ORIGINAL PAPER

Multi-technique surface characterization of bio-based filmsfrom sisal cellulose and its esters: a FE-SEM, l-XPSand ToF-SIMS approach

Bruno V. M. Rodrigues • Elina Heikkila •

Elisabete Frollini • Pedro Fardim

Received: 27 November 2013 / Accepted: 21 February 2014

� Springer Science+Business Media Dordrecht 2014

Abstract Bio-based films were prepared from LiCl/

DMAc solutions containing sisal cellulose esters

(acetates, butyrates and hexanoates) with different

degrees of substitution (DS 0.7–1.8) and solutions

prepared with the cellulose esters and 20 wt% sisal

cellulose. A novel approach for characterizing the

surface morphology utilized field emission scanning

electron microscopy (FE-SEM), X-ray photoelectron

spectroscopy (XPS), time-of-flight secondary ion

mass spectrometry (ToF-SIMS) and contact angle

analysis. XPS and ToF-SIMS were a powerful com-

bination while investigating both the ester group

distribution on the surface and effects of cellulose

content on the film. The surface coverage by ester

aliphatic chains was estimated using XPS measurements.

Fibrous structures were observed in the FE-SEM

images of the cellulose and bio-based films, most

likely because the sisal cellulose chains aggregated

during dissolution in LiCl/DMAc. Therefore, the

cellulose aggregates remained after the formation of

the films and removal of the solvent. The XPS results

indicated that the cellulose loading on the longer chain

cellulose esters films (DS 1.8) increased the surface

coverage by ester aliphatic chains (8.2 % for butyrate

and 45 % for hexanoate). However, for the shortest

ester chains, the surface coverage decreased (acetate,

42 %). The ToF-SIMS analyses of cellulose acetate

and cellulose hexanoate films (DS 1.8) revealed that

the cellulose ester groups were evenly distributed

across the surface of the films.

Keywords Sisal cellulose � Cellulose esters

films � Surface analysis � XPS � ToF-SIMS

Introduction

In recent decades, several studies have applied great

effort toward developing new applications for cellu-

lose. In particular, numerous investigations have

probed applications for cellulose, mainly focusing on

the synthesis of its derivatives for further preparation

of films, membranes, and composites. These materials

have been utilized in numerous academic and/or

industrial pursuits. The following examples demon-

strate the various possibilities for this polymer and its

B. V. M. Rodrigues � E. Frollini (&)

Macromolecular Materials and Lignocellulosic Fibers

Group, Center for Research on Science and Technology of

BioResources, Institute of Chemistry of Sao Carlos,

University of Sao Paulo, CP 780, Sao Carlos,

Sao Paulo 13560-970, Brazil

e-mail: elisabete@iqsc.usp.br

B. V. M. Rodrigues � E. Heikkila � P. Fardim (&)

Laboratory of Fibre and Cellulose Technology, Abo

Akademi University, Porthansgatana 3, 20500 Turku/Abo,

Finland

e-mail: pfardim@abo.fi

P. Fardim

Center of Excellence for Advanced Materials Research

(CEAMR), King Abdulaziz University, Jidda 21589,

Saudi Arabia

123

Cellulose

DOI 10.1007/s10570-014-0216-4

derivatives: biocompatible bacterial cellulose applied

in composites for biomedical application (Kim et al.

2010); cellulose as reinforcement agent in cellulose

ester matrices (Almeida et al. 2013; Morgado et al.

2013); modified cellulose nanocrystals used for drug

delivery (Akhlaghi et al. 2013) and cellulose compos-

ites from woven fabrics (Vo et al. 2013).

The preparation and properties of cellulose and

cellulose ester-based films are studied because of the

their outstanding properties in diverse applications

(Zhang et al. 2001; Heinze et al. 2003; Morgado et al.

2011; Ostlund et al. 2013). Cellulose acetate, propi-

onate and butyrate improve the flow properties

(viscosity), polishing ability, stability toward UV

radiation and dispersion of pigments during the

formulation of organic solvent-based paints and

coatings (Edgar et al. 2001). However to date, films

made from cellulose esters with longer chains have

received little attention related to their preparation and

application. Cellulose acetates, as well as esters with

long size chains (butyrates and hexanoates), were

studied as starting materials for bio-based films in this

report. Mixed cellulose ester-cellulose bio-based films

were prepared.

In the present study, LiCl/DMAc was chosen as the

solvent system for film preparation, in addition to the

preceding ester synthesis, because this mixture pre-

vented degradation of cellulose chains during the

derivatization (Regiani et al. 1999). Moreover, DMAc

could be efficiently recovered by distillation after the

ester synthesis (El Seoud et al. 2000) and after washing

water off of the films (Cheremisinoff 2000).

In this investigation, the use of LiCl/DMAc as

solvent system for preparing films is also connected to a

further aspect, namely, the high tendency of cellulose

chains, as well as other polysaccharides, towards

aggregation. In a prior study (Morgado et al. 2013),

the behavior of solutions of mercerized linter cellulose,

previously filtered, was investigated through viscomet-

ric measurements, from LiCl/DMAc solutions that were

prepared at a range of low concentrations

(0.002–0.007 g mL-1). The Huggins constant (kH)

obtained (1.8) indicated that even at low concentration,

the cellulose chains have a high tendency towards

aggregation, because kH � 0.55 and this can be

attributed to the presence of supramolecular structures

in solution. In the viscometric study, solutions of

cellulose acetates were also evaluated. The results

indicated that, depending on the degree of substitution

(DS) of the respective acetate, the chains of cellulose

acetate were molecularly dispersed, i.e. non aggregated,

or were less aggregated than the cellulose ones. These

results indicated that in a solution of cellulose esters/

cellulose, the chains of the polysaccharide would tend to

aggregate among themselves (Morgado et al. 2013).

