Tham khảo tài liệu 'fundamentals of polymer engineering part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 8 Measurement of Molecular Weight and Its Distribution INTRODUCTION A solid polymer is a mosaic of structures. For a crystallizable homopolymer for example we can vary the amount and nature of crystallinity and the shape and size of the crystals. In addition we can vary the orientation of the polymer chains in both the crystalline and amorphous phases. This variation can be brought about either by changing material variables or process conditions. The former include the chemical structure the molecular weight and its distribution the extent of chain branching and the bulkiness of the side groups. The latter include the temperature and the deformation rate. It is the interplay within this multitude of variables that leads to the physical structure visible in the finished product. This structure in turn determines the properties of the solid polymer. In this chapter we examine the methods of measuring the polymer s molecular weight and its distribution. These quantities were defined in Chapter 1 and knowledge thereof can be helpful to the process engineer in optimizing desired polymer properties. These properties include mechanical properties such as impact strength flow properties such as viscosity thermal properties such as the glass transition temperature and optical properties such as clarity. There are several other reasons why we might want to measure the molecular weight. The molecular weight and its distribution determine the viscous and elastic properties of the molten polymer. This affects the processi- 340 Copyright 2003 Marcel Dekker Inc. Measurement of Molecular Weight 341 bility of the melt and also the behavior of the resulting solid material see also Chapter 12 . To cite a specific example 1 a resin suitable for extrusion must have a high viscosity at low shear rates so that the extrudate maintains its integrity. To be suitable for injection molding however the same resin must have a low viscosity at high shear rates so that the injection pressure
