Thermal analysis was performed on poly(buty1ene terephthalate), PBT, between 210 and 560 K. By combination of experimental heat capacities with computations with an approximale frequency spectrum of 65 group and 19 skeletal vibrations, preliminary recommended ATHAS (1988) heat capacities are proposed for the solid state from 0 to 600 K. The Tarasov parametei s used for the computation of the skeletal vibrations were 8, = 542 K and 8, = 80 K for crystalline PBT and O3 = 40 K for amorphous PBT. The glass transition temperature of amorphoL s PBT was found on efficiently quenched samples to be 248 K, much lower than for semicrystaline PBT where a 310-325 K glass transition temperature is typical. The increase in heat capacity calculated for 100% amorphous samples is 107 J/(K . mol) at 248 K and 77 J/(K . mol) at 320 K. The equilibrium melting temperature is estimated to be 518 K. The unique existence c f rigid-amorphous fractions of the semicrystalline polymers is discussed with quantitative data for samples crystallized from the glass and from the melt between 275 and 490 K. The rigidamorphous fraction varies between above 0,9 for cold-crystallized samples to 0,3 for sampk s crystallized at 490 K . The crystallinity varied from below 0,l to 0,5. The crystallinity could t e separated into four parts, melting at high, medium, and low temperatures, and a pal t crystallized on cooling after isothermal crystallization. The sequence of crystallization c f differently melting crystals was established.

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It is shown that heat capacities of linear macromolecules consisting of all-carbon singlebonded backbones can be calculated from the appropriate contributions of substituted carbon atoms to a precision of about -0.2 + 2.5 % (155 data points), which is similar to the experimental precision. Heat capacity contributions of 42 groups are given over the full range of measurement and reasonable extrapolation. The quality of the addition scheme is tested on 16 series of measurements on homopolymers, copolymers and blends. The addition scheme works for all these different states of aggregation of the constituent groups. The basis of the addition scheme is discussed.The possible additivity of heat capacities of different constituents of linear macromolecules has been a topic of long standing interest in our laboratory. A first attempt to establish an addition scheme was published in 1969 [1]. Based on literature data on 30 polymers, it could be shown at that time that from 60 K to the glass transition temperature additivity seemed to exist with an accuracy of 4-5%. In the meantime, our collection of heat capacities of polymers has grown into the ATHAS Data Bank with data on about 100 polymers [2], During the collection of the data bank it became clear that even polymer melts may show additivity [3]. Of particular interest was that copolymer heat capacities could be generated from the heat capacities of the homopolymer constituents [1]. Heat capacities of multiphase polymers, such as partially crystallized polymers [4] and phase separated block copolymers [5] and blends [6] could also be analyzed by comparison with heat capacities derived from additivity of the components [7]. Even the increase in heat capacity at the glass transition was found to be additive and empirically predictable in terms of the rigid atomic groupings, "beads", in the macromolecule [8].