Accepted Manuscript

Highly Porous Organic-Inorganic Hybrid Fiber from Copolymers of Styrene

and Polyhedral Oligomeric Silsesquioxane-Derived Methacrylate: Syntheses,

Fiber Formation and Potential Modification

Thanarath Pisuchpen, Varol Intasanta, Voravee P. Hoven

PII: S0014-3057(14)00293-6

DOI: http://dx.doi.org/10.1016/j.eurpolymj.2014.08.017

Reference: EPJ 6545

To appear in: European Polymer Journal

Received Date: 4 May 2014

Revised Date: 6 August 2014

Accepted Date: 13 August 2014

Please cite this article as: Pisuchpen, T., Intasanta, V., Hoven, V.P., Highly Porous Organic-Inorganic Hybrid Fiber

from Copolymers of Styrene and Polyhedral Oligomeric Silsesquioxane-Derived Methacrylate: Syntheses, Fiber

Formation and Potential Modification, European Polymer Journal (2014), doi: http://dx.doi.org/10.1016/

j.eurpolymj.2014.08.017

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers

we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and

review of the resulting proof before it is published in its final form. Please note that during the production process

errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

1

Highly Porous Organic-Inorganic Hybrid Fiber from Copolymers of

Styrene and Polyhedral Oligomeric Silsesquioxane-Derived

Methacrylate: Syntheses, Fiber Formation and Potential

Modification

Thanarath Pisuchpena,b

, Varol Intasantac, Voravee P. Hoven

a*

aOrganic Synthesis Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn

University, Phayathai Road, Pathumwan, Bangkok 10330, Thailand

bCenter of Excellence on Petrochemical and Materials Technology, Chulalongkorn University,

Phayathai Road, Pathumwan, Bangkok 10330, Thailand

cNational Nanotechnology Center, National Science and Technology Development Agency,

Thailand Science Park, Phahonyothin Road, Klong Luang, Pathumthani, 12120, Thailand

Correspondence to: Voravee P. Hoven (E-mail: vipavee.p@chula.ac.th)

Abstract

Copolymers comprising styrene and polyhedral oligomeric silsesquioxane-derived methacrylate

(PS-co-PMAPOSS) of various MAPOSS:styrene ratios with Mn over 100 kDa were prepared by

concurrent ARGET ATRP-RAFT. Fibrous mats of the synthesized copolymers are then

fabricated through electrospinning process using various conditions (solvents and POSS

content). The results from SEM and EDS analysis unveiled the fibers’ physical and chemical

characteristics as a clear footprint of the influence of solvent selection (tetrahydrofuran (THF)

and dimethylformamide (DMF)). It is evident that liquid-liquid phase separation followed by

phase segregation of PS and POSS constitute to the varying degree of porosity in the electrospun

fibers. Finally, preliminary tests suggest that highly porous PS-co-PMAPOSS fiber can be

modified with high temperature, plasma and silanization for further novel applications.

Keywords: porous fibers; electrospinning; polystyrene; POSS; phase separation

2

1. Introduction

Porous materials represent a class of functional constituents in catalysis, filtration, storage,

control release, sensing, insulation and absorption. Fabrications of such structures can be

accomplished by a number of approaches. Phase separation among incompatible components at

small length scale, in particular, epitomizes one of the effective ways that help creating porosity.

Recent advances in molecular design and polymer syntheses have made possible hybridization of

not only organic and inorganic components, but also those with definite incompatibilities, into

one single polymeric chain. Through which, structural development driven by phase separation

could be tuned via the degree of incompatibilities among the choices of monomers during

syntheses and solvent combination in the course of fabrications.

Polyhedral oligomeric silsesquioxane (POSS) is an organic-inorganic hybrid compound, of

which the most well-known is that of cubic structure, also known as T8. Due to its chemical

similarity to silica, POSS is sometimes referred to as the smallest colloidal silica. Thermal and

chemical stability of POSS, as well as various functionalities can convey great potential in the

area of polymer nanocomposites.

Various routes have been developed as means to assimilate POSS functional moieties into

polymer matrix, including melt blending [1], solution blending [2] and addition of POSS to

polymer chains either through grafting [3] or copolymerization [4]. In addition to the mechanical

and thermal integrity, such incorporation of POSS molecules can affect morphological and

structural feature of the polymer itself. Polymerized POSS molecules are known to form

segregated nanosized semi-crystalline or crystalline domains, even if the functional groups on

main chains and POSS subunits are similar [5]. Random copolymers with POSS on their chains

have revealed lamellar-like POSS aggregates [6], while block copolymers containing POSS have

exhibited long-range ordered lamellar or cylindrical structures [7].

