Toughened Plastics of Isotactic Polystyrene and ... - ACS Publications

23 Toughened Plastics of Isotactic Polystyrene and Isotactic Polypropylene Blends Downloaded by UNIV OF PITTSBURGH on June 13, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0252.ch023

Guangxue Xu and Shangan L i n Institute of Polymer Science, Zhongshan University, Guangzhou 510275, China

Toughened engineered polyolefin plastics with improved heat re sistance, increased impact strength, and no loss of stiffness were prepared by blending (1) isotactic polystyrene (iPS), (2) isotac tic polypropylene (iPP), and (3) styrene-propylene diblock copoly mer (iPS-b-iPP, styrene/propylene-40/60) with crystalline isotactic structure in each block. iPS-b-iPP was synthesized by sequential copolymerization of styrene and propylene using a NdCl -modified Ziegler-Natta catalyst. The stiffness/toughness and heat resistance of the iPS-b-iPP-iPS-iPP polyblends were higher than those of typ 3

ical high-impact polystyrene or iPP when iPS-b-iPP copolymer in the blends was 25 wt%. iPS-b-iPP copolymer was proven to be an effective compatibilizer of iPS-iPP blends by differential scanning calorimetry, dynamic mechanical thermal analysis, and morpholog ical study (scanning electron microscopy). iPS-b-iPP

enhanced in

terfacial interaction between phases of blends and reduced particle dimensions of the dispersed phase, thus improving their mechanical and thermal properties.

P

OLYOLEFINS HAVE H A D AN IMPORTANT POSITION a m o n g synthetic polymers

because o f their l o w cost, versatile properties, a n d growing applications as p o l y m e r i c films, containers, a n d pipes. However, polyolefins have l i m i t e d application i n several technologically important fields because o f their extremely p o o r stiffness a n d toughness a n d l o w heat resistance. Accordingly, there has b e e n strong interest i n recent years i n the modification o f polyolefins as engin e e r e d polyolefin plastics (1-3). A m o n g the polyolefins, engineered isotactic polypropylene (iPP) is attracting more a n d more interest (4). T h e balance between impact a n d stiffness, as w e l l as the heat-distortion temperature o f iPP, is far inferior to that o f most engineering plastics. B l e n d i n g i P P w i t h high-performance engineering resins represents an attractive route to making tailor-made 0-8412-3151-6

© 1996 American Chemical Society 351

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

352

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

Downloaded by UNIV OF PITTSBURGH on June 13, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0252.ch023

materials w i t h i m p r o v e d heat resistance, rigidity, and toughness, a n d a good performance/cost ratio (5). Several PP-based blends w i t h engineering resins, such as polycarbonate, poly(ethylene terephthalate), poly(phenylene oxide), and nylon-6, have been reported, but the results indicate that they have p o o r mechanical behavior, severe delamination, a n d weak weld-line strength i n their extruded or injection-molded parts. Admittedly, poor adhesion at the i n terface, coarse phase dispersion, a n d coalescence were considered to be m a i n ly responsible for the inferior performance (6-9). Relating the final morphology o f a b l e n d to its end-use properties is an i n teresting a n d challenging aspect o f this research area. A m o n g the parameters governing phase morphology and its stability against coalescence of polyblends are interfacial tension, w h i c h generates a small phase size (10, 11); the nature o f the interactions, or interfacial adhesion, that transmit applied force effectively between the phases (12); the viscoelastic properties o f each component o f the b l e n d ; the thermal history; the b l e n d i n g procedure; and the characteristics (molecular weight a n d composition) o f the polymers (13,14). O n e possible way o f reducing interfacial tension and i m p r o v i n g phase adhesion between P P - b a s e d b l e n d phases is to use a selected copolymeric additive that has similar components to the b l e n d , as a compatibilizer i n the b l e n d system. Well-chosen diblock copolymers, w i d e l y used as compatibilizing agents i n P P - b a s e d blends, usually enhance interfacial interaction between phases o f blends (15, 16), reduce the particle dimensions o f the dispersed phase (16, 17), a n d stabilize phase dispersion against coalescence (16-18) through an "emulsification effect," thus i m p r o v i n g the mechanical properties (15-19). To our knowledge, nothing has been reported recently o n the b l e n d o f i P P a n d high-performance isotactic polystyrene (iPS) as a toughened plastic. i P S is an attractive semicrystalline polymer characterized by low cost a n d versatile performance. T h e high m e l t i n g point (-230 °C), above-ambient glasstransition temperature (100 °C), exceedingly high stiffness or rigidity, a n d excellent heat resistance (heat-distortion temperature ^ 170 °C) a d d to the favorable profile o f properties. So there is reason to believe that the i m p r o v e d balance between impact and stiffness and the i m p r o v e d heat resistance of semicrystalline i P P c o u l d be achieved by b l e n d i n g w i t h i P S . It is necessary, however, to f i n d means of (1) avoiding macroscopic phase separation o f the i P P a n d i P S , a n d (2) grafting or otherwise i m p r o v i n g the strength o f the interfaces between the i P P a n d i P S phase. As was just discussed, these two requirements are met by the addition of block copolymer emulsifiers to the hom o p o l y m e r blends; little is k n o w n about the corresponding approach for the semicrystalline i P S - i P P b l e n d system. T h e recent successful synthesis i n our laboratory of styrene-propylene d i block copolymer (iPS-fo-iPP), w i t h crystalline isotactic structure i n each block (20, 21), makes it possible to attempt toughening of the i P S - i P P b l e n d by the m e t h o d o f block copolymer emulsification o f the diblock copolymer. This


