Journal SITA, 2016, 18 (2), 14-34
ISOPHASE AND PHASE TRANSITIONS OF CELLULOSE – A SHORT REVIEW
Michael Ioelovich
Designer Energy Ltd, 2 Bergman Str., Rehovot 7670504 (ISRAEL)
E-mail: [email protected]
Abstract
Cellulose has complicated supramolecular structure consisting of nanofibrils, which are built of
ordered crystallites (CR) and low ordered non-crystalline domains (NCD) in various CR/NCD
ratios, from 4 for nanocrystalline cellulose to zero for amorphized cellulose. Moreover,
crystallites have four main allomorphs, CI, CII, CII and CIV. In this critical review isophase
temperature transitions in NCD and phase transition of cellulose crystallites, as well as
amorphous cellulose, were described and discussed. It has been shown that due to structural
heterogeneity the non-crystalline domains have three isophase temperature transitions, where the
α1 and α2 transitions are caused by the segmental mobility in dense mesomorphous and medium
packed amorphous clusters, respectively; whereas the β transition is related to the mobility of
small segments in loose packed amorphous clusters, which probably are located on the outer
surface of nanofibrils. Under the action of water and other plasticizers all three isophase
transitions are shifted to lower temperatures. Various crystalline allomorphs and amorphous
cellulose can be converted into each other as a result of phase transitions, such as
recrystallization, decrystallization, transitions between various crystalline allomorphs, etc.
Important phase transition is a sol-gel process, when cellulose is dissolved and then regenerated
from the solution. In this review mechanism of the phase transitions and their relation to
isophase transitions of cellulose was disclosed.
Keywords: Cellulose, Structure, Isophase transitions, Phase Transitions, Mechanism
INTRODUCTION
As is known, the properties of polymer materials are changed gradually in a certain
temperature range until achieving the critical temperature at with an abrupt alteration of various
characteristics is observed. This phenomenon can signify the so called isophase transition
connected with structural reorganization within the same phase, or the phase transition, when
change of the phase state occurs [1, 2]. Typical example of isophase transition is glass or α
relaxation transition caused by a development of segmental mobility, as well as β and γ
relaxation transitions, which are connected with mobility change of some units or groups in
amorphous phase. The isophase transitions appear as inflection points on temperature
dependences of volume, deformation and mechanical characteristics; jump of thermal expansion
coefficient and heat capacity; extremum of dynamic properties; but these transitions do not
change the thermodynamic characteristics of polymers.
The phase transitions include amorphization, melting (fusion), dissolving, crystallization,
recrystallization, transitions between crystalline allomorphs and some other changes, which are
accompanied by jump of volume, mechanical properties, enthalpy and other physical and
thermodynamic characteristics.
Linear polymers with a simple structure typically have one glass temperature (Tg) and one
melting point (Tm). For example, natural rubber transits from glass into viscoelastic state at cca.
200 K and melts at cca. 290 K. However, polymers with complicated structure can have multiple
isophase and phase transitions. Even such known polymer as polyethylene has three glass
transitions at 153 K (for completely amorphous domains), 240 K (for mesomorphous domains on
surface of crystals) and at 200 K (for intermediate amorphous domains) [3, 4], as well as
different melting points depending on the density and type of crystals (folded, straightened,
spherulites).
Cellulose is a renewable natural polysaccharide, which is the most abundant organic matter
on Earth [5]. This natural polymer is an inexhaustible raw material for production of paper,
fibers, films, fillers, binders, glues, explosives, drugs and others valuable materials and
substances. Since the processing and use of cellulose and cellulosic materials can be carried out
over a wide temperature range, it is important to study the isophase and phase transitions of this
polymer. Cellulose is a linear, stereoregular, semicrystalline polysaccharide composed of
anhydroglucose units (AGU) linked by chemical β-1,4-glycosidic bonds. The linear
macromolecules joined by hydrogen bonds form nanofibrils, which are built of strong crystallites
with straightened chains and weak non-crystalline domains having twisted and curved segments
[6]. Molecular chains of cellulose pass through several crystallites and non-crystalline domains
linking them by strong chemical bonds. It was also discovered that non-crystalline domains have
heterogeneous packaging and can consist of relative dense mesomorphous clusters, amorphous
clusters inside of nanofibrils and loose amorphous clusters on the surface of nanofibrils [6, 7].
Besides, crystallites of cellulose have four main allomorphs CI, CII, CIII and CIV differ by
parameters of crystalline unit cells [8-10]. Furthermore, cellulose can be of various types, such as
natural, mercerized, regenerated, microcrystalline, nanocrystalline, amorphized, etc. Therefore, it
is not surprising that such a complex polymer as cellulose has multiple isophase and phase
transitions, which are discussed in this review.
ISOPHASE TRANSITIONS OF CELLULOSE
Currently it is supposed that primary α1 glass transition of cellulose is located at high-
temperatures, above 473 K (Tg1) [11, 12]. However, direct measurement of the exact value of Tg1
is difficult due to beginning of thermal decomposition of cellulose. To evaluate Tg1, indirect
methods were used. One of these methods was determination of glass temperatures (Tg) for
plasticized cellulose samples at temperatures below temperature of thermal decomposition; then
obtained glass temperatures were extrapolated to the value corresponding to zero content of
plasticizer [13, 14], as it is shown in Fig. 1.
Figure 1: Illustration of dependence of glass transition temperature on content of plasticizer
300
350
400
450
500
0 10 20 30 40
Tg
, K
C, %
Another indirect method was determination of glass temperatures (Tg) of oligosaccharides
with various degree of polymerization (DP) [15], with the subsequent extrapolation of linear
dependence Tg = (DP-1) to zero value of DP-1 (Fig. 2).
