METHOD FOR PREPARING POLYMER THIN FILM BY GAS-LIQUID INTERFACE PLASMA POLYMERIZATION AND POLYMER THIN FILM PREPARED BY THE SAME

Abstract

The present invention relates to a method for preparing a plasma polymer thin film excellent in thermal properties and thus suitable for the matrix of a gel polymer electrolyte, a plasma polymer thin film prepared by the method, and a gel polymer electrolyte and a secondary cell using the plasma polymer thin film. More specifically, the present invention relates to a method for preparing a polymer thin film by plasma polymerization in which plasma is applied to an interface of a liquid-state monomer to perform polymerization, a polymer thin film prepared by the method, and a gel polymer electrolyte and a secondary cell using the polymer thin film.

Claims

1. A method for preparing a polymer thin film, the method comprising: applying plasma to an interface of a liquid-state monomer to perform polymerization.

2. The method as claimed in claim 1, further comprising: applying the liquid-state monomer on a substrate before the step of applying plasma; and exfoliating a plasma-polymerized polymer from the substrate after the step of applying plasma.

3. The method as claimed in claim 1, wherein the liquid-state monomer is a mixture of ionic liquid and polyethylene oxide.

4. The method as claimed in claim 3, wherein the ionic liquid is a salt comprising a cation being a substituted or unsubstituted 1-R-1-methylpyrrolidium or a substituted or unsubstituted 1-R-3-methylimidazolium, wherein R is C.sub.3-C.sub.16 alkyl, and an anion being BF.sub.4.sup.−, F.sup.−, Cl.sup.−, Br.sup.−, or I.sup.−.

5. The method as claimed in claim 3, wherein the polyethylene oxide has a molecular weight of 200 to 2,000.

6. The method as claimed in claim 3, wherein the polyethylene oxide is ##STR00001## or Tween 80, wherein n=5˜30.

7. The method as claimed in claim 6, wherein the content of the polyethylene oxide is 25 mol % or lower.

8. The method as claimed in claim 2, wherein the liquid-state monomer is a mixture of ionic liquid and polyethylene oxide.

9. The method as claimed in claim 8, wherein the ionic liquid is a salt comprising a cation being a substituted or unsubstituted 1-R-1-methylpyrrolidium or a substituted or unsubstituted 1-R-3-methylimidazolium, wherein R is C.sub.3-C.sub.16 alkyl, and an anion being BF.sub.4.sup.−, F.sup.−, Cl.sup.−, Br.sup.−, or I.sup.−.

10. The method as claimed in claim 8, wherein the polyethylene oxide has a molecular weight of 200 to 2,000.

11. The method as claimed in claim 8, wherein the polyethylene oxide is ##STR00002## or Tween 80, wherein n=5˜30.

12. The method as claimed in claim 11, wherein the content of the polyethylene oxide is 25 mol % or lower.

13. A plasma polymer thin film prepared by the method as claimed in claim 1.

14. A polymer matrix for a gel polymer electrolyte comprising the plasma polymer thin film as claimed in claim 13.

15. A gel polymer electrolyte comprising an organic electrolyte containing an ionic salt, the organic electrolyte being impregnated into the plasma polymer as claimed in claim 13.

16. A secondary cell comprising the gel polymer electrolyte as claimed in claim 15.

17. A plasma polymer thin film prepared by the method as claimed in claim 2.

18. A polymer matrix for a gel polymer electrolyte comprising the plasma polymer thin film as claimed in claim 17.

19. A gel polymer electrolyte comprising an organic electrolyte containing an ionic salt, the organic electrolyte being impregnated into the plasma polymer as claimed in claim 17.

20. A secondary cell comprising the gel polymer electrolyte as claimed in claim 19.

Description

BRIEF DESCRIPTIONS OF DRAWINGS

[0029] FIG. 1 is a picture showing a polymer thin film being formed over time in accordance with one embodiment of the present invention.

