ORIGINAL PAPER

Physical structure and thermal behavior of ethylcellulose

M. Davidovich-Pinhas S. Barbut

A. G. Marangoni

Received: 24 May 2014 / Accepted: 23 July 2014 / Published online: 1 August 2014

Springer Science+Business Media Dordrecht 2014

Abstract The physical structure and properties of

ethylcellulose (EC) powders of different molecular

weights were examined. A molecular weight in the

range of 20144 kDa with a large polydispersity was

determined. EC thermal analysis revealed a glass

transition at *130 C and a melting temperature at*180 C. Glass transition temperatures increasedwith polymer molecular weight. Wide angle (WAXS)

analysis detected an amorphous broad peak at

q = 1.5 A-1 and a distinct Braggs peak at 12.6 A,

which seems to be related to a supramolecular ordered

structure of the polymer. These observations were

confirmed using high temperature powder X-ray

diffraction analysis where the crystalline peak disap-

peared above the melting temperature of the polymer.

Ultra-small angle (USAXS) results were fitted to the

Bouacage fractal unified model and fractals with an

average size of 100600 nm with a relatively smooth

surface were predicted. This prediction was confirmed

by transmission electron microscopy (TEM) images.

According to our results, the EC polymer has a semi-

crystalline structure, with crystalline domains within

an amorphous background.

Keywords Ethyl cellulose Semi-crystalline Powder Fractal X-ray scattering

Introduction

Organisms such as plants, algae and some bacteria

produce different types of carbohydrate polymers due

to their ability to photosynthetically fix carbon diox-

ide. Approximately half of this biomass is comprised

of the cellulose biopolymer which is an important

structural component of the cell wall of many plants.

Cellulose and its derivatives are important industrial

materials used widely in the food, pharmaceutical,

plastic, textile and cosmetic industries (Perez and

Samain 2010).

Ethylcellulose (EC) is a linear polysaccharide

derived from cellulose. Its commercial production

involves the replacement of the celluloses hydroxyl

end groups with ethyl end groups. The synthesis of EC

comprises the dissolution of cellulose in an alkali

solution in order to break down the cellulose supra-

molecular structure followed by the addition of ethyl

chloride gas which interacts with the alkalized cellu-

lose (Atalla and Isogai 1998). The relative reactivity of

the cellulose hydroxyl end groups was found to be

OHC6 OHC2 [ OHC3 (Roy et al. 2009). Thefinal product is characterized by the degree of

substitution (DS) or ethoxy content. Water solubility

is achieved with DS in the range of 1.01.5 while

solubility in organic solvents is achieved with DS

values in the range of 2.42.5 (Koch 1937).

Ethylcellulose displays a variety of properties

allowing it to be used in a wide range of applications.

Its physical properties such as high flexibility,

M. Davidovich-Pinhas S. Barbut A. G. Marangoni (&)University of Guelph, Guelph, ON, Canada

e-mail: amarango@uoguelph.ca

123

Cellulose (2014) 21:32433255

DOI 10.1007/s10570-014-0377-1

thermoplasticity, considerable mechanical strength,

film forming ability, toughness and transparency allow

it to be used in coating applications in a variety of

products (Koch 1937). Its compatibility with organic

materials allows it to be used as a rheology modifier in

films, binders, adhesives, and hot blends with other

polymers and ceramic (Knill and Kennedy 1998). In

addition its tasteless, odorless, non-caloric and phys-

iological inert character, make it a suitable candidate

to be used in pharmaceutical (Rekhi and Jambhekar

1995), personal care products (Aiache et al. 1992) and

foods (Hughes et al. 2009; Zetzl et al. 2012). Several

pharmaceutical applications include using EC as a

drug carrier in dry tablets (Crowley et al. 2004; Maki

et al. 2006; Repka et al. 2007; Yu et al. 2006) where

the EC powder is either directly compressed with the

drug (Maki et al. 2006; Yu et al. 2006) or combined

with the drug using a hot-melt extrusion technique

(Repka et al. 2007). This design allows the control of

the drug release according to the tablets structure and

properties, based on pore size, glass transition and

melting temperature, which are directly related to the

powder characteristics. Thus studying the EC powder

structure and properties could potentially contribute to

better understanding EC behavior and function during

processing and handling in the above mentioned

applications.

In this study, we characterize EC powder structure

and properties using a variety of techniques. EC

molecular weight was determined using high perfor-

mance liquid chromatography (HPLC), while powder

thermal properties were analyzed by means of differ-

ential scanning calorimetry. ECs solid state structure

was examined using powder X-ray diffraction in the

wide angle (WAXS), small angle (SAXS) and ultra-

small angle (USAXS) regions. USAXS results were

analyzed using the Beaucage unified model fit and the

proposed supramolecular powder structure confirmed

by transmission electron microscopy (TEM).

Materials and methods

Materials

Ethylcellulose, EthocelTM brand of different viscosi-

ties (4 cP, 10 cP, 20 cP, 45 cP, 100 cP, 300 cP) with

degree of substitution of *2.5 were obtained fromDow Chemical Company (Midland, MI, USA) and

used as received. Viscosities of a 5 % (w/v) polymer

solution in 80 % toluene and 20 % ethanol solution

measured at 25 C are reported by the manufacturer asan indication for the average molecular weight of the

polymer (Ethylcellulose polymers technical handbook

2005).

Molecular weight determination

The molecular weights of the different EC samples

were determined using size-exclusion-high-perfor-

mance liquid chromatography (SE-HPLC).

