Random copolymers, characterization

Dielectric relaxation measurements of polyethylene grafted with acrylic acid(AA), 2-hydroxyethyl methacrylate (HEMA) and their binary mixture were carried out in a trial to explore the molecular dynamics of the grafted samples [125]. Such measurements provide information about their molecular packing and interaction. It was possible to predict that the binary mixture used yields a random copolymer PE—g—P(AA/HEMA), which is greatly enriched with HEMA. This method of characterization is very interesting and is going to be developed in different polymer/monomer systems. 

Styrene-butadiene rubber (SBR) is the most widely used synthetic rubber. It can be produced by the copolymerization of butadiene (= 75%) and styrene (=25%) using free radical initiators. A random copolymer is obtained. The micro structure of the polymer is 60-68% trans, 14-19% cis, and 17-21% 1,2-. Wet methods are normally used to characterize polybutadiene polymers and copolymers. Solid state NMR provides a more convenient way to determine the polymer micro structure. ... 

Thus, confirmation of whether the product obtained in an attempted reaction in a true random copolymer is important to clarify the mechanism of the propagation reaction and to correlate structure and reactivity in ring-opening polymerizations. Considering that apparent copolymers may be formed by reactions other than copdymerization, for example, by ionic grafting or by combination of polymer chains, characterization of cross-sequences appears to be one of the best ways to check the formation of random copolymers. 

Thus, NMR spectroscopy is useful for characterizing the nature of the cross-sequences in random copolymers. 

All poly(3HB-co-3HV)s synthesized by R. europha that were characterized for sequence distribution were found to be random copolymers as indicated by 13C NMR spectrometry. However, as noted above, the fractionation of some poly(3HB-co-3HV) copolymers revealed that the sequence distributions determined by 13C NMR might not be reliable. In that study, poly(3HB) that had a chad sequence indicative of a random poly(3HB-co-3HV), as determined by 13C NMR, was fractionated into various copolymers that had significantly different compositions. 

Conformational disorder and kink-bands structures have recently been found also in random copolymers of syndiotactic polypropylene with small amounts of ethylene.192 193 The ethylene units are included in the crystalline regions193 and induce the crystallization of the metastable form II of sPP with conformationally disordered chains characterized by kink bands. Portions of chains containing the ethylene units tend, indeed, to assume a trans planar conformation, producing the kink-band defects in chains in the prevailing twofold helical conformation.192193... 

The value of the modulus and the shape of the modulus curve allow deductions concerning not only the state of aggregation but also the structure of polymers. Thus, by means of torsion-oscillation measurements, one can determine the proportions of amorphous and crystalline regions, crosslinking and chemical non-uniformity, and can distinguish random copolymers from block copolymers. This procedure is also very suitable for the investigation of plasticized or filled polymers, as well as for the characterization of mixtures of different polymers (polymer blends). 

The characterization of block or graft copolymers is generally much more difficult than that of random copolymers (see Sect. 23.2.7). Especieilly, DSC measurements are useful for the characterization of the different segments (determination of Tg). Also dynamic-mechanical measurements are used to distinguish statistical copolymers from those with block or graft structure. 

The synthesis and characterization of water-soluble "random" copolymers containing t-valine with either A -O-hydroxypropyD-L-glutamine or / -(A-hydroxybutyD-L-glutamine are described, and the thermally induced helix-coil transitions of these copolymers in water are studied. The incorporation of /.-valine is found to decrease the helix content of the polymer at low temperatures and increase it at high temperatures. The Zimm-Bragg parameters o and s for the helix-coil transition in poly(t-valine) in water are deduced from an analysis of the melting curves of the copolymers. The values of s, computed for o = 1 x 1CI-4, are s 0.85 (273 K), 0.93 (293 K), 1.00 (313 K), 1.06 (333 Kl. 

There are, however, other possible routes to block copolymers successive addition of units of the reactive monomer to the polymer already present, Reaction 5 termination reactions between polymer molecules —side reactions of unknown nature lead to loss of reactive hydroxyl groups (18) possible reactions are ortho carbon-carbon coupling followed by dimerization, addition of amine or water to the ketal intermediate, etc. Block copolymers might even be formed by polymer-polymer redistribution assuming that such redistribution in polymers of greatly different reactivities (such as DMP and DPP), takes place almost exclusively in one type of polymer sequence—that is, that bond scission in a "mixed ketal such as IV occurs always in the same direction—to produce the aryloxy radical corresponding to the more reactive monomer. None of these possible sources of block copolymer can be ruled out on the basis of available evidence. All could produce homopolymer in addition to block copolymer. All of the polymers produced in this work, except for those characterized as completely random copolymers, probably contained at least small amount of one or both homopolymers. 

Random copolymers are characterized by the statistical placement of comonomer repeating units along the backbone chain. Alternating copolymers, as the name suggests, are characterized by the alternate placement of monomers. Graft copolymers are made of chemically linked pairs of homopolymers and resemble a comb. Block copolymers are composed of terminally connected structures. 

DMTA is a very interesting tool for characterizing heterogeneous materials in which domains of distinct Tg values coexist. The most interesting cases involve modified thermosets of different types (see Chapter 8). Examples are the use of rubbers (e.g., liquid polybutadiene and random copolymers), or thermoplastics (e.g., polyethersulphone or polyetherimide in epoxy matrices or poly(vinyl acetate) in unsaturated polyesters), as impact modifier (epoxies), or low-profile additives (polyesters). The modifier-rich phase may be characterized by the presence of a new a peak (Fig. 11.10). But on occasions there may be superposition of peaks and the presence of the modifier cannot be easily detected by these techniques. If part of the added polymer is soluble in the thermoset matrix, its eventual plasticizing effect can be determined from the corresponding matrix Tg depletion, and the... 

Miyashita et al. carried out miscibility characterization of CA blends with poly(N-vinyl pyrrolidone) (PVP), poly(vinyl acetate) (PVAc), and poly(N-vinyl pyrrolidone-co-vinyl acetate) random copolymers [P(VP-co-VAc)s] [ 104]. On the basis of thermal transition data obtained by differential scanning calorimetry (DSC), a miscibility map (Fig. 8) was completed as a function of the degree of substitution (DS) of CA and the VP fraction in P(VP-co-VAc). Figure 9 compares results of the DSC measurements between two blending pairs of CA/P(VP-co-VAc) corresponding to the polymer combinations marked as A and B in Fig. 8. In the data (Fig. 9b) for the blends of CA (DS = 2.95) with P( VP-co-VAc) of VP = 51 mol %, we can readily see a sign of poor miscibility, as is evidenced from the lack of an appreciable shift in the... 

As an example the characterization of surface grafting onto an ethylene-vinyl alcohol (33 67) random copolymer (EVA) film33 is described here. Since this water-insoluble film has hydroxyl groups on the surface, surface grafting may occur, for instance, with the so-called dialdehyde starch (DAS), whose chemical structure is given by... 

Morishima, Y., et al. (1995), Characterization of unimolecular micelles of random copolymers of sodium 2-(acrylamido)-2-methylpropanesulfonate and methacrylamides bearing bulky hydrophobic substituents, Macromolecules, 28, 2874-2881. 

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