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Application of the liquid chromatograph “milichrom” to the determination of molecular weight distribution of polymers
1(;50
YA. I. Es~IN REFERENCES
1. A. Ya. MAL'KIN, A. Ye. CHALYKH, Diffuziya i vyazkost' polimerov. Metody izmereniya (Diffusion and Viscosity of Polymers. Methods of Measurement). Moscow, 1979 2. L, M. SOROKO, Introskopiya na osnove yadcrnogo magnitnogo rezonansa (Introscopy Based on Nuclear Magnetic Resonance). Moscow, 1986 3. A. A. SAMOILENKO, D. Yu. ARTEMOV and L. A. SIBEL'DINA, Zhurn. fiz. khimii 61: :3082, 1987 4. I. Ya. SLONIM and A. N. LYUBIMOV, Yadernyi magnitnyi rezonans polimerakh (Nuclear Magnetic Resonance in Polymers). p. 252, Moscow, 1966 5. G. Ye. ZAIKOV, A. L. IORDANSKII and V. S. MARKIN, Diffuziya elektrolitov v polimerakh (Diffusion of Electrolytvs in Polymers). Moscow, 1984
Polymer Science U.S.S.R. Vol, 30, No. 7, pp. 1650-1655, 1988 Printed in Poland
0032-3950/88 $10.00+.00 1989 Pergamon Prets pie
APPLICATION OF THE LIQUID CHROMATOGRAPH "MILICHROM" TO THE DETERMINATION OF MOLECULAR WEIGHT DISTRIBUTION OF POLYMERS* YA. I. ESTRIN Department of the Institute of Chemical Physics, U.S.S,R. Academy of Sciencem
(Received22 September 1987) The liquid microcolumn chromatograph "Milichrom" has been modified by replacing the standard columns by columns of larger dimensions having the total pore volume of around 2.5 ml to enable its application in gel permeation chromatography of polymers. The columns were packed by a sorbent which ensured the shape of the calibration dependence to .be linear. Results of calibration of columns packed with silanized Lichrospher are presented together with the results of comparative measurements of several polymer samples. MOLECULAR weight distribution (MWD) is a very important characteristic which governs a whole , complex of polymer properties. Gel permeation chromatography (GPC), often called size-exclusion chromatography, is at present almost exclusively used to determine MWD of polymers. Since the knowledge of MWD is indispensable for the control of polymer quality during preparation and in the exploitation stage, it is necessary to provide plant laboratories with liquid chromatographs capable to pelfolm analyses of this kind. Unfortunately, the Soviet industry does n o t as yet produce sufficient numbers of instruments suitable for GPC. The number of produced microcolumn chromatographs KhZh-1309 is absolutely insufficient. The microcolumn liquid chromatograph "Milichrom" is not intended for use in GPC since respects it does not satisfy the necessary requirements. * Vysokomol. soyed. A30: No. 7, 1560-1564, 1988.
Application of liquid chromatograph "Milichrom '
1651
A modification of the liquid chromatograph "Milichrom" is described in this paper, which enables one to use this instrument for gel permeation chromatography. Art analysis of the characteristics of the instrument has shown that the main obstacle to its application to size-exclusion TABLE 1.
ELUTION VOLUMES OF POLYMER STANDARDS
(Dioxan at 200/d/min, 25 °) Mp~k x 10 -a
VR,/al
•v,%
Mt, e.k x 10 -a
VR' /tl
470
110
73'5
34"5
20"5
9'8
3'55
0.4
M=78 (benzen~
1033
1037 1030
0"20
1037 1041
0'20
1051 1051
0
1136 1143 1143
{
zlV, ~o
Polybutadiene
Polystyrene 165 72a
1037 1103 1005
0"10
72b 54a
1116 1123 1127
0'20
54b 25a
1135 1236 1237
0"05
25b
1245 1243 1240
0"20
0.30
1265 1259 1263
0"20
1429 1423 1426
0"10
1657 1656 1654
0.10
1418 1420
0"07
1424 1428
0"10
3.95a
t670 1674
0"10
3"90b
1692 1686
0"15
2"47a
1789 1791
0"50
2"42b
1017 1015
0'05
10'3a
1943 1943 1946
0'10
1991 2002 2003
0"25
10'3b
[
Note. Polybutadiene samples designated a are monofunctional with one hydroxyl end-group, those designated b have no functionality.
