The Thermodynamics of the Water-Retaining Properties of Cellulose

Subscriber access provided by UNIV OF TEXAS DALLAS

Article

The Thermodynamics of the Water-Retaining Properties of Cellulose-based Networks Rose-Marie Pernilla Karlsson, Per Tomas Larsson, Per Hansson, and Lars Wågberg Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b01791 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on March 3, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

1

The Thermodynamics of the Water-Retaining

2

Properties of Cellulose-based Networks

3

Rose-Marie Pernilla Karlsson1*, Per Tomas Larsson1,2, Per Hansson3, Lars Wågberg1,4**

4

1.

5

Royal Institute of Technology, Teknikringen 56, 100 44 Stockholm, Sweden

6

2.

RISE Bioeconomy, Box 5604, 114 86 Stockholm Sweden

7

3.

Department of Pharmacy, Uppsala University, Uppsala Biomedical Center, Box 580, SE-

8

75123 Uppsala, Sweden

9

4.

10

Department of Fiber- and Polymer Technology, Wallenberg Wood Science Centre, KTH

Department of Fiber- and Polymer Technology, Division of Fibre Technology, KTH

Royal Institute of Technology, Teknikringen 56, 100 44 Stockholm, Sweden

11 12 13

KEYWORDS: cellulose, wood fiber, swelling, thermodynamics, osmotic pressure, hydrogels

14 15 16 17 18 19 20

ACS Paragon Plus Environment

1

Biomacromolecules 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 43

1

ABSTRACT

2

Non-crystalline cellulose-based gel beads were used as a model material to investigate the effect

3

of osmotic stress on a cellulosic network. The gel beads were exposed to osmotic stress by

4

immersion in solutions with different concentrations of high molecular mass dextran and the

5

equilibrium dimensional change of the gel beads was studied using optical microscopy. The

6

volume fraction of cellulose was calculated from the volume of the gel beads in dextran solutions

7

and their dry content and the relation between the cellulose volume fraction and the total osmotic

8

pressure was thus obtained. The results show that the contribution to the osmotic pressure from

9

counter-ions increases the water-retaining capacity of the beads at high osmotic pressures but

10

also that the main factor controlling the gel bead collapse at high osmotic strains is the resistance

11

to the deformation of the polymer chain network within the beads. Furthermore, the osmotic

12

pressure associated with the deformation of the polymer network, which counteracts the de-

13

swelling of the beads, could be fitted to the Wall model indicating that the response of the

14

cellulose polymer networks was independent on the charge of the cellulose. The best fit to the

15

Wall model was obtained when the Flory-Huggins interaction parameter (χ) of the cellulose-

16

water system was set to 0.55-0.60, in agreement with the well-established insolubility of high

17

molecular mass β-(1,4)-D-glucan polymers in water.

18 19 20 21


2

Page 3 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

1

INTRODUCTION

2

In strive towards a more sustainable use of the assets of the planet there has been a new focus on

3

using natural raw materials and biomimetic processes. The search for renewable raw materials

4

having a low environmental impact has expanded the interest in forest-based products. Fibers

5

from wood have long been used as a bulk material in paper, packaging materials and hygiene

6

products but, during the last decades, the view of wood fibers as a bulk material has changed and

7

research has focused on the use of fibers in advanced applications where the structural

8

components of the cellulose-rich fiber wall have been used to prepare substrates for example for

9

electronic devices 1, flame-retardant materials 2, 3 and environmentally responsive products 4, 5.

10

These new applications use the cellulose-rich material not only as a substrate or a bulk material

11

but also for providing functionality to the substrate and the final devices. Regardless of the form

12

in which cellulose is used, its interaction with water originating from the hydrophilicity of the β-

13

(1,4)-D-glucan polymer will affect the properties of the products, when they are in contact with

14

humid air or liquid water, and the most energy-demanding step in the preparation of cellulose-

15

rich materials from aqueous suspensions is the drying step. It is therefore important to be able to

16

predict, prevent or enhance the cellulose-water interaction depending on the final purpose of the

17

end product and/or the optimization of the energy consumption in the production of the materials

18

and devices. Despite previous investigations of the water uptake in cellulosic materials in moist

19

conditions 6, 7 and/or liquid water 8-10 there are few studies referring to the origin and dominating

20

factors controlling the non-ionic contribution to swelling of cellulose-rich materials in liquid

21

water. Previous studies have quantified the swelling caused by the ionic contribution from

22

counter-ions in a cellulosic network and have shown the effect of charge density in different

23

environments 11. Interestingly, these studies show that when the ionic contribution is eliminated


3


Page 4 of 43

1

by increasing the ionic strength or decreasing the pH, most of the water uptake in the fiber wall

2

still remains. The remaining water uptake is thus caused by non-ionic factors controlling the

3

swelling which are often assigned to the solubility or mixing of the polymer and the liquid and

4

the structural features of the polymer network. The aim of the present study was to increase the

5

knowledge of the non-ionic factors contributing to the water-retaining capacity of a cellulose-

6

based network and to isolate the dominant factors. This knowledge is essential to be able to

7

enhance or disrupt the water-holding capacity of a cellulose-based network.

8

In an attempt to model the swelling in water of the fiber wall of cellulose-rich fibers, it was

9

decided to utilize a specially developed cellulose model system consisting of smooth, mm-sized

10

regenerated cellulose beads. The reason for using a model material was due to the complex

11

structural hierarchy, the natural variability and inhomogeneity of fiber walls, which makes it

12

difficult to obtain good repeatability and reproducibility in swelling studies with fibers. Gel

13

beads made from molecularly dispersed β-(1,4)-D-glucan carboxymethylated to different degrees

14

were used and the osmotic de-swelling was studied by immersing the gel beads in solutions

15

containing different concentrations of high molecular mass dextran. The coherent polymer

16

network of the gel bead was considered to be semipermeable, preventing the large dextran

17

molecules from entering into the gel beads while allowing free passage of water and small ions.

18

This separation gave rise to an osmotic pressure difference between the dextran-free gel bead

19

interior and the surrounding dextran solution according to the relation ∆𝜋 = 𝜋𝑔𝑒𝑙 𝑏𝑒𝑎𝑑 ― 𝜋𝑑𝑒𝑥𝑡𝑟𝑎𝑛

(1)


4

Page 5 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

1

where 𝜋𝑑𝑒𝑥𝑡𝑟𝑎𝑛 is the osmotic pressure in the dextran solution and 𝜋𝑔𝑒𝑙 𝑏𝑒𝑎𝑑 is the osmotic

2

pressure in the gel bead. When the gel beads were equilibrated with a dextran solution the

3

relation became ∆𝜋 = 𝜋𝑑𝑒𝑥𝑡𝑟𝑎𝑛 ― 𝜋𝑔𝑒𝑙 𝑏𝑒𝑎𝑑 = 0

(2)

4

i.e. the osmotic pressure inside the gel bead and that of the dextran solution being equal. This

5

means that if the osmotic pressure of the water in the dextran solution is known it is possible to

6

force the water in the gel bead to adopt a given total osmotic pressure. Relative changes in the

7

gel bead size and hence in the volume fraction of polymers at a given total osmotic pressure of

8

the gel bead were estimated from optical measurements when the gel beads were in equilibrium

9

with the dextran solution. The estimated osmotic pressure difference was then separated into the

10

different contributions (ionic, polymer-liquid mixing and network deformability) governing the

11

osmotic pressure difference by combining polymer gel swelling theories with the experimental

12

data. This method also allowed for an indirect estimated range of the chi-parameter (χ)

13

describing the polymer-liquid interactions according to the Flory-Huggins theory.

