Miscibility of polymethacrylates with poly(p-methylstyrene-co-acrylonitrile)

Eur. Polyrn. J. Vol. 27, No. 9, pp. 921-925, 1991 Printed in Great Britain. All rights reserved

0014-3057/91 $3.00 + 0.00 Copyright © 1991 Pergamon Press pie

MISCIBILITY OF POLYMETHACRYLATES WITH POLY(p-METHYLSTYRENE-CO-ACRYLONITRILE) S. H. GoH,* K. S. Slow and S. Y. LEE Department of Chemistry, National University of Singapore, Singapore 0511, Republic of Singapore (Received 7 January 1991)

Abstract--The miscibility of five polymethacrylates with poly(p-methylstyrene-co-acrylonitrile) (p MSAN) has been studied. Poly(methyl methacrylate), poly(ethyl methacrylate), poly(n-propyl methacrylate) and poly(n-butyl methacrylate) are miscible with pMSAN over certain copolymer composition ranges but poly(isopropyl methacrylate) is immiscible with pMSAN. The phase behaviour of polymethacrylate/ pMSAN blends is, in general, similar to that of the corresponding blends with poly(styreneco-acrylontrile). Various segmental interaction parameters have been evaluated and compared with the corresponding parameters involving the styrene segment.

INTRODUCTION The miscibility behaviour of polymer blends has been an active field of research during the past two decades. It is well recognized that two homopolymers are likely to form a miscible blend if there are some specific intermolecular interactions between them [1]. However, it is commonly observed that a homopolymer A is miscible with a copolymer B/C over a certain copolymer composition range although it is immiscible with either h o m o p o l y m e r B or homopolymer C. A well-known example is formed by the blends of poly(methyl methacrylate) ( P M M A ) with poly(styrene-co-acrylonitrile) (SAN) [2~]. The miscibility behaviour of such a system is explained by recent theories which take into consideration both the intermolecular and intramolecular interactions between various segments in the blend [7-9]. A strong intramolecular repulsion between the two types of segments in the copolymer is particularly important to achieve miscibility. In fact, the temperature at which a blend undergoes phase separation can be significantly increased by incorporating relatively small amounts of a suitable c o m o n o m e r into one of the component polymers [10, 11]. Poly(ethyl methacrylate) (PEMA) and poly(n-propyl methacrylate) (PnPMA) are also miscible with S A N over certain composition ranges [5]. Similar behaviour has been observed for blends of P M M A with poly(a-methylstyrene-co-acrylonitrile) (~ M S A N ) [ 12, 13]. We have recently reported the miscibility of two chlorine-containing polymethacrylates with S A N and poly(p-methylstyrene-co-acrylonitrile) (pMSAN) [14-16]. In this paper, we report the miscibility behaviour of five poly(alkyl methacrylate)s with p MSAN.

ture for 4hr using azobisisobutyronitrile (0.3wt% of monomers) as initiator. The copolymer was obtained by precipitation of the solution in excess methanol. The acrylonitrile (AN) content of the copolymer was determined by elemental analysis of nitrogen. The characteristics of various pMSAN samples are given in Table 1. The following commercial polymethacrylates were used: PMMA (Du Pont Elvacite2010, - ~ , = 120,000), PEMA (Du Pont Elvacite 2042, Mw = 310,000), PnPMA (Scientific Polymer Products, )17/w=175,000), poly(isopropyl methacrylate) (Scientific Polymer Products, [r/] = 0.33 dl/g in 2-butanone at 30°) and poly(n-butyl methacrylate) (PnBMA) (Du Pont Elvacite 2044, A,7W= 288,000). Polymer blends Binary blends ofp MSAN and polymethacrylate in weight ratios of 3:1, 1: 1 and 1: 3 were prepared by solution casting from tetrahydrofuran (THF) at room temperature. Solvent was allowed to evaporate slowly over a period of 1-2 days. The blends were then dried in vacuo at 110° for 3 days. Tg measuremenls The glass transition temperatures (Ts) of various samples were determined with a Perkin-Elmer DSC-4 differential scanning calorimeter using a heating rate of 20°/min. Except for PMMA/pMSAN blends, each of the other blends was subjected to several heating-cooling cycles and Ts was taken as the initial onset of the change of slope in the DSC curve. For PMMA/pMSAN blends, they were first heated to 150° and kept at the temperature for 5 min. The blends were then rapidly cooled to room temperature, followed by annealing at 90 ° for 10 days. The rationale for annealing is discussed in a later section.

