EUROPEAN

European Polymer Journal 41 (2005) 771–781

www.elsevier.com/locate/europolj

POLYMERJOURNAL

A novel type of Si-containing poly(urethane-imide)s:synthesis, characterization and electrical properties

Huseyin Deligoz *, Tuncer Yalcınyuva, Saadet Ozgumus

Istanbul University, Engineering Faculty, Chemical Engineering Department, Chemical Technologies Group,

34320 Avcılar-Istanbul, Turkey

Received 31 March 2004; received in revised form 4 November 2004; accepted 5 November 2004

Available online 13 January 2005

Abstract

A novel type of a Si-containing poly(urethane-imide) (PUI) was prepared by two different methods. In the first

method, Si-containing polyurethane (PU) prepolymer having isocyanate end groups was prepared by the reaction of

diphenylsilanediol (DSiD) and toluene diisocyanate (TDI). Subsequently the PU prepolymer was reacted with pyro-

mellitic dianhydride (PMDA) or benzophenonetetracarboxylic dianhydride (BTDA) in N-methyl pyrolidone (NMP)

to form Si-containing modified polyimide directly. In the second method, PU prepolymer was reacted with diaminodi-

phenylether (DDE) or diaminodiphenylsulfone (DDS) in order to prepare an amine telechelic PU prepolymer. Finally,

the PU prepolymer having diamine end groups was reacted with PMDA or BTDA to form a Si-containing modified

polyimide. Cast films prepared by second method were thermally treated at 160 �C to give a series of clear, transparent

PUI films. Thermogravimetric analysis indicated that the thermal degradation of PUI starts at 265 �C which is higher

than degradation temperature of conventional PU, confirming that the introduction of imide groups improved the ther-

mal stability of PU.

To characterize the modified polyimides and their films, TGA, FTIR, SEM and inherent viscosity analyses were car-

ried out. The dielectrical properties were investigated by the frequency–capacitance method. Dielectric constant, dielec-

tric breakdown strength, moisture uptake and solubility properties of the films were also investigated.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Poly(urethane-imide); Poly(amic acid); Thermal imidization; Dielectric properties

1. Introduction

Polyurethane (PU) is a versatile polymer and can be

easily prepared by a simple polyaddition reaction of pol-

0014-3057/$ - see front matter � 2004 Elsevier Ltd. All rights reserv

doi:10.1016/j.eurpolymj.2004.11.007

* Corresponding author. Present address: Universitat zu

Koeln, Institute fur Physikalische Chemie, Luxemburger

str.116, D-50939 Cologne, Germany. Tel.: +49 221 470 4485/

+90 212 591 2479; fax: +49 221 470 7300/+90 212 591 1997.

E-mail addresses: hdeligoz@istanbul.edu.tr, H.Deligoz@

uni-koeln.de (H. Deligoz).

yol, isocyanate and a chain extender. PUs have excellent

abrasion resistance and some properties of both rubber

and plastics [1–3]. They are becoming increasingly

important as engineering materials. Unfortunately, the

conventional PUs are known to exhibit poor thermal

stability which limits their applications. For example,

their mechanical properties rapidly deteriorate above

80–90 �C and thermal degradation takes place at tem-

peratures above 200 �C [4]. Researches focused on

improving the thermal stability of PU have attempted

in various ways. The most accepted approach for the

ed.

772 H. Deligoz et al. / European Polymer Journal 41 (2005) 771–781

improvement of thermal stability of PUs is a chemical

modification in the structure by introducing thermally

stable heterocyclic polymers like polyimides [5–13].

Polyimides (PI) are the most important members of het-

erocyclic polymers with remarkable heat resistance and

excellent mechanical, electrical, chemical and durability

properties. The term of ‘‘polyimides’’ is used for a vari-

ety of polymers containing the imide structure in the

backbone. PI has been used widely in the microelec-

tronic industry as a dielectric layer because of its low

dielectric constant and high dielectric strength [14–16].

Moreover, PI can be used as films, gas separation mem-

branes with high permeability and surface coating mate-

rials [14,16].

Although they have some superior properties as

listed above, their applications are limited since they

have high Tg and Tm values. In other words, processing

of PIs is difficult and has many problems. An effective

way for improving solubility and processibility of PI is

the introduction of some flexible groups which come

from either the diamine or the dianhydride component

(ether, carbonyl, etc.) into the polymer backbone [17].

