Kinetic investigation of photopolymerizations induced by laser beams

Makrornol. Chem. 191,963 -979 (1990) %3

Kinetic investigation of photopolymerizations induced by laser beams

Christian Decker: Khalil Moussa

Laboratoire de Photochimie Gknkrale associk au CNRS, &ole Nationale Suptrieure de Chimie, 3 rue A. Werner, 68200 Mulhouse, France

(Date of receipt: September 18, 1989)

SUMMARY: Real time infrared (RTIR) spectoscopy was used to study the kinetics of the polymerization of

acrylic monomers exposed to the 337,4 nm continuous emission of a Kr+ laser. Conversion versus time curves were directly recorded in the millisecond timescale, thus allowing a precise evaluation of the polymerization rate and of the induction period due to 0, inhibition. The influence of the type and concentration of photoinitiator and of the laser beam intensity on the polymerization profile was examined for a 20 Brn thick polyurethane-acrylate (Actilane 20) coating. The growth of the radical concentration and its dependence on the initiation rate was determined from the kinetics of the dark polymerization which develops just after the laser exposure. Pulsed laser irradiation proved to be very effective in initiating the polymerization, the overall quantum yield reaching values up to 2500 mole photon-'.

Introduction

Lasers are now being increasingly used in polymers science due to the distinct advantages of these powerful sources of coherent radiation. They proved much valuable to determine kinetic rate constants of free-radical polymerizations ' -9 and the lifetime of the excited states formed in irradiated polymers6.n, as well as to induce the ultrafast polymerization of monomers in thin film^^-'^). By using pulsed8) or continuousg) output lasers which emit in the UV range, we first succeeded in trans- forming multifunctional acrylic resins into completely insoluble materials within a few milliseconds, even in the presence of air. This laser curing technology is expected to find its main applications in areas where speed and selectivity are of prime importance, like for the ultrafast curing of optical fiber coatings or the direct writing of high-resolution polymer relief images that are needed in the manufacture of integrated circuits or printing plates.

One of the important challenges in laser-induced polymerization is to find a reliable method capable of analysing in real time and quantitatively reactions which occur in a fraction of a second. Recently, we developed a technique based on IR spectroscopy that permits for the first time to directly record conversion versus time curves for photopolymerizations taking place within a few tenths of a second, thus allowing to determine accurately the important kinetic parameter^'^). In this paper we describe the performance of this method, real-time infrared (RTIR) spectroscopy, for analysing in situ the laser-induced polymerization of some acrylic systems which are commonly used in UV-curing applications. It proved particularly well suited to study the effect on the polymerization profile of such factors as the photoinitiator efficiency, the light intensity or the 0, inhibition, as well as to evaluate the extent of the dark polymerization which continues to develop after the end of the laser exposure.

0 1990, Hiithig & Wepf Verlag, Basel CCC 0025-1 16X/90/$03.00

964 C. Decker, K. Moussa

Experimental pnrt

Muteriuk The photopolymerizable resin consisted of an aliphatic polyurethane4iaaylate (Actilane 20 from SNPE) and of an acrylic monomer used as reactive diluent: hexanediol diacrylate a) (HDDA from UCB), tripropyleneglycol diacrylate b, (TPGDA from UCB) or a monoacrylate containing a carbamate group, 2-(isopropoxycarbony1amino)ethyl acrylate (Acticryl from SNPE), with the following formula:

7% CHz=CH-C-O-CHz-CH,-NH-C-O-CH

II I 0 CH,

II 0

W. i. Formula and trade name of the photoinitiators used

Irgacure 184

Darocure 1173

PDO

Benzophenone

m-0 0 OH

CG-25-369

Kinetic investigation of photopolymerizations induced by laser beams 965

Since all these compounds are completely transparent at 337 nm, the UV emission line of the laser used in this work, a photoinitiator was added to the formulation. Different types of compounds which absorb at that wavelength were examined in order to test their initiation efficiency. Their formulae and trade names are given in lhb. 1. For most experiments, qa'dimethoxyphenyl- acetophenone (Irgacure 651 from Ciba-Geigy) was chosen because of its high efficiency in producing free radicals upon photolysis (mostly benzoyl and CH, radicals) and its widespread use in UV curing applications. 'If.pical formulations contained 5 wt.-Vo of photoinitiator and equal parts of the acrylic monomer and oligomer. The resin was applied on a KBr salt as a uniform layer, usually of 20 pm thickness.

