Comparison of Five Method to Evaluate Interfacial Tension Between Molten Polymers

7/27/2019 Comparison of Five Method to Evaluate Interfacial Tension Between Molten Polymers

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Comparison Between Five Experimental MethodsTo Evaluate Interfacial Tension Between Mo lten Polymers

NICOLE R DEMARQUETIE*,ADRIANA MARTINELLI CATELL1 DE SOUZA, GUILLERMO PALMER,and PAUL0 HENRIQUE PIERIN MACAUBAS

Mater ia l s EngineeringDepartmentUniversity of S f mPaul0Av. roJ M el o Moraes 3463

05508-900 h aul0, BrazilIn this work, an experimental comparison between five different techniques tomeasure interfacial tension between molten polymers is presented. The five tech-niques include two equilibrium methods: t he pendant drop (PD) and the sessiledrop (SD): wo dynamic methods: the breaking thread (BT) nd imbedded fiber re-traction (IF):and a rheological method based on linear viscoelastic measurementsof the blend (RM). he polymer pairs studied were polystyrene/polypropylene(PS/PP): and PP/high density polyethylene (PP/HDPE). I t was possible to determinethe interfacial tension between PP/PS with all the methods tested and the resultscorroborated within 20%. However, the interfacial tension between PP and HDPEcould be evaluated only usingrheological methods because of a too-small differenceof index of refraction between both polymers. The experimental precision increasedin the following order: R M


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Comparison Between Five Experimental Methodsof PS and three types of PP were used in this work,PP, an d PP, were used to evaluate the interfacial ten-sion between PP and PS and PP, was used to measureinterfacial tension between PP and HDPE. I t was nec-essary to use different types of PS and PP to evaluatethe interfacial tension between PP and PS to satisfythe different rheological restrictions of the differentmethods tested. The density of the polymers at a tem-perature of 200C necessary to infer interfacial ten-sion usingthe pendant drop method were inferred fromequation of state (3, ). The zero shear stress viscositynecessary to infer interfacial tension using the break-ing thread and rheological method were inferred byfitting Carreau's model (5) o plots of the complex vis-cosity against frequency. These data of complex vis-cosity were obtained using a controlled stress rheome-ter (modelSR-5000 from Rheometric Scientific).Some of the methods tested to evaluate interfacialtension between molten polymers involve long experi-ment duration during which the polymer is kept at atemperature above its melting point. Therefore, thethermal stability of the polymers was tested by GPC.The samples were kept in an argon atmosphere at atemperature of 200C for 10 hours (maximum dura-tion of the experiments performed in this work). Themolar mass of the samples were measured before an dafter this thermal treatment. It was observed that,within experimental error, neither the number aver-age molecular weight nor the polydispersity were af-fected by such a treatment (6).

INTERFACIAL TENSION MEASUREMENTSPendant drop method:The pendant drop method in-

volves the determination of the profile of a drop of onedenser liquid suspended in a less dense liquid at me-chanical equilibrium. The interfacial tension betweenboth liquids can be inferred from the resolution ofBashforth and Adams equation (7)hat relates thesurface tension to the difference of density betweenboth liquids and the geometrical profile of the drop.More details can be found in the literature (8- 11) .In this work, the interfacial tension measurementswere made using a pendant drop apparatus tha t basi-cally consists of three parts (6):n experimental cell

