Flow induced surface enrichment of poly(ethylene-block-ethylene oxide)

Korea-Australia Rheology Journal Vol. 23, No. 1, March 2011 pp. 33-40 DOI: 10.1007/s13367-011-0005-7

Flow induced surface enrichment of poly(ethylene-block-ethylene oxide) on polypropylene by capillary rheometer

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I· lhan Özen1,*, Claude Rustal2, Klaus Dirnberger3, Hans-Gerhard Fritz2 and Claus Dieter Eisenbach3

Erciyes University, Faculty of Engineering, Department of Textile Engineering 38039 Melikgazi Kayseri Turkey 2 Institut für Kunststofftechnik (IKT), University of Stuttgart, Böblinger Str. 70, 70199 Stuttgart Germany 3 Institut für Polymerchemie, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart Germany (Received July 13, 2010; final revision received December 16, 2010; accepted December 20, 2010)

Abstract In this study, we prepared polypropylene (PP) blends with commercially available amphiphilic diblock copolymer poly(ethylene-block-ethylene oxide) (P(E-b-EO)). Selective surface enrichment of the diblock copolymer used was observed only on the PP blend extrudates which were prepared using capillary rheometer with different wall shear rates showing that flow processes play an enormous role in surface migration. Keywords : polymer blends, migration, diblock copolymer, shear flow, capillary rheometer

1. Introduction The strategy of polymer blending has attracted great interest both by academia and industry since it is usually cheaper and less time-consuming than the development of new polymers by synthesis (Doudou et al., 2005). After becoming widespread commercial applications of polymer blends, intensive studies have been carried out in the last 3 decades. In most cases, polymer blends suffer from immiscibility since the blend components have different molcular structures. In order to overcome this problem, block copolymers are used as interfacial modifiers or compatibilizers to modify the interphase of the blend components (Wang and Safran, 1990; Jeon et al., 1997). They can either be added to the blend (the so-called physical compatibilization route) or can be generated during the mixing step (the so-called chemical compatibilization route). In either case, interfacial activity of the diblock copolymer added to homopolymer blends leads to the reduction of the interfacial tension between two coexisting phases caused by the accumulation of diblock copolymer at the interface between the immiscible homopolymer phases (Di Lorenzo and Frigione, 1997; Koning et al., 1998; Van Puyvelde et al., 2001; Anastasiadis et al., 1989; Noolandi and Hong, 1984; Leibler, 1982; Leibler, 1988). Surface migration and thus compatibilization mechanism of block copolymers are induced by large deformations and stresses, i.e., flow processes generated during polymer processing (Larson, 1992; Rangel-Nafaile et al., 1984).

*Corresponding author: [email protected] © 2011 The Korean Society of Rheology and Springer

Korea-Australia Rheology Journal

Actually, migration of additives dispersed in a host polymer to the host polymer’s surface has long been recognized (Garbassi et al.,1994; Brewis and Briggs, 1981; Rasmussen et al., 1977; Nuzzo and Smolinsky, 1984; Chan, 1994; Chen and Gardella, 1994; Cho et al., 2000; Lee and Archer, 2001; Lee and Archer, 2002). Nonetheless, most of the related studies have dealt with only revealing the surface accumulation of the additives and its mechanism by employing complex characterization techniques such as light scattering, small angle neutron scattering, small angle X-ray scattering, optical rheometry, and flow birefringence without emphasizing the application field. Using commodity polymers and commercially available diblock copolymers, determining the surface migration by simple characterization techniques including capillary rheometer, Fourier transform infrared spectroscopy, and contact angle instrument, and addressing its application field make the difference of our study. Enrichment of block copolymers at the interface could be used not only for compatibilization of polymer blends but also for adhesion improvement between multilayer films which are produced by coextrusion process. Normally, multilayer structures consist of at least 5 layers being one inner, two outer layers, and two tie-layers. Tie-layers are used for enhanced adhesion between film layers. The premise here is that if a surface active block copolymer is blended in small amounts with the host polymer in melt stage, physical processes such as diffusion, spontaneous surface segregation, and shear can be used to transport the block copolymer to the host polymer’s surface during normal polymer processing and thus serving the block copolymer to be migrated as tie-layer between two phases. This approach for polymer surface modification is attractive because it allows surface modification and polymer pro33

