CONFORMATIONS OF MACROMOLECULES AT THE POLYMER-SOLID INTERFACE

The conformational characteristics of macromolecules at the polymer-solid in-terface are one of the main factors determining not only the structure and prop-erties of the surface layer, but the structure and propprop-erties of a filled polymer as a whole. Theoretical concepts, which can explain chain conformations in the sur-face layers, are based on adsorption theories. At the same time, it is evident that conclusions of the adsorption theory have more theoretical meaning because the real conditions of production of filled polymers are very far from those in which adsorption layers are formed. The main concept describing the conformational properties at the interface is as follows. The surface of a solid serves as a protect-ing barrier that does not allow macromolecules to take the same number of con-formations as in the bulk. Limitations of molecular mobility in the surface layers due to conformational restrictions are connected with an entropy factor,

i.e., with the depletion of the conformational set of macromolecules near the in-terface. This permits a satisfactory explanation of a poor dependence of the ef-fects of a chemical nature and the rigidity of chains on surface mobility. In fact, the conformational set of macromolecules, already limited in its initial state, cannot vary as strongly near the interface as in flexible polymer. As a result, the effects for rigid-chain polymers will be less pronounced.

The main statements of the theory of polymer adsorption may be used for the analysis of conformations of macromolecules in filled polymers.⁸ For this case, a very simplified model of a filled polymer was used, according to which the density of a polymer between filler particles does not change. It was assumed that conformations of chains do not change, due to the interaction with a solid, and that the conformational states depend only on the entropy effects. For such a system, some general conclusions, based on the theoretical calculations, can be reached concerning the conformations in a filled system. It was established that in the border layer of a thicknessδ ≈2Rg(Rgis the radius of inertion of gaussian coil), the coils are flattened and have anisotropy, which is determined by the ex-istence of an interface. The number of contacts of chains with a flat surface of a filler depends on the distance from the surface. The average length of the chain segments bound to the surface does not depend on the number of links in the chain and is of the order of several links, whereas more than half of the links are situated at a distance from the surface. The thickness of the border layer is com-parable with the chain dimensions in the bulk (or inΘ-solvent) and growth pro-portional to M^0.5, being for flexible chains of the order of hundreds of Å.

According to the developed approach, the main contribution to the change of the structure of the layer is made by the conformational restrictions. Such a conclusion has already been reached earlier on the basis of experimental data^2,7 and is of interest as one of the first attempts to describe the effect from the theo-retical point of view.

To describe the microscopic structure and thermodynamic properties of a polymer near the interface with a solid lattice, a theory was developed⁹based on the theory of solutions. However, as distinct from a solution, the system ana-lyzed consists of a great number of chains. It is assumed that in the surface layer, various orientations of chain links are realized, which deviates from the isotropic one only in a very narrow region near the interface (approximately 6

lattice lines or 25 Å). In such conditions, the shape of polymeric chains near the interface becomes flat, whereas at a definite distance the chain has its unper-turbed dimension. In properties of the polymer near the interface, the detail of molecular structure of a polymer plays a critical role in determining the confor-mations. Effects connected with entropy loss due to the reflecting barrier and those determined by the energy of interaction at the interface should be taken into account, and especially effects connected with the difference in the interac-tions between segments of various chains and between segments and the sur-face.

Theoretical analysis in the framework of the mean field theory⁹allows the conclusion to be drawn that the idea, according to which the density in the sur-face layer sharply changes, is over simplified. The density profile is sensitive to the chemical nature as much as the structural peculiarities near the interface.

Immediately at the interface, the structure is determined by the interaction of segments with the surface atoms, all effects being strongly dependent on chain flexibility. Thus the theoretical calculations based on the general principles of conformational statistics of polymers meet the experimental data very well (which we believe the author forgot to mention) concerning the structure and properties of surface layers, which will be discussed below.

However, the problem of chain conformations in the filled polymers may be analyzed more simply, based on the principles of thermodynamics, without any simplifying assumptions about the structure of the surface layer. The approach is the following.¹⁰The heat of dissolutions of filled and unfilled polymers may be easily determined. The heat of the interaction of filled polymer with the solvent,

∆H₁₃, may be determined from the relationship:

∆H = H (1s ∆ 13 − ϕ) + H∆ 23ϕ [3.1]

where∆H₂₃is the heat of wetting of a filler with a solvent,ϕis the volume frac-tion of a filler, and∆H_sis the integral heat of interaction. Value∆H₁₃consists of two terms:

∆H = H + H₁₃ ∆ _q ∆ _p [3.2]

where∆H_q= -∆C_p(Tg- 303) is excess in relation to the equilibrium melt enthalpy of glassy polymer,∆C_pis the difference of heat capacities of a polymer in a melt and a glassy state,

∆H = H + H + Hp ∆ r ∆ ν ∆ c [3.3]

is the heat of interaction of an equilibrium melt with a solvent,∆H_ris the heat of mixing for a regular solution,∆H_vis contribution of the volume change by mix-ing,

