poly(acrylic acid) surface grafted polypropylene films ... · pdf file1. introduction owing to...

1. IntroductionOwing to its useful properties, easy workability andlow manufacturing costs, polypropylene (PP) iswidely used for the preparation of films, fibers,plates, injection-molded as well as blow moldedparts [1]. Often, insufficient surface properties pre-clude its use in an application to which bulkmechanical properties may be well-suited. Forexample dyeability, printability, paintability, adhe-sion, biocompatibility, antifogging, and gas perme-ability of PP parts can be improved by surfacemodification [2–9].Efforts have been made to develop polymer modifi-cations processes which allow the surface proper-ties to be tailored to meet a specific requirementwhile retaining beneficial mechanical properties.Surface modification can be accomplished by

increasing the polarity introducing special chemicalfunctionalities like carboxylic groups, or by coatingthe surfaces with polar thin films, for example,poly(acrylic acid) (PAA). Surface grafting tech-niques including photo-initiated grafting [10–14],living radical grafting [15], ceric ion induced graft-ing [16], layer-by-layer ionic grafting [17] andplasma polymerization [18, 19] have proven to beeffective to graft poly-functional monomers orpolymers onto PP surfaces.Among the methods to modify polymers, radicalphoto-grafting polymerisation appears as an attrac-tive method to impart a variety of functional groupsto a polymer [20, 21]. The advantages of photo-grafting over other available methods are: i) easyand controllable introduction of chains, ii) highdensity and exact localization of chains onto the

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*Corresponding author, e-mail: [email protected]© BME-PT and GTE

Poly(acrylic acid) surface grafted polypropylene films:Near surface and bulk mechanical response

L. A. Fasce1, V. Costamagna2, V. Pettarin1, M. Strumia2, P. M. Frontini1*

1Universidad Nacional de Mar del Plata, Instituto de Investigaciones en Ciencia y Tecnología de Materiales, INTEMA,J.B. Justo 4302 - B7608 FDQ - Mar del Plata, Argentina

2Departamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Argentina

Received 19 August 2008; accepted in revised form 20 September 2008

Abstract. Radical photo-grafting polymerization constitutes a promising technique for introducing functional groups ontosurfaces of polypropylene films. According to their final use, surface grafting should be done without affecting overallmechanical properties. In this work the tensile drawing, fracture and biaxial impact response of biaxially orientedpolypropylene commercial films grafted with poly(acrylic acid) (PAA) were investigated in terms of film orientation andsurface modification. The variations of surface roughness, elastic modulus, hardness and resistance to permanent deforma-tion induced by the chemical treatment were assessed by depth sensing indentation. As a consequence of chemical modifi-cation the optical, transport and wettability properties of the films were successfully varied. The introduced chainsgenerated a PAA-grafted layer, which is stiffer and harder than the neat polypropylene surface. Regardless of the surfacechanges, it was proven that this kind of grafting procedure does not detriment bulk mechanical properties of the PP film.

Keywords: mechanical properties, depth sensing indentation, surface photo-grafting, polypropylene, films

eXPRESS Polymer Letters Vol.2, No.11 (2008) 779–790Available online at www.expresspolymlett.comDOI: 10.3144/expresspolymlett.2008.91

surface, iii) covalent attachment of chains whichavoids delamination and assures long-term chemi-cal stability of introduced chains, in contrast tophysically attached layers [1].Low energy radiation, such as UV light, is fre-quently used to initiate surface modification. Theability of graft co-polymerization to quickly changethe surface properties in a tunable fashion allowsfor the development of a product that can beadapted on-demand to a particular application.Modification of PP films through grafting at rela-tively low amounts were previously demonstratedto be an effective approach to functionalize PP pro-viding a defined, stable, organic foundation forsubsequent surface modification of various types[22]. Costamagna et al. were able to modify exclu-sively the surface of bi-oriented PP films by graft-ing reaction of poly(acrylic acid) (PAA) [23]. Theyshowed that values of the permeability coefficientof oxygen, nitrogen, carbon dioxide, carbonmonoxide, argon, methane, ethane, ethylene, andpropane across the grafted films undergo a markeddrop. Their morphology investigations by AtomicForce Microscopy (AFM) suggested the initial for-mation of PAA brushes attached to the PP surfaceat short time of reaction followed by the generationof a network by cross reaction between graftedgrowing chains, brushes entanglements and stronghydrogen bonds [24]. The improvement in barrierproperties was attributed to the formation of such arigid PAA layer onto the PP surface [24].Despite it has been claimed that surface graftingtheoretically occurs without detriment of the bulkmechanical properties of the polymer [25], undesir-able changes in the mechanical performance of dif-ferent polymeric films modified by photo-graftingtechniques have been reported (see for example in[22, 26–28]). To a large extent, the tendency andlevel of concomitant changes in the material prop-erties depend on whether crosslinking or degrada-tion dominates during the irradiation of thepolymer [29, 30]. UV irradiation may induce theformation of free radicals, which promotescrosslinking. This in turn minimizes the anisotropiccharacter of the polymer leading to an increaseddegree of molecular disorder. If this occurs, bulkmechanical properties of a polymeric film would beimpaired since they are highly dependent on molec-ular orientation.

