Toughened Plastics II - American Chemical Society

22 Failure Mechanisms in Compatibilized Blends of Linear Low-Density Polyethylene and Polystyrene Tao Li , Cosimo Carfagna Jr. , Vasily A. Topolkaraev , Anne Hiltner *, Eric Baer , X.-Z. Ji , and Roderic P. Quirk 1

2

1

3

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3

Department of Macromolecular Science and Center for Applied Polymer Research, Case Western Reserve University, Cleveland, OH 44106 Sniaresearch Società consortile per Azioni, Via Pomarico, 75010 Pisticci (MT), Italy, Institute of Polymer Science, University of Akron, Akron, OH 44325

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The tensile failure of Mends of linear low-density polyethylene (LLDPE) and polystyrene (PS) compatibilized with block copoly mers of styrene (S) and butadiene (B) or hydrogenated butadiene (EB) has been studied. Compatibilizers were compared at the 5 wt% level in blends with equal amounts of LLDPE and PS by vol ume. Because of the lower viscosity of LLDPE, the morphology con sisted of spherical PS particles dispersed in an LLDPE matrix. The stress-strain curve of the compatibilized blends was composed of an initial linear region, followed by a region of decreasing slope to a second linear region with a small positive slope. The yield point was defined by the intersection of the two linear regions. A modified yield-strain approach was used to predict the yield stress of the compatibilized blends. As a result of good adhesion, yielding of the matrix was constrained by the rigid PS particles to a region deter mined by the yielding angle, Ф. Most of the blends exhibited a yield strain in the range of 1.2 to 1.6%. The calculated yielding angle of 70° or less was in accord with predictions for a rigid sphere in a plastic matrix. The higher yield stress of blends with crystalline styrene-hydrogenated butadiene and styrene-hydrogenated butadiene-styrene compatabilizers, as compared to blends with noncrys talline compatibilizers, resulted from their higher modulus. The ex ception was the blend compatibilized with Kraton G, which had a yield strain of 1.9% and a yielding angle of 79°.

H

IGH-STRAIN

PROPERTIES O F POLYMER B L E N D S , such as strength,

tensile

elongation, a n d impact strength, benefit from eompatibilization. These

*Corresponding author.

0-8412-3151-6

© 1996 American Chemical Society

Riew and Kinloch; Toughened Plastics II Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

335

336

T O U G H E N E D PLASTICS I I


advantages are specifically attributed to finer phase dispersion and i m p r o v e d interfacial adhesion. Elastic properties o f blends w i t h isotropic, dispersed morphologies are amenable to analysis w i t h mechanical models i n w h i c h good adhesion of the phases is assumed. M e c h a n i c a l models that assume good adhesion without considering the structure o f the interface are less satisfactory for describing high-strain deformation. Previously, we m o d e l e d high-strain properties o f uncompatibilized blends using an approach based o n observed microdeformation mechanisms, specifically, particle debonding and v o i d growth (see C h a p t e r 21). Approaches based on structural or morphological models that explicitly incorporate deformation and failure mechanisms of the interface or interphase are needed for compatibilized blends. T h e beneficial effects o f adding a graft or block copolymer of styrène (S) a n d butadiene (B), or a similar hydrogenated copolymer, to blends o f polyethylene and polystyrene (PS) have been demonstrated (1-8). A previous study p e r f o r m e d at this laboratory focused o n the modulus o f the compatibil i z e d b l e n d , w h i c h lay w i t h i n H a s h i n s upper modulus b o u n d and K e r n e r s equation for spherical voids (1). Two interfacial models resulted: a c o r e - s h e l l m o d e l similar to that described by others (9, 10), and an interconnected-interface model. T h e c o r e - s h e l l m o d e l w i t h a compatibilizer coating the dispersed P S particle was used to calculate the decrease i n modulus observed w i t h rubbery compatibilizers. U s i n g this model, it was possible to estimate the amount o f compatibilizer at the interface, w h i c h varied from 5 to 5 0 % , w i t h the r e m a i n i n g compatibilizer p r e s u m e d to be dispersed i n the continuous polyethylene phase. T h e modulus of blends compatibilized w i t h crystalline copolymers was calculated from an interconnected-interface m o d e l i n w h i c h the blocks selectively penetrated the polyethylene and P S phases to provide good adhesion without forming a rubbery coating on the P S particle. T h e i n terconnection p r o v i d e d i m p r o v e d stress and strain transfer between the phases, a n d as a result the modulus o f the b l e n d approached H a s h i n s upper b o u n d . These structural-mechanical models are now extended to incorporate interfacial-failure mechanisms for application to high-strain deformation and failure of compatibilized blends.

