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Hydrodynamic and Spectroscopic Measurements of Associative Polymer Solutions in Extensional Flow
Theoretical and Applied Rheology, edited by P. Moldenaers and R. Keunings Proc. Xlth Int. Congr. on Rheology, Brussels, Belgium, August 17-21, 1992 1992 Elsevier Science Publishers B.V.
492
Hydrodynamic and Spectroscopic Measurements of Associative Polymer Solutions in Extensional Flow T.D. STANTON1*, D.F. JAMES2 AND MA. WINNIK3 1,3 2 f
Department of Chemistry, University of Toronto, Toronto, Canada M5S 1A1 Department of Mechanical Engineering, University of Toronto, Toronto, Canada M5S 1A4 Present Address: Applied Chemical and Biological Sciences Department, Ryerson Polytechnical Institute, Toronto, Canada M5B 2K3
1.
INTRODUCTION Associative polymers are increasingly used in materials like latex paints where strong shearthinning is a desired characteristic. The shear rheology of solutions has been well characterized from studies like the one conducted by Jenkins (reference 1), who thoroughly tested semi-dilute solutions of a hydrophobically-modified associa tive urethane polymer, the concentrations varying up to 5% by weight. His viscometric data show that a solution has a constant viscosity at low shear rates and shear thins at high flow rates; the transition is abrupt, and the shear thinning is so severe that the shear stress is virtually constant or, equivalently, the viscosity varies inversely with shear rate. This behaviour is believed to be related to the breakup or reorganization of net works of associative clusters formed by the hydrophobic end groups. For these fluids, little is known about their extensional rheology, which plays a role in several methods of coatings applications. To gain some understanding of extensional flow behaviour, we subjected one of the associative polymers of Jenkins' study to extensional motion in a converging channel. To determine bulk stresses in the solu tions, the pressure drop in the channel was meas ured; to determine changes in molecular structure, fluorescent probes were added to the fluids and simultaneous spectroscopic measurements were made. 2.
EXPERIMENTAL RESULTS
METHOD
AND
The test section of the flow apparatus was a converging channel which had an inlet diameter of
8.7 mm and which smoothly converged over a length of 33 mm to an exit diameter of 0.42 mm. The change in diameter with axial distance was measured so that local velocities and strain rates could be determined from the measured discharge rate. Test fluids entered the channel from a pres surized reservoir and issued to atmosphere. Meas urements were made of the overall pressure drop and of the flow rate. For the spectroscopic meas urements, the quartz channel was installed in the sample chamber of an SLM fluorimeter and its excitation beam was focussed on the last 4 mm of the channel. Complete details of the experimental technique are given in reference 2. A Newtonian baseline was established with a series of water-glycerine mixtures, yielding a sin gle curve of pressure coefficient versus Reynolds number for Re from 10"2 to 104 (based on exit con ditions). To put the associative polymer data in context, solutions of a neutral linear polymer PEO with a molecular mass of about 3xl06 - were tested first. Data for one solution are shown in Figure 1, along with data for water and for a Newtonian fluid having the same viscosity as the polymer solution. The extensional stresses expected for this PEO solution are evident from the larger pressure drops. The associative polymer is composed of PEO oligomers (Mn = 8000, Mw/Mn < 1.1) connected by diisocyanate groups and terminated by hexadecyl substituents, to form a chain with a numberaveraged molecular weight of 54000. (Mw/Mn ~ 2). Solutions were prepared with concentrations from 0.1 to 1.0% by weight. Two molecular fluores cence probes, dimethyl pyrenyl ether
493 (dipyme) and pyrene, were separately added to 1% solutions to obtain information regarding the microenvironment under flow conditions. 102
T
L
|
| I | llll|
I
I I I I 1111
I
I I I II
101
Q_
I
100 b A AJZ> to°" D
0.0337.
