Stephen P. McCarthy

Biodegradable polymer blends of poly(lactic acid) and poly(ethylene glycol)

Poly(lactic acid) (PLA) and poly(ethylene glycol) (PEG) were melt-blended and extruded into films in the PLA/PEG ratios of 100/0, 90/10, 70/30, 50/50, and 30/70. It was concluded from the differential scanning calorimetry and dynamic mechanical analysis results that PLA/PEG blends range from miscible to partially miscible, depending on the concentration. Below 50% PEG content the PEG plasticized the PLA, yielding higher elongations and lower modulus values. Above 50% PEG content the blend morphology was driven by the increasing crystallinity of PEG, resulting in an increase in modulus and a corresponding decrease in elongation at break. The tensile strength was found to decrease in a linear fashion with increasing PEG content. Results obtained from enzymatic degradation show that the weight loss for all of the blends was significantly greater than that for the pure PLA. When the PEG content was 30% or lower, weight loss was found to be primarily due to enzymatic degradation of the PLA. Above 30% PEG content, the weight loss was found to be mainly due to the dissolution of PEG. During hydrolytic degradation, for PLA/PEG blends up to 30% PEG, weight loss occurs as a combination of degradation of PLA and dissolution of PEG.

Citrate Esters as Plasticizers for Poly(Lactic Acid)

Citrate esters were used as plasticizers with poly(lactic acid) (PLA). Films were extruded using a single-screw extruder with plasticizer contents of 10, 20, and 30% by weight. All of the citrate esters investigated were found to be effective in reducing the glass transition temperature and improving the elongation at break. It was observed that the plasticizing efficiency was higher for the intermediate-molecular-weight plasticizers. Hydrolytic and enzymatic degradation tests were conducted on these films. It was found that the lower-molecular-weight citrates increased the enzymatic degradation rate of PLA and the higher-molecular-weight citrates decreased the degradation rate as compared with that of unplasticized PLA.

Reactive compatibilization of biodegradable blends of poly(lactic acid) and poly(ε-caprolactone)

Abstract Poly(e-caprolactone) (PCL) was reactively blended with poly(lactic acid) (PLA) using three catalysts/coupling agents in a Haake twin screw mixing chamber. Triphenyl phosphite showed the most promising results as a coupling or branching agent. PLA and PCL were also melt blended without any catalyst/coupling agents in order to make a comparison of the properties. The transesterification reaction was monitored by measuring torque values as a function of time. 1 H NMR was used to characterize the structure of the reactive and physical blend products. The blend samples were characterized for physical properties and biodegradation. Mechanical property measurements indicated that the elongation of the PLA/PCL blends improved significantly when compared to pure PLA especially for the reactively compatabilized blends, and that a synergism was observed for certain compositions (PLA/PCL = 80/20 or 20/80). Degradation studies showed that the enzymatic degradation rate (or normalized weight loss) of the reactively compatabilized blends were much higher than that of pure PLA and PCL, while the degradation rate of physical blends are between those of pure PLA and PCL.

Miscibility and biodegradability of blends of poly(lactic acid) and poly(vinyl acetate)

Poly(lactic acid) (PLA) was melt blended with poly(vinyl acetate) (PVA) using a single-screw extruder. The extruded films obtained were characterized for miscibility, physical properties, degradation and surface morphology. Differential scanning calorimetry showed that all the as-extruded films were amorphous, and that the blends were compatible as only one glass transition temperature was observed. Results obtained from physical property testing indicated that the blends exhibit synergism in the range of 5 to 30% PVA, probably due to some interactions taking place in that region. Enzymatic degradation studies showed that there was a vast difference in the weight loss of pure PLA samples and the 95/5 PLA/PVA blend. Surface tension results showed that this was due to the vast difference in the surface tension of the pure PLA films and the 95/5 blend. Deaged-free blends showed the maximum degradation, followed by aged-extruded and then deaged-fixed. Scanning electron micrographs showed that the mode of degradation differs for the aged and deaged samples. A uniform degradation pattern was seen in the case of deaged samples while the aged samples showed a non-uniform pattern of degradation.

Further investigations on the hydrolytic degradation of poly (DL-lactide)

The hydrolytic degradation of poly(DL-lactide) (PLA50) material was investigated in order to elucidate the effects of temperature and acidity of the external medium on the degradation characteristics. It was observed that at 60 degrees C and at pH = 7.4, degradation was extremely rapid as compared with degradation at 37 degrees C. After only 2 days, heterogeneous degradation was observed due to larger internal autocatalysis. On the other hand, in the case of degradation at 37 degrees C in an acidic medium with pH = 3.7, the heterogeneous degradation was preceded by a much longer lag time. Water absorption was found to be pH dependent. At pH = 7.4, water absorption was increased due to the osmotic pressure driving the buffer solution into the polymer matrix to neutralize acidic endgroups, which was not the case for degradation at pH = 3.7. In both cases, the oligomeric stereocomplex was obtained as degradation residue at the end of the degradation period.

