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Reinforcing of expanded polymer morphology using peroxy radical initiator
Reactive & Functional Polymers 69 (2009) 353–357
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Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react
Reinforcing of expanded polymer morphology using peroxy radical initiator Krasimira Soukupová a,*, Alessandro Sassi b, Karel Jerˇábek a a b
Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, 165 02 Prague 6, Czech Republic Istituto di Scienze e Tecnologie Molecolari-CNR, Dipartimento di Processi Chimici dell’Ingegneria, Università di Padova, via Marzolo 9, 35131 Padova, Italy
a r t i c l e
i n f o
Article history: Received 27 January 2009 Received in revised form 9 February 2009 Accepted 21 February 2009 Available online 28 February 2009 Keywords: Post-crosslinking Radical initiator Polymer morphology Specific surface area
a b s t r a c t There was investigated the possibility of increasing the apparent specific surface area of polystyrene sorbents by radical-induced post-polymerization reaction using di-tert-butyl peroxide as free radical initiator. It is known that in addition to the activation of vinyl groups, tert-butyl peroxide is also able of abstraction of hydrogen atoms from aliphatic carbons and this way produces active centers for creation of post-polymerization crosslinking. Radical-induced post-polymerization morphology changes were confirmed by detailed morphological examinations of both dry and swollen materials showing distinct reinforcing of the expanded polymer morphology. However, due to the specific character of the postpolymerization reaction, the achieved additional crosslinking was not extensive and only moderate increase to the apparent BET surface area was achieved. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Accessibility of the interior of functional polymers like adsorbents or catalyst supports is the highest when the polymer is swollen by suitable solvent. After removal of the swelling solvent e.g., by drying, the polymer morphology at least partially collapses. Extent of the collapse depends on the rigidity of the polymer skeleton. For applications requiring high porosity of a functional polymer even in absence of swelling solvent there is advantageous to reinforce the polymeric skeleton against collapse by post-polymerization crosslinking. Such post-polymerization modified polymers then may exhibit extremely high apparent surface area [1]. Known procedures for enhancement of polystyrene adsorbents porosity are based mostly on chloromethylation [1] or Friedel– Crafts catalyzed alkylation involving residual double bonds [2,3]. In this contribution we report an attempt to increase the apparent specific surface area of dry polymers by treatment with di-tert-butyl peroxide radical initiator in thermodynamically good solvent facilitating softening and expansion of the polymer matrix. Swelling of styrenic polymers in a good solvent substantially decreases their glass-transition temperature [4] and therefore increases the mobility of polymer chains and reduces the sterical hindrances in the polymer skeleton. As a free radical initiator, di-tert-butyl peroxide is known for its specific mechanism of the radical polymerization initiation. Besides initiation of the polymerization of the residual double bonds, the primary t-butoxy radicals are also known for their * Corresponding author. Tel.: +420 220 390 332; fax: +420 220 920 961. E-mail address: [email protected] (K. Soukupová). 1381-5148/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2009.02.008
ability to abstract labile hydrogen atoms from an aliphatic chain, thus creating radicals able to initiate further polymerization [5]. This last scenario was used for polymerization of different low molecular weight aliphatics, a reaction known as polyrecombination, introduced by Korshak et al. [6–8]. Even if such a hydrogen abstraction process is more typical of a labile methine group, polyrecombination have been also reported for substances with methylene or even methyl groups [5,6]. The chain growth consists of multiple repeating acts of creation and recombination of radicals, forming gradually dimers, trimers etc., so the polymerization has a step-like character [9]. Since the creation of each reactive site requires one t-butyl peroxide radical, higher amount of initiator is needed. Higher amount of initiator favors the production of branched or crosslinked polymers [6–9]. We decided to investigate whether the ability of di-tert-butyl peroxide to promote beside activation of vinyl groups also other mechanisms of bond formation could be used for post-polymerization modification of morphology of porous functional polymers. In toluene, a swelling solvent for polystyrenes, di-tert-butyl peroxide breaks down reasonably rapidly at temperatures above 100 °C [10] and recombination of generated radicals may create additional crosslinks reinforcing the expanded polymer morphology and preventing its collapse during drying. 2. Experimental We have used three different starting polymers (Table 1). One of them was commercial macroreticular styrene-co-divinylbenzene (DVB) polymer with (for a macroreticular polymer) relatively low content of DVB and therefore, with a good ability to swell in
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Table 1 Description of the investigated polymers. Polymer
Description
Composition (%) Styrene
DVB
EVB
ST-DVB 10% X1-A
Commercial grade, low crosslinked, macroreticular resin (Spolchemie, Czech Republic) Laboratory-prepared resin, 80% DVB (tech. gasoline/monomers 1:1 vol.), bimodal pore size distribution (Institute of Polymer Chemistry, Nankai University, Tianjin, China) Laboratory-prepared resin, 80% DVB, (1,2-dichloroethane/monomers 1:1 vol.), monomodal pore size distribution (Institute of Polymer Chemistry, Nankai University, Tianjin, China)
