Rheological Behavior of Liquid-Crystalline PolymerPolymer Blends

Chapter 8

Rheological Behavior of Liquid-Crystalline Polymer—Polymer Blends 1

D. Dutta and R. A. Weiss

Downloaded by UNIV OF ARIZONA on August 6, 2012 | http://pubs.acs.org Publication Date: May 13, 1991 | doi: 10.1021/bk-1991-0462.ch008

Polymer Science Program and Department of Chemical Engineering, University of Connecticut, Storrs, CT 06269-3136

This paper reviews the literature on the rheological behavior of liquid crystalline polymer/polymer blends. The advantages of blending a LCP with a thermoplastic polymer is that the LCP serves as a processing aid and can also form a reinforcing phase. Rheology plays an important role in determining the morphology and hence the mechanical properties of these blends. This review also discusses the slow behavior of neat liquid crystalline polymers. In recent years, numerous studies of blends containing a l i q u i d c r y s t a l l i n e polymer (LCP) have been published. The main objective of the majority of these was to develop a f i b r i l l a r morphology of the dispersed LCP phase that could act as a reinforcement and improve the mechanical properties of the host polymer. For blends of thermotropic LCPs and thermoplastics, both phases are l i q u i d s at processing temperatures. As a consequence, the v i s c o s i t y increases usually associated with f i l l i n g thermoplastics with conventional r e i n f o r c i n g f i b e r s such as glass or graphite do not occur. In f a c t , because of the anisotropic nature of an LCP melt, the melt v i s c o s i t i e s of blends with f l e x i b l e chain thermoplastics are often lower than that of the thermoplastic. Thus, blending LCPs into thermoplastic polymers can result i n improved p r o c e s s a b i l i t y and reduced power consumption. A second consequence of blending LCPs into thermoplastic polymers i s the development of a r i g i d , r e i n f o r c ing LCP phase when the processed melt i s cooled and s o l i d i f i e d . In some cases, the orientation of the LCP phase generated during processing may be retained a f t e r s o l i d i f i c a t i o n due to the long relaxation times of LCPs. In t h i s way, one can preserve a microf i b r i l l a r morphology of a dispersed LCP phase i n the fabricated blend, and the mechanical properties of such self-reinforced blends can approach those of f i b e r g l a s s - r e i n f o r c e d p l a s t i c s . The a b i l i t y to develop these morphologies i s influenced to a great extent by the rheology of the LCP and the rheology of the blend. 1

Corresponding author 0097-6156/91/0462-0144$06.00/0 © 1991 American Chemical Society

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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Behavior ofLiquid-Crystalline Polymer-Polymer Blends 145

The purpose of t h i s a r t i c l e i s to review the published l i t e r a ture on the rheology of l i q u i d c r y s t a l l i n e polymer blends. A short section on the flow behavior of thermotropic LCPs i s presented before dealing with the rheological and mechanical properties of LCP/polymer blends.


