Schluesseltech_39 (1)

The mean square displacement in the random grows proportionally with the time, where D is the diffusion coefficient. Thus, if we replace the time t by the chain length N and the diffusion coefficient by the elementary step length we can identify the trace of a diffusing particle with a polymer conformation.

Fig. 15:(a) Trace of a random walk. (b) Jean Baptiste Perrin (Nobel Prize 1926).

In the early part of the last century Jean Perrin performed very detailed investigations of the diffusional motion of colloidal particles under a microscope. Figure 15 displays the trace which was such observed. In describing these traces Jean Perrin noted “such images in which a large number of steps are shown in one step only give a weak imagination of the extraordinary unsteadiness of the true trajectory. If we would note the positions at a hundred time shorter scale then each segment would need to be replaced by polygon which would be as intricate as the original picture and so on”. With that notion Perrin addressed the phenomena of self similarity that has been emphasized much later on in connection with the investigation and description of fractal structures. Self similarity means that a small part looks as the hole. A simple mathematical example for such a fractal structure is the Sierpinski gasket which is shown in Figure 16. The construction rule may be easily read off the figure. Each part of this Sierpinski gasket looks as a whole and will go on infinitely like that.

In terms of fractals a very important quantity is the fractal dimension. It describes for volume fractals how the mass of an object is increasing with its size. For three dimensional compact object we have m(r) ≈ rd = 3. If we consider a three dimensional polymer chain then m ≈ N, RE N1/2 or m(r) ~ r2 .Thus, the fractal dimension of an ideal polymer chain is 2. This can be generalized to the law

where df is the fractal or Hausdorff dimension. We note that for polymers in solution the fractal dimension is even smaller and we have M(r) ~ r1.6.

As an example we will now equate the fractal Hausdorff dimension of the Sierpinski gasket. Looking on the construction of this gasket, if we increase the radius by 2 then the mass increases by 3. Thus we have r1 = 2r2 and m1 = 3m2. Now the scaling law for the mass is

The Hausdorff dimension of the Sierpinski gasket is 1.58.

Fig. 16: Sierpinski gasket.

3.3 Specific chain models

We now turn to specific chain models. First we will consider a chain with fixed bond angles but free rotation around the bond angles. Under these circumstances C∞ may be calculated easily. We have to evaluate the projection angles θij between monomers that are i-j apart. For adjacent monomers θ1 is the tetraeder angle θ = 180 - 109 = 71. If we now consider free rotation then ri ri+1 = A20 cosθ and ri ri+2 = A2 cos2 θ or ri ri+k = A2 cosk θ . With Eq.[11b] we get

k=1

=NA2 1+ 2∑cosk θ − 2 ∑k cosk θ

N

For N → ∞ the last contribution in the expression 16 goes to zero and the final result for the end to end distance becomes

Its low value of 2 compares unfavourably with values between 6 and 8 found for real chains. Thus, there must be much more hindrance in the conformation of chains as expressed in the freely rotating chain. A next degree of complication would be the consideration of finite torsional angles plus the rotational potential. For this approach C∞ ≈ 3.4 is found which is still by at least a factor of 2 too small.

A real solution to the problem was provided by Flory in his rotational isomeric state model (RIS model). The solution is the consideration of further non-bonded interactions, the so called Pentane effect. Pentane is built from 5 carbons atoms along the chain and we consider a conformation, where we have a positive and negative rotation about the 2 central C-C bonds thus a g+g-conformation. In this case the final CH3 groups of the molecule come very close. The steric repulsion between these nonbonded carbon atoms will prevent such conformations. A detailed calculation needs to include such nonbonded sterical interactions and lead to the right results. In detail the rotational isomeric state model makes the following assumptions.

All bond length and angles are fixed, torsional angles have the values of the minima for the rotational potential at t, g+, g-; fluctuations are not considered.

The chain conformation is described as a sequence of states tg-tttg+tg+ttg- …

Each bond provides three states.

The whole chain then can be described by 3N-2 rotational states.

In the RIS model the different states are not equally probable but their probability depends on the energies involved.

