Poisson block copolymer

Couman W.J., Heikens D., and Sjoerdsma S.D., Dilatometric investigation of deformation mechanism in polystyrene-polyethylene block copolymer blend Correlation between Poisson ratio and adhesion. 

When these two tables are compared, there is indeed a composition distribution superimposed on the MWD in the case of Poisson block copolymer, but the range is not very wide (Table I). In volume separation, however, the range is drastically reduced. For all practical purposes,... 

Vq PB diluent concentration at the solubility limit under a standard state Vp Poisson s ratio of block copolymer composite Vp Atomic frequency factor in molecular chain scission Q Active craze front length per unit volume a Negative pressure (mean normal stress)... 

It is, of course, the anionic mechanism which is most suitable for the synthesis of block copolymers, since many of these systems are of the living polymer type, as described previously [144,150,165,194,262,263]. Thus it is possible to use organoalkah initiators to prepare block copolymers in homogeneous solution by sequential addition of monomers, where each block has a prescribed molecular weight, based on monomer-initiator stoichiometry, as well as a very narrow molecular weight distribution (Poisson) [185]. As would be expected, such block copolymers are very pure, due to the absence of any side reactions during the polymerization (e.g., termination, monomer transfer, branching). 

Even for block copolymers, in which the phase separation can be distinguished in electron micrographs, there are problems in matching parameters such as Poisson s ratios of the two components nevertheless the simple Takayanagi models, particularly when extended by a treatment to account for the finite length of the reinforcing component, can describe numerous features of static and d3mamic elastic behaviour. 

The Poisson distribution assumes that all the chains are initiated simultaneously. Growth continues at approximately the same rate in each chain, until the monomer runs out. An analogy would be a horse race, where all the horses start at the bell and finish at nearly the same time. The result is a narrow molecular weight distribution. (For anionic polymerizations following these statistics, the ends of the chains remain living, even though the monomer has run out. Addition of a second monomer then yields well-defined block copolymers see Section 4.3.9.)... 

It is a consequence of Equation (2.10) that the Mw/Mn values for an AB-block copolymer should be smaller than the values normally observed for A and B homopolymers with molar mass comparable to the blocks provided the block copolymerisation reaction proceeds in a similar manner to the homopolymerisation. The vast majority of the Mw/M data presented in the literature is based on SEC measurements. In fact SEC is problematic for the characterization of very narrow MMDs. For homopolymers the axial dispersion phenomenon is the main problem, whereas for block copolymers it is also questionable to what extent true noninteracting conditions are accessible. A development has started towards the use of alternative techniques to SEC for the characterization of diblock copolymers. Apart from the popular MALDI-TOF mass spectroscopy various newer chromatographic techniques have been used. A series of PS samples prepared under as identical conditions as possible (/CHX/sBuLi/45 C/ZCHsOH/) were analysed by SEC and TGIC and the measured Mw/M values compared with the Poisson distribution predictions. 

It turned out that many statistical properties of protein-like and random copolymers with the same HP composition are very different. In order to be able to distinguish whether this difference is due to the special sequence design described above, or just due to the different degree of blockiness, one can introduce for comparison also the random-block primary sequence. The random-block HP copolymers have the same chemical composition and the same average length L of uninterrupted H or P sequence as protein-like copolymers, but in other respects the HP sequence is random. In [18], the distribution of block length X was taken in the Poisson form /(A) = e LLl/ l. 

---