Micro-phase separated copolymer structures

In this review, we introduce another approach to study the multiscale structures of polymer materials based on a lattice model. We first show the development of a Helmholtz energy model of mixing for polymers based on close-packed lattice model by combining molecular simulation with statistical mechanics. Then, holes are introduced to account for the effect of pressure. Combined with WDA, this model of Helmholtz energy is further applied to develop a new lattice DFT to calculate the adsorption of polymers at solid-liquid interface. Finally, we develop a framework based on the strong segregation limit (SSL) theory to predict the morphologies of micro-phase separation of diblock copolymers confined in curved surfaces. 

Many polymer blends or block polymer melts separate microscopically into complex meso-scale structures. It is a challenge to predict the multiscale structure of polymer systems including phase diagram, morphology evolution of micro-phase separation, density and composition profiles, and molecular conformations in the interfacial region between different phases. The formation mechanism of micro-phase structures for polymer blends or block copolymers essentially roots in a delicate balance between entropic and enthalpic contributions to the Helmholtz energy. Therefore, it is the key to establish a molecular thermodynamic model of the Helmholtz energy considered for those complex meso-scale structures. In this paper, we introduced a theoretical method based on a lattice model developed in this laboratory to study the multi-scale structure of polymer systems. First, a molecular thermodynamic model for uniform polymer system is presented. This model can... 

Electron Microscopy. Figure 3 shows electron micrographs of ultra-thin sections of film specimens of the three kinds of block copolymers. As can be seen in the figure, TR-41-1647 and TR-41-1648 specimens have a heterogeneous structure in which the polystyrene domains are dispersed within a polybutadiene matrix and are connected to each other to form a swirl-like structure. On the other hand, TR-41-1649 specimen is seen to consist of alternating lamellar domains of the two components. Changes of the domain structure with fractional compositions of styrene and butadiene components are consistent with predictions of the current theories of micro-phase separation (12,13,14,15) for block copolymers cast from such a nearly nonselective solvent as the mixture of THF and methylethylketon (90/10 in volume ratio). 

In order to get a better insight into the formed microstructures, SAXS analysis of thick films of all PBA-P AN copolymers listed in Table 2 was performed. Most of the materials revealed cylindrical morphologies and then-different compositions affected the cylinders fi -spacing in the micro-phase separated structures. Furthermore, the SAXS studies showed that annealing of the films for 2 hours at 150°C resulted in improved phase separation (42). 

The properties of the linear material 7.27 and the network copolymer 7.28 have been studied by dynamic mechanical analysis, DSC, and transmission electron microscopy. Evidence was obtained for the formation of highly ordered micro-phase-separated superstructures in the solid state from the materials 7.27. The Cu(bipy)2 moieties appear to form ordered stacks, and this leads to thermoplastic elastomer properties. In contrast, the network structure of 7.28 prevents significant microphase separation [51-53]. By means of related approaches, dinuclear Cu helical complexes have also been used to create block copolymers by functioning as cores [54], and polymer networks have also been formed by using diiron(II) triple helicates as cores for the formation of copolymers with methyl methacrylate [55]. 

Fig. 4. Scanning electron micrographs showing the surface appearance of contact lens. a, Lens produced with dextran-MMA graft copolymer and b, PMMA. Micro-phase -separated structure appears for a.

Matyjaszewski et al. [2] patented a novel and flexible method for the preparation of CNTs with predetermined morphology. Phase-separated copolymers/stabilized blends of polymers can be pyrolyzed to form the carbon tubular morphology. These materials are referred to as precursor materials. One of the comonomers that form the copolymers can be acrylonitrile, for example. Another material added along with the precursor material is called the sacrificial material. The sacrificial material is used to control the morphology, self-assembly, and distribution of the precursor phase. The primary source of carbon in the product is the precursor. The polymer blocks in the copolymers are immiscible at the micro scale. Free energy and entropic considerations can be used to derive the conditions for phase separation. Lower critical solution temperatures and upper critical solution temperatures (LCST and UCST) are also important considerations in the phase separation of polymers. But the polymers are covalently attached, thus preventing separation at the macro scale. Phase separation is limited to the nanoscale. The nanoscale dimensions typical of these structures range from 5-100 nm. The precursor phase pyrolyzes to form carbon nanostructures. The sacrificial phase is removed after pyrolysis. 

A copolymer-homopolymer mixture provides us with a variety of domain morphology since both macro- and micro-phase separations take place simultaneously. (Equi-hbrium property has been studied extensively by a mean-field theory [1].) We consider a mixture of A-B diblock copolymer and C homopolymer assuming a short-range repulsive interaction between A and B monomers and B and C monomers. One may expect a multiple domain structure in a sense that microphase separated domains are developed in a macrophase separated domain [2, 3]. It is also expected that formation of vesicles is also possible... 

In diblock copolymer melts, the free energy of a micro-phase-separated state can be shown to favor ordered domain structures where the mutual organization of A-and B-domains form regular lattices. The equilibrium structure depends on the relative size of the respective polymer blocks, the overall polymer size and the temperature (or rather the product xN of the Flory-Huggins interaction parameter and the degree of polymerization). 

We emphasize that when the C component is absent (tj) = 0) the above model reduces to the well-known model for block copolymers [56-59] or reactive polymer blends [9,10]. In this case, the evolution of the reactive AB system is governed solely by Eq. (8.2) and the morphology of the mixture resembles the lamellar structure formed by micro phase-separated symmetric diblock copolymers [9]. (In the context of our work, we only consider reactive blends, and do not consider diblock copolymers.) When r+ = r = r for such reactive binary mixtures, a linear stability analysis gives the growth rate, w k), for the kth mode of the fluctuations of the order parameter around the homogeneous value cp = 0 as ... 

Section 2.5). In this case the blocks A and B try to separate from each other. However, a macroscopic phase separation is impossible because the A and B blocks are tightly linked to each other within the chains. As a result, we get a pattern of micro-domains which contain mainly A blocks or B blocks, separated by fairly thin interphase regions (Figures 4.8 and C4.9). This effect is known as microphase separation in block-copolymers, and the structure that emerges is called a micro-domain structure. 

The simplest are diblock copolymers, where two different polymeric chains are bound together and with an increase of block number, tri- or multiblocks with a variety of structures can be obtained [31,32]. Most block copolymers used today are prepared by living anionic polymerization, which is a feasible method to prepare block copolymers with controlled architecture. Different polymers do not mix well due to thermodynamic reasons [33], especially if their molecular mass is sufficiently high, they have a strong tendency to form separated phases. In block copolymers, this phase separation can occur only intermolecularly (micro- or nanophase separation) [34]. Those block copolymers... 

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