LiCl/DMAc solutions prepared from linter cellu-

lose, as well as from the sisal cellulose used herein,

were analyzed through static light scattering measure-

ments (SLS). Each cellulose solution was centrifuged

and the upper part of solutions were analyzed. The

results indicated aggregation of chains in the solutions

of both celluloses (Ramos et al. 2011b). Therefore,

filtration (Morgado et al. 2013) or centrifugation

(Ramos et al. 2011b) of the solutions that were

subsequently analyzed have not prevented the forma-

tion of aggregates in solution.

These supramolecular structures that can be gener-

ated by the auto-organization of cellulose chains in

solution can be a drawback for several processes.

Conversely, if films are prepared from solutions of

cellulose esters/cellulose (as in the present study),

these structures can act as reinforcement of the matrix

(cellulose esters) and improve the properties of the

respective films. In this case, nano-scale bio-based

reinforced films can be prepared in a one-pot process.

The degree of aggregation of the cellulose chains in

LiCl/DMAc solutions depends on properties such as

average molecular weight of the starting material

(Ramos et al. 2011b). However, the analyses of the

solutions of all celluloses evaluated until now have

shown that aggregates were always present (Ramos

et al. 2011b). Therefore, the reinforcement action of

these aggregates can always operate, for example,

when films are prepared from solutions of cellulose

esters/cellulose. However, as the degree of aggrega-

tion can vary depending on the characteristics of the

specific cellulose, investigation should be made for

each particular case. It should be emphasized that this

feature is not specific of the study under consideration,

that is, the characteristics of the starting cellulose may

affect any further processing, as well as the properties

of the end product.

Using cellulose as a reinforcing material in cellu-

lose acetate composites films was recently reported

(Morgado et al. 2013; Almeida et al. 2013) and the

mechanical and morphological properties of these

materials were evaluated while emphasizing the effect

of cellulose loading on these acetates films. The films

Cellulose

123

were based on cellulose acetate matrices and were

prepared from LiCl/DMAc solutions. Their mechan-

ical properties were improved with increased fiber

loading because the cellulose aggregated during its

dissolution in LiCl/DMAc (Morgado et al. 2013;

Almeida et al. 2013). Coupling cellulose ester matri-

ces with cellulose reinforcement has generated mate-

rials with highly compatible elements (Mohanty et al.

2004; Almeida et al. 2013; Morgado et al. 2013) and

the structural elements present in both chains exhibit

attractive interactions to one another.

In the present investigation, films were also

prepared from esters of longer chains than acetates,

namely, butyrates and hexanoates. The mechanical

properties of these films, prepared from the neat esters

and from esters/cellulose, will later be published. The

major aim herein was to increase the knowledge on the

properties of the surfaces of these films, seeking a

better understanding on the organization of the ester

groups and/or cellulose chains in these materials.

Over the years, several techniques have been used

to characterize films, particularly their thermal,

mechanical, microscopic and morphological proper-

ties. Although significant advances have been made

using chemical microscopy to characterize films, the

distribution of the chemical groups present across the

surface of the films and the influence of other

components, such as cellulose fibers, on these prop-

erties remain underexplored. Of the several techniques

in chemical microscopy, X-ray photoelectron spec-

troscopy (XPS) and time-of-flight secondary ion mass

spectrometry (ToF-SIMS) are two important analyti-

cal tools used for surface analyses and offer 3–10 and

0.1–1 nm depth resolutions, respectively (Fardim and

Holmbom 2005; Kovac 2011; Cunha et al. 2007a).

XPS was first utilized to analyze cellulose by Dorris

and Gray (1978) and has become an established

technique in this area (Yang et al. 2013; Brown et al.

1992; Freire et al. 2006; Kovac 2011; Guezguez et al.

2013; Cunha et al. 2007b; Orblin and Fardim 2010;

Orblin et al. 2011). The binding energy peaks in XPS

spectra can identify the chemical bonding of the

surface elements when combined with published data.

For organic materials, the carbon C1s spectrum

(energy of 282–297 eV) describes carbon binding

motifs, such as C–C/C–H, C–O, C=O, C–N, O–C–O,

O=C–O and C–F.

Combining XPS and ToF-SIMS provides a power-

ful method for analyzing both unmodified and

modified cellulosic fibers: XPS technique provides

semi-quantitative information regarding the elemental

composition of the surface, while ToF-SIMS data

characterizes the chemical surface in detail (Freire

et al. 2006; Guezguez et al. 2013; Cunha et al. 2007a;

Orblin et al. 2011; Orblin and Fardim 2010). Several

papers have reported applications of ToF-SIMS in

biomaterials, such as studies of enzymatic activity on

wood solids substrates (Goacher et al. 2012), lignin

fragmentation (Saito et al. 2005, 2006) and pectin

fragmentation (Tokareva et al. 2011) using these data.

Studies mapping the component distribution in wood

(Tokareva et al. 2007), the chemical elementals in

paper (Fardim and Holmbom 2005) and the spatial and

lateral component distributions of biopolymers in

biomass (Jung et al. 2012) are also available.

