Samuel J. Huang

Multicomponent Polymers of Poly(Lactic Acid) Macromonomers with Methacrylate Terminal and Copolymers of Poly(2-Hydroxyethyl Methacrylate)

Abstract Poly(lactic acid) macromonomers with methacrylate terminal functionality have been synthesized from the cyclic dimer of lactic acid (referred to as lactide) with 2-hydroxyethyl methacrylate (HEMA) as initiator and stannous 2-ethyl hexanoate as catalyst. The macromonomers were characterized with FT-IR, NMR, GPC, DSC, WAXS, and CD. The molecular weights of the macromonomers ranging from M n 1425 to 19,169 are predictable from the lactide/HEMA ratio in the polymerization feeds. The properties of the macromonomers vary with the stereochemistry of the lactide and the composition. Circular dichroism measurements demonstrate that there is little racemization during polymerization.

Mechanism of the Biodegradation of Polycaprolactone

The preparation of synthetic biodegradable polymers is an area of increasing interest (1–4). Disposal of nondegradable synthetic polymers after use is a problem that is growing more serious every day. The disposal of biodegradable polymers, on the other hand, is less difficult. Moreover, biodegradable polymers are becoming vital in the preparation of surgical implants, sutures, controlled release formulations of drugs and agricultural chemicals, weed suppressant covering, and mulches. Our efforts have been directed toward, a) the development of biodegradation testing methods, b) studying the factors affecting the biodegradation of synthetic polymers and the mechanisms of biodegradation, c) design and synthesis of new biodegradable polymers for special applications. During the course of our research to date we have found that both the nature of the chemical structures of the polymers and the morphology of the polymer samples affect the rates and extents of biodegradation of the polymer samples.

A second polycaprolactone depolymerase from Fusarium, a lipase distinct from cutinase

Abstract Polycaprolactone (PCL), a synthetic polyester with applications in biodegradable plastics, is degraded by a variety of microorganisms, including fungal phytopathogens. These pathogens secrete cutinase, which hydrolyzes cutin, the polyester structural component of plant cuticle, releasing ω-hydroxy fatty acids that induce cutinase synthesis. Our laboratory previously reported that growth of Fusarium solani on PCL requires cutinase, which is active as a PCL depolymerase and induced by the products of its action on PCL. A mutant strain of F. solani in which the cutinase gene is deleted was unable to grow on PCL and did not secrete PCL depolymerase activity in the media tested. It is now shown that this mutant produces a PCL depolymerase in media containing lipase inducers. Wild-type strains also produce this second PCL depolymerase, which is induced by Tween 80 and tributyrin, but not by PCL or cutin. The second depolymerase shows interfacial activation, indicating that it is a lipase. PCL may thus be a substrate but not an inducer of depolymerases that degrade it, and screening microorganisms on medium with PCL as the sole source of carbon and energy may fail to reveal strains with active PCL depolymerases, because of the absence of an inducer. Surprisingly, Tween 80 induces both cutinase and lipase activities in wild-type F. solani.

The hydrolysis of poly(lactic acid)/ poly(hexamethylene succinate) blends

Reactive melt blends of semi-crystalline oligo(hexamethylene succinate) (PHS) and highly amorphous poly(lactic acid) (PLA) have been prepared in order to study the effects of phase separation upon hydrolytic stability at pH 7.4 and 37 °C. Amorphous compatible blends were obtained at 10 and 20wt% PHS while crystalline phase separation was observed at 30 and 40wt% PHS. Gel-permeation chromatography (GPC) has shown that, in each instance, the molecular weights of PLA homopolyester and the PHS/PLA blends approach 1000 Daltons after 35 days of hydrolysis. However, major differences in the rate of hydrolysis between PLA and the blends are only seen within the first 2 weeks of degradation. Differential scanning calorimetry (DSC) has indicated that Tg minima of the materials are reached after 21 days and that the PHS crystalline phase is more susceptible to hydrolytic attack if compared with the induced PLA crystallites. In accordance with these findings, the presence of PHS and PLA crystallinity at higher concentrations of oligoester seems to complicate the degradation of the blends as the possibility exists for significant concentration fluctuations.

