenuated total reflection infrared spectra (ATR-FTIR) of the films wasobtained by using a Perkin–Elmer Paragon 500 FTIR spectrometer. A KRS-5crystal with an angle of incidence 45◦ and with a penetration depth of 5–20µm

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was used for recording the ATR spectra. In order to minimize possible shift ofIR bands, due to sample-to-sample variation and to get relative change in theabsorbance with respect to untreated film, an ATR spectrum of each piece of theuntreated film was recorded. The ATR-FTIR spectrum was recorded again afterexposure to plasma, taking care to align the film in the identical direction as thatused for the untreated film. For every ATR spectra, 350 scans were taken with aresolution of 4 cm−1 with background correction.

The morphological changes taking place at the surface of the substrate asa result of plasma processing were studied using scanning electron microscopy(SEM). SEM micrographs were recorded at Central Institute for Research in CottonTechnology (CIRCOT), Bombay, India, using SEM XL series (Model XL-30).Coating of samples was performed on Polaron Equipment Ltd., SEM CoatingUnit (model- E5100).

The standard T-peel test method29 (ASTM D 1876-72, BS 5350: PartC12:1979) was used with the commercially available adhesive tape. The treatedsurface was bonded to the scotch tape, the tape in turn adhered to the piece of paperto make ‘T’-shape. The two ends of the ‘T’ at an angle of 180◦ were clamped tothe jaws of Instron Tensile Tester 1026. An area of 2.5× 4.5 cm2 was used forthe test. The force of about 10 N was applied with the speed of moving upper jaw20 mm/min. For every treatment three specimens were prepared for peel strengthmeasurement and mean value was obtained.

3. RESULTS AND DISCUSSION

3.1. Surface Energy Measurement

The surface energy and water contact angle of the modified PP film weremeasured as shown in Fig. 2. The surface energy of the modified PP film increasedto 32 mJ/m2 for 1 min from the initial value of 22 mJ/m2. The value decreasesslightly with the exposure time and attains an equilibrium value of 26 mJ/m2

for 15 min. Similarly the water contact angle decreases for 1 min and attainssaturation. Further analysis has been carried out by expressingγ

ps and γ d

s interms of fraction of totalγs as shown in Table I. For 1 min theγ p

s fraction in-creased from 0.19 to 0.29, and for 15 min it decreased to 0.17. This observationindicates that surface modification of the PP film was achieved in two stageswhen exposed to CH2Cl2 plasma. The first step includes hydrophilic modifica-tion for shorter time, which may be due to incorporation of chloride ions onthe PP surface, and the second step is hydrophobic modification for longer timedue to the formation of cross-linked plasma-polymerized product of CH2Cl2 onPP. The observed hydrophilic modification is in accordance with the results ob-served by other researcher on surface chlorination and bromination of differentpolymers.(30–33)

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Fig. 2. Contact angle and surface energies for PP film modified in CH2Cl2.

Hydrophilic modification achieved by CH2Cl2 plasma was compared withearlier reported work of Inagakiet al. on CHCl3 and CCl4 plasma modifications.For comparison, the surface energies of CH2Cl2, CHCl3, and CCl4 plasma modifiedPP films are plotted in Fig. 3. The surface energies of CHCl3 and CCl4 plasmamodified films were found to be higher as compared to CH2Cl2 plasma modifiedone. It has been observed that CCl4 plasma makes the surface more hydrophilicthan modification in CHCl3 and CH2Cl2 plasmas. The values of theγ p

s fractionshowed sudden increase from 0.35 to 0.75 for CHCl3 and 0.52 to 0.83 for CCl4,when the exposure time was varied from 1 to 15 min. The observation indicates thatCCl4 and CHCl3 plasmas induce mainly hydrophilic modification on the surface.

Table I. Polar and Dispersive Fraction of CH2Cl2, CHCl3, and CCl4 Plasma Modified PP Films

CH2Cl2 CHCl3 CCl4

Exposure time Polar Dispersive Polar Dispersive Polar Dispersive(min) fraction fraction fraction fraction fraction fraction

0 0.19 0.81 0.19 0.81 0.19 0.811 0.29 0.71 0.35 0.65 0.52 0.483 0.25 0.75 0.49 0.51 0.58 0.425 0.25 0.75 0.55 0.45 0.60 0.40

10 0.24 0.76 0.68 0.32 0.64 0.3615 0.17 0.83 0.75 0.25 0.83 0.17

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Fig. 3. Comparison of surface energies of aged CH2Cl2, CHCl3, and CCl4 plasma-modified PP films.

