Hyperbranched polymers belong to densely branched macromolecules having a 3D dendritic structure and a large number of end groups. Dendrimers are also included in dendritic polymers which along with hyperbranched polymer molecules are made up of repeating units originating from a central core.
The functionality is given by the core, which is usually three (amine) or four (ethylenediamine), as chemical bonds are formed with the core, and, in this way, it is connected to the rest of the molecule.
These core bonds are attached to the linear chains and each of them is terminated with functional end groups. Perfect attachment of these molecules and units forms a dendrimer but in case, if there occurs any imperfect attachment to the molecules or any unit goes missing, the polymeric hyperbranched structure is formed. This is the reason that for a given amount of monomers, the formation of only one dendrimer is possible as it depends upon the perfect bonding and placement of units with molecules but the formation of different and several hyperbranched polymers can take place depending upon the different ways in which the branched and unbranched monomers are distributed.
This imparts unique physical and chemical properties to hyperbranched polymers and thus finds them diverse applications in different fields and this is the main reason the attention towards hyperbranched is increasing day by day and a lot of research is being done on this class of polymer.
Branches of Polymers
Hyperbranched polymers are considered as the fourth class of polymers which comprise linear, cross-linked, branched and dendritic structures. Polymers are further classified into six subclasses as shown in the picture below:
- Dendron and dendrimers- which have controlled molecular weight and are synthesized by convergent and divergent approaches.
- Linear dendritic hybrids
- Dendri-grafts or dendronized polymers- They are formed by linking of linear polymer with the side dendrons. To obtain these polymers direct polymerization of dendritic macromolecules can be done, or dendrons can be attached to the linear polymeric core.
- Hyperbranched polymers
- Multi-arm star polymers
- Hyper grafts
Among these subclasses, top three show perfect bonding and structures and thus show degree of branching of 1, while the other three show architecture with random branching.
Hyperbranched Polymers History
The history of hyperbranched polymers started back in the 19th century when formation of the resin from A2B2 monomer i.e. tartaric acid and B3 monomer i.e. glycerol was reported by Berzelius at the end of the 19th century after which the reaction of latent A2 monomer i.e. phthalic anhydride or A2 monomer i.e. phthalic acid with B3 monomer i.e. glycerol was reported in 1901 by Watson Smith. Kienel et al. then studied this reaction further and found out that specific viscosity of the samples of phthalic anhydride and glycerol was low when compared with synthetic linear polymers of any other kind.
Phenolic Resins, first synthetic polymer, was commercialized in 1909.
In 1952, Flory reported a theory concluding that highly branched polymers can be synthesized without gelation by polycondensation of an ABn monomer (n ≥2) in which A and B functional groups can react with each other. In 1978 Vögtle and co-workers reported the first synthesis of branched systems. Kim and Webster synthesized the first hyperbranched polymer in the form of soluble polyphenylene in 1988. Since then, hyperbranched polymers have acquired considerable attention from academics and industry due to their unique properties, they are easy to prepare and have greater availability as compared to dendrimers.
Synthesis of Hyperbranched polymers
The repeating units in hyperbranched macromolecule prepared from an AB2-type monomer are initial (I), linear (L), dendritic (D) and terminal (T). In case of AB2-type monomer, when the polymerization has taken place, the hyperbranched polymer has one A group at the initial unit which, by reaction with intramolecular group B via cyclization or by the addition of multifunctional core molecules can be converted into another bond.
The linear repeating unit is the one that has only one unreacted B group, dendritic repeating unit is the one that has two reacted B groups while terminal repeating unit is the one with two unreacted B groups.
Equation for degree of branching (DB) was first given by Fr´echet and coworkers so that the structure of hyperbranched polymers can be described and a correlation of the units in hyperbranched polymer is made possible.
DB = (No. of dendritic units) + (No. of terminal units)/ Total no. of units
DB = D + T/ D + T +L (1.1)
Here, D is the total number of dendritic units,
T is total number of terminal units,
L is the total number of linear units.
In case of the hyperbranched polymers with large molecular weights, the total number of terminal units lies very close to the total number of dendritic units. So the above mentioned equation can be simplified as:
DB=1/ (1+L/2D) (1.2)
L/D or L/T is easily calculated using nuclear magnetic resonance (NMR) spectrum but the units are difficult to find.
Properties of Hyperbranched Polymers
Properties of the hyperbranched polymers depends upon the following
- Degree of branching
- Molecular weights
Degree of Branching
Degree of branching affects polymer properties like:
- Free volume
- Chain entanglement
- Mean square radius of gyration
- Glass transition temperature (Tg)
- Degree of crystallization (DC)
- Encapsulation capability
- Mechanical strength
- Melting/ solution viscosity
- Biocompatibility
- Self-assembly behaviors
On the other hand, DB could be altered or even tuned to some extent. To increase DB, following five methods can be attempted:
- Enhancement of the reactivity of the functional group associated with linear units
- Addition of multifunctional core molecules (Bf ) to the polymerization system of
AB n - Polycondensation of dendrons without linear units
- Post modification of the formed HBPs to convert linear units to dendritic ones
- Using special catalyst
Through these techniques, DB could be obviously higher than 0.5 or even approach 1 in some cases.
