unpaid vs due

unpaid is not a noun while due is a noun.

Word	Noun	Adjective	Verb	Adverb
unpaid	No	Yes	No	No
due	Yes	Yes	No	Yes

unpaid (noun)	due (noun)
	that which is deserved or owed
	a payment that is due (e.g., as the price of membership)

unpaid (adjective)	due (adjective)
not paid	owed and payable immediately or on demand
engaged in as a pastime	suitable to or expected in the circumstances
without payment	capable of being assigned or credited to
	scheduled to arrive

unpaid (adverb)	due (adverb)
	directly or exactly; straight