Electrospinning becomes a prevalent and versatile technique to fabricate precursor polymer

solutions or melt into non-woven fibrous mat under electric-field induced repulsive coulombic

force. With sufficiently entangled polymeric networks as spinning precursor, various inorganic

materials can also be fabricated into ultrafine fibers [8-10]. Its simplicity and ability to induce

unique micro- and nanostructures bring about innumerable possibilities of applications, including

POSS-derived nanocomposites, in which electrospinning facilitates dispersion of nanoparticles

[11,12]. As Cozza and co-workers had demonstrated, electrospinning could also be used to

3

promote dispersion and prevent aggregation of POSS in cellulose acetate matrix [13].

Morphological features of electrospun fibers have also been shown to be affected by an addition

of POSS. In publication of Xue and co-workers, electrospun fibers from copolymers of

poly(methyl methacrylate) (PMMA) containing POSS exhibited nanofibrillar structures of

ordered POSS moieties which were also longitudinally aligned in the fiber direction [14].

Herein this research, we propose an approach to generate highly porous organic-inorganic

hybrid fibrous materials via chemical design, polymer syntheses, fiber formation and interplay

among solubilities and domain separation. Specifically, we would like to explore an impact of

POSS incorporation on the morphological and structural feature of polystyrene (PS) through the

dynamics of electrospining as means for fiber formation. Commercially available methacrylate-

substituted POSS, heptaisobutyl-(1-propylmethacrylate)-POSS (MAPOSS) was copolymerized

with styrene employing a controlled radical polymerization method denoted as concurrent

RAFT-ARGET ATRP (Scheme 1). With subsequent fabrication by electrospinning lined ahead,

the initial challenge was to attain copolymers with molecular weight high enough for sufficient

molecular entanglement suitable for electrospinning. This polymerization process is a

combination of Reversible Addition-Fragmentation Chain Transfer (RAFT) and Activator

Regenerated by Electron Transfer for Atom Transfer Radical Polymerization (ARGET ATRP).

Unlike conventional RAFT, which requires external source of radical, atom transfer process

provides a more consistent level of radical which is under influence of both ATRP and RAFT

equilibria. This novel approach provides a superior control over both initiation and propagation

and, as demonstrated by Matyjaszewski and coworkers, allows for narrow PDI for polymeric

chains with molecular weight even over 100 kDa [15-17]. The use of ARGET ATRP further

provides an additional simplicity to the reaction process and copolymer purification, especially

in this case where economical reducing agent like copper wire was employed. Subsequently,

fibers from the synthesized copolymers were then fabricated by electrospinning, in which several

parameters such as copolymer concentration and solvent composition were systematically

investigated.

4

S

S CN

COOH

4-cyano-4-(thiobenzoylthio)pentanoic acid

RSiO

O

O SiR

O

OSiR

O

ORSi

SiRO

SiRSi

RSi

O OOO

O O

N

NNCu(0)

CuBr2

CN

HOOC S

S

RSiO

O

O SiR

O

OSiR

O

ORSi

SiRO

SiRSi

RSi

O OOO

O O

PMDETA

styrene

MAPOSS

PS-co-PMAPOSS

n m

R =

Scheme 1. Synthesis of PS-co-PMAPOSS by concurrent RAFT-ARGET ATRP.

2. Experimental

2.1. Materials

Styrene (Sty) and copper (II) bromide (CuBr2) were purchased from Fluka. Heptaisobutyl-(1-

propylmethacrylate)-POSS (MAPOSS), pentamethyl- diethylenetri-amine (PMEDTA), 4,4′-

azobis(4-cyanovaleric acid) (ACVA), cyanovaleric acid dithiobenzoate (CVADTB) and

aluminium oxide were purchased from Sigma-Aldrich. Toluene, tetrahydrofuran (THF), N,N-

dimethylformamide (DMF) and ethanol were purchased from Lab-scan. All reagents were AR

grade and used as received.

2.2. Synthesis of PS and PS-co-PMAPOSS using concurrent RAFT-ARGET ATRP

2 mg of CuBr2, 26 µL of PMDETA, 12 mg of CVADTB (molar ratio of 1:140:10), 4.8 g

copper wire and designated amount of Sty and MAPOSS were added into a 25 mL scintillation

vial, which was then sealed with rubber septum. Purged with nitrogen gas for 15 min, the

reaction was set to carry out under nitrogen atmosphere at 90°C for 24 h. The crude products

were first dissolved in THF, and then passed through a basic alumina column to remove residual

catalysts. The resulting clear and colorless solution was then precipitated in an excessive amount

of ethanol under vigorous stirring. The solid was then dried at room temperature under vacuum

overnight to obtain a final copolymer product.

5

2.3. Electrospinning

In the preparation of spinning precursor solutions, the synthesized (co)polymers were

dissolved in various solvents including toluene, THF and mixture of THF-DMF. Each of the as-

prepared solution was loaded into a syringe equipped with a syringe pump and attached to a 1.5

cm-long blunt tip needle. A high voltage supply connected the needle with a positive voltage

cable and a receiving aluminum foil with a ground one. A polymer solution was electrospun

onto the ground aluminum foil under voltage of 20 kV, 15 cm collecting distance and flow rates

of 6.5-8.5 mL/h. The resulting electrospun fiber mats were left to dry in ambience for 24 h prior

to characterization.

