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Curing study of a preimidized photosensitive polyimide.

INTRODUCTION

Over the past decade and a half, there has been substantial interest in photosensitive polyimides (PSPIs) (1). The photosensitive behavior allows the polyimides, which are widely used in electronics applications, to be patterned photolithographically. The first type of PSPI developed was a photosensitive polyamic ester (2), which crosslinks only in the precursor state. The second type was an intrinsically photosensitive polyimide (3, 4), which is both solvent soluble and photosensitive in its fully converted state. It is the latter version of the PSPI with which we are concerned.

In general, polyimides require high temperature cures for conversion of the precursor polymer and for the removal of solvent. The chemical and physical properties of polyimide systems change substantially as they are cured, and the material is converted from precursor to polyimide. Because the intrinsically photosensitive polyimides are already in their fully converted state, and because they gel upon exposure to ultraviolet light, they show strong potential as high performance materials that can be cured rapidly and at low temperatures. One application in which rapid, low temperature curing is highly desirable is in their use as alignment films for liquid crystal display devices (5, 6).

There has been much work published regarding the performance and physical properties of intrinsically PSPI films that have undergone ultraviolet exposure and conventional high temperature thermal cures (7-14). Ree et al. (14) found that chemical crosslinks are formed by both photochemical and thermal curing. In the present work, we show how thermal and ultraviolet curing, which do not involve closure of the imide ring, physically and chemically change the material. We do so by examining a series of films that have undergone step cures from soft bake to hard bake. We report the effects of the atmosphere in which the hard bakes are performed, and the effects of ultraviolet exposure on the films.

EXPERIMENTAL

The polyimide system studied is of the Probimide 400 series produced by OCG Microelectronic Materials, Inc. The polymer contains the following photosensitive structural unit:

where R, R[prime] = alkyl group. The polyimide contains a benzophenone (Ph-CO-Ph) group in the dianhydride portion of the repeat unit. The polyimide is obtained in [Gamma]-butyrolactone (GBL) solvent, and the product used in this study, Probimide 412, has a 12.5wt% original solids content.

Bulk films of Probimide 412 on the order of 100 [[micro]meter] thick were produced. All films were amorphous, and exhibited no Bragg scattering peaks when examined by wide-angle X-ray scattering.

The polyimide solution was first dropped onto a glass plate and wiped with a metal doctor blade with a 1 mm gap. The wet films were dried overnight to a tack-free state under a dry air current at room temperature. The parent films were then systematically treated at higher temperatures. Samples were cut from the film after each curing step, The first treatment was 4.5 h at 100 [degrees] C in air. The second was 3 h at 200 [degrees] C in air. Films were then treated at 300 [degrees] C for 3 h in either air or nitrogen. Material treated at 300 [degrees] C in nitrogen was then additionally cured at 400 [degrees] C for 1 h in nitrogen. Any Probimide 412 material treated at 400 [degrees] C in air was essentially destroyed by oxidation. A separate film was treated at 140 [degrees] C for 3 h in air after the 4.5 h bake at 100 [degrees] C. These thermally cured samples were not exposed to ultraviolet irradiation.

Material that had undergone the 4.5 h treatment at 100 [degrees] C was irradiated under a Spectroline model BIB-150B ultraviolet lamp with [Lambda] = 364 nm and an area normalized power of approximately 2.5 mW/[cm.sup.2]. The film was exposed to [approximately]27 J/[cm.sup.2] over the course of 3 h, each side of the film being exposed for 90 min. In order to separate the effects of hard bake and UV cures, we did not subsequently hard bake the Irradiated film.

Thin films of Probimide 412 were spun cast on glass or KBr at 4000 RPM for 30 s. The samples were immediately softbaked at 100 [degrees] C for 15 min in air. The samples appeared dry after soft curing. The thin films were then progressively cured at 200 [degrees] C, 300 [degrees] C, and 400 [degrees] C, 1 h at each temperature. The 200 [degrees] C cures were performed in air, whereas the 300 [degrees] C cures were performed in either air or nitrogen. The 400 [degrees] C cures were performed in nitrogen only on films previously treated at 300 [degrees] C in nitrogen. Spin-cast films treated at 100 [degrees] C were irradiated under the UV lamp for various times.

A Seiko TG/DTA320 was used for thermogravimetric analysis (TGA) of the bulk films. Samples were scanned from ambient temperature to 575 [degrees] C at 10 [degrees] C/min in flowing air (300 ml/min).

