My NSString data didnt tranfer over to a view controller

My NSString data didnt tranfer over to a view controller

In my App I'm using a Navigation controller. My problem is, when I go to
previous view (using the generated navigation back button) my NSString
data doesn't transfer over. I don't know how to save these NSString values
when I navigate through the stack from navigation controller.I need to
send an email with these NSString values.
This is my original view controller before pushing back button
-(void)button{
NSString *dd = @"text";
viewcontroller.string = dd;
}
This is the previous controller after pushing the back button (view
controller) .h
@property (strong,nonatomic) NSString *string;
.m
NSString *emailBody = [NSString stringWithFormat:@"%@ ",
string];