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Polymers, Volume 6, Issue 12 (December 2014) – 9 articles , Pages 2896-3043

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Article
Evaluation of the Thickness and Oxygen Transmission Rate before and after Thermoforming Mono- and Multi-layer Sheets into Trays with Variable Depth
Polymers 2014, 6(12), 3019-3043; https://0-doi-org.brum.beds.ac.uk/10.3390/polym6123019 - 22 Dec 2014
Cited by 21 | Viewed by 6684
Abstract
During thermoforming, plastic sheets are heated and subsequently deformed through the application of mechanical stretching and/or pressure. This process directly impacts sheet properties such as material thickness in walls, corners, and bottom, crystallinity in the constituent layers, and particularly the oxygen gas permeability. [...] Read more.
During thermoforming, plastic sheets are heated and subsequently deformed through the application of mechanical stretching and/or pressure. This process directly impacts sheet properties such as material thickness in walls, corners, and bottom, crystallinity in the constituent layers, and particularly the oxygen gas permeability. The aim of this study was to quantify the impact of thermoforming on thickness and oxygen transmission rate (OTR) of selected packaging materials (polypropylene (PP); PP/ethylene-vinyl alcohol co-polymer/PP (PP/EVOH/PP); polystyrene/EVOH/polyethylene (PS/EVOH/PE); amorphous polyethylene terephtalate/PE (APET/PE); APET/PE/EVOH/PE; polyamide/PE (PA/PE); and (PE/)PA/EVOH/PA/PE). These materials were extruded in two different thicknesses and thermoformed into trays with the same top dimensions and variable depths of 25, 50, and/or 75 mm and a 50 mm tray with a variable radius of the corners. The distribution of the material thickness in the trays was visualized, showing the locations that were most affected by the deep drawn process. The OTR results indicate that the calculated OTR, based on a homogeneous material distribution, can be used as a rough approximation of the measured OTR. However, detailed analysis of crystallization and unequal thinning, which is also related to the tray design, remains necessary to explain the deviation of the measured OTR as compared to the predicted one. Full article
(This article belongs to the Special Issue Packaging Films)
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Article
Modified Weibull Distribution for Analyzing the Tensile Strength of Bamboo Fibers
Polymers 2014, 6(12), 3005-3018; https://0-doi-org.brum.beds.ac.uk/10.3390/polym6123005 - 19 Dec 2014
Cited by 48 | Viewed by 4439
Abstract
There is growing evidence that the standard Weibull strength distribution is not always accurate for the description of variability in tensile strength and its dependence on the gauge size of brittle fibers. In this work, a modified Weibull model by incorporating the diameter [...] Read more.
There is growing evidence that the standard Weibull strength distribution is not always accurate for the description of variability in tensile strength and its dependence on the gauge size of brittle fibers. In this work, a modified Weibull model by incorporating the diameter variation of bamboo fiber is proposed to investigate the effect of fiber length and diameter on the tensile strength. Fiber strengths are obtained for lengths ranging from 20 to 60 mm and diameters ranging from 196.6 to 584.3 μm through tensile tests. It is shown that as the within-fiber diameter variation increases, the fracture strength of the bamboo fiber decreases. In addition, the accuracy of using weak-link scaling predictions based on the standard and modified Weibull distribution are assessed, which indicates that the use of the modified distribution provides better correlation with the experimental data than the standard model. The result highlights the accuracy of the modified Weibull model for characterizing the strength and predicting the size dependence of bamboo fiber. Full article
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Article
MALDI-TOF and 13C NMR Analysis of Tannin–Furanic–Polyurethane Foams Adapted for Industrial Continuous Lines Application
Polymers 2014, 6(12), 2985-3004; https://0-doi-org.brum.beds.ac.uk/10.3390/polym6122985 - 17 Dec 2014
Cited by 34 | Viewed by 4332
Abstract
Mixed phenolic-polyurethane-type rigid foams were developed using tannin-furfuryl alcohol natural materials co-reacted with polymeric isocyanate in the proportions imposed by the limitations inherent to continuous industrial plants for polyurethane foams. A variety of different copolymerization oligomers formed. Urethanes appeared to have been formed [...] Read more.