Experimental Details Linear low-density polyethylene ( L L D P E ) and PS resins were the same as described previously (Chapter 21). The various block copolymers that were used as compatibilizers have also been described (1). A series of crystaUine copolymers (Q series) was prepared by hydrogénation of diblock and triblock copolymers of styrene and butadiene [styrene-hydrogenated butadiene (SEB) and styrene-hydrogenated butadtene-styrene (SEBS)j (1). Triblock copolymers of styrene and butadiene [styrene-butadiene-styrene (SBS)] and a noncrystalline hydrogenated block copolymer (SEBS) (Kraton) were supplied by Shell Chemical C o . Diblock copolymers of styrene and butadiene [styrene-butadiene (SB) (Vector)] were obtained from Dexco Polymers. The characteristics of the resins are given i n Table I.



27 130

30 21 53 75 117

60 90 139

115

150 70 130

137 154 88

S E B copolymers (crystalline) Q29 Q26 Q292 Q302 Q304

S E B S copolymers (crystalline) Q293 Q303 Q307

S E B S copolymer (noncrystalline) Kraton G-1652M

SB copolymers Vector 6000-D Vector 2320-D Vector 6010-D

SBS copolymers Kraton D ^ 1 2 2 P (plasticized) Kraton D-1101 Kraton D-1102

f l

L L D P E (GB502) PS (Styron 623)

Polymer 3

Number-Average Molecular Weiglit, M (x 1Q- )

48/52 31/69 28/72

30/70 25/75 12/88

29/71

33/67 33/67 32/68

67/33 53/47 19/81 20/80 19/81

Styrene-to-Rubber Ratio (wt/wt)

12.1 ± 0.3

12.5 ± 0.9 11.1 ±0.3 9.8 ± 0.2

8.3 ± 0.3

y

Yield Stress, a (MPa)

Table I. Properties of Resins Used

14.3 ± 1.8

7.1 ± 2.2

0.9 ± 0.1 0.2 ± 0.1 0.3 ± 0.1

22.3 ± 2.6

19.3 ± 1.6

15.8 ± 0.5 24.3 ± 0.8 29.9 ± 1.7

18.8 ± 1.0 37.3 ± 3.9

/gyigrg

Engineering Fracture Stress, or (MPa)


139 ± 8

77 ±33

1.7 ± 0.3 0.3 ± 0.1 0.5 ± 0.1

130 ± 3

88 ±24

96 ± 3 133 ± 12 174 ± 23

171 ± 25

f

Fracture Stress, a (MPa)

1109 ± 13

937 ± 238

98 ±56 110 ± 36 65 ± 11

575 ± 30

252 ± 40

346 ± 19 451 ± 15 536 ± 29

760 ± 30 1.3 ± 0.2

f

Fracture Strain, e (%)

338

TOUGHENED ΡΙΑ8Ή08

II

E q u a l amounts of L L D P E and PS by volume were dry-mixed with 5 wt% compatibilizer and 0.2 wt% antioxidant (Irganox 1076), and blended i n a Banbury mixer for about 7 m i n at 175 °C. The blend was quenched i n cold tap water and dried overnight at 40 °C i n vacuum. Plaques were compression-molded by pre heating the blend i n a press at 170 °C for about 15 min, then applying a pressure of 3.85 M P a for 8 min. The press was water-cooled to room temperature with the plaque under pressure. Type I dog-bone tensile specimens ( A S T M D-638-84) were cut from the compression-molded sheet. The stress-strain curve was obtained with a strain rate of 0.05 m i n . A gauge length of 90 m m was used to calculate the strain. After straining to fracture, some of the tensile specimens were cryogenically fractured lengthwise at liquid-nitrogen temperature, coated with 60 Â of gold, and examined using a scanning electron microscope ( J E O L J S M - 8 4 0 A ) . Downloaded by PURDUE UNIV on July 5, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0252.ch022

