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Nonrandom radiation crosslinking of SBS triblock copolymer
Radial. Phys. Chem. Vol. 41, No. 3, pp. 491—495, 1993 Printed in Great Britain. All rights reserved
0146-5724/93 $6.00 + 0.00
Copyright © 1993 Pergamon Press Ltd
NONRANDOM RADIATION CROSSLINKING OF SBS TRIBLOCK COPOLYMER YANG HuAI, Za&o ZHUDI, CHEN XINFANG, Luo YUNXIA, Li SHUHUA and Xu JuN Institute of Materials Science, Jilin University, Changchun 130023, P.R. China and Changchun Institute of Applied Chemistry, Changchun, 130022, P.R. China
(Received 13 August 1992) Abstract—Triblock copolymers, SBS (S = styrene, B = butadiene), containing 34.1 and 52.5% styrene, respectively, been studied with respect to glass the extent of crosslinking produced bywe irradiation with 60Co-rays. Byhave investigating the variation of their transition temperatures with dose, demonstrated that styrene, 1,4-butadiene and 1,2-butadiene segments have different crosslinking efficiency. Then from the method of probability, we derived the relationship, S
=
~[l
—
C’q~(1— ~b)]~’~[1 — Cq~(l— q5)]~[l — q~(1 —
q5)]Xs
and discussed the variation of sol content with dose.
INTRODUCIION
EXPERIMENTAL
Samples SBS tnblock
copolymer possesses the following
mainly structural characteristics; (1) two blocks show domain separation at a submicroscopic level; (2) 2-fold glass transition behavior, i.e. T 8 of the styrene blocks is higher than room temperature while T~of butadiene blocks is lower room temperature; there are inevitably somethan 1,2-butadiene segments (3) in large or small amounts, i.e. some side vinyl groups are attached to backbone chains. These structural charactenstics certainly result in a law of nonrandom crosslinking, different from those of homopolymers and random copolymers. Dole and Basheer (1982a,b) have done some excellent work on the radiation crosslinking of SBS. Some of their conclusions were that the crosslinking efficiency decreased with styrene content increase and gel fraction increased with dose increase. G-Value of radiation crosslinking for SBS was between those of the random copolymer and mechanical mixture of the corresponding amounts of PB and PS obtained by Witt (1959). They attributed this phenomenon to the protective effect of styrene segments on the crosslinking between the butadiene segments. However they used theclassical Charlesby— Pinner relationship when they treated the relation between sol fraction and dose theoretically; by assuming that the styrene, I ,4-butadiene and 1 2-butadiene segments had the same crosslinking efficiency. Obviously, this does not conform to reality. Since the dose and dose range they selected were low and small, their results appear to agree with the Charlesby— Pinner relationship. However, if SBS is irradiated over a wider dose range, the great difference will exhibited. In this paper, we have established a new relationship between the sol fraction and dose ~nd discussed the nonrandom crosslinking of SBS. RPC4I/3—E
491
The block copolymers studied with their characteristics are listed in Table 1. Irradiation
6°Coy-rays under The samples were irradiated with by the dose rate of vacuum at room temperature 0.78 Mrad/h. Characterization (1) Determination of gel fraction: the vacuum glass tubes were immersed in boiling water to anneal out residual free radicals as soon as irradiation was finished. ESR measurements demonstrated no free radicals left after this treatment for 15 mm. Then the gel fraction were determined by the method of solvent extraction. (2) i.r. Spectra were obtained with Perkin-Elmer 580B spectrometer. (3) Glass transition temperatures were determined with a DDV-II-EA viscoelastometer at the frequency of 3.5 Hz in the temperature range from —150 to + 150°C. EXPERIMENTAL RESULTS
The variation of glass transition temperatures with dose Owing to the thermodynamic incompatibility of PS and PB, the two blocks tend to form separation phases. However only microphase separation morphology is possible because of the existence of chemical bonds linking the styrene and butadiene segments. Such special microphase separation in SBS is also shown in its dynamic mechanical behavior. There are
492
YANG HuAI
et a!.
Table I. Composition and microstructure of block copolymers studied
% Microstructure~ Sample
Wt% PS*
cis
trans
SBS-t 34.1 32.7 59.2 SBS-2 52.5 31.4 59.6 ‘Resulted from NMR measurements. “Dete~ined by light scattering method. “l)ete~ined by film osmometry.
