(Ethylene-Propylene) Copolymer Blends - American

Chapter 18 PoIypropylene/(Ethylene-Propylene) Copolymer Blends Surface Morphology and Elasticity as Measured by A F M and FMM

B. Nysten, C. Meerman, and E. Tomasetti

Downloaded by COLUMBIA UNIV on June 15, 2013 | http://pubs.acs.org Publication Date: December 10, 1999 | doi: 10.1021/bk-2000-0741.ch018

Unité de chimie et de physique des hauts polymères, Université catholique de Louvain, place Croix du Sud, 1, B-1348 Louvain-la-Neuve, Belgium

Quantitative and qualitative methods were developed to measure the surface mechanical properties of polymers by atomic force microscopies. They were used to study the effects of molding processes and of viscosity on the surface morphology of polypropylene / (ethylene-propylene) copolymer blends (PP/EP). On compression-molded "physical blends", EP nodules are present at the outermost surface while, on injection-molded "reactor blends", they are covered by a PP layer. Resins with high viscosity ratio between EP and PP present heterogeneous surface elastic properties corresponding to the dispersion of spherical EP nodules below the surface. The low viscosity ratio resins have homogeneous surface elastic properties comparable to those measured above EP nodules on high viscosity ratio resins. This is compatible with a fine dispersion of plate-like shaped EP nodules below the surface

The surface content and the distribution of (ethylene-propylene) copolymer (EP) in toughened polypropylene (PP) resins (PP/EP) have important impact on a lot of properties such as gloss, paint adhesion, hardness, ... These surface properties are more and more important in the multiple applications of these resins, for instance for paint adhesion in the automotive industry. It has already been shown that the introduction of EP in PP provides better paint adhesion but its role remains speculative (1-4). A major drawback to the understanding of the EP influence on PP/EP surface properties is the lack of knowledge concerning the blends surface morphology (EP content, EP lateral distribution, ...). This is essentially due to the similar chemical composition of both polymers that prevents surface analysis by classical chemical surface spectroscopies.

304

© 2000 American Chemical Society In Microstructure and Microtribology of Polymer Surfaces; Tsukruk, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


305

It was previously shown that atomic force microscopy, AFM, and force modulation microscopy, FMM, can bring novel informations concerning the elastic and the viscoelastic properties of PP/EP resins at the microscopic level. They enabled the mapping of EP rubbery nodules distribution and the explanation of the macroscopic behavior of these resins (e.g. differences in the impact resistance) (5). Moreover, recent works showed that the use of AFM techniques (force-curves and force-modulation measurement, FMM cartography) can be used to characterize the surface elastic properties of PP/EP blends (6). Especially, these studies demonstrated that FMM can map the surface and the subsurface distribution of EP nodules. In the present paper, the techniques used to measure the surface elastic properties will be briefly described. Then, the application of these techniques to the study of PP/EP surfaces will be presented. First, the surface distribution and morphology of EP nodules at the surface of PP/EP compression-molded "physical blends'* and of injection-molded "reactor blends" will be compared. Second, the effect of EP vs PP viscosity ratio on the surface distribution and morphology of EP in injection-molded PP/EP will be studied. Theoretical Force-Curves Measurements. In force curve measurements, a vertical displacement of the sample, z, is imposed and the subsequent tip displacement, d, is measured. The tip-sample interaction force, F, is deduced by means of the Hooke's relation, F = -k d, where k is the cantilever stiffness. Force curves are generally divided into different regions (7). If the part where the electrostatic repulsion forces are dominant is only considered, with silicon tips much stiffer than polymers, tips penetrate the sample surface and an indentation depth, 5, equal to ζ - d, can be measured. The lower the sample elastic modulus, the greater will be the indentation depth. By using the Hertz mechanical model adapted to the geometry of the tip-sample system (8,9) surface elastic modulus could be deduced from the following equations corresponding respectively to a spherical, a paraboloid and a conical tip: c

c

(1.1) (1.2) (1.3)

z

where Κ = £ / ( l - v j is the surface elastic modulus, Ε is the Young's modulus and ν the Poisson's ratio of the material. R is the tip radius of curvature, Κ the coefficient describing the paraboloid tip, and a, the opening angle of a conical tip. When the geometry and the size of the tip is determined, it is possible to quantitatively measure the surface modulus from the force-indentation curve using the appropriate model according to the ratio between the tip dimensions and the maximum indentation depth reached during the force-curve experiment.