14 15 16 17 18 19


5


Page 6 of 43

1

THEORETICAL BASIS

2

The term swelling can be operationally defined in numerous ways depending on the property

3

used to measure it. For the gel beads used in this study, the swelling and de-swelling were

4

estimated from the changes in diameter of the gel beads exposed to different aqueous solutions.

5

The close-to-spherical shape of the gel beads was used to calculate the corresponding relative

6

volume changes. Since the water-swollen gel beads are hydrogels, the theoretical description of a

7

hydrogel 12 could be applied to the swelling behavior of the gel beads. The change in free energy

8

associated with the swelling of a hydrogel (∆Gtot) is typically decomposed into three

9

contributions; liquid-polymer mixing (∆Gmix), network deformation (∆Gdef) and the contribution

10

from ions, added ionic species or counter-ions (∆Gion) 12, 13: ∆Gtot = ∆Gmix + ∆Gdef + ∆Gion

(3)

11

This means that ∆Gtot can be balanced by using e.g., by some external agent, to affect the factors

12

on the right hand side until equilibrium is reached.

13

If the change in free energy is sufficient to deform the network structure of the gel bead during

14

swelling and de-swelling this can be monitored as a change in size of the bead caused by the

15

osmotic pressure difference across the macroscopic phase boundary of the gel bead. The

16

relationship between free energy difference and the osmotic pressure (Π) is given by 14:

Π=―

μs ― μ0s Vm,s

=―

( )

∂∆G 1 ∂ns Vm,s

(4)

17

where μs is the chemical potential of the solvent (e.g. the chemical potential of water in a

18

polymer solution), μ0s is the chemical potential of the solvent in the reference state (e.g. pure


6

Page 7 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

( ) is the partial derivative of the free ∂∆G ∂ns

1

water), Vm,s is the molar volume of the solvent and

2

energy difference with respect to the number of moles of solvent molecules. Consequently, the

3

osmotic pressure in the gel bead can be decomposed by analogy with to the decomposition of the

4

free energy difference of swelling contributions due to mixing, ion and network deformation: Πtot = Πmix + Πion + Πdef

(5)

5

The coherent polymer network of the water-swollen gel beads makes them semi-permeable when

6

immersed in aqueous solutions containing dissolved large polymers. The mesh size of the gel

7

bead network prevents large polymers from entering the gel bead, with a resulting concentration

8

gradient across the gel bead phase boundary. This becomes a source of an osmotic pressure

9

difference experienced by the bead (ΔΠtot) that vanishes at equilibrium, when the osmotic

10

pressure in the solution is balanced by the sum of the three contributions to the osmotic pressure

11

in the gel bead.

12

The mixing term, Πmix, in eqn (5) is dependent on the interaction between the polymer and the

13

solvent and can be described by the Flory-Huggins model, which describes the entropic and

14

enthalpic contributions to mixing of polymer and solvent by a mean field lattice model 12:

Πmix = ―

RT [ln (1 ― φ) + (1 ― M ―1)φ + χφ2] Vm

(6)

15

where R is the gas constant, T is the absolute temperature, Vm is the molar volume of the solvent

16

and φ and M are respectively the volume fraction and the molecular weight of the polymer (set

17

to infinity for a gel network) and χ is the polymer-solvent interaction parameter. The osmotic

18

pressure caused by the concentration difference of ionic species between the interior and exterior


7


Page 8 of 43

1

of the gel bead, Πion, is described by the van´t Hoff law 12, 13, with the special case that Csol j =0

2

describes the counter-ion contribution of charged polymers in the absence of any extra added

3

salt: N

Πion = RT

∑(C

gel j

― Csol j )

(7)

j=1

4

The network deformation contribution, Πdef, is given by the Wall theory of rubber elasticity 12, 15

5

which describes the elastic response in a network of extended polymers. The Wall model can be

6

used to describe the deformation pressure according to:

Πdef =

(

2 3

( )) φ0

RTφ 1― Vm2P φ

(8)

7

where R is the gas constant, T the absolute temperature, Vm the molar volume of the solvent, φ

8

the volume fraction of polymer in the gel bead and P the number of polymer segments between

9

cross-links. To describe stiff polymers, this number is assumed to be smaller than the actual

10

number of monomers between cross-links reducing the flexibility of the polymer chain. φ0 is the

11

volume fraction of polymer in the gel beads in a hypothetical limiting reference state.

12 13 14 15


8

Page 9 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

1

EXPERIMENTAL SECTION

2

MATERIALS

3

Never-dried wood-based dissolving fibers received from Domsjö Fabriker AB, Örnsköldsvik

4

Sweden were used as the cellulose source for preparation of the gel beads. The charge density of

5

the pulp fibers was determined to be 30 µeq/g using conductometric titration 16. Using

6

carbohydrate analysis the relative glucose content was found to be 96%.

7

The dissolving pulp was carboxymethylated using ethanol (96 vol%) (VWR International AB,

8

Sweden), monochloroacetic acid (ClCH2COOH) (ACS reagent ≥99.0 %) (Sigma-Aldrich AB,

9

Sweden), 2-propanol (ACS reagent) (VWR International AB, Sweden), sodium hydroxide

10

(NaOH) (puriss p.a. ACS reagent ≥98 %) (Sigma-Aldrich Sweden AB), methanol (ACS reagent)

11

(Thermo Fisher Scientific, Sweden) and acetic acid (CH3CO2H) (Glacial ACS grade ≥99.7 %)

12

(Thermo Fisher Scientific, Sweden).

13

When preparing the gel beads, the following chemicals were used to dissolve the pulp fibers:

14

ethanol (96 vol%) purchased from VWR International AB, Sweden, together with lithium

15

chloride (LiCl) (puriss p.a., anhydrous ≥99 %) and N,N- dimethylacetamide (DMAc) (puriss

16

p.a., ≥99.5 %) both the LiCl and DMAc were purchased from Sigma-Aldrich Sweden AB.

17

The cellulose was regenerated in ethanol (96 vol%) (VWR International AB, Sweden) and

18

hydrochloric acid (HCl) (37 % analytical reagent) (Thermo Fisher Scientific, Sweden).

19

Dextran with an average molecular weight of 1.500-2.800 kDa from Leuconostoc mesenteroides

20

was purchased from Sigma Aldrich, AB Sweden.


9


Page 10 of 43

1

Fluorescence labeling of dextran was achieved using dimethyl sulfoxide (DMSO) (GPC grade)

2

(Fisher chemical, Sweden), pyridine (≥ 99 %) (Sigma-Aldrich, Sweden AB), fluorescein

3

isothiocyanate (FITC) ( ≥ 90 %) (Sigma-Aldrich, Sweden AB) and dibutyltin dilaurate (95 %)

4

(Sigma-Aldrich, Sweden AB).

5

For pH adjustment, sodium hydroxide (NaOH) (puriss p.a. ACS reagent ≥98 %) (Sigma-Aldrich,

6

Sweden AB) and hydrochloric acid (HCl) (37 % analytic reagent) (Thermo Fisher Scientific,

7

Sweden) were used.

8

METHODS

9

Carboxymethylation

10

The pulp fibers were carboxymethylated to 300 and 600 µeq/g following the procedure described

11

by Wågberg et al 17. Never-dried dissolving pulp fibers (20 g dry weight) was reslushed in 2 L of

12

deionized water for 30 000 revs using a PTI Austria-disintegrator model 95568. The water was

13

removed through filtration followed by a solvent-exchange step using 0.8 L ethanol.

