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Molecular mobility in polyethylene after plastic flow under pressure
Molecular mobility in polyethylene
t437
REFERENCES 1. V. N. TSVETKOV, V. Ye. ESKIN and S. Ya. FRENKEL', Struktura makromolekul v rastvore (Structure of Macromolecules in Solution). Moscow, 1964 2. P. P. NEFEDOV and P. N. LAVRENKO, Transportnye metody v analiticheskoi khimii polimerov (Transport Methods in Analytical Chemistry of Polymers). Leningrad, 1979 3. B. G. BELENKII and L. Z. VILENCHIK, Khromatografia polimetov (Chromatography of Polymers). Moscow, 1978 4. E. F. CASASSA, J. Polym. Sci. BS: 773, 1967 5. E. F. CASASSA and Y. TAGAMI, Macromolecules 2: 14, 1969 6. W. HALLER, Macromolecules 10: 202, 1977 7. J, K. LEYPOLDT, R. P. FRIGON and L. W. HENDERSON, J. Appl. Polym. Sci. 29: 3533, 1984 8. J. V. DAWKINS, Eur. Polym. J. 9: 327, 1973 9. L. H. TUNG, J. Appl. Polym. Sci. 10: 375, 1966
Polymer Science U.S.S.R. Vol. 31, No. 6, 1313.1437-1443, 1989 Printed in Poland
0032-3950/89$10.00+.00 ~) 1990Pergamon Press pie
MOLECULAR MOBILITY IN POLYETHYLENE AFTER PLASTIC FLOW UNDER PRESSURE* V. A. ZI-iCRn~, O. IN. SAVRYG1~,I. I. BARASHKOVA, V. M. L1Tvn~ov and N. S. YEN]KOLOPYAN Chemical Physics Institute, U.S.S.R. Academy of Sciences (Received 25 December 1987) A reduction in the translational diffusion of radicals occurs in HDPE as a result of plastic flow under pressure (2 GPa), and is due to a decrease in molecular mobility in the HDPE amorphous phase and to rearrangement of the crystalline phase. The impulse NMR data show that ordered regions are. formed in amorphous phase of the polymer. MANY properties of polymelic materials are determined by their supermolecular structures. This accounts for the undoubted interest being shown in structmal transitions that occur in polymers subjected to mechanical treatment of various types. One intensive form o f treatment is based on the joint action of high pressure and shear stress and results in the materials studied being in the plastic flow state. Major changes occur in polymers undergoing this form of treatment carried out on a Bridgman type device. For instance, I R investigations reported in [1] revealed changes (transitions) occurring in the crystalline structure o f polyolefins; in P E a transition from an o r t h o r h o m b i c type lattice to a monoclinic one was observed. Studies of PE based on the DSC method * Vysokomol. soyed. A31: No. 6, 1311-1315, 1989.
1438
V . A . Znom,N e t al.
s h o w t h a t t h e a m o r p h o u s p h a s e c r y s t a l l i z e s w i t h t h e f o r m a t i o n o f fine crystallites. It is also k n o w n t h a t p l a s t i c flow is a c c o m p a n i e d by a c o m b i n a t i o n o f b o t h in s o m e p o l y m e r s v i a a m o r p h o u s regions [3, 4]. I t is t h e r e f o r e c l e a r t h a t v e r y significant c h a n g e s o c c u r in a m o r p h o u s p h a s e w h e n t r e a t m e n t o f p o l y m e r s l e a d s to p l a s t i c flow u n d e r pressure. O u r a i m in the p r e s e n t i n s t a n c e w a s to investigate c h a n g e s o c c u r r i n g in t h e m o l e c u l a r d y n a m i c s a n d s t r u c t u r e o f P E a m o r p h o u s p h a s e w h e n t h e p o l y m e r is s u b j e c t e d to t h e c o m b i r t e d effects o f h i g h p r e s s u r e a n d s h e a r stress. HDPE was investigated after the plastic flow on an anvil type apparatus in a pressure interval from 0.5--4.0 GPa at room temperature (the deflection angle of the anvils being 100°). Pressure treatment was carried out for films prepared by hot pressing without any special annealing, and likewise for powder-form samples. The treatment did not appear to cause any significant difference in the properties of these samples, and so all the results presented below refer to film samples. A spin probe method was used to investigate structural rearrangements and molecular dynamic features of the PE. This method is widely used by authors studying polymer systems [5]. The \ / spin probe was the stable nitroxyl radical ~ N - 0 , which was incorporated in the HDPE / \ samples from the gas phase at room temperature. To obtain an even distribution of radicals in the polymer, samples were kept for some time in sealed ampoules. The ESR spectra were recorded over a range