The mechanism of the low-temperature dielectric relaxation of polymers containing low molecular weight impurities

THE MECHANISM OF THE LOW-TEMPERATURE DIELECTRIC RELAXATION OF POLYMERS CONTAINING LOW MOLECULAR WEIGHT IMPURITIES* T. I. BORISOVA and V. N. Cn~KOV High Polymers Institute, U.S.S.R. Academy of Sciences

(Received 1 February 1972) The temperature-frequency functions for the dielectric losses a n d dielectric constants of low molecular weight substances (methyl methacrylate, anisol} present in a polymer matrix (polystyrene, polymethylmethacrylate, polyphenylsilsesquioxane) have been investigated. A small molecule was found to be a kinetmally independent u n i t of molecular movement. The fact that the dipolar polarization parameters of low molecular weight compounds presence m the polymer matmx were Identical with some of the processes in polymers containing low molecular weight lmpurltms resulted m the conclusion that the chpolar polarization mechamsms are the same m both cases.

THE effects of low molecular weight (mol.wt.) additions on the relaxation properties of polymers were studied in great detail in the range near and above the glass temperature (Tg), but very little is known about the behaviour of plasticized systems at low temperature. The studies of the latter by various relaxation methods normally centred on the effects of low mol.wt, compounds on the mobility and relaxation characteristics of polymer chain segments. Very much less is known about the effects of such compounds on the movements of kinetic units including branches, or of the corresponding local movements of the main chain. The reason is primarily t h a t the changes in the relaxation periods and in the activation energy of the process is less distinct in the glass-like polymer in the presence of the plasticizer t h a n during the transition to the highly-elastic state, and secondly that even small additions of low mol.wt. compounds to the polymer often cause the relaxation ranges of group and segmental motions to overlap, so that their separation becomes mostly impossible, even when the low-frequency measurement technique is used. Furthermore, various methods showed the relaxation processes to be present at low temperature in m a n y polymers (from --70 to --150°C) [1-11], and t h a t its distinguishing characteristic is a shift towards higher temperatures, i.e. to larger relaxation periods, and also an increase in activation energy when the content of low mol.wt, compound is increased. This behaviour is the re* Vysokomol. soyed. A15- :No. 10, 2304-2309, 1973. 2606

Low-temperature dielectric relaxation of polymers

2607

verse of normal relaxation processes, where the segmental- and group-type relaxations shift towards lower temperatures as plasticizer is added, while the activation energy normally becomes smaller. It was thought that the process progresses by an irregular ("head-head" type) addition of macromoleeular bonds [1, 2, 5] with an inhibiting action of the plasticizer on the rotation or gyration of the branches [3, 4, 6, 7, 11]. It was also assumed Chat the relaxation temperature range from --70 to --150°C could be due to the presence of impurities in the polymer [8, 9]. The study of linear polystyrene containing some polar solvent had shown [12-14] that the dipolar polarization relaxation period due to the movement of small molecules added to the polymer is situated in the same temperaturefrequency range. This will shift towards higher temperatures when the concentration of low mol.wt, substances is increased and the activation energy of the respective type of polarization will also increase. Thus there is agreement between the mechanism of the relaxation of low mol.wt, substances polarization in polymers and that of the dipolar polarization at low temperature of the polymers. This raises the question of the type of polarization mechanism existing in the system polymer-low mol.wt, compound, whether it is due to the motion of part of the polymer chain or to that of the molecule of the low tool.we, compound present in the polymer as medium. We tried to answer this question by adding low mol.wt, compounds to various polymers (polar and non-polar, with flexible and rigid branches, linear and erosslinked, etc.), while at the same time varying the reaction type between the polymer and the low mol.wt, compound. We investigated for the above purpose the dipolar polarization relaxation of methyl methacrylate (MMA) which was added to its polymer (PMMA), of anisol (An) to crosslinked polystyrene (PS) or to polyphenylsilsesquioxane (PPSO); the obtained results were then compared with those of PS-MMA [13] and PS-Au studies of dipolar phenomena [12]. EXPERIMENTAL PMMA was produced by radmal polymerization; its M ~ = 5 X 105, and it was purified by repreclpltatlon from benzene with methanol, whmh removed the low mol.wt, adrmxtures. MMA addition method to the polymer and preparation of samples for the dieleetrm measurements was the same as that used before [12]. The crosshnked PS was produced by copolymemzmg styrene and dlvmylbenzene in the presence of benzoyl peroxide between glass plates with a predetermined gap between them. The crosshnkmg denmty was varmd m the range 0.8-6.0 mole~,,. The PPSO was synthemzed m the Andrlanov laboratory. Unfractlonated samples (~ll= 3 X 10 e) were used as films, whmh had been prepared from benzene solutions. Tile films of crosshnked PS and PPSO with an An content were prepared by holding the polymer films m An vapours tmt]l the reqmred saturation was reached; thin was followed by alumlnmm fell electrodes pressed on the surface. The samples were stored several days m a hermetmally sealed vessel to obtain a homogeneous dlstrlbutmn of the plasticizer 1~1 the sample. The MMA content of PMMA was varied from 10 to 7 0 ~ w]w, t h a t of An m crosshnked PS from 17 to 50 m o l e ~ , and t h a t of An in PPSO from 6 to 1 5 ~ w/w.

