A thermal ink-jet pen cartridge having a rigid external frame member fabricated of a rigid engineering plastic, and an interior frame structure attached thereto, formed of a softer, more ductile plastic material than the rigid engineering plastic. An ink reservoir is defined by the interior structure and a pair of flexible bag membranes formed of a plastic material impervious to the ink and compatible with the interior member material, allowing the membranes to be joined to the interior member to form a leak-proof joint The interior frame structure is fabricated from a polymer blend composition comprising (A) at least one polymer selected from the group consisting of a polyethylene and a copolymer of ethylene and an alpha-olefin and (B) at least one polymer selected from the group consisting of a polypropylene and a copolymer of propylene and ethylene, wherein (A) is different from (B). The present polymer compositions provide good chemical and heat stabilities and solvent resistance when subjected to inks used in printing. Further, the present polymer compositions are flexible and have excellent compatibility and adhesion to the various polymeric components used in the ink-jet cartridge.