iterating through data and creating new data frame

I am working with a data frame that comes from the database in the following way:

username    elements
username1   """interfaces"".""dual()"""
username1   """interfaces"".""f_capitalaccrualcurrentyear"""
username2   """interfaces"".""dnow_completion"",""interfaces"".""dnow_s_daily_prod_ta"""
username2   """interfaces"".""dnow_completion"",""interfaces"".""dnow_s_daily_prod_ta"""
username2   """interfaces"".""dnow_completion"",""interfaces"".""dnow_s_daily_prod_ta"""
username4   """interfaces"".""dnow_s_downtime_stat_with_lat_long"""
username3   """interfaces"".""dnow_completion"",""interfaces"".""dnow_s_daily_prod_ta"""

So, two columns, a "username" and "elements". So there can be one element or several elements the user has used in one transaction. When multiple elements, they are separated with a comma in a transaction. I need to have the elements separated, one per row, but still tagged with the user name. At the end I'd like it to be like so:

username    elements
username1   """interfaces"".""dual()"""
username1   """interfaces"".""f_capitalaccrualcurrentyear"""
username2   """interfaces"".""dnow_completion""
username2   ""interfaces"".""dnow_s_daily_prod_ta"""
username2   """interfaces"".""dnow_completion""
username2   ""interfaces"".""dnow_s_daily_prod_ta"""
username2   """interfaces"".""dnow_completion""
username2   ""interfaces"".""dnow_s_daily_prod_ta"""
username4   """interfaces"".""dnow_s_downtime_stat_with_lat_long"""
username3   """interfaces"".""dnow_completion""
username3   ""interfaces"".""dnow_s_daily_prod_ta"""

I have been trying to iterate through the data frame, split the elements that have commas and then put them back together with the respective user name.

I have been trying the code below but it is super inefficient. I am new to "R" so my guess is that there has to be a more efficient way to do this.

interface.data <-data.frame(
    username = c(),
    elements = c()
)
for (row in 1:nrow(input)) { ##input is the frame that comes from the database
     myrowbrk<-input[row,"elements"]
     myrowelements<-chartr(",", "\n", myrowbrk)      
     user<-input[row,"username"]
     interface.newdata <- data.frame(
         username = user,
         elements = c(myrowelements)         
     )
     interface.final<- rbind(interface.data,interface.newdata )
}

output<-interface.final