14

Subsequently, for the 600 μeq/g fibers, the fibers were impregnated for 30 min with a solution

15

containing 2 g of ClCH2COOH and 100 ml isopropanol. 3.2 g NaOH was mixed with 100 mL

16

MeOH and the fibers were allowed to dissolve before 0.4 L of isopropanol was added and the

17

mixture was heated to 85 °C. The impregnated pulp fibers were oxidized in the warm solution at

18

85 °C for 1h under stirring and reflux. After the oxidation reaction, a washing step followed

19

using 4 L of deionized water, then 0.4 L of 0.1 M CH3CO2H and finally 1 L of deionized water.

20

The amount of ClCH2COOH used to obtain 300 µeq/g was 0.9 g in 100 mL isopropanol. The

21

total charge density of the pulp fibers after oxidation was determined using conductometric

22

titration 16.


10

Page 11 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

1

Preparation of cellulose-based gel beads

2

The gel beads were prepared using a procedure previously described 18, 19 with modifications

3

according to the work of Karlsson et al 20. Wet pulp fibers containing 1 g of dry mass were

4

dewatered and solvent-exchanged to ethanol and subsequently to DMAc through filtration steps.

5

The solvent-exchange steps took place over three days for each solvent, the solvent being

6

changed at least twice each per day using about 150 ml each time. After the solvent exchange,

7

100mL of DMAc was dewatered by heating it to 105 °C and keeping it warm under stirring for

8

about 30min. At the same time 7 g of LiCl was dewatered in an oven set to 105 °C. The

9

dewatered LiCl was then added to the DMAc and dissolved while the solution was allowed to

10

cool. The DMAc soaked pulp fibers were added to the DMAc/LiCl solution when the solution

11

had cooled down to a temperature below 40 °C. The fiber/DMAc/LiCl mixture was transferred to

12

a closed glass bottle and the fibers were dissolved under stirring at 5 °C. After 24 h, a clear

13

solution containing approximately 1 wt% cellulose was obtained.

14

The solution was filtered through a 45 µm acrodisc PTFE-filter (VWR, Sweden) before being

15

drop-wise precipitated in a 100 mL beaker containing about 75 mL of 0.03 M HCl (aq) and 15

16

mL 96 %-ethanol using a needle with a diameter of 0.6 mm. The gel beads formed were left in

17

the non-solvent at 5 °C for 24 h during which a DMAc phase was formed due to spontaneous

18

exchange between the non-solvent in the beaker and the DMAc contained in the gel beads. After

19

24 h, the liquid in the beaker was gradually decanted and displaced with milli-Q. The gradual

20

displacement was performed twice a day for at least one week.

21 22


11


Page 12 of 43

1

Exchanging the counter-ions in the gel beads

2

After precipitation of the beads in the acidic non-solvent and washing with milli Q water the

3

acidic groups in the beads were assumed to be protonated. To change the counter-ions from

4

hydrogen to sodium, the gel beads were immersed in a solution of 0.001 M NaHCO3 (aq) set to

5

pH 9 using NaOH (aq). The gel beads were left in the solution for at least 1 h and then washed

6

with milli Q water.

7

Fluorescence labelling of dextran

8

Dextran was fluorescein labelled using a method previously developed for dextran 21, 22. 1 g of

9

dextran was mixed in 10 ml of dimethyl sulfoxide (DMSO) together with four drops of pyridine.

10

In the absence of direct light, 21.6 mg of fluorescein isothiocyanate (FITC) was added to the

11

DMSO together with 20 mg of dibutyltin dilaurate. The amount of FITC was chosen to cover 1

12

fluorescent label per 100 glucose units. The dextran solution was then heated and kept at 95 °C

13

for 2 h. The solution was then dialyzed for a week and freeze dried before use.

14

Thermogravimetric Analysis (TGA)

15

A TGA/DSC 1 STAR Mettler Toledo system was used to determine the solid content in the gel

16

beads. The beads were rolled over a moist filter paper, placed in a 70 µl aluminum crucible and

17

weighed prior to measurement.

18

The first two temperature ramps were performed under nitrogen and ranged from 35 °C to 100

19

°C at a rate of 10 °C/min followed by a ramp from 100 °C to 220 °C at 5 °C/min after which the

20

temperature was held at 220 °C for 5 min in order to reach complete dewatering of the gel beads

21

without degrading the cellulose23. The atmosphere was switched to oxygen and kept at 220 °C


12

Page 13 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

1

for another 2 min before the cellulose degrading temperature ramp was started from 220 °C to

2

650 °C at 10 °C/min. No inorganic residue was visually detected after the measurement. The dry

3

content was calculated according to the following expression:

Dry content (wt %) =

ms

(9)

mtotal

4

where mtotal is the weighed total mass of the water-swollen gel beads prior to measurement and

5

ms is the mass read from the TGA curve at the plateau at 220 °C (see figure S1).

6

Dynamic light scattering

7

Dynamic light scattering equipment (Zetasizer ZEN3600 Malvern Instruments) with 633 nm

8

laser and a detector angle of 178° was used to measure the hydrodynamic radius of dextran

9

before and after labelling with FITC. The hydrodynamic radius was measured in dilute samples

10

containing 1.5 and 0.15 g/L dextran in pure water. For each sample, 10 measurements were

11

performed.

12

De-swelling measurements

13

The dimensional changes of the gel beads were studied immersed in the different solutions using

14

a Dino-Lite Premier AM4113TL light microscope. The DinoCapture 2.0 version 1.5.16 software

15

was used to determine the radius of the gel bead.

16

The volume fraction of cellulose was calculated using the dry content of the gel bead and the

17

measured volume of the gel bead. The dry mass in the gel bead was assumed to remain constant

18

throughout the analysis and the change in volume was thus assigned to a change in water


13


Page 14 of 43

1

content. Prior to immersion in dextran solution, the counter-ion of the carboxyl group in the gel

2

bead was changed to sodium and the gel beads were kept in water at a pH about 10 for two days.

3

Osmotic Pressure of Dextran Solutions

4

The osmotic pressure of dextran solutions has been determined in previous studies 24, 25. In this

5

study, the expression based on empirical data obtained by Vink 26 was used to calculate the

6

osmotic pressure of the dextran solutions used.

7

Confocal Laser Scanning Microscopy

8

Fluorescence-labeled dextran together with a laser scanning microscope of model Zeiss LSM 5

9

Pascal with a Zeiss objective EC Plan-Neofluar 10X/0.30 M27 was used to investigate the

10

possible diffusion of dextran into the gel beads. The dextran was detected using an Argon laser

11

source with a wavelength of 488 nm to excite the attached FITC. The transmission and detector

12

gain were set to 57.5 % and 600 respectively. Prior to the analysis, the carboxylic groups in the

13

gel beads were changed to sodium form and the beads were swollen in a pH of 10. The gel beads

14

were then equilibrated for one week with solutions of 1 % by weight of FITC-dextran prepared

15

in water at a pH of 10. Before being placed on a microscopy glass slide, the gel beads were

16

rolled on a moist filter paper and cut in half with a razor blade. The focus was set on the cross-

17

section of the gel bead.

18 19 20


14

Page 15 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

1

RESULTS

2

Material characteristics

3

Gel beads with three different charge densities were prepared. One set was made from fibers as

4

received with a charge density of 30 µeq/g (GB30) and two sets were made with the fibers

5

carboxymethylated to charge densities of 300 µeq/g (GB300) and 600 µeq/g (GB600). The solids

6

content of the gel beads immersed in milli Q water was measured using TGA as described

7

earlier. The solids contents of GB30, GB300 and GB600 were 2.7, 1.7 and 1.5 wt% respectively.

