Monomer structural asymmetry

J. Dudowicz, K.F. Freed, J.F. Douglas, Beyond Floiy-Huggins thcOTy new classes of blend miscibility associated with monomer structural asymmetry. Phys. Rev. Lett. 88(9), 955031-955034 (2002a)... 

Nonrandom mixing effects Monomer structural asymmetry... 

An additional dependence on the copolymer compositions x and y beyond that predicted by simple random copolymer FH theory appears in Eq. 24b through the front conversion factor C/S1S2 as well as through factors of 51/52 and 52/51. The main novel feature of Eq. 24b, however, lies in the presence of the temperature-independent portion of xsans. This term represents the influence of monomer structural asymmetry on the nonrandom chain packing and coincides with the leading contribution of the LCT for binary blends in the incompressible, high molecular weight, athermal, fully flexible chain limit. 

One practical example of demixing that might be attributed to a difference in crystallizability is the incompatibility in blends of polymers with different stereochemical compositions. The stereochemical isomers contain both chemical and geometrical similarities, but differ in the tendency of close packing. In this case, both the mixing energy B and the additional mixing entropy due to structural asymmetry between two kinds of monomers are small. However, the stereochemical differences between two polymers will result in a difference in the value of Ep. Under this consideration, most experimental observations on the compatibility of polymer blends with different stereochemical compositions [89-99] are tractable. For more details, we refer the reader to Ref. [86]. 

This phenomenon was originally thought to be indicative of a localised mixed-valence state, i.e. that the appearance of this mode is proof of electronic asymmetry on the infrared timescale. Flowever, control experiments with the cluster monomers that have pronounced structural asymmetry, were carried... 

J. Dudowicz, K.F. Freed, Influence of monomer structure and interaction asymmetries oti the miscibility and interfacial properties of polyolefin blends. MaOTomolecules 29(27), 8960-8972 (1996)... 

In mean field theory, two parameters control the phase behavior of diblock copolymers the volume fraction of the A block /A, and the combined interaction parameter xTak- V. where Xab is the Flory-Huggins parameter that quantifies the interaction between the A and B monomers and N is the polymerization index [30], The block copolymer composition determines the microphase morphology to a great extent. For example, comparable volume fractions of block copolymer components result in lamella structure. Increasing the degree of compositional asymmetry leads to the gyroid, cylindrical, and finally, spherical phases [31]. 

Optical activity in biopolymers has been known and studied well before this phenomenon was observed in synthetic polymers. Homopolymerization of vinyl monomers does not result in structures with asymmetric centers (The role of the end groups is generally negligible). Polymers can be formed and will exhibit optical activity, however, that will contain centers of asymmetry in the backbones [73]. This can be a result of optical activity in the monomers. This activity becomes incorporated into the polymer backbone in the process of chain growth. It can also be a result of polymerization that involves asymmetric induction [74, 75]. These processes in polymer formation are explained in subsequent chapters. An example of inclusion of an optically active monomer into the polymer chain is the polymerization of optically active propylene oxide. (See Chap. 5 for additional discussion). The process of chain growth is such that the monomer addition is sterically controlled by the asymmetric portion of the monomer. Several factors appear important in order to produce measurable optical activity in copolymers [76]. These are (1) Selection of comonomer must be such that the induced asymmetric center in the polymer backbone remains a center of asymmetry. (2) The four substituents on the originally inducing center on the center portion must differ considerably in size. (3) The location... 

Symmetric polymer blends do not exist in reality. A host erf asymmetries are present in real chemical alloys of interest These include attractive potential asymmetries (present even for isotopic blends) and specific interactions, molecular weight asymmetries and polydispersity, and single chain structural differences between the blend components (e.g., monomer shape and volume, backbone stiffness, and tacticity). Realistic accounting for most of these effects would seem to require an off-lattice description which includes local interchain density and concentration correlations, and compressibility effects [1, 2, 63, 66, 67, 80]. 

Poly(3-hexylthiophene), P3HT, is a good example to illustrate the impact of structural perfectness on fundamental properties of a polymer and performance of devices fabricated on its basis. In general, the asymmetry of 3-substituted thiophenes results in three possible coupling modes when two monomers are... 

The polymerization of butadiene monomer proceeds with chain propagation via 1,2-,, A-trans- or 1,4-cw-additions. If the polymerization is controlled to form mostly the 1,2-addition product, the polymer has a — CH2— chain with a terminal vinyl, — CH=CH2, substituent, at alternating carbon atoms. However, if 1,4-addition dominates the polymerization proceeds to form a polymer chain with a molecular structure of — (CH2 —CH=CH—CH2) —, normally with a trans configuration at the double bond. 2-Chloro-1,3-butadiene (CH2=CC1—CH=CH2 chloroprene) and 2-methyl-1,3-butadiene (isoprene) are polymerized in a similar manner. With these compounds, the asymmetry of the carbon atoms at positions 1 and 4 produces a variety of addition products with 1,2-, 1,4-cw,, A-trans, and 3,4-configurations. In the case of polyisoprene, which in nature occurs as natural rubber, the 1,4-cis configuration is the dominant structure. A summary of the polymerization products of butadiene, isoprene, and chloroprene is provided in Fig. 31. 

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