Effect of primary polymer chain rigidity on intramolecular cyclization and intramolecular crosslinking in free‐radical crosslinking monomethacrylate/dimethacrylate copolymerizations

d‐Bornyl methacrylate (BoMA) was chosen as a typical example of bulky monomethacrylate monomers, the polymerization of which led to the formation of a rigid polymer chain. To discuss the effect of primary polymer chain rigidity on intramolecular cyclization, we compared the solution copolymerization results of BoMA with 1 mol % ethylene dimethacrylate (EDMA; n = 1) and poly(ethylene glycol dimethacrylate) [CH₂?C(CH₃)CO(OCH₂CH₂)_nOCOC(CH₃)?CH₂, n = 9 (PEGDMA‐9)] with those of methyl methacrylate (MMA) with 1 mol % EDMA and PEGDMA‐9; the dependence of the weight‐average degree of polymerization on conversion for the former BoMA copolymerization systems was completely opposed to that for the latter MMA systems, and this was a reflection of a reduced occurrence of intramolecular cyclization caused by the rigidity of the primary polymer chain. The effect of primary polymer chain rigidity on intramolecular crosslinking was discussed through a comparison of both BoMA/EDMA and MMA/EDMA copolymerizations. The correlations of the intrinsic viscosity, root‐mean‐square (rms) radius of gyration, and second virial coefficient with the molecular weight were examined for both BoMA/EDMA (90/10) and MMA/EDMA (90/10) copolymerizations in a dilute solution because microgelation was observed in solution MMA/EDMA (90/10) copolymerization as a reflection of a locally extensive occurrence of intramolecular crosslinking. The logarithmic plots of both the intrinsic viscosity and rms radius of gyration versus the molecular weight for MMA/EDMA copolymerization were compared with those for the corresponding BoMA/EDMA copolymerizations. The second virial coefficients were greater than 10^?5 mol cm³ g^?2 for BoMA/EDMA copolymers, even when the conversion was very close to the gel point, whereas they were quite low, that is, less than 10^?5 mol cm³ g^?2, for an MMA/EDMA copolymer obtained at more than 15% conversion. These were ascribed to a suppressed occurrence of intramolecular crosslinking, a reflection of the lessened flexibility of the polymer main chain and a steric effect due to the bulky d‐bornyl groups. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1086–1093, 2004     

Rheological modelling of the free-radical crosslinking of PDMS rubber in the presence of TEMPO nitroxide

The aim of the present work is to study the free-radical kinetics of PDMS rubber crosslinking in the presence of 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) nitroxide. For this purpose a new method based on the relationship between the kinetics of the macro-radicals coupling [R_cc(t)] was derived from a fundamental kinetic model and the viscoelastic changes of the complex shear modulus (G′(t)_ω and G″(t)_ω). The kinetic model takes into account the initiator (Dicumyl peroxide in the present study) decomposition and the trapped PDMS macro-radicals in the presence of a radical scavenger such as TEMPO. Activation energy E_ac and collision frequency factor A_0c for the bimolecular termination reaction coefficient rate k_cc have been derived from the anisothermal DSC results according to the Kissinger method. Furthermore, it was observed that addition of TEMPO nitroxide can boost the initiator efficiency. The concentration variation of the active PDMS carbon-centred radicals [R_p(t)]_act and the [R_cc(t)] with reaction time were predicted using this kinetic model. On the other hand, the influence of TEMPO concentration in formulation ([N]₀) and effect of temperature on viscoelastic variations are studied. As a main result, the rheological modelling shows that this new method accurately predicts the time variation of complex shear modulus at any temperature and [TEMPO]/[DCP] ratio.     

Semibatch anionic ring-opening polymerization of octamethylcyclotetrasiloxane in emulsion

The semibatch anionic ring-opening polymerization of octamethylcyclotetrasiloxane in emulsion was studied under the condition of no seed particles used in polymerization system. Concentration of emulsifier was set above critical micelle concentration. Effect of monomer feed rate on the chemical kinetics, average particle size and distribution and on the polymer molar mass and distribution was investigated. During monomer addition, polymerization rate was constant and controlled by the monomer feed rate. According to the proposed mechanism for anionic polymerization in emulsion, it was assumed that also the number of active polymerization sites and monomer concentration at the particle surface, where propagation occurs, remain constant. A continuous increase of average particle size and absence of extensive condensation reactions at high conversion suggested that the freshly added monomer diffuses toward hydrophobic polymer particles, where it is consumed in propagation reaction and/or accumulated in the particle core.     

