YOU PROBABLY carry home your shopping in sheets of cold slime. I don鈥檛 mean
to be rude. It鈥檚 just that plastic carrier bags are made from a material which,
when melted, is as slimy a substance as you will ever rub between finger and
thumb.
Most slimes are tangles of long, chainlike polymer molecules. Mucus, shampoo,
fabric conditioner and green slime from a toy shop鈥攁ll of these are
polymer slimes. But the stuff in carrier bags鈥攍ow-density polyethylene, or
LDPE鈥攊s one of the slimiest of all. Molten LDPE is thick and slippery,
like all good slimes. But it also has two special properties: it hardens if you
stretch it, despite being slippery when rubbed; and if you stretch it further it
goes soft again.
LDPE鈥檚 exceptional sliminess doesn鈥檛 just make it a useful material. In the
process of understanding it, chemists and physicists have worked out how many
other polymer slimes work. Just last year, one group confirmed a theory that
explains LDPE鈥檚 peculiar properties, and using this knowledge they have begun to
develop an array of artificial slimes with everyday uses that range from hair
care to plastics to sticking plasters.
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It鈥檚 all a matter of the molecules鈥� shape. The first clue to this was a
straight and boring polymer, high-density polyethylene. HDPE doesn鈥檛 have the
weird properties of LDPE, yet it is chemically the same stuff鈥攍ong chains
of carbon atoms trimmed with hydrogen. The only difference is that molecules of
HDPE are simple strings, whereas LDPE鈥檚 are extravagantly branched tree-like
structures. How does this lead to such different properties?
Faced with the intertwined forest of LDPE trees, physicists were stumped. So
they adopted a favourite strategy: simplify. After first getting to grips with
something easy, they could work back up to LDPE through an Alphabetti Spaghetti
of increasingly complicated shapes.
The simplest polymer molecules are like strands of ordinary spaghetti. In
molten polymers and concentrated polymer solutions, the strands are tangled
together in a gloopy mess. This already seems a daunting prospect, but one
property is easy enough to understand. If you rub molten polymer between your
fingers, the motion tends to align many of the molecules, smoothing out the
tangle. That reduces the resistance to the rubbing motion, giving the substance
its characteristic slimy, slippery feel.
To understand more, you have to cut away the intractable tangle of millions
of molecules and focus instead on a single one. A polymer molecule doesn鈥檛 lie
in a straight line, it forms a floppy zigzag, writhing with its own thermal
vibrations, trying to move any which way it can. But its movement is limited by
its neighbours. You can imagine the restricting effect of all these
criss-crossing polymer chains as a tube that confines your chosen molecule
(see Diagram).
Then it is easier to see how a molecule can move: it wriggles along
the tube like a snake in a drainpipe.
Sam Edwards and Masao Doi of Cambridge University used this idea more than 20
years ago to explain the flow of linear polymers. When you deform these
substances, they try to push back because you are moving the confining tubes and
bending the chains into new shapes. But gradually, as the chains snake out of
their tubes, the whole tangle relaxes and the resistance disappears.
So linear polymer slimes behave like potty putty: if you whack them, they
bounce; if you leave them for a while, they slump into a puddle. The timescale
for this slumping depends on the weight of the molecules. Short, thin snakes
wriggle rapidly, and a melt of such molecules can slump in less than a
millisecond. Long, fat snakes move slowly, making a viscous fluid that can take
seconds to relax.
None of this explains the extension hardening or softening that is
characteristic of LDPE鈥攂ut remember, LDPE is very branched. So the next
step is to make the model a bit more sophisticated by adding some branches to
the wiggly chains. The simplest way to do this is to make chains with one
central branching point. These are star polymers鈥攖he Y, X, and * of the
Alphabetti Spaghetti.
Even the simplest of these, the Y-shaped molecule or three-armed star can鈥檛
wriggle along its tube. To see why, you need to turn to 鈥淗oudini theory鈥�, says
Tom McLeish of Leeds University. He likens the confining tube around a polymer
molecule to Houdini鈥檚 straitjacket. How a molecule escapes tells us how it is
able to move. 鈥淵ou dress your polymer up in a straitjacket, throw it in the
shark pool, start your stopwatch and say `now get out of that鈥�,鈥� he says.
