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Molecular weight and antioxidant effects on the structure of irradiated linear low density polyethylene
014&5724/91 S3.OO+O.W RrpmooFraapk
Radar. Phy~.Ckm. Vol. 38, No. I, pi. 69-94. 1Wl ht. 1. Rod&. A& Itutrm.. Part C Rimal in Gfut BritAio
MOLECULAR WEIGHT AND ANTIOXIDANT EFFECTS ON THE STRUCTURE OF IRRADIATED LINEAR LOW DENSITY POLYETHYLENE s. TRfFODI,’
J. M. Cm,
’ 0. A. Cunz~d
and E. M. VU&~’
‘Planta Pilot0 de lngcnierio Quimica (UNS-CONICET) and ‘Laboratorio de Radiois6topos (UNS). CC.717. fW0 Bahia Blanca Argentina (Received 6 May 1990; receivedfor publication28 June M!W) Abe&act-Linear copolymers of ethylene and butcne-1 with uniform chemical microstructure and very narrow molecular weight distribution arc used to study the eff~ of ionizing radiation. The well haracttnzcd copolymers arc irradiated at room temperature with y-rays from a @Co source To follow fe evolution of the molecular structure with the radiation dosea. changes in molecular weight averages
Ma and M, arc measured by membrane osmometty, light scat&ring and GPC. The influence of the original linear polymer molecular weight is cumined in the range of SO,OOO-loO.ooO. The effects of antioxidant w explored irradiating samples with and without additive.
INlIkODUCTlON
homogeneous polymers which arc cquivaknt in structure to copolymers of ethylene and butcne-I of a very narrow molecular weight distribution (M./M, c 1.1) (Stacy and Arnett. 1973; Rachapudy er al., 1979; CarclIa et al., 1984). The homogeneous chemical composition and the statistical distribution of branching points along the chains result in smaller size crystallites with a fairly uniform sixc distribution (Merino and Carclla, 1987; Romsxy lr ol., 1984; Mandelkem et al., 1986; Alamo and Mandelkem. 1989). In this work, well characteriscd model polymers were used to study the net mult of the competition bctwccn crosslinking and scission induced by irradiation in linear hydrocarbon chains with randomly distributed short branches. Both the effect of the initial molecular weight of the polymer and the iniIucncc of an incorporated antioxidant were explored.
Ionizing radiation induces at least two types of reaction in polyethylene; crosslinking and scission. These reactions proceed in parallel and both arc supposed to follow a free-radical mechanism (Randall et al., 1983). Determining the relative importance of thcsc two reactions in polyethylene is one goal that still has not he-en completely clarified. Starting with the pioneering studies of Dole (1950). and Charksby (1952). a lot of work has been done in this area. In spite of the simple chemical structure of polyethylene, the dilliculties involved in obtaining very well charactcrixcd products as starting materials have complicated considerably the analysis and the intcrprctation of the available experimental ruults. Commercially availabk low density polyethylcncs present difficulties when one tries to analyxc the structural modifications induced in the polymer by radiation. Normally an homogeneous reaction medium is assumed for the interpretation of the experimental results. However it is well known that them are at least three different sources of inhomogcneitics in LLDPE: ftrst tertiary carbon atoms along the chains that arc much more sensitive to chain scission than secondary atoms, secondly very broad molecular weight distributions that arc common in commercial polymers used for these studies and finally the prcscncc of two phases in the polymer morphology, whet-c crystallitcs are more stable to irradiation than the amorphous phase. Anionic polymerisation of butadienc. followed by hydrogenation ofTen a route to compositionally more tTo whom cotmpondena
EXPEEIMCNTAL Two linear polybutadiencs (PBS) of different molecular weights were anionicahy polymerixcd unda high purity conditions in a vacuum system. The solvent used in the polymerization was bcnxcne and the initiator tertiary butyl-lithium. To convert the two monodispcrsc PBS into ethylentbutcne I copolymers (EBC) the two lincar homopolymers wet-c hydrogenated in cyclohexane solution using a palladium catalyst. The douMe bond microstructure of the original PBS and the dcgrcc of unsaturation of the hydrogenated polybutadicncs were detcrmincd by infrared spectroscopy (IR). No residual unsaturation was dctcctcd in the hydrogenated polybutadiene (HPB), being the resolution of the IR better than
should be addressed. 89
S. TR~DI
90
