Page 71 - Carrahers_Polymer_Chemistry,_Eighth_Edition

34 Carraher’s Polymer Chemistry

TABLE 2.3
Approximate Glass Transition Temperatures (T ) for Selected Polymers
g
T g (K)
Polymer T g (K) Polymer
Cellulose acetate butyrate 323 Cellulose triacetate 430
Polyethylene (LDPE) 148 Polytetrafl uoroethylene 160, 400 *
a-Polypropylene 253 Poly(ethyl acrylate) 249
i-Polypropylene 373 Poly(methyl acrylate) 279
Polyacrylonitrile 378 a-Poly(butyl methacrylate) 339
Poly(vinyl acetate) 301 a-Poly(methyl acrylate) 378
Poly(vinyl alcohol) 358 Poly(vinyl chloride) 354
cis-Poly-1,3-butadiene 165 Nylon-66 330
trans-Poly-1,3-butadiene 255 Poly(ethylene adipate) 223
Polydimethylsiloxane 150 Poly(ethylene terephthalate) 342
PS 373
* Two major transitions observed.

These groups act as stiffening units because the groups themselves are inflexible as in the case
of 1,4-phenylene or because they form relatively strong bonding, such as hydrogen bonding between
chains as is the case of the amide linkage.
Small molecules such as water can exist in three phases: solid, liquid, and gas. Polymers do not
boil so this phase is missing for them but they do melt. But polymers undergo other transitions besides
melting. The most important of these is called the glass transition, T , which will be discussed below.
g
Before we turn to the glass transition it is important to note that polymers may undergo many other
transitions. About 20 transitions have been reported for polyethylene. PS undergoes several transitions
that have been identified. At about −230 C, the movement, often described as wagging or oscillation,
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of the phenyl groups begins. At about −140 C, movement of four-carbon groups in the PS backbone
begins. At about 50 C, torsional vibration of the phenyl groups begins. At about 100 C, long-range
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chain movement begins corresponding to the reported T value for PS (Table 2.3). It is important to
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remember that while small molecules have a precise temperature associated with its transitions, such
as 0 C for melting for water, polymer values, while often reported as a specific value, are a temperature

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range. This temperature range is the result of at least two features. First, there is a variety of polymer
chain environments at the molecular level, each with its own energy-associated features. Second, tran-
sitions that require large segment or whole chain movement will also have a kinetic factor associated
with them because it takes time for chains to untangle/tangle and rearrange themselves. Thus heating/
cooling rate affect the temperatures required to effect the changes.
As noted before, small molecules can generally exist in three phases—liquid, solid, and gas, but
polymers degrade before boiling, so they do not exist in the gas scale. Even so, polymers generally
undergo several major thermal transitions. At low temperatures polymers are brittle, glassy since

there is not sufficient energy present to encourage even local or segmental chain movement. As the

temperature is increased, at some temperature there is sufficient energy available to allow some
chain mobility. For a polymer containing both amorphous and crystalline portions or is only amor-
phous, the onset of this segmental chain mobility for the amorphous segments is called the glass
transition temperature, T . Because there is unoccupied volume in the amorphous polymer structure
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some segmental chain movement occurs. This segmental chain movement is sometimes likened to a
snake slithering “in place” within the grass. The localized chain movement causes a further increase
in unoccupied volume, and larger segments are able to move eventually, allowing the snake further
movement in the grass. As the temperature is increased, there is sufficient temperature to overcome

the forces present in the crystalline portion of the polymer, allowing a breaking up of the crystalline

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