The thermal oxidation of polypropylene octene-1 copolymers synthesised using a highly effective metallocene catalyst

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1 Plasticheskie Massy, No. 10, 2011, pp The thermal oxidation of polypropylene octene-1 copolymers synthesised using a highly effective metallocene catalyst T.V. Monakhova, 1, P.M. Nedorezova, 2 A.N. Klyamkina, 2 A.V. Chapurina, 2 A.A. Popov, 1 and L.S. Shibryaeva 1 1 N.M. Emanuel Institute of Biochemical Physics, Russian Academy of Sciences 2 N.N. Semenov Institute of Chemical Physics, Russian Academy of Sciences Selected from International Polymer Science and Technology, 39, No. 2, 2012, reference PM 11/10/17; transl. serial no Translated by P. Curtis Summary This work is devoted to a study of the properties of copolymers of polypropylene with a content of octene units of mol%. A study was made of the kinetics of oxidation of nascent specimens of polypropylene and its copolymers with octene at temperatures of C. At all temperatures, both the s and the s of oxidation depend extremally on the composition of the copolymer. It seems that the observed effects are connected with change in the chemical or physical structure of the amorphous regions. The introduction of copolymer units into the polymer chain of a homopolymer makes it possible to produce materials with a new set of properties: change in the crystallinity, the melting point, and the solubility of oxygen and other lowmolecular-weight substances, in particular antioxidants [1, 2], change in the physicomechanical properties [3], and, as a consequence of all these processes, change in the resistance to thermal oxidation [4]. Earlier it was established that the copolymerisation of propylene with octene-1 in liquid propylene has an azeotropic (ideal) nature, and the distribution of comonomer units in the copolymers is close to statistical [3]. It was shown that the modification of polypropylene even with small amounts of higher linear α-olefins affects the regularity of the polymer chain, the molecular weight characteristics of the copolymer, its melting temperature, and the degree of crystallinity, and makes it possible, in a wide range, to vary the rigidity and elasticity of the polymers. EXPERIMENTAL The specimens of copolymers of polypropylene and octene-1 that were investigated in the present work were synthesised in a laboratory reactor at 60 C in liquid propylene in the presence of a highly active isospecific homogeneous ansa-metallocene catalyst of C 2 symmetry rac-me 2 Si(4-Ph-2-MeInd) 2 ZrCl 2, activated by methylaluminoxane (MAO). Nascent copolymers in powder form were investigated. Using 13 C NMR, the composition of copolymers and the microstructure of their chains were determined. The contents of propylene and octene-1 were determined from 13 C NMR spectra of solutions of copolymers in o-dichlorobenzene. Spectra were taken on a Bruker DPX-250 instrument (frequency MHz) at 120 C. (The molecular weight characteristics of polypropylene and copolymers of propylene and octene-1 were determined on a Waters 150-C gel chromatograph at a temperature of 130 C in orthodichlorobenzene with the use of a linear HT-µ-styragel column.) X-ray measurements were carried out on a DRON-3M diffractometer in transmission mode (asymmetrical, focusing on the detector, quartz monochromator on the primary beam). CuK α radiation was used. To determine the degree of crystallinity (K), use was made of X-ray Smithers Rapra Technology T/41

2 amorphous polypropylene. The error in determining K, determined by X-ray diffraction analysis (XDA), did not exceed ±5%. The thermophysical and thermal characteristics of the polymer specimens (melting point, heat of melting, glass transition temperature) were determined on a DSC-30 calorimeter with a TS-15 processor using STAR SW software (v.8.00, Mettler). Measurements were conducted in nitrogen at a scanning rate of 10 C/min. The polymers were oxidised in the temperature range C at an oxygen pressure of 300 mmhg. The kinetics of oxygen absorption was studied using a manometric unit with the absorption of volatile reaction products by solid KOH. The errors in measuring the parameters did not exceed 10%. RESULTS AND DISCUSSION NMR investigations of the microstructure of copolymer chains, XDA investigations of the structure of the crystalline regions, and also DSC analysis of the thermal parameters of specimens of polypropylene and copolymers of propylene with octene-1 showed that the introduction into polypropylene of 1.5% octene units leads to a considerable change in the structure of the material (Table 1). Here, T melt.1 and H melt.1 are respectively the melting point and enthalpy of melting during the first heating, and T cr is the crystallisation temperature. From the data given in Table 1 it can be seen that octene-1 units introduced into the polypropylene chain, as in the case of copolymers of propylene with ethylene [7 9] and of propylene with hexene [10], are structural defects. These defects reduce the length of the regular sections of the chains in spiral 31 conformation that are capable of forming crystallites. Increase in the content of octene units not only reduces the length of the propylene sequences and consequently leads to the formation of fine crystallites with a low melting point (Table 1), it also prevents the process of crystallisation, as a result of which amorphisation of the polymer occurs. This is indicated by the fall in the degree of crystallinity according to XDA data, and by the reduction in the heat of melting of the crystallites according to DSC data (Figure 1). As is known, in the course of crystallisation of polypropylene chains, foreign units, in our case octene-1 units, are forced out into amorphous regions, thereby changing the chemical and physical structure of the latter [11 17]. According to data of 13 C NMR spectra, in the range of the studied compositions of specimens of copolymers, the distribution of octene-1 units along the polypropylene chain is statistical. Such a distribution of octene units can promote, during the crystallisation of the copolymer melt, localisation of these units in intercrystallite chains. It is evident that the latter is important for the process of thermo-oxidative degradation of the material. In fact, the introduction of octene-1 into the polypropylene chain has a considerable influence on the kinetics of thermal oxidation of the polymer. The kinetic curves of oxygen absorption by specimens of polypropylene with different contents of octene units are presented in Figure 2. These curves demonstrate the considerable increase in the initial of copolymer specimens by comparison with pure polypropylene. There is a reduction in the s. It is important to note the most significant reduction Figure 1. Dependence of the heat of melting of polypropylene (1) and the melting point (2) and crystallisation temperature of polypropylene in a copolymer (3) on the content of octene units Table 1. Structural, thermophysical, and thermal parameters of copolymers of polypropylene with octene-1 Specimen No. Content of octene-1 in Degree of crystallinity T 1 melt DH 1 melt copolymer according to XDA ( C) (J/g) (mol%) (%) G G G G G T cryst ( C) T/42 International Polymer Science and Technology, Vol. 39, No. 12, 2012

