Acid-based properties of silicas modified by organic compounds as determined by inverse gas chromatography

ELSEVIER

P{~wderTechnology95 (1998) 103-108

Acid-base properties of silicas modified by organic compounds as determined by inverse gas chromatography Adam Voelkel *, Andrzej Krysztafkiewicz Po=mo~ Universi~" of Technology. PI. M, Sklodowskiej-Curie 2, 60-965 PoznaJi, Poland

Received 18 March 1996:revised 15June 1997

A b s t r a c t

The results of silica surface modification by selected modifiers were examined by inverse gas chromatography ( IGC ). Experimentswere carried out at inlinitc dilution and Gibbs free energies and enthalpies of specific interactions were determined according to Papirer's and Dong-Donnet's approaches. K,x and KD values, calculated from enthalpies of specific interactions, as well as their ratio S~ = K.~/Ka, were used for characterization of the examined surfaces. The influence of the modifier type and its amount upon the physicoehemicalparameters investigated is presented and di,scussed. © 1998 Elsevier Science S.A. Keyword,~: Silic~tsurfacenmdification;Modiliers;Acid-hasproperties;Inversegas chromatogr:lphy;Surfacecharacterization

I. General

I. I, Silica sm;lilce~" am/dwir modilicatirm Silanol groups present o11 the surface of highly dispersed precipitated silica or silica gel have a positive influence o11 the reinfiwcing properties. However, their presence causes the surface hydmphilicity to increase due to wetting and the following water surption I I I, Moreover, silanol groups are responsible for the acidic character of silica surfaces [ 2 ]. The variety of possible reactions of silica surface active centres allows the surface properties to be changed by introduction of new groups of atoms. Surface modiiication is carried out with the use of different modifying agents, such as pro-adhesive compounds, coupling agents or adhesion promoters. The selection of the appropriate multifunctional compound having groui": reacting with the silica surface and groups with extended polymer affinity allows polymer reintbrcement to be controlled by fillers. Modification is a very complex process depending on various parameters, e.g. solvent type. modifier type, amount of modifier in the solution, pH, time of modification, additives, etc. Chemical modification of the silica surface is carried using the following substances [ 3,4]: * Correspnndingauthor. 0032-5910/98/$19.00 © 1998ElsevierScienceS.A, All rightsreserved PIIS0032-5910(97)03325-1

- compounds with oxygen-containing functional groups (alcohols, alkox,, compounds, orthoesters, aldehydes. carboxylic acids esters, bcnzenesnlfonic acid esters ); nitrogen compounds (isocyanides, amines, quaternary amine salts): surface active agents ( ionic, non-ionic): - chlorosilanes, chloromethylsilanes and rchlted compounds increasing surface hydrophobicily: proadhesive agents: - coupling agents; oxyethylene organic compounds: fatty acid salts. The degree of modification of the silica surface may be estimated by, e.g.: determination of the degree of hydmphobization from the values of the heats of immersion of the silica surface by water and/or benzene; determination of the degree of condensalion of the surface silica groups I NIR I: analysis of the tendency to agglomeration and aggt'egation using electron microscopy: inverse gas chromatography (IGC). In our experiments the tbllowing organic compoutlds were used as modifiers: ,y.methacryloxypropyltrimethoxysitane (,y-inks), dichiorodimethylsilane (2c2ms), 2-butylamino-

-

-

-

-

-

-

A. Viwlkel, A, Kt3"xzta,lkiewicz / Powder TeclmohJgy 95 ( 1998~ 103- i0,~

104

ethanol (2bae), 2-dibutylaminoethanol (2dbae), 5-butylamino-3-oxapentanol (Sba39p) and butyldiglycol (bdg). The results of surface modification are presented using silica ( in the lbrm of silica gel. 0.063-0.2 mm) a~ a model example. !.2. IGC e.rperiments

