Refractive index—A key to understanding color differences

A problem encountered in the instrumental evaluation of color is the use of integrating sphere spectrophotometers to compare materials where the standard color plaque is not constructed from the same material as the sample. This problem is encountered in the automobile industry, where polypropylene master plaques are commonly used as the standards constructed from different materials than the sample being evaluated. The paper demonstrates how these problems result from the impact of a polymer's refractive index on the reflectivity of light from its surface, and the effect of this light on the instrumental evaluations of color. The paper also discusses how the Fresnel equation can be utilized to calculate the amount of light reflected from a polymer's surface and describes a mathematical procedure that can help compensate for these differences when matching and approving colors.     

Stress development in supported polymer films during thermal cycling

A simple and in-situ bending-beam technique has been used to investigate the stress-temperature relationships for three different polymer coatings (viz, poly(methyl methacrylate), FR-4 epoxy resin, and an amide-imide polymer) during thermal cycling. With this technique, we were able to detect stress relaxation near the polymer's glass transition or caused by cracking of the polymer. Knowing the Poisson's ratio and Young's modulus for the polymer, this technique also allows calculation of the polymer's thermal expansion coefficient from measured thermal stress data. The calculated thermal expansion coefficient of poly(methyl methacrylate) (7 × 10^?5/°C) is in good agreement with literature values (5 to 9 × 10^?5/°C).     

A molecular thermodynamic model for binary lattice polymer solutions

A molecular thermodynamic model for binary lattice polymer solutions with concise and accurate expressions for the Helmholtz energy of mixing and other thermodynamic properties is established. Computer simulation results are combined with the statistical mechanics to obtain the expressions. Yan et al.'s model for Ising lattice and the sticky-point model of Cumming, Zhou and Stell are incorporated in the derivation. Besides the nearest neighbor cavity correlation function obtained from the Ising lattice, the long range correlations beyond the close contact pairs are represented by a parameter λ, the linear chain-length dependence of which is obtained by fitting the simulated critical parameters of two systems with chain lengths of 4 and 200. The predicted critical temperatures and critical compositions, spinodals and coexistence curves as well as internal energies of mixing for systems with various chain lengths are in satisfactory agreement in comparison with the computer simulation results and experimental data indicating the superiority of the model over other theories. The model can serve as a basis to develop more efficient models for practical applications.     

Effect of the introduction of polydimethylsiloxane on the foaming behavior of block‐copolymerized polypropylene

We investigated the effect of polydimethylsiloxane (PDMS) on the foaming properties of block‐copolymerized polypropylene (B‐PP) by blending different contents of PDMS with B‐PP in the extrusion process using supercritical CO₂ as the blowing agent. The experimental results indicate that the addition of PDMS greatly increased the expansion ratio of the foamed samples. At the same time, the cell population density of foams obtained from the blends also increased to a certain degree and provided a new perspective on improving B‐PP's foaming performance. The addition of PDMS also decreased the die pressure because of the reduced viscosity of the B‐PP/PDMS blends compared with that of the B‐PP matrix. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Thermal stability of a novel poly(ether ether Ketone Ketone) (PK99)

The degradation of PK99 was studied by heating the polymer in both isothermal and dynamic conditions under nitrogen flow and in a static air atmosphere. In both environments this polymer's thermal stability exceeded that of PEEK. The dynamic experiments showed that in both nitrogen and air atmospheres the polymer degraded in two stages with formation of the same residue, which was quite stable under nitrogen, while almost completely burned in air. Good agreement was observed with the isothermal experiments. Apparent activation energy values were evaluated. The double linear relationships obtained by the Kissinger equation suggested a complex degradation process.     