M~x ~
vinyl 8.1 9.0
A?, x l0~’’ 85.5
100 100
85.6
two glass transition temperatures. The one appearing at lower temperature, ~ is attributed to butadiene segments and the other, T~,is attributed to styrene
(1967), we believe they will also decrease much slower than vinyl groups. So we assume that the curved increase in variation of AT81 with dose in the initial
segments. The glass transition temperatures for irradiated SBS-l and SBS-2 are listed in Table 2. We can see that 7’~~ increases markedly with dose while T~ remains almost unchanged. This means the crosslinking occurs mainly between butadiene segments. The three-dimensional crosslinking network produced by irradiation between butadiene segments becomes denser with dose increase and increasingly restricts the motion of butadiene segments. Thus more energy is needed for the butadiene segments to reach the state of free motion and 7’~~ increases, The increment of T51 between irradiated and unirradiated samples, AT81, is plotted against dose in Fig. 1. AT81 increases curvedly and then linearly below and above 25 Mrad dose. We also see from the figure that AT81 for SBS-l is higher than that for SBS-2 at the same dose. Moreover the gel fraction of SBS-l with smaller amounts of styrene is higher than that of SBS-2 with larger amounts of styrene at the same dose as shown in Fig. 2. Figure 3 shows the i.r. spectra of SBS-1 irradiated by different doses. We compare the areas under the absorption peaks at 967 cm’ and at 911 cm ‘,these being the characteristic peaks of trans-vinylene and side vinyl groups respectively, with that of the absorption peak at 1490 cm’, this being the characteristic peak of phenyl rings and assumed to be unchanged because of its greater stability during irradiation (Charlesby, 1960; Lincoln Hawkins, 1972). Then the peak at 911 cm’ decreases markedly while the peak at 967 cm -‘ remains almost the same in the dose range from 0 to 23.08 Mrad. From 23.08 to 75.56 Mrad the former decreases very little while the decrease of the latter can be clearly observed. The detection of cis-unsaturation by i.r. spectra has been difficult because the absorption peak are obscured by the presence of that of C—H of phenyl rings. But according to the work done by Parkinson
radiation stage is caused predominantly by side vinyl groups and side vinyl groups, corresponding to 1 ,2-butadiene segments, have greater crosslinking efficiency. As discussed below, the radiation crosslinking of SBS follows the mechanism of nonrandom crosslinking. THEORY OF NONRANDOM CROSSLINRING
The relation between the so! fraction and dose The model of nonrandom crosslinking. Let us de-
scribe the chain structure of SBS as: SSS’
SBB’B
BB’BBS’’ SSS
where S represents the styrene segments; B and B’, the 1 ,4-butadiene and 1 ,2-butadiene segments, respectively. For convenience, we suppose one segment serves as a crosslinkable unit, and during irradiation, four crosslinked structures may be formed as follows: —B’— —B’—
...Jj_.
..,...~‘_
(I)
—B—
—5—
_.J~_.
~
(II)
(III)
We know that the rate of formation of structures of type (I) is the fastest, that of type (II) is less fast and that of type (III) is the slowest. We can reasonably neglect crosslinking between B and S because B and S segments form separate phases and the crosslinking efficiency of styrene segments is very low. The relation between so!1B’ fraction and crosslinked X~and X structure parameters. Let 5 be the number fractions of the unit B’, B and S in the initial block copolymer, respectively; R the dose; q5., q~and qs the number fractions of crosslinked units B’, B and 5, respectively; q the number fraction of total crosslinked units in the total number of units at R Mrad dose. Thus, by classical definition of probability, q5~, q~and q5 are the probabilities that a unit selected at
Table 2. The glass transition temperatures for SBS.1 and SBS.2 irradiated under different doses Dose (Mrad) 0.00
4.29
10.84
17.67
24.46
35.61
50.90
64.13
68.70
74.20
97.93
SBS.1 T5 T~
—82.6 98.7
—81.9 98.7
—81.2 98.6
—81.1 98.6
—80.9 98.8
—79.9 98.3
—78.4 98.7
—77.3 97.8
—76.5 97.7
—75.5 98.7
—73.3
SBS.2 T,1 T~
—83.8 104.8
—83.4 104.9
—82.8 104.7
—82.5 104.8
—82.3 104.3
—8l.~ 104.6
—80.3 103.4
—78.9 104.8
—78.4 104.1
—78.0
—75.4 104.2
103.9
98.0
Crosslinking of SBS triblock copolymer 10
493
—
8—
/ 2~/° 4-
‘I
<
. 0
20
40
60
80
100
Dose (Mrad) Fig. 1. AT
5 versus the dose for SBS-l (•) and SBS-2 (0).
