Structural Studies of Semiflexible Fluorocarbon Chains Containing an Aromatic Core
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STRUCTURAL STUDIES OF SEMIFLEXIBLE FLUOROCARBON CHAINS CONTAINING AN AROMATIC CORE A. SCHULTE, V. M. HALLMARK, R. TWIEG, K. SONG AND J. F. RABOLT IBM Research Division, Almaden Research Center, San Jose, CA 95120-6099 ABSTRACT The molecular structure of a series of perfluoroalkane oligomers having two F(CF2 ), chains attached to a phenyl group in the para position has been studied by Fourier Transform and conventional Raman spectroscopy at ambient and low temperatures. From comparison with model compounds bands attributable to both the substituted phenyl ring and the perfluoroalkane chains could be assigned. In particular, the low frequency region was investigated as a function of temperature and pressure in order to assess the impact of the rigid aromatic core on the bandshapes and frequencies. 1. INTRODUCTION The use of fluorocarbon oligomers as model systems to study semiflexible polymers is well established [1,2,3] and has provided a significant insight into the role of backbone conformation on chain stiffness. In addition to perfluorocarbon oligomers a series of diblock [4,5] and triblock [6] semifluorinated alkanes have been synthesized and their crystal and conformational structure studied extensively. Of particular interest in the triblock series was the effect of a semiflexible hydrocarbon center block on the frequency and intensity of the low frequency Raman active longitudinal acoustic, mode (LAM). Results indicate that at room temperature the center block p)articipat(es, with the fluorocarbon end blocks, in the accordion-like motion characteristic of LAM. Above, the melting point, on the other hand, the disordered hydrocarbon segment serves as a weak coupling spring perturbing the LAM frequency of each end block. It is the purpose of the present study to replace the semiflexible hydrocarbon block with a rigid aromatic core and then assess its effect on both the intensity and frequency of LAM. 2. EXPERIMENTAL METHODS The p-bisperfluoroalkylbenzenes were prepared by copper mediated conhdensation of the appropriate perfluoroalkyl iodide with p-diiodobenzene in diniethylsulfoxide. The purification of the crude products was accomplished by multiple recrystallization froim acetic acid. Differential scanning calorimetry (DSC) measurements were pirforiel on a Du Pont 910 DSC with a 1090 controller. A scanning rate of 10 'C /rain was used. Scanning Raman measurements were recorded by using a Jobiii-Yvon HG-2S double monochromator configured for photon counting and interfaced to a Nicolet 118(1 data system. Variable temperature studies were carried out in a vertical Harney-Miller cell. The sample temperature was maintained constant to within ± 3 'C . Raman experiments on samples unider pressure were pirforned ill the backscat tering geometry using a Waspalloy diamond anvil cell with metal gaskets. The pi-ssure was determined in situ from the peak position of a ruby fluorescence line with i an 'rror of ± 1 kBar.
Mat. Res. Soc. Symp. Proc. Vol. 171. ', 1990 Materials Research Society
402
Fourier transform Raman
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