Destruction of Short-Range Order in PolycarbonateâIonomer Blends

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Chapter 21

Destruction of Short-Range Order in Polycarbonate-Ionomer Blends 1

2

3

4

Ryan Tucker , Barbara Gabrýs , Wojciech Zajac , Ken Andersen , M. S. Kalhoro , and R. A. Weiss 2

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1

Department of Chemical Engineering, University of Connecticut, 91 North Eagleville Road, U-136, Storrs, CT 06269 Physics Department, Brunei University of West London, 4, Toynbee Close, North Hinksey, Oxford 0X2 9HW, United Kingdom The Henryk Hiewodniczanski Institute of Nuclear Physics, Radzikowskiego 152, 31-342 Krakow, Poland institute Laue-Langevin, Grenoble, France

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Spin polarized neutron scattering was used to study the short-range order in partially miscible blends of bisphenol A polycarbonate (PC) and lightly sulfonated polystyrene ionomers (SPS). The blends exhibited upper critical solution temperature phase behavior. In the two-phase region of the polymer blend, the short-range order for PC persisted. However, in the miscible, one-phase region, there was a significant reduction of the short-range order of the PC as a result of intimate mixing of the polymers.

Order in amorphous polymers has been debated for over 25 years. One school of thought advocates that local order in polymers is not possible if the chains assume a random Gaussian coil conformation; the other argues that a short-range is present even in melts (/). Both camps give interpretation of experiments as the ultimate proof, and in this respect, the scattering techniques are considered to provide the most direct evidence for either case. The random coil-conformation of high molecular weight bulk polymers has been demonstrated by small angle neutron" scattering (SANS) experiments (2, 3, 4). Notwithstanding the conclusions from SANS experiments, evidence for local order in amorphous polymers has also been advanced. For example, Geil, Yeh, and their coworkers (5, 6, 7) reported observations of granular or nodular structures with sizes 328

© 2000 American Chemical Society

Cebe et al.; Scattering from Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

329


on the order of 10-100 Â on surfaces of PC, polyethylene terephthalate (PET) and polystyrene glasses (7). Frank et. al. (8) observed 50-100 Â granular structures in PC. The evidence for short-range order from electron microscopy studies has been questioned, because those studies dealt with thin films or surfaces, and may not be representative of the bulk polymer. Harget and Siegmann (9), however, used small angle x-ray scattering (SAXS) measurements, which do sample the bulk polymer, to identify nodular structures in amorphous PET that were similar to the local structures seen in microscopy studies. Gabrys et. al. reported the existence of short-range order in amoprphous poly (methyl methacrylate) using wide angle neutron (WANS) with spin polarization analysis (10). Lin and Kramer (//) performed SAXS experiments on amorphous PC, and they reported large-scale electron density fluctuations that were also consistent with the nodular structures observed by electron microscopy. Amorphous PC has also been investigated by several groups using wide angle x-ray and neutron scattering (12, 13, 14, 15, 16). Cervinka et. al. (13) used WANS to study hydrogenous and deuterated derivatives of PC, and they found reasonable agreement between their experimental data and an "amorphous-cell" model developed by Suter (17), in which the polymer chains take on a trans-trans conformation and lay parallel to one another. Lamers et. al. (12) used WANS with spin polarization analysis to characterize short-range order in bulk PC and three chemically modified polycarbonates. Their results agreed with the model of Cervinka et. al.; they found that the distance between neighboring chains was 4.95 Â, and the correlation length (i.e., the distance over which the shortrange order exists) was 28 Â. Recently, Eilhard et. al. (16) combined experimental and theoretical approaches in order to determine structural properties of glassy PC. The neutron scattering experiments with spin polarization yielded a purely coherent part of the scattering, ideally suited for comparison with structure simulation models using newly developed mapping procedures. This approach gave the best agreement between simulated and measured data of all the attempts to determine the structure of PC. PC is often blended with other polymers (18). Weiss and coworkers (19, 20) reported that lightly sulfonated polystyrene ionomers (SPS) were miscible with PC as a result of the "copolymer effect" (21, 22, 23). That is, strong repulsive interactions between the ionic and nonionic segments within the ionomer favor mixing of the ionomer with PC even though there are no strong, intermolecular, attractive interactions. These blends exhibit upper critical solution temperature (UCST) phase behavior. Typical critical temperatures range from 170°C to 260°C, depending on the sulfonation level of the ionomer and the molecular weight of the polymers. The system also exhibits a miscibility window with respect to the degree of sulfonation, so that at a given composition and temperature, there is a finite range of sulfonation levels that produce miscibility. Changing the counterion used in the ionomer also affects the UCST and the miscibility window (79).


330 The objective of the work reported herein was to assess the effect of the addition of SPS on the short-range order in PC. Short-range order was determined using wide angle neutron scattering with spin polarization analysis.