Photopolymerization: The sample was placed into an IR spectrophotometer and exposed to the UV emission line at 337,4 nm of a krypton ion laser (Spectra Physics Model 2020) operated in the continuous wave mode. The power output of the laser could be varied continuously, up to a maximum value of 1W. The incident light intensity at the sample position was measured by a radiometer and could be adjusted to any value in the range of 10 to 1400 mW . cm-', which corresponds to a photon flux rate (Z& between 2,8 * lo-' and 4 * einstein * sC1 * cm-'. The amount (Za) of light absorbed per second by the sample of thickness I was determined from the absorbance A of the coating at the wavelength of the relevant laser emission:

A photocell, connected to a transient memory recorder (Schlumberger SI 8200), was used to monitor precisely the timing and duration of the laser exposure.

The irradiation was carried out at room temperature, either in the presence of air or in an N,-saturated reactor equipped with polyethylene windows which are transparent to both the laser beam and the infrared analysing beam. In order to prevent atmospheric 0, from diffusing into the sample during irradiation in the presence of air, some experiments were carried out with laminates, the resin-coated KBr disc being covered with a polyethylene film. For posteffect experiments and intermittent irradiation, an electronic shutter was used in order to expose the sample to the laser beam for a precise time, usually between 10 and 100 ms.

The method used to study in real time the kinetics of ultrafast photopolymerizations (RTIR spectroscopy) has already been described 1 5 ) . The IR spectrophotometer was operated in the absorbance mode and the detection wavelength fixed at 812 cm-' where the acrylic double bond exhibits a sharp and distinct absorption (CH,=CH twisting). Upon laser exposure, the decrease of the sample absorbance due to polymerization was monitored continuously on a transient memory recorder. The full scale response of the recorder being 0,2 ms, the time limiting factor is the response of the IR detector, which was measured to be 40 ms, thus allowing a correct kinetic analysis of reactions having 50% conversion time above 80 ms.

Since the IR absorbance at 812 cm-' is directly related to the concentration of acrylic double bonds, the rate of polymerization R, can be determined at any moment from the slope of the recorded curve:

where [MI is the concentration of acrylate double bonds and (A812)o, (&J, and AS^^)^, represent the absorbance at 812 cm-' of the sample before and after laser exposuie during time f l and f,, respectively.

Photopolymerization profiles

Fig. 1 shows a typical polymerization profile recorded by RTIR spectroscopy for a TPGDA laminate exposed to the 337,4 nm Kr + laser beam in the presence of air. After

C. Decker, K. Moussa

C 0 u)

8

0 V

._ L

50 2

20

0

Laser exposure in s

Fig. 1. Photopolymerization profile recorded by KHR spectroscopy for a TPGDA laminate exposed to the 337,4 nm emission line of a Kr' laser in the presence of air. Film thickness: 1 pm; light intensity: 50 mW * cm-'. Photoinitiator: [Irgacure 6511 = 5 wt-9'0

an induction period of 0,16 s due to the inhibition effect of oxygen on this radical- induced polymerization 'a, the chain reaction develops rapidly up to 40% conversion. The progessive slowing down observed later on is attributed to the gelation of the system which reduces the mobility of the reactive sites 17, 19). The laser-cured polymer therefore contains a certain amount of unreacted acrylic double bonds which are trapped in the three-dimensional polymer network and cannot further be polymerized, even after prolonged exposure to the laser beam. At the light-intensity used (50 mW-cm-2), the maximum rate of polymerization was measured to be 5 mole 1-1 * s-l, thus reflecting the rapidity of this process. If necessary, the polymerization speed can be increased by simply raising the laser power output.