I·lhan Özen, Claude Rustal, Klaus Dirnberger, Hans-Gerhard Fritz and Claus Dieter Eisenbach

cessing to be performed simultaneously using the same equipment enabling to reduce number of layers in coextrusion and thus reducing high costs of production. The ultimate aim of our study is to generate a mechanically stable three-layer coextruded film consisting of polypropylene (PP) / polyamide (PA) / polypropylene (PP) without a conventional tie-layer. In pursuit of our goal to achieve an improved adhesion between PP and PA film layers, a PP blend was prepared with a small amount (5 wt.%) of a commercially available amphiphilic diblock copolymer poly(ethylene-block-ethylene oxide) (P(E-b-EO)) by using extruder and capillary rheometer. The reason of choosing the P(E-bEO) diblock copolymer is related to the surface energy values being ca. 33 mJ/m2 for PE and 42.9-44.1 mJ/m2 for PEO which are close to that of the PP (γPP ≈ 29 mJ/m2) and the PA (γPA ≈ 49-57 mJ/m2), respectively (Habenicht, 1990; Immergut and Brandrup, 1989). According to our approach, the diblock copolymer should preferentially enrich on the PP surface (PE moiety being anchored in the PP and PEO moiety facing the air-side of the PP) during processing and thus reducing the interfacial tension and enabling the compability between the PP blend and polyamide during coextrusion process. Apart from rheological investigations, these blends were characterized by employing attenuated total reflection Fourier transform infrared spectroscopy (ATR/FT-IR), and contact angle instrument.

2. Experimental 2.1. Materials Random type copolymers of isotactic polypropylene (RD208CF and RB501BF with 7.5 mol and 3.6 mol% ethylene content) from Borealis were used as the matrix phase. Melt flow index (MFI) values measured at 230oC with a load of 2.16 kg are 7.1 and 2.3 g/10 min for RD208CF and RB501BF, respectively (supplier data). P(Eb-EO) with a molecular weight of 920 and 2250 g/mol from Aldrich Chemicals Co. was used as diblock copolymer. Molar ratios of EO/E are 0.62 for 920 g/mol and 2.45 for 2250 g/mol (supplier data). 2.2. Blend preparation 2.2.1. Extruder According to our concept, diblock copolymer should be pre-blended with the polyolefin used. Otherwise, it would be impractical and expensive on a processing scale to extrude diblock copolymer as a tie layer, especially when more than a few layers are to be produced. Therefore, blends of PP containing 5 wt.% diblock copolymer were pre-blended in a co-rotating intermeshing twin screw extruder (Coperion Werner & Pfleiderer ZSK 30, screw diameter D = 30 mm, screw length-to-diameter ratio L/D = 40) at 160oC and pelletized. The extrusion process was carried out with a screw rotation speed of 100 rpm and a 34

throughput of 4 kg/h. 2.2.2. Capillary rheometer We a used capillary die of flat slit type (die length, L: 180 mm, slit height, h: 0.9 mm, slit width, w: 18 mm, diameter of the storage canal, D: 20 mm). Extrudates of PP/P(Eb-EO) blends (95/5 (w/w)) blended previously in the extruder were prepared with the help of a extrusion type capillary rheometer (Rheograph 2000, Göttfert Feinwerktechnik GmbH) at 160oC. Extruder used has a diameter of 30 mm and a L/D-ratio of 20. The extruding procedure was realized under the variation of the piston speed and thus adjusting the apparent wall shear rate from 1.3 to 1293 s−1 calculated with equation 3. Using slit capillary die with very small gap height to gap width (h/w) ratio assures that the side walls of the slit capillary die don’t affect the flow behavior appreciable. Four pressure transducers were positioned along the capillary die (L1 = 30 mm, L2 = 75 mm, L3 = 120 mm and L4 = 165 mm). Pressure transducers record the pressure at the entrance and along the die. Wall shear stress, τw, wall shear rate, γ· w, and apparent shear viscosity, ηap( γ· ), were calculated according to the below equations (Pahl, 1995). Based on measurements of the piston speed Vst and the pressure drop ∆p, the apparent viscosity ηap( γ· ) is derived from the shear stress τw and shear rate γ· w at the wall: · · ηap ( γ ) = τw ⁄ γw

