On the dynamic mechanical behavior of polyimides based on aromatic and alicyclic dianhydrides.INTRODUCTION
In recent years, there has been increasing interest in the understanding of macroscopic macroscopic /mac·ro·scop·ic/ (mak?ro-skop´ik) gross (2).
mac·ro·scop·ic or mac·ro·scop·i·cal
1. Large enough to be perceived or examined by the unaided eye.
2. properties of polymers such as ductility, dielectric behavior, gas permeation character, and so on. In the field of amorphous polymers, works have shown that some specific properties appear to be dependent on chain mobility. For instance, the mechanical behavior is likely to be controlled by the cooperative character of main-chain motion. For PEEK (1) and for polycarbonate A category of plastic materials used to make a myriad of products, including CDs and CD-ROMs. (2), it has been shown that the brittle-to-ductile transition temperature is dependent on the subglass relaxation temperature. Recently for polyimides, Chauvin et al. (3) pointed out that the [beta] subglass transition temperature coincides with the temperature at which oriented chromophores starts to disorient dis·o·ri·ent
tr.v. dis·o·ri·ent·ed, dis·o·ri·ent·ing, dis·o·ri·ents
To cause (a person, for example) to experience disorientation.
Verb 1. . Accordingly, it is of interest to understand which parameters govern the polymer relaxation.
Polyimides exhibit several temperature (or frequency) regions where relaxation processes occur. Each may be associated with specific molecular motions. The molecular motions of polyimides have been examined by different techniques including dielectric relaxation (4-6), nuclear magnetic resonance nuclear magnetic resonance: see magnetic resonance.
nuclear magnetic resonance (NMR)
Selective absorption of very high-frequency radio waves by certain atomic nuclei subjected to a strong stationary magnetic field. spectroscopy (4, 7) and dynamic mechanical analysis (DMA (1) (Digital Media Adapter) See digital media hub.
(2) (Document Management Alliance) A specification that provides a common interface for accessing and searching document databases. ). Three relaxations, designed by [alpha], [beta] and [gamma], have been detected by DMA. Some controversy still remains concerning the molecular mechanisms at the origin of the two subglass transitions.
The [gamma] mechanical relaxation has been observed at about -100[degrees]C at 1 Hz (8-11) This transition obeys an Arrhenius law and the activation energy is about 50 kJ/mol (4, 9, 10). It was proposed a motion relaxation of water molecules linked to the polar group in polyimides (4, 11-13). Supported by NMR NMR: see magnetic resonance. studies, dielectric loss measurements conducted by Greenbaun et al (4, 14) have shown the presence of a second [gamma] relaxation related to small clusters of water molecules. By contrast, based on 0-13 NMR experiments, Cheng et al (10) have associated the [gamma] relaxation to oscillation and [pi] flipping motions of the phenoxy group in the diamine di·am·ine
Any of various chemical compounds containing two amino groups, especially hydrazine.
Noun 1. diamine - any organic compound containing two amino groups sequence. This hypothesis was supported by Habas et al (11) analysis.
Above room temperature, polyimides display the [beta] relaxation at 1 Hz located in a large temperature range i.e. 100-200[degrees]C (1 Hz). The activation energy of this transition is about of 100-200 kJ/mol (9-11, 15, 16). Ikeda has attributed this relaxation to interplane slippage of crystalline region (17). Because this transition is also detected in amorphous polyimides, more recent works have attributed it to motions in the polyimide Pronounced "poly-ih-mid." A type of plastic (a synthetic polymeric resin) originally developed by DuPont that is very durable, easy to machine and can handle very high temperatures. Polyimide is also highly insulative and does not contaminate its surroundings (does not outgas). chain backbone. Based on 0-13 NMR studies, Cheng et a!. conclude that the molecular motion is mainly connected to the dianhydride moiety moiety: see clan. (10). Other investigators have ascribed this relaxation to oscillations or rotations of the para-phenylene moieties In the diarnine part (12, 13, 18). The hypothesis of Sun et al. has related the [beta] relaxation to the rotation of rigid segment of p-phenylene and imide imide /im·ide/ (im´id) any compound containing the bivalent group, dbondNH, to which are attached only acid radicals.
n. groups (11, 16). More recently, Li et al. (19) and Vora et al. (6) have distinguished two [beta] processes. At low temperatures, the [[beta].sub.1] relaxation was associated w ith noncooperative motions of the diamine while the [[beta].sub.2] component is caused by motion of the dianhydride sequence. The cooperative character of this last process depends on the chemical structure (19).