Dispersive and specific in,,eractions are considered to contribute independently to the adsorption of probe molecules at the adsorbent surface, it has been shown that the adhesion of the fibre-matrix interlace depends clearly on the measured strength of acid-base interactions of both the fibre and the polymer matrix. Fowkes 15,61 also indicated that the surface of fillers can be chemically modified to enhance acid-base interaction and increase adsorption. Surface properties may be determined by means, lbr example, of inverse gas chromatography. The term 'inverse" indicates that the examined material is placed in a chromatographic column as the solid stationary phase and studied using test solutes. These carefully selected test solutes are injected into a carrier gas stream and transported over the surface of the material, which may be silica, fibre, or tiller. The retention times and peak elution profiles of standard solutes, affected by their interactions with the stationary phase, are used to estimate parameters which quantitatively express the surface properties of the examined material. Inverse gas chromatographic measurements may he cltrried out both at infinite dilution and at finite sohtte concentrations [ 7 I, In the lirst case, vapours of test solutes are injected into the column and their concentrations in tile adsorbed layer eventually became zero, The test substances interact with strong active sites on the examined surface, The retention data are then converted into, e,g., the dispersive component of the surface free energy, and the specific component of the free energy of adsorption. It has been known tbr nlany years thai acid-base interactions are an intportant component of polar forces and play a signiticant role in the adhesion of organic substances to inorganic st,bstrates 18- I I I. Acidic or basic solvents tend to compete Ibr the polymer or filler active sites. This competition between the solvents with the polymers or fillers is a means of measuring the acidity or basicity of the materials. The use of the IGC method to cilaracterize solid Inaterials hits been discussed earlier I 121 and only essential information is given below. The standard I've energy of transferring a mole of vapour fmnl the gas phase to a standard state on the surface, i.e. the adsorption energy, is given by

For a given system, B, S and g are constant and Fq ( 1 ) changes to AG" = -

]

+ const.

(2)

The value of the constant in Eq, (2) depends on the arbitrary chosen reference state of the adsorbed molecule. Saint Flour and Papirer [ 13] proposed that the specific component of the free energy of adsorption, A G ~v~, could be determined as the vertical distance between the n-alkane reference line and the point on the diagram corresponding to the polar test probe. As the physicochemical property used to define the reference state, they chose the saturated vapour pressure in the form log(P"). Theretbre, A G '~'¢ corresponds to the difference in the free energies of adsorption betwcen the given test probe and tile hypothetical n-alkane having the same vapour pressure: AG "w= "~,.,,-AG, " ' " - d = - R T I n ( V~ ,/VN ,~.t)

(3)

where AG'~, AG~p~"and AG~j denote tile tree energy of adsorption, and its specific and dispersive components, respectively, determined for the test solute x: V~., and VN.,,, refer to the net retention volumes of the test substance and the reference hypothetical n-alkane. Dong et al. [ 141 defined another reference state in terms of the molecular polarizability of n-alkanes. Examination of the temperature dependence of AG "j'' makes it possible to determine the enthalpy of specific ihv,er.'~ctions A H "w 115 I: A t/"w "=

a{ A G'P"/7 "1 a( I IT)

t4 )

Tb, enthalpy of specilic interactions between the exanlincd stu'l'ace and the test solute tnay I~c correlated to acid=l~ase properties of both species by using I)rag~¢s equation or, ill our opinion heUer, by tile following equation I 151: - AH 'w = El, × AN +/k',~ X DN

15 )

where AN and DN are the accepter and donor numhers, respectively, of tile test solute: parameters KA and K~, reflect the ability of tile examined surface to act as electron accepters and electron donors, respectively. AN denotes the accepter number on tile Gutman 1161 or Riddle-Fowkes scale 117 I. The procedure proposed by Papirer and co-workers I 13.15.18-211 was used to characterize silicas, modified silicas. oxides and minerals. ChehinJi ¢~ :d. 122,231. Panzer and Schreiber ! 241 and Voelkel and co-workers 125-28 ] used this method to characterize solid polymers.

2.

,~ (?' = - Rt ln \ ~

R T In( VN)

Experimental

11~ 2. I, MateriaLs

where B = 2,99 × I0-' ( according to the deBoers definition of the surface pressure in the adsorbed state), S is the specific area of the adsorbent (m:/g), g is the mass of the adsorbent in the column (g), and VN is the net retention volume ( m ~).