23.

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Toughened Plastics of iPS and iPP Blends

353

chapter presents our recent work on the toughened plastics of the iPS-fci P P - i P S - i P P polyblend, and the compatibilizing effect o f iPS-fe-LPP diblock copolymer o n the mechanical properties, thermal behavior, a n d morphologies o f i P S - i P P blends.

Experimental Details Synthesis of Homopolymers and Styrene-Propylene Copolymer. The iso tactic polypropylene (iPP, molecular weight M = 300,000) and isotactic poly styrene (iPS, M = 280,000; isotactic index > 99%) used i n this study were pre pared i n a laboratory-scale reactor using a modified Ziegler-Natta catalyst (MgCl /TiCl /NdCL .(OR) /Al(iBu)3, where i B u is isobutyl), which was developed in our laboratory (20). Styrene-propylene block copolymer was synthesized by se quential copolymerization of styrene and propylene using the mo dined Ziegler-Natta catalyst; the synthesis is similar to that of the corresponding homopolymer and gives a high yield. The copolymer was separated from unwanted homopolymer species i n the reaction products using a successive fractionation pro cedure by means of the difference of solubility and crystallizability in different sol vents (21). In contrast experiments on two homopolymers (corresponding ho mopolymer blends and copolymerization products), atactic polystyrene (aPS), atactic polypropylene (aPP), and iPS were separated from the pure copolymer by successive extraction fractionation using boiling methyl ethyl ketone, heptane, and chloroform, respectively, and i P P homopolymer i n copolymerization products was also isolated from pure copolymer by crystallization at 110 °C i n a-chloronaphthalene. Thus the remaining soluble fraction in α-chloronaphthalene at 110 °C was confirmed to be pure copolymer species, with 40.5% by weight of the original re action products. The copolymer contained 4 0 % PS as determined by IR spec troscopy and elemental analysis. G e l permeation chromatographic (GPC) experi ments gave the copolymer molecular weight as 340,000 g/mol. w


w

2

4

l

!

Preparation of iPS-fo-iPP-iPS-iPP Polyblends. The blends of iPS-fo-iPP, iPS, and i P P were prepared by completely dissolving all polymers and an antioxi dant into o-dichlorobenzene at 165 °C, and using a 1:1 mixture of acetone and methanol as a precipitant. After being thoroughly washed and dried under vacuum at 60 °C for 20 h, the precipitated powders were compression-molded at 300 °C into sheets or plates suitable for cutting spécimens for mechanical testing and morphologic study. Measurements. The structure of the block copolymer was determined by N M R using a J E O L F X - 9 0 Q instrument. The glass-transition temperature (T ) and melting temperature (T ) of the copolymer and the blends were measured by differential scanning calorimetry (DSC) (Perkin Elmer, D S C - 2 ) at a heating rate of 20 °C/min. The dynamic mechanical properties were determined by dynamic mechanical thermal analysis using a Rheovibron D D V - I I E A instrument. Measurements were made at an operating frequency of 110 H z and a heating rate of 3 °C/min under a liquid-nitrogen atmosphere. The morphologies of the polymer blends or the copolymer were studied with a scanning electron microscope ( S E M , Hitachi S-570). Fracture surfaces were prepared at either room temperature or liquid-nitrogen temperature. Tensile properties, Youngs modulus, Izod impact strength, and Rockwell hardness of the compression-molded specimens were determined by the standard g