In this paper, we propose to investigate the microstructural parameters, which are susceptible to govern the viscoelastic Adj. 1. viscoelastic - having viscous as well as elastic properties
natural philosophy, physics - the science of matter and energy and their interactions; "his favorite subject was physics" behavior of polyimides. In this aim, based on preliminary study (20, 21). the dynamic mechanical behavior of aromatic and semi-aromatic polyimides have been studied. The semi-aromatic polyimide is based on bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic. This dianhydride is of interest because of its high rigidity and its alicyclie character which gives rise to a soluble product (22, 23). The changes in the chemical structure correlated to the effect on the viscoelastic behavior of polyimides should also give information on the molecular processes, which are involved in the different relaxations.
Materials and Samples
The dianhydrides and the diamines used in this study are commercially available, except for the 2,6-bis-(3'aminophenox)-benzonitrile (BACN BACN Battlefield Airborne Communications Node (US Air Force)
BACN Business Advisory Centre Northumberland (Cobourg, ON, Canada) ), which was synthesized according to a previously reported procedure (24). The solvents and commercial monomers were purified prior to use. m-Oresol was distilled at 140[degrees]C under reduced pressure. The pyromellitic dianhydride (PMDA), the bicyclo(2.2.2)oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BCDA BCDA Bases Conversion Development Authority (Philippines)
BCDA British Columbia Dart Association
BCDA Buckeye Cow Dog Association, Inc (Ohio)
BCDA Business Computer Dealers Association ) and the 3,3',4,4'-benzophenonetetracarboxylic (BTDA) were recrystallized from acetic anhydride. acetonitrile acetonitrile /ac·e·to·ni·trile/ (as?e-to-ni´tril) a colorless liquid with an etherlike odor used as an extractant, solvent, and intermediate; ingestion or inhalation yields cyanide as a metabolic product. and acetic acetic /ace·tic/ (ah-se´tik) (ah-set´ik) pertaining to vinegar or its acid; sour.
pertaining to vinegar or its acid; sour. anhydride/xylene (35/65). respectively. 9,9-bis(4'-aminophenyl)fluorene (FDA) was recrystallized twice from a mixture of anisole/xylene (60/40) while 4,4' aminodiphenyl-ether (DDE (Dynamic Data Exchange) A message protocol in Windows that allows application programs to request and exchange data between them automatically.
DDE - Dynamic Data Exchange ) was purified by sublimation sublimation, in chemistry
sublimation (sŭblĭmā`shən), change of a solid substance directly to a vapor without first passing through the liquid state. at 200[degrees]C under reduced pressure. Bis[4-(4'-ammophenoxy)phenyl phenyl (fĕn`əl), C6H5, organic free radical or alkyl group derived from benzene by removing one hydrogen atom. ]sulfone sulfone /sul·fone/ (sul´fon)
1. the radical SO2.
2. a compound containing two hydrocarbon radicals attached to the —SO2— group, especially dapsone and its derivatives, which are potent antibacterials effective (BAPS BAPS British Association of Plastic Surgeons (now British Association of Plastic, Reconstructive and Aesthetic Surgeons)
BAPS Bochasanwasi Shri Akshar Purushottam Swaminarayan Sanstha
BAPS British Association of Paediatric Surgeons ) was purified through a column chromatography column chromatography
A form of partition chromatography in which a liquid phase flows down a column packed with a solid phase. on silica gel with the cyclohexan/ethyl acetate (50/50) as eluent eluent
the solution used in elution. .
