Polymer blends of polyethersulfone with all aromatic liquid crystalline co-polyester.
Research on polymer blends of thermotropic liquid crystalline polymer (TLCP) and thermoplastic polymer has been carried out by a number of researchers in recent years, and many papers and reviews have been reported (1-7). These studies have made it clear that the properties and processability of a thermoplastic polymer are improved when it is blended with TLCP. In many studies it has been reported that TLCP undergoes fibrillation within the thermoplastic polymer in situ. Many attempts to manufacture composite materials in situ have been made, and furthermore, some attempts have been made to manufacture molecular composite materials (8-13).
However, in most cases, TLCPs are incompatible with thermoplastic polymers, so that high performance alloys are not yet obtainable. We attempted to carry out a further study on blending polyethersurfone (PES) with all aromatic liquid crystalline co-polyester (LCP) in order to solve these problems.
As to the blending of PES and LCP, it was reported that the properties of blends of PES (70 wt%) and LCP (30 wt%) were measured and success was achieved in improving their tensile modulus and strength (9). James and Donald (14) have reported that blending PES with a small quantity of LCP caused a drop in dynamic viscosity.
In this report, we first studied the relationship between mechanical and viscoelastic properties resulting from the change of composition of blends of PES and one of LCP. We also studied the relationship between composition and morphology based on the results obtained using DSC and SEM.
Second, we conducted a similar study on the blends to which were added PES oligomers with reactive functional groups as a tertiary component with the aim of improving the properties of the blends. Also, we have speculated upon the mechanism of effect of compatibility of reactive PES oligomer added as tertiary component in blends of PES and LCP.
The PES used in this study was Victrex 4100 (ICI). The LCP utilized was Vectra A-950 (Hoechst Celanese), which is a copolyester of phydoroxy benzoic acid and 6-hydoroxy-2-naphthoic acid. Three PES oligomers of different average degree of polymerization n (n = 7, 15, and 25), and a reactive functional group (-ONa), which were kindly supplied by Professor Z. Wu of Jilin University of China, were used (15). Their chemical structure is as follows:
Blending of the materials was performed using a Labo Plastomill with a twin rotary mixer (Toyo Seiki Co., Japan). The mixing temperatures are shown in Table 1. The rotary speed was 50 rpm and mixing time was 5 min. Prior to blending, the materials were dried at 120 [degrees] C for at least 8 h. The ratios of PES/LCP were 100/0, 95/5, 90/10, 85/15, 80/20, 70/30, 60/40, 50/50, 40/60, 20/80, and 0/100. The ratios of PES/LCP/PES oligomer studied were 80/20/ 0, 77.5/20/2.5, 75/20/5, 70/20/10, 60/20/20, 50/20/30, and 40/20/40.
Specimens of approximate dimensions 40 by 5 by 2 mm were molded using a Mini Max Molder CS-183MMX (CSI Co.) injection machine. The barrel temperature and the mold temperature during the injection molding are shown in Table 1.
a) Flexural Properties
Flexural properties were examined using a Autograph AGS-55C (Shimazu Co., Japan) based on the standard method for testing flexural properties of rigid plastics [JIS K7203 (1982)]. The span length was 30 mm and the loading nose speed was 10 mm/min.
b) Dynamic Mechanical Properties
Dynamic viscoelastic spectra of samples were measured with a Rheovibron DDV-III (Orientech Co., Japan) with a chuck distance of 25 mm, a frequency of 110 Hz, and heating rate of 2 [degrees] C/min.
Glass transition temperature ([T.sub.g]), melting temperature ([T.sub.m]), and crystallization temperature ([T.sub.c]) were measured with a differential scanning calorimeter, MAC Science DSC-3100. A heating rate of 10 [degrees] C/min was utilized and the materials were scanned from 25 [degrees] C to 380 [degrees] C for LCP-1, and 25 [degrees] C to 340 [degrees] C for LCP-2 and its blends. At first, the [T.sub.c] of LCP was obtained by cooling at a rate of 10 [degrees] C/min and the secondary [T.sub.m] of LCP and [T.sub.g] of PES were obtained by reheating at a rate of 10 [degrees] C/min.
