Free volume microstructure of tetramethylpolycarbonate at low temperatures studied by positron annihilation lifetime spectroscopy: a comparison with polycarbonate

Free Volume Micro Probe Study of Silver Ions Implanted in Polycarbonate

Positron lifetime, UV-VIS absorption and differential scanning calorimetric (DSC) measurements were performed on pristine polycarbonate (PC) and Ag2+-ion-implanted polycarbonate before and after isochronal annealing treatment. UV absorption results indicated the formation of silver clusters in the polymer matrix. The cluster sizes and number of atoms in these clusters were evaluated from UV results. DSC scans for the sample before and after implantation revealed the diffusion of silver ions in the matrix. Further, these results reveal that the Tg of the implanted PC decreased from its pristine value before annealing whereas it increased after annealing. This was inferred to be due to the formation of cross-links by Ag2+ ions when implanted but after annealing, the cross-link seemed to decrease, which resulted in increased diffusion. Positron results as a function of annealing temperature revealed that small size clusters may have been formed in the available free volume and there was migration of silver atoms in the polymer matrix due the rearrangement of the polymer chains. From the positron results the activation energy for the diffusion of silver atoms was determined to be 0.6 eV.

Effect of chemical substituents on the structure of glassy diphenyl polycarbonates

Polycarbonates offer a wide variety of physical property behavior that is difficult to predict due to complexities at the molecular scale. Here, the physical structure of amorphous glassy polycarbonates having aliphatic and cycloaliphatic chemical groups is explored through atomistic simulations. The influence of chemical structure on solubility parameter, torsion distributions, radial distribution function, scattering structure factor, orientation distributions of phenylene rings and carbonate groups, and free volume distributions, leading to interchain packing effects, are shown. The effect of the cyclohexyl ring at the isopropylidene carbon as compared to the effect of the methyl groups positioned on the phenylene rings results in a larger reduction in the solubility parameter (δ). The interchain distance estimated for polycarbonates in this work is in the range of 5-5.8 Å. The o-methyl groups on the phenylene rings, as compared to a cyclohexyl ring, lead to higher interchain distances. The highest interchain distance is observed with a trimethylcyclohexylidene group at the isopropylidene carbon. Atomistic simulations reveal two different types of packing arrangement of nearest-neighbor chains in the glassy state, one type of which agrees with the NMR experimental data. The fundamental insights provided here can be utilized for design of chemical structures for tailored macroscopic properties.