atoms in the diamond. The most stable geometric conformation of molecules often
determines the azimuths of the packing neighbors, i.e., the angles between two axes
of the unit cell. Second, the single crystal has an intrinsicperiodicityfor the
stacking of the structural units, characterized by, for diamond, the carbon-carbon
bond lengths. The interactions between the structural units determine the axial
lengths of the unit cell. Besides the above two intrinsic factors reflecting the
common properties of the same species of single crystals, the single crystal contains
some extrinsic characteristic features on its individual structure, i.e.,the sizes,the
facetsandthe internal defects. For a piece of diamond, these extrinsic characteristic
factors mainly determine its market price. The diamond structure characterized
separately at the intrinsic and extrinsic levels facilitates a better understanding of its
relationship to performances.
One can describe polymer samples in a similar way. First of all, a polymer chain
possessessemi-flexibility, that characterizes the intra-chain interactions for the most
stable conformation persisting along the chain axis. Secondly, a polymer chain also
holdscomplex inter-chain interactions. These two intrinsic characteristic factors
dictate the basic physical behaviors of the same species of polymers. Besides these
two intrinsic factors, each individual polymer sample possesses certain extrinsic
characteristic factors, i.e.,molecular weights and their distributions,topological
architectures, andsequence irregularities. These extrinsic characteristic factors are
also important in determining the physical behaviors of the polymer samples.
The separation of chemical factors of polymer chains into the intrinsic and
extrinsic levels allows us to understand their corresponding roles in determining
the physical behaviors of polymers. The intrinsic chain structures play a primary
role in determining physical behaviors. They often serve as the thermodynamic
driving forces for structural phase transitions. In contrast, the extrinsic chain
structures play a secondary role in determining physical behaviors. They usually
serve as the external restrictions for structural phase transitions. For instance, the
anisotropic attractions represent the compact packing of polymer chains, which
drive polymer crystallization. If polymer chains contain too many randomly
distributed irregular structural units along the sequence of the chain (see Sect.2.6
for more details), the capability of polymer crystallization will be ruined. There-
fore, random copolymers often stay in the non-crystalline state and exhibit the
characteristics of amorphous polymers, such as atactic polystyrene (aPS) and
atactic poly(methyl methacrylate) (PMMA).
The following text will introduce five intrinsic and extrinsic chemical factors
above, as well as their relationships with physical properties of polymers.
2.2 Semi-Flexibility of Polymer Chains
Many factors may determine polymer semi-flexibility, such as internal rotation,
solvation, stretching, spatial confinement, surface adsorption, charge interactions,
hydrogen bonding along helix, and double helix of DNA, etc. The most common
factor is the internal rotation. One can understand the internal rotation fromthe
14 2 Structure–Property Relationships