Comparative infrared and Raman spectroscopy of energetic polymers

doi:10.1016/S0022-2860(03)00522-2

Journal of Molecular Structure

Volumes 661–662, 16 December 2003, Pages 561-566

Dedicated to Bernhard Schrader on the occasion of his 70th birthday

https://doi.org/10.1016/S0022-2860(03)00522-2 Get rights and content

Abstract

Infrared and Raman spectra of three energetic polymers, nitrocellulose, glycidyl azide polymer (GAP), and polyvinyl nitrate (PVN), are presented and compared. The bands are assigned to predominant characteristic motions by comparison to similar materials and the literature. The Raman spectra of PVN and GAP, even with 785 nm excitation, are partially obscured by native fluorescence. The inhomogeneous broadening evident in these spectra has implications for use of these energetic polymers in ultrafast spectroscopic studies of shock-induced chemistry.

Introduction

Energetic polymers are used extensively as binders in explosive materials and propellants, because they are able to add to the total energy release in addition to performing required viscoelastic and adhesion functions (i.e. for safe handling, packaging, machining, etc.). Several of these materials have been developed, including nitrocellulose (NC, also known as gun cotton), glycidyl azide polymer (GAP; HO[–CH₂–(CHCH₂N₃)–O–]_nH), and polyvinyl nitrate (PVN; [–CH₂–(CHONO₂)–]_n). We have been interested in these materials as prototypes for ultrafast spectroscopic determination of chemical reaction paths, particularly initiation reactions, in shock compressed energetic thin films. Because of the technological and chemical difficulties inherent in the production of uniform, oriented films of standard military explosives (such as HMX, TNT, RDX, TATB, etc.), the ability of energetic polymers to be spin cast into transparent, uniform thin films is very attractive for such studies.

There are spectroscopic difficulties, however, with the use of energetic polymers in studies of shocked materials. Previous studies of vibrational spectra in shock compressed materials have shown that the majority of transitions increase in frequency with shock pressure, with Δω/ΔP of the order of 1–10 cm⁻¹/GPa [1], [2]. Some transitions, including many vibrations involved in hydrogen bonds (e.g. most NO₂ groups), hardly shift with pressure. Therefore, these frequency shifts due to increased pressure behind the shock will be mostly masked by the inhomogeneous bandwidth. In addition, if the shock causes preferential orientation (due to the 1D stress) or other reduction in material symmetry during shock loading, the inhomogeneous width may change independent of pressure shifts, confusing the interpretation of the spectroscopic results [3].

Nevertheless, there are few comprehensive studies of the vibrational spectroscopy of these materials in the literature [4], [5], [6]. To facilitate future studies that utilize IR or Raman diagnostics of these materials, we present here some results from the literature as well as our own spectroscopic interrogations, including the previously unreported Raman spectra of PVN and GAP.

Section snippets

PVN synthesis

PVN was synthesized by direct nitration of polyvinyl alcohol using the procedure of Strecker and Verderame [7]. Further guidance was obtained from Ref. [8]. A solution of 2.60 g polyvinyl alcohol (Baker 99.0–99.8% OH, MW 77,000–79,000) in excess acetic anhydride was cooled to −8 °C. A slight excess of 98% HNO₃ was added dropwise over 1 h. The products were stirred a further 1 h at −8 °C and then the solution was slowly warmed to room temperature over 90 min.

The product solution was then crashed

Results

Fig. 1, Fig. 2, Fig. 3 show the infrared transmission and Raman spectra of NC, PVN, and GAP, respectively, in the format given in Schrader's compilation [11]. The Raman spectral region covered for PVN and GAP is instrumentally limited to 200–2000 cm⁻¹. Still, the important parts of the fingerprint region are covered by both techniques. Tentative assignments of the bands are shown in Table 1, and are based on comparison of the GAP and PVN spectra to literature assignments of GAP and NC infrared

Discussion

The inhomogeneous broadening of the spectral features in these polymers, due to the multiplicity of conformations in the glassy materials, is the severest drawback to use of polymer materials in studies of shock initiation, and is the reason for the desire to produce uniform oriented thin films of crystalline materials instead. However, until the experimental difficulties for production of such materials are overcome, these energetic polymers are reasonable surrogates.

It is apparent from Fig. 2

Conclusion

We have presented IR and Raman spectra of three energetic polymers: NC, GAP, and PVN. The Raman spectra of PVN and GAP are reported for the first time. Band assignments are made based on comparison to literature spectra and their assignments. The spectra for PVN show good intensities and few interfering bands in the NO₂ stretch mode region, which are beneficial attributes for the use of this material to study shock induced reactions.

Acknowledgements

The authors are very grateful to Darren Naud for assistance with the PVN synthesis, to Stephanie Hagelberg for the CHN analysis, and to My Hang V. Huynh for use of the Nicolet FTIR. This work was performed under the auspices of the US Department of Energy.

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Comparative infrared and Raman spectroscopy of energetic polymers

Abstract

Introduction

Section snippets

PVN synthesis

Results

Discussion

Conclusion

Acknowledgements

J. Mol. Struct.

Acc. Chem. Res.

J. Phys. Chem. B

Ann. Rev. Phys. Chem.

Z. Stuk. Kimii

Prop. Expl. Pyro.