Liquid-Crystalline Polymer Systems - American Chemical Society

Chapter 8

Downloaded by UNIV MASSACHUSETTS AMHERST on October 10, 2012 | http://pubs.acs.org Publication Date: July 9, 1996 | doi: 10.1021/bk-1996-0632.ch008

Fibers of Blends with Liquid-Crystalline Polymers: Spinnability and Mechanical Properties 1

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F. P. La Mantia , A. Roggero , U. Pedretti , and P. L. Magagnini 1

Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Università di Palermo, Viale delle Scienze, 90128 Palermo, Italy Eniricerche S.p.A., Via Maritano 26, 20097 San Donato Milanese, Italy Dipartimento di Ingegneria Chimica, Chimica Industriale e Scienza dei Materiali, Università di Pisa, Via Diotisalvi 2, 56126 Pisa, Italy

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Several blends of flexible polymers (PET, PC, Nylon-6, PP) with wholly aromatic or semi-aromatic liquid crystalline polymers (LCPs) were spun under different conditions and their melt viscosities, their spinnability (evaluated through the melt strength, MS, and the breaking stretching ratio, BSR), and their mechanical properties were studied. The measured MS and BSR values of some of the blends were found to be higher than expected on the basis of the viscosity reduction brought about by the LCP minor phase, and were interpreted as being the result of good phase compatibility in the molten state. Thefibermechanical properties were appreciably enhanced over those of neat polymers only when good spinnability (granting LCP droplets fibrillation, and orientation of the matrix polymer) was accompanied by sufficient interphase adhesion in the solid state.

It is well known that the addition of relatively small amounts of a thermotropic liquid crystalline polymer (LCP) into flexible thermoplastic polymers may mean a considerable energy saving, due to a strong reduction of melt viscosity (i). Moreover, a reinforcing effect can also be expected, especially if flow conditions favoring LCPfibrillationare used for processing. In particular, if a LCP/polymer blend is melt spun, the elongational flow at the die inlet, as well as that taking place as a result of the draw-down force applied to the moltenfilamentat the spinneret exit, may favor suchfibrillation.Thus, melt spunfibersdisplaying enhanced tensile properties can be expectedly obtained as a result of the in situ addition of LCP into the matrix resin, provided that the compatibility of the two phases is sufficient However, the accurate design of LCP/polymer blend fibers should be made considering that, whereas the mechanical properties of as-spunfilamentsof neat thermoplastic polymers may often be improved by furthering molecular orientation through cold drawing, this is generally not possible for LCP/polymer fibers due to the high rigidity of the LCP fibrils. Therefore, it is 0097-6156/96/0632-0110$15.00/0 © 1996 American Chemical Society In Liquid-Crystalline Polymer Systems; Isayev, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


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necessary that spinning conditions granting, not only thefibrillationof the LCP minor phase, but also a sufficient molecular orientation of the matrix resin are employed, or the advantage of LCP reinforcement may be even outweighed by the lower matrix orientation. This means that high draw-down ratios, i.e., the ratios of windup to extrusion speeds, must be used while spinning LCP/polymer blends. The melt strength (MS) and the breaking stretching ratio (BSR) are the two most important characteristics defining the spinnability of a polymer. A polymer having high MS and BSR will expectedly sustain very high draw ratios during melt spinning. For a given polymer, MS and BSR increase on increasing the molecular weight and the melt viscosity. Thus, the drawability of a molten thermoplastic might be expected to worsen as a result of the LCP addition, because this is known to bring about a strong viscosity reduction. In this work, the Theological behavior of several LCP/polymer blends has been studied. Their spinnability has also been investigated by measuring the MS and BSR, as well as the mechanical properties of as-spun fibers. The results are discussed with reference to the temperature dependence of the viscosities of the two phases, which clearly influences the blends behavior during the non-isothermal fiber drawing stage. Experimental Four thermoplastic polymers: polypropylene (PP), polycarbonate (PC), poly(ethylene terephthalate) (PET), and nylon-6 (NY), and four LCPs: Vectra A-900 (VA), Vectra B-950 (VB), SBH 1:1:2 (SBH), and SBHN 1:1:3:5 (SBHN) were used The source and some of the characteristics of the polymers, as well as the compositions of the LCPs, are shown in Table 1. Table 1 Characteristics of the polymers used for the blends preparation Sample M [η], dl/g Name Manufacturer PP 680.000 D60P Himont PC 36.000 Sinvet301 EnichemPolimeri PET 0.62 EnichemPolimeri NY 62.000 ADS40 SNIA VA Vectra A-900 Hoechst Celanese [ 4-hydroxybenzoic acid (H) (73%), 2-hydroxy-6-naphthoic acid (N) (27%) w

