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Kinetics of phase transformations in polymer blend by means of modulated temperature DSC

Friday, 14 October, 2005 - 17:00
Campus: Brussels Humanities, Sciences & Engineering campus
E
0.12
Ronny Pieters
phd defence

The main objective of this PhD work was to monitor the influence of the process
parameters (temperature and time) and the material parameters (blend composition,
chemistry, and molar mass of the constituting components) on the kinetics of the
different, often overlapping phase transformations in polymer blends, and to explore
the application range and the assets of Modulated Temperature DSC in this domain.
This information on the kinetics of the different phase transformations in polymer
blends is crucial towards the design of applications. The focus was on the MTDSC
heat capacity signal in both non-isothermal and (quasi)-isothermal conditions. The
blend system containing oligomers (PEG and PPG) was used as a model system to
test the applicability of the new MTDSC methodology, especially for studying
temperature-induced phase separation and the reverse transformation, i.e.
temperature-induced phase dissolution, before extending it to ‘real’ polymer blends.
First of all, the applicability of the MTDSC methodology for the detection of the
phase separation temperature was verified, followed by the construction of the
complete state diagram. The knowledge of the exact location of the state diagram is
crucial to define the appropriate conditions for carrying out demixing and remixing
experiments. A special focus was on the Tg-behaviour of this oligomer blend at very
low temperatures, bearing in mind the UCST-type behaviour of this blend, in contrast
with the polymer blends that show LCST-type phase behaviour. The main advantage
of this liquid-like blend is that it demixes macroscopically into two separate layers
with a minimal contact surface between both co-existing phases. This feature proved
to be very useful to interpret the MTDSC heat capacity profiles, as they can be
confronted with this morphology development towards a macroscopically demixed
state. In a next step, these findings were extended to real polymer blends by
constructing the state diagram for the different model blend systems using the
MTDSC approach explored for the oligomer blends. The polymer blend model
systems often consist of a low-Tg component and a very high-Tg component. This
combination often resulted in phase separation-induced partial vitrification of the
high-Tg co-existing phase, which can be detected by the MTDSC heat capacity signal
and divides the heterogeneous demixing region in sub-domains. Moreover, the
interference of vitrification had drastic implications on both the demixing and the
remixing kinetics, in addition to the effects of more traditional parameters as time,
temperature, and molar mass (viscosity). The main difference with the oligomer
blends is that macroscopic phase separation cannot be achieved in polymer blends,
due to intrinsically high viscosities when dealing with polymer blends, resulting in an
‘arrested’ equilibrium, which affects the observed heat capacity profiles during both
demixing and remixing. Moreover, as MTDSC is a dynamic technique, the magnitude
of both amplitude and period (frequency) can be altered and its influence on the heat
capacity effects during demixing and remixing provides additional proof of the
proposed underlying physical processes causing the heat capacity evolutions during
demixing and remixing. Moreover, an analysis of the raw modulated signals, both
under non-isothermal and (quasi)-isothermal conditions, has proven to be very
beneficial in this respect. The applicability of NMR relaxometry for the construction
of the state diagram was explored, while in situ monitoring of the relaxation times,
which are related to the compositions of the co-existing phases, provided additional
information during both demixing and remixing. Finally, for blends containing a
crystallisable component, the crystallisation kinetics of this component from the
amorphous melt, both in homogeneous and heterogeneous conditions, were studied,
with a special focus on crystallisation-induced vitrification, a feature that is not
always taken into account when studying crystallisation kinetics in polymer blends
containing a crystallisable component and an amorphous component with a higher Tg.