Determination of Long-Chain Branching Distributions of Polyethylenes

2 Determination of Long-Chain Branching

Downloaded by KTH ROYAL INST OF TECHNOLOGY on November 19, 2015 | http://pubs.acs.org Publication Date: May 5, 1990 | doi: 10.1021/ba-1990-0227.ch002

Distributions of Polyethylenes Francis M . Mirabella, Jr.,1 and Leslie Wild 1 2

2

Quantum Chemical Corporation, Rolling Meadows, IL 60008-4070 Quantum Chemical Corporation, Cincinnati, O H 45237

The long-chain branching distribution (LCBD) of polyethylene was determined from measurements obtained from an on-line viscometer detector (VD) combined with size exclusion chromatography (SEC). The SEC was calibrated according to the universal calibration method, and the VD output was used along with the concentration measurement obtained from the refractive index detector of the SEC to determine the intrinsic viscosity distribution of the eluting polymers. The SEC and VD data were then combined to yield the molecular-weight distribution (MWD) and LCBD of a variety of polyethylene resins. The SEC-VD technique was shown to be useful for determining the LCBD across the entire M W D , except at very low MW (BR

T h e ratio

M i W M i,L wih y i e l d the l o n g - c h a i n b r a n c h i n g at each e l u t i o n v o l u m e ( L C B / 1

1000C), b y t h e m e t h o d of D r o t t a n d M e n d e l s o n (2). T h u s , the l o n g - c h a i n b r a n c h i n g p e r 1000 c a r b o n atoms can be p l o t t e d as a f u n c t i o n of m o l e c u l a r w e i g h t , a n d b y s u m m a t i o n o f ( L C B / 1 0 0 0 C ) , , the w h o l e - p o l y m e r L C B / 1 0 0 0 C can b e o b t a i n e d . T h e o n l y t h e o r e t i c a l d e r i v a t i o n o f c o n s e q u e n c e that p u r p o r t s to relate measurable parameters of p o l y m e r s to the degree of L C B was p u b l i s h e d b y Z i m m a n d S t o c k m a y e r (3). T h i s d e r i v a t i o n arrives at an e x c e e d i n g l y s i m p l e relationship b e t w e e n easily measurable p o l y m e r parameters a n d L C B , b u t rests o n several tenuous assumptions. T h e results o f the d e r i v a t i o n are s i m p l y that the ratio of the i n t r i n s i c viscosity of the b r a n c h e d p o l y m e r to the i n t r i n s i c viscosity of a l i n e a r p o l y m e r o f e q u a l M W ( f t ^ R / f t D * d i r e c t l y r e l a t e d to s

the d e g r e e of L C B (g , w h e r e the exponent e has a specific value for each e

p o l y m e r ) . T h i s r e l a t i o n s h i p is expressed m a t h e m a t i c a l l y i n the f o l l o w i n g equation:

,e

*

=

NBR ML

In Polymer Characterization; Craver, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1990.

(1)

2.

MIRABELLA & W I L D

PE Long-Chain Branching Distributions

27

w h e r e the value o f e was d e t e r m i n e d to b e 0.5 for P E , as d e s c r i b e d i n d e t a i l i n a p r e v i o u s r e p o r t (2). T h u s , g is r e a d i l y accessible from m e a s u r e m e n t o f e

the i n t r i n s i c viscosity o f the b r a n c h e d p o l y m e r M B R f o l l o w e d b y calculation of the c o r r e s p o n d i n g i n t r i n s i c viscosity for a p o l y m e r w i t h e q u a l M W f r o m the M a r k - H o u w i n k e q u a t i o n : (2)