VB SBH SBHN

Vectra B-950 Hoechst Celanese [ Ν (60%), terephthalic acid (20%), 4-aminophenol (20%) ] SBH 1:1:2 Eniricerche SpA [ sebacic acid (S) (25%), 4,4'-dihydroxybiphenyl (B) (25%), Η (50%) ] SBHN 1:1:3:5 Eniricerche SpA [ S (10%), Β (10%), Η (30%), Ν (50%) ]

The LCP/polymer blends were prepared with a laboratory single screw extruder (D=19 mm, L/D=25; Brabender, Germany), equipped with a die assembly for ribbon

In Liquid-Crystalline Polymer Systems; Isayev, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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LIQUID-CRYSTALLINE POLYMER SYSTEMS


extrusion. Blends with 0, 5, 10, and 20% LCP (w/w) were prepared. The die temperatures used for the different blends were as follows: PP/SBH and NY/SBH, 250°C; PET/SBH, PET/SBHN and PET/VA, 270°C; PC/SBH and NY/VB, 290°C; PC/VB, 300°C. The shear viscosity measurements were made with a capillary (D = 1 mm, IVD = 40) viscometer (Rheoscope 1000 by CEAST). The temperatures were as follows: PP/SBH, NY/SBH, 250°C; PC/SBH, PET/SBHN, PET/VA, 270°C; PC/VB, NY/VB, 290°C. Elongational viscosity was estimated using Cogswell's analysis (2):

where η is the shear viscosity, γ is the apparent shear rate, n is the power law index, and ΔΡ is the pressure drop at the die inlet evaluated through the Bagley plot Capillaries with L/D=5,10 and 20 were used The average extension rate was evaluated by means of the relation: 4 f ε= η — 3(n+l) ΔΡ Fibers were spun using the tensile attachment of the same equipment, with a shear rate of 24 s' . The spinning temperatures were as follows: PP/SBH and NY/SBH, 250°C; PET/SBH, PET/SBHN and PET/VA, 270°C; PC/SBH and NY/VB, 290°C; PC/VB, 300°C. During the spinning tests, the MS and the BSR were measured by drawing the extruded monofilaments with a linearly increasing extension rate (acceleration: 1 rpm/s). Thefibermechanical properties were measured with an Instron 1122, at an elongational rate of 0.66 min' . 1

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Results and Discussion The shear viscosities of some of the investigated LCP/polymer blends, measured at a shear rate of 24 s' , are shown in Figure 1 as a function of the LCP content For all the blends, a considerable decrease of melt viscosity was observed as a result of the LCP addition. This is in agreement with literature data (i). A minimum is seen for some of the viscosity/composition curves (cf. NY/VB and PET/VA). This was shown to occur when the LCP's viscosity is higher than, or similar to, that of the matrix (3,4). Because the shear viscosities of thermoplastic polymers are reduced by the addition of small amounts of LCPs, one would expect their MS and, therefore, their spinnability to decrease accordingly. This is not always so. In fact, literature data show that many LCP/polymer blends can easily be spun with high draw-down ratios. On the other hand, the decrease of the shear viscosity of a polymer, due to the LCP addition, is not accompanied, in most cases, by a concomitant decrease of the elongational viscosity, as expected for materials obeying the Trouton rule (λ = 3ηο). As an example, the elongational viscosities of NY, VB, and 90/10 NY/VB are shown in Figure 2 as a function of the extension rate. It may be seen that whereas the elongational viscosity, λ, of neat NY is almost constant, and not far from the Trouton value, that of the LCP is very much higher, and 50-100 times larger than 3η . This behavior, which is in agreement with that observed by others (5, (5), has been attributed to the considerable 1

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Viscosity (Pa.s)

10000 -θ"