83 –
10 80
7 20
–
80
20
X2-A
contact with suitable solvents. The two other polymers were much more rigid materials prepared from the same technical DVB without addition of styrene as a co-monomer, but using different porogenic solvents [3]. Before use, the polymers were carefully washed with distilled water for removal of possibly present impurities like e.g. sodium salts and subsequently were dried at 100 °C overnight. Before post-polymerization procedure the polymers were preswollen overnight in toluene purified by molecular sieves and degassed. Toluene is for styrenic polymers swelling solvent able to soften the polymer network and enhance mobility of polymer chains [4]. After addition of di-tert-butyl peroxide as radical initiator in relatively high amount 0.1 g/g polymer, the modification reaction was performed at 104 °C for 14 h. The polymers were then extensively washed with THF in order to remove remnants of the initiator and low-molecular fragments possibly produced during the reaction. One part of the polymer was then dried directly from THF at 70 °C overnight and the second portion was before drying de-swollen by washing with ethanol, which for styrenic polymers is a precipitating solvent. Polymers after the post-polymerization treatment were marked as X1-A PP and X2-A PP. Concentrations of unpolymerized vinyl groups before and after the post-polymerization reaction were assessed by 13C SPE MAS NMR (Single Pulse Experiment Magic Angle Spinning Nuclear Magnetic Resonance) according approach developed by Snape and others [11] using Bruker AC200 spectrometer equipped for solid state analyses and operating at 50.26 MHz. Dry-state morphology of the examined polymers (apparent BET surface area, pore size distribution and total pore volume) was evaluated from nitrogen adsorption–desorption measurements using computerized apparatus ASAP 2010 and associated software (Micromeritics, USA). Swollen-state morphology was assessed by inverse steric exclusion chromatography (ISEC) using methodology described elsewhere [12]. For depicting of the pore size distributions we used uniformly the model of cylindrical pores. For swollen polymer this model may overestimate the pore volume [12], but the quality of steric hindrances in the pore system it depict quite acceptably and in an easily comprehensible format [13]. 3. Results and discussions Assigning of the signals of interest in the 13C SPE MAS NMR spectra is schematically depicted in Fig. 1. Those at 138 and 113 ppm corresponds to residual vinyl groups, signals for mainchain methine (39 ppm) and methylene (46 ppm) groups overlap at around 41 ppm, and the peaks at 30 and 15 ppm correspond to methylene and methyl groups of ethylvinylbenzene derivatives. Comparison of the 13C SPE MAS NMR spectra of the polymers before and after the post-polymerization crosslinking is shown in Fig. 2. Unlike the other two polymers, ST-DVB 10% contains no residual vinyl groups (Fig. 2A). Due to the low degree of crosslinking and high flexibility of its polymer backbone, all the vinyl groups have found a reaction partner already during the initial polymeri-
Fig. 1. Assignment of NMR signals to specific carbon atomtypes present in poly(divinylbenzene) network.
zation. Also, due to low content of ethylvinylbenzene co-monomer, the peaks of the methylene and methyl groups at 30 and 15 ppm are very weak. Between NMR spectra of 10% DVB polymers before and after the radical post-polymerization treatment practically no differences were observed. X1-A and X2-A are especially highly crosslinked polymers containing quite substantial amount of residual vinyl groups (about two monomer units contain one vinyl group [3]) as also pendant ethyl group side-chains substituents (Table 1). As seen in Fig. 2B and C treatment with the radical initiator had distinct influence on the polymers X1-A and X2-A in contrast to the polymer STDVB10%. However, in the course of the radical reaction not all of the residual vinyl groups was consumed. Quantitative analyses of the 13C SPE MAS NMR spectra based on the area of the de-convoluted peaks corresponding to the vinyl and aromatic carbons allowed assessment of the amount of the residual double bonds able to react during the post-polymerization treatment (Table 2). The results evidence that about one-third (or slightly more) of the pendant vinyls take part in the post-polymerization reaction. Besides the lowering of the residual vinyl group content in X1A, X2-A polymers, the radical reaction reduced also the signals of the carbons of pendant alkyl groups at 30 and 15 ppm (Fig. 2B and C). It indicates that the radical post-polymerization crosslinking most probably consists in bonding the residual double bonds with a radical derived from the pendant ethylbenzene group being previously generated by hydrogen atom abstraction (schematized in Fig. 3). The abstraction from this side-chain substituent is anticipated to take place from the CH2 groups which carry labile benzylic H atoms but surprisingly, a decreasing of 15 ppm signal is found as well for both of X1 polymers (Fig. 2). On the other hand, signals for the benzylic CH groups on the polymer backbone
K. Soukupová et al. / Reactive & Functional Polymers 69 (2009) 353–357
A 10 % DVB - original 10 % DVB - after PP treatment
160
140
120
100
80
60
40
20
0
ppm
B X1-A - original X1-A - after PP treatment
160
140
120
100
80
60
40
20
0
ppm
C X2-A - original X2-A - after PP treatment
160
140
120
100
80
60
40
20
0
ppm Fig. 2. 13C SP MAS NMR spectra of the studied polymers. Contact time = 3 ms, spin original, after PP treatment. For assignrate = 7000 Hz, D1 = 6 s, ments of the arrow annotated peaks see the text.
