Disperse structure and mechanical properties of extruded polypropylene-polystyrene blends

Structure and properties of extruded P P - P S blends

1825

t,lmir testing and also Prof. M. R. Kamal who kindly gave us the opportunity to test the viscoelastic properties of the polymers with the Rheometrix instrument. REFERENCES 1. H. VANOEN, J. Colloid Interface Sei. 40: 448, 1972 2. V. N. KULEZNEV, Polymer Blends (in Russian) p. 304, Khimiya, Moscow, 1980 :L V. N. KULEZNEV, A. V. GRACHEV and Yu. P. MIROSHNIKOV, Kolloid. zh. 38: 265, 1976 4. Yu. P. MIROSHNIKOV, A. M. GOL'MAN and V. N. KULEZNEV, 1bid. 41: 1120, 1979 .5. M. V. TSEBRENKO, T. I. ABLAZOVA, A. V. YUDIN and G. V. VINOGRADOV, Ibid. 38: 200, 1976; N. P. KRASNIKOVA, E. V. KOTOVA, G. V. VINOGRADOV and V. PELZBAUER, J. Appl. Polymer Sei. 22: 2081, 1978 t;. N. K. BARAMBOIN and B. RAKITYANSKII, Kolloid. zh. 36: 129, 1974 7. C. D. HAN and T. S. YU, Polymer Engng. Sci. 12: 81, 1972 ~. J. L. WHITE, R. S. UFFORD, K. It. SHARED and It. L. PRICE, J. Appl. Polymer Sci. 16: 1313, 1972 9. Yu. P. MIROSHNIKOV, T. N. MIEHAILOVSKAYA and V. N. KULEZNEV, Kolloid. zh. 43: 62, 1981 Io. A. Ya. MALKIN and A. Ye. CHALYKH, Diffuziya i vyazkost' polimerov (Diffusion ~md Viscosity of Polymers). p. 26, Khimiya, Moscow, 1979 it. Yu. P. MIROSHNIKOV, M. L. KAMINSKII and V. N. KULEZNEV, Kolloid. zh. 41: 1112, 14.G~.)

P,,lymerSciem.(.U.S.S.R.Vol.°4, No. 8, pp. 1825-1836,1982 Pri~,-d in Poland

0032-3950/82 $7.50+.00 © 1983PergamonPress Ltd.

DISPERSE STRUCTURE AND MECHANICAL PROPERTIES OF EXTRUDED POLYPROPYLENE-POLYSTYRENE BLENDS* Y u . P. MIROSHN[KOV a n d I-[. L. W m L I A ~ S Lomonosov Institute of Fine ChemicM Technology, Moscow l),.pa.rtm(.nt of Chemical Technology and Applied Chemistry, Toronto University, Canada

(Received 29 January 1981) The authors mlalyse the influence of the viscoelastic properties of the components, he composition of the blends, preliminary mixing, the length of the capillary and shear stress in extrusion at 200°C on the particle size of the disperse phase, the size distribut ion and the rheological and strength properties of P P - P S blends. The particle size of rl ~e disperse phase (PP) in the cross section of the extrudates falls with increase in the ratio of the capillary length to diameter, rise in shear stress ill the capillarY, fall in the content of P P in the blend a n d on transition from the one- to two-stage process of blending. The strength a n d elastic modulus of the extrudates of the blends on bending sigTlificantly depe~M on the degree of dispersity and phase structure of the composites st.udied. * Vysokomol. soyed. A24: No. 8, 1606-1615, 1982.

