ORIGINAL PAPER

The influence of thermo-hygro-mechanical treatmenton the micro- and nanoscale architecture of wood cell wallsusing small- and wide-angle X-ray scattering

Juan Guo . Harald Rennhofer .

Yafang Yin . Helga C. Lichtenegger

Received: 28 February 2016 / Accepted: 29 May 2016 / Published online: 15 June 2016

� Springer Science+Business Media Dordrecht 2016

Abstract Tracking the changes of cellulose crystal-

lites upon thermo-hygro-mechanical treatment is

essential to understand the response of wood cell

walls to steam and compression. In this paper the

influence of Compression combined with Steam (CS)

treatment on wood cellulose crystallites and pores

structure of Chinese fir (Cunninghamia lanceolata)

was studied under different steaming temperatures and

compression ratios. Small-angle X-ray scattering and

wide-angle X-ray scattering were used to investigate

the changes of cellulose crystallites dimension, aspect

ratio, fibril diameter distribution, non-crystalline

fraction, the number of chains in each microfibril, as

well as the fractal dimension and size of pores in

response to CS treatment conditions. Results indicate

that the crystallinity increased due to CS treatment, but

did not show alteration with varying CS treatment

conditions, i.e. seemed nearly unaffected by higher

temperatures or compression ratio, both for earlywood

and latewood. The cellulose crystallite diameter

depended on processing parameters: it increased with

increasing treatment temperature. No considerable

differences were found for earlywood and latewood.

We interpret our findings as a rearrangement of

adjacent cellulose chains towards higher crystalline

perfection attributing to the increase in crystallinity.

The same effect allows a larger coherence length of

crystalline order and therefore features an increasing

cross-sectional dimension. In general we can state that

the CS treatment leads to higher crystallinity and more

perfectly arranged cellulose crystals, while it does not

greatly affect the microfibril diameter but rather the

amorphous regions of the microfibrils and the sur-

rounding hemicellulose and lignin.

Keywords Cellulose crystallites � Compression

combined with steam treatment � Small-angle X-ray

scattering �Wide-angle X-ray scattering � Earlywood �Latewood

Introduction

Wood is the most important natural and renewable

resource, while its utilization is restricted by the lack

of dimensional stability (i.e. changes of wood dimen-

sions following environmental changes, especially

moisture change), low resistance to decay and poor

durability. As an eco-friendly wood modification

J. Guo � Y. Yin (&)

Department of Wood Anatomy and Utilization, Research

Institute of Wood Industry, Chinese Academy of Forestry,

Beijing 100091, China

e-mail: yafang@caf.ac.cn;

yingyafangcaf@gmail.com

H. Rennhofer � H. C. Lichtenegger (&)

Institute of Physics and Materials Science, Department of

Material Sciences and Process Engineering, University of

Natural Resources and Life Sciences (BOKU), Vienna,

Austria

e-mail: helga.lichtenegger@boku.ac.at

123

Cellulose (2016) 23:2325–2340

DOI 10.1007/s10570-016-0982-2

method, thermo-hygro-mechanical (THM) treatment

can significantly improve the physical properties of

wood, including the mechanical behaviour and size

stability (Navi and Heger 2004; Skyba and Schwarze

2009; Navi and Sandberg 2011; Sandberg et al. 2013).

A promising THM technique is Compression com-

bined with Steam (CS) treatment, as compressing

wood in the transverse direction under the steaming

environment allows to densify wood samples without

fractures. Anatomical, structural, chemical, microme-

chanical and hygroscopic properties of wood under CS

treatment have been intensely investigated (Inoue

et al. 1993; Ito et al. 1998; Navi and Girardet 2000;

Navi and Heger 2004; Yin et al. 2011; Guo et al.

2015). However, the response mechanism of wood cell

walls to CS treatment has not been fully understood,

due to the rather complicated chemical and structural

changes in the wood cell wall upon the steaming and

compression treatment.

The wood cell wall is principally composed of

cellulose microfibrils embedded in a matrix composed

of lignin and hemicelulloses (Fengel and Wegener

1984). Cellulose, the main biomacromolecule in

wood, is synthesized in plant cell walls as crystalline

nanosized fibrils, consisting of dozens of unidirec-

tionally aligned molecular chains (Wada et al. 2010).

These microfibrils contain both ordered and disor-

dered domains, where the disordered portion is often

referred to as amorphous. The structure, size and

arrangement of the cellulose fibrils and the proportion

of crystalline cellulose are key factors determining the

mechanical performance of wood materials (Reiterer

et al. 1999; Barnett and Bonham 2004; Burgert and

Fratzl 2009). In addition, the proportion of crystalline

cellulose and the arrangement of cellulose molecules

in the cellulose microfibrils influence the accessibility

to water or other molecules (Inagaki et al. 2010;

Kulasinski et al. 2015). Therefore, tracking the

changes of cellulose upon CS treatment can provide

important information for understanding the response

mechanism of wood cell wall to the CS treatment.

Several parameters describing the cellulose crystal-

lites inwood, i.e. crystallinity, crystalline dimensions and

arrangement of the crystallites correlate with the treat-

ment conditions, e.g. heat (Kim et al. 2001; Andersson

et al. 2005), steaming (Inagaki et al. 2010; Nishiyama

et al. 2014), or strain (Peura et al. 2006). Cellulose

molecules in the wood cell wall undergo different

cellulose degradation processes, reorientation and/or

co-crystallization processes under CS treatment. It was

reported that CS treated wood showed similar crys-

tallinity but increased regularity of the (200) lattice

distance compared with untreated wood (Dwianto et al.

