Positron lifetime spectroscopy was used to study the compatibilization effect of maleic-anhydride (maH) grafted copolymers and its ionomers in metallocene ...
Jpn. J. Appl. Phys. Vol. 41 (2002) pp. 2146–2149 Part 1, No. 4A, April 2002 #2002 The Japan Society of Applied Physics
Compatibilization of Metallocene Polyethylene/Polyamide Blends with Maleic Anhydride Studied by Positron Annihilation Zhi Quan C HEN* , Akira U EDONO, Yu Ying LI1 and Jia Song H E1 Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan 1 State Key Laboratory of Engineering Plastics, Institute of Chemistry, Center for Molecular Science, The Chinese Academy of Sciences, Beijing 100080, P. R. China (Received November 20, 2001; accepted for publication January 17, 2002)
Positron lifetime spectroscopy was used to study the compatibilization effect of maleic-anhydride (maH) grafted copolymers and its ionomers in metallocene polyethylene/poly-amide blends. The ortho-positronium (o-Ps) lifetime 3 showed a decrease with increasing maH content from 0 to 0.5%, then remained constant. A larger decrease was observed for the Naþ -neutralized ionomer containing blends. From the analysis of continuous positron lifetime distribution, we found that in the blend with no or smaller maH content, the o-Ps lifetime distribution was composed of two peaks, which can be attributed to the phase separation in these blends. When the maH content was equal to or higher than 0.5%, the phase separation disappeared, which is due to the decrease of dispersed polyethylene (PE) phase size. In the blends with ionomer, the phase separation disappeared at a lower content of maH. These results proved that the maH grafted copolymer and ionomer can be very good compatibilizers for the PE/PA blends. [DOI: 10.1143/JJAP.41.2146] KEYWORDS: positron annihilation, polymer blend, free volume, compatibilizer
1.
Introduction
Polymer blends provide many useful properties which combine the properties of individual components. They can exhibit higher performance at a lower cost, therefore they have been attracting increasing attention in recent years.1) But for thermodynamic reasons, most of the blends are immiscible, only a few are miscible. However, sometimes it is desirable to mix immiscible blends to combine several properties from each homopolymer of the blend. Polyamide/polyethylene (PA/PE) is an immiscible system. Polyamide is a polymer of great industrial importance, it shows good strength and resistance to hydrocarbon solvents, but it has disadvantages such as brittleness, high moisture sorption, poor processability and poor dimensional stability. To overcome these disadvantages, polyamide is often blended with polyethylene, as it contributes lowtemperature toughness and low moisture sorption. In fact, while PA is a polar polymer, PE is not polar, thus PA and PE do not present segmental chemical similarity. Therefore blending of PA/PE leads to a thermodynamically immiscible two-phase system. Such immiscible blends often exhibit poor mechanical properties because of a lack of adhesion between the blend components, and have an unstable phase morphology during melt processing. The improvement of compatibility can be achieved by using interfacial agents (called compatibilizer), for example, block or graft copolymer which preferentially resides at the polymer-polymer interfaces. It was reported that grafting of maleic anhydride (maH) onto the polyethylene can improve the impact strength markedly and reduce the dispersed phase dimensions in these blends.2–6) This could be attributed to the chemical reactions between maH unit and the terminal amine group of PA. Other effective interfacial agents are ion neutralized ionomers. This is due to the ion-dipole interaction between the ion group in ionomers and the polar polymers. Complete knowledge of the microstructural change at the
molecular level is necessary to understand the underlying mechanism of miscibility of blends. However the conventional methods cannot fulfill this need. Positron annihilation has proved to be a new powerful tool to study the microstructural properties of polymers.7,8) In polymers a positron will prefer to be localized in free volume holes and form a positronium (Ps) atom by capturing one electron. There is a semi-empirical relationship between orthopositronium (o-Ps) lifetime 3 and the free volume hole radius R:9–11) 1 1 R 1 R ð1Þ 1 þ sin 2 3 ¼ 2 R þ R 2 R þ R where R is the thickness of electron layers, which can be obtained by empirical fitting. In polymers and molecular According to this relationship, we solids, it is about 1.66 A. can obtain the average size of free volume holes probed by o-Ps (Vf ¼ 4 R3 =3). On the other hand, the o-Ps intensity I3 contains information about the number of free volume holes. Therefore the annihilation characteristics of o-Ps reflect the information of the free volume in polymers. It is believed that the free volume in polymer systems plays an important role in determining the viscoelastic properties of polymers, transport phenomena and so forth. The viscosity of polymers () has a direct relationship with the fractional free volume fv : 1 ¼ A exp ð2Þ fv where A is a constant which depends on molecular weight. Therefore study of the free volume and its correlation with mechanical properties in polymers is a subject of great interest. In this study, we investigated the effect of maH grafted copolymer and its Naþ -neutralized ionomers on the compatibility of metallocene polyethylene/polyamide blends (mPE/PA) by studying the free volume properties using positron annihilation spectroscopy. 2.