RHEOLOGY OF THERMOTROPIC LIQUID CRYSTALS In thermotropic l i q u i d c r y s t a l s the mesophase t r a n s i t i o n s are brought about by changing temperature. Porter and Johnson (1,2) reviewed the rheology of l i q u i d c r y s t a l s (LC). The general v i s c o s i t y behavior of a thermotropic LC as a function of temperature i s represented schematically i n Figure 1. A decrease i n the v i s c o s i t y , associated with the transformation from an isotropic melt to an anisotropic mesophase i s usually observed when temperature i s lowered. The v i s c o s i t y again increases as the temperature of the nematic mesophase i s decreased. In the mesophase the molecules orient along the flow d i r e c t i o n and s l i d e past one another, while i n the i s o t r o p i c melt, they can be highly entangled. The orientat i o n of the molecules i s responsible f o r the lower v i s c o s i t y of the nematic mesophase r e l a t i v e to the i s o t r o p i c melt. Baird (3) and Wissbrun (4) reviewed the l i t e r a t u r e on LCP rheology. Onogl and Asada (5) proposed that the v i s c o s i t y vs. shear rate behavior for LCPs can be represented by three d i s t i n c t regions: 1. a shear thinning region at low shear rate 2. a Newtownian (plateau) region i n an intermediate shear rate region and 3. a power-law shear thinning region at high shear rate (Figure 2 ) . Very few sets of data show a l l the three regions i n a single polymer. However, i n h i s review a r t i c l e , Wissbrun (4) was able to i d e n t i f y the three flow regions by analyzing the published data of a number of authors f o r both thermotropic and lyotropic LCPs. An important c h a r a c t e r i s t i c of polymeric l i q u i d c r y s t a l s i s that they have longer relaxation times compared with f l e x i b l e c o i l polymers ( 4 ) . For example, Suto et a l . (6) studied the b i r e f r i n g ence decay of hydroxypropyl c e l l u l o s e (HPC) and ethyl c e l l u l o s e (EC) after cessation of shear flow and found that whereas the birefringence decayed on the order of seconds f o r f l e x i b l e c o l l polymer melts l i k e polystyrene and polypropylene, the LCPs had relaxation times of the order of ten minutes or more. S i m i l a r l y , Jackson (7) reported that polyethylene terephlhalate (PET)/parahydroxybenzoic acid (PHB) copolyesters have much longer terminal relaxation times than PET. In these polymers, the relaxation times increased as the PHB content i n the copolyester increased due to decreased chain f l e x i b i l i t y . Jerman and Baird (8) proposed that for LCP melts, two relaxation times were important: 1. f o r the stresses and 2. f o r the orientation. While these phenomena were related through the s t r e s s - o p t i c a l law f o r conventional f l e x i b l e c o i l polymers, they were independent f o r LCPs. The o r i e n t a t i o n a l relaxation time was longer than the stress relaxation time, and as a consequence, the orientation of thermotropic LCPs achieved during processing was retained i n the s o l i d state more e a s i l y than f o r f l e x i b l e chain polymers. The type of deformation (shear or extension) plays an important r o l e i n determining the orientation, texture and morphology of


146

POLYMERS AS R H E O L O G Y MODIFIERS


η

Nematic Mesophase

Isotropic Phase

Temperature Figure 1. Schematic representation of the viscosity (rj) versus temperature relationship for a nematic liquid crystal.

Log Shear Rate

Figure 2. Schematic of the three distinct regions of flow behavior for thermotropic LCPs. (Reproduced with permission from reference 5. Copyright 1980 Plenum.)



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Behavior of Liquid-Crystalline Polymer-Polymer Blends 147

