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PFG Alumni: Dora Medina

Dora Medina

The Development of Microcapillary Films and Microcapillary Monolith Structures to produce controlled capillary diameters and voidage

Introduction 

Dora's research concerned the manufacture and hole size reduction of Microcapillary Films (MCFs), a novel type of plastic film containing one-dimensional microchannel arrays with original diameters between 30 and 500 µm. Microcapillary film (MCF) extrusion is a new, innovative, product from the Polymer Fluids Group at the Department of Chemical Engineering, University of Cambridge. The overall aim of this project was to evaluate the melt drawing characteristics of MCFs. 

In addition, she also investigated the formation of Microcapillary Monoliths (MCMs). These are solid polymer blocks with two-dimensional arrays of microcapillaries that are manufactured by heat melding together multiple layers of MCFs. The optimal conditions for interface adhesion of the MCFs to create MCMs without compromising capillary integrity and the influence of compatibility between MCFs were studied.

LLDPE Rheology

The melt processing properties of the linear low density polyethylene (LLDPE) that the MCFs were prepared from was investigated by performing shear rheometry. An Advanced Rheometric Expansion System rheometer (ARES-LC model 5450DT) was used to carry out these experiments. In order to establish the temperature dependence of the viscoelastic properties in the solid and the melt, temperature sweep experiments were performed at strains of 1% and 10% respectively, at a frequency of 10 rad/s. The experiment performed in the solid state used torsional rectangular fixtures, while those conducted in the melt state used 25mm parallel plates. The measurements were carried out within the linear viscoelastic region of the material.

Variation of the dynamic rheology of LLDPE with temperature
Plot showing variaition of the variation of the dynamic rheology with temperature between 10 and 160°C for linear low density polyethylene NG5056E

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Melt drawing characteristics of MCFs

The drawing of MCFs was investigated to optimise their properties. The drawing process is a function of temperature, polymer (molecular weight distribution and degree of crystallinity), drawing rate, geometry and tensile load dimensions. In order to find the temperature dependence of the viscoelastic properties in the solid and the melt, rheological studies were carried out (see above). These were crucial to understanding quantitatively the deformation of the structures during melting and in the solid state. X-ray diffraction and thermal analysis studies were conducted to determine the molecular orientation of the drawn films.

Schematic diagram of mcf drawing process
Schematic diagram showing the continuous drawing process

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Heat Melding of MCFs to form MCMs

Interfacial bonding is important in relation to the adhesion of polymers and bonding of polymer interlayers. Investigation of the influence of compatibility of MCFs on melt processing, micro-structure and mechanical properties and their mutual interactions are critical for interfacial bonding. In this work, a model for the welding process was jointly developed with Professor F. Chinesta of ENSAM, Paris, starting from a fundamental formulation of the reptation of polymer chains.

The multilayer structure of MCMs is created by hot pressing layers of MCFs together at a temperature close to the melting point of the polymer. Experiments on formation of high voidage MCMs and standard MCMs were carried out to investigate the effect of temperature and compression ratio variation. The thickness of the MCM is a function of the number of bonded microcapillary films. The MCMs were produced using a commercial grade of linear low density polyethylene (LLDPE).

Diagram showing the optimum conditions for MCM formation
A schematic diagram showing the optimum MCM formation conditions for pressing time of 5 minutes.

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