Ind. Eng. Chem. Res. 2003, 42, 5819-5826
5819
Chain-Exchange Dynamics at a Polymer-Solid Interface: Effects of Polydispersity and Shear Stress on Linear Low-Density Polyethylene Flow Maria Cecilia A. Esperidia˜ o† Instituto de Quı´mica, Departamento de Fisico-Quı´mica, Universidade Federal da Bahia, rua Bara˜ o de Geremoabo, Campus de Ondina s/n, Ondina, CEP 40170-290, Salvador-Bahia, Brazil
Polymer-surface interactions in flowing systems were studied using an attenuated total reflectance Fourier transform infrared spectroscopy technique. The absorbance of deuterated polyethylene, d-PE66 (Mw ) 6.60 × 104) and d-PE112 (Mw ) 1.12 × 105), near the surface of a flat zinc selenide crystal was followed as it was replaced by linear low-density polyethylene (Mw ) 1.07 × 105). The experiments were performed under wall stresses of 0.090, 0.134, and 0.155 MPa using a rectangular flow channel formed by a ZnSe crystal and an aluminum block. The decay profiles depended on the molecular weight distribution, the chemical composition distribution, and the shear stress. For d-PE112, the decay profiles suggest that slip is due to a cohesive failure. For d-PE66, the slip mechanism appeared to be due to adhesive failure. For d-PE112 with larger and regular chains, an immobile layer forms whose thickness (12-17 nm) depended on the wall shear stress. Introduction Wall slip in polymer melts has been extensively studied. Comprehensive accounts of these phenomena have been given in a recent review by Denn.1 Until now, it is not clear if slip is due to adhesive failure on the polymer-surface interface2,3 or cohesive failure a few molecular distances away4-6 or if there is a lubricated layer on the wall. The microscopic origin of the slip has been attributed to a possible reversible coil-stretch transition induced by the flow. Two main mechanisms have been proposed: detachment/desorption of the polymer chains on the wall (adhesive failure)7,8 and entanglement/disentanglement among polymer chains.5,6,9-14 It has been generally accepted that sudden slip occurs by a disentanglement mechanism. In their studies Brochard and de Gennes6,13 proposed that under the effect of the shear stress the polymer chains end grafted onto a solid surface can undergo a transition from a random coil to a highly stretched state. Above a given shear stress, these chains remain in this state of deformation and they do not entangle with the bulk chains; because of this, the bulk chains can slide over the surface chains with appreciable velocity. The dynamics of chain exchanges near a surface in flowing conditions2,5,9-11,15-18 has been studied by using different techniques. Some experiments have been executed by Le´ger and co-workers5,9,10 using a technique involving evanescent wave-induced fluorescence and pattern photobleaching of the fluorescently labeled chains, by Mhetar and Archer11 using the tracer velocimetry technique, and by Wise et al.16-18 through the attenuated total reflectance Fourier transform infrared spectroscopy technique (ATR-FTIR). Their results are † Mailing address: rua Monsenhor Gaspar Sadock 120, Ed. Renato Franco, ap 202, Jd. Armac¸ a˜o, CEP 41750-200, SalvadorBahia, Brazil. Tel.: (71) 343-5924. Fax: (71) 237-4117. Email:
[email protected].
