Surface-initiated polymerisation

Polymer brushes are polymers tethered to a surface via one end. The connection to the surface can be covalent or non-covalent, and the brushes can be made via grafting to or grafting from the surface. In the past few years, there has been considerable interest in the growth of polymer brushes via surface-initiated polymerisations from (patterned) initiator-functionalised SAMs.62,63 For example, we have recently shown that surface confined Atom Transfer Radical Polymerisations (ATRP) in aqueous solvents leads to rapid and controlled... 

Taking a more radical approach to controlling morphology, studies have demonstrated the production of what are called polymer brushes on conducting substrates. These polymers are grown vertically from the substrate using a surface-initiated polymerisation technique (Snaith et al, 2005) and a layer of acceptor... 

Jiang, G., Walker, G.S., Jones, I.A., Rudd, C.D., 2005b. XPS identification of surface-initiated polymerisation during monomer transfer moulding of poly(epsilon-caprolactone)/ Bioglass fibre composite. Applied Surface Science 252, 1854-1862. 

This polymer first appeared commercially in 1965 (Parylene N Union Carbide). It is prepared by a sequence of reactions initiated by the pyrolysis of p-xylene at 950°C in the presence of steam to give the cyclic dimer. This, when pyrolysed at 550°C, yields monomeric p-xylylene. When the vapour of the monomer condenses on a cool surface it polymerises and the polymer may be stripped off as a free film. This is claimed to have a service life of 10 years at 220°C, and the main interest in it is as a dielectric film. A monochloro-substituted polymer (Parylene C) is also available. With both Parylene materials the polymers have molecular weights of the order of 500 000. 

PolyHIPE has found a successful application in the field of solid phase peptide synthesis (SPPS), where the highly porous microstructure acts as a support material for a polyamide gel [134]. The polystyrene matrix is functionalised to give vinyl groups on its internal surfaces, and is then impregnated with a DMF solution of N, JV -dimethylacrylamide, acryloylsarcosine methyl ester, crosslinker and initiator. Polymerisation grafts the soft gel onto the rigid support, giving a novel composite material (Fig. 16). 

Initiation for grafting The free radicals formed on the surface of fibre by exposure to plasma can be used directly to initiate polymerisation, with new polymer bonded firmly to the surface by carbon linkages. Thus, a new polymer can be applied in the following manner ... 

In the grafting from approach, a surface, that was previously activated e.g. by plasma treatment, is exposed to a monomer solution (Huang et al. 2003). A more simple, one-step procedure is to inadiate a polymeric surface like TCP, which is covered with the monomer solution, by an electron beam (Yamada et al. 1990). Alternatively, ultraviolet light and a photosensitiser can be utilised to initiate polymerisation and cross-linking (Curti et al. 2005). A completely different route to prepare thin SRP coatings with good adhesion to solid substrates is plasma polymerisation (Biederman and Osada 1992). In this case, NIPAAm is used as a precursor in a plasmachemical thin film deposition process (Cheng et al. 2005 Pan et al. 2001). 

McArdle, C., Burke, J., and McGettrick, B., Characterisation of cure distribution in surface initiated redox polymerisation by DM and di-electric thermal analyses. Plastics. Rubber Composites Processing Applications. 16, 245-253 (1991). 

Intensity of the illumination falling upon the surface of polymerising composition of UV-illumination (Jq) has been calculated by taking into account it s power Wq (Wlm ) (approximation), that it concentrated on the conditional wavelength X = 340 nm Jq = 2.83 X 10 Wq mole x quant/m3 x s. The starting concentrations of photoinitiator Cq = (% by mass) = pJM and monomer in the block [Mq] = pJM (where p = 1.04 x lO glm -density of monomer, = 256 and M =142 g/mole - the molecular masses of the initiator and monomer, respectively), expressed in the units mole/m. The given choice of the dimensions Jq, Cq also [Mq] determines the numerical values, k2 and k, as represented in Table 4.2. 

High-energy radiation such as ultraviolet (UV) radiation, X-rays, y rays and electron beams can initiate polymerisation. Radicals formed into polymeric materials during sterilisation by such radiation have a long life, especially in the bulk of the material, and can initiate new polymerisation. Such an initiation process has been used widely to modify polymeric surfaces by grafting another polymer. 

The primary distinction between miniemulsion and conventional emulsion polymerisation is the nucleation mechanism. In miniemulsion polymerisation (104,132, 193, 361) radicals from the water phase enter the dispersed monomer droplets directly to initiate polymerisation (i.e., the droplets act as individual reactors). This nucleation mechanism is referred to as droplet nucleation. Because of the small size and large surface area of the miniemulsion droplets, they are competitive for radicals relative to the homogeneous and micellar nucleation mechanisms. The monomer droplets polymerise to become polymer particles (275). Miniemulsion latex particles are typically prepared in the size range of 50 to 500 nm in diameter. 

The curing process depends on the radical polymerisation of the acrylic vinyl group giving total liquid-to-solid conversion. In anaerobic variants, this may be by the same mechanism described in Section 5.1.2 but all respond to a catalytic primer - either a surface initiator or mixed directly into the adhesive. 

Barbey, R., Lavanant, L., Paripovic, D., Schuwer, N., Sugnaux, C., Tugulu, S., Klok, H. A. 2009. Polymer brushes via surface-initiated controlled radical polymerisation Synthesis, characterisation, properties, and applications. Chem. Rev. 109 5437-5527. 

Sonochemistry is also proving to have important applications with polymeric materials. Substantial work has been accomplished in the sonochemical initiation of polymerisation and in the modification of polymers after synthesis (3,5). The use of sonolysis to create radicals which function as radical initiators has been well explored. Similarly the use of sonochemicaHy prepared radicals and other reactive species to modify the surface properties of polymers is being developed, particularly by G. Price. Other effects of ultrasound on long chain polymers tend to be mechanical cleavage, which produces relatively uniform size distributions of shorter chain lengths. 

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