Activation of radical polymerization by microwaves—I. Polymerization of 2-hydroxyethyl methacrylate

Eur. Polym. J. Vol. 19, No. 6, pp. 543 549, 1983

0014-3057/83/060543-07503.00/0 Copyright © 1983 Pergamon Press Ltd

Printed in Great Britain. All rights reserved

ACTIVATION OF RADICAL POLYMERIZATION BY MICROWAVES--I POLYMERIZATION

OF

2-HYDROXYETHYL

METHACRYLATE

M. TEFFAL* and A. GOURDENNE** Laboratoire de Physico-Chimie des Hauts Polymeres, Ecole Nationale Sup6rieure de Chimie de Toulouse, 118, route de Narbonne-31077 Toulouse Cedex, France (Received 13 M a y 1982; in revisedJorm 7 December 1982)

Abstraet--2-Hydroxyethylmethacrylate(HEMA) in bulk without any radical initiator is treated with a polarized microwave beam (2.45 GHz) inside a wave guide at various electrical powers (P0)- The fluid monomer polymerizes finally to form a solid material. The various steps of the reaction of polymerization are described through study of the variation with time of the temperature of the sample and of the part (P.) of the electrical power (Po) degraded in the polymerizable medium because of dielectric loss.

INTRODUCTION

EXPERIMENTAL

Microwave heating of suitable materials can be used in many cases to activate the reactions of polymerization. This process is mainly studied in industrial laboratories with a view to preparing composite material involving glass or graphite with epoxy or polyester resins, and so the only information on this curing technique is collected in patents which are not useful to define the conditions for good reproducibility, especially as the temperature of the samples during the microwave treatment is not measured. During the last three years, we have tried to record the temperature of the samples and to find the part of the electrical power dissipated by dielectric loss. The results have been partially published: they are related to the activation of cross-linking under microwaves at 2.45 G H z of epoxy resins (step-growth polymerization) I-1, 2] and of unsaturated polyesters/styrene mixtures (radical polymerization) [-1, 3]. In the latter case no initiator was necessary because the polarity of the unsaturated polyester chains was sufficient to reach a temperature suitable for initiation of the thermal copolymerization of the vinyl m o n o m e r with unsaturated groups in the macromolecules. This paper concerns the polymerization of 2-hydroxyethylmethacrylate (HEMA); the polymer is designated P H E M A .

The experimental microwave apparatus is indicated in Fig. 1. A generator of microwaves delivers an electrically polarized electromagnetic beam at a frequency of 2.45 GHz and a given power (Po) in the range 0-1 kW, which is forced to propagate inside a wave-guide, according to the TEot mode, where it meets the reactor which consists of a cylindrical pyrex-glass pill-box with good transparency to the microwaves and filled with HEMA. The initial beam is then divided into three parts: (i) a part is reflected by the reactor and deviated through a ferrite system or circulator towards a load charge (1) which absorbs the radiation; (ii) a part is absorbed inside the sample because of the dielectric moss; for microwaves at 2.45 GHz, the polarizable entities are the dipoles; (iii) a part is transmitted and absorbed by the load charge (lI).

CH3

CH2:C

l

C=O

i

O--CH 2--CH

2--OH

(HEMA) * Permanent address: Ecole Normale Sup6rieure de Rabat (Maroc). ** To whom all correspondance should be addressed. 543

The use of the two charges prevents any stationary wave system inside the wave-guide and the waves propagate in a progressive mode. Various electrical powers can be associated with the different beams and are measured through wattmeters: (Po), (Pr) and (P,) are the powers of the initial, reflected and transmitted beams; (P,) corresponds to the dielectric loss when the microwaves cross the samples. On the other hand the loss of energy along the walls inside the apparatus, although low, should be taken into account: its magnitude is usually 2 or 3~',~iof the power (P0). The variations of the temperature of the samples treated by the microwaves are measured through a thermistor inside a pyrex-glass tube filled with silicone oil which is transparent to the electromagnetic beam and which is partially immersed in the polymerizable materials, and held perpendicular to the polarized electric field so as to limit any coupling with the waves. We shall see later that such an arrangement leads to reproducible results and that consequently the perturbation of the propagation of the beam due to the thermistor is not significant. The variations with time, at given P0 values, of the experimental parameters Pr, P,, P, ( = Po - Pr - P,) and T are stored inside the memory of a multichannel recorder using a microprocessor. The derivatives (T)' = d T / d t and (P,)' = d P . ' d t are also calculated.