In Microstructure and Microtribology of Polymer Surfaces; Tsukruk, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

306 The elastic model was modified by taking into account the adhesion force acting between the tip and the surface using the Johnson, Kendall and Roberts theory (JKR model) (10). This leads to the following relations for the different tip geometries (spherical, paraboloid and conical):

(2.1)


(2.2) (2.3) where W dh is the work of adhesion that can be determined by measuring the pull-off force on the force-indentation curves (10). a

Force-Modulation. In force modulation, the sample and the tip are previously placed in contact with a interaction force kept constant (F ). A modulation of the sample vertical position is added to its contact equilibrium position and the subsequent modulation movement of the cantilever deflection is measured by means of a lock-in amplifier. The amplitude and the phase shift of the cantilever response are influenced by the viscoelastic properties of the surface. In FMM, the sample vertical position is modulated during the sample scanning and simultaneous acquisition of topographical and viscoelastic maps of the surface can be realized. The sample-tip-cantilever system can be modeled as a mechanical system with springs and dash-pots (11,12). Solving the motion equations of this model at low frequency (i.e. below the cantilever resonance frequency) and neglecting the damping constants (i.e. neglecting viscoelastic effects in polymers) leads to the following relation for the ratio between the sample modulation amplitude, z\, and the tip response amplitude, d\ also called the dynamic elastic response: 0

%

Z\

(3)

k +k t

c

In this relation, k is the cantilever stiffness and is the tip-surface effective stiffness given by dF/dS . From the relations (1.1 to 1.3), it can be seen that depends on the static contact force, the tip geometry and the surface elastic modulus. Knowing the cantilever stiffness, the static contact force and the tip geometry and dimensions, it is thus theoretically possible to determine the surface elastic modulus from the dynamic response. c


307 Experimental Materials, For the calibration of force-curves and force-modulation measurements, experiments were performed on various standard homopolymers (polycarbonate, PP, polyethylene and EP) with Young's modulus ranging between 10 and 3000 MPa. Plates (50x30x2 mm ) of these polymers were compression-molded on polyimide (Kapton, Du Pont de Nemours) during 3 min under a pressure of 2 MPa. For the measurement of the bulk elastic modulus, rods (50x5x2 mm ) were cut in these plates. For AFM experiments, small sample pieces were fixed with cyanoacrylate glue (Ara glue, Sodiema) on AFM sample holders. The main characteristics of the PP/EP resins studied here are listed in table I. The PP/EP "physical blend" (PPEP1) was obtained by mixing pellets of PP (LY from Shell, E= 1500 MPa) and of EP (Vistalon 805 from Exxon, Ε = 20 MPa) in a Brabender mixer at 60 rpm and 180°C during 5 min. The materials was ground in liquid nitrogen and then compression-molded on polyimide (Kapton, Du Pont de Nemours) during 2.5 min at 220°C under a pressure of 450 kPa. The PP/EP "reactor blend" (PPEP2) (Hifax SP179) was supplied by Montell (Ferrara, Italy). Plates of 10 χ 15 cm were injected with a DK300T Codim by Renault (Rueil-Malmaison, France). The material at a temperature of 270°C was injected in a mould at 30°C with an injection speed of 20 mm sec-1 and a holding pressure of 270 bars. 3


3

2

Table I: Main characteristics of the resin used in the present study; elastomer content, resin viscosity in the melt (Melt Flow Index) and viscosity ratio between the xylene soluble and the xylene insoluble fractions. Resin MFI Type EP wt.% (g/10 min) PPEP1 Physical blend 20.0 PPEP2 Reactor blend 23.0 3.2 PPEP3 Reactor blend 4.7 1.63 13.8 PPEP4 Reactor blend 12.1 5.2 0.76

The resins used to study the influence of EP/PP viscosity ratio were provided by SOLVAY. Samples, PPEP3 and PPEP4, are "reactor blends" produced in gasphase by a two-stage polymerization process. They differ by the viscosity ratio between the EP and the PP phases which corresponds to the ratio of the solution intrinsic viscosities of the xylene-soluble fraction (mainly EP) and the xyleneinsoluble fraction (mainly crystalline PP). Disks of these resins were injection molded and analyzed by TEM and by AFM and FMM. DMA Measurements. Elastic tensile modulus, E\ of the standard polymers was measured by dynamic mechanical analysis, DMA, on a Rheometries RSAII DMTA apparatus. Measurements were done at 1 Hz. The deformation amplitude was limited to 0.02% for the stiffer polymers and to 0.1% for the softer ones.


