xxiii πανελληνιο ÏƒÏ Î½ÎµÎ´ÏÎ¹Î¿ Ï†Ï ÏƒÎ¹ÎºÎ·Ï‚ στερεας καταστασης & επιστημης ...

Viscoelastic Response of Micelles with Chemically Cross-linked Cores
Pamvouxoglou A. 1,2* , Van Ruymbeke E. 2 , Petekidis G. 1,2 , Vlassopoulos D. 1,2 , Mountrichas G. 3 and Pispas S. 3
1 University of Crete, Department of Materials Science & Technology, Heraklion, Crete, Hellas
2 Foundation for Research & Technology, Institute of Electronic Structure & Laser, Heraklion, Crete, Hellas
3 Theoritical & Physical Chemistry Institute, National Hellenic Research Foundation, Athens, Hellas
*pamvou@iesl.forth.gr
The past decades, a lot of progress has been made in the understanding of the viscoelastic behaviour of concentrated polymer
solutions and colloidal suspensions. Besides these two classes of materials, there exist intermediate situations which combine
the existence of topological constraints and long-range interactions, leading to a different behaviour. Block copolymer
micelles are a popular representative. Understanding the consequences of the interplay between polymer-like and colloid-like
behaviour on the dynamic properties is of the great importance for the design of materials with desired properties. In the last
few years there has been a lot of effort in characterizing suspensions of star-like micelles with a responsive fixed core which
is chemically cross-linked. The reason is that solvent quality or temperature plays an important role in the dynamics and
rheology of such soft colloids. By changing the quality of the solvent we can have in situ variation of the core size through an
appropriate choice of the solvent and/or temperature [1]. Responsive cores alter the static and dynamic properties of the
colloidal suspensions in a controlled way. In this work we investigated micelles with a fixed core (via crosslinking) which
exhibits the above mentioned responsiveness while the system remains stable.
A highly asymmetric PS – PI block copolymer micelle consisting of polyisoprene (9% PI) cross-linked core and
polystyrene (91% PS) hairs (shell) was used as model responsive soft colloids, where the core swelling can be tuned by
changing solvent quality [1]. Dynamic and static light scattering measurements were used to study the dynamics of the
micelles and the size. The study was carried out in dioctyl – phthalate (DOP) which is theta for polystyrene at 22 o C [2] and
for polyisoprene at 25 o C [3]. The effect of DOP solvent in our system was to swell the micelle up to 170 nm at 80 o C when at
20 o C is almost 95 nm.
Our primary goal was to investigate the concentration and temperature dependence of the linear viscoelastic
properties. Specifically we investigated the dependence of the relative on the effective volume fraction in good solvent and
compared it with linear and star polymers. Below 10% wt our system exhibited a liquid-like (Newtonian) behavior and as the
concentration increased (20% wt) it became viscoelastic (see figure 1). Evidence of core deswelling is found in the nonmonotonic
dependence of the relative viscosity with the effective volume fraction. The response of these micelles with long
hairy corona was dominated by their polymeric nature (see figure 2). The dependence of the relative viscosity on φ eff was
different depending on whether it was tuned by increasing the temperature or by increasing the mass concentration,
suggesting that the micelles have different effective interactions in the two cases.
G' ( ) , G" ( ) (Pa)
10 3
10 2
10 1
10 0 Concentration
20 % wt
2.65 % wt
10 -1
η 0
/η s
T 3
PMMA in Decalin (640 nm)
T
10 5 Star 32/80
2
Star 64/07
Star 128/07
10 4 Star 128/80
linear polymer
Eintein's equation
10 3 1 % wt
2,65 % wt
5.08 % wt
10 2 10.2 % wt
20 % wt
10 1
T 1
10 -2
10 -2 10 -1 10 0 10 1 10 2
ϖ (rad/s)
10 0
10 -1 10 0 10 1
φ eff
=c/c*
Figure 1: Dynamic frequency sweep tests at 2 different
concentrations (2.65% and 20% wt) at 20 o C for PS-PI.
All strains are in the linear regime.
Figure 2: Relative viscosity with the effective volume
fraction for spheres, stars, linear polymers and star-like
micelles. Our experimental data from PS-PI are with red
color. T 1 , T 2 , and T 3 are the temperatures 20 o , 35 o and
50 o C respectively.
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