1999). Nevertheless, the changes of cellulose crystallites

and pore structures in wood upon CS treatment have not

been fully characterized and understood.

Small-angle X-ray scattering (SAXS) as a method

capable of averaging over the volume in the X-ray

beam is one of the very few techniques that can

provide statistical information about the morphology,

porosity and specific surface area of materials on the

nanometre scale (Lichtenegger et al. 2002, 2003;

Rennhofer et al. 2014; Cheng et al. 2015). SAXS

patterns fromwood can be regarded as arising from the

electron density contrast in a two-phase composite,

cellulose fibrils in a hemicellulose–lignin matrix, and

with contributions from pores and other cavities at

very small scattering angles (Jakob et al. 1996). SAXS

has been used to investigate the microfibril structure,

diameter of cellulose element fibril and pore structures

in wood (Lichtenegger et al. 1999; Farber and

Lichtenegger 2001; Kennedy et al. 2007; Nishiyama

2009; Cheng et al. 2011; Fernandes et al. 2011). In

addition, Wide-angle X-ray scattering (WAXS) has

provided very important results on the crystallinity

and cellulose crystallite dimension in wood (Penttila

et al. 2010; French 2014; Song et al. 2014; Thomas

et al. 2013; Toba et al. 2013). Therefore, SAXS/

WAXS were chosen as appropriate techniques to

monitor the changes of cellulose crystalline structure

under CS treatment.

Softwood consists of much simpler wood cell types

as compared with hardwood. In the softwood, the main

wood cells are tracheids. Earlywood tracheids with a

thinner wall and wider lumen, and latewood tracheids,

with a much thicker wall and narrower lumen, can be

observed in an integrated growth ring (Panshin and

Zeeuw 1970). In our previous studies (Yin et al. 2011,

Guo et al. 2015), it is reported that cell walls in

earlywood and latewood undergo different biomacro-

molecules degradation and show different hygroscop-

icity and mechanical properties after steaming

treatment and CS treatment, deriving from their

different chemical and physical properties (Winandy

1994). Therefore, it is of interest to examine the

cellulose crystallites and the pore structure in the cell

wall of earlywood and latewood after CS treatment

and gain information that can be used to understand

2326 Cellulose (2016) 23:2325–2340

123

the degradation mechanisms in the individual cell

walls.

In this paper, to elucidate how the influences of CS

treatment on wood cellulose crystallites and the pore

structure proceed, CS treatment on Chinese fir (Cun-

ninghamia lanceolata), an important commercial

conifer species, was conducted under different steam-

ing temperature (140, 160 and 180 �C) and two

compression ratios (25 and 50 %). SAXS was used

to investigate the variations in the diameter of

cellulose crystallites, the fibril diameter distribution

and the pore structure of CS treated wood. WAXS was

used to study the changes of crystallinity and dimen-

sions of the cellulose crystallites, and the number of

chains in each microfibril during CS treatment. Our

results contribute to a deeper understanding of the

effect of steaming and compression on the cellulose

crystalline structure and also provide a scientific basis

for the development of eco-friendly and high value

added wood products.

Experimental

Material

Raw material

Chinese fir trees were harvested from a 28-aged

plantation located in Jiangxi Province of China. A tree

with good growth, health, stem straightness and vigor

was selected for this study. Mature wood was used as

the raw material for CS treatment. Small specimen

(dimensions 20 9 20 9 25 mm in the tangential (T),

radial (R), and longitudinal (L) directions, respec-

tively) containing an integrated 25th growth ring on

the surface of the specimen were cut from wood

samples (Fig. 1).

Compression combined with Steam (CS) treatment

CS treatment was conducted in a laboratory-scale

autoclave at Kyoto University. All treated specimens

were placed in the pre-heated autoclave and pressur-

ized steam was applied and regulated to the corre-

sponding prescribed temperature. Small specimens

were treated with a 25 % or 50 % radial compression

ratio (the percentage of the decrease in thickness as

compared to the initial thickness of the specimen) at

110 �C for 6 min followed by a steaming process at

different steam temperatures (140, 160 or 180 �C) for30 min, respectively (Fig. 1). The treated specimens

were then cooled down to room temperature inside the

autoclave and conditioned to an equilibrium moisture

content (EMC) of approximately 12 %, by storing in a

constant environment room maintained at a constant

20 �C, 65 % relative humidity for at least 20 days. For

simplicity, the wood specimens treated with a 25 %

radial compression ratio combined with steam treatment

at 140, 160 or 180 �C were labelled S14025, S16025 and

S18025, respectively, and the wood specimens treated

with a 50 % radial compression ratio combined with

steam treatment at 140, 160 or 180 �C were labelled

S14050, S16050 and S18050, respectively. The control wood

specimen without treatment was labelled Sun.

Sample preparation for SAXS/WAXS measurements

All slices of wood specimen were cut directly by hand

without softening treatment to avoid the influence of

water or other solvents on the structure of the wood

cell wall. The thickness of the wood specimen slices

was around 200 lm. For each treatment condition,

three different positions on the tracheids in the

earlywood and latewood of the 25th growth ring were

chosen for the SAXS/WAXS measurements,

respectively.

Instruments and methods

Small-angle X-ray scattering/wide-angle X-ray

scattering

Small-angle X-ray scattering (SAXS)/wide-angle

X-ray scattering (WAXS) measurements were carried

out using a Rigaku S-Max 3000 SAXS/WAXS system

equipped with a copper-target micro-focus X-ray tube

MicroMax-002? (45 kV, 0.88 mA), collimated

through three pinholes to achieve a beam diameter at

the sample position of 160 or 210 lm, respectively

and a Triton 200 2D multi wire gas-filled X-ray

detector (200 mm diameter of active area, spatial

resolution 195 lm). Taking the wavelength k of the

X-rays and the scattering angle 2h into account the

length of the scattering vector q is described by the

Bragg equation: q = 4p sin(h)/k.