random from the units of B’, B and S is crosslinked, respectively; q the probability of choosing a unit from all the units being crosslinked. According to the followingofrelationship: method probability, we can obtain easily the
I
I
1500
1300
1)900
1100
700
Wavenumber (cm-
Fig. 3. i.r. Spectra for SBS-l irradiated by different doses: spectrum 1 (0.00 Mrad); spectrum 2 (4.90 Mrad); spectrum
S
~ W
=
1(l — q~.+ x (1
3 (23.08 Mrad); spectrum 4 (75.56 Mrad).
q8~)lXB~
— q~ + t7B~)~(1 — q5 + q5~)~5 (1)
Since the polydispersity of SBS (M~~/A?,) equals approx 1, SBS follows a uniform distribution. Let n
where W1 is the probability that a unit of weight selected randomly from the system belongs to i-mers, i.e. the weight fraction of i-mers; 4.’ is the probability that a crosslinked unit selected at random links it with the finite chains which belong to the sol part through a crosslinking bond. 4 Can be described as follows:
represent the degree of number average polymerization, then 4.’ = S’ — (3) supposing, in a certain range of dose, that q5~= qoB’ R = q08R qs=q~R
5”
W1((l —q8~+q8.4i)°’~
q~=~
(4)
x (1 — q~+ q 84)(I_ ‘~“(1— q5 + q54,)(i_ 1)1s 10
q0~Iq0s C’
(2)
~
-
(5)
q08/q~—C
units B’, B and S (per unit dose absorbed by the system) to q~.,q~and qs respectively; C’ and C reflect
a
0
the characteristics of nonrandom crosslinking. From equations (1—5), we can get
/T
/
where ~
q08 and q05 represent the contribution of 4,)]Xw
S=4[l —C’q05R(l
x [1 — Cq~R(l — q5)]xs[1 — q~R(l— ~ 5
10
15
20
25
Dose (Mrad) Fig. 2. Gel fraction as a function of dose forSBS-l (~)and SBS-2 (0).
~)JXs
(6)
THE COMPAR~ONBE~(EENThEOR~ICALAND EXPERIMENTAL RESULTS Table 3 lists some parameters we need, which are
calculated from Table 1. We deduce that the G-values of radiation crosslinking for polystyrene having a
494
YANG HUAI
et a!.
Table 3. The structural parameters for SBS-1 and SBS-2 Sample
X
SBS-1 SBS-2
5
A’5
X~
n
m
0.064 0.057
0.724 0.578
0.212 0.365
1324 1185
64.6 72.2
uniform distribution and polybutadiene (cis—trans—
3
~
0.5
-
vinyl, Then 45.4. (6), Table weSubstituting 324/73/3) q05 obtain and 5.2the different as xofcan curves 2.38 lO_6, these C’ xvalues q05 of lO_2 parameters, = as so! 2.36 and into fraction 2.10, x equations 10~ those asrespectively. aand listed function (3) and = in of 5, dose for= SBS-l It and be SBS-2 seen from shown the 2in figures Figs 4Cthat and therespectively. so! fraction SBS-1 is lower than that of SBS-2 at the same dose. This is an obvious result. Because
Fig.
