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Organosolubility and optical transparency of novel polyimides derived from20,70-bis(4-aminophenoxy)-spiro(fluorene-9,90-xanthene)

ShuJiang Zhang, YanFen Li,* Tao Ma, JiuJiang Zhao, XiangYang Xu, FenChun Yang and Xiao-Yan Xiang

Received 5th November 2009, Accepted 13th December 2009

First published as an Advance Article on the web 11th January 2010

DOI: 10.1039/b9py00339h

A novel spiro(fluorene-9,90-xanthene) skeleton bis(ether amine) monomer, 20,70-bis(4-aminophenoxy)-

spiro(fluorene-9,90-xanthene), was prepared through a simple acid-catalyzed condensation reaction of

9-fluorenone with resorcinol to form the spiro framework through an sp3 carbon atom. Subsequent

nucleophilic substitution reaction of spiro[fluorene-9,90-(20,70-dihydroxyxanthene)] with 1-fluoro-

4-nitro-benzene in the presence of potassium carbonate in N,N-dimethylacetamide, followed by

catalytic reduction with hydrazine and Pd/C in ethanol. A series of new polyimides were synthesized

from the diamine with various commercially available aromatic tetracarboxylic dianhydrides via

a conventional two-stage process with the thermal or chemical imidization of the poly(amic acid)

precursors. Most of the polyimides obtained from both routes were soluble in many organic solvents

such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide and m-cresol. All the polyimides could

afford transparent, flexible, and strong films with low moisture absorptions of 0.36–0.73% and low

dielectric constants of 2.65–3.12 at 1 kHz. Thin films of these polyimides showed an UV-vis

absorption cutoff wavelength at 352–409 nm, and those of polyimides from 4,40-oxydiphthalic

dianhydride and 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) were

essentially colorless. The polyimides exhibited excellent thermal stability, with decomposition

temperatures (at 10% weight loss) above 510 �C in both air and nitrogen atmospheres and glass

transition temperatures (Tg) in the range 296–346 �C.

1. Introduction

Aromatic polyimides are well known as high performance

polymeric materials because of their good chemical resistance,

high mechanical strength, and excellent thermal stability.1–3

Incorporation of specific functionality into the polyimide back-

bones leads to various advanced functional materials that exhibit

certain advantageous properties, such as electrochromic, gas

separation, charge-transporting, nonlinear optical, highly

refractive, and photosensitive properties.4–9 However, their

applicability has been limited because aromatic polyimides are

normally insoluble and fusible in the fully imidized form because

of their rigid-chain characteristics, leading to processing diffi-

culties. Thus, polyimide processing is generally carried out with

a poly(amic acid) intermediate and then is converted to a poly-

imide via rigorous thermal treatment. Problems often arise

because the poly(amic acid)s are thermally and hydrolytically

unstable. To overcome these problems, polymer structure

modification becomes necessary. Therefore, several modification

of the chemical structure have been made to enhance their

processability and solubility while other advantageous polymer

properties are retained either by introducing flexible linkages,10–16

bulky substituents,17–20 noncoplannar structures,21–23 and

spiroskeletons24,25 into the polymer backbone.

State Key Laboratory of Applied Organic Chemistry, Institute ofBiochemical Engineering & Environmental Technology, College ofChemistry and Chemical Engineering, Lanzhou University, Lanzhou,730000, China. E-mail: liyf@lzu.edu.cn

This journal is ª The Royal Society of Chemistry 2010

It has been demonstrated that incorporating a spiroskeletons

linkage into the structure of small molecules, as well as polymeric

materials, leads to amorphous materials with an improvement in

both solubility and thermal stability.26–28 In the spiro-segment,

the rings of the connected spiroskeletons are orthogonally

arranged and connected via a common tetra-coordinated

carbon,29,30 and the polymer chains were twisted at an angle of 90�

at each spiro-center. This structural feature was predicted to

restrict the close packing of the polymer chains, thereby reducing

the probability of interchain interactions, resulting in higher

polymer solubility. Moreover, for the spiro-annulated segment,

the rigidity of the polymer backbone would be preserved. Such

spiro structures have also been applied to polymeric materials

such as polyfluorenes,31–33 polyquinolines,34 and polyimides35–40

to reduce crystallization tendency and to enhance their solubility,

glass-transition temperature (Tg), and thermal stability. More

importantly, the fluorene containing polymers have been proven

to exhibit high refractive indices due to their high aromatic

contents, so some of the fluorene-containing polymers have been

successfully utilized to make optical lenses.41 On the other hand,

the incorporation of bulky fluorene groups, which sterically

hinder the intermolecular interaction of the PI chains, reduces the

packing density, thus increases the transparency of the PIs.42,43

In this study, therefore, we attempted to synthesize a novel

diamine monomer by combining spiro fluorene and xanthene

moieties to afford good solubility and good thermal properties,

as well as good dielectric and optical properties. A novel mono-

mer, 20,70-bis(4-aminophenoxy)-spiro(fluorene-9,90-xanthene) (3),

was synthesized. This monomer was used to prepare polyimides

Polym. Chem., 2010, 1, 485–493 | 485

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with various commercially available aromatic tetracarboxylic