Effects of physical aging, crystallinity, and orientation on the enzymatic degradation of poly(lactic acid)

The effects of physical aging, degree of crystallinity, and orientation of poly(lactic acid) (PLA) were studied using differential scanning calorimetry (DSC) and wide angle X-ray scattering (WAXS). The samples of PLA with 96% [L] and 4% [D] contents were prepared by injection molding. The physical aging of PLA strongly depended on time and temperature. The change of rate of physical aging was very fast initially and slowed down as time increased. The enzymatic degradation of PLA was carried out with proteinase K at 37°C at a pH value of 8.6 in a Tris/HCl buffer solution. The enzymatic degradation rate was found to decrease as a function of physical aging (i.e., excess enthalpy relaxation). The rate of enzymatic degradation of PLA decreased with the increase in crystallinity. A threshold was observed when the heat of fusion was less than 20 J/g. The weight loss of PLA with a low level of crystallinity had no apparent change during any period of testing time. The rates of enzymatic degradation of stretched and injection-molded specimens were comparable.

Electrospun silk material systems for wound healing

The functional properties of six distinct electrospun silk material groups were evaluated to assess conformational and biocompatible characteristics related to wound dressings. In a hydrated state, all six silk matrices exhibited absorption, water vapor transmission, oxygen permeation and enzymatic biodegradation suitable for full-thickness wound sites. Employing constrained drying techniques, silk concentration was a determinate factor influencing material structural properties related to the storage and distribution of such wound dressing systems. Subsequently, three electrospun silk models demonstrated ideal biomaterial properties with potential utility for wound dressings.

Cellulose acetate biodegradability upon exposure to simulated aerobic composting and anaerobic bioreactor environments

Cellulose acetate (CA) films with degree of substitution (d.s.) values of 1.7 and 2.5 were exposed to biologically active in-laboratory composting test vessels maintained at approximately 53 °C. The CA 1.7- and 2.5-d.s. films (thickness values of ∼0.5–1.0 and 2.0 mil, respectively) had completely disappeared by the end of 7- and 18-day exposure time periods in the biologically active bioreactors, respectively. The relatively small CA film weight loss observed in the poisoned control test vessels allows the conclusion that CA film erosion during the composting exposures resulted, at least in part, from biologically mediated processes. Under strictly anaerobic conditions, an active methanogenic inoculum was developed by acclimation of a sewage sludge to a synthetic municipal solid waste (SMSW) mixture at 42°C. The CA 1.7-d.s. film samples (0.5- to 1.0-mil thickness) were exposed in anaerobic serum bottles containing a 25% solids loading of SMSW in which methanogenic activity was rapidly established after introducing of the developed inoculum. For exposures of 30 days only small visually distinguishable fragments of the CA 1.7-d.s. films were recovered. In contrast, exposure of the CA 1.7-d.s. film to a poisoned control test vessel resulted in negligible weight loss. Therefore, degradation of the CA 1.7-d.s. films upon exposure to the anaerobic bioreactors was due, at least in part, to biologically mediated processes.

Liquid crystalline, rheological and thermal properties of konjac glucomannan

Konjac glucomannan (KGM) exhibited liquid crystalline (LC) behaviour in aqueous solutions above 7% (w/w) concentrations as was determined by polarized optical microscopy and circular dichroism. The rheological properties of the concentrated LC solutions of KGM exhibited pseudoplastic behaviour. The fibrous extrudates retained a significant degree of flow-induced orientation as was determined by wide angle X-ray scattering, thereby indicating potential applications of KGM as fibres and films. Differential scanning calorimetry experiments showed that a significant degree of interaction occurred between KGM and water and that the KGM gels produced in our study cannot be classified as thermoreversible.

The effect of polymer surface on the wetting and adhesion of liquid systems

Youngs equation describes the wetting phenomenon in terms of the contact angle between a liquid and a solid surface. However, the contact angle is not the only parameter that defines liquid–solid interactions, an additional parameter related to the adhesion between the liquid drop and the solid surface is also of importance in cases where liquid sliding is involved. It is postulated that wetting which is related to the contact angle, and interfacial adhesion, which is related to the sliding angle, are interdependent phenomena and have to be considered simultaneously. A variety of models that relate the sliding angle to the forces developed along the contact periphery between a liquid drop and a solid surface have been proposed in the literature. Here, a modified model is proposed that quantifies the drop-sliding phenomenon, based also on the interfacial adhesion that develops across the contact area of the liquid/solid interface. Consequently, an interfacial adhesion strength parameter can be defined depending on the mass of the drop, the contact angle and the sliding angle. To verify the proposed approach the adhesion strength parameter has been calculated, based on experimental results, for a number of polymer surfaces and has been correlated with their composition and structure. The interaction strength parameter can be calculated for any smooth surface from measurements of the contact and the sliding angles.