Effect of thermotropic copolyesteramide on the properties of polyamide-66/liquid crystalline copolyester composites.

X.L. Xie (l.2*)

R.K.Y. Li (2)

Y.W. MAI (3)

S.C. Tjong (2)

Liquid ciystalline copolyester-polyamide 66 (LOPES/PA66) composites compatibilized by liquid crystalline copolyesteramide (LCPEA) were prepared by injection molding. The LCPES employed was a commercial copolyester, Vectra A950, and the LCPES was a semiflexible thermotropic copolyesteramides based on 30 mo1% of pamino benzoic acid (ABA) and 70 mol% of poly(ethylene terephthalate) [PET). Thermal analysis, mechanical characterization, and morphological investigations were conducted on the blends. The dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) tests showed that LCPEA is an effective compatibilizer for the LCPES/PA66 composites. The mechanical measurements showed that the stiffness, tensile strength and Izod impact strength of the in-situ composites are improved by adding LCPEA because of the compatibilization and reinforcement to LCPES/PA66 composites. However, the properties improvement vanished when LCP content reached 10 wt%. The drop weight dart impact test was also applie d to analyze the impact fracture characteristics of these composites. The results showed that the maximum impact force ([F.sub.max]), crack initiation and propagation energy all improved with the addition of a small percent of LCPEA. From these results, it appeared that LCPEA prolongs the time for crack initiation and propagation. It also increases the energies for crack initiation and propagation, thereby leading to toughening of the LCPES/PA66 in-situ composites. Finally, the correlation between the mechanical properties and morphology of the composites is discussed.

INTRODUCTION

In recent years, in-situ polymer composites based on liquid crystalline polymers (LOPs) and conventional thermoplastic polymers have attracted considerable attention (1). The major objective of these studies was to obtain new structural polymers with improved mechanical properties. In most cases, LCPs and thermoplastics are incompatible owing to their fundamentally different structures. Therefore, LCP and the matrix component of in-situ composites exhibit a low interfacial adhesion, thereby resulting in poor mechanical properties, particularly the tensile strength. The adhesion can be improved by increasing the interactions between the phases, physically or chemically via the addition of compatibilizers or the modification of the structure of the LCPs. Compatibilizers based on block or graft copolymers are commonly used to improve the adhesion of the components for the LOP/thermoplastic blends (2-4). Compatibilization can also take place by specific interactions between the LCP and the matrix polymer. e. g., ion-dipole interaction (5,6), chemical interaction (7-9), hydrogen bonding (10), etc. In the case of modifying the LCP structure for enhancing the compatibility, several workers have reported that the introduction of a long flexible spacer in the main chain of LCP enhances the adhesion between the LCP and thermolplastic matrix as well as improving their processabili ty (11-16). Little information is available on the properties of in-situ composites compatibilized by other LCP.

More recently, the authors (17-19) have studied in-situ composites consisting of a semiflexible LCP with either polyamide or polyester as the matrix. Semiflexible LCP is a copolyesteramide based on p-amino benzoic acid (ABA) and poly(ethylene terephthalate) (PET) (designated as ABA30/PET). It was reported that the semiflexible LCP is partially miscible with poly(butylene terephthalate) (PBT) and polyamide 66 (PA66) in the solid state, but are miscible in the molten state because of the presence of flexible PET chains segment and amide bonds in ABA30/PET. Moreover, some specific interactions between the LCP and the PA66 and PBT matrices have been observed during blending.

In the present work, we attempt to prepare the ternary in-situ composites consisting of liquid crystalline copolyester (LCPES) and PA66, and compatibilized with ABA30/PET liquid crystalline copolyesteramide (LCPEA), and to study their morphological and mechanical properties. As the semiflexible ABA30/PET molecules consist of ester and amide segments, it is expected that the semiflexible LCP can function as a compatibilizer for the LCPES/PA66 composites and act as an additional reinforcing agent.