The silica examined was supplied by Merck in tile form of silica gel (0.063-0.2 ram). The physicochemical properties of file examined silica are as tbllows: pH of water suspension

A. VoelkeL A. Ko',vztqlkieu'icz I Powth,r l"echmdogy 95 t 19981 103-108

7.0; bulk density 550-650 g/dmS; specific heat 0.22 cal/g °C: specific surface area (BET) 420-500 m-~/g; pore size 150-160 nm. The following modifiers were used in the derivation procedure: ",/-methacryloxypropyltrimethoxysilane (~/-mks), Union Carbide: dichlorodimethylsilane (2c2ms), Fluka; 2-butylaminoethanoi (2bae), 2-dibutylaminoethano! (2dbae), 5-butylamino-3-oxapentanol (5ba3op) and butyldiglycol (bdg), Bergakademie Freiberg, Germany. In order to modify the silica surface ( 10 g), organosilanes (besides dichlorodimethylsilane) (1-3 parts by weight of these compounds in suitable solvent per 100 parts by weight of silica gel ) were dissolved in a methanol-water mixture (ca. 3 cm s, 4:1 vol./vol.). The process was carried out for ! hour in a mixer of 100 em 3capacity, which has been described earlier 1291. The silica modification was carried out in the presence of a minimal amount of silane-methanol-water solution and only the surface of the silica particles was wetted. Modification by dichlorodimethylsilane was performed in a reversible cooler. Excess modifier was distilled out. The silicas were then dried for2 hours at 110°C. All silane solutions were prepared immediately before their use in the tnodificalion process. The amount of silylation after modilication was calculated from the Ibllowing fm'mula: N=

I HI ~ ),,, - ( HI ~),,

( HI ~),,,

x 100~,

w~'tt:rc (HI~),,, is the heat of immersion ill benzene of the n,odilied silica surface and ( ~ ),, is the heat of immersiml in I~enzene of the unmodilied silica surface. The heat of immersion of the silica surface in benzene was detern|ined by Ihe calorimetric method. By using the KRM alloritueter, it is possible to delermine the heat effects by a dynanlic method under conditions close lu Miahatic. The values ol' the heat of inltuersion in benzene Ibr the modilied lind itnniodilJed surfaces of the silica gel wcre nlelisured ( Table I ), 'l'ahlc I The tleats of Jnllllcrsion ill I'lOll/Cllealld Ihe ,'lnltillnl of sJlylatkm flu silica gel unnlodilied and modilied ~ilh orgunic ¢OlllpOUtlds T) pe of org;mi¢ cotl|potll|d

Unmoditied ,,,ilica gel 'y-rnks 2c2ms 2blie 2dhae 5ba3op bdg

Amount of ,Ioditier wt./wt.I

I 3 I 3 I 3 I 3 I 3 I 3

ItI ~ ( J/g I

24. I 28.5 33.3 33.11 39.9 25.0 27.2 26.7 29.2 27.9 32.7 25.5 26.9

Amoun! or' silylalion ('~ I

15.4 27.t~ 27.0 39.b 7.0 II .4 9.7 17.5 13.(~ 26.3 5.5 10.4

105

2.2. Test compounds told IGC experiment condilioll.~

The following compounds were used as the test solutes in the IGC experiment: n-alkanes C~-Ct,, benzene, diethyl ether, chloroform, methylene chloride, toluene, ethylbenzene, acetone, ethyl acetate, butanone-2. Approximately _.,9 ,, of each silica was used as the chromatographic column fill,:ng. Columns were conditioned at 190°C overnight. Other conditions of the IGC experiment were as follows: gas chromatograph Chrom 5 (Czech Republic) equipped with an SRI data acquisition program; column temperature: isothermally at 150, 170 and 190°C; detector: FID, stainless steel columns. 3 mm i.d., length 0.5 m; carrier gas: He.40 ml/min.; injection volume: vapours of solute to achieve a region of intinite dilution. 2.3. Calculation af sucface palzmwtersJ)'on, the IGC data

The values of the specific component of the free energy of adsorption, A G ~°', were calculated from retention data (net retention volume) according to Eq. (3). Two sets of these values were found owing to the use of two reference states according to Papirer's and Dong-Donnet's procedures. The specific components of the enthalpy of adsorption were calculated from the ,'.'.&G ~r~values from Eq. (4). Acid-base ci'aracteristics were evaluated using Eq. (5) and the set of A H~o" values for polar lest solutes. We used the procedure proposed by Saint Flour and Papirer [ 13 I.