(1)

The shear stress τw at the wall is obtained from the pressure drop ∆p measured by the transducers along the die length: τw = ( h * ∆p ) ⁄ 2l

(2)

h corresponds to the slit height and l the slit length. Pressure drop ∆p was measured within the die meaning that there is no need for Bagley correction. The shear rate γ· w for a Newtonian fluid is calculated as shown in equation 3 · 2 2 γw = ( 1.5 * π * D * Vst ) ⁄ ( l * h )

(3)

D is the diameter of the storage canal. The piston speed Vst can be derived from flow rate V · using the following equation: . 2 V = ( π * D * Vst ) ⁄ 4

(4)

Actually, the Weissenberg/Rabinowitsch correction is subsequently applied in order to take into account the non-Newtonian behaviour of polymers. However, we didn’t make this correction, since apparent values suffice in our case.

2.3. Surface analysis 2.3.1. Attenuated total reflection fourier transform infrared spectroscopy (ATR/FT-IR) Attenuated total reflection spectra of the samples were Korea-Australia Rheology Journal


recorded at a fixed incident angle of 45o using a Bruker IFS 66/S FT-IR spectrometer with single-reflection ATR unit (Golden Gate) equipped with a diamond crystal. Just one side of the blends were taken into consideration for evaluation. The penetration depth dp in the ATR measurement can be estimated to be 1.76 µm at 1105 cm−1 using the formula dp = λ/(2πnc(sin2θ − (ns/nc)2)1/2) (Harrick, 1979) Here nc = 2.4175 and ns = 1.49 are the refractive indices of the ATR crystal and polymer sample, respectively. 2.3.2. Contact angle instrument The advancing contact angle measurements were done with a Krüss instrument (Dataphysics OCA 20) according to sessile drop method by using distilled water as wetting agent at ambient conditions. Measurements using this method are sensitive to chemical changes within the first 5 Å of a materials surface and thus making this method more sensitive than ATR/FT-IR. Measurements were made with a volume of 4 µl drops and reported values were the average of ten measurements. Contact angles were determined just for one side of the blends. 2.3.3. Scanning electron microscope Morphologies of the blends were examined by employing Leo G34-Supra 35VP Scanning Electron Microscope (SEM) operating with an accelerating voltage of 15 kV. Surfaces were coated with the help of a carbon evaporator (Emitech K950X) prior to SEM examination.

3. Results and Discussion We prepared the PP/P(E-b-EO) blends by using a block copolymer content of 5 wt.% since we revealed in our study (Özen, 2006) that the diblock copolymer content lower than 5 wt.% showed no appreciable enrichment on the PP surface. And in order to observe a surface accumulation of the P(E-b-EO) explicitly, the longest (L = 180 mm) capillary die present was used. This is due to the fact that shear deformation γ , and thus the phase separation are dependent upon the shear rate γ· , and residence time ts(y) according to γ = γ· * ts(y ) (Ohlsson et al., 1998). Using long capillary die enables long residence and thus long shearing time for the accumulation to be observed.

