Quantitative analysis of random scission and chain-end scission in the thermal degradation of polyethylene

The thermal degradation of polyethylene includes two different kinds of pathways. These are random and chain-end scissions which include β-scission on the chain end and radical transfer scission. We conducted a quantitative analysis on these pathways by Pyrolysis-GC/MS and computer simulation. Two different distributions of scission products of polyethylene were observed at different temperatures. They are determined by the relationship between rate of reaction and that of volatilisation. Furthermore, a characteristic distribution was observed in lower molecular weight. It could be explained by direct scission and one to five-step radical transfer scissions. The pathway possibilities calculated with the accumulated schemes showed that the direct scission and one-step-radical transfer increased with the temperature. This indicates that β-scission occurs on the chain end before the radical transfer because the rate of the β-scission becomes faster as the temperature rises.     

Inclusion of the effects of polydispersity and volatility on the random chain scission statistical analysis for polymer degradation

The statistical product distribution for a linear polydisperse polymer of finite molecular weight was included into the statistical analysis for a system undergoing random chain scission showing the effect of volatilization of species other than monomer. Two sets of equations were derived. One set is for the nonvolatile fraction; the other is for the volatile fraction. Within each set there are three equations, one for the number of polymer molecules, the second for the molar (or number) fraction, and the third for the weight fraction of polymer molecules containing a specific number of repeat units. As degradation proceeds the polydispersity index should converge to a value of 1 rather than 2, which has been reported previously. The expected effects of polydispersity, number‐average degree of polymerization, and volatility were treated individually, and we determined that the molecular weight of a polymer has no theoretical influence on the product distribution. As for the effect of volatility, we determined that only the volatile product distribution would change. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3690–3696, 2000     

Composition of aromatic products in the catalytic degradation of the mixture of waste polystyrene and high-density polyethylene using spent FCC catalyst

In this chemical recycling process, spent FCC catalyst used had an advantage with an economical and environment aspect, such as a low catalyst price in liquid-phase reaction and a reuse of waste catalyst. The characteristics of oil product and its aromatic product distribution, as a function of reaction time in the reactor and also proportion of HDPE and PS in the mixture, were compared. Main products obtained were light hydrocarbons within the gasoline range that were mainly produced during initial reaction time. The formation of aromatic products such as styrene and ethylbenzene as major components depended appreciably on the reaction time, as well as the composition of HDPE and PS in the mixture used for degradation. For the distribution of C₉-C₁₂ alkylaromatic components as by-products, methylstyrene (C₁-styrene) and isopropylbenzene (C₃-benzene) components were the main products formed by β-scission and hydrogen transfer of PS, while the rest of alkylaromatic products showed very low fraction being 1% or less.     

Metallocene ethylene-1-octene copolymers: Influence of comonomer content on thermo-mechanical, rheological, and thermo-oxidative behaviours before and after melt processing in an internal mixer

The influence of the comonomer content in a series of metallocene-based ethylene-1-octene copolymers (m-LLDPE) on thermo-mechanical, rheological, and thermo-oxidative behaviours during melt processing were examined using a range of characterisation techniques. The amount of branching was calculated from ¹³C NMR and studies using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were employed to determine the effect of short chain branching (SCB, comonomer content) on thermal and mechanical characteristics of the polymer. The effect of melt processing at different temperatures on the thermo-oxidative behaviour of the polymers was investigated by examining the changes in rheological properties, using both melt flow and capillary rheometry, and the evolution of oxidation products during processing using infrared spectroscopy.The results show that the comonomer content and catalyst type greatly affect thermal, mechanical and oxidative behaviour of the polymers. For the metallocene polymer series, it was shown from both DSC and DMA that (i) crystallinity and melting temperatures decreased linearly with comonomer content, (ii) the intensity of the β-transition increased, and (iii) the position of the tan δ_max peak corresponding to the α-transition shifted to lower temperatures, with higher comonomer content. In contrast, a corresponding Ziegler polymer containing the same level of SCB as in one of the m-LLDPE polymers, showed different characteristics due to its more heterogeneous nature: higher elongational viscosity, and a double melting peak with broader intensity that occurred at higher temperature (from DSC endotherm) indicating a much broader short chain branch distribution.The thermo-oxidative behaviour of the polymers after melt processing was similarly influenced by the comonomer content. Rheological characteristics and changes in concentrations of carbonyl and the different unsaturated groups, particularly vinyl, vinylidene and trans-vinylene, during processing of m-LLDPE polymers, showed that polymers with lower levels of SCB gave rise to predominantly crosslinking reactions at all processing temperatures. By contrast, chain scission reactions at higher processing temperatures became more favoured in the higher comonomer-containing polymers. Compared to its metallocene analogue, the Ziegler polymer showed a much higher degree of crosslinking at all temperatures because of the high levels of vinyl unsaturation initially present.     

Highly sensitive and quantitative analysis of polyeptides using a new gradient system based on an abrupt change in adsorption of polypeptide to the reversed-phase column packing

During a study of 100 microL aliquots of urocortin containing various acetonitrile contents, we hypothesized that a change in the acetonitrile content in the solution across a specific content of acetonitrile (critical threshold) causes an abrupt change in adsorption capacity to the column packing. Circular dichroism measurements suggest that the conformational change induced by acetonitrile in the solution causes the abrupt change in adsorption capacity, and this solvent-induced conformational change is reversible across the critical threshold. This hypothesis can apply to various polypeptides with molecular weights range from 1007 to 6789 and to other organic solvents. A new gradient system utilizing the instant recovery of the adsorption capacity across the critical threshold was designed, and applied to the analysis of a 100 microL aliquot of various polypeptide solutions. The results suggest that use of a solution containing organic solvents more than the critical threshold allows successful dilution of polypeptides up to picomolar concentration range without any loss due to its adsorption during handling procedures and loading onto the LC system, and that a new gradient system enables quantitative analysis of polypeptides at picomolar concentrations in such solutions.