chromatography is the small column volume (small total pore volume Vp) which amounts to about 80 al, and also the insufficient separation efficiency of columns, where the maximum number of theoretical plates is ~- 5000. On the other hand, the design of both the injection port and the detector ensures that the extracolumn peak spreading is very small and these parts belong to the distinct advantages of the instrument. The edficiency of columns for GPC must be at least 10,000 theoretical
plates; tO 013tainresults of satisi'acto ryprecision the retention volumes rfiust be reproducible to Within 0.5 % from the value of Vp. Since the instrument "Milichrom" ensures the error of retention volumes to lie between 2 and 5/zl, the total pore volume should be at least 5O0 ,ul. Both the efficiency and the pore volume can be raised by increasing the column length. As the maximum eiution volume in GPC does not surpass Vo+ lzp (where Vo is the interstitial volume), the maximum volume V of the empty column (taking into account a reserve for ctiromatogram tailing), limited by the volume Of the syringe, should be around 25O0/~1, so that one chromatogram would require the full available volume of the syringe (it is assumed that Vo-0"42 V). A column of length ~=2O0 mm and diameter of 4 mm aiready satisfies these requirements (it is not easy to prepare and pack columns of still larger length and smaller diameter; to Say nothing of the excessive pressure drop idvolved):
•
"V~.
\ I
I
I
l
Fio. 1. Calibration curves for PB (1) and polystyrene (2). Starting from these considerations, a stainless steel column with length 205 mm and internal diameter 4 rnm (V=2.5 ml) has been devised. Both the input (injection) and the end fitting were of identical design as those in the original "Milichrom" columns, only their dimensions were change& S
2
/0
3
5
10
t8
22
FIG. 2. Chromatogram of a mixture of PS standards. M x I0 -a =470 (I), 34.5 (2), 20.5 (3), 9.8 (4), 3.55 (~, and 0.4 (6). The colunm was placed in a glass thermostating jacket provided with inlet and outlet tubes for circulating water. As the column was intended for the analysis of polymer samples with molecular weight between 103 and 5 x 104, Lichrospher SilO0 wasused as the packing (particle diameter -~ 5 #m)
AppliCation of liquid Chromatograph "Milichrom"
1~53
[l ]. The packed columns exhibited ti'i"efollowing parameters: V6= 1035/zl, V0+ V~=--2000/,l, 10,000 theoretical plates at a flowrate of 200 ~ul/min. With decreasing fl0wrate the efficiency was found to decrease. To obviate adsorption effects the sorbent was silanized directly in the cblurfln by purging with hexamethyldisilazane at 95°C for 10 hr at a flowrate of 5/~l/min
2 3b
I0
14
18
~'2
v~, ~o-2,/~z
i=IG. 3. Chromatograms of PS and PB samples from Table 2.
Purified dioxan containing less than 0.01% of impurities (according to gas-liquid chromatography) and exhibiting absorption at 210 nm not higher than 2 tel. units/cm served as the eluent; dioxan Of this purity was found to preserve its optical characteristics at room temperature for at least 2 months.
10
1#
18
g2
FIG. 4. Chromatograms of the sample PB-2 before (l, 2) and after reprecipitation (1', 2') registered at two wavelengths: 1, 1"-210 nm, 2, 2 ' - 2 2 0 nm.
Chromatograms were stored and processed in a computer Iskra 226 interfaced to the digital output of the chromatograph. The column was calibrated by means of polystyrene standards purchased from Waters Ass. and also by linear polybutadiene (PB) samples characterized by means of the Waters GPC 200 instrument. Parameters of PB standards were verified by measuring the content of OH end groups in
1654
YA. I. ESTgI~
monofunctional polymers containing less than 2 ~o of nowfunctionalized chains. The concentration of PS solutions was ~ 1 mg/ml, that of PB standards was _~5 mg/ml; the injected volume was 8/~1. The UV detector was adjusted to 2=210 nm. The column was kept at 25°C, the eluent flow rate was 200/~l/min. Under these conditions the scatter of retention volumes between repeated injections of the same sample did not exceed 6/zl (Table 1). Results of calibration experiments are shown in Fig. 1. TABLE 2.
