Robust Phase Behavior of Model Transient Networks

Abstract

In order to study the viscoelastic properties of certain complex fluids which are described in terms of a multiconnected transient network we have developed a convenient model system composed of microemulsion droplets linked by telechelic polymers. The phase behavior of such systems has two characteristic features: a large monophasic region which consists of two sub-regions (a fluid sol phase and a viscoelastic gel phase) separated by a percolation line and a two phase region at low volume fraction with separation into a dilute sol phase and a concentrated gel phase. From the plausible origin of these features we expect them to be very similar in different systems. We describe here the phase behavior of four different systems we prepared in order to vary the time scale of the dynamical response of the transient network; they consist of the combination of two oil(decane) in water microemulsions differing by the stabilizing surfactant monolayer (Cetyl pyridinium chloride/ octanol or TX100/TX35) and of two telechelic polymers which are end-grafted poly (ethylene oxide) chains, differing by the end-grafted hydrophobic aliphatic chains (C 12 H 25 or C 18 H 37 ). We first summarize the characterization of the structure of the four systems by small angle neutron scattering: the size of the microemulsion droplets is found to be constant in a given system upon addition of a telechelic polymer.In the CPCl systems we find a mean radius of the microemulsion droplets = 62± 1Å and a very narrow size distribution and in the TX systems we find a mean radius = 84± 2Å and a somewhat larger size ____________________________________________________________________ 2 06/01 Paper "Robust phase behaviour..." Filali et al distribution. We can then calculate precisely the number of polymers per microemulsion droplet and compare the phase behavior of the four systems in consistent units. As expected we find very similar phase behavior in the four systems.

Entropic networks in colloidal, polymeric and amphiphilic systems

“…Here, we review the theory of the structure and thermodynamic properties of equilibrium networks. This paper unifies concepts that have been previously discussed in several publications, each in different experimental contexts such as gels [1], worm-like micelles [2], microemulsions [2,3], dipolar liquids [4] and colloids [5,6]. In all these systems the networks consist of selfassembling, one-dimensional objects (e.g.…”

“…Due to the hydrophobic interaction, it is energetically preferable for the chain ends to reside inside the hydrophobic core of the microemulsion droplets, and to a first approximation one can neglect the very small number of free hydrophobic chain ends in contact with water and assume that all the polymer ends reside within the droplets. Thus, a given polymer chain can either connect two neighbouring droplets (which we shall term a 'bonding chain'), or loop on a single droplet, so that both of its ends reside inside the same drop [5]. The free energy difference between a looped polymer and a bonding one is largely determined by the difference in the number of spatial configurations available to a looped polymer compared with one that forms a bond between two drops.…”

“…It has been found experimentally that as the ratio of the number of the polymers to the number of droplets is changed, the system undergoes two independent transitions: (i) a continuous fluid to gel transition, attributed to a formation of an infinite network (ii) a first-order thermodynamic phase transition, where the system separates into a dense system of tightly connected droplets, in equilibrium with a phase of dilute droplets, decorated with polymer loops [5].…”

One-step RAFT synthesis of well-defined amphiphilic star polymers and their self-assembly in aqueous solution

“…Depending on the length of the alkyl end groups incorporated, hydrophobic aggregation is expected as it is known for previously studied linear and star like hydrophobically end-capped polymers. 1,3,16,35,[63][64][65][66] SANS experiments were exemplarily done on 1 wt% and 5 wt% solutions of polymer samples with dodecyl stickers (entries 4, 10 and 15, see Table 1) in order to learn about their structure in aqueous solution. The scattering curves (Fig.…”

“…Typical rheology modifiers used are a,u-double hydrophobically end-capped hydrophilic polymers. [1][2][3][4][5][6][7][8][9][10][11] So far, these have been mostly polyethylene oxides (PEOs) end-capped with n-alkyl ethers, n-alkyl urethanes (HEURs), and fatty acid esters or, alternatively, PEOs bearing short hydrophobic polymer end blocks. 8 The thickening effect of these compounds has been studied intensely.…”