three-dimensionally ordered macroporous syndiotactic polystyrene: preparation and characterization

Three-Dimensionally Ordered Macroporous

Syndiotactic Polystyrene: Preparation and

Characterization

Weidong Yan,* Haiqing Li, Xiaoli Shen

Institute of Polymer Science and Engineering, Hebei University of Technology, Tianjin 300130, ChinaFax: þ86-022-26582421; E-mail: [email protected]

Received: November 24, 2004; Revised: February 1, 2005; Accepted: February 3, 2005; DOI: 10.1002/marc.200400581

Keywords: characterization; inverse opal; ordered macroporous polymer; syndiotactic polystyrene; templates

Introduction

Three-dimensionally ordered macroporous (3DOM) mate-

rials with pore size in the submicrometer to micrometer

range comprise of matrix and uniform air balls that are

closed-packed with a high degree of order and are intercon-

nected to each other by small channels. Owing to this

special structure, 3DOM materials can be utilized in absorp-

tion and separation process, as catalytic supports and

surfaces.[1,2] These materials are also useful for many inte-

resting applications, such as photonic crystals, sensors,

hierarchic battery electrodes etc.[3–6]

The use of colloidal crystals as template has been proven

to be a promising approach for the preparation of 3DOM

materials. Using this simple and effective method, a wide

range of 3DOM materials are created, including metals, inor-

ganic oxides, semiconductors and polymers.[7–10] 3DOM

polymeric materials have developed rapidly in recent years.

The polymeric materials used as soft materials can overcome

some shortcomings of rigid inorganic materials. In addition,

a wide range of multifunctional materials can be created by

introducing functional groups onto the surface of the porous

polymer easily. As far as the preparation of 3DOM polymer

is concerned, most of the previous studies have focused on

the radical polymerization or crosslink by thermal treat-

ment or exposure to UV light.[11,12] In another approach,

3DOM polymer have been fabricated by infiltrating col-

loidal crystals with solution of preformed polymers.[13]

However, there have been few reports on coordination poly-

merization which is sensitive to water and oxygen. As it is

well known, polymer with high stereo-regularity polymer-

ized via coordination polymerization have excellent pro-

perties that are also different from other polymers. For

example, syndiotactic polystyrene (sPS) has high melting

temperature, fast crystallization rate, complex polymorphic

behavior, low dielectric constant and permeability

Summary: Three-dimensionally ordered macroporous(3DOM) syndiotactic polystyrenes with pore size in therange of 71–286 nm were fabricated by means of silicatemplates using (dbm)2Ti(OPh)2/MAO catalytic system. Theresulting materials were characterized by NMR, SEM, pow-der X-ray diffraction, GPC and DSC. The results indicatedthat the 3DOM syndiotactic polystyrenes were highly syn-diotactic, and the pore contraction increased when the averagepore diameter decreased. Compared with bulk syndiotacticpolystyrenes, 3DOM syndiotactic polystyrenepossessed low-er molecular weight, narrower molecular weight distribution,and lower crystallinity and melting temperature.

SEM image of 3DOM syndiotactic polystyrene, the insetshows the detail of the cavities.

Macromol. Rapid Commun. 2005, 26, 564–568 DOI: 10.1002/marc.200400581 � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

564 Communication

resistance to gas, and good chemical resistance. It may be

significant to introduce polymer with stereo-regularity into

the preparation of 3DOM polymer, which could endow the

3DOM materials some extraordinary properties. Conse-

quently, the use of some special materials under some

special conditions may be expected, for instance, 3DOM

sPS might be used as photonic materials at high temperature

or in the presence of solvent.

Therefore, in this study, sPS was used as the candidate

polymer to fabricate 3DOM sPS with different pore size by

means of silica colloidal crystal template using b-

diketonate titanium complex/methylaluminoxane (MAO).

The resulting materials were characterized by NMR, SEM,

powder X-ray diffraction, GPC and DSC.

Experimental Part

Materials

Ethanol, 25 wt.-% aqueous ammonia, 40 wt.-% hydrofluoricacid purchased from Tianjin Chemical Reagent Factory wereof reagent quality and used without further purification.Styrene from Beijing Chemical Reagent Factory was driedover calcium hydride under reflux for 24 h, and distilled underreduced pressure before use. Tetraethoxysilane was commer-cially obtained from Beijing Chemical Limited Company.Methylaluminoxane (MAO, 10% solution in toluene) was pur-chased from Arbemarle Company. The synthesis of bis(di-benzoylmethane) phenoxyl titanate, (dbm)2Ti(OPh)2, ab-diketonate titanium catalyst, has been reported in ref.[14]

Preparation of Colloidal Crystal Templates

Monodispersed silica spheres with diameter in the range 114–371 nm were synthesized according to Stober-Fink-Bohntechnique.[15] Three dimensionally interconnected colloidalcrystal pellets were obtained by gravitational sedimentationmethod. The resulting pellets were sintered at 400 8C for 2 h toenhance the connectivity between the spheres and then cooleddown to ambient temperature for further use as templates.

