The free-radical polymerization of ethylene leads to formation of both short and long branches off the main polymer chain via side reactions of the growing polymeric radical. These side reactions occur during homopolymerization of ethylene to form low density polyethylene (LDPE) and they also occur during free-radical copolymerization of ethylene with comonomers such as vinyl acetate (VA). Here I will generally describe two major side reactions involved and how they affect the polymer architecture and some of its resultant properties.
The first side reaction shown below is often called “back-biting” and occurs when the active, polymeric carbon radical at the chain end abstracts a hydrogen atom from, typically, the third or fourth carbon atom away from it along its own chain. If you took organic chemistry, you may recall that five or six-membered cyclic transition states are favored in many reaction mechanisms. In any case, this intra-molecular reaction leads to a “short” branch of typically four, sometimes three or five, carbon atoms when the transferred radical adds another monomer unit and the chain grows from that “new” starting point. It should be noted that while these intra-molecular reactions can occur near or among the VA units, they most often occur within sets of contiguous ethylene segments due to the more favorable energetics of both the reacting and resulting radicals as well as that of the cyclic transition state.
Alternatively, the active polymer radical can react with a second, separate polymer chain by abstracting a hydrogen atom from any carbon anywhere along that second chain. This “transfers” the chain growth process to that second polymer molecule. That can occur anywhere along that pre-existing long chain, and the new chain formed from that point will, on average, also be relatively long, then this inter-molecular reaction leads to a so called “long chain branch” off the second polymer. You can think of the polymer molecule that has branched one time as being shaped like the letter “Y” with different lengths for each “leg”. The same process can occur again anywhere along one of those legs to produce a molecule with two long branch points, and so on to produce multiply-branched molecules.
These two types of branching, “long chain” and “short chain”, will occur randomly along any given polymer backbone. Each have effects on polymer properties, principally due to their effects on crystallinity and melt flow behavior.
Both branch types disrupt the ability of that portion of the chain to crystallize and thereby reduce the degree of crystallinity of the polymer sample. To answer a question posed in an earlier post, this is why a homopolymer of ethylene produced by free-radical polymerization (“low density” polyethylene, LDPE) is limited to a maximum degree of crystallinity of about 48 to about 52%, depending on the reaction conditions. Enough branches, both short and long, are formed to inhibit crystallization to that degree. This impact on crystallinity in turn affects other properties including density, clarity, flexibility, and melting range. While these branching effects also apply to EVA copolymers, the most significant factor affecting crystallinity in EVA is still, by far, the VA content. One can consider the crystallinity of LDPE as a starting point from which the crystallinity of EVA copolymers decline with VA content.
Long chain branches affect the melt flow behavior, especially the viscosity at very low shear rate and the degree of shear thinning behavior. By contrast, short chain branches have practically no impact on melt viscosity behavior. Shear thinning is the phenomenon in which the viscosity decreases with increasing shear rate. Think of shear rate as proportional to flow rate if the geometry of the flow channel is kept constant. The consequence of shear thinning is that the material will flow easier (exhibit lower viscosity) than expected as the flow rate is increased. Branched materials require less pressure to pump at the same rate than an equivalent polymer having no long chain branches. Higher viscosity at lower shear rate for a polymer with long chain branching can enable easier fabrication via processes like profile, blown film, or foam extrusion. We will discuss rheological aspects of EVA copolymers in more detail later on.