Study of molecular packing density in boundary layers of some polymers

Study of molecular packing density in boundary layers of some polymers Yuri S. Lipatov, E. G. Moisya and G. M. Semenovich Institute o f Macromolecular Chernistrl/, Academv o f Sciences o f the Ukrainian SSR, 252160, Kiev 160, USSR (Received 9 September 1974; revised 11 November 1974)

Packing density of macromolecules of some amorphous polymers, poly(methyl methacrylate), polystyrene and polydimethylsiloxane, in boundary layers formed at the solid-polymer interface, was studied by a 'molecular probe' technique. It was shown that the boundary layers of the above polymers have a complex structure composed of regions with different densities of molecular packing: a surface region of thickness 2 to 4/am, and a transient and loosely-packed region of thickness of the order of 30 to 60/am. It was established that both the structure and the thickness of boundary layers for each of the polymers depends both on polymer chain flexibility, and on cohesion energy density of a polymer.

INTRODUCTION Current interest in the phenomena occurring at the solidpolymer phase interface is readily explained by their role in the properties of filled polymers. Experimental data accumulated during recent years leads one to conclude that in the vicinity of the polymer-solid interface a boundary layer is formed, the properties of which markedly differ from those of polymer in bulk ~-4. Physico-chemical investigations of the layer have shown that the polymer-solid interaction results in the net decrease of segmental mobility of macromolecules near the interface, and may lead in some cases to the shift of glass transition temperature of a polymeric matrix and/or change of molecular packing density in the latter. However, studies of packing density in filled polymers in different laboratories have given seemingly discordant results. For example, experimental data obtained by methods which permit the estimation of the overall effect of a solid-polymer interface on the properties of a filled polymer, suggest ~ that packing efficiency of macromolecules in filled systems is worse than that in pure samples. This was attributed by some authors ~ to the loosening of molecular packing in boundary layers. On the other hand, investigation of properties of very thin polymeric films on various fillers reported by Nesterov and Lipatovs indicates that boundary layers have a rather complex structure and that the packing density change with distance from the filler surface is not smooth. The purpose of the present paper is to study the packing density of macromolecules near the polymer-solid interface as a function of chain flexibility, cohesion energy density (CED) of a polymer and energetic properties of a filler surface. We have chosen for this study polystyrene (PS), poly(methyl methacrylate) (PMMA) and polydimethylsiloxane (PDMS). As shown in Table 1, the first two polymers have similar values of a flexibility parameter o*, while their CED's are different, and the last two (PS and * O = (r2)o/(rZ)f, where (r2)oand q'2)/'are mean-square end-to-end

dimensions of aft'unperturbed' macroinoleculeand of a model freely rotating chain respectively.

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PDMS) possess approximately equal CED values, but markedly different o a'6. This choice allows us to elucidate the contribution of polymer chain flexibility and its energetic interaction with the substrate to the change of packing density at the solid-polymer interface. EXPERIMENTAL We used in this study the 'molecular probe' (MP) technique 7- lo which is especially suitable for estimation of magnitude of the packing density change in polymeric matrix under various environmental conditions. This information is provided by measurements of the magnitude of the absorption spectral shifts, Av, or luminescence spectra of impurity molecules dissolved in a matrix polymer with respect to the spectra of these molecules in the 'free' state (vapour). Since the magnitude of spectral shifts for 'probe' molecules in non-polar or weakly polar media exhibits quadratic dependence on the density of a medium, the equation for a molecular packing density change in polymer boundary layers with respect to polymer density in the bulk phase ~° is: 2 Pboundary _ AVboundary pb2ulk

(1)

AVbulk

where Pboun"dary and Pbulk are polymer 'apparent' densities in boundary layers and in the bulk, A~ooundary is the difference Vvapour- Vboundary, and APbulk is the difference Uvapour -- Ubulk, Pboundary and Pbulk are band positions for a

Table I Valuesof cohesionenergydensity, CED (cal/¢m3) and flexibility parameter, a

