Changes In Polymer Structure Research Articles

Thermal and mechanical properties of ultrahigh molecular weight polyethylene/high-density polyethylene/polyethylene glycol blends

Abstract Blends of ultrahigh molecular weight polyethylene (UHMWPE) with high-density polyethylene (HDPE) provide adequate mechanical properties for biomedical application. In this study, the mechanical and thermal properties of UHMWPE/HDPE blends with the addition of polyethylene glycol (PEG) prepared via single-screw extruder nanomixer were investigated. The UHMWPE/HDPE blends exhibit a gradual increase in strength, modulus, and impact strength over pure polymers, suggesting synergism in the polymer blends. The elastic and flexural modulus was increased at the expense of tensile, flexural, and impact strength for the blends containing PEG. The degradation temperature of UHMWPE was improved with the incorporation of HDPE due to good thermal stability of HDPE. HDPE improved the dispersibility of PEG in matrix, consequently reduced the surface area available for the kinetic effects, and reduced the degradation temperature. The morphology analysis confirmed the miscibility between UHMWPE and HDPE and the changes in polymer structure with the presence of PEG modify the thermal behavior of the blends. The mechanical properties of the blends that are underlying values for the design of implant material show the potential used as biomedical devices.

Probing structure–antifouling activity relationships of polyacrylamides and polyacrylates

We have synthesized two different polyacrylamide polymers with amide groups (polySBAA and polyHEAA) and two corresponding polyacrylate polymers without amide groups (polySBMA and polyHEA), with particular attention to the evaluation of the effect of amide group on the hydration and antifouling ability of these systems using both computational and experimental approaches. The influence of polymer architectures of brushes, hydrogels, and nanogels, prepared by different polymerization methods, on antifouling performance is also studied. SPR and ELISA data reveal that all polymers exhibit excellent antifouling ability to repel proteins from undiluted human blood serum/plasma, and such antifouling ability can be further enhanced by presenting amide groups in polySBAA and polyHEAA as compared to polySBMA and polyHEA. The antifouling performance is positively correlated with the hydration properties. Simulations confirm that four polymers indeed have different hydration characteristics, while all presenting a strong hydration overall. Integration of amide group with pendant hydroxyl or sulfobetaine group in polymer backbones is found to increase their surface hydration of polymer chains and thus to improve their antifouling ability. Importantly, we present a proof-of-concept experiment to synthesize polySBAA nanogels, which show a switchable property between antifouling and pH-responsive functions driven by acid–base conditions, while still maintaining high stability in undiluted fetal bovine serum and minimal toxicity to cultured cells. This work provides important structural insights into how very subtle structural changes in polymers can yield great improvement in biological activity, specifically the inclusion of amide group in polymer backbone/sidechain enables to obtain antifouling materials with better performance for biomedical applications.

Quantitative method for evaluating fingernail induced mar damage of coatings

A traditional method of coating qualification, known as fingernail mar resistance, historically has been practiced in industry where the back of a fingernail is “flicked” across a coating surface. Any resulting permanent mark from this qualitative analysis is deemed a failure. Lack of quantitative analysis and control over experimental parameters in this type of test limits its utility as an analytical test. Fundamental understanding of this specific type of mar damage is hindered, and research efforts to find unique solutions based on changes in polymer structure are frustrated by the binary response of the test. In an attempt to rectify the situation, a high speed mar tester was developed to mimic and quantify the effects of the fingernail flick test. The work described in this report demonstrates the applicability of this tester to produce similar damage to that of the fingernail test. In addition, a quantitative method enabling differentiation of the degree of mar damage of different coatings is described. Finally, the method is demonstrated by differentiating mar resistance of coatings that show similar qualitative results in fingernail flick testing.

