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Rheological, mechanical, and adhesive properties of thermoplastic-LCP blends filled by glass fibers.

INTRODUCTION

The prospect of using liquid crystal polymer (LCP) as a reinforcing component in two-phase polymer blends involves the possibility of a reinforcing framework formation into the matrix polymer at certain processing conditions (1-4). There are not only fibers of the LC-phase distributed in a commodity thermoplastic, but also formation of a LC-skin, as a consequence of its partial migration to the periphery of the stream. Both these processes lead to a melt blend viscosity decrease and to enhanced solid state mechanical characteristics of the extruded products.

The traditional reinforcing agents for polymers are particulate and fibrous fillers. From general conclusions (5-7) concerning the change of filled thermoplastic properties, one presumes that introducing reinforcing fillers into the LC-matrix and a binary matrix (LCP-thermoplastic blend) could improve the properties of such materials. In doing so for the binary matrix, it is necessary to account for the possible influence of fillers on the fiber- and skin-formation processes of the LC-phase.

We previously investigated the rheological and mechanical properties of a filled LC-copolyester of PET/ HBA (8, 9) and its blends with a thermoplastic poly-sulfone (10, 11). Both active (carbon black, silica) and inactive (talc) particulate fillers were used. The main result of these works concluded in decreasing LCP melt viscosity (see also Ref. 12) and in increasing the elastic modulus of solid extrudates at low filler content, compared with the neat polymer.

Despite the production of LCPs with fibrous fillers (13), the number of papers devoted to physical-chemical properties of such composites is not high (Ref. 14 is a rare exception). The anisotropy in such systems is defined basically by the LCP-matrix. The presence of high-modulus fibers leads to an increase not only in the elasticity modulus, but also in the strength in the fiber direction. From a rheological point of view (15), there exists a principal difference in properties even for isotropic polymers filled by particulate and fibrous fillers in comparison with neat polymers.

So, there are two levels of anisotropy: molecular ordering inherent to LC-melts and macroscopic orientation of anisometric filler particles. The relationship between the two levels of anisotropy can lead either to a reinforcing or to a weakening effect depending on their superposition. The question becomes much more complicated for polymer blends containing LCPs filled by anisometric fillers. That is why one of the aims of this work is the investigation of rheological and mechanical properties of the thermoplastic--LCP blend with fibrous filler.

It stands to reason that the rheological properties of such systems strongly depend on the interfacial phenomena in the heterophase melt and their mechanical properties - on the adhesion between the components. In filled polymer blends one has to distinguish two types of adhesion: between the components of the blend, and between them and the solid filler surface. Some thermodynamic relations are proposed to describe the work of adhesion of polymer blends to solids (16). The thermodynamic work of adhesion, [W.sub.A], for incompatible polymers may be presented as

[W.sub.A] = [[Phi].sub.1][W.sub.1] + [[Phi].sub.2][W.sub.2] (1)

where [W.sub.1] and [W.sub.2] are the adhesion work of the blend components to the solid and [[Phi].sub.1] and [[Phi].sub.2] are their volume fractions. Such an approach is based on the assumption that both blend components take part in the formation of the adhesive joints at the interface with solid. The failure of such joints depends on the ratio of the adhesion strength of each component, their volume fractions, moduli, and adhesion between them (17). The modified Takayanagi model allows one to predict the behavior of the system under deformation, depending on the properties of components and their adhesive characteristics, if the adhesion between the components is high compared with the adhesion of each component to the surface. However, it is more probable that adhesion between the two components of the polymer blend does not exceed the adhesion of any component to the surface. When one of components has a higher surface tension, it would interact selectively with the surface and displace the component with lower surface tension.

Theoretically and experimentally, such a case has been considered (18). It was found that it is possible to form a wetting-like layer in a binary blend at the interface with a solid. This layer is formed by one of the two polymer phases. If the mixture components have different surface tensions, to minimize the interfacial tension of the system as whole, the component with the higher surface tension should transfer to the surface. There are many data concerning the selective adsorption and segregation from polymer blends (19) that confirm this view. Thus one can suppose that at a definite conditions, only one phase is in direct contact with the solid surface. This implies the possibility of the formation of adhesion joints by only one of the components (phases) of the polymer blend. In this case the failure of the adhesive joints will depend only on the adhesion strength of this component to the surface, independent on the blend composition. We can suppose the formation of some kind of bilayer at the boundary with the solid surface, being formed by the two phases with different surface tensions. However, in such a situation the failure of the adhesive joints will be determined by the adhesion strength of the weaker layer, and may occur either at the polymer-solid interface or at the polymer-polymer interface.

