ethylene-octene copolymer blends

European Polymer Journal 45 (2009) 1485–1492

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Transmission electron microscopy study of phase morphology in polypropylene/ethylene-octene copolymer blends Petr Svoboda a,*, Dagmar Svobodova a, Petr Slobodian a, Toshiaki Ougizawa b, Takashi Inoue c a b c

Faculty of Technology, Tomas Bata University in Zlin, Nam. TGM 275, 762 72 Zlin, Czech Republic Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1-S8-33, Ookayama, Meguro-ku, Tokyo 152-8552, Japan Department of Polymer Science and Engineering, Yamagata University, Yonezawa 992-8510, Japan

a r t i c l e

i n f o

Article history: Received 20 October 2008 Received in revised form 15 December 2008 Accepted 25 January 2009 Available online 4 February 2009

Keywords: Polypropylene Ethylene-octene copolymer Phase morphology TEM DSC Partial miscibility

a b s t r a c t Blends of polypropylene (PP) and ethylene-octene copolymers (EOC) were investigated by transmission electron microscopy (TEM) and by differential scanning calorimetry (DSC). The EOC contained 28, 37, 40 or 52 wt% of octene. Only the 50/50 PP/EOC ratio was used for all blends. None of the blends was fully miscible, there was always two-phase morphology. TEM observation followed by image analysis by ImageJ software revealed that the largest particles were in blend containing EOC-28 and the smallest were in blend with EOC-52. The coarsening rate at 200 °C was evaluated by TEM. The glass transition temperature (Tg) shift indicated partial miscibility. Partial miscibility was then confirmed by direct observation of bright PP lamellae in EOC dark phase. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Polypropylene (PP) is one of the most versatile commodity polymers because it possesses exceptional properties including excellent chemical and moisture resistance, good ductility and stiffness, and low density. It is also easy to process and relatively inexpensive. It is well known that good properties of PP as an engineering polymer are seriously limited by its low impact resistance, especially at low temperatures. To improve the impact resistance of the PP matrix, rubbers have been used as impact modifiers. Extensive research has been published on the blends of PP with ethylene-propylene rubber (EPR), ethylene-propylene diene copolymer (EPDM), styrene-ethylene-butylene-styrene copolymer (SEBS). Recently interest have centered on the use of ethylene-octene copolymer (EOC) [1–29]. Many researchers have observed the improvement of impact strength of the PP with the addition of EOC * Corresponding author. Tel.: +420 576 031 335; fax: +420 577 210 172. E-mail address: [email protected] (P. Svoboda). 0014-3057/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2009.01.032

[3,5–8,12,17,22,25]. Size of spherulites is also affecting the impact strength, and therefore some researchers have investigated crystallization of these PP/EOC blends in detail [4,16,19,23,26,28], some have used a nucleation agent to decrease the spherulite size and thus increased the impact strength [6,26]. Other researchers have investigated how the addition of various EOCs would affect the rheology of these blends [7,14–17,20]. Dow Chemicals is nowadays producing quite large number of EOCs with trade name ENGAGEÒ with a range in density and melt flow index. Carriere et al. [21] has shown that increasing octene content in EOC has lowered the interfacial tension in PP–EOC systems. How would that affect the actual phase morphology? To our best knowledge such systematic detailed transmission electron microscopy (TEM) study has not been done yet, and therefore it was investigated in this research work. The softness, the high temperature resistance, and the impact strength should be closely related to the morphology which may be affected by the miscibility and the crystallization. In this paper, we deal with the basic aspect of