from 30-70 °, using an EPR-V type spectrometer. Translational diffusion coefficients were determined on the basis of the radical desorption kinetics [6]. Study of molecular mobility in the PE was also carried out using an N M R relaxation method involving an analysis of the shape of free induction decay. At temperatures above Ts there are three regions that may be identified. Relaxation times relating to these regions characterize (1) molecular mobility in the amorphous phase T~2, (2) molecular mobility in the crystalline phase T~ and (3)molecular mobility in the intermediate phase T~"t[7-9l. Moreover the shorter the relaxation time T2, the more limited is the molecular mobility. Weight fractions of the components are in line with contents of these phases in the sample. An analysis of these data enables one to monitor changes in the phase composition and the molecular mobility in the PE when exposed to the effects of high pressure and shear stress. ACTIVATION CHARACTERISTICS OF THE ROTATIONAL ( r ) AND TRANSLATIONAL
(t)
MOBILITY OF THE
PROBE IN P E
E, +_.2, kJ/mole
log Do,
Et +_17, kJ/mole
log Dot
Initial PE
32
14
67
2.1
Probe introduced before treatment Probe introduced after treatment
26 24
12.6 12-6
88 138
4.6 12.0
Samples
Free induction decay measurements were carried out on a multipulse N M R spectrometer (Bruker model SXR) at a proton resonance frequency of 88.08 MHz; the 90 degree pulse length was 2"5 ~sec, the "dead" time of the receiver was 6/~sec, and the temperature 30°. The pulse sequence investigated was ( - 2 r - 9 0 ° x ) , - z - 9 0 ° y [10], the time T being 3/~sec. By using this sequence one is able to avoid systematic errors in determination of the phase composition of the polymer. The last pulse in the sequence is followed by the formation of an echo, making it possible to plot the initial portion of the curve and to eliminate the effect due to the "dead" time of the receiver. Amplification of the signal/noise ratio was obtained by means of an accumulation of signals with a timelag of 3 sec between every consequent accumulation.
1439
Molecular mobility in polyethylene
The Table and Fig. 1 show temperature dependences and activation parameters (the activation energy E and diffusion coetficient D) for processes of rotational and translational diffusion of the probe. It is seen from Fig. 1 that plastic flow leads to a change in the rotational and translational mobility of the probe. The mobility is a function of the mode of incorporation of the radical; the changes are more marked for the samples where the radical was incorporated before treatment under pressure than for those where it was incorporated after this treatment. This finding applies equally for rotational or translational mobility of the radical. ~T~ (
~ec
I0-8 Dr, sec
10-1~ lOe
2.9
3.7
3.3
2.9
3.1 (f0~T),K -~
FIG. 1. Temperature dependences of coefficients of rotational (a) and translational (b) diffusion after plastic flow under a pressure of 2 GPa in the initial PE (1) and in samples in which the probe was introduced after treatment under pressure (2) or before it (3). Changes in the rotational mobility of the probe due to high pressure treatment of PE are the result of a change in the segmental mobility of the macromolecules [5]. When introduced prior to high pressure tleatment of the polymer the probe is surrounded by less mobile chain segments, while the probe introduced after this treatment enters preferentially into less compact mobile regions. The translational diffusion coefficient is a function of amorphous phase mobility, and equally a function of incleased complexity of the transport route appearing as a result of the fact that the radical, moving in the PE has to find its way round crystals that are impenetrable for it. The experimentally determined coefficient of translational diffusion in semicrystalline polymers Dte~pare effective values and are related to coefficients of diffusion in amorphous regions by the formula D x p - FIn/,, (1) t ~ut/¥~ where ? is a coefficient that takes lengthening of the transport route into account [11]. The relationship between rotational diffusion coefficients Dr and Dt for amorphous polymers above TB may be determined by the equation
Dt=uD~ where for the initial PE we have log ~=-34"6; p=2"13.
(2)
1440
V . A . ZHOR~N et al.
