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A method for studying micellar aggregates in block and graft copolymers
European Polymer Journal, 1973, Vol. 9, pp. 827-833. Pergamon Press. Printed in England.
A M E T H O D F O R S T U D Y I N G MICELLAR A G G R E G A T E S IN BLOCK A N D G R A F T COPOLYMERS C. PRICE and D. WOODS Chemistry Department, University of Manchester, Manchester, M13 9PL, England (Received 11 January 1973)
Abstraet--A method has been developed for studying the presence of micellar aggregates in block and graft copolymers. Specimens for examination in the electron microscope were prepared using a freeze-etching/replication technique. Four well-characterized copolymers were studied: a polystyrene-polyisoprene two-block copolymer, a (polystyrene-polyisoprene-)4Sistar-block copolymer, and two polystyrene-g-polyisoprene graft copolymers. Technical white oil, which is a very poor solvent for polystyrene, was taken as the solvent for the copolymers. For the purpose of comparison, a study was also made of specimens prepared from polystyrene homopolymer dissolved in a 1 : 1 mixture by vol of technical white oil and toluene. INTRODUCTION THE POSSIBILITYof using electron microscopy in the determination of the molecular weights of high polymers has been the subject of numerous studies. "-4) Specimens for investigation may be prepared by allowing very dilute polymer solutions to evaporate on supporting films. It has been shown that the tendency for the polymer coils to aggregate on removing the solvent may be reduced if the solution is in the form of microdroplets sprayed from an atomizer. Siegel et al. estimated the molecular weight of polystyrene samples from studies on specimens prepared by a spraying and evaporation technique/2) Micrographs were obtained of shadow-cast molecular dispersions mounted on colloidon membranes and of preshadowed replicas. To calculate the molecular weight from the observed particle sizes, it was assumed that on removing the solvent the polymer coils collapse until their density is the same as that of the bulk polymer. Quite good agreement was obtained between results obtained by electron microscopy and by membrane osmometry; the electron microscopy method was found to have a useful lower limit of 1-5 × 106. Other workers have also successfully estimated molecular weight from the volume of collapsed coils as observed by electron microscopy; these include Nasini et al. for poly(methyl methacrylate), (5) Richardson for polystyrene, polyacrylonitrile, poly(vinyl acetate) and polyacrylamide(~} and Ruscher for cellulose nitrate and a polycarbonate37~ One of the main limitations of the above techniques is that, in order to avoid aggregation during evaporation, it is necessary to work with extremely dilute solutions. In an attempt to overcome these problems, several groups of workers ~s.9) have developed specimen preparation techniques involving a combination of spraying and freeze drying. Recently, Cuniberti and Fernando ~1°~ reported the use of such a technique in the study of supermolecular structures in moderately dilute solutions of polyethylene oxide. In our present contribution we report an alternative approach 827
828
C. PRICE and D. WOODS
for studying the presence o f aggregates in dilute a n d moderately dilute solutions. Specimens for e x a m i n a t i o n in the electron microscope were prepared using a freezeetching/replication technique. Freeze-etching has been developed in the biological field almost exclusively for a q u e o u s systems311~ Recently, however, Riegelhuth a n d W a t k i n s ~12) used a n o n - a q u e o u s solvent for m e a s u r e m e n t of microdispersed particles i n overbased additives. W e have extended the latter work somewhat further to permit the study of micellar aggregates in block copolymers. EXPERIMENTAL
Polymer samples The investigated polymers are listed below:
Type Polystyrene-polyisoprene (Polystyrene-polyisoprene-)4Si Polyst yrene--g-polyisoprene Polystyrene-g-polyisoprene Polystyrene
Two-block Star-block Graft Graft Homopolymer
Sample no. 1DI 1P4 1G 1 1G3 1S1
The polystyrene was purchased from Pressure Chemicals Co. All the copolymer samples were synthesized in our laboratory. Details of the method of synthesis, fractionation and structural characterization are to be found elsewhere,cla,~4) Briefly the polystyrene--polyisoprene and (polystyrene-polyisoprene-)4Si copolymers were prepared by terminating polystyrene-polyisoprenyllithium polymers with methanol and SiCI4, respectively. The graft copolymers were prepared by adding polyisoprenyllithium (1~1, = 12,500) to chloromethylated polystyrene (lql. = 190,000). In the syntheses, n-butyl lithium (Koch-Light, Ltd.) was used as the primary initiator. In all cases liquidliquid separation was used to remove traces of homopolymer and uncoupled block copolymers; gel permeation chromatography was used to check the homogeneity of the samples. The relevant characterization results are summarized in Table 1.
