2684 Macromolecules 1985, 18, 2684-2688 Differential Scanning Calorimetry Studies of Ethylene-Propylene Copolymers Seng-Neon Gant and David R. Burfield*$ Centre of Foundation Studies in Science and Department of Chemistry, University of Malaya, Kuala Lumpur 22-11, Malaysia Kazuo Soga Research Laboratory of Resources Utilization, Tokyo Institute of Technology, Nagatsuta 4259, Yokohama 227, Japan. Received May I, 1985 ABSTRACT: A DSC examination of a series of ethylene-propylene copolymers together with their parent homopolymers has been made. Introduction of ethylene leads to a sudden drop in the glass transition temperature (T,) of the polypropylene homopolymer (262 K) to values in the region of 214-222 K, depending on the overall composition. NMR analysis shows that the incorporation of ethylene and propylene into the copolymers is essentially random although the Al/Cr ratio in the catalyst does affect the tendency to block formation. Whereas only a few copolymers exhibit well-definedmelting endotherms, the process of crystallization is observed in all samples. By cross correlationwith NMR data, it is possible to assign the various exotherms to the crystallization of either ethylene or propylene blocks. The crystallization temperature (T,) and the enthalpy of crystallization (AHc) decrease in step with the reduction in sequence length of the monomer blocks. A logarithmic plot of AHc vs. either the mole fraction of ethylene or the mole fraction of ethylene triads is linear and indicates a minimum ethylene sequence length of between 9 and 10 for crystallization to occur. Introduction Polyethylene and isotactic polypropylene are rigid plastics at normal temperatures due to the fact that the linear sequences of either type of unit have a strong tendency to crystallize. However, the two units are not able to cocrystallize satisfactorily in copolymers. Thus each type of unit is able to cause disruption in the crystalliza- bility of the other, thereby increasing the inherent rota- tional flexibility of the two types of main-chain segments. This is manifested in the elastomeric character of the bulk material. As the glass transition temperature (T,) is a fundamental polymer characteristic and generally has a determining influence on the bulk properties of the material, the measurement of the Tg of EP rubbers has attracted much attention over the years. Thus Tg studies by techniques such as dilat~metry,l-~ coefficient of thermal expansibil- ity,"' NMR,s differential thermal analysis,*" and DSC12 have been reported. While there is a great variation of Tg with composition for copolymers containing more than 65 mol % propylene, most commercial EP rubbers contain 30-65 mol % propylene and tend to show a very small variation of Tg over the composition range for copolymers of comparable molecular weights. Thus Tg alone does not provide much information about the actual composition and monomer sequence distributions in the copolymers. Since the contrasting elastomeric properties of the co- polymers as compared to the homopolymers have been attributed to reduced crystallinities, the melting and crystallization of the polymers have become the subjects of studies over the years.12J3 The crystallinity of the polymers is very much dependent on the chain length and composition, as well as the sequence distribution of the two types of monomers. Correlation of the various liter- ature results of density and X-ray crystallinity data pro- duces rather scattered graphs.I2 This may in part stem from the failure to characterize the polymers with respect to both molecular weight and monomer sequence distri- bution. In light of the above, we now report the results Centre of Foundation Studies in Science. * Department of Chemistry. of DSC measurements for a series of well-characterized EP copolymers. Experimental Section Polyethylene, polypropylene, and the various EP copolymers were prepared with a chemically modified chromium catalyst in combination with AlEt,Cl at 40 "C as previously described.14J5 All the samples, except the homopolymers, were prepared at the same temperature and total monomer pressure. The experimental conditions of copolymerization are summarized in Table I. The samples of series A were prepared at a constant Al/Cr ratio of 10, while the composition of the monomer mixture was varied progressively. The B series was designed to probe the effect of variation in Al/Cr ratio on copolymer composition at constant monomer ratio. 13C NMR spectra were recorded on a JEOL JNM PS-100 spectrometer, equipped with a PET-100 FT system, operating at 25.14 MHz with full proton noise decoupling. Spectra were recorded at a temperature of 120 "C with pulse at 13.5 ps, a repetition rate of 7.0 s, and a sweep width of 5000 Hz. The monomer sequence distributions in the copolymers were deter- mined from NMR data by the method of Doi et a1.I6 The molecular weight distributions of polypropylene and the various copolymerswere measured at 140 "C with a Showo Denko gel permeation chromatograph fitted with a Shodex 80M column (ASOM/SX2), using 1,2-dichlorobenzene as solvent. The poly- ethylene sample was not soluble in this solvent and could not be measured. Calorimetric measurements were made with a Perkin-Elmer DSC-2C instrument equipped with a liquid nitrogen subambient accessory. The temperature scale was calibrated at 20 K/min against indium (429.78 K), water (273.15 K), and mercury (234.28 K). The heat of fusion of indium (6.80 cal/g) was used as a calorimetric calibration. Melting temperatures, crystallization temperatures, heats of fusion, and heats of crystallization were calculated with the Perkin-Elmer standard TADS program. Unless otherwise stated, all cooling and heating runs were carried out at a scan rate of 20 K/min on samples of 5-10 mg encapsulated in standard aluminum pans. Melting and crystallization temperatures are quoted as the maximum and minimum values, respectively, and not as onset temperatures. Results and Discussion Molecular Weight Distribution. The results of the GPC studies on the polymers are summarized in Table I. The molecular weight distribution as characterized by the 0024-9297/85/2218-2684$01.50/0 0 1985 American Chemical Society