Photoelastic properties of plasticized polymers in the glassy state

Polymer 8ctence U.S.S.R. Vol. 28, No. 10, pp. ~22-2529, 1 9 8 1 Printed in Poland

00~-8950/81/102522-0&907.50]0 © 1982 Pergamon Prem Ltd.

PHOTOELASTIC PROPERTIES OF PLASTICIZED POLYMERS IN THE GLASSY STATE* Y~. V. CmSTYAXOV, B. M. ZuY~.V, E. F. GmaANov and E. P. Dn~on~rKO A. Ye. Arbuzov Institute of Organic and Physical Chemistry, U.S.S.R. Academy of Sciences (Received 11 August 1980) A study was made of the effect of natural anisotropy of tim polarizability of plastieizers on the overall photoelastic effect of the polyraer-plasticiser system in the glassy state. Results were presented of investigating birefringence of linear and crosslinked polymers plasticized by esters of different chemical structure. EXP~.m:m~TAT. investigations show t h a t plastieizers have ~ m a r k e d effect on photoelastic properties o f p o l y m e r s [1]. T h e i r effect, however, on the optical sensitivity o f t r a n s p a r e n t materials has so far been little e x a m i n e d . A s t u d y is m a d e in this p a p e r using results t h a t h a v e been derived from investigating plasticized model systems using the polarization-optical m e t h o d in the t e m p e r a t u r e range well below Tg. I n the glassy state the m e c h a n i s m of stress distribution in the p o l y m e r is m a i n l y d e t e r m i n e d b y molecular interaction. Therefore, s t u d y i n g the v a r i a t i o n of birefringence for this state, according to chemical s t r u c t u r e a n d the a m o u n t o f plasticizer added, the effect of the n a t u r a l a n i s o t r o p y o f polarizability of plasticizers on the overall photoelastic effect of the p o l y m e r - p l a s t i c i z e r s y s t e m m a y first be established a n d secondly, the role of m o l e c u l a r i n t e r a c t i o n in this effect, determined. F o r polarization-optical m e t h o d s of s t u d y i n g deformation properties of polymers these d a t a are of definite practical a n d theoretical interest. A study was made of PMMA plasticized using dibuty] esters of oxalic acid (]:)BOA), succinic acid (DBSA), adipie acid (DBAA), suberie acid (DBSuA), ortho- (DBOPA), iso(DBIPA) and terephthalic acids (DBTPA) and polydiallyltorephthalate (PDTP), plasticized using dipropyl esters of succinic (DPSA), ortho- (DPOPA) and terephthalie acids (DPTPA). Furthermore, polydiallylorthopthalate (PDOP) plasticized by DPOPA was used for comparative evaluation of photoelastie properties. Selection of objects of investigation was based on the wish to observe the effect of plasticizers on thc birefringence of both linear and crosslinked polymers. To obtain more general infornmtion, PMMA--one of the few polymers with negative birefringence in the glassy state--was included axaong linear samples. Physical and chemical constants of monomers and saturated esters corresponded to results in the literature [2-4]. The monomer-plasticizer mixture was polymerized in glass arapoules in argon. The arapoules had previously been treated with a benzene solution of diraethyldichlorosilane. Polymerization of PMMA samples was started in the presence of * Vysokemol. soyed. A23: No. 10, 2321-2327, 1981. 2522

Photoolastic properties of plastioized polymers

252~

{~5o/o benzoyl peroxide by heating for 24 hr at 40 °, temperature was then raised by 10 ~ daily ~o 80 °. To complete the process of hardening, the contents of arapoules were kept at 120 ° for 2 days. P D P T and P D O P samples were polymerized in the presence of 2~o tertiary

butyl peroxide. Minimum residual unsaturation (up to 10~/o on average according to IR spectra) was obtained using the following conditions of heating: 70° - 2 days, 100° - 2-5 days. 120°--2 days and 130°--0-5 day.

