Langmuir 1993,9, 2326-2329
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A Study of Chemisorption of Poly(hydrogen methylsiloxane) Using Neutron Reflectometry and Small-Angle Neutron Scattering T. Cosgrove,* A. Patel, J. A. Semlyen,? J. R. P. Webster,f and A. Zarbakhsh Department of Physical Chemistry, School of Chemistry, Cantock’s Close, Bristol BS8 1TS,U.K. Received December 30,1992. I n Final Form: June 7, 1993 Neutron reflectivity and small-angle neutron scattering have been used to study the chemisorption of poly(hydrogen methylailoxane), on quartz and porous silica. The results indicate an increase in layer thickness and density near the solid interface upon increasing the grafting reaction temperature. The results are compared qualitatively with adsorption data obtained from FTIR experiments on alumina and with theoretical predictions.
Introduction The dynamics of the polymer adsorption process have been the subject of several publications, and a recent review covers much of this information.’ The time evolution of the structure of a polymer layer is of paramount importance in many practical applications. Thisis evident in processes such as the formation of stable colloidal dispersions and the modification of crystal growth. In terms of the time scale of polymer adsorption, several different regimes can be envisaged: (i) the diffusion to the surface, (ii) the adsoiption in some initial conformation, and (iii) slow rearrangement, desorption-adsorption as the system approaches thermodynamic equilibrium. These time scales are dependent on the chemical and physical structure of the system and any imposed mechanical constraints. In most systems the diffusion-limited step is fast and in a typical dispersion may be complete in a few milliseconds. The latter processes, however, may be rather long and can take several hours to complete. Only techniques such as neutron scattering or reflection can give detailed structural information on the adsorbed layer. However, to study the dynamics of the adsorption process with neutrons is rather difficult because relatively long experimental runs are required to obtain a satisfactory signal to noise ratio. Typically these times are -1 h. This problem may be overcome in part by ”freezing” the system such that its structure may be evaluated at different stages during the adsorption process. This can be achieved by studying a system in which a progressive chemical reaction between the adsorbing polymer and substrate occur^.^^^ One example is the chemisorption of poly(hydrogen methylsiloxane) (PHMS)on alumina from tetrachloromethane, which has been studied by FTIR.3 The progress of this reaction waa monitored by the disappearance of the Si-H moiety as it reacted with the surface hydroxyl groups. This chemisorption progressively “locks” the chain conformation and together with the normal physisorption process determines the layer structure and adsorbed t
Present address: Chemistry Department, University of
Heslington, York YO1 5DD, U.K.
York,
t Present address: Neutron Division, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 OQX, U.K. (1)Cohen Stuart, M.; Cosgrove,T.; Vincent, B.Adu. Colloidlnterface
Sci. 1986, 24, 143. ( 2 ) Akutin, M. S.; Lebedeva, E. D.; Mainikova, N. F.; Chuduk, N. A.; El’tekov, Yu. A. Colloid J. USSR (Engl. Trawl.) 1980, 42, 87. (3) Cosgrove, T.; Prestidge, C. A.; Vincent, B. J. Chem. SOC.,Faraday Trans. 1990,86 (9), 1383.
0743-’746319312409-2326$04.00/0
amount. The rate at which this chemisorption of the polymer takes place is influenced by temperature or by the presence of a catalyst. In this study both the reflection and small-angle scattering of neutrons have been used to determine the structure of chemisorbed PHMS on quartz and porous silica at two different temperatures. The results are compared with parallel FTIR studies of the same polymer chemisorbed on alumina from tetrachloromethane.
Experimental Section Materials. Linear poly(hydrogen methylsiloxane) (PHMS) was prepared by the fractionation of a commercial sample supplied by Dow Corning (Barry, U.K.) using preparative gel permeation chromatography (GPC). The purity of the sample was rigorously checked by various spectroscopictechniques.UV absorption waa used to detect any traces of benzaldehyde and benzoic acid in the sample, the byproducts from the oxidation of toluene which is used in the preparationlpurification. The infrared spectrum for PHMS is rather simple, and there is a characteristic Si-H stretching band at 2170 cm-l. Traces of polystyrene in the samples are a sign that the GPC columns have degraded,but this can be easily detected by its characteristic IR absorptions which lie in the region 1475-1600 cn-l. lH NMR spectroscopy waa also used to detect any other possible contamination. Once these fractions were shown to be spectroscopically pure, they were then characterizedby analytical GPC which waa calibrated with monodisperse standard polystyrene samples obtained from Polymer LaboratoriesLtd. The analysis gave a number-average molecular weight M. = 12200 and polydispersity M a n = 1.09 0.05. The substrates used were single crystalline quartz blocks, polished to 20 nm for the reflectivityexperiments,and porous silica (Vycorglass with pore diameter 1250A)for the small-angle neutron scattering (SANS) experimenb. The Vycor glass is known to have a very narrow pore size distribution.‘ The advantageof using a porous substrate is that the internal surface area is much greater than the external area, and hence most of the polymer adsorbed is inside the substrate. This means that problems such as settling and bridging flocculation, which may affect the adsorbed amount and the layer structure, are minimized. Given that the radius of gyration of the polymer is of the order of nanometers,no pore trapping or bridging of the polymer is expected to occur. The y-alumina (PREST,Grenoble, France)consisted of small aggregates of -9 nm-diameter primary particles and did not form a stable dispersion. However, by choosing a consistent method for preparing the samples, reproducible results for
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(4) Haller, W. Nature 1965,206, 693.