an LCP (3). Ide and Ophir (9,10) showed that i n extensional flow, the rod-like domains of an LCP were stretched and the molecules aligned along the flow d i r e c t i o n . Shearing d i d not orient the molecules i f the LCP domains were stable. For i n j e c t i o n molded LCPs, the skin region was generally highly oriented because of extensional flow near the walls, but the core regions that experienced mainly shear flow were r e l a t i v e l y unoriented. Baird et a l . (11) reported similar r e s u l t s and obtained electron micrographs that showed that the highly oriented skin layer had a fibrous texture while the core layer that experienced shear flow did not. Polymer melt extrudates generally tend to swell as a consequence of recoverable stress related to the deformation i n the d i e entrance. In contrast, l i q u i d c r y s t a l l i n e PET/PHB copolymers exhibited n e g l i g i b l e extrudate swell and i n some cases, contraction of the extrudate was reported (8,12-13). Jerman and Baird (8) reported that extrudate swell increased with increasing temperature. The n e g l i g i b l e extrudate swell was attributed to y i e l d i n g of the LCP melt, similar to what i s observed i n f i b e r - f i l l e d isotropic melts. Another possible explanation involved the correspondence of the extruded swell with the negative f i r s t normal stress difference (N^) that has also been reported for LCPs. RHEOLOGY OF LCP/POLYMER BLENDS LCPs can be used as processing aids by blending with thermoplastic polymers. The LCP phase i s p r e f e r e n t i a l l y oriented i n the d i r e c t i o n of flow such that oriented LCP domains translate without entanglements. In t h i s way, they can be viewed as a lubricant f o r the polymer melt, which reduces the e f f e c t i v e v i s c o s i t y of the blend. The lowering of v i s c o s i t y not only reduces the energy consumption during processing but f a c i l i t a t e s the f i l l i n g of large and complex molds. The temperature at which the blend i s melt processed plays an important r o l e i n determining the effectiveness of the LCP as a processing a i d . According to Cogswell et a l . (14-16), the temperature range at which the thermoplastic polymer i s melt processed must overlap the nematic temperature zone of the LCP. Most studies of LCP/polymer blends d i d f i n d a lowering of v i s c o s i t y by the addition of an LCP. Other factors such as the shear rate, temperature and melt morphology also affected the v i s c o s i t y of the blend. In the published studies of LCP/polymer blends, rheological characterization has r e l i e d mainly on c a p i l l a r y viscometry, though several researchers also used cone and plate and p a r a l l e l plate rheometers to get data at lower shear rates. More important than the d i f f e r e n t shear rate regimes investigated, however, i s the fact that i n c a p i l l a r y viscometry the shear region i n the c a p i l l a r y i s preceeded by extensional flow i n the entrance. This can perturb the blend melt morphology and y i e l d s i g n i f i c a n t l y d i f f e r e n t v i s c o s i t y r e s u l t s than one would achieve i n shear alone. One l i q u i d c r y s t a l l i n e copolyester that has been used extens i v e l y i n blends i s the copolyester of 6-hydroxy-2-naphthoic acid (HNA) and parahydroxybenzoic acid (PHB) (17-25). Several grades of t h i s LCP have been commercialized by Hoechst-Celanese Corp. under the trade name Vectra and by Imperial Chemical Industries L t d .

American Chemical Society Library 1155 16th St., N.W. In Polymers as Rheology Modifiers; Schulz, D., et al.; Washington, D.C 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


148


under the name Vitrex . Some of these grades also contain terepht h a l i c acid (TA) and hydroquinone (HQ) comonomers. Nearly a l l researchers have observed that the addition of t h i s LCP r e s u l t s i n a decrease of v i s c o s i t y of the blend. Siegmann and coworkers (17) studied blends of HNA/PHB LCP with an amorphous polyamide and observed a large reduction i n v i s c o s i t y , as measured with a c a p i l l a r y viscometer, with the addition of as l i t t l e as 5 wt. % LCP. The melts exhibited non-Newtonian behavior, which could be described by a power-law c o n s t i t u t i v e equation over a limited shear rate region. Swaminathan and Isayev (19) observed similar behavior f o r blends of t h i s type of LCP and polyether sulfone (PES). James et a l . (24) and Froix et a l . (25) also reported a reduction of v i s c o s i t y of polyether sulfone (PES) due to addition of PHB/HNA LCP. The flow curves of blends containing up to 20 wt. % LCP resembled that of pure PES while the 50% LCP blends showed shear thinning behavior that was s i m i l a r to that of the pure LCP. James and co-workers (24) also observed a four-fold decrease i n v i s c o s i t y with the addition of j u s t 2% LCP. Malik et a l . (22) found that blends of HNA/PHB LCP with polycarbonate (PC) had lower v i s c o s i t i e s and were more shear thinning than PC. Solid-state relaxation measurements indicated that the relaxation modulus also increased with the addition of LCP. Isayev and Modic (18) studied similar blend systems and observed a cross-over point i n the flow curves of the LCP and PC, Figure 3. The cross-over can be taken as the point where the v i s c o s i t y r a t i o of the neat components i s approximately equal to unity. Maximum f i b r i l l a t i o n of the LCP during flow occurred at t h i s cross-over point. K o h l i et a l . (23) investigated blends of PC and a lower melting point LCP based on HNA/PHB/TA/HQ. Blend compositions ranging from 5 wt % LCP to 80 wt % LCP were investigated and compared with the pure components. In contrast to the r e s u l t of Isayev and Modic (18), they found that the v i s c o s i t y decreased with increasing LCP content over the entire range of composition. (Figure 4). The shape of the flow curves were determined by the rheology of the polymer that made up the continuous phase. At low LCP content the PC constituted the continuous phase while a phase inversion occurred around 40-50 wt. % LCP content. Above t h i s composition the flow curves were similar to that of the LCP. The blends were pseudoplastic and no Newtonian region was observed. Blend studies with the copolyester of PET and PHB as the LCP component also showed a reduction i n v i s c o s i t y by the addition of LCP. B l i z a r d and Baird (26) investigated blends of PET/PHB LCP with Nylon 66 and polycarbonate. Dynamic o s c i l l a t o r y and steady shear data showed a s i g n i f i c a n t reduction of v i s c o s i t y with the addition of the LCP, Figure 5. Similar r e s u l t s were also reported by Acierno et a l . (27) for blends of PET/PHB and PC and by Zhuang et a l . (28) for blends of PET/PHB with PC, PET or polystyrene (PS). Several researchers have found that f o r c e r t a i n blends the v i s c o s i t y d i d not decrease monotonically with increasing LCP content (29-37). This was attributed to factors such as the melt morphology and temperature. Chung (29,30) studied blends of HNA/PHB LCP and Nylon 12 and found a minimum i n the v i s c o s i t y at 10 wt. %