- 1

Results and Discussion S t r e s s - S t r a i n B e h a v i o r . T h e effect o f various compatibilizers at the 5 % level was examined i n blends w i t h equal amounts o f L L D P E a n d P S o n a v o l u m e basis. T h e following description o f the b l e n d morphology o f the c o m pression-molded L L D P E - P S blends is summarized from o u r previous study, w h e r e the focus was o n the low-strain mechanical response (J). Because o f the lower viscosity o f L L D P E , the morphology consisted o f spherical P S particles dispersed i n an L L D P E matrix. T h e large, somewhat elongated PS domains o f the u n c o m p a t i b i l i z e d b l e n d became smaller a n d more spherical w i t h the addition o f compatibilizer. T h e particle size was reduced from - 2 8 μηι to ~4 μηι w i t h 5 % K r a t o n G . T h e crystalline S E B and S E B S compatibilizers (Q series) w e r e also effective i n reducing the particle size. Unsaturated S B a n d S B S copolymers w e r e not as effective as the hydrogenated compatibilizers; they de creased the particle size only slightly, to - 2 0 μηι. Tensile deformation o f the uncompatibilized b l e n d w i t h 5 0 % P S was characterized b y the appearance o f several regions o f localized stress-whiten i n g i n the gauge section without global necking. F r a c t u r e occurred at one o f these regions at a relatively l o w strain, about 3.2%. This behavior is character i z e d as quasi-brittle rather than brittle, because some level o f plastic deforma tion precedes fracture even though the fracture strain is l o w (Chapter 21). T h e stress-strain curves o f most o f the compatibilized blends consisted o f an initial linear region, followed b y a region o f decreasing slope to a second l i n ear region w i t h a small positive slope (Figure 1). Stress-whitening was notice able at about 2 % strain, and deformation proceeded w i t h uniform extension o f the entire gauge section. T h e slope of the initial linear region o f the stress-strain curve reflected the tensile modulus, and it c o u l d either increase or decrease w i t h respect to the uncompatibilized b l e n d . Noncrystalline c o m patibilizers lowered the tensile modulus o f the b l e n d by as m u c h as 5 0 % . C a l culations based o n a c o r e - s h e l l m o d e l w i t h the rubbery compatibilizer coating the P S particle satisfactorily described the decrease i n modulus and made it


22.

L i E T AL.

339

Failure Mechanisms in Compatibilized Blends ~i—r—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—r—1—r


0307 (SEBS)

Strain Figure 1. Effect of 5 wt% compatibilizer with equal volumes of LLDPE and PS.

(S) on the stress-strain curve of a blend

possible t o estimate the fraction o f compatibilizer i n the interfacial coating. T h e increased modulus o f blends compatibilized w i t h crystalline, nonrubbery S E B a n d S E B S copolymers approached H a s h i n s upper b o u n d . A n intercon nected interface m o d e l was proposed i n w h i c h the blocks selectively penetrate the L L D P E a n d P S phases to provide good adhesion a n d i m p r o v e d stress a n d strain transfer between the phases (I). T h e general shape o f the stress-strain curve is the same for all the blends w i t h Q-series and K r a t o n compatibilizers. This shape is described b y two tan gent lines drawn from the initial elastic region a n d the plastic region, respec tively. T h e intersection o f the lines is defined as the y i e l d point a n d is d e scribed b y a y i e l d stress (a ) and a n apparent y i e l d strain (e ). T h e stress ( σ ) a n d strain (e) i n the plastic region are related b y y

y

a = a + E (€-€ ) y

p

y

(1)

where E is the slope o f the stress-strain curve i n the plastic region. A l l the S E B a n d S E B S copolymers w e r e effective i n increasing the y i e l d stress a n d i m p r o v i n g the fracture stress and strain o f the b l e n d (Table II). C o m p a t i b i l i z a t i o n w i t h the S B S copolymers resulted i n some increase i n the y i e l d stress a n d fracture stress, b u t the increases were not as large as those achieved w i t h the S E B and S E B S copolymers. T h e r e was enough variation i n the Q-series copolymers to suggest some relationships between the styrene fraction i n the copolymer and the properties o f the compatibilized b l e n d . T h e S E B copolymers w i t h the highest styrene fraction (Q29 a n d Q 2 6 ) f o r m e d blends w i t h slightly higher y i e l d stress and fracture stress, a n d lower fracture p



6.3 ± 0.1 4.7 ± 0.1 4.9 ± 0.1

15 18 20

20 20 20

SB copolymers Vector 6000D Vector 2320D Vector 6010D

SBS copolymers Kraton D ^ 1 2 2 P Kraton D-1101 Kraton D-1102

fc

"From reference 1. Area under the stress-strain curve.

10.1 ± 0.4

SEBS copolymer (noncrystalline) Kraton G-1652M 4

8.6 ± 0.1 8.9 ± 0.5 8.1 ± 0.6

14.9 ± 0.4 16.2 ± 0.6 15.6 ± 0.9

SEBS copolymers (crystalline) Q293 6 Q303 8 Q307 9

0.4 0.6 1.0 0.2 0.8

7.5 ± 0.5

y

a (MPa)

16.4 ± 16.0 ± 12.9 ± 13.5 ± 13.7 ±

0

SEB copolymers (crystalline) Q29 5 Q26 1 Q292 3 Q302 15 Q304 15

None

Compatibilizer (5wt%)

Average PS Domain Size (pm)

± ± ± ± ±

0.1 0.1 0.2 0.1 0.1

1.2 ± 0.1 1.4 ± 0.2 1.5 ± 0.1

2.0 ± 0.1 2.6 ± 0.2 2.7 ±0.1

2.6 ± 0.6

1.9 ± 0.1 1.7 ±0.1 1.6 ± 0.1

1.4 1.4 1.6 1.6 1.6

0.9 ± 0.1

*,(%)

