Constantinos Tsitsilianis

pH-Tunable rheological properties of a telechelic cationic polyelectrolyte reversible hydrogel

Steady shear properties and linear and nonlinear viscoelastic behaviors of a poly(methyl methacrylate)-poly(dimethyl amino ethyl methacrylate)-poly(methyl methacrylate) polymer, (PMMA-PDMAEMA-PMMA), telechelic polymers in salt-free aqueous solution have been investigated as a function of concentration and pH. Above a critical concentration, a transient physical network is formed through an association mechanism between hydrophobic end groups, leading to a gel-like behavior. The gel-like polymer solutions were shown to exhibit a peculiar flow behavior, associated with time fluctuation of the transient first normal stress difference, attributed to orientation effects of the stiff charged polymer chains. The viscoelastic behavior was shown to be governed by two pH dependent time scales: a short time scale controlled by the lifetime of the hydrophobic associative junctions and a long time scale corresponding to the network relaxation time. All rheological results show strong evidence that Coulomb interactions, which control both macromolecular chain rigidity and inter-chain interactions, lead to specific pH-tunable properties of great potential interest.

Amphiphilic heteroarm star copolymers of polystyrene and poly(ethylene oxide)

Abstract Amphiphilic heteroarm star copolymers bearing polystyrene (PS) and poly(ethylene oxide) (PEO) branches have been synthesized by sequential anionic ‘living’ copolymerization. These samples have been characterized adequately and shown to exhibit rather well-defined structures. The functionality of the stars is influenced mainly by the molar ratio of divinylbenzene per living ends and the molecular weight of the linear PS precursor. These star copolymers exhibit association phenomena not only in water but also in tetrahydrofuran, which is not a selective solvent for the different arms. Finally, it has been shown that the PEO arms can be crystallized, forming well-defined spherulites. This ability is strongly affected by the PEO content and the thermal history of the samples.

pH-Responsive Hydrogel/Liposome Soft Nanocomposites For Tuning Drug Release

A novel liposome/hydrogel soft nanocomposite was explored as a controlled drug delivery system. A P2VP-PAA-PnBMA biocompatible, pH-responsive triblock terpolymer was used as an injectable gelator, entrapping PC/Chol liposomes loaded with calcein as hydrophilic model drug. The composite hydrogel was formed in vitro through a pH-induced sol-gel transition by dialysis against buffer under physiological conditions and at polymer concentration as low as 1 wt %. Excellent control of the calcein release was achieved just by adjusting the gelator concentration; that is, from 1 to 1.5 wt %, the drug release period was significantly prolonged from 14 to 32 days.

Polystyrene/poly(2-vinyl pyridine) heteroarm star copolymer micelles in toluene: morphology and thermodynamics

Abstract Polystyrene/poly(2-vinyl pyridine) heteroarm star copolymer (i.e. star-shaped polymers consisting of a central poly(divinyl benzene) core bearing an equal number of polystyrene and poly(2-vinyl pyridine) arms, PS 6 P2VP 6 ) and the corresponding diblock copolymer were investigated in toluene, which is a selective solvent for PS. The star copolymers are associated in polymolecular micelles following the closed association model. The characteristics of micelles were determined by means of static light scattering, viscometry and cryo-transmission electron microscopy. The micelles adopt a core-shell structure of spherical shape with core radius almost equal to the corona thickness. Comparing their solution behaviour with that of the corresponding diblock copolymer, having the same block lengths as those of the star arms, significant differences were observed. The A n B n exhibit cmc which is three orders of magnitude higher and an aggregation number which is about one order of magnitude lower with respect to the AB copolymer. The thermodynamics of micellization were also studied. Δ G 0 , Δ H 0 and Δ S 0 were found to be negative for both systems. Δ G 0 is less negative for the star copolymer implying a smaller driving force for micellization. This is due to a pronounced loss of the combinatorial entropy of the star polymer arms.

pH responsive self assemblies from an An-core-(B-b-C)n heteroarm star block terpolymer bearing oppositely charged segments

We report on the association capability of a novel multisegmented, multiarm star terpolymer to form a diversity of pH-responsive amphoteric micellar nanostructured self-assemblies.