[0030] FIG. 2 shows SEM images of the cross-section of the polymer thin film produced according to the reaction time in one embodiment of the present invention and a graph plotting the thickness of the thin film as a function of the reaction time.

[0031] FIG. 3 shows an SEM image of the cross-section of the polymer thin film produced according to the proportion of Triton X-100 in one embodiment of the present invention and a graph plotting the thickness of the thin film as a function of the proportion of Triton X-100.

[0032] FIG. 4 is an enlarged graph plotting the thickness of the thin film as a function of the proportion of Triton X-100 in a region having a low content of Triton X-100.

[0033] FIG. 5 shows (a).sup.13C MAS-NMR, (b).sup.1H-MAS-NMR (at 15 kHz) and (c) FTIR spectra of the polymer thin film according to one embodiment of the present invention.

[0034] FIG. 6 shows an IR spectrum of the polymer thin film as a function of the content (mol. %) of Triton X-100 according to one embodiment of the present invention.

[0035] FIG. 7 shows an XPS spectrum of the polymer thin film according to one embodiment of the present invention and a graph plotting the ratio of elements in the polymer thin film as calculated from the XPS spectrum.

[0036] FIG. 8 shows spectra enlarging peaks corresponding to the is electrons of C and F as a function of the content (mol. %) of Triton X-100 in the polymer thin film according to one embodiment of the present invention.

[0037] FIG. 9 shows a spectrum presenting the simulation results of C1s peaks of FIG. 8.

[0038] FIG. 10 shows DSC and TGA spectra of the polymer thin film according to one embodiment of the present invention.

[0039] FIG. 11 is a graph plotting the impedance of a pouch cell prepared using the polymer thin film according to one embodiment of the present invention.

[0040] FIG. 12 is a graph plotting the ion conductivity of the polymer thin film calculated from the impedance of the pouch cell of FIG. 11.

DETAILED DESCRIPTION

[0041] Reference will now be made in detail to the accompanying drawings, prior experiments and embodiments of the present invention. It will be understood that the accompanying drawings and embodiments are provided to give the better understanding on the contents and scope of the technical conceptions of the present invention and not to limit or change the technical scope of the present invention. It is apparent to those skilled in the art that various modifications and changes may be covered within the scope of the technical conceptions of the present invention based on the examples of the present invention.

Example 1: Preparation of Plasma Polymer Thin Film

[0042] (1) Preparation of Plasma Polymer Thin Film from Polyethylene Oxide and Ionic Substance

[0043] Triton X-100 (Sigma-Aldrich, USA) was added to [BMIM]BF.sub.4 (1-butyl-3-methylimidazolium tetrafluoroborate, Sigma-Aldrich) to reach the final Triton X-100 concentration of 6 mol. %, and the mixture was agitated with a vortex mixer (Vortex Mixer-KMC-1300V) for 5 minutes. 0.5 ml of the resultant solution was spin-coated on a 20×20 mm glass plate at 500 rpm for 15 seconds using a spin-coater (SPIN-1200D, MIDAS). Subsequently, a room-pressure plasma system (Ar, 150 W, 3 lpm) was used to perform polymerization for 10 minutes. The distance between the plasma electrode and the spin-coated thin film was 2 mm. The plasma-treated glass plate was immersed in ethanol to separate the thin film from it, and the thin film was washed with acetone and distilled water in sequence and dried out at 60° C. for one hour.

[0044] FIG. 1 is a picture showing a polymer thin film being formed over time. A visual observation shows that the thicker opaque thin film is created as the plasma application time increases.

[0045] Table 1 presents the results of the plasma polymerization under the above-specified conditions using different types of ionic substances and polyethylene oxide. In Table 1, [BMIM]BF.sub.4 is 1-butyl-2,3-dimethylimidazolium tetrafluoroborate, and EMPyrr BF.sub.4 is 1-ethyl-1-methylpyrrolidinium tetrafluoroborate.

