Published: August 30, 2011

r 2011 American Chemical Society 12630 dx.doi.org/10.1021/la2023055 | Langmuir 2011, 27, 12630–12635

ARTICLE

pubs.acs.org/Langmuir

Effect of Hydrophobicity of Monomers on the Structures andProperties of 1,3:2,4-Dibenzylidene-D-sorbitol Organogels andPolymers Prepared by Templating the GelsWei-Chi Lai,*,†,‡ Shen-Jhen Tseng,† and Yu-Sheng Chao†

†Department of Chemical and Materials Engineering and ‡Energy and Opto-Electronic Materials Research Center,Tamkang University, No. 151, Yingzhuan Road, Danshui District, New Taipei City 25137, Taiwan

bS Supporting Information

’ INTRODUCTION

Porous polymer synthesis, using “template” techniques, hasbeen investigated for the past decade.1�5 The easiest and mostgeneral method uses a template as a structure-directing agent.For example, the nanoscale templates are directly mixed with theinitiators and monomers. The thermal or photopolymerizationof monomers results in nanostructured, solid polymers, contain-ing templates. The porous materials are obtained by the solventextraction of the templates from polymers. These porous poly-mers find useful applications, such as mesoporous catalysts,membranes, and stationary phases for chromatography andchemical sensors.6�8

Usually, the template materials consist of low molecular weightmolecules, colloids, micelles, and self-assemblies, due to their easeof extraction by the solvents.1 The techniques of template synthe-sis of porous polymers are as follows: molecular imprinting,9,10

colloid crystal templating,11 micellar templating,12�14 and orga-nogel templating.15�17 In this study, we chose the organogels asour templates to prepare the porous polymers. The organogelsresulted from the formation of network structures, by hydrogenbonding or other interactions. Other template systems, suchas micelles, were unsuitable because of their lack of strong

interactions between themolecules. The structures of the nanoscaledorganogel templates are easily maintained during polymerization.18

1,3:2,4-Dibenzylidene sorbitol (DBS) is an amphiphilic mole-cule, which has hydrophobic phenyl rings and hydroxyl groups.It is derived from the sugar alcohol, D-glucitol. The chemicalstructure of DBS is shown in Figure 1. DBS is an inexpensivesugar derivative that could self-assemble into nanofibrils, throughhydrogen bonding, at relatively low concentrations in a variety oforganic solvents and low molecular weight polymers to produceorganogels.19�22 The DBS amount was typically <1 wt %, andthe gel would form.Moreover, DBS organogels have a spherulite-like morphology by polarizing optical microscopy (POM), whichis very special in organogel systems. The average diameter offibrils ranged from 10 nm to 1 μm depending on the solvent andpolymer matrix.

In our previous studies,18 DBS organogels were used as temp-lates to prepare porous polystyrene (PS) materials. The DBSorganogels, which consisted of nanofibrils in the PS matrix, acted

Received: June 20, 2011Revised: August 15, 2011

ABSTRACT: The effects of hydrophobicity of monomers onthe structures and properties of 1,3:2,4-dibenzylidene-D-sorbi-tol (DBS) organogels and nanostructured polymers preparedby templating the self-assembled organogels were investigatedin this study. Hydrophobic styrene (St), hydrophilic methyl(methacrylate) (MMA), and their mixtures were chosen as themonomers. Though the gelation time varied, the averagediameters (around 10 nm) of DBS nanofibrils found in theresulting organogels did not change significantly, for monomersof different hydrophobicity, as observed by transmission elec-tron microscopy (TEM). Nonetheless, new structures, DBSmicroaggregates, appeared when the MMA content in themonomers was high enough. These irregular, micrometer-sized DBS structures (microaggregates) may have formed because theaggregated DBS molecules were influenced by the MMA monomers, due to the hydrogen bonding between DBS and MMA. Thiswas confirmed by Fourier transform infrared (FTIR) spectroscopy and could also explain the differences in the gelation time of theDBS organogels: gels form more slowly in MMA than in St because of the competing interaction, hydrogen bonding, between DBSand MMA. Subsequently, we thermally initiated the free-radical polymerization of these St/MMA co-monomers. PS/PMMAcopolymers were obtained, and nomacroscopic phase separation occurred after the polymerization. Finally, the porous structures ofthe polymers produced by the solvent extraction of the DBS templates were observed, using TEM.

