Chapter 26
Phase Separation Kinetics during Shear in Compatibilized Polymer Blends Alan I. Nakatani Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD 20899
The effect of a diblock copolymer on the phase separation behavior of a polymer blend during shear was examined by light scattering. The phase separation process in the mixtures was initiated by a slow cooling of the sample from the one-phase region into the two-phase region and various shear rates were applied to the samples simultaneously. These studies were designed to imitate behavior which may be encountered during injection molding or other manufacturing processes where mechanical deformation and phase separation may be superimposed on one another, thereby influencing the resultant blend morphology. Results were obtained for a model polystyrene and polybutadiene blend as a function of shear rate for two different final temperatures (shallow and deep quenches) in the two-phase region. Similar experiments were performed on a blend with a mass fraction of 2.5 percent of a symmetric diblock copolymer added. For the shallow quench, the pure blend exhibited an enhancement of the coarsening rate during shear over the quiescent coarsening rate while the modified blend showed a suppressed coarsening rate during shear compared to the zero shear coarsening behavior. For the deeper quench, the pure blend exhibited a crossover in the coarseningfromshear induced droplet breakup at low shear rates to shear enhanced coalescence at higher shear rates. The opposite behavior was observed for the modified blend, namely, shear enhanced coalescence at low shear rates, and shear induced droplet breakup (or suppressed coalescence) at higher shear rates.
© U.S. government work. Published 2000 American Chemical Society
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Introduction Polymer blends and compatibilizing agents historically have been the subject of a wide variety of studies and an extensive body of literature on these materials exists. Without specific chemical interactions between dissimilar polymers, most polymer mixtures tend to phase separate due to the unfavorable entropy of mixing between the polymer chains. Efforts to control or retard the phase separation process have led to the research and development of compatibilizing agents for polymer blends. For a variety of systems the dispersed phase particle size has been found to decrease with increasing copolymer concentration. Above a critical concentration of copolymer, the size of the dispersed phase remains constant. An analogy is often drawn between block copolymers as compatibilizing agents in polymer blends and amphiphilic molecules utilized for stabilization of emulsions. A considerable amount of research on employing block copolymers to modify the phase behavior and morphology of polymer blends has been conducted over the last twenty years. Block copolymers have also been found to improve mechanical properties of polymer blends by improving the interfacial adhesion between phases. Majumdar and coworkers have examined the effects of compatibilizing agents on the morphology of polymer blends as well as the mechanical properties of the compatibilized blends. Optimization of processing conditions to produce the best material properties have also been examined. Recent studies in polymer blend behavior have focussed on the influence of simple shear fields on polymer blend morphology. The final morphology resulting from processing of polymer blends has a direct influence on the bulk mechanical properties of the materials. Similarly, extensive research concerning the effect of block copolymers on the resultant size of the domain morphology in polymer blends during processing has been conducted. Polymer blend behavior during shear and the effect of compatibilizers have been examined most commonly by utilizing extruders or other mechanical mixing devices and quenching the extrudate to stabilize the blend morphology. Through subsequent microscopic examination of the extrudate, the particle size distribution within the blend can be measured. These quenching studies are limited to the final morphology of the extrudate and the in-situ behavior in the mixing chamber, where the interplay of droplet breakup in the high shear region and droplet coalescence in the low shear region, has not been studied. Shear light scattering is a technique which has been developed in recent years which may be able to address the issue of temporal evolution of polymer blend morphology during simple shear. The work cited below has particular relevance to the experiments conducted in this report. For more general reviews of the area of polymer blends and block copolymers as compatibilizing agents the interested reader is directed to the cited work and the references listed therein. Most studies of the behavior of block copolymers as compatibilizing agents consider two opposing effects during deformation: a reduction in critical droplet size due to a reduction in the interfacial tension (droplet breakup) proposed by Taylor, and an increase in droplet size due to increased collisionfrequencybetween droplets (droplet coalescence) studied by Smoluchowski. The problem of droplet breakup in a 1
2,3
4,5
407 6,7
flow field was addressed by Taylor in the 1930's. Taylor predicted a critical droplet size, Rc, as a function of shear rate, γ, the interfacial tension, σ, the viscosity of the medium, η, and the critical capillary number, Ca*: Rc = C a V y T | ) .
