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Polymerization under electron beam: stability problem of grafted polymer systems
Rachat Phys Chem Vol 31, Nos 4-6, pp 599-605, 1988 Int J Radmt Appl Instrum Part C Printed m Great Bntmn
0146-5724/88 $3 O0 + 0 O0 Pergamon Journals Ltd
POLYMERIZATION UNDER ELECTRON BEAM: STABILITY PROBLEM OF GRAFTED POLYMER SYSTEMS
I. Yu. Babkin, S. B. Burukhin Karpov Institute of Physical Chemistry,
107120 Moscow, USSR
ABSTRACT In order to clarify the stability of grafted polymer systems the permeability and diffusion coefficient behavior of low-molecular substances have been investigated as a function of temperature treatment and organic solvent attacks. It is shown that stability of some grafted acrylic polymers can be improved significantly by varying irradiation regime and preliminary cross-linking of polyethylene matrix. KEYWORDS Accelerator, irradiation, grafting, distribution, grafted polymer, polyacryllc acid, polyethylene, cross-linking, stability, permeability, structure, dose. INTRODUCTION Though radiation grafting of polymers does not allow in principle to solve the incompatibility problem it provides in a number of cases the polymer combinations quite stable for practical use. Nevertheless the problem of grafted systems stability is urgent enough because it is retaining wider application of grafting for producing of filters, membranes, composite materials, films for packing and so on. The variety of grafted polymer systems is specified either by the chemical nature of the components or by structural features which can be developed quite intentionally. The capability of meterial to withstand without loss of valuable properties the heating procedure, organic solvent attacks, mechanical elongations must be considered as an important characteristics. It is known that properties are much dependent on the type of component interdistribution such as full uniformity, surface layers formation, filling by microglobules, diffuse distribution of component concentration, combination of those. The object of the present investigation is to demonstrate the ability of radiation grafting to rule the permeability and diffusion of low-molecular substances in polymers and to search the solution of instability problem by radiation. EXPERIMENTAL Polyethylene (PE) - based systems with grafted polyacrylic acid (PAA), pclyacrylonitrile (PAN), polyvinylacetate (PVA) end divinylbenzene (DVB) were investigated. The grafted polymer distribution is characterized by a well-known Odian's criterion 0( , when o( = O. I and o ( ~ 3 correspond to uniform distribution throughout the film thickness (U structure) and localization in near-surface barrier region (B structure), respectively. Low density polyethylene 5 0 ~ films of 45% orystallinity have been irradia~od in monomer vapors at relative'pressure near 0.9. Electron accelerators and Co e° ~-irradiation were used to produce B and U structures, respectively. In some experiments PAA has been grafted to PE films under gradual increase of dose rate from 5 to 170 Gy/s. Dose absorbed and weight-gain is shown in Fig. I. In such a way G structures were obtained. Gross-linking of PE film before grafting was carried out at doses of 0.2-4.0 MGy in vacuum followed by 2 hours annealing at 80°C. Transfer of grafted PAA into Zn-salt was carried out by 30 hours exposing to 0.2 M ammoniate zinc hydroxide solution. The conversion of PAA into salt was not less than 30%. Before measurements the samples were dried till constant weight. To estimate grafted polymer distribution the microinterfercmetry method has been used. Relative shift of interference bands can be considered as a measure of PAA concentration. Those
599
600
I Yu BABKn~ and S B BURUKH~
4,0
B-& 50
lu. 0 , 5
10 1-26 O |
'I~)
~
450
O
|
0,2
|
I
0,4
DOSF. ~ K(3y Fig. I.
Programme of dose rate increase (1) and PAAweight-gain (2) as a function of dose absorbed.
Fig. 2.