where the pendant drop of the polymer was formed,an optical system to monitor the evolution of the pend-ant drop and a data acquisition system to infer theinterfacial tension from the geometrical profile of thedrop. A proportional temperature controller with aprecision of kO.5"C was used to maintain the sampleat the desired temperature. The experimental cell wasmaintained in an argon atmosphere in order to avoiddegradation. The drop insertion device consisted of aspecially designed syringe to avoid problems of neck-ing and capillary effects (9).The drop profile analysiswas done using algorithms based on a robust shapecomparison between the experimental profile and the-oretical profile of the drop (6). ore details about theexperimental procedures can be found in Demarquetteand Kamal (9).Neumann Triangle: The Neumann triangle or ses-sile drop method is very similar to t he pendant dropmethod, consisting of the study of the profile of a dropof one liquid resting on a flat plate surrounded by an-other liquid of smaller density (in the case of the deter-mination of interfacial tension) or by air (in the case ofdetermination of surface tension) at mechanical equi-librium. The shape of the drop is determined by a bal-ance between gravily (or buoyancy forces) and surfaceforces. It is possible to infer the value of surface or in-terfacial tension from the shape of the drop at mechani-cal equilibrium. However, because of long equilibrationtimes, it is very ditficult to be u sed in molten polymers.A new variation of the sessile drop method consistsof using the Neumann triangle (12, 3). o evaluatethe interfacial tension between two liquids. When adrop of a molten polymer rests on a plate formed byanother polymer, two contact angles el and e,, can bemeasured as shown in Flg. 1. Using the values of thesetwo contact angles and the values of surface tension ofboth polymers determined by another method it ispossible to determine the interfacial tension betweenboth polymers using the followingEq 1:

=ylz cosez+y2 case, (11where yl, ,, are the surface tension of both polymers,ylz is the interfacial tension between both polymers,e l is the contact angle formed between the horizontal

Table 1. Materials Used in This Work.

~ ~

75.200 4.65 1.5 4.64 - 0.751 Polibrasil20 0.1 - 0.751 Polibrasil76.000 4.5 8 - 1.07 Polibrasil91.200 2.5 2.2 3.39 - 0.970 Estirenodo Brasil76.600 2 1.85 0.970 BASF44.000 3.2 8 0.850 lpiranga Petroquimica

PP,pp2pp3PSIps2HDPE,HDPE, 17.000 9.0 - 86.40 AldrichMFI Melt Flow Index,q,,. zero shearstressviscosity, p: density.

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Hg. 1 . Sessile drop geometry.

line and the &/polymer surface of polymer 1 and 8,is the contact angle formed between the horizontalline and the polymer 1 polymer 2 interface.In this work, drops of polystyrene were formed on aplate of polypropylene at a temperature of 200C.Thesamples were left in an argon atmosphere until me-chanical equilibrium was reached (around 8 hours).The sample was the n cooled, encapsulated in acrylicresin, cut with a diamond disc rotary cutter, and ob-served with a reflected light microscopy. The contactangles could be subsequently measured.BreakingThread method:The breaking thread meth-od involves the observation of the evolution of theshape of a long fluid thread imbedded in another. Be-cause of Brownian motion, small distortions of arbi-trary wavelength are generated at the surface of thethread; this leads to a pressure difference between theinside and the outside of th e thread, which inducesmore important deformations caused by the effect ofthe interfacial tension that tends to reduce the inter-facial tension. It i s possible to infer interfacial tensionbetween the polymers forming the thread and thematrix from the s tudy of the evolution of the disturb-ances and the zero shear stress viscosity of the poly-mers. ' l b o theories have been developed to infer inter-facial tension between both polymers from the studyof the evolution of the thr ead the theory of Tomokita(14, 15)an d the theory of Tjahjadi et aL (16).More de-tails about the calculation of interfacial tension fromthe study of the evolution of the thread can be foundin Luciani et aL (17)andTjahjadi et aL (16).In this work, based on the melting and glass transi-tion temperatures, PS was chosen to make the films(matrix) and PP to produce fibers. PP fibers were ob-tained by melt spinning of molten pellets from a hotplate. Fibers diameters varied from 30 p,m to 110 pm.The fibers were annealed during 12 hours at 150Cunder vacuum to avoid residual stresses. The fibersused were cut in 1.0 cm pieces prior to annealing. Theaspect ratio bf /Dbf (where and D are respectively