3. R e s u l t s a n d d i s c u s s i o n

The values of tile ft'ee energy of adsorplion corresponding to the specilic intcr:~ctions i'o1' unmoditicd silica and the ~urfaces nlodilied hy three parts of ~/.mks are presented in Tables 2 :rod 3, respectively, in each case two series of data correspoMing to Papt 'er s (A G~P,' Papirer) and Dong=I)onnet's (A G "~'' Dong-Donnetl approaches are given. The values of the specific component of the free energy of adsorption are higher than those found by Papirer et al. 1191 for Aerosil 300. They reported A G"p~"values for benzene ,'rodchloroform equal to 1.25 and 0.6 kJ/mol, respectively. Significantly strong interactions were observed between a number of test solutes and the unmodified silica, Therefore. it was impossible to collect retention data for diethyl ether, acetone or ethyl acetate due to the very long retention times and significant skewing of the chromatographic peaks. The same observation was reported by Papirer et al. 1301. Specific interactions between the examined surfaces and tile test solutes decrease in the order 3,-inks > bdg > 2dbae > 5ba3op > 2bae. Modilication by ",/-inks is less effective when surface hydrophobization is taken into account. A G~'' values determined using Dong-Donnet's model are. generally, higher. Signilicant differeqces (up to 100%) were found among the low values of the free energies of adsorption. A G~°~values decrease with increase of the temperature of the experiment. Temperature

A. Voelkel,A. Kla'szt~lk~ewicz/ Pow~h,rTechnology95119~8j I03-108

I06

Table 2 Specific interactions for unmodified silica on Papir¢~ 's and Dong-D~)nnet scales ( 'kJ/ tool ) Test solute

Benzene Chloroform Methylene chloride Toluene Ethylbenzer.e

- AG "r"Papirer

-

A G "~" Dong-Donne!

IS0°C

170°C

190°C

1500C

170°C

I90oc

3.823 2.188 3.314 4.904 5.219

3,389 1.910 3.065 4.390 4,532

2.985 1.446 2.597 3.512 3.813

5.873 4.903 6.170 7.108 7.375

5.226 4.334 5.485 6,388 6,501

4.584 3.549 4.574 5.278 5.571

Table 3 Spc¢,lic interactions for silic,t moditied by three parts of 'y-inks on Papirer's and Dong-Donnet's scales I kJtmol ) Test solute

Benzene Dicthyl ether Chloroform Methylene chloride Toluene F,thylbenzene Acetone Ethyl acclale

Bulanonc-2

- A G '~" Papirer

- .1 G 'F Dong-Donnet

150~C

170°C

190°C

150°C

170°C

190°C

2.777 12,180 1.818 2.577 3,227 3.437 12.640 12.970 12,8 I0

2.394 10.150 1,652 2,549 2.769 2.953 I 1,5(~) I 1.720 I 1.520

2.070 9.020 1.547 2.486 2.362 2,506 I 1).361) 10.660 11).440

4.770 12.840 4.482 5.525 5.369 5.532 16.560 15.860 16.570

4.203 10.720 4.041 4.934 4.736 4.890 14.gtx) 14.281) 14.891)

3.716 9.400 3.712 4.523 4.178 4,312 13.470 12,910 13,450

gradients are similar in both models. Increase of the amount of rood[tier signilicantly decreases the magnitude ol" specilic interactions. The decrease in mugnitu,Je of specific interactions expressed by the decreasing val,e ot'A G 'l" was also observed by Papirer et al, 1191 for Aen~sil 300 modilied by polyeth. ylene glycols. They also reported that the decrease of A G'~" for diethyl ether and tetrahydmfurane was accompanied by an increase of this parameter Ior chloroform. We have not observed such a situation with our modified silicas, The AG 'r~ values are similar to those reported in other work by Papirer~t al. 1301, A G.;'~ values for benzene found for Spberosii XOB 75 modified by tridecanofluorooctyldimethylchlorosilane, octanol anti hexadecanol were equal Io 6,4, 2,25 and 1.4 kJ/mol, respectively, compared with 2,78 kJ/tool for our silica modified by ",/-inks. The corresponding values for chloroform were equal to 3.7, 1.4, 0.95 and !,82 kJ/tool, respectively. However, one should take into account that Papirer's retention data were collected at 6(}°C and that AG't'' decreases with increasing tempcrature of the IGC experiment, ThereFore,the surface of the examined ~ilica may be assumed to be more active than those of Aerosil A 300 [ 191 and Spherosil XOB 75 130 I, It is clear that modification of the silica surface by selected modifiers produces an extremely high decrease in the accep[or ability K^, and a lower, but significant, decrease in the donor ability Kr)oftbe examined material (Table 4). K^ and ~V values decrea~ witli the amount of modified silica. The

Tahlc 4

A¢id=hase characlerislics l~.wthe ex ,, ~ :d silic,so. l~apirei"sscale

Modilicr

Amtatn[ ( ~,vl,/ ~,l. I

t Iluilodilicd

"~-Iiik~

3

2haq.'

