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Online Light Scattering Measurements:

Method To Assess Morphology Developmentof Polymer Blends in a Twin-Screw Extruder

G. SCHLAITER C. SERRA, M. BOUQUEY, R. MULLER, and J. TERRISSE

Laboratoire des Prockdks et Mat&iaux PoZyn-kresEcok de Chimie PoZydres et M a t k r i a u x

25 rue BecquereZ, F-67087 t rasbourg C e d e x 2 , France

Light scattering ha s proved itself an efficient technique to determine particle di-ameters in heterogeneous dilute dispersions in t he micrometer range. Extrusion ofpolymer blend s is expected to give rise to very small particles, typically in the rangefrom hundreds of nanometers to tens of micrometers. A light scattering devicedeveloped in OUT laboratory ha s been used to s tu dy the morphology of polymerblends obtained in a twin-screw extruder. The main advantage of this technique isthe imm ediate response obtained without any su rface or interface modification thatcan occur during the sample preparation by using more conventional techniqueslike electron microscopy. To show the possible applications of this light scatteringdevice, preliminary tests have been carried out. First, we present a comparison be-tween experimental measurements and theoretical results for dilute systems. Sec-ond, we have investiga ted the effect of sh ear flow on the droplet deformation. Fi-nally, we have studied th e variations of the light scattering pa ttern for a reactiveblend.

1. INTRODUCTION designed to be connected to high-p ressure zones along

nterest in twin-screw extrusion of polymer blen dsI a s been growing in recent years because of the rel-ative ease of producing materials having new proper-ties. As a consequence, many studies concerning themorphology of immiscible polymer blends have beencarried out since the 1970s 1-5). Study of morphol-ogy evolution along the twin-screw (6, 7) is generallyinvestigated by scanning electron microscopy SEM)or transmission electronic microscopy TEM). Thesemethods give a direct pictu re of the morphology, bu tthey are time-consuming as they require the polymerblend to be quenched rapidly. After that, each sampleha s to be prepared (curing, ultramicrotome cutting, ...before observation by microscopy. But morphologycan also be studied in the reciprocal space by usingthe light scattering principle. This technique has beenwidely used to character ize the morphology of polymerblends und er simple she ar flow 8-10). Recent pape rs

11-13) have demonstrated the use of this techniquea t the exit of a n extruder.

This paper deals with this technique and the first re-sul ts obtained with a n original light scattering devicethat allows online morphological characterization ofpolymer blends. The instrument has been especially

the &-screw extruder. Consequently, it can be usedto stu dy the influence of material and processing pa-rameters. Furthermore, the instrum ent allows one tomonitor the process and its stability.

2. LIGHT SCATTERING THEORY

When the size of a particle is comparable with thewavelength of th e light and when the refractive indicesof particle and its surrounding are different Fig. I ) ,the light is scattered. Two theories describe physicallythe phenomenon, the Mie and the Rayleigh Debyetheories.

2.1. Mie Theory for Spherical Diffuser

The theory of Mie (14, 15) gives a n exact solution ofthe Maxwell equatio ns governing the problem of th elight scattered by a n isotropic sphe re of radius a intoan isotropic medium . The light, of wavelength A is as-sumed to be monochromatic and plane. There is norestriction concerning th e respective refractive indicesof sphere, 5 nd surrounding m edium, ns. The am-plitude of the scattered light h 0) for a sphere locatedbetween two parallel polarizers is given by the follow-ing equation:

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G. Schlatter, C . Serra , M . Bouquey, R . MuUer, a n d J . Terrisse

Incident electricfield E i)

Direction of kipropagation 2

-a

Fig. 1 . Elastic scattering by a phere.

where the coefficients eBt and 'Bt are built w ith Riccatian d Bessel functions and depend o n both refractive in-dices of the diffuser and th e medium, on th e numbe r27ra-, and Pfos,) the first order Legendre functions of

degree i?

2.2. Rayleigh Debye Theory

When the polarizability does no t occur, the theory ofRayleigh Debye 16) gives a simpler solution of theMaxwell equations und er t he two following conditions:

27ra - 1 << 13)

Assumption 2 means that one can neglect the anom-alous diffraction due to refraction occurring when theindices are too different. Restriction 3 assumes thatno phase difference inside the particle occurs. TheRayleigh Debye theory tak es ad van tage of these a s -sumptions to describe scattering phenom ena for non-spherical particles. Consequently, th e intens ity of th escattered light follows the equati on:

47rrr e) = I , k4cos20

where i is the incident intensity, k the norm of thewave vector, V the volume of the particle and R(@, heRayleigh form factor 16).