in agreement with many of the features of the Brochard-de Gennes6,13 model. This study intends to investigate the chain-exchange dynamics of linear low-density polyethylene (LLDPE) within a distance of 283 nm from a surface of ZnSe. The ATR-FTIR technique was used to record the velocity of substitution of deuterated polyethylene (d-PE) previously deposited on the surface, by flowing LLDPE. The factors that instigated this study are as follows: (1) we wished to know if the slow near-surface dynamics and the cohesive slip mechanism observed by Wise et al.16-18 using a permanently amorphous polymer (polybutadiene, Mw g 8.0 × 104) are also observed with polyethylene, a highly crystallizable polymer; (2) we wished to compare the LLDPE behavior flowing on two kinds of d-PE with different molecular weight distributions (MWDs) and chemical composition distributions (CCDs). The influence of the shear stress on the velocity profile was also investigated. The polymers were characterized in terms of the MWD, CCD, viscosity-average molecular weight, melting temperature, density, viscosity, flow curves, and degree of short-chain branching. Experimental Section Materials. LLDPE denoted DJM 1810 is a linear lowdensity polyethylene copolymer with 1-butene produced by Union Carbide and provided in granular powder; it contains only a storage stabilizer. The deuterated polyethylenes (ethylene-d4, 98%) were supplied by Cambridge Polymer Laboratory and were named d-PE66 and d-PE112 according to their weight-average molecular weights. Polymer Characterization. The viscosity-average molecular weight of LLDPE was determined using n-decaline as the solvent at 135 °C and the KuhnMark-Houwink19 constants K ) 6.2 × 10-4 and a ) 0.7. The determination of the quantity of 1-butene (number of CH3/1000 C atoms) was made by FTIR19 (Perkin-Elmer, model Spectrum BXII). The film for the
10.1021/ie030106d CCC: $25.00 © 2003 American Chemical Society Published on Web 10/07/2003
5820 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 Table 1. Physical Properties of the Polymers
polymer LLDPEa d-PE66 d-PE112 d-PE112/66b
peak melting temp, °C
density (22 °C), g/cm3
density (190 °C), g/cm3
viscosity (190 °C), Pa‚s
〈REE2〉, Å2
122 125 127
0.921 ( 0.001 1.089 ( 0.002 1.087 ( 0.001
0.750 0.765 0.765
12165 (1.2 s-1) 1460 (13.5 s-1) 8820 (2.2 s-1)
59 700 30 600 51 000
diffusivities × 1011, cm2/s Me ) 828 Me ) 1250 ref 23 1.8 7.2 2.6 4.9
1.0 4.0 1.4 2.7
11 3.7 7.9
a [η] ) 1.6862 dL/g, M ) 8.1 × 104, CH /1000 C atoms ) 18. b Mixture of d-PE66 and d-PE112, 50% (by weight); 〈R 2〉 ) average v 3 EE quadratic radius of gyration; Me [g/mol].
Figure 1. Flow curve of LLDPE (Mw ) 1.07 × 105) at 190.0 °C. The critical shear stress (σw,c) is the stress at which the curve slope starts to change. Shear stress is in MPa and shear rate in s-1.
infrared measurements was prepared by pressing the sample on a hot plate. The melt index of LLDPE at 190 °C was measured using an extrusion plastometer (model MP933, Tinius Olsen; Table 1). The LLDPE flow curve (Figure 1) was determined by capillary extrusion using a home-built piston-driven rheometer.16 The peak melting temperatures (Tm) of LLDPE and deuterated polyethylenes were obtained by differential scanning calorimetry by heating at 10 °C/min from room temperature up to 200 °C (Shimadzu). Densities were measured at 22 °C in a gradient column of n-propanol, water, and sucrose (F1 ) 0.8852 g/cm3 and F2 ) 0.9468 g/cm3 for LLDPE and F1 ) 0.9464 g/cm3 and F2 ) 1.1169 g/cm3 for d-PE). The diffusivities (D) at 190 °C were calculated based on Pearson et al.’s equation.20 For this, the average quadratic radius of gyration, 〈REE2〉, was obtained from the literature16 and the viscosities and densities at 190 °C were measured using the extrusion plastometer. The viscosities and densities of the deuterated polymers were considered to be equal to that of high-density polyethylene (HDPE) of the same molecular weight. The entanglement molecular weights (Me), 82821 and 1250 g/mol,22 were obtained from the literature. Assuming that log(diffusivity) varies linearly with log(average molecular weight)23 and ignoring the diffusivity dependence on the polydispersity, we estimated the diffusivity for a 50/50 wt % d-PE66/d-PE112 mixture. We also estimated the diffusivities by extrapolation of Bartels et al.’s data23 for diffusion of polydisperse linear polyethylene chains in a high molecular weight polyethylene matrix. The results are given in Table 1. Average molecular weights and MWDs of LLDPE and d-PE were determined by size-exclusion chromatography (SEC) in a Waters 150C apparatus at 135 °C using 1,2,4,trichlorobenzene (TCB) as the solvent, detector of the refraction index, and polystyrene standards. The values for d-PE are for equivalent hydrogen-containing
Figure 2. Schematic representation of (a) the flow cell and (b) the velocity profile model. ω is the angle of incidence, ∆X is the measurement region, xobs is the position in the flow direction where the infrared beam reflects at the crystal-polymer interface, z is the perpendicular distance from the crystal surface, V is the velocity, and H is the slit height.