M L = kM*

w h e r e M is r e a d i l y o b t a i n e d w i t h the V D - S E C t e c h n i q u e as j u s t d e s c r i b e d a n d k a n d a are the M a r k - H o u w i n k constants. To calculate the L C B , the value of e i n equation 1 m u s t b e i n c l u d e d also. Values o f e r a n g i n g f r o m 0.5 to 1.5 have b e e n p r o p o s e d for P E (4). O u r w o r k shows that the value s h o u l d b e closer to 0.5, a n d this value has b e e n u s e d p r e v i o u s l y . H o w e v e r , recent data (5) w e r e u s e d to p r o d u c e the p l o t i n F i g u r e 1, w h i c h indicates that an average value of 0.65 i n the L C B / 1 0 0 0 C r e g i o n o f 0 to 10 is most reasonable. T h i s value was u s e d along w i t h other values of e, a n d the best agreement of calculated a n d certificate values o f

0.9

0.8

e 0.7

0.6

10

30

20

40

50

60

LCB/1000 C Figure 1. Dependence of e value in g = [T\]BRI[T\]L on LCB. A value of 0.65 appears to be the best average value in the 0 to 10 LCB/1000C region. Data are from Grinshpun et al. (5). e


28

POLYMER CHARACTERIZATION

L C B for a series o f standards was o b t a i n e d w i t h e — 0.65. T h u s , this value has b e e n a d o p t e d for P E . A t this p o i n t , t h e value o f g can b e u s e d to calculate t h e degree o f L C B from s i m p l e equations. H o w e v e r , b y a s s u m i n g that b r a n c h points are t r i fimctional, the b r a n c h d i s t r i b u t i o n has a u n i f o r m n u m b e r o f branches p e r m o l e c u l e , a n d the branches have a r a n d o m d i s t r i b u t i o n o f lengths, the f o l l o w i n g t w o equations c a n b e d e r i v e d (3). H e r e g is r e p l a c e d b y g to signify trifunctional b r a n c h i n g .


3

T h e e q u a t i o n for polydisperse p o l y m e r s is

where B is the weight-average number of long branches per molecule. The equation for polymer fractions monodisperse in MW is w

-V4 (g B„) =

1 + V 7/

3

where B

n

9ir

J

(4)

is t h e n u m b e r - a v e r a g e n u m b e r of l o n g branches p e r m o l e c u l e .

E q u a t i o n s 3 a n d 4 are p l o t t e d i n F i g u r e 2. I f g has b e e n d e t e r m i n e d , B

n

and B

c a n b e r e a d i l y o b t a i n e d from F i g u r e 2. F o r e x a m p l e , i f g = 0 . 5 ,

w

then B

n

= 16.5 a n d B

w

= 6.5 from F i g u r e 2. I f the value o f M =

t h e n t h e L C B / 1 0 0 0 C is o b t a i n e d

L

C

B

1000C

100,000,

from

= I L « (14,000) M

(5)

w h e r e x == n o r w. T h e r e f o r e , from t h e n u m b e r - a v e r a g e equation L C B / 1 0 0 0 C w o u l d b e 2.31, a n d f r o m t h e weight-average equation i t w o u l d b e 0.91. T h e q u e s t i o n i m m e d i a t e l y arises as to w h i c h value is correct. T h e answer w o u l d p r e s u m ably b e 2.31 i f the p o l y m e r w e r e monodisperse i n M W o r 0.91 i f the p o l y m e r w e r e p o l y d i s p e r s e i n M W . A t this p o i n t , a suspicion m a y arise that t h e equation i n v o l v i n g B w o u l d n e v e r b e appropriate for a " r e a l " p o l y m e r because e v e n a " n a r r o w " p o l y m e r fraction, fractionated o n t h e basis o f t h e u n i f o r m h y d r o d y n a m i c v o l u m e o f the p o l y m e r chains, w o u l d b e suspected to contain a c o l l e c t i o n o f molecules v a r y i n g from u n b r a n c h e d to some maxi m u m b r a n c h i n g l e v e l , a l l h a v i n g approximately e q u a l h y d r o d y n a m i c v o l ume. n

Because t h e V D - S E C m e t h o d measures i n t r i n s i c viscosity for s m a l l " s l i c e s " o v e r an e l u t i o n d i s t r i b u t i o n for a p o l y m e r sample, it may b e a s s u m e d that t h e n u m b e r - a v e r a g e calculation (for monodisperse p o l y m e r ) s h o u l d b e




