Synthesis and Characterization of PLA-g-hydroxyethyl cellulose based polyurethanes

Abstract

In this work, a new method for the synthesis of HEC-g-PLA was carried out successfully by graft copolymerization of HEC with LA using ammonia water as environmentally safe catalyst. Highest degree of grafting was achieved with 1:9 HEC:LA mole ratio, 25 % NH3.H2O and 14 % urea solution. The DS and degree of grafting enhanced by increasing activation temperature and activation time up to a certain limit and then decreased due to production of more hydrogen bonds of HEC with urea. Esterification of HEC with LA increased by increasing the reaction temperature up to 90°C and above this temperature grating decreased due to hydrolysis of LA. The presence of absorption band at 1723 cm−1 and 1H SS NMR spectral study provides evidence for the successful copolymerization reaction between LA and HEC. The resultant HEC-g-PLA exhibits mainly amorphous structure due to random distribution PLA side chains on to HEC backbone. The TGA analysis showed weight loss between 200 to 300°C due to loss of moisture and scission of PLA side chain from HEC backbone followed by the decomposition of the side chains and 300 to 500°C due to thermal degradation of HEC residues. HEC-g-PLA had significant antimicrobial activities and 26.19 and 16.59 % Escherichia coli and Bacillus subtilis inhibition, respectively. In the second part, synthesis of PUs was carried out by reacting HTPB with excess IPDI to form NCO terminated polyurethane prepolymer which was then extended with 100 % BDO (ANPU1) or varying composition of BDO/HEC-g-PLA blends (ANPU2-ANPU5) or 100% HEC-g-PLA (ANPU6) to form a range of PUs. Disappearance of the -NCO peak and the appearance of characteristic NH peak in FT-IR spectra of all PU samples confirmed the completion of reaction and formation urethane linkage. The static 1H SS NMR of synthesized PUs also confirmed the formation of urethane linkage. The XRD diffraction peak became broader with lower intensities by increasing of HEC-g-PLA content into PU backbone implying that crystallization of PU had been suppressed by HEC-g-PLA. Tg of PUs increased by increasing HEC-g-PLA content due to decrease in chain mobility. The SEM images of PUs provide the knowledge about the incorporation of HEC-g-PLA. By increasing the ratio of HEC-g-PLA in HEC-g-PLA/BDO blended PU samples there was a gradual 143 increase in antimicrobial activity and biofilm inhibition due to the fact that the hydrophilicity of PU samples is increased by increasing the amount of HEC-g-PLA and hydrophilic surfaces provide close contact with aqueous microbe suspension. Result indicated that cytotoxicity decreased with increasing the ratio of HEC-g-PLA and sample ANPU6 showed minimum toxic behavior due to higher HEC-g-PLA content due to synergistic biocompatibility of HEC-g-PLA as well as PU The effect of molecular weight of HTPB (Mn = 2293, 2896, 3614, 4069 and 4400 g/mol) on various properties of polyurethanes (ANPU4, ANPU7-ANPU10) was studied. Results showed that in HEC-g-PLA/BDO blend based PUs the peak intensity and glass transition temperature decreased by increasing the molecular weight of HTPB because higher molecular weight polyols impart additional flexibility to PU while weight loss enhanced by increasing molecular weight indicating that thermal stability of PU samples increased by increasing molecular weight of HTPB. Antibacterial activity against pathogenic bacteria decreased by increasing the molecular weight of HTPB due to nonpolar nature of HTPB which leads to more phase separation, lower H-bonding between SS and HS and higher resistance to hydrolysis. All PU samples showed biocompatible and toxic behavior. Among all PU samples with varying molecular weight of HTPB, HTPB (Mn = 2293) based PU (ANPU7) showed better crystallinity, biocompatibility and non-toxicity. Variation in the structure of diisocyanate greatly influenced the properties of PUs (ANPU7, ANPU11-ANPU14). Aromatic diisocyanate based PU samples structure exhibited the higher peak intensities due to greater degree of chain orientation. TDI and IPDI based PU samples exhibited more crystallinity than H12MDI, MDI and HMDI due to smaller molecular size, less distance between NHCOO groups with more H-bonding and restricted mobility of SS and HS. The order of crystallinity is TDI> IPDI>MDI>H12MDI >HDI. The order of antibacterial activity and biofilm inhibition of diisocyanate based PUs is HDI>IPDI>H12MDI>TDI>MDI. Aliphatic/cycloaliphatic diisocyanates based samples, ANPU7, ANPU11 and ANPU12, showed non-toxicity while aromatic diisocyanates based PUs (ANPU13 and ANPU14) displayed slightly toxic behavior towards the living cells. IPDI and TDI based samples showed better crystallinity as compared to others. IPDI based PU displayed more hydrophilic and biocompatible behavior compared to TDI based PU. 144 Therefore, among all diisocyanate based PUs, IPDI based PU sample (ANPU7) showed better crystallinity, biocompatibility and non-toxicity. Alkane diol chain length (1,2-ethane diol, 1,3-propane diol, 1,4-butane diol, 1,5-pentane diol, 1,6-hexane diol) specifically affects the properties of HEC-g-PLA based PUs (ANPU7, ANPU15-ANPU18). Therefore, the corresponding peaks became broader with lower intensities by increasing the chain length of alkane diol because an increase in the number of methylene ( CH2) units resulted in an increase in better phase separation between the hard and soft segments. PU elastomer extended with EDO (ANPU15) was thermally more stable than all of the other samples. Thermal stability of the PU samples was decreased with increase of alkane diol chain length while the glass-transition temperature (Tg) decreased with increase of alkane diol chain length due to increase in chain flexibility and decrease in hard segment content and decrease in H-bonding which in turns lower the thermal stability. Antibacterial activity and biofilm inhibition of PU samples decreased with increase of alkane diol chain length because hydrophilicity of prepared samples decreased with an increase in the number of methylene carbons (hydrophobic) in the chain extender. The % hemolysis of ANPU15, ANPU16, ANPU7, ANPU17 and ANPU18, was within the range of non-toxicity and no sample displayed any toxic behavior towards the living cells. Effect of incorporation of HEC-g-PLA in PDMS based PU was studied varying its composition from 0 to 100% (ANPU19–ANPU23). Vanishing of the isocyanate peak and the appearance of urethane peak in FT-IR spectra of all HEC-g-PLA/PDMS blended PU samples confirmed the completion of reaction and formation of carbamate linkage. All PU samples exhibited improved biocompatibility and non-mutagenicity than the control due to inherent biocompatible and non-mutagenic behavior of HEC-g-PLA as well as PU and their levels increased by incorporating the increased amount of modified HEC in PDMS based PU. Overall, it is concluded that HEC-g-PLA based PUs exhibited biocompatibility and non-cytotoxicity due to inherent biocompatibility and non-mutagenicity of HEC-g-PLA as well as PU which makes it good candidate for development of biomaterial which can be used for various biomedical applications.