Modification of mechanical properties of oriented polypropylene by ozone oxidation

Modification of mechanical properties of oriented PP by ozone oxidation

17"]

16. A. V. RAGIMOV, B. A. MAMEDOV and B. I. LIOGON'KII, Vysokomol. soyed. AI9: 2538, 1977 (Translated in Polymer Sci. U.S.S.R. 19: 11, 2922, 1979) 17. A. V. RAGIMOV, S. S. SULEIMANOVA, B. I. LIOGON'KII and S. I. SADYKHZADE, Vysokomol. soyed. A16: 1222, 1974 (Translated in Polymer Sci. U.S.S.R. 16: 6, 1413, 1974) 18. I.L. STOYACHENKO, Ye. I. SHKLYAROV, A. M. KAPLAN, V. B. GOLUBEV, V. P. ZUBOV and V. A. KABANOV, Vysokomol. doyed. A18: 1420, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 6, 1628, 1976)

Polymer Science U.S.S.R, Vol. 30, No. 1, pp. 177-182, 1988 Printed in Poland

0032-3950/88 $10.00+.00 72, 1989 Pergamon Press pie

MODIFICATION OF MECHANICAL PROPERTIES OF ORIENTED POLYPROPYLENE BY OZONE OXIDATION* A. A. PoPov, At. V. RUSSAK,YE. S. POPOVA,N. N. K.OMOVAand G. YE. ZAIKOV Institute of Chemical Physics, U.S.S.R. Academy of Sciences

(Received 24 July 1986) The variation of mechanical, structural and dynamic parameters and MD of oriented PP were examined during ozone oxidation. It is assumed that the improvement of mechanical properties at the initial stage of oxidation is due to the crystallization of macromolecules which undergo further structure-formation after termination. Intensification of molecular interaction as a consequence of the biuld-up of functional groups containing oxygen has a certain effect on the improvement of mechanical properties. At more advanced stages of oxidation factors promoting the reinforcement of the polymer are overlapped by macromolecular terminations, which reduce mechanical parameters.

OXIDATION may be a promising method of modifying polymer materials. Oxidative breakdown normally causes a deterioration in operating properties of polymers. Therefore, main efforts of scientists are now aimed at developing compositions resistant to oxidation by introducing various additives and by chemical and structural modification of the polymer. However, increasing interest has recently been shown in the possibility of imparting new valuable properties of polymer materials by oxidative breakdown. Transition from experimental research to developing scientific foundations for this trend requires a complex approach which, apart from controlling variations in operational properties, also incorporates information that characterizes the variation of structural, dynamic and molecular parameters during oxidation. It is precisely this * Vysokomol. soyed. A30: No. 1, 159-163, 1988.

178

A. A. POPOVet al.

complex approach which is, in fact, missing from studies dealing with the modification of polymers by the action of breakdown. This study has been carried out to make up for the omission in this field. &inhibited isotactic PP (i&.=2.86 x lo5 and M,/M, =4*6) was used in the study. Films were prepared from powder by compression at 190’ and under a pressure of 15 MPa on a cellophane substrate, followed by rapid cooling in iced water. Orientation was carried out by local hating to 132’ to an elongation of 10. The films tested were 15 + 1 pm thick, which does not exceed the thickness, within which oxidation takes place under kinetic conditions [l, 21. Oxidation with an ozoneoxygen mixture was carried out in a box with temperature control at 25’ and an ozone concentration of @5 mole/m3. The degree of oxidation D A was determined by IR spectrometry (using a carbony1 band at 1710 cm-’ and a reference band at 1375 cm-’ in polarized light). Segmental mobility was studied by ESR spectroscopy using 2,2,6,6-tetramethylpiperidine-l-oxide as paramagnetic probe, which was introduced into the polymer film by diffusion from vapours of a concentration of up to 1O-3 mole/l. Correlation time r,, which characterizes the rotary mobility of the probe, was calculated by a method described previously [3]. MD was determined by temperature fractionation from a dilute solution with [PP] = 5 x 10m4 g/cm3 in diphenyloxide (&solvent) [4]. Calorimetric tests were carried out using a DSM-2M device (rate of scanning 16 deg/min, sample (3kO.l) mg, calibration for indium with T,=156.6”). When determining crystallinity the heat of melting of crystalline PP was used (146 J/g). The accuracy of determining T, of the polymer was not less than +0*2”. Physical and mechanical tests were carried out using a universal tensile testing machine (Instron1122) at 20°C. Polymer samples were shaped as rectangular bands 3.5 mm wide with an operating part 15 mm long. The rate of elongation was 10 mm/min. From the initial section of the dependence of o-e (3 4 1% according to deformation) the initial modulus E1 was calculated and the modulus of viscoelasticity E2 was determined from the linear section of this dependence up to e=12f 2%. Tensile strength e and breaking elongation e were also calculated. 13-15 individual measurements were made to derive each point.

