Shear Degradation of Very High Molecular Weight Polymers in Gel

15 Shear Degradation of Very High Molecular Weight Polymers in Gel Permeation Chromatography D. McINTYRE, A. L. SHIH, J. SAVOCA, R. SEEGER, and A. MACARTHUR

Downloaded by IOWA STATE UNIV on March 28, 2017 | http://pubs.acs.org Publication Date: March 30, 1984 | doi: 10.1021/bk-1984-0245.ch015

Institute of Polymer Science, The University of Akron, Akron, OH 44325

The degradation of very high molecular polymers in GPC is demonstrated to occur in the gel columns, to begin at a critical molecular weight depending on the polymer structure, and to follow a power law dependence on MW after the onset of degradation, A loop model of entanglement is advanced to explain the degradation, and guidelines to minimize degradation are explicitly described. An e a r l i e r experiment i n these l a b o r a t o r i e s reported that very h i g h molecular weight p o l y s t y r e n e (PS) was degraded i n g e l perme a t i o n chromatography (GPC) columns o p e r a t i n g a t r e l a t i v e l y low pressures (125 p s i ) and low e l u t i o n r a t e s (lml/min) ( 1 ) . The de graded very h i g h molecular weight p o l y s t y r e n e (MW 4 4 x l 0 ) was r e covered from the e l u e n t , and i t s molecular weight was determined by i n t r i n s i c v i s c o s i t y measurements. The molecular weight o f the o r i g i n a l polymer, 4 4 x l 0 , had been decreased t o 1 9 x l 0 . Thus t h e o r i g i n a l polymer chain had on the average been cut t o l e s s than one-half i t s s i z e i n i t s passage through the GPC column. When the degraded molecular weight was used as the c o r r e c t molecular weight, the degraded polymer n e a r l y f i t the GPC c a l i b r a t i o n curve o f e l u t i o n volume-molecular weight that had been e s t a b l i s h e d w i t h much lower molecular weight p o l y s t y r e n e s . Since e a r l i e r work (2) had shown that a l O x l O MW p o l y s t y r e n e d i d obey the GPC c a l i b r a t i o n curve, the onset o f measurable degradation had t o occur a t a mol e c u l a r weight g r e a t e r than l O x l O . I t seemed worthwhile t o explore the g e n e r a l i t y of the e a r l i e r f i n d i n g o f chain degradation i n PS a t very h i g h molecular weights, s i n c e the degradation only had been shown t o occur w i t h p o l y s t y rene i n a given set of columns, u s i n g a conventional mechanical c o n f i g u r a t i o n , w h i l e o p e r a t i n g a t a low shear r a t e (or e q u i v a l e n t l y e l u t i o n r a t e ) . Consequently, both the p h y s i c a l set-up o f the GPC columns and the chemical s t r u c t u r e of the chromâtographically separated polymers were v a r i e d i n t h i s study. High molecular 6

6

6

6

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0097-6156/ 84/0245-0227506.00/0 © 1984 American Chemical Society

Provder; Size Exclusion Chromatography ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


