Vibrational Dynamics, Phonon Dispersion and Specific Heat in Gas Permeable Poly(4-Methyl-2-Pentyne) Anuj Kumar, * 1 Sapna Pathak, 1 Mahendra Singh, 2 Poonam Tandon, 3 V. D. Gupta 4 Summary: Poly(4-methyl-2-pentyne) (PMP) is an amorphous glassy disubstituted acetylene based polymer. The excellent gas-separation and mechanical properties of these polymers have stipulated their use as membrane material for vapor and gas separation. PMP is among the hydrocarbon disubstituted polyacetylenes which have been synthesized to date. This polymer combines excellent gas and vapor per- meability with good resistance to organic solvents. As was shown recently, PMP offers promise in the manufacture of nanocomposite membranes for the separation of various hydrocarbon mixtures. It is also of importance as its monomer, 4-methyl- 2-pentyne, can be easily derived from commercial compounds, 4-methyl-2-pentene or methyl isobutyl ketone, produced on a large scale. It is known that PMP exists in cis and trans configurations. Synthetic conditions, e.g., the used catalyst, temperature, solvent etc., of substituted polyacetylenes decide percentage of different configur- ations (cis or trans). Different geometries of macromolecules can influence the supramolecular structure of the polymer, which primarily defines its properties, such as solubility, permeability, sorption, etc. Qualitative assignments of few bands of IR spectra are reported earlier. We present here, complete normal mode analysis and dispersion curves for PMP using Wilson GF matrix method modified by Higgs using Urey-Bradley force field. Dispersion curves for PMP are drawn and salient features are discussed. Predicted values of specific heat via density-of-states are also reported. Keywords: disubstituted polyacetylenes; normal modes; permeability; supramolecular structure; Urey-Bradley force field Introduction Polyacetylene is the best-known conjugated polymer, but its intractability and instabil- ity have greatly limited its scope of practical applications. [1] Attachments of appropriate substituents or pendants to the polyene backbone can help improve its processa- bility and stability as well as applications. These substituted PAs with appropriate backbone-pendant combinations show var- ious functional properties such as liquid crystallinity, photoconductivity, light emis- sion, ionic susceptibility, photo resistance, chromism, helical chirality, optical nonli- nearity, and gas permeability etc. [2] The separation of hydrocarbons from permanent gases is of considerable impor- tance in the chemical industry. Due to it’s extreme permeability to hydrocarbons and high hydrocarbon/permanent gas selectivity, Poly(1-trimethylsilyl-1-propyne) [PTMSP] is studied the most. [3] However, the poor chemical resistance of this material limits its use as a membrane for industrial applications. To overcome this problem, an alternative acetylene-based polymer, Macromol. Symp. 2009, 277, 51–61 DOI: 10.1002/masy.200950307 51 1 Jaypee Institute of Engg. & Technology, Guna, MP, 473226 India E-mail: anuj.kumar@jiet.ac.in 2 Department of Physics, Brahmanand P.G. College, Kanpur 224 008, India 3 Department of Physics, University of Lucknow, Lucknow 226 007, India 4 Department of Physics, Integral University, Lucknow, -226026 Copyright ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim