I want to synthesize a polyurethane elastomer based on pTHF (1000g/mol), MDI and BDO via two-step method. The molar ratio are 0.3 / 1 / 0.7 respectively. There is someone that can calculate the hard segment content for this ratio, and that can show me an equation to obtain %HS for any reagent ratio?
Thank you for all the answers.
Hi fabio. You can check this web page of dow chemical. the best.
http://dowac.custhelp.com/app/answers/detail/a_id/13359/~/dow-polyurethanes---functionality%2C-oh-number%2C-and-equivalent-weight-definitions
I am assuming that your MDI is almost purely 4,4'-MDI (since you're doing a two-step process), which has a functionality of 2. Many people would just say that the BDO portion and the MDI portion combined are the hard segments. So %HS = (m_BDO + m_MDI) / (m_BDO + m_MDI + m_pTHF). This would be the upper bound for the ideal case. In reality, the %HS would be slightly lower due to the following factors:
1) For your first step (NCO prepolymer synthesis by reacting pTHF with MDI), there is always some small amount of chain extension of pTHF by MDI -- that is, a pTHF-MDI-pTHF linkage. That linkage would stay in the soft segment (you can confirm by reacting a little bit (stoichiometrically) of MDI with lots of pTHF, and you'll still get a liquid, and should see no phase separation in any technique). So that small amount would have to be subtracted from the %HS. How do you do that? You'd have to analyze your NCO prepolymer. --> I'm not sure anyone does this however.
2) Your shortest MDI-BDO hard segment may not be part of the "hard domain" but rather dissolve with pTHF and be part of the "soft domain" -- the linkage I have in mind is pTHF-MDI-BDO-MDI-pTHF. This isn't long enough to cause phase separation according to Christenson et al. (reference link below)
So those two factors would cause a discrepancy between the %HS calculation and the actual %HS in the hard domain. It's probably fine to report the %HS calculation (but explain in your manuscript how you calculated it -- many papers do not state it, which is the source of many moments of confusion).
http://onlinelibrary.wiley.com/doi/10.1002/polb.1986.090240703/full
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