Dynamic mechanical analysis (DMA) was performed in tension mode on bulk films using a Seiko DMS200. Samples were scanned under nitrogen at 2 [degrees] C/min from room temperature to the temperature above softening at which breakage occurred. Frequencies 1, 2, 5, 10, and 20 Hz were used. Only the 1 Hz data is presented here.

Index of refraction at [Lambda] = 632.8 nm was measured using a Metricon PC-2000 prism coupler. The refractive index was measured by determInation of the waveguide propagation modes. Both in-plane ([n.sub.TE]) and out-of-plane ([n.sub.TM]) Indices were measured. The average refractive index [n.sub.avg] was calculated as shown below.

[n.sub.avg] = (1/3)(2[n.sub.TE] + [n.sub.TM]) (1)

Refractive index measurements were taken for the free-standing films and the films spin-coated on glass.

Fourier transform infrared spectroscopy (FTIR) was performed using a Nicolet 510P. FTIR spectra were taken for the Probimide 412 films spin-coated on KBr.

RESULTS AND DISCUSSION

Coloration and Brittleness

The changes in coloration and brittleness in Probimide 412 thick films that occur with subsequent cures are summarized in Table 1. Probimide 412 is slightly yellow and flexible when cured at 100 [degrees] C. These characteristics change little upon curing to 200 [degrees] C. The material becomes dark orange and brittle when cured at 300 [degrees] C in air but remains yellow and flexible in nitrogen. However, the polymer turns brown and brittle when treated at 400 [degrees] C in nitrogen. The dark coloration and brittleness in the films cured 300 [degrees] C in air and at 400 [degrees] C in nitrogen suggest that chemical crosslinking is taking place. Ultraviolet irradiation after thermally curing at 100 [degrees] C substantially increases the yellowness of the film. The yellowing of the UV irradiated film is indicative of the formation of chemical crosslinks.

Thermogravimetric Analysis

Thermogravimetric analysis was used to track the presence of solvent as the bulk films were cured. Thermogravimetric scans in Figs. 1 and 2 also indicate vaporization of absorbed water and, at high temperatures, decomposition of the polymer. All of the films lose [approximately]1% of their original weight below 150 [degrees] C owing to removal of water from the polymer, while the films lose varying amounts of weight, depending upon the cure conditions, between [approximately]150 [degrees] C and 300 [degrees] C because of removal of residual solvent. The TGA scans reach a plateau until the temperature exceeds [approximately]400 [degrees] C, when the films begin to degrade.
Table 1. Color and Brittleness in Films Cured at Progressively
Higher Temperatures.

Treatment Environment Color Brittleness

100 [degrees] C air light yellow flexible
200 [degrees] C air light yellow flexible
300 [degrees] C [N.sub.2] light yellow flexible
300 [degrees] C air dark orange brittle
400 [degrees] C [N.sub.2] brown brittle
100 [degrees] C + UV air yellow barely brittle


In Fig. 1a through e, five thermogravimetric scans of Probimide 412 films are compared. Weight loss (left vertical axis) and derivative of weight loss (right vertical axis) are shown as functions of temperature. Each sample has a different highest curing temperature, which is indicated by the vertical marker. None of the films in Fig. 1 has been UV cured. The amount of residual solvent vaporized between 150 [degrees] C and 300 [degrees] C decreases substantially as the cure temperature increases. That is, each cure at a higher temperature progressively removes more solvent from the film. There is no residual solvent in the film cured at 300 [degrees] C [ILLUSTRATION FOR FIGURE 1E OMITTED].

The temperature at which the solvent boris off during thermogravimetric analysis also increases with the increased cure temperature. This can be more clearly seen in the plots of the first derivative of weight loss (with respect to scan time) in Fig. 1. The temperature at which maximum solvent vaporization occurs increases from [approximately]200 [degrees] C to 300 [degrees] C as the cure temperature of the film is increased from ambient temperature to 200 [degrees] C.

The observed distribution of solvent removal temperatures, ranging from under [less than]150 [degrees] C up to [approximately]300 [degrees] C, is most like the result of polymer-solvent binding. Loosely bound solvent molecules readily evaporate below the boiling point of the solvent, while tightly bound solvent molecules require much higher temperatures in order to evaporate. As the material is cured, loosely bound species vaporize first, and the distribution of removal temperatures of residual solvent moves to higher temperatures.

In Fig. 2, thermogravimetric scans of unirradiated and UV irradiated film cured at 100 [degrees] C are compared. Not only is there less solvent in the UV cured film, but also the volatilization of the residual solvent occurs at higher temperatures after UV exposure. It is thus apparent that the UV curing, by crosslinking the polymer chains, affects the solvent removal dynamics of the thick film.