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the morphology generation. We have chosen 4 EOC grades with various octene content – 28, 37, 40 and 52 wt% to be tested in the 50/50 blend with PP, to see the effect of octene content on phase behavior and miscibility. 2. Experimental The isotactic polypropylene (PP) was commercial polymer supplied by Mitsui Chemicals Inc. (J3HG, Mw = 3.5  105 g mol 1 and Mn = 5  104 g mol1). Ethylene-octene copolymers, polymerized using metallocene initiator, were special samples prepared by Dow Chemicals. Table 1 shows the octene content in weight and molar%, ethylene/octene molar ratio and molecular weight Mn. The PP and EOC were melt-mixed (charge 0.7 g) at 200 °C for 7 min at 100 rpm in a miniature mixer, Mini– Max Moulder (model CS-183MMX, Custom Scientific Instruments, Inc.). Only one blend ratio was used – 50/50 wt%. The melt-mixed blend was extruded and the extruded string was quenched in ice-water (0 °C). For one experiment the extruded string was put into annealing box set at 200 °C for various times (1, 3 and 15 min). For the transmission electron microscopy (TEM) analysis, the specimens were microtomed to an ultrathin section of about 70 nm thick using a Reichert-Jung ultracryomicrotome with a diamond knife at 80 °C and then the section was stained with RuO4 vapor at room temperature for 2 h. The structure was observed by electron microscope, JEM 100CX (100 kV). We have used two differential scanning calorimeters: (1) Seiko Instruments – EXSTAR 6000 and (2) Perkin–Elmer DSC 1 Pyris. In both cases the specimens were heated in nitrogen atmosphere at the rate 10 °C min1. In case of EXSTAR 6000 the instrument’s output was time (min), temperature (°C), heat flow curve called ‘‘DSC” (lW) and first derivative of the DSC trace with respect to time called ‘‘DDSC” (lW/min). Cooling was performed with help of liquid nitrogen, so that we could start measurement from negative 95 °C. In case of Perkin–Elmer the instrument’s output was temperature (°C), heat flow (mW) and first derivative of the DSC trace with respect to temperature (mW/K). Cooling was performed with help of cooling unit capable of negative 34 °C. The temperature and heat flow of the apparatus was calibrated on indium standard. 3. Results and discussion First of all it should be mentioned that diameters of particles obtained from TEM images are in many cases smaller than in the 3D reality. TEM shows a 2D projection of a very Table 1 List of used ethylene/octene copolymers. Name

wt% octenea

Mna g/mol

mol% etylene

mol% octene

et/oct ratio

EOC-28-131 EOC-37-115 EOC-40-228 EOC-52-130

28 37 40 52

131,000 115,000 228,000 130,000

91.14 87.20 85.71 78.69

8.86 12.80 14.29 21.31

10.3 6.8 6.0 3.7

a

Data were supplied by the manufacturer.

