20 Size Exclusion Chromatography with Low-Angle Laser Light-Scattering Detection
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Application to Linear and Branched Block Copolymers
R. C. JORDAN, S. F. SILVER, R. D. SEHON, and R. J. RIVARD 3M-3M Center, P.O. Box 33221, St. Paul, ΜΝ 55133
Linear block polymers of styrene-co-isoprene and styrene-co-butadiene were prepared via anionic polymerization and subsequently were coupled with divinyl benzene to give multi-arm macro -molecules. Low-angle laser light scattering (LALLS) was used for molecular weight measure ment both in the stand-alone (static) mode and as a detector coupled to SEC. No dependence of the weight average molecular weight (M ) on solvent was found, which is consistent with light scattering theory for compositionally homogeneous block polymers. Comparison of SEC/LALLS data for both the linear and branched species shows the strong effect of branching upon the hydro dynamic volume/molecular weight relationship. The data indicate the multi-arm samples are of relatively small molecular weight polydispersity, with a weight-average branching functionality of 16-18. Use was made of the universal calibration and SEC/LALLS data to calculate the branched/ linear intrinsic viscosity ratio through the molecular weight distribution of the multiarm samples; an anomalous dependence on molecular weight was found. Difficulties with the universal calibration procedure or sample viscosity effects are discussed as possible causes. w
The d e l i b e r a t e i n t r o d u c t i o n of m u l t i f u n c t i o n a l b r a n c h i n g i n t o anionically prepared polydiene a n d poly ( diene-co-styrene) polymers produces materials with unique morphological a n d viscoelastic properties ( 1 - 3 ) . Work h a s i n c l u d e d s y n t h e s i s of symmetric star polymers produced by reaction of living polyanionic "arms" with multi-functional chlorosilane (4-9), 0097-6156/ 84/0245-0295507.50/0 © 1984 American Chemical Society
In Size Exclusion Chromatography; Provder, Theodore; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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and less well defined s t r u c t u r e s have been o b t a i n e d using d i v i n y l benzene (DVB) as the l i n k i n g agent (10-16). Dilute solution studies on such m a t e r i a l s have been carried out i n a n attempt to g a i n some u n d e r s t a n d i n g o f m o l e c u l a r s t r u c t u r e / p r o p e r t y r e l a t i o n s h i p s ( 6 , 7* 1 4 , 1 7 - 1 9 ) , and the work h a s benefited from elegant theoretical frameworks developed b y several workers (20-24). Molecular weight c h a r a c t e r i z a t i o n i s c r i t i c a l to f u n d a m e n t a l investiga tions, as well a s to d e v e l o p m e n t of c o n t r o l l e d production processes i n commercial a p p l i c a t i o n s . S e v e r a l s t u d i e s h a v e been p u b l i s h e d w h i c h u t i l i z e size exclusion chromatography (SEC) f o r c h a r a c t e r i z a t i o n of the molecular weight distribution of multi-arm s t r u c t u r e s of polystyrene, polyisoprene, a n d b l o c k c o p o l y m e r s of s t y r e n e / butadiene a n d styrene/isoprene ( 1 , 2 , 8 , 17, 25-26). A n interesting phenomenon of direct consequence to work presented here w a s recorded b y B i a n d Fetters (1) a s a r e s u l t of t h e i r work on D V B - l i n k e d " s t a r " b l o c k copolymers of poly ( styrene-co-polybutadiene) a n d poly ( styrene-coisoprene) : they found that the u n i v e r s a l c a l i b r a t i o n procedure gave anomalous m o l e c u l a r weight v a l u e s for s e v e r a l samples. These workers noted that the sample i n t r i n s i c v i s c o s i t y ( [ η ] ) w a s i n s e n s i t i v e to t h e number of a r m s , w h e n t h e a r m c o m p o s i t i o n and molecular weight was held constant, and it was suggested that this c h a r a c t e r i s t i c might underlie the u n u s u a l chromatographic behavior. H o w e v e r , i t w a s n o t e d t h a t some d e g r e e of S E C s e p a r a t i o n b y n u m b e r of a r m s w a s a c h i e v e d . These a u t h o r s suggest that u n i q u e h y d r o d y n a m i c properties c o n f e r r e d b y a " c o r e / s h e l l " s t r u c t u r e m a y be r e s p o n s i b l e for the S E C b e h a v i o r , a n d also i t i s not c l e a r what relative effects the number of arms a n d a r m l e n g t h h a v e on t h e SEC separation. The o b s e r v a t i o n s a n d q u e s t i o n s r a i s e d b y these w o r k e r s helped stimulate the experiments which are presented here. In t h i s w o r k , size e x c l u s i o n chromatography with low angle laser light scattering detection (SEC/LALLS) w a s used to probe the d i s t r i b u t i o n of b r a n c h i n g f u n c t i o n a l i t y i n m u l t i a r m macromolecules of styrene/isoprene a n d styrene/butadiene block copolymers a n d to e v a l u a t e the a p p l i c a b i l i t y of u n i v e r s a l c a l i b r a t i o n to a n a l y s i s of s u c h m a t e r i a l s . The SEC process separates v i a hydrodynamic volume, not molecular weight, so t h a t use of c o n v e n t i o n a l calibration methods ( e . g . , c a l i b r a t i o n w i t h l i n e a r p o l y s t y r e n e standards) suffers from the inherent hydrodynamic volume/molecular weight differences between sample a n d c a l i b r a n t . I n g e n e r a l , branching decreases the h y d r o d y n a m i c volume of a m a c r o m o l e c u l e r e l a t i v e to i t s l i n e a r h o m o l o g , so t h a t c o n v e n t i o n a l SEC a n a l y s i s of b r a n c h e d s p e c i e s i s e s p e c i a l l y precarious. Use o f L A L L S w i t h S E C c a n c i r c u m v e n t s u c h d i f f i c u l t i e s s i n c e connection of a L A L L S detector i n series w i t h a c o n c e n t r a t i o n detector a l l o w s d e t e r m i n a t i o n of t h e c o r r e c t m o l e c u l a r w e i g h t
In Size Exclusion Chromatography; Provder, Theodore; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 12, 2016 | http://pubs.acs.org Publication Date: March 30, 1984 | doi: 10.1021/bk-1984-0245.ch020
20.
