Effect of stabilizer on scission and crosslinking rate changes during photo-oxidation of polypropylene

Chain scission and crosslinking rates have been derived from molecular mass distributions obtained by gel permeation chromatography at different stages during photodegradation of polypropylene samples exposed to ultraviolet irradiation (UV). Results for rubber-toughened polypropylene (PP) containing no photostabilizer are compared with those for the same polymer stabilized using a commercial photostabilizing package (PPS). The samples were in the form of 3 mm thick bars and measurements were obtained at various depths from the exposed surface after different exposure times. The depth profiles for PP and PPS were very different. Reaction rates in the interior of PP showed oxygen diffusion limited behaviour and after prolonged exposure, the rates in the interior of PPS were higher than those in PP. The ratio of scission rate/crosslink rate fell when reaction rate increased. Crosslinking became relatively more likely when reaction rates were low. The low degradation rates obtained with stabilized polymer coupled with the sensitivity of the method of analysis enabled detection of inhibition of photodegradation attributed to residual moulding stresses in the samples.     

H2NNO2裂解与重排反应的理论热力学动力学研究

硝胺化合物的N—N键断裂和硝基-亚硝基重排(>N—NO_2→>N—ONO)是一类重要的化学反应。最近Golden等重新测定了DMNA[(CH_3)_2NNO_22]消失的速度常数,认为其分解应包括N—N键断裂和硝基-亚硝基重排后的再分解等过程,并得到分子束实验的确认,但理论研究尚不多见。Melius和McKee等都曾对该类体系进行过量化计算,但结果并不令人满意。Saxon等取其模型体系H_2NNO_2进行量化计算,似可定性说明其断裂、重排     

The degradation in solid state of polyvinyl alcohol by gamma-irradiation

The viscosity of polyvinyl alcohol (PVA) (Powder form) falls after irradiation. Gel does not occur after a dosage of above 1000 kGy. The G-values for scission under various conditions were measured. Irradiation of PVA brings about an increase in the intensity of a number of bands in the UV spectrum. It is shown that formation of unsaturated bonds and carbonyl groups occur under irradiation. The other one of radiolysis products is organic acid. After irradiation, the hydrolysis degree is unchanged.     

Determination of scission, crosslinking and branching parameters of electron beam irradiated methacrylate-acrylamide copolymer

The aim of this work was to investigate the impact of electron beam irradiation at different dose rates on the molecular structure of linear methacrylate-acrylamide copolymer. In the first part, the radiation chemical yields of scission (G_s) and crosslinking (G_x) have been determined after irradiation for various initial molecular weights CL1 (40?000 g/mol), CL2 (90?000 g/mol) and CL3 (425?000 g/mol). Based on this calculation, solvent (ethanol) was found to increase the impact of irradiation especially at low concentration of copolymer. In the second part, the presence of branching in samples before and after e-beam irradiation was explored, and branching calculation was performed.     

Macromolecular scission and crosslinking rate changes during polyolefin photo-oxidation

Chain scission and crosslinking rates have been derived from molecular mass distributions obtained by gel permeation chromatography at different stages during photodegradation of various thermoplastics exposed to ultraviolet irradiation (UV). Results are given for a high density polyethylene (HDPE); a low density polyethylene (LDPE); a linear low density polyethylene (LLDPE); a polypropylene homopolymer (PPHO); and a polypropylene copolymer (PPCO). As the oxidation progressed, it was observed that the scission rate for HDPE, LLDPE, PPHO and PPCO increased near to the exposed surface whereas for LDPE the rate remained almost unchanged. The crosslink rate fell near to the surface with HDPE and LDPE but increased with PPHO and PPCO. The reaction rates near to the bar centre (∼1.5 mm from the exposed surface) were low for HDPE, PPHO and PPCO; this is attributed to oxygen starvation, caused by consumption of oxygen by rapid reaction near the surface. Reaction was observed in the interior with LDPE and LLDPE, presumably because of a combination of a higher oxygen diffusion rate than for HDPE and a lower rate of consumption of oxygen near the surface than with the polypropylenes.     

Mechanochemistry in Block Copolymers: New Scission Site due to Dynamic Phase Separation

Mechanochemistry can lead to the degradation of the properties of covalent macromolecules. In recent years, numerous functional materials have been developed based on block copolymers (BCPs), however, like homopolymers, their chains could undergo mechanochemical damage during processing, which could have crucial impact on their performance. To investigate the mechanochemical response of BCPs, multiple polymers comprising different ratios of butyl acrylate and methyl methacrylate were prepared with similar degree of polymerization and stressed in solution via ultrasonication. Interestingly, all BCPs, regardless of the amount of the methacrylate monomer, presented a mechanochemistry rate constant similar to that of the methacrylate homopolymer, while a random copolymer reacted like the acrylate homopolymer. Size-exclusion chromatography showed that, in addition to the typical main peak shift towards higher retention times, a different daughter fragment was produced indicating a secondary selective scission site, situated around the covalent connection between the two blocks. Molecular dynamics modeling using acrylate and methacrylate oligomers were carried out and indicated that dynamic phase separation occurs even in a good solvent. Such non-random conformations can explain the faster polymer mechanochemistry. Moreover, the dynamic model for end-to-end chain overstretching supports bond scission which is not necessarily chain-centered.