macromolecules. The chemical composition of d-PE samples dispersed in KBr disks was investigated by FTIR. The CCD was investigated by a solution calorimetry technique24 using TCB as the solvent at a constant cooling and heating rate between 2 and 20 °C/h. This technique fractionates according to the polymer molecular structure; homogeneous linear chains crystallize more easily than branched chains or heterogeneous chains and present higher crystallization and dissolution temperatures. Flow Cell. The experiments were conducted in the same ATR flow cell as that used by Wise et al.,16-18 Dietsche and Denn,25,26 and David et al.27 Because the experimental apparatus has been described in detail elsewhere,16-18 only a brief description of the flow cell will be given (Figure 2). The flow channel is formed by a flat surface of a hemicylindrical ZnSe crystal superimposed on a stainless steel block of the ATR accessory. The dimensions of the rectangular section of the channel were 4.9 mm in length (L), 5.6 mm in width (W), and 0.41 mm in height (H). We used a stainless steel mask to limit the measurement region (∆X) to 2.9 mm. The angle of the incidence (ω) was 54°, and the penetration depths of the beam (dp) for the wavenumbers of 2198 and 2928 cm-1 were 283 and 214 nm, respectively. In accordance with Wise et al.’s data,16-18 the nominal rootmean-square roughness of the crystal over a 1 µm2 area was equal to 6 Å; however, there may have been a slight modification after his experiments. Flow Experiments. A d-PE film with a diameter equal to 4.0 mm and an average thickness of 0.19 ( 0.01 mm was previously deposited on the ZnSe crystal by hand pressing using a plunger and a Teflon mold at 160 °C. The LLDPE contained in a reservoir was then forced to flow through the channel under a constant pressure drop (∆P) using a pressure system with N2 gas (not shown).
Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 5821
Figure 3. MWD obtained by SEC.
Figure 4. CCD obtained by solution calorimetry.
The cell, the LLDPE reservoir, and the flow channels were heated using rods, cords, and heating tapes, and their temperatures were controlled by four thermocouples placed along the flow path. The temperature of the cell changed within (0.2 °C. The experiments were performed in duplicate at 190.0 °C and consisted of measurement of the decrease in the intensity of the absorption by C-D groups at 2198 cm-1 as a function of time when d-PE was substituted by flowing LLDPE. The absorbance values (At) at any time t were divided by the absorbance at zero time (A0), obtaining the normalized absorbances (At/A0). The measurements were taken using a Mattson Polaris spectrophotometer with a mercury-cadmium-telluride detector. We also measured the increase in the intensity of the absorption by C-H groups at 2928 cm-1 with the time in the experiment at 0.155 MPa. The velocity profile model in the absence of slip is shown schematically in Figure 2b: xobs is the position in the flow direction where the infrared beam reflects at the crystal-polymer interface, and z is the perpendicular distance from the crystal surface. The apparent wall shear stress (σw) was calculated from the equation obtained based on the geometry of the flow cell:16 σw ) ∆P (bar)/33. The values used in the experiments were 0.090, 0.134, and 0.155 MPa. The apparent wall shear rate (γ˘ w,a) was calculated using the equation γ˘ w,a ) 6φ/ WH2, where the volumetric flow rate (φ) was measured during the experiments. Flow rates were determined by catching and measuring extrudates using a digital balance and a stopwatch. The flow rates ranged from 0.11 to 0.30 g/min (standard deviation between 0.01 and 0.03 g/min), corresponding to apparent wall shear rates between 15.0 and 42.6 s-1. For shear thinning fluids such as the LLDPE used in this work, the true wall shear rate (γ˘ w) is related to the apparent wall shear rate3 by γ˘ w ) [(3n + 1)/4n]γ˘ w,a, where n is the power law parameter evaluated under conditions of no slip from the flow curve (Figure 1), n ) 0.54. This was consistent with the literature3 value for a LLDPE with Mw ) 1.1 × 105 and n ) 0.56 at 215 °C.