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as reinforcing materials to increase the mechanical propertiesof PS. DBS nanofibrils were also found in some crystallinepolymers, such as polypropylene (PP) and poly(L-lactic acid)(PLLA).23,24 Our previous study24 showed that DBS nanofibrilscould form in PLLA after different heat treatments (crystallizationprocesses) of PLLA.When PLLA crystallizes, DBSmolecules areexcluded from the crystals, leading to an obvious increase in theDBS concentration and the formation of DBS nanofibrils. Theporous structure of PLLAwas produced by the extraction of DBSnanofibrils.

Before this study, we attempted to blend DBS and theamorphous polymers such as PS and poly(methyl methacrylate)(PMMA), by the solution casting method (the preparationmethod was the same as that used in our previous study24).The DBS/PS and DBS/PMMA samples were also subjected todifferent heat treatments. However, DBS nanofibrils were notfound in PS and PMMA systems. It was assumed that the numberof aggregated DBS molecules was not large enough because DBSmolecules were dispersed by the polymer matrix. Thus, the DBSnanofibrils did not form in the amorphous polymers.

The formation of nanofibrils is much easier in crystallinepolymers, due to the greater aggregation of DBS moleculesduring the crystallization (no DBS organogel forms). In compar-ison, only when organogel template synthesis was employedcould nanofibrils form in amorphous polymers. In this study, itwas found that DBS could produce the organogels in styrene (St)and methyl methacrylate (MMA) monomers, so we inducedDBS organogels in St and MMA samples and then thermal-initiated the free-radical polymerization. The final porous struc-ture polymers were produced by templating these self-assembledorganogels.

This study mainly focuses on the effect of hydrophobicity ofmonomers (St/MMA) on the structures and properties of DBSorganogels and porous polymers prepared by templating the gels.Fourier transform infrared (FTIR) spectroscopy was used todetermine whether there was molecular interaction, such ashydrogen bonding, between DBS and St, or MMA. Our resultsindicate that there was hydrogen bonding between DBS andMMA. The gelation time and dynamic rheological properties oforganogels were quite different for different St/MMA composi-tions. Transmission electron microscopy (TEM) observationsshow that the samples containing MMA demonstrated a newlydiscovered DBS “microaggregate” structure. While DBS nanofi-bril structures have been found in several organic solvents andpolymers, this is, to our knowledge, the first paper to reportthe intriguing morphology. Furthermore, unlike our previousstudy,18 which focused on the structure of homopolymerssynthesized in the presence of DBS, here St and MMA wereblended to alter the monomer hydrophobicity, and we managed

to observe the porous structures of polymers after the extractionof the templates and compared these structures to those of thegel samples. There are several reports concerning the differencesin template structures, in the gel and solid states.15,16 Tan et al.15

found that the hollow replica was obtained after the removal ofthe gels in the bis(2-ethylhexyl) sodium sulfosuccinate (AOT)organogel systems. There was not much difference in the gel andsolid states for the AOT fiber bundle structures. George andWeiss16 reported that the density of aggregates increased andthat phase separation microscopically occurred after the polym-erization of the organogels with conjugated diyne units. Incomparison, our results indicate that the structures of thepolymerized samples after the extraction of DBS componentswere different from those in the gel state and in the sampleswithout extraction. Moreover, no phase separation occurred afterthe polymerization. This paper discusses these differences andstudies the effect of hydrophobicity of monomers on thestructures and properties of organogels and polymers.

’EXPERIMENTAL SECTION

Materials. DBS (1,3:2,4-dibenzylidene-D-sorbitol) was purchasedfromMilliken Chemical. Styrene (St) and methyl methacrylate (MMA)were of reagent grade and were purchased from Aldrich. Benzoylperoxide (BPO) and ethanol were of reagent grade and were purchasedfrom Fluka Cpany.Sample Preparation. The DBS organogel samples were prepared

by dissolving various amounts (0.5�3 wt %) of DBS in St/MMA (v/v)(0/100, 25/75, 50/50, 75/25, and 100/0) co-monomer mixtures at80 �C on a hot plate under constant agitation. After the DBS wascompletely dissolved, the clear solution was removed from the plate andcooled to room temperature, to induce gelation. The samples werestored at 25 �C for 1 week prior to conducting experiments.

