Cellulose Based Composites (New Green Nanomaterials) || Hydrolytic Degradation of Nanocomposite Fibers Electrospun from Poly(Lactic Acid)/Cellulose Nanocrystals

117

6Hydrolytic Degradation of Nanocomposite Fibers Electrospunfrom Poly(Lactic Acid)/Cellulose NanocrystalsChunhui Xiang and Margaret W. Frey

6.1Introduction

Advanced technology in petrochemical-based polymers has brought many benefitsto mankind [1]. However, it is evident that the ecosystem is disturbed as aresult of the use of nondegradable plastic materials for disposable items. Theenvironmental impact of persistent plastic wastes is a growing global concern,and alternative disposal methods are limited. As the petroleum resources arefinite, there is an urgent need to develop renewable-source-based environmentallybenign polymeric materials, especially in short term, packaging and disposableapplications, which do not involve the use of toxic or noxious components in theirmanufacture, and can be composted to biodegradable products. Poly(lactic acid)(PLA) is of increasing commercial interest because it can be made from completelyrenewable agricultural products, has comparable properties to many petroleum-based plastics, and is readily biodegradable [2, 3]. High-molecular-weight PLA isgenerally produced by the ring-opening polymerization of lactide monomer, whichin turn is obtained from the fermentation of sugar feed stocks, corn, and so on [4].Even when burned, PLA produces no nitrogen oxide gases; only one-third of thecombustible heat generated by polyolefins and does not damage the incinerator,thus providing a significant energy savings [1].

PLA has been widely used in various biomedical applications because of itsbiodegradability, biocompatibility, good mechanical properties, and solubilityin common solvents for processing [5]. However, PLA has a slow biodegra-dation rate even in the noncrystalline form of poly (d,l-lactide) as well as inenantiomeric semicrystalline forms of poly(d-lactide) and poly(l-lactide). Thusdegradation of PLA-based materials may take too long for many biomedicalapplications.

The degradation of aliphatic polyester is based on a hydrolytic reaction [5].When water molecules attack ester bonds in the polymer chains, the averagelength of the degraded chains decreases. Eventually, the process results in shortfragments of chains with carboxyl end groups that become soluble in water. Veryoften, the molecular weights of some fragments are still relatively large so that

Cellulose Based Composites: New Green Nanomaterials, First Edition.Edited by Juan P. Hinestroza and Anil N. Netravali.c© 2014 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2014 by Wiley-VCH Verlag GmbH & Co. KGaA.

118 6 Hydrolytic Degradation of Nanocomposite Fibers Electrospun

the corresponding diffusion rates are slow. As a result, the remaining oligomerswill lower the local pH value, catalyze the hydrolysis of other ester bonds, andspeed up the degradation process. This mechanism is termed autocatalysis, and isfrequently observed in thick biodegradable implants. However, if the dimensionof the implant is small and the diffusion path is short, the hydrophilic oligomerscan quickly escape from the surface. This is the case with electrospun scaffolds inwhich the dimension of the nanofibers is small and the diffusion length for thedegraded by-products (hydrophilic oligomers) is short. As a result, the possibilityof autocatalysis in electrospun scaffolds is very limited [5].

The degradation of PLA is primarily due to the hydrolysis of the ester linkages,which occurs more or less randomly along the backbone of the polymer. Hydrolysisrequires the presence of water according to the following reaction [6]:

PLA

OPLAPLA

CH3

CH3 CH3

CH3

H2OC

C

C

C

O

OPLAO

O

O O

CH

CH

CH

CHOH OH

+

+

The rate of hydrolysis is determined by its intrinsic rate constant, waterconcentration, acid or base catalysis, temperature, and morphology [6]. Two majorchallenges to the stabilization of PLA with regard to hydrolysis are the fact that itis quite permeable in water and that the hydrolysis reaction is autocatalytic. Theautocatalytic hydrolysis reaction is as follows:

R COOH R COO− H+ka

kh

+

PLA

PLA

OPLA + H2O + H+

OPLA + H+

CH

CH

CH

CH+ OHOH

CH3

CH3 CH3

CH3

C

C

C

C

O

O

O

O

O

The following equation describes the decrease in ester concentration [E] overtime:

d[E]𝑑𝑡

= k[−COOH][H2O] =d(1∕Mn)

𝑑𝑡(6.1)

For a random chain scission, [–COOH]∝ 1/Mn and the product [H2O][E] isconstant. Rearranging to

6.2 Experiments 119

Mnd

(1

Mn

)= 𝑘𝑑𝑡 (6.2)

the integrated form becomes

ln Mn,t = ln Mn,0 –𝑘𝑡 (6.3)

where Mn,t is the number average molecular weight at time t, Mn,0 is the numberaverage molecular weight at time zero, and k is the hydrolysis rate constant. Thekinetics were derived by Pitt et al. [7] and were again supported by Tsuji [8].

