Ultrafine cellulose acetate fibers with nanoscale structural features

RESEARCHARTICLE

Copyright © 2008 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol. 8, 1–9, 2008

Ultrafine Cellulose Acetate Fibers withNanoscale Structural Features

Lifeng Zhang and You-Lo Hsieh∗

Fiber and Polymer Science, University of California, Davis, California 95616, USA

Nano-structural features were introduced to ultrafine cellulose acetate (CA) fibers by electrospinningof its mixtures with either poly(vinyl pyrrolidone) PVP or �-cyclodextrin (�-CD) in DMF, followedby dissolution of the added PVP or �-CD. The presence of the charge-holding PVP enabled fiberformation from CA below its entanglement chain length and improved the electrospinning efficiencyto produce bicomponent fibers with wide ranging diameters from 30 to 650 nm. At up to 50%contents, the PVP in the bicomponent fibers was phase-separated from CA and, upon removal,resulting in highly angulated fiber surfaces with nanometer-size spherulites and sub-micron sizeridges and grooves. Adding �-CD to CA enabled fiber formation at concentrations below the chainentanglement concentration Ce (16.5%). Hydrogen bonding between �-CD and CA, as evidentby FTIR, helped to distribute �-CD as individual molecules in the CA matrix and producing moreuniform and finer (130–150 nm in diameters) fibers, irrespective of their �-CD contents. Removalof �-CD from the fibers originally containing 40% �-CD, generated nanoporous fibers with 2-nmnanopores and 70% increase in specific surface and doubled pore volume.

Keywords: Cellulose Acetate, Poly(vinyl pyrrolidone), �-Cyclodextrin, Bicomponent Nanofiber,Nanostructures, Electrospinning.

1. INTRODUCTION

Producing submicrometer or even tens nanometer fibers byelectrospinning of solutions or melts of single polymershas been extensively reported. More recently, some atten-tion has been paid to fiber formation from polymer sys-tems containing two or more components that could not beelectrospun alone1–3 and those with combined properties.4

Others have electrospun polymer solutions in two coax-ial capillaries to generate different fiber structures, such assheath-core,5�6 hollow6�7 or porous6 fibers. These coaxiallyelectrospun fibers were larger than those typically elec-trospun fibers. For instance, poly(ethylene oxide) (PEO)-sheath and poly(dodecylthiophene)-core fibers had outsidediameters about 1 �m and core diameter about 200 nm.5

We have demonstrated that electrospinning of binarypolymer mixtures followed by physical treatments is asimple approach to create new nanofibers with uniquechemical8–10 and physical11 features. Hydrogel nanofibrousmembranes were generated from electrospinning aqueousmixtures of poly(acrylic acid) (PAA) and poly(vinyl alco-hol), followed by heat treatments to induce crosslinking.8

Due to the ultra-high specific surfaces of these nano-fibers, their swelling responses were instantaneous and

∗Author to whom correspondence should be addressed.

could be tuned to pH and could be further expandedin electric fields.9–10 Nanoporous polyacrylonitrile (PAN)fibers were created by dissolution of the nanometer size,phase-separated PEO domains in electrospun PAN/PEObicomponent fibers.11

Fiber formation from cellulose is challenging frommany perspectives, including its insolubility in most org-anic media. We have generated cellulose nanofibers byelectro spinning of cellulose acetate (CA) and convertingback to cellulose via either aqueous or alcoholic hydro-lysis.12 In the attempt to create new structures on cellulose-based fibers, PEO was paired with CA in the electrospindope to generate PEO sheath and CA core fiber structure.13

The fact that PEO phase separated into much largerdomains, i.e., complete sheath-core phase separation atlarger than 100 nm, was attributed to their distinctly differ-ent chain structures, i.e., very rigid sugar rings of CA andhighly flexible PEO chains. To promote better mixing andto reduce the dimensions of the phase separated domainsin CA-based bicomponent fibers, molecules that are morerigid or more similar to CA may be necessary.

