Oscillatory shear rheometry

Various characterization methods both in vitro and in vivo can provide information to understand, predict, and improve the performance of drug delivery systems. Selection of methods depends on the material properties and their applications. Viscoelastic properties can be measured using both DMA and oscillatory shear rheometry. DSC is a most useful method of measuring thermal transitions. Various microscopic methods are available to obtain the microstrac-ture and shape of the materials. Amorphous and crystaUine materials have different packing patterns of molecules, and these properties can be determined from XRD or density measurements. Surface properties such as surface elemental composition and material porosity can be obtained from various spectroscopic methods as well as from BET measurements. The biocompatibility of the material can be determined from both in vitro and in vivo assays. In vitro dissolution testing can be utilized to correlate with the in vivo performance of polymeric drug delivery systems. All these characterization methods can provide valuable information... 

In this chapter, we first present the oscillatory shear rheometry of microphase-separated block copolymers in terms of molecular weight, block composition, and... 

Oscillatory Shear Rheometry of Microphase-Separated Block Copolymers Exhibiting Upper Critical Order—Disorder Transition Behavior... 

Among several different experimental techniques reported in the literature, radiation scattering (SAXS or SANS) and oscillatory shear rheometry have been used most widely to determine phase transition temperatures of block copolymers. In this section, we present how oscillatory shear rheometry can be used to determine phase transition temperatures of a block copolymer exhibiting UCODT. Following the tradition in the... 

Oscillatory Shear Rheometry of Symmetric or Nearly Symmetric Block Copolymers... 

In this section, we present the effect of thermal history on the oscillatory shear rheometry of block copolymers. We will show that the occurrence of a minimum in G in the isochronal dynamic temperature sweep experiment does not necessarily signify OOT for highly asymmetric block copolymers instead, it sometimes reflects imperfect bcc spheres, as determined by SAXS and TEM, due to an insufficient annealing of a specimen. Here, we will show that a minimum in G, observed for an unannealed specimen in the isochronal dynamic temperature sweep experiment, may disappear completely when the specimen is annealed for a sufficiently long time at an elevated temperature below the TmoT of highly symmetric SI diblock copolymer. 

Using oscillatory shear rheometry, SAXS, and SANS, Ryu et al. (2003) investigated phase transition in symmetric dPS-Wocfe-poly(n-pentyl methacrylate) (PwPMA) copolymers having two different molecular weights (dPS-ft/ocfc-IViPMA-L with = 4.5 X 10 and dPS-( /ocA -PwPMA-H with = 5.33 x 10 ) and three binary mixtures of the two block copolymers. Figure 8.32 shows the results of their isochronal dynamic... 

Figure 5 (a) Elastic and viscous moduli as functions of shear stress obtained from oscillatory shear dynamic rheometry (see text and Ref 80 for details) performed at the oil-water interfece. The system consisted of 40 ml of water on to which 35 ml of an oil phase was layered, consisting of 0.75% (w/v) B6 asphaltenes dissolved in 50% (v/v) toluene in heptane. The asphaltenes were allowed to adsorb for 8 h at which point the rheometrical measurements were performed. The frequency of oscillation of the biconical hob was 1 rad/s. (b) Elastic and viscous moduh as functions of frequency. The same system and experiment as described for Fig. 5a were used with the exception that the shear stress was fixed at 1 mPa and the frequency was varied. Asphaltenes were allowed to adsorb for a period of 8 h. 

Figure 8 Elastic modulus as a function of frequency obtained from oscillatory shear dynamic rheometry (see text) performed at the oil-water interface. The proportions of oleic and aqueous phase are the same as described for Fig. 5. Concentrations of 0.25, 0.75, and 1.5% (w/v) B6 asphaltenes were dissolved in the oleic phase (50% yh toluene in heptane) before adsorption. Samples were aged for 8 or 24 h. Shear stress was held constant at 1 mPa.

Because polymers have viscoelastic properties, rheometry is often employed to simultaneously measure both elastic and viscous parameters. To measure the viscoelastic properties using a rheometer, an oscillatory shear force/shear stress is applied to the material. Because of the viscoelasticity, the sample s deforma-tion/shear strain exhibits a phase lag to the shear force/shear stress. When the applied shear stress is ... 

