- Email: [email protected]
Glass transition point of plasticized polymers determined by different methods
1988
Yu. V. OVCRth'mKOVand Y~,. Yo. OvcmNNIKOV
9. V . P . SHABOLDIN, A. G. S U K H O M U D R E N K O , A. I. K R A S H E N I N N I K O V and V. A. M O R O ZOV, Vysokomol soyed. A14: 1462, 1972 (Translated in Polymer Sci. U.S.S.R 14: 7, 1638, 1972) 10. Ye. A. BEKTUROV, L. A. B I M E N D I N A , V. V. R O G A N O V and S. R. RAFIKOV, Ibid A14343, 1972 (Translated in Polymer Sci. U.S S.R. 14: 2, 384, 1972) 11. S. M. K O C H E R G I N and V. P. BARANOV, Ibid. 4: 135, 1962 (Not translated in Polymer Sci.
U.S.S.R.)
Polymer Science U.S.S.R. Vol. 29. No 9, pp 1988-1993, 1987 Printed in Poland
0032-3950/87 $10 00+.00 © 1988 Pergamon Press 01c
GLASS TRANSITION POINT OF PLASTICIZED POLYMERS DETERMINED BY DIFFERENT METHODS* YU. V. OVCHINNIKOV and YE. Yu. OVCHINNIKOV
(Received 12 February 1986)
The reasons for the difference in the value of the glass transition point of plasticized polymers determined by different methods are considered in relation to PVC-plasticlzer systems. IT IS well known [1 ] that vitrification of polymers is a relaxatlonal process of the transition of the polymer on cooling from the viscous flow or highly elastic state to the glassy. Therefore, the glass transition point T, essentially depends on the rate of coohng (or heating on devitrification) and the method of determining it. The glass transition point determined by dynamic methods, as a rule, IS always higher than T~ determined by static methods. However, for many amorphous polymers with similar rates of temperature increase (,-,1-3 degree/min) and extremely at low frequencies the glass transition points determined by dynamic and static methods differ by not more thart 3-5 K. In particular, the T, values of such polymers as PVC, P M M A and PS amounting to 353, 373 and 378+3 K respectively are recognized. For plasticized polymers the picture is much more complex. The Ts values determined for the same polymer-plasticizer systems by different methods even for similar rates of rise in temperature appreciably differ. This difference no longer amounts to 3-5 K as for the starting homopolymers but is greater by art order, i.e. 30-50 K. The validity of all this is convincingly confirmed by Fig. 1 giwng the dependences of Tu of plasticized PVC t on the plasticizer content, obtained by different methods. * Vysokomol. soyed. A29: No. 9, 1814--1818, 1987.
t PVC-plasticizer systems are chosen for analys~s for the reason that the most extensive experimental material exists for them.
1989
Glass transition point of plasticized polymers
As plasticlzers we used dioctylphthalate (DOP), dioctyl sebacate (DOS) and tncresyl phosphate (TCP). A l t h o u g h these experimental data are known the physical essence of the reasons for such a d~fference m the T wvalues determined by different methods has in substance not been considered.
~K
L 350
350
a
~ 6 250
150
50 [DOB],t. oO
T,K 35O
I
I
50 [Dos],wt~I00
C
I ? 50 fog I-TCP], w t %
Fzo I T~ as a funct]on of the content of plasticizer in the systems PVC-DOP (a), PVC-DOS (b) and PVC-TCP (c) obtained by different methods a. l-Measurement of the speed of sound [2]; 2-adiabatic calorimetry [3]; 3 - D S C [4]; 4-thermomechamcal [5], 5-thermomechanical [6]; 6 - ddatometry [7]; 7-d~electrlc losses [8], b: 1 - adlabauc calorimetry [9, p. 95]; 2-torsional vibrations [10, p 119]; 3 - d a t a of [11]; 4-thermomechamcal [6]; 5-dilatometry [12]; 6 - D S C [12]; 7-Aleksandrov-Gayev instrument [13]; c- 1-refractometry [10, p 86], 2-torsional vibrations [14], 3-refractometry [15]; 4-dmlatometry [7]; 5-dielectric changes [16, p 162]; 6-adiabattc calorimetry [17].
