Mathf.Pingpong without the repetition!?

Using this to change materials in a transtition....but would like it to stop reverting back to the main material1 after. Is there another way other then Mathf.Pingpong or a way to stop if after getting material2???

   // Blends between two materials
     
     var material1 : Material;
     var material2 : Material;
     var duration = 5.0;
     
     function Start () {
     
         // At start, use the first material
         renderer.material = material1;
         
     }
     
     function Update () {
     
          // ping-pong between the materials over the duration
          var lerp : float = Mathf.PingPong (Time.time, duration) / duration;
          renderer.material.Lerp (material1, material2, lerp);
         
     }