The morphology of PLA (i.e., size and shape) plays an important role in itshydrolytic degradation [9]. If the size is very small as in the case of microparticles,slim fibers, or thin films, the degradation should be slower than for large-sized materials because no autocatalytic degradation occurs in the former casebecause of the easier diffusion of oligomers and the neutralization of carboxyl endgroups [9].

The objective of the present work is to investigate the influence of cellulosenanocrystals on the hydrolytic degradation of electrospun PLA/cellulose nanocom-posite fibers. Cellulose nanocrystals occur naturally in the cell wall of plants and havebeen shown to increase the crystallinity of PLA when incorporated in PLA/cellulosenanocomposite fibers [10]. The presence of cellulose nanocrystals at the surfaceof the electrospun PLA fibers was confirmed, and the quantity of cellulose avail-able at the surface was enriched compared to the bulk composition. Cellulosenanocrystals acted as nucleation sites during the electrospinning process resultingin increased crystallinity of electrospun PLA nanocomposite fibers. Influence ofcellulose nanocrystal content on the hydrophilicity of the electrospun nonwovenfabrics was studied by measuring the water absorption and water contact angleof PLA/cellulose nonwoven fabrics. The hydrolytic degradation behaviors of theelectrospun PLA/cellulose nanocomposite fibers spun from solutions containing0, 1, and 10% suspended cellulose nanocrystals in phosphate buffer saline (PBS,pH 7.4) at 37 ◦C were investigated, and the degraded nanocomposite fibers wereexamined. The morphological changes of the electrospun PLA/cellulose nanocom-posite fibers were observed by field emission scanning electron microscopy (KeckSEM). And the molecular weight of PLA from the nanocomposite fibers dur-ing hydrolytic degradation was investigated by size exclusion chromatography(SEC).

6.2Experiments

6.2.1Materials

Microcrystalline cellulose powder (MCC, extra pure, average particle size is 90 μm)was purchased from Acros Oganics (Geel, Belgium). PLA (Mw = 211 000 Da,


Mn = 109 000 Da) was supplied by Cargill Dow (Minnetonka, MN) and PBS(p-5368, pH 7.4) was purchased from Sigma-Aldrich (St. Louis, MO). N,N-dimethylformamide (DMF) was purchased from Mallinckrodt Laboratory Chemicals(Phillipsburg, NJ). Cellulose nanocrystals were prepared from MCC by acidhydrolysis [10]. All other reagents were used without further purification.

6.2.2Methods and Techniques

6.2.2.1 Elevated Temperature Electrospinning ProcessingPolymer suspensions, consisting of PLA and cellulose nanocrystals, were preparedin DMF solvent. The concentration of the final suspension used for electrospin-ning was 22 wt% PLA in DMF containing cellulose nanocrystals contents of 0,1, and 10% based on the weight of PLA. The suspensions were then electro-spun at 70 ◦C. During electrospinning, the polymer suspension was introducedinto a 5 ml glass syringe (VWR Scientific, West Chester, PA). The syringe wasattached with a metal needle (ID= 0.60 mm) and put into a shielded heatingunit that was preheated to 70± 5 ◦C and controlled by a Watlow controller (St.Louis, Missouri). After about 10 min thermal equilibration, electrospinning wasstarted at 15 kV, which was supplied by a high voltage supply (Gamma HighVoltage Research Inc., FL), and at 10 μl/min feed rate driven by a programmablesyringe micropump (Harvard Apparatus, MA). A rotating aluminum plate (diame-ter= 20 cm) covered with aluminum foil was used to collect nanocomposite fibersat a 10 cm distance away from the needle tip. Each sample was collected for5 h.

6.2.2.2 Water Contact Angle MeasurementsThe contact angle of water on the electrospun PLA/cellulose nonwoven fabricswas measured by the sessile drop method [11] using a contact angle ana-lyzer (Imass, Model CAA2). Smooth surface (spin-coated) films were cast fromPLA/DMF solution with 0, 1, and 10% cellulose nanocrystals suspended usinga spin processor (Model Ws-650sx – 6NPP/A1/AR1Laurell Technologies Corpo-ration) at a speed of 500 rpm. The contact angle of water on the spin-coatedfilms was measured using the same method as the electrospun nonwoven fabrics.The final result for each sample was obtained by averaging at least 10 separatemeasurements.

