46399-AC7
Nonlinear Rheology of Branched Polymers
The primary goal of this project is to develop an understanding of the rheology of branched polymers. Although branched polymers are of great commercial importance, their rheology, and especially the relationship between the polymer “architecture” and its macroscopic physical properties is still not well understood. This means that it is currently not possible to “reverse design” a polymer and/or a polymeric blend based upon some prescribed set of required properties. The focus of our research is the nonlinear rheology; namely, the mechanical flow properties under strong flow conditions characteristic of processing or other applications. The starting point for these objectives is a study of monodisperse polymers with an idealized chain architecture. We are considering the class of linear combs—namely, a linear chain with a large number of equal length arms or branches distributed along its contour. This is a reasonable model system for commercial systems such as branched polyethylene that contain multiple branch points. We have made tremendous progress over the past year, starting with a series of classical step-shear strain experiments carried out at the FORTH Institute in Greece in collaboration with Prof. Dimitris Vlassopoulos, and continuing with the subsequent analysis of this data. We have considered a spectrum of materials including systems with long, highly entangled arms and much shorter barely entangled arms, and with a range in the number of arms from a few to O(20). The data demonstrates the applicability of time-temperature superposition even in the nonlinear regime (itself an important result), and this has allowed measurements to be taken over approximately a seven order of magnitude range of relaxation times, thus encompassing the relaxation for the arms, which occurs on the shortest time scales, and the backbone, which occurs much more slowly. A typical example of the relaxation data is shown in the TOC graphic for a 70% solution of polybutadiene comb in PBd-1k with a backbone MW of 50 kg/mol, and 17 arms each with a MW of 7 kg/mol. This data can be superposed to find the damping function for both the backbone and the arms, both of which are distinct from theoretical expectations based upon the well-known theory of entangled polymers from Doi and Edwards. The data we have obtained is unique, and it’s interpretation has exposed a number of puzzles that we are in the process of analyzing. First, we have demonstrated that the arm relaxation occurs by mechanisms that are equivalent to the arms of star polymers (branched polymers with a number of arms emanating from a single branch point). Once the arms have relaxed, we expect that the backbone should relax as a linear chain in a dilated “tube”—with the dilation occurring because of the release of entanglements of the backbone with the arms. However, this expectation is not consistent with the data, and we are currently attempting to understand why. This probes the relaxation process for multiple branched polymers at the most fundamental level. The next step in this project is to measure the response to steady shear flow where the additional relaxation mechanism of “convective constraint release” is expected to play an important role. We will make rheo-optical measurements using a phase modulated birefringence system, for both start-up of and relaxation of steady shear flow. We believe that this project will lead to a major advance in understanding the processing behavior of important commercial polymers, such as polyethylene, which appear in a number of configurations depending on the number and molecular weights of the arms.
