Chapter 10
Blends of Poly(amide imides) and Liquid-Crystalline Polymers Avraam I. Isayev and T. R. Varma Institute of Polymer Engineering, University of Akron, Akron,OH44325-0301
Poly(amide imides) (PAI) are high performance polymers which are capable of replacing metals. Though they are thermoplastics, they need highly specialized machinery for processing in order to achieve maximum property development. The difficulty in their processing is mainly due to high viscosity and melt reactivity. Attempts have been made to process the PAI precursor material using conventional thermoplastic processing methods by blending it with a liquid crystal polymer (LCP) which is in itself a high performance polymer with easy processability and self-reinforcing capabilities. The LCP is also capable of reducing the melt viscosity when blended with other thermoplastics. The PAI precursors were melt blended with the LCP using two methods: a corotating twin screw extruder, and a single screw extruder attached to a static mixer. The blends were injection molded and characterized, both before and after heat treatment. Studies were done to determine the thermal, flow and mechanical properties of the blends and the pure components.
Blending two or more characteristically different polymers resulting in a new material with improved properties is a very old practice. As an alternative to synthesizing new polymers, blending can be used to modify the thermal, mechanical, and processing characteristics of various existing polymers. High performance engineering thermoplastics, a class of polymers with high load bearing characteristics, resistant to chemical attack and having high continuous use temperatures have been modified by blending to suit specific application needs. Polyimides are engineering thermoplastics which are used when high quality parts and performance are required (1). The main drawback of these materials,
0097-6156/96/0632-0142$19.75/0 © 1996 American Chemical Society
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which overshadows the excellent properties of the fabricated parts, is their intractability. The intractability makes it very difficult, if not impossible, to melt process this class of materials and hence limits the mass production of polyimide parts. Therefore, attempts were made to develop a more tractable material without serious loss in properties. By using the synthesis route, polyimides have been modified by incorporating flexible functional groups, such as ether, ester or amide, into the main chain. This has led to the development of modified polyimides such as poly(bismalein imides), pory(ester imides), poly(ether imides) and poly(amide imides). Poly(amide imide) precursors have excellent mechanical properties after imidization, at temperatures up to 260 °C. It was first synthesized in the 60's as a material for high temperature wire enamel (2). This high performance thermoplastic was developed from research done by James Stephens of Amoco Chemicals (see Ref. 3). It became available as an injection molding resin in 1976. These materials were first applied to make burn-in electrical pin connectors. At present the usage has spread into the aerospace, transportation, chemical processing and electronics industries. The highly intractable chemical structure which imparts the outstanding mechanical properties also makes the PATs very difficult to process (4, 5). In the fully imidized form PAI is not processable hence a poly(amic acid) (PAA) precursor is the usual form in which they are supplied and fabricated. The precursors themselves have very high viscosities in the melt state and hence the flow characteristics tend to be very poor. Semicrystalline and amorphous polyamides (6) and aromatic sulfone polymers such as poly(phenylene sulfide), pory(ether sulfone) and polysulfone (7) have been blended with the precursor to PAI, to obtain better flow characteristics. Thermotropic liquid crystal polymers (LCPs) are a special class of engineering thermoplastics which form a highly ordered structure in molten states (8). Their rigid rod-like molecular conformation and the stiffness of the backbone chains impart a high degree of