Differential thermal analysis (DTA) of some commercial nylons has disclosed some anomalous phenomena with respect to the glass transition, generally considered to occur at 40–50°C. On the first heat cycle the transition occurs normally. On cooling, however, no corresponding transition occurs, and on an immediate rerun the transition has disappeared. If another DTA thermogram is made after a few hours, the transition begins to reappear, but at a temperature lower by a few degrees. After about five days rest, the transition is again normal in size and temperature. On annealing at 75°C, the 43°C transition is pushed up to about 92°C. On resting after annealing, transitions appear at both 40 and 92°C. These phenomena are explained in terms of the slow formation of a hydrogen‐bonded network in the amorphous regions of the polymer. It is the disruption of this network that is normally considered to be the glass transition in nylons. The network is slow in re‐forming because of problems involved in matching up potential hydrogen‐bonding sites, which are, of course, distributed at intervals along the polymer chain. The temperature at which the network is disrupted is apparently dependent not so much on the ratio of bonding to nonbonding sites, as on the temperature at which it was formed.

Highly crosslinked polymers of varying structure were produced by the reaction of poly(glycidyl acrylate) (PGA) and methacrylate (PGM) with a variety of anhydride crosslinking agents: chlorendic (CA), glutaric (GA), maleic (MeA), succinic (SA), and polyrnalonic (pMnA) anhydrides. Topology was also varied by the use of a diluent and a comonomer in the backbone chain. Oxygen permeation measurements were made on these polymers coated onto a polypropylene film substrate before crosslinking. The crosslinking process greatly reduced the O2 permeability which, however, was dependent not only on the degree of crosslinking (yield of the crosslinking reaction), but also on the crosslink density, the chemical nature of the structural elements, and the topology of the polymer network. Thus the most impermeable coating (XPGA/CA) was made not from the stiffest and bulkiest components (PGM and CA), but by the reaction of the bulkiest anhydride (CA) with the more flexible polymer backbone chain (PGA). This is explained in terms of the need for chain flexibility to produce a crosslinked structure of optimum space filling character and network tightness.

Oxygen permeation has been measured in flat sheet as a function of degree of orientation, and oxygen and water transport have been measured in oriented polyester bottles. O2 permeability in flat sheets decreases gradually with orientation on either side of an abrupt decrease by about a factor of 2 after moderate orientation. The bottles tested were all more highly oriented than that critical decree; no significant effect of orientation on water or O2 transport in bottles could be found. Only container intrinsic viscosity (IV) (which is determined by molecular weight, and is therefore a convenient measure of polymer degradation) was significantly related to the transport properties of the bottles. It was found that O2 transport is increased with increasing IV while H2O transport decreased. The explanation for these seemingly contradictory data can be found in the chemistry of degradation of the polyester.

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