Bulk modification of polymers by reactive extrusion represents a technologically attractive and relatively simple method. Modification is achieved during the forming process (extrusion or injection moulding) without the need of an additonal modification step. It is important to choose reactive modifiers showing high compatibility with the polymer matrix in order to avoid agglomeration and negative effects on the mechanical properties of the bulk. Reactive extrusion of e.g. stabilizers containing a polymerizable group could be a very interesting approach to overcoming the problem of the physical loss of these additives. The grafted stabilizers cannot be detached from the polymer matrix except through severance of chemical bonds and should, therefore, lead to highly substantive and effective stabilization systems, especially in an aggressive leaching environment. Among other things we grafted functional methacrylates and maleimides to polyolefines by reactive extrusion. Choosing appropriate functional groups enhanced compatibility and increased light stability were achieved. Basic research was done on grafting of maleimides to oligomers such as squalane, eicosane and 2,4,6,8-tetramethylnonane, a new model compound for polypropylene.

By reactive extrusion low-cost commercially available polymers such as PE, PP or PS can be converted into materials with tailor-made properties of increased value. They are upgraded by chemical reactions during processing. Extruders act simultaneously as conventional polymer processing machinery and as pressure vessels for chemical synthesis in highly viscous media. Unlike organic chemistry, where the final product very often is obtained by multi-step synthesis, reactive extrusion requires high-yield one-step synthesis with very short reaction times. Only volatile unreacted monomers [e.g. maleic anhydride (MA)] and by-products can be removed in the venting section. Additional purification and separation processes are uneconomical.

The modification of conventional reactor polypropylene, usually having a relatively high molecular weight (MW) and broad molecular weight distribution (MWD), in an extruder through the action of peroxides is a well-established method nowadays. In this so-called controlled rheology polypropylene, random chain scission reactions reduce MW and narrow MWD, thus reducing viscosity and elasticity. This type of PP shows improved processing characteristics for specialized applications (e.g. lower melt processing temperatures and higher speeds in melt spinning of very thin fibers). Controlled degradation at elevated temperatures leads to low-molecular waxes. By reactive extrusion with monomers such as maleic anhydride polar functional groups are covalently bonded to the PP matrix.

Blending PP-g-MA with nylon 6 results in the generation of a compatibilizing agent containing segments of PP and segments of nylon 6, which can enhance the compatibilization of PP an nylon 6, two normally incompatible polymers. In the blending process in an extruder in the melt the compatibilizing agent becomes located at the interface of the incompatible polymers and a multiphase system is established in which particles of the polymer present in the lower concentration are dispersed through the matrix of the polymer which is present in the higher concentration. In lieu of mixing incompatible polymers and a compatibilizing block or graft copolymer, the latter may be generated in situ by the presence of reactive functionality ('reactive blending'). Carboxylated polymers are particularly useful for this purpose.

There are many problems associated with the analysis of polymers, particularly because of insufficient solubility in organic solvents. The use of low molecular model compounds is a suitable method to overcome these problems. Typical model compounds for PE or EPR are eicosane and squalane, both of them commercially available. To investigate grafting of maleimides onto PP, we synthesized 2,4,6,8-tetramethylnonane (TMN) as a novel model compound.

The products were analysed by NMR spectroscopy, GC, SEC, UV spektroscopy and elemental analysis. It was found that, contrary to maleic anhydride, where onyl single units are grafted, single units as well as short side chains are formed. As expected, maleimides are preferrably grafted onto the tertiary carbons of TMN due to the higher reactivity of tertiary hydrogen atoms in hydrogen abstraction processes. Grafting in dichlorobenzene resulted in higher yields than grafting in bulk.

Photografting has been widely applied as a simple and versatile method to tailing physical-chemical properties of various surfaces. Multiphoton grafting provides additional benefit of spatial and temporal control of the process. Since multiphoton-induced reactions are confined only within the focal spot, more accurate functionalization with high resolution in a 3D volume is possible. Our interest is to design and synthesize novel arylazides possessing large multiphoton absorption combined with high photografting efficiency. After writing with the focused laser beams, 3D grafting patterns within the transparent hydrogel matrix could be obtained with micrometer resolution. The levels of immobilization can be spatially controlled by simply altering the irradiation exposure time or laser intensity during the photopatterning process. Such simple and versatile 3D grafting method show great potentials in studies of cell-surface interactions, drug screening, sensing applications and microarray-based proteome analysis.

Aleksandr Ovsianikov, Zhiquan Li, Jan Torgersen, Jürgen Stampfl and Robert Liska; Advanced Functional Materials, 2012, 22, 3429-3433.

Aleksandr Ovsianikov, Zhiquan Li, Aliasghar Ajami, Jan Torgersen, Wolfgang Husinsky, Jürgen Stampfl and Robert Liska; Applied Physics: Materials Science & Processing, 2012,108, 29-34.