Study of the Molecular Mobility of Polysaccharide Solid Thin Layers by Dielectric Relaxation Spectroscopy
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ABSTRACT Temperature dependence of measured dielectric relaxation spectra (DRS) in the frequency range 20 Hz - 1 MHz of hydroxyethylcellulose (HEC) are in the temperature range 100 - 350 K of Arrhenius character with one relaxation process at 150 - 250 K. This process reflects most probably 13-relaxation of the side chain groups. Calculated activation energy of this process was 5730 kJ/mole. Four types of polysaccharides were studied at 293 K temperature: hyaluronic acid (HA), chondroitin sulfate (CHS), HEC and carboxymethylcellulose (CMC), in the low-frequency range 10-5 - 100 Hz. Measured dielectric spectra were interpreted as sum of one A.C. conductivity process and of up to two relaxation processes. The relaxation processes were described by means of the Havriliak--Negami formula and their parameters were related to the molecular structure of the polymers. The low value of a in CHS is related to its strong coupling due to the presence of two polar groups in its monomeric unit, whereas low values of a x
13are
interpreted as being due to the strong steric hindrances caused by long pendants present in HEC.
157 Mat. Res. Soc. Symp. Proc. Vol. 500 ©1998 Materials Research Society
INTRODUCTION From the physicochemical point of view it is desirable to know, to understand and to quantify the molecular mobility of the polymer main chain, its side-chain groups and its different skeletal segments. For the application as coatings of the affinity sensors, it is desirable to cover the surface of the measuring device with the membrane having both controllable pores size distribution as well as the specific affinity to various diffusants. Suitable materials, which find increasing application for these purposes seems to be polysaccharides, particularly hyaluronic acid. Molecular mobility properties of polysaccharides may be studied by means of the Dielectric Relaxation Spectroscopy (DRS), as both the main chain and the side chain groups are of a polar character. DRS method is particularly suitable for the analysis of polymers with a high degree of structure similarity, as the dielectric relaxation spectra reveal even minor differences in the dipolar constitution.
EXPERIMENT Dielectric spectra were obtained from the time-domain measurements. Measuring apparatus consisted of a digital electrometer Keithley 617, stabilized D.C.-power supply, relay control circuitry, and personal computer interconnected via IEEE-488 bus. For the majority of the measurements, the charging period was set to be 24 h. For temperature dependent measurements in the frequency range of 20 Hz - 1 MHz a digital electrometer HP4284 was used. Prior to charging, samples were kept short-circuited for at least 12 h in order to remove any stray charges. Measured samples were of the circular or rectangular shape with graphite electrodes of the size about 12 mm x 20 mm pasted onto their surface. Samples were fixed in the sample holder and placed into the thermostatic chamber. The measuring chamber was provided with a simple installation based on the oversatura
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