Intrinsic viscosity dendrimer

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... 

However, dendrimeric and hyperbranched polyesters are more soluble than the linear ones (respectively 1.05, 0.70, and 0.02 g/mL in acetone). The solution behavior has been investigated, and in the case of aromatic hyperbranched polyesters,84 a very low a-value of the Mark-Houvink-Sakurada equation 0/ = KMa) and low intrinsic viscosity were observed. Frechet presented a description of the intrinsic viscosity as a function of the molar mass85 for different architectures The hyperbranched macromolecules show a nonlinear variation for low molecular weight and a bell-shaped curve is observed in the case of dendrimers (Fig. 5.18). 

Dendrimers have distinctive properties, such as the ability to entrap small molecules in their core region and very low intrinsic viscosities in solution. Such properties require molecules to have achieved a particular size, and not all molecules with branches radiating from a core are large enough to develop the characteristic properties of true dendrimers. Branched molecules below this critical size are called dendrons and are the equivalent in dendrimer chemistry of oligomers in polymer chemistry. 

The reason for the low intrinsic viscosities in solution is that dendrimers exist as tightly packed balls. This is by contrast with linear polymers, which tend to form flexible coils. The effect of this difference is that, whereas polymer solutions tend to be of high viscosity, dendrimer solutions are of very low viscosity. In fact, as dendrimers are prepared, their intrinsic viscosity increases as far as the addition of the fourth monomer unit to growing branches (the so-called fourth generation), but this is the maximum value that the viscosity reaches, and as the side chains grow beyond that, the viscosity decreases. 

Mourey, T.H., Turner, S.R., Rubenstein, M., Frechet, J.M.J., Hawker, C.J., and Wooley, K.L. (1992) Unique behavior of dendritic macromolecules Intrinsic viscosity of polyether dendrimers. Macromolecules 25, 2401-2406. 

Parallel studies on PAMAM dendrimers, the Frechet type polyether den-drons, and other dendrimer families have generated an extensive list of unique properties driven by the dendritic state/ Figure 1.18 compares several significant physical property differences between the linear and dendritic topologies related to conformations, crystallinity, solubilities, intrinsic viscosities, entanglement, diffusion/mobility and electronic conductivity. 

Figure 1.19 Comparison of intrinsic viscosities (log (f/)) versus molecular weight (log M) for (A) linear, (B) random hyperbranched, (C) dendrimers and (D) dendrigraft topologies. Data for A, B, C adapted from Frechet etal.. Ref. 49.

Figure 14.2 Dependence of intrinsic viscosity, [ /], on generation for PAMAM, PPI and PBzE dendrimers. Data for PBzE monodendrons are included for comparison...

The viscosity of the polypropylene imine) dendrimers is investigated both in THF (intrinsic viscosity, Figure 26.3) and in the neat form. For the intrinsic... 

Figure 26.3 Intrinsic viscosity of the various nitrile-terminated polypropylene imine) dendrimers in THF as a function of molar mass...

Fig. 9. Generalized description of the intrinsic viscosity as function of molar mass for linear polymers, hyperbranched polymers and dendrimers as described by Frechet [33]...

Dendrimers have a star-like centre (functionality e.g. 4) in contrast to a star however, the ends of the polymer chains emerging from the centre again carry multifunctional centres that allow for a bifurcation into a new generation of chains. Multiple repetition of this sequence describes dendrimers of increasing generation number g. The dynamics of such objects has been addressed by Chen and Cai [305] using a semi-analytical treatment. They treat diffusion coefficients, intrinsic viscosities and the spectrum of internal modes. However, no expression for S(Q,t) was given, therefore, up to now the analysis of NSE data has stayed on a more elementary level. 

Keywords. Dendrimers, Hyperbranched macromolecules. Block copolymers. Surface functionalization, Solvatochromism, Intrinsic viscosity... 

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]. 

Mansfield ML, Klushin LI. Intrinsic viscosity of model starburst dendrimers. J Phys Chem 1992 96 3994-3998. 

Mouiey TH, Turner SR, Rubinstein M, Fiechet JMJ, Hawker CJ, Wooley KL. Unique behavior of dendritic macromolecules intrinsic viscosity of polyether dendrimers. Macromolecules 1992 25 2401-2406. 

Figure 1. General behavior of the intrinsic viscosity as a function of molar mass for dendrimers, linear, and hyperbranched polymers.

Unlike that of linear polymers, the intrinsic viscosity of dendrimers does not increase linearly (Fig. 1.15) with molar mass, but reaches a maximum at a certain generation (limiting generation), only to decrease again at high generations... 

Fig. 1.15 Intrinsic viscosity of dendrimers - compared with that of polymers (schematic) [10,30]...

More extensive data are available for the hydrodynamic radii of dendrimers than for the radii of gyration, because they can be derived from intrinsic viscosity measurements according to... 

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