DSC Crystallinity Calculator
Convert a melting enthalpy into degree of crystallinity, subtracting cold crystallisation and correcting for inert filler.
Learn more
What it does
Crystallinity controls stiffness, barrier performance, opacity and shrinkage in a semicrystalline polymer, and DSC is the cheapest way to measure it. The calculation is one division, but two corrections sit on either side of it and both are routinely skipped. Skipping them is how the same pellet of resin can be reported as 26 percent crystalline by one laboratory and 34 percent by another.
How it works
Crystallinity is the ratio of the measured melting enthalpy to the enthalpy a fully crystalline sample would show: Xc = (ΔHm − ΔHcc) / ΔHm°. The cold crystallisation term matters because a quenched sample is largely amorphous on arrival, and the crystals it grows during the heating scan appear as an exothermic peak below the melting endotherm. Those crystals are new rather than pre-existing, so their enthalpy has to come off the top. The filler correction is simpler: divide by the polymer mass fraction, because the instrument weighs the whole sample.
Worked example
A PET sample gives ΔHm = 48.0 J/g and ΔHcc = 12.0 J/g. Against a reference of 140 J/g the net enthalpy is 36.0 J/g and Xc = 25.71 percent. Ignoring the cold crystallisation peak gives 34.29 percent, an error of 8.6 percentage points in a single step. Now add 25 percent glass fibre, which never melts: the same raw numbers become 64.0 J/g and Xc = 34.29 percent, because the polymer makes up only three quarters of the pan.
When to use it
Use it to convert a raw DSC trace into the number a datasheet quotes, to compare a quenched and a slow-cooled variant of the same resin, or to check whether a claimed crystallinity is even reachable at the filler loading involved. It is also the fastest way to catch an integration mistake, since a cold crystallisation enthalpy larger than the melting enthalpy is physically impossible.
FAQ
- Why do I have to subtract the cold crystallisation enthalpy?
- Because that peak represents crystals formed during the scan rather than crystals that were present when the sample went into the instrument. A quenched sample arrives largely amorphous; as it is heated past the glass transition it crystallises, releasing heat, and only then melts. Dividing the melting peak by the reference enthalpy without subtracting counts those new crystals as though they had always existed, so the crystallinity comes out too high.
- How much difference does the cold crystallisation correction actually make?
- For a PET sample with a melting enthalpy of 48.0 J/g, a cold crystallisation enthalpy of 12.0 J/g and a 140 J/g reference, the correct answer is 25.71 percent while the uncorrected answer is 34.29 percent, a gap of 8.6 percentage points. The gap scales with thermal history: a slowly cooled part has almost no cold crystallisation peak and the correction is negligible, whereas a quenched film can show a gap of twenty points or more.
- Why do published reference enthalpies differ between sources?
- Because the value depends on which crystal form is present and on how perfect the crystals are. Isotactic polypropylene has alpha, beta and gamma forms with different reference enthalpies, and polyamide 6 differs between its alpha and gamma forms. Reported values also carry the systematic error of whichever calibration standard was used. The practical consequence is that a crystallinity measured against one reference is only strictly comparable with another measurement that used the same one, and a measured value for your own grade beats any table.
- My filler-corrected crystallinity is higher than the uncorrected one. Is that right?
- Yes, and it is not an artefact. The instrument reports enthalpy per gram of the whole sample. If a quarter of that sample is glass fibre, then a quarter of the mass contributes to the denominator while contributing no crystallinity at all, so dividing by the polymer mass fraction raises the result. The uncorrected number is not wrong as a property of the composite, it simply answers a different question from degree of crystallinity of the polymer.