2.3. Intensity of the Light Scatteredby a Distribution of Particles

In the case of a highly dilute system, th e light scat-tering intensity of a distribution of N particles corre-

sponds to the s um of the intensities s cattered by each

particle. Using this additivity rule, we have developeda n inverse method to obtain to the particle size distri-bution from an experimental light scattering pattern.The normalized intensity profile is then given by thefollowing relations 17)

where:

and:

(5)

(7)

j =

I @ ) is the intensity profile for a particle of diameter di ,N i s the number of particles of diameter d i , class thenumber of classes corresponding to the different diam-eters and rbw 9) the normalized total light intensity.

For a given set of Nchs droplet diameters (0.2 kmup to 10 km) and for a given experimental normalizedintensity profile (0). the method consists in com-puting each N i with a least square method and a non-negativity constraint. The inverse method has beennumerically tested. First, intensity profiles were calcu-lated for different kinds of chosen distributions. Theinverse method has then been used to re-calculatethese distributions and to compare them with the ini-tial ones. The results show a good correlation betweenthe initial and t he re-calculated distributions.

Furthermore, E q 4 hows th at t he intens ity of lightscattered by a large particle is mu ch higher than thatscattered by a smaller particle. As an example, a atioof 2 for the radius results in a ratio of 64 or the scat-tered intensity at 0 = 0 . However, if one considers thenormalized intensity I f3)/I 00), small particles scatterlight at wider angles (see also Fig. 6).

2.4. Multiple Light Scattering

For highly concentrated systems, multiple light scat-

tering occurs. This means that the intensity scattered

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Online Light Scattering Measurements

by a particle reaches its neighbors. Therefore, suchsystems scatter the light at wide angles. In this case,the two previous theories are not applicable anymore.Rusu (17) ha s proposed th e following criteria to evalu-ate when multiple light scattering can b e neglected:

h

8a

C ,[ 0.2

where cW1 s the volume fraction of spherical particlesand h the thickness of the observed system.

As a consequence, the us e of an inverse m ethod willgive smaller droplet sizes that one really gets.

3. EXPERIMENTAL

3.1. The Light Scattering Device

The light scattering device has been especially de-signed and built for this study . It cons ists of three dif-ferent parts Fig. 2).

The ight source: It is constituted of three He/Nered laser beams of which the wavelength is 633 nm.Each laser can b e individually translated a nd rotatedin and around two directions to facilitate the align-ment with the t wo other parts. The first laser is in thecenter of the optical bench, where as the second a ndthe third lasers are respectively at 35 a n d 70 inorder to observe the scattered light at wide angles. Inthis study, only the centerline laser ha s been used.

The Light scattering ceU: This part is directlyconnected to the e xtrude r by mean s of special connec-tors: therefore, a small flow of molten po lymer is sepa-rated from the main flow inside the extruder to thelight scattering device. The latter consists of a thermo-regulated barrel in which a 10-mm-diameter glasssphere is strictly aligned with the axis of th e cell. Intothe glass sphere a 0.8-mm hole is drilled to let thepolymer blend flow through t. This is the heart of thedevice since th e s ma ll volume of blend inside th esphere is illuminated by the laser beam an d expectedto scatter the light. A set of two distributors leads to aswitch between the purge an d the analysis mode (m.3 . n the former, the dead volume beneath the sphere

is flushed away an d fresh blend is therefore ready foranalysis. The latter permits the fresh blend to flowthrough the sphere and lets evacuate it from the de-vice. Furthermore, on e can observe the scattered lightwhen the polymer blend flows or not through the holeof the glass sphere (respectively dynamic or staticmode). The ratio of flow through the scattering device

to the main flow in the extruder is typically around1/100 to 1/1000.

The opticcrL bench: An aspheric condenser lens, anachromatic doublet lens, and a diaphragm are used toget back the scattered pattern on a semi-transparentplate. The pattern is recorded with a CCD camera (SonyXC ST70). he bench can be translated and rotated inand around two directions for alignment purposes.Furthermore, the achromatic doublet lens and the di-aphragm can be independently centered. The CCDcamera is linked to a computer for data acquisition(Matrox MeteorLI) and the pictures are processed with acommercial software (Matrox Inspector).

