# Polymer field theory

Polymer field theory

A polymer field theory within the framework of statistical mechanics is a statistical field theory, describing the statistical behavior of a neutral or charged polymer system within the field-theoretic approach.It can be derived by transforming the partition function from itsstandard many-dimensional integral representation over the particle degrees of freedom in a functional integral representation over an auxiliary field function, using either the route via the so-called Hubbard-Stratonovich transformation or the delta-functional transformation. Computer simulations based on polymer field theories have already been shown to deliver useful results in many cases, like e.g. to calculate the structures and properties of polymer solutions (Baeurle 2007, Schmid 1998), polymer melts (Schmid 1998, Matsen 2002, Fredrickson 2002) as well as thermoplastics (Baeurle 2006). The basic concept of the field-theoretic approach on the example of the standard continuum model of flexible polymers and related calculation techniques will be presented in the following.

Canonical ensemble

Particle representation of the canonical partition function

The standard continuum model of flexible polymers, introduced by Edwards(Edwards 1965), treats a solution composed of $n$ linear monodisperse homopolymers as a system of coarse-grained polymers, in which the statistical mechanics of the chains is described by the continuous Gaussian thread model (Baeurle 2007) and the solvent is taken into account implicitly. The Gaussian thread model can be considered as the continuum limit of the discrete Gaussian chain model, in which the polymers are described as continuous, linearly elastic filaments. The canonical partition function of such a system, kept at an inverse temperature and confined in a volume $V$, can be expressed as: where is the potential of mean force given by,:representing the solvent-mediated non-bonded interactions among thesegments, while $Phi_0 \left[mathbf\left\{r\right\}\right]$ represents the harmonic binding energy of the chains. The latter energy contribution can be formulated as:$Phi_0 \left[mathbf\left\{r\right\}\right] = frac\left\{3 k_B T\right\}\left\{2 N b^2\right\} sum_\left\{l=1\right\}^n int_0^1 dsleft| frac\left\{d mathbf\left\{r\right\}_\left\{l\right\} \left(s\right)\right\}\left\{d s\right\} ight|^2,$where $b$ is the statistical segment length and $N$ the polymerization index.

Field-theoretic transformation

To derive the basic field-theoretic representation of the canonicalpartition function, one introduces in the following the segment density operatorof the polymer system:$hat\left\{ ho\right\} \left(mathbf\left\{r\right\}\right) = N sum_\left\{j=1\right\}^n int_0^1 ds delta left\left( mathbf\left\{r\right\} - mathbf\left\{r\right\}_j \left(s\right) ight\right).$Using this definition, one can rewrite Eq. (2) as:Next, one converts the model into a field theory by making use of the
Hubbard-Stratonovich transformation or delta-functional transformation:$int D ho ; delta left \left[ ho - hat\left\{ ho\right\} ight\right] Fleft \left[ ho ight\right] = F left \left[ hat\left\{ ho\right\} ight\right] , qquad \left(4\right)$where $F left \left[ hat\left\{ ho\right\} ight\right]$ is a functional and$delta left \left[ ho - hat\left\{ ho\right\} ight\right]$ is the delta functional given by:$delta left \left[ ho - hat\left\{ ho\right\} ight\right] = int D we^\left\{i int d mathbf\left\{r\right\} w \left(mathbf\left\{r\right\}\right) left \left[ ho \left(mathbf\left\{r\right\}\right) - hat\left\{ ho\right\} \left(mathbf\left\{r\right\}\right) ight\right] \right\}, qquad \left(5\right)$with $w \left(mathbf\left\{r\right\}\right) = sum olimits_\left\{mathbf\left\{G w \left(mathbf\left\{G\right\}\right) exp left \left[ i mathbf\left\{G\right\} mathbf\left\{r\right\} ight\right]$ representing the auxiliary field function. Here we note that, expanding the field function in a Fourier series, implies that periodic boundary conditions are applied in all directions and that the $mathbf\left\{G\right\}$-vectors designate the reciprocal lattice vectors of the supercell.

