tailieunhanh - Mechanical properties of polymers and composites-Nielsen Episode 11

Tham khảo tài liệu 'mechanical properties of polymers and composites-nielsen episode 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Creep and Stress Relaxation 93 This form is also known as the Williams-Watts function 145 . It is a powerful yet simple form to use in fitting data since it can accommodate any slope in the transition region. However equation 40 cannot describe a complete master curve from glassy to rubbery state with a single value of p Instead p or is taken to be time or temperature dependent. As the time scale of observation increases the response of increasingly large segments is being observed. As drops below 10 to 10A Pa two new mechanisms of behavior control the response. The local structure no longer impedes the chain segmental motion and the entropy takes over as the restoring force. That is the chain segments have a rubberlike elasticity with each segment of molecular weight having a modulus of kTIM r When chains are disturbed from their equilibrium position as a step strain is imposed all segments initially respond with this elastic restoring force but then begin immediately to diffuse under Brownian motion toward a new equilibrium position. This motion is impeded by the local viscosity of the surrounding chains. A description or model of this response was first developed by Rouse 13 and Zimm 14 for dilute solutions but the Rouse treatment is applicable to undiluted polymer as well 17 18 21 . As noted earlier a discrete spectrum of relaxation times emerges see equations 16 and 17 . For molecular weight below the entanglement molecular weight these equations describe the complete course of the relaxation process and the flow behavior. If the molecular weight is above a critical value the chain segments impede each other s motion. This entanglement effect has long been known 146 and was formerly attributed to the looping of chains around each other. The current picture is that adjoining chains and chain segments impede the lateral diffusion while diffusion along a chain s contour length is largely unmodified. The local constraints inhibiting lateral motion can be thought .