Cellulose (2016) 23:2325–2340 2327

123

For SAXS measurements the sample to detector

distance was 520 and 1400 mm, respectively, for data

acquisition in a wider q-range from 0.007 to 0.7 A-1.

For WAXS measurements a Rigaku R-AXIS IV??

system (a 150 9 150 mm image plate) was used

26 mm away from the sample with a higher spatial

resolution of 100 lm and to access higher angles, than

the TRITON 200 detector in the setup would allow.

Scattering images were azimuthally averaged to

obtain the scattering intensity I(q) in dependence of

the scattering vector q and then background corrected

before further data evaluation.

For better statistics three different positions in the

25th growth ring for earlywood and latewood of each

sample were chosen, measured and evaluated. The

final results were averaged, thus values given in the

tables and displayed in the diagrams are mean values

of three individual measurements.

Crystallinity from WAXS

The crystallinity CrI of wood samples was determined

following the area method (Eq. 1) (Park et al. 2010).

CrI ¼ A1�10 þ A110 þ A102 þ A200

A1�10 þ A110 þ A102 þ A200 þ Aam� 100%

ð1Þ

where Ahkl denotes the peak areas of crystallographic

reflections in wood denoted by theMiller incides (hkl).

Hence the crystallinity CrI was calculated from areas

Ahkl of the reflections (1–10), (110), (102) and (200),

to the total area of both crystalline and amorphous

contributions Aam in the diffraction profile given by a

broad peak centered around 1.33 A-1. The peak fitting

program (Origin 8.0, OriginLab Corporation, UK) was

used to fit the measured data, assuming Gaussian

functions for all peaks.

Cellulose crystalline dimensions from WAXS

In addition to the above mentioned peaks (1–10),

(110), (102), (200) also the (004) was fitted with

Origin 8.0 (OriginLab Corporation, UK). The peak

positions of (1–10), (110), (102) and (200) were fixed

at 1.05, 1.15, 1.43 and 1.59 A-1 during the curve

fitting procedure. Also the widths of the (1–10), (110),

and (200) reflections were restricted to be the same

value during the curve fitting for each scattering

profile, since the values of the crystal sizes for these

reflections are almost identical in wood (Jakob et al.

1995).

The average dimension (Dhkl) of the cellulose

crystallites was calculated from the width of the Bragg

peaks using Scherrer’s formula (Eq. 2), with k the

wavelength of the X-rays, with the integral breath of

the corresponding diffraction peak, i.e. the full width

at half-maximum (FWHM) of a Gaussian peak, bi the

instrumental broadening and 2h the angle where the

diffraction peak occurred (Penttila et al. 2010):

Dhkl ¼0:89� k

cosHffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

FWHM2 � b2i

q ð2Þ

Dimension of element cellulose fibril by SAXS

The position of the first minimum of I(q) in the SAXS

curve of wood (see Fig. 6b) is defined by the diameter

Fig. 1 Schematic pictures

indicating the preparation of

samples for a the raw

material for Compression

combined with Steam (CS)

treatment; b the wood

specimen slice for SAXS/

WAXS measurements; c themicroscopy image of a wood

specimen slice with the

indications ‘‘L’’ and ‘‘E’’ at

example measurement

points for latewood and

earlywood in the 25th

growth ring, respectively

2328 Cellulose (2016) 23:2325–2340

123

of the cellulose fibrils (Jakob et al. 1995). Thus a fit of

the SAXS curve with a function describing the

scattering of fibrils allows determining the fibrillary

diameter. Considering a Gaussian distribution of fibril

diameters about a mean diameter D0 with a width ofr,the scattering curve is given by

I q;D0ð Þ �Z

1

0

I0

q� 4

J1q2� D

� �

q� D

� �2

� 1

rffiffiffiffiffiffi

2pp e�

D�Doð Þ2

2r2 dDþ Bgr

ð3Þ

where I0 is an intensity constant, q the scattering

vector, Bgr the background, J1 is the Bessel function of

the first kind and D the fibril diameter.

The fitting process was carried out using the

software Wolfram Mathematica 8 (Wolfram Research

Corporation, UK) in order to obtain values D0 and r.In order to speed up the fitting process, the integral was

approximated by a weighted sum for diameters D

between 1 and 5 nm.

Radii of gyration for pores in wood samples

The SAXS intensity at very low scattering angles was

attributed to the pore structure and evaluated in two

ways. For curves with straight slopes in the logarith-

mic plots at small q, the slope was determined in order

to obtain the fractal dimension from the relation

I(q) = A 9 q-m, with m the fractal dimension and

A being a constant.

For a small number of scattering curves a Guinier

region was detectable at small q and the Guinier radius

of pores could be determined according to (Eq. 4)

ln I qð Þð Þ ¼�R2

gq2

3þ ln I0ð Þ ð4Þ

with Rg the radius of gyration, which is proportional to

the mean pore radius in the wood samples.

Data statistical analysis

The Analysis of Variance (ANOVA), a particular form

of statistical hypothesis testing for comparing three or

more groups for statistical significance (Shapiro and

Wilk 1965, Yin et al. 2011; Guo et al. 2016), was also

applied to our data using an SAS program (SAS

Institute 9.0, USA). Least-significant-difference tests

were used. If the P value is less 0.05, it indicates that it

is significantly different at a significance level of 5 %.

