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Anti-plasticization of polymethylmethacrylate
Polymer Scieace U.S.S.R. Vol. 26, No. 8, pp. 1806-1"813, 1984 Printed in Poland
0032-3950/84 $10.00+ .00 C 1985 Pergamon Press Ltd.
ANTI-PLASTICIZATiON OF POLYMETHYLMETHACRYLATE* I. N. RAZINSKAYA,B. P. SHTARKMAN,V. A. IZVOZCHIKOVA, N. Yu. AVERBAKH, I. M. MONICH, L. P. BUBNOVA and N. I. PUPUKINA (Received 22 December 1982) Using PMMA to which were added phosphoric acid esters the authors have studied the anti-plasticization of a linear amorphous polymer. They measured the temperature dependenee of the elasticity modulus, the molecular mobility of the system, its p-v-T-diagram and the rheological properties and calculated the free volume. It is shown that the reason for the appearance of the anti-plasticization effect in PMMA is the inhomogeneity of structure and, as a result, the uneveness of the distribution of the plasticizer in the polymer.
RELATIVELY small amounts of plasticizers (down to 10-15 ~ by weight) in a number o f cases simultaneous with fall in the glass transition point Tg raise the rigidity and brittleness of the system [1-3]. This phenomenon has been termed the anti-plasticization effect [4]. The causes of its manifestation and mechanism have not been fully clarified and have given rise to lively discussion. There is dual interest in this problem. On the one hand, this is due to the fact that small additions of plasticizers are often used to solve tasks of practical importance associated with modifications of varied nature of polymers (improving their processing qualifies, reducing combustibility, etc.) and, on the other, the causes of the anti-plasticization effect concern fundamental aspects of the organization of polymers and therefore their proper evaluation is of great importance. Anti-plasticization has, in the main, been studied in polymers prone to crystallization, which largely influences the mechanism of raising rigidity. It was therefore of interest to examine this effect in non-crystallizing polymers. The aim of the present work was to study the phenomenon of anti-plasticization in relation to such a typical amorphous glassy linear polymer as PMMA. The author of [5] was the first to refer to the anomalous behaviour of the strength o f plasticized PMMA. Later, the extremal dependence of the mechanical properties o f P M M A when small additions o f phthalic acid esters were introduced into it was observed in studies [6]. However, the reasons for this phenomenon'in relation to P M M A have not been studied in detail. In this work some phosphoric acid esters served as plasticizers. Their choice was dictated primarily by their use as anti-pyrenes for imparting fire resistance to P M M A [7]. In addition their plasticizing effect was studied in detail in reference [8]. * Vysokomol. soyed. A26: No. 8, 1617-1622, 1984. 1806
Anti-plasticization of polymethylmethacrylate
1807
TABLE 1. CHARACTERIIgTIC$OF PHOSPHORIC ACID ESTERS
Plasticizer Tris-(2-chloroethyl) phosphate (TCEP) Tris-(2-chioropropyl) phosphate (TCPP) Tris-(1,3-dichloroisopropyl) phosphate (TCIPP) Bromoethylbromopropylchloropropyl phosphate (BECP) 2-Bromoethyl-(1,3 -dichloroisopropyl) (1,3-bromochloroisopropyl) phosphate (BECIP)
M
Vx 103, nms
Tp, K
AE, MPa
285.5 327.5
136.6 186.0
355-385 379-393
280 490
431.0
163-6
398--415
370
402.5
170.0
429-447
390
471.5
155.0
425--449
300
Note. The values of Tp and the intrinsic volume of the molecules V are taken from [8].
Table 1 lists the plasticizers studied and some of their properties. The plasticized compositions were obtained on mixing these plasticizers with PMMA of LSOM grade ( M = 1.02 × l 0 s) in the m e l t at 453-463 K for 15 rain in the mixing chamber of the Brabender plastograph. The elasticity modulus E was determined from the compression curves obtained with a modernized relaxometer [9] at different temperatures; the rate of loading was 2.5/zm/sec. The dielectric measurements were made with a two-electrode measuring cell on the TR-9701 bridge over the frequency range 55-2 × l0 s Hz and temperature range 123-433 K. The compressibility of plasticized PMMA w a s s t u d i e d in the isothermal regime in a cell o f t h e c o m p r e s s i o n type with the a p p a r a t u s described in [10]. The viscous properties of the melts were evaluated with the Flowtest capillary viscometer in the shear rate range 10-104 sec -1 in the temperature region 453-530 K. The P M M A compositions studied with the listed phosphates in the glassy state display an extremal concentration dependence of E in the region 3-5 ~o by wt. of the plasticizers (Fig. 1), i.e. an anti-plasticization effect is observed. The Tg values of P M M A monotonically fall with rise in the content 9 of the plasticizers in the system. Taking as the measure of anti-plasticization the difference between the m a x i m u m elasticity modulus of plasticized P M M A and that of the initial polymer zfE, then for the phosphates studied we get fundamentally different positive values of AE amounting to 1018 ~o of the initial E values (Table4lt). The value of the anti-plasticization effect for the objects chosen by us was found not to be related to the degree of affinity of the plasticizer for the polymer which was characterized by the temperature of dissolution Tp (Table 1) and was directly dependent on the intrinsic volume of the plasticizer molecules the larger the molecule the greater the rise in the elasticity modulus of the system. On passing from the glassy to the highly elastic state the elasticity modulus sharply dropped by more than two orders of magnitude (Fig. 2). It is known that this transition is of a relaxation character and the temperature interval of the transition A T reflects the cooperativeness of the relaxation processes [11]. I f it is considered that the introduction of a plasticizer lessens the intermolecutar interaction in the polymer than rise in .4 T on introducing up to 5 Yo T C E P by wt. into P M M A (Fig. 3) points to increase in the size of the kinetic units the movement of which determines the transition to the highly elastic state.