Table 2 Quantitative assessment for the consumption of residual vinyl groups after the postpolymerization treatment (from 13C SPE MAS NMR) and resulting variations in the apparent BET specific surface area and pore volume of polymers (data obtained by nitrogen adsorption measurements).
Residual vinyl group content, mmol/g Apparent BET surface area, m2/g Dried from THF Dried from ethanol Pore volume of polymers dried from THF, cm3/g
X1-A
X1-A PP
X2-A
X2-A PP
3.7
2.2
3.4
2.2
408.2 483.2 0.89
527.6 533.1 0.94
686.0 689.6 0.52
765.6 778.6 0.59
(41 ppm) also possessing labile hydrogen atoms decrease weakly for X2-A and negligibly for X1-A. This may be due to the stereochemical restriction of the H atoms from CH backbone group hampering the generation of a radical (Fig. 3). Besides the reaction of propagation (i.e. reaction of generated radical and a pendant vinyl group), radical–radical coupling may also happen but for the highly crosslinked and immobilized polymer chains this reaction is less likely. Incomplete consumption of the residual vinyls during the radical reaction is in sharp contrast with results of our previously
355
reported post-polymerization modification by Friedel–Crafts alkylation [3], when in the polymers X1-A and X2-A were consumed all the residual double bonds. This difference in the vinyl group consumption probably stems from differences in the availability of reaction partners in radical polymerization and Friedel–Crafts alkylation. While for the creation of additional crosslinks by Friedel–Crafts catalyzed alkylation occurrence of a benzene ring close to the residual vinyl should be no serious problem, effective postcrosslinking using t-butyl peroxide radical initiator is possible only if radicals derived from the ethyl groups are located in close proximity to pendant double bonds. Because of the inhomogeneous distribution of DVB and EVB units in the polymer due to their different reactivity [14], it is not likely that each radical encounters a double bond. Effects of this modification on morphology of the investigated polymers were examined both in the swollen and dry states. In the Fig. 4 are shown swollen state morphologies of the examined polymers determined by ISEC modeled as set of discrete pore fractions, each consisting of pores of a single size only. Pore system of the swollen polymers consists of ‘‘true” pores which is possible to describe using the conventional model of cylindrical pores and pores in swollen polymer gel for which would be more appropriate the model devised by Ogston [15] depicting pores as spaces between cylindrical solid bodies. However, for simplicity we used the model of cylindrical pores in the whole range of the polymer sizes. This modeling may overestimate the volume of the narrow pores in the polymer matrix but offers more comprehensible overview of the pore structure. As it is seen in the Fig. 4, the radical post-polymerization treatment did not change the basic features of the polymer morphologies but the additional crosslinking emphasizes its existing features. The increased fraction of narrowest pores and the diminished dispersity of the pore sizes in the mesopore region were distinctly apparent in both of examined polymers carrying free double bonds (Fig. 4). Collapse of the polymer gel during drying changes the polymer morphology substantially and the extent of these changes depends on the rigidity of the polymer gel. Morphological modifications induced by the radical post-polymerization treatment can be most simply detected by variations of the apparent surface area of dried polymers. Treatment with diverse solvents before polymer drying makes possible to assess rigidity of the polymer skeleton. Swelling by a ‘‘good” solvent (e.g. THF) softens the polymer skeleton and capillary forces generated in the menisci of liquid filled pores may then induce collapse of the pores. Porous structure of macroreticular polymers can be better preserved if before drying the polymer is de-swollen by washing with a non-swelling solvent (e.g. ethanol), which precipitates the polymer and increases its resistance to the collapse during drying [4]. This effect of solvent on the polymer skeleton collapsibility best manifested for resins prepared with very low DVB content [16] may also serve as an indicator of structural stability for higher crosslinked polystyrenes, by the difference of their dry-state morphologies achieved after using the two solvent types. Comparison of apparent BET surface areas of the examined polymers determined before and after the post-polymerization treatment and dried from different solvents is shown in Table 2. The effect of the post-polymerization treatment on the morphologies of the polymers X1-A and X2-A was distinctly apparent. In the polymer X1-A the treatment resulted both in a moderate increase of the apparent BET surface area and lowering of the sensitivity of the morphology to the type of the solvent treatment before drying, indicating certain increase of the rigidity of the polymer skeleton (Table 2). Morphology of the polymer X2-A was insensitive to the type of the solvent from which was dried even before the post-polymerization treatment and hence, it remained insensitive to it also after. As it is shown in Table 2, the increase
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K. Soukupová et al. / Reactive & Functional Polymers 69 (2009) 353–357
Fig. 3. Abstraction of labile H atoms from ethyl-group side-chain substituent (path a, easier) or from the methine group on the polymer backbone (path b) generates free radicals able of further reaction with the pendant double bonds.
