Vibrational spectroscopy of amphiphilic biomaterials
Amphiphilic polymers or hydrogels as they are commonly called are three dimensional crosslinked polymers. Such polymers are insoluble in water and the crosslinks can be either physical, chemical or of the van Der Waal’s type. Such materials were first synthesised in 1960 and have been the scope of research and increasing number of applications in the last four decades. We have synthesised such materials in situ on the surface of an ATR crystal of an FTIR instrument and deduced characteristic changes occurring during polymerisation and crosslinking by interpreting the spectra resulting therein. Diffusion experiments with distilled water have been performed and water polymer interactions have been observed. The effect of crosslinker on the diffusion has been studied. Further during the course of our work, we aim to synthesise temperature sensitive polymers incorporating NIPAAM fractions. Protein diffusions and insitu degradation of such materials is also aimed in the coming months. Variations in the monomeric ratios during the synthesis of such materials and the possible trends likely to be followed will be established.
Experimental
Amphiphilic copolymers of glycerol monomethacrylate (GMMA) and lauryl methacrylate (DM4) crosslinked with ethylene glycol dimethacrylate (EGDMA) have been synthesised in situ by us by free radical polymerisation using azoisobutyronitrile (AIBN) as an initiator. The reaction is as shown below in Figure 1.
Figure 1 The monomers used for our work
An in-house designed cell was been used for carrying out polymerisations nd diffusions and spectra recorded at specific time intervals during the course of the experiments. The experimental setup is shown in Figure 2.
Figure 2 Experimental setup for polymerisation and diffusion
Results
Figure 3 below shows a thin film of the hydrogel obtained on the surface of the ATR crystal. All films measured about 250 – 300 µm in thickness.
Figure 3 Thin film obtained on the surface of the ATR crystal
Figure 4 below shows the changes occurring during polymerisation in the fingerprint region of the IR spectrum.
Figure 4 Spectral changes in the fingerprint region during polymerisation
Two bands have been identified one at 750 cm-1 and the other at 815 cm-1. The 750 band, attributed to the CH rocking vibrations is found to increase successively indicating that polymerisation takes place by breaking of the vinyl double bonds. The 815 band which arises from the =CH2 out of plane vibrations of the vinyl group decreases as polymerisation progresses indicating once more that the vinyl double bonds are being broken. The 750 and the 815 bands are actually complementary to each other.
A plot of percentile conversion of the polymer and the percentile depletion of the monomer as calculated from the integrated areas of the 750 and the 815 bands is shown below in Figure 5.
Figure 5 Percentile conversions and depletions from 750 and 815 cm-1 band
Evidence of crosslinking was indicated at the oxygen of the ester group. Some changes in the region between 1400 – 1500 cm-1 indicated ether formation and hence the crosslinking. Increase in intensity of the band between 1020 – 1080 cm-1 could also indicate the formation of an anhydride crosslink.
Diffusion experiments
Successive increase in the OH stretch region was found as the concentration of water increased during the course of diffusion. Delamination was also found to occur towards the end of the diffusion processes. Evidence of hydrogen bonding was obvious from the band shifting to lower wavenumbers. The spectra ratioed against the clean crystal are shown below in Figure 6.
Figure 6 Spectral changes during diffusion
Evidence of swelling of the gel was obtained when ratioed against the dry polymer film. These spectra are shown below in Figure 7.
Figure 7 Evidence of swelling of the gel during diffusion