Comparison of our results on CH2Cl2 with that of Inagakiet al. leads to aconclusion that as the number of chlorine substituents increases from CH2Cl2,CHCl3 to CCl4, polarity of the solvent changes, and as a result fragmentation ofthe molecules in the plasma state gets affected (i.e., requires more power for disso-ciation; see Section 2). CH2Cl2 contains dichloro substituted alkyl group that canrecombine easily after losing first chlorine from the molecule, whereas in the caseof CHCl3 plasma, recombination of alkyl groups will be relatively hindered due tothe presence of higher number of chlorine atoms and less favourable energetics ofradical species.(34) On the other hand, in CCl4 plasma due to the presence of a stillhigher number of chlorine substituents, propagation of radicals is more hinderedand plasma polymerization cannot be achieved. Radicals generated by CHCl3

and CCl4 plasmas are short lived(34) and do not give any plasma polymerizationproduct. As a result, CCl4 and CHCl3 plasma modifications mainly contribute forsurface chlorination and etching of the PP film.

Figure 4 shows percent weight gain of the PP film modified in CH2Cl2 plasma.Initially, the weight increased by 0.96% for 3 min and thereafter it increasedsignificantly. This could be possibly due to the dissociation of heavy chloride ionsfrom CH2Cl2 molecules, which bombard the PP surface resulting in etching in theearly stage together with simultaneous deposition; however, the weight loss due to

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Fig. 4. Percent gain of PP film treated in CH2Cl2 plasma.

the etching process was counterbalanced by weight gain. After 3 min free radicalsformed due to dissociation of molecules may lead to deposition of cross-linkedplasma product on PP surface. Table II shows the solubility of modified PP filmdeposited by a thin film of plasma-polymerized product of CH2Cl2. Solubility testperformed on the film shows continuous decrease with increase in time of plasmatreatment. This signifies that as the time of exposure increases, formation of thincross-linked, insoluble plasma-polymerized product of CH2Cl2 takes place on thePP surface, leading to decrease in solubility.

3.2. ATR-FTIR Analysis of Modified PP Film

ATR-FTIR spectra of modified PP film were recorded immediately after treat-ment. ATR-FTIR spectra were recorded for 1 and 10 min of exposure time in

Table II. Solubility of the Plasma-PolymerizedCH2Cl2 Film

Exposure time Solubility g/100 g @ 100◦C(min) for 5 min

1 8.53 7.85 7.1

10 5.615 2.9

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Fig. 5. ATR-FTIR spectra of (a) untreated PP film, (b) 1 minand (c) 10 min CH2Cl2 plasma modified PP film.

CH2Cl2 plasma as shown in Fig. 5. It was observed that films treated for longerduration (10 min) gave only broad C----Cl stretching band due to thick deposi-tion. Hence, an ATR spectrum of PP film treated for 1 minute in CH2Cl2 plasmawas further analysed. It was found that apart from broad C----Cl stretch band at755 cm−1, there were spectral changes for C----H stretch vibrations. Hence it wasthought important to analyse C----H stretch bands by calculating change in relativeintensity. Due to plasma modification, following changes in the intensity of C----Hstretch bands could be expected:

1. Surface chlorination of PP film when exposed for shorter duration leadingto decrease in intensity of C----H stretch band due to chemical interaction.

2. Broadening in C----H stretch vibration, shifting and increase in intensitydue to formation of new organic deposit.

3. Weakening of C----H stretch bands, decrease in intensity due to masking oforiginal C----H stretch of PP surface.

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4. Multiple scattering of IR beam if the surface is nonuniform, leading todiminishing intensity at far end.

Cumulative results of these possible consequences are reflected in decreasein overall relative intensity of C----H stretch vibration. Changes in the relativeintensity are mainly attributed to surface chlorination, when modified for shorterduration (i.e., reason 1). However, when a film is exposed for longer duration,simultaneous sputtering and formation of nonuniform deposition takes place (i.e.,reason 2, 3, and 4). Therefore, calculation of relative intensity of C----H stretchband could possibly reflect the site of chemical interaction on the PP surface forshorter exposure time. Experiments were repeated three times in order to check thereproducibility of method, and it was found to be reliable and can be used to projectthe surface chemical changes. This type of analysis can be used to determine thechemical changes occurring on the polymeric surface.

