The effect of molecular weight and crystallite structure on yielding in ethylene copolymers

Graham, Jasmin; Alamo, Rufina G.; Mandelkern, L.

doi:10.1002/(sici)1099-0488(19970130)35:2<213::aid-polb2>3.0.co;2-t

Cited by 46 publications

(26 citation statements)

References 20 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…As both the (log) modulus and the yield stress were directly related to the crystallinity, the yield stress-(log) modulus relationship should be simple [6,18,[33][34][35][36]. The yield stress-(log) modulus relationship is often studied on polyethylene [33][34][35], however in these studies the PE crystallite thickness is frequently changed at the same time as the PE crystallinity and probably also the aspect ratio of the crystallites. Since the T6A6T segments of the studied segmented block copolymers were mono-disperse in length, the crystallite thickness in these copolymers was expected to be constant and also the crystallinity of the T6A6T segments was constant.…”

Section: Dmtamentioning

confidence: 99%

Tensile properties of segmented block copolymers with monodisperse hard segments

2008

View full text Add to dashboard Cite

The tensile properties of segmented block copolymers with mono-disperse hard segments were studied with respect to the hard segment content (16-44 wt.%) and the temperature (20-110°C). The copolymers were comprised of poly(tetramethylene oxide) segments with the molecular weights of 650-2,900 Da and of mono-disperse bisester-tetra-amide segments (T6A6T) based on adipic acid (A), terephthalic acid (T) and hexamethylene diamine (6). An increasing content of T6A6T gave rise to an increased modulus, yield stress and fracture stress. The modulus could be modeled by a composite model. Moreover, a strain-softening was observed well below the yield stress, due to the shearing of the T6A6T crystallites. At strains [200%, a strain-hardening of the PTMO segments took place and this even for PTMO segments that were amorphous in the isotropic state. The strain hardening increased the tensile properties. An increase in temperature had little effect on the modulus of the copolymers, but was found to lower the yield and fracture stresses. At temperatures above the melting temperature of the oriented PTMO, no strain-hardening took place. The yield stress as a function of temperature could be described by the Eyring relationship, but a modulus-yield stress relationship could not be established.

show abstract

Section: Dmtamentioning

confidence: 99%

Tensile properties of segmented block copolymers with monodisperse hard segments

2008

View full text Add to dashboard Cite

show abstract

“…Polyethylenes with lower crystallinity are normally expected to exhibit double yielding due to an interlamellar deformation mechanism combining fine chain slip with lamellar fragmentation. [22,23] For the quenched blends, the yielding region became more diffused with addition of C 8 VLDPE, whereas for the SC blends, the yielding region remained approximately unchanged with increasing C 8 VLDPE, illustrating different effects of the slow cooling and quenching treatments on the crystallite perfection.…”

Section: Tensile Propertiesmentioning

confidence: 99%

“…Transmission electron microscopy (TEM) studies performed by Graham and coworkers have shown that for quenched ethylene-butene copolymers, crystallites are much smaller in the lateral direction and lamellae become shorter, highly curved and segmented, whereas much longer and straighter lamellae can be found for a slowly cooled sample. [23] Therefore, there is a very strong correlation between the crystallite structure and the changing yield behaviour.…”

Section: Tensile Propertiesmentioning

confidence: 99%

Structural and Mechanical Properties Changes of Ethylene‐α‐olefin Copolymer Blends Induced by Thermal Treatments and Composition

Chen

Shanks

Amarasinghe

2004

Macro Materials & Eng

View full text Add to dashboard Cite

Summary: Blends of single‐site catalysed ethylene‐α‐butene (C4VLDPE) and ethylene‐α‐octene (C8VLDPE) copolymers were prepared by melt extrusion. The phase morphology, thermal and mechanical properties of the blends have been investigated by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), tensile test and dynamic mechanical analysis (DMA). Depending on the composition and thermal history, significant differences in structure and behaviour were found. It was also found that some degree of co‐crystallization occurred for quenched blends; whereas most of the oven slowly cooled blends showed two well‐defined melting peaks, indicating that the slow cooling favoured partial segregation of the fractions with different degrees of branching to form two morphologies. Moreover, SEM revealed morphology of the thinner crystals distributed in‐between the thicker sheaf‐like crystals for the slowly cooled blends with 20–50% C8VLDPE. Therefore, the synergism in mechanical properties for the blends with 20–50% C8VLDPE is due to a combination of larger crystal size, more complete phase separation and interfacial interaction produced by the segregation effect of the slow cooling treatment. DMA studies showed that the storage modulus increased as the addition of C8VLDPE and modulus for the slowly cooled blends are about twice those measured for the quenched ones, indicating higher stiffness of the blends. The smooth shift of β relaxation temperature with addition of C8VLDPE for both sets of blends confirmed the miscibility in the amorphous phase.SEM image of the C4VLDPE‐C8VLDPE (50/50) blend after oven slow cooling treatment.magnified imageSEM image of the C4VLDPE‐C8VLDPE (50/50) blend after oven slow cooling treatment.

show abstract

“…Mandelkern et al [85,86] have discussed the occurrence of double yield points in PE and ethylene copolymers in relation to two populations of crystalline lamellae having different size distributions. The smaller and less perfect crystals undergo straininduced melting, and then the fraction of recrystallized material can undergo yielding again at higher strain level.…”

Section: Double Yield Phenomenonmentioning

confidence: 99%

Plasticity of semi-crystalline polymers: crystal slip versus melting-recrystallization

Séguéla¹

2007

E-Polymers

View full text Add to dashboard Cite

Semi-crystalline polymers can be drawn up to very high draw ratios that involve a complete molecular rearrangement of the chain-folded lamellar morphology into a more or less chain-unfolded fibrillar microstructure. This metamorphosis is likely to take place through an intermediate state of high molecular disorder at a local scale. This is the reason why some authors talk of a strain-induced melting-recrystallization process. In contrast, several structural features occurring at moderate plastic strains are relevant to strictly crystallographic processes. A critical discussion of experimental findings and theoretical approaches is made for pointing out the strength or the deficiency of the different author argumentations. No doubt that both phenomena can occur: crystallographic processes are active at all strain levels whereas meltingrecrystallization is restricted to the post-yield stage accompanied with chain unfolding. Melting-recrystallization is rather a corollary of chain unfolding than a basic mechanism of the plastic deformation. Besides, it may concern only a part of the crystalline phase and can be inhibited under adequate deformation conditions.

show abstract

The effect of molecular weight and crystallite structure on yielding in ethylene copolymers

Cited by 46 publications

References 20 publications

Tensile properties of segmented block copolymers with monodisperse hard segments

Tensile properties of segmented block copolymers with monodisperse hard segments

Structural and Mechanical Properties Changes of Ethylene‐α‐olefin Copolymer Blends Induced by Thermal Treatments and Composition

Plasticity of semi-crystalline polymers: crystal slip versus melting-recrystallization

Contact Info

Product

Resources

About