This study reports the surface properties of bio-

based films prepared from LiCl/DMAc sisal cellulose

ester solutions. Cellulose esters with different chain

sizes (acetates, butyrates and hexanoates) and degrees

of substitution (DS 0.7–1.8) were prepared. In addi-

tion, bio-based composites-type films were prepared

from esters/sisal cellulose (20 wt%). The surface

morphology was assessed using FE-SEM (Field

Emission Scanning Electronic Microscopy), while

the surface chemistry was probed using XPS and ToF-

SIMS. Measurements of contact angle were performed

looking for a possible correlation between them and

the other results. Therefore, by combining these

particular techniques, this investigation elucidated

the surface distribution of ester groups on the bio-

based films. Additionally, these results might reveal

the contributions of the chain length and the degree of

substitution, as well as the presence or lack of

cellulose fibers, establishing a correlation between

them.

Materials and methods

Materials

The sisal pulp used in this investigation was gently

provided by Lwarcel Company (Lencois Paulista, Sao

Paulo, Brazil), where it was obtained by Kraft pulping

process, using Elemental Chlorine Free (ECF) bleach-

ing sequence (Lacerda et al. 2013). The pulp had

degree of polymerization (DPv) of 743, cristallinity of

72 % and a-cellulose content of 83 %, which were

Cellulose

123

determined as described elsewhere (Ass et al. 2006).

The sisal pulp was submitted to a mercerization

process (Ramos et al. 2011b; Morgado et al. 2013),

and the mercerized pulp (DPv 680; 63 % as cristallin-

ity; 91 % as a-cellulose content) was used for the next

steps, namely, dissolution, acylation and preparation

of the films.

The chemical reagents used for cellulose dissolu-

tion/acylation were N,N dimethylacetamide (DMAc,

Vetec), lithium chloride (LiCl, Vetec), acetic anhy-

dride (Synth), butyric anhydride (Sigma-Aldrich) and

hexanoic anhydride (Sigma-Aldrich). DMAc was

purified by distillation with CaH2 and the acylating

agents were distilled with P2O5 prior to use. After

distillation, the chemicals were stored over 4 A

molecular sieves. LiCl was dried under vacuum for

4 h at 160 �C, and stored in a desiccator before using.

Methods

Sisal cellulose dissolution and acylation

The dissolution conditions for cellulose used during

this investigation were published previously (Ramos

et al. 2005), as was the homogeneous acylation

previously described by Ass et al. (2006). After

dissolution, the desired amount of acylating agent was

added in molar ratios relative to the anhydroglucose

units (AGUs), generating cellulose esters with specific

degrees of substitution (DS) (the molar ratios were

established in a parallel study to be published soon).

Cellulose acetate, butyrate and hexanoate DS1.8,

as well as cellulose butyrate DS0.7 and 1.4, were

prepared under homogenous conditions in LiCl/

DMAc and posteriorly purified in methanol and

characterized regarding DS by 1H-NMR (Ramos

et al. 2011a; Almeida et al. 2013; Ass et al. 2006).

These esters were used as the matrices during the bio-

based film preparation, as described below. The DS

were selected to study the properties of films prepared

from cellulose esters with the same DS (1.8) and

varied chain lengths (acetate, butyrate, hexanoate), as

well as those with the same chain length (butyrate) and

varied DS (0.7, 1.4 and 1.8).

Bio-based film preparation

The bio-based films were prepared in LiCl/DMAc

following a previously described experimental

procedure (Almeida et al. 2013; Morgado et al.

2013). Cellulose film, films made from cellulose

esters (without cellulose) and bio-based composites-

type films (cellulose ester mixed with 20 wt% of

cellulose) were considered.

The solutions were prepared as described elsewhere

(Almeida et al. 2013; Morgado et al. 2013) and then

filtered through 47 mm disks of a glass microfiber filter

paper (MGC grade, particle retention in liquids:

1.2 lm) under positive pressure and then immediately

cast on glass petri dishes (diameter 5.8 cm), and stood

overnight at room temperature (*25 �C) and humidity

(*50 %). During this time, a gel-like material with a

consistency suitable for washing had formed, which

enabled the formation of films by removal of the solvent

system, rather than by volatilization, as usually occurs

in the preparation of films from solutions. Therefore,

the gel-like materials were exhaustively washed with

distilled water until the solvent system was entirely

removed. To verify the complete elimination of LiCl,

the conductivity of the washing water was measured for

comparison with the running water with a Schott

Handylab LF1 conductometer. Afterward, the LiCl

removal was confirmed by atomic absorption (Morgado

et al. 2013). The elimination of the DMAc was verified

by N elemental analysis (Morgado et al. 2013). The

films were dried at room temperature for 1 day and

subsequently dried at 60 �C under reduced pressure

until their weights were constant.

Surface characterization

To remove any residual impurities, the samples were

extracted with acetone (Soxhlet apparatus, acetone/

water 9:1, 4 h) and subjected to surface analyses.

Field emission scanning electron microscope (FE-

SEM) experiments were undertaken to examine the

surface morphology of the films using a Leo Gemini

1530 with an In-Lens detector. The samples were

coated with carbon with a Temcarb TB500 sputter

coater; the optimal accelerating voltage for the FE-

SEM experiments was 5.00 kV. The magnification

was 500–10,0009.