8

Dextran with a molecular weight ranging between 1.500-2.800 KDa (supplier information) was

9

used as received without further purification. The hydrodynamic diameter (Dh) of dextran was

10

measured using dynamic light scattering (DLS) in pure water and the average hydrodynamic

11

diameter was 94 (±2) nm.

12

The diffusion of dextran into the gel beads was studied with dextran molecules fluorescence

13

label before contact with the gel beads. The hydrodynamic diameter increased slightly to 107

14

(±3) nm as a result of the fluorescence label. The investigated gel beads were placed in a solution

15

of 1 wt% fluorescence-labeled dextran for one week before they were cut in half and placed on a

16

thin microscope glass and monitored in the laser scanning microscope. The images captured by

17

the microscope are shown in Figure 1. The images show a slight detection of dextran in the low

18

charged gel beads which faded out as the charge density increased. Laser scanning microscopy is

19

not a quantitative method and thus cannot tell how much dextran is present. As a complement,

20

the average mesh-size of the network structure based on the SAXS-data in previous work 20 and

21

the dry content of the gel beads were used to assess the significance of the diffused dextran. The

22

calculations given in detail in Supporting Information, section 2, show that the average distance


15


Page 16 of 43

1

between scattering entities for GB30 in 1 wt% dextran was 116 nm which shows that the dextran

2

molecules were not able to penetrate into the beads and could therefore safely be used in the

3

osmotic deswelling experiments. In order to calculate the osmotic pressure the relation between

4

dextran concentration and osmotic pressure, obtained by Vink 26 was used, and the osmotic

5

pressure of the prepared dextran solutions ranged from 0.3 to 754.3 kPa

6

Figure 1. Laser Scanning Micrographs of gel bead cross-sections showing the diffusion of fluorescence labeled dextran into gel beads with different charge densities. From left to right: GB30, GB300 and GB600. The gel beads were given one week to reach equilibrium in contact with 1wt% dextran solution. The scale-bar indicates 200 μm.

7

Osmotic (de-) swelling of gel beads in dextran solutions

8

The volume of the gel beads in equilibrium with solutions of different dextran concentrations

9

was measured optically and used together with the dry content of cellulose to calculate the

10

volume fraction of cellulose inside the gel bead. The volume fraction was plotted against the

11

osmotic pressure induced by the dextran concentration in contact with the beads (Figure 2). For

12

GB30 an initial increase in volume fraction was seen at an osmotic pressure between 14 kPa to


16

Page 17 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

1

60 kPa while for GB300 and GB600 the increase in volume fraction occurs between 60 kPa to

2

297 kPa with the steepest change being for GB300. At the highest osmotic pressure, GB30,

3

reached a volume fraction of 14.6 % and GB300 and GB600 reached 10.3 % and 10.9 %

4

respectively.

Figure 2. The volume fraction, φ of cellulose in gel beads in equilibrium with dextran solutions plotted against the osmotic pressure of the dextran solution. The gel beads were pre-swollen to their maximum in a pH of 10. The dextran-gel bead system was allowed to equilibrate for one week. GB30- black circles, GB300- red stars and GB600- blue squares.

5

Degree of volume recovery when releasing the external osmotic pressure

6

After the gel beads had been equilibrated and measured in the different dextran solutions, they

7

were transferred back to water at a pH of ca. 10. After one week the recovered volume of the gel

8

beads was measured. Figure 3 shows the relative volume before dextran solution, in dextran

9

solution and when placed back in water (pH 10) after dextran solution for four different dextran

10

concentrations. The volume of the gel beads equilibrated with dextran solution decreased with

11

increasing dextran concentration, and the decrease was greater for gel beads with low charge


17


Page 18 of 43

1

density. The recovered volume after immersion in dextran solution was dependent on both the

2

dextran concentration and the charge density. Exposure to a high dextran concentration i.e. a

3

higher osmotic pressure, reduced the volume recovery and this effect was larger for the gel beads

4

with lower charge density. GB30 had a recovered volume of 76 % after exposed to osmotic

5

pressure of 60 kPa but the recovered volume was only 12 % after a pressure of 754 kPa. For

6

GB600, the corresponding figures were 100 % and 73 % for 60 kPa and 754 kPa respectively.

7

Additional data can be seen in Figure S3 in supporting information.


18

Page 19 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

Figure 3. Relative gel bead dimension recovery (Vf/V0) after release of the osmotic stress, for gel beads of different charge densities exposed to different levels of osmotic stress. The bars represents the relative gel bead volume; before dextran in pH 10 (light grey), in dextran (dark grey) and after dextran in pH 10 (pattern). The graphs show the results for four different osmotic pressures, 0.6 kPa (a), 60 kPa (b), 297 kPa (c) and 754 kPa (d).

1

The different factors contributing to the water-retaining capacity of the gel beads

2

Swelling of gel beads due to dissociation of counter-ions

3

The ionic contribution to the swelling of the gel beads was investigated extensively by Karlsson et al 20

4

who quantified how the swelling was affected by the pH and ionic strength. The swelling of the gel beads

5

in the present work was studied by measuring the gel beads in water at a pH of about 2 and subsequently

6

at pH 10. The contribution to the swelling caused by the dissociation of counter-ions that occured when

7

the pH was increased from 2 to 10 is shown in Figure 4 as the increase in water uptake, ΔWupt (g water/g

8

cellulose), between the two different states for the three different gel beads as a function of the charge

9

density.

Figure 4. The increase in water uptake, ΔWupt (g water/g cellulose), when dissociating the counter ions in


19


Page 20 of 43

the gel beads at different charge densities. Calculations based on the volume increase of the beads measured by optical microscopy. The coefficient of determination for the fit was R2=0.97 and the broken line is only a guide to the eye.

1

Network model and interaction parameter

2

The different contributions to the water-retaining capacity were quantified using the

3

experimental total osmotic pressure and the volume fraction of cellulose at that pressure. The

4

theoretical models applied were the Flory-Huggins model (Equation. 6) describing the mixing of

5

polymer and water using an interaction parameter,𝜒, which has been changed between 0.5-0.9,

6

and the Van´t Hoff expression (Equation. 7) describing the ionic contribution to the water-

7

retaining capacity. These two models together with the total osmotic pressure, 𝛱𝑇𝑜𝑡 and network

8

pressure, 𝛱𝑑𝑒𝑓 were combined in the expression:

𝑅𝑇 [ ― ln (1 ― 𝜑) ― (1 ― 𝑀 ―1)𝜑 ― 𝜒𝜑2] + 𝑅𝑇 𝑉𝑚

9 10

𝑁

∑(𝐶

𝑔𝑒𝑙 𝑗

― 𝐶𝑠𝑜𝑙 𝑗 ) + 𝛱𝑑𝑒𝑓 = 𝛱𝑇𝑜𝑡

(10)

𝑗=1

Equation 10 can be rearranged so that the contribution from the network, 𝛱𝑑𝑒𝑓, is left as a rest term without applying any specific model.

𝑅𝑇 𝛱𝑑𝑒𝑓 = 𝛱𝑇𝑜𝑡 ― [ ― ln (1 ― 𝜑) ― (1 ― 𝑀 ―1)𝜑 ― 𝜒𝜑2] ― 𝑅𝑇 𝑉𝑚

𝑁

∑(𝐶

𝑔𝑒𝑙 𝑗

― 𝐶𝑠𝑜𝑙 𝑗 )

(11)

𝑗=1

11

Figure 5 shows the term 𝛱𝑑𝑒𝑓 obtained from this equation plotted against the inverse volume

12

fraction of the cellulose in the gel beads. Only plots for 𝜒 ≥ 0.55 are shown; for 𝜒 < 0.5 the data

13

for the different charge densities did not collapse on a single curve (see SI). The elastic network,

14

Wall model (Equation 8), was fitted to these data points and the fit was evaluated for different


20

Page 21 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

1

interaction parameters (Figure 5 a-d and S4). The initial volume fraction (𝜑0) was chosen as

2

0.007 (the volume fraction of polymer solution before crosslinked) and P was set to a value of

3

10. The best fit between the 𝛱𝑑𝑒𝑓 data and the Wall model was obtained using an interaction

4

parameter of 0.55, judged by calculating the sum of all squares (SSR) which showed that the

5

discrepancy between the experimental data and the Wall model became larger as the interaction

6

parameter increased. The application of other 𝜒-values is shown in SI to demonstrate the

7

sensitivity of this parameter for matching the Wall theory.