Experimental The metallocene polyethylene was obtained from Re-
*
On leave from Wuhan University, P. R. China. 2146
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Z. Q. C HEN et al.
3.
1.75
24 mPE/PA (5/95)
3
o-Ps lifetime (ns)
1.70 22 1.65
21 20
1.60 19 1.55
0.0
0.5
1.0
1.5
2.0
18
maH content (%) Fig. 1. o-Ps lifetime and intensity as a function of the maH content in the PE/PA (5/95) blend.
24
1.75 mPE/PA (5/95)
3
22 21
1.65
20 1.60
o-Ps intensity (%)
23
I3
1.70
19
Result and Discussion
All the positron lifetime measurements were performed at room temperature of about 300 K. The spectra were first analyzed by the PATFIT program. For most of the spectra, three lifetimes can be resolved with the best 2 , but in pure mPE and mPE/PA blend without maH grafting, it is also possible to perform analysis with four lifetime components, but with large standard deviation. Thus we still used results from analysis with three components. In this paper, we will only discuss the longest lifetime components, 3 and I3 , which are associated with o-Ps annihilation in free volume holes. In the pure mPE, the o-Ps lifetime 3 was 2.38 ns, and its intensity was around 22%, while in PA, the o-Ps lifetime was much shorter, which was about 1.56 ns, but with the same intensity as in mPE. In the mPE/PA blends, the lifetime 3 was between that of mPE and PA. Figure 1 shows the o-Ps lifetime and its intensity as a function of the maH content in mPE/PA blends. 3 first showed a fast decrease with increasing maH content, then remained constant when the maH content was higher than 0.5%. Whereas its intensity was independent of the maH content. This result suggested that the free volume properties changed as a result of the maH grafting of polyethylene in the blends. Figure 2 shows the variation of 3 and I3 with maH content after neutralization by Na salt. Compared with the result in Fig. 1, the drop of 3 was slightly larger. The o-Ps intensity still exhibited no change with ionomer content. In a polymer blend, the free volume is not simply addictive of each individual free volume, instead it can be expressed as follows:14) fv ¼ fv1 1 þ fv2 2 þ ð fv1 fv2 Þ1=2 1 2
o-Ps intensity (%)
23
I3
o-Ps lifetime (ns)
search Institute of Petroleum Processing, SINOPEC, its density was 0.937 g/cm3 , branch degree was 6.5 (/1000C), Mn was about 21000 and Mw was about 46000. Polyamide (K123GL, Nylon6) was obtained from DSM Engineering Plastics. MaH was of reagent grade purity. The grafted metallocene polyethylene was prepared by melt graft polymerization by putting the maH into melt-state mPE. The reaction was held for 10 min. The maH content varied from 0 to 2 mol%. For ionomer, it was prepared by putting a stoichiometric amount of NaOH into maH grafted mPE solution (dissolved in dimethylbenzene xylene), then the grafted mPE was neutralized by Naþ ion. Blends of mPE/PA were prepared by melt-mixing them at 235 C twice in a Cs-194 Mini-Max extruder. The ratio of mPE/PA in the blend was 5/95 in weight percentage. A conventional fast-fast coincidence lifetime spectrometer was used to measure the positron lifetime spectra. The temporal resolution was about 250 ps. A 1:8 105 Bq 22 Na positron source was used in this study, which was deposited and sealed in a 7-m-thick kapton foil. The positron source was sandwiched between two identical pieces of polymer sample with the size of 10 10 2 mm3 for the lifetime measurement. For each spectrum, it contained one million total counts, and each sample was measured twice. The measured lifetime spectra were analyzed by the computer programs PATFIT12) and MELT.13)
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ð3Þ
1.55
0.0
0.5
1.0
18
ionomer content (%) Fig. 2. o-Ps lifetime and intensity as a function of the ionomer content in the PE/PA (5/95) blend.
