- Gradient copolymers
Copolymers are polymers that are synthesized with more than one kind of repeat unit (or monomer). A gradient copolymer has gradient in repeat units arranged from mostly monomer A to mostly monomer B along much or all of the copolymer chainRef|1Ref|2. (See Figure 1)
Development of controlled radical polymerization synthesis in the 1990’s sparked the interest of many researchers to study the concepts and properties of gradient copolymers because the synthesis of this group of novel polymers is more straightforward ever since.
Method of Synthesis
Gradient copolymers could not be synthesized until the development of controlled radical copolymerization in the 1990’sRef|3. Due to the gradient nature of copolymer, gradient copolymers could not be prepared using “living” free radical polymerization due to the random addition of monomers to radical chain. There are several common ways to synthesize gradient copolymers via several controlled radical polymerization (CRP) techniques such as Atom Transfer Radical Polymerization (ATRP), nitroxide-mediated CRP, and Reversible Addition-Fragmentation Transfer (RAFT) . The CRP techniques are possible since the radical chain is a function of the instantaneous co-monomer composition in the system. The sequence distribution is controlled through the gradual addition of second monomer to a reaction vessel containing first monomer.
Atom Transfer Radical Polymerization is one of the most powerful, versatile, simple and inexpensive methods. ATRP typically uses alkyl halides as initiator and simple transition metals as the catalysts4. The reversible deactivation by atom transfer allows gradient copolymer to be formed. (Refer to Figure 2)
Figure 2: Reversible Deactivation by Atom Transfer
Another method synthesize is by using Reversible Addition-Fragmentation Chain transfer (RAFT) polymerization. In RAFT polymerization where RAFT reagents are added, the termination reaction of polymer radicals is substituted by the reversible deactivation mechanism. Hence, this allows the formation of gradient copolymer. (Refer to Figure 3)
Figure 3: Degenerative Transfer Methods such as RAFT
Gradient copolymers can also be prepared by nitroxide-mediated controlled radical polymerization. This polymerization is controlled via reversible deactivation by coupling . (Refer to Figure 4)
Figure 4: Reversible Deactivation by coupling
Other methods also include living or pseudo-living polymerization methods such as ring-opening metathesis polymerization and cationic polymerization.
Properties of Gradient Copolymers
The wide spectrum of composition within the gradient copolymer chain contributes to the wide temperature range for segmental relaxation that relates to the
glass transition temperature(Tg). By controlling the molecular weight, composition and the shape of the gradient along the polymer backbone, phase transitionsin a gradient copolymer can be manipulated. Changing the gradient along the polymer backbone can alter the volume fraction that resides within the interphase of a phase separating copolymer. As a result, the gradient copolymer will have a very broad segmental relaxation. This will subsequently change the composition continuously across the interphasial boundary and the volume fraction of the material within the boundary layer can be controlled.
Applications of Gradient Copolymers
Some applications of gradient copolymers are listed below:
*Compatibilizing Phase-Separated Polymer Blends A small amount of gradient copolymer (i.e.styrene/4-hydroxystyrene) is added to a polymer blend (i.e. polystyrene/polycaprolactone) during melt processing leads to an interfacial copolymer which stabilizes the dispersed phase. This is due to the hydrogen-bonding effects of hydroxylstyrene with the polycaprolactone ester group.
*Impact Modifiers and Sound or Vibration DampenersThe behavior of gradient copolymers at the interface can be controlled
*Pressure Sensitive AdhesivesThis is because gradient copolymers has the ability to prepare materials with broad and selectively adjustable range of glass transition temperatures (Tg)
*Wetting or Leveling Additives for Coatings or InksSimilar reasoning as pressure sensitive adhesives
#Wong C.L.H; Kim J.; Torkelson, J.M. “Breadth of Glass Transition Temperature in Styrene/Acrylic Acid Block, Random and Gradient Copolymers: Unusual Sequence Distribution Effects”, J. Polym. Sci. Part B: Polym. Phys. 45, 2842 - 2849 (2007).
#Mok, M.M.; Kim J.; Torkelson, J.M. “Gradient Copolymers with Broad Glass Transition Temperature Regions: Design of Purely Interphase Compositions for Damping Applications”, J. Polym. Sci. Part B: Polym. Phys. 46, 48-58 (2008).
#Webpage of Carnegie Mellon, the Matyjaszewski Polymer Group Research Areas: Properties of Well-Defined Novel Copolymers Prepared by CRP, http://www.chem.cmu.edu/groups/maty/about/research/11.html, Dec 5,2007
#Webpage of Carnegie Mellon, the Matyjaszewski Polymer Group Research Areas: Development of Controlled/Living Radical Polymerization, http://www.chem.cmu.edu/groups/maty/about/research/02.html, Dec 5, 2007
#Kim, J.; Zhou, H.; Nguyen, S.T.; Torkelson, J.M; “Synthesis and application of styrene/4-hydroxystyrene Gradient Copolymers Made by Controlled Radical Polymerization: Compatibilization of Immiscible Polymer Blends via Hydrogen-Bonding Effects”, Polymer 47, 5799-5809 (2006).
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