Non-crosslinked Polymers Research Articles

Update of quality control tests for new PV encapsulation materials

Although ethylene vinyl acetate copolymer (EVA) is still the dominant PV encapsulant, polyolefins (PO) have gained market share in recent years. Like EVA, PO can be mixed with a cross-linking agent and chemically reacted during the lamination process to form polyolefin elastomers (POE). The cross-linked and therefore insoluble fraction of the encapsulation can be determined by extracting the soluble part of the material with a solvent. The degree of cross-linking, or gel content (GC), is determined from the mass ratio before and after extraction of the samples. The analytical methods for determining the GC value are described for example in IEC 62788-1-6 and the Chinese national standard GB/T29848-2018. We observe a significant difference between the test results of these two standards, with the GC values resulting from the GB standard test being significantly higher. This is particularly true in the low GC range. The main reason for this difference is the extraction time of 5 h specified in the GB standard, which is insufficient to ensure complete extraction of the non-crosslinked polymers at lower GC values. However, for certain POE materials the required minimum GC (50%-65%) is exactly in this range.

In-depth analysis of reaction kinetics parameters of phenolic resin using molecular dynamics and unsupervised machine learning approach

Reaction kinetics parameters of a material depend on energy dynamics based on breaking and formation of all the types of bonds in the system. While employing molecular dynamics simulation, it can become tedious and a complicated job to use all the bond information for extracting reaction kinetics parameters. With this understanding, in the current study, the use of an unsupervised machine learning technique is demonstrated for extracting the reaction kinetics parameters from the molecular dynamics simulation of an ablative material. Molecular dynamics simulations are performed on crosslinked and non-crosslinked polymers in temperature regime where they would undergo pyrolysis decomposition. Non-negative Matrix Factorization (NMF) technique is used to reduce the bonding environment, obtained during the simulations, to the concentration profiles of a few principal components. A comparative analysis performed with polymers having different degrees of crosslinking reveals that the activation energy reduces with increase in the degree of crosslinking. The effect of heating rate on the reaction kinetics of phenolic polymers during the pyrolysis simulation is investigated in detail. The assumption of chemical equilibrium between gases and porous solid domain is frequently made in continuum level thermal response solvers. It is unknown if this assumption significantly affects the calculated reaction kinetics parameters. To understand the same, a molecular dynamics simulation, which eliminates the generated gas molecules in a systematic manner throughout the pyrolysis process, is carried out. Furthermore, to demonstrate the usefulness of reaction kinetics parameters extracted after manifestation of chemical equilibrium at microscale level, a one-dimensional heat conduction analysis is performed. The results obtained by not considering the gas particles in reaction modeling agree well with experiments. At the end, a multiscale thermal response analysis is performed over an axi-symmetric geometry for which, a relation is derived between pyrolysis gas species and solid material density evolution from MD simulations. Based on the relation, the axi-symmetric domain is segregated into different regions for their contribution in changing pyrolysis gas composition by either adding or consuming gas species.

Small molecule diffusion in poly-(olygo ethylene glycol methacrylate) based hydrogels studied by fluorescence correlation spectroscopy

Understanding just how solutes such as small molecules, polymers or proteins diffuse in “smart” hydrogels is a key factor in developing potential applications. In this work, we used fluorescence correlation spectroscopy (FCS) to study how structural parameters (length of side groups, network density) influence the diffusion of small dye molecules in poly (oligo (ethylene glycol) methyl ether methacrylates) (POEGMAs) hydrogels. Two diffusion components were found and attributed to (i) Fickian-like diffusion slowed only by steric effects, and (ii) diffusion slowed by interactions with the polymer. The relationship between the diffusion component (i) and the polymer concentration showed two regimes with different slopes in both hydrogels and solutions of non-crosslinked polymers, additionally studied for reference. The threshold between these regimes depended on the polymer hydrophilicity/hydrophobicity and the presence of cross-links. The release time of the model drug (ibuprofen) proved to be shorter in the case of denser hydrogel networks than for looser ones. This means that the drug affinity to the polymer network is the crucial parameter determining release processes, while the diffusion factor is of secondary and minor importance.

