课后习题答案(英文版)
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2021年01月28日 19:26
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are linear and branched polymers? Can thermoplastics be branched?
A linear polymer consists of spaghetti-like molecular chains.
In
a
branched
polymer,
there
are
primary
polymer
chains
and
secondary
offshoots
of
smaller
chains that stem from these main chains.
(a) a thermoplastic,
(b ) thermosetting plastics,
(c) elastomers, and
(d) thermoplastic elastomers.
Thermoplastics
are
composed
of
long
chains
produced
by
joining
together
monomers;
they
typically behave in a plastic, ductile manner. The chains may or may not have branches.
Thermosetting polymers
are composed of long chains (linear or branched) of molecules that are
strongly cross-linked to one another to form three-dimensional network structures.
Elastomers
These are known as rubbers. They have an elastic deformation >200%. The polymer
chains consist of coil-like molecules that can reversibly stretch by applying a force.
Thermoplastic
elastomers
are
a
special
group
of
polymers.
They
have
the
processing
ease
of
thermoplastics and the elastic behavior of elastomers
electrical and optical applications are polymers used for? Explain using examples.
We use polymers in photochromic lenses.
Plastics are often used to make electronic components because of their insulating ability and low
dielectric constant.
More recently, significant developments have occurred in the area of flexible electronic devices
based on the useful piezoelectricity, semiconductivity, optical and electro-optical properties.
are the major advantages associated with plastics compared to ceramics, glasses, and
metallic materials?
Commercial polymers are lightweight, corrosion-resistant material with low strength and stiffness,
and they are not suitable for use at high temperatures.
These polymers are relatively inexpensive and are readily formed into a variety of shapes, ranging
form plastic bags to mechanical gears to bathtubs.
do the terms condensation polymerization, addition polymerization, initiator mean ?
Addition
polymerization
and
condensation
polymerization
are
the
two
main
ways
to
conduct
“polymerization”.
The
polymers
derived
from
these
processes
are
known
as
addition
and
condensation polymers, respectively.
The formation of the most common polymer, polyethylene (PE), from ethylene molecules is an
example of addition or chain-growth polymerization.
Ethylene,
a
gas,
is
the
monomer
(single
unit).
The
two
carbon
atoms
are
joined
by
a
double
covalent bond.
Each carbon atom shares two of its electrons with the second carbon atom,
and two hydrogen
atoms are bonded to each of the carbon atoms.
In
presence
of
an
appropriate
combination
of
heat,
pressure,
and
catalysts,
the
double
bond
between the carbon atoms is broken and replaced with a single covalent bond.
The double bond is an
unsaturated bond
. After changing to a single bond, the carbon atoms are