1、PDF外文：http:/ 中文 2345字 出处： Polymer testing, 2004, 23(7): 835-838 A new methodto estimate pore volume of porousstyrenedivinylbenzene copolymers MuhammadArif Malik , Munir Ahmed, Muhammad Ikram Abstract： A new method to estimate the pore volume of porous styrenedivinylbenzene copolymer beads is p
2、roposed.The estimatedpore volume（ PVe） is basedon the solubility parameters （ i） andthe corresponding weight fractions（ fi） of styrene, divinylbenzenes and diluents in the polymerization mixture. The equation used is: PVe=1.136（ i fi） 2 31.193 i fi 98.825. Regression analysis results sho
3、w that an ideally close match of the predictedpore volume andthe pore volume determined by mercury porosimeter is achievedwith R2=0.99, when non-polarand moderately polar liquids are employed as diluents. In the case of strongly polar (H-bonding) liquids as diluents,the method should be used only to
4、 predict a trend towards an increase or decrease in pore volume, which occurs as aresult of variation in the composition of polymerization mixture. Keywords: Styrene-divinylbenzene； Pore volume estimation； Solubility parameters； Macroporous resins； Polymeric adsorbents 1. Introduction Porous styrene
5、 divinylbenzene (S-DVB) copolymerbeads are produced on an industrial scale as well asin laboratories throughout the worldby o/w suspensionpolymerizationandthen convertedto ionexchangers,chelating resins, supports for catalysts andenzymes, etc. . The monomers are diluted with someinert organic liquid
6、that introduces porosity in thecopolymer . The porosity is determined by amountandsolvating property of the diluents andthe degree ofcross-linkage in the copolymer . This has been asubject of research interest in the past 50 years, as isevident from review articles published from time totime , andso
7、me recent research articles .Porous copolymers from a number of relatedmonomershave also been synthesizedby the same technique,e.g., methacrylamide t styrene divinylbenzene ,acrylonitrile divinylbenzene , glycidyl methacrylateethyleneglycol dimethacrylate , methacrylicacid triethyleneglycol dimethac
8、rylate , acrylicacid triethyleneglycol dimethacrylate, acrylamide ethyleneglycol dimethacrylate , 4-vinylpyridine divinylbenzene, etc. Some important properties of the derived productsdepend on the porous structure of the S-DVB basecopolymer . Pore volume is an important parameterthat is relatedto t
9、he porous structure of thecopolymer. The pore volume in the dry state of thecopolymer is usually determined by nitrogen desorption(BET) or mercury penetration methods. These methodsrequire expensive instruments. We have proposedasimple, inexpensive, yet reasonably accurate methodofpore volume determ
10、ination, which is based on the densityof the dried copolymer beads . Here, we proposea simple empirical relation which can be usedtopre-determine pore volume of the S-DVB copolymersbefore they are actually produced. The relation will behelpful in deciding the appropriate composition of thepolymeriza
11、tion mixture requiredfor obtaining a copolymerof desired porosity. 2. Experimental Solubility parameter （ ） values of liquids (in Hildebrandunit(H) were taken from the CRC Hand Book. When the value was not listedin the reference, itwas calculatedby using the following formula: （ d（ Hv RT） /M）
12、1/2(where d， Hv and M are density,heat of vaporization andmolecular weight,respectively, for the liquidand R and T are gas constantandabsolute temperature, respectively, takenfrom Ref. ). The solubility parameter can also beestimatedbasedon the group contribution methodofHoftyzer Van Krevelen andthe
13、 methodof Hoy .The d values of styrene,divinylbenzene and diluent/swere multipliedwith their corresponding weight fraction（ fi） in their mixture andsummedto get i fi. Asample of pore volume values, determined by mercuryporosimetry （ PVm） andreportedby different researchgroups at different time
14、s, was taken from the publishedliterature. The PVm values were plottedas afunction of Pdifi and a second-degree polynomial wasfittedto the data using Microsoft Excel software in theform: PVe=A（ i fi） 2 B i fi C. In this equation,PVe is the pore volume estimatedby the best-fit equationand A, B and C
