Thermodynamics and structure of self-assembled networks

We study a generic model of self-assembling chains which can branch and form networks with branching points (junctions) of arbitrary functionality. The physical realizations include physical gels, wormlike micells, dipolar fluids and microemulsions. The model maps the partition function of a solution of branched, self-assembling, mutually avoiding clusters onto that of a Heisenberg magnet in the mathematical limit of zero spin components. The model is solved in the mean field approximation. It is found that despite the absence of any specific interaction between the chains, the entropy of the junctions induces an effective attraction between the monomers, which in the case of three-fold junctions leads to a first order reentrant phase separation between a dilute phase consisting mainly of single chains, and a dense network, or two network phases. Independent of the phase separation, we predict the percolation (connectivity) transition at which an infinite network is formed that partially overlaps with the first-order transition. The percolation transition is a continuous, non thermodynamic transition that describes a change in the topology of the system. Our treatment which predicts both the thermodynamic phase equilibria as well as the spatial correlations in the system allows us to treat both the phase separation and the percolation threshold within the same framework. The density-density correlation correlation has a usual Ornstein-Zernicke form at low monomer densities. At higher densities, a peak emerges in the structure factor, signifying an onset of medium-range order in the system. Implications of the results for different physical systems are discussed.Comment: Submitted to Phys. Rev.

Complexity of Outsourcing Contracts and "Ex Post" Transaction Costs: An Empirical Investigation

In this article, we use Transaction Cost Economics (TCE) and the Resource-Based View (RBV) of the firm to study outsourcing agreements. We develop an original approach of contract complexity and analyse the links among exchange hazards (i.e. specificity and environmental uncertainty), the contractual aspects of outsourcing (control, incentives, penalties, price and flexibility clauses) and the level of "ex post" transaction costs. Both contract complexity and "ex post" transaction costs are operationalized and measured. Our empirical research analyses 82 outsourcing contracts. This article uses three different dimensions (proximity to the core business, switching costs and adaptation costs) to assess the strategic importance of an outsourced activity. Our findings extend TCE's validity for the outsourcing of activities with a strategic value. Finally, this study offers an indirect measurement of "ex post" transaction costs. In short, to restrict vendor opportunism, contracts must contain incentives and penalties, as well as pricing and monitoring clauses. Copyright Blackwell Publishing Ltd 2006.