On the use of PET-LCP copolymers as compatibilizers for PET/LCP blends

On the Use of PET-LCP Copolymers as Compatibilizers for PET/LCP Blends

GIOVANNA POLL MASSIMO PACI, and PIERLUIGI MAGAGNINI*

Dipartimento di Ingegneria Chirnica Chimica Industriale e Scienza dei MateriQli

Universita di Pisa 56126 Pisa, Italy

and

ROBERTO SCAFFARO and FRANCESCO P. LA MANTIA

Dipartimento di Ingegneria Chirnica dei Processi e dei Materiali Universita di Palermo 901 28 Palermo, Italy

Copolyesters of poly(ethy1ene terephthalate) (PET) with a liquid crystalline polymer (LCP), SBH 1:1:2, have been synthesized by the polycondensation, carried out in the melt at temperatures up to 300°C of sebacic acid (S), 4,4'-dihydroqbiphenyl (B), and 4-hydroqbenzoic acid (H) in the presence of PET. The PET-SBH copolyesters have been characterized by differential scanning calorimetry, scanning electron microscopy, X-ray diffraction, etc., and the relationships between properties and preparation conditions are discussed. The copolyesters show a biphasic nature, which is more evident for the products synthesized with a thermal profile comprising relatively lower temperatures (220-230°C) in the initial stages of the polycondensation. Another procedure, whereby the addition of PET to the monomer charge was made at a later stage of the reaction, has also been devised to prepare copolyesters with enhanced blockiness. The compatibilizing effect of the PET-SBH copolymers toward PET/SBH blends has been investigated. PET/SBH blends (75/ 25, w/w) have been prepared in a Brabender mixer at 270°C and 30 rpm, with and without the addition of appropriate amounts (2.5, 5, and lo%, w/w) of 50-50 PET-SBH copolyesters. Different blending techniques have been used according to whether the three components were fed into the mixer at the same time, or one of them was added at a later stage. The effect of the type and the amount of added copolyester has been studied through morphological, thermal, and mechanical characterizations. The results show that the addition of small amounts -5 wt% of copolyesters leads to improved dispersion and adhesion of the minor SBH phase. Moreover, while the tensile modulus of the blends is practically unaffected by the addition of the copolymer, a substantial increase of both tensile strength and elongation to break is found for a concentration of added copolyester of -5wt%. Slightly better results were apparently obtained by the use of a block copolyester.

INTRODUCTION

lends of liquid crystalline polymers (LCPs) with B flexible thermoplastics attract considerable attention in academic and industrial research laboratories ( 1). However, their practical application in product design and engineering has raised concerns because of the difficulties in controlling morphology and me-

' To whom correspondence should be addressed.

chanical properties. Phase incompatibility, that is the rule for polymer/LCP blends, is one of the main rea- sons for this. A minor LCP phase often displays rough dispersion and poor adhesion to the matrix, which thwarts the hope of achieving synergistic property interactions between the blend components. In princi- ple, a high level of interaction in the melt can be expected to promote network formation and reduce flow, i.e., to inhibit or to limit the processing aid ability of the LCP. Despite of this, there is no doubt that many

1 244 POLYMER ENGINEERING AND SCIENCE, MID-MAY 1- Vol. 3s, NO. 9

Use of PET-LCP Copolymers as Compatibilizers

common polymer/LCP blends could benefit by enhanced interfacial adhesion.

Phase incompatibility is of course higher the more dissimilar are the chemical structures of the two polymers being blended. From this view, the blends of LCPs with polyesters such as poly(ethy1ene terephthalate) (PET) are expected to be less affected by the draw- backs of poor compatibility. This may be one reason for the wide attention paid to PET/LCP blends during the last decade. PET has been blended with a variety of LCPs, either commerciai or laboratory products.

