Reduction In Polymer Molecular Weight Research Articles

Additive Manufacturing of Anatomical Poly(d,l-lactide) Scaffolds

Poly(lactide) (PLA) is one of the most investigated semicrystalline polymers for material extrusion (MEX) additive manufacturing (AM) techniques based on polymer melt processing. Research on its application for the development of customized devices tailored to specific anatomical parts of the human body can provide new personalized medicine strategies. This research activity was aimed at testing a new multifunctional AM system for the design and fabrication by MEX of anatomical and dog-bone-shaped PLA samples with different infill densities and deposition angles. In particular, a commercial PLA filament was employed to validate the computer-aided design (CAD) and manufacturing (CAM) process for the development of scaffold prototypes modeled on a human bone defect. Physical-chemical characterization of the obtained samples by 1H-NMR spectroscopy, size exclusion chromatography (SEC), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) demonstrated a small reduction of polymer molecular weight (~5%) due to thermal processing, as well as that the commercial polymer employed was a semicrystalline poly(d,l-lactide). Mechanical characterization highlighted the possibility of tuning elastic modulus and strength, as well as the elongation at break up to a 60% value by varying infill parameters.

Backbone Degradation of Polymethacrylates via Metal-Free Ambient-Temperature Photoinduced Single-Electron Transfer.

Polymeric materials comprised of all-carbon backbones are ubiquitous to modern society due to their low cost, impressive robustness, and unparalleled physical properties. It is well-known that these materials often persist long beyond their intended usage lifetime, resulting in environmental accumulation of plastic waste. A substantial barrier to the breakdown of these polymers is the relative chemical inertness of carbon-carbon bonds within their backbone. Herein, we describe a photocatalytic strategy for cleaving carbon-based polymer backbones. Inclusion of a low mole percent of a redox-active comonomer allows for a dramatic reduction in polymer molecular weight upon exposure to light. The N-(acyloxy)phthalimide comonomer, upon reception of an electron from a single-electron transfer (SET) donor, undergoes decarboxylation to yield a backbone-centered radical. Depending on the nature of this backbone radical, as well as the substitution on neighboring monomer repeat units, a β-scission pathway is thermodynamically favored, resulting in backbone cleavage. In this way, polymers with an all-carbon backbone may be degraded at ambient temperature under metal-free conditions.

Design of Pt-Supported 1D and 3D Multilayer Graphene-Based Structural Composite Electrodes with Controlled Morphology by Core-Shell Electrospinning/Electrospraying.

Platinum (Pt)-decorated graphene-based carbon composite electrodes with controlled dimensionality were successfully fabricated via core–shell electrospinning/electrospraying techniques. In this process, multilayer graphene sheets were converted into the three different forms, fiber, sphere, and foam, by tailoring the polymer concentration, molecular weight of polymer, and applied voltage. As polymer concentration increased, continuous fibers were produced, whereas decreasing polymer concentration caused the formation of graphene-based foam. In addition, the reduction in polymer molecular weight in electrospun solution led to the creation of three-dimensional (3D) spherical structures. In this work, graphene-based foam was produced for the first time by utilizing core–shell electrospraying technology instead of available chemical vapor deposition techniques. The effect of morphologies and dimensions of carbonized graphene-based carbon electrodes on its electrochemical behavior was investigated by cyclic voltammetry and galvanostatic charge–discharge methods. Among the three different electrodes, Pt-supported 3D graphene-based spheres showed the highest specific capacitance of 118 F/g at a scan rate of 1 mV/s owing to the homogeneous decoration of Pt particles with a small diameter of 4 nm on the surface. After 1000 cycles of charging–discharging, Pt-decorated graphene-based structures showed high cyclic stability and retention of capacitance, indicating their potential as high-performance electrodes for energy storage devices.

The Influence of Pulsed Electron Beam Treatment on Properties of PLLA Nonwoven Materials Produced by Solution Blow Spinning

One of the most important aspects in nonwoven materials fabrication in case biodegradable scaffolds is the control of their degradation rate which must correlate with a speed of tissue regeneration. Radiation treatment based on electron beam irradiation is one of the promising methods for control over polymer degradation. The effect of nanosecond pulsed electron beam irradiation on properties of poly-L-lactic acid (PLLA) nonwoven materials produced by solution blow spinning (SBS) method was investigated. Modification of nonwoven materials was performed in electron beam accelerator with absorbed dose from 26 to 260 kGy. Properties of the nonwoven materials were examined by means of viscosity measurement, X-ray diffraction (XRD) analysis, Fourier transform infrared spectroscopy (FTIR) spectroscopy, and scanning electron microscopy (SEM). It was shown that pulsed electron beam exposure leads to the reduction of polymer molecular weight and recrystallization. It was shown that pulsed electron beam modification of PLLA nonwoven materials changes their physical and chemical properties that allows considering this method as a potential technology for the manipulation of polymer degradation rate and fabrication of polymer materials with required properties.

