Abstract: The alignment angle and distribution of thermal conduction pathways are key structural parameters for regulating the thermal conductivity of polymer composites. However, their intrinsic quantitative influence on thermal conductivity remains unclear, limiting the rational structural design and performance optimization of thermally conductive composites. Herein, two-dimensional (2D) copper wire (Cw)/PLA composites were fabricated via 3D printing, in which Cw thermal conduction pathways were embedded within the poly(lactic acid) (PLA) matrix with controllable pathway alignment angle and distribution. The results show that the in-plane thermal conductivity coefficient (λ_//) of 2D Cw/PLA composites is positively correlated with the Cw pathway alignment angle θ (0° ≤ θ ≤ 90°) and the distribution uniformity factor K (0 ≤ K ≤ 1). When the Cw volume fraction is 12.6 vol%, and the Cw pathways are orthogonally aligned (θ = 90°) with uniform distribution (K = 1), the λ_// increases to 3.65 W/(m K), which is 40.9 % higher than that of the composites (2.59 W/(m K)) with parallel Cw pathways (θ = 0°) and completely non-uniform distribution (K = 0), and approximately 14 times that of pure PLA (0.26 W/(m K)). Based on the optimized geometric mean thermal conductivity model, a quantitative relationship was established linking θ and K with the λ_// of the 2D Cw/PLA composites. The average ratio of the theoretically predicted λ_// to the experimentally measured λ_// is 1.04, validating the quantitative influence of thermal conduction pathway alignment angle and distribution uniformity on the thermal conduction behavior of the composites. This study deepens the understanding of how thermal conduction pathway geometry governs heat conduction in polymer composites and provides both theoretical and practical guidance for the rational design of thermally conductive composites.

Key words: 3D printing, Thermal conduction pathway, Thermally conductive composites, Thermal pathway alignment angle, Thermal pathway distribution