Wholly aromatic LCPs;, such as Vectra-A, a copolyester of 4-hydroxybenzcic acid (H), and 2-hydroxy-6- naphthoic acid (N) produced by Hoechst Celanese have been shown to display no miscibility and poor compatibility with PET 12-12). However, because it is known that the need of interfacial adhesion is lower the higher the aspect ratio of the LCP fibrils, good reinforcing effect of Vecl ra has been observed in cases where the PET/Vectra blends were processed by the use of techniques involving high extensional flow such as extrusion or spinning with high draw ratios (2-7). A deeper insight into the interfacial interactions in these blends is provided by the accurate morphological investigation of PET/Vecl.ra fibers spun with a take-up speed of 85 m/min, carried out by Li et al. (4). These authors have shown that the aspect ratio L /D of the LCP fibrils was 10, 29, and @, for LCP concentrations of 35, 60, and 85%, respectively, and that the initial modulus and the strength of the fibers obeyed the rule of mixtures only for LCP contents as high as 85 and 96%. The appreciably lower values found for the fibers with 35 and 60% LCP could be accounted for by ana- lytic models applicable to composites with short aligned fibers. The critical L I D ratio of the Vectra fibrils (the ratio below which fibril pull-out takes place) was calculated to be 67, i.e., much higher than the aspect ratio found experimentally for the blends with 35 and 60% LCP. Thus, although some kind of interaction between Vectra and PET has been demonstrated by some authors (2, 131, the interphase adhesion appears nevertheless unsatisfactory. Attempts at the compatibilization of PET/Vectra blends by the addition of organofunctional silane coupling agents have been made recently (1 +).

The compatibility of PET with semiflexible LCPs, such as the PET-H copolyesters first synthesized by Jackson and KuhfUSS (151, or the LCPs containing longer aliphatic spacers either in the main chain or as side groups appears to be slightly better (11, 16-37). Thus, partial miscibility of PET with the PET-rich phase of the PET-H copolyesters has been demonstrated to take place (16-24). Although this partial miscibility is suspected to hamper LCP fibrillation to some extent (22). good reinforcement has been observed for as-spoon fibers of PETIPET-H blends (25). At any rate, a very striking demonstration that PET is more compatible with PET-H than with wholly aromatic LCPs, is provided by the recent work of DiBenedetto (38) who successfully used the 40-60 PET-H copolyester as a compatibilizer for blends of

PET with K161 (a wholly aromatic LCP produced by Bayer A.G.). However, even the compatibility of PET with the 40-60 PET-H copolyester is not the best that one can achieve. It has been shown (39) that the latter LCP displays a definitely stronger adhesion to the flexible 70-30 PET-H copolyester than to neat PET.

The literature gives concordant support for the idea that a PET-LCP copolymer having appropriate composition and sequence distribution can act as a compatibilizer for PET/LCP blends. This may be obvious, because it is known (40) that, e.g., block or graft copolymers made up of branches of the two polymers being blended can play a compatibilizing effect. How- ever, to our knowledge, the synthesis of copolymers containing the repeat units of both matrix and filler polymers, carried out with the idea of using them as compatibilizing additives for polymer / LCP blends, has not been considered previously.

In our laboratories an investigation of the compatibilizing ability of PET-LCP copolymers for PET/LCP blends is being carried out. The LCP we have chosen is a copolyester of sebacic acid ( S ) , 4,4'-dihydroxybiphenyl (B) and 4-hydroxybenzoic acid (H) in the molar ratio 1:1:2, disclosed by Eniricerche SPA, Milan (41- 43). and referred to as SBH. The preliminary results described in this article appear promising. They also show, however, that further studies are needed for the accurate optimization of the procedures for the synthesis of the copolymers and, thereby, of their structures and morphologies.

EXPERIMENTAL,

Materials

PET was a commercial pelletized material produced by Shell. Its intrinsic viscosity, measured in tetrachlo- roethane/phenol (50/50 v/v) at 25"C, was 171 = 0.82 dL / g.

Sebacic acid (S) , 4,4'-dihydroxybiphenyl (B) and 4-hydroxybenzoic acid (H), were supplied by Merck, Schuchardt and Aldrich, and were used without further purification. The hydroxylated monomers B and H were acetylated and purified by crystallization before being used for the polymer synthesis, as described elsewhere (42, 43).

The LCP was a laboratory sample (LCPlB/024) of SBH 1: 1:2, supplied by Eniricerche SPA, Milan. This semiflexible copolyester was synthesized from the above monomers as described before (41-43). The sequence distribution was shown to be random by NMR analysis (43). The inherent viscosity, measured in a pentafluorophenol solution (T = 60"C, c = 0.1 g/dL), was 1.20 dL/g. The LCP will be indicated as SBH in the following. The chemical structures of PET and SBH are illustrated in the scheme and the codes used to identify the repeat units are also indicated.

All materials were dried before being used for the copolyester synthesis or the blends preparation.

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Giovanna Poli et al.