A framework to predict and experimentally evaluate polymer-solute thermodynamic affinity for two-phase partitioning bioreactor (TPPB) applications

BACKGROUND Selection of a polymer for two-phase partitioning bioreactor (TPPB) applications has previously been limited to heuristic approaches. However, recent interest has focused on first principles' selection methods based on polymer crystallinity, glass transition temperature and polymer–solute thermodynamic affinity. In this work, a framework is proposed to evaluate and predict polymer–solute thermodynamic affinity via the polymer-phase activity coefficient. RESULTS Polymer screening via thermodynamic affinity was shown to be most effective at very dilute concentrations, where partition coefficients can be estimated using infinite dilution activity coefficients. In the absence of published values, UNIFAC-vdW-FV or Flory–Huggins based activity models can provide very good predictions for the polymer-phase activity coefficient, significantly improving upon previous approaches using Hildebrand and Hansen solubility parameter differences. For non-dilute systems, however, the activity models failed to consider the full effects of concentration on partition coefficient. Additionally, a reduction in polymer molecular weight resulted in improved partition coefficients, a phenomena well described by the activity models. CONCLUSION Predicting and experimentally quantifying polymer–solute thermodynamic affinity at very dilute concentrations will aid future attempts at TPPB polymer selection. Furthermore, experimental partition coefficient data at a range of operational concentrations will indicate how TPPB effectiveness will change throughout the fermentation course. Finally, reduction of polymer molecular weight to improve solute partitioning should be investigated further for a range of polymers. © 2014 Society of Chemical Industry

Medical grade sterilization affects synthetic polymer film properties intended for peripheral nerve repair: an in vitro study

In vitro characterization of synthetic scaffolds in the laboratory commonly employs sterilization techniques, such as, ultraviolet light or ethanol (EtOH) soaking. These sterilization methods are not sufficient, however, to gain Regulatory approval for therapeutic use. Neglecting the effects medical grade sterilization may have on material properties could lead to years of research never translating to the clinic. The objective of this study was to ascertain whether medical grade gamma irradiation affected the properties of solvent-cast poly(ε-caprolactone)/poly(D, L-lactic acid) blend films for peripheral nerve repair. Scaffolds were sterilized at eight incremental doses of Gamma radiation (0-45 kGy). With increasing radiation dose, tensile testing identified significant reductions in both maximum tensile strength (>40 %) and strain (>90 %); gel permeation chromatography showed a dose-dependent reduction in polymer molecular weight (>46 %) and differential scanning calorimetry highlighted an increase in crystallinity. NG108-15 nerve cells were cultured up to 7 days on gamma irradiated and EtOH soaked films (control). Scanning electron microscopy showed cells proliferated on all films during this time and confirmed cell morphology was unaffected by sterilization method. However, cellular proliferation and number were greater for gamma irradiated films compared to EtOH soaked. Despite material properties being significantly altered, the in vitro response was encouraging and gamma irradiation may prove effective for medical grade sterilization of films intended for peripheral nerve repair.

Osteoblast function on electrically conductive electrospun PLA/MWCNTs nanofibers

The electrospinning process was utilized successfully to fabricate the random oriented and aligned electrically conductive nanofibers of biodegradable poly-DL-lactide (PLA) in which multiwalled carbon nanotubes (MWCNTs) were embedded. The topographical features of the composite nanofibers were characterized by SEM. The dispersion and alignment of MWCNTs in nanofiber matrix were observed by TEM. The in vitro degradation was characterized in terms of the morphological change, the mass loss and the reduction of polymer molecular weight as well as the decrease of pH value of degradation media. In particular, these conductive nanofiber meshes offered a unique system to study the synergistic effect of topographic cues and electrical stimulation on osteoblasts outgrowth as a way of exploring their potential application in bone tissue engineering. The results of obsteoblasts assay unstimulated showed that the aligned nanofibers as topographic cues could enhance the extension and direct the outgrowth of obsteoblasts better than random fibers. In the presence of direct current (DC) of 100 μA, the obsteoblasts on all samples grew along the electrical current direction. The cellular elongation and proliferation were mainly dependent on the electrical stimulation whereas the topographical features played a minor role in them. Therefore, electrical stimulation with an appropriate DC value imparted on conductive substrate had great potential in application of bone tissue engineering.