Another possible choice is the preparation of some

copolymers such as poly(ether-imide) [12], poly(urea-

imide) [18], poly(amide-imide) [19], poly(urethane-

imide) [5–13].

PIs are typically chosen for high temperature applica-

tions such as coating materials and insulators for elec-

tronic parts [16]. This is due to their good thermal

stability, their high planarization, low leakage current

density, high dielectric breakdown strength, good pro-

cessibility and high glass transition temperatures. The

method generally used for lowering the dielectric con-

stant, is the incorporation of fluorine atoms or other

groups into the polyimide backbone [20,21]. Another

alternative is the copolymerization of PIs with other

polymers [20]. There is only one literature that gives lim-

ited information on the electrical properties of PUIs [22].

Various attempts have been made to prepare

poly(urethane-imide) (PUI). Reaction of an isocyanate

terminated PU prepolymer with acid dianhydride is

the most widely utilized method (direct method) [5–

12]. Another method is blending of an isocyanate termi-

nated PU prepolymer with poly(amic acid) [5,6,9,12,13].

Intermolecular Diels–Alder (DA) reaction of 4-methyl-

1,3-phenylenebis(2-furanyl carbamate) with various

bismaleimides is also reported to give poly(urethane-

imide) [23–25]. Recently, it was reported that an imide

function was introduced into PU backbone through a

different synthetic strategy. This route involves reaction

of an amine telechelic PU prepolymer with dianhydride

and cyclodehydration of the intermediate polyamic acid

in a sequential way [8]. Ghatge and Jadhav have re-

ported that preparation of Si-containing polymers can

lead to an improvement of the thermal stability of the

polymers due to incorporation of a silicon–carbon bond

in the polymer backbone [26]. Moreover, although there

are lots of paper on the synthesis of PUI, no study has

been reported on dielectric properties of Si-containing

PUI by now.

In this work, for the first time, Si-containing

poly(urethane-imide)s were synthesized by using two dif-

ferent methods. In the method 1, isocyanate terminated

PU prepolymer was directly reacted with tetracarboxylic

dianhydride. In order to prepare isocyanate terminated

PU prepolymer, diphenylsilanediol (DSiD) compound

was reacted with TDI. This method has some disadvan-

tages such as moisture sensitivity of the isocyanate

groups and side reactions such as the trimerization of

isocyanate compounds or amide formation. In the

method 2, reactions were carried out in two steps over

amine telechelic precursor and poly(amic acid) forma-

tion in order to avoid these problems. The chemical

and physical properties of Si-containing poly(urethane-

imide) films produced by the direct method with those

obtained by the two step method are compared. Further,

preliminary studies on the electrical properties such as

dielectric constant and dielectrical breakdown strength

of silicon containing PUI films have been carried out.

2. Experimental

2.1. Materials

Diphenylsilane diol (DSiD) was synthesized by C.A.

Buckhard�s method (mp: 147 �C) [27]. Toluene diisocya-nate (TDI, mixture of 2,4- and 2,6-isomer), diaminodi-

phenylether (DDE), diaminodiphenylsulfone (DDS),

pyromellitic dianhydride (PMDA) and benzophenone

tetracarboxylic dianhydride (BTDA) were used as re-

ceived from Merck, Germany. N-Methyl pyrolidone

(NMP) and dimethyl acetamide (DMAc), were supplied

from Merck-Germany and then were purified by distilla-

tion under reduced pressure, and NMP was stored over

5 A molecular sieve. Methylene chloride (MeCl2) was

used as received from Merck-Germany, toluene was

used as received from Carlo Erba-Italy and petrol ether

was used as received from Riedel de Haen.

2.2. Reactions

2.2.1. Preparation of PU prepolymer

Reaction was carried out in a 250 ml five necked

round bottomed flask equipped with a thermometer,

condenser, magnetic stirrer, oil bath and nitrogen

inlet–outlet system. TDI (0.13 mol) was added onto di-

phenyl silane diol (0.065 mol) and reacted for 9 h at

80 �C under nitrogen atmosphere. TDI and DSiD were

mixed vigorously. After 1 h, 100 ml toluene was added

to the mixture. The temperature was kept constant at

80 �C and the reaction was followed by the determina-

Table 1

Formulation and viscosity of a prepolymer with isocyanate end

groups

Product DSiD

(mmol)

TDI

(mmol)

Isocyanate

content (%)

gred (dl/g)a

Calc. Theo.