30


u s e d . H o w e v e r , this w o r k s h o w e d that the n u m b e r - a v e r a g e (equation 4) calculation y i e l d s L C B / 1 0 0 0 C values about 3 times h i g h e r t h a n the w e i g h t average (equation 3) calculation. F o r several p o l y m e r s the L C B / 1 0 0 0 C was d e t e r m i n e d b y alternate means, such as C N M R spectroscopy. T a b l e I presents a c o m p a r i s o n o f L C B / 1 0 0 0 C values for t w o p o l y m e r s for w h i c h this p a r a m e t e r was d e t e r m i n e d b y alternate means. T h e s e data show that the


1 3

weight-average (equation 3) e q u a t i o n y i e l d e d values i n agreement w i t h the alternate t e c h n i q u e s , b u t the n u m b e r - a v e r a g e (equation 4) e q u a t i o n y i e l d e d values about 3 times too h i g h . Table I. LCB/1000C Calculated with Equation 3 (Weight-Average) and Equation 4 (Number-Average) Branching Equations Compared to Certificate and C NMR Values 13

Polymer

Certificate

N B S S R M 1476 N P E 350

1.2

-

C NMR

13

a

1.3 4.5

Number Average 3.0 9.4

Weight Average 1.3 3.3

"NMR spectroscopy overestimates L C B because it counts all branches greater than six carbons as long branches.

N M R values are always overestimates o f L C B because a l l branches o v e r six c a r b o n atoms are c o u n t e d as l o n g branches. T h u s , o n l y the l o w e r w e i g h t average L C B values are i n agreement w i t h N M R values. T h e p r o p o s i t i o n that the n u m b e r - a v e r a g e calculation y i e l d s fallaciously h i g h values of L C B has b e e n o b s e r v e d b y others. W e s t e r m a n a n d C l a r k (6) a n d D r o t t a n d M e n delson (7) d i s c o u n t e d the use of the n u m b e r - a v e r a g e equation because it y i e l d e d fallaciously h i g h L C B values; t h e i r values w e r e most c l e a r l y too h i g h for some fractions for w h i c h the L C B values e x c e e d e d the short-chain b r a n c h i n g values d e t e r m i n e d b y I R spectroscopy, w h i c h is clearly i m p o s s i b l e . O n the basis of this analysis, the weight-average equation was j u d g e d to b e appropriate a n d was u s e d for the L C B calculations i n this w o r k . A reexa m i n a t i o n o f the d e r i v a t i o n o f these equations was b e y o n d the scope o f this study. A c o m p i l a t i o n of data for " s t a n d a r d " a n d c o m m e r c i a l p o l y e t h y l e n e resins is p r e s e n t e d i n T a b l e I I . T h e agreement b e t w e e n the o b s e r v e d data a n d the certificate data is generally good. (Certificate values w e r e d e t e r m i n e d b y A m e r i c a n P o l y m e r Standards C o r p o r a t i o n . ) T h e o b s e r v e d data agree w i t h past e x p e r i e n c e w i t h l o w - d e n s i t y p o l y e t h y l e n e ( L D P E ) a n d h i g h - d e n s i t y p o l y e t h y l e n e ( H D P E ) resins; that is, c o m m e r c i a l L D P E resins t y p i c a l l y e x h i b i t e d L C B o f 2 to 5 a n d H D P E resins of < 1 . F o r the H D P E resins, the values of L C B / 1 0 0 0 C of several tenths m a y arise from o t h e r effects, s u c h as short-chain b r a n c h i n g , because L C B is assumed to be zero i n H D P E resins. H o w e v e r , the l o w levels of L C B m a y b e real. T h i s m e t h o d is p r o b a b l y the most d i r e c t m e t h o d to o b t a i n L C B values a n d , therefore, is expected to y i e l d the most accurate values. T h e D r o t t a n d M e n d e l s o n approach suffers from the fact that the w h o l e - p o l y m e r L C B value is o b t a i n e d iteratively, b u t