Extraction of Thermal Properties of Organic Ablative Materials Using Molecular Dynamics Simulations

In this study, molecular dynamics method is used for extracting thermal properties of materials relevant to the ablative type of thermal protection system. The methodology and results are discussed for extracting thermal conductivity and specific heat capacity of ablative materials for their virgin and char states in a manner that is defined in the experimental data sets. To select a suitable nonreactive force field, Green–Kubo simulations are performed over phenol, toluene, and benzene materials using Amber, Dreiding, and all-atom optimized potentials for liquid simulations (OPLSAA) force fields. Next, crosslinked and non-crosslinked polymers are created, and pyrolysis simulations are performed over them using ReaxFF to obtain virgin and char states of the polymers. Thermal conductivity is extracted using the Green–Kubo method, whereas specific heat capacity is extracted by using the enthalpy gradient obtained by performing isobaric simulations over virgin and char materials. Later, by introducing the concept of an artificial composite material, the extracted thermal properties are used in macroscale one-dimensional heat conduction simulations with pyrolysis using an in-house finite element method (FEM)-based thermal response solver.

An efficient preparation of porous polymeric microspheres by solvent evaporation in foam phase

Abstract This paper reports an efficient method of preparing porous polymeric microspheres by solvent evaporation in foam phase, in which phase separation between polymer and porogen occurs in foam phase instead of that in water phase by using the traditional solvent evaporation method. The method provides outstanding features, including being time-saving, of high-yield and able for continuous production, in which formation of porous polymeric microspheres finished within 3 min with a high production yield up to approximate 95 wt% and the process was able to be developed into a continuous process for production of porous polymeric microspheres. It was also universal to non-crosslinked polymers since the method is a development on the traditional emulsion solvent evaporation method. The new method is efficient and can be used potentially on the industrial scale for continuous production of porous polymeric microspheres.

Morphology control of shell-crosslinked polymer particles prepared by photo-induced shell-selective crosslinking approach via dispersed state control

HypothesisMorphology control of polymer particles is of great importance for creating functional particulate materials. Photoinduced shell-selective crosslinking approach with spherical polymer particles has been previously developed in a dispersed state, in which the polymer particles could rotate freely, yielding hollow polymer particles after removal of non-crosslinked polymers because photoinduced crosslinking occurred for the entire particle surface. This insight prompted the question of whether non-spherical particles can be obtained if particle rotation is restricted and photoirradiation occurs for only part of the shell region. ExperimentsHerein, we developed a facile approach for morphology control of polymer particles prepared by photoinduced shell-selective crosslinking of spherical polymer particles bearing photoreactive cinnamoyl groups by regulating the dispersed state. The morphology of the polymer particles obtained from the photo-induced shell-selective crosslinking approach in dispersed or non-dispersed states was confirmed. In-situ real time observation of forming the non-spherical polymer particles was also performed by optical microscope. FindingsHollow and bowl-shaped polymer particles were successfully obtained from same spherical polymer particles bearing photo-crosslinking groups in dispersed and non-dispersed states, respectively. The versatility of the approach was confirmed by using various comonomers. Furthermore, post-functionalization for the polymer particles was also successfully performed using functional groups derived from the comonomers.

Molecular alignment, large surface deformations and hysteresis effects in polydomain LC polymer films under an in-plane DC electric field

The response of multidomain liquid crystal polymer films to prolonged exposure to an in-plane DC electric field is investigated. Non-crosslinked polymers show a rotation of the mesogenic units along the field lines, but the rotation is asymmetric between the positive and negative electrodes. Crosslinked films respond by undergoing large surface deformations, with deformation profiles that correspond with the observed alignment profiles in non-crosslinked films, including asymmetry. Additionally, a strong hysteresis effect is observed, where subsequent exposures to the field lead to stronger and faster deformations.

Quartz crystal microbalance electrode modified with thermoresponsive crosslinked and non-crosslinked N-isopropylacrylamide polymers. Response to changes in temperature

Electrochemical quartz crystal microbalance (EQCM) electrode was modified with environmentally sensitive polymers. The polymer layers on the electrode were composed of either crosslinked or non-crosslinked thermoresponsive poly(N-isopropylacrylamide). For anchoring thin gel films on the electrode surface, the electrochemically induced free radical polymerization (EIFRP) was employed. The electroreduction of the peroxydisulfate anion led to generation of free radical. This radical initiated the free radical polymerization process that led to the formation of a thin gel layer attached to the electrode surface. To monitor in situ the growth of the polymer film the changes in resonant frequency of the quartz crystal were recorded. A significant decrease in the frequency (growth of the layer) was seen in the potential range where the reduction of peroxydisulfate anion took place. The morphology of the layers was examined with a scanning electron microscopy (SEM). The phenomenon of volume phase transition (shrinking/swelling process) in the gel layers initiated by a change in temperature was investigated. Unexpected big, sharp, and of negative sign minima appeared at the change-in-frequency vs. temperature plots. These minima were not seen in the plots obtained for the polymers without linker. That situation should be useful in the investigation of chemical interactions proceeding under the conditions of volume phase transition. The influence of volume phase transition on the transport of a member of a model red-ox system, ferro- and ferricyanide couple, and therefore on height of its voltammetric response was examined. The changes in electrochemical properties of the layers induced by volume phase transition were monitored with electrochemical impedance spectroscopy.