Electron beam effects on polymers. II. Surface modification of bis-GMA substrates by functionalized siloxane oligomers

Surface modification of polymer films via electron beam irradiation was studied using the methacrylic acid derivative of the diglycidyl ether of bisphenol A, commonly called bis-GMA, as a curable substrate. Functionalized polydimethylsiloxane (PDMS) oligomers were utilized as surface modifiers. Considerable changes in the wetting characteristics were observed for the siloxane modified bis-GMA surfaces by critical surface tension measurement. For dosages up to 5 Mrads, the dosage level strongly affects the critical surface tension of the polymer. This result implies differences in the concentration of the attached PDMS oligomers to the bis-GMA substrate. The methacrylate-terminated PDMS was observed to be more effective in lowering the critical surface tension than a similar vinyl-terminated PDMS. Higher molecular weight and multifunctional PDMS coatings resulted in somewhat lower critical surface tensions in the dosage range applied in this study. The surface thickness of the functionalized PDMS coatings which were bonded to the substrate surface depended on the molecular weight of the surface modifiers as obtained by XPS analysis. Peel tests of the uncoated and PDMS coated bis-GMA clearly resulted in agreement with the critical surface tension data. Chemical inertness and poor wettability of PDMS provided the PDMS coatings with enhanced resistance to chemical degradation caused by a 24-h exposure to aqueous nitric and acetic acid.     

The Thermal Degradation Behaviour of a Series of Siloxane Copolymers - a Study by Thermal Volatilisation Analysis

The thermal degradation behaviour of novel high number average molecular mass polysilalkylenesiloxanes is reported. These have been synthesised using anionic ring-opening polymerisation of 1,1,3,3,14,14,16,16-octamethyl-2,15-dioxa-1,3,14,16-tetrasilacyclohexacosane and octamethylcyclotetrasiloxane (D4) mixtures. The thermal degradation behaviour of these materials was evaluated by a combination of thermogravimetric analysis (TGA) and thermal volatilisation analysis (TVA) and compared with a commercial sample of PDMS. The results demonstrated that the thermal degradation of the polysilalkylenesiloxanes is more complex than the PDMS, with the polysilalkylenesiloxanes exhibiting a lower degradation peak maximum temperature. The major volatile degradation products evolved from the PDMS were identified as D3 to D6 cyclic siloxane oligomers, in addition to higher molecular mass cyclic siloxane oligomers. The polysilalkylenesiloxanes, on the other hand, evolved short chain aliphatic hydrocarbons, cyclic and linear siloxane oligomers and silanes. The TVA results indicate that the polysilalkylenesiloxanes degrade mostly by random chain scission of the polymer backbone, whereas the commercial PDMS degrades by the accepted depolymerisation reaction which involves “back-biting” reactions.     

Heat capacity of polymer melts from the polymer chain-of-rotators equation of state

Recently, the chain-of-rotators equation of state derived from the rotational partition function was extended to polymers. Values of the three equation of state (EOS) parameters were obtained from fitting with experimental pressure–volume–temperature data and the parameters were correlated with the structure of the polymer repeat unit. In this article, the residual molar heat capacity derived from an EOS is added to the ideal gas heat capacity from Benson's group contribution method to obtain the polymer molar heat capacity at constant pressure, C_p. Predictions from the polymer chain-of-rotators (PCOR) using correlated parameters are compared with those obtained from PCOR, Sanchez–Lacombe, Flory–Orwoll–Vrij, and the perturbed-hard-sphere chain equations of state using parameters fitted from experimental data. Deviations of calculated C_p from the formula of van Krevelen for liquid polymers are likewise presented. With the correlations developed for its parameters, the PCOR offers the advantage of predicting the C_p for polymer melts from just the knowledge of the polymer's structure. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:841–848, 1998     