the length and diameter of the fiber) was chosen sothat it would be higher than a critical value that de-pends on the viscosity ratio A = qd/qom (where qofisthe zero shear viscosity of the fiber and qom s the zeroshea r viscosity of the matrix) 16).The fibers had theirextremities fixed during annealing to avoid sigmficantdistortions of the diameter. The films used in the ex-periments were obtained by compression molding andhad a thickness of 0.25 mm. This thickness was opti-mized to minimize eventual problems with air bubblesan d to promote a melting process fa s t enough to avoidthe fibers distortion to start before complete melting ofthe film. The width and length of the filmwere 1.5 cm.These dimensions are much larger than those of thefiber, preventing edge effects to have an influence onthe capillary instability (17, 18).The experiments werecarried out placing the PP ibers between two PS films.The "sandwich" formed was then placed between twoglass sheets and heated in a hot stage (MettlerFP-90).The temperature was raised at a rate of 2OoC/s to150OC.The system was maintained at 150C until allth e air bubbles were able to escape. Then, the tem-perature was raised to the temperature at which theexperiment was performed (200C).Photos of the break-up process were taken using a CCD camera. More de-tails about the experimental procedures can be foundin another work (18).Imbedded Fiber Retraction method. The imbeddedfiber retraction method is very similar to the breakingthread method, except that the fiber is shorter. Themethod involves the observation of the fiber that re-tracts into a sphere. From the study of the evolution of'the fiber and the knowledge of the zero shear viscosityof both polymers, it is possible to infer the interfacidtension between those both polymers. Two theorieshave been developed to infer interfacial tension betweenboth polymers from the study of the evolution of thefiber: the theory of Carriere and Cohen (19, 20) andtheory of Tjahjadi et al. (16).Both theories were usedin this work.

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ComparisonBetween Five Experimental M e t W sThe experimental procedures to perform an imbed-ded fiber retraction experiment were similar to theones adopted for the breaking thread method exceptfor the length of the fiber which was, in the case of theimbedded fiber method considerably shorter.Rheological methods: A continuous effort has beenapplied in the last fifteen years in order to improve the

understanding of the relationship between the v i s -coelastic properties of polymer blends, their morphol-ogy and the interfacial tension between the polymersforming the blend (21-26). in particular, it has beenshown that immiscible blends have a higher elasticity,in the low frequency range, than the individual com-ponents of the blend. The higher value of elasticity ofthe blend results in the presence of a secondary pla-teau in the curve of the storage modulus G versusfrequency, w , for low frequencies, that can be used toinfer the interfacial tension from the rheological prop-erties of the blend (21, 22). Two emulsion models havebeen developed to predict the l i near viscoelastic behav-ior of polymer blends: a) Paliemes (21);b) Bousminas(22). Those models correlate the dynamic response ofpolymer blends to their morphology, composition andinterfacial tension between the components. Two typesof analysis can be performed to infer interfacial ten-sion from small amplitude oscillatory shear measure-ments: a) comparison between the complex modulusof the blend measured experimentally to the emul-sions models in the plateau region;b) identification ofa relaxation time relative to the relaxation of the dis-persed phase in the relaxation spectrum of the blend(23, 27). Those two types of analysis were used here toinfer interfacial tension between PP and PS and be-tween HDPE and PP polymer.The small amplitude oscillatory shear experimentswere carried out using a controlled stress rheometer(model SR-5000 from Rheometric Scientific) under ni-trogen atmosphere. A parallel-plate configuration wasused. Dynamic frequency sweeps were performed forPP,/PS, blends , HDPE,/PP, blends an d pu re poly-mers.Blends of PP,/PS, Blends were prepared in 90/10concentration an d PP,/HDPE, were prepared in sixdifferent weight concentrations ranging from 95/5 to70/30. The blends were prepared in a Werner & Pflei-derer twin-screw extruder, model ZSK -3 0 , with sixzones of temperatures, ranging from 170 to 210Calong the barrel of the extruder. The morphology ofthe blend was characterized by scanning Electron Mi-croscopy (SEW using a Cambridge microscope, modelStereoscan 240. The samples were fractured in liquidnitrogen and then covered with gold using a Bakerssputter coater, model SCD-050. The average diameterand volume fraction of the minor phase were calcu-lated using the SEM photomicrographs. About 300particles were used to calculate these parameters. Forthe calculation of average size of the minor phase,Saltikovs correction (28) was used. This correctiontakes into account the polydispersity of the samplesand the fact that the fracture in the sample does not

always occur at the maximum diameter of the dis-persed phase droplets.RESULTS AND DISCUSSION