Thus another aim of this work is to investigate the adhesion of the polymer blend with components that vary in their surface tension and in their adhesion to the solid surface.

EXPERIMENTAL

The LCP used in the study was a copolyester (CPE) of PET and p-hydroxybenzoic acid (HBA) containing 60 mol% of HBA. The intrinsic viscosity of this polymer in a mixture of trifluoroacetic acid and chloroform was 0.51 dl/g. According to the literature, CPE has two main relaxation transitions at 343 and 433 K, corresponding to manifestation of a large-scale mobility of chain segments enriched either in PET or HBA. The CPE softening (melting) point is 453 K.

The second component of blend was isotactic polypropylene (PP) with the melt index 3.2 g/10 min, produced by the Shell Company.

The filler was glass fibers (GF). The content of GF in CPE, PP, and PP/CPE blends was of 2, 5, and 10 wt%. GF was cut in sections 2 to 2.5 mm in length. During mixing, they were crushed, and the final dimensions of the glass fibers in the mixture with polymers were: d = 5.6 mkm, l = 100-280 mkm and l/d = 18-50. For an example of GF distribution in the CPE, see the micrograph in Fig. 1. In this case, the content of GF was 20%.

The mixing of polymers with and without filler was performed in a laboratory micromixer of rotor-plunger type at 468 K (the mixing volume is 4 [cm.sup.3], a gap between rotor and outer cylinder is 0.25 mm, and the shear rate into the gap is 500 [s.sup.-1]). At this mixing temperature both polymers have identical viscosities; this is essential for a uniform distribution of filler particles in the blend. The homogeneity of mixing was controlled by optic and electron microscopy.

For rheological measurements at 463 and 493 K, a capillary microviscometer MV-2 (20) of melt indexer type (capillary diameter is 1.26 mm and length is 8.3 mm) was used.

Mechanical measurements were performed on extrudates prepared at a fixed shear stress. The tenacity of the extrudates at room temperature was obtained using an Instron 1122 tensile machine (extension rate 10 mm/min).

The surface tension of solid CPE, measured by the Wilgelmi method from the contact angles (21), was found to be 42 dyne/cm. The surface tension of PP is 30 dyne/cm, according to Privalko (22). Adhesive contact between PP, CPE, and PP/CPE blends and glass was formed at 480 K over 10 min and that between PP and CPE at 453 over 15 min. Figure 2 shows the scheme of the formation of the adhesive contacts and the method of measurement of the adhesive joint strength.

RESULTS AND DISCUSSION

Rheological Properties of the PP/CPE/GF Blends

The temperature dependencies of CPE and PP melt viscosity are presented in Fig. 3. The dependence of log [Eta](1/T) for CPE has two regions below and over 473 K, where flow activation energies are sharply different: at T [less than] 473 K, [E.sub.CPE] [[similar to] 1000 kJ/mol, whereas in a range of 483-513 K, [E.sub.CPE] = 65 kJ/mol. In a temperature range of 453-513, [E.sub.pp] = 45 kJ/mol. To elucidate such a dependence of [Eta] on T for CPE, let us remember that one contains blocks of HBA and PET sequences. This gives rise to at least two relaxation transitions in the temperature range of 343-433 K, melting of the major portion of crystallites at 453 K, and melting of residual crystallites enriched in HBA sequences at over 483 K.

From literature data (23-26) and Fig. 3, it may be assumed that although the CPE softens at 453 K, local crystallites resulting from HBA sequences are preserved to above 483 K (the second melting point). Consequently, below 473 K the CPE melt is heterogeneous or two-phase and its viscosity is high. Above 483 K a homogeneous nematic liquid crystal structure is formed with viscosity reaching 400-100 Pas in the range 483-513 K.

Figure 4 shows flow curves of CPE, PP, and their blends at temperatures of 463 K (solid lines) and 493 K (broken lines). The addition of CPE to PP in the first case leads to decreasing and in the second case to increasing volume flow rates of the PP/CPE blends. The reason is that at 463 K, the CPE viscosity is higher than that of PP, and at 493 K, in contrast, the PP viscosity is more than that of CPE.

The dependencies of blend viscosity on CPE concentration are shown in Fig. 5. As seen from this plot, in the low temperature (heterogeneous structure of the CPE melt) and high temperature (homogenous nematic state of CPE) ranges, the blends display different rheological behaviors. In the first case, blend viscosity monotonously increases with an increase in CPE content, whereas in the second case the PP viscosity prominently decreases at a CPE content exceeding 10 wt%. In the same Figure, the dependence of the PP/CPE/10% GF blend viscosity on the CPE concentration is shown. We note that addition, for example, of 30% CPE and simultaneously 10% GF to the PP does not increase its viscosity at 493 K.