The thickness of the acrylic coating studied in Fig. 1 was only I pm, which is typically the thickness of the photoresists used in microelectronic applications. WIR spectroscopy thus appears as the only technique presently available which permits to follow quantitatively and in real time the ultrafast polymerization of such thin coatings, from the very beginning of the reaction up to its ultimate stage. Furthermore, the great sensitivity of IR spectroscopy allows very small changes in the monomer concentration to be detected; by expanding the ordinate scale, we were able to analyse the early stages of the polymerization (Fig. 1, curve (b)) and detect as little as 19'0 conversion in the 1 pm thick coating, a performance unchallenged so far.

It should be noticed that, in the absence of the 0, barrier polyethylene film, no polymerization occurred in such thin coating; indeed, under those conditions, the oxygen consumed by reaction with the initiator radicals is replaced very rapidly by the atmospheric 0, which diffuses into the 1 pm coating, thus preventing the monomer to compete effectively with oxygen for the scavenging of the initiator radicalsm).


Influence of the photoinitiator

When lasers are used as radiation source, the choice of the photoinitiator is crucial because of the monochromatic character of the emission. Photoinitiators must both absorb effectively the incident light and generate initiating species with a high quantum yield. The rate of polymerization will depend directly on these two factors, as shown by the classical rate equation ’I):

R, = - kp [Z, (1 - e-2s3A) &I0*’ [MI (2 kt)OP5

where A is the absorbance at 337,4 nm of the sample, Zo the photon flux rate and & the initiation quantum yield, i. e., the number of initiating radicals generated by each photon absorbed.

Fig. 2 shows the polymerization profiles recorded by RTIR spectroscopy for a polyuretane-acrylate + HDDA resin containing various commercial photoinitiators, upon exposure to the 337,4 nm radiation of a Krc laser, in the presence of air. The best results were obtained with the morpholino-type photoinitiator CG-25-369, newly developed by Ciba-GeigyZZ), which combines a very high absorbance at the laser emission wavelength and a fair initiation efficiency. The large variation observed with the type of photoinitiator used partly results from differences in the absorbance at 337,4 nm, as shown by lhb. 2, which reports the values of the fraction f of incident light absorbed by a 20 pm thick film (f = 1 - e - ’ ~ ~ ~ ) , together with the values of the induction period, ti, and of the maximum polymerization rate, (R,),,.

In order to compare more precisely the performance of these compounds, we have calculated the ratio Ri/(Zo - f * [MI’), which is directly related to the initiation quantum yield

$100,

Fig. 2. Influence of the photoinitiator (5 wt.-To) on the laser-induced polymerization of a polyurethane-diacrylate + hexanediol diacrylate resin, in the presence of air. Light intensity: 200 mw . cm -z; fiim thickness: 20 pm

0.5 1.0 lrradation time in s


'Rib. 2. Comparison of the performance of various photoinitiators in the laser-induced polymerization of a polyurethane-acrylate + HDDA coating. [Photoinitiator] = 5 wt.-Yo; [Acrylate] = 5,2 mol .1- ' . Light intensity at 337,4 nm: 200 mW an-'

Photoinitiator a) Induction R p b, I, c, S d )

2 kt period in s mol. I - i . s - i lo-z E. s - i

Benzophenone 0,17 0,16 497 497 18 (+5 wt.-Yo amine) PDO 0909 0,13 990 2S 122 Darocure 1173 0,075 0,09 995 291 164 Irgacure 184 0909 0906 11,5 2,s 200 Irgacure 65 1 020 0,06 14.8 5,6 153 Irgacure 907 0,70 0.06 15,l 19,5 48 CG-25-369 190 0,03 21 ,o 28,O 58

a) Fraction of incident laser radiation absorbed by the 25 pm thick film. b, ~ a x i m u m rate of Dolvmerization. ') Absorbed light in&nsity (E = einstein).

d, @i: initiation quantum yield; @i -E = k2 R; 2kt I; [MIz

The values of @i k;/(2k3, which are reported in lhb. 2, thus permit to classify accurately the various photoinitiators according to their intrinsic efficiency, since the ratio k;/(2kJ remains constant for a given monomer system. On that basis, the 8- hydroxyalkylphenones (Irgacure 184 and Darocure 1173) appear to be the most efficient photoinitiators.