1808
I . N . RAZINSKAYA et al.
As is clear from Fig. 3 (curve 1), AT/A log E passes through a maximum in the region of 5 % TCEP by wt. after which the interval of the transition narrows and remains unchanged to ,-~ 15 % by wt. of the plasticizer. Further increase in AT occurs in that region of composition where the "free" plasticizer appears [12]. Apparently, in this case it serves as a kind of lubrication allowing larger elements of structure to take part in the movement. E.IO~ MPa
tosE fMPaJ
I
+++~
3
2
! 10
20 333
FIO. 1
373
413 ~ K
FIG. 2
FIG. I. Concentration functions of the elasticity moduli of PMMA plasticized with TCEP (1), TCPP (2), TCIPP (5), BECP (4) and BECIP (5). FIG. 2. Temperature dependence of the elasticity moduli of initial PMMA (1) and polymer containing 3 (2), 5 (.3), 7 (4), 10 (5), 15 (6) and 20% (7) TCEP by wt.
The character of such an extremal dependence also persists for another plastic i z e r - BECP (Fig. 3, curve 2). Consequently, it ~ be assumed that this phenomenon is of a general nature and indicates that small additions of plasticizer produce clearly perceptible changes in the structure of PMMA. It was, therefore, of interest to study the molecular mobility of plasticized P M M A characterized by the dielectric method. Measurements of the temperature-frequency dependence of the factor of dielectric losses e" showed that at a frequency 55-200 Hz two relaxation transitions are resolved corresponding to the 0~ and fl processes in PMMA. The temperatures of the maxima Tin,= of the ~t and ff transitions are presented in Table 2. For the 0t process monotonic fall in Tm=x is observed which corresponds to fall in T= of the system while the introduction of up to 15 % TCEP by vol. into P M M A does not affect the fl process and its value and T=,= do not change as compared with the initial PMMA. Similar results have been obtained for the PMMA-dibutyl phthalate system.
Anti-plasticizatiort of potymethylmethacrylate
1809
'Consequently, the phenomenon of anti-plasticization in P M M A is not associated with mobility of the side groups and portions of the macrochains governing//-relaxation in this polymer. Let us now look at the parameters of molecular motion of the portions of the macro~ehains determining the high temperature relaxation process in P M M A depending on fits TCEP content. Relaxation z and the activation energy of the process U serve as such parameters. These parameters were determined from the temperature-frequency functions e", U was calculated from the temperature dependence of the frequency fma, and z from the ratio z = 1/2nfma, where fm,x is the frequency at which e" passes through a maximum. The activation energies are presented in Table 2. Figure 4 gives .the dependence of r at different temperatures on the content of TCEP in PMMA. As may be seen, U passes through a maximum in the region of content of 3 % TCEP b y vol. As for z in the region of the glassy state (333-343 K) it passes through a maximum in the same regiofi of compositions and on passing to the highly elastic state begins t o fall appreciably only after introducing 8-10 % TCEP by vol. Thus, investigation of intramolecular mobility shows that the addition of small ~amounts of plasticizer leads to straightening and enlargement of the portions of the AT AlogE
tc~ 'r ~sec]
.9 -4'
7 -S 5
i
t~%%bgwt.
5
5
7~
FIG. 3
es~o,*~bgvoZ
FZG. 4
Fro. 3. Concentrat ion funct ions of A T/A log E for PM MA plasticized with TCEP (1) and BECP (2). FIG. 4. Concentration functions of the relaxation times v for P M M A with TCEP at 333 (1), 343 (2), 353 (3), 363 (4) and 483 (5) K. T A B L E 2. CHARACTERISTICS OF THE RELAXATION PROCESSES OF
PMMA
AS A FUNCTION OF THE CONTENT
~a OF TCEP (frequency 55 Hz) ¢p,% by vol.
!
T~,K
r~,K
b
I i
!
387 386 376
320 320 323
U, k J/mole 87 106 84
% by vol. 10 15
364 356
319 321
86 85
1810
,
I. N . RAZ~NSKAYA et al.
maerochain responsible for the =-transition in PMMA. This conclusion agrees with the above results of study of the temperature interval of vitrification of the system. To study the structural state of the polymer matrix on adding small amounts o f plasticizers we examined compressibility i.e. the p--v-T-diagram of PMMA plasicized with small amounts of TCEP. Figure 5 presents the dependence of Tg determined from the dilatometrie curves on pressure for mixtures of PMMA with different contents o£ TCEP.