thin section (about 50–100 nm thick). When the particle is not cut through its center (or equator of the particle) then the apparent diameter of a particle on the 2D image is smaller than its real diameter. As a result in real 3D world many of the particles are somewhat bigger than what we observe by TEM [30]. In Fig. 1 four TEM pictures of the PP/EOC (50/50) are compared. We could not achieve full miscibility, always we observed a two-phase morphology of a matrix/particles type. Particles had various complex shapes (not only elliptical). Bearing in mind that the mixing conditions (time, temperature, rotor speed) were the same for all of them, one can observe the influence of octene content on resulting morphology. The blend containing EOC with 28 wt% of octene seems to have the largest particles (Fig. 1(a)). The RuO4 preferably stains EOC, so the dark phase can be assigned to be the EOC. The blends with 28 and 37 wt% of octene, see Fig. 1(a) and (b), have continuous phase the ethylene-octene copolymer, however, in case of the two blends containing EOC with 40 and 52 wt% of octene it is just the opposite – the continuous phase is polypropylene (bright phase), see Fig. 1(c) and (d). Knowledge of the fact which phase is continuous is very important because it is related to mechanical properties. It is generally accepted that the properties of blend are governed mainly by the properties of the matrix. In our case the PP is a hard polymer while EOC is a soft rubbery polymer. The elastic properties should be affected by the morphology – if the matrix is hard PP or elastic EOC. The morphology also depends on the blend concentration. It was not the subject of this study but it is important to mention that. At low concentration of either component the dispersed phase forms nearly spherical drops, then, at higher loading, cylinders, fibers, and sheets are formed. Thus, one may classify the morphology into dispersed at both ends of the concentration scale, and co-continuous in the middle range. The maximum co-continuity occurs at the phase inversion concentration, /I, where the distinction between the dispersed and matrix phase vanishes [31]. The /I depends also strongly on viscosity ratio. This was practically shown on PP/EOC blend by Xu [11] who found that when the concentration of PP increased continuously, there was an onset of interpenetrating co-continuous morphology at 50–60 wt% of PP. Looking at the four TEM pictures (Fig. 1(a–d)) with bare eyes it seems that the more octene in the EOC the finer morphology we can obtain in blend with PP. To compare the morphologies quantitatively we have used a special image analyzing software ‘‘ImageJ” [32]. The treatment of a TEM picture with this ImageJ software is shown in Fig. 2. The example is taken from Fig. 1, magnified lower left part. Fig. 2(a) is pure untreated TEM picture. At first it is necessary to increase the contrast between the phases, make the image 8-bit type and finally make it binary (we can see only black and white areas), see Fig. 2(b). Then we had to erase some artifacts from cutting (needle like shapes). The next step is particle analysis, see Fig. 2(c). Each particle receives a number and the software calculates area of each individual particle. We had to input the scale bar so that the area could be in lm2. Further we can obtain mean and circularity of each individual

P. Svoboda et al. / European Polymer Journal 45 (2009) 1485–1492

Fig. 1. TEM micrographs of PP/EOC 50/50 blends after mixing at 100 rpm for 7 min at 200 °C and quenching to 0 °C water. (a) 28, (b) 37, (c) 40, (d) 52 wt% of octene in EOC.

particle, then the summary and a histogram. This treatment would be sufficient in case of spherical and ellipsoi-

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Fig. 2. Image analysis procedure shown on PP/EOC 50/50 blend (EOC-28131). Zoom to lower left part of Fig. 1a.

dal shapes. In our case some particles had very complex shape far from the circle so we had to use more appropriate image analysis described by Xu [29] which is schemat-

P. Svoboda et al. / European Polymer Journal 45 (2009) 1485–1492

ically shown in Fig. 2(d). The image is scanned by many lines across the picture in many angles. Then one can obtain ‘‘characteristic length in real space, L” defined as a span from one boundary to another. The edge particles are not included in calculation. By counting L one is able to obtain at first a histogram and then also ‘‘average characteristic length, Lm”. In Fig. 3 the result of above described analysis is shown. For each blend more than 400 characteristic lengths were obtained to create a histogram. The blend containing EOC-28 has the broadest particle size distribution with the maximum being around 0.7 lm, see Fig. 3(a). By increasing the octene content in EOC the distribution is narrower and also the maximum of the histogram is gradually moving towards the smaller characteristic length L. In Fig. 4 the average characteristic length, Lm, is plotted as a function of octene content. It is clear that with the increasing octene content the characteristic length is decreasing. To help to imagine the chemical structure in Fig. 4 we have listed also the ethylene/octene molar ratio numbers. The EOC with only about 1 octene per 10 ethylene monomer units has about 2.5 times larger particles in blend with PP compared to EOC with 1 octene per about 4 ethylene units. The EOCs with 6–7 ethylene units per octene lay somewhere in the middle of the two extremes. It is important to know the stability of a polymer blend at quiescent conditions at elevated temperature above Tm. Fig. 5 shows the structure development of PP/EOC-40-228 blend in time at elevated temperature (200 °C). It is clear that at elevated temperature coalescence takes place very rapidly and the particles are growing enormously without

50

(a)

EOC-28-131

Count

40 30 20 10 0 EOC-37-115

(b)

EOC-40-228

(c)

EOC-52-130

(d)

Count

60 40 20 0

Count

80 60 40 20 0 100 Count

80

1.2

1.0

0.8

6.8 6.0

0.6

3.7

0.4

0.2

0.0 25

30

40 20 0 0

1

2

3

Characteristic length L / μ m

4

5

Fig. 3. Histograms of characteristic lengths for PP/EOC 50/50 blends.