tions were observed in the region between 2000 and 2200 cm-1 attributed to CD2 and CDH stretching,28 those at 1085 cm-1 due to CD2 bending, and those at 660 and 520 cm-1 due to CDH29 and (CD2)n>4 rocking, respectively. The absorbance ratios A660/A1085 and A2140/ A1085 (νCDH at 2140 cm-1) for d-PE66 were 2 times greater than those for d-PE112, indicating that d-PE66 has a larger quantity of isolated CDH groups than d-PE112. In the d-PE112 spectrum, as compared with the d-PE66 spectrum, the intensity of the absorption by CH2 groups (νCH at 2885 cm-1) relative to the absorption by CD2-CD2 groups was 4 times greater. These results together suggest that in the d-PE66 the H atoms are preferentially connected to the C-containing deuterium (CDH groups) while in the d-PE112 there is a larger quantity of H atoms connected to the C-containing hydrogen (CH2 groups). The CCD of dPE112 as compared with d-PE66 (Figure 4) is narrower and presents a higher crystallization temperature, indicating less heterogeneity in the molecular structure. The CCD of LLDPE is broad, evidencing the heterogeneity of the comonomer distribution in the polymer chains. These results were consistent with the higher melting temperature of d-PE112 as compared with that of d-PE66 and LLDPE (Table 1). The phase behavior of the polymers was investigated from literature data. According to Nicholson and Crist,29 χHDN e 2.1 for a 50/50 HDPE/d-PE mixture (Mw,(d-PE) ) 1.12 × 105 and 6.6 × 104), where χ is the interaction parameter and N is the degree of polymerization. In work by Graessley et al.,30 χDD for d-PE66/d-PE112 mixtures should be smaller than χHD, so we can expect single-phase behavior for the d-PE66/d-PE112 mixture at 190 °C. Tashiro et al.’s experiments28 have demonstrated that phase segregation does not occur in 50/50 mixtures of d-HDPE (Mw ) 8.0 × 104) and LLDPE (Mw ) 7.5 × 104) with 17 CH3 groups/1000 C atoms and LLDPE (Mw ) 1.2 × 105) with 18 CH3 groups/1000 C atoms at room temperature. In their studies on the solubility of a series of blends of fully hydrogenous and partially deuterated polydienes (Mw between 8.0 × 104 and 1.1 × 105), Graessley and co-workers30 have found single-phase behavior above 167 °C in almost all blends investigated. So, phase separation between LLDPE and d-PE used in this work is not expected to occur at 190 °C under no-flow conditions. Chain-Exchange Dynamics at 0.090 and 0.134 MPa. The resolution of the transport equation for our system has been described by Wise et al.16-18 The flow
Results and Discussion Polymer Characterization. The MWDs of the dPE66 and d-PE112 samples are broad and are superimposed in the low molecular weight region (Figure 3). The MWD of LLDPE is narrow. In the infrared spectra of both deuterated polymers (not shown), three absorp-
5822 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003
Figure 5. Decay profile of d-PE when substituted by LLDPE at σw ) 0.090 MPa. d-PE112-corrected represents the curve of d-PE112 whose absorbance was subtracted from the absorbance of the immobile layer.
