Activation functions of photodegraded carboxymethyl cellulose in solution: Viscosity measurements

photodegradation technology has been used, and the data were attained viscometrically. The decrease in macromolecular mass and concentration brought down the reduction in solution viscosity, while the temperature had the opposite effect. Eyring's parameters as functions of concentration and molar mass were then illustrated. The enthalpy and Arrhenius factor have increased with increasing concentration and molar mass, while the entropy has decreased. The polyelectrolytic CMC fractions in salt solutions behaved like polymers in neutral solutions. The photodegraded solution of CMC had better solvation quality than the native one.


Introduction
The energetic behaviour of viscous flow is an essential parameter in nowadays applications, ranging from the rheological industrial to the biocompatible domain for polysaccharides in solution [1].Eyring [2] has interpreted the viscous flow of polymers as an activated rate process according to the following equation:  =    −∆  /  ∆  / 1 where η, ΔHη, Aη, and ∆Sη are the solution viscosity, the appeared enthalpy of activation, the Arrhinus proportionality constant, and the entropy of activation, respectively.
Moore [4] has pointed out that the ∆Sη, of dilute solutions of flexible chain polymers, decreases with increasing the polymer concentration (C) and its molar mass (Mη).In contrast, ∆Sη for stiff and extended cellulose derivatives in dilute solutions generally increases with an increase in C and Mη.
Carboxymethyl cellulose (CMC) is a water-soluble polyelectrolyte, that belongs to the cellulose ethers family [5].
Although many studies of the thermal conductivity of CMC have been done [6] yet there is no published study on the thermodynamic conformational behaviour of CMC in semidilute solutions in the temperature range of 20-60 °C.Our aim in this work is to use the Equations from 1 to 8 to investigate such behaviour.

Material
The native CMC powder in Figure 1 was a gift from Jowfe Company, Benghazi, Libya, and was used as received.
Figure 1.Monomer molecular structure of a CMC sodium salt [5].Reproduced with permission of John Wiley and Sons.

Preparation and measuring methods
The native solution was photodegraded by ultraviolet (UV) irradiation at room temperature.The non-degraded (high Mη) and degraded (low Mη) CMC solutions were prepared with concentrations ranging from 0.5 to 1.00 mg cm -3 .The nondegraded solutions (highly viscous samples) were labeled as HV-CMC, while the degraded solutions (low viscous samples) were labeled as LV-CMC.Monitoring the change in each sample was done immediately after irradiation, and at intervals afterwards for each sample, by using the Ostwald-viscometric technique [7].The viscosity measurements were taken at the temperature range of 20-60 °C, ±0.02.

Results and discussion
The results are shown in the forthcoming Fig. 2 to Fig. 5. Fig. 2 illustrates the logarithmic form of Equation 1. Fig. 2a demonstrates a typical Huggins concentration dependence of viscosity for photodegraded CMC solution [8].The deviation from the linearity, in Fig. 2b, can be attributed to the excluded volume effect, due to the higher Mη of CMC in the native solution [9, 10] Fig. 3-5 show the activation functions of CMCs in NaCl solutions.Their activation parameters, like ∆Hη, Aη, and ∆Sη, were obtained by applying Eyring's viscosity theory on the plots of Fig. 2, via Equations 2-8 [2,4].All the functions, in Fig. 3-5, show typical plots of flexible chains in solution [4].
The increase in ∆Hη, with C and Mη, in Fig. 3, indicates that the CMC chains require more energy to change their equilibrium positions at higher concentration values.This would naturally lead to the formation of aggregated chains at such values [11].

Fig. ‫׃5‬
Fig. ‫׃5‬ CMC-Concentration dependence of ∆Sη, (a) is the HV-CMC and (b) is the LV-CMCConclusionPhoto-irradiation of carboxymethyl cellulose in the salt solution is expected to reduce their molecular weight, and improve the solvation quality of the system.Relative viscosity/temperature relationships can be conveniently employed to investigate these phenomena.