# extrapolation of the orthorhombic n-paraffin melting properties to very long chain lengths

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Extrapolation of the Orthorhombic nParaffin Melting Properties to Very LongChain LengthsMartin G. Broadhurst Citation: The Journal of Chemical Physics 36, 2578 (1962); doi: 10.1063/1.1732337 View online: http://dx.doi.org/10.1063/1.1732337 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/36/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Calculation of Physicochemical Properties for Short- and Medium-Chain Chlorinated Paraffins J. Phys. Chem. Ref. Data 42, 023103 (2013); 10.1063/1.4802693 Disorder in paraffin chains of urea adducts and nparaffins J. Chem. Phys. 92, 6867 (1990); 10.1063/1.458274 Theory of phase transitions in crystals of longchain molecules and the application to nparaffins J. Chem. Phys. 72, 353 (1980); 10.1063/1.438856 Infrared Spectra of Polyethylene and Long Chain nParaffins J. Chem. Phys. 24, 1115 (1956); 10.1063/1.1742709 Premelting Anomalies of Some Long Chain Normal Paraffin Hydrocarbons J. Chem. Phys. 10, 686 (1942); 10.1063/1.1723645

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THE JOURNAL OF CHEMICAL PHYSICS VOLUME 36, NUMBER 10 MAY 15,1962

Extrapolation of the Orthorhombic n-Paraffin Melting Properties to Very Long Chain Lengths

MARTIN G. BROADHURST

National Bureau of Standards, Washington, D. C.

(Received December 26, 1961)

An analysis of the equation T M = To (n+a)/(n+b) is made in order to determine the ~onveq~ence te:n-perature To of the melting temperatures of the n-paraffins from C44

0

to ClOD. A g~od fit IS obtamed u~mg To=414.3K, a= -1.5, and b=5.0. This value of To (414.3K= 141.1 C) has an estImated total uncertamty of 2.4K and is proposed as the correct value of the equilibrium melting t.emperature of polyethylene. It is shown that presently available data do not permit accurate extrapolatIOn of the n-paraffin heats and entropies of fusion to the polymer limit.

1. INTRODUCTION

CORRELATIONS of the behavior and properties of polyethylene with those of the n-paraffins are common in current literature and it is generally agreed that the change in physical properties accompanying an increase in chain length from moderate to very long chains is reasonably smooth and free of serious anom-alies. This conclusion is a natural consequence of the view that a -CH2- unit experiences the effects of its immediate surroundings only and hence its contribution to the properties of a -CH2- chain molecule is reasonably insensitive to the length of the chain. Chain ends small non-CH2 groups in the chain, and chain folds (in the case of polyethylene lamellas) might well be handled as perturbations on the basic structure and behavior patterns established by the -CH2-units.

This paper reports part of a study that is aimed at a better understanding of the basic nature of the -CH2-chain. After reviewing some previous work on melting point equations we shall establish within narrow limits the validity of the equation TM=To(n+a)/(n+b), where T M and n are the melting temperatures and corresponding numbers of carbon atoms in the paraffins, and To, a, and b are constants. This relationship is fitted to the relevant n-paraffin data and the results discussed. In particular, To (commonly called the convergence temperature) is determined for the orthorhombic n-paraffins and this value is proposed as the equilibrium melting temperature of polyethylene, which also has an orthorhombic structure. The heats and entropies of fusion for the n-paraffins, and the difficulties in extending these quantities to the polymer limit are discussed. Mathematical considerations of the melting point equation are included in the Appendixes at the end of the paper.

II. MELTING TEMPERATURES

have been proposed. For most of these equations there exists a value To defined as

It is this constant which represents the equilibrium melting point of polyethylene, i.e., the melting point of a large crystal (of long -CH2- chains) that contains an equilibrium number of defects.

