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Rheology of Blends of a Thermotropic Liquid Crystalline Polymer With Polyphenylene Sulfide.

WOO NYON KIM [*]

DAVID W. GILES [**]

Blends of thermotropic liquid crystalline polymer (LOP) and polyphenylene sulfide (PPS) were studied over the entire composition range using Rheometrics Stress Rheometer, capillary rheometer, and differential scanning calorimeter. There is no molecular scale mixing or chemical reaction between the components, as evidenced by melting and crystallization points in the PPS phase. From the strain scaling transients test at low-rate, LOP and the blends require approximately 60 strain units to obtain steady state shearing results. The large recoveries in the strain recovery test, magnitude 3 to 3.3 strain unit, are likely the results of texture present in LOPs. With increasing PPS content in LCP/PPS blends, the total recovery declines. Scaling of the transient strain rate remains, but the magnitude of the transients is reduced. At low-rate, when the LOP is added to the PPS, the pure melts have similar viscosity: 500 Pa * s for LOP and 600 Pa * s for PPS, but the viscosity of the blends goes through a maximum with c oncentration that is nearly three times the viscosity of the individual melts. At high-rate, a significant depression of the viscosity is observed in the PPS-rich compositions and this may be due to the fibrous structure of the LOP at high shear rates.

INTRODUCTION

Blends of thermotropic liquid crystalline polymers (LOPs) with flexible-chain melts offer potential of improved and unique properties [1] such as fibrous structure of the LOP [2, 3] and viscosity reduction of the blends [4-6] when the small amount of LOP is added to the matrix. Several researchers have studied blends of thermotropic LOPs and flexible polymers [7, 8] including polyphenylene sulfide (PPS) [9-12], poly (ether imide) (PEI) [13-19], poly (ether ether ketone) (PEEK) [19-26], polysulfone [27-29], polycarbonate (PC) (2, [30-36]), poly(ethylene terephthalate) (PET) (4, [37, 38]), polyarylate (PAr) (39) and a copolymer of tetrafluoroethylene and hexafluropropylene (40). In our previous work (18, 30, 39, 41) we applied the lattice theory to blends of a thermotropic liquid crystalline polymer and flexible chain polymers such as PET, PEEK, PAr, and P0 and reported the polymerpolymer interaction parameter ([[chi].sub.12]) and equilibrium degree of disorder (y/[x.sub.1]) of the blend at melting processing tempe rature, where [x.sub.1] is the axis ratio of each of the m, the number of freely rotating joint in submolecule [x.sub.1], rods comprising the molecules, and y denotes disorientation.

We report here on the rheology of blends of a thermotropic liquid crystalline copolyester with PPS over the entire compositional range. The blends have been characterized using Rhoemetrics Stress Rheometer (RSR) and capillary rheometer. Transient strain rate and strain recovery of the blends have been investigated at the low shear rates. Detailed experimental procedures of the low-rate rheological measurements have been discussed.

EXPERIMENTAL

Polymers

The liquid crystalline polymer (LCP) was Vectra A900 by Hoechst-Celanese, which is a random liquid crystalline copolyester containing 73 mol% 4-hydroxybezoic acid (HBA) 27 mol% 6-hydroxy-2-naphthoic acid (HNA). The sample of polyphenylene sulfide (PPS) was supplied by Tohpren Co. (Japan). The glass transition temperature ([T.sub.g]) and melting points ([T.sub.m]) reported in Table 1 and thermal properties shown in this work were measured by differential scanning calorimetry (DSC), using procedure reported elsewhere (4).

Blend Preparations

Blends with weight fraction of LCP from 0.1 to 0.9 in increments of 0.1 were prepared using a Brabender Plasti-Corder PL 2000 twin-screw extruder and mixer. The polymer sample were dried under vacuum ([less than] 1 mmHg) at 120[degrees]C for 24 hours before use. The temperature of the extruder was set at 310[degrees]C in the barrel zones, and the screw speed was set at 30 rpm. The temperature of the mixer was set at 310[degrees]C, and the mixing time was 10 min. For both the extruder and mixer blending, nitrogen was purged into the hopper to minimize the chemical modifications of the polymers at high temperature. Samples were compression molded using a hot press at 310[degrees]C and 40 psi for 5 min. These were used for thermal analysis and scanning electron micrograph measurement.