The rate of polymerization depends on the absorbance A, and thus on the photoinitiator concentration [In], since A = E - I * [In], as calculated from the equation

The experimental values of R,, determined by RTIR spectroscopy for various concentrations of the photoinitiator (Irgacure 651), show a perfect fit with the theoretical curve (Fig. 3).

As expected, an increase of the photoinitiator concentration yields a concomitant decrease of the induction period, as more radicals are produced to react with, and thus consume, the oxygen dissolved in the sample (Fig. 3, curve (b)). It also leads to a decrease of the amount of residual unsaturation in the cured polymer, which dropped from 22% to 8% as the initiator concentration was increased from 1 qo to 5% (Fig. 3, curve (c)).

Influence of the laser beam intensity

When the power output of the laser is increased, the light intensity and thus the initiation rate are raising concomitantly, so that the polymerization develops more

Kinetic investigation of photopolymerizations induced by laser beams %9

Fig. 3. Influence of the photoinitiator concentration on the rate of polymerization R, (curve (a)), the induction period ti (curve (b)) and the amount of residual unsaturation (curve (c)), upon laser irradiation in the presence of air of a polyurethane-acrylate + Acticryl coating. Light intensity = 100 mw * cm-'

rapidly, as expected from Eq. (I). Fig. 4 shows the variation of the maximum rate of polymerization, determined from the recorded RTIR curves, as a function of the light intensity I,. Above a certain d u e of I, (= 100 mW cm-'), a slowing down is observed and R, reaches finally a maximum value of 17 mol.l-'.s-' for the polyurethane-acrylate + Acticryl formulation. This saturation effect is apparently not due to some limitation inherent to the technique used, since the response time of the IR detector (40 ms at half scale) is significantly lower than the 50% conversion time of the fastest system examined: t,, = 130 ms for I, = 1,4 W * ern-'. It is neither likely to arise from a depletion of the photoinitiator, since after 100 ms of exposure, the time at which R, is measured, it remain still 80% of the photoinitiator molecules which have not been excited by the laser radiation when operating at a light intensity of 1,4 W cm -', and as much as 97% at 200 mW * cm-' where the saturation effect becomes already apparent.

I

i I Fig. 4. Dependence of the rate of polymerization R, on the light intensity for a polyurethane acrylate + Acticryl coating exposed to the 337,4 nm laser beam in the presence of air

1 1 5 -

Kr' Laser 337.1 nrn

200 300 Laser intensity in mW * cn-*

970 C. Decker. K. Moussa

The reasons for this unusual behaviour are still not clear, but one may consider that, as the polymerization rate reaches such very high values, the limiting factor then becomes the actual growth of the polymer chains, which will depend, amongst other parameters, on the viscosity of the resin. Indeed, with the formulation based on polyurethane-acrylate + HDDA, which is less viscous, R, reached a maximum value of 22 mole 1-’ * s- l . Further experiments are in progress in order to confirm and elucidate this saturation effect.

It should be mentioned that we reported in some earlier work8*23*24) much higher values of R, for the photopolymerization of acrylic monomers under intense irradiation. They were determined from discrete IR measurements after a given exposure and therefore include an important posteffect (see next section). RTIR spectroscopy presents the unique advantage of allowing the actual polymerization profile to be determined, even for ultrafast processes.

