try to work with http://codex.wordpress.org/Function_Reference/get_adjacent_post;
example:
$next_post_link_url = get_permalink( get_adjacent_post(false,'',false)->ID );
$prev_post_link_url = get_permalink( get_adjacent_post(false,'',true)->ID );
try to work with http://codex.wordpress.org/Function_Reference/get_adjacent_post;
example:
$next_post_link_url = get_permalink( get_adjacent_post(false,'',false)->ID );
$prev_post_link_url = get_permalink( get_adjacent_post(false,'',true)->ID );