dianhydrides via a conventional two-step process of poly(amic

acid) preparation, which was followed by the thermal or chem-

ical imidization. The polyimides synthesized were characterized

by Fourier transform infrared (FTIR), NMR, gel permeation

chromatography (GPC), differential scanning calorimetry

(DSC), and thermogravimetric analysis (TGA), and their solu-

bility, water absorption and dielectric constant were also

measured.

2. Experimental

2.1 Materials

9-Fluorenone (Fluka), 1-fluoro-4-nitrobenzene (TCI, Japan),

resorcinol (TCI), potassium carbonate (K2CO3) (Fluka), 10%

palladium on charcoal (Pd/C) (Fluka), and hydrazine mono-

hydrate (Acros) were used as received. Commercially available

aromatic tetracarboxylic dianhydrides such as pyromellitic

dianhydride (PMDA) (4a) (Aldrich) and 3,30,4,40-benzopheno-

netetracarboxylic dianhydride (BTDA) (4c) (Aldrich) were

purified by recrystallization from acetic anhydride. 3,30,4,40-

Biphenyltetracarboxylic dianhydride (BPDA) (4b) (Oxychem),

and 4,40-oxydiphthalic dianhydride (ODPA) (4d) (Oxychem) and

2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride

(6FDA) (4e) (Hoechst Celanese) were heated at 250 �C under

vacuum for 3 h prior to use. N,N-Dimethylacetamide (DMAc)

was purified by distillation under reduced pressure over calcium

hydride and stored over 4 �A molecular sieves.

2.2 Characterizations

The inherent viscosities of the polymers were measured with an

Ubbelohde viscometer at 30 �C. Weight-average molecular

weights (Mw) and number-average molecular weights (Mn) were

obtained via gel permeation chromatography (GPC) on the basis

of polystyrene calibration using Waters 2410 as an apparatus and

tetrahydrofuran (THF) as the eluent. FT-IR spectra (KBr) were

recorded on a Nicolet NEXUS670 fourier transform infrared

spectrometer. 1H NMR and 13C NMR spectra were measured on

a JEOL EX-300 spectrometer using tetramethylsilane as the

internal reference. Elemental analyses were determined by

a Perkin–Elmer model 2400 CHN analyzer. Testing by differ-

ential scanning calorimetry (DSC) was performed on a Perkin–

Elmer differential scanning calorimeter DSC 7 or Pyris 1 DSC at

a scanning rate of 20 �C min�1 in flowing nitrogen (30 cm3 min�1),

and glass transition temperatures (Tg) were read at the DSC

curves at the same time. Thermogravimetric analysis (TGA) was

conducted with a TA Instruments TGA 2050, and experiments

were carried out on approximately 10 mg of sample in flowing air

(flowing rate ¼ 100 cm3 min�1) at a heating rate of 20 �C min�1.

The ultraviolet-visible (UV-vis) spectra were recorded on

a Hitachi U-3210 spectrophotometer. Dielectric property of the

polymer films was tested by the parallel-plate capacitor method

using a HP-4194A Impedance/Gain Phase Analyzer. Gold elec-

trodes were vacuum deposited on both surfaces of dried film.

Experiments were performed at 25 �C in a dry chamber.

Elemental analyses were performed on a Yanaco MT-6 CHN

recorder elemental analysis instrument. Wide-angle X-ray

diffraction measurements were performed at room temperature

486 | Polym. Chem., 2010, 1, 485–493

(about 25 �C) on a Siemens Kristalloflex D5000 X-ray diffrac-

tometer, using nickel-filtered Cu-Ka, radiation (l ¼ 1.5418 �A,

operating at 40 kV and 30 mA). Tensile properties were deter-

mined from stress-strain curves with a Toyo Instron UTM-III-

500 with a load cell of 10 kg at a drawing speed of 5 cm min�1.