Two-Fluid Model for the Interpretation of Quartz Crystal Microbalance Response: Tuning Properties of Polymer Brushes with Solvent Mixtures

A new approach, involving a two-fluid model, has been developed to interpret the quartz crystal microbalance response of adsorbed viscoelastic polymers. The model utilizes the Navier–Stokes–Brinkmann equation to describe the motion of a porous, semirigid, viscoelastic polymer-brush film with a viscous solvent flowing through it. The two phases, solid (polymer brush) and liquid (solvent mixture), hydrodynamically interact with each other, as represented by means of a Darcy term with a characteristic correlation factor. The two-fluid model is used to estimate structural changes in polymer brushes consisting of the copolymers poly(l-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) or poly(l-lysine)-graft-dextran (PLL-g-dextran) adsorbed on an amorphous SiO2-coated quartz surface in aqueous solutions of glycerol, ethylene glycol (EG), and dimethyl sulfoxide (DMSO). Layer thickness, polymer volume fraction, and shear modulus of the polymer films with varying co-solvent concentration are determined with this approach. It was found that preferential hydrogen-bonding interactions of solvent mixtures with the polymers leads to variation in the structural properties of the polymer brushes upon changing the co-solvent composition. Furthermore, the conformation of polymer brushes in solvent mixtures is influenced by the solvent–solvent interactions, which can be explained in terms of the free energy of solvent mixing.

Correlation between in vitro Elastomeric Force Degradation and Glass Transition Temperature determined by Dynamic Mechanical Analysis

Objective: To determine the relation between force degradation of elastomeric chains and its glass transition temperature (Tg) by dynamic mechanical analysis (DMA). Materials and methods: Elastomeric products from two different manufacturers (Groups A and B) were compared for their force decay in an in vitro experiment for 4 weeks. Force values (F) were measured at 0 hour, 1 hour, 24 hours, 1, 2, 3 and 4 weeks. Samples were subjected to DMA for its corresponding Tg. Mean force and Tg were compared with one-way analysis of variance (ANOVA) and post-hoc Duncan multiple range test. Two-way ANOVA was used to assess the effects between Groups and different time periods. Relationship between force and Tg were studied with regression analysis. Results: Force values decreased by 48% within 1 hour and then showed a continuous decrease to cease with only 24.5 and 20.2% for Groups A and B respectively. Tg also paralleled a decrease (Group A: –19.21 ± 1.73 to –26.10 ± 1.72 and Group B: –21.09 ± 2.14 to –27.30 ± 1.82). Beta and gamma subtransitions other than main alpha transitions (Tg) were noted. Loss of rigidity and deformation of samples were thus attributed to the structural changes in polymer. A strong relationship was noticed for force values and Tg (r = 0.89 for Group A and 0.86 for Group B). Conclusion: Tg can be considered as a dependable parameter in envisaging the force properties of an elastomeric chain. Clinically, it suggested

Characterization of Electron Beam Irradiated Polyvinylpyrrolidone-Dextran (PVP/DEX) Blends

In the past years an increased interest to create new polymeric blends with application in the medical area for development of new types of biomaterials has appeared. Electron beam irradiation is well known as a method of producing important changes in polymer structure, being an alternative to chemical synthesis of biomaterials based on polymeric materials. The aim of the present study was to investigate the behaviour of some polyvinylpyrrolidone-dextran (PVP/DEX) blends under electron beam irradiation. Aqueous solutions of PVP with molecular weights of 360 000 Da (PVP 360), 40 000 Da (PVP 40), and DEX with molecular weight of 500 000 Da (DEX), were mixed as to obtain 50:50 blends of PVP40/DEX and PVP360/DEX. The obtained blends were irradiated with electron beam at different radiation doses and after irradiation treatment were processed by freeze-drying. PVP/DEX blends were characterized by infrared spectroscopy (FT-IR) and thermal analysis. The analyses were conducted in order to establish the relation between radiation dose and changes of structural and thermal properties.

