This model also accounts
fortheroleoffreevolumeandvolumechangeonmixing.Amodelthatexplicitly
accounts for polydispersity in molecular weight, heterogeneity in chemical
composition, and stiffness of the main chain has been developed by Koningsveld
and Staverman (1968). Sanchez modified the Flory Huggins model to include
negative heat and volume change on mixing (Sanchez and Lacombe, 1978).