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Dynamic Mechanical Analysis part 2

Tham khảo tài liệu 'dynamic mechanical analysis part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 1 TK FIGURE 1.7 Isothermal cures for AEa. Isothermal runs allow the development of models for curing. Plotting the log of the measured viscosity h against time for each temperature gives the true initial viscosity ho and the rate constant k. Then we obtain the two activation energies AEa and AEh by plotting the initial viscosities and rate constants against the inverse temperature 1 T . This approach is discussed in Chapter 6. 1999 CRC Press LLC FIGURE 1.8 Frequency scans. Frequency scans are one of the less often used methods in DMA. Frequency responses depend on molecular structure and can be used to probe the molecular weight and distribution of the material. Properties such as relative tack stickiness and peel resistance to removal responses can also be studied. Log E 1999 CRC Press LLC material as a function of temperature we can also use DMA to quickly look at the effect of shear rate or frequency on viscosity. For example a polymer melt can be scanned in a DMA for the effect of frequency on viscosity in less than 2 hours over a range of 0.01 Hz to 200 Hz. A capillary rheometer study for similar rates would take days. For a hot melt adhesive we may need to see the low frequency modulus for stickiness or tack as well as the high frequency response for peel resistance .23 We need to keep the material fluid enough to fill the pores of the substrate without the elasticity getting so low the material pulls out of the pores too easily. By scanning across a range of frequencies Figure 1.8 we can collect information about the elasticity and flow of the adhesive as E and h at the temperature of interest. The frequency behavior of materials can also give information on molecular structure. The crossover point between either E and h or between E and E can be related to the molecular weight24 and the molecular weight distribution25 by the Doi-Edwards theory. As a qualitative assessment of two or more samples this crossover point allows a fast comparison of samples that .