The interfacial tension between PP and PS was eval-uated using the five different methods studied in thiswork. It was possible to evaluate the interfacial ten-sion between PP and HDPE only using the rheologicalmethod; the pendant drop and dynam~cmethods testedin this work are based on the visualization of a dropor fiber of one of the two polymers in a matrix formedby the other. For that, both polymers should have adifference of index of refraction high enough to enablethe visualization of the profile of the drop. This wasnot the case for PP and HDPE.The experimental results found using each methodtested are reported below. The different experimentaltechniques are then compared in terms of experimen-ta duration, reliability and experimental difficulty.Theadvantages and limitations of each method are thendiscussed.Pendant Drop

Figure 2 shows the interfacial tension calculatedfrom the shape comparison as a function of time of atypical drop of PS, in PP, at a temperature of 200C.It can be seen from Fig. 2 that after 10 hours, thevalue of interfacial tension is constant. Typically, ittakes eight to ten hours for a drop of PS in a matrix ofPP to reach equilibrium. The time to reach mechanicalequilibrium depends on the viscosity of the samplesinvolved in the measurement, i.e., on the temperatureat which the experiment is performed and also on themolecular weight of the sample. The interfacial ten-sion between PP, and PS, at a temperature of 200Cwas found to be equal to 5.52 2 0.2 mN/m, corrobo-rating the values obtained by other authors (27, 29).Naumaxu~Triangle

F@ue 3 shows a typical cut of a sessile drop of PP,on a plate of PS, imbedded in acrylic resin. The con-tact angles 8, and 8, as well as the values of surfacetension of PP, and 5, t a temperature of 200C nec-essary for the calculation of the interfacial tensionbetween PP, and PS, using E q I are reported in Table2. The values of surface tension of PP, a nd PS, wereobtained using the pendant drop method. Using thevalues reported in Tabte 2, the interfacial tension be-tween PP, and PS, was found to be equal to 6.65 t 1mN/m.Breaking Thread

In order to determine the interfacial tension betweentwo polymers using the breaking thread method, thefiber should be formed with the material with thelower viscosity (30, 31) and with the highest meltingor glass transition temperature. If the fiber is formedby the material with the highest viscosity, phenomena

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0 200 400 600 800 1000 1200 1400 1600 1800Time (min)

m.2. Interfacial ension betweenPSI nd PP l at a ternperahre of 200C asa-n of t i me.

such as end pinching and retraction could occur. Ifthe fiber is formed by a material with the lowest melt-ing or glass temperature, its distortions will start be-fore it is completely imbedded in the matrix and airbubbles will occur at the interface between both poly-mers (18). If both conditions are not satisfied at thesame time, it is possible to use a fiber with a lowermelting temperature than the matrix using appro-priate thermal treatment during the experiment (18).However, the difference between the melting or glasstransition temperatures should not exceed 20C. here-fore, it was not possible to measure the interfacial ten-sion between PP. and PS, usin@ h e breaking thread

the theory of Tjahjadi et at (I6) llowed a better evd-uation of the dynamic behavior of the maximum andminimum instabilities of the fiber during the breakingphenomenon, facilitatingthe discarding of bad experi-ments.