m


354


procedures described i n A S T M D638, D790, D256, and D785, respectively. The heat-distortion temperature ( H D T ) was measured under 980 and 9.8 χ 10^ P a at heating rates of 2 and 20 °C/min, respectively.

Results and Discussion


Characterization o f the Styrene-Propylene Copolymer.

The

synthesis o f styrene-propylene block copolymer w i t h crystalline isotactic structure i n a polystyrenic block has been an interesting research topic from b o t h academic a n d practical points o f view. However, little is found about styrene-propylene block copolymer synthesis i n the literature (22), and the structure a n d properties o f the block copolymer have not yet been clarified. T h u s , p r i o r to studying its effect as a compatibilizer o n i P S / i P P blends, exten sive structural characterization was carried out o n the p u r i f i e d styrene-propy lene copolymer. F i g u r e 1 presents the D S C scan of the copolymer. T h e two glass transitions at T = - 2 and 98 °C indicate segregated P P a n d P S microdomains. Also, the D S C analysis o f the copolymer exhibits the two strong first-order transitions at 162 a n d 220 °C, suggesting that considerable amounts o f erystallinity exist i n re spective i P P a n d i P S blocks or segments. T h e melting temperature (T ) o f each segment i n the copolymer was found to be lower than those o f corresponding isotactic homopolymers d e t e r m i n e d under similar conditions. O n the basis o f the D S C analysis a n d wide-angle X-ray scattering, we estimate the crystalhnity o f P P a n d P S blocks i n the copolymer to be 35.1 a n d 14.6%, respectively. g

m


23.

Xu AND LIN.

δ 01-146.72(146.88) δ 02-127.60(128.00) δ (33-41.60(41.78) δ C4-44.04(44.08)

Ô 06-28.92(29.00) δ 06-21.82(21.79) δ 07-46.62(46.63)


355


6

0H

a

1

-(CB-CH,)* -(CH-CH.)* 3 4 6 7

25

PPM

Figure 2. C-ΝMR spectrum of the pure copolymer (iPS-h-iPP) at 130 °C in odichlorobenzene. Numbers in parentheses denote corresponding carbon peaks in the homopolymer. 13

As shown i n F i g u r e 2, C - N M R analysis o f the copolymer gave u n a m biguous signals o f a block copolymer w i t h two long sequences o f essentially p u r e i P S a n d i P P (iPS-fe-iPP). Isotactic triads o f each block were about 9 8 % . Because of little measurable shift of peak locations, w h i c h might be interpret e d i n terms of the repeat unit interaction o f a styrene-propylene sequence as c o m p a r e d w i t h respective homopolymers, the copolymer characterized i n this study is considered to be a simple mixture o f the two homopolymers. B u t the results o f successive fractionation, as discussed i n the section " E x p e r i m e n t a l D e t a i l s , " c o u l d eliminate the possibility of homopolymer mixtures. I n the transmission electron microscopic ( T E M ) studies, we found that it was exceedingly difficult to obtain ultramicrotomed sections o f i P S - i P P blends, whereas the iPS-fc-iPP diblock copolymer c o u l d b e cut w i t h relative ease. This result exhibits one major difference between the diblock copolymer a n d the corresponding homopolymer blend. Unfortunately, owing to the diffi culty of finding a selective staining technique, the sample of diblock copolymer d i d not display visible contrast or obvious structural features i n the T E M stud ies. However, the results o f S E M studies do reveal a clear difference between the b l e n d a n d the diblock copolymer; the macrophase separation is revealed o n the etched surface o f the b l e n d and is not present i n the copolymer (Figure 3). T h e diblock copolymer exhibits only a finely dispersed a n d continuous subm i c r o n structure throughout the field of view, as expected. 1 3



