How much of a polymer is rigid
Many plastics are semi-crystalline: ordered, rigid crystals are embedded in disordered, mobile material. The proportion of the two decides stiffness, strength, clarity and processing. In polypropylene, the soluble fraction, which is mostly mobile material, is a classic quality parameter. Time-domain NMR separates rigid and mobile material by how quickly their signals fade, so the ratio is read directly.
Where it is used
- Polypropylene: the soluble fraction, which is classically measured with xylene
- Polyethylene: crystallinity and density
- Blends of an elastomer with a rigid polymer, such as rubber in polystyrene or ABS
- Plastics with plasticisers, additives or residual monomer, as far as they are mobile
- Films, pellets, powders and moulded parts
Typical tasks are checking incoming resins, controlling production against a specification, comparing grades, and following how processing changes the structure.
A calibration against the wet method
The sample is placed in a tube. The signal right after the pulse separates the rigid part, which fades almost at once, from the mobile part, which lasts. A calibration with samples that were also measured by the traditional wet method turns the signal into the quantity you use, for example the xylene-soluble fraction.
In-house methods and their wet references
These measurements are application methods that are set up and calibrated for each material. They are not international standards. The wet reference for the soluble fraction of polypropylene is xylene extraction, for example as described in ASTM D5492, and the NMR result is calibrated against it.
What it adds
- A short measurement instead of dissolving and precipitating the polymer
- No solvent, no heating of solvents and no waste
- Non-destructive, so the sample can be used again
- Suitable for routine control next to the production line
Points that matter in practice
- The NMR result is calibrated against the wet method. It correlates with it but is not identical, so each material and grade needs its own calibration with a few well-characterised samples
- Mobile means mobile at the measurement temperature, so the temperature must be controlled
- Fillers and glass fibres contain no signal-giving hydrogen, so account for the sample mass or the filler content
- Take representative samples. Pellets, powders and films can differ in their structure
- Results are only as comparable as the calibration is stable, so check it regularly with reference samples
From your polymer to a routine method
Send or bring representative samples. MSY Consulting runs feasibility measurements, plans the calibration with you against your wet reference, and trains your team until the method is part of your routine.
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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