Samples for testing were cut from cylindrical units shaped as discs which after polishing had a diameter d = l S + 0 . 0 1 and a thickness b = 2 × 0 - 0 1 mm. After thermo-mechanical treatment [5] the initial optical path difference $ in the sample was less than 1/50 ~s4s. Methods of stressing and measurement of optical and mechanical characteristics did not differ from those described [4]. Temperature dependences of £=Ad/d=f(t) and 5=fp(t) were measured in the range 20--120 ° . The scale division of the tail spindles which secure the displacement of the horizontal diameter ztd was 20 nm and of the compensator for optical, path difference, 1 nm. 'Pests were repeated 2-3 times for samples of each series deviation of individual measurements from the arithmetic mean not exceeding 6%. The constant of optical stress sensitivity (OSS) for disc samples deformed using a compressive load in the direction of the vertical diameter was determined from the equation [6]

Ct=5gd/8P, and the elasticity modulus at --120 ° was calculated from the formula [7]

E=P(v+O.273)/Adb, where v=0.3-0.32 is the Poisson coeffcient, P - - l o a d , which maintained at 318 1~~ in all experiments. Temperature conditions of "pure" elastic deformation of the samples examined were selected by comparative analysis of curves showing the variation of Tg and density p according to the concentration of plasticizer in the system and results of measurements of e=f(t) and 5~(t) in the glassy state. The effect of plasticizer on the first two characteristics is shown in Fig. 1". It was established by experimental investigation t h a t the second two hmctions decreased with a reduction in temperature and in the temperature range --80 to --120 ° showed a linear variation; at a temperature of --120 ° deformation and the path difference after keeping the sample for 10 sec under load and with " i n s t a n t a n e o u s " relief showed no difference in practice. Comparing these results and data of Fig. ] it m a y be assumed t h a t at a temperature of --120 °, i.e. 170220 ° below Tg samples undergo deformation close to the "pure" elastic mechanism. Figure 2 shows dependence of the elasticity modulus and OSS on the molar fraction of plastieizers for PMMA samples at 120 °. The Figure (curve 8) indicates t h a t an increase in plasticizer content to 0.1 molar fractions reduces the elastic modulus by 20~/o on average; on changing the type of plasticizer deviaticm * Tg was deter~nined from temperature/deformation curves (P=~3 MPa), and p by hydrostatic weighing.

YEo V. CHISTYAKOV et 02.

2524

from the average modulus does not exceed ~6~o. P a y i n g attention now to OSS it m a y be noted t h a t this parameter is affected by the t y p e of plasticizer a n d its content. I t follows from curves first t h a t the addition of esters of homologous a]iphatie acid series a n d isomeric phthalic acid series has a positive influence o n OSS of the system arid the extent of this influence depends on the natural anisotropy of polarizability of plasticizer molecules. The dependence of OSS on the molar fraction of aliphatic plasticizers is curvilinear and t h a t of the aromatic ones is satisfactorily described by a linear furiction. H a v i n g a low anisot r o p y of polarizability, plasticizers of aliI)h,~tic series also have a slight effect on the birefringence of the system. For example, the effect of DBOA is 2.8

1-7

150~ 7-o

PdO'a~k~/

r

n

3

m

O4

3

1.22~

z r" " Plo.~,;,tzer ~ mole fpctchons

O.2

G 1

I.ILI

I ii i

I

l

J

••

O.lO 0.20 PIQsticizep, mo(e fraclions Fie. I. Dependence of

glass temperature (u,) "rod density (b) or~ the composition

of mixtures, a--PMMA-DBSA (1), PMMA--DBOPA (2); PDOP-DPOPA (3). Curves of mixtures of PMMA with DBOA, DBAA, DBSuA, DBIPA, DBTPA are arranged insido the shaded part; b--PMMA-DBSuA (1), PMMA-DBTPA (2); PDTP-DPSA (3), PDTP-DPOPA (4), PDTP-DPTPA (5); PDOP-DPOPA (6). Curves of mixtures of PMMA with ])BOA, DBSA, DBAA, DBOPA and DBIPA are inside tho shadod part.