TA

B Scheme 1

Techniques

Copolyester Synthesis

The required amounts of S, B, H, and PET were charged into a Pyrex round bottomed cylindrical flask equipped with a stainless steel stirrer and attached to a vacuum line. After careful purging with pure nitrogen, the flask was immersed in a salt bath preheated to the selected temperature (220-290°C). The SBH monomers melted and gave rise to a clear mobile liquid when used alone. If PET was added at the beginning with the monomers, the pellets rapidly dissolved in the liquid to give a clear solution whose viscosity decreased strongly during the initial reaction. Byproduct acetic acid was distilled off and condensed in a trap cooled with liquid nitrogen. Thereafter, the viscosity started increasing again and the reaction mixture became opaque. The pressure was then gradually reduced and the polycondensation was completed under vacuum (-0.01 mm Hg) for 30-60 min. This treatment led to the fmal evolution of acetic acid and of other low molar mass substances which sublimed on the inner wall of the reactor cover. At the end of the polycondensation, pure nitrogen was admit- ted to the reactor and the latter was opened under nitrogen blanket. The polymer was extracted with a stainless steel spoon, while hot, and was then cooled and ground. The powder was finally washed with boiling acetone to extract any low molar mass contaminants.

The reaction was carried out over a period of 3.5 to 6 h, following different thermal profiles. Moreover, PET was either charged together with the SBH monomers or at a later stage of the polycondensation, when the viscosity of the charge (SBH oligomers) was already high. Typical polycondensation procedures, designated as no. 1, 2, or 3, are illustrated schematically in Fig. 1. Proce- dures 1 and 3 closely resemble those commonly used for the synthesis of neat SBH (41-43). except for the fact that PET is added either at the beginning (Procedure no. 1) or close to the end of the polycondensation (no. 3).

Procedure 2, in contrast, was similar to that of Jackson and Kuhfuss (15) for the synthesis of their

I0

PET addition

n

250 Vacuum

0 1 2 3 4 5 6

Time (h)

Rg. 1 . Schematic representation of the typical polymerization procedures adopted for synthesizing the copolyesters.

PET-H copolyesters, although the temperature used by us was 15°C higher. The copolyesters produced by the different procedures will be designated in the following as COP1, COP2 and COP3, whereas the composition will be indicated as, e.g., (70-30), where the first figure is the w/w content of PET (cf. Table 1 ).

Blend Preparation

Binary and ternary blends of PET, SBH, and COP (if any) were prepared in a Brabender Plasticorder apparatus Mod. PL330, equipped with a 50 mL mixing unit

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Use of PET-UP Copolymers as Compatibilizers

Table 1. PET-SBH Copolyesters Synthesized in this Work.

Copolymer Procedure* PET TA, EG S, B H Code Number (wYo) (mol%) (mol%) (molY0)

COP-1 (30-70) 1 30 19.89 15.05 30.11 COP-1 (50-50) 1 50 30.33 9.84 19.67 COP-2 (30-70) 2 30 19.89 15.05 30.11 COP-2 (50-50) 2 50 30.33 9.84 19.67 COP-2 (70-30) 2 70 39.12 5.44 10.88 COP-3 (30-70) 3 30 19.89 15.05 30.11 COP-3 (50-50) 3 50 30.33 9.84 19.67

cf. 5. 7.

at 270°C and 30 rpm. The w/w ratio of PET to SBH was held constant at 3:1, unless otherwise stated. Some of the 75/25 PET/SBH blends were compatibilized by the addition O F COP, whose concentration is expressed as the weight added to 100 g of the 75 / 25 PET/SBH blend. The ccincentration of COP was 0, 2.5, 5, and 10 wt%. The copolyesters used in this study had always a 50:50 composition. The addition of, e.g., 1Owt% COP to a 75/'25 blend changed the actual PET/SBH ratio to 72.7/27.3. However, this composi- tional change was assumed to be negligible in view of the comparison of morphology and the properties of this blend with those of the reference binary 75/25 blend.

Four different compounding techniques were used for the preparation of the ternary blends, according to whether the three components were fed to the mixer at the same time, or one of them was added at a later stage. According to the first method (A), the dry blend of the three components was fed to the mixer, preheated to 270°C and running at 3 rpm; the speed was then increased to 30 q m and mixing was carried out until constant torque was measured (-6 min). In the second (B) and the third (C) methods only PET and COP, or, respectively, PET and SBH, were mixed as described. After torque stabilization, the preweighed amount of SBH or COP, respectively, was added under a nitrogen blanket to the mixer running at 30 rpm, and mixing was continued for an additional 4 to 6 min period (until constant torque was again obtained). The fourth method (D) consisted of the addition of PET to the SBH/COP blend. This could not be realized as for B and C because the oterall amount of SBH and COP of ternary blends is too small to grant satisfactory filling of the mixer during the first blending stage. Thus, a masterbatch was first prepared by mixing SBH with COP as described for A. The masterbatch was then ground, dried overnight in a vacuum oven at 100°C and blended with PET with the usual tech- nique. It should be appreciated that the overall mixing time was appreciably higher (10-12 min) for methods B, C and D than for method A (-6 min.).

