Polyethylene Melt Adsorption and Desorption during Flow on High

the polymer processing and rheology communities.1,2. Highly entangled polymeric liquids may exhibit a stick- slip transition (SST) upon reaching a cri...
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Polyethylene Melt Adsorption and Desorption during Flow on High-Energy Surfaces: Characterization of Postextrusion Die Wall by Laser Scanning Confocal Fluorescence Microscopy J. R. Barone and S. Q. Wang* Department of Macromolecular Science, Case Western Reserve University, Cleveland, Ohio 44106-7202

J. P. S. Farinha† and M. A. Winnik‡ Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3H6 Received January 24, 2000. In Final Form: April 24, 2000 In this paper we report results that delineate the state of polymer adsorption on high-energy surfaces following no-slip and slip flow. The experiments were performed by precoating a slit die wall made of a silicon wafer with a thin layer of fluorescent dye-labeled-polyethylene (FPE) and then extruding through the slit die a nonfluorescent polyethylene both below and above the interfacial stick-slip transition (SST). Using a laser scanning confocal fluorescence microscope, the postextrusion die walls were examined to check for the existence of the preadsorbed FPE. It is shown that the preadsorbed fluorescent polyethylene (PE) chains were still present after the stick-slip transition. The presence of adsorbed polymer chains following slip flow supports the previously proposed “cohesive” mechanism for the SST of PE on highly adsorbing surfaces.

Introduction Polymer/wall interactions are generally not very well understood, especially at the level of chain adsorption. It is believed that the state of polymer chain adsorption at a melt/solid interface might affect the hydrodynamic boundary condition of the melt at the wall. Therefore, it becomes necessary to characterize polymer adsorption in the presence of flow and to show how the adsorption state relates to wall slip. The subject of molecular mechanisms for wall slip has long been a topic of intense interest among the polymer processing and rheology communities.1,2 Highly entangled polymeric liquids may exhibit a stickslip transition (SST) upon reaching a critical stress at the wall, σc, when the flow rate jumps up discontinuously. Beyond the critical wall stress, complete slip flow prevails and the extrudate may appear smooth once again in the absence of any entry flow instability. The occurrence of melt-flow instabilities has also been associated with features of the stick-slip behavior.3,4,5 Under controlled piston speed, a regular extrudate distortion may occur owing to the alternating stick and slip flows in the window defined by the magnitude of the stick-slip transition.6 The molecular processes involved in wall slip have been proposed to be either adhesive or cohesive, depending on * To whom correspondence should be sent. Address after July 2000: Department of Polymer Science, University of Akron, Akron, OH 44325. † On leave from: Centro de Quı´mica-Fı´sica Molecular, Portugal. E-mail: [email protected]. ‡ E-mail: [email protected]. (1) Denn, M. M. Annu. Rev. Fluid. Mech. 1990, 22, 13 and references therein. (2) Larson, R. G. Rheol. Acta 1992, 31, 213 and references therein. (3) Barone, J. R.; Plucktaveesak, N.; Wang, S. Q. J. Rheol. 1998, 42, 813. (4) Wang, S. Q.; Drda, P. A. Macromol. Chem. Phys. 1997, 198, 673. (5) Wang, S. Q. Adv. Polym. Sci. 1999, 138, 227. (6) Kalika, D. S.; Denn, M. M. J. Rheol. 1987, 31, 815.