Figure 2: Illustration of dependence of glass transition temperature (Tg) on degree of
polymerization (DP) of cellulose oligomers (oligosaccharides)
The third is a calculation method based on the ratio: Tg/Tm = 0.66 [1, 16]; where the
melting point, Tm ≈ 720-770 K, was estimated from experiment of flash fusion of cellulose
crystallites by laser beam [17]. As a result of indirect evaluations, the following primary glass
temperature of cellulose was obtained: Tg1 ≈ 490-500 K (Table 1). The apparent activation
energy of this transition was above 200 kJ/mole [18], which is typical for glass transition.
Below Tg1 at the temperatures 390-410 K, the relaxation transition was discovered by
various experimental methods (Table 1): sharp decrease of dynamic modulus [11], peak of
mechanical absorption [18], sharp increase of deformation [19], jumps of thermal expansion
coefficient [19, 20] and heat capacity [21] (see for example Fig. 3, 4).
Figure 3: Dilatometry of dry film of regenerated cellulose
350
400
450
500
0 0.1 0.2 0.3 0.4 0.5 0.6
Tg
, K
1/DP
β
α2
Figure 4: Temperature dependence of volume expansion coefficient for dry samples of cotton
cellulose (CC) and regenerated cellulose (RC) with different crystallinity degrees (X)
The apparent activation energy of this transition was above 100 kJ/mole, which is higher
than for β transition. Therefore, most researchers attribute this transition to secondary α2 glass
transition [12, 18-21].
The third relaxation transition was detected at the temperatures 280-300 K by methods of
thermomechanics [19], calorimetry [24] and electron paramagnetic resonance [25], linear and
volume dilatometry [19, 20, 22, 23] (see for example Fig. 3, 4). However, changes of various
properties at transition temperature were relative small. Furthermore, the energy of activation of
this transition was about 80 kJ/mole [26], which is typical for β transition.
The fourth relaxation transition was found at 180-200 K using dynamic method of
dialectical absorption at low temperatures [26-28]. It has been proven that it is ϒ transition with
activation energy of 40-50 kJ/mole and caused by the mobility of hydroxymethyl groups in non-
crystalline domains.
Table 1: Isophase relaxation transitions of cellulose
Transition temperatures, K Interpretation Methods
490-500 Primary α1 glass transition Indirect and calculation
methods [13-15]
390-410 Secondary α2 glass transition Dynamic methods [11,
18], dilatometry [19, 20],
thermomechanics [19],
calorimetry [21].
280-300 β transition of small segments Dilatometry [19, 20, 22,
23], thermomechanics 19],
EPR [25]
180-200 ϒ transition of hydroxymethyl groups Dynamic dielectric
methods [26-28]
With decrease of crystallinity degree, the intensity of α, β and ϒ transitions increases; thus all
these transitions occur in non-crystalline domains of cellulose (see for example Fig. 4).
It is important to note that in contrast to elastomers, cellulose materials even above glass
transition temperatures show a high modulus, low deformation, as well as small changes of
various properties at transition temperatures. Therefore to discover isophase temperature
transitions in this polymer, special high-precise research methods should be used.
To explain multiple isophase transitions of cellulose the structural organization of non-
crystalline domains (NCD) of this polymer should be studied in detail. However, these
investigations are hampered by the presence of highly ordered crystalline regions, as well as by
the lack of reliable methods for the study of non-crystalline components. For instance, various
variants of NMR method gave a little information on structure of NCD. So, method CP/MAS 13C
NMR showed only the presence in NCD two “amorphous” components, non-accessible and
accessible clusters [29].
To study the structure of NCD by WAXS-method, samples of amorphized cellulose are
required. These samples can be prepared by ball-milling, saponification of cellulose acetate in
non-water alkali solutions and some others methods. The typical X-ray diffractogram of
amorphous cellulose has a wide peak with maximum at 2θ ≈ 20o (Fig. 5). Using the modified
Scherer’s equation for amorphous polymers with the shape factor K = 1.8, an average size of
ordered mesomorphous clusters of NCD can be estimated [7], Dms = 1.9 nm; whereas the average
Bragg’s distance between planes in such mesomorphous cluster, dms, was 0.45 nm.
Figure 5. X-ray scattering of amorphous cellulose
Further, the function of radial distribution of electron density, F, was also calculated for
precision of the cluster structure:
where i(s) is corrected reduced intensity; s = 4π sin/.
The F-function has five maximums of electron density at distances r of 0.1; 0.45; 0.9; 1.4 and
1.9 nm. The first maximum of F-distribution at 0.1 nm is intramolecular and relates to
superposition of lengths between various atoms in AGU of the polymer. Other maximums of
electron density are intermolecular and relate to average distance dms = 0.45 nm between layers
of AGU in mesomorphous cluster, while the third and fourth maximums – to the same distance
dms increased in two and three times. The radius of electron density at 1.9 nm is correlated with
average size of the cluster Dms. As is follows from calculations, the cluster consists of about five
AGU. These clusters are serves as centers of growth of crystallites at recrystallization process
of amorphized cellulose.
dsrsssirF )sin()(2 (1)
dms
b a
Figure 6. Model of mesomorphous unit cell of the clusters
(Projection)
As a result of analysis, a model of mesomorphous unit cell was proposed for the cluster.
This unit cell has parameters a and b about 0.9 nm, c = 1.034 nm and -angle about 120o (Fig.
6). Only one distance between layers in the cluster is nearly constant: dms = 0.45 nm. Therefore
only one maximum at 2θ ≈ 20o appears on the X-ray diffractogram of amorphous cellulose. The
specific gravity of the mesomorphous cluster is 1.48-1.50 g/cm3, while the average specific
gravity of NCD or the amorphous cellulose is 1.45 g/cm3 (Table 2).
