高分子材料
2. MOLECULAR STRUCTURE OF POLYMERS;
POLYMER MORPHOLOGY;
CHARACTERIZATION OF MOLECULAR WEIGHT
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a. MOLECULAR STRUCTURE OF
POLYMER
We will explore the types of bonds formed in polymers, and focuses on how weak bonds
can impact material properties. The arrangement of these bonds and side groups along the
polymer backbone also lead to a wide range of possible isomers (same chemical formula,
different arrangement of atoms) that are also important in determining polymer properties
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3.1 TYPES OF BONDS
• Various types of bonds hold together the atoms in polymeric materials,
unlike in metals, for example, where only one type of bond (metallic)
exists. These types are:
(1) primary covalent,
(2) hydrogen bond,
(3) dipole interaction,
(4) van der Waals, and
(5) ionic.
• The latter four are known collectively as secondary (or weak) bonds.
• The distinctions are not always clear-cut, that is, hydrogen bonds may
be considered as the extreme of dipole interactions.
• The secondary bonds are generally weaker bonds and are responsible
for many of the bonds between different polymer chains (intermolecular
bonds).
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3.2 BOND DISTANCES AND STRENGTHS
• Regardless of the type of bond, the potential energy of the
interacting atoms vary as a function of the separation
between them.
• As the interacting centers are brought together from large separation,
an increasingly great attraction tends to draw them together
(negative potential energy).
• Beyond the separation rm, as the atoms are brought closer together,
their electronic “atmospheres” begin to interact and a powerful
repulsion is set up.
• At rm, the system is at a minimum potential energy, its most
probable or equilibrium separation, rm being the equilibrium
bond distance.
• The “depth” of the potential well e is the energy required to
break the bond, separating the atoms completely.
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• The primary covalent bonds are much stronger than the others.
• As the material’s temperature is raised and its thermal energy (kT) is
thereby increased, the primary covalent bonds will be the last to dissociate
when the available thermal energy exceeds their dissociation energy.
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3.3 BONDING & RESPONSE TO TEMPERATURE
• In linear and branched polymers, only the secondary bonds hold the individual polymer
chains together (neglecting temporary entanglements). Thus, as the temperature is raised,
a point will be reached where the holding the chains together become insignificant, and
the chains are then free to slide past one another upon the application of stress. Therefore,
linear and branched polymers are generally thermoplastic.
• The network polymer, on the other hand, are held together by the same covalent bonds as
are the main chains. Therefore, no interchain sliding is enhanced by increased
temperature. Furthermore, when the thermal energy exceeds the dissociation energy of
the covalent bonds, both main-chain and crosslink bonds randomly fail, and the polymer
degrades. Hence, crosslinked polymers are thermosetting.
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• Some exceptions: It is occasionally possible for secondary bonds to make up for in quantity what they lack
in the quality (or strength) of a single bond. For example, polyacrylonitrile
is capable of
strong dipole interactions between the pendent nitrile groups (腈基).
• If these secondary bonds could be broken one by one (i.e., “unzippped”), polyacrylonitrile would behave as a
typical thermoplastic. This is impossible, of course, due to the random nature of polymer configurations.
• By the time enough of flow, the dissociation energy of some carbon–carbon backbone covalent bonds will
have been exceeded and the materials will degrade. Extreme stiffness of the polymer chain also contributes to
this sort of behavior.
• Cellulose has a bulky, complex repeat unit that contains three hydroxyl groups. Though linear, its chains are
therefore stiff and strongly hydrogen bonded, and, thus, it is not thermoplastic. If the hydroxyls are reacted
with acids, such as nitric, acetic, or butyric, the resulting derivative of cellulose (a cellulose ester) behaves as
a typical thermoplastic largely because of the reduced hydrogen bonding:
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• Here, three -OH groups are converted to acetate esters and the acetate esters simply
modify side groups, making the structure of these cellulose derivatives more
“workable.” They are commonly used in pharmaceutical coatings and a number of
foodstuffs, even being a component in some soft-serve ice creams.
• Polytetrafluoroethylene (Teflon, TFE) with the repeat unit -(CF2-CF2)- is another
example of a thermosetting material, as the close packing and extensive secondary
bonding of the main chains prevents flow when the polymer is heated.
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3.4 ACTION OF SOLVENTS
• The action of solvents on polymers is in many ways similar to that of heat.
