Here's a very simple and concise bit of JS that I use to animate my collapsible sets.
$(document).on('pageinit', function(){
$("[data-role='collapsible']").collapsible({
collapse: function( event, ui ) {
$(this).children().next().slideUp(150);
},
expand: function( event, ui ) {
$(this).children().next().hide();
$(this).children().next().slideDown(150);
}
});
});
Here's a demo in JSFiddle.