Figure 5. Calculated network pressure using different interaction parameters; (a) 0.55, (b) 0.60, (c) 0.70 and (d) 0.80 plotted against the inverse volume fraction (1/φ) of cellulose. Data fitted to the Wall model,


21


Page 22 of 43

equation (8), using an initial volume fraction of 0.007 and P = 10. GB30- circles, GB300- stars, GB600Squares the Wall model- solid line.

1

Quantification of the osmotic pressure from ionic, mix and network factors

2

The osmotic pressures caused by each factor (Equations 6, 7 and 11) are summarized in Table 1.

3

The data are based on calculations using the interaction parameter 0.55. The data obtained using

4

other interaction parameters are summarized in the supporting information (section 4). Table 1

5

shows that the dominating factor contributing to the water-retaining capacity shifts from the

6

ionic contribution at a low volume fraction of cellulose (below 2 vol%) to the network

7

deformation contribution as the volume fraction of cellulose increases.

8 9 10 11 12 13 14 15 16


22

Page 23 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Biomacromolecules

Table 1. The three factors, Πion, Πmix, and Πdef for different volume fractions of cellulose.

Charge (μeq/g)

30

30

30

30

30

30

30

1.8

1.8

1.9

1.9

3.0

6.6

14.6

Πion (kPa)

2.0

2.0

2.1

2.2

3.4

6.5

18.6

Πmix (kPa)

-1.9

-1.9

-2.0

-2.2

-4.8

-15.8

13.3

Πdef (kPa)

0.2

0.5

4.2

14.2

61.4

304.9

722.4

Πtot (kPa)

0.3

0.6

4.3

14.2

60

295.6

754.3

300

300

300

300

300

300

300

1.1

1.2

1.1

1.2

1.3

6.8

10.3

Πion (kPa)

12.5

12.8

12.4

12.7

14.3

79.6

124.7

Πmix (kPa)

-0.79

-0.83

-0.79

-0.82

-1.00

-16.2

-18.3

Πdef (kPa)

-11.4

-11.3

-7.39

2.20

46.7

232.3

647.9

Πtot (kPa)

0.3

0.6

4.3

14.2

60

295.6

754.3

600

600

600

600

600

600

600

1.0

1.0

1.0

1.2

1.2

3.3

10.9

Πion (kPa)

22.6

22.0

22.9

26.0

26.6

73.8

249.2

Πmix (kPa)

-0.64

-0.62

-0.68

-0.85

-0.90

-5.71

-16.8

Πdef (kPa)

-21.5

-20.8

-18.0

-11.0

34.2

227.5

521.9

Πtot (kPa)

0.3

0.6

4.3

14.2

60

295.6

754.3

Volume fraction (%)

Charge (μeq/g) Volume fraction (%)

Charge (μeq/g) Volume fraction (%)

The volume fraction of cellulose increases due to the de-swelling of the gel beads in equilibrium


23


Page 24 of 43

with solutions having different concentrations of dextran. An interaction parameter of 0.55 was used.

1 2

DISCUSSION

3

Gel bead characteristics

4

The gel beads used in this work have previously been subjected to careful structural

5

characterization 20. Small angle X-ray scattering in combination with Solid State NMR showed

6

that the gel beads consist of a non-crystalline molecularly dispersed polymer network.

7

Furthermore, as the charge density of the gel beads was increased the network became more

8

stretched and showed typical properties of a polyelectrolyte network regarding the water uptake

9

at different pH and different ionic strengths.

10

It was assumed that the oxidation occurred on the surface of the fibrils during

11

carboxymethylation of the fibers prior to dissolution, leaving the polymers in their crystalline

12

core unmodified27. When the oxidized fibers were dissolved, the solution thus contained

13

uncharged and charged polymers, and the final gel bead contained a network consisting of a

14

mixture of those two components. In this work, the polymer in the gel beads, consisting of β-

15

(1,4)-D-glucan polymers carboxymethylated to different degrees, will be simply referred to as

16

the “polymer”.

17

As shown with the CLS images in Figure 1 and based on the calculations in the supporting

18

information, section 2, the average distance between scattering objects (or crosslinks) was about

19

116 nm. This is somewhat an overestimation of the typical ‘pore size’ of the gel beads, since the

20

25 nm scattering objects observed by SAXS measurements are tethered to a sparser network of


24

Page 25 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

1

polymers 20. The network of the gel bead can be seen as semipermeable excluding most of the

2

dextran molecules. This hypothesis is based on studies showing that a pore size having a

3

diameter 3-5 times larger than the macromolecule was needed for rapid penetration into the

4

network 28. The reason why the gel beads with high charge density (GB300 and GB600) were

5

better at excluding dextran than GB30 was probably, as shown by SAXS20, the more stretched

6

polymer conformation of the highly charged cellulose, which created a denser network, and

7

hence increased the steric hindrance to the penetration of dextran molecules.

8

Contraction of cellulose-based polymer networks using osmotic pressure

9

Figure 2 shows how the volume fraction of cellulose in the gel beads changed when they were

10

equilibrated with dextran solutions of different concentrations. It is clear that the increasing

11

dextran concentrations, i.e. increasing osmotic pressure, were sufficient to de-swell the beads

12

when a sufficient pressure was reached. It is also clear that the concentration of dextran

13

determined the degree of de-swelling, since the de-swelling is due to the difference in chemical

14

potential of the water inside the gel bead and in the dextran solution. Since the dextran molecules

15

cannot penetrate the network of the gel bead to any significant extent, water flows out from the

16

gel bead into the dextran solution in order to equalize the chemical potential (i.e. the osmotic

17

pressure difference) between the two compartments. This release of water, leads to a contraction

18

of the gel bead, until the water inside the polymer network has the same chemical potential as the

19

water on the outside.

20

In general, the ability of a polymer network to retain water is dependent on how prone the

21

polymer is to interact with the water, the deformability of the polymer chains under load and, if

22

present, the ability of the counter ions to associate with the immobile network charges to increase


25


Page 26 of 43

1

the osmotic pressure inside the network. In Figure 2, the curves for GB30, GB300 and GB600

2

show that the gel beads with low charge density (GB30) started to contract at a lower pressure

3

(14 kPa to 60 kPa) than GB300 and GB600 (60 kPa to 297 kPa) that the contraction for GB300

4

was more severe than for GB600 in the 60 kPa to 297 kPa range. One possible explanation of the

5

slightly higher water holding capacity of the gel beads with high charge density may be that the

6

ionic species is able to decrease the chemical potential of the water and consequentially to

7

increase the osmotic pressure. This statement was indicated when comparing the gel beads with

8

different charge densities at a specific total osmotic pressure. In the case of a total osmotic

9

pressure of e.g. 296 kPa it was seen that the ionic pressure was 11-12 times higher in GB300 and

10

GB600 compared to GB30 while the relative network pressure was not changed as much. In all

11

cases the gel beads were swollen to a maximum size at a pH of about 10 before they were

12

immersed in the dextran solutions at pH 10 so that it was possible to predict the contribution of

13

the counter-ions of the carboxyl groups to the swelling as they are all expected to be dissociated

14

at pH 10. Since the mobile ionic species in the network gel, i.e. the dissociated counter-ions of

15

the carboxylate group, are considered to be confined to the gel beads due to the need for electro-

16

neutrality, an increase in carboxylate groups increases the osmotic pressure caused by the ionic

17

species. This slightly increases the resistance to dewatering because of its ability to lower the

18

osmotic pressure difference between the water inside the gel and the water in the dextran

19

solution, as can be seen for GB300 and GB600 in Figure 2.

