Compositional Heterogeneity of Copolymers by ... - ACS Publications

15 Compositional Heterogeneity of Copolymers by Coupled Techniques Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 18, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/ba-1995-0247.ch015

with Chromatographic Columns and Multidetectors J o h n V. D a w k i n s D e p a r t m e n t of C h e m i s t r y , L o u g h b o r o u g h U n i v e r s i t y o f T e c h n o l o g y , L o u g h b o r o u g h , Leicestershire LE11 3 T U , E n g l a n d

For copolymers having composition and molar mass distributions, it is shown that characterization with one or more concentration detectors on-line to a chromatographic system based on size-exclusion chromatography (SEC) produces average composition data. For more detailed information on composition heterogeneity, two approaches are reviewed. An SEC method involving on-line concentration detection together with on-line low-angle laser light scattering is described to demonstrate how heterogeneity parameters permit a distinction between block copolymers and polymer blends. Coupled column chromatography with two chromatographic systems in which fractions from an SEC column are injected into a second column containing a polymer-based packing where retention is determined by nonexclusion mechanisms is described.

SIZE-EXCLUSION CHROMATOGRAPHY (SEC) is w e l l established as a t e c h nique for d e t e r m i n i n g the molar mass distribution ( M M D ) of h o m o p o l y mers. Heterogeneous copolymers contain distributions i n b o t h molar mass (M) and copolymer composition. C o p o l y m e r characterization based on S E C is often performed w i t h on-line selective concentration detectors (I, 2). F o r heterogeneous copolymers this S E C - b a s e d m e t h o d is only capable of p r o d u c i n g average composition data across a chromatogram, because c o p o l y m e r chains having the same molecular size i n solution w i l l have variations i n molar mass and composition (3). 0065-2393/95/0247-0197$12.00/0 © 1995 American Chemical Society

Provder et al.; Chromatographic Characterization of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1995.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 18, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/ba-1995-0247.ch015

198

CHROMATOGRAPHIC CHARACTERIZATION OF POLYMERS

Off-line light scattering has been developed to determine data for M and compositional heterogeneity for copolymers (4-7). T h e param eters used to quantify the compositional heterogeneity of a copolymer sample, as determined from light-scattering data, are F , representing the effect of M o n compositional heterogeneity, and Q, w h i c h represents the overall compositional drift. Therefore, addition of on-line low-angle laser light-scattering ( L A L L S ) detection to an S E C system w i t h dual concentration detection can permit for some types of copolymers the calculation of compositional heterogeneity at each elution volume to gether w i t h overall heterogeneity parameters (8, 9). W h e n copolymers contain composition heterogeneities, some types of cross-fractionation procedure, involving separating b y composition fractions previously separated by size, can be attempted, but the experimental work involving transfers between techniques is quite time-consuming. I n coupled c o l u m n chromatography ( C G C ) , on-line transfer can be automated, and Balke and Patel (10-13) demonstrated copolymer separations w i t h two chromatographic systems i n w h i c h copolymer is separated first b y S E C and second by nonexclusion mechanisms. H e r e , some aspects of copolymer characterisation b y S E C w i t h cou pling are reviewed considering, first, concentration detectors w i t h L A L L S detection and, second, concentration detection w i t h on-line transfer to an interactive column system i n C C C . Investigations of C G C indicate h o w nonexclusion separations dependent on copolymer com position in the second column can be influenced b y choice of stationary and mobile phases (14, 15). T h e examples of statistical and block co polymers are selected to illustrate not only heterogeneity w i t h i n co polymer chains but also homopolymer contamination w i t h i n copolymer samples. T h e presence of residual homopolymer is important to the production of comb graft copolymers b y grafting-on and grafting-through processes (16).

Experimental Chromatographic data for copolymers obtained by S E C with on-line dual concentration detectors was gathered with a gel permeation chromatograph (model 301, Waters Associates, Milford, M A ) with refractive index (RI) (thermostatted at 298 K) and U V (254 nm) detection (17). Elutions were performed with tetrahydrofuran ( T H F ) (distilled before use) at a flow rate of 1 c m m i n at room temperature. A series arrangement of four S E C columns (Styragel, Waters Associates) was used. Solution concentrations were i n the range of 0.1-0.3% (wt/vol). Calibrations of detector propor tionality constants were established according to methodology described previously (17). Details of the chromatographic system with on-line L A L L S were de scribed previously (8, 9). After a series arrangement of four S E C columns (300 X 7 mm P L g e L , 10 μπι, Polymer Laboratories, Church Stretton, 3