A
i
10-1
equiv. Newtonian
o
i i i mil
water
■ ■ ■ ■ "in
1 00
10-1
i
i T-rW
102
1 01
V(m-s-i)
Figure 1. Hydrodynamic data for a linear, high MM PEO polymer (0.033% concentration) com pared to water and to a Newtonian fluid of equivalent low-shear viscosity. 10000 t= i iimii|
ι iiiniii
ι i mini
ü A 1000
ι MIMMI
I M MI
1 7. associative equiv. Newtonian water
t
£ 100 y w
Ό CL
< 5i
10
00
t
1 Ë 0.1 10-3
o o o o o o ^,*** i 11 mill 10-2
i 11 um! 10-1
κ an
D«#
ι ι mini
ι ι unni
ι ι mm
100
101
102
V(rrvs-1)
Figure 2. Hydrodynamic data for a hydrophobically-modified associative polymer (MM 54000, 1.0% w/w) compared to water and to a Newtonian fluid of equivalent low-shear viscos ity.
The hydrodynamic data for the 1% solution are shown in Figure 2, along with data for water and for a Newtonian fluid having the same lowshear viscosity as the solution. The ordinate in the figure is the ratio of the pressure drop to the velo city. In terms of the pressure drop itself, the figure indicates that the drop departs from the Newtonian curve and increases only gradually with flow rate. The point of departure corresponds to a wall shear rate of 200 s"1, which, as our own viscometric tests showed, is the critical shear rate when the viscosity drops off suddenly and the clusters (presumably) start to break up. With break-up occurring first near the wall in the channel, the fluid there had a lower viscosity and thus effec tively lubricated the central flow and enhanced the flow rate. A further consequence of the thin boun dary layer was that the bulk of the flow was sub jected primarily to extensional motion. The effect of this motion on the associative networks was detected from the fluorescent probes. These probes, hydrophobic like the hexadecyl end groups, solubilize primarily in the clus ters formed by the end groups. Measurements showed that the hydrodynamic data were not altered by the addition of either probe. Pyrene and dipyme (which contains pyrene groups) are attrac tive probes because their vibronic fine structure (peak intensity ratio \XHZ) is very sensitive to the polarity of the probe environment. In addition, at high local concentrations, a new excimer emission Ie can be observed in the fluorescence spectrum along with a concomitant decrease in monomer emission Im. The ratio Ie/Im is a measure of the ease of excimer formation and is sensitive to the microfluidity of the local environment. Fluores cence results for dipyme are displayed in Figures 3 and 4. The values of I ^ , measured at λ378ηπι/λ388ηιη, in Figure 3 confirm that the probe is in a micelle-like environment. This figure shows that lxllz did not change for extensional rates up to 1700 s"1. The extensional rate is defined here as the average over the length of the channel, the maximum being about three times the average. It should be noted that the corresponding residence time of the fluid in the 4-mm optical detection length was about 0.2 ms. Similarly, Fig ure 4 illustrates that Ic/Im, measured at X495nmA398nm, also did not change with exten sional rate. These probe data suggest that the
494 molecular structure of the clusters was not re organized by the extensional motion, despite the fact that the strain rates were almost an order of magnitude higher than the 200 s"1 rate necessary for break-up in shearing motion. 1
z.uu
1
—r- —
i — i — l
1
I
J
1.80
H
1.60
4
1.40
«J
S[I i
I
Σ
A
1.20
1 nn
H 1
500
i
I
1000
ii
1
1500
i
2000
έ(β-ι)
Figure 3. Fluorescence vibrational bands ratio measured as a function of strain rate in the chan nel, for the 1.0% associative polymer solution containing the dipyme probe. Excitation wavelength: 348 nm. 0.30
0.25
0.20 h
0.15
500
1000
1500
2000
e(s-i) Figure 4. Excimer-to-monomer fluorescence ratio monitored as a function of strain rate in the chan-
nel, for the 1.0% associative polymer solution containing the dipyme probe. Excitation wavelength: 348 nm. 3.
CONCLUDING REMARKS
Since the networks remained intact, exten sional stresses may have been generated. How ever, it is difficult to determine these stresses because they cannot be extracted straightforwardly from the hydrodynamic data. A finite-element program is currently being developed to predict the pressure drop in the channel based on inertia and the shear properties of the solutions. A com parison of the experimental data to the predicted pressure drop should indicate whether extensional stresses were developed in the flow. REFERENCES 1. R.D. Jenkins, Ph.D. Thesis, Lehigh Univer sity, 1990. 2.