I 130 60 13.9 14.9 0. 1

I: isocyanate end group having prepolymer.a 0.5 g/dl in NMP at 30 �C.

H. Deligoz et al. / European Polymer Journal 41 (2005) 771–781 773

tion of the –NCO content with the dibutyl amine meth-

od [ASTM D1638-74] and found to be closed to the ex-

pected theoretical value. At the end of the reaction, the

PU prepolymer was purified by precipitation in petro-

leum ether, then filtered and dried in a vacuum oven

at 50 �C for 4 h. Characterization of PU prepolymer

was carried out by FTIR spectroscopy, –NCO end

group analysis and inherent viscosity. Results are given

in Table 1.

2.2.2. Preparation of Si-containing poly(urethane-imide)

film by direct method (method 1)

The PU prepolymer was placed into a 100 ml three

necked round bottomed flask equipped with a thermom-

eter, magnetic stirrer, oil bath and nitrogen inlet–outlet

system. PU prepolymer (1 g) was dissolved in dry

NMP under nitrogen atmosphere. An equivalent

amount of dianhydride (PMDA) (0.18 g) was added step

by step in 5 min by stirring at 0 �C and temperature was

kept at 0 �C for 30 min. Then the temperature was in-

creased stepwise to 40 �C and held there for 2.5 h, subse-

quently it was heated to 90 �C for 2 h for evaluation of

carbon dioxide. Finally the reaction mixture was heated

to 130 �C and held there for 20 h. The brownish polymer

was precipitated by pouring the solution into cold water.

Precipitated polymer was filtered and washed twice with

methanol and dried in a vacuum oven at 110 �C. Thesolution of the resulting mixture was spread onto a

well cleaned glass in order to obtain modified polyi-

mide film. Then it was dried at 80 �C for 5 h to remove

the solvent. Finally free standing modified PI film

was obtained by immersing the glass plate into the boil-

ing water. The chemical structures are depicted in

Scheme 1.

2.2.3. Preparation of Si-containing poly(urethane-imide)s

by using amine telechelic precursors (method 2)

2.2.3.1. Preparation of diamine terminated PU. The sim-

ilar reaction system described in Section 2.2.2 was used.

Amine terminated PU was prepared by the reaction of

an equivalent amount of isocyanate terminated PU pre-

polymer with aromatic diamine in DMAc (isocyanate–

amine equivalent ratio:1/2). Isocyanate terminated PU

prepolymer was added within 1 h into aromatic diamine

solution dissolved in DMAc. The reaction was carried

out at 10 �C under nitrogen atmosphere. After the addi-

tion of the PU prepolymer, the temperature was slowly

raised to 40 �C and held at this temperature for another

1 h to complete the reaction. Amine terminated PU pre-

polymer was isolated by precipitation in water, and then

it was filtered and dried at 60 �C for 2 h in a vacuum

oven. The product was characterized by FTIR spectros-

copy, amine content analysis and inherent viscosity

determination. The results are presented in Table 2

(Scheme 2).

2.2.3.2. Preparation of poly(urethane-amic acid) (PU-

AA). PU-AA was prepared by the reaction of an equiv-

alent amount of dianhydride with diamine terminated

PU prepolymer in DMAc at 10 �C under nitrogen atmo-

sphere (isocyanate–amine equivalent ratio:1/2). An

equivalent amount of PMDA (0.064 g) was added into

diamine terminated PU (1.5 g) solution in DMAc for

2 h under nitrogen atmosphere. After the addition of

PMDA, the temperature was increased to 40 �C and

held there for another 1 h to complete amic acid forma-

tion. The readily prepared amic acid (PU-AA) solution

was kept in a refrigerator to prevent the imidization

and hydrolysis reactions for long term usage. Polymer

was isolated by precipitation with methanol. The precip-

itated polymer was washed with methanol and dried

under vacuum at 60 �C for 2 h. The dried PU-AA was

characterized by FTIR spectroscopy.

