Thermal properties and 1H NMR relaxations for liquid crystalline polymer blends with PBT

Pergamon

Solid State Communications, Copyright Vol. 100.0 No. 199610,Elsevier pp. 731-736, Science 1996 Ltd Printed in Great Britain. All rights reserved 0038-1098/96 %12.00+.00

PII:SOO38-1098(96)00436-X

THERMAL

PROPERTIES

AND ‘H NMR RELAXATIONS FOR LIQUID CRYSTALLINE POLYMER BLENDS WITH PBT

Ae Ran Lim,” Kwan Soo Hong’ and Jin-Hae Chang’

‘Polymer

‘Department of Physics, Jeonju University, Chonju 560-759, Korea ‘Seoul Branch, Korea Basics Science Institute, Seoul 136-701, Korea Science and Engineering Department, Kum-Oh University of Technology, Kumi 730-701, Korea (Received 18 December 1995; accepted 18 June 1996 by A. Pinczuk)

Blends of thermotropic liquid crystalline polymer (TLCP) with poly (butylene terephthalate) (PBT) were prepared by coprecipitation from a common solvent. Their glass transition temperatures (T,) and melting transition temperatures (T,,,) of the blends were not affected depending on the LCP weight percent in the PBT matrix, respectively, while the isotropic temperatures (Ti) decreased with increasing LCP composition. The measurements of the line-width and spin-lattice relaxation time for ‘H have been investigated on LCP/PBT blends at different LCP contents. The linewidth monotonically decreased with increasing LCP content. The decrease of the line-width with increasing temperature is due to the motional narrowing effect. The inverse T, relaxation rate increases with increasing the temperature. Copyright c! 1996 Elsevier Science Ltd Keywords:

A. liquid crystals, A. polymers,

terephthalate), PBT. And we measured the line-width and spin-lattice relaxation time (T,) according to the amount of LCP in PBT matrix. In addition to knowledge about the molecular orientation, information concerning the molecular motion is expected to give in sight into the nature of liquid crystalline copolyesters with PBT.

1. INTRODUCTION The blends of main chain thermotropic liquid crystalline polymers (TLCPs) with commercial thermoplastics have studied systematically in many laboratories, studies on blends of LCPs with other polymers focus on the chemical structures, characterization of rheology, phase behavior, morphology, mechanical properties [l-4]. In view of above interest, many studies have appeared in the scientific literature that aim to understand the intimate structure of the blends. Such investigations rely on the results obtained by using spectroscopic techniques (IR and NMR) [5], differential scanning calorimetry (DSC) [6], and dynamic mechanical analysis (DMA) [7]. In order to understand the effect of chemical structure on the molecular motion, it is essential to study the relaxation behavior of the blends. The molecular motion and relaxation behavior have been studied by broad line NMR [8], dielectric dispersion [9] and ESR studied [lo]. In particular, NMR is a useful tool to observe molecular motion in solid polymers. NMR measurements are sensitive to microscopic motions. This paper deals with the thermal characterizations of new side-group TLCP blends with poly(butylene

E. nuclear resonances.

2. EXPERIMENTAL 2.1. Materials The polymer, THE-3, which has a flexible side group on a mesogenic ring in the main chain was prepared by solution polymerization of the trioxy-ethylene substitute hydroquinone, ethylene glycol and terephthaloyl chloride. The TLCP is a copolymer consisted of 6Omol% THE-3 and 40 mol % PET. Typical photomicrographs of nematic textures observed through a polarizing microscope for the liquid crystalline polymer melts. The chemical structure of the THE-3, PET and TLCP are as follows:

731

-oQ-&*E-

THE-3

732

LIQUID CRYSTALLINE POLYMER BLENDS WITH PBT

Table 1. Properties of THE-3, TLCP and PBT

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Table 2. Thermal properties of pure polymers and LCP/ PBT blends

a

Polymer

qinh

T,,,(K)

TiCKI

tin

THE-3 TLCP PBT

1.07 0.65 -

468 501 497

521 543 -

33 000

a Inherent viscosity was measured at 323 K at 0.5 g dL-’ in 4-chlorophenol.

