Research group

Group publications

Research Objectives/Activities

Research focuses on micromolecular sorption and transport in polymeric materials by a combination of theoretical and experimental approaches. The aim of this work is to help create the basic scientific background for the optimization of the design of polymeric materials for important applications (controlled release systems, permselective membranes, packaging, chemical sensors etc).

Current research activities include

• Polymer-based controlled release systems

• Mechanisms of Micromolecular Non-Fickian Transport Kinetics in Glassy Polymers

• Polymer-based Chemical sensors

• Permeability properties of composites

An overview of each one of these activities is given below

• Polymer-based controlled release systems

Development of controlled release devices aims at the regulated, prolonged delivery of drugs, agrochemicals or other bioactive agents. Matrix-type controlled release (MCR) devices consist of a swellable polymer matrix incorporating the requisite bioactive solute and are activated by the ingress of water when placed in an aqueous environment (Fig 1). In comparison with other types of controlled release systems (e.g. reservoir systems or osmotic pumps), MCR devices are very attractive commercially, due to their structural simplicity and low cost of manufacture. However, simple monolithic devices, operating by diffusion-controlled release kinetics (t^1/2 kinetics) are characterized by a continuously declining dose rate. A major part of our research in this area aims at the optimization of the design of these devices, in order to alleviate their main drawback of continuous decline of dose rate. 

Fig. 1a. Simple monolithic matrix type controlled release (MCR) devices are characterized by an undesirable declining release rate	Fig 1b. Evolution of  solvent (W) and solute (N )  concentration profiles during solute release from an MCR device

Modeling of MCR performance

We have developed advanced, realistic models, simulating the release performance of single-layer as well as multilayered MCR devices. These models can serve as a tool for the (i) elucidation of the effect of individual MCR design factors on the rate of release and (ii) predictive evaluation of the performance of proposed specific CR devices

The model takes account for the effect of each diffusing species (solvent-solute) on the diffusivity and/or solubility of the other and can simulate the release performance of devices functioning under conditions of both fast solvent penetration as well as under conditions of comparable rates of solvent ingress and solute release. Moreover the model takes into account the cases of: (i) the solute present in the layers in dissolved (mobile) and dispersed (immobilized) state and (ii) enhanced water uptake of the polymer due to the solute’s osmotic action. Finally it can be parameterized on the basis of information relating to the sorption and diffusion properties of the polymer-solvent-solute system, derived from the literature or from independent experimental measurements.

Models have been so far validated against experiment in model, as well as pharmaceutical, systems (Fig 2)

Fig 2. Monolithic (single-layer) controlled release matrices with uniform permeability properties and uniform drug load usually suffer from a declining dose rate. One method to stabilise the rate of release is the construction of multilayer devices with layers of different polymeric materials and/or different concentration of the embedded drug. The figure presents validation of our MCR model against experimental release rates of theophylline from single-, and from three-layer, PDMS-based devices (Ind. Engin. Chem. Res. 2012, 51, 7126)

Experimental work includes

• Validation of the developed models against experiment in model, as well as pharmaceutical, systems (Fig. 2)

• Study of complex release mechanisms of osmotic solutes: In many practical cases, particles of strongly osmotic solutes are embedded in a hydrophobic polymer matrix and may exert a strong osmotic action during the release process. Due to osmotically induced ingress of water there is a marked enhancement of the release rate. A previously proposed mechanism involving mechanical rupture of the polymer matrix has been studied in detail for hydrophobic elastomers. Additionally, we proposed a more general mechanism involving the formation of “zones of enhanced hydration” applicable to rigid polymers of limited hydrophilicity (Fig. 3).

• Modification of release kinetics from hydrogels by varying the swelling capacity of the matrix. This may be achieved by chemical (e.g. crosslinking) or physical (e.g. annealing) treatment of the polymer (PVA)

• Hydrophilic modification of elastomers: Hydrophobic silicone elastomers are suitable for the release of lipophilic drugs. However, hydrophilic modification by physical or chemical methods is sought in order to increase their biocompatibility or to facilitate transport of hydrophilic drugs or proteins from silicone elastomer-based controlled release devices.

 Fig. 3. Mechanism of osmotically enhanced solute release via (a) crack formation or (b) proposed more general mechanism of zones of excess matrix hydration (Int J Pharm 437, 2012, 178)

Selected publications (2005-)

• A.I Panou K.G. Papadokostaki, P.A. Tarantili, M. Sanopoulou, “Effect of hydrophilic inclusions on PDMS crosslinking reaction and its interrelation with mechanical and water sorption properties of cured films” Eur. Polym. J. 49 (2013) 1803-1810

• J. H. Petropoulos, K. G. Papadokostaki, M, Sanopoulou “Higuchi’s equation and beyond: Overview of the formulation and application of a generalized model of drug release from polymeric matrices” Int. J. Pharm. 437 (2012) 178– 191