system (Figure 2) was simplified by considering the velocity profile uniform in the flow direction and ignoring any dependence on the vorticity (y) direction. In absence of wall slip and other near-surface anomalies, the near-surface concentration profiles among experiments should be identical if distance and time are scaled as D-1/3γ˘ w1/3 and D1/3γ˘ w2/3, respectively, where D is the diffusivity of the melt in the shear direction. In Figure 5 the decay profiles of the normalized absorbance (At/A0) with time are shown for d-PE66 and d-PE112 flowing at 0.090 MPa, where little or no wall slip is expected. The time was scaled by γ˘ w2/3 and by the diffusivities ratio (Dd-PE66/Dd-PE112)1/3. It can be observed that the curves overlap until the value of At/ A0 is equal to 0.60, which corresponds to a distance (z) perpendicular to the crystal surface of 144 nm. For values below 144 nm, d-PE66 moves faster than dPE112, and the decay rate increases with time. The d-PE112 decay rate is less, and the normalized absorbance converges to a level of At/A0 ) 0.07 and z ) 17 nm. While the absorbance (At) for long times of d-PE66 tended toward 0.0001, the absorbance of d-PE112 tended toward 0.0030. This means that a thin layer of d-PE112 (equivalent thickness of 17 nm) remained attached at the surface, as will be discussed later. Because the whole ATR signal is due to contribution of the constant signal of the immobile layer and the signal of the chains in the bulk, the true decay rate of the bulk chains should be better represented by subtracting from the absolute absorbance the value of the average absorbance of the layer (Figure 5). Now the velocity profile resembles that of d-PE66. This means that the lesser decay rate observed in d-PE112 was only due to the contribution of the immobile layer. In Figure 6, the velocity profiles for d-PE112, d-PE66, and a mixture of these polymers (50/50) at σw of 0.134 MPa are shown. When the d-PE66 curve is compared with that of d-PE112, it can be seen that the divergence between them begins below At/A0 ) 0.80 (z ) 192 nm) and the value of z at that level was 12 nm. As in the former case, d-PE112 moves slower than d-PE66. In Figure 6, we have shown two runs for d-PE112 to evidence the reproducibility of the measurements. Effect of the Polydispersity on the Polymer Mobility. With the objective of investigating the rea-
Figure 6. Decay profile of d-PE when substituted by LLDPE at σw ) 0.134 MPa. d-PE(112+66) is a 50/50 mixture of d-PE112 and d-PE66.
Figure 7. Infrared spectra of d-PE112 and d-PE112/d-PE66 taken during the experiments at 0.134 MPa in the time indicated.
sons for the formation of the thin layer, an experiment was performed using a 50/50 mixture of d-PE112 and d-PE66. In Figure 6, it can be seen that the curves of d-PE112 and the mixture overlap until the At/A0 value of 0.09 (z ) 22 nm) is reached. However, for z < 22 nm, the mixture did not present the level. Its behavior was similar to that of d-PE66. In Figure 7, the FTIR spectra of d-PE112 and a d-PE112/d-PE66 mixture obtained during the experiments are shown to evidence the presence of the deuterated polymer in the first case after 92 min and no deuterated polymer in the mixture after 77 min. In this case, the absorbance signal is comparable to noise. It is evident from these results that the increase in the proportion of the labeled fractions with lower molecular weight and larger heterogeneity of molecular structure altered the chain dynamics near the wall. Chain-Exchange Dynamics at 0.155 MPa. Decay profiles to experiments at 0.155 MPa are given in Figure 8. Below At/A0 ) 0.6 (z ) 144 nm), the d-PE112 absorbance decay is faster than the d-PE66 decay, contrary to what was observed at 0.09 and 0.134 MPa. To distances of less than 144 nm, the apparent decay
Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 5823
Figure 8. Decay profile of d-PE when substituted by LLDPE at σw ) 0.155 MPa. The arrows indicate the point at which the LLDPE concentration reaches a constant value.