The present paper is focused on determining themechanical properties of a bi-oriented PP filmbefore and after surface photo-grafting with PAAchains. To this aim the bulk mechanical behaviorwas characterized by evaluating uniaxial tensiledeformation, quasi-static fracture and dart impactproperties. In addition depth sensing indentationexperiments [31–36] were carried out in order togain a deeper insight into differences in surfacemechanical characteristics of PAA-grafted and neatfilms.

2. Experimental section

2.1. Film grafting procedure

A commercial polypropylene film (PP) (Conver-flex S.A., Buenos Aires, Argentina), was surfacemodified by radical grafting polymerisation initi-ated by UV light. The appropriate reaction condi-tions were previously investigated [23]. Theprocedure was as follows: Acrylic acid was used asgrafting co-monomer. The initiator benzophenone(Fluka AG, Buchs, Switzerland) was dissolved inthe acrylic acid (BASF, New Jersey, USA) and dis-tilled water was added. The solution in contact withthe PP film was enclosed into a photo-reactor andirradiated with UV light (medium pressure UVlamp Engenlhard-Hanovia, Slough, England) undernitrogen atmosphere at room temperature for10 min. The grafted film was extensively washedwith a NaOH solution (pH = 8) in order to removetraces of un-reacted monomer and formedhomopolymer as well as to extract rests of the ini-tiator.Before characterization, surface modified filmswere left in a pH 8 solution for 24 h in order to dis-rupt the hydrogen bonds.The thickness (t) of the neat PP film determinedwith a micrometer was 34±1 μm. The differencebetween the thickness values of the neat and thePA-grafted films was determined with a Mitutoyocoordinate measuring machine. Measurementsmade on at least ten different locations for eachsample yielded an averaged value of 4.5 μm for thePAA layer.Hereafter, unmodified films and surface modifiedfilms are designated as PP and smPP films, respec-tively.

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Fasce et al. – eXPRESS Polymer Letters Vol.2, No.11 (2008) 779–790

2.2. Physical and chemical characterization

2.2.1. Chemical structure

Fourier transform infrared spectroscopy (FTIR)measurements of the surface modified films wereobtained from a Nicolet 5 SXC FTIR spectropho-tometer operating in the transmission mode in orderto characterize the chemical modification. Eachspectrum was collected by cumulating 32 scans at aresolution of 8 cm–1.Gravimetric measurements of films before andafter grafting reactions were carried out to deter-mine the grafting percentage by means of the Equa-tion (1) [37, 38]:

(1)

The weight of grafted layer was also estimated byvolumetric titration of the COOH groups graftedonto the PP surface with a 0.01 M NaOH solution.

2.2.2. Surface feature, topography androughness

Scanning electron microscopy (SEM) observationswere made by using a Jeol JSM-6460LV scanningelectron microscope operating at an acceleratedvoltage of 15 kV. Sample surfaces were coatedwith a thin layer of gold.Scanning probe microscopy (SPM) analyses weremade using the SPM module of the TriboindenterHysitron. Sample surfaces were explored with animaging type tip at a scan rate of 0.5 Hz and a set-point force of 1 μN. The arithmetic mean surfaceroughness (Ra) was calculated from the SPMimages as the average of the absolute values of thesurface height deviations measured from the meanplane within a box of 400 μm2.