Characterization Techniques

NMR spectra were recorded on a Bruker AMX300 spectrometer using a 5 mm 'H selective probe.

The thermal behavior of the polymers was moni- tored by differential scanning calorimetry (DSC) using

a Perkin-Elmer DSC4 apparatus. Scanning rates of 20"/min were normally used.

Powder X-ray diffraction patterns were taken on a Siemens D500 diffractometer using the Ni filtered CuKa radiation.

Morphology investigation was made by scanning electron microscopy (SEMI with a Jeol T300 apparatus. The SEM micrographs were taken on gold coated fracture surfaces produced under liquid nitrogen.

For mechanical testing, strip specimens 0.4 mm thick, 5.0 mm wide were cut out from compression molded sheets. The specimens were mounted on an Instron Mod. 1122 apparatus with a gauge length of 30 mm and drawn with a crosshead speed of 50 mm/ min. As a rule, seven measurements were made for each material and the results were averaged.

The compression molded sheets were normally prepared with a Carver laboratory press at 270°C. The sheets were cooled by water circulation within the press plates (estimated cooling rate =l"C/s). In a number of experiments the slabs were prepared with a closed mold equipped with a thermocouple connected to a recorder. At the end of the molding cycle the mold was immersed into ice-water. The initial cooling rate was -4O"C/s.

RESULTS AND DISCUSSION

Synthesis of PET-SBH Copolyesters

It is known that the reaction of PET with 4-acetoxy- benzoic acid (15) gives rise to PET-H copolyesters characterized by a bimodal sequence distribution (2 1, 44). It has been demonstrated that, depending on the reaction conditions, the structural heterogeneity, which is thought to be due to the energetically favored formation of homopolymeric H sequences, may be more or less pronounced (45). At the beginning, we had anticipated that the reaction of PET with a mixture of S, B, and H would lead to copolyesters with sequence distributions ranging from that of a random copolymer to that of a block copolymer made up of PET and SBH segments. We also thought that, with respect to the reaction of PET with H as the only monomer, there would be a lesser tendency toward the formation of blocky structures, because the SBH segments are probably less stable than the homopolymeric H segments from an enthalpy point of view. On the other hand, a PET-SBH block copolymer was expected to display even better compatibilizing perfor- mance with respect to the random isomer. Thus, when designing the research, we devised to carry out the synthesis by the two procedures indicated by 1 and 3 in Fig. I. Procedure 1 was expected to lead, with no special difficulty, to a random copolyester and we were prepared to concentrate our efforts on the optimization of procedure 3 for the synthesis of block PET-SBH copolymers. Experiments showed that the reaction of PET with a mixture of S, B and H is far more intricate than thought before, and therefore we undertook to carry out a separate study of the copolyesters synthe-

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Giouanna Poli et al.

Fg. 2. SEM micrographs of the fracture surfaces of blends and copolyesters specimens: (a) 50/ 50 PET/ SBH blend; (b) COP-3 (50501: (c) COP-1 (50-50): (d) COP-2 (70-301: (e) COP-2 (50-50); and If) COP-2 (30-70).

sis the results of which will be discussed in a future geneous materials and, surprisingly, even more het- paper. Here we will just summarize the preliminary erogeneous than those obtained by procedure 3. results of the characterization of the copolyesters syn- Therefore, we tried to increase the copolyester ran- thesized. Procedure 1 turned out to produce hetero- domness by using procedure 2.

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14 16 18 20 22 24 26 28 FUJ. 3. Powder X-ray dgractionpatterns of COP-2 (50-50) (a), and its toluene insoluble 'b) and soluble [c) fractions.

The PET-SBH copolyesters synthesized in this work are indicated in Table I .