Measurement were performed at 28 �C with film specimens

(about 0.1 mm thick,1.0 cm wide and 5-cm long) and average of

at least five individual determinations was used. The equilibrium

water absorption was determined by the weighing of the changes

in vacuum-dried film specimens before and after immersion in

deionized water at 25 �C for 3 days.

2.3 Monomer synthesis

2.3.1 Synthesis of spiro[fluorene-9,90-(20,70-dihydroxyxanth-

ene)](1). A mixture of 9-fluorenone (3.60 g, 20 mmol), rensorcinol

(8.80 g, 80 mmol), and ZnCl2 (1.11 g, 9.01 mmol) was stirred and

heated at 140 �C for 3 h. The melt was then heated with

concentrated HCl (aqueous, 50 mL), under refux for another 2 h.

The reaction mixture was poured into water (500 mL). The

precipitate was washed with water, dried, and purified by column

chromatography (EtOAc–hexane, 1 : 4) to yield 1(4.88 g, 67.0%);

mp¼ 266–268 �C (onset to peak top temperature), by differential

scanning calorimetry (DSC) at a scan rate of 2 �C min�1.1H NMR (acetone-d6): d6.13 (d, J ¼ 8.7 Hz, 2H, H3), 6.31

(dd, J ¼ 6.0, 2.7 Hz, 2H, H4), 6.67 (d, J ¼ 2.4 Hz, 2H, H1), 7.08

(d, J ¼ 7.8 Hz,2H, H9), 7.21 (dd, J ¼ 7.2, 7.6 Hz, 2H, H10), 7.41

(dd, J ¼ 7.2, 7.6 Hz, 2H, H11), 7.88 (d, J ¼ 7.8 Hz, 2H, H12),

8.49(s, 2H,–OH). 13C NMR (acetone-d6): 63.4 (C7), 113.3 (C1),

121.8 (C3), 126.4 (C5), 130.5 (C12), 135.9 (C11), 138.2 (C9), 138.8

(C10), 139.1 (C4), 150.1 (C13), 162.7 (C8), 166.1 (C6), 167.8 (C2).

FT-IR (KBr): 3504–3404 cm�1 (O–H), 3063–3033 cm�1(C–H),

1168 and 1260 cm�1(C–O). EI-MS (m/z):[M]+ calcd for

C25H16O3, 364; Found 364. Anal. Calcd for C25H16O3: C, 82.40;

H, 4.43; Found: C, 82.29; H, 4.71.

2.3.2 Synthesis of 20,70-bis(4-nitrophenoxy)-spiro(fluorene-

9,90-xanthene)(2). In a three-necked flask equipped with

a nitrogen inlet and a condenser, 7.28 g (20 mmol) of 1 was first

dissolved in 80 ml of DMAc, 5.78 g (42 mmol) of 1-fluoro-4-

nitrobenzene and 5.38 g (42 mmol) of potassium carbonate were

added subsequently. The mixture was heated at 110 �C for 12 h

with stirring under nitrogen, then was poured into 400 mL of

solution consisting of equal volumes of ethanol and water, and

yellowish solids precipitated out of the solution overnight. After

filtration, the residual reactants and potassium carbonate were

eliminated from the solids by washing with water, methanol and

ethanol consecutively. Finally, 20,70-bis(4-nitrophenoxy)-spiro-

(fluorene-9,90-xanthene) (2) with solids was collected and dried at

100 �C for 12 h. The crude 2 was purified by recrystallized from

acetone to afford white microcrystals (10.54 g, 87%), mp ¼ 163–

165 �C by DSC (2 �C min�1).

This journal is ª The Royal Society of Chemistry 2010

Scheme 1 Synthesis of the diamine monomer 3.

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1H NMR (acetone-d6): d6.46 (d, J ¼ 8.8 Hz, 2H, H3), 6.69

(dd, J ¼ 2.4,2.8 Hz, 2H, H4), 7.08 (d, J ¼ 2.4 Hz, 2H, H1), 7.18

(d, J ¼ 8.4 Hz, 4H, H15), 7.21 (d, J ¼ 7.2 Hz, 2H, H9), 7.31 (dd,

J¼ 7.6,7.2 Hz, 2H, H10), 7.45 (dd, 7.6,7.2 Hz, 2H, H11), 7.99 (d, J

¼ 7.2 Hz, 2H, H12), 8.25 (d, J ¼ 8.4 Hz, 4H, H16). 13C NMR

(acetone-d6): 80.4 (C7), 109.2 (C1), 116.6 (C3), 118.7 (C15), 121.2

(C12), 124.4 (C5), 126.2 (C11), 126.8 (C16), 129.2 (C10), 129.5 (C9),

130.4 (C4), 140.5 (C17), 144.1 (C13), 153.0 (C8), 155.5(C6), 155.8

(C2), 163.4 (C14). FT-IR (KBr): 3088–2956 cm�1 (C–H stretch),

1519 and 1348 cm�1(–NO2 stretch), 1260 and 1206 cm�1(C–O

stretch). EI-MS (m/z):[M]+ calcd for C37H22N2O7, 606; Found

606. Anal. Calcd for C37H22N2O7: C, 73.26; H, 3.66; N, 4.62;

Found: C, 71.29; H, 4.51; N, 4.41.

