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Thermal and electrical behavior of polyimide/silica hybrid thin films.


High-performance polymers are used in applications demanding service at enhanced temperatures while maintaining their structural integrity and an excellent combination of chemical, physical, and mechanical properties. Among them, aromatic polyimides represent an increasing important class of materials in aerospace, microelectronics, and other industrial applications. Polyimides are one of the important classes of polymers used as inter-layer dielectrics for advanced printed circuit boards and multichip module packing. Dielectric materials used for thin film multichip modules must meet a number of material and electrical requirements including a low-dielectric constant, low-dissipation factor, minimal moisture absorption, and high thermal stability (1), (2). Many desirable properties of polyimides, including good thermooxidative stability and excellent mechanical properties, contribute to their success (3), (4). It has been generally recognized that aromatic ether linkages inserted into aromatic main chain polymers provide them with a significantly lower energy of internal rotation. In general, such a structural modification leads to lower glass transition temperature and crystalline melting temperatures as well as significant improvement in solubility and other processing characteristics of the polymers without greatly sacrificing other advantageous polymer properties (5-8). However, the polyimides exhibit relatively high water absorption and high-dielectric constant and large coefficient of thermal expansion. A way to obtain materials with improved properties is to form organic-inorganic hybrids by incorporating reinforced inorganic fillers. The hybrid polyimides represent a class of new generation of materials that combine the properties of the ceramic phase with those of organic polymers. The incorporation of various metallic additives, such as [TiO.sub.2], [BaTiO.sub.3], [A1.sub.2][O.sub.3] and ZnO, into polyimides has been reported to improve the properties of the resulted materials (9-14).

Of the organic/inorganic hybrid composites investigated, polyimide/silica hybrid films are of interest (15-27). The sol-gel technique is an excellent method to produce hybrid polyimide/silica polymers. This method consists in a hydrolysis of an alkoxysilane and a polycondensation process (15), (20). Therefore, the processing of the polyimides is generally carried out via soluble poly(amic acid) precursors, which are cast onto various substrates, and then they are converted into polyimide films by thermal treatment. The good solubility of the poly(amic acid) precursors in amidic solvents makes possible the introduction into their solutions of metal oxide precursors and of the water required for the hydrolysis process. Also, the excellent thermal stability of polyimides makes possible the required thermal treatment up to high temperatures (300-400[degrees]C) without inducing appreciable degradation in the organic phase. It was found that phase separation occurred at higher silica content, and the mechanical strength of the resulting film has decreased relative to pure polyimide. Therefore, the introduction of chemical bonds between the polyimide and silica should improve their compatibility and mechanical properties (15).


In this article, we report the preparation of new silica-containing polyimide films. The polyimide matrix contains ether linkages both in the dianhydride and diamine segments. To improve the affinity of silica phase with the polymeric matrix, 3-aminopropyltriethoxysilane was used as coupling agent. The properties of these films, such as water vapors sorption capacity, dynamic contact angles, and contact angle hysteresis, thermal and electrical behavior, have been evaluated with respect to their structure. Effects of inorganic particles on local (secondary) relaxations and on the overall dielectric behavior were also investigated.



l,3-Bis(4-aminophenoxy)benzene (DAB), 4,4'-oxydiphthalic anhydride (ODPA), 3-aminopropyltriethoxysilane (APTES), and tetraethoxysilane (TEOS) were obtained from Aldrich and used without further purification. N-methyl-2-pyrrolidone (NMP) was obtained from Aldrich and distilled over [P.sub.2][O.sub.5].

Preparation of the Poly (Amic Acid) Solution 2

The poly(amic acid) solution 1 was prepared by poly-condensation reaction of diamine DAB with ODPA, in solution using NMP as solvent (Scheme 1). The molecular ratio between DAB and ODPA was 10/11. A typical polycondensation reaction was run as shown in the following example: In a 100 mL three-necked flask equipped with mechanical stirrer and nitrogen-inlet and outlet were introduced DAB (2.92 g, 0.01 mol) and NMP (40 mL). The mixture was stirred under nitrogen to complete dissolution. Then ODPA (3.41 g, 0.011 mol) was added to the resulting solution and stirring was continued for 3 h (Scheme 1).