M O L E C U L A R W E I G H T DISTRIBUTION ANALYSIS OF POLYSTYREN E A N D POLYBUTADIENE SAMPLES
Sample* Polystyrene (1)* Polystyrenet Polybutadiene (2) Polybutadiene (3)§
GPC-200
"Milichrom" 2300 8630 8600 8660 6400 31,950 5640 6630 31,360
2690 9750 9780 9810 10,600 32,030 6850 11,020 32,030
5740
6900
2800 9490 9610 9710 34,700 6210
m
m
w
9200
12,300
12,400
5420 46,900
12,300 53,160
51,500
4850
6676
5662
33,000 602O
* N u m b e r s in parentheses refer to c h r o m a t o g r a m s in Fig. 3. * N o m i n a l molecular weisht o f PS s t a n d a r d W a t e r s 2350. : N o m i n a l molecular weight o f PS standard 9800. § Parameters o f t h e wh01e c h r n m a t o g r a m and o f t w o peaks in the bimodal sample: n u m e r o a t o r - peak a, d e n o m i n a t o r peak b. Note. Results o f two parallel experiments are presented for P B (2) and PB (3).
It is apparent that the PS calibration is linear from M ~ 2 x 10a to 4.5 x 10"; for PB the upper limit of calibration linearity was ---3 x 104, the lower limit was not determined owing to the absence of suitable standards. The mean square deviations (PS samples) were 1.6% for molecular weight (on the linear part of the calibration dependence) and 3/zl for Va; the corresponding values for PB were 6.5% and 15/zl. The larger relative error in the case of PB can be attributed to less accurate characteristics of the non-functionalized PB calibration standards. The instrumental spreading coefficient (determined from the chromatogram of a monodisperse sample-benzene) was 1"01, the asymmetry was 1.05. Figure 2 shows the chromatogram of a mixture of six polystyrene standards with M between 400 and 4.7 x 105; the chromatograms of individual polymers are in Fig. 3. Chromatograms of a PB sample (identical to that corresponding to chromatogram 2 in Fig. 2), registered at two wavelengths (210 and 220 nm) before and after reprecipitation, are shown in Fig. 4; it is apparent from the disappearance of the low-molecular-weight peak from the chromatogram of the reprecipitated sample that this peak does not belong to the polymer but to some low-molecular-weight admixture which adsorbs strongly at 220 nm (in all probability-to an antioxidant), since PB practically do~s not absorb at 220 nm. Table 2 collects some data on MWD of several polymers; the agreement with the results obtained on the GPC 200 instrument is satisfactory. The somewhat larger discrepancy of results pertaining to the first peak in the bimodal sample is probably related to the fact that the elution volume of this peak lies outside the linear calibration region. One experiment under these conditions requires about 15 min, including the time necessary for filling of styringe and sample injection, i.e., the instrument provides results very rapidly. One analysis requires about 2 to 2.5 ml of solvent and about 10 to 50 ng of polymer sample. Making use of the multiple-wavelength detection facility one can accomplish analyses that would normally require a multi-detector instrument.
Application of liquid chrom~ttograph "Milichrom"
1655
Thus, by a simple replacement of the standard column by a column of larger volume the chromatograph "Milichrom" becomes a gel chromatograph with fully satisfactory parameters. Its performance is only limited by the requirement that the analysed polymer absorbs in the UV region and/or by its solubility. The molecular weight range is obviously limited only by the type of sorbent employed. The principles underlying the selection of sorbent combinations that ensure the linearity of the calibration dependence over a broad interval of molecular weight (covering four or even five orders of magnitude) are described in the literature [2]. The column efficiency may be improved by employing a more suitable mode of packing, by using sorbents of either smaller particle diameter (e.g. 3 llm particles) and/or with a narrower size distribution than Lichrospher. Switching of columns including the preparation of the new column for analysis requires only 1 hr, so that one instrument may be effectively used both in the normal mode and as a chromatograph for GPC.
