Procedimiento para calificar los árboles en campo

When a waterborne mixture of colloids is frozen, if not entrapped in the ice- crystals, the colloids are confined in the region between them. The ice-crystals can be removed by subsequent freeze-drying. The result is a porous composite network showing the reciprocal structure. In the case of self-supported materials, when the structure does not collapse, the porosity can be estimated from the percentage of solids whereby the structure of the foams mirrors the morphology of the ice-crystals (see Table VI-1). Control on the freezing

temperature, and hence the size and morphology of the ice-crystals, is therefore of crucial importance to template the structured material.

The samples were frozen in a slush of liquid/solid nitrogen (ca. -210 °C). Low temperature is employed in order to generate micron-sized ice-crystals. The ice-crystals typically have the shape of columns, dendrites or “ice fingers”.18, 20 Subsequently, the ice-crystals were sublimated by exposing samples to a reduced pressure of 1 mbar (or 100 Pa). The temperature of sublimation was estimated atca.250 K (or -23 ºC) from the pressure-temperature phase diagram of H2O.18 Highly porous materials were obtained and could be imaged at room

temperature using scanning electron microscopy, see Figure VI-1 (A and B). It is interesting to note that, at atmospheric pressure and room temperature, the ice can melt resulting in damage to the foam structure, see Figure VI-1 (C and D). Collapse was logical due to the importance of capillary forces at this length scale; the cohesion of the structured material was not strong enough.21

Figure VI-1. SEM images of composite foams obtained after subsequently freezing a waterborne mixture of colloids and, freeze-drying (A and B) or drying (C and D) the sample (“hard” silica nanoparticles (Ludox TM-40) and larger “soft” PVL latex: mass ratio of silica/polymer = 4.33 and overall solid contents of the samples = 24.7 wt%).

Morphology controlled by heat transfer

The porous structure of the foams mirrors the morphology of the formed ice crystals. The obtained cellular structures were clearly affected by heat transfer and mass transfer experienced during the freezing of the water. Notably, for portions of the samples located at sufficient distance from its boundaries, elongated and parallel sheets of foam are observed, while perpendicularly to the elongated-cell axe, a more cellular morphology is found (see e.g. Figure VI-2.B). In these zones, equiaxed crystal growth according to an orientation parallel to, or as close as possible to, the main heat flux direction occurred,18 leading to parallel needle-shaped ice crystals.22-23 This is in accordance with results obtained from directional freezing experiments carried out by Zhang and

10μm 2μm 1μm 10μm B A D C

coworkers16and the works by Devilleet al..24As the result of restrictions in heat transfer, the clear increasing gradient in average pore size from the outer regions of the sample, its boundaries being directly exposed to slush of nitrogen, towards its inner content is observed.25-26 The arrows on the SEM images of Figure VI-2 show the effect of the phenomenon of heat transfer. From the bottom of the arrows, where the sample was in contact with the silicon wafer, and therefore the initial contact with the nitrogen slush, very small pores are formed and the pore size increases along the arrows.

Figure VI-2. SEM images of composite foams obtained after freeze-drying a waterborne mixture of colloids (“hard” silica nanoparticles (Ludox TM-40) and larger “soft” PVL latex: mass ratio of silica/polymer = 4.33 and overall solid contents of the samples = 24.7 wt%). The red arrows show the phenomenon of

heat transfer; the direction going from smaller to larger pores. Scale bars 10 μm.

VI.3.3. Reinforcement with “small-hard” nanoparticles Reinforced structure and high porosities

A series of polymeric foams were prepared using mass ratios of Ludox TM- 40 colloidal silica nanoparticles and poly(vinyl laurate) particles of 0, 0.22, 0.38, 0.46 and 0.69, respectively at corresponding overall solids contents of the waterborne mixtures of colloids of 9.3, 12.1, 13.0, 13.6, and 19.7 wt%. SEM analysis at room temperature of the fabricated materials is shown in Figure VI-3.A-E.

Figure VI-3. SEM images of polymeric foams obtained after freeze-drying a waterborne mixture of “hard” silica nanoparticles (Ludox TM-40) and larger “soft” poly(vinyl laurate) latexes at increasing amounts of silica nanoparticles. Mass ratios of silica/polymer are A = 0, B = 0.22, C = 0.38, D = 0.46, E = 2.22. Overall solid contents of the samples are: A = 9.3 wt%, B = 12.1 wt%, C = 13.0 wt%, D = 13.6 wt%, E = 19.7 wt%.

As shown in Figure VI-3.A, absence of “hard” silica nanoparticles resulted in the collapsing of the polymer foam at ambient temperatures. PVL has a glass transition temperature far below ambient temperature, around -60 ºC,27 which explains the lack of mechanical strength and deformation of the foam. Upon addition of silica nanoparticles of increasing amounts a porous structure representing the reciprocal template of the ice crystals emerges (Figure VI-3.B- E). These templated composite polymer foams become well-defined when ratios of “hard” nanoparticles to large “soft” polymer latex particles in excess of 0.38 are used. From the quantities of PVL, solid nanoparticles, and water used, the overall calculated porosities ranged from 85% upwards. A series of samples are reported in Table VI-1 in the experimental part, VI.2.5.

Enrichment in silica nanoparticles

Looking at a high magnified image (Figure VI-4) it becomes evident that the cell walls of the composite polymer foam are covered with nanoparticles.

Figure VI-4. Cryo-SEM image of nanocomposite polymer foam obtained after freezing a mixture of a PVL latex and Ludox silica nanoparticles at -210 °C with silica/polymer ratio and total solids content (wt%) of 0.38 and 13.0, respectively. The image was taken after full sublimation of the ice. Scale bar 200 nm.

These armoured structures are a consequence of the difference in size between the large “soft” polymer latex particles and the “hard” silica nanoparticles. The inability of larger particles (~270 nm diameter polymer particles) to approach the solid-liquid interface of the interstitial regions with the channel walls of the growing ice crystals compared to smaller particles (~25 nm diameter silica nanoparticles) generates an enrichment of the nanoparticles near the walls. Similar effects have been observed in composite polymer latex films formed from a blend of silica nanoparticles and a poly(methyl methacrylate-co- butyl acrylate) latex.28

We envisage that the armoured structure not only provides an enhanced mechanical stability of the composite polymer foam, but also introduces a

unique and tailored surface functionality. This can be of great value when these foams are used in areas that require adhesion or adsorption, such as chromatographic applications. Due to the surface excess of the nanoparticles a near saturated functionalisation is achieved at mass ratios of Ludox TM-40 colloidal silica nanoparticles and poly(vinyl laurate) particles of 0.46 and higher. Increasing the silica content to very high ratios generated composite polymer foams of a more brittle nature.