Styrene-Siloxane ABA Block Copolymers Synthesis and Dilute Solution Properties
W. Grenville Davies' and David P. Jones Research Department, Midland Sdicone5 Ltd , B u r n Glamorgnn, l - K
Narrow molecular weight distribution (M,/M,
-
1.3) polystyrene (A)-polydimethyl-
siloxane (8) A8A block copolymers have been prepared using anionic polymerization techniques. These copolymers are structurally similar to polystyrene-polybutadiene block copolymers, as they possess well-defined block segments. The narrowness of their molecular weight distribution was confirmed b y direct measurement of M, a n d
3, a n d
by
gel permeation chromatography. Light scattering, using the Benoit a n d Krause approach to copolymer solutions, osmometry, a n d viscometry were used to study the molecular parameters of these block copolymers. The evidence from both light scattering a n d viscometry suggests that the preferred configuration of the copolymers in solution i s that of randomly interpenetrating coils.
T h e preparation of narrow molecular weight distribution polystyrene-polybutadiene A BA block copolymers and the demonstration of their useful and unique thermoplastic elastomeric properties (Milkovich, 1964, 1965; Milkovich et al., 1966) prompted an investigation of similar block copolymers in which the rubbery segment (B) was polydimethylsiloxane. As part of the study of these novel A B and ABA block copolymers, an investigation of their dilute solution properties was undertaken. The solution properties of wholly organic block copolymers were discussed principally by Dondos et al. (1969), Inagaki (1965), Unvin (1969), and Utracki et al. (1968). These authors demonstrated the influence of segment A-segment B interactions on the solution properties of the copolymers, bct so far have been unable to present a unified model for the interpretation of the complex situation that exists within these solutions. Polydimethylsiloxane-polystyrene (PDMS-PS) copolymers represent a new series of copolymers in which the glassy and rubbery segments are. much more incompatible than their organic counterparts. The difference in compatibility may be exemplified by solubility parameter ( u ) data (Brandrup and Immergut, 1966) uPb = 9.1 and upukl:, = 7.3, compared to upoi\huiadlenr = 8.6. The discussion presented here is restricted t o ABA block copolymers in which the segments are A' polystyrene and B' polydimethylsiloxane, and both segments possess narrow molecular weight distributions. Experimental
Materials. Hexamethylcyclotrisiloxane was distilled at 134"C under an inert atmosphere before use. The styrene was purified by reduced pressure distillation. n-Butyl lithium, as a 2.3M solution in n-hexane was analyzed by the Gilman (1964) double titration method. Tet'
To whom correspondence should be addressed.
168
Ind. Eng. Chern. Prod. Res. Develop., Vol. 10, No. 2, 1971
rahydrofuran (THF) was purified by distillation from sodium into a flask containing sodium and naphthalene; it was then redistilled from this complex immediately before use. Other solvents were redistilled and dried over Linde molecular sieve 4A, where necessary. Copolymerizations. Solutions of monomers in T H F , etc. were thoroughly dried by standing in contact with Linde molecular sieve. type 4A, and calcium hydride for at least 48 hr before polymerizing. All polymerizations were conducted in an inert atmosphere in preflamed apparatus. Solution transfers were made via syringes through silicone rubber serum caps. ABA block copolymers were prepared by the sequential addition of D,, in T H F , to polystyryl lithium, followed by the addition of the stoichiometric amount of coupling agent (based on the initiator concentration). Techniques. A Waters gel permeation chromatograph (gpc), operated at 20°C, with toluene as the eluent a t a flow rate of 1 ml per min, was used to study molecular weight and molecular weight distribution changes. Four columns packed with polystyrene gel of porosities 2 x lo,', 3 x lo', 2 x 1 0 , and 8 x lo3were used in series. Sample solutions contained 0.25 wt polymer. The instrument was calibrated with narrow molecular weight distribution PDMS and PS fractions. Number average molecular weights, ( R j 2were ) , measured using a Mechrolab high speed membrane osmometer (Model 501) with toliuene at 37°C. Light scattering studies were conducted using a Sofica ipstrument, at 20°C, employing the Hg green line (5461 A); dwdc measurements of the copolymers in the various solvents were done using a Polymer Consultants Ltd. differential refractometer. Viscosity data were obtained by means of calibrated Ubbelohde suspended-level viscometers: kinetic energy corrections being made. Intrinsic viscosities were determined graphically using the Huggins equation.