Figure 1 shows the dependence of physical and mechanical parameters (a, E, El and I&) on the duration of oxidation of PP samples. At the initial stage of ozonization (< 2 hr) a marked improvement is observed in mechanical properties: o and E2 increase by -30x, E, by -70%. The value of e also increases somewhat (by -20 %). However, more prolonged action results in a sudden deterioration of physical and mechanical properties-with oxidation for 3 hr the parameters mentioned decrease considerably. It is well known [5] that physical and mechanical indices of isotropic PE may be increased by radiation as a consequence of the formation of intermolecular crosslinks. At the initial stage of radiation M of the polymer increases, while further radiation effect rest&s in the foi mation of a crosslinked, three-dimensional structure. Breakdown takes place in parallel with crosslinking. Competition of two processes results in’the appearance of a maximum on curves showing the dependence of tensile and breaking elongation on the absorbed dose (most often with doses of 5040-250 kGr) [5]. It is interesting to note that the orientation of HDPE noticeably changes the ratio of rates of breakdown and crosslinking ,slcr, which for isotropic PE varies between 0.18 and 0.30, showing a linear increase with the degree of elongation and reaching four when 3,= 12 [6]. This means that PE a polymer crosslinked by the action of high energy radiation, under the influence of orientation elongation becomes a polymer undergoing breakdown.

Modification of mechanical properties of oriented PP by ozone oxidation

179

As regards PP it is known [5] that the rate of macro-chain breakdown in isotropic samples is close to the rate of crosslinking (fl]~=0.75-1.0). Photochemical modification of polyolefins is in many ways similar to radiation treatment, particularly variations in heat resistance and main physical and mechanical characteristics as a result of the effect of both m e t h o d s - a r e very similar [5].

wi,% lO0

G

-

5

z~ 3

50

/3

I I 60 120 Time, m L~ FIG. 1

i

1

q

5

I

lo9 M[6

FIG. 2 FIG. 1. Dependences of Ex (1), £'2 (2), a (3) and e (4) on the duration of oxidation of oriented PP with ),= 10, [03]=0"5 mole/m3, at 25°. F~G. 2. Integral curves of MD of initial PP (1) and samples treated with an ozone-oxygen mixture for 12 (2), 34 (3), 69 (4), 130 (5) and 180 min (6). These results in the literature made it possible to carry out crosslinking of oriented PP macrochains during ozone action. Variations of M D were therefore examined during oxidation (Fig. 2). However, results do not enable us to draw such conclusion. The integral curve of MD, as ozone action proceeds, is more and more displaced to the range of low M. It is possible that crosslinking takes place, but it takes place at a lower rate in comparison with breakdown and filet > 1. Therefore, reinforcement of polymer samples during ozonization cannot be explained from the point of view of three-dimensional erosslinking as a result of the formation of intennolecular crosslinks. To explain causes of the pattern observed in Fig. 1, a study was made of the dependence of molecular mobility, crystallinity and melting point on the degree of oxidation of oriented PP samples (Fig. 3). It can be seen that kinetics of the formation of oxygen-containing groups C = O are linear (curve 1). The correlation time of the rotalion of the radical-probe increases continuously (curve 2), which proves that the molecular mobility of macrochains decreases in the amorphous phase. The increase in the rigidity of polymer matrix is evidently due to the formation of polar groups, increasing molecular interaction. The dependence of Tm on the duration of oxidation is similar