1
X1-A
me,
0.8
pore volu 3 cm /g
0.6 0.4 0.2 PP original
0 1
2
3
5
10
20
40
60
pore diameter, nm 1.5
X2-A
lum pore vo 3 m c /g
e,
1.2
0.9 0.6 0.3
PP original
0 1
2
3
5
10
20
40
60
pore diameter, nm Fig. 4. ISEC determined swollen state morphology of the examined polymer before and after the post-polymerization treatment.
of the rigidity of the polymer gel by the post-polymerization crosslinking also slightly increased the total pore volume of the polymers (also determined from nitrogen adsorption–desorption measurements).
More detailed picture offers the evaluation of pore-size distribution from the results of nitrogen adsorption–desorption measurements (Fig. 5). The influence of the post-polymerization modification on pore size distribution in dry polymers has similar
K. Soukupová et al. / Reactive & Functional Polymers 69 (2009) 353–357
frequency function
2
methodologies however do not allow to generate polymers with surface area as high as the Davankov-type hypercrosslinked materials one, up to 2000 m2/g [1,18].
X1-A
1.5
4. Conclusions
1
0.5
0 10
1
100
pore diameter, nm 6
frequency function
357
X2-A
4
2
Results of 13C SPE MAS NMR confirmed the ability of di-tert-butyl peroxide initiator to induce post-polymerization crosslinking of poly(divinylbenzene) resins by creating reactive radicals after splitting a labile hydrogen atoms from the aliphatic chains. The hydrogene splitting happens most readily at the pendant mobile ethylbenzene residues and the generated radical then reacts with a neighbor residual vinyl group. This radical-induced post-polymerization treatment was able to moderately increase the apparent dry-state surface area of highly crosslinked X1-A and X2-A polymers. The generated changes have basically similar character as the modifications induced by reaction we investigated previously [3], i.e. reinforcing of the existing morphology features by increasing the rigidity of already created polymer domains rather than creation of new pores. The achieved extent of the morphology modifications is substantially smaller than the efficiencies of Davankov hypercrosslinking methodology [1,18] or the Friedel–Crafts catalyzed alkylation [3], possibly due to limited probability of existence of suitable reaction partners in correct mutual distance. We believe nevertheless that the knowledge of this post-polymerization effect can be useful. References
0 1
3
5
pore diameter, nm Fig. 5. Pore size distribution in the dried polymers before ( ) and after the ) as determined by nitrogen adsorption– post-polymerization treatment ( desorption porosimetry.
pattern as it was found in swollen polymers by ISEC that is maintaining the basic shape of the pore size distribution with slight increasing (reinforcing) of the amount of pore fractions available in dry-state. Apparently, there were created no new pores but the existing pore-size distribution characters were more enhanced both in X1-A with the bimodal pore size distribution and X2-A with the monomodal distribution. Similar extent of increasing the apparent BET surface area and porosity of poly(divinylbenzene) resins was found when during the initial polymerization in addition to the thermodynamically good solvent porogen low levels of oligomeric coporogen was used, finely tuning their porous morphology [17]. This and present
[1] V. Davankov, M. Tsyurupa, React. Polym. 13 (1990) 27. [2] K. Ando, T. Ito, H. Kusano, in: M. Streat (Ed.), Ion Exchange in Industry, Ellis Horwood Ltd., Chichester, 1988, p. 232. [3] K. Aleksieva, J. Xu, Li min Wang, A. Sassi, Z. Pientka, Z. Zhang, K. Jerabek, Polymer 47 (2006) 6544. [4] H. Galina, B.N. Kolarz, P.P. Wieczorek, M. Wojczynska, Br. Polym. J. 17 (2) (1985) 215. [5] E.H. Farmer, C.G. Moore, J. Chem. Soc. (1951) 131. [6] V.V. Korshak, S.L. Sosin, V.P. Alekseeva, J. Polym. Sci. 52 (1961) 213. [7] V.V. Korshak, E.M. Morozova, S.L. Sosyn, Vysokomol. Soedin. VI (7) (1964) 1228. [8] V.V. Korshak, S.L. Sosyn, V.P. Alekseeva, Dokl. Acad. Nauk SSSR 132 (2) (1960) 360. [9] V.V. Korshak, S.L. Sosin, et al., Vysokomol. Soedin. 1 (7) (1959) 937. [10] J. Brandrup, E.H. Immergut, A.A. Grulke, Polymer Handbook, fourth ed., II/25. [11] R.V. Law, D.C. Sherrington, C.E. Snape, Macromolecules 30 (1997) 2868. [12] K. Jerˇábek, Anal. Chem. 57 (1985) 1598. [13] A. Biffis, A.A. D’Archivio, K. Jerˇábek, G. Schmid, B. Corain, Adv. Mater. 12 (2000) 1909. [14] A. Guyot, M. Bartholin, Prog. Polym. Sci. 8 (1982) 277. [15] A.G. Ogston, Trans. Faraday Soc. 54 (1958) 1754. [16] S.M. Howdle, K. Jerabek, V. Leocorbo, P.C. Marr, D.C. Sherrington, Polymer 41 (2000) 7273. [17] F.S. Macintyre, D.C. Sherrington, Macromolecules 37 (2004) 7628. [18] J.-H. Ahn, J.-E. Jang, C.-G. Oh, et al., Macromolecules 39 (2006) 627.