2.4. Chemical modification

To test the tolerance to modification of the electrospun fibers, several procedures were carried

out. First, tetraethoxysilane (TEOS) (5% of polymer weight) was added to polymer solution prior

to electrospinning. Next, silanol groups were introduced to surface of the electrospun fiber mats

through 2 methods, calcination at 400oC for 1 h and oxygen plasma treatment at 100 W for 10

min. Finally, electrospun fiber mats with silanol groups were treated with vapor phase of

trichloromethylsilane (MeSiCl3) following a published procedure [18]. Treated fiber mats were

characterized by a contact angle goniometer (Ramé-Hart, Model 200-F1, USA) for water contact

angle.

2.5. Nuclear magnetic resonance spectroscopy (NMR)

1H-NMR spectra was recorded in CDCl3 using Varian, model Mercury-400 nuclear magnetic

resonance spectrometer operating at 400 MHz. Chemical shifts (δ) were reported in part per

million (ppm) relative to the reference signals of tetramethylsilane (TMS) or the residual

protonated solvent.

2.6. Gel permeation chromatography (GPC)

Molecular weight and molecular weight distributions of the synthesized (co)polymers were

determined by gel permeation chromatography (GPC) using Water 600 controller and pump,

Waters E600 column connected to Waters 2140 refractive index detector, and THF as eluent.

The flow rate was 1 mL/min. PS standards were employed to construct a calibration curve.

6

2.7. Thermogravimetric analysis (TGA)

Thermal degradation behavior of all (co)polymer samples and percent ash were investigated

by thermogravimetric analysis (TGA) (Mettler Toledo, model TGA/SDTA 851, USA) over a

temperature range of 30-600 °C at a heating rate of 10 °C/min under ambient condition. The

data were analyzed with STARe SW program version 9.30.

2.8. Scanning electron microscopy (SEM) and energy dispersive x-Ray spectrometry (SEM-EDS)

The morphological appearances of the as-spun fibers were investigated using a scanning

electron microscope (SEM, JEOL, Model JSM-6480LV, Japan). Each sample was placed on the

holder with an adhesive tape and coated with a thin sputtered layer of gold. The scanning

electron images were obtained by using an acceleration voltage of 15 kV. The average fiber

diameter of the electrospun fibers was measured by Semafore software directly from SEM

images. Elemental analyses were performed under SEM model JSM-5800LV (JEOL, Japan)

under an EDS mode.

2.9. X-ray diffractometry (XRD)

The crystallinity of the synthesized (co)polymers and their respective as-spun fibers were

investigated using an X-ray diffractometer (model Rigaku TTRAX III, 18 kW, Japan). Each

sample was grounded into powder prior to analysis.

3. Results and discussion

3.1. Synthesis of PS and copolymers by concurrent RAFT - ARGET ATRP

For a polymer to be able to form fibers by itself via electrospinning, molecular weight of the

polymer has to be high enough to promote sufficient chain entanglement. To ensure

electrospinnability, this research aimed to synthesize PS-co-PMAPOSS with molecular weight

well above 100 kDa. A successful synthetic protocol of the copolymer with Mn (131.7 kDa)

reaching the expected target (130 kDa) employed 0.96 g of copper wire. As can be seen from

1H-NMR analysis in Fig.1, the increases in intensity of peaks at 0.9-1.0, assigned to methyl

protons on isopropyl groups of MAPOSS, and at 0.6 ppm, assigned to methylene protons on

isopropyl groups of MAPOSS (b and c, respectively in Fig 1A-C) as a function of MAPOSS

content suggested that the MAPOSS compositions in the copolymers were strongly correlated

7

with and proportional to those in the feed. Furthermore, there were no peaks in 5.5-6.0 ppm, a

range assigned to methylene protons of the monomers, indicating the absence of residual

unreacted monomers after purification. The weight percent of MAPOSS (%MAPOSS) in

copolymers was calculated from 1H NMR data using relative intensity of the peak at 0.6 ppm,

assigned to methylene protons on isopropyl groups of MAPOSS (b, Fig 1A-C), and that of the

peak at 6.2-7.2, assigned to phenyl protons on phenyl group of styrene (a, Fig 1A- C) (data

shown in Table 1). The %MAPOSS calculated from 1H-NMR, molecular weights of the

(co)polymers obtained from GPC analysis, and the thermal characteristics of all (co)polymer

samples (shown in Fig S1 in Supplementary Material and 2) are summarized in Table 1. As

illustrated, the molecular weights of the copolymers closely resembled the target molecular

weight of 130 kDa. In addition, %MAPOSS by 1H-NMR progressively increased with the

theoretical %MAPOSS in the feed. As the numbers calculated from 1H-NMR data appeared

greater than the theoretical values, it was hypothesized that MAPOSS was more reactive towards

copolymerization than styrene.

