Described herein are polymer compositions including a polyamide blend and glass fibers. The polyamide blends include an aliphatic polyamide and a semi-aromatic polyamide. The aliphatic polyamide is obtained by a ring-opening polymerization. It was surprisingly discovered that the polymer compositions had improved mechanical performance (e.g. tensile and impact performance), relative to corresponding polymer compositions where the aliphatic polyamide is obtained by polycondensation. Due at least in part to the improved mechanical performance, the polymer compositions can be desirably incorporated into mobile electronic devices.