Thermal Imidization and Structural Evolution of Thin Films of Poly(4

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13894

J. Phys. Chem. B 2007, 111, 13894-13900

Thermal Imidization and Structural Evolution of Thin Films of Poly(4,4′-oxydiphenylene p-pyromellitamic diethyl ester) Tae Joo Shin*,† and Moonhor Ree*,†,‡ Pohang Accelerator Laboratory, Pohang UniVersity of Science & Technology, Pohang, 790-784, Republic of Korea, and Department of Chemistry, National Research Lab for Polymer Synthesis and Physics, Center for Integrated Molecular Systems, Polymer Research Institute, and BK21 Program, Pohang UniVersity of Science & Technology, Pohang 790-784, Republic of Korea ReceiVed: June 29, 2007; In Final Form: October 9, 2007

The evolution of chemical composition and structure during the thermal imidization of an ester-type polyimide precursor, poly(4,4′-oxydiphenylene p-pyromellitamic diethyl ester), in micrometer scale films were studied for a heating rate of 2.0 °C/min with time-resolved synchrotron X-ray diffraction, in-situ infrared spectroscopy, and modulated differential scanning calorimetry. Our analyses show that the precursor polymer undergoes imidization in a two-step process. In the first step, the precursor polymer is decomplexed from the residual solvent molecules, and in the second step, it undergoes imide ring formation with the release of ethanol as a byproduct. The imidization reaction starts around 210 °C and continues up to 320 °C. The thermal imidization reaction induces the structural evolution of the film. As the imidization reaction proceeds, the coherent length along the polymer chain axis increases. This imidization-induced structural evolution was found to occur via three steps: (i) initiation, (ii) the first crystallization, and (iii) the second crystallization. The initiation step is necessary prior to the evolution of the crystalline structure to increase the chain mobility of the precursor polymer chains, and it requires thermal heating up to at least 238 °C at which point 22.5% of the imidization is complete. Thereafter, the first crystallization occurs up to 310 °C, at which point 98.3% of the imidization is complete. In the range 310-380 °C, the second crystallization occurs and produces almost complete imidization of the polymer chains.

Introduction Aromatic polyimides generally have excellent mechanical, electrical, and thermal properties primarily because of their aromatic heterocyclic structures.1-3 However, sometimes greater thermal stability and mechanical strength than those of pure polyimides are required for various applications. The fabrication of polyimide nanocomposite systems, such as hybrids with organoclays,4 mesoporous silica,5 and zirconium,6 is a new approach to obtaining materials with improved properties. The use of polyimides as dispersants for carbon nanotubes, in which nanotubes are homogeneously dispersed in the polyimide matrix, has also been investigated.7,8 Nevertheless, detailed investigations of the thermal imidization reactions and structural evolution of pure polyimides are still required if we are to fully understand and utilize the “processing-structure-property relationship” in the design of further hybridized polyimide systems. In general, in the fabrication of polyimides, poly(amic acid) (PAA) or poly(amic dialkyl ester) (PAE) is synthesized as a precursor polymer. These precursor polymers with amide bond linkages are soluble in organic polar solvents, and thin films can be obtained by simple spin casting of their solutions. The precursor films are then thermally converted to polyimides, a process that involves ring closing reactions. One shortcoming of PAA as a precursor is the instability of its solutions, i.e., its * To whom all correspondence should be addressed. Fax: +82-54-2791599. E-mail: [email protected] (T.J.S.), [email protected] (M.R.). † Pohang Accelerator Laboratory. ‡ Department of Chemistry, National Research Lab for Polymer Synthesis and Physics, Center for Integrated Molecular Systems, Polymer Research Institute, and BK21 Program.