The following instrumentation was used: Spectra

Systems (model SCM100, Providence, RI, USA),

degasser, pump (P1000), auto sampler (AS3500) and

control unit (SN4000). A Sedex75 Evaporative Light

Scattering Detector (SEDERE, Alfortville Cedex,

France) was connected to a computer through a

NCI900 Network Chromatography Interface (Perkin-

Elmer, Woodbridge, ON, Canada). For an effective

SE-HPLC procedure, four Styragel columns (Waters

Corporation Mississauga, ON, Canada) were con-

nected in the following order: (1) Guard column (2)

HR 5E (effective Mw range 2 9 1034 9 106), (3)

HR 5E and (4) HR 4E (effective Mw range 5 9

101105). EC samples were dissolved in toluene to a

final concentration of 0.20.3 % (w/v) as recom-

mended by the column manufacturer. The amount

injected was 20 ll; the flow rate was 0.3 ml/min; thecolumns were operated at room temperature. The same

sample was injected three (n = 3) times in order to

check the reproducibility of the technique.

A calibration curve was obtained using polystyrene

standards (ReadyCal Standards, Polymer Standards

Service, Amherst, MA, USA). The kit consists of 3

vials (named Green, Red and White), each containing

4 polystyrene standards with a narrow molecular

weight range. The standards were prepared according

to the manufacturers instructions and analyzed using

the above mentioned instrument. A calibration curve

was obtained from the ReadyCal standards chromato-

grams, based on the elution time and the peak

molecular weight (Mp). An equation Mp = A exp

(-kt) was constructed and further used for the

determination of EC peak molecular weight. The

polydispersity index (PDI) was calculated from

the ratio PDI = Mw/Mn. The mass molecular weight,

Mw, and the number molecular weight, Mn, were

calculated using the following equations,

3244 Cellulose (2014) 21:32433255

123

Mn P

i hiP

ihi=Mi

1

Mw P

i hi MiPi hi

2

here hi is the height (from baseline) of the chromato-

gram curve at the ith elution increment and Mi is the

molecular weight of species eluting at this increment.

Mi is calculated from the calibration curve.

Differential scanning calorimetry (DSC)

Powder thermal behavior was analyzed using Mettler-

Toledo DSC 1 instrument (Mississauga, ON, Canada).

67 mg of EC powder was placed in a sealed

aluminum pan. Experiments were conducted using

5 min-1 heating/cooling rate with 3040 ml/minnitrogen flow rate. Two heatingcooling runs from 25

to 200 C were carried out for each sample.

Room temperature X-ray analysis

Wide angle X-ray scattering (WAXS) data was

collected using a Rigaku Multiplex Powder X-ray

diffractometer (Rigaku, Tokyo, Japan). The apparatus

was set with a 1/2 divergence slit, a 1/2 scatter slitand 0.3 mm receiving slit. The accelerating voltage

and current of the X-ray copper tube were set at 40 kV

and 44 mA, respectively. Scans were performed from

1.135 at a scanning rate of 0.3 min-1. Intensity as afunction of 2h data was converted to q using q = 4psin(h)/k, where k = 1.54 A for copper. Powder sam-ples were mounted on a glass sample holder and

introduced into the instrument at room temperature.

Ultra Small Angle X-ray Scattering (USAXS) and

Small Angle X-ray Scattering (SAXS) experiments

were conducted in the APS Argonne synchrotron

facility (Chicago, IL, USA) sector 15 (ChemMat-

CARS). The USAXS instrument included optional

SAXS camera connected to a Pilatus 100 k detector

(Dectris, Switzerland). USAXS experiments were

performed with 24 keV X-ray energy (Ilavsky et al.

2009, 2012). SAXS experiments were performed with

30 keV X-ray energy and 1.8 m sample-detector

distance (Freelon et al. 2013). Data reduction and

analysis was done using Nika (Ilavsky 2012) and Irena

(Ilavsky and Jemian 2009) software. EC powder was

mounted within a Grace-Bio silicone sample holder

(Grace Bio-Labs, Bend, Oregon) and sandwiched

between two glass cover slits.

The scattering pattern from the empty cell was

subtracted from all samples.

High temperature X-ray analysis

High temperature X-ray analysis was performed in the

APS Argonne synchrotron facility (Chicago, IL, USA)

sector 15 (ChemMatCARS). The USAXS instrument

included optional SAXS and WAXS cameras each

connected to Pilatus 100 k detector (Dectris, Switzer-

land). USAXS experiments were performed with

24 keV X-ray energy (Ilavsky et al. 2009, 2012). Data

collection for the SAXS and WAXS region was

performed using two different detectors with 0.5 and

0.2 m sample-detector distance, respectively. Data

reduction and analysis was done using Nika (Ilavsky

2012) and Irena (Ilavsky and Jemian 2009) software.

High temperature experiments were performed

using a temperature control stage (Linkam Scientific

Instruments, Tadworth, Surrey, UK). EC 10 cP

powder was mounted within a Grace-Bio silicone

sample holder (Grace Bio-Labs, Bend, Oregon) and

sandwiched between two glass cover slits. Experi-

ments were conducted using 10 C/min cooling/heat-ing rate and the data was collected at 25, 120, 150 and

200 C during the heating and at 25 C after cooling.The WAXS/SAXS data was analyzed using Peakfit

software (System Software, San Jose, CA, USA)

where the area of each peak was determined. The

peaks were fitted using exponential baseline correc-

tion and Gaussian convolution smoothing. An asym-

metric double Gaussian cumulative peak type with

varied width and shape was used. The ratio between

the Braggs peak at q = 0.5 A-1, ABragg, and the

amorphous peak at q = 1.5 A-1, Aamorphous, were

determined.