(1)
From Equation 1, it is apparent that by reducing the interfacial tension, or by increasing the shear rate, the critical droplet size decreases. Since the Taylor approach is only applicable to a single droplet suspended in a Newtonian fluid, multiple droplet interactions and non-Newtonian viscosities are not considered. Despite these drawbacks, the Taylor approach remains one of the most commonly used models for predicting the sizes of dispersed droplets during shear. According to Smoluchowski, ' the collision frequency between droplets, J ^ , ignoring effects due to Brownian motion of the droplets and density driven diffusion is given as: 8 9
J = (4/3)n n y(a +a ) 12
1
2
1
3
(2)
2
where nj are the droplet concentrations of a given size, aj. Therefore, the collision frequency between droplets increases as the shear rate, concentrations, or the sizes of the droplets increase. Milner and X i have addressed the balance between droplet breakup and collision induced coalescence in a twin screw extruder for a polymer blend with block copolymer formed by a grafting reaction between the two homopolymers. They concluded that the principal mechanism for copolymers to promote compatibilization in polymer blends is suppression of droplet coalescence during collisions as opposed to droplet breakup due to a reduction in the interfacial tension. Milner and X i also concluded that droplet breakup was prevalent in the high shear regime and droplet coalescence dominated in the low shear regime of the extruder. S0ndergaard and Lyngaae-J0rgensen have utilized a rheometer equipped with light scattering instrumentation to examine the behavior of polystyrene and poly(methyl methacrylate) blended with a diblock copolymer of the two homopolymers. They found that coalescence increases with shear rate up to a critical shear rate, which is in agreement with the predictions of Milner and X i . Experimental work by Sundararaj and Macosko, nicely contrasted the competing effects of droplet breakup and droplet coalescence in both Newtonian and non-Newtonian mixtures. They concluded that the extent of interfacial tension reduction due to the presence of block copolymer was insufficient to be the primary reason for the reduction of the droplet size, and the primary effect of the copolymer was to prevent droplet coalescence through steric stabilization of the droplets. Sundararaj and Macosko also noted that the droplet size as a function of shear rate for a pure blend decreased to a minimum value, then increased at higher shear rates. No data was given for the compatibilized blend and they referred to this shear rate dependence of the dispersed phase size for the pure blend as "anomalous". Sundararaj and Macosko noted that this anomalous behavior has been observed previously by 1 0
n
12
408 13,14,15
14
other researchers. The work by Favis and Chalifoux suggested that the viscosity ratio between the two phases was a factor. This result is opposite to the results and predictions for the compatibilized blends discussed in the previous paragraph. A study on polypropylene/ polycarbonate blends in twin screw extrusion by Favis and Therrien found no effect of screw speed on the size of the dispersed phases. This result also appears to conflict with the results described above, however the range of screw speeds utilized by Favis and Therrien only varied by a factor of two, and it may be possible that even wider variations in the screw speed would result in a variation of the dispersed phase size. Most commercial polymer blends are considered to be completely immiscible systems, therefore, the small shifts in the phase boundary due to the addition of the copolymer and the subsequent narrowing of the temperature gap between the phase boundary and typical processing temperatures are not very important. However, in some reactive polymer blends, such as the polycarbonate/polyester and polycarbonate/poly(methyl methacrylate) blends studied by Yoon et al., the phase boundary and reaction temperatures are separated by a few degrees. The relationship between the phase separation kinetics and the mechanical deformation during processing becomes much more important in such cases. If a sample is undergoing phase separation and shear is applied during the process, what is the interplay between the driving force for phase separation and the mechanical force balance between droplet breakup and collision induced coalescence? The influence of the copolymer on the dynamics of morphological development under these conditions is also of interest. Scott and Macosko have addressed the development of morphology of immiscible and reactive polymer blends in the initial stages of mixing. The phase boundaries of the blends were not specified. The critical time scale for observation of morphological changes occurred during the first one or two minutes of mixing. Favis has drawn similar conclusions examining the effects of processing parameters on polypropylene/polycarbonate