544

M. TEFFAL and A. GOURDENNE

1 I i I

(

GENERATOR

REACTOR

I ,.o,o ]

!

I

(II)

WATTMETER

Fig. 1. Apparatus for the microwave treatment at 2.45 GHz.

able material and the external medium which consists, in our case, of air at room temperature surrounding the reactor. First, H E M A under microwaves receives energy from the electromagnetic beam because of the dielectric loss and loses energy by heat transfer by convection towards the external medium. The energy transferred from the waves per second is the power P. which is assumed until now to be degraded as heat according to the dipolar relaxation of the chemical species. If P. is sufficient, the material can polymerize. W h e n exothermic reactions, such as polymerization of H E M A , take place in the medium, the heat which is

Monomer (MERCK-Schuchardt; Ref. 800-588) was previously distilled under vacuum and carefully degassed before use. All samples consisted of 20 g of HEMA and no radical initiator was added.

RESULTS AND DISCUSSION

Before analysing

the

various

curves

T = T(t),

P= = P.(t), ( T ) ' = dT/dt and ( P , ) ' = dP,/dt, is is necessary to describe the major exchanges of energy which can occur between the samples of polymeriz-

Toe/IT, o

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Pu

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( W / min) 7

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40 30

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0 -0

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I0

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30

40

50

60

70

80

90

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I10

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Fig. 2. Radical polymerization of HEMA. Polymerization under microwaves at P0 = 4 2 W : (1) T = T(t); (2) P, = Pu(t); (1') (T)' = dT/dt; (2') (P=)' = dP~/dt. Microwave treatment at Po = 42 W of a sample polymerized at the same power: (3) T = T(t); (4) P. = P=(t). Thermal polymerization in an oven at 103°: (5) T = T(t).

Polymerization of HEMA generated must be taken into account; the resulting increase of temperature increases the dielectric loss and the heat transfer to the external medium which is proportional to the thermal gradient between the samples and the external medium, at least in the range of temperature (~200 °, involved in our experiments. Figure 2 shows a series of curves for polymerization under microwaves at Po = 4 2 W (curves 1-2-3-4-1' and 2') and a polymerization activated by classical heating in an oven at 103 ° using the same type of reactor and amount of monomer (curve 5). Curve 1 shows the variations of T with time for polymerization under microwaves. From the thermodynamic point of view, it can be divided in three parts: (a) a pre-heating period, when the temperature rises from 0 to 104 '~, corresponding to the first inflexion at 10.4 min; (b) an exothermic period where the temperature increases from 104 ° to a maximum T = 178 ° at 18.9 min, through a second inflexion point (14.7 rain; 135°); (c) a cooling period spreading from this maximum to a plateau (T = 114 °) stabilized in time. The initial liquid monomeric medium becomes solid, rigid and transparent. The final product is insoluble in all usual solvents, but swells in water; it is certainly chemically cross-linked. In order to compare microwave heating and classical thermal heating for activation of the polymerization of HEMA, a thermal polymerization has been done at 103" (Fig. 2, curve 5). This experiment refers to a well-known type of reaction. Three steps can be identified, viz. pre-heating, exothermic period and cooling beyond the maximum. The thermal plateau at long times corresponds to the temperature of the oven. Moreover, the pre-heating period is longer in time. First, the temperature of the monomer solution increases. When it is high enough, radical species are formed from the monomeric units. Before long their concentration becomes sufficient to start the exothermic conversion of HEMA which can be considered as the second step of the thermal polymerization of the monomer : the temperature of the samples increases to a maximum (57.17rain; 170 °) through an inflexion point (55.04 rain; 152'), passing through the temperature of the oven. It has to be noticed that the temperature of 103" is a key-value from the point of view of the thermodynamics, because it corresponds to a reversal in the direction of heat transfer between the sample and the external medium which, for example, loses energy in the exothermic stage. It is also admitted that most of the monomer is converted during the exothermic period which, furthermore, sees the change in the physical structure of the chemical medium from a viscous state around the first inflexion point (34.5 rain; 94 °) into a glassy state, probably a chemically cross-linked network, at the maximum of temperature. The cooling period after the maximum of temperature corresponds to the transfer of remaining chemical energy still stored in the chemical material from the sample to the external medium. However, slight exothermic conversion cannot be excluded beyond the maximum at least when the sample