2.3.3 Synthesis of 20,70-bis(4-aminophenoxy)-spiro(fluorene-

9,90 -xanthene)(3). A mixture of the purified dinitro compound 2

(9.1 g, 15 mmol), 10% Pd/C (0.15 g), ethanol (150 mL), and

hydrazine monohydrate (15 mL) was heated at reflux tempera-

ture for about 12 h. The resulting clear, darkened solution was

filtered hot to remove Pd/C, and the filtrate was then distilled to

remove the solvent. The crude product was purified by recrys-

tallization from acetone/water to give pale-white powdery crys-

tals (7.04 g, 86%); mp ¼ 233–235 �C by DSC (2 �C min�1).1H NMR (CDCl3): d3.54 (br, 4H, –NH2), 6.26 (d, J ¼ 8.8 Hz,

2H, H3), 6.46 (dd, J¼ 6.0,2.4 Hz, 2H, H4), 6.61(d, J¼ 2.4 Hz, 2H,

H1), 6.64 (d, J ¼ 8.4 Hz, 4H, H16), 6.84 (d, J ¼ 8.4 Hz, 4H, H15),

7.14 (d, J ¼ 7.2 Hz, 2H, H9), 7.21(dd, J ¼ 7.6,7.2 Hz, 2H, H10),

7.33 (dd, J ¼ 7.6,7.2 Hz, 2H, H11), 7.75 (d, J ¼ 8.0 Hz, 2H, H12).13C NMR(CDCl3): d69.3 (C7), 104.4 (C1), 112.6 (C3), 116.1 (C15),

118.1(C12), 119.8 (C5), 121.5 (C16), 125.6 (C11), 127.7 (C10), 128.3

(C9), 128.7 (C4), 139.5 (C17), 143.6 (C13), 147.8 (C8), 151.9 (C14),

155.1 (C6), 158.8 (C2). FT-IR (KBr): 3428, 3352 cm�1 (N–H

stretch), 1248 and 1206 cm�1(C–O stretch). EI-MS (m/z): [M]+

calcd for C37H22N2O3, 546; Found 546. Anal. Calcd for

C37H22N2O3: C, 81.30; H, 4.79; N, 5.21; Found: C, 79.69; H,

4.31; N, 5.01.

2.3.4 Synthesis of polyimides. The polyimides were synthe-

sized from various dianhydrides and the diamine 3 via

a conventional two-step method. The synthesis of polyimide 6d is

used as an example to illustrate the general synthesis route used

to produce the polyimides. To a solution of 0.8199 g (1.50 mmol)

of diamine 3 in 15 mL of CaH2–dried DMAc in a 50 mL flask,

0.4653 g (1.50 mmol) of dianhydride ODPA was added in one

This journal is ª The Royal Society of Chemistry 2010

portion. Thus, the solid content of the solution is approximately

12 wt%. The mixture was stirred at room temperature overnight

(for about 24 h) to afford a highly viscous poly(amic acid)

solution. The inherent viscosity of the resulting poly(amic acid)

5d was 1.02 dL/g, measured in DMAc at a concentration of

0.5 g/dL at 30 �C. The poly(amic acid) was subsequently con-

verted to polyimide by either thermal or chemical imidization

process. For the thermal imidization process, about 4 g of the

obtained poly(amic acid) solution was poured into a 5 cm glass

culture dish, which was placed overnight in a 90 �C oven to

slowly release the casting solvent. The semi-dried poly(amic acid)

film was further dried and transformed into polyimide 6d by

sequential heating at 150 �C for 30 min, 200 �C for 30 min, and

250 �C for 1 h. The polyimide film was stripped from the glass

substrate by being soaked in water. The inherent viscosity of the

polyimide 6d was 1.09 dL/g in DMAc at a concentration of 0.5

g/dL at 30 �C. For the tensile test, dielectric and thermogravi-

metric analyses, the polyimide films were heated at 300 �C for

another 1 h. For the chemical imidization process, 3 mL of acetic

anhydride and 1 mL of pyridine were added to the remaining

poly(amic acid) solution, and the mixture was heated at 100 �C

for 1 h to effect a complete imidization. The resulting polyimide

solution was poured slowly into 250 mL of methanol giving rise

to a fibrous precipitate, which was washed thoroughly with

methanol and water, collected by filtration, and dried. The

inherent viscosity of chemically imidized 6d is 0.81 dL/g in

DMAc, measured at a concentration of 0.5 g/dL at 30 �C. FT-IR

(film): 1780, 1724 (imide C]O stretch), 1374 (imide C–N

stretch), 1261 and 1236 cm�1 (C–O stretch).