T.D. Stanton, Ph.D. Thesis, University of Toronto, 1992.
492
Hydrodynamic and Spectroscopic Measurements of Associative Polymer Solutions in Extensional Flow T.D. STANTON1*, D.F. JAMES2 AND MA. WINNIK3 1,3 2 f
Department of Chemistry, University of Toronto, Toronto, Canada M5S 1A1 Department of Mechanical Engineering, University of Toronto, Toronto, Canada M5S 1A4 Present Address: Applied Chemical and Biological Sciences Department, Ryerson Polytechnical Institute, Toronto, Canada M5B 2K3
1.
INTRODUCTION Associative polymers are increasingly used in materials like latex paints where strong shearthinning is a desired characteristic. The shear rheology of solutions has been well characterized from studies like the one conducted by Jenkins (reference 1), who thoroughly tested semi-dilute solutions of a hydrophobically-modified associa tive urethane polymer, the concentrations varying up to 5% by weight. His viscometric data show that a solution has a constant viscosity at low shear rates and shear thins at high flow rates; the transition is abrupt, and the shear thinning is so severe that the shear stress is virtually constant or, equivalently, the viscosity varies inversely with shear rate. This behaviour is believed to be related to the breakup or reorganization of net works of associative clusters formed by the hydrophobic end groups. For these fluids, little is known about their extensional rheology, which plays a role in several methods of coatings applications. To gain some understanding of extensional flow behaviour, we subjected one of the associative polymers of Jenkins' study to extensional motion in a converging channel. To determine bulk stresses in the solu tions, the pressure drop in the channel was meas ured; to determine changes in molecular structure, fluorescent probes were added to the fluids and simultaneous spectroscopic measurements were made. 2.
EXPERIMENTAL RESULTS
METHOD
AND
The test section of the flow apparatus was a converging channel which had an inlet diameter of
8.7 mm and which smoothly converged over a length of 33 mm to an exit diameter of 0.42 mm. The change in diameter with axial distance was measured so that local velocities and strain rates could be determined from the measured discharge rate. Test fluids entered the channel from a pres surized reservoir and issued to atmosphere. Meas urements were made of the overall pressure drop and of the flow rate. For the spectroscopic meas urements, the quartz channel was installed in the sample chamber of an SLM fluorimeter and its excitation beam was focussed on the last 4 mm of the channel. Complete details of the experimental technique are given in reference 2. A Newtonian baseline was established with a series of water-glycerine mixtures, yielding a sin gle curve of pressure coefficient versus Reynolds number for Re from 10"2 to 104 (based on exit con ditions). To put the associative polymer data in context, solutions of a neutral linear polymer PEO with a molecular mass of about 3xl06 - were tested first. Data for one solution are shown in Figure 1, along with data for water and for a Newtonian fluid having the same viscosity as the polymer solution. The extensional stresses expected for this PEO solution are evident from the larger pressure drops. The associative polymer is composed of PEO oligomers (Mn = 8000, Mw/Mn < 1.1) connected by diisocyanate groups and terminated by hexadecyl substituents, to form a chain with a numberaveraged molecular weight of 54000. (Mw/Mn ~ 2). Solutions were prepared with concentrations from 0.1 to 1.0% by weight. Two molecular fluores cence probes, dimethyl pyrenyl ether
493 (dipyme) and pyrene, were separately added to 1% solutions to obtain information regarding the microenvironment under flow conditions. 102
T
L
|
| I | llll|
I
I I I I 1111
I
I I I II
101
Q_
I
100 b A AJZ> to°" D
0.0337.
