Chem. Mater. 2002, 14, 4619-4623
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Composites Prepared by the Polymerization of Styrene within Supercritical CO2-Swollen Polypropylene Zhimin Liu, Zexuan Dong, Buxing Han,* Jiaqiu Wang, Jun He, and Guanying Yang Center for Molecular Sciences, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China Received March 29, 2002. Revised Manuscript Received September 18, 2002
Monomer styrene and initiator 2,2′-azobis(isobutyronitrile) (AIBN) dissolved in supercritical (SC) CO2 were impregnated into SC CO2-swollen polypropylene (PP) matrix at 35.0 °C, and the monomer were then polymerized within the PP substrates at 70 °C, resulting in polypropylene/polystyrene (PP/PS) composites. The Young’s modulus and tensile strength of the PP were improved significantly in the presence of PS. Transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and infrared spectroscopy were used to characterize the morphology and microstructure of the composites. The results showed that the PS was more homogeneously dispersed in the blends and its phase size was in the range of nanometers. Some of the PS entangles with PP in the composites. The special microstructures and morphology of the blends result in the enhanced mechanical performances of PP/PS composites.
Introduction Polymer blends have gained an increasing popularity in the field of polymer science and industry1. Among them, polypropylene (PP) and polystyrene (PS) blends have been investigated widely in order to combine the properties of these two polymers in a product2-10. As known, PP and PS are immiscible, and it is difficult to mix them with very fine domain size. To improve their performance, interfacial agents such as polystyrene/ polypropylene (PP/PS)-based block or grafted copolymers were added into PP/PS blends3-10. These copolymers can change interfacial tension and interfacial adhesion between the phases, resulting in better compatibility. However, the effects of interfacial agents on the morphology and mechanical properties of the PP/ PS blends are very complicated, and sometimes good performance is not obtained by adding these interfacial agents. In recent years there has been increasing interest in using supercritical carbon dioxide (SC CO2) as a solvent * Corresponding author. E-mail:
[email protected]. Tel: (861062562821). Fax: (8610-62559564). (1) Paul, D. R.; Bucknall, C. B. Polymer Blends; John Wiley & Sons: Canada; 2000. (2) Krause, S. In Polymer Blends, Vol. 1; Paul, D. R., Newman, S., Eds.; Academic Press: New York, 1978; Chapter 2. (3) Utracki, L. A.; Dumoulin, M. M. In Polypropylene: Structure, Blends and Composites, Vol. 2; Karger-Kocsis, J., Ed.; Chapman and Hall: London, 1995; Chapter 3. (4) Macau´bas, P. H. P.; Demarquette, N. R. Polymer 2001, 42, 2543. (5) D’Orazio, L.; Guarino, R.; Mancarella, C.; Martuscelli, E.; Cecchin, G. J. Appl. Polym. Sci. 1999, 72, 1429. (6) Radonjic, G. J. Appl. Polym. Sci. 1999, 72, 291. (7) Radonji, G. , Musil, V.; Sˇ mit, I. J. Appl. Polym. Sci. 1998, 69, 2625. (8) Hora´k, Z.; J. Kolak, Sˇ pek, M.; Hynek, V.; F. Veerka J. Appl. Polym. Sci. 1998, 69, 2615. (9) Fujiyama, M. J. Appl. Polym, Sci. 1997, 63, 1015. (10) Fortelny´, I.; Micha´lkova´, D.; Mike×f0ova´, J. J. Appl. Polym. Sci. 1996, 59, 155.