0

c

b

a

2.0 0.25 3.8 0.25 0.79

— — — — —

0.978 0.931

0.923 0.918 0.923 0.955 0.960

— — — — —

M„

17.3 28.2 34.0 24.2 26.0 19.7 14.1 28.5 34.6 26.9 20.6 11.0

Density 52.4 79.8 91.6 90.1 107 124 122 120 181 140 103 91.1

In T C B at 140 °C from on-line Viscotek detector. In TCB at 130 °C. American Polymer Standards Corp.

0

0

0

0

MI

2.07 1.19

Sample

NBS 1475 H D P E NBS 1476 L D P E Standard Standard Standard Standard Standard NPE353 NPE940 USI 1016 USI 5602A USI 6009

58.3 101 127 123 149 187 190 162 273 275 128 116

z

122 248 360 346 479 683 744 473 855 1370 467 490

M

D 3.4 3.6 3.7 5.1 5.9 9.5 13.4 5.7 7.9 10.2 6.2 10.5

z+1

M 204 420 711 631 891 1236 1441 845 1434 2450 898 1043

Observed Values x 10~

1.01 0.91 0.86 0.89 0.84 0.87 0.92 0.83 1.06 0.93 1.55 1.43

0.4 1.3 1.8 1.6 2.7 3.2 2.4 3.3 2.6 2.0 0.3 0.3

LCB/ 1000C n

— — — — —

18.3 23.7

M

Table II. M W D and L C B Data for H D P E and L D P E Resins

w

53.1 102 110 83.9 143 202 243

M

z

— — — — — —

138

M

— — — — —

2.9 4.3

D

— — — — —

1.01 0.90

Certificate Values X 10^


0 1.2 1.6 1.7 3.4 2.5 4.5

LCB/ 1000C

GO

3

C

3".

Qta

C5

3

to

dp a

P 0

5 m r

to

32



the V i s c o t e k detector makes the L C B of each m o l e c u l a r species over the M W D accessible. A s i n d i c a t e d , the N M R m e t h o d tends to overestimate LCB. A n o v e l result o f this w o r k is the characteristic shapes of the l o n g - c h a i n b r a n c h i n g d i s t r i b u t i o n s ( L C B D ) of t y p i c a l L D P E resins. F i g u r e s 3 a n d 4 show the L C B D for N P E 353 a n d N P E 940. T h e L C B decreases w i t h increasing M W a n d r e m a i n s constant to h i g h M W . T h i s b e h a v i o r is t y p i c a l o f all c o m m e r c i a l L D P E resins so far o b s e r v e d . T h e opposite b e h a v i o r was r e p o r t e d b y W a g n e r a n d M c C r a c k i n (8) as s h o w n i n F i g u r e 5. H o w e v e r , m o r e r e c e n t l y R u d i n et a l . (9) r e p o r t e d the same b e h a v i o r as r e p o r t e d h e r e , o b s e r v e d b y o n - l i n e low-angle laser light-scattering detection. T h e L C B D for N B S 1476 d e t e r m i n e d b y R u d i n et a l . is s h o w n i n F i g u r e 6. T h e r e are two L C B D i n F i g u r e 6. R u d i n et a l . c l a i m e d that the dissolution p r o c e d u r e affected the d e t a i l e d shape o f the L C B D a n d that a l o w e r d i s s o l u t i o n t e m perature left p o l y m e r "aggregates" i n solution that cause fallaciously h i g h L C B at l o w M W . T h e r e f o r e , h i g h e r d i s s o l u t i o n temperatures w o u l d b e r e q u i r e d to totally dissociate these "aggregates" so that correct L C B is o b tained.

20.0 • 18.0 •• 16.0 14.0 -

.000 -I 4.00

1 4.50

1 5.00

LogM

1 5.50

Figure 3. LCB/1000C versus log Mfor NPE 353 LDPE.


