Kinetics of Phase Segregation in Thermotropic Liquid-Crystalline

Chapter 31

Kinetics of Phase Segregation in Thermotropic Liquid-Crystalline Copolyester and Polyether Imide Blends

Downloaded by VIRGINIA TECH on February 27, 2015 | http://pubs.acs.org Publication Date: August 24, 1990 | doi: 10.1021/bk-1990-0435.ch031

Johnny Q. Zheng and Thein Kyu Institute of Polymer Engineering, University of Akron, Akron, OH 44325 Phase segregation in the mixtures of polyethylene terephthalate and polyhydroxybenzoic acid copolymer (PET-PHB) and polyether imide (PEI) has been examined by means of differential scanning calorimetry, optical microscopy and laser light scattering. The solvent cast blends from mixed solvents of phenol and tetrachloroethane (60/40) show a single glass transition (Tg) and is optically clear. Thermally induced phase separation takes place upon heating above the Tg of PEI. A phase diagram was established on the basis of cloud point determination which is reminiscent of a lower critical solution temperature (LCST). However, we are unable to confirm the reversibility of the phase diagram. Several temperature jump (T-jump) experiments were carried out from ambient to two-phase temperature regions. Phase separation occurs via spinodal decomposition and is dominated by non-linear behavior. The time-evolution of scattering curves were analyzed in the context of non-linear and dynamical scaling theories. In a previous paper (1), phase segregation by spinodal decomposition in mixtures of polyethylene terephthalate and polyhydroxybenzoic acid copolymer (PET-PHB) and polycarbonate (PC) has been investigated. It was shown that thermally induced phase segregation takes place above the Tg of PC and exhibits a lower critical solution temperature (LCST). However, the phase separated domains do not grow until the temperature exceeds 255°C. Some disclinations developed within the liquid crystal rich regions. Even in the pure PET-PHB component, four dark brushes with negative sense of disclinations form around 240°C, indicating the presence of nematic liquid crystals. Paci and coworkers (2) claimed that a smectic-nematic transition exists near 270°C in this liquid crystalline copolyester. 0097-6156/90/0435-O458$O6.00/0 © 1990 American Chemical Society In Liquid-Crystalline Polymers; Weiss, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

31. ZHENG & KYU

Kinetics ofPhase Segregation

459

In this paper, we continue our study on the kinetic aspects of thermally induced phase segregation in PET-PHB/polyether imide (PEI) blends. We select PEI because it is easier to prepare a thicker miscible film of PET-PHB blended with PEI as compared to that with PC. Solvent induced crystallization of PC (3,4) often presents added difficulty in the analysis of phase segregation. This problem can be avoided in the present case because PEI is amorphous under the present experimental conditions. Several temperature (T)-jump experiments were carried out from ambient to 260, 270 and 280°C. The kinetic results are compared with those obtained for polymer blends (5-9). molecular composites (10), and thermotropic liquid crystals containing polymer alloys (11,12). Downloaded by VIRGINIA TECH on February 27, 2015 | http://pubs.acs.org Publication Date: August 24, 1990 | doi: 10.1021/bk-1990-0435.ch031

EXPERIMENTAL Copolyester of p-hydroxybenzoic acid with ethylene terephthalate (PHB-PET, 60/40) was supplied by Tennessee Eastman Kodak Co., whereas polyether imide (PEI) was provided by General Electric. Co. (Ultem 1000). These polymers were dissolved together in a mixed solvent of phenol and tetrachloroethane in the ratio of 60/40 by weight at 80°C for about a week. The polymer concentration of the solution was 2 wtX. Various PHB-PET/PEI films were cast on glass slides at ambient temperature, then dried in a vacuum oven at 60°C for two weeks. Thicker films were prepared in Petri dishes for differential scanning calorimetric (DSC) studies. A cloud point phase diagram was established at l°C/min, using small-angle light scattering (SALS). The SALS set-up consists of a He-Ne laser light source with a wavelength of 632.8 nm. The scattered intensity was monitored by means of a two-dimensional Vidicon camera (Model 1254, EG k G Co.) interlinked with an Optical Multichannel Analyzer (OMA III). SALS pictures were photographed with a Polaroid instant camera (Land film holder 545). Optical micrographs were obtained on a Leitz optical microscope (Laborlux 12 Pol). Several temperature jump (T-jump) experiments were carried out from room temperature to 260, 270 and 280°C. Differential scanning calorimetric runs were conducted on a Du Pont Thermal Analyzer (Model 9900) at an arbitrary heating rate of 10°C/min. Indium standard was used for temperature calibration. RESULTS AND DISCUSSION Miscibility Phase Diagram. The solvent casting films of PHB-PET/PEI mixtures were transparent and scattered no light, suggestive of miscible character. Figure 1 shows DSC thermograms for various blend compositions. The neat PHB-PET reveals a small Tg in the vicinity of 65°C and a small endotherm around 195°C. There is no clearcut view on this melting transition. Viney and Windle (13) postulated that this peak represents melting out of regions of local order which, at low temperature, pin down the microstructure. The authors noted that the polymer has gained molecular mobility above this transition. The disclinations with four-fold brushes were observed under polarized microscope which rotates in the same direction of the polarizers, suggesting

In Liquid-Crystalline Polymers; Weiss, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