A review on the importance of nanocomposite processing to enhance electrical insulation

Study of the experimental literature regarding nanodielectrics indicates numerous inconsistencies in the results obtained. In many cases, the likely cause is due to a lack of quality control during nanocomposite processing. By examining examples from the literature along with an alumina/polyamideimide nanocomposite and silica/crosslinked polyethylene nanocomposite, this contribution seeks to shed light on some of the likely causes for these inconsistencies. Measurements of the dielectric breakdown strength and voltage endurance confirm that poor dispersion can lead to poor material performance and the use of quantitative techniques is highlighted. Good dispersion alone is not sufficient to achieve improved properties. The addition of nanoparticles can alter the resulting structure of the nanocomposites, can introduce water into the system resulting in cavity formation and can also result in degradation of the polymer if the processing parameters are not carefully selected. In this review, understanding the effects of nanoparticle addition requires not only characterization of relevant dielectric properties, but also careful control of the processing parameters and characterization of changes in polymer structure, particle dispersion, and water content.

Analysis of UV absorbers and stabilizers in polypropylene by liquid chromatography/atmospheric pressure chemical ionization-mass spectrometry

Polymer degradation by outdoor weathering, heat and ultraviolet (UV) light causes chemical changes in polymer structure and affects the mechanical properties and service lifetime of the polymer. Both UV absorbers (UVAs) and stabilizers (UVSs) are used to enhance the lifetime of polymeric materials. Tinuvin 234, Tinuvin 329, and Tinuvin 360 are commonly used as UVAs, while Tinuvin 770 and Tinuvin 123 are commonly used as UVSs. A sample mixture of the three UVAs and two UVSs was separated using high performance liquid chromatography (HPLC) and the separated species were quantified using atmospheric pressure chemical ionization-mass spectrometry (APCI-MS). Separation was performed using a C18-column and gradient elution system of methylene chloride/ethanol (80/20) and methanol. Polypropylene (PP) composite containing the five additives was aged at 90 °C for 5–20 days in a convection oven. The additives remaining in the sample after the thermal aging were extracted and analyzed using LC/APCI-MS to measure the amounts of consumed additives. By using this method, the degree of consumption of UVAs and UVSs in a polymer composite by thermal aging can be estimated.

Structural, Optical And Gas Evolution Studies Of 60 MeV Si5+ Ion Irradiated PoT-PVC Blends

Swift Heavy Ion (SHI) irradiation induces chemical and structural changes in polymers by evolving various gases and gaseous fragments. The evolution of gases as a result of chain scissoring and bond breaking leads to cross-linking and cluster formation. Study of the evolved gases helps in understanding the various chemical and structural changes occurring within the polymer under the effect of SHI irradiation. In the present work, Poly (o-toluidine) (PoT), a derivative of polyaniline, is prepared by chemical oxidation polymerization and is blended with polyvinylchloride (PVC) to achieve self supported films. These PoT-PVC blend films were irradiated by 60 MeV Si 5+ ions at different fluences and evolved gases were monitored on-line by Residual Gas Analyzer (RGA). Pre and post irradiation FTIR, UV-Visible absorption and XRD studies have been carried out on these films to observe the changes in chemical/structural and optical properties. An effort has been made to correlate the evolved gases and structural properties after irradiation.Copyright © 2011 VBRI press.

A method for the determination of the thermally induced optical and structural changes of polymers used to fabricate lightpipes integrated in CMOS image sensor arrays

In this article, we will present a method to be able to determine the thermally induced optical and structural changes of polymers used to realize lightpipes integrated in CMOS image sensor pixel arrays. The reduction of the pixel size to the visible wavelength scale increases the diffraction of the transmitted light, and lightpipes can compensate this effect by improving light confinement in each pixel enhancing the resulting device external quantum efficiency. Thus, the measurements of the polymer structural and optical changes are the key parameters for the optimization of the guiding and transmission properties of the lightpipe. Spectroscopic ellipsometric measurements allow users to determine the variations of the polymer film refractive index and thickness as a function of the thermal budgets for temperatures ranging from 250 °C to 500 °C. On the other hand, thermogravimetric analysis permits users to relate the variations of these parameters to the polymer structural modifications as a function of the increasing temperature.