356


Figure 3. SEM images of cast films of (a) iPS-h-iPP copolymer (40/60), and (b) iPS-iPP blend (40/60). The surface of the casting films was etched with allylamine vapor at room temperature for 1 h. The scale is 20 pm. A s c o m p a r e d w i t h the homopolymers a n d the corresponding homopoly m e r b l e n d , the copolymer also shows significant differences i n dynamic me chanical behavior (Figure 4). T h e b l e n d shows two distinct transitions at 0 a n d 98 °C, w h i c h result from the glass transitions o f the respective h o m o p o l y m e r components. T h e b l e n d exhibits a rapid drop i n modulus w i t h increasing t e m perature, whereas the copolymer exhibits a more gradual drop i n modulus, suggesting that the phases o f two blocks or segments are w e l d e d together to a greater extent i n the copolymer.

ι / ι

»

-60

ι

I

1

V

1

L

20 100 Temperature^)

Figure 4. Dynamic mechanical spectrum of (a) iPS, (b) iPS-b-iPP (40/60), (c) iPS-iPP (40/60) blend, and (d) iPP.

copolymer


23.

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357



Significant differences i n mechanical properties are also seen i n the copolymer b y c o m p a r i n g corresponding homopolymers or h o m o p o l y m e r blends (Table I). As seen i n the table, the diblock copolymer has superior tensile strength and hardness to iPP. T h e I z o d impact strength o f the copolymer is higher than that o f homopolymers and homopolymer blends. T h e results are perhaps interrelated w i t h the stereoregular structure, crystallinity, a n d m i crophase morphologies o f the copolymer. E v i d e n c e that the phases are w e l d e d together and "interpenetrated" or cross-linked to a greater extent i n the copolymer may be inferred from the increased modulus, I z o d impact strength, a n d ultimate elongation. Taking all the fact presented i n this section into account, together w i t h the synthesis m e t h o d a n d fractionation results, we conclude that the p u r i f i e d copolymer separated from reaction products is an iPS-fe-iPP diblock copolym e r consisting o f i P S and i P P blocks; it is definitely not a simple b l e n d o f homopolymers. O n the other hand, the distinctive characteristics o f the copolym e r s crystallization kinetics also indicate that, compared w i t h homopolymers a n d the i P S - i P P b l e n d , the p u r i f i e d copolymer is a true iPS-è-iPP diblock copolymer (23). T o u g h e n e d Plasties o f i P S - f e - i P P - i P S - i P P P o l y b l e n d s . The molecular weight and block composition o f the diblock copolymer are certainly key thermodynamic criteria i n designing the most efficient compatibilizer for p o l y m e r blends as toughened plastics (24). Accordingly, we deal here w i t h the iPS-fo-iPP diblock copolymer w i t h a composition (styrene/propylene, S/P) o f 40/60 a n d a molecular weight o f 340,000 g/mol. To establish a connection between mechanical or thermal properties and observed morphological changes, ternary blends were made by following the isopleth o f the triangular phase diagram (Figure 5). A l o n g the isopleth, all b l e n d materials contain equal weight percentages of propylene and styrene repeat units, while the copolym e r content varies from 0 to 100%.

Table I. Mechanical Properties of the Styrene-Propylene Diblock Copolymer Unnotched Tensile Young's Izod Impact Strength Hardness Ultimate Modulus Strength at Yield (Rockwell Elongation (MPa) (KJ/M ) (MPa) R Scale) (%)

Sample

2

iPS iPS-fc-iPP iPS-iPP blend (40/60) iPP

HeatDistortion Temperature (°C)

0

2250 1890 1595

2.5-4.0 38.5 4.1

42.9 35.8

—

102 80 84

3-5 302 2-3

210 168 150

1050

27.5

23.4

70

898

140

NOTE: The molecular weight of the copolymer is 340,000 g/mol (by G P C ) . "Measured under 980 Pa at a heating rate of 2 °C/min.


358



iPS-b-iPP

Figure 5. Triangular phase diagram showing the composition of the iPS-b~ iPP-iPS-iPP blends examined.

Mechanical

Properties.