"'
A
~00,
3
I
t
I,
170
2~0
3£0
FIO. 5
60
120
180 240 p, MPo
FIO. 6
FIG. 5. Dependence of T= on pressure for initial P M M A (1) and P M M A containing 3 (2), 5 (3) and 10~, (4) TCEP. FIG. 6. Free volume fraction f , as a function of pressure for initial P M M A (1) and polymer c o n taining 3 (2) and 5 ~0 (3) TCEP.
The presence of a horizontal portion of the curve zip for PMMA is related to the inhomogeneity of the polymer as a result of which it is primarily compressed through the loose regions, which does not lead to change in the intermolecular interactions and. therefore does not influence T~ [13]. Thus, zip serves as a kind of criterion of the degree of homogeneity of the polymer. Addition of TCEP to the system leads to substantial change in alp: zlp passes through a maximum in the region 3 ~o TCEP by wt., then falling on adding the plasticizer. Consequently, 3 ~o TCEP by wt. raises the degree of inhomogeneity of the system as compared with the initial PMMA. Calculations of the free volume fractionfc for Ts (after Simkha and Boyer) showed that f~ for the system with 3 ~ TCEP by wt. is somewhat higher and 5 ~ lower than in the initial PMMA (Fig. 6). According to the ideas of Lipatov and Privalko [14] increase in the free volume fraction is linked with rise in chain rigidity. It may be assumed that small additions of TCEP (,-~3 ~o by wt.) help the polymer macrochains to assume more elongated conformations raising the rigidity of the system. Thus, the results of study of compressibility are in complete accord with the conclusions which follow from evaluation of the parameters of molecular mobility presented above.
Anti-plasticization of polymethylmethacrylate
181I
Consequently, increase in the degree of inhomogeneity of P M M A on adding small portions of plasticizer is linked with the uneven distribution of the latter. Apparently, the small additions of plasticizer primarily enter the loosely packed regions of t h e polymer in which the relatively free chains on interaction with a good plasticizer ( T C E P ) acquire more elongated conformations and therefore become more rigid.
P'toJ, ks/'
/
log, [Pa.sec.]
ii
1.2E / 1
//
r /~jf i i
,,i
I
I 1.18
i
~
i
I
J
ao Fio. 7
FIG. 8
Flo. 7. Concentration functions of density p of PMMA plasticized with. TCEP (1) and TCPP (2). Broken lines, additive density values. F~G. 8. Concentration functions of viscosity r/of plasticized PMMA at 483 K and shear rates 0 (1, 2), 102"s (3) and 103"s (4) see-L A-PMMA with TCEP, B-with TCPP. However this is not the only cause of the anti-plasticization effect. Let us look at the problems of the packing of the polymer when small additions of plasticizers are introduced into it. Figure 7 presents the dependence of the density of P M M A on its content of two plasticizers- T,~EP and TCPP. As may be seen, the experimental points in both cases lie above the values of additivity. If it is borne in mind that the free volume fraction of P M M A on introducing plasticizers into it increases [8] then rise in density must be related to a certain pre-ordering of the polymer structure tlarough passage of some of the fragments of the macrochain to the dense regions. This statement is also supported by the results of rheological investigations. O n introducing 1-2% TCEP and TCPP by wt. into P M M A the viscosity values pass through a distinct maximum the value of which falls from 0 to 3 × 103 sec- ~ with rise in the shear rate (Fig. 8). The extremal character of the concentration dependence o f viscosity persists in the whole temperature interval from 473 to 503 K and is smoothed out on exposure to high shear stresses. Thus, the ~auses of the anti-plasticization effect in P M M A are associated with changes in the structure of the polymer under the influence of small additions of plasti-
1812
I . N . RAZINSKAYA et aL
¢izer. It is known that the structure of amorphous polymers varies. These inhomoge= neities are of a dual character: regions exist more tightly packed apparently as a result o f the local parallel packing of the chain segments and looser portions between them with relatively free and disordered packing of the segments of the macrochains. It is therefore pertinent to speak of a density spectrum in polymers. The differences in the local densities for P M M A obtained from the data on small angle X-ray scatter amount