1826

Yu. P. MmOSH~mOV and H. L. WW.T.TA~S

T ~ p r e v i o u s c o m m u n i c a t i o n [1] a n a l y s e d t h e d e p e n d e n c e o f t h e t y p e o f p h a s e structure of PP-PS blends on the ratio of the viscoelastic characteristics of the individual polymers, the ratio of the components of the blends, the shear stress, length of capillary and the initial degree of disporsity of the composites. T h e a i m o f t h i s w o r k is t o e x p l o r e t h e i n f l u e n c e o f t h e s a m e v a r i a b l e s o n t h e d e g r e e of dispersity and certain mechanical properties (strength limit and elastic modulus on bending) of the oxtrudates. The characteristics of the polymers a n d the conditions of obtaining the blends were indicated in [1]. The phase structure of the blends was analysed with the Stereoscan scamfing electron microscope of Cambridge Instruments. The surface of the chips of the extrudates was subjected to vacuum coating with gold. The quantitative analysis of the microstructure of the blends was carried out with t h e Tectronix 4051 microcomputer fitted with an a t t a c h m e n t for analysing the disperse structure. The computer enabled us to p u t out on the display screen the curve of the particle size distribution, the arithmetical mean and m a x i m a l a n d minimal equivalent diameters of the particles. The image from the screen was a u t o m a t i c a l l y copied on paper. I n this way weanalysed over 200 microfilms of the structure of the blends. The mechanical properties of the extrudates in the three-point bending regime were investigated with the Instron universal test machine at 20°C. The distance between the bearing points on which the extrudate was placed was 38 mm. The elasticity modulus on bending was estimated from the formula P

L

E~= -3T" 4--g' where P is the load; ~ is the deflexion of the extrudate; I=Trd~e/64is the moment of inertia o f the cylinder; d e is the diameter of the extrudate; L is the distance between supports. T h ~ strength of the extrudates on bending was estimated as 8Pd

where Pb is the breaking load of the sample. The theological properties of the P P - P S blends at 200°C were studied with the Brabender single-screw extruder fitted with a pressure sensor at the inlet to the capillary a n d a Koch static mixer placed between the capillary and the screw. We used three capillaries of d i a l n e t e r 2.0 m m and with ratio of length to diameter l/d=2, 11 and 20. The corrections for the pressure losses at the inlet to the capillary were calculated b y the differential m e t h o d [2] using the d a t a for three capillaries. I n the work we a d o p t e d the following notations for the samples. The first letter denotes: A blends with one-stage; B blends with two-stage blending. The second numerical index indicates the blend number in line with Table 2 in [1]: 1 - - t h e P P - 1 / P S blend; ratio of viscosities of the phases (for a shear stress on the capillary wall vn-----2× 104 Pa), a=/~pp//~es = 5. 6; 2 - - P P - 2 / P S , /,=1.0; 3 - - P P - 3 / P S , /,=0.25; 4 - - P P - 4 / P S , /,=0.09. The t h i r d index denotes the content of P P in the blend ((Oppvol.%); the fourth (where present) the value of the shear stress in the capillary during extrusion (rn× 10-*Pa); 1~<1.2; 2 - - ~ 2 . 0 ; 3 - - ~ 1 0 . 0 ; 4 - - > 10. We shall analyse the influence of the variables studied on the degree of dispersitya n d mechanical properties of the P P - P S blends.