the die wall surface condition.4,5 The adhesive mechanism is associated with stress-induced polymer desorption on weakly adsorbing (typically organic) surfaces. de Gennes theorized that adhesive slip may be detectable for polymers.7 The “cohesive” mechanism involves disentanglement of the unbound chains from the adsorbed chains on most metallic and inorganic surfaces, which are of high surface energy and apparently strongly adsorbing.4,5 Such a picture presented a coherent understanding of various flow instability phenomena.4,5 Previously, the adhesive failure mechanism had been thought to govern the stick-slip transition.6,8-11 The surface energy of the die wall and, therefore, the state of polymer adsorption seem to be the defining factor in determining whether the slip is cohesive or adhesive in nature. On high-energy surfaces such as steel, aluminum, or glass, the SST for polyethylene (PE) occurs at a stress ca. σ ) 0.35 MPa. Polyethylene will not show a SST on weakly adsorbing organic surfaces, such as Teflon, in the same applied stress range.4,5 Apparently, only a minimum amount of applied shear stress is needed to cause massive wall slip on a low-energy surface. In this case, adhesive wall slip occurs at a critical stress for desorption, σd, where σd , σc. The question remains as to whether the process governing the SST on high-energy surfaces at σc is cohesive or adhesive. Lack of specific knowledge about the state of polymer adsorption in flow has led researchers to speculate that adsorbed polymer chains on a die wall would completely detach to cause wall slip on high-energy surfaces such as steel. Ramamurthy asserted that this adhesive mechanism was indeed responsible for the slip behavior of polyeth(7) de Gennes, P. G. C. R. Acad. Sci. 1979, 288B, 219. (8) Hill, D. A.; Hasegawa, T.; Denn, M. M. J. Rheol. 1990, 34, 891. (9) Anastasiadis, S. H.; Hatzikiriakos, S. G. J. Rheol. 1998, 42, 795. (10) Yarin, A. L.; Graham, M. D. J. Rheol. 1998, 42, 1491. (11) Mackay, M. E.; Henson, D. J. J. Rheol. 1998, 42, 1505.

10.1021/la000094q CCC: $19.00 © 2000 American Chemical Society Published on Web 07/19/2000

Polyethylene Melt Adsorption and Desorption

ylene.12 Since a die with brass walls was found to remove sharkskin after a period of initial PE extrusion, Ramamurthy claimed that an aged brass promoted adhesion to suppress wall slip.12 Dhori et al. adopted this argument to explain the sharkskin phenomenon in linear low-density polyethylene extrusion as an oscillation between wetting and dewetting of the die exit surface.13 It has been challenging to develop appropriate experimental techniques to probe molecular-level details of wall slip. Migler et al. developed a technique employing fluorescence labeling of chains to monitor their response in shear flow.14 A silanation surface treatment eliminated many poly(dimethylsiloxane) (PDMS) adsorption sites and allowed the authors to observe a greater velocity within 70 nm of the silicon wall than that permitted by the noslip boundary condition. Unfortunately, the nature of the inferred wall slip could not be determined by this pioneering technique. Recently, Legrand et al. have developed an in situ technique building on the idea of Migler’s apparatus.15 This work involved coating a slit die wall with a layer of labeled chains before extruding unlabeled polymer over the preadsorbed layer at high rates. Concurrently, the authors monitored the interface with a fluorescence detector. Legrand et al. were able to detect an adsorbed polymer layer at the die wall in the presence of slip. Wise et al. used attenuated totalreflectance infrared (ATR-IR) spectroscopy to monitor the diffusion behavior of deuterated polybutadiene molecules near a polymer/solid interface.16 However, this experiment was done below the stick-slip transition, where there is negligible wall slip. Consequently, a macroscopic thickness of the preadsorbed labeled layer was still detectable at relatively long times. The more recent data of Wise et al. used many unknown parameters that were adjustable to fit the ATR-IR data.17 Therefore, many different interpretations were possible. The intent of this paper is to obtain more direct information about the molecular mechanism for the stickslip transition on highly adsorbing surfaces. To interrogate the state of polymer adsorption before and after wall slip, we precoat molecularly smooth silicon wafer substrates with thin films of a fluorescent-dye-labeled polyethylene (FPE). These coated silicon substrates form the walls of a slit die and are exposed to pressure-driven flow of nonfluorescent polyethylene at various flow rates. We monitor the state of polymer chain adsorption before and after slip by examining the postextrusion silicon wafers with a laser scanning confocal fluorescence microscope (LSCFM).18-20 Experimental Section Silicon Substrate. A fluorescence-labeled linear low-density polyethylene (FPE) was synthesized at the University of Toronto. Maleated linear low-density polyethylene from DuPont Canada (Mw ) 115 000, Mw/Mn ) 4.77) was purified by removing the antioxidant. Fluorescence dye was then attached to this func(12) Ramamurthy, A. V. J. Rheol. 1986, 30, 337. (13) Dhori, P. K.; Jeyaseelan, R. S.; Giacomin, A. J.; Slattery, J. C. J. Non-Newtonian Fluid Mech. 1997, 71, 231. (14) Migler, K. B.; Hervet, H.; Leger, L. Phys. Rev. Lett. 1993, 70, 287. (15) Legrand, F.; Piau, J. M.; Hervet, H. J. Rheol. 1998, 42, 1389. (16) Wise, G. M.; Denn, M. M.; Bell, A. T. AIChE J. 1998, 44, 701. (17) Wise, G. M.; Denn, M. M.; Bell, A. T.; Mays, J. W.; Hong, K.; Iatrou, H. J. Rheol. 2000, 44, 549. (18) Wilson, T. Confocal Microscopy; Academic: London, 1990. (19) Cheng, P. C. Multidimensional Spectroscopy; Wiley: New York, 1994. (20) Corle, T. R.; Kino, G. S. Confocal Scanning Optical Microscopy and Related Imaging Systems; Academic: New York, 1996.