Concentration P A A p r o file in terms of optical shift depending on grafting regime: high dose rate (B-8), gradual dose rate increase (G-26). U-26 is taken as a base.
s h ~ s as a function of depth in bulk depend on the manner of grafting and dose rate increase (~ig. 2). Characteristics of the samples investigated are given in Table I. To estimate the stability the samples were exposed to acetone (A), heptane (H), diethyl ether (E), water (W) and dimethylformamide (F) at 20°0 or heating during one hour at each temperature. Structural changes were observed by measuring permeability coefficient P(mole.m/m=.s.Pa) of oxygen and organic solvent vapors at room temperature. Diffusion method as an instrument of stability investigation or structure characterisation proved itself valid. The method offered a possibility of identifying the specificity of structures resulted from diffusion-controlled grafting (B structures) and monomer sorption equilibrium (U structures). Relative permeability change was calculated as follows: ~ o = (P~ untreated ~rafted film (P) polyethylene film ~ I
5
= (P) treated grafted film (P) polyethylene film ~ ~o
(P~ treated ~rafted film (P) untreated grafted film
RESULTS AND DISCUSSION Barrier La~ers Pormation and Permeabilit 2 Control Relative permeability of vapors for U and B systems is given in Pig. 3. Permeability monotonously decreases with system composition change for U structures. The effect of permeability decline is not large. On the other hand, electron beam grafting decreases permeability factor by several decimal orders at low content of grafted polymer. Barrier properties are improved in a narrow concentra-
6th Internattonal Meeung on Radiatton Processing
601
=
-I m o~
~-2
E
I¢
8 --I
I /....
of-2
-3 .~
e-4-
!
!
I
10
o
m , mass Fig. 3.
Log L~o for n-hexane (1-4) and pethroleum ether (5) plotted against total mass increase. Structures: B (I); U (2); U-PAN (3); B-PAN (4); B-PVA (5). Grafting rates of PAN to PE as a function of mass increase (6) and (7) correspond to structures (4) and (3), respectively.
TEMPERATURE , °C 50,
"10¢~I g
/f
-d
Fzg. 4.
@___.O-~ &
O-I
-2
0-2
As
A-3 x-@
0-S 0-6
@-7 A-&
50
-4
-5
IOO 'I'30
Log ~ for heptane (1-5) and oxygen (6-8) after heat treatment of B-8 (1); B-5 (6); B-8-Zn (4); G-26 (5) and preliminary irradiated B-8-R (2,7), B-5-R (3,8) samples at doses of 1,0 and 4,0 MGy, respectively.
602
I Yu BAar~ and S B. BURUKHm
tion range which corresponds to near-surface layer formation. It seems convincing when compared to the kinetics grafting curves. Three parts can be distlnguished on the curves corresponding to I) range of high-rate reaction proceeding to some extent in bulk, 2) "exhausture" of reaction volume and displacement of monomer concentration front to the surface, 3) surface grafting range. High dose-rate grafting is characterized by peculiarities as follows: I) high grafting rate leads to sharp increase of medium viscosity which complicates the relaxation of propagating grafted chains, 2) nonequilibrium monomer distribution affects localization of grafted chains on both macro- and microlevels, 3) shorter grafted chains, while their concentration is higher, 4) low average monomer concentration and consequently weak plastification effect by un~eacted monomer. TABLE I
Characteristics
Sample
Grafted polymer
U-... B-... U-26-R(0.2.4.0) I U-45-Zn 4 B-5-R(1.0;4.0)~ B-8-R(0.2.4.0) B-8-Zn G-26
PAA PAA PAA PAA PAA PAA PAA PAA
U-PAN B-PAN B-PVA B-DVB B-6-DVB B-(PAA+DVB)
I
of the Samples
Grafted polymer content (weight %)
Dose rate (Gyfs)
Distribution thoughout the thickness
0.45.0 O. 8.0 26.0 45.0 5.5 8.0 8.0 26.0
0.03 55 0.03 0.03 55 55 55 6-170
uniform barrier uniform uniform barrier barrier barrier diffusion
PAN PAN PVA
0.25.0 0.20.0 0.30.0
1.8 370 370
uniform barrier barrier
DVB PAA;DVB PAA+DVB
3.2 6.3;0.3 12.0
55 55 55
barrier barrier barrier
Note
PAA in salt form PAA in salt form gradual increase of dose rate T is higher than T G of PVA DVB grafted to B-6 grafting from monomer mixture
Preliminary dose of matrix irradiation is given in MGy.