»— 6.00


.000 -I4.00

1

1

1

I

'

4.50

5.00

5.50

6.00

5.50

Log M

Figure 4. LCB/1000C versus log M for NPE 940 LDPE.

Figure 5. Number of branch points per carbon atom for NBS 1476. (Reproduced with permission from ref. 8. Copyright 1977 Wiley.)



34


«

10

5

10

Molecular weight

6

10

Figure 6. Long-chain branch frequency-molecular-weight relation for NBS 1476. Estimate was made with e = 0.5. A, polymer dissolved and analyzed in trichlorobenzene at 145 °C; B, polymer dissolved at 160 °C and analyzed at 145 °C. (Reproduced with permission from ref. 9. Copyright 1984, Marcel Dekker.)

T h i s c l a i m was tested b y d i s s o l v i n g t w o L D P E resins o v e r t h e t e m p e r atures range 1 3 0 - 1 6 0 ° C . T h e data from this study are c o m p i l e d i n T a b l e III. T h e L C B D for t h e N B S 1476 a n d U S I 1016 resins s t u d i e d are s h o w n i n F i g u r e s 7 a n d 8, respectively. T h e s e t w o figures show that there is no correlation b e t w e e n t h e dissolution t e m p e r a t u r e a n d the shape o f the L C B D c u r v e ; that i s , that t h e l o w - M W side of the c u r v e does not b e c o m e larger as dissolution t e m p e r a t u r e decreases, as c l a i m e d b y R u d i n et a l . T h e source o f this v a r i a b i l i t y was suspected to b e not a dissolution t e m p e r a t u r e effect, b u t a v a r i a b i l i t y d u e to t h e base l i n e chosen for t h e V D o u t p u t data. V a r i -


2.

MIRABELLA & W I L D


35

Table III. Comparison of L C B and M W D Data for L D P E Resins Dissolved at Different Temperatures


Dissolution Temperature (° C)

M„

NBS 1476 160 160 150 140 130

28.2 28.3 18.9 25.8 25.6

USI 1016 160 160 150 140 130

22.5 26.9 23.9 22.7 30.0

M

v

79.8 80.1 76.5 78.7 76.7

139 140 137 141 148

M

w

101 100 94.4 101 93.9

281 275 238 273 281

M

3

M

3 + I

D

LCB I 1000C

248 239 237 253 208

420 396 426 442 318

3.6 3.6 5.0 3.9 3.7

1.3 1.3 1.3 1.1 1.4

1440 1370 1020 1200 1330

2650 2450 1700 1970 2340

12.5 10.2 10.0 12.0 9.4

1.9 2.0 2.4 1.9 2.5

N O T E : In all cases, the dissolution time was 1 h. In all cases, the equilibrium time was 15 min at 140 ° C . All M W D values are X 1 0 " . 3

ability is often seen in M W D data as a result of the variability of drawing base lines for S E C peaks. This suspicion was tested by varying the base line under the peak for the USI 1016 resin dissolved at 130 °C. Figure 9 shows a series of L C B D curves for this resin, with the base line being progressively moved further toward higher elution volume. The further the base line is moved toward higher elution volume (that is, the l o w - M W end) the smaller the M becomes, and this decrease results in a dramatically smaller l o w - M W side of the L C B D curve, as seen in Figure 9. Therefore, this base-line effect probably is the cause of the shape change in the L C B D curve and not the presence of undissolved polymer "aggregates". n

The fact that the base-line drawing procedure results in major changes in the shape of the l o w - M W portion of the L C B D curve implies that this shape may be an artifact. This circumstance is considered to be likely because the V D is insensitive to l o w - M W species. The R I D is more sensitive to lowM W species. Therefore, at l o w - M W the V D signal approaches zero while the R I D signal remains finite. This behavior can be appreciated from Figure 10, which shows that the V D signal goes to zero at high elution volume (low M W ) significantly before the R I D signal reaches zero. The Viscotek software uses an extrapolation procedure in this vicinity and this procedure may give rise to the characteristic shape of the L C B D curves observed. The question, however, may be asked as to the reason that Rudin et al. observed the same shape of the L C B D for branched polyethylenes by online low-angle laser light-scattering ( L A L L S ) detection (9). The answer may be for the same reason as noted earlier because L A L L S is even less sensitive at low M W than V D . Thus, both techniques may yield this rapidly decreasing