Rheology

The tests were conducted at 310[degrees]C using the Rheometrics Stress Pheometer (RSR) with cone and plate fixture of 25 mm diameter and 0.1 radian cone angle. Before molding or testing, the polymer samples were dried in vacuum at 120[degrees]C for 24 hours. The blended material was cut for loading into a cylindical vacuum mold (9.8 mm diameter and 130 mm long). Samples 6.3 to 100 mm long were molded under vacuum at 310[degrees]C for two minutes. The ramp-up to 310[degrees]C was at 50[degrees]C per minute and the ramp-down was at 30[degrees]C per minute for temperatures above 200[degrees]C, and at 20[degrees]C per minute for temperatures below 200[degrees]C. The longer molded samples were cut into 6.3 mm long cylindrical pieces for testing in the rheometer. The samples were sized to have 10% to 30% excess. Blends of LOP/PPS with weight ratios of 0/10, 2/8, 4/6, 6/4, 8/2, and 10/0 were tested. All blend ratios, including 0/10 and 10/0, were processed identically in the extruder to give equivalent thermal and mechanical treatment to all samples.

In loading the rheometer, the following procedure was used: Rheometer was heated to 310[degrees]C and set the zero gap. Then the sample was loaded (the sample was cylindrical, approximately 9.8 mm in diameter and 6.3 mm long) placing it near center on the lower plate. Note the time: Allow the sample to heat for 2 minutes. After the sample was loaded on the plate, the upper fixture (the cone) was lowered to contract the melting samples and squeeze it halfway (to 4.9 mm thickness); rotate the rheometer fixture 2 to 5 revolutions (1 to 2 rpm is a good speed) by applying a small adjustable stress command. This is to evenly distribute and center the sample. The fixture may be lowered further while it is rotating, but stop the rotation (cancel the stress command) when the fixture is at least 0.5 mm from the final test position. Then lower the fixture the rest of the way, setting the gap. And last, wait 2 minutes after setting the gap and then run the test. The entire loading procedure takes about 11 minutes from i nitial loading to the start of the test.

The rotation procedure used during loading centers the sample and also provides a preshearing that evens out any "pellet" morphology present in the sample from molding the discrete pieces in the vacuum mold. The rotation and preshearing during loading spreads out and mixes the pieces to give a "standard" morphology prior to the shearing tests.

The shearing tests were of the step stress type: A constant stress was applied to the sample at time [t.sub.1] and removed at time [t.sub.2] and the strain and time were recorded. The strain rate versus time is obtained from the derivative of this data. The strain was further monitored from time [t.sub.2] to time [t.sub.3], to recorded the strain recovery transient. These strain rate and recovery test were performed on a sample. Tests of the same type were conducted with the same sample. Several tests at different stresses were run on each different sample.

Rhoelogical measurements at high rates were carried out on a Instron capillary rheometer Model 3211. Samples were loaded in pellet form at 310[degrees]C. Approximately 10 mm was required following loading for the system to reach thermal equilibrium. Nitrogen was purged into the sample loading inlet to minimize the chemical modifications of the polymers. Three capillaries of diameter 0.762 mm (0.03 inch, L/D = 10.3, 33.3, and 99.4) were employed to cover a shear rate 30 to 4 X [10.sup.3] [S.sup.-1]. The Rabinowitsch correction was applied to all data and only the true shear rate is shown.

Scanning Electron Microscopy (SEM)

The morphology of the cross section of the extrudate prepared by cryogenic fracturing was examined by Cambridge scanning electron microscopy (Model 250-MK3) at 15 kV accelerating voltage after gold sputter coating (500A).

RESULTS AND DISCUSSION

Low-Rate Rheology

The results for the 10/0 blends (pure LCP) are shown in Fig. 1. The transient strain rate response is plotted versus strain rather than time and the transients scale with strain, as shown previously in Giles and Denn [42]. The scaling is actually better than that shown in the previous work, where the transients shifted to slightly lower strain as the stress was lowered. The better scaling seen in Fig. 1 is attributed to the preshearing procedure used in this work, and the elimination thereby of the pellet morphology has also been presheared at the higher stress of the preceding test.

From the strain scaling transients of Fig. 1a, this LCP requires approximately 60 strain units to obtain steady shearing results. The requirement of large strains to reach steady state and the scaling of the transient response with strain has been observed for other LCP systems and in other types of tests [43-47]. The steady state typically required on the order of 100 strain units in these reports.