Contents lists available at ScienceDirect
Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react
Reinforcing of expanded polymer morphology using peroxy radical initiator Krasimira Soukupová a,*, Alessandro Sassi b, Karel Jerˇábek a a b
Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, 165 02 Prague 6, Czech Republic Istituto di Scienze e Tecnologie Molecolari-CNR, Dipartimento di Processi Chimici dell’Ingegneria, Università di Padova, via Marzolo 9, 35131 Padova, Italy
a r t i c l e
i n f o
Article history: Received 27 January 2009 Received in revised form 9 February 2009 Accepted 21 February 2009 Available online 28 February 2009 Keywords: Post-crosslinking Radical initiator Polymer morphology Specific surface area
a b s t r a c t There was investigated the possibility of increasing the apparent specific surface area of polystyrene sorbents by radical-induced post-polymerization reaction using di-tert-butyl peroxide as free radical initiator. It is known that in addition to the activation of vinyl groups, tert-butyl peroxide is also able of abstraction of hydrogen atoms from aliphatic carbons and this way produces active centers for creation of post-polymerization crosslinking. Radical-induced post-polymerization morphology changes were confirmed by detailed morphological examinations of both dry and swollen materials showing distinct reinforcing of the expanded polymer morphology. However, due to the specific character of the postpolymerization reaction, the achieved additional crosslinking was not extensive and only moderate increase to the apparent BET surface area was achieved. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Accessibility of the interior of functional polymers like adsorbents or catalyst supports is the highest when the polymer is swollen by suitable solvent. After removal of the swelling solvent e.g., by drying, the polymer morphology at least partially collapses. Extent of the collapse depends on the rigidity of the polymer skeleton. For applications requiring high porosity of a functional polymer even in absence of swelling solvent there is advantageous to reinforce the polymeric skeleton against collapse by post-polymerization crosslinking. Such post-polymerization modified polymers then may exhibit extremely high apparent surface area [1]. Known procedures for enhancement of polystyrene adsorbents porosity are based mostly on chloromethylation [1] or Friedel– Crafts catalyzed alkylation involving residual double bonds [2,3]. In this contribution we report an attempt to increase the apparent specific surface area of dry polymers by treatment with di-tert-butyl peroxide radical initiator in thermodynamically good solvent facilitating softening and expansion of the polymer matrix. Swelling of styrenic polymers in a good solvent substantially decreases their glass-transition temperature [4] and therefore increases the mobility of polymer chains and reduces the sterical hindrances in the polymer skeleton. As a free radical initiator, di-tert-butyl peroxide is known for its specific mechanism of the radical polymerization initiation. Besides initiation of the polymerization of the residual double bonds, the primary t-butoxy radicals are also known for their * Corresponding author. Tel.: +420 220 390 332; fax: +420 220 920 961. E-mail address: [email protected] (K. Soukupová). 1381-5148/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2009.02.008
ability to abstract labile hydrogen atoms from an aliphatic chain, thus creating radicals able to initiate further polymerization [5]. This last scenario was used for polymerization of different low molecular weight aliphatics, a reaction known as polyrecombination, introduced by Korshak et al. [6–8]. Even if such a hydrogen abstraction process is more typical of a labile methine group, polyrecombination have been also reported for substances with methylene or even methyl groups [5,6]. The chain growth consists of multiple repeating acts of creation and recombination of radicals, forming gradually dimers, trimers etc., so the polymerization has a step-like character [9]. Since the creation of each reactive site requires one t-butyl peroxide radical, higher amount of initiator is needed. Higher amount of initiator favors the production of branched or crosslinked polymers [6–9]. We decided to investigate whether the ability of di-tert-butyl peroxide to promote beside activation of vinyl groups also other mechanisms of bond formation could be used for post-polymerization modification of morphology of porous functional polymers. In toluene, a swelling solvent for polystyrenes, di-tert-butyl peroxide breaks down reasonably rapidly at temperatures above 100 °C [10] and recombination of generated radicals may create additional crosslinks reinforcing the expanded polymer morphology and preventing its collapse during drying. 2. Experimental We have used three different starting polymers (Table 1). One of them was commercial macroreticular styrene-co-divinylbenzene (DVB) polymer with (for a macroreticular polymer) relatively low content of DVB and therefore, with a good ability to swell in
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K. Soukupová et al. / Reactive & Functional Polymers 69 (2009) 353–357
Table 1 Description of the investigated polymers. Polymer
Description
Composition (%) Styrene
DVB
EVB
ST-DVB 10% X1-A
Commercial grade, low crosslinked, macroreticular resin (Spolchemie, Czech Republic) Laboratory-prepared resin, 80% DVB (tech. gasoline/monomers 1:1 vol.), bimodal pore size distribution (Institute of Polymer Chemistry, Nankai University, Tianjin, China) Laboratory-prepared resin, 80% DVB, (1,2-dichloroethane/monomers 1:1 vol.), monomodal pore size distribution (Institute of Polymer Chemistry, Nankai University, Tianjin, China)