As Fig I shows for all the PVC-plasticizer systems studied the same character or change in T, is observed Two n o t e w o r t h y features in the change in 7'= may be singled out, firstly, for the non-plasticized polymers in the experimental conditions chosen T, practically does not depend on the m e t h o d of its determinaUon or at any rate is
1990
Ytr. V. OVCHINNIKOVand Yz. Ytl. OVCHINNIKOV
within the limits of error of the method. Secondly, the difference in the Tz values (Ts~ - T~.j) determined by different methods begins to grow immediately the content of the plasticizer in PVC increases reaching a maximum value for most systems in the region 30-50 wt. ~o of the plasticizer. In our view, it appears most logical to explain the last effect, the main subject of the present analysis, from the concept of the structural heterogeneity of amorphous polymers governing the wide set of relaxation times. As for PVC its structural hetelogeneity both at molecular and supramolecular levels [18, 19] is particularly clearly marked. For this reason the plasticizer introduced into the polymer is distributed in the polymer matrix at microlevel irregularly in view of the different "partial" solubihty of the low molecular weight component in the PVC structural elements. But since the molecules of the components of the jointly supercooled solutions form a single system with one mean relaxation time [20] naturally addition of plasticizer must lead to widening of the relaxation time spectrum in the polymer-plasticizer system in the transitional region as compared with the pure polymer. This statement is confirmed experimentally by the widening of the maxima of tan 5 of dipole-elastic and mechanical losses in the region of the ~-transltion on introducing plasticizer into the polymer [21] as the appearance of diffuse maxima due to the presence of different relaxational processes as is known is governed by a set of relaxation times [22]. And since different methods of investigation "sense" the mobility of different structural elements we encounter different T~ values of the plasticized polymers. The fact that different methods may "sense" the mobility of different structural elements in virtually all physical states of an amorphous polymer is well known [23] and there is nothing surprising in the fact that in the temperature region of vitrification, too, different methods reflect the mobility of different segments and aggregates of the macromolecules. A graphic example of this is provided by the results of study of the structural plasticization of polymers presented in a number of studies [24, 25]. A substantial fall in T~ on introducing into the polymer negligible amounts of poorly compatible plasticizers (0.05-0.1 wt. ~o) is recorded by the thermolnechanical method. At the same time, such methods of investigation as measurement of the dielectric losses, dilatometry, calorimetry, etc. show insignificant change in T~ or do not "sense" at all the shift in Twon introducing into the polymer neghbigle amounts of poorly compatible low molecular weight compounds. The widening of the relaxation time spectrum, in turn, leads to considerable widening of the temperature interval of the transitional region dT(Tg) (Fig. 2). It should be emphasized that the widening of the temperature interval of dev]trification (vitrification) dT(T~) is also observed for other plasticized polymers [27-30] and this for all methods of investigation fixing the temperature region of vitrification (calorimetric method of measuring thermal capacity, DSC, measurement of the tangent of the angle of dielectric and mechanical losses, dielectric constant, etc.) and such widening must be assumed typical on plasticization of all amorphous and amorphocrystalline polymers.
