Behaviour of two cellulosic derivatives in aqueous solutions: charged carboxymethylcellulose

and neutral methylcellulose

M.Rinaudo,Eme

Emeritus Professor at Joseph Fourier University,CERMAV, Grenoble (France)

www.cermav.cnrs.frmarguerite.rinaudo@cermav.cnrs.fr

Lotz, 17th March 2011-STEP ITN - FP7

Main characteristics of polysaccharides.

-From natural sources with different original chemicalstructures (ionic or neutral)

-Rich in –OH groups forming dense H-bond network (intra and interchains)

-Neutral polysaccharides (such as starch, cellulose, chitin…) are insoluble in many organic solvents (and in water!)

-They are not thermoplastic so difficult to process

-They are semi-cristalline polymers and react more generallyheterogeneously which gives irreproducible characteristics.

-Renewable, biocompatible,..

-Main properties for the soluble polysaccharides are: film-forming, thickener, gelling, suspending, emulsifier….

*

* *

*-OH 2, -OH 3, -OH 6 able to be modified

Cellulose derivatives

with ClCH2COONa -O-CH2COONa (CMC with 0 < DS < 3)

Or CH3Cl , ICH3 -O-CH3 (MeCell with 0 < DS < 3)

but there are different distributions of substituents along the cellulosicchain which control the solubility

Reactivity of cellulose

*Intra- and inter-H bonds network

* Influence of the cellulose fibermorphology (amorphousand crystalline phases)

Carboxymethylcelluloses

Methods of preparation:

-ClCH2COONa + NaOH heterogeneous (in presence of isopropanolor not)

-Two steps:

*Na in liquid ammonia - ONa homogeneous ClCH2COOH

*Solubility occurs for DS > 0.5 (under sodium form)

*Obtention of 0.5 < DS < 3 to test the ionic properties indepenton the molecular weight

They are polyelectrolytes

Introduction to Introduction to PolyelectrolytesPolyelectrolytes::CharacterisationCharacterisation and original and original propertiesproperties

in the in the dilutedilute regimeregime

--General model :electrostatic potential, General model :electrostatic potential, pKpK and and counterionscounterions interactionsinteractions--Conformation of polyelectrolyte (persistence length Conformation of polyelectrolyte (persistence length

–– local stiffness) and local stiffness) and viscometricviscometric behaviourbehaviour--Macromolecular characterizationMacromolecular characterization

Characteristics of a polyelectrolyte

∅, γ, f•controlled by λ, the charge parameter•depending on the valency of the counterions•pKa, pK0

*ionic selectivity (ion pairs)*long range e.s. interactions (κ-1, the Debye length) κ-1 when Cs or Cp

κ-1 = 3A° Ct-1/2

extension of the coil (Rg)

exclusion from gel in SEC

increase of η (intra + inter e.s.)

λ linear charge density, independant on DP (when DP > 20), dependent on the degree of titration for weak acid or base.

-

-

-

-

-

+

+

+

+

+

+

+ is the counterion(ν)

ν charges on length h= α/b

λ= (νe2)/DhkT=(α e2)/DbkT

electrostatic potential

Polyelectrolyte in absence of external salt(Cp) or low salt content(Cs)

*Total ionic concentration Ct=Cp+Cs

*Electrostatic interactions (κ-1 value, Debye length)

A) Intra-chains

Polyion-counterions interactions

Conformation and viscosity

B) Interchain e.s.interactions (e.s.network)

peak for viscosity and radiation scattering

Polyion-counterions interactions.

γ+ ~ 1/2 λ

Example for CMC DS=1 :

λ=1.38 and γ+ ~ 0.36

np

Ionic selectivity and charge density

when λ>1

Polyion-divalent counterions interaction.

Evidence of ion pair formation and ion selectivity

Experiment of ultrasoundabsorption

Li>Na>K>Rb>Cs ~TMA

Evidence of ion pair formation and ion selectivity ( with λ)

on CMC with differentcharge parameter (DS)

B-Long range e.s.interactions (absence of external salt or low salt content)

interchain interactions:

-Viscosity peak

-Neutron or light scattering peak

- intrachain interactions:

-polyelectrolyte conformation

macromolecular characterization (MW & dimensions determination)????

Intrinsic viscosity of polyelectrolytes???

Over Cs~0.05M NaCl

Electrostatic influence on viscosity

WHY??

Electrostatic expansion and interchain interactions (depend on Cp, Cs,

pH, lower effet of temperature) .