Information about hydrogen bonds in NCD can be obtained using the method of IR
spectroscopy in combination with deuteration. It was found that the separated area of stretching
vibrations of OH-groups is located in a wide frequency range from 3000 to 3650 cm-1; this
indicates about the presence in NCD of H-bonds with different energies. After deuteration of
cellulose, a wide area of stretching vibrations of OD-groups in NCD appeared in the range 2300-
2680 cm-1 having maximum at 2500 cm-1. Energy of H-bonds can be calculated from OD-
frequency by the equation [30]:
EH = Eo + K √Δν (2)
where Eo ≈ 8 kJ/mol is minimum base energy; K = 0.81.
As a result it was discovered that the energy of H-bonds in NCD varies from 8 to 26 kJ/mol
(Table 2).
Table 2: Structural characteristics of non-crystalline domains of cellulose*
Cluster WP d, g/cm3 V, cm3/mol EH, kJ/mol Transitions DMC 0.6 1.48-1.50 108-110 23-25 α1 glass transition MAC 0.3 1.41-1.43 113-115 16-18 α2 glass transition LAC 0.1 1.29-1.31 124-126 8-10 β transition
Bulk NCD 1 1.45 112 20 α1, α2 and β
*WP is estimated weight part of a cluster; d is specific gravity; V is molar volume
Thus, the detailed studies showed that non-crystalline domains (NCD) of cellulose have
heterogeneous structural organization, which consists of dense mesomorphous clusters (DMC),
medium packed amorphous clusters (MAC) and loose packed amorphous clusters (LAC).
Consequence of structural heterogeneity of NCD of cellulose is the presence of three isophase
transitions, where the α1 and α2 transitions are caused by the segmental mobility in dense
mesomorphous and medium packed amorphous clusters, respectively; whereas the β transition is
related to the mobility of small segments in loose packed amorphous clusters, which probably
are located on the outer surface of nanofibrils.
Plasticization of cellulose causes a decrease of the temperatures of α1, α2 and β transitions
[12]. The most widespread and active plasticizer of cellulose is water. Therefore, it is advisable
to test the effect of this plasticizer on the relaxation transitions in NCD of cellulose.
Unfortunately, water is easily volatile substance, so its plasticizing effect can be studied only at
d
m
s γ
moderate and low temperatures. As a result, it was found lowering of temperature of β transition
under effect of water [19, 31, 32]. The activation energy of the β relaxation involves an entropy
contribution that varies with the water content [32]. However, effect of water on temperatures of
α1 and α2 transitions can be detected only at a high content of the plasticizer close to saturation
point [14, 19].
The largest number of papers was devoted to the study of the influence of water on the
glass transition. It has been shown that at high relative humidity of 80 to 100% temperature of
glass transition (Tg) for moist cellulose is located below room temperature [33]. To determine the
theoretical Tg of cellulose-water system over a wide range of water content, several calculation
methods were proposed. For this purpose, Salmen and Back [34] used Kaelbe’s equation [35]:
Tg = [Tg2 X2 + (h1/h2) Tg1 X1]/[X2 + (h1/h1) X1] (3)
where T1 and T2 are glass transition temperatures of water and amorphous cellulose; X1 and X2
are mole fraction of water and polymer; while h1 and h2 are constants of water and polymer,
respectively. Paes et al. [36] succeeded obtain Tg of amorphous cellulose with a certain moisture content,
and calculate the dependence of Tg on moisture content by Couchman–Karasz equation [37]:
Tg = (W1 ΔC1Tg1 + W2 ΔC2Tg2)/(W1 ΔC1 + W2 ΔC2) (4)
where Tg1 and Tg2 are glass transition temperatures of water and amorphous cellulose (AC); W1
and W2 are weight fractions of water and AC; ΔC1 and ΔC2 are change in heat capacity of
water at Tg1 and AC at Tg2, respectively.
In our investigations, we used the Fox equation [38]:
Tg-1 = W1Tg1
-1 + W2Tg2-1 (5)
where Tg1 and Tg2 are glass transitions temperatures of water and amorphous cellulose (AC); W1
and W2 are weight fractions of water and AC, respectively.
The results showed that calculations by means of all three equations gave the equal values
of theoretical glass transition temperature for wet amorphous cellulose, assuming that glass
transition temperature of dry polymer is 493 K and water is 136 K (Fig. 7).
Figure 7. Theoretical dependence of glass transition temperature on weight part of water in
amorphous cellulose or in NCD calculated by Eq. 3-5 and experimentally obtained points [36]
200
300
400
500
0 0.1 0.2 0.3
Tg
, K
W1, w.p.
Eq. 3
Eq. 4
Eq. 5
[36]
It should be noted that really it is difficult to obtain the correct results for samples of
amorphous or amorphized cellulose because of recrystallization of such samples under action of
water, or water and temperature [36]. In fact, also for semicrystalline cellulose samples the
experimental values of Tg cannot be obtained at temperatures more than 313-323 K, because at
higher temperatures the evaporation of water from the wet cellulose is enhanced resulting in
significant errors.
Consequently, a problem arises regarding extrapolation of the experimental values of Tg
for the wet samples to the value corresponding to dry cellulose. In Figure 8, we tried to
reproduce the experimental values of Tg experimentally obtained for wet cellulose samples in
some papers [33, 34, 39, 40].
Figure 8. Experimental values of Tg of wet cellulose samples with different crystallinity indexes
(CrI) obtained in papers [33, 34, 39, 40]. The dashed curve shows an exponential extrapolation to
Tg,α1 = 220°C for cellulose sample with CrI=70%
As can be seen from Fig. 8, all the experimental Tg are obtained in the temperature range -
50 to 80°C (223-353 K), i.e. far off 220-230oC attributed to temperature of primary α1 glass
transition of dry cellulose. To reach the primary glass transition temperature of dry sample, the
researchers carried out an arbitrary exponential extrapolation of the experimental points to Tg,α1
= 220°C (e.g. as it is shown in Fig. 8), which seems unjustified.