Langmuir, Vol. 16, No. 17, 2000 7039 tionalized LLDPE. The maleated LLDPE used here had 0.68 mol % of maleic anhydride. Some extra fluoresceineamine dye was added to the maleated LLDPE in xylene to get nearly 100% yield. The final product was used to prepare solutions from which films were cast on the substrates. The FPE was dissolved in xylene at 110 °C to give a 1 wt % solution. The silicon substrate was degreased by boiling in xylene and rinsing in acetone. It was then placed in a vacuum for 24 h to remove any residual solvent. The purpose of using silicon is to have a molecularly smooth substrate to study polymer chain adsorption. Any surface roughness might allow polymer molecules to become trapped at the interface. Accordingly, care was taken not to pit or damage the integrity of the silicon surface. The film thickness for flow experiments was ca. 10 µm. The silicon substrates were 8 mm by 9 mm in surface area. Therefore, m ) FV ≈ (1 g/cm3)(0.8 cm)(0.9 cm)(1 × 10-3 cm) ) 7.2 × 10-4 g of material would be needed to cover the entire surface. A drop of solution has a volume of 0.02 cm3. Thus four drops of the 1% solution were used for total surface coverage. The casting was usually carried out at room temperature; i.e., the 110 °C PE solution was deposited onto the substrate, which was at room temperature. The solution droplet spread over the entire surface, and the solvent evaporated after several minutes. The cast substrate was then put under vacuum and annealed at 180 °C for 30 min. A higher substrate temperature during casting caused solvent to evaporate too quickly to cover the entire substrate. If the solvent did not evaporate fast enough, the PE in solution tended to migrate toward the rim of the drop and collect on the edge of the substrate. To determine whether thermal degradation of the fluorescent dye occurred, some samples were annealed at lower temperatures. However, no difference was observed in the signal intensity between the two. The films covered the entire substrate surface, but further examination revealed a rough surface topography suggestive of nonuniform thickness. Regardless, the labeled polymer adsorbed quite strongly to the substrate and was therefore sufficient for our purposes. The experiments on silicon wafer substrates involved extrusion of a metallocenecatalyzed high-density polyethylene, denoted here as mHDPE, which was not fluorescent. The mHDPE from BP Chemicals had Mw ) 165 000 and Mw/Mn ) 2. The silicon wafers were used in a slit die designed to fit a Go¨ttfert Rheo-tester 1000 rheometer. There were two sides to the slit die, each containing a small steel frame to hold an 8 mm by 9 mm silicon substrate. One side contained a silicon wafer covered with an FPE film, while the other side of the die had a bare silicon wafer used as a control sample. Between the silicon holders were two spacers that form the two narrow side walls (H × L ) 0.25 × 10.5 mm2) of the slit die. The other two sides of dimension W × L ) 2 × 10.5 mm2 were formed by the silicon wafers. This flexible design allowed us to remove the silicon walls after subjection to flow and examine the state of polymer adsorption. Although the entire silicon substrate was coated with FPE, only a 2 mm wide portion at the middle of the substrate was subjected to flow. This allowed us to look at the predeposited FPE film before and after flow all in the same experiment. A typical procedure was to assemble the slit die and attach it to the rheometer. The rheometer barrel was then filled with the mHDPE. The mHDPE first filled the slit die and then was allowed to stay quiescent for 5 min to ensure that the melt and the adsorbed chains intermingle effectively. Subsequently, there was sufficient interentanglement between the labeled and unlabeled polymers as evidenced by the presence of no-slip flow in the slit die. Experiments were conducted using the constant-speed mode of the rheometer. The die entry pressure was monitored as a function of time. As the piston began to travel down at a preset speed, the melt compressed in the barrel and reached steady state. The beginning of steady-state flow was chosen as the initial time. There was a finite amount of time involved in stopping the flow as well. Flow experiments were conducted both below and above the stick-slip transition for different flow times. A steady flow time of 1 s actually involved about 5 s if the transient periods were included. After flow, the slit die was removed from the rheometer and disassembled while the die was still hot. Disassembly involved separating the slit die into halves and peeling off the extraneous mHDPE on the silicon wafer upon separation.