Figure 8. LCB/1000C versus log Mfor USI 1016 dissolved at the indicated temperatures. Points symbols were added to help distinguish curves.



« 4.00

• 4.50

Log Af

» 5.00

• 5.50

1

6.50

1

6.00

1

7.00

Figure 9. LCB/1000C versus log Mfor USI 1016 dissolved at 130 °C. Each curve was_obtained with a different base line. As the base line was extended toward the lower-MW end of the curve, the M „ decreased as noted on each curve.

. 0 0 0 -I 3.50


2.

MIRABELLA & W I L D

39



10.0

Figure 10. Raw data output from RID and VD showing that the VD signal goes to zero at high elution volume (low MW) while the RID signal remains finite to higher elution volume. Conditions: 130 °C for 1 h and 140 °C for 15 min. L C B f u n c t i o n at l o w M W for t h e same reason, that is, a n artifact d u e to t h e insensitivity o f b o t h detectors at l o w M W . T o further investigate t h e accuracy o f the shape o f the L C B D o f t y p i c a l L D P E resins as p r o d u c e d b y t h e V D - S E C t e c h n i q u e , t w o L D P E resins w e r e fractionated a c c o r d i n g to m o l e c u l a r w e i g h t a n d y i e l d e d a series o f n a r r o w - M W fractions. T h e m o l e c u l a r w e i g h t , l o n g - c h a i n b r a n c h i n g , a n d ancillary data for these t w o L D P E resins are p r e s e n t e d i n Tables I V a n d V . Table IV. Molecular Weights and Branching Parameters of Fractions of NA205 Solution Viscosity (dL/g)

Fraction 4 5 8 10 12 14 15 16 Whole polymer

Melt Viscosity (poise)

0.345 0.439 0.646 0.873 1.095 1.351 1.561 1.785

8.82 1.97 1.97 2.37 2.05 1.02 7.10

0.896

3.7 x 104

x x x x x x x

10 10 10 10 10 10 10

2

3

4 5 6 7 7

M» x 10~

LCB/ 1000C

2.0 2.7 6.2 9.9 18 31 51 81

4.2 2.7 2.5 1.6 1.6 1.4 1.4 1.4

30.0

2.8

4

N O T E : The L D P E resin N A 2 0 5 was lot 12162.


40

P O L Y M E R CHARACTERIZATION

Table V. Molecular Weights and Branching Parameters of Fractions of NA102


Fraction

Solution Viscosity (dL/g)

Melt Viscosity (poise)

4

0.29

5

0.40

5.5

8

0.74

2.38

10

0.97

7.01

12

1.31

1.84

14

1.88

4.81

x x x x x

Whole polymer

0.896

6.35

x

M

LCB/ 1000C

x 10~

4

w

102 103

1.71

5.7

2.45

3.2

6.41

1.6

104

11.1

1.4

106

23.6

1.2

107

66.7

1.0

104

21.3

1.3


.500 A 3.50

1

1

1

4.00

4.50

5.00 LogM

1

5.50

1

6.00

1

6.50

Figure 11. LCBD of USI NA205 and average LCBD of narrow-MW fractions.

The L C B D was determined by the V D - S E C technique for each whole L D P E resin, and then the average L C B was determined for each set of corresponding narrow-MW fractions. These values are plotted in Figures 1 1 and 1 2 for USI N A 2 0 5 and USI N A 1 0 2 , respectively. In each case Figures 11 and 1 2 show that the average L C B of the fractions confirm the shape of the whole-polymer L C B D . To further investigate this shape of the L C B D of typical L D P E resins,



2.

