Structure and Dynamics
of Networks in Mixtures of
Hydrophobically Modified Telechelic Multiarm Polymers and Oil in Water
Microemulsions
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Abstract
The structural and dynamical properties of oil-in-water
(O/W) microemulsions
(MEs) modified with telechelic polymers of different functionality
(e.g., number of hydrophobically modified arms, <i>f</i>) were studied by means of dynamic light scattering (DLS), small-angle
neutron scattering (SANS), and high frequency rheology measurements
as a function of the
polymer architecture and the amount of added polymer. For this purpose,
we employed tailor-made hydrophobically end-capped poly(<i>N,N</i>-dimethylacrylamide) star polymers of a variable number of endcaps, <i>f</i>, of different alkyl chain lengths, synthesized by the
reversible addition–fragmentation chain transfer method. The
addition of the different end-capped polymers to an uncharged
ME of O/W droplets leads to a large enhancement of the viscosity of
the systems. SANS experiments show that the O/W ME droplets are not
changed upon the addition of the polymer, and its presence only changes
the interdroplet interactions. The viscosity increases largely upon
addition of a polymer, and this enhancement depends pronouncedly on
the alkyl length of the hydrophobic sticker as it controls the residence
time in a ME droplet. Similarly, the high frequency modulus <i>G</i><sub>0</sub> depends on the amount of added polymer but
not on the sticker length. <i>G</i><sub>0</sub> was found
to be directly proportional to <i>f</i> – 1. The
onset of network formation is shifted to a lower number of stickers
per ME droplet with increasing <i>f</i>, and the network
formation becomes more effective. Thus, the dynamics of network formation
are controlled by the polymer architecture. The effect on the dynamics
seen by DLS is even more pronounced. Upon increasing the polymer concentration,
slower relaxation modes appear that become especially pronounced with
increasing number of arms. The relaxation dynamics are correlated
to the rheological relaxation, and both are controlled by the polymer
architecture