The porous polymer samples were prepared by templating the DBSorganogels. First, different amounts of DBS were dissolved in St/MMAco-monomers at 80 �Con a hot plate under constant agitation.When theDBS had dissolved completely in the co-monomers, BPO (1.0 wt %),was added to these solutions, as a thermal initiator. At this hightemperature (80 �C), the BPO dissolved easily, in a few seconds, inthe co-monomers. The samples were then removed from the hot plateand very quickly cooled to room temperature to suppress the poly-merization. The resulting gel samples were synthesized using thermallyinitiated polymerization at 60 �C for 3 days. The porous polymers wereprepared by the solvent extraction of the DBS organogels. Afterpolymerization, the solid samples were immersed in warmed ethanol(50 �C) for 1 week. The ethanol was changed every day.Transmission Electron Microscopy (TEM). The microstruc-

tures of the DBS organogels and porous polymers prepared by templat-ing the gels were observed using field emission transmission electronmicroscopy (JEOL JEM-2100F). The organogel samples were preparedby dissolving different amounts of DBS in St/MMA co-monomers at80 �C on a hot plate under constant agitation. When the DBS dissolvedcompletely, the solutions were quickly dripped onto carbon-supportedTEM grids to form the gel samples. The samples were stored at 25 �C for1 week. The samples were then washed with solvent (methanol) andsubjected to a vacuum to remove the monomers. Finally, some sampleswere treated with RuO4 vapors, while others were not. The porouspolymer samples were prepared by solvent extraction of the DBScomponents. The polymerized samples were ground in a ball-millingmachine and then immersed in warm ethanol for 1 week. The powderwas dried and then put in ethanol again. The suspensions were thendripped onto carbon-supported TEM grids and dried in a vacuum ovento remove residual ethanol.

Figure 1. Chemical structure of 1,3:2,4-dibenzylidene-D-sorbitol (DBS).

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Fourier Transform Infrared (FTIR) Spectroscopy.The chemi-cal structures and molecular interactions of samples were determinedusing FTIR spectroscopy (Nicolet Magna-IR 550 spectrometer). Theorganogel samples and monomers were cast onto CaF2 plates for the IRexperiment.Gelation Time. The gelation time (time required to form gels) for

the DBS organogel samples was determined by inverting the vials everyminute, once heating was halted, to determine when the samples werecapable of holding weights, without flowing down. The formation of gelsamples,e 1wt %DBS ande50 volume percentage of St, was very slow,so they were inverted every hour.Dynamic Rheological Measurement. The dynamic rheological

properties of the organogel samples were measured using an oscillatoryviscometer (Paar Physica RheolabMC-100). The frequency (ω) spectraof the elastic and viscous moduli (G0 and G00) were collected, from 0.01to 100 rad/s, at 25 �C, with constant strain amplitude of 0.5%.

’RESULTS AND DISCUSSION

Self-Assembly of DBS Organogels. DBS self-assembles intothree-dimensional (3-D) nanofibrillar networks, through hy-drogen bonding, in a range of organic solvents and liquidpolymers.18�22 In this study, we focus on the self-assemblybehavior of DBS organogels in monomers of different hydro-phobicity. The co-monomers weremixtures of St andMMA. Thevolume ratios for the different co-monomers were St/MMA0/100, 25/75, 50/50, 75/25, and 100/0. MMA is more hydro-philic due to the presence of an ester group.25 First, we examinedthe phase behaviors of these samples. The samples were stored at25 �C for 1 week before observation. The formation of the gelscould be qualitatively observed with the naked eye. Upon inver-sion of a vial, it can be seen that the solution does not flow, due toits high viscosity, and can therefore be termed a gel. The quan-titative analysis of the gels was performed later, in the rheologicalmeasurement. In all samples, it was found that gels formed as theDBS concentration reached 0.5 wt %. However, if the DBSamounts exceeded 3 and 3.5 wt %, the MMA and St samplesbecame heterogeneous gels. Here heterogeneous means that theDBS precipitation, or macroscopic phase separation, is observa-ble by optical microscopy. In the process of sample preparation,the dissolution time for DBS in St was much shorter than that inMMA. The hydrophobic phenyl rings of DBS seem to facilitatethe dissolution in the more hydrophobic St monomer. For thecomparison of the different hydrophobicity of monomers, in thisstudy, the organogels with 0.5 �3 wt % DBS were chosen forfurther investigation.Selective TEMmicrographs of St, St/MMA 50/50, and MMA