A
i
10-1
equiv. Newtonian
o
i i i mil
water
■ ■ ■ ■ "in
1 00
10-1
i
i T-rW
102
1 01
V(m-s-i)
Figure 1. Hydrodynamic data for a linear, high MM PEO polymer (0.033% concentration) com pared to water and to a Newtonian fluid of equivalent low-shear viscosity. 10000 t= i iimii|
ι iiiniii
ι i mini
ü A 1000
ι MIMMI
I M MI
1 7. associative equiv. Newtonian water
t
£ 100 y w
Ό CL
< 5i
10
00
t
1 Ë 0.1 10-3
o o o o o o ^,*** i 11 mill 10-2
i 11 um! 10-1
κ an
D«#
ι ι mini
ι ι unni
ι ι mm
100
101
102
V(rrvs-1)
Figure 2. Hydrodynamic data for a hydrophobically-modified associative polymer (MM 54000, 1.0% w/w) compared to water and to a Newtonian fluid of equivalent low-shear viscos ity.
The hydrodynamic data for the 1% solution are shown in Figure 2, along with data for water and for a Newtonian fluid having the same lowshear viscosity as the solution. The ordinate in the figure is the ratio of the pressure drop to the velo city. In terms of the pressure drop itself, the figure indicates that the drop departs from the Newtonian curve and increases only gradually with flow rate. The point of departure corresponds to a wall shear rate of 200 s"1, which, as our own viscometric tests showed, is the critical shear rate when the viscosity drops off suddenly and the clusters (presumably) start to break up. With break-up occurring first near the wall in the channel, the fluid there had a lower viscosity and thus effec tively lubricated the central flow and enhanced the flow rate. A further consequence of the thin boun dary layer was that the bulk of the flow was sub jected primarily to extensional motion. The effect of this motion on the associative networks was detected from the fluorescent probes. These probes, hydrophobic like the hexadecyl end groups, solubilize primarily in the clus ters formed by the end groups. Measurements showed that the hydrodynamic data were not altered by the addition of either probe. Pyrene and dipyme (which contains pyrene groups) are attrac tive probes because their vibronic fine structure (peak intensity ratio \XHZ) is very sensitive to the polarity of the probe environment. In addition, at high local concentrations, a new excimer emission Ie can be observed in the fluorescence spectrum along with a concomitant decrease in monomer emission Im. The ratio Ie/Im is a measure of the ease of excimer formation and is sensitive to the microfluidity of the local environment. Fluores cence results for dipyme are displayed in Figures 3 and 4. The values of I ^ , measured at λ378ηπι/λ388ηιη, in Figure 3 confirm that the probe is in a micelle-like environment. This figure shows that lxllz did not change for extensional rates up to 1700 s"1. The extensional rate is defined here as the average over the length of the channel, the maximum being about three times the average. It should be noted that the corresponding residence time of the fluid in the 4-mm optical detection length was about 0.2 ms. Similarly, Fig ure 4 illustrates that Ic/Im, measured at X495nmA398nm, also did not change with exten sional rate. These probe data suggest that the
494 molecular structure of the clusters was not re organized by the extensional motion, despite the fact that the strain rates were almost an order of magnitude higher than the 200 s"1 rate necessary for break-up in shearing motion. 1
z.uu
1
—r- —
i — i — l
1
I
J
1.80
H
1.60
4
1.40
«J
S[I i
I
Σ
A
1.20
1 nn
H 1
500
i
I
1000
ii
1
1500
i
2000
έ(β-ι)
Figure 3. Fluorescence vibrational bands ratio measured as a function of strain rate in the chan nel, for the 1.0% associative polymer solution containing the dipyme probe. Excitation wavelength: 348 nm. 0.30
0.25
0.20 h
0.15
500
1000
1500
2000
e(s-i) Figure 4. Excimer-to-monomer fluorescence ratio monitored as a function of strain rate in the chan-
nel, for the 1.0% associative polymer solution containing the dipyme probe. Excitation wavelength: 348 nm. 3.
CONCLUDING REMARKS
Since the networks remained intact, exten sional stresses may have been generated. How ever, it is difficult to determine these stresses because they cannot be extracted straightforwardly from the hydrodynamic data. A finite-element program is currently being developed to predict the pressure drop in the channel based on inertia and the shear properties of the solutions. A com parison of the experimental data to the predicted pressure drop should indicate whether extensional stresses were developed in the flow. REFERENCES 1. R.D. Jenkins, Ph.D. Thesis, Lehigh Univer sity, 1990. 2.
T.D. Stanton, Ph.D. Thesis, University of Toronto, 1992.