83 –
10 80
7 20
–
80
20
X2-A
contact with suitable solvents. The two other polymers were much more rigid materials prepared from the same technical DVB without addition of styrene as a co-monomer, but using different porogenic solvents [3]. Before use, the polymers were carefully washed with distilled water for removal of possibly present impurities like e.g. sodium salts and subsequently were dried at 100 °C overnight. Before post-polymerization procedure the polymers were preswollen overnight in toluene purified by molecular sieves and degassed. Toluene is for styrenic polymers swelling solvent able to soften the polymer network and enhance mobility of polymer chains [4]. After addition of di-tert-butyl peroxide as radical initiator in relatively high amount 0.1 g/g polymer, the modification reaction was performed at 104 °C for 14 h. The polymers were then extensively washed with THF in order to remove remnants of the initiator and low-molecular fragments possibly produced during the reaction. One part of the polymer was then dried directly from THF at 70 °C overnight and the second portion was before drying de-swollen by washing with ethanol, which for styrenic polymers is a precipitating solvent. Polymers after the post-polymerization treatment were marked as X1-A PP and X2-A PP. Concentrations of unpolymerized vinyl groups before and after the post-polymerization reaction were assessed by 13C SPE MAS NMR (Single Pulse Experiment Magic Angle Spinning Nuclear Magnetic Resonance) according approach developed by Snape and others [11] using Bruker AC200 spectrometer equipped for solid state analyses and operating at 50.26 MHz. Dry-state morphology of the examined polymers (apparent BET surface area, pore size distribution and total pore volume) was evaluated from nitrogen adsorption–desorption measurements using computerized apparatus ASAP 2010 and associated software (Micromeritics, USA). Swollen-state morphology was assessed by inverse steric exclusion chromatography (ISEC) using methodology described elsewhere [12]. For depicting of the pore size distributions we used uniformly the model of cylindrical pores. For swollen polymer this model may overestimate the pore volume [12], but the quality of steric hindrances in the pore system it depict quite acceptably and in an easily comprehensible format [13]. 3. Results and discussions Assigning of the signals of interest in the 13C SPE MAS NMR spectra is schematically depicted in Fig. 1. Those at 138 and 113 ppm corresponds to residual vinyl groups, signals for mainchain methine (39 ppm) and methylene (46 ppm) groups overlap at around 41 ppm, and the peaks at 30 and 15 ppm correspond to methylene and methyl groups of ethylvinylbenzene derivatives. Comparison of the 13C SPE MAS NMR spectra of the polymers before and after the post-polymerization crosslinking is shown in Fig. 2. Unlike the other two polymers, ST-DVB 10% contains no residual vinyl groups (Fig. 2A). Due to the low degree of crosslinking and high flexibility of its polymer backbone, all the vinyl groups have found a reaction partner already during the initial polymeri-
Fig. 1. Assignment of NMR signals to specific carbon atomtypes present in poly(divinylbenzene) network.
zation. Also, due to low content of ethylvinylbenzene co-monomer, the peaks of the methylene and methyl groups at 30 and 15 ppm are very weak. Between NMR spectra of 10% DVB polymers before and after the radical post-polymerization treatment practically no differences were observed. X1-A and X2-A are especially highly crosslinked polymers containing quite substantial amount of residual vinyl groups (about two monomer units contain one vinyl group [3]) as also pendant ethyl group side-chains substituents (Table 1). As seen in Fig. 2B and C treatment with the radical initiator had distinct influence on the polymers X1-A and X2-A in contrast to the polymer STDVB10%. However, in the course of the radical reaction not all of the residual vinyl groups was consumed. Quantitative analyses of the 13C SPE MAS NMR spectra based on the area of the de-convoluted peaks corresponding to the vinyl and aromatic carbons allowed assessment of the amount of the residual double bonds able to react during the post-polymerization treatment (Table 2). The results evidence that about one-third (or slightly more) of the pendant vinyls take part in the post-polymerization reaction. Besides the lowering of the residual vinyl group content in X1A, X2-A polymers, the radical reaction reduced also the signals of the carbons of pendant alkyl groups at 30 and 15 ppm (Fig. 2B and C). It indicates that the radical post-polymerization crosslinking most probably consists in bonding the residual double bonds with a radical derived from the pendant ethylbenzene group being previously generated by hydrogen atom abstraction (schematized in Fig. 3). The abstraction from this side-chain substituent is anticipated to take place from the CH2 groups which carry labile benzylic H atoms but surprisingly, a decreasing of 15 ppm signal is found as well for both of X1 polymers (Fig. 2). On the other hand, signals for the benzylic CH groups on the polymer backbone
K. Soukupová et al. / Reactive & Functional Polymers 69 (2009) 353–357
A 10 % DVB - original 10 % DVB - after PP treatment
160
140
120
100
80
60
40
20
0
ppm
B X1-A - original X1-A - after PP treatment
160
140
120
100
80
60
40
20
0
ppm
C X2-A - original X2-A - after PP treatment
160
140
120
100
80
60
40
20
0
ppm Fig. 2. 13C SP MAS NMR spectra of the studied polymers. Contact time = 3 ms, spin original, after PP treatment. For assignrate = 7000 Hz, D1 = 6 s, ments of the arrow annotated peaks see the text.