XPS experiments were recorded with a PHI Quantum

2000 Spectrometer equipped with monochromatized

AlKa radiation and charge neutralization. The photo-

electrons were collected in a 300 lm 9 200 lm area

with a 187.85 eV pass energy over wide scans (3.5 min);

23.50 eV was used for high resolution C1 s scans

Cellulose

123

(9 min). The measurements were carried out at four

different locations per sample. A similarly extracted ash-

free filter paper composed of cellulosic fibers was used as

the internal reference for pure cellulose during the XPS

measurements. The oxygen-to-carbon (O/C) ratios were

obtained from the low-resolution XPS spectra. The

high-resolution C1s spectra were deconvoluted into

C1, C2, C3 and C4 partial curves using the software

provided by the instrument manufacturer. The following

binding energy shifts relative to the C–C (that is, C1)

position were employed for the respective chemical states

of carbon: ?(1.7 ± 0.2) eV for C–O (C2), ?(3.1 ±

0.3) eV for C=O or O–C–O (C3), and ?(4.6 ± 0.3) eV

for O=C–O (C4) groups.

ToF-SIMS spectra were obtained using a Physical

Electronics ToF-SIMS TRIFT II spectrometer. The

primary ion beam utilized a 69Ga? liquid metal ion

source with a 25 kV applied voltage in both positive

and negative modes. ToF-SIMS spectra in the 2–4000

mass range were collected at a 200 lm 9 200 lm

raster size. In addition, one ToF-SIMS image was

collected at a 100 lm 9 100 lm raster size. At least

three different locations were measured on each

sample. Charge compensation was achieved using an

electron flood gun pulsing out of phase with the ion

gun. The scanning time per image was 8 min. No

programs were used to modify or improve the images.

Contact angle measurements

The dynamic contact angle between a deionized water

droplet and the bio-based films was measured (TAPPI

1994). A contact angle measurement device (model

CAM 2008, KSV) equipped with a camera and a

recording system was used. A deionized water drop (3

lL) was deposited onto the surface of various bio-

based films. The resulting angle was calculated using

300 measurements collected in three random posi-

tions. Table 1 lists the films/bio-based composites

prepared and their analyses.

The films and bio/based composites were strategi-

cally chosen based on the following criteria: (1) series

of cellulose ester matrices analogues (acetate; butyrate

and hexanoate); (2) a common and intermediate

degree of substitution (DS 1.8) for the different

cellulose ester chains; (3) a varied degree of substi-

tution (DS 0.7–1.8) for cellulose butyrate (intermedi-

ate chain size relative to acetates and hexanoates); (4)

a constant cellulose content (20 wt%) for the bio-based

composites. Consequently, the surface properties were

evaluated to determine the influence of the chain size

in films prepared from different cellulose esters (DS

1.8), across a range of DS (cellulose butyrate, DS

0.7–1.8), while varying the cellulose content (0–20

wt%).The cellulose esters films with the shortest and

longest chains (Ac1.8 and Hex1.8, respectively), as

well as the bio-based composite for the latter

(Hex1.8Cell20), were chosen for ToF-SIMS analyses.

Results and discussion

Surface morphology of films as analyzed

by FE-SEM

The FE-SEM image revealed that the surface of

cellulose films was irregular and wrinkled (Cell)

(Fig. 1), suggesting that aggregated cellulose chains

were most likely generated during the dissolution step

and, despite careful filtration after the end of the

dissolution process (using glass microfiber filter paper,

Table 1 Films prepared and respective analyses

Film Analysis Abbreviation

FE-

SEM

XPS ToF-

SIMS

Contact

angle

Acetate DS

1.8

X X X X Ac1.8

Acetate DS

1.8 20 wt%

cellulose

X X – X Ac1.8Cell20

Butyrate DS

0.7

X X – X But0.7

Butyrate DS

1.4

X X – X But1.4

Butyrate DS

1.8

X X – X But1.8

Butyrate DS

1.8 20 wt%

cellulose

X X – X But1.8Cell20

Hexanoate

DS 1.8

X X X X Hex1.8

Hexanoate

DS 1.8

20 wt%

cellulose

X X X X Hex1.8Cell20

Cellulose

film

X X X X Cell

X = done; — = not done

Cellulose

123

1.2 lm particle retention in liquids), smaller aggregates

may not have been removed by filtering the solution,

which may have generated larger aggregates during the

formation of the films. In addition, the production of

new aggregates may have started in this step.

As emphasized in the introductory part of this

paper, previous research indicated that is very difficult

to prevent the aggregation of cellulose chains in

solution. However, in the general study that embodies

the present work, the presence of aggregates is used

under a positive perspective, which is linked to the

possibility of these aggregates act as nano-scale

reinforcements of the ester matrices, as also mentioned

in the introductory part. The mechanical properties of

these films, prepared from the neat esters (in this case

acting as control samples) and from esters/cellulose,

will later be published.

Figure 2 presents surface micrographs of the cel-

lulose esters films and bio-based composite films

prepared from sisal cellulose esters/sisal cellulose (20

wt%). For Ac1.8 (Fig. 2a) and Ac1.8Cell20 (figure not

shown), the images revealed 2–3 lm microspheres on

the surfaces (evaluated using the ImageJ Software).

