Nonlinear rheology of long-chain branched polymers

Entangled polymers show significant nonlinear rheological behaviors. Those studies on the mechanical behaviors of polymer melts and solutions not only promote our understanding on polymer dynamics, but also guide the application of polymers and establish principals to design polymeric materials. Recently emerged interpretations on nonlinear rheology of linear polymers proposed by Dr. Shi-Qing Wang emphasize the network nature of entangled polymers.;This dissertation studies the nonlinear rheology of long-chain branched (LCB) polymers. A new synthetic method is developed and implemented to overcome the limitation of previous methods for not being able to synthesize long enough branches. This method can produce ultra-high molecular weight LCB polymers with branches of identical length and uniform spacing between branch points.;Polymers with multiple long branches show remarkable resistant to the elastic-driven decohesion comparing to linear polymers. In startup uniaxial extension, they are extraordinarily more stretchable. An empirical rule shows that the failure of entangled network, as characterized by the overshoot of engineering stress, is proportional to the square root of number of entanglements. Polymers with LCB are also more resistant to failure in stepwise extension (withstand a larger stretching ratio), which would be part of film blowing process.;Historically, strain hardening stands for the upward deviation of transient extensional viscosity comparing to zero-rate transient viscosity, which typically shows up on branched polymers. Under the newly emerged conceptual framework, such behavior is due to three factors: firstly, the shrinking cross-section area leads to a factor of extension ratio in calculating true stress and transient extensional viscosity; secondly, the introduction of branches suppresses the breakdown of entangled network; lastly, the entanglement network is strengthened at sufficient high Hencky rates during extension.;Entangled polymeric liquids have so far only shown strain softening upon startup shear, signified by stress overshoot. However, solutions of polystyrenes with LCB exhibit strain hardening upon startup shear at high shear rates, undergoing non-Gaussian chain stretching and reaching finite extensibility limit. The stronger than linear increase of the shear stress ends with a sharp decline, forming a cusp. At intermediate shear rates, stress overshoots always occur at the same strain, which is explained also by the length of backbone. The LCB polymers show a rich variety of transient responses to startup shear at different rates and open a large window of dynamics to meet practical applications.