308 AFM Analyses. Force curves, local force modulation, AFM and FMM images were realized in air with an Autoprobe CP from Park Scientific Instruments (Sunnyvale, CA) using a 100 pm scanner. The cantilevers were triangular (180 pm length, 25 μπι wide and 2 pm-thick) silicon Ultralevers (Park Scientific Instruments) with k ranging between 10 and 20 Ν nr . The apex geometry and dimensions of the tips were determined by high resolution FEG-SEM. Measured radius of curvature, R, ranged between 10 and 30 nm and cone opening angles, a, varied between 10 and 20°. The cantilevers stiffness was determined within 10% by the measurement of the free resonance frequency of the cantilever in air (13). The conversion factor of the voltage measured on the photodetectors into tip displacement was calibrated by performing a force-curve measurement on a rigid sample (silicon) and by assuming that, in this case, the tip displacement is equal to the sample displacement. This calibration procedure was repeated each time the cantilever was changed or the laser beam refocussed on the cantilever. For force modulation measurements and imaging, the sample vertical position was modulated by adding a sinusoidal signal to the z-voltage applied to the piezoelectric scanner. Unless specified otherwise, the modulation amplitude, z\, was equal to 7 Â and the frequency was equal to 2 kHz, i.e. at least two order of magnitude lower than the cantilevers resonance frequency. The deflection signal of the cantilever, du was measured with a dual phase lock-in amplifier (EG&G Princeton Applied Research, Model 5210). The static force, F , varied between 10 and 200 nN. The cut-off frequency of the feedback loop was maintained below 300 Hz. For AFM and FMM imaging, the scanning frequency was equal to 0.4 lines sec . The fast-scan direction was always perpendicular to the cantilever long axis. To measure force-curves or to perform quantitative measurements of the dynamic elastic response in force modulation the sample raster scan was stopped and the tip was positioned at various location at the sample surface. c


1

0

-1

T E M Analyses. For the TEM analyses, the samples were stained with RuCl .xH 0 in NaClO. 90-nm-thick sections were then cut at low temperature (-40°C). For both resins, transverse sections parallel to the injection direction were studied. For the PPEP4 resin, additional sections were made perpendicular to the injection direction. 3

2

Results and Discussion Force-Curves and Force-Modulation Calibration. In figure l(a & b), typical forceindentation curves obtained respectively on a rigid (£"=610 MPa) and a soft (E = 27 MPa) polymer are presented. The elastic modulus derived from the analysis of the force-indentation curves is compared to the bulk elastic modulus measured by DMA in figure 1(c). For this analysis, the used tip geometry was adapted to the maximum indentation depth reached during the experiment, δ . For 5 « R, the spherical geometry was considered while, for ômax » R, the conical one was used. For intermediate values, the paraboloid model was used. Using the Hertz model, a good quantitative agreement is obtained for stiffer polymers (£>500MPa). For the softer polymers, the elastic modulus is systematically underestimated. The results of the analysis performed on polymers ηιαχ

nmx


309

with Young's modulus ranging between 10 and 500 MPa show that taking into account the adhesion force improves the quantitative measurement of the surface modulus on "soft" polymers (Fig. 1(c)). Surprisingly, on stiffer polymers, the results are better with the Hertz model than with the JKR model. This could be in agreement with the fact that this second model is better suited for soft materials (14).

