Class II includesflexible macromolecules. They stay only in the states of liquid
and solid, in order to reserve the integrity of chemical bonds. Evaporation of
such macromolecules requires so high level of thermal energy that the chemical
bonds are actually broken before reaching that level. The molecular flexibility in
the liquid mainly comes from the internal rotation of the main-chain C-C bonds.
This class includes structural materials of synthetic polymers such as Nylon,
PVC, PET, and PC, adhesives such as PVA, epoxy resins and Glue 502,
elastomers such as natural rubber, polyurethane, SBS and EPDM (rubber
could be regarded as the cross-linked liquid polymers.), biomaterials such as
celluloses, starch, silks and wools, and even bio-macromolecules such as DNA,
RNA and proteins. The class of flexible macromolecules corresponds to the soft
matter defined above.
Class III includesrigid macromolecules. They stay only in the solid states for
reserving the integrity of chemical bonds. Examples of this class include metals,
oxides, salts, ceramics, silicon glasses, diamond, graphite, and some conductive
polymers without any solvent or melting point. The class of rigid
macromolecules corresponds to the hard matter defined above.
In this classification, chain-like structures, as the major reason for polymers
belonging to the soft matter, have received a highlight.
In fact,chain-like structuresare mainly responsible for those unique physical
behaviors of polymers in our study. This kind of structures exhibits anisotropic
properties, i.e. strong covalent bonds along the backbone of the chain, and much
weaker sub-valence interactions on the normal directions of the chain. In thermal
fluctuations and Brownian motions of condensed matter macromolecules, the
strong correlation along the chain dominates physical properties of polymers,
especially in their amorphous states. Polymer chemistry mainly concerns the
preparation of chain-like structures, or using them as building blocks to construct
more complicated macromolecules. In contrast, polymer physics mainly concerns
those physical behaviors brought by chain-like structures, although as building
blocks the latter may construct the more complicated topological architectures of
macromolecules.
Conventionally, we categorize the chain structures of polymers according to their
spatial length scales. The primary structures, also called the short-range structures on
the polymer chain, mainly characterize the chemical microstructures or the chemical
configurations (note that this configuration is different from that defined in the
physics of classical mechanics, where the configuration space means all the possible
combinations of spatial coordinates and momentums.). The primary structures can
only be modified by chemical reactions for making specific sequences of structural
units and their connections along the chain. The secondary structures, also called the
long-range structures on the polymer chain, mainly reflect the chain conformations,
such as the conventional random coils of polymers, the alpha-helices and the
beta-sheets of proteins, etc. The secondary structures are changed with thermal
fluctuations or phase transitions. The tertiary structures mainly describe the steric
assembly of secondary structures in the single protein molecules. The spontaneous
4 1 Introduction