∆H = [4∆ ( + 1)] (1 )

(1+ )

c 2 2

2 2

23

εσ σ α

α σ

 −

 

 [3.4]

is the configurational contribution which is determined by the expansion or compression of the coil during its transition from the state in a bulk into solu-tion,α is the expansion coefficient, given by h^{2 1 2/} / hθ^{2 1 2/} ,σ = h_θ^{2 1 2/} / h_o^{2 1 2/} is the parameter of thermodynamic flexibility, h_θ^{2 1 2/} , h^{2 1 2/} , and h_o^{2 1 2/} are mean-square distances between the chain-ends in ideal, non-ideal solvent, and for the model of the freely-joined chain,∆εis the difference in energy between two rotational isomers.

Using these equations, polystyrene, PS, filled with fumed silica was stud-ied. It was found that calculated values∆H < 0_p , and they decrease with increas-ing amount of filler until they reach some constant value,∆H′_p. It means that the heat of interaction between polymer and a filler,∆H = H₁₂ ∆ _p −∆H′_p found experimentally, reaches its limiting value, which corresponds to the saturation of interactions at the interface due to realization of a maximum number of con-tacts of chain segments with active centers, N*, of the surface. The number of contacts may be calculated as:

N *= H S H *

∆ 12

∆ [3.5]

where S = S₀ϕis the surface available for interaction, S₀is the specific surface of the filler,∆H* is the energy of a single contact. Calculations have shown that the

fraction of PS segments interacting with the surface is about 15-20% of the total number of segments. Therefore, the majority of segments is present in the sur-face layer as loops. It is easy to establish that the thickness of the sursur-face layer (i.e., the height of the loops),δ ≈ R_g , is of an order of radius inertion of an unper-turbed chain. The value may be considered for a filled system as a half-thickness of the interlayer between two filler particles (at saturation),δ = L / 2. Calcula-tion ofα shows that macromolecules in the surface layer have more extended conformation, compared with the bulk, which may be the result of realization of the maximum possible number of polymer-filler contacts. This effect should de-pend on the chain flexibility, which determines the possibility of changing con-formations in the surface layer. Such a simple experimental approach allows one to make some qualitative conclusions concerning the conformation of chains at the interface.

More detailed information may be gained from application of the method of the attenuated total inner reflection in IR-region (ATR). It is known that for at-tenuated total inner reflection, the depth of penetration of radiation,∆P, into the sample depends on the indices of refraction of element and sample, n1and n2, and on the wavelength of radiation,λ, and angle of incidence,θ:

dP = becomes possible to investigate the polymer structure at various distances from the surface.

For thin layers of PMMA on the surface of crystal KRS-5, it was established that at changing depths of radiation penetration, the spectra vary in regard to the intensity of some bands. Complex changes in spectra and the intensity of conformation-sensitive bands are observed at the distances of 1.5-2µm from the surface. The analysis of spectral data allows us to understand that in the surface layer a narrowing of conformational set of macromolecules takes place and a more regular arrangement of some chain fragments arises. The solid surface

stabilizes a more stable conformation of the ester group of PMMA, which leads to the densening of a polymer and to the redistribution of conformations in such a way that in the surface layer, the concentration of more stable conformers in-creases. For polymers capable of crystallization, the isomeric composition of macromolecules in the surface layers also changes.^12,13The structure of surface layers of polypropylene was studied using this method.¹⁴It was found that the structure of a layer is heterogeneous. At the surface, macromolecules are in a conformationally-regular state, whereas at some distance from the surface the structure is more irregular. Application of the ATR method has shown¹⁴that the degree of the conformational ordering in amorphous regions near the surface in-creases. The perfection of packing of macromolecules is higher, the closer the layer is to the surface.

The degree of the order of macromolecules in the surface layers depends on the nature of the polymer. For polymers capable of crystallization, the range of changes is broader compared with amorphous polymers, and the degree of per-fection of packing is higher. The latter may be explained by the formation of one-dimensional or two-dimensional isomorphic structures. All peculiarities of the conformational state of macromolecules in the surface layer depend on the conditions of the layer formation. Therefore, by changing these conditions, one can vary the degree of conformational ordering of macromolecules in amorphous regions. The phenomenon of epitaxy is of special interest, i.e., the oriented growth of crystalline structures on the surface of crystalline substrate. The face having a regular structure may induce the orientation. In this case, the sur-face directly influences conformational changes and the ordering. The investigation of the epitaxial crystallization of some polyurethanes^14-18 has shown that changes in the structure of a polymer are observed in layers up to 4 µm and have their essential representation in IR-spectra, due to conformational restrictions. As distinct from the free films of the same polymers, for epitaxial films the half-width of the adsorption bands in IR-spectra depends on the film thickness. It was also established that during epitaxial crystallization in sur-face layers changes take place in the orientation of macromolecules in relation to the plane of a solid, macromolecules being oriented preferentially, perpendic-ular to the surface.