3. Results and discussion

3.1 Monomer synthesis

Three steps were used to synthesize the new diamine 3 which

contained a spiro(fluorene-9,90-xanthene) skeleton from 9-fluo-

renone as shown in Scheme 1. The key intermediate spiro-

(fluorene-9,90-(20,70-dihydroxyxanthene) (1) was prepared

through a simple acid catalyzed condensation reaction 9-fluo-

renone with resorcinol, using ZnCl2/HCl as a condensing agent,

to form the spiro framework.44,45 Subsequent the dinitro

compound 2 was synthesized by nucleophilic aromatic

substitution of 1 with 1-fluoro-4-nitrobenzene in the presence of

Polym. Chem., 2010, 1, 485–493 | 487

Fig. 2 1H and 13C NMR spectra of bisphenol compound 1 in acetone-d6.

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postassium carbonate as base in DMAc. The diamine 3 was

obtained in high purity and high yields by the catalytic reduction

of intermediate dinitro compound 2 with hydrazine monohydrate

and Pd/C catalyst in refluxing ethanol. Elemental analysis, IR,

and NMR spectroscopic techniques were used to identify struc-

tures of the intermediate compounds (1 and 2) and target diamine

monomer (3). The FT-IR spectra of compounds 1, 2 and 3 are

shown in Fig. 1. Bisphenol compound 1 displayed characteristic

absorption bands corresponding to –OH stretching at 3504–3404

cm�1 and –C–O–C– stretching band around 1260 cm�1. Dinitro

compound 2 displayed characteristic absorption bands corre-

sponding to asymmetric and symmetric NO2 stretching at 1519

and 1348 cm�1, respectively, which disappeared after reduction.

Diamine compound 3 exhibited typical N–H stretching bands at

3428 and 3352 cm�1. Both the dinitro and diamine compounds

showed the –C–O–C– stretching band around 1240 cm�1 con-

firming the presence of the aromatic ether linkage. Fig. 2 display

the 1H and 13C NMR spectra of bisphenol compound 1. In the

case of Fig. 2, all the aromatic protons of 1 resonated in the region

of 6.13–7.88 ppm and the protons of –OH were appeared at

farther downfield (8.49 ppm). In 13C NMR spectra, the central

spiro carbon (C7) signal resonate at 63.4 ppm indicative of the

presence of a spiro skeleton in 1 and the 13 main signals appeared

and the number of carbons was consistent with the structure.

Fig. 3 and 4 display the 1H and 13C NMR spectra of compounds 2

and 3, respectively. The NMR spectra confirmed that the nitro

groups had been converted into amino groups by the high field

shift of the aromatic protons and carbons. The signal at 3.54 ppm

corresponds to the primary aromatic amine protons. All the

spectroscopic data obtained were in good agreement with the

expected structures. The structure of the diamine was designed to

impart several desirable properties to polyimides.

3.2 Polymer synthesis

The polyimides 6a–6e were prepared from five commercially

available dianhydrides 4a–4e and the diamine 3 by a conven-

tional two-step synthesis method as shown in Scheme 2. As

Fig. 1 FTIR spectra of bisphenol compound 1, dinitro compound 2 and

diamine 3.

488 | Polym. Chem., 2010, 1, 485–493

shown in Table 1, the inherent viscosities of the intermediate

poly(amic acid)s ranged from 0.87 dL/g to 1.02 dL/g, as

measured in DMAc at 30 �C. The molecular weights of all the

poly(amic acid)s were sufficiently high to permit the casting of

flexible and tough poly(amic acid) films, which were subse-

quently converted into tough polyimide films by extended

Fig. 3 1H and 13C NMR spectra of dinitro compound 1 in acetone-d6.

This journal is ª The Royal Society of Chemistry 2010

Fig. 4 1H and 13C NMR spectra of diamine compound 3 in CD3Cl.

Scheme 2 Synthesis of the polyimides.