Following IR bands(35–39)of PP were analysed in order to understand surfacechemical reaction:

2950 cm−1 corresponding toνas ----CH3 (1◦) group,2918 cm−1 corresponding toνanti ----CH2 (2◦) group,2906 cm−1 corresponding toν CH (3◦) group appear as a shoulder peak,2876 cm−1 and 2868 cm−1 of νs ----CH3 (1◦) and2837 cm−1 of νs ----CH2 (2◦).

A band at 2906 cm−1 was fitted with Gaussian peaks of equal full widthat half maximum (FWHM) using a Marquardt minimization computer program.C-H bending vibration of----CH3 group corresponding to 1376 cm−1 was chosenas standard for calculating relative intensity. It is known that substitution of elec-tronegative group affects the C-H stretching vibration,(39) and hence calculationof relative intensity can indicate the site of attachment of chlorine due to plasmatreatment.

Figure 6 shows C----H stretching bands of untreated and CH2Cl2 plasma mod-ified PP film. Table III shows the relative intensity of C----H stretch bands, when PPfilm was treated in CH2Cl2 plasma. In order to understand the site of attachment ofchlorine on PP, relative intensity changes of only symmetric bands are considered,since symmetric bands are more sensitive to chemical change. Among the sym-metric stretching vibrations, a band at 2906 cm−1 corresponding toν CH (3◦) wasfound to have decreased further. This finding is in accordance with Inagakiet al.,who investigated the plasma treatment of PP in the vapors of CHCl3 and CCl4 andconcluded that chlorination takes place at 3◦ carbon. In addition, absorption bandsat 2950 cm−1 corresponding toνas of ----CH3 (1◦) and 2918 cm−1 correspondingto νanti of ----CH2 (2◦) also showed decrease in their respective relative intensityvalues.

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Fig. 6. ATR-FTIR spectra of (a) untreated and (b) 1 min CH2Cl2plasma modified PP film. Note the absorption bands at 2950–2840 cm−1 due to C----H stretching vibration.

It is important to note that band corresponding toν CH (3◦) carbon atomwas better resolved and it has shifted to lower wave number side, possibly dueto the formation of new 3◦ carbon of plasma-polymerized product. On the basisof relative intensity change, following surface chlorination reaction is expected

Table III. Relative Intensity of C----H Stretch Bands of Untreated andCH2Cl2 Plasma Modified PP Film

Relative intensity Relative intensityBands (cm−1) before treatment after treatment

2950νas of ----CH3 (1◦) 0.64 0.332918νanti of ----CH2 (2◦) 0.71 0.332906ν of CH (3◦) 0.54 0.322876νs of ----CH3 (1◦) 0.43 0.282868νsof ----CH3 (1◦) 0.43 0.292837νs of ----CH2 (2◦) 0.45 0.30

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where (a) is more predominant than (b) and (c).

3.3. Nature of Plasma-Polymerized Products of CH2Cl2

Generally ionization, fragmentation and polymerization are expected to occurin plasma of organic vapors. In the case of CH2Cl2, it was thought importantto investigate what possible reaction would be. On the basis of surface energy,percent weight gain, obvious plasma chemistry, and energetics of the precursor,initially ionization and fragmentation occur followed by polymerization and majordepositions for longer time of exposure.

For longer exposure time, it could be expected that more number of radi-cal/ionized species of CH2Cl2 molecules might combine to give oligomeric deposi-tion, which was indeed observed as a brown powdery substance near the walls of thereactor. In order to characterize the final plasma-polymerized product of CH2Cl2,FTIR spectrum was recorded (a) by depositing thin film on KBr pellet in CH2Cl2plasma and (b) by mixing the powder obtained with KBr separately. Figure 7 showstwo C-H stretch bands at 2928 cm−1 of νas ----CH2(2◦)and 2854 cm−1 of νs ----CH2

