Solution Properties of Some Amphiphilic Polysaccharide Derivatives

Chapter 4

Solution Properties of Some Amphiphilic Polysaccharide Derivatives J . Desbrieres and M. Rinaudo

Downloaded by COLUMBIA UNIV on September 17, 2012 | http://pubs.acs.org Publication Date: February 15, 2001 | doi: 10.1021/bk-2001-0786.ch004

Centre de Recherches sur les M a c r o m o l é c u l e s V é g é t a l e s , C E R M A V - C N R S , affiliated with the University Joseph Fourier, B P 53, 38041 Grenoble Cedex 9, France

Amphiphilic polymers give very interesting properties in aqueous solution; some of them give a large increase of the viscosity in semi-dilute regime or some form a noncovalent gel. The hydrophilic-hydrophobic balance plays a large role in the interchain interactions, depending on charge density of the polymer (electrostatic character), the temperature, the nature of hydrophobic substitution or external salt concentrations. In our work we have investigated two systems : * methylcelluloses which form gel in given thermodynamic conditions when highly substituted hydrophobic zones are present in the molecules (block-like copolymers), * alkylchitosans obtained by grafting alkyl chains on a poly-D-glucosamine backbone. The relation between the chemical structure of the modified polysaccharides and the physical properties of the solution will be discussed.

Introduction For many years, our work has concerned the extension of the knowledge of natural polymers and especially polysaccharides. Our goal is to develop new ways to extend the use of these important sources of polymers such as starch, cellulose or chitin. Due to the difficulty to process these polymers in their native form, it is useful 72

© 2001 American Chemical Society

In Biopolymers from Polysaccharides and Agroproteins; Gross, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.


73 to perform chemical or enzymic modifications to open new applications. Specifically in the field of water soluble polymers, the cellulose derivatives found a lot of developments. For this purpose we have proposed to perform homogeneous modifications of these polysaccharides such as to be able to relate the chemical modifications with the physical properties in solution or solid state (1). One of the chemical modifications we have recently investigated is the introduction of hydrophobic character in these usually hydrophilic polysaccharides. Hydrophobic associating water soluble polymers represent a new class of industrially important macromolecules. They possess unusual rheological characteristics which are thought to arise from the intermolecular association of neighboring hydrophobic substituants (2) which are incorporated into the polymer molecule through chemical grafting (3,4) or suitable copolymerisation procedures (5). The hydrophobic associations give rise to a three-dimensional polymer network. For synthetic polymers, the large variety of behaviours observed are now qualitatively understood in terms of nature, number and length of the hydrophobic substituents and also in terms of distribution along the backbone. In contrast natural polysaccharides suffer from a lack of fundamental studies. The difficulty arises mainly in the absence of model polymers with a precise distribution of substituents. The hydrophilic-hydrophobic balance plays a large role in the interchain interactions, depending on charge density of the polymer (electrostatic character) or the solubility for the neutral polymers, the temperature, the nature of hydrophobic substitution or external salt concentrations. In this work we have investigated two types of polymers, one being neutral and block-like copolymer (methylcellulose) and the other one ionic and having grafted hydrophobic chains (alkylchitosan). The hydrophilic-hydrophobic balance may be adjusted under controlled chemical modifications using numerous parameters such as the charge density, the length and the number of hydrophobic groups. The related properties will be discussed.

Experimental Products Commercial samples of methylcelluloses were kindly supplied by Dow Chemical company under the trade name Methocel A 4 C . Laboratory made methylcellulose samples ( M l 2 , M18) were prepared according to an original procedure (6) based on a previously proposed methylation method (7). The cellulose was dissolved in D M A c (dimethylacetamide) / 6 wt% L i C l from a swelling procedure followed by solvent exchange. Then the dimsyl solution (NaH - DMSO) was added to the cellulose solution for activation of the reactive sites of the cellulose and finally iodomethane was added to the mixture, stirred at room temperature for different times to reach different substitution degrees (8).