Table 1. Characteristics of pMSAN samples Sample M, M'w T8(°C) 7MSAN4.6 25,000 50,000 108 ~MSAN 7.7 21,000 34,000 107 9MSAN 10.2 28,000 34,000 110 ~MSAN 13.6 21,000 42,000 105 ~MSAN 15.8 28,000 46,000 105 ~MSAN 19.6 27,000 43,000 109 ~MSAN 21.3 28,000 52,000 105 ~MSAN26.5 27,000 44,000 105 9MSAN 29.1 34,000 56,000 105 ~MSAN 32.3 38,000 62,000 104 ~MSAN 35.9 31,000 50,000 104 aMSAN 39.6 33,000 59,000 110

EXPERIMENTAL PROCEDURES

Materials pMSAN samples of various compositions were prepared by solution polymerization in 2-butanone at reflux tempera-

*To whom all correspondence should be addressed. 921

922

S.H. Gorl et al. Table 2. Characteristicsof PMMA/pMSANblends AN content(wt%) Clarity Ts LCSTbehaviour

/-

t

7.7 10.2 13.6 19.6 21.3 26.5 29.1 32.3 35.9 39.6

35.9 % AN

29.1

Hazy Hazy Clear Clear Clear Clear Clear Clear Hazy Cloudy

2 2 I I 1 1 1 1 2 2

--No No No No No No ---

21.3 RESULTS

P M M A / p M S A N blends

13.6

7.7

•

2.7

1 40

I

I

I

I

60

80

100

120

Temp

140

(*C)

Fig. 1. DSC curves of various annealed PMMA/pMSAN (1 : 1) mixtures. Cloud-point measurements All the miscibile blends were examined for the existence of lower critical solution temperature (LCST) behaviour using the method described previously [17].

The measurement of Ts of a blend by DSC is widely used to determine the miscibility of the blend. Provided the Ts s of the two component polymers are sufficiently far apart (>20°), the appearance of a composition-dependent Tg indicates the formation of a miscible blend. Since the Ts of PMMA (100 °) is slightly lower than those of p MSAN samples, it is difficult to use conventional DSC measurement to ascertain the miscibility of PMMA/pMSAN blends. However, it has been shown that the enthalpy recovery of an annealed blend can be used to ascertain the miscibility of a blend containing polymers with very close Tss [18-20]. An immiscible blend is characterized by the appearance of two enthalpy recovery peaks. To investigate the applicability of the enthalpy recovery method, several 1:1 physical mixtures of PMMA and p M S A N were prepared. Each mixture was thermally treated in the calorimeter at 150° for 5 min and then annealed at 90° for 10 days. The DSC curves of annealed PMMA/p MMSAN mixtures are shown in Fig. 1. Each mixture shows two enthalpy 280

39.6% AN 240 3,5.9

32.3

29.1

21.3

o" "o

o

160

3 P

12o

One phase

Q.

13.6

E

80 10.2__

__ 40

7.7

40

e

I

I

I

I

60

80

100

120

140

Temp ( * C )

Fig. 2. DSC curves of various annealed PMMA/FMSAN (l : I) blends.

•

I 10

o

oo

I 20

o o

I 30

•

•

I 40

wt % AN in pMSAN

Fig. 3. Phase diagram of PMMA/pMSAN blends. O, Miscible blends; @, immiscible blends.

