Special Local Need labels

The net result is shown in Fig. II. 10. The intensity now has a broad maximum at some intermediate wave vector qm. When peaks of this type were first observed on partly labeled chains, there was a natural temptation to ascribe them to some sort of local segregation of the deuterated species. In fact, as shown in Fig. IX.4 there is very good agreement between the data and precise calculations on the correlation hole (described in Chapter IX). No special segregation effect needs to be involved (at least for the molecular weights 10 which have been used in practice). 

All IR methods for measuring copolym compositions are relative and need calibration with suitable standards. Three calibration m hods are usually used radiochemical standards, calibration with the homopolymer mixtures, and calibration with model compounds (33-36). Calibration with radiochemical standards is very precise and accurate but is limited by the availability of labelled olefins. Calibration with homopolymer mixtures is also very popular (33,37-41,43). It is a very simple method but has many drawbacks, the main one being the constitutional difference between such mixtures and real copolymers (32). Nevertheless, its use is justified if the chosen analytical bands are highly localized and if the copolymers examined have a block structure (rj rj > 1) similar to that of the polymer mixtures. The third calibration method consists in using model compounds for the determination of absorption coefficients. These model compounds are either homopolymers (42,44-47), in which case the method is in priiKiple the same as calibration with polymer mixtures, or special compoimds with structures resembling those of characteristic groups in copolymers (13,49,51). 

---