(2◦) for the (a). On the other hand, (b) shows four C-H stretching vibrations at 3070cm−1 and 3016 cm−1 for substituted vinyl group and 2932 cm−1 of gνas ----CH2 (2◦)and 2853 cm−1 of νs ----CH2 (2◦). Apart from C----H stretching vibration, some morebands were observed that were common in both the spectra. Two major bands corre-sponding to C--------CH2C stretching vibration of substituted alkene at 1725 cm−1andcis-dichloro substituted ethylene peak at 1598 cm−1 were also observed. Althoughpossibility of the formation oftrans chlorinated ethylene cannot be denied thatcould be more favourable, but due to the presence of the centre of symmetry the

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Fig. 7. FTIR spectra of plasma-polymerizedCH2Cl2product (a) deposited on KBr pellet, (b) KBrpellet of CH2Cl2 powder.

molecule is not IR active and hence cannot be detected. The peaks at 1725 cm−1 and1598 cm−1 were also observed in ATR-FTIR spectra of CH2Cl2 plasma modifiedPP film (see Fig. 5). In the case of (a), broad absorption was observed from 1430 to1300 cm−1 (Fig. 7a). Similar absorption was obtained for (b), but the peaks werewell separated and appeared at 1429, 1384, and 1343 cm−1 due to respective CH inplane and CH out of plan deformation of CH and CH2 groups of vinyl.(39) A bandat 1213 cm−1 was observed in both the samples, which is due to C----H bending ofchlorinated hydrocarbon.(36) A band at 935 cm−1 was observed corresponding toC----H deformation vibration of----CH--------CHCl. A band at 806 cm−1 was observeddue to C----H out of plane vibration of alkene. Apart from C-H stretch bands broadC----Cl stretch was also observed at 763 cm−1. PC (i.e. primary carbon substitutedby chlorinetrans to another primary carbon atom(39,40)) generated due to the re-combination reaction of CH2Cl2 radicals (Reaction 2c). SC (i.e., secondary carbonsubstituted by chlorinetransto another secondary carbon atom) generated due tothe dehydrogenation reaction of recombined CH2Cl2 radicals (Reactions 2b andd). All these complex reaction products (i.e., formation of PC and SC), may lead

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Fig. 8. Comparison of the peel strength of CH2Cl2, CHCl3, and CCl4 plasma-modified PP films.

to the observed broad C----Cl stretch band to higher wave number side (normallyobserved at 720–730 cm−1 and in the case of CH2Cl2 liquid, it is expected at 750cm−1)(39) in ATR spectra of PP film and FTIR spectra of KBr pellets.

Thus on the basis of the FTIR spectral interpretation, the following plasmadissociation and cross-linking reaction of CH2Cl2 molecules could be expected:

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3.4. T-Peel Test

Adhesion strength of PP film modified in CH2Cl2 plasma was studied byT-Peel test method. Peel strength was measured for the exposure time of 15 and30 s to understand the minimum plasma modification required for achieving goodbonding strength. Figure 8 shows the peel strength of CH2Cl2 plasma modified PPfilm, steadily increasing with the exposure time.

Fig. 9. Comparison of the surface energies of aged CH2Cl2, CHCl3, and CCl4 plasmamodified PP films.

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It is interesting to compare this result with similar studies carried out for CCl4

and CHCl3 plasma modifications. Peel strength observed for PP film modified inCH2Cl2 plasma is much greater than that for CCl4 or CHCl3 plasmas. The observeddifference may be due to the higher surface cohesive energy of CH2Cl2 plasmamodified film as compared to CCl4 or CHCl3. Also one important thing observedwas that the modification of PP film just for 30 s improves the bonding strengthto such an extent that it transfers all the adhesive material from scotch tape to thetreated surface. Such observation was not noticed in the case of CCl4 and CHCl3plasma modified films. These results show that modification in CH2Cl2 plasmagives better bonding strength than do CCl4 and CHCl3 plasmas.

3.5. Effect of Aging

It is well known that the modification of hydrophobic surface with chemi-cal, flame, corona, and glow discharge leads to an improvement in wettability ofthe surface.(41) These processes are widely used on industrial scale to enhancethe adhesion of resins and inks. On the other hand, it is also well known that thewettability of surfaces introduced by these processes decays with time and maybe almost completely lost with longer storage.(42) Although other factors such ascontamination of surfaces, type of material, electrical properties, etc. also maycontribute to the overall decay process, usually it is the polar groups that playa major role in the decay phenomenon. Details of the phenomena of molecularmobility with storage time are discussed by Yasudaet al.(43)