Miscibility of polymethacrylates with p MSAN Table 3. Characteristics of PEMA/pMSAN blends AN content (wt%) Clarity Ts LCSTbehaviour 4.6 Cloudy 2 -7.7 Cloudy 2 -10.2 Cloudy 2 -13.6 Clear 1 No 21.3 Clear 1 No 26.5 Clear 1 Yes 29. l Clear 1 Yes 32.3 Clear 1 Yes 35.9 Cloudy 2 -39.6 Cloudy 2 recovery peaks, indicating the applicability of the method. Various P M M A / p M S A N blends were simlarly anneaied and the D S C curves of the annealed blends are shown in Fig. 2. The appearance of a single enthalpy recovery peak for some blends indicates that P M M A is miscible with p M S A N over a certain copolymer composition range. Table 2 summarizes the optical appearance, the number of glass transitions and the L C S T behaviour o f the blends. Those miscible blends as judged by their glass transition behaviour show a certain degree of haziness. The origin of haziness is not certain. It has been reported that some miscible T H F - c a s e P M M A / S A N blends are hazy and the haziness has been attributed to some artifact of the blend preparation method [5]. The phase diagrams of PMMA/pMSAN blends is shown in Fig. 3.

923

Table 4. Characteristics of PnPMA/pMSAN blends AN content (wt%) Clarity Ts LCSTbehaviour 4.6 Cloudy 2 -7.7 Clear 1 No 13.6 Clear 1 No 21.3 Clear 1 Yes 26.5 Clear 1 Yes 29.1 Cloudy 2 -32.3 Cloudy 2 -shown in Fig. 4. The appearance of rather low cloud point for P E M A / p M S A N 32.2 blends suggests that the blends are near the miscibility limit.

PnPMA /pMSAN blends The low Tg of P n P M A also enables the use o f conventional DSC measurements to ascertain the miscibility of the blends. Table 4 summarizes the characteristics o f the blends and the phase diagram is shown in Fig. 5.

PiPMA /pMSAN blends The T s of P i P M A is about 20-25 ° lower than those o f p M S A N samples. The Tg values are sufficiently far apart to allow the use o f conventional D S C measurement to ascertain the miscibility. All the blends were cloudy and each blend showed two distinct glass transitions. Figure 6 shows the D S C curves o f various P i P M A / p M S A N (1:1) blends. Thus, P i P M A is immiscibile with all the p M S A N samples.

PnBMA /pMSAN blends PEMA /pMSAN blends The T 8 of P E M A is about 40 ° lower than those of p M S A N samples and, therefore, annealing o f the blends is not necessary. Table 3 summarizes the characteristics of the blends and the phase diagram is

Blends of P n B M A with p M S A N 13.6 were transparent. Each of the blends showed a single composition-dependent T s and underwent phase separation at high temperatures. The Tg-composition curve and the cloud point curve of the blends are shown in

280

280

I 240

240

200

o

\

160

P

160 One phase

D One phase

a. 120

I

E

120

E

80

80

40

40

• •

•

I 10

0

0

I 20

0

0

[ 30

o

o

0

•

•

0

I 4o

w t % AN in pMSAN

Fig. 4. Phase diagram of PEMA/pMSAN blends.

0

I

i

I

I

lO

20

30

4o

wt % AN in pMSAN

Fig. 5. Phase diagram of PnPMA/pMSAN blends.

S . H . G O H e t al.

924

Table 5. Characteristics of PnBMA/pMSAN blends

I 32.3

AN content (wt%)

Clarity

Ts

LCST behaviour

7.7 10.2 13.6 15.9 19.6 26.5

Cloudy Cloudy Clear Cloudy Cloudy Cloudy

2

--

2

--

1 2 2 2

Yes ----

/ " DISCUSSION

29.1

/

Table 6 summarizes the miscibility ranges of various polymethacrylates with pMSAN. Also included in the table are the corresponding miscibility ranges for blends with SAN. PMMA, PEMA and PnPMA are miscible with both p M S A N and SAN over certain coplymer composition ranges. PiPMA is immiscible with both p M S A N and SAN. However, PnBMA is miscible with p M S A N over a very narrow copolymer composition range but it is immiscible with SAN. As mentioned earlier the miscibility behaviour of a hompolymer/copolymer blends depends on various segmental interactions. For the present blend systems, the overall interaction parameter Xb~e,dis given by the equation:

(, o" "o

19.6

15.9 13.6

7.7

40

/

I

L

I

I

60

80

100

120

Xbknd = YZM/AN "~- (1 140

Temp (eC)

Fig. 6. DSC curves of various PiPMA/p MSAN (1 : 1) blends.

