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Ethylene–butadiene (PEBU) copolymer with low percentage of butadiene was previously investigated by van der Waals potential energy calculations. The trans HCCH group (TDB) was found to show fold-supporting behavior, so that it was supposed that many TDB's were located on the surface of PEBU single crystals under favorable conditions of TDB concentration and of thickness of crystal platelets. In order to verify this hypothesis, three PEBU samples with a butadiene/(ethylene + butadiene) molar ratio of 2.3% (B), 3.6% (C), and 4.6% (D), by assuming that the HCCH groups are all trans, were studied and compared with a polyethylene (PE) sample (A). Samples A, B, C, and D were grown isothermally at temperatures of 50, 55, 60, 61, 70, 71, 75, and 80°C from dilute xylene solutions. Infrared, differential scanning calorimetry (DSC), small-angle x-ray scattering (SAXS), bromination kinetics, and density measurements were performed. B, C, and D lamellar crystals show a long spacing L nearly equal to that of A at crystallization temperatures Tc of 50 and 60°C. For Tc ≥ 70°C, the fold period of samples B, C, and D increases more rapidly than that of A in the order D > C > B. The melting temperature Tm and the enthalpy of fusion ΣHf decrease with increasing content of butadiene, except for B and C, which show nearly equal Tm and ΔHf values. Two peaks appear in the fusion curves at temperatures T1 and T2, T1 > T2. The area of the peak at T2 increases with the butadiene content, and, in the same sample, with Tc. Results of bromination in suspension of B, C, and D single crystals, crystallized in the range 50–71°C, indicate that a high percentage of TDB's react on the fold surfaces. The chemical attack is more successful with crystals grown in the range 60–66°C, and, in the case of sample C, as much as 81% of the TDB's are consumed. The density of the samples increases with both Tc and TDB percentage. On the basis of these observed results a simple model for PEBU is proposed. It is supposed that the copolymerization reaction leads to the formation of defects, consisting of one (b), two (bb), three (bbb), or more sequential molecules of butadiene, almost homogeneously distributed along a macromolecular chain. At very small percentage of butadiene, b defects prevail. The addition of further butadiene gives rise mainly to bb defects, and so on. Thus each kind of defect characterizes PEBU copolymers with a certain butadiene/ethylene molar ratio. The experimental data, valid for low butadiene/ethylene molar ratios, agree with this reasonable model and with the potential energy calculations. |
