Photonics polymerisation

Light has been an important resource in polymer chemistry since the introduction of photo-initiated polymerisations. As this is, however, not the topic of this section, we will focus on advanced applications of light in polymer science including two-photon polymerisation and light-responsive materials. 

An interesting processing technique applying light in material science is two-photon polymerisation [2PP] [127, 148]. Using 2PP, a photo-curable material is subjected to spatially well-defined laser irradiation [Fig. 21.19]. 

Figure 21.19 Two-photon polymerisation set-up curing a photo-curable resin at the focal point.

Wu S, Serbin J, Gu M. Two-photon polymerisation for three-dimensional micro-fabrication. J Photochem Photohiol A Chem 2006 181 1-11. 

Bhuian, B., Winfield, R.J., O Brien, S., and Crean, G.M. (2006) Investigation of the two-photon polymerisation of a Zr-based inorganic-organic hybrid material system. Appl. Surf. Set, 252 (13), 4845-4849. 

Figure 4.18 Two-photon initiated polymerisation of methyl acrylate.

A special instance of photoinitiation was already di us in Sect. VI-B, within the context of bare cation formation. In those sterns in fact, h -energy photons were used to produce the ejection of an electron from the moiK>mer molecule. In this chapter we will briefly review other photochemical tediniques involving the framation of electronically excited intermediates, which in turn generate suitable species for the initiation of cationic polymerisation. Crivello has recently reviewed this topic and other authors have published more specialised monographs on some specific aspects of cationic photo-initiation. We will therefore reduce our coverage to the basic premises on whidi the various methods are founded and to a few comments on the more recent contributions and medianistic interpretations. 

Our, as well literature, data show that when the intensity of the used photo source is-1016+17 photons / (cm2, s) non linear effects concerning light absorption by matter, are not observed. This means that in such cases classic photochemistry laws can be used and laser sources in the frame of mentioned intinsities of light emission also may be used to initiate polymerisation without distortion of the initiation mechanism. 

Since two photons are required to initiate the polymerisation, curing will only take place in the focal point of the laser [i.e. where the concentration of photons is the highest]. The microscope objective used to focus the laser beam can be moved relative to the sample. After removal of the uncured material, complex three-dimensional structures can be achieved via direct laser writing [127, 149]. Interestingly, biodegradable polyester oligomers have... 

There are many reports on azide-alkyne click polymerisation (a subject not discussed in the present chapter) but research on thiol-yne click polymerisation is in its early stages. New reaction types, novel catalyst systems other than those existing (i.e., photon, heat, organic base, transition-metal complexes) and new functionalities of the ensuing polymers are waiting to be developed [51]. 

A 75/25 butadiene-styrene latex with a total sohds content of 68%, produced by cold emulsion polymerisation reaction and subsequently concentrated and evaporated, was incrementally fractionated through the technique of fractionated creaming with sodium alginate. The fractions were analysed in relation to the average particle size by photon correlation spectrometry. 13 refs. 

PS latices were produced by dispersion polymerisation of the monomer in an ethanoFwater mixture. Polyacrylic acid, PVAl or polyvinyl pyrrolidone were used as steric stabilisers with a disperse-phase soluble initiator, i.e. AIBN. Transparencies of the films fonned from these PS latices were studied by measuring the transmitted photon intensities using a UV-visible spectrophotometer. Monte Carlo simulations were performed to calculate the transmitted photon intensities to simulate the latex fihn-formation process. The method developed by Prager and Tirrell was used to investigate the healing processes at the junction surfaces. 35 refs. 

In consideration of the intrinsic characteristics of the laser emission, these powerful light sources present many advantages which make them very attractive for curing applications. After absorbing a laser photon, the photoinitiator will split into radicals which will act on the monomer double bond, initiating in a fraction of millisecond the polymerisation that will develop in three-dimensions [105]. 

Cationic latex particles with surface amino groups were prepared by a multi-step batch emulsion polymerisation. Monodisperse cationic latex particles to be used as the seed were synthesised first. Then the amino-functionalised monomer, aminoethylmethacrylate hydrochloride, was used to synthesise the final functionalised latex particles. Three different azo initiators were used 2,2 -azobisisobutyramidine dihydrochloride, 2,2 -azobisdimethyleneisobutyramidine dihydrochloride, and 2,2 -azobisisobutyronitrile. Hexadecyltrimethylammonium bromide was used as the emulsifier. The latices were characterised by photon correlation spectroscopy to study the mean particle diameters, transmission electron microscopy to deteimine the particle size distributions, and hence the number- and weight-average diameters and the polydispersity index. The conversion was determined gravimetrically, the surface density of the amino groups was detemiined by conductimetric titrations, and the... 

Readers interested in polymer science should be familiar with the photochemistry of polymers. Photochemistry plays a role in polymerisation reactions, in the degradation of the backbone chain, or in cross-Hnking of different chains, all of which can be initiated by light Further, photochemistry has been used in a variety of different ways to change and probe polymer structure and intermolecular interactions. However, there are a number of primary physical processes, which take place between the photon impinging on the polymer and the commencement of chemical reaction. It is these primary physical processes that are the subject of this chapter. 

Formation and Growth of Primary Particles The first readily observable particles which precipitate in the monomer phase are the so-called primary particles. Most authors agree on the size of primary particle nuclei (eiO.lvm), which seems to be independent of the polymerisation temperature or rate of initiation Primary particles are formed by the coagulation of basic particles this appears to be a rapid process after the basic paticles are formed An attempt to study the aggregation of basic particles to form primaries was reported by Ranee and Zichy (39) in which the nucleation of polymer in the monomer phase was followed by Photon Correlation Spectroscopy (PCS) ... 

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