blends. Virtually all of the reports cited indicate very little change in the size of the dispersed phase with longer mixing times. Work by Shih et al. which incorporated temperature variations during processing examined the effects of softening and melting, however, the behavior relative to the phase boundary was not discussed. In this work, we address the issue of competition between phase separation and mechanical mixing utilizing shear light scattering techniques. We have examined the phase separation behavior of a polymer blend during shear and compared the behavior to the same blend with a symmetric diblock copolymer added as a compatibilizing agent. The component polymers of the block copolymer were the same as the blend homopolymers, and the concentrations of copolymer utilized were sufficiently low that aggregates of the copolymer did not form independently. The phase separation process was initiated by changing the temperature of the samplesfromthe one-phase region to the two-phase region. Concurrent with the change in temperature, a simple shear field was applied to the sample and the light scattering patterns were monitored as a function of time. From the intensities and scattering patterns, the relative coarsening rates in the samples as a function of shear rate and final temperature were determined. 16
17,18
19
20
21
409
Materials 22
The model blend for these experiments consisted of low molecular weight polystyrene (PS) ( M = 2.0x10 g/mol, M / M = 1.19) and polybutadiene (PB) (M = 2.8xl0 g/mol, M /M„ = 1.08). For all blends a fixed mass ratio of PS:PB of 60:40 was utilized. The model compatibilizing agent was a symmetric diblock copolymer of perdeuterated polystyrene (PSD) and PB. The diblock was synthesized by coupling the homopolymer precursors ( M of each precursor = 5.0xl0 g/mol) which were prepared using high vacuum, anionic polymerization techniques by Dr. J. W. Mays of the University of Alabama, Birmingham. Similar methods were used to synthesize the PS, while the PB was purchased commercially from Goodyear Tire and Rubber Co. A single diblock copolymer concentration (copolymer mass fraction of 2.5%) was used in these experiments for comparison against the pure blend behavior. The mass ratio of the PS:PB remained constant at 60:40 for the modified blend. The samples were prepared by melting the polymers together at 120 °C in an oven with occasional stirring. Due to the low molecular weights of the polymers, the low glass transition temperatures (T ) of the polymers, and the low cloudpoint temperature (T = 113 °C for the pure blend, 107 °C for the modified blend), the oven conditions were sufficient to have the samples in the one phase region and mechanical mixing was sufficient to produce a homogeneous sample. 3
w
w
n
n
3
w
3
n
23
g
c
Experimental All the shear experiments were conducted on a shear light scattering apparatus with transparent cone and plate fixtures. Details of the apparatus have been published previously. The experiments were conducted as shown schematically in Figure 1. Steady shear with cooling is a dynamic, non-equilibrium experiment which is a simple attempt to model manufacturing behavior such as injection molding, where a well mixed sample is injected into a cooler cavity, so that deformation and phase separation are occurring concurrently. In these experiments, the sample was allowed to equilibrate at an initial temperature, T in the one phase region of the phase diagram for 30 minutes. The oven controls for the shear light scattering apparatus were then set to a final temperature, T in the two phase region of the phase diagram. Simultaneously, a shear rate was applied to the sample as the sample cooled and began to phase separate. During the shearing and change in temperature, light scattering images were taken at uniformly spaced intervals for the duration of the mechanical deformation. After cessation of the shear rate, the oven was set back to Tj and the sample was allowed to remix in the one-phase region before the process was repeated with the next desired shear rate. Due to the large thermal mass of the light scattering instrumentation, the change in temperature was not instantaneous as in temperature jump light scattering experiments typically used to study phase separation kinetics. Therefore, instead of a sharp, temperature quench, the sample experiences a slow cooling profile. The temperature profile is reproducible as demonstrated in 24
it
fl
25
410
Time
Figure L Schematic of experimental procedure. The top trace indicates the time dependence of the temperature profile while the bottom trace indicates the time dependence of the shear rate profile. The initial temperature, Tu is in the one phase region of the phase diagram. To start the experiment the oven settings are changed to afinaltemperature, Tp in the two phase region and shear rate is simultaneously applied to the sample. The arrows schematically represent the data acquisition points during the experiment.