and a swelling agent in polymer processing and polymer chemistry11-26. CO2 is nonflammable, nontoxic, and relatively inexpensive, and has moderate critical conditions (Tc ) 31.1 °C, Pc ) 73.8 bar), which make it convenient to use11. The interaction of supercritical CO2 with some solid polymers has been studied in detail12. Although CO2 is a poor solvent for most polymers, it swells most polymers, including those that are generally considered solvent-resistant. Thus, a solid polymer sample can be soaked in a SC CO2/monomer/initiator solution at a temperature at which the initiator decomposes very slowly. The reactor is then heated after vented or in the presence of the CO2/monomer/initiator solution to promote polymerization in the matrix. This approach has proven to be very successful for the preparation of some types of polymer-polymer compos(11) Eckert, C. A.; Knutson, B. L.; Debenedetti, P. G. Nature 1996, 383, 313. (12) McHugh, M. A.; Krukonis, V. J. Supercritical Fluids Extraction: Principle and Practice, 2nd ed.: Butterworth-Heinemann: Boston, 1994. (13) DeSimone, J. M.; Guan, Z.; Elsbernd, C. S. Science 1992, 257, 945. (14) Kendall, J. L.; Canelas, D. A.; Young, J. L.; DeSimone, J. M. Chem. Rev. 1999, 99, 543. (15) Cooper, A. I. J. Mater. Chem. 2000, 10, 207. (16) Canelus, D. A.; DeSimone, J. M. Macromolecules 1997, 30, 5673. (17) Goel, S. K.; Beckman, E. J. Polym. Eng. Sci. 1994, 34, 1137. (18) Watkins, J. J.; McCarty, T. J. Macromolecules 1994, 27, 4845. (19) Watkins, J. J.; McCarty, T. J. Macromolecules 1995, 28, 4067. (20) Kung, E.; Lesser, A. J.; McCarty, T. J. Macromolecules, 1998, 31, 4160. (21) Hayes, H. J.; McCarty, T. J. Macromolecules 1998, 31, 4813. (22) Arora, K. A.; Lesser, A. J.; McCarty, T. J. Macromolecules 1999, 32, 2562. (23) Kung, E.; Lesser, A. J.; McCarty, T. J. Macromolecules 2000, 33, 8192. (24) Li, D.; Han, B. X. Macromolecules 2000, 33, 4550. (25) Cooper, A. I.; Holmes, A. B. Adv. Mater. 1999, 11, 1270. (26) Giles, M. R.; Hay, J. N.; Howdle, S. M.; Winder, R. J. Polymer 2000, 41, 6715.
10.1021/cm0203215 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/31/2002
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ites, including those consisting of incompatible polymers. McCarthy and co-workers have prepared different polymer composites using this method18-23. Polyethylene/ PS composites,20 in particular, showed very interesting phase morphologies when compared to conventional melt-blended systems, which resulted in enhanced mechanical performance. Li et al.24 also fabricated polymerpolymer composites using a similar method, and the poly(vinyl chloride)/PS blends showed improved mechanical properties24. In this paper, we prepared PP/PS composites using the method discussed above at different conditions. We focused on (1) how the composition of the PP/PS composites changes with various factors, such as soaking time, pressure of CO2, and monomer concentration; and (2) the effect of the composition, morphology, and microstructure of the composites on their mechanical properties. Experimental Section Materials. PP substrate samples in the form of film were obtained from Yanshan Plastic Institute. The thickness of the film was 100 µm. Styrene (Beijing Chemical Reagent Center, A. R. grade) was vacuum distilled from calcium hydride. AIBN (Beijing Chemical Plant) was recrystallized twice from acetone. Tetrahydrofuran (THF, Beijing Chemical Reagent Center, A. R. grade) was used as received. CO2 with a purity of 99.95% was supplied by Beijing Analytical Instrument Factory and used as received. Composites Synthesis. PP samples were sealed in a highpressure stainless steel vessel together with styrene/AIBN solution (0.3 mol % AIBN). The air in the vessel was replaced by CO2. The vessel was put into a constant-temperature water bath of 35.0 °C, and CO2 was charged into the vessel to a desired pressure. CO2 was released after suitable equilibration time. Then the samples were put into another vessel and heated to the polymerization temperature of 70 °C under the protection of N2. The samples were taken out after a reaction time of 20 h. The weight of the samples was determined by an electronic balance (Shanghai 100) with a sensitivity of 0.0001 g. The PP/PS composite samples were Soxhlet-extracted using hot THF for 24 h to remove the homopolymer of PS in the blends. Experiments showed the extractable PS could be removed completely because the weight of the blends was independent of extraction time after 24 h. After dried, the extracted PP/PS composite was weighed again. Characterization. FTIR (Perkin-Elmer 180) was used