- Email: [email protected]
Acceleration of relaxational processes in polymers under radiation
Polymer ScienceU.S.S.R. Vol. 30, No. 6, lap. 1336-1340, 1988 Printed in Poland
ACCELERATION
0032-39.50188 $I0.00+.00 ~ 1989 Pergamon Press pie
OF RELAXATIONAL
IN POLYMERS
UNDER
PROCESSES
RADIATION*
V. V. MALAYEV a n d V. F. STEPANOV L. Ya. Karpov Physicochemical Scientific Research Institute
(Received 5 January 1987) Irradiation with electrons and UV light increases the rate of spontaneous contraction of unloaded polymer specimens. This phenomenon is associated with the accleeration of relaxational processes of reverse slipping of elastically strained chain polymer macromolecules re, lative to the main bulk of unstretched molecules during specimen unloading.
PROC~SS~ associated with the transition of any system from the non-equilibrium to the equilibrium state are called relaxational, and such processes play an important part in application of polymer materials, since these are often in the non-equilibrium state [1, 2]. In this work the relaxational compression of previously loaded polymer materials is studied. When the load is removed from the polymer specimen it returns to its original shape because of relaxation, and deformation under tension is reduced, i.e. reverse creep associated with intermolecular regrouping, with slipping of the supermolecular structures relative to their environment, and with restoration of the equilibrium shape by the rupture and restoration of intermolecular bonds. Fracture of the remaining stressed section of the polymer chains results in an increase in specimen deformation, i.e. it acts in a direction opposite to the direction of action of relaxational shortening of the specimen length. Acceleration of relaxational processes can be associated with heating of the polymer specimens on irradiation. Overall heating of a specimen under irradiation, if it occurs, results in expansion and an increase in linear dimensions, i.e. it also acts in a direction opposite to the direction of the relaxational processes. Stepanov [3] evaluated the heating of polymer specimens on irradiation by electrons. Polyacrylamide and polyethylene terephthalate films of thickness 100/tm were'heated under a radiation dose of b =2 kGr/sec by about 1 K. Threads of the s'ame materials and consisting of 80 monofibres, diameter 0.02 ram, overall thread diameter about 0.08 mm, were heated up less than I K under the same electron radiation dose. When the temperature of the air or nitrogen sweeping the specimen was deliberately increased by 1 K, no appreciable increase in rate of the relaxational process could be detected, within the limits of the measurement error. In this work the heating up of polymer specimens by UV light was determined from the reverse thermal expansion of an unloaded specimen and from * Vysokomol. soyed. A30: No. 6, 1275-1278, 1988. 1336
Acceleration of relaxational processes in polymers under radiation
1337
the j u m p in deformation on irradiating or ceasing to irradiate the specimen. The specimen temperature before UV irradiation was thus deliberately lowered by an amount equal to the heating up produced by light, i.e. was held constant. The creep rate in irradiation of polymer specimens was studied on a setput intended l\~r loading the specimens through blocks. The lower specimen clamp was stationary, whereas the upper moving clamp was linked kinematically to the rotation of the rotor of a selsyn inductive pick-up. An electrical signal proportional to the elongation or compression of the specimen was fed to the input of an automatic recorder. The method is described in detail by Stepanov [3]. It is of interest to confirm whether or not irradiation accelerates relaxational restoration of the dimensions of a fractured polymer, when the number of stressed chemical bonds in the specimen is markedly decreased, i.e. whether or not irradiation accelerates the breaking of inter-molecular bonds. The w o r k was thus concerned with demonstrating the increased rate of relaxational processes in polymers subjected to the action of ionizing and UV irradiation, i.e. with demonstrating that the acceleration of relaxational processes under the action of irradiation, is similar to their acceleration on raising the temperature. The radiation dose was 2 kGr/sec, and the energy of the accelerated electrons at the maximum distribution was 300 keV. The UV light source was a DRSh-1000 lamp. The radiation was focussed on the specimen by means of quartz lenses. The region 250-313 nm was separated from the lamp spectrum by means of a CoSO4-NiSO4 liquid filter. The UV irradiation quanta density (3 × 10~ cm2.sec - t) was controlled by means of an F-7 photoelement. The specimens were irradiated in a special thermostat swept with a nitrogen current,to decrease heating up of the specimens under irradiation and to eliminate possible oxidation reactions. The temperature close to the specimen was recorded by means of Cu-constantan thermocouples. The heating up of the specimens was determined from the jump in temperature under the action of the UV radiation. Regel et al. [4] showed that UV irradiation breaks only the chemical bonds in polymers, but does not apprecially affect the rate of intermolecular regroupings and the breaking of intermolecular chains.'Acceleration of relaxational creep on irradiation was not observed against the background of the considerable creep acceleration due to breakage of the chemical bonds in the macromolecule chains. It was thus interesting to study and compare the laws of relaxational processes in unloaded polymers subjected to irradiation with electrons and UV radiation. With this in view, the following tests were made. Specimens of various polymers, i.e. polyacrylamide and PTFE threads, polyethylene, polymethyl methacrylate, polystyrene, PTFE, and polycarbonate films were loaded for 5 rain to stresses amounting to 50-70 ~5~iof the fracture stress. This resulted in a specific deformation of the polymer specimens, i.e. e , = ( l - l o ) / l o, where/o is the length of the original specimen, and l is the length of the specimen after deformation. The specimens were then rapidly unloaded to a stress less by two orders to magnitude. Decrease in the elongation produced then started, i.e. a process of deformation relaxation. A certain time after the relaxational process of shortening of the unloaded polymer specimen had started, the specimen was subjected
1338
V. V. MALAYEVand V. F. SI"ZI+ANOV
to irradiation with electrons or UV light. Relaxational compression of the specimens accelerated sharply from the moment o f irradiating the specimens, and when irradiation was stopped it slowed down to the original rate. Figure 1 shows a loading and unloading diagram for an oriented polycaproamide specimen and the corresponding relaxational processes. The diagram shows the change in relative elongation e of a specimen subjected to different stresses. It can be seen that relaxational compression, which starts on unloading the specimen from the moment t2, s reversibly accelerated by irradiation with electrons.
~,£,% 0.3'
10-
O'Z
+I .!'; r-..
,
,
0"1
f
t21N~ i TM, 300 T¢o~"....,~.~ Time, sec
,
zlO0
.-:'me. sec
800
Flo. 1 Fro. 2 FIG. 1. Dia~gram showing the loading and unloading and deforma)ional relaxation of standard polyeaproamide threads: at=13, a2ffi400 MPa, where tl is the moment of loading from ~1 to tr2, t2 is the moment of unloading from a2 to.ax, t3 is the moment of starting, and t+ is the moment of finishing the electron irradiation. FIo. 2. Change in deformation ,J8 of PTFE threads at 298 K as a function of time: 1 - without irradiation on unloading from #1 =250 to a=8 MPa; 2, 3--tL--the moment of starting UV irradiation (2) and electron irradiation (3), t2- the moment of stopping electron irradiation. Figure 2 (curve 1) shows, on an increased scale, the relaxational compression of oriented PTFE, which starts from the moment of its unloading and is recorded automatically. The quantity Ae indicates the difference A~=8--81=
l-1 o
It--I o
I-1 t
AI
Io
1o
1o
1o
where It is the larger part of the specimen length after deformation. Zero deformation (,48 =0) corresponds to the lower edge of the automatic recorder diagram, and on attaining it the scale was shifted to the other edge of the diagram in order to follow the whole process on an enlarged scale. Curve 2 corresponded to relaxational compression of the P T F E on UV inadiation, and curve 3 to it~ relaxational compression on irradiation with accelerated electrons. In this Figure the acceleration of relaxational compression of a specimen under the action of both types o f radiation can be seen more clearly, so that the data can be differentiated graphically and evaluated quantitatively.