1991
Glass transition point of plasticized polymers
The widening of the set of relaxation times in the transitional region and henc~ also the difference in the Tg values determined by different methods generally speaking occurs not only on introducing a plasticizer into a polymer but on the contrary on introducing a polymer into a plasticizer. A7-(7 o
/4
~t"
60
60
q
3
qO rio 20
50 w, w~.%
tOO
I
I
50 w, wt'.%
tO0
FIG 2 FIG. 3 FiG 2 Temperature interval of devJtr~ficatlon m the PVC-plastic~zer system as a function of plastlcLzer content for the P V C - D O P systems according to the findings of [21] (1), [26] (3) and (4) and P V C - D O S systems according to [9, p. 94] (2). FiG 3 Dependence of the difference in the glass transition points m a PVC-plastlc~zer system termined by different methods on the plasticizer content 1-T~ [6]-T~ [3]; 2-Tg [7]-Tg
the [3] de-
13],
3 - T ~ [ 8 ] - T K [ 3 ] ; 4 - T g [ 9 ] - T j [6]
For systems with high plasticizer content gradual transition from weakly concentrated polymer solutions to polymer gels is observed. The latter, as indicated by numerous published findings, are macroscopically (morphologically) homogeneous systems but heterogeneous at microlevel. In them, two phases of saturated solutions co-existpolymer m the plasticizer and plasticizer in the polymer uniformly distributed in volume with a corresponding different set of structural elements which apparently also governs the extremal change in AT(Ts)=f(wpl ) and ( T ~ , - T ~ j ) = J (wpl) (Figs. 2 and 3). The second reason for the considerable difference in the Twvalues determined by different methods is purely methodological in character. Those of a methodological character may, m particular, include the results of investigation of polymer-plasticizer systems with a high content of crystalhzable plasticizer. Depending on the method of investigation the moment~ of vitrification of the PVC plasticizer system and crystallization of the plasticizer may be readdy evaluated separately and one may commit an error by taking as the glass transition point the temperature of crystalhzation of the plasticizer (on cooling) or the metllng point of the crystallized plasticizer (on heating). Such a natural error arises from the fact that the changes m many physical properties of polymers close to TB in their character match the changes in the corresponding physical properties on crystallization. For
1992
Yu. V. OVCHINNIKOVand YE. Yu. OVCnlNNIKOV
example, if for the investigation one uses a dynamomechamcal method then in all cases a sharp change m mechanical properties may be observed primarily in the capacity for deformation on exposure to particular loads [31]. In other words, this method does not show which effect is responsible for change in the mechanical properties. At the same time, the above noted features of the behaviour of the poylmer-crystalhzable plasticizer systems very clearly stand out with use of the precision calorimetric method of investigating the temperature dependence of thermal capacity. Let us consider, for example, two systems: PVC-DOS and PVC-dioctyl ester of 1,10-decane&carboxylic acid (DODA). These low molecular weight compounds are freeze-resistant PVC plastlcizers [32] and are usually used m blends with dlalkyl phthalates for imparting better freeze resistance to PVC materials. In pure form and also in amounts exceeding the limit of thermodynamic compattbihty with PVC both DOS and DODA are able to vitrify and crystallize. As shown by investigation of the temperature dependence of the thermal capacity of these plastic~zers the glass transition point of DOS is 161 K [9, p. 69] and of D O D A 175 K [33]; the melting points of the crystalhzed plasticlzers are respectively 228 and 248 (the first crystalline modification of DODA) and 229 K (the second crystalline modification of DODA). In [13] where these systems were investigated by the AleksandrovGayev method ~t was pointed out that the Tg values of pure