Kinetics of network formation via free-radical mechanisms — Polymerization and polymer modification

The study of polymeric networks synthesized via free-radical polymerization and/or prepolymer modification have become an attractive research area of both fundamental and commercial interest. Recently, a research/development program was initiated to investigate the fundamentals and applications of polymer networks. The following aspects are being emphasized: the elucidation of crosslinking mechanisms and kinetics, the characterization of network microstructures, and the development of kinetic gelation models. This paper reports recent activities towards these goals.     

Photo-crosslinked acrylates degradation kinetics

A series of crosslinked polyurethane acrylate solids with glass transition temperatures ranging from –49 to +65 °C was prepared by photopolymerization of specially formulated solvent-free resins. The kinetics of thermooxidative and thermal (in N₂) degradation of these crosslinked acrylate networks at temperatures ranging from 100 to 400 °C was studied as a function of crosslink density using thermogravimetry. The polyacrylate network degradation rate decreased with the increase of crosslink density, while apparent activation energy of degradation increased. Polyacrylate thermal stability increase with crosslinking was explained by decreased rate of oxygen and volatile products diffusion and/or slowing of depolymerization due to increased radical recombination rate, and decreased chain segments mobility in systems with higher crosslink density.     

Polymer coupling and theory of on-random crosslinking: an analytical solution

Gel formation is an important feature in free-radical polymer coupling. Due to the different possible combination reactivities of each polymer backbone radical, polymer chains are crosslinked in a non-random manner. Equations of the moments have been derived to predict the pregel molecular weight development and the crosslink density at gel point. This work provides an analytical solution for the differential equations. The model agrees with the Flory-Stockmayer gelation theory under the condition of random crosslinking. The magnitude of deviations from the classical theory for non-random crosslinking depends on the product of the radical termination reactivity ratios (r₁r₂), the ratio of the rate constants of backbone radical generation (k), the ratio of the weight-average chain lengths of primary polymers (y), and the polymer weight fractions (w₂).     

Preparation and properties of two-component hydrogels based on 2-acrylamido-2-methylpropane sulphonic acid

Xerogels comprising 2-acrylamido-2-methylpropane sulphonic acid (AMPS) and acrylic acid as well as AMPS and acrylamide crosslinked with hexafunctional crosslinking agent have been prepared by catalytic initiation polymerization to complete conversion. Different percentages of hexafunctional crosslinker, 1,1,1-trimethylolpropane trimethacrylate, were used to prepare crosslinked copolymers having different degree of crosslink densities. The crosslinked copolymers were swollen in water to equilibrium. The volume fraction of polymer, the swelling capacity and the equilibrium water content were obtained. Low conversion polymerization was used to determine the reactivity ratios of both monomer pairs. Fourier transform spectroscopy was utilized to measure the molar ratio of copolymers constituents. © 1998 John Wiley & Sons, Ltd.     

Mechanical properties of composite heterogeneous gels

Composites with a matrix of poly(2-hydroxyethyl methacrylate) (PHEMA) and 10% by volume of various crosslinked PHEMA polymer fillers (prepared by copolymerization with 0.1, 0.4, 1.0, and 20.0% by weight of ethylenedimethacrylate) of particle size about 1 μm were prepared. Some polymer matrixes were prepared from soluble branched PHEMA (Hydron S), and others by copolymerization, in the presence of the filler with 0.4 and 1.0% of ethylenedimethacrylate as a crosslinking agent. In the case of the uncrosslinked matrix, a linear polymer–crosslinked polymer system, resulted; in the case of the crosslinked matrix, a composite heterogeneous network was formed (in the latter case, the particles of the filler were swollen with monomer during the crosslinking polymerization). Stress–strain, equilibrium, and ultimate characteristics were measured at 3, 10, 25, 40, 60, and 80°C on samples swollen to equilibrium in water (T_g ≈ ?50°C) and at 80, 110, and 140°C on dry samples (T_g ≈ 100°C). Depending on experimental conditions, above all on the distance from the main transition region and on whether the polymer is dry or swollen, it was found that the measured hydrophilic composite systems behaves as a filled system (with the polymer filler acting mostly as solid particles, irrespective of the crosslink density) or as a system with crosslink density fluctuations (where both networks, the matrix and the filler, contribute roughly additively to the properties of the system), or finally as defect heterogeneous systems (where the properties depend primarily on the character of the polymer–filler interface).     