Acceleration of relaxational processes in polymers under radiation
1339
The rate of the relaxation process (c_ t) is expressed by differentiating the deformation with respect to time, which gives the differential equation v = - d ~ / d t = - d ( d e ) / d t = - d (e - e t ) / d t = - ( d l / d t ) / l o .
In Fig. 3 the data of Fig. 2 are represented in differential form, i.e. the rate of relaxational compression of P T F E v is plotted against the time t (the notation on the curves in Fig. 3 corresponds to that on the curves in Fig. 2). It can be seen that for the selected intensities of electron and UV irradiation, the accelerated electrons produce a large increase in the rate of the relaxational processes. Moreover, under the action o f the UV radiation there is a constant fall in relaxation rate (Fig. 3, curve 2), whereas under the action of accelerated electrons (Fig. 3, curve 3) the relaxation rate remains constant up to the moment of stopping irradiation, and then falls to the original rate, i.e. the effect is reversible. Irradiation with accelerated electrons results in greater elongation of loaded and oriented polycaproamide specimens than does UV irradiation, as shown in Fig. 4, which shows a plot of the product r v = e (where r is the radiation time, v is the creep rate, and e is the total deformation up to fracture under irradiation) for polycaproamide against the stress tr under the action of electron and UV irradiation. Electron radiation
/.5 !
t
- u , l O ~ s e c "r
lO
I
qO0
1
]
8OO
T[nTe , sec
FIG. 3 FIG. 4 FI~. 3. Rate of deformation relaxation of unloaded PTFE threads as a function of time without irradiation (1) and under UV irradiation (2) and electron irradiation (3); tt is the moment of starting irradiation and t2 is the moment of stopping electron irradiation. FIo. 4. Product rv for polycaproamide threads at 298 K as a function of a on electron (1) and UV irradiation (2).
is much more effective in accelerating relaxation in polycaproamide than UV radiation, i.e. the accelerated electrons promote slipping of the polymer chains without fracture under load, thus increasing the overall deformation of the polymer specimen on irradiation.
1340
V. V. MALAYEV and V. F. Sr~rANOV
Accordingly, in this work direct acceleration of relaxation processes in unloaded polymers by the action o f radiation is observed, The observed laws provide confirmation that it is impossible to ignore the radiation induced kinetic acceleration of intermolecular regrouping, and the accelerated slipping of macromolecules and supermolecular structures relative to their surroundings. The mechanism ot' this phenomenon can be associated with radiation induced generation of additional vibratory excitations in solids, arising on nonradiative degradation of electron excitation into thermal vibrations. These additional vibratory excitations must accelerate any kinetic processes in solids: diffusion, gas desorption, etc., as has been observed by various authors [5--7]. The radiational acceleration o f kinetic intermolecular regrouping processes can be used in practice for radiational modification and technological treatment of polymer materials. Translated by N. STANDI~N REFERENCES
1. A. A. TAGER, Fizikokhimiya polimerov (Physical Chemistry of Polymers). Moscow, 1978 2. V. Ye. GUL and V. N. KULEZNEV, Struktura i mekhanicheskie svoistva polimerov (Structure and Mechanical Properties of Polymers). Moscow, 1979 3. V. F. STEPANOV, Diss. Kand. Fiz. Mat. Nauk, L. Ya. Karpov Physical Chemistry Research Institute, Moscow, 1971 4. V. R. REGEL, A. I. SLUTSKER and E. Ye. TOMASHEVSKII, Kineticheskaya priroda prochnosti tverdykh tel (Kinetic Nature of the Strength of Solids). Moscow, 1974 5. O. A. KLIMOVA and O. R. NIYAZOVA, Fiz. tverd, tela, 12, 2199, 1970 6. E. F. LAZNEVA, T. G. BYKOVA, L. A. SERGEYEVA and Yu. A. KHARLAMOV, Voprosy elektroniki tverdogo tela (Problems in the Electronics of Solids). Collection 4, Leningrad, 1974 7. Yu. I. BELYAKOV, M. P. DATSIEV and T. N. KOMPANIETS, Voprosy elektroniki tverdogo tela (Problems in the Electronics.of Solids). Collection 4, Leningrad, 1974
ACCELERATION
0032-39.50188 $I0.00+.00 ~ 1989 Pergamon Press pie
OF RELAXATIONAL
IN POLYMERS
UNDER
PROCESSES
RADIATION*
V. V. MALAYEV a n d V. F. STEPANOV L. Ya. Karpov Physicochemical Scientific Research Institute
(Received 5 January 1987) Irradiation with electrons and UV light increases the rate of spontaneous contraction of unloaded polymer specimens. This phenomenon is associated with the accleeration of relaxational processes of reverse slipping of elastically strained chain polymer macromolecules re, lative to the main bulk of unstretched molecules during specimen unloading.