plasticizers are 230 and 238 K, i e. evidently the melting points of crystallized DOS and D O D A are taken as the T B. As rightly noted in [31] to avoid erroneous conclusions in comparing the T~ values it is necessary to allow (standar&ze) not only for the conditions of conductmg the experiment but also the method of determining Tg. Since &fferent methods even in standard experimental conditions give different values of the glass transition points of plasticized polymers it is in order to ask which methods of investigation should be used for the evaluation of T~. We think that the answer to th~s question is equivocal If m study of plasticization of a polymer the task is essentially of an exploratory nature then preference should apparently be given to more mformatwe methods which allow one to judge together with the glass transition pomt the features of the behaviour of the polymer-plasticizer system over a wide temperature and concentration range. If the evaluation of TB pursues essentially practical goals ~t ~s better to use those methods which most closely reflect the conditions of use of the plasticized materml. Translated by A. CROZV REFERENCES
1 Entsiklopediya pohmerov (Polymer Encyclopae&a). Vol 3, p 489, Moscow, 1977 2. I. I. PEREPECHKO, L. A. USHAKOV and R. S. BARSHTEIN, Vysokomol soyed A14: 2553, 1972 (Translated in Polymer Sct U S S_R 14: 12, 2971, 1972) 3 L. Ya. MARTYNENKO, L. B. RABINOVICH, Yu. V. OVCHINNKOV and V. A. MASLOVA, 1bid. AI2: 841, 1970 (Translated m Polymer Scl U.S.S.R. 12.4, 952, 1970)
Glass transition point of plasticized polymers
1993
4 L. H. DUNLAP, J. Polymer Scl A-2, 4: 673, 1966 5 T. B. Z A V A R O V A , K. S. MINSKER, G. T. F E D O S E Y E V A , B. F. TEPLOV, V. D. A R D A S H NIKOVA and A. I. KUTSENKO, Vysokomol. soyed. 8: 599, 1966 (Translated m Polymer Sci. USSR 8: 4, 657, 1966) 6 ~v. A. KARGIN and Yu. M. MALINSKII, Dokl Akad Nauk SSSR 43: 967, 1950 7. T. C Z E H A J and J. KAPKO, Europ Polymer J_ 17: 1227, 198l 8 I. N. RAZINSKAYA, P. V. K O Z L O V and B. P. S H T A R K M A N , V>sokomol soyed. 6: 516, 1964 (Translated in Polymer Scl U S S R 6: 3, 570, 1964) 9 Ye. Yu. O V C H I N N I K O V , D]ssert Cand. Chem Scl., 200 p p , G G U , GorkJ, 1983 10 K. TIN1US, Plastifikatory (Plastlcizers). 916 p p , Moscow, 1964 l l O. LEUCHS, Kunststoffe 58: 375, 1968 12 L. H. DUNLAP, J. llkolymer Scl 13: 561, 1970 13 A. 1. SUVOROVA, V. M. ANDREYEVA, T. V. INANINA, L. K. ZYRYANOVA, 1. I. SOROKINA and A. A. TAGER, Vysokomol soyed B22: 910, 1980 (Not translated m Polymer Scl U S S . R ) 14 A. COEN and P. PARRINI, Mater Plast 25: 357, 1959 15 W. KNAPE and A. S C H U L Z , Kunststoffe 41: 321, 1951 16. Elektrtchesktyc svolstva pohmcrov (Electrical Propeltles of Polymers) (Ed B. I. Sazhm), p 162, Leningrad, 1977 17. V. A. P O D G O R N O ~ , Ye. N. MOSEYEVA and Ire. Yu. OVCHINNIKOV, Tez IV Vsesoyuz konf. po tcrrnodmamlke organ soyedmenH (SummaNcs of Reports to Fourth All-Umon Conference on the Thermodynamtcs of OrgaMc Compounds) p 212, Kmbyshev, 1985 18 V. P. LEBEDEV and B. P. S H T A R K M A N , Poluchenlye i svotstva pohvmilkhlorida (Synthesis and Ploperttes of Polyvmyl Chloride) (Ed Ye. N Zd'berman), p 199, Moscow, 1968 19 V. P. LEBEDEV, D]ssert Cand. Phys Math ScJ IKhF, Akad Nauk SSSR, Moscow, 1969 20 P. P. KOBEKO, Arnorfaye veshchcstva (Amorphous Substances) p 97, Moscow, 1952 21 A . M . BISHAI, F. A. J A M I L , F. ~. A W N I and H. F. F. AL-KHAYAAT, Plaste u. Kautschuk 32: 58, 1985 22 V. A. KARGIN and G. L. SLONIMSKII, Kratkiye ocherki po fizlko-khlmil pohmetov (Short Essays on the Physical Chemistry of Polymers) p 69, Moscow, 1960 23 G. M. BARTENEV, Struktura i telaksalslonnye svolst,,a elastomerov (Structure and RelaxatJonal Properties of Elastomers) p 73, Moscow, 1979 24. P. V. KOZLOV, V. G. T I M O F E Y E V A and V. A. KARGIN, Dokl Akad. Nauk SSSR 148: 886, 1963 25 P. V. KOZLOV, Z h V K h O ]m D 1 Mendeleyeva 9: 660, 1964 26 M. KISBENYI, J Polymer Scl, No 33, 113, 1971 27 L. ~. FAMINSKAYA, V. A. MASLOVA, L N. RABINOVICH, V. N. Z I B O R O V A , V. F. UR'YASH, B. P. S H T A R K M A N and L N. RAZINSKAYA, Trud po khtmH I khlm tekhnologH (Wolk on C h e m [ s t r y a n d ChemlcalTechnology) Iusse4, p 71, GolkH. 