Basic field-theoretic representation of canonical partition function

Using the Eqs. (3), (4) and (5), we can recast the canonical partition function in Eq. (1) in field-theoretic representation, which leads to:where:can be interpreted as the partition function for an ideal gas ofnon-interacting polymers and:is the path integral of a free polymer in a zero field with elasticenergy:$U_0 \left[mathbf\left\{R\right\}\right] = frac\left\{k_B T\right\}\left\{4 R_\left\{g0\right\}^2\right\} int_0^1 dsleft| frac\left\{d mathbf\left\{R\right\} \left(s\right)\right\}\left\{d s\right\} ight|^2.$In the latter equation the unperturbed radius of gyration of a chain $R_\left\{g0\right\}=sqrt\left\{N b^2/\left(2 d\right)\right\}$, where the space dimension $d=3$. Moreover, in Eq. (6) the partition function of a single polymer, subjected to the field $w \left(mathbf\left\{R\right\}\right)$, is given by:

Grand canonical ensemble

Basic field-theoretic representation of grand canonical partition function

To derive the grand canonical partition function, we use its standardthermodynamic relation to the canonical partition function, given by:where $mu$ is the chemical potential and is given by Eq. (6). Performing the sum, this provides the field-theoretic representation of the grand canonical partition function,:where:is the grand canonical action with $Q \left[ i w \right]$ defined byEq. (8) and the constant:Moreover, the parameter related to the chemical potential is given by:where $Z\text{'}$ is provided by Eq. (7).

Mean field approximation

A standard approximation strategy for polymer field theories is the
mean field (MF) approximation, which consists in replacing the many-bodyinteraction term in the action by a term where all bodies of the systeminteract with an average effective field. This approach reduces anymulti-body problem into an effective one-body problem by assuming that thepartition function integral of the model is dominated by a single fieldconfiguration. A major benefit of solving problems with the MF approximation,or its numerical implementation commonly referred to as the self-consistentfield theory (SCFT), is that it often provides some useful insights into theproperties and behavior of complex many-body systems at relatively lowcomputational cost. Successful applications of this approximation strategycan be found for various systems of polymers and complex fluids, like e.g.strongly segregated block copolymers of high molecular weight, highlyconcentrated neutral polymer solutions or highly concentrated block
polyelectrolyte (PE) solutions (Schmid 1998, Matsen 2002, Fredrickson 2002). There are, however, a multitude of cases for which SCFT provides inaccurate or even qualitatively incorrect results (Baeurle 2006a). These comprise neutral polymer or polyelectrolyte solutions in dilute and semidilute concentration regimes, block copolymers near their order-disorder transition, polymer blends near their phase transitions, etc. In such situations the partition function integral defining the field-theoretic model is not entirely dominated by a single MF configuration and field configurations far from it can make important contributions, which require the use of more sophisticated calculation techniques beyond the MF level of approximation.

Higher-order corrections

One possibility to face the problem is to calculate higher-order corrections to the MF approximation. Tsonchev et al. developed such a strategy including leading (one-loop) order fluctuation corrections, which allowed to gain new insights into the physics of confined PE solutions (Tsonchev 1999). However, in situations where the MF approximation is bad many computationally demanding higher-order corrections to the integral are necessary to get the desired accuracy.

Renormalization techniques

An alternative theoretical tool to cope with strong fluctuations problems occurring in field theories has been provided in the late 1940s by the concept of renormalization, which has originally been devised to calculate functional integrals arising in quantum field theories (QFT's). In QFT's a standard approximation strategy is to expand the functional integrals in a power series in the coupling constant using perturbation theory. Unfortunately, generally most of the expansion terms turn out to be infinite, rendering such calculations impracticable (Shirkov 2001). A way to remove the infinities from QFT's is to make use of the concept of renormalization (Baeurle 2007). It mainly consists in replacing the bare values of the coupling parameters, like e.g. electric charges or masses, by renormalized coupling parametersand requiring that the physical quantities do not change under thistransformation, thereby leading to finite terms in the perturbationexpansion. A simple physical picture of the procedure of renormalizationcan be drawn from the example of a classical electrical charge, $Q$,inserted into a polarizable medium, such as in an electrolyte solution. At a distance $r$ from the charge due to polarization of the medium, its Coulomb field willeffectively depend on a function $Q \left(r\right)$, i.e. the effective (renormalized) charge, instead of the bare electrical charge, $Q$. At the beginning of the 1970s, K.G. Wilson further pioneered the power ofrenormalization concepts by developing the formalism of renormalization group (RG) theory, to investigate critical phenomena of statistical systems (Wilson 1971).