3.1. Rheological behavior of the PP/P(E-b-EO) blends Pressure gradients and derived viscosity curves as a function of apparent shear rate of the neat PP and its blends with 5 wt.% P(E-b-EO) (920 and 2250 g/mol) are given in Figs. 1 and 2. Almost a linear pressure drop along the die length has been observed which is typical as depicted in Fig. 1.a, b, c and 2.a, b, c. RD208CF blends have pressure gradients (Fig. 1. b, c) different from that of the neat PP (Fig. 1.a). Pressure drop changes according to the piston speed applied which is observed distinctly in RD208CF Korea-Australia Rheology Journal

blends with 2250 g/mol P(E-b-EO) (Fig. 1.c). Increasing the piston speed from 5 to 6.7 mm/sec leads to an enormous jump of the starting pressure values, i.e., 120 to 350 bar. As explained in the experimental part, firstly shear stress values were calculated with the measured pressures which change depending on the piston speed (Eq. 2). Apparent shear rates were calculated with the help of piston speeds (Eq. 3). Finally, viscosity values were determined by employing Eq. 1. Plotting the viscosity values (y-axis) vs shear rate values (x-axis) delivers the viscosity functions, as displayed in Fig. 1.a1, b1, and c1. It is noticed that the viscosity functions change depending on the pressure drops which are related to the addition of the diblock copolymer (Fig. 1.b1 and c1). In the shear rate region between 900 and 1300 s−1, either a levelling-off (P(E-b-EO) 920 g/mol) or an increase (P(E-bEO) 2250 g/mol) has been observable in viscosity values which are attributed to the pressure gradients depending upon the piston speed. Generally, both blends exhibit much lower apparent viscosity values than the neat PP (RD208CF). Using the diblock copolymer with higher molcular weight of 2250 g/mol leads to a more distinctive reduction in apparent viscosity than the low molcular weight one. This could be attributed to the slip effect of the molecules of the diblock copolymer used. The values on the wall were used for the calculations as mentioned above. The slip effect of the blends with 2250 g/mol diblock copolymer is larger than that of the blends with 920 g/mol because of the higher ethylene oxide moiety present in the copolymer with 2250 g/mol leading to a reduction in viscosity. The blends consisting of high viscous PP type of RB501BF exhibit qualitative identical relationships (Fig. 2). Much higher shear viscosity values were attained when used RB501BF as the matrix polymer which can be explained in terms of MFI values. As given in the experimental part, RB501BF has a low MFI value which means high viscosity. Because of its high viscosity, larger piston speeds were not able to be realized in the blends containing RB501BF. Therefore, the viscosity function was not able to be observed at high shear rates (>1000 s−1). As a result, any increase could not have been detected in these blends containing both 920 and 2250 g/mol P(E-b-EO). It is obvious from the literature (Katsaros et al., 1986) that the flow processes influence the miscibility of polymer blends to a large extent. Either mixing or demixing could occur depending on the blend system, which can be detected through rheological measurements. Besides, increase of the viscosity is known to be closely related to the phase separation (Takahashi et.al., 1994). According to these facts, increase of the viscosity observed at high shear rates in RD-type PP blends points out a possible demixing.

3.2. Surface properties of the PP/P(E-b-EO) blends It is known that high-stress regions are generated near the

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Fig. 1. Pressure gradient and apparent viscosity curves of the neat PP and the PP/P(E-b-EO) blends (95/5 w/w) (a, a1: RD208CF; b, b1: 95/5 RD208CF/P(E-b-EO) 920 g/mol; c, c1: 95/5 RD208CF/P(E-b-EO) 2250 g/mol; test temperature 160oC).

walls of a capillary rheometer meaning that the surface accumulation of low molecular weight materials is favored (Aganval et al., 1994; Jones and Richards, 1999; Kumar and Russell, 1991; Mhetar and Archer, 1998; Larson, 1992). Considering this fact led to the expectation that the diblock copolymer would migrate preferentially to the polymer surface provided that sufficient time is allowed. 36

Band extrudates of the PP/P(E-b-EO) blends (95/5 (w/w)) blended previously in the extruder were prepared with the help of a capillary rheometer for studying the effects of flow processes on surface migration. These extrudates are the ones which were used for calculating the viscosity functions. In other words, the data of the extrudates during processing on the capillary rheometer were used for revealKorea-Australia Rheology Journal


