Combinatorial polymer phase behaviour

Mixing of two or more polymers has long been known as an inexpensive and effective tool to alter the physical, rheological and mechanical properties of polymer products.  In recent years, an increased number of block-copolymer, (nano)particle and surfactant mixtures have been employed to improve the interfacial properties (adhesion, tension) and to control the morphology of polymer composites.  Phase behaviour and separation and morphology of polymeric systems remains of fundamental importance and has triggered a wide interest among both theoreticians and industrial researchers.  One of the major challenges in elucidating polymer phase behaviour phenomena is the significant amount of experimental work required to generate data.  Normally, data for a single blend system takes several weeks to collect.  In this project, we seek to develop a novel high throughput combinatorial method which facilitates rapid, parallel studies of polymer phase behaviour.

Combinatorial polymer mixtures are generated with a programmable liquid dispenser and deposited onto custom-made microwell fabricated by contact photolithography. The dimensions and volume of the microarrays are chosen in function of the measurement technique. Optical turbidity and light scattering are employed to determine the phase boundaries and the mixture microstructure (nucleation & growth and spinodal decomposition). The location of the phase boundaries, the mechanisms and kinetics of phase separation are therefore readily determined.

The large parameter space defining multi-component polymer stability requires high-throughput approaches if significant progress is expected. In addition, the slow kinetics of polymer phase separation permits sequential (scattering) experimentation, since the process evolves over minutes-hours thereby allowing ample time between individual measurements. One of the challenges faced concerns automation of the data analysis and definition of phase separation criteria. For turbidity measurements, the onset of intensity increase/decrease is an adequate criterion for the (apparent, rate-dependent) location of the phase boundaries but for scattering (e.g., polydisperse droplets) automation of the structure factor could be seriously complex.

The turbidty setup is extremely simple, involving only a hot stage, CCD camera (high bit-depth) and a flood light source. The data acquisition and analysis is done with LabVIEW, involving taking a picture synchronously with heating/cooling rate and defining individual regions-of-interest (ROIs) where the intensity data is individually intergrated for each microwell. That data is then plotted as a function of temperature and the onset defines the location of the phase boundary for that rate. The location of the binodal line is extrapolated to zero heating/cooling rate (equilibrium) using a series of curves obtained at slow rates (below approximately 1 °C/min).

References

A. Chan and J. T. Cabral, in preparation.

J. T. Cabral and A. Karim, "Discrete Combinatorial Investigation of Polymer Mixture Phase Boundaries", Meas. Sci. Technol. 16, 191 (2005). - Cover article.

A. I. Norman, J. T. Cabral, A. Karim, and E. J. Amis, "Scattering Measurements for High Throughput Materials Science Research", Macromol. Rapid Comm. 25, 307, (2004).