TABLE-US-00001 TABLE 1 Triton X- Triton X- Terpineol 100 200 Tween 20 [BMIM]BF.sub.4 No rxn Film formed Film formed Film formed [BMIM]Cl No rxn Film formed Film formed Film formed [BMIM]TFSI No rxn No rxn No rxn No rxn [BMIM]Br No rxn Film formed Film formed Film formed [BMIM]BF.sub.4 No rxn Film formed Film formed Film formed EMPyrr BF.sub.4 No rxn No rxn No rxn No rxn HCl No rxn Polymerized Polymerized Polymerized HAuCl.sub.4 No rxn Polymerized Polymerized Polymerized Triton X- No rxn No rxn No rxn No rxn 100

[0046] As can be seen from Table 1, terpineol other than polyethylene oxide did not participate in the polymerization reaction irrespective to the type of the ionic substance. And no polymerization reaction occurred irrespective to the type of the polyethylene oxide when the ionic substance was not added. EMPyrr BF.sub.4, which is an inorganic salt other than an ionic substance did not participate in the polymerization reaction, either. When the ionic substance was an inorganic acid HCl or an inorganic salt HAuCl.sub.4, a polymer was formed by the polymerization reaction but obtained in the form of powder or cake rather than a film. In contrast, the use of an imidazolium salt, that is, an ionic liquid together with polyethylene oxide resulted in polymerization reaction to form a polymer thin film. [BMIM]TFSI did not participate in the polymerization reaction, which is assumedly due to the presence of TFSI that eliminates radicals participating in the polymerization reaction.

[0047] (2) Preparation of Plasma Polymer Thin Film According to the Varied Reaction Time

[0048] The procedures were performed in the same manner as described in the preparation method (1), excepting that the reaction time was controlled between 1 to 30 minutes. Triton X-100 and [BMIM]BF.sub.4 were plasma-polymerized. The polymer thin film thus obtained was separated and its cross-section was observed with a scanning electron microscope (SEM, JEOL, JSM-7000F, USA). The observation results are presented in FIG. 2. Referring to FIG. 2, (a) to (d) show SEM images of the polymer thin film formed by the plasma polymerization reaction for 1, 2, 6, and 10 minutes, respectively; and (e) is a graph plotting the thickness of the thin film as a function of the reaction time.

[0049] As can be seen from the images and graph of FIG. 2, the thickness of the plasma polymer thin film increased in proportion to the reaction time in the early stage and then remained constant even with an increase in the reaction time once the spin-coated precursor was all polymerized with an elapse of the reaction time.

[0050] (3) Preparation of Plasma Polymer Thin Film According to the Different Ratios of Ionic Liquid to Polyethylene Oxide

[0051] The procedures were performed in the same manner as described in the preparation method (1), excepting that the content (mol. %) of Triton X-100 was controlled between 0.3 to 48 mol. %. Triton X-100 and [BMIM]BF.sub.4 were plasma-polymerized for 6 minutes (at air flow of 5 lpm). The polymer thin film thus obtained was separated and its cross-section was observed with a scanning electron microscope (SEM). The observation results are presented in FIG. 3. Referring to FIG. 3, (a) to (g) show SEM images of the cross-section of the polymer thin film formed by the 6-minute plasma polymerization reaction using 0.3, 0.7, 1.5, 3, 6, 12, and 24 mol. % of Triton X-100, respectively; and (h) is a graph plotting the thickness of the thin film as a function of the proportion of Triton X-100. As can be seen from FIG. 3, the molar ratio of the ionic liquid to polyethylene oxide affects the thickness of the thin film. FIG. 4 is a graph enlarging the interval where the content of Triton X-100 is 0 to 3 mol. %, revealing that the thickest film can be produced when the content of Triton X-100 is lowest as much as 1.5 mol. %.

