Crosslinking polymerization leading to interpenetrating polymer network formation. II. Polyaddition crosslinking reactions of poly(methyl methacrylate‐co‐2‐methacryloyloxyethyl isocyanate) with various diols

As part of our continuing studies concerned with the elucidation of the crosslinking polymerization mechanism leading to interpenetrating polymer network (IPN) formation, in which IPNs consist of both polymethacrylates and polyurethane (PU) networks, this article explores the polyaddition crosslinking reactions of multifunctional poly(methyl methacrylate‐co‐2‐methacryloyloxyethyl isocyanate) [poly(MMA‐co‐MOI)] [MMA/MOI = 90/10] with various diols leading to PU network formation. Thus, the equimolar polyaddition crosslinking reactions of poly(MMA‐co‐MOI) with ethylene glycol (EG), 1,6‐hexane diol, and 1,10‐decane diol (DD) were carried out in N‐methyl pyrrolidone at a 0.25 mol/L isocyanate group concentration at 80 °C. The second‐order rate constants decreased from EG to DD. The deviation of the actual gel point from the theoretical one was smaller from EG to DD. The intrinsic viscosity of resulting prepolymer demonstrated almost no variation with progressing polymerization for the EG system, whereas it gradually increased with conversion for the DD system. Close to the gel point conversion it increased rather drastically for both systems. The swelling ratio of resulting gel was higher from EG to DD. These are discussed mechanistically in terms of the significant occurrence of intramolecular cyclization and intramolecular crosslinking reactions leading to shrinkage of the molecular size. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3243–3248, 2003     

Surface‐attached polymer networks through carbene intermediates generated from α‐diazo esters

We report on the generation of surface attached polymer networks through C,H insertion reactions based on carbene intermediates. To this copolymers based on N,N‐dimethyl acrylamide, which contain α‐diazo ester groups, are generated and coated onto a solid substrate covered with a self‐assembled monolayer of a silane. After deposition, films having thicknesses of approximately 120 nm are irradiated with UV light having a wavelength of 254 nm or 360 nm or heated to temperatures between 80 and 180 °C. During the light or heat‐induced activation process, carbenes are formed, which react via C—H insertion with any neighboring polymer chain and/or with the self‐assembled monolayer on the substrate, thus forming surface‐attached polymer networks. To follow the kinetics of the crosslinking process, the conditions are adjusted, so that they range between 5 and 10 min for the UV‐crosslinking and 5 to 15 min for the heat‐induced crosslinking. UV‐patterning of the surface‐attached polymer networks through chrome contact masks allows generation of microstructures in the micrometer range. We investigate the layer formation process and describe the formation of microstructured surfaces based on these surface‐attached polymer networks. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3276–3285     

Crosslinking polymerization leading to interpenetrating polymer network formation. I. Polyaddition crosslinking reactions of poly(methyl methacrylate‐co‐2‐methacryloyloxyethyl isocyanate)s with ethylene glycol resulting in polyurethane networks

At the start of our research program concerned with the elucidation of the crosslinking polymerization mechanism leading to interpenetrating polymer network (IPN) formation, in which IPNs consist of both polymethacrylates and polyurethane (PU) networks, this article deals with the polyaddition crosslinking reaction leading to PU network formation. Therefore, 2‐methacryloyloxyethyl isocyanate (MOI) was radically copolymerized with methyl methacrylate (MMA) in the presence of CBr₄ as a chain‐transfer agent. The resulting poly(MMA‐co‐MOI)s, having pendant isocyanate (NCO) groups as novel multifunctional polyisocyanates, were used for polyaddition crosslinking reactions with ethylene glycol as a typical diol. The second‐order rate constants depended on both the functionality of poly(MMA‐co‐MOI) and the NCO group concentration. The actual gel points were compared with the theoretical ones calculated according to Macosko's equation; the deviation of the actual gel point from the theoretical value became more remarkable for a greater functionality of poly(MMA‐co‐MOI) and at a lower NCO group concentration or at a lower poly(MMA‐co‐MOI) concentration. These are discussed mechanistically, with consideration given to the significance of intramolecular cyclization and intramolecular crosslinking reactions leading to the shrinkage of the molecular size of the prepolymer, along with the data of the intrinsic viscosities of resulting prepolymers and the swelling ratios of resulting gels. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 606–615, 2003     

Radiation‐induced fabrication of polymer nanoporous materials from microphase‐separated structure of diblock copolymers as a template