Renormalization group theory

The RG theory makes use of a series of RG transformations, each of which consists of a coarse-graining step followed by a change of scale (Wilson 1974). In case of statistical-mechanical problems the steps are implemented by successively eliminating and rescaling the degrees of freedom in the partition sum or integral that defines the model under consideration. De Gennes used this strategy to establish an analogy between the behavior of the zero-component classical vector model of ferromagnetism near the
phase transition and a self-avoiding random walk of a polymer chain of infinite length on a lattice, to calculate the polymer excluded volume exponents (de Gennes 1972). Adapting this concept to field-theoretic functional integrals, implies to study in a systematic way how a field theory model changes while eliminating and rescaling a certain number of degrees of freedom from the partition function integral (Wilson 1974).

Hartree renormalization

An alternative approach is known as the "Hartree approximation" or "self-consistent one-loop approximation" (Amit 1984). It takes advantage of Gaussian fluctuation corrections to the $0^\left\{th\right\}$-order MF contribution, to renormalize the model parameters and extract in a self-consistent way the dominant length scale of the concentration fluctuations in critical concentration regimes.

In a more recent work Efimov and Nogovitsin showed that an alternative renormalization technique originating from QFT, based on the concept of "tadpole renormalization", can be a very effective approach forcomputing functional integrals arising in statistical mechanics ofclassical many-particle systems (Efimov 1996). They demonstratedthat the main contributions to classical partition function integrals areprovided by low-order tadpole-type Feynman diagrams, which account fordivergent contributions due to particle self-interaction. The renormalizationprocedure performed in this approach effects on the self-interactioncontribution of a charge (like e.g. an electron or an ion), resulting from thestatic polarization induced in the vacuum due to the presence of that charge(Baeurle 2007). As evidenced by Efimov and Ganbold in an earlier work(Efimov 1991), the procedure of tadpole renormalization can be employed very effectively to remove the divergences from the action of the basic field-theoretic representation of the partition function and leadsleads to an alternative functional integral representation, called theGaussian equivalent representation (GER). They showed that the procedureprovides functional integrals with significantly ameliorated convergenceproperties for analytical perturbation calculations. In subsequent works Baeurle et al. developed effective low-cost approximation methods based on the tadpole renormalization procedure, which have shown to deliver useful results for prototypical polymer and PE solutions (Baeurle 2006a, Baeurle 2006b, Baeurle 2007a).

Numerical simulation

Another possibility is to use Monte Carlo (MC) algorithms and to sample the full partition function integral in field-theoretic formulation. The resulting procedure isthen called a polymer field-theoretic simulation. In a recent work, however, Baeurle demonstrated that MC sampling in conjunction with the basic field-theoretic representation is impracticable due to the so-called numerical sign problem (Baeurle 2002). The difficulty is related to the complex and oscillatory nature of the resulting distribution function, which causes a bad statistical convergence of the ensemble averages of the desired thermodynamic and structural quantities. In such cases special analytical and numerical techniques are necessary to accelerate the statistical convergence (Baeurle 2003, Baeurle 2003a, Baeurle 2004).

Mean field representation

To make the methodology amenable for computation, Baeurle proposed to shift the contour of integration of the partition function integral through thehomogeneous MF solution using Cauchy's integral theorem, providing its so-called "mean-field representation". This strategy was previously successfully employed by Baer et al. in field-theoretic electronic structure calculations (Baer 1998). Baeurle could demonstrate that this technique provides a significant acceleration of the statistical convergence of the ensemble averages in the MC sampling procedure (Baeurle 2002, Baeurle 2002a).