The objective of this study was to create nano-scalestructures in cellulose fibers by mixing poly(vinyl pyrroli-done) (PVP) and �-cyclodextrin (�-CD) with CA (Fig. 1).Both PVP and �-CD are more rigid than PEO and are

J. Nanosci. Nanotechnol. 2008, Vol. 8, No. 10 1533-4880/2008/8/001/009 doi:10.1166/jnn.2008.285 1

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Ultrafine Cellulose Acetate Fibers with Nanoscale Structural Features Zhang and Hsieh

-[CH2-CH2-O]n–

PVP

CA PEO

β-CD

Fig. 1. Molecular structure of CA, PEO, PVP and �-CD.

expect to phase separate into smaller domains. Both arealso water soluble, making their removal from the bicom-ponent fibers easy. Furthermore, both PVP and �-CDexhibit unique physical and chemical properties that havepotential for additional functional properties. The lone pairelectrons on the N and O atoms of the PVP moleculesenabled the molecules to hold surface charges to serveas anti-flocculating agent or colloidal stabilizer14 as wellas to form PVP-metal composite fibers via complex withmetal ions.15–17 In the latter case, calcination of PVP fromthe composite fibers produced ceramics and semiconduc-tors fibers. The seven-gluclose ring structure of �-CDis best known for its inclusive complex ability for turn-ing hydrophobic compounds into water-soluble by hostingthem in its hydrophobic central cavity.18 We have utilizedthe water-solubility and mixibility of �-CD with PAA toproduce pH-responsive hydrogel fibrous membranes where�-CD served as a crosslinker.19

N,N-dimethylformamide (DMF) was selected as pri-mary solvent for CA/PVP and CA/�-CD binary systemsbecause it is a common solvent for CA and the addedcomponents. DMF is also favorable for efficient and con-tinuous electrospinning due to its high dielectric constantand boiling point as demonstrated in electropsinning ofCA.13 The effects of the second components, i.e., PVP and�-CD, on the fiber formation process and phase-separationbehavior in the CA-based fibers were studied with respectto their molecular weights and contents. The structural fea-tures on the cellulose based fibers after selective removalof water-soluble PVP and �-CD were also probed.

2. EXPERIMENTAL DETAILS

2.1. Materials

Cellulose acetate (CA) (Mn = 30 kDa, DS = 2�45)was purchased from Aldrich Chemical Company Inc.,CA (Mn = 50 kDa and 60 kDa, DS = 2�45) were

provided by Eastman Chemical Company. Poly(vinylpyrrolidone) (PVP) (Mw = 55 kDa and 360 kDa)were purchased from Sigma-Aldrich Inc. �-cyclodextrinhydrate (�-CD, 99%) was purchased from Acros Organ-ics. N,N-dimethylformamide (DMF) was purchased fromEMD Chemicals Inc. All materials were used as received.

2.2. Electrospinning and Selective Dissolution

CA and PVP solutions as well as binary CA/PVP andCA/�-CD solutions at prescribed mass ratios were pre-pared in DMF at 40 �C with stirring. All concentrations inthis paper were weight percentages. These polymer solu-tions were electrospun to an Al foil collector 8 inches awayusing 8–12 kV voltage with high voltage power supply(Gamma high voltage research Inc., ES 30-0.1P) at ambi-ent temperature. The obtained fibrous membranes werevacuum dried for at least 24 h, detached from collector andimmersed in 40 �C and 50 �C deionized water to removePVP and �-CD, respectively. The water-treated fibrousmembranes were then dried in 60 �C oven overnight andkept under vacuum thereafter for further analysis.

2.3. Characterization

The morphology of electrospun fibers were examined byusing scanning electron microscope (SEM, Philips, FEIXL30s FEG) and transmission electron microscope (TEM,Philips, CM120). All samples were sputter coated withgold beforehand for SEM examination. The average fiberdiameters were analyzed by 30–50 measurements fromSEM images using analysis® pro software from Soft Imag-ing System GmbH. The thermal properties of electrospunfibers were measured in N2 at a 10 �C/min heating ratefrom 30 to 500 �C by using a Shimadzu DSC-60 differen-tial scanning calorimeter (DSC). Fourier transform infrared(FTIR) spectroscopic measurements of KBr-sample pelletswere performed by using Nicolet 6700 FTIR spectrome-ter at ambient temperature. The pore volume and surfacearea of fibrous membranes were characterized by nitro-gen adsorption using a surface area and porosity analyzer(Micromeritics, ASAP 2020).