The hydrophilicity of the electrospun PLA/cellulose nanocomposite fibers wasstudied by measuring the water absorption of the electrospun nonwoven fabrics.The water absorption was investigated by measuring the weight change withtime when the electrospun PLA/cellulose nonwoven fabrics were in contact withwater using a Sigma 700 (KSV Instruments) wettability apparatus. The electrospunnonwoven fabrics that were cut into 0.5 cm× 3 cm rectangles were attached to smallcopper wire hooks with an adhesive and allowed to dry at room temperature forat least 12 h. Four specimens were tested for each sample. The pore size of theelectrospun nonwoven fabrics cut into two-inch-diameter circles was measured with

6.2 Experiments 121

an 1100-AEHXL capillary flow porometer (Porous Media, Inc.). Three specimensfor each sample were measured.

6.2.2.3 Hydrolytic Degradation of Electrospun Nanocomposite Fibers

The hydrolytic degradation of the electrospun PLA/cellulose nanocomposite fiberswas conducted following the method by Tarvainen [12]. The electrospun non-woven fabrics (30× 30 mm2) were immersed in 10 ml PBS (pH 7.4) in closedbottles and shaken constantly (100 rpm) in a water bath at 37 ◦C. The hydrolyticdegradation procedure was set to 15 weeks. One specimen was withdrawn ateach week. The degraded electrospun PLA/cellulose nanocomposite fibers werevacuum dried at 25 ◦C for a week before being subjected to various analy-ses.

6.2.2.4 Microscopy

The morphology of the electrospun PLA/cellulose nanocomposite fibers duringhydrolytic degradation was observed using a field emission scanning electronmicroscope (FESEM, LEO 1550). The fibers were sputter coated with a 2–3 nmlayer of gold and palladium for imaging using a Desk Π cold sputter/etch unit(Edwards S150 Sputter Coater). The fiber diameters were determined using imageprocessing and analysis in Java software (ImageJ). The structural study of PLAnanocomposite fibers spun from solutions containing 10% cellulose nanocrystalswas performed through transition transmission electron microscopy (TEM, TechnaiT12). In the TEM study, to obtain a sectional image, the electrospun fibers weremicrotomed at room temperature using a diamond knife and to get whole fibermorphology, the nanocomposite fibers were directly collected on the TEM grids forabout 2 s. The TEM grids with nanocomposite fibers were stained with rutheniumtetraoxide (RuO4) vapors overnight before TEM observation to improve contrastbetween PLA and cellulose nanocrystals. TEM images of the fibers were takenusing Technai T12 at an accelerating voltage of 120 kV.

6.2.2.5 Size Exclusion Chromatography (SEC)

The molecular weight of as-received and hydrolytically degraded PLA samples wasdetermined by SEC (a Waters 486 UV detector and a Waters 2410 differentialrefractive index detector, Waters Corporation), using polystyrene standards for cali-bration and tetrahydrofuran (THF) as the carrier solvent at 40 ◦C with a flow rate of0.5 ml min−1. For SEC measurements, the electrospun PLA/cellulose nanocompos-ite fibers were dissolved in THF. Cellulose nanocrystals were removed by filtration(pore size= 0.45 μm, Millipore) before the molecular weight measurements.

6.2.2.6 Thermogravimetric Analysis (TGA)

Thermogravimetric measurements were carried out using a thermogravimetricanalyzer (TGA 2050, TA Instruments Inc.). The temperature range was 25–600 ◦Cat 10 ◦C min−1 ramp under nitrogen flow.


6.3Results and Discussion

6.3.1Distribution of Cellulose Nanocrystals in the Electrospun PLA/CelluloseNanocomposite Fibers

The morphology of the electrospun PLA nanocomposite fibers spun from solutionscontaining 10% cellulose nanocrystals is shown in Figure 6.1a. The distributionof cellulose nanocrystals within the PLA/cellulose nanocomposite is shown inFigure 6.1b. Darker portions in both the photomicrographs are identified ascellulose nanocrystals, which have been stained with RuO4 for contrast. Cellulosenanocrystals appear to be well dispersed within the PLA fibers; however, theapparent size of cellulose nanocrystals at the surface and in the interior of the fibersis much shorter than the 100 nm length measured for the original nanocrystals[10]. Although cellulose nanocrystals are reported to have high modulus [13],flexibility of these crystals is low. During the course of the stretching and whippingprocesses occurring during electrospinning, the cellulose nanocrystals appear tohave fractured into shorter lengths. For the purposes of this study, the quantityof cellulose in the fibers is more critical than the fiber length. Incorporation ofcellulose has been confirmed earlier via X-ray photoelectron spectroscopy [10] andis further confirmed by thermogravimetric analysis (TGA) below.

