Block copolymer incremental

Clear impact-resistant polystyrene is a commercial plastic with the desirable combination of toughness and exceptional clarity. It is a styrene-1,3-butadiene multiblock copolymer containing more than 60% styrene. Most of these products are mixtures of block copolymers formed by incremental additions of initiator and monomers followed by coupling (Sec. 5-4c). The products generally have a tapered and multiblock composition with branching (due to the coupling agent). 

So far the discussion was focused on copolymers derived from a mixture of styrene and a diene. In view of the "living" nature of organolithium polymerization, it is also possible to synthesize block polymers in which the sequence and length of the blocks are controlled by incremental (or sequential) addition of monomersr This general method of preparing block polymers is readily adaptable to commercial production, and, indeed, a number of block copolymers are manufactured this way. Those that have received the most attention in recent years are the diene-styrene two-phase... 

Refractometry can be used to determine the composition of a copolymer. In addition, differential refractometry has been used to study micellization in dilute block copolymer solutions (Tfizar and Kratochvfl 1972). The refractive index (n) is obtained in an Abbe refractometer via measurements of the critical angle for external reflection. The refractive index increment dn/dc, where c is the polymer concentration, can be related to the molecular weight of particles in solution. Further details of the method are provided by Pepper and Samuels (1989). 

Most controlled/living cationic systems provide well-defined polymers of relatively low molecular weight, typically in the range M = 5,000 to 20,000. The degrees of polymerization are equal to the ratio of concentrations of the reacted monomer to that of the introduced initiator (DP = d[M]/[I]0), and the polydispersities remain relatively low, MJMn < 1.2. The addition of new portions of monomer (sometimes called incremental monomer addition, IMA) [5] leads to the expected increase of molecular weights, and the addition of another comonomer results in block copolymers, whereas the addition of terminating reagents provides end-functionalized macromolecules (cf. Chapter 5). 

To achieve optimal properties in an AB block copolymer, it is important to control the molecular weight of the blocks, and minimize the amount of A homopolymer produced on addition of the second monomer. Termination reactions do occur in these systems [20], but the rate is fairly slow, particularly at temperatures below about 100 °C. In a practical sense, protic impurities present a much greater challenge. In a two-reactor system it is common practice to prepare the first block in one reactor, titrate out impurities in the B monomer charge in a second reactor by adding small increments of butyllithium to a solution of the B monomer until the first sign of color or exotherm, and then the transfer poly(A)Li solution to the second reactor. 

Various workers have developed analyses for physical mixtures and block copolymers based on the ratio of the incremental methylene area to the total polymer proton... 

Various workers have developed analyses for physical mixtures and block copolymers based on the ratio of the incremental methylene area to the total polymer proton absorption. This concept has been tested by Barrall and co-workers [12, 13] using PMR analyses on a series of physical mixtures and block copolymers synthesised with C-labelled propylene and others with C-labelled ethylene. A most important feature of this analysis is that the methylene peaks A and B have virtually the same relative heights in PP with a variety of tacticities (Figure 6.3(a)). This is also true for PMR spectra given by Satoh and others for a tactic series of PP [14,15]. This suggests that PMR analyses for ethylene are independent of tacticity because the area increment of peak A above peak B has been used for analysis. 

Figure 1. SAKS (left) and WAXD (right) from the SEL-4.7/20/1.8 star block copolymer taken in increments of 60s following a temperature jump from 383 K to 313 K. The SAKS invariant Q and the WAXD intensity of the most intense reflection of the PEO are also compared.

Zhu et al. studied polyurethane foams from soy reinforced with cellulose microfibers. They found an increase on the onset degradation temperature of the thermal degradation of polyurethane with the addition of 2 wt % cellulose fibers. They attributed this fact to the insulator effect of cellulose fibers [52]. Navarro-Baena et al. studied shape memory PU based on PLA-PCL-PLA block copolymer and reinforced with both CNCs and PLA grafted CNCs [72]. Aside the increment on the shape memory behavior of the polyurethane-based nanocomposites, they reported an increase on the thermal stability of the PU matrix in particular, they reported that, although CNCs improved the thermal stability of both PCL and PLEA blocks, in particular the thermal stability of the PCL block was improved in the nanocomposites increasing the maximum degradation temperature of about 40 with respect to the PCL block of the neat PU-matrix [72]. 

An increasing amount of styrene-butadiene rubber is being manufactured by solution processes using alkyllithium catalysts. Production techniques resemble those used for the polymerization of isoprene (section 20.3.3) and butadiene (section 20.4.3). There is a tendency for alkyllithium initiation to lead to block copolymers since the butadiene in the mixture polymerizes first to the virtual exclusion of the styrene. In order to obtain random copolymers it is necessary to add the butadiene incrementally so that the molar ratio of... 

A series of high-cis B-I-B triblock copolymers were prepared with lanthanide catalysts by incremental monomer addition. TTie hydrogenated products lead to E-(E/P)-E triblock copolymers with hard (semi-crystalline) ethylene block at both chain ends and soft (amorphous rubbery) alternating E/P block in the center segment. 

With macromolecular systems consisting of more than one type of monomer units (statistical, block, or graft copolymers), the experimenter may be confronted with the problem of the compositional fluctuation of the macromolecules. In such a case the molar mass values measured are only apparent, if the increment of refractive index of the copolymer (dhldCcopo ) is deduced from the weight average of the two components A and B. Benoit suggested instead to carry out the measurements in three different solvents and to use the following relation ... 

---