Thus, the synthesis of B systems provides thermodynamically nonequilibrium state characterized by incomplete phase separation. The temperature of grafting is of importance for suppressing of relaxation processes in the course of B s%ructure formation. It can be seen from B-PVA sample grafted at the temperature higher than that of PVA glass-transition. Different behavior of U and B systems under subsequent treatment is accounted for by their structure differences. The change of relative permeability in grafted samples after alternate treatment in water and diethyl ether is given in Fig. 4. Similar dependence has been obtained for other solvent pairs. For U samples complete reversibility of ~ parameter is observed and its change is specified by the grafted macromole~ules conformation (Babkin and co-workers, 1984). Materials with controlled characteristics, ruled permeability for example, can be developed on the base of this perculiarity. Significant permeability change in B systems makes their contact with solvents doubtful. The initial drastic jump of permeability corresponds to irreversible change of phase organization. Next alterations are rover@ibis and related mainly to chain conformation. Heat treatment effect on the change of barrier properties is seen in Fig. 4, sample B-8, curve I. Similar functions have been obtained for PE - grafted polyacrolein (Burukhin and co-workers, 1982) and polytetrafluoroet~ylene - grafted PAN systems. Drastic property change caused by relaxation processes in the region of glass-transition of grafted polymer is the peculiarity of all systems. Salt Pormation Effect on PE-PAA System Stability Instability of U system can be partially overcome by cross-linking of PAA phase. It can be done by PAA transfer into Zn-salt. As follows from Table 2 stabilization of the grafted chain conformation in U systems suppresses solvent effect on their properties. Sorption investigations showed (Babkin and co-workers, 1985) that permeability decrease in these systems was due to macromolecular filling of polymeric matrix. Transfer to Zn-salt occured in the presence of good solvent for grafted polymer, so phase destruction took place inevitably leading to substan-
6th Internauonal Meeting on Radiation Proceum8
603
DOSE ,
IvlOy
~L
4
e3
&
-4
09
10
.gb
5 5
\\ "1
2
3
4
5
6
7
PROCEDURES
Fig. 5.
®-2
IL
a" •°
o- s
~ ~ O-5 ~ ~,,~- •
[
o
Change of relative permeability of oxygen (I), acetone (2), heptane (3, 4) in U-45 (I-3) and B-5 (4) samples as a result of alternative exposing to water (procedures I, 3, 5, 7) and diethyl ether (procedures 2, 4, 6).
0
I
2
3
4
DOSE , MGy Pig. 6.
Change of relative diffusion (solid line) and ermeability coefficient dashed llne) for oxygen (1, 2), heptane (3, 4), acetone (5, 6) in B-8-R (a) and U-26-R (b) as a function of preliminary irradiation dose of PE matrix.
~
tial loss of barrier ability. However, PAA Zn-salt transfer results in the heat stability (Fig. 4, curve 4).
TABLE 2 Charge o~ ~4 for Heptane (Numerator} and Acetone (Denominator) in Samples after Different Solvent Attacks Solvent
U-45
U-45-Zn
untreated
0.09/1.33
A
.
P
E W
0.32/2.39 0.14/0.80
B-6
B-6-Zn
0.03/0.28
0.03/0.59
0.24/0.56
0.03/0.28
0.21/0.79
0.30/0.58.
0.03/0.28 0.03/0.28
0.48/1.00 0.26/0.80
0.25~0.58
0.03/0.28
0.48/1.00
0.30/0.58
Chan~e of Concentration Gradient for Grafted Pol~mer The distribution of grafted polymer in B structures can be too sharp and unfavourable to provide good stability. It seemed reasonable to decrease concentration gradient using a more complicated regime of irradiation. Gradual increase of dose
! Yu B ~
and S B BURUT..mN
rate in the course of irradiation enabled us to form desirable profile of grafted polymer concentration and to make a wider collection of supermolecular structures formed in near-surface region. The values of (-ig ~ ) in Table 3 have been obtained for U and B structures and for the samples irradiated at dose rate increase (G structure). Permeability comparison shows substantial increase of barrier ability for G structure. TABLE 3 C h a ~ e of (-Ig ~4 ) for Oxygen (Numerator) and Heptane (Denominator~ in Samples after Solvent Attack Solvent
G-26
B-8
U-26
untreated A F W
2.75/4.90 1.75/3.00 0.50/1.25 0.50/2.00
0.80/2.40 0.60/0.85 0.03/0.60 0.19/0.80
0.27/0.72 0.24/0.47 0.22/0.48 0.14/0.54
Though temperature effect (Pig. 4, curve 5) and solvent attacks (Table 3) result in barrier property decrease, permeability remains at the initial level of B systems and it is much lower compared to U systems with the same integral composition. It is obvious that irradiation conditions considered in the paper are not most favourable. However, the data presented show that optimization of concentration dradient is an effective instrument to improve barrier ability both before and after solvent attacks. DOSE o
4
2
, MGy
3
Compensation Effect
/.