EXPERIMENTAL

Materials

The ABA30/PET liquid crystalline copolyesteramide (LCPEA) used in this work was synthesized from 30 mol% of ABA and 70 mol% of PET according to the procedures reported by Jackson and Kuhfuss (20), and Xie et al. (21,22). Its intrinsic viscosity was 0.57dl/g. The glass transition and melting temperatures were 100[degrees]C and 256[degrees]C, respectively. The liquid crystalline copolyester (LCPES) is Vectra A950 from Hoechst Celanese Company (Chatham, N.J.) based on 2,6-hydroxynaphthoic acid and p-hydroxybenzoic acid. PA66 pellets (Novamid) supplied by Mitsubishi Engineering-Plastics Corp. (Taiwan) were used as the matrix material. The LCPEA, Vectra A950 and PA66 pellets were dried in an oven at 120[degrees]C for 48 h before mixing.

Sample Preparation

An LCPES/PA66 blend containing 25 wt% of LCPES was prepared in a twin-screw attached to a Brabender Plasticorder PL2000 at 290[degrees]C and 50 rpm, in which the screw diameter (D) was 19.05 mm, and the length-diameter ratio (L/D) was 20. The resulting binary blend was then further melt blended with 0. 2.5, 5 and 10 wt% of LCPEA using the same conditions as above. The extrudates were injection molded into the plaques with dimensions of 200 X 80 X 3.2 [mm.sup.3] by means of Cosmo injection-molding machine (Welltec industrial Equipment Ltd., Hong Hong). The injection barrel temperature profile was set at 280, 285 and 280[degrees]C. These plaques were cut into dogbone-shaped tensile bars (ASTM D638). Notched Izod impact specimens (ASTM D256) were also prepared from the plaques. Both longitudinal and transverse specimens were used for the tensile and impact tests. For the longitudinal specimens, the length direction was parallel to the flow direction, while it was perpendicular to the flow direction for the tran sverse specimens.

Dynamic Mechanical Analysis

Dynamic mechanical analysis (DMA) was conducted with a TA Instruments' dynamic mechanical analyser (model 983) at a fixed frequency of 1 Hz and an oscillation amplitude of 0.15 mm. Samples with dimensions of 50 x 15 x 3.2 [mm.sup.3] were prepared from compression molded plates. The temperature range studied was from -10[degrees]C to 170[degrees]C with a heating rate of 2[degrees]C [min.sup.-1].

Differential Scanning Calorimetry (DSC)

DSC measurements were conducted in a Perkin-Elmer DSC-7 instrument at a heating rate of 10[degrees]C [min.sup.-1] under a dry nitrogen atmosphere. Prior to the DSC recording. all samples were heated to 300[degrees]C, and kept at this temperature for 3 min to eliminate the influence of their previous thermal histories. They were then quenched to an ambient temperature. For the non-isothermal crystallization measurement, the samples were heated to and kept at 300[degrees]C for 3 min, and then cooled at a cooling rate of 10[degrees]C [min.sup.-1].

Mechanical Properties

The tensile behavior was determined using an Instron tensile tester (model 4206) at room temperature under a crosshead speed of 1 mm [min.sup.-1]. Izod and instrumented drop-weight dart impact specimens with dimensions of 65 x 13 X 3.2 [mm.sup.3] were cut from the plaques, and were respectively tested by a Ceast impact pendulum instrument and Ceast Fractovis instrumented drop weight impact tester with a wedge tip (tip diameter = 20 mm). These specimens were sharply notched with a Ceast cutter with a notch tip radius of 0.25 mm. The force-time curves exhibit various noises due to the force oscillations during the drop weight test. In this work, we made use of Fourier transforms to filter the noise from the experimental signals (9).

Morphology Observations

The morphologies of the fracture surfaces were observed in a scanning electron microscope (JEOL JSM 820). The specimens were cut from the injection molding plaques along the flow direction. They were fractured after immersion in liquid nitrogen. All fractured surfaces were coated with a thin layer of gold prior to SEM examination.