DSC Characterizatioia of the PET-SBH Copolyesters

That polymerization of the SBH monomers, carried out in the presence of PET, actually leads to a chemical interaction with this polymer has already been concluded from the marked reduction of the viscosity of the reaction mixture observed during the initial stage of synthesis (cf. Experimental). This means that extensive degrada!ion of PET to low polymers or oligomers containing carboxylic and acetate end groups takes place, and that these fragments build up new macromolecular chains during the subsequent polycondensation stage. Thus, it is not surprising that the DSC traces of the copolyesters prepared by procedures 1 and 2 are completely different from those of the PET/SBH blends of equal composition. From this view, it is perhaps moi-e interesting that significant differences exist between the DSC traces of the blends and those of the copolymers obtained by procedure 3, even when, for their synthesis, PET was added to the reactor - 1 h after vacuum application, when the mixture was already viscous.

The DSC traces of a 50/50 PET/SBH blend, run at 20°C / min show an endothermic peak at 253°C on the first heating run (245°C on second heating), clearly due to the fusion of the PET phase, plus a smaller partially overlapping peak at -230°C (which reduces to a shoulder on second heating), due to the crystal-nematic transition of the SBH phase. On cooling, a clean separation of the crystallization peaks of the two phases is observed. In fact, the nematic-mystal transition of SBH is characterized, as expected, by small undercooling, as seen in the trace as a small exothe-m at 212°C. i.e., practically

the same temperature measured for neat SBH. Thl crystallization of PET appears as a strong exothern peaking at -165°C. The temperature, the shape, an( the enthalpy change of the crystallization peak of thc PET phase of the blend, as compared to that of nea PET, demonstrate that blending with SBH enhancer the crystallization rate of this polymer. The nucleating effect played by many LCPs for the crystallization 01

PET had already been demonstrated by several authors (9, 16, 17, 22, 27, 33, 35, 46).

The DSC traces of COP-3 (50-50) show a broad melting endotherm peaking at 220°C, on heating, and a broad exotherm around 170"C, on cooling. These peaks, which can be associated with the fusiodcrys- tallization of the PET-rich phase of this copolymer are, however, much broader and less intense than those of the corresponding blend. The DSC traces of the copolyesters prepared by procedures 1 and 2, on the other hand, display no strong endotherm/exotherm at all, although two very broad and weak phenomena could be observed for some of the COP-1 and COP-2 samples at temperatures around 120-150 and 270- 3 10°C. As described below, the higher temperature transition was clearly visible in the DSC traces of the toluene insoluble fractions of these copolymers.

SEM Investigation of the PET-SBH Copolyesters

The SEM micrographs of some typical copolyesters are compared in Fig. 2 with that of a 50150 PET/SBH blend. The morphology of the blend (Fig. 2a) is typical for a biphasic system characterized by inadequate dispersion of the phases and poor interfacial adhesion. COP-3 (50-50) (Fig. 2b) is also distinctly biphasic. However, a much finer dispersion and a slightly improved adhesion can be observed for this material, thus confirming that a chemical interaction has in fact taken place between PET and the SBH oligomers during the preparation of this allegedly blocky copolyester.

The morphology of COP-1 (50-50) reveals a homo- geneous isotropic matrix with large (100-500 pm) droplets of a highly fibrous dispersed phase (Fig. 2c). The interphase adhesion is fairly good: no droplet pullout can be noticed and the fracture is seen to travel across the two phases without unsticking them. The two phases of COP-1 could generally be separated without difficulty by extraction with CHCI, or toluene.

The morphology of COP-2 is also biphasic, indepen- dent of composition (Fig. 2d-f 1. Here also a homoge- neous matrix with dispersed fibrous droplets is observed. The relative content of the matrix increases with increasing the concentration of PET. However, with respect to COP-1, the phase dispersion is strongly improved: the droplets dimensions are in the 1-25 pm range. The interfacial adhesion appears good.

X-ray and Nllw Analysis of the Copolyesters

As mentioned before, the biphasic nature of COP- 1 and COP-2 was surprising because, rather superfi-

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Gwvanna Poli et al.

H H H H

\ A k I I I I I I I I

ppm 8 7 6 5 4 3 2 1

Q. 4. 'H NMR spectrum of the toluene soluble fraction of COP-2 (50-501.

Table 2. Molar Concentrations of the Monomer Units in COP-2 (50-50) and in its Toluene Soluble (37%) and

Insoluble (63%) Fractions.

