Compatibility of poly(ethylene 2,6-naphthalate) and poly(butylene 2,6-naphthalate) blends.

INTRODUCTION

There has been a great interest in blends of polyesters because of their growing industrial importance. One of the most important properties of polymer blends is their mechanical behavior. In crystallizable polymer blends, the mechanical behavior is affected by properties of the individual constituents, mode of dispersion, degree of crystallinity, morphology, and compatibility in the amorphous state.

Studies (1-3) indicate that significant improvement in properties of poly (ethylene terephthalate) (PET) can be achieved by blending with poly (butylene terephthalate) (PBT). Escala and Stein(4) have shown that these polymers are compatible in the amorphous state, however, they crystallize separately according to their own unit cell.

Paul and Barlow (5-8) examined extensively the mechanical behavior of various polymer blends. They attempted to establish a compatibility criterion based on the tensile properties.

In this study, we have tried to describe the compatibility of poly (ethylene 2,6-naphthalate) (PEN) and poly (butylene 2,6-naphthalate) (PBN) blends based on the glass transition temperatures and the tensile properties. The rheology of the melts was also measured to see the morphological dependence on composition.

EXPERIMENTAL

Polymer Preparation

PEN was prepared by melt polymerization. Dimethyl naphthalate (DMN), kindly supplied by Kolon Ind., was reacted in a nitrogen environment with ethylene glycol in the presence of Mn[(OAc).sub.2] [center dot] 4[H.sub.2]O catalyst in a small scale batch reactor. The reactor was first heated to 230 [degrees] C in a silicone oil bath. This temperature was maintained for 4 h. After that, S[b.sub.2][O.sub.3] catalyst was introduced into the reactor and the reaction temperature was progressively increased to 295 [degrees] C. Then the pressure was reduced to a specified level and maintained for 1 h. PBN was prepared similarly with different reaction temperatures (190 and 260 [degrees] C) and different catalyst (titanium tetrabutoxide) with DMN and butylene glycol. The inherent viscosity, glass transition temperature ([T.sub.g]) and melting temperature ([T.sub.m]) of polymerized PEN and PBN are listed in Table 1.

Blend Preparation

All blends were prepared by melt mixing in a Twin Screw Extruder (Model ZCM 32/36-8G, Automatik Ltd.) at 270 [degrees] C for 6 min. The chips obtained from extruder were dried in a vacuum oven at 150 [degrees] C for 16 h. The pure homopolymers were also subjected to the same process.

Table 1. Properties of Polymers.

               Inherent            [T.sub.g]        [T.sub.m]
Sample    Viscosity (dl/g)(a)    ([degrees] C)    ([degrees] C)

PEN              0.53                 125              268
PBN              0.83                  48              247

a Values measured with a mixed solvent of phenol/o-dichlorobenzene
(60/40, v/v) at 35 [degrees] C.

Differential Scanning Calorimetry 

Thermal analysis was performed using a DuPont 910 differential scanning calorimeter (DSC) equipped with a mechanical cooling accessory. Samples were first melted at 280 [degrees] C for 5 min, and then quenched into liquid nitrogen. The quenched samples were heated from -30 to 300 [degrees] C at a heating rate of 20 [degrees] C/min to measure [T.sub.g] and [T.sub.m].

Tensile Testing

Tensile properties such as initial modulus measured at 1% elongation, tensile strength at yield, and elongation at maximum tensile strength were measured using an Instron Model 4204 universal instrument. Measurements were made at room temperature at a constant crosshead speed 50 mm/min on specimens (ASTM D-638) which were injection molded in Fanuc Autoshot series (Model 50, Fanuc Ltd.). The data were taken as averages of at least five measurements.

Rheological Measurements

A parallel plate rheometer (PHYSICA-RHEOLAB, MC20/UM/TEK 500) was used to measure complex viscosity (Pa [center dot] s) as a function of angular frequency (rad/s) at 280 [degrees] C in the oscillatory shear mode. The gap size was 1 mm and the diameter of the plate was 50 mm. To avoid the oxidative degradation, nitrogen was continuously purged during the measurements.

Degree of Crystallinity

The degree of crystallinity of molding specimens was determined by X-ray diffraction analysis and are equal to the ratio of the integrated crystalline scattering to the total scattering, both crystalline and amorphous, and is given by

[[Chi].sub.c] = [integral of] [s.sup.2][I.sub.c](s)ds between limits of [infinity] and 0/[integral of] [s.sup.2] I(s) ds between limits of [infinity] and 0 (1)

where s is the magnitude of the reciprocal-lattice vector and is given by

s = 2 sin [Theta]/[Lambda] (2)

[Theta] is one-half the angle of deviation of the diffracted rays from the incident X-rays, [Lambda] is the X-ray wavelength, I(s) and [I.sub.c](s) is the intensity of coherent X-ray scatter from a specimen and the crystalline region, respectively.

RESULTS AND DISCUSSION

The glass transition temperatures of PEN/PBN blends are shown in Fig. 1. All blends have two glass transition temperatures and those of PEN are decreased nearly linearly with increasing PBN content. It indicates that all blends are separated into two amorphous phases. One phase may be relatively rich in PEN and the other in PBN. As content of PBN is increased, the PEN-rich phase seems to incorporate more PBN and the PBN-rich phase also more PEN, which means that the difference in composition between two phases is decreased. Therefore, it may be said that PEN/PBN blends shows the only partial miscibility throughout compositions.