20

Quantification of factors controlling the water-retaining capacity by applying theoretical

21

models to experimental data

22

To calculate the osmotic pressure contributions, several assumptions were made and the values

23

of several parameters had to be fixed to calculate the mixing, the ion and the network


26

Page 27 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

1

contributions. In the mixing term (Flory-Huggins model, Equation 4) the molar volume of the

2

solvent, 𝑉𝑚, was assumed to be that of pure water. The apparent or effective molecular weight of

3

the polymer (M) was assumed to be infinite. This was motivated by the loss of translational

4

entropy and molecular individuality in a highly cross-linked polymer network. One of the

5

challenges when applying the Flory-Huggins model to a cellulose water system is to set a value

6

for the interaction parameter. This was particularly challenging since the gel beads contained a

7

non-homogeneous distribution of ionic groups on the cellulose polymer chain. The interaction

8

parameter measures the unwillingness of the polymer to mix with the solvent rather than with

9

itself. Since cellulose is known to be insoluble in water 29 a range of interaction parameters

10

between 0.50 and 0.90 were therefore tested, representing a system in which the components

11

tend to phase separate to different degrees.

12

Figure 3 shows the re-swelling of the gel beads as the applied osmotic pressure is released i.e.

13

when the gel beads were transferred from the dextran solutions into alkaline (pH 10) water. The

14

measurement was made 7 days after the osmotic pressure was released. Already after being

15

subjected to an osmotic pressure of 60 kPa, the gel beads with low charge density (GB30, Figure

16

3 b) was unable to fully recover their initial volume within the chosen time frame. The inability

17

to quickly recover the volume was seen for all the charge densities with increasing osmotic

18

pressure of the dextran solution but it was always more pronounced for GB30. After being

19

subjected to the highest osmotic pressure used in this work, 754 kPa, GB30 was able to reach a

20

volume of 12 % of the initial gel bead. This shows that the characteristic time of swelling of the

21

gel bead was dependent on the charge content of the gel bead and also that 7 days was not a

22

sufficiently long time for the gel beads to recover from the possible polymer-polymer

23

interactions induced as the chains approach each other. In order to be able to rule out the


27


Page 28 of 43

1

possibility that the slow re-swelling was a consequence of crystallization resulting from the

2

interaction between cellulose chains, solid state NMR was used to study gel beads that were

3

dried and subsequently re-swollen in water (see Figure S8). No signal corresponding to

4

crystalline regions for cellulose was detected. The fact that the gel beads with the lowest charge

5

density, GB30, re-swelled the least could indicate that the interaction between the cellulose

6

chains upon contraction in GB30 were stronger compared to GB300 and GB600. The change in

7

the structure of cellulose during oxidation (i.e. introduction of carboxyl groups) could be the

8

reason why the cellulose chains in GB300 and GB600 were hindered to interact to the same

9

extent as GB30. The ability to re-swell after drying, depending on charge content of the gel

10

beads were previously shown 20 and the results were similar to the results shown in this work.

11

Model fitting of elastic network and interaction parameter

12

In Figure 5, the osmotic pressure from the elastic deformation of the network, 𝛱𝑑𝑒𝑓, calculated

13

from Equation 11, was plotted against the inverse volume fraction of cellulose in the beads. The

14

calculated values represent the osmotic pressure needed to balance the osmotic pressure from the

15

ionic and Flory-Huggins mixing contributions. The interaction parameter, 𝜒 in Equation 11 was

16

changed over a range of values and Figure 5 shows the results of using an interaction parameter

17

of 0.55, 0.60, 0.70 and 0.80. In the figure, the data from the three different gel beads GB30,

18

GB300 and GB600 representing 𝛱𝑑𝑒𝑓 all collapse on a single curve, indicating that the response

19

from the network at a given volume fraction of cellulose was independent of the charge density.

20

The similarity of 𝛱𝑑𝑒𝑓 for all gel beads indicated that the deformation contribution to the osmotic

21

pressure could easily be separated from the other contributions to the osmotic pressure. The

22

change in water uptake as a function of charge density (Figure 4) indicates that there is a relation


28

Page 29 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

1

between the gel bead expansion and charge density (counter-ion dissociation) of the gel beads,

2

suggesting that the response from the network structure is the same.

3

In order to fit the Wall model to the 𝛱𝑑𝑒𝑓 values (Figure 5), it is necessary to make assumptions

4

regarding P and 𝜑0. The parameter P, related to the number of polymer segments between cross-

5

links, was set to a value smaller than the number of monomers (the value 10 was used), because

6

of the relatively high rigidity of the -(1,4)-D-glucan polymer 30. The limiting volume fraction of

7

polymer in the reference state (𝜑0) is an estimate of the polymer volume fraction in a state where

8

the network chains have the same entropy as free chains, except for a correction for the entropy

9

of crosslinking the chains 12 (the value 0.007 was used, corresponding to the volume fraction in

10

the gelling solution). The sensitivity to the assumptions regarding the number of chain segments

11

and initial volume fraction was evaluated to some extent, and the results are shown in figures S4-

12

S7 in the Supporting Information. By comparing the four different fittings, it was concluded that

13

the agreement with the Wall model (using φ0=0.007 and P=10) was best fitted when calculated

14

using an interaction parameter of 0.55, as judged by the residual sum of squares.

15

The dominating factor controlling the water-retaining capacity in the gel beads

16

The osmotic pressures due to cellulose-water mixing, 𝛱𝑚𝑖𝑥, to the immobile ions, 𝛱𝑖𝑜𝑛, and to the

17

cellulose network, 𝛱𝑑𝑒𝑓, were all calculated from Equations 6, 7 and 10 in order to determine

18

which of these has the dominant influence on the water-retaining capacity of the cellulose-based

19

network. Based on the fitting of the Wall model (Figure 5), the different factors controlling the

20

osmotic pressure; Πmix, Πion and Πdef were calculated using an interaction parameter of 0.55.

21

The results are summarized in Table 1, which shows how the distribution between the different

22

factors changes when the volume fraction increases. In general, based on the data in table 1, the


29


Page 30 of 43

1

water-retaining capacity is dominated by 𝛱𝑖𝑜𝑛 and 𝛱𝑑𝑒𝑓. The dominance shifted from 𝛱𝑖𝑜𝑛to 𝛱𝑑𝑒𝑓

2

at volume fractions above 2 vol%, regardless of the charge density. In most cases, within the

3

investigated range of volume fractions, 𝛱𝑑𝑒𝑓 was the dominant factor for water holding capacity.

