Materials for engineering, 3rd Edition - (Malestrom)

Organic polymeric materials 183
such models to predict polymer properties. This type of modelling attempts
to establish quantitative relationships between processes at the molecular
level and the behaviour at the macroscopic level. This approach can be used
as a design tool when it has been successfully tested against experimental
data, permitting the identification of the chemical constitution and the synthesis
or processing conditions required to yield a material with certain prescribed
characteristics.
Given a set of performance requirements, it is necessary to identify the
monomers to be used, the process of synthesis to be adopted, and the processing
conditions to which the material has to be subjected in order to meet the
specified properties. The first two of these stages fall largely within the
fields of organic chemistry and chemical engineering, and the third will
include finite-element calculations of melt flow in processing equipment or
of stress fields in deformed structures. The final stage is to describe relationships
between chemical constitution, polymer morphology (e.g. the type and size
of crystallites in semicrystalline polymers, or the size and distribution of
domains in others), and macroscopic properties.
Molecular modelling is employed to provide this final link, by deriving
values of properties or constitutive laws based on fundamental principles of
molecular science. Figure 5.12 illustrates schematically the basis of molecular
modelling approaches to the establishment of structure–property relations in
polymers.
Quantum mechanics can be used to derive the bond lengths, bond angles,
and intermolecular interactions in polymers, while statistical mechanics
provides a link between macroscopically observable behaviour and molecular
geometry and energetics, elucidating molecular mechanisms that govern
material properties. Commercial software packages are available which
incorporate theoretical and simulation techniques, so that modelling methods
(in conjunction with experimental research) are now employed for the
development of new polymeric products in industry.
Morphology
Chemical
constitution
Quantum
mechanical
calculations
Intermolecular and
intramolecular
interactions
Statistical
mechanics based
theories and
computer
simulations
Values of
material properties
constitutive laws
Chain organization
microscopic mechanisms
of molecular processes
5.12 Information flow involved in molecular modelling for the
establishment of structure–property relationships.