The chemical structures of investigated polyimides are shown in Fig. 1. Polyimides based on BCDA dianhydride were synthesized in m-cresol while N-methyl pyrrolidinone (NMP NMP New Millennium Program (NASA)
NMP National Military Park (National Park Service)
NMP Network Management Protocol
NMP Not My Problem ) was used for the synthesis of BTDA- and PMDA-based polyimides. The polyimides were obtained via a two-step process by thermal imidization method as shown in Scheme 1. Excepted for PMDA-DDE, the poly(amic acids) were not isolated and all the polymers were synthesized by thermal imidization method in solution. In a thoroughly dried 250 ml three-necked round bottom flask equipped with a mechanical stirrer, a nitrogen inlet tube and a reflux condenser with a drying tube, the diamine was dissolved in the anhydrous an·hy·drous
Without water, especially water of crystallization.
adj without water.
containing no water. solvent. A slow nitrogen flow through the flask was continuously employed throughout the reaction. A solution of the dianhydride In anhydrous solvent was rapidly added. The mixture was stirred at room temperature, then at 160-180[degrees]C for at least 5 hours and finally cooled down. Approximately 20% of the polymer solution was remo ved, poured into an excess of methanol and homogenized with a laboratory mixer. The precipitate was filtered off and soaked in excess methanol during 2h. The precipitate was again filtered. The obtained powder was finally dried in an oven at 80[degrees]C under reduced pressure for 4h. Intrinsic viscosity of the polymers was measured using Ubbelhode viscosimeter viscosimeter
an apparatus used in measuring viscosity of a substance. at 30[degrees]C (Table 1) with 1% wt. solutions in NMP or m-cresol. These viscosities indicate that the molecular weights of our synthesized polymers were high enough to obtain polymeric films.
Typically, the polyimide films were prepared by casting 20 wt% polymer solution onto clean glass plate and then dried in an oven. Two thermal programs were used. For BCDA-polyimides, it was imposed 70[degrees]C for 4h, 100[degrees]C for 4h and 250[degrees]C for 7h. For BTDA-, and PMDA-polyimides, the thermal cycling is 100[degrees]C for 1h, 200[degrees]C for 1h and 300[degrees]C for 3h. Solvent content was determined by thermogravimetric analysis to be less than 1% (w/w). Completed imidization of all soluble polymers was confirmed by NMR analysis. Then, films were kept under room atmosphere before analysis.
Glass transition temperatures of polyimides were measured by differential scanning calorimetry Differential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference are measured as a function of temperature. using a Perkin Elmer DSC-7 instrument purged with nitrogen gas. DSC (1) (Digital Signal Controller) A microcontroller and DSP combined on the same chip. It adds the interrupt-driven capabilities normally associated with a microcontroller to a DSP, which typically functions as a continuous process. See microcontroller and DSP. thermograms were recorded from +200[degrees]C + 500[degrees]C at 20[degrees]C/min. Densities of the film were measured by floating method. The density of n-hexane was adjusted by mixing with carbon tetrachloride carbon tetrachloride (tĕ'trəklôr`īd) or tetrachloromethane (tĕ'trəklôr'əmĕth`ān), CCl4, colorless, poisonous, liquid organic compound that boils at 76. until the films reached the flotation equilibrium at 20[degrees]C. This density method was validated by using calibrated density floats. The accuracy of the density measurement was of [+ or -]0.001. The Wide-angle X-ray scattering (WAXS WAXS Wide-Angle X-Ray ) measurements were performed on a Siemens D500 X-ray diffractometer A Diffractometer (Main Entry: dif·frac·tom·e·ter Pronunciation: di-"frak-'tä-m&-t&r Function: noun) is a measuring instrument for analyzing the structure of a usually crystalline substance from the scattering pattern produced when a beam of radiation or particles (as X rays or (CuK[alpha]l, Nickel-filtered radiation) with 0.02[degrees] (2[theta Theta
A measure of the rate of decline in the value of an option due to the passage of time. Theta can also be referred to as the time decay on the value of an option. If everything is held constant, then the option will lose value as time moves closer to the maturity of the option. ]) scan increments in reflexion mode. Film transmission spectra were measured at room temperature using a UV-visible spectrophotometer spectrophotometer, instrument for measuring and comparing the intensities of common spectral lines in the spectra of two different sources of light. See photometry; spectroscope; spectrum. (Perkin-Elmer). The curves were then normalized to a 20 [micro]m thickness according to the Beer-Lambert Law. From these spectra, the [[lambda].sub.0] parameter was determined by the wavelength of transmission threshold. Water contents of films were estimated by weighing the sample at a heating rate of 50[degrees]C/min under air, using a TA2050 thermogravimetric analyzer.