Results

Results from the WAXS Measurements

The WAXS patterns of CS treated and untreated wood

samples showed the characteristic diffraction signal

for native cellulose (Fig. 2). The observed patterns are

anisotropic, since the elementary cellulose microfib-

rils are aligned parallel to each other and oriented

roughly in the vertical direction, which corresponds to

a small microfibril angle, i.e. the tilt angle of cellulose

fibers with respect to the cell axis (Fengel and

Wegener 1984). At such a low level of resolution it

is not possible to distinguish the Ia and Ib forms by

crystallographic means. Since the Ib form is dominant

in wood (Moon et al. 2011), for clarity all reflections

are indexed here on the Ib lattice.

The position of the (1–10) and (110) reflections was

found to be very close at around q = 1.13 A-1 but

were separable in the fit applied to the data. The

(1–10), (110) and (200) peaks did not show any shift

after CS treatment, neither for earlywood, nor for

latewood, indicating that the lattice constant remained

the same and no distortion took place under CS

treatment.

Crystallinity

The crystallinity CrI of the samples was evaluated

from the WAXS patterns as described with the peak

area method above. Figure 2 shows an example of a

typical scattering curve and the fit functions for the

individual peaks and the sum curve approximating the

data closely.

The results of the fit are given in Table 1 and the

crystallinity is displayed in Fig. 3. The crystallinity of

earlywood and latewood clearly increased due to CS

treatment with respect to the untreated samples. For

the untreated samples a value for CrI of 46 % was

calculated for earlywood and 49 % for latewood,

respectively, in agreement with previous studies

(Andersson et al. 2005; Xu et al. 2013). After CS

treatment, the CrI for both earlywood and latewood in

all samples increased to higher than 50 %, similar to

Cellulose (2016) 23:2325–2340 2329

123

Fig. 2 a WAXS pattern

from latewood in S18025indicating the different

Bragg peaks. b Example fit

to the data with the different

crystalline and the one

assumed amorphous

Gaussian peaks and the sum

curve of the fit functions.

The integration and

evaluation for (200) and

(004) were done in the

equatorial spectrum and the

meridional spectrum,

respectively

Table 1 Results of the WAXS evaluation for earlywood (EW) and latewood (LW): crystallinity CrI evaluated by the area method,

the crystallite size (D200 and D004) and the crystallite aspect ratio (D004/D200), which is the relation of the two

Sample Crystallinity CrI/% D200/A D004/A D004/D200

EW LW EW LW EW LW EW LW

Sun 46 (1) 49 (3) 29.0 (0.3) 31.6 (1.0) 197 (6) 185 (22) 6.8 (0.2) 5.8 (0.7)

S14025 51 (2) 55 (3) 32.8 (0.6) 33.5 (0.3) 226 (22) 197 (21) 6.9 (0.7) 5.9 (0.6)

S14050 54 (2) 52 (1) 32.9 (1.2) 33.5 (0.9) 176 (22) 162 (18) 5.4 (0.7) 4.8 (0.6)

S16025 51 (2) 55 (2) 33.7 (0.3) 34.4 (0.5) 234 (54) 300 (41) 6.9 (1.6) 8.7 (1.2)

S16050 53 (1) 53 (2) 35.0 (0.5) 35.1 (0.8) 203 (15) 242 (51) 5.8 (0.4) 6.9 (1.5)

S18025 50 (1) 56 (1) 38.2 (0.5) 38.1 (1.7) 240 (44) 292 (29) 6.3 (1.2) 7.7 (0.8)

S18050 52 (3) 50 (2) 37.1 (1.2) 37.2 (1.0) 239 (2) 255 (23) 6.4 (0.2) 6.9 (0.6)

The errors of the crystallinity, D200 and D004 are calculated from the standard deviation of three individual measurements (only two

measurements for the D004 values of S18025), respectively and accordingly the error of D004/D200. Errors are given in brackets

Fig. 3 The effect of CS treatments on the crystallinity in wood samples treated at a compression ratio of 25 % (left) and 50 % (right).

Filled square earlywood, open square latewood

2330 Cellulose (2016) 23:2325–2340

123

the variations in CrI after heating under moist

conditions (Bhuiyan et al. 2000; Ito et al. 1998).

Moreover, the crystallinity of latewood in CS treated

wood was still higher than that of earlywood, except

for the samples compressed by 50 %, where the

crystallinity of earlywood approached and partly

slightly exceeded that of latewood.

The two-factor ANOVA analysis was further used

to investigate the statistical significance for the

crystallinity derived from different CS treatments

and wood type, i.e. earlywood/latewood (Table 2).

Analysis showed that both CS treatment (P = 0.0014)

and the wood type, i.e. earlywood/latewood

(P = 0.0049) had a significant influence on the

crystallinity at a significance level of 5 %. While,

there was no considerable dependence on both the

treatment temperature and the compression ratio for

the crystallinity of CS treated wood, as shown in

Table 2 by the same symbol letter.

Cellulose crystalline dimensions in wood

The radial width of the (200) reflection in cellulose Ib,

has been widely used as a guide to the diameter of the

crystalline part of microfibrils (D200). The (004)

reflection is used to measure the length of the

crystalline domains (along the microfibrillar axis)

(D004) (Garvey et al. 2005).