As Figs. 1 and 2 show, at [approximately]400 [degrees] C, Probimide 412 begins to decompose. The decomposition temperature, arbitrarily defined as the temperature at which the time derivative of weight loss equals 3% of the polymer per minute (scanning at 10 [degrees] C/min), of our Probimide 412 samples is [approximately]520 [degrees] C in flowing air. Other authors (10, 11) previously report decomposition at 527 [degrees] C in flowing nitrogen.

Dynamic Mechanical Analysis

The evolution of mechanical properties during the cure of Probimide 412 bulk films was investigated by dynamic mechanical analysis. The dynamic mechanical spectra at 1 Hz for Probimide 412 samples progressively treated at 100 [degrees] C, 200 [degrees] C, 300 [degrees] C, and 400 [degrees] C are shown in Fig. 3a through d respectively. Young's modulus and dissipation factor, tan[Delta], are shown as functions of temperature. For Fig. 3, the 300 [degrees] C and 400 [degrees] C cures were performed in nitrogen. The DMA scans show both glass transition [T.sub.g] and secondary transition behavior.

The room temperature Young's modulus does not appear to be dependent upon cure conditions. The glassy moduli of our films are 2.2 to 2.5 GPa. Other authors previously reported a modulus of 2.6 to 2.9 GPa for hard-baked Probimide 400 series films (10, 13, 14).

For the samples cured in air at 100 [degrees] C [ILLUSTRATION FOR FIGURE 3A OMITTED] and at 200 [degrees] C [ILLUSTRATION FOR FIGURE 3B OMITTED], Young's modulus decreases by about an order of magnitude at 375 [degrees] C. Tan[Delta] shows a strong maximum at 375 [degrees] C, which is the [T.sub.g]. Several broad and weak maxima occur near 100 [degrees] C and 250 [degrees] C.

In the film cured in nitrogen at 300 [degrees] C [ILLUSTRATION FOR FIGURE 3C OMITTED], the glassy modulus decreases slightly at 280 [degrees] C and sharply at 375 [degrees] C, Above 425 [degrees] C, the modulus exhibits an upturn due to the thermal induction of covalent crosslinks. The dissipation factor, tan[Delta], shows a strong [T.sub.g] at 375 [degrees] C and a weaker, but distinct, secondary relaxation at 280 [degrees] C.

In the film cured at 400 [degrees] C in nitrogen [ILLUSTRATION FOR FIGURE 3D OMITTED], above room temperature, the modulus declines with increasing temperature. There is a secondary relaxation at 280 [degrees] C, and a broad relaxation above the 375 [degrees] C [T.sub.g] seen in the other samples. Tan[Delta] shows several broad steps. There is no clear [T.sub.g] peak seen in this sample.

The [T.sub.g]s of all of the samples are near 375 [degrees] C. However, while the samples treated at 100 [degrees] C, 200 [degrees] C, and 300 [degrees] C have sharp [T.sub.g]s at 375 [degrees] C, the sample treated at 400 [degrees] C has a broad relaxation shifted to higher temperatures. Ree et al. (14) used dynamic mechanical analysis to show that thermal cures from 350 [degrees] C to 400 [degrees] C induce chemical crosslinks in Probimide 412. Our results confirm that the 400 [degrees] C treatment in nitrogen causes the polymer to crosslink, reinforcing the rubbery modulus and shifting the softening transition to higher temperatures. We also show that the thermal crosslinking does not occur in the films cured at 300 [degrees] C and below.

Secondary relaxation also differs among the samples. Ree et al. (14) observed broad and weak [Beta] relaxation at low temperatures and attributed the phenomenon to the relaxation of phenyl moieties or to absorbed moisture. Our samples cured at 100 [degrees] C and 200 [degrees] C have weak and broad secondary relaxations. On the other hand, the samples cured at 300 [degrees] C and 400 [degrees] C, which have been shown by TGA to be free of solvent, have well defined relaxations at 280 [degrees] C.

Ultraviolet irradiation crosslinks the Probimide 412 thick films, affecting its relaxation behavior. In Fig. 4, unirradiated and UV irradiated samples initially cured at 100 [degrees] C are compared. The crosslinking causes an elevation of the glass transition temperature by a few degrees, reinforces the rubbery modulus, and broadens the tan[Delta] peak. Ree et al. (14) observed that UV exposure reinforced the rubbery modulus in samples that had been subsequently hard baked at 350 [degrees] C in nitrogen. They found, however, that thermally induced crosslinks created by subsequent curing at 400 [degrees] C dominated over the UV induced crosslinks such that no effect of the UV could be observed.