Transmission electron microscopy (TEM)

The powder sample was fixed by embedding it in Epon

resin (Canemco & Marivac Ltd, Lakefield, Quebec,

Canada), creating 80 nm thick sections and mounting

it on a 200 mesh copper/formvar grids (Canemco &

Marivac Ltd, Lakefield, Quebec, Canada). TEM

images were recorded using Tecnai G2 F2 instrument

(FEI, Hillsboro, OR, USA) at 120 keV equipped with

a Gatan 4 K CCD camera (Gatan Inc., Warrendale,

Cellulose (2014) 21:32433255 3245

123

PA, USA) connected to Digital Micrograph software

(Gatan Inc., Warrendale, PA, USA).

Results and discussion

Molecular weight determination

Most experimental techniques used to determine EC

molecular weight have been based on the Mark-

Houwink equation (Moore and Brown 1959; Morris

et al. 1981). This equation relates the polymers

intrinsic viscosity to its molecular weight using the

parameters a and k,

g k Ma 3Both a and k are determined experimentally for

specific polymer, solvent and temperature.

Here we use high performance liquid chromatog-

raphy (HPLC) to determine EC molecular weight. The

chromatograms of six types of EC are presented in

Fig. 1. As can be seen from the resolution of the peaks,

the HPLC system was capable of separating the EC

polymers effectively.

Overall, as evidenced by the broad peak and

calculated PDI (Table 1), the samples are very poly-

disperse. Polydispersity has a value of 1 for macro-

molecules with a single molecular weight

(monodisperse), and the value increases with increas-

ing polydispersity. Large value and wide range of PDI

values can be found for various synthetic polymers

(Ward 1981).

The peak molecular weight of the EC was deter-

mined using the standards chromatograms. A cali-

bration curve was obtained from the standards elution

time and the peak molecular weight (Mp),

Mp 9:931 109 e0:624t 4The elution time (Table 1) corresponding to the

peak maximum for each sample was determined

manually from the data and converted to MP using

the above equation.

Thermal analysis

Figure 2 presents typical DSC thermogram obtained

for EC 45 cP. Results suggest the existence of two

reversible thermal events occurring at approximately

130 and 180 C during heating and at 120 and 180 C

Fig. 1 Chromatograms obtained for the various ethylcellulosesamples

Table 1 Estimated peak molecular weight (Mp) and polydis-persity index (PDI) calculated from HPLC data

Ethyl

cellulose

type

Peak elution

time (min)

Estimated peak

molecular weight,

Mp (kDa)

PDI

4 cP 32.19 0.13 19 1.5 59.5 24.8

10 cP 31.55 0.37 28.6 6.2 51.8 13.5

20 cP 30.59 0.33 51.9 10 56.0 4.5

45 cP 30.05 0.32 72.8 15 81.3 3.6

100 cP 29.91 0.45 80.8 24 102.5 10.5

300 cP 28.95 0.28 144.1 24.4 229.7 122

Values represent the average and standard deviation of three

runs of each sample

Fig. 2 Typical DSC thermogram obtained for EC 45 cP. Firstheating/cooling step (solid line), second heating/cooling step

(dash line)

3246 Cellulose (2014) 21:32433255

123

during cooling. The broad endothermic peak seen at

temperature of up to 100 C during the first heatingrun arises from water loss which typically occurs in

this temperature range (Soares et al. 2004). This peak

was absent from the second heating run.

The thermal event observed at*130 C representsthe EC glass transition temperature which is in

agreement with the manufacturers data sheet and

other published data on EC, reporting transition

temperature in the range of 120135 C (Crowleyet al. 2004; Rowe et al. 1984; Sakellariou et al. 1985;

Tarvainen et al. 2003). A glass transition can occur in

many types of polymers, from linear chain amorphous

polymers to complex grafted co-polymers, as well as

for partially crystalline polymers (Overney et al.

2000). The reversible event, termed the vitrification

process, was identified during cooling at approxi-

mately 120 C for all EC samples, suggesting thermalhysteresis.

The second endothermic thermal event corresponds

to the melting phase transition of EC. According to the

manufacturer datasheet, the melting temperature of

this polymer is in the range of 165173 C. Thereversible exothermic transition, or crystallization

phase transition, was detected at a lower temperature

during cooling, suggesting thermal hysteresis for this

transition as well. It would seem that the material

needs to be undercooled for nucleation and crystal

growth to commence.

Figure 2, reveals reversible phase behavior found

during two cycles of cooling/heating runs. Such

behavior suggests thermal stability of the EC polymer

backbone up to, at least, 200 C. Several studies havesuggested that EC decomposition process begins

around 200 C (Cavalcanti et al. 2004; Follonieret al. 1994). It should be noted that the relatively high

thermal stability of EC contributes to its use in

application such as plastics, food and ceramics; all

require processing at high temperatures.

Figure 3 shows the DSC thermograms obtained for

each EC sample. As mentioned above, the heating

process reveals two thermal events, or phase transi-

tions, the glass transition and the melting transition

while the cooling process reveals two reversible

transitions as well for vitrification and crystallization,

respectively. During the heating stage we identified a

shift in the glass transition and melting peak maxima

with increasing polymer molecular weight. A decrease

in crystallization transition temperature peak with

decrease in polymer molecular weight was identified.

This behavior was found in both the first and second

heating/cooling runs.