460

LIQUID-CRYSTALLINE POLYMERS

positive sense of 1. In a similar temperature region (240°C), Shiwaku et a l . (12) observed the Schlieren textures with the strength of -1/2 and 1. On the other hand, Paci et a l . (2) claimed a smectic-nematic transition around 270°C. Hence, there is no agree upon opinion on the nature of the mesophase region, but this melting transition may be associated with the crystal to mesophase transition (13). In their blends with PEI, the melting transition of PHB-PET is no longer discerned. Instead, a single Tg appears in the intermediate blends and moves systematically with blend compositions. The movement of Tg with composition, as shown in Figure 2, appears to follow the prediction of Fox (14), indicating that PHB-PET and PEI may be a miscible pair or they merely form a kinetically entrapped single phase. When the blend films were annealed at 250°C, the films turned translucent within a minute, suggesting the occurrence of thermally induced phase segregation. Tiny, but interconnected domain structures developed under optical microscope. A large, but very weak scattering halo was discerned in light scattering studies. These features are familiar characteristics of spinodal decomposition. As can be seen in Figure 3, the average size of the phase separated domains does not get larger with longer annealing time up to 10 h, implying that phase growth is prohibited at that annealing temperature. However, when the temperature is raised instantaneously from ambient to 260°C, thermally induced phase separation occurs within a few second and the domain size increases subsequently. Figure 4 shows a typical evolution of domain structure of the 50/50 blends as a function of annealing time at 270°C. Within the liquid crystalline polymer rich regions, birefringent entities can be discerned. The Vv (vertical polarization with vertical analyzer) scattering shows a scattering halo corresponding to the periodic composition fluctuations of phase separated domains (Figure 5). The scattering halo becomes smaller with elapsed time as a result of phase growth. According to Viney and Windle (13), a small endotherm was observed around 250°C which is difficult to be reproduced within 5°C. We occasionally observe such a DSC peak in some scans of pure PHB-PET or its blends with PEI, but it is absent in the second runs. However, Paci and coworkers (2) observed a transition around 270°C and attributed to a smectic-nematic transition. The cloud point measurement was undertaken for 40/60 PET-PHB/PEI at a heating rate of l°C/min by measuring the scattered intensity at a scattering wavenumber q = 2. Here q is defined as (4T/A) sin(0/2) with X and 0 being the wavelength of light and the scattering angle measured in the medium. The intensity begins to change its slope slightly above the Tg of PEI due to phase segregation. However, the increase of intensity is insignificant below 250°C. This is consistent with the optical microscopic investigation that the phase separated domains do not grow for some i n i t i a l period, e.g. for about 2 h. At present, it is unclear to us why the domains are unable to grow below 250°C. It may be speculated that the moment the mixture undergoes phase separation, the LCP molecules tend to self-associate due to the large aspect


31. ZHENG & KYU



P E T - P H B / PEI

T (°C) Figure 1. DSC thermograms of varous blends of PET-PHB/PEI, displaying a single glass transition for intermediate compositions.

250

200 y

u o

150

h

100

0

20

40

LCP

60 (wt

80

100

%)

Figure 2. The variation of Tg as a function of blend composition.


461

LIQUID-CRYSTALLINE POLYMERS


462

Figure 3. Optical micrographs of 50/50 blends for various isothermal annealing time at 250°C.



Figure 4. The evolution of domain structure of 50/50 blends as a function of annealing time at 270°C.


2


Figure 5. The change of Vv s c a t t e r i n g r i n g as a function of temperature.


3g

g

0

1

1

S 0

E o

31. ZHENG & KYU


465


ratio of rigid molecules, thus aligning themselves to form anisotropic regions. This ordering process may suppress the molecular mobility of LCP regions, thereby restricting the phase growth process. When the temperature increases further beyond 260°C, the system gains additional mobility which would permit molecular transport so that the domains can grow further. Similar experiments were conducted for other compositions and a cloud point phase diagram was established as shown in Figure 6. As expected the cloud point phase diagram resembles a lower critical solution temperature (LCST) in character. The formation of liquid crystal phase makes it difficult to confirm the reversibility of the phase diagram; thus it should be regarded as a virtual one. Dynamics of Spinodal Decomposition. Several temperature (T)-jump experiments were undertaken for 50/50 PET-PHB/PEI from ambient to 250, 260, 270, and 280°C. At 250°C, a very weak scattering ring is observed, but the intensity does not appreciably increase for a considerable period of approximately 10 h. The peak position remains virtually constant, indicating that the phase separated regions are not getting larger with time which is exactly what was observed in the optical microscopic investigation. Figure 7 shows the time-evolution of scattering curves for 260 to 280°C, in which the scattering peak appears at relatively low wavenumbers, implying the large average size of concentration fluctuations. There is no initial period where the peak position is stationary (15). The lack of linear regime may be understandable because phase separated domains are already formed at relatively low temperatures, say 240°C or less. Hence, we are probably detecting the late stages of the growth process exclusively at such high T-jumps. In this regime, it is difficult to distinguish whether the growth process occurs via spinodal decomposition or nucleation-growth. Judging from the optical micrographs and the scattering halo, although by no means conclusive, the SD mechanism may be appropriate. This non-linear growth character may be best explained in terms of power law relations of maximum scattering wavenumbers (qm) and the corresponding maximum scattered intensity (Im) versus phase separation time (t) as follows, q.(t)-t-P

(i)

I.(t) - t '

(2)

and

where the subscript m stands for the maximum values. The kinetic exponents (p and ij) are predicted to have various values depending on the time scale. On the basis of the non-linear statistical consideration, Langer, Bar-on, and Miller (LBM) (16), obtained a value of ip = 0.21. Binder and Stauffer (17) postulated a relationship $ = 3

Kinetics of Phase Segregation in Thermotropic Liquid-Crystalline

Recommend Documents