orientation during melt processing and forms fibrous structures in the final product. They are very easy to process and possess outstanding mechanical properties and high chemical resistance. However, they show a high degree of anisotropy in properties (9) which may be overcome by blending with flexible polymers (10). LCPs have also shown that they can impart fiber reinforcement when melt blended with other engineering thermoplastics (1014). Brief reviews on studies of blends of isotropic polymers and thermotropic LCFs are given by Brostow (15), by Isayev and Limtasiri (16) Dutta et al. (17), and Handlos and Baird (18). Since the LCP forms thefibersin the melt state, there is considerably lower wear and tear on the processing niachinery in comparison to conventional reinforcing glass fibers. There is also a viscosity reduction reported in the above studies which imparts better flow properties to the blends. Present authors are aware of only two studies reported in open literature (19, 20) where PAI blended with LCP. In these papers some results on thermal stability, processability and shear stability of these blends have been reported. The present study seeks to improve the flow properties of the PAA precursor in the melt state by melt-blending it with an LCP. It is hoped that this results in a
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LIQUID-CRYSTALLINE POLYMER SYSTEMS
blend which can be melt processable using conventional equipment with thermoplastic screws as opposed to highly specialized equipment with thermoset screws that are currently being used. The present work also seeks to understand the effect of the LCP on the imidization process in order to improve the mechanical properties of the PAI by utilizing the in-situ fibre forming capability, during processing, of the liquid crystal polymer with the intrinsic strength of the irnidized PAA matrix. Materials and Methods of Investigation The PATs used were Torlon 4000Tf-40 (PAI-1) and Torlon 4203L (PAI-2) supplied by Amoco Performance Products. The thermotropic liquid crystal polymer (LCP) used was Vectra A950, which is a random copolyester delivered from HBA-HNA (Hoechst Celanese). The PAI-1 was in fine powder form with no additives while the PAI-2 was in pellet containing 0.5% PTFE and 3% Ti0 . PAI-2 was melt-blended with the LCP using a six-element static mixture (Koch Engineering) attached to the exit of a 1" single screw extruder (Killion Inc.) at the screw speed of 30 rpm and temperature of 350 °C. The PAI-1 and PAI-2 were melt-blended with the LCP using a modular, corotating, fully mtermeshing twin-screw extruder with mixing elements (ZSK-30, Werner & Pfleiderer Corp.) at the screw speed of 200 rpm, the barrel temperature of 330 °C. The static mixer and the twin-screw blends are referred below as STM and TS blends, respectively. Molding was carried out using an injection molding machine (Boyl5S) at barrel and mold temperature 330 °C and 220 °C, respectively, and at maximum injection speed. The molding conditions are listed in Table 1. End-gated dumbbell shaped test bars of two sizes were molded: mini-tensile bars (MTB's) (0.155m χ 0.013m χ 0.0033m), and standard tensile bars (MTB's) (0.065m χ 0.0031m χ 0.0052m). Only TS blends were molded since the STM blends showed discoloration and voids in the extrudate indicating degradation. From the twin-screw mixed blends involving PAI-2, only the higher concentration (90 and 75% PAI-2) were molded since the other blend compositions (50, 25 and 10% PAI-2) showed distinct phase separation and delamination due to poor mixing. This was visible to the naked eye by the green color of PAI and the cream color of LCP. All blends involving PAI-1 were molded. Some of the molded samples of each blend were heat treated (cured) according to the recommended procedure by Amoco. The curing cycle was as follows: 166 °C, 216 °C, and 243 °C for 1 day each, and 260 °C for 2 days in a vacuum oven at -28 in. Mercury. PAI-1 powder was not injection molded since it cannot be molded using conventional machines. Molding of PAI-2 was also tried unsuccesfulry. An Instron capillary rheometer (Model 3211) was used to measure the rheological properties. A capillary die of diameter D=0.00127m and a length to diameter ratio, L/D, of 28.7 was used. Measurement was done at 330 °C and 350 °C for all PAI-1/LCP blends, but only for the 10% and 25% LCP concentration blends with PAI-2 since at all other concentrations a poor mixing was noticeable. 2