3 in in the specimen of polypropylene with 1.5% octene-1. At the same time, the dependence of the maximum s on the copolymer composition is extremal in nature (Figures 3 to 5). The relations between the parameters of oxidation of the copolymers and pure polypropylene change according to the oxidation temperature (Table 2). During solid-phase oxidation (90 C) of copolymers, the s of all specimens are below that of pure polypropylene. The minimum value of t ind is found in the specimen with 1.5% octene-1 (Table 2). The highest rate is found in the specimen containing 2.1% octene-1, equal to 4 x 10-5 mol/kg s, an order of magnitude higher than the rate of pure polypropylene (W max O2 = 0.14 x 10-5 mol/ kg s). Here, for the specimen containing 1.5% octene-1, W max O2 = 2.4 x 10-5 mol/kg s, and for copolymers with a greater ( %) content of octene it varies within the limits of error from 1.3 x 10-5 mol/kg s to 1.5 x 10-5 mol/kg s. These values are again greater than for pure polypropylene. Thus, the rate of solid-phase oxidation of the copolymers is considerably higher than the oxidation rate of the pure polypropylene. Increase in the of polypropylene in macromolecules with foreign units may be due to change in the chemical or physical structure of the amorphous regions. Figure 2. Kinetic curves of oxidation of specimens of isotactic polypropylene and its copolymers with octene: 1 pure polypropylene; 2 copolymer containing 1.5% octene units; 3 copolymer containing 2.1% octene units; 4 copolymer containing 3.6% octene units; 5 copolymer containing 4.5% octene units; 6 copolymer containing 8.4% octene units; 130 C, oxygen pressure 300 mmhg Figure 3. Change in the (A) and in the of oxidation (B) of copolymers of copolymer. 130 C, oxygen pressure 300 mmhg Table 2. Induction s and s of polypropylene and copolymers Test No. (batch) Content of octene-1 (mol%) 130 C 130 C 120 C 120 C 90 C 90 C G0 (398) G G G G (409) Smithers Rapra Technology T/43

4 The chemical structure of the macrochains determines the reactivity of the bonds leading the kinetic chains of oxidation, and consequently the reactivity of the polymer in relation to oxygen. The reactivity is determined from the magnitude of the kinetic parameter b, which includes the ratio of the rate constants of elementary stages of oxidation [18 20]: b = (ασk 2 2 k 4 [RH] 3 ) 0.5 /(8k 6 ) 0.5 Figure 4. Change in the (A) and in the of oxidation (B) of copolymers of copolymer. 120 C, oxygen pressure 300 mmhg where k 2 and k 6 are the rate constants of oxidation kinetic chain continuation and rupture, k 4 is the rate constant of decomposition of the hydroperoxide, α is the hydroperoxide yield per mole of absorbed oxygen, and σ is the probability of degenerate branching of the kinetic chains of oxidation. For polyolefins it is known that, in establishing a stationary concentration of radicals, owing to bimolecular chain rupture and auto-initiation by a first-order reaction, the kinetics of oxidation at the stage of leaving the to a depth of oxidation of the polymer of 1 mol/kg is described by a parabolic law DN O2 ~ t 2. At this stage of oxidation of the polymer, the concentration of monomer units remains unchanged, the rate of the process is determined by the rate of accumulation of the hydroperoxide, and the relation DN O2 = b 2 (t - t 0 ) 2 is fulfilled. In this equation, t is the oxidation time, t 0 is the, and b is the sought kinetic parameter. To determine the values of parameters b, the kinetic curves of oxidation at 90 C were transformed in coordinates of the equation (N O2 ) 1/2 = b(t - t 0 ). The obtained values of b for all specimens studied are presented in Figure 6 in the form of the curve of the dependence of the sought parameter on the content of octene units. As can be seen, the indicated curve passes through a maximum: with increase in the content of octene units to 2.1% the value of b increases sharply, and with subsequent increase in the content of comonomer it falls sharply. From this it can be seen that the introduction of Figure 5. Change in the (A) and in the of oxidation (B) of copolymers of copolymer. 90 C, oxygen pressure 300 mmhg Figure 6. Change in the kinetic parameter b on the composition of the copolymer at 90 C T/44 International Polymer Science and Technology, Vol. 39, No. 12, 2012

