The radiation chemistry of poly(dimethylsiloxane)

The radiation chemistry of poly(dimethyl siloxane) has been investigated with respect to identification of the nature of the small molecule chain scission products. Low molecular weight linear and cyclic products have been identified through the use of Si-29 solution NMR, GPC and MALDI-TOF mass spectrometry. It has been suggested that the low molecular weight cyclic products are formed by back-biting depolymerization reactions.

Electrical and chemical diagnostics of transformers insulation .B. Accelerated aged insulation samples

This paper describes the analysis of accelerated aged insulation samples to investigate the degradation processes observed in the insulation from aged transformers. Short term accelerated ageing experiments were performed on paper wrapped insulated conductors and on pressboard samples. The condition of aged insulation samples was investigated by two relatively new diagnostic techniques: (a) measurements of interfacial polarization spectra by a DC method (b) measurements of molecular weight and its distribution by gel permeation chromatography. Several other electrical properties of the paper/pressboard samples were also studied. Possible correlations have been investigated among the different measured properties. The GPC results have been used to predict how molecular weights change with temperature and time.

The radical homopolymerization of N-phenylmaleimide, N-n-hexylmaleimide and N-cyclohexylmaleimide in tetrahydrofuran

The kinetics and mechanisms of thermally initiated (using 2,2'-azobisisoburyronitrile (AIBN) as initiator) radical homopolymerizations of a series of maleimides, including N-phenymaleimide (PHMI) [l-phenyl-1H-pyrrole-2,5-dione]; N-n-hexylmaleimide (nHMI) [l-(n-hexyI)-1H-pyrrole-2,5-dione]; and N-cyclohexylmaIeimide (CHMI) [l-cyclohexyl- 1H-pyrrole-2,5-dione] have been investigated in THF solution by an on-line FT-NIR technique. It was found that the order of the activation energies for the three N-sub-MIs is: E-a PHMI < E-a (PHMI) < E-a (CHMI). The overall polymerization rate parameter k and the pre-exponential factor A were calculated. The kinetic order with respect to the N-sub-MIs was in the range of 0.71 < m < 0.75 for the initiator and n = 1.0 for the monomer. Radical transfer to solvent was found to be the key factor in determining the apparent order with respect to the initiator. All of the homopolymers had a relatively low molecular weight. The end groups of the polymer chains were characterized by MALDI-TOF, GPC and NMR methods and the results clearly indicate that the polymerization was initiated by THF radicals, and that the termination reaction is mainly controlled by chain transfer to solvent through an hydrogen abstraction mechanism. (C) 2001 Elsevier Science Ltd. All rights reserved.

Free radical copolymerization of methyl methacrylate and allyl acetate Part 2 structure and properties

The structure of the product from the free radical bulk copolymerization of methyl methacrylate (MMA) and allyl acetate (AAc) was investigated. The mole fraction of AAc plays an important role in the copolymerization of these two monomers. Molecular weight (MW) and molecular weight distribution (MWD) are completely altered when the feed composition is dominantly AAc. NMR spectroscopy confirmed the incorporation of AAc into the polymer. However, no allyl-allyl linkages were observed at low conversions. T-g was found to be affected by the incorporation of AAc into the polymer. (C) 2001 Society of Chemical Industry.

Molecular weight changes and scission and crosslinking in poly(dimethyl siloxane) on gamma radiolysis

The molecular weight changes which occur on the gamma -radiolysis of poly(dimethyl siloxane) under vacuum between 77 and 373 K and in air at 303 K have been investigated using triple detection GPC to obtain the complete molecular weight distributions for the irradiated samples and to determine the number and weight average molecular weights. The results have been interpreted in terms of the relative yields of scission and crosslinking. The total yields for crosslinking predominate over those for scission at all the temperatures investigated for radiolysis under vacuum. Based on a solid-state Si-29 NMR analysis of PDMS irradiated under vacuum at 303 K, which yielded a value of G(Y) of 1.70, the values of G(S) = 1.15 +/-0.2 and G(H) = 1.45 +/-0.2 were obtained for radiolysis under vacuum at 303 K. For radiolysis in air at 303 K, crosslinking was also predominant, but the nett yield of crosslinking was much less than that observed for radiolysis under vacuum. Under the conditions of the radiolysis in air at 303 K, because of the low solubility of oxygen in PDMS, it is likely that the radiation chemistry is limited by oxygen diffusion. (C) 2001 Elsevier Science Ltd. All rights reserved.