Table 1 Inherent viscosity of poly(amic acid)s and polyimides and thin film

Poly(amic acid) Polyimide GPC data for polyimides Ten

Code hinha (dL/g) Code hinh

a (dL/g) Mn Mw Mw/Mn Ten

5a 0.87 6a —b —c 765b 0.79 6b 0.92 —c 805c 0.97 6c 1.13 23,000 45,000 1.95 1125d 1.02 6d 1.09 24,000 47,500 2.07 1245e 0.92 6e 0.69 21,000 41,500 1.97 92

a Measured at a polymer concentration at 0.5 g/dL in DMAc at 30 �C. b Ins

This journal is ª The Royal Society of Chemistry 2010

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heating at elevated temperatures. Most of the thermally cured

polyimides exhibited excellent solubility in polar solvents such

as NMP and DMAc. Therefore, the characterization of solution

viscosity was carried out without any difficulty, and the inherent

viscosities of the organosoluble polyimides were recorded in the

range of 0.69–1.13 dL/g, Table 1 also shows the molecular

weights of 6a–6e prepared by the thermal imidization method.

The weight-average molecular weights (Mw) and number-

average molecular weights (Mn) were recorded in the ranges of

41,500–47,500 and 21,000–24,000, respectively, relative to

polystyrene standards. The transformation from poly(amic

acid) to a polyimide was also carried out via chemical cyclo-

dehydration by using acetic anhydride and pyridine. The

chemical structures of the polyimides were characterized by IR,

NMR and elemental analysis. Fig. 5 demonstrates a typical set

of IR spectra for poly(amic acid) 5d and polyimide 6d. All the

polyimides exhibited characteristic imide absorptions at around

1780 cm�1 and 1724 cm�1 (typical of imide carbonyl asymmet-

rical and symmetrical stretches), 1374 (C–N stretch), and 1102

and 747 (imide ring deformation), together with some strong

absorption bands in the region of 1100–1300 cm�1 due to the C–

O stretchings. The disappearance of amide and carboxyl bands

indicates a virtually complete conversion of the poly(amic acid)

precursor into polyimide. The typical 1H NMR spectrum of

polyimide 6d are shown in Fig. 6. In the 1H NMR spectrum, all

the protons resonated in the region of 6.26–8.23 ppm. The

tensile properties of the polyimides

sile properties of the polyimide films

sile strength (MPa) Elongation to break (%) Initial modulous (GPa)

7 1.787 1.7610 2.0611 2.119 2.10

oluble in DMAc. c Insoluble in THF.

Fig. 5 FT-IR spectra of poly(amic acid) 5d and polyimide 6d.

Polym. Chem., 2010, 1, 485–493 | 489

Fig. 6 1H NMR spectra of polyimide 6d.

Table 3 Solubility behavior of the polyimides prepared via thermalimidization and chemical imidization a,b

Solvent c

Polyimides

6a 6b 6c 6d 6e

NMP +h(+)d +h(+) + (+) +(+) +(+)DMAc +h(+h) +h(+h) +h(+) +(+) +(+)DMF +h(+h) �(+h) +h(+) +(+) +(+)DMSO �(�) �(�) +h(+h) + (+) +(+)m-Cresol �(+h) +h(+) +h(+) +(+) +(+)o-Chorophenol �(�) +h(+) +(+) +(+) +(+)THF �(�) �(�) +(+) +(+) +(+)Pyridine �(�) �(�) �(+) +h(+) +h(+)1,4-Dioxane �(�) �(�) �(+) +(+) +(+)chloroform �(�) �(�) +h(+) +h(+) +(+)CH2Cl2 �(�) �(�) �(h+) +h (+h) +h (+)Acetone �(�) �(�) �(�) �(+h) �(+h)

a The symbol +: soluble at room temperature; +h: soluble on heating at100 �C or boiling temperature; �: insoluble even on heating. b Thesolubility was determined by using 10 mg sample in 1 mL of stirredsolvent. c NMP ¼ N-methyl-2-pyrrolidone; DMAc ¼ N-dimethylacetamide; DMF ¼ N-dimethylformamide; DMSO ¼ dimethylsulfoxide; THF ¼ tetrahydrofuran. d Data in parentheses are those forchemical imidization polyimides.

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protons H11 close to the imide ring appeared at the farthest

downfield region of the spectrum because of the resonance. The

protons H2 and H1 shifted to a higher field because of the

electron-donating property of aromatic ether. All the spectro-

scopic data obtained were in good agreement with the desires

polymer structure. The results of the elemental analyses of all

the thermally cured polyimides are listed in Table 2. The values

found were in good agreement with the calculated ones of the

proposed structures.