Fig. 3 DSC thermogramsobtained for all EC samples

during first (a) and second(c) heating stage and first(b) and second (d) coolingstage using 5 C/mincooling/heating rates

Cellulose (2014) 21:32433255 3247

123

Figure 4 shows both glass transition and melting

temperatures (during cooling and heating stages). The

analysis reveals an increase in glass transition tem-

perature, Tg, with increasing molecular weight regard-

less of the heating/cooling stages. According to the

Flory-Fox model, an increase in Tg value with

increasing polymer molecular weight is predicted

(Fox and Flory 1950). Closer look at the Tg behavior,

during the heating/cooling stages, reveals higher Tgvalues at the first heating stage compare to the second

stage. It appears from the results (Fig. 4a) that the Tgdecreases after the first run and remains constant in all

subsequent runs. It is possible that the first heating run

eliminated local inhomogenities which induce a

higher Tg value (Overney et al. 2000; Roudaut et al.

2004). It is possible that due to the hydrophobic nature

of EC the water present in the powder is responsible

for this inhomogeneity which disappears after the first

heating run due to water loss.

The melting/crystallization transitions also display

a positive relationship with polymer molecular weight.

Increase in melting/crystallization temperatures with

polymer molecular weight were detected regardless of

the heating/cooling stages. Previous work done with

synthetic semi-crystallite polymers has shown an

increase in the melting temperature with increasing

polymer molecular weight (Flory and Vrij 1963;

Gopalan and Mandelkern 1967). Similar behavior was

observed by Roos and Karel (1991) who worked with

various food polysaccharides. The melting or crystal-

lization transition appears to be reversible with respect

to the first and second runs, where the same melting

temperature and crystallization temperature were

obtained for both runs. However, higher melting

temperatures, compared to crystallization tempera-

tures, were detected in all EC samples. Such hysteresis

behavior was also observed in gellan, kappa carra-

geenan and agar gels (Sandford et al. 1984).

The presence of both a reversible glass transition as

well as melting transition in all EC samples suggests a

semi-crystallite polymer behavior. Therefore it can be

concluded that according to the thermal data, the EC

consist of both ordered crystallite and dis-

ordered amorphous areas. This observation will be

further discussed in the following section.

Structure analysis

Figure 5 shows the X-ray diffraction spectra obtained

from all three techniques (WAXS, SAXS and

USAXS) for EC 10 cP (5a) and EC 100 cP (5b) at

room temperature. Both samples have similar scatter-

ing patterns suggesting similar powder structures. The

data spans a wide range of q values

(10-5 \ q \ 10?1). Due to the wide range of lengthscales involved, the data will be analyzed separately.

The SAXS/WAXS region (q [ 0.1 A-1) revealstwo characteristic peaks located at approximately

q = 0.5 A-1 and q = 1.5 A-1 for both EC samples.

Previous work on cellulose and methylated celluloses

assigned the peak centered at around q = 1.5 A-1 to

the presence of amorphous polymer chains (Kondo

and Sawatari 1996). In this study, fully amorphous

samples were prepared by dissolving cellulose and its

derivatives in different organic solvents (i.e., metha-

nol, chloroform or N,N-dimethylacetamide) for few

days in order to destroy the polymers supra-molecular

structure. Other studies on amorphous cellulose also

correlated this peak to the presence of disordered

amorphous regions (Jeffries 1968; Nelson and

Fig. 4 Glass transition temperatures, Tg (a) and meltingtemperatures, Tm (b) obtained from the DSC thermograms

3248 Cellulose (2014) 21:32433255

123

OConner 1964). The broad Bragg peak at q = 0.5

A-1 corresponds to a lattice parameter of 12.6 A.

These results suggest some level of order within the

amorphous background, in agreement with DSC

results mentioned above, suggesting the existence of

both amorphous and crystalline regions within the EC

polymer.

According to the manufacturers data sheet, EC

synthesis involves alkaline treatment of the native

cellulose, in order to create alkali cellulose, followed

by the addition of ethyl chloride gas, leading to the

final EC product. Several studies have focused on the

effects of alkaline treatment on cellulose structure

with an emphasis on the order/disorder transition

(Isogai and Atalla 1998; Jeffries 1968; Zuluaga et al.

2009). This transition occurs due to hydrogen-bond

breaking which leads to rearrangement of the inter-

molecular network in order to stabilize the conforma-

tion, even in the amorphous state (Nelson and

OConner 1964). It is possible that this treatment

contributed to the dis-ordered structure demonstrated

by the wide and broad peaks obtained in the EC XRD

patterns. Furthermore, it was suggested that this

treatment interferes with chain association responsible

for the formation of cellulose crystalline structure.

Cellulose crystalline structure is based on a 3D

arrangement of the polymer chains via intra- and inter-

chain hydrogen bonds (French 2012; Osullivan

1997). Different crystallite unit cell dimensions were

determined for different cellulose types and sources.

However, an agreement was reported for the cellulose

subunit length along the polymer chain of 10.4 A

(dimer length), regardless of the polymer source or

type (Osullivan 1997). Hydrogen bonding interac-

tions are possible between the three free hydroxyl end

groups present in the cellulose backbone. More

specifically, intra-chain association can take place

between OH end-group at the C2 with OH end-group at

the C6 position of neighboring rings or OH end-group

at the C3 position with the O in neighboring rings from

the same polymer backbone. Inter-chain interactions

are available between OH end-group at the C2, C3, and

C6 positions from different polymer chains.

It was found that the substitution of each OH end-

group in the EC glucose monomers has different

effects on the final ability of the polymer to crystallize.

More specifically, it seems that the C6 position may be

favorably involved in inter-chain hydrogen bonding,

which leads to chain association (Kondo 1998).