The average dimensions (Dhkl) of cellulose crys-

tallites were calculated. The results are given in

Table 1 and displayed in Fig. 4. In the untreated wood,

the average D200 and D004 for earlywood were 29.0

and 197 A, respectively, and those for latewood were

31.6 and 185 A, respectively. With CS treatment, both

D200 and D004 in earlywood and latewood increased

(Fig. 4), as reported for woods after hydrothermal

treatment (Andersson et al. 2005; Inagaki et al. 2010);

for D200 (27.9 and 17.7 % increase for the earlywood

and latewood in S18050, respectively, compared with

that in untreated wood) and for D004 (21.1 and 38.1 %

increase for earlywood and latewood in S18050,

respectively, compared with that in untreated wood).

The cellulose crystalline dimensions in both early-

wood and latewood were comparable (Fig. 4).

In accordance with this, statistical analysis also

showed that CS treatment (P\ 0.0001 for D200 and

P = 0.0006 for D004) resulted in a significant increase

of the cellulose crystalline dimensions (Table 2). The

difference between the behavior of latewood and

earlywood with respect to the development of the

crystalline dimensions is, however, not significant—

D200 (P = 0.0429) and D004 (P = 0.2008). D200 and

D004 for 25 and 50 % compression ratio were similar.

Besides, statistical results showed that there was no

considerable difference of the crystallite aspect ratio

between CS treated wood and untreated wood (Fig. 5

and Table 2). This indicates that the increase of

cellulose crystallite dimension might proceed at a

similar rate for both the cross sectional dimension and

the longitudinal dimension. In addition, the influence

Table 2 Two-factor ANOVA results for the effects of earlywood/latewood and CS treatment conditions on the crystallinity,

cellulose crystallite dimensions D200 and D004, and crystallite aspect ratio (D004/D200), which is the relation of the two

Sun S14025 S14050 S16025 S16050 S18025 S18050

Crystallinity Earlywood (a) B A A A A A A

Latewood (b)

D200 Earlywood (a) D C C BC B A A

Latewood (b)

D004 Earlywood (a) CD CD D AB BC A BC

Latewood (a)

D004/D200 Earlywood (a) BC BC C A BC AB C

Latewood (a)

* a, b for wood type (earlywood and latewood): different letters indicate that there is a significant difference between the earlywood

and latewood at P\ 0.05 (least-significant-difference test)

A–E for treatment conditions: different letters indicate that there is a significant difference between different treatment conditions at

P\ 0.05 (least-significant-difference test)

Cellulose (2016) 23:2325–2340 2331

123

of earlywood or latewood on the aspect ratio was

insignificant (P = 0.3792).

Results from the SAXS measurements

A typical SAXS pattern is displayed in Fig. 6a. The

scattering curves obtained by integration and back-

ground subtraction are shown in Fig. 6c, d. First the

fibrillar diameter of the cellulose fibrils was deter-

mined by the fit with Eq. (3), as described above. In a

second step the fit function was subtracted from the

data in order to access the nano pores in the structure

under the simplifying assumption that pores and fibrils

are not correlated.

Cellulose fibrils

A typical fit function is displayed in Fig. 6b, the results

of the fit for the mean cellulose fibril diameter D0 and

the Gaussian diameter distribution r are listed in

Table 3 and displayed in Figs. 7 and 8. The mean

diameter of the fibrils of untreated wood was deter-

mined with 25 ± 2 and 25 ± 1 A for earlywood and

latewood, respectively. And their diameter distribution

Fig. 4 The effects of CS treatment on the cellulose crystalline

dimension (D200 and D004) in untreated wood samples and

treated at a compression ratio of 25 % (left) and 50 % (right).

Filled square: D200 for earlywood, open square D200 for

latewood, filled circle D004 for earlywood, open circle D004 for

latewood

Fig. 5 The changes of crystallite aspect ratio D004/D200 in untreated wood samples and treated at a compression ratio of 25 % (left) and

50 % (right). Filled square earlywood, open square latewood

2332 Cellulose (2016) 23:2325–2340

123

was 2.1 ± 0.7 and 1.4 ± 0.4 A, respectively. After CS

treatment, a slight increase of the mean diameter for

both earlywood and latewood, except the wood treated

at 140 �C, was found. Moreover, the fibril diameter

distribution also became broader, except for the wood

treated at 140 �C. For the sample treated at 140 �C, thediameter of fibrils and its distribution were the same as

those for untreated wood, respectively.

Statistical analysis showed that CS treatment

(P\ 0.0001) results in a significant increase of the

diameter of cellulose fibrils, except the wood treated at

a low steam temperature about 140 �C (Table 4). The

diameter of cellulose fibrils increased by 3.6 and

9.4 % for earlywood and latewood in S18050, compared

with that in untreated wood respectively (Table 3).

The fibril diameter for earlywood and latewood were

Fig. 6 a SAXS pattern of

the latewood in S18050. b TheSAXS curve fit results using

Eq. (3) and the Wolfram

Mathematica 8 software.

The dots are the measured

data of the scattering

intensity of earlywood in

S14025, whereas the line is

the corresponding curve

fitting profile. D0 = 24.5 A

and r = 2.3 A was the best

fit. Comparison of the SAXS

profiles for earlywood c andlatewood d in untreated

wood and CS treated wood

Table 3 Results of the SAXS evaluation for earlywood (EW) and latewood (LW): D0 is the mean cellulose fibril diameter and r the

Gaussian diameter distribution of the fibrils. m is the fractal dimension of the pores obtained by the power law

Sample D0/A r/A m

EW LW EW LW EW LW

Sun 25.0 (1.7) 24.5 (1.1) 2.1 (0.7) 1.4 (0.4) 3.77 (0.09) 3.42 (0.13)

S14025 23.9 (0.3) 23.9 (0.2) 2.1 (0.3) 2.1 (0.2) 3.38 (0.10) 3.31 (0.04)

