Polystyrene-Graft-Poly(Ethylene Oxide) Copolymers Prepared by Macromonomer Technique in Dispersion

Home , Styrene

Polystyrene-Graft-Poly(Ethylene Oxide) Copolymers Prepared by Macromonomer Technique in Dispersion. I. Liquid Chromatographic Separation of Product Mixtures

SON H. NGUYEN,1 DUSˇAN BEREK,1 IGNA´ C CAPEK,1 OSCAR CHIANTORE2 1 Polymer Institute of the Slovak Academy of Sciences, 842 36 Bratislava, Slovakia

2 Department of Inorganic, Physical and Materials Chemistry, University of Turin, 10125 Turin, Italy

Received 30 November 1999; accepted 22 March 2000

ABSTRACT: Products of the radical dispersion copolymerization of methacryloyl-terminated poly(ethylene oxide) (PEO) macromonomer and styrene were separated and characterized by size exclusion chromatography (SEC), full adsorption-desorption (FAD)/SEC coupling and eluent gradient liquid adsorption chromatography (LAC). In dimethylformamide, which is a good solvent for PEO side chains but a poor solvent for polystyrene (PS), amphiphilic PS-graft-PEO copolymers formed aggregates, which were very stable at room temperature even upon substantial dilution. The aggregates disappeared at high temperature or in tetrahydrofuran (THF), which is a good solvent for both homopolymers and for PS-graft-PEO. FAD/SEC procedure allowed separation of homo-PS from graft-copolymer and determination of both its amount and molar mass. Effective molar mass of graft-copolymer was estimated directly from the SEC calibration curve determined with PS standards. Presence of larger amount of the homo-PS in the ﬁnal graft-copolymer products was also conﬁrmed with LAC measurements. The results indicate that there are at least two or maybe three polymerization loci; namely the continuous phase, the particle surface layer and the particle core. The graft copolymers are produced mainly in the continuous phase while PS or copolymer rich in styrene units is formed mostly in the core of monomer-swollen particles. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2284–2291, 2000 Keywords: macromonomer; dispersion copolymerization; liquid chromatography; polymer adsorption; separation; size exclusion chromatography