Figure 9. Increase in the LLDPE absorbance with time when it replaces d-PE at 0.155 MPa. The arrows indicate the point at which the LLDPE concentration reaches a constant value.
rate of d-PE112 decreases with time and converges to a level (z ) 12 nm) as observed at 0.09 and 0.134 MPa. We registered the absorbance at 2928 cm-1 (absorptions by C-H groups) relative to the absorbance of pure d-PE (absorptions at 2198 cm-1) at zero time as a function of the time (Figure 9). The ratio A(2928 cm-1)t/A(2198 cm-1)t)0 reaches a level when t ) 27 and 35 min for d-PE112 and d-PE66, respectively. These values are represented in Figure 8 by the points indicated by the arrows and correspond at At/A0 ) 0.09 and 0.02, z ) 22 and 5 nm, for d-PE112 and d-PE66, respectively. It can be concluded that when d-PE66 is replaced by LLDPE, the LLDPE concentration increases with time and becomes constant when the quantity of d-PE66 tends toward zero. These results suggest that LLDPE slides at the surface. In the case of d-PE112, the LLDPE concentration saturates in less time but at the same time in which the d-PE112 quantity tends to the constant value at the surface (no flow of the deuterated chains). These results evidence that the remaining signal of d-PE112 was due to chains attached to the surface and that LLDPE slides over an xy plane at a maximum of 22 nm far from the crystal surface. Effect of the Flow on the Chain Mobility at the Surface. With the aim of verifying the influence of the
Figure 10. Decay profile of d-PE112 when substituted by LLDPE at σw ) 0.155 MPa and desorption kinetics without flowing of the thin layer adsorbed at the crystal surface at 190.0 °C.
flow on the velocity of substitution of d-PE112 by LLDPE, an experiment was performed without flow. After the formation of the thin layer at σw ) 0.155 MPa (thickness of 12 nm), the flow was stopped and the cell was suddenly cooled to room temperature. After 48 h, the absorbance decay profile was registered without any flow at 190.0 °C (Figure 10). It was observed that At/A0 fell from 1 to 0.65 in the first 25 min. The value of absolute absorbance tended toward 0.0001 after 150 min, indicating that the deuterated polymer spread in the LLDPE completely. The absorbance decay followed the equation At ) 0.00227e-t/74.430. In the flow experiment, At/A0 of the thin layer remained constant for more than 25 min and the value of absolute absorbance tended toward 0.0020. Because there was a concentration gradient near the crystal surface, the mixing process should be spontaneous under static conditions as observed (phase separation is not expected as described before). However, under flow, the d-PE112 chains remained attached to the surface. These results demonstrate that the formation of the immobile layer is related to the flow of the bulk chains. From the experiment without flow, we have calculated the diffusivity, assuming that there was no motion in the x and y directions. The initial conditions and boundary conditions were C(z,0) ) C0 at z ) 0, C(z,0) ) 0 at z * 0, and C(0,t) ) C0, where C0 is the initial concentration of d-PE112 on the surface (C0 ) 1, pure polymer). The resulting equation was
C(z,t)/C0 ) 1 - erf[z/2(Dt)1/2]
(1)
To calculate C(z,t)/C0 from ATR absorbance, we should define the maximum distance (zmax) starting from the crystal surface and continuing until the point at which the ATR measurement is still significant. On the basis of work by Dietsche,26 we have considered that it is 3dp (849 nm). So, C(z,t)/C0 is proportional to decreases in the normalized absorbance with time, [1 - At/A0] at zmax. We have used Harrick’s27 equation to obtain the relationship between the concentration and absorbance, assuming that C(zmax,t) was uniform. Using the data from Figure 10, in the range between 0 and 90 min, we found D ) (2.4 ( 0.3) × 10-12 cm2‚s-1, which is 6-15 times less than the diffusivity found in the literature (Table 1). For D ) 3.7 × 10-11 cm2‚s-1, for example, zmax
5824 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003
