Thermal Properties polymers

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ISSN 1070-4272, Russian Journal of Applied Chemistry, 2007, Vol. 80, No. 9, pp. 1575 1578. Pleiades Publishing, Ltd., 2007.

Original Russian Text B.B. Troitskii, L.V. Khokhlova, V.N. Denisova, M.A. Novikova, A.E. Golubev, A.V. Arapova, 2007, published in Zhurnal

Prikladnoi Khimii, 2007, Vol. 80, No. 9, pp. 1543 1546.

MACROMOLECULAR CHEMISTRY

AND POLYMERIC MATERIALS

Thermal Properties of Poly(Methyl Methacrylate)Prepared by Emulsifier-Free Emulsion Polymerization

B. B. Troitskii, L. V. Khokhlova, V. N. Denisova, M. A. Novikova,A. E. Golubev, and A. V. Arapova

Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences, Nizhni Novgorod, Russia

Received January 22, 2007

Abstract Emulsifier-free emulsion polymerization of methyl methacrylate in the presence of potassium

persulfate initiator, taken in several different concentrations, at various pH values was studied with the aim to

obtain colloidal crystals. The thermal properties of poly(methyl methacrylate) prepared by emulsifier-freeemulsion polymerization, as the starting material for fabrication of photonic crystals, were examined in rela-

tion to the synthesis conditions.

DOI: 10.1134/S107042720709025X

An important feature of emulsifier-free emulsionpolymerization is the possibility of preparing mono-disperse polymeric particles with strictly orderedpacking, which can serve as a basis for forming collo-idal crystals [1, 2]. In turn, from colloidal crystals itis possible to prepare various kinds of photonic crys-tals opening wide prospects for the development ofphotonics [3] and showing promise for applications inmedicine and optics, as materials for light guides andchemical sensors.

It appeared possible to control the properties ofcolloidal crystals by using various monomers. In par-ticular, polymeric colloidal crystals based on poly-(methyl methacrylate) (PMMA) were studied [2]. Itwas found [2] that the size of particles formed byemulsifier-free polymerization of methyl methacrylate(MMA) depends on the polymerization conditions andcomposition of the reaction medium.

At the same time, thermal properties of latex par-ticle powders and of colloidal crystals prepared fromthese particles have been studied inadequately.

The degradation mechanism of poly(methyl meth-acrylate) at elevated temperatures has been studied insufficient detail [4]. Thermal degradation of PMMAoccurs by the mechanism of radical unbranched chainreactions. Thermal oxidative degradation of PMMAoccurs by the mechanism of branched chain reactionswith degenerate chain branching. The main degrada-tion process is depolymerization. The monomer isvirtually the only product of PMMA degradation

(99% in thermal and 95% in thermal oxidative de-gradation) [5].

It was shown by mass spectrometry [6] that thermaldegradation of atactic PMMA prepared by radicalpolymerization occurs in three steps. Shibaev et al. [6]assumed that the first degradation step occurring at120 200 C involves a chain reaction initiated by thedecomposition of head-to-head structures in macro-molecules. In the second step, at 200

300

C, the de-gradation is initiated by decomposition of terminal

double bonds. The major fraction of the polymerdegrades at 300 420 C; the process is initiated byrandom scission of the macromolecule with the forma-tion of macroradicals which subsequently undergodepolymerization.

It was interesting to examine the thermal propertiesof PMMA prepared by emulsifier-free emulsion poly-merization, as this polymer is a raw material for pre-paring colloidal crystals. In this study, using DSC andTGA, we examined how the polymerization condi-tions (initiator concentration, pH) affect the thermalproperties of emulsion PMMA.

EXPERIMENTAL

Emulsion polymerization of MMA was performedin an aqueous medium with vigorous stirring at a rateof 1200 rpm. The pH of the medium was adjusted us-ing a pH meter, by adding to double-distilled water therequired amount of 0.1 N NaOH or HCl. Water withthe adjusted pH was heated to 80

C with continuous


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RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 80 No. 9 2007

1576 TROITSKII et al.

Glass melting, 131.5Peak

PeakPeak

Exo

T, C

Fig. 1. DSC curve of emulsion PMMA prepared at an initi-

ator concentration of 0.1% and pH 10 (run no. 3). (T) Tem-

perature; the same for Fig. 2.