Table II shows the mechanical properties (i.e.,

Youngs modulus, I z o d impact strength, tensile strength, R o c k w e l l hardness, elongation at break, a n d H D T ) o f several i P S - Z ? - i P P - i P S - i P P polyblends. T h e incorporation of the diblock copolymer i n a 5 % amount does make the impact strength o f the i P S - i P P b l e n d rise enormously. I n addition to noting the clear t r e n d o f increased impact resistance w i t h increased copolymer content, we note that the impact strength o f the i P S - i P P b l e n d containing 2 5 % copolymer already exceeds that o f high-impact polystyrene ( H I P S ) , a n d at high copolym e r contents the impact strength is approximately double that o f a typical H I P S . These results indicate that the dispersed phase may not only adhere to

Table II. M e c h a n i c a l Properties o f i P S - f c - i P P - i P S - i P P Polyblends Weight ratio ofiPS-b-iPPiPS-iPP 0/40/60 5/38/57 10/36/54 25/30/45 50/20/30 75/10/15 100/0/0 0/100/0

Unnotched Youngs Izod Impact Modulus Strength (MPa) (KJ/M ) 2

1590 1601 1586

Tensile HeatStrength Hardness Ultimate Distortion at Yield (Rockwell Elongation Temperature" (MPa) R Scale) (°C) (%)

1580

4.1 8.4 16.0 24.3

23.3

1595

29.1

29.0

82

38.2

155

1608 1880

33.5 38.5

30.0

82

48.2

160

80 102

168 210

0/0/100

3.5 27.5

35.8 42.9

302.2

2120 1030

HIPS

1160

18.5

B

6.3 12.1 16.2

84 84 84 82

23.4

70

19.2

65

2-3 7.6 16.5 27.5

3-5 808 25.5

N O T E : The weight ratio of two blocks in iPS-2?-iPP copolymer is 4 0 / 6 0 (iPS/iPP).

Measured under 9 8 0 Pa at a heating rate of 2 °C/min. ^From Jinling Chemical Co., Ltd., China. a


150 150 150 153

140 80

23.

Xu AND LIN.

359



the surrounding glassy polymer but also be cross-linked (25). T h e i m p r o v e d adhesion i n blends w i t h a small amount of diblock copolymer retards mechanically i n d u c e d slippage between the i P S and i P P phases, thus enhancing the impact strength o f the polyblends. E n h a n c e m e n t o f mechanical properties is of interest only i f it is not acc o m p a n i e d by a loss o f other important properties o f the b l e n d . O f particular concern for such polymer blends is stiffness, because most means of increasing impact strength also reduce stiffness (14-19). B u t this is not the case for the i P S - i ? - i P P - i P S - i P P blends studied here as seen i n Table II. It is clear that the enhancement i n toughness just described is not accompanied by a loss o f stiffness, but it is essentially unaffected by the compatibilizer. A n d the stiffness of i P S - f e - i P P - i P S - i P P is higher than that of i P P and H I P S . T h e impact-modulus behavior seems to be due to the " t o u g h " (or rigid) characteristics, morphologies o f phases, and semicrystalline isotactic structure of each block i n the i P S fc-iPP diblock copolymer. T h e addition o f the diblock copolymer to the i P S - i P P blends also significantly enhances b o t h tensile strength and ultimate elongation (Table II). A s b o t h i P S and i P P have higher tensile strengths than the 40/60 iPS/iPP blends, it is feasible to suppose that the phase boundary is the weakest spot i n these blends. T h e fact that the addition o f the copolymer contributes to enhancing tensile strength a n d ultimate elongation also suggests i m p r o v e d interfacial adhesion o f the b l e n d through the "emulsification effect" of the diblock copolymer. O f considerable interest is the fact that added i P S i n P P resin can improve the heat resistance o f the materials and thus upgrade their endurance at elevated temperature. Also, as seen i n Table II, the H D T of i P S - i P P blends is, as expected, higher than that o f c o m m o n H I P S and iPP. T h e results seem to be interrelated w i t h the isotactic structure and crystallinity o f the i P S component. T h e iPS-fo-iPP that is added contributes to a further improvement i n the H D T o f the i P S - i P P binary blend. F i g u r e 6 shows that, at lower measured pressure, the H D T s increase slowly w i t h increasing iPS-Z?-iPP copolymer. T h e H D T s o f the blends, however, increase rapidly at higher measured pressure as iPS-foi P P copolymer is increased. Thermal Properties. Modifications o f the thermal behavior o f p o l y m e r systems, particularly i n the temperature and breadth of various transitions o f state, are often used to show changes i n their morphology and miscibility (26). Differential scanning calorimetric ( D S C ) thermograms o f i P S - f o - i P P - i P S - i P P ternary blends are shown i n F i g u r e 7. Binary blends ( i P S - i P P ) are clearly i m miscible, as evidenced by the presence of two distinct glass-transition temperatures (Tgs) o f the respective homopolymer. I n F i g u r e 7 it is seen that, owing to the addition o f iPS-fc-iPP diblock copolymer i n the blends, T of i P S decreases, whereas T o f i P P increases slightly, and then T and T t e n d to bec o m e closer as the amount of the diblock copolymer is increased. These results g l