Polymer

CED3

a6

PMMA PS PDMS

132.0 72.8 77.0

2.14 2.22 1.47

Molecular packing density in boundary layers of polymers: Y. S. Lipatov et aL pure electron transition or a position of one band of vibrational electronic sequence in spectra of absorption or luminescence of probe molecules, incorporated into the boundary layers or bulk phase respectively, and Vvapour is the position of the same band in the vapour spectrum of the impurity molecules. Having determined experimentally the values/)boundary and/)bulk and knowing l)vapour , w e may estimate the deviation of packing density in the boundary layers from that in the bulk phase from equation (1). In this work change of packing density of macromolecules in the boundary layers was estimated from the shift of position of(u00 + 2 x 1405) cm -1 band of luminescence spectra of anthracene probe molecules which are in a molecularly dispersed state in studied polymers. Luminescence spectra of anthracene molecules in polymeric matrices were recorded at the liquid nitrogen temperature (77 K) using the spectrometer model D F S - 12. Luminescence was excited with a high pressure mercury lamp model DRS-500 equipped with a glass filter Xmax = 365 run. Since both excitement and recording spectra were effected from the substrate side, the latter had to satisfy the following requirements: (i) the substrates had to have different surface energies (see above); (ii) they should be transparent in the u.v. region. These requirements seemed to be met by the fused silica and Teflon polymer. Prior to use the fused silica slides were annealed at 600°C for 1.5 to 2 hours, while Teflon fdms were carefully washed with the same solvents which were used for casting of polymer samples. The following polymers with a rather narrow molecular weight distribution were used: atactic PS of high_ purity (productofWaters Associates Co., USA) with Mw = 867 000 and Mn = 773 000; atactic PMMA (product of suspension polymerization supplied by Miss N. Averbakh, Dzerzhinsk) with the 'viscosity-average' molecular weight (chloroform, 25°C) 590 000, which was obtained by a step-by-step precipitation fractionation in acetone-water and subsequent solvent evaporation; and commercial sample of PDMS (trademark SKT, supplied by Dr Yu. Godovsky, Moscow) with the viscosity molecular weight 237 000 purified by a double precipitation. It is pertinent to note that sudden quenching of PDMS in liquid nitrogen virtually excludes any possibility of its crystallization, so that it forms a glass below its glass transition temperature 11. This means that all the data obtained in the present work refer to glassy polymers. Films from the above polymers were cast from 'good' solvents (chloroform, dichloroethane and benzene for PMMA, PS and PDMS respectively) on a substrate. All solvents used were carefully purified, dried and fractionally distilled according to standard methods 12. To insure the homogeneous distribution of impurity molecules in the solution, an amount of anthracene corresponding to desired concentration in polymer was added to a solvent before a polymer. The polymer concentration in solution was always 0.2 g/100 ml. Polymer films were formed at room temperature on the substrates and then dried in a vacuum oven to a constant weight. The thickness of the polymer films on the substrates remained constant, while the thickness of the layer from which the information was obtained was varied by changing the concentration of impure probe molecules in the range from 10 -2 to 10 -4 g/g. This method allowed us to follow the packing density change in the polymer matrix at different distances from the substrate, since all other conditions being equal, the higher the concentration of probe molecules in a matrix, the lower the depth of penetration of exciting radiation into the polymer surface layers. Hence, we obtain information on the den-

sity of an environmental phase of impurity molecules which refers only to those polymer layers which are in the immediate vicinity of a solid surface. On the other hand, when the probe molecules concentration is low, the radiation penetrates to greater depth (Lambert-Beer law), and we obtain information on the density of the medium in the more distant layers, the thickness of which is greater, the lower the concentration of impurity molecules in a polymer. Using eight different anthracene concentrations (see Figure 1) we calculated from a knowledge of the absorptive characteristics of anthracene 13, the dependence of an 'apparent' polymer layer thickness on impurity concentration. The experimental error (i.e. resolution of an adjacent weak band on the wing of a principal luminescence band) is about 10%, and therefore our calculations were limited to those thicknesses corresponding to a 90% absorption at a given concentration. Noticeable differences between impurity molecules spectra in surface layers and in the bulk or in the loosely packed regions would imply that the former might be superimposed on the latter. This effect, however, should not be taken into account because transition from higher concentrations (10 -2 g/g) to the lower ones is accompanied by a rather large decrease of relative intensity of probe molecules in the surface layers, and will not exceed about 10% at a concentration of 10 -3 (10% on the band wing will be close to the experimental accuracy limit), while at a concentration of 10 -4 g/g it will drop to only 1% with respect to the overall intensity of anthracene spectra.