35

40

45

50

55

wt% of octene Fig. 4. Average characteristic length Lm (from the image analysis of TEM pictures) as a function of octene content. The molar ratio of ethylene/ octene is also listed.

mixing share stress. To preserve very fine morphology the blend must be quenched as fast as possible. Again we have analyzed the TEM pictures by image analysis and the result is shown in Fig. 6. In the 0–3 min time-frame the growth of the particles is almost linear with time, but later (3–15 min) it slows down. The image analysis of the Fig. 5(a) revealed that the composition on the picture was 72/28. That is quite different from the 50/50 charge. This suggests partial miscibility. In the field of polymer blends people many times evaluate miscibility also with help of Tg (glass transition temperature) measurement. Two Tg values located exactly in position of pure polymers indicate complete immiscibility. On the other hand, only one Tg somewhere between original two Tg values indicates full miscibility on molecular level. The third case represents partial miscibility when we find two Tg values which depend on composition. Therefore we have run the DSC test of the blends to see if there is any Tg shift. The result of the Tg measurements is shown in Fig. 7(a) and (b). From the DSC measurement we have used the ‘‘DDSC” curve instead of the typical heat flow curve because evaluating of peak position from a DDSC peak seems to be more precise than evaluating Tg from inflection point of the heat flow curve. On Fig. 7(a) the DDSC curve for pure EOC-40-228 is compared with its 50/50 blend with PP. The blend’s Tg is shifted about 3.5 °C to higher temperature. The Tg of pure PP was found to be 7.7 °C. According to Fox equation [33]

1 wA wB ¼ þ T g;b T g;A T g;B

60

ethylene/octene molar ratio

10.3

L m/ μm

1488

ð1Þ

where Tg,b is the glass transition of the blend, wi is the weight fraction of component i and Tg,i is the glass transition point of the pure component i, the Tg of fully miscible 50/50 blend would be at 32.6 °C. The Tg of the blend that was experimentally found (49.5 °C) according to Fox

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8

7

6

Lm / μm

5

4

3

2

1

0 0

2

4

6

8

10

12

14

16

Time / min Fig. 6. Average characteristic length Lm (from the image analysis of TEM pictures in Fig. 5) as a function of annealing time at 200 °C.

Fig. 5. TEM observation of structure development of PP/EOC-40-228 (50/ 50) blend during annealing at 200 °C. (a) 0 min, (b) 1 min, (c) 3 min, (d) 15 min.

equation gives composition about 90/10 (EOC/PP). On Fig. 7(b) the effect of EOC on the Tg of PP is shown. The

Tg of PP has decreased for about 2.3 °C. It is a little bit smaller change than in case of EOC but it is still proving the existence of partial miscibility. Fig. 8 (a) and (b) show the effect of PP on crystallization of various EOC’s. EOC-28-131 has the highest melting temperature (about 87 °C). Blending of this EOC-28 with PP did not decrease the Tm. The opposite behavior was observed for EOC-40. There seems to be a fraction of the EOC that has melting point decreased almost 10 °C. Melting point depression of EOC-40 is probably connected with less perfect crystalline structure caused by presence of PP that is mixed in EOC due to partial miscibility. The effect of EOC on PP is shown in Fig. 8(c). The melting point of PP has decreased about 1.2 °C. Isotactic PP has a relatively high melting point (about 163 °C) that allows rather high service temperatures, and EOC gives this blend softness (rubber-like feeling) and increases impact strength of PP. The only disadvantage of EOC is a low melting point (40–90 °C) that could limit the service temperature of the blend. This shortcoming can be overcome by silane grafting followed by water crosslinking [34,35]. Such thermoplastic vulcanizates have a great potential in automotive industry [27]. Fig. 9(a) and (b) show the PP/EOC blends after slow isothermal crystallization near the melting point of PP. The PP lamellae had enough time to nicely develop so that they can be clearly seen by TEM. The development of PP lamellae in EOC-rich region strongly support that PP chains had existed in EOC-rich melt. It suggests clearly the partial miscibility between PP and EOC. Fig. 10 shows the crosshatch PP lamellae in bright PP areas (inside the ellipses). It is generally difficult to take a TEM picture of crosshatch PP lamellae in case of pure PP. However, in our case the presence of EOC stainable impurity made it possible to see them. One could consider that as a proof of small amount of EOC in PP and thus another proof of partial miscibility between them.