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Figure 1. Measured fluorescence intensity, I, vs as-cast film thickness, a, where the straight line is an arbitrary fit to guide the eye. Intensity is in arbitrary units described by the scale overlaid onto each image, and film thickness is in nm. The fast peeling might create sufficient stress to damage the precoating and is avoided. In addition, a set of control FPE films was cast to establish the relationship between the measured fluorescence intensity, I, and the film thickness, a. Films of thickness 1, 5, 10, 50, 100, and 500 nm were cast from dilute solution in a manner similar to that described above. However, it was difficult to get uniform films so thin. In such dilute solutions, the polymer molecules tended to aggregate if the solvent did not evaporate fast enough. For the control samples, the substrates were heated to T ) 150 °C to avoid precipitation of FPE before solvent evaporation. The solution did not spread over the entire substrate surface. Instead, the solvent evaporated almost instantaneously. Therefore, the actual surface area of the film was that of the solution droplet, and the film thicknesses were calculated on the basis of the actual area. Upon solvent evaporation, the films were annealed under vacuum at T ) 180 °C for 30 min. Laser Scanning Confocal Fluorescence Microscopy (LSCFM). A Zeiss laser scanning confocal fluorescence microscope (LSM 410) was used to detect residual fluorescent chains on the silicon wafers after their exposure to the slit die flow both below and above the interfacial stick-slip transition. The fluorescence dyed films were excited at 488 nm using an Ar-Kr laser. All the images were obtained in the fluorescence mode using a 488 nm interference filter for the excitation and a cutoff filter at 515 nm for signal collection. LSCFM relies on the confocal effect to enhance both the lateral and in-depth resolutions with respect to conventional optical microscopy.18-20 The laser beam was focused on the sample, and the light emitted was directed back through the same optical path and passed through a small aperture (confocal pinhole) before reaching the photomultiplier. Only light emitted from the focal plane was detected because the light coming from other planes cannot pass the pinhole. The images were obtained with wet objectives, 10×, 40×, and 100×. The images shown in this paper have the contrast inverted for illustrative purposes. Accordingly, black corresponds to a positive fluorescence signal while white corresponds to no signal at all.

Results Correlation between Measured Intensity and Film Thickness. First, the control FPE films of anticipated thickness 1, 5, 10, 50, 100, and 500 nm are examined with the LSCFM to determine a correlation between measured fluorescence intensity and film thickness. The open circles in Figure 1 show how measured intensity varies with thickness. The measured intensity is in arbitrary units described on the scale overlaid onto each image. Intensity values represent the average peak height across the entire surface area scanned with the laser. It is important to note that there is a large amount of fluctuation in the measured intensity. This partially arises from a nonuniform film thickness, or more accurately, a nonuniform adsorbed FPE density. The fluctuation may also be inherent in part. For the thin films cast from dilute solution, some chains aggregate resulting in nonuniform surface coverage. It is difficult to cast thin PE films because

