midi volume single note

[Gianmaria Lari]

I have a long note that I would like it starts playing \ffff and it ends \pppp (in the midi).

I tried this but without success:

\version "2.19.80"

right = \relative c'' { c1 ~ c1 }

dynamics = { s4\fffff\> s4 s4 s4

             s4 s4 s4 s4\! \ppppp }

\score {

  \new Staff \with { midiInstrument = "accordion"} 

    \new Voice <<\right \dynamics>>

  \midi {}

  \layout {}

}

Did I make any error or does exist any escamotage to do it?

Thank you, g.