Hygromechanical mechanisms of wood cell wall revealed by molecular modeling and mixture rule analysis

Abstract

Despite the thousands of years of wood utilization, the mechanisms of wood hygromechanics remain barely elucidated, owing to the nanoscopic system size and highly coupled physics. This study uses molecular dynamics simulations to systematically characterize wood polymers, their mixtures, interface, and composites, yielding an unprecedented micromechanical dataset including swelling, mechanical weakening, and hydrogen bonding, over the full hydration range. The rich data reveal the mechanism of wood cell wall hygromechanics: Cellulose fiber dominates the mechanics of cell wall along the longitudinal direction. Hemicellulose glues lignin and cellulose fiber together defining the cell wall mechanics along the transverse direction, and water severely disturbs the hemicellulose-related hydrogen bonds. In contrast, lignin is rather hydration independent and serves mainly as a space filler. The moisture-induced highly anisotropic swelling and weakening of wood cell wall is governed by the interplay of cellulose reinforcement, mechanical degradation of matrix, and fiber-matrix interface.

Fig. 1.. Uniaxial hygroscopic swelling strain (ε) of the polymer systems as a function of moisture content (m).

The MD measurements are represented by symbols and the fit by lines. (A) AGX, uLGN, and M1. (B) GGM, cLGN, and M2. (C) M1, M2, and S2.

Fig. 2.. Mechanical moduli as a function of moisture content.

(A) Schematics of mechanical measurement. (B and C) Bulk moduli, where MD measurements are represented by symbols and fitted with Eq. 7 represented by lines, similarly hereinafter. (D to G) Young’s moduli. (H to J) Shear moduli. (K) Consistency check of transverse shear moduli between MD measurement and prediction.

Fig. 3.. Mixture rule analyses.

(A) Density of compounds and interphase predicted by mixture rules. (B) Uniaxial swelling strains of M1 and M2, comparison of MD measurements with RoM predictions without (thin lines) and with interphase (bold lines), similarly hereinafter. (C) Uniaxial swelling strains of S2 on the longitudinal and transverse directions. Elastic moduli of M1: (D) bulk moduli, (E) Young’s moduli, and (F) shear moduli. Elastic moduli of M2: (G) bulk moduli, (H) Young’s moduli, and (I) shear moduli. Elastic moduli of S2: (J) longitudinal Young’s moduli, (K) transverse Young’s moduli, and (L) shear moduli.

Fig. 4.. Structure, distribution of components, interface mechanics, and hydrogen bonding of the softwood cell wall S2 layer atomistic model.

(A) Three-dimensional view of dry S2 composite. Color denotes different components, namely, CC in black, GGM in red, cLGN in blue, AGX in green, and uLGN in orange. The locations of CC and (B) M2 and (C) M1. Spatial density distribution of (D to F) matrix and (G to I) water at three moisture contents. Note that the CC is white-colored for clarity and is not representing its density. (J) Density of matrix and water as a function of distance d_CC to the CC surface for three moisture contents, 0.02, 0.16, and 0.3. (K) Maximum shear stress as a function of moisture content. (L) Absolute number of HBs, and (M) rescaled number of HBs within the S2 system in function of moisture content. (N) Number of HBs normalized by dry volume: dry condition in green line, m ~ 0.3 in blue line, and the difference in brown bars.

Fig. 5.. Preparation of the S2 layer atomistic model and characterization of fiber-matrix interface.

The chemical structure of (A) CC, (B) AGX, (C) coniferyl unit for cLGN and uLGN, and (D) GGM. (E) General procedure for the preparation of a bulk or mixture material system of polymer chains. (F to L) Assembling procedure of S2 layer model. (M) Schematic of pulling test setup. (N) Interfacial stress as a function of time of dry S2 system, where the maximum shear stresses associated each with a displacement event are marked with triangles.