JORDAN ET AL.
Low-Angle
Laser Light-Scattering
Detection
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at each increment in the polymer chromatogram without recourse to use of calibration techniques (27-29). The S E C / L A L L S m e t h o d h a s p r o v e d to be a v a l u a b l e t o o l f o r t h e s t u d y of b r a n c h e d p o l y m e r s , w i t h s e v e r a l e x c e l l e n t publicat i o n s a p p e a r i n g on t h i s s u b j e c t (17, 26, 3 0 - 3 3 ) . Synthetic copolymers often present a fundamental d i f f i c u l t y for m o l e c u l a r w e i g h t measurement b y l i g h t s c a t t e r i n g , s i n c e c o m p o s i t i o n a l h e t e r o g e n e i t y c a n be s u p e r i m p o s e d o n the d i s t r i b u t i o n of molecular weights. Whereas a dilute solution light scattering measurement from a homopolymer which is polydisperse in molecular weight yields the weight-average molecular weight of t h e s a m p l e ( M w ), a compositionally heterodisperse polymer sample gives an a p p a r e n t m o l e c u l a r w e i g h t w h i c h depends on the s o l v e n t (34). The copolymer difficulty arises from the change of the specific refractive index increment ( dn/dc ) with polymer c o m p o s i t i o n , w h i l e a v a l u e of d n / d c m e a s u r e d o n t h e e n t i r e sample is used to c a l c u l a t e molecular weights from light scattering data. In cases without compositional variation among p o l y m e r c h a i n s , the l i g h t s c a t t e r i n g m o l e c u l a r weight i s i n d e p e n d e n t of s o l v e n t i d e n t i t y . The experimental samples used i n this work should, however, be amenable to s t r a i g h t f o r w a r d light scattering analysis since the constituent polymers possess backbone block microstructures which guarantee compositional uniformity throughout the sample. The synthetic route utilizes "living" anionic polymerization of p o l y s t y r e n e , followed by isoprene or butadiene addition; divinyl benzene ( DVB) linkage of these species gives multiarm structures (MA) containing arms which are homogeneous with respect to composition and molecular weight. The light scattering average molecular weight of both the linear block (LB ) c o p o l y m e r a n d MA s h o u l d be i n d e p e n d e n t of s o l v e n t i d e n t i t y , w i t h a n i d e n t i c a l dn/dc for both m a t e r i a l s . The SEC/LALLS method was 1) used to study a commercially available linear block copolymer: K r a t o n 1107 b r a n d elastomer (Shell Chemical Company), a n d 2) to s t u d y the s t a r t i n g " a r m " b l o c k c o p o l y m e r a n d r e s u l t a n t D V B - l i n k e d MA of s e v e r a l e x p e r i m e n t a l s a m p l e s .
Theory Dilute Solution Light Scattering: Homopolymers and Copolymers. The theoretical basis for polymer molecular weight measurement by light scattering has been developed in d e t a i l , a n d o n l y t h e c o n c e p t s r e l e v a n t to S E C / L A L L S s t u d i e s of c o p o l y m e r s a r e p r e s e n t e d h e r e . Proper use of LALLS to measure polymer molecular weight requires a dilute solution of optically isotropic flexible macromolecules whose dimensions are of t h e same
In Size Exclusion Chromatography; Provder, Theodore; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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o r d e r a s t h e w a v e l e n g t h of s c a t t e r i n g r a d i a t i o n (35-37). F o r chains which are compositionally and molecular-weight monod i s p e r s e , i n t h e l i m i t of v a n i s h i n g l y s m a l l o b s e r v a t i o n angle ( Θ) :
jf • D ^