The silane-terminated oligomer 2 was synthesized from the poly(amic acid) 1 and APTES. APTES (0.442 g, 0.02 mol) was added to the solution of 1 and the stirring was continued for 6 h.

Preparation of Hybrid Polyimide Films

For the preparation of polyimide/silica hybrids, a calculated amount of water and TEOS, in the ratio [[H.sub.2]O]/[TEOS] = 6, diluted in NMP, was added to the silaneterminated oligomer 2 and stirred for an additional 12 h at room temperature. The resulting homogenous solution was cast onto a glass plate and after being dried at 50[degrees]C for 12 h, the resulting film was thermally imidized at 100[degrees]C for 2 h, 150[degrees]C for 2 h, 200[degrees]C for 2 h, and 300[degrees]C for 1 h. Table 1 presents details about the preparation of hybrid polyimide films.
TABLE 1. Preparation of polyimide PI-0 and hybrid polyimides PI-10,
PI-20, and PI-40.

Sample  Poly(amic acid) (g)  TEOS (g)   Theoretic content of TEOS
                                       in poly(amic acid) film (%)

PI-0           1.122          0                      0
PI-10          1.122          0.124                 10
PI-20          1.122          0.280                 20
PI-40          1.122          0.748                 40

PI-0: FT-IR ([cm.sup.-1]): 3480, 1776, 1713, 1590, 1504, 1370, 1260, 743.

PI-10: FT-IR ([cm.sup.-1]): 3478, 1776, 1716, 1591, 1505, 1371, 1262, 744.

PI-20: FT-IR ([cm.sup.-1]): 3478, 1776, 1716, 1591, 1505, 1371, 1262, 744.

PI-40: FT-IR ([cm.sup.-1]): 3481, 1776, 1714, 1590, 1504, 1372, 1261, 744.


The inherent viscosity ([[eta].sub.inh]) of the polymer was determined with an Ubbelohde viscometer, by using polymer solution in NMP, at 20[degrees] C, at a concentration of 0.5 g/dL

Infrared spectra were recorded on a FT-IR Bruker Vertex 70 analyzer, by using KBr pellets or polymer films.

Atomic force microscopy (AFM) images were taken in air, on a SPM SOLVER Pro-M instrument. A NSG10/Au Silicon tip with a 35 nm radius of curvature and 255 kHz oscillation mean frequency was used. The apparatus was operated in semicontact mode, over a 5 x 5 [micro]m scan area, 256 x 256 scan point size images being thus obtained.

Microscopic investigations were performed on an environmental scanning electron microscope (ESEM) type Quanta 200 operating at 30 kV with secondary and back-scattering electrons in high vacuum mode. Before analysis, the investigated films were covered with a thin layer of gold by sputtering (EMITECH K550X). The coupled dispersive X-ray spectroscope (EDX) permitted to perform the elemental analysis on the film surface.

Dynamic contact angles (DCA) and contact angle hysteresis were measured by using a Sigma 700 tensiometer system, a modular high performance computer controlled (running by a Windows TM software) surface tension/contact angle meter. Water was used as measure liquid. The DCA runs were performed on rectangular films of about 10 x 0.15 mm. The measurement parameters were: advancing-receding speed: 5 mm/min; start depth: 0 mm; immersion depth: 6 mm; cycles number: 3. The average values were taken into consideration.

Water vapors sorption capacity of the samples has been measured by using the fully automated gravimetric analyzer IGAsorp supplied by Hiden Analytical, Warrington (UK). An ultrasensitive microbalance measures the weight change as the humidity is modified in the sample chamber at a constant regulated temperature. System measurements are fully automated and controlled by a user-friendly software package running on Microsoft[R] Windows[TM].

