Trong-Ming Don

Free radical degradation of chitosan with potassium persulfate

A thermal dissociation initiator, potassium persulfate (KPS), is added to the chitosan solution at 70 � C; immediately, the solution viscosity and the molecular weight of chitosan decrease in a very short time. Size exclusion chromatography, nuclear magnetic resonance and electron spin resonance were used to study the degradation mechanism. A free radical degradation mechanism of chitosan by KPS is then proposed. When KPS is thermally dissociated into anionic radicals, they are attracted to the cationic amino group in the chitosan ring. Subsequently, the anionic radical attacks the C-4 carbon and transfers the radical to the C-4 carbon by subtracting the hydrogen from it. The presence of free radical at C-4 carbon eventually results in the breakage of the glycosidic C– O–C bond in the chitosan main chain. According to this mechanism, the concentrations of KPS, total free radicals and the degraded chitosan chain at different degradation times are all calculated by solving the rate equations. Finally, the calculated average molecular weights of the degraded chitosan chains at different reaction times agree with the experimental values. # 2001 Elsevier Science Ltd. All rights reserved.

Preparation and characterization of poly(hydroxyalkanoate) from the fermentation of Haloferax mediterranei.

In this study, fed-batch fermentation of Haloferax mediterranei using glucose and yeast extract as carbon and nitrogen source, respectively, was carried out to produce poly(hydroxyalkanoate) (PHA). After fermentation for 117 h, the concentration of H. mediterranei and PHA content reached 85.8 g/l and 48.6%, respectively. 1H- and 13C-NMR spectra proved that the produced PHA was poly(3-hydroxybutyrate-co-3-hydroxyvalerate) P(3HB-co-3HV) co-polymer. However, further fractionation using chloroform/acetone revealed that the produced PHA consisted of at least two compositionally different co-polymers (P1 and P2). One P(3HB-co-3HV) co-polymer (P1, 93.4 wt%) contains 10.7 mol% of 3-HV unit in the chain structure and has a high molecular weight of 569.5 kg/mol. The other one (P2, 6.6 wt%) has a slightly higher 3-HV content, ca. 12.3 mol%, but its molecular weight is relatively low, 78.2 kg/mol. Both fractions exhibit two overlapped melting peaks measured by differential scanning calorimetry when the heating rate is at and below 20°C/min. For example, at a heating rate of 10°C/min, the two melting peaks occur at 134.8°C and 144.3°C for P1, and 131.1°C and 140.6°C for P2. Through observing the variation of relative intensity of these two melting peaks by changing the heating rate, it was proven that the phenomenon is caused by a melt/recrystallization process. Glass-transition temperature, crystallization temperature and thermal degradation behavior of these co-polymers were also discussed.

Spherulitic morphology and crystallization kinetics of melt-miscible blends of poly(3-hydroxybutyrate) with low molecular weight poly(ethylene oxide)

Abstract The spherulitic morphology and crystallization kinetics of the blends of poly(3-hydroxybutyrate) (PHB) with the low molecular weight poly(ethylene oxide) (PEO) prepared by the solution casting have been investigated by differential scanning calorimetry and polarized optical microscopy. The blend was a crystalline/amorphous system when temperatures lie between the melting point of PEO (T m PEO = ca . 60 ° C ) and that of PHB (T m PHB = ca . 170 ° C ), while it became a crystalline/crystalline system below TmPEO. With PHB crystallization at the crystallization temperature (Tc) of 70 °C, the blends showed PHB banded spherulitic texture, which could be perturbed by the subsequent PEO crystallization in PEO-rich compositions upon further cooling to room temperature. When PHB and PEO were allowed to crystallize upon quench from melt to room temperature, fairly competitive crystallization emerged where crystallization and segregation of PHB and PEO may occur simultaneously, leading to a complicated spherulitic morphology. As for the results of crystallization kinetics, the PHB crystallization exothermic temperature (TfPHB) on cooling, PHB spherulitic growth rate (GPHB), and overall crystallization rate of PHB (kn) exhibited maxima in their dependences of PEO composition. This was attributed to the coupling between enhanced chain mobility and depression in equilibrium melting point (Tm0), since the low molecular weight PEO has a very low glass transition temperature (Tg) over the Tc range studied, GPHB increased with Tc but kn was found to decrease. The drop of kn with increasing Tc was therefore predominately governed by the large depression in nucleation rate at higher Tc.