-4
Up to now three-dimensional network and grafted polymer phase formation proceeded simultaneously. Naturally it effected PE cross-linking in the grafting zone, but some portion of the links in PE matrix was not formed due to steric hindrances caused by swelling. In other words PE network formed by itself is less stressed and strained as compared to the grafting onto the network formed preliminary. The same conclusion follows from data in Fig. 6. Increased dose of preliminary matrix irradiation results in the decrease of diffusion coefficient which can be accounted for by suppressed macromolecular mobility in diffusion medium and free volume decrease in the grafted system. The change of relative permeability for B-8-R sample after solvent attacks is given in Fig. 7. Though changeability cannot be overcome completely (follows from comparison ~ith the data in Fig. 5), battler properties of the solvent attacked systems are better compared to the samples with graft-gained network. Similar to U structures maximum preirradiation effect is observed at a dose of I MGy. The change of l g ~ depending on temperature in irradiated PE based samples is glven in Fig. 4. It is seen that increased dose of preliminary matrix irradiation reduces barrier changeability and ~I growth interval shifts to higher temperatures.
Fig. 7.
Change of relative permeability of oxygen (a), heptans (b) and acetone (dashed line) in B-8-R samples after P (1), W (2), E ~3), A (4) solvent attack.
Most probably the shrinking of preliminary formed and strained matrix caused by solvents or heating can compensate to a certain degree the free volume and some defects developing in a system as phase separation and relaxation proceed.
6th InternaUonal MeeUng on Radlatton Processing
TEMPERATURE
30 O _c~..._---
100 --o----,---I
~--
,
605
°C
1~0 ~ ,_~
.J
•
•
•
•
5
-2
Fig. 8. Thermal stability of B-DVB (I), B-6-DVB (2), B- (PAA+DVB) (3) in terms of lg ~4 for oxygen. The Third Component Because additional separate crosslinking of both PE matrix and grafted polymer leads to a limited success in stabilization it would be resonable to try to increase the phase interaction using the third component. DVB is a monomer capable both to copolymerize with acrylic acid into three-dimensional network and cross-link the PE matrix. For better solubility in PE than in PAA it is valid presumably to form transitional microphase between PAA and PE as polymerization proceeds. In other words DVB can help us to reduce the gradient of medium property on the microlevel. Permeability change as a function of heat treatment temperature in Fig. 8 has been obtained for the films with the barrier layer containing PAA and DVB as a third component. It is significant that DVB introduced separately results neither in barrier properties formation nor in the increased stability of the grafted system. However, when combined with PAA it produces a barrier of greater stability. CONCLUSION Electron beam graft polymerization blocks or at least suppresses relaxation processes in macromolecules. This enables us to produce the systems with high barrier properties. On the other hand, it creates a certain instability of materials if one does not take special precautions. As can be seen radiatlon synthesis of multicomponent polymeric systems can be developed with regard to stability increase. And this method is quite competitive, if not more effective, as compared to chemical procedures. The possibilities of radiation method to control polymer structure at the specific organization levels covering network formation, interphase and diffusion tracks have been considered on the samples presented. ~T rl p•~FER~LvES
Babkin, I. Yu., S. B. 3urukhin, ~u. M. Gordgev, and A. I. ICrasnogorov (198¢). The stability problem of some polymer systems grafted under irradiation. In J. Schmidt (Ed.), Abstracts of papers of third workin~ meetin£ on radiation interaction, Vol. I, Academy of Sciences of GDR, Leipzig, pp. 333-338. Babkin, I. Yu., S. B. Burukhin, and A. I. Krasnogorov (1985). Stability of radiation-grafted polyethylene-polyacrylic acid system. V2sokomol. Soed., A27, No. 8, 1703-~708. Burukhin, S. B., A. I. Krasnogorov, and V. A. Sutyagin (1982). Change of gas permeability of radiation-grafted films after heat treatment. V~sokomol. Soed., B2~, No. 9, 678-682.