PROC~SS~ associated with the transition of any system from the non-equilibrium to the equilibrium state are called relaxational, and such processes play an important part in application of polymer materials, since these are often in the non-equilibrium state [1, 2]. In this work the relaxational compression of previously loaded polymer materials is studied. When the load is removed from the polymer specimen it returns to its original shape because of relaxation, and deformation under tension is reduced, i.e. reverse creep associated with intermolecular regrouping, with slipping of the supermolecular structures relative to their environment, and with restoration of the equilibrium shape by the rupture and restoration of intermolecular bonds. Fracture of the remaining stressed section of the polymer chains results in an increase in specimen deformation, i.e. it acts in a direction opposite to the direction of action of relaxational shortening of the specimen length. Acceleration of relaxational processes can be associated with heating of the polymer specimens on irradiation. Overall heating of a specimen under irradiation, if it occurs, results in expansion and an increase in linear dimensions, i.e. it also acts in a direction opposite to the direction of the relaxational processes. Stepanov [3] evaluated the heating of polymer specimens on irradiation by electrons. Polyacrylamide and polyethylene terephthalate films of thickness 100/tm were'heated under a radiation dose of b =2 kGr/sec by about 1 K. Threads of the s'ame materials and consisting of 80 monofibres, diameter 0.02 ram, overall thread diameter about 0.08 mm, were heated up less than I K under the same electron radiation dose. When the temperature of the air or nitrogen sweeping the specimen was deliberately increased by 1 K, no appreciable increase in rate of the relaxational process could be detected, within the limits of the measurement error. In this work the heating up of polymer specimens by UV light was determined from the reverse thermal expansion of an unloaded specimen and from * Vysokomol. soyed. A30: No. 6, 1275-1278, 1988. 1336
Acceleration of relaxational processes in polymers under radiation
1337
the j u m p in deformation on irradiating or ceasing to irradiate the specimen. The specimen temperature before UV irradiation was thus deliberately lowered by an amount equal to the heating up produced by light, i.e. was held constant. The creep rate in irradiation of polymer specimens was studied on a setput intended l\~r loading the specimens through blocks. The lower specimen clamp was stationary, whereas the upper moving clamp was linked kinematically to the rotation of the rotor of a selsyn inductive pick-up. An electrical signal proportional to the elongation or compression of the specimen was fed to the input of an automatic recorder. The method is described in detail by Stepanov [3]. It is of interest to confirm whether or not irradiation accelerates relaxational restoration of the dimensions of a fractured polymer, when the number of stressed chemical bonds in the specimen is markedly decreased, i.e. whether or not irradiation accelerates the breaking of inter-molecular bonds. The w o r k was thus concerned with demonstrating the increased rate of relaxational processes in polymers subjected to the action of ionizing and UV irradiation, i.e. with demonstrating that the acceleration of relaxational processes under the action of irradiation, is similar to their acceleration on raising the temperature. The radiation dose was 2 kGr/sec, and the energy of the accelerated electrons at the maximum distribution was 300 keV. The UV light source was a DRSh-1000 lamp. The radiation was focussed on the specimen by means of quartz lenses. The region 250-313 nm was separated from the lamp spectrum by means of a CoSO4-NiSO4 liquid filter. The UV irradiation quanta density (3 × 10~ cm2.sec - t) was controlled by means of an F-7 photoelement. The specimens were irradiated in a special thermostat swept with a nitrogen current,to decrease heating up of the specimens under irradiation and to eliminate possible oxidation reactions. The