Toughness and Brittleness of Plastics

25 Impact Modification of Polysulfone with Polysulfone/Poly(dimethylsiloxane) Block Copolymers Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0154.ch025

A. NOSHAY, M. MATZNER, B. P. BARTH, and R. K. WALTON Research and Development Department, Chemicals and Plastics, Union Carbide Corp., Bound Brook, N. J. 08805

Polysulfone/poly(dimethylsiloxane) block copolymers (PSF/ PSX) of controlled structure and composition are very effective additives for increasing the notched impact strength of polysulfone. This is attributed to a unique combination of PSF/PSX characteristcis: morphology, compatibility, elastomeric nature, and thermal stability. The optimum effect is achieved with copolymers containing 5000 M PSF and PSX blocks at block copolymer concentrations of 5 wt % and with blending conditions that produce a PSF/ PSX maximum particle size of 0.5-3.0 μ. Under these conditions good quality injection molded parts can be produced with notched Izod impact strengths as high as 22.0 ft-lbs/ inch, compared with 1.3 ft-lbs/inch for unmodified polysulfone. The other properties of polysulfone are affected to only a minor extent by this modification. n

*Tphe impact strength of thermoplastic resins can be improved by blending with flexible polymeric additives or by synthesizing the resin in the presence of a rubber modifier to produce an in-situ graft copolymer. Since the latter technique is more amenable to addition polymers such as polystyrene (1) than to step-growth condensation polymers, impact modification of polysulfone was investigated via the additive blending approach. Polysulfone/poly(dimethylsiloxane) block copolymers are ideal additives for this purpose. Other block copolymers have also been reported as useful additives to improve the properties of the corresponding homopolymers. For example styrene/butadiene block copolymers, especially when used with peroxide crosslinking agents, improve the impact 302 In Toughness and Brittleness of Plastics; Deanin, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0154.ch025

25.

NOSHAY E T A L .

Impact

303

Modification

strength of polystyrene (2), and polycarbonate/siloxane block copolymers reduce the wettability and frictional properties of polycarbonate resins (3). Polysulfone is a high performance engineering thermoplastic which displays good toughness in the form of flat sheets. However it is notch sensitive—i.e., it displays reduced toughness in molded items containing sharp corners, holes, etc. The notched Izod impact strength has been improved from 1.3 ft-lbs/inch to as high as 22.0 ft-lbs/inch by incorporating 5% of a polysulfone/poly(dimethylsiloxane) block copolymer (hereafter abbreviated as PSF/PSX). This improvement has been achieved without appreciably sacrificing processability and physical properties because of the unique morphology, compatibility, elastomeric nature, and thermal stability of PSF/PSX block copolymers. Many other additives including dimethylsiloxane homopolymers were investigated and found to be inferior to PSF/PSX because of the absence of one or more of these characteristics. This paper successfully demonstrates the concept of impact modification of a thermoplastic resin via the tailoring of a block copolymer structure designed to achieve a difficult combination of requirements. Experimental The synthesis of polysulfone/poly(dimethylsiloxane) block copolymers was reported earlier (4, 5). Blends of the block copolymer with CH

H

CH

3

°-OtO( ^ ^ D - ? - O ) 0

CH

S0

3

0H

CH

+

(CH ) N - SSii 4 -^OSi-^-N C3 H)^ 0 S i - + N ( C(H I \ I /b CH C H 3

a

0H

3

w

2

2

3

70—I20°C

3

+

Figure 1.

3

(CH ) NH 3

2

Synthesis of polysulfone-poly(dimethylsiloxane) block copolymers

In Toughness and Brittleness of Plastics; Deanin, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.