styrene blocks, which are difficult to crosslink be-
ments, we may be able to explain it better. We have
tween them, will certainly lead to the increase of so! fraction. The results of the two figures show that c’ values of the theoretical curves which the experimental data plotted are far higher than 45.4 and the theoretical curves are far away from the one whose C’ and C equal to 45.4 in the early stage of radiation crosslinking, i.e. in the dose range close to gel point, This means that G-value of 1,2-butadiene segments is much higher than that of 1 ,4-butadiene segments and crosslinking of type (I) takes place predominantly in the region of low dose. The reason why C’ values of
measured the glass transition temperatures of butadiene segments of SBS-l and SBS-2 as well as that of pure polybutadiene (cis—trans—vinyl), 24/73/3; = 6.2 x l0~)and their values are —82.6, —83.8 and —96.0°C, respectively, i.e. the glass transition temperature of pure polybutadiene is lower than those of butadiene segments of the two block copolymers. This is because the styrene segments at room temperature below 7’~restricts the motion of butadiene segments by their presence at both ends of the butadiene blocks, resulting in the increase of the I’~ of butadiene segments. So the mobility acquired by
their molecular weights and molecular weight distributions are the same, the increase of the content of
the curves on which the experimental data fall become smaller with dose increase is that the decrease of content of 1,2-butadiene segments with dose results in the decrease of G-values of 1 ,2-butadiene segments. We can also see that the so! fraction measured is even higher than the calculated yield on the curve whose C’ equals 0 at the same dose in the region of high dose. Considering the protective effect of styrene segments on the crosslinking between butadiene seg-
0 5.
_____________________________________ 5 Dose 10 (Mrad) 15 20 25 Same as Fig. 4 but for SBS-2. Curve 1 (C’ = 0); curve 2 (C’ = 45.4); curve 3 (C’ = 460).
free radicals on the butadiene segments decreases and the styrene segments produce a protective effect on the crosslinking between butadiene segments. On the other hand some energy will be transferred from the butadiene blocks to styrene segments due to the existence of phenyl rings. This will also produce a protective effect on the butadiene segments. But we only consider the simple addition of the styrene units and butadiene units when we derive the relationships above. So sol fraction measured is higher than the theoretical yields at the same dose. In particular in the to react completely. The 1,4-butadiene away from styrene blocks are being crosslinked gradually. The styrene segments will produce a greater protective
0.5
-~ ~
—
~ 0
5
10
15
20
25
Dose (Mrad) Fig. 4. Sol fraction as a function of dose for SBS-1; The
solid lines are the theoretical curves with different values of C’: curve 1 (C’ = 0); curve 2 (C’ = 45.4); curve 3 (C’ = 300); curve 4 (C’ = 1500). The dotted line is the experimental curve.
effect blockson region of and high the thedose, butadiene difference the 1,2-butadiene between segmentssol near fractions segments the styrene which tends are calculated and measured respectively will become more distinct. The reason why the glass transition temperature of butadiene segments of SBS-l is higher ity between the styrene and butadiene segments in the than that of SBS-2 is that there is a bettercompatibilsample containing a lower proportion of styrene than in that containing a medium proportion of styrene according to the effect of domain-boundary mixturing (Hashimoto et al., 1983). The increase of compatibility between the two blocks leads not only to the decrease of the mobility of the butadiene segments decided by its higher glass transition temperature but also an increase of the energy transferred from the
Crosslinking of SBS triblock copolymer
butadiene blocks to the styrene segments because the interfacial surface contact per unit mass of polystyrene between the polystyrene spheres and the polybutadiene matrix gets greater. So the protective effect of the styrene segments on the crosslinking between butadiene segments is greater in SBS-l than in SBS-2. But to compare the protective effects between SBS-l and SBS-2 from the 2 figures is difficult because factors affecting the sol fraction are not only the protective effects; the effect of the ratio of the two blocks can be more important. In addition, G-value of l,4-butadiene segments, like that of 1,2-
butadiene segments, will vary with dose and the ratio of S/B due to the existence of protective effect. Therefore we assume that the G-value of 1 ,4-buta-
diene segments remains unchanged in order to discuss the problem conveniently.
495
As discussed above, the styrene, 1 ,4-butadiene and I ,2-butadiene segments have different crosslinking
efficiency and SUS follows the mechanism of nonrandom crosslinking. REFERENCES Basheer R. and Dole M. (1982a) Macromol. C/scm. 183, 2141.
Basheer R. and Dole M. (l982b) Polym. Prepr. 23, 311. Witt E. (1959) J. Polym. Sci. 41, 507. Charlesby A. (1960) Atomic Radiation and Polymers. Pergamon, Oxford. Hashimoto T., Tsukahara Y. et a!. (1983) Macromolecules 16, 648. Lincon Hawkins W. (1972) Polymer Stabilization. Wiley, New York. Parkinson W. W. and Sears W. C. (1967) Adv. Chem. Ser. 66, 57.
