Polymer blends INDEX

To support this hypothesis, the OBC sample can be fractionated by the TREF experiment. TREF fractionation of the OBC, followed by evaluation of the octene content by 13C NMR, reveals the data shown in Fig. 21. For a polymer blend, each molecule dissolves and elutes according to its comonomer content. The results invariably fall on the line in Fig. 21 labeled random copolymer line. The triangles reveal the comonomer content of the TREF fractions from an OBC. At any given temperature, the polymer eluting has much more comonomer than would be expected for a random distribution. The only explanation is that the comonomer is blocked, as expected from the chain shuttling mechanism. The extent of deviation can even be quantified, and a new method was recently invented to determine the block index for a given polyolefin [46],... 

Glass transition temperature is one of the most important parameters used to determine the application scope of a polymeric material. Properties of PVDF such as modulus, thermal expansion coefficient, dielectric constant and loss, heat capacity, refractive index, and hardness change drastically helow and above the glass transition temperature. A compatible polymer blend has properties intermediate between those of its constituents. The change of glass transition temperature has been a widely used method to study the compatibility of polymer blends. Normally, the glass transition temperatme of a compatible polymer blend can be predicted by the Gordon-Taylor relation ... 

Coleman et al. 2471 reported the spectra of different proportions of poly(vinylidene fluoride) PVDF and atactic poly(methyl methacrylate) PMMA. At a level of 75/25 PVDF/PMMA the blend is incompatible and the spectra of the blend can be synthesized by addition of the spectra of the pure components in the appropriate amounts. On the other hand, a blend composition of 39 61 had an infrared spectrum which could not be approximated by absorbance addition of the two pure spectra. A carbonyl band at 1718cm-1 was observed and indicates a distinct interaction involving the carbonyl groups. The spectra of the PVDF shows that a conformational change has been induced in the compatible blend but only a fraction of the PVDF is involved in the conformational change. Allara M9 250 251) cautioned that some of these spectroscopic effects in polymer blends may arise from dispersion effects in the difference spectra rather than chemical effects. Refractive index differences between the pure component and the blend can alter the band shapes and lead to frequency shifts to lower frequencies and in general the frequency shifts are to lower frequencies. 

Hydrogels and amphiphilic membranes Poly(carbophosphazenes) and poly(thiophosphazenes) New condensation syntheses NLO and high refractive index polymers Microencapsulation of mammalian cells (PCPP) Polyphosphazene polymer blends and IPN s Borazine derivatives Poly(phosphazophosphazenes)... 

Figure 3.6 Variation of retention with the composition of the stationary phase in GLC. Stationary phase styrene-butadiene polymer blends and copolymers, the butadiene fraction is plotted on the horizontal axis, (a) Specific retention volumes for three n-alkanes and benzene. V is proportional to the capacity factor, (b) the retention index for benzene. The solid line is calculated from the straight lines in figure 3.6a. The circles (polymer blends) and triangles (copolymers) represent experimental data. Figure taken from ref. [310], Reprinted with permission.

Several implications can be drawn directly from Eq. (2-39). First, A // is always positive. Thus, the rule like attracts like, inferred from Eq. (2-30) for molecular mixtures, should also hold at the continuum level. Second, when dispersion forces are dominant, the Hamaker constant is small when ha= b—that is when the dispersed phase (A) has an index of refraction close to that of the medium (B), These rules also apply to molecular mixtures. Nevertheless, small molecules with a significant difference in index of refraction often mix because of the large entropy thereby gained. But particles lose too little entropy on coagulation to resist doing so when there is an attractive van der Waals interaction, and so particle-particle clumping is the norm in suspensions, unless countermeasures are taken to stop it (see Section 7.1). Analogous considerations explain the prevalence of phase separation in polymer blends (see Section 2.3.1.2). 

Fig. 22. (a) Snapshot of an interface between two coexisting phases in a binary polymer blend in the bond fluctuation model (invariant polymerization index // = 91, incompatibihty 17, linear box dimension L 7.5iJe, or number of effective segments N = 32, interaction e/ksT = 0.1, monomer number density po = 1/16.0). (b) Cartoon of the configuration illustrating loops of a chain into the domain of opposite type, fluctuations of the local interface position (capillary waves) and composition fluctuations in the bulk and the shrinking of the chains in the minority phase. Prom Miiller [109]... 

First information on the morphology of polymer blends is simply obtained by visual inspection. Blending two transparent, colorless amorphous polymers that have different refractive indexes usually leads to an opaque material, in which the size of phases exceeds the wavelength of visible light (>500 nm). It is usually assumed that to have opacity the difference of the refractive indexes should be larger than 0.003 [Paul, 1978]. 

The following index of trade names and suppliers is based on a choice of commercial polymer blends and alloys. No claim is made for completeness. Detailed lists can be found in the source cited. 

One can assume that blends of polymers will be more difficult to color than any component by themselves. DiflPuse reflection can increase due to internal light reflection or scattering at phase interfaces if the polymers are at least partially immiscible or their refractive indexes are significantly different. Blends of translucent polymers are typically more opaque than either resin alone. Furthermore, colorant stability (thermal or chemical) can be adversely affected by the presence of the other polymer(s). As in the case of neat polymers, both circumstances will result in a restricted achievable color gamut for the polymer blend. An example of a prominent polymer blend is GEs Noryl (PS/PPO) which certainly colors much differently than the polystyrene component by itself... 

The data shown in Figure 3 also illustrates the tunable nature of a material property in PVC/PS composite particles - namely the dielectric constant manifested in the refractive index. Both Re(n) and Im(n) for the polymer-blend microparticles are intermediate between the values determined for pure single-component particles (PVC Re(n) = 1.4780, lm(n) = 10- PS Re(n) = 1.5908, Im(n) = 2 x lO- ) and can be controlled by adjusting the weight fractions of polymers. Interestingly, the measured refractive index for composite particles are very close to estimates obtained from a simple mass-weighted average of the two species. 

The miscibility of a polymer blend is usually ascertained by studying the optical, morphological, and glass transition behavior of the blend. When two amorphous polymers with different refractive indices mix intimately to form a miscible blend, the refractive index of the blend is uniform, and the blend appears transparent. On the other hand, when the two polymers do not mix intimately, the resulting blend is opaque. It must be cautioned that a two-phase immiscible blend may appear transparent if the refractive indices of the two polymers are closely matched or the domain size is smaller than the wavelength of the visible light. For a blend containing a crystallizable polymer, the blend may appear opaque even when the amorphous phases of the two polymers mix intimately. 

At this point the integral viscosity has to be expressed as a function of the volume fraction solid. A number of relationships have been proposed to describe the increase in viscosity with volume fraction solid. Good reviews are incorporated in the two-volume book on polymer blends by Paul and Newman [263] some other references are 264 through 267. A useful expression is the Maron-Pierce relationship, which for a power law fluid results in the following expression for the consistency index ... 

The amount of scattered light increases as the square of the difference in refractive index of the two phases. Thus, polymer blends, grafts, and IPNs will be clear if the refractive indexes nearly match. 

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