Monomer COP-2 Toluene Toluene unit (9-50) soluble insoluble

TA 30.33 26.61 32.77 EG 30.33 37.72 25.46 S 9.84 13.69 7.30 B 9.84 2.59 14.61 H 19.67 19.39 19.86 Degree of 45.09% 34.72% 51.42%

aroma ticity*

Calculated according to Ref. 47.

cially, we expected that random copolyesters would be obtained under the conditions of procedures 1 and 2. Even more surprising, however, was that the two phases came out to be more cleanly separated for the copolyesters, such as COP-1, synthesized with the conditions apparently best suited to yield random copolymers, than, e.g., for COP-3, which was expected to be blocky (Fig. 2b-c). To shed more light on this point we fractionated COP-2 (50-50) and analyzed the two phases. To this end, 1 g of this material was stirred for 4 h in 100 mL toluene at -100°C. This treatment led to a complete disintegration of the polymer and a fine suspension was obtained. The suspended powder was separated by centrifugation, washed repeatedly with fresh toluene, and finally dried and weighed. The yield was 61 % . The soluble polymer was recovered from the toluene solution by precipitation into methanol, centrifugation and drying. The yield was 33%.

The X-ray diffraction patterns of unfractionated COP-2 (50-50) and of the soluble and insoluble fractions are shown in Fig. 3. The sharp reflection at 26 = 20.3" displayed by the insoluble fraction is typical for a LCP with parallel aligned, longitudinally disordered macromolecules. On the contrary, the X-ray pattern of the soluble fraction is typical for an isotropic material. Thus, the X-ray analysis fully confirms the conclusion of the SEM investigation.

The 'H NMR spectrum of the toluene soluble fraction of COP-2, in CDCl,, is shown in Fig. 4 together with the assignments. From the integrals of the reso- nance peaks, the molar concentrations of the different repeat units in the soluble and the insoluble fractions of COP-2 (50-501, taken as amounting to 37% and 63%. respectively, were calculated. The relevant values are collected in Table 2 with the degrees of aromaticity calculated as the ratios of the number of aromatic carbons in the chain to the total number of atoms, according to Calundann and Jaffe (47). The NMR information of Table 2 teaches that to define the two phases of COP-2 as "PET-rich" and "SBH-rich," according to a custom set in for the PET-H copolyesters, would be a misleading over-simplification. Close examination of the molar composition of the two phases reveals that the soluble one contains an excess of S units which appear to have replaced almost one third of the TA units of PET. Moreover, this fraction contains few B units. On the other hand, the insoluble fraction has a relatively high proportion of TA and B units. The H units, on the contrary, appear to be evenly distributed between the two phases. It is also

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90 120 i5a1 180 210 240 T,OC

Fig. 5. DSC cooling traces of: (a) 75/25 PET/SBH blend: [b) same with 5% COP-3 (50-50); and (c) same with 10% COP-3 (50-50).

interesting that the degi-ee of aromaticity of the soluble fraction (34.7%) is even lower than that of PET (40.0), whereas the degree of aromaticity of the insoluble fraction (51.4%) is higher than that of pure SBH (50.0%). Thus, COP-2 cannot be considered as a PET- SBH block copolymer, with the two phases containing more (and/or longer) segments of either polymer but, rather, as a random copolymer with bimodal distribution of the sequences of the repeat units. Although a discussion of the relations between synthesis conditions and copolymer structure is behind the scope of the present article and will be made elsewhere, it may be pointed out that, as ii was anticipated on the basis of the SEM investigation, the difference between the chemical structures of the two phases of the copolyesters prepared by procedure 1 is even stronger than that described before for COP-2. It may just be useful to mention here that the above conclusions were supported by the DSC ana.lysis, too. The DSC heating trace of the insoluble fraction of COP-2 (50-50) showed a distinct endothermic transition at -270°C. This temperature is much higher than the -230°C resulting from the crystal+nematic transition of neat SBH (41-43).

Fig. 6. SEM micrographs of the fracture surfaces of: [a) 75/25 PET/SBH blend: [b) same with 2.5% COP-3 (5060). prepared by procedure C [Experimental, Blends Preparation).

DSC Characterization of the PETICOPISBH Blends

The addition of COP to binary blends of PET and SBH (75125, w/w) brings about visible changes of the shape of the crystallization peak of the PET phase. It has been mentioned that, for the binary blends, the PET crystallization start just after the SBH nematic-rystal transition and is characterized by a strongly asymmetric peak. The low temperature side is steep and the crystallization is over at -150°C. Under the same conditions neat PET displays a much broader and symmetric endotherm spanning from 200°C down to 110°C. It is also interesting that, contrary to neat PET, the rerun heating traces of the blends show no PET cold crystallization. The effect of the addition of COP to the blends is illustrated by the DSC cooling traces of 75/25 PET/SBH blends, containing 0, 5, and 10% of COP-3 (50-501, shown in Fig. 5. The progressive sharpening of the exothermic peak is evident. The blend with 10% COP-3 displays a clear separation of the two peaks at 190 and 160°C attributed to the crystallization of SBH and PET, respectively. The peak sharpening appears to result from a delayed start of the PET crystallization which might be

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attributed to miscibility of the PET phase with the isotropic phase of the copolymer. It is also noteworthy that the addition of COP lowers the SBH crystallization temperature by 15-20°C suggesting that SBH may be partially miscible with the toluene insoluble phase of COP.