The effects of primary structure on the degradation of poly(ɛ-caprolactone)/poly(l-lactide) block copolymers

Abstract Poly( l -lactide- block -ɛ-caprolactone- block - l -lactide)s have been formulated to study the effects of structure upon resistance to hydrolysis and biodegradation. The rate of hydrolysis of the copolyesters at pH 7.4 and 37 °C depends upon a sensitive combination of morphology and composition with the initial chain cleavage (0–7 days) suppressed most by the those systems with the highest ɛ-caprolactone (CL) crystallinity. In addition, microorganisms that secrete PCL depolymerase (cutinase), such as Fusarium solani and Fusarium moniliforme , show the ability to degrade those systems that possess longer CL sequence lengths.

Inactivation of polycaprolactone depolymerase (cutinase) in Fusarium cultures by an extracellular protease

Cultures of Fusarium moniliforme grown on polycaprolactone (PCL) or on cutin as a sole source of carbon and energy had low levels of detectable PCL depolymerase (cutinase) activity in the supernatant medium. A small peak of depolymerase activity was observed after hyphal accumulation had ceased, but this activity soon declined. The low level of the peak of activity and its decline were attributable to proteolytic inactivation of the depolymerase. A decrease in the pH of cultures coincided with the appearance of protease activity in the supernatant at about the same time as the appearance of the transient peak of depolymerase activity. Addition of protease substrates (bovine serum albumin, casein) to the culture at this time caused a dramatic although temporary increase in PCL depolymerase activity. The same effect was seen for cultures of F. solani pisi. Use of a different buffer system for the medium prevented a drop in pH and resulted in higher and stable levels of PCL depolymerase activity.

Aliphatic polyester blends based upon poly(lactic acid) and oligomeric poly(hexamethylene succinate)

Abstract In an attempt to gain a degree of control over the mechanical and degradation properties of poly(lactic acid) [PLA], large-scale efforts are underway to alter the phase morphology of PLA through chemical and physical modification. Consistent with this theme, our work aims to adjust the molecular architecture of highly amorphous PLA with an increasing concentration of hydroxy-terminated oligomeric poly(hexamethylene succinate) [PHS]. Gel-permeation chromatography (GPC) verifies the enhanced presence of PHS in the blends with a concomitant reduction in number-average molecular weight as the weight fraction of PHS is raised from 0.10 to 0.40. Differential scanning calorimetry (DSC) indicates amorphous phase compatibility between PHS and PLA at weight compositions of 10/90 and 20/80. However, as the amount of PHS approaches 30 and 40 wt%, the PHS exhibits the ability to crystallize independently from the induced PLA crystalline phase. Dynamic mechanical thermal analysis (DMTA) illustrates variable be...

The Synthesis and Characterization of Polyesters Derived from L-Lactide and Variably-Sized Poly(Caprolactone)

In an age of increasing societal concerns for an environmentally conscious method of disposing end-use materials, poly(lactic acid)[PLA] offers a viable means to potentially alleviate the repercussions that accompany this complex issue. With biocompatible degradation by-products and physical properties that rival those of polyethylene, polystyrene and polyvinyl chloride, PLA appears to be an attractive alternative to these high-volume commodity thermoplastics from a somewhat superficial perspective.1 However, with its relatively high cost, PLA does not reside in the same marketability category as the aforementioned commodities. A possible resolution to this problem of economics stems from the origin of its lactic acid(LA) building blocks; that is, LA is produced from fermentation of naturally abundant carbohydrates.2 In any event, prior to addressing the ever-present question of cost, the fact that PLA is deficient in certain areas of performance must not be overlooked. With a glass transition temperature(~55–65°C) that resides in a precarious regime if considering biodegradable, consumer product applications, less than adequate thermal and hydrolytic stability, and toughness that is highly dependent upon stereoregularity, it is obvious that much attention must be given to these limitations.

A Study of Polymers Using Fluorescent Dyes

Fluorescence spectroscopy is often used to study polymer properties which are difficult to learn about or incompletely understood by the use of more traditional techniques. Fluorescent dyes have been used in the study of solid homopolymers1, polymer blends2, polymer colloids3 and polymers in solution4. Block copolymers are a specific group of polymers which have not been studied using fluorescence techniques. In this work polyurethane elastomer block copolymers have been chosen and two dyes, 1, a fluorescent probe which can be dissolved or blended in a polymer film, and 2, a fluorescent label, which can react through the hydroxy functional groups to become part of the polymer chain.