However, the C6 hydroxyl has the higher relative

reactivity for substitution (Roy et al. 2009). In

addition, according to the manufacturers handbook,

EC production involves the random addition of ethoxy

groups to the native OH groups, inducing a very high

degree of substitution (2.5 out of 3.0). These will

translate to a large decrease in the hydrogen bonding

ability of the polymer. However, it appears that the EC

polymer chains still have the ability to form a semi-

crystalline structure via the remaining H-bonds and

possibly via van der Waals interactions (Kondo 1998).

Previous XRD and computational molecular modeling

work has demonstrated the important role of van der

Waals forces in cellulose crystallization (Agarwal

et al. 2011; Cousins and Brown 1995). Therefore, we

can assume that EC molecules arrange into a different

crystalline structure compared to a native cellulose

crystal.

Previous work on the effects of high temperature on

cellulose structure demonstrated a lateral expansion of

the unit cell upon temperature increase, meaning

Fig. 5 USAXS (solid), SAXS (dash) and WAXS (dark graysolid) data obtained for EC 10 cP (a) and EC 100 cP (b)

Cellulose (2014) 21:32433255 3249

123

increase in the distance between chains due to

disruption of inter-chain hydrogen bonds. However,

changes in the repeating length along the polymer

chain, i.e., the dimer dimension, were not detected

(Agarwal et al. 2011; Wada 2002; Wada et al. 2010).

This observation could explain the presence of only

one characteristic lattice parameter in the EC samples,

where the derivatization process leads to a reduction in

hydrogen bonding ability and lateral association

between the chains. We obtained a characteristic

length of 12.6 A which has a similar order of

magnitude as the dimer length in a native cellulose

chain (Osullivan 1997).

The USAXS/SAXS q region (q \ 0.1 A-1) revealsa complex system having three characteristic slopes

(Fig. 5). Such a system can be described by the unified

model developed by Beaucage (1995, 1996). The

unified model describes systems over a wide range of q

values in terms of structural levels. A structural level

in scattering is described by the Guiniers law, and a

power law, which on a loglog scale is reflected by a

knee and a linear region, respectively. In this model,

both Guinier and Porod regimes are combined in a

single equation which describes the scattering I(q) of

any system containing a random distribution of

structures,

Iq Xn

i1G exp q

2R2g;i

3

!

B exp q2R2g;i13

!erf 3

qRg;i6

p

q

2

4

3

5

Pi 5

The first term describes the Guinier region which

represents scatterers having approximately spherical

structure characterized by an average radius of gyra-

tion, Rg. The second term describes the power-law

region characterized by an exponent P which provides

information regarding the nature of the particles

described by the Guinier region. G and B are the

Guinier and the power law scale factors, respectively.

Figure 6 shows the USAXS/SAXS data and corre-

sponding model fits obtained by assuming three levels

in the unified model using the Irena package software

(Ilavsky and Jemian 2009) implanted in Igor (Wave-

Metrics, Portland, OR, USA). The parameters

obtained from the model fit are summarized in

Table 2. From the fit parameters, it appears that both

samples share similar characteristic length scales and

slopes.

The radius of gyration obtained for both samples

suggests structures having an average length scale of

between 1,000 to 6,000 A. Such a length scale could

represent either large particles or large ensembles of

smaller particles (aggregates or clusters) where the

primary particles were not detected in the X-ray

results.

The power law slope values are analyzed with

respect to the q region obtained. In the higher q regime

the scattering intensity decays according to Porod law

for surface fractal structure (Schaefer and Hurd 1990):

I q q6Ds 6where Ds is the surface fractal dimension. We have

obtained a slope of -3.8 which can be referred to as

surface fractal dimension, Ds, of 2.2 suggesting a

relatively smooth surface (Schaefer and Hurd 1990).

Fig. 6 USAXS data and the unified fit (solid line) obtainedusing Irena package (Ilavsky and Jemian 2009) for EC 10cP

(a) and EC 100cP (b)

3250 Cellulose (2014) 21:32433255

123

In the intermediate q-value regime the scattering

curve of mass fractal structure can be described as

(Schaefer and Hurd 1990):

I q qDf 7where Df is the mass fractal dimension. Fractal

dimensions can be non-integer or fractional and can

vary between 1 and 3 for an object embedded in a 3D

space (Schaefer and Hurd 1990). We have obtained a

mass fractal dimension of 2.8 for both samples,

suggesting a spherical structure. Skillas et al. (2002)

reported a fractal dimension of 2.5 and 2.7 for two

different organic pigment powder samples embedded

in poly(methyl-methacrylate) matrix.

In conclusion, the USAXS analysis suggests a

fractal structure of smaller particles having an average

aggregate dimension of 1,0006,000 A with a semi-

smooth surface for both EC 10cP and EC 100cP

samples. In order to verify these results TEM images

of EC 100 cP powder were taken (Fig. 7).

Transmission electron microscopy images of the

EC powder are presented in Fig. 7. Micrographs

suggest that the EC primary powder particles are

spherical and form aggregates with a relatively smooth

surface, with effective dimension of a couple hundred

nm. This result is in agreement with the USAXS

analysis. Previous work on EC particles reported the

presence of aggregates of primary particles with a

mean size of approximately 4 lm, similar to ourresults (Duarte et al. 2006).

According to the data obtained from both X-ray

analysis and TEM imaging, it seems that the EC

powder forms density inhomogeneous colloidal aggre-

gates. Due to the shape of the aggregates seen by TEM

and the fractal dimension of 2.8 obtained from the

USAXS experiments, it seems that EC powder aggre-

gates could have been formed via a particle-cluster

diffusion limited aggregation process in 3-dimensions,

followed by some densification (Jullien 1987). Witten

and Sander (1983) developed the diffusion limited

aggregation model for particle-cluster aggregation.