Photoelastio properties o f plasticized polymers

2526

TPa-*, while t h a t of DBSA is less* than ~ 2.65 TPa -1, The latter value is somewhat unexpected since two ethylene units form part of the acid residue of DBSA; as shown by Fig. 2 on further increasing --CH2-- units, the effect of plasticizer on OSS of the system increases and in the case of DBSuA, becomes 3.6 TPa -1. This is probably due to the fact that the combination of two carbonyl groups with a different number of methylene units results in conformations, for which the overall anisotropy of polarizability does not conform to the rule of additivity. EGPa

C~ , TPa-'

- 7

I

I

0"05 0-10 Plasticizep , mole fraoffons

FIG. 2. Dependence of optical and mechanical properties on the composition of mixtures at --120 °. OSS: P M M A - D B O A (1), P M M A - D B S A (2); P M M A - D B A A (3), PI~L%IA-DBSuA (4), P M M A - D B O P A (5), P M M A - D B I P A (6), P M M A - D B T P A (7); average elastic modnlus (8).

In contrast with esters of aliphatic series, the addition of a plasticizer with an aromatic acid residue of high anisotropy of polarizability, considerably increases OSS of the system (Fig. 2, curves 5-7). This characteristic is also influenced by the position of ester groups on phenyl radical. The Figure indicates t h a t a change of the ester group from the or~ho- into the para-position, the absolute value of OSS of samples changes in an increasing sequence of 6.5; 8.6 and 10 TPa-1. * Systems eontaining (> 1 mole fraction plasticizer are considered for comparison,

quaJltitative g

9526

YE. V. CHISTYAKOV e$ ~...

A similar effect was observed previously when investigating the isomeric series of polydiallylphthalates (PDP) [4] and the result obtained should therefore be regarded regular. The fact that the sequence mentioned in relative units (1; 1.32; 1.54} changes less markedly than described previously [4] (1; 1.84; 2.05) is unexpected. This m a y possibly be due to the special interaction of the plasticizer with the polymer, as a result of which the effect of the natural anisotropy of polarizability of DPOP on the overall birefringenee of the system is more marked (compared with PDP) than that of its isomers.

Co', T'Pa -t !

2~

--

0.05 0.I5 Plasticizer, mole fcucfions FIef. 3. Dependence of OSS on the composition of PDTP-DPTPA (1), PDTPDPOPA (2), PDTP-DPSA (3), PDOP-DPOPA (4) mixtures at --120 °. This problem will be examined in detail. Let us now analyse the type of variation of OSS of plasticized systems based on P D T P (Fig. 3, curves 1-3). I t is easy to see that for a polymer showing positive birefringence in the glassy state, the addition of DPSA, DPOPA, D P T P A plastieizers with positive natural anisotropy results in a reduction of OSS of the system. This result is generally understandable for samples with DPSA and DPOPA, since the natural anisotropy of polarizability of these esters is lower than the natural anisotropy of polarizability of the inter-unit chain of PDTP. The reduction of OSS of the system on introducing D P T P A is unexpected, if only because this ester is a hydrogenated monomer of D T P - - a low molecular weight analogue of the inter-unit chain. Its presence in the glassy state has no marked effect on the anisotropy of polarizability of the polymer-plasticizer mixture. We therefore carried out further investigations into the "anomaly" noted using erosslinked PDOP samples plasticized using DPOPA. Figure 3 (curve 4) indicates that even in this case OSS of the system decreases with an increase in plasticizer content. At the same time, comparing curves 1 and 4 it m a y be noted t h a t with the same molar content (0.2 mole) the hydrogenated monomer in the P D T P sample reduces OSS by 16%, while in PDOP, by only 12%. This result agrees with the view expressed that the effect on overall DVLP of the

