Dynamics of branched polymers: motion, scattering and rheology

Here, molecular models for the description of the dynamics and flow properties of melts of entangled branched polymers are developed. Their predictions are compared against
data from MD simulations, NSE spectroscopy, and rheological measurements.

Following an introductory chapter, in chapters 2 and 3 the attention is drawn to local branch point motion. Expressions for the MSD correlation functions are derived. The expression for the segmental MSD is compared against MD results [1], obtained from simulations in which arm ends are motionless, i.e. standard CR events are suppressed.
This comparison suggests an apparent slow relaxation of the branch point localisation at early times; here, this process is referred to as “early tube dilation” (ETD). Standard CR events are also taken into account by utilizing the dynamic dilution hypothesis [2]. It is shown that the theoretical expression matches MSD data from simulations in which chain ends are mobile provided that CR and ETD are accounted for in the model. The theoretical MSD correlation functions are also used, in the context of a dynamic version
of the RPA, for the calculation of the scattering signal from the branch point; the predicted signal is compared against NSE data [3].

Chapters 4 and 5 deal with the flow properties of pom-pom melts. FSR and cross-slot flow measurements [4, 5, 6, 7], from industrial melts, indicate a viscosity overshoot in
extension. In chapter 4 this phenomenon is modelled by introducing the overshoot model, a variant of the pom-pom model of McLeish and Larson [8]. Following the approach of
Inkson et al. [9], a multimode version of the overshoot model is employed to fit the FSR data for the industrial resin DOW150R [6, 7]. In chapter 5, CR events are incorporated. They are modelled by means of Rouse-like hops in common with Refs. [10, 11, 12, 13]. In analogy with Refs. [14, 15], the physical picture of thin and fat tubes is adopted in the description of the dynamics of the system. The model predicts strain hardening (thinning)
at extension (shear) during start up of the flow. The maximum stretch, however, becomes dependent on flow-rate.