g 2

g l

g 2



360


C

I 20

1

IPS-b-iPP

Figure 6. Heat-distortion

I 60

1

1

100

copolymer(wt%)

temperature of iPS-b-iPP-iPS-iPP

polyblends.

suggest the occurrence o f m u t u a l "dissolution" or cross-linking o f i P S a n d i P P segments o f the homopolymers w i t h the diblock copolymer i n the blends. It is w e l l k n o w n that the t h i r d components, although small i n quantity, might also b r i n g about other significant changes such as changes i n crystalline behavior, morphology, and dynamic mechanical properties, w h i c h influence the b l e n d properties for e n d use. Table III lists the data obtained b y D S C for i P S - f c - i P P - i P S - i P P ternary blends. T h e results indicate that P P crystallization

J

0

/i

'

1

1

40

70

L-

100

Temperature('C) Figure 7. DSC thermograms of iPS-b-iPP-iPS-iPP ternary blends: (a) 0/40/60; (b) 5/38/57; (c) 10/36/54; (d) 25/30/45; and (e) 50/20/30.


23.

Xu AND LIN.


361

Table III. DSC Melting and Crystallization Results for Homopolymers and iPS-fc-iPP-iPS-iPP Blends Melting Point, T f°c;

Crystallization Peak, T (°C)

Degree of Undercooling fT -TJ

iPP

165.0

107.0

58.0

i P P - i P S (90/10) i P P - i P S (75/25) i P P - i P S (60/40) iPP-iPS-iPS-fc-iPP iPP-iPS-iPS-fc-iPP iPP-iPS-iPS-fc-iPP iPP-iPS-iPS-fc-iPP

164.2

107.0

57.2

164.7

107.1

57.6

163.5

106.8

56.7

164.0

116.8

47.2

164.2

120.2

44.0

164.1

122.4

41.7

164.3

124.0

40.3


Sample and Composition

TO

(80/15/5) (57/38/5) (45/30/25) (30/20/50)

m

c

is not affected by the presence o f an i P S phase i n the binary b l e n d , but that P P crystallization is affected b y the presence o f an i P S phase i n ternary blends. That is, the melting temperature (T ) as w e l l as the crystallization temperature (T ) o f the P P phase i n a binary b l e n d are unchanged w i t h increasing i P S c o n tent, w h i c h indicates that there is no miscibility or entanglement between the two phases. However, the addition o f iPS-Z?-iPP copolymer to the blends can affect the P P crystallization: T o f the P P phase increases significantly, while T is almost unchanged, resulting i n a lower degree o f supercooling a n d h i g h er crystallization. It is believed that iPS-fo-iPP copolymer can function as a fair nucleation agent to cocrystallize w i t h the P P phase and then increase the P P crystallization temperature. O f course, the diblock copolymer c o u l d also possi b l y act as a nucleating agent i n the role o f an i m p u r i t y to increase T o f the P P a n d reduce the degree o f undercooling o f P P crystallization. A s seen from Table III, at a given amount of iPS-fo-iPP copolymer added, T of the P P phase also tends to increase w i t h increasing i P S content. These results provide i n sight into the level o f interaction between the b l e n d components and may also suggest the m u t u a l interaction o f the i P S and i P P segments o f the homopoly m e r w i t h the i P S - b - i P P diblock copolymer. m

