Rigid-flexible sequencing

Copolymerization of rigid and flexible sequences in the same molecule, for example ... 

The thermoplastic polyurethanes consist of rigid isocyanate sequences and flexible polyester or polyether sequences. The properties, of which hardness is often used for the denomination of the grades, vary broadly as functions of ... 

Rigid segments can be, for example, a semicrystalline polybutylene terephthalate, and flexible sequences can be polyesters or polyethers - polyetherglycol, for example. 

The maleimide cycle is a rigid structure whose C - C bond participates in the chain backbone. On each side of the rigid cycle are flexible sequences of MMA linked to the cycle either directly by the quaternary carbon or through the methylene group. 

Flory, P. J. Conformational rearrangement in the nematic phase of a polymer comprising rigid and flexible sequences in altemating succession orientation-dependent interactions included, MRSSymp. Proc., (Adams, W. W., Eby, R. K., and McLemore, D. E., Eds.), 134,3-9 (1989). 

The components effectively incorporated into the polyassociation depend in particular on the nature of the core groups and on the interactions with the environment, so that supramolecular polymers possess the possibility of adaptation by association/growth/dissociation sequences. The selection of components may occur on the basis of size commensurability [18], of compatibility in chemical properties, in charge, in rigidity/flexibility, etc. An example is given by the formation of homochiral helical fibers with chiral selection from a racemic mixture of monomeric tartaric acid derivatives LU2 = LP2 -I- DU2 -I- DP2(LU2LP2)n + (DU2DP2)n (see Section II.C) [16],... 

The work by Ciferri and coworkers [128-130] focused on the supramolecular organization of aromatic rod-coil copolyamides in diluted (isotropic) and moderately concentrated (lyotropic) phases. Their diblock copolymers were based on a rigid block of poly(p-benzamide) (PBA) having a DP about 100, and different comparable lengths of flexible blocks, such as poly(m-phenylene isophthalamide) (MPD-I), poly(/n-benzamide) (MBA), or poly(ethylene glycol) (PEG). The use of end-capped prepolymers and selective extraction techniques assured a strict two-block sequence, the absence of free homopolymers not strongly bound to the copolymers, and a fractionation in terms of the rigid/flexible compositional distribution ratio. 

In the case of the elastomers, formulations that use diisocyanates (O Fig. 14.16) and mixtures of long diols (oligomers of polybutadiene, polycaprolactone, or polaxyethylene) and short diols lead to thermoplastic elastomer behavior, with physical cross-links (rigid sequences) finding themselves associated, dispersed in micro-doinains, with a continuous elastomer phase (flexible sequences). 

For polymers in which the incorporation of flexible sequences between the rigid mesogenic moieties allows for lower transition temperatures some recent X-ray data also indicate a prominence of smectic, layered structures (23, 25). In ad-... 

We realize that the rigid reinforcement in traditional composite materials acts at the macroscopic level. As suggested in Ref. 19, we will call such materials heterogeneous composites. Problems inherent to their use can be eliminated in at least two ways. Helminiak, Hwang and their colleagues developed materials which they called molecular composites (MCs) rigid chain molecules dispersed at the molecular level in flexible chain polymers. The other way is to use PLCs—in which, in most cases, each chain already contains rigid and flexible sequences connected by primary chemical bonds. 

In a PLC we can expect somewhat lower anisotropy factors, but we should realize that the rigid sequences impart orientation to flexible sequences between. Again Escher has provided something that can be used to illustrate a scientific point. Look at Fig. 1.3 and assume that the black lizards represent rigid PLC sequences and white animals the flexible sequences. It is easy to see that the flexible sequences necessarily become oriented to a certain extent whether they like it or not . 

Much fewer experiments are available in solution where the few reported data are generally more concerned about the effect of molecular structure than about bond dissociation energy. In simple shear, it is generally agreed that chain flexibility dominantly influences the rate of bond scission, with the most rigid polymers being the easiest to fracture [157]. The results are interpreted in terms of the presence of good and poor sequences in the chain conformation. 

The rheological behaviour of polymeric solutions is strongly influenced by the conformation of the polymer. In principle one has to deal with three different conformations, namely (1) random coil polymers (2) semi-flexible rod-like macromolecules and (2) rigid rods. It is easily understood that the hydrody-namically effective volume increases in the sequence mentioned, i.e. molecules with an equal degree of polymerisation exhibit drastically larger viscosities in a rod-like conformation than as statistical coil molecules. An experimental parameter, easily determined, for the conformation of a polymer is the exponent a of the Mark-Houwink relationship [25,26]. In the case of coiled polymers a is between 0.5 and 0.9,semi-flexible rods exhibit values between 1 and 1.3, whereas for an ideal rod the intrinsic viscosity is found to be proportional to M2. 

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