Table 2 Quantitative assessment for the consumption of residual vinyl groups after the postpolymerization treatment (from 13C SPE MAS NMR) and resulting variations in the apparent BET specific surface area and pore volume of polymers (data obtained by nitrogen adsorption measurements).
Residual vinyl group content, mmol/g Apparent BET surface area, m2/g Dried from THF Dried from ethanol Pore volume of polymers dried from THF, cm3/g
X1-A
X1-A PP
X2-A
X2-A PP
3.7
2.2
3.4
2.2
408.2 483.2 0.89
527.6 533.1 0.94
686.0 689.6 0.52
765.6 778.6 0.59
(41 ppm) also possessing labile hydrogen atoms decrease weakly for X2-A and negligibly for X1-A. This may be due to the stereochemical restriction of the H atoms from CH backbone group hampering the generation of a radical (Fig. 3). Besides the reaction of propagation (i.e. reaction of generated radical and a pendant vinyl group), radical–radical coupling may also happen but for the highly crosslinked and immobilized polymer chains this reaction is less likely. Incomplete consumption of the residual vinyls during the radical reaction is in sharp contrast with results of our previously
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reported post-polymerization modification by Friedel–Crafts alkylation [3], when in the polymers X1-A and X2-A were consumed all the residual double bonds. This difference in the vinyl group consumption probably stems from differences in the availability of reaction partners in radical polymerization and Friedel–Crafts alkylation. While for the creation of additional crosslinks by Friedel–Crafts catalyzed alkylation occurrence of a benzene ring close to the residual vinyl should be no serious problem, effective postcrosslinking using t-butyl peroxide radical initiator is possible only if radicals derived from the ethyl groups are located in close proximity to pendant double bonds. Because of the inhomogeneous distribution of DVB and EVB units in the polymer due to their different reactivity [14], it is not likely that each radical encounters a double bond. Effects of this modification on morphology of the investigated polymers were examined both in the swollen and dry states. In the Fig. 4 are shown swollen state morphologies of the examined polymers determined by ISEC modeled as set of discrete pore fractions, each consisting of pores of a single size only. Pore system of the swollen polymers consists of ‘‘true” pores which is possible to describe using the conventional model of cylindrical pores and pores in swollen polymer gel for which would be more appropriate the model devised by Ogston [15] depicting pores as spaces between cylindrical solid bodies. However, for simplicity we used the model of cylindrical pores in the whole range of the polymer sizes. This modeling may overestimate the volume of the narrow pores in the polymer matrix but offers more comprehensible overview of the pore structure. As it is seen in the Fig. 4, the radical post-polymerization treatment did not change the basic features of the polymer morphologies but the additional crosslinking emphasizes its existing features. The increased fraction of narrowest pores and the diminished dispersity of the pore sizes in the mesopore region were distinctly apparent in both of examined polymers carrying free double bonds (Fig. 4). Collapse of the polymer gel during drying changes the polymer morphology substantially and the extent of these changes depends on the rigidity of the polymer gel. Morphological modifications induced by the radical post-polymerization treatment can be most simply detected by variations of the apparent surface area of dried polymers. Treatment with diverse solvents before polymer drying makes possible to assess rigidity of the polymer skeleton. Swelling by a ‘‘good” solvent (e.g. THF) softens the polymer skeleton and capillary forces generated in the menisci of liquid filled pores may then induce collapse of the pores. Porous structure of macroreticular polymers can be better preserved if before drying the polymer is de-swollen by washing with a non-swelling solvent (e.g. ethanol), which precipitates the polymer and increases its resistance to the collapse during drying [4]. This effect of solvent on the polymer skeleton collapsibility best manifested for resins prepared with very low DVB content [16] may also serve as an indicator of structural stability for higher crosslinked polystyrenes, by the difference of their dry-state morphologies achieved after using the two solvent types. Comparison of apparent BET surface areas of the examined polymers determined before and after the post-polymerization treatment and dried from different solvents is shown in Table 2. The effect of the post-polymerization treatment on the morphologies of the polymers X1-A and X2-A was distinctly apparent. In the polymer X1-A the treatment resulted both in a moderate increase of the apparent BET surface area and lowering of the sensitivity of the morphology to the type of the solvent treatment before drying, indicating certain increase of the rigidity of the polymer skeleton (Table 2). Morphology of the polymer X2-A was insensitive to the type of the solvent from which was dried even before the post-polymerization treatment and hence, it remained insensitive to it also after. As it is shown in Table 2, the increase
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Fig. 3. Abstraction of labile H atoms from ethyl-group side-chain substituent (path a, easier) or from the methine group on the polymer backbone (path b) generates free radicals able of further reaction with the pendant double bonds.