Thermal degradation behavior and flammability of polyurethanes blended with poly(bispropoxyphosphazene)

Polyurethanes containing different amount of flame retardant, poly(bispropoxyphosphazene), were synthesized by a two-step polymerization. The thermal degradation behaviors of these polyurethanes were then studied by the thermal gravimetric analysis (TGA), TGA coupled with Fourier transform infrared analysis and elemental analysis. A limiting oxygen index was used to evaluate the flammability of these polyurethanes. For these modified polyurethanes under nitrogen, a two-stage thermal degradation behavior was observed. The first stage was caused by the degradation of hard segments, whereas the soft segments were responsible for the second-stage degradation. The thermal degradation activation energies were calculated by using Ozawas method. It was found that the addition of flame retardant caused a decrease of the activation energy in the first stage, but an increase in the second stage, which was probably due to the formation of a thermal stable structure. As for the flame retardancy, the modified polyurethanes have a higher char yield at 550°C, and a higher limiting oxygen index than the neat polyurethane.

Preparation of water-resistant antifog hard coatings on plastic substrate.

A novel water resistant antifog (AF) coating for plastic substrates was developed, which has a special hydrophilic/hydrophobic bilayer structure. The bottom layer, acting both as a mechanical support and a hydrophobic barrier against water penetration, is an organic-inorganic composite comprising colloidal silica embedded in a cross-linked network of dipentaethritol hexaacrylate (DPHA). Atop this layer, an AF coating is applied, which incorporates a superhydrophilic species synthesized from Tween-20 (surfactant), isophorone diisocyanate (coupling agent), and 2-hydroxyethyl methacrylate (monomer). Various methods, e.g., FTIR, SEM, AFM, contact angle, and steam test, were employed to characterize the prepared AF coatings. The results indicated that the size and the continuity of the hydrophilic domains on the top surface increased with increasing added amount of T20, however, at the expense of hardness, adhesiveness, and water resistivity. The optimal T20 content was found to be 10 wt %, at which capacity the resultant AF coating was transparent and wearable (5H, hardness) and could be soaked in water for 7 days at 25 °C without downgrading of its AF capability.

Preparation and antibacterial test of chitosan/PAA/PEGDA bi-layer composite membranes.

Chitosan/poly(acrylic acid)/poly(ethylene glycol) diacrylate (PEGDA) composite membranes with a bi-layer configuration were prepared and their potential application as an antibacterial material was examined. A two-step process was adopted. A dope consisting of PEGDA, acrylic acid (AA) and a photoinitiator was cast and then UV-cured on a glass substrate to form a mechanically strong, dense membrane. Subsequently, the membrane was coated with a layer of solution composed of chitosan, AA and water. As the majority of AA diffused downwards into the supporting layer underneath, chitosan coagulated with residual AA to form a nano-layer on the top surface by means of UV irradiation. Low-voltage field-emission scanning electron microscopy was used to observe the membrane morphology and to measure the thickness of the top layer. Contact angle measurements indicated a top layer mixed with chitosan and poly(acrylic acid), which was confirmed by chemical composition analysis of X-ray photon spectroscopy. The antibacterial activities of the formed membranes were tested both with respect to a Gram-negative (Escherichia coli) and a Gram-positive (Staphylococcus aureus) bacteria.