6.3.2Thermogravimetric Analysis of Electrospun PLA/Cellulose Nanocomposite Fibers

Figure 6.2 shows the TGA measurements of nanocomposite fibers electrospun fromPLA spun from solutions containing 0, 1, and 10% cellulose nanocrystals. The lack

100 nm 500 nm

(a) (b)

Figure 6.1 TEM images of the electrospun PLA/cellulose nanocomposite fibers spun fromsolutions containing 10% cellulose nanocrystals. (a) whole fiber morphology and (b) sec-tional structure of the nanocomposite fiber.

6.3 Results and Discussion 123

0 50 100 150 200

Temperature (°C)

250 300 350 400

0% Cellulose–PLA

1% Cellulose–PLA

10% Cellulose–PLA0

20

40

We

igh

t p

erc

en

t 60

80

100

Figure 6.2 TGA results of electrospun nanocomposite PLA fibers with 0, 1, and 10% cellu-lose nanocrystals.

of mass loss at temperatures below 200 ◦C confirms that samples were well dried.At 1% cellulose nanocrystal loadings, no obvious degradation was observed at about250 ◦C. The 10% cellulose nanocrystal loadings showed a bimodal distribution withthe expected ∼250 ◦C cellulose nanocrystal degradation step, and the ∼350 ◦C PLAstep occurring relatively independently at degradation temperatures expected for thepure components. Cellulose nanocrystals degraded at a relatively low temperaturefor pure cellulose material because of the residual acidity from the acid hydrolysisnanocrystal preparation method [14, 15]. Additionally, cellulose nanocrystals havebeen shown to leave ∼30% of their mass as ash after hydrolysis. On the basisof the weight loss percentage from each degradation step and the mass afterincineration relative to the original nanocomposite fiber mass, the mass loss forthe nanofibers with 10% cellulose nanocrystals incorporated into the electrospunPLA nanocomposite fibers was consistent with the nominal loading.

6.3.3Hydrophobicity/Hydrophilicity of Electrospun Non-woven Fabrics

A nanocomposite fiber combining hydrophobic (PLA) and hydrophilic (cellulosenanocrystals) components is expected to have wetting and water absorbance (wick-ing) behavior intermittent between the two components. Wettability is commonlymeasure by contact angle. A water drop on a solid surface will completely spreadout on a hydrophilic surface and the contact angle will be close to 0◦. Less stronglyhydrophilic solids will have a contact angle of up to 90◦. If the solid surface is


Table 6.1 The initial water contact angle of electrospun PLA/cellulose nonwoven fabrics andPLA/cellulose spin-coated films.

Water contact angle (◦) ES nonwovens Spin-coated film

0% cellulose PLA 128± 2 91± 21% cellulose PLA 127± 2 91± 210% cellulose PLA 115± 3 77± 1

ES, electrospun.

hydrophobic, the contact angle will be larger than 90◦ [16]. The surface in theseexperiments is a nonwoven mesh of multiple fibers that is both rough and porous.In the case of a hydrophobic sample, surface roughness is expected to increasethe observed contact angle [17, 18]. A hydrophilic material will rapidly absorb theapplied droplet.

Generally, surface chemistry and the surface roughness affect contact angle[19, 20]. Namely, the contact angle increases as the surface roughness increases[21] and nonwoven fabrics composed of small fibers are more hydrophobic thanfilms prepared from the same polymer. Table 6.1 shows the initial contact angleof water on the electrospun nonwoven fabrics and the spin-coated films. Theinitial contact angle is defined as the contact angle measured within 1 min ofplacement of the drop on the substrate. As the cellulose nanocrystal contentincreased, the water contact angle of the electrospun nonwoven fabrics and spin-coated films decreased. The addition of 1% w/w cellulose did not significantlydecrease the contact angle of water on the surface of the nonwoven fabrics orspin-coated films. When cellulose nanocrystal loading was increased from 0 to10% w/w, the water contact angle was decreased by more than 10◦. The decreaseof the initial contact angle of the electrospun nonwoven fabrics and spin-coatedfilms indicated that the incorporation of cellulose nanocrystals improved thehydrophilicity of PLA.