Swelling studies of crosslinked poly(p-phenylene terephthalamide) copolymers in sulfuric acid

In an attempt to improve the mechanical properties of extended chain polymers such as poly(p-phenylene terephthalamide) (PPTA), a crosslinkable terephthalic acid derivative (XTA) has been developed which can be incorporated into copolymers in various concentrations and activated after polymerization. The crosslinking of PPTA-co-XTA copolymer particles was investigated through a series of swelling experiments in concentrated H₂SO₄. The data show a systematic decrease in equilibrium swelling with increasing XTA content, indicating the XTA units are in fact acting as crosslink sites. Values for crosslink density were calculated from the Flory-Rehner theory of polymer swelling and compared with previous findings on crosslinked rigid polymer network systems. The effective number of crosslinks per XTA unit (efficiency) predicted by the Flory-Rehner theory increases and then decreases with % XTA. The decrease in crosslinking efficiency at high XTA concentrations is consistent with differential scanning calorimetry data which show the enthalpy of XTA reaction decreasing slightly with % XTA. The deviations at low % XTA may represent a failure of the Flory-Rehner theory to properly describe the rubbery elasticity of extended chain polymers. © 1994 John Wiley & Sons, Inc.     

Gelation in the Copolymerization of Diallyldimethylammonium Chloride with Acrylamide

Diallyldimethylammonium chloride (DADMAC) was free-radical copolymerized with acrylamide (AA) in water at a total monomer concentration of 4 mol/L and 40°C with different monomer feed compositions. Gelation occurred only for 20/80 DADMAC/AA monomer feed although crosslinking was observed for all monomer feed compositions. The gel point was at 51% conversion, and the swelling ratios of the resulting gels were quite high, from 1400 to 700. Addition of 2‐propanol as a chain‐transfer reagent reduced crosslinking and prevented gelation. These results are mechanistically discussed in connection with the cyclopolymerizability of DADMAC, and significant allylic hydrogen abstraction by the growing polymer radical characteristic of allyl polymerization is proposed.     

Well-defined crosslinked materials via ring opening metathesis polymerisation

The work reported here is part of our ongoing programme of work directed. towards the synthesis and characterisation of polymeric materials via ROMP-RIM and ROMP-RTM. It describes the synthesis and characterisation of well-defined linear and crosslinked polymeric materials via ROMP-RTM. The process involves in-mould polymerisation of monofunctional imidonorbornene monomers, with different alkyl side chain lengths, to give a range of linear polymers. The process also involves in-mould copolymerisation of monofunctional imidonorbornene monomers, with different alkyl side chain lengths and difunctional monomers with different alkylene spacer lengths, to produce well-defined crosslinked polymers. The glass transition temperature (T_g) of the linear polymers was found to depend on the length of the alkyl side chain. For the crosslinked materials the results show that as the percentage of the difunctional, crosslinking unit, is increased (1, 5 and 10 molar percentage of the difunctional monomer) the glass transition shifts to a higher temperature, the height of the tanδ peak decreases and the plateau shear modulus above T_g increases. These results are as expected for an increase in the crosslink density of a polymer.     

不同交联剂对甲基丙烯酸-β-羟乙酯/E-51双甲基丙烯酸酯聚合物水凝胶性能的影响

以水溶性单体甲基丙烯酸-β-羟乙酯(HEMA)与大分子交联剂E-51双甲基丙烯酸酯(E-51-DMA)(质量比HEMA/E-51-DMA=90/10)为主要原料,分别引入了5种小分子交联剂:N,N′-亚甲基双丙烯酰胺(MBA)、二乙烯基苯(DVB)、双甲基丙烯酸乙二醇酯(EDMA)、1,1,1-三(丙烯酰氧甲基)丙烷(TAP)和2,2,2-三(丙烯酰氧甲基)乙醇(TAE),采用本体聚合方法合成了5个系列的聚合物水凝胶.研究了小分子交联剂的类型及用量对水凝胶溶胀性能、杨氏模量以及有效交联密度ve和聚合物-水相互作用参数χ的影响,并比较了不同交联剂的交联效率.结果表明,随着小分子交联剂用量的增大,水凝胶平衡含水量EWC逐渐降低,聚合物体积分数2逐渐增大,反映聚合物网络结构的有效交联密度ve以及热力学参数聚合物-水相互作用参数χ值也随之增大.通过理论交联密度和有效交联密度的线性拟合,得到所选用的5种小分子交联剂在E-51-DMA10/HEMA90水凝胶体系中的交联效率,其顺序为DVB>EDMA>TAE>MBA≈TAP.     