The polymer also crosslinks when it is cured at 300 [degrees] C in an oxygen-containing environment. This is demonstrated in Fig. 5, where the mechanical properties of samples cured at 300 [degrees] C in air and nitrogen are compared. While the sample baked in nitrogen has a sharp [T.sub.g] at 375 [degrees] C, the sample baked in air has a broad [T.sub.g] and a highly reinforced rubbery modulus. It is apparent that the 300 [degrees] C cure in air has a similar effect on the mechanical properties as does the 400 [degrees] C cure in nitrogen. A 400 [degrees] C cure in air, however, severely oxidizes and degrades the specimen.

Index of Refraction

The index of refraction of the polyimide changes as it is cured. Measurements of the refractive indices ([Lambda] = 632.8 nm) of spin-coated Probimide 412 films are shown in Fig. 6. Average indices obtained for bulk films are quite similar to those obtained for the spin-coated films, and thus, only the results for the thin films are presented. The average index is about 1.61 to 1.62 for samples treated at 100 [degrees] C and 200 [degrees] C in air, and for the sample treated at 300 [degrees] C in nitrogen. However, the index of refraction increases after the films are treated at 300 [degrees] C in air or at 400 [degrees] C in nitrogen. We attribute the large increase in refractive index to thermally induced crosslinking that takes place at 300 [degrees] C in air, but at 400 [degrees] C in nitrogen. These results are consistent with the observed color changes and with changes in the dynamic mechanical relaxation spectra. Previous work (8) reports an in-plane index ([Lambda] = 1.06 [[micro]meter]) for Probimide 400 series films of approximately 1.61, although the authors did not observe an increase in refractive index upon hard curing to 400 [degrees] C.

Ultraviolet curing also affects the optical properties of the Probimide 412 film. Fig. 7 shows the index of refraction of 100 [degrees] C soft-baked samples as a function of UV irradiation time. Refractive index increases with UV exposure as a result of crosslinking. Gelation occurs rapidly at first but slows with larger exposure times.

Fourier Transform Infrared Spectroscopy

We used FTIR spectroscopy to determine how thermal and UV curing changes the chemical structure of the polyimide. Figs. 8 through 10 show the effects of curing conditions on the FTIR spectra of Probimide 412. Fig. 8a through d demonstrates the effect of UV exposure, while Figs. 9 and 10 show the effects of thermal cures. Figures 8 and 10 are scaled to three times the height of the N - C imide peak at 1370 [cm.sup.-1]. Figure 9, which shows only the 2000 to 4000 [cm.sup.-1] region, is scaled to one-third the height of the 1370 [cm.sup.-1] peak. Table 2 lists the main absorption bands observed and the bond motion tentatively assigned to the vibrational frequency.

Several IR peak are of particular interest. Polyimides are well known to absorb in the infrared strongly at 1720 [cm.sup.-1] and weakly at 1770 [cm.sup.-1] because of carbonyl absorption (15). According to Higuchi et al. (16), weak absorption can be observed at 1680 [cm.sup.-1] because of benzophenone carbonyl. This was based upon the observation that only benzophenone-containing polyimides absorb at this frequency (17). The broad absorption regions observed near 3000 [cm.sup.-1] and 3500 [cm.sup.-1] are also of interest. The absorption near 3000 [cm.sup.-1] is attributed to a series of overlapping hydrocarbon stretching bands (18, 19), while the broad absorption near 3500 [cm.sup.-1] is attributed to the stretching of highly hydrogen-bonded hydroxyl groups (18, 19).

UV Effects

The chemical mechanism of UV crosslinking in inherently photosensitive polyimides has been studied (16, 20-22). UV crosslinking occurs as follows. Hydrogen is first abstracted from an alkyl group (see Scheme I). The resulting free radical attacks the benzophenone carbonyl group (in preference to the imide ring carbonyls), altering the bond to a carbon-hydroxyl group (see Scheme II).

Probimide 412 films spun on KBr were soft baked at 100 [degrees] C for 15 min and cured under ultraviolet. The FTIR spectra at different UV exposure times are shown in Fig. 8a through d. Significant absorption band changes that occur with increased UV exposure are observed in Fig. 8. First, broad absorption near 3500 [cm.sup.-1] systematically increases with UV exposure, indicative of an increase in hydroxyl groups. Second, the broad absorption near 3000 [cm.sup.-1] declines, indicative of a decrease in hydrocarbon bonds. Third. the peak at 1680 [cm.sup.-1] due to the benzophenone carbonyl group (16) systematically recedes into a shoulder. This is accompanied by a decline in carbonyl absorption at 1720 [cm.sup.-1]. We also note the peak at 1770 [cm.sup.-1] due to the imide ring carbonyl bonds does not seem to be affected.
Table 2. IR Absorption Regions of Probimide 412.