PP/SBH

PC/SBH

PC/VB

ΡΕΤΛΑ

NY/SBH

NY/VB

-B-

PET/SBHN


1000

100

10

16

80

LCP (%) Figure 1 Viscosity/composition curves measured at a shear rate of 24 s"\ and at the following temperatures: 250°C (PP/SBH, NY/SBH); 270°C (PC/SBH, PET/SBHN, PET/VA); 290°C (PC/VB, NY/VB). amount of energy required for deforming and orienting the LCP domains in the convergent flow at the die entrance. Thus, it is not surprising that, contrary to shear viscosity, the elongational viscosity of the 90/10 NY/VB blend is considerably higher than that of the pure matrix. In order to get more direct information on the spinnability of LCP/polymer blends, measurements of MS and BSR, carried out under non-isothermal conditions on the spinning line, have been made. As it is shown in Figure 3, a decrease of MS with increasing the LCP content is actually observed only for some of the blends, namely: PC/VB, PC/SBH, PET/SBH, PET/SBHN and NY/SBH. For the PP/SBH, PET/VA and NY/VB blends, on the contrary, an increase of MS is obtained. These results should be interpreted by the consideration of the different factors influencing the tensile strength of the molten filament, i.e., the viscosity, the viscosity changes taking place along the spinning line while the extruded filament rapidly cools down in the draw region (7), etc. Actually, under non-isothermal conditions, the viscosity of some blends may come out to be higher than that of the neat thermoplastic matrices, in some temperature intervals, if the viscosities of the two components have different activation energies. As for the BSR values of the LCP/polymer blends, i.e., the ratios of the windup speeds to the extrusion speeds corresponding tofilamentbreaking, the values measured at a shear rate of 24 s" for the investigated blends are collected in Table 2. It may be observed that the BSR of all the investigated blends decreases upon increasing the LCP content, as it might be expected on the basis of the triphasic nature of the blends. However, the extent of BSR reduction, which depends mainly on the interphase adhesion in the molten state, changes markedly with the chemical structure of the two 1


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Elong. Viscosity (Pa.s)


10000

-J

1

I

I

*

ι ι ι ι

_J

I

I I I I I

10

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Extension Rate (1/s) Figure 2 Elongational viscosity of VB, NY, and NY/VB 90/10.

MS (cN) ~ & - PP/SBH "β-

PET/VA

"A" PC/SBH

PC/VB

NY/SBH

NY/VB

10

PET/SBHN

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LCP (%) Figure 3 Melt strength of some LCP/polymer blends as a function of the LCP content



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components of the blends. For PET, for example, the decrease of BSR is very small when it is blended with semi-aromatic LCPs, such as SBH and SBHN, whereas a more significant drop of stretchability is found when it is blended with a wholly aromatic LCP, such as VA. On the contrary, both PC and NY experience almost the same BSR reduction when added with aromatic or semi-aromatic LCPs. The spinning behavior of the PP/SBH blend is peculiar in that an increase of the LCP content leads to a marked MS increase, accompanied by a fairly small BSR reduction. This means that, in the molten state, the adhesion between the PP and the SBH incompatible phases is not bad. From these experimental data, it may be concluded that the spinnability of PP is not severely worsened by the addition of a semi-aromatic LCP, such as SBH. Table 2 Breaking stretching ratios (BSR) of the LCP/polymer blends, measured at a shear rate of 24 s* 1

LCP (%) 0 5 10 20

PP/ SBH 650 610 500 450

PC/ SBH 730 520 400 200

PC/ VB 1400 1100 650 400

PET/ SBH 710 710 700 660

PET/ SBHN 710 700 660 620

PET/ VA 710 620 400 300

NY/ SBH 1400 950 670 540

NY/ VB 2000 1000 850 750

The elastic modulus of the fibers, prepared with different windup speeds, was also measured in order to get an idea of the reinforcing ability of the different LCPs, toward the different thermoplastics. The tensile modulus increases with the draw-down ratio. In Figure 4, the ratios of the maximum moduli measured for the blends and the neat thermoplastics are plotted versus the LCP content, for the different blends. For the correct interpretation of the data shown in Figure 4, it must be kept in mind that the values of the tensile moduli whose ratios are plotted in thefigureare those measured on fibers prepared with the highest windup speeds that could be used for either the blends and the neat polymers. Thus, the ratio of the maximum tensile moduli accounts also for the orientation of the matrix, due to thefilamentattenuation induced by drawing. The strongest modulus improvement is found for NY, especially when a wholly aromatic copolyesteramide (VB) is added into it (E ratio = 2.75, for an LCP content of 20%). However, even with a semi-aromatic copolyester (SBH), NY experiences fairly good reinforcement. The behavior of the PC based blends is very similar the comparatively lower values of the Ε ratios of the PC blends, with respect to the NY blends, can be explained considering that the tensile modulus of neat PC fibers, prepared under comparable conditions, is much higher than that of neat NYfibers(6.0 GPavs. 0.8 Gpa). As for the PET based blends, apparently opposite results have been obtained. In fact, whereas the addition of a semi-aromatic LCP, such as SBH or SBHN, leads to a remarkable modulus enhancement, the effect of the addition of an intrinsically much stiffer wholly aromatic LCP, such as VA, is surprisingly negligible. This is even more unexpected because, for compression molded specimens, the modulus improvement was found to be higher for PET/VA blends than for, e.g., PET/SBH blends (8).