1
X1-A
me,
0.8
pore volu 3 cm /g
0.6 0.4 0.2 PP original
0 1
2
3
5
10
20
40
60
pore diameter, nm 1.5
X2-A
lum pore vo 3 m c /g
e,
1.2
0.9 0.6 0.3
PP original
0 1
2
3
5
10
20
40
60
pore diameter, nm Fig. 4. ISEC determined swollen state morphology of the examined polymer before and after the post-polymerization treatment.
of the rigidity of the polymer gel by the post-polymerization crosslinking also slightly increased the total pore volume of the polymers (also determined from nitrogen adsorption–desorption measurements).
More detailed picture offers the evaluation of pore-size distribution from the results of nitrogen adsorption–desorption measurements (Fig. 5). The influence of the post-polymerization modification on pore size distribution in dry polymers has similar
K. Soukupová et al. / Reactive & Functional Polymers 69 (2009) 353–357
frequency function
2
methodologies however do not allow to generate polymers with surface area as high as the Davankov-type hypercrosslinked materials one, up to 2000 m2/g [1,18].
X1-A
1.5
4. Conclusions
1
0.5
0 10
1
100
pore diameter, nm 6
frequency function
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X2-A
4
2
Results of 13C SPE MAS NMR confirmed the ability of di-tert-butyl peroxide initiator to induce post-polymerization crosslinking of poly(divinylbenzene) resins by creating reactive radicals after splitting a labile hydrogen atoms from the aliphatic chains. The hydrogene splitting happens most readily at the pendant mobile ethylbenzene residues and the generated radical then reacts with a neighbor residual vinyl group. This radical-induced post-polymerization treatment was able to moderately increase the apparent dry-state surface area of highly crosslinked X1-A and X2-A polymers. The generated changes have basically similar character as the modifications induced by reaction we investigated previously [3], i.e. reinforcing of the existing morphology features by increasing the rigidity of already created polymer domains rather than creation of new pores. The achieved extent of the morphology modifications is substantially smaller than the efficiencies of Davankov hypercrosslinking methodology [1,18] or the Friedel–Crafts catalyzed alkylation [3], possibly due to limited probability of existence of suitable reaction partners in correct mutual distance. We believe nevertheless that the knowledge of this post-polymerization effect can be useful. References
0 1
3
5
pore diameter, nm Fig. 5. Pore size distribution in the dried polymers before ( ) and after the ) as determined by nitrogen adsorption– post-polymerization treatment ( desorption porosimetry.
pattern as it was found in swollen polymers by ISEC that is maintaining the basic shape of the pore size distribution with slight increasing (reinforcing) of the amount of pore fractions available in dry-state. Apparently, there were created no new pores but the existing pore-size distribution characters were more enhanced both in X1-A with the bimodal pore size distribution and X2-A with the monomodal distribution. Similar extent of increasing the apparent BET surface area and porosity of poly(divinylbenzene) resins was found when during the initial polymerization in addition to the thermodynamically good solvent porogen low levels of oligomeric coporogen was used, finely tuning their porous morphology [17]. This and present
[1] V. Davankov, M. Tsyurupa, React. Polym. 13 (1990) 27. [2] K. Ando, T. Ito, H. Kusano, in: M. Streat (Ed.), Ion Exchange in Industry, Ellis Horwood Ltd., Chichester, 1988, p. 232. [3] K. Aleksieva, J. Xu, Li min Wang, A. Sassi, Z. Pientka, Z. Zhang, K. Jerabek, Polymer 47 (2006) 6544. [4] H. Galina, B.N. Kolarz, P.P. Wieczorek, M. Wojczynska, Br. Polym. J. 17 (2) (1985) 215. [5] E.H. Farmer, C.G. Moore, J. Chem. Soc. (1951) 131. [6] V.V. Korshak, S.L. Sosin, V.P. Alekseeva, J. Polym. Sci. 52 (1961) 213. [7] V.V. Korshak, E.M. Morozova, S.L. Sosyn, Vysokomol. Soedin. VI (7) (1964) 1228. [8] V.V. Korshak, S.L. Sosyn, V.P. Alekseeva, Dokl. Acad. Nauk SSSR 132 (2) (1960) 360. [9] V.V. Korshak, S.L. Sosin, et al., Vysokomol. Soedin. 1 (7) (1959) 937. [10] J. Brandrup, E.H. Immergut, A.A. Grulke, Polymer Handbook, fourth ed., II/25. [11] R.V. Law, D.C. Sherrington, C.E. Snape, Macromolecules 30 (1997) 2868. [12] K. Jerˇábek, Anal. Chem. 57 (1985) 1598. [13] A. Biffis, A.A. D’Archivio, K. Jerˇábek, G. Schmid, B. Corain, Adv. Mater. 12 (2000) 1909. [14] A. Guyot, M. Bartholin, Prog. Polym. Sci. 8 (1982) 277. [15] A.G. Ogston, Trans. Faraday Soc. 54 (1958) 1754. [16] S.M. Howdle, K. Jerabek, V. Leocorbo, P.C. Marr, D.C. Sherrington, Polymer 41 (2000) 7273. [17] F.S. Macintyre, D.C. Sherrington, Macromolecules 37 (2004) 7628. [18] J.-H. Ahn, J.-E. Jang, C.-G. Oh, et al., Macromolecules 39 (2006) 627.