Wavenumber Tentative
([cm.sup.-1]) Assignment Strength(*)

3600-3400 O - H w
3000-2800 C - H w
1770 C = O (imide) m
1720 C = O (imide and benzophenone) s
1680 C = O (benzophenone) w
1610 C - C (phenyl) w
1600 C - C (phenyl) w
1490 C - C (phenyl) m
1420 C - C (phenyl) m
1370 N - C (imide) s
1300 C - C - C (benzophenone) m
1240 C - F m
1100 N - C (imide) m
850 phenyl substitution w
710 phenyl substitution m

* s = strong, m = medium, w = weak.


Thermal Effects

The effects of thermal cures on infrared absorption are shown in Fig. 9 (2000 to 4000 [cm.sup.-1] range) and in Fig. 10 (400 to 2000 [cm.sup.-1] range). Changes in the FTIR spectra occur when the polymer is cured at 300 [degrees] C in air [ILLUSTRATION FOR FIGURE 9B AND C OMITTED] and [ILLUSTRATION FOR FIGURE 10B AND C OMITTED], or at 400 [degrees] C in nitrogen [ILLUSTRATION FOR FIGURES 9D AND 10D OMITTED]. Under these conditions, the IR spectra from 2000 to 4000 [cm.sup.-1] in Fig. 9 show a decrease in absorbance from C - H bonds (near 3000 [cm.sup.-1]) and an increase in absorbance from O - H groups (near 3500 [cm.sup.-1]). This suggests that the mechanism of thermally induced crosslinking may be similar to that of the UV induced crosslinking. The IR absorption spectra from 400 to 2000 [cm.sup.-1] in Fig. 10 Indicate that, as in the UV exposed samples, the carbonyl peak at 1720 [cm.sup.-1] declines and the benzophenone carbonyl peak at 1680 [cm.sup.-1] becomes slightly flatter. We also notice that there is general broadening of all peaks across the finger print region of the spectra in the thermally crosslinked samples. Virtually all of the peaks appear wider and less resolved under conditions of thermal crosslinking. Thus, there are probably many random, nonspecific degradation and crosslinking reactions simultaneously taking place during the thermal cure. The aliphatic-group attack upon carbonyl may be one of many reactions.

CONCLUSIONS

Cures above 200 [degrees] C are required to remove solvent from thick films of Probimide 412. However, ultraviolet curing also drives out solvent. Regardless of cure treatment, however, the films contain [approximately]1% absorbed water. Thermogravimetric analysis reveals a decomposition temperature [approximately]520 [degrees] C.

Probimide 412 undergoes glass transition softening at [approximately]375 [degrees] C and a secondary relaxation at [approximately]280 [degrees] C. Although residual solvent does not appear to affect the primary glass transition, It may induce secondary relaxation at lower temperatures.

Ultraviolet curing at [Lambda] = 364 nm crosslinks the polymer. The crosslinks reinforce the rubbery modulus and broaden the distribution of relaxation times. The UV exposure yellows the polymer and increases its refractive index. The crosslinking mechanism is highly specific and occurs by hydrogen abstraction and attack of the free radical at the benzophenone carbonyl.

The material also undergoes substantial thermally induced crosslinking, which is especially favored in an oxygen-containing environment. We found extensive crosslinking to occur when films are baked at 400 [degrees] C in nitrogen or at 300 [degrees] C in air. A nitrogen environment appears to inhibit crosslinking at 300 [degrees] C. The thermal crosslinking process embrittles the polymer, reinforces its rubbery modulus, and broadens its distribution of relaxation times. Also, it gives the material a deep yellow to brown color and increases the index of refraction by [approximately]20%. Although one possible crosslinking mechanism may be similar to that of the UV process, the thermally induced crosslinking is probably not limited to one mechanism.

ACKNOWLEDGMENTS

This research was supported by Minnesota Mining and Manufacturing Company. The authors thank Dr. R. Wenz of 3M for discussions, OCG Microelectronic Materials, Inc., for Probimide 412, and Prof. S. Senturia for use of the Metricon PC-2000.

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Author:Rich, David C.; Sichel, Enid K.; Cebe, Peggy
Publication:Polymer Engineering and Science
Date:Sep 15, 1996
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