Thermogravimetric analysis was performed on a MOM derivatograph (Hungary) in air, at a heating rate of 10[degrees]C/min. The initial decomposition temperature is characterized as the temperature at which the sample achieves a 5% weight loss. The temperature of 10% weight loss ([T.sub.10]) was also recorded.

The dynamic mechanical analysis (DMA) was conducted using a Perkin-Elmer Diamond apparatus provided with a standard tension attachment at a frequency of 2 Hz. The apparatus was heated between 0[degrees]C and 300[degrees]C at 2[degrees]C/min, in a nitrogen atmosphere. The films (20 x 10 x 0.05 mm) were longitudinally deformed by small sinusoidal stress and the resulting strain was measured. The value of storage modulus E', the loss modulus E" and tension loss tangent (tan [delta] = E"/E') were obtained as a function of temperature.

The dielectric measurements were carried out using a Novocontrol system composed from an Alpha frequency response analyzer and Quattro temperature controller. The samples were prepared in the form of films with thickness of about 50 /[micro]m. The polymer films were sandwiched between two copper electrodes of diameter of 20 mm and placed inside temperature controlled sample cell. The complex permittivity: [epsilon]*([Florin]) = [epsilon]'([Florin]) + i[epsilon]"([Florin]) has been determined in the frequency ([Florin]) range of [10.sup.-1] - [10.sup.6] Hz and at temperature interval from - 100 to +200[degrees]C. The AC voltage applied to the capacitor was equal to 1.5 V. Temperature was controlled using a nitrogen gas cryostat and the temperature stability of the sample was better than 0.1[degrees]C.


General Characterization

The hybrid polyimide films containing various proportions of silica were obtained using the sol-gel process. First, a poly(amic acid) 1 was obtained by the reaction of DAB with ODPA, using NMP as solvent. To the solution of poly(amic acid) 1 was added APTES resulting the polyamic acid 2 having the inherent viscosity of 0.45 dl/g. Different quantities of TEOS and [H.sub.2]O in NMP were introduced into the solution of poly(amic acid) 2 (Scheme 1). The resulted mixtures were stirred for 12 h, and then they were cast onto glass plates, followed by a special treatment up to 300[degrees]C, to obtain polyimide/silica hybrids films.

FT-IR spectroscopy was used to study the chemical structure of the hybrid polyimide films containing various proportions of silica. The hybrid films presented characteristic absorption bands at 1776 [cm.sup.-1] and 1716 [cm.sup.-1] because of the symmetrical and asymmetrical vibrations of the carbonyl of imide rings, at 1371 [cm.sup.-1] because of C--N stretching, and at 744 [cm.sup.-1] because of the vibration of imide rings. The characteristic absorption peak of poly(amic acid) at 1650 [cm.sup.-1] disappeared, indicating that the imidization reaction was complete. Characteristic absorption bands appeared at 1262 [cm.sup.-1] for aromatic ether linkages and at 1591 [cm.sup.-1] and 1505 [cm.sup.-1] due to the vibration of aromatic rings in the polyimide. A very weak absorption appeared at around 3478 [cm.sup.-1] due to the presence of silanol groups (Si--OH) that were formed during the hydrolysis of alkoxy groups in APTES or TEOS. The characteristic band of Si--O--Si at 1040--1090 [cm.sup.-1] is overlapped by the absorption band of polyimide.

Atomic force microscopy was used to examine the film surface and to measure its surface topography. Figure 1 shows the bi- and three-dimensional structures of both reference and hybrid films PI-0 and PI-20. The surface of polyimide film PI-0 showed a low root-mean-square roughness value of 2.25 nm, over an area of 5 x 5 [micro][m.sup.2]. The surface of hybrid film PI-20 showed a higher root-mean-square roughness, of 5.34 nm. In a 2D image, Fig. 1c, bright domains refer to the inorganic particles. To investigate more precisely the film surface, the 2D image was reconstructed as a 3D image, which shows that inorganic particles seem to be embedded in the film surface (Fig. 1d). For the hybrid film PI-20, the diameter of the inorganic particles was lower than 1.5 [micro]m. Figure 2 shows the SEM images of PI-0 and PI-20 films. The EDX method able to identify the nature of the atoms present in the sample at a depth of 100--1000 nm from the surface reveals the distribution of the C, O, and Si. From the Fig. 2, it can be observed the dispersion of silica particles in polyimide that have irregular shapes.



