Effects of density fluctuations on the fracture behavior of amorphous polymers investigated by time-resolved small-angle and wide-angle X-ray scattering

Abstract

We investigated the influence of nanometer-scale density fluctuations on fractures in two amorphous polymers, poly(methyl methacrylate) (PMMA) and polystyrene (PS), using time-resolved small-angle X-ray scattering (SAXS)/wide-angle X-ray scattering (WAXS) combined with mechanical tests. Three-point bending and tensile tests showed different fracture behaviors: PMMA formed a relatively uniform plastic zone with internal crazes before an abrupt fracture, whereas PS produced abundant surface crazes and fractures at lower displacements. The elastic moduli indicated that both materials have bulk moduli greater than their shear moduli, with the bulk modulus of PMMA exceeding that of PS. Static SAXS revealed nanoscale density fluctuations in both polymers: PMMA exhibited small-amplitude, short-correlation-length Debye–Bueche-type fluctuations, whereas PS showed much stronger scattering with fractal-like power-law behavior. In situ SAXS during tensile deformation revealed that PMMA developed anisotropic, Ornstein-Zernike-Debye-type fluctuations along the tensile axis, with markedly increased low-q intensity near the necking. PS exhibited abrupt Porod-type scattering and fibrillar signatures of crazing (average fibril diameter ≈ 6.0 nm) with minimal WAXS change. We concluded that the amplitude and spatial scale of the initial density fluctuations govern the local strain concentration and energy dissipation pathways, thereby accounting for the distinct brittle-like fracture modes of PMMA and PS. Time-resolved SAXS/WAXS reveals that nanoscale density fluctuations govern the fracture behavior of amorphous polymers. PMMA exhibits DB-type fluctuations with small amplitude that evolve into anisotropic OZD-type fluctuations under deformation. In contrast, PS shows large fractal-like fluctuations that transform into Porod-type scattering with fibrillar signatures, resulting in numerous surface crazes. These findings demonstrate that nanoscale density fluctuations govern the distinct brittle-like fracture modes of PMMA and PS.