2.2.3.3. Preparation of Si-containing poly(urethane-

imide) film by cyclodehydration of poly(urethane-amic

acid). PU-AA solution was cast on a well cleaned glass

plate by doctor blade to form a thin film. It was heated

in an oven under nitrogen atmosphere. The films

obtained by method 2, were cured at 160 �C for 10 h

in order to remove solvent and to cure PAA. All pre-

pared films were obtained as clear, light brown to yellow

colored and transparent. Thicknesses of all films are in

the range of 5–30 lm. All chemical structures are shown

in Scheme 2.

2.3. Measurements (instruments)

Inherent viscosities of modified polymers and inter-

mediates were determined for solutions of 0.5 g/dl in

NMP at 30 �C with an Ubbelohde viscometer. Thermal

analyses were carried out with Linseis thermal analyzer

with a heating rate of 5 �C/min in air. The sample mass

was approximately 10 mg. Infrared spectra of the sam-

ples were recorded on a Perkin Elmer Spectrum 2000

model with an ATR (attenuated total reflectance) unit

in the range of 400–4000 cm�1. Dielectric constants

and loss factors were measured using HP 4192 A LCR

meter Impedance Analyzer at various frequencies and

at room temperature. After coating both surfaces of

Table 2

Formulation and viscosity of a prepolymer with amine end

groups

Product Precursor

(g)

DDE

(g)

DDS

(g)

Amine

content (%)

gred(dl/g)a

Calc. Theo.

A-1 I (1.5) 0.6 – 61.6 58.4 0.11

A-2 I (1.0) – 0.5 52.4 54.5 0.12

A: amine end group having intermediates.a 0.5 g/dl in NMP at 30 �C.

CH3

NCO

NCO

+Si OHOH2

NH CO Si O CO NH

Toluene diisocyanate Diphenylsilanediol

polyurethane prepolymer with isocyanate end group

C

O

CCC

C

O

OO

O

Benzophenonetetracarboxylic dianhydride (BTDA)

- CO2

at 90 OC for 2 hours and

at 130 OC for 20 ho

Silicon containing poly(urethane-inide)

urs .

CH3

NCOOCN

3HC

NH CO Si O CO NH

CH3

N

3HC

C

O

C

C

O

O

N

n

O O

CC

O

O

N

O

O

Scheme 1. Synthesis of PUI by method 1.

774 H. Deligoz et al. / European Polymer Journal 41 (2005) 771–781

the specimen films with gold, the contacts were applied

using indium oxide. The dielectric constant were calcu-

lated from the measured capacitance data as follow:

e1 ¼Cde0A

where C is the capacitance, e1 is the dielectric constant,

eo is the permittivity of the free space (8.85 · 10�12

MKS unit), d is the film thickness and A is electrode

area. Dielectric breakdown strengths of the films were

measured with an Electrotechnic Laboratorium D-7015

‘‘Insulation Breakdown Tester’’ UH 270, Resolution

50 V for 2.5 kV and 100 V for 5 kV. Thicknesses of the

films were measured with Mitutoyo Model, 0–25

micrometer with 0.001 mm resolution. Moisture uptake

properties were investigated by the following method:

films firstly were dried at 80 �C for 10 h and kept in a

confined container under a humidity of 55% for a week.

Following this procedure, the weight changes were

determined. SEM analyses were carried out by using a

Jeol JXA 840A Scanning Electron Microscope. The sol-

ubility tests were carried out by immersing the films in

various solvents for one week at room temperature.

Hardness values of the films coated on a glass were

determined by Konig Pandulum [DIN 53 157] and the

results were given as Konig second in Table 6. Adhesion

test of the films performed by cross-cut technique

[ASTM 3359-76]. For –NCO and amine analysis, we re-

peated the measurements at least two times. The differ-

ences are in the range of 1–5%.