T. I. BoRisovA and V. N. CHIRKOV

2608

The dmlectrm constant and dmlectrm loss measurements were carrmd out at 0.4200 kHz frequencies m the range from --180 to 20°C. The temperature-frequency dependence of tan 5 was used to calculate the most probable relaxation period v and the activation energy U of the respective polarization processes. RESULTS

Figure l a illustrates as an e x a m p l e t h e t a n 5 = ~ (T) functions of systems P M M A - M M A a n d P S - M M A c o n t a i n i n g similar m o n o m e r concentrations. As t o composition, t h e effects of dipolar polarization of t h e p o l y m e r molecules c a n be

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Fro. 1. The temperature dependence of tan 5 of system: a--PMMA-MMA (1) and PS-MMA (2); b--crosshnked PS-An (1), and PS-An (2) in the low temperature range of dipolar polarization relaxation at frequencms of a--1 kHz; b--2.5 kHz. The MMA content in PMMA and PS was 19~, that of An m the erosslinked PS 2 5 ~ and in PS 28~.

seen on the high t e m p e r a t u r e side as the rise on t h e curves a t - - 3 0 a n d 0°C for t h e P M M A a n d P S s y s t e m s respectively. A t m o n o m e r c o n t e n t s greater t h a n 30 m o l e % the dipole losses due to the p o l y m e r m a t r i x a p p e a r e d as a m a x i m u m of t a n 5 in this t e m p e r a t u r e - f r e q u e n c y range. T a n 5max shifted t o w a r d s lower t e m p e r a t u r e s on increasing the MMA c o n t e n t (Fig. 2, c u r v e 2) a n d decreased at the same time (curve 1), while the a c t i v a t i o n e n e r g y of polarization was ind e p e n d e n t of the m o n o m e r content, i.e. was 20-22 kcal/mole. The low t e m p e r a t u r e dipolar polarization r e l a x a t i o n is well resolved in syst e m s P S - M M A a n d P M M A - M M A a t greater t h a n 10 m o l e % MMA c o n c e n t r a -

Low-temperature dmleet,rm relaxation of polymers

2609

tions and is visible m t h e range - - 7 0 to --150°C as a t a n 6max. The l a t t e r a n d t h e U-value of t h e low t e m p e r a t u r e range b e c o m e larger as t h e MMA c o n t e n t is increased (Fig. 3). The c o n c e n t r a t m n d e p e n d e n c e of t h e most probable ~-values is e x t r e m a l in n a t u r e (Fig. 3) a n d similar to t h a t of the o t h e r systems [12-14] ~QX ~O~

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Fro. 2. The dependence of tan ~ax (1) and of its temperature posltmn 'T~a~ at 1 kHz (2) on the MMA c~ntent by weight m the PMMA-MMA system (dipole losses of the polymer matmx). T a n 6max for b o t h systems fall on a single curve for the 0-60 mole°,~) MMA c o n c e n t r a t i o n range. The p a r a m e t e r s characterizing t h e molecular mobility of the m o n o m e r , i.e. U and r, d e p e n d on t h e p o l y m e r t y p e . The U-value of P M M A MMA is larger b y l O - 1 5 % t h a n t h a t of s y s t e m P S - M M A over almost the whole