Figure 5 shows the time for complete breakup of afiber of polystyrene (PS,) in polypropylene (PP,). as

1 -- I --- - - - - - ~ - -_ _ _ - - - _ _ - -method. In order to test the breaking thread methodfor polypropylene/polystyrene polymer pair, it wasnecessary that the zero shear viscosity of the polypro-pylene be lower than that of polystyrene. Therefore ahigher melt index PP, PP,, w as used. Similar procedurewas necessary for the imbedded fiber experiments.Figure 4 shows a typical evolution of a fiber of PP,imbedded in a matrix of PS, at a temperature of 200C.It can be seen that the fiber is completely imbedded,before any significant growth of instabilities can beobserved. Also during the experiments, no matrix thin-ning was observed. The average values of interfacialtension obtained in this work for PP, and PS, poly-mer pair using the theories of Tomotika (14, 15)andTjahjadi et al (16)were respectively8.28 r. 0.79 mN/mand 7.83 5 0.57 mN/m showing that both methodslead to a similar result. It was noticed, however, that674

Fig.3. Sessile drop of PPl on PSI.POLYMER ENGlNEERlMG AND SCIENCE, MARCH 2003, Vol .43,No. 3


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ComparisonBetween Flue Experimental MethaisTable 2. Interfacial Tension Between PP, and PS, Evaluated Using the Sessi le Drop Method.

PP, Ips,

yps an d ypp , are the surface tension of PS an d PP,. y, * is he interfacial ension between PS, an d PP,. 0, and O2 are defined in Fig. 6

calculated by Elemans (3 1, at a temperature of 200Cas a function of fiber radius for different viscosity ra-tios, A. A = 0.059 corresponds to the viscosity ratio ofthe polymers used in this study. The range of fiber ra-dius studied corresponds to the radiuses used for thedifferent experiments. It can be seen that the time forcomplete breakup increases with increasing fiber ra-dius and wi th increasing viscosity ratio, A. However, itcan be seen that for A =0.059 (case studied here) thetime for complete breakup is less than three hours,for radiuses ranging from 15 Fm to 55 pm. Therefore,the time to perform a breaking thread experiment isconsiderably smaller than the time to perform a pend-ant drop experiment.Figure 6 shows the time for complete breakup offiber of polystyrene (PS,) in polypropylene (PP,) at atemperature of 200C as a function of matrix viscos-ity or fiber viscosity. When the value of either the fiberor matrix were varied, the viscosities of the comple-mentary pha se s were considered as the viscositiesreported in Table 1. The values of interfacial tensionand fiber diameter taken in the calculation were re-spectively 6.5 mN/m an d 60 pm. Assuming fourhours as a reasonable time for an experiment withoutthermal degradation, it can be seen that the breakingthread suffers limitation as f a r as viscosity ofboth thematrix an d fiber are concerned. For a diameter of 60pm an d a n interfacial tension of 6.5 mN/m, the vis-cosity of the matrix should not exceed 69,200 Pa.s(when the zero shear viscosity of the fiber is 1100Pa.s) and the viscosity of the fiber should not exceed

5590 Pa.s (when the zero shear viscosity of the matrixis 18,583Pa.$. These upper limits of viscosity can beincreased to 110,000 Pa.s and 18,583 Pa.s respec-tively if the diameter of the fiber is reduced to 30 Fm,which the lower limit for a breaking thread experi-ment (18). These upper limits of viscosity are lower ifthe interfacial tension between both polymers is lower,which is the case of compatibilized blends.Imbedded Fiber Retraction

Figure 7 presents a typical evolution of a fiber ofPP, imbedded in a matrix of PS, at a temperature of200C. The interfacial tension between PP, and PS,was measured using both the methods of Carriere andCohen (20, 21) and Tjahjadi (16).According to Carriereand Cohen the evolution of an imbedded fiber as afunction of time canbe described by:

where,f ( x )=1.5Ln{( l t .l- x2)0'5>

+1 .5 f i t an - ' (2+x - 0 . 5 ~ 4x-2 (3)where x is an hydrodynamic coefficient, Re is the ra-dius of a sphere having the same volume as the fiber,

FYg. 4. Optical micrographs of aP P 2 ~mbedded in aPS2matrix at 200C.POLYMER ENGINEERING AND SCIENCE, MARCH2003, Vol. 43, No. 3 675


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17 I Iw-

14 -12-10-8 -A25

13+ 6 -

8 -

6 -

4 -

2-

0-

10 20 30 40 50 60Ro, (P-0

FQ.5.Breahng time of ajtber of PP, in a matrixof PS, at a temperatureof 200C as afunctionoffiber radius.

