S. P. Chum

Classification of homogeneous ethylene‐octene copolymers based on comonomer content

Ethylene-octene copolymers prepared by Dows INSITE™ constrained geometry catalyst technology present a broad range of solid-state structures from highly crystalline, lamellar morphologies to the granular morphology of low crystallinity copolymers. As the comonomer content increases, the accompanying tensile behavior changes from necking and cold drawing typical of a semicrystalline thermoplastic to uniform drawing and high recovery characteristic of an elastomer. Although changes in morphological features and tensile properties occur gradually with increasing comonomer content, the combined body of observations from melting behavior, morphology, dynamic mechanical response, yielding, and large-scale deformation suggest a classification scheme with four distinct categories. Materials with densities higher than 0.93 g/cc, type IV, exhibit a lamellar morphology with well-developed spherulitic superstructure. Type III polymers with densities between 0.93 and 0.91 g/cc have thinner lamellae and smaller spherulites. Type II materials with densities between 0.91 and 0.89 g/cc have a mixed morphology of small lamellae and bundled crystals. These materials can form very small spherulites. Type I copolymers with densities less than 0.89 g/cc have no lamellae or spherulites. Fringed micellar or bundled crystals are inferred from the low degree of crystallinity, the low melting temperature, and the granular, nonlamellar morphology.

Classification of ethylene-styrene interpolymers based on comonomer content

Copolymerization of ethylene and styrene by the INSITETM technology from Dow presents a new polymer family identified as ethylene–styrene interpolymers (ESI). Based on the combined observations from melting behavior, density, dynamic mechanical response, and tensile deformation, a classification scheme with 3 distinct categories is proposed. Polymers with up to 50 wt % styrene are semicrystalline and are classified as type E. The stress–strain behavior of low-crystallinity polymers at ambient temperature exhibits elastomeric characteristics with low initial modulus, a gradual increase in the slope of the stress–strain curve at higher strains, and large instantaneous recovery. The structural origin of the elastomeric behavior is probably a network of flexible chains with fringed micellar crystals serving as multifunctional junctions. Polymers with more than 50 wt % styrene are amorphous. Because the range of glass transition temperatures encompasses ambient temperature (nominally 25°C), it is useful to differentiate ESIs that are above the glass transition as type M and those that are below the glass transition as type S. Type M polymers behave as rubber-like liquids. They have the lowest modulus and lowest stress levels. Some elastic characteristics are attributed to the entanglement network. Type S polymers exhibit large strain rate sensitivity with glassy behavior at short times and rubbery behavior at longer times. The term “glasstomer” is coined to describe these polymers. The division between type M and type S is based on chain dynamics, rather than solid state structure, and thus depends on the temperature of interest. At ambient temperature, ESIs with 50 to 70 wt % styrene are classified as type M; polymers with more than 70 wt % styrene are classified as type S.

Blends of homogeneous ethylene-octene copolymers

Abstract Blends of ethylene-octene copolymers prepared by Dows INSITE™ constrained geometry catalyst and process technology were characterized. (INSITE™ is a trademark of The Dow Chemical Company.) A previously described classification scheme based on density, or comonomer content, was the basis for the choice of blend components. The blends combined a low density Type I copolymer (0.865 g cm−2) with a higher density copolymer. The second component was either another Type I copolymer (0.887 g cm−3), a Type II copolymer (0.901 g cm−3), or a Type III copolymer (0.913 g cm−3). The melting and crystallization behaviour suggested that the components crystallized separately in all the blends. However, dynamic mechanical analysis indicated that the noncrystalline portions of the Type I blends formed a single phase, whereas the noncrystalline regions of blends with the Type II or Type III copolymer appeared to be phase separated in the solid state. The stress-strain behaviour at ambient temperature correlated with density, or total crystallinity, regardless of whether the material was a copolymer or a blend.