Preparation of 3DOM sPS

The silica templates were treated under vacuum at 120 8C toremove water and air in the templates completely. Afterwards,the mixture of styrene, MAO and the catalyst was fully pouredinto the template by means of ultrasonification. Coordinationpolymerization was carried out at 80 8C for 24 h under argonatmosphere, to prepare silica/sPS composites. The bulk poly-mer, polymerized outside the template was peeled off. Silicatemplate in the composites was removed with 40 wt.-%aqueous hydrofluoric acid under ultrosonification and thensoaked in HF overnight. Thus, 3DOM sPS was obtained. Theresulting 3DOM sPS was washed with distilled water severaltimes and dried at 80 8C under vacuum. In this paper, threesilica templates with microsphere in the diameter range 114–341 nm were used to prepare the 3DOM syndiotacticpolystyrenes.

Characterization

The vacuum-sputtered samples with Au were characterizedusing scanning electron microscopy (Hitachi S-530 SEM). Inthe SEM images, over 100 spheres or pores candidates weremeasured to determine their average diameter. 13C-NMR spec-trum of the 3DOM sPS was run on a DMX-300 Bruker instru-ment at 100 8C in 4 wt.-% solution of tetrachloro-1,2-dideuteroethane. Differential scanning calorimetric (DSC)analysis was performed on Perkin-Elmer DSC-7 at a heatingrate of 20 8C/min under nitrogen from ambient temperature to290 8C, then cooled down to the room temperature at the samerate. GPC measurements were carried out via PL-GPC-210 intrichlorobenzene (TCB). X-Ray diffraction patterns were ob-tained with an automatic Philips powder diffractometer using anickel-filtered CuKa radiation.

Results and Discussion

Figure 1 illustrates the 13C-NMR spectrum of 3DOM sPS

prepared with (dbm)2Ti(OPh)2/MAO catalytic system. The

peaks at 125.6, 127.9 and 128.0 ppm can be assigned to the

chemical shift of the three phenyl carbon. The peaks at 41

and 44 ppm correspond to the CH and CH2 carbons of sPS.

The spectrum displays a sharp singlet at 145.1 ppm corres-

ponding to phenyl C1 carbon, which revealed that 3DOM

sPS sample was highly syndiotactic.[16]

Figure 2 shows SEM images of the fractured silica tem-

plates and corresponding 3DOM syndiotactic polystyrenes.

In Figure 2a, 2b and 2c, the average diameter of the silica

microspheres of highly ordered templates were 341, 214

and 114 nm respectively. In Figure 2a0, 2b0 and 2c0 showing

the corresponding 3DOM syndiotactic polystyrenes, the

uniform pores were arranged in the same highly ordered

fashion as the corresponding templates. However, the aver-

age diameters of the pores were only 286, 169 and 71 nm

respectively, which indicated that the pores contracted when

the silica templates were removed, and the contraction was

16.1, 21.0 and 37.3% respectively. The results revealed that

the contraction increased with decreasing average diameter

of silica microspheres. The phenomenon of the pore dia-

meter contraction after removal of the rigid template is

common in the preparation of 3DOM polymers,[17,18] and

can be explained in terms of confinement effect of the

polymer chains.[17,19] When the coordination polymeriza-

tion occurs in the small space between the rigid silica micro-

spheres, the polymer chains are strongly confined, which

results in accumulation of internal stress. Once the con-

finement effect are weakened by the removal of template,

the internal stress is released accompanied by the elastic

recovery of the polymer chains. This leads to decrease of the

pore size. With the decrease of the diameter of the silica

microsphere, the confinement effect on the polymer chains

gets more drastic, and the contraction of the pore becomes

more remarkable and corroborates our experimental

observations.

Three-Dimensionally Ordered Macroporous Syndiotactic Polystyrene: Preparation and Characterization 565

Macromol. Rapid Commun. 2005, 26, 564–568 www.mrc-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 1. 13C NMR spectrum of ordered macroporous syndiotactic polystyrenewith an average pore diameter of 71 nm.

Figure 2. SEM images of the templates with varied sphere size: (a) 361 nm, (b) 214 nm,(c)114 nm, and the corresponding 3DOM syndiotactic polystyrenes with varied pore size: (a0)286 nm, (b0) 169 nm, (c0) 71 nm. The inset in (a0), (b0) and (c0) are higher magnification ofthe connecting windows formed between the pores.