Rameshwaram et al. [6] investigated the structure-property relationships and the effects of a viscosity ratio on the rheological properties of polymer blends using oscillatory and steady shear rheometry and optical microscopy. 

Mutel, A.T. Kamal, M.R. (1986). Characterization of the Rheological Behavior of Fiber-Filled Polypropylene Melts under Steady and Oscillatory Shear using Cone-and-Plate and Rotational Parallel Plate Rheometry. Polymer Compwsites, Vol.7, No.5, pp. 283-294ISSN0272-8397... 

With dynamic rheometry, the measurement of the dynamic moduli G and G" in small amplitude oscillatory shear is exploited. The gelation point is reported to be the intersection point of the curves of storage and loss moduli, i.e. the moment at which tan 8 equals one [49]. However, the crossover is observed to correspond to the gel point only for stoichiometrically balanced network polymers and networks with excess crosslinking agent at temperatures much above Tg [50]. 

Malmberg, A., Gabriel, C., Steffi, T., Munstedt, H., Lofgren, B. Long-chain branching in metallocene-catalyzed polyethylenes investigated by low oscillatory shear and uniaxial rheometry. MacromoL (2002) 35, pp. 1038-1048. 

Oscillatory shear measurements provide more precise data than capillary rheometry and require smaller samples. In addition, capillary viscometers are hmited to use at moderate to high shear rates and do not provide accurate results at shear rates below 10 s for most materials. It would thus be useful to be able to estimate the viscosity curve using data from oscillatory shear measurements. Based on their data for two polystyrenes, Cox and Merz [133] reported that the curve of apparent viscosity versus shear rate, as determined using a capillary viscometer, lay very close to the curve of complex viscosity versus frequency. While the apparent viscosity measured in a capillary viscometer is usually defined as the apparent wall shear stress divided by the apparent wall shear rate, it is likely that the entrance pressure drop was nearly negligible for the polystyrenes studied by Cox and Merz. Therefore, what they observed can be expressed as follows ... 

A.T. Mutel, M.R. Kamal. Characterization of the rheological behavior of fiber-filled polypropylene melts imder steady and oscillatory shear using cone-and-plate and rotational parallel plate rheometry. Polym. Compos., 7 (5), 283-294, 1986. 

However, ar rheometry presents an important drawback, i.e.. the sample microsmicture may be destroyed during the test run. This is especially true for weak gels or pastes, even at low shear rates or shear stresses. Much information about the fluid microstnicture is lost in this manner, in this case, creep and oscillatory rheometry are more convenient. The creep and viscoelastic parameters may be related to the strength of flocculation or aggregation, and a better knowledge of the microstructure is obtained. Useful information about the application of these methods for the study of suspensions of clay paniclc.s. creams, and concentrated suspensions of latex particles can be found in Refs. 12.49. and 50. respectively. 

Phase angle n. In many cychc processes, particularly ones at high frequency, there is usually a time lag between the impulses driving the processes and the responses to those impulses. The lag time divided by the cycle period and multiplied by 2n is the phase angle (radians). This occurs in oscillatory rheometry, where the shear applied... 

Elastic Modulus. The elastic shear modulus of swollen superabsorbent polymer is commonly measured by means of oscillatory rheometry, using any of a variety of commercially available instruments (31). The sample to be analyzed is screened to the desired particle size cut such that the swollen particle size is smaller than the gap between the measuring plates. The sample is swollen in the desired liquid, usually 0.9 wt% NaCl solution or synthetic urine. Excess liquid is removed from the swollen hydrogel by blotting to minimize interparticle slippage. The hydrogel typically is placed onto the lower circular plate. An upper conical plate is positioned at the proper sample gap, which is packed with the gel. An oscillating torque is applied at a known frequency to the upper plate and the... 

Investigations on moduli and shear viscosity are accomplished by oscillatory and continuous flow rheometry, while droplet size of emulsions is determined by flow particle image analysis (FPIA). 

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