To compare the results of various samples obtained in [33, 34, 39, 40], we calculated the
moisture content (WNCD) in non-crystalline domains (NCD) of cellulose; furthermore, we
calculated Tg,c (K) of dry cellulose from the Eq. 5 in the form:
Tg,c-1 = [Tg,s (1- WNCD)]-1 – [Tg,w (1- WNCD)/(WNCD)]-1 (5a)
where Tg,s is experimental glass transition temperature of wet sample; Tg,w = 136 K is glass
transitions temperature of water; WNCD is weight fraction of water in NCD.
The calculations showed that the real Tg,c of dry cellulose is about 408 K, i.e. it is the
temperature of secondary α2 glass transition of dry cellulose (Table 3). Using this result, we
performed also the reverse calculation Tg,s = f(WNCD), shown in Fig. 9.
-50
0
50
100
150
200
0 5 10 15 20
Tg,
oC
Moisure, % (db)
CrI=80%
CrI=66%
CrI=55%
CrI=35%
Table 3: Calculated glass transition temperature (Tg,c) of dry cellulose
WNCD, w.p. Tg,s, K Tg,c, K
0.1 345 416
0.2 300 420
0.3 255 408
0.35 235 386
Average 408
As follows from the obtained results (Table 3, Fig. 9), in fact the experimental data
presented in [33, 34, 39, 40] describe the dependence of secondary glass transition temperature
on the water content, i.e. Tg,α2 = f(W), instead of Tg,α1 = f(W).
Figure 9. Experimental and calculated values of Tg,s of wet cellulose samples
Some techniques, e.g. dilatometry, allow conduct the experiments in sealed ampoules filled
with aqueous solutions of high boiling liquid such as low-molecular PEG; thus the evaporation
of water from the sample can be avoided [14]. This technique allows study both the α1 and α2
glass transitions, and also β transition, in cellulose-water system [12, 14, 19]. Various studies
have revealed that with increase the water content in NCD, temperatures of all three transitions
decrease (Fig. 7, 9, 10).
Another problem is that for cellulose samples with various crystallinity degrees, different
dependences of transition temperature on the water content are observed. To get the same results
for various cellulose materials it is advisable to use the relative humidity (RH) instead of water
content in cellulose (Fig. 11).
200
250
300
350
400
450
0 0.1 0.2 0.3 0.4
Tg
,s, K
WNCD, w.p.
Exp
Calc
Figure 10. Dependences of transition temperatures on weight part of water in NCD
Figure 11. Dependences of transition temperatures in cellulose-water system on relative
humidity
Like water, also other polar liquids such as triethylphenyl-ammonium hydroxide [13],
monoethanolamin, ethylenediamine, ethylene glycol, glycerol, and some others show plasticizing
effect on NCD of cellulose and cause the decrease of all transition temperatures [12, 14].
PHASE TRANSITIONS OF CELLULOSE
As known, various cellulose samples contain non-crystalline (amorphous) phase and
different crystalline allomorphs (CI, CII, CIII and CIV), which can be converted into each other
as a result of phase transitions, such as recrystallization, decrystallization, transitions between
various crystalline allomorphs, etc. In should be noted that in contrast to synthetic crystalline
polymers, crystallites of cellulose cannot melt or crystallize from melt, since theoretical melting
point is significantly higher than the temperature of chemical decomposition [17]. Some phase
200
300
400
500
0 0.1 0.2 0.3 0.4
Tt,
K
WNCD, w.p
1 - α1
2 - α2
3 - β
[36]
[12]
[12, 19]
1
2
3
200
300
400
500
0 20 40 60 80 100
Tt, K
RH, %
α1
α2
β
transitions of cellulose can be carried out within a solid state, when NCD turn into viscoelastic
state under action of temperature and/or plasticizers, or into swollen state under action of special
swelling agents. Other phase transitions may be of sol-gel type, when cellulose is dissolved and
then regenerated from the solution.
Model of non-crystalline phase, such as amorphous cellulose (AC), can be prepared using
ball-milling of semicrystalline cellulose samples, saponification of amorphous cellulose acetate
in non-aqueous medium, regeneration cellulose from solution in SO2-amine-organic solvent
system or some others methods [36, 41]. Amorphous cellulose has unstable thermodynamic
phase state, which nevertheless can be maintained for a long time in the glassy state where the
segmental motion is kinetically hindered. However, when the ambient temperature (Ta) is above
Tg (or Tg is below Ta) it makes possible the development of cooperative segmental motion and
implementation of recrystallization process within a solid viscoelastic amorphous state.
Hatakeyama et al. [42, 43] established that the wide peak of X-ray diffractogram of dry AC
became narrower after annealing at temperature of about 400 K, which is a first sign of
recrystallization. In the presence of such plasticizer as water, the crystallization of AC is
observed at room temperature (293-300 K). Study of the effect of relative humidity (RH) at room
temperature (RT) showed that the first sign of recrystallization of AC was observed at RH of 65-
75 % [44].
Paes et al. [36] reported on starting of recrystallization process after conditioning of AC at
54% RH and RT followed by drying. On the other hand, it has been found that recrystallization
of AC is caused by moisture, but not by drying [44]. In all mentioned cases the experiments were
carried out in such conditions, where the ambient temperature, Ta, exceeded the temperature of
secondary α2 glass transition, whereas the temperature of primary α1 glass transition was higher
than Ta (Fig. 11).
To achieve the temperature of primary α1 glass transition at RT, the relative humidity
should be above 80%. At these conditions, the recrystallization process is carried out more
intensive and leads to perceptible increase in crystallinity of cellulose [44]. Liquid water causes
intensive recrystallization of AC in a wide temperature range, above melting point [45]. After
wetting, the completely amorphized cellulose recrystallizes in CII crystallites; while for
crystallization in the CI allomorph, the amorphized sample must contain residual CI crystallites
[46, 47].