Photoehstic properties of plasticized polymers

2527

PMMA-DBPA system of an aromatic plasticizer with an ester group in the or$ho-position is stronger than that of its isomers. Let us now deal with the physical interpretation of experimental results. In the temperature range studied the external field of force applied so the sample is counteracted by internal forces of valence t / a n d intermolecular ~ bonds and forces of elasticity of valence bonds ? of the macromolecule. However, the level of elastic forces in the polymer system varies (t/~r~p). I f to a first approximation we assume that tl/7,~lO and 7/P~2.5 [8], it may be assumed that stress in the physical bond will be at least 2.5 times lower than in a chemical bond. Based on these considerations two types of stress transfer may be considered in the polymerplasticizer system: transfer through the polymer-plasticizer bond and through the chemical bond of a continuous macromolecule.* On transferring stress through the physical bond the main effect on deformation is exerted by displacement of atoms of the physical bond and in the macromolecule and plasticizer molecule stress causes comparatively slight distortion of valence angles and even slighter variation in covalent bond length. With tho second method of stress transfer through the chemical bond deformation in the continuous macromolecule is only selected as a result of angular distortion and deformation of the chemical bond, which in this case arc significant since the stress level in the chemical bond is higher than in the physical bond. Therefore, by the action of stress in both physical and chemical bonds the distance between nuclei of atoms increases, as a result of which the cloud of electrons is mechanically polarized and a dipole moment is induced in the bonds. Furthermore, stress causes distortion of the valence angle, which changes the direction of chemical bonds in the molecular system of coordinates, as reflected by the dipole moment of the bond and the type of hybridization. Therefore, the elastic deformation of the polymer-plasticizer system cause~ controlled mechanical polarization of electrons in the macromolecule and plasticizer molecule. The effect on the birefringence of mechanical polarization is higher by an ord¢~r of magnitude than the effect on optical polarizability. This follows from the fact that for polymers the optical sensitivity factor to elastic deformation is higher by an order of magnitude on average than is the coefficient of optical sensitivity to high-elastic deformation, which is known to be proportional to the difference of optical polarizability values (a1--~2)t of the macromolecular segment [9]. It is clear from the foregoing that in deformation of the physical bond optical anisotropy is caused not only by the marked polarization of this bond, but also by polarization of electron clouds of the macromolecule and plasticizer molecule. The effect caused by a change in the state of electron clouds of the macromolecule and plasticizer molecule evidently depends on the natural anisotropy of their units. Therefore, in a PMMA-plasticizer system with negative natural anisotropy * Tho maeromolecule-macromoleculephysical bond does not provide further in-formation. t When ~ ~'s--~a.

2528

YE. V. C~xs~rtwov eta/.

o f t h e polarizability of PMMA even a slight distortion of valence angles a n d bonds o f t h e plasticizer molecule as a result of stress o f t h e physical b o n d has a m a r k e d effect on t h e overall birefringence of t h e system*. I n t h e light of these ideas it becomes clear w h y in systems plasticized b y h y d r o g e n a t e d m o n o m e r s overall birefringence is lower t h a n t h a t observed in non-plasticized polymers. Although it has the same s t r u c t u r e as t h e m o n o m e r r e s i d u e of the i n t e r - u n i t chain, interacting w i t h the n e t w o r k via physical units t h e plasticizer molecule is u n a b l e to compete with this residue in r e g a r d to t h e effect on birefringence. A t the same t i m e in t h e isomer series of a r o m a t i c plastieizers D P O P A has a more m a r k e d effect on the birefringence of the system t h a n D P T P A since w h e n ester groups in a benzene nucleus are a t a n angle of 60 ° stress via the physical b o n d deforms this angle, whereas in t h e case of D P T P A t h e possibility of this d e f o r m a t i o n is excluded. I t follows therefore, t h a t the plasticizer i n t r o d u c e d into the p o l y m e r is, no doubt, a c o m p o n e n t which actively p a r t i c i p a t e s in stress distribution via molecular interaction a n d is capable of influencing t h e overall birefringence of po]ymers u n d e r stress, t h e e n d result of this effect largely d e p e n d e n t on t h e ratio between a n i s o t r o p y values of the polarizability o f t h e plasticizer a n d units forming the macromolecule. Translated by E. SEME~ REFERENCES

