Reptation in langmuir polymer monolayers Armando Maestro, ab Hani M. Hilles, a Francisco Ortega, a Ram on G. Rubio, a Dominique Langevin b and Francisco Monroy * ab Received 13th April 2010, Accepted 1st June 2010 DOI: 10.1039/c0sm00250j We report the existence of reptation motion typical of entangled polymer chains in dense polymer monolayers spread at the air–water interface. Ellipsometry determinations of the layer thickness reveal a thickening of the layers in the semi-dilute region, enabling the existence of entanglements. Relaxation measurements were performed using compression and shear surface deformations. The resulting dependence on the chain concentration and size are compatible with the existence of diffusion reptation motions controlling molecular transport inside the monolayer. As in the bulk, these features are observed above a critical number of monomers, N e , for which chain entanglements become possible. Introduction The great success of scaling concepts is grounded in their simplicity and they have had an enormous influence on the development of polymer physics. 1–3 They describe the chain conformation and dynamics of polymers in solution in terms of two parameters, the number of monomers in the chain, N, and the concentration, c. Scaling concepts can be extended to describe polymers near surfaces, and both conformation and dynamics of polymer chains are predicted to differ significantly from the behaviour in the bulk. Based on the argument that adsorption energy outweighs the entropy loss from a bulk conformation, de Gennes theoretically postulated that a fully adsorbed chain might adopt a pancake conformation with most monomers adsorbed at the interface. 1 In this case, the chains take conformations of SAWs (self-avoiding walks) for which chain intersections are forbidden. 1,4 Such strong adsorption conditions do not prevail when the adsorbed chains are in contact with a good solvent. In this case, de Gennes showed that adsorbed chains adopt a self similar conformation with a density decaying algebraically away from the interface. 5 These self-similar profiles were experimentally evidenced later. 6,7 De Gennes also predicted that reptation-like motions could occur in these adsorbed layers, and derived a scaling law for the relaxation time, s z N 3 logN, that differs from the law established in the bulk (s bulk z N 3 ) by a logarithmic correction. 8 Experimental evidence for this prediction is still lacking. One major difficulty comes from the fact that non-equilibrium effects are dominant whenever monomer–surface sticking energies are somewhat larger than k B T, a common case for polymer layers supported on solid surfaces. Polymer relaxation kinetics within the layer are then severely retarded, leading to non-equilibrium layers, whose structure and dynamics depend on their adsorption kinetics and layer ageing. 9 Fluorescence correlation spectroscopy was used extensively by Granick and collaborators to study the diffusion of synthetic polymer chains weakly adsorbed at solid surfaces avoiding aggregation or crystallization in the adsorbed state. 10 Despite these precautions, no well-defined power laws have yet been observed. Fluid interfaces offer an interesting option for probing polymer dynamics in the weak adsorption regime where slowing-down effects typical of solid surfaces are avoided, since the substrate does not impose topological constraints to chain motion. They appear ideal to probe ensemble-averaged equilibria and dynamical scaling through changes in surface free energy, easily accessible from the surface pressure observable. 11 In this paper, we present an extensive body of experimental evidence on the structure and mechanics of monolayers of flex- ible polymer spread at the air–water interface (Langmuir layers). A crossover from Rouse- to reptation-like behaviour was observed for highly monodisperse polymers when increasing chain size above a critical entanglement value N e . Our results show that long enough flexible chains can entangle in the monolayer, evidencing reptation-like motion. Equilibrium behaviour We have studied poly(ter-butyl-acrylate) (PTBA) of different molecular weights spread onto the air/water interface at increasing surface concentration, G. In the dilute (G < G*) and semi-dilute (G*<G < G**) regimes, the surface layer thickness is well below the typical sizes of the polymer coil, hence these layers can be considered as ‘‘monolayers’’ (for vinyl polymers with a bulk density, r z 1 g cm 3 , if G # 10 mg m 2 , the layer thickness h (z G/r) # 10 nm). Twelve different PTBA samples were chosen, spanning a quite broad range of molecular weights (10 3 –10 6 Da) with a low polydispersity (M w /M n z 1.1) (see caption in Fig. 1 for details). Fig. 1a shows the three usual equilibrium concentration regimes, as identified from the surface pressure curves 11 (see caption for details). The shift observed towards lower concentrations at increasing polymerisation degree clearly manifests the influence of chain size and concen- tration on monolayer energetics (see Fig. 1a–c). This makes the system particularly suitable for mechanical studies, since these variables can be easily modulated. At the overlapping state (*), the system crosses over from the isolated-chain dilute regime to a dense regime structurally dominated by the collective behav- iour of a strongly packed arrangement of polymer chains. The overlapping concentration is of fundamental importance, as it a Departamento de Qu ımica F ısica I, Universidad Complutense de Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spain b Laboratoire de Physique des Solides, B^ atiment 510, Universit e Paris-Sud 11, 91450 Orsay, France. E-mail: monroy@quim.ucm.es This journal is ª The Royal Society of Chemistry 2010 Soft Matter , 2010, 6, 4407–4412 | 4407 PAPER www.rsc.org/softmatter | Soft Matter Downloaded by UNIVERSITE PARIS SUD on 24 January 2011 Published on 04 August 2010 on http://pubs.rsc.org | doi:10.1039/C0SM00250J View Online