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A Dupont DSC 9900 Thermal Analysis System was used to analyze granulated extrudates of the blends and pure resins, both before and after heat treatment. The heating rate was 20 °C/min. A Monsanto Tensiometer (Model T-10) was used with an extensiometer to measure the tensile properties (ASTM D648) of the MTB's, both before and after heat treatment, at a crosshead speed of 5mrn/min. To measure the dynamic mechanical properties, a Dynamic Mechanical and Thermal Analyzer (DMTA, Polymer Laboratories) interfaced with a HP300PC was used. A single cantilever, and a heating rate of 4 °C/min. was used at the frequency of 1 Hz. and strain of 4%. An Impact Tester (Testing Machines, Inc.) was used to conduct the Izod impact test (ASTM D235C) on notched STB's of both cured and uncured samples. A Scanning Electron Microscope (SEM), Model ISI-SX-40m was used in the morphological study of the surface of MTB's broken in liquid nitrogen. Gold-palladium plasma coating technique was utilized to make the surfaces conductive. The skin and core morphologies of both cured and uncured samples were studied. Rheology Fig. 1 gives a comparison of flow curves of pure PAI-1 and PAI-2 and their blends with LCP. From that figure it follows that PAI-1 melt show higher viscosity than PAI-2 melt. That could be due to the presence of PTFE in PAI-2 which is used as processing aid for PAI. The viscosity of the 90/10 PAI/LCP blend is significantly lower than that of pure PAI. Possibly, the lower viscosity LCP melt in the blend migrates to region of higher shear rate leading to drop in viscosity of the blend. The drop is more significant in the case of the TS blends than the STM blends as indicated by Fig. 2 for PAI-2/LCP blends. The lower viscosity LCP acting as an external lubricating agent in combination with the lower residence time in the twinscrew extruder could cause the 90/10 PAI-2/LCP blend viscosity to be lower than that of the corresponding STM blend. The long residence time in the static mixersingle screw extruder system causes an increase in the viscosity by chain extension in the reactive PAI. In the case of the 75/25 PAI-2/LCP blend, the higher concentration of the LCP (lower concentration of PAI) causes the viscosity increase due chain extension to be lower. Various workers have proposed theoretical models to predict the flow behavior of polymer blends. Einstein studied the shear flow behavior of a suspension of rigid spheres in Newtonian fluids. Taylor (21, 22) extended this concept to include dispersions of one liquid in another liquid based on their shear viscosities and also accounted for circulation in the droplets. According to his model, for a component 2 which is dispersed in a component 1, the blend viscosity is given by the following equation: ^b = ^i[l+{{(^i+2.577 )/(/ +77 )}^ }] 2
7l
2
2
(1)
The main drawback of Taylor's equation is that it only predicts an increase in viscosity with addition of component 2 even if the viscosity of component 2 is lower
LIQUID-CRYSTALLINE POLYMER SYSTEMS
Table 1 - Injection Molding Conditions for PAI/LCP Blends
Barrel Temperature Mold Temperature Max. Shot Size Back Présure Nozzle Mold Cooling Time Injection Pressure Injection Speed Screw Speed Clamp Pressure
= = = = = = = = = =
330 °C 220 °C 1.24 oz OPa 100% 15 sec 13.75 MPa Max. 250 RPM 220 kN
Injection Molding Screw Characteristics Compression Ratio Length/Diameter Ratio Diameter Back Flow Valve
= = = =
3.2 17 24 mm Yes
Note * All barrel zones were kept at the same temperatures.
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^
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—
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100
—ι 10 SHEAR RATE, 1 / S
1 100
1000
Figure 1. Shear stress as a function of shear rate at 350 °C: Comparison of twin-screw mixed PAI-2/LCP (PEL) and PAI-1/LCP (PDR).
