Block copolymer melts 220 Subject

The fascinating thermodynamics of block copolymers that results from microphase separation are the subject of the parts 2.2,2.3, and 2.4 of Chapter 2. Part 2.4 is concerned with the complex kinetic processes that accompany phase transitions, and the dynamic processes controlled by the structure of the block copolymer melt. 

There is no comprehensive theory for crystallization in block copolymers that can account for the configuration of the polymer chain, i.e. extent of chain folding, whether tilted or oriented parallel or perpendicular to the lamellar interface. The self-consistent field theory that has been applied in a restricted model seems to be the most promising approach, if it is as successful for crystallizable block copolymers as it has been for block copolymer melts. The structure of crystallizable block copolymers and the kinetics of crystallization are the subject of Chapter 5. 

Finally, we draw attention to a topic somewhat neglected in this review, namely the interplay between concentration inhomogeneities ( > p,z near the surfaces and interfaces and the local configurational properties of the polymer coils (enrichment of chain ends, orientation and possibly distortion of polymer coils, etc.). The reason for this omission was that not so much general features are known about these questions. Clearly, the subject of phase transitions of polymer blends and block copolymer melts in thin film geometry will remain a challenge in the future. 

In a seminal paper, Leibler [43] presented the first mean-field-like theory of the ODT transition and the phase diagram of block copolymer melts in the weak segregation limit. This work still is the basis for more elaborate theories [58-64] and for the discussion of recent experiments (e.g. [317-323]). As shown in Fig. 42, the quantitative details of the resulting predictions are still subject of current research, but nevertheless we try to sketch this theory here, since this derivation gives a good insight into the relevant physical aspects of this problem. 

The interest in the phase behaviour of block copolymer melts stems from microphase separation of polymers that leads to nanoscale ordered morphologies. This subject has been reviewed extensively [1 ]. The identification of the structure of bicontinuous phases has only recently been confirmed, and this together with major advances in the theoretical understanding of block copolymers, means that the most up-to-date reviews should be consulted [1,3]. The dynamics of block copolymer melts, in particular rheological behaviour and studies of chain diffusion via light scattering and NMR techniques have also been the focus of several reviews [1,5,6]. 

A nnmber of texts covering general aspects of block copolymer science and engineering have appeared in the last 30 years eg. References 1 and 2. More recently specialized reviews have appeared on block copolymer melts and block copol5uner solntions, and these are cited in appropriate following sections. The bnrgeoning interest in block copolymers is illustrated by contributions covering varions aspects of the subject in a review journal (3) and in an edited book (4). 

The orientation of crystalline stems with respect to the lamellar interface in block copolymers is a subject of ongoing interest and controversy. In contrast to homopolymers, where folding of chains occurs such that stems are perpendicular to the lamellar interface, the parallel orientation has been observed for block copolymers crystallized from the heterogeneous melt. It is not yet clear whether this is always the preferred orientation, or whether chains can crystallize perpendicular to the lamellar plane, for example when crystallization occurs from the homogeneous melt or from solution. 

Our understanding of the physics of block copolymers is increasing rapidly. It therefore seemed to me to be timely to summarize developments in this burgeoning field. Furthermore, there have been no previous monographs on the subject, and some aspects have not even been reviewed. The present volume is the result of my efforts to capture the Zeitgeist of the subject and is concerned with experiments and theory on the thermodynamics and dynamics of block copolymers in melt, solution, and solid states and in polymer blends. The synthesis and applications of these fascinating materials are not considered here. 

Thermoplastic elastomers (TPEs) with blocks of polydiene rubber are subject to degradation at the carbon-carbon double-bond sites and require proper stabilization. In SIS block copolymers, chain scission is the predominant degradation mechanism. In an SIS block copolymer, the addition of a more effective stabilizer, AO-3, alone or blended with a secondary antioxidant, PS-1, can provide a significantly superior performance over AO-1 alone or with PS-1. Resistance to discoloration after static oven aging at 80°C (176°F) is improved dramatically (Fig. 5). Viscosity stabilization (melt flow index stability) (Fig. 6) is also improved drastically using AO-3/PS-1. 

The mechanical synthesis of block and graft copolymer is a method of sizable versatility. It can be performed (as already stated) during polymer processing and in standard equipment. The reaction, also, can be carried out by subjecting a mixture of two or more polymers to mechanical degradation in either the solid, elastic-melt, or solution states. It is, also, possible to induce reaction mechanically between polymers and monomers. 

The combination of monomers to form copolymers can be compared with the mixing of metals to form solid solutions, which is the basis of alloy formation. The chemical engineer by small variations in copolymer composition can synthesize polymers with subtly different properties. The properties which are controlled by changes in copolymer composition include elastic modulus, toughness, melt viscosity, and thermal stability (l.N.ll). We return to this subject again in Chapters 4 and 5. Copolymers are also polymerized with block or graft structures (see Fig. l.S) for specific purposes (1.N.12). 

El Fray and Altstadt [12] used MTA to study the relationship between morphological features of semi-crystalline and multi-block polymeric materials and their thermal properties. Samples of semi-crystalline polybutylene terephthalate and its copolymer were crystallised from the melt showing a spherulitic morphology. The surface of the spherulitic shapes was subjected to L-TA at selected regions of different thermal conductivity (at the centre of the spherulite and at its outer surface). This reveals information, which cannot otherwise be obtained. 

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