Gradient copolymer

File:1-s2.0-S0009250917307546-fx1 lrg.jpg

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 $g(X)$ is described by molar fraction of monomer 1 in the copolymer $(F\_1)$ and degree of polymerization $(X)$ and its relationship is as follows:

$g(X)=\backslash frac\{dF\_1(X)\}\{dX\}$

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:

$F^\{(loc)\}\_1(X)=\backslash frac\{1\}\{N\}\backslash sum\_\{i=1\}^NF\_\{1,i\}(X)$

The $F^\{(loc)\}\_1(X)$ refers ensemble average of the local chain composition, $X$ refers degree of polymerization, $N$ refers number of polymer chains in the sample and $F\_\{1,i\}(X)$ refers composition of polymer chain i at position $X$.

This second equation identifies the average composition over all present polymer chains at a given position, $X$.

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{{Cite book|last=Davis|first=Kelly|author2=Krzysztof Matyjaszewski|year=2002|title=Statistical, Gradient, Block, and Graft Copolymers by Controlled/Living Radical Polymerizations|pages=1–13| doi= 10.1007/3-540-45806-9_1|series=Advances in Polymer Science|volume=159|isbn=978-3-540-43244-9}} including atom transfer radical polymerization and Reversible addition−fragmentation chain transfer polymerization as well as other living polymerization 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 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. Contrast matching SANS experiments revealed that the outer part of the core of the gradient copolymer micelles has a distinctly higher density than the middle of the core. This finding was attributed to back-folding of chains resulting from hydrophilic-hydrophobic interactions, leading to a new type of micelles that was referred to as micelles with a "bitterball-core" structure.{{Cite journal |last=Filippov |first=Sergey K. |last2=Verbraeken |first2=Bart |last3=Konarev |first3=Petr V. |last4=Svergun |first4=Dmitri I. |last5=Angelov |first5=Borislav |last6=Vishnevetskaya |first6=Natalya S. |last7=Papadakis |first7=Christine M. |last8=Rogers |first8=Sarah |last9=Radulescu |first9=Aurel |last10=Courtin |first10=Tim |last11=Martins |first11=José C. |last12=Starovoytova |first12=Larisa |last13=Hruby |first13=Martin |last14=Stepanek |first14=Petr |last15=Kravchenko |first15=Vitaly S. |date=2017 |title=Block and Gradient Copoly(2-oxazoline) Micelles: Strikingly Different on the Inside |url=https://pubs.acs.org/doi/10.1021/acs.jpclett.7b01588?src=getftr&utm_source=scopus&getft_integrator=scopus& |journal=The Journal of Physical Chemistry Letters |volume=8 |issue=16 |pages=3800–3804 |doi=10.1021/acs.jpclett.7b01588}}

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.

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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.{{cite journal|last=Ramic|first=Anthony J.|author2=Julia C. Stehlin |author3=Steven D. Hudson |author4=Alexander M. Jamieson |author5=Ica Manas-Zloczower |year=2000|title=Influence of Block Copolymer on Droplet Brekup and Coalescence in Model Immiscible Polymer Blends|journal=Macromolecules|volume=33|issue=2|pages=371–374| doi=10.1021/ma990420c|bibcode = 2000MaMol..33..371R }} 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.{{cite journal|last=Kim|first=Jungki|author2=Maisha K. Gray |author3=Hongying Zhou |author4=SonBinh T. Nguyen |author5=John M. Torkelson |date=Feb 22, 2005|title=Polymer Blend Compatibilization by Gradient Copolymer Addition during Melt Processing: Stabilization nof Dispersed Phase to Static Coarsening|journal=Macromolecules|volume=38|issue=4|pages=1037–1040| doi=10.1021/ma047549t|bibcode = 2005MaMol..38.1037K }}

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.{{Cite journal |last1=Muzammil |first1=Iqbal |last2=Li |first2=Yupeng |last3=Lei |first3=Mingkai |year=2017 |title=Tunable wettability and pH-responsiveness of plasma copolymers of acrylic acid and octafluorocyclobutane |journal=Plasma Processes and Polymers |volume=14 |issue=10 |pages=1700053 |doi=10.1002/ppap.201700053|s2cid=104161308 }} Recent studies have evaluated gradient copolymers as potential drug delivery carriers.{{Cite journal |last=Sedlacek |first=Ondrej |last2=Bardoula |first2=Valentin |last3=Vuorimaa-Laukkanen |first3=Elina |last4=Gedda |first4=Lars |last5=Edwards |first5=Katarina |last6=Radulescu |first6=Aurel |last7=Mun |first7=Grigoriy A. |last8=Guo |first8=Yong |last9=Zhou |first9=Junnian |last10=Zhang |first10=Hongbo |last11=Nardello-Rataj |first11=Véronique |last12=Filippov |first12=Sergey K. |last13=Hoogenboom |first13=Richard |date=2022 |title=Influence of Chain Length of Gradient and Block Copoly(2-oxazoline)s on Self-Assembly and Drug Encapsulation |url=https://onlinelibrary.wiley.com/doi/10.1002/smll.202106251 |journal=Small |language=en |volume=18 |issue=17 |pages=2106251 |doi=10.1002/smll.202106251 |issn=1613-6829|url-access=subscription }} Additionally, gradient copolymers have been shown the best ¹⁹F MRI signal-to-noise ratio in comparison with the analogue block copolymer structures, making them most promising as ¹⁹F MRI contrast agents.{{Cite journal |last=Kaberov |first=Leonid I. |last2=Kaberova |first2=Zhansaya |last3=Murmiliuk |first3=Anastasiia |last4=Trousil |first4=Jiří |last5=Sedláček |first5=Ondřej |last6=Konefal |first6=Rafal |last7=Zhigunov |first7=Alexander |last8=Pavlova |first8=Ewa |last9=Vít |first9=Martin |last10=Jirák |first10=Daniel |last11=Hoogenboom |first11=Richard |last12=Filippov |first12=Sergey K. |date=2021 |title=Fluorine-Containing Block and Gradient Copoly(2-oxazoline)s Based on 2-(3,3,3-Trifluoropropyl)-2-oxazoline: A Quest for the Optimal Self-Assembled Structure for 19F Imaging |url=https://pubs.acs.org/doi/pdf/10.1021/acs.biomac.1c00367?utm_source=scopus&getft_integrator=scopus |journal=Biomacromolecules |language=en |volume=22 |issue=7 |pages=2963-2975 |doi=10.1021/acs.biomac.1c00367|url-access=subscription }}

• Mechanically gradient polymers

• http://torkelson.northwestern.edu/Research/GCP/gcp.html
• http://www.cmu.edu/maty/materials/index.html

Category:Copolymers