I W

8 -

6 -

4 -;0-in I I I T- 7 I I

0

/=1-C- q, =18583 Pa.s-+- , =1100 Pa.s7;.nrW+

/=7

1I I I I I fI I0 20000 40000 60000 80000 100000 120000

ro P a4Flg.6.Breaking time of of PP, in a matrix of PS, at a temperatureof 200C as afunctionof matrix andfiber viscosity.

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Cornparison Between Five Experimentai Methods

Fig. 7. Optical micrographsof a PP,jiber hbeddedin a PS, matrix at 200C. he UnpertUrlEdradius of them a,= 26 krn andviscostty ratio p =0.059. he dimensionless tirnes,frornhe top, are t = 660,840,960, 1 10 and 1560 s , respectiuely.

Rv s the radius of the fiber as a function of time andqe is the effective viscosity, which is a function of theviscosities ofboth polymers, given by:q o m +1.7TofX T e = 2.7 (4)

where qoms the zero shear stress viscosity of the ma-trix and qof is the zero shear stress viscosity of thefiber.

FTgure 8 showsf (2)f(4)s a function oftime for the fiber presented in6.. It can be seenthat the data lead to a straight line from which theinterfacial tension can be inferred using E q 2. Usingqe defined by Eq 4 and ROiJ= 32.01 p,m, the inter-facial tension was found to be equal to 15.85mN/m.Tjahjadi et aL (16)resented a method to determinethe interfacial tension between two Newtonian fluids

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Nicole R . Demarquette et al.from short fiber retraction. The method uses curve-fit-ted polynomials to describe the decrease in length ofthe fiber as a function of the viscosity ratio, X, and di-mensionless computational time as :

where the five coefficients, k,- k,, for different viscos-ity ratios and initial aspect ratios, AR, an be found inthe article of Tjahjadi et al. (16),L(T)s the half lengthof the fiber at time T , and 7 a dimensionless computa-tional time defined as:7 = t/tclf (6)

where t is the real time of the experiment and tCiJischaracteristic time for the interfacial tension drivenmotions in the experiment given by:

where y is the interfacial tension, %ifis nitial the ra-dius of the fiber, -qom s zero shea r matrixviscosity.To infer interfacial tension from the retraction of afiber using Tjahjadi's method, two images of the evo-lution of the fiber at two different times t, and t, (withAt, = t , - l ) are analyzed and the half length of thefiber for both images are measured. Independently,L(T) /R,s plotted using E q 5 and the coefficient k,, orthe appropriate viscosity ratio. Using the graph andthe experimental values of L( t ) /Re or both images,it is possible to determine AT^^ which correspondsto the theoretical time interval between both images.

AT^^ is then compared to At- and the interfacialtension can be inferred using Eqs 6 and 7.Figure 9 presents __(7) as a function of time for =

for t, =.03.Using the experimental value of ~540 s and & = 600 s , can be measured fromFig. 9. It was found equal to 0.74. sing Eqs 6 nd 7,the value of interfacial tension was found equal to7.34mN/m.Figures 8 and 9 show that the values of interfacialtension obtained, from the study of the retraction ofPS, in a matrix of PP,, using Carriere and Cohen'sanalysis is considerably larger than the one obtainedusing Tjahjadi's analysis using the same fibers. Thevalues obtained using Carriere and Cohen's analysisis also considerably larger than the one obtained withthe pendant drop or imbedded fiber method. Similarbehavior was observed for all the experiments. Thesehigh values may be due to the use of E q 4 to deter-mine the effective viscosity of the polymer pair. Thisexpression has been derived empirically for polysty-rene/polymethyl methacrylate polymer pair and maybe not adapted to polypropylene/polystyrene olymerpair.

