Experimental chain block

Fig. 8. Number of repeat units per polymer chain as a function of conversion. Dashed curves are from Ref. and were calculated on the basis of the elastic chain theory for different ratios of the elastic constants E and Ep of monomer and polymer, respectively, and normalized to fit the experimental data near X = 0.1. The full curve is calculated according to Eq. (9) (Sect. 6.1) ignoring chain blocking effects. Data points are experimental...

The three theoretically predicted forms and their dependence on the block-length ratio are actually experimentally found (Figure 5-31). The experimentally found block-length ratio of 40 60 for lamellae does not, of course, correspond to the theoretically required ratio of 50 50. This result, however, can be traced to the fact that the films were formed from evaporation of solutions. Solvents alter the chain dimensions and thus the morphology. 

Use of D-amino acids in the synthesis of a hairpin loop portion from the CD4 receptor provides a stable CD4 receptor mimic, which blocks experimental allergic encephalomyelitis (144). This synthetic constmct is not simply the mirror image or enantiomer of the CD4 hairpin loop, but rather an aH-D-constmct in the reverse sequence, thus providing stereochemicaHy similar side-chain projections of the now inverted backbone (Fig. 11). This peptide mimetic, unlike its aH-L amino acid counterpart, is resistant to en2yme degradation. As one would expect, the aH-D amino acid CD4 hairpin loop, synthesi2ed in the natural direction, the enantiomer of the natural constmct, is inactive. 

The main experimental techniques used to study the failure processes at the scale of a chain have involved the use of deuterated polymers, particularly copolymers, at the interface and the measurement of the amounts of the deuterated copolymers at each of the fracture surfaces. The presence and quantity of the deuterated copolymer has typically been measured using forward recoil ion scattering (FRES) or secondary ion mass spectroscopy (SIMS). The technique was originally used in a study of the effects of placing polystyrene-polymethyl methacrylate (PS-PMMA) block copolymers of total molecular weight of 200,000 Da at an interface between polyphenylene ether (PPE or PPO) and PMMA copolymers [1]. The PS block is miscible in the PPE. The use of copolymers where just the PS block was deuterated and copolymers where just the PMMA block was deuterated showed that, when the interface was fractured, the copolymer molecules all broke close to their junction points The basic idea of this technique is shown in Fig, I. 

The generic features of these approaches are known from experience in anionic polymerization. However, radical polymerization brings some issues and some advantages. Combinations of strategies (a-d) are also known. Following star formation and with appropriate experimental design to ensure dormant chain end functionality is retained, the arms may be chain extended to give star block copolymers (321). In other cases the dormant functionality can be retained in the core in a manner that allows synthesis of mikto-arm stars (324). 

Here, the chains are expected to be stretched, as indicated by the 2/3-power dependence of L on N, but less strongly than in solvent. The experimental evidence available to examine this argument is discussed in the section on block copolymer melts. 

Clearly Fig. 7 must actually have a maximum at high asymmetry since this corresponds to negligible anchor block size and therefore to no adsorption (ct = 0). The lattice theory of Evers et al. predicts this quantitatively [78] and is, on preliminary examination, also able to explain some aspects of these data. From these data, the deviation from power law behavior occurs at a number density of chains where the number of segments in the PVP blocks are insufficient to cover the surface completely, making the idea of a continuous wetting anchor layer untenable. Discontinuous adsorbed layers and surface micelles have been studied theoretically but to date have not been directly observed experimentally [79]. 

Experimentally, the stretching of block copolymer chains has been addressed in two ways by measuring L as a function of N, and by measuring the components of Rg of the block chains both parallel and perpendicular to the interface. The domain dimensions have been studied most extensively for styrene-isoprene and styrene-butadiene block copolymers X-ray and neutron scattering are the methods of choice. The predicted SSL scaling of L N2/3 has been reported for spheres, cylinders and lamellae [99,102-106], but not in all cases. For example, Bates et al. found N0 37 for styrene-butadiene spheres [100], and Hadziioannou and Skoulios observed N0 79 for styrene-isoprene lamellae [107], In the sphere case, kinetic limitations to equilibration were felt to be an important factor [100],... 

The thermostability of siloxane-silazane copolymers of both random and block structure is found to be much higher (i.e. 100-200°C) with respect to polysiloxanes. This effect is brought about by introducing only a few silazane entities into the polymer chain. The reasons for the effect are not clear and the mechanism of thermal degradation of polysilazoxanes will require further experimental studies. 

Comparison of solution pH with the pKa of a side chain informs about the protonation state. A unique pKa, termed the standard or model pKa, can be experimentally determined for each ionizable side chain in solution when it is incorporated in a model compound, often a blocked amino acid residue [73] (Table 10-1). In a protein environment, however, the pKa value of an ionizable side chain can substantially deviate from the standard value, due to desolvation effects, hydrogen bonding, charge-charge, charge-dipole, and other electrostatic interactions with the... 

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