Morgado et al. (2013) and Almeida et al. (2013) also

observed microspheres on bio-based films prepared

from linter and sisal cellulose acetates (DS *1,5),

respectively (Almeida et al. 2013). Generating micro-

spheres of this size might be important when applying

these films in different areas, such as controlled drug

release (Edgar 2007; Almeida et al. 2013; Morgado

et al. 2013; Berthold et al. 1996; Bodmeier et al. 1995).

Many studies have used similar materials for drug

delivery in recent decades, including the report by

Wang et al. (2002); Meier et al. (2004) applying

cellulose acetate-based membranes as drugs delivery

vehicles.

The cellulose butyrate film with a lower DS (But0.7,

Fig. 2b) exhibited a more compact and smoother

morphology in its SE-SEM image relative to the

cellulose butyrate films with a higher DS (But1.4 and

But1.8, Fig. 2g, i). Cellulose esters with lower DS

contain more free hydroxyl groups able to interact with

LiCl/DMAc, most likely leading to films with a higher

homogeneity. During an earlier viscometric study, low

DS acetate (0.8) exhibited no tendency towards aggre-

gation in LiCl/DMAc solutions. However, increasing

the number of acetate groups favored aggregation. The

larger volume of the acetate relative to the hydroxyl

group might lead to expanded chains due to steric

repulsion, facilitating interchains interactions. Further-

more, decreasing the number of free hydroxyl groups

might affect the interactions of cellulose chains with the

LiCl/DMAc, further favoring interactions between the

acetate chains (Morgado et al. 2013). In the current

study, the same effect was observed in the cellulose

butyrate films with a lower DS (Fig. 2b). The cellulose

butyrate films with a higher DS exhibited a morphology

that suggested fibrous structures were present (But1.4,

Fig. 2d) alongside the microspheres (But1.8, Fig. 2f).

After adding cellulose to But1.8, the resultant bio-based

composite (But1.8Cell20, Fig. 2g) revealed a surface

containing 1–3 lm microspheres almost exclusively

(evaluated with ImageJ Software). Moreover, some

voids were detected in But0.7 (Fig. 2c, indicated by

arrows), possibly indicating that the microspheres

erupted during the consolidation of films with a lower

DS.

The irregular surface of Hex.1.8Cell20 (Fig. 2i)

suggested that structures generated by aggregates were

present; these structures may also be observed to a

lesser extent in images of neat Hex1.8 (Fig. 2h). The

longer ester chain length (relative to the other studied

Fig. 1 FE-SEM micrographs of the surface of cellulose sisal film (Cell)

Cellulose

123

esters) most likely participated in different intermo-

lecular interactions,

XPS

The elemental composition of the films and bio-based

composites was detected via XPS wide scan spectra. The

deconvolution of the high-resolution C1 peak was

responsible to provide the relative contents of C1 (C–H,

C–C), C2 (C–O), C3 (O–C–O or C=O) and C4 (O–C=O).

In addition to the C and O molecular constituents

present in cellulose and its esters, small amounts of Si

(0.3–1.6 %) were detected in the samples (excepting

But0.7 and But1.4) despite carefully avoiding contact

with any plastics or other siloxane contamination

sources during sample preparation. The presence of Si

in plants is usually attributed to protection from the

environment. Trees use bark for protection, while

annual plants generate silica that acts as a ‘‘skin’’.

Non-wood plants generally exhibit higher ash contents

than wood; wood contains silica as a primary constit-

uent, although different plants exhibit large variations

in both ash and silica contents (Smith 1997). For some

plants, such as sisal, the total ash content is low and

comparable to wood (\1 %); Total silica content is

negligible in wood and only slightly higher in sisal. The

content of silica (SiO2) in sisal fibers was gravimetri-

cally reported by Sibani et al. (2012) as 0.33–0.47 %.

Therefore, in the present study, Si was most likely

present in the raw material used during film prepara-

tion. Furthermore, sodium silicates are widely used

during pulp processing as de-foaming agents. The sisal

pulp utilized in this work was prepared using the Kraft

pulping process and elemental chlorine-free (ECF)

bleaching sequence (Lacerda et al. 2013).

The neat cellulose film (Cell) differed somewhat

from the pure cellulose reference (cellulose ref., filter

paper free of Ash). Cell exhibited a smaller O/C ratio

Fig. 2 FE-SEM micrographs of the surface of a Ac1.8, b and c But0.7, d and e But1.4, f But1.8, g But1.8Cell20, h Hex1.8 and

i Hex1.8Cell20

Cellulose

123

(0.73, 6.5 % lower), traces of Si, a higher C1 content

(14.9, 30 % higher) and a smaller C2 content (53.8,

5 % lower, high-resolution XPS spectra) that could be

attributed to the non-cellulosic impurities in the Cell

surface.

For the cellulose ester films (without cellulose),

trends in the O/C ratios and the C1 contents were

observed, related to the chemical structure and the DS

of ester side chain. The ester side chains were located

in the surface zone of the films detectable by XPS

(Fig. 3). When the same ester side group (butyrate)

was used, a decrease in the O/C ratio (Fig. 4a) and an

increase in the C1 contribution attributed to the

increased DS was observed (Fig. 4b).

The C1 content reflects the extension of the

coverage of the fibrous surface by modifying agents;

in this case, these agents are ester chains. This C1

contribution is a combination of the ester chain length

and the DS (reflected by the density of the side chain).

