"I wasn't able to divide the data into time intervals, as it was already quite statistics-limited. Also, the space radiation environment is not that bad, giving something well under 1 milliGy/day, so even over 2 years the plastic has probably integrated something less than 1 Gy from GCRs. Solar particles will have added to that, but the sun has been pretty quiet."
1 Gy of radiation is not a whole lot compared to the 25 kGy that is typically used to sterilize medical devices, but at the same time, it is enough to cause some damage. I need to do some more digging. Radiation damage of different plastics has been studied over the past decades and the results have been tabulated, so it is not necessary to hit the lab just yet. Off to the Library! I'll report back on what I find.