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Melting points were determined for several series of linear polyesters containing substituents either in the acid or glycol unit. The number of substituents was progressively increased by preparing copolyesters containing both unsubstituted and substituted glycol or acid units. The following systems were studied: (1) Those with trimethylene and 2‐methyltrimethylene glycols show decreasing melting points as the number of methyl groups is increased, because they introduce tetrahedral asymmetry. If the structrual unit is relatively short, crystallinity is completely destroyed. (2) Those with trimethylene and 2,2‐dimethyltrimethylene glycols show eutectic type melting point curves, since the substituent groups do not introduce asymmetry. The completely substituted polyesters have new crystal structures which give sharp X‐ray patterns. Those from short acids (C2–4) have higher melting points than the unsubstituted polyesters, while the others have lower melting points. The polyesters with an odd number of chain atoms in the unit have higher melting points than the even ones. The ability to pack closely into the crystal lattice appears to be important, a view supported by melting points of certain polyesters of β,β‐dimethylgultaric acid. (3) Those with trimethylene and 2‐methylenetrimethylene glycols and sebacic acid give a eutectic type curve, because of the appearance of a new crystal structure. (4) Those with succinic and α‐methylsuccinic acids and decamethylene glycol give progressively decreased melting points, decreased breaks in heating curves, and an increasingly distorted lattice as the number of methyl groups is increased, because random “head‐to‐head” and “head‐to‐tail” polymerization makes the completely substituted polyester irregular. (5) The polyester of d‐α,β‐dimethoxysuccinic acid and tetramethylene glycol is highly crystalline because the substituent groups are oriented in a regular manner. The meso‐polyester did not crystallize because of more random orientation of substituents. Decamethylene d‐tartrate similarly melts higher than the meso‐tartrate.

By using an existing cut-growth test in a new way it has been possible not only to increase the reproducibility of the results, but also to show important relations between cut-growth resistance and modulus, or the elongation at break of cold SBR tread stocks. The logarithm of the cut-growth resistance of cold SBR vulcanizates as measured on a modified De Mattia machine, is shown to be a linear function of the 300 per cent modulus, except for certain undercured stocks. The slope of the curve is dependent on the type and the amount of black, but is relatively independent of the type of curing system, amount of antioxidant, and time of Geer aging. An equation has been found which relates cut-growth resistance at 150° F with the elongation at break of the vulcanizate. Except for stocks having black loadings in excess of about 55 phr and having in addition modulus values less than 800 psi, this relation appears to be nearly independent of black loading, type of black, curing system, time or temperature of cure, antioxidant concentration, and extent of Geer aging. The elongation at break of an SBR tread stock thus appears to be a primary factor governing its groove-cracking behavior. The relation between cut-growth resistance and modulus is of secondary significance and evidently depends upon the interrelation between the modulus and breaking elongation of these formulations. The breaking elongation, however, cannot be the sole factor governing the groove-cracking behavior; otherwise there would be no restrictions on the validity of the breaking elongation equation.

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