I 3 I 3 I 3 I 3

2dl~ae 5ba3op hdg

KI;

/x'a

S, ~, Ki ,,' K,x

(|,438 0.237 0.113 lI.3t)9 11.21}2 0.292 U.282 0.307 U.237 0.337 0.30.'~

34.,~38 U.3(lO II. I,~7 ll.340 0.217 11.355 [I.229 0.331 0,094 0,367 (I,279

0.013 0.789 U.719 I.17?, 1.35o 0.822 1.257 0?)29 2535 11.918 1.082

~urface character chauges from having a high acceptor ability to ainphoteric. The nzagnitude of the change depends on the amount of modifier. The addition of a higher amount of rood[tier decreases the values of the acceptor-donor paralneters. Ka and Kit, which corresponds well with the decrease in the tree energy of adsorption of specilic interactions ( Figs. I and 2), However, the donor character (S,,) generally increases with increase oftbe amount of modifier added. A very specific effect was observed in the case of the modifier 2c2ms whose addition caused a significant decrease in the donor ability of the examined surface. In this case, Ka is also low (0.157)

A. VoelkeI. A. K)3"szt(~lkiewicz l l)owder Techuoh)~,oy 95 1998~

1()7

I03-~ I!1,~

Table 5 Acid-base characteristics li.)r the examined s Icas on Dimg-l)onnctn scale 6!.

Modilier

Amollnl

K=)

K ~

S = K=)/ K ~

0.917 0.667 0.389 0.779 0.742 0.748 0.738 @742 0.688 0.778 0.716

46.515 0.626 0.163 0.684 0.567 0.711 0.579 0.668 0.441 0.7f)8 0.596

0.020 1.065 2.394 1.140 1.310 1.050 1.275 1,110 1.558 1.099 1,200

( wt,Iwt, )

0

E

5"

.-j

•

Unmodified ~-mkx 2c2ms 2bae

4

0


3 I 3 I 3 1 3 1 3

2dbae .....

S~ 5ba3op

0

1

2

3

amount of modifier [w/w]

bdg

Fie. I. - A G "w vah,es for silica modilied by 2dbae usillg Papircr's and Dong-Donnet's rtmdels: ~, C), benzene:/Z, Q, methyleqe chloride: l..q,~Z. Papirer's model: O. ~ . I)ong-Donnel's model.

and as a result this surface is inactive with a slight acceptor character. Similar relations were found when the fi'ee energy of adsorption was expressed according to Dong-Donnet's (Table 5) proposal. However. in this case modified surfaces are described as having a more donor character than previously. 4. Conclusions

Results of the IGC experiments indicate that unmodified silica exhibits a s t r o n g ucceptor character due to the presence

of silanol groups = S i - O H on the surface. It is wortll noting that it was possible to determine the retention data for a relatively Im~e group of polar test solutes. Most often the

surface characteristics have been based on the results collected for only a few test solutes, owing to problems with retention on rather active materials. The other advantage is the comparison of the surface characteristics derived by the use of two reference states in the determination of the specific component of the free energy of adsorption. A G ")'~'.However, the specific component of the fi'ee energy of adsorption is the parameter describing a given system, solute-stationary phase

35° i"....................................................................... (a) %,,-Z .

' ~i~,

5 50

5O0

--

iI

=,~

g

[

4150

Lr

400

3 50 ....................................~ 0

I

0

2

amountof modifier[w/w]

350

E

3OO

(c) 0

E

",.~,'~,~.. ~---._,. ~r'~..

2

. . . . . . . . . . . ~*.................................. =

I

amoum of modifier [w/w]

O

L~

"~

~

..

"~"<~-'-.~-

"

•~

g

.

I

t

50

0<1

<

,500

200 I

2

amountof modifier[w/w]

3

................

i I

...............

~ ............... 2

3

amount of modifier [w/w]

Fig. 2. - A G 'w values for modilied silicas al 150°C: (a) and (¢) Fapirer's model: (b) and (d) Dong-I)o,,ael's model: (a) and (b) benzene: (c) and (d* methylene chlorkle ( ~ . bae. A, 2dbae: O. 5ba3op: O, bdg ).