Postpolymerization study

Kinetic profiles

RTIR spectroscopy was shown recently to be a technique particularly well suited to study the dark polymerization which takes place in acrylic monomers just after the UV light has been switched off 25). Fig. 5 shows the polymerization profiles recorded for a photoresist made of a 50/50 (by wt.) mixture of a polyurethane-diacrylate and Acticryl, with 5 wt.-070 Irgacure 651 (formulation I). The 20 pm thick coating was exposed to the 337,4 nm laser beam for various durations, between 10 and 100 ms, in the presence of air. After the irradiation, the polymerization appears to continue for about one second, although initiating radicals are no more produced. It indicates that, even in the presence of air, propagation can proceed in such viscous systems for quite a long time, compared to the short laser exposure, until the living polymer chains stop growing due to termination reactions and 0, scavenging of the radicals. It can be seen from Fig. 5

Laser exposure time in m s Kr’ Laser

337,Lnm _ _ _ - - -

/ 10

2 Time in s

Fig. 5. Kinetic profiles recorded by RTIR spectroscopy after laser exposure for various durations (10 to 100 ms) of a polyurethane- acrylate + Acticryl coating, in the presence of air. Light intensity: 250 mW cm-’; (- - -): continuous irradiation

Kinetic investigation of photopolymerizations induced by laser beams 97 1

that the polymerization develops almost as fast and as effectively after 100 ms of UV exposure than upon continuous irradiation; the polymer formed during the posteffect then accounts for more than 80% of the total polymer obtained. The contribution of the dark reaction was found to increase with the light intensity and, as expected, to be more important in the absence of oxygen.

Growth of the radical concentration

The initial rate of the dark polymerization (It;) is directly related to the concentration of radicals [P7 present at the end of the laser exposure:

where [MI, is the concentration of acrylic double bonds which have not polymerized after irradiation time t. Since the propagation rate constant kp of these acrylic monomers was recently shownw to remain essentially constant for degrees of conversion up to 35%, the ratio R;/[M], will be a measure of the radical concentration in the irradiated system. When this ratio was plotted as a function of the duration of the laser exposure (Fig. 6), it was found to grow rapidly with irradiation time and to level off to a constant value after about 70 ms and a degree of conversion of 10%. The radicals have then reached their stationary concentration (d[P']/dt = 0) where the rate of termination equals the rate of initiation. The amount of initiating radicals formed during that 70 ms period was calculated to be 1,6 - lo-' mol radicals - 1-', taking a Gi value of 0,42'). Essentially the same profile was obtained by plotting the maximum conversion, which is reached after 2 s, as a function of the laser exposure time (Fig. 6).

337,Lnm

50 100 15C Laser exposure time in ms

100 C ._

75 : L - c a, 0

50 c

E .-

25 0 V

0

Fig. 6. Growth of the radical concentration upon laser irradiation of a polyurethane-acrylate coating (formulation 1) in the presence of air. Light intensity: 250 mW * cm-'. (- - -): degree of conversion after 2 s posteffect


Effect of the light intensity

According to the conventional polymerization mechanism based on bimolecular termination reactions, the stationary concentration of radicals should depend on the square root of the initiation rate ri or of the light intensity I,, since ri = @i I,, (1 - e - ' ~ ~ ~ ) . Posteffect experiments were thus carried out at various light intensities, in the range of 50 to 300 mW - cm-', in order to determine the dependence of [ P O ] on I,. The sample, a 20 vm thick coating of formulation 1, was exposed to the laser beam at a given intensity until a degree of conversion of 30% was reached, and the dark polymerization was monitored by RTIR spectroscopy. From the kinetic profiles thus recorded at different light intensities (Fig. 7), one can evaluate the initial rate of the dark reaction (Z?;). When the ratio R',/[A],, which is directly proportional to the radical concentration, was plotted as a function of a straight line was obained (Fig. 7):

where [A], is the concentration of acrylate at time t . The square root dependence of the radical concentration on the rate of initiation is

in good agreement with a polymerization mechanism based on bimolecular termina- tions.