NH CO Si O CO NH

polyurethane prepolymer with isocyanate end group

CH3

NCOOCN

+ NH2-Ar-NH2Ar: O

Diaminodiphenyether (DDE)

NH CO Si O CO NH

CH3

NH - Ar - NH2H2N - Ar - HN

O

H3C

H3C

O

polyurethane prepolymer with diamine end group

C

CCC

O

OO

O

O O

NH CO Si O CO NH

CH3

NH - Ar - NHN

H3C

O

C

CC

CH

OO

O

OHNH

n

O

HO

NH CO Si O CO NH

CH3

NH - Ar -NHN

H3C

O

CCC

C

OO

O

N

n

O

160 oC in Nitrogen

- H2O

poly(urethane amic acid)(PU-AA)

Pyromellitic dianhydride (PMDA)

Silicon containing poly(urethane-inide)

Scheme 2. Poly(urethane-imide) (PUI) synthesis over amine telechelic precursor (method 2).

H. Deligoz et al. / European Polymer Journal 41 (2005) 771–781 775

3. Results and discussion

In this study, the modified polymers in the form of

PUI were obtained by two different methods and the

film properties were compared. One of the methods is

the reaction of a diisocyanate compound with a dianhy-

dride compound at relatively lower temperatures under

evaluation of CO2. The other method and also the most

commonly used route for preparation of polyimides is

the reaction between a diamine and a dianhydride in a

polar solvent to provide a PAA that loses water and

forms the PI structure on subsequent heating. The prep-

aration of the corresponding PI from this precursor

should be conducted at a high temperature that will

evolve the water molecules resulting the formation of

void in the final product. The inherent viscosities of

the prepared PAAs are listed in Table 3.

No gelation or precipitation has occurred in the prep-

aration of modified PI solutions. According to two

different methods, modified PI films 2 (obtained by

Table 3

Formulation of Si-containing PUIs and viscosity of PAA and

PI

Polymer Precursor (g) PMDA (g) BTDA (g) gred (dl/g)a

P1 I (1.0) 0.18 – 0.13

P2 I (1.16) – 0.31 0.15

P3 A-1 (0.6) 0.06 – 0.14

P4 A-1 (0.5) – 0.06 0.15

P5 A-2 (0.5) 0.05 – 0.15

P6 A-2 (0.13) – 0.02 0.16

a 0.5 g/dl in NMP at 30 �C.

776 H. Deligoz et al. / European Polymer Journal 41 (2005) 771–781

method 1) and 4 (obtained by method 2) were obtained

respectively.

Most of the polymer solutions could be cast into flexi-

ble, tough, transparent, films from NMP or DMAc solu-

tions at ambient temperature and under atmospheric

pressure. Many commercially available polyurethanes

start to decompose at 150–200 �C, their decomposition

temperature range is 200–250 �C [4]. Compared with

the conventional PU, the decomposition temperatures

of PUIs prepared by our method were improved. By

using diisocyanate instead of diamine, the formation of

imide groups is achieved at lower temperatures and

4000 3000 2000

Tra

nsm

itta

nce

PU-I prepared by method I

Polyurethane prepolymerhavingisocyanate end group

2259

3354

Wave num

Fig. 1. FTIR spectra of a prepolymer with iso

polymerization occurs under milder conditions. In addi-

tion, the resulting small molecule, CO2 which would eas-

ily get out of the reaction system during the

polymerization (8).

3.1. FTIR spectra

The isocyanate terminated PU prepolymer is charac-

terized by FTIR which shows the characteristic absorp-

tion band at 2259 cm�1 due to the presence of the

isocyanate groups and also exhibits a NH peak at

1660 cm�1. In the method 1, formation of imide groups

is characterized by FTIR. The formation of imide groups

is connected with the disappearance of the characteristic

absorption band of the isocyanate group. Moreover

an IR absorption maximum showed up at 1778 and

1718 cm�1 due to the imide–carbonyl, at 1373 cm�1

due to C–N stretching and at 720 cm�1 due to imide ring

deformation. Furthermore, FTIR spectra of modified

polyimides do not show any peak near 2250 cm�1 which

indicates the absence of isocyanate groups. The spectra

for the products obtained by method 1 are given in

Fig. 1.

In the method 2, Si–urethane prepolymer with

diamine end group is also characterized by FTIR. FTIR

1500 1000

1

,

0

1660

1778

1718 13

73

1660

811

720

ber (cm-1)

650

cyanate end groups and PUI (method 1).

