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Reactively formed ENPT copolymers as compatibilizers in ternary blends of poly(ethylene naphthalate)/poly(pentamethylene terephthalate)/poly(ether imide).

INTRODUCTION

Various polymers can be blended to develop novel materials, with a range of favorable characteristics. However, most such blends are thermodynamically immiscible because the components have different solubilities [1, 2]. The morphology of the samples with a heterogeneous structure is typically poor, because their morphology is unstable and the adhesion between the phases is poor [3, 4]. In such systems, the compatibility between the blends and the characteristics of the interfaces among the components can be improved using a suitable compatibilizer that is compatible with each component. Previous studies confer that a polymer blend is most effectively compatibilized using appropriate copolymers, which comprise various species of polymers chemically bonded by a covalent bond [3-8]. The copolymers were present at the interface between the phase domains in the blends. These copolymers strengthen the interface, reduce the interfacial tension between the phases, and increase the compatibility of the blend. Additionally, reactive compatibilization improves the interfacial characteristics of the polymer blends [4, 8]. In the ternary blends, a third component is typically added as a compatibilizer to an immiscible pair to yield a homogeneous phase in which the third component is miscible with each of the other two polymers, by hydrogen bonding or van der Waals forces [9, 10]. Moreover, chemical interactions in the ternary blends promote compatibility and the phase homogeneity of polymer blends [11].

Ternary blends that comprise semicrystalline polymers and amorphous polymers have been extensively studied and may provide a synergistic balance of polymer characteristics [9-11]. The interaction parameters [chi] of ternary blends (A/B/C) can be assumed to be parameters associated with three binary pairs ([[chi].sub.AB], [[chi].sub.AC], and [[chi].sub.BC]) [7]. The miscibility of the three binary pairs (PEN/PEI, PPT/PEI, and PPT/PEI) associated with three polymers in poly(ethylene naphthalate)/poly(pentamethylene terephthalate/poly(ether imide) (PEN/PPT/PEI) blends has been discussed in the last few decades [12-23]. PEN/PEI blends have been reported to be completely miscible over the entire range of compositions [12, 13]. Polyester/PEI blends, poly(ethylene terephthalate) (PET)/PEI [14, 15], poly(trimethylene terephthalate) (PTT)/PEI [16, 17] and poly(butylene terephthalate) (PBT)/PEI [18, 19] have been reported in recent years. These cited studies have improved our understanding of the morphology, crystal lization, and characteristics of miscible blend systems. The interaction parameter ([[chi].sub.12]) of aryl polyester/PEI blends increases with the number of methylene moieties in the repeating unit of the aryl polyester, indicating that increasing the number of methylene moieties in the repeating unit of the aryl polyester weakens the interaction between the aryl polyester and PEI. Additionally, increasing the methylene moieties changes the morphology of the aryl polyester/PEI blends from homogeneous to heterogeneous. Both PPT/PEI and poly(hexamethylene terephthalate) (PHT)/PEI blends exhibit phase heterogeneity because the systems have positive interaction parameters [20]. Blends of aryl polyester and PEN obtained by precipitation from the solvent are immiscible. When the extent of transesterification reaches 50% in the completely randomized state, the blends have a single glass transition temperature ([T.sub.g]) between two initial polymer [T.sub.g]'s [21-23]. Although the process is not completely understood, the literature proposes three potential mechanisms of transesterification-alcoholysis, acidolysis, and direct ester exchange [22, 23]. Regardless of the mechanism, the interchange reactions are typically considered to lead initially to the formation of block copolymers, and then of random copolymers that increase the compatibility of the blend [23].

[GRAPHIC OMITTED]

An earlier work [11] reported transesterification of the PEN/PPT blend. Scheme 1 shows the six possible sequences of the copolymer based on glycol units of PEN/PPT = 50/50 following heating at 300[degrees]C for a specified period. As PEN reacted with PPT, an increasing number of terephthalate-ethylene-naphthalate ([A.sub.2]-[B.sub.1]-[A.sub.1]), terephthalate-ethylene-terephthalate ([A.sub.2]-[B.sub.1]-[A.sub.2]), naphthalate-pentamethylene-naphthalate ([A.sub.1]-[B.sub.2]-[A.sub.1]), and naphthalate-pentamethylene-terephthalate ([A.sub.1]-[B.sub.2]-[A.sub.2]) residues were present in the blend; the concentrations of PEN and PPT decreased with the reaction time. This work examines how reaction-induced ENPT copolymers formed in situ as a compatibilizer impacts the compatibility of the ternary blends PEN/PPT/PEI. The effect of ENPT copolymers as a compatibilizer on the compatibility and morphology of the PEN/PPT/PEI blends was studied in detail by controlling the concentration of PPT in the reactively formed copolymers.