3. RESULTS AND DISCUSSION

3.1. Fiber Formation

3.1.1. PVP and CA Single Component Fibers

Electrospinning of either PVP or CA alone was studied attwo molecular weights, i.e., PVP at 55 kDa and 360 kDaand CA at 30 and 50 KDa, all from DMF. PVP, at eithermolecular weight, was readily soluble in DMF at the ambi-ent temperature. Both the 40% 55 kDa and 20% 360 kDaPVP solutions could be electrospun efficiently to producestraight and smooth fibers (Fig. 2). The fibers generated

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Zhang and Hsieh Ultrafine Cellulose Acetate Fibers with Nanoscale Structural Features

Fig. 2. PVP fibers from DMF solutions (5 �m bar): (a) 360 kDa, 20%;(b) 55 kDa, 40%.

from 360 kDa PVP had diameters in two distinct ranges of60–120 nm and 200–400 nm (Fig. 2(a)). The electrospunproducts from the 55 kDa PVP were mostly fibers with70–150 nm diameters, similar to the smaller fibers from360 kDa PVP, but also included few large beads with upto 2.5 �m diameters (Fig. 2(b)). This fiber-bead coexistedstructure suggests the chain length of 55 kDa to be barelyabove the molecular entanglement threshold for PVP tosustain continuous jet for fiber generation.

Our previous work showed that CA fiber formation fromelectrospinning was influenced by a combination of fac-tors involving molecular chain lengths and solvents.12�13

At 20% concentration in DMF, the threshold CA molec-ular weight to produce uniform fibers (50–200 nmdiameters) was 50 kDa.13 Although the 30 kDa CA pro-duced only clustered beads (200 nm–1.5 �m diameters)from the 20% DMF solution, uniform fibers with a200 nm average diameter were generated when 1:1 DMF/dioxane co-solvent was used. Similarly, the 2:1 acetone/dimethylacetamide (DMAc) mix-solvent allowed electro-spinning of 30 kDa CA into fibers from a wide rangeof concentrations (12.5%–20%).12 However, the CA fiberselectrospun from the 2:1 acetone/DMAc mixture weremuch larger (0.83 �m to 2.5 �m diameters) than thosefrom the 1:1 DMF/dioxane at the same 20% concentration.

1 100.1

1

10

100

1000

10000

100000

Ce

C*

CA(30 kDa)

CA(50 kDa)

η sp

CA concentration, %

50

C*

Ce

Fig. 3. Concentration dependence of specific viscosity of CA solutionsin DMF.

Viscosities of CA in DMF at concentration up to 22%were measured for both 30 kDa and 50 kDa to con-struct the logarithmic plot of specific viscosity (�sp) versusconcentrations (C). Two slope increases in each of the�sp-C plots were clearly observed, i.e., 6.1% and 19.6%for 30 kDa CA and 3.8% and 16.5% for 50 kDa CA(Fig. 3). The two slope changes in the �sp-C plot separatethe viscosities into three regions. The linear regression ofeach region in these plots gives the �sp-C correlation. For30 kDa CA, the linear regressions yielded Log�sp = 1�89Log C−0�02, Log�sp = 3�59 Log C−1�36 (r = 0�99829),and Log�sp = 4�59 Log C−2�54 for the <6.1%, 6.1–19.6%and >19.6% regions, respectively. For 50 kDa CA, thelinear regressions gave Log�sp = 1�70 Log C+ 0�32 (r =0�99524), Log�sp = 3�70 Log C−0�85 (r = 0�99816), andLog�sp = 5�58 Log C−3�13 for the <3.8%, 3.8–16.5% and>16.5% regions, respectively.