Dynamic mechanical experiments have been carried out using a Rheometrics MKIII analyzer. The instrument was operated in a tensile mode at three frequencies (1, 3 and 10 Hz). Dynamic strain amplitude of 10 [micro]m (0.1%) was superimposed on a static deformation. Isochronal i·soch·ro·nal or i·soch·ro·nous
1. Equal in duration.
2. Characterized by or occurring at equal intervals of time. scans for were recorded at 1[degrees]C/min from - 150[degrees]C to up to the glass transition under nitrogen gas. A storage modulus E', a loss modulus E" and tan[delta] were recorded. The relaxation temperature (T) is determined at the maximum of tan[delta] peak. The frequency (f) dependence of the relaxation temperature (T) of the subglass relaxation is assumed to obey to the Arrhenius law.
f = [f.sub.0] * exp (-[E.sub.a]/RT) (1)
where [E.sub.a] is the activation energy and [f.sub.0] is the preex-ponential factor.
RESULTS AND DISCUSSION
All the studied polyimides form transparent and yellow films. Toughness of these films allows mechanical handling. The physical characteristics of the synthesized polyimide films derived from various analyses are listed In Table 2.
The wide-angle X-ray diffraction curves of all polyimide films exhibit a broad peak, which is consistent with an amorphous character. Nevertheless, some differences between the amorphous structures of the polyimide are detected as the comparison of the WAXS patterns suggests (Fig. 2 and 3). According to the literature, the top of broad peaks on each X-ray pattern for noncrystalline polymers Is attributed to intersegmental interference. It Is thus representative of the average intersegmental distance, called d-spacing and calculated from the Bragg's equation. The intermolecular Adj. 1. intermolecular - existing or acting between molecules; "intermolecular forces"; "intermolecular condensation" chain packing can also be quantified by the free volume fraction (FVF FVF Federación Venezolana de Fútbol (Venezuelan Football Federation)
FVF Final Value Fee (eBay auctions fee - based on a percent of the final selling price)
FVF Formation Volume Factor ) determined from density measurements ([rho]) according to the equation.
FVF = 1 - [rho]/M [V.sub.0] (2)
where M is the molar weight and [V.sub.0] is the occupied chain volume. Based the Bondi group contribution method (25, 26). [V.sub.0] was estimated to be 1.3 times the Van der Waals volume ([V.sub.w]). The d-spacing and FVF values are listed in Table 2. Excepted for PMDA-DDE, no comparison with literature value can be made for these microstructural parameters. For PMDA-DDE, the density value reported in the literature lies in the 1.395 (273-1.431 (28) g.[cm.sup.-3] range. Both density and d-spacing values are also consistent with those reported by Stern et al. (29). The d-spacing measured for BCDA-based polyimides is higher than that of other polyimides. Moreover, the highest FVF were observed for polyimides based either on BCDA dianhydride or on FDA diamine. These high values of both microstructural parameters relating to these BODA-based polyimides can be attributed to the alicyclic structure, which induces steric steric /ste·ric/ (ster´ik) pertaining to the arrangement of atoms in space; pertaining to stereochemistry.
ster·ic or ster·i·cal
n. hindrance for chain packing.
The [[lambda].sub.0] parameter value is mainly attributed to charge transfer complex (CTC) formation (30, 31), i.e. charge-transfer between electron-acceptor diimide fragments and electron-donor aromatic units (32, 33). The polyimides based on BODA dianhydrides are colorless as [[lambda].sub.0] ranges from 275 to 315 nm. These results are consistent with Chun study (22), which indicates that BODA does not favor inter-molecular interaction on both steric and electronic point of view because of its non-planar and non-aromatic structure. Now, it is of interest to examine the correlation between [[lambda].sub.0] and FVF (Fig. 4) for our polyimides. No data from literature is available for comparison. Good correlation between FFV FFV
First Family of Virginia and [[lambda].sub.0] can be observed except for both FDA-based polyimides. This induces that a decrease in chain packing could be attributed to the reduction of intermolecular CTC Interaction, as the decrease of [[lambda].sub.0] value suggests. However, both BTDA-FDA and BCDA-FDA films seem to e xhibit free volume fraction higher than expected from transmission measurements. This could be due to a [V.sub.w] underestimation of fluorene bridge because of the additive group method (26). This hypothesis could explain why representative points for 9,9-fluorenylene bridge polyimides deviate from the logarithmic logarithmic
pertaining to logarithm.
when the logs of two variables plotted against each other create a straight line. relationship of oxygen permeability versus FVF (34).