- 1


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199

England), the on-line detectors were in sequence light scattering (model K M X - 6 , Chromatix Thermo Separation Products, Riviera Beach, F L ) , i n frared (model 1A, Wilks-Miran), and RI (model R 4 0 1 , Waters Associates). Separations were performed with tetrachloroethylene as eluent at 353 K . Solution concentrations were 5 mg/cm and toluene (0.1%) was added as internal marker. The C C C instrumentation consisted of two independent chromato graphic systems joined together via a switching valve (14, 15). System one contained in sequence an SEC column (300 X 7 mm, mixed P L g e L , 10 μπι), six port-switching valve (model 7010, Rheodyne, Cotati, C A ) , and R I de tector (type 98.00, Knauer, supplied by Polymer Laboratories, C h u r c h Stretton, England). System two had pump and column linked through the same switching valve and contained in sequence a single column of either mixed P L g e L , 10 μπι, or P L Aquagel P 3 type, 10 μπι (both 300 X 7 mm), U V detector (Pye Unicam), and RI detector (type 98.00, Knauer, supplied by Polymer Laboratories, Church Stretton, England). A l l separations were performed at ambient temperature. T H F was always used as eluent in system one. Isocratic elutions were performed with the second system with mixtures of either T H F - h e p t a n e (HEP) or T H F - i s o p r o p a n o l (IP) as mobile phase. Solution concentrations in T H F injected into the first column system were 0.4% (wt/vol). Polymers and copolymers were laboratory-prepared samples. Samples W 4 and W 7 of the diblock copolymer A B poly(styrene-fo-tetramethylene oxide) (PS-PT) were synthesized by producing a polystyrene prepolymer whose terminal group was transformed to a macroinitiator for the poly merization of T H F . Samples B 1 3 and B 1 6 of the diblock copolymer A B poly[styrene-fo-(dimethyl siloxane)] (PS-PDMS) were prepared by sequential anionic polymerization. Samples of statistical copolymers of styrene and nbutyl methacrylate (PSBMA) were produced by radical copolymerization. Details of synthetic and characterization methods have been reported else where (15,17-19).


3

Results and Discussion C o m p o s i t i o n D r i f t . Determinations of copolymer composition distribution b y S E C w i t h dual U V and R I detectors were developed b y several researchers (1, 20-22). T o exemplify that this methodology may only be capable of producing average composition data across a chromatogram as a function of retention volume V , results for samples of the diblock copolymer P S - P T are presented. T h e block lengths i n sam ples of P S - P T were chosen such that the S E C peak for copolymer was w e l l resolved from the PS prepolymer peak. T h e response / ι (V) of the U V detector as a function of V depends only on the weight w of styrene units i n P S - P T , whereas the response h (V) of the R I detector depends on both w and weight w of tetramethylene oxide units i n the copolymer. T h e detector responses are given b y υ ν

s

m

s

T

^uv(V) = K w s

h (V) = K (w m

c

(1)

s

s

+ w) T


(2)

200


where K is a proportionality constant dependent on the U V extinction coefficient for styrene units i n the copolymer, and K is a proportionality constant related to the R I increment of the P S - P T diblock copolymer i n the S E C eluent. This increment is usually assumed to be represented in terms of the values for the corresponding homopolymers by a linear equation, so that K is given i n terms of weight fractions by s

c

c


K

c

=

WK S

(1

+

A

-

(3)

W )K S

B

where K and K are the refractometer proportionality constants for PS and F T , respectively. T h e weight fraction of styrene units i n the copolymer is A

B

W

s

= w /(w + w ) s

s

(4)

T

It follows that substitution of equations 1 and 2 into equation 4, and elimination of K w i t h equation 3, gives after rearrangement an expression for W as a function of V c

s

W S { V

) =

W s ( V )

*BMV)/MV)

( 5 )

K -(K -K )h (V)/h (V) s

A

B

uv

K

m

!