to ~1.5% [15]. From the results it may be concluded that small additions of plasticizer affect both types of iniaomogeneities of the P M M A structure. In the loose regions the relatively free chains on interaction with a good plasticizer assume more elongated conformations a n d become more rigid with simultaneous pre-ordering of the structure as a result of the passage of some of the fragments of the macromolecules to the denser regions with mobility operating at the level of the segments. Consequently, the prime cause of the appearance of the anti-plasticization effect in amorphous linear polymers is the inhomogeneity o f their structure and as a result t h e uneveness of the distribution of the plasticizer in the polymer matrix. Local rise in the mobility of the portions of the macrochains leads to structural rearrangements producing extremal change in the physicomechanical properties. Translated by A. CROZY
REFERENCES
1. B. P. SHTARKMAN, Plastifikatsiya polivinilkhlorida (Plasticization of Polyvinyl Chloride). p. 215, Khimiya, Moscow, 1975 .2. P. V. KOZLOV and S. P. PAPKOV, Fiziko-khimicheskiye osnovy plastifikatsii polimerov (Physicochemical Bases of Polymer Plasticization). p. 124, Khimiya, Moscow, 1982 3. I. I. PEREPECHKO, Akusticheskiye metody issledovaniya polimerov (Acoustic Methods of Investigating Polymers). p. 128, Khimiya, Moscow, 1973 -4. W.J. JACKSON and J. R. CALDWRLL, Advances Chem. Ser. 48:185, 1965; J. Appl. Polymer Sci. 11: 211, 1967 5. P. GHERSA, Modern Plast. 36: 135, 1958 6. A. V. YEFIMOV, P. V. KOZLOV and N. F. BAKF~YEV, Dold. Akad. Nauk SSSR 230: 639, 1976: A.V. YEFIMOV, Z. S. BELOKON' and N. Ya. GI~KOVA, Vysokomol. soyed. B17: 171, 1975 (Not translated in Polymer Sci. U.S.S.R.) "7. I. N. RAZINSKAYA, V. A. AGEYEVA, N. I. YERMEJ.INA, I. K. RUBTSOVA and B. P. SHTARKMAN, Plast. massy, 1, 27, 1977 .'8. I. N. RAZINSKAYA, V. A. IZVOZCHIKOVA, B. P. SHTARKMAN, L. V. ADAMOVA, B. I. LIROVA and A. A. TAGER, Vysokomol. soyed. A23: 2738, 1981 (Translated in Polymer Sci. U.S.S.R. 23: 12, 2969, 1981) 9. V.R. REGEL', G. V. BEREZHKOVA and G. A. DOBOV, Zavodsk. lab. 25 : 101, 1959 10. B. P. SHTARKMAN, I. M. MONICH, S. A. ARZHAKOV and N. Yu. AVERBAKH, Vysokomol. soyed. A18: 1047, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 5, 1203, 1976) 11. A. A. ASKADSKE[,Deformatsiya polimerov (Deformation of Polymers). p. 33, K.himiya, Moscow, 1973 12. N. V. GENKIN, M. S. KITAI, V. A. AGEYEVA,I. N. RAZINSKAYA and B. P. SHTARKMAN, Vysokomol. soyed. B2,0: 670, 1978 (Not translated in Polymer Sci U.S.S.R.) t 3. N. Yn. AVERBAKH, Dissert. Cand. Chem. Sci., p. 74, NII khim. i tekhnolfpoiimerov, Dzerzhinsk, 1981
Thermal degradation of copolymers of VC with propylene
1813
14. Yu. S. L I P A T O V and V. P. PRIVALKO, Vysokomol. soyed. A I 5 : 1517, 1973 (Translated in Polymer Sci. U.S.S.R. 15: 7, 170, 1973) 15. E. FISHER, J. WENDORFF, M. D E T T E N M A I E R , G. L E I S E R and J. V O I G H T - M A R T I N , .l. Macromolec. Sci. 12: 41, 1976
sPolymerScienceU.S.S.R.Vol. 26, No. 8, pp. 1813-1818. 1984 Printed in Poland
0032-3950/84 $10.00+,00 ~) 1985 Pergamon Press Ltd.
THERMAL DEGRADATION OF COPOLYMERS OF VINYL CHLORIDE WITH PROPYLENE* V. V. LISITSKll, V. N . BRONNIKOVA, L. F. MULLOYANOVA, N. I. KUBOVSKAYA, N . N . ZAVODCHIKOVA a n d D . M . YANOVSKII Bashkir 40th October Anniversary State University
(Received 27 December 1982) The influence of propylene units on the thermal degradation of vinyl chloride copolymers, containing up to 9.2 mole ~ propylene units has been evaluated. The ratio of the rate constants \ / of the formation of single double b o n d s / C = C \ k, : k~ : k~'= 1 : 6-5 : 45 indicates the weak destabilizing effect of the CH3 groups of the propylene units accelerating the elimination of HC1 from the adjacent vinyl chloride units.
THE processing capacity of PVC and the quality of the products obtained on the basis of it are determined not only by its M, MD, supramolecular structure and physical properties but also by the chemical structure of the macromolecules [1] which governs such very important technological properties of PVC as thermal stability, the temperature of the start of decomposition, heat resistance, etc. In practice to improve the thermal stability and physical properties of PVC often vinyl chloride is copolymerized with a small quantity of ~-olefines. The copolymers obtained are distinguished by improved physicomechanical properties and are readily processed into rigid and plasticized products [2, 3]. However it was recently shown [4] that the introduction into the PVC macromolecules of units other than of a C H 2 - CHCI,-~ nature has a destabilizing effect on the breakdown of the adjacent VC units. The present study is the first to examine the effect of propylene units on the kinetics of the reactions of elimination of HCI on thermal degradation of VC copolymers with propylene of uniform composition. In the work we used samples of uniform composition containing from 2"0 to 9"2 mole % propylene in the eopolymer. The polymer products were obtained at 326 K by suspension polymerization o f the monomer blend in presence of lauryl peroxide (initiator) and methycellulose (protective colloid) * Vysokomol. soyed. A26: No. 8, 1623-1627, 1984.