140
600
800
1400
Time (s)
Figure 2, Example of the oven cooling reproducibility. Three separate cooling profiles are shown indicating good reproducibility of the time dependence of the temperature profile. Temperatures were monitored at the wall (upper trace) and the middle of the oven (lower trace).
412
Figure 3. Light scattering patterns for the pure blend at a shallow quench depth (AT - 0.5 °C). Data are in column format with each applied shear rate (0, 5.22, and 52.2 s' ) at the top of each column. The elapsed time of the experiments are given to the 1
413
left. Theflowdirection in all patterns is along the vertical axis. Grayscale has been adjusted to provide the best contrast. Within eachfigurethe same grayscale lookup table has been utilized for ease of comparison.
414 Figure 2, showing data from three separate cooling runs between the initial (Tj =125 °C) and final (T = 112 °C) temperatures. The temperature was monitored on the interior surface of the oven walls as well as in the interior of the oven next to the cone and plate fixtures. f
Results The steady shear during cooling experiments were conducted for both the pure and modified blends at two different final temperatures, to examine the effect of quench depth. As mentioned in the Experimental section, the sample cooling was not instantaneous, however, the term "quench depth" (ΔΤ) will be used here and defined as the difference between the cloudpoint temperature, T , and thefinalexperimental temperature, T . The final temperatures were selected so that approximately equivalent quench depths could be compared for the pure and modified blends. For the pure blend, at a very shallow quench depth (ΔΤ = 0.5 °C), the time dependence of the scattering at zero shear and two shear rates are shown in Figure 3. The images are organized into columns, with the shear rates (0, 5.22 and 52.2 s ) listed across the top of thefigure.The elapsed times are given along the left of the figure, so that the time evolution of the scattering pattern at each shear rate can be followed down each column in the figure. The quench depth is very shallow, as evidencedfromthe zero shear data, showing a very slight increase in the low angle scattering with increasing time indicating domain growth. The zero shear time dependence shows no evidence of a spinodal-like peak developing in the scattering intensity as observed typically in temperature jump light scattering (TJLS) experiments. This is attributed to the slow cooling rate of these experiments compared to the instantaneous temperature change of the TJLS experiments. At 5.22 s"\ a greater degree of low angle scattering is observed at equivalent times after initiation of the experiment compared to the zero shear case. At 52.2 s*, an even greater degree of low angle scattering is observed at each time. This enhanced low angle scattering with shear, is attributed to the presence of large size scale droplets. The fact that the scattering intensity during shear is greater at equivalent elapsed times is indicative of a higher rate of phase separation (coarsening rate) during shear. Hence, as phase separation proceeds, the applied shear rate appears to promote coalescence due to the increased collisionfrequencyof the droplets. Since the changes in the scattering patterns shown are often subtle, sector averages (± 15°) of the scattering images were obtained along the axis normal to the flow direction (the horizontal plane in the images) from which the intensity as a function of scattering angle is determined. The growth rate could be determined as a function of scattering vector, q = (4fcn/X)sin(0/2), where η is the refractive index of the sample, λ is the wavelength of the incident radiation (0.6328 μπι), and θ is the scattering angle. From the theories of Cahn-Hilliard and Cook for phase separation in binary mixtures, the q-dependent growth rate can be obtainedfromthe slopes of plots of ln[I(q,t)] versus time. For the sector averaged data of these experiments, the data was corrected for a baseline contribution to the scattering intensity and normalized to c
f
l
1