1 Initiation: In R' ri = $i I, (1 - e-2*3A

Propagation: R' + M ___* RM' __* P' R , = kp [p '][MI

Termination: P' + P' Products r, = 2k, [Po]'

hv

M

Time in s

Fig. 7. Influence of the light intensity on the polymerization profiles and on the radical concentration for a polyurethane-acrylate coating (formulation 1) exposed to the 337,4 nm a+ laser emission, in the presence of air; (- - -): continuous irradiation at 300 mw . cm -'; (-): irradiation up to 30Vo conversion and posteffect


Under steady state conditions, ri = r,, and the expression of [PT becomes:

It should be mentioned that in previous work%) on the laser-induced polymerization of these acrylic monomers, which was performed at very large light intensities (1 to 10' W - ern-,) and in the presence of air, we found the kinetic law R , = K * Zo~8s instead of the expected square root relationship. This discrepancy may result from two factors: (i) R, values were determined from discrete IR measurements (which include the postpolymerization effect) and not by a time-resolved technique; (ii) 0, inhibition is less pronounced at the very high intensities used, so that R, values are close to those found in N,-saturated systemsm); as I, is decreased, 0, inhibition becomes more important, thus depressing the R, value, which makes the exponent of the kinetic law increase concomitantly.

Influence of oxygen on the postpolymerization effect

Previous work on the photopolymerization of acrylic monomers has shown the dark reaction to develop more efficiently and longer in an inert atmosphere than in the presence of airzs). In order to make the inhibition effect of oxygen more apparent, the following experiment was devised a 20 pm thick coating of formulation 1 was first exposed during 50 ms to an intense beam (1 W * cm-') of the Kr+ laser and then covered, after about 0,l s of posteffect, with a transparent polyethylene film. Based on our work on laminateszs), this should prevent atmospheric oxygen from diffusing into the coating and enhance the polymerization rate.

The RTIR curve recorded under those conditions is shown in Fig. 8 (curve (a)), in comparison to the continuous irradiation (curve (b)) and the 50 ms laser exposure followed by posteffect in the presence of air (curve (c)). It clearly appears that the dark polymerization, which was progressively slowing down, continued at a fast pace as soon as the coating was covered with the polyethylene film, so that the system reached

Fig. 8. Influence of oxygen on the postpolymerization of a polyurethane-acrylate (formulation 1). (a): 50 ms of laser polymerization + polyethylene f i i after 100 ms posteffect; (b): continuous irradiation in the presence of air; (c): 50 ms of laser irradiation + posteffect in air

- 50ms Laser

Kr' Laser 337,Lnm

Laser Time in s


high degrees of conversion, like in N,-saturated experiments. The final polymer contained only 5% residual unsaturation instead of 20% in the reference sample. By preventing 0, from diffusing further into the coating, the polyethylene barrier reduces the scavenging of the free radicals by oxygen, and allows the polymer chains to continue to grow effectively in the dark.

Intermittent laser irradiation

In consideration of the important postpolymerization effect shown in this study, it might be of interest to submit repeatedly the monomer samples to short laser flashes. Indeed, by taking full measure of the dark polymerization, one might expect to reduce the exposure time and thus consume less energy to get a fully cured polymer. By means of an electronic shutter, the acrylic coating (formulation I) was exposed to several 10 ms wide laser flashes with a time interval between successive pulses of 2 s, in order to let the posteffect develop completely.

The step-like kinetic profile recorded (Fig. 9) clearly shows the progressive decrease of the polymerization efficiency with increasing number of pulses. The extent of the dark polymerization (At) drops from 24% after the first flash to less than 5% after the fifth. This effect becomes even more apparent when the ratio At/[M],, which is directly related to the radical concentration at time t , is plotted as a function of the degree of conversion of the system just before the laser is fired (Fig. 9). The concentration of reactive radicals remains constant up to 30% conversion and then decreases steadily as the degree of conversion increases, most probably because of mobility restrictions in the crosslinked polymer formed.