0032-3950/84 $10.00+ .00 C 1985 Pergamon Press Ltd.
ANTI-PLASTICIZATiON OF POLYMETHYLMETHACRYLATE* I. N. RAZINSKAYA,B. P. SHTARKMAN,V. A. IZVOZCHIKOVA, N. Yu. AVERBAKH, I. M. MONICH, L. P. BUBNOVA and N. I. PUPUKINA (Received 22 December 1982) Using PMMA to which were added phosphoric acid esters the authors have studied the anti-plasticization of a linear amorphous polymer. They measured the temperature dependenee of the elasticity modulus, the molecular mobility of the system, its p-v-T-diagram and the rheological properties and calculated the free volume. It is shown that the reason for the appearance of the anti-plasticization effect in PMMA is the inhomogeneity of structure and, as a result, the uneveness of the distribution of the plasticizer in the polymer.
RELATIVELY small amounts of plasticizers (down to 10-15 ~ by weight) in a number o f cases simultaneous with fall in the glass transition point Tg raise the rigidity and brittleness of the system [1-3]. This phenomenon has been termed the anti-plasticization effect [4]. The causes of its manifestation and mechanism have not been fully clarified and have given rise to lively discussion. There is dual interest in this problem. On the one hand, this is due to the fact that small additions of plasticizers are often used to solve tasks of practical importance associated with modifications of varied nature of polymers (improving their processing qualifies, reducing combustibility, etc.) and, on the other, the causes of the anti-plasticization effect concern fundamental aspects of the organization of polymers and therefore their proper evaluation is of great importance. Anti-plasticization has, in the main, been studied in polymers prone to crystallization, which largely influences the mechanism of raising rigidity. It was therefore of interest to examine this effect in non-crystallizing polymers. The aim of the present work was to study the phenomenon of anti-plasticization in relation to such a typical amorphous glassy linear polymer as PMMA. The author of [5] was the first to refer to the anomalous behaviour of the strength o f plasticized PMMA. Later, the extremal dependence of the mechanical properties o f P M M A when small additions o f phthalic acid esters were introduced into it was observed in studies [6]. However, the reasons for this phenomenon'in relation to P M M A have not been studied in detail. In this work some phosphoric acid esters served as plasticizers. Their choice was dictated primarily by their use as anti-pyrenes for imparting fire resistance to P M M A [7]. In addition their plasticizing effect was studied in detail in reference [8]. * Vysokomol. soyed. A26: No. 8, 1617-1622, 1984. 1806
Anti-plasticization of polymethylmethacrylate
1807
TABLE 1. CHARACTERIIgTIC$OF PHOSPHORIC ACID ESTERS
Plasticizer Tris-(2-chloroethyl) phosphate (TCEP) Tris-(2-chioropropyl) phosphate (TCPP) Tris-(1,3-dichloroisopropyl) phosphate (TCIPP) Bromoethylbromopropylchloropropyl phosphate (BECP) 2-Bromoethyl-(1,3 -dichloroisopropyl) (1,3-bromochloroisopropyl) phosphate (BECIP)
M
Vx 103, nms
Tp, K
AE, MPa
285.5 327.5
136.6 186.0
355-385 379-393
280 490
431.0
163-6
398--415
370
402.5
170.0
429-447
390
471.5
155.0
425--449
300
Note. The values of Tp and the intrinsic volume of the molecules V are taken from [8].
Table 1 lists the plasticizers studied and some of their properties. The plasticized compositions were obtained on mixing these plasticizers with PMMA of LSOM grade ( M = 1.02 × l 0 s) in the m e l t at 453-463 K for 15 rain in the mixing chamber of the Brabender plastograph. The elasticity modulus E was determined from the compression curves obtained with a modernized relaxometer [9] at different temperatures; the rate of loading was 2.5/zm/sec. The dielectric measurements were made with a two-electrode measuring cell on the TR-9701 bridge over the frequency range 55-2 × l0 s Hz and temperature range 123-433 K. The compressibility of plasticized PMMA w a s s t u d i e d in the isothermal regime in a cell o f t h e c o m p r e s s i o n type with the a p p a r a t u s described in [10]. The viscous properties of the melts were evaluated with the Flowtest capillary viscometer in the shear rate range 10-104 sec -1 in the temperature region 453-530 K. The P M M A compositions studied with the listed phosphates in the glassy state display an extremal concentration dependence of E in the region 3-5 ~o by wt. of the plasticizers (Fig. 1), i.e. an anti-plasticization effect is observed. The Tg values of P M M A monotonically fall with rise in the content 9 of the plasticizers in the system. Taking as the measure of anti-plasticization the difference between the m a x i m u m elasticity modulus of plasticized P M M A and that of the initial polymer zfE, then for the phosphates studied we get fundamentally different positive values of AE amounting to 1018 ~o of the initial E values (Table4lt). The value of the anti-plasticization effect for the objects chosen by us was found not to be related to the degree of affinity of the plasticizer for the polymer which was characterized by the temperature of dissolution Tp (Table 1) and was directly dependent on the intrinsic volume of the plasticizer molecules the larger the molecule the greater the rise in the elasticity modulus of the system. On passing from the glassy to the highly elastic state the elasticity modulus sharply dropped by more than two orders of magnitude (Fig. 2). It is known that this transition is of a relaxation character and the temperature interval of the transition A T reflects the cooperativeness of the relaxation processes [11]. I f it is considered that the introduction of a plasticizer lessens the intermolecutar interaction in the polymer than rise in .4 T on introducing up to 5 Yo T C E P by wt. into P M M A (Fig. 3) points to increase in the size of the kinetic units the movement of which determines the transition to the highly elastic state.