± ± ± ± ±

0.8 0.3 0.2 0.3 0.6

10.2 ± 0.8 9.6 ± 0.5 8.8 ± 0.6

5.7 ± 0.3 3.9 ± 0.4 3.4 ± 0.7

12.2 ± 1.4

16.1 ± 0.4 16.4 ± 0.6 16.3 ± 0.7

17.3 17.3 15.6 13.7 14.4

7.0 ± 0.6

f

a (MPa)

± ± ± ± ±

0.3 0.4 2.4 1.4 2.0

16.2 ± 1.1 10.1 ± 1.6 7.6 ± 0.6

3.3 ± 0.8 9.4 ± 6.2 5.2 ± 1.2

20.4 ± 1.7

12.8 ± 1.4 9.4 ± 0.7 9.6 ± 1.3

4.2 4.5 13.7 8.2 8.9

3.2 ± 1.0

f

€ (%)

Table II. Properties of the Blends Studied

3

b

1.29 1.00 0.60

2.44

1.62 1.25 1.35

0.56 0.61 1.67 0.92 1.05

6

Toughness (J/m χ 10- )


1.6 0.7 0.7

2.1

1.2 0.2 0.7

0.9 1.3 2.7 0.2 0.7

Γ

σσ (MPa) E

P 2

0.11 0.08 0.11

0.12

0.11 0.03 0.09

0.32 0.42 0.22 0.03 0.10

(MPa χ 10- )

H Ω

D

z

M

3

ο ο ce

ο


22.

L i ET AL.

Failure Mechanisms in Compatibilized

Blends

341

strain, than the copolymers w i t h a lower styrene fraction (Q304, Q 3 0 2 , a n d Q292). Increasing the molecular weight o f S E B copolymers w i t h 20/80 (S/EB) composition caused the copolymer y i e l d stress to decrease and the fracture stress a n d strain to increase significantly. These trends d i d not carry over to the blends compatibilized w i t h these copolymers. T h e only difference i n the p r o p erties o f the blends that might be significant was the slightly higher fracture strain o f the b l e n d compatibilized w i t h Q 2 9 2 , the copolymer w i t h the lowest molecular weight. C o m p a r i n g S E B to S E B S copolymers as compatibilizers d i d not reveal any striking differences between the diblock and triblock architecture. A d d i n g a second styrene block to an S E B copolymer d i d not affect the b l e n d properties except for a slight increase i n the yield stress and possibly also the fracture stress, effects that were also consistent w i t h increasing styrene fraction. Blends w i t h the S B copolymers h a d the lowest y i e l d stress and fracture stress, even lower than those o f the uncompatibilized control. I n this i n stance, the low strength o f the S B copolymers carried over to the poor properties o f the blends. T h e parameter E was greater than zero, and it was greatest for blends w i t h the two low-molecular-weight S E B copolymers w i t h high P S content. A range o f values, from 0.03 to 0.22, was obtained for blends w i t h the other crystalline S E B a n d S E B S copolymers; no clear trends w i t h molecular weight or differences between diblock and triblock copolymers were seen. Blends w i t h the noncrystalline S E B S and S B S all h a d E values near the m i d d l e of the range cited. p

p

T h e higher fracture stress and strain i n the compatibilized b l e n d , together w i t h the positive slope i n the plastic region, indicated that i m p r o v e d adhesion between P S particles and the L L D P E matrix suppressed the processes of d e b o n d i n g and subsequent v o i d growth that were observed i n the uncompatib i l i z e d blends (Chapter 21). W i t h good adhesion o f the P S particles to the m a trix, the local strains at the interface w o u l d have been high because of the c o n straint i m p o s e d b y the undeformed P S particles. T h e fractured tensile specimens were cryogenically fractured lengthwise to reveal the internal morphology o f the deformed b l e n d . Two examples, i n Figures 2 a n d 3, o f blends c o m p a t i b i l i z e d w i t h 5 % K r a t o n G a n d 5 % Q 2 9 2 , respectively, show that the P S particles were not d e b o n d e d from the matrix. Instead, they remained connecte d to the matrix b y microfibrils that formed w h e n the interfacial region was drawn out d u r i n g tensile deformation. Subtle differences i n the texture of the particle surface are discerned at higher magnification. T h e P S particles o n the fracture surface of the b l e n d w i t h K r a t o n G appear to have a coating of the compatibilizer, w h i c h is suggestive of adhesive fracture between a rubbery shell a n d the matrix. I n contrast, the P S particles i n the b l e n d w i t h the crystalline S E B copolymer have a rougher texture, as i f a fibrous interface has fractured. T h e processes o f interfacial stretching and microfibrillation that accomp a n i e d tensile deformation are shown schematically w i t h the stress-strain



342


Figure 2. Scanning electron micrographs of a blend with 5% Kraton G-1652M that was drawn to fracture and cryogenically fractured lengthwise. The direction of stress is shown by the arrow.