Experimental Details


Materials SPS was prepared by solution sulfonation of polystyrene with acetyl sulfate following the procedure of Makowski et. al. (24). This method substitutes a sulfonic acid group at the para-position of the phenyl ring, randomly along the chain. The weightaverage molecular weight of the starting polystyrene as determined by gel permeation chromatography (GPC) was M = 280,000 and the polydispersity was 2.8. The sulfonation level was determined by titration of the sulfonic acid derivative, HSPS, in a mixed solvent of toluene/methanol (90/10 v/v) with methanolic sodium hydroxide. SPS with lithium and sodium counterions were prepared by neutralizing a solution of HSPS with the corresponding metal hydroxide. The nomenclature used for the ionomers was xj/MSPS, where x.y, and M denote the sulfonation level in mol % of styrene substituted and the counterion, respectively. Bisphenol A polycarbonate was obtained from General Electric Co. and had a M = 48,000. w

n

Blend Preparation Blends were prepared by adding a 3% (w/v) PC solution in tetrahydrofuran (THF) dropwise to a stirred 4% solution of MSPS in THF. The blend solution was then cast into Teflon dishes at 60°C and dried under vacuum.

WANS with Spin Polarization Analysis Neutron scattering experiments were performed on the D7 instrument at the high flux reactor of the Institute Laue-Langevin (Grenoble, France). In hydrogen-rich samples, strong incoherent scattering is a dominant effect, therefore, the incoherent scattering contribution must be treated and accounted for in some fashion. Typically, incoherent scattering is treated by means of arbitrary "background corrections" which can be unreliable. Spin polarization analysis allows one to separate coherent and incoherent scattering contributions, which provides a means of obtaining a reliable intensity calibration. Thus, the measured structure factors can be directly compared with model calculations without arbitrary adjustments (25). To measure the coherent and incoherent scattering intensity, scattering from non-flipped and spin-flipped


331 neutrons were detected and measured. The non-flip (NF) scattering intensity refers to scattered neutrons that maintain their spin direction. Spin-flip (SF) scattering refers to scattered neutrons that change their magnetic spin. The relation between non-flip and spin-flip intensities determine the coherent and incoherent scattering contributions (//):


X

NF

%

= 1

=

(1)

c o h +-Mn

(2)

I

| inc

The coherent structure factor S(q), in absolute units, can be derived from equations. 2 and 3;

S(q) =

dcj άΩ

2 ^SF

^NF

(3)

ISF

where (da/dO) is calculated from the chemical composition of the sample. The scattering wavevector range of interest was 0.5 < q (Â* ) < 2.5, with q = (47i/X)sin(0/2). The sample holder was equipped with a furnace to control temperature. inc

1

Results and Discussion It is now widely accepted that the presence of the short-range order in an amorphous polymer produces a peak in the WANS profile, such as peaks appearing at q = 0.6 Â" and q = 1.26 Â" shown in Figure 1. The first peak is interpreted as due to the correlation between the carbonate groups, with added influence of the isopropylidene groups (16), The more prominent, amorphous halo peak at q = 1.26 Â" is due to the correlation between the adjacent chains (12, 13, 16). The spacing between chains (D) and the correlation length (ξ) can be calculated from the peak position (q ) and the width (Aq) of a Gaussian fit to the scattering data using the following two equations. 1

1

1

p

D=

Y

2%

q

(4)

P

4%

Aq


(5)

332


For the second peak, analysis of Figure 1 yields for PC, D = 4.99 Â and ξ = 35.25 Â, which are in excellent agreement with the values of D = 4.95 Â and ξ = 28 À reported by Lamers et. al. (12) and those reported by Eilhard et. al. (16): D = 4.93 Â and ξ = 27.6 Â. The simulated values by Eilhard are D = 5.19 Â and ξ = 22.43 Â. (16). The scattering profile of 9%LiSPS is also included in Figure 1. In contrast to the PC data, the 9%LiSPS did not produce a sharp, intense peak, which indicates that the ionomer had little short-range order (27).

20

A

-I

• /

/

·

\

\

— P C 9%LISPS

·

—,—,—,—,—,—,—,—,—,—,— 0.5

1.0

1.5

2.0

2.5

q(AM)

Figure 1. WANS profiles of PC and 9%LiSPS.

