Transcript POLYMERS

ΠΟΛΥΜΕΡΗ
1. Εισαγωγή.
2. Διαμόρφωση των μακρομοριακών αλυσίδων.
3. Θερμοδυναμική των μακρομοριακών διαλυμάτων.
4. Σκέδαση του φωτός από διαλύματα πολυμερών
5. Δυναμική των αραιών διαλυμάτων των πολυμερών.
6. Υαλώδης μετάπτωση
7. Κρυσταλλικά πολυμερή
Βοήθημα
Paul C. Hiemenz and Timothy P. Lodge, Polymer Chemistry, CRC Press, Taylor & Francis
Group, Boca Raton, 2007.
POLYMERS
1
Introduction to chain molecules
1.1 Introduction
“I am inclined to think that the development of polymerization is perhaps the biggest
thing chemistry has done, where it has had the biggest impact on every day life”
Lord Todd, 1980 (Nobel Laureate in Chemistry, 1957)
There is hardly an area of modern life in which polymer materials do not play an
important role. Applications span the range from packaging, toys, fabrics to
bulletproof vests and stealth aircrafts.
What is impressive is the breadth of applications for polymers. Not only they
continue to encroach into the domains of “classical” materials such as metal,
wood and glass, but they also play an important role in many emerging
technologies. Examples include “plastic electronics”, gene therapy, artificial
prostheses, optical data storage, electric cars, and fuel cells.
During the early years of the 20th century, the idea that polymers were some
sort of association between low molecular weight constituent molecules
prevailed for a long while.
Staudinger is generally credited as being the father of modern polymer
chemistry. In 1920 Staudinger proposed the chain formulas that we accept
today, maintaining that structures are held together by covalent bonds. There
was a decade of controversy before this “macromolecular hypothesis” began to
experience widespread acceptance.
1.2 How Big is Big
polymer = poly and meros (many parts).
macromolecule = “large (or long) molecule”.
All polymers are macromolecules, but not all macromolecules are polymers.
A protein is a macromolecule, but not a polymer.
Nevertheless, the terms are usually used interchangeably.
1.2.1 Molecular weight.
Molecular weight (or molar mass) is a term that has to do with the number of repeat units
of a polymer molecule.
The degree of polymerization is also commonly used in this context.
These expressions should always be modified by the word average.
The degree of polymerization, N, of a polymer is simply the number of repeat units in a
molecule, given by the ratio of the molecular weight of the polymer, M, to the molecular
weight of the repeat unit, M0,
N= M
M0
1.2.2 Spatial Extent
A fully extended hydrocarbon molecule will have the familiar all-trans zig-zag
profile with an angle of 109.5o between successive carbon-carbon bonds
(tetrahedral geometry).
The chain may be pictured as a row of triangles corresponding to the pairs of carbon
atoms. The base of each triangle, corresponding to the length of a monomer (repeat
unit of two carbon atoms, e.g. vinyl polymers), equals to 0,154 nm x sin54,75 x 2 =
0,252 nm = 2,52 Å
(0,154 nm, being the length of the carbon – carbon single bond)
For a polymer with N = 104, this corresponds to 2.52 μm.
Because of the possibility of rotation around carbon – carbon bonds, a fully
extended molecular length is not representative of the spatial extension that a
molecule actually displays. It usually takes a coiled conformation, and sometimes
a rodlike or a semiflexible chain.
Molecules for which N < 10 are called oligomers.
1.3 Linear and Branched Polymers, Homopolymers, and Copolymers
1.3.1 Branched Structures
While linear polymers are important, they are not the only type of molecules possible:
branched and cross-linked molecules are also common.
When we speak of a branched polymer, we refer to the presence of additional
polymeric chains issuing from the backbone of a linear molecule.
Branching can arise through several routes.
One is to introduce into the polymerization reaction some monomer with the capability
of serving as a branch, e.g. a polyester from trifunctional acids or alcohols.
A second route is through adventitious branching, for example, as a result of an atom
being abstracted from the original linear molecule, … in the free-radical
polymerization of ethylene, for example.
A third route is grafting, whereby pre-formed but still reactive polymer chains can be
added to sites along an existing backbone (grafting to), or where multiple initiation
sites along a chain can be exposed to monomer (grafting from).
The amount of branching is an additional variable for the molecule to be fully
characterized.
If a molecule has ν branches, it has ν + 2 chain ends. Two limiting cases are combs
and stars.
If the concentration of junction points is high enough, a point can be reached at which
the polymer molecule becomes a giant three dimensional network, and it is said to be
cross-linked. However, it is also possible to suppress cross-linking such that the
highly branched molecules remain as discrete entities, known as hyperbranched
polymers. Another important class of highly branched polymers are dendrimers, or
tree-like molecules, made by the successive condensation of branched monomers.
A final class of nonlinear polymers to consider are cycles or rings, where the two ends
of the molecule react to close the loop.
1.3.2 Copolymers
Homopolymer, copolymer, terpolymer, multicomponent copolymer.
Starting from monomers A and B, the following distribution patterns can be obtained:
Random (or statistical), poly(A-stat-B) or poly(A-ran-B), Alternating, poly(A-alt-B),
Block, poly(A-block-B), Graft, polyA-gaft-polyB.
1.4 Addition, Condensation and natural polymers
1.4.1 Addition and Condensation polymers
Addition type polymers:
1. The repeat unit in the polymer and the monomer has the same composition
2. The mechanism of these reactions places them in the category of chain reactions
3. The product molecules often have an all-carbon chain backbone.
In contrast, for condensation polymers:
4. The polymer repeat unit arises from reacting two different functional groups, it
is different from either of the monomers and small molecules are often eliminated
during the condensation reaction.
5. The reactions occur in steps.
6. The product molecules have the functional groups formed by the condensation
reactions: -C-C-Y-C-C-Y-
1.4.2 Natural Polymers
Since they are of biological origin, they are also called biopolymers.
Polysaccharides, proteins and nucleic acids.
Polysaccharides: cellulose and starch, hydrolyzed to D-glucose, the repeat unit in
both polymer chains, while the configuration of the glycoside linkage is different,
being a β-acetal and an α-acetal respectively.
Proteins are polyamides, (-NH-CH-CO-), also called polypeptides, when M ≤ 10.000
R
Since some of the aminoacids carry substituent carboxyl or amino groups, protein
molecules are charged in aqueous solutions.
Ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) are polymers in which the
repeat units are substituted esters of phosphoric acid and D-ribose (RNA) or D-2deoxyribose (DNA).
1.5 Polymer nomenclature
A polymer formed from a single monomer is named by attaching the prefix poly to
the name of the monomer.
The polymers which are the condensation products of two different monomers are
named by attaching the prefix poly to the name of the repeat unit.
1.6 Structural Isomerism
Positional isomerism, stereo isomerism, and geometrical isomerism.
1.6.1 Positional isomerism
Considering the polymerization of a vinyl monomer, head-to-tail or head-toHead orientations are possible.
For most vinyl polymers head-to-tail addition is the dominant mode of addition.
1.6.2 Stereo isomerism (tacticity)
We consider the number of ways a singly substituted vinyl monomer can add to
a growing polymer chain. Three different situations are distinguished:
1. Isotactic. All substituents lie on the same side of the extended chain.
2. Syndiotactic. Substituents lie on alternating sides of the backbone.
3. Atactic. Substituents are distributed at random along the chain.
1.6.3 Geometrical Isomerism
This type of isomerism is illustrated by the various possible structures that
result from the polymerization of 1,3-dienes, e.g. 1,3-butadiene and 1,3isoprene.
1.7 Molecular Weights and Molecular Weight Averages
Polymers show polydispersity with respect to molecular weight or degree of polymerization.
1.7.1 Number-, Weight-, and z-Average Molecular Weights
Suppose we have a polymer sample containing many molecules with a variety of degrees of
Polymerization. If we choose a molecule at random, the probability of obtaining an i-mer (a
molecule with degree of polymerization i) is given by
n
x
(1.7.1)
n
where the probability xi is the number fraction, or mole fraction of i-mer.
i
i
i
i
- ni is the number of i-mers, and Σini is the total number of molecules.
We can use this quantity to define the number average molecular weight, Mn.
Mn = i xi M i 
 nM
n
i
i
i
i
i
 M0
 in
n
i
i
i
i
(1.7.2)
As the total number of monomers in a sample is Σiini , the chance of picking a particular
i-mer will be given by the weight fraction or mass fraction of i-mer in the sample
in
(1.7.3)
w
 in
i
i
i
i
Accordingly , we define the weight-average molecular weight of the sample, Mw, by
(1.7.4)
2
M  w M
w
i
i
i