The tensometric method was used for measuring dynamic contact angles, by the Wilhelmy plate technique. Values obtained for advancing and receding contact angles with water for the samples PI-0, PI-10, PI-20, and PI-40 are presented in Table 2. It can be seen that advancing contact angles increased by increasing the concentration of silica in the hybrid polyimide films.
TABLE 2. The main parameters of the water dynamic contact angle

              Water dynamic contact angles ([degrees])

Sample  [[theta].sub.a] (a)  [[theta].sub.r] (b)  H (c)

PI-0           82.86                34.13         48.74
PI-10          85.57                39.55         46.02
PI-20          86.99                49.64         37.35
PI-40          88.79                44.22         44.57

(a) Maximum advancing contact angle value, determined as average of the
three measurements.

(b) Maximum receding contact angle value.

(c) H = [[theta].sub.a] - [[theta].sub.r].

Another surface property of interest is the sorption, the process of interaction between the solute and the surface of an adsorbent. The forces involved can be strong (for example, hydrogen bonds) or weak (van der Waals forces). Water absorption of polymers heavily influences their dielectric constants and limits their application in the electric and microelectronic industry. It can increase the dielectric constant and promote the corrosion of metal conductors. Water vapors uptaking capacity for the samples PI-0, PI-10, PI-20, and PI-40 at 25[degrees]C in the relative humidity range RH 0-90% was investigated by using the IGAsorp equipment. The vapor pressure was increased in 10% humidity steps, every having a preestablished equilibrium time between 30 and 40 min (minimum time and time out, respectively). At each step, the weight gained was measured by electromagnetic compensation between tare and sample when equilibrium is reached. An anticondensation system was available for vapor pressure very close to saturation. The cycle was ended by decreasing the vapor pressure in steps to obtain also the desorption isotherms. The drying of the samples before sorption measurements was carried out at 25[degrees]C in flowing nitrogen (250 ml/min) until the weight of the sample was in equilibrium at RH <1%. The sorption/desorption isotherms for PI-0 and PI-40 are presented in Fig. 3. Maximum water vapor sorption values for the samples, at 25[degrees]C and RH = 90% are presented in Table 3. The polyimide PI-0 had the highest maximum water vapor sorption values (1.97%). Introducing of TEOS slightly decreased this property. It is considered that during the preparation of hybrid polyimide, porous silica is formed. Some of the pores are in the closed form, and the condensation of moisture inside the pores becomes difficult, thus reducing the water absorption (27).

TABLE 3. Maximum water vapor sorption values.

Sample  Weight change, % (Total water adsorption at RH = 90%,
                          T = 25[degrees]C)

PI-0                            1.97
PI-10                           1.94
PI-20                           1.80
PI-40                           1.82

Thermal Characterization of Hybrid Polyimide Films

The thermal oxidative stability was investigated by thermogravimetric analysis (TGA). The initial decomposition temperature and the temperature of 10% weight loss, the maximum decomposition temperature, and the residue at 700[degrees]C were listed in Table 4. All the hybrid polyimide films exhibited excellent thermal stability having the initial decomposition temperature in the range of 450-468[degrees]C and the temperature of 10% weight loss in the range of 500-515[degrees]C. The maximum polymer decomposition temperature was between 550[degrees]C and 570[degrees]C. As it can be seen from Table 4, the thermal stability did not decrease to a great extent by increasing the inorganic content. The percent of carbonaceous residue at 700[degrees]C slightly decreased with the increase of inorganic content, probably due to the incomplete silica conversion at processing temperature. At high temperature, the volatile products from the condensation reactions of the silicon alkoxide were eliminated (21).
TABLE 4. The thermal properties for samples PI-0, PI-10, PI-20, and