organogel samples containing 2 wt % DBS, without and withexposure to RuO4 vapor, are shown in Figure 2. In this figure, itcan be seen that the network structures are formed. This provesthat the organogels result from the formation of DBS nanofibi-brillar networks. The average diameter of the DBS nanofibrilswas around 10 nm, which is very similar to that for DBS inpoly(propylene glycol) (PPG), poly(ethylene glycol) (PEG),tetrahydrofuran/benzene, silicone polymers, and so on.19�22

The most interesting result, which is very different from accountsin literature, is the occurrence of new structures, which we called“DBS microaggregates” (the dark regions in Figure 2c�f), in thesamples with more than 50 volume percentage of MMA. ThisDBS aggregated structure is irregular and quite different from theDBS fibrils. It appears that the DBS molecules are aggregatedinto a large domain and the aggregated size, or diameter, is more

than 440 nm, with some close to micrometer magnitude. More-over, the size of aggregated DBS microaggregates increases withthe increase of MMA amounts. Figure SI 1 of the SupportingInformation shows a photograph of St and MMA organogelswith 2 wt %DBS. It can be seen that the sample containingMMAcontent was less transparent, whichmay be due to light scatteringcaused by the DBS microaggregates, indicating their presence inthe gel state.Table 1 lists the average diameters of DBS nanofibrils andDBS

microaggregates for gel samples with different composition ratiosof St/MMA co-monomers. Theses diameters were determinedusing Power Image Analysis System (PIA) software. It was foundthat the average diameter of DBS nanofibrils varied widely insamples not stained with RuO4 vapor. For example, the averagediameter of DBS nanofibrils for St samples without staining wasaround 10 nm, while that for the MMA samples was nearly30 nm. Since the phenyl rings are easily stained by RuO4 vapor,the obtained DBS nanofibril size should be more precise in thesamples with staining, especially in the MMA samples. As canbe seen, the average diameter of DBS nanofibrils was more

Figure 2. Selective TEM micrographs of (a) St (without RuO4 vapor)2000�, (b) St (without RuO4 vapor) 4000�, (c) St/MMA 50/50(without RuO4 vapor) 2000�, (d) St/MMA 50/50 (withoutRuO4vapor) 4000�, (e) MMA (without RuO4 vapor) 4000�, and(f) MMA (with RuO4 vapor) 4000� organogel samples containing 2 wt %DBS. The dark regions in (c)�(f) are the “DBS microaggregates”.

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consistent in the stained samples. In comparison, the averagesizes of the relatively large microaggregates showed no significantdifference whether or not the samples were stained withRuO4 vapor.To summarize, the sizes of DBS nanofibrils (around 10 nm)

are not significantly affected by the hydrophobicity of monomers,but the samples with MMA components show micrometer-sizedDBS irregular structures (microaggregates), of which the sizeincreases with the increase ofMMA contents. We believe that thereason for these different structures may be the intermolecularinteraction between DBS and MMA.IR spectroscopy was used to determine whether intermole-

cular interaction, such as hydrogen bonding, occurred betweenDBS and St, or MMA. Figure SI 2 of the Supporting Informationshows the IR spectra for St, DBS, and a St organogel with 2 wt %DBS. In this figure, the positions and shapes of characteristicpeaks of styrene, in both the spectra of the St monomer and thatof the gel sample, are very similar, with respect to the aromaticrings’ C�H bands at 3065, 3048, and 3027 cm�1, the aromaticrings’ CdC stretching bands at 1601 and 1573 cm�1, and thealkenes’ CdC stretching band at 1630 cm�1. This implies thatthe intermolecular interaction between DBS and St was small inthe St organogel samples. Moreover, the characteristic peaks ofneat DBS, such as the�OHband at 3000�3500 cm�1 andC�Obands at 1015 and 1095 cm�1, were not found in the gel sample,due to the small amount of DBS present (2 wt %).Figure SI 3 of the Supporting Information shows the IR