tendency to spontaneously decompose due to hydrolysis and exchange reactions.9 As a result, there is a rapid drop in the solution viscosity, especially at elevated temperatures. PAE precursors have some advantages over PAA, in spite of their relatively complicated synthetic pathways (see Scheme 1); in particular, they have better solution properties and hydrolytic stability due to the absence of carboxylic acid groups that cause monomer-polymer equilibration in solution.10-13 Further, they can be obtained in meta- or para-catenated form in powder without the presence of residual solvent, which can permit easier characterization and comparison of the effect of catenation on the chain dimensions. A considerable amount of research into the thermal imidization of poly(4,4′-oxydiphenylene pyromellitamic acid) (PMDA4,4′-ODA PAA) has been carried out.14-18 In comparison, research into its PAE precursor, poly(4,4′-oxydiphenylene pyromellitamic diethyl ester) (PMDA-4,4′-ODA PAE), has been limited. Kramer and co-workers19-20 measured the extent of imidization as a function of depth in films of deuterated PMDA4,4′-ODA PAE using ion beam analysis techniques. They also investigated solvent and isomer effects on the imidization using the same techniques.21 Wunder et al.22 reported their study of the thermal imidization reactions of meta-, para-, and 50/50 mixed isomers of PMDA-4,4′-ODA PAE using mass spectroscopy, Fourier-transform Raman spectroscopy, and density gradient column analysis. However, the structural evolution of PAE precursors during the thermal imidization reaction has rarely been investigated. In the present study, we chose the para-isomer of PMDA4,4′-ODA PAE precursor as our model PAE precursor, and we

10.1021/jp075067o CCC: $37.00 © 2007 American Chemical Society Published on Web 11/23/2007

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SCHEME 1: Synthetic Scheme of p-PMDA-4,4′-ODA PAE Precursor and Its Polyimide

investigated its structural evolution and imidization during thermal heating by using modulated differential scanning calorimetry (MDSC), in situ Fourier-transform infrared (FTIR) spectroscopy, and synchrotron X-ray diffraction (XRD). Experimental Section Materials and Sample Preparation. Powdery p-PMDA-4,4′ODA PAE (0.926 dL/g intrinsic viscosity and 20000 weight average molecular weight) was dissolved in anhydrous N-methyl-2-pyrrolidone (NMP: boiling point 202 °C) to be 13.5 wt %, and then stirred for 2 h to be fully mixed. In synthesis of this precursor, the nucleophilic substitution reaction on the dianhydride carbonyl results in the addition of the diamine in the meta- or para-position. The pyromellitate acid was suspended in ethyl acetate, yielding ethyl acetate soluble metarich isomer (45%) and ethyl acetate insoluble para-rich isomer (55%). Recrystallization of meta-rich and para-rich isomers from butyl acetate yielded greater than 99% isomeric purity, respectively.21 The precursor solution was spin-coated onto silicon wafers and then dried on a hot plate at 80 °C for 2 days in ambient air, resulting in thin PAE precursor films 1.0-4.0 µm in thickness. The dried precursor films were determined to contain residual solvent of approximately 7 wt % by using proton nuclear magnetic resonance spectroscopy. These films were used for MDSC, IR, and WAXD measurements. MDSC Measurements. MDSC is now recognized as a very powerful analytical technique in polymer science to deal with detection of weak glass transitions, determination of heat capacities in quasi-isothermal mode, and separation of superimposed phenomena.23-27 Thermal behavior of the p-PMDA4,4′-ODA PAE in thin films was studied by using a Seiko calorimeter (model 220CU DSC, Osaka, Japan). About 5.2 mg of pieces of thin films was put into a DSC sample cell and heated up to 380 °C. In the measurement, 2.0 °C/min heating rate, 5.0 °C oscillation amplitude, and 60 s oscillation period were employed. In Situ FT-IR Spectroscopy Measurements. FT-IR spectra were acquired in a transmission mode using an FT-IR spectrometer (Mattson Research Series, Madison Instruments, Inc., Madison, WI) equipped with an MCT (mercury-cadmiumtelluride) detector cooled by liquid nitrogen and a heating sample block with nitrogen blowing holes. The spectrometer was