Freeze-etching technique The block copolymer samples were all made up as 2 percent (by weight) solutions in technical white oil. Since polystyrene is insoluble in the latter, it was made up as a 2 per cent (by weight) solution in a 1 : 1 by volume mixture of toluene and technical white oil. Each solution was treated as follows: 1. A drop of solution was shock-cooled in liquid freon to liquid nitrogen temperature. The cold specimen was then quickly transferred to the cooled pedestal of a low temperature microtome in a vacuum chamber and the pressure reduced to 10-5-10-6 torr. 2. After the temperature had been allowed to rise to - 9 0 ° in order to minimize splintering under the impact of the cutting edge, a layer of the specimen was cut away. 3. Solvent was allowed to evaporate off from the cleanly cut surface of the specimen (ca. 15-60 sec at 10-~ torr). 4. A carbon replica was then made of the surface in the usual way. When necessary the replica was C/Pt shadowed to enhance contrast. 5. The solution was allowed to warm up to room temperature and the vacuum released. 6. After cleaning with solvent, the replica of the specimen was examined in an electron microscope. A schematic outline of steps 1-4 is given in Fig. 1.
Specimandrop
l
'Cu ' pl
LI iJJ
Liquid~ Ne
CiPt beam ~
Removereplica fromsurface -90"C Ulframicrotomeetch
Fio. 1. Schematic outline of the freeze-etching/replication procedure.
Micellar Aggregates in Block and Graft Copolymers
829
RESULTS Electron micrographs obtained using the above procedure are shown in Figs. 2-6 for the five samples listed in Table 1. The polystyrene homopolymer and (polystyrene-polyisoprene-),Si star-block copolymer both form particles monodisperse in size. A striking feature with the copolymer is the presence of large clusters formed by the hexagonal packing of particles. With the polystyrene-polyisoprene two-block copolymer, the observed particles are also fairly monodisperse; with this system there is a frequent occurrence of paired particles. The two graft copolymers form particles which appear very polydisperse in size. For all five samples we believe the basic particles are spherical in shape; we base this conclusion on the fact that the plan view of all particles is circular and the method of preparation should lead to a statistical orientation of particles. TABLE 1. CHARACTERIZATIONRESULTSFOR THE FOUR COPOLYMERSAND THE POLYSTYRENEHOMOPOLYMER lff'l, x 10 -5
(1Ul,,),p~
Polymer
No.
S
I
Total
(1VIw),p, × 10 -n
I(-ln
Styrene (wt 9/oo)
S-I (S-I-),Si S-g-I S-g-I S
ID1 1P4 1G1 IG3 1S1
0"13 0'125 1-9 1'9 --
0"38 0"375 0"125 0'125 --
0"51 1"88 5"5 4'2 6'7
0-57 2"08 6'5 5'1 --
1 "12 1'10 1"18 1"22 --
25"2 24"9 29-2 38"1 --
The method chosen for analysing the micrographs was similar to that used by Riegelhuth and Watkins312) The average size of particles was determined from measurements on enlarged prints using an optical microscope at 10 × magnification. It has been pointed out (ASTM E20-68) that the accuracy of such determinations expressed as a percentage of the particle size is increased proportional to the square root of the number of particles counted. In the present study, 300 particles were counted for each specimen, giving an estimated accuracy for graft copolymer 1G1 of zk 5 per cent and for the polystyrene homopolymer and the other copolymers of 4- l0 per cent. The data obtained were substituted into the following relationships, where n~ is the number of particles having diameter di.