INTRODUCTION tion of the macromonomers can be easily controlled, particularly of those prepared by ionic Interest in various techniques for preparation of polymerization, the graft copolymers having de- graft copolymers with deﬁned structure grows sirable side chain lengths can be obtained with due to wide range of applications of these materials. this method. Macromonomer and comonomer One route is called macromonomer method.1–3 may have similar or different polarities. If mac- Radical copolymerization of a macromonomer romonomer is hydrophilic and comonomer is hy- bearing polymerizable end group with a suitable drophobic, very interesting amphiphilic graft co- low molecular comonomer generates graft copol- polymer is created. Amphiphilic graft copolymers ymer. Since molar mass and molar mass distribu- exhibit typical properties of nonionic surfactants, such as micellization and interfacial tension re- duction. This fact can be utilized in surfactant- Correspondence to: D. Berek free dispersion and emulsion synthesis of graft Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 38, 2284–2291 (2000) 4,5 © 2000 John Wiley & Sons, Inc. copolymers. The dispersion copolymerization of 2284 PS-GRAFT-PEO COPOLYMERS 2285 a hydrophilic macromonomer with a low molar tal problems.12 Further, it is so far not completely mass hydrophobic comonomer is considered to clear what is the conformation, and consequently, proceed as follows: macromonomer, comonomer, adsorptivity as well as hydrodynamic volume of and polymerization initiator are completely dis- “chromatographically invisible” chains in block solved in an appropriate polar solvent. Copoly- and graft copolymers. If the side chains in a graft merization starts in the homogeneous system and copolymers are made “invisible”, the homopoly- grafted macromolecules are generated. The solu- mer will most probably co-elute with graft copol- bility of these species is a function of their both ymer due to similar hydrodynamic volumes of molar mass and composition. Macromolecules both species. with molar mass larger than a certain critical In eluent gradient liquid adsorption chroma- value begin to coagulate and particles of a new tography (LAC)6 sample is injected into an ad- phase are formed. These particles are stabilized sorptive LC column ﬂushed with appropriate ad- by the macromolecules of amphiphilic graft copol- sorption promoting liquid (adsorli). Macromole- ymers. cules are retained near column inlet. Next, a Presently, size exclusion chromatography gradient with increasing amount of desorbing liq- (SEC) is a dominating method in molecular char- uid (desorli) is applied. Macromolecules start acterization of products from most polymer syn- moving along the column and elute at retention theses including macromonomer technique for volume, which depends on their chemical compo- graft copolymer preparation. SEC separates mac- sition but not on their molar mass as far as they romolecules on the basis of their hydrodynamic are excluded from the pores of the column pack- volume. Macromolecules of different molar mass, ing.13 LAC was successfully applied to separation chemical composition and physical architecture of numerous random copolymers6,14 as well as may exhibit similar hydrodynamic volume and, as graft copolymers.15–19 Very promising is the com- a result, their SEC separation is impossible. For bination of LAC with SEC, which represents an the same reason, the molar mass values for copol- approach to a two-dimensional separation of co- ymers determined with direct SEC measure- polymers.14,20 ments should be considered only semiquantita- Recently, combination of full adsorption-des- tive. Additionally, SEC fails to separate ho- orption with size exclusion chromatography mopolymer(s) from polymacromonomer and graft (FAD/SEC) has been developed to separate and copolymer if their hydrodynamic volumes are characterize multicomponent polymer blends.21 similar. This is often the case because the hydro- This method is based on a full retention of mac- dynamic volume of branched macromolecules romolecules within adequately designed column does not substantially differ from that for the and on subsequent stepwise desorption of species linear analogues of their backbone though the differing in their adsorptivity. Desorption of mac- latter possess much lower molar mass. These are romolecules is controlled by, for example, compo- the reasons why alternative methods are de- sition of adsorli/desorli (displacer) mixtures. It signed for discrimination of polymer components has been shown that utilizing optimized adsorp- in graft copolymerization reaction mixture and tion and desorption of polymer species at liquid– for their molecular characterization. solid interfaces, FAD/SEC procedure can be ap- Liquid chromatography at critical adsorption plied to deformulation of blends containing minor point (LC CAP) is based on compensation of ex- components at concentration as low as 1%.22 It clusion and adsorption of macromolecules in an was anticipated that FAD/SEC procedure could appropriately chosen system of mixed eluent, give valuable information on the presence and temperature, and column packing. At the critical characteristics of homopolymers in graft copoly- adsorption point (CAP) macromolecules of given mer reaction mixture. composition and/or architecture elute irrespec- In this work, we are mainly concerned with the tively of their molar mass: they are “chromato- separation and characterization of the products graphically invisible.”6,7 Other kinds of polymer yielded by dispersion copolymerization of styrene chains that are present in the system elute under with polyethyleneoxide methacryloyl terminated SEC conditions and can be independently charac- macromonomer applying FAD/SEC and LAC ap- terized in the conventional way. This approach proaches. The results will be discussed in terms of has been applied to various binary polymer the multiple reaction loci mechanism for the dis- blends,7 block,8,9 and graft copolymers.10,11 LC persion copolymerization of PEO macromonomer CAP, however, suffers from numerous experimen- and styrene in the second part of this series. 2286 NGUYEN ET AL.

EXPERIMENTAL

Materials Commercially available monomer (styrene), initiator (dibenzoyl peroxide, DBP), solvents [(water, ethanol, toluene, dichloroethane, (DCE) tetrahydrofuran (THF), dimethylformamide (DMF))] were puriﬁed by usual methods.23,24 Methacroyl terminated polyoxyethylene [PEO-MA, Mn ϭ 1000] was supplied by NOF Corp., Ltd., Japan. PS standards were purchased from Pressure Chemicals (Pittsburgh, PA).