RESULTS AND DISCUSSION

Dynamic Mechanical Properties

Figure 1 shows the variation of the tan [delta] with temperature for the LCPES/LCPEA/PA66 blends with different LCPEA content. It can be seen that the LOPES/ PA66 blend possesses two glass transitions (refer to the curve labeled LCPEA = 0%). This means that the blends are heterogeneous, i.e., they consist of distinct PA66 and LOPES phases. The glass transition temperatures ([T.sub.g]) of PA66 and LCPES are located at about 65[degrees]C and 108[degrees]C, respectively. The peak located at 35.0[degrees]C is associated with the [beta]-transition of the LCPES. However, with increasing LCPEA concentration, the above glass transition peaks shift toward each other. At same time, the [beta]-transition of the LOPES also shifts to higher temperatures. These results indicate that LCPEA can effectively compatibilize PA66 and LOPES components in the LCPES/LCPEA/PA66 ternary blends.

Thermal Properties

The melting temperature ([T.sub.m]). crystallization temperature ([T.sub.c]] and heat of fusion ([DELTA]Hm) of the LCPES/ LCPEA/PA66 blends with different LCPEA content are determined from the DSC heating and cooling thermograms. The degree of crystallinity of the PA66 phase (Xc) in LCPES/LCPEA/PA66 blends can be determined from their heats of fusion normalized to that of the PA66 homopolymer. The heat of fusion of 100% crystalline PA66 is estimated to be 194.69 J/g according to the literature (23). The results are listed in Table 1. Apparently, [T.sub.m], [T.sub.c] and [X.sub.c] of the PA66 phase in LCPES/LCPEA/PA66 blends all decrease with increasing LCPEA concentrations. In our previous studies (17-19), we found that the LCPEA is miscible with PA66 and thermoplastic polyester (such as PBT) in the molten state owing to hydrogen bonds and physical entanglements. For the same reasons, the introduction of LCPES into the LCPES/PA66 blends leads to improved compatibility between LOPES and PA66 phases. [T.sub.m], [T.sub.c] and [X.sub.c] of the PA66 phases in the LCPES/PA66 blends are therefore reduced. Even though the LCPES can enhance the crystallization of PA66 because of their mutual incompatibility, the addition of LCPEA reduces the crystallinity of the PA66 phase dramatically. These observations are in accordance with the crystallinity behavior of compatibilized blends (24-27).

It is generally known that the degree of supercooling, [DELTA]T, can be used to characterize the crystallization behavior of polymer melts. [DELTA]T is defined by the difference between the peak of the melting temperature and onset of the crystallization temperature. An increase in [DELTA]T generally indicates that the crystallization rate of the crystallizing polymer is retarded. From Table 1, it can be seen that [DELTA]T of the LCPES/LCPEA/PA66 blends shows almost no variation with increasing LCPEA content. This result indicates that the LCPEA has almost no effect on the crystallization rate of the PA66 phases in LCPES/LCPEA/PA66 blends, possibly because there are some balances between the compatibilization and nucleation of LCPEA to the LCPES/PA66 system.

Mechanical Properties

The variation of Young's modulus and tensile strength with LCPEA content for the ternary LCPES/ LCPEA/PA66 composites in both the longitudinal and transverse directions are shown in Figs. 2 and 3. respectively. Apparently, the Young's modulus (E) and tensile strength (TS) of the longitudinal specimens are much higher than those of the transverse samples. This is a typical behavior of short fiber reinforced composites. It can also be seen from these figures that the addition of only about 2.5 wt% LCPEA to the binary LCPES/PA66 matrix blend results in an obvious increase in both Young's modulus and tensile strength of the longitudinal and transverse samples. When the LCPEA content exceeds 2.5 wt%, the stiffness and tensile strength of the longitudinal and transverse samples tend to decrease with increasing LCPEA content.

