Average Polymerisation degree of polymethylmethacrylate was measured by several methods, (viscosity method osmotic measurements, quantitative analyst of the end group of moleculars and Wijs' method etc.) but those values were different by each method (Table 1) .
Viscosity method in chloroform solution has been discussed by Japanes two makers (Mitubisi Rayon Co. and Fuji Kasei Co.) and our research institute.
The authors measured average polymerisation degree of denture base and direct resins by means of viscosity method under the following condition: Ostwald's viscosimeter was used at 20°C, And chloroform solution was placid more than for 48 hrs, at room temperature, since polymer had begun to resolve by the quantitative chloroform. Concentration of solution was 1g/l to 10g/l and then average polymerisation degree was calculated by Schulz's equation.
Intrinsic viscosity was obtained by the following four methods:
i) graphic method (plotting relation of ηsp/c-c),
ii) graphic method (plotting relation of lnηrel/c-c),
iii) Schulz's equation, and
iv) Sakurada's equation.
Intrinsic viscosity and average polymerisation degree are shown in Tables 2 and 4.
As shown in Table 4, it could be concluded as follows:
When the authors made the determination of average polymerisation degree of polymethylmethacrylate for dental use by viscosity method in chloroform solution under the same experimental condition by the use of the same equation calculating average polymerisation degree, difference of the average polymerisation degree is about 10 per cent by difference of method getting intrinsic viscosity.
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