0146-5724/88 $3 O0 + 0 O0 Pergamon Journals Ltd
POLYMERIZATION UNDER ELECTRON BEAM: STABILITY PROBLEM OF GRAFTED POLYMER SYSTEMS
I. Yu. Babkin, S. B. Burukhin Karpov Institute of Physical Chemistry,
107120 Moscow, USSR
ABSTRACT In order to clarify the stability of grafted polymer systems the permeability and diffusion coefficient behavior of low-molecular substances have been investigated as a function of temperature treatment and organic solvent attacks. It is shown that stability of some grafted acrylic polymers can be improved significantly by varying irradiation regime and preliminary cross-linking of polyethylene matrix. KEYWORDS Accelerator, irradiation, grafting, distribution, grafted polymer, polyacryllc acid, polyethylene, cross-linking, stability, permeability, structure, dose. INTRODUCTION Though radiation grafting of polymers does not allow in principle to solve the incompatibility problem it provides in a number of cases the polymer combinations quite stable for practical use. Nevertheless the problem of grafted systems stability is urgent enough because it is retaining wider application of grafting for producing of filters, membranes, composite materials, films for packing and so on. The variety of grafted polymer systems is specified either by the chemical nature of the components or by structural features which can be developed quite intentionally. The capability of meterial to withstand without loss of valuable properties the heating procedure, organic solvent attacks, mechanical elongations must be considered as an important characteristics. It is known that properties are much dependent on the type of component interdistribution such as full uniformity, surface layers formation, filling by microglobules, diffuse distribution of component concentration, combination of those. The object of the present investigation is to demonstrate the ability of radiation grafting to rule the permeability and diffusion of low-molecular substances in polymers and to search the solution of instability problem by radiation. EXPERIMENTAL Polyethylene (PE) - based systems with grafted polyacrylic acid (PAA), pclyacrylonitrile (PAN), polyvinylacetate (PVA) end divinylbenzene (DVB) were investigated. The grafted polymer distribution is characterized by a well-known Odian's criterion 0( , when o( = O. I and o ( ~ 3 correspond to uniform distribution throughout the film thickness (U structure) and localization in near-surface barrier region (B structure), respectively. Low density polyethylene 5 0 ~ films of 45% orystallinity have been irradia~od in monomer vapors at relative'pressure near 0.9. Electron accelerators and Co e° ~-irradiation were used to produce B and U structures, respectively. In some experiments PAA has been grafted to PE films under gradual increase of dose rate from 5 to 170 Gy/s. Dose absorbed and weight-gain is shown in Fig. I. In such a way G structures were obtained. Gross-linking of PE film before grafting was carried out at doses of 0.2-4.0 MGy in vacuum followed by 2 hours annealing at 80°C. Transfer of grafted PAA into Zn-salt was carried out by 30 hours exposing to 0.2 M ammoniate zinc hydroxide solution. The conversion of PAA into salt was not less than 30%. Before measurements the samples were dried till constant weight. To estimate grafted polymer distribution the microinterfercmetry method has been used. Relative shift of interference bands can be considered as a measure of PAA concentration. Those
599
600
I Yu BABKn~ and S B BURUKH~
4,0
B-& 50
lu. 0 , 5
10 1-26 O |
'I~)
~
450
O
|
0,2
|
I
0,4
DOSF. ~ K(3y Fig. I.
Programme of dose rate increase (1) and PAAweight-gain (2) as a function of dose absorbed.
Fig. 2.
Concentration P A A p r o file in terms of optical shift depending on grafting regime: high dose rate (B-8), gradual dose rate increase (G-26). U-26 is taken as a base.