c

c

m

c

c

D y n a m i c mechanical thermal analysis ( D M T A ) is often more sensitive than D S C i n identifying the glass-transition temperature (T ) o f blends (27). T h e D M T A data can b e assumed to closely reflect the phase behavior o f blends. F i g u r e 8 shows the results o f D M T A for the 40/60 i P S - i P P b l e n d . T h e complex modulus i n D M T A ( Ε ' ) o f the unmodified b l e n d is slightly enhanced w i t h increasing iPS-fe-iPP copolymer content. A t 2 5 % copolymer, the modulus decreases slightly but remains at the modulus o f the unmodified b l e n d . A s shown i n F i g u r e 8, two distinct tan δ peaks are discernible. T h e lower one is due to T o f iPP, while the peak at 98 °C is due to T o f i P S . T h e diblock copolymers slightly modify the two transition temperatures. As the i P S - & - i P P copolymer content increases, the tan δ peaks o f two individual components g

g

g



362


I

1

1

1

1

1

-60

-20

20

60

100

U

TemperatureCC) Figure 8. Dynamic spectra of (a) a binary blend of iPS-iPP (40/60), (b) a polyblend of iPS-b-iPP-iPS-iPP (5/38/57), and (c) an iPS-b-iPP-iPS-iPP blend (25/30/45).

shift progressively toward each other, and the peaks become broader. This ef fect may be attributed to emulsification of the diblock copolymer, thus p r o m o t i n g partial miscibility o f i P S - i P P blends. Morphology. T h e change i n the thermal and mechanical properties o f the blends containing the diblock copolymer may be closely related to their morphology. F i g u r e 9 shows S E M images of fracture surfaces of (1) the i P S - i P P binary b l e n d , and (2) the i P S - f e - i P P - i P S - i P P (5/38/57) ternary blend). B o t h samples were prepared by impact fracture at room temperature and then etched w i t h allylamine vapor for 1 h to remove the PS phase. A s seen i n F i g u r e 9a, the P S domains have a defined spherical shape w i t h diameters ranging from 1 to 40 μηι. I n addition, through selective etching, many domains are p u l l e d away from their previous positions and the surfaces of holes left b y the P S phases removed appear to be clear. T h e lack of adhesion between i P S and i P P is obvious i n F i g u r e 9, where dispersed particles do not adhere to the m a trix a n d leave cavities w i t h a smooth surface. T h e morphology is strikingly modified by adding as little as 5 w t % iPS-fc-iPP copolymer (Figure 9b). T h e component i P P no longer forms dispersed particles but a continuous thin net w o r k (entanglement) or fine lamellar bundle firmly anchored into the i P S m a trix by emulsification of iPS-è-iPP copolymer. This result indicates that the d i block copolymer exhibits a p r o n o u n c e d interfacial activity that reduces the particle size of individual phases and enhances interfacial adhesion between i P S and iPP.



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Figure 9. SEM images of the fracture surface of (a) a binary blend of iPS-iPP (40/60), and (b) a polyblend of iPS-b-iPP-iPS-iPP (5/38/57). Both specimens were etched with allylamine vaporfor 1 h. The scale is 20 pm.