415 the scattering intensity at t=0. As noted previously, the scattering patterns during these experiments showed no evidence of a spinodal-like ring, and with the gradual temperature change used here instead of a sharp temperature quench, it is not clear that mis type of analysis is entirely appropriate. Also the Cahn-Hilliard and Cook analysis is only strictly applicable to early stages of spinodal decomposition. However, the analysis provides a more quantitative evaluation of the scattering patterns and is capable of providing some revealing insight into the later stages of phase separation during the experiments. Of particular interest are the slope values between the last two data points of the ln{[I(q,t)-Baseline]/[I(q,t=0)-Baseline]} versus time plots, which are referred to as the terminal slopes. The ln{[I(q,t)-Baseline]/[I(q,t=0)-Baseline]} versus time plots for the conditions shown in Figure 3 are shown in Figure 4. Data for every twentieth point are plotted on the figures. The difference in q varies between points due to the sine dependence of q with scattering angle. It is interesting to note that there is very little q dependence in the growth rates, in contrast to the strong q dependence which is typically observed during spinodal decomposition. The greatest changes are seen in the data for the highest q value plotted, indicated by the filled symbols for ease of comparison. For the pure blend, at the shallow quench depth, the quiescent growth is very slight. However, the coarsening rate increases with shear rate as indicated by the greater slope at longer elapsed times. The results for the pure blend, at a ΔΤ of 2 °C, are shown in Figures 5 and 6. At this deeper quench depth, the quiescent coarsening proceeds at a much faster rate, demonstrated by the higher scattering intensities at equal times after the start of the experiment compared with the shallow quench. The data taken at 5.22 s" shows a lower total scattering intensity compared with the zero shear case, as well as some anisotropy in the scattering at the longest times. This relative decrease in scattering intensity is ascribed to fewer and smaller phase separated domains being present. Thus for the deeper quench, the applied shear rate appears to suppress the coarsening of the sample. By comparison, the data at 52.2 s show a greater amount of total scattering at equivalent times than the 5.22 s" data, but the scattering is still less than the scattering observed without shear. The degree of anisotropy in the scattering at 52.2 s* also appears at much earlier times than in the 5.22 s" case. Hence, with increasing shear rate, a crossover in behavior appears to occur, from droplet breakup (or shear inhibited coalescence) to shear enhanced coalescence (or droplet growth). However, higher shear rates are necessary to confirm this crossover behavior. The corresponding ln{[I(q,t)-Baseline]/[I(q,t=0)-Baseline]} versus time plot for the scattering shown in Figure 5 is shown in Figure 6. For the pure blend at this deeper quench depth, the terminal slope is the smallest for an applied shear of 5.22 s* and greater for an applied shear rate of 52.2 s" . The highest slope is observed under quiescent conditions. Application of shear rate also appears to delay the onset of growth under all conditions compared to the quiescent growth. The behavior for the modified blend is shown in Figures 7 and 9. The corresponding ln{ [I(q,t)-Baseline]/[I(q,t=0)-Baseline]} versus time plots are shown in Figures 8 and 10, respectively. Figure 7 shows the data for a shallow quench (AT = 2.5 °C). The elapsed time necessary for the sample to coarsen at zero shear is much 1
1
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1
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1
416
Figure 4. Time dependence of the scattering intensities at various q values for pure blend at a shallow quench depth (AT = 0.5 °C) corresponding to Figure 3 Filled symbols are the data for the highest q value (4.72 pm ). l
417
418
Figure 5. Light scattering patterns for the pure blend at a deeper quench depth (AT = 2.0 °C). The band of "specks" on the images at 5.22 and 52.2 s" are due to ice 1
419
formation on the active surface of the charge couple device detector and not part of the scattering patterns.
420
1
5.22 β"
600
800
1000 1200 1400 1600
Time (s)
Figure 6. Time dependence of the scattering intensities at various q values for the pure blend at a deeper quench depth (AT = 2.0 °C) corresponding to Figure 5.