From the 20% DMF solution, CA at 30 kDa pro-duced only beads (Fig. 4(a)) but uniform fibers at 50 kDa(Fig. 4(d)). Based on the observations that few fibersamongst beads were observed with 30 kDa CA at 22%(Fig. 4(b)) and 50 kDa CA at 18% (Fig. 4(c)) and the�sp-C relationships, the second slope changes at 19.6% and16.5% were assigned as the starting entanglement concen-tration (Ce) for the 30 kDa and 50 kDa CA, respectively.The first slope changes at 6.1% and 3.8% were the onsetconcentration (C∗) of the semidilute unentangled solutionsfor the 30 kDa and 50 kDa CA, respectively. The two slopechanges of each �sp-C plot separated the viscosities intothree regions corresponding to dilute (<6.1% for 30 kDaCA and <3.8% for 50 kDa CA), semidilute unentangled(6.1–19.6% for 30 kDa CA and 3.8–16.5% for 50 kDaCA) and semidilute entangled concentrations (>19.6% for30 kDa CA and >16.5% for 50 kDa CA). Thus �sp wasproportional to C1�89, C3�59, and C4�59 for 30 kDa CA and toC1�70, C3�70 and C5�58 for 50 kDa CA in the dilute, semidi-lute unentangled and semidilute entangled of concentrationranges, respectively.

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Fig. 4. Electrospun products (5 �m bar) of CA from DMF: 30 kDa: (a) 20%; (b) 22%; 50 kDa: (c) 18%; (d) 20%.

The �sp-C relationships in the dilute and semidiluteunentangled regions are similar for CA at 30 kDa and50 kDa. The �sp dependence on concentration in thesemidilute entangled regions is, however, much higher for50 kDa CA, indicating greater chain entanglement. CAfiber formation from DMF was more efficient with 50 kDaat 20% concentration that was significantly higher than theCe (16.5%).

3.1.2. CA/PVP Bicomponent Fibers

Previous results showed that neither 30 kDa CA nor55 kDa PVP alone was fiber-forming at 20% in DMF.This pair was not expected to be fiber forming at theprescribed condition, thus not included. The other threeCA/PVP pairs, with at least one at high molecular chainlength, were studied.

Electrospinning of three equal-mass CA/PVP mixtures,all at 20% total polymer concentrations, proceeded con-tinuously with high efficiency at a rate greater than 0.6 gsolution/h. This was similar to electrospinning of PVPalone, but much faster than that of CA alone. The high effi-ciency is attributed to the charge holding ability of the lonepair electrons on the N and O atoms in PVP, increasing theelectric driving force of the mixtures. The highest molecu-lar weight pair, i.e., 60 kDa CA and 360 kDa PVP, yieldeduniform fibers whose diameters were bimodally distributed

in the 30–90 nm and 120–300 nm ranges (Fig. 5(a)).This bimodal fiber size distribution is similar to that of360 kDa PVP fibers, but smaller. At a lower 12% for thesame pair, the fibers became smaller (60–150 nm) whilesome very large beads (0.7–4.5 �m) appeared. Pairing thishigher molecular weight CA (60 kDa) with the shorterPVP (55 kDa) produced fibers with 20–120 nm diam-eters that were also accompanied by some large beadswith up to 2 �m diameters (Fig. 5(b)). The opposite pairof 30 kDa CA and 360 kDa PVP produced uniformedfibers efficiently as the highest molecular pair (60 kDaCA and 360 kDa PVP). The average fiber diameter was530 nm (Fig. 5(c)), significantly larger than the same PVPalone (Fig. 2(a)) or its binary mixture with 60 kDa CA(Fig. 5(a)). In fact, the average fiber sizes from the 360kDa PVP and 30 kDa CA mixtures increased from 300 nmto 650 nm with the increasing PVP contents from 30% to70% (Fig. 5(d)).

These results showed that fiber formation from theCA/PVP binary mixtures was most efficient with the360 kDa PVP, which enabled fiber formation with as muchas 70% 30 kDa CA. Although incorporating longer chainof one polymer in the pair improved fiber formation withthe shorter chain of the other in general, the much longerPVP chain length was more effective than the longer CAchain. The greater impact of the 360 kDa PVP has also todo with its charge capacity.


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Fig. 5. Electrospun products (2 �m bar) from equal mixtures of CA/PVP in 20% DMF solutions: (a) CA (60 kDa)/PVP (360 kDa); (b) CA(60 kDa)/PVP (55 kDa); (c) CA (30 kDa)/PVP (360 kDa); (d) CA (30 kDa)/PVP (360 kDa) = 30/70.