180

A.A. PoPovet at.

to the variation of zc but with opposite sign. This indicates a correlation between zc

and TInIt is possible that increased molecular interaction as a result of polar groups has considerable effect on increasing strength parameters of PP samples. However, this

,i01°,sec 1

3 1

75

2~

l

176

FO-

f8 ,16', I

i

1~0 Time, rain

60

1

j, I

/7o

I

I

130 7"*

Fie. 3 Fie. 4 FIG. 3. Dependences of the duration of oxidation of PP of the degree of oxidation D,et (1), correlation time rc (2), crystallinity x (3) and melting point .T= (4). FIG. 4. Calorimetric curves of heat absorption of initial PP (1) and samples oxidized for 34 (2), 69 (3) and 180 min (4). factor cannot, apparently, remain decisive as macromolecular ruptures proceed. This means that the result of competition of two processes - increased rigidity of the polymer matrix and terminations- represent one method of explaining the extremal dependence in Fig. 1. The dependence of the variation of e according to the time o f oxidation (Fig. 1, curve 4) is, to some extent, in conflict with this interpretation. It seems that the increase in the rigidity of the polymer matrix and the number of terminations result in lower from the very beginning of ozone action. However, the opposite pattern is observed experimentally - t h e value of e increases (although slightly) with a duration of oxidation < 2 hr. The form of variation in crystallinity K is significant (Fig. 3, curve 3) which, in fact, agrees with the form of variation of physical and mechanical parameters (Fig. 1). This agreement is hardly accidental. The mechanism of propagation of K with ozone action on the polymer is not clear enough. In the case of radiation exposure of PE no increase is observed in crystallinity [5]; on the contrary, a reduction in x is recorded. This difference in two methods of modification is evidently due to macrochain crosslinking. The formation of this chain in radiation treatment impedes pre-crystallization. In fact, macrochain termination in the amorphous phase gives a further degree o f freedom to terminal fragments formed and probably makes further structure formation possible, which could be termed "chemical annealing".

Modification of"mechanical properties of oriented PP by ozone oxidation

181

However, in contrast with temperature annealing, "chemical annealing" does rtot improve the previous crystalline phase, but only adds to the latter a new more directive part. This is indicated by the variation of the peak form of absorption of heat by oxidized samples-in accordance with DSC data (Fig. 4). It can be seen that in proportion to oxidation the peak intensity of heat absorption decreases on the background of considerable widening and displacement to the low temperature range, i.e. the value of ~c is subject to integral increase due to the readily melting defective part of the crystalline phase. We note that the problem remains open whether partial rearrangement of a previously existing crystalline phase takes place as a result of surface oxidation of crystallites, macromolecular termination and consequent recrystallization. This problem was raised by a reduction of the area of the high temperature part under the curve of heat absorption on increasing the extent of oxidation of PP samples (Fig. 4). Another cause of shifting the entire curve to the low temperature region may be the increase of surface energy of crystallites as a result of the appearance on it of oxygen-containing functional groups. According to the Gibbs-Thomson equation, first applied to polymers in [7]: Tin= TO(1