calibrated using a polystyrene standard film. A 1.4 µm-thick p-PMDA-4,4′-ODA PAE film on silicon substrate was inserted into a heating sample block. IR spectra were collected as a function of temperature up to 380 °C at a 2.0 °C/min heating rate. Each IR spectrum was recorded every 1 min with 42 scans at 4.0 cm-1 spectral resolution. Time-Resolved WAXD Measurements. Wide-angle X-ray diffraction (WAXD) measurements were carried out at 4C1 beamline of the Pohang Light Source (PLS) facility;28-29 the X-ray beam was monochromated at wavelength λ ) 1.608 Å. Diffraction patterns were acquired by using a position sensitive one-dimensional silicon-photodiode array detector (Model X/PDA2048, Princeton Instruments, Trenton, NJ). PAE precursor films were cut into pieces, stacked to be ca. 0.8 mm in thickness, and then inserted into a square-shaped sample cell with a 2 mmdiameter central hole for X-ray beam path. The sample cell was put into the heating sample block with nitrogen blowing holes. Two temperature controllers were equipped: one for temperature control of the heating block and the other for temperature monitoring of a stack of films. During thermal imidization, WAXD data were collected for 30 s as a function of temperature. A peak-fitting program (Peakfit, Jandel Scientific) was used for the deconvolution of amorphous and crystalline peaks in the measured WAXD profiles. All the diffraction peaks were deconvoluted and fitted with Gaussian functions on a single baseline. The d-spacing of each diffraction peak was calculated using the Bragg equation. In general, a diffraction peak from single crystals or crystalline powders becomes broader as the crystal size decreases and the crystal defect level increases. This phenomenon provides an experimental method for determining the size of, and defect level in, submicroscopic crystals. For a sample composed of relatively perfect crystallites, the mean crystallite dimension (or coherence length) Lc perpendicular to a (hkl) plane, Lhkl, is estimated by the Scherrer equation30

Lhkl )

Kλ β0 cos θ

where β0 is the full-width at half-maximum (fwhm) of the (hkl) peak in radian, θ is one-half the angle between incident and diffracted X-rays, and K is a constant commonly assigned to unity.

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Figure 1. Reversing and nonreversing heat flows of p-PMDA-4,4′ODA PAE thin films.

Figure 2. Representative IR spectra during thermal treatment of a p-PMDA-4,4′-ODA PAE thin film at 2.0 °C/min heating rate.

Results and Discussion MDSC Analysis. Figure 1 shows the reversing and nonreversing heat flows of p-PMDA-4,4′-ODA PAE films. Generally, the nonreversing heat flow contains information about the enthalpy relaxation, crystallization, thermal decomposition, thermal curing, and evaporation of the solvent. In this study, this process involves the evaporation of the residual NMP solvent and the release of ethanol as a byproduct. The nonreversing heat flow contains exothermic heat flow in the range 116-166 °C, indicating that rearrangement and ordering of the PAE polymer chains, which is caused by the thermal increase in mobility due to the evaporation of residual NMP solvent, has occurred. Characteristically, there is an exothermic peak near 334 °C, which is indicative of the crystallization of the polyimide chains. This exothermic peak does not arise for the meta-isomer of PMDA-4,4′-ODA PAE (data not shown here). In the reversing heat flow, a change in the heat capacity is observed near 208 °C, reflecting the conversion of the PAE to its polyimide with the release of ethanol. The PAE solid state is converted to a rubbery state at higher temperatures due to thermal gains in mobility, and it then undergoes imidization to reach a glassy state. Thus, this vitrification process results in the change in heat capacity. In Situ FT-IR Spectroscopic Analysis. Representative IR spectra obtained during the heating process are shown in Figure 2. The characteristic IR absorption peaks were assigned with the aid of previous results.31-36 The IR absorption peak of the precursor polymer at 1722 cm-1 (see the arrow at 30 °C in Figure 2) is assigned to the CdO stretch of the carboxylic acid group, and the peak at 1662 cm-1 is assigned to the CdO stretch

