摘要:

The degradation of a peroxide‐cured polydimethylsiloxane (PDMS) elastomer subjected to γ‐irradiation at various temperatures has been studied by chemorheological techniques. Preliminary investigations of the thermal degradation of mechanisms operative in the elastomer utilising continuous and intermittent stress relaxation measurements revealed that degradation took place by a chain interchange reaction. The cleavage reaction occurred randomly along the main chain and was a hydrolytic scission reaction catalyzed by the presence of peroxide decomposition products in the material—namely, 2, 4‐dichlorobenzoic acid. Subsequent chain reformation took place by the condensation of silanol chain end groups. The reaction was found to have an activation energy of15±4 kJ mol−1.Intermittent stress relaxation techniques revealed that the net crosslink density of the elastomer increased as thermal degradation proceeded. This phenomenon has not been reported in any previous studies of the thermal degradation of PDMS elastomers. The effect has been attributed to the reformation of network chains cleaved during the post‐cure operation prior to the commencement of the thermal degradation studies. This reaction which is also believed to take place by the condensation of silanol groups, occurs simultaneously with the conventional interchange reaction. Continuous stress relaxation studies during γ‐irradiation indicated that the rate of the radiation induced scission reaction increased with increasing temperature. The temperature dependence of this rate did not obey the Arrhenius relationship suggesting a possible change in the scission kinetics at higher temperatures. For the particular sample geometry in the investigations, it was discovered that there was no significant difference between the response of elastomers tested in air and in an inert atmosphere.

ISSN:0148-6055    

DOI:10.1122/1.549947

出版商:The Society of Rheology    

年代:1987

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.549927

出版商:The Society of Rheology    

年代:1987

数据来源: AIP

摘要:

Velocity profiles were measured up to a minimum distance of 0.15 μm from the wall for tube flow of aqueous solutions of anionic Polyacrylamides (Praestol from the Stockhausen Corporation, at a concentrationc0=0.25percentage by weight). The profiles measured could be recalculated on the basis of the flow curve. Extrapolation of the profile to the wall resulted in finite velocity values (slip‐velocityvs). This slip phenomenon could not be calculated in advance on the basis of the flow curve alone. Within the range of wall shear stressτwexamined, the dependence ofvxonτwwas linear for a given polymer concentration and exhibited no threshold. The slip velocityvsdecreased if capillary active substances or strong electrolytes were added to the solution; divalent cations(Ca+ +) being ten times more effective than mono‐valent ones(Na+).The increment of flow velocity fromv=0tov=vswith increasing distance from the wall occurs within a fluid layer which borders the wall (or a layer of adsorbed macromolecules resp.) and which possesses a width smaller thanδ=0.15 μm(i.e. the axial resolution of the microanemometer). When assuming a value of δ between 0.022 μm and 0.1 μm, the viscosityηsof the fluid within the slip interval is calculated to be 1 cP up to 4.57 cP, respectively. The value ofηsis shear independent and at least seven times smaller than the viscosity measured by means of the viscosimeter. The results indicate that the Polyacrylamide solution does not glide directly along the wall but, instead, by mediation of a low viscous Newtonian fluid lining the wall.

ISSN:0148-6055    

DOI:10.1122/1.549952

出版商:The Society of Rheology    

年代:1987

数据来源: AIP

摘要:

Dynamic oscillatory shear testing has been used to study the rheology of xanthan gum in the moderate concentration regime(500

ISSN:0148-6055    

DOI:10.1122/1.549953

出版商:The Society of Rheology    

年代:1987

数据来源: AIP