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Investigation of diffusion mechanisms in polyamide/solvent systems, Florentina Preda LPMA, Lyon

par Salmon Jean-Baptiste - publié le

Polyamides are a family of semi-crystalline thermoplastic polymers widely employed in the
automotive industry due to its excellent thermal stability and mechanical properties. However,
polyamide can be significantly affected by the absorption of low molecular weight penetrants
like water from the air humidity or ethanol in biofuels. The rate of sorption and amount of
absorbed solvent depend on the mechanisms of interaction between solvent and polyamide,
along with sorption and diffusion mechanisms.

Diffusion and sorption of solvents in polymers can be very complex because of the existence
of specific interactions (non-polar or polar), dynamic heterogeneities in the amorphous phase,
modification of the polymer dynamics induced by the solvents and different crystalline phases.
In polyamide/solvent systems, all of these factors have to be taken into account. To begin with,
solvents can interact strongly with amide groups and lead to the decrease of the glass transition
temperature, or plasticization of the amorphous phase, i.e. an increase in the mobility of the
polymer chains.
Secondly, although the crystalline phase is considered impermeable to the
solvent, its existence induces a gradient of mobility in the amorphous phase, which has often
been depicted as a Rigid and Mobile Amorphous Fraction.

To begin with, this seminar will focus on the comparison between water and ethanol diffusion in two polyamides with slightly different ratios of amide/methylene groups (PA6,6
and PA6,10). Although water and ethanol both decrease the glass transition temperature of
polyamide, ethanol diffusion follows a strongly non-Fickian mechanism, whereas water dif-
fusion can follow either a Fickian (PA6,10) or non-Fickian (PA6,6) mechanism. Based on the
variation of diffusion coefficients with solvent concentration, we have proposed an explanation
for this phenomenon.

A second part of this study focused on the relationship between water diffusion and the
dynamics of the amorphous phase in polyamide. The dielectric characteristic relaxation times
of the α relaxation, associated to the glass transition, and of the β relaxation, related to more local
dynamics, have been measured by Broadband Dielectric Spectroscopy. A simple comparison
with the timescale of diffusion suggests that diffusion and polyamide α relaxation should not
be directly correlated. However, diffusion is correlated to the secondary β relaxation, which
encompasses the local chain dynamics of hydrogen bonded amide groups in the presence of
water. A mechanism of diffusion based on the trapping of water molecules between neighboring sorption sites (amide groups) is proposed in these strongly interacting polymers. It is
suggested that diffusion is limited by the relaxation time of hydrogen bonds between water molecules and amide groups and the change in conformation of these amide groups present in
polyamides