The PBT for this work was obtained from AKZO company and synthetic methods for TLCP in elsewhere [ 111.The general properties of the polymers are tabulated in Table 1. 2.2. Blend preparation Both TLCP which was composed THE-3 and PET (60: 40 mol ratio) and PBT were dissolved in a trifluoroacetic acid at room temperature. The concentration of the solution was 5% by weight to avoid aggregation of LCP itself in a solvent. Thereafter the polymer solution was coprecipitated in acetone drop by drop with vigorous agitation, and washed the white precipitants with 5% NaHCOs and water, twice respectively. The blends were dried in a vacuum oven at 380K for 2-3 days. For simplicity, the blends will be described as 0% LCP/PBT, 5% LCP/PBT, 10% LCP/PBT and so on. Where LCP and PBT represent the polymer components used to prepare the blend, the numbers denote the amount of LCP in the blend in weight percent.

LCP/PRT(wt.%) 100/o

mmb (J/g) Tic W

Blends (LCP %)

Tga (K)

Tm (K)

100 (pure LCP) 90 80 50 20 10 5

312

501

4.8

543

311 311 311 311 309 309 310

498 498 497 496 496 495 497

18.5 38.6 46.7 50.2 58.8 53.9 48.5

533 535 536 537 -

(pie PBT) a Determined using quenched samples. b Enthalpy change of fusion. ’ Isotropization temperature. 2.3. Characterization Thermal properties of the blends were studied under Nz atmosphere on a Du Pont 910 differential scanning calorimeter (DSC). Heating and cooling rates of 20 Kmin-’ were used. The spin-lattice relaxation time of ‘H in the LCP/ PBT was investigated using a Bruker MSL 200 FT NMR spectrometer with an external magnetic field of 4.7T. The various temperatures were established, within an accuracy of +0.5 K, by regulating the heat-current in steady dry-air flow over the room temperature and in steady cold nitrogen gas flow from the liquid nitrogen dewar below the room temperature. 3. RESULTS AND DISCUSSION 3.1. Thermal properties

30/10

80/20

50/50

1OAO

bure

NT) I

I

350

400

I

450

I

1

500

550

TEMPERATURE

(K)

Fig. 1. DSC heating thermograms for the pure polymers and their blends.

Figure 1 shows the DSC traces of the pure polymer (PBT and LCP) and their blends as obtained by solution precipitation. The crystalline nematic transition (K - N) (T,) for the pure LCP was observed at about 501 K, while T,,, of PBT appeared at about 497 K. Each thermogram of 5% and 10% LCP/PBT blends in Fig. 1 is very similar to that of pure PBT. But that of 20% blend shows small peak at 537K, which was considered to be isotropic temperature (Ti) of LCP. In the case of pure LCP, isotropic transition point was observed at 543 K. An endothermic peak corresponding to mesophase-to-isotropic liquid transition is observed in the blends containing LCP amounts higher than 20%. The transition temperatures and the enthalpy changes corresponding to melting transition are listed in Table 2. The values of the glass transition temperature (T,) of the

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LIQUID CRYSTALLJNE POLYMER BLENDS WITH PBT

733

Fig. 2. Optical micrographs of solvent casting films at room temperature (200x). (a) 0% LCP/PBT, (b) 5% LCP/PBT, (c) 10% LCWPBT, (d) 20% LCP/PBT.

LIQUID CRYSTALLINE

POLYMER

h

BLENDS WITH PBT

Vol. 100, No. 10

1.0

:: 3

A (O/100)

/

2.0

I

/

I

2.5

3.0

3.5

1000/T Fig. 3. Linewidth of LCP/PBTas

0

( 3/97)

v

(10/W)

Y --

D

(10/90)

cl (20/80)

‘I

cw9~)

.