2.2.3. Wettability

Static contact angle measurements were performedby the sessile drop method at room temperature.The ‘NIH’ image software was used with a MV-50camera and a magnification of 6×. The contactangles of water droplets on PP and surface modi-fied PP surfaces were used to determine the change

in hydrophilicity of the film surface. Five measure-ments were recorded and an average contact anglewas calculated for each material [39].

2.2.4. Transparency

Light transmittance measurements were carried outby using a Leica DMLB microscope incorporatinga photo-detector in its optical path. The light trans-mittance signal (IT) measured for PP and smPPsamples were used to estimate the loss of filmtransparency according to the following definition(Equation (2)):

(2)

2.2.5. Microstructure and orientation

The main factors that determine the mechanicalproperties are the molecular orientation and thecrystalline level.The anisotropy developed during the planar defor-mation that films underwent during blowing wasnot identifiable from the simple naked eye inspec-tion. The main film orientation directions wereidentified from the birefringence using polarizingmicroscopy. Machine (MD) and transverse (TD)directions of biaxially oriented films coincide withthe two orthogonal axes defined by the four posi-tions in which light extinguishes when the film isrotated under cross polarizers [40].The crystalline fraction (Xc) was determine usingthermal analysis. DSC measurements were per-formed in a Perkin Elmer Pyris 1 equipment at10°C/min. For the sake of simplicity, samples weredirectly put in the DSC capsules. In Xc calculations,the enthalpy of fusion of 100% crystalline PP wastaken as 209 J/g [41].

2.3. Near Surface mechanical properties

Near surface mechanical properties of the filmswere investigated up to the micron scale using aHysitron Triboindenter® testing system [http://www.hysitron.com]. While a diamond indenterdrives into the film surface, the system is able tocollect the applied force and displacement data.Samples were indented with a conospherical dia-

100·)PP(

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mond tip of nominal radius of curvature equal to1 μm. Indentations were performed using trape-zoidal loading curves in order to minimize the‘nose effect’ caused by the viscoelastic nature ofpolymers [33]. Maximum applied loads were var-ied between 4000 and 9000 μN at a constant load-ing rate of 250 μN/s. The holding time betweenloading and unloading stages was 20 s. Indenta-tions were load-controlled and repeated at least20 times for every load on different locations of thefilms surfaces.The Oliver-Pharr method [42], which proposes theestimation of the slope of the unloading curve byfirst fitting the entire unloading data, was employedto determine reduced elastic modulus (Er) and thehardness (H) of the materials. The reduced elasticmodulus is related to the elastic modulus of thesample (E) and the contact stiffness (S) by Equa-tions (3) and (4):

(3)

(4)

where ν is the Poisson’s ratio and subscript idenotes the indenter material. Amax is the surfacecontact area at the maximum displacement. Thecontact stiffness (S) is the slope of the unloadingcurve taken as the first derivative in the maximumdepth of a fitted power law function of the unload-ing segment of the curve.The material hardness (H) is defined as the maxi-mum load, Pmax, divided by the projected area ofthe indentation under this load, see Equation (5):

(5)

The surface contact area versus contact depth func-tion of the tip A(hc) was empirically determined byperforming multiple indentations on a polycarbon-ate block of known Er as suggested in [33]. Theobtained calibrated area function was used in thecomputation of Er and H values (Equations (3) and(4)).

2.4. Bulk mechanical propertiesBoth PP and surface modified PP films were testedalong with (MD) and transversely to the machinedirection (TD) at room temperature. Experimentswere carried out in an Instron 4467 universal test-ing machine at a crosshead speed of 10 mm/min.Uniaxial tensile tests were performed on dumbbell-shaped specimens, which were die-cut from thefilms. Longitudinal strain was measured with theaid of a video extensometer in which the deforma-tion of the material is assessed from the current dis-tortion of a close array of two ink dots printed ontothe samples prior to deformation [43]. Yield stress(σy) was determined as the stress where two tan-gents to the initial and final parts of the load-elon-gation curve intersect [44]. Elastic modulus wascalculated from the stress–strain curves. Propertieswere averaged from at least five tests.Fracture experiments were performed using Deeplydouble edge-notched samples (DENT, Mode I) pre-pared by cutting the sheets into rectangularcoupons of total length Zt 80 mm (with a lengthbetween the grips of Z = 60 mm) and a width W =40 mm. A special device was used to perform thetwo notches, allowing them to be perfectly aligned.Notches were then sharpened by pushing throughthe material a fresh razor blade into the tip to adepth of 1 mm. The length of each notch a was10 mm.Fracture toughness was determined by using theJ-integral approachl, as defined by Equation (6):

