Spectroscopic Studies of the Diffusion of Water and Ammonia in

Chapter 22

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Spectroscopic Studies of the Diffusion of Water and Ammonia in Polyimide and Polyimide-Silica Hybrids 1

1

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P.Musto ,G.Ragosta ,G.Scarinzi ,and G. Mensitieri 1

Institute of Chemistry and Technology of Polymers, National Research Council of Italy, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy Department of Materials and Production Engineering, University of Napoli Federico II, P.le Tecchio 80, 80125 Napoli, Italy 2

Mass transport of low molecular weight penetrants in polyimide and silica/polyimide hybrids has been investigated using time-resolved FTIR spectroscopy and gravimetric analysis. In particular, transport of reacting (ammonia) and non-reacting (water) penetrants has been studied as a function of penetrant concentration, evidencing peculiar features related to the presence of the inorganic phase in the hybrid systems. For the case of water, diffusivity and sorption equilibrium have been evaluated in an activity range between 0.1 and 0.75. Free water as well as molecular aggregates have been detected in both systems. In the case of ammonia, its reactivity with polyimide has been directly observed, and the rection mechanism elucidated. Furthermore, it has been possible to discriminate diffusion and reaction phenomena due to the different time scales of the two processes.

296 In New Polymeric Materials; Korugic-Karasz, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.


297 Polyimides (PFs) represent an important class of high-performance polymeric materials, widely used because of their outstanding thermal-oxidative stability, excellent mechanical properties and very high glass transition temperature. These materials find applications as electrical and electronic insulators, and in separation-membrane technology (1-3). More recently, they have been proposed as matrices for organic/inorganic (O-I) hybrid systems prepared by the sol-gel route (4-7, 9). Hybrid systems are an interesting class of new-generation materials which combine the relevant properties of a ceramic phase (heat resistance, high-temperature mechanical performances, low thermal expansion) and those of organic polymers (toughness, ductility and processability). Polyimides are among the few matrices well suited for use in connection with sol-gel technology, which is a complex process accomplished in aqueous media and with formation of water and alcohols as by-products. In the present study the diffusion behaviour in a polyimide has been compared with that of a polyimide-silica hybrid having a nanoscale morphology. As penetrants, two low molecular weight compounds were selected, namely water and ammonia. Water sorption has obvious implications with respect to the performances of the materials as insulators, as well as in their aging behaviour. Of particular relevance, in this respect, is the state of aggregation of the absorbed water molecules and the molecular interactions they form with the polymeric matrix and/or with the inorganic phase in the case of the hybrid system. These issues have been addressed by a careful analysis of the infrared spectra of the penetrant in the V H frequency range. Ammonia was selected as the second molecular probe to be investigated since it has been reported in earlier investigations (8) that the polyimide, which is otherwise highly stable to solvents, displays a certain reactivity when contacted with basic reagents. These findings were based on gravimetric measurements which showed a marked non-Fickian behaviour characterized by a prolonged sorption process without approaching sorption equilibrium. The subsequent desorption evidenced that a substantial amount of ammonia was irreversibly retained in the polymer as a consequence of reaction of ammonia with imide linkages. Information on the reaction and its possible mechanism and kinetics were tentatively inferred from a Danckwerts analysis of sorption data (8). Time-resolved FTIR spectroscopy applied to diffusion studies offers the unique opportunity to monitor in real time the reagent diffusion and the chemical reaction of the penetrant with the polymeric substrate. The data presented here confirm the propensity of the polyimide to undergo a chemical attack by ammonia and the reaction mechanism is elucidated. The same measurements carried out on the hybrid system revealed that the inorganic phase accelerates the reaction. 0

In New Polymeric Materials; Korugic-Karasz, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

298

Results and Discussion Sorption of Water in Poiyimide and the Hybrid In figures 1 and 2 are reported the water spectra in the V H stretching region collected at sorption equilibrium (a = 0.4) for the poiyimide and the hybrid, respectively. 0


0.25

3645

r

0.2 i

0.15 -

1 1

0.1 0.05 'oL 4000

3750

3500

3250

3000

1

Wavenumber [cm" ] Figure 1: Curvefittinganalysis of the spectrum representative of water absorbed at equilibrium (a = 0.4) in neat poiyimide. Thefiguredisplays the resolved components (thin lines) and the experimental profile (thick line).