Two-stage crystallization process of AC takes place (Fig. 12) after treatment also with
other plasticizers such as ethylene glycol (EG) and glycerol (GL) [45].
Figure 12. Dependence of crystallinity (X) of ball-milled cellulose on heating temperature in
medium of EG (1) and GL (2)
1
2
When AC is heated in the medium of liquid plasticizer above temperature of secondary α2
glass transition in this medium, the formation of crystalline nuclei with lateral size (Ln) of 1.5-
1.8 nm is observed. On the second stage, after heating of the sample in the liquid medium above
temperature of primary α1 glass transition in this medium, an intensive recrystallization of AC
on the nuclei occurs into crystallites with lateral size (Lcr) of 3.5-4.0 nm, which is accompanied
by an increase in crystallinity (X) to 50-56% (Table 4). Non-polar or weakly polar liquids, such
as low-molecular polyethylene glycol (PEG), lower alcohols, esters, ethers and hydrocarbons do
not plasticize and do not recrystallize AC.
Table 4: Recrystallization of AC in various liquid media*
Medium Tg,α2, K Tn, K Ln , nm Tg,α1, K Tcr, K Lcr , nm X, % PEG 400 410 1.5 (493) - - 20
GL 380 400 1.8 460 470 4.0 51
EG 330 350 1.6 375 400 3.5 50
H2O 240 <273 - 260 <273 3.5 56
Mechanism of recrystallization of the completely amorphized cellulose under action of
water is, probably, in: hydration of the sample, transformation of the plasticized amorphous
phase into viscoelastic state with developed cooperative segmental motion; growth of hydrated
crystallites on nuclei - mesomorphous clusters of AC; and finally the conversion of hydrated
crystalline into CII crystallites after drying. In the case of recrystallization of partially
amorphized natural cellulose under action of water and other plasticizers, energetically more
advantageous is the crystallization of viscoelastic NCD on the existing CI-matrix with forming
of CI crystallites.
Important type of phase transition is a sol-gel process, which occurs in the production of
regenerated fibers and films using various solvents such as cuprammonium (Cupra process),
NaOH/CS2 (Viscose process), N-Methylmorpholine N-oxide (Lyocell process), etc. As a result
of regeneration, a cellulose solution turns into amorphous or mesomorphous hydrogel, which
after stretching and drying at increased temperatures transforms into regenerated cellulose with
crystallinity of 35-40% and containing small CII-crystallites. The hydrogel isolated from N,N-
dimethylacetamide/LiCl solvent system followed by stretching was converted into a dry film
with low crystallinity less than 20%, having predominantly a nematic mesomorphous structure
[41]. Further, the nematic cellulose was mercerized with 7% NaOH, treated with 50% EDA or
heated in water at 453 K; as a result, the CII, CIII and CIV allomorphs respectively were
obtained.
As known, crystallites of various cellulose samples can be in different crystalline
allomorphs. Crystallites of natural celluloses have the allomorph type CI. Furthermore it was
found that crystalline unit cell of CI can be in two distinct crystalline forms: triclinic CIα of P1-
space group and monoclinic CIβ of P21-space group; where CIα form is characteristic for algae
and bacterial celluloses, while more stable CIβ form is dominant in higher plants and tunicate [6,
8-10]. Additional crystalline allomorphs: CII, CIII1, CIII2, CIV1 and CIV2, have been identified,
which are attributed to structural-modified celluloses. Various allomorphs have different
parameters and shape of crystalline unit cells.
Different crystalline allomorphs can be converted into each other under certain conditions. It is
well known how to convert one crystalline allomorph of cellulose to another. For example,
cellulose samples containing CII-crystallites can be obtained by mercerization, i.e. treatment of
natural cellulose with 4-5 M sodium hydroxide at RT, or by regeneration from cellulose
solutions. Cellulose samples with CIII1 and CIII2 crystalline allomorph are derived from samples
of CI or CII, respectively, by treatment with liquid ammonia, primary amines or
ethylenediamine. Samples with CIV1 and CIV2 crystallites can be prepared usually by heating of
other allomorphs in hot glycerol at 533 K. When processing cellulose in the hot glycerol, this
liquid medium isolates the sample from the air atmosphere, thereby preventing the development
of thermo-oxidative destruction.
Due to existence of distinct crystalline polymorphs and amorphous cellulose, various
studies were performed in order to estimate the phase stability of different allomorphs. The
amorphous phase state is regarded as a labile, because the amorphous cellulose can easily
recrystallize in any crystalline allomorph under certain conditions [44-47]. This conclusion is
also confirmed by the results of thermodynamic and thermochemical investigations [48-50]. In
the case of crystalline polymorphs of cellulose, the problem regarding the relative stability of the
phase state is not completely resolved and remains open. Study of thermodynamic and
thermochemical characteristics gave reason to believe that phase state of CII is more stable than
CI [48-50]. However, in another study it was concluded that the stability of phase state of
various crystalline allomorphs cannot be estimated with a reasonable degree of certainty due to
uncertainties in the measured characteristics [50].
As is known, the completely crystalline cellulose is absent. Therefore, to study the phase
state of a crystalline allomorph, it requires the use of several samples with different crystallinity
degrees. Unfortunately, the existing methods for determining the crystallinity of cellulose such
as XRD and 13C-NMR, give unreliable indexes of crystallinity (CrI) [6], which cannot be used in
accurate thermodynamic calculations. To avoid the use of uncertain CrI, a precise
thermochemical method was proposed to determine the actual degree of crystallinity of cellulose
samples. This thermochemical method is based on the measurement of standard enthalpy of
cellulose wetting with water (ΔwHo) [6, 51]. The actual degree of crystallinity (X) of cellulose
was calculated by the equation:
X= 1- (ΔwHo/ΔwHoam) (6)
where ΔwHoam = - 27.16 kJ/mol is enthalpy of wetting of amorphous cellulose.