(4
Results and Discussion
A B A block copolymers can be unambiguously prepared by the controlled, sequential anionic copolymerization of styrene and hexamethylcyclotrisiloxane (D J. T h e copolymerization may be carried out in an aprotic solvent or in an aprotic solvent mixture, such as tetrahydrofuranbenzene, a t room temperature. D 3 was used because it has significantly higher strain energy than any higher cyclic siloxanes (octamethylcyclotetrasiloxane), and can be rapidly polymerized in aprotic solvents by n-butyl lithium (Davies et al., 1969), without the formation of ohgomeric products by deterimental chain cleavage reactions. In this manner, it is possible t o prepare polydimethylsiloxane segments of narrow molecular weight distribution by the initiation of D 3 by polystyryl lithium (Bostick, 1969; Saam et al., 1970). Unfortunately, the reactivity of the siloxanolate anion is much less than that of the styryl anion and so S i 0 cannot be used to reinitiate styrene polymerization. As a consequence of this, a coupling reaction (Greber, 1963, 1964), using a dichlorosiloxane, has t o be employed finally to form the A B A copolymer. The reaction is illustrated in the scheme below.
CfiHs
I
CfiH,
Mix t u r e of
Homopolymers
AB
-t-
MOLECULAR WEIGHT INCREASE
Figure 1 . Gel permeation chromatograms
n-BuLi + m C H * = C H + Bu-(CHL-CH),
I
6)
Copolymers
I
CHlC HLi
I
I
CsHs
CsHs
I
I
CHI
I
I
CsHs CHy
CH?
ClSi (CH,) ,OSi (CH?),Cl
The effectiveness of the coupling reaction was studied using gel permeation chromatography. Typical A B , A B A , and mixed homopolymer chromatograms are shown in Figure 1. The shift of the complete chromatogram to higher molecular weights on coupling together with the doubling of the peak molecular weights of the corresponding A B A , (24A) relative to the A B , (24B) copolymer is indicative of a highly efficient coupling reaction. Values of the coupling efficiency for a number of copolymer syntheses are listed in Table I where coupling efficiency = [MgpcA B A I 2 (Mmc A B ) ]x 100%. Molecular weight data for some A B A copolymers prepared by the above route are listed in Table I1 where M , = [ M O ] / [ ~x , wt I ] fraction of monomer polymerized. M o is the initial monomer concentration, and l o is the initiator concentration. Mi is the calculated molecular weight assuming each initiator molecule initiates a single polymer chain. The discrepancies between Mi, and M , are attributed to the loss of catalyst by reaction with impurities present in the system.
Table 1. Coupling Reaction Efficiency AB MgpcX 10
Polymer
22 24 25 48
'
ABA
4.9 5.7 4.6 2.4
Coupling
x 10
M,,,
efficiency,
9.5 11.0 8.5 4.8
Yo
97 97 92 100
Table II. Composition and Molecular Weight Data for ABA Copolymers Mol Copolymer
styrene
16A 25A 24A 27A
14.8 30.8 11.9 2.2
Oh
MI X
1.6 9.5 10.0 12.6
p,X
M,,X
IO-'
10
2.05 8.2 10.5 13.1
M ,,,
X
'
M,/M,
IO-'
10.6 14.6
1.29 1.39
2.0 8.5 11.0 12.5
Mu/Mn.
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 2 , 1971
gpc
1.06 1.35 1.35 1.52
169
[a] = KM'
0
I sin2 €&
2 t kc
Figure 2. Schematic representation of the3Zimm plot obtained for 25A in toluene a t 20°C using 5461 A Hg line
The light scattering method proposed by Bushuk and Benoit (1958) and Krause (1961), was used for the direct determination of M u . Four solvents (benzene, toluene, tetrahydrofuran, and ethyl acetate) were used to obtain values of the apparent molecular weight (Mapp)to solve Equation 1 for M u .
