Characterization of polyester degradation using tapping mode atomic force microscopy: exposure to alkaline solution at room temperature

Characterization Techniques of Polymer Aging: From Beginning to End

Polymers have been widely applied in various fields in the daily routines and the manufacturing. Despite the awareness of the aggressive and inevitable aging for the polymers, it still remains a challenge to choose an appropriate characterization strategy for evaluating the aging behaviors. The difficulties lie in the fact that the polymer features from the different aging stages require different characterization methods. In this review, we present an overview of the characterization strategies preferable for the initial, accelerated, and late stages during polymer aging. The optimum strategies have been discussed to characterize the generation of radicals, variation of functional groups, substantial chain scission, formation of low-molecular products, and deterioration in the polymers' macro-performances. In view of the advantages and the limitations of these characterization techniques, their utilization in a strategic approach is considered. In addition, we highlight the structure-property relationship for the aged polymers and provide available guidance for lifetime prediction. This review could allow the readers to be knowledgeable of the features for the polymers in the different aging stages and provide access to choose the optimum characterization techniques. We believe that this review will attract the communities dedicated to materials science and chemistry.

Dynamic Surfaces-Degradable Polyester Networks that Resist Protein Adsorption

We synthesized a series of novel degradable alternating copolyesters composed of diglycolic anhydride (DGA) and two epoxides, epoxymethoxytriethylene glycol (ETEG) and a photoactive crosslinking agent epoxy benzophenone (EBP). After UV crosslinking, soaking the films in a good solvent (tetrahydrofuran) removed uncrosslinked material, and the resulting film gel fractions were calculated. These network films were then degraded in buffer solutions of varying pH values. The degradation of networks with lower gel fraction (fewer crosslinks) was faster and followed first-order kinetics. In contrast, the denser network degraded slower and followed zeroth-order kinetics. The lower gel fraction networks possess a higher swelling ratio and resist bovine serum albumin (BSA) adsorption better by entropic shielding and faster degradation. In comparison, higher gel fraction networks with higher EBP mole fractions adsorb more BSA due to hydrophobic interactions and slower degradation.

Unravelling the Immunotoxicity of Polycaprolactone Nanoparticles-Effects of Polymer Molecular Weight, Hydrolysis, and Blends

Poly-ε-caprolactone (PCL) is a biodegradable polyester that has FDA and CE approval as a medical device. Nonetheless, the lack of toxicity exhibited by the polymer cannot be extrapolated to its nanomaterial conformation. Despite PCL-based NPs being widely studied in the biomedical field for their advantages as controlled drug delivery systems, little data describe PCL NPs' toxicity, particularly immunotoxicity. This work assessed different PCL-based delivery systems intended for protein delivery regarding their immunotoxicity and hemocompatibility. Two different molecular weight PCL polymers were used, as well as blends with chitosan and glucan. Results showed that the presence of NaOH during the production of PCL₂ NPs and PCL₂/glucan NPs induced PCL alkali hydrolysis, generating more reactive groups (carboxyl and hydroxyl) that contributed to an increased toxicity of the NPs (higher reduction in peripheral blood mononuclear cell viability and lower hemocompatibility). PCL₂/glucan NPs showed an anti-inflammatory activity characterized by the inhibition of LPS stimulated nitric oxide (NO) and TNF-α. In conclusion, generalizations among different PCL NP delivery systems must be avoided, and immunotoxicity assessments should be performed in the early stage of product development to increase the clinical success of the nanomedicine.

Mechanical properties and degradation studies of poly(mannitol sebacate)/cellulose nanocrystals nanocomposites

Two pre-polymers with ad-mannitol : sebacic acid 1 : 1 and 1 : 2 ratios respectively were combined with cellulose nanocrystals (CNCs) and crosslinked applying different time–temperature profiles to obtain PMS/CNC nanocomposites with different properties.

Effect of High Temperature Sodium Hydroxide Immersion on Fusion Bond Epoxy Coating

Fusion Bond Epoxy (FBE) coating system was exposed to 5% sodium hydroxide at elevated temperature for 30 days. The result of exposure showed formation of adhere deposit layer, a discolored zone underneath and remaining un-affected bulk of the coating. The deterioration of the coating was characterized using analytical techniques like scanning electron microscopy (SEM), energy-dispersive X-ray (EDAX) spectroscopy, attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), differential scanning calorimetry (DSC), pull-off adhesion, and electrochemical impedance spectroscopy (EIS). Results obtained indicated chemical deterioration of the coating in the discolored zone and leaching of low molecular weight coating component forming deposit layer. Although the adhesion strength and barrier property were not affected, the polymer matrix in the affected zone undergoes severe changes in its surface microstructure, primary chemical structure, and glass transition temperature. This may inflict serious impairment of the coating functional properties and premature failure of the coating in long term exposure.