Sample  [T.sub.5] (a)  [T.sub.10] (b)  [T.sub.max]   Char yield at
        ([degrees]C)    ([degrees]C)       (c)       700[degrees]C

PI-0         468             515             566           44.0
PI-10        465             510             570           43.6
PI-20        465             505             555           40.0
PI-40        450             500             550           37.6

Sample  [T.sub.g] (d)  tan [delta] at  [T.sub.[beta]]  tan [delta] at
        ([degrees]C)     [T.sub.g]           (e)       [T.sub.[beta]]

PI-0         223            1.41             112.0          0.076
PI-10         --              --                --             --
PI-20        226            1.34             107.0          0.054
PI-40        228            0.86             106.5          0.045

(a) Initial decomposition temperature = the temperature of 5% weight

(b) Temperature of 10% weight loss.

(c) The maximum polymer decomposition temperature.

(d) Glass transition temperature, determined from DMA curves.

(e) Temperature of maximum [beta] relaxation process.

Figure 4a and b present the dynamic storage modulus E' and the loss tangent tan [delta] versus temperature, respectively, for the polyimide films PI-0, PI-20, and PI-40. An increase of E' can be observed both in the glassy region and at temperatures above the glass transition by introducing inorganic particles into the polyimide matrix due to an increase of the macromolecular chain rigidity.


The tan [delta] plots reveal the occurrence of three relaxation processes: the low and the medium temperature relaxations are defined, respectively, as the [gamma] and [beta] transitions (Fig. 4b). The high-temperature peak is an [alpha]-relaxation process and corresponds to the glass transition. The magnitude of tan [delta] at [T.sub.g] slightly decreased with the introduction of inorganic compound. In general, decreasing magnitude of tan [delta] with increasing particle concentration is a typical behavior for a particle filled system. The magnitude of tan [delta] at [T.sub.g] is a measure of the energy damping characteristic of a material and is related with the impact strength (28). The impact strength increased with the increase of tan [delta] value at [T.sub.g]. The value of tan [delta] at [T.sub.g] for PI-0 was about 1.410; however, that for PI-40 was only about 0.86. The results indicated that the energy damping characteristic of PI-0 was superior. The introduction of inorganic compounds increased the [T.sub.g] of the material from 223[degrees]C, in the case of PI-0, to 228[degrees]C, in the case of polymer film PI-40 indicating the restriction effect of the silica on polyimide molecule motion (Table 4).

The [beta] relaxation occurs below the [T.sub.g] and is associated with local bond rotation and molecular segment motion along the polymer backbone, and the magnitude of this relaxation is proportional with the concentration of segments contributing to the relaxation. By increasing the silica content the breadth of the [beta] relaxation was also characterized by broadening and a decrease of the tan [delta] at maximum [beta] relaxation appeared. This suggested that the number of polymer segments in motions decreased with the increase of silica content. The temperature of the [beta] transition slightly decreased by increasing the inorganic content (Table 4).