0.1% molar. This means that the original double bonds of the narrow polydisperse PB were completely reacted to obtain the EBC with composition equivalent to that of linear low density polyethylene (approx. 20 CH,/lOOO C). Details of the synthesis procedure and the characterization techniques of the polymers can be found in previous publications (Carella et al., 1984; Kringas et al., 1985; Andreucetti et al., 1988). After hydrogenation of the two PBS the resulting EBC samples were labelled as polymers B and C. To study the influence of a commercial antioxidant on the radiation effects, part of the low molecular weight EBC (polymer B), was separated after the hydrogenation when it was still in cyclohexene solution at 70°C and slowly poured over cold methanol containing 2000ppm of Irganox 1010 from Ciba-Geigy. The precipitated polymer contained about 1OOOppm of trapped antioxidant. This sample was labelled as polymer A. Molecular weights and molecular weight distributions of the polymers were measured by several methods. Gel permeation chromatography (GPC) was performed in a Waters 150 C ALC/GPC equipped with five micro styragel columns with nominal pore sizes 106, 10s, 104, 10’ and 500 A. The solvent used for the PB was tetrahydrofuran (THF) at room temperature. The hydrogenated PB was run in 1,2, 3 trichlorobenzene (TCB) at 140°C. The flow rate of the solvents was always 1.0 ml/min. The polymer concentration was always less than 0.1%. The universal calibration curve was used, with linear polyethylene standards (NBS) and polystyrene standards (Pressure Chemicals) (Grubisic et al., 1967). No branching or column spreading corrections were applied. Low angle laser light scattering (LALLS) was used to determine weight average molecular weights (M,). The instrument used was a Chromatrix KMX-6. The PBS were run in cyclohexane at 30°C and the EBCs in TCB at 90°C. Number average molecular weights (MJ were measured by membrane osmometry (MO). The instrument used in this case was the Knauer model 01 .OO.The solvent was also TCB at 90°C. The molecular weight characterisation of the three EBCs is reported in Table 1. The copolymer samples were compression molded under vacuum into disk shapes at a molding temperature of 130°C. Once cooled back to room temperature the samples were sealed under vacuum in Pyrex tubes. The radiation of the polymers was performed at room temperature by a @Co y-source. After the polymer samples were irradiated they were annealed for 2 h at Table 1. Moleadar weights of the non-irradiated polymers Polymer c
Polymers A and B
Mw
GPC MO LALLS
43,ooo 42,ooo
50,ooo 50,ooo
MWIM” 1.16
M” 88,000
Mlv %,OOO 109,ooo
MVIM” 1.09
ef al.
140°C to allow for complete recombination of the free radicals formed. The radiation doses applied to each material were the following: polymer A-S, 10, 15, 20, 25, 30, 35, 40, 60, 80 and 100 kGy; polymer B-5, 10,20,30,35,40,45,50,55,60,75,80,95, 119, 130 and 160 kGy; polymer C-20, 30, 35,40,45, 50, 55 and 96 kGy. Finally the Pyrex glass tubes were cut open and the disks were extracted in soxhlet hot-vapour-jacketed extractors with xylene at 140°C to obtain the soluble fractions. Nitrogen was bubbled continuously in the lower flask of the soxhlet to avoid oxidation of the samples. GPC measurements were then performed on all samples irradiated at doses below the critical gelation dose (CGD) and on the soluble fraction of the samples above the CGD. RESULTS
AND DISCUSSION
Theoretical calculations for the evolution of IU. and h4, of the irradiated samples as a function of the radiation doses were performed using the equations reported in a previous publication (Andreucetti et al., 1988). L, =
1
1 - u1/2 + /I1 ;
and
(2) where L, and L, are the number and weight average degree of polymerization, I is the initial degree of polymerization, a the average degree of crosslinking and /I the average degree of scissions. It was considered in these calculations that an average of 6% of the total energy of ionising radiation absorbed by the polymer was used in chain scission. This value was calculated from GPC experimental curves as is discussed below. Figure 1 compares the evolution of the theoretical and experimental molecular weight values for polymer A. Aside from certain scattering in the experimental results, the agreement between the calculated and the measured molecular weights is very good. Only for the sample irradiated with 15 kGy we detected that the measured Mn value from MO was lower than expected, and even lower than the molecular weight of polymer A with no irradiation. This result is consistent with the hypothesis that: (1) The most noticeable effect of the addition of the antioxidant is the deactivation of the free radicals induced by the irradiation process. This favours chain scission and inhibits crosslinking lowering the molecular weight. (2) The influence of the antioxidant is more prominent at the beginning of the radiation process when its concentration is higher.
