Positron porosimetry study of mesoporous polymer–s

Authors: Radosław Zaleski Marek Gorgol Agnieszka Kierys Jacek Goworek

Publish Date: 2016/02/01

Volume: 22, Issue: 4-6, Pages: 745-754

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Abstract

The polymer–SiO2 composites prepared using polymers with different chemical composition and porosity ie XAD4 XAD16 and XAD7 as well as series of composites synthesized with different contents of silica species and at different pH were investigated The study was conducted to compare the results of nitrogen adsorption and positron porosimetry for these three sets of samples The possible causes of the inconsistency between the results of both methods are discussed The impact of the chemical character of the polymer support surface on the final properties of the polymer–silica composite was found to be greater than its porosity ie the shape of pores and their connectivity Changing the SiO2 content influences mostly the pore volume of composites To significantly modify their pore size distribution more than 30 wt of silica species are required The composite synthesized at pH 5 has the widest pore size distribution among all samples supported by XAD7 The discrepancies concerning the porosity derived with both methods are the smallest for composites where XAD7 was used as the support This allows determination of the relation V = 338 VPALS−003 cm3/g which may be useful for estimation of pore volumes corresponding to those found by nitrogen adsorptionCrosslinked polymers having excellent porosity and unusually high bulk density especially these in the form of spherical beads are very attractive for many applications including adsorption and separation processes supports for exchange of ionic compounds and catalysts or carriers for drugs and immobilization of enzymes etc Kolarz et al 2002 Oh et al 2008 Li and Chase 2010 Grochowicz et al 2013 Maciejewska and Kołodyńska 2015 Obviously their application is closely related to both their chemical composition and microscale morphology Therefore much effort was directed towards the design and synthesis of new polymers with tailored properties Horák et al 2004 Grochowicz et al 2008 Wu et al 2012 Naghash and Shafie 2013 Zaleski et al 2015 A very interesting approach to the modification of the properties of preformed porous polymers is combining them with various inorganic species with technological importance such as silica gel titania zirconium oxide and others Suzuki et al 2000 Kierys et al 2010 2013 Koubková et al 2014 Polymer beads play a critical role in such polymerinorganic composite materials by acting as supports templates or matrices They may be assumed as a universal phase that allows deposition of an inorganic component with different appearance and physical properties Furthermore the interior of polymer beads can be considered as a reaction system ensuring a specific microenvironment where transformation of precursors of inorganic species takes place Therefore it might be expected that such a polymersupported system in which significant isolation of space is achieved prevents inorganic nanoparticles from aggregation which is very important for preserving their morphological properties Since such polymersupported systems possess a highly developed and complex internal pore structure the determination of changes in the porosity of composite materials in the micro and mesoscale in comparison to the employed polymer support is a crucial issue for their successful separation and applicationPositron porosimetry based on positron annihilation lifetime spectroscopy PALS is a highly suitable technique for thorough investigation of porosity changes in such polymersupported systems since it provides information about the system in conditions where it is used Unlike the most common adsorption methods ie the lowtemperature gas sorption method or mercury porosimetry positron porosimetry makes it possible to investigate materials without cooling thereof In addition a great advantage of this technique is undoubtedly the possibility to investigate any free volumes without using any adsorbate molecules This is especially important in the case of polymersupported samples because of their ability to swell in different solvents both in gases and liquids Moreover this technique provides reliable information concerning changes in any free volumes with diameters below 2 nm up to several tens of nanometers Its usefulness for investigation of various free spaces which include mesopores He et al 2013 Hill et al 2009 micropores Kullmann et al 2010 and even intermolecular free volumes Zgardzińska and Goworek 2015 has already been recognizedThis work is focused on the correlation between structural parameters derived from the PALS technique and from the lowtemperature nitrogen sorption for a series of polymer–silica composites synthesized by the swelling of the preformed porous polymers in a silica precursor tetraethylsilane TEOS Polymer–SiO2 composites were prepared using crosslinked polymers with different chemical composition and porosity ie shape of pores and their connectivity as a polymer support The series of composites was synthesized at different pH and with different contents of silica species Application of different polymer supports and processing conditions makes it possible to investigate their influence on the resulting properties of composite materials Variation of such conditions changes the pore structure and arrangement of SiO2 deposits within a polymer supportPolymer resins Amberlite XAD7 ROHM HAAS denoted hereafter by the abbreviation XAD7 XAD4 and XAD16 were supplied by Sigma Aldrich in the form of white insoluble beads and they served as polymer supports Prior to use the beads were rinsed with distilled water and dried in air at 80 °C for at least 12 h Tetraethoxysilane TEOS 98  serving as a silica source was supplied by SigmaAldrich and used as receivedPolymer–SiO2 composites were synthesized according to the procedure described elsewhere Krasucka et al 2015 To synthesize the composites dry polymer beads were soaked with TEOS so as to yield TEOSswollen polymer beads The maximum amount of absorbed TEOS was 21 19 and 08 g of TEOS per 1 g of XAD16 XAD7 and XAD4 respectively Next the TEOSswollen beads were submerged in a 2 M HCl solution 100 mL/1 g and left for 24 h at room temperature The solid product was separated rinsed with water dried at 80 °C and labeled as XAD4SiO2 XAD16SiO2 and XAD7SiO2 respectivelyXAD7SiO2 composite samples with different SiO2 contamination were obtained by soaking with the TEOS solution at different concentrations of anhydrous ethanol Ethanol was used as a good solvent for TEOS to facilitate total filling of pores and homogeneous distribution of TEOS within the polymer beads The maximum amount of TEOS introduced into 1 g of the polymer support by swelling was as follows 00024 00048 00072 and 00091 mol After 6 h of equilibration swollen XAD7 beads were dried for 6 h at 80 °C Next gelation of SiO2 was conducted in a water solution of HCl at pH 044 Then the beads were rinsed with water and again dried at 80 °C The resulting XAD7SiO2 composites were denoted as XAD7SiO212wt XAD7SiO222wt XAD7SiO230wt and XAD7SiO235wt The number in the name of the composites presents the SiO2 contamination wt in a given sample taking into account the total mass of the polymer–SiO2 compositeThe procedure was begun with soaking the XAD7 beads with TEOS Next the TEOSswollen XAD7 beads were divided into four parts and each group was immersed submerged in a water solution of HCl with an appropriate pH ie 044 2 5 and 9 and left for 24 h at room temperature After 24 h the solid product was rinsed with water and dried at 80 °C The composites were labelled as follows XAD7SiO2pH044 XAD7SiO2pH2 XAD7SiO2pH5 and XAD7SiO2pH9

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