545

starts cooling. When the loss of energy as heat ends, the sample acquires the temperature of the oven (103°). Because of the results from the thermal polymerization and of the similarity of curves 1 and 5 of Fig. 2 (respectively related to activation by microwave and classical thermal heating), the microwave T curve can be grossly interpretted. Nevertheless the interpretation will be easier after having analysed the variations with time of the dielectric loss i.e. the power P, during the microwave treatment of a polymerizable sample (Fig. 2, curve 2). HEMA is polar because of its ester function, and this more especially as it bears an alcohol group bound to a flexible segment - - O ~ C H z - - C H 2 - - . This disposition is maintained in the chemical structure where the polarizable entities at 2.45 GHz are the dipoles carried by the side-group esters: at such a high frequency the relaxation of the dipoles belonging to the main chain cannot be detected. One can understand that the capability of relaxation of the dipoles depends on their mobility i.e. on the Brownian motion of the species which carry them, depending on the temperature and the chemical structure of the reactive medium. The observed results, at least in the early stage, from the oscillatory break of Van der Waals interactions between the species which carry the relaxing dipoles that tend to fall into line with the electrical field. So, from these comments, the curve P , = P,(t) which shows the variations of the dielectric loss, on a proportional factor, with time for a given Po value of 4 2 W can be interpretted. First P, starts growing rapidly from an initial value at 0 of 20W representing a significant loss, although HEMA is at 0 , until a maximum P, = 23W at 2.20rain and which corresponds to a temperature of 53, much lower than that of the first inflexion point (T = 104 at 10.4rain) in the microwave curve 7"= T(t). The growth is due to the enhancement of the motion of the HEMA species which become more and more capable of dissipating the energy of the incident beam. If the process continued, the temperature T and the power P, should simultaneously increase, as for a normal material, to reach ultimate values at the thermal equilibrium. But such a correlation is not observed: beyond 5 3 , the temperature still increases, whereas P. starts dropping to a constant value of 10.5 W. The only explanation of this divergence in the variations of T and P, is as follows: the chemical structure of the HEMA solution changes very early, before the first inflexion point of the curve T = T(t), in a way which reduces the motion of the dipoles i.e. partial polymerization of HEMA with possible crosslinking, which lowers the dielectric loss. This involves the production of radical species from the monomer as soon as its temperature increases fiom the initial value under irradiation. It might be also objected that the change in the time variation of P,, is related to the formation of a network from HEMA and ethylene dimethacrylate (DME) which often remains as an impurity in HEMA even it is carefully distilled. But such such an assumption has to be rejected since many grades which have been tested (among them, some were DME free and the rate of appearing of the P, maximum is not in favour of thermal dimerization of HEMA under irradiation) lead to the same ordering.

546

M. TEFFALand A. GOURDENNE

250

zoo

/,............................................................................

150

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i

4

/

z

I... I00

50

o

lo

2o

30

4o

50

6o

7o

eo

9o

loo

t lo

t (min) Fig. 3. Polymerization under microwaves at various Po values. Curves T = T(t). (1) 31 W; (2) 40 W; (3) 63 W; (4) 78 W; (5) 96W.