S14050 24.1 (0.4) 24.1 (0.4) 2.1 (0.5) 2.1 (0.4) 3.61 (0.06) 3.58 (0.08)

S16025 26.3 (1.2) 27.3 (0.1) 2.8 (0.2) 2.8 (0.1) 3.55 (0.02) 3.41 (0.15)

S16050 26.2 (1.0) 26.4 (0.6) 2.9 (0.3) 3.0 (1.1) 3.67 (0.07) 3.38 (0.19)

S18025 27.8 (0.4) 27.8 (0.1) 2.6 (0.3) 2.6 (0.1) 3.17 (0.11) 2.99 (0.08)

S18050 25.9 (0.4) 26.8 (0.3) 3.3 (0.3) 3.7 (0.1) 3.20 (0.04) 3.31 (0.03)

The errors are calculated from the standard deviation of three individual measurements and are given in brackets

Cellulose (2016) 23:2325–2340 2333

123

Fig. 7 The changes of cellulose fibril diameter D0 evaluated via SAXS measurements in untreated wood samples and treated at a

compression ratio of 25 % (left) and 50 % (right). Filled square earlywood, open square latewood

Fig. 8 The changes of Gaussian diameter distribution of the fibrilr evaluated via SAXSmeasurements in untreated wood samples and

treated at a compression ratio of 25 % (left) and 50 % (right). Filled square earlywood, open square latewood

Table 4 Two-factor ANOVA results for the effects of earlywood/latewood and CS treatment conditions on the mean cellulose fibril

diameter (D0), Gaussian diameter distribution (r), and fractal dimension of the pores (m)

Sun S14025 S14050 S16025 S16050 S18025 S18050

D0 Earlywood (a) C C C AB B A B

Latewood (a)

r Earlywood (a) D CD CD B AB BC A

Latewood (a)

m Earlywood (a) A BC A AB A D C

Latewood (a)

A–E for treatment conditions: different letters indicate a significant difference between different treatment conditions at P\ 0.05

(least-significant-difference test)

* a, b for wood type (earlywood and latewood): different letters indicate a significant difference between the earlywood and latewood

at P\ 0.05 (least-significant-difference test)

2334 Cellulose (2016) 23:2325–2340

123

comparable (P = 0.3891). Besides, the diameter dis-

tribution is rather influenced by CS treatment

(P\ 0.0001) than by wood type—earlywood/latwood

(P = 0.9705). Furthermore, the diameter distribution

significantly increased with raising steaming temper-

ature while the compression ratio influenced only the

diameter distribution of cellulose fibrils at 180 �C.

Pore size

In the SAXS pattern of wood samples, the total

scattering intensity consists of contributions from the

cell wall (cellulose fibrils in a hemicellulose–lignin

matrix) and from pores and other cavities (Jakob et al.

1996). Depending on the way in which fiber and pore

structure is correlated, different approaches can be

chosen. In this paper we choose the simplest approx-

imation in assuming that the pores are not correlated

with the fibril arrangement. In this case both contri-

butions can be regarded as independent and contribute

as a sum to the full scattering profile. Following this

approach, we subtracted the microfibrillar contribu-

tion from the curve fit from the full scattering curve to

obtain the scattering curve of the pore structure. Only

three of these curves showed a Guinier region at small

q values that could be evaluated. The obtained radius

of gyration Rg of the pores in earlywood in S18025 and

latewood in S18025was 104 ± 1 A and Rg in latewood

in S16025 was 106 ± 1 A. In the other samples Rg may

be much larger and the Guinier region may therefore

be out of q-range accessible by our instrument.

Another explanation might be a very large pore size

distribution resulting in a smearing of the correspond-

ing signal and therefore vanishing of any discernible

Guinier region.

Pore structure

The fractal dimension of the porous structure is

accessible via the power law behavior of the scattering

curve in the low q range I(q) decays with q-m. The

exponent m was determined via a fit of the scattering

curves in the q-range of 0.009–0.1 A-1. The exponent

for untreated wood was 3.8 ± 0.1 and 3.4 ± 0.1 for

the earlywood and latewood samples, respectively.

Considerably different behavior for earlywood and

latewood was found. For earlywood all values for CS

treated samples were smaller compared to the

untreated samples. The 25 % compression had a

stronger effect than the 50 % compression, but both

show a clearly reduced m for CS treatment at 180 �C.Latewood only shows significant changes for the CS

treatment at 180 �C. After CS treatment at 180 �C, theexponent for both earlywood and latewood displayed a

significant decrease to 3.20 and 3.31 for earlywood

and latewood in S18050 respectively and comparable

low values for the S18025 sample.

An exponent of 4 usually is attributed to smooth

surfaces, while a decreasing value towards 3 indicates

rougher surfaces. Thus the CS treatment on earlywood

for all temperatures and for latewood only for 180 �Cfeatures an increase of surface roughness.

Discussion

We found that the cellulose crystallinity of cell walls

in both earlywood and latewood increased signifi-

cantly after CS treatment, similar to those variations

after heating under moist conditions (Bhuiyan et al.

2000; Ito et al. 1998). In addition, cellulose crystallite

dimensions increased significantly (Andersson et al.

2005; Inagaki et al. 2010). In accordance to this the

SAXS results also show a slight increase in the

microfibril diameter and the spread of the microfibril

diameter with increased treatment temperature. It has

to be mentioned that the SAXS evaluation of the

fibrillary diameter very much depends on the stability

of the fit of the first minimum of the Bessel function of

first kind. This could give rise to an additional

uncertainty in the fibrillary diameter values. But even

calculating a mean value from all fibrillary diameters

after CS treatment which is 25.7 A for the earlywood

and 26.1 A for the latewood an increase with respect to

the untreated sample value of 25.0 A is visible for both

wood types.