curve in Figure 4. In the elastic region, Stage I, the interface is intact. As the yield point is approached, in Stage II, the interface (or interphase) begins to undergo large strains that culminate in microfibrillation. In Stage III, the microfibrils stretch out until fracture of the interface leads to catastrophic failure of the blend. This concept is thought to be applicable to both the core-shell model and the interconnected-interface model of compatibilization. Microfibrils in the blend compatibilized with Kraton G would have formed by drawing of the rubbery shell of the core-shell particle. When the blend fractured, a rubbery coating remained on the PS particles. In the blend compatibilized with Q-292, with the interconnected interface, it was more likely that microfibrils formed by drawing of the L L D P E matrix connected to the PS parti-



22.

L i E T AL.

Failure Mechanisms in Compatibilized Blends

343

Figure 3. Scanning electron micrographs of a blend with 5Ψο Q292 that was drawn to fracture and cryogenically fractured lengthwise. The direction of stress is shown by the arrow.

cles. T h e n e e d for significantly higher loads to deform blends compatibilized w i t h Q-series copolymers supports this hypothesis. P r e d i c t i o n o f Y i e l d Stress. A modified yield-strain approach was used to predict the y i e l d stress o f the compatibilized blends. Instead o f assum i n g interfacial d e b o n d i n g as i n t h e previous analysis o f uncompatibilized blends (Chapter 21), we assumed that the P S particles were well-connected to the L L D P E matrix. A s a result, yielding o f the matrix was constrained b y the r i g i d P S particles to a region determined b y the yielding angle ( φ ) , as defined i n F i g u r e 5. N i e l s e n s approach was used to obtain the effective deformation length o f the matrix ( L ) w i t h the constraint from the rigid P S particles (11): eff



344


Θ

Interphase "intact"

Void growth

Debonding and void nudeation

Figure 4. Schematic of the stress-strain behavior of a compatibilized blend with good adhesion.

L

e f f

=L [l-a(V 0

P S

)^]

(2)

where L is the unit cell dimension o f the cubic array o f particles, V is the volume fraction o f P S , and α is a geometric factor that depends o n the yielding angle. T h e factor a is given by 0

P S

a = 2|

_3_\l/3 -7— I cos φ

(3)

4TT)

Figure 5. Schematic of constrained yielding: (a) interpenetrating-interface mod el; and (b) core-shell interface model with a rubbery shell. h ^is the effective de formation length of the matrix. t


22.

345


L i E T AL.

A s s u m i n g further that the matrix has the same effective yield strain as L L D P E (€y ~ 2.4%), the y i e l d strain o f the compatibilized blends is given b y € = €^^£![ = ° [ l - a ( V y

e

P

s

n

(4)

T h e y i e l d strain calculated from equation 4 is plotted i n F i g u r e 6a for several y i e l d i n g angles. A yielding angle o f 90° corresponds to the unconstrained c o n dition. D e p e n d i n g o n the yielding angle, the decrease i n y i e l d strain can b e


(a)

3.0

τ

1

1

1

r

1

1

1

1

1

1

1

* 2.6 #

r

1

Yielding Angle φ go

:

0

5 2.0 %m

—

1.5

2

>

7

:

0

1.0

Ό C

— —

-

CD 0.0 J

1

1

1

0.0

!..

1

1

1

20.0

PS Concentration (b)

1

40.0

1

ι

J

l.

ι

40.0

1

1

1

_l_

.,

_r

60.0

(vol*) 1

1

1

1 τρ>—r

Yielding Angle φ: gn°

Ο

0.

VI VI

30.0

«

20.0 ft. in 5

"Ό

^

10.0

^

^

^

45°

C

il 0.0 0.0

20.0

40.0

PS Concentration Figure 6. Predictions of the constrained-yielding yield stress.

60.0

(vol*) model: (a) yield strain; and (b)


346


dramatic. Analysis o f the stress a n d strain field around a r i g i d sphere i n a plas tic matrix suggests that the yielding angle should be about 70° (12). T h e y i e l d stress i n the idealized representation o f the stress-strain curve is then given b y a

y

= Ee

(5)

y

w h e r e Ε is the modulus o f the compatibilized b l e n d a n d e is the y i e l d strain from equation 4. T h e y i e l d stress calculated from equation 5 for several y i e l d i n g angles is plotted i n F i g u r e 6b using H a s h i n s upper modulus b o u n d for spherical P S particles dispersed i n L L D P E . I n equation 5, the P S particles have two opposing effects o n the y i e l d stress: they constrain yielding, w h i c h decreases the y i e l d strain, a n d they also have a reinforcing effect that increases the modulus. A t a l o w P S content, the effect o f the constraint is more i m p o r tant a n d the y i e l d stress decreases; however, as the P S content increases, the reinforcing effect dominates, a n d w i t h a reasonable value for the yielding a n gle (70°), significant increases i n the y i e l d stress are predicted for blends o f L L D P E and PS.