4

It can also be seen that 𝛱𝑑𝑒𝑓 shifts from negative to positive when the volume fraction is

5

increased, meaning that there is a shift from resisting further swelling expansion to resisting

6

further deswelling compression for this term. A change from a negative to a positive value can

7

be seen at low volume fractions of GB300 and GB600. The lack of data points in the same range

8

of volume factions for GB30 means that this shift in sign is not shown in table 1. However, by

9

combining the 𝛱𝑑𝑒𝑓 values in the graphs in Fig 5 and the decrease in 𝛱𝑑𝑒𝑓value from high to

10

lower polymer volume fractions, it is plausible to assume that this shift also occurs at about the

11

same volume fraction as for GB300 and GB600. All this means that if 𝛱𝑑𝑒𝑓 is negative the

12

network is constraining the swelling of the network caused by the polymer-solvent mixing and

13

the swelling caused by the charged groups in the gel beads. This constraint is due to the inability

14

of the polymer chains and/or polymer network to extend further. When 𝛱𝑑𝑒𝑓 is positive, the

15

network is working in favor of retaining the water due to its inability to deform under stress. The

16

resistance of the network to deformation is probably due to the rigidity of the cellulose polymer

17

chains and that the crosslinks (entanglements) are formed with the chains in an expanded state.

18

The value representing the cellulose water interaction 𝛱𝑚𝑖𝑥 was constantly low, since for

19

interaction parameters above 0.5 the cellulose chains prefer to interact with each other rather

20

than to interact with the water and this in turn means that they are working against retaining the

21

water.

22


30

Page 31 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

1

The relevance of using cellulose-based gel beads as a model for the swelling behavior of

2

delignified wood fibers

3

The cellulose-based gel beads were used as a model material for studying the swelling of a

4

cellulose-rich fiber wall since gel beads have previously shown a water uptake at different pH

5

and different salt concentrations similar to that of delignified wood fibers 20. The beads have also

6

shown the same irreversible loss of re-swelling (i.e. hornification) after drying. These similarities

7

make the gel beads a suitable model material for a delignified fiber wall. However, the gel beads

8

consist of a molecularly dispersed cellulose polymer network held together by physical

9

entanglement whereas a fiber wall is a supramolecular structure based on aggregates of cellulose

10

nanofibrils 31, 32. The differences in structural characteristics between the gel beads and a

11

cellulose rich fiber wall together with the limitations of the Flory-Huggins model makes direct

12

comparison on an absolute scale difficult. However, a qualitative comparison of gel beads and a

13

cellulose rich fiber wall will indeed shed some light on phenomena that are mainly coupled to

14

polymer-polymer and polymer-water interactions in both systems. The structural difference

15

affects the ability of the network to take up water. The gel beads with a less hierarchal structure

16

than the fiber wall, can thus take up more water than the fiber wall; the low charged gel bead,

17

GB30, in acidic conditions takes up about 42.6 g/g 20 while a low charged fiber wall takes up

18

about 1 g/g 27. The volume fraction under acidic condition is thus 1.6 vol% and 40 vol%

19

respectively for the gel bead and the fiber wall. This study includes experimental values up to

20

about 15 vol% so the experimental data must be extrapolated to reach the typical volume

21

fractions for a fiber wall. The extrapolation was performed for GB600 (Figure S9). To

22

extrapolate the data, the Wall model was applied as the contribution from the deformability of

23

the network and the chi-parameter in the Flory-Huggins model was chosen to be 0.55. Figure S9


31


Page 32 of 43

1

shows the relative contributions to the osmotic pressure of 𝛱𝑚𝑖𝑥, 𝛱𝑑𝑒𝑓 and 𝛱𝑖𝑜𝑛. In the range of

2

volume fractions for which the experimental data was obtained and up to about 35 vol%, 𝛱𝑑𝑒𝑓

3

was dominating at most of the volume fractions but when the volume fraction increased to a

4

level corresponding to a fiber wall (above 35 vol%), the dominance shifted from 𝛱𝑑𝑒𝑓 to 𝛱𝑚𝑖𝑥.

5

This shift was most likely due to the decreased entropy of mixing when the volume fraction of

6

cellulose was increased. It hence needs to be stressed that 𝛱𝑖𝑜𝑛 will only have dominating

7

influence on the swelling pressure at low volume fractions of cellulose and the critical volume

8

fractions of cellulose the dominance is changed from 𝛱𝑑𝑒𝑓 to 𝛱𝑚𝑖𝑥 is very dependent on the value

9

of the χ parameter.

10 11

CONCLUSION

12

The water-retaining factors governing the swelling of a cellulose-based network, 𝛱𝑚𝑖𝑥, 𝛱𝑖𝑜𝑛 and

13

𝛱𝑑𝑒𝑓, were quantified by applying polymer gel swelling theories to experimental de-swelling

14

data of model cellulose gel beads, used as a model material for the delignified wood fiber wall.

15

In contact with high molecular mass dextran solutions, creating a deswelling pressure on the gel

16

beads, it was possible to control the swelling of the beads and hence to adjust the specific volume

17

fraction of cellulose within the beads. These data were then used in classical swelling theories to

18

calculate the distribution of the osmotic pressure between 𝛱𝑚𝑖𝑥, 𝛱𝑖𝑜𝑛 and 𝛱𝑑𝑒𝑓. The results

19

showed that gel beads with a higher charge density, 600 µeq/g, were, as expected, able to

20

withstand the contraction to a greater extent than the lower charged beads at 30 and 300 µeq/g.

21

The effect of charge density was also shown as a linear increase in the swelling with an

22

increasing charge density which should be expected as long as the cellulose network is in the


32

Page 33 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

1

linear elastic regime. It was found that an interaction parameter of about 0.55-0.70 gave rise to

2

𝛱𝑑𝑒𝑓 values that best fitted the elastic Wall model when φ0=0.007 and P ranged between 5-15

3

(Figure S5). The calculated 𝛱𝑑𝑒𝑓 values also collapsed on a single curve, indicating that the

4

elastic features of the cellulose network were independent of charge density. When the three

5

factors 𝛱𝑚𝑖𝑥, 𝛱𝑖𝑜𝑛 and 𝛱𝑑𝑒𝑓 were quantified using an interaction parameter of 0.55, it was found

6

that at volume fractions between 2 % and 15 % 𝛱𝑑𝑒𝑓was dominating in hindering the cellulose-

7

network to collapse and as a consequence retaining the water. When the Wall model together

8

with the Flory-Huggins (Chi=0.55) and Van´t Hoff theories were extrapolated to cellulose

9

volume fractions higher than those achieved experimentally, it was shown that above about 35 %

10

volume fraction of cellulose the dominance shifted from 𝛱𝑑𝑒𝑓 to 𝛱𝑚𝑖𝑥 owing to the increase in

11

entropy of mixing when the cellulose content increased. Interestingly, the 𝛱𝑖𝑜𝑛was dominating

12

only at very low volume fractions (below 2 vol%), clearly revealing that it is important to obtain

13

more knowledge of the non-ionic contribution to the water uptake in cellulose-rich networks.

14

AUTHOR INFORMATION

15

Corresponding authors

16

* Email: [email protected]

17

** Email: [email protected]

18

ACKNOWLEDGEMENT

19

This work was financially supported by the Knut and Alice Wallenberg foundation. Domsjö

20

Fabriker AB, Örnsköldsvik Sweden is thanked for supplying the dissolving pulp used in this

21

study. Albanova University Center is thanked for the use of the confocal laser scanning


33


Page 34 of 43

1

microscope. Prof. Lars Ödberg at KTH, Stockholm is acknowledged for the fruitful discussions

2

around this work.