In a comparative study between continuous and pulsed laser irradiation, the total polymerization extent after UV exposure and posteffect during 2 s was plotted as a function of the cumulative exposure time (Fig. lo), for both the intermittent (curve (a))

Time in s 0 2 4 6 a 10

I I I

Degree of conversion

Fig. 9. RTIR polymerization profile recorded upon intermittent irradiation of formulation I with 10 ms wide laser flashes. Dependence of the conversion increase (Ar/[MJ,) on the degree of conversion


Fig. 10. Total polymerization extent after UV exposure and posteffect during 2 s as a function of the cumulative laser exposure time, for pulsed and continuous irradiation of formulation 1. Pulse width 10 ms; frequency: 0.5 Hz

s .- c 7 5 - - * 0 0) -

Kr* Laser 337.L nm

.- 5 7 5 1 - la) * 0 0) - L

5 0 - c u) 0

+ a

5 2 5 - C Kr* Laser u) 337.L nm 0

& & '0 10 20 30 LO 50 60

v Irradiation time in ms

and the continuous (curve (b)) irradiation. Degrees of conversion appear to be only slightly higher in the intermittent irradiation, most probably because the more extensive posteffect is partly offset by a stronger 0, inhibition which takes place between successive pulses. This holds no more true when irradiation is carried out in an N, atmosphere or on laminates where pulsed exposure proved to be a more energy efficient process.

When the time interval between pulses is shortened to 100 ms or less, the step-like profile smoothes down into a continuous curve (Fig. 11). From these curves, one can evaluate the overall quantum yield of the polymerization, @,, defined by the expression:

Number of acrylate groups polymerized Number of photons absorbed

9, =

It was calculated from the ratio

[MI,. 51

$100 C .-

Fig. 11. Influence of the pulse frequency on the polymerization profile of a polyurethane-acrylate (formulation I) exposed to a + laser beam in the presence of air

0 0.5 10 Time in s


lilb. 3. Influence of the repetition rate on the quantum yield of polymerization (@J upon intermittent laser irradiation (337,4 nm) of a polyurethane-acrylate (formulation 1). I , = 100 rnW.cm-,

Continuous 28 14 500 irradiation 100 10 10 14 9 650 10 10 100 2.5 6 1 2400 1 to 1 OOO 0,28 - 2 500 0 s 10 2 OOO 0,14 - 2600

a) Absorbed light intensity (E = einstein).

where rt is the conversion at time t and ti and t represent the irradiation time and the total time (UV + posteffect), respectively. 'hb. 3 summarizes the values of R , and $, obtained under the different experimental conditions. It shows the increase of the polymerization quantum yield (pp with the duration of the posteffect, up to a constant value of about 2500 mol photon-1 when the pulse interval exceeds 100 ms, i. c, the time needed for the polymer radicals to reach their steady state concentration.

Photopolymerization by a pulsed N, laser

Instead of chopping the laser beam with an electronic shutter, one can also use pulsed lasers which emit their coherent radiation during an extremely short timeg). Polymerization experiments were thus carried out by exposing the polyurethane- acrylate film (formulation 1) to the UV emission at 337,l nm of a pulsed nitrogen laser.

Number of laser shots 0 5 10 15 20

s .5; 75 -

Nitrogen Laser

0.5 1.0 1.5 2.0

Fig. 12. Oxygen inhibition effect on the photopolymerization of a polyurethane-acrylate (formulation 1) exposed to a pulse nitrogen laser emitting at 337,l nm. Pulse width 8 ns; pulse energy: 4 mJ

Laser exposure time in s


Since this emission line is very close to the UV emission line of the Kr + laser (337,4 nm), the photoinitiator classification found previously remains valid in the present case. The 5 mJ laser pulse lasts only 8 ns, which is about the time needed to generate the initiating radicals, so that the polymerization itself will develop here only in the dark, after the laser shot. Oxygen diffusion will therefore play an important role, acting both on the induction period and on the polymerization rate.

