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Authors
Advisor(s)
Abstract(s)
Polycarbonate/poly(butylene terephthalate)/impact modifier (PC/PBT/IM) blends are a
commercially important type of polymer blend. In recent years, pigmented PC/PBT/IM blends
have been produced for specific applications, with particular attention being given to the method of
mass coloration. The useful properties of these pigmented blends are determined, to a large extent,
by the phase separation and phase preference phenomena that occur in these multi-component
polymeric systems. In this study, the thermodynamic origins of the phase separation and phase
preferences that exist in pigmented PC/PBT/IM blends have been assessed by means of inverse gas
chromatography (IGC). Subsequently, in order to characterise these blends both physically and
chemically, and to assess the influence of the pigment (C. I. Pigment Blue 28) on the physical
properties, on the mechanical properties, and on the morphology of the blends, several analytical
techniques and mechanical tests were used. These analytical techniques and mechanical tests, along
with the controlled surface modifications of the pigment (which were achieved by means of a
photo-sensitised grafting procedure), allowed for a rationalisation of the interactions that exist
between the components of the pigmented blends as encountered in the phase separation, the phase
preferences, the physical properties and the mechanical properties of these polymeric systems.
The Lewis acid/base interaction between the major components of these blends has been
proven to influence decisively the physical properties and the mechanical properties of the
pigmented PC/PBT/IM blends. Phase separation exists in PC/PBT/IM blends as the PBT molecules
are preferentially involved in specific intermolecular, and intramolecular, interactions with
themselves and other PBT molecules. Partial miscibility between the PC and the PBT has been
interpreted on the basis of the Lewis acid/base attraction between these polymers, with
contributions from the repulsion effect that exists between the Lewis basic centres in PBT. The
impact modifier is shown to interact preferentially with the PC phase rather than with the PBT
phase. This is due to the preference of the PBT molecules to interact with PBT molecules and to
the strong Lewis base/base repulsion between the impact modifier PMMA shell and the PBT
molecules. The fast crystallisation of PBT, favoured by the strong Lewis amphoteric character of
this polymer, also contributes to the expulsion of the IM particles, and of the PC, from the PBT
domains. The pigment interacts favourably with both the PBT and the PC, but preferentially with
the PBT phase. This is because of the Lewis amphoteric properties of C. I. Pigment Blue 28.
The pigment under study, C. I. Pigment Blue 28, influences significantly the physical
properties and the mechanical properties of the PC/PBT/IM blends. These effects differ for blends
that have been processed in equipment and/or under conditions that lead to a lesser or greater
degradation of the molecular weight of PC and of PBT. Also, the importance of the PBT-rich phase
and of the PC-rich phase, in relation to the viscoelastic properties of the PC/PBT/IM blends, depends on the magnitude of the molecular weights of PC and of PBT. The influence of the
pigment on the physical and mechanical properties of the pigmented blends has both direct and
indirect consequences. The direct consequences arise from the physical properties of the pigment
(particle size and particle size distribution, surface area) and from the chemical properties of the
pigment (inorganic nature, surface chemical composition). In particular, the surface chemical
composition and the surface area determine the interaction potential of the pigment with the other
components of the PC/PBT/IM blends. The indirect consequences stem from the influence the
pigment has on the occurrence of transesterification, on the crystalline properties of PBT, and on
the molecular weights of PC and of PBT. The influence of these factors on the physical properties
and on the mechanical properties of the pigmented blends has been rationalised.
C. I. Pigment Blue 28 enhances the impact resistance of the blends by means of altering the
mechanisms of absorption of the impact energy. The pigment decreases the crystallisation
activation energy and increases the rate of crystallisation of PBT. The crystallinity degree is not
directly affected by the presence of the pigment. At low loadings, the pigment enhances the
transesterification reactions that occur between PC and PBT and the thermal degradation of the
molecular weight of PBT. At greater pigment loadings, the pigment particles act as an inhibitor of
the transesterification reactions and of the polymer chains thermal scission. The effect that C. I.
Pigment Blue 28 has on the transesterification reactions is thought to be due mainly to the thermal
conductivity differences between the inorganic pigment particles and the polymers.
Microscopic evaluations have established that the pigment is preferentially located at the
PC/PBT interphase. This finding is in line with the predictions made from the IGC study and is
substantiated by thermal analytical techniques and by mechanical testing of the pigmented blends.
Control of the Lewis surface acid/base properties of C. I. Pigment Blue 28, by means of a photosensitised
surface modification procedure, allows one to modify the preferential location of the
pigment particles in the PC/PBT/IM blend. Accordingly, the physical properties and the
mechanical properties of these pigmented blends are influenced. When the Lewis acidity of the
pigment is significantly enhanced, the interaction of the inorganic particles with the PBT is
improved. This results in a more significant nucleating effect of the pigment, an increased rate of
crystallisation and an increased crystallinity degree of PBT. On subsequent reduction of the surface
Lewis acidity of the modified pigment by neutralisation of the surface carboxylic acid groups, the
interaction of the pigment particles with the PC phase is enhanced. The surface modifications lead
to improved adhesion between C. I. Pigment Blue 28 and the polymeric matrix. This improvement
in the adhesion, along with the changes to the phase preferences of the pigment, result in: 1) lower
viscosity of the pigmented blends, due to improved dispersion of the modified pigments in the
polymeric matrix; 2) very significant reduction of the transesterification reactions and of the
thermal scission of the polymers, namely at the lower pigment loadings; 3) more consistent
viscoelastic behaviour of the blends with varying pigment loading, and 4) less pronounced
dependence of the impact resistance on temperature.
Description
Keywords
Phase separation PC/PBT blends