3

SUPPORTING INFORMATION

4

The supporting information includes:

5



TGA curve

6



Calculations of estimated pore size of gel beads

7



Additional data on volume recovery after deswelling in dextran solutions

8



Additional values on mix, ion and deformation terms using different interaction

9 10

parameter 

11

Deformation values fitted to the Wall model alternating the P and 𝜑0values in the Wall model

12



Solid state NMR spectra for dried beads re-swollen in water

13



Extrapolation of theoretical models

14 15


34

Page 35 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

Biomacromolecules

REFERENCES 1. Nyström, G.; Marais, A.; Karabulut, E.; Wågberg, L.; Cui, Y.; Hamedi, M. M., Selfassembled three-dimensional and compressible interdigitated thin-film supercapacitors and batteries. Nat. Commun. 2015, 6, 7259. 2. Köklükaya, O.; Carosio, F.; Wågberg, L., Superior Flame-Resistant Cellulose Nanofibril Aerogels Modified with Hybrid Layer-by-Layer Coatings. ACS Appl. Mater. Interfaces 2017, 9, (34), 29082-29092. 3. Kandola, B. K.; Horrocks, A. R.; Price, D.; Coleman, G. V., Flame-Retardant Treatments of Cellulose and Their Influence on the Mechanism of Cellulose Pyrolysis. J. Macromol. Sci., Part C 1996, 36, (4), 721-794. 4. Chang, C.; He, M.; Zhou, J.; Zhang, L., Swelling Behaviors of pH- and Salt-Responsive Cellulose-Based Hydrogels. Macromolecules 2011, 44, (6), 1642-1648. 5. Annamalai, P. K.; Dagnon, K. L.; Monemian, S.; Foster, E. J.; Rowan, S. J.; Weder, C., Water-Responsive Mechanically Adaptive Nanocomposites Based on Styrene–Butadiene Rubber and Cellulose Nanocrystals—Processing Matters. ACS Appl. Mater. Interfaces 2014, 6, (2), 967976. 6. Berthold, J.; Rinaudo, M.; Salmeń, L., Association of water to polar groups; estimations by an adsorption model for ligno-cellulosic materials. Colloid. Surf., A 1996, 112, (2), 117-129. 7. Morrison, J. L.; Dzieciuch, M. A., The Thermodynamic Properties of the System Cellulose-Water vapor. Can. J. Res. 1959, 37, (9). 8. Mantanis, G. I.; Young, R. A.; Rowell, R. M., Swelling of compressed cellulose fiber webs in organic liquids. Cellulose 1995, 2, (1), 1-22. 9. Grignon, J.; Scallan, A. M., Effect of pH and Neutral Salts upon the Swelling of Cellulose Gels. J. Appl. Polym. Sci. 1980, 25, 2829-2843. 10. Scallan, A. M.; Tigerström, A. C., Swelling and Elasticity of the Cell Walls of Pulp Fibres. J. Pulp pap. Sci. 1992, 18, (5), 188-193. 11. Lindström, T.; Carlsson, G., The Effect of chemical Environment on Fiber Swelling. Sven. Papperstidn. 1982, 85, (3), 14-20. 12. Flory, P. J., Principles of Polymer Chemistry. 12 ed.; Cornell University Press: 1983. 13. Katchalsky, A.; Michaeli, I., Polyelectrolyte Gels in Salt Solutions. J. Polym. Sci. 1955, 15, 69-86. 14. Atkins, P.; de Paula, J., Atkin´s Physical Chemistry. 8 ed.; Oxford University Press: 2006. 15. Wall, F. T.; Flory, P. J., Statistical Thermodynamics of Rubber Elasticity. J. Chem. Phys. 1951, 19, (12), 1435-1439. 16. Katz, S.; Beatson, R. P.; Scallon, A. M., The determination of strong and weak acidic groups in sulfite pulps. Sven.Papperstidn. 1984, 87, (6), 48-53. 17. Wågberg, L.; Decher, G.; Norgren, M.; Lindström, T.; Ankerfors, M.; Axnäs, K., The Build-Up of Polyelectrolyte Multilayers of Microfibrillated Cellulose and Cationoic Polyelectrolytes. Langmuir 2008, 24, (3), 784-795. 18. Carrick, C.; Pendergraph, S. A.; Wågberg, L., Nanometer Smooth, Macroscopic Spherical Cellulose Probes for Contact Adhesion Measurements. ACS Appl. Mater. Interfaces 2014, 6, (23), 20928-20935. 19. Berthold, F.; Gustafsson, K.; Berggren, R.; Sjo, E.; Lindström, M., Dissolution of Softwood Kraft Pulps by Direct Derivatization in Lithium Chloride/N,N- Dimethylacetamide. J. Appl. Polym. Sci. 2004, 94, 424-431.


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Page 36 of 43

20. Karlsson, R.-M. P.; Larsson, P. T.; Yu, S.; Pendergraph, S. A.; Pettersson, T.; Hellwig, J.; Wågberg, L., Carbohydrate gel beads as model probes for quantifying non-ionic and ionic contributions behind the swelling of delignified plant fibers. J. Colloid Interface Sci. 2018, 519, 119-129. 21. de Belder, A. N.; Granath, K., Preparation and properties of fluorescein-labelled dextrans. Carbohydr. Res. 1973, 30, (2), 375-378. 22. Horvath, A. T.; Horvath, A. E.; Lindström, T.; Wågberg, L., Adsorption of Low Charge Density Polyelectrolytes to an Oppositely Charged Porous Substrate. Langmuir 2008, 24, (13), 6585-6594. 23. Scheirs, J.; Camino, G.; Tumiatti, W., Overview of water evolution during the thermal degradation of cellulose. Eur. Polym. J. 2001, 37, 933-942. 24. Pusch, W., Osmotic pressure of dextran T10 solutions. Desalination 1988, 68, (1), 69-73. 25. Livney, Y.; Ramon, O.; Kesselman, E.; Cogan, U.; Mizrahi, S.; Cohen, Y., Swelling of dextran gel and osmotic pressure of soluble dextran in the presence of salts. J. Polym. Sci., Part B: Polym. Phys. 2001, 39, 2740-2750. 26. Vink, H., Precision measurements of osmotic pressure in concentrated polymer solutions. Eur. Polym. J. 1971, 7, (10), 1411-1419. 27. Sjöstedt, A.; Wohlert, J.; Larsson, P. T.; Wågberg, L., Structural changes during swelling of highly charged cellulose fibres. Cellulose 2015, 22, (5), 2943-2953. 28. Alince, B.; van de Ven, T. G. M., Porosity of swollen pulp fibers evaluated by polymer adsorption. In The Fundamentals of Papermaking Materials, C.F.Baker, Ed. Pira International: United Kingdom, 1997; Vol. 2, p 771. 29. Lindman, B.; Karlström, G.; Stigsson, L., On the mechanism of dissolution of cellulose. J. Mol. Liq. 2010, 156, (1), 76-81. 30. Wang, D.; Ámundadóttir, M. L.; van Gunsteren, W. F.; Hünenberger, P. H., Intramolecular hydrogen-bonding in aqueous carbohydrates as a cause or consequence of conformational preferences: a molecular dynamics study of cellobiose stereoisomers. Eur. Biophys. J. 2013, 42, (7), 521-537. 31. Duchesne, I.; Daniel, G., The ultrastructure of wood fibre surfaces as shown by a variety of microscopical methods - A review. Nord. Pulp pap. Res. J. 1999, 14, (2), 129-139. 32. Hult, E.-L.; Larsson, P. T.; Iversen, T., A comparative CP/MAS 13C-NMR study of cellulose structure in spruce wood and kraft pulp. Cellulose 2000, 7, (1), 35-55.

33 34


36

Page 37 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

1 2

TOC image

3


37



Page 38 of 43

Page 39 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

248x182mm (300 x 300 DPI)




Page 40 of 43

Page 41 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

82x60mm (300 x 300 DPI)




Page 42 of 43

Page 43 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules


The Thermodynamics of the Water-Retaining Properties of Cellulose

Recommend Documents