Antimicrobial activity of poly(vinyl alcohol)-poly(acrylic acid) electrospun nanofibers

Electrospun nanofibers were prepared from blends of poly(acrylic acid) (PAA) and poly(vinyl alcohol) (PVA). The fibers were stabilized by heat curing at 140°C via anhydride and ketone formation and crosslinking esterification. The antimicrobial effect was assessed using strains of Escherichia coli and Staphylococcus aureus by tracking their capacity to form colonies and their metabolic impairment upon contact with PAA/PVA membranes. Membranes containing >35wt.% PAA displayed significant antibacterial activity, which was particularly high for the gram-positive S. aureus. All membranes were negatively charged, with surface ζ-potential in the (⿿34.5)⿿(⿿45.6)mV range, but the electrostatic interaction with the negatively charged cells was not the reason for the antimicrobial effect. Neither pH reduction nor the passing of non-crosslinked polymers to the solution affected microbial growth. The antibacterial activity was attributed to the chelation of the divalent cations stabilizing the outer cell membrane. The effect on gram-positive bacteria was attributed to the destabilization of the peptidoglycan layer. The sequestration of divalent cations was demonstrated with experiments in which calcium and a chelating agent were added to the cultures in contact with membranes. The damage to bacterial cells was tracked by measuring their surface charge and the evolution of intracellular calcium during the early stages after contact with PAA/PVA membranes.

Synthesis, characterization, and polymerization of a novel benzoxazine based on diethyltoluenediamine

ABSTRACTA novel aromatic diamine‐based benzoxazine monomer (PDETDA) was successfully prepared from diethyltoluenediamine (DETDA), phenol, and paraformaldehyde through a simple one‐step solvent‐less method. The structure of PDETDA was confirmed by FTIR, 1H NMR, and 13C NMR. The curing behavior of PDETDA was studied by DSC, FTIR, and rheological measurement. The results showed that the alkyl substituents on the benzene ring in DETDA not only facilitated the synthesis of PDETDA by effectively hindering the formation of triazine network, but also endowed PDETDA with the advantage of low viscosity (1 Pa s at 90°C). However, steric hindrance of the substituents made PDETDA difficult to form a crosslinked network through ring‐opening polymerization, and therefore only oligomers and noncrosslinked polymers were obtained. The curing kinetics of PDETDA was studied by nonisothermal DSC, and the results revealed that the curing of PDETDA displayed autocatalytic characteristic. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41920.

Synthesis, oxidation and crosslinking of tetramethyl bisphenol F (TMBPF)-based polymers for oxygen/nitrogen gas separations

Amorphous, high glass transition, crosslinkable poly(arylene ether)s for gas purification membranes have been synthesized. The polymers include a moiety capable of several oxidation reactions and UV crosslinking. Structural identification was confirmed by 1H NMR and IR spectroscopy and molecular weights were determined by SEC. Two oxidation reactions of the polymers were identified, one by chemical treatment using Oxone and KBr and one by elevated thermal treatment in air. DSC and TGA were used for thermal characterization, and TGA, 1H NMR and ATR-FTIR revealed the progress of the thermal oxidation reactions. Both polymers produced tough, ductile films and gas transport properties of the non-crosslinked linear polymers and crosslinked polymer are compared. The O2 permeability of one exemplary non-crosslinked poly(arylene ether) was 2.8 Barrer, with an O2/N2 selectivity of 5.4. Following UV crosslinking, the O2 permeability decreased to 1.8 Barrer, and the O2/N2 selectivity increased to 6.2.

Study on the Reaction of CCl2F2with CH4in a Dielectric Barrier Discharge Nonequilibrium Plasma

The reaction of CCl2F2 (CFC-12) with methane (in an argon bath gas) in a dielectric barrier discharge (DBD) non-thermal plasma reactor at atmospheric pressure has been investigated. The reaction (where oxygen and nitrogen are excluded from the feed gas) produced a range of gas phase products, including H2, HCl, HF, CHF3, CF3CH2F, CHClF2, CH3Cl, CH2ClF, CH2Cl2, CHCl2F and CHCl3 as well as solid products which were primarily non-crosslinked polymers. The conversion of CCl2F2 increased, from 44 to 71%, when the input energy density of the plasma was raised from 3 to 13 kJ · L−1 (applied voltage range is 13.4–14.8 kV, peak-peak). The relatively low input feed concentration of CCl2F2 and methane in the argon bath gas modifies the strong filamentary appearance of the pure argon discharge to a more diffuse appearance.