Number-average particle volume
arti ,o
=
amotor On :
[ ,X n,
]"
.
Weight-average particle volume Vw = ~ _-----d, a n, ~
l
830
C. PRICE and D. WOODS Weight-average particle diameter Dw = [~--~] ÷. Dispersion index DI =
Vw/1I,.
The results are recorded in Table 2; for the two graft copolymers, histograms are given in Figs. 7 and 8 to illustrate the distribution of sizes. TABLE 2. RESULTS OF PARTICLE SIZE DETERMINATIONSFOR THE FOUR COPOLYMERSAND THE POLYSTYRENE HOMOPOLYMER
V. Sample no. 1DI IP4 1G1 1G3 1S1
x 10 -4 (nm) 3
D. I (nm)
0"46 0"15 3'42 3"25 0'09
V~, × 10-'* (nm) s
Dw (nm)
Vw/V.
0"46 0"15 4"63 5.70 0"09
21.0 14.0 45"0 48"0 12"0
1"I 1"I 1"3 1.7 1.1
21"0 14"0 40.0 39"0 12.0
Histogram
IGI
8O
70
60
5O f(D) 4O
30
20
I0
P 0
V,
I,,
14'4
28"8
l
l 4312
Diameter,
I 57-6
l
l
l
I
72"0
nm
FIG. 7. H i s t o g r a m o f n u m b e r - d i s t r i b u t i o n o f particle diameters for 1GI.
DISCUSSION If we assume that the particles, whose replicas we have studied, contain no solvent molecules and have a density similar to that of the bulk polymer, then it is possible to calculate the average molecularweights of the particles from the dimensionsgiven in Table 2. The results of such calculations are compared in Table 3 with number-
A M E T H O D F O R S T U D Y I N G MICELLAR A G G R E G A T E S IN BLOCK A N D G R A F T COPOLYMERS C. PRICE and D. WOODS Chemistry Department, University of Manchester, Manchester, M13 9PL, England (Received 11 January 1973)
Abstraet--A method has been developed for studying the presence of micellar aggregates in block and graft copolymers. Specimens for examination in the electron microscope were prepared using a freeze-etching/replication technique. Four well-characterized copolymers were studied: a polystyrene-polyisoprene two-block copolymer, a (polystyrene-polyisoprene-)4Sistar-block copolymer, and two polystyrene-g-polyisoprene graft copolymers. Technical white oil, which is a very poor solvent for polystyrene, was taken as the solvent for the copolymers. For the purpose of comparison, a study was also made of specimens prepared from polystyrene homopolymer dissolved in a 1 : 1 mixture by vol of technical white oil and toluene. INTRODUCTION THE POSSIBILITYof using electron microscopy in the determination of the molecular weights of high polymers has been the subject of numerous studies. "-4) Specimens for investigation may be prepared by allowing very dilute polymer solutions to evaporate on supporting films. It has been shown that the tendency for the polymer coils to aggregate on removing the solvent may be reduced if the solution is in the form of microdroplets sprayed from an atomizer. Siegel et al. estimated the molecular weight of polystyrene samples from studies on specimens prepared by a spraying and evaporation technique/2) Micrographs were obtained of shadow-cast molecular dispersions mounted on colloidon membranes and of preshadowed replicas. To calculate the molecular weight from the observed particle sizes, it was assumed that on removing the solvent the polymer coils collapse until their density is the same as that of the bulk polymer. Quite good agreement was obtained between results obtained by electron microscopy and by membrane osmometry; the electron microscopy method was found to have a useful lower limit of 1-5 × 106. Other workers have also successfully estimated molecular weight from the volume of collapsed coils as observed by electron microscopy; these include Nasini et al. for poly(methyl methacrylate), (5) Richardson for polystyrene, polyacrylonitrile, poly(vinyl acetate) and polyacrylamide(~} and Ruscher for cellulose nitrate and a polycarbonate37~ One of the main limitations of the above techniques is that, in order to avoid aggregation during evaporation, it is necessary to work with extremely dilute solutions. In an attempt to overcome these problems, several groups of workers ~s.9) have developed specimen preparation techniques involving a combination of spraying and freeze drying. Recently, Cuniberti and Fernando ~1°~ reported the use of such a technique in the study of supermolecular structures in moderately dilute solutions of polyethylene oxide. In our present contribution we report an alternative approach 827
