Chemistry Reference
In-Depth Information
bonds tend to be flexible and have low glass transitions. Insertion of an aromatic
ring in the main chain causes an increase in
T
g
, and this is of importance in the
application of amorphous glassy polymers like poly(phenylene oxide) (1-14) and
polycarbonate (1-52).
Bulky, inflexible substituents on chain carbons impede rotations about single
bonds in the main chain and raise
T
g
. Thus, the
T
g
of polypropylene and poly
(methyl methacrylate) are respectively higher than those of polyethylene and poly
(methyl acrylate). However, the size of the substituent is not directly related to
T
g
; a flexible side group like an alkyl chain lowers
T
g
because a segment contain-
ing the substituent can move through a smaller unoccupied volume in the solid
than one in which the pendant group has more rigid steric requirements. Larger
substituents prevent efficient packing of macromolecules in the absence of crys-
tallization, but motion of the polymer chain is freed only if the substituent itself
can change its conformation readily. The interplay of these two influences is
shown in
Table 4.2
for the methacrylate polymers.
Stronger intermolecular attractive forces pull the chains together and hinder
relative motions of segments of different macromolecules. Polar polymers and
those in which hydrogen bonding or other specific interactions are important
therefore have high
T
g
. Glass transition temperatures are in this order: polyacrylo-
nitrile
.
poly(vinyl alcohol)
.
poly(vinyl acetate)
.
polypropylene.
Polymers of vinylidene monomers (1,1-disubstituted ethylenes) have lower
T
g
's than the corresponding vinyl polymers. Polyisobutene and polypropylene
comprise such a pair and so do poly(vinylidene chloride) and poly(vinyl chloride).
Symmetrical disubstituted polymers have lower
T
g
's than the monosubstituted
macromolecules because no conformation is an appreciably lower energy form
than any other (cf. the discussion of polyisobutene in Section 1.13).
For a given polymer type,
T
g
increases with number average molecular weight
according to
T
g
2
T
g
5
u
=
M
n
(4-4)
where
T
g
is the glass-to-rubber transition temperature of an infinitely long poly-
mer chain and
u
is a constant that depends on the polymer. Observed
T
g
's level
off within experimental uncertainty at a degree of polymerization
be
tween 500
and 1000, for vinyl polymers. Thus
T
g
is
88
C for polystyrene with
M
n
.
;
10
000
and 100
C for the same polymer with
M
n
.
;
000.
Cross-linking increases the glass transition temperature of a polymer when the
average size of the segments between cross-links is the same or less than the
lengths of the main chain that can start to move at temperatures near
T
g
. The glass
transition temperature changes little with the degree of cross-linking when the
crosslinks are widely spaced, as they are in normal vulcanized rubber. Large
shifts of
T
g
with increased cross-linking are observed, however, in polymers that
are already highly cross-linked, as in the “cure” of epoxy (Section 1.3.3) and phe-
nolic (Fig. 8.1) thermosetting resins.
50