Dielectric Spectroscopy Analyses of Hybrid Polyimide Films

Electrical insulating properties of polyimide films were evaluated on the basis of dielectric constant and dielectric loss and their variation with frequency and temperature. In an alternating electric field, the dielectric constant is a complex quantity, [epsilon]* = [epsilon]' - i[epsilon]", where [epsilon]' is the relative permittivity or dielectric constant, and [epsilon]" is an imaginary component called dielectric loss or dissipation factor. Figure 5 presents the dependence of real and imaginary parts of complex permittivity on frequency, for the samples PI-0, PI-10, PI-20, and PI-40, at three chosen temperatures. From Fig. 5, it can be seen that [epsilon]' slightly decreased with increasing frequency. A good dielectric should exhibit low variation of the dielectric constant with frequency and temperature. The dielectric constant of polymers decreased gradually with increasing frequency because the response of the electronic, atomic, and dipolar polarizable units varies with frequency. It depends on the ability of the polarizable units in a polymer to orient fast enough to keep up with the oscillations of an alternating electric field. When frequency increases the orientational polarization decreases because the orientation of dipole moments needs a longer time than electronic and ionic polarization. For all the films, the dielectric constant increased with the increase of the temperature. With increasing temperature, the chain segment mobility increases and polar groups start to move in response to the applied electrical filed, which increases the orientation of polymer and dielectric constant (29). The dielectric constants for the samples PI-0, PI-10, PI-20, and PI-40 at 0.1 Hz, 100 Hz, and 10 kHz, at room temperature, are shown in Table 5. The values of the dielectric constant at 10 kHz were in the range of 2.64 - 3.16. Dielectric constant of dense silica (4.0) is higher than that of the polyimide PI-0 (3.16) as was measured by dielectric spectroscopy. Therefore, the addition of silica in PI should raise the dielectric constant of hybrid film. As shown in Table 5, the dielectric constant of hybrid films decreased by introducing silica into polyimide. The lowest dielectric constant about 2.64 is achieved in the case of PI-20. The decrease of dielectric constant in the case of hybrid polyimide films is attributed to the presence of inorganic silica in the polyimide. It reduced the humidity absorption of the material and increased the free volume thus decreasing the dielectric constant. Also, the presence of the segments derived from APTES tends to decrease the hydrophilicity of hybrid polyimides (30). The formation of porous silica may play an important role in the suppression of dielectric constant of hybrids. However, it was not found a clear dependence between the inorganic content and the dielectric constant. The sample PI-40, having a higher concentration of silica, exhibited higher dielectric constant than PI-20 probably due to the presence of Si--OH groups in higher concentration.

TABLE 5. Dielectric constant at selected frequency, at 25[degrees]C.
and activation energies of relaxation phenomena for samples PI-0.
PI-10. PI-20. and PI-40.

          Dielectric constant at

Sample  1 Hz  100 Hz  10 kHz  1 MHz  [E.sub.a] of  [E.sub.a] of
                                       [gamma]        [beta]
                                      relaxation    relaxation
                                       (kJ/mol)      (kJ/mol)

PI-0    3.20   3.17    3.16    3.12      43.1          57.6
PI-10   2.70   2.72    2.71    2.67      43.2          53.9
PI-20   2.69   2.66    2.64    2.61      42.2          54.9
PI-40   2.84   2.81    2.80    2.75      43.6          51.0

The dielectric loss for hybrid polyimide films at different temperatures taken in the range from - 100[degrees]C to +200[degrees]C is shown in Fig. 5. Two secondary [beta] and [gamma] relaxation processes connected with local movements of polymer chain appeared. For polyimide film PI-0 and hybrid polyimide films PI-10, PI-20, and PI-40, the dielectric loss exhibited low values in the interval of measured frequency and temperature. Low values of the dielectric loss are indicative of minimal conversion of electrical energy to heat in the dielectric material. It is advantageous to have low values for both dielectric constant and dielectric loss because electrical signals will loss less of their intensity in the dielectric medium (29). By introduction of silica into the polyimide, an increase of the dielectric loss appeared at high temperatures and low frequencies. PI-10, PI-20, and PI-40 exhibited broader [beta] relaxation peaks when compared with PI-0.

