Effect of chemical crosslinking on the decay of macroradicals in crosslinked poly(methyl methacrylate)

Radiat. Phys. Chem. Vol. 29, No. 3, pp. 237-240, 1987 Int. J. Radiat. Appl. Instrum. Part C

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EFFECT OF CHEMICAL CROSSLINKING ON THE DECAY OF MACRORADICALS IN CROSSLINKED POLY(METHYL METHACRYLATE) M A R T A KtJuOVA and FERENC SZ6CS Institute of Polymers, Chemical Research Centre, Slovak Academy of Sciences, 842 36 Bratislava, Czechoslovakia (Received 6 M a y 1986)

Abstract--Free radicals were generated in cromlinked poly(methyl methacrylate) by y irradiation with 16kGy at 195 K. Decay of macroradicals was observed as a function of the concentration of ethylene glycol dimethacrylate used as a crot.slinking agent by the ESR method. The rate constants for free radical decay were found to decrease with the increasing content of the crosslinking agent. The overall activation energies determined from the temperature variations of the rateconstantswere red,__~d__with the increasing amount of ethylene glycol dimethacrylate in the copolymer system. The results are discussed from the point of view of molecular motions dominating in the temperature interval examined and controlling the decay process in croufinked poly(methyl methacrylate).

INTRODUCTION Considerations about the crosslinking effect on the local dynamics of a polymer chain show that the restricted mobility can be due either to the existence of kinematic and dynamic hindrances along the chain or to the change in the local environment of the kinetic motional unitsY) Correlation between the mobility and reactivity of free radical centres can be done by phenomenology through comparison of the kinetic reaction parameters and of the relaxation parameters of mobility. Connection between the reactivity and inner mobility is evident from t3C N M R , dielectricand mechanical relaxations as well as from ESR data. Molecular motions of the main chain and side segments were investigated in crosslinked poly(methyl methacrylate) by dynamic-mechanical relaxation measurements, (2,3~ dielectric relaxations~4) and nuclear magnetic relaxations. ¢5-s)Asakura ~6~studied the motion of covalent crosslinked gels o f P M M A with triethylene glycol dimethaerylate and found a decrease in the values T~ with increasing crosslinking. However, he observed slightly larger values of Ti when the content o f crosslinking agent was I tool%, compared with uncrosslinked P M M A . A slight increase of the activation energies AE for internal motions of-OCHs, --CH3 and skeletal --CH,- groups with increasing crosslink densities was observed in crosslinked P M M A ( 0 . l - l . 0 m o l % of ethylene dimethacrylate) by the M A R - N M R method/s) Connections between the free radical decay and the effect of the external high pressure in crosslinked P M M A have been studied earlier/9~ The effect of crosslinking on the rate o f macroradical decay has so far been examined in crosslinked poly(glycol meth-

acrylate)°°) and in crosslinked polyolefins i-PP, em LDPE v2) by the ESR method. The aim o f this paper was to show the effect of erosshnking on the rate of free radical decay in crosshnked PMMA. In addition, we tried to compare at least qualitatively the effect of increased external pressure and steric hindrances caused by crosshnking on the rate of free radical decay.

EXPERIMENTAL

Crosslinked P M M A samples were prepared by bulk polymerization o f a methyl metha~rylate and crosslinking monomer of ethylene glycol dimethacrylate with the contents of 2.10 and 20 wt%. Polymerization was initiated by dibenzoyl peroxide (0.5 wt%) at 338 K for 5 h and then at 358 K for 3 h. (9~ After further increasing the temperature to 398--403 K, polymerization was completed in 3 days and then cooled to 298 K. Free radicals in samples were generated/n vacuo by 3' irradiation with a e°Co source at 195 K, with a total dose of 16 kGy. The ESR spectra were recorded on an E-4 X band Varian spectrometer. Kinetic measurements of the decay of macroradicals were performed at various temperatures using a temperature device Varian E257 with gaseous N2. Absolute concentrations o f free radicals were calculated on a Varian 620-100 computer. The method of high pressure annealing of in'a. diated samples with 2 wt% of crosslinking agent at 373 K over the pressure region between 200 and 400 MPa was the same as described earlier.~9~ Rate constants for radical decay shown in Fig. 4 were

237

238

MARTAKLIMOV.~and FERENCST_J~CS

determined from the difference in absolute concentrations before and after pressure temperature processing for 20 min.