RSiO

O

O SiR

O

OSiR

O

ORSi

SiRO

SiRSi

RSi

O OOO

O O

n m

R =

HHH

HH

H

H

H

H HH

0.50.51.01.01.51.52.02.02.52.53.03.03.53.54.04.04.54.55.05.05.55.56.06.06.56.57.07.07.57.58.08.0

a b c

C

B

A

a

b

b

b

c

δ (ppm)

Fig. 1. 1

H NMR spectra of PS-co-PMAPOSS with %MAPOSS in monomer feed of (A) 5%, (B)

10%, and (C) 25%.

8

TABLE 1

Molecular weight (Mn), composition of PS and PS-co-PMAPOSS (%MAPOSS by mass), and

decomposition temperature (Td).

%MAPOSS

in feed

Mn (kDa) %MAPOSS

obtained by

1H-NMR

Td (oC)

Initial Derivative

Peak

0 158.9 0 281 391

5 111.1 9 281 350a,390

10 129.6 14 284 394

25 131.7 36 295 409

a Peak shoulder

As revealed in Fig. 2, the derivative of TGA thermograms suggested that the decomposition

of the PS-co-PMAPOSS with %MAPOSS of 9% occurred at slightly lower temperature than that

of the PS. On the other hand, those with %MAPOSS of 14 and 36% decomposed at higher

temperature than the PS. This phenomenon could be explained after a morphological

investigation of MAPOSS, PS, and the copolymers by XRD as shown in Fig.3. As for the

spectrum of PS, the lack of any sharp peaks and the presence of two broad peaks at 2θ = 10° and

19° (Fig. 3b) indicated that a majority of PS was amorphous, yet, with a certain degree of chain

packing. Nevertheless, at low %MAPOSS (9%), the spectral intensity (Fig. 3c) dropped

significantly implying that the addition of small amount of MAPOSS apparently disrupted the

packing of PS chains.

9

250 350 450 550

d)

c)

b)

a)

Temperature (oC)

Fig. 2. Derivatives of TGA thermograms of (a) PS and PS-co-PMAPOSS with %MAPOSS of

(b) 9%, (c) 14%, and (d) 36%.

0 10 20 30 400 10 20 30 40

a)

b)

c)

d)

e)

2θ (Degree)

Fig. 3. XRD spectra of (a) MAPOSS (b) PS and PS-co-PMAPOSS with %MAPOSS of (c) 9%,

(d) 14%, and (e) 36%.

This particular observation coincided very well with the TGA data and helped explain that the

reduced decomposition temperature could possibly originate from this disorder promoted by

MAPOSS inclusion. At higher ratio of MAPOSS (14 and 36%), however, the peak intensity

became once again comparable to that of the PS, inferring that chain ordering had been

reinstalled. Finally, an increase in the intensity ratio between the peaks at 2θ = 9° (from

MAPOSS +PS) and at 2θ = 19° (from PS) of the two copolymers (14 and 36%PMAPOSS)

implied that there was higher detected MAPOSS content at higher MAPOSS feed ratio which

was in good agreement with the results from 1H-NMR.

10

3.2 Electrospinning of (co)polymers

Previously, it has been shown in literatures that solvent has a strong impact on morphological

features of electrospun fiber [19-20]. In this research, single solvents (toluene, THF, DMF) and

mixtures of THF and DMF with various volume ratios were employed as media to solubilize the

copolymers for electrospinning. The concentration used was 20% in all cases. In terms of

solubility, PS could be dissolved in all three solvents, while the copolymers were soluble in

toluene and THF, but could only swell in DMF. As an addition of THF to DMF could increase

the solubility of the copolymers, the materials became completely soluble in THF/DMF mixture

as the ratio between THF and DMF reached 1:2. Although toluene was a good solvent for both

PS and copolymers, electrospinning of the polymer solution in toluene resulted in spraying of

large droplets without any fiber formation, presumably due to low dipole moment and

conductivity of toluene. Therefore, it was concluded that toluene was not a practical solvent for

electrospinning of these (co)polymers.

SEM micrographs of electrospun PS fibers are shown in Fig. S2 (Supplementary Material).

Through variation of solvents, it was found that, while PS was soluble in both THF and DMF,

using THF alone resulted almost exclusively in beads with very few fibers (Fig. S2a). Using

DMF and mixture of THF and DMF (Fig. S2b-d) as solvents gave fibers with very small number

of beads, which could be explained as a result of higher dipole moment and conductivity of DMF

than those of THF. Average fiber diameters and standard deviations of those fibers obtained

from DMF were higher than those from the solution in mixed solvents. There was, however,

very little difference in fiber diameters among those from solvent mixtures of different ratio (Fig.

S2b-c). The results were in good agreement with a previous report by Jarusuwannapoom and co-

workers, although the molecular weight of PS in the current case (158.9 kDa) was lower than

that of the aforementioned report [21].

