Internet Explorer - Powershell - Get document for a newly opened tab

I am trying to figure out how to login to web-pages in Internet Explorer 9 using PowerShell version 2 on Windows 7 x64. When I run these lines one by one in PowerShell, I get exactly what I want - The page opens and I am able to login.

$internet_explorer = new-object -com "InternetExplorer.Application"
$internet_explorer.visible = $true
$internet_explorer.navigate2("https://abcd.efgh.com/ijkl/Login.jsp")
$this_document = $internet_explorer.document
$username_field = $this_document.getElementByID("usernameField")
$password_field = $this_document.getElementByID("passwordField")
$login_button = $this_document.getElementByID("SubmitButton")
$username_field.value = "username"
$password_field.value = "password"
$login_button.click()

But when I try to open another tab and try to login to the new webpage by executing the below lines one-by-one,

$internet_explorer.navigate2("https://mnop.qrst.com/uvwx/Login.jsp", 0x0800)
$this_document = $internet_explorer.document
$username_field = $this_document.getElementByID("usernameField")
$password_field = $this_document.getElementByID("passwordField")
$login_button = $this_document.getElementByID("SubmitButton")
$username_field.value = "username"
$password_field.value = "password"
$login_button.click()

I get the following errors :

Property 'value' cannot be found on this object; make sure it exists and is settable.
At line:1 char:17+ $username_field. <<<< value = "username"+ CategoryInfo          : InvalidOperation: (value:String) [], RuntimeException+ FullyQualifiedErrorId : PropertyNotFound

Property 'value' cannot be found on this object; make sure it exists and is settable.
At line:1 char:17
+ $password_field. <<<< value = "password"+ CategoryInfo          : InvalidOperation: (value:String) [], RuntimeException+ FullyQualifiedErrorId : PropertyNotFound

You cannot call a method on a null-valued expression.
At line:1 char:20
+ $login_button.click <<<< ()+ CategoryInfo          : InvalidOperation: (click:String) [], RuntimeException+ FullyQualifiedErrorId : InvokeMethodOnNull

So when I open a new tab, how can I get Document of that newly opened tab? Please help me solve this

EDIT : Alternative methods will do. Please feel free to suggest any other way to login to multiple web pages automatically (like what was attempted above) if they exist