The study of mechanical properties and thickness of the interphase in polymer-matrix composites is necessary for a deep understanding of the strength and reliability of the composite material. The interphase is an intermediate layer between different phases, has a significant effect on the stress distribution inside the material and, as a result, affects the mechanical and operational characteristics of the composite. The selection of the interphase parameters can improve the elastic, strength and fatigue properties of the composite. The purpose of this work is to investigate the influence of external curing pressure on the formation of interphase thickness and its elastic properties in a fiberglass plastic composite. Based on data obtained using dynamic force microscopy, experiments demonstrate that increasing external curing pressure from 0 to 10 MPa leads to a decrease in interphase thickness from 1232 ± 95 nm to 212 ± 71 nm, and this decrease is described by an exponential dependence. The computational mesomechanical model of the composite monolayer shows that increasing pressure from 0 to 2 MPa increases the elastic modulus of the interphase from 6.3 GPa to 7.9 GPa. At the same time, external pressure does not affect the Poisson’s ratio of the interphase, which remains in the range of 0.18–0.20. The Poisson’s ratio of the monolayer does not change with increasing external curing pressure and constitutes 0.24 ± 0.02, which is related to a compensatory effect caused by redistribution of volume fractions of structural components: an increase in the fiber fraction with low Poisson’s ratio (0.23) and a decrease in the matrix fraction with high Poisson’s ratio (0.33) are compensated by changes in the contribution of the interphase. The study also shows that the elastic modulus of the interphase exceeds the elastic modulus of the matrix, while the Poisson’s ratio is lower. In addition, the work determines the elastic modulus and Poisson’s ratio of single glass fibers based on micromechanical tests.
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