Table 1. Conditions of PMMA synthesis

Run

pH Initiator concentra-

Polymerization

no.

tion, %

time, h

1 3 0.1 5

2 7 0.1 5

3 10 0.1 5

4 3 0.5 5

5 7 0.5 5

6 10 0.5 5

7 3 1.0 38

7

1.0

3

9

10

1.0 3

Table 2. Results of DSC study

Run

Tg, C

Onset of

Peaks in DSC curve

no. decomposi- I II III

tion, C

T, C S, % T, C S, % T, C S, %

1 130.6 154.4 184.0 0.7 293.5 8.4 368.3 90.9

2 129.3 154.6 169.0 11.0 297.4 4.5 370.2 84.53

131.5

156.0

180.6

6.0

297.2

6.8

373.1

87.2

4

127.6

144.6 178.3

25.7

292.3

13.5

377.7

60.8

5 126.9 145.4 179.5 18.9 293.6 10.1 376.7 71.0

6

125.6

136.7

177.6

22.9

291.6

9.3

379.7

67.87 129.4 162.2 183.1 22.4 291.1 15.2 381.4 62.4

8 128.8 148.2 176.2 27.3 294.0 7.3 385.7 65.4

9 126.2 147.8 173.3 26.0 294.8 6.0 384.5 68.0

7a* 66.0 176.7 20.4 293.6 8.0 381.8 71.6

8a* 69.2 176.9 25.2 291.0 9.3 380.8 65.59a*

90.5

173.5

5.3

295.8

17.9

380.8

76.8

* Films prepared from the corresponding powders.

argon bubbling. Then the monomer purified by vacu-um distillation was added in an amount of 10 wt %relative to the total weight of the mixture. The mix-ture was heated for 5 min, after which the requiredamount of potassium persulfate (analytically puregrade) dissolved in 1 ml of water was added. Thereaction was performed at 80

C for 3

5 h depending

on the initiator concentration. In the first 10 min themixture was purged with argon, after which the argonsupply was stopped to avoid foaming. The conditionsof preparing polymer samples are given in Table 1.

The polymer obtained was filtered off on a mem-brane filter with a pore size of 0.2

m under slightlyreduced pressure (200 400 mm Hg) and dried in avacuum oven to constant weight at 60 80 C.

The thermal properties of the samples obtainedwere studied by DSC and TGA. The DSC studieswere performed with a DSK 204 F1 Phoenix differen-tial scanning calorimeter (Netzsch, Germany) in anargon flow (flow rate 20 ml min 1), with linear heat-ing from 20 to 450

C at a rate of 2 or 10 deg min 1.The sample (powder or film) weight was 2.4 6.5 mg.The onset temperatures of particular processes weredetermined with an accuracy of

1

C. The glasstransition point Tg was determined as an inflection

point on the baseline in the region of glass transition(Fig. 1) and was averaged over three replicate meas-urements (repeatability 1 C).

The rate of thermal oxidative degradation (TOD) ata constant temperature (302

C) was determined from


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THERMAL PROPERTIES OF POLY(METHYL METHACRYLATE) 1577

(a)

(b)

(c)

T, C

T, C

T, C

Exo

Exo

Exo

Fig. 2. DSC curves of emulsion PMMA in the form of(a, b) powder and (c) film. Heating rate, deg min 1: (a) 10

and (b, c) 2. Heating run: (1) first and (2) second, after

cooling to room temperature.

the weight loss in an atmosphere with oxygen partialpressure of 200 mm Hg. Samples in the form of 100- m-thick films were prepared from a 7 8 wt %solution of emulsion PMMA powder in methylenechloride by casting onto a glass surface followed byvacuum drying at 60

80

C.

The DSC curve (Fig. 1) exhibits the glass transitionregion and three endothermic peaks corresponding to

the degradation of different structures in the PMMAmacromolecule. The glass transition points are import-ant characteristics of the emulsion PMMA samples.They appreciably exceed the glass transition points offilms cast from powder solutions, and the highestglass transition point is exhibited by samples preparedat the lowest initiator content (Table 2, run nos. 1 3).