s h ~ s as a function of depth in bulk depend on the manner of grafting and dose rate increase (~ig. 2). Characteristics of the samples investigated are given in Table I. To estimate the stability the samples were exposed to acetone (A), heptane (H), diethyl ether (E), water (W) and dimethylformamide (F) at 20°0 or heating during one hour at each temperature. Structural changes were observed by measuring permeability coefficient P(mole.m/m=.s.Pa) of oxygen and organic solvent vapors at room temperature. Diffusion method as an instrument of stability investigation or structure characterisation proved itself valid. The method offered a possibility of identifying the specificity of structures resulted from diffusion-controlled grafting (B structures) and monomer sorption equilibrium (U structures). Relative permeability change was calculated as follows: ~ o = (P~ untreated ~rafted film (P) polyethylene film ~ I
5
= (P) treated grafted film (P) polyethylene film ~ ~o
(P~ treated ~rafted film (P) untreated grafted film
RESULTS AND DISCUSSION Barrier La~ers Pormation and Permeabilit 2 Control Relative permeability of vapors for U and B systems is given in Pig. 3. Permeability monotonously decreases with system composition change for U structures. The effect of permeability decline is not large. On the other hand, electron beam grafting decreases permeability factor by several decimal orders at low content of grafted polymer. Barrier properties are improved in a narrow concentra-
6th Internattonal Meeung on Radiatton Processing
601
=
-I m o~
~-2
E
I¢
8 --I
I /....
of-2
-3 .~
e-4-
!
!
I
10
o
m , mass Fig. 3.
Log L~o for n-hexane (1-4) and pethroleum ether (5) plotted against total mass increase. Structures: B (I); U (2); U-PAN (3); B-PAN (4); B-PVA (5). Grafting rates of PAN to PE as a function of mass increase (6) and (7) correspond to structures (4) and (3), respectively.
TEMPERATURE , °C 50,
"10¢~I g
/f
-d
Fzg. 4.
@___.O-~ &
O-I
-2
0-2
As
A-3 x-@
0-S 0-6
@-7 A-&
50
-4
-5
IOO 'I'30
Log ~ for heptane (1-5) and oxygen (6-8) after heat treatment of B-8 (1); B-5 (6); B-8-Zn (4); G-26 (5) and preliminary irradiated B-8-R (2,7), B-5-R (3,8) samples at doses of 1,0 and 4,0 MGy, respectively.
602
I Yu BAar~ and S B. BURUKHm
tion range which corresponds to near-surface layer formation. It seems convincing when compared to the kinetics grafting curves. Three parts can be distlnguished on the curves corresponding to I) range of high-rate reaction proceeding to some extent in bulk, 2) "exhausture" of reaction volume and displacement of monomer concentration front to the surface, 3) surface grafting range. High dose-rate grafting is characterized by peculiarities as follows: I) high grafting rate leads to sharp increase of medium viscosity which complicates the relaxation of propagating grafted chains, 2) nonequilibrium monomer distribution affects localization of grafted chains on both macro- and microlevels, 3) shorter grafted chains, while their concentration is higher, 4) low average monomer concentration and consequently weak plastification effect by un~eacted monomer. TABLE I
Characteristics
Sample
Grafted polymer
U-... B-... U-26-R(0.2.4.0) I U-45-Zn 4 B-5-R(1.0;4.0)~ B-8-R(0.2.4.0) B-8-Zn G-26
PAA PAA PAA PAA PAA PAA PAA PAA
U-PAN B-PAN B-PVA B-DVB B-6-DVB B-(PAA+DVB)
I
of the Samples
Grafted polymer content (weight %)
Dose rate (Gyfs)
Distribution thoughout the thickness
0.45.0 O. 8.0 26.0 45.0 5.5 8.0 8.0 26.0
0.03 55 0.03 0.03 55 55 55 6-170
uniform barrier uniform uniform barrier barrier barrier diffusion
PAN PAN PVA
0.25.0 0.20.0 0.30.0
1.8 370 370
uniform barrier barrier
DVB PAA;DVB PAA+DVB
3.2 6.3;0.3 12.0
55 55 55
barrier barrier barrier
Note
PAA in salt form PAA in salt form gradual increase of dose rate T is higher than T G of PVA DVB grafted to B-6 grafting from monomer mixture
Preliminary dose of matrix irradiation is given in MGy.