This RIS model proves to be very successful in the calculation of chain dimensions. As an example for n-pentane where we have four bonds we would have to consider 32 = 9 states.

Finally we like to present another extreme case namely the worm like chain that represents a very stiff polymer like DNA. Such a stiff chain may be considered as a continuously flexible isotropic rod. Its characteristic quantity is the contour length L and the direction of the tangent

vector t(s) at any given point s (0, L). The tangent vector is obtained by the derivative of the spatial coordinate r(s) with respect so s. For the end to end distance we then obtain

For the orientation correlation function between the tangent vectors at different positions along the chain we have

where P is the persistence length of the chain. P is the length at which the correlation between the tangent vectors has decayed to 1/e. The mean square end to end distance can be calculated from Eq.[18] to

For the example of DNA P = 50nm. Using Eq.[12b] we can then also calculate the Kuhn length which comes out to b = 2P = 100nm.

Finally we like to introduce the notion of the radius of gyration that also characterises the size of a coil but in a more general fashion that it can also be used for any branched polymer. The radius of gyration is defined as

where Rcm is a centre of mass coordinate of the chain. For an ideal chain Eq.[21] reduces to

3.4 Distribution functions

Now we ask the question how the intra chain distances rij2 are distributed. For that purpose

we define the probability density function to find a monomer at a distance r . For a statistical coil this is probability density P(r) is isotropic. 4πr2 P(r) is then the probability to find a monomer between r and r + dr. Following the central limit theorem for a random walk the distribution function is Gaussian

r2 is the average distance between the two considered monomers. P(r) is a very good approximation. However, P(r ≠ 0) for r > Rmax = AN. This deficiency is minute. Consider the example of a long chain N = 104 then P(Rmax) = 10-26.

Such a Gaussian distribution is of very general importance. It gives the distribution of statistical errors, it describes the distribution of distances in random walks and it also describes the solution of the diffusion equation. The distribution is directionally independent and can be written as a product from the contributions along the three Cartesian coordinates.

Each end to end distance vector RE may be realised by a number of conformations of a chain with N monomers. The probability distribution function P3d may be written in terms of these conformation numbers Ω as follows.

This notion relates to the conformational entropy of the chain that is given by the logarithm of the number of conformations of a chain with N monomers and RE.

S(RE , N ) = kBAn (Ω(N , RE ))

= kBAn P(N , RE ) + kBAn (∫ΩdRE )

If we now neglect the monomermonomer interaction that in a polymer melt is well screened, the internal energy U(N,RE ) is independent of RE. Then the Helmholtz free energy becomes

The free energy shows a quadratic increase with the absolute value of the end to end distance. Thus, we have derived an entropic elasticity. If we stretch or compress a chain, then in response an entropic force acts to restore the ideal conformation.

fx is the force to hold a chain at a fixed radius R = Rx. This force increases with temperature and the spring constant is given by the prefactor in Eq.[28].

3.5 Correlation function and scattering

The pair correlation function that describes the probability to find a monomer at a given distance from another monomer is a very important quantity that describes the spatial cloud of monomers. It is also directly measured in any scattering experiment. Let n be the number of

monomers within a radius r. From Eq.[11b] follows n ≈ (r/A)2. The number density within the

same volume r3 is proportional to n/r3. Then the probability to find a monomer in a volume r is given by

the exact solution for an ideal chain gives g (r ) = πr1A2 . Eq.[29] may be generalised for fractal objects where n ≈ rd or

Eqs. [29] and [30] tell us that large polymer coils are nearly empty.