Mixed phenolic-polyurethane-type rigid foams were developed using tannin-furfuryl alcohol natural materials co-reacted with polymeric isocyanate in the proportions imposed by the limitations inherent to continuous industrial plants for polyurethane foams. A variety of different copolymerization oligomers formed. Urethanes appeared to have been formed with two flavonoid tannin sites, mainly at the flavonoid hydroxyl group at C3, but also, although less, on the phenolic hydroxyl groups of the flavonoid oligomers. Urethanes are also formed with (i) glyoxal in the formulation, be it pre-reacted or not with the tannin; (ii) with phenolsulfonic acid and (iii) with furfural. This latter one, however, greatly favors reaction with the A-ring of the flavonoids through a methylene bridge rather than reaction with the isocyanate groups to form urethanes. All of the materials appeared to have co-reacted in a manner to form urethane and methylene bridges between all of the main components used. Thus, the tannin, the furfuryl alcohol, the isocyanate, the glyoxal and even the phenol sulfonic acid hardener formed a number of mixed species linked by the two bridge types. Several mixed species comprised of 2, 3 and even 4 co-reacted different components have been observed. Full article
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Article
Fabrication of Chitin/Poly(butylene succinate)/Chondroitin Sulfate Nanoparticles Ternary Composite Hydrogel Scaffold for Skin Tissue Engineering
Polymers 2014, 6(12), 2974-2984; https://0-doi-org.brum.beds.ac.uk/10.3390/polym6122974 - 16 Dec 2014
Cited by 18 | Viewed by 5477
Abstract
Skin loss is one of the oldest and still not totally resolved problems in the medical field. Since spontaneous healing of the dermal defects would not occur, the regeneration of full thickness of skin requires skin substitutes. Tissue engineering constructs would provide a [...] Read more.
Skin loss is one of the oldest and still not totally resolved problems in the medical field. Since spontaneous healing of the dermal defects would not occur, the regeneration of full thickness of skin requires skin substitutes. Tissue engineering constructs would provide a three dimensional matrix for the reconstruction of skin tissue and the repair of damage. The aim of the present work is to develop a chitin based scaffold, by blending it with poly(butylene succinate) (PBS), an aliphatic, biodegradable and biocompatible synthetic polymer with excellent mechanical properties. The presence of chondroitin sulfate nanoparticles (CSnp) in the scaffold would favor cell adhesion. A chitin/PBS/CSnp composite hydrogel scaffold was developed and characterized by SEM (Scanning Electron Microscope), FTIR (Fourier Transform Infrared Spectroscopy), and swelling ratio of scaffolds were analyzed. The scaffolds were evaluated for the suitability for skin tissue engineering application by cytotoxicity, cell attachment, and cell proliferation studies using human dermal fibroblasts (HDF). The cytotoxicity and cell proliferation studies using HDF confirm the suitability of the scaffold for skin regeneration. In short, these results show promising applicability of the developed chitin/PBS/CSnps ternary composite hydrogel scaffolds for skin tissue regeneration. Full article
(This article belongs to the Collection Polysaccharides)
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Article
Interfacial Properties of Methylcelluloses: The Influence of Molar Mass
Polymers 2014, 6(12), 2961-2973; https://0-doi-org.brum.beds.ac.uk/10.3390/polym6122961 - 12 Dec 2014
Cited by 12 | Viewed by 2988
Abstract
The interfacial interactions of four methylcelluloses having the same average degree of substitution and distribution of methyl groups, but different molar masses, are studied at ambient temperature and at very low polymer concentrations. Firstly, the surface tension σ at the water/air interface is [...] Read more.
The interfacial interactions of four methylcelluloses having the same average degree of substitution and distribution of methyl groups, but different molar masses, are studied at ambient temperature and at very low polymer concentrations. Firstly, the surface tension σ at the water/air interface is determined for the progressive addition of methylcellulose up to 100 mg/L; σ starts to decrease over 1 mg/L up to the critical aggregation concentration (CAC) at 10 mg/L. The curves describing the influence of polymer concentration on σ are independent of the molar mass at equilibrium. Secondly, the adsorption of methylcellulose on silica particles is estimated from ζ-potential measurements. The data are interpreted in terms of an increase of the adsorbed layer thickness at the interface when the molar mass of methylcellulose increases. It is concluded that methylcellulose is adsorbed, forming trains and loops at the interface based on the equilibrium between surface free energy and solvent quality. Full article
(This article belongs to the Collection Polysaccharides)
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Article
Charge Inversion Effects in Electrophoresis of Polyelectrolytes in the Presence of Multivalent Counterions and Transversal Electric Fields
Polymers 2014, 6(12), 2942-2960; https://doi.org/10.3390/polym6122942 - 04 Dec 2014
Cited by 1 | Viewed by 2977
Abstract
By molecular dynamics simulations we investigate the transport of charged polymers in confinement, under externally applied electric fields, in straight cylinders of uniform diameter and in the presence of monovalent or multivalent counterions. The applied electric field has two components; a longitudinal component [...] Read more.