The decrease in hydrophobicity of the electrospun PLA/cellulose nonwovenfabrics was also measured by the rate of water absorption (Figure 6.3). Againthe behavior of the sample spun from solutions containing 1% w/w cellulosenanocrystals was not significantly different from the neat PLA sample. The electro-spun PLA/cellulose nonwoven fabrics containing 10% w/w cellulose nanocrystalsabsorbed six times more water than the PLA containing 0 and 1% w/w cellulosenanocrystals. The electrospun nonwoven fabrics had typical wicking behavior,with initial rapid absorbance followed by slower absorbance, which is consistentwith the results reported in our laboratory [10]. Overall, the incorporation of 1%w/w cellulose nanocrystals did not decrease the hydrophobicity of the electrospunnonwoven fabrics significantly. But there was an obvious decrease in hydropho-bicity as 10% w/w cellulose nanocrystals added into the electrospun PLA/cellulosenanocomposite. Nanocomposite fibers spun from solutions containing 10% cel-lulose nanocrystals absorbed three times the initial sample weight of water. For


0 100 200 300 400 500 600

0.0

0.5

1.0

1.5

2.0

2.5

3.0W

ate

r absorp

tion (

g/g

fabric)

Time (s)

0% Cellulose–PLA

1% Cellulose–PLA

10% Cellulose–PLA

Figure 6.3 Water absorption of the electrospun PLA/cellulose non-woven fabrics with dif-ferent cellulose nanocrystal loading as a function of time.

comparison, 100% cellulose electrospun fabrics have been reported to absorb morethan 10 times the initial sample weight of water [22]. During absorption processes,pores act as capillaries, pulling liquid into the fabric via capillary action. Thecapillary action can be described in the following equation [23]:

H = 2T cos 𝜃𝜌𝑔𝑟

(6.4)

where h is the height (in m), T is the surface tension (in J m−2 or N m−1), 𝜃 is thecontact angle, 𝜌 is the density of the liquid (in kg m−3), g is the acceleration due togravity (in m s−2), and r is the radius of the tube (in m). The pore size correspondsto the radius of the tube, and the compatibility between the liquid and the fiber isexpressed as cos 𝜃. When the liquid wets the fiber surface, cos 𝜃 will approach 1. Acombination of a compatible liquid and a small pore radius is expected to result inincreased liquid absorbance. Figure 6.4 shows the mean pore size of the electrospunnonwoven fabrics of PLA containing 0, 1, and 10% cellulose nanocrystals. At a 0.05significant level, there is no difference in the mean pore size among the nonwovenfabrics containing different proportions of cellulose nanocrystals. Hence, thereis no significant difference in the effective pore radius (r). On the basis of Eq.(6.4), cos 𝜃 is the parameter that determines the liquid absorption. The higherthe cos 𝜃, the higher the absorption. Our results showed that the electrospunnonwoven fabrics of PLA containing 10% cellulose nanocrystals had the highestcos 𝜃, and it also had the highest water absorption. The water absorption results areconsistent with the predicted results. On the basis of the contact angle and waterabsorption results, incorporation of cellulose nanocrystals is expected to increasethe interaction between water and the PLA/cellulose nanocomposite fibers duringthe hydrolytic degradation processes.


00 0

0.20.40.60.8

11.21.41.61.8

2

0.5

1

1.5

2

2.5

Cellulose nanocrystals content (%)

Mean p

ore

siz

e (μm

)

0

0.5

1

1.5

2

2.5

Mean p

ore

siz

e (μm

)

Mean p

ore

siz

e (μm

)

1 1


10

1


10

(a) (b)

(c)

Figure 6.4 Mean pore size of the electrospun nonwoven fabrics. Values are given asmean± SD. (a) P = 0.361; (b) P = 0.286, and (c) P = 0.07.

6.3.4Morphologies of the Electrospun PLA/Cellulose Nanocomposite Fibers duringHydrolytic Degradation

Over the course of the hydrolytic degradation study, the bulk morphology ofthe electrospun PLA nonwoven fabrics deteriorates. Digital camera images ofthe electrospun nonwoven fabrics of PLA/cellulose nanocomposite fibers spunfrom solutions containing 10% cellulose nanocrystals clearly exhibit the effectsof degradation (Figure 6.5). The original sample was a single piece of nonwovenfabric (Figure 6.5a). After 8 weeks of hydrolytic degradation, the nonwoven fabricwas broken. There is an obvious embrittlement and cracking on the nonwovenfabric (Figure 6.5b). The nonwoven fabric fell into parts after 15 weeks of hydrolyticdegradation (Figure 6.5c). Embrittlement, cracking, and general loss of physicalproperties are frequently associated with the degradation of polymeric materials[24] as the polymer molecular weight decreases.

