HPMC was dispersed in deionized hot water (90 °C) by slowly agitating for 20 min, then it was cooled to room temperature by stirring at 30 rpm for 40 min to prepare 10% ( w / w ) HPMC, followed by the addition of CL to prepare the HPMC/CL system with 10% HPMC ( w / w ) and 0%, 2%, 4%, 6%, and 8% CL ( w CL / w (water+HPMC) ), respectively. Finally, the above suspensions were continuously stirred at 60 rpm for 1 h and at 40 rpm for another 1 h at room temperature before a rheological test. HC0, HC2, HC4, HC6, and HC8 were used to mark the five suspensions, respectively.
A strain sweep, frequency sweep, temperature sweep, and shear rate sweep were used to measure the rheological properties of this HPMC/CL blend system by using a controlled stress Rheometer (Kinexus Pro+, Malvern Instruments, Worcestershire, UK). The plate rotor PU40 SR1343 SS, with a diameter of 40 mm and a gap of 1.0 mm, was chosen for the strain sweep, frequency sweep, and temperature sweep, while the cylindrical rotor C25 SW1114 SS, with a diameter of 25 mm and a gap of 1.0 mm, was chosen for the shear rate sweep. Except for the temperature sweep, pretreating was done for all other sweeps. For pretreating, all samples were heated to 95 °C and maintained for 5 min in the sample holder; they were then cooled to 82 °C and 25 °C, respectively. The sweep began when the sample stabilized. All the samples tested at higher temperatures were sealed with silicone oil to prevent water evaporation. The specific methods are described below:
The strain sweep was mainly carried out to obtain the linear viscoelastic region (LVR) [ 20 21 ]. For the strain sweep, samples were tested in the range of 0.01–100%, with the tested frequencies of 1 Hz at 25 °C and 82 °C, respectively. The fixed amplitude of 0.1% was chosen for the frequency sweep and temperature sweep because, with this constant amplitude, the linear region was not surpassed in all cases, as is shown later.
The frequency sweep was tested in the range of 0.1–10 Hz, with the tested strains of 0.1% at 25 °C and 82 °C, respectively.
σ
) and shear rate values (γ) into the corresponding fluid equations. The fluid equation corresponded to the shear thinning fluid and the yield-shear thinning fluid, as displayed below, respectively:σ = K γ n
(1)
σ = K γ n + σ y
(2)
K
), the flow behavior index (n
), and yield stress (σy
) can be calculated.The shear rate sweep was tested using the procedure of the Toolkit_V001–1 Table of Shear Rates/Equilibrium Flow Curve in the range of 1–500/s at 40 °C and 82 °C, respectively. The flow pattern could be ascertained by fitting the shear stress () and shear rate values (γ) into the corresponding fluid equations. The fluid equation corresponded to the shear thinning fluid and the yield-shear thinning fluid, as displayed below, respectively:where, the fluid consistency index (), the flow behavior index (), and yield stress () can be calculated.
The temperature sweep was tested in the range of 25–90 °C. A heating rate of 2 °C/min, a strain of 0.1%, and a frequency of 1 Hz were set in the experimental work. The samples were sealed with silicone oil to prevent water evaporation. Immediately after the first sweep, a repeated second temperature sweep was performed under the same conditions. The two temperature sweeps were differentiated as −1 and −2, for example, HC0-1 for the first time and HC0-2 for the second time. For HC8, another temperature sweep in the range of 25–95 °C was carried out twice as a comparison to prove that pretreating temperature greatly affected the thermal-irreversible gel formation. The second temperature sweeps of HC8, after the first temperature sweep heating to 90 °C and 95 °C, were recorded and written as H90-2 and H95-2, respectively.
All the rheological tests were tested three times, and the data used in the figures are the average data of the three tests.
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