15、are the constants determining theshape of the curve. Some S-DVB copolymers were synthesizedto experimentallyverify the proposedpore volume estimationmethod. Styrene, divinylbenzenes (comprising of 60%divinylbenzene isomers and 40% ethylvinylbenzene isomers)andd iluents were mixedat 26.6%, 13.3% and6
16、0% ratio by volume, respectively. Benzoylperoxide(1% by weight) was dissolved in the mixture to initiatepolymerization. The mixture was suspended in water ata 1:5 volume-to-volume ratio, under mechanical stirringandat room temperature. Gum Arabic (3% byweight) was pre-dissolved in the water. The tem
17、peraturewas raisedto 80C andmaintained at 80C for20 h. Then, the copolymer beads were filtered out on aBuchner funnel andwashedwith hot water andacetone.The beads were sieved and the fraction havingdiameter 60 160 lm was selectedfor further studies.The beads were treated with acetone in a Soxhletapp
18、aratus for a few hours, then left in acetone atroom temperature for around16 h. Excess acetonewas removedby centrifugation and the volume of theacetone-swollen beads (Vs) was recorded. The beadswere dried at 110C until constant weight and theweight (Wd) andvolume (Vd) of the dried beads wasrecorded.
19、 Density (d) of the dried beads was calculatedby using the following formula: d=Wd/Vd. The swellabilitycoefficient in acetone (SCa) was calculatedbyusing the following formula: SCa =(Vs-Vd)100/Vd.Pore volume of the dried beads (PVm) was determinedby mercury porosimeter, Autopore II 29220 fromMicrome
20、ritics. 3. Results and discussion Fig. 1 illustrates the plot of PVmversus i fi.Several types of curves were fittedto the data andasecond-degree polynomial was found to fit betterthan others. The second-degree polynomial was refinedby excluding the data points with i fi>9.7. Theexcluded data poin
21、ts involved H-bonding liquids asdiluents, where strong intermolecular forces make itdifficult to predict their solubility properties. Including the data points obtained experimentally in the presentwork further refinedthe polynomial. The final relationshipwas foundto be the following:PVe=1.136（ i fi
22、）2 31.193 i fi 98.825. The R2valuewas foundto be 0.96 for the data points taken fromliterature andit improved to 0.99 for the data points ofthe present study. The difference is explained by thefact that different research groups reported the datapoints in different time periods, whichincreases the e
23、lement of random error in the data. TheR2values andthe visual observation from Fig. 1 showthat pore volume can be estimatedwith reasonablygoodaccuracy when i fi 9.7H. It covers almostall the non-polar and moderately polar liquidsemployed as diluents. In the case of H-bonding diluents,the proposedmet
24、hodmay not predict the porevolume accurately, but it can predict a trend towardsan increase or decrease in pore volume as a resultof variation in the composition of the polymerizationmixture. The methodis basedon the polymerization mechanismproposedby many researchers . Accordingto the mechani
25、sm, the copolymer molecules stay swollenif their value is close to the value of the ambientliquid. When the diluents are removed at the end, thecopolymer matrix shrinks showing no pores in the drystate. The copolymers may still have large spacesbetween copolymer chains andcross-links, i.e., microret
26、icularporosity, which may reappear upon swellingwith some goodsolvent. On the other hand, if the value of the copolymer differs from the value of theambient liquid, the copolymer precipitates and acquiresspherical shapes callednuclei. The nuclei grow intomicrospheres as the polymerization progresses
27、. Themicrospheres agglomerate with each other resulting ina honeycomb like structure. When the diluents areremoved, the permanent pores or macroreticularporosity is left behind, which, in fact, is the void spacebetween microsphere agglomerates. Pore volume in thedry state of the copolymer is a measure of its macroreticularporosity. The larger the difference between thed values of the copolymer andthe ambient liquid, thestronger will be the phase separation effect betweenthe copolymer andambient liquidfor microsphericalagglomeration, resulting in higher pore volume. This isthe