Molecular weight and antioxidant effects
The gel fraction measured by soxhlet extractions on the irradiated samples of the three polymers is shown in Fig. 2. The gel fractions for the sets of samples obtained irradiating polymers B and C, i.e. those corresponding to the polymers with no incorporated antioxidant, were fitted with the calculated curve for the fraction of gel (W,) reported by Andreucetti et al. (1988). These calculations were also performed considering that 6% of the energy was spent in chain scission. The intercepts of the curves with the 0% gel fraction give the critical gelation doses (CDG), for polymers B and C. They were 74 and 36 kGy respectively. The agreement between experiments and the theoretical curves is remarkably good. Also the proper relation M, x CGD equal to a constant is verified for these two samples. From these results we have calculated the G-values for crosslinking and scission for polymers B and C. They are G, = 1.40 and G, = 0.05 for polymer B and G, = 1.36 and G, = 0.09 for polymer C. This procedure was not used for polymer A because only two data points were available. Even with this limited amount of data the gel fraction of polymer A seems to grow at a lower rate with the radiation doses than in the case of polymers that do not contain antioxidant. These results confirm the hypothesis discussed above that postulates that antioxidants such as Irganox 1010 are substances which react very fast with any type of free radicals. The lower slope of the curve for the growth of W, with irradiation and the delay in reaching the CDG of about 10% is also consistent with the hypothesis that the effect of the antioxidant delays the growth of the molecular structure through crosslinking (Randall et al., 1983; Chapiro, 1982). A more direct evidence of the effect of the antioxidant is seen when GPC results from polymers A and B exposed at
A
Mwmeasurements
l
Mn measurements
20
40
80
Doses (kGy1
Fig. 1. Evolution of molecular weight with radiation doses for polymer A. Lines calculated from equations (1) and (2).
91
loo -
10 0
20
40
60
80
100120
140
so
180
Daes (kGy) Fig. 2. Gel fraction from soxhlet extractions. Solid lines correspond to theoretical calculations from equation (7) in Andreucctti er al. (1988).
different doses of radiation are closely examined as we explain below. The GPC outputs for the three starting materials are shown in Figs 3-5. All three depict very narrow molecular Poisson-type weight distributions @4,/M, < 1.2). For this type of starting materials the width and the shape of the chromatographic traces become very sensitive to the combined processes of crosslinking and s&ion. Compared to the traces of the original polymer the scission process generates variable amounts of low molecular weight material (widening of the low molecular side of the chromatographic traces). The higher molecular weight material resulting from the crosslinking process widens the left hand side of the original GPC traces. The interpretation of the chromatograms is in fact more complicated because the crosslinking process can conceivably link small species (produced by scission) to larger ones, and this causes them to move in the GPC traces from the low molecular weight side to the high molecular weight part of the chromatogram. Also the product of the scission process is shown at the lower molecular side of the original curve only when the size of the scissioned species Sissmaller than that of the original non-irradiated polymer molecules. !&&ion from molecules with larger hydrodynamic volume than the original polymer molecules may result in large species that also appear at the left-hand side of the chromatogram. Another aspect expected to complicate the GPC trace analysis is branching. The increase of molecular weight is caused by random branching and therefore the hydrodynamic volume for the irradiated polymer cannot be related to the molecular weight in a reasonably simple manner. For this reason a quantitative interpretation of the GPC is only possible at low doses of radiation where the effects described in these last two paragraphs will
92
S. lkfmcu
lo5
104
Molecular
weight
Molecdar
weig ht
et al.
lo3 Molecular weight
106
105
104
10
Molecular weight
Fig. 3. Caption opposite.
Fig. 4. Caption opposite.
be minimized, since at low doses most of the polymer available for radiation is the original monodisperse material. At higher doses a qualitative interpretation of the traces is possible and it is still very useful to understand the mechanics of the radiation effects on the polymer. Figures 3-5 show the result of the GPC measurement for polymers A, B and C respectively. Part (a) of the figures corresponds to the pregel samples and part (b) to the soluble fractions extracted from the post gel samples. In all cases the chromatogram of the original non-irradiated polymer is drawn with full line as a reference. As is evident from the figures the broadening of the peaks at the low molecular weight side is very small. For the doses of radiation that were used in this study it would have been almost impossible to detect any change at this side of the GPC trace if we would have worked with a regular wide molecular distribution polymer.
By evaluating the widening of the low molecular weight side of the chromatogram for polymer A at low doses of radiation the proportion of energy employed in chain scission was estimated in 6%. At the high molecular weight side of the GPC’s of the three polymers it is possible to observe the formation of the dimer and the widening of the trace up to the gel point. GPC from common polyethylenes frequently show irregular traces at the high molecular weight side making this type of analysis almost useless on commercial polyethylenes. We will now discuss the main differences between polymers A and B, i.e. the low molecular weight polymer with and without antioxidant. If we look at the right hand side of the GPC’s for both polymers it is possible to conclude that the amount of scissioned materials at low doses of radiation grows faster for polymer A than for polymer B. Even if this is not evident from the plots in Figs 3a and 4a the
Molecular weight and antioxidant effects
Molecular weight
106
105
104
103
Molecular weight
Fig. 5 Figs 3-5. GPC cbromatograms from polymers A (Fig. 3), B (Fig. 4) and C (Fig. 5). Part (a) corresponds to GPC from samples irradiated below the CGD. Part (b) to GPC of the soluble fraction above the CGD. In all the figures the trace in full line correspcnds to chromatograms of the original non-irradiated polymers. Doses (kGy): (-) 0; (A) 5; (0) lo; (0) 20; (0) 30; (a) 35; (A) 40; (0) 45; (A) 50; (0) 60; (0) 75; (0) 80; (0) 96; (V) loo; (W) 119.