The complex spectral profiles were resolved by curve fitting analysis and the resulting components are reported in the same figures. These spectra have been interpreted on the basis of a simplified association model (10, 14, 15), whereby three different water species (So, Si and S ) could be spectroscopically distinguished. In particular, with S we indicate water molecules which do not establish any Η-bond, while with Si and S we designate, respectively, water molecules with one Η atom or two Η atoms involved in Η-bonding with a proton acceptor. In the spectra reported in figs. 1 and 2 the peaks at the highest frequency (3645, 3638 cm" ) can be associated with So molecules, those located at the next lowerfrequency(3570,3593 cm" ) to Si molecules, while the various components appearing below 3570 cm" can be associated to S2 species. The observation that in the spectrum of Fig. 1 there is only one S) component, while 2

0

2

1

1

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299 in Fig. 2 three distinct peaks are identified, implies the presence of a single S adduct in pure polyimide and of a series of Η-bonding complexes in the polyimide/silica system, characterized by different interaction strengths (the lower thefrequencyof the component, the higher the strength of the H-bond). 2

3638 3593 Downloaded by UNIV OF NORTH CAROLINA on July 20, 2013 | http://pubs.acs.org Publication Date: September 29, 2005 | doi: 10.1021/bk-2005-0916.ch022

3474

3500 3000 2500 Wavenumber [cm*] Figure 2: Curvefittinganalysis of the spectrum representative of water absorbed at equilibrium (a = 0.4) in the hybrid sample. Thefiguredisplays the resolved components (thin lines) and the experimental profile (thick line). 1

It is likely that S and Si species are characterised by high molecular mobility. S molecules should be confined into excess free volume (microvoids) or molecularly dispersed with no Η-bonding interactions (bulk dissolution), while Si may either interact moderately by Η-bonding or may represent selfassociated dimers. On the other hand, S molecules are expected to be characterized by a much lower mobility; these species should be firmly bound to specific sites present in the matrix or be involved in clusters of more than two water molecules. When comparing the two investigated samples, significant differences emerge in the species distribution at sorption equilibrium. A higher amount of sorbed water is evident in the case of the hybrid sample, as well as an increase of the contribution of strongly interacting species. Thesefindingscan be interpreted by assuming that, in the case of the hybrid, the inorganic phase, which contains a significant amount of interacting OH groups, promotes strong Η-bonding interactions between penetrant molecules and the silica phase, which results in an enhanced solubility. Conversely, in the case of pure polyimide, due to the low tendency of the matrix to form H-bpnding, the number of water molecules involved in molecular interactions is much lower and most likely 0

0

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300 related to water clustering, which increases as the concentration of sorbed water increases. Sorption isotherms evaluated gravimetrically and reported in figure 3 are consistent with the proposed interpretation. In fact, solubility in the hybrid is higher and a significant difference in the shape of the isotherms is observed. In the case of poiyimide the upward concavity at higher activities can be related to the occurrence of water clustering which contributes to an increase of solubility with activity. Conversely, in the case of the hybrid, the slightly downward concavity of the isotherm suggests water adsorption on specific sites present in the inorganic phase. No upturn is evident in this case, owing to the increased hydrophilicity of the substrate which prevents waterfromclustering.

Water vapor activity Figure 3: Sorption isotherms at 30 °C expressed as grams of sorbed water per gram of matrix vs water vapour activity for neat poiyimide (open squares) and hybrid sample (open circles).

Time-resolved ΈΉΚ spectroscopy allowed a precise monitoring of sorption/desorption kinetics: examples of such curves, evaluated from the absorbance area of the stretching band of water are reported in Figure 4a for the poiyimide and in Figure 4b for the hybrid. For both materials at all the investigated activities, desorption kinetics is slower than sorption, which is a typical feature of Fickian systems where mutual diffusivity is an increasing function of penetrant concentration. When mutual diffusivity increases with


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concentration, the shape of the sorption curve does not depend significantly on the functional form of the diffusion coefficient.

or 0

20

40

60

(time) (s) ' 05

80

100

05

Figure 4: Sorption and desorption kinetics (a = 0.4) in terms ofA(t)/Aaofor sorption, and offA^- A(t)]/A for desorption.(a): pure polyimide; (b): hybrid. w

The diffusion coefficient, £>, evaluated from the Fick's relationship (16) is reported, as a function of water vapour activity, in Figure 5, which shows that diffusivities are very close in both materials. In this analysis D represents a


302 diffusivity averaged over the whole range of water concentrations established during the sorption test inside the sample. Thus, although comparison of sorption and desorption kinetics points to a slight dependence of diffusivity on water concentration, these averaged values result to be independent.