(6)

with Utot the overall fracture energy i.e. the totalarea under the load-deflection curve, B the thick-ness of tested specimens, and η a geometry factorexpressed as Equation (7) [45]:

(7)

In order to complete the bulk characterization, dartimpact experiments were carried out since it hasbeen shown that such tests are very sensitive to sur-face modifications in polymeric systems [46]. Dartimpact experiments were conducted on a FractovisCeast falling weight type machine at room temper-ature at 3 m/s, using an instrumented high-speed

32

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dart with hemispherical end onto disk specimens of80 mm of diameter. The modified surface wasplaced opposite to the impact tip in order to inducetension loads on the modified surface. The thick-ness related energy (U/t) was determined bynumerical integration of the experimental load-dis-placement data. Disc maximum strength (σd) wascomputed from the recorded traces through the fol-lowing relationship (see Equation (8)) [47]:

(8)

3. Results and discussion

3.1. Surface modification

FTIR spectra of the films are shown in Figure 1, inwhich an intense –C=O absorption (1718 cm–1)characteristic of polyacrylic acid is seen in thecurve of the surface modified PP film.The grafting percentage obtained by both tech-niques (gravimetric measurements and volumetrictitration) was 12.9% (Equation (1)).PP and surface modified PP films displayed melt-ing endotherms of very similar shape: besides themain melting peak at 168°C a small peak appearedat about 160°C (Figure 2). During heating, the dis-orientation process that occurs in crystalline ori-ented polymers is often accompanied by dimen-sional shrinkage. For isotactic polypropylene, Sunand Magill [48] demonstrated that shrinkage andmelting take place concomitantly producing multi-ple melting peaks. Since any slight difference in Xc

values of PP and surface modified PP samples iswithin experimental error, PP crystalline fraction,Xc, is about 40%. Based on the DSC results, it

appears that the grafting process did not affectsupramolecular structure of the neat PP film.SEM and SPM images of both films surfaces areshown in Figures 3 and 4, respectively. It is clear

2

max5.2t

Pd =σ

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Figure 1. FTIR spectra of PP and surface modified PPfilms

Figure 2. DSC thermograms showing the melting behav-ior of PP and surface modified PP films

Figure 3. SEM microgrpahs showing the topography ofPP (a) and surface modified PP (b) films

that the surface topography of PP and grafted PPfilms is significantly different. The PP film exhibitsa smooth surface pattern free of defects and a sur-face roughness, Ra, equal to 21 nm. On the con-trary, the surface of modified film is rougher (Ra =357 nm) showing a pin-hole free, textured densepattern, composed of a number of clusters distrib-uted along the substrate in a ‘patchy’ fashion. Thepatches join together to form a continuous struc-ture.The transmitted light intensity measured throughthe surface modified PP film was 16% lower thanthe one measured through the neat PP film. Theadditional scattering arises from the enlarged sur-face roughness displayed by grafted films sincecrystalline fraction was not altered.Figure 5 shows images of a water droplet on thesurface of PP and grafted PP films. The liquid con-tact angles on both films were markedly different:79.7±1.1° for the PP film surface and 18.6±3.5° forthe PAA grafted surface. The smaller contact angleof water droplet on the surface grafted film indi-cates a stronger hydrophilicity nature on the surfacegiven for grafting modification. This increase in thehydrophilicity and wettability is due to the presenceof a hydrophilic grafted chain of the PAA and it ismarkedly increased by the electrostatic repulsion

between the negatively charged carboxylic groups(–COO–) present in the grafted chains.