Various cellulose samples were studied. Samples with CIβ allomorph were: chemical Kraft
pulp (KP), cotton cellulose (CC), Avicel MCC PH-101 (MCC-1) and PH-301 (MCC-2)
Amorphization of cellulose was carried out by ball-milling of Kraft pulp with ceramic balls for 2
(AC-1) and 8 h (AC-2). Samples of KP and CC were treated with 20% NaOH at room
temperature overnight to obtain mercerized samples with CII allomorph - KPM and CCM,
respectively. Besides, fibers of regenerated cellulose (RC) with CII allomorph were used. To
prepare CIII from KP (KPA) and CC (CCA), the cellulose samples were treated with anhydrous
liquid ammonia at 240 K for 3 h. Samples with CIV allomorph – KPAG and CCAG, were
prepared by treatment of KPA and CCA samples in glycerol at 533 K for 30 min.
The combustion reaction of one glucopyranose unit of cellulose is written as:
C6H10O5 (s) + O2 (g) = 6CO2 (g) + 5H2O (l) + ΔcHo (7)
Based on this reaction, the standard enthalpy of formation of cellulose can be calculated by the
equation:
ΔfHo = 6ΔfH
o (CO2, g) + 5ΔfHo (H2O, l) – ΔcH
o (8)
where standard enthalpies of formation of carbon dioxide and water are:
ΔfHo (CO2, g) = -393.51 kJ/mol and ΔfH
o(H2O, l) = -285.83 kJ/mol.
Gibbs free energy of formation was calculated, as follows:
ΔfGo = ΔfH
o –To(So - ∑𝑆𝑖) (9)
where ∑𝑆𝑖 is sum of standard entropies of carbon atoms (graphite), molecules of H2 and O2
needed for forming one glucopyranose unit of cellulose; To = 298.15 K.
The thermodynamic characteristics of cellulose samples having different structural
characteristics are given in Table 5.
Table 5: Standard thermodynamic characteristics of cellulose samples [52]
Abbreviation Allomorph X -ΔcHo, kJ/mol -ΔfHo, kJ/mol -ΔfGo, kJ/mol MCC-1 CI 0.72±0.02 2821.0±2.0 969.2±2.0 667.0±2.3
MCC-2 CI 0.75±0.02 2819.8±1.8 970.4±1.8 668.1±2.1
CC CI 0.70±0.02 2821.2±1.7 969.0±1.7 666.8±2.0
KP CI 0.65±0.02 2823.6±2.2 966.6±2.2 664.6±2.5
AC-1 CI 0.52±0.02 2828.7±2.3 961.5±2.3 659.9±2.6
AC-2 CI 0.28±0.02 2837.4±2.1 952.8±2.1 652.0 ±2.4
CCM CII 0.55±0.02 2823.3±1.8 966.9±1.8 664.7±2.1
KPM CII 0.53±0.02 2824.2±2.3 966.3±2.3 664.4±2.6
RC CII 0.40±0.02 2830.0±1.7 960.2±1.7 659.0±2.0
CCA CIII 0.37±0.02 2836.4±2.3 953.8±2.3 652.7±2.6
KPA CIII 0.35±0.02 2837.0±2.0 953.2±2.0 652.2±2.3
CCAG CIV 0.60±0.02 2825.0±2.1 965.2±2.1 662.4±2.4
KPAG CIV 0.57±0.02 2826.1±1.9 964.1±1.9 662.2±2.2
As can be seen from Figures 13 and 14, with the decrease of crystallinity degree all linear
dependences ΔfHo = f(X) and ΔfG
o = f(X) converge at one common point, ΔfHoam = -942.4
kJ/mol and ΔfGoam
= -642.6 kJ/mol, corresponding to X = 0. This evidences that the non-
crystalline (amorphous) phase in different cellulose samples has identical thermodynamic
characteristics. On the other hand, the linear extrapolation of these dependences to the values
corresponding to X = 1 gives the enthalpy and free energy of formation of different crystalline
allomorphs, ΔfHocr and ΔfG
ocr (Table 6). Furthermore, melting enthalpy (ΔHo
m) of the cellulose
crystallites was calculated:
ΔHom = ΔfH
oam - ΔfH
ocr (10)
Figure 13. Dependence of enthalpy of cellulose formation on degree of crystallinity
940
950
960
970
980
0 0.2 0.4 0.6 0.8
-ΔfH
o, k
J/m
ol
X
C1
CII
CIII
CIV
Figure 14. Dependence of Gibbs energy of cellulose formation on degree of crystallinity
Table 6: Thermodynamic characteristics of various allomorphs [52]
Allomorph -ΔfHo, kJ/mol -ΔfGo, kJ/mol ΔHom, kJ/mol
CI 979.6 676.4 37.2
CII 986.9 683.7 44.5
CIII 973.2 670.0 30.8
CIV 980.4 677.2 38.0
CA* 942.4 642.6 0
*CA is cellulose in noncrystalline (amorphous) phase state
Based on obtained thermodynamic characteristics, as well as thermochemical and
physicochemical properties [44-50] it can be concluded that the relative thermodynamic stability
of the various allomorphs decreases in the following order:
CII > CIV ≥ CI > CIII > CA (11)
On the other hand, the relative reactivity of the different allomorphs decreases in the reverse
order:
CA > CIII > CI ≥ CIV > CII (12)
Increased phase stability of the CII crystallites leads to some problems at the cellulose
application. It is known that after transformation of crystalline structure CI of natural cellulose
into CII, e.g. by mercerization, a decrease in solubility and reactivity of the sample is observed
[53]. In particular, transparence and filterability of solutions of CII-samples declines due to
forming of gel-particles. The presence of CII crystallites is a main factor that causes the low
reactivity of cellulose at acetylation, nitration and forming of viscose [54, 55]. On the other
hand, the transformation of crystalline structure CI of chemical pulp to labile CIII1 allomorph
increases the reactivity of cellulose [55].