Strain scaling transients necessitate progressively longer test times as the stress and thus the shear rate Is lowered. Failure to allow sufficient time for lower stress (rate) transients is a not uncommon problem in studies of LCP rheology. Low-rate tests require long times to accumulate much strain, and the tendency may be to limit low-rate tests to some fixed time (and thus to smaller strain) to reduce concerns about sample degradation or other changes that may occur at times [42]. This rationale has indeed been explicitly used in Kalika's work [48], and in the present study.

Recovery from each stress test at various compositions of LCP and LOP/PPS blends Is shown in Fig. 1b and Fig. 2. The recoveries are plotted versus time, but scale if the time axis is multiplied by the steady shear rate in effect prior to recovery, as first reported by Larson and Mead [44] for two lyotropic LCPs, and by Giles and Denn [42] for the same LCP used here. The large recoveries, of magnitude 3 to 3.3 strain units independent of stress, are likely the result of the texture present in LCPs due to the large number of defects in the nematic order, and to the evolution of that texture in response to shearing. Texture evolution is also indicated for the strain transients seen during the stress test in Fig. la. In Fig. 1b and Fig. 2, with increasing PPS content, the total recovery declines.

Data from the strain rate transient plots at large strain give approximations of the steady state results. These flow curves for the different blend ratios are shown in Fig. 3. Rather similar, mildly shear thinning behavior is seen for all blend ratios. In the low rate region, the blends have higher viscosity than the individual melts. For the high-rate region, results are discussed in the next section (High-Rate Rheology). The dependence of viscosity on composition at 0.5 [S.sup.-1] is shown in Fig. 4: Filled circles represent the steady shear viscosity from RSR and broken lines represent the connection between two points. The pure melts have similar viscosity: 500 Pa s for LCP and 600 Pa s for PPS, but the viscosity of the blends goes through a maximum with concentration that is nearly three times the viscosity of the individual melts. This result is generally similar to that obtained recently [49] for mixtures of two immiscible Newtonian fluids of nearly identical viscosity and density. In that work, the maximum in viscosity was more than double that of the pure fluids. The concentration dependence for LOP and PPS shown in Fig. 4 contrasts with other published results for blends of LOPs with flexible polymers such as nylon-6 (50), polystyrene [31], and another LCP (51), in which minima were seen.

The LCP/PPS blend is not miscible. Therefore, when the LCP is added to the PPS, the PPS structure is disrupted and a viscosity increase is expected. This is in contrast to the results in the literature where addition of a miscible liquid crystalline component increases the orientation of the non-LCP matrix phase [52]: this necessarily lowers the viscosity. A further addition of the LCP component must in either case lead to the LCP limit [53], which is when the LOP phase is formed (54). Addition of the LOP component beyond the LOP limit causes an increase in the overall orientation of the material and lowering of the viscosity (Fig. 4). One can start from the LCP-rich side and state that the addition of PPS lowers the extent of LCP orientation and increases the viscosity, and this happens until PPS becomes the matrix phase; these are two ways of describing the same situation.

High-Rate Rheology

Figure 3 shows also the capillary measurements of the viscosity of the LCP/PPS blends when the L/D is 33.3. Data for all the blend compositions show thinning behavior over approximately three decades of shear rate. For the LCP, the value of viscosity is in good agreement with that of reported by Kalika and co-workers [55] and Nuel and Denn [56]. For the blends a significant drop in viscosity is observed at 0.1 and 0.3 weight fractions LCP in the shear rate above [10.sup.3] [s.sup.-1]. At the shear rate about 3 X [10.sup.3] [s.sup.-1], the viscosity of the 0.1 and 0.3 weight fractions LCP is about 70% and 45% of that of the pure PPS, respectively.

Figure 5a and 5b show the micrographs of the 1/9 LCP/PPS blend extruded from the capillary rheometer when L/D is 33.3 and the shear rate is 32 [s.sup.-1] and 3200 [s.sup.-1], respectively. The decrease in viscosity is perhaps due to the fibrous structure of the LCP, which is shown in Fig. 5b. When the shear rate low, the fibrous structure is not observed (Fig. 5a). The fibrous structure is not the cause but only one of the manifestations of liquid crystallinity. The diameter of these fibrils shown in Fig. 5b is observed to be 1 to 3 [micro]m. This fibrous structure of the LCP may act as a reinforcement for the matrix such that for 0.2 and 0.3 weight fractions the LCP/PPS blends [9], mechanical properties such as tensile strength and elastic strength and elastic modulus were increased 2.0- and 2.6-fold, respectively. When the L/D is 10.3 and 99.4, similar morphologies was observed as can be seen in Fig. 5.