The glass transition points of the films are appre-ciably lower than those of the powders from whichthese films were prepared. Additional examination ofthe glass transition region (repeated heating after cool-ing of powder no. 1 from room temperature to 140 C

at a rate of 10 deg min

1

) revealed a decrease in thetemperature of the inflection point in the DSC curvefrom 129.6 to 127.1 C (Fig. 2a). The decrease is stillmore pronounced (from 125.0 to 121.4

C) at a heatingrate of 2 deg min 1 (Fig. 2b). At the same time, infilm sample no. 7a, on repeated heating at a rate of2 deg min 1 after cooling, the glass transition point(97.5 and 98.0 C) remains essentially unchanged(Fig. 2c). This fact indicates that the high glass tran-

(a) m, %

, min

m, %

, min

(b)

m, % (c)

, minFig. 3. Curves of weight loss m in the course of thermal

oxidation degradation of PMMA film. 302

C, oxygen pres-

sure 200 mm Hg. Samples prepared at initiator concentra-

tions of (a) 1.0, (b) 0.5, and (c) 0.1%. (

) Time. pH: (1) 3,

(2) 7 , a n d (3) 10.

sition point of the powdered samples may be causedby the ordered structure of the emulsion PMMA inwhich the macromolecular motion is hindered. Thesestructures are partially broken on heating. In the filmsamples, the polymer occurs in the glassy state andhas no ordered structure, and therefore Tg of the filmsis considerably lower than that of the powders.


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1578 TROITSKII et al.

As for the endothermic peaks, the temperatureranges of their occurrence suggest that they belongto three steps of thermal degradation of PMMA [6].The peak areas Sare proportional to the total thermaleffects of the processes occurring in the polymer onheating.

The first DSC peak corresponds to the thermaleffect of macromolecule degradation initiated byscission of weak bonds formed by head-to-head re-combination of macroradicals. For the samples pre-pared in neutral medium, the peak area linearly growswith an increase in the initiator concentration. At theminimal initiator concentration (0.1%), the peak areais minimal at all pH values. This fact indicates thatunder these conditions the rate of macroradical recom-bination is low.

The second peak in the DSC curve corresponds toinitiation of the degradation by cleavage of the termi-nal double bonds. For the samples prepared at the

maximal initiator concentration (1.0%), the peak areagrows with decreasing pH. Under these conditions ofthe polymerization, the disproportionation responsiblefor the formation of terminal double bonds plays asignificant role.

The largest thermal effect is associated with the laststep of the degradation, corresponding to randomscission of the macromolecules. It is proportional tothe area of the third peak. Its intensity grows witha decrease in the catalyst concentration.

A TGA study of thermal oxidative degradation ofemulsion PMMA films showed (Figs. 3a 3c) that the

polymer samples prepared at the highest initiator con-centration (1%, Fig. 3a) were the most heat-resistant,with the highest degradation rate observed with thesample prepared at pH 3 (Fig. 3a, curve 1). At 0.1%initiator concentration, the dependence of the PMMAheat resistance on pH of the polymerization mediumis inverse.

CONCLUSIONS

(1) Increased glass transition points of poly(methyl

methacrylate) samples prepared by emulsifier-free

emulsion polymerization, compared to the film sam-

ples, are consistent with the ordered structure of the

latex particles.

(2) Endothermic processes of degradation of emul-

sion poly(methyl methacrylate) in DSC analysis in an

inert medium occur in three steps. Their temperature

ranges correspond to the degradation of macromole-

cules initiated by terminal defective structures and

random scission of the backbone.

(3) Thermal oxidative degradation of poly(methyl

methacrylate) prepared by emulsifier-free emulsion

polymerization occurs more slowly in samples pre-

pared at a higher initiator concentration, which may

be due to stabilization of the polymer by terminal

sulfate groups of the macromolecules.

REFERENCES

1. Tauer, K., Deckwer, R., Kuhn, I., and Schellenberg, C.,

Colloid Polym. Sci., 1999, vol. 277, pp. 607 626.

2. Egen, M. and Zentel, R., Macromol. Chem. Phys.,

2004, vol. 205, pp. 1479 1488.

3. Photonic Crystal Advances in Design, Fabrication,

and Characterization, Busch, K., Lolkes, S., Wehr-

spohn, R., and Foll, H., Eds., Wiley, 2004.

4. Grassie, N. and Scott, G., Polymer Degradation andStabilisation, Cambridge: Cambridge Univ., 1985.

5. Song, J., Fischer, Ch.-H., and Schnabel, W., Polym.

Degrad. Stab., 1992, vol. 36, pp. 261 266.

6. Shibaev, L.A., Antonova, T.A. Vinogradova, L.V.,

et al., Pisma Zh. Tekh. Fiz., 1997, vol. 23, no. 18,

pp. 81 86.

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