Because of the results to be presented to illustrate composition drift, we prefer to define the average weight fraction of styrene W ( V ) i n equation 5. Determinations of K , K , and K were reported previously (17). It was demonstrated that for the range of molar masses studied there was no dependence of refractometer proportionality constants on chain length or end group structure. C o p o l y m e r composition data for S E C peaks corresponding to two samples of P S - P T diblock copolymer are displayed i n F i g u r e 1. Sample W 4 appears to be close to monodisperse both i n terms of M M D , p o l y dispersity computed to be 1.04 for chains eluting over the range 2 1 . 0 26.5 counts, and i n terms of composition distribution, because the i n crease i n W ( V ) above 0.12 at the peak of the chromatogram corresponds to the low molar mass tail of the S E C chromatogram where the accuracy of the dual detector method w i l l decrease. O n the other hand, sample W 7 is much more polydisperse both i n terms of M M D , polydispersity computed to be 1.65 for chains eluting over the range 1 9 . 5 - 2 7 . 5 counts, and i n terms of composition distribution, because there is considerable increase i n W ( V ) across the chromatogram from the value of 0.1 at the peak. These very different composition distributions for samples W 4 and W 7 may be explained by the type of chemistry used i n the transformation reaction to produce a macroinitiator for the polymerization of T H F i n formation of P S - P T diblock copolymer (17). A t the i t h elution volume interval i n the elution of copolymer by S E C , a detector having a c e l l volume A V w i l l provide a response corS

s

A

B

S

S


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201

0-5 0-4


~ > ^

0-3 0-2 0-1 0 20

22

24

26

28

V(counts)

Figure 1. Dependence of composition of styrene units in PS-PT diblock copolymers on retention volume. , sample W4; , sample W7.

responding to chains w i t h almost identical sizes i n solution i n the c e l l , as j u d g e d by hydrodynamic volume as universal calibration parameter. If there is a drift of composition across a chromatogram, as exemplified by sample W 7 i n F i g u r e 1, then it can be expected that there has to be a compositional heterogeneity for chains of almost identical sizes i n the detector cell volume AV at a particular elution time. F r o m universal calibration considerations (17), the experimental S E C calibrations for PS and P T homopolymers are related at a given elution volume by log Μρτ-log M p = log 0.55

(6)

S

Therefore, heterogeneous copolymer chains i n a detector cell volume AV w i l l have different molar masses. It follows that for a definite h y drodynamic volume of chains i n solution these chains may be constituted by a range of structures with variations i n block lengths and composition. Consequently, for heterogeneous copolymers, S E C w i t h concentration detectors is only capable of producing average composition data, and more detailed studies of compositional heterogeneity require additional characterization methodology, that is, by L A L L S for some polymer types or by cross-fractionation. L A L L S D e t e c t i o n . Consideration of the treatment of light scat tering for heterogeneous copolymers (6) permits the dependence of the apparent molar mass M* at the i t h elution volume interval i n a S E C on-line L A L L S experiment to be represented i n terms of P* and Q by f

M

;

= M

wi

+

2 P

i

( ^ )

+

Q ^f^J i


(7)

202


where v is the R I increment for all the components (having weight average molar mass M ) at the i t h elution volume interval. If one assumes a linear relation between R I increment and copolymer composition W (determined for chains at the i t h elution volume interval from peak responses from on-line concentration detectors), it is easy to calculate Pi from the homopolymer R I increments v and v and the measured W value by analogy w i t h the method defined by equation 3. T h e apparent molar mass M* is determined w i t h equation 8, {

w

f


A

KW/Rii

=

B

(1/M0ERROR*)

+

t

2A A

(8)

2

w h i c h defines the excess Rayleigh factor due to scattering from solute alone (concentration c ) at the i t h elution volume interval. In this equation the term containing the second virial coefficient A can be neglected for S E C experiments at low values of c , and K* contains the usual constants i n light scattering. D i b l o c k copolymers of P S - P D M S were chosen for study because PS and P D M S homopolymers i n good solvents have the same molar mass calibrations i n S E C (23). F o r PS and P D M S homopolymers i n tetrachloroethylene, it can be shown from data for intrinsic viscosity that the M a r k - H o u w i n k exponent for both of these polymers is near 0.8 (9). Equations for universal calibration (24) indicate that an M ( P S - P D M S ) diblock copolymer calibration should therefore follow that for the corresponding homopolymers. Consequently, there should be a narrow range of masses at each elution volume, so that the term containing P in equation 7 can be ignored and M can be replaced by M giving t