H. Deligoz et al. / European Polymer Journal 41 (2005) 771–781 777

results show that the isocyanate terminated PU prepoly-

mer reacted with diamines (diaminodiphenyl ether or

diaminodiphenyl sulfone) which is indicated by the dis-

appearance of the isocyanate band at 2259 cm�1. More-

over, all isocyanate groups have reacted to give the

urea–amine structure which shows characteristic peaks

at 1593 cm�1 (urea–carbonyl) and at 3300–3600 cm�1

(NH-amine stretching vibrations).

Reaction of diamine terminated Si–urethane prepoly-

mer with PMDA results in the corresponding poly(ure-

thane-amic acid) PU-AA which is characterized by

FTIR. The poly (amic acid) shows carboxylic amide

and characteristic amide absorption bands at approxi-

mately 3300 cm�1 and 1500–1600 cm�1. Modified PU-

AAs are turned into the corresponding polyurethane-

3000 20004000

Wave

3250

Prepolymer withisocyanate end group

PAA Intermediates

Prepolymer withdiamino end group

PU-I prepared by method II

Tra

nsm

ittan

ce

3333

2259

3357

Fig. 2. FTIR spectra of a prepolymer with amin

imides by thermal imidization at 100 �C for 1 h, at

200 �C for 1 h and at 300 �C for 1 h. Formed modified

films contain characteristic bands of the imide and ure-

thane groups. The bands at around 1780 cm�1 are due

to the imide–carbonyl group, bands at 1710–1740 cm�1

are due to the imide and urethane carbonyl linkages.

In addition, an N–H stretching band of the urethane

group is observed. The FTIR spectra of PUIs prepared

by method 2 are given in Fig. 2.

3.2. Thermal properties of Si-containing (polyurethane-

imide)

The thermal properties of the polymers were

investigated by TGA under air atmosphere. All TGA

1500 1000 650 0,

1776

,

1

number(cm-1)

1606

1720

1680

1374

720

1593

1776

e end groups, PAA and PUI (method 2).

Table 4

Thermal properties of Si-containing PUI films

Polymer T 10% (�C)a T 50% (�C)b Char yieldc

P1 260 475 16.9

P2 260 490 11.0

P3 260 465 24.8

P4 255 460 8.6

P5 245 415 9.7

P6 240 410 n.d.

a The temperature where 10% weight loss occurred.b The temperature where 50% weight loss occurred.c Weight of residue polymer at 750 �C.

778 H. Deligoz et al. / European Polymer Journal 41 (2005) 771–781

thermograms of PUIs were identical and shown in Figs.

3 and 4. We observed that the decomposition of PUI

films take place in two stages at a heating rate of 5 �C/min in thermogravimetric analysis. All films that are

produced with method 1 and 2 exhibit more or less the

same thermal and oxidative degradation stability. In

addition, PMDA derived films have a higher thermal

stability than those obtained from BTDA. The first

decomposition stage starts at roughly 260 �C, indicatingthe decomposition of the most thermally labile urethane

component. The second stage corresponding to the

decomposition and carbonization of the imide compo-

nents occurs above 400 �C in each TGA curve. These re-

sults are in very good agreement with Refs. [5,13,20,28].

0 100 200 300 400 500 600 700 800 9000

20

40

60

80

100

Wei

ght

(%)

P1(TDI+DSiD+PMDA)P2(TDI+DSiD+BTDA)

Temperature ( C)o

Fig. 3. TGA curves of Si-containing PUIs prepared by method

1 under atmospheric condition.

0 100 200 300 400 500 600 700 800 9000

20

40

60

80

100

Temperature ( C)o

Wei

ght (

%)

P3 (TDI+DSiD+DDE+PMDA)P4 (TDI+DSiD+DDE+BTDA)

Fig. 4. TGA curves of Si-containing PUIs prepared by method

2 under atmospheric condition.

Hence, it can be concluded that PU was introduced into

the main chain of PI. We believe that the introduction of

Si linkages (Si–C bonds) would improve the thermal sta-

bility. However interestingly, we have not observed a

significant improvement effect. Comparing the conven-

tional PU, the decomposition temperature of PUIs pre-

pared by our method increased, showing that this

approach was very efficient. TG curves in all polyimides

show temperature resistant residue due to silicon dioxide

formation and this result was in good agreement with

Ref. [26]. Thermal properties of the Si-containing PUI

films are given in Table 4.