Nanostructured anti-bacterial poly-lactic-co-glycolic acid films for skin tissue engineering applications

Major issues faced with the use of today's skin grafts are infection, scar tissue formation, insufficient keratinocyte (or skin producing cells) proliferation and high production costs. To overcome these limitations, we propose here for the first time, a nanofeatured poly(lactide-co-glycolide) (PLGA) membrane as a next generation antibacterial skin graft material. An alkaline surface treatment method was used to create random nanofeatures on PLGA membranes where sodium hydroxide (NaOH) concentration and exposure times were altered to control surface morphology. Most significantly, and without the use of antibiotics, results showed a decrease in Staphylococcus aureus (a dangerous pathogen infecting skin grafts) growth for up to ∼40% after 2 days of culture on nanofeatured PLGA membranes compared to untreated controls. Results also showed that while bacteria growth was stunted, mammalian cell growth was not. Specifically, cell culture results showed an increase in human epidermal keratinocyte density, while the density of scar tissue forming human dermal fibroblasts, did not change on nanofeatured PLGA surfaces compared to the untreated controls after 3 days of culture. These findings indicate that the alkaline treatment of PLGA membranes is a promising quick and effective manner to limit scar tissue formation and bacterial invasion while increasing skin cell proliferation for improving numerous wound-healing applications.

Surface modification and adhesion improvement of polyester films

Facile surface modification of polyester films was performed via chemical solutions treatment. Surface hydrolysis was carried out by means of sodium hydroxide solutions, leading to the formation of carboxylate groups. Three commercial polyester films of 100 μm in thickness were used in this work: AryLite™, Mylar™, and Teonex™, hydrolysis time being the main modification parameter. FTIR-ATR analysis, topography and contact angle (CA) measurements, surface free energy (SFE) and T-Peel adhesion tests were carried out to characterize the modified films. A quantitative estimate of the carboxylates surface coverage as a function of treatment time was obtained through a supramolecular approach, i.e. the ionic self-assembly of a tetracationic porphyrin chromophore onto the film surface. The surface free energy and critical surface tension of the hydrolyzed polyesters was evaluated by means of Zisman, Saito, Berthelot and Owens-Wendt methods. It was shown that NaOH solution treatment increases roughness, polarity and surface free energy of polymers. As a result, T-Peel strengths for modified Mylar™ and Teonex™ films were respectively 2.2 and 1.8 times higher than that for the unmodified films, whereas AryLite™ adhesion test failed.

Nanocharacterization of Surface and Interface of Different Epoxy Networks

The effect of network changes on the surface and interface properties of amine-cured epoxy has been investigated. Samples of different crosslinked epoxies are prepared by mixing stoichiometrically pure diglycidyl ether of bisphenol A (n=0.03) with different ratios of 1,3-bis(aminomethyl)cyclohexane (terafunctional amine) and cyclohexylmethylamine (difunctional amine). All samples are cured in CO2-free air. Both the film surface in contact with air and that in contact with the silicon substrate (the interface) are analyzed using atomic force microscopy (AFM) and nanoindentation. Small angle neutron scattering (SANS), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, and contact angle measurements, are used to assist in the interpretation of AFM results. Substantial morphological and mechanical differences are observed between the surface and the interface for different crosslinked epoxies. The findings have strong implications on the wettablity, adhesion, and durability of amine-cured epoxies.

Effects of Surface Functionality and Humidity on the Adhesion Force And Chemical Contrast Measured with AFM

The ability to probe chemical heterogeneity with nanometer scale resolution is essential for developing a molecular–level understanding of a variety of phenomena occurring at surfaces of materials. One area that could benefit greatly from nanoscale chemical measurement is an understanding of the degradation mechanisms of polymeric materials exposed to the environment. For example, the degradation (photo and hydrolytic) of polymers and polymeric materials has been observed to occur non-uniformly in which nanometer pits form locally, which deepen and enlarge with exposure (1, 2). The pitting has been postulated to initiate in the hydrophilic degradation-susceptible regions of the films (3). However, due to the lack of spatial resolution of the most current surface analytical techniques, the chemical nature of the degradation-initiated locations has not been identified. The use of a chemically-functionalized probe in an AFM (chemical force microscopy CFM) (4) has been shown to be capable of discriminating chemically-different domains of self-assembled monolayer (SAM) surfaces at the nanoscale spatial resolution. This study provides data to demonstrate that, by using proper RH at the tip-sample environment, the contrast between the hydrophilic and hydrophobic domains in SAM and polymer samples can be discerned, and presents results on the effects of RH on tipsample adhesion forces for different substrates.