3.2. M a t e r i a l s

Two kinds of polymer blends have been used:PSIPP blend: polystyrene is a Styron 6783 from

Dow and polypropylene a Stamylan P 48M10 fromDSM. The PP is the minor phase whereas the PS isthe matrix. They were chosen for their high refractiveindex contrast. As a consequence, even at low concen-tration we had sufficient turbidity to give adequatelight scattering. At room temperature, the respectiverefractive indices of PS and PP are nPs = 1.6 and npp= 1.49. Successive masterblend s have been preparedin order to analyze a low concentrated blend of 99.2%PS and 0.8 PP in weight. First of all, a masterblendof 80 PS and 20% PP in weight is extruded at 200°C.A second ste p consisted in the preparation of a mas-terblend of 96%/4% in weight obtained a t 200°C fromthe 80%/20% blend and PS. The blend of 96?40/4%was analyzed at the same time. In a final step, we ex-truded and analyzed the 99.2%/0.8% blend obtainedat 200°C from the 96%/4% and PS. The dependenceof the viscosity on the shea r rate was determined on a

Fig. 2. Schematic overviau of the @hi scattering device.

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bezqaser lightttered

Flow of themolten polymer

Analysis underflow Analysisw ~ t h o u t ~ o w

Upper distributor Glass sphere\ I

ation

Purge mode

Lower/- distributor

I

\ Arrival of the mo lten polymer from the extruder

b)Flg. 3. Schematic views of 4) the glass sphere and b] Operating modes of the light s c a t t e r i ng ceU.

single-screw extruder equipped with a capillary die.Figure 4 shows the viscosities and the extrapolatedviscosity ratio between the PS and the PP at 200°C.One can observe that in the range of shea r rates from102 s-l to lo4s-l, he viscosity ratio ypp/qPs varies onlybetween 0.7 and 1.1.

E M F A 6 and R E M PA 6 blends: PA6 is a n Ul-tramid I35 from BASF. EBA is a copolymer of ethylenean d bu thy l acrylate (Lotry1 17BAO7 from Elf Atochem)while REBA is the EBA for which 4 .6 molecules of

1000

epvf

b

x>

.e 100

R

10

maleic anhydrid MAH) have been grafted per chain(Lotader AM3410 rom Elf Atochem). MAH of the REBAcan react with the terminal m i n e function of th e PA6and give rise to a reactive polymer blend. Both blendswere extruded at 250°C.

3.3. Co-rotating Twin-Screw Extruder

The co-rotating twin-screw ext ruder (Werner Pflei-derer ZSK30) has a length to diameter ratio of 40. Fig-ure 5 shows the screw profile used. The optical cell is

100

1.2

5z1/1c,

d

z.

0.8 tzam

W

0.61000 10000

Shear rate s-1)Flg. 4. Shear rate dependence of the viscosity and the extrapolated viscosity ratio of PP to PS at 200°C. P: -0-1 PS: -0-1TlPP/qPs: -A].

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P2a P2b P3a P3b P4a P4b P5a

Fig, 5. Screw p r o m of the b i n s c r m extruder. ma, 2 b , P 3a , P 3 b , P 4 q P 4 b and P 5 a are the dtzwent possible locations of the lightscattering device. h light scattering cell is mounted on P 4 b .

located at th e point P4b at th e end of the mixing zone.For all experiments, the feed throughput was 3 kg.h-'an d screw rotational speed was set to 150 RPM.

3.4. Procedures for LightScatter ing Experiments

I m a g e processing: The data acquisition needs astatistical treatment to eliminate the noise resultingfrom variations in the scattered pattern. Therefore, astream of 400 pictures (x, y patterns, see Figs. 2 and3 for the x and y axis definition) was recorded at a fre-quency of 25 pictures per second. These 400 frameswere averaged to get an image from which the varia-tion of intensity as a functi on of the a ngl e was ex-tracted and compared. A statistical study has shownthat one has a Gauss ian distribution of th e scatteredintens ity for each pixel of the CCD camera.