228

SIZE EXCLUSION CHROMATOGRAPHY

weight p o l y d i m e t h y l s i l o x a n e (PDMS) and PS over a range o f molec u l a r weights were examined. Benzene was used as a s o l v e n t . The f l o w r a t e and mechanical c o n s t r i c t i o n s i n the t u b i n g were v a r i e d w h i l e attempting t o measure degradation i n the GPC. The change i n flow r a t e i s r e l a t e d to the pressure drop and t h e r e f o r e to the shear r a t e i n the columns. The operating pressure was v a r i e d only over a narrow range (50 p s i t o 150 p s i , o r an equiv a l e n t f l o w r a t e of lml/min t o 0.25 ml/min). Severe c o n s t r i c t i o n s to the f l o w o f l i q u i d s i n the column occur i n the 10 ym f r i t t e d f i l t e r a t both the i n l e t and the o u t l e t of each packed column and a l s o i n the i n t e r s t i c e s of the packing i n the column. E i t h e r o f these c o n s t r i c t i o n s might be the source of the shearing s t r e s s e s f o r polymer degradation. Since a 44 m i l l i o n MW p o l y s t y r e n e has an unperturbed r a d i u s o f g y r a t i o n of 0.25 m i c r o n O ) and t h e r e f o r e would have some instantaneous c h a i n segment end-to-end d i s t a n c e s that would approach the s i z e of some of the pores i n the f r i t t e d f i l t e r , the e f f e c t of the f i l t e r on the degradation was c a r e f u l l y examined f i r s t . PDMS was chosen t o determine i f polymers other than p o l y s t y rene degrade d u r i n g GPC a n a l y s e s , and, i f so, a t what molecular weights. PDMS was chosen because i t i s an even more f l e x i b l e c h a i n and a l s o has a l a r g e chemical d i f f e r e n c e i n the c h a i n back bone s t r u c t u r e . Although the exact r e l a t i o n between c h a i n f l e x i b i l i t y , c h a i n entanglements, and shear degradation i s not w e l l understood, these experiments use d i l u t e polymer s o l u t i o n s so t h a t the entanglements ought t o be r e l a t e d to the c h a r a c t e r i s t i c par ameter (or r e l a t i v e unperturbed s i z e ) of the s i n g l e polymer c h a i n . Consequently the degradation of h i g h molecular weight PDMS i n GPC columns ought t o be d i f f e r e n t from the degradation of the l e s s f l e x i b l e and p u r e l y hydrocarbon backbone of PS. A l s o , i t was f e l t that the PDMS backbone rupture would not i n v o l v e a f r e e r a d i c a l mechanism and subsequent c h a i n t r a n s f e r r e a c t i o n s . These f i n d ings are p a r t i c u l a r l y t i m e l y now because there has r e c e n t l y been s p e c u l a t i o n t h a t there i s e x t e n s i v e degradation of a l l polymer chains i n the newer and f a s t e r , high-pressure GPC instruments(3,4). Other polymers w i t h a g r e a t e r range of f l e x i b i l i t y were a l s o studied. Experimental Polymers - The PS, PDMS, p o l y h e x y l i s o c y a n a t e (PHIC), and p o l y i s o prene (PI) samples had been e x t e n s i v e l y c h a r a c t e r i z e d t o determine molecular weights, molecular s i z e s , and thermodynamic parameters (5, 6, 7 ) . The samples were a n i o n i c a l l y polymerized u s i n g b u t y l l i t h i u m as the i n i t i a t o r . The p e r t i n e n t data are shown i n Table L P o l y i s o b u t y l e n e / P I B polymers were obtained by f r a c t i o n a t i o n o f commercial polymers and t h e i r molecular weights were measured (8). S o l v e n t s . Benzene - Baker, reagent grade; Cyclohexane - Matheson, Coleman and B e l l (MCB), reagent grade; Tetrahydrofuran - F i s h e r S c i e n t i f i c , reagent grade.


15.

McINTYRE ET AL.

229

Shear Degradation of Polymers in GPC

Table I . I d e n t i f i c a t i o n and M o l e c u l a r Weight o f Polymers Polymer


PS

PIB

M w

13 18 11 9 25166 61970 25167 41995 Β Ε F PIIA

PDMS

5-1 5 A Β A-l A-2 A-3

PHIC

PI

Source 7

4.4 χ 1 0 2.72 χ 1 0 9.6 χ 1 0 4.5 χ 10 4.11 χ 1 0 2.6 χ 10 8.67 χ 1 0 9.82 χ 10*

Ref. 5

7

6

6

5

6

Waters A s s o c i a t e s

5

1.5 χ 1 0 1.2 χ 1 0 6.5 χ 1 0 1.5 χ 1 0

6

2.0 1.2 6.8 4.4 1.46 5.5 5.5

7

Ref. 8

6 5

5

χ 10 χ 10 χ 10 χ 10 χ 10 χ 10 χ 10

Ref. 6

7

6 6

6

6

5

%

11 22 33 44 66

4.24 χ 1 0 5.8 χ 10* 1.33 χ 1 0 2.30 χ ; 0 1.31 χ 1 0

2E7 20M 7E5

7.2 χ 1 0 1.8 χ 1 0 7.6 χ 1 0

Ref. 7

5

5

6

6

Ref. 7

6 5


230



GPC Instrument Operation 1. High M o l e c u l a r Weight Polymers i n Routine Degradation E x p e r i ments. Waters A s s o c i a t e s Ana-Prep and 501 GPC were used f o r s e p a r a t i o n of h i g h molecular weight PS, PDMS, P I , and PIB f r a c t i o n s . F i v e f o u r - f o o t S t y r a g e l columns were connected i n the f o l l o w i n g sequences (Set A) u s i n g a d i f f e r e n t i a l r e f r a c t o m e t e r as the d e t e c t o r . Set A one: one: one: one: one:

7 χ 7 χ 7 χ 1.5 5 χ

5

10 to 5 χ 10 to 5 χ 10 to 5 χ χ 10 to 7 10* t o 1.5 s

5

s

6

10 A 10 A 10 A χ 10 £ χ 10 A 6

6

0

5

5

The s i z e d e s i g n a t i o n s a r e those g i v e n by Waters A s s o c i a t e s . T h i s set had a p l a t e count of 680 PPF when o-dichlorobenzene was the s o l u t e . Samples were prepared on a weight-to-volume b a s i s . Each sample was run a t s e v e r a l d i f f e r e n t concentrations i n the range of 0.05 - 0.2 g / d l i n order t o e x t r a p o l a t e the peak p o s i t i o n t o zero concent r a t i o n . F u l l loop i n j e c t i o n s were used f o r a l l s o l u t i o n s . A 2.5 ml siphon was used a t the e l u t i o n end. PS 13 and PS 18 were a l s o run through Set A at a reduced flow r a t e of 0.5 ml/min and reduced concentrât i o n . No s i g n i f i c a n t changes occurred i n the peak p o s i t i o n and i n t h e shapes o f the curves. 2.

High M o l e c u l a r Weight Polymers i n Cyclohexane and a l s o i n S p e c i a l Column Arrangements. Waters A s s o c i a t e s Ana-Prep and 501 GPC were used. One f o u r - f o o t S t y r a g e l column of 5 x l 0 pore s i z e was connected t o a pump and a d i f f e r e n t i a l r e f r a c t o m e t e r de t e c t o r t o determine the e f f e c t of f r i t t e d d i s c s on degradation. S i n g l e columns of d i f f e r e n t pore s i z e were used t o determine the e f f e c t of g e l pore s i z e on degradation. S i n g l e columns were used t o determine the e f f e c t of solvent power on degradation. Samples were prepared on a weight-to-volume b a s i s . F u l l loop i n j e c t i o n s were used f o r a l l s o l u t i o n s , and polymer from the GPC eluent was recovered f o r c h a r a c t e r i z a t i o n by t a k i n g a l l eluent s o l u t i o n 2 counts b e f o r e and 2 counts a f t e r the polymer e l u t i o n peak. 6

V i s c o s i t y Measurements. A Ζimm-Couette type low shear viscometer was used. The i n t r i n s i c v i s c o s i t i e s were estimated from s i n g l e c o n c e n t r a t i o n v i s c o s i t y measurements u s i n g the equations f o r the c o n c e n t r a t i o n dependence of the s p e c i f i c v i s c o s i t y (5,6). The Mark-Houwink equation was used t o determine My (5,6). Experimental Design a) Measurement of Degradation. The experiments were c a r r i e d out to e l u c i d a t e the r o l e s of both p h y s i c a l and chemical v a r i a b l e s i n


15.

McINTYRE ETAL.


231

the GPC degradation o f h i g h molecular weight polymers t o lower molecular weight polymers. Therefore, a measure o f degradation had t o be chosen t h a t was independent o f GPC. Although v i s c o s i t y , l i g h t s c a t t e r i n g , and sedimentation measurements of m o l e l c u l a r weight have been made, only the v i s c o s i t y measurements are repor ted here. Although the whole molecular weight d i s t r i b u t i o n i s d e s i r a b l e f o r a n a l y s i s , only the s i n g l e v i s c o s i t y - average moment of the molecular weight d i s t r i b u t i o n was determined. A simple measurement of degradation was determined as:


% Dégradâtion=%D = (100-% Decrease MW)=100 1 - ^

before^PC

b) P h y s i c a l V a r i a b l e s . The e f f e c t of shear r a t e on degradation was evaluated by changing f l o w r a t e s , pore s i z e , packing geometry, column l e n g t h , s o l u t i o n v i s c o s i t y , and f r i t s i n the columns. c) Chemical V a r i a b l e s . The e f f e c t o f the backbone bond s t r e n g t h s and the f l e x i b i l i t y o f the polymeric c h a i n was evaluated by study ing the degradation o f polymers o f d i f f e r e n t backbone s t r u c t u r e s [{C-C}, {Si-0>], of f l e x i b l e polymers w i t h d i f f e r e n t c h a i n f l e x i b i l i t i e s a t constant backbone s t r u c t u r e [PIB, PS], and o f r i g i d polymers [PHIC]. d) Physico-Chemical E f f e c t s . Polymer c o n c e n t r a t i o n s were kept low i n order to reduce the s o l u t i o n v i s c o s i t i e s and measure only the e f f e c t o f the GPC on s i n g l e polymer chains. At the h i g h e s t MW s the c o n c e n t r a t i o n s were always where degradation i n GPC f i r s t b e g i n s . A l s o the d i s c u s s i o n o f P I leads t o a v a l u e o f (M ) p i n the r e g i o n of ( 2 - 5 x l 0 g/mol. The d i r e c t r e l a t i o n s h i p between ( M ) y j s C < * ( M ) p i s d i s c u s s e d elsewhere (12) and ap pears to be approximately ( M ) * ç * ( M ) . As a consequence o f t h i s model i t i s q u a l i t a t i v e l y easy t o a n t i c i p a t e when degradation w i l l occur i f (M )yyg£ i s known. That is, (M ) p i s (M ) yigC A rough dependence o f degradation on Μ|χρ or M^keo H not be f a r from the c o r r e c t r e s u l t f o r PS o r PIB i f ( M C ) Q P C i s estimated c o r r e c t l y . At h i g h shear s t r e s s e s the ( M ) Q P C f o r μ-Styragel i s lower and the power law dependence i n M i s lower ( ^ M * ) . A more exact d e s c r i p t i o n of these phenomena i s c u r r e n t l y under i n v e s t i g a t i o n , t h e o r e t i c a l l y and experimenta11^ 15). 3