differences are noticeable in a higher scale plot. The contrast in behaviour between these two polymers becomes more evident on the high molecular weight aide of the chromatograms. The growth of the molecular weight for polymer B is much faster (compare for example the trace correspondent to 40 kGy in the same figures). The shape of the high molecular weight side for sample B shows a distinct shoulder slightly below 10’ that is not present in sample A. A simple calculation indicates that the shoulder elution volume corresponds exactly to the star-shaped dimer formed from the original linear polymer. Also for samples from polymer A, the high molecular weight side shows a smoother shape, as expected when a higher
93
random scission rate causes larger molecules to be broken apart faster than smaller ones. The post. gel results from the soluble fractions of polymers A and B (Figs 3b and 4b) show a common pattern of behaviour. Increasing the radiation doses steadily narrows the high molecular weight side of the trace. This is also a consequence of the random crosslinking process which favours larger molecules to be linked to the gel. A closer examination of the traces also reveals that the soluble fraction from polymer B is of considerably higher molecular weight than polymer A. The traces correspondent to 75 and 80 kGy in Fig. 4b show a second shoulder close to IO6 which corresponds to the resolution limit of the columns. At this point the hydrodynamic volume of the high molecular weight molecules in solution is so big that they are not retained by the GPC columns. The high molecular weight shoulder is only barely noticed at equivalent doses of radiation in polymer A. We will compare now the GPC results from polymers B and C, i.e. those correspondent to two initial polymers of different molecular weights with no antioxidant added. Figure 5a shows the GPC traces from polymer C prior to gelation. At equivalent doses of radiation the changes in the traces for the high molecular weight polymer are more evident than in polymer B. This is expected since each crosslinking reaction results in species of higher molecular weight than those from the lower molecular weight polymer. The gel point as calculated from the soxhlet extractions at about 36 kGy is conBrmed from the GPC measurements that show the broader peak at 35 kGy. The post gel chromatograms for polymer C are shown in Fig. 5b. They exhibit the characteristic narrowing of the bands in the high molecular weight side with increasing doses of radiation. The trace from the soluble fraction of the polymer irradiated at 96 kGy shows an additional feature; a broad low molecular weight tail. This is not unexpected since 96 kGy corresponds to a dose equivalent to 2.7 times the CGD. At this extent of radiation, less than 5% of the original polymer remains soluble and most of the high molecular weight molecules have been already attached to the gel by the crosslinking process (observe that the high molecular weight part of the trace is almost the same than in the original polymer). The remaining soluble fraction is mostly composed of non-reacted or scissioned chains that have a lower probability of reaction. CONCLUSIONS The results from the preceding sections show that the use of copolymers with a narrow molecular weight distribution is very useful to study the effect of ionizing radiation because it allows direct qualitative analysis from the GPC traces. The partial results obtained from this study demonstrate that s&ion is present in irradiated EBC. The percentage of energy consumed in this
94
s.
TRtpODI
process is about 6%. Addition of antioxidant favours chain scission and inhibits crosslinking as a consequence of the radical deactivation promoted by the latter. The effect of the antioxidant seems to be more prominent at the initial stages. These preliminary results will be confirmed in the ongoing work in our laboratory.
near
future
by
Acknowledgements-We
wish to thank Lit. Cristina Frova for performing part of the GPC measurements of this work; the Comision National de Energia Atomica (CNEA) for making their irradiation facilities available and to CIC, SECYT and CONICET for financial support.
REFERENCES Alamo R. G. and Mandelkern L. (1989) Macromolecules 22, 1273.
Andreucetti N. A., Curzio 0. A., Valles E. M. and Carella J. M. (1988) Radial. Phys. Chem. 31, 663.
et al.
Carella J. M., Graessley W. W. and Fetters L. J. (1984) Macromolecules 17, 2775.
Chapiro A. (1982) Radiation Chemistry of Polymeric S&terns. Pergamon Press, Oxford. Charlesbv A. (1952) Proc. R. Sot. Land A 215. 187. Dole M.- (1950) Rep. 9th Symp. Chemistry physics and Radiation Dosimetry.
Grubisic Z., Rempp P. and Benoit H. (1967) J. Polym Sci. Bs, 753.