Gaussian equivalent representation

In subsequent works Baeurle et al. (Baeurle 2002, Baeurle 2002a, Baeurle 2003, Baeurle 2003a, Baeurle 2004) applied the concept of tadpole renormalization, leading to the "Gaussian equivalent representation"of the partition function integral, in conjunction with advanced MC techniques in the grand canonical ensemble. They could convincingly demonstrate that this strategy provides a further boost in the statistical convergence of the desired ensemble averages (Baeurle 2002).

References

* cite journal |
url = http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TXW-4NXHCCV-1&_user=616165&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000032338&_version=1&_urlVersion=0&_userid=616165&md5=c46d37a250d912279b63702d2dd8826e
last = Baeurle
first = S.A.
coauthors = Nogovitsin, E.A.
title = Challenging scaling laws of flexible polyelectrolyte solutions with effective renormalization concepts
journal = Polymer
volume = 48
pages = 4883
year = 2007

* cite journal |
url = http://www.iop.org/EJ/article/0953-8984/10/37/002/c837r1.pdf?request-id=dc51bfac-c186-4d9f-9c07-1e0214fb6c1c
last = Schmid
first = F.
title = Self-consistent-field theories for complex fluids
journal = J. Phys.: Condens. Matter
volume = 10
pages = 8105
year = 1998

* cite journal |
url = http://www.iop.org/EJ/article/0953-8984/14/2/201/c202r1.pdf?request-id=a6852406-ba65-4af9-b4f4-b1c473f32913
last = Matsen
first = M.W.
title = The standard Gaussian model for block copolymer melts
journal = J. Phys.: Condens. Matter
volume = 14
pages = R21
year = 2002

* cite journal |
url = http://www.mrl.ucsb.edu/~ghf/ghfgroup/pubs/pub156.pdf
last = Fredrickson
first = G.H.
coauthors = Ganesan, V.; Drolet, F.
title = Field-Theoretic Computer Simulation Methods for Polymers and Complex Fluids
journal = Macromolecules
volume = 35
pages = 16
year = 2002

* cite journal |
url = http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TXW-4M9H3TS-1&_user=616165&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000032338&_version=1&_urlVersion=0&_userid=616165&md5=dd2810c8b689c9762244517c80cab949
last = Baeurle
first = S.A.
coauthors = Usami, T.; Gusev, A.A.
title = A new multiscale modeling approach for the prediction of mechanical properties of polymer-based nanomaterials
journal = Polymer
volume = 47
pages = 8604
year = 2006

* cite journal |
url = http://www.iop.org/EJ/abstract/0370-1328/85/4/301/
last = Edwards
first = S.F.
coauthors =
title = The statistical mechanics of polymers with excluded volume
journal = Proc. Phys. Soc.
volume = 85
pages = 613
year = 1965

* cite journal |
url = http://www.iop.org/EJ/article/0295-5075/75/3/378/epl9473.html
last = Baeurle
first = S.A.
coauthors = Efimov, G.V.; Nogovitsin, E.A.
title = Calculating field theories beyond the mean-field level
journal = Europhys. Lett.
volume = 75
pages = 378
year = 2006a

* cite journal |
url = http://prola.aps.org/abstract/PRE/v60/i4/p4257_1
last = Tsonchev
first = S.
coauthors = Coalson, R.D.; Duncan, A.
title = Statistical mechanics of charged polymers in electrolyte solutions: A lattice field theory approach
journal = Phys. Rev. E
volume = 60
pages = 4257
year = 1999

* cite journal |
url = http://cerncourier.com/cws/article/cern/28487
last = Shirkov
first = D.V.
coauthors =
title = Fifty years of the renormalization group
journal = CERN Courier
volume = 41
pages = 14
year = 2001

* cite journal |
url = http://prola.aps.org/abstract/PRB/v4/i9/p3184_1
last = Wilson
first = K.G.
coauthors =
title = Renormalization Group and Critical Phenomena. II. Phase-Space Cell Analysis of Critical Behavior
journal = Phys. Rev. B
volume = 4
pages = 3184
year = 1971