Polymerization Procedure Figure 1. SEC chromatograms of samples C1–C4 in The dispersion polymerization technique used to DMF at 25 °C using refractometer. prepare the graft copolymers has been described in detail previously.23,25 Monomer and macromonomer were dissolved in ethanol/water (4:1) mixture together with different DBP concentra- Cergy St. Christophe, France) and HPLC-FTIR tions and polymerized at 60 °C (Samples C1–C4). interface (100 LC-Transform, Lab Connections, Unreacted macromonomer and other low molecu- Marlborough, MA). Rear-surface-aluminized ger- lar substances were removed from polymer dis- manium (Ge-Al) discs were used for sample col- persion by precipitation in methanol and then by lection under following conditions of FTIR evapo- dialysis. The feed weight ratios of S/PEO-MA to- rative interface: nitrogen nebulizer pressure and gether with DBP concentration of four samples ﬂow rate was 60 psi and 4.5 L/min, nitrogen C1–C4 are given in Table I. The PEO composition sheath ﬂow rate and temperature was 55 mL/min of the reaction mixtures after precipitation was and 110 °C and 20 mm height of nozzle above the determined to be about 17 wt % with help of 1H collection disc. The FTIR scanning was carried NMR.11 For comparison, samples C5 and C6 pre- out on PerkinElmer FTIR 1710 (Norwalk, CT) pared in homogenous system by the solution po- using Spectra Calc Collect Arithmetic C2.23 soft- ware (Galatic Industries Corp., Salem, NH). FTIR lymerization in THF using DBP initiator at 70 °C Ϫ1 were also used in this study (Table I). The poly- spectra at 8 cm resolution were collected by meric products were twice precipitated from averaging 20 scans. methanol. RESULTS AND DISCUSSION Liquid Chromatographic Instruments Size Exclusion Chromatography Simple full adsorption-desorption (FAD)/size exclusion chromatography (SEC)21 and gradient liq- PL-gel was found to be highly interactive for poly- uid adsorption chromatography (LAC) setups mers of medium polarity like poly(methyl methac- were used. FAD (30 ϫ 3.3) column was packed rylate) in nonpolar eluent toluene.26 Tetrahydro- with nonporous silica and LAC column (150 ϫ 4.6 furan effectively suppressed adsorption. On the mm) was packed with microporous (30 Å pore other hand, highly polar polymers as poly 2-pyri- size, 5 ␮m diameter) spherical silica particles (De- dine were retained from THF and eluted in velosil, Nomura Chemical Co., Ltd., Japan). SEC DMFA.27 Therefore, preliminary SEC measure- column was linear PL-gel mixed bed 300 ϫ 7.5 ments of the samples were performed in DMF. mm (Polymer Laboratories, Church Stretton, SEC chromatograms for samples C1–C4 in DMF UK). Various LC detectors were employed: refrac- at 25 °C (Fig. 1) showed peaks at very low reten- tometer Waters 410 Waters (Milford, MA), UV tion volumes. This indicated that copolymers ei- detector for PS in DCE at 264 nm (Knauer, Ber- ther contained fractions of very large macromol- lin, Germany), evaporative light scattering detec- ecules or they created large aggregates in DMF tor (ELSD) model DDL-21 (Eurosep Instruments, solution. The sizes of these macromolecular self- PS-GRAFT-PEO COPOLYMERS 2287

eluent was used in further SEC measurements. Ten percent of DMF added to THF effectively suppressed adsorptive retention of poly 2-pyri- dines with most polystyrene/divinylbenzene SEC column packings under study27 and we believed that this may be the case also with our graft copolymers. It is worth noting that all copolymer samples still contained some unreacted macromonomer causing small peaks at high retention time.

