Chemistry Reference
In-Depth Information
salt, for cationization, and deposited on a probe surface. A UV laser is pulsed at
the mixture, vaporizing a layer of the target area. Collisions between cations and
polymer in the cloud of debris form charged polymer molecules. These are
extracted and accelerated to a fixed kinetic energy by application of a high poten-
tial. They are diverted into a field-free chamber, where they separate during flight
into groups of ions according to their mass/charge ratios. The output of a detector
at the end of the drift region is converted to a mass spectrum on the basis of the
time elapsed between the initiation of the laser pulse and the arrival of the
charged species at the detector. Lighter ions travel faster and reach the detector
earlier. Responses from a multiplicity of laser shots are combined to improve the
signal/noise character of the mass spectrum.
MALDI-TOF is a useful technique at present for low-molecular-weight poly-
mers. Application to most commercially important polymers is problematic at the
time of writing, however, because these materials have high mean molecular
weights and broad molecular weight distributions
[28]
.
Appendix 3A: Multigrade Motor Oils
[29]
Certain polymers act to improve the
viscosity index
(VI) in crankcase lubricants.
The principles involved are those described in
Section 3.3
. Most internal combus-
tion engines are designed to function most efficiently by maintaining approxi-
mately constant engine torque over the wide temperature range that the
lubricating oil may experience. If the crankcase oil is too viscous at low tempera-
tures, the starting motor will have difficulty in cranking the engine and access of
the lubricant may be impeded to all the components it is designed to protect. On
the other hand, if the oil is too fluid at the engine's operating temperature, which
can exceed 200
C, it may fail to prevent wear of metal parts and may be con-
sumed too fast during running of the vehicle. Oil-soluble polymers are used as VI
improvers to counteract the tendency for the lubricant's viscosity to drop with
increasing crankcase temperature.
VI improvers require oxidation resistance without generation of corrosive by-
products, thermal stability, compatibility with the other additives in the lubricant
package, shear stability and solubility, or, rather, absence of separation over the
operating range of the engine. This balance of properties is achieved by use of
certain polymers at the 0.5
3.0% level. The commercially important VI impro-
vers are polymethacrylates, ethylene-propylene copolymers, and hydrogenated
styrenediene copolymers. At low temperatures the oil is a relatively poor solvent
for the polymer and the macromolecules tend to shrink into small coils which add
very little to the viscosity of the mixture. At high temperatures, however, the oil
is a better solvent and swells the polymer coils. The result is a greater hydrody-
namic volume of the macromolecular solute, higher viscosity of the solution
(cf.
Eq. 3-63
), and compensation for the increased fluidity of the base oil.