This process assumes that the rate-limiting step in the

aggregation by Brownian motion is the diffusion of the

particles to the surface of the already existing aggre-

gate or to the initial seed particle (Witten and

Sander 1983). Based on their model, a fractal dimen-

sion of 2.5 is predicted for three dimensional diffusion

limited particle-cluster fractal structures. Further

studies have demonstrated an increase in the fractal

dimension for compact structures due to particle

rearrangement (Jullien 1987; Meakin and Jullien

1985) or denser fractal structures in powder samples

(Sinha et al. 1984). It was also suggested that

aggregation taking place under shear, yields results

that are different than aggregation occurring solely

due to diffusion. This results in the formation of

aggregates with more compact structures where the

fractal dimension can approach a value of 3 (Torres

et al. 1991).

The semi-crystalline nature of EC

In order to verify the semi-crystalline nature of EC a

high temperature X-ray experiment was performed on

EC 10 cP. Figure 8a shows the SAXS/WAXS results

obtained using elevating temperatures. It is evident

from the results that the peak located at q = 0.5 A-1,

corresponds to the ordered polymer region, disap-

pears during the temperature increase. According to

the DSC results, EC 10 cP melt at *178 C meaningthe disappearance of the peak can be correlated to the

melting of the crystalline regions. This result confirm

the assumption that the peak located at q = 0.5 A-1 is

a result of the crystalline structure of the polymer.

Figure 8b shows the USAXS data obtained at the

same temperature range for EC 10 cP. As can be seen

the microstructure illustrated by the powder and

analyzed by the unified model at room temperature

disappears after increasing the temperature above the

glass transition (i.e.,*130 C). All slopes presented inthe unified model converge to a slope of approximately

-4 during the temperature increase, indicating a larger

Table 2 The unified model fit parameters obtained for EC10cP and EC 100cP

Sample Parameter Level 1 Level 2 Level 3

EC 10 cP G 2.88e?06 3.26e?08

Rg [A] 1,106 5,984

B 4.63e-05 0.036 1.84e-05

P 3.79 2.76 3.65

EC 100 cP G 2.86e?07 3.03e?08

Rg [A] 1,935 4,785

B 7.95e-05 0.05 0.18e-03

P 3.78 2.8 3.42

Cellulose (2014) 21:32433255 3251

123

particle/aggregate size out of the USAXS detection

range. Therefore it can be concluded that the EC

microstructure collapses due to the heat treatment. It is

interesting to note that the microstructure starts to

change after the glass transition and completely

collapses beyond the melting transition while the

atomic scale structure, i.e., crystallinity, changes only

above the melting temperature.

The ratio between the Braggs peak and amorphous

peak areas, ABragg/Aamorphous, was determined from

SAXS/WAXS curves before and after the melting

using PeakFit software. The results, Fig. 8a, suggest a

ratio of crystalline ordered region to amorphous of

around 0.2 which decreases to 0.07 above the melting

temperature. It seems that the polymer crystalline

structure destroyed during melting and transform to

amorphous state.

Conclusions

Ethylcellulose powder characterization was carried

out by means of size-exclusion HPLC, thermal

analysis, X-ray scattering and TEM imaging.

Ethylcellulose molecular weight was determined in

the range of 20144 kDa with a large polydispersity.

EC thermal analysis revealed a glass transition at

*130 C and a melting temperature at *180 C forall EC samples (4, 10, 20, 45, 100, 300 cP). Reversible

vitrification and crystallization transitions were also

observed in all samples. An increase in the glass

transition, vitrification, melting and crystallization

temperatures with increasing polymer molecular

weight was observed. The presence of both a glass

transition as well as melting transition suggests a semi-

crystallite polymer structure.

Fig. 7 TEM micrographs of EC 100 cP powder embedded in Epon resin

3252 Cellulose (2014) 21:32433255

123

Ethylcellulose powder structure was analyzed by

means of X-ray scattering and electron microscopy. At

the atomic scale, we detected an amorphous broad

peak at q = 1.5 A-1 and a distinct Braggs peak at

12.6 A, which seems to be related to a supramolecular

polymer ordered structure. At higher length scales, a

fractal aggregate formed from the aggregation of

primary roughly spherical EC particles was proposed

for both EC 10 cP and EC 100 cP. The USAXS results

were fitted to the Bouacage fractal unified model and

fractals with an average size of 1,0006,000 A were

predicted. This prediction was confirmed using TEM

images. A fractal dimension of 2.8 was observed in

both samples. In addition, a smooth particle surface

was observed in the model fits and TEM images.

High temperature X-ray analysis demonstrated the

disappearance of the Braggs peak above the melting

temperature verifying its crystalline nature. Moreover,

the microstructure of the polymer collapsed during the

same temperature treatment. The degree of crystallinity

suggests a ratio of crystalline ordered region to

amorphous of around 0.2.

According to the results, the EC polymer has a

semi-crystalline structure, with a degree of order

within an amorphous content. The exact nature of the

ordered or crystalline regions is not clear and

therefore additional studies are required.

Acknowledgments Research supported by the OntarioMinistry of Agriculture and Food (OMAF) and the Natural

Sciences and Engineering Research Council of Canada (NSERC).

We acknowledge the technical assistance of Fernanda Peyronel

for setting up experiments and data analysis. The author wish to

thank Dr. Jan Ilavsky from the APS sector 15ID-D USAXS/SAXS

facility for his help conducting both SAXS and USAXS

experiments. ChemMatCARS Sector 15 is principally supported

by the National Science Foundation/Department of Energy under

grant number NSF/CHE-0822838. Use of the Advanced Photon

Source was supported by the U. S. Department of Energy, Office

of Science, Office of Basic Energy Sciences, under Contract No.