Appropriate solvents, that is, those that can form strong secondary bonds
with the polymer chains, can penetrate, replace the interchain secondary
bonds, and thereby pull apart and dissolve linear and branched polymers.
• The polymer–solvent secondary bonds cannot overcome primary covalent
crosslinks, however, so crosslinked polymers are not soluble, although they
may swell considerably. (An entropic effect of the “heated” polymer.)
• The amount of swelling is a convenient measure of the extent of crosslinking.
• A lightly crosslinked polymer, such as the rubber band or superabsorbent polymers in
diapers, will swell tremendously, while one with extensive crosslinking, for example,
an ebonite (“hard rubber”) bowling ball, will not swell noticeably at all.
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3.5 BONDING AND MOLECULAR STRUCTURE
• It is obvious that the chemical nature of a polymer is of considerable importance in
determining the polymer’s properties. Of comparable significance is the way the
atoms are arranged geometrically within the individual polymer chains.
• A look into protein structure illustrates the different types of bonds and their effects
on the three-dimensional organization of a polymer.
• The bonds that are found in proteins (polypeptides) are the same as those listed here for
polymers, yet weak bonds play a very important role in determining the three-dimensional
organization and biological activity of proteins.
• These bonds are divided into interactions at four levels.
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Primary structure:
the amino acid
sequence. (The
chemical configuration)
Secondary structure:
regularly repeating
local structures
stabilized by hydrogen
bonds. (Conformation
of the molecular
segments, local
conformation)
Tertiary structure: the
overall shape of a single
protein molecule; the spatial
relationship of the secondary
structures to one another.
(Conformation of the whole
protein molecule)
Quaternary structure: the
structure formed by
several protein molecules
(polypeptide chains),
usually called protein
subunits in this context,
which function as a
single protein complex. (
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1. Primary Bonds:
• the amino acid sequence. (The
chemical configuration)
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2. Secondary Bonds:
• These are weak bonds between near-neighbor
repeat units, covering a range of perhaps 10–20
repeat units. Hydrogen bonds are common here
and cause an impact on the formation of coils or
zig-zag structures.
• Regularly repeating local structures stabilized by
hydrogen bonds (Conformation of the molecular
segments, local conformation)
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3. Tertiary Bonds:
• These are weak intramolecular bonds (those
between a polymer and itself) between repeat
units that are widely separated in the onedimensional structure These bonds hold the
three dimensional structure of an individual
polymer in place.
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4. Quaternary Bonds:
• These are weak intermolecular bonds (those between two different polymers).
They stabilize the three-dimensional structure of a polymeric material,
linking the different chains without forming covalent bonds (which would
cause a network to form).
The crystal structure of the chaperonin
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• This breakdown of bonding in proteins is essentially the same for all polymers, although
commercial homopolymers do not display the same structural complexity of proteins.
• The carbon atom is normally (exclusively, for our purposes) tetravalent.
• In compounds such as methane (CH4) and carbon tetrachloride (CCl4), the four identical substituents surround
the carbon in a symmetrical tetrahedral geometry.
• If the substituent atoms are not identical, the symmetry is destroyed, but the general tetrahedral pattern is
maintained.
• This is still true for each carbon atom in the interior of a linear polymer chain, where two of the substituents are
the extensions of the polymer backbone.
• If a polyethylene chain (in normal spaghetti-type coil) were to be stretched out, for example, the
carbon atoms in the chain backbone would lie in a zigzag fashion in a plane, with the hydrogen
substituents on either side of the plane (Figure 3.4).
• In the case of polyethylene, in which all the substituents are the same, this is the only arrangement possible.
With vinyl polymers, however, there are several possible ways of arranging the side groups.
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3.6 STEREOISOMERISM IN VINYL POLYMERS
• The monomers (H2C=CHX, where X can be Cl, -OH, an organic group, etc.)
polymerize almost exclusively in a head-to-tail fashion, placing the X groups on
every other carbon atom along the chain.
• Although head-to-head (or tail-to-tail)
connections are possible, steric hindrances
between successive X groups (particularly if the
X group is bulky) and electrostatic repulsion
between groups with similar polarities generally
keeps most of the linkages as head-to-tail.
• Ignoring head-to-head connections, there are then
three possible ways in which the X side group
may be arranged with respect to the carbon
backbone plane.
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• These arrangements represent three types of
stereoisomers:
1. Atactic: A random arrangement of the X groups.
Its lack of regularity has important consequences.
2. Isotactic: The structure in which all the X
groups are lined up on the same side of the backbone plane.