•a » » » ι

150 Approach Retraction

ι » ι» ι ι I t ι ι

100 +

ι i

:

/·" 1/

I Downloaded by COLUMBIA UNIV on June 15, 2013 | http://pubs.acs.org Publication Date: December 10, 1999 | doi: 10.1021/bk-2000-0741.ch018

t

: : — Retraction

/

•50 4r

:

11111

-80

-40

0

40

Indentation (nm)

f

ι ι ι I ι ι ι

-400

-200

ι ι ι I ι ι ι I ι τ 0

200

:

400

Indentation (nm)

4

β

β

2

1000 DMA (MPa)

(c) Figure 1. Typical force-indentation curves obtained respectively (a) on a rigid polymer (£ = 610 MPa) and (b) on a soft one(£= 27 MPa). (c) Comparison between the surface Young's modulus deduced from the analysis of the forceindentation curves and the volume modulus measured by dynamic mechanical analysis, DMA, using the Hertz elastic model (•) and using the JKR model (A).

In figure 2, the results obtained in force modulation on various polymers are presented. As expected, the elastic response increases with the bulk modulus. Using Hertz models, the elastic modulus has been derived. For rigid polymers, the agreement between the surface modulus and the bulk modulus is quantitatively good. For softer polymers, a large discrepancy is observed, probably due to the fact that the adhesion force and the viscoelasticity are neglected. This could also be explained by


310


the well known fact that, in AFM experiments, the assumption of pure vertical indentation is not valid. Lateral deformation should normally be taken into account in the models (Mazeran, P.E.; Loubet, J.-L. Tribology Lett, in press). However, the results show that the dynamic response measured by force modulation is qualitatively correlated to the Young's modulus and may be used to evaluate and compare the elastic properties of polymer surfaces.

ε DMA (MPa)

ε A (MPa) DM

(a) (b) Figure 2. (a) Variation of the dynamic elastic response, d\lzu measured by local force modulation as a function of the bulk elastic modulus, (b) Comparison between the surface Young's modulus deduced from force modulation experiments and the volume modulus calculated using Hertz model.

Compression-Molding vs Injection-Molding. The FMM elastic images obtained on the surface of the PP/EP "physical blend" and in the volume and at the surface of the "reactor blend" are presented in figure 3. The volume analyses were performed on 5 pm thick cut realized at - 40°C. The FMM image reveals soft regions in dark (EP nodules) embedded in a rigid matrix in bright (PP). The results of local measurements of the dynamic response on the PP matrix and on the EP nodules are given in table Π.

Table II: Elastic response measured on the pure PP & EP, on the PPEP1 sample, in the volume and at the surface of PPEP2 resin. Dynamic elastic response PP EP Pure components 0.60 ±0.04 0.25 ±0.02 « Physical blend >• surface 0.60 ±0.04 0.25 ±0.01 « Reactor blend » volume 0.60 ±0.04 0.38 ±0.04 « Reactor blend »surface 0.60 ±0.04 0.43 ±0.09

These measurements reveal that the PP matrix and EP nodules of the "physical blend" surface have the same elastic properties as the surface of the respective pure


311


samples. This interpretation is confirmed by studying the histogram of the dynamic response (Fig. 4(a)) where two peaks are observed at 0.25 and 0.6. These ratios are very close to the values obtained by local measurements on EP and PP respectively (Tabl. Π). These results suggest that, in this case, EP is present at the outermost surface.

Figure 3. Elastic response FMM images obtained on the surface of the compression-molded "physical blend" PPEP1 (a) and in the volume (b) and at the surface (c) of the injection-molded "reactor blend" PPEP2. Gray scales are calibrated in d\lz\ units. For the injection-molded "reactor blend", comparison between figures 3(b) and 3(c) reveals that, in the bulk, a large contrast exists between the elastic response on the EP nodules and on the PP matrix while, at the surface, this contrast is weaker and nodules with various gray levels are observed. This qualitative analysis is confirmed by the quantitative local measurements of the άχίζχ ratio on the PP matrix and on the EP nodules in the bulk and at the surface (Tabl. Π). These measurements show that PP has the same elastic properties in the bulk and at the surface. They also suggest that surface EP nodules have higher stiffness values than bulk nodules. Finally, the


312


elastic response values measured on the surface nodules are more spread than on the bulk nodules. The histograms of the dynamic responses (Fig. 4(b & c)) confirm these observations.