Synthesis of polymeric core–shell particles using surface‐initiated living free‐radical polymerization

An easy and novel approach to the synthesis of functionalized nanostructured polymeric particles is reported. The surfactant‐free emulsion polymerization of methyl methacrylate in the presence of the crosslinking reagent 2‐ethyl‐2‐(hydroxy methyl)‐1,3‐propanediol trimethacrylate was used to in situ crosslink colloid micelles to produce stable, crosslinked polymeric particles (diameter size ～ 100–300 nm). A functionalized methacrylate monomer, 2‐methacryloxyethyl‐2′‐bromoisobutyrate, containing a dormant atom transfer radical polymerization (ATRP) living free‐radical initiator, which is termed an inimer (initiator/monomer), was added to the solution during the polymerization to functionalize the surface of the particles with ATRP initiator groups. The surface‐initiated ATRP of different monomers was then carried out to produce core–shell‐type polymeric nanostructures. This versatile technique can be easily employed for the design of a wide variety of polymeric shells surrounding a crosslinked core while keeping good control over the sizes of the nanostructures. The particles were characterized with scanning electron microscopy, transmission electron microscopy, optical microscopy, dynamic light scattering, and Raman spectroscopy. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1575–1584, 2007     

聚氨酯的后交联研究进展

通过交联可以将线型聚合物连接成三维网状结构,适度交联后聚氨酯的拉伸强度、模量、耐溶剂性、耐高温性等许多性能都能得到大幅提高,而交联方法、交联的种类及交联密度的大小都是重要的影响因素,因此对交联方法、交联密度的测定、交联密度与聚氨酯性能之间的关系研究对于聚氨酯的质量控制非常重要。本文首先详细介绍了聚氨酯后交联的常用方法,然后对聚氨酯交联密度的测定方法进行了分析比较,在此基础上进一步介绍了国内外关于聚氨酯交联密度与其性能的关系方面的研究进展。     

Characterization of ring‐opening polymerization of genipin and pH‐dependent cross‐linking reactions between chitosan and genipin

In this study, a novel chitosan‐based polymeric network was synthesized by crosslinking with a naturally occurring crosslinking agent—genipin. The results showed that the crosslinking reactions were pH‐dependent. Under basic conditions, genipin underwent a ring‐opening polymerization prior to crosslinking with chitosan. The crosslink bridges consisted of polymerized genipin macromers or oligomers (7 ～ 88 monomer units). This ring‐opening polymerization of genipin was initiated by extracting proton from the hydroxyl groups at C‐1 of deoxyloganin aglycone, followed by opening the dihydropyran ring to conduct an aldol condensation. At neutral and acidic conditions, genipin reacted with primary amino groups on chitosan to form heterocyclic amines. The heterocyclic amines were further associated to form crosslinked networks with short chains of dimmer, trimer, and tetramer bridges. An accompanied reaction of nucleophilic substitution of the ester group on genipin by the primary amine group on chitosan would occur in the presence of an acid catalysis. The extent in which chitosan gels crosslinked with genipin was significantly dependent on the crosslinking pH values: 39.9 ± 3.8% at pH 5.0, 96.0 ± 1.9% at pH 7.4, 45.4 ± 1.8% at pH 9.0, and 1.4 ± 1.0% at pH 13.6 (n = 5, p < 0.05). Owing to the different crosslinking extents and different chain lengths of crosslink bridges, the genipin‐crosslinked chitosan gels showed significant difference in their swelling capability and their resistance against enzymatic hydrolysis, depending on the pH conditions for crosslinking. These results indicated a direct relationship between the mode of crosslinking reaction, and the swelling and enzymatic hydrolysis properties of the genipin‐crosslinked chitosan gels. The ring‐opening polymerization of genipin and the pH‐dependent crosslinking reactions may provide a novel way for the preparation and exploitation of chitosan‐based gels for biomedical applications. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1985–2000, 2005     

