Gradient copolymers
Encyclopedia
Copolymers are polymer
s that are synthesized with more than one kind of repeat unit (or monomer
). It exhibits a gradual change in monomer composition from predominantly one species to predominantly the other, unlike with block copolymers, which have an abrupt change in composition, and random copolymers, which have no continuous change in composition (see Figure 1).
In the gradient copolymer, as a result of the gradual compositional change along the length of the polymer chain less intrachain and interchain repulsion are observed.
The development of controlled radical polymerization as a synthetic methodology in the 1990s allowed for increased study of the concepts and properties of gradient copolymers because the synthesis of this group of novel polymers was now straightforward.
Due to the similar properties of gradient copolymers to that of block copolymers, they have been considered as a cost effective alternative in applications for other preexisting copolymers.
is described by molar fraction of monomer 1 in the copolymer 
and degree of polymerization 
and its relationship is as follows:
The above equation supposes all of the local monomer composition is continuous. To make up for this assumption, another equation of ensemble average
is used:
The
refers ensemble average
of the local chain composition,
refers degree of polymerization, 
refers number of polymer chains in the sample and 
refers composition of polymer chain i at position 
.
This second equation identifies the average composition over all present polymer chains at a given position,
.
techniques including anionic addition polymerization
and ring-opening polymerization
have been used to synthesize gradient copolymers.
The gradient can be formed through either a spontaneous or a forced gradient. Spontaneous gradient polymerization is due to a difference in reactivity of the monomers. The resulting change in composition throughout the polymerization creates an inconsistent gradient along the polymer. Forced gradient polymerization involves varying the comonomer composition of the feed being throughout the reaction time. Because the rate of addition of the second monomer influences the polymerization and therefore properties of the formed polymer, continuous information about the polymer composition is vital. The online compositional information is often gathered through automatic continuous online monitoring of polymerization reactions
, a process which provides in situ information allowing for constant composition adjustment to achieve the desired gradient composition.
The composition can be determined by gel permeation chromatography
(GPC) and nuclear magnetic resonance
(NMR). Generally the composition has a narrow polydispersity index
(PDI) and the molecular weight increases with time as the polymer forms.
For the compatiabilization of immiscible blends, the gradient copolymer can be used by improving mechanical and optical properties of immiscible polymers and decreasing its dispersed phase to droplet size. The compatibilization has been tested by reduction in interfacial tension and steric hindrance against coalescence. This application is not available for block and graft copolymer because of its very low critical micelle concentration
(cmc). However, the gradient copolymer, which has higher cmc and exhibits a broader interfacial coverage, can be applied to effective blend compatibilizers.
A small amount of gradient copolymer (i.e.styrene/4-hydroxystyrene) is added to a polymer blend (i.e. polystyrene/polycaprolactone) during melt processing. The resulting interfacial copolymer helps to stabilize the dispersed phase due to the hydrogen-bonding effects of hydroxylstyrene with the polycaprolactone ester group.
The gradient copolymer have very broad glass transition temperature (Tg) in comparison with other copolymers, at least four times bigger than that of a random copolymer. This broad glass transition is one of the important features for vibration and acoustic damping applications. The broad Tg gives wide range of mechanical properties of material. The glass transition breadth can be adjusted by selection of monomers with different degrees of reactivity in their controlled radical polymerization (CRP). The strongly segregated styrene/4-hydroxystyrene (S/HS) gradient copolymer is used to study damping properties due to its unusual broad glass transition breadth.
There are many possible applications for gradient copolymer like pressure sensitive adhesives, wetting agent, coating, or dispersion. However, these applications are not proved about its practical performance and stability as gradient copolymers.
http://www.cmu.edu/maty/materials/index.html
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
s that are synthesized with more than one kind of repeat unit (or monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
). It exhibits a gradual change in monomer composition from predominantly one species to predominantly the other, unlike with block copolymers, which have an abrupt change in composition, and random copolymers, which have no continuous change in composition (see Figure 1).
In the gradient copolymer, as a result of the gradual compositional change along the length of the polymer chain less intrachain and interchain repulsion are observed.
The development of controlled radical polymerization as a synthetic methodology in the 1990s allowed for increased study of the concepts and properties of gradient copolymers because the synthesis of this group of novel polymers was now straightforward.
Due to the similar properties of gradient copolymers to that of block copolymers, they have been considered as a cost effective alternative in applications for other preexisting copolymers.
Polymer Composition
In the gradient copolymer, there is a continuous change in monomer composition along the polymer chain (see Figure 2). This change in composition can be depicted in a mathematical expression.The local composition gradient fraction is described by molar fraction of monomer 1 in the copolymer and degree of polymerization and its relationship is as follows:The above equation supposes all of the local monomer composition is continuous. To make up for this assumption, another equation of ensemble average
Ensemble average
In statistical mechanics, the ensemble average is defined as the mean of a quantity that is a function of the micro-state of a system , according to the distribution of the system on its micro-states in this ensemble....
is used:
The
refers ensemble averageEnsemble average
In statistical mechanics, the ensemble average is defined as the mean of a quantity that is a function of the micro-state of a system , according to the distribution of the system on its micro-states in this ensemble....