* cite journal |
url = http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TVP-46SPHW2-C1&_user=616165&_coverDate=08%2F31%2F1974&_alid=801769368&_rdoc=1&_fmt=high&_orig=search&_cdi=5540&_sort=d&_docanchor=&view=c&_ct=1&_acct=C000032338&_version=1&_urlVersion=0&_userid=616165&md5=72f6431d18699534d964f5a297c47f3f
last = Wilson
first = K.G.
coauthors = Kogut J.
title = The renormalization group and the ε expansion
journal = Phys. Rep. C
volume = 12
pages = 75
year = 1974

* cite journal |
url = http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TVM-46RW3FS-G5&_user=616165&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000032338&_version=1&_urlVersion=0&_userid=616165&md5=d86970beb2e29d4b7c0253950fb6571b
last = de Gennes
first = P.G.
coauthors =
title = Exponents for the excluded volume problem as derived by the Wilson method
journal = Phys. Lett.
volume = 38 A
pages = 339
year = 1972

* cite journal |
last = Amit
first = D.J.
coauthors =
title = Field theory, the renormalization group, and critical phenomena
journal = Singapore, World Scientific
year = 1984

* cite journal |
url = http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TVG-3VSFGYD-14&_user=616165&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_version=1&_urlVersion=0&_userid=616165&md5=b9ed135b46486bd4336d9963a9bb3792
last = Efimov
first = G.V.
coauthors = Nogovitsin, E.A.
title = The partition functions of classical systems in the Gaussian equivalent representation of functional integrals
journal = Physica A
volume = 234
pages = 506
year = 1996

* cite journal |
url = http://www3.interscience.wiley.com/journal/112454630/abstract?CRETRY=1&SRETRY=0
last = Efimov
first = G.V.
coauthors = Ganbold, G.
title = Functional Integrals in the Strong Coupling Regime and the Polaron Self-Energy
journal = Phys. Stat. Sol.
volume = 168
pages = 165
year = 1991

* cite journal |
url = http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JCPSA6000124000022224110000001&idtype=cvips&gifs=yes
last = Baeurle
first = S.A.
coauthors = Efimov, G.V.; Nogovitsin, E.A.
title = On a new self-consistent-field theory for the canonical ensemble
journal = J. Chem. Phys.
volume = 124
pages = 224110
year = 2006b

* cite journal |
url = http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLEEE8000075000001011804000001&idtype=cvips&gifs=yes
last = Baeurle
first = S.A.
coauthors = Charlot, M.; Nogovitsin E.A.
title = Grand canonical investigations of prototypical polyelectrolyte models beyond the mean field level of approximation
journal = Phys. Rev. E
volume = 75
pages = 011804
year = 2007a

* cite journal |
url = http://prola.aps.org/abstract/PRL/v89/i8/e080602
last = Baeurle
first = S.A.
title = Method of Gaussian Equivalent Representation: A New Technique for Reducing the Sign Problem of Functional Integral Methods
journal = Phys. Rev. Lett.
volume = 89
pages = 080602
year = 2002

* cite journal |
url = http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WHY-47MKK8M-2&_user=616165&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000032338&_version=1&_urlVersion=0&_userid=616165&md5=60e4b8712970cf7e07a14b05c5f0e609
last = Baeurle
first = S.A.
title = Computation within the auxiliary field approach
journal = J. Comput. Phys.
volume = 184
pages = 540
year = 2003

* cite journal |
url = http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TJ5-48XCNNY-3&_user=616165&_coverDate=08%2F01%2F2003&_alid=745709877&_rdoc=6&_fmt=high&_orig=search&_cdi=5301&_sort=d&_docanchor=&view=c&_ct=8&_acct=C000032338&_version=1&_urlVersion=0&_userid=616165&md5=c5a0e9b03b74325c0dc7bf7d88406ecf
last = Baeurle
first = S.A.
title = The stationary phase auxiliary field Monte Carlo method: a new strategy for reducing the sign problem of auxiliary field methodologies
journal = Comput. Phys. Commun.
volume = 154
pages = 111
year = 2003a

