1.2 Polymers in the Eyes of Physicists.......................
In 1990, the Nobel Physics Prize laureate P.-G. de Gennes delivered his Nobel
lecture titled with “Soft Matter” (de Gennes 1992 ). He used polymers as one of
examples of soft matter. Another commonly used term for soft matter is “complex
fluids”.
The hardness, or softness, of matter is normally characterized by their cohesive
energy density, i.e. the interaction energy of particles in each unit of volume,
Ee/a^3 , whereeis the interaction energy between two particles andais the
inter-particle distance.
The conventional hard matter includes metals, glasses and ceramics. The atoms
are connected by strong chemical bonds with the interaction energy of the order
of 10^18 J. The bond lengths or atomic spacing are at the level of angstrom,
a¼ 10 ^10 m. Therefore, the cohesive energy density of hard matter is estimated
as 10^12 N/m^2 , which is about the Young’s modulus of diamond.
In contrast, the soft matter includes polymers, liquid crystals, colloids, nano-
particles, self-assembled or hybrid materials, foams, foods (Fortunately our teeth
are some sort of hard matter!), and even the life systems (Our body is unfortunately
not as hard as the superman in science fictions!). The building blocks of soft matter
typically interact via the sub-valence bonds, with the interaction energy at the order
of magnitude 10^20 J that is much lower than the chemical bonds, and their spacing
ranges from nanometers to micrometers,a¼ 10 ^8 ~10^6 m. Therefore, the cohe-
sive energy density of soft matter is as low as 10^2 ~10^4 N/m^2 , much lower than
that of hard matter! The connection between particles is liable to break as a
response to thermal fluctuations near room temperatures or to weak mechanical
disturbances. Therefore, the soft matter that we daily encounter can undergo a
gigantic structural change around the ambient conditions, and some of its phase
transformations are even driven by the entropic changes. From this perspective, soft
matter can be described as materials “comprising all physicochemical systems
which have large response functions.” (de Gennes’ words (de Gennes 2005 )).
Current classification of chemical substances is also limited in reflecting the
structural characters of polymer compounds. Chemical substances can be divided
into pure substances and their mixtures. The pure substances can be further divided
into elements and compounds. Polymer compounds, as a typical soft matter, can
change their molecular shapes (conformations) to a large extent, and may contain
multiple chemical components in each macromolecule to behave like a mixture.
Accordingly, as pure substances, they are more complicated than normal small
molecular compounds. In 1990, Wunderlich proposed to divide all chemical
compounds into three classes (Wunderlich 1990 ):
Class I includes conventionalsmall molecules. They stay in all three states of
gas, liquid and solid, reserving the integrity of chemical bonds. Examples of this
class are the molecules of oxygen, hydrogen, nitrogen and methane, respec-
tively. There are currently more than 10^7 kinds of small molecules.1.2 Polymers in the Eyes of Physicists 3