3.1.3. CA/�-CD Bicomponent Fibers

The CA/�-CD bicomponent fibers were electrospun fromDMF solutions of 50 kDa CA with 20%, 40% and 50%of �-CD (Table I). At a fixed 25% total CA/�-CD con-centration, the mixtures at all �-CD levels generated fiberscontinuously. All these CA/�-CD fibers appeared similarin shapes and their 130–150 nm diameters (Fig. 6), indi-cating fiber morphology to be independent of the �-CDcontents or CA concentrations. It is interesting to note thatthe equal-mass CA/�-CD binary mixture contains only12.5% 50 kDa CA, which is below the Ce of the CA(16.5% in DMF) and is not expected to be fiber form-ing. The fact that the addition of �-CD small molecules

Table I. Electrospinning of CA/�-CD in DMF solutions.

CA Total CA:�-CD ES ES Fiber(kDa) concentration (%) (mass ratio) Viscosity voltage (kV) process formation

30 25 4:1 Low 8 kV Continuous No50 30 2:1 Very high N/A N/A No

30 1:1 Very high N/A N/A No25 4:1 High 12 kV Continuous Yes25 3:2 High 12 kV Continuous Yes25 1:1 Medium 12 kV Continuous Yes20 1:1 Low 12 kV Continuous No20 1:1 Low 8 kV Continuous No

enabled fiber formation of CA below its Ce concentrationsuggested enhanced CA chain entanglement due to inter-actions between CA and �-CD molecules. The interactionis thought to be hydrogen bonding between CA and �-CD,to be verified in the following section.

3.2. Structures of Bicomponent Fibers

The phase separation behavior in the CA/PVP and CA/CDbicomponent fibers were discerned by comparing theirFTIR and DSC with those of the individual componentsand their physical mixtures. The FTIR of the PVP (360kDa) fibers exhibited the overlapping vibrations of C Oand C N at 1660 cm−1 (Fig. 7(a)) while the CA (30 kDa)


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fibers had the characteristic carbonyl stretching vibrationat 1752 cm−1 (Fig. 7(d)). The FTIR of the equal massCA/PVP bicomponent fibers showed the characteristicpeaks of each polymer at the same wavenumbers(Fig. 7(b)) and, in fact, identical to that of the physicalmixtures of the CA and PVP single component fibers(Fig. 7(c)). This observation indicates no interaction

Fig. 6. CA (50 kDa)/�-CD bicomponent fibers (2 �m bar) from 25%DMF solutions with �-CD at: (a) 20%; (b) 50%.

Fig. 7. FTIR-KBr absorbance spectra of fibers electrospun from 20%DMF solutions: (a) 360 kDa PVP; (b) 50/50 CA (30 kDa)/PVP(360 kDa); (c) equal mass physical mixture of fibers of (a) and (d);(d) 30 kDa CA.

between CA and PVP in the bicomponent fibers. Mean-while the DSC of these CA and PVP bicomponent fibersexhibited overlapped thermal transitions of pure CA andPVP, an indication of phase separation between the twopolymers.

The FTIR of the CA/�-CD fibers showed character-istic peaks of both (Fig. 8). �-CD exhibited a strong

16001700

a

b

c

d

16361660

1800

Wavenumbers (cm–1)

(I)

(II)

Fig. 8. FTIR-KBr absorbance spectra of (a) CA (50 kDa) fibers elec-trospun from 20% DMF solution; (b) 50/50 CA (50 kDa)/�-CD fiberselectrospun from 25% DMF solution; (c) equal mass physical mixture offibers in (a) and (d); (d) �-CD cast film from 10% DMF solution.

Fig. 9. FTIR-KBr absorbance spectra of fibers electrospun from 20%DMF solutions: (a) water treated (40 �C, 24 h) 50/50 CA (30 kDa)/PVP(360 kDa); (b) PVP (360 kDa).


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characteristic peak at 1660 cm−1 (Fig. 8(d)), assignedto H O H bending of the absorbed water.20�21 Thishydrated water peak was much weakened for the bicompo-nent fibers (Fig. 8(I)), suggesting much reduced hydratedwater. The broad peak at 3396 cm−1 was attributed to

OH vibration in �-CD (Fig. 8(II)). The OH vibrationwas also found in CA fibers due to incomplete substitution,but at a much higher wavenumber of 3481 cm−1. This indi-cated that the CA hydroxyls were less hydrogen bondedthan those in �-CD. The equal mass physical mixture ofthe two solids showed OH vibration at 3406 cm−1, veryclose to that of �-CD, expected from the abundant �-CDhydroxyl groups and their lack of interaction with CA.The OH vibration of the bicomponent electrospun fibers,however, was present at a significant higher wavenumberof 3425 cm−1 than that of the physical mixture, indicatinghydrogen bond formation between hydroxyls of CA and�-CD. This observation strongly supports the hypothesisthat CA and �-CD associate with each other in the solutionand enhance CA chain entanglement to form CA/�-CDbicomponent fibers at a concentration below the Ce of CA.

