8 . Figure 2. Schemes of structure of polymer macromolecule: a) linear,
branched, c) lattice.
According to their spatial state, polymers can be in amorphous and crystalline
states.
The structure of polymers has a great influence on their physico-mechanical
and chemical properties. Due to their large molecular mass, polymers do not exist in
the gaseous state, and they do not even soften when heated.
As the molecular weight increases, the polymer becomes more difficult to
dissolve under the influence of solvents. If higher in high molecular weight polymer
in the case of polar groups, such a polymer is insoluble in any solvent.
Polydispersity in polymers causes them to spread all their properties.
Mechanical properties of polymers (plasticity, strength) depend on their
structure, physical condition, temperature and other factors.
Polymers can be in three physical states: glassy, highly elastic and viscous
(sticky) states ( Fig . 8 . 3).
95
8 . Figure 3. Thermomechanical graphs of non-crystalline (1), crystalline (2)
and reticulated (3) polymers:
T
sh
- glass transition temperature;
T
k
- crystallization temperature;
T
t
- temperature of adhesiveness;
T
k
- chemical decay (destruction) temperature;
I, II, III - areas of glassy (1), high elasticity (II) and viscous (III) states.
High elasticity is only high characteristic of polymers, these materials can
undergo large elastic deformations.
A sticky state is similar to a liquid with high viscosity.
Linear or branched polymers can change from one physical state to another
with temperature changes.
Polymers with a spatial structure (with a network structure) exist only in the
glassy state. Orientation can be strengthened. In this case, the polymer in a highly
elastic or viscous state is gradually deformed and a certain order appears in the
structure of the polymer. In uniaxial deformation, fibrous, pipe-like products are
obtained, in biaxial deformation, films are obtained. In the process of orientation,
the strength of the polymer increases.
The strength of amorphous polymers
decreases over time, because the phenomenon of disorientation occurs over time
under the influence of temperature.
The mechanical properties of polymers are also affected by the time and rate
of exposure to the powder. Under the influence of an external force, the
macromolecule of the polymer is straightened, elongated, the links shift and twist.
The passage of such changes depends on time, therefore, when evaluating the
properties of polymers, it is necessary to take into account the effect of temperature
and time on them, in other words, the relaxation property of the polymer should be
taken into account.
|
