Dynamic processes in block copolymer melts

These conclusions were later supported by time-resolved SAXS experiments by Stiihn et al. (1994) who studied the ordering of a PS-PI diblock with /PS = 0.44 following quenches from the disordered phase into the lamellar phase. They found that the relaxation times of the structure factor were wavevector dependent, and consistent with the Cahn-Hilliard from (Cahn and Hilliard 1958) 

The kinetics of ordering in block copolymer melts have been studied using cell dynamics simulations of the TDGL equation (Bahiana and Oono 1990 Chakrabarti et al. 1991 Puri and Oono 1988 Qi and Wang 1996, 1997 Shiwa et al. 1996). Here, the time evolution of the order parameter ip is followed  

Here M is a mobility coefficient, which is assumed to be constant and r/(r.t) is the random thermal noise term, which for a system in equilibrium at temperature T satisfies the fluctuation-dissipation theorem. The free energy functional is taken to be of a Ginzburg-Landau form. In the notation of Qi and Wang (1996,1997) it is given by 

The evolution of a system described by an equation related to eqn 2.26 was studied using cell dynamics simulations by Oono and co-workers (Bahiana and Oono 1990 Puri and Oono 1988). In the CDS method the continuous order parameter is discretized on a lattice and at time t is denoted where n labels 

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 

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The dynamics of block copolymers melts are as intriguing as their thermodynamics leading to complex linear viscoelastic behaviour and anisotropic diffusion processes. The non-linear viscoelastic behaviour is even richer, and the study of the effect of external fields (shear, electric. ..) on the alignment and orientation of ordered structures in block copolymer melts is still in its infancy. Furthermore, these fields can influence the thermodynamics of block copolymer melts, as recent work has shown that phase transition lines shift depending on the applied shear. The theoretical understanding of dynamic processes in block copolymer melts is much less advanced than that for thermodynamics, and promises to be a particularly active area of research in the coming years. 

The combination of careful chemical synthesis with NSE and SANS experiments sheds some light on the fast relaxation processes observed in the collective dynamics of block copolymers melts. The results reveal the existence of an important driving force acting on the junction points at and even well above the ODT. Modelling the surface forces by an expression for the surface tension, it was possible to describe the NSE spectra consistently. The experimental surface tension agrees reasonably well with the Helfand predictions, which are strictly valid only in the strong-segregation hmit. Beyond that, these data are a first example for NSE experiments on the interface dynamics in a bulk polymer system. 

This chapter deals almost exclusively with neat, or pure, diblock copolymer melts. Polymer blends are discussed in Chapter 9, micellar solutions in Chapter 12, and stabilized suspensions in Chapter 6. In the following, Section 13.2 briefly reviews the thermodynamics of block copolymers, and Section 13.3 describes the rheological properties and flow alignment of lamellae, cylinders, and sphere-forming mesophases of block copolymers. More thorough reviews of the thermodynamics and dynamics of block copolymers in the liquid state have been written by Bates and Fredrickson (1990 Fredrickson and Bates 1996). The processing of block copolymers and mechanical properties of the solid-state structures formed by them are covered in Folkes (1985). Biological applications are discussed in Alexandridis (1996). 

A unique strength of solid-state NMR is its ability to probe molecular dynamics with site selectivity.9132 In this section, we present some specific examples that illustrate the considerable insight into dynamic processes provided by advanced solid-state NMR experiments applicable to as-synthesized samples, i.e., without the requirement for isotopic labeling. These examples focus on well-defined processes that are fast as compared to the time scale of the H DQ MAS experiment, this being on the order of 10-6 to 10 4 s. In addition, it should be noted that the extraction of dipolar couplings by following the buildup of DQC in a H DQ MAS experiment has been shown to provide insight into the complex dynamic processes in polymer melts,172 block copolymers,173 and elastomers.174... 

In contrast, crystallization of one or both components of a block copolymer is accompanied by profound structural and dynamic changes. The fundamental process in crystallization of chains in a crystallizable block copolymer is the change in block conformation, i.e. the adoption of an extended or a folded structure rather than a coiled configuration found in the melt or solution (see Fig. 1.5). 

The synthesis of siloxane-urea block copolymers can be performed either in solution or in the melt at elevated temperatures up to 200 °C. We favor the use of dynamic mixers or extruders to ensure intensive mixing of the components. Surprisingly, adding aminosiloxane and isocyanate to get the block copolymers (2K process) is not the only possibility. We can also add the silanol fluid, the cyclic aminosilane, and the diiosocyanate consecutively to the extruder for a successful solventless preparation of siloxane-urea block copolymers (3K process). 

Several reviews are devoted to SANS applications to TPEs and block copolymers [113,114] and almost 300 reports describe SANS studies of block copolymers. As far as TPEs are concerned, it is a very valuable technique, particularly efficient in the analysis of disorder in the melt [115] or of macroscopic phase separation [116] it is also used in blends [117,118] and in processing operations, such as dynamic vulcanization [119] or co-molding [120]. 

At suitable temperatures between the glass transition and the melting point, semictystalline polymers trivially give a H NMR response that is qualitatively the same as in the block copolymers discussed in the preceding section, ensuring applicability of the discussed NMR approaches. In some polymers, however, additional complications arise due to the often fast chain motion within the crystallites.Examples for crystal-mobile polymers are isotactic polypropylene, poly(ethylene oxide), and most prominently PE. The dynamics of the local heUcal-jump processes within the aystaUites can easily... 

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