1.5-10


o poiyimide • Hybrid

1.0-10 J, Q

•

ο ο

9

5.0-10-

_Ι

•

'

L_

0 0.01 0.02 average water concentration [g„ /g] 20

Figure 5. Diffusion coefficient, D, as a function ofaverage water concentratio for pure poiyimide and the hybrid.

Sorption of Ammonia in Poiyimide and the Hybrid Time-resolved FTIR spectroscopy is particularly useful whenever the diffusion phenomenon is accompanied by changes in the molecular structure of the polymeric substrate since, in principle, both phenomena can be concurrently monitored. As an example of this particular application, we report preliminary results obtained for the case of sorption of ammonia in poiyimide and poiyimide hybrids. This analysis is also relevant in techonlogical applications in view of possible use of the investigated materials in aggressive environments. Time evolution of the sample spectrum in the 3480-3000 cm" range is reported in figure 6 with reference to the sorption test conducted on the poiyimide at 100 Torr and 30°C. The diffusing ammonia is clearly discernible, giving rise to two well resolved peaks at 3400 and 3306 cm" due, respectively, to the asymmetric and symmetric stretching vibrations of the diffusing molecule. A gradual absorbance increase in the whole 3480-3000 cm" range, which is observed in the same time frame, is due to concurrent reaction of ammonia with imide moieties. After a short time (about 5 minutes) the intensity 1

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of both ammonia peaks reaches a plateau value, indicating the attainment of an apparent sorption equilibrium.

3470 3400 3300 3200

3100 3000 2930 1

Wavenumber (cm") 1

Figure 6: FTIR transmission spectra in the 3480-3000 cm' range, collec increasing times in thefirst5 minutes of the sorption of ammonia in ne polyimide [p(NH )=100 Ton, T=30°CJ. 3

At longer times, the spectrum of the matrix displays conspicuous changes, clearly indicating the reactivity of the system. As an example, in figure 7, are compared the spectra collected at 0, 14 and 28 hours, in the range 2000 - 500 cm" for the test carried out on pure polyimide at 30°C and 760 Torr. This comparison evidences an extensive reduction of the peaks arising from the imide moiety (1780, 1725, 1377 and 1170 cm' ) and the concurrent increase of absorptions at 1670, 1606 and 1408 cm" . The latter can be attributed to the normal modes of the amide functionality which represents the reaction product. 1

1

1

2.20

2000

1800

1600

1400

1200

1000

800

600

1

Wavenumber (cm*) 1

Figure 7: FTIR transmission spectra in the 2000-1000 cm' range, collec three times (0,14 and 28 hours) during the sorption ofammonia in nea polyimide [p(NH )=760 Torr, T=30°CJ. 3


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According to the above evidences, the likely mechanism is the following:

1

The evolution with time of the peaks at 3400 and 3306 cm' in terms of normalized absorbance (i.e. A(t)/Aoo, with A(t) absorbance at time t and A» absorbance at sorption equilibrium) is reported in figure 8. The interpretation of sorption kinetics using ideal Fickian behavior (16) gave a calculated ammonia diffusivity equal to 2.2 · 10" cm /s. A gravimetric analysis of ammonia diffusion in Kapton® polyimide at 30°C has been previously reported by lier et al. (8) for the same experimental conditions adopted in the present study. These investigators interpreted sorption kinetics curves using an approach proposed by Danekwerts (17) for the case of diffusion-reaction processes with a pseudo-first order reaction kinetics. They calculated diffusivity values for ammonia which were at least two orders of magnitude lower than those expected on the basis of the size of the diffusing molecule. This is shown in figure 9, where the diffusion coefficients at infinite dilution for various penetrants in Kapton® are reported as a function of the van der Waals volume, b, of the penetrant (8). Her et al. attributed the deviation to strong molecular interactions occurring between ammonia and the matrix. Actually, the value we have estimated in this study using time-resolved FTIR spectroscopy is very close to that expected on the basis of the molecular size of the penetrant, the slight positive deviation being reasonably related to the increase of diffusivity with penetrant concentration. 9

2



305

1.2 r

Figure 8: Aty/A^/or the sorption test ofammonia in neat poiyimide [p(NH3)=100 Ton, T=30°CJ.


306 10 -8 HOB

\

θΝΗ

3

10"

• •

data from Liter at al. [8] this study


ι CO. 10

V

10"

r

ΙΟ"" 10

NH

10 -13

I

20 40 60 80 van der Waals volume [cmVmole]

Figure 9: Correlation of diffusion coefficient at infinite dilution for various penetrants in Kapton® at 30°C with van der Waals volume, b, of the penetrant Full squares refer to datafromHer et al (8), open squares refer to ammonia and water diffusivities evaluated in the present study.

