Applications – Case studies

Phase Separated Polymers
Most polymer mixtures tend to phase separate at a critical degree of polymerization and temperature during their production. This effect can be used to create nanostructured phase separated polymeric materials where nanosized domains of one material are embedded in a matrix of the other. The formation of the nanosized domains in two component systems has been studied extensively over a number of decades, traditionally focusing on well-defined polymer molecules, as it enables control the nanoconfiguration and therefore the properties of the final material. Theory of phase separated materials dealt primarily with ideal systems, monodisperse diblock, triblock and multi-block copolymers. In contrast, industry deals with multi-component systems, synthesised by free radical or condensation polymerisations.
The rheological and mechanical properties of phase separated materials are largely influenced by their nano-scale structure, which is the result of chemical and physical processes taking place during material formation. Atomistic and coarse grained molecular dynamics methods can in principle deal with these phenomena but in practice particularly the gradual change in mixing thermodynamic properties of the reactants during polymerisation and its consequences for the final product are neglected.

Separation Membranes
Nano-composite membranes for separation applications show significant advantages over the conventional ones because they hold promise for improved selectivity thanks to their impregnation with highly selective inclusions, like zeolites or metal-organic frameworks. However, they exhibit significantly lower efficiency than expected due to limitations that have not been fully investigated or understood. Simplified expressions are used in the membrane literature [1, 2] to facilitate the design of improved materials and membrane processes, but often fail to predict the overall permeability or the selectivity of the membrane, the main design parameters for separation applications.

The use of multi-purpose mixed matrix filter membranes (MMM) for various molecular sizes using a combination of diffusion and absorption would significantly reduce costs to European health systems. The combination of diffusion and adsorption in the MMMs for removing uremic toxins from blood plasma by using activated carbon adsorptive particles in macroporous membranes was recently shown to be a promising way [3, 4]. The effective removal of small molecular weight solutes is achieved by diffusion, the transport of larger molecular weight toxins such as ß2-microglobulin and protein-bound uremic toxins, such as the phenol compound p-cresol is limited. Sorbents may be very effective in removing toxins insufficiently cleared by conventional dialysis technique, however, the use of uncoated sorbents is associated with poor biocompatibility due to direct blood contact.

Conductive Composites
Multi-functional polymer based nanocomposites have seen ongoing interest of the scientific community over the past 20 years [5, 6]. During the production and processing of polymeric nanocomposites, the dispersion of the filler particles in the polymer melt and partial reagglomerations due to subsequent processing steps are the main challenges. Today´s conviction is that an excellent deagglomeration and dispersion of the nanostructures and sometimes, i.e. for electrical conductivity, subsequent reagglomeration results in optimum material properties. The use of suitable surface functionalizations can enhance dispersion and filler-matrix interactions determining performance. The optimum dispersion level for specific applications is still hard to define or to predict.

The most decisive parameters during compounding are the screw configuration of the extruder [7], the location of nanofiller feeding to the extruder [8], the melt viscosity [9], the residence time and a good thermodynamic affinity between the filler and the polymer. Most of this knowledge is empirical, without a sound theoretical base. Therefore the relationships between the morphology induced during processing and the final properties are ambiguous. Regarding EMI (Electromagnetic Interference) shielding, the efficiency of micron sized stainless steel fibre composites is fairly well known, but CNTs and graphenes as recent developments with a high potential are not yet widely used in commercial products as the effect of processing on shielding properties is widely unknown. Mechanical and electrical properties of CNT filled polymers can be highly enhanced by a purposeful spatial orientation of the nanofillers [10, 11].

[1] D. Noble, J. Membr. Sci. 378 (2011), 393-397
[2] R. Mahajan, C.M. Zimmerman, W. J. Koros, in: B.D. Freeman, I. Pinnau (Eds.), Polymer Membranes for Gas and Vapor Separation, ACS, 1999, pp. 277-286
[3] M. S.L. Tijink, M. Wester, J. Sun, A. Saris, L.A.M. Bolhuis-Versteeg, S. Saiful, J. A. Joles, Z. Borneman, M. Wessling, D. F. Stamatialis, Acta Biomaterialia, 8 (2012) 2279-2287.
[4] M. Tijink, M. Wester, G. Glorieux, K.G.F. Gerritsen, J. Sun, P. Swart, Z. Borneman, M. Wessling, R. Vanholder, J.A. Joles, D. Stamatialis, Biomaterials, 34 (2013) 7819-7828.
[5] M.T. Byrne and Y.K. Gun'ko, Advanced Materials, 22: 1672–1688, (2010)
[6] G.G. Tibbetts, M.L. Lake, K.L. Strong and B.P. Rice, Comp. Sci. Tech. 67, p. 1709, (2007)
[7] Dennis HR, Hunter DL, Chang D, Kim S, White JL, Cho JW, et al. 2001.Polymer; 42:9513
[8] Chavarria K, Shah RK, Hunter DL, Paul DR. 2007. Polym Eng Sci;47:1847
[9] Kim SW, Jo WH, Lee MS, Ko MB, Jho JY. 2002. Polymer;34(3):103–11
[10] Schaefer DW, Justice RS. 2007. Macromolecules;40(24):8501–17
[11] Vaia RA, Maguire JF. 2007. Chem Mater;18:2736–51