The photoelastic properties of oriented polymethylmethacrylate

The photoelastic properties of oriented polymethylmethacrylate

Polymer ScienceU.S.S.R.Vol. 26, 'No. 9, pp. 2116-2121, 1984 Printed in Poland THE PHOTOELASTIC 0032-3950184 $10.00+ .00 © 1985 PergamonPress Ltd. ...

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Polymer ScienceU.S.S.R.Vol. 26, 'No. 9, pp. 2116-2121, 1984 Printed in Poland

THE

PHOTOELASTIC

0032-3950184 $10.00+ .00 © 1985 PergamonPress Ltd.

PROPERTIES

OF

ORIENTED

POLYMETHYLMETHACRYLATE* YE. V. CHISTYAKOV, O. S. ARKHIREYEV a n d B. M. ZUYEV A. Ye. Arbuzov Institute of Organic and Physical Chemistry, Kazan Branch of the U.S.S.R. Academy of Sciences

(Received 29 March 1983) Bireflingence caused by loading uniaxially oriented P M M A samples in the glassy state, has been studied. The increase in the anisotropic properties of the polymer with increasing extent of stretching was evaluated from the coefficient of optical sensitivity to stress. A substantial effect on the value and sign of the birefringence of chemical and physical bonds, related to the stress distribution in the sample volume, has been found. The nature of the negative path.length difference during elastic deformation of P M M A and the reason for changes in the picture of frozen bands in the material, with a decrease in temperature, are discussed.

PMMA is one of the few polymeric materials whose macromolecules exhibit negative sign birefringence in the glassy state. Many workers have studied PMMA birefringence, but up to the present, no clear explanation of this phenomenon has appeared in the literature [1, 2]. t The study of birefringence of the polymer under model stress conditions, when previously oriented and frozen samples with different stresses are strained, may clear up this question. Evidently loading a frozen sample along the orientation axis causes birefringence predominantly by chemical bond straining and in the case of loading in a direction perpendicular to this, causes physical bond straining. Thus, by following the birefringence value of a loaded sample in the above directions, one can study the change in its anisotropic polarizability and directly evaluate the roles of the main chain and intermolecular interaction in this effect, which as is known, is proportional to the stresses acting on the system. It should be pointed out that up to the present, oriented PMMA has been widely used as a constructional material, therefore the study of its optical-mechanical properties presents a practical interest. P M M A was prepared by bulk polymerization in fiat glass moulds, previously treated with a benzene solution of dimethyldichlorosilane. Polymerization was canied out in presence of 1 wt. To of benzoyl peroxide for 12 days at room temperature, after which the temperature was gradually raised by 10"/day to 373°K. To complete the polymerization, the P M M A laminae were separated from the moulds and heated at 393°K for 2 days. The intrinsic viscosity of P M M A solutions in benzene was [*/]=0.252 m-~/kg, corresponding to M = 1.4 × 106. The temperatures of the physical transitions, determined by a thermomechanical melhod at a loading of I N had the following values, Tz=385 and Tr=481°K. Slices cut with a 2-sided blade (with an allowance for subsequent treatment) were annealed from residual stress using the temperature pattern, recommended in [3]. The annealed samples * Vysokomoi. soyed. A26: No. 9, 1893--1897, 1984. It should be stated that the authors have studied P M M A . plasticized by dibutyl phthalatc. 2116

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were reduced to nominal dimensions and submitted to uniaxial stretching in a thermo.stat at 393°K under a load ensuring a given extension, for 5 rain. Then the strained samples were cooled under load at room temperature. The degree of extension varied from 60 to 300 ~. In all the frozen samples, (matrices) the optical pathlength difference, relative to unit thickness was proportional to the degree of stretching. For studies of stress distribution relative to the bonds, samples in the form of rectangular parallelopipeds, size 2 x 2 x 18 mm were cut from the matrices. A variation of + 0"1 mm was allowed in the smallest dimension. The samples were so cut that in one lot (series A) the largest edge coincided with the stretching axis of the matrix and in the other (B series) it was perpendicular to this axis. The samples were further submitted to uniaxial compression in respect of the surface of the small side, in the direction of the largest edge of the parallelopiped; in all the experiments the load P was 76 N. The birefringence caused by the load was measured by a Soleil compensator with a graduation value of 1 nm at 153°K. To determine the photoelastic constant, the experiment was repeated on 3-4 samples and the results averaged. The translucence of each sample was examined in 2 directions, through the face coincident with the matrix plane (directions A1 and B1) and through the matrix thickness (directions A2 and B2). Moreover, the pathlength, difference was noted in the central part of the faces where the stress condition was homogeneous. The optical sensitivity, in respect of a stress Co, was calculated from the equation