Closer inspection (Fig. 4) shows that these electrospun PS fibers had coarse surfaces, with

roughness appearing more pronounced with the increase of DMF content. The mechanism of

roughness evolution has been proposed in a publication by Lu and Xia, which involved

condensation of miniscule water droplets on the surface of fibers during evaporation of solvent,

leaving pores to develop on the surface during the course of subsequent dehydration [22].

11

Fig. 4. SEM micrographs of surfaces of the fibers electrospun from 20% solution of PS in mixed

solvent of THF:DMF with the volume ratio of 2:1 (a) and 1:2 (b), and DMF (c).

SEM micrographs of electrospun fibers from PS-co-PMAPOSS copolymers with %MAPOSS

= 36% are shown in Fig. 5. Unlike PS, using THF alone as a solvent resulted in bead-less fibers

with larger average fiber diameters (Fig. 5a), which could inevitably be a consequence of

PMAPOSS incorporation. Since POSS intrinsically tended to self-aggregate, its presence

should, to a certain extent, increase polymer chain entanglement and, thus, improve ability of

polymeric network to stretch upon electrospinning, leading to formation of fibers with relative

larger sizes. The enhanced chain entanglement would also explain the increase in average fiber

diameter in PS-co-PMAPOSS fibers as opposed to that of the PS with comparable molecular

weights. However, addition of DMF to the solvent caused a greater variation in fiber diameters,

as evidenced by the resulted broader standard deviation values (Fig. 5b-c). Since DMF could not

solubilize PS-co-PMAPOSS copolymers, it was hypothesized that its presence induced

solution’s inhomogeneity, affecting the electrospinning jet formation and resulting in poorer

consistency of fiber diameters.

12

Fig. 5. SEM micrographs of fiber mats (a-c), SEM micrograph demonstrating surface texture (d-

f), inner porosity (g-i), and TEM micrographs (j-l) of electrospun fibers obtained from PS-co-

PMAPOSS (%MAPOSS = 36%) solution at 20% concentration in THF (a,d,g,j) and mixed

solvent of THF:DMF with the volume ratio of 2:1 (b,e,h,k) and 1:2 (c,f,i,l).

13

SEM analyses showed that electrospun fiber obtained from the THF solution of copolymers

(Fig. 5d) gave relatively smooth fibers, with little roughness. However, the surface of fibers

electrospun from the polymer solutions in mixed solvents showed fibrillar structures with

directional grooves along the fibers’ axis, resembling a “wood-like” feature. An increase of

DMF content in the solvent mixture further promoted the formation of fibril structures with even

more distinct grooves. Since the roughness of the fiber surface was different from that of PS, it

was hypothesized that structural development in the two cases progressed via different routes.

For this reason, peering into the internal structures of the fibers could shed light into the

mechanisms through which the observed roughness manifested. The interior of the fibers were

examined as in SEM cross-sectional images (Fig. 5g-i) and TEM micrographs of electrospun

fibers (Fig. 5j-l). The results revealed that the fibers fabricated from THF solution were mostly

dense with largely miniscule pores present, whereas those from mixed solvents exhibited

numerous pores of mixed and relatively larger sizes. The fibers from 2:1 THF:DMF mixture

possessed noticeable “sheath” and hollow pores, which resembled the cross-section of lotus root

(Fig. 5h). TEM micrographs (Fig. 5k) also confirmed that the outer part of the fibers consisted of

nanofibrillar structures while the inner part consisted of interconnected pores. In contrast, the

fibers from 1:2 THF:DMF mixture showed no discernible “sheath” but illustrated a large number

of small hollow pores similar to sponges (Fig. 5i). TEM micrograph shown in Fig. 5l further

revealed that the inner part of the fibers not only consisted of nanofibrillar structures similar to

those on the surface, but also accommodated numerous small pores throughout. As, incidentally,

the internal nanofibrillar structures observed in this study were similar to those reported

previously by Xue and co-workers as mentioned earlier, in our case the fibers contained many

more visible pores [22].

To further investigate the influence of MAPOSS as the physical origin of the observed

roughness and pores, additional fiber fabrications were performed with PS-co-PMAPOSS having

%MAPOSS = 9% and 14% under the same procedure as that done with the copolymer sample of

36%MAPOSS. SEM micrographs of the electrospun fibers from PS-co-PMAPOSS with

%MAPOSS = 9 and 14% are displayed in Fig. 6 and 7, respectively. The average diameter of

fibers with 9%MAPOSS (Fig. 6) was comparable to that of the electrospun PS fibers shown

earlier. The morphological features on the surface of these fibers were also similar to those on

the PS fibers, but with small pores instead of fibrillar structures. Nonetheless, their porosity

14

followed the same trend as the electrospun fibers obtained from the copolymer with higher POSS

described previously, being more prominent with the increase of DMF content in the mixed

solvent.