640
650
660
670
680
0 0.2 0.4 0.6 0.8
-ΔfG
o,
kJ
/mo
l
X
C1
CII
CIII
CIV
It should be noted that the found order of the stability or reactivity is valid only for
macrocrystals or for crystallites with the close sizes. If the sizes of nanocrystallites of two
allomorphs are very different, this order can be changed due to contribution of Gibbs-Thomson
free surface energy (ΔGos) to free energy of conversion (ΔGo
c) of nanocrystallites of allomorph 1
into crystallites of allomorph 2:
ΔGoc = ΔfG
o2 - ΔfG
o1 + ΔGo
s (13)
ΔGos = 2σ1,2 V(L1 - L2)/L1L2 (13a)
where L1 and L2 are lateral sizes of rod-like nanocrystallites for initial and converted allomorph; σ1,2 is interface energy between allomorphs 1 and 2; V is molar volume; while experimental
coefficient K= 2σ1,2 V ≈ 100 (kJ x nm/mol) if L is in nm.
For example, consider the conversion of CII into CIV. If crystallites of CII and CIV have
the same size, then ΔGos =0, and the conversion order is determined by ΔfG
o values of Table 6,
i.e. free energy of conversion will be:
ΔGoc = ΔfG
o (CIV) - ΔfGo (CII) = 6.5 (kJ/mol)
Since ΔGoc > 0, the conversion of CII to CIV cannot be implemented. Indeed, the
mercerized CII cellulose cannot be converted into CIV because the both CII and CIV crystallites
have the similar lateral size of 5.6-6.0 nm. This result corresponds to the order of stability (11),
wherein CII is more stable than CIV.
On the other hand, if lateral size of CII-crystallites (LCII = 4 nm) is less than the lateral size
of CIV-crystallites (LCII = 6 nm), then order of stability will be reverse due to contribution of
ΔGos = - 8.3 kJ/mol, and namely:
ΔGoc = ΔfG
o (CIV) - ΔfGo (CII) + ΔGo
s = 6.5 - 8.3 = - 1.8 (kJ/mol)
In this case ΔGoc < 0. Thus, the conversion of small CII-crystallites into larger CIV-crystallites is
possible, which is confirmed by experiment.
General scheme of transformation of various crystalline allomorphs of cellulose is shown
on the Figure 15 [6].
Figure 15. Scheme of phase transitions between various crystalline allomorphs of cellulose
CIV1 CIII1
CI
CIII2 CIV2
CII
Some allomorphs, such as CI, CIII1 and CIV1, as well as CII, CIII2 and CIV2 can be
converted into each other reversible, while conversions CI into CII or CI into CIV are
irreversible. An example of reversible phase transition is transformation of CI into CIIII after
treatment by liquid ammonia and reverse transformation of CIII1 into CI after boiling in water or
by high-temperature treatment above Tg,α1 [56].
To form CIII1 allomorph, molecules of ammonia penetrate into crystalline lattice of CI and
create crystalline ammonia-cellulose complex, which turns into CIII1 while drying. Mechanism
of reverse transformation of CIII1 into CI consists in transition of NCD of CIII1 into viscoelastic
state at heating in the plasticizing medium; crystallization of viscoelastic NCD on residual CI-
nuclei with forming of stable CI crystallites, which is accompanied by breaking of small unstable
crystallites of CIII1.
Mechanism of irreversible transformation of CI into CII during mercerization consists in
penetration of hydroxyl anions into crystalline lattice of CI with creation of crystalline alkali-
cellulose, which turns into crystallites of cellulose hydrate after washing, and then into CII
crystallites after drying [52].
Unfortunately, the scheme 15 is simplified and only partially describes the real phase
transformations. Especially, it concerns the formation mechanism of CIV allomorph. For
example, small CII-crystallites (lateral size Lcr = 4.0 nm) of regenerated cellulose (RC) actually
are transformed into CIV after treatment in a hot glycerol at the conventional conditions, 533 K
for 30 min (Fig. 16). However, at the same treatment conditions, larger CII-crystallites (Lcr = 5.7
nm) of mercerized cotton cellulose (CC) are not turned into CIV allomorph (Fig. 17).
Another example is related to phase transition of CI to CIV after treatment in a hot glycerol
at the conventional conditions. As it was shown in the paper [56], CI-crystallites of natural
cellulose samples (cotton cellulose, wood pulp, etc.) cannot be transformed directly into CIV,
which is contrary to scheme 15. Nevertheless, after partial decrystallization of natural cellulose
samples e.g. by ball-milling, the remained small crystallites of CI (Lcr = 3.4 nm) are easily
converted into CIV allomorph after treatment in hot glycerol (Fig. 18). Thus, the experimental
data confirm the scheme 15 concerning formation of CIV allomorph, but only for small
crystallites having lateral size Lcr <4.3 nm.
Figure 17. X-ray diffractograms of
mercerized CC (Lcr = 5.7 nm) (1) and this
cellulose treated in hot glycerol (2)
Figure 16. X-ray diffractograms of
RC (Lcr = 4.0 nm) (1) and this
cellulose treated in hot glycerol (2)
Figure 18. X-ray diffractograms of ball-milled cellulose I (Lcr = 3.4 nm) (1) and this cellulose
treated in hot glycerol (2)
After study of crystalline structure of various cellulose samples we found that the main
structural factor responsible for transformation of any of three crystalline allomorphs, CI, CII and
CIII into CIV after treatment in the hot glycerol is a lateral size of crystallites (Lcr). As can be
seen from Table 7 [45], the conversion of any crystalline allomorph into CIV-structure at 533 K is
possible, when the lateral size of crystallites is ≤ 4.3 nm; whereas the larger crystallites cannot be
transformed directly into the CIV allomorph.