FESEM observations (Figures 6.6–6.8) revealed that original electrospunPLA/cellulose nanocomposite fibers exhibited smooth surfaces. After 8 weeks ofhydrolytic degradation, the electrospun PLA/cellulose nanocomposite fibers spunfrom solutions containing 0, 1, and 10% cellulose nanocrystals were still in fibershape. However, small cracks and white agglomerates were observed at the surfaceof the fibers. After 15 weeks of hydrolytic degradation, broken nanocomposite fibers(Figure 6.8, 10% 15 weeks) were found in the nonwoven fabrics of PLA/cellulosenanocomposite fibers spun from solutions containing 10% cellulose nanocrystals.The morphological changes were greater in PLA/cellulose nanocomposite fibersspun from solutions containing 10% cellulose nanocrystals indicating a greater


(a)

(c)

(b)

Figure 6.5 Digital camera images of the electrospun PLA/cellulose nanocomposite fibersspun from solutions containing 10% cellulose nanocrystals during hydrolytic degradation.(a) Original electrospun nonwoven fabrics; (b) degraded for eight weeks; and (c) degradedfor 15 weeks.

degree of hydrolytic degradation than apparent for the nanocomposite fibers spun

from solutions containing 0 and 1% cellulose nanocrystals. The average fiber

diameter of the electrospun nanocomposite fibers increased during the hydrolytic

degradation (Figure 6.9). The fibers swelled during the hydrolytic degradation.

Fiber swelling, however, occurred in all samples regardless of cellulose nanocrystal

content.

6.3.5Molecular Weight Change of PLA in the Electrospun Nanocomposite Fibers duringHydrolytic Degradation

Figure 6.10 shows the changes of molecular weight of PLA from the electrospun

PLA/cellulose nanocomposite fibers during hydrolytic degradation. The molecular

weight decreased exponentially over the degradation process, indicating almost

simultaneous degradation on the surface and in the interior of the material. This

is a typical characteristic of bulk degradation mechanism of polymers [25]. The


Keck SEMMag = 20.00 KX Mag = 20.00 KX

200 nm

(a)

(c)

(b)

200 nmWD = 5 mmFile name = 0%_05.tif

WD = 7 mmFile name = 0%cell_week8_05.tif

EHT = 3.00 kV EHT = 3.00 kVAperture size = 30.00 μm

Signal A = InLensAperture size = 20.00 μm

Signal A = InLens

Date = 5 Oct 2007

Time : 16:52:46Date = 28 Mar 2008

Time : 12:00:01

Keck SEM

Mag = 20.00 KX

200 nm WD = 6 mm

File name = 0%cellpla_15kweek8_11.tif

EHT = 3.00 kV Aperture size = 20.00 μm

Signal A = InLens

Date = 7 May 2008

Time : 16:17:09Keck SEM

Figure 6.6 FESEM images of electrospun PLA nanocomposite fibers spun from solu-tions containing 0% cellulose nanocrystals during hydrolytic degradation. (a) 0% 0 week,(b) 0% 8 weeks, (c) 0% 15 weeks.

slopes of the curves are considered as the hydrolytic degradation rate. PLA/cellulosenanocomposite fibers spun from solutions containing 10% cellulose nanocrystalsdegraded faster than the nanocomposite fibers spun from solutions containing 0and 1% cellulose nanocrystals degraded. Huang et al. [26] reported that the in vitrodegradation of PLA started with the absorption of water, followed by the hydrolyticcleavage of ester bonds, which generates chain fragments with acidic end groups.The absorption of water is the first step and hydrolytic cleavage of ester bondsis the second step in polymer degradation. Hydrolytic degradation results followthe same pattern as the contact angle and moisture absorbance results above.Incorporation of 1% wt cellulose did not significantly change the wetting behavioror the degradation behavior of the electrospun PLA/cellulose nanocomposite fibersas compared to the neat PLA fibers. The electrospun PLA/cellulose nanocompositefibers spun from solutions containing 10 wt% cellulose nanocrystals absorbedmore water than nanocomposite fibers spun from solutions containing 0 and1% cellulose nanocrystals and also degraded at a significantly more rapid rate.Although swelling was observed for all fibers, the addition of cellulose nanocrystals


Mag = 20.00 KX

200 nm WD = 5 mm

(a) (b)

(c)

File name = 1%_04.tif


Signal A = InLens

Date :5 Oct 2007


Mag = 20.00 KX

200 nm WD = 9 mm

File name = 1%_8weeks._06.tif


Signal A = InLens

Date :21 May 2008

Time : 15:06:10

Keck SEM

Mag = 20.00 KX

200 nm WD = 7 mm

File name = 1%cell.pla_15weeks._13.tif


Signal A = InLens

Date :13 May 2008

Time : 14:03:28

Keck SEM

Figure 6.7 FESEM images of electrospun PLA/cellulose nanocomposite fibers spun fromsolutions containing 1% cellulose nanocrystals during hydrolytic degradation. (a) 1% 0week, (b) 1% 8 weeks, (c) 1% 15 weeks.