1975 28 A . G . BABINKOV, Dissert. Cand Chem. Scl, p 151, Gorkll, 1982 29 F. EMF, Dleletrichesktye lzmeremya (Dielectric Measurements) p 176, Moscow, 1967 30 I B. RABINOVICH, T. B. K H L ~ U S T O V A and A. N. M O C H A L O V , Vysokomol soyed A27: 525, 19~5 (Translated m Polymer Sc[ U S S R 27: 3, 586, 1985) 3[ V. ~te. GUL' and V. N. K U L E Z N E V , Struktura i mekhamchesklye SVOlStVa pollmerov (Structure and Mechamcal Properties of Polymers) p_ 151, Moscow, 1966 32. R. S. BARSHTEIN, V. I. K I R I L L O V I C H and Yu. Ye. NOSOVSKII, Plastifikatory dlya pohraerov tPlasticlzers for Polymers) p 86, Moscow, 1982 33 1. B. RABINOVICH, A. G. BABINKOV, Ye. Yu. OVCHINNIKOV, Ye. M. MOSEYEVA, V. A. M A S L O V A and L. A. F A M I N S K A Y A , Tez IX Vsesoyuz konf. po kalorimetril i khim. termodmamlke (Summaries of Reports to Ninth AlI-Umon Conference on Calorimetry and Chemical Thel modynamlcs), p 116, Tbdlsl, 1982
Yu. V. OVCRth'mKOVand Y~,. Yo. OvcmNNIKOV
9. V . P . SHABOLDIN, A. G. S U K H O M U D R E N K O , A. I. K R A S H E N I N N I K O V and V. A. M O R O ZOV, Vysokomol soyed. A14: 1462, 1972 (Translated in Polymer Sci. U.S.S.R 14: 7, 1638, 1972) 10. Ye. A. BEKTUROV, L. A. B I M E N D I N A , V. V. R O G A N O V and S. R. RAFIKOV, Ibid A14343, 1972 (Translated in Polymer Sci. U.S S.R. 14: 2, 384, 1972) 11. S. M. K O C H E R G I N and V. P. BARANOV, Ibid. 4: 135, 1962 (Not translated in Polymer Sci.
U.S.S.R.)
Polymer Science U.S.S.R. Vol. 29. No 9, pp 1988-1993, 1987 Printed in Poland
0032-3950/87 $10 00+.00 © 1988 Pergamon Press 01c
GLASS TRANSITION POINT OF PLASTICIZED POLYMERS DETERMINED BY DIFFERENT METHODS* YU. V. OVCHINNIKOV and YE. Yu. OVCHINNIKOV
(Received 12 February 1986)
The reasons for the difference in the value of the glass transition point of plasticized polymers determined by different methods are considered in relation to PVC-plasticlzer systems. IT IS well known [1 ] that vitrification of polymers is a relaxatlonal process of the transition of the polymer on cooling from the viscous flow or highly elastic state to the glassy. Therefore, the glass transition point T, essentially depends on the rate of coohng (or heating on devitrification) and the method of determining it. The glass transition point determined by dynamic methods, as a rule, IS always higher than T~ determined by static methods. However, for many amorphous polymers with similar rates of temperature increase (,-,1-3 degree/min) and extremely at low frequencies the glass transition points determined by dynamic and static methods differ by not more thart 3-5 K. In particular, the T, values of such polymers as PVC, P M M A and PS amounting to 353, 373 and 378+3 K respectively are recognized. For plasticized polymers the picture is much more complex. The Ts values determined for the same polymer-plasticizer systems by different methods even for similar rates of rise in temperature appreciably differ. This difference no longer amounts to 3-5 K as for the starting homopolymers but is greater by art order, i.e. 30-50 K. The validity of all this is convincingly confirmed by Fig. 1 giwng the dependences of Tu of plasticized PVC t on the plasticizer content, obtained by different methods. * Vysokomol. soyed. A29: No. 9, 1814--1818, 1987.
t PVC-plasticizer systems are chosen for analys~s for the reason that the most extensive experimental material exists for them.
1989
Glass transition point of plasticized polymers
As plasticlzers we used dioctylphthalate (DOP), dioctyl sebacate (DOS) and tncresyl phosphate (TCP). A l t h o u g h these experimental data are known the physical essence of the reasons for such a d~fference m the T wvalues determined by different methods has in substance not been considered.