Structure and properties of extruded PP-PS blends

1827

Ratio of viscoelastic properties of components. The dependence of the meanweighted probable size of the disperse phase on the ratio of the viscosities of the phases p for the blends obtained on rollers or in a mixer of the closed t y p e is usually described b y a curve with a minimum in the region of/2 close to unity (3, 4]. The authors of [3, 4] used extrusion of ah'eady prepared composites with a single goal--to orient the particles alor~g the axis of the extrudate and thereby obtahl information on the volumetric size of the particles b y preparing slices in the direction of fl~w. In this case the question concerns the particle size in the <,ross section of the extrudate (thickvess) not givir~g an idea of the volumetric particle size of the disperse phase. The extruder with capillary at the outlet is not a suitable instrument for explori~ g the dependence of particle size on the parameter /2 since change in the v'scosity of the bler.d and the length of the capillary is accompanied b y charge in the pressure in the chamber of the extruder ent aili~g a correspondir g change in the conditions of blending and degree of dispersity of the composites. In blends where the main element of the phase structure is represented b y very lovg fibres only the diameter of the fibre can be measured; it is even more difficult to measure the particle s'ze in blends with a reticular or occluded stl~lcture [1]. All these factors governed the complex chai'acter of the change in mean D and probable Dh (from the maximum in the distribulion curve) of the particle diameters in the cross section of the extrudates with (,hange in the ratio of viscosities of the phases of the P P - P S blends. Depending on the length of the capillary, prel'minary agitation and shear stress the curves D(p) wore characterized b y the presence of a minimum, maximum or both at the same time. These findings appear to be of some importance from the practical standpoint. They indicate that the results of laboratory tests on the influence of the viscoelastic parameters of the components on the degree of dispersity of the polymer blends obtained with use of capillary viscometors can hardly be transferred to processes of extrusion of such muterials without additional checking and analysis. Since charge in the viscoelastic characteristics loads to change in the type of structure in the blends it was of interest to analyse the rheological properties of the composites. It is known [5] that on flow of a viscoelastic emulsion the particles of the disperse phase accumulate large highly elastic deformations. Therefore, it. m a y be supposed that the inlet pressure losses APln are associated with the elasticity of both phases, the ratio of the components in the blend and possibly the phase structure of the composites. Figure la indicates the dependence of the value of the pressure losses at the inlet to the capillary determined b y the differential method [2] on the value 12 in the different blends. From comparison of curves 1 and 2 one m a y evaluate the influence on API, of the sheur rate yR on the capillary wall. Characteristic of blonds of both compositions is fall in zlPln with fall in viscosity (and elasticity) of the disperse phase and its content in the blond. The position of point 3 belonging to blend 2 obtained b y two-stage blending

1828

Yu. P. MmOSHNZKOVand H. L. WILLIAMS

indicates that the improvement in the quality of blending (rise in the degree of dispersity) is accompanied b y fall in the elastic losses in the capillary. Thus, fall both in the total number and mean size of the particles of the disperse phase loads to reduction in APln.

a

y

t.6

b zog/~.s~j Z.6

I

|

-~'0

I

I

]

I

I

0

I

I

I

I

I'0 -I.0

0

log ~

~'0

l~o. 1. Pressure losses at the inlet to the capillary (a) and effective viscosity (b) as a function of the ratio of the viscosities of the phases for the PS :PP blends at 200°C: a-- PS: PP = 75: 25 (1-3) and 85:15 (4). Shear rate in capillary 316 (1) and 100 (2-4) see-l; 1, 2, 4--one-sbage; 3--two-stage blending, b--PS:PP=75:25 (1, 3, 4) and 85:15 (2); zR=2× 104 (1-3) and 4.5× l0 t (4) Pa; 1/d=ll (1, 3); 20 (2) and 2 (4); 1, 2, 4--one-stage; 3--two-stage blending I f the absence of the influence of the phase structure on the viscosity of the polymer blends is postulated then the dependence ~/(/~) must evidently monotonically fall with fall in the viscosity of the disperse phase. However as indicated b y Fig. lb, this dependence is distinguished b y appreciable non-monotonicity, the character of the curves ~ (/~) with rare exceptions being practically the same for all the cases studied. It is worth noting the sharp fall in the rate of drop in viscosity for the blend 2 (/~=1) and even the subsequent substantial rise in viscosity for blend 3 (/L=0.25). The answer to the problem of such a change in viscosity apparently must be sought in the phase structure of the blends. Confirmation of this assumption m a y be provided b y the fact that the viscosity of the blonds depends on the capillary le:-gth (with corrections introduced). Figure 2 presents the photomicrographs of the P P - P S blends the function (~) for which is described b y curve 2 in Fig. lb. Analysis of a large number of microfilms and comparison of the structure of the blends 2 and 3 with the phase structure of other composites [1] showed that the most characteristic distinguishing feature of these samples is the presence in them of branched and occluded structures. From Fig. 2 (Fig. 3 in [1]) it follows that the structure of blends 1

Structure and propertioa of extruded PP-PS blends

1829.

and 4 is characterized, in the main, by a fibrous of fibrous-ribbon morphology. Possibly the flow of the branched structures in the c~pil'ary calls for heavier energy expenditure t h a n the flow of the "regular" fibrous structure. The result is t h a t the introduction int~ PS cf the much more viscous PP-1 (~--~8000 Pa.sec) gives a blend half as v'scous as on introducir.g PP-3 w:th a viscosity of 370 Pa.sec (Fig. lb, curve 4). It is hardly possible to attribute the effect ibund to thermo- or mechano-degradation of P P since it is characteristic of blends ()brained both by one- and two-stage ble:,.dir~g.

