Thermodynamic properties of dendrimers compared with linear polymers: General observations

The pressure-volume-temperature and thermal properties of dendrimers based on benzyl ether were measured and compared with literature values for monod isperse, linear polystyrenes. In addition, property measurements are presen ted for an exact linear analogue to the fifth-generation dendrimer. The the rmodynamic properties' molecular weight behavior for the dendrimers is uniq ue when compared with that of linear polystyrene. All of the evidence prese nted in this work suggests that some form of structural transition occurs i n the bulk at a molecular mass near that for the fourth-generation dendrime r. No such transition is seen for polystyrene. Dendrimers exhibit an increa sed packing efficiency as evidenced by a decreased specific volume (increas ed density) as compared with an exact linear analogue of the fifth-generati on dendrimer analogue, and the dendrimer highlights the entropic difference s between the two molecules. In addition, differences in the change in heat capacity with temperature for the two systems further allude to their entr opic differences. A crystalline state can be formed for the lower generatio n dendrimer and linear analogue. This crystalline state is not seen in dend rimers above the third generation. These behaviors compiled with the differ ence in the glass-transition temperature for the linear analogue suggest th at the dendrimers' microstructure has a significant influence on the bulk t hermodynamic state of the material. The Tait equation was fitted to the vol ume data for comparative purposes; the Tait equation has known limitations but was selected because of its widespread application. (C) 2001 John Wiley & Sons, Inc.