Table 1. Processing Temperatures. PES PES/LCP Mixing temperature 350 [degrees] C 320 [degrees] C Barrel temperature 350 [degrees] C 320 [degrees] C Mold temperature 100 [degrees] C 90 [degrees] C
The morphology of the blends was determined by scanning electron microscopy (SEM), using a JSM-840 instrument (Japan Electron Co., Japan). All samples were fractured perpendicular in air. The fractured surfaces were sputter-coated with gold to provide enhanced conductivity.
e) Torque Viscosity
The torque viscosity of melts was measured with a twin rotary mixer Labo Plastomill (Toyo Seiki Co., Japan). The temperature was 290 [degrees] C and the speed of the rotary mixer was 50 rpm.
RESULTS AND DISCUSSION
Mechanical Properties of Blends
Flexural modulus (E) and strength ([Sigma]) were measured as mechanical properties of blends. Figure 1 represents the effects on flexural modulus of blending PES with LCP. PES and LCP had flexural moduli of 3100 and 7700 MPa, respectively. Thus, the flexural modulus of LCP was roughly twice that of PES. As shown in Fig. 1, the flexural modulus of the blends increased almost linearly with increasing LCP content, indicating the additive nature of blending.
Figure 2 shows the effect on flexural strength of blending PES with LCP. PES was 135 MPa in flexural strength, which was lower than that of LCP (157 MPa). The flexural strength of the blends of PES with LCP decreased as LCP content increased to 20 wt%. It was essentially constant thereafter until LCP content was increased to 60 wt%. The flexural strength started to increase, eventually overtaking that of PES. These results were different from those of Kiss (9), but we could not interpret the results.
Next, an attempt was made to add PES oligomers with reactive functional groups (-ONa) at both terminal ends of the monomer chain to the PES/LCP blend. The oligomer content were 2.5, 5, 10, 20, 30, and 40 wt%. The LCP content was kept at 20 wt%. This was done in order to follow their flexural modulus and strength. Three types of PES oligomer were used, which had average degrees of polymerization (n), of 7, 15, and 25.
Figure 3 shows the effects of flexural modulus of the average degree of polymerization and content of the PES oligomer. The addition of the PES oligomer had the effect of increasing flexural modulus slightly. No significant effect caused by varying the polymerization degree was noted.
Figure 4 shows the effects of PES oligomer degree of polymerization and these contents on flexural strength of PES/LCP/PES oligomer blends. Significant effects were observed on flexural strength, which increased as oligomer content increased from 10 to 20 wt% and reached a maximum at 20 wt% of oligomer content, and decreased thereafter. The effects of degree of polymerization were not clear. However, it was observed that the blends with the oligomers of polymerization degree 7 and 25 shared a common trend. The flexural strength of the blend with 20 wt% oligomer was higher than that of PES, and near to that of LCP. It is interesting to note that the addition of the oligomer, which is itself of low strength because of its low molecular weight, increases the strength of the blend. This indicates the presence of some compatibility-related effects caused by the oligomers.
Dynamic Viscoelastic Properties of Blends
Dynamic viscoelasticity of the PES, LCP, and blends were measured. Figure 5 shows the results for PES, LCP before blending. E[prime] is the dynamic storage elastic modulus, and tan [Delta] is the loss tangent, which is the ratio of storage elastic modulus (E[prime]) to loss elastic modulus (E[double prime]).
Wu (16) has studied the dynamic viscoelastic properties of a PES. We were able to obtain the same results. As PES is a noncrystalline polymer, it has an essentially constant E[prime] value of approximately 2 MPa at temperatures up to its transition temperature ([T.sub.g]) of 217 [degrees] C. In contrast, E[prime] of the LCP, although 3.5 times higher than that of the PES at room temperature (2 vs 7 MPa), decreased with increasing temperature. As the result, E[prime] of the LCP was higher than PES by approximately 150 [degrees] C, but this relationship was reversed at higher temperatures. And, E[prime] of the LCP decreased sharply at approximately 250 [degrees] C because of the melting of LCP crystals.
As shown in Fig. 5, the tan [Delta] spectra of the LCP had one broad peak at approximately 100 [degrees] C. The peaks found near 100 [degrees] C are characteristic of aromatic liquid crystalline polymers (17) and are believed to be associated with ring flip motions.
Figure 6 shows the effect on E[prime] of varying the composition of the PES/LCP blend. The values of E[prime] of these blends were mostly intermediate between those of the PES and LCP, being higher than that of PES at about 125 [degrees] C and lower than those of PES at temperatures above 125 [degrees] C.