3.3. SEM (scanning electron microscope)

Surfaces of the films were investigated by SEM in

order to characterize surface properties. According to

SEM results, the film surfaces are dense, smooth and

do not show any cracking or delamination.

3.4. Electrical properties of Si-containing

(polyurethane-imide)

3.4.1. Dielectric constants of PUI films

The dielectric properties in polymers are most often

studied by the capacitance method. Polyimides are suit-

able for dielectric interlayers in the electronic industry

due to their high dielectric breakdown and low dielectric

constant and dissipation factor (16). Low values of tandare indicative of minimal conversion of electric energy to

heat. It is very advantageous to have low values for both

the dissipation factor and the dielectric constant because

electric signals will loose less of their intensity in the

dielectric medium.

The dielectric constants (e1) of PUI films were calcu-

lated according to the usual parallel plate capacitor for-

mula at different frequencies (1 kHz to 13 MHz). The

dielectric constant dependence on frequency for PUI

films at different frequencies is shown in Fig. 5. One

can see from the figure that e1 decreases with increasing

frequency when temperature is kept constant. Dielectric

constants of PIs, in general, are known to decrease grad-

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,43,0

3,1

3,2

3,3

3,4

3,5

3,6

3,7

3,8

3,9

4,0

Die

lect

ricC

onst

ant

Frequency (x10 )KHz-3

P3 (TDI+DSiD+DDE+PMDA)P4 (TDI+DSiD+DDS+BTDA)

Fig. 5. Dielectric constant vs frequency for PUI films prepared

by method 2.

H. Deligoz et al. / European Polymer Journal 41 (2005) 771–781 779

ually with increasing frequency [29]. This behavior can

be attributed to the frequency dependence of the polariz-

ation mechanisms. The dielectric constant depends upon

the ability of the polarizable units in a polymer to orient

fast enough to keep up with the oscillations of an alter-

nating electric field. When the frequency increases, the

orientational polarization decreases since the orientation

of dipole moments need a longer time than electronic

and ionic polarizations. This causes the dielectric con-

stant e1 to decrease.

The dielectric constants of PUIs prepared by method

2 are in the range of 3–3.5 at 100 kHz frequency. More-

over, dielectric constant does not exhibit significant

change with a wide range of frequency. The materials

with this properties are highly preferred for devices that

will be used in microelectronic industry. As seen from

Fig. 5, PI 3 and PI 4 contain the polar ether and sulfone

groups. It is observed that sample 4 has a higher dielec-

tric constant than sample 3. This can be explained by the

fact that sample 4 contains the highly polar sulfone

group. These PUIs exhibit lower dielectric constants

Table 5

Solubility tests of Si-containing PUI films

Polymer Solubilitya

NMP THF MeCl2

P1 ++ �� � ��P2 ++ �� � ��P3 +� + �� � ��P4 �� + �� � ��P5 �� + �� � ��P6 �� + �� � ����: insoluble at room temperature; �: insoluble in hot solvent; ++: so

soluble at room temperature; �+: partly soluble in hot solvent.a NMP: N-methyl pyrolidone, THF: tetrahydrofuran, MeCl2: dich

than those prepared by method 1. Dissipation factors

of the films are in the range between 0.014 and 0.07 at

1 kHz and 13 MHz respectively.

3.4.2. Dielectric breakdown strength of PUI films

The dielectric strength is an important parameter for

selecting an appropriate electrical insulation material to

avoid short circuiting, especially at high temperature

operations. In particular, the dielectric breakdown

strength of materials depends on temperature, specimen

thickness, surface condition and humidity. The dielectric

strength of the Si-containing PUI is in the range of 25–

150 kV/mm. The dielectrical breakdown strengths are gi-

ven in Table 6. For comparison, electrical porcelains,

typically have a dielectric strength of 25 kV/mm, sol–

gel ceramics have a dielectric strength of 24–22 kV/mm

while dielectric breakdown strength for the films are

about 40–50 kV/mm.

3.5. Solubility and water uptakes of the Si-containing

(polyurethane-imide) films

Solubility properties of the films were investigated.

