Predicting and Measuring the Sequence Distribution of Condensation Polymers - Mathematical Approaches to Polymer Sequence Analysis and Related Problems - page 234

T m , which is often above 200 ı C. In some binary polymer melts
(e.g., polyester-polyester, polyester-polyamide, polyester-polycarbonate, and
polyamide-polyamide) exchange reactions occur. Depending on the chemical
groups found along the macromolecular chain, various types of exchange reactions
in polymers occur, namely ester-ester exchange [ 23 ], ester-carbonate exchange [ 24 ],
carbonate-carbonate exchange [ 25 ], amide-ester exchange [ 26 ], amide-carbonate
exchange [ 27 ], and amide-amide exchange [ 28 ]. When the blend is heated, the two
homopolymers can react together to form a copolymer. The above process can be
described as follows:

temperature,

AAAAAAAA C BBBBBBBB ! AAABBBBB C AAAAABBB (11.12)

The copolymer has initially long AAAAAA and BBBBB blocks but, as the reaction
goes on, the length of the blocks decreases and it becomes random. This second
process can be described as follows

AAAAABBBBB ! AAABBBBBAA (11.13)

Most condensation polymers blends are incompatible at room temperature. In fact,
a phase-separation quickly occurs in the blend, with A-rich and B-rich phases. The
dimension of these two phases can be characterized by a “phase average diameter”
(PHAD). Above the melting temperature, the segmental mobility is high and the
blend is compatible. However, once the blend is cooled, two phases are generated
and PHAD increases slowly with time, which implies that the blend is unstable.
Phase-separation must be avoided, otherwise the blend's characteristics are utterly
depleted. In order to overcome (at least in part) this drawback, one can add a compat-
ibilizer. The latter are usually interfacially active copolymers (for instance, block or
graft copolymers) which possess at least two different types of segments, and each
type is capable of specific interactions with one of the blend's components. The ac-
tion of a compatibilizer can be depicted as follows. There are two phases (I and II)
separated by a phase boundary. The compatibilizer joins the two phases, since it lies
in partly in phase I and partly in phase II. Thanks to the compatibilizer, the overall
energy of the blend is reduced and this has benefits on the mechanical properties.
In some cases, one uses macromolecular compatibilizer that are totally different
from the blend's component. In the case of condensation copolymers, however, this
strategy has a most evident shortcoming. In fact, condensation copolymers are of-
ten used as engineering thermoplastics and high compatibilizer concentrations must
be avoided, otherwise the presence of additional repeat units may cause unwanted
effects. Vice versa, it is quite apparent that the best compatibilizer for a blend of
A C B is an AAAAABBBB block copolymer. In fact, the system will evolve in such
a way to minimize its overall energy, and it is intuitive that AAAA blocks will lie
in phase I and BBBB blocks will lie in phase II. Furthermore, the best compatibi-
lizers are those that have a relatively high molar mass so that the average length of
AAAA blocks is high enough to form entaglements in the phase I. It is apparent
that the exchange reaction depicted in ( 11.13 ) yields a block copolymer which is