to the electrospun PLA/cellulose nanocomposite fibers accelerated the hydrolyticdegradation. If the increased PLA degradation rate is related to residual acidity at thecellulose surface, the well-known autocatalytic PLA degradation could be expectedfor this system. Biomodal profiles of SEC images of PLA aging in phosphate bufferevidence the occurrence of autocatalysis [24]. The monomodal SEC patterns ofPLA confirmed that no autocatalysis occurred during hydrolytic degradation ofelectrospun PLA/cellulose nanocomposite fibers.

There is a linear relationship between LnM and degradation time, which fitsPitt’s equation [7]. The linear relationship between LnM and the degradationtime suggests that the hydrolytic degradation of the electrospun PLA/cellulosenanocomposite fibers in PBS (pH 7.4) proceeded via a random chain scissionreaction. According to the following exponential relationship between molecularweight and degradation time,

lg M = lg M0 –𝐾𝑡 (6.5)

the apparent degradation rate, K, can be obtained. The degradation half-time, t1/2

can further be calculated by Eq. (6.6) [26]:


Mag = 20.00 KX

200 nm WD = 7 mm

(a) (b)

(c)

File name = 10%cell.pLA_orig._03.tif


Signal A = InLens

Date :13 May 2008


Mag = 20.00 KX

200 nm WD = 5 mm

File name = 10%cell.PLA_8weeks._23.tif


Signal A = InLens

Date :13 May 2008

Time : 16:31:42

Keck SEM

Mag = 20.00 KX

200 nm WD = 7 mm

File name = 10%cell.pla_15weeks._16.tif


Signal A = InLens

Date :7 May 2008

Time : 14:56:08

Keck SEM

Figure 6.8 FESEM images of electrospun PLA/cellulose nanocomposite fibers spun fromsolutions containing 10% cellulose nanocrystals during hydrolytic degradation. (a) 10% 0week, (b) 10% 8 weeks, (c) 10% 15 weeks.

0

100

200

300

400

500

600

700

800

0 1 10Cellulose content (%)

Fib

er

dia

me

ter

(nm

)

0 weeks

8 weeks

15 weeks

Figure 6.9 Average fiber diameters of electrospinning PLA/cellulose nanocomposite fibersduring hydrolytic degradation. Values are given as mean± SD.


0

20000

40000

60000

80000

100000

120000

140000

160000

180000

200000

0 2 4 6 8 10 12 14 16 18

Degradation time (weeks)

Mw o

f P

LA

0% Cellulose–PLA

1% Cellulose–PLA

10% Cellulose–PLA

Figure 6.10 Molecular weight changes of poly(lactic acid) as a function of degradationtime in phosphate buffer solution (pH 7.4) at 37 ◦C.

t1∕2 = ln 2K

(6.6)

The apparent degradation rates for the electrospun PLA/cellulose nanocom-posite fibers without cellulose nanocrystals incorporated in were calculated tobe 0.0068 LnM per week (R2 = 97.77%) based on the weight-average molecularweight of PLA, for the electrospun PLA/cellulose nanocomposite fibers spunfrom solutions containing 1% cellulose nanocrystals were calculated to be0.0084 LnM per week (R2 = 96.72%), and for the electrospun PLA/cellulosenanocomposite fibers spun from solutions containing 10% cellulose nanocrystalswere calculated to be 0.0128 LnM per week (R2 = 97.67%). Degradation half-timesderived from molecular weight of PLA were about 44, 36, and 24 weeks forthe electrospun PLA/cellulose nanocomposite fibers spun from solutionscontaining 0, 1, and 10% cellulose nanocrystals, respectively. The hydrolyticdegradation rate of the electrospun PLA/cellulose nanocomposite fibers spunfrom solutions containing 10% cellulose nanocrystals was faster than thatof nanocomposite fibers spun from solutions containing 0 and 1% cellulosenanocrystals.