spectra of MMA and a MMA organogel with 2 wt % DBS. It isobvious that the spectra for the MMA monomer and the MMAorganogel sample are quite different. The peaks for the MMAorganogel sample, such as the alkanes’ C�H bands at 2985 and2948 cm�1, the alkenes’CdC stretching band at 1630 cm�1, andthe CdO stretching band at 1722 cm�1, are much broader thanthose for the MMA monomer sample. Reference 26 mentionsthat hydrogen bonding affects the molecular vibrations of nearbygroups. In our previous study, the CdO stretching band (ataround 1700�1750 cm�1) for a PLLA sample was much broad-er, as DBS was added to the DBS/PLLA system.24 This resultsuggests that there is hydrogen bonding between PLLA andDBS,

so we believe that there is intermolecular interaction, such ashydrogen bonding, between MMA and DBS in the MMAorganogel samples. Therefore, the microaggregates observedcould be explained as follows. While DBS molecules aggregatetogether, due to hydrogen bonding between their hydroxylgroups, to form nanofibrils, DBS molecules may also interactwith MMA monomers through hydrogen bonding. Thus, theaggregated DBS molecules may be further influenced by theMMAmonomers, leading to the formation of large aggregates ofDBS nanofibrils (microaggregates).Figure 3 shows the gelation time of organogels with different

concentrations of DBS and volume percentages of St. Thegelation time of DBS organogels decreases as the DBS contentsincrease for all samples. Thismeans that the gels formmore easilyat higher DBS concentrations. The gelation time of the samplesdecreases with the increase of St contents. The gelation time ofDBS in the more hydrophilic MMA monomer is much longerthan that in St. Gels form slowly due to competing interactions,such as hydrogen bonding, between DBS and MMA. Thisbehavior is also found in DBS/PEG systems, which exhibit alonger gel formation time for DBS in polar PEGmatrix (PEG endgroup is OH).19 In addition, temperature seems to be a para-meter controlling the gelaion time. Additional gel samplesprepared at lower temperatures (below 25 �C) had shortergelation time.The rheological experiment characterized the dynamic rheo-

logical properties of DBS organogels. Dynamic rheological data(elastic modulus, G0, and viscous modulus, G00, as functions offrequency,ω) for theMMAorganogels containing 1 wt%DBS at25 �C (see Figure SI 4 of the Supporting Information) demon-strated that G0 is independent of ω, and G0 is greater than G00 atall ω. Therefore, this system can be termed a “gel”. Similarrheological results (not shown) were recorded for all sampleswith 0.5�3 wt %DBS. Figure SI 5 of the Supporting Informationdisplays the elastic modulus (G0) at frequency (ω) =1 (rad/s) forsamples with different concentrations of DBS and volumepercentages of St. It can be seen that G0 increases when theDBS concentration increases, which implies that the stiffness ofthe sample increases.27 The increased stiffness may be due to theincrease in the aggregated sizes of DBS nanofibrils, increasing thecross-linking density of the samples.18 Therefore, G0 increases

Figure 3. Gelation time of organogels with different DBS concentra-tions and St/MMA composition ratios.

Table 1. Average Diameters of DBS Nanofibrils and DBSMicroaggregates for Gel Samples with 2 wt % DBS forDifferent Composition Ratios of St/MMA Co-monomers

average diameters of DBS nanofibrils

samples without RuO4 staining with RuO4 staining

St/MMA 100/0 13.4 ((0.9 nm) 13.2 ((0.6 nm)

St/MMA 75/25 25.1 ((1.8 nm) 9.98 ((0.5 nm)

St/MMA 50/50 11.6 ((0.9 nm) 7.86 ((1.2 nm)

St/MMA 25/75 14.4 ((1.2 nm) 12.3 ((0.5 nm)