Thus, the synthesis of B systems provides thermodynamically nonequilibrium state characterized by incomplete phase separation. The temperature of grafting is of importance for suppressing of relaxation processes in the course of B s%ructure formation. It can be seen from B-PVA sample grafted at the temperature higher than that of PVA glass-transition. Different behavior of U and B systems under subsequent treatment is accounted for by their structure differences. The change of relative permeability in grafted samples after alternate treatment in water and diethyl ether is given in Fig. 4. Similar dependence has been obtained for other solvent pairs. For U samples complete reversibility of ~ parameter is observed and its change is specified by the grafted macromole~ules conformation (Babkin and co-workers, 1984). Materials with controlled characteristics, ruled permeability for example, can be developed on the base of this perculiarity. Significant permeability change in B systems makes their contact with solvents doubtful. The initial drastic jump of permeability corresponds to irreversible change of phase organization. Next alterations are rover@ibis and related mainly to chain conformation. Heat treatment effect on the change of barrier properties is seen in Fig. 4, sample B-8, curve I. Similar functions have been obtained for PE - grafted polyacrolein (Burukhin and co-workers, 1982) and polytetrafluoroet~ylene - grafted PAN systems. Drastic property change caused by relaxation processes in the region of glass-transition of grafted polymer is the peculiarity of all systems. Salt Pormation Effect on PE-PAA System Stability Instability of U system can be partially overcome by cross-linking of PAA phase. It can be done by PAA transfer into Zn-salt. As follows from Table 2 stabilization of the grafted chain conformation in U systems suppresses solvent effect on their properties. Sorption investigations showed (Babkin and co-workers, 1985) that permeability decrease in these systems was due to macromolecular filling of polymeric matrix. Transfer to Zn-salt occured in the presence of good solvent for grafted polymer, so phase destruction took place inevitably leading to substan-
6th Internauonal Meeting on Radiation Proceum8
603
DOSE ,
IvlOy
~L
4
e3
&
-4
09
10
.gb
5 5
\\ "1
2
3
4
5
6
7
PROCEDURES
Fig. 5.
®-2
IL
a" •°
o- s
~ ~ O-5 ~ ~,,~- •
[
o
Change of relative permeability of oxygen (I), acetone (2), heptane (3, 4) in U-45 (I-3) and B-5 (4) samples as a result of alternative exposing to water (procedures I, 3, 5, 7) and diethyl ether (procedures 2, 4, 6).
0
I
2
3
4
DOSE , MGy Pig. 6.
Change of relative diffusion (solid line) and ermeability coefficient dashed llne) for oxygen (1, 2), heptane (3, 4), acetone (5, 6) in B-8-R (a) and U-26-R (b) as a function of preliminary irradiation dose of PE matrix.
~
tial loss of barrier ability. However, PAA Zn-salt transfer results in the heat stability (Fig. 4, curve 4).
TABLE 2 Charge o~ ~4 for Heptane (Numerator} and Acetone (Denominator) in Samples after Different Solvent Attacks Solvent
U-45
U-45-Zn
untreated
0.09/1.33
A
.
P
E W
0.32/2.39 0.14/0.80
B-6
B-6-Zn
0.03/0.28
0.03/0.59
0.24/0.56
0.03/0.28
0.21/0.79
0.30/0.58.
0.03/0.28 0.03/0.28
0.48/1.00 0.26/0.80
0.25~0.58
0.03/0.28
0.48/1.00
0.30/0.58
Chan~e of Concentration Gradient for Grafted Pol~mer The distribution of grafted polymer in B structures can be too sharp and unfavourable to provide good stability. It seemed reasonable to decrease concentration gradient using a more complicated regime of irradiation. Gradual increase of dose
! Yu B ~
and S B BURUT..mN
rate in the course of irradiation enabled us to form desirable profile of grafted polymer concentration and to make a wider collection of supermolecular structures formed in near-surface region. The values of (-ig ~ ) in Table 3 have been obtained for U and B structures and for the samples irradiated at dose rate increase (G structure). Permeability comparison shows substantial increase of barrier ability for G structure. TABLE 3 C h a ~ e of (-Ig ~4 ) for Oxygen (Numerator) and Heptane (Denominator~ in Samples after Solvent Attack Solvent
G-26
B-8
U-26
untreated A F W
2.75/4.90 1.75/3.00 0.50/1.25 0.50/2.00
0.80/2.40 0.60/0.85 0.03/0.60 0.19/0.80
0.27/0.72 0.24/0.47 0.22/0.48 0.14/0.54
Though temperature effect (Pig. 4, curve 5) and solvent attacks (Table 3) result in barrier property decrease, permeability remains at the initial level of B systems and it is much lower compared to U systems with the same integral composition. It is obvious that irradiation conditions considered in the paper are not most favourable. However, the data presented show that optimization of concentration dradient is an effective instrument to improve barrier ability both before and after solvent attacks. DOSE o
4
2
, MGy
3
Compensation Effect
/.