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~IG. 3. [['he tan 6max dependence at: 1, 1'--1 kttz; 2, 3--the aetlvatmn energy; d, 5--the relaxatmn permd at --105°C, on the MMA content m system: 1, 2, d--PMMA-MMA, 1' 3, 5--PS-MMA (low temperature relaxatmn range). c o n c e n t r a t i o n range. The c o n c e n t r a t i o n dependences of the most probable z of b o t h systems calculated for a c o n s t a n t t e m p e r a t u r e (--105°C) showed t h a t u p to 35 m o l e ° MMA in PMMA the values of w were a b o u t 10 times greater t h a n for t h e PS-MMA. A t a b o v e 35 m o l e ~ MMA content, w h e n w begins to decrease as the m o n o m e r c o n t e n t increases in t h e medium, the r e l a x a t i o n periods of b o t h the systems b e c o m e similar. J u d g i n g f r o m t h e half-width of t h e t a n 6 = 9 (T) curves, the r e l a x a t i o n t i m e s p e c t r u m of P M M A - M M A is slightly broader.

T. I. BORISOVAand V. N. CHYRKOV

2610

The dipolar relaxation parameters of the MMA molecules in the PMMA or PS matrix are thus fairly similar (e.g. the ~ n 5max curves as functions of the molar content of monomer x2), or hardly distinguishable from each other (U and v). One can therefore assume that the dipolar polarization of MMA is due to the movements of the monomer molecules arid not to complexes with groups belonging to the branches on the macromoleeular chains. The movements of the chain segments in the matrix are frozen under the observation conditions of the low-temperature dipolar relaxation process. The kinetic units retaining movement at these temperatures can be produced b y the appendages (or b y its par~ when the branch is flexible), or b y main chain sections smaller than the kinetic segment. I f polarization relaxation had been due to the motion of a MMA-branch complex, the different nature of the latter in PS or PMMA would cause a large difference of the parameters in the range of relaxation studied, which is not the case according to our results.

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FzG. 4. The tan 5=ax dependence at: 1 - 3 - - 2 . 5 kHz; 4-6--the actlvatmn energy; 7-8--the relaxatmn permd at --94.5°C, on the An-content of systems: 1, 4, 7--crosshnked PS-An; 2, 5, 8--PS-An; 3, 6--PPSO-An (low temperature range of relaxatmn). The possibility of a participation, in the dipolar polarization, of complexes formed between the low mol.wt, compound and the groups making up the main chain of the macromoleeule, was studied on systems PS-An, crosslinked PS-An, and PPSO-An. All three polymers have the same branches b u t differing kinetic properties of the main chain. The shoulder in the range --130 to --180°C for the low temperature process and the systems of erosslinked P S - A n and of P S - A n is due to the intramoleeular movements in anisol [12], as Fig. lb shows. Similar dependences for the system P P S O - A n have the same qualitative character. Tan (~max of system P S - A n at 10°C is linked with the segmental dipole processes in the plasticized PS. In the crosslinked PS tan 5max of the segmental dipole losses is displaced towards temperatures above 20°C and is not shown in Fig. lb.

