novel polymer monolith prepared from a water-soluble crosslinking agent

Novel Polymer Monolith Prepared from a Water-SolubleCrosslinking Agent

TAKUYA KUBO, NAOMI KIMURA, KEN HOSOYA, KUNIMITSU KAYA

Graduate School of Environmental Studies, Tohoku University, Aoba 6-6-20, Aramaki, Aoba-Ku,Sendai 980-8579, Japan

Received 28 September 2006; accepted 15 March 2007DOI: 10.1002/pola.22130Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: In this study, we described the fundamental properties of novel polymermonoliths that were prepared from a water-soluble crosslinking agent. Each monolithwas evaluated by scanning electron microscope (SEM) and scanning probe microscope(SPM) to observe the monolithic structure, and the polymer films that were preparedfrom several monomers were evaluated by the contact angle of water. As results ofevaluations, the polymer prepared from a water-soluble crosslinking agent had highhydrophilicity. Furthermore, SEM evaluations suggested that polymer porogenic sol-vent (PEG) was contributed to the construction of monolithic structure, and the poly-merization degree of PEG was also taken effect of the structural changing by the var-iation of phase separation. Additionally, the results of SPM evaluations and the dif-ferences of monolithic structure were also reflected under water condition althoughthe swelling of polymer was observed. VVC 2007 Wiley Periodicals, Inc. J Polym Sci Part A:

Polym Chem 45: 3811–3817, 2007

Keywords: hydrophilicity polymer; liquid chromatography; morphology; phase separa-tion; polymer monolith; water-soluble crosslinking agent

INTRODUCTION

The monolithic materials based on silica, otherinorganic compounds, and organic polymershave been developed on the separation field dur-ing the last few decades.1–11 Especially, the or-ganic polymer materials have been taken noticeas novel separation media because they havechemical stabilities and the easiness of chemicalsurface modification with remaining the throughpore structure for the high through-put elution.At an early stage on the development of mono-lithic materials, the hydrophobic octadecyl groupsbonded silica as well as hydrophobic-methacry-

late-based polymers had been mainly studied.However, the hydrophilic materials have beentaken notice, especially the hydrophilic interac-tion (HILIC). In the analysis fields for drugs,biomolecules, environmental, and natural com-pounds, the hydrophilicity of the separationmedia is much significant because almost all thecompounds are water-soluble. Recently, somepolymer monoliths having hydrophilic propertyhave been reported.12–14 However, the mono-lithic materials reported previously had a certainlevel of hydrophobicity. Therefore, we believe thatthe novel separation media, showing the highlyhydrophilic interaction and absolutely no hydro-phobicity, is required in the monolithic materials.

In this study, we describe the fundamentalproperties (physical appearance, hydrophilicity,porous structure) of a novel polymer monolithprepared from the water-soluble crosslinking

Correspondence to: Takuya Kubo; (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, 3811–3817 (2007)VVC 2007 Wiley Periodicals, Inc.

3811

agent by the contact angle of water, SEM, andSPM. Previously, we reported about the molecu-larly imprinted polymers (MIPs) prepared withthe water-soluble crosslinking agent and water15

and the MIPs showed high hydrophilicity. In thistime, two kinds of poly(ethylene glycol)-diacry-late-based monomers were utilized as the water-soluble crosslinking agents (shown in Fig. 1).

EXPERIMENTAL

Chemicals

Polyethylene glycol (PEG), ethylene glycol dime-thacrylate (EDMA), pure water, and a radicalpolymerization initiator (2,20-azobis [2-(2-imida-

zolin-2-yl) propane dihydrochoride]) (ABIP) werepurchased from Wako Chemicals (Osaka, Ja-pan). Water-soluble crosslinking agents (poly(ethylene glycol)-diacrylate-based monomer) andglycerol dimethacrylate (GDMA) were providedfrom NOF (Tokyo, Japan).

Preparation of Polymer Monoliths

Polymer monoliths were prepared from poly(eth-ylene glycol)-diacrylate-based monomers (PEDA-9 or OH-PEDA-4 shown in Fig. 1). The mixtureof porogenic solvent and crosslinking agent werepacked into glass tube (50 mm 3 5.0 mm I.D.)with radical polymerization initiator. Then, theglass tubes were sealed-up and polymerized bythe photo or thermal polymerization using 2,20-Azobis [2-(2-imidazolin-2-yl) propanedihydrochor-ide (ABIP) (1.0 wt %) as a initiator. The composi-tions, numbers of polymers, and polymerizationconditions are shown in Table 1.

Morphological Evaluations for Each Polymer

Contact angle of water against each polymerfilm evaluated with CA-DS (Kyowa Interface,Japan). Scanning electron microscope (SEM, S-4200, Hitachi, Japan) images of polymer mono-liths were observed and scanning probe micro-scope (SPM, Shimadzu, Japan) was also utilizedfor the observation polymers in water.

