Nuclear Instruments and Methods in Physics Research B 105 (199.5) 282-284
Beam Interactions with Materials&Atoms
Time resolved study of the high energy radiation initiated polymerization E. Takhzs *, L. Wojnhovits Institute of Isotopes of the Hungarian Academy of Sciences, P.O. Box 77, H-1525 Budapest, Hungary
Abstract The EB (electron beam) initiated polymerization of ethyl acrylate (EA) and ethyl methacrylate (EMA) in dilute cyclohexane solution was studied by pulse radiolysis technique with transient spectroscopic detection. The maximum in the absorption spectrum of the intermediates lies at A = 290-320 nm in EA solution and at A = 285 in EMA solution. The extinction coefficients of the intermediates at the maximum of the spectrum were calculated to be .e2ssnm - 700 mol-’ dm3 cm-’ in EMA solution and ~~~~~~~~~ - 300 mol-’ dm3 cm -’ in EA solution. From the decay curves rate parameters of termination (2k,) were calculated. The decays were generally found to be second order with a rate coefficient of 1.4 X lo9 mol-’ dm3 s-l in EMA solutions and 6 X lo8 mol-’ dm3 s-l in EA solutions. In some cases deviation from a single second order was observed. This was interpreted in terms of the hindered mobility of the growing radicals.
1. Introduction Different esters of acrylic and methacrylic acid are the main components of EB curable coatings. Therefore it has both scientific and industrial importance to study the polymerization kinetics of these monomers. After-effect experiments are mostly applied for determining termination rate parameters. In photo initiated polymerization EPR spectroscopy is used to follow free radical concentration directly [l-3]. In EB initiated polymerization pulse radiolysis with kinetic spectroscopic detection can be used for direct determination of the radical concentration on a ps time scale . Previously the rate parameters of ethyl acrylate were determined in aqueous solutions . Our results on the determination of termination rate parameter of both ethyl acrylate and ethyl methacrylate in cyclohexane solution are discussed in this paper. Cyclohexane was chosen as a solvent because its radiolytic process is well known
[d. 2. Experimental Ethyl acrylate (EA) and ethyl methacrylate (EMA) monomers of Scientific Polymer Products were purified by removing the inhibitor using the chromatographic column
* Corresponding author. Tel. 361 155 0520, e-mail: [email protected]
supplied. Cyclohexane used as solvent was Fluka product and was purified by distillation. Pulse radiolysis using optical detection was performed at room temperature with a 4 MeV Linac. Pulse length was 2600 ns, dose/pulse value was varied between 30-100 Gy. For controlling both the instrument and collecting the measured data a computer was connected on line. The doses were determined by KSCN dosimetry. The deaerated solution was continuously flown through Suprasil quartz measuring cell. The absorbances calculated from the oscilloscope traces were converted to GE,, values by dividing the absorbances by the dose and multiplying by a factor of 9.65 X lo6 (100 eV)-’ mol-’ dm3 cm- ‘. G is the radiation chemical yield in number of species/100 eV energy absorbed, en is the extinction coefficient in mol-’ dm3 cm-‘. The GEM values are proportional to the concentration of intermediates.
3. Results and discussion Cyclohexyl radicals are produced during the radiolysis of cyclohexane with a yield of G = 5 and in the absence of radical acceptors such as iodine or compounds containing double bonds the radicals decay in the second order reactions of 2c - C,H,,
with a rate coefficient of 2k, = 2 X lo9 mol-’ dm3 s-l. Applying 100 or 200 Gy dose pulses the initial concentration of radicals is estimated as 4 X 10e5 and 8 X 10h5 mol dmp3 and the half-lives are ca. 12.5 and 6.2 ps.
0168-583X/95/$09.50 0 1995 Elsevier Science B.V. At1 rights reserved SSDIO168-583X(95)00554-4
E. Takbcs, L. WojnriroLlits/Nucl.
Instr. and Meth. in Phys. Res. B 105 (1995) 282-284
B - EA 0.02
20 30 microseconds
When acrylates are present the radicals are expected react in the following reactions:
c - C,H,,
. + CH, = CR,(COOR,)
+ c - C,H,,
- CH, - . CR,(COOR,)
- CH, -
+ CH,=CR,(COOR2) + c - C,H,,
- CH, - CR,(COOR,)
- CH, - . CR,(COOR,),
In Figs. 1 and 2 absorbance-time profiles are shown taken in 100 mmol dme3 solutions of EMA and EA at a dose/pulse value of w 100 Gy and wavelength of 285 and 293 nm, and solutions of EMA at dose/pulse values of 210 and 43 Gy at wavelength of 285 nm. These absorbances on the basis of analogies in the literature are attributed to the carboxyalkyl radicals produced in reactions (2). The buildup of the absorbance needs _ 3 ps, therefore the reaction between the cyclohexyl radical and
20 30 microseconds
Fig. 1. Absorbance-time curves in 0.1 mol dmw3 cyclohexane solutions of EMA (curve A, measured at 285 nm) and BA (curve B, measured at 293 nml.