3.2. Near surface mechanical properties

Depth sensing indentation experiments have beenperformed to characterize the mechanical proper-ties of the near surface layer of the neat and graftedPP films. Typical curves are shown in Figure 6while parameters obtained from indentation testsare shown in Figure 7 as a function of maximumapplied load. The measured load-depth curves forthe grafted films appeared widely scattered (Fig-ure 6b) in contrast to the neat film traces. Reducedelastic modulus (Figure 7a) as well as hardness(Figure 7b) of PP and surface modified PP films arenot dependent on penetration depth, which actuallyincreased as the maximum load applied increased(Figure 7c).The values of nanomechanical parameters of neatPP films are in coincidence with the ones reportedin literature for other PP grades [49–51].Er and H values of the modified film correspond tothe PAA grafted layer since the indentation depthsachieved during the experiments were lower thanthe thickness of the grafted layer (Figures 7a to 7c).These values appeared widely scattered, but theywere always larger than their homologues corre-sponding to neat films. The scattering arises from

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Figure 4. Typical SPM images of PP (a) and surface mod-ified PP (b) film surfaces used in roughnessmeasurements

Figure 5. Pictures of typical sessile droplets found in con-tact angle experiments for PP (a) and surfacemodified PP (b) films

the large surface roughness of the PAA layer,which was a consequence of the grafting process(see section 3.1.). It is well known that if the sur-face roughness is too large the actual penetrationdepth of the indenter may be smaller than the

sensed depth [52]. This yields large errors in thecalculated contact area values (Amax), which gener-ates a wide scattering in the parameters calculatedthrough Equations (3) and (4) from multipleindents in a material. Note that as H is proportional

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Figure 6. Typical load and depth curves obtained in indentation experiments for a) PP samples at different maximumapplied loads and b) smPP samples at the same load

Figure 7. Reduced elastic modulus (a), universal hardness (b), contact depth (c) and P/S2 (d) parameters arisen fromnanoindentation experiments

to 1/Am and Er is proportional to 1/Amax1/2, scatter-

ing in hardness values is wider than in elastic mod-ulus values.Joslin and Oliver [52] proposed an alternativemethod to analyze nanoindentation data for sam-ples that are less than ideal surfaces. By combiningEquations (4) and (5), it emerges that, according toEquation (9):

(9)

The ratio of the maximum load to the stiffnesssquared parameter, Pmax/S2, is a mechanical prop-erty that describes material’s resistance to plasticdeformation. When the indenter is forced a certaindistance beyond the initial contact point, the inter-ference between the indenter and the specimen isaccommodated in two ways: elastic deformationand plastic (or permanent) deformation. For a givenhardness, the lower the modulus, the greater theelastic accommodation and the smaller the perma-nent damage to the specimen when the indenter isremoved. On the contrary, for a given modulus, ifthe hardness is increased, the plastic strain isreduced. The Pmax/S2 ratio is a directly measurableexperimental parameter that is independent of thecontact area provided the hardness and elastic mod-ulus do not vary with depth.Indentation data were re-analyzed following Equa-tion (9) and plotted in Figure 7d. The scattering inthe Pmax/S2 parameter is now negligible, confirmingthat the sole source of scattering is the large surfaceroughness of the surface modified PP film.H/Er

2 data determined for materials displaying dif-ferent mechanical characteristics are plottedtogether versus the Pmax/S2 parameter in Figure 8.Note that the resistance to plastic deformation ofthe PAA layer falls close to the values displayed byamorphous glassy polymers, specially polymethyl-methacrylate (PMMA). This result is consistentwith the chemical similarity between PAA andPMMA and the extra stiffness usually shown bypolymer brushes subjected to neighboring chainsconfinement [53].From depth sensing indentation experiments itemerges that the graft amorphous PAA layer isstiffer, harder and more resistant to mechanicalcompression and indentation than the neat surfacethanks to its glassy nature.

3.3. Bulk mechanical properties and fracturebehavior

Typical uniaxial tensile stress-strain curves areshown in Figure 9 while the derived uniaxial tensileparameters are listed in Table 1. PP and surfacemodified PP films showed the same behavior underuniaxial tensile deformation. In each curve thestress increases continuously with strain, showingonly a change of slope near the yield point. Asexpected, films show themselves to be highlyanisotropic displaying large differences in Young’smodulus and yield point between TD and MDdirections. The recognition that molecular orienta-tion produces important variations in the mechani-cal properties of polymers has long been estab-lished [54].