For the composites of CPE?GF, PP/GF, and PP/ CPE/GF, the relative viscosity of the first system depends weakly on the GF content, a little more for the second system, and for the ternary system the dependence of [[Eta].sub.r]([c.sub.GF]) Occupies an intermediate position.

The specific fiber-formation of the disperse LC-phase into the PP-matrix (concentration range 5% to 70%) at the capillary flow takes place above 488 K, and perfect fibrillation is observed at CPE concentrations exceeding 20%. In the low-temperature range the fiber-formation of the disperse LC-phase is absent, since the CPE viscosity is much more than for PP, and its droplets do not deform in the stream (they act as the solid inclusions). These morphological peculiarities lead to the following consequences: a) the blend viscosity monotonously increases over all concentrations at 463 K, and b) at 493 K the blend viscosity decreases distinctly at a CPE content exceeding 10%.

As a rule, the reinforcing effect in the direction of extrusion is achieved for perfect fibrillation of the LCP phase in blend extrudates (1, 2). The same occurred for PP/LCP blends. But the question is how the fiber-like filler can influence mechanical properties (the strength and the initial elasticity modulus) of such extrudates.

Mechanical Properties of the Extrudates

Figure 6 is a photograph of the composite extrudates PP/CPE = 7/3 (left) and PP/CPE = 7/3 + 10% GF (right). They were prepared at 493 K and the constant shear stress, [Tau] = 2.34 x [10.sup.4] Pa. The result of an extraction of the PP-matrix by toluene is at the top of the extrudates. Before consideration of the mechanical properties, some features should be noted on the micrographs:

1) For the PP/CPE = 7/3 blend without GF, prominent die-swell takes place, which is practically absent for the extrudates of the PP/CPE blend produced at 463 K. The presence of 10% GF depresses this effect completely;

2) In the first case {without GF) the diameter of CPE fibers is equal to or greater than 10[Mu], whereas in the second case (in presence of GF) it is less than 10[Mu] and fibers are more uniform in size. The probable reason for this effect is the more effective dispersing of the LC-phase in the presence of the GF;

3) The specific density of CPE fibers (in the extrudate cross section) in PP/CPE blends is higher in the presence of GF.

The dependencies of the strength and the initial modulus of elasticity of the polymer components and their blends on the GF concentration are shown in Fig. 7. As seen, the mechanical characteristics of these systems increase with GF content.

Figure 8 shows the dependencies of the same characteristics of PP/CPE composites on the concentration of the CPE in the absence (curve 1) and presence of 10% GF (curve 2). The weak increasing of the strength and the modulus takes place in the first case and the pronounced increasing of these characteristics is observed in the second case. The strong reinforcement of the PP with 30% CPE and 10% GF is likely explained by morphology (more intensive and homogeneous fiber-formation of the disperse LC-phase in the presence of the fibrous filler).

The strength of composites containing uniaxially oriented fibers depends (at the same other parameters) on an angle, [Theta], between the direction of the load action and an axis of fiber orientation. It may be calculated from (28):

[[Sigma].sub.0] = [[Sigma].sub.T]/sin [Theta] (2)

where [[Sigma].sub.T] is the transversal strength of fibrous composites.

A theoretical equation describing the effect of the adhesive strength, [[Sigma].sub.A], on the transversal strength of fibrous composites has been proposed (29):

[[Sigma].sub.T] = [[Sigma].sub.1][1 - 2[([[Phi].sub.1]/[Pi]).sup.1/2]] + 2[[Sigma].sub.A][([[Phi].sub.2]/[Pi]).sup.1/2] (3)

where [[Sigma].sub.1] is the strength of the matrix, and [[Phi].sub.1] and [[Phi].sub.2] are the volume fractions of the matrix and disperse phase, respectively.

The calculation of the strength [taking into account the measured values [[Sigma].sub.A] (see below)] and [[Sigma].sub.1] by Eqs 2 and 3 indicates a satisfactory agreement with the experimental values at [Theta] = 10 [degrees] and at CPE concentrations [greater than]50% (see the broken line in [ILLUSTRATION FOR FIGURE 8 OMITTED]).

ADHESIVE PROPERTIES

Estimates of the strength of the adhesive joints between various components of the blend are presented in Table 1. As indicated, the adhesion strength at the CPE-glass interface is 100 times the adhesion between glass and PP. The higher surface tension of LCP predetermines the higher mechanical properties of the composites.