The diameter of cellulose fibrils calculated by

SAXS in untreated Chinese fir was peaked at a value

consistent with previous studies on Norway spruce

(Jakob et al. 1995). However, it was somewhat smaller

than the value obtained for D200 by WAXS. This may

be due to the above mentioned uncertainty in the

evaluation of the SAXS curve. Even a real discrepancy

in SAXS and WAXS diameters is conceivable due to

the different materials accessible to SAXS and

WAXS; SAXS depends on the contrast of the electron

density with the surrounding matrix and provides

information about the outline of the microfibrils, while

Cellulose (2016) 23:2325–2340 2335

123

WAXS originates from crystalline structures. In many

cases the crystalline dimensions as measured by

WAXS will therefore be smaller than the overall size

determined by SAXS, because the crystal dimension

determined from the Scherrer width is decreased by

any potential imperfections in the crystal (Jakob et al.

1995; Kennedy et al. 2007). This is in contrast to our

current results, where we observe somewhat larger

WAXS derived crystallite diameters than SAXS

derived fibril diameters. This may be due to uncer-

tainties in the Scherrer evaluation. It is known that

WAXS somewhat overestimates the crystallite size by

the application of the Scherrer formula. In addition,

the Scherrer dimension represents not the overall

diameter but the weighted-mean volume length in the

direction normal to the lattice plane concerned, and

thus depends on the shape of the microfibrils (Fernan-

des et al. 2011). Furthermore different fibril sections

along the fibril length may contribute to SAXS and

WAXS signal, respectively: highly or partially crys-

talline sections will yield the major contribution to the

WAXS signal while less ordered or amorphous

sections that may occur along the microfibril axis will

not contribute. These will be visible exclusively in

SAXS and may shift the mean diameter to a slightly

different value, depending on the exact fibril diameter

in such regions. CS treatment induces heat and stress

into the fibrillary wood cell wall structures, which

could allow misaligned cellulose sheets to realign and

for imperfections in the stacking to be optimized. This

would clearly increase the crystallinity as well as the

measured crystallite size, while the fibrillary diameter

determined by SAXS is much less affected, as

observed in our study.

In our previous study (Guo et al. 2015), it was

reported that the degradation of hemicellulose and

lignin in wood cell walls could be accelerated by

raising steaming temperature and/or compression

ratio. This might cause a relative increase of the

measured crystallinity, assuming that amorphous

hemicellulose and lignin contribute to the amorphous

scattering in the WAXS region of the cellulose crystal

peaks. However, in this paper, statistical results

showed that steaming temperature and compression

ratio had insignificant influences on the crystallinity.

This means that the increase of crystallinity after CS

treatment was mainly due to the increase or reordering

in the crystalline region of cellulose, but not from the

degradation of amorphous domains, which is

consistent with results reported by Ito’s group (Ito

et al. 1998). Besides, statistical results show that

earlywood/latewood have a significantly different

degree of crystallinity.

In contrast to the higher crystallinity for latewood

in untreated wood and CS treated wood at 25 %

compression, the crystallinity of earlywood in samples

compressed by 50 % approached that of latewood.

Also the crystallite dimensions for CS treated early-

wood and latewood with 50 % compression were

similar.

This may be due to alignment of cellulose chains in

wood cell wall under CS treatment, especially for the

higher compression ratio. Thus the disordered cellu-

lose chains that are more prominent in untreated

earlywood would end up in a similar state of order as

latewood after CS treatment with 50 % compression.

Regarding the changes of cellulose crystalline

dimensions under CS treatment, transverse contrac-

tion (Peura et al. 2006) or thermal expansion effects

(Nishiyama 2009; Wada et al. 2010) can be excluded,

since both would result in a change of d-spacing and

consequentially shift of peak positions. However, the

peak positions of (1–10), (110), (200) and (004) were

maintained during CS treatment. The d-spacing of the

(200) reflection for both untreated wood and CS

treated wood was 0.39 nm. The cross-sectional area of

one cellulose chain in the cellulose Ib lattice occupies

0.317 nm2 (Fernandes et al. 2011), therefore the

diameter of cellulose crystallites (D200) as determined

by our measurements corresponded to 24 chains and a

maximum of 36 chains in each microfibril for

untreated wood and CS treated wood (S18050), respec-

tively. This means that the increase of the cellulose

crystallites dimension can probably be attributed to the

rearrangement of cellulose molecules in the partially

detached chains and the adjacent microfibrils. It is

known that the glass transition temperature (Tg) of dry

cellulose was determined to be 220 �C (Salmen and

Back 1977), while it decreased in the presence of the

moisture (Szezesniak et al. 2008). Therefore, under CS

treatment, the movement of cellulose chains in the

amorphous domains is promoted. In addition, the

release of strains within the crystals may play an

important role (Sugiyama and Okano 1990), since the

relaxed monoclinic Ib form has a larger diameter of

cellulose crystallites. Since there are intermolecular

hydrogen bonding interactions along the [200] direc-

tion, both effects are sensitive to temperature rather

2336 Cellulose (2016) 23:2325–2340

123

than compression. This is compatible with our

results—the fibril diameter and its distribution showed

a significant increase after CS treatment (Scheme 1). It

should be mentioned that statistical results showed

that there was no significant difference for D0 (as

determined by SAXS) between CS treated wood at

140 �C and untreated wood, however significant

difference for D200. This indicates that the increase

of D200 in CS treated wood at 140 �C or higher

temperatures, might not be due to the rearrangement of

cellulose molecules in the partially detached chains

and the adjacent microfibrils, but rather a change in the

nature of surface chains. As indicated by Kennedy’s

work, the surface monolayer thickness is 0.4 nm when

the surface chains lie flat on the surface of the

microfibrils, while it is 0.563 nm when the surface

monolayer is a continuation of the crystalline lattice

(Kennedy et al. 2007). The increased value of D200 for

wood CS treated at 140 �C is consistent with this

finding. Our results thus hint towards a higher ordering

of the surface chains of the cellulose crystallites under

CS treatment at temperatures 140 �C or higher.