Selective Dispersion of Single-Walled Carbon Nanotubes with Specific Chiral Indices by Poly(N-decyl-2,7-carbazole)

Physico-chemical methods to sort single-walled carbon nanotubes (SWNTs) by chiral index are presently lacking but are required for in-depth experimental analysis and also for potential future applications of specific species. Here we report the unexpected selectivity of poly(N-decyl-2,7-carbazole) to almost exclusively disperse semiconducting SWNTs with differences of their chiral indices (n - m) ≥ 2 in toluene. The observed selectivity complements perfectly the dispersing features of the fluorene analogue poly(9,9-dialkyl-2,7-fluorene), which disperses semiconducting SWNTs with (n - m) ≤ 2 in toluene. The dispersed samples are further purified by density gradient centrifugation and analyzed by photoluminescence excitation spectroscopy. All-atom molecular modeling with decamer model compounds of the polymers and (10,2) and (7,6) SWNTs suggests differences in the π-π stacking interaction as origin of the selectivity. We observe energetically favored complexes between the (10,2) SWNT and the carbazole decamer and between the (7,6) SWNT and the fluorene decamer, respectively. These findings demonstrate that subtle structural changes of polymers lead to selective solvation of different families of carbon nanotubes. Furthermore, chemical screening of closely related polymers may pave the way toward simple, low-cost, and index-specific isolation of SWNTs.

Removal of Heavy Metals from Multicomponent Metal Mixtures by Polymer Enhanced Ultrafiltration: Effects of pH, Ionic Strength and Conformational Changes in Polymer Structure

Fractional separation of industrially important heavy metals (cadmium, nickel, zinc) from binary and ternary metal mixtures by continuous mode polymer enhanced ultrafiltration was studied. Polyethyleneimine (PEI) was used as a complexation polymer. Effects of pH and ionic strength on process efficiency were investigated. To gain insight about the characteristics of binding between highly branched PEI and metal ions and effect of salt concentration on this binding were investigated by performing dynamic and static light scattering measurements. It was observed that with optimum pH and salt concentration, fractional separation of metals can be achieved.

Requirement of NMB0065 for connecting assembly and export of sialic acid capsular polysaccharides in Neisseria meningitidis

Capsule expression in Neisseria meningitidis is encoded by the cps locus comprised of genes required for biosynthesis and surface translocation. Located adjacent to the gene encoding the polysialyltransferase in serogroups expressing sialic acid-containing capsule, NMB0065 is likely a member of the cps locus, but it is not found in serogroups A or X that express non-sialic acid capsules. To further understand its role in CPS expression, NMB0065 mutants were created in the serogroups B, C and Y strains. The mutants were as sensitive as unencapsulated strains to killing by normal human serum, despite producing near wild-type levels of CPS. Absence of surface expression of capsule was suggested by increased surface hydrophobicity and confirmed by immunogold electron microscopy, which revealed the presence of large vacuoles containing CPS within the cell. GC–MS and NMR analyses of purified capsule from the mutant revealed no apparent changes in polymer structures and lipid anchors. Mutants of NMB0065 homologues in other sialic acid CPS expressing meningococcal serogroups had similar phenotypes. Thus, NMB0065 (CtrG) is not involved in biosynthesis or lipidation of sialic acid-containing capsule but encodes a protein required for proper coupling of the assembly complex to the membrane transport complex allowing surface expression of CPS.

