Chemistry Reference
In-Depth Information
poly(4-oxybenzoyl) blocks in the melt-transesterification, the control of the
segment distribution of the copolyesters is di
cult. This is especially so
when a high content of HBA is required. For example, Economy (1989)
and coworkers found that PET/60-HBA samples contained a chloroform
soluble fraction and an insoluble fraction. Yet the ratio and composition
of the two fractions were different for different batches of the samples. For
one batch the soluble portion was 17% with an average composition of
56/44 PET/HBA which was rich in PET. For another batch the soluble
portion was 28% which was made of 50/50 PET/HBA. Thus the apparent
composition was only an average of at least two fractions of different copoly-
mers. Meesiri
et al
. found there were two glass transitions of PET/HBA
copolyesters. One was at
80
◦
C for the PET-rich segments. It was essen-
tially independent of the apparent composition up to 63 mol-% HBA. The
other was at
∼
167
◦
C for polymers of 30-100 mol-% HBA. This high glass
transition was attributed to a separate phase. In contrast to the constant
glass transition temperature, the melting temperature of the copolyesters
changed with HBA content. The PET/60-HBA had the lowest melting point
of
∼
200
◦
C, while the melting point of the homopolymer of HBA was found
to be 345
◦
C. However, according to Economy
et al
. (1988), the transi-
tion at 340
◦
C was the transition from crystals to plastic-crystals which
transformed to a nematic phase at about 445
◦
C.
One significant result of the nonuniform distribution of PET and HBA
segments in the copolyester chains is the low HDT as shown in Table 5.10.
The HDT for the copolymer containing 60 mol-% HBA (PET/60-HBA) was
only 64
◦
C which was about the same as that of PET. Obviously this low
value was attributed to the presence of PET-rich phase. Another result
is the presence of highly melting particles which may come out unmelted
during process leading to structural defects and lower properties.
The high tendency to homopolymerization of the 4-acetoxybenzoic acid
relative to transesterification is responsible for the nonuniform distribu-
tion. In order to obtain copolymers with HBA units randomly distributed
along the chains, the monomer 4-acetoxybenzoic acid may be added in por-
tions rather than in a singla batch. Unitika (Suenaga and Okada, 1989)
advanced the production process so that the HBA and PET moieties
are uniformly distributed in the copolymers. With the advanced process
the resins (“Rodrun LC-5000”) showed almost 100% solubility in a hot
(150
◦
C-160
◦
C) 50/50 mixed solvent of tetrachloroethane and phenol, while
that produced by the old (Jackson and Kuhfuss, 1973) method has an insol-
uble residue of about 30 wt-%. The
13
C-NMR spectra also showed that
∼