FIG. 2. Microstructure of the blends A1-25 (a); A2-25 (b); A3-25 (c) and A4-25 (d); = 2 × 104 Pa.

rR-

Comparison of curves 2 and 3 in Fig. lb p)ints t ) a fall in the viscosity of the composibes over the whole range of values of/z studigA oil pass!t)g from one- to two-stage blending. Shear stress. For all the cases analysed increase iu the shear stress in the capillary during extrus'on was accompanied by rTse in the degree of dispersity of the composites. As an example Fig. 3a gives the dependence of the mean (empty circles) and probable Da (filled circles) of the pzrticle diameters of the disperse phase in the cross section of the extrudates over a d;stance of 1/2 radius of

1830

Yu. P. MZROSHZC~OVand H. L. Wrrx.T~s

the extrudate (Da was definecl from the position of the maximum in the distribution curves). For shear stresses of the order 1.5 × 105 Pa the curves D(vR) pass to a plateau and in some cases (Fig. 3, curve 1) a tendency to enlargement of the P P particles is noted. This m a y be due to rise in the intensity of the coalescence of the particles of the disperse phase in the inlet zone of the capillary [4]. These findings indicate that intens'fication of the regime of polymer blending is not always accompanied b y the expected rise in the degree of dispersity. To obtain composites with a limiting particle size moderate shear stresses suffice.

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.

3

6

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i

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i

I

I

i

5

I0

15

5

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I5

F~o. 3. Effect of shear stress on capillary wall in the course of extrusion on the mean (1-3) and probable diameter (1'-3') of the PP particles (a) and also the elastic modulus (1-3) and strength on bending of extrudates (1'3') (b) for l/d-~ 11. 1--A2-25; 2--B2-25 and 3-B2-15.

Reduction of the thickness of the fibres, layers and other elements of the ,disperse phase with increase in the shear stress in the capillary is accompanied b y corresponding declirc in stre:lgth and elastic modulus on bending (Fig. 3b). This effect m a y be explained b y the dilution of the rigid glassy PS matrix b y the less high modulus component. For a constant P P : P S ratio in the blend fall in the s:ze of the P P particles in the extr'ader channel signifies increase in their number and the number of fibres and particles of another shape formed in the capillary. This leads to a more uniform distribution of the low modulus compor~e~,t in the PS matrix and its more effective weakening. Preliminary blending ratio of componeuts. In tile previous communication it was shcw_l that fall in the content of P P in the blond or transition from the one-to two-stage process of blendirg has a similar (,fleet on the phase structure of the composites. Th's also applies to the disperse stracturc: change in the parameters along these lines is accompanied b y rise in the degree of disporsity of the blends. From Fig. 3a it m a y be seen that preliminary blending loads to t h e formation of more highly disperse compos.ites (curve 2) than blending in ~)ne procedure (curve 1). Fall in the content of P P from 25 to 15 vol.°/o led to

Structure and properties of extruded PP-PS blends

1831

further rise in the degree of dispersity (Fig. 3a, curve 3) of the composites obtained by the two-stage regime. The resistance to bending and the elastic modulus of the composites obtained by extrusion of the initially mixed components (Fig. 3b, curve 2) are less than the, corresponding values for the composites obtained in one stage (Fig. 3b, curve 1). Fall in the content of P P to 15% was accompanied b y fall in the norabet of particles in the volume of the matrix and some rise in the values af and E] (Fig. 3b, curve 3). Length of capillary. The use of three capillaries enabled us to analyse the influence of the magnitude lid on the particle size of the disperse phase and the mechanical properties of the extrudates in the bending regime (Fig. 4). As follows from the data presented the degree of dispersity of the composites rises with increse in the length of the capillary. We have the results for the shortest capillary only for four blends. For the other composites we give the data obtained with capillaries with a length of 22 and 40 mm in order to illustrate the general ten(le:lcy of the influence of the value of the ratio lid on the value Da and a! (Fig. 4, broken lines).

