As Chris' note suggested, this problem is related to the default properties under Glassfish Resources. In my case, the default connection pool, DerbyPool, comes pre-set with certain properties that appear to have superseded those specified in the persistence.xml file. To see the properties, log in to the Glassfish Admin Console and go to Resources> JDBC> Connection Pools> DerbyPool >Additional Properties.
After a failed attempt to delete the properties in glassfish and use the properties in my persistence.xml instead, I ended up deleting many of the properties in my persistence.xml and just using the glassfish properties. I also found this tutorial to be of assistance:
http://wiki.eclipse.org/EclipseLink/Examples/JPA/GlassFishV2_Web_Tutorial#GlassFish_Configuration_Changes
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