Using tho experimental values for D in the samples after high-pressure treatment we now calculate on the basis of equation (2) what the value of the translational diffusion coefficient Dt 'will be if it is related solely to an amorphous-phase change in segmental mobility. In so doing we are assuming that treatment of the PE leads to no change in or/~, and for the initial polymer we will say that 7= 1. The Dt values calculated in this way are then compared with the experimental ~tr~'xP,and equation (1) is used to determine the value of 7 (Fig. 2). A study of the data in Fig. 2 shows the changes in 7 for samples that underwent the treatment (pressure 2 GPa), introducing the probe before the pressure treatment, and after it. Introduction of the probe after the treatment leads to an increase in the value of 7. This means that the polymer contains an increased amount of regions of dense1 structure which the radical cannot enter and which it has to circumvent. This could be the result of a fragmentation of PE crystailites and to a reorganization of amorphous phase that may involve partial crystallization of PE chains. In the case of the samples into which the probe was introduced ahead of pressure treatment there
M(~)/M(O) Z~
I0-2
G
"°. 2
1
2 \ "..%.3
I~
2 l 40
I 6O
I
.,
50
To
Fro. 2
I
700 Fxo.
I
150 f,#sec
3
l~o. 2. Temperature dependences of the coefficient of transport path lengthening for the probe in PE samples after plastic flow under a pressure of 2 GPa with introduction of the radical before (1) or after treatment under pressure (2). Fmi 3. Free radical decay M(t)/M(O) for the initial sample (1) and for samples treated under pressure of 2 GPa (2), annealing after pressure at 100 ° for 1.5 hr (3), remehing of the sample after pressure at lO0 ° for ~0 rain (4).
Molecular mobility in polyethylene
1441
is a wider range of 3' values. This difference in the ~ values means that the probe added to the PE samples prior to their treatment has to go on a further path or route in the desorption process compared with that taken by the radical added to samples after their treatment. It follows that in the course of plastic flow the radical finds itself in regions that it does not enter when it is added to samples that have already undergone pressure treatment. So the behaviour of the probe in samples into which it was introduced prior to the pressure treatment allows a more detailed determination of the change that takes place in the amorphous phase of the PE in the course of plastic flow. One could say that it involves a process whereby more compact regions have been formed, each region in turn consisting of parts having diverse properties from the standpoint of their effect on the kinetics of desorption of the probe. Further information on the change in molecular mobility occurring in PE under the action of pressure combined with shear stress was obtained through an analysis of the free induction decay curves. The data for the initial HDPE are presented in Fig. 3. The curves consist of three portions: an initial one (with t~20--30/zsec), which relates to relaxation of crystalline phase; a portion with t~>70/zsec, relating to relaxation of amorphous phase; thirdly, a middle portion (30
1442
v.A. ZHORINet aL
mobility o f polymer units in amorphous does not increase in the course of annealing, which is probably further evidence of orientation of chain fragments occurring in amorphous regions. The recovery o f amorphous phase mobility takes place only after remelting o f the samples. At the same time mobility in the newly formed amorphous phase becomes even a little greater than in the initial sample, since T~ is increased to 52/tsec. In view of the experimental results it may therefore be said that plastic flow under pressure loads to a change in the properties of both crystalline and amorphous phases o f the polymer. Mobility in the crystalline phase increases and approximates to the less structurally ordered intermediate phase. This could be due both to a polymorphic transition and to formation of a more defective crystalline structure under pressure. There is reduced mobility in the amorphous phase. The data obtained by the two experimental methods point to formation, in the amorphous phase, of regions whose density exceeds that o f amorphous phase in the initial PE. In view of these results we propose that the process o f plastic flow is accompanied by the formation of ordered regions, i.e. a sort of "crystallization" o f amorphous phase takes place. Similar conclusions regarding ordered regions emerging in amorphous phase were reported by Zhorin [2] and coworkers who carried out a DSC analysis o f PE and of some other polymers, as well as by Badayev and coweIkers [13] who investigated the viscoelastic properties of PE following plastic flow under pressure. Translated by R. J. A. HENDRY