2 I

{

f

wi

i 5

(9) Because M is k n o w n from S E C calibrations w i t h PS and P D M S homopolymer standards and because M* can be determined from on-line L A L L S and concentration detectors w i t h the conventional light-scattering equation containing the excess Rayleigh factor (equation 8), sufficient information is available to compute Q across a chromatogram. These values can then be averaged to obtain the heterogeneity parameter Q for the overall sample. T o facilitate comparisons among samples, it is convenient to use another heterogeneity parameter, H , defined as t

{

max

(10)

where Q is the value obtained for a b l e n d of two homopolymers. T h e range of H values is from zero (homogeneous sample) to unity (maximum heterogeneity, i.e., a blend). max


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Table I. Molar Mass and Heterogeneity Data for a Blend and a Diblock Copolymer


Blend of PS + PDMS Homopolymers (Sample 4) Characterization Parameter

Expected Value

PS (or styrene) content (%)

75.9°

M(PS) M(PDMS)

43,600 35,800 1.00

Η

c c

Diblock Copolymer PS-PDMS (Sample Β16) Expected Value

Value SEC/LALLS

Value SEC/LALLS 74.8

75.5

76.1

44,800 43,500 1.06

43,600 13,400 0

b

39,700 13,400 0.25

c

d

° B l e n d composition w e i g h e d out. C a l c u l a t e d from Si analysis. ° Independent S E C characterization o f h o m o p o l y m e r s . C o m p u t e d from b and c. b

d

Representative results for a blend and a diblock copolymer are shown in Table I. It is evident for both samples that the on-line infrared and R I concentration detectors provide excellent estimates o f overall com position, component molar masses, and the heterogeneity parameter H. Data for blends i n Table I were obtained to assess the proposed methods. F u r t h e r evidence for the capability of the S E C - L A L L S procedure can be seen i n F i g u r e 2, w h i c h shows plots of data for W and Η computed across the M M D for b l e n d sample 4. I n this figure the expected results s

1

άΞ 1

ο 3

10

4

5

10

10

6

10

Mi

Figure 2. Variation of composition of PS and compositional heterogeneity across MMD for PS-PDMS blend sample 4. , composition W (V); , heterogeneity parameter H{. S



204

for W and H from Table I are obtained for the M* range from 1 0 to 1 0 g m o l " . This indicates that possible errors due to volume offset among the three detectors have been m i n i m i z e d by analysis of marker peaks. T h e greatest errors i n the plots i n F i g u r e 2 are at the tails of the distribution, w h i c h are partly due to the P S - r i c h b l e n d , the greater polydispersity of the PS component than P D M S (4 and 2, respectively) and possible distortion of tails by band-broadening. T h e results for copolymer sample B 1 6 i n Table I suggest that it is a good homogeneous diblock copolymer w i t h minimal levels of contam inating homopolymers. This is confirmed by plots of data for W$ and H computed from outputs from the three on-line detectors across the M M D and displayed i n F i g u r e 3. T h e weight fraction of styrene is close to 0.75 over about a decade of M , and it is only at the l o w molar mass tail of the distribution, where the multidetector approach w i l l have lowest accuracy, that W decreases significantly below the mean value quoted in Table I. Characterization results for a much more polydisperse sample B 1 3 , i n terms of range of molar masses, are also shown i n F i g u r e 3. This sample, although synthesized b y a sequential monomer addition process to produce a diblock copolymer, exhibits a fluctuating trend i n terms of 4

s


5

1

s

Figure 3. Variation of composition of styrene units and compositional het erogeneity across MMD for PS-PDMS copolymer samples; (a) sample B16 and (b) sample Β13. , composition W (V); , heterogeneity param eter H i . S


15.

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205

W , indicating fractions having styrene-rich and styrene-deficient com ponents. It is difficult to rationalize these composition data w i t h the method of block copolymer synthesis, and it has to be considered that sample B 1 3 contains substantial contamination b y one or both homo polymers. T h e interpretation that this sample is largely a poly disperse b l e n d of polymers rather than based o n a copolymer is supported b y the plot of H that lies i n the range 0 . 8 0 - 1 . 0 6 across the peak of the M M D i n F i g u r e 3. This deduction w o u l d not have been possible b y examination of average composition data alone without the application of light scattering to determine heterogeneity parameters. T h e overall information obtained from the three detectors enables molar masses of components to be determined, and for the two samples i n Table I the good agreement between expected and S E C - L A L L S results indicates that the methods proposed are reasonably accurate.


s

C C C . A schematic diagram of the C C C system is shown i n F i g u r e 4 [this technique also has been named orthogonal chromatography (1013)]. Cross-fractionation on the polymer solution injected into the S E C column was performed on the solution passing through the switching valve i n the time interval 8 3 0 - 8 5 0 s. This fraction was separated b y isocratic elutions w i t h column two.

pumpi

•

injection vcdvel

SEC switching , valve

column

pump 2

column 2

Ϋ

s

)

RI detector

•waste

data system

UV detector

RI detector

waste

Figure 4.