Viscoelastic spectra of the BTDA-DDE polyimide are shown in Fig. 5. The BTDA-DDE polyimide exhibits three relaxations. The low-temperature process, designed as [gamma] relaxation, is observed at -95[degrees]C for 1 Hz. The activation energy of this subglass transition determined according the Arrhenius law is about 50 kJ/mol. The pre-exponential factor is about [10.sup.+14] Hz, characteristic of transition related to non-cooperative localized motions. The second subglass transition ([beta] relaxation) located at 118[degrees]C (1 Hz) displays higher activation energy and pre-exponential factor, respectively of about 130 kJ/mol and [10.sup.+18] Hz, respectively. The high-temperature transition ([alpha] relaxation) observed at 290[degrees]C (1 Hz) is related to the glass transition.
The changes of these three relaxations depending on the polyimide chemical structure are now being explored. Therefore the tensile tan[delta] profile for the different polymers are shown in Fig. 6-8. Table 3 summarizes the [alpha], [beta] and [gamma] relaxation temperatures. The activation energy and preexponential factor are also reported for subglass transitions.
The [alpha] Relaxation
The [alpha] relaxation temperature of theses polyimides ranges from 305 to 420[degrees]C. The Tg have been also determined from DSC thermograms. Nevertheless, the characteristic gap in heat capacity is not detected for PMDA-DDE and BCDA-FDE owing to the beginning of thermal degradation. A good correlation is found for Tg and T[alpha]. It is interesting to note that polyimides based on BACN have the lowest T[alpha]. This could be related to both the ether flexible bond and the meta link (33). The fluorene group gives rise to the highest T[alpha] whatever the dianhydride group is. This result can be attributed to steric hindrance of the fluorene linkage. Moreover, for flexible diamine as DDE and BACN, the T[alpha] of BTDA-based polyimides is the lowest. Thus, T[alpha] can not be related to the chain packing but it is mainly controlled by the monomer rigidity.
The [beta] Relaxation
As can be seen in Fig. 6-8, the [beta] relaxation is quite broad compared with both of the other mechanical relaxations. For BTDA-based polyimides, this relaxation is quite well defined from -50[degrees]C to the beginning of the glass transition temperature The glass transition temperature is the temperature below which the physical properties of amorphous materials vary in a manner similar to those of a solid phase (glassy state), and above which amorphous materials behave like liquids (rubbery state). . The activation energy is of the same order as those reported in the literature (15). The activation energy calculated from Starkweather theory (9, 35, 36) for the lower limit of Ea for non-cooperative motion is of about 95 kJ/mol. As this value is close to the experimental activation energy and considering the preexponential factor of [10.sup.19] Hz, the [beta] relaxation would originate from quite local motions. It is worth noting that cooperative motions in poly(aryl ar·yl
An organic radical derived from an aromatic compound by the removal of one hydrogen atom. ether ether ketone ketone (kē`tōn), any of a class of organic compounds that contain the carbonyl group, C=O, and in which the carbonyl group is bonded only to carbon atoms. ) is associated to a preexponential factor of [10.sup.30] Hz (37). No [beta] relaxation can be detected for PMDA- and BCDA-based polyimides, excepted for BCDA-BACN. Nevertheless, the tan[delta] value in 0 to 250[degrees]C-temperature range is of about 0.02. This value can not result from to the apparatus contribution. This could be attributed a very broad [beta] relaxation located from at least--150[degrees]C to the beginning of the glass transition temperature. Thus no characteristic parameter of this transition can be determined for these polyimides. These observations are consistent with the results of Sun et al. (16). Indeed, they have concluded that the [beta] relaxation is related to rotations of rigid segment of para-phenylene and imide rings due to phenyl-nitrogen conjugation conjugation, in genetics
conjugation, in genetics: see recombination.
conjugation, in grammar
conjugation: see inflection. .
However, this hypothesis does not explain why [beta] viscoelastic behavior of BCDA-BACN is the same as that of displayed by BTDA-BACN and differs from those exhibited by other BCDA-based polyimides (Fig. 6 and 8). No difference in microstructural parameters listed in Table 2 can be found to explain such a difference. However, the BACN unit differs from other diamine groups in that it is a meta diamine. Based on [N.sup.15] NMR chemical shift, Ando et al. (30, 38) have pointed out that the meta-linked diamines exhibit lower electron-donating property than para-linked one. Thus, the intra-molecular charge transfer in BACN-based polyimides can be strongly decreased in comparison to those of polyimides based on para-diamme. In these conditions, the [beta] relaxation of BACN-based polyimides likely arises from both oscillations of the imide ring and motions of the phenyl ring in the diamine moieties.