Thermal Behavior

The experimental melting temperature ([T.sub.m]) of LCP was found to be 280.4[degrees]C (Table 1) as previously reported elsewhere [4]. The [T.sub.m] of PPS was found to be 280.0[degrees]C, which is close to the [T.sub.m] of LCP. Therefore the [T.sub.m]s of the LCP and PPS could not be determined separately. Since the heat of fusion of PPS (34.7 J/g) was very large compared to that of the LCP (1.2 J/g), the [T.sub.m] of the blends was considered as the mostly melting of the PPS. All blend compositions showed a single glass transition in the DSC, which was the value between 93[degrees]C and 98[degrees]C associated with the LCP and PPS phase, respectively. The [T.sub.g]s could not be detected separately since the two [T.sub.g]s were so close.

The effect of annealing on the PPS melting and crystallization temperature ([T.sub.c]) for all the compositions is shown in Fig. 6. The blend samples used in this study were prepared by screw extruder and mixer. Blend samples were annealed in DSC at 310[degrees]C for 10 min and 30 min. No significant depression of [T.sub.m] (PPS) and [T.sub.m] (LCP) is observed for all the blend compositions with different annealing time in DSC. In the blend of the LCP and PPS, Seppala and co-workers [12] reported the melting point of PPS in the range 283[degrees]C to 285[degrees]C, which is close to the [T.sub.m] of pure PPS. From the thermal behavior of the LCP and PPS, it is suggested that there is no molecular scale mixing or chemical reaction between the components.

CONCLUSIONS

Blends of a thermotropic liquid crystalline polymer (LCP) and polyphenylene sulfide (PPS) were studied using Rheometrics Stress Rheometer, capillary rheometer, and differential scanning calorimeter.

From the strain scaling transients test at low-rate, LCP and the blends require approximately 60 strain units to obtain steady state shearing results. The large recoveries in strain recovery test, of magnitude 3 to 3.3 strain units, are likely the results of the texture present in LCPs. With increasing PPS content in the LCP/PPS blends, the total recovery declines. Scaling of the transient strain rate remains, but the magnitude of the transients Is reduced.

At low-rate, when the LCP is added to the PPS, the pure melts have similar viscosity: 500 Pa * s for LCP and 600 Pa * s for PPS, but the viscosity of the blends goes through a maximum with concentration that is nearly three times the viscosity of the individual melts. At high-rate, a significant depression of the viscosity is observed In the PPS-rich compositions, and this may be due to the fibrous structure of the LCP at high shear rates. There is no molecular scale mixing or chemical reaction between the components, as evidenced by the melting- and crystallization-point In the PPS phase.

ACKNOWLEDGMENTS

This work was supported in part by the Center for Advanced Functional Polymers under contract No. 97K3-1005-03-11-3 through the Korea Science and Engineering Foundation. Also, this work was supported in part by the Director, Office of Energy Research, Office of Basic Energy Science, Materials Science Division of the U.S. Department of Energy under Contract No. DE-ACO3-76SF00098.

(*.) To whom correspondence should be addressed. E-mail address: kimwn@ mail. korea.ac.kr

(**.) Present address: Department of Chemical Engineering and Materials Science, University of Minnesota. 421 Washington Avenue S.E., Minnespolls. MN 55455-0132

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Table 1

Characteristics of Polymer
Samples Used in the LCP/PPS Blends.


 [T.sub.m.]([degrees]C) [T.sub.g]([degrees]C)


Vectra A900 280.4 92.9
PPS 280.0 98.3



 [delta][C.sub.p]
 (J [g.sup.-1][K.sup.-1]

Vectra A900 0.031
PPS 0.081
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Author:HAN, MINSOO; KIM, WOO NYON; GILES, DAVID W.
Publication:Polymer Engineering and Science
Article Type:Statistical Data Included
Geographic Code:1USA
Date:Sep 1, 2001
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