1808
I . N . RAZINSKAYA et al.
As is clear from Fig. 3 (curve 1), AT/A log E passes through a maximum in the region of 5 % TCEP by wt. after which the interval of the transition narrows and remains unchanged to ,-~ 15 % by wt. of the plasticizer. Further increase in AT occurs in that region of composition where the "free" plasticizer appears [12]. Apparently, in this case it serves as a kind of lubrication allowing larger elements of structure to take part in the movement. E.IO~ MPa
tosE fMPaJ
I
+++~
3
2
! 10
20 333
FIO. 1
373
413 ~ K
FIG. 2
FIG. I. Concentration functions of the elasticity moduli of PMMA plasticized with TCEP (1), TCPP (2), TCIPP (5), BECP (4) and BECIP (5). FIG. 2. Temperature dependence of the elasticity moduli of initial PMMA (1) and polymer containing 3 (2), 5 (.3), 7 (4), 10 (5), 15 (6) and 20% (7) TCEP by wt.
The character of such an extremal dependence also persists for another plastic i z e r - BECP (Fig. 3, curve 2). Consequently, it ~ be assumed that this phenomenon is of a general nature and indicates that small additions of plasticizer produce clearly perceptible changes in the structure of PMMA. It was, therefore, of interest to study the molecular mobility of plasticized P M M A characterized by the dielectric method. Measurements of the temperature-frequency dependence of the factor of dielectric losses e" showed that at a frequency 55-200 Hz two relaxation transitions are resolved corresponding to the 0~ and fl processes in PMMA. The temperatures of the maxima Tin,= of the ~t and ff transitions are presented in Table 2. For the 0t process monotonic fall in Tm=x is observed which corresponds to fall in T= of the system while the introduction of up to 15 % TCEP by vol. into P M M A does not affect the fl process and its value and T=,= do not change as compared with the initial PMMA. Similar results have been obtained for the PMMA-dibutyl phthalate system.
Anti-plasticizatiort of potymethylmethacrylate
1809
'Consequently, the phenomenon of anti-plasticization in P M M A is not associated with mobility of the side groups and portions of the macrochains governing//-relaxation in this polymer. Let us now look at the parameters of molecular motion of the portions of the macro~ehains determining the high temperature relaxation process in P M M A depending on fits TCEP content. Relaxation z and the activation energy of the process U serve as such parameters. These parameters were determined from the temperature-frequency functions e", U was calculated from the temperature dependence of the frequency fma, and z from the ratio z = 1/2nfma, where fm,x is the frequency at which e" passes through a maximum. The activation energies are presented in Table 2. Figure 4 gives .the dependence of r at different temperatures on the content of TCEP in PMMA. As may be seen, U passes through a maximum in the region of content of 3 % TCEP b y vol. As for z in the region of the glassy state (333-343 K) it passes through a maximum in the same regiofi of compositions and on passing to the highly elastic state begins t o fall appreciably only after introducing 8-10 % TCEP by vol. Thus, investigation of intramolecular mobility shows that the addition of small ~amounts of plasticizer leads to straightening and enlargement of the portions of the AT AlogE
tc~ 'r ~sec]
.9 -4'
7 -S 5
i
t~%%bgwt.
5
5
7~
FIG. 3
es~o,*~bgvoZ
FZG. 4
Fro. 3. Concentrat ion funct ions of A T/A log E for PM MA plasticized with TCEP (1) and BECP (2). FIG. 4. Concentration functions of the relaxation times v for P M M A with TCEP at 333 (1), 343 (2), 353 (3), 363 (4) and 483 (5) K. T A B L E 2. CHARACTERISTICS OF THE RELAXATION PROCESSES OF
PMMA
AS A FUNCTION OF THE CONTENT
~a OF TCEP (frequency 55 Hz) ¢p,% by vol.
!
T~,K
r~,K
b
I i
!
387 386 376
320 320 323
U, k J/mole 87 106 84
% by vol. 10 15
364 356
319 321
86 85
1810
,
I. N . RAZ~NSKAYA et al.
maerochain responsible for the =-transition in PMMA. This conclusion agrees with the above results of study of the temperature interval of vitrification of the system. To study the structural state of the polymer matrix on adding small amounts o f plasticizers we examined compressibility i.e. the p--v-T-diagram of PMMA plasicized with small amounts of TCEP. Figure 5 presents the dependence of Tg determined from the dilatometrie curves on pressure for mixtures of PMMA with different contents o£ TCEP.
