열방성 폴리이미드 액정고분자의 미세구조와 결정화 속도에 관한 연구

화학공학의 이론과 응용 제8권 제1호 2002년

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

고영관, 김상욱, 구종민, 정인재, 정희태

한국과학기술원 생명화학공학과

Microstructure and Crystallization Kinetics of Monotropic Liquid Crystalline Poly(ester-imide)s with a Decamethylene Spacer

Young Koan Ko, Sang Ouk Kim, Chong Min Koo, In Jae Chung and Hee-Tae Jung

Department of Chemical & Biomolecular Engineering, KAIST

Abstract

The phase transition, crystallization kinetics and morphology of monotropic liquid crystalline poly(ester-imide)s are investigated by synchrotron X-ray diffraction, differential scanning calorimetry (DSC), polarized light microscopy, and transmissionelectron microscopy (TEM). Synchrotron X-ray diffraction and DSC results show that the poly(ester-imide) with methyl side pendant group (10M) exhibits monotropic nematic and smectic A phases upon cooling from the isotropic melt, followed by a crystalline phase (crystalE structure). The results of isothermal crystallization kinetics show that the crystallization rate is faster in smectic phase than in nematic phase and the slowest in isotropic phase. It is found that the crystallization in nematic phase leads to the lamellar decorated disclination structures upon TEM observation, indicating that the enhancement of nucleation dominates the acceleration of crystallization. Poly(ester-imide) without side pendant group (10H) forms only crystalline phase upon cooling. However, the air-quenched sample shows a thread-like texture. TEM observation further reveals the trace of nematic disclination structures, confirming that the crystallization proceeds through nematic phase. In contrast, the sample, cooled with a slow cooling rate of 5 C/min, exhibits the spherulite structure of. The morphological variation of the poly(ester-imide)is discussed based on the competition between primary nucleation and growth of crystal.

Introduction

The liquid crystalline phase of a monotropic liquid crystalline polymer is thermodynamically stable below its crystal melting temperature. The metastable liquid crystalline phase is temporarily observable only when crystallization is bypassed below the mesophase forming temperature with an adequate cooling rate. 1,2

Because the monotropic liquid crystalline polymers crystallize from both monotropic liquid crystalline phase and supercooled isotropic phase, they provide good opportunities to study the effect of liquid crystalline ordering on the crystallization behaviors of polymeric materials.1 The changes of crystal melting temperature,3-5 Avrami exponent values for the crystallization,6,7 and crystal structure6(b)in the presence of liquid crystalline ordering have been reported for various monotropic liquid crystalline polymers. MacKnight and coworkers7 reported for a monotropic liquid crystalline polyurethane thatthe crystallizations from monotropic liquid crystalline phase and isotropic melt exhibit different Avrami exponent values, suggesting different crystallization mechanisms. Cebe and coworkers3 demonstrated that the melting temperature of the crystal formed in the monotropic nematic phase is higher than that in isotropic melt for a monotropic liquid crystalline polycarbonate, whichis interpreted by a model that nematic phase serves as nuclei for crystallization.8 Cheng and coworkers conducted precise characterizations of the crystallization behavior for a series of monotropic liquid crystalline poly(ester-imide)s with smectic A phase.6 They showed that Avrami exponent value for crystallization in smectic phase is two, whereas the value in isotropic phase is three, and the wide angle X-ray diffraction (WAXD) patterns of the crystal formed in monotropic smectic phase and isotropic phase were different for the poly(ester-imide)s with odd number of methylene spacers. In our previous articles, we synthesized and characterized the monotropic nematic phase behaviors of poly(ester-imide)s with rheological methods.5,9 Avrami exponent value for crystallization was four both in isotropic and nematic phases and the WAXD patterns of the crystallized samples are the same regardless of the precursor phase. The crystal formed from nematic phase showed the higher melting temperature than that from isotropic melt. From the previous crystallization studies on monotropic liquid crystalline polymers, the most significant conclusion is that the crystallization accelerates in the presence of liquid crystalline ordering.1,3,5,6(a),(c),7-10 However, the mechanism for the accelerationof crystallization has been unknown.

In this article, we investigate the phase behaviors, crystallization kinetics, and crystal morphologies of monotropic liquid crystalline poly(ester-imide)s. Synchrotron X-ray diffraction experiments determines the precise phase transition behaviors of the poly(ester-imide)s. The morphological characterization of crystallized samples by TEM gives some insight into the origin of the enhancement of crystallization in liquid crystalline phase. The methyl side pendant group in the poly(ester-imide) strongly influences phase behavior, crystallization kinetics and morphology.

Experimental

The chemical structures of the monotropic liquid crystalline poly(ester-imide)s with a decamethylene flexible spacer are shown below. They are polymerized from N-(10-carboxydecanyl) trimellitic imide and hydroquinone or methylhydroquinone by melt transesterification reaction with MgO catalyst, as described in our previous publications.5,9 We denote the poly(ester-imide) with methyl side pendant group as 10M, and  the polymer without methyl side pendant group as 10H, respectively. The inherent viscosities in m-cresol at 20 C at a concentration of 0.5 g/dL, are measured to be 0.71 for 10M and 0.25 for 10H.

The thermal transition behaviors and crystallization rates were performed on a Dupont 2010 Thermal Analyzer under N2 flow at various scan rates. The crystallization rates were measured from isothermal crystallization experiments. Before each measurement, DSC was pre-equilibratedat the crystallization temperature. The samples, annealed above their melting temperature for several minutes to erase previous thermal history, were shifted to DSC and the crystallization exotherm was recorded.

<Chemical Structures of Monotropic Liquid Crystalline Poly(ester-imide)s>

Syncrotron X-ray scattering measurement was conducted with a wavelength of 1.54 at the 4C2 XRD beamline in Pohang Accelator Laboratory (PAL), consisting of 2 GeV LINAC accelerator, storage ring, Si (111) doublecrystal monochromator, ion chambers, and one-dimensional position-sensitive detector with 2048 pixels. A small amount of sample was sealed with imide films, and each sample was heated above the isotropic melt and then cooled to a specified temperature. The thermal history of the samples is kept the same as those in the DSC experiment. The temperature was controlled with an accuracy of  1 C in the experimental temperature range.

The anisotropic optical textures were observed on a polarized optical microscopy (A Leitz, Model Laborlux 12 Pols), equipped with a hot stage (Mettler FP82 HT). The thermal history of each sample was controlled as follows: The texture evolution of 10M was observed on the isothermal conditions. The sample was cooled from the isotropic melt to each isotropic annealing temperature with a rate of 5 C/min. The thermal history of 10H was controlled with dynamic temperature protocol. Each sample wascooled from isotropic melt, varying the cooling rate (5 C/min, 20 C/min and air-quenching).

For the TEM observation, thin films were prepared by surface tension spreading on hot phosphoric acid above their melting temperatures.11 After the spreading, the specimen was poured into cold water, which results in quenched thin films floating on the water surface. The films, showing the color of silver or weak gold, were carefully recovered on the carbon-supported Cu grids. These samples were examined at a 120 kV PHILIPS CM-20 TEM. The thermal history of the thin films was controlled in the same way as the samples for the polarized light microscopy observation. It has to be noted that all the thermal history control in this study started from the isotropic melt. Thus, all the morphology formations are spontaneous in each condition.

Acknowledgements

This work was supported by the Advanced Backbone IT Technology Development Project, CUPS and the Brain Korea 21 Project.

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