R u n of a n experiment: First, a diffraction gratingfilm (140 grooves/mm) was used to correlate scatter-ing angle with the coordinates of each pixel of theCCD came ra. Sec ond, the polymer blen d was ex-truded for a t least half a n hour to reach a steady stateafter some process parameters were changed. Then,the dead volume beneath the optical glass sphere isflushed away an d t he fresh blend was allowed to flowthr ou gh the hole of the glass sphere for measure-men ts. Then, the 40 0 pattern s were recorded andprocessed i n the dynamic or static mode.

3.5. Microscopy

A scanning electron microscope (JEOL SM 5900LV) operated at 20 kV was used to take micrographsof the blends. The samples were taken at the exitof th e light scattering cell an d rapidly quenched inwater. After that, PS was selectively etched out usingTHF or PS/PP blend s and PA6 was selectively etchedou t u si ng form ic acid for EBA/PA6 an d REBA/PA6blends.

4. RESULTS AND DISCUSSION

4.1. Characterization of h eMorphology for Diluted Blend

Figure 6 shows experimental and theoretical lightscattering profiles in the y direction for the 99.2%/0.8% PS/PP blend (see Figs. 2 and 3 for th e y axis def-inition). The experimental y profile was obtained inthe static mode. Several measurements, carried out

for 2 hours, showed a very stab le profile of th e scat-tered light. Theoretical curves were obtained with theMie model. No mono-disperse distribution w as foundto fit the experimental profile, whereas a bi-dispersedistribution of particle size allows a f a i r fitting of theexperimental curve. The distrib ution highlights a largenumber of small particles (90% of the total particlevolume, which corresponds to 23 0 particles of 1.5-pmdiameter for only 1 particle of 4.5-pm diameter). The

respective intensity profiles for the 1.5-pm and the4.5-pm diameter are also plotted to demonstrate theimportance of each class of diam eter .

Figure 7 shows a rep resentat ive SEM micrograph for99.2%/0.8% PS/PP blend. One observes that PP isdispersed in PS matrix. The micrograph shows thatthe droplet diameters are in th e range from 0.2 pm to4 p.m, with a mean value around 0.8 pm. This resultis in agreement with the size distribution obtainedfrom light scattering if one takes into account the pre-cision of the experim ental measurem ents.

4.2. Study Under Shear F l o w

Figure 8 shows the x pat terns (averages of 400 re-corded frames) respectively in the static ( Rg . 8a) andin the dynamic modes (Rg. 8b) for the 96% /4% PS/PPblend. One observes that the scattered light patternfor the dynamic mode is smoother than the one ob-tained for the static mode. The flow through t he g lasssphere acts as a filter for th e signal by averaging theintensity during time. For the static mode, Fig. 8 ashows an isotropic pattern, while Q. 8 b shows a n el-liptical pattern corresponding to an ellipsoidal defor-mation of the droplets inside th e glass sphere.

Figure 9 shows the x and y intensity profiles ob-tained for the dynamic mode (flow through t he glasssphere) and for the 96%/4% PS/PP blend. I t shouldbe noticed that the x profiles are available only u p toan angle of 15 . The anisotro py of the ligh t scatter ingpattern shows that th e particles are stretched underthe flow during the dynamic mode. In this case, athroughput of 0.3 g.min-' was measured, which leadsto a wall shear rate around 140 s-l (radius of th e hole0.4 mm and local power law index for the melt viscos-ity n = 0.35 (see Fig. 4 ) . f one considers that shearflow leads to a n ellipsoidal sha pe, Glatter an d Hofer

18) have shown that the mean length of ellipsoidalparticles can be estimated from the light scatteringprofile observed in the axial direction x of t he flow. We

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0.8

0.6

0.4

0.2

05

G. Schlatter, C. S e r a , M . Bouquey, R. Mullet-, and J. Terrisse

I

10 15Angle ( O )

20

Rg. 6 . Horizontal normalized light intensity p r o m I / I e = 541. Comparison of experimental results and theoretical calculations basedon the Mie model Blend 99.2 /0.8 , noflow through the g l a s s sphere (static mode). 0: xperiment : heory 4 = 4.5 prn (10UOL . = 1.5 prn (9090 oLJI: - B e : Theory (+ = 1.5 prn (100 uoLJ A : Theory (4 4.5 prn (100 ~ 0 1 . .