566 W. Yan, H. Li, X. Shen


Figure 3 shows WXRD patterns of the bulk and porous

sPS sample. The both diffraction patterns contain typical

peaks at 2yffi 8, 10, 17, 20 and 238 corresponding to 010,�2210, 111, �3321-301, and �4411 reflections respectively, which

were generally attributed to the sPS clathrates.[20–22] From

the two diffractograms, it was observed that the bulk sPS

had a higher crystallinity than porous sPS. Therefore, the

coordination polymerization within silica templates voids

affect the crystallinity of the sPS samples, but had little

effect on the crystalline forms of the sPS.

GPC results are shown in Table 1. It revealed that the

3DOM syndiotactic polystyrenes possessed lower molecu-

lar weight and narrower distribution than that of bulk sPS,

and there were no obvious differences among the three

3DOM syndiotactic polystyrenes. This fact indicated that

the polymerization in the template affected MW and MWD

of the resulting polymer, which may be related to the active

chains propagation, transfer and termination processes. The

mechanism needs further investigation.

Figure 4 shows DSC heating and cooling scans of bulk

sPS and three syndiotactic polystyrenes with varied pore

sizes. The melting temperature (Tm) of the bulk sPS is

261.4 8C (curve B), while the melting temperatures of three

3DOM macroporous syndiotactic polystyrenes are 256.0 8C(curve P1), 255.0 8C (curve P2) and 255.4 8C (curve P3),

respectively. It was observed that the bulk syndiotactic

polystyrene exhibited higher Tm than the macroporous

syndiotactic polystyrenes, which could be related with the

lower crystallinity and molecular weight of the macropor-

ous syndiotactic polystyrenes than that of bulk sPS. The

melting temperatures of the 3DOM syndiotactic polystyr-

enes varied slightly with the pore size, which indicated that

the melting temperatures of 3DOM syndiotactic polystyr-

enes were not sensitive to the pore size in the range of 71–

286 nm. The DSC thermograms of the bulk sPS and the

three 3DOM syndiotactic polystyrenes obtained by non-

isothermal crystallization from 290 8C to ambient temper-

ature are also shown in Figure 4 (curve Bc, P1c, P2c, P3c). In

all cases, only one single crystallization exotherm is

presented. Compared with bulk sPS, the crystallization

temperatures of samples P1 and P2 with average pore

diameter larger than 100 nm were higher, but the case was

opposite when the average pore size of P3 was smaller than

100 nm. Studies on the mechanism are in progress.

Conclusion

Three-dimensionally ordered macroporous syndiotactic

polystyrenes with pore size in the range of 71–286 nm were

prepared by means of silica templates using (dbm)2Ti(OPh)2/

MAO catalytic system. The resulting polymers were

characterized. It was shown that both silica spheres within

the templates and pores in the macroporous syndiotactic

polystyrenes were arranged in highly ordered fashion, and

the contraction of the pores in the macroporous sPS got more

remarkable with the decrease of the corresponding silica

Table 1. GPC results of syndiotactic polystyrene samples.

No. Ba) P1b) P2

b) P3b)

Mw(�104)/g �mol�1 21.69 15.85 14.38 16.88MWD(Mw=Mn) 3.35 2.77 2.59 2.34

a) B: bulk syndiotactic polystyrene.b) P1, P2, P3: 3DOM syndiotactic polystyrenes with average pore

diameter of 286, 169, 71 nm, respectively.

Figure 3. X-Ray diffraction patterns of bulk syndiotacticpolystyrene and ordered macroporous syndiotactic polystyrenewith an average pore diameter of 169 nm. Figure 4. DSC heating and cooling scans of bulk syndiotactic

polystyrene and three 3DOM syndiotactic polystyrenes withvaried pore size, (P1) 286 nm, (P2) 169 nm and (P3) 71 nm. Note:(B) bulk sPS polymerized outside of templates; (Bc), (P1c), (P2c)and (P3c) obtained by non-isothermal crystallization of (B), (P1),(P2) and (P3) sample from 290 8C to ambient temperaturerespectively.

Three-Dimensionally Ordered Macroporous Syndiotactic Polystyrene: Preparation and Characterization 567


microspheres size. GPC curves showed that the porous sPS

possessed lower MW and MWD compared with bulk sPS,

and no obvious differences were observed among the three

3DOM syndiotactic polystyrenes. Meanwhile, the crystal-

linity and molecular weight of macroporous sPS were lower

than bulk sPS, which resulted in the lower melting tem-

perature of the macroporous material. DSC results revealed

that three 3DOM syndiotactic polystyrenes exhibited similar

melting temperatures, and 3DOM syndiotactic polystyrenes

with pore size larger than 100 nm had a higher crystalliza-

tion temperature than bulk sPS. However, the inverse was

observed when the pore size of the macroporous sPS was

smaller than 100 nm.

Acknowledgements: This work is supported by the NationalNature Science Foundation of China (Grant No. 50273009).

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three-dimensionally ordered macroporous syndiotactic polystyrene: preparation and characterization

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