Post-fracture analyses of polyethylene—metal interfaces

Pergamon

Chemical

POST-FRACTURE

Department

Engineering

Science, Vol.49, No. 5, pp. 655 658, 1994 Copyri@ 0 1994 Ekvier Science Lid Printed in Great Britain. All rights reserved oco-2509/94 s6.00 + 0.w

ANALYSES OF POLYETHYLENE-METAL INTERFACES?

DAVIDE A. HILL* and MORTON M. DENN* of Chemical Engineering, University of California at Berkeley and Materials Science Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720, U.S.A. and

MIQUEL Q. SALMERON Materials Science Division, Lawrence. Berkeley Laboratory, (Receivedjor

publication

Berkeley, CA 94720, U.S.A.

18 August 1993)

Ahstrati-We have analyzed the surfaces created by peeling deuterated polyethylene films from copper, aluminum, and brass substrates. The polymer was analyzed for metal residues by X-ray photoelectron spectroscopy (XPS), while secondary-ion mass spectroscopy (SIMS) was used to detect deuterium on the metal. Optical microscopy ahvays indicated apparent adhesive (i.e. interfacial) failure for all polymer-metal samples, but SIMS consistently showed extensivecoverage by polymer at all exposed metal surfaces. XPS of the polymer revealed traces of metal on samples peeled from copper and brass, but not from aluminum. Ellipsometry and efectron microscopy confirmed the presence of a 60 f 20 8, polymer layer on the metal surface.

try to probe post-fracture surfaces of both the polymer and the metal following separation of polyethylene films from metal substrates. XPS is sensitive to traces of metal on tbe polymer, but it cannot discriminate between naturally adsorbed aliphatic hydrocarbons and polymer chains at the metal surface. Analysis of the metal surfaces for traces of polymer was implemented by negative-ion SIMS with the aid of deuterated polymer; the presence of deuterium at the surface enabled us to focus on the analysis of species with very low mass-to-charge ratio (i.e. H-, D-), where the number of possible combinations of mass and charge with the same ratio is minimal. When polymer was detected at the metal surface, ellipsometry and SEM were employed to characterize film thickness and morphology.

INTRODUCTION

Understanding the mechanism of fracture at a polymer-metal interface is of interest for a wide range of applications, from the synthesis of polymer-metal adhesives (Wu, 1982) to the onset of instabilities and wall slip in polymer melt extrusion [e.g. Denn (1992)]. Interfacial phenomena involving poIymers are complex, and they depend on both chain conformations and specific interaction potentials. The theoretical understanding of strong polymer-metal interfacial interactions has been recently reviewed by Chakraborty (1992). One of the core issues in applications is whether fracture between a polymer and a metal is purely adhesive (interfacial), cohesive (completely in the polymer phase), or a mixture of the two. Ramamurthy (1986) has demonstrated that the onset and severity of flow instabilities during melt extrusion of linear low-density polyethylene depends on the materials of construction of the die, and that the instability is associated with apparent melt slip at the wall. Hill et al. (1990) developed a theoretical framework for instability and wall slip based on adhesive failure at the interface, and they showed that data obtained from solid-state adhesion measurements could be combined with melt rheological data and a knowledge of interfacial energies to predict the critical stress and measured melt slip velocities. It is this work which motivates the present study. We have used X-ray photoemission spectroscopy secondary-ion mass spectroscopy (SIMS), (XIV, scanning-electron microscopy (SEM), and ellipsome-

MATERIALS

AND METHODS

Commercially available sheets of all metal samples were machined into disks of approximately 1 in. diameter (the size of an ultra-high vacuum, or UHV, sample holder), and both surfaces were then polished to a mirror-like appearance with diamond paste (0.4 pm), cleaned with acetone, and finally with methanol. Deuterated polyethylene (DPE) in the form of a fine powder with molecular weight of approximately 2 x lo4 but unknown molecular-weight distribution, and density of approximately 0.93 g/ml, was purchased from Polymer Laboratories. A relatively thick coating of DPE was formed on the metal substrate by first immersing the disk into a 5 (wt) percent solution of the polymer in toluene at 13O”C, and then slowly withdrawing the disk from the solution, after which solvent evaporation and polymer crystallization occurred rapidly. The samples were then annealed under vacuum at 130°C for about 12 h to ensure complete

‘Dedicated to Gianni Astarita on his 60th birthday. ‘Present address: Department of Chemical Engineering, University of Notre Dame, Notre Dame, IN 46556, U.S.A. ‘Author to whom correspondence should be addressed. 655