St/MMA 0/100 29.9 ((1.5 nm) 9.98 ((0.8 nm)

average diameters of DBS microaggregates

samples without RuO4 staining with RuO4 staining

St/MMA 100/0 not found not found

St/MMA 75/25 not found not found

St/MMA 50/50 440 ((20 nm) 500 ((10 nm)

St/MMA 25/75 580 ((20 nm) 610 ((15 nm)

St/MMA 0/100 760 ((15 nm) 720 ((10 nm)

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with the increase of DBS. In comparison, G0 decreased with theincrease of St contents. This suggests that higher hydrophobicityof the monomers leads to higher mobility. These results are quitedifferent from those of Wilder et al., which showed that the elasticmodulus is less dependent on matrix polarity.19 The TEMobservations could also explain why the elastic modulus (G0) ofsamples increases as the MMA component increases. The micro-meter-sizedDBSmicroaggregates apparently lowered themobilityof the monomers and caused the increase in G0 as a result.The gel dissolution temperatures (Td) of samples with differ-

ent concentrations of DBS and volume percentages of St areshown in Figure 4. The values of Td were derived from therheological data as the intersection of the tangents from low tohigh temperatures for the plot of G0 versus temperature (notshown), at a heating rate of 5 �C/min. The strain amplitude was0.5%, and the frequency maintained 1 rad/s. Fahrlander et al.28

reported that gel dissolution or gel formation (Tf) is a first ordertransition. The first order transition temperature is given by Ttr =Htr/Str, where Htr and Str are the enthalpy and entropy of thephase transition, respectively. The enthalpy is the dominantfactor influencing Td and Tf.

27 In this study, the gels requiredmore thermal energy for disruption as the amount of DBSincreased. The enthalpy of gel dissolution increases when theDBS content increases. Thus, Td increases as the DBS concen-tration increases, as seen in Figure 4. In addition, Td increaseswith the increase of St amounts, though theTd's for St/MMA25/75 samples show a slight decrease. The hydrophilic MMAcomponents may interact with the hydroxyl groups of the DBSmolecules, through hydrogen bonding, and hinder the self-assembly of DBS molecules. Hence, the enthalpy of gel dissolu-tion of the samples was reduced, and Td was decreased when theMMA content was increased.Polymers Prepared by Templating the DBS Organogels.

Polymerization.We polymerized the monomers, using thermallyinitiated polymerization, with different compositions of St/MMAand different amounts of DBS. The thermal initiator, BPO (allfixed with 1 wt %), was added to initiate the polymerization. Thepolymerization temperature was chosen to be 60 �C because thistemperature is lower than the gel dissolution temperature(80�120 �C), as determined by rheological measurements

(Figure 4). Moreover, the G0 values were independent of ω at25 and 60 �C (see Figure SI 6 of the Supporting Information).Since the properties were still typical of a gel at 60 �C, thesamples, we believe, were in their gel states when the polymer-ization was initiated.In this study, we successfully synthesized a series of solid

samples with different St/MMA composition ratios (volumeratio: 0/100, 25/75, 50/50, 75/25, and 100/0) and differentamounts of DBS (0, 1, 2, and 3 wt %). The resident monomersand small molecules were washed by methanol. The percentageof monomer conversion was calculated by dividing the weight ofthe dried samples with that of the samples before immersion. Theconversion of monomers to polymers was calculated to bearound 95% for all samples after polymerization for 72 h. Thesesamples were found to be mostly transparent, and no macro-scopic phase separation occurred after the polymerization, im-plying that the nanoscaled structure (DBS nanofibrils) ismaintained within the polymers. It is worthy of note that thesamples withmoreMMA content were slightly more translucent.It is possible that the “DBS microaggregate” structures appearbecause of the addition of MMA.Nanostructure. Finally, we extracted the DBS components

from the PS/PMMA copolymers by washing with ethanol.Figure 5 shows the TEM photographs of PS, PS/PMMA50/50, and PMMA samples after the template (2 wt % DBScomponent) extraction. It can be seen that the shape of thepores is similar to that of the DBS nanofibrils. The minimumdiameter size of the nanofibrils, observed by TEM, was around10 nm, and the maximum diameter was around 100 nm. The aver-age diameter of the pores in all the samples after the extraction ofDBS nanofibrils was around 40 nm, which was larger than theaverage diameter of the nanofibrils of the gel samples (seeTable 1). Compared to our previous study,18 the pore sizes ofthese samples were also slightly larger than the average nanofibrildiameter (around 30 nm) observed in the polymerized sampleswithout the extraction of DBS.