Krigas T. M., Carella J. M., Struglinski M. J., Crist B., Graessley W. W. and Schilling F. C. (1985) J. Polym. Sci. Polym. ihys. 23, 509.
-
.
Mandelkem L.. Alamo R.. Mattice W. L. and Snvder R. G. (1986) Macromolecules i9, 2404. Merino J. J. and Carella J. M. (1987) Latin Am. J. Chem. Engng 17, 1. Rachapudy H., Smith G. G., Raju V. R. and Graessley W. W. (1979) J. Polym. Sci. Polym. Phys. 17, 1211. Randall J. C., Zoepfl F. J. and Silverman J. (1983) J. Makromol. Chem. Rapid. Commun. 4, 149.
Romszy R. C., Alamo R., Mathiew P. J. M. and Mandelkem L. (1984) J. Polym. Sci. Polym. Phys. 22, 1727. Stacy C. J. and Amett R. L. (1973) J. Phys. Chem. 77, 78.
Radar. Phy~.Ckm. Vol. 38, No. I, pi. 69-94. 1Wl ht. 1. Rod&. A& Itutrm.. Part C Rimal in Gfut BritAio
MOLECULAR WEIGHT AND ANTIOXIDANT EFFECTS ON THE STRUCTURE OF IRRADIATED LINEAR LOW DENSITY POLYETHYLENE s. TRfFODI,’
J. M. Cm,
’ 0. A. Cunz~d
and E. M. VU&~’
‘Planta Pilot0 de lngcnierio Quimica (UNS-CONICET) and ‘Laboratorio de Radiois6topos (UNS). CC.717. fW0 Bahia Blanca Argentina (Received 6 May 1990; receivedfor publication28 June M!W) Abe&act-Linear copolymers of ethylene and butcne-1 with uniform chemical microstructure and very narrow molecular weight distribution arc used to study the eff~ of ionizing radiation. The well haracttnzcd copolymers arc irradiated at room temperature with y-rays from a @Co source To follow fe evolution of the molecular structure with the radiation dosea. changes in molecular weight averages
Ma and M, arc measured by membrane osmometty, light scat&ring and GPC. The influence of the original linear polymer molecular weight is cumined in the range of SO,OOO-loO.ooO. The effects of antioxidant w explored irradiating samples with and without additive.
INlIkODUCTlON
homogeneous polymers which arc cquivaknt in structure to copolymers of ethylene and butcne-I of a very narrow molecular weight distribution (M./M, c 1.1) (Stacy and Arnett. 1973; Rachapudy er al., 1979; CarclIa et al., 1984). The homogeneous chemical composition and the statistical distribution of branching points along the chains result in smaller size crystallites with a fairly uniform sixc distribution (Merino and Carclla, 1987; Romsxy lr ol., 1984; Mandelkem et al., 1986; Alamo and Mandelkem. 1989). In this work, well characteriscd model polymers were used to study the net mult of the competition bctwccn crosslinking and scission induced by irradiation in linear hydrocarbon chains with randomly distributed short branches. Both the effect of the initial molecular weight of the polymer and the iniIucncc of an incorporated antioxidant were explored.
Ionizing radiation induces at least two types of reaction in polyethylene; crosslinking and scission. These reactions proceed in parallel and both arc supposed to follow a free-radical mechanism (Randall et al., 1983). Determining the relative importance of thcsc two reactions in polyethylene is one goal that still has not he-en completely clarified. Starting with the pioneering studies of Dole (1950). and Charksby (1952). a lot of work has been done in this area. In spite of the simple chemical structure of polyethylene, the dilliculties involved in obtaining very well charactcrixcd products as starting materials have complicated considerably the analysis and the intcrprctation of the available experimental ruults. Commercially availabk low density polyethylcncs present difficulties when one tries to analyxc the structural modifications induced in the polymer by radiation. Normally an homogeneous reaction medium is assumed for the interpretation of the experimental results. However it is well known that them are at least three different sources of inhomogcneitics in LLDPE: ftrst tertiary carbon atoms along the chains that arc much more sensitive to chain scission than secondary atoms, secondly very broad molecular weight distributions that arc common in commercial polymers used for these studies and finally the prcscncc of two phases in the polymer morphology, whet-c crystallitcs are more stable to irradiation than the amorphous phase. Anionic polymerisation of butadienc. followed by hydrogenation ofTen a route to compositionally more tTo whom cotmpondena
EXPEEIMCNTAL Two linear polybutadiencs (PBS) of different molecular weights were anionicahy polymerixcd unda high purity conditions in a vacuum system. The solvent used in the polymerization was bcnxcne and the initiator tertiary butyl-lithium. To convert the two monodispcrsc PBS into ethylentbutcne I copolymers (EBC) the two lincar homopolymers wet-c hydrogenated in cyclohexane solution using a palladium catalyst. The douMe bond microstructure of the original PBS and the dcgrcc of unsaturation of the hydrogenated polybutadicncs were detcrmincd by infrared spectroscopy (IR). No residual unsaturation was dctcctcd in the hydrogenated polybutadiene (HPB), being the resolution of the IR better than
should be addressed. 89
S. TR~DI
90