y

T h e y i e l d stress dictions i n F i g u r e 7. y i e l d i n g angle o f 70° F i g u r e 7 is obtained

ο

25.0

CL 3^ m m

«

20.0

taken from the stress-strain curves is compared w i t h p r e T h e u p p e r b o u n d is the prediction from equation 5 for a a n d H a s h i n s upper modulus b o u n d . T h e lower b o u n d i n from N i e l s e n s approach for completely d e b o n d e d parti-

- ,

1

1

«

ι

·

•

1

I

1

5% ;•· A5X

;•

15.0

•

1

Ό

J»

1

r-^

5X SEB (Q series) LLOPE-PS SEBS (Q series) SEBS (Kraton G) L • 5X SBS 5% SB •

10.0

>· Ό C

©

.

5.0

CD 0.0

a — - ι

0.0

1

1

•

Q

1

20.0

1 — — ι

1

—11

1

ι,

40.0

PS Concentration

•

ι

1 ι C ri 60.0

(vol*)

Figure 7. Yield stress of the compatibilized blends compared with calculations. Key: —, a yielding angle of 70° and Hashin s upper modulus bound; —, Nielsens approach for debonded particles; and , Nielsens approach with a debond ing angle of 45°. Data for the uncompatibilized blends are from Chapter 21.


22.

L i E T AL.

Failure Mechanisms in Compatibilized

347

Blends

cles b y considering the decrease i n load-bearing cross section that results from interfacial debonding. A modification o f N i e l s e n s approach, developed to c o n sider partial debonding o f the spherical particles, describes the y i e l d stress o f u n c o m p a t i b i l i z e d blends o f L L D P E a n d P S (Chapter 21). T h i s prediction, w h i c h incorporates a debonding angle o f 45°, is i n c l u d e d as the solid curve. The uncompatibilized blend with V o f 0.5 falls slightly above the solid curve because the morphology o f this composition d i d not conform w i t h the assumptions o f the m o d e l as w e l l as compositions w i t h a lower P S fraction; specifically, the P S particles tended to be elongated rather than spherical. T h e y i e l d stress o f blends w i t h S B copolymers falls o n the same curve as the uncompatibilized blends. T h i s result is attributed to interfacial failure before yielding i n blends w i t h the low-strength S B copolymers. Blends w i t h the Q-series compatibilizers are the closest to the u p p e r b o u n d ; blends w i t h S B S compatibilizers are intermediate between the uncompatibilized b l e n d a n d blends w i t h Q-series c o m patibilizers. Differences i n the y i e l d stress among these compatibilized blends p r i m a r i l y reflect the effect o f compatibilization o n the b l e n d modulus.


P S

T h e y i e l d strain obtained from the point o f intersection o f the two linear regions o f the stress-strain curve is compared i n Table III w i t h the y i e l d strain calculated from the y i e l d stress a n d the modulus. T h e r e are slight differences between measured a n d calculated values; the calculated values are considered

Table III. Yield Parameters of Compatibilized Blends Compatibilizer (5 wt%)

Yield Stress, a (MPa) y

Modulus (GPa)

0

S E B copolymer (crystalline) Q29 16.4 ± 0.4 Q26 16.0 ± 0.6 Q292 12.9 ± 1.0 Q302 13.5 ± 0.2 Q304 13.7 ± 0.8

1.25 1.06 0.90 0.92 1.03

S E B S copolymer (crystalline) Q293 14.9 ± 0.4 Q303 16.2 ± 0.6 Q307 15.6 ± 0.9

Measured Calculated Yielding Yield Strain, Yield Strain, Angle, 6y(%) é> (%) φ(ώ%) y

0.1 0.1 0.2 0.1 0.1

1.3 1.5 1.4 1.5 1.3

62 68 65 66 63

0.94 ± 0.02 1.09 ± 0.04 1.03 ± 0.02

1.9 ± 0.1 1.7 ± 0.1 1.6 ± 0.1

1.6 1.5 1.5

70 67 68

S E B S copolymer (noncrystalline) Kraton G-1652M 10.1 ± 0.4

0.52 ± 0.07

1.9 ± 0.2

1.9

79

SBS copolymer Kraton D-4122P Kraton D-1101 Kraton D-1102

0.71 ± 0.04 0.67 ± 0.04 0.52 ± 0.03

1.2 ± 0.1 1.4 ± 0.2 1.5 ± 0.1

1.2 1.3 1.6

59 62 69

8.6 ±0.1 8.9 ± 0.5 8.1 ± 0.6

± ± ± ± ±

0.07 0.06 0.04 0.03 0.06

1.4 1.4 1.6 1.6 1.6

± ± ± ± ±

" F r o m Reference 1.