Computational Study of the Structure and Charge-Transfer Parameters in Low-Molecular-Mass P3HT

Using classical molecular dynamics simulations and quantum chemical calculations, the structure and charge-transfer parameters in crystalline poly(3-hexylthiophene) (P3HT) are investigated. The changes in polymer structure with temperature are studied and, by performing DFT calculations on configurations found from MD, the changes in the charge-transfer characteristics are investigated. The system is found to adopt a structure consistent with X-ray diffraction experiments on the so-called type-II polymorph of the poly(3-alkylthiophenes). Upon increasing temperature, a conformational change in the polymer side chains occurs, which is found to lead to increased disorder in the interring torsions, which modulates the charge transfer along the polymer backbone. The intrachain transfer integrals are found to decrease slightly with temperature, while their distribution broadens considerably due to increased thermal motion of the rings. The interchain transfer integrals are found to be appreciable for both nearest and next-nearest neighbor rings. This, taken with the fact that the positions of rings on neighboring chains are strongly correlated, has consequences for the development of more accurate phenomenological charge-transport models, such as variable range hopping models.

PH-sensitive immunoliposomes specific to the CD33 cell surface antigen of leukemic cells

A promising avenue in cancer therapy using liposomal formulations is the combination of site-specific delivery with triggered drug release. The use of trigger mechanisms in liposomes could be relevant for drugs susceptible to lysosomal hydrolytic/enzymatic degradation. Here, we propose a polymeric pH-sensitive liposome system that is designed to release its content inside the endosomes through a polymer structural change following receptor-mediated internalization. Specifically, pH-sensitive immunoliposomes (ILs) were obtained by including a terminally alkylated copolymer of N-isopropylacrylamide (NIPAM) in the liposome bilayer and by coupling the anti-CD33 monoclonal antibody to target leukemic cells. In vitro release of encapsulated fluorescent probes and cytosine arabinoside (ara-C) revealed that pH-sensitivity of the vector was retained in the presence of the antibody upon incubation in plasma. Flow cytometry and confocal microscopy analyses demonstrated that the pH-sensitive ILs were efficiently internalized by various CD33+ leukemic cell lines while limited interaction was found for liposomes decorated with an isotype-matched control antibody. Finally, the pH-sensitive ILs-CD33 formulation exhibited the highest cytotoxicity against HL60 cells, confirming the role of the NIPAM copolymer in promoting the escape of intact ara-C in the endosomes. These results suggest that this pH-sensitive liposomal formulation could be beneficial in the treatment of acute myeloid leukemia.

UV resonance Raman determination of molecular mechanism of poly(N-isopropylacrylamide) volume phase transition.

Poly(N-isopropylacrylamide) (PNIPAM) is the premier example of a macromolecule that undergoes a hydrophobic collapse when heated above its lower critical solution temperature (LCST). Here we utilize dynamic light scattering, H-NMR, and steady-state and time-resolved UVRR measurements to determine the molecular mechanism of PNIPAM's hydrophobic collapse. Our steady-state results indicate that in the collapsed state the amide bonds of PNIPAM do not engage in interamide hydrogen bonding, but are hydrogen bonded to water molecules. At low temperatures, the amide bonds of PNIPAM are predominantly fully water hydrogen bonded, whereas, in the collapsed state one of the two normal CO hydrogen bonds is lost. The NH-water hydrogen bonding, however, remains unperturbed by the PNIPAM collapse. Our kinetic results indicate a monoexponential collapse with tau approximately 360 (+/-85) ns. The collapse rate indicates a persistence length of n approximately 10. At lengths shorter than the persistence length the polymer acts as an elastic rod, whereas at lengths longer than the persistence length the polymer backbone conformation forms a random coil. On the basis of these results, we propose the following mechanism for the PNIPAM volume phase transition. At low temperatures PNIPAM adopts an extended, water-exposed conformation that is stabilized by favorable NIPAM-water solvation shell interactions which stabilize large clusters of water molecules. As the temperature increases an increasing entropic penalty occurs for the water molecules situated at the surface of the hydrophobic isopropyl groups. A cooperative transition occurs where hydrophobic collapse minimizes the exposed hydrophobic surface area. The polymer structural change forces the amide carbonyl and N-H to invaginate and the water clusters cease to be stabilized and are expelled. In this compact state, PNIPAM forms small hydrophobic nanopockets where the (i, i + 3) isopropyl groups make hydrophobic contacts. A persistent length of n approximately 10 suggests a cooperative collapse where hydrophobic interactions between adjacent hydrophobic pockets stabilize the collapsed PNIPAM.