Figure 1. Structure of the water-soluble crosslink-ing agents.

Table 1. Composition of Polymer Monoliths

CrosslinkingAgent

Porogen(wt %)

Mw ofPEG Polymerization

Ratio(Monomer/

Porogen, w/w)

1 OH-PEDA-4 H2O – Thermala 1:12 PEDA-9 H2O – Thermal 1:13 OH-PEDA-4 H2O – Thermal 3:74 OH-PEDA-4 H2O – Photob 1:15 OH-PEDA-4 10% PEG aq. 20,000 Thermal 1:16 OH-PEDA-4 10% PEG aq. 20,000 Photo 1:17 OH-PEDA-4 1% PEG aq. 20,000 Thermal 1:18 OH-PEDA-4 1% PEG aq. 20,000 Photo 1:19 OH-PEDA-4 10% PEG aq. 20,000 Thermal 3:7

10 OH-PEDA-4 10% PEG aq. 200 Photo 1:111 OH-PEDA-4 10% PEG aq. 400 Photo 1:112 OH-PEDA-4 10% PEG aq. 2,000 Photo 1:113 OH-PEDA-4 10% PEG aq. 6,000 Photo 1:1

a At 508C for 10 H.b 365 nm for 2H.

3812 KUBO ET AL.

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

RESULTS AND DISCUSSION

Hydrophilicity of Polymers

At first, to clarify the potential hydrophilicity ofpolymers, the contact angle of water was eval-uated. To obtain the data of contact angle, thepolymer films of PEDA-9, OH-PEDA-4, glyceroldimethacrylate (GDMA), and ethylene glycoldimethacrylate (EDMA) were prepared withoutporogenic solvent using photo radical polymer-ization. EDMA is one of the popular crosslinkingagents and usually utilized for MIPs, the pack-ing materials of liquid chromatography and thepolymer monoliths. Moreover, GDMA is alsoused for the packing materials, which have thehydrophilic property. Photos and results of thecontact angle of water against each polymer filmare shown in Figure 2. The film of EDMA andGDMA, which is water-insoluble, showed highvalue for contact angle of water because of theirhydrophobic property, although GDMA hashydrophilicity based on the OH groups. Corre-spondingly, the film of PEDA-9 showed rela-

tively lower angle compared with EDMA andGDMA. In contrast, the film of OH-PEDA-4showed much higher hydrophilicity, the waterdrop was immediately expanded on the surfaceof the film so that the contact angle could not bemeasured. Although both PEDA-9 and OH-PEDA-4 can be easily dissolved in water, thehydrophilicity of these polymer films was muchdifferent. According to the calculated solubilityparameter of each monomer,16 the parameter ofPEDA-9 and OH-PEDA-4 are 18.9 and 22.7 sothat the hydrophilicity of OH-PEDA-4 is higherthan that of PEDA-9. Consequently, the hydro-philicity of the prepared polymers was also dif-ferent. Accordingly, the results suggested thatthe existence of hydrophilic groups (such as OHgroup) as well as the water-soluble is very im-portant for ensuring the hydrophilicity of mono-lithic materials after polymerization.

Evaluation of Monolithic Structure bySEM and SPM

Several polymer monoliths were prepared inglass tube under each condition using PEDA-9

Figure 2. The contact angle of water on each polymer film.

POLYMER PREPARED FROM A CROSSLINKING AGENT 3813


or OH-PEDA-4. It was easily observed that thephysical appearance was much varied by the dif-ference of crosslinking agent and ratio of mono-mer/porogen. Furthermore, to examine aboutthe differences on the microscale structure ofpolymer monoliths, each polymer was evaluatedby the SEM. SEM images of some polymermonoliths are shown in Figures 3 and 4.

As shown in Figure 3, the polymerizationmethod (thermal or photo) was not much relatedwith the construction of the porous structure.On the other hand, the results clearly suggestedthat the porous structure could not be con-structed without polymer (PEG) porogenic sol-vent. It was easily understood that the phaseseparation was controlled with PEG. Actually,according to the comparison between 1 (OH-PEDA-4 + H2O, 1/1), 5 (OH-PEDA-4 +10%PEG-T, 1/1), and 7 (OH-PEDA-4 +1%PEG-T, 1/1), the porous structure was clearlyvaried with the content of PEG. On the other

hand, the comparison between 5 and 9 (OH-PEDA-4 + 10%PEG-T, 3/7) suggested that theratio of porogenic solvent containing PEG didnot affect the changing of porous structure. Con-sequentially, it is assumed that the additiveswere more important than the simply control ofratio of monomer/solvent to obtain a certainlevel of porous structure.