348


more accurate because they are based o n careful determinations o f modulus. T h e y i e l d i n g angle obtained from equation 4 w i t h a V o f 0.5 is also i n c l u d e d i n Table III. M o s t o f the blends exhibit a y i e l d strain i n the range o f 1.2 to 1.6% a n d a y i e l d i n g angle o f 70° or less. T h e exception is the b l e n d compatibil i z e d w i t h K r a t o n G , where a y i e l d strain of 1.9% gives a yielding angle o f 79°.


P S

T h e Q-series compatibilizers are crystalline S E B and S E B S block copolymers. F r o m analysis o f the modulus o f L L D P E - P S blends compatibilized w i t h Q-series copolymers, a n d examination of electron micrographs, these compatibilizers are thought to impart good adhesion to the interface between L L D P E and P S b y interpenetrating both phases without creating a rubbery interphase (1). Because they provide good adhesion, the stress and strain fields a r o u n d the particle are expected to resemble those obtained analytically (12). This expectation was confirmed w h e n the calculated yielding angle i n blends compati b i l i z e d w i t h Q-series copolymers was found to be close to the 70° p r e d i c t e d angle. Therefore, the y i e l d stress o f blends w i t h Q-series compatibilizers is det e r m i n e d b y the b l e n d modulus. I n contrast to the Q-series compatibilizers, the noncrystalline K r a t o n compatibilizers are thought to provide good adhesion by forming a rubbery shell a r o u n d the P S particles (I). O n e manifestation o f the rubbery shell is the lower modulus o f blends compatibilized w i t h the Kratons. T h e Kratons differ i n the amount o f compatibilizer i n the shell; i n a b l e n d w i t h 5 % compatibilizer, it is estimated that about 5 0 % o f K r a t o n G is i n the shell, whereas only 5 - 1 5 % of a K r a t o n D coats the particles, w i t h the rest assumed to be dispersed i n the L L D P E matrix. W i t h only a t h i n shell o f K r a t o n D coating the P S particle, the stress and strain fields should not be affected as m u c h as w i t h a thicker shell o f K r a t o n G . This is borne out i n the yielding characteristics; the y i e l d strain a n d corresponding y i e l d i n g angle calculated for blends compatibilized w i t h K r a t o n D s are similar to those w i t h Q-series compatibilizers. O n the other hand, the higher y i e l d strain a n d yielding angle o f the b l e n d compatibilized w i t h K r a t o n G are consistent w i t h a rubbery, deformable shell o n the PS particle. F r a c t u r e Stress a n d S t r a i n . Y i e l d i n g and plastic deformation i n the schematic representation o f tensile deformation were associated w i t h m i crofibrillation at the interface and stretching o f the microfibrils. Because this representation was assumed to apply to both the core-shell and interconnected-interface models o f compatibilization, the constrained-yielding approach was u s e d without specific reference to the microstructure o f the interface. I n extending the discussion to fracture, however, it is useful to consider the interfacial-deformation mechanisms. Tensile deformation culminated i n catastrophic fracture w h e n the microfibrillated interface failed. This was inferred from the quasi-brittle fracture behavior of the uncompatibilized b l e n d w i t h V o f 0.5, w h i c h indicated that the reduced load-bearing cross section after interfacial d e b o n d i n g c o u l d not support plastic deformation. Accordingly, the ultimate properties o f the compatibilized b l e n d d e p e n d e d o n interfacial charP S


22.

L i E T AL.


349

acteristics such as adhesive strength, ductility o f the fibrillating material, and strength o f the microfibrils. These were controlled b y the microstructure o f the interface.


M i c r o f i b r i l s i n the b l e n d compatibilized w i t h K r a t o n G probably formed b y drawing o f the rubbery shell o f the c o r e - s h e l l particle. Important factors w o u l d have been the amount o f rubber i n the shell, the strength o f the rubber, a n d the strength o f adhesion to L L D P E . A l l these factors may have c o n t r i b u t e d i n some degree to the high fracture stress and strain o f the b l e n d w i t h K r a t o n G . T h e amount o f compatibilizer i n the shell differed for the various Kratons; the thicker coating was certainly one o f the reasons K r a t o n G gave better properties to the compatibilized b l e n d than the K r a t o n D compatibiliz ers. Microfibrils formed at the interconnected interface o f the Q-series c o m patibilizers b y drawing out o f the attached L L D P E matrix. Interconnection, possibly involving cocrystallization o f the E B block (13), was thought to create a m o d i f i e d L L D P E region around the P S particles. This modified L L D P E , w h i c h probably deformed more easily than L L D P E , w o u l d have drawn out into microfibrils. E v e n the lowest molecular weight E B block, nominally 10,000 i n Q 2 6 and Q 2 9 , was sufficient to give good interconnections between the phases. Blends w i t h these compatibilizers exhibited the largest values o f E , the slope o f the stress-strain curve i n the plastic region, and the fracture stresses were at least as high as i n blends w i t h other Q-series compatibilizers. T h e blends w i t h Q 2 6 and Q 2 9 d i d have fairly low fracture strains, possibly be cause the m o d i f i e d L L D P E layer was relatively thin. T h e molecular weights o f the crystallizable Ε Β blocks i n the other Q-series compatibilizers were n o m i nally 40,000, 60,000, a n d 95,000. T h e blends w i t h these compatibilizers showed modest effects o f the molecular weight o f the E B block. T h e S E B and S E B S w i t h E B blocks that h a d a molecular weight o f 40,000 i m p a r t e d slightly higher fracture strains to the blends, but there were no differences between diblock a n d triblock architectures. Possibly, the molecular weight o f the Ε Β blocks was too h i g h to show the effect o f architecture. p