temperature close to the specimen was recorded by means of Cu-constantan thermocouples. The heating up of the specimens was determined from the jump in temperature under the action of the UV radiation. Regel et al. [4] showed that UV irradiation breaks only the chemical bonds in polymers, but does not apprecially affect the rate of intermolecular regroupings and the breaking of intermolecular chains.'Acceleration of relaxational creep on irradiation was not observed against the background of the considerable creep acceleration due to breakage of the chemical bonds in the macromolecule chains. It was thus interesting to study and compare the laws of relaxational processes in unloaded polymers subjected to irradiation with electrons and UV radiation. With this in view, the following tests were made. Specimens of various polymers, i.e. polyacrylamide and PTFE threads, polyethylene, polymethyl methacrylate, polystyrene, PTFE, and polycarbonate films were loaded for 5 rain to stresses amounting to 50-70 ~5~iof the fracture stress. This resulted in a specific deformation of the polymer specimens, i.e. e , = ( l - l o ) / l o, where/o is the length of the original specimen, and l is the length of the specimen after deformation. The specimens were then rapidly unloaded to a stress less by two orders to magnitude. Decrease in the elongation produced then started, i.e. a process of deformation relaxation. A certain time after the relaxational process of shortening of the unloaded polymer specimen had started, the specimen was subjected
1338
V. V. MALAYEVand V. F. SI"ZI+ANOV
to irradiation with electrons or UV light. Relaxational compression of the specimens accelerated sharply from the moment o f irradiating the specimens, and when irradiation was stopped it slowed down to the original rate. Figure 1 shows a loading and unloading diagram for an oriented polycaproamide specimen and the corresponding relaxational processes. The diagram shows the change in relative elongation e of a specimen subjected to different stresses. It can be seen that relaxational compression, which starts on unloading the specimen from the moment t2, s reversibly accelerated by irradiation with electrons.
~,£,% 0.3'
10-
O'Z
+I .!'; r-..
,
,
0"1
f
t21N~ i TM, 300 T¢o~"....,~.~ Time, sec
,
zlO0
.-:'me. sec
800
Flo. 1 Fro. 2 FIG. 1. Dia~gram showing the loading and unloading and deforma)ional relaxation of standard polyeaproamide threads: at=13, a2ffi400 MPa, where tl is the moment of loading from ~1 to tr2, t2 is the moment of unloading from a2 to.ax, t3 is the moment of starting, and t+ is the moment of finishing the electron irradiation. FIo. 2. Change in deformation ,J8 of PTFE threads at 298 K as a function of time: 1 - without irradiation on unloading from #1 =250 to a=8 MPa; 2, 3--tL--the moment of starting UV irradiation (2) and electron irradiation (3), t2- the moment of stopping electron irradiation. Figure 2 (curve 1) shows, on an increased scale, the relaxational compression of oriented PTFE, which starts from the moment of its unloading and is recorded automatically. The quantity Ae indicates the difference A~=8--81=
l-1 o
It--I o
I-1 t
AI
Io
1o
1o
1o
where It is the larger part of the specimen length after deformation. Zero deformation (,48 =0) corresponds to the lower edge of the automatic recorder diagram, and on attaining it the scale was shifted to the other edge of the diagram in order to follow the whole process on an enlarged scale. Curve 2 corresponded to relaxational compression of the P T F E on UV inadiation, and curve 3 to it~ relaxational compression on irradiation with accelerated electrons. In this Figure the acceleration of relaxational compression of a specimen under the action of both types o f radiation can be seen more clearly, so that the data can be differentiated graphically and evaluated quantitatively.
Acceleration of relaxational processes in polymers under radiation
1339
The rate of the relaxation process (c_ t) is expressed by differentiating the deformation with respect to time, which gives the differential equation v = - d ~ / d t = - d ( d e ) / d t = - d (e - e t ) / d t = - ( d l / d t ) / l o .