It is well known that polyester blends can be transesterified at high temperatures and this reaction leads to increase of the miscibility. To know the effect of transesterification during melt processing of PEN/PBN blends, [T.sub.m] depression and [T.sub.g] were measured with isothermal reaction time at 280 [degrees] C and these results were reported in our previous study (9). In case of these blends, for compositions less than 80 wt% PBN, transesterification did not occur, while for compositions more than 80 wt%, it occurred under the experimental conditions (during melt mixing, 6 min and residence time in the DSC, 5 min). Therefore the partial miscibility shown in Fig. 1 does not result from increase of miscibility due to transesterification.

The typical stress-strain curves for the blends of PEN/PBN are shown in Fig. 2. It is seen that the elongation of blends at break increases with PBN content and the mode of failure is changed from brittle fracture below 20 wt% of PBN to ductile or yielding one above 20 wt% of PBN. The elongation of blends at peak is shown in Fig. 3. The elongation up to 10 wt% of PBN is relatively low and that [greater than] 20 wt% of PBN is relatively high, which exhibits a brittle-ductile transition around PBN 20 wt%.

One of the most important parameters that distinguish the compatible blends from the incompatible ones is the modulus. The modulus represents the intermolecular interaction energy. Nielson (19) proposed that the modulus can be represented by rule of mixtures, which gives the upper bound and lower bound in the modulus for a multiphase system.

For a two-phase system, the equations are as follows:

Upper bound: [E.sub.b] = [[Phi].sub.1] [E.sub.1] + [[Phi].sub.2] [E.sub.2]

or [E.sub.b] = [w.sub.1] [E.sub.1] + [w.sub.2] [E.sub.2] (3)

Lower bound: [E.sub.b] = [[[[Phi].sub.1]/[E.sub.1] + [[Phi].sub.2]/[E.sub.2]].sup.-1] (4)

where [E.sub.b], [E.sub.1], and [E.sub.2] are the modulus of the blend, first component and second one, respectively. [Phi] is the volume fraction and [Omega] is the weight fraction.

If there is any interaction between two components, then the equation has the following form: (11)

[E.sub.b] = [w.sub.1] [E.sub.1] + [w.sub.2] [E.sub.2] + [[Beta].sub.12] [w.sub.1] [w.sub.2] (5)

where interaction parameter, [[Beta].sub.12], expresses the deviation from linearity. A positive [[Beta].sub.12] represents a nonlinear synergism, in other words, an indication of compatibility.

Nolley and Paul (12) predicted the nature of interaction from strength data. The total strength [[Sigma].sub.b] of a binary blend can be given by:

[Mathematical Expression Omitted]

where [[Sigma].sub.11] and [[Sigma].sub.22] refer to the adhesive strength of the pure components to themselves, and [[Sigma].sub.12] is the adhesive strength between polymer 1 and 2. In the limit of poor adhesion, strength of the blend is equal to

[Mathematical Expression Omitted]

Figure 4 shows the effect of blend composition on modulus and the calculated values of modulus [Eqs 3 and 5] are applied with 140 kg/[cm.sup.2] of [[Beta].sub.12]. There are large variations of modulus with composition. It reaches a maximum [approximately] 20 wt% of PBN, indicating positive synergism and mechanical compatibility.

Figure 5 shows the effect of blend composition on strength and the calculated values of strength [Eqs 6 and 7] are applied with 778 kg/[cm.sup.2] of [[Sigma].sub.12]. Strength of blends also suggests some positive synergism at 20 wt% of PBN like modulus.

In crystallizable polymer blends, the mechanical properties is affected by the degree of crystallinity. The degree of crystallinity measured by using Eq 1 is shown in Fig. 6. The degree of crystallinity of molding specimens decreases with increasing PBN content, and it reaches a minimum at 60 wt% PBN and increases and reaches a maximum at 90 wt% PBN. These results can be explained by the crystallization rate. The crystallization rate of PBN is relatively fast and that of PEN slow under the same experimental condition (9). Therefore the crystallinity of PBN-rich blends is higher than that of PEN-rich blends. It is thought that the mechanical properties of these blends in Figs. 4 and 5 is not affected directly by the degree of crystallinity.

Mishra and Deopura (13) have reported that 98/2 PET/PBT blend shows a higher modulus and strength value, indicating a synergistic effect. They showed that the rheological behavior for their blend has higher chain entanglement. The entanglement may result in higher crystalline links as changes introduced in the supermolecular structure of a polymer melt may be reflected in the solid state (14). This may account for high modulus and strength.

To see the entanglement effect of PEN/PBN blends, rheological behavior was measured and shown in Fig. 7. One of the interesting observations is the properties of the 20 wt% of PBN sample. The higher viscosity may be due to entanglement density as discussed by Han and Kim (15), Porter (16), and Bueche (17).

CONCLUSION

The compatibility of poly(ethylene 2,6-naphthalate) (PEN) and poly(butylene 2,6-naphthalate) (PBN) blends was observed from the [T.sub.g]s and the tensile properties. The results are as follows:

1. PEN/PBN blends have at least partial miscibility in the whole range judged from [T.sub.g] behavior.

2. The stress-strain behavior is changed from brittle [less than] 20 wt% of PBN to ductile [greater than] 20 wt% of PBN.

3. Both modulus and strength values of the blends are within the values proposed by Kleiner (11) and Paul (12) with proper [[Beta].sub.12] and [[Sigma].sub.12] except [approximately] 20 wt% of PBN that exhibit synergistic effect, which represents that blends have mechanical compatibility in the whole range.

4. PBN 20 wt% blend shows both a high modulus and strength, and a high melt viscosity.

ACKNOWLEDGMENT

One of the authors (K. H. Yoon) acknowledges the financial support provided by Korea Science & Engineering Foundation in 1993.

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