tailieunhanh - Mechanical properties of polymers and composites-Nielsen Episode 7

Tham khảo tài liệu 'mechanical properties of polymers and composites-nielsen episode 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Elastic Moduli 53 The morphology of the crystallite structures present is also important in determining the mechanical properties. D. Copolymerization and Plasticization Plasticizer and Copolymerization change the glass transition temperature as discussed in Chapter 1. Plasticixers have little effect on . Copolymerization can change A although less strongly than 7 x. As a result the basic modulus-temperature and modulus-time curves are shifted as shown in Figure 8 for different compositions. The shift in the modulus-temperature curve is essentially the same as the shift in TK. The shift in the modulus-time curve includes this plus the effect of any change in jr Copolymers and plasticized materials often show another _ effect a broadening of the transition region with a decrease in the slope of the modulus curve in the region of the inflection point. This effect is most pronounced with plasticizer which are poor solvents for the polymer 8084 . Heterogeneous copolymers in which there is a change in overall chemical composition in going from one molecule to another also have broad transition regions 85-87 . This heterogeneity can result from the fact that in most copolymerizations the first polymer formed terrds to be richer in one component than the other while during the last part of the polymerization reaction the molecules become richer in the other comonomer. This results in a mixture with a distribution of glass transitions and friction factors. Polymer mixtures of different chemical composition tend to be insoluble in each other and this tendency for phase separation is another cause of the broadening of the transition region 87 . If the molecules of different composition are completely soluble in one another and if the mixture is homogeneous in the sense of being thoroughly mixed there is little if any broadening of the transition 88 . The magnitude of the rubbery modulus does not depend strictly speaking on Mf but on the number concentration of chains between