Modification of a microbial co-polyester by graft copolymerisation
The principal objective of this project was to introduce specific properties in to a polymer
required by the sponsor company, Bonded Fibre Fabric, namely appropriate thermal
characteristics coupled with biodegradability. Microbial polyesters are biodegradable, but
no commercial grade was available that met the required thermal criteria detailed below.
Therefore it was decided to attempt to lower the thermal softening temperature by grafting
chains of poly(methyl acrylate), (which softened at the required temperature) to the
microbial polyester backbone selected for this work was poly(3-hydroxybutyrate-co-3-
PHV is very stable chemically and of the three methods attempted, that employing gamma
irradiation to produce polymer peroxides gave the highest grafting add-on.
Fundamental work was carried out on the interaction of gamma rays with the polymer
substrate. These interactions were monitored via esr spectroscopy. A G(radical) value for
the polymer was determined as 1.7 (heV)'1 by comparison to an irradiated standard,
Initial reaction conditions for mutual irradiation graft copolymerisation were established by
swelling followed by subsequent extraction of monomer from the film and quantitative
The inevitable production of homopolymer during irradiation was suppressed by the
inclusion of cupric chloride. Graft copolymerisationw as effected via mutual irradiation and
monitored by gravimetric and nmr analysis. A maximum grafting add-on of 17wt% was
A second graft copolymerisation technique involving irradiation, namely preirradiation
peroxide grafting was also attempted. The formation and stability of the radicals produced
on irradiation of the polymer in vacuo were confirmed by esr, as was radical decay on
exposure to air. Peroxide groups were also qualitatively determined chemically. Both
techniques gave a strong indication that peroxide groups were formed on irradiation in air.
In grafting experiments the peroxidised PHV was found to produce grafting add-on, which
was again confirmed by gravimetry and nmr. The extent of grafting was much greater via
this technique when compared to mutual irradiation. Grafting add-on figures in excess of
200wt% were obtained with minimal production of homopolymer.
The level of grafting was shown to depend on the monomer concentration in the medium,
the preirradiation dose, reaction time and temperature.
Thermal analysis showed that grafting occurred in both the crystalline and amorphous
regions for higher grafting add-on, but only in the amorphous region for lower add-on.
It was also demonstratedth at this technique could be applied to other acrylic monomers,
namely n-butyl acrylate, ethyl acrylate and also to methyl methacrylate. Grafting was once
again confirmed by gravimetric and nmr analysis.
Biodegradation results were disappointing on grafted materials when compared to those on
an ungrafted sample. It appeared that the grafted chains were interfering with biological
activity. It was thought that the hydrophobicity of the grafted chains inhibited the take up
of water essential for microbial growth.
In addition to graft copolymerisation employing gamma irradiation techniques, the route
exploiting chain transfer reactions was also explored. However, although graft copolymers
were obtained, they were accompanied by the innevitable production of homopolymer.
Finally, a short section is included that details overall conclusions and suggestions for