Fig. 6. SEM micrographs demonstrating overall morphology (a,b), surface texture (c,d), and

inner porosity (e,f) of electrospun fibers obtained from PS-co-PMAPOSS (%MAPOSS = 9%)

solution at 20% concentration in mixed solvent of THF:DMF with the volume ratio of 2:1 (a,c,e)

and 1:2 (b,d,f). Below the top row is the average diameter of the corresponding electrospun

fibers.

In the cases of fibers from PS-co-PMAPOSS copolymers with 14%MAPOSS (Fig. 7), the

resulting diameter was comparable to that of the sample with 36%MAPOSS. The morphological

features on surface and inner pore structures also followed the same predisposition as unveiled in

the fibers with higher POSS content, with fibrillar surface structure and porosity became more

distinct with the increase in DMF content. These results implied that in order to form

15

nanofibrillar structures, the amount of POSS content in the copolymers must be higher than a

certain threshold (about 14% in this case).

Fig. 7. SEM micrographs demonstrating overall morphology (a,b), surface texture (c,d), and

inner porosity (e,f) of electrospun fibers obtained from PS-co-PMAPOSS (%MAPOSS = 14%)

at 20% concentration solution in mixed solvent of THF:DMF with the volume ratio of 2:1 (a,c,e)

and 1:2 (b,d,f). Below the top row is the average diameter of the corresponding electrospun

fibers.

It could be seen from the comprehensive sets of phenomena illustrated so far that the POSS-

containing random copolymers currently explored led to fibers with unique and highly

advantageous morphologies. As the influences of POSS segregation, solubility and materials

compatibility were more or less relevant, our desire for better understanding directed towards the

mechanisms by which the constituents (copolymer and solvents) went through to reach the final

structures. As such, it was hypothesized that phase separation was mainly responsible for the

16

phenomena observed. Phase separation of polymers and formation of porous structure in

polymers were investigated by many research groups in the past.

Over a decade ago, Megelski et al. reported their pioneering work on pore generation in

electrospun nanofibers. Void in nanofibers manifested in various structures-from densely packed

and well-defined nanopores to relatively larger flat micropores. Considered generic in fibers

constituting a broad range of polymers, the case was hypothesized to correlate with high

volatility of solvents used in the processes [23]. In addition, Casper et al. investigated the effect

of molecular weight and humidity on fiber surface characteristics during their experiments with

electrospinning of polystyrene solutions [24]. The authors revealed that the increase in relative

humidity and molecular weight led to such profound effects as the increase in pore size, pore size

distribution and overall porosity. Dayal et al. theoretically peered into the internal structures of

nanofibers formed by solution-based electrospinning [25]. They have suggested that solvent

evaporation affecting the phase diagram of amorphous polymer solution was the morphological

determining factor for the internal pores.

Within the electrospinning scheme, Medeiros et al. investigated on the physical origin of the

relevance of humidity in their inquiries with a number of polymers such as PVA, PMMA, PVC,

PS and PLA in various solvents including toluene, DMF [26]. While pore manifestation was

varied with and influenced by types of polymers, component solubility and molecular weights,

the final structure was a genuine footprint of the competitive kinetics of phase separation and

solvent evaporation. As per end-use applications of solution-based phase separation, solvent

compatibility was taken for granted and explored to a great extent. Under DMF solvent system

and high humidity, Pai et al. attributed the presence of internal porosity in PS electrospun

nanofibers to the solvent’s miscibility with water [27]. This inherent characteristics investigated

diffusion of airborne water into the fiber, stimulating liquid (DMF)-liquid (water) phase

separation which preceded solidification of PS from DMF and the birth of voids in the vicinity

once apprehended by water. Suggested by the reported findings discussed above, phase

separation in polymer solutions leading to inner pores and structured surface and interfaces was

arguably one of the most complex phenomena. To grasp that under the dynamics of

electrospinning was even more challenging.

To further comprehend the unique structures of PS-co-PMAPOSS fibers and their

development, we carefully examined the samples’ surfaces utilizing SEM/EDS technique. As

17

seen in Table 2, the elemental composition of the PS-co-PMAPOSS fiber surface changed

accordingly with the solvent. With THF, a good solvent for both PS and PS-co-PMAPOSS, the

elemental composition of fiber almost matched the theoretical estimation calculated from that of

the bulk composition. In contrast, the addition of DMF caused a decrease in silicon signal, a

prominent footprint of MAPOSS moiety. Despite increase of oxygen content, oxidation of POSS

was unlikely to take place due to its oxidative stability [28]. While both PS and PMAPOSS

phases were well dispersed in THF, PS phase seemed to be more available at the surface than its

PMAPOSS counterpart when DMF was added.

Table 2

Elemental composition of the surface of fiber electrospun from PS-co-PMAPOSS (36 %

MAPOSS) obtained by SEM/EDS analysis compared to calculated data from 1H NMR.