"'
A
~00,
3
I
t
I,
170
2~0
3£0
FIO. 5
60
120
180 240 p, MPo
FIO. 6
FIG. 5. Dependence of T= on pressure for initial P M M A (1) and P M M A containing 3 (2), 5 (3) and 10~, (4) TCEP. FIG. 6. Free volume fraction f , as a function of pressure for initial P M M A (1) and polymer c o n taining 3 (2) and 5 ~0 (3) TCEP.
The presence of a horizontal portion of the curve zip for PMMA is related to the inhomogeneity of the polymer as a result of which it is primarily compressed through the loose regions, which does not lead to change in the intermolecular interactions and. therefore does not influence T~ [13]. Thus, zip serves as a kind of criterion of the degree of homogeneity of the polymer. Addition of TCEP to the system leads to substantial change in alp: zlp passes through a maximum in the region 3 ~o TCEP by wt., then falling on adding the plasticizer. Consequently, 3 ~o TCEP by wt. raises the degree of inhomogeneity of the system as compared with the initial PMMA. Calculations of the free volume fractionfc for Ts (after Simkha and Boyer) showed that f~ for the system with 3 ~ TCEP by wt. is somewhat higher and 5 ~ lower than in the initial PMMA (Fig. 6). According to the ideas of Lipatov and Privalko [14] increase in the free volume fraction is linked with rise in chain rigidity. It may be assumed that small additions of TCEP (,-~3 ~o by wt.) help the polymer macrochains to assume more elongated conformations raising the rigidity of the system. Thus, the results of study of compressibility are in complete accord with the conclusions which follow from evaluation of the parameters of molecular mobility presented above.
Anti-plasticization of polymethylmethacrylate
181I
Consequently, increase in the degree of inhomogeneity of P M M A on adding small portions of plasticizer is linked with the uneven distribution of the latter. Apparently, the small additions of plasticizer primarily enter the loosely packed regions of t h e polymer in which the relatively free chains on interaction with a good plasticizer ( T C E P ) acquire more elongated conformations and therefore become more rigid.
P'toJ, ks/'
/
log, [Pa.sec.]
ii
1.2E / 1
//
r /~jf i i
,,i
I
I 1.18
i
~
i
I
J
ao Fio. 7
FIG. 8
Flo. 7. Concentration functions of density p of PMMA plasticized with. TCEP (1) and TCPP (2). Broken lines, additive density values. F~G. 8. Concentration functions of viscosity r/of plasticized PMMA at 483 K and shear rates 0 (1, 2), 102"s (3) and 103"s (4) see-L A-PMMA with TCEP, B-with TCPP. However this is not the only cause of the anti-plasticization effect. Let us look at the problems of the packing of the polymer when small additions of plasticizers are introduced into it. Figure 7 presents the dependence of the density of P M M A on its content of two plasticizers- T,~EP and TCPP. As may be seen, the experimental points in both cases lie above the values of additivity. If it is borne in mind that the free volume fraction of P M M A on introducing plasticizers into it increases [8] then rise in density must be related to a certain pre-ordering of the polymer structure tlarough passage of some of the fragments of the macrochain to the dense regions. This statement is also supported by the results of rheological investigations. O n introducing 1-2% TCEP and TCPP by wt. into P M M A the viscosity values pass through a distinct maximum the value of which falls from 0 to 3 × 103 sec- ~ with rise in the shear rate (Fig. 8). The extremal character of the concentration dependence o f viscosity persists in the whole temperature interval from 473 to 503 K and is smoothed out on exposure to high shear stresses. Thus, the ~auses of the anti-plasticization effect in P M M A are associated with changes in the structure of the polymer under the influence of small additions of plasti-
1812
I . N . RAZINSKAYA et aL
¢izer. It is known that the structure of amorphous polymers varies. These inhomoge= neities are of a dual character: regions exist more tightly packed apparently as a result o f the local parallel packing of the chain segments and looser portions between them with relatively free and disordered packing of the segments of the macrochains. It is therefore pertinent to speak of a density spectrum in polymers. The differences in the local densities for P M M A obtained from the data on small angle X-ray scatter amount