DE-AC02-06CH11357.

References

Agarwal V, Huber GW, Conner WC Jr, Auerbach SM (2011)

Simulating infrared spectra and hydrogen bonding in cel-

lulose Ib at elevated temperatures. J Chem Phys 135:113Aiache JM, Gauthier P, Aiache S (1992) New gelification

method for vegetable oils I: cosmetic application. Int J

Cosmetic Sci 14:228234

Atalla RH, Isogai A (1998) Recent developments in spectro-

scopic and chemical characterization of cellulose. In:

Dumitriu S (ed) Polysaccharides: structural diversity and

functional versatility, 2nd edn. Marcel Dekker, New York,

pp 123157

Beaucage G (1995) Approximations leading to a unified expo-

nential/power-law approach to small-angle scattering.

J Appl Cryst 28:717728

Beaucage G (1996) Small-angle scattering from polymeric mass

fractals of arbitrary mass-fractal dimension. J Appl Cryst

29:134146

Cavalcanti OA, Petenuc B, Bedin AC, Pineda EAG, Hechen-

leitner AAW (2004) Characterisation of ethylcellulose

films containing natural polysaccharides by thermal ana-

lysis and FTIR spectroscopy. Acta Farm Bonaerense

23:5357

Cousins SK, Brown RM (1995) Cellulose I microfibril assem-

bly: computational molecular mechanics energy analysis

favours bonding by van der Waals forces as initial step in

crystalization. Polymer 36:38853888

Crowley MM, Schroeder B, Fredersdorf A, Obara S, Talarico

M, Kucera S, McGinity JW (2004) Physicochemical

properties and mechanism of drug release from ethyl cel-

lulose matrix tablets prepared by direct compression and

hot-melt extrusion. Int J Pharm 269:509522

Duarte ARC, Gordillo MD, Cardoso MM, Simplicio AL, Duarte

CMM (2006) Preparation of ethyl cellulose/methyl

Fig. 8 High temperature SAXS/WAXS (a) and USAXS(b) patterns obtained for EC 10 cP

Cellulose (2014) 21:32433255 3253

123

cellulose blends by supercritical antisolvent precipitation.

Int J Pharm 311:5054

Ethylcellulose polymers technical handbook (2005). Dow

Chemical Company, http://www.dow.com/dowwolff/en/

pdfs/192-00818.pdf

Flory PJ, Vrij A (1963) Melting points of linear-chain homologs,

the normal paraffin hydrocarbons. J Am Chem Soc

85:35483553

Follonier N, Doelker E, Cole ET (1994) Evaluation of hot-melt

extrusion as a new technique for the production of poly-

mer-based pellets for sustained release capsules containing

high loading of freely soluble drugs. Drug Dev Ind Pharm

20:13231339

Fox TG, Flory PJ (1950) Second order transition temperatures

and related properties of polystyrene. I. Influence of

molecular weight. J Appl Phys 21:581591

Freelon B, Sutharb K, Ilavsky J (2013) A multi-length-scale

USAXS/SAXS facility: 1050 keV small-angle X-ray

scattering instrument. J Appl Cryst 46:15081512

French AD (2012) Combining computational chemistry and

crystallography for a better understanding of the structure

of cellulose. Adv Cacbohyd Chem Biochem 67:1993

Gopalan M, Mandelkern L (1967) The effect of crystallization

temperature and molecular weight on the melting temper-

ature of linear polyethylene. J Phys Chem 71:38333841

Hughes NE, Marangoni AG, Wright AJ, Rogers MA, Rush JWE

(2009) Potential food applications of edible oil organogels.

Food Sci Tech 20:470480

Ilavsky J (2012) Nika: software for two-dimensional data

reduction. J Appl Cryst 45:324328

Ilavsky J, Jemian PR (2009) Irena: tool suite for modeling and

analysis of small-angle scattering. J Appl Cryst 42:347353

Ilavsky J, Jemian PR, Allen AJ, Zhang F, Levine LE, Long GG

(2009) Ultra-small-angle X-ray scattering at the advanced

photon source. J Appl Cryst 42:469479

Ilavsky J, Allen AJ, Levine LE, Zhang F, Jemianc PR, Longa

GG (2012) High-energy ultra-small-angle X-ray scattering

instrument at the advanced photon source. J Appl Cryst

45:13181320

Isogai A, Atalla RH (1998) Dissolution of cellulose in aqueous

NaOH solutions. Cellulose 5:309319

Jeffries R (1968) Preparation and properties of films and fibers

of disordered cellulose. J Appl Polym Sci 12:425445

Jullien R (1987) Aggregation phenomena and fractal aggre-

gates. Contemp Phys 28:477493

Knill CJ, Kennedy JF (1998) Cellulosic biomass-derived pro-

ducts. In: Dumitriu S (ed) Polysaccharides: structural

diversity and functional versatility. Marcel Dekker, New

York, pp 937956

Koch W (1937) Properties and uses of ethylcellulose. Ind Ing

Chem 29:687690

Kondo T (1998) Hydrogen bonds in cellulose and cellulose

derivatives. In: Dumitriu S (ed) Polysaccharides: structural

diversity and functional versatility, 2nd edn. Marcel Dek-

ker, New York, pp 6998

Kondo T, Sawatari C (1996) A Fourier transform infra-red

spectroscopic analysis of the character of hydrogen bonds

in amorphous cellulose. Polymer 37:393399

Maki R, Suihko E, Korhonen O, Pitkanen H, Niemi R, Lehtonen

M, Ketolainen J (2006) Controlled release of saccharides

from matrix tablets. Eur J Pharm Biopharm 62:163170

Meakin P, Jullien R (1985) Structural readjustment effects in

clustercluster aggregation. J Phys France 46:15431552

Moore WR, Brown AM (1959) Viscosity-temperature rela-

tionships for dilute solutions of cellulose derivatives II.