(50/50)

0

(SO/SO)

A

(80/D)

n

(eo/zol

A

(lOO/Ol

0

(100/O) I

4.0

0.1

-

quenched samples in range of 309-312K, respectively. Although the amount of LCP increased from 5 to 90 wt %, the melting temperatures of all samples were also shown at 495-498 K. It can be noted that the glass transition and the melting temperature of the samples were not affected significantly by the LCP composition in the PBT matrix. To examine the definite miscibility, solvent casting films from trifluoroacetic acid were examined by polarized optical microscope (Fig. 2). For pure PBT, big domains were observed. But to increase the amount of LCP 5% to 20%, the domain size of PBT becomes smaller. According to these pictures, it may be that the LCP acts as a nucleating agent for PBT matrix and there is no macrophase separation between two phases [12, 131; in other words, the LCP is well dispersed into the PBT matrix. The heat of fusion (A&,), in Table 2, was shown its maximum value at 10% LCP and thereafter decreased with the increase in the LCP content in the blend. Some authors [13] have suggested the possibility that the nucleation of the LCP phase may affect the heat of fusion of the matrix polymer. 3.2. NMR analysis Temperature dependencies of the full width at half maximum according to the amount of LCP and PBT are shown in Fig. 3 as a function of the inverse of temperature. The line-width gradually decreases with increasing temperature. The tendencies of the change with respect to the temperature are similar with each other, but the line-width is varied with temperatures more drastically as lowering the LCP weight percent. Near the temperature varied most drastically, the intramolecular dipolar interaction becomes partially averaged by motion, and the

;

I

I

I

1

2.8

3.2

3.6

4.0

1000/T

(l/K) a function of temperature.

0

Fig. 4. Temperature

dependence

(1 /‘I<) of l/T, for LCP/PBT.

temperature increases with lowering the LCP ratio. But these temperatures are not associated with the glass transition temperature. Also, the decrease of the linewidth with increasing temperature is due to the motional narrowing effect [14]. The T1 measurement was made in the temperature range 160-420 K with an inversion recovery sequence of (180”-t-90”) pulse [ 151. All the traces of the recovery at six different temperatures are fitted by the following single exponential function M,(tj = MO[l - 2 exp (-t/T,)],

(1)

where i&f,is the projection of the spin magnetization. The relaxation rate T;l in equation (1) is determined directly from the slopes; the TI values were determined from log [(Me - M,(t))I2i&] vs time plot, where Ma is the nuclear magnetization of ‘H in thermal equilibrium. In general, the single exponential relaxation is expected only for blends where the domain size is less than spin diffusion length (-200 A). From the data given by scanning electron microscope (SEM) in our cases [ 16 1, LCP! were shown fine dispersion with domain sizes 400-600A in diameter. The LCP domain sizes of all samples are greater than 400A, thus it is expected that the relaxation is multi-exponential. However, the relaxations which we obtained are shown single-exponential. The origin of the single-exponential is not properly understood. Temperature dependence of l/T, is shown in Figure 4. The spin-lattice relaxation rate l/T1 increases, except for the case of pure LCP, as the temperature increases. The relaxation time T1 for pure LCP is shortest than any other samples, therefore the relaxation for blends is dominantly governed by PBT. The value of T1 for LCP/PBT blends are longer than that of pure LCP, and the relaxation is single exponential. Molecular dynamics

LIQUID CRYSTALLINE

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1

1

POLYMER

735

activation energy decrease with the increase in the LCP concentration in the blends.

,

I

BLENDS WITH PBT

'--I -1 'O-

1

8-

ii

4 i

2

4 4

2 .

1

!

.___1_i._ 25

0

LCP

Fig. 5. Activation

i

)

50 Ratio

energy according

75

100

(Z)

to the LCP ratio.

of LCP for LCP/PBT blends is very different from that of pure LCP. When the relaxation rate of LCP in blends is shorter than that of PBT, the spin energy of LCP can be well transferred to PBT by means of spin diffusion. The temperature dependence of relaxation rate l/T, of blends are different from each other, but we can separate with two groups of blends: 3%, 5% and 10% LCP and 20%, 50% and 80% LCP. In Fig. 4, blend containing 3% and 5% LCP exhibit drastic changes in the relaxation times. The morphology of LCP and PBT blends for films was examined by observing their fracture surfaces by SEM [16]. Based on the SEM micrographs, 5% LCP/PBT morphology consisted of spherical LCP domains, about 400-600A in diameter, dispersed in the PBT continuous phase. It is conceivable that the very well dispersed LCP domain changes the relaxation time of the blends significantly. But the LCP phase in the 10% and 20% LCP/f’BT blends was much bigger due to agglomeration. It is clearly seen that the particle size of the dispersed LCP phase increases with LCP content. From the trends of the relaxation times as a function of LCP composition in Fig. 4, it is suggested that LCP domain size in PBT matrix affects the relaxation times. We know that the dominant mechanism of the relaxation is related to the changes in motion on blending. The behavior of l/T, is nearly exponential with l/T, which indicates that the relaxation is mostly determined by the contribution of the thermal fluctuation with correlation frequencies lower than 200.13 MHz [17]. This TI behavior can be analyzed with the equation given by [18]

