Image Copyright © 2001 Greg Ford

The experimental techniques employed in this project included dilatometry, reaction sampling and gel permeation chromotography. A new dilatometer was constructed for me by the CSIRO Molecular Science workshops. Here are some photos of the new dilatometer and some information about how it works. This technique is coupled with gamma initation of the polymerisation to determine the kinetic parameters of the system.

Dilatometry

Put simply, dilatometry is the process of measuring the volume change of a sample. In this project, the volume change of an emulsion polymerisation was measured by following the height of a meniscus in a capillary with a computer controlled tracker. The principle used in these experiments is that the polymer being formed is more dense (to takes up less volume) than the monomer from which it is being made. Following the rate of volume decrease thus gives you a way of measuring the progress of the reaction.

The meniscus is detected using a light source (red LED) and a phototransistor as shown below. The signal detected is considerably different above and below the meniscus, allowing the position to be accurately measured. A stepper motor, gear box and threaded rod are used to move the platform on which the light detector sits. The precision of these controls allows the position of the meniscus to be measured to the nearest 0.001 mm. That's pretty accurate...

Finally, an electronics box is used to convert the signal from the light detector into something usable by the controlling computer (i.e. "above" or "below") and also to control the stepper motor.

The entire assembly (from left to right in the picture below) consists of

• the water bath to control the temperature of the reaction
• the computer to control the movement of the light sensor platform and to log the data being collected
• the mechanical rig to measure the meniscus height (with the small, glass reactor, capillary and magnetic stirrer in place)
• the electronics box to interface to the computer

Gamma relaxation experiments

In gamma-initiated polymerisations, the gamma radiation is used to generate OH and H radicals which can then initiate polymerisation as in a regular chemically initiated reaction. The advantage of the gamma experiment is that initiation can be turned off at will by removing the sample from the gamma cell. In this project, this was done using a mechanical lift device.

The usual experimental approach is as follows: the reaction mixture is assembled in a thermostatted dilatometer as for the chemically iniated experiments. The dilatometer was lowered into a gamma cell to initiate polymerization. Once the reaction appeared to have reached a constant rate of polymerization, the dilatometer was removed from the gamma source. Once the out-of-source steady state had been reached, the dilatometer was re-inserted into the gamma source and the relaxation procedure was repeated up to eight times.

Dilatometry data for gamma-relaxation experiments can be designed to provide extensive data: numerous relaxations, approaches to steady states, as well as both the in-source and out-of-source steady states may be observed.