The [gamma] Relaxation
The temperature and the magnitude of the [gamma] relaxation are strongly dependent on the chemical structure of polyimide (Fig. 6-8). Nevertheless, after drying polyimide films in the dynamic mechanical setup, the magnitude of the [gamma] relaxation sharply decreases and it seems that it disappears as it is shown in Fig. 9. Furthermore, the height of the [gamma] relaxation tan[delta] peak ([H[gamma]) tends to increase when increasing the water content (Tables 2 and 3). Thus, unlike 13 and a relaxations, the [gamma] relaxation seems not to be only related to motions of the macromolecular mac·ro·mol·e·cule
A very large molecule, such as a polymer or protein, consisting of many smaller structural units linked together. Also called supermolecule. backbone.
The comparison of the viscoelastic behavior for DDE-based polyimides in the -150[degrees]C to 0[degrees]C-temperature range is shown in Fig. 6. As reported in Table 3, the [gamma] relaxation temperature ([T.sub.[gamma]]) strongly depends on the dianhydride part. Nevertheless, the activation energy and the preexponential factor of this relaxation are in the same value range for all these polyimides. The [gamma] relaxation occurs 20[degrees]C lower for BTDA-DDE than for PDMA-DDE and it is located 35[degrees] higher than for BCDA-DDE. The [gamma] transition of the BTDA-based polyimides is located at higher temperature than those of the BCDA-based polyimides (Fig. 8). The tan[delta] spectra for BTDA-based polyimides are shown as a function of temperature at 1 Hz in Fig. 7. A small difference between [T.sub.[gamma]] for polyimide based on the same dianhydride part can be observed when changing the diamine structure. Such differences do not exceed 10[degrees]C (Table 3). Nevertheless, polyimides based on FDA diamine exhibit the lowest [T.sub.[gamma]] whatever the dianhydride group can be.
For the studied chemical structures, the [gamma] relaxation temperature seems to be more sensitive to the dianhydride moieties than to the diamine chemical structure. The comparison of the [gamma] process at 1 Hz induces the following classifications:
[T.sub.[gamma]]-BCDA [less than or equal to] [T.sub.[gamma]]-BTDA [less than or equal to] T-PMDA
[T.sub.[gamma]]-FDA [less than or equal to] [T.sub.[gamma]]-DDE [less than or equal to] [T.sub.[gamma]]-BACN
Replacement of BTDA dianhydride that can be considered as flexible groups by rigid dianhydride as BCDA and PDMA structure does not induce a systematic change on the [gamma] relaxation. Such a result is not consistent with conclusion reported by Hirayama et al. (39).
Xu et al. (4) have observed that the [gamma] relaxation temperature of PMDA-DDE is not dependent on the water content. It results that the modifications in the [gamma] relaxation temperature with chemical structure cannot be attributed to the differences in water content. To point out parameters, which govern the [gamma] relaxation temperature, dianhydride structures can be first compared. PMDA displays a planar and conjugated structure. Moreover, PMDA diimide has a high electron affinity (33). Thus PMDA-ODA film for which [gamma] relaxation is located at the highest temperature exhibits the lowest FVF and the highest [[lambda].sub.0]. In contrast, the presence of the cyclohexene in BCDA dianhydride avoids the interaction by conjugation and, thus, reduces the electron affinity of the diimide. In addition, BCDA displays a non-planar structure. Thus, the replacement of PDMA unit by the BCDA element could weaken the interaction between the polyimide chains as the decrease of [[lambda].sub.0] and the increase in both FVF and d-spacing suggest (Table 2). This decrease in the interchain interaction makes the packing chain state looser and promotes the molecular process at the origin of the [gamma] relaxation. Then, this induces a shift of the [gamma] relaxation towards lower temperatures (Table 3).