The increase of temperature, in spite of the decay of which is of the radical type, with three steps (preP,, is then mainly due to the production "in situ" of heating, main exothermal conversion and cooling); chemical heat from the exothermal polymerization of (ii) the analysis of the time variation of the dielectric loss P. = P.(t) confirms the results on the time HEMA. The curve P. = P.(t) also shows that the decay with variations of the temperature on the structural change time of P, leads to a plateau (P, = 10.5 W) typical of of the HEMA material; in addition, it shows, through the behaviour under microwaves of a material with a a maximum, that the polymerization starts very early defined structure. That result involves that the with the so-called pre-heating period. sample, known to be solid, has been polymerized to To confirm that the activation of the HEMA polyits maximum. It has to be noted that P. maintains a constant value as soon as the temperature starts de- merization under microwaves is of the thermal type, through the degradation of the electrical power as creasing from its maximum. Now, in order to determine the efficiency of the heat, some experiments have been done at different P0 microwaves towards the radical polymerization and values. Figure 3 shows a series of curves T = T(t) also to understand better the electromagnetic process drawn at P0 = 31-40-63-78-96 W. They are similar in shape and when Po increases, i.e. more heat is of activation, a "polymerized" sample has been brought into the polymerizable medium, the exothertreated at the same power P0 = 42W. The corremal peaks are greater and shifted to shorter times, sponding variations of T = T(t) and P, = P,(t) are whereas the intensities of their maxima grow (Table respectively represented by the curves 3 and 4: no 1), because the thermal activation is more and more peak is observed in any curve, T and P. grow conefficient. Furthermore, and in step with this tendency, tinuously up to plateau which is similar in both cases one observes that the plateau heights increase with P0 with that previously recorded for the single irvalues. This result is significant since all the final radiation. This means that only a single microwave products have the same weight and, of course, the treatment at Po = 42W is sufficient to polymerize same number of side-chains active in the dipolar HEMA: a residual amount of monomer should have relaxation. induced some maxima or shoulders in the temperaCurve 5 of Fig. 3 (dotted line) drawn for Po = 96 W ture and power curves. seems not to have a regular thermal behaviour in The curves of the derivative, functions ( P . ) ' = with the other curves of the series, dP./dt and (T)' = dT/dt, respectively curves 1' and 2' comparison because its plateau is too high. Inspection of the corof Fig. 2, for a single treatment at P o - - 4 2 W, do not help in the understanding of the microwave polymerization though they help to define better the Table 1. Curves T = T(t) at different Po values position in the various singular points in the curves T(t) = P.(t); for example, they show that P. is stable Po T (°C) T (°C) with time (dP./dt = 0) when the temperature rises up Curve (W) maximum plateau to its maximum. From the preceding results, some preliminary con1 31 190 110 clusions on the HEMA polymerization activated at 2 40 193 125 2.45 GHz microwaves can be made: 3 63 195 135 4 78 200 150 (i) the microwave polymerization is similar to the 5 96 203 186 thermal polymerization carried out in an oven and

Polymerization of HEMA

50

547

5 /--..\

40,

30

20

~

°

°

4

s,;

I0

0

5

I0

15

20

120

t [rain)

Fig. 4. Polymerization under microwaves at various Po values. Curves P, = P.(t). (11 31 W; (2) 40 W; (3) 63W; (4) 78W; (5) 96W.

responding sample gives the explanation: many bubbles, probably due to the boiling of the monomer, are trapped in the polymer matrix, mainly around the thermistor and act as a thermal insulator. The effect of P0 on the time variation of the dielectric loss can be seen in Fig. 4, where the corresponding curves Pu = P.(t) are drawn: when Po drops, the peaks are widened in much the same way as in the case of the series T = T(t), with their maxima reduced and shifted to longer times; then, beyond those maxima, the curves form plateaux typical of the behaviour of materials with permanent structure. Moreover, Pu and Po are correlated through the simple relation P,/Po = 0.27, and independent of temperature and Po (Table 2); curve 5 is excluded because of the bubbles trapped inside the sample. This linear relation between Po and P, cannot be easily interpretted. Indeed, when the ratio Pu/Po should increase with the temperature, and of course with Po since the dielectric loss is expected to be raised with the temperature for a given structure, it remains more or less constant. This abnormal behaviour of the materials is under study in our laboratory. A more understandable result is obtained when the ratio P,/Po is calculated for the monomer at 0 ° (Table 2), as soon as the microwave treatment starts: Pu/Po remains constant (0.43) and shows that the dielectric loss is directly proportional to Po.