• D. N. Soulas, M. Sanopoulou, K. G. Papadokostaki “Performance of three-layer controlled release devices with uniform or non-uniform material properties: Experiment and computer simulation”, J. Membrane Sci. 372 (2011) 1-10

• D.N. Soulas, M. Sanopoulou, K.G. Papadokostaki, “A comparative study on the release kinetics of osmotically active solutes from hydrophobic elastomeric matrices, combined with the characterization of the depleted matrices”, J. Appl. Polymer Sci., 113 (2009) 936-949

• K.G. Papadokostaki, A. Stavropoulou, M. Sanopoulou, J.H. Petropoulos, “An advanced model for composite planar three-layer matrix-controlled release devices. Part I. Devices of uniform material properties and non-uniform solute load” J. Membrane Sci. 312 (2008) 193-206

• K.G. Papadokostaki, M. Sanopoulou, J.H. Petropoulos, “An advanced model for composite planar three-layer matrix-controlled release devices. Part II. J. Membrane Sci. 343 (2009) 128-136

• Hasimi, A. Stavropoulou, K.G. Papadokostaki, M. Sanopoulou “Transport of water in polyvinyl alcohol films: Effect of thermal treatment and chemical crosslinking”, Eur. Polymer J. 44 (2008) 4098-4107

• Mechanisms of micromolecular non-Fickian transport kinetics in glassy polymers

Sorption kinetics in glassy polymer systems exhibits a variety of deviations from normal Fickian behaviour, attributable to either (i) slow viscous relaxations of the swelling polymer, or (ii) differential swelling stresses generated by the constraints imposed on local swelling during sorption. Our group develops models based on both mechanisms, capable of simulating all basic features of observed non-Fickian kinetic behaviour, including Case II kinetics. Experimental work includes

(i) sorption from the vapour phase. Carefully designed experimental sorption protocols, supplemented by measurement of longitudinal swelling kinetics of the polymer film, enable us to study various types of non-Fickian behaviour. On the basis of the models mentioned above, we develop general diagnostic criteria for distinguishing between the underlying mechanisms responsible for the observed experimental behaviour (Figs 4, 5).

(ii) sorption from the liquid phase. Combination of various optical techniques enables us to study in detail various types of non-Fickian penetration such as stress-dependent diffusion and Case II kinetics. 

Fig. 4. Water sorption kinetics in a poly(vinyl alcohol) hydrogel. Non-Fickian (deviating form t^1/2 law) kinetics results from slow molecular relaxations of the swelling glassy polymer matrix that occur on time scales comparable to those of water diffusion. The line represents fitting of the experimental data to our relevant model, with output the diffusion coefficient of water (Eur. Polymer J. 2008, 44, 4098)

Fig. 5. Non-Fickian (deviating form t1/2 law) kinetics of methylene chloride vapor sorption () in a cellulose acetate film. Analysis of the concurrent longitudinal (□), and thickness () dilation kinetics of the polymer film, reveal the operation of the Differential Swelling Stresses (DSS) during the sorption process. Based on model predictions (Macromolecules, 34, 2001, 1400) the correlations of sorption-longitudinal dilation kinetics can be used as a diagnostic tool for identifying the viscoelastic behaviour of the glassy polymer in response to the DSS operating during a sorption experiment. (Diffusion-fundamentals.org 11 (2009) 10, pp 1-2)

Selected publications (2005-)

• J.H. Petropoulos, M. Sanopoulou, K.G. Papadokostaki, “Physically insightful modeling of non-Fickian kinetic regimes encountered in fundamental studies of isothermal sorption of swelling agents in polymeric media” Eur. Polymer J. 47 (2011) 2053-2062

• V. Dimos, M. Sanopoulou, “Experimental and theoretical investigation of anomalous sorption kinetics in the methanol vapor – poly(methyl methacrylate) system at 35^oC", J. Polym. Sci., Part B: Poyml Phys., 44 (2006) 3173-3184

• A. Alentiev, S.I. Semenova, M. Sanopoulou “Non-Fickian Vapor Sorption Kinetics in Rubbery Poly(dimethylsilamethylene) and the Effect of Radiation –Induced Crosslinking”, J. Applied Polym. Sci., 95 (2005) 226-230

• Polymer-based Chemical sensors (collaboration with the laboratory of Mechanical and Chemical Sensors, Dept of Microelectronics in Demokritos)

The quantitative detection and monitoring of VOCs and moisture by chemical sensors is based on changes of a physicochemical property of the polymeric sensing layer due to absorption of the target vapour analyte. The operation of the low cost, low energy consumption capacitive- type sensors (Fig. 6), is based on changes in the dielectric properties of the polymer layer due to sorption of the vapour analyte. For a particular geometrical design of a capacitive sensor, sorption properties determine not only the sensitivity of the sensor to a particular VOC but also its selectivity for the target VOC in real complex environments.