EXPERIMENTAL PROCEDURES

Materials

PEN was obtained from Aldrich, with [M.sub.w] = 43,000. PPT was synthesized from 1,5-pentanediol and dimethyl terephthalate, using 0.1% butyl titanate as a catalyst by two-step polymerization [20]. The molecular weight of PPT was determined by GPC (Waters), using polystyrene as a standard. The weight-averaged molecular weight ([M.sub.w]) of PPT is 16,600 g/mol. PEI was purchased from Polysciences, with [M.sub.w] = 30,000. The chemical structure of the repeating units of PEI was as follows:

[GRAPHIC OMITTED]

In this study, all of the blends that comprised PEN and PPT were produced by precipitation out of solution at room temperature to prevent transesterification during the preparations of the blends. Polymers of various fractions were codissolved in dichloroacetic acid at 120[degrees]C to yield a 4% (w/w) solution and then were precipitated in an excessive volume of methanol. The blends were washed with a large amount of hot water, and allowed to dry at 60[degrees]C for several days. Residual solvent was then removed from the blends after it had been kept in a vacuum oven at 80[degrees]C for around 1 week. Additionally, the PEN/PPT blends with various compositions (1/9, 3/7, and 5/5) were preheated at 300[degrees]C for 360 min to yield the random copolymer first, and then the products of the ENPT copolymer were blended with PPT and PEI by coprecipitation.

Apparatus

The thermal behaviors of blend samples were measured with a differential scanning calorimeter (PerkinElmer DSC-7) equipped with an intracooler. Notably, the samples were heated at 300[degrees]C for I min to eliminate thermal history in the blends. Heating, however, did not alter the morphology of the samples in such a short time. The glass transition temperature ([T.sub.g]), cold-crystallization temperature ([T.sub.cc]), and melting temperature ([T.sub.m]) were measured at 20[degrees]C/min after heat treatment at 300[degrees]C for a desired period of time. The obtained [T.sub.g] values were taken as the onset of the glass transition (i.e., change in the specific heat) in the DSC thermograms.

A polarized-light optical microscope (POM) (Nikon Optiphot-2 POL) with UFX-DX automatic exposure was used to examine and confirm the phase structure of the polymer mixtures. Samples for microscopy were placed between two micro glass slides, then heated and gently pressed by hand to reduce film thickness for the microscopic heating stage (Linkam THMS-600 with a TP-92 temperature programmer).

RESULTS AND DISCUSSION

Phase Behavior in Ternary Blends of PEN/PPT/PEI

The phase behavior of ternary PEN/PPT/PEI systems depends on the process of annealing. In the ternary blend. PEI is an amorphous polymer ([T.sub.g] = 215.6[degrees]C), whereas PPT ([T.sub.m] = 128.0[degrees]C and [T.sub.g] = 8.1[degrees]C) and PEN ([T.sub.m] = 337.0[degrees]C, and [T.sub.g] = 117.0[degrees]C) are crystalline polymers. Table 1 lists the [T.sub.g]s of PEN/PPT/PEI blends heated at 300[degrees]C for 1 and 60 min. Before annealing, each blend had two [T.sub.g]'s, indicating the presence of a heterogeneous phase and the immiscibility of the blends. Additionally, one of the [T.sub.g] values of the blends was found to be extremely close to that of the pure PPT ([T.sub.g,ppt] = 8.1[degrees]C); this [T.sub.g] was associated with the PPT-rich regions in the blends. In contrast, PEN-rich regions in the blends had a relatively high [T.sub.g], which is between that of PEI and that of PEN. The [T.sub.g] value of the PEN-rich regions increased with the PEI content of the blend, indicating that PEN was miscible with the PEI component in the ternary blends. This result is consistent with the findings that PEN and PEI were completely miscible over the entire range of compositions [7]. Conversely, the [T.sub.g] of PPT was almost independent of the concentration of PEI in the ternary blends. This finding indicates that only a weak interaction occurred between PPT and PEI, which is consistent with a previous study [20]. During annealing at 300[degrees]C for 60 min, a single glass transition was observed; the morphology of the blends changed from heterogeneous to homogeneous. Transesterification in PEN and PPT improved the compatibility of PEN/PPT/PEI blends that were annealed at 300[degrees]C for more than 60 min. The extent of transesterification in PEN and PPT set a threshold of compatibility for PEN/PPT/PEI blends in the exchange reaction. Additionally, transesterification at 300[degrees]C for 60 min reduces the melting temperature of PEN and PPT because the crystallinity of PEN and PPT is destroyed. The amorphous state of the blend exhibited improved compatibility of the PEN/PPT/PEI system.