828
C. PRICE and D. WOODS
for studying the presence o f aggregates in dilute a n d moderately dilute solutions. Specimens for e x a m i n a t i o n in the electron microscope were prepared using a freezeetching/replication technique. Freeze-etching has been developed in the biological field almost exclusively for a q u e o u s systems311~ Recently, however, Riegelhuth a n d W a t k i n s ~12) used a n o n - a q u e o u s solvent for m e a s u r e m e n t of microdispersed particles i n overbased additives. W e have extended the latter work somewhat further to permit the study of micellar aggregates in block copolymers. EXPERIMENTAL
Polymer samples The investigated polymers are listed below:
Type Polystyrene-polyisoprene (Polystyrene-polyisoprene-)4Si Polyst yrene--g-polyisoprene Polystyrene-g-polyisoprene Polystyrene
Two-block Star-block Graft Graft Homopolymer
Sample no. 1DI 1P4 1G 1 1G3 1S1
The polystyrene was purchased from Pressure Chemicals Co. All the copolymer samples were synthesized in our laboratory. Details of the method of synthesis, fractionation and structural characterization are to be found elsewhere,cla,~4) Briefly the polystyrene--polyisoprene and (polystyrene-polyisoprene-)4Si copolymers were prepared by terminating polystyrene-polyisoprenyllithium polymers with methanol and SiCI4, respectively. The graft copolymers were prepared by adding polyisoprenyllithium (1~1, = 12,500) to chloromethylated polystyrene (lql. = 190,000). In the syntheses, n-butyl lithium (Koch-Light, Ltd.) was used as the primary initiator. In all cases liquidliquid separation was used to remove traces of homopolymer and uncoupled block copolymers; gel permeation chromatography was used to check the homogeneity of the samples. The relevant characterization results are summarized in Table 1.
Freeze-etching technique The block copolymer samples were all made up as 2 percent (by weight) solutions in technical white oil. Since polystyrene is insoluble in the latter, it was made up as a 2 per cent (by weight) solution in a 1 : 1 by volume mixture of toluene and technical white oil. Each solution was treated as follows: 1. A drop of solution was shock-cooled in liquid freon to liquid nitrogen temperature. The cold specimen was then quickly transferred to the cooled pedestal of a low temperature microtome in a vacuum chamber and the pressure reduced to 10-5-10-6 torr. 2. After the temperature had been allowed to rise to - 9 0 ° in order to minimize splintering under the impact of the cutting edge, a layer of the specimen was cut away. 3. Solvent was allowed to evaporate off from the cleanly cut surface of the specimen (ca. 15-60 sec at 10-~ torr). 4. A carbon replica was then made of the surface in the usual way. When necessary the replica was C/Pt shadowed to enhance contrast. 5. The solution was allowed to warm up to room temperature and the vacuum released. 6. After cleaning with solvent, the replica of the specimen was examined in an electron microscope. A schematic outline of steps 1-4 is given in Fig. 1.
Specimandrop
l
'Cu ' pl
LI iJJ
Liquid~ Ne
CiPt beam ~
Removereplica fromsurface -90"C Ulframicrotomeetch
Fio. 1. Schematic outline of the freeze-etching/replication procedure.