Example 2: Analysis of Structure of Plasma Polymer Thin Film

[0052] The plasma polymer thin film prepared in Example 1 was analyzed in regards to the structure with solid-NMR (Agilent 400 MHz, 54 mm, NMR DD2, USA), IR (Nicolet 670, USA) and XPS (Thermo Scientific MultiLab 2000) and to the thermal properties with a thermogravimeter (TGA/DSC1, Mettler-Toledo Inc.). In the following embodiment, the plasma polymers of Triton X-100 and [BMIM]BF.sub.4 were used as species in the analysis. Unless otherwise stated, the species for the analysis are the polymers obtained from a 10-minute plasma polymerization reaction using 6 mol. % of Triton X-100 and [BMIM]BF.sub.4 according to the preparation method described in section (1) of Example 1. The instruments used for the analysis are as follows.

[0053] (1) Structure Analysis Using Solid NMR and FT-IR

[0054] FIG. 5 shows (a).sup.13C MAS-NMR, (b).sup.1H-MAS-NMR (at 15 kHz) and (c) FT-IR spectra of the polymer thin film. It can be seen from the IR spectrum that the C—H peak for the imidazolium ring of the plasma polymer is weak and that C═C and C═O bonds are formed.

[0055] FIG. 6 is an IR spectrum of the polymer thin film as a function of the content (mol. %) of Triton X-100. In FIG. 6, the peak areas for C═O and C═C bonds are enlarged. Table 2 presents the relative intensity (I.sub.1660/I.sub.1725) of peaks representing C═C bond (1660 cm.sup.−1) and C═O bond (1725 cm.sup.−1) according to the content (mol. %) of Triton X-100.

TABLE-US-00002 TABLE 2 Triton X-100 (mol. %) I.sub.1660/I.sub.1725 1.5 0.92 3 1.06 6 1.15 12 1.88 24 1.86

[0056] As can be seen from FIG. 6 and Table 2, the proportion of the C═O bond relative to the C═C bond in the polymer decreases with an increase in the content of Triton X-100, and the red-shift of the C—O—C bond implicitly shows the presence of a double bond capable of being conjugated into the C—O—C bond.

[0057] (2) Structure Analysis Using X-Ray Photoelectron Spectroscopy (XPS)

[0058] Referring to FIG. 7, (a) shows an XPS spectrum of the polymer thin film and (b) presents a graph plotting the ratio of elements in the polymer thin film as a function of the content (mol. %) of Triton X-100 as calculated from the XPS spectrum. The proportions (%) of the elements and their ratios in the plasma polymer according to the content (mol. %) of Triton X-100 are presented in Tables 3 and 4, respectively.

TABLE-US-00003 TABLE 3 Atomic percentage (%) Triton X-100 (mol. %) C N O F B 1.5 69.91 2.55 22.55 2.78 2.21 3 73.18 1.28 21.65 1.69 1.28 6 74.51 1.24 21.96 1.39 0.90 12 75.56 0.92 21.75 1.04 0.74 24 75.30 0.99 21.27 1.20 1.23

TABLE-US-00004 TABLE 4 Triton Atomic percentage (%) X-100 (mol. %) O/C F/C N/C B/C 1.5 0.322558 0.039765 0.036475 0.031612 3 0.295846 0.023094 0.017491 0.017491 6 0.294726 0.018655 0.016642 0.012079 12 0.287851 0.013764 0.012176 0.009794 24 0.282470 0.015936 0.013147 0.016335

[0059] It can be seen that as the content of [BMIM]BF.sub.4 decreases relatively with an increase in the content of Triton X-100, the contents of F, N and B in the polymer decrease, where F, N and B are contained only in [BMIM]BF.sub.4. Further, the reduction of the O/C ratio with an increase in the content of Triton X-100 assumedly has a close relation with the C═C/C═O bond ratio in FIG. 6 and Table 2. Namely, the cross-linking between Triton X-100 and Triton X-100 rather than between the ionic liquid and Triton X-100 increases with an increase in the content of Triton X-100, so it can be assumed that the oxygen atoms are eliminated in the form of CO or CO.sub.2 during the cross-linking process to reduce the O/C ratio.