1

r

hybrid

Figure 10: Relative conversion of the imide groups at 30 °C as a function of time for the neat poiyimide and the hybrid sample. [p(NH )=760 Torr T-30°C] 3


t

307 The kinetics of the chemical reaction with time can be followed by monitoring the absorbance of several peaks related to the imide ring. In particular we have measured the peak centered at 726 cm" , since it is well resolved both for the neat polyimide and the hybrid. The behavior of the two materials is compared in figure 10 at 770 Torr. It is found that the polyimide reactivity in the examined conditions is conspicuous, leading to final conversion of imide rings exceeding 60 %. The reaction rate and the final conversion are considerably higher in the case of the hybrid, and this effect may be related to a catalytic activity of the inorganic phase towards the ammonolysis of imide rings. A similar effect has been documented in a recent publication (9) in terms of the increased rate of imidization of poly(amic acid) in the presence of silica nanophase.


1

Concluding Remarks Water sorption and transport in polyimide and polyimide/silica hybrids has been investigated by combining time-resolved FTIR spectroscopy measurements and gravimetric analysis. For the neat polyimide, the spectropic analysis evidenced that the prevailing penetrant species is monomeric water along with minor contribution from self-associated species (dimers and multimers). Conversely, in the hybrid system the amount of sorbed water increases with respect to the control material due to a substantial increment of Η-bonded water, likely because of molecular interaction occurring with the inorganic phase. The transport behavior is substantially Fickian in all cases investigated with diffusivity essentially independent of concentration. The same experimental approach has been employed to study transport of ammonia in both types of materials. A significant reactivity of the penetrant with the organic phase has been evidenced by time-resolved FTIR spectroscopy. Analysis of the time evolution of the IR spectra allowed us to propose a likely reaction mechanism, an appropriate selection of experimental conditions allowed a wide difference between the characteristic times for diffusion and reaction of ammonia, thus making it possible to monitor concurrently both processes. A considerable catalytic activity of the inorganic phase towards the ammonolysis was evidenced and its possible origin was discussed.


308


References 1. Polyimides; Fundamentals and Applications, Gosh M.K.; Mittal K.L., Eds., Marcel Dekker, New York, 1996. 2. Polyamic Acids and Polyimides: Synthesis, Transformation and Structure, Bessonov M.I.; Zubkov V.A., CRC Press, Boca Raton, FL, 1993. 3. Polymers for Microelectronics: Resists and Dielectrics, Thompson, L.F.; Wilson, C.G.; Tagawa S. Eds., ACS Symposium Series 537, ACS, Washington DC, 1994. 4. Mascia, L. Trends Polym. Sci., 1995,3,61. 5. Morikawa, Α.; Iyoku, Y.; Kakimoto M.; Imal, Y. Polym. J., 1992, 24, 107. 6. Nandi, M.; Conklin, J.A.; Salvati L. Jr.; Sen, A. Chem. Mater., 1991, 3, 201. 7. Kioul Α.; Mascia, L. J. Non-Cryst. Solids, 1994, 175, 169. 8 Polyimides. Synthesis, Characterization and Applications, Mittal K.L. Ed., Volume 1, p.443. Plenum Press, New York, 1984. 9. Musto, P.; Ragosta, G.; Scarinzi G.; Mascia, L.; Polymer, 2004, 45, 1697. 10. Cotugno, S.; Larobina, D.; Mensitieri, G.; Musto, P.; Ragosta, G. Polymer, 2001, 42, 6431. 11. Perry's Chemical Engineers Handbook, 7 edition, Perry, R.H.; Green D.W.; Maloney J.O. Eds, McGraw Hill, New York, 1999. 12. Marquardt, D.W. J. Soc. Ind. Appl. Math., 1963, 11, 441. 13. Maddams, W.F. Appl. Spectroscopy, 1980, 34, 245. 14. Musto, P.; Ragosta, G.; Mascia, L. Chem. Mater., 2000, 12, 1331. 15. Musto, P.; Ragosta, G.; Scarinzi, G.; Mascia, L. J. Polym. Sci. Part B: Polym. Phys. Ed., 2002, 40, 922. 16. The Mathematics of Diffusion Crank,. J. 2 edition, Clarendon Press, Oxford, 1975. 17. Danckwerts, P.V. Trans. Farad. Soc., 1951, 47, 1014. th

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Spectroscopic Studies of the Diffusion of Water and Ammonia in

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