108

A. V~wlkeLA. Kryszt~t[iewi('z/ Powder T~,clmoh)gy96 f I~/98J 103-1(h~

( here modified silica). The value of this parameter depends on the properties of each component of the system and the temperature. Therelbre, much more interesting is the charactedzation of the examined material by parameters characterizing only this material, such as coefficients describing its ability to interact as an electron donor, KD, or an electron aeceptor, Ka, and their ratio K~/KA, Addition of the modifier causes a decrease of the specific interactions between the test solutes and the silica surfaces. It is exhibited by changes in the values of the free energies of adsorption corresponding to the specific interactions and parameters describing the acceptor/douor ability. The acceptor ability of the surface may be reduced, even by two orders or magnitude, which indicates significant hydrophobization. The addition of modifier and increase of the amount of modifier cause the character of the silica surface to change from a strong acceptor to amphoteric in character. The best results (hydrophobization) were observed when dichlorodimethylsilane (2c2ms) was used as the modifier.

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191 J.C. Bolger and A.S. Michaels, in P. Weiss (ed. I, Interface Conversion for Polymer Coatings, Elsevier, New York, 1968. ! I01 Yu,S. Lipatov and L,M, Scrgeeva, Adsorption of Polymers, Halsted, New York, 1974. I 1 ! I P. Sprensen, J, Paint Technol., 47 ( 1975 ) 3 I. I 121 A, Voelkel, Inverse gas chromatography in the examination of acidbase and some olher properties of solid materials, in A. D~browski and V,A. Tertykh (eds.), Adsorption on New and Modified Inorganic Sorbents, Elsevier, Amsterdam, 1996, pp, 465--477. 1131 C, Saint Flour and E. Papirer, J. Colloid Interface Sci., 91 ( 1983 J 69. [ 141 S, Dong, M. Brendle and J.B. Donnet, Chromatographia, 28 (1989) 469, 1151 M. Nardin. H. Balard aud E. Papirer, Carbon, 28 (1990) 43, I 161 V. Gutman, The Donor-Acceptor Approach to Molecular Interactions, Plenum, New York, 1978. 1171 F.L. Riddle and F.M. Fowkes, J. Am. Chem. Sot., 112 ( 19901 3259. I 181 H. Balard. M. Sidqi, E. Papirer, J,B. Donner, A. Tuel, H, Hommel and A.P. Legraud, Chromatogral)hia, 25 ( 1988 ) 712. 1191 E. Papirer, H. Balard, Y. Rahmani, A.P. Legrand, L. Facchini and H. Hommel, Chromatographia, 23 119871 639. 1201 M. Sidqi, H, Balard, E. Papirer, A, Tuel, H, Homrnel and A.P. Legrand, Chromatographia, 27 ( 1989 ) 31 I. 1211M. Sidqi, G. Ligner, J. Jagiello, H, Balard and E. Papirer, Chromatographia, 28 (1989) 588. 1221 E. Pigois-Landureau and M.M. Chehimi, J. Appl. Polym. Sci., 49 ( 19931 183. 1231 M.M. Chehimi :rod E. Pigois-i,andureau, J. Mater. ('hem., 4 ( 19941 741, 1241 U. Panter and H.P, Schreiber. Macromolecules. 25 t I~J921 3633. 1251 A. Voelkel, E. Andrzejewska, R. Maga and M. Andrzejewski, Polymer, 33 ( 19931 31()9. 12(11 A. Voelkel, E, Andrzejew,,,ka, R. Maga and M. Andrz(~e~ ski, Polymer, 35 ( 10941 178(I. 1271 A. Voelkel, E. Andrzeje~v~ka, R. Maga and M. Andr/ejewski, Polymer, 37 ( 19t161 455. 1281 E Antlrz¢ie~ ~ki., A. Vtwlkel, M. Andrzejc~ski and R. M~lga, I)olymer. 37 (199h) 4]33. 12()1 I,, I)ontk~l, A. Kl'ystlalkiewi¢,, ~qltdW, Kl'y~ltalkicwi¢l, Ih)lish I)alelll l ltl }58 ( I(1821, 1301 I!, I)~q)irer. H, Ilalard aml M. Sidqi. ,I, ('(dh)id Inlerf~)ve SL'i,. 15L) I Iq~1,~.,) 218,