[FIGURE 1 OMITTED]

Figure 1 shows how annealing time affects the glass transition behavior of the PEN/PPT/PEI = 1/1/1 blend. Initially, the [T.sub.g] values of the blends increased with the reaction time. After 60 min, the [T.sub.g] of the blends remained constant and independent of annealing time. The glass transition range initially narrowed quickly with the reaction time. In the late stages of the reaction, the width of the range of glass transition temperatures remained constant and independent of the reaction time. These results indicate that transesterification of PEN and PPT increased the compatibility of the ternary blends, and that this reaction was almost completed in its early stages. Restated, the compatibility of the ternary blends increased with the concentration of the product. ENPT copolymer, which is formed by the copolymerization of PEN and PPT. Therefore, [T.sub.g] was constant and the glass transition range was narrow when the blends were heated at 300[degrees]C for more than 60 min.

Miscibility of Binary Blends of ENPT Copolymer/PPT and ENPT Copolymer/PEI

In another work [11], proton nuclear magnetic resonance ([.sup.1]H NMR) imaging showed that when the samples were heated at 300[degrees]C for 60 min, a random ENPT copolymer was present in the PEN/PPT/PEI blends, improving their compatibility. Therefore, the miscibility of the ENPT copolymer blended with PPT or PEI is of interest. Figure 2 shows DSC thermograms of the ENPT/PPT blend, the ENPT/PEI blend, and three polymers (PPT, PEI, and ENPT copolymer). Notably, the ENPT copolymer was the transesterification product of the PEN/PPT = 5/5 blend. The thermograms indicate that the ENPT/PPT and ENPT/PEI blends exhibited a clear and single [T.sub.g]. The experimental results indicate that both the ENPT/PPT and ENPT/PEI blends are miscible. The transesterification product of PEN/PPT = 5/5 blends, ENPT copolymer, was not only miscible with PEI, but also miscible with PPT. An optical microscope was utilized preliminarily to study both the ENPT/PPT and the ENPT/PEI blends. The samples were plain and transparent without any detectable phase domain/boundary at 800x.

Figure 3 shows the miscibility between the transesterification product of PEN/PPT = 5/5 (ENPT) and PEI. In Fig. 3, the effect of composition on the [T.sub.g] of the ENPT/PEI blends was determined from the Fox equation [24]:

1/[T.sub.g] = [[[omega].sub.1]/[T.sub.g1]] + [[[omega].sub.2]/[T.sub.g2]] (1)

where [[omega].sub.i] is the weight fraction and [T.sub.gi] is the glass transition temperature of component i. As shown in Fig. 3, the experimental [T.sub.g] at each composition deviates negatively from the corresponding expected [T.sub.g] value obtained from the Fox equation, indicating that the miscibility of the blends was related to the interactions between the two components [1, 25]. Additionally, the relationship between [T.sub.g] and the composition of ENPT/PEI blends was most accurately given by the Gordon-Taylor equation [26, 27]:

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[T.sub.g] = ([T.sub.g1][[omega].sub.1] + k[T.sub.g2][[omega].sub.2])/([[omega].sub.1] + k[[omega].sub.2]) (2)

where k is the ratio of the thermal expansion coefficient of ENPT to that of PEI. The Gordon-Taylor equation with an adjustable parametric constant seems to most accurately specify the [T.sub.g] values of the ENPT/PEI blends. The parameter, k, was determined to be 0.56. These [T.sub.g] data indicate that the ENPT/PEI blends were indeed a homogeneous, single phase. The phase of all ENPT/PEI blends, as observed by OM, was domain-free and transparent.