In Fig. 3 the data of Fig. 2 are represented in differential form, i.e. the rate of relaxational compression of P T F E v is plotted against the time t (the notation on the curves in Fig. 3 corresponds to that on the curves in Fig. 2). It can be seen that for the selected intensities of electron and UV irradiation, the accelerated electrons produce a large increase in the rate of the relaxational processes. Moreover, under the action o f the UV radiation there is a constant fall in relaxation rate (Fig. 3, curve 2), whereas under the action of accelerated electrons (Fig. 3, curve 3) the relaxation rate remains constant up to the moment of stopping irradiation, and then falls to the original rate, i.e. the effect is reversible. Irradiation with accelerated electrons results in greater elongation of loaded and oriented polycaproamide specimens than does UV irradiation, as shown in Fig. 4, which shows a plot of the product r v = e (where r is the radiation time, v is the creep rate, and e is the total deformation up to fracture under irradiation) for polycaproamide against the stress tr under the action of electron and UV irradiation. Electron radiation
/.5 !
t
- u , l O ~ s e c "r
lO
I
qO0
1
]
8OO
T[nTe , sec
FIG. 3 FIG. 4 FI~. 3. Rate of deformation relaxation of unloaded PTFE threads as a function of time without irradiation (1) and under UV irradiation (2) and electron irradiation (3); tt is the moment of starting irradiation and t2 is the moment of stopping electron irradiation. FIo. 4. Product rv for polycaproamide threads at 298 K as a function of a on electron (1) and UV irradiation (2).
is much more effective in accelerating relaxation in polycaproamide than UV radiation, i.e. the accelerated electrons promote slipping of the polymer chains without fracture under load, thus increasing the overall deformation of the polymer specimen on irradiation.
1340
V. V. MALAYEV and V. F. Sr~rANOV
Accordingly, in this work direct acceleration of relaxation processes in unloaded polymers by the action o f radiation is observed, The observed laws provide confirmation that it is impossible to ignore the radiation induced kinetic acceleration of intermolecular regrouping, and the accelerated slipping of macromolecules and supermolecular structures relative to their surroundings. The mechanism ot' this phenomenon can be associated with radiation induced generation of additional vibratory excitations in solids, arising on nonradiative degradation of electron excitation into thermal vibrations. These additional vibratory excitations must accelerate any kinetic processes in solids: diffusion, gas desorption, etc., as has been observed by various authors [5--7]. The radiational acceleration o f kinetic intermolecular regrouping processes can be used in practice for radiational modification and technological treatment of polymer materials. Translated by N. STANDI~N REFERENCES
1. A. A. TAGER, Fizikokhimiya polimerov (Physical Chemistry of Polymers). Moscow, 1978 2. V. Ye. GUL and V. N. KULEZNEV, Struktura i mekhanicheskie svoistva polimerov (Structure and Mechanical Properties of Polymers). Moscow, 1979 3. V. F. STEPANOV, Diss. Kand. Fiz. Mat. Nauk, L. Ya. Karpov Physical Chemistry Research Institute, Moscow, 1971 4. V. R. REGEL, A. I. SLUTSKER and E. Ye. TOMASHEVSKII, Kineticheskaya priroda prochnosti tverdykh tel (Kinetic Nature of the Strength of Solids). Moscow, 1974 5. O. A. KLIMOVA and O. R. NIYAZOVA, Fiz. tverd, tela, 12, 2199, 1970 6. E. F. LAZNEVA, T. G. BYKOVA, L. A. SERGEYEVA and Yu. A. KHARLAMOV, Voprosy elektroniki tverdogo tela (Problems in the Electronics of Solids). Collection 4, Leningrad, 1974 7. Yu. I. BELYAKOV, M. P. DATSIEV and T. N. KOMPANIETS, Voprosy elektroniki tverdogo tela (Problems in the Electronics.of Solids). Collection 4, Leningrad, 1974