Shin and Ree of the amide linkage (amide-I). The peaks at 1545 and 1536 cm-1 are assigned to the CNH bend-stretches of the amide linkage (amide-II). The spectrum of the fully imidized film at 380 °C contains peaks at 1775 and 1722 cm-1, which are assigned to the CdO in-phase and out-of-phase stretching vibrations of the imide ring, respectively. The peak at 1380 cm-1 is assigned to the CN stretch of the imide ring, and the peak at 726 cm-1 originates from the CdO bending mode of the imide ring. Finally, the asymmetric stretch of Car-O-Car in the 4,4′ODA unit is clearly evident at 1251 cm-1. As shown in Figure 2, the intensities of the characteristic peaks of the PAE groups (ethyl ester, amide-I, and amide-II) change with increasing temperature. On commencement of the imidization, the intensities of the absorption peaks of PAE start to decrease with increasing temperature, while those of the polyimide appear and become stronger. In order to examine the imidization behavior quantitatively, the characteristic vibrational bands were deconvoluted from the IR spectrum, and the individual peak positions and integrated absorbances were further analyzed. Monitoring the NH stretching peak during thermal treatment can supply useful information about the imidization process because NH stretching is directly involved in the ring closure reaction. Figure 3a shows the changes in the integrated absorbance and position of the peak due to NH stretching as a function of temperature. The drastic drop in the integrated absorbance from 210 °C onward indicates the commencement of thermal imidization. The intensity of the NH stretching peak becomes very weak when the temperature nears 286 °C, and the rate of decrease slows above this temperature because the imidization reaction is restricted in the vitrified system. The peak position abruptly shifts to higher wavenumbers above 222 °C. Initially, it appears at 3305 cm-1, which is approximately 60 cm-1 lower than the free NH stretching peak, which is suggestive of the presence of hydrogen bonded amide linkages in the precursor state. At 264 °C, which is almost in the middle of the imidization process, this peak appears at 3367 cm-1, corresponding to free NH stretching. This result suggests that the later stages of imidization occur when the NH group is free, i.e., in a decomplexed state, but it is not clear from the results for the NH stretching peak whether the initial state is free or complexed. Figure 3b shows the changes in the positions of the integrated absorption peaks of the CdO in-phase stretch (in-plane), C-N stretch, and CdO bend (out-of-plane) of the imide ring with increases in temperature. These imide peaks appear at approximately 210 °C and gradually increase in intensity, becoming saturated at approximately 320 °C. The slight changes in these peaks above 320 °C are attributed to the reorientation of the polyimide chains. Characteristically, the intensity of the peak due to the CdO in-phase stretch (in-plane) increases slightly above 330 °C, and the intensity of the CdO bend (out-of-plane) peak decreases slightly, reflecting the improvement in the in-plane orientation of the imide rings. This result is consistent with the presence of the exothermic peak in the MDSC nonreversing heat flow plot (Figure 1). The behavior of the peaks due to imide carbonyl groups in PAA and PAE precursors are readily distinguishable because of their different imidization onset temperatures. For the PAA precursor, the imide carbonyl symmetric stretching and bending peaks partially overlap with those of the anhydride carbonyl groups that form transiently during heating.37 Thus, estimating the degree of imidization of PAA from the imide carbonyl peaks may cause errors. In contrast, for PAE precursors the imidization onset temperature is above 210 °C, and there is no sign of

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Figure 4. Variations of the absorbance of -CNH- bend-stretch (amide-II) and aromatic ring-breathing vibrations observed in the range 30-370 °C.

Figure 5. Variations of the integrated absorbance of “-CH- in-phase bending vibrations mixed with C-C vibrations in benzene” as a function of temperature. The centrosymmetric C-H bending vibration absorbs at 1167 cm-1, and the centroantisymmetric C-H bending vibration absorbs at 1013 cm-1.

SCHEME 2: Peaks Due to the Mixing of -CHIn-Plane Bending Vibrations with C-C Benzene Vibrations Appearing at 1013 and 1167 cm-1 a,b Figure 3. (a) Variations of the normalized absorbance (area) and position of NH stretching vibration as a function of temperature. (b) Variations of the normalized absorbance (area) and positions of CdO in-phase stretch (in-plane), C-N stretch, and CdO bend (out-of-plane) vibration as a function of temperature. (c) Degree of imidization (XImidization) and its derivative determined from C-N stretch vibration in imide-ring.

anhydride formation, so the imide carbonyl peaks can be usefully utilized in the estimation of the degree of imidization, particularly when combined with results for the NH and CN stretching peaks. The degree of imidization (XImidization) was determined as a function of temperature from the imide C-N absorption peak and plotted in Figure 3c; imidization commences at 210 °C and gradually increases before reaching saturation at 320 °C. Note that the maximum rate of imidization in the d(XImidization)/dT profile occurs at 240.9 °C. The CNH bend-stretches (the amide-II band) at 1545 and 1536 cm-1 were used to investigate the imidization mechanism of the precursor film (peak a in Figure 4). The transition of the conjugated amide linkages to nonconjugated linkages is evident in the shift in the position of the peak due to the NH stretch during thermal treatment, as mentioned above. The examination of the amide-II band below the imidization onset temperature can also indicate the status of the imidization-ready precursor. The amide-II band appears as two split peaks at 1545 and 1536 cm-1, as shown in Figure 4. The relative intensities of these peaks vary with temperature. The amide linkages in the precursor polymer form complexes with residual NMP solvent via hydrogen-bonding. The high wavenumber peak of the amideII band, 1545 cm-1, is attributed to complexed amide linkages,

a The centrosymmetric C-H bending vibration. bThe centroantisymmetric C-H bending vibration.