Mapp= iAiB.MiL+ [ i A ( i -~ i ~ ) / i i ] x + ~M~ [ i B ( i B - iA)/iLC](l- xA)MB (1) ia, LB, and ic are the refractive index increments for homopolymers A and B, and the copolymer, respectively; x 4 is the weight fraction of component A present; MA and M B are representative of the weight average molecular weight of segments A and B. Comparison of M u and M, shows that the copolymers have narrow molecular weight distributions; this is also confirmed by calculation from gpc data. Analysis of the distribution within each segment revealed that the majority of polydispersity arises in the polystyrene segments. For example, copolymer 24A has values of Ma and M B (calculated from 1) of 37,500 and 121,800, respectively, whereas the values based on composition are 25,000 and 121,000. The broadening of the PS segments arose from the use of high concentrations of tetrahydrofuran, which were required for the subsequent rapid polymerization of D B . In a study of the light scattering behavior of the copolymers, large excluded volume effects were observed a t concentrations greater than 0.5 wt %. These effects resulted in marked curvature and distortion of the Zimm plots (Figure 2 ) , and were most pronounced in solvents which were good solvents for both A and B segmentsLe., toluene. Structural ordering effects in solution, which could give rise to micelle formations, were initially considered to be the main contributing factors. However, a study of the change of refractive index with concentration through this concentration region showed no deviation or inflection. This observation suggested the lack of micelle formation. Similarly, the Zimm plots obtained showed no dissymmetry effects before or after the curvature, again demonstrating the lack of association. T o further elucidate the conformation of the triblock copolymers in dilute solution, their viscosity behavior was investigated. In good solvents the intrinsic viscosity, [ q ] , of a homopolymer can be related to the short ( K ) and long (B') range interactions by the Stockmayer-Fixman (1963) equat'ion: 170
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 2, 1971
' + B'M
(2)
The equation has very limited use for block copolymers as interactions between chain segments of blocks A and B must be considered. In a solution of an ABA block copolymer there are five types of interaction possible. As [ q ] is measured at infinite dilution it will only be influenced by intramolecular interactions and as such, will only depend on interactions involving near neighbors within the coordination sphere of the considered chain segment. In the ABA case there will be only two pure A-B links per molecule and unless an A-chain segment enters the coordination sphere of a B-chain segment, the A-B interaction will contribute little to the total interaction energy. If this assumption is correct, it should be possible to calculate [ q ] from existing Mark-Houwink data of the homopolymers. By considering the copolymer as a binary mixture of homopolymers (Bohmer and Berek. 1970) the empirical expression: [ a ] * = n ~ [ o+~ n] ~ [ 7 may be written. This equation can be further expanded with the Mark-Houwink relationship t o give:
where [ q ] * is the calculated intrinsic viscosity, na and n~ are the mole fractions of A and B present, and A4 is the overall number average molecular weight for the copolymer. Using Equation 3, a qualitative estimation of the A-B interaction, and hence, chain configuration of the copolymer, was obtained. The results listed in Table I11 give a comparison of [q]' with that obtained experimentally and those calculated for P D M S of an equivalent molecular weight, [PDMS], as the copolymer. In Table 111, A = 1 ( [ q ] - [ a ] * ) / [ ? ] " ) x 100, and [ q ] is expressed in dl. per gram. Table IV lists the relevant Mark-Houwink data (Brandrup and Immergut, 1966) from which values of [a]* were obtained and compared with the experimentally determined values, [ q ] . The solvents used were toluene a t 25.0" C, a good solvent for both segments, MEK a t 2O.O0C, and cyclohexane a t 34.0°C. The latter two are theta solvents for P D M S and PS, respectively. For all three solvents, higher [ q ] values were observed than calculated using Equation 3.
([VI),
Table Ill. Copolymer Viscosity Data ?A
Toluene, 25.OCC
25A 24A 27A 25A 24A 21A 25A 24A 27A
MEK, 20.0" C
Cyclohexane, 34.0" C
0.308 0.119 0.022 0.308 0.119 0.022 0.308 0.119 0.022
i?I 0.491 0.493 0.545 0.351 0.334 0.354 0.427 0.529 0.666
[VI'
0.455 0.458 0.487 0.227 0.261 0.293 0.362 0.467 0.630
1
[ PDMS]
7.9 7.6 11.9 54.6 27.9 20.8 17.9 13.3 5.7
0.350 0.411 0.478 0.232 0.262 0.293 0.418 0.494 0.579
Table IV. Mark-Houwink Data for Homopolymers Toluene, 25.0" C Polymer
K
a
MEK, 20.0" C
K
PDMS 2.0 x lo-' 0.66 8.1 x lo-' PS 4.4 x lo-' 0.65 7.03 x lo-'
Cyclohexane, 34.0' C a
K
0.50 1.59 x lo-: 0.71 8.2 x 10
a
0.70 0.50
~ ]
Also, the observed values were considerably higher than those calculated for pure PDMS. These results suggest expansion of the polymer coils owing to A - B interactions. I n toluene, the differences, as expressed by A, are small and represent only 6 to 12% of the calculated value. They are also independent of molecular weight and composition. I n M E K , however, the theta solvent for PDMS, values of A are up to 54c; greater than those calculated and are proportional to the mole fraction of polystyrene. Cyclohexane, a theta solvent for the PS segment, also caused expansion of the coil configuration, but t o a smaller extent than M E K . All these data show expansion of the predicted coil configuration caused by large contributions of A - B interactions. These findings cannot be reconciled with those of Dondos e t al. (1969), who, from a study of polystyrene-polymethylmethacrylate block copolymers, conclude that in dilute solution, in any solvent, the blocks are segregated and therefore, have distinct locations. The interpretation which most readily lends itself to our results is that the copolymers form randomly interpenetrating coils. In good solvents the coils are expanded by polymersolvent interactions and A - B interactions are a minimum, whereas, in a theta solvent for one of the blocks, repulsive A - B interactions contribute appreciably toward coil expansion. These large repulsive interactions are to be expected if the incompatibility of segment A with segment B is considered.