It should be noticed that even if the degree of branching is 1, unlike dendrimers, hyperbranched polymers may have many isomers with varying molecular weights. For tuning this degree of branching, four methods can be attempted:
- Copolymerization of AB2 and AB monomers with different feed ratios
- Changing the polymerization conditions such as temperature, feed ratio of monomer to catalyst, and solvent
- Host– guest inclusion of AB2 or multifunctional monomer
- Combination of the above ones
Molecular Weight
Molecular weight is also an important parameter for hyperbranched polymers. Following are the equations 1.3–1.5. [40, 41] for the polymers prepared from ABg-type monomers (g≥1)
Number average polymerization Pn =1/ (1−x) (1.3)
Weight average polymerization Pw= (1−x2/g) (1−x) 2 (1.4)
Polydispersity Index (PDI) PDI=Pw/Pn = (1−x2/g)/ (1−x) (1.5)
Here, x is the conversion of A group. If g=1 or 2, we obtain the corresponding equations of linear polymers prepared by polycondensation of the AB monomer or the HBP prepared from the AB2 monomer. This shows that for linear polymers, PDI increases linearly in case of linear polymers but for hyperbranched polymers PDI exponentially increases with increasing the conversion (x) so, PDI for hyperbranched polymers will be much higher near completion of reaction. During purification, the residual monomers are removed so the actual value of PDI is smaller than the calculated value.
For hyperbranched polymers to be used as plasticizers in order to improve processability of polymers, PDI should be broad. On contrary, PDI could be narrowed by:
- Slow addition of monomers during polymerization
- Polymerization in the presence of core molecules
- Classification of HBPs via precipitation or dialysis.
Hyperbranched polymers have greater intrinsic viscosity than the dendrimers but lower than the linear analog as shown by the Table.
Lower Viscosity Compared to Linear Polymers
Hyperbranched polymers have very different viscosity characteristics when compared to linear polymers due to their branched architecture. Solution of hyperbranched macromolecules reaches maximum intrinsic viscosity with the increase in molecular weight. This is because the shape of macromolecules becomes compact globular structure at high molecular weights.
But the conditions where the intrinsic viscosity of the hyperbranched solution reaches maximum is still not clear. In case of linear polymers, the melt viscosity has a direct relation to a critical molar mass as a result of entanglement of polymer chains with the increase in critical molar mass. This does not hold for dendrimers or hyperbranched polymers because they show minimal entanglement in the branched chains. The interesting characteristics of hyperbranched polymers are due to their conformation and degree of branching. According to the results shown by X-ray and small angle neutron scattering, dendrimers show special conformations whereas hyperbranched polymers have globular structures.
The flexibility of the branching components and the intrinsic viscosity of the polymers show the degree of branching, which reflects that polymers that have lower viscosity show a higher degree of branching and the polymer’s relative solubility in different media is affected by the degree of branching. This clearly indicates why hyperbranched polymers show enhanced solubility and high chemical reactivity as compared to linear polymers. Blends studies have shown that hyperbranched polymers show greater compatibility with other polymers.
Because of the presence of compact highly branched structures, hyperbranched polymers show excellent mechanical properties i.e. tensile strength, initial and compressive moduli.
Hyperbranched polymers have ellipsoid-like 3D architecture having randomly branched structure with DB<1.0, a wide PDI >3.0, less entanglement, low viscosity, high solubility and plenty of functional groups at the linear and terminal units, these undergo one-step polymerization and are more close to conventional polymers with molecular weight (MW) and DB distributions. Whereas dendrimers have globular architecture with perfect branched regular structure with DB=1 and show narrow polydispersity of MW, no entanglement, low viscosity, high solubility, several functional units at terminal units and are controlled via multistep polymerization and are closely related to pure molecules with exact chemical units and bonds and hence exact molar mass.
Hyperbranched Polymers are combined to create aramid blends which overall enhances the functionality of aramid. For Blending, Plasticizers are added in Blend to improve processibility. You can study Aramids Blend formation, which can clear concepts.
Some similarities like low viscosity, high solubility, weak strength, highly reactive functional groups and good capacity of encapsulation for foreign molecules are also present between hyperbranched polymers and dendrimers.
As HBPs can be produced on a large scale using one step polymerization this makes them cost-effective too, they find greater applications and use in industry as compared to dendrimers.
References
Kevlar Aramid Fiber
By Wiley.
A very interesting book for readers interesting in commercially used Aramid Fibers and Bullet proof vests of KEVLAR.