增塑化学微交联聚氯乙烯中化学交联和物理交联的协同作用

由氯乙烯／ 邻苯二甲酸二烯丙基酯（ＶＣ／ＤＡＰ） 悬浮共聚合成了化学微交联聚氯乙烯（ＰＶＣ） 树脂,并进行增塑加工．共聚得到的化学交联ＰＶＣ 具有溶胶／ 凝胶分配特性,交联密度较低;化学交联ＰＶＣ 的溶胶和凝胶均存在分子链缠结作用,尤其当凝胶含量较高时,物理缠结对凝胶交联密度有较大贡献．化学交联对增塑ＰＶＣ 结晶性的影响较小,因此在增塑化学微交联ＰＶＣ 中同时存在化学交联网络和以分子链物理缠结点和微晶为交联点的物理交联网络,两者协同影响增塑ＰＶＣ 材料的性能．     

Swelling Behaviour of Isotropic Poly(n-butyl acrylate) Networks in Isotropic and Anisotropic Solvents

Summary: The swelling properties of photochemically crosslinked poly(n-butyl acrylate) (PABu) networks in isotropic and anisotropic solvents were investigated experimentally. The purpose of this study was to examine the swelling kinetics of PABu networks in isotropic solvents and to compare the results obtained which those observed in the case of the low molecular weight liquid crystal 4-cyano-4′-n-pentyl-biphenyl known as 5CB. The phase diagrams were established in terms of composition and temperature for isotropic solvents, as toluene, acetone, cyclohexane, and methanol, and 5CB, using the plateau values corresponding to equilibrium states of swelling. The polymer networks were prepared via free radical polymerization/crosslinking processes by ultraviolet (UV) radiation of initial mixtures made up from a monomer, a crosslinker, and a photoinitiator. PABu networks with several crosslinking densities were formed using different quantities of difunctional monomer hexanedioldiacrylate (HDDA). Immersion of these networks in excess solvent allows measuring the solvent uptake by determination of the weight in isotropic solvents and diameter in an anisotropic solvent (5CB). Swelling data were rationalized by calculating weight and diameter ratios considering swollen to dry network states of the samples.     

Simulation model for network formation in free-radical crosslinking copolymerization: Pregelation period

A new simulation model for network formation in free-radical copolymerization of vinyl and divinyl monomers is proposed. This model is based on the crosslinking density distribution of the primary polymer molecules that results from a kinetically controlled network formation. The crosslinking density distribution provides information on how each chain is connected to other chains and therefore, a detailed analysis of the kinetics of network formation becomes possible by application of Monte Carlo simulations. In this method, not only averages but also various distributions, such as molecular weight distribution and distribution of crosslinked units as well as of unreacted pendant double bonds among various polymer molecules, can be calculated. The present theory is a direct solution for the Bethe lattice formed under nonequilibrium conditions, and therefore, it can be used to examine the applicability of the earlier theories of network formation to kinetically controlled systems. The present method is quite general and can be applied to various complex reactions systems that involve crosslinking, branching, cryclization and degradation in a nonequilibrium system.     

Uniform nonspherical latex particles as model interpenetrating polymer networks

Polystyrene/polystyrene latex interpenetrating polymer networks (IPNs) were prepared by seeded emulsion polymerization of styrene–divinylbenzene mixtures in crosslinked monodisperse polystyrene particles. The resulting latexes comprised uniform nonspherical particles, e.g., ellipsodal and egg-like singlets, symmetry and asymmetric doublets, and ice cream cone-like and popcorn-like multiplets. The nonspherical particles, which were formed by separation of the second-stage monomer from the crosslinked seed network during swelling and polymerization, are excellent models for studying phase separation in IPN's. The degree of phase separation increased with increasing degree of crosslinking of the seed particles, monomer/polymer swelling ratio, polymerization temperature, and seed particle size, and with decreasing divinylbenzene concentration in the swelling monomer. The results were consistent with a thermodynamic analysis based on the elastic-retractile force of the polymer network, the monomer/polymer mixing force, and interfacial tension force.     