K?YT)Re

RheologicaI Me th o dFigures 10 and 1 1 show the storage modulus for

PP,/PS, (90/10)lend at a temperature of 200C andfor different PP,/HDPE, blends at a temperature of

0 - 2 4 6 8 10 12 14 16 18(s)

FYg. 9. Analysis of a P, imbedded iber n P S, according to"ahjadi's analysis.678 POLYMER ENGINEERING AND SCIENCE, MARCH 2003, Vol. 43, No. 3


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Comparison Between Five Experimental Methods000000

1000007

100007

100051

100 z

10

1 j 1 I I I I I , I 1 I 1 , , , 1 , , , , , , , , ,,, , , , , , , , , , ,, IA * *. n $ W

A 6 3, e P -18es -

G' PP/PS 90/10 :0 G" PP

U A A G" PS0 x B

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I 1 , , . , I 1 I I I , , , , I I I , , , , i I I I , 1 1 1 , ( I I i I l 1 . l I , , , , 1 1 1 I

Frequency (rad/s)Q. 10. Storage f G f w ) ] moduli of the PPl /P SI (90/101 lends and ofpure phas es at 200C.

100000

10000ncaa.Wv) 1000

O 1002S=73-a,m02 10G

1

0.10.01 0.1 1 10 100

Frequency (rad/s)Q. 1 1 . Storage (G ' fw) ) moduli of the P P 3 / H D P E , blendsfor different compositionsand of the pure phases at 220C.

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Nicole R . Demarquette et al.160000 I I I i l l l 1 I I I I l l . 1 1 1 1 k 1 1 # I I 1 1 1 1 8 1 I I 1 1 1 1 1 1 1 I

TI=35 s-*P/PS 90/10140000- -C-PP -120000-100000-

- L P S -nrT)


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ComparisonBetweenthe relaxation spectra of the different blends of PP,/HDPE,. It can be seen that the values of interfacialtension between PP, and HDPE, vary from 1.01 to2.32 mN/m at 220C. It can also be seen that forcomposition range from 85/15 to 75/25, the interfa-cial tension seems to be constant. These results seemto indicate that there is a range of compositions forwhich it is possible to use the rheological method toinfer interfacial tension between polymers.Comparison of the MethodsTable4 hows a comparison of the values of surfacetension between PP and PS and between HDPE andPP using the different experimental methods tested inthis work. The difference between the highest and thelowest value of interfacial tension between PP and PSis around 20% but is acceptable within experimentalerror. The higher values obtained with the dynamicmethods may be due to the fact that the theories ofthese methods do not take into consideration th e vis-

coelastic character of the polymers. Also, the valuesobtained using the dynamic methods are directly pro-portional to the zero shear stress viscosity of the poly-mers , which is a parameter that is diffcult to measurewith high accuracy. The interfacial tension betweenPP and HDPE could only be evaluated using RM dueto a too small difference of index of refraction betweenboth polymers.Table4 lso shows the duration, experimental errorand experimental difficulty for each method. The ex-perimental methods are ranked from 1 to 5 accordingto those characteristics. Table 5 shows the advan-tages, limitations for each method tested in this work.It also presents the parameters necessary for the eval-uation of interfacial tension. The pendant drop methodis the most precise of all the methods tested in thiswork. I t s precision is limited by the precision withwhich the density of the polymers is determined. Nowa-days, this density can be determined with a good pre-cision because of the development of PVT apparatus(3). he pendant drop method is, however, limited asf a r as duration of experiment is concerned, as thetimes to reach mechanical equilibrium of the drop canbe up to five times the ones needed for other methods.The time for a pendant drop to reach mechanical equi-librium can reach ten hours whereas the time to per-form a rheological experiment does not exceed twohours, an imbedded or breaking thread experiment

Five Experimental Methodsthree hours . The experimental precision of the break-ing thread an d imbedded fiber methods is limited be-cause the interfacial tension is directly proportional tothe zero shear stress of the polymers involved, a diffi-cult parameter to be determined accurately (5).More-over, the breaking thread and imbedded fiber experi-ment need to be done under a precise temperaturecontrol, owing to the large variation of viscosity withtemperature. Any small variation of the temperatureduring the experiment could lead to erroneous resultsof interfacial tension. The interfacial tension, as deter-mined using the Neumann triangle, depends on thevalues of surface tension of the other polymers evalu-ated using another method and on the preparation ofthe samples. At las t, the rheological method is the onethat involves the largest imprecision in t he determina-tion of interfacial tension, as the value obtained de-pends on a precise characterization of the morphologyand on the values of zero shea r str ess of the polymersinvolved.