Polymer nanoporous materials with periodic cylindrical holes were fabricated from microphase‐separated structure of diblock copolymers consisting of a radiation‐crosslinking polymer and a radiation‐degrading polymer through simultaneous crosslinking and degradation by γ‐irradiation. A polybutadiene‐block‐poly(methyl methacrylate) (PB‐b‐PMMA) diblock copolymer film that self‐assembles into hexagonally packed poly(methyl methacrylate) cylinders in polybutadiene matrix was irradiated with γ‐rays. Solubility test, IR spectroscopy, and TEM and SEM observations for this copolymer film in comparison with a polystyrene‐block‐poly(methyl methacrylate) diblock copolymer film revealed that poly(methyl methacrylate) domains were removed by γ‐irradiation and succeeding solvent washing to form cylindrical holes within polybutadiene matrix, which was rigidified by radiation crosslinking. Thus, it was demonstrated that nanoporous materials can be prepared by γ‐irradiation, maintaining the original structure of PB‐b‐PMMA diblock copolymer film. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5916–5922, 2007     

Epoxy‐functionalized,low‐glass‐transition‐temperature latex. II. Interdiffusion versus crosslinking in the presence of a diamine

This article describes the results of experiments examining the competition between the polymer diffusion rate and the crosslinking rate in low‐glass‐transition‐temperature, epoxy‐containing latex films in the presence of a diamine. We examined films formed from donor‐ and acceptor‐labeled poly(butyl acrylate‐co‐methyl methacrylate‐co‐glycidyl methacrylate) copolymer latex and studied the influence of several parameters on the growth rate of gel content and the rate of polymer diffusion. These factors include the molecular weight of the latex polymer, the presence or absence of a diamine crosslinking agent, and the cure protocol. The results were compared to the predictions of a recent theory of the competition between crosslinking and polymer diffusion across interfaces. In the initially formed films, polymer diffusion occurs more rapidly than the chemical reaction rate. Therefore, these films fall into the fast‐diffusion category of this model. In our system (unlike in the model), the latex polymer has a broad distribution of molecular weights and a distribution of diffusivities. The shortest chains contribute to the early time diffusion that we measure. At later stages of our experiment, slower diffusing species contribute to the signal that we measure. The diffusion time decreases substantially, and we observe a crossover to a regime in which the chemical reaction dominates. The increases in chain branching and gel formation bring polymer diffusion to a halt. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4098–4116, 2002     

Isobutylene‐rich macromonomers: Dynamics and yields of peroxide‐initiated crosslinking

New isobutylene‐rich elastomers bearing multiple pendant styrenic, acrylic, maleimidic, vinylic, and allylic functional groups have been prepared and examined in the context of peroxide‐initiated crosslinking. Halide displacement from brominated poly(isobutylene‐co‐isoprene) (BIIR) by the requisite carboxylate nucleophiles in homogeneous toluene solutions provide the desired esters in quantitative yield without complications from dehydrohalogenation or premature crosslinking. Heating the resulting macromonomers with dicumyl peroxide to 160 °C under solvent‐free conditions gives thermoset derivatives, with reaction rates and yields depending markedly on functional group structure. In general, high cure extents can only be achieved using highly reactive pendant functional groups, owing to the competitive balance between crosslinking through C?C oligomerization, and degradation through β‐scission of backbone macroradical intermediates. Independent control of crosslinking rates and cure extents is gained through the use of nitroxyl radical traps bearing acrylate functionality. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 123–132     

Interfacial crosslinking of self‐assembled triblock copolymer nanoparticles via alkoxysilane hydrolysis and condensation

The use of amphiphilic triblock copolymers bearing a reactive alkoxysilane middle block as polymeric stabilizers is reported in this work. A series of poly(ethylene glycol) methyl ether methacrylate‐b‐(3‐trimethoxysilyl)propyl methacrylate‐b‐benzyl methacrylate (PEGMA‐b‐MPS‐b‐BzMA) triblock copolymers were prepared by RAFT solution polymerization and polymerization‐induced self‐assembly (PISA), respectively, where the various block lengths and overall composition were varied. The copolymers prepared by solution polymerization were employed as oil‐in‐water stabilizers where upon application of a catalyst, the 3‐(trimethoxysilyl)propyl methacrylate (MPS) block at the droplet interface was crosslinked to yield capsule‐like structures. The effectiveness of interfacial crosslinking was validated by dynamic light scattering and electron microscopy. In situ self‐assembly by the PISA method resulted in spherical nanoparticles of controllable size that were readily crosslinked by addition of base, with significant enhancement of colloidal stability. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1897–1907     