Figure 4 shows the Izod impact strength ([S.sub.1]) versus the LCPEA content for both longitudinal and transverse specimens. This Figure indicates that the impact strengths of these composites increase with increasing LCPEA content up to 5 wt%. Thereafter, they decrease with increasing the LCPEA content. From these results, it is clear that the addition of LCPEA enhances the compatibility and interfacial adhesion between the LCPES and PA66 phases, thereby greatly improving the mechanical stiffness, strength and impact toughness of the LCPES/PA66/LCPEA composites.

It is worth mentioning that the anisotropy of LCPES/ PA66/LCPEA composites depends upon the addition of LCPEA. From Table 2, we can see that anisotropy of the LCPES/PA66/LCPEA composites increases with increasing LCPEA content. With a further increase in LCPEA content, the anisotropy of the LCPES/LCPEA/ PA66 composites decreases.

Finally, the instrumented drop weight dart impact test was used to analyze the fracture process and to better understand the toughening mechanics more clearly. Figure 5 shows the force-time curves filtered by Fourier transforms for the LCPES/LCPEA/PA66 composites without and with LCPEA. From Fig. 5, a number of parameters can be easily identified: (a) the maximum force [F.sub.max]; (b) the energy absorbed up to the maximum force, which is defined as the initiation energy [E.sub.init]; (c) the propagation energy [E.sub.prop]: (d) the total fracture energy [E.sub.T]; (e) the load rise time up to the force peak position, [t.sub.r]; (f) the initiation time, [t.sub.init]; (g) the propagation time, [t.sub.prop]. It is apparent that [F.sub.max] [E.sub.init], [E.sub.prop], [E.sub.T], [t.sub.r], [t.sub.init], and [t.sub.prop] increase with increasing LCPEA content. When the LCPEA content is above 2.5 wt%, these values tend to decrease. These behaviors are in accordance with the above tensile and Izod notched impact pr operties. However, from the increasing scale. the effect of LCPEA on the crack initiation is larger than for the crack propagation. From these results, we believe that LCPEA prolongs the crack initiation and propagation time, and increases the energies for crack initiation, propagation and impact fracture, and so it leads to the toughening of LCPES/PA66/LCPEA in-situ composites.

Morphology

Figures 6a-d are low-magnification SEM fracto-graphs of the LCPES/LCPEA/PA66 composites with different LCPEA contents. From Fig. 6a. it can be seen that the binary LCPES/PA66 composite containing 25 wt% LCPES exhibits a typical skin-core structure. Such a structure disappears by adding 2.5 wt% LCPEA (Fig. 6b). However, the skin-core structure reappears with further addition of LCPEA. Figures 7. 8 and 9 are higher-magnification views of the skin and core regions for some of the in-situ composites with 0%, 2.5% and 5% of LCPEA, respectively. It is apparent that fine and elongated LCP fibrils are developed in the skin region of the LCPES/PA66 25/75 composite (Fig. 7a), and only LCP droplets with diameters of about 2-15 [micro]m are observed in the core region of the composite (Fig. 7b). However, the fine and elongated LCP fibrils are well developed in the skin and core sections by adding 2.5 wt% LCPEA in the LCPES/PA66 composite (Figs. 8a, b). As the reinforcement becomes finer and has higher aspect ratio, the m echanical properties increase. Wiff and Weinert (28) found that the uniform-dispersed and nano-scale fibrous liquid crystalline polymer domains can obviously improve the mechanical properties of in-situ composites by means of special processing techniques. This indicates that LCPEA improves the fibrillation of LCPES in the PA66 matrix. Moreover, the surface of these micro fibrils is coarser than the above. With a further increase in the LCPEA content, some micro fibrils are pulled out from PA66 matrix in the skin section (Fig. 9a). and many LCP droplets form again in the core section (Fig. 9b). These are related to the compatibilization effect of LCPEA to the LCPES/PA66 blends. When the LCPEA content is increased above 5 wt%, the number of LCPEA domains increases, thereby forming interlocked domains that link themselves via hydrogen bonding. Then, fewer interactions occur between LCPEA-LCPES and LCPEA-PA66, ie., the intermolecular bonding to link LCPEA molecules themselves prevails over the LCPEA-LCPES and LC PEA-PA66 interactions. In a word, the compatibility between LCPES and PA66 becomes weak with further increasing LCPEA content. The behavior is similar to that of LCPEA/PA66 and LCPEA/PBT in-situ composites (17-19).

