Flow birefringence and strain-induced hardening of cycloolefin copolymers under elongational flow

Strain-induced hardening behavior and flow birefringence were studied on copolymers of ethylene–tetracyclododecene (E–TD) and ethylene–norbornene (E–NB) of various compositions subjected to uniaxial elongation with constant Hencky strain rate between 0.01 and 1.0 s−1 in the temperature range from +40 to +60°C above their respective glass transition Tg. When compared at the corresponding temperature of, e.g. Tg+50°C, the E–TD copolymer exhibited stronger tendency of strain-induced hardening than the E–NB copolymer of the corresponding comonomer content. The stress optical rule (SOR) was obeyed for the E–NB copolymer in the whole range of its composition, while for the E–TD copolymer, the SOR was valid only for that of low TD content (<30 mol%), but invalid for that of high TD (≥30 mol%) content. The results suggest that the increase in the population of bulkier TD comonomer may be a molecular origin of the strong strain-induced hardening as well as a reason for the invalidity of SOR for the E–TD copolymer of high TD content. For the two copolymers that satisfy SOR, the stress optical coefficient C decreases exponentially with the comonomer content, and the tendency is stronger for E–TD than for E–NB, reflecting the comonomer size. The stress optical coefficient C vs. composition plots of the E–NB (block type) and E–TD copolymers are extrapolated at zero E limit to 5.5×10−10 and 7.5×10−12 Pa−1, respectively, which are expected to be the C for the NB and TD homopolymers if such polymers do exist.