Table 7: Structural characteristics of cellulose samples and their ability to conversion into CIV*
Cellulose CRA Lcr , nm CIV Index Cotton cellulose (CC) CI 8 0
Partially decrystallized CC CI 3.6 1
Mercerized CC CII 5.7 0
Kraft pulp (KP) CI 6.7 0
Partially decrystallized KP CI 3.4 1
Mercerized KP CII 5.6 0
KP treated by liquid NH3 CIII 3.3 1
Regenerated cellulose fibers CII 4.0 1
Regenerated cellulose film CII 4.3 1
*Note: If the CIV index is zero it means the absence of phase transformation; If CIV index is one, it means
complete transformation into CIV.
Prediction of phase transformation to form of CIV allomorph was shown above by
contribution of free surface energy to free energy of conversion (see eq. 13). Quantitative
calculations of this phase transition can be performed by means of thermodynamic equation of
Gibbs and Thomson, which describes the dependence of melting point (Tm) of small rod-like
crystallites on their lateral size, Lcr [57]:
Tm = Tm,o – (2σc-a V Tm,o/ΔHm Lcr) (14)
where Tm,o is equilibrium melting point of large crystals; ΔHm is melting enthalpy; V is molar
volume of crystals; and σc-a is crystal – melt interface energy.
Thermal analysis of various cellulose samples in inert atmosphere revealed that in the
temperature range of 520 to 620 K the decay of crystalline structure occurs until complete
amorphization, which triggers the subsequent chemical decomposition [58]. Moreover, it was
found that an average temperature of thermal amorphization (Ta) is inversely proportional to
lateral size of crystallites (Fig. 19):
Ta = Ta,o – n/Lcr (15)
where coefficient n = 893 K, if Lcr is expressed in nm.
Figure 19. Dependence of average temperature of thermal amorphization on lateral size of
crystallites
The linear extrapolation of dependence Ta= f(Lcr-1) to Lcr
-1 = 0, gives value of Ta,o ≈ 740 K,
which is close to theoretical melting point of large crystals of cellulose [17]. The experimentally
obtained Eq. (15) is an analogue of theoretical Eq. (14), where Ta,o = Tm,o ≈ 740 K; and
coefficient n = 2σc-a V Tm,o/ΔHm.
As is follows from investigations, crystallites of CI, CII or CIII with lateral size Lcr > 4.3
nm (Lcr-1 < 0.23 nm-1) do not decrystallize at the treatment temperature of 533 K, and hence they
cannot be transformed into CIV after treatment in hot glycerol (Fig. 19). On the other hand, a
high-temperature treatment of cellulose having small crystallites with lateral size of Lcr ≤ 4.3 nm
(Lcr-1 > 0.23 nm-1) causes thermal amorphization (melting) of these crystallites. Then, an unstable
viscoelastic amorphous phase recrystallizes into CIV-crystallites with lateral size of Lcr ≈ 6.0 nm
(Lcr-1 = 0.17 nm-1), which remain intact at the processing temperature of 533 K.
FINAL REMARKS
Cellulose is a renewable natural polysaccharide, which is the most abundant organic matter on
Earth. This natural polymer is an inexhaustible raw material for production of paper, fibers,
films, fillers, binders, glues, explosives, drugs and others valuable materials and substances.
Since the processing and use of cellulose and cellulosic materials can be carried out over a wide
temperature range, it is important to study the isophase and phase transitions of this polymer.
Cellulose has complicated supramolecular structure consisting of nanofibrils, which are built of
ordered crystallites (CR) and low ordered non-crystalline domains (NCD) in various CR/NCD
ratios, from 4 for nanocrystalline cellulose to zero for amorphized cellulose. Moreover,
crystallites have four main allomorphs, CI, CII, CII and CIV.
It has been shown that due to structural heterogeneity the non-crystalline domains have
three isophase temperature transitions, where the α1 and α2 transitions are caused by the
occurrence of segmental mobility in dense mesomorphous and medium packed amorphous
clusters, respectively; whereas the β transition is related to the mobility of small segments in
loose packed amorphous clusters, which probably are located on the outer surface of nanofibrils.
Under the action of water and other plasticizers all three isophase transitions are shifted to lower
temperatures. Various crystalline allomorphs and amorphous cellulose can be converted into
each other as a result of phase transitions, such as recrystallization, decrystallization, transitions
between various crystalline allomorphs, etc. In this review mechanism of the phase transitions
and their relation to isophase transitions of cellulose was disclosed.
Amorphous cellulose has unstable thermodynamic phase state, which nevertheless can be
maintained for a long time in the glassy state where the segmental motion is kinetically hindered.
However, when the ambient temperature (Ta) is above Tg (or Tg is below Ta) it makes possible
the development of cooperative segmental motion and implementation of recrystallization
process within a solid viscoelastic amorphous state. Important phase transition is a sol-gel
process, when cellulose is dissolved and then regenerated from the solution.
Different crystalline allomorphs can be converted into each other under certain conditions.
Mechanism of phase transitions of these allomorphs was described. In particular, the mechanism
of CIV formation consists in thermal amorphization (melting) of small crystallites (Lcr ≤ 4.3 nm)
at the treatment temperature of 533 K in glycerol, and the following recrystallization of unstable
viscoelastic amorphous phase into relative large CIV crystallites.
This review addresses only main basic research. The complexity and diversity of
mechanisms of isophase and phase transitions in cellulose requires further more detailed
investigations.
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