~K
L 350
350
a
~ 6 250
150
50 [DOB],t. oO
T,K 35O
I
I
50 [Dos],wt~I00
C
I ? 50 fog I-TCP], w t %
Fzo I T~ as a funct]on of the content of plasticizer in the systems PVC-DOP (a), PVC-DOS (b) and PVC-TCP (c) obtained by different methods a. l-Measurement of the speed of sound [2]; 2-adiabatic calorimetry [3]; 3 - D S C [4]; 4-thermomechamcal [5], 5-thermomechanical [6]; 6 - ddatometry [7]; 7-d~electrlc losses [8], b: 1 - adlabauc calorimetry [9, p. 95]; 2-torsional vibrations [10, p 119]; 3 - d a t a of [11]; 4-thermomechamcal [6]; 5-dilatometry [12]; 6 - D S C [12]; 7-Aleksandrov-Gayev instrument [13]; c- 1-refractometry [10, p 86], 2-torsional vibrations [14], 3-refractometry [15]; 4-dmlatometry [7]; 5-dielectric changes [16, p 162]; 6-adiabattc calorimetry [17].
As Fig I shows for all the PVC-plasticizer systems studied the same character or change in T, is observed Two n o t e w o r t h y features in the change in 7'= may be singled out, firstly, for the non-plasticized polymers in the experimental conditions chosen T, practically does not depend on the m e t h o d of its determinaUon or at any rate is
1990
Ytr. V. OVCHINNIKOVand Yz. Ytl. OVCHINNIKOV
within the limits of error of the method. Secondly, the difference in the Tz values (Ts~ - T~.j) determined by different methods begins to grow immediately the content of the plasticizer in PVC increases reaching a maximum value for most systems in the region 30-50 wt. ~o of the plasticizer. In our view, it appears most logical to explain the last effect, the main subject of the present analysis, from the concept of the structural heterogeneity of amorphous polymers governing the wide set of relaxation times. As for PVC its structural hetelogeneity both at molecular and supramolecular levels [18, 19] is particularly clearly marked. For this reason the plasticizer introduced into the polymer is distributed in the polymer matrix at microlevel irregularly in view of the different "partial" solubihty of the low molecular weight component in the PVC structural elements. But since the molecules of the components of the jointly supercooled solutions form a single system with one mean relaxation time [20] naturally addition of plasticizer must lead to widening of the relaxation time spectrum in the polymer-plasticizer system in the transitional region as compared with the pure polymer. This statement is confirmed experimentally by the widening of the maxima of tan 5 of dipole-elastic and mechanical losses in the region of the ~-transltion on introducing plasticizer into the polymer [21] as the appearance of diffuse maxima due to the presence of different relaxational processes as is known is governed by a set of relaxation times [22]. And since different methods of investigation "sense" the mobility of different structural elements we encounter different T~ values of the plasticized polymers. The fact that different methods may "sense" the mobility of different structural elements in virtually all physical states of an amorphous polymer is well known [23] and there is nothing surprising in the fact that in the temperature region of vitrification, too, different methods reflect the mobility of different segments and aggregates of the macromolecules. A graphic example of this is provided by the results of study of the structural plasticization of polymers presented in a number of studies [24, 25]. A substantial fall in T~ on introducing into the polymer negligible amounts of poorly compatible plasticizers (0.05-0.1 wt. ~o) is recorded by the thermolnechanical method. At the same time, such methods of investigation as measurement of the dielectric losses, dilatometry, calorimetry, etc. show insignificant change in T~ or do not "sense" at all the shift in Twon introducing into the polymer neghbigle amounts of poorly compatible low molecular weight compounds. The widening of the relaxation time spectrum, in turn, leads to considerable widening of the temperature interval of the transitional region dT(Tg) (Fig. 2). It should be emphasized that the widening of the temperature interval of dev]trification (vitrification) dT(T~) is also observed for other plasticized polymers [27-30] and this for all methods of investigation fixing the temperature region of vitrification (calorimetric method of measuring thermal capacity, DSC, measurement of the tangent of the angle of dielectric and mechanical losses, dielectric constant, etc.) and such widening must be assumed typical on plasticization of all amorphous and amorphocrystalline polymers.