to characterize the PP/PS composites and the possibility of grafting PS onto PP substrate in the composites. The phase morphological characteristics of the samples were observed by means of TEM (Hitachi, H-800). Prior to the examination, cryoultrathin sections were cut using an ultramicrotome and they were chemically stained in OsO4 vapor for 24 h. The thermal properties of PP/PS composites were investigated using differential scanning calorimetry (DSC, Perkin-Elmer DSC-7). Samples of about 8 mg were scanned from room temperature to 190 °C with a heating rate of 10 °C/min under an atmosphere of dry nitrogen. The melting point (Tm) and the apparent enthalpy of melting (∆H*) were obtained from the maximum and the area of the peak, respectively. The tensile properties of the PP/PS composites were investigated on a universal tensile tester (Instron 1122) using a load of 20 kg. The fracture elongation and tensile strength were measured at a crosshead speed of 50 mm/min. The average of five tests was reported.
Results and Discussion Composites Synthesis. To prepare PP/PS composites with different compositions, effects of the soaking
Liu et al.
Figure 1. Effect of the soaking time on the mass uptake of PP substrates before and after extraction of PS by refluxing THF. (The soaking conditions were 35.0 °C, 120 bar, and the concentration of monomer in the fluid phase was 0.80 mol/L.)
Figure 2. Effect of SC CO2 pressure on the composition of PP/PS composites. (The other soaking conditions are the same as those presented above for Figure 1.)
Figure 3. Dependence of PS contents of PP/PS composites on the monomer (styrene) concentration in the fluid phase. (The other soaking conditions are same as Figure 1.)
time, pressure of SC CO2, and monomer concentration in the fluid phase on the mass uptake of PP substrates were studied, and the results are shown in Figures 1-3. Figure 1 shows the dependence of mass uptake on the soaking time at 35 °C and 120 bar. The concentration of the monomer in the fluid phase is 0.80 mol/L. The
Preparation of Polypropylene/Polystyrene Composites
Figure 4. Young’s modulus of the composites as a function of PS content.
figure illustrates that the mass uptakes before extraction by refluxing THF is larger than those after extraction, indicating that there are some extractable PS homopolymers in the composites. However, some of the PS is not extractable in the blends because the mass uptake is still larger than zero after extraction. The two curves in the figure have a similar trend, i.e., the mass uptakes increase initially with soaking time, and become independent of soaking time after about 8 h. In other words, soaking equilibrium can be reached after 8 h. To study the effect of SC CO2 pressure on the composition of PP/PS composites, the pressure was varied from 8 to 15 MPa, and the results are shown in Figure 2. It can be observed that the mass uptake has a peak at 11 MPa. We think two opposite factors relative to the soaking process can result in this phenomenon. An increase in pressure leads to increased solvent power of CO2, which is not favorable to increasing the mass uptake because styrene distributes between CO2 phase and PP matrix. On the other hand, increasing the pressure of CO2 induces a larger degree of swelling of PP substrate, which should enhance the mass uptake. The two competing factors result in the maxima in the curves. Figure 3 demonstrates the dependence of PS contents in the PP/PS composites on the monomer (styrene) concentration in the fluid phase during the soaking process. The PS content increases with increasing monomer concentration in the studied concentration range. This is easy to understand because larger concentration of the monomer in the fluid phase favors the absorption of the monomer in the PP matrix. Composite Characterization. After incorporation of PS, the specimens were found to retain their original shape and clearness. The blends prepared were characterized by different techniques. Mechanical Test. The dependence of the Young’s modulus, tensile strength, and the elongation-at-break of the composites on PS content is plotted in Figures 4-6, respectively. Obviously, Young’s modulus, tensile strength, and the elongation-at-break of the blends are higher than those of the PP substrate. It is interesting to note that the Young’s modulus, tensile strength, and elongation-at-break of the blends first increase with PS content, and then decrease with PS content. To explain
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Figure 5. Tensile strength of the composites as a function of PS content.