REFERENCES
1. V. A. ZHORIN, Yu. V. KISSIN, Yu. V. LUIZO, N. M. FRIDMAN and N. S. YENIKOLOPYAN, Vysokomol. soyed. A18: 12, 2677, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 12, 3057, 1976) 2. V. A. ZHORIN and Yu. K. GODOVSKII, Vysokomol. soyed. A24: 5, 953, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 5, 1073, 1982) 3. V. A. ZHORIN, N. A. MIRONOV, V. G. NIKOL'SKII and N. S. YENIKOLOPYAN, Dokl. AN SSSR 244: 5, 1153, 1979 4. V. A. ZHORIN, I. A. MIRONOV, T. A. ALEKSANDROVA, A. I. KRYUKOV, V. G. NIKOL'SKII and N. S. YENIKOLOPYAN, Vysokomol. soyed. B23: 8, 606, 1981 (Not translated in Polymer Sci. U.S.S.R.) 5. A. M. VASSERMAN and A. L. KOVARSKII, Spinovye metki i zondy v fizikokhimii polimerov (Spin Labels and Spin Probes in the Physical Chemistry of Polymers. p. 245, Moscow, 1986 6. Diffusion in Polymers (Eds. J. Crank and G. S. Park), p. 452, London and New York, 1968 7. M. ITO, T. MANAMOTO, T. TANAKA and R. S. PORTER, Macromolecules 14: 6, 1779, 1981 8. V. D. FEDOTOV and N. A. ABDRASHITOVA, Vysokomol. soyed. A27: 2, 263, 1985 (Translated in Polymer Sci. U.S.S.R. 27: 2, 287, 1985) 9. K. BERGMAN, J. Polymer Sci. Polymer Phys. Ed. 16: 9, 1611, 1978 10. H. SCHNEIDER and H. SCHMIDEL, Phys. Letters A30: 1, 298, 1969 11. S. A. ROITLINGER, Pronitsayemost' polimernykh materialov (Permeability of Polymeric Materials). p. 272, Moscow, 1974
1443
Thermodynamic compatibility with polyvinylnitrate
12. V. A. ZHORIN, A. V. MAKSIMYCHEV, M. Ya. KUSHNEREV, D. P. SHASHKIN and N. S. YENIKOLOPYAN, Zhurn. fiz. khim. 53: II, 2772, 1979 13. A. S. BADAYEV, V. A. ZHORIN, I. I. PEREPECHKO and N. S. YENIKOLOPYAN, Dokl AN SSSR 289: 5, 1148, 1986
Polymer Science U.S.S.R. Vol. 31, No. 6, pp. 1443-1448, 1989 Printed in Poland
0032-3950/89 $10.00+ .00 (~) 1990 Pergamon Press pie
EFFECT OF THE DEGREE OF SUBSTITUTION OF CELLULOSE NITRATE ON ITS THERMODYNAMIC COMPATIBILITY WITH POLYVINYLNITRATE* A. A. TAGER, N. I. SHIL'NIKOVA,V. F. SoPm and G. N. MARCHENKO Gorkii State University of the Urals
(Received 15 January 1988) Gibbs energies of mixing have been determined for two cellulose nitrate samples with differing degrees of substitution with polyvinylnitrate. It was found that the CN sample containing 13.4~ nitrogen is incompatible with polyvinylnitrate over the entire range of compositions, while the sample containing 1270 nitrogen is compatible with the polyvinylnitrate only if its concentration in the mixture does not exceed 60~. As EARLYas 1947 the system C N - P V A was one of the first of the thermodynamically compatible polymer-polymer systems that were investigated. Subsequent studies [2, 3] showed that C N containing 12 ~o nitrogen is compatible with PVA and the Gibbs energy is reduced. The mixing o f the polymers is accompanied by high negative values for the enthalpy and entropy and the Gibbs energy of mixing such as appear typical for thermodynamically stable polymer-polymer systems [4-45]. Our aim in the present investigation was to examine the thermodynamic compatibility o f C N with polyvinylnitrate (PVN) and to determine the extent to which the degree o f substitution of C N affects the thermodynamic compatibility. A study was made of CN samples containing 13.4 and 12~/~nitrogen with 39/nequal to 6.5 × 10~ and 9"5x 104 respectively. PVN had 37/,=3 x 106. Evaluation of the thermodynamic compatibility of the polymers was based on the sign of the Gibbs energies of mixing determined by the method proposed by Tager and coworkers [2-4], for which an experimental study of the sorption of acetone vapours was carried out at 298 K in the pressure range 10-3-10 -4 Pa with samples of NC, PVN and their blends, using a sorption device fitted with a quartz spring balance, its sensitivity being (0.2-0.3) x 103 m/kg. By means of the * Vysokomol. soyed. A31" No. 6, 1316-1319, 1989.