Coupled Liquid Chromatographic Techniques As mentioned, SEC separates macromolecules ac- Figure 2. SEC chromatograms of samples C1–C4 in cording to their sizes in solution, and it is there- DMF at 100 °C using ELSD. fore hardly possible to make quantitative conclusions as to the molar mass and chemical composition heterogeneity of the prepared graft assemblies exceeded exclusion limit of the SEC copolymers from a simple SEC analysis. Am- column packing used. To verify this observation, phiphilic graft copolymers consist of monomer we carried out SEC measurements with the same units, which differ substantially in adsorption af- samples in DMF at elevated temperatures. The ﬁnity toward column packing surface (styrene peaks with very low elution volume were still and ethylene oxide in our case). As a consequence, present in SEC chromatograms at 65 °C but to- homopolymer(s), if any, could be readily sepa- tally disappeared at 100 °C (Fig. 2). This sug- rated from copolymer samples using adsorption- gested that the early eluted species were aggre- based liquid chromatographic techniques. Our gates rather than macromolecules with extremely previous work21 showed that PS did not adsorb high molar masses. The aggregation of graft co- onto nonporous silica surface from most solvents polymers can be explained by the fact that DMF is while PEO was strongly attached onto SiO2 sur- a thermodynamically good solvent for PEO side face not only from toluene, which is a typical chains but a poor solvent for PS backbone. The adsorli for many polymers, but also from pure thermodynamic quality of DMF for PS could be THF. On the contrary, DMF quantitatively de- even diminished as this solvent possibly absorbed sorbed PEO homopolymers from silica surface. trace amount of water due to its high hygroscop- Toluene is a good solvent for both kinds of chains icity. The macromolecular aggregates did not dis- in PS-graft-PEO29 and the aggregation of graft sociate at very low sample concentration and, consequently, their peaks appeared on SEC chromatograms even when the concentration of the injected sample solutions was lowered from 1.0 to 0.1 mg/mL. Since THF is a good solvent for PS, PEO, and also for PS-graft-PEO,28 attempt was made to increase solubility of copolymers by adding THF to the DMF eluent. Shoulders corresponding to the aggregates still appeared on SEC chromatograms at low retention volumes in a mixed eluent DMF/THF (50/50) (Fig. 3). Eventually, the SEC peaks in mixture DMF/THF (15/85) were similar to those in DMF at 100 °C suggesting that the aggregates were disassembled at this high content of THF in the eluent (results not shown). Thus we arrived at the eluent composition 15/85 in which both aggregation and adsorption of mac- Figure 3. SEC chromatograms of samples C1–C4 in romolecules seem to be fairly suppressed. This mixed eluent THF/DMF (50/50) at 25 °C using ELSD. 2288 NGUYEN ET AL.

of graft copolymer in the course of LAC separation. Each LAC chromatogram showed two dis- tinct peaks (Fig. 4). The first one eluted even in pure toluene that is without applying gradient elution. Its elution volume corresponded to void volume of the column. This peak can be attributed to homo-PS and possibly also to PS-graft-PEO species with very low content of EO. However, much less polar statistical copolymer poly(styrene-stat-methyl methacrylate) containing only 5% of methyl methacrylate was found to be fully retained within FAD column packed with the same nonporous silica from toluene. Therefore, it was likely that the first peak corresponded to homo-PS. The second peak appeared in the course of the gradient elution. The retention volumes at peak maximum only slightly increased in a se- quence C1ϽC2ϽC3ϳC4 and they differed substantially from that for PEO45k (Fig. 4). Higher DBP initiator concentration gave higher ratio of peak areas PS/copolymer, which indicated larger portion of homo-PS in the reaction mixture. This is in agreement with the peak position in LAC, which separates graft copolymers according to chemical composition:15–19 samples C3 and C4 exhibit the highest content of homo-PS, thus the Figure 4. LAC chromatograms of raw graft copoly- lowest styrene content in graft copolymer (Table mer samples C1–C6 and of PEO45K, at 30 °C using I). The selectivity of LAC separation can be en- ELSD. Gradient from 100% toluene to 100% toluene/ hanced by optimization of eluent gradient profile. DMF (60/40) in 30 min. Due to the lack of well-characterized PS-graft- PEO model samples, the chemical composition distributions of the graft copolymers could not be copolymers in toluene as observed in DMF should assessed. not be present. LAC with linear gradient from Further, we tried to obtain at least some mo- pure toluene to toluene/DMF (60/40) mixture in lecular characteristics of homo-PS and the graft 30 min was applied to all six samples which were copolymer applying the FAD/SEC approach.21 dissolved in toluene in order to avoid aggregation Polymer samples were dissolved in toluene and