Translated by M. KUBIN
REFERENCES 1. E.L. STYSKIN, L. B. ITSIKSON and E. V. BRAUDE, Prakticheskaya vysokoeffektivnaya zhidkostnaya khromatografiya (Practice of High-Performance Liquid Chromatography). p. 274, Moscow, 1986 2. L. Z. VILENCHIK, O. I. KURENBIN, T. P. ZHMAKINA and B. G. BELEN'KII, Vysokomol. soyed. "-A22: 2801, 1980 (Translated in Polymer Sci. U.S.S.R. 22: 12, 3074, 1980)
YA. I. Es~IN REFERENCES
1. A. Ya. MAL'KIN, A. Ye. CHALYKH, Diffuziya i vyazkost' polimerov. Metody izmereniya (Diffusion and Viscosity of Polymers. Methods of Measurement). Moscow, 1979 2. L, M. SOROKO, Introskopiya na osnove yadcrnogo magnitnogo rezonansa (Introscopy Based on Nuclear Magnetic Resonance). Moscow, 1986 3. A. A. SAMOILENKO, D. Yu. ARTEMOV and L. A. SIBEL'DINA, Zhurn. fiz. khimii 61: :3082, 1987 4. I. Ya. SLONIM and A. N. LYUBIMOV, Yadernyi magnitnyi rezonans polimerakh (Nuclear Magnetic Resonance in Polymers). p. 252, Moscow, 1966 5. G. Ye. ZAIKOV, A. L. IORDANSKII and V. S. MARKIN, Diffuziya elektrolitov v polimerakh (Diffusion of Electrolytvs in Polymers). Moscow, 1984
Polymer Science U.S.S.R. Vol, 30, No. 7, pp. 1650-1655, 1988 Printed in Poland
0032-3950/88 $10.00+.00 1989 Pergamon Prets pie
APPLICATION OF THE LIQUID CHROMATOGRAPH "MILICHROM" TO THE DETERMINATION OF MOLECULAR WEIGHT DISTRIBUTION OF POLYMERS* YA. I. ESTRIN Department of the Institute of Chemical Physics, U.S.S,R. Academy of Sciencem
(Received22 September 1987) The liquid microcolumn chromatograph "Milichrom" has been modified by replacing the standard columns by columns of larger dimensions having the total pore volume of around 2.5 ml to enable its application in gel permeation chromatography of polymers. The columns were packed by a sorbent which ensured the shape of the calibration dependence to .be linear. Results of calibration of columns packed with silanized Lichrospher are presented together with the results of comparative measurements of several polymer samples. MOLECULAR weight distribution (MWD) is a very important characteristic which governs a whole , complex of polymer properties. Gel permeation chromatography (GPC), often called size-exclusion chromatography, is at present almost exclusively used to determine MWD of polymers. Since the knowledge of MWD is indispensable for the control of polymer quality during preparation and in the exploitation stage, it is necessary to provide plant laboratories with liquid chromatographs capable to pelfolm analyses of this kind. Unfortunately, the Soviet industry does n o t as yet produce sufficient numbers of instruments suitable for GPC. The number of produced microcolumn chromatographs KhZh-1309 is absolutely insufficient. The microcolumn liquid chromatograph "Milichrom" is not intended for use in GPC since respects it does not satisfy the necessary requirements. * Vysokomol. soyed. A30: No. 7, 1560-1564, 1988.
Application of liquid chromatograph "Milichrom '
1651
A modification of the liquid chromatograph "Milichrom" is described in this paper, which enables one to use this instrument for gel permeation chromatography. Art analysis of the characteristics of the instrument has shown that the main obstacle to its application to size-exclusion TABLE 1.
ELUTION VOLUMES OF POLYMER STANDARDS
(Dioxan at 200/d/min, 25 °) Mp~k x 10 -a
VR,/al
•v,%
Mt, e.k x 10 -a
VR' /tl
470
110
73'5
34"5
20"5
9'8
3'55
0.4
M=78 (benzen~
1033
1037 1030
0"20
1037 1041
0'20
1051 1051
0
1136 1143 1143
{
zlV, ~o
Polybutadiene
Polystyrene 165 72a
1037 1103 1005
0"10
72b 54a
1116 1123 1127
0'20
54b 25a
1135 1236 1237
0"05
25b
1245 1243 1240
0"20
0.30
1265 1259 1263
0"20
1429 1423 1426
0"10
1657 1656 1654
0.10
1418 1420
0"07
1424 1428
0"10
3.95a
t670 1674
0"10
3"90b
1692 1686
0"15
2"47a
1789 1791
0"50
2"42b
1017 1015
0'05
10'3a
1943 1943 1946
0'10
1991 2002 2003
0"25
10'3b
[
Note. Polybutadiene samples designated a are monofunctional with one hydroxyl end-group, those designated b have no functionality.