Influence of processing and sterilisation on properties of poly-ϵ-caprolactone

Abstract

Poly-?-caprolactone (PCL) is a significant member of a group of polymers regarded as bioabsorbable, having been the focus of extensive research for use as a diffusion controlled drug delivery system. Degradation of PCL proceeds through hydrolysis of the ester bonds in the polymer chains and is influenced significantly by the polymer's initial molecular weight and crystallinity. To evaluate the effects of processing and sterilisation on these properties, PCL pellets (CAPA 6400) were either injection moulded or extruded and sterilised by ethylene oxide gas (EtO). Procedures were used to evaluate mechanical properties, molecular weight and crystallinity. Upon processing and sterilisation the molecular weights of the injection moulded and extruded materials did not differ significantly from that of the PCL pellets, suggesting processing and sterilisation did not initiate chain scission of the polymer's ester bonds. However, the crystallinity of PCL proved to be sensitive to injection moulding with an increase of approximately 5% observed after processing, with sterilisation by EtO gas causing annealing of the PCL pellets, injection moulded and extruded material. After sterilisation the crystallinity of the PCL pellets and extruded material increased by approximately 10% with a 4% increase observed for the injection moulded material. The mechanical properties of both the injection moulded and extruded material where not influenced by sterilisation. The results from this investigation suggest that PCL's molecular weight is insensitive to processing and sterilisation. However, sterilisation by EtO gas in the temperature range of 38-48°C used in this study does result in annealing of the polymer.     

Effects of solvent and thermal annealing on the dielectric properties and morphology of dimethyl siloxane/α-methylstyrene block copolymers

The dielectric and morphological properties of a series of 9/1 poly(dimethyl siloxane) PDMS)/poly(α-methylstyrene) (PαMS) block copolymers and a 6/4 PDMS/PαMS block copolymer have been determined as a function of solvent casting and thermal treatment. Transmission electron microscope (TEM) results show better phase separation as a function of thermal annealing and casting from cyclohexane, a PDMS preferential solvent. Dielectric studies in the temperature region of the PDMS glass transition are consistent with the TEM results and are interpreted in terms of a most probable distribution of PDMS/PαMS mixed states. When the PDMS segment molecular weight is less than the critical molecular weight, thermal annealing of the solvent cast samples produces a phase separated sample exhibiting the T_g of PDMS as well as a mixed phase. Thermal annealing of samples with the PDMS segment molecular weight greater than the critical molecular weight produces little change in the mixed structure and the dielectric data. All samples have PαMS segment molecular weights less than the critical molecular weight. The observed changes are interpreted in terms of kinetic effects associated with polymer melt viscosity (critical molecular weight) and expected mixing of the two components.     

Functionalizing polymer surfaces by field‐induced migration of copolymer additives—role of shear fields

Flow‐induced migration polyethylene‐co‐methacrylic acid (PE‐co‐MA) and polystyrene‐b‐polydimethylsiloxane (PS‐b‐PD MS) copolymer additives in commercial long‐chain branch polyethylene (PE) and narrow‐molecular distribution polystyrene (PS) hosts was investigated in a capillary flow device. Attenuated Total Reflection Fourier Transform Infrared (ATR‐FTIR) spectroscopy and Dynamic Contact Angle (DCA) measurements were used to characterize surface composition of polymer specimen following extrusion through metallic dies with various length‐to‐diameter (L/D) ratios, (1100 ? L/D ? 3000). Results from experiments covering a broad range of shear rates and polymer residence times in the dies are reported. Provided that the polymer residence time in the die is sufficiently long, shear is found to increase the concentrations of low molecular weight copolymer additives on the host polymer's surface. The surface composition of copolymer additive is found to vary strongly with the wall shear rate and die L/D ratio. Decreasing the die diameter at fixed flow rate is found, for example, to be a more effective method for enhancing transport of additive to a polymer's surface than increasing shear rate at fixed diameter. A mechanism based on shear‐induced diffusion is proposed to explain the observed migration.     

