FTIR spectroscopy

For the better understanding of plasma polymerisation processes and impact of process parameters some samples were further characterised by FTIR spectroscopy. Since the sampling depth of the FTIR in attenuated total reflection (ATR) mode is too intensive (1-3 µm) compared to the thickness of deposited films of only about 100 nm, background emission of PET was visible on the spectra collected by the spectrometer. To eliminate the background noise allylamine was plasma

polymerised on KBr tablets and transmission spectra were collected.

IR-spectrum of plasma polymerised allylamine (PPAa) and the monomer indicates that the monomer has undergone a reorganisation during the plasma polymerisation (fig.

4.23). One can see after comparison that some bands were significantly broadened, some disappeared while new bands appeared also.

ittan

Double peaks of primary amine N-H stretching vibration at 3380-3290 cm-1 (A1) were well resolved on

1000

Figure 4.23 FT-IR spectrum of the monomer allylamine and a plasma deposited polyallylamine (PPAa)

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the spectra of the monomer but a wide absorption band was found on the polymer at 3250-3600 cm-1, which originates from a primary amine, a secondary amine, an imine, might have a small contribution from a hydroxyl group as well. The deformation vibration of primary amines was observed in both spectra, but was considerably broadened in the spectrum of PPAa, which indicates the presence of alkene groups, or imines. A new band appeared for the polymers – but very week when a sample was treated with pulsed plasma at low energy - at 2200 cm-1 (N1), which was associated with the stretching vibration of nitrile (C≡N) groups.

The multiple absorption peak at 2960, 2940, and 2880 cm-1, (C1) is due to the stretching vibration of aliphatic C-H groups, was also detected in the polymer, but the deformation of the vibration band located at 1460, 1380 cm-1 (C2) rose up [Krishnamurthy 1989, Fally 1995, Zhao 1999, Gancarz 2002].

Samples were compared first based on their relative amine content obtained by IR spectroscopy (fig. 4.24). At first glance it is visible in the diagrams that the IA1 related to IC1 thus the relative amine content is significantly higher in films deposited at high energetic plasmas compared to the ones produced at 400 W. The pulse plasma operation reduced the amine content considerably either. It should be noted as well that at high energetic plasma exposure the elimination of hydrogen from C-H bonds occurred (IC1 decreases) this – not only a higher IA1 - contributes also to a higher IA1/IC1.

In figure 4.25 the intensity ratios of IN1 to IA1 for polymers obtained at different process parameters are depicted. Either for shorts such as for longer residence times the effect of temperature was similar namely that the higher the sample temperature was the lower the nitrile content was related to the amine content. This could be explained assuming that at higher temperatures double or triple bounds are more likely to react thus nitrile groups are degraded to other nitro groups building up various cross linked polymeric structures [Yasuda 1985]. The plasma polymerisation of allylamine exposed to high energetic plasmas led to a significantly higher IN1/IA1 ratio. Furthermore it was observed that longer residence time (smaller flow rate) of the monomer at the same plasma caused a higher degradation – higher IN1/IA1.

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0

400 puls 800 1200 puls

Figure 4.24. Relative intensity of IR absorption of IA1/IC1 in PPAa films deposited at different process parameters. Full columns 20 ºC, striped columns 40 ºC, empty ones 60 ºC.

Tg.= 50 %. Allylamine flow rate left 15 g/h, right 30 g/h.

Figure 4.25. Relative intensity of IR absorption of IN1/IA1 in PPAa films deposited at different process parameters. Full columns 20 ºC, empty columns 40 ºC, striped ones 60 ºC.

Allylamine flow rate left 15 g/h, right 30 g/h.

4.2.5 XPS measurements

Six samples with diverse ammine contents modified at relatively different plasma parameters - only substrate temperature was fixed to 60 ºC - (table 4.7) were selected for further characterisation regarding their chemical structure physical properties and cell adhesive properties at the end either. Results of XPS measurements are shown in table 4.8.

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Table 4.7. Process parameters of selected samples.

Sample MW-power (W) φAllylamine(sccm) Duty cycle (%)

PPAa-1 400 15 -

PPAa-2 1200 15 -

PPAa-3 400 30 -

PPAa-4 400 30 50

PPAa-5 1200 30 -

PPAa-6 1200 30 50

Table 4.8. Elemental composition of deposited layers obtained by XPS.

C O N [N]/[C] C1sI/[C] C1sII/[C] C1sIII/[C]

Sample

cc at% atomic ratio (x100)

PPAa-1 72,6 6,0 21,4 29,5 80,8 10,3 8,8

PPAa-2 67,6 5,5 27,0 39,9 69,4 18,7 11,9

PPAa-3 69,7 5,9 24,4 35,0 72,1 14,7 13,2

PPAa-4 69,6 6,7 23,6 33,9 77,2 13,9 8,8

PPAa-5 62,8 7,1 30,1 47,9 60,4 23,7 15,9

PPAa-6 66,5 7,4 26,1 39,2 69,7 16,5 13,8

An enrichment of nitrogen (see N/C ratio) occurred as a result of high energetic plasmas, whereas on the contrary the N/C-ratio for polymers produced by low energetic plasma was lower than that of the monomer (33 %). These results correspond well with the results obtained by other analytical tools, as indicated already are not commonly observed. In general for “in plasma” modifications namely less functional groups are formed at high plasma power is typical [Inagaki 1996].

Other research groups already observed the effect of enrichment of nitrogen during the polymerisation of allylamine, in the postdischarge region [Nowak 1989, Fally 1994, 1995].

An exact explanation could not be given but implementing the deposition scheme (fig 4.26) proposed by Yasuda [1985] followings could be taken into considerations. At high energies numerous fragmentation reactions of the monomer occur in the plasma. Some nitrogen containing and relatively stable activated species react possibly not right in the glow-discharge, but with substrates placed downstream. Furthermore low molecular weight carbon containing fragments could leave the reactor as non-reactive gases reducing this way the N/C

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Figure 4.26. Plasma polymerisation mechanism of allylamine [Yasuda 1985]

ratio either. Another reason could be that the ion density at high energies in the post discharge region is similar to that for low energies in the plasma, thus less fragmentation occurs. Based on results presented – namely that parallel to the increase of N/C ratio an enrichment of nitrogen compared to the monomer was also observed we assume that both phenomena were involved.

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5-7% oxygen was incorporated in the polymers which originates presumably from residual oxygen adsorbed on the reactor wall, could also be a result of oxidation reaction after treatments.

Figure 4.27. C1s, N1s and O1s spectra of allylamine plasma deposited film (PPAa-5 )

The C1s spectrum of PPAa was resolved into three peaks: C1sI at 284,5 eV typical peak corresponds to carbon present in CHx groups, C1sII at 286 eV due to carbon atoms singly bounded to nitrogen (C-NH2, C-NH-C, C=N-C …), and C1sIII (286,8-287,4 eV) which could be associated to imine (C=N) or nitrile (C≡N) groups [Beamson 1992, Fally 1995, László 2000]. Because of the oxygen contamination present in the polymers C1sII and C1sIII includes C-O and C=O functionalities respectively (Fig. 4.27). The intensities of C1sI peaks were significantly lower at high energetic polymerisation, which indicates that aliphatic bond scission and CHx elimination occurred (refer fig 4.26). Pulsed plasma operation seemed to reduce that in contrast to the same continuous plasma. Based on the intensities of peak II and III of C1s spectrum we consider that more than 50% of the nitrogen was singly bounded to carbon, whereas the rest was involved in imine or nitrile groups as confirmed by IR-spectroscopy also.

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