A similar argument can be proposed for the comparison of BTDA-based polyimides with PMDA-DDE. The BTDA diimine has a lower electron affinity than the PMDA moieties. i.e. 1.55eV (38)-1.64eV (40) for BTDA and 1.90eV (40) for PMDA. These differences arise from the introduction of carbonyl group carbonyl group (kär`bənĭl), in chemistry, functional group that consists of an oxygen atom joined by a double bond to a carbon atom. The carbon atom is joined to the remainder of the molecule by two single bonds or one double bond. in the dianhydride, which isolates the powerful electron withdrawing effect of imide groups from each other (33). Moreover, the carbonyl group induces a torsion torsion, stress on a body when external forces tend to twist it about an axis. See strength of materials. angle between the imide group. Therefore the charge transfer interaction and, thus, the [T.sub.[gamma]] of BTDA-DDE are lower than those exhibited by PMDA-DDE. The comparison with polyimides based on 2,2-bis(3,4-decarboxyphenyl) hexafluoropropane dianhydrides (6FDA) agrees with this interpretation. As a matter of fact, the perfluorinated group breaks up the electron conjugation between the imide groups (22, 41), then distorts the linearity of the polyimide chain and induces steric hindrance for chain interactions. These effects produce a weak CTC formation. In suc h conditions, it is not surprising that Hirayama et al. have observed the [gamma] relaxation of 6FDA-DDE at -116[degrees]C for 1 Hz (39).
The effect of the diamine part on the [gamma] relaxation temperature can also be explained according to intermolecular considerations. Like 6FDA, the FDA diamine induces steric hindrance, which limits the interchain CTC formation (34). The increase in the free volume fraction for the polyimides based on 9,9-bis(4'-aminophenyl)fluorene (FDA) compared to the DDE-based polyimides (Table 2) seems to induce a shift of the [T.sub.[gamma]] towards the lower temperature.
In order to give evidence for the intermolecular contributions to the [gamma] relaxation temperature variation, [T.sub.[gamma]] was plotted versus microstructural parameters (Figs. 10 and 11). In Fig. 10, the points 1 and 2 represent data of FDA-based polyimides, which FFV is likely overestimated. As shown in Fig. 10, [T.sub.[gamma]] increases when the free volume fraction decreases. In spite of the fact that this correlation cannot be confirmed by the result of Hirayama et al. (39, 42, 43), this tendency was consistent with previous [T.sub.[gamma]]/FVF relationship reported by Aitken et al. (44) for polysulfone (Fig. 10). Such an evolution is in agreement with the increase of the [gamma] relaxation temperature when the average interchain distance decreases (Fig. 11). In addition, the gamma relaxation appears at higher temperature when the wavelength of transmission threshold increases. Thus, the improvement of inter-molecular charge transfer which results in an increase of macromolecular chain packing makes the molecular process at the origin of the [gamma] relaxation more difficult and implies a shift of this relaxation towards the higher temperatures.
The linear correlation coefficients ([r.sup.2]) between the microstructural parameters and the [gamma] relaxation temperature are calculated based on N measurements. According to both [r.sup.2] and N values, the probability of linear correlation (P) is determined (Table 4). Based on the P values, statistical correlation can be discussed. Thus, it is likely that [T.sub.[gamma]] and [[lambda].sub.0] are correlated with a probability of 99%. In addition, the chance that free volume fraction and polyimide [T.sub.[gamma]] dependence is high (94%). Nevertheless, the probability for correlation between FVF and [T.sub.[gamma]] is lower than that of [T.sub.[gamma]] and [[lambda].sub.0] because of the additive group method for the Van der Waals volume of cyclic chemical function. In addition, correlation between [T.sub.[gamma]] and d can be assumed In spite of the fact that the statistical value is the lowest.
Relationships between the dynamic mechanical behavior of polyimides and the chemical structure as well as the microstructure mi·cro·struc·ture
The structure of an organism or object as revealed through microscopic examination.
a structure on a microscopic scale, such as that of a metal or a cell were investigated by studying film specimens. Polyimides were synthesized with aromatic and non-aromatic dianhydride monomers. Both [beta] and [alpha] relaxations of polyimides related to chain motions appear to be governed by different parameters. Thus, the [alpha] relaxation temperature related to the glass transition is controlled by the monomer rigidity while the broadness of the [beta] relaxation is mainly governed by the intensity of the intramolecular in·tra·mo·lec·u·lar
Within a molecule.
intra·mo·lec charge transfer between diamine and diimide fragments. Moreover, the [gamma] relaxation temperature related to bound-water molecules is very sensitive to changes in the chemical structure. A statistical correlation concludes that the gamma relaxation temperature of polyimides is dependent on the microstructural characteristics such as the interchain distance, fractional free volume and color index. Therefore the location of gamma re laxation could be considered as a microstructural probe.
