From what I've been reading, US aid to Ukraine has never really been that much money. Maybe a few billion USD at best. So I don't believe the US will bail out Ukraine if it starts to default.
Also, the EU said last October that they wanted to speak to a new government in Kiev before throwing any more money at Ukraine. They weren't happy with Kiev's "unrealistic" 2014 budget.
The article also noted that Kiev was spending way too much on it's military operations.
Ukraine unlikely to receive IMF loan tranche this year: finance minister | Reuters

So yeah, if the conflict continues much longer then I do think the Ukraine will default. I'm only surprised it hasn't defaulted already since the EU only has so much money it can offer in loans.
Also, Kiev has already forced as much money as it can from the Ukrainian citizens. Poroshenko was booed in Lviv over all the huge taxes and the other Ukrainians probably aren't very happy either.