Chemistry Reference
In-Depth Information
Owing to a hairpin conformation both end groups were directly neighboring each
other on the surface of the crystalline lamellae. When heated to the molten state,
equilibration reduced their concentration, and the cyclic tetramer formed the top of
the frequency distribution.
Closely related to SSP is the so-called chemical healing. This term includes all
chemical reactions which go on at the interphase when two pieces of a polymer are
pressed together. Both, condensation steps and trans reactions are usually involved
[
36
]. When pieces of identical polymers or miscible polymers brought together
migration of polymer chains from one particle to another one may also take place,
a process called physical healing. However, chemical healing may also occur
between two immiscible polymer such as a polyamide and a polyester and then
may called heterochemical healing [
37
]. A broader discussion of healing processes
is presented in Ref. [
38
].
In the case of high melting polyesters such as PET and PBT SS-postconden-
sation is again a standard method to enhance the molecular weights [
24
,
25
]. Most
studies of polyesters concern SSP of PET [
24
,
25
,
39
-
48
]. Most PET samples used
for these studies had a relatively high molecular weight, meaning an inherent
viscosity in the range of 0.5-0.7 dL/g. This is the maximum usually achieved in
melt polycondensations of dimethyl terephthalate and ethanediol [
45
]. Such
inherent viscosities are sufficient for many applications, but for bottle-blowing
viscosities [1.0 are desirable, and such high viscosities can easily be achieved by
SSP as demonstrated in the early work of Ravens and Ward [
39
] or by Karayanidis
et al. [
45
].
A comprehensive and detailed study of kinetic and property parameters was
published by Jabarin and Loffgren in 1986 [
43
]. They developed equations
describing relationships of time, temperature, and final molecular weight for
specific catalyst and precursor systems. Moisture concentration, oxygen exposure,
and nitrogen flow were also taken into account. Measurements of inherent vis-
cosities carboxyl end group concentration and residual acetaldehyde and Tm were
conducted. In another paper [
46
] the influence of Sb
2
O
3
, a particularly useful
catalyst for the production of PET, was investigated. Furthermore, the influence of
microwave irradiation was studied [
47
]. Measurements of the diffusion of small
molecules in the amorphous phase were performed by Chang as early as 1960 [
40
].
A mathematical modeling of all relevant processes (but ignoring the role of cyc-
lics) was later reported by Kwon [
48
].
Further studies of the SSP of PET were performed with oligoesters as pre-
cursors [
41
,
42
,
49
,
50
]. Miyagi and Wundrlich [
41
] obtained linear oligomers free
of cyclics by etching the chain folds from the surface of lamellar crystals. Droe-
scher and Wegner [
42
] prepared their oligoesters by polycondensation of bis(2-
hydroxyethyl)terephthalate, BEHT, with terephthaloyl chloride in solution, a
method certainly involving cyclization. Nonetheless, both research groups agreed
in that the SSP of oligoesters may enhance the molecular weights of oligoesters by
a factor of 10. Changes in the crystal lattice during SSP of PET or PEN oligoesters
were analyzed by Sivaram and coworkers [
49
].