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Table 1 Characterization of Polyimide Intrinsic viscosity (dL.[g.sup-1] in NMP (a) or in Polyimide m-cresol (b) Tg([degrees]C) % [H.sub.2]O PMDA-DDE 0.91 (a) * >390 0.3 BTDA-DDE 1.02 (a) 280 0.2 BTDA-FDA 0.98 (a) 370 0.4 BTDA-BACN 0.77 (a) 240 0.0 BCDA-DDE 0.89 (b) 390 3.0 BCDA-FDA 0.63 (b) >390 3.1 BCDA-BACN 0.72 (b) 270 0.2 BCDA-BAPS 0.41 (b) 320 2.3 * Measured for poly (amic acide) solution. Table 2 Microstructural Parameters of Polyimide Films Polyimide P (g.[cm.sup.-3]) FVF (%) d-spacing (A) PMDA-DDE 1.417 11.3 4.2 BTDA-DDE 1.373 12.2 4.0 BTDA-FDA 1.280 14.6 4.0 BTDA-BACN 1.359 12.1 4.2 BCDA-DDE 1.339 13.6 4.6 BCDA.FDA 1.251 15.7 6.3 BCDA-BACN 1.329 13.2 5.3 BCDA-BAPS 1.350 14.3 5.2 Polyimide [[lambda].sub.0](nm) PMDA-DDE 449 BTDA-DDE 439 BTDA-FDA 413 BTDA-BACN 407 BCDA-DDE 275 BCDA.FDA 310 BCDA-BACN 315 BCDA-BAPS 298 Table 3 [gamma] relaxation [T.sub.[gamma]] ([degrees]C) [H.sub.[gamma]] Polyimide at 1 Hz (x [10.sup.3]) PMDA-DDE -78 13 BTDA-DDE -95 9 BTDA-FDA -100 5 BTDA-BACN -90 7 BCDA-DDE -122 15 BCDA-FDA -130 19 BCDA-BACN -115 11 BODA-BAPS -120 18 [gamma] relaxation [beta] relaxation Ea (a) [f.sub.0] [T.sub.[beta]]([degrees]C) Polyimide (kJ.[mol.sup.-1]) (Hz) (b) at 1 Hz PMDA-DDE 54 [10.sup.14] BTDA-DDE 49 [10.sup.14] 118 BTDA-FDA 38 [10.sup.12] 111 BTDA-BACN 54 [10.sup.15] 131 BCDA-DDE 55 [10.sup.19] BCDA-FDA 60 [10.sup.22] BCDA-BACN 47 [10.sup.18] 123 BODA-BAPS 74 [10.sup.25] [beta] relaxation [alpha] relaxation Ea [T.sub.[alpha]]([degrees]C) Polyimide (kJ.[mol.sup.-1]) [f.sub.0](Hz) at 1 Hz PMDA-DDE -- 380 BTDA-DDE 132 [10.sup.18] 295 BTDA-FDA 135 [10.sup.18] 420 BTDA-BACN 139 [10.sup.18] 245 BCDA-DDE -- 370 BCDA-FDA -- 415 BCDA-BACN 143 [10.sup.19] 305 BODA-BAPS -- 335 * (a)[+ or -] 4 (b)[+ or -] [10.sup.2] * Determined at the E" maximum. Table 4 Probability (%), P, That Both Parameters Are Non-Correlated Polyimides (39, 42) FVF [rho] [[lambda].sub.0] FVF [rho] T[gamma] 6 10 1 95 13 FVF * 28 33 * 28 p * * 21 * * Polysulfones (44) FVF T[gamma] <1 FVF * p -** ** Data not available.
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Corine Bas *
Thierry Pascal #
* Corresponding author. Tel: 33-479-75-86-24; Fax: 34-479-75-86-14 E-mail address: firstname.lastname@example.org
# Present address. Elf Atochem-Centre de recherche Rhode-Alpes-BP63-69493 Pierre-Benite (France)