A universal method for determining the modulus of the interlamellar amorphous phase of semicrystalline polymers was proposed. The basics of the method were presented on the example of high-density polyethylene (HDPE). The local deformation of the amorphous component was induced by the introduction of a swelling agent (hexane). The swelling-induced local strain and local stress of the interlamellar amorphous phase were estimated based on the changes of the long period and the yield stress, respectively. The determined modulus of the interlamellar amorphous phase of HDPE (≈40 MPa) was an order of magnitude higher than the modulus of the bulk rubbery amorphous phase of HDPE (≈3 MPa). The modulus of the interlamellar amorphous phase of other semicrystalline polymers, such as polypropylene (PP), low-density polyethylene (LDPE), and an ethylene−octane copolymer (EOC), was also determined and amounted to 50.3, 11.8, and 4.2 MPa, respectively. For polyethylene materials (HDPE, LDPE, and the EOC), a linear increase in the modulus of the interlamellar amorphous phase with increasing of the degree of crystallinity was observed. This effect was correlated with a decrease in "deformability" of the amorphous layers (a decrease in the swelling-induced local strain of amorphous layers for samples with higher crystallinity). The decrease in "deformability" of the amorphous component at low strains was probably caused by an increase in the density of stress transmitters (entanglements and tie molecules), which affected the mechanical properties (modulus) of the interlamellar regions. In the case of the material with the lowest degree of crystallinity (EOC, 26.2 vol%), the modulus of the interlamellar amorphous phase was similar to the modulus of the bulk rubbery amorphous phase, which indicated a very limited influence of the lamellar crystals on the stiffness of the amorphous regions.

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