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there are issues of film rupture and dewetting in very thin films of any polymer,21,22 let alone a polymer that must undergo crystallization. Therefore, the measured intensity fluctuates throughout each film resulting in a distribution of intensities. Figure 2a shows a 10 nm ascast FPE film indicating variations in the adsorbed film density. Figure 2b shows an as-cast 100 nm FPE film depicting better surface coverage. Flow Curves on Silicon Die Walls. To determine the state of adsorption in flow under various boundary conditions, the flow curve of the mHDPE was first obtained using the slit die with bare silicon wafers as the die walls. Figure 3 shows the steady-state flow curve for the mHDPE on silicon. As the applied stress at the die wall increases, the mHDPE exhibits very pronounced sharkskin roughening of its extrudate followed by a large stick-slip transition. Under controlled piston speed, the SST manifests as pressure oscillation, as shown by the filled circles, which can be approximately viewed as a hysteresis loop. Table 1 shows the experimental flow conditions used for the fluorescence experiments with silicon wafer die walls, which are also indicated in Figure 3 by the arrows and the triangles. The flow curves for mHDPE vary slightly between different runs owing to a small difference in the dimension of the slit die as it is assembled, disassembled, and reassembled each time to load silicon walls. Fluorescence Spectroscopic Imaging. Control Samples. The mHDPE used for the flow experiments does not show any fluorescent signal upon excitation. This was confirmed by imaging the bare silicon wafers taken out of the slit die after the mHDPE extrusion. The baseline, given on the scale superimposed over each image to be presented below, corresponds to zero intensity from the control samples, i.e., bare silicon and the mHDPE on bare silicon. Below the Stick-Slip Transition. This situation represents melt flow when the no-slip boundary condition holds. The data were obtained on 10 µm films after 10 s of flow at γ˘ ) 70 s-1. Figure 4 shows a representative picture of the fluorescence film after extrusion. A large fluorescence signal is detected at the surface over that of bare silicon. Referring to the calibration curve in Figure 1, the film thickness after 10 s at γ˘ ) 70 s-1 is at least on the order of a g 1000 nm ) 1 µm, which is consistent with the rheological indication of the no-slip condition at the silicon wall, as to be discussed later. Above the Stick-Slip Transition. (1) Flow Time ) 1 s. The results presented here were obtained after subjecting 10 µm FPE films to slip flow for 1 s. Figure 5 shows the border between the no-flow and the flow region of the precoated Si wafer extruded at γ˘ ) 4000 s-1, and the flow direction is downward on the image. The signal intensity is appreciable over that of bare silicon in the flow region and very strong in the no-flow region. The original FPE film in the no-flow region is intact and serves as a reference. Overlaid onto the image is the signal intensity profile obtained from the region represented by the line under “Intensity Profile” running across the middle of the image. The baseline is the reference of zero intensity. From Figure 1, it can be deduced that the intensity measured from the flow region corresponds to a film thickness around a ≈ 67 nm. (2) Flow Time ) 5 s. In this instance, 10 µm FPE films were subjected to slip flow (γ˘ ) 4000 s-1) for 5 s. In Figure (21) Orts, W. J.; van Zanten, J. H.; Wu, W.-l.; Satija, S. K. Phys. Rev. Lett. 1993, 71, 867. (22) Zhao, W.; Rafailovich, M. H.; Sokolov, J.; Fetters, L. J.; Plano, R.; Sanyal, M. K.; Sinha, S. K.; Sauer, B. B. Phys. Rev. Lett. 1993, 70, 1453.

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Figure 2. (a, top) 10 nm as-cast FPE film on silicon; (b, bottom) 100 nm as-cast FPE film on silicon.

Discussion Legrand et al.15 studied polymer melt adsorption in slip

Figure 3. Flow curve for mHDPE at T ) 180 °C in the slit die with bare silicon walls. Closed circles indicate region of applied flow rates where pressure oscillations occur. Triangles indicate mHDPE flow on FPE-coated silicon walls. Table 1. Duration and Flow Conditions for Si Die Walls below transition: above transition:

σ (MPa)

γ˘ (s-1)

t (s)

0.22 0.30 0.30

70 4000 4000

10 1 5

6 only the flowed region is imaged. The film has thinned even more in the flow region. However, a distinct signal remains at the substrate surface, as shown in Figure 6. Referring to Figure 1, the film thickness is on the order of a ≈ 47 nm.