Figure 6.11 shows the polydispersity indices (I =Mw/Mn) of PLA in theelectrospun PLA/cellulose nanocomposite fibers as a function of degradationtime in PBS (pH 7.4) at 37 ◦C. The polydispersity indices of PLA did notchange significantly during hydrolytic degradation. The molecular weightof PLA decreased with hydrolytic degradation but the polydispersity indicesremained nearly unchanged, which suggests a random chain cleavagerather than an unzipping process [27]. The polydispersity indices of PLA


0

0.5

1

1.5

2

2.5

0 2 4 6 8 10 12 14 16 18

Degradation time (weeks)

PD

I (M

w/M

n)

0% Cellulose–PLA

1% Cellulose–PLA

10% Cellulose–PLA

Figure 6.11 Polydispersity indices (I=Mw/Mn) of poly(lactic acid) as a function of degra-dation time in phosphate buffer solution (pH 7.4) at 37 ◦C.

during hydrolytic degradation further confirmed a random chain scissionreaction.

Figures 6.12–6.14 show the molecular weight distribution (MWD) of PLAcontaining 0, 1, and 10% cellulose nanocrystals via SEC measurement duringthe hydrolytic degradation, respectively. The MWD indicated a homogeneouslyhydrolytic degradation of the electrospun PLA nanocomposite fibers spun from

0 200000 400000 600000 800000 10000001200000

Molar mass (g mol−1)

0%_0 weeks

0%_4 weeks

0%_8 weeks

0%_13 weeks

0%_16 weeks

Figure 6.12 Molecular weight distribution of pure electrospun PLA during hydrolyticdegradation.


0 200000 400000 600000 800000 1000000 1200000


1%_0 weeks

1%_4 weeks

1%_8 weeks

1%_13 weeks

1%_16 weeks

Figure 6.13 Molecular weight distribution of electrospun PLA with 1% cellulose nanocrys-tals during hydrolytic degradation.

0 200000 400000 600000 800000 1000000


10%_0 weeks

10%_4 weeks

10%_8 weeks

10%_13 weeks

10%_16 weeks

Figure 6.14 Molecular weight distribution of electrospun PLA with 10% cellulose nanocrys-tals during hydrolytic degradation.


solutions containing 0, 1, and 10% cellulose nanocrystals. As expected, no auto-catalysis was seen in these samples.

6.4Conclusions

Cellulose at the surface of PLA fibers decreased the hydrophobicity of the resultingelectrospun nonwoven fabrics as evidenced by the water contact angle and waterabsorption of the fabrics. TGA suggested strong PLA/cellulose nanocrystal inter-actions at 1% filler loading and agglomeration or poor PLA/cellulose nanocrystalinteractions at 10% filler loadings. The electrospun PLA/cellulose nanocompositefibers became rougher and swelled during hydrolytic degradation in PBS (pH 7.4)at 37 ◦C. The apparent degradation rates based on the molecular weight of PLAwere calculated to be 0.0068, 0.0084, and 0.0128 LnM per week for the electrospunPLA/cellulose nanocomposite fibers spun from solutions containing 0, 1, and 10%cellulose nanocrystals, respectively. Degradation half-times derived from molecularweight of PLA are about 44, 36, and 24 weeks for the electrospun PLA/cellulosenanocomposite fibers spun from solutions containing 0, 1, and 10% cellulosenanocrystals, respectively. Although the addition of 1% cellulose nanocrystalsresulted in an insignificant difference in wetting behavior of the resulting nonwo-ven fabric, a significant increase in the hydrolytic degradation rate was achieved.Increasing the cellulose nanocrystal composition from 1 to 10% in the nanocompos-ite fibers further increased the PLA degradation rate. The increase in degradationrate cannot be explained by the increased interaction between water and PLAalone and may also be attributed to the influence of cellulose nanocrystals on fiberdiameter. A linear relationship between LnM and the degradation time suggeststhat the hydrolytic degradation of the electrospun PLA/cellulose nanocompositefibers in PBS (pH 7.4, 37 ◦C) were a random chain scission reaction. Polydispersityindices of PLA did not change significantly. The polydispersity indices of PLAduring hydrolytic degradation further confirmed a random chain scission degra-dation mechanism. The MWD indicated a homogeneously hydrolytic degradationof the electrospun PLA nanocomposite fibers spun from solutions containing 0, 1,and 10% cellulose nanocrystals. No autocatalytic degradation occurred during thehydrolytic degradation of electrospun PLA/cellulose nanocomposite fibers.

Acknowledgment

This research was supported by the Cornell University Agricultural ExperimentStation federal formula funds, Project No. NYC-329415 received from the Coop-erative State Research, Education, and Extension Service, U.S. Department ofAgriculture. We would like to thank the Cornell Center for Materials Research(CCMR), a materials research science and engineering center of the NationalScience Foundation.

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Cellulose Based Composites (New Green Nanomaterials) || Hydrolytic Degradation of Nanocomposite Fibers Electrospun from Poly(Lactic Acid)/Cellulose Nanocrystals