SEM and X-ray Investigation of the Blends The SEM micrographs of the fracture surface of

compression molded sheets of the 75/25 PET/SBH blends show that the addition of COP generally leads to a finer distribution of the LCP droplets. As an example, the micrograph of the blend prepared by the addition of 2.5% COP-3 (50-50) by method C (cf. Ex- perimental, Blends Preparation) is compared with that of the uncompatibilized blend in Fig. 6. The lower dimensions (52 pm vs. 0.5-5 pm) of the SBH droplets in the ternary blend are evident, although no definite indication of an improved adhesion can be drawn from these micrographs.

A clearer indication of specific interactions between COP and the two main components of the blends, as well as of the effect of the particular blending method employed, is offered by the X-ray diffraction patterns. The relative intensities of the reflection at 28 = 20.3" assigned to the SBH phase and those assigned to PET depend to some extent on the preparation procedure and change with annealing. As an example, the patterns of the 75/25 PET/SBH blends with 2.5% COP-3 (50-501, prepared by methods A and B and having the same thermal history (cooling between the press plates with a rate of - l"C/s) show that blending PET with COP and adding SBH to a later stage (method B), rather than blending the three components at the same time [method A), leads to a lower degree of crystallinity of the PET phase (Fig. 7a-b). This supports the hypothesis of a partial miscibility of PET with the isotropic phase of COP. On the other hand, Fig. 7b-c show that a prolonged annealing at 200°C manifests itself, not only through the expected increase of PET crystallinity, but also through the significant reduction of the intensity of the SBH reflection. The com- prehensive interpretation of the effects of the thermal history and of the concentration, microstructure and way of addition of the compatibilizers on the X-ray diffraction patterns of the PET/SBH blends is not easy. The results, nevertheless, demonstrate that the PET-SBH copolyesters do interact to some extent with the blend components.

Mechanical Properties of the PETICOPISBH Blends

The tensile modulus, the tensile strength and the elongation to break of 75/25 PETISBH blends specimens, cut out from compression molded sheets cooled within the press plates with a rate of -l"C/s, are plotted in Fig. 8 through 10 as a function of the concentration of added compatibilizer. The copolyesters used for these experiments were COP-1 (50-50) and COP-3 (50-50). Some of the blends, especially those

I 14 16 18 20 22 24 26 26

Fig. 7. Powder X-ray dgraction patterns of compression molded 75/25 PET/SBH blends with 2.5% COP-3 (5050). [a) blend prepared by method A {Experimental, Blends Prepara- tion); (b) blend prepared by method B; and (c) same as (b), annealed 17 h at 200°C.

Tensile Modulus (GPa)

@ -8 --____

L@ 8

COP-1 (50-50) 0 COP-3 (50-50)

0 2,s 5 7 s 10 COP (w%)

Fig. 8. Tensile modulus of strips cut out from compression molded sheets of 75/25 PET/SBH blends, as a function of the amount of added copolyesters.

prepared by method A (i.e., by mixing the three components at the same time), gave brittle sheets under these preparation conditions and it was sometimes impossible even to cut the specimens out them. For this reason the mechanical properties of the blends prepared by method A are not included in the Figures. As for the other blending techniques (B, C, and D), they generally led to small mutual differences and for this reason no distinction is made in this respect among the experimental points in Fig. 8-1 0. The three points shown for each composition represent the max- imum, minimum, and average values. Different mark- ers were used in the plots for the two compatibilizers, COP-1 and COP-3, because there appears to be a consistent, though small, positive difference in favour

1252 POLYMER ENGINEERING AND SCIENCE, MID-MAY 1- Vol. 36, NO. 9

Use of PET-LCP Copolymers as Cornpatibilizers

50

40-

30

20

10

0 ~

0 8

F /’ 8

-d -

COP-1 (50-50) ’ COP-3 (50-50) -

0 2S 5 7,5 10 COP (W%)

Fg. 10. Elongation to break of strips cut outfrom compression molded sheets of 75/25 P € T / S B H blends, as a function of the amount of added copolyes ters.

of the second one, at le,& for the tensile strength and the elongation to break.