* cite journal |
last = Baeurle
first = S.A.
title = Grand canonical auxiliary field Monte Carlo: a new technique for simulating open systems at high density
journal = Comput. Phys. Commun.
volume = 157
pages = 201
year = 2004

* cite journal |
url = http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JCPSA6000109000015006219000001&idtype=cvips&gifs=yes
last = Baer
first = R.
coauthors = Head-Gordon, M.; Neuhauser, D.
title = Shifted-contour auxiliary field Monte Carlo for ab initio electronic structure: Straddling the sign problem
journal = J. Chem. Phys.
volume = 109
pages = 6219
year = 1998

* cite journal |
url = http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JCPSA6000117000007003027000001&idtype=cvips&gifs=yes
last = Baeurle
first = S.A.
coauthors = Martonak, R.; Parrinello, M.
title = A field-theoretical approach to simulation in the classical canonical and grand canonical ensemble
journal = J. Chem. Phys.
volume = 117
pages = 3027
year = 2002a

* [http://www-dick.chemie.uni-regensburg.de/group/stephan_baeurle/index.html Particle and Polymer Field Theories]

Wikimedia Foundation. 2010.

### См. также в других словарях:

• Field theory (physics) — There are two types of field theory in physics:*Classical field theory, the theory and dynamics of classical fields. *Quantum field theory, the theory of quantum mechanical fields.ee also*Field (physics)External links* [http://www dick.chemie.uni …   Wikipedia

• Statistical field theory — A statistical field theory is any model in statistical mechanics where the degrees of freedom comprise a field or fields. In other words, the microstates of the system are expressed through field configurations. It is closely related to quantum… …   Wikipedia

• Partition function (quantum field theory) — In quantum field theory, we have a generating functional, Z [J] of correlation functions and this value, called the partition function is usually expressed by something like the following functional integral::Z [J] = int mathcal{D}phi e^{i(S… …   Wikipedia

• Field-theoretic simulation — A field theoretic simulation is a numerical strategy to calculate structure and physical properties of a many particle system within the framework of a statistical field theory, like e.g. a polymer field theory. A convenient possibility is to use …   Wikipedia

• Field (physics) — The magnitude of an electric field surrounding two equally charged (repelling) particles. Brighter areas have a greater magnitude. The direction of the field is not visible …   Wikipedia

• Polymer science — or macromolecular science is the subfield of materials science concerned with polymers, primarily synthetic polymers such as plastics. The field of polymer science includes researchers in multiple disciplines including chemistry, physics, and… …   Wikipedia

• Polymer — Appearance of real linear polymer chains as recorded using an atomic force microscope on surface under liquid medium. Chain contour length for this polymer is 204 nm; thickness is 0.4 nm.[1] A polymer is a large molecule (macromolecule …   Wikipedia

• Auxiliary field Monte Carlo — is a method that allows the calculation, by use of Monte Carlo techniques, of averages of operators in many body quantum mechanical (Blankenbecler 1981, Ceperley 1977) or classical problems (Baeurle 2004, Baeurle 2003, Baeurle 2002a).The… …   Wikipedia

• Flory-Huggins solution theory — is a mathematical model of the thermodynamics of polymer solutions which takes account of the great dissimilarity in molecular sizes in adapting the usual expression for the entropy of mixing. The result is an equation for the Gibbs free energy… …   Wikipedia

• Scheutjens-Fleer theory — is a lattice based self consistent field theory that is the basis for many computational analyses of polymer adsorption. References * Polymers at Interfaces by G.J. Fleer, M.A. Cohen Stuart, J.M.H.M. Scheutjens, T. Cosgrove, B. Vincent. ISBN… …   Wikipedia

### Поделиться ссылкой на выделенное

##### Прямая ссылка:
Нажмите правой клавишей мыши и выберите «Копировать ссылку»