Studying the degradation of polyhydroxybutyrate‐co‐valerate during processing with clay‐based nanofillers

AbstractPolyhydroxybutyrate‐co‐valerate (PHBV) is attracting interest as a new material for packaging applications and nanoparticulate layered silicates are being increasingly explored as a way to improve PHBV film properties. In this context, it is essential to understand how different types of nanofillers could influence polymer properties. PHBV was processed with three‐layered clay types using different mixing methods, and we examined the effect of processing time, clay type, and clay content on polymer molecular weight and composite morphology. PHBV molecular weight (Mw) decreased by 38% after extrusion processing and was further reduced in the presence of montmorillonite (MMT). However, when PHBV was processed with kaolinite as the additive, no further reduction in polymer molecular weight was observed. Molecular weight also decreased as the MMT clay content increased from 1 to 5 wt %. The results suggest that release of tightly bound water from clay surfaces at elevated temperature may be responsible for PHBV degradation during processing. Evidence also points to the possibility that the surface modifier present in organically modified MMT may catalyze PHBV degradation in some way. X‐ray diffraction studies indicated an intercalated morphology in the presence of modified montmorillonite but good dispersion was also achieved when unmodified kaolinite was blended with PHBV. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Polymer molecular weight and chain transfer during the photopolymerization of an aliphatic monoacrylate in a smectic liquid crystal

Polymer/liquid crystal (LC) composites offer a unique opportunity to study polymerizations in ordered media, specifically the potential effect mesophase order can have on polymer properties including molecular weight. To develop successful polymer/LC composites for display applications, it is important to understand the effect of mesophase order on polymer molecular weight in order to optimize the electro-optic (EO) properties of the polymer/LC composite. Polymer molecular weight may be influenced in a LC by changes in polymerization rate as LC order is modulated and by chain transfer. This work focuses on the photopolymerization of an aliphatic monoacrylate monomer, decyl acrylate (DA), both in the ordered LC phases of 8CB as well as in isotropic solutions with LC and co-solvent. When DA is polymerized using the LC as the solvent, enhanced polymerization rates and polymer molecular weights are observed in the highly ordered smectic phase compared to the less ordered nematic and isotropic phases. When conducted strictly in an isotropic environment using a co-solvent with increasing 8CB percentages, a dramatic decrease in the polymerization rate and a significant reduction of the polymer molecular weight is observed, implying degradative chain transfer to the LC. NMR results show that this chain transfer is a result of hydrogen abstraction from the liquid crystals, which leads to the reduction in the polymerization rate with increasing 8CB concentration. The most likely site of hydrogen abstraction is from the benzyl hydrogens of the alkyl chain of 8CB. This chain transfer also plays a role for polymerizations performed in the ordered phases of the LC. Chain transfer appears to be less significant when polymerizations are conducted in the smectic phase due to the anti-parallel association of the LC molecules. When polymerizations occur in the less ordered phases, chain transfer dominates leading to a large reduction in polymer molecular weight and polymerization rate.

Temperature-induced transformations of peroxide radicals formed in polytetrafluoroethylene with γ-ray irradiation

At 200/sup 0/C, temperature-induced transformations of peroxide radicals from -CF/sub 2/CF(OO.)CF/sub 2/- (radical I) to -CF/sub 2/CFCF/sub 2/- (radical 1) and -CF/sub 2/CF/sub 2/OO. (radical II) to -CF/sub 2/CF/sub 2/ (radical 2) in vacuo, and from I to II in oxygen atmosphere were observed. In these thermal transformations, the evolution of CO/sub 2/ with a small amount of CO and CF/sub 2/O was observed. Further, measurements of the heat of crystallization of the treated polytetrafluoroethylene suggest that a reduction of polymer molecular weight occurs during the transformations from I to 1 and I to II.

Kinetic identification of individual reactions determining molecular weight variation of polymers during thermal oxidative degradation

IT IS well known tha t thermal oxidative degradation of most polymers is a radical chain process with degenerate branching. During degradation, as well as formation of functional groups containing oxygen, the molecular weight and molecular weight dis tr ibut ion of polym e r s is modified. Even in the case of inhibited thermal oxidation of polymers subject to random degradation, a marked molecular weight variat ion is observed during the induction period [1]. Therefore, the problem of identifying the main individual reactions determining this change is very essential. Kinetic equations based on the generally adopted system of oxidation satisfactorily describe oxygen absorption and the accumulation of some of the products of oxidation. However, the kinetics of variat ion of polymer molecular weight during degradat ion remain obscure. Assumptions have boon made in the l i terature concerning reactions resulting in variat ion of molecular weight during degradation [2]. Reduction of polymer molecular weight in part icular has boon related to isomerization in the decomposit ion of macroradicals. These assumptions, however, have not boon confirmed by kinetic analysis.