6 f" 10-~f t,lPa .Dh,,~

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FIG. 4. Probable particle diameter of disperse phase (a) and bending strong%h (b) as a function of the lid ratio: 1--A2-25-2; 2--A4-15-2; 3--A2-15-3; 4--A4-25-3; 5--A4-25-2; 6--A2.15-2; 7--A3-25-2; 8--B2-25-2; 9--A1-25-2. In the inlet zone of the capillary a slightly viscous drop of the disperse phase is heavily deformed although the subsequent flow in the capillary may load to almost complete relaxation of its form (elastic restoration) [5]. Probably this effect is realized in the case of low viscous emulsions (for example, as in [5] for 2% solution of an incompatible polymer blend) when the relaxation times are short and the shear stress in the capillary insufficient to maintain the elongated fi)rm of the drop. In the case of condensed polymer emulsions relaxation of the

Yxy. P. M~esKmxov and H. L. Wrr.T.TA~S

1832

drop in the capillary is loss effective promoted b y high shear stresses and low interphase tension. Probably therefore anisometric particles are present in all extrudatos irrespective of the value of the lid ratio for the capillary. At the same time rise in the degree of disporsity of the composites with increase in the lid ratio indicates the appreciable role of the residence time in the capillary. Other things being equal, the larger the value of the lid ratio t h e longer the time of action of the shear forces on the drop and the smaller the transverse particles size.

F7 I I I I I

3O

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-7

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I0

I I

0"5

1"5

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Fro. 5. Histograms of the particle diameter distribution for the blend A1-25-2 (l/d=ll) in the central (continuous lines} and peripheral (broken) parts of the cross section of ex. trudate. The degree of dispersity of the composites depends not only on the l[d ratio b u t also the radial disposition of the element of the volume of the blend in the capillary. In line with the shour stresses in the longitudinal section of the capillary the particle diameter must be less in the zone of raised TR, i.e. closer to the capillary walls. Figure 5 presents the histograms of the distribution of the P1> particles over the transverse dimension for the blend A1-15-2 (1/d~ll) at the centre and periphery of the oxtrudate (continuous and broken lines, respectively). For much the same probable particle diameter the blend at the centre of the extrudato is characterized by a less intense main maximum and has a second maximum of smallcr height in the rogio:l 1 pm. At the periphery of the oxtrudate the particles are more uniform in s'ze. In line with the special features of the distribution curves the moan particle sizes at the centre and periphery of the. extrudates are respectively 0.72 and 0.58/zm (Fig. 5).