t437
REFERENCES 1. V. N. TSVETKOV, V. Ye. ESKIN and S. Ya. FRENKEL', Struktura makromolekul v rastvore (Structure of Macromolecules in Solution). Moscow, 1964 2. P. P. NEFEDOV and P. N. LAVRENKO, Transportnye metody v analiticheskoi khimii polimerov (Transport Methods in Analytical Chemistry of Polymers). Leningrad, 1979 3. B. G. BELENKII and L. Z. VILENCHIK, Khromatografia polimetov (Chromatography of Polymers). Moscow, 1978 4. E. F. CASASSA, J. Polym. Sci. BS: 773, 1967 5. E. F. CASASSA and Y. TAGAMI, Macromolecules 2: 14, 1969 6. W. HALLER, Macromolecules 10: 202, 1977 7. J, K. LEYPOLDT, R. P. FRIGON and L. W. HENDERSON, J. Appl. Polym. Sci. 29: 3533, 1984 8. J. V. DAWKINS, Eur. Polym. J. 9: 327, 1973 9. L. H. TUNG, J. Appl. Polym. Sci. 10: 375, 1966
Polymer Science U.S.S.R. Vol. 31, No. 6, 1313.1437-1443, 1989 Printed in Poland
0032-3950/89$10.00+.00 ~) 1990Pergamon Press pie
MOLECULAR MOBILITY IN POLYETHYLENE AFTER PLASTIC FLOW UNDER PRESSURE* V. A. ZI-iCRn~, O. IN. SAVRYG1~,I. I. BARASHKOVA, V. M. L1Tvn~ov and N. S. YEN]KOLOPYAN Chemical Physics Institute, U.S.S.R. Academy of Sciences (Received 25 December 1987) A reduction in the translational diffusion of radicals occurs in HDPE as a result of plastic flow under pressure (2 GPa), and is due to a decrease in molecular mobility in the HDPE amorphous phase and to rearrangement of the crystalline phase. The impulse NMR data show that ordered regions are. formed in amorphous phase of the polymer. MANY properties of polymelic materials are determined by their supermolecular structures. This accounts for the undoubted interest being shown in structmal transitions that occur in polymers subjected to mechanical treatment of various types. One intensive form o f treatment is based on the joint action of high pressure and shear stress and results in the materials studied being in the plastic flow state. Major changes occur in polymers undergoing this form of treatment carried out on a Bridgman type device. For instance, I R investigations reported in [1] revealed changes (transitions) occurring in the crystalline structure o f polyolefins; in P E a transition from an o r t h o r h o m b i c type lattice to a monoclinic one was observed. Studies of PE based on the DSC method * Vysokomol. soyed. A31: No. 6, 1311-1315, 1989.
1438
V . A . Znom,N e t al.
s h o w t h a t t h e a m o r p h o u s p h a s e c r y s t a l l i z e s w i t h t h e f o r m a t i o n o f fine crystallites. It is also k n o w n t h a t p l a s t i c flow is a c c o m p a n i e d by a c o m b i n a t i o n o f b o t h in s o m e p o l y m e r s v i a a m o r p h o u s regions [3, 4]. I t is t h e r e f o r e c l e a r t h a t v e r y significant c h a n g e s o c c u r in a m o r p h o u s p h a s e w h e n t r e a t m e n t o f p o l y m e r s l e a d s to p l a s t i c flow u n d e r pressure. O u r a i m in the p r e s e n t i n s t a n c e w a s to investigate c h a n g e s o c c u r r i n g in t h e m o l e c u l a r d y n a m i c s a n d s t r u c t u r e o f P E a m o r p h o u s p h a s e w h e n t h e p o l y m e r is s u b j e c t e d to t h e c o m b i r t e d effects o f h i g h p r e s s u r e a n d s h e a r stress. HDPE was investigated after the plastic flow on an anvil type apparatus in a pressure interval from 0.5--4.0 GPa at room temperature (the deflection angle of the anvils being 100°). Pressure treatment was carried out for films prepared by hot pressing without any special annealing, and likewise for powder-form samples. The treatment did not appear to cause any significant difference in the properties of these samples, and so all the results presented below refer to film samples. A spin probe method was used to investigate structural rearrangements and molecular dynamic features of the PE. This method is widely used by authors studying polymer systems [5]. The \ / spin probe was the stable nitroxyl radical ~ N - 0 , which was incorporated in the HDPE / \ samples from the gas phase at room temperature. To obtain an even distribution of radicals in the polymer, samples were kept for some time in sealed ampoules. The ESR spectra were recorded over a range from 30-70 °, using an EPR-V type spectrometer. Translational diffusion coefficients