3.3 Properties of polyimides

3.3.1 Organo-solubility and film property. The solubility of

the polyimides was tested qualitatively in various organic

solvents. The solubility properties of the thermally cured poly-

imides are reported in Table 3. All the polyimides were soluble

in dipolar amide-type solvents, such as NMP, DMAc, and

DMF, at room temperature or upon heating. Some of them also

were soluble in DMSO and phenol solvents like m-cresol and

o-chlorophenol, as well as chlorinated solvents like chloroform.

The polyimides (6d–6e) derived from less stiff dianhydride

components were also soluble in low boiling point organic

solvents such as THF and 1,4-dioxane. In general, the present

spiroskeletons linkage 6 series polyimides revealed an enhanced

solubility, and this could be attributed to the presence of kinked

Table 2 Elemental analysis of the polyimides prepare via thermal imidizatio

PolyimideFormula of the repeat unit(formula weight)

C (%)

Calcd. Fo

6a C47H24N2O7(728.71) 77.25 766b C53H28N2O7(804.81) 78.91 786c C54H28N2O8(832.82) 77.72 776d C53H28N2O8(820.74) 77.36 776e C56H28F6N2O7(954.84) 70.31 70

490 | Polym. Chem., 2010, 1, 485–493

spirofluorene units, with flexible aryl ether linkages along the

polymer backbone, which increased the disorder in the chains

and hindered dense chain packing, thus, reducing the interchain

interactions to enhance the solubility. The solubilities of the

resulting polyimides 6a–6e by chemical imidization were also

investigated, and the result is presented in Table 3. The poly-

imide prepared by chemical imidization exhibited higher solu-

bility than those by thermal curing. The less solubility of the

latter is possibly due to the presence of partial interchain

crosslinking or denser aggregation of the polymer chains during

the thermal imidization process. All of the spiroskeletons

linkage polyimides afforded good quality and tough films with

light color. These films were subjected to a tensile test, and their

tensile properties are also summarized in Table 1. The films

exhibited ultimate tensile strengths of 76–124 MPa, elongations

to break of 7–11%, and initial moduli of 1.76–2.11 GPa, indi-

cating that they are strong and tough polymeric materials. The

crystallinity of the polyimides was characterized by wide-angle

X-ray diffraction (WAXD) studies. Two of the PI 6 series, 6a

and 6b, displayed slightly semi-crystalline WAXD patterns,

whereas all of the others showed amorphous patterns. The

amorphous nature of the polyimides 6a–6e is attributed to the

spiroskeletons structure and flexible ether linkage coming from

the diamine monomer.

n

H (%) N (%)

und Calcd. Found Calcd. Found

.78 3.42 3.21 3.91 3.78

.56 3.73 3.54 3.45 3.55

.41 3.58 3.46 3.37 3.21

.17 3.64 3.38 3.41 3.53

.13 3.08 3.24 2.97 3.68

This journal is ª The Royal Society of Chemistry 2010

Table 4 Thermal behavior data and cutoff wavelength (l0) from UV-Visspectra of polyimides

Polymer Tga/�C

T5b/�C T10

b/�C

Char Yieldc (%) l0/nmIn N2 In air In N2 In air

6a 346 548 540 557 555 54 4096b 331 565 560 582 570 58 3786c 311 541 535 563 551 58 3626d 296 536 524 542 540 54 3566e 304 527 514 544 536 56 352

a Glass transition temperature (Tg) was measured by DSC at heating rateof 20 �C min�1 in nitrogen. b Decomposition temperatures recorded byTGA at a heating rate of 20 �C min�1. c Residual weight percentage at800 �C in nitrogen. Fig. 8 TGA curves of polyimide 6d at a heating rate of 20 �C min�1.

Fig. 9 UV-visible spectra of thermally imidized polyimides.

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3.3.2 Thermal properties. The thermal properties of the

polyimides were determined by using DSC and TGA (Table 4).

DSC experiments were conducted at a heating rate of 20 �C

min�1 in nitrogen. Rapid cooling from 400 �C to room temper-

ature produced predominantly amorphous samples, so the Tg of

almost all the polyimides could be easily taken from the second

DSC heating traces. DSC curves for the polyimides are repro-

duced in Fig. 7. The Tg values of the PI 6 series ranged from 296�C to 345 �C, which increased in the order of PMDA > BPTA >

BTDA > 6FDA > OPDA. 6d showed the lowest Tg because of

the presence of two flexible ether linkages between the phthal-

imide units. As in Table 4, the Tgs of PI 6 series are high,

demonstrating the Tg-enhancement effect of spiro framework,

which will hide the rotation of polymer chains. This indicates

that introducing the bulky fluorene groups, does not sacrifice the

glass transition even though the free volume of PI 6 series is

larger. The thermal stability of the polyimides was evaluated by

TGA measurements in both air and nitrogen atmospheres.

