Polymer, branched Miscibility

Other dilute solution properties depend also on LCB. For example, the second virial coefficient (A2) is reduced due to LCB. However, near the Flory 0 temperature, where A2 = 0 for linear polymers, branched polymers are observed to have apparent positive values of A2 [35]. This is now understood to be due to a more important contribution of the third virial coefficient near the 0 point in branched than in linear polymers. As a consequence, the experimental 0 temperature, defined as the temperature where A2 = 0 is lower in branched than in linear polymers [36, 37]. Branched polymers have also been found to have a wider miscibility range than linear polymers [38], As a consequence, high MW highly branched polymers will tend to coprecipitate with lower MW more lightly branched or linear polymers in solvent/non-solvent fractionation experiments. This makes fractionation according to the extent of branching less effective. 

Silicone Resins. Sihcone resins are an unusual class of organosdoxane polymers. Unlike linear poly(siloxanes), the typical siUcone resin has a highly branched molecular stmcture. The most unique, and perhaps most usehil, characteristics of these materials are their solubiUty in organic solvents and apparent miscibility in other polymers, including siUcones. The incongmity between solubiUty and three-dimensional stmcture is caused by low molecular weight < 10, 000 g/mol) and broad polydispersivity of most sihcone resins. 

The presence of a large number of chain-ends in the fully synthesized dendrimer molecules makes them highly soluble and also readily miscible, for example with other dendrimer solutions. The solubility is controlled by the nature of the end-groups, so that dendrimers with hydrophilic groups, such as hydroxyl or carboxylic acid, at the ends of the branches are soluble in polar solvents, whereas dendrimers with hydrophobic end-groups are soluble in non-polar solvents. The density of the end-groups at the surface of the dendrimer molecule means that they have proportionately more influence on the solubility than in linear polymers. Hence a dendritic polyester has been shown to be more soluble in tetrahydrofuran than an equivalent linear polyester. 

The chemical features that prohibit crystallinity are main chain flexibility (e.g., rotation), branching, random copolymers or low inter-polymer chain attraction. Normally, polymers are not miscible with each other and on cooling from the melt will separate into different phases. When miscibility is exhibited, e.g., poly(phenylene oxide) (PPO) and PS, crystallisation does not take place. 

Blend solutions. Solutions of blends comprising immiscible polymers Pj and P2 in a nonselective solvent have miscibility gaps as shown schematically in Fig. 14. When the polymer concentration increases by solvent evaporation the polymer coils start to interpenetrate above a certain concentration. As a consequence, interactions between the polymers become operative and phase separation must start above a critical polymer concentration p. The composition of the new phases will be situated on the branches of the coexistence curve. Finally, the unmixing process is arrested owing to enhanced viscosity. This simple scheme reveals the factors directing morphology evolution in blend solutions ... 

LCT (originally developed for di-block copolymers) was found to be particularly useful to explain miscibility of polyolefin blends where the two resins differ in the type and size of short chain branching. The stractural units of a polymer with two carbons in the main chain can be written as PE = (CH,-CH,), PP = [CH,-CH (CHj)], poly-2-butene (P2B) = [CH (CH3)-CH (CHj)], PIB = [CH -C (CHj) ]jj, poly(4,4-dimethyl 1-butene) (PDMB) = [CH -CH (C Hg)], etc. Three structural parameters (ratio of end to interior groups) have been used to distinguish PO structure r, p, and q. Their values for the model macromolecules discussed above are listed in Table 2.7. 

A similar branching effect on the miscibility was observed in the HDPE/ LLDPE-O blends, which were prepared from HDPE (49,400 and 3.60 PDI) and LLDPE-O polymers (69,200-104,000 M and 1.8-3.40 PDI) with 2-87 hexyl branches per 1000 backbone carbons (42). It was observed that the critical branch number in the LLDPE-O component capable of causing immiscibility in the HDPE/ LLDPE-O blend was 50 branches per 1000 backbone carbons, as determined using inverse gas chromatography, rather than the SANS technique. 

Kyu et al. (2) and Ree et al. (3,5) studied blends of LLDPE-B (114,000 M , 4.50 PDI, and 18 ethyl branches per 1000 backbone carbons) and LDPE (286,000 M, 15.98 PDI, and 26 short and 1.6 long branches per 1000 backbone carbons) using in situ small-angle light scattering (SALS) and DSC and found that these blends are miscible across the whole composition range. Similar miscibihty results were reported for LDPE blends with LLDPE-B and LLDPE-O polymers by other research groups. The DSC and DMTA analysis of Lee et al. (43) confirmed that the blends of LLDPEs (LLDPE-B 89,300 M, 3.8 PDI, and 15-16 branches per 1000 backbone carbons LLDPE-O 93,100 M, 3.6 PDI, and 15-16 branches per 1000 backbone carbons) and LDPEs (73,000-98,000 and 8.7-9.2 PDI 32-34 branches per 1000... 

As reviewed above, there is still no consensus on the miscibility of LLDPE/ LDPE blends, or the effects of short and long branches on the miscibility. Furthermore, the lack of detailed SANS analysis on LLDPE/LDPE blends is surprising, although the miscibility specifications of these blends are highly demanded, particularly in relation to their various applications in the polymer industry. 

Hameed and Hussein (2007) studied blends of m-LLDPE with HDPE varying the MW and branch content (BC) in m-LLDPE. No influence of MW on miscibility was observed for polymers with low BC ( 2/100 C). However, at high-BC levels (ca. 4/100 C) MW did affect miscibility of m-LLDPE/HDPE blends. Low-MW m-LLDPE/HDPE blends were miscible at aU compositions, while high-MW phase segregated into layered morphology. The HDPE-rich blends co-crystallized, whereas m-LLDPE-rich phase showed separate ciystaUization. Mechanical properties of these blends strongly depended oti blend miscibility and properties of components. It is noteworthy that the high-BC pairs had poor mechanical properties, caused by weak interphase. 

Y. Y. Chen, T.P. Lodge, E.S. Bates, Inllutatce of Istg-chain branching on the miscibility of poly (ethylene-r-ethyl-ethylene) blends with diffsent miCTostructures. J. Polym. Sci. B Polym. Phys. 40,466-477 (2002)... 

Z.J. Fan, M.C. Williams, P. Choi, A molecular study of the effects of branching characteristics of LDPE on its miscibility with HDPE. Polymer 43, 1497-1502 (2002)... 

K.F. Freed, J. Dudowicz, Influence of short chain branching rai the miscibility of binary polymer blends application to polyolefin mixtures. Macromolecules 29(2), 625-636 (1996)... 

---