c - C,H,,
300 330 Wavelength,
Fig. 2. Decay curves measured in 0.1 mol dmm3 cyclohexane solutions of EMA at dose rates of 210 Gy (curve A) and 43 Gy (curve B) measured at 285 nm.
Fig. 3. Absorption spectra taken in 0.1 mol dm -3 cyclohexane solutions of EMA (curve Al and EA (curve Bl; in the insert the spectrum of pure cyclohexane (curve Cl. Dose/pulse 200 Gy.
the acrylates is fast and the kz rate coefficient is estimated in the range of 10’ mol-’ dm3 s-‘. In Fig. 3 the absorption spectra of deaerated 0.1 mol dmP3 cyclohexane solution of EMA (curve A) and EA (curve B) are shown. The spectrum of cyclohexyl radicals is inserted (curve C) to show that although the GE* of the cyclohexyl radical is relatively high at the maximum of the spectrum, at about 250 nm, above 280 nm the absorption of cyclohexyl radicals is negligible therefore the absorption of the monomer and oligomer radicals (N CH, CR,(COOR,) - CH2 - CR,(COOR2)) will not be effected by the absorption of the solvent radicals significantly. The extinction coefficients of the carboxyalkyl radicals are calculated to be: EA solution at A = 290-320 nm, .e = 300 mol-’ dm3 cm-‘; EMA solution at A = 285 nm, E = 700 mol-’ dm3 cm-‘. The difference found between the extinction coefficients of the monomers can be explained on the basis of the difference in their chemical structures. The higher level of conjugation of the electrons in the case of ethyl methacrylate results in a higher extinction and higher absorbance. As Figs. 1 and 2 show, the decay of the radicals is considerably different in EA and EMA solution and also it depends on the dose/pulse value, and therefore on the initial concentration of radicals. The decays were generally found to be second order with a rate coefficient of 1.4 X lo9 mol-’ dm3 SK’ in EMA solutions and 6 X lo8 mall’ dm’ s-l in EA solutions. The value found in EMA solution is close to the termination rate coefficient of cyclohexyl radicals , 2 X lo9 mall ’ dm3 s-l. This rate coefficient is suggested to be basically determined by diffusive motion of radicals, but endproduct formation occurs only from the singlet encounter pairs (spin statistical factor l/4). Since the larger radicals have lower diffusion coefficient, their termination rate coefficients are expectedly smaller. The high rate coefficient of termination of carboxyalkyl radicals found in EMA solutions suggest that the termination oc-
E. Takhcs. L. Wojncirouits/Nucl. Instr. and Meth. in Phys. Res. B 105 (1995) 282-284
Intensity, 200 ,,
Fig. 4). In such cases good fits were obtained by combination of two second order decays, the second one, that described the curves at longer times was much smaller than the first one. Such a phenomenon may be interpreted in terms of growing radicals, that move more slowly, and probably the radicals site is hidden by other parts of the radical. Such deviation is expected to become more important with the increasing monomer concentration and also with the decreasing dose/pulse value. From the experiments conducted so far, such a behaviour was not obvious. Further works are needed to clarify the situation.
200 400 600 800 1000 Time, microseconds
Fig. 4. Decay curve measured in 0.1 mol dm-3 solution of EA and second order fit. Dose 210 Gy.
curs between monomer radicals or oligomer radicals containing only a few acrylate units. Based on the low rate coefficient found in EMA solutions, one may think that the reaction instead of the diffusion is basically controlled by chemical activation. However the high rate coefficients measured for some acrylic ester radicals, e.g. in aqueous solutions of methyl acrylate radical , 2.3 X 10’ mol-’ dm” s-l, contradict this suggestion. Probably the termination occurs between two oligomer radicals in the present case. The higher rate of olygomerization in acrylates than that of methacrylates is a well known phenomenon. In some cases on the millisecond time scale deviation from a single second order dependence was observed (see
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