chromatography is the small column volume (small total pore volume Vp) which amounts to about 80 al, and also the insufficient separation efficiency of columns, where the maximum number of theoretical plates is ~- 5000. On the other hand, the design of both the injection port and the detector ensures that the extracolumn peak spreading is very small and these parts belong to the distinct advantages of the instrument. The edficiency of columns for GPC must be at least 10,000 theoretical
plates; tO 013tainresults of satisi'acto ryprecision the retention volumes rfiust be reproducible to Within 0.5 % from the value of Vp. Since the instrument "Milichrom" ensures the error of retention volumes to lie between 2 and 5/zl, the total pore volume should be at least 5O0 ,ul. Both the efficiency and the pore volume can be raised by increasing the column length. As the maximum eiution volume in GPC does not surpass Vo+ lzp (where Vo is the interstitial volume), the maximum volume V of the empty column (taking into account a reserve for ctiromatogram tailing), limited by the volume Of the syringe, should be around 25O0/~1, so that one chromatogram would require the full available volume of the syringe (it is assumed that Vo-0"42 V). A column of length ~=2O0 mm and diameter of 4 mm aiready satisfies these requirements (it is not easy to prepare and pack columns of still larger length and smaller diameter; to Say nothing of the excessive pressure drop idvolved):
•
"V~.
\ I
I
I
l
Fio. 1. Calibration curves for PB (1) and polystyrene (2). Starting from these considerations, a stainless steel column with length 205 mm and internal diameter 4 rnm (V=2.5 ml) has been devised. Both the input (injection) and the end fitting were of identical design as those in the original "Milichrom" columns, only their dimensions were change& S
2
/0
3
5
10
t8
22
FIG. 2. Chromatogram of a mixture of PS standards. M x I0 -a =470 (I), 34.5 (2), 20.5 (3), 9.8 (4), 3.55 (~, and 0.4 (6). The colunm was placed in a glass thermostating jacket provided with inlet and outlet tubes for circulating water. As the column was intended for the analysis of polymer samples with molecular weight between 103 and 5 x 104, Lichrospher SilO0 wasused as the packing (particle diameter -~ 5 #m)
AppliCation of liquid Chromatograph "Milichrom"
1~53
[l ]. The packed columns exhibited ti'i"efollowing parameters: V6= 1035/zl, V0+ V~=--2000/,l, 10,000 theoretical plates at a flowrate of 200 ~ul/min. With decreasing fl0wrate the efficiency was found to decrease. To obviate adsorption effects the sorbent was silanized directly in the cblurfln by purging with hexamethyldisilazane at 95°C for 10 hr at a flowrate of 5/~l/min
2 3b
I0
14
18
~'2
v~, ~o-2,/~z
i=IG. 3. Chromatograms of PS and PB samples from Table 2.
Purified dioxan containing less than 0.01% of impurities (according to gas-liquid chromatography) and exhibiting absorption at 210 nm not higher than 2 tel. units/cm served as the eluent; dioxan Of this purity was found to preserve its optical characteristics at room temperature for at least 2 months.
10
1#
18
g2
FIG. 4. Chromatograms of the sample PB-2 before (l, 2) and after reprecipitation (1', 2') registered at two wavelengths: 1, 1"-210 nm, 2, 2 ' - 2 2 0 nm.
Chromatograms were stored and processed in a computer Iskra 226 interfaced to the digital output of the chromatograph. The column was calibrated by means of polystyrene standards purchased from Waters Ass. and also by linear polybutadiene (PB) samples characterized by means of the Waters GPC 200 instrument. Parameters of PB standards were verified by measuring the content of OH end groups in
1654
YA. I. ESTgI~
monofunctional polymers containing less than 2 ~o of nowfunctionalized chains. The concentration of PS solutions was ~ 1 mg/ml, that of PB standards was _~5 mg/ml; the injected volume was 8/~1. The UV detector was adjusted to 2=210 nm. The column was kept at 25°C, the eluent flow rate was 200/~l/min. Under these conditions the scatter of retention volumes between repeated injections of the same sample did not exceed 6/zl (Table 1). Results of calibration experiments are shown in Fig. 1. TABLE 2.
