Microstructural Interpretation of Influences of Molecular Weight on the Tensile Properties of High-Density Polyethylene Solids Using Rheo-Raman Spectroscopy

Abstract

The influence of the weight-average molecular weight (M w) on the structure–properties relationship was investigated by preparing high-density polyethylenes (HDPEs) of various M w but constant polydispersity index (M w/M n) values. The thickness of the lamellar crystalline layer increased with increasing M w, whereas the amorphous characteristics, such as the amorphous layer thickness and tie-molecule fraction, remained unchanged. The microscopic deformation of these HDPEs was investigated by in situ Raman spectroscopy. The orientation parameter ⟨P 2⟩ showed a power law to M w with an exponent of −3/4 in the strain-hardening region. The M w dependence of the molecular orientation was interpreted as the realignment of the lamellar cluster units, the size of which was determined by the end-to-end distance of the polymer chains in their melt state. It was found that higher stretching stress is applied to the crystalline chains for HDPEs of lower M w, implying that a larger amount of taut-tie chains is formed.

Raman Spectroscopic Analyses of Structure–Mechanical Properties Relationship of Crystalline Polyolefin Materials

“…For high-density PE (HDPE) with a crystallinity of more than 60 vol%, the values of 〈P 2 〉 and 〈P 4 〉 during the yield deformation are in the region (iv): the values of 〈P 4 〉 decrease in the yield region, whereas 〈P 2 〉 increases monotonously with increasing strain. This result suggests that the crystalline chains orient to the oblique direction (θ = 30-50°) even during uniaxial stretching [38][39][40][41] . Oblique orientation of the crystalline chains has been observed in both PE and PP 43) , and is caused by the suppression of the molecular orientation due to the excluded volume effect of the rigid and bulky lamellar structure 44) .…”

“…The orientation degree of crystalline chains depends on its molecular weight and molecular weight distribution: samples with lower M w or higher M w /M n values have a high degree of molecular orientation in the stretching direction with uniaxial stretching behavior. 40,41) On the other hand, the values of 〈P 2 〉 and 〈P 4 〉 lie in the dotted curve above 60 °C, which represents an ideal uniaxial orientation, as shown in Fig. 3(c) 42) .…”

“…The crystalline chains orient markedly to the stretching direction beyond the yield point, whereas the orientation function of the amorphous chains immediately increases in the elastic region and reaches an asymptotic value in the strain-hardening region 37) . Kida et al measured the strain dependences of both orientation parameters and the orientation distribution function of crystalline chains during the uniaxial elongation of various PE samples [38][39][40][41] at various temperatures 42) . For high-density PE (HDPE) with a crystallinity of more than 60 vol%, the values of 〈P 2 〉 and 〈P 4 〉 during the yield deformation are in the region (iv): the values of 〈P 4 〉 decrease in the yield region, whereas 〈P 2 〉 increases monotonously with increasing strain.…”

Application of solubility parameters in the preparation of PMMA with permanent antistatic, high toughness, and excellent optical properties

“…It can be seen from Figure 3 that by improving the preparation process of PMMA, the tensile strength of pure PMMA has increased from 56.6 to 69.6 MPa. It can attribute to the increase in molecular weight, which increases the chain entanglement between the crystals, thereby enhancing the connection between the longitudinal and transverse microfibers 31 . During the tensile test, chain entanglement inhibits the relative sliding of microfibers and thus increased tensile strength.…”

Visualizing molecular weights differences in supramolecular polymers

“…The number-average molecular weight ( M n ) of polymeric materials has a direct effect on their mechanical properties ( 1 – 3 ). Typically, the higher the molecular weight of the polymer, the greater its hardness and strength, as exemplified by polyethylene ( 4 , 5 ). Therefore, determining the molecular weight of a polymeric material is often a first key step in its characterization.…”