0 1993 American Chemical Society
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Chemisorption of Poly(hydrogen methylsitoxone) adsorbed amounts were obtained. It is therefore apparent that the preparation procedure did not lead to any dramatic increase in surface area and hence no substantial breakdown of the aggregates. The advantage of the porous substrate in this regard is clear. However,it should be noted that carryingout adsorption kinetics studies with the porous materials will be rather difficult. The solvents used were tetrachloromethane and toluene (spectroscopicgrade, Aldrich ChemicalCo.) and were dried over molecular sieves prior to use. Polymer solutions of 500 ppm were initiallyprepared in tetrachloromethane. The quartz block and porous silica substrate were thoroughly washed and dried overnight under vacuum in order to minimize the contribution of bound water to the overall neutron reflectivity and scattering. The quartz blocks were sealed in flasks, and the polymer solution was injected through a septum. Experiments were carried out at room temperature (nominally 298 K)and at 333 K, and the samples were left to equilibrate overnight to ensure that the adsorption process had reached completion. The upper (maximum) temperature was chosen so that it was below the boiliig point of tetrachloromethane(350K). Similar steps were followed for the silica and alumina substrates. Once the polymer was ”locked”onto the substrates (i.e., chemisorbed) it was necessary to change the solvent to toluene in order to perform the contrast variation experiments. This change in the solvent neutron scattering length density cannot be achieved with tetrachloromethane, and suitable combinations of deuterated and protonated toluene were used. No desorption of the polymer is likely to occur as a result of this change in solvent because the polymer is irreversibly chemisorbed. Sufficient time was allowed for a completeexchange of the solvents. To contrast match the quartz, a mixture of 69.7% toluene-h and 30.3 % toluene-d was required. The off-contrast experimentswere carried out in pure toluene-d. Kinetic Study of PHMS by FTIR Spectroscopy. To undertake kinetic studies, relatively large samples are required, so that a small aliquot can be removed for analysis without changing the composition of the sample being studied. The dispersion sample was prepared in a 25 cm3round-bottom flask; 15 g of alumina dispersion (& = 0.01) and 15 g of polymer solution (M,= 12 200) were mixed together; the final concentrationwas 440 ppm. The sample was stirred to maintain controlled hydrodynamicconditionsfor adsorptionto occur and to minimize flocculation and settling. The flasks containing the alumina dispersion and the polymer solution were sealed with a suba-seal and left to equilibrate at the reaction temperaturecontrolled water bath. At predetermined times, equal amounts of the dispersion were taken out with a hypodermic syringe, and were divided into two equal fractions, one of which was centrifuged at 4000 rpm. For each aliquot, infrared spectra of both the supernatant and the dispersion were recorded. From these spectra the concentration of nonadsorbed and adsorbed PHMS was determined. Reflectivity Experiment. In a specularreflection experiment the neutron reflectivity is measured as a function of the momentum transfer 8.5 The CRISP reflectometer at the ISIS pulsed neutron source at the Rutherford Appleton Laboratory was used. A time-of-flight technique was employed to measure the reflectivity as a function of wavelength at a fiied angle of incidence. The Tied sample geometry ensuresa constant sample illumination, and the wave vector resolution is therefore essent i d y constant over the entire reflectivity profile. The experiments were carried out with a wavelength range of 2-6.5 A with angles of incidence at 0.4O and 0 . 8 O in order to obtain as wide a Q range as possible (0.014-0.085 A-I). For the purpose of the reflectivityexperiment,the quartz blocks were placed in a PTFE trough. The initial alignment of the trough with the detector was performed using a laser alignedalongthe neutron flight path. A series of reflectivities were measured as a function of sample height, which were then integrated over a fiied wavelength range. In this way the optimum sample height was determined. The raw data were intially ratioed to the incident spectrum and corrected for the detector efficiencies. The intensity of the neutron beam passing directlythrough the quartz was measured, and the measured reflectivity profiles were set to an absolute (5) Russell, T.P.Mater. Sci. Rep. 1990, 5, 171.
