Strictly twodimensional selfavoiding walks: Thermodynamic properties revisited
 N. Schulmann,
 H. Xu,
 H. Meyer,
 P. Polińska,
 J. Baschnagel,
 J. P. Wittmer
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The density crossover scaling of various thermodynamic properties of solutions and melts of selfavoiding and highly flexible polymer chains without chain intersections confined to strictly two dimensions is investigated by means of molecular dynamics and Monte Carlo simulations of a standard coarsegrained beadspring model. In the semidilute regime we confirm over an order of magnitude of the monomer density ρ the expected power law scaling for the interaction energy between different chains e _{ int } ∼ ρ ^{21/8}, the total pressure P ∼ ρ ^{3} and the dimensionless compressibility gT = lim_{ q→0} S(q) ∼ 1/ρ ^{2}. Various elastic contributions associated to the affine and nonaffine response to an infinitesimal strain are analyzed as functions of density and sampling time. We show how the size ξ(ρ) of the semidilute blob may be determined experimentally from the total monomer structure factor S(q) characterizing the compressibility of the solution at a given wave vector q . We comment briefly on finite persistence length effects.
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 Being truncated and shifted at the minimum of the full LJ potential our excludedvolume potential is continuous and differentiable everywhere. As shown in ref. paptrunc, this is of relevance for calculations of elastic moduli using a stress fluctuation relation, such as eq. (eq_KRowlinson), which involves derivatives of the interaction potentials.
 This clearly separates the bonded and nonbonded interactions which is of importance for the various thermodynamic contributions investigated in sect. sec_res. Note that some implementations of the KG model, as the recent version of the LAMMPS code, allow to view the LJ interactions between bonded monomers as intrachain contributions.
 The bond potential being harmonic, various conformational and thermodynamic properties can easily be calculated if the nonbonded potential is thought to be switched off or known to be irrelevant. Under this assumption the equipartition theorem [37] tells us, e.g., that the average bonding energy e _{b} per bond should be k _{B} T/2. As a consequence the relative deviation from the reference distance l _{b} = 0.967 is given by \(\left\langle {\left( {r/l_b  1} \right)^2 } \right\rangle ^{1/2} = \sqrt {{{k_B T} \mathord{\left/ {\vphantom {{k_B T} {k_b l_b^2 }}} \right. \kern0em} {k_b l_b^2 }}} \approx 0.0398\) . This gives an excellent approximation for the data in the dilute and semidilute regimes where 〈r〉 ≈ 0.9692 and l ≡ 〈r ^{2}〉 ≈ 0.9700 (table 1).
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 For noninteracting phantom chains we have k _{B} T for the pressure contribution per bond as one confirms by integration by parts of \(\smallint drru'_b (r)e^{  \beta u_b (r)}\) . Summing over all bonds we thus have P _{b} β/ρ = −(N − 1)/N = −1 + 1/N and, hence, P = P _{id} + P _{b} = k _{B} Tρ/N.
 Plotting P _{int} β/ρ as a function of chain length N reveals the same power law exponents −νθ = −19/16 and 3/8 for the dilute and dense limits as seen in fig. 5 for interchain interaction energy e _{int}(N).
 Since the nonbonded interactions get more important at higher densities, these numerical problems become irrelevant for ρ ≥ 0.25. The data points given in fig. 6 and the main panel of fig. 7 all refer to the best δtindependent thermodynamic relevant values available.
 This scaling has been directly tested by tracing N Pβ/ρ as a function of x = ρ/ρ ^{*} ∼ ρN ^{1/2>}. This plot is not presented since the related dilutesemidilute crossover scaling for the compressibility is given in the inset of fig. 8.
 We have additionally checked that similar values are obtained from the volume fluctuations δV in an isobaric ensemble with imposed pressure P using K = k _{B} T〈V〉/〈δ ^{2} V〉 [37]. While we find again that this method is straight forward for polymer melts (ρ > 0.5), K(t) is seen to converge increasingly slowly with decreasing density to the asymptotic longtime plateau —just as the compression moduli computed using the stress fluctuation formula, eq. (16), for the canonical ensemble presented in fig. 11.
 The presented numerical results suggest to express quite generally the difference η _{B} − η _{F}of the different potential contributions in terms of the “distinct stress fluctuation correlation” \(\eta _{F,dist} \equiv \frac{\beta } {{d^2 V}}\sum\limits_{l \ne l'} {\left\langle {w(r_l )w(r_{l'} )} \right\rangle .}\) (This can be readily done by integration by parts.) Unfortunately, this expression is quadratic with respect to the total particle number and the direct computation of η _{F,dist} is, hence, not a practical route either.
 A similar deviation of the RPA formula in the crossover regime at q ≈ 2π/ξ has also been seen for threedimensional bulks [51].
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 Title
 Strictly twodimensional selfavoiding walks: Thermodynamic properties revisited
 Journal

The European Physical Journal E
35:93
 Online Date
 September 2012
 DOI
 10.1140/epje/i201212093x
 Print ISSN
 12928941
 Online ISSN
 1292895X
 Publisher
 SpringerVerlag
 Additional Links
 Topics
 Keywords

 Soft Matter: Polymers and Polyelectrolytes
 Industry Sectors
 Authors

 N. Schulmann ^{(1)}
 H. Xu ^{(2)}
 H. Meyer ^{(1)}
 P. Polińska ^{(1)}
 J. Baschnagel ^{(1)}
 J. P. Wittmer ^{(1)}
 Author Affiliations

 1. Institut Charles Sadron, Université de Strasbourg & CNRS, 23 rue du Loess, BP 84047, 67034, Strasbourg Cedex 2, France
 2. LCPA2MC, Institut Jean Barriol, Université de Lorraine & CNRS, 1 bd Arago, 57078, Metz Cedex 03, France