For one arm of a Y to snake out of the open end of its sleeve, it would have
to pull the other two arms into the tube behind it. This would seriously
restrict their random wriggling, so they resist. Because of this, the centre of
a star molecule should be pinned in place.
Yet star polymers somehow manage to flow. The puzzle of how they do this was
solved in the early 1980s by Doi, Pierre-Gilles de Gennes at the Coll猫ge
de France in Paris and Dale Pearson and Gene Helfand of Bell Laboratories. They
started by considering the motion options available to a star鈥檚 arm stuck in its
straitjacket. All it can do, they reasoned, is wriggle randomly inside the
sleeve. This wriggling will occasionally send the arm snaking inwards along its
sleeve towards the centre of the star. When it snakes back out again, it doesn鈥檛
have to go back up the old sleeve鈥攊t can find another way to poke between
the surrounding confining molecules, in effect making itself a new sleeve.
So when you squeeze a glob of star-polymer slime, bending the sleeves and the
arms inside them, they want to bounce back just like the linear polymer. But
give the arms time to rearrange themselves, and the resistance fades away. So
under the persistent force of gravity, a blob of star polymer will slump into a
pool.
If this can happen with a Y-shaped molecule it can happen with any other star
polymer鈥攖he number of arms doesn鈥檛 matter. A Y behaves much like an X or
the hundred-armed giants made by Jacques Roovers of the National Research
Council in Ottawa, Canada. What counts is the time it takes for the longest arm
to snake back to the branch point, which increases exponentially with the weight
of the arm. Because of this more roundabout method of movement, star polymers
generally make much thicker slime than linear polymers of the same molecular
weight.
Glacial flow
This still doesn鈥檛 take us where we want to go. Although stars make good
thick slime, they鈥檙e still not stringy and stretchy like LDPE. But try adding a
second branch point, to make an H-shaped molecule, and it鈥檚 a different
story.
McLeish鈥檚 Houdini theory can explain what happens here, too. 鈥淒ress up the H
in its straitjacket, and throw it in the shark pool,鈥� he says. 鈥淎t first, the
middle section looks to be completely stuck.鈥� Though the four side branches
wriggle and reform themselves like the arms of a star, the middle section can鈥檛
move. But if you wait a while, one of the arms will bunch in all the way for a
moment, so that its branch point is free to make a little step. Over time, as
this happens again and again, the centre bar snakes along its sleeve鈥攍ike
a linear polymer, but much more slowly. 鈥淭he H polymer is like a glacier,鈥� says
McLeish. 鈥淲e can walk about on the glacier and play hockey, but over the years
it flows like a river.鈥�
What鈥檚 really different about the H is that if you stretch a dollop of H
polymer, the two branch points on each molecule pull the crossbar taut and
straight. The crossbar resists, and the further you pull, the more it resists.
This is what we have been seeking, the extension hardening that makes LDPE
stringy and stretchy.
鈥淣either linear nor star polymers do this, because every segment has a free
end,鈥� says McLeish. When you stretch out a glob of star polymer, you pull on the
constraining tubes, but the arms stay coiled up inside them. 鈥淭hink of Houdini
in his straitjacket. If you pull on his sleeve, his arm doesn鈥檛 stretch to twice
its original length, it just slips back inside the sleeve.鈥� The crossbar of the
H doesn鈥檛 have this option, because it sits between two branch points that get
pulled apart with the fluid. If you pull the branch points, the crossbar has to
move.
But then the second odd property of LDPE appears. When a crossbar has reached
twice its original length, the tension becomes so great that it pulls the
protesting side-arms into its tube, and all the tension is released. Your slab
of slime suddenly goes floppy.
Having developed his theory, McLeish decided to test whether this is how
real-life H polymers behave. To do that, he needed a sample of the stuff, free
from mutant molecules that might interfere with the flow properties. H polymers
aren鈥檛 easy to make, especially in large amounts, and making one pure enough for
this experiment is even harder. 鈥淚t鈥檚 not your standard wet-bucket chemistry,鈥�
says Ron Young of Sheffield University. But in 1996, Young and his colleague
J眉rgen Allaier succeeded in growing some pure H polymers using
polyisoprene, a natural rubber (Macromolecules, vol 32, p 6734).
