This paper describes the processing conditions needed to produce low density nanocellular polymers based on polymethylmethacrylate (PMMA) with relative densities between 0.45 and 0.25, cell sizes between 200 and 250 nm and cell densities higher than 10 14 cells/cm 3 . To produce these nanocellular polymers, the foaming parameters of the gas dissolution foaming technique using CO 2 as blowing agent have been optimized. Taking into account previous works, the amount of CO 2 uptake was maintained constant (31% by weight) for all the materials. Foaming parameters were modified between 40˝C and 110˝C for the foaming temperature and from 1 to 5 min for the foaming time. Foaming temperatures in the range of 80 to 100˝C and foaming times of 2 min allow for production of nanocellular polymers with relative densities as low as 0.25. Cellular structure has been studied in-depth to obtain the processing-cellular structure relationship. In addition, it has been proved that the glass transition temperature depends on the cellular structure. This effect is associated with a confinement of the polymer in the cell walls, and is one of the key reasons for the improved properties of nanocellular polymers.

from the gas dissolution foaming process, but previous studies carried out with silica aerogels (nanostructured materials presenting transparent character [8] ) suggest that pore sizes should be around ten times smaller than the wavelength of light, and in addition the cell size distribution should be very narrow.Previous papers have dealt with the reduction of cell sizes below 50 nm following different strategies. Some of them are the use of self-assembled order copolymers that can provide nanocellular structures from 100 [9] to 10 nm, [10] the use of homopolymers based on polycarbonate (PC) that can produce materials with cell sizes of 20-30 nm [7] or the use of polyphenylsulfone (PPSU) as raw material that allows producing cellular polymers with cell sizes of 20-30 nm [11] However, the authors of these papers did not analyse the possible transparency of these systems. On the other hand, one of the most studied systems for the production of nanocellular materials has been polymethylmethacrylate (PMMA) due to its high affinity with CO 2 . In order to reduce the cell size of this system the solubility of PMMA homopolymer have been improved by means of two different strategies, increasing the saturation pressure (p sat ) [12] or decreasing saturation temperature (T sat ) (reaching cell sizes of 35 nm). [13] But the combination of the two previous strategies at the same time has not been explored.Taking the previous information in mind, the objective of this work is to produce transparent nanocellular materials based on a PMMA homopolymer. In order to do that the CO 2 solubility in the material has been maximized by using T sat of −32 °C and p sat of 6, 10, and 20 MPa simultaneously, exploring these limits for the first time. This method results in an improvement in cell nucleation densities of two orders of magnitude and smaller cell sizes compared with results obtained up to date. Moreover, the materials with cell sizes below 40 nm show a signficant degree of transparency.

Low-density nanocellular polymers based on PMMA/MAM blends are produced. • Low MAM copolymer contents, as low as 0.1 wt%, are used to produce such materials. • The physical mechanisms that allow this reduction of the density are discussed.

A B S T R A C TIn this work, a new system based on poly(methyl methacrylate) (PMMA) sepiolite nanocomposites that allow producing nanocellular polymers by using the gas dissolution foaming technique is described. Nanocomposites with different nanoparticle types and contents have been produced by extrusion. From these blends, cellular materials have been fabricated using the so-called gas dissolution foaming method. An extensive study of the effect of the processing parameters (saturation pressure and foaming temperature) on the cellular materials produced has been performed. Results showed that among the three sepiolites used, only those modified with a quaternary ammonium salt are suitable for being used as nucleating agents in PMMA. With these nanoparticles bimodal cellular polymers, with micro and nanometric cells, have been produced. Cell sizes in the range of 300-500 nm and cell densities of the order of 10 13 -10 14 nuclei/cm 3 have been obtained in the nanocellular region. A foaming temperature of 80°C and a wide range of saturation pressures (between 10 and 30 MPa) and low particle contents (between 0.5 and 1.5 wt%) allow obtaining these materials. Furthermore, it has been found that cell size in the nanometric population can be controlled by means of the particles content; a reduction in the cell size is obtained when the particles content increases. Finally, results indicate that an increase in the foaming temperature leads to cellular nanocomposites with lower relative densities (below 0.21) and larger cell sizes (above 450 nm).