The dependence of the adhesion strength to the glass surface on the blend composition is presented in Fig. 9. Up to CPE content [approximately]70%, adhesion strength is low, by one order of magnitude, and coincides with the adhesion between PP and CPE. After 70% the sharp increase in adhesion is observed up to the value inherent for pure CPE. Thus the dependence of adhesion on blend composition cannot be described by the simple relation in Eq 1. The reason for such behavior may lie in the difference in the surface tension of two components in the melt state. It seems reasonable to expect the selective wetting of the surface by the component with higher surface tension, because in this case the minimization of interfacial energy is achieved.

We may consider the structure of the interface region as consisting of two layers, one formed by the component with higher surface tension and the second with the lower one. The first layer may be pure CPE or blend enriched in CPE, whereas the second may be the PP/CPE blend of various composition [so-called effect of surface segregation (19)]. Because of the different adhesion strength at the CPE-glass interface and between CPE and PP one can expect that the failure of the adhesion joints will take place in the weakest boundary layer [according to Bikerman (10)], namely between CPE and PP. Such a supposition may explain the lower adhesion in the composition interval up to 70% CPE. At this concentration, the phase inversion is expected. Up to 70%, PP forms the matrix with embedded droplets of CPE. After inversion, CPE forms the matrix with PP inclusions. In this case, the sharp boundary between the wetting layer of CPE and the blend disappears and the failure of adhesion joints proceeds near the interface with the solid. Thus the location of the failure of the adhesion joints changes from the interface between two incompatible polymers to the interface between solid and polymer.
Table 1. Adhesion Strength Between Various Components.

System Adhesion Strength, kg/[cm.sup.2]

PP-CPE 12.5
PP-glass 1.2
CPE-glass 120.0




We believe that in the same manner, the results (27) obtained for the adhesion of the blends of a random copolymer of ethylene and acrylic acid with paraffin and polystyrene may be explained. In the case of polymer blends, the formation of the adhesion joints proceeds under non-equilibrium conditions. One can expect that, depending on temperature of adhesion joint formation and duration of the process, the adhesion strength may vary because of structural changes in the interface.

Comparing these results with data on mechanical properties [ILLUSTRATION FOR FIGURE 8 OMITTED], one can see that the shape of dependencies of the mechanical characteristics and the adhesion on the blend composition are identical. These data support the concept of a phase inversion in the system with increasing amounts of CPE. Up to 70% CPE, the matrix is formed by PP and the modulus and tensile strength are not high. Phase inversion makes a matrix formed by CPE. The poor adhesion between PP and CPE determines the lack of the effect of blending on the properties up to 70% CPE. The effect is more pronounced for the modulus, but not the tensile strength, because the latter is an ultimate property, which depends on defects. Introduction of the GF reinforces the system in a usual way, this reinforcement being more pronounced after the phase inversion, because the adhesion between the glass surface and LC-CPE is much higher.

CONCLUSIONS

The mechanical properties of two-phase polymeric blends are strongly dependent on the interfacial phenomena. Investigation of the adhesive strength between the blend of CPE with PP and the glass surface shows that the composition dependence of the adhesion may be presented as consisting of two parts. Up to a CPE content [approximately]70% the adhesive strength is of the same order of a magnitude as the adhesion of pure PP to the glass surface, whereas after 70% the adhesion reaches the values typical for the adhesion of pure LC-CPE to the same surface. Taking into account the remarkable difference in the surface tension of two blend components, and, correspondingly, in wetting, one can suppose that these effects are connected with the peculiarity of the structure of the interface. We suppose that interface is formed by two layers: a layer of LCP in direct contact with the solid surface and a layer of the blend in contact with the wetting layer of CPE. When the matrix is formed by PP, the failure of adhesion joints proceeds between CPE and the blend, being rather weak, whereas the adhesion between CPE and the solid surface is high. After phase inversion, there occurs a transition in the zone of the failure to the interface between glass and CPE. The transition from the adhesion failure between the components of the blend to the adhesion failure between the blend and a solid is a specific feature of the adhesion of two-phase blends of components strongly different in surface tension and wetting ability.

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Title Annotation:liquid crystal polymer
Author:Kulichikhin, V.G.; Parsamyan, I.L.; Lipatov, Yu S.; Shumskii, V.F.; Getmanchuk, I.P.; Babich, V.F.;
Publication:Polymer Engineering and Science
Date:Aug 1, 1997
Words:3876
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