-4
Up to now three-dimensional network and grafted polymer phase formation proceeded simultaneously. Naturally it effected PE cross-linking in the grafting zone, but some portion of the links in PE matrix was not formed due to steric hindrances caused by swelling. In other words PE network formed by itself is less stressed and strained as compared to the grafting onto the network formed preliminary. The same conclusion follows from data in Fig. 6. Increased dose of preliminary matrix irradiation results in the decrease of diffusion coefficient which can be accounted for by suppressed macromolecular mobility in diffusion medium and free volume decrease in the grafted system. The change of relative permeability for B-8-R sample after solvent attacks is given in Fig. 7. Though changeability cannot be overcome completely (follows from comparison ~ith the data in Fig. 5), battler properties of the solvent attacked systems are better compared to the samples with graft-gained network. Similar to U structures maximum preirradiation effect is observed at a dose of I MGy. The change of l g ~ depending on temperature in irradiated PE based samples is glven in Fig. 4. It is seen that increased dose of preliminary matrix irradiation reduces barrier changeability and ~I growth interval shifts to higher temperatures.
Fig. 7.
Change of relative permeability of oxygen (a), heptans (b) and acetone (dashed line) in B-8-R samples after P (1), W (2), E ~3), A (4) solvent attack.
Most probably the shrinking of preliminary formed and strained matrix caused by solvents or heating can compensate to a certain degree the free volume and some defects developing in a system as phase separation and relaxation proceed.
6th InternaUonal MeeUng on Radlatton Processing
TEMPERATURE
30 O _c~..._---
100 --o----,---I
~--
,
605
°C
1~0 ~ ,_~
.J
•
•
•
•
5
-2
Fig. 8. Thermal stability of B-DVB (I), B-6-DVB (2), B- (PAA+DVB) (3) in terms of lg ~4 for oxygen. The Third Component Because additional separate crosslinking of both PE matrix and grafted polymer leads to a limited success in stabilization it would be resonable to try to increase the phase interaction using the third component. DVB is a monomer capable both to copolymerize with acrylic acid into three-dimensional network and cross-link the PE matrix. For better solubility in PE than in PAA it is valid presumably to form transitional microphase between PAA and PE as polymerization proceeds. In other words DVB can help us to reduce the gradient of medium property on the microlevel. Permeability change as a function of heat treatment temperature in Fig. 8 has been obtained for the films with the barrier layer containing PAA and DVB as a third component. It is significant that DVB introduced separately results neither in barrier properties formation nor in the increased stability of the grafted system. However, when combined with PAA it produces a barrier of greater stability. CONCLUSION Electron beam graft polymerization blocks or at least suppresses relaxation processes in macromolecules. This enables us to produce the systems with high barrier properties. On the other hand, it creates a certain instability of materials if one does not take special precautions. As can be seen radiatlon synthesis of multicomponent polymeric systems can be developed with regard to stability increase. And this method is quite competitive, if not more effective, as compared to chemical procedures. The possibilities of radiation method to control polymer structure at the specific organization levels covering network formation, interphase and diffusion tracks have been considered on the samples presented. ~T rl p•~FER~LvES
Babkin, I. Yu., S. B. 3urukhin, ~u. M. Gordgev, and A. I. ICrasnogorov (198¢). The stability problem of some polymer systems grafted under irradiation. In J. Schmidt (Ed.), Abstracts of papers of third workin~ meetin£ on radiation interaction, Vol. I, Academy of Sciences of GDR, Leipzig, pp. 333-338. Babkin, I. Yu., S. B. Burukhin, and A. I. Krasnogorov (1985). Stability of radiation-grafted polyethylene-polyacrylic acid system. V2sokomol. Soed., A27, No. 8, 1703-~708. Burukhin, S. B., A. I. Krasnogorov, and V. A. Sutyagin (1982). Change of gas permeability of radiation-grafted films after heat treatment. V~sokomol. Soed., B2~, No. 9, 678-682.