4. CONCLUSIONS New thermotropic LCP showed nematic behavior and LCPs highly dispersed in the PBT matrix. Transition temperatures, Tg and T,, were not affected by the presence of the LCP in the PBT matrix, respectively. But Ti progressively shifted to lower temperature with increasing the LCP content. Also, we have been investigating by line-width and T, of ‘H according to the LCP concentration of the polyblends in the wide temperature range. Although the FCP domain sizes of all samples are greater than 400A, the single-exponential relaxation times are obtained. The activation energy is decreased as the LCP ratio increases. Activation energy is estimated to be 8.3 kJ mol-’ for the pure LCP. The value of pure LCP are larger than that of 50 and 80% LCP/PBT. Acknowledgements-This work supported by the Korea Science and Engineering Foundation (KOSEF) through the Research Center for Dielectric and Advanced Matter Physics (RCDAMP) at Pusan National University (19947). It was also supported in part by the Faculty Research Grant to A.R. Lim of Jeonju University. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

11. 12.

(2)

13.

where E, is activation energy for the modulation and k is Boltzmann constant. The activation energy vs LCP ratio, as shown in Fig. 5, is obtained by equation (2). The

14.

l/T1 CCexp (-E,/kT),

Friedrich, K., Hess, M. and Kosfeld, R., Mukromol. Chem., Macromol. Symp. 16, 1988, 251. Ameno, M. and Nakagawa, K., Polymer 28, 1987, 263. Ko, CU. and Wilkes, G.L., J. Appl. Polym. Sci. 37, 1989, 3063. Dutta, D., Fruitwala, H., Kohli, A. and Weiss, R.A., Polym. Eng. Sci. 30, 1990, 1005. Linder, M., Henrichs, P.M., Hewitt, J.M. and Massa, D.J., .I. Chem. Phys. 82, 1985, 1585. Nassar, T.R., Paul, D.R. and Barlow, J.W., J. Appl. Polym. Sci. 82, 1985, 1585. Chen, X.Y. and Birley, A.W., Br. Polym. J. 17, 1985, 347. Mitchell, G.R. and Ishii, F., Polym. Commun. 26, 1985, 34. Attard, G.S., Williams, G., Gray, G.W., Lacey, D. and Gemmel, P.A., Polymer 27, 1986, 186. Meurisse, P., Friedrich, C., Dvolaitzky, M., Laupretre, F., Noel, C. and Monnerie, L., Macromolecules 17, 1984, 72. Lenz, R.W., Furukawa, A., Bhowmik, P., Garay, R.O. and Majnusz, J., Polymer 32, 1991, 1703. Joseph, E.G., Wilkes, G.L. and Baird, D.G., Polymeric Liquid Crystals (Edited by A. Blumstein). Plenum Press, New York, 1985. Joseph, E.G., Wilkes, G.L. and Baird, D.G., Am. Chem. Sot., Div. Polym. Chem., Polym. Prepr. 24, 1983, 304. Abragam, A., The Principle of Nuclear Magnetism, p. 451. Oxford University Press, London, 1961.

736 15. 16.

LIQUID CRYSTALLINE POLYMER BLENDS WITH PBT Fukushima, E. and Roeder, S.B.W., Experimental Pulse NMR, p. 169. Addison-Wesley, New York, 1981. Chang, J.-H. and Farris, R.J., Polymer J. 27, 1995, 780.

17.

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Bloembergen, N., Purcell, E.M. and Pound, R.V., Phys. Rev. 73, 1948, 679.

18.

Gerstein, B.C. and Dybowski, C.R., Transient Techniques in NMR of Solids, p. 84, 1985.