Blends of polystyrene (PS) and PC are immiscible at all temperatures. Figures 2 and 3 show the WANS from PS/PC blends of several compositions. The weak peak at % = 0.6 Â* disappeared, but the peak at % = 1.26 Â" persisted and increased in intensity with increasing PC blend content, indicating that the short-range order of the PC was unaffected by the addition of the immiscible PS. 1

1

The 9%LiSPS ionomer forms a UCST with PC with a critical temperature of ~170°C (19). WANS curves of various blend compositions of 9%LiSPS and PC at 25°C, which is in the two-phase region of the phase diagram, are shown in Figures 4 and 5. The scattering from the two-phase 9%LiSPS/PC blends was similar to that for the immiscible PS/PC system. The characteristic, short-range order peak of PC was present at each blend composition, and the intensity of the peak scaled linearly with the PC content (Figure 6). Those results support the conclusion that the short-range order in PC is unaffected in a two-phase blend. WANS of blends of 5.1%NaSPS, 2%ZnSPS, and 8.45%HSPS with PC in the two-phase region were also measured, and for all compositions the peak characteristic of the short-range order in PC was present and depended linearly on the blend composition.


333


16

1

H 0.0

1

0.5

•

1

1.0

•

1

1.5

•

1

2.0

'

1

2.5

q (AM) Figure 2. WANS ofPS/PC blends: A, 75/25; B, 50/50.


334

35-^

i 3025-

/1


20-

CO

1510-

50-

o.o

1.0

1.5

q (AM)

Figure 3. WANS of (25/75) PS/PC blend.

181614-

12

A

10-

CO

864 21.0

1.5 A

q(A -1)

Figure 4. Scattering of9%LiSPS/PC blend in two-phase region, 75/25 composition.


335

!\ I \


-S

15J

ι

1.0

1.5

2.0

2.5

qfAM)

B

/i io-| 5-

w

00.0

0.5

1.0

1.5

2.0

2.5

A

q (A -1)

Figure 5. Scattering of 9%LiSPS/PC blends in two-phase region: A, 50/50; B, 25/75.


336

&

25


c

2 0 3 0 4 0 5 0 6 0

70

8 0 9 0

100

Weight Percent (%)

Figure 6. Peak intensity vs. PC weight percent.

The 9%LiSPS ionomer was miscible with PC above 170°C. Samples with different compositions were heated to 210°C, which is well within the one-phase region, and held at that temperature for 40 min. before the scattering was measured in order to allow sufficient time for the polymers to mix. Figure 7 compares the WANS data for a (50/50) 9%LiSPS/PC blend in the two-phase and one-phase regions. A clear and significant change in the scattering profile occurred between two-phase blend and the

30-,

H

0.0

,

, 0.5

,

,

,

,

1.0

1.5

,

, 2.0

. r2.5

q (A -1) A

Figure 7. (50/50) 9%LiSPS/PC blend. Scattering in one- and two-phase regions.


337


1

one-phase blend. The intensity of the PC short-ranger order peak at % = 1.27 Â' decreased by a factor of almost 2 and the peak increased in width. Figure 8 shows similar results for 75/25 and 25/75 blends, and in each case the short-range order peak decreased in intensity and broadened when the blend was moved to the miscible region of the phase diagram. Note that for the (75/25) 9%LiSPS/PC blend, the reduction of the peak intensity upon moving from the two-phase to the one-phase region is small. This small reduction is probably just a consequence of the low concentration of PC, but in may also be that the high concentration of the LiSPS disrupts the ability of the PC to form domains with short-range order. The results in Figures 7 and 8 indicate a decrease in the short-range order of the PC in the onephase region of the blends.

\ , , . , . , • , 0.0

0.5

1.0

1.5

2.0

r2.5

q (AM)

Figure 8. 9%LiSPS/PC scattering, 1- and 2-phase region: A, 25/75; B, 75/25.



338 The width of the WANS peak for the blends provides a measure of the level of mixing between the ionomer and PC, and more importantly, the level or "amount" of order in PC. The correlation length, ξ, is the measure of the length scale over which the short-range order exists in the blend. Table 1 shows the correlation length values for the 9%LiSPS/PC system in both the two-phase and one-phase region. For each composition, the correlation length decreases from the two- to the one-phase region, indicating that the length scale over which the PC short-range order exists is reduced when the polymer components are miscible. The 50/50 and 25/75 blends, which have the sharpest short-range order peak in the two-phase region, have the largest correlation length values, as well as the largest decrease in correlation length when the blends are taken into the miscible region. This significant change in correlation length for the 50/50 and 25/75 9%LiSPS/PC blends shows the extent of mixing that is occurring between the polymer components.

