Exploring polymer behavior: from crystallization to filament breakup

With her PhD research Jessica Pepe marked a significant advancement in polymer research. Pepe’s pioneering work focuses on understanding polymers in extensional flow, encompassing processes from crystallization to filament breakup. Her findings shed light on fundamental aspects of polymer behavior, paving the way for improved processing techniques and applications in various industries.

Pepe developed two experimental setups that allow in-situ characterization in extensional flow. One enables in-line microstructural characterizations while following the rheological response to a controlled uniaxial extensional deformation. This opens new horizons in the extensional flow induced crystallization field and helps verifying existing theories as well as introducing new fascinating questions. The other allows to perform experiments on the actual processing flow and to investigate the macrostructure of the dispensed fluid. The results help building new knowledge about the use of high viscous and viscoelastic materials for jetting and provide a better awareness of the challenges as well as of new possibilities.

Understanding polymer behavior

Polymers are widely used in various applications and their behavior and final properties strongly depend on the processing conditions such as temperature, presence of pressure and type of flow. Among these, flow is one of the most difficult parameters to fully understand and control. Two model types of flow are shear and extension. They can be present separately in different parts of the process or simultaneously, generating, in this case, a complex flow. When semicrystalline polymers are processed, crystallization during flow becomes a dominant factor in determining the final properties. In order to characterize and understand the effect of flow on the final properties of a polymer, its rheological behavior (i.e. response to an applied flow field) and structure development in model flows should first be studied.

Challenges in characterization

Shear flow is relatively easy to apply, therefore rheometers have been developed wherein controlled shear experiments with well-controlled temperature profiles can be applied and the material response can be studied. Innovative setups have been developed that capture simultaneously the rheological response as well as the evolution of the crystalline microstructure, thereby utilizing in-situ X-ray characterization. On the other hand, a pure and controlled uniaxial extensional flow is difficult to realize and therefore the challenge to design suitable measurement devices is higher, especially when in-situ structure characterization is required. The existing extensional rheometers do not allow for both controlled uniaxial extensional flows and in-situ structure characterization generating a lack of knowledge in the field.

Investigating material jetting

In free surface flows the interplay between flow, interfacial properties and extensional rheology, for instance during filament stretching and breakup, becomes crucial for understanding and accurately controlling the process. Material jetting is one of such processes during which tiny droplets of a material are deposited, on demand, on a specific position of a substrate. The material choice will affect the droplet formation as well as the deposition mechanism. Despite being a mature and consolidated technique used in different fields, it has a strong limitation in handling very highly viscous fluids like polymers with a consequent scarcity of studies on the matter despite the benefit they could offer for specific applications.

Research goals and findings

Given the limitations of the current technologies to fully characterize uniaxial extensional flow induced crystallization, the first goal of this research is to provide a tool to study the effect of extensional flow on the rheology and crystalline microstructure evolution of semi-crystalline polymers. A filament stretching extensional rheometer has been designed and built in-house with the capability of applying well controlled uniaxial extensional deformations while performing in-situ X-ray experiments. After validation of the measurement capability of the novel setup, the flow enhanced crystallization of a branched semicrystalline polymer has been investigated with a systematic study of the effect of flow parameters (i.e. strain and strain rate) under isothermal conditions. A clear correlation between flow strength, degree of orientation of the polymer chains and crystallization rate was found. Moreover, fitting the experimental scattering data with a model for shish-kebab growth, a detailed analysis of the microstructure development during crystallization was performed.

In the second part of the research, the jetting and deposition process of highly viscous polymers have been investigated with a new generation dispensing machine using a piezoactivated plunger above the dispensing nozzle. The effects of crucial parameters such as fluid viscosity, distance between nozzle and substrate and substrate properties have been studied. A rheological characterization of the fluids together with optical visualization of the filament breakup process and spreading on the substrate reveals new insights of drop on substrate deposition when highly viscous fluids are used. An understanding of the interplay between filament stretching and spreading is key for a full control of the process thereby opening new possibilities in the jetting field.

Title of PhD thesis: Polymers in extension: from crystallization to filament breakup. Supervisors: Prof. Patrick Anderson, Prof. Ruth Cardinaels, and Em. Prof. Gerrit Peters.