Disclosed is a polyester polymer composition comprising at least one polyethylene terephthalate polyester in an amount ranging from 94.0 weight percent to 99.5 weight percent and at least one additive in an amount ranging from 0.5 weight percent to 6 weight percent, each based on the total weight of the polyester composition. The at least one additive comprises a diamide molecule. The composition of the present invention is useful in producing shaped articles such as, for example, sheeting, films, tubes, bottles, preforms, and profiles. These articles can exhibit improved gas barrier properties. The process for making the polymer composition and the shaped articles is also disclosed.