656

DAVIDE A. HILL

evaporation of the solvent. (In all cases, once the sample was in place, the vacuum oven was first flushed several times by alternating vacuum and nitrogen, and then it was evacuated and brought up to temperature.) The deuterated layers dried to a thickness of approximately 100 pm. Unipol linear-low-density polyethylene (LLDPE) of molecular weight 1.15 x 10’ was molded into flat sheets approximately 1 mm in thickness. Fracture specimens were prepared by sandwiching the DPEcoated disks between two “backing” layers of LLDPE in a spring-loaded mold under vacuum at 160°C for about one day; the sample was then allowed to cool slowly to room temperature under vacuum for about six hours. The final thickness of the composite polymer layer adhering to the metal substrate was about 250 pm. The films were peeled rapidly in air immediately prior to analysis in UHV. The XPS analysis was performed on a PHI 5300 instrument equipped with Mg and Al X-ray sources. The spectra reported here were obtained with the Mg source. SIMS was performed on a PHI 660; this instrument could be used also for scanning Auger microscopy and for secondary electron imaging (SEM). Depth profiling and SIMS were performed using Ar ions. The ellipsometer was fabricated at the Lawrence Berkeley Laboratory; an earlier version of the instrument is described in Muller and Farmer (1984). SIMSEXPERIMENTS A positive-ion SIMS spectrum of a brass substrate fractured from DPE is shown in Fig. 1. The strong peak at mass/charge (m/e) = 2 cannot be attributed exclusively to D+, since Hi is also possible. The negative spectrum, by contrast, can confirm the presence of deuterium, since the ion H; is highly unstable and would quickly dissociate before detection. Figure 2 shows a negative-ion spectrum of a brass substrate fractured from DPE. These experi-

r II+

Hi D*

rnd others

D;

Mass/charge

Fig. 1. Positive-ion SIMS spectrum of brass fractured from deuterated polyethylene. Possible secondary ions are noted.

et al.

0

I

I

I

I

I

I

1

2

3

4

5

Mass/charge Fig. 2. Negative-ion SIMS spectrum of brass fracturedfrom deuteratedpolyethylene.Note the absence of peaks at m/e of 3 and 4. ments were run in a static mode; i.e. at low ion fluxes. The D- peak at m/e = 2 is at least one order of magnitude higher than expected from the natural abundance of deuterium relative to hydrogen, thus confirming the presence of deuterated polymer at the metal surface. All other metal surfaces exposed to DPE showed negative-ion spectra which were essentially identical to that in Fig. 2. SIMS experiments were run in a variety of modes to assure reliability with respect to surface homogeneity, as follows: with standing beam and low ion fluxes (“static”), with standing beam and high ion fluxes (“dynamic” depth profiling), and by scanning the beam along the surface of the substrate with high ion fluxes. The fluxes in the various modes were consistent. Neither SEM nor XPS was ever used before SIMS to avoid risk of surface damage. Post-SIMS electron micrographs on areas unexposed to the ion beam were often performed to verify surface homogeneity, however. The presence of polymer at the metal surface caused the SEM signal to pass from an initially bright/lowcontrast image to a regular image. This transient of 5 to 30 s, depending on magnification, indicated the evolution from a highly-charged, insulating polymer surface to a severely damaged, conductive film. The initial image was uniformly bright for all magnifications to 20,000 x, and the decrease in image brightness was homogeneous across the surface (i.e. there were no bright or dark spots coexisting at any time). The latter observation seems to indicate homogeneous coverage of the metal surfaces by a thin polymer film of uniform thickness. By contrast, the SIMS spectra always showed a strong signal due to the substrate metal, indicative of inhomogeneous surface coverage by the polymer. The existence of irregularities of a size smaller than the minimum scale observable by SEM, which is about 0.5 pm, can reconcile these seemingly paradoxical observations. Figure 3 shows the results of a depth profiling (-) SIMS of a copper substrate separated from DPE, with

Post-fracture

2

4

analyses of polyethylene-metal

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12

14

16

Time (rain) Fig. 3. Negative-ion SIMS depth profiling of a copper substrate fractured from brass. The analysis focuses on the H- (upper) and D- (lower) peaks.

a beam diameter of about 3 mm. The analysis focused on m/e ratios of - 1 (H-) and -2 (D-). While the H- signal persists for long times, the D- surface population decays rapidly to nearly below detection levels. The ratio of the signals increases monotonically in time to a plateau characteristic of the natural abundance of H and D. This result confirms the presence of a thin film of deuterated polymer at the metal surface. Because of the unknown SIMS-crosssection (rate of etching) of deuterium bound to the polyethylene backbone, depth profiles could not be used to estimate the thickness of the polymer film. The estimate of film thickness obtained by ellipsometry was 60 f 20 A. (The range reflects uncertainty in the value of the film refractive index, n.) Literature values for polyethylene increase with density, ranging from 1.51 to 1.54 (Bandrup and Immergut, 1975). Larger values would be expected for a crystalline, highly oriented polymer. The best fit to the ellipsometry data

9

DPE

from

XPS scan

gave n = 1.94, which seems too high. Ellipsometry cannot resolve the uncertainty regarding the layer homogeneity.