0146-5724/93 $6.00 + 0.00
Copyright © 1993 Pergamon Press Ltd
NONRANDOM RADIATION CROSSLINKING OF SBS TRIBLOCK COPOLYMER YANG HuAI, Za&o ZHUDI, CHEN XINFANG, Luo YUNXIA, Li SHUHUA and Xu JuN Institute of Materials Science, Jilin University, Changchun 130023, P.R. China and Changchun Institute of Applied Chemistry, Changchun, 130022, P.R. China
(Received 13 August 1992) Abstract—Triblock copolymers, SBS (S = styrene, B = butadiene), containing 34.1 and 52.5% styrene, respectively, been studied with respect to glass the extent of crosslinking produced bywe irradiation with 60Co-rays. Byhave investigating the variation of their transition temperatures with dose, demonstrated that styrene, 1,4-butadiene and 1,2-butadiene segments have different crosslinking efficiency. Then from the method of probability, we derived the relationship, S
=
~[l
—
C’q~(1— ~b)]~’~[1 — Cq~(l— q5)]~[l — q~(1 —
q5)]Xs
and discussed the variation of sol content with dose.
INTRODUCIION
EXPERIMENTAL
Samples SBS tnblock
copolymer possesses the following
mainly structural characteristics; (1) two blocks show domain separation at a submicroscopic level; (2) 2-fold glass transition behavior, i.e. T 8 of the styrene blocks is higher than room temperature while T~of butadiene blocks is lower room temperature; there are inevitably somethan 1,2-butadiene segments (3) in large or small amounts, i.e. some side vinyl groups are attached to backbone chains. These structural charactenstics certainly result in a law of nonrandom crosslinking, different from those of homopolymers and random copolymers. Dole and Basheer (1982a,b) have done some excellent work on the radiation crosslinking of SBS. Some of their conclusions were that the crosslinking efficiency decreased with styrene content increase and gel fraction increased with dose increase. G-Value of radiation crosslinking for SBS was between those of the random copolymer and mechanical mixture of the corresponding amounts of PB and PS obtained by Witt (1959). They attributed this phenomenon to the protective effect of styrene segments on the crosslinking between the butadiene segments. However they used theclassical Charlesby— Pinner relationship when they treated the relation between sol fraction and dose theoretically; by assuming that the styrene, I ,4-butadiene and 1 2-butadiene segments had the same crosslinking efficiency. Obviously, this does not conform to reality. Since the dose and dose range they selected were low and small, their results appear to agree with the Charlesby— Pinner relationship. However, if SBS is irradiated over a wider dose range, the great difference will exhibited. In this paper, we have established a new relationship between the sol fraction and dose ~nd discussed the nonrandom crosslinking of SBS. RPC4I/3—E
491
The block copolymers studied with their characteristics are listed in Table 1. Irradiation
6°Coy-rays under The samples were irradiated with by the dose rate of vacuum at room temperature 0.78 Mrad/h. Characterization (1) Determination of gel fraction: the vacuum glass tubes were immersed in boiling water to anneal out residual free radicals as soon as irradiation was finished. ESR measurements demonstrated no free radicals left after this treatment for 15 mm. Then the gel fraction were determined by the method of solvent extraction. (2) i.r. Spectra were obtained with Perkin-Elmer 580B spectrometer. (3) Glass transition temperatures were determined with a DDV-II-EA viscoelastometer at the frequency of 3.5 Hz in the temperature range from —150 to + 150°C. EXPERIMENTAL RESULTS
The variation of glass transition temperatures with dose Owing to the thermodynamic incompatibility of PS and PB, the two blocks tend to form separation phases. However only microphase separation morphology is possible because of the existence of chemical bonds linking the styrene and butadiene segments. Such special microphase separation in SBS is also shown in its dynamic mechanical behavior. There are
492
YANG HuAI
et a!.
Table I. Composition and microstructure of block copolymers studied
% Microstructure~ Sample
Wt% PS*
cis
trans
SBS-t 34.1 32.7 59.2 SBS-2 52.5 31.4 59.6 ‘Resulted from NMR measurements. “Dete~ined by light scattering method. “l)ete~ined by film osmometry.