Structure a n d properties of extruded P P - P S blends

1833

STBUtr±'U~tAL AND MEC~A~rIOA~p A ~ t ~ m ~ a S OF P P - P S BLENDS •on X X 10 -~

Blend

lid

D/Dn

L*'*

~n

Pa

2

o'1x ,I E.,.x X

10-11I X 10 -3 MPa

A4-15 Al-15 A2-15

(~09 5.50 1.00

11 11

1.44/1.0 0. 7/0. 5 0. 9/0. 7

4.2 3.6 3.3

A2-15

~50

2

O. 9/(~ 6

3-3

A2-25

1.00

1.4/0.9

3.3

A1-25 A4-25

5.50 0.09

1. o/o. 6

3"0 3.2

Al-15

5.50

0-7/0.5

A1-25 A2-15

5.50 1.35

0.9/0.5 0.9/~6

AI-15

4-10 0.55

0.7/0.5

A2-25

1" 1/0. 8

A3-25

9.6

1.4/0.4

A1-25 A1-25

10.0 9.7

o.5/o.4 ~5/~3

A4-25

9.5

0.7/0.4

A4-25

9.8

0.9/0-4

B2-15 B2-15

1.2 2.2

o. 9/0. 5 0. 8/0. 7

B3-25 B1-25

1.8 1.9

8/o- 6

B2-25 B4-15 B3-15 B2-25 B2-25 B2-25

0.7 10.7 11.8 5.9 8.9 11.0

3'6 3.6

8/0. 7

F o r m of particles of disperse phase Mostly fibres Fibres, r a r e l y ribbons Fibres, rarely ribbons, layers Fibres, irregular ribbons branched structures, rarely layers Fibres, large particles of irregular form Fibres, ribbons, layers Fibres, branched and occluded particles Fibres, rarely ribbons, layers Fibres, ribbons, layers Fibres, more rarely ribbons, layers Lamellar-fibrous Bent thick ribbons, branched and occluded structures, fibres Fibres, branched and occluded structures Fibres, ribbons, layers Bent fibres, layers of irregular form Fibres, ribbons, occluded structures Fibres, gross layers of irregular form, branched particles Mostly fibres Fibres, more rarely, layers, ribbons Mostly fibres Fibres, ribbons, occluded structures

2 11 11 11 11 11

0.9/0.8 0.9/0.4

o. 510. 3 0.7/~3 1.1/0.3 0.5/ff 3

0.5 0.37 0.36 0.3

2-9 2.8 2.5 2.5 2.6 2.6

Mostly fibres Fibres, ribbons, layers Fibres, rarely ribbons Fibres, ribbons Many ribbons, fibres Distorted ribbons and fibres

1834

Y v . P. MIROSHNIEOV and H. L. WILLIAm/IS

The above analysis shows the fundamental influence of the variables studied on the disperse structure and the mechanical properties of the P P - P S blends. Rise in the shear stress and ratio of the length of the capillary to its diameter in the course of extrusion and transition from the one- to two-stage process of blending are accompanied by rise in the degree of disporsity of the composites and declirc in the strength and elastic modulus or bending. Thus it m a y be considered that the transverse size of the fibres, ribbons~ layers and other elements of phase structm'e is one of the m.~in factors in the formation of the mechanical characteristics considered. C~nfirmation of this assumption m a y be provided by the data in Fig. 6 and representing the dependence of the strength on bending on the probable particle diameter of the disperse phase in the cross section of the extrudates, including here all the data irrespective of the value of the variable parameters. It is clear that the strength increases with increase in the diameter of the P P particlbs. For a fixed part;cle diameter the height of the zone bounded b y the broken lines evidently characterizes the influence on the value al of the other variables (such as, for example, cha~ge in the modulus of the disperse phase on passing from one P P sample to another, change in the degree of molecular orientation of the phases with change in the shear rate and stress on extrusion, polydispersity of the blends, etc.).

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o

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:FIG. 6. B e n d i n g s t r e n g t h as f u n c t i o n of the probable particle diameter of the disperse phase in P P : I~S b l e n d s 25 : 75 (1) a n d 85 : 15 (2).

It is clear that the mechanical properties et' tile composites depend not only on the particle size but also the tbrm of the particles of the disperse phase. This may be judged from the data in ttle Tab'e. It prese-lts the values of #, TR, lid and D/Da, the widths of the dis;ribut:on curve of particles by diameters at 0-3 height and the values ~ri, El and also gives a brief description of the elements of the phase structure charact<,r'stic of the particular blend. The Table includes only those blends the bending strength of which is greater than 80 and less than

Structure and properties of extruded PP-PS blends

1835

50 MPa. Thus, it is possible to analyse the special aspects of the formation of the strongest and least strong composites. From the Table it follows that the strongest blend is A4-15 (p-~0.09) obtained on the basis of the lowest molecular weight P P sample with use of a capillary with I/d.~ 2. This composite is characterized b y a predominantly fibrous structure, a fairly large diameter of the fibres and relatively narrow distribution curves of the fibres b y diameter. The blend A1-15 (second line in Table) made up of the highest molecular weight P P and PS sample is also characterized by a fibrous structure (with a low content of ribbons) and considerable strength although the particle size in this case is h a l f that in the preceding. Possibly the fall in size Gf the P P particles is compensated in this case by their higher modulus. When the AI-15 blend was extruded through the capillary with 1/d~-20 at ~R-~10.6 X 104 P~ the strength was about halved, which is apparently connected with the formation in the latter case of a less strong lamello-fibrous structure.