3. Syndiotactic: Alternating placement of the X
group on either side of the plane.
• These three terms were coined by Dr. Giulio Natta, who
shared the 1964 Nobel Chemistry Prize for his work in
this area.
• Although atactic polymers are certainly most common, the arrangement and packing (and
thus the properties) of stereoregular (syndiotactic and isotactic) polymers makes them
important for certain applications. Methods to synthesize the stereoregular isomers will be
discussed later in this course.
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• The planar zigzag arrangement of the mainchain carbon atoms is not always the one preferred by nature,
that is, it is not necessarily the minimum free-energy configuration.
• In the case of polyethylene, it is, but for isotactic and syndiotactic polypropylene (which has a pendent methyl, -CH3,
group), the preferred (minimum-energy) configurations are quite regular, with the backbone forming helical twists to
maximize the distance between consecutive -CH3 groups.
• The atactic polymer, however, lacks regular twists and has an irregular shape.
• Atactic polypropylene has a consistency somewhat like used chewing gum, whereas the stereoregular
forms are hard, rigid plastics. The reason why this regularity (or lack of it) has a profound effect on
mechanical properties is discussed in the next chapter of polymer crystallization.
• The type of stereoregularity described above is a direct result of the dissymmetry of vinyl monomers. No
amount of twisting and turning the chain about its bonds can convert the three-dimensional geometry of one
stereoisomer into another, it is established in the polymerization reaction.
• The situation is even more complex for monomers of the form HXC=CHX’ , where X and X’ are different
substituent groups. This is discussed by Natta, but is currently of no commercial importance.
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Example 3.1
• Both isotactic and atactic polymers of propylene oxide have been prepared by ring
scission polymerization.
(a) Write the general structural formula of the polymer.
(b) Indicate how the atactic, isotactic, and syndiotactic structures differ.
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Solution:
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3.7 STEREOISOMERISM IN DIENE POLYMERS
• Another type of stereoisomerism arises in
the case of poly-1,4-dienes because
carbon– carbon double bonds are rigid
and do not allow rotation.
• The substituent groups on the doublebonded carbons may be either on the
same side of the chain (cis) or on the
opposite sides (trans), as shown in Figure
3.6 for 1,4 polyisoprene.
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• The chains of cis-1,4-polyisoprene assume a tortured, irregular configuration
because of the steric interference of the substituents adjacent to the double bonds.
• This stereoisomer is familiar as natural rubber (made by the rubber tree) and is used in rubber
bands.
• Trans-1,4- polyisoprene chains assume a regular structure. This polymer is known
as gutta-percha, a tough but not elastic material, long used as a golf-ball cover
• The stereoisomerism in poly-1,4-dienes does not depend on the dissymmetry of
the repeating unit. Atactic, syndiotactic, and isotactic isomers are possible with
butadiene, even though all of the carbon substituents are hydrogen, as shown in
Example 3.2.
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Example 3.2
• Identify all possible structural and stereoisomers that can result from
the addition polymerization of butadiene:
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Solution:
• Recall from Chapter 2 that butadiene can undergo 1,4-addition or 1,2-addition (3,4addition results in the same structure as 1,2-addition). The 1,4-polymer can have cis
and trans isomers like the 1,4-polyisoprene in Figure 3.6. The 1,2-polymer can have
atactic, isotactic, and syndiotactic stereoisomers like any vinyl polymer.
1,4-polymer
1,2-polymer
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Example 3.3
• Identify all the possible structural and stereoisomers that can result
from the addition polymerization of chloroprene (this rubbery polymer
is commercially known as Neoprene and is used in wetsuits).
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Solution:
• As in Example 3.2, the 1,4-polymer can have cis and trans stereoisomers (two
different possibilities). This unsymmetrical diene monomer can also undergo both
1,2-and 3,4-addition:
C
• Each of these structural isomers may have atactic, isotactic, or syndiotactic
stereoisomers (making for six more possible isomers). Thus, in principle at least,
there are eight different isomers of polychloroprene possible.
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3.8 SUMMARY
• Bonding in polymers is significantly more complex than in metals.
The variety of bonds and the way side groups are organized in isomers
make for the wide range of properties polymers can have, as will be
demonstrated in later chapters.
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PROBLEMS
• Draw chemical structures of isotactic and syndiotactic oligomers of
vinyl alcohol with five repeat units; include hydrogen atoms for the
end groups.
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