D. E. Powers, W. C. Harris, and V. F. Kalasinskyl Furman University Greenville, South Carolina 29613
Laboratory Automation in the Undergraduate Curriculum
I
Determination of polyethylene chain branching by computerized IR methods
One of the experiments offered in our advanced, integrated lahoratory course is the preparation and characterization of polyethylene. The synthetic route using his (pentahaptocyclopentadieny1)titanium dichloride, (h5-C5H5)2TiC12,is essentially as discussed by Kranhuehl e t a L 2hut we have been able to use this exneriment to familarize students with computer-controlled laboratory instrument. Polvmeri~ationsusing the Zeigler.iiatta catalyits result in composed of very long chains with a minimum of chain branchina.Vhe number of methyl groups per 100 carbon atoms is &ed as a measure of t h e degree of chain branching, and this can he conveniently monitored by using infrared absorption spectroscopy. An absorption at 1378 cm-' is associated with the bending vihrations of the methyl groups in the oolvmer samole. Althoueh this neak " . .orovides the basis for the infrared study, there are two complications that preclude a straiehtforward analvsis: (a) methvl groups in amorphous a n d crystalline regions4 of the poly&leie absorb a t 1378 cm-1. and (h) there are overlapping .. .hands a t 1368 and 1352 cm-'which arise from methylene groupsin amorphous regions. Since a band a t 1304 cm-' is attributed to methylene vihrations for amorphous regions only, it is possible to partially compensate for the amorphous regions by using a conventional houhle-beam spectrophotometer. The annealed polymer sample under investigation is placed in the "sample" beam and an amorphous (quenched) sample in the "reference" beam and the relative ahsorbances a t 1378 and 1304 cm-' are measured. A series of difference spectra showing partial compensation of the 1304 cm-1 band can he ohtained if amornhous samoles with varvinz thicknesses are used. A plot of the relative absorhances a t 1% and 1304 cm-I can he-extranolated to find the point of exact compensation, and this a gaud PSIlmRW of the absorpextrapdated \,aI~tcpr~~vides ti\,ity;a>iociatedwith the methyl g n u p s in cryztalline regims of is romnartd with that - - the ~~~-nolveth\flene.This :~hsor~tivitv for a hexadecane'standard to determine the numher of methyl erouos oer hundred carbon atoms.2 " wk have recently acquired a Perkin-Elmer 580 spectroohotometer which is interfaced to an Interdata 6/16 computer.5Since the data are in digital form, spectral subtraction can he used to simnlifv the analysis because a difference spectrum for which thd 1304 cm" band has been exactly compensated can be ohtained as indicated below. For the annealed sample the ahsorhances a t 1378 and 1304 cm-' can he given by
.
A
ANNEALED
-
~
.
~
