BRANCHING IN POLYETHYLENE
The principal purpose of this paper is to present the current status, as viewed by the authors, of our knowledge of branching in polyethylene and the effects of branching on its properties. Another purpose is to present some speculative material, and some challenging observations, in an attempt to stimulate, in some measure, future work on the structure and chemistry of polyethylene. In endeavoring to achieve these aims, the authors have drawn heavily upon their own work and liberally from the literature, particularly the published results of R. B. Richards and his collaborators. Polyethylene resins are made by polymerizing ethylene at elevated temperatures and high pressures.’, The polymer molecules formed under these conditions are branched hydrocarbon chains3* 4 . 6 * and their average molecular size varies from approximately 50 carbon atoms per molecule, for the greasy products, to more than 1000 carbon atoms per molecule for rigid resins. The degree of branching in polyethylene has been shown to play an important role in determining the properties of the resin.’ In fact, it is now generally recognized that degree of branching and molecular weight are the most important molecular variables in polyethylene resins. In this paper, degree of branching is defined as the number of branches per 100 carbon atoms, and the number of branches per molecule is defined as the number of -CH3 groups per molecule more than two. Although some chains and branches are terminated by vinyl groups (-CH=CHJ, the concentration of these is very small compared to the concentration of methyl groups. Side chain methylene groups (RR’C=CHZ) may be regarded as short branches but, as such, represent not more than 15 per cent, and generally a much smaller proportion, of the total number of branches. As a result, this configuration is not considered significant for the purposes of this paper.
Infrared spectrophotometric studies on polyethylene. I. Structure
A series of polyethylene resins made by making progressive changes in temperature, pressure, and catalyst content has been studied by infrared absorption spectroscopy. From the methyl content and the number-averages molecular weight of each resin, it was found that as molecular weight increases degree of branching decreases. Also revealed was the fact that each of the three types of double bond groups present, i. e., RHCCH2, RHCCHR, and RRCCH2, decreases in concentration with increasing molecular weight. Using polarized radiation, high resolution grating spectra were obtained on polyethylene films which had been highly oriented by cold drawing 500%. The results show that the CH3 terminated branches are not predominantly oriented either parallel or perpendicular to the direction of stretch. It is the authors' view that the chain branches vary in length with a considerable fraction short enough to be essentially at right angles to the long chain axis while others are sufficiently long to be parallel to the long chain axis in stretched polyethylene. When a stretched polyethylene film is heat relaxed (110–115°C.), its spectra using polarized radiation reveal the presence of crystallites in which the long carbon-to-carbon chain axes are predominantly oriented at right angles to the direction of stretch.
RATE THEORY AND SOME PHYSICAL AND CHEMICAL PROPERTIES OF HIGH POLYMERS
The formation of high polymers from their respective monomers has been the subject of much experimental investigation, much of which has been subject to conflicting interpretations. It is generally conceded that formation of t.he “addition” type of polymer is a chemical chain reaction. Those molecules which form “addition” polymers are all olefinic or conjugated molecules, and their reactivity is to be associated with the structure of the double bond. Staudingerl first proposed that the chain-initiating substance in polymerization is an active monomer molecule in which the double bond has “opened” and can react rapidly. The activation energy for chain initiation would be, on this view, the energy required to form a radical or ion from the monomer. A consideration of the electronic structure of the double bond lends greater precision to our picture of the mechanism by which a double bond 66 opens.’’ The electronic states of olefins, ethylene in particular, have been considered by Mulliken,2 H u ~ k e l , ~ and Lennard-Jones4 by the method of molecular orbitals. This theoretical treatment indicates that, whereas ten of the twelve electrons in ethylene are localized by pairs, taking up positions between the carbon and hydrogen nuclei and between the two-carbon nuclei, the remaining two electrons are distributed over the entire molecule. Thus they cannot be said to “belong” to any one carbon atom, for which r e w n they have been distinguished by the name “unsaturation electrons.” Being less tightly bound, these are the electrons which account for the reactivity of the double bond. The energy states of these electrons are shown in FIGURE 1, where energy is plotted as a function of the angular rotation of one methylene