Fig. 2. Pressure gradient and apparent viscosity curves of the neat PP and the PP/P(E-b-EO) blends (95/5 w/w) (a, a1: RB501BF; b, b1: 95/5 RB501BF/P(E-b-EO) 920 g/mol; c, c1: 95/5 RB501BF/P(E-b-EO) 2250 g/mol; test temperature 160oC).

ing the viscosity functions and at the exit of the capillary die the extrudates were collected for further characterizations such as ATR/FT-IR and contact angle measurements. In our study (Özen, 2006), we have shown that COC/CH2 integral band ratio obtained from spectroscopic measurements can be used for quantitative evaluation. Here, C-OKorea-Australia Rheology Journal

C band observed at 1105 cm−1 is assigned to the stretching band of the diblock copolymer, while CH2 band (1000 cm−1) belongs to the PP. The results of the blends prepared by extruder and capillary rheometer were given successively, in order to reveal any difference between preparation methods. All extruded


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Fig. 3. Integral band ratios and contact angle values of the band extrudates (PP/P(E-b-EO) 920 g/mol, 95/5 (w/w)) generated using capillary rheometer depending on the shear rate by using flat slit capillary die (L=180 mm) ((●;---),(○;---): RD208CF blends; (■;---), (□;---): RB501BF blends; (●;--), (■;---): values of integral band ratios; (○;---), (□;---): contact angle values).

Fig. 4. Integral band ratios and contact angle values of the band extrudates (PP/P(E-b-EO) 2250 g/mol, 95/5 (w/w)) generated using capillary rheometer depending on the shear rate by using flat slit capillary die (L = 180 mm) ((●;---), (○;---): RD208CF blends; ( ■;---), ( □;---): RB501BF blends; (●;---), (■; ---): values of integral band ratios; (○ ;---), (□; ---): contact angle values).

blends (two types of PP and two types of P(E-b-EO)) show insignificant increases in the integral band ratio changing between 1 and 3 (not shown here). Contact angle measurements, however, display relatively low values which are closely related to the sensitivity of this method. Contact angle values of the neat polypropylenes are 90 ± 3o (RD208CF) and 94.1 ± 2.1o (RB501BF) and those of the blends being 81 ± 2.8o (RD208CF/P(E-b-EO) 920 g/mol), 80 ± 2.1o (RD208CF/P(E-b-EO) 2250 g/mol), 83 ± 1.8o (RB501BF/P(E-b-EO) 920 g/mol), 84 ± 1.4o (RB501BF/ P(E-b-EO) 2250 g/mol). The results of the contact angle and ATR/FT-IR measurements of the band extrudates prepared with P(E-b-EO) 920 g/mol were plotted in Fig. 3. Although depending on the shear rate the contact angle values become low changing between 76 ± 3.1o and 58 ± 1.2o for RD208CF blends (cp. curve ○;---), 50 ± 4.8o and 58 ± 2.2o for RB501BF blends (cp. curve □;---), there is no appreciable increase in integral band ratios determined by ATR/FT-IR results ((● ;---), (■;---)). On the other hand, band extrudates prepared with the higher molecular weight block copolymer, i.e., 2250 g/mol show high COC/CH2 band ratios which are scattered between 5 and 32 for RD208CF blends, 25 and 35 for RB501BF blends. Accordingly, contact angle values are also low (60±1.1o-75±1o and 55±6.5o-67±1.9o) (Fig. 4). Using block copolymer with different molecular weights has resulted in different behaviors. Although results of the contact angle measurements exhibit almost the same trend being lower Θ values depending on the shear rate which is related to the sensitivity of this method, integral band ratios obtained with ATR/FT-IR measurements were appeared to be higher when used the P(E-b-EO) with a molecular weight of 2250 g/mol. The EO/E ratios of the diblock

copolymers with 920 and 2250 g/mol are 0.62 and 2.45, respectively which means that the ethylene oxide moiety of the P(E-b-EO) 2250 g/mol is high. The possibility of being immiscible, i.e., phase separated or surface migrated of the PP blends containing higher molecular weight diblock copolymer would be accordingly high. Particularly, the blends consisting of the high viscous type random PP, i.e., RB501BF delivered pronounced COC/CH2 band ratios which could be attributed to the viscosity differences between the PP and the P(E-b-EO). This difference has apparently paved the way for migration of the diblock copolymer to the surface of the PP matrix phase. In the case of RD208CF blends containing P(E-b-EO) 2250 g/ mol, lack of viscosity difference between the blend components leads to the low surface enrichment of the diblock copolymer which however was balanced with increasing the shear rate. Although the results of the blends prepared in the extruder seemingly points out a surface enrichment of the diblock copolymer, it would be obvious that this is only a bulk property when compared these results with those of the band extrudates by capillary rheometer. Since the shear rate and the residence time for shearing are low in extruder, particles of the diblock copolymer are only distributed in the PP matrix evenly in the course of blending. Therefore, a small fraction of the diblock copolymer was observed in the blends generated by the extruder. On the other hand, surface enrichment reveals itself on the band extrudates prepared by using capillary rheometer. According to these findings, the diblock copolymer accumulates on the PP surface with increasing shear rate when worked on the capillary rheometer with a long enough capillary die. A careful consideration of the results obtained

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(P(E-b-EO)). Flow-induced migration of the P(E-b-EO) in PP prepared using capillary rheometer was investigated by attenuated total reflection Fourier transform infrared spectroscopy, contact angle measurements and SEM analysis. Selective enrichment of copolymer additives at the PP polymer surface was observed in the polymer blend systems studied. It was shown in this study that not only suitable materials but also their proper molecular weights have to be chosen for a specific application field. Realization of surface accumulation with the help of shear flow processes leads to the expectation of adhesion improvement between PP and PA layers during coextrusion in which an efficient shear flow also exists.

Acknowledgments

Fig. 5. SEM micrographs of the RD208CF/P(E-b-EO) 2250 g/ mol, (95/5 (w/w)) band extrudates generated using capillary rheometer. (a) wall shear rate: 6,46 1/sec, (b) wall shear rate: 64,6 1/sec, (c) wall shear rate: 646 1/sec.

This work was supported by the DFG (German Research Foundation) under Grant No. FR 562/29-1 and EI 147/301 and by the DAAD (German Academic Exchange Service). The authors would like to thank the FPL (Research Institute for Pigments and Coatings) for their kind help.

References with RB501BF indicates a strong enrichment of the copolymer in spite of low shear rates (Fig. 4). This result is originated from the components belonging to shear deformation · γ , i.e., shear rate γ , and residence time ts(y) according to · γ = γ∗ ts ( y ) (Ohlsson et al., 1998) as mentioned above. Accordingly, alone residence time could suffice for achieving the surface accumulation of the P(E-b-EO) on the PP surface. Observations made with the blends prepared with the help of capillary rheometer underscore the enormous importance of the flow processes in migration. In order to verify the phase separation at high shear rates, morphological characterization was carried out on the samples by employing scanning electron microscope. As an example, the SEM micrographs of the RD208CF/P(E-bEO) 2250 g/mol (95/5 w/w) were shown in Fig. 5, which were generated on the capillary rheometer by applying different wall shear rates. Spherical and ellipsoidal poly(ethylene-block-ethylene oxide) particles were observed on the surface of the PP matrix phase (Fig. 5. a, b, and c) and increased shear rates (from 6.46 to 646 1/sec) led to pronounced phase separation between the blend components. Complementary results of the morphological analysis show explicitly that a phase separation occurs in PP/P(E-b-EO) blends with increasing wall shear rates.

4. Conclusions We have turned the immiscibility problem of polymer blends to an advantage by surface modification of polypropylene (PP) with commercially available amphiphilic diblock copolymer poly(ethylene-block-ethylene oxide) Korea-Australia Rheology Journal

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