Further, the ester bonds in the cellulose esters are

detected as contribution to the C4 in XPS high-

resolution spectra. In pure cellulose materials, the

initial C4 content should be zero. In practice, small

amounts of C4 are always detected because the local

charging of insulating samples causes peak broaden-

ing despite the applied charge neutralization or minor,

ubiquitous contamination occurs (Carlsson and

Stroem 1991). In the samples with higher C1 values,

specifically longer side chains (butyrates and hexano-

ates), the increase in the C4 content was negligible.

However, the influence of ester carbon was clearly

observed during the XPS measurements of Ac1.8.

Regarding the bio-based films containing cellulose,

the O/C ratios were higher compared to the corre-

sponding neat films (Fig. 5), and there was an

expected decrease in the O/C ratio as the chain size

increased (Ac. [ But. [ Hex.).

The increase in C1 (Ac \ But \ Hex) and the

decrease in C2 were evident when 20 % cellulose was

added to the bio-based composites (Fig. 6).

The surface coverage (%hfa) by aliphatic chains of

the ester of the films and bio-based composites was

estimated using the relative C1 area (C–C, C–H)

(Freire et al. 2006).

%hfa ¼ CFE1 � CFCell1 ð1Þ

CFE1 and CFCell1 correspond to the C1 obtained for

the films/bio-based composites and the neat cellulose

film (Cell), respectively. Figure 7 presents the esti-

mated surface coverage by ester aliphatic chains of the

films prepared from different cellulose esters with

different DS.

The surface coverage of the films by ester aliphatic

chains increases as the DS increases when the chain

length is constant (butyrate). Moreover, when the DS

(1.8) is constant, the surface coverage increases as the

chain length increases (acetate in relation to butyrate

and hexanoate).

Figure 8 reports the estimated surface coverage for

the bio-based films prepared using different cellulose

esters (DS 1.8), both with (20 %) and without

cellulose.

The surface coverage by ester aliphatic chains at

DS 1.8 was estimated, exhibiting different trends

(Fig. 8). When comparing the neat films (DS1.8) with

those containing cellulose (DS1.8Cell20), cellulose

acetate, butyrate and hexanoate exhibited a significant

Fig. 3 Representation of the surface of cellulose esters films

detectable by XPS analysis

Fig. 4 Variation in a O/C

ratio and b C1 for the

cellulose esters films

Cellulose

123

decrease (42.4 %), a slight increase (8.2 %), and a

significant increase (45 %) in the surface coverage,

respectively. These changes were most likely due to

the size differences between the cellulose ester chains.

As the ester chain length increases (Ac \ But \ Hex),

the hydrophobic character of the ester groups also

increases, causing a gradual loss of affinity with

cellulose (highly hydrophilic). More hydrophobic

groups tend to project themselves toward the surface,

as observed when the surface coverage increased due

to the longer esters chains. In contrast, the surface

coverage decreased after cellulose was loaded into the

cellulose acetate, demonstrating strong affinity of

cellulose for this short ester group and possibly

justifying the larger amount of cellulose on the

surface.

ToF-SIMS

The cellulose film and Ac1.8 and Hex1.8 bio-based

films (lower and higher size of chains) and the bio-

based composite Hex1.8Cell20 were characterized by

ToF-SIMS spectrometry and imaging.

In the ToF-SIMS positive spectra for Ac1.8 and

Hex1.8 (the latter was less intense, Figures not

shown), the fingerprint of poly (dimethyl siloxane)

was observed (peaks 28, 73, 147, 191, 207, 222 and

282 Da), verifying the presence of Si, as prior

mentioned (XPS discussion). The characteristic peaks

for cellulose at 127 and 145 Da were also detected at a

reasonable intensity.

The Ac1.8 film exhibited a more intense mass peak

at 43 (C2H3O?) in positive ion mode. This mass

fragment is common in cellulosic samples, but acet-

ylation increased this amount 360 % relative to Cell

(normalized intensities). This increase confirmed that

the side groups were present on the outer surface. The

mass peak at 43 was chosen to map the acetate groups

on the sample surface using positive ionization mode

(Fig. 9a, b).

Fig. 6 Variation in C1 and C2 for the bio-based films

Fig. 7 Surface coverage by ester aliphatic chains for the films

prepared from different cellulose esters with different DS

Fig. 8 Surface coverage by ester aliphatic chains for bio-based

films prepared from different cellulose esters (DS 1.8) with

(20 wt%) and without cellulose

Fig. 5 O/C ratios for the cellulose esters films and their

respective bio-based films (Cell20 %)

Cellulose

123

The signal from the acetate fragment (Fig. 9a, b)

was observed as increased brightness in the pixel in

question. Some intensity differences were due to the

topographical effect and could be envisaged in the

total ion image. The detected acetate group distribu-

tion was fairly even at the experimental scale.

The Hex1.8 film positive mass spectrum exhibited

an intense mass peak at 99 that was not detected in

Cell. This peak was attributed to the C6H11O?

fragment originating from cellulose hexanoate after

ester bond cleavage. The distribution of the 99 peak is

presented in Fig. 9c, d); the hexanoate groups were

evenly distributed on the surface.

In negative ionization mode, these fragments

contained an ether oxygen as well; the peaks were

intense for both Ac1.8 (m/z 59, C2H3O2) and Hex1.8

(m/z 115, C6H11O2), as displayed in Fig. 10.

Distribution maps were obtained also for negative

ionization mode; the distributions were similar to

those in the positive ion images, Fig. 11.

