Authors:R. Barreiro-Iglesias, C. Alvarez-Lorenzo, and A. Concheiro
The thermodynamics of the interaction of chitosan and sodium dodecylsulfate, SDS, was characterised by titration microcalorimetry to gain an insight into the binding process of amphiphilic molecules to this biocompatible polymer and its consequences on the behaviour of the solutions and chemically cross-linked hydrogels of chitosan. 0.2 M acetic acid was used as solvent medium, without or with 0.9% NaCl, in order to evaluate the influence of the ionic and hydrophobic interactions with two chitosans of different molecular mass and degree of deacetylation, DD. The critical micellar concentration, CMC, of SDS was ten times lower in the presence of the salt (0.35 vs. 3.5 mM, as estimated by surface tension measurements). Binding to chitosan (at 0.25%) began at concentrations significantly lower than CMC (critical aggregation concentration, CAC=0.035-0.17 mM) and saturation was reached at around 10 mM SDS, which corresponds to a positive/negative charges ratio of about 1. The process was in all cases enthalpy-driven (strongly exothermic) and, in the absence of the salt, also entropically favourable. The Gibbs free energy of interaction values were slightly greater for the chitosan with lower DD but greater molecular mass. The addition of increasing amounts of SDS resulted in a continuous decrease in the viscosity of chitosan solutions above the CAC, which ended in a macroscopic coacervation when around 1/3 of the positive charges were neutralised. In the same range of SDS concentrations, the hydrogel beads showed a continuous decrease in the swelling degree and a final collapsed state. The scarce tendency to redissolution or hydrogel reswelling in the presence of greater SDS concentrations can be attributed to that the binding process is mainly caused by the ionic interaction and did not go beyond the neutralisation point.
Authors:M. Mayo-Pedrosa, C. Alvarez-Lorenzo, and A. Concheiro
The miscibility of poly(N-isopropylacrylamide) (PNIPA) with poly(vinyl pyrrolidone) (PVP) and a cross-linked poly(acrylic
acid) (Carbopol 971P) was evaluated from the rheological data of aqueous dispersions and the temperature of glass transitions of films made
of binary mixtures. PNIPA has a low critical solubility temperature (LCST) of about 33C, below which 1% dispersion behaves
as a viscous system. At temperatures above LCST, the hydrophobic interactions among the isopropyl groups initially provide
transient networks of greater elasticity. The LCST of PNIPA as well as its Tg (144C, estimated by DSC and MTDSC of films) were not modified by the presence of PVP. The immiscibility of PNIPA and PVP
was confirmed by the absence of interaction between both polymers as shown by FTIR analysis of the films. In contrast, PNIPA
and carbopol were miscible and the behaviour of their mixtures differed significantly from that of the parent polymers; i.e.
a strong synergistic effect on the viscoelasticity of the dispersions was observed below the LCST. As temperature increased,
the blends showed a decrease in the loss and storage moduli, especially those with greater PNIPA proportions. The fall was
smoother as the PNIPA proportion decreased. This behaviour may be explained as the result of the balance between PNIPA/carbopol
hydrogen bonding interactions (as shown in the shift of C=O stretch in FTIR spectra) and PNIPA/PNIPA hydrophobic interactions.
The Tg values of the films of the blends showed a positive deviation from the additivity rule; the mixtures containing more than
1:1 amide:carboxylic acid groups have a notably high Tg (up to 181C). This increase is related to the stiffness induced in
the films by the PNIPA/carbopol interactions.
Authors:A. Gómez-Carracedo, C. Alvarez-Lorenzo, J. Gómez-Amoza, and A. Concheiro
Glass transitions of several non-ionic cellulose ethers differing in molecular mass and nature and amount of substituents
were analyzed (as compressed probes) by differential scanning calorimetry (DSC), modulated temperature differential scanning
calorimetry (TMDSC@®), and oscillatory rheometry. In general, the low energy transitions accompanying the Tg of methylcellulose (MC), hydroxypropyl methylcellulose (HPMC), and hydroxypropylcelluloses of low (L-HPC) or medium-high
(HPC) degree of substitution were difficult to characterize using DSC. Non-reversing heat flow signals obtained in TMDSC experiments
were more sensitive. However, the best resolution was obtained using oscillatory rheometry since these cellulose ethers undergo
considerable changes in their storage and loss moduli when reaching the Tg. Oscillatory rheometry also appears as a useful technique to characterize the viscoelastic behavior and thermal stability
of pharmaceutical tablets. Tg values followed the order HPC (105°C)<HPMC (170-198°C)<MC (184-197°C)<L-HPC (220°C). For HPMCs, the Tg increases as the methoxyl/hydroxypropoxyl content ratio decreases. The results indicate that Tg depends strongly on the structure of the cellulose ethers. In general, increasing the degree of substitution of cellulosic
hydroxyls, the hydrogen bonding network of cellulose decreases (especially when the substituents cannot form hydrogen bonds)
and, in consequence, Tg also decreases.