M~x ~
vinyl 8.1 9.0
A?, x l0~’’ 85.5
100 100
85.6
two glass transition temperatures. The one appearing at lower temperature, ~ is attributed to butadiene segments and the other, T~,is attributed to styrene
(1967), we believe they will also decrease much slower than vinyl groups. So we assume that the curved increase in variation of AT81 with dose in the initial
segments. The glass transition temperatures for irradiated SBS-l and SBS-2 are listed in Table 2. We can see that 7’~~ increases markedly with dose while T~ remains almost unchanged. This means the crosslinking occurs mainly between butadiene segments. The three-dimensional crosslinking network produced by irradiation between butadiene segments becomes denser with dose increase and increasingly restricts the motion of butadiene segments. Thus more energy is needed for the butadiene segments to reach the state of free motion and 7’~~ increases, The increment of T51 between irradiated and unirradiated samples, AT81, is plotted against dose in Fig. 1. AT81 increases curvedly and then linearly below and above 25 Mrad dose. We also see from the figure that AT81 for SBS-l is higher than that for SBS-2 at the same dose. Moreover the gel fraction of SBS-l with smaller amounts of styrene is higher than that of SBS-2 with larger amounts of styrene at the same dose as shown in Fig. 2. Figure 3 shows the i.r. spectra of SBS-1 irradiated by different doses. We compare the areas under the absorption peaks at 967 cm’ and at 911 cm ‘,these being the characteristic peaks of trans-vinylene and side vinyl groups respectively, with that of the absorption peak at 1490 cm’, this being the characteristic peak of phenyl rings and assumed to be unchanged because of its greater stability during irradiation (Charlesby, 1960; Lincoln Hawkins, 1972). Then the peak at 911 cm’ decreases markedly while the peak at 967 cm -‘ remains almost the same in the dose range from 0 to 23.08 Mrad. From 23.08 to 75.56 Mrad the former decreases very little while the decrease of the latter can be clearly observed. The detection of cis-unsaturation by i.r. spectra has been difficult because the absorption peak are obscured by the presence of that of C—H of phenyl rings. But according to the work done by Parkinson
radiation stage is caused predominantly by side vinyl groups and side vinyl groups, corresponding to 1 ,2-butadiene segments, have greater crosslinking efficiency. As discussed below, the radiation crosslinking of SBS follows the mechanism of nonrandom crosslinking. THEORY OF NONRANDOM CROSSLINRING
The relation between the so! fraction and dose The model of nonrandom crosslinking. Let us de-
scribe the chain structure of SBS as: SSS’
SBB’B
BB’BBS’’ SSS
where S represents the styrene segments; B and B’, the 1 ,4-butadiene and 1 ,2-butadiene segments, respectively. For convenience, we suppose one segment serves as a crosslinkable unit, and during irradiation, four crosslinked structures may be formed as follows: —B’— —B’—
...Jj_.
..,...~‘_
(I)
—B—
—5—
_.J~_.
~
(II)
(III)
We know that the rate of formation of structures of type (I) is the fastest, that of type (II) is less fast and that of type (III) is the slowest. We can reasonably neglect crosslinking between B and S because B and S segments form separate phases and the crosslinking efficiency of styrene segments is very low. The relation between so!1B’ fraction and crosslinked X~and X structure parameters. Let 5 be the number fractions of the unit B’, B and S in the initial block copolymer, respectively; R the dose; q5., q~and qs the number fractions of crosslinked units B’, B and 5, respectively; q the number fraction of total crosslinked units in the total number of units at R Mrad dose. Thus, by classical definition of probability, q5~, q~and q5 are the probabilities that a unit selected at
Table 2. The glass transition temperatures for SBS.1 and SBS.2 irradiated under different doses Dose (Mrad) 0.00
4.29
10.84
17.67
24.46
35.61
50.90
64.13
68.70
74.20
97.93
SBS.1 T5 T~
—82.6 98.7
—81.9 98.7
—81.2 98.6
—81.1 98.6
—80.9 98.8
—79.9 98.3
—78.4 98.7
—77.3 97.8
—76.5 97.7
—75.5 98.7
—73.3
SBS.2 T,1 T~
—83.8 104.8
—83.4 104.9
—82.8 104.7
—82.5 104.8
—82.3 104.3
—8l.~ 104.6
—80.3 103.4
—78.9 104.8
—78.4 104.1
—78.0
—75.4 104.2
103.9
98.0
Crosslinking of SBS triblock copolymer 10
493
—
8—
/ 2~/° 4-
‘I
<
. 0
20
40
60
80
100
Dose (Mrad) Fig. 1. AT
5 versus the dose for SBS-l (•) and SBS-2 (0).