Figure 5 which shows the time variation of the derivatives ( T ) ' = dT/dt at given Po values, provides some information on the kinetics of the polymerization. First, (T)', when positive, can be considered as the rate of heating of the samples. One observes that the maximum of the sharp peak which corresponds to the inflexion point in the ascending part of the exothermic peak in the curve T --- T(t), is shifted to longer times, while its intensity is reduced with decreasing Po values. Now, when (T)' takes the zero value, a series of points is obtained, giving the position in time of the various maxima of temperature. Then, as soon as (T)' becomes negative, the cooling step takes place because no significant heat is produced from the conversion of HEMA. Figure 6 presents the time variation of the derivatives (P,)' = dPu/dt. Although the curves look simple, they permit some comments on the HEMA polymerization under microwaves: (i) the rate of degradation of the electrical power Po or the rate of increasing of the dielectric loss P. in the early steps of irradiation increases, as expected, with P0; this tendency is reversed when HEMA starts polymerizing before Pu goes through its maximum i.e. (P°)' = 0;

(ii) the shift towards longer times of the maxima of P,, is the larger as Po is low;

Table 2. Curves Pu = P,(t) at different P0 values

Pu Curve

Po (W)

(P.)o (W)*

(Pu)o/Po

1 2 3 4 5**

31 40 63 78 96

12.9 17.4 26.3 34.0 42.4

0.42 0.44 0.42 0.44 0.44

P. maximum P. plateau (W) (W) 16.5 20,5 31,4 41.4 51.2

* (Pu)o: degraded power at 0 ° and given Po value. ** Sample with bubbles.

8.5 11.0 17.8 19.5 21.0

plateau/' Po 0.27 0.28 0.28 0.25 0.22

M. TEFFALand A. GOURDENNE

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i O

5

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I/

-5 iJO

o

I

i

ZO

,

30

,

40

t

i

50

J i~.------

60

70 JlO

IzO

(rain)

Fig. 5. Polymerization under microwaves at various P0 values. Curves (T)' = d T/dt. (1) 31 W; (2) 40 W; (3) 63W; (4) 78W; (5) 96W.

15

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Fig. 6. Polymerization under microwaves at various Po values. Curves (P,)' = dP=/dt. (1) 31W; (2) 40W; (3) 63W; (4) 78W;(5) 96W.

(iii) the negative values of (P~)' indicate that P, is now decreasing; the time variation of (P.)' gives a curve with a minimum corresponding to the inflexion point in the curve P. = P,(t), which can be divided in two parts: (a) in the first, from zero to the minimum, corresponds to the polymerization of HEMA which reduces the dielectric loss; (b) in the second, from the minimum to zero, the rate of decreasing of P~ to the polymerization is now compensated by the increasing of P, due to the increase of the motion of sidegroups of the polymer under the thermical effect; (P,)' tends to zero when the thermal equilibrium is reached.

CONCLUSION

All the results show that the radical polymerization can be activated by 2.45 GHz. Such a process does not need any initiator and so will be of interest if it can be extended to other vinyl monomers that have sufficient polarity. The prepared polymerized materials, are chemically cross-linked because of reactions of transfer and termination under the given experimental conditions, viz. radical polymerization in bulk, no thermal regulation and probably too energetic beam. New conditions have to be defined in order to synthesize linear macromolecules by radical polymerization under microwaves. This technique

Polymerization of HEMA should provide good results because the energy of the microwave p h o t o n is too low for photolysis to occur.

REFERENCES

1. A. Gourdenne, A. H. Maassarani, P. Monchaux, S.

549

Aussudre and L. Thourel Polym. Prepr. 20(2), 471 (1979). 2. Q. Le Van, P. Monchaux and A. Gourdenne, International Symposium on Macromolecules I.U.P.A.C. A(1), 128 (1981). 3. S. Aussudre, A. H. Maassarani, L. Thourel and A. Gourdenne, 9th International Congress on Electroheat, Sect. 4, Comm. III A 3 (1980).

R~um6---Le m6thacrylate de 2-hydroxy6thyle (HEMA) ne contenant aucun amorceur radicalaire est soumis b, Faction d'un rayonnement micro-ondes polaris6 (2,45 GHz) de puissance 61ectrique (P0) variable ~t rint&ieur d'un guide d'onde. Le monom&e fluide polym&ise au bout d'un certain temps pour former un mat6riau solide. Les diverses Stapes de la r6action de polym6risation sont d6crites grace h l'analyse des variations en fonction du temps de la temp&ature des 6chantillons d'HEMA et de la partie (Pu) de la puissance 61ectrique (P0) d6grad6e dans le milieu polym6risable h cause des pertes di61ectriques.