Now we consider the scattering form a chain performing a random coil conformation. The scattering cross section is evaluated by summing up the phase factors of all monomers at positions exp[iQrj] where the wave vector Q = ki - kf in the momentum transfer during scattering (ki and kf are the wave vectors of the incoming and scattered radiation). For elastic scattering Q = 4π/λ sin θ where λ is the wave length of the radiation and θ the scattering angle. The sum of the phase factors gives the scattering amplitude. The measured intensity reflects the square of this scattering amplitude

where bi, bj are the scattering lengths of the monomers. Assuming bi = bj Eq.[31] reduces to

Here Φ is a polymer volume fraction Vw the polymer volume and Kp the scattering contrast factor. We now define the form factor

where the pair correlation function g(r) describes the probability to find a second monomer at distance r if there is one at r = 0. For a Gaussian chain Eq.[33] may be evaluated further. In such a chain, as we have seen above, all the distances are Gaussian distributed. Under these circumstances we can take the ensemble average into the exponent and have

The last part of this equation follows from the isotropy of the problem. Taking random walk statistics, we have rij2 = A2 i − j . With that the form factor becomes

Now we have to evaluate the sum. We do this in two steps first we take the parts where i = j and then consider the relations i < j. With that the sum of Eq.[35] becomes

with z = Q2 Rg2 . Figure 17a displays the Debye function as a function of QRg. Figure 17b displays the same function in form of a Kratky representation that is achieved if D(z) is

multiplied by Q2. The Kratky representation emphasises the high Q regime. For Gaussian chains with an asymptotic Q-2 behaviour the high Q regime then assumes a plateau.

Small and high Q regimes can also be explicitly obtained from an expansion of Eq.[38]. For

Thus, in the low Q regime we obtain information on the chain volume VW and the radius of gyration Rg. For high Q Eq.[38] gives

From Eq.[40] the radius of gyration may also be obtained. However, since data at high Q, where the background plays an important role, are involved, these data are sensitive to background corrections.

Fig. 17:Graphical representation of the Debye function. (a) Linear plot. (b) Kratky representation D(Q) Rg Q 2.

Fig. 18: Polymer form factors measured on polyethylene propylene in the melt at two different temperatures (A. Zirkel et al., Macromolecules 25, 954 (1992).

We now turn to an experimental example. Using small angle neutron scattering, the chain form factor of polyethylenepropylene in the melt with a molecular weight of 8.3 104g/mol was studied. Figure 18 displays experimental form factors for different polymer concentrations and two different temperatures. A Zimm analysis of these data is displayed in Figure 19. The extrapolation towards Q = 0 displays a horizontal line. There is basically no interaction between the labelled chains – the second virial coefficient A2 ≈ 0. The extrapolation to φ = 0 gives the radius of gyration for each temperature.

Fig. 19: Zimm representation of the SANS data revealing the second virial coefficient, the radius of gyration and the polymer volume.

Figure 20 displays the result for the temperature dependence of Rg and we realise that with increasing temperature Rg shrinks. At T ≈ 25°C Rg = 106Å is found, while at 275°C Rg shrinks to about 95Å. Using Eq.[11b] in connection with Eq.[22] C∞ = 6.2 is found at room temperature.

Fig. 20: Temperature dependence of the radius of gyration for polyethylenepropylene.

4 Real chains: Flory theory

So far we have neglected any long range monomer-monomer interaction. We will now consider the effect of this long range interaction in terms of a simple model, the Flory approach. For further reading we refer for example to De Gennes’ book.

We begin with an estimation of the number of monomer-monomer contacts, in 3-dimensional space. Thereby we approximate the N monomers of a chain by an ideal gas. We take a mean field approximation for the contact probability that is proportional to the overlap volume

fraction. With a monomer volume of A3, the number density φ* amounts to

large N, the number density within the volume of one chain is very small. With the approximation of an ideal gas the total number of contacts within one chain is then Nφ* = N1-1/2 = N½. For long chains the total number of contacts is much larger than one and therefore the contact energy will effect the chain conformation. We note that the net effect is important. That means the energetic difference between the monomer-monomer interaction and the interaction energy E with other molecules, for example, the solvent molecules if we consider chains and solution.

In a mean field approximation Flory considered the balance between the effective repulsion between the different monomers in a chain and the entropy loss that occurs due to swelling. We assume:

a uniform distribution of monomers in R3.

the probability of a second monomer to be within the excluded volume υ = A3 of a given

monomer is then the product of the excluded volume υ and the number density of monomers in the pervaded volume of the chain N/R3.