Cell wall penetration, polymers

Chemical modification will be defined for this chapter as any chemical reaction between some reactive part of a wood cell wall component and a simple single chemical reagent, with or without catalyst, that forms a covalent bond between the two components. This excludes in situ polymerizations of monomers in the lumen structure of the wood and those reactions that result in cell wall-penetrating polymer systems that do not result in any cell wall attachment. It is well known that lumen-filling polymer treatment results in large improvements in mechanical properties, but these are mainly a result of the properties of the new polymer introduced [ 1 ]. 

A solution of styrene in methanol to impregnate wood samples, followed by polymerization, was used by Furuno and Goto (1979). Penetration of the monomer into the cell wall was determined by solvent extraction of samples after polymerization. This removed lumen located polymer, whilst leaving the cell wall bound polymer in place. This showed that the concentration of cell wall bound polymer increased in proportion to the monomer content in methanol, up to a maximum of 80% of the monomer in the solvent. No cell wall penetration was observed for treatment with neat monomer. This was also found for bulking of the wood, as determined from external dimensions of the samples. Improvements in ASE were obtained as a result of the presence of cell wall bound polymer. To achieve similar ASE values with lumen located polymer required very high polymer loadings. 

Many monomers have been studied, including acrylonitrile, acrylates, methacrylates, styrene, and t-butylstyrene (ii). Most of the research done with these monomers has shown that the formed polymer is in the lumen rather than in the cell wall. Swelling, cell-wall-penetrating solvents can be used to achieve cell wall penetration, so that the polymers that form are in both the cell wall and the lumen. 

Although a few mechanisms have so far been proposed to explain the antimicrobial properties exhibited by proanthocyanidins (e.g., inhibition of extracellular enzymes) [86], Jones et al. [83] postulated that their ability to bind bacterial cell coat polymers and their abihty to inhibit cell-associated proteolysis might be considered responsible for the observed activity (Table 1). Accordingly, despite the formation of complexes with cell coat polymers, proanthocyanidins penetrated to the cell wall in sufficient concentration to react with one or more ultra-structural components and to selectively inhibit cell wall synthesis. Decreased proteolysis in these strains may also reflect a reduction of the export of proteases from the cell in the presence of proanthocyanidins [83]. 

There are at least five types of phenylpropanoid related reactions which appear to occur in plant cell walls. Two are UV-mediated photochemical reactions, and hence may be restricted only to the first few layers of cells under the plant surface due to poor penetrability of the light (3). The other reactions appear to be enzymatically mediated, and result in the formation of dimers or polymers from the corresponding monomeric units. 

In certain instances, however, factors other than the cell wall polymers of the phellem may be important in the protection provided by the secondary surface. Rosellinia desmazieresii inoculated in a food base onto the underground stems of a resistant Salix repens hybrid (5. x Friesiana) exhibited greatly reduced epiphytic growth and cord formation compared with inoculations onto susceptible S. repens itself. Attempted penetration was not observed on the resistant hybrid (30). This behaviour suggests that diffusible chemical inhibitors at the stem surface may be important in resistance to this pathogen, which has a demonstrated ability to degrade suberin and penetrate the surface periderm (30). 

Some research has been done on the addition of polar solvents to the nonpolar monomer in an attempt to swell the cell wall structure and anchor it in a swollen state (9). This can be done and the antishrink efficiency (ASE) does increase, but after the solvent evaporates, the wood is only partially loaded which in turn decreases the physical properties. Wood-polymer composites normally have about 10-15% ASE, which means that there is some penetration of the cell wall structure to reduce the swelling over that of untreated wood. 

The mechanism of adhesion is also an important factor in failure analysis in composites [31]. Some adhesives work due to a physical entanglement of the resin into the wood structure whereas others require a free hydroxyl group on one of the cell wall polymers to participate in a chemical reaction with the resin. Substitution of hydroxyl groups was shown to decrease adhesion between chemically modified veneers due to the loss of hydroxyl functionality [32]. Resins that are water-soluble and depend on a hydrophilic substrate for penetration will be less efficient in chemically modified wood due to the decreased hydrophilic nature of the celt wall resulting from modification [33]. 

To improve dimensional stabiUty, low molecular weight chemicals are used that penetrate the cell walls and either bond to the cell wall polymers or polymerize in the cell wall. Improvements in dimensional stabiUty are measured by antishrink efficiency (ASE) ... 

In order to measure the fibre saturation point Feist and Tarkow (1967) used a sufficiently large water soluble polymer to preclude it penetrating the cell wall. Stone and Scallan (1968) reversed this approach and used a series of much smaller... 

H. Tarkow In answer to a previous question regarding your work and ours, I would agree that our conclusions are quite similar. Your work is more quantitative. Both stress the difficulty encountered by enzymes in penetrating cell walls of wood substance because of limited submicroscopic pore sizes. We considered dextrans initially as our probe material. We found, however, that commercially available dextran preparations, such as dextran 10 and dextran 20, were highly dispersed as revealed by gel chromatography down a Sephadex column. Since one of the requirements for the solute exclusion procedure is a uniform molecular size, we sought another polymer system and found the polyethylene glycols to be suitable. ... 

FIG. 2—Models to illustrate the difference between an interfacial bond and those involving adhesive penetration into the wood cell wall, including interdigitation, adlayer, and a fidly interpenetrating polymer network. 

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