flow using fluorescence labeled PDMS on a rough steel surface. The high degree of surface roughness on their substrate made it impossible to discern whether a monolayer of preadsorbed fluorescence-labeled poly(dimethylsiloxane) (PDMS) was still present after the stick-slip transition. Legrand et al. estimated that the polymer layer that remained adsorbed after slip was ca. 0.17 ( 0.04 µm, which is much larger than a polymer monolayer would be. They attributed this high value to the fact that a lot of preadsorbed fluorescence-labeled PDMS remained trapped in the furrows. They postulated that the slip plane lay just above the polymer molecules in the furrows, which is a picture originally presented by Blyler and Hart.23 Hill et al. detected a layer of polyethylene on a copper surface after peeling.24 The copper surface used was also fairly rough. From ellipsometry experiments they estimated that this layer thickness was ca. 6 ( 2 nm, which would be on the order of a monolayer of polymer chains. Since the action of peeling involves an unknown amount of shear and extensional stresses, both polymer debonding and cohesive failure may occur. The problem of surface roughness is avoided in the present work by using a microscopically smooth single(23) Blyler, L. L.; Hart, A. C. Polym. Eng. Sci. 1970, 10, 193. (24) Hill, D. A.; Denn, M. M.; Salmeron, M. Q. Chem. Eng. Sci. 1994, 49, 655.

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Figure 4. Flow condition: mHDPE at γ˘ ) 70 s-1, σ ) 0.22 MPa for t ) 10 s on silicon. The observed signal is strong and indicative of a film ca. 1 µm thick in the flow path. The spatial variation of the fluorescence intensity is given for the line just below “Intensity Profile” running across the image. The scale bar is 25 µm.

Figure 5. Flow condition: mHDPE at γ˘ ) 4000 s-1, σ ) 0.30 MPa for t ) 1 s on silicon. The micrograph shows the border between the flow and no-flow regions of a 10 µm thick FPE film. A strong signal can still be seen inside the flow path. The scale bar is 25 µm. The intensity profile corresponding to the line going across the image is also shown.

crystal silicon surface. However, we still detected a considerable intensity from the silicon surface after slip flow. In other words, using the calibration curve in Figure 1, what is left on the substrate surface after slip flow is as much as a monolayer. It is apparent that there is a difference between a cast monolayer and a slip-produced monolayer. Attempting to cast a monolayer from very dilute solution results in polymer chain aggregation and therefore incomplete surface coverage. Conversely, we can saturate the entire substrate surface by casting from a more concentrated solution to produce thicker films. The entire substrate surface is covered although the film topography is nonuniform. After subjection to slip flow, the film thickness will decrease but the entire surface may remain saturated with polymer chains. In other words, a monolayer produced by slip may constitute a more densely packed layer of chains than that prepared by casting. There are several important points to comment on. For the mHDPE flowing on silicon below the transition, at an apparent shear rate of γ˘ ) 70 s-1, we can estimate the time, t, required to convect the 10 µm thick preadsorbed

film out of the die in the absence of slip. The velocity of the chains at a distance h from the wall is given by v ) hγ˘ , where γ˘ is the apparent shear rate. The length of the die is L ) 1.05 cm. Therefore, for h ) 10 µm, t ) L/(hγ˘ ) ) (1.05 cm)/(10 × 10-4 cm × 70 s-1) ) 15 s. This is a conservative estimate because the time to convect the FPE chains out of the die would increase significantly as the film thickness, h, becomes smaller. Thus it is anticipated that within 10 s of flow time a significant amount of the precoating should survive. This is confirmed by the strong signal in Figure 4. Although it is possible for the postextrusion peeling to damage the original FPE coating, it apparently has a negligible effect. The FPE film that remains after no-slip flow is thick and dense and yields a very strong signal. Any peeling here would involve polymer/polymer failure and not polymer/die wall failure. As is the case for all the images, any intensity fluctuations involve film nonuniformities due primarily to the crude film casting technique and inherent factors. Above the stick-slip transition (SST), it is easy to show that only adsorbed chains may be found on the substrate after a flow time of 1 s. The time required to convect all

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Figure 6. Flow condition: mHDPE at γ˘ ) 4000 s-1, σ ) 0.30 MPa for t ) 5 s on silicon. There is still signal at the substrate surface. The scale bar is 25 µm. The signal intensity is also shown for the dark line going across the image.