The effects of PES oligomer content on E[prime] were investigated for the oligomer-containing PES/LCP blends, where the LCP content was set to 20 wt% and the average polymerization degree (n) of the oligomer was 7. The results are shown in Fig. 7. The presence of the oligomer appeared to have no significant effect on the E[prime]-temperature relationship at temperatures below the [T.sub.g] of PES.
The major effect of the oligomer was observed in the reduction of E[prime] with oligomer content, conceivably resulting from a changed [T.sub.g] of the PES phase. The addition of the oligomer decreased the [T.sub.g], with the result that the temperature at which the value of E[prime] started to decrease was rapidly lowered.
Figure 8 illustrates the effects of polymerization degree (n) of the PES oligomer on the E[prime]-temperature relationship for the blend containing 20 wt% LCP and 20 wt% oligomer. No significant effect of the degree of polymerization was observed.
Next, the effects of composition and degree of polymerization (n) of PES oligomer on the tan [Delta] peak temperature of the PES/LCP/PES oligomer blends were studied. The tan [Delta] peak temperature was considered to correspond to the [T.sub.g] of the blend. The results are given in Fig. 9. The degree of polymerization appeared to have no significant effect, as was the case with the E[prime] and E[prime]-temperature relationships, as shown in Figs. 8 and 9. However, it was observed that the addition of oligomer resulted in a rapid decrease in the peak temperature. The peak temperature decreased by more than 20 [degrees] at a 2.5 wt% oligomer content, but changed little as the content increased from 2.5 to 20 wt%. For blends with more than 20 wt% oligomer content, the peak temperature decreased again. These phenomena may not be sufficiently explained by the reduced [T.sub.g] alone. Since there is an increase in the compatibility of the oligomer with PES, it is necessary to consider the changed morphologies of the blend composition, such as those resulting from changed dispersion of LCP.
Morphologies of Blends
The morphologies of the blends was investigated using DSC to examine their thermal behavior, and scanning electron microscopy (SEM) of the cross sections of the injection molded blend specimens.
Figure 10 shows the dependency of [T.sub.g] (PES) and [T.sub.m] and [T.sub.c] (LCP) blends. The measured values of [T.sub.m] of the LCP was 273.0 [degrees] C and [T.sub.c] was 236.2 [degrees] C. [T.sub.g] of the PES was 217 [degrees] C.
As shown in these Figures, the melting point and crystallization temperature of LCP in the blends were essentially constant, irrespective of composition. On the other hand, the [T.sub.g] of PES decreased at a rate of 1.8 [degrees] C for every 10 wt% increase in LCP. The decrease in [T.sub.g] suggests some minor interactions between PES and LCP.
Figure 11 shows the dependence of [T.sub.g] of the PES on the composition and degree of polymerization (n) of the PES oligomers in PES/LCP/PES oligomer blends, where the amount of LCP has been set to 20 wt%. Only one [T.sub.g] peak of the PES was observed with each blend, suggesting that PES and its oligomer were soluble in each other. PES's [T.sub.g] decreased almost linearly with increasing oligomer content, approaching the oligomer's [T.sub.g] of 155 [degrees] C. The DSC-analyzed [T.sub.g] decreased in a different manner with increasing oligomer content, depending on the average degree of polymerization (n) of the oligomer. The oligomer with n = 7 caused the most notable decrease in [T.sub.g].
All the [T.sub.g]s analyzed by DSC varied linearly with oligomer content. This trend was clearly different from that of the temperature-dependent change of the tan [Delta] peak, which was considered to correspond to the [T.sub.g] determined from the viscoelastic properties. We considered that the peak temperature of tan [Delta] decreased more sensitively at low oligomer content because of the morphological changes, such as those changes in the particle size and shapes of dispersed LCP.
The fracture surface of the cross sections of the injection molded specimens were analyzed using SEM to observe their morphologies.
Representative results are given in Fig. 12 for the PES/LCP (90/10) composition, where Fig. 12a shows the SEM micrograph of the skin and Fig. 12b shows that of the core. LCP was fibrous in the skin and spherical in the core. Figure 13 shows the SEM micrographs of the fracture surface of the cross section of the PES/LCP (60/40) composition, where Fig. 13a shows the skin and Fig. 13b shows the core. In this case also, LCP is fibrous in the skin and spherical in the core. Thus, LCP was fibrous only in the skin in the blends with LCP as the minor ingredient.