Figure 4. Gel dissolution temperatures (Td) for samples with differentDBS concentrations and St/MMA composition ratios.

Figure 5. TEM photographs of (a) PS, (b) PS/PMMA 50/50, and (c)PMMA samples after the template (2 wt %DBS component) extraction.The pores after the extraction of DBS microaggregates are indicated bycircles.

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Because the samples were washed by the solvent, ethanol, someunreacted monomers, small molecules, or oligomers in thesamples could also be removed during the extraction of DBSnanofibrils. Therefore, the average pore diameter of the samplesslightly increased. In addition, the porous structures, formed by theextraction of DBS microaggregates, were also found in the PS/PMMA 50/50 (see Figure 5b) and neat PMMA (see Figure 5c)samples. The respective average diameters of the pores (indicatedby circles in the Figures) were around 380 and 500 nm, whichweresmaller than the sizes of the aggregates observed in the gel samples(see Table 1). It is possible that the DBS microaggregates in thegels consisted of not only DBS but also a considerable amount ofMMA, because of the existence of hydrogen bonding betweenthem. Polymerized MMA remained, while the DBS componentswere leached out of the polymers during the washing for thepreparation of the porous samples. Therefore, the resulting poreswere smaller than the DBS microaggregates.

’CONCLUSIONS

The effects of hydrophobicity of monomers on the structuresand rheological properties of DBS organogels were investigated.The gelation time of DBS organogels in more hydrophilic MMAmonomer is much longer than that in St. FTIR results demon-strated that hydrogen bonding occurs between DBS and MMA,so the gel formation time for DBS in more hydrophilic MMAmonomer was much slower than that in St. DBS nanofibrils, withaverage diameters of around 10 nm, were found in all gel samplesusing TEM. The samples containing MMA monomers exhibitedmicrometer-sized DBS irregular structures (microaggregates).The average sizes of these microaggregates increased with theincrease of MMAmonomer. One reason for the formation of theDBS microaggregates could be that the aggregated DBS mol-ecules are affected by the MMA monomers, due to the hydrogenbonding between DBS and MMA. Dynamic rheological mea-surements showed that G0 decreased with the increase of Stcontents. The DBS microaggregates may increase the cross-linking density of the samples, so G0 increased as the amount ofhydrophilic MMA monomer increased.

The effect of hydrophobicity ofmonomers on the structures ofporous polymers prepared by templating the DBS organogelswas observed. The polymerized samples were transparent, andno macroscopic phase separation happened after the thermallyinitiated polymerization. The porous structures of PS-PMMAcopolymers were produced by the extraction of DBS compo-nents from polymers. The average pore diameters of samples,due to the removal of DBS nanofibrils, were around 40 nm.These were slightly larger than the average diameters of thenanofibrils of the gel samples. Some unreacted monomers, smallmolecules, or oligomers of samples were also removed during theextraction of DBS, so the average pore diameter of these sampleswas slightly larger than the average diameter of the nanofibrils ofthe gel samples. The porous structures of samples produced bythe extraction of DBS microaggregates were also found in thesamples containingMMA. The pore diameters were smaller thanthe sizes of the aggregates observed in the gel samples. It isassumed that the aggregates of gel samples consisted of not onlyDBS but also a considerable amount of MMA, due to theexistence of hydrogen bonding between DBS and MMA. TheDBS components were leached out of the polymers during thewashing, and polymerizedMMA remained, so the resulting poreswere smaller than the DBS microaggregates of the gel samples.

’ASSOCIATED CONTENT

bS Supporting Information. Photographs, IR spectra, andrheological data (Figures SI 1�6). This material is available freeof charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*Telephone: +886-2-2621-5656 ext 3516. Fax: +886-2-2620-9887.E-mail: wclai@mail.tku.edu.tw.

’ACKNOWLEDGMENT

We gratefully acknowledge financial support from the TamkangUniversity and TaiwanNational Science Council (NSC 98-2621-M-032-001).

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