Nanostructured polymer blends with CO2-philic domains can be used to produce nanocellular materials with controlled nucleation. It is well known that this nanostructuration can be induced by the addition of a block copolymer poly(methyl methacrylate)-poly(butyl acrylate)-poly(methyl methacrylate) (MAM) to a poly(methyl methacrylate) (PMMA) matrix. However, the effect of the block copolymer molecular weight on the production of nanocellular materials is still unknown. In this work, this effect is analysed by using three types of MAM triblock copolymers with different molecular weights, and a fixed blend ratio of 90 wt% PMMA and 10 wt% of MAM. Blends were produced by extrusion. As a result of the extrusion process, a non-equilibrium nanostructuration takes place in the blends, and the micelle density increases as MAM molecular weight increases. Micelle formation is proposed to occur as result of two mechanisms: dispersion, controlled by the extrusion parameters and the relative viscosities of the polymers, and self-assembly of MAM molecules in the dispersed domains. On the other hand, in the nanocellular materials produced with these blends, cell size decreases from 200 to 120 nm as MAM molecular weight increases. Cell growth is suggested to be controlled by the intermicelle distance and limited by the cell wall thickness. Furthermore, a theoretical explanation of the mechanisms underlying the limited expansion of PMMA/MAM systems is proposed and discussed.

a b s t r a c tPolymer foams with controlled and templated pore size have been obtained for the first time by CO 2 gas dissolution foaming from poly(methyl methacrylate) (PMMA) films. This kind of materials, with a variable porous structure, mimic some high-performance natural materials and could present significant interest in many applications. However, up to now their controlled fabrication has not been successfully achieved. Herein, we present a method to achieve a fine control in the production of such materials. Thermal in situ synthesis of ZnO nanoparticles from Zn(OAc) 2 was proposed to obtain PMMA nanocomposites, in which the ZnO nanoparticles induce heterogeneous nucleation that leads to formation of pores with size below the micron, upon CO 2 foaming. Starting from templated solid PMMA samples with well-differentiated regions, presenting or not ZnO nanoparticles, it was possible to obtain PMMA-based foams with well-defined areas of different pore sizes.

a b s t r a c t Nanocellular foams have been produced by means of a gas dissolution process using polyphenylsulfone (PPSU) as matrix polymer. Cell sizes in the range 20-30 nm and cell nucleation densities higher than 10 15 cm À 3 have been achieved for materials with relative densities in the range 0.65-0.75. The influence of both saturation pressure and foaming temperature has been studied. On the one hand, it has been proved that there is a large influence of the amount of gas (CO 2 ) absorbed in the final cellular structure, in fact it has been found a critical CO 2 uptake between 9% and 9.5% at which the cell sizes evolve from the micro to the nanoscale. On the other hand, it has been found that there is a wide range of foaming parameters (foaming time and foaming temperature) in which nanocellular foams can be produced.

The evolution of technology means that increasingly better materials are needed. It is well known that as a result of their interesting properties, nanocellular polymers perform better than microcellular ones. For this reason, the investigation on nanocellular materials is nowadays a very topical issue. In this paper, the different approaches for the production of these materials in our laboratory are explained, and results obtained by using polymethylmethacrylate (PMMA) are shown. Homogeneous nucleation has been studied by using raw PMMA, while two different systems were used for heterogeneous nucleation; adding nanoparticles to the system and using nanostructured polymers as solid precursors for foaming. The effects of the different parameters of the production process (gas dissolution foaming process) have been evaluated for all systems being possible to establish a comparison between the materials produced by different approaches. Moreover, the limitations and future work to optimise the materials produced are also discussed.