~1, N. I. PRIGOROVSItll and Ye. N. FII~IMONOVA, (book) Trudy VII Vsesoyuznoi konferentsil po polyarizatsionno-optichcskomu metodu issledovanii napryazhenii (Proceedings of the VII All-Union Conference on the Polarization.Optical Method of Investigating Stress}. edited by Kh. K. Pben, Tanlu, Izd. AN ESSR, vol. 2, 95, 1971 2. Slovar' organieheskikh soyedinenii (Dictionary of Organic Compounds). edited by I. Kheil'broni and G. M. Banbury, 322, 475, 647, 664, Izd-vo inostr, lit., Moscow, vol. 3, 1949 3. Spravochnik khimika (Chemist's Handbook). 398, 778, Khimiya, Moscow-Leningrad, vol. 2, 1965 4. B. M. ZUYEV, Vysokomol. soyed. A12: 730, 1970 (Translated in Polymer Sci. U.S.S.R. 12" 4, 822, 1970) 5. M. M. FROKHT, Fotouprugost' (Photoelasticity). Gostekhizdat, Moscow-Leningrad, 1: 351, 1948 6. B. M. ZUYEV, S. G. STEPANOV and A. A. KORGOV, (book} Issledovaniya po teorii plastin i obolochek (Studies of the Theory of Sheets and Shells}. p. 550, Izd. KGU, Kazan, 1966 7. B. M. ZUYEV, Dis. na soiskaniye ueh. st. kand. tekhn, nauk (Discussion concerning * It should be noted that the cause of negative path difference for PM:MAin the temperature range below T~is not explained in the literature. It may be assumed, however, that this effect arises as a result of the disorientation of the angle between the direction of the overall component of the dipole moment of the macroraolecular unit and the direction of maximum optical polarizability [10].

2529

Interaction of sols of P S A with P V P

a Competition for a P o s t . g r a d u a t e Degree in Technical Sciences), p. 107, K K h T I ira. S. M. Kirova, Kazan, 1968 8. W. H. STOCVKMAYER and C. E. HECHT, J. Chem. Phys. 21: 1953, 1954 9. A. V. TURAZYAN, V. P. NETREBKO and A. L. RABINOVICH, Mekhanika polimerov, 923, 1975 10. M. V. VOLKENSHTEIN, Stroyeniye i fizioheskiye svoistva molekul (Structure and

Physical Properties of Molecules), p. 312, Izd. A N SSSR, Moscow-Leningrad, 1966

:PolymerScience U.S.S.R. Vol. 28, No. 10, pp. 2529-2544, 1981

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INTERACTION OF SOLS OF POLYSILICIC ACID WITH QUATERNIZED POLY-4-VINYLPYRIDINES* L. N. Y~A~OVA, Yr. G. FROLOV,V. A. KAmAr~r~, A. B. Zxzn~ and V. A. K A ~ O V M. V. Lomonosov State University, Moscow (Reck/red 15 August 1980) A s t u d y was made of the interaction of sols of polysilicic acid (PSA) with polyN.ethyl-4-vinylpyridinium bromide (Q-PVP) in dilute solutions. I t was shown t h a t in weak alkaline and neutral m e d i a (7 ~ p H ~ 9 ) all Q-PVP fractions studied form insoluble polycomplexcs with P S A sol, while in acid media (pH 3) t h e solubility of polyeomplexes depends on the molecular weight of t h e polycation. A s t u d y was made of the composition of polycomplexes formed and the t y p e of variation of these compositions according to medium pH. I t was established b y potentiometric titration t h a t t h e interaction between Q-PVP and P S A sol is electrostatic, the number of salt bonds form e d b y quatel~aary nitrogen atoms of Q-PVP and ionized silanol groups of P S A sol, increasing with an increase in pH. The dependence on m e d i u ~ p H of the degree of conversion in the intermacromolecular reaction of Q - P V P - P S A sol was calculated for various polycation fractions. Equilibrium systems were proposed and described for t h e association of t!m polyeation and PSA sol particles, resulting in the formation of polycomplex particles.

The study of interaction of polysilicic acid sols with polymer cations is mainly of interest from the point of view of understanding processes taking place during the adsorption of macromolecules on interphase boundaries. Similar investigations are also important from an applied point of view, as they are closely related to effects such as flocculation and stabilization of colloids examined within the framework of the stability theory of colloid systems. On the other hand, * Vysokomol. soyed. A28: No. 10, 2328-2341, 1981.