0.1% molar. This means that the original double bonds of the narrow polydisperse PB were completely reacted to obtain the EBC with composition equivalent to that of linear low density polyethylene (approx. 20 CH,/lOOO C). Details of the synthesis procedure and the characterization techniques of the polymers can be found in previous publications (Carella et al., 1984; Kringas et al., 1985; Andreucetti et al., 1988). After hydrogenation of the two PBS the resulting EBC samples were labelled as polymers B and C. To study the influence of a commercial antioxidant on the radiation effects, part of the low molecular weight EBC (polymer B), was separated after the hydrogenation when it was still in cyclohexene solution at 70°C and slowly poured over cold methanol containing 2000ppm of Irganox 1010 from Ciba-Geigy. The precipitated polymer contained about 1OOOppm of trapped antioxidant. This sample was labelled as polymer A. Molecular weights and molecular weight distributions of the polymers were measured by several methods. Gel permeation chromatography (GPC) was performed in a Waters 150 C ALC/GPC equipped with five micro styragel columns with nominal pore sizes 106, 10s, 104, 10’ and 500 A. The solvent used for the PB was tetrahydrofuran (THF) at room temperature. The hydrogenated PB was run in 1,2, 3 trichlorobenzene (TCB) at 140°C. The flow rate of the solvents was always 1.0 ml/min. The polymer concentration was always less than 0.1%. The universal calibration curve was used, with linear polyethylene standards (NBS) and polystyrene standards (Pressure Chemicals) (Grubisic et al., 1967). No branching or column spreading corrections were applied. Low angle laser light scattering (LALLS) was used to determine weight average molecular weights (M,). The instrument used was a Chromatrix KMX-6. The PBS were run in cyclohexane at 30°C and the EBCs in TCB at 90°C. Number average molecular weights (MJ were measured by membrane osmometry (MO). The instrument used in this case was the Knauer model 01 .OO.The solvent was also TCB at 90°C. The molecular weight characterisation of the three EBCs is reported in Table 1. The copolymer samples were compression molded under vacuum into disk shapes at a molding temperature of 130°C. Once cooled back to room temperature the samples were sealed under vacuum in Pyrex tubes. The radiation of the polymers was performed at room temperature by a @Co y-source. After the polymer samples were irradiated they were annealed for 2 h at Table 1. Moleadar weights of the non-irradiated polymers Polymer c
Polymers A and B
Mw
GPC MO LALLS
43,ooo 42,ooo
50,ooo 50,ooo
MWIM” 1.16
M” 88,000
Mlv %,OOO 109,ooo
MVIM” 1.09
ef al.
140°C to allow for complete recombination of the free radicals formed. The radiation doses applied to each material were the following: polymer A-S, 10, 15, 20, 25, 30, 35, 40, 60, 80 and 100 kGy; polymer B-5, 10,20,30,35,40,45,50,55,60,75,80,95, 119, 130 and 160 kGy; polymer C-20, 30, 35,40,45, 50, 55 and 96 kGy. Finally the Pyrex glass tubes were cut open and the disks were extracted in soxhlet hot-vapour-jacketed extractors with xylene at 140°C to obtain the soluble fractions. Nitrogen was bubbled continuously in the lower flask of the soxhlet to avoid oxidation of the samples. GPC measurements were then performed on all samples irradiated at doses below the critical gelation dose (CGD) and on the soluble fraction of the samples above the CGD. RESULTS
AND DISCUSSION
Theoretical calculations for the evolution of IU. and h4, of the irradiated samples as a function of the radiation doses were performed using the equations reported in a previous publication (Andreucetti et al., 1988). L, =
1
1 - u1/2 + /I1 ;
and
(2) where L, and L, are the number and weight average degree of polymerization, I is the initial degree of polymerization, a the average degree of crosslinking and /I the average degree of scissions. It was considered in these calculations that an average of 6% of the total energy of ionising radiation absorbed by the polymer was used in chain scission. This value was calculated from GPC experimental curves as is discussed below. Figure 1 compares the evolution of the theoretical and experimental molecular weight values for polymer A. Aside from certain scattering in the experimental results, the agreement between the calculated and the measured molecular weights is very good. Only for the sample irradiated with 15 kGy we detected that the measured Mn value from MO was lower than expected, and even lower than the molecular weight of polymer A with no irradiation. This result is consistent with the hypothesis that: (1) The most noticeable effect of the addition of the antioxidant is the deactivation of the free radicals induced by the irradiation process. This favours chain scission and inhibits crosslinking lowering the molecular weight. (2) The influence of the antioxidant is more prominent at the beginning of the radiation process when its concentration is higher.