were determined on the basis of the radical desorption kinetics [6]. Study of molecular mobility in the PE was also carried out using an N M R relaxation method involving an analysis of the shape of free induction decay. At temperatures above Ts there are three regions that may be identified. Relaxation times relating to these regions characterize (1) molecular mobility in the amorphous phase T~2, (2) molecular mobility in the crystalline phase T~ and (3)molecular mobility in the intermediate phase T~"t[7-9l. Moreover the shorter the relaxation time T2, the more limited is the molecular mobility. Weight fractions of the components are in line with contents of these phases in the sample. An analysis of these data enables one to monitor changes in the phase composition and the molecular mobility in the PE when exposed to the effects of high pressure and shear stress. ACTIVATION CHARACTERISTICS OF THE ROTATIONAL ( r ) AND TRANSLATIONAL
(t)
MOBILITY OF THE
PROBE IN P E
E, +_.2, kJ/mole
log Do,
Et +_17, kJ/mole
log Dot
Initial PE
32
14
67
2.1
Probe introduced before treatment Probe introduced after treatment
26 24
12.6 12-6
88 138
4.6 12.0
Samples
Free induction decay measurements were carried out on a multipulse N M R spectrometer (Bruker model SXR) at a proton resonance frequency of 88.08 MHz; the 90 degree pulse length was 2"5 ~sec, the "dead" time of the receiver was 6/~sec, and the temperature 30°. The pulse sequence investigated was ( - 2 r - 9 0 ° x ) , - z - 9 0 ° y [10], the time T being 3/~sec. By using this sequence one is able to avoid systematic errors in determination of the phase composition of the polymer. The last pulse in the sequence is followed by the formation of an echo, making it possible to plot the initial portion of the curve and to eliminate the effect due to the "dead" time of the receiver. Amplification of the signal/noise ratio was obtained by means of an accumulation of signals with a timelag of 3 sec between every consequent accumulation.
1439
Molecular mobility in polyethylene
The Table and Fig. 1 show temperature dependences and activation parameters (the activation energy E and diffusion coetficient D) for processes of rotational and translational diffusion of the probe. It is seen from Fig. 1 that plastic flow leads to a change in the rotational and translational mobility of the probe. The mobility is a function of the mode of incorporation of the radical; the changes are more marked for the samples where the radical was incorporated before treatment under pressure than for those where it was incorporated after this treatment. This finding applies equally for rotational or translational mobility of the radical. ~T~ (
~ec
I0-8 Dr, sec
10-1~ lOe
2.9
3.7
3.3
2.9
3.1 (f0~T),K -~
FIG. 1. Temperature dependences of coefficients of rotational (a) and translational (b) diffusion after plastic flow under a pressure of 2 GPa in the initial PE (1) and in samples in which the probe was introduced after treatment under pressure (2) or before it (3). Changes in the rotational mobility of the probe due to high pressure treatment of PE are the result of a change in the segmental mobility of the macromolecules [5]. When introduced prior to high pressure tleatment of the polymer the probe is surrounded by less mobile chain segments, while the probe introduced after this treatment enters preferentially into less compact mobile regions. The translational diffusion coefficient is a function of amorphous phase mobility, and equally a function of incleased complexity of the transport route appearing as a result of the fact that the radical, moving in the PE has to find its way round crystals that are impenetrable for it. The experimentally determined coefficient of translational diffusion in semicrystalline polymers Dte~pare effective values and are related to coefficients of diffusion in amorphous regions by the formula D x p - FIn/,, (1) t ~ut/¥~ where ? is a coefficient that takes lengthening of the transport route into account [11]. The relationship between rotational diffusion coefficients Dr and Dt for amorphous polymers above TB may be determined by the equation
Dt=uD~ where for the initial PE we have log ~=-34"6; p=2"13.
(2)
1440
V . A . ZHOR~N et al.