Co=ab/P where a = (ao - ax) is the instantaneous-path.length difference, obtained by subtraction of the orientational component d~x from the total effect ao; b is the sample thickness. The results of these measurements are given in Fig. 1. As is evident, the optical sensitivity at instantaneous strain, in all cases, has a negative sign and changes linearly with an increasing degree of stretching of the matrix. In series A samples, it hardly depended on which face the birefringence was observed, which confirmed that uniaxial matrix stretching was being accomplished. According to the experimental conditions, in series A, as the degree of stretching increased in a transverse section, b o t h the c o n c e n t r a t i o n of orientated m a c r o m o l e c u l e s p r o d u c e d grew a n d also the m e a n cosine of the angle between the strong field d i r e c t i o n a n d the ellipsoids of polarizing chain segments. This means that the m a c r o m o l e c u l e s , with increasing stretching, in large part take u p a direction, which relieves the i n t e r m o lecular bundles, the n u m b e r of which is decreased due to orientation.* I n this case, prel i m i n a r y or ientation of the macromolecules, a n d the redistribution of stresses in the b o n d s due to it, create the c o n d i t i o n s for a more distinct photoelastic effect, arising f r o m the d i s t o r t i o n of valency angles a n d straining of covalent b o n d s of the skeletal chains. J u d g i n g f r o m the slope of line I (see Fig. 1), it can be said that as stretching increases, this photoelastic effect decreases the optical sensitivity to stress. It ist herefore n a t u r a l to suppose that the optical sensitivity to stress of the P M M A skeletal chains has a positive sign. The m a i n c o n t r i b u t i o n to strain in series B samples is evidently due to m o v e m e n t of atoms in the dipole-dipole type physical b o n d s between the ester chainlets. If atten* C i s - t r a n s conformational transitions are not considered here, since pathlength differences were measured 10 sec after loading. Under such conditions, constrained elastic strain was insignificant and did not contribute noticeably to birefringence. This contribution cannot be great and therefore the optical sensitivity in respect of stressed elastic origin is an order higher than the sensitivity arising from redistribution of the segmental conformations of the chain.

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YE. v. CHISXYAKOVet al.

tion is turned to the system geometry, one may be persuaded that the ellipsoidal polarizability of carbonyl groups in the ester chainlets must cause a negative contribution to the birefringence of PMMA. This conclusion is confirmed experimentally: the negative value of the optical sensitivity to stress of series B samples grows in direct proportion to the extent of stretching (see Fig. l, lines 2 and 3). Line 3 was constructed from studies on B2 samples, in which the orientation direction of the macrochains and the polarized light path coincided. As can be seen, the optical sensitivity to stress of these samples

-C : ~TPa -7

5-

lO0

200

oc~%300

FIG. I. Dependence of optical sensitivity to stress of PMMA samples on degree of stretching and direction of translucence at 153°K. /-Translucence in direction A, 2 - B I , 3-B2, 4 - in all directions. changed with stretching more than that of BI samples (line 2). Therefore in BI and B2 sample volumes, the number of ester chainlets, linked in physical bundles and the number of non-oriented macromolecules is the same and it is natural to suppose that the difference in birefringence of these samples arises from the orientation of the macromolecules in B1. The same fact, relative to the birefringence of oriented molecules in BI samples, prompts two important conclusions. Firstly, both in B1 and B2 samples, orientated macromolecules (even in the case where a large part is perpendicular to the stretching axis) actively, share in stress distribution through intermolecular interaction of the ester chainlets which frame them. However, in B2 samples the macromolecules are "laid" in the direction of light propagation and therefore cannot add a notable contribution to the birefringence. Secondly, the difference between the slopes of lines 2 and 3 once more tends to convince one that the optical sensitivity to stress of the P M M A skeletal chains has a positive sign: this is the reason for a reduction of total bJrefringence in BI samples. At the same time, the positive contribution to birefringence of the oriented chains of series B samples is appreciably lower than of series A samples (line 1). The latter indicates that the main portion of the stress in series B samples is taken up by intermolecular reactions and non-oriented macromolecules.

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In concluding a disscussion of the experimental results in Fig. 1, it should be said that the arilhmetical mean value of the optical sensitivity to stress for the 3 faces of the samples studied remained constant, independently of the extent of matrix stretching (line 4), and was equal to the sensitivity of unoriented PMMA. We will now consider the change in frozen pathlength difference in P M M A in relation to temperature. Figure 2 presents curves for 6x=f(T) for samples, previously stretched by 60, 120, 215 and 300~. An overall view of the curves confirms the viewpoint that orientation of P M M A macromolecules in a highly elastic state causes a negative pathlength difference, which preserves its sign even after freezing [2]. However, the main point is that as temperature is reduced, the frozen pathlength difference grows to an absolute value. Thus for example, a temperature reduction from 293 to 153°K raises the negative pathlength difference by 70 ~o on average. This is observed under conditions when the strong field is cut off.

-~

nm

_/~

14001

37a

In 9

4

i 15cTi'r,K

zaa

q

FIG. 2

Fie.