Table 1 : Correlation lengths for the packing of PC neighbored chains from experiment. 9%LiSPS/PC, ξ (Â) composition

75/25

50/50

25/75

2-phase region

20.79

49.61

55.11

1-phase region

16.86

11.48

7.62

To ensure that the results described above were indeed due to changes in the shortrange order of the PC and not simply temperature effects, e.g., thermal density fluctuations, scattering data were obtained on an immiscible blend, a (25/75) 5.1%NaSPS/PC blend, at 25°C and 200°C (see Figure 9). In this case, the change in the morphology of the blend upon heating is minor compared with that when a phase change takes place The short-range order peaks for the two temperatures in Figure 9 were nearly identical, which indicates that the phase changes and not temperature were responsible for the changes in the short-range order peak in Figures 7 and 8. That is, the reduction of short-range order of the PC occurred when the ionomer and the PC were miscible. Based upon the picture for the short-range order perfected by Eilhard et. al. (16), i.e., PC chain segments aligned in parallel trans-trans conformations, the ionomer appears to disrupt the parallel packing, which is consistent with intimate mixing of the two polymers.


339


35 η

—,

0.0

,

,

0.5

,

,

,

1.0

,

,

1.5

,

2.0

,

1—

2.5

q (AM)

Figure 9. 25/75 5.1 %NaSPS/PC at temperatures of 25 °C and 200 °C.

Conclusions Amorphous PC exhibits short-range order, which is evident from a WANS peak at % = 1.27 Â* . In two-phase blends of PC with either polystyrene or sulfonated polystyrene ionomers, the short-range order is retained by the PC. However, in the one-phase region, the PC order is diminished, as evident from a reduction in the WANS peak intensity and increase in the peak broadness. The reduction of the PC short-range order in the miscible blends is believed to result from the intimate mixing of the two polymers, which disrupts the parallel packing of PC chain segments with trans-trans conformations. 1

Acknowledgment This work was supported by a grant from the Polymers Program of the National Science Foundation (DMR 97-12194).

Literature Cited 1. 2. 3. 4.

Gabrýs, B; TRIP. 1994, 1, 2. Kirste, R.G; Kruse, W.A.; Schelten, J; Makromol. Chem. 1972, 162, 299. Benoit, H; Nature. 1973, 245, 13. Wignall, G.D; Schelten, J; Ballard, D.G.H.; Eur. Polym. J. 1973, 9, 965.


340


5. 6. 7. 8.

Yeh, G.S.Y.; Geil, P.H; J. Macromol. Sci. 1967, 1, 235. Carr, S.H.; Geil, P.H.; Baer, E; J. Macromol. Sci. 1968, 2, 13. Siegmann, A; Geil, P.H.; J. Macromol. Sci. 1970, 4, 239. Frank, W; Goddar, H; Stuart, H.A.; J. Polym. Sci., Part B: Polym. Lett. 1967, 5, 711. 9. Harget, P.J.; Siegmann, A; J. Appl. Phys. 1972, 43, 4357. 10. Gabrýs, B; Higgins, J.S.; Schärpf, O; J. Chem. Soc. Far. Trans.I.1986, 82, 1929. 11. Lin, W; Kramer, E.J.; J. Appl. Phys. 1973, 44, 4288. 12. Lamers, C; Schärpf, Ο; Schweika, W; Batoulis, J; Sommer, Κ; Richter, D; Physica B. 1992, 180, 515. 13. Červinka, L; Fischer, E.W.; Hahn, K; Jiang, B.Z.; Hellman, G.P.; Kuhn, K.J.; Polymer. 1987, 28, 1287. 14. Mitchell, G.R.; Windle, A.H.; Colloid Polym. Sci. 1985, 263, 280. 15. Schubach, H.R.; Heise, B; Colloid Polym. Sci. 1986, 264, 335. 16. Eilhard, J; Zirkel, A; Tschoep, W; Hahn, O; Kremer, K; Schärpf, Ο; Richter, D; Buchenau, U; J. Chem. Phys. 1999, 110, 1819. 17. Suter, U.W.; Theodoru, D.N.; Macromolecules. 1985, 18, 1467. 18. Lu, X ; Weiss, R.A.; Proc. Annu. Tech.Conf.,Soc. Plast. Eng. 1993, 684 - 686. 19. Lu, X ; Weiss, R.A.; Macromolecules. 1996, 29, 1216. 20. Xie, R; Weiss, R.A.; Polymer. 1998, 39, 2851. 21. Kambour, R.P.; Bendler, J.T.; Macromolecules. 1983, 16, 753. 22. Paul, D.R.; Barlow, J.W.; Polymer. 1984, 25, 487. 23. ten Bricke, G; Karasz, F.E.; Macknight, W.J.; Macromolecules. 1983, 16, 1827. 24. Makowski, H.S.; Lundberg, R.D.; Singhal, G.H.; U.S. Patent 3,870,841, 1975. 25. Schärpf, Ο; Gabrýs, B; Peiffer, D.G.; ILL Report 90SC26T (PTA), Institute Laue-Langevin. 1990. 26. Gabrys, B, Schärpf, Ο; Peiffer, D. G.; J.Polym.Sci.:Part B: Polymer Physics, 1993, 31, 1891.


Destruction of Short-Range Order in PolycarbonateâIonomer Blends

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