25

RESULTS AND DISCUSSION

Figure 1 shows the ESR spectra of free radicals in crosslinked PMMA samples where free radicals were generated by T irradiation at 195 K in vacuo by total radiation dose of 16kGy. The spectra 1-3 were recorded at 353 K. The basic shape of the ESR spectra in crosslinked samples with various E G D M A content is nonet. The signal is assigned to the propagating end PMMA radicals. The presence of other types of free radicals cannot be eliminated. With regard to the identity of the ESR spectra throughout the series of samples we have not studied spectral analysis in detail. The total absolute radical concentration was obtained on a computer. The decay of propagating macroradicals in 7 irradiated uncrosslinked P M M A at the temperatures lower than TI has been thoroughly studied previously.¢13) A kinetic study of the decay process of free radicals at temperatures between 333 and 373 K was performed in crosslinked PMMA. As is obvious from Fig. 2, the time dependence in crosslinked PMMA. As is obvious from Fig. 2, the time dependence of the concentrations of macroradicals at 353 K documents the validity of the second order kinetics for the crosslinked samples studied. Figure 3 shows a plot of the rate constants for free radical decay vs the amount of the crosslinking agent at 353-373 K. As is obvious, significant retardation of the decay rate is observed in crosslinked samples

i.~ 1.5 m

i

o~

I

lo

20

I

30

Time (rain)

Fig. 2. Second order plot of the decay of 353 K of macro-

radicals in ~osslink~i PMMA containing: O--2wt%; A--10 wt%; O--20 wt% EGDMA. containing 2-10 wt% EGDMA. The plot shows that a further increase of the content of crosslinking agent will not remarkably change the rate of decay. The activation energies determined from the Arrhanius temperature dependence of the rate constants as well as frequency factors are listed in Table 1. As the portion of E G D M A increases in the copolymerization system, the overall activation energies for decay decrease. On condition that the activation energies for uncrosslinked P M M A are identical with that for minimally crosslinked PMMA (2 wt% EGDMA), about ll7.0kJ.mol -~, we can compare the effect of increased pressure and crosslinked structure on the rate of free radical decay. At 373 K the decay rate constant is !.459x 10-2°g.spin-m-s -~ for minimum crosslinking (Fig. 3). 1.5-

Itn

~c

lo

o o O.5

,2mT ,

0

Fig. 1. ESR spectra of free radicals of ¢roulinked poly(methyl metham~late) 7 irradiated in vacuo at 195 K with various EGDMA content (wt%) measured at 353 K: 1, 2%; 2, 10%; 3, 20%.

2

10 20 EGDMA (wt %)

Fill. 3. Rate ¢omaants for free radical decay is crmslinked PMMA vs the content of the crosslinking component EGDMA for temperatures: O--353 K; 0--363 K; A--373 K.

Decay of macroradicals in crosslinked PMMA Table I. Activation energies and frequency factors for free radical decay in crosslinked P M M A at the temperatures between 333 K and 373 K Crossfinking agent E G D M A wt%

Activation energy E (kJ mol -~)