In contrast to the weaker intermolecular hydrogen

bonding interaction along the [200] direction, there are

the stronger glycosidic bonds in the longitudinal [004]

direction. Steaming treatment at low temperatures

Scheme 1 a Possible arrangements of cellulose chains in the

cross-section of an approximately square microfibril in Sun,

S14025, S14050, S16025, S16050, S18025 and S18050. With increasing

treatment temperature the adjacent molecules feature higher

order and thus contribute to the crystallinity. This is reflected in

a larger crystal diameter as measured by WAXS, while the

SAXS fibril diameter is hardly affected. bModel for the change

of the average crystalline regions in longitudinal direction. The

observed increased correlation length is reflected in the

increased dimension of D004. Since both ordered and unordered

domains are present along a cellulose fibril, it is assumed that the

ordered domains increase in length at the expense of the

unordered domains. The average cellulose crystallites dimen-

sions displayed in (a, b) correspond to the averaged values

measured in this study for both earlywood and latewood at each

treatment temperature. Note that the observed increase of crystal

dimensions as deduced from the decreased width of the WAXS

reflections can also partly be due to strain relaxation during heat

treatment or increased order within the crystals. The parameter

actually observed in WAXS is the coherence length in a given

crystallographic direction (i.e. the length over which the order is

maintained), which does not always concur with the crystal size

Cellulose (2016) 23:2325–2340 2337

123

should not affect the [004] direction verymuch. In fact,

our results showed that the longitudinal dimension of

cellulose crystallites only increased significantly for

steaming temperatures higher than 140 �C. This indi-cates that CS treatment at 140 �C mainly led to the

above described higher degree of orientation of the

surface chains of the cellulose crystallites, resulting in

an increase of D200 and at the same temperature

unchanged D004. CS treatment at higher temperatures,

enables rearrangement of cellulose molecules in the

partially detached chains and the adjacent microfibrils,

resulting in the increase of the crystallite dimension in

both the [200] and the [004] direction. In accordance

with this interpretation, the cellulose aspect ratio (D004/

D200) showed no considerable difference between CS

treated wood and untreated wood. Besides, cellulose

crystallite dimensions were mainly influenced by

steaming temperature rather than compression.

The surface fractal of the pore structure indicates a

development towards rougher surfaces for CS treat-

ment at 180 �C while the exponent of the power law

and therefore the pores are not affected at lower

temperatures, although there was also a small change

in earlywood at 140 �C and at 160 �C. The rougher

surface might be ascribed to the degradation of matrix

in the wood cell wall, which was proven by our

previous study. The hemicellulose and lignin were

degraded seriously under CS treatment at 180 �C (Guo

et al. 2015).

Conclusions

We investigated the influence of CS treatment on the

crystal and microfibrillar structure in earlywood and

latewood of Chinese fir. The crystallinity and the

dimensions of the cellulose crystallites in the [200]

and [004] directions were determined by WAXS, the

cellulose microfibril diameter with its Gaussian

diameter distribution and some pore size values were

determined by SAXS.

The crystallinity increased due to CS treatment, but

did not show alteration with varying CS treatment

conditions, i.e. seemed nearly unaffected by higher

temperatures or compression ratio, both for earlywood

and latewood. The cellulose crystallite dimension D200

and D004 was mainly affected by CS treatment and

depended on processing parameters: it increased with

increasing treatment temperature while compression

displayed insignificant influence on crystallite dimen-

sion. No considerable differences were found between

latewood and earlywood. SAXS measurements also

displayed only a very moderate increase of the fibril

diameter after CS treatment.

Since the treatment temperature was above the

cellulose softening temperature, we interpret our

findings as a rearrangement of adjacent cellulose

chains towards higher crystalline perfection attribut-

ing to the increase in crystallinity at 140 �C. The same

effect allows a larger coherence length of crystalline

order and therefore features an increasing cross-

sectional dimension as determined from WAXS. CS

treatment also leads to degradation in lignin and

hemicellulose and affects the amorphous background

of crystallinity calculation. While the measured crys-

tal dimension increases with temperature, the degra-

dation is the same for different treatment temperatures

and therefore the crystallinity does not significantly

increase any more, while the surface roughness still

increases at 180 �C. At 160 �C and at 180 �C also the

length of the crystallites increased.

In conclusion, the CS treatment leads to higher

crystallinity and more perfectly arranged cellulose

crystals, while it does not greatly affect the microfibril

diameter but rather the amorphous regions of the

microfibrils and the surrounding hemicellulose and

lignin.

Acknowledgments The authors would gratefully like to

acknowledge the financial supports the Chinese National

Natural Science Foundation (No. 31370559) and the Austrian

Federal Ministry of Science, Research and Economy

(BMWFW) within the framework of the EURASIA PACIFIC

UNINET. The authors thank Dr. Toshiro Morooka from

Research Institute for Sustainable Humanosphere, Kyoto

University, Japan for the Compression combined with Steam

treatment of wood samples.

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