Typical TGA curves for polyimide 6d is reproduced in Fig. 8. The

decomposition temperatures (Td) at 5% and 10% weight loss in

nitrogen and in air atmospheres were determined from the

original TGA thermograms and are also given in Table 4. The Td

at 10% weight loss of the polyimides (6a–6e) in nitrogen and air

stayed in the range of 542–582 �C and 536–570 �C, respectively.

They left more than 54% char yield at 800 �C in nitrogen. The

Fig. 7 DSC thermograms of polyimides.

This journal is ª The Royal Society of Chemistry 2010

TGA data indicated that these polyimides had fairly high

thermal stability with the introduction of the spiro framework

structure, even though they revealed high solubility and optical

transparency.

3.3.3 Color and optical transparency. Thin films were

measured for optical transparency with UV-visible sepectro-

scopy. Fig. 9 shows the UV-visible absorption spectra of the PI

film prepared via thermal imidizations, and cutoff wavelength

(absorption edge, l0) value are listed in Table 4. In agreement

with the results obtained from the colorimeter, all the polyimides

revealed low l0 and high optical transparency, with a percentage

of transmittance higher than 80% at 450 nm. Because of the

highly conjugated aromatic structures and intermolecular

charge-transfer complex (CTC) formation of PI, most polymers

between the UV and visible area have strong absorption.

However, these PIs which have bulky spiroskeletons structure

and flexible ether linkage in the center of diamine reduced the

intermolecular CTC between alternating electrondonor

(diamine) and electron-acceptor (dianhydride) moieties. The

polyimides 6d and 6e produced from ODPA and 6FDA were

essentially colorless and showed relatively lower l0 values in

contrast to other dianhydrides, and these can be explained by the

decreased intermolecular interactions.46 The decrease in chain–

chain CTC formation also was understandable from the

Polym. Chem., 2010, 1, 485–493 | 491

Table 5 Moisture-absorption and dielectric constants of polyimides

PolymerFilmthickness/mm

Moistureabsorption(%)a

Dielectricconstant1 KHz

6a 45 0.61 3.106b 53 0.68 3.046c 32 0.73 3.126d 51 0.54 2.926e 36 0.36 2.65

a Moisture absorption of polyimide films was measured immersing thefilms in distilled water at 25 �C for 100 h.

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significant solubility of the polyimides prepared from the spi-

roskeletons structure diamine 3.

3.3.4 Dielectric constant and water absorption. The dielectric

constants and water absorption of the 6 series were measured,

and the results were summarized in Table 5. The 6 series showed

dielectric constants in the range 2.65–3.12 at 1 KHz, which were

much lower than the standard PI such as Kapton films (3.5 at

1 kHz) and Ultem (3.5 at 1 kHz).47 6c had a higher dielectric

constant for the reason that the carbonyl bridge (C]O) had less

free volume, more polarizability in the PI backbone when

compared to the –O– bridge (6d). The low dielectric constant of

the 6 series is mainly attributed to the fact that the introduction

of the spiro framework and noncoplanar structure loosened the

polymer packing, subsequently reducing their density and

dielectric constants. As expected, the 6 series also exhibited low

water absorptions (0.36–0.73%) due to the water proofing fluo-

rene groups. Because of the proofing effect of the trifluoromethyl

groups, 6e had significantly lower moisture absorption than the

other 6 series.

4. Conclusions

A novel spiro(fluorene-9,90-xanthene) skeleton bis(ether amine)

monomer, 20,70-bis(4-aminophenoxy)-spiro(fluorene-9,90-xan-

thene), was successfully synthesized and characterized in the

present work, which was employed in polycondensation with

various aromatic dianhydrides, to prepare a series of organo-

soluble aromatic polyimides. The resulting polyimides could be

cast into flexible and strong films with high solubility, high

optical transparency, excellent thermal stability, moderate to

high glass transition temperatures (296–346 �C), and low

dielectric constants. Thus, this series of polyimides exhibits

a good combination of properties required for high-performance

materials and demonstrates a promising potential for future

applications.

Acknowledgements

The authors are grateful for the research support from the

Natural Science Foundation of Gansu Province (No.

096RJZA047) and the State Key Laboratory of Applied Organic

Chemistry of People’s Republic of China.

492 | Polym. Chem., 2010, 1, 485–493

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