T h e S E M results further support these findings (Figure 10). T h e observed surface morphologies, obtained by fracturing specimens i n l i q u i d nitrogen, reveal that smooth surfaces o f i P S - i P P b l e n d are seen o n the larger p r o trusions a n d craters formed d u r i n g fracturing o f the surface (Figure 10a). After the addition o f i P S - b - i P P copolymer, the surface morphologies o f blends reveal a different behavior; a pattern o f ever-smaller scale or roughness is observed as copolymer content increases (Figure l O b - e ) . A t higher concentrations o f the diblock copolymer (25 and 50%), the surface of these materials exhibits a level of roughness (finer scale) that is not seen at lower copolymer contents. This roughness or smaller particle size results from the stretched and b r o k e n fibrils o f material, w h i c h form d u r i n g the fracturing process. Stretched a n d b r o k e n " f i b r i l s " o f materials appear to span the interfaces between regions o f i P S a n d iPP. T h e dispersed particles have been broken i n the plane o f fracture, b u t no slippages have occurred. Apparently, the sequences are firmly anc h o r e d into the domains they penetrate. W h e n the percentage of diblock copolymer increases, it is impossible to identify individual phases i n Figures 9 a n d 10. Similar observations have been made b y Heikens a n d co-workers (28) for a P S - P S - g ( f c ) - P E - L D P E b l e n d and b y D e l G i u d i c e et al. (29) for an i P S - i P S - f c - i P P - i P P b l e n d system. These results demonstrate the compatibilizi n g effect o f iPS-fe-iPP o n i P S - i P P b l e n d . M o r p h o l o g i c a l study, together w i t h D M T A and D S C results, confirms the expectation o f miscibility of the diblock copolymer w i t h each component o f the b l e n d . This miscibility occurs at the interphases between the components o f blends, allowing enhanced interphase interactions and better stress transfer i n the b l e n d system. T h i s is probably due to the anchoring o f each sequence o f the b l o c k w i t h its corresponding component o f the b l e n d , w h i c h is i n good




364

Figure 10. SEM images of the fracture surface of (a) a binary blend of iPS-iPP (40/60), (b) a polyblend ofiPS-b-iPP~iPS-iPP (5/38/57), (c) an iPS-b-iPP-iPS-iPP blend (10/36/54), (d) an iPS-b-iPP-iPS-iPP blend (25/30/45), and (e) an iPS-biPP-iPS-iPP blend (50/20/30). The scale is 20 pm.


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agreement w i t h what is inferred from the mechanical properties of the iPS-fci P P - i P S - i P P polyblends.

Summary and Conclusions B y sequential copolymerization o f styrene a n d propylene using a m o d i fied Z i e g l e r - N a t t a catalyst, M g C l / T i C l / N d C l ( O R ) / A l ( i B u ) 3 , w h i c h was de 2

4

c

!

v e l o p e d i n our laboratory, a styrene-propylene block copolymer is obtained. After fractionation b y successive solvent extraction w i t h suitable solvents, the copolymer was subjected to extensive molecular a n d morphological character ization using C - N M R , D S C , D M T A , and T E M . T h e results indicate that the


1 3

copolymer is a crystalline diblock copolymer o f i P S a n d i P P (iPS-fe-iPP). T h e diblock copolymer contains 4 0 % i P S as d e t e r m i n e d by F o u r i e r transform i n frared spectroscopy a n d elemental analysis. A toughened polyolefin-engineered plastic w i t h i m p r o v e d heat resistance a n d increased impact strength, and w i t h no loss of stiffness, was prepared b y b l e n d i n g (1) i P S , (2) iPP, a n d (3) iPS-fc-iPP diblock copolymer

(styrene/propy-

lene ratio is 40/60). T h e mechanical properties a n d stiffness o f the iPS-fei P P - i P S - i P P polyblends were higher than those o f i P P a n d H I P S w h e n iPS-fci P P copolymer i n the blends was 25 wt%. Because of the outstanding tensile modulus a n d heat resistance o f i P S , added i P S i n the i P P resin containing a given amount o f iPS-fc-iPP diblock copolymer can enhance the heat resistance. T h e t h e r m a l properties are also superior to those o f H I P S a n d iPP. E n h a n c e d interphase interactions, deduced from thermal a n d dynamic mechanical properties a n d morphology observed by S E M , demonstrate the ef ficient compatibilizing effect o f iPS-fe-iPP copolymer o n i P S - i P P blends. E a c h sequence o f the iPS-fc-iPP diblock copolymer can probably penetrate or easily anchor its h o m o p o l y m e r phase a n d provide important entanglements, improv i n g the miscibility a n d interaction between the i P S and i P P phases. T h i s is i n good agreement w i t h what is inferred from the mechanical properties o f the i P S - i ? - i P P - i P S - i P P polyblends.

Acknowledgments T h i s w o r k was supported b y the National N a t u r a l Science F o u n d a t i o n o f C h i na a n d the F o u n d a t i o n o f Specialties O p e n e d to D o c t o r a l Studies o f the State Education Committee, China.

References 1. 2. 3. 4.

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