Studies of the crystallization, melting, and morphology of random ethylene/1-octene copolymers by a combination of differential scanning calorimetry and atomic force microscopy are presented. Two different crystallization mechanisms prevalent in separate temperature ranges are inferred from the effect of cooling rate on the temperature dependence of crystallinity, from the reversibility of crystallization/melting phenomena at the lowest temperatures, and from the temperature dependence of kinetic parameters describing isothermal crystallization and melting. Morphological studies of these copolymers demonstrate the coexistence of two distinct crystalline superstructures (i.e., lamellae and fringed-micellar or chain cluster structures) which we tentatively associate with the two crystallization mechanisms. The multiple melting behavior of these copolymers is associated with the existence of separate morphological entities and is not explained by a mechanism of melting−recrystallization−remelting. Finally, the upward shift of the melting endotherm of secondary crystals (i.e., these formed by the low-temperature mechanism) with longer crystallization times is explained by a decrease in the molar conformational entropy of the remaining amorphous fraction as a result of secondary crystallization.

The phase behavior and miscibility of poly(c-caprolactone) (PCL)/polycarbonate (PC) blends have been investigated with DSC, cloud-point measurement, TGA, FTIR, NMR, and small-angle neutron scattering (SANS). Thermal analysis results indicated that the PCL-rich blends are semicrystallinel semicrystalline at room temperature. At about 30% PC incorporation, the PCL crystallinity showed amarked reduction, whereas the PC crystallinity approached amaximum. The composition dependence of Tgexhibited a discontinuity (cusp) and was critically analyzed using the classical equations of Gordon-Taylor and Fox and the free volume theory of Braun-Kovace. Combination of the Fox and Braun-Kovace equations accurately reproduced the T,-composition dependence. Thermal stability of the blends, as measured by the onset degradation temperature in air, increased with increasing PC. FTIR results coupled with lSC NMR findings supported the hypothesis that the blends primarily degraded via thermally-induced chain scission of PCL as evidenced by the evolution of COS. SANS studies on the deuterated PC-rich blends revealed that the scattering intensity first remained fairly constant with increasing temperature and then increased sharply at temperatures above the blend Tp The sudden rise in the scattering intensity was attributed to crystallization of PC resulting from prolonged annealing. Results derived from the RPA analysis of the SANS profiles measured at 30 "C for the deuterated PC-rich blends and those obtained from the melting point depression analysis of the PCL-rich blends suggested favorable blend interactions as reflected by the negative sign of the x parameter.

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Crystalline morphologies of poly(e-caprolactone) (PCL) and deuterated polycarbonate (d-PC) blends were studied by small-angle neutron and X-ray scattering (SANS and SAXS). Measurements were conducted at both room temperature and temperatures above the melting point (60 °C) of PCL. Due to the different contrast between the phases for neutrons and X-rays, SANS exhibited a monotonic drop in intensity with increasing scattering angle while SAXS showed lamellar (peak) scattering. A two-correlation length model provided an excellent fit for the SANS data over the entire composition range. This model reproduced not only the shape but also the absolute magnitude of the scattering curves. The long range correlation length (~10s A) and the short range correlation length (-10 A) derived from this model are inferred to be associated with the crystalline PC domain and the local cluster in the amorphous phase (possibly resulting from crystallization-induced phase separation), respectively. Both the long range correlation length obtained from SANS and the long period measured from SAXS showed identical composition dependence. This further supports the applicability of the two-correlation length model and our interpretation.

Evolution of the poly(ecapro1actone) (PCL) lamellae in blends of PCL/PC (polycarbonate) was monitored by synchrotron small-angle X-ray scattering (SAXS). Theeffecta of crystallization temperature, PC concentration, and PC crystallinity on the PCL lamellar growth in the PCL-rich blends were investigated. The half-crystallization time derived from the temporal change of the peak intensity increased with crystallization temperature and generally increased with the addition of PC. For a given blend composition, the lamellar growth rate increased with increasing PC crystallinity. The interlamellar spacing initially varied with time and then approached a plateau value at the later stage of crystallization. An insertion mechanism is proposed in which the PCL is crystallized in the amorphous intralamellar phase. This model is also consistent with the quantitative SAXS results? which suggested that random mixing of PCL and PC lamellae occurred in the semicrystalline (PCL)/semicrystalline (PC) state.

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The stress-strain response of low-crystallinity ethylene-octene (EO) and ethylene-styrene (ES) copolymers with 7-20 mol % comonomer was compared over a temperature range that spanned the glass-transition and crystal melting regions. Above the onset temperature of the glass transition, the copolymers exhibited elastomeric behavior with low initial modulus, uniform deformation to high strains, and high recovery after the stress was released. In the glass-transition range, an initial lowstress elastomeric response was followed by a distinct "bump" in the stress-strain curve. On the basis of the temperature and rate dependence of the stress-strain curve, local strain-rate measurements, local temperature changes, and recovery characteristics, the "bump" was identified as high strain yielding. Hence, the stress-strain curve sequentially exhibited the features of elastomeric and plastic deformation. Following high strain yielding, strain hardening dramatically increased the fracture strength. This behavior was defined as elastomeric-plastic. Elastomeric-plastic behavior in the broad glass-transition range constituted a gradual transition from elastomeric behavior at higher temperatures to low-temperature plastic behavior with high modulus and macroscopic necking. Because of the lower glass-transition temperature of EO, Ϫ40°C as compared with Ϫ10°C for ES, the onset of elastomeric-plastic behavior occurred at a significantly lower temperature. The concept of a network of flexible chains with fringed micellar crystals serving as the multifunctional junctions that provides the structural basis for elastomeric behavior of low-crystallinity ethylene copolymers was extended to elastomeric-plastic behavior by considering a network with a fraction of rigid, glassy chains.

ABSTRACT:The effects of styrene content, layer thickness, and temperature on the adhesion of ethylene-styrene copolymers (ES) to low-density polyethylene (LDPE) were examined by measuring the delamination toughness of LDPE/ES microlayers in the T-peel test. Delamination toughness at ambient temperature decreased with increasing styrene content. A linear correlation between delamination toughness and styrene content in ES was observed. Extrapolation predicted that a copolymer with 72.5 wt % styrene would have no adhesion to LDPE. Experiments on microlayers with relatively thin (8 -18 m) ES layers demonstrated that delamination toughness was proportional to ES layer thickness. In situ observations of the damage zone showed stretching throughout the entire thickness of the ES layer. For higher adhesion systems, ES deformation occurred concurrently with localized stretching and crazing of the LDPE layer at the interface. For two representative systems, the effect of temperature on delamination toughness was studied. Major transitions in delamination toughness and fracture mode were observed at the primary thermal transitions of the ES copolymer. The contribution by stretching of the ES layer to measured delamination toughness was approximated by relating the damage zone to the ES engineering stress-strain curve and was estimated to be about half. The remaining contribution to delamination toughness was assumed to be due to LDPE deformation in the damage zone.