1991
Glass transition point of plasticized polymers
The widening of the set of relaxation times in the transitional region and henc~ also the difference in the Tg values determined by different methods generally speaking occurs not only on introducing a plasticizer into a polymer but on the contrary on introducing a polymer into a plasticizer. A7-(7 o
/4
~t"
60
60
q
3
qO rio 20
50 w, w~.%
tOO
I
I
50 w, wt'.%
tO0
FIG 2 FIG. 3 FiG 2 Temperature interval of devJtr~ficatlon m the PVC-plastic~zer system as a function of plastlcLzer content for the P V C - D O P systems according to the findings of [21] (1), [26] (3) and (4) and P V C - D O S systems according to [9, p. 94] (2). FiG 3 Dependence of the difference in the glass transition points m a PVC-plastlc~zer system termined by different methods on the plasticizer content 1-T~ [6]-T~ [3]; 2-Tg [7]-Tg
the [3] de-
13],
3 - T ~ [ 8 ] - T K [ 3 ] ; 4 - T g [ 9 ] - T j [6]
For systems with high plasticizer content gradual transition from weakly concentrated polymer solutions to polymer gels is observed. The latter, as indicated by numerous published findings, are macroscopically (morphologically) homogeneous systems but heterogeneous at microlevel. In them, two phases of saturated solutions co-existpolymer m the plasticizer and plasticizer in the polymer uniformly distributed in volume with a corresponding different set of structural elements which apparently also governs the extremal change in AT(Ts)=f(wpl ) and ( T ~ , - T ~ j ) = J (wpl) (Figs. 2 and 3). The second reason for the considerable difference in the Twvalues determined by different methods is purely methodological in character. Those of a methodological character may, m particular, include the results of investigation of polymer-plasticizer systems with a high content of crystalhzable plasticizer. Depending on the method of investigation the moment~ of vitrification of the PVC plasticizer system and crystallization of the plasticizer may be readdy evaluated separately and one may commit an error by taking as the glass transition point the temperature of crystalhzation of the plasticizer (on cooling) or the metllng point of the crystallized plasticizer (on heating). Such a natural error arises from the fact that the changes m many physical properties of polymers close to TB in their character match the changes in the corresponding physical properties on crystallization. For
1992
Yu. V. OVCHINNIKOVand YE. Yu. OVCnlNNIKOV
example, if for the investigation one uses a dynamomechamcal method then in all cases a sharp change m mechanical properties may be observed primarily in the capacity for deformation on exposure to particular loads [31]. In other words, this method does not show which effect is responsible for change in the mechanical properties. At the same time, the above noted features of the behaviour of the poylmer-crystalhzable plasticizer systems very clearly stand out with use of the precision calorimetric method of investigating the temperature dependence of thermal capacity. Let us consider, for example, two systems: PVC-DOS and PVC-dioctyl ester of 1,10-decane&carboxylic acid (DODA). These low molecular weight compounds are freeze-resistant PVC plastlcizers [32] and are usually used m blends with dlalkyl phthalates for imparting better freeze resistance to PVC materials. In pure form and also in amounts exceeding the limit of thermodynamic compattbihty with PVC both DOS and DODA are able to vitrify and crystallize. As shown by investigation of the temperature dependence of the thermal capacity of these plastic~zers the glass transition point of DOS is 161 K [9, p. 69] and of D O D A 175 K [33]; the melting points of the crystalhzed plasticlzers are respectively 228 and 248 (the first crystalline modification of DODA) and 229 K (the second crystalline modification of DODA). In [13] where these systems were investigated by the AleksandrovGayev method ~t was pointed out that the Tg values of pure plasticizers are 230 and 238 K, i e. evidently the melting points of crystallized DOS and D O D A are taken as the T B. As rightly noted in [31] to avoid erroneous conclusions in comparing the T~ values it is necessary to allow (standar&ze) not only for the conditions of conductmg the experiment but also the method of determining Tg. Since &fferent methods even in standard experimental conditions give different values of the glass transition points of plasticized polymers it is in order to ask which methods of investigation should be used for the evaluation of T~. We think that the answer to th~s question is equivocal If m study of plasticization of a polymer the task is essentially of an exploratory nature then preference should apparently be given to more mformatwe methods which allow one to judge together with the glass transition pomt the features of the behaviour of the polymer-plasticizer system over a wide temperature and concentration range. If the evaluation of TB pursues essentially practical goals ~t ~s better to use those methods which most closely reflect the conditions of use of the plasticized materml. Translated by A. CROZV REFERENCES
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