Sure enough, when McLeish squeezed and stretched the new polyisoprene H
polymer, he found that it showed all the slimy subtlety his theory predicted.
And by scattering a beam of neutrons off the molecules last year, he could
actually see them move. At last McLeish had solved the problem: LDPE is so odd
because it is complicated enough to have crossbars.
This is what makes it such a useful plastic. Extension hardening stops it
from splattering loose droplets onto the inside of a mould, while extension
softening helps it squidge into all the tiny corners. Hardening also makes LDPE
ideal for bubble extrusion, the process used to make the thin sheets of polymer
that get turned into plastic carrier bags. Here, the melt is squirted out
through a ring and inflated into a huge cylinder. A linear or star melt would
just break into blobs.
LDPE molecules are actually a lot more complicated than the H polymer. They
are actually more like a tree than an H, with many branches and twigs, all
irregularly placed. But the extra complication isn鈥檛 too hard to deal with.
鈥淭his time Houdini needs a carefully stitched straitjacket,鈥� says McLeish.
His fingers wriggle quickly, bunching up occasionally to let his arms move in
little hops. Eventually that allows his torso to move, step by painful step,
along its own tube. Each level of the hierarchy moves much more slowly than the
one below.
Plastic fantastic
Understanding LDPE isn鈥檛 the end of the story. If you鈥檝e ever had a week鈥檚
groceries splatter on the ground as the plastic carrier bag gives way, you won鈥檛
need telling that solidified LDPE could benefit from some beefing up. The
trouble is that its molecules are too branchy. When plastics solidify, some bits
of polymer crystallise into hard blobs called spherulites, which remain embedded
in a sea of amorphous polymer tangles. But a highly branched molecule like LDPE
finds it difficult to crystallise. 鈥淵ou get too much cement and not enough
bricks鈥� says Rudi Koopmans of Dow Benelux in Terneuzen, the Netherlands.
So chemists are looking for ways to control the arrangement of branches, to
produce plastics that have processing properties as good as LDPE鈥檚, but are much
tougher when they solidify. Koopmans says that shape tailoring could also lead
to more environmentally friendly plastics, based on renewable resources. His
company is developing plastics based on polylactic acid, a polymer derived from
maize, while Monsanto are producing a biopolymer made by bacteria. Without shape
tailoring, these plastics might be weak or hard to process.
It鈥檚 not just plastics that might benefit from tailored slime. Polymers are
already used to give hair gel a satisfying feel. And daily-wash shampoos, which
are only a few per cent soap, feel pleasingly viscous because some of the soap
molecules assemble into giant tubes which act like linear polymers. Meanwhile
Smith & Nephew, which makes Airstrip and Elastoplast dressings, is
interested in using branched-polymer glues for sticking plasters. The aim is to
make a plaster that sticks firmly to the skin but can be whipped off again
without leaving lumps of goo behind. In a glue full of H polymers, the fibres
connecting plaster to skin would get stronger as you pull, and leave clean skin
behind.
So have scientists found the general theory of slime? Well, not quite.
Although biological goos like mucus get much of their sliminess from this
entanglement of polymers, they are more complicated. Slug slime, for example, is
extra slippery because it arranges itself into an ordered state called a liquid
crystal, according to Christopher Viney of Heriot-Watt University in Edinburgh.
The gunk made by Hagfish is weirder still
(see 鈥淢onstrous mucus鈥�).
And there is one more letter hiding itself in the Alphabetti Spaghetti: the
enigmatic O. With the O we鈥檙e in a different world. 鈥淓verything so far has been
essentially a theory of ends鈥攂ut the O has no ends鈥� says McLeish, 鈥渟o what
does a melt of O-shaped rings do?鈥� In slow-flow experiments, O-shaped molecules
seem to behave like linear polymers. Michael Rubinstein of the University of
North Carolina at Chapel Hill thinks they collapse into skinny amoebas, throwing
out little pseudopodia from time to time. 鈥淢ichael is brilliant, so he鈥檚
probably right,鈥� says McLeish. 鈥淏ut whether they鈥檒l be any use to anyone I鈥檝e no
颈诲别补.鈥