Bio-based composite Hex1.8Cell20 qualitatively

exhibited ToF-SIMS spectra similar to the neat film.

The hexanoates group distribution in Hex1.8Cell20 is

presented in Fig. 12 in positive ion mode. An even

distribution was observed.

The positive 99 mass fragment’s peak intensity in

Hex1.8Cell20 was lower than was observed for the

neat Hex1.8. The normalized peak intensities were

calculated (Table 2).

The peak intensity decrease in mass fragment 99 for

Hex1.8Cell20 was attributed to the lower concentra-

tion of cellulose hexanoate in the bio-based composite

relative to the film prepared using only the cellulose

ester (Hex1.8). Because this fragment corresponds to

the C6H11O? fragment (through cleavage of the ester

bond), its intensity was close to zero for the Cell film.

Contact angle measurements

Contact angle measurements were performed for the

cellulose and bio-based films prepared from sisal

cellulose and its esters. The following discussion will

mainly focus on the contact angle hysteresis, or the

difference between the maximum and minimum

contact angle values. This property describes the

hydrophobicity, roughness and heterogeneity of the

surface (Gomes et al. 2007; Grundke et al. 1996). The

advancing (maximum) and receding (minimal) angles

for the bio-based films, as well their respective contact

angle hysteresis (difference between advancing and

receding angles), were obtained (Table 3) from the

curves describing the water contact angle over time

(not shown).

Higher advancing contact angles were observed for

the cellulose esters films (Table 3; 95.6�, 97.1� and

101.4� for Ac.18, But1.8 and Hex1.8, respectively)

relative to the cellulose film (Cell, 76.5�), indicating

an increasing hydrophobic character. This result was

attributed to the cellulose esters groups on the bio-

based films’ surface that provided greater hydropho-

bicity as the ester chain length increased relative to the

highly polar hydroxyl groups present in the cellulose

film. Crepy et al. (2009) reported contact angles with a

maximum between 100� and 109� for cellulose fatty

esters (12–18 carbons).

For the Ac1.8 film, the advancing angle (95.6�) was

higher than that of cellulose film (76.5�, Table 3)

because the water drop interacted with the hydropho-

bic domains on the Ac1.8 surface. However, the

contact between the Ac.18 film’s surface and the water

drop led to a projection of the surface toward the drop,

generating a protuberance on the surface; the water

drop was completely absorbed toward the inside of

Fig. 9 The distributions of acetates (a, b) and hexanoates groups (c, d) on Ac1.8 and Hex1.8 samples in positive mode. Total ion

images. Raster size 200 9 200 lm2 (a, c) and 100 9 100 lm2 (b, d)

Cellulose

123

this protuberance. Therefore, because the water was

absorbed, the contact angle decreased quickly, leading

to a high contact angle hysteresis (79.4�) for Ac1.8

over a short interval (Table 3). Therefore, the water

drop initially interacted with the hydrophobic domains

on the Ac1.8 surface; directly after the first contact

between the surface and the water drop, the drop

interacted with the hydrophilic domains, generating a

larger contact angle hysteresis relative to Cell (14.4�,

Table 3). In the cellulose film (Cell), the hydroxyl

Fig. 11 The distributions of acetates (a, b) and hexanoates groups (c, d) on Ac1.8 and Hex1.8 samples in negative mode. Total ion

images. Raster size 200 9 200 lm2

Fig. 12 The distribution of hexanoate groups on Hex1.8Cell20 mapping the positive ion 99 Da (a, b Total ion images). Raster size

200 9 200 lm2 (a) and 100 9 100 lm2 (b)

Fig. 10 Negative ion ToF-SIMS spectra mass range 30–130 Da for Ac1.8 and Hex1.8 collected on the raster size 200 9 200 lm2

Cellulose

123

groups participated in both intra- and intermolecular

interactions, and the contact angle did not change

significantly over time. Conversely, the droplet’s

interaction with the acetate’s hydrophilic domains

(Ac1.8) involved both free hydroxyl and carbonyl

groups, increasing the interaction with water since

shorter ester chain lengths may also favor the affinity

for water.

The contact angle hysteresis for But1.8 and Hex1.8

(Table 3, 27.3� and 17.8�, respectively) decreased

relative to Ac1.8 (79.4�). This decrease was attributed

to the increase in the hydrophobic character as the

ester chain length increased, as well as to the

significant steric hindrance that the ester groups

imparted to the hydroxyl and carbonyl groups,

hindering their interactions with the water.

The cellulose butyrate film with a lower DS (0.7)

[But0.7] presented the highest advanced contact angle

(52.8�, Table 3) relative to those with a higher DS

(But1.4 and But1.8, Table 3), revealing a lower

hydrophilic character (contrary to expectations).

However, this film (But0.7) also exhibited the largest

contact angle hysteresis (53�, Table 3) compared to

But1.4 and But1.8, possibly supporting its higher-

affinity for water. A water drop’s initial contact with

the surface renders the advanced contact angle, while

being strongly influenced by the surface morphology.

As observed in the SEM Images (Fig. 2), the cellulose

butyrate film with a lower DS (But0.7) exhibited a

more compact and smooth surface compared to the

others (But1.4 and But1.8) with fibrous structures and

microspheres on their surfaces. These elements

appeared on the surfaces of But1.4 and But1.8 and

may have influenced their interactions with water; the

advanced contact angles may not accurately reflect the

affinity of these surfaces for water.