Table 2 compares the glass transition temperatures of the ENPT/PEI and the PEN/PPT/PEI blends. Notably, the ENPT/PEI blends included the ENPT copolymer, which is a complete transesterification product of PEN/PPT = 5/5 blend. All PEN/PPT/PEI blends for which PEN/PPT = 5/5 were annealed at 300[degrees]C for 60 min. The [T.sub.g] values of the ENPT/PEI blends were close to those of the annealed PEN/PPT/PEI blends, indicating that the PEN/PPT/PEI blends yielded a similar product, the ENPT copolymer, formed by the transesterification of PEN and PPT when the samples were annealed at 300[degrees]C. The transesterification of PEN and PPT was critical in improving the compatibility of ternary systems.

[FIGURE 4 OMITTED]

The miscibility between the transesterification products of the PEN/PPT blends (ENPT) and PEI was studied for various compositions of ENPT in the PEN/PPT blends. Figure 4 shows DSC traces of ENPT/PEI = 50/50, where ENPT represents various PEN/PPT ratios. The insets present the OM morphology of the blends. The various PEN/PPT blends were preheated at 300[degrees]C for 360 min to yield the random copolymer initially; the products of the ENPT copolymer with various concentrations of PPT were then blended with PEI by coprecipitation. The DSC thermogram in Fig. 4A clearly shows two [T.sub.g] values (at the onset positions and indicated by arrows). The inset presents the morphology with apparent phase domains in the blends, as showed by OM. The experimental results concerning the PPT-rich (>90 wt%) ENPT copolymer indicate phase immiscibility in the ENPT/PEI = 50/50 blend. In contrast, DSC traces showed a clear and single [T.sub.g], and the morphology was domain-free and transparent as shown in Fig. 4B and 4C. The phase was homogeneous in the blends when the concentration of PPT in the ENPT copolymer fell to 70 wt%. In the ENPT/PEI = 50/50 blends, the phase homogeneity of the blends decreased as the concentration of PPT in the ENPT copolymers increased, because the PPT and PEI components in the blends are immiscible. Figure 5 shows reactively formed ENPT copolymers as compatibilizers in PEN/PPT/PEI blends. A homogeneous phase was found in the ENPT/PEI = 50/50 blends when the ENPT copolymers contained PEN-rich sequences (>90 wt%). Conversely, the ENPT/PEI = 50/50 blends exhibited phase heterogeneity when the concentration of PPT in the ENPT copolymer exceeded 90 wt%.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Effect of ENPT Copolymers on Compatibility of ENPT/PPT/PEI Blends

Transesteritication in PEN and PPT in the PEN/PPT/PEI blends increased phase homogeneity, as shown in Fig. 1 and Table 1. The polymer-polymer interaction parameter ([[chi].sub.12]) between the aryl polyester and the PEI in the binary PPT/PEI blend was positive, indicating phase separation [20]. However, the effect of the ENPT copolymer on the thermal transition and morphology of the PPT/PPI blends was studied. Figure 6 shows the effect of the composition on the thermal behavior of the ENPT/PPT/PEI blends, in which the ENPT copolymer was the transesterification product of the PEN/PPT = 5/5 blend. The thermograms of the ENPT/PPT/PEI blends showed that PPT had a crystallization temperature [T.sub.c,PPT] = 93.0[degrees]C and a melting temperature [T.sub.m,PPT] = 128.0[degrees]C. Additionally, one of the [T.sub.g] values of the blends was found to be extremely close to that of the pure PPT, and was associated with the PPT-rich regions in the blends. In contrast, PEI had a relatively high [T.sub.g], associated with PEI-rich regions in the blends. As the ENPT copolymer content of the blend increased, the [T.sub.g] value of the PEI-rich regions decreased and the [T.sub.g] of the PPT-rich regions increased. The DSC traces of the ENPT-rich blends (>50 wt%) revealed a clear and single [T.sub.g]. Restated, the compatibility of the PPT/PEI blends increased with the concentration of the ENPT copolymer, which was produced by the copolymerization of PEN and PPT. Therefore, the [T.sub.g] was clear and the glass transition range was narrow when the concentration of the ENPT copolymer exceeded 50 wt%. This result shows that the ENPT copolymer is a good compatibilizer in PPT/PEI blends. The effect of the ENPT copolymer as a compatibilizer on the morphological changes of the ENPT/PPT/PEI blends was determined using OM.