M O L E C U L A R W E I G H T DISTRIBUTION ANALYSIS OF POLYSTYREN E A N D POLYBUTADIENE SAMPLES
Sample* Polystyrene (1)* Polystyrenet Polybutadiene (2) Polybutadiene (3)§
GPC-200
"Milichrom" 2300 8630 8600 8660 6400 31,950 5640 6630 31,360
2690 9750 9780 9810 10,600 32,030 6850 11,020 32,030
5740
6900
2800 9490 9610 9710 34,700 6210
m
m
w
9200
12,300
12,400
5420 46,900
12,300 53,160
51,500
4850
6676
5662
33,000 602O
* N u m b e r s in parentheses refer to c h r o m a t o g r a m s in Fig. 3. * N o m i n a l molecular weisht o f PS s t a n d a r d W a t e r s 2350. : N o m i n a l molecular weight o f PS standard 9800. § Parameters o f t h e wh01e c h r n m a t o g r a m and o f t w o peaks in the bimodal sample: n u m e r o a t o r - peak a, d e n o m i n a t o r peak b. Note. Results o f two parallel experiments are presented for P B (2) and PB (3).
It is apparent that the PS calibration is linear from M ~ 2 x 10a to 4.5 x 10"; for PB the upper limit of calibration linearity was ---3 x 104, the lower limit was not determined owing to the absence of suitable standards. The mean square deviations (PS samples) were 1.6% for molecular weight (on the linear part of the calibration dependence) and 3/zl for Va; the corresponding values for PB were 6.5% and 15/zl. The larger relative error in the case of PB can be attributed to less accurate characteristics of the non-functionalized PB calibration standards. The instrumental spreading coefficient (determined from the chromatogram of a monodisperse sample-benzene) was 1"01, the asymmetry was 1.05. Figure 2 shows the chromatogram of a mixture of six polystyrene standards with M between 400 and 4.7 x 105; the chromatograms of individual polymers are in Fig. 3. Chromatograms of a PB sample (identical to that corresponding to chromatogram 2 in Fig. 2), registered at two wavelengths (210 and 220 nm) before and after reprecipitation, are shown in Fig. 4; it is apparent from the disappearance of the low-molecular-weight peak from the chromatogram of the reprecipitated sample that this peak does not belong to the polymer but to some low-molecular-weight admixture which adsorbs strongly at 220 nm (in all probability-to an antioxidant), since PB practically do~s not absorb at 220 nm. Table 2 collects some data on MWD of several polymers; the agreement with the results obtained on the GPC 200 instrument is satisfactory. The somewhat larger discrepancy of results pertaining to the first peak in the bimodal sample is probably related to the fact that the elution volume of this peak lies outside the linear calibration region. One experiment under these conditions requires about 15 min, including the time necessary for filling of styringe and sample injection, i.e., the instrument provides results very rapidly. One analysis requires about 2 to 2.5 ml of solvent and about 10 to 50 ng of polymer sample. Making use of the multiple-wavelength detection facility one can accomplish analyses that would normally require a multi-detector instrument.
Application of liquid chrom~ttograph "Milichrom"
1655
Thus, by a simple replacement of the standard column by a column of larger volume the chromatograph "Milichrom" becomes a gel chromatograph with fully satisfactory parameters. Its performance is only limited by the requirement that the analysed polymer absorbs in the UV region and/or by its solubility. The molecular weight range is obviously limited only by the type of sorbent employed. The principles underlying the selection of sorbent combinations that ensure the linearity of the calibration dependence over a broad interval of molecular weight (covering four or even five orders of magnitude) are described in the literature [2]. The column efficiency may be improved by employing a more suitable mode of packing, by using sorbents of either smaller particle diameter (e.g. 3 llm particles) and/or with a narrower size distribution than Lichrospher. Switching of columns including the preparation of the new column for analysis requires only 1 hr, so that one instrument may be effectively used both in the normal mode and as a chromatograph for GPC.
Translated by M. KUBIN
REFERENCES 1. E.L. STYSKIN, L. B. ITSIKSON and E. V. BRAUDE, Prakticheskaya vysokoeffektivnaya zhidkostnaya khromatografiya (Practice of High-Performance Liquid Chromatography). p. 274, Moscow, 1986 2. L. Z. VILENCHIK, O. I. KURENBIN, T. P. ZHMAKINA and B. G. BELEN'KII, Vysokomol. soyed. "-A22: 2801, 1980 (Translated in Polymer Sci. U.S.S.R. 22: 12, 3074, 1980)