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Momentum Transfer Figure I. Normalized neutron reflectivity profiies for PHMS adsorbed on quartz at two temperatures (333 K for upper trace and room temperature for lower trace) and at two contrasts in toluene. The fitted solid curves have been obtained using the volume fraction profiies shown in Figure 4 for (a) on contrast and (b) off contrast. scale by dividing them by the straight through beam profile. After data reduction, the resultant reflectivity profiies for both angles of incidence were combined in Q and a good overlap was obtained. The normalized reflectivity profiies on contrast and off contrast at 333 K and room temperature are shown in parta a and b of Figure 1. The analysis of the neutron reflectivity data was carried out using an iterative fitting model based upon the optical matrix calculation, which is an exact method? In this method the interfacestructure is described as a series of layers each of which is defiied in terms of layer thickness, scattering length density (mean density) within the layer, and the interfacial roughness between the layers. A series of simulationswere carried out until a reasonable fit was obtained. The parameters which appeared to give the best fit were then used as a starting point for an iterative calculation using a least-squaresanalysisuntil avolume fraction profile was obtained which best fitted the experimental data. In order to minimize the possible ambiguity when fitting the volume fraction profile to the reflectivity data, both the onand off-contrast experimental data were fitted simultaneously to the same profile. SANS Experiment. The small-angle neutron scattering experiments were carried out using the LOQ diffractometer at the Rutherford Appleton Laboratory. The sampleswere placed in standard 2-mm quartz cells which were mounted behind cadmium windows,attached to a samplechanger with a horizontal travel. Eight million neutron counts were accumulatedfor each sample, and a complete normalized differential scattering crosssection was obtained. The transmissionswere measured for each sample, and a linear fitting of the transmission as a function of wavelength was carried out prior to being used in the data correction. The scatteringcross-sections for the adsorbedPHMS on porous silica for conditionswhere the silica is contrast matched to toluene are shown in Figure 2 for both the 333 K and room temperature samples. Due to the relativelylow molecular weight
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2328 Langmuir, Vol. 9, No.9, 1993 1
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temperature. Assuming firsborder kinetics, rate contanta for the interactionlreaction can be calculated from a plot of ln(I', - I') against time. The analysis gave values of ~ R= T 5.39 X 10-5 8-l and k 3 3 3 ~ 1.36 X 10-4 E-'. The higher adsorbed amount which results from the faster adsorption rate can be understood in terms of chain conformations. When chemisorption is rapid, the adsorbed chains retain much of their solution conformation because of the rapid "locking" of the chain segments to the surface which precludes further rearrangements. When the rate of chemisorption is slower than the surface rearrangementtime, substantial chain reconformation can take place.' The final state in this case, even though chemisorbed, will resemble the structure of a pure physically adsorbed polymer that has reached adsorption equilibrium. The FTIR results of the above system can be compared qualitatively with the results obtained from the neutron experiments. The normalized neutron reflectivityprofiles for the chemisorbed PHMS a t room temperature and at 333 K at two contrasts with respect to the solvent are shown in Figure 1. The reflectivity profiies at 333 K are higher and steeper than at room temperature, indicating changes in the polymer layer thickness and volume fraction at the interface. The two data sets (on contrastloff contrast) at each temperature were fitted simultaneously, i.e., keeping the structure the same but changing only the known scattering length density of the solvent. A multiblock model profile was used as described before, and the results are shown graphically in Figure 4, in terms of the average volume fraction of each block. The results indicate that the layer thickness and density near the solid interface have increased upon increasing the reaction temperature to 333 ~
(6) Auroy, P.; Auvray, L. J. Phys. II1998,3,227.
(7) Cosgrove, T.;Prestidge, C. A.; King,S. M.; Vincent, B. Lmgmuir 1992,8,2206.
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Chemisorption of Poly(hydrogen methylsilorane) Table I. Results Obtained from the Neutron and FTIR Experiment# neutron reflectivity SANS FTIR 0.64 0.09 0.66 0.08 0.49 0.06 rRT/rWk 0.58 0.07 0.68* 0.11 N/A WtT/C+.Sk
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K. A value for the ratio of the adsorbed amounts was obtained by integrating the volume fraction profiles, and the result compares well with the FTIR result (see Table 1).
The low Q region of the SANS shown in Figure 2 can be analyzed using a Guinier-type approximation:8 I(Q) = I(0) exp[-u2Q21/Q2
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where Q is the second moment about the mean of the volume fraction profile of the adsorbed polymer and I(0) is the intensity at Q = 0. This approximation can be used when u2Qz
