Polyolefin properties range from liquidlike to rigid solids, and are primarily determined by their molecular weight and degree of crystallinity. Polyolefin degrees of crystallinity range from 0% (liquidlike) to 60% or higher (rigid plastics). Crystallinity is primarily governed by the lengths of polymer's crystallizable sequences established during
polymerization. Examples include adding a small percentage of
comonomer like
1-hexene or
1-octene during the polymerization of
ethylene, or occasional irregular insertions ("stereo" or "regio" defects) during the polymerization of
isotactic propylene. The polymer's ability to crystallize to high degrees decreases with increasing content of defects. Low degrees of crystallinity (0–20%) are associated with liquidlike-to-elastomeric properties. Intermediate degrees of crystallinity (20–50%) are associated with ductile thermoplastics, and degrees of crystallity over 50% are associated with rigid and sometimes brittle plastics. Polyolefin surfaces are not effectively joined together by
solvent welding because they have excellent
chemical resistance and are unaffected by common solvents. They inherently have very low
surface energies and don't wet-out well (the process of being covered and filled with
resin). They can be adhesively bonded after surface treatment, and by some superglues (
cyanoacrylates) and reactive (meth)
acrylate glues. They are extremely
inert chemically but exhibit decreased strength at lower and higher temperatures. As a result of this, thermal welding is a common bonding technique. Almost all polyolefins that are of any practical or commercial importance are '
poly-alpha
-olefin (or poly-α-olefin or polyalphaolefin, sometimes abbreviated as PAO'), a polymer made by polymerizing an
alpha-olefin. An '''
alpha-olefin''' (or α-olefin) is an
alkene where the carbon-carbon
double bond starts at the α-carbon atom, i.e. the double bond is between the #1 and #2 carbons in the
molecule. Alpha-olefins such as
1-hexene may be used as co-monomers to give an
alkyl branched
polymer (see
chemical structure below), although
1-decene is most commonly used for lubricant base stocks. Many poly-alpha-olefins have flexible alkyl branching groups on every other carbon of their polymer backbone chain. These alkyl groups, which can shape themselves in numerous
conformations, make it very difficult for the polymer
molecules to line up side-by-side in an orderly way. This results in lower contact
surface area between the
molecules and decreases the
intermolecular interactions between molecules. Therefore, many poly-alpha-olefins do not crystallize or solidify easily and are able to remain oily,
viscous liquids even at lower
temperatures. Low molecular weight poly-
alpha-olefins are useful as synthetic
lubricants such as
synthetic motor oils for vehicles and can be used over a wide temperature range. Even
polyethylenes
copolymerized with a small amount of alpha-olefins (such as
1-hexene,
1-octene, or longer) are more flexible than simple straight-chain high-density polyethylene, which has no branching. The
methyl branch groups on a
polypropylene polymer are not long enough to make typical commercial polypropylene more flexible than polyethylene. == Uses ==