Molecular weight and antioxidant effects
The gel fraction measured by soxhlet extractions on the irradiated samples of the three polymers is shown in Fig. 2. The gel fractions for the sets of samples obtained irradiating polymers B and C, i.e. those corresponding to the polymers with no incorporated antioxidant, were fitted with the calculated curve for the fraction of gel (W,) reported by Andreucetti et al. (1988). These calculations were also performed considering that 6% of the energy was spent in chain scission. The intercepts of the curves with the 0% gel fraction give the critical gelation doses (CDG), for polymers B and C. They were 74 and 36 kGy respectively. The agreement between experiments and the theoretical curves is remarkably good. Also the proper relation M, x CGD equal to a constant is verified for these two samples. From these results we have calculated the G-values for crosslinking and scission for polymers B and C. They are G, = 1.40 and G, = 0.05 for polymer B and G, = 1.36 and G, = 0.09 for polymer C. This procedure was not used for polymer A because only two data points were available. Even with this limited amount of data the gel fraction of polymer A seems to grow at a lower rate with the radiation doses than in the case of polymers that do not contain antioxidant. These results confirm the hypothesis discussed above that postulates that antioxidants such as Irganox 1010 are substances which react very fast with any type of free radicals. The lower slope of the curve for the growth of W, with irradiation and the delay in reaching the CDG of about 10% is also consistent with the hypothesis that the effect of the antioxidant delays the growth of the molecular structure through crosslinking (Randall et al., 1983; Chapiro, 1982). A more direct evidence of the effect of the antioxidant is seen when GPC results from polymers A and B exposed at
A
Mwmeasurements
l
Mn measurements
20
40
80
Doses (kGy1
Fig. 1. Evolution of molecular weight with radiation doses for polymer A. Lines calculated from equations (1) and (2).
91
loo -
10 0
20
40
60
80
100120
140
so
180
Daes (kGy) Fig. 2. Gel fraction from soxhlet extractions. Solid lines correspond to theoretical calculations from equation (7) in Andreucctti er al. (1988).
different doses of radiation are closely examined as we explain below. The GPC outputs for the three starting materials are shown in Figs 3-5. All three depict very narrow molecular Poisson-type weight distributions @4,/M, < 1.2). For this type of starting materials the width and the shape of the chromatographic traces become very sensitive to the combined processes of crosslinking and s&ion. Compared to the traces of the original polymer the scission process generates variable amounts of low molecular weight material (widening of the low molecular side of the chromatographic traces). The higher molecular weight material resulting from the crosslinking process widens the left hand side of the original GPC traces. The interpretation of the chromatograms is in fact more complicated because the crosslinking process can conceivably link small species (produced by scission) to larger ones, and this causes them to move in the GPC traces from the low molecular weight side to the high molecular weight part of the chromatogram. Also the product of the scission process is shown at the lower molecular side of the original curve only when the size of the scissioned species Sissmaller than that of the original non-irradiated polymer molecules. !&&ion from molecules with larger hydrodynamic volume than the original polymer molecules may result in large species that also appear at the left-hand side of the chromatogram. Another aspect expected to complicate the GPC trace analysis is branching. The increase of molecular weight is caused by random branching and therefore the hydrodynamic volume for the irradiated polymer cannot be related to the molecular weight in a reasonably simple manner. For this reason a quantitative interpretation of the GPC is only possible at low doses of radiation where the effects described in these last two paragraphs will
92
S. lkfmcu
lo5
104
Molecular
weight
Molecdar
weig ht
et al.
lo3 Molecular weight
106
105
104
10
Molecular weight
Fig. 3. Caption opposite.
Fig. 4. Caption opposite.
be minimized, since at low doses most of the polymer available for radiation is the original monodisperse material. At higher doses a qualitative interpretation of the traces is possible and it is still very useful to understand the mechanics of the radiation effects on the polymer. Figures 3-5 show the result of the GPC measurement for polymers A, B and C respectively. Part (a) of the figures corresponds to the pregel samples and part (b) to the soluble fractions extracted from the post gel samples. In all cases the chromatogram of the original non-irradiated polymer is drawn with full line as a reference. As is evident from the figures the broadening of the peaks at the low molecular weight side is very small. For the doses of radiation that were used in this study it would have been almost impossible to detect any change at this side of the GPC trace if we would have worked with a regular wide molecular distribution polymer.