Fourier transform infrared analysis of polycarbonate/epoxy mixtures cured with an aromatic amine

In our previous article, we established that polycarbonate (PC) can react with the diglycidyl ether of bisphenol-A (DGEBA) at 200°C through transesterification and addition reactions, resulting in degraded PC chains with phenolic end groups and also in PC/DGEBA copolymers. However, these reactions can be minimized or eliminated at lower temperatures, below 160°C. In this article, Fourier transform infrared analysis (FTIR) was used to study the curing kinetics of epoxies in the presence of PC. The curing agent was an aromatic amine, diaminodiphenyl methane (DDM). FTIR results showed that the presence of a small amount of PC promoted the amine–epoxide reactions, probably due to the catalytic effect of the phenolic end groups in the PC chains. However, the PC did not alter the epoxy cure reaction mechanism. Two different blending processes were used to premix the PC and DGEBA, namely, solution-blending and melt-blending processes, in order to give different extents of prereactions. If a solution-blending process was used, PC tended to undergo crystallization during an early stage of cure. When a melt-blending process was used, no melting peak was observed in the thermograms of the differential scanning calorimeter (DSC) for the modified epoxies; PC chains bonded to DGEBA during prereaction at 200°C, thus inhibiting the crystallization of PC during cure.

Kinetic model of hyperbranched polymers formed by the polymerization of AB2 monomer with a substitution effect

Hyperbranched polymers obtained by the polymerization of AB2-type monomer with a substitution effect on the B2 groups were studied by means of the kinetic model. In this polymerization with the substitution effect, if one of the B2 group reacts first, the reactivity of the remaining unreacted B group will be changed. The profiles of the degree of polymerization, polydispersity, degree of branching, and structural units of the hyperbranched polymers with the conversions were all calculated by the generating function method. It is shown that the weight-average degree of polymerization and the degree of branching of the hyperbranched polymers having substitution effect differ from that with equal reactivity of the B2 groups. If the substitution effect causes an increase in the rate constant after one of the B2 groups has reacted, a broader molecular weight distribution and a higher degree of branching are observed.

Preparation and properties of chitosan/clay (nano)composites: a silanol quaternary ammonium intercalated clay

Chitosan/clay (nano)composites were prepared by using a special quaternary ammonium intercalating agent coupled with a silanol group to facilitate the organic clay formation. Exfoliated clay in the chitosan matrix was attained at the higher intercalant dosages through X-ray diffraction (XRD) and transmission electron microscope (TEM) analyses. Optical transmittance for the (nano)composites increased slightly with increasing the amount of intercalants in the clays. In light of the hydrophobic component on the intercalant and the effective clay content, the interfacial interaction between chitosan and modified clay may not be strong enough to render higher mechanical properties, even though the partially exfoliated clays were achieved to provide high interfacial area for the dispersed phase and the matrix. An optimum Young’s modulus was thus found for (nano)composites using modified clay at a medium dosage of intercalant, which resulted from the balance of the dispersion status and interfacial interaction. This outcome indicated high dispersion of modified clay may not guarantee high mechanical properties of (nano)composites. The antimicrobial property of chitosan against Escherichia coli (E. coli) increased further with the addition of modified clays, in which the intercalant exhibiting the antimicrobial function. The modified clay at an optimum dosage of modifier to balance the mechanical properties and antimicrobial property was attained.

Influence of Hydroxyvalerate Content on the Crystallization Kinetics of Poly(hydroxybutyrate-co-hydroxyvalerate)

Influence of hydroxyvalerate (HV) content on the crystallization kinetics of hydroxybutyrate (HB) units in random poly(hydroxybutyrate-co-hydroxyvalerate) (P(HB-co-HV)) copolymers has been studied by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). It was found that the crystallization kinetics of HB units was strongly affected by the presence of the ethyl side chain of HV units whether under the non-isothermal or isothermal crystallization conditions. The spherulitic growth rate (G) and overallall crystallization rate (kn) of HB units decreased with increasing HV content. Both G and kn exhibited the maxima, Gmax  and knmax , which showed a gradual shift toward lower temperatures with increasing HV content, which may be attributed to the depression in the equilibrium melting point (Tm0) and nucleation factor (Kg). The temperature corresponding knmax  was different from Gmax  due to the depression of nucleation rate with the degree of undercooling was more susceptible than that of the growth rate. According to the Lauritzen–Hoffman theory of secondary nucleation, the crystallization of HB units in P(HB-co-HV) copolymers was similar to that of neat PHB as regime III and n=4 growth process.