From liquid to network
In a liquid, molecules move freely and the NMR signal lasts. As a network forms, movement is restricted and the signal fades faster. A relaxation time is therefore a measure of how mobile the molecules are, and it can be followed while the sample changes.
Where it is used
- Gels and hydrogels, including those for biomedical use
- Polymer networks and elastomers
- Biopolymer powders and films that take up water
- Biological samples and food-like soft materials
Typical tasks are following the gelling, swelling or ageing of one sample over time, comparing formulations, and relating mobility to mechanical or release properties.
Rigid and mobile parts, and several kinds of water
The signal right after the pulse separates a rigid part, which fades almost at once, from a mobile part, which lasts. The ratio of the two is a direct measure of how rigid the sample is. Water in the sample often shows several populations with different mobility, from water held tightly by the polymer to water that moves almost freely.
One dimension, then two
When the signal contains several components, an inverse Laplace transform turns the decay into a distribution of relaxation times, with one peak for each population of water or polymer. This is the one-dimensional version, and it already shows how many states are present and how much of the sample is in each.
Measuring T1 and T2 together and inverting in two dimensions gives a map. The position of a peak shows how fast the molecules move, and its distance from the diagonal, where T1 equals T2, shows how strongly their motion is held back. Water that is tightly bound to a polymer sits far from the diagonal, and free water close to it.
What it adds
- Non-destructive, so the same sample can be followed from start to end
- No labels, dyes or special sample preparation
- Works on opaque, coloured and thick samples
- Different formulations are compared on the same scale
Points that matter in practice
- Relaxation times depend on temperature, concentration and pH, so compare samples under the same conditions
- Decays with several components need careful analysis. The number of water populations is partly a matter of the model used
- The inverse Laplace transform needs good signal quality and uses smoothing, so peaks should be judged together with the settings that produced them. Maps in two dimensions need longer measurements and suitable software
- Confirm the interpretation with a second technique, such as rheology or calorimetry, when the result will be published
From your question to a routine method
Describe your materials and your question. MSY Consulting runs feasibility measurements, shows what the technique can and cannot tell you in your case, and helps you set up the analysis and the sample handling.
Let's talk about your application
Send an email describing your instrument and your application, and you will get a reply.
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