UV‐responsive degradable polymers derived from 1‐(4‐aminophenyl) ethane‐1,2‐diol

A UV‐responsive polymer was prepared via condensation polymerization of 2‐nitrobenzyl(4‐(1,2‐dihydroxyethyl)phenyl)carbamate and azalaic acid dichloride. When the polymer was irradiated with UV light, the nitrobenzyl urethane protecting group was removed and the deprotected aniline underwent spontaneous 1,6‐elimination reactions, resulting in degradation of the polymer. Nanoparticles with encapsulated Nile Red were formulated with the degradable polymer and triggered burst release of Nile Red was observed when the nanoparticles were irradiated by UV light. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1161–1168     

Tailored rheology of a metallocene polyolefin through silane grafting and subsequent silane crosslinking

Polymer modification through silane grafting and its subsequent crosslinking allows the rheological properties of a polymer to be tuned from those of a viscous melt to those of a crosslinked elastic network. In this study, a metallocene polyolefin resin is grafted with vinyl trimethoxy silane (VTMS) using dicumyl peroxide (DCP) as the initiator and is subsequently crosslinked in an oxidative environment. Dynamic rheological experiments are conducted to elucidate the effects of DCP and VTMS concentrations on the grafting and ensuing crosslinking processes. We find that the addition of VTMS alone to the polymer produces no grafting. In contrast, the presence of DCP by itself leads to direct crosslinking between polymer chains as suggested by an increase in elastic modulus and complex viscosity. Samples containing both DCP and VTMS undergo silane grafting, with the extent of grafting increasing with increasing DCP concentration. This conclusion is borne out by both rheological and Fourier transform infrared measurements. The grafted samples undergo silane crosslinking only in an oxidative environment and at temperatures equal to or greater than 190 °C. During crosslinking, the samples undergo a transition from a viscous melt with frequency‐dependent moduli to a gel exhibiting frequency‐independent moduli with the elastic modulus exceeding the viscous modulus. However, the kinetics of crosslinking and the extent of the modulus increase are a function of the DCP concentration, with both exhibiting a maximum at a specific DCP and VTMS combination. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2468–2479, 2000     

Local deformation in photo‐crosslinked polymer blends monitored by Mach‐Zehnder interferometry

Local deformation of a polymer mixture crosslinked by irradiation with ultraviolet light was in situ monitored by using a Mach‐Zehnder interferometer. In combination with the refractive index data obtained from independent measurements, the deformation in the nanometer scales of the crosslinked blends was calculated by using the difference in optical path length of the blend measured before and after irradiation. Upon varying the crosslink density of the blend by changing the light intensity, it was found that the local deformation well correlates with the crosslink density obtained from the reaction kinetics experiments. Furthermore, the strain relaxation of the blends was also monitored in situ and analyzed after irradiation over different time intervals. The results obtained in this study reveal the possibility of monitoring the nanometer‐scale deformation in polymers during radiation curing. These data also provide important information on the correlations between the irradiation‐induced elastic strain and the resulting morphology of reacting polymer blends. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2898–2913, 2005     

A comparative study of calorimetric methods to determine the crosslinking degree of the ethylene‐Co‐vinyl acetate polymer used as a photovoltaic encapsulant

EVA copolymer foils are widely used as encapsulants for photovoltaic (PV) cells in PV modules. These foils need to be crosslinked during module manufacturing, to enhance their properties. Accurate and reliable methods for the determination of their crosslinking degree are thus very important. In this work, two semi‐empirical calorimetric methods are considered and compared to a scientifically sound rheological method used as a reference. The main purpose of this work is to reveal the chemical and physical fundamentals on which the two calorimetric methods are based, to allow for a scientific understanding of their advantages, and limitations. For one of these calorimetric methods, the so‐called “Melt‐Freeze” method, we have sought for a deeper understanding of the underlying crystallization physics through the use of successive self‐nucleation and annealing experiments, which give access to the crystallite size distribution of EVA, and to the influence of crosslinking upon it. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 866–876     

Dynamics of cyclodimerization and viscoelasticity of photo‐crosslinkable PVA

We investigated the interfacial properties of poly(vinyl alcohol) carrying UV‐crosslinkable pendant quaternized stilbazole (styrylpyridinium), PVA‐SbQ. The extent and dynamics of PVA‐SbQ cyclodimerization reactions and crosslinking induced by UV irradiation were monitored in situ and in real time by quartz crystal microgravimetry (QCM). Sensograms reflecting time‐dependent changes in density and viscoelasticity of crosslinking films followed a Boltzmann sigmoidal model, depending on precursor film composition and irradiation power. The shifts in QCM frequency and energy dissipation upon PVA‐SbQ cyclodimerization correlated with three photo‐crosslinking phases involving soft‐to‐rigid transitions, namely, induction (initiation), main crosslinking (interaction), and termination. PVA‐SbQ films crosslinked to different degree were used as protein carriers and a slower release profile was determined for the films that underwent more extensive crosslinking. Overall, this study demonstrates for the first time the dynamics of PVA‐SbQ crosslinking and its impact in system viscoelasticity and protein release. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 345–355     