Diagram of apparatus for CCC.


206

CHROMATOGRAPHIC CHARACTERIZATION OF POLYMERS THF/HEP

Vis)

800

1000


35-5/ 64-5

Figure 5. Chromatogram obtained with column system 2 containing PLgeL PS packing for a mixture of homopolymers of PBMA (peak b) and PS (peak a) with isocratic elution ofTHF-HEP (35.5:64.5) as mobile phase.

Cross-fractionation i n C C C requires the establishment of separation conditions for homopolymers for the second column system. A m i n i m u m requirement is to produce nonoverlapping chromatograms by identifying mobile-phase compositions for resolution of PS and P B M A homopolymer peaks. This separation is obtained by introducing H E P or I P , as nonsolvent component for PS, and resolution of homopolymer peaks for the P L g e L PS packing was obtained with mobile-phase compositions of 35.5: 64.5 ( T H F - H E P ) , as shown i n F i g u r e 5, and 55:45 ( T H F - I P ) , as shown in Figure 6. A n increase i n nonsolvent concentration i n the mobile phase markedly shifts PS elution to longer retention times, whereas P B M A exhibits little or no change i n elution volume. T h e behavior of PS is consistent w i t h separations of PS i n poor and theta solvents w i t h crossl i n k e d PS gels (25), that is, nonexclusion interaction mechanisms are similar for poor solvents that are both more polar (IP) and less polar

V(s)

800

1000

THF/IP

55 / 4 5 ·

Figure 6. Chromatograms obtained with column system 2 containing PLgeL PS packing for homopolymers of PBMA (peak b) and PS (peak a) with isocratic elution of THF-IP (55:45) as mobile phase.



15.

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207

(HEP) than PS. It is proposed that size exclusion continues to dominate separations of P B M A for the mobile phases given previously. F o r P L Aquagel P 3 packing i n the second column, resolution of homopolymer peaks was obtained w i t h mobile-phase compositions of 55:45 ( T H F H E P ) and 30:70 ( T H F - I P ) . H o w e v e r , these results are not as easy to interpret as the observations for P L g e L because both P S and P B M A homopolymer peaks appear to be influenced b y adsorption on P L A q u a gel P 3 as the H E P concentration is increased and because there is a l o w difference i n peak retention volumes between these peaks for elutions w i t h T H F - I P , requiring a very high fraction of I P to achieve peak resolution. Consequently, separation of P S B M A copolymers according to styrène composition ought to be possible i n the second column system containing a P L g e L packing. Separations of three different P S B M A copolymers i n mixtures with P B M A are shown i n F i g u r e 7. Chromatogram a shows the P B M A peak eluting first and the peak due to copolymer P S B M A 8 / 2 merging w i t h the solvent peak at 1200 s. A s the styrene composition decreases, the copolymer is less retained exhibiting decreased V R as shown b y chromatograms b and c. T w o peaks are observed in each case, due to P B M A w i t h V near 9 0 0 s and copolymer eluting later. Therefore, this C C C method has potential not only for separating copolymers on the basis of composition b u t also for isolating residual homopolymer s from copolymers. T h e latter problem is of relevance to the production of comb graft copolymers b y grafting-on and graftingthrough processes (Slark, A . T . ; A z a m , M . ; Branch, M . G . ; Dawkins, J . V . , Loughborough University of Technology, L o u g h b o r o u g h , U n i t e d K i n g d o m , unpublished results.) R

α

b

c

Figure 7. Chromatograms obtained with column system 2 containing PLgeL PS packing with THF-HEP (composition 30:70) as mobile phase, (a) PBMA homopolymer and PSBMA copolymer (65 mol/% styrene) ; (b) PBMA homopolymer and PSBMA copolymer (51 mol/'% styrene) ; and (c) PBMA homopolymer and PSBMA copolymer (36 mol/'% styrene).