5 octene leads to a change in the rate of the elementary stages of the radical chain process of oxidation. The extremal nature of the dependence of parameter b on the content of octene units most likely indicates change in the factors determining the kinetics of radical reactions in specimens with a low and a high concentration of octene in the polypropylene chain. Increase in the concentration of foreign units statistically distributed along the polypropylene chain seems to lead to an increase in the structural defectiveness of the amorphous regions, and consequently to increase in the free volume and segmental mobility of the chains. The former and latter facilitate the transfer of kinetic chains of oxidation. As a result there is an increase in constant k 2, in parameter b, and consequently in the. The high concentration of side branches can retard or block the process of transfer of free valency to the R H bond either of their own or of the neighbouring macromolecule. This leads to a reduction in the constant k 2, in parameter b, and consequently in the oxidation rate of the polymer. As, in our copolymers, synthesised using a metallocene catalytic system, the octene units are distributed evenly along the chain, the modification of polypropylene even with small amounts of octene-1 affects the regularity of the polymer chain and the molecular weight characteristics of the copolymer, leads to appreciable changes in thermal behaviour, and lowers its temperature and the heat of melting and crystallisation, and the resistance of the copolymers to the action of oxygen changes. ACKNOWLEDGEMENT The authors are grateful to B.F. Shklyaruk for investigating the thermal properties of the polymers. REFERENCES 1. Monakhova T.V. et al., The solubility of antioxidants and oxygen in copolymers of ethylene with propylene. Vys. Soed., B18:90 (1976). 2. Kalinina I.G. et al., The solubility of lowmolecular-weight substances in copolymers. Vys. Soed., B48:1523 (2006). 3. Nedorezova P.M. et al., Vys. Soed., A43:605 (2001). 4. Monakhova T.V. et al., The thermal oxidation of copolymers of propylene with ethylene, obtained on a metallocene catalytic system. Plast. Massy, (3):10 (2008). 5. Kissin Yu.V., Isospecific Polymerisation of Olefins. Springer-Verlag, New York/Berlin/ Heidelberg/Tokyo (1985). 6. Kim Il, Macromol. Rapid Commun., 19(6):299 (1998). 7. Koval chuk A.A. et al., Polym. Bull., 56:145 (2006). 8. Nedorezova P.M. et al., Vys. Soed., A49(2):1 (2007). 9. Dubnikova I.L. et al., Vys. Soed., A37(12):2025 (1995). 10. Rishina L.A. et al., The copolymerisation of propylene and hexene-1 in the presence of homogeneous metallocene catalysts. Vys. Soed., A46(9): (2004). 11. Mandel kern L., The Crystallisation of Polymers. Khimiya, Leningrad, 336 pp. (1966). 12. Wunderlich B., The Physics of Macromolecules. Vol. 2. Mir, Moscow (1979). 13. Godovskii Yu.K., Thermal Methods for Investigating Polymers. Khimiya, Moscow, 280 pp. (1982). 14. Wilkinson R.W. and Dole M., Specific heat of synthetic high polymers. X. Isotactic and atactic polypropylene. J. Polym. Sci., 58(7): (1962). 15. Turner-Jones A., Development of the γ-crystal form in random copolymers of propylene and their analysis by DSC and X-ray method. Polymer, 12(8): (1971). 16. Piccarolo S. et al., Crystallisation of Polymers, ed. by Dosiere P. Kluwer Academic, Dordrecht, The Netherlands, p. 475 (1993). 17. Gen D.E. et al., Investigating copolymers of propylene with higher olefins by Raman spectroscopy. Proc Conf on Raman Scattering 80 Years of Research, 8 10 October 2008, Moscow (2009). 18. Emanuel N.M. and Buchachenko A.L., The Chemical Physics of the Molecular Breakdown and Stabilisation of Polymers. Nauka, Moscow (1988). 19. Shlyapnikov Yu.A. et al., Anti-oxidative Stabilisation of Polymers. Khimiya, Moscow (1986). 20. Popov A.A. et al., The Oxidation of Oriented and Stressed Polymers. Khimiya, Moscow (1987) Smithers Rapra Technology T/45