Electron spin resonance study on chemical crosslinking reaction mechanisms of polyethylene using a chemical agent. V. Comparison with polypropylene and ethylene‐propylene copolymer

We studied the chemical reaction process of polypropylene (PP), ethylene‐propylene copolymer (EPM), and ethylene‐propylene‐diene copolymer (EPDM) crosslinking induced by dicumyl peroxide (DCP) using electron spin resonance (ESR). Free radicals appeared at an elevated temperature of around 120 °C and the behavior and kinetics of the reaction process were observed at 180 °C. The radical species detected in PP were alkyl type radicals, formed by the abstraction of hydrogen atoms from the tertiary carbon of polymer chains. For EPDM containing a diene component, the radicals were trapped at double bonds in this diene component to form allyl radicals. The resolutions of these spectra were extremely clear; hence, isotropic spectra of these polymer radicals were obtained. We measured the ESR at high temperatures and confirmed that the process of crosslinking induced by DCP was a free radical reaction. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3383–3389, 2000     

Enzymatic‐ and light‐degradable hybrid nanogels: Crosslinking of polyacrylamide with acrylate‐functionalized Dextrans containing photocleavable linkers

Enzymatically cleavable and light‐degradable hybrid nanogels were prepared by free radical inverse miniemulsion copolymerization of acrylamide (AAm) with a newly synthesized functional dextran crosslinker containing acrylate moieties attached to the backbone via a photolabile linker, that is, dextran‐photolabile linker‐acrylate (Dex‐PL‐A). The Dex‐PL‐A/AAm feed ratio was systematically varied to investigate the influence of the particle composition on the gel properties. The resulting hydrogel nanoparticles were examined with regard to their degradation behavior upon the appliance of the two orthogonal stimuli by turbidity measurements in combination with dynamic light scattering. Although continuous photolytic cleavage of the photolabile linkers between polyacrylamide chains and dextran molecules was found to proceed fast and quantitatively yielding completely disintegrated networks, stepwise irradiation resulted in partial degradation of crosslinking points. Thus, nanogels of a desired specific degree of swelling (DGS) can be obtained by adjusting the irradiation time accordingly. Partial enzymatic cleavage of the dextran backbones of the Dex‐PL‐A crosslinking molecules resulted in an increase in the DGS of the nanogels up to a constant value. Subsequent irradiation of those swollen hydrogel particles was used to fully degrade the network structure in a second step. Hence, a two‐step degradation profile was realized by the combination of the two orthogonal stimuli. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Synthesis of telechelic anthraquinone‐functionalized polybutadiene via ROMP and study of its photo‐oxidation and UV crosslinking behaviors

Telechelic anthraquinone‐functionalized polybutadiene (AQ‐PB‐AQ) was synthesized by ring opening metathesis polymerization to achieve homogeneous dispersion of AQ groups in the polymer matrix. It was observed that the AQ end groups act as a sensitizing group resulting in photo‐induced crosslinking and oxidation reaction at the olefin groups in the PB. The process was confirmed by comparing the polymer's mass loss and Fourier transform infrared (FTIR) spectra with those of telechelic diacetoxy‐functionalized PB (AcO‐PB‐OAc) which did not show any difference before and after ultraviolet irradiation. Homogeneity of the AQ groups in PB matrix results in rapid crosslinking of PB in a short period of time (<4 min) while a simple blend of AQ methyl ester in AcO‐PB‐OAc matrix did not show such behavior. Photo‐oxidative reaction has concurrently occurred during the crosslinking reaction. Generation of  OH,  OOH, and carbonyl groups in the matrix during the photo‐oxidation was verified by FTIR, elemental analysis, and water contact angle measurements. A plausible mechanism for the process was proposed in which the photo‐sensitized AQ groups abstract hydrogen from the PB chain to yield reactive radical species, initiating oxidative crosslinking, and degradation of PB or reduction of AQ to hydroanthraquinone species. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1249–1258     

Sorption and diffusion of normal alkanes through poly(ethylene‐co‐vinyl acetate) membranes

