Starch-poly(vinyl alcohol) (PVA) blends with different compositions were prepared and crosslinked with borax by in situ and posttreatment methods. Various amounts of glycerol and poly(ethylene glycol) with a molecular weight of 400 were added to the formulations as plasticizers. The pure starch-PVA blends and the crosslinked blends were subjected to differential scanning calorimetry, thermogravimetry, and X-ray photoelectron spectroscopic studies. Broido and Coats-Redfern equations were used to calculate the thermal decomposition kinetic parameters. The tensile strengths and elongation percentages of the films were also evaluated. The results suggested that the glass-transition temperature (T g ) and the melting temperature strongly depended on the plasticizer concentration. The enthalpy relaxation phenomenon was dependent on the starch content in the pure blend. The crosslinked films showed higher stability and lower T g 's than pure PVA and starch-PVA blends, respectively. High-resolution X-ray photoelectron spectroscopy provided a method of differentiating the presence of various carbons associated with different environments in the films.
Computer simulations play an important role in designing new polymers as well as in predicting properties of existing polymers. In this paper, the blend compatibility of poly(vinyl alcohol) (PVA) with poly(methyl methacrylate) (PMMA) was studied over the wide range of compositions allowed by the atomistic and mesoscopic simulation methods. The Flory-Huggins interaction parameter, chi, of the blends computed using the atomistic simulation confirmed the blend compatibility for compositions containing >60 wt % of PVA. This observation was further supported by differential scanning calorimetric experiments. Solubility parameters of the polymers obtained from the simulation procedure were in good agreement with those of the literature data. Simulation results were further supported by the spectral and solution property measurements. From the atomistic simulations, chi versus concentration plots were constructed, which showed trends similar to those experimentally measured melting temperature versus concentration. The chi values for the blends, which satisfied the criteria of miscibility of two polymers by the atomistic simulation, agreed quite well with the solubility criteria related to order parameters calculated from the mesoscopic simulation. Kinetics of phase separation was examined via density profiles calculated using the MesoDyn approach for incompatible blends. The length and time scales spanned by these simulations were found to be relevant to the real application scales. The free energy computed in the mesoscopic simulation for blends reached equilibrium, particularly when the simulation was performed at a higher time step, indicating the stability of the blend system at certain compositions.
Molecular modeling simulations are the most important tools to predict blend compatibility of polymers that are otherwise difficult to predict by experimental means. Conflicting reports have been reported on the blend compatibility of poly(vinyl alcohol), PVA, and chitosan, CS polymers. Since both the polymers are widely used in pharmaceutics as drug-loaded particulates and as separation membranes, we felt it necessary to investigate their compatibility over the practical range of compositions. In this paper, we attempt to study the compatibility of PVA and CS polymers using molecular modeling strategies to understand the interactions between CS and PVA polymers to predict their compatibility from atomistic simulations. Flory-Huggins interaction parameter, chi, was computed at 298 K to assess the blend compatibility at different ratios of the component polymers. Miscibility was observed for blends below 50% of PVA, while immiscibility was prevalent at compositions between 50 and 90% PVA. Computed results confirmed the experimental findings of dynamic mechanical thermal analysis, suggesting the validity of modeling strategies employed. Plots of Hildebrand solubility parameter and cohesive energy density calculated at 298 K supported these findings. The chi values for blends, which satisfied the criteria of miscibility of polymers computed by atomistic simulations, agreed with the solubility criteria related to order parameters calculated from mesoscopic simulations. Miscibility between PVA and CS polymers is attributed to hydrogen bond formation and to an understanding of which of the interacting groups of CS, i.e., -CH2OH or -NH2, are responsible in blend miscibility. This was further confirmed by molecular dynamics simulations of radial distribution functions for groups or atoms that are tentatively involved in interactions. These results are correlated well to obtain more realistic information about interactions involved as a function of blend composition. Computed free-energy from the mesoscopic simulation for blends reached equilibrium, particularly when the simulation was performed at higher time step, indicating stability of the blend system at certain compositions.
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