random from the units of B’, B and S is crosslinked, respectively; q the probability of choosing a unit from all the units being crosslinked. According to the followingofrelationship: method probability, we can obtain easily the
I
I
1500
1300
1)900
1100
700
Wavenumber (cm-
Fig. 3. i.r. Spectra for SBS-l irradiated by different doses: spectrum 1 (0.00 Mrad); spectrum 2 (4.90 Mrad); spectrum
S
~ W
=
1(l — q~.+ x (1
3 (23.08 Mrad); spectrum 4 (75.56 Mrad).
q8~)lXB~
— q~ + t7B~)~(1 — q5 + q5~)~5 (1)
Since the polydispersity of SBS (M~~/A?,) equals approx 1, SBS follows a uniform distribution. Let n
where W1 is the probability that a unit of weight selected randomly from the system belongs to i-mers, i.e. the weight fraction of i-mers; 4.’ is the probability that a crosslinked unit selected at random links it with the finite chains which belong to the sol part through a crosslinking bond. 4 Can be described as follows:
represent the degree of number average polymerization, then 4.’ = S’ — (3) supposing, in a certain range of dose, that q5~= qoB’ R = q08R qs=q~R
5”
W1((l —q8~+q8.4i)°’~
q~=~
(4)
x (1 — q~+ q 84)(I_ ‘~“(1— q5 + q54,)(i_ 1)1s 10
q0~Iq0s C’
(2)
~
-
(5)
q08/q~—C
units B’, B and S (per unit dose absorbed by the system) to q~.,q~and qs respectively; C’ and C reflect
a
0
the characteristics of nonrandom crosslinking. From equations (1—5), we can get
/T
/
where ~
q08 and q05 represent the contribution of 4,)]Xw
S=4[l —C’q05R(l
x [1 — Cq~R(l — q5)]xs[1 — q~R(l— ~ 5
10
15
20
25
Dose (Mrad) Fig. 2. Gel fraction as a function of dose forSBS-l (~)and SBS-2 (0).
~)JXs
(6)
THE COMPAR~ONBE~(EENThEOR~ICALAND EXPERIMENTAL RESULTS Table 3 lists some parameters we need, which are
calculated from Table 1. We deduce that the G-values of radiation crosslinking for polystyrene having a
494
YANG HUAI
et a!.
Table 3. The structural parameters for SBS-1 and SBS-2 Sample
X
SBS-1 SBS-2
5
A’5
X~
n
m
0.064 0.057
0.724 0.578
0.212 0.365
1324 1185
64.6 72.2
uniform distribution and polybutadiene (cis—trans—
3
~
0.5
-
vinyl, Then 45.4. (6), Table weSubstituting 324/73/3) q05 obtain and 5.2the different as xofcan curves 2.38 lO_6, these C’ xvalues q05 of lO_2 parameters, = as so! 2.36 and into fraction 2.10, x equations 10~ those asrespectively. aand listed function (3) and = in of 5, dose for= SBS-l It and be SBS-2 seen from shown the 2in figures Figs 4Cthat and therespectively. so! fraction SBS-1 is lower than that of SBS-2 at the same dose. This is an obvious result. Because
Fig.
