JOLIRNAL OF POLYMEI< SCIICNCli VOI,. XXXII, PAGES 27-31 (1958)

Effect of Composition on the Photoelastic Behavior of Methyl Methacrylate-Diallyl Phthalate

(MMA-DAP) Copolymers*

IiOZO IL4WATA, Scientific Research Institute, Komagome, Bunkyo-Ku. Tolqo, Japan

INTRODUCTION

For an ideal, crosslinked rubber in the rubbery region, the following can he theoretically derived:

(1) stress

abs. temp. X birefringence = constant

This fact has been verified experimentally for various p ~ l y m e r s . ~ - ~ Ideal, crosslinked rubber having Gaussian chains satisfies relation (l), and its stress-optical coefficient An/a is independent of the degree of crosslinking. For the polymers having degrees of crosslinking varying over a wide range, it does not seem to be verified whether this relationship is obeyed. Be- sides, little is known of the effect of copolymerization on the stress-optical coefficient in the glassy state.

In a previous paper,7 the results for styrene-divinylbenzene (DVB) copolymers nith various DVB contents were reported. Now, in order to make clearer the effect of crosslinking on photoelastic behavior in a nide range, results of a series of measurenieiits 011 the photoelastic behavior of hCIILIA-DAP copolymers are reported.

EXPERIMENTAL

The nionochroniatic fringe method8 was chosen for nieasuring stress- optical coefficients. The photoelastic properties a t various temperatures between 15 and 190°C. were studied by tension tests of samples in the form of plates, 10 min. wide and 5.5 nini. thick, in an air thermostat in circularly polarized light (A = 5461 A.). The opt)icnl system of this apparatus is shown in Figure 1. Tht: friiige method is coiiveiiient, for measuring the coefficient of highly seiisitivc nxiterinls, but not so for m:tt,erials of low sen- sitivit’y.

Sarnia, Ontario, Canada, on November 9, 1956.

Tardy’s method9 is applied for the lxtter materials.

* Read before the meeting of the Seventh Canadinn High 1’olynic:r Forum held in

27

28 I(. KAWAI’A

The coefficient measurements were carried out after keeping the test piece for 30 minutes a t each temperature. The fringe order \\-as observed 1.5 seconds after application of load, and the observations were repeated 11 ith varied loads by adding weights in succession. In the measurement, the de- gree of elongation is kept within a quarter of its breaking elongation.

Fig. 1. The optical system of the photoelastic apparatus. F: monochromatic filter (A = 5461 A.).

S: mercury lamp. C1, C p : condenser lenses. PI, P p : Polaroid plates. Q1, Q2: quarter wave plates. FLI, FL2: field lenses. B: air thermostat. T: test piece. CL: camera lens. SC: screen.

The test specimens of MMA-DAP copolymers \\-ere prepared from M R i I A monomer aiid DAP monomer in the weight percentages of 100, 90, 70, 60, 50, 20, and 0% of DAP monomer.

RESULTS AND DISCUSSION

With the MMA-DAP copolymers, the time behavior of the birefringence under constant stress is as follows. Optical creep is slight in the glassy re- gion and is found to be marked in the transition region. I n the rubberlike region, the birefringence reaches a limiting value instantly after loading, and, thereafter optical creep is practically zero. With the polymethyl methacrylate, (PRIMA), the time behavior is the same as that with the MMA-DAP copolymers in the glassy and trailsition regions. In the higher temperature region above the transition, it is found that the optical creep is not so unusual, though the mechanical creep is rather large.

The experimental results on the relation of stress-optical coefficient ver- sus MMA content in mole per cent are shon-n in Figure 2 . Typical results on the variation of the coefficient with temperature are shown in Figure 3.

In Figure 3, three different types of behavior are found. Pure DAP polymer does not show perfect entropy elasticity up to 190°C. On the other hand, MMA-DAP copolymers crosslink suitably, such that when pre- pared in the weight percentage of 80 : 20, they exhibit transitions to entropy elasticity at higher temperatures. Lastly, anomalous behavior is observed for the MMA polymer. The stress-optical coefficient reaches a maximum a t about 127°C. and decreases abruptly with increasing temperature above the maximum point.

The origin of these types in the relatioil of stress-optical coefficient versus temperature seems attributable to the degree of crosslinkage; this is too high in DAI’ polymer, suitable in several MMA-DAI’ copolymers, such as that 80 : 20 in weight percentage, and zero in the PPtlMA. Anomalous be-

METI IYI, MlWT IACRYLATE-DIALLYL PI I'I'LIALATE 29

haoior. such as that exhibited by the l'i\'IRIA, seems to be a result of flow in the higher temperature region and should be studied further for other linear polymers.

I t is observed, further, that the transition region becomes lower with in- creasing MMA content (lYg. 3 ) . The follon-iiig facts are kiioicn from Figure 2. At 20°C. (glassy region), the stress-optical coefficient varies ap- proximately linearly from the value of DAP polymer(+) to the value of

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Fig. 2. The variation of the stress-optical coeffirient with h111.4 content in mole per cent.