Monodisperse polystyrene particles crosslinked with poly(dimethyl siloxane) diacrylate using dispersion polymerization and their monomer swelling capability

A flexible poly(dimethyl siloxane) diacrylate (PDMSDA) crosslinker was synthesized using different molecular weights of poly(dimethyl siloxane) (PDMS, M _n =550, 1,700, 4,000 g/mol). The monodisperse polystyrene (PS) particles crosslinked with various contents of PDMSDA were prepared by dispersion polymerization, and applied as seed particles in the seeded polymerization. The crosslinking density of the PS particles was determined from the rate of transport of the monomer molecules to the crosslinked seed particles. It was confirmed that the monomer swelling capacity of seed particles and final morphological changes of polymer beads were determined significantly by the crosslinking density of the seed particles. In addition, the morphological change was not observed without the oligomer swelling step in the seeded polymerization due to the hydrophobic property of PDMS. When highly crosslinked seed particles were used in the seeded polymerization, peculiar morphology (doublet structure) of polymer beads appeared.     

Effect of Fatty Acid Polyunsaturation on Synthesis and Properties of Emulsion Polymers Based on Plant Oil-Based Acrylic Monomers

This study demonstrated that polymerization behavior of plant oil-based acrylic monomers (POBMs) synthesized in one-step transesterification reaction from naturally rich in oleic acid olive, canola, and high-oleic soybean oils is associated with a varying mass fraction of polyunsaturated fatty acid fragments (linoleic (C18:2) and linolenic (C18:3) acid esters) in plant oil. Using miniemulsion polymerization, a range of stable copolymer latexes was synthesized from 60 wt.% of each POBM and styrene to determine the impact of POBM chemical composition (polyunsaturation) on thermal and mechanical properties of the resulted polymeric materials. The unique composition of each plant oil serves as an experimental tool to determine the effect of polyunsaturated fatty acid fragments on POBM polymerization behavior and thermomechanical properties of crosslinked films made from POBM-based latexes. The obtained results show that increasing polyunsaturation in the copolymers results in an enhanced crosslink density of the latex polymer network which essentially impacts the mechanical properties of the films (both Young’s modulus and toughness). Maximum toughness was observed for crosslinked latex films made from 50 wt.% of each POBM in the monomer feed.     

Crosslinking in Acetoacetoxy Functional Waterborne Crosslinkable Latexes

Chain crosslinking through the different stages of the production of acetoacetoxy ethyl methacrylate (AAEMA) containing acrylic latexes was studied; namely, during the synthesis step, after latex neutralization and after crosslinking with diamines. The gel content of the latex increased with increasing amounts of the functional AAEMA monomer likely due to dimethacrylate impurities contained in the monomer. Moreover, the neutralization of the latex with ammonia had important implications in the microstructure of the polymer. A base catalyzed Michael addition reaction between acetaocetoxy groups and terminal double bonds present in the polymer particles produced chain pre-crosslinking that affected the further crosslinking reaction with diamines. The compatibility of the diamines with the polymer matrix and the ratio diamine/acetoacetoxy used to crosslink the latex during film formation also played an important role in the crosslinking efficiency as evaluated by solvent resistance of crosslinked films.     

Simulation of Non‐Linear Free‐Radical Polymerization Using a Percolation Kinetic Gelation Model

A new kinetic gelation model that incorporates the kinetics of representative non‐linear free‐radical polymerization is presented. Specifically, free‐radical homopolymerization, polymerization in the presence of a chain‐transfer agent (CTA, CTA‐induced polymerization), and copolymerization of a mixture of the bi‐ and tetrafunctional monomer is used to simulate kinetic effects on polymerization statistics and microstructures. An algorithm for random next‐step selection in a self‐avoiding random walk and efficient mechanisms of a component's mobility are introduced to improve the generality of the predictions by removing commonly occurring deficiencies due to early trapping of radicals. The model has the capability to take into account into several free‐radical polymerization mechanisms such as crosslinking, branching, and transfer reaction, and also to predict the onset of the sol–gel transition, and the effect of chemical composition on the transition point. It is shown that a better understanding of microstructure evolution during polymerization and chemical gelation is attained. Lastly, one important benefit of the model is to simulate very highly packed random chains or microgels within a polymer network.