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450

EOC-28-131

EOC-40-228 400

Heat Flow Endo Down (μW)

EOC EOC/PP

DDSC / μW min-1

350

300

250

200

150

EOC EOC/PP

-5000

-6000

-7000

-8000

100 -9000 20

50 -65

-60

-55

-50

-45

-40

30

40

50

60

70

80

90

100

110

120

Temperature / °C

-35

Temperature / ºC EOC-40-228

0.116

Heat Flow Endo Down (μW)

-800

PP/EOC-40 ΔTg=2.3°C

DDSC / mW K-1

0.114

0.112

EOC EOC/PP

-900

-1000

-1100

-1200

-1300

0.110

PP -1400 0

0.108

10

20

30

40

50

60

70

80

90

100

Temperature / ºC PP/EOC-28

0.106 -15

-10

-5

0

Temperature / ºC Fig. 7. Tg evaluation by DDSC curves of: (a) pure EOC-40-228 vs. PP/EOC (50/50) blend, (b) pure PP vs. PP/EOC-40-228 (50/50) blend.

4. Conclusion TEM study has proved that the morphology of the PP/EOC blends depends greatly on octene content. With increasing octene content the particles in the blend are getting smaller. This observation is in agreement with Carriere et al. [21] who has shown that increasing octene content in EOC has lowered the interfacial tension in PP–EOC systems. From the practical point of view, choosing EOC with low octene content (or higher density) would result in morphology with larger particles that could be detrimental for some mechanical properties. On the other hand the EOC with low octene content has quite high melting point (about 90 °C) which will render higher temperature resistance. To achieve very fine morphology one has to choose EOC with high octene content. However, there is a practical problem with this high octene EOC – the melting point is

Heat Flow Endo Down (mW)

-20

28

PP/EOC-40

30

PP 32

34

ΔTm=1.2°C

36

38 135

140

145

150

155

160

165

170

175

180

Temperature / ºC Fig. 8. DSC curves of pure polymers vs. PP/EOC 50/50 blends: (a) and (b) focus on melting point of EOC, (c) focus on melting point of PP.

very low (40–50 °C). This could very seriously limit the service temperature. Fortunately, one could crosslink the rubber phase and improve the thermal resistance in this way.

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References

Fig. 9. Penetration of PP lamellae into dark EOC-rich regions in PP/EOC40-228 (50/50) blend. TEM picture after 5 min pre-heating to 200 °C followed by 20 min isothermal crystallization at Tc = 130 °C.

Fig. 10. Cross-hatch lamellar structure in PP/EOC-40-228 (50/50) blend. TEM picture after 5 min pre-heating to 200 °C followed by 5 min isothermal crystallization at Tc = 120 °C.

TEM of the interfacial area revealed bright PP lamellae in the dark EOC regions. This confirmed visually the existence of partial miscibility in PP/EOC blends. Acknowledgement This work has been supported by the Ministry of Education of the Czech Republic as a part of the project No. VZ MSM 7088352102.

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