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Ε ratio


2.5 -

1,5

0.5

-Θ-

PP/SBH

~ B - PET/SBHN

" A - pc/SBH

PC/VB

" β - PET/VA

NY/SBH

10

PET/SBH ~ 4 - NY/VB

15

80

LCP (%)

Figure 4 Ratio of the maximum tensile moduli measured for blend and neat thermoplastic fibers, as a function of the LCP content

Figure 5 Tensile moduli of 80/20 PET/SBH and PET/VA fibers, as a function of the draw-down ratio.



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However, as shown in Table 2, the stretehability of the PET/VA blends is approximately the half of those of the PET/SBH and PET/SBHN blends. It must be emphasized that, in order to avoid PET degradation, the PET/VA blends were spun at a temperature (270°C) lower than that of the crystal-nematic transition of VA. Therefore, it is possible that the LCP melting was uncompleted under the spinning conditions, thus preventing the use of the high draw-down ratios needed in view of a pronounced orientation of the PET macromolecules. The plots of the tensile moduli of the PET/VA and PET/SBH blends with 20% LCP, against draw-down ratio, shown in Figure 5, demonstrate that the PET/VAfiberscould not be spun with draw ratios higher than ca. 250, and that their modulus remained much lower than the hypothetical asymptotic value. On the other hand, an increase of the spinning temperature worsened the MS of the blends besides leading to initial PET degradation. The modulus improvement of the PPfibersresulting from the addition of SBH is much lower than it might be anticipated in view of the good spinnability of the PP/SBH blends. The apparent conflict between the very good MS and BRS values of these blends and their disappointing mechanical properties can perhaps be explained assuming that the interphase adhesion between PP and SBH, which has been shown to be fairly good in the molten state, is strongly depressed in the solid state, as a result of the matrix crystallization. Conclusions Despite the fact that the LCPs are known to reduce the shear viscosity of molten flexible polymers, the MS and the BSR of the LCP/polymer blends may still be high enough to grant good spinnability. High MS and BSR values may be taken as an indication of good phase compatibility in the molten state. On the other hand, the LCP phase may actually play a considerable reinforcing effect only if good spinnability (granting the fibrillation of the LCP particles and the orientation of the matrix macromolecules) is accompanied by good interphase adhesion is good in the solid state. Acknowledgments This work wasfinanciallysupported by C.N.R., by the Italian Ministry of University and Scientific and Technological Research (MURST), and by Eniricerche S.p.A. Literature Cited

1. Thermotropic Liquid Crystal Polymer Blends; La Mantia, F. P., Ed.; Technom Publ. Co. Inc., Lancaster, U.S.A., 1993, Chapters 4, 5, and references therein 2. Cogswell, F. N. Polym. Eng. Sci., 1972, 12, 64. 3. La Mantia, F. P.; Valenza, A. Makromol. Chem., Macromol. Symp., 1992, 56, 151. 4. La Mantia, F. P.; Valenza, A. Polym. Networks & Blends, 1993, 3, 125. 5. Beery, D.; Kenig, S.; Siegmann, A. Polym. Eng. Sci., 1992, 32, 14. 6. Turek, D .E.; Simon, G. P.; Smejkal, F.; Grosso, M.; Incarnato, L.; Acierno, D. Polymer Commun., 1993, 34, 204. 7. La Mantia, F. P.; Valenza, Α.; Scargiali, F. Polym. Eng. Sci., 1994, 34, 799. 8. La Mantia, F. P.; Pedretti, U.; Città, V.; Geraci, C. Polym. Networks Blends, 1994, 4, 151.


Liquid-Crystalline Polymer Systems - American Chemical Society

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