Changing an Animation duration has no effect

import QtQuick 2.6
import QtQuick.Window 2.2

Window {
    visible: true
    width: 640
    height: 480

    SequentialAnimation {
        id: mainAnim
        running: true
        ScriptAction {
            script: {
                targetAnim.duration = 10000
            }
        }

        PropertyAnimation {
            id: targetAnim
            target: aRectangle
            property: "width"
            from: 100
            to: 1000
            duration: 1000
            onDurationChanged: console.log(duration)
        }
    }

    Rectangle {
        id: aRectangle
        width: 100
        height: 100
        color: "yellow"
    }
}