1
52.2 s" 1
1
- • - 0 . 5 1 μπι" β
-
~
-
—*~3.17 μπι"
-
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Γ
1
~ 1.44 μηι" 2.33 μπι"
3.98 μπι"
0
200
400
600
800 1000 Time (s)
Figure 6.
Continued.
1200
1400
1600
422
1
Figure 7. Light scattering patterns for the modified blend at a shallow quench depth (AT = 2.5 °C)for applied shear rates ofO, 0.093, 0.93 and 9.3 s .
424
1
AT = 2.5°C
Os" τ
1
Γ
Time (s) Figure 8. Time dependence of the scattering intensities at various q values for the modified blend at a shallow quench depth (AT = 2.5 °C) corresponding to Figure 7.
425
0.093 s
200
400
600
1
800 1000 1200 1400 1600 Time (s)
9.3 s "τ
1
1
1
r
1
2.5 -
-*~0.51 μπι" 1
W 1
g
1.44 μπι" 2• 1.5 1 1 -
«
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ii
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1
"^"3.17 μιη"
1
~^-3.98μιη"
1
~*~ 4.72 μπι"
1
-0.5 200
400
600
800 1000 1200 1400 1600 Time (s)
Figure 8.
Continued.
426
Figure 9. Light scattering patterns for the modified blend at a deeper quench depth (AT = 5.0 °C)for applied shear rates ofO, 2.93, 29.3 and 52.2 s . 1
427
Figure 9.
Continued.
428
AT = 5°C
.q 5 #
Os
I
ι
ι
Ο
200
400
ι
ι
ι
1
1
1
600 800 1000 1200 1400 1600 Time (s)
Figure 10. Time dependence of the scattering intensities at various q values for the modified blend at a deeper quench depth (AT = 5.0 °C) corresponding to Figure 9.
429
1
3.5
ι
_0.5 I Ο
1
1
1
ι
ι
I
200
400
600
1
1
52.2 s" 1 τ
1
1
1
1
1
800 1000 1200 1400 1600 Time (s)
Figure 10.
Continued.
430 longer for the modified blend compared to the pure blend, even for a deeper quench depth. To demostrate these long coarsening times, Figure 7 contains two matrices of scattering images as a function of time after quench and shear rate. The left hand portion of Figure 7 shows the behavior over early times as in Figures 3 and 5, while the right hand side of Figure 7 shows the behavior for much longer times after the temperature quench. The quiescent behavior is shown in both halves of the Figure for comparison. The decrease in the rate of phase separation is qualitatively in agreement with the TJLS results of Sung and Han showing that the kinetics of phase separation in a diblock copolymer modified polymer blend are much slower than the pure blend at equivalent quench depths. The shear rates utilized to obtain Figure 7 were 0.093,0.93 and 9.3 s", which are all lower shear rates than were utilized for the pure blend. At all shear rates, the scattering intensity is less than the zero shear scattering intensity. The effect is most obvious at the longest elapsed times (1513 and 1597 s). For the data taken at 9.3 s"\ the growth in scattering is completely suppressed over the entire time scale of the experiment. The results are indicative of suppression of the phase separation by shear in the modified blend. This is the opposite behavior observed in the pure blend at shallow quench depths. For a deeper quench (ΔΤ = 5 °C), the modified blend again shows very different behavior than the pure blend. The shear rates utilized in Figure 9 are 2.93, 29.3 and 52.2 s". As was the case for the pure blend, without shear being applied to the sample, the phase separation is much faster in the modified blend at the deeper quench depth. When a low shear rate (2.93 s") is applied to the sample, the scattering intensity at all elapsed times is much higher than the quiescent scattering intensity. Similar behavior is observed at 29.3 s*. However, at 52.2 s'\ the scattering intensity is lower at equivalent elapsed times relative to the unsheared sample. Therefore, a crossover in behavior is also observed in the modified blend at deep quench depths, from shear enhancement of coarsening at low shear rates, to a suppression of coarsening at high shear rates. This behavior is also opposite to the results obtained on the pure blend at a deeper quench depth. For a shallow quench of the modified blend (Figure 8), applied shear rates of 0.093 and 0.93 s* also demonstrate a delay in the onset of coarsening compared with the quiescent case. The difference in the terminal slope values for applied shear rates of 0.093 and 0.93 s" are difficult to distinguish, however, the terminal slopes appear to be greater than for the quiescent case. In contrast, the slopes for an applied shear rate of 9.3 s* are much less than the quiescent slopes. For the deeper quench of the modified blend (Figure 10), the applied shear rates of 2.93 and 29.3 s"\ demonstrate a higher growth rate than the quiescent case, while the highest shear rate of 52.2 s" demonstrates a slower growth rate than the sample at rest. 1
1
1
1
!