Table I. (Effective) Molar Mass and Polydispersity Values of Homo-PS and Graft Copolymer Fractions

Homo- Homo-PS Graft Copolymer PS [DBP] ϫ ϫ Ϫ3 ϫ Ϫ3 100 Feed Mw 10 Mw 10 Sample mol/L (%S) (g/mol) Mw/Mn (g/mol) Mw/Mn (wt %)

C1 0.23 20 371 2.63 851 4.15 22 C2 0.56 20 218 2.61 377 3.94 30 C3 1.13 20 145 2.84 170 3.46 62 C4 2.25 20 79.0 2.82 92.0 3.08 61 C5 1.98 4.76 12.1 2.73 13.5 1.82 42 C6 1.98 16.7 7.26 2.36 15.4 1.95 13

Homo-PS content in each sample was calculated from ELSD peak area. PS-GRAFT-PEO COPOLYMERS 2289 injected into FAD/SEC system. Homo-PS fraction unretained in the FAD column was forwarded directly into SEC column for molecular characterization using PS calibration in the same solvent toluene. The adsorbed fraction of sample was then released from FAD column into SEC with appropriate desorli. As shown before, pure DMF is a very strong desorli for PEO but it could not be used as eluent for SEC separation of the graft copolymers. The aggregation of PS-graft-PEO in DMF occurred again here: SEC peaks of adsorbed fractions that were released from FAD with pure DMF exhibited shapes similar to these observed before with raw copolymers (Fig. 1). On the other hand, the mixture DMF/THF (15/85), which was Figure 5. Typical IR spectra of PS, PS-graft-PEO shown to prevent aggregation was sufficiently sample C2 and macromonomer PEO-1000 collected on strong to displace entire adsorbed copolymer frac- Ge-Al disc. Absorbance band around 700 and 1105 tion into SEC column. cmϪ1 are characteristic for styrene and ethyleneoxide, The apparent molar mass values for the frac- respectively. tions containing graft copolymers were calculated employing PS calibration in THF. The homo-PS content in each sample was calculated using CONCLUSIONS ELSD peak area versus PS amount calibration. Results are given in Table I. From the presented results it turns out that am- Similar results were also obtained using a less phiphilic PS-graft-PEO copolymers aggregate in effective adsorli DCE instead of toluene or a DMF while they seem to be molecularly dispersed larger and/or more adsorption-active FAD col- in THF. The aggregates are supposed to consist of umn. In other words, the same amount of nonad- a more compact core of poorly soluble PS chains sorbed homo-PS passed the FAD column unre- surrounded by a corona of PEO soluble chains. At tained when the sample solutions were injected elevated temperature the aggregates dissociate to into FAD/SEC system under these conditions. unimers and/or transform to more loosely orga- Chemical compositions of polymer species were nized structures. The presence of homopolymer also determined with the online HPLC-FTIR in- PS together with graft copolymer in the final co- terface. Typical IR spectra for PS, PEO, and raw polymerization product was taken as an indica- graft copolymer sample C2 obtained with KBr tion that there are three (the continuous phase, technique show two characteristic bands at about the particle surface layer, and the particle core) or Ϫ1 700 and 1115 cm for styrene and ethylene ox- at least two (continuous phase and the polymer ide, respectively (Fig. 5). IR spectra at particular particle core) polymerization loci. Full adsorption/ elution volumes for samples C1, C2, and C3 col- desorption procedure represents a powerful tool lected in the course of SEC separation in DMF for discrimination of parent homopolymer (“poly- (Fig. 1) are shown in Figure 6. It is evident that comonomer”) from graft copolymer prepared by the PEO content steeply increased with retention macromonomer technique. The online SEC in- volume suggesting compositional heterogeneity of strument provides molar mass and molar mass the samples C1–C4. Similar heterogeneity was distribution data for parent homopolymer. The found also with polydimethylsiloxane-graft-poly- complete separation of graft copolymer by FAD methylmethacrylate prepared by macromonomer procedure according to its composition will be technique.19 More importantly, IR spectra for most probably complicated with molar mass in- nonadsorbed fractions in toluene did not show terference because desorption of macromolecules any detectable absorbance band characteristic for from solid surfaces is governed not only with ethylene oxide unit (Fig. 7). This observation con- chemical structure but also with molar mass of firmed the above conclusions concerning the ho- polymer species.29 Therefore, the simple FAD/ mopolymerization of styrene in heterogeneous SEC combination will be the method of choice (dispersion) copolymerization system. only when determination of amount and molecu- 2290 NGUYEN ET AL. lar characteristics of parent homopolymer(s) are of interest. Liquid adsorption chromatography easily separates parent homopolymer from graft copolymer