Fig. 7. Blends of PnBMA with other pMSAN samples were cloudy and the two-phase nature of these blends was also confirmed by their glass transition behaviour. Thus, PnBMA is miscible with p M S A N over a very narrow copolymer composition range around 13.6% AN (Table 5). 250

240

• -

o/°

2,3O

1oo

o

IIJ

E

40

20,

-

Y)XpMS/M -- y ( 1

Y)~pMS/AN

Table 6. Miscibility ranges of various blends p MSAN

[ o

-

where y is the volume fraction of AN in p M S A N and the subscripts M, p MS and AN denote methacrylate, p-methylstryrene and acrylonitrile segments. The criterion for miscibility is that Zblend< Xc.t where Zcnt= 1/2 ( N ; ~/2+ N f ]/2)2with N] and N 2 the degrees of polymerization of the two polymers. The miscibility behaviour of homopolymer/copolymer blends enables the evaluation of various segmental interaction parameters [6, 8, 14, 15]. For the present systems, the five polymethacrylates have higher molecular weights than the pMSAN samples. Therefore, the Xc,t values depend mainly on the degrees of polymerization of p M S A N samples. We assume Xcnt to be 0.005 based on the N values of 400 for both polymers in the blends. From the phase boundaries and a value of 0.91 for XpMS/AN[15], various segmental interaction parameters were evaluated and are summarized in Table 7. For comparison purposes, the X values for the corresponding SAN blends are also shown in Table 7. The X values for PMMA/SAN blends were reported by Cowie and Lath [6] and those for PEMA/SAN and PnPMA/SAN blends were estimated using the results reported by Fowler et al. [5]. The interactions between various methacrylates with styrene and p-methylstyrene are only weakly repulsive as shown by the small positive X values, and the interaction decreases slighly in magnitude with increasing size of the pendant group of the methacrylate. On the other hand, the repulsive interactions between various

25

I

I

50

7%

PMMA

12-34 wt% AN

8-30 [51

lOO

pMSAN (%)

Fig. 7. T=-composition curve ( 0 ) and cloud point curve (O) of miscible PnBMA/pMSAN 13.6 blends.

SAN 9-35 wt% AN [4]

PEMA PnPMA PnBMA

12-33 6-28 12-14

9-39 [6] 5-30 [5] 5.5-20 [5] Immiscible [5]

Miscibility of polymethacrylates with pMSAN

925

Table 7. Segmental interaction parameters pMSAN blends

SAN blends

ZMM^/pMS= 0.037, ZEMA/pMS = 0.036,

ZMMAIAN =

Xnr~^/pMS = 0 . 0 1 8 ,

Z.r~^/AN = 0 . 6 4

~EMA/AN =

0.56 0.56

ZMMA/S= 0.03, ZMUA/AN = 0.46 ZEMA/S= 0.016, ~EMA/AN = 0.57 ;.e.PMA/S = 0 . 0 1 3 , ~*PU^/^N = 0.64

;C,aMA/pMS= 0.019, Z*BMA/AN = 0.70 methacrylates with acrylonitrile are m u c h stronger, a n d the interaction becomes m o r e intense with increasing bulkiness o f the methacrylate. F u r t h e r more, the various methacrylate/acrylonitrile interaction p a r a m e t e r s based o n the present work are in good a g r e e m e n t with those based o n the S A N systems. A n o t h e r interesting p o i n t is t h a t the m e t h a crylate/p-methylstryene interaction p a r a m e t e r s are slightly larger t h a n the c o r r e s p o n d i n g m e t h a crylate/styrene interaction parameters. This result is consistent with o t h e r recent studies t h a t the segmental interaction p a r a m e t e r between p - m e t h y l styrene a n d a n o t h e r segment is larger t h a n t h a t between styrene a n d the same reference segment [15, 16, 21-26]. In s u m m a r y , P M M A , P E M A , P n P M A a n d P n B M A are miscible with p M S A N over certain c o p o l y m e r c o m p o s i t i o n ranges. T h e phase b e h a v i o u r o f the p M S A N blends is, in general, similar to t h a t o f the S A N blends.

Acknowledgements--The authors thank the National University of Singapore for financial support of this research and Miss C. S. Lee for assistance in GPC measurements of p M S A N samples. REFERENCES

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