MIRABELLA

.500 -I 3.50

—

6r

WILD


—HI

1

4.00

1

1

4.50

41

5.00 LogM

1

5.50

»

6.00

6.50

Figure 12. LCBD of USI NA102 and average LCBD of narrow-MW fractions.

Table VI. Molecular Weights and Branching Parameters of Fractions of NA205 Fraction M „ X 10~

4

4 5 12 16 Whole resin

LCB

a

Amyl Butyl Ethyl

Total

2.0 2.7 18 81

3.0 3.0 2.8 3.0

2.8 2.6 2.8 2.5

6.7 7.2 7.4 7.0

1.3 1.2 1.3 1.4

13.8 14.0 14.3 13.9

3.0

2.9

2.6

7.2

0.5

13.2


All values given are branches per 1000 carbons determined by N M R spectroscopy. Defined as any branch of six or more carbons long.

13

C

a

w e sought an i n d e p e n d e n t t e c h n i q u e . B o t h short- and l o n g - c h a i n b r a n c h i n g may b e s i m u l t a n e o u s l y d e t e r m i n e d i n L D P E resins b y the use o f C N M R (10-12). T h e s e e x p e r i m e n t s assume that the N M R resonance assigned to h e x y l branches is, i n fact, d u e o n l y to l o n g - c h a i n branches (10). A l t h o u g h some controversy exists as to the v a l i d i t y of this assumption (II), the h e x y l resonance is g e n e r a l l y u s e d to d e t e r m i n e L C B i n L D P E resins. 1 3

Results of the N M R e x p e r i m e n t s are s h o w n i n Table V I . T h e w h o l e p o l y m e r L C B D a n d the V D - S E C a n d N M R fraction-average L C B data are In Polymer Characterization; Craver, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1990.

42



p l o t t e d together i n F i g u r e 13. F e w , i f any, differences are n o t e d i n e i t h e r the short-chain o r the l o n g - c h a i n b r a n c h i n g content across the M W range. I f h e x y l branches are actually L C B s , the N M R data indicate little v a r i a t i o n i n L C B across the M W range. T h e data r e p o r t e d h e r e are consistent w i t h the N M R w o r k of B u g a d a a n d R u d i n (12), w h o report L C B s o n the o r d e r of 2 . 4 - 3 . 0 i n a v a r i e t y of L D P E resins. T h e s e N M R results c o n f i r m the suspicion that the L C B d e t e r m i n e d at l o w M W b y the V D - S E C t e c h n i q u e is p r o b a b l y fallacious because of the l o w sensitivity o f the viscosity detector to l o w - M W p o l y m e r species. H o w ever, the qualitative t r e n d of a flat (zero slope) L C B D across the m a j o r i t y of the M W range (about 3 X 1 0 to 1 0 M W ) of U S I N A 2 0 5 L D P E as d e t e r m i n e d b y the V D - S E C t e c h n i q u e (see T a b l e IV) is s u p p o r t e d b y the C N M R results i n T a b l e V I a n d F i g u r e 13. F u r t h e r , the values of the L C B of U S I N A 2 0 5 o f about 3.0 across this same M W range as d e t e r m i n e d b y N M R spectroscopy are i n reasonable agreement w i t h the values of about 1.5 to 2.0 as d e t e r m i n e d b y the V D - S E C t e c h n i q u e , especially w h e n it is c o n s i d e r e d that the N M R t e c h n i q u e overestimates L C B . 4

6

1 3

BRANCHING FREQUENCY

0

SEC-VD

A

NMR

7.00 6.00

-

3.50

4.00

4.50

5.00

5.50

6.00

6.50

Log M Figure 13. LCBD of USI NA205 whole-polymer (—) and molecular-weight (#, A) fractions.


2.