Figure 7. IR spectra of nonadsorbed (homo-PS) (C1F1, C2F1, C3F1 and C4F1) and adsorbed fractions C1F2, C2F2, C3F2 and C4F2) using FAD procedure.

as well, but it does not provide molar mass data. LAC supposedly separates graft copolymers ex- clusively according to their composition. The two- dimensional LC, viz. an online combination of LAC with SEC appears to be an appropriate procedure for complete characterization of all components of reaction mixture. Such combination, however, suffers from

(1) small quantities of polymer fractions leav- ing LAC column as very diluted solutions, and (2) possible incompatibility of the LAC eluent with the SEC system.

The above problems can be overcome by inclusion of a full adsorption-desorption column into the liquid chromatographic system. FAD allows to reconcentrate the LAC efﬂuent30 and even to combine corresponding LAC fractions from re- peated elutions. Further, FAD step enables the appropriate exchange of sample matrix. In this way the SEC separation can be done in a single

Figure 6. (a) IR spectra of SEC slices for sample C1 in DMF collected on Ge-Al disc. The above indicated numbers are retention times (sec) in the chromatogram shown in Figure 1. (b) IR spectra of SEC slices for sample C3 in DMF collected on Ge-Al disc. The above indicated numbers are retention times (sec) in the chromatogram shown in Figure 1. (c) IR spectra of SEC slices for sample C4 in DMF collected on Ge-Al disc. The above indicated numbers are retention times (sec) in the chromatogram shown in Figure 1. PS-GRAFT-PEO COPOLYMERS 2291 eluent so that refractive index, photometric, light 15. Teramachi, S. Macromol Symp 1996, 110, 217. scattering, and viscometric detection is facili- 16. Tanaka, S.; Uno, M.; Teramachi, S. Polymer 1995, tated. Eventually, we arrive at a three-step two- 36, 2219. dimensional separation, viz. LAC/FAD/SEC. 17. Teramachi, S.; Sato, S.; Shimura, H.; Watanabe, S.; Tsukahara, Y. Macromolecules 1995, 28, 6183. 18. Teramachi, S.; Hasegawa, A.; Matsumoto, T.; Ki- This work was supported by a grant of the Slovak tahara, K.; Tsukahara, Y.; Yamashita, Y. Macro- Grant Agency VEGA (Projects No. 2/4012/97 and molecules 1992, 25, 4025. 2/5005/98), as well as in the framework of the US-SK 19. Schunk, T. C.; Long, T. E. J Chromatogr A 1995, Scientiﬁc-Technical Cooperation (Project No. 007-95) 692, 221. and the bilateral cooperation between the Italian Con- 20. Schunk, T. J Chromatogr 1994, 661, 215. siglio Nazionale delle Ricerche and the Slovak Acad- 21. (a) Janco, M.; Berek, D.; Prudskova, T. Polymer emy of Sciences. 