 in M
 in
i
i
i
i
i
 M0
in
 in
i
i
i
i
Comparison of the last expressions in Eq. 1.7.2 and Eq. 1.7.4 suggests a trend. A new average
is defined by multiplying the summation terms in the numerator and the denumerator, by i.
The z-average molecular weight, Mz, is constructed in this way:
(1.7.5)
i 3n
M M 

z
i
0
i
2
i
i ni
1.7.2 Polydispersity Index
Information about the breadth of the distribution of the molecular weight in a polymer sample
is given by the polydispersity index (PDI) or just polydispersity:
PDI =
M
w
M
n
(1.7.6)
The PDI is always greater than 1, unless it is monodisperse, in which case the PDI=1.
Typical polymerizations (radical and polycondensation) give PDIs close to 2. In industrial
practice side reactions often lead to PDIs as large as 10 or more. The so-called living
polymerizations give PDIs of 1,1 or smaller.
1.8 Measurement of Molecular Weight
1.8.1 General Considerations
-the most important step in characterizing a polymer sample
- a variety of experimental techniques have been developed and employed for this purpose
(Table 1.5)
- we will describe end group analysis in this section, while deferring size exclusion
chromatography (SEC), osmotic pressure, light scattering, and intrinsic viscometry to
subsequent chapters.
1.8.2 End Group Analysis
- the end groups of polymers, inherently different in chemical structure from the
repeat units of the chain, provide a possible means of counting the number of
molecules in a sample.
- this approach, will yield a measure of the number-average molecular weight,
Mn.
- the experiment will involve preparing a known mass of sample, m0, probably in
solution, which corresponds to a certain number of repeat units.
- the number of end groups is directly proportional to the number of polymers
and the ratio of the repeat units to the number of polymers is the number
average degree of polymerization.
- as an example consider condensation polymers, polyesters and polyamides,
naturally having unreacted carboxyl or amine groups at each end, which can be
titrated.
- nuclear magnetic resonance (NMR) is the most commonly used analytical
method for end-group analysis, especially proton (1H) NMR.