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Discussion A summary of the experimental observations from these studies is shown by the bar chart in Figure 11. For various temperature ranges and shear rates, the bars
431
Pure Blend
Blend with Block
Enhancement
Suppression
suppi^mi^mi
Shear Rate
Shear Rate
Figure 11. Summary of coarsening behavior as a function of shear rate and temperature for the pure blend and the blend with block copolymer.
432 designate the observed behavior with the shading indicative of the degree of shear enhanced coalescence (droplet growth) (black) or shear induced droplet breakup (white). For shallow quench depths, the steady shear experiments during cooling demonstrated a shear enhancement of droplet growth in the pure blend, while shear suppression of droplet growth was observed in the modified blend. We therefore conclude that at shallow quench depths the principal effect of the copolymer is to prevent shear enhanced droplet coalescence. For the deeper quench depths, the behavior in both samples showed a transition with increasing shear rate. In the pure blend, shear suppression of droplet growth was observed at low shear rates, while shear enhanced droplet growth was indicated at higher shear rates in agreement with the work on two component blends reported by Sundararaj and Macosko. The opposite behavior was observed in the modified blend, where shear enhancement of droplet growth was noted at low shear rates, and shear suppression of droplet growth was indicated at high shear rates. This behavior is the same as predicted by Milner and X i and observed by S0ndergaard and Lyngaae-J0rgensen for three component mixtures. The bar chart emphasizes the striking contrast in behavior between the pure and modified blends. For each category of behavior studied, the exact opposite behavior is observed in the modified blend relative to the pure blend. The suppression of coarsening in the pure blend followed by an increase in coarsening has been addressed by Sundararaj and Macosko. They attributed the behavior to the balance between coalescence at high shear rates predicted by Smoluchowski and the critical droplet size of Taylor which can exist at low shear rates. The arguments presented for the critical droplet size were based on a viscoelastic fluid, while the sample we have examined displays Newtonian viscosity behavior, indicating the presence of viscoelasticity is not necessary to observe the crossover from droplet breakup to coalescence. The model of Milner and X i invokes the relationship between the interdroplet layer draining time and the applied shear rate as an explanation for the modified blend behavior as a function of shear rate. At low shear rates, there is sufficient time for the layer to drain and fusion of the droplets can occur, while at higher shear rates, the interdroplet layer cannot drain away fast enough for fusion to occur. Milner and X i also proposed that the copolymer must migrate toward the backs of the droplets before coalescence can occur and suggest that this migration time also plays a role in the shear rate dependence of the coalescence. We have no evidence from this study to support that hypothesis. It is important to note that the shear rates applied at these quench depths are insufficient to completely suppress coarsening, since for all shear rates, the scattering intensity is varying with time. Therefore, one should not confuse the final equilibrium and steady shear morphologies with these non-equilibrium, transient structures. Certainly one does not expect the equilibrium phase size during mixing of a pure blend to be larger than the phase sizes in an unmixed blend, since at extremely long times, the unmixed blend will form two distinct layers. The experiments must be extended to longer times to ascertain if a steady state scattering pattern is ever achieved. Similar studies of the steady shear droplet sizes in the two phase region would also help to determine the effects of copolymer on the equilibrium droplet size. 12
1 0
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433 Acknowledgments I would like to acknowledge Mr. D. S. Johnsonbaugh for his assistance in the sample preparation and determination the cloudpoint temperatures. I would also like to acknowledge Drs. L. Sung, J. F. Douglas, C. L. Jackson, and C. C. Han for their enlightening discussions of this work.
