Page 92 - Carrahers_Polymer_Chemistry,_Eighth_Edition

3 Molecular Weight of Polymers

3.1 INTRODUCTION
It is the size of macromolecules that give them their unique and useful properties. Size allows poly-
mers to act more as a group so that when one polymer chain moves surrounding chains are affected
by that movement. Size also allows polymers to be nonvolatile since the secondary attractive forces
are cumulative (e.g., the London dispersion forces are about 8 kJ/mole per repeat unit), and, because
of the shear size, the energy necessary to volatilize them is greater than the energy to degrade the
polymer.
Generally, the larger the polymer, the higher is the molecular weight. The average molecular
weight (M) of a polymer is the product of the average number of repeat units or mers expressed as
the degree of polymerization, DP, times the molecular weight for the repeating unit. Thus, for poly-
ethylene, PE, with an average DP of 100 the average molecular weight is simply 100 units times
28 daltons (Da)/unit = 2,800 Da. Note that amu and Da are often used interchangeably as units.
Polymerization reactions, which produce both synthetic and natural (but not for all natural
materials, such as proteins and nucleic acids) polymers, lead to products with heterogeneous
molecular weights, that is, polymer chains with different numbers of mers. Molecular weight dis-
tribution (MWD) may be rather broad (Figure 3.1), or relatively narrow, or may be mono-, bi-,
tri-, or polymodal. A bimodal curve is often characteristic of a polymerization occurring under
two different environments. Polymers consisting of chains of differing lengths are called polydis-

perse while polymers containing only one chain length, such as specific nucleic acids, are called
monodisperse.
Some properties, such as heat capacity, refractive index, and density, are not particularly sensi-
tive to molecular weight but many important properties are related to chain length. Figure 3.2 lists
three of these. The melt viscosity is typically proportional to the 3.4 power of the average chain
3.4
length; so η is proportional to M . Thus, the melt viscosity increases rapidly as the chain length
increases, and more energy is required for the processing and fabrication of large molecules. This is
due to chain entanglements that occur at higher chain lengths. However, there is a tradeoff between
molecular weight–related properties and chain size such that there is a range where acceptable
physical properties are present but the energy required to cause the polymers to flow is acceptable.

This range is called the commercial polymer range. Many physical properties, such as tensile and
impact strength (Figure 3.2), tend to level off at some point and increased chain lengths give little
increase in that physical property. Most commercial polymer ranges include the beginning of this
leveling off threshold.
While a value above the threshold molecular weight value (TMWV; lowest molecular weight
where the desired property value is achieved) is essential for most practical applications, the addi-
tional cost of energy required for processing extremely high polymer molecular weights is seldom

justified. Accordingly, it is customary to establish a commercial polymer range above the TMWV
but below the extremely high molecular weight range. However, it should be noted that some prop-
erties, such as toughness, increases with chain length. Thus, extremely high molecular weight poly-
mers, such as ultrahigh molecular weight polyethylene (UHMPE), are used for the production of
tough articles such as waste barrels.
Oligomers and other low molecular weight polymers are not useful for applications where high
strength is required. The word oligomer is derived from the Greek work oligos, meaning “a few.”

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