Marino Guaita

Changes in degree of polymerization in the thermal degradation of polystyrene

Abstract The chain scission and volatilization processes which occur in the thermal degradation of polystyrene have been simulated by Monte Carlo procedures and the results have been compared with the degrees of polymerization, determined experimentally, of polystyrene samples degraded to different extents. The results of the calculations agree with random scission of the polymer chains at temperatures at which volatilization of simple molecules does not occur and with a degradation mechanism involving extensive deactivation of radicals through hydrogen abstraction from the body of polymer chains at temperatures at which volatilization does occur. The calculations also show that the molecularity index, i.e. the ratio between the weight average and the number average degrees of polymerization, tends to 2, independently of the degradation mechanism.

Refractive index of poly(thiocarbonate)s and poly(dithiocarbonate)s

Abstract The refractive indices and densities of recently synthesized poly(thiocarbonate)s and poly(dithiocarbonate)s were measured, and were used to determine the group contributions of thiocarbonate (OCOS) and dithiocarbonate (SCOS) groups to the molar refraction. The high refractive indices observed make these polymers, particularly the poly(dithiocarbonate)s, potentially interesting for optical applications (i.e. lenses and optical fibres).

THERMAL OXIDATIVE-DEGRADATION OF AES

Abstract The thermal oxidative degradation of an AES blend formed by a mixture of ethylene-propylene-diene (EPDM) rubber, styrene-acrylonitrile (SAN) copolymer and SAN grafted on EPDM chains has been investigated both in conditions of total volatilization and at temperatures below decomposition. During degradation, chemical interactions between the different polymeric structures take place, particularly in the case of the graft copolymer. Under decomposition conditions the effect of the interactions is the reduction of the overall volatilization rate. The oxidation reactions start on the EPDM chains at temperatures below those at which the formation of volatile compounds takes place, with a mechanism typical of hydrocarbon oxidation. Higher reactivity of EPDM has been found in the graft copolymer and attributed to the presence of the SAN chains attached to diene units. During oxidation, crosslinking takes place at the same time with chain scissions and partial destruction of the graft structure.

Evidence of random chain scissions at low temperatures in the thermal degradation of anionic polystyrenes

Abstract In the thermal degradation of some anionic polystyrene samples chain scission was found to occur in the temperature range 180–220°C. The resulting polymer molecules thereafter remain stable up to about 280°C, above which strong reductions occur in the degree of polymerization. It is shown that the low temperature degradation is due to the presence of a small fraction of weak links, which could be tentatively attributed to some oxidation of the polymers.

Thermal degradation of ABS

Abstract Thermal degradation of polymers and copolymers of acrylonitrile, styrene, and butadiene heated separately or blended (ABS) is studied by means of thermo-gravimetry and thermal volatilization analysis.

Changes of degree of polymerisation in the thermal degradation of poly(methyl methacrylate)

Abstract The experimental decrease in the degree of polymerisation of three poly(methyl methacrylate) samples heated at 320°C to different conversions to volatile products has been reproduced by simulating the thermal degradation process by Monte Carlo procedures. The results show that the tertiary and primary radicals formed by polymer chain scission depolymerise to different extents, then preferentially abstract a hydrogen atom or, less frequently, combine by coupling. Some evidence has been obtained that the degradation of the largest molecules may be influenced by diffusion phenomena.

High Performance Size Exclusion Chromatography with Microparticulate Porous Silica Spheres. Influence of Flow Rate, Salute Mass Transfer and Polydispersity of Samples

Abstract The contributions to peak broadening in Size Exclusion Chromatography with microparticles of porous silica spheres having narrow size distributions have been determined by measuring the plate height dependence on flow rate for toluene and for polystyrene standards covering a wide range of molecular weights. From these contributions, the diffusion coefficients of the macromolecules in the pore matrix and the polydispersities of the samples could be evaluated. It is shown that for permeating polymers the band broadening is determined by the eddy diffusion in the mobile phase, by the slow mass transfer of the solute in the stationary phase and by the polydispersity of the standards. In properly packed columns the eddy diffusion term is of minor importance compared to the other effects, whereas the solute mass transfer, which is a velocity dependent process, can be minimized only at extremely low flow rates.

Thermal degradation of model linear and star-shaped polyisoprene molecules

The thermal degradation behavior, in the absence of air, of model star-shaped and linear polyisoprenes with silyl alkyl groups, prepared using anionic polymerization techniques and chlorosilanes as linking agents, has been investigated and compared with that of model linear polyisoprenes. Thermal stabilities were examined by thermogravimetry and differential scanning calorimetry, while analysis of degradation products and of polymer residues obtained after different thermal treatments was performed by gas chromatography-mass spectrometry, 1H-NMR, FTIR and size exclusion chromatography. The temperature at which polymer decomposition takes place, the shape of the weight loss curves and the nature of the compounds formed during the degradation seems not to be influenced by the structural differences or the presence of silyl alkyl groups in the polyisoprene molecules. However, the star-shaped polymers show a higher tendency for coupling and cross-linking during degradation.

Size exclusion chromatography of polymer molecules on microparticulate silica gels: A comparative study

Abstract Inverse size exclusion chromatographic procedures have been applied to silica gel packings with different pore size distributions and the resulting pore dimensions have been compared with those obtained by nitrogen capillary condensation. It is shown that the evaluation of pore sizes depends on the model which is assumed to describe the real pore texture. The empirical method developed by Halasz seems to be useful in checking the adequacy of the pore model adopted. The differences in the results obtained from the chromatographic and absolute methods are discussed.

Simulation of Size Exclusion Chromatograms from Viscometry and Refractometry Detectors: An Analysis of the Influence of Concentration Errors on Reliability of Estimated Parameters

Abstract Size exclusion chromatograms of polymer samples of known molecular weight distribution (MWD), in solutions of known Mark-Houwink constants, as obtained from viscometry and refractometry detectors, have been simulated to check the influence of some errors in the solution concentration on the evaluation of the sample molecular characteristics and of the solution properties. It has been found that concentration errors arising from uncorrect estimate of the weight of injected polymer yield proportional errors on the average molecular weights and the Mark-Houwink K constant, but do not affect the MWD and the a exponent Concentration errors arising from uncorrect estimate of the difference between the times occurring to identical macromolecules to reach the detectors strongly modify the MWD, with consequences on the evaluation of both molecular characteristics and Mark-Houwink constants, and give rise to curvatures in the Mark-Houwink plots, thus suggesting a check, during the treatment of the chromato...