The films were immersed into various solvents such as

NMP, THF, MeCl2 and toluene for one week. Results

are shown in Table 5. PI 1 and 2 (obtained by method

1) show better solubility in NMP than PI 3–6. This is

probably due to the strong hydrogen bonding occurs

during thermal treatment at 160 �C.None of the PUI films dissolved or swelled in THF

and MeCl2. Hereby, it is shown that PUI films prepared

by our method have excellent solvent resistance. The

reason for the excellent solvent resistance is considered

to be the formation of network structure between the

PU and PI.

Water absorption of polymers heavily influences their

dielectric constants and limits their application in the

electric and microelectronic industry. Moisture absorp-

tion can increase the conductivity of the dielectric and

promote the corrosion of metal conductors, which can

potentially lead to device failure. For this reason, it is

Toluene Water uptake (%)

� �� �+ 1.22

�+ �� �+ 1.53

� �� � 1.41

� �� � 2.90

� �� � 1.27

� �� � 2.50

luble at room temperature; +: soluble in hot solvent; +�: partly

loro methane.

Table 6

Dielectric breakdown strength and some physical properties of PUI films

Polymer Dielectric

breakdown

strength (kV/mm)

Physical properties

Hardness

(Konig second)

Adhesion

(%)

Appearance

P1 n.d. 242 100 Homogen, transparent and dark brown, brittle

P2 146.6 218 100 Homogen, transparent and dark brown, brittle

P3 62.13 243 100 Homogen, transparent and dark yellow, flexible

P4 58.36 240 100 Homogen, transparent and light yellow, flexible

P5 25.7 245 100 Homogen, transparent and dark yellow, flexible

P6 n.d. 240 100 Homogen, transparent and light yellow, flexible

780 H. Deligoz et al. / European Polymer Journal 41 (2005) 771–781

important to prepare PIs with low water absorption and

this may be managed by chemical structure modifica-

tion. Our modified polyimides have moisture uptakes

in the range of 0.95–2.9%. Moisture uptake data are gi-

ven in Table 5. The modified PI films that were synthe-

sized according to method 1 have lower moisture uptake

than the other films. This may be explained by the fact

that the presence of some flexible groups such as ether

and sulfone in the backbone. BTDA derivated PUI have

exhibited higher water uptake values.

Adhesion of the polyimide films is known to be excel-

lent. Our results indicate that all films exhibit superior

adhesions to the glass substrate. The data are given in

Table 6. For all PUI films adhesion were 100%. The

determined hardness values are very close to glass hard-

ness. The color of final PUI films depends on type of

aromatic diamine component. While the color of

DDE-based PUI film was dark yellow, DDS-based one

was light yellow.

4. Conclusion

In this work, Si-containing poly(urethane-imide)

have been synthesized by using two different methods.

All products were clearly identified by FTIR. TGA anal-

yses showed that all PUI films exhibited two step ther-

mal decomposition pattern. The first stage of the

thermal degradation of Si–PUI was attributed to the

cleavage of labile linkages of urethane groups except

imide groups. The thermo-oxidative stability of the

polyimide was estimated by the initial weight loss of

the polyimide at 350 �C in air. The incorporation of

polyimide into polyurethane backbone was the effective

method for improving its thermal stability. Introduction

of Si into the PUI network, has not significantly im-

proved the thermal stability. Cyclized polymer showed

significant decrease in solubility when compared with

their corresponding noncyclic precursors.

Preliminary studies on the electrical properties of the

silicon containing PUI were carried out. The film that is

produced by method 2 has exhibited lower dielectric

constants. The dielectric constants of PUI films prepared

by method 2 were roughly 3–3.5 at 100 kHz and electri-

cal properties of the PUI were close to the ones of con-

ventional PI. In addition dielectric constant has not

changed with a wide range of frequency. Using the same

technique, we are presently working on the detailed elec-

trical properties including ac, dc conductivity and relax-

ation process of conventional and modified PI films. On

the other hand, the hardness and adhesion properties of

the prepared films were found to be excellent.

Acknowledgments

The authors wish to express their sincere thanks to

Prof. Dr. Christian Mayer, Universitat Duisburg-Ger-

many, for helpful discussions and suggestions. We are

also grateful to Mr. Ismet Cakar, general director of

POLIYA company, for financial support to this project.

We also thank to Dr. Saffettin Yıldırım for his coopera-

tion in the electrical analysis.

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