Summary Structural-mechanical models o f compatibilization o f L L D P E blends w i t h P S have been extended to take interfacial-failure mechanisms into account. T h e c o r e - s h e l l m o d e l w i t h a coating o f the rubbery compatibilizer on the P S parti cle is exemplified b y blends w i t h K r a t o n G . T h e interconnected-interface m o d e l w i t h the blocks selectively penetrating the L L D P E and P S phases to provide good adhesion without a rubbery coating o n the P S particle apply to the Q-series compatibilizers. Y i e l d i n g o f both interfacial models is described b y processes o f interfacial stretching and microfibrillation. Analytically, a m o d ified yield-strain approach was successful. Y i e l d i n g o f the matrix was assumed to be constrained b y the rigid P S particles to a region determined b y a yielding


350


angle, φ. Y i e l d i n g o f blends w i t h an interconnected interface conformed w e l l w i t h predictions for rigid spheres i n a plastic matrix. T h e presence o f a rubbery coating relaxed the constraint somewhat to allow for a larger yielding angle a n d hence a higher y i e l d strain. Ultimate properties o f the compatibilized blends w e r e d e t e r m i n e d b y fracture o f the microfibrillated interface. This i n turn was controlled b y interfacial characteristics such as adhesive strength, ductility o f the fibrillating material, a n d strength o f the microfibrils. A l t h o u g h these fac tors w e r e expected to d e p e n d o n compatibilizer structure, i n c l u d i n g molecular weight o f the blocks a n d diblock versus triblock architecture, the expected variations i n ultimate properties w e r e not observed.


Acknowledgments T h i s research was generously supported b y the National Science F o u n d a t i o n (Grant E E C 9 3 - 2 0 0 5 4 ) a n d the E d i s o n P o l y m e r Innovation C o r p . ( E P I C ) o f Brecksville, O H .

References 1. Li, T.; Topolkaraev, V. Α.; Hiltner, Α.; Baer, E.; Ji, X.-Z.; Quirk, R. P.J. Polym. Sci.: Part B: Polym. Phys. 1995, 33, 667. 2. Fayt, R.; Jérôme, R.; Teyssié, Ph. J. Polym. Sci.: Polym. Phys. Ed. 1982, 20, 2209. 3. Fayt, R.; R. Jérôme, R.; Teyssié, Ph. J. Polym. Sci: Part B: Polym. Phys. 1989, 27, 775. 4. Fayt, R.; Jérôme, R.; Teyssié, Ph. Makromol. Chem. 1986, 187, 837. 5. Lindsey, C. R.; Paul, D. R.; Barlow, J. W. J. Appl. Polym. Sci. 1981, 26, 1. 6. Heikens, D.; Hoen, N.; Barentsen, W.; Piet, P.; Ladan, H . J. Polym. Sci. Symp. 1978, 62, 309. 7. Barentsen, W. M.; Heikens, D . Polymer 1973, 14, 579. 8. Heikens, D.; Barentsen, W. Polymer 1977, 18, 69. 9. Theoearis, P. S. The Mesophase Concept in Composites; Springer-Verlag, Berlin, Germany, 1987; Chapter III. 10. Maurer, F.H.J. In Polymer Composites; Sedlacek, B., Ed.; W. D e Gruyter: Berlin, Germany, 1986; p. 399. 11. Nielsen, L. E. J. Appl. Polym. Sci. 1966, 10, 97. 12. Zuck, Α. V.; Gorenberg, A. Ya.; Topolkaraev, V. Α.; Oshmyan, V. G . Mech. Compos. Mater. 1988, 5, 533. 13. Duvall, J.; Sellitti,C.;Myers, C.; Hiltner, Α.; Baer, E. J. Appl. Polym. Sci 1994, 52, 195.


Toughened Plastics II - American Chemical Society

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