whereas the low wavenumber peak at 1536 cm-1 is assigned to free amide linkages.37 When the temperature is increased, the higher wavenumber peak (complexed amide linkages) weakens and becomes negligible, whereas the low wavenumber peak (free amide linkages) becomes more intense, and then this peak (peak b in Figure 4) shifts further to slightly lower wavenumbers, 1532 cm-1 (peak c in Figure 4), at 210 °C. When imidization occurs above 210 °C, peak c weakens and disappears due to the conversion of amide linkages to imide rings. Thus, we conclude that the amide linkages undergo imidization through a two-step process: first “precursor polymer-NMP solvent” decomplexation occurs, and then imide ring formation occurs. The changes in the integrated absorbance for the peaks due to the mixing of C-H in-plane bending vibrations with C-C benzene vibrations (see Scheme 2) at 1013 and 1167 cm-1 are plotted in Figure 5. These two peaks are assigned to the centroantisymmetric (1013 cm-1) and centrosymmetric (1167 cm-1) modes of the para-disubstituted aromatic ring. With

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Figure 6. Representative time-resolved WAXD patterns during 2.0 °C/min of thermal heating from 50 to 390 °C.

increases in temperature, the intensity of the peak due to the centroantisymmetric mode decreases slightly up to 210 °C, and then drops drastically in the range 210-306 °C. In the centroantisymmetric mode, the benzene ring is stretched or compressed following the centroantisymmetric movement of carbon and hydrogen (Scheme 2) because the 2- and 3-positioned hydrogens in the ring tend to move clockwise while the 5- and 6-positioned hydrogens tend to move counterclockwise, a motion that is favorable in the flexible PAE precursor. Thus, once the imide ring is formed this motion will be restricted. In contrast, the integrated absorbance of the centrosymmetric mode is very weak from the beginning and remains unchanged until imidization commences at 220 °C. This weak absorption peak is due to the para nitrogen and oxygen substituents on the aromatic ring. Indeed, this mode is infrared inactive in parasubstituted benzene with identical substituents. Above 220 °C, the intensity of this peak increases abruptly up to 320 °C. Because the carbon and hydrogen atoms of the benzene ring tend to move centrosymmetrically in this mode, the increase in the intensity of this absorption peak could result from the improved delocalization of electrons along the imidized polymer chains. Time-Resolved WAXD Analysis. With the elevation of the temperature up to 390 °C at 2.0 °C/min, time-resolved WAXD measurements in transmission mode were carried out for the p-PMDA-4,4′-ODA PAE precursor. Representative diffraction patterns are shown in Figure 6. At room temperature, the PAE precursor film produces two characteristic diffraction peaks (peak-1 and an amorphous halo), centered at 2θ ) 6.97° (dspacing ) 13.8 Å) and 2θ ) 20.52° (d-spacing ) 4.5 Å), respectively. The d-spacing of peak-1 corresponds to the chemical repeat unit of the precursor polymer, depending on the molecular conformation. This result indicates that shortranged molecular ordering along the molecular chain is present. This ordering may result from a conformation of the paraisomeric precursor polymer that is favorable to the formation of an ordered structure. The other broad peak at 2θ ) 20.52° is an amorphous halo, with a d-spacing that arises from the mean distance between the molecular chains. These results suggest that the precursor polymer consists of two phases, i.e., shortrange-ordered and amorphous phases. A similar structure has been reported previously for PMDA-4,4′-ODA PAA in dried films.38 At 390 °C, the polyimide film produces several semicrystalline diffraction peaks in the range 3-30° (2θ): 5.56°, 10.93°, 14.91°, 16.09°, 20.60°, and 25.65°. These diffraction peaks can be assigned according to the structural refinement analysis used previously.39 The crystalline structure of PMDA4,4′-ODA polyimide is known to be orthorhombic. In particular, the (002) diffraction at 5.58° (peak-2, d-spacing ) 16.52 Å)

Shin and Ree

Figure 7. Selected WAXD patterns in low angle region during thermal imidization, where the characteristic diffraction peaks (namely, Peak-1 and Peak-2) of p-PMDA-4,4′-ODA PAE and its polyimide are shown.