PMMA(-). The fact that the maximum points are observed a t 120", 140", 160", and 180°C. seems to be attributable to the variatioii of the transition region and also to the fact that an increase in the flexibility of molecular chains, obtained by iiicreasing the MMA content, compensates the decrease in the optical ailisotropy resulting from ail increase jn the MMA content.

The author wishes to eqress sincere thanks to Ur Z. Tuzi, I l r . 11. Xisida, and Mr. S. Suzuki for their kind guidance and encouragement, arid to I)r. S. Sakumi and Mr. I. Hori for their instruction and assistanre in carrying out the experiment.

30 K . KAW7ATA

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Temperature ("0 - Fig. 3. The variation of the stress-optical coefficient, with temperature. The parametrr

is the ratio of MLZI.4 to DAP in weight per cent.

References (1) W. Kuhn and F. Griin, Kolloid-Z., 101, 248 (1942). (2) 1,. R. G. Treloar, Trans. Furacla!y Soc., 43, 2 i 7 (1947). ( 3 ) R. Kubo, J . Phys. Soc. Japan, 2, 8.2 (194i). (4) R. S. Stein, S. Krimm, and A. V. Tobolsky, Textile Research J . , 19, 8 (10.29). (5) R. S. Stein and A. V. Tobolsky, J . Polymer Sci., 14, 443 (1951). ( 6 ) K. Kawata and Z. Tuzi, J. Sci. Research Inst. (Jnptcn), 47, 12 (1953). (T) K. Kawata, J . Polymer Sci., 19, 359 (1956). (8) Z. Tuzi, Sci. Papers Inst. Ph?ls. Chem. ZZesearch (7 'ok! /o) , 7, i 9 (192T). (9) H. I,. Tardy, Rev. opt., 7, 59 (1929).

Synopsis The photoelastic behavior of methyl methacrglnte-diall3-1 phthalatc copolymers has

I J ~ W investigated in a wide range of temperature :~nd copolymerization ratio to study the

effect of crosslinking. In the relation of the stress-optical coefficient versus temperature, three different types of behavior, corrcspondirig to the three degrees of crosslinkage are found. The first is that a t pure DAP polymer, for which the degree of crosslinkage is too high; the second is seen in several MMA-DAP copolymers for which the degree is suitable, and the third is shown by pure MMA polymer, for which the degree of crosslinking is zero. The first does not come to show perfect entropy elasticity in the higher temperature region. The second group changes to perfect entropy elasticity in the higher temperature region. In the third, the coefficient shows a maximum in the higher temperature region and decreases abruptly with increasing temperature above the maximum point. The effect of copolymerization ratio in the glassy region is quite different from that in the higher temperature region.

The birefringence was measured under constant stresses.

Le comportement photoitlastiqne de copolymkres de mkthacrglate de mitthyle-phtha- late de diallyle a B t C 6tudi6 sur un large domaine de temperature et de rapport de co- polymkrisation afin d’etudier I’effet du pontage. La biritfringence a C t E mesurge sous tension constante. En rapport avec le coefficient optique de tension en fonction de la temperature, on a trouve trois types differents correspondants A trois degres de pontage. Le premier est le polymbre DAP pur pour lequel le degrC de pontage est trop 6lev6, le second est celui de nombreux copolymkres MMA-DAP pour lesquels le degre est con- venable, et le troisikme est le polym&re MMA pur dans lequel le degr6 de pontage est nul. Le premier ne montre pas d’blasticit6 entropique parfaite dans la region des tempbratures 61evCes. Le second groupe change en elasticit6 parfaite aux tempkratures Blevkes. Dans le troisikme, le coefficient montre un maximum dans la rkgion thermique supkrieure, et decroit brusquement B temperature croissante au dessus du point maxi- mum. L’effet du rapport de copolymdrisation dans la region vitreuse est absolument different de celui dans la ritgion thermique supbrieure.

Zusammenfassung Das photoelastische Verhalten von Methylmethacrylat-Diallylphthalat-Copolymeren

wird in einem weiten Temperatur- und Copolymerisationsverhaltnisbereich untersucht, um den Vernetzurigseffekt keniien zu lernen. Die Doppelbrechung nird bei konstanter Spannung gemessen. F u r die hbhangigkeit des spannungs-optischen Koeffizienten von der Temperatur, nerden drei verschiederie Typen gefunden, die drei Vernetzungsgraden entsprechen. Der erste Type ist der des reinen DAP-Polymeren, dcssen Vernetzungs- grad zu hoch ist, zum zxeiten gehoren mehrere MMA-DAP-Copolymere, die einen geeigneten Vernetzungsgrad aufweiseri und den dritten bildet reines MMA-Polymeres, fur nelches der Vernetzungsgrad Xu11 ist. I>er erste Typ zeigt auch im hoheren Tem- peraturbereich noch keine vollkommene Entropie-Elastizitat. Die zneite Gruppe erreicht in dem hoheren Temperaturbereich vollkommene Entropie-Elastizitat. In der dritten zeigt der Koeffizient im Bereich hoherer Temperatur ein Maximum und fallt jah ab, sobald die Temperatur uber die des Maximums erhoht wird. Der Einfluss des Copolymerisationsverhaltnisses ist im Glashereich vollig verschiederi von dem im Bereich der hoheren Temperatur.

Received Xovember 8, 1957