To analyze the relaxation of polymer films, the complex permittivity was converted to the complex dielectric modulus M *([omega])) = [1/[epsilon]*]([omega]) according to an equation described in the literature (31). The real (M') and imaginary (M") parts of the dielectric modulus can be calculated from [epsilon]' and [epsilon]":

M' = [[epsilon]'/[[[epsilon]'.sup.2] + [[epsilon]".sup.2]]]; M" = [[epsilon]"/[[[epsilon]'.sup.2] + [[epsilon]".sup.2]]] (1)

Figure 6 presents the dependence of M', M" on the frequency, at different selected temperatures for sample PI-40. On the M" diagrams two relaxation processes, [beta] and [gamma], appeared. The activation map is the best for a comparison of all samples and is necessary for calculation of activation energy. Relaxation times ([tau]) of these two processes at various temperatures have been determined from maxima position of M" at frequency scale ([tau] = [1/[[omega].sub.max]]). The relaxation times ([tau]) were calculated from the equation [tau] = [1/2[pi][[Florin].sub.max]], where [[Florin].sub.max] is the frequency of the relaxation peak at a given temperature T. The activation plots are shown in Fig. 7. In Table 5, the activation energies of relaxation processes calculated on the bases of these diagrams are presented.

The [gamma] relaxation is assigned to the small scale local oscillations of imide rings (25). The value of activation of [gamma] relaxation for PI-0 was of 43.1 kJ/mol. The hybrid films PI-10, PI-20, and PI-40 exhibited similar values (43.2 kJ/mol for PI-10, 42.1 kJ/mol for PI-20 and 43.6 kJ/mol for PI-40) (Table 5) suggesting the existence of the same process for this transition. Similar results have been reported in the literature (32). The magnitude of the [gamma] relaxation of hybrid films slightly increased with increasing silica content.



Although the exact description is still uncertain, the [beta] transition generally associated with local bond rotations along the polyimide backbone. In general, these motions are considered to be primarily a function of the polyimide structure, and their presence and magnitude have been ascribed to several material properties. About the [beta] relaxation, there are many proposed explanations. It was proposed that the main type of motion was a rotational vibration of small segments of a chain around quasi-equilibrium positions. The motion of phenylene and imide rings contributed to the vibration (33). Sun et al. (34) also reported that [beta] relaxation related with rotation of phenylene and imide groups around hinges such --O--, --[CH.sub.2]-- linkages. A decrease of the maximum of the peak that characterized the [beta] relaxation appeared by increasing the silica content in the hybrid polyimide film. The activation energy of [beta] relaxation was of 57.6 kJ/mol for PI-0 and slightly decreased by increasing the silica content (Table 5), probably due to an increase of free volume resulting in an increase of molecular mobility, as a consequence of a change of the intermolecular interactions that play a role in the relaxation process.


Hybrid polyimide films having different content of silica were prepared starting from a poly(amic acid) and tetraethoxysilane. The films having the thickness in the range of tens of micrometers were flexible and were used for dynamic mechanical analysis and dielectric relaxation spectroscopy. The introduction of silica into polyimide films increased the hydrophobicity and reduced the humidity absorption and dielectric constant. The presence of silica produced change in the [alpha] relaxation process and slightly shifted [T.sub.g] to higher values. Two subglass transitions, [gamma] and [beta], were evidenced by dielectric spectroscopy. The nature of the [gamma] transition process was not influenced to a great extent by the presence of silica in the polyimide matrix. The temperature of the [beta] transition process shifted to lower values with increasing of silica content.


The authors thank Dr. M. Cazacu and Dr. M. Cristea from "Petru Poni" Institute of Macromolecular Chemistry, Iasi, Romania, for DCA, water vapors sorption measurements and DMA analyses.


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Elena Hamciuc, (1) Corneliu Hamciuc, (1) Marius Olariu (2)

(1) "Petru Poni" Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41A, lasi-700487, Romania

(2) Department of EMEM, Electrical Engineering Faculty, "Gh. Asachi" Technical University, B-dul D. Mangeron 53-55, lasi-700050, Romania

Correspondence to: Elena Hamciuc; e-mail:

Contract grant sponsor: Romanian Ministry of Education and Research

(Program PN II-IDEA); contract grant number: 654/2009.

DOI 10.1002/pen.21562

Published online in Wiley InterScience (

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Author:Hamciuc, Elena; Hamciuc, Corneliu; Olariu, Marius
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:4EXRO
Date:Mar 1, 2010
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