16-

12-

8

q

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0"5

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I~G. 7. Histograms of the particle diameter dis4ribiltion for the blends. B2-15 obtained by" extrusion for rR~-l.2X 104 Pa and l/d----2 (a); for rR~10"7× 104 and l/d=20 (b).

1836

YU. P. MIROSHNIKOVand H. L. WILLIAMS

The second part of the Table gives in the same order the corresponding propertios of the blends obtained by two-stage blending. I t was of interest to compare the particle s!ze distribution curves for the blends with d~fferent bending strength. Figure 7 presents the histograms for the B2-15 blend obtained for zR= 1.2 × 10* Pa, l/d=2 (a) and for zR=10.7 x 10' Pa, I/d=20 (b). For the first blend b/Dh=0"9/0"5 pm and ai=-98 MPa, for the second respectively 0.5/0.3/ml and 52 M:Pa. Such a difference in the strength of the two blends considered is primarily due to the peculiarities of their particle size distributions. While the presence of fibres over a wide interval of diameters with the most probable s:zo Da----0.53/~nl is characteristic of the first, the second contains very thin fibres (Da=0.31/lm) in a very narrow interval of D values (Fig. 7b). Thus, the results of study of the dependence of the disperse structure and mechanical properties of the P P - P S blends on the ratio of the viscoelastic characteristics of the phases, shear stress on extrusion, the length of the capillary and the preliminary blending shed light on the following. The ratio of the viscoelastic properties of the phases influences the type of structme, the form of the particles of the disperse phase and their size, which, in turn, is reflected in the mechanical properties of the blends. Thus, the formation in the blends 2 and 3 of occluded and reticular structures led to appreciable rise in the viscosity of these composites. Increase in the length of the capillary (with its diameter unchanged) was accompanied by rise in the degree of dispersity of the blends and decline in strength and the modulus on bending. Similar effects are given by increase in the shear stress in the capillary on extrusion and transition from the one- to two-stage process of blending. Composites firmest in the bending regime form with use of 1)1)-4, the shortest capillary and low shear stresses. A relatively coarsely disperse structure forms with particles in the form of fibres. The lamellar structures are inferior in strength characteristics to the fibrous. The authors are grateful to V. N. Kulezaev for taking part in the discussion of the results of the work and express their sincere thanks to Profs. R. T. Woodhams and C. I. Cheify for assistance in organizing the work, members of Shell Canada Limited Research Centre for the P P samples made available and their testing and also Prof. M. R. Kamal who kindly gave us the opportunity to test the viscoelastic properties on the Rheometrix instrument. REFERENCES

1. Yu. P. MIROSHNIKOV and H. L. WIIJ.TA.VIS, Vysokomol. soyod. A$4: 1594, 1982 (Translated in Polymer Sci. U.S.S.R. A$4: 8, 1811, 1982) 2. A. Ira. MAI~f][N and A. Ye. CHALYKH, Diffuziya i vyazkost' polimorov (Diffusion and Viscosity of Polymers). p. 26, Kbimlya, Moscow, 1979 3. V.N. KULEZNEV, A. V. GRACHEV and Yu. P. MIROSHNIKOV, Kolloid. zh. 38: 265, 1976 4. Yu. P. MIROSH2fIKOV, A. M. COL*MAN and V. N. KULEZNEV, Ibid. 41: 1120, 1979 5. C. D. HAN, l~eologiya v protsessakh porerabotki polimerov (Rheology in Polymer Processing), p. 184, Khimiya, Moscow, 1979