Figure 6. Elongation-at-break of the composites as a function of PS content.
Figure 7. Infrared spectra of the original PP substrate (a); PP/PS composite (∼12 wt % PS) (b); sample b extracted with refluxing THF for 24 h (c); and precipitant of sample c solution in 1,2-dichlorobenzene precipitated in THF (d).
this phenomenon, we studied the morphology and the microstructure of the composites using different methods. Infrared Spectroscopy. Figure 7 shows the IR spectra of the original PP substrate (a), a PP/PS composite (∼12 wt % PS) (b), and the same composite sample after extraction with refluxing THF (c). The spectrum of the composite (b) indicates significant PS incorporation in the composite (aromatic C-C stretching at 1600-1900 cm-1, and aromatic C-H out-of-plane
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Table 1. Parameters of PP and the Blends crystalline degree/% sample
a
Tg/°C
Cp/J‚g-1 °C-1
162.6 162.7
114.4
0.042
163.4 165.3
110.0
0.066
∆Hm/J/g
Tm/°C
PP substrate
92.28
162.5
sample A extracted A
79.6 88.0
sample B extracted B
74.2 87.0
PSb-Cp/J‚g-1 °C-1
PSb-Tg/°C
PP/PS
PP portion
0.189
106.59
38.1 42.1
44.7 43.5
0.193
108.03
35.5 41.6
42.9 43.0
44.2
a Sample A is a PP/PS composite containing 14.75 wt % PS, and the treated A with residual PS of 3.11 wt %; sample B is a PP/PS composite with PS content of 18.16 wt %, and the treated B with residual PS of 3.26 wt %. b Extractable PS.
bending at 698 cm-1). The spectrum of the composite (c) shows that some PS still exists in the composite after extraction by THF. As discussed above, some unextractable PS exists in the blends. This portion of PS may play an important role for the improvement of the mechanical properties of the blends. At least two cases can result in residue of PS after extraction by refluxing THF. The first is that the PS molecules entangle with the PP molecules in the blends. This may result from the special blending process. As discussed above, the monomer and the initiator are first impregnated into the PP matrix, and then polymerized in the matrix. It is easy to imagine that some monomers may joint together surrounding PP molecules, resulting in the entanglement of some PS molecules with PP. Second, PS could not be removed in the extraction process if PS is grafted onto PP substrates. However, this can be ruled out by IR study, which is discussed in the following. The PP/PS blend with 18.16 wt % PS was first dissolved into 1,2-dichlorobenzene, then the solution was casted into THF, which resulted in the precipitation of PP and the grafted PP/PS copolymers (if existed), and left PS in the solution. Then the IR spectrum of the precipitate was determined. The absorption bands of PS cannot be observed in the spectrum of the precipitate, as can be known from curve d in Figure 7. This confirms that PS could not be grafted onto PP substrates at our experimental conditions. On the basis of IR spectra and the analysis above, it can be deduced that PS in the PP/PS composite exists in two different states: free PS homopolymers which can be extracted by THF, and those entangling with PP molecules, which can result in better compatibility of PP and PS, thus improving the performance of PP/PS composites. DSC Measurement. The thermal properties of the virgin PP and PP/PS composites were examined by means of DSC. The melting endotherms of the virgin PP and the PP/PS blend (with 18.16 wt % PS) before and after extraction are shown in Figure 8. Obviously, there is only one peak in each curve which corresponds to the melting temperature (Tm) of the polymer. Magnifying the endotherm curve of the unextracted specimen between 95 °C and 120 °C, an inflection at about 110 °C can be observed, which corresponds to the glass transition temperature (Tg) of PS phase in the blend. It appears that there is still an inflection in the curve of the composite after extraction. This is because that the PS phase is incompatible with PP phase. The melting points (Tm), apparent enthalpies of melting (∆Hm), and heat capacities Cp of the samples were obtained from DSC measurements, and the data
Figure 8. Melting endotherms of virgin PP and PP/PS (18.16 wt % PS) composite before and after extraction (3.26 wt % PS left).
