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Background

In my answer to another question, I repeated a claim made to my class by my high school chemistry teacher. I didn't reference it, because the relevant FAQ suggests high school science can generally be assumed. However, the claim was, to my surprise, pooh-poohed. This is a follow-up question to allow it to be properly scrutinised.

Claims

  • Plastic ice-cream tubs are typically made of low-density polyethylene (LPDE). [Ref]

  • Polyethylene has a lot of cross-branching between the chains. Wikipedia states:

As with any polymer, the structure of the resulting substance defies molecular description due to cross branching of the chains

  • The key claim: There is so much cross-branching, that a typical ice-cream tub (excluding the separate lid) consists of one single molecule of plastic. [Reference: My Year 11 Chemistry Teacher, whose name I have shamefully forgotten.]

I never had any reason to doubt these claims until recently. Are they true?

  • 1
    Doesn't "defies molecular description" pretty much sum it up? It's not one molecule, nor is it many - the naive molecular model of physics is not the correct paradigm to study its bonding properties. – user792 Oct 1 '11 at 16:36
  • The WP link suggests an easy test: can you dissolve the tub? If it gradually dissolves (one molecule at a time), then obviously there must have been multiple molecules. – MSalters Oct 3 '11 at 13:34
  • @MSalters, what solvent will dissolve a substance with a molecular weight of 90,000+? – Oddthinking Oct 3 '11 at 13:38
  • @Oddthinking: according to WP, toluene or trichlorobenzene. (A clear case of "don't try this at home"; that stuff is seriously carcinogenic) – MSalters Oct 3 '11 at 13:42
  • I've locked this - my own question! - because I don't think it meets our notability requirements and I have been using it as (bad) precedent to justify other "my teacher said" questions. See a meta-question on the subject. – Oddthinking Dec 12 '13 at 0:46
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This is a very good question but one where it isn't the principle that's wrong just the detail.

Polymerization reactions typically produce very large molecules but not so large that a single molecule would make up as much material as a whole ice cream tub. One data sheet on LDPE tells us that the typical molecular weight of the industrial product is 90,000 (some polymers are much bigger than this). (see here for an illuminating description relevant to LDPE).

For non-chemists molecular weights are (simplifying slightly) the weight relative to a single hydrogen atom. Carbon is 12 on this scale and ethylene (the monomer from which the plastic is made which consists of 2 carbons and 4 hydrogens) is 32. This mean that a typical molecule of the plastic is (crudely) made from a little under 3,000 ethylene molecules. The other fact you need to know is that there are about 6 x 10^23 hydrogen atoms in a gramme of hydrogen (see the wikipedia definition of Avogadro's number). Simple mathematics tells us that a gramme of LDPE will contain roughly 6.7 x 10^18 molecules so that is the order of the number in a tub for ice cream which, I expect, weighs a few grammes.

12

While the claim is certainly false for thermoplastic polymers like polyethylene, it's far more likely to be true for thermoset polymers such as polyurethanes, epoxies or phenolics such as Bakelite (which, being the first widely used synthetic plastic, may well be the original source of this claim).

Specifically, a thermoplastic polymer, by definition, consists of moderately large (that is, large on an atomic scale, but still microscopically small from a human perspective) chains or branched "trees" of monomer units, each (mostly) linked to two other neighboring units in the chain.

(In polyethylene, for example, these chains generally have molecular weights ranging from a few hundred to a few million AMU,[1] with an average typically somewhere around 10,000 AMU (≈ 1.6 × 10-23 kilograms) depending on the manufacturing process. For comparison, a single ethylene monomer weighs about 28 AMU, so a 10,000 AMU polyethylene molecule has about 350 ethylene units, or about 700 carbon atoms.)

These long chains stick to each other by intermolecular forces (mainly van der Waals forces) and also simply due to the chains getting tangled with each other (steric effects, in chemistry jargon) rather like a bowl of sticky noodles. When heated and/or mechanically stretched, however, the chains can slip past each other, allowing the material to be reshaped (= thermoplasticity).

In a thermoset polymer like Bakelite, however, the monomer units can (and typically do) link up with more than two other monomers. Thus, instead of forming chains, they form a complex three-dimensional network structure. As long as the average number of links per monomer exceeds a critical percolation threshold (which varies depending on the structure of the material, but is typically not far above two), most of the monomers will link up together into a single giant network that spans the whole object.

Once the monomers have joined together into such a network, they can no longer be separated without destroying the material. Thus, thermoset polymers cannot be re-molded after curing, and they tend to be relatively hard and brittle compared to thermoplastics (although exceptions exist).

(Another way to make a thermoset polymer is to start with a suitable thermoplastic polymer and somehow, e.g. by rapid heating and/or chemical treatment, create additional cross-links between the chains than join them up into a single network. Examples of such thermoset polymers include cross-linked polyethylene and vulcanized rubber. An advantage of such polymers is that they can be shaped like thermoplastics before curing them into their final shape and consistency; the degree of cross-linking can also be controlled to adjust their flexibility and elasticity properties.)

Thus, if your ice cream tub happens to be made of, say, polyurethane instead of polyethylene, it probably does consist mostly of a single giant molecule. While there might be some small isolated clusters that are disconnected from the main network, those will generally make up a relatively insignificant fraction of the material.


Ps. The best sources for this stuff are probably textbooks on polymer chemistry like [2] or introductory online sources like [3] or [4]; it's considered basic enough that you won't really find it discussed in primary sources like scientific papers these days, except maybe as background material.

In a lot of sources, you'll find somewhat weaselly wording like "the entire molecular network can be considered to be a single molecule" ([2], emphasis mine). One reason for such wording is that, while it's easy enough to conclusively demonstrate that a thermoplastic like polyethylene does not consist of a single molecule just by melting it, there's no way to definitively prove that any given piece of thermoset polymer really consists of a single network, and not, say, two or three or 10,000 interpenetrating giant networks.

While a single network comprising most of the material is indeed expected on theoretical grounds, provided that the amount of cross-linking is sufficiently high, and while it is consistent with observed evidence, we don't really have any way to directly show that, say, this atom and that atom at the opposite ends of the object are really connected by a chain of covalent bonds.

Another reason for such weasel wording is simply that many chemists tend to feel vaguely uncomfortable using the word "molecule" for things so huge that you can see them with the unaided eye, instead preferring to call such materials e.g. "network covalent solids". Such macroscopic chunks of matter simply don't have much in common with the kind of small groups of atoms (from a few AMU up to say, a few thousand AMU; maybe a few million for polymer chemists and biochemists) that most chemists normally refer to when they say "molecule", even if they technically do fall under the same definition.

References:

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