Download MME 4713 Polymers D3

POLYMER CRYSTALLINITY
The amorphous nature of polymers is analogous to a plateful of spaghetti --- loose
and randomly coiled.
The crystalline state is more like the uncooked spaghetti in the box --- the chains
are all tightly bundled and ordered in the same direction.
Electron micrograph of
a single crystal of
polyethylene
The structure of the crystalline regions may be deduced by examination
of polymer single crystals, which may be grown from dilute solutions:
These crystals are regularly shaped, thin platelets (or lamellae),
approximately 10 to 20 nm thick, and on the order of 10 μm long.
Frequently, these platelets will form a multilayered structure, like that
shown in the electron micrograph of a single crystal of polyethylene.
The chain-folded
structure for a plate
shaped polymer
crystallite.
Chain-folded model: The molecular chains within each platelet fold
back and forth on themselves, with folds occurring at the faces. Each
platelet will consist of a number of polymer molecules.
Many bulk polymers that are
crystallized from a melt are
semicrystalline
and form a
spherulite structure. Figure on
the left shows the individual chainfolded lamellar crystals that are
separated by amorphous material.
 Each spherulite may grow to
be roughly spherical in shape.
 The spherulite consists of
ribbon-like
chain-folded
crystallites
(lamellae)
that
radiate outward from a single
nucleation site in the center.
 Tie-chain molecules act as
connecting
links
between
adjacent lamellae.
Transmission electron
micrograph (TEM) showing
the spherulite structure in a
natural rubber specimen.
Chain-folded lamellar
crystallites approximately
10 nm thick extend in radial
directions from the center;
they appear as white thin
lines in the micrograph.
30,000X
Spherulites are considered to be the polymer analogue of grains in
polycrystalline metals and ceramics. However, each spherulite is composed of
many different lamellar crystals and, in addition, some amorphous material.
PE, PP, PVC, polytetrafluoroethylene, and nylon form a spherulitic structure
when they crystallize from a melt.
A transmission
photomicrograph (using
cross-polarized light)
showing the spherulite
structure of polyethylene
(525X)
Linear boundaries form
between adjacent spherulites
and a characteristic Maltese
cross pattern appears within
each spherulite.
Polystyrene
Polyethylene
Nylon 6,6
PET - Poly(ethylene terephthalate)
*The polar ester groups make for strong
crystals.
*In addition, the aromatic rings like to
stack together in an orderly fashion,
making the crystal even stronger.
Polyketons
These polar carbonyl groups are attracted to each other, and very
strongly at that. This attraction is so strong that while polyethlyene
melts at ~140 °C, the polyketone doesn't melt until 255 °C!
This makes polyketone very strong but very brittle.
Shell has put the polyketone made with ethylene,
carbon monoxide, and a little bit of propylene on the
market and sells them under the name Carilon.
This polymer is tougher and less brittle.
Carilon Thermoplastic Polymers
SRI (nonprofit research institute) International offers Carilon thermoplastic
polymers for multiple applications in the engineering thermoplastic and fiber
markets. Originally developed by Shell Oil Company and now available for
license exclusively through SRI, Carilon polymers offer superior strength, wear
and low permeability. These features make them ideal for use in automotive
parts; electrical and electronics systems; business machines and consumer
appliances; film, fiber and protective coatings; laboratory supplies, and industrial
applications.
Representing the next wave in high-performance polymeric materials, Carilon
plastics are based on a semicrystalline thermoplastic technology, exhibiting
performance characteristics that are maintained even at high temperatures.
Carilon polymers offer a broad range of features:
• Outstanding chemical resistance and low permeability
• Superior strength, wear and friction characteristics
• High resistance to fatigue, creep, swelling and repetitive deformation
• Excellent balance of stiffness and toughness over a wide temperature range
• High-quality moldings at short cycle times
• Resistance to a variety of fuels, organic solvents and aggressive aqueous
media
• Flame retardancy
Article: Shell Abandons Carilon, Sells Polyurethanes
(Company Business and Marketing)(Brief Article)
Article from: Chemical Week Article date: February 23, 2000
Kevlar
Kevlar
Kevlar is the trademark for a para-aramid synthetic
fiber, developed at DuPont in 1965. This high
strength material was first commercially used in the
early 1970s as a replacement for steel in racing tires.
Currently, Kevlar has many applications, ranging
from bicycle tires and racing sails to body armor
because of its high tensile strength-to-weight ratio;
It is 5 times stronger than steel on an equal weight
basis.
Nomex is a trademark for flame-resistant meta-aramid material developed in
the early 1960s by DuPont.