THF:DMF (v/v)

Elemental Composition (%)

C O Si

calculated from 1H

NMR data

88.16 7.53 4.30

3:0 87.78 7.60 4.62

2:1 85.15 11.08 3.77

1:2 84.73 12.20 3.06

This result pointed out that the phase morphology of electrospun PS-co-PMAPOSS fibers was

influenced by the solvent composition. Furthermore, the unveiled influence of solvent

composition on the phase morphology of electrospun PS-co-PMAPOSS fibers might have hinted

at possible chain re-orientation upon solubility change. As a schematic representation, Fig. 8

depicted our proposed mechanism for fiber formation illustrating development of internal pores

and structured surfaces. In particular, when THF was used as a solvent (Fig. 8A), its rapid

evaporation rate caused the charged jet to shrink and kinetically trap both PS and PMAPOSS

18

together without phase separation. This mechanism gave rise to dense fiber with little pores left

from the evaporation of THF. In the cases of mixed solvents with THF: DMF of 2:1 (Fig. 8B),

the solvent would evaporate more slowly due to the high boiling point of DMF, causing

difference in solvent composition between what remained at the outer layer and that resided

within the core of the fibers. As a result, this led to the formation of fibrillar “sheath” on the

outer part. Subsequent evaporation of the remaining solvent (mainly DMF) drove the formation

of internal pores. As for the case of mixed solvent with THF: DMF of 1:2 (Fig. 8C), solvent

evaporation would be even more hindered, leaving larger DMF fraction trapped inside the fiber

core, originating the fibrillar structure throughout.

To further reveal the unique potential applications of the obtained organic-inorganic hybrid

fibers, chemical transformation was performed with thermal and plasma treatment. With its

organic-inorganic hybrid nature, POSS and POSS-containing fibrous composites were

hypothesized to possess an ability to convert to full silicon dioxide after high temperature

treatment. Even though the fibers electrospun solely from PS-co-PMAPOSS (36% MAPOSS)

could not tolerate the high heat (the fibrous structure was destroyed upon calcination at 400oC as

shown in Fig. S3 (Supplementary Material), addition of TEOS (5% of polymer weight) into the

spinning precursor solution led to fibers with better physical integrity after calcination as shown

in Fig. 9a.

Furthermore, an electrospun fiber mesh of PS-co-PMAPOSS was transferred into an oxygen

plasma chamber. After a given condition of plasma treatment, the resulting fibers were

examined under electron microscopy. Fig. 9b illustrated SEM micrograph of the plasma treated

nanofibers with their fibrous structures remained intact. Furthermore, after plasma treatment, the

water receding contact angle plummeted to almost 0°, implying emergence of hydrophilic silanol

groups in the samples. The results hinted at the potential of these hybrid materials after further

chemical modifications as backbones for a broad range of functional surfaces.

To enhance the functionality of the treated samples for even broader range of applications,

silanization by MeSiCl3 was carried out to chemically modify the surface characteristics of the

samples. First, fibers of PS-co-PMAPOSS/TEOS before plasma treatment showed high contact

angle (θ = 144°) and decrease to almost 0° after plasma treatment. Yet, after silanization, the

contact angle rose again (θ = 134°). Despite their preliminary nature, the results inexorably

19

suggested possibilities of surface modification on the developed hybrid structures for a broad

range of end use applications.

Fig. 8. Schematic illustration of proposed mechanism of formation of pores and roughness in PS-

co-PMAPOSS fibers electrospun from THF solution (A) and mixed solvent of THF and DMF

with THF:DMF volume ratio of 2:1 (B) and 1:2 (C). The left column shows charged jet of the

polymer solution. The middle column shows evaporation of solvent from the charged jet to form

fiber. The last column indicates the morphological features of the fiber.

20

Fig. 9. SEM micrographs of modified electrospun fibers obtained from PS-co-PMAPOSS (36%

MAPOSS): (a) after an addition of TEOS and calcination and (b) followed by plasma treatment.

Conclusions

Highly porous organic-inorganic hybrid fibers were fabricated via a combination of

electrospinning and solvent-driven phase separation. First, PS-co-PMAPOSS were successfully

synthesized with target molecular weight higher than 100 kDa to ensure electrospinnability via

concurrent ARGET ATRP-RAFT. The composition of MAPOSS in the copolymer calculated

from 1H NMR data progressively increased with the MAPOSS composition in the feed. PS-co-

PMAPOSS fiber mats were then fabricated using electrospinning with mixed THF and DMF as

solvents. Addition of DMF in solvent caused the as-spun fibers to display various degrees of

roughness and porosity. Investigation by SEM/EDS technique showed that elemental

composition of PS-co-PMAPOSS fiber surface changed according to solvent used. The decrease

in silicon content (MAPOSS moiety) on the surface was found together with the increase in

DMF content. This implied that the morphology of PS-co-PMAPOSS fiber was driven by DMF-

induced phase separation. Finally, preliminary tests suggested that highly porous PS-co-

PMAPOSS fiber could be modified with high temperature, plasma treatment, and silanization for

further novel applications.