the precoated FPE except the adsorbed chains is t ) L/vs, where the slip velocity at the wall above the SST can be estimated according to vs ) bγ˘ , with b being the extrapolation length5,25 and γ˘ the true shear rate at the wall. For example, b can be calculated to be over 1 mm from the flow curve in Figure 3 for mHDPE, and γ˘ can be read to be around 100 s-1. Thus, we have t ) 1.05 cm/(0.1 cm × 100 s-1) ≈ 0.1 s. This is why we have chosen flow times as long as 1 and 5 s to allow all unbound chains in the die to be displaced by the new nonlabeled chains from the barrel. Longer flow time above the SST was not considered because chain desorption is expected to occur gradually over time at such high stresses. In principle, the layer of adsorbed chains only needs to survive for as long as it takes for the disentanglement to occur between the adsorbed and unbound chains, thus producing the stickslip transition. Indeed, slip occurs nearly instantly upon reaching σc. At finite times after reaching σc, it may be possible for desorption to occur owing to the high stress state. But, we observe the cohesive slip mechanism even after slip flow for a finite time; i.e., we observe approximately a monolayer of adsorbed chains on the die wall after slip flow. Figure 6 shows that a layer of adsorbed chains survives above the SST even for 5 s. This is indeed impressive because the preadsorbed FPE film has survived not only slip flow but also the postflow peeling involved in disassembling the die. We can estimate the unperturbed end-to-end distance of the polymer chain, R0, to be R0 ≈ 38 nm for the FPE.26 The FPE film thicknesses remaining after slip flow, i.e., a ≈ 67 nm in Figure 5 and a ≈ 47 nm in Figure 6, are quite close to the chain dimensions. Therefore, what is left on the substrate surface after slip flow is on the order of a polymer monolayer. This would lend credence to an entanglement-disentanglement mechanism for macroscopic wall slip on high-energy surfaces. Brochard and de Gennes initiated the notion of a coil-stretch/disentanglement transition, beyond a critical condition, for walls grafted with entangling chains.25,27 Borrowing the conceptual content of the Brochard-de Gennes theory, we

have been inspired to interpret an extensive set of our experimental data by envisioning macroscopically negligible wall slip (or at best molecular level slip) below a critical applied stress and a stick-slip transition involving disentanglement between adsorbed and flowing-by chains at a critical applied stress.4,5 The residual signals in Figures 5 and 6 show that as much as a monolayer of FPE remains adsorbed on the clean silicon substrate even after flowing beyond the SST for 1 and 5 s, respectively. This is indeed consistent with the idea4,5 that the commonly observed SST in PE extrusion in dies made of high-energy materials involves cohesive slip between adsorbed chains on the substrate and unbound chains. Before the SST, in the no-slip region of flow, bulk chains are fully entangled with the adsorbed chains at the die wall. Upon reaching the SST, slip occurs because the bulk polymer chains disentangle from the tethered chains at the wall. The remaining chains, as observed in our experiment, indicate that desorption did not occur after the SST, pointing to a cohesive mechanism for wall slip associated with the SST. Conclusions We have reported the use of a fluorescence technique to characterize the adsorption state of polymer in flow at a melt/wall interface. It was found that a layer of adsorbed chains still remains after slip flow, as shown by the fluorescence signal attainable from the preadsorbed fluorescent polyethylene on the silicon wafer. The present observations support the proposed molecular mechanism4,5 for the stick-slip transition: that it is cohesive in nature for polyethylene on high-energy surfaces, involving adsorbed chains disentangling from flowing-by chains. Acknowledgment. The authors gratefully acknowledge the award of a BP fellowship from BP Chemicals and support of the National Science Foundation under Grants CTS-9632466 and CTS-9819704. J.P.S.F. acknowledges the support of FCT-PRAXIS XXI. In addition, the comments of the reviewers are greatly appreciated. LA000094Q

(25) Brochard, F.; de Gennes, P. G. Langmuir 1992, 8, 3033. (26) Fetters, L. J.; Lohse, D. J.; Richter, D.; Witten, T. A.; Zirkel, A. Macromolecules 1994, 27, 4639.

(27) Brochard-Wyart, F.; Gay, C.; de Gennes, P. G. Macromolecules 1996, 29, 377.