Estimation of the oxygen‐limiting amount consumed by polymeric melt in the thermooxidative degradation process

Fractal analysis representations were shown to elucidate quantitative relationships between the parameters of thermooxidative degradation and polymeric melt structure. The process of oxidative consumption during degradation was defined by both the polymer's chemical constitution and the structure of the macromolecular coil. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2840–2844, 2004     

Study on droplet deformation of an immiscible fluid system by selective radiation heat using infrared laser

The droplet deformation's mechanism in a simple shear flow is well understood, but most studies only considered a uniform temperature field of immiscible droplet systems. This research investigates the possible droplet deformation and visualizes its evolution under selective radiation heating, which is a novel method to modify droplet dispersion via thermo-physical mechanism. Polybutenes and PDMS silicone oils were employed as the dispersed droplet phase and matrix phase, respectively. Results showed that the mean value of the droplet's modified deformation parameter was limited to 0.12 and 0.08 at an isothermal ambient room temperature having a viscosity ratio of 4.32 and 17.69, respectively. The droplet under selective radiation was visualized. Experimental investigation showed that the selective radiation method enhanced the droplet deformation, as confirmed by the obtained droplet image during laser irradiation process, even allowing droplet breakup. At a moderate level of viscosity ratio (4.32), selective radiation can be applied effectively, whereas at a higher level of viscosity ratio (17.69), droplet elongation-breakup and droplet retraction were observed.     

Relationship between molecular structure and zero‐shear viscosity of polymers

The relationship between molecular structure and zero‐shear viscosity of polymers was studied. In this study we propose a new equation, which is based on Berry and Fox's equation. This new equation is constructed from some molecular parameters, such as mean square length and average molecular weight of statistical skeletal unit, characteristic ratio, entanglement molecular weight, glass‐transition temperature, free volume fraction at glass‐transition temperature, and thermal expansion coefficient of free volume. It is proposed that some of these molecular parameters could be predicted by group contribution methods, except for the free volume parameters. We also propose new empirical relations between free volume parameters and molecular structures of polymers, which make it possible for free volume parameters to be obtained from molecular structure. Using these relationships, it is possible that the zero‐shear viscosity and its temperature dependence are obtainable from the molecular structure of polymers. We applied this formula to some polymers, including both amorphous and semicrystalline polymers. Comparison between the measured and calculated zero‐shear viscosity showed quite good agreement. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1609–1618, 2001     

The role of surfactant structure and monoethanolamine in the environmental stress cracking of polycarbonate

The critical strains required to initiate cracking of polycarbonate exposed to a number of poly(oxyethylene)ethoxylate surfactants were determined. Solubility parameters of the surfactants were calculated from knowledge of the molecular structure. A model proposed by Jacques and Wyzgoski that uses the square of the solubility parameter difference of the surfactant and polycarbonate and the surfactant molar volume was determined to be useful for predicting critical strains for polycarbonate. A major assumption in this model is that stress cracking is related to swelling or plasticization of the polymer by the cracking agent, which ultimately leads to the polymer's failure. However, the model does not predict the observed strong stress cracking of polycarbonate by monoethanolamine. In this investigation it was determined that polycarbonate is chemically degraded by monoethanolamine. This degradation is sufficient to initiate stress cracking at lower strains than would otherwise be predicted by solubility parameter and molar volume concepts. With the knowledge obtained from this investigation, it is possible to predict which poly(oxyethylene)ethoxylate surfactants are stress cracking agents for polycarbonate.     

The viscosity effect on autoacceleration of the rate of free radical polymerization

The reaction kinetics at 70°C were investigated for the suspension polymerization of methyl methacrylate initiated by benzoyl peroxide in the presence of variable amounts of dodecyl mercaptan. A dilatometric method designed to follow a suspension polymerization was used. It showed that the autoacceleration of the rate of polymerization begins at higher conversions and becomes less pronounced as the concentration of chain transfer agent is increased. The investigations focused on the determination of the viscosities of the reaction mixtures at the onset of autoacceleration. It was concluded from the flow curves obtained for different reaction mixtures that there exists a critical solution viscosity at which the autoacceleration begins, which supports the accepted theory about the nature of this phenomenon. Measured at a shear rate of 10,000 s^?1 and at 25°C this viscosity was found to equal 0.6 poise regardless of the molecular weight of the growing polymer.     