Fig. 10. Water treated (40 �C, 48 h) CA (30 kDa)/PVP (360 kDa) fibers electrospun from 20% DMF solutions with PVP at 30% (a, c) and 50%(b, d): (a, b) SEM (2 �m bar); (c, d) TEM (50 nm and 20 nm bar, respectively).

The DSC of �-CD film cast from a DMF solutionshowed two endotherms around 116 �C and 321 �C thatwere attributed to dehydration and melting of �-CD,respectively. The dehydration peak was not observed inthe bicomponent fibers, indicating �-CD was not hydratedwhen mixed with CA. This is consistent with the FTIRresult. The absence of �-CD melting peak gave evidencethat �-CD in the bicomponent fibers was not crystalline.Therefore, both the FTIR and DSC results support thenotion that �-CD molecules do not aggregate and likelydisperse in the CA matrix as individual molecules.

3.3. Dissolution of PVP and �-CD on Fiber Structures

The water-soluble PVP and �-CD in the biocomponentfibers were dissolved by water immersions. PVP and �-CDare soluble in water at 40 �C in 30 minutes and 50 �Cin two hours, respectively. Therefore, the bicomponentfibrous membranes were immersed in water at the pre-scribed elevated temperatures but at much longer time.


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3.3.1. Water Immersion on CA/PVP BicomponentFiber Structures

Immersion of the CA (30 kDa)/PVP (360 kDa) bicom-ponent fibrous membranes in 40 �C water caused themto shrink in dimensions and become stiffened after onlya few minutes. Extended immersion for 48 h resulted in18%, 21% and 67% mass reduction of CA/PVP bicompo-nent fibers that originally contained 30%, 50% and 70% ofPVP, respectively. These data clearly showed incompleteremoval of PVP from these bicomponent fibers, especiallyin those with 30% and 50% PVP. The FTIR of the water-immersed fibers still exhibited the PVP characteristic peakat 1660 cm−1 (Fig. 9), further confirming the presence ofPVP or its incomplete removal from bicomponent fibers.

The surface morphology of these water treated fiberswas examined by SEM (Figs. 10(a, b)) and TEM(Figs. 10(c, d)). Those fibers originally containing30% PVP generally retained their fiber shapes andsizes (Fig. 10(a)) while beads covered with nanopores(30–80 nm) on the surfaces were also observed. The fibersthat originally contained 50% PVP became flattened andadhered to each other (Fig. 10(b)) while those fibers thatoriginally contained 70% PVP lost their fiber appear-ance and totally collapsed into film-like structure. TheTEM of the water treated fibers containing 30% and 50%PVP showed highly irregular surfaces with nanometer-sizespherulitic bumps and sub-micron size ridges and grooves(Figs. 10(c, d)). While the fibrous membranes originally

Fig. 11. Water treated (50 �C, 24 h) CA (50 kDa)/�-CD fibers electrospun from 25% DMF solutions with �-CD at 20% (a, c) and 50% (b, d):(a, b) SEM (2 �m bar); (c, d) TEM (50 nm and 20 nm bar, respectively).

containing 70% PVP lost most of their fibrous forms, thefew remaining fibers exhibited elongated nanopores.

Observations from these SEM and TEM images suggestthat, in the bicomponent fibers with 30% and 50% PVP,the phase-separated PVP distributed in a continuous CAmatrix, both in the bulk and on the fiber surfaces. Some ofthe PVP molecules inside the fibers might not be as acces-sible, thus were not completely removed. With increasingPVP as in the case of 50% content, partial removal of PVPcaused the fibers lost their shapes and became flattened andmerged in places. As the major component, e.g., 70%, PVPformed larger and inter-connected domains in the bicom-ponent fibers that, upon PVP removal, the remaining CAcould not support the cylindrical fiber shape, collapsed andmerged into a film-like structure.

