Dendritic polymers intrinsic viscosity

Due to dieir compact, branched structure and to die resulting lack of chain entanglement, dendritic polymers exhibit much lower melt and solution viscosity dian their lineal" counterparts. Low a-values in die Mark-Houwink-Sakurada intrinsic viscosity-molar mass equation have been reported for hyperbranched polyesters.198 199 Dendrimers do not obey diis equation, a maximum being observed in die corresponding log-log viscosity-molar mass curves.200 The lack of chain entanglements, which are responsible for most of the polymer mechanical properties, also explains why hyperbranched polymers cannot be used as diermoplastics for structural applications. Aldiough some crystalline or liquid... 

While it can be expected that a number of physical properties of hyperbranched and dendritic macromolecules will be similar, it should not be assumed that all properties found for dendrimers will apply to hyperbranched macromolecules. This difference has clearly been observed in a number of different areas. As would be expected for a material intermediate between dendrimers and linear polymers, the reactivity of the chain ends is lower for hyperbranched macromolecules than for dendrimers [125]. Dendritic macromolecules would therefore possess a clear advantage in processes, which require maximum chain end reactivity such as novel catalysts. A dramatic difference is also observed when the intrinsic viscosity behavior of hyperbranched macromolecules is compared with regular dendrimers. While dendrimers are found to be the only materials that do not obey the Mark-Houwink-Sakurada relationship, hyperbranched macromolecules are found to follow this relationship, albeit with extremely low a values when compared to linear and branched polymers [126]. 

The intrinsic viscosities of the dendritic polymers are extremely small compared with those of linear polymer of the same MW [86,91]. Furthermore, the dendritic polymers expand very little in going from a 0 solvent to a good solvent [92]. This is to be expected. When steric congestion forces the polymer chains to expand in a 0 solvent, further expansion in a good solvent is limited. In this regard it is important to note that the 0 condition must be carefully specified. It is known that branched polymers have different 0 conditions to the linear counterpart [93]. 

The unique architecture of dendritic polymers affords distinct physical properties to these molecules as compared to their linear polymer analogues. For example, as shown in Figure 30.2, branched molecules have a much lower intrinsic viscosity than their linear analogues. Their compact and dense molecular configuration also leads to enhanced solubility at high molecular weights. 

Figure 30.2 Molecular weight dependence of intrinsic viscosity [/ ] for polymers with (a) linear, (b) hyperbranched, (c) dendrimer, and (d) dendrigraft architectures. Source Reproduced with permission from Tomalia DA, Frechet JMJ. Introduction to the dendritic state. In Tomalia DA, Frdchet JMJ, editors. Dendrimers and Other Dendritic Polymers. West Sussex Wiley 2001. p 3 [14]. Copyright 2001 John Wiley and Sons.

Fig. 37 Schematic drawing of a dendrimer (a) and hyperbranched polymers (b). D, B, and T denote dendritic, linear, and terminal units, respectively, (c) niustratirai of an n-butane molecule for the explanation of Wiener index, (d) Intrinsic viscosity [tj grows non-mraiotraiically as a function of generation G...

Aerts, J. Prediction of intrinsic viscosities of dendritic, hyperbranched and branched polymers. Computational and Theoretical Polymer Science, 8,49-54 (1998). 

The exploration of new polymer architectures has been the focus of significant recent research, motivated by the fundamental hypothesis that a polymer s properties are intimately related to its structure. This concept has led to the development and optimization of synthetic techniques for the preparation of graft, star, dendritic, ladder, and hyperbranched polymers as well as a variety of hybrid and more complex architectures. Cyclic polymers are of particular interest because their circular shape and lack of end groups has a profound effect on their physical properties such as intrinsic viscosity and hydrodynamic volumes (/). 

A comparison of the physical properties of hyperbranched and dendritic macromolecules with linear polymers and the linear analogs of these 3-dimensional polymers is presented. It is found that thermal properties, such as glass transition temperature and degradation, are the same regardless of the macromolecular architecture but are very sensitive to the number and nature of chain end functional groups. However, other properties, such as solubility, melt viscosity, chemicd reactivity, intrinsic viscosity were found to be very dependent on the macromolecular architecture. 

Dendritic polymers exhibit very different properties compared with their linear analogs. For instance, they exhibit extremely high solubility in various organic solvents and low intrinsic viscosity in comparison to their linear analogs. These differences are probably a reflection of the large number of chain end functional groups as well as the influence of architectural differences [59]. 

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