Morphological evolution of polymorphic crystalline structures during 3D-printing of polypropylene

Abstract

This study establishes a quantitative framework for morphological evolution during three-dimensional (3D) printing of polypropylene, a commodity thermoplastic, using spatially resolved, in-situ synchrotron micro-focused wide-angle X-ray scattering (WAXS). Measurements at varying bed temperatures and local heights within the printed road enabled direct quantification of polymorph selection, crystallinity, lattice parameters, orientation, and crystallization kinetics. WAXS patterns remained isotropic under all conditions, indicating the absence of preferred crystalline orientation induced by printing. Crystallization proceeded from the amorphous melt through abrupt formation of α and γ phases, followed by lattice densification and progressive crystallinity growth. Morphological parameters depended systematically on bed temperature and spatial height. Height-dependent crystallinity confirms a thermal gradient across the printed road, arising from low polymer thermal conductivity and asymmetric boundary conditions imposed by the heated bed and ambient air. A proposed kinetic model captures the full crystallization pathway. Primary crystallization dominates and precedes secondary crystallization (regime-I and II), which remains minor. Both processes are strongly temperature dependent: increasing bed temperature suppresses crystallization rates due to reduced supercooling and enhanced diffusion-controlled chain mobility. Primary crystallization occurs via sporadic nucleation with mixed two- and three-dimensional growth and impingement, whereas secondary crystallization is governed by diffusion-limited chain rearrangement within interstitial regions of the primary crystalline framework. Despite significant spatial variation in absolute crystallinity, normalized crystallization kinetics show negligible height dependence, indicating that the thermal gradient does not alter the underlying nucleation and growth mechanisms. Overall, these results demonstrate that in-situ synchrotron WAXS provides a rigorous quantitative platform for correlating thermal history, spatial position, and crystallization kinetics in polymer additive manufacturing, thereby offering fundamental guidance for morphological control in additively manufactured PP components.