Conformation and Structure of Hydroxyethyl Cellulose Ether with a Wide Range of Average Molar Masses in Aqueous Solutions

Abstract

The solution properties of a water-soluble chemically modified cellulose ether, hydroxyethyl cellulose (HeC), were examined using static light scattering (SLS), dynamic light scattering (DLS), small-to-wide-angle neutron scattering (S-WANS), small-to-wide-angle X-ray scattering (S-WAXS) and viscometric techniques at 25 °C. The examined HeC samples had average molar substitution numbers ranging from 2.36 to 2.41 and weight average molar masses (M_w) that fell within a wide range from 87 to 1500 kg mol^-1. Although the relationship between the determined radius of gyration (R_g) and M_w was described as R_g ∝ M_w^~0.6, as is observed usually in flexible polymer solutions in good solvents, the observed scattering vector (q) dependencies of excess Rayleigh ratios were well interpreted using a rigid rod particle model, even in high-M_w samples. Moreover, the ratios of the formed particle length (L) evaluated assuming the model for rigid rods to the determined R_g showed the relationship LR_g^-1 ~ 3.5 irrespective of M_w and were close to those theoretically predicted for rigid rod particle systems, i.e., LR_g^-1 = 12. The observed SLS behavior suggested that HeC molecules behave just like rigid rods in aqueous solution. As the L values were not simply proportional to the average molecular contour length calculated from the M_w, the chain conformation or structure of the formed particles by HeC molecules in aqueous solution changed with increasing M_w. The q dependencies of excess scattering intensities observed using the S-WANS and S-WAXS experiments demonstrated that HeC molecules with M_w less than 200 kg mol^-1 have a diameter of ~1.4 nm and possess an extended rigid rod-like local structure, the size of which increases gradually with increasing M_w. The observed M_w dependencies of the translational and rotational diffusion coefficients and the intrinsic viscosity of the particle suspensions strongly support the idea that the HeC molecules behave as rigid rod particles irrespective of their M_w.

Keywords: dynamic light scattering; hydroxyethyl cellulose; radius of gyration; rotational diffusion coefficient; small-to-wide-angle X-ray scattering; small-to-wide-angle neutron scattering; static light scattering; translational diffusion coefficient; weight average molar mass.

Figure 1

q² dependencies of the ${\left(\sqrt{Kc{\left(R_{\theta }\right)}^{-1}}\right)}_{c=0}$ data (a) and c dependencies of the ${\left(\sqrt{Kc{\left(R_{\theta }\right)}^{-1}}\right)}_{q=0}$ data (b) for aqueous solutions of the HeC170 sample at 25 °C.

Figure 2

M_w dependencies of R_g (a) and A₂ (b) for HeC samples in aqueous solution at 25 °C.

Figure 3

q dependencies of the R_θ(Kc)⁻¹_c=0 data and fitted M_wP(q) curves for HeC samples in aqueous solution at 25 °C.

Figure 4

M_w dependencies of L, LR_g⁻¹ and lL⁻¹ for HeC samples in aqueous solution at 25 °C. Lines are drawn in this figure as a guide for the eye.

Figure 5

(a) q dependencies of the ΔI_N(q)c⁻¹ (S-WANS) data for D₂O solutions of HeC87 at c = 0.005 and 0.010 g mL⁻¹ at 25 °C; (b) q dependencies of the mΔI_X(q)c⁻¹ (S-WAXS) data for aqueous HeC87 solutions at c = 0.0025, 0.0050 and 0.010 g mL⁻¹ at 25 °C and that of ΔI_N(q)c⁻¹ data for the same HeC87 sample shown in (a). The SLS data obtained for the same system, fR_θ(Kc)⁻¹, and the two fit curves, fM_wP(q), obtained assuming f = 6.5 × 10⁻⁴ cm² g⁻² mol, are also shown in (a). The numerical constant of m = 0.17 is used for all the mΔI_X(q)c⁻¹ data in (b). The places of * and ** in the figures mean the positions of the broad interference-type peaks.

Figure 6

q dependencies of the mΔI_X(q)c⁻¹ (S-WAXS) data obtained for HeC1000 at c = 1.0 × 10⁻³ g mL⁻¹ (a) and those obtained for HeC1500 at c = 0.8 × 10⁻³ g mL⁻¹ at 25 °C (b). The same numerical constants as those used to generate the fitted curves shown in Figure 5b, f = 6.5 × 10⁻⁴ cm² g⁻² mol and m = 0.17, were employed.

Figure 7

M_w dependence of [η] data for HeC samples in aqueous solution at 25 °C. The solid line labeled [η]_cal1 in this figure shows the M_w dependence of [η]_cal assuming the first simple relationship, L_η = L and d_η = d, and the broken line labeled [η]_cal2 represents the M_w dependence obtained assuming the second relationship, L_η = 0.87L and d_η = d.

Figure 8

q² dependencies of the Γ₁ data for the shortest (HeC87, (a)) and the longest (HeC1500, (b)) samples. The broken line in (b) demonstrates the relationship Γ₁ = 6D_r + D_t q²; thus, the D_r value can be evaluated from the intercept of the broken line at q² = 0 nm⁻².

Figure 9

(a) M_w dependencies of D_t and D_r data for the HeC samples in aqueous solution at 25 °C. The solid lines show D_{t cal1} and D_{r cal1} calculated assuming L_h = L and d_h = d, and the broken lines show D_{t cal2} and D_{r cal2} calculated assuming L_h = 0.87L and d_h = d, as in the M_w dependencies of the [η] data seen in Figure 7. (b) Dependence of ρ (= R_hR_g⁻¹) on ln(L_hd_h⁻¹) for all the HeC samples in aqueous solution at 25 °C assuming the first condition, L_h = L and d_h = d (circular symbols), and the second condition, L_h = 0.87L and d_h = d (square symbols).