Q. 7. SEM of 99.2 /0.8 PS/PP lend


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a) (b)

Fig. 8. Light scattering patterns. a): Static mode. b): Dynamic mode Flow through the glass sphere = 0.3 g.miK’ . P S / P P 96 /4 .

1

0.8

0.6

0 4

0.2

0

10Angle ( O )

15

Fg. 9. Dynamic mode, anisotropy of the scattered pattern. PS/PP 96%/4%. Flow through the glass sphere = 0.3 g . rn in l the dvec-tion of the&w is xJ. -A-: y experimental pro@ of the n o d i z e d intensity : x txperimental pro-: -0-: theoretical pro m(+ = 5.5 pm (22 vol) , = 2.5 prn (38 voL), = 1.5 pm (40% vol .

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have therefore fitted the experimental x scatteringprofile by a theoretical curve obtained with the inversemethod using the Mie model. Calculations show awide distribution of partic le size, from 1.5 pm to 5.5pm (see Fig. 9) . t should be noticed that the size of5.5 pm, which is underestimated because of multiplelight scattering, is nevertheless higher than the one

obtained for the 99.2%/0.8% blend, which also high-lights the particle stretching.

The flow through the capillary of the glass spherehas been modeled in order to estimate whether d ropletbreakup is possible in th is flow section. A Carreau lawhas been chosen to characterize the rheological be-havior of 96% /4% PS/PP blend. Velocity and shearrate fields have been calculated for a throughput of0.3 g.rnin-l, a capillary radius of 0.4 mm and a lengthof 5 mm. Figure 10 shows the local residence time andthe critical time tb versus radial position in the capil-lary. Time tb corresponds to the time that a dropletneeds to break up under flow corresponding to the

critical capillary number C h t given by Grace (19) andMeijer (20). Figure 10 shows tha t only droplets locatedbetween r = 0.27 mm and the Capillary wall (r = 0.4mm) are able to break up. Figure 1 1 shows minimumand maximum radii of the d roplets that are broken forr = 0.27 mm up to 0.4 mm. The minimum radius iscalculated from th e critical capillary number given byGrace (19) and Meijer (20). The maximum radius iscalculated for a droplet that breaks up after a timeequal to th e local residence time. Rgure 1 1 shows thatbreakup do es not occur at the center of the capillary

(from r = 0 up to 0.27 mm), while it occu rs for particlediameters from 0.5 pm up to 3 pm (see the breakupzone in Fig 11. This phenomenon should lead to a bi-dispe rse distri bution of the di ameters near t he wall ofthe capillary.

4.3. Nonreactive and Reactive Blends

Figure 12 shows t he intensity profiles obtained forreactive blend (REBA/PAG 80%/20%) and its corre-sponding nonreactive blend (EBAIPAG 80%/20%).Such concentrated blen ds generate of course multiplelight scattering. Nevertheless, one c an have a correla-tion between the scattered light and th e morphology;see Cielo (21) and Belanger (22). For EBA/PA6 blend ,all measurements, which were carried out for morethan 2 hours, have shown a high stability of the lightscattering intensity profile. These results are in agree-ment with a previous work of Serra (23). A s a matterof fact, Serra (23) h as shown for a simi lar blend (EBA/Platamid 80%/2O%) that there i s no effect of process-

ing parameters s uch as feed throughput and screwrotational speed. Nevertheless, one has observed highrandom fluctuations of the light scattering profiles inthe reactive case. F'igure 12 shows the two curves Minand M a x corresponding to the envelope of all profilesobtained during 2 hours of measurements withoutvariation of processing parameters. These randomfluctuatio ns highlight, surprisingly, a non-constantmorphology of the reactive REBA/PAG blend. How-ever, the level of th e normalized intens ity profile Fig .12) was always higher for the reactive blend than for

0 E+OO 1 E-04 2 E-04 3 .E-04 4 .E-04

radial position (m)

Fig 10. Comparison of the local residence t i m and the breakup time inside the optical sphere. : Critical time bA h a t esidencetime


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2 .OE-06

SE-06

EW

1.OE-063d

5 .OE-07

O.OE OO

I

.. ... ...:;;;j

0 .E+OO 1.E-04 2 .E-04 3 .E-04 4 .E-04radial position (m)

Fig. 11. Minimum and mcuciTnum radius of the droplets that are broken. €t: M a x radius (ginen by the local residence time : inradius @wen by C ).