1995, 36, 3295; (b) Janco, M.; Prudskova, T.; Berek, D. Int J Polym Anal Charact 1997, 3, 319; (c) Nguyen, S. H.; Berek, D. Chromatographia 1998, REFERENCES AND NOTES 48, 65; (d) Nguyen, S. H.; Berek, D. In Chromatog- raphy of Polymers: Hyphenated and Multidimen- 1. Pitsikalis, M.; Pispas, S.; Mays, J. W.; Hadjichris- sional Techniques; Provder, T., Ed.; ACS Sympo- tidis, N. Adv Polym Sci 1998, 135, 1. sium Series; American Chemical Society: Washing- 2. Ito, K.; Kawaguchi, S. Adv Polym Sci 1999, 142, 130. ton, DC, 1999; Vol. 731, Chapter 15; (e) Nguyen, 3. Capek, I. Adv Polym Sci 1999, 145, 1. S. H.; Berek, D. Colloid Polym Sci 1999, 277, 318. 4. Hansen, F. K.; Ugelstad, J. In Emulsion Polymer- 22. Nguyen, S. H.; Berek, D. Int J Polym Anal Charact, ization; Piirma, I., Ed.; Academic Press: New York, accepted for publication. 1982. 23. Capek, I.; Riza, M.; Akashi, M. Makromol Chem 5. Fitch, R. M.; Tsai, C. H. J Polym Sci Polym Chem 1992, 193, 2843. Ed 1970, 8, 703. 24. Tanrisever, T.; Okay, O.; Sonmezoglu, I. C. J Appl 6. (a) Glockner, G. Gradient HPLC of Copolymers and Polym Sci 1996, 61, 485. Chromatographic Cross-Fractionation; Springer- 25. Capek, I.; Riza, M.; Akashi, M. Polym J 1999, 24, 959. Verlag: Berlin, 1991; Chapter 6; (b) Pasch, H.; 26. Berek, D. In Column Handbook for SEC; Wu, C-S., Trathnigg, B. HPLC of Polymers; Springer-Verlag: Ed.; Academic Press: New York, 1999; Chapter 15, Berlin, 1998; Chapter 6. p 445. 7. Pasch, H. Polymer 1993, 34, 4095. 27. Berek, D.; Tarbajovska, J. Separation and Charac- 8. Gankina, E.; Belenkii, B.; Malakhova, I.; Mele- terization of Macromolecules. Proceedings of the nevskaya, E.; Zgonnik, V. J. Planar Chromatogr 13th Bratislava Conference on Polymers, 1991, 4, 199. Bratislava, Slovakia, July 1999, p 97. 9. Zimina, T. M.; Kever, J. J.; Melenevskaya, E. Y.; 28. Brandrup, J.; Imergut, E. H.; Grulke, E. A. Poly- Fell, A. F. J. Chromatogr. 1992, 593, 233. mer Handbook, 4th ed.; Wiley: New York, 1999; 10. Murgasova, R.; Capek, I.; Lathova, E.; Berek, D.; Chapter 5. Florian, S. Eur Polym J 1998, 34, 659. 29. (a) Berek, D.; Nguyen, S. H. Macromolecules 1998, 11. Murgasova, R. Ph.D. Thesis, Polymer Institute, 31, 8243; (b) Nguyen, S. H.; Berek, D. Colloids The Slovak Academy of Sciences, Bratislava, Slo- Surfaces A: Physicochem Eng Aspects 2000, 162, vakia, 1998 75; (c) Nguyen, S. H.; Berek, D. Colloid Polym Sci 12. Berek, D. Macromol Symp 1996, 110, 33. 1999, 277, 318. 13. Berek, D. Macromolecules 1999, 32, 3671. 30. (a) Nguyen, S. H.; Berek, D.; Chiantore, O. Polymer 14. Mori, S. Anal Chem 1986, 58, 303; Anal Sci 1988, 4, 1998, 39, 5127; (b) Nguyen, S. H.; Berek, D. J 365; J Appl Polym Sci 1989, 43, 65. Polym Sci A: Polym Chem 1999, 37, 267.