corresponds to the chemical repeat unit of polyimide. The calculated Lc of the (002) diffraction is 97 Å according to the Scherrer equation, which corresponds to approximately 6 chemical repeat units. Furthermore, the (004) and (210) diffraction peaks indicate the presence of a semicrystalline phase in the polyimide film. From the diffraction peaks, the orthorhombic unit cells of the crystalline phase were estimated to have a lattice dimension, a ) 6.23 Å, b ) 4.71 Å, and c ) 33.04 Å. The polyimide film cooled to room temperature was found to reveal almost the same lattice parameters as those observed for the film at 390 °C. The determined lattice parameters are slightly different from those of the oriented films and fibers prepared from the PMDA-4,4′-ODA PAA precursor.39 When we combined these results with the structural information presented above, the time-resolved WAXD patterns were analyzed further to investigate the structural evolution occurring in the precursor polymer during the thermal treatment. Figure 7 shows the diffraction profiles for the range 4.5-11° (2θ), in which peak-1 is decreasing in intensity with increasing temperature while peak-2 is increasing in intensity. The intensity of peak-1 becomes stronger up to 194 °C, which is indicative of the increased molecular rearrangement and ordering of precursor chains due to the thermal increase in mobility. Indeed, in the MDSC nonreversing heat flow measurements (see Figure 1), exothermic heat flow was observed in the range 116166 °C, which is interpreted as due to the ordering of the precursor polymer chains. Above 194 °C, peak-1 decreases in intensity and returns to its original level at 220 °C, and further decreases in intensity above that temperature. In the range 278301 °C, there is little change in the diffraction pattern. Above 301 °C, the intensity of peak-2 increases steeply, reflecting the crystalline structure evolution of the polyimide chains. The formation of crystalline structure at very high temperatures seems to be significantly influenced by the catenation of the precursor polymer. For example, the meta-isomer of PMDA4,4′-ODA poly(amic diethyl ester), which has a highly kinked structure,40 does not exhibit a crystallization peak in the MDSC nonreversing heat flow or a big increase in the intensity of the X-ray diffraction pattern at high temperatures (data not shown here). The two characteristic diffraction peaks (peak-1 and peak-2) of the precursor polymer and polyimide were separated from the WAXD patterns, and their integrated intensity is plotted as a function of temperature in Figure 8. The intensity of peak-1 increases with increasing temperature up to 180 °C and then decreases; during this increase, it exhibits two different slope regions, one up to 126 °C and the other up to 180 °C. This growth in peak-1 also indicates that enhancement of the structural ordering in the precursor polymer is occurring due to the thermal increase in mobility of the polymer chains. In fact,

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J. Phys. Chem. B, Vol. 111, No. 50, 2007 13899 Lc is 74 Å at 238 °C, increases very slowly with increases in temperature up to 290 °C, and thereafter rapidly increases, reaching 96.7 Å at 380 °C. Then, it remains at that level up to 390 °C. Values for the d-spacing and Lc are retained through almost the entire cooling process. Conclusions

Figure 8. Variations of the diffraction peaks (area) representing p-PMDA-4,4′-ODA PAE (Peak-1) and its polyimide (Peak-2) as a function of temperature.

Figure 9. Variations of the d-spacing and coherence length Lc of Peak-2 during thermal imidization and subsequent cooling process.

the precursor film contains residual NMP (8.3 wt %) solvent molecules complexed with the reactive groups of the precursor polymer. On heating, the NMP molecules partly decomplex from the precursor polymer and are evaporated out, so the precursor chains gain sufficient mobility for structural ordering to occur. Above the imidization onset temperature, 210 °C, the intensity of peak-1 drops sharply with increasing temperature and levels off at approximately 254 °C, at which point 51.3% of imidization has occurred. Above this temperature, the peak intensity slowly declines further up to approximately 380 °C. However, this peak is still detectable even near 390 °C, which suggests the persistence of some short-range ordered structure of the precursor polymer resulting from incomplete rearrangement to the crystalline structure. In contrast, the characteristic diffraction peak of the polyimide, peak-2, first appears at 238 °C, which is 28 °C higher than the imidization onset temperature, at which point 22.5% of imidization has occurred. However, the intensity of peak-2 increases very slowly up to 310 °C at which point XImidization ≈ 98.3%. Thereafter, this peak drastically increases in intensity with increases in temperature up to 370 °C. Apart from these changes in the peak intensity, the peak shape becomes narrower and the peak position shifts to the low angle region, particularly in the range 310-380 °C. The d-spacing and coherence length Lc of peak-2 were calculated and plotted as a function of temperature in Figure 9a,b, respectively. The d-spacing is 15.62 Å at 238 °C, and then it increases slowly up to 310 °C and thereafter rapidly increases with increasing temperature, reaching 16.57 Å at 374 °C. Then, it remains unchanged up to 390 °C.