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Behavior of Liquid-Crystalline Polymer—Polymer Blends 149

PC - HBA/HNA


10' 0 9 Ψ 0 • α A

10'

100/0.0 97.5/2.5 95.0/5.0 90.0/10.0 75.0/25.0 50.0/50.0 0.0/100

4μ 10'

10*

J

10

w

10

10*

%

10

Figure 3. Shear stress ( σ ) versus shear rate data for HBA/HNA—PC blends showing the crossover point of the LCP and PC flow curves at -7 = 44/s. (Reproduced with permission from reference 18. Copyright 1987 Society of Plastics Engineers.) 1 2


150



SHEAR RATE «rt

Figure 4. Melt viscosity versus shear rate at 270 ° C for LCP/PC blends: • , PC; O , 95% PC/5% LCP; 90% PC/10% LCP; 0 , 80% PC/20% LCP; · , 60% PC/40% LCP; Δ , 40% PC/60% LCP; • , 20% PC/80% LCP; and V , LCP. (Reproduced with permission from reference 23. Copyright 1989 Society of Plastics Engineers.)

PC BLENDS (L/D-75.II, 2 6 0 C , 1000/sec) e

I

l

I

I

I

I

I

0

10

20

30

40

50

I

I

60

70

ι

'

80

90

'

100

PERCENT 60PHB-PET Figure 5. Melt viscosity of LCP/PC blends as a function of LCP content. (Reproduced with permission from reference 26. Copyright 1987 Society of Plastics Engineers.)



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Behavior of Liquid-Crystalline Polymer-Polymer Blends 151

LCP concentration and a maximum at 20 wt. % LCP content. He proposed that below 10% LCP content, the LCP domains were well dispersed and i n the Nylon 12 matrix and acted as a lubricant, but at 20 wt. % LCP an interconnected morphology formed, thereby increasing the v i s c o s i t y of the melt. Lorenzo et a l . (31) found that temperature had a s i g n i f i c a n t effect on the melt flow index (MFI) and complex v i s c o s i t y η*, of blends of PET/PHB and styrene-butadiene copolymer. When the blends were extruded below the melting point of the LCP, η* was independ ent of the LCP content. But when the extrusion was done above the melting temperature of the LCP, MFI and η* showed minima at 10 wt. % LCP concentration. The v i s c o e l a s t i c behavior of blends of PET/PHB LCP and PC was investigated^by Nobile et a l . (32). At low shear frequency rates (below 0.3s~ ) the complex v i s c o s i t y η* increased with increasing LCP content. At higher frequencies, however, η* decreased with increasing LCP concentration. S i m i l a r l y , Weiss et a l . (33-37) observed that the addition of a thermotropic LCP based on A^'-dihydroxyjaja'-dimethylbenzalazine to PS raised the steady shear and dynamic v i s c o s i t i e s at r e l a t i v e l y low shear rates (

Rheological Behavior of Liquid-Crystalline PolymerPolymer Blends

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