Molecular dynamics simulation of diffusion of gases in pure and silica-filled poly(1-trimethylsilyl-1-propyne) [PTMSP]
Introduction
Recently, organic–inorganic hybrid materials have received tremendous attention, for they seemingly offer a relatively accessible means for altering the properties of existing materials without requiring synthesis of entirely new chemical species. In molecular separations, ongoing efforts continue to explore the utility of incorporating porous particles, such as highly selective zeolites, into dense polymeric membranes to improve gas separation properties [1], [2], [3], [4], In theory, such materials might combine the excellent size selectivity of zeolites with the desirable mechanical and processing attributes of polymers. In practice, however, attaining this synergy is difficult. Often, hybrid membrane performance is compromised by the formation of relatively non-selective defects caused by dewetting of polymer chains from the surface of micrometer-sized zeolite particles. These problems with zeolite/polymer adhesion, as well as inadequate particle dispersion in the polymer phase, have hindered the development and implementation of such membranes.
In addition to porous zeolite particles, many types of non-porous, inorganic fillers are added to polymers for a variety of reasons. Traditionally, the incorporation of non-porous fillers, such as metal oxides, silica, or carbon blacks, into polymers reduces gas or vapor permeability. This decreased permeability is the result of a reduction in the amount of polymer through, which transport may occur and an increase in the diffusion path length that penetrant molecules experience [5] as they are forced to take a tortuous course around filler particles to traverse a film.
However, recently incorporation of nanoscale, non-porous fumed silica (FS) particles into high-free-volume, poly(4-methyl-2-pentyne) [PMP] have been discovered to increase gas permeability organic vapor [6], [7], and this highly unusual and counterintuitive result suggests that the volume filling and tortuosity effects are offset by the ability of these tiny particles to disrupt packing of rigid polymer chains, thereby subtly increasing the amount of free volume in the polymer. Such nanoscale hybridization represents an innovative means to tune the separation properties of glassy polymeric media through systematic manipulation of molecular packing. Poly(4-methyl-2-pentyne) [PMP] is the member of a family of substituted acetylene polymers that exhibit poor polymer chain packing in part due to rigid backbones, low interchain cohesion, and bulk substituents. These glassy polymers are characterized by low densities, high fractional free volumes, high gas permeabilities, and in some cases, highest selectivity for organic vapors over permanent gases. Poly(1-trimethylsilyl-1-propyne) [PTMSP] is also the member of a family of substituted acetylene polymers, and it has the similar attributes with PMP, and the people implemented a lot of research on PMP and PTMSP in experiment and theory [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18].
Although experimental works have pointed that gas permeability was increased for silica–PMP and silica–PTMSP nanocomposites, at the molecular level, the interaction between silica nanoparticle and polymer, and the influence of silica particle on the mobility, structure, free volume of polymer was unknown. For the above reasons, it was interesting to undertake a study of such a system using a molecular dynamics simulation technique. In present paper, we selected silica–PTMSP as research object. PTMSP has the highest gas permeability in known glass polymers. It has been proved that the addition of silica particle to PTMSP had little effect on the solubility of gases in PTMSP in experiment, so we only have simulated the diffusion of several gases (H2, O2, N2, CO2, CH4, n-C4H10) through pure and filled PTMSP systems, and have discussed the effect of the concentrations and the sizes of silica particles on diffusion coefficients and free volume of PTMSP, at the same time, have researched the interaction between silica nanoparticle and PTMSP and the influence of the addition of silica to PTMSP on the flexibility of the backbone of PTMSP.
Section snippets
Simulation details
According to the literatures [8], [11], an atactic chain of 50 repeat units with a 50:50 probability for the occurrence of monomers with trans and cis configurations (Fig. 1) was build. To create some roughly spherical silica particles, a α-quartz supercell crystal structure was first created, then all Si and O atoms outside a radius of 9 Å from the center were discarded. The resulted cluster contained 78 Si atoms and 156 O atoms and was electrically neutral (Fig. 1). However, it should be borne
Diffusion of gases through pure and filled PTMSP
Firstly, every model was minimized, then equilibration runs for 500 ps were carried out, at last the density of unit cell for pure and filled PTMSP was fluctuated within the range of ±2% of the experiment density (0.75 g/cm3 for pure PTMSP and 0.80 g/cm3 for filled PTMSP), for example, the change of the unit cell density during the equilibrium stage for pure and one silica particle filled PTMSP was shown in Fig. 3 and the average of the density of the last 100 ps for pure and filled PTMSP unit cell
Conclusions
Fully atomistic equilibrium MD simulations of pure and filled PTMSP systems were performed, and we draw several conclusions as follows:
- (1)
Si atoms of PTMSP play an important role in non-bond interaction between gases and PTMSP, and the treating method of non-bond interaction can affect the diffusion coefficients of the gases.
- (2)
The diffusion coefficients of gases (H2, O2, N2, CO2, CH4, n-C4H10) are increased to different degree.
- (3)
The diffusion coefficients are increased with the enhancement of the
References (22)
- et al.
J Membr Sci
(1991) J Membr Sci
(1993)- et al.
J Membr Sci
(1994) Polymer
(2000)- Kulprathipanja S, Neuzil RW, Li N. US Patent No. 4,740,219,...
J Rubber Chem Technol
(1964)- Pinnau I, He Z. US Patent No. 6,316,684,...
- et al.
Science
(2002) - et al.
J Polym Sci, Part B: Polym Phys
(1998) - et al.
Chem Mater
(2003)
Macromolecules
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