The transport behavior of uncrosslinked and crosslinked poly(ethylene‐co‐vinyl acetate) membranes has been investigated using normal alkanes as probe molecules, in the temperature range of 30–60 °C. Benzoyl peroxide was used for crosslinking the matrix. It has been observed that, a critical concentration of crosslinker is necessary for maximum solvent uptake, followed by a decrease at higher concentration. The effect of free volume on liquid transport was investigated by positron annihilation lifetime spectroscopy. The mechanism of transport has been found to deviate from the regular Fickian behavior. The dependence of the transport coefficients on crosslink density, nature of penetrants, and temperature was studied. The polymer–solvent interaction parameter, enthalpy, and entropy of sorption have also been estimated from the transport data. The affine and phantom models for chemical crosslinks were used to predict the nature of crosslinks. Finally, the experimental sorption data were compared with theoretical predictions. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2470–2480, 2007     

Solid‐state NMR studies of cis‐1,4‐polyisoprene crosslinked with dicumyl peroxide in the presence of triallyl cyanurate

The network of dicumyl peroxide (DCP)/triallyl cyanurate (TAC) crosslinked cis‐1,4‐polyisoprene was studied by solid‐state NMR techniques such as direct‐polarization (DP), cross‐polarization (CP), and proton T₂ experiments. Line broadening and cis/trans isomerization of mobile carbons were observed in the DP experiments. The information on rigid carbons of network structures was observed with the CP technique. Motional heterogeneity was examined by proton T₂ relaxation experiments. Decreases in long T₂ (T_2L) values from the mobile non‐network structures and short T₂ (T_2S) values from the rigid network structures were observed with an increase in peroxide or coagent concentration. The percentage of T_2S in T₂ relaxation, which is related to network density, was observed to increase with peroxide and coagent addition. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1417–1423, 2000     

Formation of pore‐filled ion‐exchange membranes with in situ crosslinking: Poly(vinylbenzyl ammonium salt)‐filled membranes

Robust, polyelectrolyte‐filled, microporous membranes were prepared by the introduction and crosslinking of a preformed polymer within the pores of a poly(propylene) host membrane. Specifically, poly(vinylbenzyl chloride) (PVBCl) was reacted with piperazine or 1,4‐diaminobicyclo[2.2.2]octane in an N,N‐dimethylformamide (DMF) solution contained in the pores of the microporous base membrane. The remaining chloromethyl groups were reacted with an amine, such as trimethylamine, to form positively charged ammonium sites. This simple two‐step procedure gave dimensionally stable, anion‐exchange membranes in which the degree of crosslinking and the mass loading were determined by the concentration of PVBCl and crosslinker in the starting DMF solution. The incorporated polyelectrolyte gel was evenly distributed within the pores of the host membrane with no surface layers present. The membranes are fully characterized. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 807–820, 2001     

Oxidation and epoxidation of poly(1,3‐cyclohexadiene)

Poly(1,3‐cyclohexadiene) (PCHD) derivatives were synthesized via facile chemical modification reactions of the residual double bond in the repeat unit. The oxidation and degradation of PCHD was investigated to enable subsequent controlled epoxidation reactions. PCHD exhibited a 15% weight loss at 110 °C in the presence of oxygen. The oxidative degradation, demonstrated by gel permeation chromatography (GPC) and ¹H NMR spectroscopy, was attributed to main‐chain scission. Aldehyde and ether functional groups were introduced into the polymer during the oxidation process. PCHD was quantitatively epoxidized in the absence of deleterious oxidation with meta‐chloroperoxybenzoic acid. ¹H and ¹³C NMR spectroscopy confirmed that polymers with controlled degrees of epoxidation were reproducibly obtained. Epoxidized PCHD exhibited a glass‐transition temperature at 154 °C, which was slightly higher than that of a PCHD precursor of a nearly equivalent molecular weight. Moreover, GPC indicated the absence of undesirable crosslinking or degradation, and the molecular weight distributions remained narrow. The thermooxidative stability of the fully epoxidized polymer was compared to that of the PCHD precursor, and the epoxidized PCHD exhibited an initial weight loss at 250 °C in oxygen, which was 140 °C higher than the temperature for PCHD. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 84–93, 2003     

Melt grafting of polymethyl methacrylate onto poly(ethylene‐co‐1‐octene) by reactive extrusion: Model compound approach

The grafting of poly(methylmethacrylate) onto poly(ethylene‐co‐1‐octene) by in situ radical polymerization of methyl methacrylate is a process where the side reactions are difficult to characterize. To increase the understanding of both the nature and the extent of such reactions, products resulting from the same chemical system, where polymer is replaced by squalane and/or pentadecane, are analyzed. The influence of the temperature, the nature of peroxides (used as radicals generators) and the monomer concentration are investigated toward the chain length of the grafts. The resulting grafted PMMA and PMMA homopolymer are qualitatively analyzed by MALDI‐TOF spectroscopy and size exclusion chromatography. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5215–5226, 2007