By molecular dynamics simulations we investigate the transport of charged polymers in confinement, under externally applied electric fields, in straight cylinders of uniform diameter and in the presence of monovalent or multivalent counterions. The applied electric field has two components; a longitudinal component along the axis of the cylinder and a transversal component perpendicular to the cylinder axis. The direction of electrophoretic velocity depends on the polyelectrolyte length, valency of the counterions present in solution and transversal electric field value. A statistical model is put forward in order to explain these observations. Full article
(This article belongs to the Special Issue Polyelectrolytes 2014)
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Article
Functionalization of Liquid Natural Rubber via Oxidative Degradation of Natural Rubber
Polymers 2014, 6(12), 2928-2941; https://0-doi-org.brum.beds.ac.uk/10.3390/polym6122928 - 03 Dec 2014
Cited by 39 | Viewed by 4462
Abstract
Natural rubber (NR) is a high molecular weight natural polymer and can be degraded to liquid natural rubber (LNR) leaving certain functional groups at the end of chains. In this study, LNR samples prepared via oxidative degradation using H2O2 and [...] Read more.
Natural rubber (NR) is a high molecular weight natural polymer and can be degraded to liquid natural rubber (LNR) leaving certain functional groups at the end of chains. In this study, LNR samples prepared via oxidative degradation using H2O2 and NaNO2 as reagents were found to have different end groups depending on the pH of the reaction medium. In an acidic medium, LNR with hydroxyl terminal groups was formed as the degradation reaction was initiated by hydroxyl radicals produced from decomposition of peroxynitrite acid. In contrast, a redox reaction took place in an alkaline medium to yield LNR with carbonyl terminal groups. The mechanisms of reaction are discussed and proposed to explain the formation of different end groups when reaction carried out in acidic and alkaline media. Chain degradation in an acidic medium seems to be more effective than in an alkaline medium, and thus yields LNR with lower Mn. Full article
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Article
The Effect of Multiple Extrusions on the Properties of Montmorillonite Filled Polypropylene
Polymers 2014, 6(12), 2912-2927; https://0-doi-org.brum.beds.ac.uk/10.3390/polym6122912 - 01 Dec 2014
Cited by 38 | Viewed by 4075
Abstract
Nanocomposites have attracted a great deal of interest during recent years. Much research has been conducted towards the incorporation of clay particles in a polypropylene matrix. However, the effect of extrusion reprocessing on the material properties has not been studied in depth. In [...] Read more.
Nanocomposites have attracted a great deal of interest during recent years. Much research has been conducted towards the incorporation of clay particles in a polypropylene matrix. However, the effect of extrusion reprocessing on the material properties has not been studied in depth. In this study, composites of polypropylene (PP) reinforced with organic modified montmorillonite (MMT) (4 wt%) and coupling agent were subjected to 15 extrusion cycles. The materials were characterized by melt flow index (MFI), plate-plate rheometry, tensile testing and impact measurements, differential scanning calorimetry (DSC), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed an improved intercalation of the MMT clay in the first few extrusions, thus improving the mechanical properties. Increasing the processing steps furthermore lead to a decrease in elastic modulus and impact strength, which was attributed to a reduction in matrix-filler interaction, most probably caused by organoclay degradation as shown by thermogravimetric analysis (TGA). The results of this study provide a useful insight in either the recycling of PP/MMT waste streams or the use of clay particles as a strengthening filler agent in the recycling process of different polymer waste streams. Full article
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Article
On the Relation between Natural and Enforced Syneresis of Acidic Precipitated Silica
Polymers 2014, 6(12), 2896-2911; https://0-doi-org.brum.beds.ac.uk/10.3390/polym6122896 - 26 Nov 2014
Cited by 10 | Viewed by 4944
Abstract
Silica in industrial production processes is precipitated by mixing an acid and an inorganic precursor. In this aqueous solution, silica particles form due to a polymerization reaction and agglomeration and, finally, build a gel. Thereafter, the reaction continues, and the gel network shrinks [...] Read more.
Silica in industrial production processes is precipitated by mixing an acid and an inorganic precursor. In this aqueous solution, silica particles form due to a polymerization reaction and agglomeration and, finally, build a gel. Thereafter, the reaction continues, and the gel network shrinks with the expulsion of the enclosed pore liquid. This slow process is known as “natural syneresis” and strongly affects the product properties, such as the agglomerate size, specific surface or porosity of the silica produced. In order to investigate the influence of process parameters, such as temperature, pH or ionic strength, on the shrinkage in shorter time-scales, we propose an acceleration of this process and define it as “enforced syneresis”. The acceleration is performed by applying a mechanical external force to the gel by means of a plunger and measuring the shrinkage behavior under these conditions. Thereby, the conceptual idea is the prediction of the shrinkage due to natural syneresis based on the results of enforced syneresis. We are now able to predict the natural syneresis behavior from enforced syneresis data by the development of a correlative model. Using this prediction model, we can show the influence of temperature on the maximum shrinkage and on its rate in a significantly shorter time of about 12 h instead of several days. Full article
(This article belongs to the Special Issue Polymer Colloids)
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