6ª. FEIRA 12/12/2014
Auditório Novo 1, às 11h.
Gels based on Molecular Self-assembly
Combined Studies of Structure and Rheology
Kell Mortensen
Niels Bohr Institute, University of Copenhagen, Denmark. kell@nbi.dk
A variety of gels are based on the self-assembly potential of copolymers of which only parts of the polymers are hydrophilic. Poly(ethylene oxide) based copolymers, PEO, constitute such class of materials when covalently coupled to hydrophobic polymer blocks [1]. An example is PEO polymer chain coupled to a block of hydrocarbon chains (CH)n. A large variety of studies and applications are based on such systems, and with significant variation in molar sizes ranging from oligomer amphiphiles, usually abbreviated as EC, to high molar mass polymers. Only the latter gives the basis for real network and gel structures. The micellar cores act as knot-points in the gels. Details of the network structure and mechanical properties depend on the specific copolymer architecture. AB and ABA type of linear block copolymers and BAn star-polymers form typically independent micelles when A is the hydrophilic block. Such systems may still have gel-like characteristics, as resulting from inter!
-micellar correlations. BAB-type of architecture, on the other hand, form true network structures when self-assembled into micelles, where individual A-polymer blocks can interlink different micellar cores. It is by chemistry possibly to control whether the gel structures are thermal stable or will have major temperature dependence. PEO-PS type of hydrogels (PS equal polystyrene), for example, shows basically no temperature dependence within relevant T-ranges, while PEO-PPO (poly(propylene oxide)) and PEO-(CH)n shows significant temperature dependence. In the former the polymer gel dissolve upon cooling, while in the latter it dissolves upon heating. There is accordingly a large variety of design possibilities to control the hydrogel properties for specific applications.
The micellar cores, constitute very often an ordered structure even for relative low polymer concentrations. The texture of this ordered structure can further be controlled by simple mechanical treatments, revealing near single crystalline properties with orientation that can be effectively controlled by appropriate shear (γ,γ ̇ ) [2]. Further control and variations in the gel properties can be obtained by adding other molecular or nano-particle constitutes into the systems. Non-perfect copolymerization may act as polymeric additives, and have been shown to significantly affect the ordered structure in some systems. We found that commercial F127 Pluronics (PEO-PPO-PEO) contains roughly 20% PEO-PPO diblocks and 80% PEO-PPO-PEO triblock copolymers. Acqueous solutions of F127 as received form fcc ordered micellar structures, while copolymers depleted with respect to the diblock impurities form bcc [3]. Organic-inorganic hybrid hydrogels can potentially be tailore!
d to combine the advantages of organic polymers with those of inorganic components. We have studied such nano-composite materials composed of hydrophobically-modified poly(ethylene glycol) and silica based OCAPS nano-particles [4]. The OCAPS nano-particles are very well dispersed given rise to fully transparent materials.
Kell Mortensen. Colloids & Surfaces A: Physicochem. Engineering Aspects, 183, 277, 2001.
Kell Mortensen, J.Pol.Sci.B. Pol.Phys. 42, 3095, 2004.
Kell Mortensen, Walter Batsberg, Søren Hvidt. Macromolecules 41, 1720, 2008.
M Annaka, K Mortensen, T Matsuura, M Ito, K Nochioka, N Ogata. Soft Matter 8, 7185, 2012.