By far the most popular and successful of the proposed melting-point equations1- s can be put in the following form:

TM= To(n+a)/(n+b), (1)

where a and b are constants. This relationship was first derived by Garner, Van Bibber, and King l on the basis of the proposed linearity of the heats and entropies of fusion of four even numbered paraffins. Grey2 arrived at (1) by way of a melting theory which equated the vibrational energy of the chains to the lattice energy at TM Meyer and Van der Wyk3 found empirically that a form of Eq. (1) provided a good fit of the melting points of several paraffins. StaJlberg et al.4

found that the melting points of their C82 and ClOO were best predicted by the empirical equation of Etessam and Sawyer.s The equation of these latter authors [derived from the apparently ad hoc assumption that dM/dT= (aM+b)2, where M=14n+2 is the molecular weight and T is the corresponding melting temperature] has not only the form of Eq. (1) but nearly the same values for the constants as those found in this current study. None of the above references gives a very convincing derivation of Eq. (1) and hence its justification is one of the necessary concerns of this paper.

I W. E. Garner, K. Van Bibber, and A. M. King, J. Chern. Soc. 1931, 1533.

2 C. G. Gray, J. lnst. Petrol. 29, 226 (1943). 3 K. H. Meyer and A. Van der Wyk, Helv. Chim. Acta 20,

Many relationships TM= TM(n) between the melting 1313 (1937). temperatures (TM ) of the n-paraffins and the corre- 4 G. StiilIberg, S. StiilIberg-Stenhagen, and E. Stenhagen, Acta.

Chern. Scand. 6, 313 (1952). sponding numbers (n) of carbon atoms in the molecule 5 A. H. Etessam and M. F. Sawyer, J. lnst. Petrol. 25, 253 (or equivalently the molecular length or weight) (1939).

2578

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n-PARAFFIN MELTING PROPERTIES 2579

There are in fact several important aspects to the problem of calculating a significant value for To, which must be considered. (1) The validity of the functional form of Eq. (1) must be established not only over the range of chain lengths covered by experimental data but also up to the limit of infinite chain length. (2) The n-paraffin melting points used in fitting the function TM(n) and correctly evaluating the constants therein must themselves be accurate temperatures of equilibrium between the liquid and crystal phases. (3) These liquid and crystal phases must be the ones that persist up to the very long chain limit. That is, the extrapolated melting points of one phase would obviously not predict the melting points of some different phase.

The proposed validity of Eq. (1) is based on the following argument: In Appendix I, a general thermo-dynamic treatment based on the linearity of the enthalpy and entropy with number of carbons in any given n-paraffin phase at any given temperature, leads to

T M = To(n+a)/\ (n+b)-k[(nal+i3l) In(To/TM ) p q

+ L1naT/(r-1) ](TMT-L Tor-I) + LU3T/(r-1)] r=2 r=2

The term in boldface brackets converges to a constant as n becomes large for any values of p, q, and the a's and i3's. Hence if Eq. (1) is satisfactory at short chain lengths, it becomes even a better approximation at longer chain lengths. It will be shown below that Eq. (1) does fit the appropriate experimental data within the limits of experimental error.

Before selecting a series of data with which to fit Eq. (1), one must be certain that the solid and liquid structures of the chosen paraffins are the same structures which exist in very long -CH2- chains. Polyethylene is known to crystallize with an orthorhombic subcell which is apparently stable at all temperatures at which the material is crystalline.6 7 Thus it seems reasonable that the large, long chain length, ideal crystal would similarly melt from the same ortho-rhombic structure, and we are thus limited to con-sidering only those n-paraffins which also melt from the orthorhombic crystalline phase. Whereas the paraffins below C44 melt from a hexagonal or triclinic phase, it is known that above C44 the orthorhombic subcell structure (i.e., either the orthorhombic or monoclinic phase) is stable at and below the melting point.8

The question of the similarity of the liquid structures of the paraffins and very long -CH2- chain liquids is

6 C. W. Bunn, Trans. Faraday Soc. 35, 482 (1945). 7 E. A. Cole and D. R. Holmes, J. Polymer Sci. 46,147 (1960). 8 M. G. Broadhurst, J. Research Nat!. Bur. Stand

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