RESULTS AND DISCUSSION The observed changes in the position of fluorescence spectra of probe molecules at different concentrations of the latter are shown in Figure 1. The ordinate shows the values of the r a t i o Pboundary/Pbulk(in %) calculated from the experimental values of spectral shifts of impurity molecules and the abscissa log C from which the thickness of the surface layer was calculated. It can be seen that increase of packing density of macromolecules in the immediate vicinity of the solid surface is a common feature for all polymers and substrates used. Since this effect is observed both for high energy, as well as low energy solid surfaces, it seems reasonable to propose that the orientation of macromolecules under the influence of the solid is dominating 14. The corresponding packing density increment for this surface layer with respect to the bulk polymer for the three polymers amounts to 3 to 5%. Comparison of the influence of high energy and low energy substrates on the packing density of macromolecules in the surface layers shows that the difference was most pronounced for PMMA, which has a high CED value (132 cal/cm 3) and rather stiff chain (o = 2.14), somewhat less for PS which has similar chain stiffness but lower CED, whereas for PDMS which possesses the lowest values of o and CED, the nature of the substrate plays a rather minor role. This comparison alone shows that the mode of molecular packing in the polymer boundary layers depends significantly on values of o and CED. Moreover, the whole spectrum of changes of packing density (i.e., quantitative differences between packing densities in boundary layers and in the bulk, thickness of the layer where structural rearrangement due to the solid surface effect is still observed, and differences between effects of a high energy and low energy solids, etc.) was also most noticeable for PMMA with the high CED and o, while it was the least for PDMS with

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Molecular packing density in boundary layers of polymers: Y. S. Lipatov et al. IO4

a

IO2

of the boundary layers on the high energy substrate where the change of molecular packing density compared to that of the bulk phase is still observed, was estimated as 30/am for PS and 60/am for PMMA.

CONCLUSIONS IO(

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The results indicate that the surface layers have a complex structure. The layer nearest to the solid exhibits enhanced packing density of macromolecules as compared with the bulk, and this can be tentatively attributed to the orienting effect of the solid surface. At larger distances from the solid the orientation induced by a substrate surface greatly weakens, especially in the case of low energy solids and/or polymers with high chain flexibility and low cohesion energy density, the effect being more pronounced the lower the flexibility parameter, o. For polymers with stiffer chains and stronger interactions with a substrate, however, such structural changes are far more evident. In this case and for films cast on a high energy substrate the 'transient' layer is followed ,by a 'loosely-packed' region with the packing density lower than that for bulk polymer. This effect is observable in the more remote regions, the higher the CED of the polymer.

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ACKNOWLEDGEMENTS IO0

We wish to express our gratitude to Miss N. Averbakh and Dr Yu. 'Godovsky for the gift of PMMA and PDMS samples used in the present study.

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3'10 4-O -Log C Figure 1 Changeof packing density of macromolecules in boundary layers as function of the distance from the polymer-solid interface. (a) PDMS; (b) P$; (c) PMMA; - - , boundary layers on fused silica; - - - , those on Teflon

REFERENCES 1 2 3 4

minimum values of CED and o, PS exhibiting an intermediate effect. Structural rearrangement in the surface layers of films cast on Teflon was limited to formation of a densified surface layer with thicknesses of 2 to 3/am for PDMS and 3 to 4/am for PS and PMMA. In the more remote regions for the most flexible polymer (PDMS) the orienting effect of both high energy and low energy substrates virtually disappears and the packing density at distances of more than 3/am from the solid corresponds to that for a bulk polymer. In the case of PMMA and PS the structure of the layer formed on the high energy substrate (fused silica) is rather complex: here the regions in the immediate vicinity of interface that show an enhanced packing density are followed presumably by regions where packing density is close to that in the bulk ('transient' layers), after which packing of macromolecules becomes looser than in the bulk ('loosely-packed' regions). The thickness

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5 6 7 8 9 10 11 12 13 14

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