bend vs unbend

bend is a noun but unbend is not a noun.

Word	Noun	Adjective	Verb	Adverb
bend	Yes	No	Yes	No
unbend	No	No	Yes	No

bend (noun)	unbend (noun)
curved segment (of a road or river or railroad track etc.)
diagonal line traversing a shield from the upper right corner to the lower left
movement that causes the formation of a curve
an angular or rounded shape made by folding
a circular segment of a curve

bend (verb)	unbend (verb)
cause (an object) to assume a crooked or angular form	release from mental strain, tension, or formality
bend a joint	become less tense, rest, or take one's ease
form a curve	make less taut
change direction	free from flexure
turn from a straight course, fixed direction, or line of interest	unfasten, as a sail, from a spar or a stay
bend one's back forward from the waist on down	straighten up or out; make straight