P-01-421
POLYMER/POLYMER INTERFACIAL SLIP BETWEENPOLYPROPYLENE/POLYSTYRENE MELTS IN MULTI-LAYER FLOW
Ryohei Komuro, Sathish K. Sukumaran, Masataka Sugimoto, Kiyohito Koyama*
Graduate School of Science and Engineering, Yamagata University, Japan – koyama@yz.yamagata-u.ac.jp
Abstract - At the interface between two immiscible polymers, chemically different chains are likely to be weakly
entangled leading to poor interfacial adhesion. Early observations of unusally high flow rates under coextrusionsuggested the existence of liquid-liquid interfacial slip. To understand the behavior at the interface, we investigatedinterfacial slip using Polypropylene(PP)/Polystyrene(PS) with matched shear viscosity and moduli in co-extrusion
process. As seen from this study, We found that the interfacial slip velocity of PP/PS/PP and PS/PP/PS could beevaluated by eq.(2), and the interfacial slip velocity depended on the interfacial shear stress. Furthermore, wedemonstrated the existence of polymer/polymer interfacial slip in co-extrusion process by the visualization technique.
Keywords: polymer-polymer interfacial slip, multi-layers, coextrusion
Introduction
In multi-layer film processing, two or morematerials are extruded through a single die. Especially,in case of the immiscible polymer systems, theinterface has low entanglement densities and weakinteractions. Therefore, the interface between polymerpolymer cannot sustain high shear stress. So, it isthought that the interfacial slip is believed to occur
under this situation.
Past studies about the slip at polymer/polymer
interface, some scientists have investigated via
rheological measurements and/or the opticalobservation [1-8]. When two or more polymers are
coextruded, at the interface between two immiscible
polymers liquid, the chemically different chains arelikely to be weakly entangled leading to poorinterfacial adhesion. Hence when two immiscible
polymers are coextruded, one might expect slip at theliquid/liquid interface. Han et al. [1] found that thepressure gradient in multi-layer flow was decreased ascompared with the individual component at same flow
rate. These result suggested the existence ofpolymer/polymer melts interfacial slip during multilayer flow. Recently, Lee et al. [2] and Park et al. [3]estimated the interfacial slip velocity using therotational plate rheometer and the sliding platerheometer. However, the polymer/polymer interfacialslip in co-extrusion has not been well understood.
Because, the method to estimate the polymer/polymer
interfacial slip in co-extrusion has not been wellestablished.
Recently, we succeeded to estimate thePolypropylene(PP)/Polystyrene(PS) interfacial slip
verocity with matched shear viscosity and moduli in
two-phase concentric flow [9]. In this study, weinvestigated PP/PS interfacial slip in two-layer andthree-layer flow using the slit die. Then, we comparedwith our earlier results from capillary rheometry[9].
Moreover, we investigated the flow velocity
distribution by the visualization technique to confirm
the existence of. polymer/polymer interfacial slip.
Because If interfacial slip occurs at the interface of themultilayer structure, it will be reflected directly by theexistence of velocity discontinuity at the interface.

Experimental
Materials
In order to avoid the effect of viscoelastic ratio on
the interfacial slip behavior, we used isotactic-PP andatactic-PS having the quite similar viscoelastic
properties over a wide range of shear rates as shown inFig.1. The materials chosen for the experimental study
were PP (Mw=345,000 g/mol, Mw/Mn=5.02) and PS(Mw=312,000 g/mol, Mw/Mn=2.66). PP was kindly
supplied by JAPAN POLYPROPYLENE Co.. PS waskindly supplied by PS JAPAN Co..

Fig.1 (a) G’, G’’, and (b)|η*| as function of ω at 230 oC.

Proceedings of the Polymer Processing Society 28th Annual Meeting ~ PPS-28 ~ December 11-15, 2012, Pattaya (Thailand)Rheological measurements
The dynamic small angle oscillatory shear
measurements were performed using a rotational platerheometer (TA instruments, ARES) under a nitrogen
atmosphere at 190-250oC. The geometry of jig isparallel-parallel plate with diameter of 25mm. The gap
was set to be 1mm. Circular disks of each sample wereprepared by compression molding at 230oC. Fig.1
shows the master curve of (a) storage modulus G’ andloss modulus G’’ and (b) the complex viscosity |η*|and the steady shear viscosity η s of PP and PS at thereference temperature Tr=230 oC.
The steady shear viscosity η s at the higher shear
rate range (γ& > 10 s −1 ) was measured using a capillaryrheometer (Toyo Seiki Seisaku-sho Co, Capilograph).
The wall shear stress σw and the wall shear strain rate
γ&w were corrected using the Bagley correction and theQno-slip was evaluated using eq.(1). Since the wall slip
does not occur within our experimental condition, Vs-i
was estimated by eq.(2).
Results and Discussion
Fig.2 shows Vs-i as σa-i for PP/PS (capillary),
PS/PP/PS (slit), and PP/PS/PP (slit). From Fig.2, it isfound that the interfacial slip velocity of PP/PS/PP andPS/PP/PS were rather close and increase confidence
the determined interfacial slip velocities, and thedependence of Vs-i on σa-i was a power-law. the powerlaw exponent were approximately 2.2 at an interfacialstress. These results suggested that Vs-i depended on σa-i
although the Vs-i was not affected by the shape of die.

