Simple analytical method for determination of microstructures of poly(ethylene-co-vinyl acetate) using the melting points

Microstructures of poly (ethylene-co-vinyl acetate) (EVA) are dependent on the distribution of VA units as well as the VA content. Simple analytical method to determine microstructure of EVA was established using melting points of EVA and correlationship with those of wax and its ester. EVAs with the VA contents of 9.5–40 wt% (3.3–17.8 mol%) were used. The melting points were analyzed using differential scanning calorimetry (DSC). EVAs had two melting points of T_m1 and T_m2 at 96–44 and 56 - 36 °C, respectively. The melting points were found to be closely related to the minimum carbon number participating in the crystal formation (CN_min). The crystalline size could be estimated by equation of CN_min = - ln [(106.7 - T_m)/214.8]/0.055 + 15. The CN_min1 and CN_min2 were 68–38 and 41–35, respectively. The CN_min1 decreased by about 4.5/mol% until 6.7 mol% of the VA content, while the CN_min2 slightly decreased by about 0.4/mol%. It was found that EVA would be generally composed of three ethylene block sizes of ~VA-(CH₂CH₂)_c1>20-VA-(CH₂CH₂)_15<c2<20-VA-(CH₂CH₂)_a<15-VÃ (c1 and c2: crystalline regions, a: amorphous region). Size of the amorphous region became smaller by increasing the CN_mins. EVAs deviated from the average trends could be explained by difference in the microstructures such as the ethylene block sizes and branched structures. It was possible to explain difference in the microstructures of EVAs with the same VA content by difference in the CN_mins obtained using the T_ms.