Modification of Wood Surfaces via controlled Polymerization Methods
von Martin Königsmann
Datum der mündl. Prüfung:2018-09-27
Betreuer:Prof. Dr. Philipp Vana
Gutachter:Prof. Dr. Philipp Vana
Gutachter:Prof. Dr. Kai Zhang
EnglischThe present study was conducted to explore the possibilities of wood modification by reversible-deactivation radical polymerization (RDRP) techniques in order to tailor surface properties. Surface-initiated reversible addition–fragmentation chain transfer (SI-RAFT) polymerization was carried out on bulk wood to obtain a non-leaching polymer coating on the wood surface, which offers enhanced hydrophobicity and the possibility to easily cleave the polymer coating from wood. Furthermore, surface-initiated activators regenerated by electron transfer atom transfer radical polymerization (SI-ARGET ATRP) was used to graft a hydrophobic polymer on wood flour. The grafted wood was used as a filler material in a thermoplastic polymer matrix to obtain a composite material that maintains the ductility of the polymer matrix but possesses higher strength and toughness. A xanthate was immobilized on the wood surface via the Z-group approach through esterification of the superficial hydroxyl groups in a one-pot reaction with minimal reaction steps on the surface. Afterwards, SI-RAFT polymerization of vinyl acetate and methyl acrylate was conducted to obtain a tailored polymer layer. Water contact angle (WCA) measurements showed a significant change in surface hydrophobicity indicated by a smaller contact angle compared to unmodified wood. Thermogravimetric analysis (TGA) of modified wood depicted combined properties of both polymer and wood, which demonstrated a successful surface-initiated polymerization. The grafted polymer was cleaved from the wood surface by a radical induced single addition-fragmentation chain transfer step. The cleaved polymer showed the characteristics of a controlled polymerization. The immobilization of an ATRP initiator on wood was conducted using -bromoisobutyryl bromide. Surface-initiated ARGET ATRP of methyl acrylate was performed in the absence of a sacrificial initiator using ascorbic acid as an environmentally friendly reducing agent. Attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy and differential scanning calorimetry (DSC) measurements revealed the successful grafting of poly(methyl acrylate) (PMA) on the wood surface. Through TGA experiments, it was possible to assess the amount of grafted polymer, which increased with progressing polymerization time. Furthermore, the wetting properties were examined via WCA and dynamic vapor sorption (DVS) measurements demonstrating a greatly increased hydrophobicity. To examine the properties of the grafted polymer, control experiments were performed using wood flour covered with a cleavable ATRP initiator bearing a disulfide moiety. Size-exclusion chromatography (SEC) analysis of the detached polymer confirmed the living character of the polymerization. After cleavage, the amount of resulting surface-located thiol groups was determined quantitatively via UV/vis spectroscopy using Ellmann's reagent to assess the grafting density of the initiator. Wood flour-reinforced thermoplastics consisting of PMA as polymer matrix were produced by solvent casting. Within these composites, functionalized wood particles with varying amount of grafted polymer were incorporated into the polymer matrix in a constant mass fraction. In a second series of measurements, grafted wood particles with constant amount of grafted polymer were incorporated into the polymer matrix in varying mass fractions. Tensile testing showed that a longer polymerization time led to a higher reinforcing effect on the resulting composite and revealed an optimum of added grafted wood particles at 7 wt%. The Young's modulus, yield point and tensile toughness were increased up to 150 % compared to the polymer matrix. Dynamic mechanical analysis (DMA) revealed decreased viscous behavior of the composite when wood with a low amount of grafted polymer was incorporated and increased viscous behavior with longer polymerization times. The higher the amount of added grafted wood, the higher the elastic behavior of the composite. The glass transition temperature of the composites was observed to be hardly affected by the incorporation of wood particles. The results presented here show that addition of grafted wood particles into a polymer matrix results in composites with increased strength and ductility when compared to the pure polymer while maintaining the same thermal range of application.
Keywords: ATRP; wood; composite; surface modification; graft polymerization; mechanical properties; tensile testing; dynamic mechanical analysis; poly(methyl acrylate) (PMA); poly(vinyl acetate) (PVAc); RAFT polymerization