d/z

f

Figure 4. Histograms of the dynamic elastic response corresponding to the FMM images (Fig. 3). (a) Surface of the PPEP1 sample, (b) Volume and (c) surface of the PPEP2 sample. In the bulk, a sharp peak corresponding to PP is observed close to 0.6 and a broad one between 0.15 and 0.4. This broad peak can be explained by the complex microstructure of EP nodules of "reactor blends" leading to inhomogeneous


313 mechanical properties (5). At the surface, a single peak appears close to 0.6 with a shoulder extending towards the low d\lz\ values. These observations can be explained by the presence of a surface PP skin with varying thickness covering the EP nodules close to the surface. The presence of the PP skin at the surface of injection molded PP/EP blends have been already proposed in the literature (3,15,16). It can be explain by the fact that the injection molding process induces high shear rate at the surface; the phase having the lower viscosity (PP) tends thus to go to the surface in order to minimize friction forces during molding process (17).


Effect of EP/PP Viscosity Ratio. TEM images obtained on the PPEP3 and PPEP4 samples are presented in figure 5.

Figure 5. TEM images obtained on sections parallel to the injection direction of the PPEP3 resin (a) and of the PPEP4 resin (b) and on sections perpendicular to the injection direction of the PPEP4 sample (c).

An increased deformation of the EP nodules in the injection direction with the decrease of the viscosity ratio is observed. For the PPEP3 resin (Fig. 5(a)), the EP nodules are almost spherical, even close to the surface, while, for the PPEP4 resin



314 (Fig. 5(b)), the EP nodules are elongated in the injection direction. The deformation is more important close to the surface where the shear rate during injection is more pronounced. A higher concentration of EP nodules near the surface also seems to appear when the viscosity ratio decreases. TEM images of sections perpendicular to the injection direction (Fig. 5(c)) reveal that the EP nodules below the surface are in fact plate-like shaped. Very few rod-like shaped EP nodules can be observed. These observations confirm that the EP vs PP viscosity ratio strongly influences the EP nodules morphology close to the surface. At high viscosity ratio, the nodules are almost not deformed. At low viscosity ratio, the high shear rate close to the mould surface strongly deforms and orients the nodules in the injection direction. The TEM observations also reveal that the outermost surface is composed of a more or less thin pure PP skin in all cases which confirms the hypothesis made for the PPEP2 sample. On the PPEP3 resin, the surface morphology appears slightly oriented in the injection direction (Fig. 6(a&b)). The EP nodules that appear in the FMM images are

Figure 6. AFM and FMM images obtained on the PPEP3 and the PPEP4 samples: PPEP3 topography (a) and FM elastic response (b), PPEP4 topography (c) and FM elastic response (d). The injection directions for both samples are indicated by the arrows.



315 almost not deformed. On the PPEP4 sample, the surface morphology is clearly oriented in the injection direction (Fig. 6(c&d)). However, few EP nodules or even not could be observed at or below the surface as revealed by the weak contrast on the FMM images. The measured values of the elastic modulus and of the dynamic elastic response measured at different spots on the surface of both samples are summarized in table ΙΠ. The value of the surface elastic modulus of the matrix is of the order of 100 MPa. Surprisingly, this value is one order of magnitude lower than what is expected for pure PP (approximately 1000 MPa). The very rapid crystallization conditions near the surface "freezing" both PP and EP amorphous phases in disorganized micro-domains could perhaps explain this observation. The calculated modulus for EP is around 25 MPa, i.e. almost one order of magnitude lower than that measured on the matrix. The surface elastic modulus of the low viscosity ratio resin is estimated to vary between 20 and 35 MPa, i.e. around the value measured above EP nodules on the high viscosity ratio resin. While for the high viscosity ratio, the elastic dynamic response is very different on the matrix and on the nodules, for the low viscosity ratio resin, it is almost the same everywhere on the surface and its value varies around that measured above the EP nodules of the PPEP3 resin. These results combined with those obtained by TEM show that the resin with the highest EP/PP viscosity ratio present heterogeneous surface elastic properties corresponding to the rough dispersion of almost spherical EP nodules below the surface. On the contrary, the low viscosity ratio resin presents homogeneous surface elastic properties at the resolution of our measurements (> 100 nm). The measured surface rigidity is comparable to that measured above EP nodules on the high viscosity ratio resin. This could be explained by the easier deformation of the EP nodules into platelets and by theirfinedispersion below the thin PP surface layer.