Next, the comparisons of structure of mono-liths that were prepared with PEG having sev-eral molecular weights are shown in Figure 4.These SEM images indicated very interestingresults. Using PEG having low molecular weight(less than 1000) (10, 11), the narrow skeletonsize and the small through-pore were con-structed although the monoliths had close tomonolithic structure. On the other hand, usinghigh molecular weight (more than 5000) (13, 6),the monoliths showed the agglomeration of par-ticles so that the through-pore could not beobtained. In contrast, the monolith 12, which

Figure 3. SEM images of the polymer monoliths in each polymerization condition(310,000). Aberrations: OH; T, thermal; P, photo, weight/weight.

3814 KUBO ET AL.


was prepared with PEG (molecular weight¼ 2000) showed the better monolithic structureand a certain level of through-pore. As reportedpreviously,17–20 these results suggested that theviscosity of porogen as well as the affinitybetween the crosslinking agent and porogen,which contributes to the rate of phase separa-tion, were strongly concerned with the changingof monolithic structures. In our study, we uti-lized PEG having each molecular weight as theporogenic solvent at the same weight percent.Therefore, the total number of hydroxyl groupsof PEG became higher according to the decrease ofmolecular weight of PEG so that the affinity ofporogenic solvent against OH-PEDA-4 would bechanged. Herein, it is assumed that the porogencontaining low molecular weight PEG have goodaffinity for OH-PEDA-4 and low viscosity,whereas the porogen containing high molecularweight PEG have poor affinity and high viscos-ity. Therefore, when PEG (molecular weight

¼ 2000) was utilized as the additive of porogen,which has medium properties in point of the af-finity and viscosity, the optimized monolithicstructure was constructed.

As shown in SEM evaluations, the skeletonsize of prepared polymer monoliths was verysmall. Therefore, we expected that the physicalstrength of the prepared polymer monoliths wastoo low. In fact, we could not carry out theevaluation by mercury intrusion porosimetrybecause of the breakage failure of polymers.Depending on this term, it was assumed thatthe drastic changing of monolithic structure wasobserved in water or in vacuum. Therefore, thestructural observation of cross-sectional surfaceof polymer monoliths in water was carried outusing SPM. 3D SPM images and the result ofanalysis for roughness are shown in Figure 5and Table 2. Figure 5 indicated that the polymer1 prepared without PEG had porous structurealthough SEM images suggested the nonporous

Figure 4. SEM images of the polymer monoliths prepared with the different molec-ular weight of PEG (310,000).



structure. Additionally, the polymer 12 hadmore obvious porous structure and larger scalepores. Results of analysis for roughness (Table 2)also showed the differences of pore scale be-tween 1 and 12. These results suggested thatthe differences of pore scale was observed evenin water, and also strongly supported that theexistence of PEG as an additive of porogenic sol-vent was very important for the construction ofmonolithic structure. However, these resultsclarified the shrinkage of polymer in water or invacuum, and the shortness of physical strengthof prepared polymer monoliths. Probably, itdepended on the characterization of crosslinkingagent (which has long spacer between vinylgroups). Therefore, this problem may be solvedby using the other water-soluble crosslinkingagent, which has shorter spacer. But, we believethat the polymer monoliths prepared in thisstudy has possibility of novel separation mediafor several hydrophilic compounds and novel

monolithic column of liquid chromatography af-ter additional examinations.

CONCLUSIONS

We described a possibility of the development ofa novel hydrophilic polymer monolith preparedfrom a water-soluble crosslinking agent andwater containing PEG. The results suggestedthat we can easily control the monolithic struc-ture by changing the affinity and viscosity of theporogenic solvent. Furthermore, we clearlyshowed the differences of pore structure of poly-mer monoliths even in water by SPM evalua-tions. Additionally, the polymer monoliths havefree OH groups on the surface of the porousmonolithic structure. Therefore, the additionalsurface modification by the chemical reactionscan be easily carried out so that several applica-tions are expected.

Figure 5. 3D images of cross-sectional surface of polymer monoliths in water bySPM. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Table 2. Analysis for Roughness of Cross-Sectional Surface of Polymer Monolithsin Water

PolymerRa(nm)

Rz(nm)

Rzjis(nm)

Rq(nm)

Rp(nm)

Rv(nm)

1: OH-PEDA-4+H2O, 1/1 52.3 551.5 269.9 65.9 329.7 221.812: OH-PEDA-4+10%PEG (2,000)-P, 1/1

264.7 1911 953.1 330.6 924.8 989.1

Ra, arithmetical mean deviation; Rs, maximum height; Rzjis, 10-point mean roughness; Rq,Root-mean-square roughness; Rp, Average height; Rv, Average depth.

3816 KUBO ET AL.


This research was partly supported by Nanotechnol-ogy Project of the Ministry of Environment andGrant-in-Aid for Basic Scientific Research (No.18685009) from the Ministry of Education, Science,Sport, and Culture of Japan. We thank the NOFCorp. (Japan) for providing the crosslinking agent.The authors would like to show their appreciation toMr. N. Edo of Shimadzu Corp. for his excellent SPMwork and discussion.

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novel polymer monolith prepared from a water-soluble crosslinking agent

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