XPS

EXPERIMENTS

XPS was used to detect metals left on the polymer after fracture. All samples were scanned for three elements: Al, Cu, and Zn. Figures 4 and S show the Cu and Zn spectra, respectively, of a DPE film peeled from brass. The signal is barely detectable above the noise, indicating that these elements were present only in trace amounts. No Al was observed. The XPS results are summarized in Table 1. The polymer peeled from copper showed traces of Cu but no Zn or Al; the copper signal from this sample *was much weaker than that from the polymer fractured from brass. No traces of any metal were found on the polymer peeled from aluminum. The presence of Cu

cu

brass for cu

2P3

I

cu 2PI 4 3 2 1 0 970.0

961.6

953.2

944.8

936.4

9:

Binding energy (eV)

Fig. 4. XPS analysis of deuterated polyethylene peeled from brass. The energy window focuses on the Cu 2Pl and Cu 2P3 peaks.

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HILL et al.

8

0 1050.0

1044.3

1033.0

1038.7

Binding

energy

1027.4

1021.7

(eV)

Fig. 5. XPS analysis of the same. polymer sample as in Fig. 4. The energy window here focuses on the Zn 2Pl and Zn 2P3 peaks.

Table 1. Summary of results of surface analyses

Substrate Aluminum Brass Copper

Metal present on polymer surface

Polymer present on metal surface

Al

CU

Zn

: Y

2 N

N Y Y

N Y N

the issue of extrusion instabilities and the validity of the no-slip condition in polymer melt flow, which is the subject which first motivated the study at hand. In particular, when failure does occur at a polymermetal interface it appears that the crack does not propagate solely (or perhaps at all) along the phase boundary; rather, failure is a mechanically complex process involving adhesion and cohesion, and is influenced by interfacial chemistry.

Y = present; N = absent.

and Zn on polymer samples separated from brass, and of Cu on samples peeled from copper, may reflect limited chemical reaction between the metal substrates and the polymer. The fact that only traces of metal were present on the polymer substrates, while substantial amounts of deuterated polymer were found on the metal, indicates that the primary mode of failure was cohesive within the polymer, and at most to a minor extent interfacial. The absence of metal on the polymer peeled from aluminum can be rationalized in terms of unfavorable energetic interactions between the aliphatic moieties on the polymer and the native oxide/hydroxide at the Al surface. CONCLUSION The results of this study show the complexity

of the

interfacial interactions between a metal or metal oxide and a polymer, even a polymer with a structure as simple as that of polyethylene. It appears that a model of interactions resulting from dispersion forces alone is inadequate. This conclusion has a direct bearing on

Acknowledgements-This work was supported by the Director, Office of Energy Research, Office of Basic Energy Science, Materials Science Division of the U.S. Department of Energy under Contract No. DE-AC03-76SFOOO98. We are grateful to Robert Cracker for carrying out the ellipsometry measurements. REFERENCES Bandrup, J. and Immergut, E. H., 1975, Eds, Polymer Handbook. 2nd Edition. Wiley-Interscibnce, New York. Chakraborty, A. K., 1992, Progress and future directions in the theory of strongly interacting polymer-solid interfaces, in Polymer-Solid Interfaces (Edited by J. J. Pireaux, P. Bertrand and J. L. Bredas), pp. 3-35. IOP Publishing Ltd, Bristol. Denn, M. M., 1992,Surface-induced effects in polymer melt flow, in Theoretical and Applied Rheology (Edited by P. Moldenaers and R. Keunings), p. 45. Else&r, Amsterdam. Hill, D. A., Hasegawa, T. and Denn, M. M., 1990, On the apparent relation between adhesive failure and melt fracture. .I. Rheology 34, 891. Muller, R. H. and Farmer, J. C., 1984, Fast, self-compensating spectral-scanning ellipsometer. Rev. Sci. Instrum. 55, 371. Ramamurthy, A. V., 1986, Wall slip in viscous fluids and influence of materials of construction. ./. Rheology 30, 337. Wu, S., 1982, Polymer Interjkce and Adhesion. Marcel Dekker, New York.