of some PP/PS composites and PP matrix are listed in Table 1. The heat of fusion for PP of 100% crystallinity is known to be 209 J/g27. Thus, the crystallinity of the composites and the matrix can be easily calculated considering the fact that PS is amorphous and the related parameters are also listed in Table 1. It is known from the data in Table 1 that the apparent enthalpies of melting (∆Hm) decrease with increasing PS content in the composites, thus resulting in decreased crystallinity of the composites. However, the reduction in sample crystallinity is mainly due to dilution by the addition of amorphous PS, because the crystallinity in the PP portion is nearly the same as that of the PP matrix. The Tm of the PP also remains unchanged. These results indicate that the styrene polymerization occurs mainly within the amorphous regions of the polymer. TEM Measurements. The samples were microtomed in cross section so that the distribution of PS from the surface to the center of the samples could be examined. Figure 9 displays TEM micrographs of the blends with different PS contents. The OsO4-stained PS is the dark phase. As expected, phase separation is observed. However, PS is dispersed homogeneously in the PP matrix and the PS domain size is very small. This can be attributed to the special properties of SCFs and can be explained as follows. It is known that SCFs have high diffusivity and low viscosity compared to that of liquids, and have near zero surface tension. Thus, the monomer and the initiator molecules can diffuse into any interchain spaces in the PP matrix with a faster rate, provided that the size of the interchain space is larger than the molecules. Thus, these molecules are more uniformly distributed in the PP matrix before polymerization. SC CO2 is a swelling (27) Brandrup, S.; Immergut, E. M. Polymer Handbook; Interscience: New York, 1975; Vol. 5.
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that the entanglement can enhance the interface adhesion, which also contributes to the improvement of the mechanical performance of the composites. Figure 9 shows clearly that the size of PS phase increases with the PS content in the composites. After the PS content reaches a certain value, the increase in the total amount of PS mainly increases the size of PS phase, which is not favorable to improving the interfacial adhesion. This can explain the maxima on the curves of Young’s modulus, tensile strength, and the elongation-at-break of the composites as shown in Figures 4-6. Conclusion
Figure 9. TEM micrographs of a PP/PS composite with different PS contents.
agent for PP; however, it mainly swells the amorphous regions of PP matrix. Because of the existence of crystalline domains in PP matrix, SC CO2 cannot create large spaces within the PP substrate, and it can only swell the polymer to some extent. Thus, the size of the PS phase within PP matrix is small and PS is distributed more uniformly. This contributes to improving the mechanical performance of PP/PS composites. The phase boundary between PP phase and PS phase is very ambiguous, as can be seen from Figure 9. This may result partially from the entanglement of the PS and PP molecules near the phase boundary. We believe
The PP/PS composites can be fabricated using the free radical polymerization of styrene within CO2-swollen PP matrix. The Young’s modulus, tensile strength, and the elongation-at-break of the PP are improved significantly by the blending. The size of the PS phase in the composites is in the range of nanometers, and is distributed uniformly. Some of the PS molecules entangle with PP in the composites. The special microstructure and morphology result in the enhanced mechanical performances of the PP/PS composites. Acknowledgment. This work was financially supported by the China National Natural Foundation of Sciences (50173030) and the Chinese Academy of Sciences. CM0203215