to ~1.5% [15]. From the results it may be concluded that small additions of plasticizer affect both types of iniaomogeneities of the P M M A structure. In the loose regions the relatively free chains on interaction with a good plasticizer assume more elongated conformations a n d become more rigid with simultaneous pre-ordering of the structure as a result of the passage of some of the fragments of the macromolecules to the denser regions with mobility operating at the level of the segments. Consequently, the prime cause of the appearance of the anti-plasticization effect in amorphous linear polymers is the inhomogeneity o f their structure and as a result t h e uneveness of the distribution of the plasticizer in the polymer matrix. Local rise in the mobility of the portions of the macrochains leads to structural rearrangements producing extremal change in the physicomechanical properties. Translated by A. CROZY
REFERENCES
1. B. P. SHTARKMAN, Plastifikatsiya polivinilkhlorida (Plasticization of Polyvinyl Chloride). p. 215, Khimiya, Moscow, 1975 .2. P. V. KOZLOV and S. P. PAPKOV, Fiziko-khimicheskiye osnovy plastifikatsii polimerov (Physicochemical Bases of Polymer Plasticization). p. 124, Khimiya, Moscow, 1982 3. I. I. PEREPECHKO, Akusticheskiye metody issledovaniya polimerov (Acoustic Methods of Investigating Polymers). p. 128, Khimiya, Moscow, 1973 -4. W.J. JACKSON and J. R. CALDWRLL, Advances Chem. Ser. 48:185, 1965; J. Appl. Polymer Sci. 11: 211, 1967 5. P. GHERSA, Modern Plast. 36: 135, 1958 6. A. V. YEFIMOV, P. V. KOZLOV and N. F. BAKF~YEV, Dold. Akad. Nauk SSSR 230: 639, 1976: A.V. YEFIMOV, Z. S. BELOKON' and N. Ya. GI~KOVA, Vysokomol. soyed. B17: 171, 1975 (Not translated in Polymer Sci. U.S.S.R.) "7. I. N. RAZINSKAYA, V. A. AGEYEVA, N. I. YERMEJ.INA, I. K. RUBTSOVA and B. P. SHTARKMAN, Plast. massy, 1, 27, 1977 .'8. I. N. RAZINSKAYA, V. A. IZVOZCHIKOVA, B. P. SHTARKMAN, L. V. ADAMOVA, B. I. LIROVA and A. A. TAGER, Vysokomol. soyed. A23: 2738, 1981 (Translated in Polymer Sci. U.S.S.R. 23: 12, 2969, 1981) 9. V.R. REGEL', G. V. BEREZHKOVA and G. A. DOBOV, Zavodsk. lab. 25 : 101, 1959 10. B. P. SHTARKMAN, I. M. MONICH, S. A. ARZHAKOV and N. Yu. AVERBAKH, Vysokomol. soyed. A18: 1047, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 5, 1203, 1976) 11. A. A. ASKADSKE[,Deformatsiya polimerov (Deformation of Polymers). p. 33, K.himiya, Moscow, 1973 12. N. V. GENKIN, M. S. KITAI, V. A. AGEYEVA,I. N. RAZINSKAYA and B. P. SHTARKMAN, Vysokomol. soyed. B2,0: 670, 1978 (Not translated in Polymer Sci U.S.S.R.) t 3. N. Yn. AVERBAKH, Dissert. Cand. Chem. Sci., p. 74, NII khim. i tekhnolfpoiimerov, Dzerzhinsk, 1981
Thermal degradation of copolymers of VC with propylene
1813
14. Yu. S. L I P A T O V and V. P. PRIVALKO, Vysokomol. soyed. A I 5 : 1517, 1973 (Translated in Polymer Sci. U.S.S.R. 15: 7, 170, 1973) 15. E. FISHER, J. WENDORFF, M. D E T T E N M A I E R , G. L E I S E R and J. V O I G H T - M A R T I N , .l. Macromolec. Sci. 12: 41, 1976
sPolymerScienceU.S.S.R.Vol. 26, No. 8, pp. 1813-1818. 1984 Printed in Poland
0032-3950/84 $10.00+,00 ~) 1985 Pergamon Press Ltd.
THERMAL DEGRADATION OF COPOLYMERS OF VINYL CHLORIDE WITH PROPYLENE* V. V. LISITSKll, V. N . BRONNIKOVA, L. F. MULLOYANOVA, N. I. KUBOVSKAYA, N . N . ZAVODCHIKOVA a n d D . M . YANOVSKII Bashkir 40th October Anniversary State University
(Received 27 December 1982) The influence of propylene units on the thermal degradation of vinyl chloride copolymers, containing up to 9.2 mole ~ propylene units has been evaluated. The ratio of the rate constants \ / of the formation of single double b o n d s / C = C \ k, : k~ : k~'= 1 : 6-5 : 45 indicates the weak destabilizing effect of the CH3 groups of the propylene units accelerating the elimination of HC1 from the adjacent vinyl chloride units.
THE processing capacity of PVC and the quality of the products obtained on the basis of it are determined not only by its M, MD, supramolecular structure and physical properties but also by the chemical structure of the macromolecules [1] which governs such very important technological properties of PVC as thermal stability, the temperature of the start of decomposition, heat resistance, etc. In practice to improve the thermal stability and physical properties of PVC often vinyl chloride is copolymerized with a small quantity of ~-olefines. The copolymers obtained are distinguished by improved physicomechanical properties and are readily processed into rigid and plasticized products [2, 3]. However it was recently shown [4] that the introduction into the PVC macromolecules of units other than of a C H 2 - CHCI,-~ nature has a destabilizing effect on the breakdown of the adjacent VC units. The present study is the first to examine the effect of propylene units on the kinetics of the reactions of elimination of HCI on thermal degradation of VC copolymers with propylene of uniform composition. In the work we used samples of uniform composition containing from 2"0 to 9"2 mole % propylene in the eopolymer. The polymer products were obtained at 326 K by suspension polymerization o f the monomer blend in presence of lauryl peroxide (initiator) and methycellulose (protective colloid) * Vysokomol. soyed. A26: No. 8, 1623-1627, 1984.