Influence of salt concentration on the conformation

of a polyelectrolyte.Behave as a neutral polymer

e.s. interchaininteractions(e.s network?)

Isolatedchains

+ Expansion of the chain due to electrostatic repulsions

1010--55 MM

1010--44 MM

155mL/g for L=500A155mL/g for L=500A

C-Macromolecular characterization:

- In excess of neutral salt (1-1 electrolyte0.05-0.1 M)

-Usual techniques: SEC, Viscometry….

Analysis by the same treatments as for neutral polymers

*in dilute regime (C<C*)

*Cs controls the dimensions of the chains and extrapolation to infinite Cs gives a good estimate of Φ-state.

-Importance of working in the dilute domain (C<C*)

C*~[η]-1

-Two different regimes: absence of external saltor excess of salt

Which is C* for a polyelectrolyte (↔conformation)??

Conclusions.

Polyelectrolytes have very original properties in diluteregime and in absence of external salt

*Due to electrostatic intrachain (extension) and interchaininteractions (viscosity & diffusion peaks)

*Electrostatic controls activity coefficient of counterions, apparent pK, ionic selectivity, osmotic pressure

* influence of the charge parameter allows to predict theseproperties

Characterization of these polymers needs to screen atleast long range e.s. repulsion and to work in the presenceof external salt (0.1M 1-1 electrolyte)

Viscoelastic behaviour to gel like system for lowerdegree of substitution of abaca cellulosic fibers.

Example of Carboxymethylation of non-wood pulps

Abaca 1:DS=0.95, 43% insoluble

Abaca 2:DS=2.4, 13% insoluble

Rheological behaviour isdirectly related with the fraction of insoluble fibers.

Influence of the « quality » of the sample on the physical properties of aqueous solutions and especially on thickening characteristics (favouredby heterogeneities).

Interesting application in food applications of methylcellulose which forms a gel whentemperature increases.

Methylcellulose, an amphiphilic polysaccharide

OO

ORRO

OR

O

OR

ORORO

n

R= R= --H or H or ––CHCH33

Preparation of methylcelluloses.

Objective: to study the mechanism of physical gelation in relation withthe microstructure

*Commercial samples are obtained by heterogeneous modification givetypical gels at high temperature (T>60°C)

*We have prepared methylcelluloses in homogeneous conditions withdifferent DS (cellulose is soluble in DMAc)

Preparation of aqueous solution solutions

-Heterogeneous Mecells are soluble at 5°C for DS ~ 1.7

-Homogeneous Mecells are soluble at 5°C for DS ~ 0.9

13 C NMR Characterization of Mecell DS=1.7

in DMSO-d6 at 353K

HPLC of Mecell DS=1.7 after hydrolysis

*absence of reduction (α/β anomers)

Characterization:

-Average DS (by NMR), Substitution on C-2,C-3, C-6

-Unsubstituted glucose(1), monosubstituted(x3),disubstituted(x3), trisubstituted (1) (by HPLC)

-Mw and [η](mL/g) obtained by SEC at low temperature

Example:

*A4C from Dow Chemical: DS=1.7, DS2=0.8, DS3=0.4, DS6=0.5

*10% glucose, 29% monosubsti.,39% disubstit., 22% trisubstitut.

*Mw=181,000, [η](mL/g) =509

Methylcellulose DS=1,7; Mv=150,000

Methylcellulose DS=1,8; Mv=409,000; C=15 g/L.

MethylcelluloseMethylcellulose behaviourbehaviour in water: in water: amphiphilicamphiphilicpolymerpolymer formingforming gel gel atat T>60T>60°°CC

TwoTwo types of gel types of gel dependingdepending on the on the temperaturetemperatureand and methylmethyl distribution distribution alongalong the chain.the chain.

Clear gel Turbid gel

Influence of the Influence of the methylmethyl groups distributiongroups distribution

Heterogeneous MeCell

Homogeneous MeCell

Influence of the methyl groups distribution along the chain

DSC in aqueous solution

Phase Phase diagramdiagram for for methylcellulosemethylcellulose A4C*A4C*

LCST ~29LCST ~29°°C Cp~45 g/LC Cp~45 g/L

**coll.M.Axeloscoll.M.Axelos & & C.ChevillardC.Chevillard

Conclusions

Methylcellulose is an amphiphilic polymer forminggel in a two steps process:

-a clear gel over 35°C related with the existence of highly methylated zones followed by turbid gel – phase separation over 60°C

-heterogeneous distribution of the methyl groups is necessary for gelation.

ReferencesReferences

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