Acknowledgements

Financial support for this work from the Thailand Graduate Institute of Science and

Technology (TGIST) sponsored by the National Science and Technology Development Agency

(NSTDA), Thailand, awarded to TP, VI, and VPH, the Higher Education Research Promotion

and National Research University Project of Thailand, Office of the Higher Education

Commission (Project Code AS505A), the Ratchadaphiseksomphot Endowment Fund of

Chulalongkorn University (RES560530126-AM), and the Thai Government Stimulus Package 2

21

(TKK2555), under the Project for Establishment of Comprehensive Center for Innovative Food,

Health Products and Agriculture are acknowledged.

References

[1] Fina A, Tabuani D, Frache A, Camino A. Polymer 2005:46;7855.

[2] Yen Y, Kuo S, Huang C, Chen J, Chang F. J Phys Chem B 2008:112;10821.

[3] Kim C, Kim B, Sheikh FA, Lee U, Khil M, Kim H. Macromolecules 2007:40;4823.

[4] Romo–Uribe A, Mather PT, Haddad TS, Lichtenhan JD. J Polym Sci Part B: Polym

Phys 1998:36;1857.

[5] Waddon AJ, Zheng L, Farris RJ, Coughlin EB. Nano Lett 2002:2;1149.

[6] Hirai T, Leolukman M, Jin S, Goseki R, Ishida Y, Ka-kimoto M, Hayakawa T, Ree M,

Gopalan P. Macromolecules 2009:42;8835.

[7] Zheng L, Hong S, Cardoen G, Burgaz E, Gido SP, Coughlin EB. Macromolecules

2004:37; 8606.

[8] Jamil H, Batool SS, Imran Z, Usman M, Rafiq MA, Willander M, Hassan MM. Ceram

Int 2012:38;2437.

[9] Pirzada T, Arvidson SA, Saquing CD, Shah SS, Khan SA. Langmuir 2012:28;5834.

[10] Baek J, Park J, Kang J, Kim D, Koh S, Kang Y. Bull Korean Chem Soc 2012:33;2694.

[11] Kim H, Lee H, Knowles JC. J. Biomed. Mater. Res. A 2006:79A;643.

[12] Mazinani S, Ajji A, Dubois C. Polymer 2009:50;3329.

[13] Cozza ES, Monticelli O, Marsano E. Macromol Mater Eng 2010:295;791.

[14] Xue Y, Wang H, Yu D, Feng L, Dai L, Wang X, Lin T, Chem Commun 2009:6418.

[15] Nicolaÿ R, Kwak Y, Matyjaszewski K. Angew Chem 2010:122;551.

[16] Kwak Y, Nicolaÿ R, Matyjaszewski K. Macromolecules 2008:41;6602.

[17] Magenau AJD, Kwak Y, Matyjaszewski K. Macromolecules 2010:43;9682.

[18] Pisuchpen T, Chaim-ngoen N, Intasanta N, Supaphol P, Hoven VP. Langmuir

2011:27;3654.

[19] Wannatong L, Sirivat A, Supaphol P. Polym Int 2004:53;1851.

[20] Uyar T, Besenbacher F. Polymer 2008:49;5336.

22

[21] Jarusuwannapoom T, Hongrojjanawiwat W, Jitjaicham S, Wannatong L, Nithitanakul

M, Pattamaprom C, Koombhongse P, Rangkupan R, Supaphol P. Eur Polym J

2005:41;409.

[22] Lu P, Xia Y. Langmuir 2013:29;7070.

[23] Megelski S, Stephens JS, Chase DB, Rabolt JF. Macromolecules 2002:35;8456.

[24] Casper CL, Stephens JS, Tassi NG, Chase DB, Rabolt JF. Macromolecules

2004:37;573.

[25] Dayal P, Liu J, Kumar S, Kyu T. Macromolecules 2007:40;7689.

[26] Medeiros ES, Mattoso LHC, Offeman RD, Wood DF, Orts WJ. Can J Chem

2008:86;590.

[27] Pai C, Boyce MC, Rutledge GC. Macromolecules 2009:42;2102.

[28] Baney RH, Itoh M, Sakakibara A, Suzuki T. Chem Rev 1995:95;1409.

23

GRAPHICAL ABSTRACT

Highly Porous Organic-Inorganic Hybrid Fiber from Copolymers of Styrene and

Polyhedral Oligomeric Silsesquioxane-Derived Methacrylate: Synthesis, Fiber Formation

and Potential Modification.

Thanarath Pisuchpen, Varol Intasanta, Voravee P. Hoven

24

Highlights

• POSS-containing copolymers can be synthesized by concurrent ARGET ATRP-RAFT

• Thermal decomposition and solubility of PS was altered by PMAPOSS incorporation

• Characteristic of fibers electrospun from the copolymers was influenced by solvent

• Phase separation of the copolymer constitutes to the degree of fiber porosity

• The copolymer fibers can be modified with high temperature, plasma and silanization