74 The original chitosan samples were from Aber Technologies (Plouguerneau, France) and Protan (Norway). The alkylated derivatives were obtained by reductive amination following the procedure described by Yalpani (9). It is a versatile and specific method for creating a covalent bond between a substrate and the amine function of the chitosan. They are called CCx, χ being the number of C atoms of the hydrophobic chain. The degree of substitution is defined as τ. In any case we have developed procedures for performing chemical reactions in homogeneous phase. This allows a better accessibility of the reactive sites to the reagent and hence a regular repartition of substituents compared with the commercial samples which have a heterogeneous repartition of substituents leading to block-like copolymers.

The physicochemical characteristics of the samples are given in table I.

Methods The rheological measurements were performed with a Couette type rheometer (Contraves Low Shear 40) or a stress-controlled rheometer (CarriMed CS50) according to the concentration and the temperature of the sample. The calorimetric experiments were carried out with a Micro DSC III calorimeter from Setaram (France). The temperature rate was 0.5 deg/min. The fluorescence spectra were obtained using a LS50B luminescence spectrometer from Perkin-Elmer. The experiments were carried out in the temperature and concentration domains where no turbidity was observed. The pyrene concentration was 10" M due to its low solubility in water and the excitation wavelength was 334 nm. The studied parameter is the 1 /1 ratio of the intensities of first and third peaks of fluorescence spectrum of pyrene in chitosan derivative solutions. 7

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3

Gelation of methylcelluloses Commercial methylcellulose (MC) is a heterogeneous polymer consisting of highly substituted zones called "hydrophobic zones" and less substituted ones called "hydrophilic zones" (10) as block-like copolymers. It has the ability to form a gel on heating which melts again on cooling. Due to these unusual properties, most of the experimental work reported in the literature was dedicated to the evolution of the viscosity and turbidity of MC solutions during heating and cooling cycles in a small polymer concentration range (10-25 g/L) (11-14). The behaviour of this polymer is not monotonous when the temperature varies : the viscosity of a semi-dilute solution decreases when temperature is increased up to a critical value over which the viscosity increases. Then, the formation of a gel may be observed and this



1 3

A4C 1.7 10 29 39 22 149,000

M12 1.5 5 51 29 15

1.50

1.52

765** 0.35

k

DA : acetylation degree τ : degree of substitution * : solvent AcOH 0.3M / AcOH 0.05M, T=200C ** : chlorhydrate form in water, T=5^C

H

v

0.12 1250*

CC8 2

0.04 1990**

CC12 12

DA M τ [η] (mL/g)

Chitosan 12 190,000

Table l b : Characterization of chitosan and alkyl derivatives

Determination by C n.m.r. in DMSO-d6 (353K) DS is the average degree of substitution per glucose residue

a

w

a

DS % Non S % MonoS % DiS % TriS M

Table l a : Characterization of methylcelluloses

1.92

0.12 1200*

CC10 2

M18 2.2 9 12 36 43



76 phenomenon is associated with a turbidity, indicating phase separation (11). A l l these phenomena are reversible. We have studied the evolution of M C solutions with the temperature using different techniques such as n.m.r., rheometry (15), calorimetry (8) and fluorescence spectroscopy (16). In-depth studies were carried out on the commercial sample A 4 C . The phase diagram of A 4 C sample is very complex (figure 1). A cloud point is observed indicating that the A 4 C aqueous solution presents a LCST-type phase separation and this curve can be considered as the binodal curve with a minimum corresponding to the critical point (17). Below this curve a clear homogeneous phase was present, while above, the phase separation was incomplete due the superposition of a gelation process. The gelation was all the more slowed down because the gap to the binodal curve was small. It was demonstrated that there is a competition between the phase separation leading to polymer-rich regions, and the gelation which prevents the mobility of chains, hence the growth of dense zones. Moreover, the homogeneous domain appeared also to be very complex. It is constituted by two gel phases separated by a sol phase. In the dilute regime there was no gel formation, or for very high temperatures there was direct phase separation. For low concentration solutions (c

Solution Properties of Some Amphiphilic Polysaccharide Derivatives

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