Amphiphile micelles formed by polystyrene/poly(2-vinyl pyridine) heteroarm star copolymers in toluene

Abstract We report the micellar behavior of polystyrene/poly(2-vinyl pyridine) heteroarm star copolymers (PSnP2VPn) in toluene which is a selective solvent of polystyrene. A series of PSnP2VPn were synthesized by anionic polymerization and their micellar characteristics such as critical micelle concentrations (cmc), aggregation numbers, hydrodynamic dimensions and core–corona sizes were determined by the means of static and dynamic light scattering, viscometry and transmission electron microscopy. All these parameters were studied as a function of the insoluble arm length and the corresponding scaling relations were established. Our system seems to belong to the class of amphiphile micelles characterized by strong dependencies of the aggregation number and the microdomain sizes on the insoluble arm length. This behavior is also characteristic of strongly segregated diblock and triblock copolymer systems.

Viscometric study of aggregation phenomena in polymer dilute solutions and determination of the critical concentration c

Abstract A crossover is observed in the variation of the reduced viscosity, η sp c , as a function of the concentration, c , for the systems poly(methyl methyacrylate)/nitromethane and poly(vinyl-2 pyridine)/benzene. This crossover is correlated with the critical concentration of the polymer c∗∗ in which the macromolecular coils start their contacts. The critical concentration c∗∗ is related with the molecular weight and the intrinsic viscosity of the polymer through the scaling laws: c∗∗∝ M −α and c∗∗∝[η] −1 .

Self-assembly and drug delivery studies of pH/thermo-sensitive polyampholytic (A-co-B)-b-C-b-(A-co-B) segmented terpolymers

We investigated the behavior of the pH and thermo-responsive poly(2-(diethylamino)ethyl methacrylate-co-methacrylic acid)-b-(ethylene glycol methyl ether methacrylate)-b-poly(2-(diethylamino)ethyl methacrylate-co-methacrylic acid) (P(DEAEMA-co-MAA)-b-PEGMA-b-P(DEAEMA-co-MAA)) triblock terpolymers in aqueous solution by means of static and dynamic light scattering, transmission electron microscopy and rheology. Association phenomena were observed at the isoelectric point region of the polyampholyte outer blocks leading to flower-like micelles and at higher polymer concentrations, to a three dimensional transient micellar network, exhibiting a compex thermo-response thanks to the thermo-sensitivity of the PEGMA middle block. Doxorubicin (DOX) drug was loaded in the terpolymer micelles and drug release studies were performed. Controlled drug delivery could be achieved by tuning pH. The terpolymer micelles exhibited satisfactory cytocompatibility towards human umbilical vein endothelial cells.

Heteroarm Star‐Like Micelles Formed from Polystyrene‐block‐poly(2‐vinyl pyridine)‐block‐poly(methyl methacrylate) ABC Triblock Copolymers in Toluene

Polystyrene-block-poly(2-vinyl pyridine)block-poly(methyl methacrylate) ABC triblock copolymers were synthesized by sequential living anionic polymerization. Their solution properties were investigated in toluene, which is a bad solvent for the middle block. Spherical micelles are formed, which consist of a poly(2-vinyl pyridine) dense core bearing polystytene and poly(methyl methacrylate) soluble chains at the corona. These micelles exhibit the architecture of heteroarm star copolymers obtained by living polymerization methods. The aggregation numbers strongly depend on the length of the insoluble P2VP middle block thus remarkably attecting the size of the micelles.

Perfect mixing of immiscible macromolecules at fluid interfaces

The difficulty of mixing chemically incompatible substances--in particular macromolecules and colloidal particles--is a canonical problem limiting advances in fields ranging from health care to materials engineering. Although the self-assembly of chemically different moieties has been demonstrated in coordination complexes, supramolecular structures, and colloidal lattices among other systems, the mechanisms of mixing largely rely on specific interfacing of chemically, physically or geometrically complementary objects. Here, by taking advantage of the steric repulsion between brush-like polymers tethered to surface-active species, we obtained long-range arrays of perfectly mixed macromolecules with a variety of polymer architectures and a wide range of chemistries without the need of encoding specific complementarity. The net repulsion arises from the significant increase in the conformational entropy of the brush-like polymers with increasing distance between adjacent macromolecules at fluid interfaces. This entropic-templating assembly strategy enables long-range patterning of thin films on sub-100 nm length scales.