Poly(dimethylsiloxane) : blends with poly(urethane) & radiation chemistry

Preston, Christopher M. L. (2001). Poly(dimethylsiloxane) : blends with poly(urethane) & radiation chemistry PhD Thesis, School of Molecular and Microbial Sciences, The University of Queensland.

       
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Author Preston, Christopher M. L.
Thesis Title Poly(dimethylsiloxane) : blends with poly(urethane) & radiation chemistry
School, Centre or Institute School of Molecular and Microbial Sciences
Institution The University of Queensland
Publication date 2001
Thesis type PhD Thesis
Supervisor A/Prof David Hill
Dr. Andrew Whittaker
Total pages 272
Collection year 2001
Language eng
Subjects L
250604 Radiation and Matter
780103 Chemical sciences
Formatted abstract The role of poly (dimethylsiloxane) (PDMS) in blends with poly(urethane) (PU) was investigated with respect to their abrasion resistance and mechanical properties. Two different thermoplastic poly(urethane) elastomers, Ela85A and Pel55D, were blended with a total of eight poly(dimethylsiloxane) viscosity fractions. The molecular mobility of the polymers in the blends was examined, and related to the morphological model responsible for the observed physical behaviour.

The radiation chemistry of pure poly(dimethylsiloxane) was also investigated, resulting in the identification of the radiation products by direct spectroscopic techniques. The associated changes in molecular weight of poly(dimethylsiloxane) upon exposure to γ -radiation, based upon the developed crosslinking model, were also studied.

The macroscopic behaviour of the poly(urethane)/ poly(dimethylsiloxane) blends was studied by a range of techniques, including Dry-Sand Rubber- Wheel abrasion testing, kinetic coefficient of friction measurements, hardness and tensile testing. The morphology of the blends was studied by thermal analysis, small-angle x-ray scattering and 1H NMR spin-diffusion measurements.

The dilute poly(urethane) / poly(dimethylsiloxane) blends were found to demonstrate greatest wear resistance at high PDMS viscosity (> 100 Pa s), at which point the Ela85A based blends performed better than, and the Pel55D based blends about equal to, the unblended elastomers: Severe degradation of abrasion resistance and mechanical properties was observed at low PDMS viscosities in all samples. This behaviour was related to the observed disruption of the poly(urethane) elastomer soft-segment by low molecular weight PDMS. The crystalline hard-segments of the PU elastomer were found to be largely unaffected by the addition of PDMS. The mobility of the low molecular weight poly(dimethylsiloxane) species, and their enhanced surface effect, was also considered to contribute to the decreased wear performance.

The molecular mobility and chain dynamics of poly(dimethylsiloxane) in the pure molten state and in a blend with poly(urethane) were investigated by NMR transverse relaxation measurements. The calculated spin-spin relaxation times were found to be significantly shorter in the blends than in the pure PDMS fluids. This was related to partial solubilisation of the low molecular weight fractions of the polydisperse PDMS in the PU soft-segment, with the remainder of the PDMS residing in phase-separated domains. The size of the phase-separated domains was measured by PFG-NMR self-diffusion measurements, and was found to be approximately 5 μm in diameter.

The inter-domain spacing of the poly(urethane) hard-segments was measured by 1H NMR spin-diffusion measurements, and was found to be approximately 110 Å in Ela85A, and 65 Å in Pel55D. Excellent agreement between the spin-diffusion measurements and SAXS was observed for Ela85A.

The radiation chemistry of poly (dimethylsiloxane) was investigated by solution and solid-state NMR. This was the first study to provide direct spectroscopic evidence on the mechanism of crosslinking of γ-irradiated poly(dimethylsiloxane). It was revealed that the primary method of crosslinking in γ -irradiated poly(dimethylsiloxane) is through a previously unidentified trioxygenated silicon Y-type crosslink. Smaller amounts of H-type silmethylene and silethylene bridge crosslinks were also identified. Other new chain-end and side-chain functionalities formed during γ -irradiation were also identified.

The total crosslinking yield, which is the sum of new structures due to observable H- and Y-links, G(X), was found by solid-state NMR to be 2.54 ± 0.3, and the scission yield, G(S), to be 1.2 ± 0.26. The effect of oxygen on the radiation chemistry was determined to be limited by the rate of diffusion of oxygen into the crosslinked polymer. The formation of additional methoxvtype end-groups during oxygenated γ -irradiation were, however, identified.

The change in molecular weight of PDMS as a function of absorbed dose was investigated by triple-detection GPC, based upon a developed mixed Hand Y-linking model. Equations were developed to describe the mixed system, and were found to adequately describe the experimental data. A ratio of Hlinks to the total crosslink population of 0.45 was determined for irradiations performed at 303 K, with the G-value for formation of H-links found to be 1.38, and hence the total G(X) to be 3.08. Acceleration of scission and crosslinking processes with temperature, from 77 to 373 K, was found, although the ratio of H- to Y-links did not appear to remain constant. This was a novel use of the combined techniques of NMR and GPC to characterise a complex radiation induced crosslinking system.

Low molecular weight products of the γ -radiolysis of PDMS were identified by NMR, GPC and MALDI-TOF MS. Small cyclic siloxanes with four, five and six members were identified in the sol-fraction of the irradiated polymer by solution-state 29Si NMR, with larger linear and cyclic siloxanes identified by GPC and MALDI-TOF MS. A preferred size of 14 and 15 members was identified for the linear and cyclic species, respectively. The mechanism of the formation of these low molecular weight species was uncertain, and warrants further investigation.

In essence, this study revealed that the production of wear resistant poly (urethane) / poly(dimethylsiloxane) materials would be enhanced by using high molecular weight poly (dimethylsiloxane), rather that the less viscous fluids used in this study. A study into the wear and mechanical behaviour of such materials based on very high molecular weight linear poly (dimethylsiloxane) elastomers (> 200 Pa s) which are commercially available, may reveal the PDMS properties required for optimal blend performance. The study into the radiation chemistry of γ -irradiated poly (dimethylsiloxane) revealed previously unreported radiation products, with the mechanism of crosslinking being elucidated more thoroughly than ever before.

Keyword Polymers -- Effect of radiation on
Radiation chemistry -- Industrial applications
Additional Notes Variant title: Polydimethylsiloxane : blends with polyurethane & radiation chemistry

 
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