Intrinsic viscosities of ethyl cellulose. J Colloid Sci

14:343353

Morris ER, Cutler AN, Ross-Murphy SB, Rees DA (1981)

Concentration and shear rate dependence of viscosity in

random coil polysaccharide solutions. Carbohyd Polym

1:521

Nelson ML, OConner RT (1964) Relation of certain infrared

bands to cellulose crystallinity and crystal lattice type. Part

I. Spectra of lattice types I, 11, I11 and of amorphous

cellulose. J Appl Polym Sci 8:13111324

Osullivan AC (1997) Cellulose: the structure slowly unravels.

Cellulose 4:173207

Overney RM, Buenviaje C, Luginbuhl R, Dinelli F (2000) Glass

and structural transitions measured at polymer surfaces on

the nanoscale. J Therm Anal Cal 59:205225

Perez S, Samain D (2010) Structure and engineering of cellu-

loses. Adv Cacbohyd Chem Biochem 64:26116

Rekhi GS, Jambhekar SS (1995) Ethylcellulose: a polymer

review. Drud Dev Ind Pharm 21:6177

Repka MA et al (2007) Pharmaceutical applications of hot-melt

extrusion: part II. Drug Dev Ind Pharm 33:10431057

Roos Y, Karel M (1991) Water and molecular weight effects on

glass transitions in amorphous carbohydrates and carbo-

hydrate solutions. J Food Sci 56:16761681

Roudaut G, Simatos D, Champion D, Contreras-Lopez E, Meste

ML (2004) Molecular mobility around the glass transition

temperature: a mini review. Innov Food Sci Emerg Tech

5:127134

Rowe RC, Kotaras AD, White EFT (1984) An evaluation of the

plasticizing efficiency of the dialkyl phthalates in ethyl

cellulose films using the torsional braid pendulum. Int J

Pharm 22:5762

Roy D, Semsarilar M, Guthrie JT, Perrier S (2009) Cellulose

modification by polymer grafting: a review. Chem Soc Rev

38:20462064

Sakellariou P, Rowe RC, White EFT (1985) The thermo

mechanical properties and glass transition temperatures of

some cellulose derivatives used in film coating. Int J Pharm

27:267277

Sandford PA, Cottrell IW, Pettitt DJ (1984) Microbial poly-

saccharides: new products and their commercial applica-

tions. Pure Appl Chem 56:879892

Schaefer DW, Hurd AJ (1990) Growth and structure of com-

bustion aerosols fumed silica. Aerosol Sci Tech

12:876890

Sinha SK, Freltoft T, Kjems J (1984) Observation of power law

correlations in silica-particle aggregates by small-angle

neutron scattering. In: Family F, Landau DP (eds) Kinetics

of aggregation and gelation. North-Holland Physics Pub-

lishing, Amsterdam, pp 8790

Skillas G et al (2002) Relation of the fractal structure of organic

pigments to their performance. J Appl Phys 91:61206124

Soares JP, Santos JE, Chierice GO, Cavalheiro ETG (2004)

Thermal behavior of alginic acid and its sodium salt. Eclet

Quim 29:5763

Tarvainen M et al (2003) Enhanced film-forming properties for

ethyl cellulose and starch acetate using n-alkenyl succinic

3254 Cellulose (2014) 21:32433255

123

anhydrides as novel plasticizers. Eur J Pharm Sci

19:363371

Torres FE, Russel WB, Schowalter WR (1991) Simulations of

coagulation in viscous flows. J Colloid Interface Sci

145:5173

Wada M (2002) Lateral thermal expansion of cellulose I and I III

Polymorphs. J Polym Sci B: Polym Phys 40:10951102

Wada M, Hori R, Kim U-J, Sasaki S (2010) X-ray diffraction

study on the thermal expansion behavior of cellulose Ib and

its high-temperature phase. Polym Deg Stab 95:13301334

Ward TC (1981) Molecular weight and molecular weight dis-

tributions in synthetic polymers. J Chem Edu 58:867879

Witten TA, Sander LM (1983) Diffusion-limited aggregation.

Phys Rev B 27:56865697

Yu DG, Yang XL, Huang WD, Liu J, Wang YG, Xu H (2006)

Tablets with material gradients fabricated by three-

dimensional printing. J Pharm Sci 96:24462456

Zetzl AK, Marangoni AG, Barbut S (2012) Mechanical properties

of ethylcellulose oleo gels and their potential for saturated

fat reduction in frankfurters. Food Funct 3:327337

Zuluaga R, Putaux JL, Cruz J, Velez J, Mondragon I, Ganan P

(2009) Cellulose microfibrils from banana rachis: effect of

alkaline treatments on structural and morphological fea-

tures. Carbohyd Polym 76:5159

Cellulose (2014) 21:32433255 3255

123

Physical structure and thermal behavior of ethylcelluloseAbstractIntroductionMaterials and methodsMaterialsMolecular weight determinationDifferential scanning calorimetry (DSC)Room temperature X-ray analysisHigh temperature X-ray analysisTransmission electron microscopy (TEM)

Results and discussionMolecular weight determinationThermal analysisStructure analysis

The semi-crystalline nature of ECConclusionsAcknowledgmentsReferences