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than that of the matrix polymer, component 1. This equation assumes that no chemical interaction occurs between the dispersed and continuous phases, the dispersed phase forms uniform spherical droplets, and that the droplets retain their spherical shape in the shear flow field. This equation does not include the effect of droplet size on the viscosity. According to Chuang and Han (23), if no chemical interaction occurs between the phases, the experimentally observed viscosities would generally be lower than the theoretically predicted values. Figure 3 compares Taylor's prediction to the observed shear viscosities at two shear rates: 3.93 and 118.1 s . If PAI is taken as component 2 most of the predicted values, though lower, closely follow the experimental values. At PAI concentration of 90% the under-prediction is substantial. The predictions are better for higher shear rates at all concentrations. Blend viscosities show an increase in value with increasing PAI concentration which indicates a lack of interaction between the two phases. According to Han (24), studies on HDPE - EVA blends have shown that when deformation of the droplets is absent, the mixture gives viscosities greater than the viscosity of the less viscous of the two polymers. Since no maximum is observed in the case of the PAI/LCP system studied here, it is safe to presume that no interaction between the phases have taken place. Han also theorized that the viscosity of the droplet phase is much larger than that of the suspending medium. He also claimed that an inflection takes place when there is a phase inversion. A minimum of the viscosity occurs when elongated droplets are present givingriseto threadlike fibrils. In the PAI/LCP system, this is not observed. Figure 4 shows the predicted viscosity values when LCP is taken as component 2. As can be seen, the values are substantially over-predicted and at variance with the observed values. Hekmiller et al. (25) derived an expression for the viscosity of a mixture based on an inverse volume-weighted rule and assuming concentric layers of component 1 in component 2. Assuming a large number of layers the viscosity of the mixture is given as: -1
\Ιη =φ Ιη }>
ι
λ
+ φ Ιη 2
2
(2)
According to Ηβΐίτηί1ΐ6Γ·8 equation, the blend viscosity varies monotonically with the volume fraction ^ a n d ^ (φι+φ =ΐ) · As in the case of Taylor's prediction, Heitmiller's prediction also under-predicts the viscosity values, but, the predicted values compare better at higher, rather than lower, shear rates. This is shown in Figure 5 which gives the comparison of the curves of the experimental viscosity values and the predicted viscosity values using the Heitmiller equation. Hashin (26) extended the equations used successfully for the prediction of the upper and lower bounds for elastic modulus of a composite material and obtained a viscosity envelope for polymer blends. In case of mixture of Newtonian fluids the viscosity equation is: 2
2
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LIQUID-CRYSTALLINE POLYMER SYSTEMS
1E7 yf I
— ^ I
E]
Ο—Ο
1E6
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, o î s
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73/23 SU
φ
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73/23 ,o
V —V °/ °
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1E4--
1000
100 10 100 SHEAR RATE, 1 / S
1000
Figure 2. Shear stress as a function of shear rate at 350 °C: Comparison of twin-screw and static mixer mixed PAI-2/LCP blends.
1E7 1E6
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_Δ
Δ
EXPERIMENTAL 3.93 1 / S
1/S
#
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A
EXPERIMENTAL 118
1/S 1/S
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10 0 0 Η
100, 10»
-+10
20
—I 30
1 40
PAI-1,
1— 50
60
70
80
90
100
WT.
Figure 3. Viscosity as a function of PAI-1 concentration at 330 °C at two shear rates: Comparison of observed viscosity and viscosity predicted by Taylor's equation [η = LCP, η = PAI -1). λ
2
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1E7-|
— Ο
]£5_Δ
Ο
PREDICTED 3.93 L / S
Δ
EXPERIMENTAL 3.33 1/S
φ
φ
PREDICTED 118 1/S
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A
EXPERIMENTAL 118 1/S
to 1 E 5 - -
Ο
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10
20
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70
80
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\
90 100
PAI-1, WT.% Figure 4. Viscosity as a function of PAI-1 concentration at 330 °C at two shear rates: Comparison of observed viscosity and viscosity predicted by Taylor's equation (77! = PAI-1,77 = LCP). 2
10-4
0
1
1
1
1
1
1
1
i
10
20
30
40
50
60
70
80
1
90 100
PAI-1, WT% Figure 5. Viscosity as a function of PAÎ-1 concentration at 330 °C at two shear rates: Comparison of observed viscosity and viscosity predicted by Heitmiller's equation for twin-screw mixed blends.
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LIQUID-CRYSTALLINE POLYMER SYSTEMS
Upper Bound:
% = % + [