where fv1 and fv2 are the fractional free volume of polymer 1 and polymer 2, respectively, and 1 and 2 are the volume fractions of the two polymers in the blend. is a parameter that may be related to the interaction between the dissimilar polymer chains.14) > 0 indicates positive deviation from free volume additivity, and < 0 indicates negative deviation. In our results, because we observed the same oPs intensity in the blends, we can simply rewrite the above equation as: Vf ¼ Vf1 1 þ Vf2 2 þ ðVf1 Vf2 Þ1=2 1 2
ð4Þ
Figure 3 shows the calculated in PE/PA blends as a function of maH or ionomer content. It is interesting to note that was positive in the blend without any compatibilizer. After adding maH, it decreased and became negative, changing from 1.3 to 0:4. In the blends with ionomer, showed a further decrease, and it dropped to 0:8. The effect of maH itself on the o-Ps annihilation can be excluded, because we did not observe any change of 3 or I3 when the maH content is larger than 0.5%. Therefore the change of is possibly due to the interaction between PE and PA polymer chains, enhanced by the interfacial agent, such as maH grafted PE and its ionomers. Wang et al.15) revealed that in a polypropylene/ethylene-propylene-diene monomer blend (PP/EPDM), the free volume showed negative
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1.5 mPE/PA (5/95)
without Na with Na
1.0
(a)
mPE/PA (5/95)
0.20
+
+
Probability
0.15 0.5 0.0
2.0% 1.5%
0.10
1.0% 0.5%
0.05 -0.5 -1.0
0.2% maH: 0%
0.00 0.0
0.5
1.0
1.5
0
2.0
1
3
4
Lifetime (ns)
maH content (%) Fig. 3. Interaction parameter in the PE/PA (5/95) blend calculated using eq. (4).
0.15
Probability
deviation due to the phase interaction. Sood et al.14) observed negative interaction parameters in a series of miscible polymer blends from the measurement of the viscosity. Liu et al.16) also found that the interaction parameter is negative in miscible blends, while it is positive in immiscible blends. Our result then suggested that the system became partly or completely compatible after introducing the maH grafted copolymers and its ionomers which strengthens the interaction between PE and PA chains. Strong interactions result in good interfacial adhesion and efficient stress transfer from the continuous to the dispersed polymer phase in the blends. This will improve its mechanical properties. All these results reflected that the compatibilizer had a strong effect on the free volume properties in the blend. However, the information about the free volume which we obtained from the finite term analysis of positron lifetime spectra is limited, because it gives only a few averaged lifetimes. In another words, what we obtain from this analysis is the average free volume size. But most of the time, the free volume size has a distribution, and obtaining this information will be more helpful for us to analyze the detailed microstructural information in polymer and polymer blends. Recently, Shukla et al.13) successfully applied the maximum entropy principle and Bayes’ theorem to extract positron lifetime distributions. They introduced a program called MELT. In this study, we used the MELT program to analyze the measured spectra. The lifetime distribution data are shown in Fig. 4. As shown in Fig. 4, in the mPE/PA blend without maH compatibilizer, the o-Ps lifetime has two clear peaks, one was located at about 1.4 ns, and another at about 2.3 ns. The former lifetime peak is close to the o-Ps lifetime in polyamide, and for the latter one, it is very close to the 3 in mPE. Therefore we can conclude that the two peaks correspond to o-Ps annihilation in two different materials, that is mPE and PA. The small difference between the first lifetime peak and the 3 in PA may be due to the analysis program. We tried to analyze the lifetime spectrum in this sample by PATFIT using four lifetime components. By fixing 3 and 4 to the o-Ps lifetime in pure PA and mPE, respectively, we obtained the I3 and I4 as 20.4% and 2.3%.
2
(b)
mPE/PA (5/95)
0.10 1.0% 0.5%
0.05
0.2%
0.00
ionomer: 0%
0
1
2
3
4
Lifetime (ns) Fig. 4. Positron lifetime distribution in the PE/PA (5/95) blend as a function of the maH and ionomer content.