Literature Cited
Bohmer, B., Berek, D.. Eur. Poiym. J . . 6. 471 (1970). Bostick, E. E. (to General Electric Co.. Ltd.). U.S.Patent 3,483,270 (1969). Brandrup, J . , Immergut, E. H.. Eds.. “Polymer Handbook,” Interscience, New York, S.Y., 1966. Bushuk, W., Benoit, H., Can. J . Chem., 36, 1616 (1958). Davies, W. G., Elliott, B., Kendrick, T. C., (to Midland Silicones Ltd.), U. S.Patent 3,481,898 (1969). Dondos, A., Rempp, P., Benoit, H., Makromol. Chem., 130, 233 (1969). Gilman, H., J . Organometal. Chem., 2, 447 (1964). Greber, G., Balciunas, A., Makromol. Chem., 69, 193 (1963). Greber, G., Balciunas, A., ibid., 79, 149 (1964). Inagaki, H . , ibid., 86, 289 (1965). Krause, S.,J . Phys. Chem., 65, 1618 (1961). Milkovich, R. (to Shell Oil Co.), South African Patent Application 642,271 (1964). Milkovich, R . (to Shell Oil Co.), British Patent 1,000,090 (1965). Milkovich, R., Holden, G., Bishop, E. T., Hendricks, W. R., British Patent 1,035,873 (1966). Saam, J. C., Gordon, D. J., Lindsey, S., Macromolecules, 3, 1 (1970). Stockmayer, W . H., Fixman, M., J . Polym. Sci.,Part C, 1, 137 (1963). Urwin, J. R., Aust. J . Chem., 22, 1649 (1969). Utracki, L. A., Simha, R., Fetters, L . J.: J . Polym. Sci., Part A - 2 , 6, 2051 (1968).
Yellow D e n t Corn Starches Effects of Chlorine Oxidation David M. Hall‘, Eric Van Patten’, John L. Brown3, Grady R. Harmon, and Gordon H. N i x
School of Engineering, Auburn Uniuersity, Auburn, Ala. 36830
C o r n starch is perhaps the major product used for the surface size application to paper and in warp sizing of textiles. The type of starch used is usually dictated by the desired solids content (add-on) and the viscosity of the paste desired. Preconverted starches, such as oxidized starches, can easily meet the low viscosity and free flowing characteristics a t high solids required for most paper and textile applications. The oxidized starches retain the granule structure and have the same general appearance under both the polarizing and ordinary light microscope as unmodified starch. They are insoluble in cold water and show the typical starch-iodine coloration upon exposure to iodine (Scallet and Sowell, 1967). In this study we are attempting t o correlate the surface T o whom correspondence should be addressed. address, American Maize-Products Co., Roby, Ind. 46326. Present address, Analytical Instrumentation Laboratories. Georgia Institute of Technology, Atlanta, Ga. 30332.
’ Present
structure of the starch with the cooking properties associated with the oxidized starch. Starch is a transparent spherulite. We have found that light diffraction through the starch can often give spurious images which are manifestations of internal details but which might appear as a surface structure. This can be convincingly seen by comparing the light micrographs with the images seen using the scanning electron microscope ( S E M ) . The starch granules, being nonconductive, must be shadowed with a metal coating; hence, only surface details are observed. From a comparison of the light microscope and SEM micrographs, some speculations concerning internal details are possible (Hall, 1968; Hall and Sayre, 1969, 1970a.b). We have extended the use of this instrument to study the changes in the surface structure of the starch upon oxidation. Since the starches were all treated identically in preparation for the scanning electron microscopical study, any observed difference in surface structure should be due only to the oxidation treatment. Chlorite oxidation of starch has been extensively studied. Some of the aspects Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 2, 1971
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