Thermal Degradation Behavior and Product Speciation in Model Poly(dimethylsiloxane) Networks

The thermal degradation behavior of a series of well defined poly(dimethylsiloxane) (PDMS) model networks has been studied using a combination analytical thermal analysis techniques and multivariate statistical analysis in order to probe the influence of network architecture on degradation chemistry. The aim of this research has been to determine the effect differing network architectures: mono and bimodality, a range of crosslink density, inter-chain molar mass and percentage of free chain ends on the mechanisms of PDMS thermal degradation. A series of model PDMS networks have been formulated using of tin catalyzed condensation cure chemistry and a range of linear precursors to yield a matrix of model network systems. The thermal degradation chemistry of these model networks have been characterized in relation to their structure by means of pyrolysis gas chromatography mass spectrometry (Py-GCMS), thermal gravimetric analysis (TGA) and multivariate statistical analysis. The results clearly demonstrate that the structural architecture of (chemically similar) PDMS networks has a significant impact on the mechanisms of PDMS thermal degradation. Notability, with decreasing inter-crosslink chain length, larger cyclic siloxane species (>D5) become more abundant degradation products and that there is a relationship between inter-chain molar mass, degree of crosslinking and the thermal stability on the mechanisms of degradation. This work effectively demonstrates that quantifiable relationships exist between basic network architectures and the distributions of degradation derived species in PDMS networks.     

Cyclization dynamics of polymers: 7. Applications of the pyrene excimer technique to the internal dynamics of poly(dimethylsiloxane) chains

Poly(dimethylsiloxane) (PDMS) polymers end-capped on both ends with pyrene chromophores have been synthesized. Rate constants for end-to-end cyclization 〈 k₁ 〉 have been determined for dilute solutions of these polymers in toluene solution using a combination of fluorescence decay and steady-state fluorescence measurements. While precise values of the critical exponent for the chain length (N¯) dependence of 〈 k₁ 〉 are not yet available, these results are consistent with the N¯^{? 3/2} dependence predicted by Wilemski-Fixman theory. PDMS chains cyclize somewhat more than two times faster than polystyrene chains of the same length in solvents of similar solvating power and viscosity. These results provide strong support for similar predictions made several years ago by Perico and Cuniberti, who used intrinsic viscosity data to parametrize the Rouse-Zimm model for analysis of polymer cyclization dynamics.     

Stapled Peptide Inhibitors of Autophagy Adapter LC3B

A growing body of evidence suggests that autophagy inhibition enhances the effectiveness of chemotherapy, especially in difficult-to-treat cancers. Existing autophagy inhibitors are primarily lysosomotropic agents. More specific autophagy inhibitors are highly sought-after. The microtubule-associated protein 1A/1B light chain 3B protein, LC3B, is an adapter protein that mediates key protein-protein interactions at several points in autophagy pathways. In this work, we used a known peptide ligand as a starting point to develop improved LC3B inhibitors. We obtained structure-activity relationships that quantify the binding contributions of peptide termini, individual charged residues, and hydrophobic interactions. Based on these data, we used artificial amino acids and diversity-oriented stapling to improve affinity and resistance to biological degradation, while maintaining or improving LC3B affinity and selectivity. These peptides represent the highest-affinity LC3B-selective ligands reported to date, and they will be useful tools for further elucidation of LC3B's role in autophagy and in cancer.     

Nylon-6 containing metal halides II: Tensile properties

Following the reported pronounced effects of metal halides incorporated into nylon-6 on its glass transition temperature, melting temperature, melt viscosity, crystallization rate and final crystallinity, the tensile mechanical properties and crystallinity of nylon-6 as affected by the absorption of metal halides were studied. At low salt content, some of the mechanical properties are substantially altered and at higher salt concentrations the ductile polymer becomes brittle. There is no marked difference between the salts studied (CuCl₂, CuBr₂, FeCl₃) regarding their effects on the polymer mechanical properties. In the concentration range studied, absorbed Cu salts do not significantly change the polymer's degree of crystallinity, whereas absorbed FeCl₃ results in a pronounced reduction, indicating its effect also on the crystalline phase.