[FIGURE 7 OMITTED]

Figure 7 shows optical micrographs (A-F) of coprecipitated ENPT/PPT/PEI samples with six compositions. The ENPT copolymer in the samples was the transesterification product of the PEN/PPT = 5/5 blend. The experimental results reveal a homogeneous phase in copolymer-rich ENPT (>50 wt%) and phase separation in blends with less of the ENPT copolymer ([less than or equal to]50 wt%). The size of the phase domains in the incompatible compositions depended on the composition. The binary blends, PEN/polyester (prepared by coprecipitation) [21-23], and PPT/PEI [20] are immiscible, regardless of composition, but PEN/PEI [12, 13] is miscible for all compositions, indicating that the ENPT copolymer increases the compatibility of the PPT/PEI blends.

CONCLUSIONS

ENPT copolymers formed in situ as a compatibilizer in a reaction increased the compatibility of PEN/PPT/PEI blends. The transesterification of PEN and PPT in the PEN/PPT/PEI blends increased the compatibility of the PEN/PPT/PEI blends that had been annealed at 300[degrees]C for over 60 min. The initially incompatible blends became compatible after transesterification had proceeded to a critical extent. The extent of transesterification in PEN and PPT governs the threshold of compatibility of PEN/PPT/PEI blends in the exchange reaction. Although the ternary blends were incompatible, a single [T.sub.g] was identified after the reaction lasted for a sufficient period, indicating a homogeneous phase in the ternary systems. Additionally, the transesterification product of PEN/PPT = 5/5 blends, ENPT copolymer, was miscible with PEI and PPT in ENPT/PPT = 50/50 and ENPT/PEI = 50/50 blends. DSC and OM showed that the ENPT/PEI = 50/50 blends exhibited phase-separated morphology when the ENPT copolymer contained PPT-rich components (>90 wt%). In contrast, a homogeneous phase with a clear [T.sub.g] was observed in blends when the concentration of PPT in the ENPT copolymer fell to 70 wt%. Moreover, the ENPT copolymer is a compatibilizer in ENPT/PPT/PEI systems, increasing the compatibility between PPT and PEI. The ENPT-rich blends contained a homogeneous phase. The compatibility of the PEN/PPT/PEI ternary blends increased not only with the concentration of the ENPT copolymer, but also as the concentration of PPT in the ENPT copolymer decreases.

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Chean Cheng Su, Chih Kuan Shih

Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan, Republic of China

Correspondence to: C.C. Su; e-mail: ccsu@nuk.edu.tw

Contract grant sponsor: National Science Council of Taiwan; contract grant number: NSC 92-2216-E-390-003.
TABLE 1. The glass transition temperatures of PEN/PPT/PEI = 1/1/1 blends
heated at 300[degrees]C for 1 and 60 min, respectively.

 [T.sub.g] ([degrees]C)
 (300[degrees]C, 1 min) [T.sub.g] ([degrees]C)
 Heterogeneous phase (300[degrees]C, 60 min)
 PPT-rich PEN-rich Homogeneous
PEN/PPT/PEI regions regions phase

100/0/0 -- -- 117.0
 0/100/0 -- -- 8.1
 50/50/0 8.0 -- 50.0
 40/40/20 9.6 Obscure 71.3
 33/33/33 12.0 134.6 81.0
 25/25/50 14.4 138.5 105.4
 20/20/60 14.2 145.2 125.8
 10/10/80 Obscure 169.2 161.9
 0/0/100 -- -- 215.6

TABLE 2. Comparison of glass transition temperatures between ENPT/PEI
and PEN/PPT/PEI blends.

 Glass transition temperature [T.sub.g] ([degrees]C)
 PEN/PPT/PEI
PEI (wt%) (300[degrees]C, 60 min) ENPT/PEI

 0 50.0 50.5
 20 71.3 72.6
 33 81.0 83.2
 50 105.4 109.5
 60 125.8 126.3
 80 161.9 162.1
100 215.6 215.6
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Author:Su, Chean Cheng; Shih, Chih Kuan
Publication:Polymer Engineering and Science
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Date:Mar 1, 2006
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