Figure 14 shows the SEM micrographs of the PES/LCP (20/80) composition, where Fig. 14a shows the fracture surface of a cross section of an injection molded specimen and Fig. 14b shows an LCP fibril of specimen from which PES had been removed using DMF (dimethylformamide). LCP was generally fibrous in the blend that had LCP as the major ingredient, and the skin and core were not clearly distinguishable. It was also observed that the LCP fibers were monodirectionally arranged, and that PES occupied the spaces between the fibers.
Figure 15 shows the SEM micrographs of the fracture faces of a cross section of the injection molded species of the blends (PES/LCP/PES oligomer = 60/20/20) containing PES oligomer of average degree of polymerization n = 7 (Fig. 15a) and n = 15 (Fig. 15b). These blends had the highest bending strength, as shown in Fig. 4. As shown, LCP was finely fibrous even in the core of each blend. We concluded that the improved mechanical properties (flexural strength and modulus) were due to an even dispersion of the fine fibers.
Mechanisms by Which PES Oligomers Act as Compatibilizers
It has been found that the PES oligomers, when added to the PES/LCP blends, worked to facilitate the dispersion of LCP and to make it fibrous, thereby contributing to an improvement in the mechanical properties. These results indicated that the oligomer served as a compatibfiizer in the PES/LCP blend.
The reactivity of the oligomer with PES and LCP was investigated in order to understand the mechanisms by which PES oligomers act as compatibilizer. Figure 16 shows the change in torque viscosity with time of the molten PES, LCP, and PES/LCP (80/20) blend, and Fig. 17 shows the change in torque viscosity with time of the molten PES, LCP, and PES/LCP (80/20) blend containing 20 wt% PES oligomer. These specimens were in a molten state at 320 [degrees] C. Each sample was melted and kneaded for 3 min before the oligomer was added. The PES oligomer was in a liquefied state when it was added to the blend. Hence, the torque dropped rapidly because of slipping in the melt. The viscosity of the mixture was stabilized after the oligomer was absorbed by the blend. The torque viscosity of each mixture was measured before and after the addition of the PES oligomer. The viscosity of the PES decreased greatly when the oligomer was added, so that no effect of the addition of the oligomer was observed. However, the torque viscosity of the LCP returned to its original level after the oligomer was added. The PES/LCP blend behaved intermediately.
One possible interpretation of these results is that ester bonds of the LCP were disconnected by the reactions of -- ONa at the terminals of the oligomer molecules. Bonds could then recombined to form a block copolymer with the PES oligomer in the LCP chains, and this copolymer worked as a compatibilizer for the LCP/PES blends, but we believe that more work is needed to prove the mechanism by which the PES oligomer acts as compatibilizer in the PES/LCP blend.
Polymer blends of commercial PES with LCP were investigated. In addition, PES oligomers with reactive functional groups at their terminals were added as a third component to the above blends in order to improve mechanical properties.
1) Regarding flexural properties, the flexural modulus of the PES/LCP blends increased almost linearly as LCP content increased. The flexural strength of the PES/LCP blends decreased most notably when LCP content was in the range of 20 to 60 wt%, but increased again for higher LCP content. In contrast, the addition of the PES oligomers had little effect on flexural modulus, but flexural strength was clearly improved.
2) Regarding dynamic viscoelastic properties, E[prime] of the blends was intermediate between those of PES and LCP. However, the oligomer-containing blends showed complex behavior as oligomer content increased; a transition peak temperature of tan [Delta] decreased sharply at low oligomer content, then remained constant at up to 20 wt% oligomers, and decreased again thereafter.
3) The morphologies of each blend were analyzed by DSC. [T.sub.m] and [T.sub.c] of both LCPs remained constant when blended with PES. However, the [T.sub.g] of PES decreased slightly upon blending. The addition of the PES oligomers caused a decrease in the [T.sub.g] of PES, suggesting that they were miscible in PES.
4) SEM analysis revealed that LCP was fibrous in the skin but not in the core in the blends containing 40 wt% or less LCP. The addition of the PES oligomer resulted in the LCP becoming fibrous even in the core, in the blends with LCP as a minor ingredient.
5) It was concluded that the PES oligomers with reactive functional groups acted as a compatibilizer in polymer blends of PES/LCP.
PES oligomers were kindly supplied by Professor Z. Wu of Jilin University of China. Special thanks to Mr. Y. Tsubouchi for his useful contributions and discussions.
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|Author:||Yazaki, Fumihiko; Kohara, Atsushi; Yosomiya, Ryutoku|
|Publication:||Polymer Engineering and Science|
|Date:||Jul 1, 1994|
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