Rabinowitsch correction using three types of capillary
dies having D=1mm, L=5, 10, 20mm, respectively.
From Fig. 1(b), it is found that the data of |η*| and η swere in good agreement with Cox-Merz rule [10].
The Carreau-Yasuda equation is found to fit theshear viscosity versus shear rate data quite well [11]:{η = η 0 1 + (λγ& )α}( )n −1 α(1)ω = γ&

where η0, λ, n, and α are the zero-shear viscosity, time
constant, power-law exponent, and dimensionlessparameter, respectively. The solid lines in Fig.1 are theCarreau-Yasuda prediction for PP and PS.
Co-extrusion measurements
Thepolymer/polymer
interfacialslip
measurements in the multi-layer flow were performed
using the two twin-screw extruder (TECHNOVEL Co,ULT nano) and the slit die (width/height=10/0.5mm).
This slit die contain the glass window at the bottom ofdie. In order to investigate the velocity profile forheight direction, we used the confocal scanning laser
microscope (Yokogawa Electric Co., CSU-22). The
scanning data was imported by high speed camera(RED LAKE Inc., Motion Pro HS-4), Ar-laser(MELLES GRIOT Inc., 543-BS-A02). Fluorescenceparticle 0.05wt% (average diameter is 3μm) werepremixed with PP and PS. Then, the velocity profilefor height direction was evaluated to scan the velocity
of fluorescence particle.
Polymer/polymer interfacial slip velocity in threelayer flow
In case of three-layer flow, if we assumed that h,w, Q, Qno-slip, a,b, and Vs-i were the height of slit die, thewidth of slit die, the total volumetric flow rate, the total
volumetric flow rate in case of no-slip boundary
condition, the interfacial slip velocity, Vs-i is asfollowing equation:(2)
V s −i = Q − Q no −slip (b − a )W(Fig.2 Interfacial slip velocity Vs-i as a function ofinterfacial shear stress σs-i for ○ PP/PS(capillary), ■PS/PP/PS (slit), and ✠PP/PS/PP(slit).
Fig.3 shows the velocity profile for PP, PS, and
PS/PP/PS at almost similar pressure gradient. In thisstudy, PP and PS have quite similar viscoelastic
properties over a wide range of shear rates as shown inFig.1. Namely, the velocity profile of PP and PS atsame pressure gradient must be same. If the interfacialslip does not occur in PS/PP/PS, the velocity profile ofPS/PP/PS would be same with PP and PS (at samepressure gradient). However, As seen from Fig.3, it isfound that the velocity profile of PS/PP/PS wasdiscontinued at PP/PS interface although the velocity
profile of PP and PS were almost same. We
demonstrated that the existence of polymer/polymer
interfacial slip in co-extrusion process by thevisualization technique)

Proceedings of the Polymer Processing Society 28th Annual Meeting ~ PPS-28 ~ December 11-15, 2012, Pattaya (Thailand)

Acknowledgements
This research was supported by JSPS KAKENHI(24350114) and the Dissemination of Tenure Tracking
System Program of Ministry of Education, Culture,Sports, Science and Technology -- Japan.

Fig.3 Velocity profile for PP, PS, and PS/PP/PS atconstant pressure gradient.
Conclusions
We investigated interfacial slip using PP/PS withmatched shear viscosity and moduli in co-extrusion
process. As seen from this study, We found that theinterfacial slip velocity of PP/PS/PP and PS/PP/PS
could be evaluated by eq.(2), and the interfacial slip
velocity depended on the interfacial shear stress.
Furthermore, we demonstrated the existence ofpolymer/polymer interfacial slip in co-extrusion
process by the visualization technique.

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Proceedings of the Polymer Processing Society 28th Annual Meeting ~ PPS-28 ~ December 11-15, 2012, Pattaya (Thailand)