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Thermal Analysis of Polymeric Materials Part 8

Tham khảo tài liệu 'thermal analysis of polymeric materials part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 406 4 Thermal Analysis Tools Change of Young s Modulus with Temperature _375_425_475 K__ Fig. 4.145 The present graph refers to a time scale of about 10 s for any one measurement and is to be compared for example to a DSC scan at 6 K min 1. On the left-hand side of the graph the high moduli of the glassy and semicrystalline solid are seen and can be compared to the table of bulk moduli in Fig. 4.144. At the glass transition Young s modulus for amorphous polystyrene starts dropping towards zero as is expected for a liquid. The applied stress causes the molecules to flow so that the strain increases without a limit when given enough time. Before very low values of E are reached however the untangling of the macromolecular chains requires more time than the 10 s the experiment permits. For a given time scale the modulus reaches the so-called rubber-elastic plateau. Only at higher temperature 450 K is the molecular motion fast enough for the viscous flow to reduce the modulus to zero. Introducing permanent cross-links between the macromolecules by chemical bonds the rubberelastic plateau is followed with a gradual increase in E. The data for a semicrystalline sample of polystyrene are also shown in the graph of Fig. 4.145. They show a much higher Ethan the rubbery plateau. Here the crystals form a dense network between the parts of the molecules that become liquid at the glass transition temperature. This network prohibits flow beyond the rubber elastic extension of the liquid parts of the molecules. Unimpeded flow is possible only after the crystals melt in the vicinity of 500 K. 4.5.2 Instrumentation of TMA A schematic diagram illustrating a typical thermomechanical analyzer is shown in Fig. 4.146. This instrument was produced by the Perkin-Elmer Co. Temperature is controlled through a heater and the coolant at the bottom. Atmosphere control is possible through the sample tube. The heavy black probe measures the .