3.3.2. Water Immersion on CA/�-CD BicomponentFiber Structures

Water immersion (50 �C for 24 h) of the CA(50 kDa)/�-CD bicomponent fibers with 20%, 40% and 50% �-CDlost 20.9%, 38.3% and 49.8% mass, respectively. Theclosely matched mass losses to the �-CD mass fractionsin original bicomponent fibers indicated excellent �-CDremoval by dissolution. The FTIR spectrum of the watertreated bicomponent fibers appeared nearly identical to thatof pure CA, confirming nearly complete removal of �-CD.


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Following water treatment, bicomponent fibers origi-nally containing 20% and 40% �-CD retained their shapesbut became slightly wider (Figs. 11(a, b)). Bicompo-nent fibers originally containing 50% �-CD, however, par-tially lost their cylindrical shapes, became flattened anddensely packed. TEM showed that the surfaces of watertreated fibers was very rough and contained nanoporeswith diameters around 2 nm (Figs. 11(c, d)). Suchnanopores appeared to increase in numbers with increas-ing �-CD contents. The fibers originally containing 40%�-CD increased their specific surface from 10.9 m2/g to18.4 m2/g, a 70% increase, and doubled pore volume from0.027 cm3/g to 0.058 cm3/g.

The significant increases in the total pore volume andthe numbers of 2 nm size pores with increasing �-CD con-tents suggested that most, if not all, of the �-CD was dis-tributed in CA matrix as individual molecules. This 2 nmpore size is consistent with outer diameter of �-CD cone(1.54 nm).

4. CONCLUSION

Cellulose acetate based bicomponent fibers were suc-cessfully generated by electrospinning CA (30 kDa and60 kDa) with PVP (360 kDa) and CA (50 kDa) with �-CDfrom DMF solutions at 20% and 25% concentrations,respectively. The starting entanglement concentration (Ce)for the 30 kDa and 50 kDa CA were determined to be19.6% and 16.5%, respectively. The CA (50 kDa) fibersproduced from the 20% DMF solution had 50–200 nmdiameters, similar to those (30 kDa) electrospun fromDMF/dioxane but much smaller than those from ace-tone/DMAc mixtures. The fiber formation process fromthe CA/PVP binary mixtures was most efficient with the360 kDa PVP, which enabled fiber formation of mixturescontaining up to 70% of 30 kDa CA that was not fiberforming alone. The diameters of the bicomponent fibersranged from 30–90 nm (60 kDa CA/360 kDa PVP) to300–650 nm (30 kDa CA/360 kDa PVP). The presenceof PVP improved the electrospinning efficiency, attribut-ing to the ability of PVP to hold charges and increase theelectric driving force on the polymer jet. The PVP in thebicomponent fibers was phase-separated from CA as con-firmed by FTIR and DSC and could be partially removedby dissolution in water (40 �C, 48 h). Following watertreatment, the CA (30 kDa)/PVP (360 kDa) fibers origi-nally containing 30% and 50% PVP showed highly irreg-ular surfaces with nanometer-size spherulitic bumps and

sub-micron size ridges and grooves while those originallycontaining 70% PVP lost most of their fibrous form, witha few remaining fibers showed elongated nanopores.

All CA/�-CD bicomponent fibers had similar morphol-ogy and average fiber sizes (130–150 nm), irrespectiveof the �-CD contents between 20% and 50%. Hydro-gen bonding between CA and �-CD as evident by FTIRenhanced CA chain entanglement and enabled fiber forma-tion at 12%, a concentration below its chain entanglementconcentration Ce (16.5%). Removal of �-CD from all threebicomponent fibers by dissolution in water (50 �C, 24 h)was complete while leaving similar fiber sizes. Follow-ing water treatment, the fibers originally containing 40%�-CD had a significantly increase (70%) in specific sur-face and doubled pore volume. Observation of the 2 nmnanopores suggests that �-CD distributed in the CA matrixas individual molecules.

Acknowledgments: The supports from the NationalTextile Center (project C04-CD06) and the Universityof California, Davis (Jastro-Shields Graduate ResearchAward) are greatly appreciated.

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Received: 11 July 2007. Accepted: 7 January 2008.


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