0 2 10 20

117.3 116.4 93.5 76.2

Frequency factor /co (g spin -~ s - ' ) 4.0x 3.5 x 1.3 x 1.05 x

10 -17 10 -17 10 -18 10 -t9

Figure 4 shows isothermal dependence of the rate of free radical decay for uncrosslinked P M M A . In the pressure interval between 200 and 400 MPa the effect of pressure qualitatively corresponds to the effect of the crosslinked structure at about 300 MPa. The correlation between mobility and reactivity of radical eentres is found when the decay is controlled only by the respective molecular motion dominant for the given temperature region. The activation parameters characterizing the mobility of the polymer chain are identical with the reactivity determining parameters. Indirect connection between mobility and reactivity occurs more often, when molecular motions form, in combination with chemical processes, suitable conditions for the decay of free valency. Comparison of activation parameters shows that decay reactions proceed in temperature intervals of molecular motions but activation energies for decay reaction are different from those for mobility. An important factor of the decay o f free radicals is their localization to the site of crosslinking. Rotational mobility of bonds was found to be influenced only in the close vicinity of the crosslinking elements/ ]) On the other hand, the overall limitation of

3.5

";~ 2.5

'.g %

~ 0.1 200

300

I 400

Pressure (MPo)

Fig. 4. Pressure dependence of the rate constant for free radical decay in uncrosslinked PMMA at 373 K; O----the value of the decay rate constant in crosslinked PMMA containing 2 wt% EGDMA at 373 K and atmospheric pressure.

239

the mobility will also depend on the structure of the network,~th© !~ngth and flexibility of the crosslinking. An interesting fact was observed in a study of the molecular structure of copolymers of M M A and E G D M A in view of the effect of the crosslinking concentration on glass temperature/~ In addition to an increase of Ts values with crosslinking, an inflexion point was identified on the curve of the glass temperature at a concentration of about 8 wt% of E D M A . The authors ascribe this fact to the establishment of the critical molecular weight between crosslinks. They assume that at high concentrations of crosslinks more than one chain segment will be in motion. This is interesting from our point of view because above the E G D M A concentration of 10 wt% no remarkable changes in the decay rate are observed. Isothermal compressibility of the copolymers M M A and E G D M A containing 3, 10 and 20 wt% E G D M A was studied in a paper. ~]4~Besides densities, Ts values and specific volumes, the molecular weight of chain segments between nodal points were measured. The average molecular weight between nodal points in crosslinked P M M A decreases from the value of 1000 at 3 w t % to 140 for 20wt% of E G D M A . °41 The method of dielectric relaxation~') was used to determine the existence of the complex supermolecular structure of chemically crosslinked P M M A from relaxation characteristics of ,, and transitions. This paper showed on this basis of the character of dielectric losses that the activation energy does not vary with crosslinking. As has been reported in the introduction, in a series of cross-linked P M M A gels swollen in chloroform, activation energies for motions of various proton groups between 0.1 and !.0 tool% E G D M A increasd s~ slightly. In our ESR study we used much higher concentrations of the crosslinking agent since the overall activation energies for free radical decay determined by us in uncrosslinked and minimally cross-linked P M M A are almost identical. By the characterization of peroxide crosslinked LDPE from the point of view of the crossfinking effect on the rate of free radical decay we observed an increase in the effective activation energies.~2~ With the increasing number of crosslinkings between the chain, an average cohesion energy of the structure units in crosslinked polyethylene was increased. We assume that in the case of crosslinked P M M A indirect correlation between mobility and reactivity will occur. The slight decrease of the total activation energy for free radical decay can be qualitatively explained as follows. The activation energy for part of the decay process of radicals will increase as a result of the restricted segmental motion of macromolecules. By higher activation energy the rapid decay reaction of crosslinked samples will be almost stopped. It can be deduced from the decrease of both the rate constants of free radical decay (Fig. 3) and the frequency factors (Table 1) as a consequence of crosslinking. This will lower the overall activation

240

MARTA KLIMOVJ~and Fv.m~c SzOcs

energy of this multicomponent process by a contribution from the segmental approach of radical centres. In such a case, slower mechanisms could take place based on successive chemical reactions but with lower activation energies than is necessary for the segmental diffusion o f macromolecules in the u-transition region. REFERENCES

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