ah}l)' 2o'~

where ~e is the free surface energy per unit surface, dh°y is the melting enthalpy of a perfect crystal, l is the linear size of a lamella, T° and Tm are the thermodynamical equilibrium temperature and the melting points observed, respectively. The melting point of the crystal decreases when increasing its surface energy. One cannot exclude the partial disruption of the crystalline phase itself as a result of some ozone penetration. It follows from the variation described that the experimental variation of the type of melting curve may be due to various causes, or their combination. However, independent of the causes of this variation, structural modification improves mechanical properties. It is possible that the phase is formed by the formation of crystalline sections in the amorphous phase. These sections are evidently bridges linking individual crystallites. The presence of crystals of various dimensions should widen the curve of melting heat since it is known [8] that the dimensional polydispersion of crystallites results in this widmling. This is also observed experimentally (Fig. 4). The fact that crystalline fragments formed by the action of initial macrochain terminations- are in the amorphous phase, also follows from the condition that prolonged ozonization causes the simultaneous reduction of crystallinity and mechanical properties. Therefore, tile extremal form of physical and mechanical properties observed during ozone modification of oriented PP, may possibly be due to the formation in the amorphous phase of crystalline bridges their formation being initiated by macrochain terminations in the amorphous phase accessible to ozone. More intensive oxidation results in the breakdown of these bridges. Increased rigidity of the amorphous phase may have some influence at the initial stage of ozone action upon the improve

182

T. P. SMIRNOVAet al.

ment .of mechanical properties because of intensified molecular interaction and higher density, according to increased concentration of oxygen-containing functional groups. However, even this factor, which contributes to a reinforcement of the polymer- is overlapped by macromolecular terminations during oxidation, Data obtained suggest that with ozone modification-unlike radiation and UV action - crosslinking is not significant. Translated by E. SEMERE REFERENCES 1. A. A. POPOV, B. E. KRISYUK and G. Ye. ZAIKOV, Vysokomol. soyed. A22: 1366, 1980 (Translated in Polymer Sci U.S.S.R. 22: 6, 1501, 1980) 2. B. E. KRISYUK, A. A. POPOV and G. Ye. ZAIKOV, Vysokomol. soyed. B27: 266, 1985 (Not translated in Polymer Sci. U.S.S.R.) 3. A. L. BUCHACHENKO and A. M. VASSERMAN, Stabil’nyye radikaly (Stable Radicals) p. 349, Moscow, 1973 4. N. N. KOMOVA, V. M. GOLDBERG, M. S. AKUTIN and Ye. D. LEBEDEVA, Vysokomol. soyed. A27: 654, 1985 (Translated in Polymer Sci. U.S.S.R. 27: 3, 734, 1985) 5. A. G. SIROTA, Modifikatsiya struktury i svoistv poliolefinov, p. 150, Leningrad, 1984 6. A. G. SIROTA, A. P. VERKHOVETS and L. Ye. UTEVSKII, Vysokomol. soyed. B18: 661, 1976 (Not translated in Polymer Sci. U.S.S.R.) 7. J. D. HOFFMAN and J. J. WEEKS, J. Res. Nat. Bur. Standards A66: 13, 1962 8. N. M. TITOVA, V. P. POPOV, Yu. M. MALINSKII and Yu. K. GODOVSKII, Kompozitsionnyye materialy, No. 21, 11, 1982

Polymer Science U.S.S.R. Vol. 30, No. 1, pp, 182-188,1988 Printed in Poland

0032-3950/88 510.00+.00 0 1989 Pergamon Press plc

PREPARATION OF POLYMER FILMS FROM HEXAMETH3?LCYCLOTRISILAZANE IN THE PLASMA OF HIGH-FREQUENCY DISCHARGE* T. P. SMIRNOVA,L. V. KI-IRAMOVA, V. I. BELYI, A. P. SOLOV’YEVand I. V. TARANOVA Institute of Inorganic

Chemistry,

U.S.S.R.

Academy of Sciences

(Received 24 July 1986)

The effect of the HF discharge power and substratum temperature on the rate of propagation of hexamethylcyclotrisilazane films has been studied. Intense fragmentation of monomer molecules takes place in the plasma of the HF discharge with hydrogen abstraction, * Vysokomol.

soyed. A30: No. 1, 164-169, 1988,