Regarding the butyrate films, as the DS increased

[0.7, 1.4, 1.8], the contact angle hysteresis decreased

(Table 3, 52.8�, 12.2� and 27.3�, respectively) because

the hydrophobicity of the films’ surface increased.

Different effects regarding the cellulose loading on

the cellulose esters films could be observed, with a

strong dependence on the ester’s chain size. For the

cellulose acetate film (Ac1.8Cell20) adding cellulose

increased its hydrophilicity, lowering the advancing

contact angle (Table 3, 95.6�–79.8�) and contact angle

hysteresis (Table 3, 79.4�–69.1�) relative to the neat

film. Ac1.8Cell20 exhibited the same behavior as

described for the neat film (Ac1.8); specifically, a

protuberance was formed immediately after contact

between the surface and the water drop that immedi-

ately absorbed the drop. Accordingly, this film exhib-

ited the largest contact angle hysteresis of every film

containing cellulose, confirming our observations for

the neat films (Table 3).

Regarding the cellulose butyrate and hexanoate

films, adding cellulose generated films with a higher

advancing contact angle (Table 3, 97.1�–99.9� and

101.4�–132.4�, respectively) and significantly

decreased the hysteresis (Table 3, 27.3�–19.9� and

17.8�–6.7�, respectively), indicating their surfaces’

hydrophilic character decreased. The trend observed

here was validated by the XPS analyses that described

the cellulose esters chains’ surface coverage in terms

of C1. Among the cellulose acetate chains, a decreased

surface coverage was observed for Ac1.8Cell20

relative to the neat Ac1.8film (Fig. 8); however, the

butyrate film (But1.8Cell20) exhibited an increased

surface coverage relative to But1.8. Adding cellulose

led to an even higher increase in the cellulose ester

Table 2 Comparison of the count intensity for the 99 mass

fragment representing hexanoate group in the positive ToF-

SIMS spectrum

Sample Normalized intensity of m/z 99

Cell 0.4

Hex1.8 3.9

Hex1.8Cell20 3.1

The intensities were normalized by dividing the amount of

counts by the total counts and presented with one decimal place

Table 3 Advancing and receding angles for the bio-based

films and respective contact angle hysteresis

Film Angle (�)

Advancing Receding Hysteresis

Ac1.8 95.6 22.3 73.4

Ac1.8Cell20 79.8 19.5 60.2

But0.7 103.8 50.9 52.8

But1.4 77.8 65.6 12.2

But1.8 97.1 69.8 27.3

But1.8Cell20 99.9 80.0 20.0

Hex1.8 101.4 83.6 17.8

Hex1.8Cell20 132.4 125.7 6.7

Cell 76.5 62.1 14.4

Cellulose

123

chains’ surface coverage in Hex1.8Cell20 compared

to Hex1.8 (Fig. 8). These findings suggest more

cellulose was present on the cellulose acetate film’s

surface, leading to a more hydrophilic surface. How-

ever, increasing the cellulose esters chains surface

coverage (for the butyrate and hexanoate films)

indicates a strong hydrophobic character that was

confirmed by the contact angle measurements.

Conclusions

In this paper, we characterized the surface of bio-

based composites from sisal cellulose and its esters.

FE-SEM, contact angle analysis, XPS and ToF-SIMS

were utilized with the last two combined, accessing

detailed information concerning the surfaces’ elemen-

tal composition. The FE-SEM images revealed cellu-

lose aggregation and fibrous structures in the films and

some of the bio-based composites prepared from

cellulose esters. The XPS results revealed an increased

contribution of aliphatic carbon as the DS increased

when the side chain was constant (butyrate), as well as

a decreased in O/C ratio, confirming the expected

cellulose fiber modifications. Moreover, the XPS

results revealed that the O/C ratios for the bio-based

films including cellulose (DS 1.8, 20 wt %) were

higher relative to the films with same DS and no

cellulose. Concurrently, the O/C ratio decreased

when the chain size increased (Acetates [ Butyrates [Hexanoates). According to the ToF-SIMS data, the

cellulose ester groups were evenly distributed across

the bio-based composites’ surfaces. Moreover, adding

cellulose produced a cellulose acetate film that was

more hydrophilic, generating a lower advancing

contact angle. Therefore, for the cellulose butyrate

and hexanoate films, adding cellulose increased the

contact angles and lowered the hysteresis, indicating a

decreased hydrophilicity on the films’ surfaces; the

decreased hydrophilicity occurred because the ali-

phatic ester chains covered more of the surface

(verified by XPS). The results presented here have

enhanced our understanding of the interactions

between cellulose and cellulose ester chains in films

prepared from these macromolecules. In addition,

these results provide important and helpful informa-

tion regarding the distribution of these chains across

the films’ surfaces.

Acknowledgments The authors gratefully acknowledge

FAPESP (The Sate of Sao Paulo Research Foundation, Brazil)

for the fellowships of B. V. M. R. (proc. 2010/00005-4 and

2012/00813-9) and financial support, as well as the CNPq

(National Research Council, Brazil) for the research

productivity fellowship of E.F. and financial support. We also

thank Top Analytica Ltd (Turku - Finland) for providing us with

the XPS and ToF-SIMS instruments and M. Sc. Linus Silvander

(Research Assistant at Abo Akademi Process Chemistry

Centre c/o Combustion and Materials Chemistry) for taking

the FE-SEM measurements.

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