CONCLUSION

Liquid crystalline copolyester-polyamide 66 (LCPES/ PA66) in-situ composite was compatilized by a thermotropic liquid crystalline copolyesteramide (LCPEA). The ternary blends were prepared by melt blending, followed by injection molding. Their properties and morphology were systematically studied. The DMA and DSC results showed that LCPEA is an effective compatibiizer for the LCPES/PA66 composites. The mechanical measurements showed that the stiffness. tensile strength and toughness of the in-situ composites are improved by adding LCPEA because of the compatibilization and reinforcement to the LCPES/ PA66 composites. However, the above mechanical properties deteriorated considerably. when the LCPEA content was above 2.5-5 wt%. The instrumented drop weight dart impact test was also applied to analyze the toughening behavior of these composites. The results showed that the maximum impact force ([F.sub.max]), crack initiation and propagation energy increase with increasing LCPEA. From these results, it appeared that LCPEA prolongs the crack initiation and propagation, and increases the energies for crack initiation, propagation and impact fracture, thereby leading to toughening of the LCPES/LCPEA/PA66 in-situ composites.

ACKNOWLEDGMENT

This work was supported by the National Natural Scientific Foundation of China (Grant No.50003005) and the City University of Hong Kong (Strategic Grant No. 7000607).

[Figure 1 omitted]

[Figure 2 omitted]

[Figure 3 omitted]

[Figure 4 omitted]

[Figure 5 omitted]

Table 1.

The Thermal Properties of the PA66 Phase in the LCPES/LCPEA/PA66
Blends.

LCPES/LCPEA/PA66   [T.sub.m] ([degrees]C)  [T.sub.m] ([degrees]C)
                          (onset)                 (peak)

25/0/75                    252.6                   260.8
24.375/2.5/73.125          248.9                   257.6
23.75/5/71.25              244.4                   254.1
22.5/10/67.5               238.4                   248.0

LCPES/LCPEA/PA66   [DELTA][H.sub.m]  [T.sub.c] ([degrees]C)
                        (J/g)               (onset)

25/0/75                 57.04                231.3
24.375/2.5/73.125       51.98                228.6
23.75/5/71.25           45.56                225.4
22.5/10/67.5            43.81                219.2

LCPES/LCPEA/PA66   [DELTA]T ([degrees]C)  Xc (%)


25/0/75                    29.5            29.3
24.375/2.5/73.125          29.0            26.7
23.75/5/71.25              28.7            23.4
22.5/10/67.5               28.8            22.5
Table 2.

The Anisotropy of LCPES/PA66/LCPEA Composites.

LCPES/LCPEA/PA66   [E.sub.L]/[E.sub.T]  [(TS).sub.L]/  [S.sub.IL]/
                                        [(TS).sub.T]   [S.sub.IT]

25/0/75                   1.07              1.07          1.21
24.375/2.5/73.125         1.14              1.25          1.25
23.75/5/71.25             1.17              1.29          1.44
22.5/10/67.5              1.10              1.04          1.09

(1.) Department of Chemistry Huazhong University of Science and Technology Wuhan 430074 P. R. China

(2.) Department of Physics and Materials Science City University of Hong Kong Tat Chee Avenue, Kowloon, Hong Kong, P. R. China

(3.) Department of Manufacturing Engineering and Engineering Management City University of Hong Kong Tat Chee Avenue, Kowloon. Hona Kona. P. R. China

(*.) Correspondence to: X. L. Xie. E-mail address: [e-mail]

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