Fig. 12 shows that, in the presence of air, it takes 2 laser shots to consume the 0, dissolved in the sample and allow the polymerization to start slowly. After as much as 20 pulses, only 30% of the acrylate groups have yet polymerized. By contrast, in an inert atmosphere the polymerization proceeds immediately at a fast pace, with an overall rate of 5 mol 1-' * s-l for a repetition rate of 10 Hz. In order to reduce the drastic 0, inhibition effect, one can either increase the laser frequency to shorten the pulse interval during which oxygen diffuses in the coating, or cover the sample with a polyethylene film that will act as an 0, barrier. When such laminates were irradiated in the presence of air, the induction period was reduced and the polymerization rate much enhanced (Fig. 12).

One of the advantages of pulsed lasers is that the time interval between successive shots can be easily changed by acting on the repetition rate. Fig. 13 shows the increase of the degree of conversion with the number of laser shots, i. e., the energy received by the sample, for 4 frequencies (2,5, 10, 25 and 40 Hz) corresponding to pulse interval of 400, 100,40 and 25 ms, respectively. As the repetition rate is increased, the multiple jump profile progressively smoothes down into a regular and monotonous variation, while the overall polymerization extent dropped concomitantly, e. g., from 73% conversion after 6 laser shots at a frequency of 2,5 Hz to 30% at a frequency of 25 Hz (Fig. 13). A similar effect of the laser repetition rate on the reaction efficiency was observed recently by Hoyle et al. for the photopolymerization of acrylic monomers. It was explained on the basis of a premature termination of the living polymer chains by the next pulses, each of which generates extremely high concentrations of initiating radicals. The effect of termination of growing polymer chains by radicals formed in subsequent flashes has been already treated quantitatively by Olaj et al. l).

Fig. 13. Influence of the laser repetition rate on the photopolymerization of formulation 1 exposed as laminate to the 337.1 nm emission line of an N, laser. Pulse energy: 4 mJ

0 1 2 3 1 5 6 7 8 Number of laser shots


Conclusion

As a result of this study, it should be apparent that both continuous wave and pulsed laser beams are capable of initiating very efficiently the photopolymerization of multifunctional monomers. The main advantages of this technique arise from the inherent characteristics of the laser emission:

- the large power output that allows very high speeds of cure to be reached; - the directivity of the beam that can be sharply focused to draw high-resolution

images; - the narrow bandwith that can be matched with the maximum absorption of the

initiator and thus eliminates undesirable secondary photochemical reactions.

In order to study the kinetics of laser-induced photopolymerizations which proceed within a fraction of a second, we used the newly developed real-time infrared (RTIR) spectroscopy, which proved to be particularly well suited owing to its short response time and high sensitivity. From the conversion versus time curves, which are directly recorded, one can determine in a single run the important kinetic parameters, such as the rate of polymerization and the quantum yield, and assess the performance of new photoinitiators or monomers.

During the photopolymerization of multiacrylic monomers in the presence of air, the polymer chains were found to continue to grow for about one second after the laser exposure. By studying the kinetics of this postpolymerization, we were able to obtain the actual growth profile of the radical concentration and to show its square root dependence with the light intensity, as expected from a mechanism based on bimolecular termination. The dark polymerization was shown to be most important in the early stage of the reaction, accounting for more than 80% of the total polymer formed. This explains why intermittent or pulsed laser irradiation proved to be so effective in achieving a fast and extensive polymerization of these acrylic monomers.

Potential applications of laser-induced polymerization are expected to concentrate mostly in microelectronics and in the graphic arts for the production of high-resolution polymer relief images. Other possible uses of intense laser irradiation include three- dimension modelling, the curing of thick sections of polymers or of composites, and the high speed surface treatment of optical fibers by UV curable coatings.

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Kinetic investigation of photopolymerizations induced by laser beams