should be 3225 nm (13.4dp), which is a much larger value than the infrared observation region. Lin et al.,31 Frank et al.,32 and Zheng and coworkers,33 in their experiments about chain mobility in thin films, proposed that the diffusivity depends on the thickness of the layer adsorbed. It is an effective segment diffusivity (De) that is less than the bulk diffusivity. For films with thicknesses between 1 and 2 〈REE〉 on an attractive surface, Lin et al.31 found De values 1 order of magnitude less than those of their thickest films. For films with thickness less than 1 〈REE〉, De was 2 orders of magnitude less. For less attractive surfaces, De was only 3 times less than the bulk diffusivity.34 The effective segment diffusivity also depends on the difference in the interaction energy between the deuterated and hydrogenated polymer segments, the polymer-surface interactions, and the conformations of the chains. When the deuterated polymer preferentially segregates to the surface or when the interactions with the surface are strong, its concentration does not significantly change, even after several hours31 of contact with the hydrogenated polymer. In our experiment, the deuterated polymer concentration rapidly decreased with time at the beginning of the experiment (Figure 10), indicating that the LLDPE chains readily replace the deuterated segments. We conclude that there is no significant segregation and the polymer interaction with the surface is weak. Low attractive interactions with the ZnSe crystal are expected because the LLDPE segment-ZnSe interaction energy should be lesser than that of polybutadieneZnSe (1.2 kT/segment).17 The near-surface dynamics only become slowed when the interaction energy is at least 2 kT/segments.35 So, we conclude that the reduced mobility observed is mainly related to the chain conformation at the surface. Discussion Wise et al.16,18 have considered three near-wall velocity profiles to model their ATR experiments with polybutadiene: (a) classical slip, (b) cohesive slip, and (c) lubrification. Our results suggest that, in the LLDPE/ d-PE66 system, LLDPE slides at the surface, indicating adhesive failure. Similarly, the velocity profiles resemble those of the Wise et at.16 model for low stress, in which slip occurs at the surface (classical slip). For the LLDPE/d-PE112 system, we observed that the chains at the surface become immobile, leading to the formation of a thin layer on the crystal surface. This behavior was attributed to cohesive slip occurring on a plane with a distance of up to 22 nm away from the surface. At low stress (0.09 MPa), Wise et at.16,18 have proposed a cohesive slip model to describe the behavior of d-PB/h-PB. However, the near-surface region of reduced mobility found was around 100-150 nm (4-6 times 〈REE〉). The difference of behavior between LLDPE/d-PE112 and d-PB/h-PB may be due to differences of the chemical structure, which cause differences in the polymer crystallizability and polymer conformation. Polybutadiene was a mixture of fractions with cis and trans configurations. Because of its irregular structure, it is permanently amorphous even at room temperature. So, the chain orientation at the surface is more difficult than that in the case of LLDPE/d-PE112. The experiments performed with the 50/50 d-PE66/ d-PE112 mixture demonstrated how the chain-exchange dynamics can be modified by the variation in the
molecular structure and size of the chains. We propose that d-PE112 became immobile at the surface and d-PE66 did not because d-PE112 has homogeneous and higher molecular weight fractions. Polydispersive polymers can display molecular-weight-driven self-exchange, increasing the fraction of larger chains on the surface.36 These chains can be more easily stretched and oriented in the flow direction than d-PE66 and the bulk chains.14 We propose that, as a result of the conformation variation and of the increase in the molecular weight, the interaction energy and the number of contacts of segments-surface are greater and De is smaller in the case of d-PE112. So, the amount of energy needed to remove an entire chain is large. With the addition of d-PE66 to d-PE112, the environment around the d-PE112 segments changes, the number of contacts of segments-surface of the regular and larger fractions of d-PE112 decreases (effect of dilution), and the average De increases. Because of this, the energy needed to remove these segments decreases, facilitating its removal. This can be visualized considering the Boltzmann factor31 (τ) for the removal of adsorbed chains. τ is proportional to exp(-NcEa/kT), where Nc is the number of monomer contacts with the surface, Ea is the ZnSe-polymer segment interaction energy (