304

TOUGHNESS A N D BRITTLENESS

O F PLASTICS

polysulfone were prepared by solid or solution blending techniques. Solid blending was carried out for 5-10 min in a Brabender plastograph at ~ 2 8 0 ° C or in a steam-heated Banbury mixer. Solution blending was carried out in a Marshall mill to mix and to strip off the solvent simultaneously. Compression molding was carried out at 290°C, and injection molding was performed at 3 1 5 ° - 3 7 0 ° C . Tensile properties, notched Izod impact strengths, and tensile impact strengths were determined via ASTM Methods D638, D256, and D1822. Discussion The synthesis of PSF/PSX block copolymers described earlier (4) is carried out by the interaction of hydroxyl-terminated polysulfone oligomers with dimethylamino-terminated poly (dimethylsiloxane) oligomers (see Figure 1). The length of the PSF and PSX blocks in the copolymer, which are controlled by varying the molecular weight of the oligomers, greatly influences the composition and thus the properties of the block


25.

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Impact Modification

copolymer (4,5,6,7,8). Copolymers of short block length display single phase morphology, but those with block molecular weights (M ) of 5000 or greater display two-phase morphology. This is demonstrated by the presence of two glass transitions in the two-phase systems characteristic of each of the segments (see Figure 2). Mechanical properties, which depend on the weight fraction of the segments in the copolymer, are summarized in Table I. Compositions ranging from rigid to elastomeric can be achieved by varying the PSX content from 10-79 wt % . The elastomeric nature of the high PSX-content materials is attributed to a physical network which is an outgrowth of the two-phase morphology of the systems (4,5,6,7,8). The preferred compositions for impact modification of polysulfone are those containing 5000 M PSF and PSX blocks since they exhibit the best balance of required properties. A proper degree of additive compatibility with a matrix resin is necessary to achieve optimum impact modification (9). Excessive compatibility reduces energy absorbing capacity and therefore causes poor impact strength. On the other hand gross incompatibility leads to "cheezy" blends and inefficient utilization of the impact modifier. The PSF/PSX block copolymers display just the proper degree of compatibility with PSF homopolymer. The PSX segments are highly incompatible with the polysulfone matrix resin because of a large difference in their solubility parameters, 7.3 and 10.6 (8). However the PSF segments of the block copolymers are of course soluble in the chemically identical polysulfone matrix resin. This provides an ideal mechanism for achieving a high degree of additive dispersion and adhesion between the resulting dispersed particles and the surrounding matrix resin. This is shown schematically in Figure 3. As a result, the PSF/PSX block copolymer disperses very easily in polysulfone when compounded via solid or solution blending techniques. These blends, after molding, contain PSF/PSX particles ranging from < 0.5 to 8.0 jx (see Figure 4). By way of comparison, the optimum particle size in impact modified polystyrene is 1-5 fi (1).


n

n

Table I. Block M

Effect of Block M on Block Copolymer Properties n

n

Polysulfone

Poly(dimethylsiloxane)

4,700 4,700 9,300 4,700 4,700 6,500

350 1,700 4,900 5,100 9,200 25,000

Wt % Siloxane

10 28 41 55 67 79

Tensile Modulus (psi)

240,000 170,000 29,000 20,000 2,000 300

Tensile Strength

6,000 4,700 2,700 2,400 1,300 900

ElongaHon (%)

5 12 150 350 500 550

(°(7) +125 + 140 -110; +170 •120; +160 •120; +160 •120; +160


TOUGHNESS A N D BRITTLENESS


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O F PLASTICS

PSF BLOCK W

^ Figure 3.

A

^

A

A

psx ^PSF

BLOCK H0M0P0LYER MATRIX

Compatibility of PSF blocks with PSF matrix

Another attribute that makes the PSF/PSX block copolymer a good impact modifier for polysulfone is its highly elastic nature. This arises from the unique rheological behavior of the material. Its melt viscosity and melt elasticity are so high that it cannot be extruded into filaments even at temperatures (e.g., > 3 0 0 ° C ) that are well above the glass transition temperature of the PSF segment ( 1 6 0 ° C ) . This unusual behavior for linear, soluble polymers is believed to arise from an extraordinarily stable physical network structure which persists even in the melt. This, in turn, is attributed to the high degree of phase separation in the PSF/PSX block copolymer which results from the large differential (A) in segment solubility parameter (8). All attempts to extrude the block copolymer led to extreme melt fracture resulting in "fluffy" extrudates similar in appear-

Figure 4.