Using tho experimental values for D in the samples after high-pressure treatment we now calculate on the basis of equation (2) what the value of the translational diffusion coefficient Dt 'will be if it is related solely to an amorphous-phase change in segmental mobility. In so doing we are assuming that treatment of the PE leads to no change in or/~, and for the initial polymer we will say that 7= 1. The Dt values calculated in this way are then compared with the experimental ~tr~'xP,and equation (1) is used to determine the value of 7 (Fig. 2). A study of the data in Fig. 2 shows the changes in 7 for samples that underwent the treatment (pressure 2 GPa), introducing the probe before the pressure treatment, and after it. Introduction of the probe after the treatment leads to an increase in the value of 7. This means that the polymer contains an increased amount of regions of dense1 structure which the radical cannot enter and which it has to circumvent. This could be the result of a fragmentation of PE crystailites and to a reorganization of amorphous phase that may involve partial crystallization of PE chains. In the case of the samples into which the probe was introduced ahead of pressure treatment there
M(~)/M(O) Z~
I0-2
G
"°. 2
1
2 \ "..%.3
I~
2 l 40
I 6O
I
.,
50
To
Fro. 2
I
700 Fxo.
I
150 f,#sec
3
l~o. 2. Temperature dependences of the coefficient of transport path lengthening for the probe in PE samples after plastic flow under a pressure of 2 GPa with introduction of the radical before (1) or after treatment under pressure (2). Fmi 3. Free radical decay M(t)/M(O) for the initial sample (1) and for samples treated under pressure of 2 GPa (2), annealing after pressure at 100 ° for 1.5 hr (3), remehing of the sample after pressure at lO0 ° for ~0 rain (4).
Molecular mobility in polyethylene
1441
is a wider range of 3' values. This difference in the ~ values means that the probe added to the PE samples prior to their treatment has to go on a further path or route in the desorption process compared with that taken by the radical added to samples after their treatment. It follows that in the course of plastic flow the radical finds itself in regions that it does not enter when it is added to samples that have already undergone pressure treatment. So the behaviour of the probe in samples into which it was introduced prior to the pressure treatment allows a more detailed determination of the change that takes place in the amorphous phase of the PE in the course of plastic flow. One could say that it involves a process whereby more compact regions have been formed, each region in turn consisting of parts having diverse properties from the standpoint of their effect on the kinetics of desorption of the probe. Further information on the change in molecular mobility occurring in PE under the action of pressure combined with shear stress was obtained through an analysis of the free induction decay curves. The data for the initial HDPE are presented in Fig. 3. The curves consist of three portions: an initial one (with t~20--30/zsec), which relates to relaxation of crystalline phase; a portion with t~>70/zsec, relating to relaxation of amorphous phase; thirdly, a middle portion (30
1442
v.A. ZHORINet aL
mobility o f polymer units in amorphous does not increase in the course of annealing, which is probably further evidence of orientation of chain fragments occurring in amorphous regions. The recovery o f amorphous phase mobility takes place only after remelting o f the samples. At the same time mobility in the newly formed amorphous phase becomes even a little greater than in the initial sample, since T~ is increased to 52/tsec. In view of the experimental results it may therefore be said that plastic flow under pressure loads to a change in the properties of both crystalline and amorphous phases o f the polymer. Mobility in the crystalline phase increases and approximates to the less structurally ordered intermediate phase. This could be due both to a polymorphic transition and to formation of a more defective crystalline structure under pressure. There is reduced mobility in the amorphous phase. The data obtained by the two experimental methods point to formation, in the amorphous phase, of regions whose density exceeds that o f amorphous phase in the initial PE. In view of these results we propose that the process o f plastic flow is accompanied by the formation of ordered regions, i.e. a sort of "crystallization" o f amorphous phase takes place. Similar conclusions regarding ordered regions emerging in amorphous phase were reported by Zhorin [2] and coworkers who carried out a DSC analysis o f PE and of some other polymers, as well as by Badayev and coweIkers [13] who investigated the viscoelastic properties of PE following plastic flow under pressure. Translated by R. J. A. HENDRY