In situ FT-IR, synchrotron WAXD, and MDSC analyses were carried out in detail for p-PMDA-4,4′-ODA PAE thin films during their thermal imidization and subsequent cooling. These analyses provide important information about the thermal imidization mechanism of the precursor polymer and the mechanism of imidization-induced structure evolution in the thin precursor film. The p-PMDA-4,4′-ODA PAE precursors in the film were found to undergo a two-step imidization reaction; that is, the amide linkages in the precursor polymer decomplex from the residual NMP solvent molecules first, and then undergo imide ring formation. The imidization reaction of the precursor polymers commences at 210 °C at a heating rate of 2.0 °C/min and continues up to 320 °C. The maximum rate of imidization was found to occur at 240.9 °C. The thermal imidization reaction was found to result in structural evolution in the film via a three-step process, namely initiation, the first crystallization, and the second crystallization. The initiation step, which is necessary to increase the chain mobility of the precursor polymer chains prior to crystalline structure evolution, requires thermal heating to at least 238 °C, at which point 22.5% imidization has occurred. Thereafter, the first crystallization commences at 238 °C and continues slowly up to 310 °C, at which point XImidization ) 98.3%. Above 310 °C, the second crystallization begins and continues up to 380 °C, at which point the imidization reaction is almost complete. The second crystallization seems to be related to the glass transition of the imidized film, and further to the catenation of the precursor polymer. Thus, although the polymer chain motion is still restricted by chain stiffness above the glass transition temperature of the polyimide, the second crystallization occurs with the aid of the intrinsic ordering of the paraisomeric precursors. Acknowledgment. This study was supported by Korea Science and Engineering Foundation (National Research Laboratory Program and Centre for Integrated Molecular Systems) and by the Ministry of Education (BK 21 Program). Synchrotron WAXD measurements at the Pohang Accelerator Laboratory were supported by the Ministry of Science and Technology and the POSCO. References and Notes (1) (a) Czornyj, G.; Chen, K. J.; Pradasilva, G.; Arnold, A.; Souleotis, H.; Kim, S.; Ree, M.; Volksen, W.; Dawson, D.; DiPietro, R. Proc. Electron. Compon. Technol. Conf. 1992, 42, 682. (b) Pyo, S. M.; Kim, S. I.; Shin, T. J.; Ree, M.; Park, K. H.; Kang, J. S. Macromolecules 1998, 31, 4777. (c) Kim, S. I.; Ree, M.; Shin, T. J.; Lee, C.; Woo, T.-H.; Rhee, S. B. Polymer 2000, 41, 5173. (d) Chang, H.; Kim, K.; Ree, M.; Lee, K.-W. Macromol. Chem. Phys. 1999, 200, 422. (e) Kim, Y.; Ree, M.; Chang, T.; Ha, C. S.; Nunes, T. L.; Lin, J. S. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 2075. (f) Shin, T. J.; Lee, B.; Youn, H. S.; Lee, K.-B.; Ree, M. Langmuir 2001, 17, 7842. (g) Yu, J.; Ree, M.; Shin, T. J.; Wang, X.; Cai, W.; Zhou, D.; Lee, K.-W. Polymer 2000, 41, 169. (h) Pyo, S. M.; Kim, S. I.; Shin, T. J.; Park, Y. H.; Ree, M. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 937. (i) Ree, M.; Kim, K.; Woo, S. H.; Chang, H. J. Appl. Phys. 1997, 81, 698. (j) Kim, K.; Ree, M. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 1755. (2) (a) Matsuda, S.; Yasuda, Y.; Ando, S. AdV. Mater. 2005, 17, 2221. (b) Matsuura, T.; Ando, S.; Sasaki, S.; Yamamoto, F. Macromolecules 1994, 27, 6665. (c) Ando, S.; Matsuura, T.; Sasaki, S. Macromolecules 1992, 25,

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