Diels-Alder Polymers

The DA-polymer is a thermoreversible polymer network formed by a DA-cross-linking reaction, between a furan rings, present on the synthesized four-functional furan compound, with the maleimide rings, present on the bi-functional maleimide compound.


The synthesis allows tuning of the mechanical properties of the DA-polymers by varying the furan spacer length, which is the chain length of the poly(propylene oxide) chain of the Jeffamine used. A series of three DA-polymers, noted DPBM-FGE-J400, -J2000 and -J4000, with diverse mechanical properties was synthesized. 

Properties of Diels-Alder polymers

Classification of DA-polymers

The three DA-materials, -J400, -J2000 and -J4000, can be classified in two groups: the ‘reversible glassy thermosets’ and the ‘reversible elastomeric thermosets’, based on their viscoelastic behavior at ambient temperature (Tambient). 

To explain this classification, the glass transition temperature (Tg) has to be introduced. Tg is the temperature at which an amorphous material undergoes a reversible transition from a hard and relatively brittle glassy state into a molten or rubbery state (upon heating). On the one hand, -J400 contains furan compounds with short polymer chain length. Due to these short chains, the network has a high (reversible) crosslink density, which raises the Tg of the polymer above Tambient, leading to (brittle) glassy thermosets at Tambient. -J2000 and -J4000 on the other hand are built up out of furan compounds with longer polymer chain length, limiting the cross-link density. The latter materials are elastomeric thermosets, with ductile characteristics, and having a Tg lower then Tambient. In the table, the Tg, and the densities of the three DA-polymer materials are presented.

SPAs are constructed from hyperelastic polymers. Therefore, both elastomer DA-materials, -J2000 and -J4000, are combined in the construction of a first SH-soft pneumatic actuator (SH-SPA), exploiting the difference in mechanical properties of these two materials in the mechanical design of the SH-SPA. The brittle, -J400 material cannot be used in SPAs, however, it can be used in other robotic applications.

Recently, we used  this brittle -J400 material to develop a self-healing mechanical fuse (SH-MF), which can be integrated in a series elastic actuator. This SH-MF protects the system from damaging overloads. Upon an overload on the system, potentially damaging one of the actuator components, the fuse fractures sacrificially and will be healed after removal of the overload. Using this principle all components are protected and there is no need for large over-dimensioning. 

Mechanical properties of the DA-series

In order to determine the viscoelastic behavior of the self-healing polymer series at ambient temperature, Dynamic Mechanical Analysis (DMA) was carried out on the three materials. In the table the Storage modulus, Loss modulus and their ratio, the tan(delta) are presented. At 25 °C -J400 is in the glassy region (Table I), where the storage modulus (2280 MPa) is high compared to the loss modulus (111 MPa), which results in a low tan(delta), indicating that there is almost no viscous contribution. -J400 behaves like an elastic solid, almost all the energy required to deform sample is elastically recoverable. Therefore, we will consider -J400 as an elastic material (instead of viscoelastic) at ambient  temperature, with a Young’s modulus equal to the storage modulus. This however is not the case for the other two materials: their viscoelastic behavior makes it impossible to neglect the viscous contribution at 25 °C and attention should be given to this in further applications. To derive the fracture stress and strain, a static stress-strain test until fracture was carried out in tension.

Mixing ability

The studied polymers differ only in spacer length, which makes it possible to mix furan compounds having a different degree of polymerization, during the synthesis of the SHmaterial. Using this mixing method a DA-polymer is obtained with a mixture of spacer lengths. In this way a polymer with desirable, intermediate material properties lying in the broad interval between the two extremes (J400 and J4000) can be obtained. This mixing ability is a great advantage since it provides a certain degree of freedom in the design of future self-healing actuator applications.


Synthese of Diels-Alder polymers


The reactants, the monomers, used in the synthesis of the reversible DA-polymer network system were purchased from Sigma-Aldrich:

  • ˆ(A): 2 Jeamine D-series: Poly(propylene glycol) bis(2-aminopropyl ether) with average degree of polymerization n (determined by nuclear magnetic resonance spectroscopy): 
    • n = 44.2: J2000, Mn = 2640 g/mol
    • n = 71.1: J4000, Mn = 4200 g/mol
  • ˆ(B): furfuryl glycidyl ether (FGE, 96 %)
  • (C): 1,1'-(methylenedi-1,4-phenylene)bismaleimide (DPBM, 95%)

Reagens used for the polymerization of the Diels-Alder network


In this section it is explained how the DA-polymers are synthesized. J4000 and J2000 material is synthesized into sheets while J400 is synthesized into powder. For each step the chemical reaction is explained followed by a description of the practical actions required to preform this step. The following amounts of DA-polymer material was made:

  • DPBM-FGE-J4000: 100x100 mm sheet with a 0.75 mm  thickness.
  • DPBM-FGE-J2000: 100x100 mm sheet with a 0.50 mm  thickness.

Step 1: epoxy-amine reaction

In a first step, FGE was irreversibly bonded to Jx (x = 400, 2000, and 4000) through an epoxy-amine reaction, yielding a furan functionalized compound. This reaction was performed at 60 °C for minimum 7 days after which the reaction was completed at 90 C° (to speed up the reaction) for 2 days.


In practice: Pour the FGE and the Jeffamine together. The following amounts were used: 

Step 1

  • Heat the solution using an oil bath to 60°C  for minimum 7 days. Stir continuously using a magnetic stirrer. 
  • Increase the temperature to 90°C for another 2 days. 

Step 2: Network formation

In a second step, the furan functionalized compound (FGE-Jx) was mixed with DPBM in a stoichiometric ratio of one (r= nMaleimide / nFuran = 1) to obtain the reversible polymer network through DA reaction. Prior to the DA reaction, the DPBM was dissolved in chloroform (a 20 w% solution) to obtain a homogeneous reversible network. 

Network formation

In practice: Pour the FGE and the Jeffamine together. The following amounts were used: 

Step 2


  • Pour the chloroform at the FGE-Jx
  • Add the DPBM to the solution
  • Stir at room temperature for at least 2 hours, using a magnetic stirrer, until a completely homogeneous solution is obtained

Step 1: practical

Step 3

The chloroform has to be evaporated out of the solution and the thermo-reversible network has to be formed. The chloroform is extracted by increasing the temperature to 60°C at vacuum. The thermo-reversible network is formed by slowly cooling down the sheet after the chloroform has been completely extracted.

In practice: The following amounts were used to develop the DPBM-FGE-Jx sheets:

  • DPBM-FGE-J4000: 28.1 ml
  • DPBM-FGE-J2000: 18.8 ml

Step by Step:

  •  1 hour at 30°C and atmospheric pressure
  • gradually increase the temperature up to 660°C
  • gradually decrease the pressure up to vacuum.
  • remain at 60°C and vacuum until all bells are removed
  • remain for 3 days at 50°C and vacuum
  • cool down slowly (5°C/hour) to ambient temperature after which the synthesis is completed