Lo~v-temperature dmlectrmrelaxatmn of polymers

2611

The dependences of tan 5 and U of the low temperature process on concentration is shown in Fig. 4 for the system crosslinked PS-An and is compared with the same dependence of the linear PS-An system [12]. The differences in crosslinking density (from 0.8 to 8.0 mole%) were found not to affect the parameters of the relaxation process connected with the molecular motion of anisol (entire molecule) at identical plasticizer content. The dependence tan 5 ~ q (x2), and also lJ'~ ~ (x2) (x2--molar fraction of An) was identical for both the systems (Fig. 4). This relaxation process in crosslinked PS-An at low An concentrations is situated at slightly higher temperatures than that for system PS-An. This difference, however, is less distinct in this case than that between the relaxation parameters of the PMMA-MMA and PS-MMA systems. The existence of transverse crosslinkages between macromolecules in the polymer matrix (below 8~o) does not affect the mobility of molecules in the low mol.wt, compounds as a whole, because the values of v and U are the same for the systems with linear and crosslinked PS. The values of ~, U and tan 5max in the range of dipolar polarization relaxation of An in PPSO (the mol.wt, of the repetitive PPSO unit is taken to be 129 and corresponds to the part of molecule associated with one branch) also falls on the curves for PS-An and crosslinked PS-An at the corresponding concentrations. Regardless of the obvious difference in the kinetic properties of the main chain, the dielectric relaxation parameters of the An molecule in the matrices of the particular polymers were found to be the same. Kinetically independent complexes between solvent molecules and groups within the main chain of the polymer matrix thus do not form. Where hydrogen bonds are likely to form in the systems between the macromolecules of the polymer matrix and the molecules of the low mol.wt, compounds, their cooperative movements will possibly be substantially larger. An analysis of the quantitative characteristics of the dielectric relaxations in systems PMMAMMA, PS-MMA, and also PS-An, crosslinked PS-AN and PPSO-An support the view that the polymer matrix does not participate in the movements of the low mol.wt, compound molecules. Some difference in the values of the activation energies (for systems PSMMA and PMMA-PMA), the relaxation periods, and the width of the polarization spectrum of low mol.wt, compounds is evidence for the structure of the polymer matrix also having some effect on the movements of small molecules. The latter will be most noticeable where the plasticizer content is low (less than 30-35~/o), as an increase of the concentration of small molecules will cause the formation of solvating layers in the polymer and the spontaneous aggregation of the low mol.wt, compound molecules [13]. The difference in the reaction between the polymer and the low mol.wt, compound can be considerable in this concentration range. The higher values of U and ~ and the wider r-distribution in system PMMA-MMA, compared with PS-MMA, could be the result of a dipole-dipole interaction of the polar plasticizer with the polar polymer. The similar values of U and ~ in the cases of PS-An, erosslinked PS-An, and PPSO-

2612

T. I. BoRIsovA and V. :N. CHI~xov

A n could indicate a d o m i n a n c e of t h e p h e n y l ring reactions in p o l y m e r molecules with those of An. A t larger t h a n 35% c o n t e n t of small molecules, a t which t h e low mol.wt. c o m p o u n d s could f o r m associates, t h e s t a t e a n d t h e properties of t h e s y s t e m could d e p e n d on t h e macroscopic properties of t h e f o r m e r [15]. T h e same low mol.wt, substances will h a v e similar r e l a x a t i o n periods in different polymers, a n d these will decrease as t h e c o n t e n t of t h e f o r m e r increases in t h e system. This change of t h e m e c h a n i s m of r e a c t i o n is p r o b a b l y due to t h e f r a c t u r e of t h e physical n e t w o r k of bonds present in t h e polymer. Earlier studies of various polar low mol.wt, c o m p o u n d s in non-polar PS h a d shown t h a t t h e r e l a x a t i o n characteristics of polar molecules h a v i n g differing s t r u c t u r e s d e p e n d on t h e i r chemical composition a n d shape [13, 14]. T h e same was also established for t h e values of U a n d T in t h e studies of low t e m p e r a t u r e processes, as well as for t h e respective peaks of t h e r e l a x a t i o n losses, a l t h o u g h t h e s t r u ( t u r e of t h e p o l y m e r also p l a y e d a p a r t here [3, 4]. One can assume on t h e basis of identical m e c h a n i s m s of t h e dielectric r e l a x a t i o n process e x a m i n e d here in tim p o l y m e r s a n o m a l o u s l y affected b y t h e plasticizer with respect to r e l a x a t i o n periods, a n d t h e r e l a x a t i o n of low mol.wt, molecules present in t h e p o l y m e r m a t r i x , t h a t t h e p r o p o r t i o n a l i t y of t a n 5max to t h e a m o u n t of low mol.wt. c o m p o u n d added, the identical t y p e of effect of the low mol.wt, c o m p o u n d a n d of p o l y m e r s t r u c t u r e are due to t h e m o v e m e n t of t h e low mol.wt, molecules witho u t macromolecules participating, b u t h a v i n g an inhibiting effect on t h e movem e n t , which depends on t h e s t r u c t u r e of t h e p o l y m e r as well as on t h a t of t h e low mol.wt c o m p o u n d . Translated by K. A. ALLE~REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

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