Micellar Aggregates in Block and Graft Copolymers
829
RESULTS Electron micrographs obtained using the above procedure are shown in Figs. 2-6 for the five samples listed in Table 1. The polystyrene homopolymer and (polystyrene-polyisoprene-),Si star-block copolymer both form particles monodisperse in size. A striking feature with the copolymer is the presence of large clusters formed by the hexagonal packing of particles. With the polystyrene-polyisoprene two-block copolymer, the observed particles are also fairly monodisperse; with this system there is a frequent occurrence of paired particles. The two graft copolymers form particles which appear very polydisperse in size. For all five samples we believe the basic particles are spherical in shape; we base this conclusion on the fact that the plan view of all particles is circular and the method of preparation should lead to a statistical orientation of particles. TABLE 1. CHARACTERIZATIONRESULTSFOR THE FOUR COPOLYMERSAND THE POLYSTYRENEHOMOPOLYMER lff'l, x 10 -5
(1Ul,,),p~
Polymer
No.
S
I
Total
(1VIw),p, × 10 -n
I(-ln
Styrene (wt 9/oo)
S-I (S-I-),Si S-g-I S-g-I S
ID1 1P4 1G1 IG3 1S1
0"13 0'125 1-9 1'9 --
0"38 0"375 0"125 0'125 --
0"51 1"88 5"5 4'2 6'7
0-57 2"08 6'5 5'1 --
1 "12 1'10 1"18 1"22 --
25"2 24"9 29-2 38"1 --
The method chosen for analysing the micrographs was similar to that used by Riegelhuth and Watkins312) The average size of particles was determined from measurements on enlarged prints using an optical microscope at 10 × magnification. It has been pointed out (ASTM E20-68) that the accuracy of such determinations expressed as a percentage of the particle size is increased proportional to the square root of the number of particles counted. In the present study, 300 particles were counted for each specimen, giving an estimated accuracy for graft copolymer 1G1 of zk 5 per cent and for the polystyrene homopolymer and the other copolymers of 4- l0 per cent. The data obtained were substituted into the following relationships, where n~ is the number of particles having diameter di.
Number-average particle volume
arti ,o
=
amotor On :
[ ,X n,
]"
.
Weight-average particle volume Vw = ~ _-----d, a n, ~
l
830
C. PRICE and D. WOODS Weight-average particle diameter Dw = [~--~] ÷. Dispersion index DI =
Vw/1I,.
The results are recorded in Table 2; for the two graft copolymers, histograms are given in Figs. 7 and 8 to illustrate the distribution of sizes. TABLE 2. RESULTS OF PARTICLE SIZE DETERMINATIONSFOR THE FOUR COPOLYMERSAND THE POLYSTYRENE HOMOPOLYMER
V. Sample no. 1DI IP4 1G1 1G3 1S1
x 10 -4 (nm) 3
D. I (nm)
0"46 0"15 3'42 3"25 0'09
V~, × 10-'* (nm) s
Dw (nm)
Vw/V.
0"46 0"15 4"63 5.70 0"09
21.0 14.0 45"0 48"0 12"0
1"I 1"I 1"3 1.7 1.1
21"0 14"0 40.0 39"0 12.0
Histogram
IGI
8O
70
60
5O f(D) 4O
30
20
I0
P 0
V,
I,,
14'4
28"8
l
l 4312
Diameter,
I 57-6
l
l
l
I
72"0
nm
FIG. 7. H i s t o g r a m o f n u m b e r - d i s t r i b u t i o n o f particle diameters for 1GI.
DISCUSSION If we assume that the particles, whose replicas we have studied, contain no solvent molecules and have a density similar to that of the bulk polymer, then it is possible to calculate the average molecularweights of the particles from the dimensionsgiven in Table 2. The results of such calculations are compared in Table 3 with number-