The influence of molecular weight and processing conditions on the crystallization kinetics of isotactic polypropylene is studied using rheometry. Flow-induced crystallization experiments are performed with shear rates at which molecular stretch of the longest chains is expected. Depending on the molecular weight, a saturation of pointlike nuclei is observed with increasing shear time. In most cases, the process accelerates after sufficient flow time, and this change in kinetics is due to the occurrence of fibrillar nucleation resulting in the formation of row structures and/or shishes. The number of pointlike nuclei is derived from the rheometry experiments by modeling the system as a suspension. This method has some important advantages, i.e., (1) it is applicable to systems where optical microscopy does not work (i.e., colored systems) and (2) it is much easier, faster, and more accurate than optical methods.

SynopsisA model is presented to describe flow-induced crystallization in isotactic polypropylene at high shear rates. This model incorporates nonlinear viscoelasticity, compressibility, and nonisothermal process conditions due to shear heating and heat release due to crystallization. Flow-induced nucleation occurs with a rate coupled to the chain backbone stretch associated with the longest mode relaxation time of the polymer melt, obtained from a viscoelastic constitutive model. Flow-induced nuclei propagate in flow direction with a speed related to shear rate, thus forming shish, which increase the viscosity of the material. The viscosity change with formation of oriented fibrillar crystals (known as "shish") is implemented in a phenomenological manner; shish act as a suspension of fibers with radius equivalent to the radius of the shish plus the attached entangled molecules? The model is implemented in a 2D finite element code and validated with experimental data obtained in a channel flow geometry. Quantitative agreement is observed in terms of pressure drop, apparent crystallinity, parent/daughter ratio, Hermans' orientation, and shear layer thickness. Moreover, simulations for lower flow rates are performed and the results are compared, in a qualitative sense, to experiments from literature. V C 2015 The Society of Rheology.

No abstract

Using dilatometry combined with shear flow at conditions comparable to realistic processing conditions, the flow‐induced crystallization of polymers is modelled. The model describes the kinetics of quiescent nucleation, flow‐enhanced point nucleation, fibrillar growth, and the time evolution of the dimensions of the resulting crystalline structures. The growth rate of nuclei is coupled to the backbone stretch of a mode with a relaxation time representative of the average of the molecular weight distribution. The eXtended Pom‐Pom (XPP) model is used to calculate the backbone stretches from flow conditions. Three important model parameters are determined over a broad range of temperatures, pressures, and shear rates for a fixed shear time; a prefactor to the creation rate of flow‐induced nucleation, a prefactor to the shish growth rate, and the critical molecular stretch defining the transition between flow‐enhanced nucleation and flow‐induced crystallization of oriented fibrilar structures. Excellent agreement is obtained between calculated and experimentally determined crystallization kinetics of iPP. Moreover, the extended experimental dataset leads to an important adaption of the model, i.e., a new criterion for the initiation of shish growth.

A model for flow‐enhanced nucleation is presented based on the concept of a polymer melt containing a fixed number of nucleation precursors with a fixed size distribution. Depending on the size, precursors can either be active (i.e. susceptible to nucleation, the characteristic time scale of which is governed by the deformation rate) and grow into a spherulite or remain dormant. The size distribution of precursors is derived by combining nucleation theory and experimentally determined quiescent spherulite number densities. Longitudinal precursor growth, causing activation of dormant precursors, is a function of molecular deformation: the stretch of high molecular weight chains. Both the eXtended Pom‐Pom and the Rolie‐Poly model are tested to calculate the molecular deformation. A quantitative agreement is found between simulations and experimental results. magnified image