PSF/PSX

dispersion in polysulfone (5% injection-molded blend observed at 172X)

Left: < 0.5 /*; Middle: < 3.0 p; Right: < 8.0



25.

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307

ance to the material before extrusion (see Figure 5). These rheological characteristics however do not adversely affect the melt processability of blends of PSF/PSX with polysulfone since the block copolymer is present in the dispersed phase. On the other hand the high melt viscosity and elasticity within the dispersed particles of linear PSF/PSX block copolymer results in rheological behavior which is similar to that displayed by the crosslinked grafted rubber modifiers present in impact-modified polystyrene. An important consequence of this behavior is retention of particle size integrity during high shear melt processing. Since polysulfone is often melt processed at high temperatures (up to 3 7 0 ° C ) , another requirement of a successful impact modifier is that it display adequate thermal stability at these temperatures. As expected from the known good thermal stability of polysulfone and silicones (10, 11), the PSF/PSX block copolymer displays excellent thermal stability. This is illustrated by the thermogravimetric analysis curves shown in Figure 6.

Figure 5. Highly melt-fractured extrudate of PSF/ PSX. Left: before extrusion; right: after extrusion at 340°C. Impact strength and melt processability are affected by the composition, concentration, and molecular weight of the PSF/PSX block copolymer. Low notched Izod impact values were obtained with 5% concentrations of high modulus block copolymers that contained short PSX blocks (e.g., M = 1 7 0 0 ) . However uniformly high values (16-20 ft-lbs/inch) were obtained with copolymers containing 5000-10,000 M PSF and PSX blocks (see Table II). Although some copolymers had high impact strengths at 3% concentration levels, the 5% level provided consistently high values (see Table III). The range of PSF/PSX block copolymer molecular weight capable of producing high impact strengths is quite broad, covering reduced viscosity levels of 0.5-1.3 (see. Table IV). n

n



308

TOUGHNESS A N D BRITTLENESS O F PLASTICS

20 10 0

I

1

0

100

l 200

I 300

I 400

1

1

1

500

600

700

1—

800

TEMPERATURE (°C)

Figure 6. TGA of polysulfone-poly(dimethylsiloxane) block copoly mers ana homopolymers (in nitrogen at 10° C/min) Table II.

Effect of PSF/PSX Composition on Impact Strength Block M

PSF

1

0

n

wt% PSX

PSX

6,600 5,000 5,000 10,000 10,000

Notched Izod Impact Strength of 5% Blend (ft-lbs/inch)

1,700 5,000 10,000 5,000 10,000

20 50 67 33 50

1.6 20.4 20.5 15.9 18.6

Compression molded specimens. Table III.

Effect of PSF/PSX Concentration on Impact Strength

% PSF/PSX Concentration

Notched Izod Impact (ft-lbs/inch) 5,000/5,000 PSF/PSX

3 4 5

11.0 16.0 20.4

Strength

0,

10,000/10,000 PSF/PSX

2.0 3.1 18.6

° Compression molded specimens.


NOSHAY E T A L .

25.

Table IV.

Effect of PSF and PSF/PSX Molecular Weight

PSF Melt Flow (dgm/min


a

309

Impact Modification

44 psi @ S4S°C)

5,000/5,000 PSF/PSX Reduced Viscosity (0.2% Soln in CH Cl )

Notched Izod Impact Strength of 5% Blend (ft-lbs/inch)

7 7 7 7 17

0.3 0.5 1.0 1.3 1.0

1.7 17.6 17.8 18.5 17.4

2

2

a

Injection molded specimens. Table V . 5,000/5,000 PSF/PSX Maximum Particle Size in Molded 5% Blend (/x) a

Effect of PSF/PSX Degree of Dispersion Injection Molded Notched Izod Impact Strength (ft-lb/inch)

Toughness and Brittleness of Plastics

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