Methods for preparing thermally stable lignin fractions

A method of fractionating a lignin sample into stable high purity lignin fractions having a consistent glass transition temperature, the method comprising:

• heating the lignin sample in an organic solvent for 0.5 to 24 hours to form a resulting biphasic mixture, wherein the mixture comprises:
• a) a remainder solid designated as solvent insoluble (SI) lignin fraction comprising a first fraction of the lignin sample, and
• b) a liquid solution comprising the organic solvent and a second fraction of the lignin sample, wherein the second fraction is designated as solvent soluble (SS) lignin fraction;
• spatially separating the SI lignin fraction from the SS lignin fraction;
• evaporating the organic solvent from the SS lignin fraction;
• and
• drying each fraction to obtain a dry solid SS lignin fraction and a dry solid SI lignin fraction;
• wherein the two dry solid lignin fractions have different mass average molar masses (MW) as measured by gel permeation chromatography (GPC), and
• wherein at least one of the lignin fractions has the consistent glass transition temperature (Tg) as determined by two differential scanning calorimetry (DSC) runs of a same lignin portion in a single day according to DIN 53765-1994,
• wherein a first Tg is measured during the first DSC run, a second Tg is measured during the second DSC run, and the difference between the first Tg and the second Tg is less than 5° C.