We report the detailed study of mechanically induced free radical (mechanoradical) formation of glucosebased polysaccharides such as cellulose and amylose based on electron spin resonance (ESR) on its comparison with plasma-induced radicals of polysaccharides. The observed ESR spectra of mechanically fractured samples by ball milling at room temperature have shown the multicomponent spectra, which differ in pattern from those of plasma-irradiated cellulose but are similar to those of plasma-irradiated amylose. The systematic computer simulations disclosed that the observed spectra of cellulose consist of three kinds of spectral components, an isotropic doublet (I) assigned to a hydroxylalkyl-type radical at C 1 , an anisotropic doublet of doublets (II) assigned to an acylalkyl-type radical at C 2 and/or C 3 as discrete components, and a singlet spectrum (III) assigned to dangling-bond sites (DBS), while those of amylose consist of two kinds of spectral components, I and III. One of the most intriguing facts is that the component radicals are all glucose-derived mid-chain alkyl-type radicals as in the case of plasma irradiation, although it is known that mechanoradicals are produced by the polymer main-chain scission. It can be reasonably assumed, therefore, that the mechanoradicals primarily formed by 1,4-glucosidic bond cleavage of polysaccharides at room temperature underwent a hydrogen abstraction from the glucose units to give rise to the glucose-derived mid-chain alkyl-type radicals. Furthermore, spectrum III was a major component in the simulated spectra of both cellulose and amylose, unlike those in the case of plasma irradiation, suggesting that cross-linking reactions simultaneously occur accompanied by a decrease in the molecular weight in the course of vibratory milling.

The nature of peroxy radical formation from plasma-induced surface radicals of polyethylene (PE), both low-density polyethylene (LDPE) and high-density polyethylene (HDPE), was studied by electron spin resonance with the aid of systematic computer simulations. It was found that peroxy radical formation varies with the structure of component radicals of plasma-irradiated PE, both LDPE and HDPE:  Among three plasma-induced radicals of PE, dangling bond sites (DBS) undergo an instant conversion into the corresponding peroxy radicals in contact with oxygen, while the midchain alkyl radical is of very low reactivity with oxygen in both LDPE and HDPE. Computer simulations disclosed that ESR spectra of peroxy radicals are similar to each other in LDPE and HDPE, both being composed of two types of spectra, a partial g-averaging anisotropic spectrum and a nearly isotropic single line spectrum due to different molecular motional freedom at the trapping sites of peroxy radicals.

Plasma-induced low-density polyethylene (LDPE) radicals were studied in detail by electron spin resonance (ESR) by its comparison with ESR of high-density polyethylene (HDPE). The observed ESR spectra of plasma-irradiated LDPE are largely different in pattern from those of HDPE. The systematic computer simulation disclosed that such observed spectra consist of three kinds of radicals, midchain alkyl radical (1), allylic radical (2) as discrete radical species, and a large amount of dangling bond sites (DBS) (3) at an intra- and intersegmental cross-linked region. All these component radicals are essentially identical to those of HDPE. One of the most special features unique to plasma-irradiated LDPE, however, is the fact that thermally stable DBS (3) is a major component radical instead of a midchain alkyl radical in HDPE. This can be ascribed to the difference in polymer morphology between LDPE and HDPE:  branched structure with a large amount of amorphous region for LDPE and linear structure with a large amount of crystalline region for HDPE. Since one of the characteristics of plasma irradiation is the fact that it is surface-limited, LDPE would undergo the radical formation preferentially on the surface-branched structural moiety followed by facile cross-link reactions resulting in the formation of DBS. Thus, the nature of radical formation of PE was found to be affected by the polymer morphology in a very sensitive manner.

No abstract

The effects of laser energy and atmosphere on the emission characteristics of laser-induced plasmas were studied with the use of a Q-switched Nd: YAG laser over a laser energy range of 20 to 95 mJ. Argon, helium, and air were used as surrounding atmospheres, and the pressures were changed from atmospheric pressure to 1 Torr. The experimental results showed that the maximum spectral intensity was obtained in argon at around 200 Torr at a high laser energy of 95 mJ, whereas the line-to-background ratio was maximized in helium at around 40 Torr at a low energy of 20 mJ. The results are discussed briefly on the basis of the temporal and spatial observations of the laser-induced plasmas.