Buoyancy-Induced Convection Driven by Frontal Polymerization

In this Letter, we study the interaction between a self-sustaining exothermic reaction front propagating in a direction perpendicular to that of gravity and the buoyancy-driven convective flow during frontal polymerization (FP) of a low-viscosity monomer resin. As the polymerization front transforms the liquid monomer into the solid polymer, the large thermal gradients associated with the propagating front sustain a natural convection of the fluid ahead of the front. The fluid convection in turn affects the reaction-diffusion (RD) dynamics and the shape of the front. Detailed multiphysics numerical analyses and particle image velocimetry experiments reveal this coupling between natural convection and frontal polymerization. The frontal Rayleigh (Ra) number affects the magnitude of the velocity field and the inclination of the front. A higher Ra number drives instability during FP, leading to the observation of thermal-chemical patterns with tunable wavelengths and magnitudes.

Percolation Implications in the Rheology of Polymer Crystallization

The rheology of polymer crystallization is an old problem that often defies explanation due to the complex interrelationships between crystallization and flow properties. Although separate measurements of rheology and crystallinity can give some information on their relationship, it is only through simultaneous measurements that ideas on the rheology of polymer crystallization can be tested and developed. This Perspective details recent experimental developments in simultaneous crystallinity and rheology measurements as well as continuum modeling efforts for the case of quiescent and isothermal crystallization. Experimental results reveal that the rheology is dominated initially by growth of individual spherulites that evolve into spherulitic superstructures that eventually span the measurement geometry. A generalized effective medium model based on this concept of percolation can explain both the growth of the viscoelastic modulus during crystallization and the changes in the relaxation spectrum of the crystallizing polymer, including a critical gel response at percolation. The success of the combined measurement techniques and percolation concepts motivate research to extend the semicrystalline polymer materials space where these methods are applied as well as further develop novel techniques to gain additional insight into the evolution of structure and relaxation dynamics during crystallization.

Rheology of crystallizing polymers: The role of spherulitic superstructures, gap height, and nucleation densities

A longstanding goal in polymer rheology is to develop a physical picture that relates the growth of mechanical moduli during polymer crystallization to that of a structure. Here, we utilize simultaneous mechanical rheology and optical microscopy, with augmentation by deterministic reconstruction and stochastic simulations, to study isothermal crystallization in isotactic polypropylene. We observe the nucleation and growth of the surface and bulk spherulites, which are initially isolated and then impinge to form clusters and superstructures that eventually span the gap. We find that spherulitic superstructures play a critical role in the rheology, especially in the characteristic sharp upturn in moduli. Both the rheology and the spherulitic superstructures show pronounced gap dependencies, which we explain via finite-size effects in percolation phenomena and via surface-induced nucleation. The modulus-crystallinity relationship can be described through a general effective medium theory. It indicates that for thicker gaps, the viscoelastic liquid to solid transition can be described via percolation, whereas for our thinnest gap, it is best described by the linear mixing rule. We describe our results in terms of dimensionless nucleation rates and spherulite size, which enable the estimation of when gap-dependent superstructure effects can be anticipated.

Analysis of the No-Flow Criterion Based on Accurate Crystallization Data for the Simulation of Injection Molding of Semi-Crystalline Thermoplastics

It is well known in practice that the shape and dimensions of injected parts are highly dependent on the packing-holding stage. A major problem in semi-crystalline polymers is the prediction of the solidified layer, whose thickness has an important effect on shrinkage and warpage. We propose a pragmatic approach based on the concept of no-flow temperature. This temperature should be related to crystallization temperature, but the choice is not easy because it depends on cooling rate and pressure which are functions of time and position. The objective of the work is to evaluate the sensitivity of an injection molding computation to the no-flow temperature and to evaluate the relevance of its choice. The crystallization temperature of an isotactic polypropylene is determined as a function of cooling rate and pressure in laboratory experiments. The pressure dependence is measured using the original Cristapress cell. As a case study, we simulate the filling and post-filling of a plate mold using Rem3D, a 3D code for injection molding. Three no-flow temperatures and two sets of parameters for temperature dependence of viscosity are tested. Their respective influences on the pressure evolution are shown, and the crystallization temperature calculated a posteriori using the experimental material data is compared to the “arbitrary” no-flow temperature.

Crystallization of Polymers in Processing Conditions: An Overview

In polymer processing, crystallization generally occurs in complex, inhomogeneous and coupled mechanical (flow, pressure), thermal (cooling rate, temperature gradient) and geometrical (surface of processing tools) conditions. A first route to understand crystallization in processing conditions is to design model experiments to isolate the specific influence of a given parameter. The emphasis will be laid here on the influence of: (i) shear flow through rheo-optical measurements using the commercial RheoScope module, (ii) high cooling rates obtained with the modified hot stage Cristaspeed (up to 2 000 °C min−1) and (iii) high pressures in the original Cristapress cell (up to 200 MPa). Numerical simulation is also a useful tool to understand and predict the coupled phenomena involved in crystallization. Based on Avrami's ideas and equations, a general differential formulation of overall crystallization kinetics has been proposed by Haudin and Chenot (2004). It is able to treat both isothermal and non-isothermal cases, and has been extended to crystallization in a limited volume without and with surface nucleation inducing transcrystallinity.