The effect of the secondary relaxation on mechanical properties of PET/PCT copolymers.

Abstract

The sub-T$\sb{\rm g}$ relaxations of a series of copolymers based on poly(ethylene terephthalate) (PET) and poly(1,4-cyclohexylenedimethylene terephthalate) (PCT) were investigated by dynamic mechanical spectroscopy. The secondary loss peak $\rm(-70\sp\circ C$ at 1 Hz) only increases in amplitude as the mole% of 1,4-cyclohexylene dimethanol (CHDM) monomer is increased. The activation energies increase from 11 (PET) to 16 (PCT) kcal/mole. Comparisons of the peak temperatures and activation energies between these materials and poly(cyclohexyl methacrylate) (PCHMA) show that the secondary relaxation of the copolyesters involves motions of the cyclohexylene ring. Solid state $\rm\sp{13}C$ NMR detected large amplitude molecular motions of the cyclohexylene rings and of the main chain methylene groups of PCT. Chair and twist- boat conformational transitions may be possible which impose translational movements on the terephthalate linkages. The cyclohexylene motions may behave cooperatively giving rise to longer range motions which create greater volume fluctuations. Such volume fluctuations were also evidenced by positron annihilation lifetime spectroscopy (PALS). The yield stress decreases with increasing CHDM content by 20 MPa as observed throughout temperatures ranging from $\rm-40\sp\circ C$ to $\rm60\sp\circ C$ and strain rates from 0.00036 to $\rm0.22s\sp{-1}.$ We attribute the decrease to the lubrication effect by conformational transitions of the cyclohexylene group. Lateral excursions produced by conformational changes of the cyclohexylene rings reduce the resistance against chain slippage and facilitates the onset of yielding. The craze stress of the copolyesters was measured using Hill's slip-line field theory for notched plane-strain specimens. The craze stress increases with increasing CHDM content due to factors affecting craze initiation based on a homogeneous nucleation theory. In the case of high CHDM content, the formation of stable voids is impeded by local chain fluctuations which keep the voids at a subcritical size. Enhanced molecular motions increase the craze stress by preventing craze initiation until stress levels are high enough to attain stable nanovoids. The ductile/brittle transition of these materials can be understood as a competition between yielding and crazing with changes in the transition temperature attributed ultimately to molecular motions of the cyclohexylene groups.