styrene blocks, which are difficult to crosslink be-
ments, we may be able to explain it better. We have
tween them, will certainly lead to the increase of so! fraction. The results of the two figures show that c’ values of the theoretical curves which the experimental data plotted are far higher than 45.4 and the theoretical curves are far away from the one whose C’ and C equal to 45.4 in the early stage of radiation crosslinking, i.e. in the dose range close to gel point, This means that G-value of 1,2-butadiene segments is much higher than that of 1 ,4-butadiene segments and crosslinking of type (I) takes place predominantly in the region of low dose. The reason why C’ values of
measured the glass transition temperatures of butadiene segments of SBS-l and SBS-2 as well as that of pure polybutadiene (cis—trans—vinyl), 24/73/3; = 6.2 x l0~)and their values are —82.6, —83.8 and —96.0°C, respectively, i.e. the glass transition temperature of pure polybutadiene is lower than those of butadiene segments of the two block copolymers. This is because the styrene segments at room temperature below 7’~restricts the motion of butadiene segments by their presence at both ends of the butadiene blocks, resulting in the increase of the I’~ of butadiene segments. So the mobility acquired by
their molecular weights and molecular weight distributions are the same, the increase of the content of
the curves on which the experimental data fall become smaller with dose increase is that the decrease of content of 1,2-butadiene segments with dose results in the decrease of G-values of 1 ,2-butadiene segments. We can also see that the so! fraction measured is even higher than the calculated yield on the curve whose C’ equals 0 at the same dose in the region of high dose. Considering the protective effect of styrene segments on the crosslinking between butadiene seg-
0 5.
_____________________________________ 5 Dose 10 (Mrad) 15 20 25 Same as Fig. 4 but for SBS-2. Curve 1 (C’ = 0); curve 2 (C’ = 45.4); curve 3 (C’ = 460).
free radicals on the butadiene segments decreases and the styrene segments produce a protective effect on the crosslinking between butadiene segments. On the other hand some energy will be transferred from the butadiene blocks to styrene segments due to the existence of phenyl rings. This will also produce a protective effect on the butadiene segments. But we only consider the simple addition of the styrene units and butadiene units when we derive the relationships above. So sol fraction measured is higher than the theoretical yields at the same dose. In particular in the to react completely. The 1,4-butadiene away from styrene blocks are being crosslinked gradually. The styrene segments will produce a greater protective
0.5
-~ ~
—
~ 0
5
10
15
20
25
Dose (Mrad) Fig. 4. Sol fraction as a function of dose for SBS-1; The
solid lines are the theoretical curves with different values of C’: curve 1 (C’ = 0); curve 2 (C’ = 45.4); curve 3 (C’ = 300); curve 4 (C’ = 1500). The dotted line is the experimental curve.
effect blockson region of and high the thedose, butadiene difference the 1,2-butadiene between segmentssol near fractions segments the styrene which tends are calculated and measured respectively will become more distinct. The reason why the glass transition temperature of butadiene segments of SBS-l is higher ity between the styrene and butadiene segments in the than that of SBS-2 is that there is a bettercompatibilsample containing a lower proportion of styrene than in that containing a medium proportion of styrene according to the effect of domain-boundary mixturing (Hashimoto et al., 1983). The increase of compatibility between the two blocks leads not only to the decrease of the mobility of the butadiene segments decided by its higher glass transition temperature but also an increase of the energy transferred from the
Crosslinking of SBS triblock copolymer
butadiene blocks to the styrene segments because the interfacial surface contact per unit mass of polystyrene between the polystyrene spheres and the polybutadiene matrix gets greater. So the protective effect of the styrene segments on the crosslinking between butadiene segments is greater in SBS-l than in SBS-2. But to compare the protective effects between SBS-l and SBS-2 from the 2 figures is difficult because factors affecting the sol fraction are not only the protective effects; the effect of the ratio of the two blocks can be more important. In addition, G-value of l,4-butadiene segments, like that of 1,2-
butadiene segments, will vary with dose and the ratio of S/B due to the existence of protective effect. Therefore we assume that the G-value of 1 ,4-buta-
diene segments remains unchanged in order to discuss the problem conveniently.
495
As discussed above, the styrene, 1 ,4-butadiene and I ,2-butadiene segments have different crosslinking
efficiency and SUS follows the mechanism of nonrandom crosslinking. REFERENCES Basheer R. and Dole M. (1982a) Macromol. C/scm. 183, 2141.
Basheer R. and Dole M. (l982b) Polym. Prepr. 23, 311. Witt E. (1959) J. Polym. Sci. 41, 507. Charlesby A. (1960) Atomic Radiation and Polymers. Pergamon, Oxford. Hashimoto T., Tsukahara Y. et a!. (1983) Macromolecules 16, 648. Lincon Hawkins W. (1972) Polymer Stabilization. Wiley, New York. Parkinson W. W. and Sears W. C. (1967) Adv. Chem. Ser. 66, 57.