[0060] Referring to FIG. 8, (a) and (b) show the spectra enlarging the peaks corresponding to the is electrons of C and F as a function of the content (mol. %) of Triton X-100, respectively.

[0061] The peak for the is electron of C shifts to the lower energy according to the content of Triton X-100. Hence, the composition of the C1s peak of the plasma polymer prepared using 1.5 mol. % or 24 mol. % of Triton X-100 is analyzed in consideration of the types of the bonds forming the polymer (Plasmas and Polymers, Vol. 7, No. 4, p311-325, December 2002). FIG. 9 shows a spectrum presenting the simulation results of peaks. The proportions of the peaks are presented in Table 5. It can be seen from the analytical results that the C═O and C—F bonds greatly decreases and the proportion of the C—C bond increases with an increase in the content of Triton X-100. In addition, the ratio of C═C/C═O is almost doubled and becomes in agreement with the results obtained from the ratio of peak intensities from the IR spectrum.

TABLE-US-00005 TABLE 5 Triton X-100 % (mol. %) C═C C—C C—O—C C═O C—F C═C/C═O 1.5 5.07 27.48 40.67 14.02 12.76 0.36   (286 eV) (286.7 eV) (288.2 eV) (288.9 eV) (290 EV) 24 5.08 41.44 36.88  8.08  7.82 0.72 (285.6 eV) (286.4 eV) (287.9 eV) (288.2 eV) (288.7 eV) 

[0062] (3) Thermal Analysis

[0063] The polymer thin film obtained from the plasma polymerization was analyzed with a thermogravimeter. FIG. 10 presents DSC and TGA spectra of the polymer thin film heated from 25° C. up to 1,000° C. at a rate of 10° C./min, showing that the polymer has such a high thermal stability as to display a degradation temperature of 200° C. or above. Further, the Tg and Tm values measured from the DSC spectrum were 3.11° C. and 279.50° C., respectively.

[0064] The conventional gel polymer electrolytes display poor durability at high temperature, as they have such a low Tm value as much as 40 to 50° C. for PEO and 160° C. for PVDF or PMMA. But the plasma polymer of the present invention has a Tm value as high as about 300° C., so the driving temperature of the equipment using it increases relative to that of equipment using the conventional gel polymer electrolyte.

Example 3: Analysis on Electrical Properties of Plasma Polymer Thin Film

[0065] The plasma thin film prepared in Example 1 was inserted into a nickel-ion battery to complete a thin film type cell, in order to measure its electrical properties. 0.5 ml of 1M LiPF6/DMC as an electrolyte was added, and the tightly sealed specimen was stabilized at 150° C. for 3 seconds before use. With the cell connected to a potentiostat (IVIUMSTAT, Ivium Technologies) using a lead wire, the resistance value of the specimen was measured according to the alternating current impedance method. FIG. 11 is a graph plotting the impedance measurements. The ion conductivity (.Math.ò) was determined from the resistance value (R.sub.b) and thickness (,) of the specimen calculated from the graph and the area (A) of the polymer electrolyte according to the following equation. The results are presented in Table 12.

[00001]$\sigma =\frac{l}{R_b.Math.A}$

[0066] FIG. 12 shows that the electrical conductivity decreases with an increase in the content of Triton X-100. This is in good agreement with the predictions induced from the analytical results of the IR and XPS spectra that the C═C/C═O ratio increases with an increase in the content of Triton X-100 and those of the XPS spectrum that the proportion of polar bonds such as C═O or C—F reduces with an increase in the content of Triton X-100. In addition, the ion conductivity was so high as 10.sup.4 or greater when the content of Triton X-100 was 6% or below.