6 tO3/r,K-' 3

FIG. 2. Dependence of frozen path.length difference on temperature and extent of sample stretching, equal to 60 (1), 120 (2), 215 (3) and 300~ (4). Fro. 3. Graph of ratio In y~ 1/T for four PMMA samples with stretching of 60, 120, 215 and 300~. In order to explain the nature of this change in the interference picture, it seemed expedient to use the thermal activation concept [4]. To describe the functional dependence of the experimental results by the latter, the most suitable equation appears to be: 1

6 = 6 o [ 1 - e --~ ( ~-- ~ ) ]

(1)

which was reduced to the form U

v=ae rR

(2)

where y = 6 o - 6 / 6 0 , 6 is the pathlength difference for unit sample thickness, 60 is the limiting value of the increment in pathlength difference, determined by a method recommended in [5], a = e x p (U/RTg) and Tg is the glass transition point of PMMA.

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Yr,. V. CHISTYAKOVet al.

By graphical representation of equation (2) in In v ~ I / T coordinates, there was obtained a line with a break at 233°K (Fig. 3). It was seen from the Figure that the experimental points, independently of the degree of stretching the matrix, satisfactorily lie along this line. It is easy to determine from the slopes of the linear sections, the apparent activation energy of the observed changes in frozen pathlength difference with temperature. Since the activation energy does not remain constant, it may be concluded that this process is determined by 2 mechanisms, of which one predominates below 233°K with activation energy of 3 k J/mole and the other at above 233°K, with activation energy of 6-3 k J/mole. Let us examine these results, in connection with a molecular model of P M M A which was constructed as follows. P M M A macromolecules are framed by methyl groups and ester lateral chainlets, part of which share m intermolecular reactions, whilst another part does not do so (passive part). The quantitative ratio of active and passive chainlets is determined by the temperature. Direct contact of main (skeletal) chains is impeded by steric obstacles in the framework. Besides this, in the "'frozen" state, the segmental mobility of the skeletal chains is excluded, and thermal change of birefringence occurs only as a result of redistribution of lhe conformations of the ester chainlets.* It is readily seen that in the adopted model, the most mobile structural elements are the active and passive ester chainlets. As regards the methyl groups on the skeletal chain, they do not change their conformation and their thermal oscillations relative to - C - C - bonds cannot notably affect the observed effect. The passive chainlets are in a convoluted state and as lateral pendants contribute negatively to the total anisotropic polarizability of orientated PM MA. However, during temperature reduction these passive chainlets are capable of intermolecular interaction, forming physical bundles of the following structure crq I ~cIt2-c~

o=c .... /

CHa--O

O--CHs o=c

l

~C--CH~ CH3

As can be seen, the intermolecular bond in the bundle results from dipole-dipole interaction of carbonyl groups. Moreover, the same carbonyl oxygen and the - O - CH3 fragment in linked chainlets appear in the role of lateral pendants, whose M M amounts to 47 ~ of the mass of the monomer unit. it may be assumed that (and this is confirmed by the results in Fig. 2) in the passive state, the ester chainlets of P M M A have a lower anisotropic polarizability than in a physically bonded bridge. Transfer of these chain* Segmental mobility causes irreversible changes in the "frozen" bands' picture. In our experiments, the pathlength difference for a given temperature remained constant independently of which way-cooling or heating-the sample was brought to the above temperature.

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212l

lets into the active state leads to formation of lateral pendants which have significant mass and anisotropic polarizability and which also make the experimentally-noted additional positive contribution to the'total birefringence of the system. Rotational isomerization of the ester chainlets from the convoluted to tile active state occurs through rolation relative to the C - C and - O bonds. The necessary activation energies for these processes determine the results of Fig. 3. It is evident t h a t the least value of the activation energy corresponds to the rotation of the - O - C H 3 fragment, relative to the ester bond. Thus the experimental results permit the conclusion that the skeletal chain of P M M A is characterized by a small positive anisotropic polarizability and the framework elements of the chains play a basic role in the birefringence of the system. The specific structure and features of the physical interaction of the fragments of the framework also determine the nature of the negative pathlength difference during elastic straining o f PMMA. Trtmslated by C. w , CAPP REFERENCES

I. 2. 3. 4.

O. N. TRAPEZNIKOVA and M. ZHURINA, Zh. fiz. khimii 24: 1471, 1950 M. F. MILAGIN and I. I. SHISHKIN, Fizika tverdogo tela 4: 2681, 1962 M. M. FROKHT, Fotouprugnost' (Photoelasticity). vol. I. p. 351, OGIZ, Moscow, 1948 P. P. KOBEKO, Amorfnye veshchestva (Amorphous Substances). p. 204, lzd. AN SSSR, Moscow-Leningrad, 1952 5. N. N. BRONSHTEIN and K. A. SEMENDYAYEV, Spravochnik po matematike (Mathematics Handbook), p. 578, 10th edn., Nauka, Moscow, 1964