The rheological method is the simplest method to beused to measure interfacial tension between moltenpolymers. The main experimental difficulties encoun-tered when using the pendant drop, breaking thread,imbedded fiber and Neumann triangle methods rely inthe preparation of the samples. In the case of the pen-dant drop method, it is very difficult to form a polymerdrop avoiding the necking and capillary effect: lots ofcare has to be taken in the image analysis to avoiderrors such as dependence of interfacial tension withdrop volume, which contradicts the Laplace equation(10): he analysis of the pendant drop is relatively com-plex. The Neumann triangle presents a n intrinsic dif-ficulty since the measurement of the contact anglesdepends on the abilily of cutting the sample in the mid-dle section of the drop and at the same time perpen-dicular to the base of the drop (12). n the case of thebreaking thread a nd imbedded fiber methods, thefibers have to be annealed to avoid residual stressesthat could affect the breaking or retraction process,the thicknes s of the film th at sur rou nds the dropshould be carefully lailored to avoid problems such asmatrix thinning. This matrix thinning squeezes thefiber and affects the dynamic phenomena (18).

CONCLUSIONSIn this work, five rq er im en ta l methods were testedand compared to evaluate the interfacial tension

Table 4. Summary of the Experimental Results.Pendant Neumann Breaking Imbedded Fiber RheologicalDrop Triangle Thread Retraction MethodPolymer Pair (P.D.) (N.T.) (B.T.) (I.F.R.) (R.M.)

PP,/PS, (200C) 5.522 0.2 6.65L 1 Impossible Impossible 6.25 0.87PPJPS, (200C) 5.06 % 0.4HDPE,/PP, (220C) lmpossible No t tested lmpossible lmpossible 1.63 % 0.16Duration 4 5 3 2 1Error 1 4 3 2 5Experimental difficulty 2 3 4 4 1

- 7.83% 0.57 7.82% 0.43 -

POL YMER ENGINEERING AND SCIENCE, MA RCH 2003, Vol. 43,No . 3 681


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Nicole R. Demarquette et al.

PO

NT

Very good accuracyRequires little amount of materialCan be usedof LCP

Theory developed for all types of fluids(10 mg for the drop and 3 g for the other) The difference of index of refraction of thetwo

-

of the three polymer pairs(10 mg for the drop and 3g for the other)Theory developed for all types of fluids

Simple experimental set-up and analysisRequires small amount of material

BT Theory developed for Newtonian fluidsPolymer matrix (with higher viscosity)must be transparentThe difference of melting or glass transitiontemperature between both polymers shouldexceed 10CThe viscosity of the polymers nvolved should notexceed an upper value that depends on viscosityratio and interfacial tension

IFR

Zero shear viscosityof both polymersShort time duration of experimentRequires very small amountof bothpolymers (


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Comparison Between Five Experimental Methods5) The pendant drop and dynamic methods relyon the visualization of one phase into anotherand therefore can be used only for polymerpairs presenting a large difference of refractiveindexes of their components; the methods can-not be used either the polymer forming the ma-

trix (lower density in the case of pendant dropmethod and higher viscosity in the case of dy-namic methods) are opaque in the molten state.

6) The pendant drop method requires the knowl-edge of the density of the polymer in the moltenstate, a difficult parameter that can now be ob-tained using PVT apparatus (3). he dynamicand rheological methods require the knowledgeof zero shear stress viscosity of the polymers, adifficult parameter to be determined accurately(5) he rheological method requires an accuratequantitative determination of the morphology ofthe blends formed by the two polymers.

7) The static and breaking thread methods are l imited to low-medium range viscosities of the poly-mers (

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Comparison of Five Method to Evaluate Interfacial Tension Between Molten Polymers