Polyterpenes by ring opening metathesis polymerization of caryophyllene and humulene

Ring opening metathesis polymerization of the natural sesquiterpenes caryophyllene and humulene, optionally complemented by exhaustive post-polymerization hydrogenation, yields non-crosslinked linear polymers with unprecedented microstructures reflecting the specific scaffolds of the feedstocks and with low glass transition temperatures in the range from −15 to −50 °C.

Mean field theory for a reversibly crosslinked polymer network

We present a mean field theory for melts and solutions of reversibly crosslinked polymers. In our model, crosslinks are considered as local bonds between two monomers. For a blend of A+B+AB polymers, we assume reversible crosslinks between the copolymers AB with a crosslink strength z and interaction weights ω(A) and ω(B) for monomers of type A and B, respectively. The usual mean field model for polymer blends without reversible crosslinks is recovered if z vanishes. With or without crosslinks, the A+B+AB blend can form a lamellar phase with A and B rich regions. If reversible crosslinks are enabled and ω(A) differs strongly from ω(B), the lamellar nanophase separation of A and B monomers is accompanied by a similar segregation of crosslinked and noncrosslinked polymers. If ω(A) and ω(B) are equal, crosslinked copolymers are well mixed with the homopolymers. For a homopolymer solution with reversible crosslinks between the polymers, our calculations show that polymers and solvent molecules are separated macroscopically if the Flory-Huggins interaction parameter and the crosslink strength are suitably high or if the volume fraction of polymers or the chain length are suitably low.

Design principles for cytokine-neutralizing gels: Cross-linking effects

Constructs composed of cytokine-neutralizing antibodies conjugated to high-molecular-weight hyaluronic acid have been shown to be effective at controlling inflammatory responses in vivo. A critical question in the development of this new class of biomaterial is whether crosslinked conjugates have similar anti-inflammatory effects, which would open up a broad range of tissue engineering applications in which the material would have intrinsic inflammation-controlling function. To test this, high-molecular-weight hyaluronic acid was conjugated with monoclonal antibodies to the pro-inflammatory cytokines interleukin-1β and tumor necrosis factor-α in two forms of the material: viscous, non-crosslinked polymer-antibody conjugates and crosslinked, elastomeric polymer-antibody conjugates. The cytokine affinities of both constructs were validated using molecular characterization methods, and the biological activities were tested through subcutaneous implantation in Sprague-Dawley rats. In vitro, both forms of these constructs are capable of binding cytokines, but in vivo only the non-crosslinked polymer significantly reduces markers of acute inflammation compared to controls that lack the antibodies. We propose that these materials function by retarding cytokine diffusion, with the non-crosslinked polymers being capable of retarding the diffusion of cytokines in the extracellular matrix and preventing engagement with receptors. In contrast, crosslinked materials have long diffusion lengths into the gel compared with those between cells on the surface of the material, which may make them ineffective at sequestering pro-inflammatory cytokines on biologically relevant timescales. These results suggest an important design principle for preparing cytokine-regulating materials based on consideration of transport phenomena.

The effect of molecular weight and temperature on tack properties of model polyisobutylenes

The effect of molecular weight and temperature on the tack of non-crosslinked soft polymers was investigated using blends of low and high molecular weight polyisobutylene as model systems. Molecular weight between entanglements M e was varied systematically by changing the mixing ratio. Video-optical imaging confirms that these model systems exhibit the characteristic debonding features of pressure-sensitive adhesives, including cavitation as well as formation and deformation of fibrils. The initial peak in the stress–strain diagram is a function of both the cohesive strength and ability to wet the probe. It contributes significantly to the work of adhesion at low molecular weight, low temperature and high contact force. Beyond a critical molecular weight, debonding is dominated by fibril deformation. In this regime, the work of adhesion is essentially independent of temperature, molecular weight and the details of the fibrillation pattern (more but thinner fibrils are observed as molecular weight increases). The minimum molecular weight needed for significant fibrillation is around 850 kg/mol (≈3 M e) both at −10°C and 25°C.