22

max

rE

H

S

P ∝

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Figure 8. H/Er2 parameter for several materials tested by

depth sensing indentation: Al (aluminum), Qz(fused Quartz), PC (polycarbonate), PMMA(polymethymethacrylate), UHMWPE (ultra highmolecular weight polyethylene)

Figure 9. Typical stress-strain curves of PP and surfacemodified PP samples under uniaxial drawing

SEM inspection of stretched surface modified PPsamples (Figure 10) revealed that during deforma-tion disruption of the clustered structure of thePAA-grafted layer occurred without transferringcracks to the substrate suggesting weak interactionsbetween chains.Typical load-displacement traces displayed byDENT samples are shown in Figure 11. Again, thefeature of the curves depended on stretching direc-tion but not on surface modification. All samplesexhibited initially stable crack propagation fol-lowed by sudden unstable crack growth up to finalfracture. The feature of the fracture surfaces in thestable propagation regime is illustrated in Fig-ure 12. No striking differences are evidenced in

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Figure 10. SEM micrograph of uniaxial tensile deformedPP (a) and surface modified PP (b) samples.Red arrows indicate drawing direction.

Figure 11. Typical normalized load vs. displacementcurves of PP (a) and surface modified PP(b) DENT samples

Table 1. Summary of bulk mechanical properties

Test Uniaxial tensile Fracture mechanics Dart impactParameter E [GPa] σσy [MPa] Jc [N/mm] U/t [J/mm] σσd [GPa]

PPMD 3.6 ± 0.8 35.6 ± 0.7 40.9 ± 1.1

32.1 ± 6.4 115.6 ± 17.9TD 1.1 ± 0.2 24.2 ± 0.7 20.3 ± 1.9

smPPMD 3.7 ± 0.2 34.3 ± 1.7 41.7 ± 2.8

30.6 ± 4.8 113.9 ± 23.9TD 1.2 ± 0.3 22.6 ± 2.6 22.6 ± 2.0

Figure 12. SEM micrographs of fracture surfaces of PP (a)and surface modified PP (b) DENT samplestested in MD

both fracture surfaces. Fracture toughness parame-ters, Jc, reported in Table 1, show that the resist-ance to unstable propagation is twice when thecrack propagates passing through the machine ori-entation direction (MD) than when it propagatesalong with it. The observed fracture behavior

reflects the imbalanced properties between themachine direction and the comparatively weaktransverse direction.Typical dart impact load-displacement curves areshown in Figure 13 while the derived dart impactproperties are reported in Table 1. TOM images ofthe dart impacted region of PP and smPP speci-mens are shown in Figure 14. Both films behave inthe same way exhibiting a semi-brittle fracture pat-tern as judged from the load deflection recordscharacteristic (Figure 13) and the feature of the bro-ken samples (Figure 14). Load–displacement curvesdropped to zero instantaneously upon reaching themaximum load. A punch-like fractured surfacewith a punch-out-cap still attached to the specimenand two radial main cracks responsible for the finalfailure of the specimen are observed in the brokensamples.Both the fracture patterns and the derived parame-ters point out that the surface modification has notinfluenced the biaxial impact response of the neatPP film.

4. Conclusions

Through this paper a complete investigation of themechanical response of surface modified polypropy-lene films by photo-grafting with acrylic acid wascarried out. The principal findings are:– the degree of grafting induced in the films was

high enough to alter the topographical patternand physico-chemical properties of the surface,such as roughness, wettability, hidrophilicity andtransparency of PP films. Besides, the presenceof specific functional groups offers the opportu-nity to use them as anchorage sites for furtherderivatization. They could be used to bond cova-lently specific organic molecules (dye, antimi-crobial or antifungal drugs, etc.)

– the near surface indentation behaviour displayedby the grafted surface indicate that a glassy layerof PAA, which can be viewed as a sort of lami-nate was formed onto the PP surface.

– uniaxial deformation properties, fracture behav-ior and biaxial impact performance of the neatfilms were retained after chemical modification.It is evident that the mechanical response of sur-face modified film is governed only by PP resist-ance to deformation, thus, confirming thehypothesis that the PAA chains which were

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Figure 13. Typical load-displacement curves of PP (a) andsurface modified PP (b) samples under biaxialimpact deformation

Figure 14. Failure feature of PP (a) and surface modifiedPP (b) falling dart impacted specimens

strongly attached to the PP film, only interactamong each other through the formation of phys-ical entanglements and secondary bonds withoutconstraining the PP surface.

– the beneficial original degree of molecular orien-tation of neat films was not altered after photo-grafting. This means that the UV-irradiationinvolved in the grafting procedure induces negli-gible cross-linking and degradation in the neatPP.

AcknowledgementsThis work was financial supported by CONICET projectnumber PIP 6253 and by ANPyCT Project number PICT2004 N°25530.

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