MIRABELLA & WILD

43


Conclusions T h e results of this study indicate the V D - S E C t e c h n i q u e is suitable for the d e t e r m i n a t i o n of the L C B a n d L C B D of p o l y e t h y l e n e resins. H o w e v e r , these data w e r e r e l i a b l y d e t e r m i n e d o n l y above a M W of about 3 X 1 0 b y this 4

t e c h n i q u e , because of the l o w sensitivity of the viscosity detector to l o w M W p o l y m e r species. A n i m p o r t a n t observation of this w o r k was that the Downloaded by KTH ROYAL INST OF TECHNOLOGY on November 19, 2015 | http://pubs.acs.org Publication Date: May 5, 1990 | doi: 10.1021/ba-1990-0227.ch002

L C B of a large n u m b e r of t y p i c a l c o m m e r c i a l L D P E resins was constant across the majority of the M W D a n d does not increase at h i g h M W . A c o m m o n b e l i e f of w o r k e r s s t u d y i n g p o l y e t h y l e n e is that the

often-observed

h i g h - M W " h u m p " (see F i g u r e 14) o n the M W D of these resins is an i n d i c a t i o n of h i g h l y l o n g - c h a i n - b r a n c h e d species. T h i s type of species w o u l d y i e l d a sharp increase of the frequency of L C B at h i g h M W . S u c h a n u p t u r n i n L C B at h i g h M W was not observed i n a large variety of typical c o m m e r c i a l L D P E resins. T h u s , the large " h u m p " o n the m o l e c u l a r - w e i g h t d i s t r i b u t i o n of t y p ical L D P E resins, as seen i n F i g u r e 14, is a p p a r e n t l y due not to an increased frequency of L C B relative to the rest of the r e s i n , b u t rather to a s i m p l e bimodal molecular-weight distribution. 6.00 5.00

(m_

=

3.23 x 10

4.00

\ 4

M ]

n

= 2.99 x 10 M

/

g 3.00

1

n

5

=

1.30 x 10

6

1.00 .000 2.50

3.50

4.50 Log M

6.50

5.50

Figure 14. Molecular-weight distribution of a typical commercial LDPE resin (USI NA205).

Acknowledgments T h e authors are grateful to D a v i d B a i l e y for m a k i n g the

1 3

C N M R meas-

u r e m e n t s , K u r t K l e b e for the V D - S E C measurements, a n d M i r i a m A . C r a n d a l l for the t y p i n g of the m a n u s c r i p t .

References 1. Kamath, P. M . ; Wild, L . , paper presented at 22nd Annual Technical Conference of the Society of Plastics Engineers, Montreal, Canada, March 7-10, 1966; SPE Tech. Papers 1966, 12, XVII-6.



44 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.


Drott, E . E.; Mendelson, R. A. J. Polym. Sci. 1970, A-Z, 8, 1361, 1373. Zimm, B. H.; Stockmayer, W. H . J. Chem. Phys. 1949, 17(12), 1301. Berry, G. C. J. Polym. Sci. 1971, A-Z, 9, 687. Grinshpun, V.; et al. J. Polym. Sci. Polym. Phys. Ed. 1986, 24, 1171; Table III. Westerman, L . ; Clark, J. C. J. Polym. Sci. Polym. Phys. Ed. 1973, 11, 559. Drott, E . E.; Mendelson, R. A. J. Polym. Sci. 1970, A-Z, 8, 1373. Wagner, H . L.; McCrackin, F. L . J. Appl. Polym. Sci. 1977, 21, 2833. Rudin, A.; Grinshpun, V.; O'Driscoll, K. F.J. Liq. Chrom. 1984, 7, 1809. Usami, T.; Takayama, S. Macromolecules 1984, 17, 1756-1761. Bugada, D.; Rudin, A. Eur. Polym. J. 1987, 23, 809-818. Bugada, D . ; Rudin, A. J. Appl. Polym. Sci. 1987, 33, 86-93.

RECEIVED

1989.

for review February 14, 1989.

ACCEPTED

revised manuscript August 14,


Determination of Long-Chain Branching Distributions of Polyethylenes

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