Elastomeric blends of homogeneous ethylene-octene copolymers

Abstract An elastomeric ethylene-octene copolymer was compared with binary blends of ethylene-octene copolymers formulated to have the same crystallinity as the target copolymer. Copolymers having narrow molecular weight distribution, homogeneous comonomer distribution and homogeneous long chain branching structure were prepared by Dows INSITE ™ constrained geometry catalyst and process technology (INSITE ™ is a trademark of The Dow Chemical Company). A copolymer of higher comonomer content than the target was blended with the appropriate amount of a lower comonomer content copolymer to obtain the target level of crystallinity. Thermal analysis indicated that the components crystallized separately in all the blends. The stress-strain behaviour of the copolymers and their blends was evaluated as a function of temperature. At ambient temperature, the total amount of crystallinity primarily determined the stress-strain relationship regardless of whether the material was a single copolymer or a copolymer blend. Any effects of phase morphology were subtle at ambient temperature. At higher temperatures, where the network junctions started to melt, miscibility of the non-crystalline regions produced a synergistic effect on the tensile strength. However, if the branch concentration of the blended copolymers was too different, the non-crystalline regions were immiscible, and the copolymer blend had a lower tensile strength than the target at higher temperatures.

Creep Behavior of Amorphous Ethylene-Styrene Interpolymers in the Glass Transition Region

The viscoelastic behavior of amorphous ethylene-styrene interpolymers (ESIs) was studied in the glass transition region. The creep behavior at temperatures from 15°C below the glass transition temperature (Tg )t oTg was determined for three amorphous ESIs. These three copolymers with 62, 69, and 72 wt % styrene had glass transition temperatures of 11, 23, and 33°C, respectively, as determined by DMTA at 1 Hz. Time-temperature superposition master curves were constructed from creep curves for each polymer. The temperature dependence of the shift factors was well described by the WLF equation. Using the Tg determined by DMTA at 1 Hz as a reference temperature, C1 and C2 constants for the Williams, Landel, and Ferry (WLF) equation were calculated as approximately 7 and 40 K, respectively. The master curves were used to obtain the retardation time spectrum and the plateau compliance. The entan- glement molecular weight obtained from the plateau compliance increased with in- creasing styrene content as 1,600, 1,870, and 2,040, respectively. The entanglement molecular weight of the ESIs was much closer to that of polyethylene (1,390) than to that of polystyrene (18,700); this was attributed to the unique chain microstructure of these ESIs with no styrene-styrene dyads.

Mechanisms of ductile tear in blown film from blends of polyethylene and high melt strength polypropylene

Deformation processes associated with ductile tear in blown films of polyethylene blended with up to 30 wt% high melt strength polypropylene (hmsPP) were studied. The tear resistance was determined with a reinforced trouser tear test. During stable crack growth, a crack-tip damage zone was transformed into a continuous yielded zone at the fractured edge. The relationship to lamellar morphology was probed with atomic force microscopy. Balanced tear characteristics of polyethylene film reflected the nearly isotropic lamellar morphology. In contrast, the highly oriented shish-kebab morphology of hmsPP domains in the blend films resulted in increasingly anisotropic behavior as the amount of hmsPP increased. The most important manifestation was a significant reduction in machine direction (MD) tear. Good adhesion of polyethylene and hmsPP in blend films prevented interfacial failure and provided stress transfer to the dispersed phase at high strains. In MD tear, extension of the matrix by the normal processes of lamellar breakup and fibrillation caused rotation of hmsPP domains into the loading direction and in a later stage shear displacement of reoriented hmsPP lamellae. Locally, hmsPP domains constrained deformation of polyethylene lamellae. Factors that increased constraint on the polyethylene matrix such as increasing the amount of hmsPP or increasing the aspect ratio of hmsPP domains reduced the MD tear resistance. In transverse direction (TD) tear, the oriented hmsPP domains deformed by lamellar shear processes concurrently with lamellar breakup and fibrillation of the polyethylene matrix. As a result, the blend film preserved good TD tear resistance.