To understand the durability of CH2Cl2 plasma modified PP film, surfaceenergy was measured after two months of storage. Figure 9 shows surface energyplot before and after aging. The surface energy of CH2Cl2 plasma modified filmdecreases by lesser extent as compared to that in the case of CHCl3 and CCl4plasma modifications. It can be seen from Table IV that after aging CH2Cl2plasmamodified film shows marginal decrease inγ d

s fractions. However, CCl4 and CHCl3plasma modified films show significant drop ingγ p

s fractions. The observation

Table IV. Polar and Dispersive Fraction of CH2Cl2, CHCl3, and CCl4 Plasma Modified PP FilmAfter Aging

CH2Cl2 CHCl3 CCl4

Exposure time Polar Dispersive Polar Dispersive Polar Dispersive(min) fraction fraction fraction fraction fraction fraction

0 0.19 0.81 0.19 0.81 0.19 0.811 0.25 0.75 0.21 0.79 0.37 0.633 0.23 0.77 0.32 0.68 0.34 0.665 0.20 0.80 0.30 0.70 0.33 0.67

10 0.19 0.81 0.31 0.69 0.33 0.6715 0.19 0.81 0.30 0.70 0.30 0.70

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Fig. 10. Comparison of the peel strength of aged CH2Cl2, CHCl3, and CCl4plasma modified PP films.

can be explained by the phenomena of orientation of surface mobile group. AsCH2Cl2 plasma gives cross-linked plasma-polymerized product, orientation ofpolar functional groups is restricted and as a result surface energy decreases tolesser extent. This can be evident from the observed closeness between the gγ

ps

fraction of fresh and aged film for 15 min of exposure time, where the modifiedsurface has retained its property. The observed marginal improvement in polarfraction may be attributed by orientation of chlorinated deposit. In the case ofCCl4 and CHCl3 plasma modifications, the gγ p

s fraction decreases due to similarreason.

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Fig. 11. SEM micrograph of control PP.

Figure 10 shows the peel strength of CH2Cl2, CCl4, and CHCl3 plasma mod-ified films before and after aging. The peel strength of CH2Cl2 plasma modifiedfilm shows maximum bonding strength till 3 min, after that it decreases gradually.In the case of CHCl3 plasma, the modified PP film bonding strength does notshow a significant decrease possibly due to the improvement in disperse fractionon the surface, which contributes for more surface cohesive bonding. In the caseof CCl4 plasma modified PP film, peel strength decreases to a great extent afteraging. These results indicate that improvement in bonding strength of CH2Cl2plasma-treated film lasts longer.

3.6. Morphological Studies

Figure 11 shows the micrographs of control PP film having some sphericalgrowths embedded on the surface. It may be recalled that PP films when drawnfrom melt and quenched, usually result having a large number of spherulitic andpartially crystalline growths (known as transcrystallization). The smallest grainlike dot observed was found to have a size of about 0.2µm with a range extendingup to 2.5µm. Some smaller grains were observed to have a definite square-like or

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Fig. 12. SEM micrograph of CH2Cl2 plasma modified PP films.

diamond-like shape with an average size of about 2µm. They appear like a singlecrystal growth and it is quite likely that these features develop from low molecularweight fractions forming a small crystallite.

SEM studies of CH2Cl2 plasma modified PP film reveal that there is a uniformdeposition of globular particles on the surface (as shown in Fig. 12). The size ofthe globules varies from 0.25 to 1.12µm and the shape was found to be identicalfor all the particles resembling a sphere. The observation by naked eye revealedthe coloration of the film that varied from pale yellow to light brown as the time ofdeposition was increased. In addition, it is noteworthy that the gravimetric studiesalways revealed gain in the weight for the treated film ranging from 0.46 to 5.59%for exposure time of 1 min to 15 min in CH2Cl2 plasma.

4. CONCLUSIONS

PP film treated in CH2Cl2 plasma induced hydrophilic and hydrophobic mod-ifications and improved the surface energy to 32 mJ/m2. CH2Cl2 was polymerizedin plasma and gave thin cross-linked network, which was verified by solubilitytest. On the other hand, film treated in CCl4 and CHCl3 plasma gave greater hy-drophilic modifications. CH2Cl2 was found to incorporate chloride radicals mainly

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at CH (3◦) carbon atom. Modification of PP film in CH2Cl2 plasma showed gooddurability and bondability when compared with that in CCl4 and CHCl3 plasmas.

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