REFERENCES 1. Matos, M.; Favis, B.D.; Lomellini, P. Polymer 1995, 36, 3899-3907. 2. Majumdar, B.; Keskkula, H.; Paul, D. R.; Harvey, N. G.; Polymer 1994, 35, 4263-4279. 3. Majumdar, B.; Keskkula, H.; Paul, D. R. Polymer 1994, 35, 3164-3172. 4. Majumdar, B.; Keskkula, H.; Paul, D. R. J. Appl. Polym. Sci. 1994, 54, 339-354. 5. Majumdar, B.; Keskkula, H.; Paul, D. R. Polymer 1994, 35, 5453-5467. 6. Taylor, G. I. Proc. Roy. Soc. London 1932, A138, 41-48. 7. Taylor, G. I. Proc. Roy. Soc. London 1934, A146, 501-523. 8. Smoluchowski, M. Phys. Z. 1916, 17, 557-571, 585-599. 9. Smoluchowski, Μ. Z. Phys. Chem. 1917, 92:129-168. 10. Milner, S. T.; Xi, H. J. Rheol. 1996, 40, 663-687. 11. Søndergaard, K., Lyngaae-Jørgensen, J. In Flow Induced Structure in Polymers; Nakatani, A. I.; Dadmun, M. D., Eds.; ACS Symposium Series 597; American Chemical Society: Washington, DC, 1995; pp. 169-187. 12. Sundararaj, U.; Macosko, C. W. Macromolecules 1995, 28, 2647-2657. 13. Plochocki, A. P.; Dagli, S. S.; Andrews, R. D. Polym. Eng. Sci. 1990, 30, 741752. 14. Favis, B. D.; Chalifoux, J. P. Polym. Eng. Sci. 1987, 27, 1591-1600. 15. Huneault, Μ. Α.; Shi, Z. H.; Utracki, L. A. Polym. Eng. Sci. 1995, 35, 115-127. 16. Favis, B. D.; Therrien, D. Polymer 1991, 32, 1474-1481. 17. Yoon, H.; Feng, Y.; Qiu, Y.; Han, C. C. J. Polym. Sci.: Part B.: Polym. Phys. 1994, 32, 1485-1492. 18. Yoon, H.; Han, C. C. Polym. Sci. Eng. 1995, 35, 1476-1480. 19. Scott C. E.; Macosko, C. W. Polymer 1995, 36, 461-470. 20. Favis, B. D. J. Appl. Polym. Sci. 1990 39, 285-300. 21. Shih, C.-K.; Tynan, D. G.; Denelsbeck, D. A. Polym. Eng. Sci. 1991, 31, 16701673. 22. According to ISO 31-8, the term "Molecular Weight" has been replaced by "Relative Molecular Mass", symbol M . Thus, if this nomenclature and notation were to be followed in this publication, one would write M instead of the historically conventional M for the number average molecular weight, with similar changes for Mw, Mz, and Mv, and it would be called the "Number Average r
r,n
n
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23.
24. 25. 26.
Relative Molecular Mass". The conventional notation, rather than the ISO notation has been employed for this publication. Certain equipment and instruments or materials are identified in this paper in order to adequately specify the experimental details. Such identification does not imply recommendation by the National Institute of Standards and Technology nor does it imply the materials are the best available for the purpose. Nakatani, A. I.; Waldow, D. Α.; Han, C. C. Rev. Sci. lnstrum. 1992, 63, 35903598. Sato, T.; Han, C. C. J. Chem. Phys. 1988, 88, 2057-2065. Sung, L.; Han, C. C. J. Polym. Sci.: Part B.: Polym. Phys. 1995, 33, 2405-2412.