1

b O . 8

2 .6

7 0.4E

cI

3.II

zNIL

i?0.2

05 10 15

Angle ( O )

Fig. 12. Comparison of horizontal y normalized light intensity prom I / l O = 5 ) static mode) of nonreactive blend [- EBA/PA680 /2096) and reactive blend [REBA/PAG 0 /20 ). ?he M a x [O nd Min (.) profiles correspond to the envelope of d m ? sobtained during the experiments for REBAIPAG blend

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the nonreactive one, which means that PA6 dropletsare smaller for the reactive blend. This result wasquite expected since the reaction at the interface ofthe two polymers decreases th e su rface tension, lead-ing to smaller droplets.

Figure 13 shows the SEM pictures respectively forEBA/PA6 (Fig. 13a) nd for REBA/PA6 Fig . 13b)

taken at th e sam e scale, where th e dark areas repre-sent the PA6 phase. For each case, one observes thatPA6 is dispersed in EBA or REBA matrix. The micro-graphs confirm, without any doub t, t ha t th e size ofthe d roplets i s muc h lower for the reactive blend.

5. CONCLUSION

We have developed a new app ara tus for the purposeof following th e morphology of polymer blen ds alongan extruder by online light scattering m easurements.The frst tests have shown the capabilities of this de-vice to as se ss th e developm ent of polymer blend mor-phology duri ng their extrusion.

These preliminary results suggest:

For dilute blends, deconvolution of the inten-sity profile gives access to the droplet size dis-tribution a nd makes it possible to study the in-fluence of processing parameters such as feedthroughput, FWM, screw profile, temperature, ...The anisotropy of the light scattering patterninduced during the s hea r flow through t he op-tical cell can be used to analyze the relaxationof th e stretched drop lets when th e shear flowis suddenly stopped.

The important difference in terms of light scat-tering intensity profile between reactive andnonreactive blends will provide deeper insightinto how the morphology is affected by thefunctionality (n umber of reactive functions perchain) and the processing parameters.

Moreover, the us e of the second and third la ser willimprove the sensitivity to polymer blends having verysmall particles by analysis at wide angles. Finally, thisdevice could be probably used as a control processapparat us to follow the stability of the process and t hequali ty of the product.

ACKNOWLEDGMENTThe authors wish to thank Christophe Melart and

Patrice Simon for their help in running the experi-ments an d designing the apparatus.

REFERENCES

1. C. D. Han a nd T. C. Yu , Polym Erg. Sci., 12. 1 (1972).2. B. D. Favis an d D. Themen , Polymer, 32 474 (1991).3. N . C h ap leau an d B. D. Favis, J Mat. Sci., 3 0 , 142

4. D. B o u n y and B. D. Favis, Polymer, 39 851 (1998).5. J . K. Lee a nd C. D. Han, Polymer. 4 0 6277 (1999).6. L. Delamare, Ph.D. Thesis, Ecole des Mines de Paris

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14. H. C. van de Hulst, Light Scattering by Small Particles,Wiley, New York (1964).

15. M. Born and E. Wolf, Principles of Opt i c s , PergamonPre ss, London (1975).

16. G. G. Fuller, Optical Rheometry of Complex Fluids, Ox-ford University Press, New York (1995).

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1995).

(1995).

484 (1988).

a> b)Fig. 1 3 . d: EM of the nonreactiue blend (EBA/PAG 8@ /20 ). b): SEM of the reactiue blend [REBA/PAG 80 /20 ].


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19. H. P. Grace. C h e m Eng. C o r n , 14 225 (1982).20. H . E. H. Meijer and J. M. H. Janssen, in I. Manas-Zloc-

zower and Z . Tadrnor. eds., Mixing and C o m p o d i n g -Theory and Practical Progress, Vol. 4 in Progress in Poly-mer Process Series, Hanser Verlag, Munich (1993).

21. P. Cielo, B. D. Favis, and X . Maldague, PoZym Eng. Sci,2 7 , 1601 (1987).

22. C. Belanger, P. Cielo. B. D. Favis. and W. I. Patterson,Polym Eng. M, 3 0 , 1094 (1990).

23. C. Serra, G. Schlatter, M. Bouquey, R. Muller, and J .Terrisse, t h e 17th Annual Polymer Processing SocietyMeeting, Montreal (May 2001).

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