By evaluating the widening of the low molecular weight side of the chromatogram for polymer A at low doses of radiation the proportion of energy employed in chain scission was estimated in 6%. At the high molecular weight side of the GPC’s of the three polymers it is possible to observe the formation of the dimer and the widening of the trace up to the gel point. GPC from common polyethylenes frequently show irregular traces at the high molecular weight side making this type of analysis almost useless on commercial polyethylenes. We will now discuss the main differences between polymers A and B, i.e. the low molecular weight polymer with and without antioxidant. If we look at the right hand side of the GPC’s for both polymers it is possible to conclude that the amount of scissioned materials at low doses of radiation grows faster for polymer A than for polymer B. Even if this is not evident from the plots in Figs 3a and 4a the
Molecular weight and antioxidant effects
Molecular weight
106
105
104
103
Molecular weight
Fig. 5 Figs 3-5. GPC cbromatograms from polymers A (Fig. 3), B (Fig. 4) and C (Fig. 5). Part (a) corresponds to GPC from samples irradiated below the CGD. Part (b) to GPC of the soluble fraction above the CGD. In all the figures the trace in full line correspcnds to chromatograms of the original non-irradiated polymers. Doses (kGy): (-) 0; (A) 5; (0) lo; (0) 20; (0) 30; (a) 35; (A) 40; (0) 45; (A) 50; (0) 60; (0) 75; (0) 80; (0) 96; (V) loo; (W) 119.
differences are noticeable in a higher scale plot. The contrast in behaviour between these two polymers becomes more evident on the high molecular weight aide of the chromatograms. The growth of the molecular weight for polymer B is much faster (compare for example the trace correspondent to 40 kGy in the same figures). The shape of the high molecular weight side for sample B shows a distinct shoulder slightly below 10’ that is not present in sample A. A simple calculation indicates that the shoulder elution volume corresponds exactly to the star-shaped dimer formed from the original linear polymer. Also for samples from polymer A, the high molecular weight side shows a smoother shape, as expected when a higher
93
random scission rate causes larger molecules to be broken apart faster than smaller ones. The post. gel results from the soluble fractions of polymers A and B (Figs 3b and 4b) show a common pattern of behaviour. Increasing the radiation doses steadily narrows the high molecular weight side of the trace. This is also a consequence of the random crosslinking process which favours larger molecules to be linked to the gel. A closer examination of the traces also reveals that the soluble fraction from polymer B is of considerably higher molecular weight than polymer A. The traces correspondent to 75 and 80 kGy in Fig. 4b show a second shoulder close to IO6 which corresponds to the resolution limit of the columns. At this point the hydrodynamic volume of the high molecular weight molecules in solution is so big that they are not retained by the GPC columns. The high molecular weight shoulder is only barely noticed at equivalent doses of radiation in polymer A. We will compare now the GPC results from polymers B and C, i.e. those correspondent to two initial polymers of different molecular weights with no antioxidant added. Figure 5a shows the GPC traces from polymer C prior to gelation. At equivalent doses of radiation the changes in the traces for the high molecular weight polymer are more evident than in polymer B. This is expected since each crosslinking reaction results in species of higher molecular weight than those from the lower molecular weight polymer. The gel point as calculated from the soxhlet extractions at about 36 kGy is conBrmed from the GPC measurements that show the broader peak at 35 kGy. The post gel chromatograms for polymer C are shown in Fig. 5b. They exhibit the characteristic narrowing of the bands in the high molecular weight side with increasing doses of radiation. The trace from the soluble fraction of the polymer irradiated at 96 kGy shows an additional feature; a broad low molecular weight tail. This is not unexpected since 96 kGy corresponds to a dose equivalent to 2.7 times the CGD. At this extent of radiation, less than 5% of the original polymer remains soluble and most of the high molecular weight molecules have been already attached to the gel by the crosslinking process (observe that the high molecular weight part of the trace is almost the same than in the original polymer). The remaining soluble fraction is mostly composed of non-reacted or scissioned chains that have a lower probability of reaction. CONCLUSIONS The results from the preceding sections show that the use of copolymers with a narrow molecular weight distribution is very useful to study the effect of ionizing radiation because it allows direct qualitative analysis from the GPC traces. The partial results obtained from this study demonstrate that s&ion is present in irradiated EBC. The percentage of energy consumed in this
94
s.
TRtpODI
process is about 6%. Addition of antioxidant favours chain scission and inhibits crosslinking as a consequence of the radical deactivation promoted by the latter. The effect of the antioxidant seems to be more prominent at the initial stages. These preliminary results will be confirmed in the ongoing work in our laboratory.
near
future
by
Acknowledgements-We
wish to thank Lit. Cristina Frova for performing part of the GPC measurements of this work; the Comision National de Energia Atomica (CNEA) for making their irradiation facilities available and to CIC, SECYT and CONICET for financial support.
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