of the local chain composition,
refers degree of polymerization, refers number of polymer chains in the sample and refers composition of polymer chain i at position .This second equation identifies the average composition over all present polymer chains at a given position,
.Synthesis
Prior to the development of controlled radical polymerization (CRP), gradient copolymers (as distinguished from statistical copolymers) were not synthetically possible. While a "gradient" can be achieved through compositional drift due to a difference in reactivity of the two monomers, this drift will not encompass the entire possible compositional range. All of the common CRP methods including atom transfer radical polymerization and Reversible addition−fragmentation chain transfer polymerization as well as other living polymerizationLiving polymerization
In polymer chemistry, living polymerization is a form of addition polymerization where the ability of a growing polymer chain to terminate has been removed. This can be accomplished in a variety of ways. Chain termination and chain transfer reactions are absent and the rate of chain initiation is...
techniques including anionic addition polymerization
Anionic addition polymerization
Anionic addition polymerization is a form of chain-growth polymerization or addition polymerization that involves the polymerization of vinyl monomers with strong electronegative groups. This polymerization is carried out through a carbanion active species. Like all addition polymerizations, it...
and ring-opening polymerization
Ring-opening polymerization
In polymer chemistry, ring-opening polymerization is a form of chain-growth polymerization, in which the terminal end of a polymer acts as a reactive center, where further cyclic monomers join to form a larger polymer chain through ionic propagation...
have been used to synthesize gradient copolymers.
The gradient can be formed through either a spontaneous or a forced gradient. Spontaneous gradient polymerization is due to a difference in reactivity of the monomers. The resulting change in composition throughout the polymerization creates an inconsistent gradient along the polymer. Forced gradient polymerization involves varying the comonomer composition of the feed being throughout the reaction time. Because the rate of addition of the second monomer influences the polymerization and therefore properties of the formed polymer, continuous information about the polymer composition is vital. The online compositional information is often gathered through automatic continuous online monitoring of polymerization reactions
Automatic Continuous Online Monitoring of Polymerization Reactions
Automatic continuous online monitoring of polymerization reactions was developed at Tulane University by Prof. Reed in 1998, with the first publication appearing in 1998. Polymerization reactions are difficult to monitor in real-time, so characteristics and properties of the polymers aren't...
, a process which provides in situ information allowing for constant composition adjustment to achieve the desired gradient composition.
Properties
The wide range of composition possible in a gradient polymer due to the variety of monomers incorporated and the change of the composition results in a large variety of properties. In general, the glass transition temperature (Tg) is broad in comparison with the homopolymers. Micelles of the gradient copolymer can form when the gradient copolymer concentration is too high in a block copolymer solution. As the micelles form, the micelle diameter actually shrinks creating a "reel in" effect. The general structure of these copolymers in solution is not yet well established.The composition can be determined by gel permeation chromatography
Gel Permeation Chromatography
Gel permeation chromatography is a type of size exclusion chromatography , that separates analytes on the basis of size. The technique is often used for the analysis of polymers. As a technique, SEC was first developed in 1955 by Lathe and Ruthven. The term gel permeation chromatography can be...
(GPC) and nuclear magnetic resonance
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
(NMR). Generally the composition has a narrow polydispersity index
Polydispersity index
In physical and organic chemistry, the polydispersity index , is a measure of the distribution of molecular mass in a given polymer sample. The PDI calculated is the weight average molecular weight divided by the number average molecular weight. It indicates the distribution of individual...
(PDI) and the molecular weight increases with time as the polymer forms.
Applications
- Compatibilizing Phase-Separated Polymer Blends
For the compatiabilization of immiscible blends, the gradient copolymer can be used by improving mechanical and optical properties of immiscible polymers and decreasing its dispersed phase to droplet size. The compatibilization has been tested by reduction in interfacial tension and steric hindrance against coalescence. This application is not available for block and graft copolymer because of its very low critical micelle concentration
Critical micelle concentration
In colloidal and surface chemistry, the critical micelle concentration is defined as the concentration of surfactants above which micelles form and almost all additional surfactants added to the system go to micelles....
(cmc). However, the gradient copolymer, which has higher cmc and exhibits a broader interfacial coverage, can be applied to effective blend compatibilizers.
A small amount of gradient copolymer (i.e.styrene/4-hydroxystyrene) is added to a polymer blend (i.e. polystyrene/polycaprolactone) during melt processing. The resulting interfacial copolymer helps to stabilize the dispersed phase due to the hydrogen-bonding effects of hydroxylstyrene with the polycaprolactone ester group.
- Impact Modifiers and Sound or Vibration Dampers
The gradient copolymer have very broad glass transition temperature (Tg) in comparison with other copolymers, at least four times bigger than that of a random copolymer. This broad glass transition is one of the important features for vibration and acoustic damping applications. The broad Tg gives wide range of mechanical properties of material. The glass transition breadth can be adjusted by selection of monomers with different degrees of reactivity in their controlled radical polymerization (CRP). The strongly segregated styrene/4-hydroxystyrene (S/HS) gradient copolymer is used to study damping properties due to its unusual broad glass transition breadth.
- Potential applications
There are many possible applications for gradient copolymer like pressure sensitive adhesives, wetting agent, coating, or dispersion. However, these applications are not proved about its practical performance and stability as gradient copolymers.
Resources
http://torkelson.northwestern.edu/Research/GCP/gcp.htmlhttp://www.cmu.edu/maty/materials/index.html