Characterisation methods stabilisation

A review on the most versatile methods for studying surface properties of solids is available [12]. Takeguchi etal. [13] have recently reviewed progress in surface microanalysis for various polymer additives, such as stabilisers, softeners, fillers, etc. More extensive information on surface characterisation methods of polymers can be found in various recent books [4,5,14-16]. For quantitative surface analysis of materials, cfr. Chp. 6 and refs. [17,18]. 

Kim el al. [582] have described maleimide-based antioxidants melt grafted onto low-MW PE. IR spectroscopic methods and titration were used for the quantitative determination of the extent of grafting of the monomeric antioxidant. Smedberg el al. [583] have characterised polymer-bound stabilisers by FTIR and NMR. The binding of antioxidants and photostabilisers to polyurethanes was verified by tJV/VIS spectroscopy [584]. 

Acyl-a-chymotrypsins may be isolated if the acyl function can be made sufficiently unre-active by structural variation several have been recrystallised at low pH, conditions which stabilise the acyl enzyme. The stabilised acyl enzymes can be characterised by physical methods and the first example of an X-ray crystallographic study of an enzyme intermediate is that of indolylacryloyl-o -chymotrypsin [21]. 

Further progress has been reported in the chemistry of CT X, -p -bonded systems. Full details of such systems stabilised by intramolecular coordination, as in, e.g., 334, have been described. The kinetically stable system 335 has been prepared and its solid state structure determined . The P-halobis(imino)-CT -phosphoranes 336 have also been prepared, and detailed NMR studies of bis(imino) phosphoranes reported . Quin s group has continued studies of the generation and characterisation of reactive a X -systems, e.g., 337 ". Methods for the generation of monomeric metaphosphate esters in solution have been investigated. A theoretical approach has been used to probe the mechanism of the reaction between phosphanylnitrenes 338 and boranes. The thiophosphonic anhydride 339 behaves as a source of the dithioxophosphorane... 

The crystal structure of zirconia and the catalytic properties of SZ generally depend on the synthesis method and thermal treatment adopted. In particular zirconia crystallises in three different polymorphs characterised by monoclinic, tetragonal and cubic symmetry. Among them only the tetragonal SZ phase displays significant catalytic properties [5-7]. Unfortunately, the synthesis of the pure tetragonal polymorph is difiBcult and, in the absence of promoted oxides [8], it could be stabilised only through an accurate control of the synthesis parameters, with particular attention to the thermal treatments. 

The complex containing the D1 and D2 polypeptides was shown to contain the primary electron donor and acceptors, P680 and pheophytin and it now seems clear that also present is the tyrosine moiety which most probably acts as Z, the immediate donor to P680 (8,9). However there is a complete absence of water oxidation activity and lack of ability to stabilise charge separation by secondary electron acceptors. Two experimental approaches can be used to determine the components essential for full PS2 activity. Comparison of different types of PS2 preparation has proved a valuable method, although detailed characterisation of different size samples, or those from mutants, can be a problem. The second approach is that of reconstitution and this is now more attractive because we can take advantage of the simple composition of the photochemically active D1-D2 complex. Thus, we have attempted to add specific thylakoid components, assess their interaction with the D1-D2 complex and measure the recovery of various PS2 functions. As essential prerequisits to this reconstitution type of experiment we have also determined the precise composition of the reaction centre preparation and overcome the inherent instability of the sample above 0 C so as to allow the manipulations and incubations needed for reconstitution and subsequent analysis. 

Potassium ionisation of desorbed species (K IDS) with mass spectrometric detection is an extremely useful tool for the characterisation of high performance organic coatings. K IDS uses a commercial rapid heating probe to desorb intact molecules which are then ionised by potassium cation attachment. Based upon the molecular ions, which appear as [M]K, coatings components can be qualitatively and quantitatively analysed. In this work K IDS was selected as a method of soft ionisation, (i.e., producing molecular ions) because of its simplicity, wide applicability, low cost and compatibility with the quadrupole mass spectrometer. Simonsick [76] reports the application of K IDS to polymer additives (UV stabilisers and antioxidants), catalysts (organotin), reactive diluents (vernonia oil and aliphatic epoxides) and polyurethane precursors (polyesters and isocyanates). Tikuisis and co-workers [77] also discussed this technique. 

Ruddle and Wilson [66] have described a UV spectroscopic method for the characterisation of phenolic stabilisers in solvent extracts of polymer compositions. 

These workers point out that usually the additive must be separated in a pure state from co-extracted additives usually by thin-layer chromatography (TLC) and then identified by measurement of the UV, IR, nuclear magnetic resonance (NMR) and mass spectra of the compound. This full treatment is required only for new stabilisers - for a characterisation of well known compounds the simplest method is by direct comparison of the UV absorption spectra with those of a series of known stabilisers. For some compounds this will probably be sufficient, but many substituted phenols have similar spectra, and for three of the most frequently used antioxidants the UV spectra are identical. Topanol OC, lonox 330 and Binox M (see Table 2.11 for their chemical constitution) in ethanolic solution all have = 277 nm, with a shoulder at 282 nm. To extend this procedure Ruddle andWilson [66] prepared the spectra of alkaline solutions of the phenols, which were then measured either directly against a solvent blank or as difference spectra measured against the neutral solution. This still gives almost identical spectra for the three compounds mentioned previously. 

DPC provides a means of characterising polymers supplementing information obtainable by traditional thermal methods, e.g., the physical properties of the photopolymer can be measured before and after exposure to light. The combined physical and light-sensitive properties provide information on rate of cure, as well as the effect of cure on the physical properties of the material, e.g., time-heat flow plots with different pigments, stabilisers, antioxidants, and plasticisers. 

Along with the indirect chemical methods (radical acceptors and monomers), the mechano-chemical mechanism is sustained by the data obtained using electronic paramagnetic resonance spectroscopy (EPR) for the characterisation of free radicals [907], The identification of the new end-functional groups, obtained by radicals stabilisation has been performed by FTIR spectroscopy [908], Complementary results were obtained using mass spectroscopy, MS, and small angles X-ray diffraction techniques. 

The essential feature of a radical quencher or trap is the availability of a radical state of low energy which is therefore relatively stable to further reaction. There are a wide range of radical quenchers available commercially as stabilisers, particularly polymer stabilisers. Electron spin resonance (ESR) is the obvious method of radical characterisation but some radical traps can be identified spectrophotometrically. Radical sensitisers, quenchers and traps are discussed in further detail in Chap. 8. 

---