Beyond Molecular Beacons: Optical Sensors Based on the Binding‐Induced Folding of Proteins and Polypeptides

Many polypeptides and small proteins can be readily engineered such that they only fold upon binding a specific target ligand. This approach couples target recognition with a considerable change in polymer structure and dynamics. Recent years have seen the development of a number of biosensors that couple these large changes to readily measurable optical (fluorescent) outputs. These sensors afford the detection of a wide variety of macromolecular targets including proteins, polypeptides, and nucleic acids. Here we describe the design of such biosensors, from the first iterations as protein engineering experiments, to the development of biosensors targeting a range of protein and nucleic acid targets.

Effect of pH on Water Proton NMR Relaxation in Agmatine-Containing Poly(amidoamine) Hydrogels

Transverse and longitudinal relaxation times of water protons were measured and magnetization transfer experiments were performed on cross-linked amphoteric poly(amidoamine)s containing different proportions of agmatine side groups swollen in buffer media with pH values in the 1.4-8.5 range in order to obtain information on water/polymer interactions as a function of pH and polymer composition. The experimental results indicated that water interacts with polymer chains mainly via hydrogen bonding to carboxylate groups and via proton exchange with amino groups. The introduction of agmatine in the polymer disfavors water/polymer interactions possibly because of polymer structural changes and competing electrostatic interactions with carboxylate groups.

Raman spectroelectrochemical study on the kinetics of electrochemical degradation of polyaniline

The kinetics of electrochemical degradation of polyaniline and a copolymer of aniline and metanilic acid have been studied by in situ Raman spectroscopy at a gold electrode. It has been concluded that probably no drastic changes in polymer structure occur on prolonged electrochemical treatment of polymer films at a high electrode potential (0.8 V vs. Ag/AgCl). Instead, most prominent changes relate to a gradual decrease of an overall intensity of spectra, viz. to gradual degradation of a polymer layer. The degradation proceeds faster at pH 1.0, compared to pH 7.0. The kinetic results obtained have been analyzed following simple 2- or 3-parameter exponential decay equations, and compared with the known degradation rate constants.

Stabilisation of metallocene ethylene-1-octene copolymers during multiple extrusions

The melt stabilising efficiency of antioxidants with different structures based on hindered phenols, phosphite esters, phosphonite and a lactone was examined during multi-pass extrusions at 265 °C in three metallocene ethylene-1-octene copolymers (m-LLDPE) having different extent of short chain branching (SCB) and one Zeigler copolymer (z-LLDPE) containing the same level of SCB corresponding to one of the m-LLDPE polymers. The effect of the different antioxidants, when used separately and in combination, was investigated by characterising the changes in the polymer's rheological behaviour, colour formation and structural changes based on unsaturated groups and carbonyl content during five multi-pass extrusions. The results showed that all stabilisation systems examined offered higher efficiency in the metallocene polymers compared to the Zeigler. The effect of the extent of SCB in the metallocene polymers on the stabilising efficacy of the antioxidant systems was also examined, and it was shown that it had a significant effect, with both single and combinations of antioxidants giving higher efficiency in the m-LLDPE polymer containing higher extent of SCB. The presence of the lactone HP136 in mixtures containing hindered phenol–phosphite antioxidant systems gave a higher melt stabilisation efficiency than in its absence and this has been attributed to a co-operative antioxidant reaction steps that take place between the antioxidants resulting in the possible regeneration of the lactone antioxidant through a redox reaction. In all the metallocene PE polymers examined, the biologically hindered phenol, Irganox E, was shown to be more effective than the conventionally hindered phenol Irganox 1076, when examined alone or in combination with phosphite esters.