Our collaboration with the laboratory of Mechanical and Chemical Sensors, Dept of Microelectronics in Demokritos, includes development and evaluation of (i) optical methodologies for fast screening the sorption properties of polymeric materials for gas sensor applications (ii) simulation methodologies for the prediction of chemocapacitors performance and (iii) sensor arrays, developed at the Dept of Microelectronics, for specific applications concerning the detection and continuous monitoring of VOCs and/or moisture in complex vapor environments (e.g. industrial installation using solvents) (Fig. 7).

Fig. 6. Schematic side view of an InterDigitative Chemocapacitive (IDC) sensor. Vapor detection and measurement by Chemocapacitive sensors, is based on changes in the dielectric properties of the sensing polymeric material due to sorption of analytes

Fig. 7. Experimental capacitance changes (ΔC/Co) of an IDC sensor, fabricated in the Dept. of Microelectronics, NCRS ”Demokritos”, upon exposure to four vapor analytes of varying dielectric constant (ε). The ΔC/Co values are compared with the predictions on dielectric constant changes (Δε/εo) of the polymer-analyte system, calculated by the Clausius-Mosotti mixing rule. The calculation was based on the corresponding experimental vapor-induced swelling (ΔL/Lo) of the polymeric films by use of white light reflectance spectroscopy (IEEE Sensors 2008, Lecce, Italy, Book of Abstracts, p.423-426)

Selected publications (2005-)

• Botsialas A, Oikonomou P, Goustouridis D, , Ganetsos, T., Raptis, I., Sanopoulou, M.. “A miniaturized chemocapacitor system for the detection of volatile organic compounds” Sens and Act B: Chemical 177 (2013) 776-784.

• K, Manoli, P. Oikonomou, E. Valamontes, I. Raptis, M. Sanopoulou “Polymer-BaTiO₃ composites: dielectric constant and vapor sensing properties in chemocapacitor applications” J. Appl. Polym. Sci., 125 (2012) , 2577-2584

• K. Manoli, D. Goustouridis, S. Chatzandroulis, I. Raptis, E. S.Valamontes, M. Sanopoulou “Vapor sorption in thin supported polymer films studied by white light interferometry”, Polymer, 47 (2006) 6117-6122

• Permeability properties of composites

The proper theoretical description of the permeability or analogous properties (thermal and electrical conductivity, electrical or magnetic permittivity, elastic modulus, etc.) of composite polymeric materials is of great interest, particularly in view of the growing technological importance of these materials and the significant improvement of their properties when the dispersed particle phase is of the nanoscale. The polymer-particle interface modifies the polymer structure and should be taken into account in modeling, especially in the case of nanocomposites, due the high surface to volume ratio of the nanoparticles (Fig. 8). Ongoing research is performed in collaboration with the University of Bologna.

Fig. 8. Permeability (P/P_B) of two-phase composite material as a function of the volume fraction (v_A) of the dispersed phase A embedded in phase B. Results for cubic inclusions of phase A and various permeability ratios (a =P_A/P_B ). Numerical calculations (points) and corresponding predictions by Maxwell's equation (Chem Eng Sci, 104, 2013, 630).
 

Selected publications

• M. Minelli, F. Doghieri, K.G. Papadokostaki, J.H. Petropoulos, A fundamental study of the extent of meaningful application of Maxwell's and Wiener's equations to the permeability of binary composite materials. Part I: A numerical computation approach, Chem. Eng. Sci. 104 (2013) 630-637.

• J.H. Petropoulos, K.G. Papadokostaki, M. Minelli, F. Doghieri, “On the role of diffusivity ratio and partition coefficient in diffusional molecular transport in binary composite materials with special reference to the Maxwell equation”, J. Membrane Sci., 2014, in press

Funded Projects (2005- )

1. “Computer aided molecular design of multifunctional materials with controlled permeability properties-Multimatdesign”, FP6-NMP-STREP, 2005-2008.

2. “Facing pathogenic conditions by combined use of bio-medical methods and nanotechnology” Infrastructure, Measure 4.5, Action «Consortia of research and technological development in sectors of National priorities», 2050-4/2, 2005-2008.

3. “Development of innovative bio-active magnetic nanomaterials for diagnosis and monitoring of pathogenic conditions by magnetic tomography”, PEP Attikis, 2006-2008.

4. “Morphological control of polymer blend nanofilms for organic (opto-) electronics" Joint research and technology programmes, Greece -Poland, 2006 – 2008.

5. “Autonomous and integrated system for in-situ and continuous contaminant gases monitoring in industrial environments- ALEPOU” - General Secretariat for Research & Technology, 2012-2014

Infrastructure

• Dissolution tester equipped with fraction collector and UV-Vis spectrophotometer (Jasco).

• Thermal analysis instruments (Temperature Modulated DSC),

• Home-made apparatuses for kinetic release measurements

• Vacuum apparatuses for sorption and longitudinal dilation kinetic measurements on polymer samples including electronic microbalances (Cahn 2000 and MK2-M5 CI Electronics) or quartz spring balances

• Polarizing and interferometric microscopes,

• Tensile tester in conjunction with optical setup,

• Abbe refractometer.

 Laboratory services