Table III: Elastic modulus and dynamic elastic response measured on the PP matrix and the EP nodules of the PPEP3 resin and on the surface of the PPEP4 sample. Elastic modulus Dynamic elastic (MPa) response PP matrix on PPEP3 100 0.71 EP nodules on PPEP3 25 0.32 Surface of PPEP4 20 to 35 0.21 to 0.42

Conclusions The use of a simple elastic model taking into account adhesion forces enables the quantitative measurement by AFM of the elastic modulus of polymer surfaces. Local force-modulation measurement also permits to qualitatively compare the local stiffness of polymers. These techniques associated to the imaging abilities of AFM



316 and FMM could be thus used as powerful tools to characterize the surface morphology (EP nodules shape and distribution) of PP/EP blends. The analysis of the effect of the molding process show that, at the surface of compression-molded "physical blends", the EP nodules are present at the outermost surface. On the contrary, FM measurements reveal that the surface EP nodules in injection-molded "reactor blends" are covered by a thin PP surface layer. The EP/PP viscosity ratio also strongly influences the surface morphology and elasticity of injection-molded blends. TEM and FMM show that the resin with the higher viscosity ratio presents heterogeneous surface elastic properties corresponding to the rough dispersion of almost spherical EP nodules below the surface PP skin. The low viscosity ratio resin has homogeneous surface elastic properties at the scale of the FMM resolution. The measured rigidity is comparable to that measured above EP nodules on the high viscosity ratio resin. This is compatible with a fine dispersion of plate-like shaped EP nodules below the thin surface PP layer. Acknowledgments B.N. is Research Associate of the Belgian National Funds for Scientific Research (F.N.R.S.). E.T. thanks the Belgian Funds for Industrial and Agricultural Research (F.R.I.A.) for its financial support. The authors also gratefully acknowledge the Department of Scientific Policy (P.A.I.), Montell, Renault and Solvay for their support. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Bonnerup, C.; Gatenholm, P. J. AdhesionSci.Technol. 1993, 7, 247 Clemens, R. J.; Batts, G. N.; Lawniczak, J. E.; Middleton, K. P.; Sass, C. Progress in Organic Coatings 1994, 24, 43 Prater, T. J.; Kaberline, S. L.; Holubka, J. W.; Ryntz, R. A. J. Coatings Technol. 1996, 68, 83 Ryntz, R. Α.; Xie, Q.; Ramamurthy, A. C. J. Coatings Technol. 1995, 67, 45 Nysten, B.; Legras, R.; Costa, J.-L. J. Appl. Phys. 1995, 78, 5953 Tomasetti, E.; Nysten, B., Legras, R. Nanotechnology 1998, 9, 305 Burnham, Ν. Α.; Colton, R. J.; Pollock, Η. M. Nanotechnology 1993, 4, 64 Hertz, H. J. Reine Angew. Math. 1882, 92, 156 Sneddon, I. N. Int. J. Eng. Sci. 1965, 3, 47 Johnson, K.L.;. Kendall, K.; Roberts, A.D. Proc. Roy. Soc. London A 1971, 324, 301 Burnham, Ν. Α.; Kulik, A. J.; Gremaud, G.; Gallo, P. J.; Oulevey, F. J. Vacuum Sci. Technol. Β 1996,14,794 Burnham, Ν. Α.; Gremaud, G.; Kulik, A. J.; Gallo, P. J.; Oulevey, F. J. Vacuum Sci. Technol. Β 1996,14,1308 Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64, 403 Dejarguin, B.V.; Müller, V.M. ; Toporov, Y.P. J. Coll. Interf. Sci. 1975, 53, 314 Ryntz, R. Α.; Ramamurthy, A. C.; Mihora, D. J. J. Coatings Technol. 1995, 67, 35 Ryntz, R. Α.; McNeight, Α.; Ford, A. Plastics Engineering 1996, 52, 35 Bhatia, Q. S.; Burell, M. C.; Chera, J. J. J. Appl. Polym. Sci. 1992, 46, 1915


(Ethylene-Propylene) Copolymer Blends - American

Recommend Documents