1 0

3

β

n

5

3

5

a

s

3

Α

5

c

G

C

a n

c

c

G

C

c

S

C

G P C ;

c

2

c

G

C

e

c

w

i

C

0

6




MclNTYRE ET AL.

F i g u r e 2. Loop entanglements that could l e a d t o " l o c k i n g under s t r e s s : (a) l o c k i n g of f r e e loops; b) l o c k i n g of loops by a d s o r p t i o n on s u b s t r a t e s ; c) l o c k i n g of loops by interpénétration i n t o loops on s u b s t r a t e .


11

240


F u r t h e r experiments a r e i n progress to l o c a t e the ( M ) Q P Q f o r the onset of c h a i n degradation i n G P C and a l s o t o determine the q u a n t i t a t i v e r e l a t i o n s h i p s between f l e x i b i l i t y , entanglement, and backbone bond s t r e n g t h s on the G P C degradation. However, i t i s c l e a r i n the experiments so f a r t h a t no s i g n i f i c a n t c h a i n deg r a d a t i o n occurs i n s a t u r a t e d hydrocarbon polymers unless the mol e c u l a r weights are 5-10 m i l l i o n . Therefore, most MW measurements on polymers l e s s than ( 5 - 1 0 ) x l 0 i n MW can be s a f e l y c a r r i e d out w i t h a low pressure G P C apparatus, unless (1) the polymer i s known from other o b s e r v a t i o n s t o be e s p e c i a l l y s u s c e p t i b l e t o shear degradation, o r (2) the MW's must be determined t o accu r a c i e s b e t t e r than 3%. But a t very h i g h molecular weights i t i s c l e a r t h a t the estimated G P C MW can e a s i l y be one-half o r l e s s t h a t of the t r u e MW. For unsaturated chains the secondary chem i c a l r e a c t i o n s hasten the degradation as soon as the ( M ) G P C t h r e s h o l d i s passed. Q u a l i t a t i v e l y a l l o f the observed G P C degradation c h a r a c t e r i s t i c s can be r a t i o n a l i z e d by the above loop model. Reasonable estimates of the onset of degradation i n G P C can be made, and estimates o f the percent degradation can be made c a u t i o u s l y . c


6

c

Literature Cited 1. Slagowski, E.L.; Fetters, L.J.; McIntyre, D.; Macromol. 1974, 7, 394. 2. McIntyre, D.; Fetters, L. J.; Slagowski, E. L . ; Science 1972, 176, 1042. 3. Huber, C.; Lederer, Κ. H.; Polymer Letters 18, 535 (1980). 4. Rooney, J. G . ; VerStrate, G . ; in "Liquid Chromatography of Polymers", Cazes, J., Ed.; Dekker, New York, 1981; p. 207. 5. Slagowski, E. L . ; Ph.D. Thesis, The University of Akron (1972). 6. Shih, A. L . ; Ph.D. Thesis, The University of Akron (1972). 7. Kuo, C. C.; Ph.D. Thesis, The University of Akron (1980). 8. Shih, A. L . ; M.S. Thesis, The University of Akron (1968). 9. Ambler, M. R.; McIntyre, D.; Polymer Letters, 13, 589 (1975). 10. MacArthur, A.; M.S. Thesis, The University of Akron (1978). 11. MacArthur, A.; Stephens, H. L.; J. Appl. Polymer Sci., 1983, 28, 1561. 12. MacArthur, A.; McIntyre, D.; Rubber Division, Toronto, May (1983), Paper #7. 13. Flory, P. J.; "Statistical Mechanics of Chain Molecules"; Wiley: New York, 1968. 14. Ferry, J . S.; "Viscoelastic Properties of Polymers" (second ed.); Wiley: New York, 1970. 15. McIntyre, D.; MacArthur, A.; Polymer Preprints, 24, August 1983, p. 102. RECEIVED October

13, 1983


Shear Degradation of Very High Molecular Weight Polymers in Gel

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