Solutions, block copolymers

The presence of flexible PEO and PPO blocks increases the viscosities of block copolymer solutions, this tendency is manifesting itself the stronger the greater is the PEO and PPO content in block copolymers. 

Hashimoto T., Shibayama M., and Kawai H., Ordered structure in block copolymer solution. 4. Scaling rules on size of fluctuations with block molecular weight, concentration temperature in segregation and homogeneous regimes. Macromolecules, 16, 1093, 1983. 

Linse, P Mahnsten, M, Temperature-Dependent MiceUization in Aqueous Block Copolymer Solutions, Macromolecules 25, 5434, 1992. 

Figure 67 shows Q QVQ2 vs. Q for both systems. As expected from Eqs. (142) and (143) their behavior is completely different. One can see that a pronounced divergency occurs at small Q-values in the semi-dilute block copolymer solution. If Qi(Q)/Q2 is analyzed in terms of a generalized mobility ji(Q) [see Eq. (94)], Fig. 68 results from the different concentrations of the diblock copolymer solution. Q(Q) varies both with Q and with c. In particular, the Q-dependence is indicative of the non-local character of the mobility and incompatible with the assumption of a pure Rouse type of dynamics. The... 

Fig. 10a Average hydrodynamic radii ((Rh)agg) and b weight-average molar mass ((Mw)agg) of the aggregates in aqueous block copolymer solutions at different polymer concentrations (c given in moles of PNIPAM blocks) at 45 °C a NE-A, b NE-B, c NE-C, d NE-1, e NE-2, and/ NE-3. (Reprinted with permission from Ref. [169] copyright 2002 American Chemical Society)... 

Alexandridis P (2004) Nanoparticle Synthesis and Colloidal Stabilization using Amphiphilic Block Copolymer Solutions. Abstracts, 32nd Northeast Regional Meeting of the American Chemical Society, Rochester, NY, USA, October 31-November 3, GEN-119... 

Alexandridis P, Sakai T (2004) Amphiphilic block copolymer solutions as media for the facile synthesis and colloidal stabilization of metal nanoparticles. Abstracts of Papers, 228th ACS National Meeting, Philadelphia, PA, USA, August 22-26, 2004,... 

Sakai T, Alexandridis P (2004) Single-step synthesis and stabilization of metal nanoparticles in aqueous Pluronic block copolymer solutions at ambient temperature. Langmuir 20 8426-8430... 

Polyethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) (Pluronic) block copolymer is a very efficient reducing agent and nanoparticle stabilizer. Au NPs of about 10 nm can be stabilized with PEO-PPO-PEO block copolymer solutions in water and at room temperature and using HAuC14 as precursor. The formation of gold nanoparticles is controlled by the overall molecular weight and relative block length of the block copolymer [118]. 

Nuclear magnetic resonance (NMR) has been used to study segmental motions in block copolymer solutions. The mobility of protons in polymer chains in dilute solutions has been probed using high-resolution H NMR. Association of chains into micelles leads to a reduction in mobility in the core, which leads to a broadening of the respective NMR lines that has been studied for a number of systems, as described by Tuzar and Kratochvil (1993). The sol-gel transition in concentrated solutions has been located via ]H transverse relaxation time experiments, as outlined in Chapter 4. 

In a solution containing both unimer and micelles, Ma, which is by definition more sensitive to low molar mass particles, is always less than the weight-average molecular weight, Mv. Further details on osmometry can be found in the review by Adams (1989), whilst examples of its application to micellar block copolymer solutions are given in Chapter 3. 

Rheology has also been used to locate sol-gel transitions in concentrated block copolymer solutions, as described in Chapter 4. Gels exhibit a finite yield stress (i.e. they are Bingham fluids), which can be measured in steady shear experiments. 

The critical micelle concentration (cmc) in block copolymer solutions can be determined by measurement of the surface tension (y) as a function of concentration. The method detects completion of the Gibbs monolayer at the air/water interface, and is a secondary indicator of the onset of micellization. The cmc for solutions of monodisperse polymers is indicated by a fairly sharp decrease in y versus log(c). 

Turbidometry is simply the quantitative measurement of light transmission in turbid solutions, and is employed to locate the cloud point (i.e. onset of macrophase separation) in block copolymer solutions, as discussed in Chapter 3. 

Here kH is the Huggins coefficient. The intrinsic viscosity decreases and the Huggins coefficient increases, as micelles become smaller. On micellization, ijsp/c has been observed to increase for some systems but to decrease for others, and unfortunately there are no firm rules governing which case will prevail for a given block copolymer solution. The viscosities of polymer solutions are measured in capillary flow viscometers, which are described in detail by Macosko (1994). 

Refractometry can be used to determine the composition of a copolymer. In addition, differential refractometry has been used to study micellization in dilute block copolymer solutions (Tfizar and Kratochvfl 1972). The refractive index (n) is obtained in an Abbe refractometer via measurements of the critical angle for external reflection. The refractive index increment dn/dc, where c is the polymer concentration, can be related to the molecular weight of particles in solution. Further details of the method are provided by Pepper and Samuels (1989). 

Dynamic light scattering has traditionally been applied to polymer solutions, and DLS results for block copolymer solutions are discussed in Chapters 3 and 4. A number of recent papers have described the application of the technique to disordered block copolymer melts (Anastasiadis et al. 1993a,6 Boudenne et al. 1996 Floudas et al. 1995 Fytas et al. 1993 Jian et al. 1994a Stepanek and Lodge 1996 Vogt et al. 1994). Due to the limited range of dynamic time-scales that can... 

Fig. 3.1 Illustration of the critical micelle concentration (cmc) and critical gel concentration (cgc) in a block copolymer solution.

This chapter is organized as follows. The thermodynamics of the critical micelle concentration are considered in Section 3.2. Section 3.3 is concerned with a summary of experiments characterizing micellization in block copolymers, and tables are used to provide a summary of some of the studies from the vast literature. Theories for dilute block copolymer solutions are described in Section 3.4, including both scaling models and mean field theories. Computer simulations of block copolymer micelles are discussed in Section 3.5. Micellization of ionic block copolymers is described in Section 3.6. Several methods for the study of dynamics in block copolymer solutions are sketched in Section 3.7. Finally, Section 3.8 is concerned with adsorption of block copolymers at the liquid interface. 

The mass-average molar mass of the block copolymer solute, M , and the second virial coefficient, A2, can be obtained from SLS. These quantities can be determined from the concentration dependence of the scattered light intensity using the relationship (cf. Section 1.4.10)... 

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