The results illustrated in Figs. 8 through 10 show that, under the adopted conditions, a measurable and reproducible increase of both tensile strength and break elongation was obtained in the presence of very small amounts of copolyesters, while the tensile modulus was hardly affected or decreased slightly. It is not clear whether the slight decline of the modulus may be attributed to the relatively low mechanical properties of the copolyesters (for COP-1 (50-50) we found E = 0.3 GPa, TS = 10.6 MF’a, and EB = 10%).

Despite the increase of break elongation, specimens’ fracture was always fragile, under the conditions employed. Therelore, the results illustrated by the plots in Fig. 8 through 10 cannot be taken as a definite evidence that ]:he PET-SBH copolymers synthesized in this work behave in fact as efficient compatibilizers.

Another set of experiments was made using rapidly cooled (cf. Experlmenlal) compression molded specimens of 75/25 PET/SBH blends containing 5% COP-2 (50-50). Out of seven specimens, four dis-

played ductile behavior, as shown in Fig. 11. Under the same conditions, the specimens prepared from uncompatibilized blends showed brittle fracture with initial necking (break elongation of ca. 8-12%). The SEM micrograph of the neck zone of the broken spec- imen of compatibilized blend (elongation 250%). longitudinally fractured under liquid nitrogen, is shown in Fig. 12. The adhesion between the matrix and the LCP droplets appears definitely improved.

CONCLUSION

We have presented the results of an exploratory investigation of the synthesis of PET-LCP copolyesters and of their use as compatibilizing agents for PET/ LCP blends. A semiflexible LCP, SBH 1: 1:2, was chosen for our study, and the PET-SBH copolyesters were synthesized by the melt polycondensation of the S , B, and H monomers camed out in the presence of appropriate amounts of PET. The effect of some of the reaction parameters was studied, including the thermal profile and the time of addition of PET to the mixture. It was found that whatever the polycondensation conditions, biphasic materials were obtained as the products. Interestingly, the two phases of some of the copolyesters were shown to contain an excess of aliphatic and, respectively, aromatic moieties, with respect to neat PET and SBH. The characterization of the copolyesters showed that further studies are needed to shed more light on the relationships between the synthesis conditions and the microstructure and the properties of the products, especially if the latter are touted for their potential as compatibilizers.

The results of the preliminary experiments of compatibilization of PET/SBH blends with small amounts of PET-SBH copolyesters seem promising. The fact that the LCP droplets dispersion is improved by the copolyester addition may be considered as estab- lished. As far as the ability of the copolymers to en- hance the interfacial adhesion, thereby improving the mechanical properties of the blends, the present re-

Stress (MPa) 50 I--

10’

1 -u- OM 0 30 60 90 120 150 180 210 240 270

Elongation (%)

Fig. 11. Stress-strain curves of four rapidly cooled, compression molded strips of a 75/25 PETISBH blend compatibilized with 5% COP-2 150601.

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Giovanna Poli et al.

1 254 POLYMER ENGINEERING AND SCIENCE, MID-MAY 1- Vol. 36, No. 9

Fcg. 12. SEM micrograph of the neck zone (longitudinal fracture) of one of the specimens of Fig. 1 I .

sults appear encouraging yet not allowing enthusias- tic conclusions. The finding that a PET blend with as much as 25% LCP may be made to be ductile, and that this result may be achieved by the addition of only 5% of a PET-LCP copolyester, has never been reported before. Nevertheless, there are good grounds for cau- tion before generalizing the data at hand. In particular, it is important to shed more light on the effect of the cooling rate on resultant properties. It cannot be excluded, that phase interactions and interfacial adhesion characterizing these PET/LCP blends may be adversely affected by the crystallization of the two phases, induced by slow cooling or annealing. Further studies are being carried out in our laboratories along these lines.

ACKNOWLEDGMENTS This work was financially supported by C.N.R., and

by the Italian Ministry of University and Scientific and Technological Research (MURST). The authors wish to thank Eniricerche S.p.A., Milan, for providing the LCP sample used in this study. Sincere thanks are due to Dr. C. Forte of the I.Q.C.E.M. of the National Research Council, Pisa, for her help in recording the NMR spectra. Mr. P. Narducci and Mr. M. Masseti of the Depart- ment of Chemical Engineering, University of Pisa, gave a valuable contribution to this work by taking the SEM micrographs and carrying on the DSC measurements, respectively.

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On the use of PET-LCP copolymers as compatibilizers for PET/LCP blends