208


Conclusions T h e results demonstrate that c o u p l e d chromatographic techniques w i t h multiple detectors permit the determination of average composition data, heterogeneity parameters, and separations of h o m o p o l y m e r s and

co

polymers. T h e methodology r e v i e w e d here enables a distinction to

be


made b e t w e e n copolymers and p o l y m e r blends.

Acknowledgments I thank T. D u m e l o w , S. R. H o l d i n g , A . M . C . M o n t e n e g r o , and G . T a y l o r for helpful discussions. Support for part of this work was kindly p r o v i d e d b y the P o l y m e r S u p p l y and C h a r a c t e r i z a t i o n C e n t r e at R u b b e r and Plas tics Research Association and b y the Science and E n g i n e e r i n g Research Council.

References 1. Runyon, J. R.; Barnes, D . E . ; Rudd, J. F . ; Tung, L . H . J. Appl. Polym. Sci. 1969, 13, 2359. 2. Quivoron, C. In Steric Exclusion Liquid Chromatography of Polymers; Janca, J., E d . ; Marcel Dekker: New York, 1984; pp 213-280. 3. Hamielec, A. Pure. Appl. Chem. 1982, 54, 293. 4. Bushuk, W.; Bemoit, H . Can. J. Chem. 1956, 36, 1616. 5. Krause, S. J. Phys. Chem. 1961, 65, 1618. 6. Leng, M . ; Benoit, H . J. Polym. Sci. 1962, 57, 263. 7. Benoit, H . ; Froelich, D . In Light Scattering from Polymer Solutions; Huglin, M . B., E d . ; Academic: London, 1972; p 467. 8. Dumelow, T. Ph.D. Thesis, Loughborough University of Technology, Loughborough, United Kingdom, 1984. 9. Dumelow, T.; Holding, S. R.; Maisey, L . J . ; Dawkins, J. V. Polymer 1986, 27, 1170. 10. Balke, S. T.; Patel, R. D . In Size Exclusion Chromatography (GPC); Provder, T., E d . ; ACS Symposium Series 138; American Chemical Society: Wash ington, D C , 1980; pp 149-152. 11. Balke, S. T.; Patel, R. D. J. Polym. Sci. Β 1980, 18, 453. 12. Balke, S. T.; Patel, R. D . In Polymer Characterization—Spectroscopic, Chro matographic and Physical Instrumental Methods; Craven, C., E d . ; A C S A d vances in Chemistry 203; American Chemical Society: Washington, D C , 1983; pp 281-310. 13. Balke, S. T. In Detection and Data Analysis in Size Exclusion Chromatography; Provder, T., E d . ; ACS Symposium Series 352; American Chemical Society: Washington, D C , 1987; pp 59-77. 14. Montenegro, A. M . C. Ph.D. Thesis, Loughborough University of Tech nology, Loughborough, United Kingdom, 1986. 15. Dawkins, J. V.; Montenegro, A . M . C. Br. Polym. J. 1989, 21, 1. 16. Rempp, P.; Lutz, P. J. In Comprehensive Polymer Science; Allen, G . ; Bevington, J. C., Eds.; Pergamon: Oxford, England, 1989. 17. Burgess, F . J.; Cunliffe, Α. V.; Dawkins, J. V . ; Richards, D . H . Polymer 1977, 18, 733.



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18. Taylor, G. Ph.D. Thesis, Loughborough University of Technology, Lough borough, United Kingdom, 1977. 19. Dawkins, J. V.; Taylor, G. Polymer 1979, 20, 559. 20. Adams, H . E . Sep. Sci. 1971, 6, 259. 21. Grubisic-Gallot, Z.; Picot, M . ; Gramain, P. R . ; Benoit, H . J. Appl. Polym. Sci. 1972, 6, 2931. 22. Harmon, D . J.; Folt, V. L . Rubber Chem. Technol. 1973, 46, 448. 23. Dawkins, J. V. J. Macromol. Sci.-Phys. Β 1968, 2, 623. 24. Dawkins, J. V.; Guest, M . J.; Jeffs, G. M . F . J. Liq. Chromatogr. 1984, 7, 1739. 25. Dawkins, J. V.; Hemming, M . Makromol. Chem. 1975, 176, 1795. RECEIVED for review January 6, 1994. ACCEPTED revised manuscript July 21, 1994.


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