REFERENCES
1. V. A. ZHORIN, Yu. V. KISSIN, Yu. V. LUIZO, N. M. FRIDMAN and N. S. YENIKOLOPYAN, Vysokomol. soyed. A18: 12, 2677, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 12, 3057, 1976) 2. V. A. ZHORIN and Yu. K. GODOVSKII, Vysokomol. soyed. A24: 5, 953, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 5, 1073, 1982) 3. V. A. ZHORIN, N. A. MIRONOV, V. G. NIKOL'SKII and N. S. YENIKOLOPYAN, Dokl. AN SSSR 244: 5, 1153, 1979 4. V. A. ZHORIN, I. A. MIRONOV, T. A. ALEKSANDROVA, A. I. KRYUKOV, V. G. NIKOL'SKII and N. S. YENIKOLOPYAN, Vysokomol. soyed. B23: 8, 606, 1981 (Not translated in Polymer Sci. U.S.S.R.) 5. A. M. VASSERMAN and A. L. KOVARSKII, Spinovye metki i zondy v fizikokhimii polimerov (Spin Labels and Spin Probes in the Physical Chemistry of Polymers. p. 245, Moscow, 1986 6. Diffusion in Polymers (Eds. J. Crank and G. S. Park), p. 452, London and New York, 1968 7. M. ITO, T. MANAMOTO, T. TANAKA and R. S. PORTER, Macromolecules 14: 6, 1779, 1981 8. V. D. FEDOTOV and N. A. ABDRASHITOVA, Vysokomol. soyed. A27: 2, 263, 1985 (Translated in Polymer Sci. U.S.S.R. 27: 2, 287, 1985) 9. K. BERGMAN, J. Polymer Sci. Polymer Phys. Ed. 16: 9, 1611, 1978 10. H. SCHNEIDER and H. SCHMIDEL, Phys. Letters A30: 1, 298, 1969 11. S. A. ROITLINGER, Pronitsayemost' polimernykh materialov (Permeability of Polymeric Materials). p. 272, Moscow, 1974
1443
Thermodynamic compatibility with polyvinylnitrate
12. V. A. ZHORIN, A. V. MAKSIMYCHEV, M. Ya. KUSHNEREV, D. P. SHASHKIN and N. S. YENIKOLOPYAN, Zhurn. fiz. khim. 53: II, 2772, 1979 13. A. S. BADAYEV, V. A. ZHORIN, I. I. PEREPECHKO and N. S. YENIKOLOPYAN, Dokl AN SSSR 289: 5, 1148, 1986
Polymer Science U.S.S.R. Vol. 31, No. 6, pp. 1443-1448, 1989 Printed in Poland
0032-3950/89 $10.00+ .00 (~) 1990 Pergamon Press pie
EFFECT OF THE DEGREE OF SUBSTITUTION OF CELLULOSE NITRATE ON ITS THERMODYNAMIC COMPATIBILITY WITH POLYVINYLNITRATE* A. A. TAGER, N. I. SHIL'NIKOVA,V. F. SoPm and G. N. MARCHENKO Gorkii State University of the Urals
(Received 15 January 1988) Gibbs energies of mixing have been determined for two cellulose nitrate samples with differing degrees of substitution with polyvinylnitrate. It was found that the CN sample containing 13.4~ nitrogen is incompatible with polyvinylnitrate over the entire range of compositions, while the sample containing 1270 nitrogen is compatible with the polyvinylnitrate only if its concentration in the mixture does not exceed 60~. As EARLYas 1947 the system C N - P V A was one of the first of the thermodynamically compatible polymer-polymer systems that were investigated. Subsequent studies [2, 3] showed that C N containing 12 ~o nitrogen is compatible with PVA and the Gibbs energy is reduced. The mixing o f the polymers is accompanied by high negative values for the enthalpy and entropy and the Gibbs energy of mixing such as appear typical for thermodynamically stable polymer-polymer systems [4-45]. Our aim in the present investigation was to examine the thermodynamic compatibility o f C N with polyvinylnitrate (PVN) and to determine the extent to which the degree o f substitution of C N affects the thermodynamic compatibility. A study was made of CN samples containing 13.4 and 12~/~nitrogen with 39/nequal to 6.5 × 10~ and 9"5x 104 respectively. PVN had 37/,=3 x 106. Evaluation of the thermodynamic compatibility of the polymers was based on the sign of the Gibbs energies of mixing determined by the method proposed by Tager and coworkers [2-4], for which an experimental study of the sorption of acetone vapours was carried out at 298 K in the pressure range 10-3-10 -4 Pa with samples of NC, PVN and their blends, using a sorption device fitted with a quartz spring balance, its sensitivity being (0.2-0.3) x 103 m/kg. By means of the * Vysokomol. soyed. A31" No. 6, 1316-1319, 1989.