The ratio of I3 : I4 is about 9 : 1, which is smaller than the volume ratio of PA and mPE (16 : 1). This means that the formation of Ps in PE phase is likely to be enhanced. When the mPE was grafted with maleic anhydride, the oPs lifetime distribution became narrower. The two peaks moved closer to each other, and it was very clear that the second peak shifted leftward. But we can still observe the two peaks. When the maH content increased to 0.5%, the two peaks merged into one peak, which was around 1.6 ns. With increasing maH content above 0.5%, the lifetime distribution showed almost no further change. The two clear o-Ps lifetime peaks in ungrafted mPE/PA blends showed strong evidence of phase separation. Because this blend is immiscible, the dispersed PE tends to have as large a domain size as possible, and tries to avoid any interaction with polyamide chains. Therefore o-Ps will form and annihilate in both the phases. The maleic anhydride grafted copolymer was then used as a compatibilizer to strengthen the interaction. The anhydride unit will react with the amine end groups of polyamide. As a result, the dispersed PE phase will become smaller in size and the interface area between the two phases will increase. Hobbs et al.2) reported that in a PE/PA blend compatibilized by maH, the average size of PE particles decreased from 2.4 m to nearly 0.4 m. In another PP/PA blend compatibilized with maleated PP, Gonza¨lez-Montiel et al.17) observed a similar change of dispersed PP particle size, it even decreased from 4 m to less than 0.1 m. As a consequence
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of the decreased PE phase, the o-Ps has a lower possibility to form and annihilate in the PE phase, therefore the contribution from PE becomes smaller, and finally it will be so small that only one o-Ps lifetime peak can be resolved, which is the o-Ps annihilation lifetime in homogeneous polymer blends. From the results of refs. 2 and 17, it is also interesting to note that the dispersed phase had a sharp decrease at a low maH content (lower than 0.4–0.5%), and subsequently the decrease slowed down or even stopped thereafter. This showed good agreement with our result, in which we only observed a change of o-Ps lifetime distribution with maH content lower than 0.5%. Our result suggested that measurement of positron lifetime distribution can also be a very useful method for studying the phase behavior of polymer blends. When the Naþ -ion-neutralized ionomers were added to the blend, with a maH content of only 0.2%, the separation of o-Ps lifetime disappeared, and we could only see one o-Ps lifetime peak. This might be due to the stronger interaction between the ion group and the amine end group of polyamide because of the stronger polarity of the ion group. As a result, the dispersed PE particle size decreased faster, and larger amounts of Ps annihilated in the PA phase, thus the second peak disappeared at a lower maH content. However, this needs to be further confirmed by other methods. 4.
Conclusion
Free volume properties in maleated polyethylene/polyamide blends were studied by positron annihilation spectroscopy. Interaction between PE and PA chains was found in the blends containing maH grafted copolymers and their ionomers. Direct evidence of the decrease of dispersed PE phase size was also observed in the blends with maH and ionomers from the analysis of continuous o-Ps lifetime
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distribution, which suggested good compatibility. Acknowledgements One of the authors (Z. Q. Chen) would like to appreciate the support from Japan Society for the Promotion of Science (JSPS). This work is supported by the Grant-in-Aid of JSPS fund and the Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan. 1) L. A. Utracki: Polymer Alloys and Blends (Hanser, Munich 1989). 2) S. Y. Hobbs, R. C. Bopp and V. H. Watkins: Polym. Eng. Sci. 23 (1983) 380. 3) C. C. Chen, E. Fontan, K. Min and J. L. White: Polym. Eng. Sci. 28 (1988) 69. 4) B. K. Kim, S. Y. Park and S. J. Park: Eur. Polym. J. 27 (1991) 349. 5) B. Jurkowski, K. Kelar and D. Ciesielska: J. Appl. Polym. Sci. 69 (1998) 719. 6) R. A. Kudv, H. Keskkul and D. R. Paul: Polymer 40 (1999) 6003. 7) Y. C. Jean: Microchem. J. 42 (1990) 72. 8) Y. C. Jean: Positron Spectroscopy of Solids, eds. A. Dupasquier and A. P. Mills Jr. (IOS Press, Amsterdam, 1995) p. 563. 9) S. J. Tao: J. Chem. Phys. 56 (1972) 5499. 10) M. Eldrup, D. Lightbody and J. N. Sherwood: Chem. Phys. 63 (1981) 51. 11) H. Nakanishi, S. J. Wang and Y. C. Jean: Positron Annihilation Studies of Fluids, ed. S. C. Sharma (World Scientific, Singapore, 1988) p. 292. 12) P. Kirkegaard, N. J. Pederson and M. Eldrup: PATFIT-88, Ris National Laboratory, DK-4000 Roskilde, Denmark, 1989. 13) A. Shukla, M. Peter and L. Hoffmann: Nucl. Instrum. Methods A 335 (1993) 310. 14) R. Sood, M. G. Kulkarni, A. Dutta and R. A. Mashelkar: Polym. Eng. Sci. 28 (1988) 20. 15) C. L. Wang, S. J. Wang, W. G. Zheng and Z. N. Qi: Phys. Status Solidi A 141 (1994) 253. 16) J. Liu, Y. C. Jean and H. Yang: Macromolecules 28 (1995) 5774. 17) A. Gonza¨lez-Montiel, H. Keskkula and D. R. Paul: J. Polym. Sci. Part B 33 (1995) 1751.
