Macromechanical and micromechanical properties of polymers with reduced density of entanglements

Abstract

The macromechanical and micromechanical properties of polypropylene, polylactide, and polystyrene with different entanglement densities and different crystallinities were tested. The mechanical properties were characterized by macroscale compression tests and microindentation hardness measurements. The entanglement density modified the mechanical properties only in the high-deformation region, above the yield point, when the strain-hardening occurred, and ca 10% lower stresses were measured for disentangled polymers. In the low-deformation region, the influence of the entanglement density was negligible. The stiffness of semicrystalline polymers increased with their crystallinity and the stiffness of amorphous polymers increased with their glass transition temperature. For example, the elastic modulus was equal to 1.59 MPa for entangled polylactide, 1.68 MPa for partially disentangled polylactide, and 2.20 MPa for semicrystalline, disentangled polylactide. The stiffness-related properties in both macroscale (compressive modulus, E, and yield stress, Y) and microscale (indentation modulus, EIT and indentation hardness, HIT) showed very similar trends, which was in perfect agreement with theoretical predictions and confirmed the reliability of our results. The supplementary, viscosity-related properties from microindentation experiments (indentation creep, CIT, and elastic part of indentation work, ηIT) supported the above-mentioned conclusions in the sense that also these two low-deformation region properties were independent on the entanglement density.