A copolymerized dodecacarborane anion as covalently attached cation exchanger in ion-selective sensors.

The traditional cation exchangers used in ion-selective electrodes and optodes are tetraphenylborate derivatives, which are generally adequate for most analytical applications but may in some cases suffer from decomposition by acid hydrolysis, oxidants, and light. Recently, halogenated dodecacarboranes were found to be improved cation exchangers in terms of lipophilicity and chemical stability. This forms the basis for the convenient covalent attachment of the cation exchanger to the polymeric backbone of the sensing material. This is a challenge that has not satisfactorily been solved and which is especially important in view of developing ultraminiaturized sensing arrays. Here, a C-derivative of the closo-dodecacarborane anion (CB(11)H(12)(-)) with a polymerizable group was synthesized as a chemically stable cation exchanger. This new derivative was copolymerized with methyl methacrylate and decyl methacrylate (MMA-DMA) to fabricate a plasticizer-free polymer with cation-exchange properties. This polymer could be conveniently blended with traditional plasticized poly(vinyl chloride) or with noncrosslinked methacrylic polymers to give solvent cast films that appear to be clear and homogeneous and that could be doped with ionophores. Optode leaching experiments supported the covalent grafting of the carborane anions. Ion-selective membranes and optode thin films were evaluated in terms of response function, response time, and selectivity. In all cases, the new material exhibited behavior similar to free tetraphenylborate derivative-based membranes. As a result of these studies, an all-polymeric plasticizer-free calcium-selective membrane was fabricated on the basis of the covalently attached carborane, a recently introduced grafted calcium ionophore, and an MMA-DMA polymer matrix. The resulting ion-selective electrodes showed Nernstian response slopes and rapid response times, demonstrating that covalent grafting of all sensing components is a feasible approach to the development of ion sensors.

Synthesis and characterization of polymethacrylate-based nitric oxide donors.

A synthetic path for the preparation of methacrylic homo- and copolymers containing secondary amine groups that can be converted into nitric oxide (NO) releasing N-diazeniumdiolates is described. The polymers are obtained by a multistep procedure involving synthesis of methacrylate monomers containing boc-protected secondary amine sites, free radical benzoyl peroxide initiated polymerization, deprotection of the amine sites, and subsequent reaction of the polymers with NO in the presence of sodium methoxide. Monomers with both linear and cyclic pendant secondary amines are examined as polymer building blocks. In most cases, polymers are obtained for both types with compositions that agree well with initial monomer ratios and with number average molecular weights (M(n)) ranging from 1.69 to 2.58 x 10(6) Da. The final N-diazeniumdiolated methacrylic amine polymers are shown to release NO for extended periods of time with "apparent" t(1/2) values ranging from 30 to 60 min when suspended in phosphate buffer, pH 7.4. Total NO loading and release for these materials can reach 1.99 micromol per mg of polymer and is proportional to the amine content of the polymer. It is further shown that by using a dimethacrylate cross-linking agent in conjunction with the various methacrylate amines, suspension polymerization methods can be employed to create small (100-200 microm) polymeric methacrylate microbeads. Such microbeads that can be sequentially deprotected and converted to NO release particles via in-situ diazeniumdiolate formation as carried out for the non-crosslinked polymers.

Photo-crosslinkable polymers as hole-transport materials for organic light-emitting diodes

A bis(diarylamino)biphenyl-functionalised methacrylate has been synthesised and polymerised to give a hole-transport polymer; copolymerisation with a chalcone- or cinnamate-functionalised methacrylate affords photo-crosslinkable hole-transport polymers that become insoluble upon exposure to ultra-violet irradiation. Organic light-emitting diodes (OLEDs) were fabricated based on both crosslinked and non-crosslinked hole-transport polymers, and the performances of the devices were evaluated.

Friction studies of hydrogel contact lenses using AFM: non-crosslinked polymers of low friction at the surface

The surface of soft contact lenses made of crosslinked poly(2-hydroxyethyl methacrylate), pHEMA, has been investigated with atomic force microscopy in contact mode. The friction force and adhesive force measurements were able to differentiate the non-crosslinked pHEMA chains from the surface of the crosslinked pHEMA networks. These non-crosslinked pHEMA chains at the surface were anchored to the crosslinked pHEMA network, most likely by entanglement and their surfaces were about 2–4 nm higher than the surrounding surface in a dehydrated state. In saline solution, the surface friction and adhesive force of the contact lens were significantly reduced compared to those measured for the surface-dehydrated contact lens.