Ideal crosslinked-polymers and their characterization in free-radical monovinyl-divinyl copolymerizations

The critical conditions in which the classical Flory-Stockmayer gelation theory (F-S theory) is applicable to monovinyl-divinyl copolymerizations were pursued in detail. The resulting prepolymers or precursors of ideal crosslinked-polymers were characterized as standard polymers for the discussion of network formation in free-radical monovinyl-divinyl copolymerizations. Methyl methacrylate was copolymerized with a small amount of ethylene dimethacrylate, butylene dimethacrylate or nonapropyleneglycol dimethacrylate in the presence of lauryl mercaptan, a chain transfer agent to reduce the occurrence of a thermodynamic excluded volume effect and intramolecular crosslinking as the primary and secondary factors, respectively, for the greatly delayed gelation in the free-radical monovinyl-divinyl copolymerizations and, moreover, to keep the primary chain length constant by inhibiting a gel effect. The ratio of the actual gel point to the theoretical one reached 1.1, supporting the validity of F-S theory. The resulting prepolymers were subjected to SEC-MALLS analysis to determine the molecular weights, the molecular-weight distributions and the radii of gyration; the correlations of molecular weight vs. elution volume and radius of gyration vs. molecular weight were useful for the characterization of the precursors of ideal network-polymers.     

Effects of cyclization and electrostatic interactions on the termination rate of macroradicals in free-radical crosslinking copolymerization

Summary Effects of cyclization and ionic group contents on the termination rate of macroradicals formed at zero monomer conversion were investigated. For this purpose, the pregel regime of free-radical methyl methacrylate / ethylene glycol dimethacrylate (MMA/EGDM) and acrylamide / N,N'-methylenebisacrylamide (AAm/BAAm) copolymerization systems was studied by means of the dilatometric technique. To eliminate the chain-length dependent variation of the termination rates, different sets of experiments were carried out each at a fixed monomer and initiator concentration. At low crosslinker contents, the termination rate of zero-conversion macroradicals was enhanced in crosslinking copolymerizations compared to linear polymerization. This is due to the cyclization reactions which reduce the size of the macroradical coils and thus, enhance the termination rates due to the lowering of the thermodynamic excluded volume effect. As the amount of the crosslinker increases, an enhancement in the initial rate of polymerization is observed in all series of experiments, indicating that steric effects on segmental diffusion dominate at high crosslinker contents. The results also indicate a slower rate of termination of ionic macroradicals compared to the non-ionic radicals of the same molecular weight and points the significance of the thermodynamic excluded volume effect on rising the ionic group content. Received: 15 December 1997/Accepted: 13 February 1998     

空气湿固化含硅丙烯酸酯的制备与表征

通过甲基丙烯酸甲酯、丙烯酸丁酯、γ-甲基丙烯酰氧丙基三甲氧基硅烷制备了空气湿固化胶黏剂,使其在pH=7时水解交联。采用H1NMR及FTIR和气象色谱对其结构交联情况做了详细分析,并对溶液膜的性能做了一系列的测试比较。     

Polymer crosslinking controlled by release of catalyst encapsulated in polycarbonate micro-spheres

EVA crosslinking by ester-silane exchange reaction, controlled by catalyst released from polycarbonate micro-particles, was studied from rheological experiments. Firstly, dibutyltin dilaurate in polycarbonate (PC) (DBTDL) catalyst was encapsulated in polycarbonate micro-particles by an emulsion process. These PC micro-particles have a trimodal size distribution with a mean diameter around 25 μm. Furthermore, scanning electronic microscopy analysis revealed that the DBTDL catalyst is homogeneously dispersed in PC micro-particles. This was verified by DSC experiments which showed a T_g equal to 72 °C for the DBTL micro-particles. Secondly, the deformation and break-up studies of PC micro-particles in molten EVA matrix showed that PC micro-particles can only break up in EVA matrix of high molecular weight (EVA28-03) and for temperatures higher than 200 °C. Thirdly, rheological experiments showed that the crosslink reaction is controlled by the diffusion rate of DBTDL from PC micro-particles. However, this diffusion rate was observed to be independent on either the temperature or the PC micro-particles size.     

Radiation-induced crosslinking of UHMWPE in the presence of co-agents: chemical and mechanical characterisation

Radiation-induced crosslinking has been shown to have a beneficial effect on the wear resistance of ultra high molecular weight polyethylene (UHMWPE). Since we postulate that crosslinking takes place through reactions involving terminal double bonds, unsaturated additives were added to UHMWPE in this study to enhance crosslinking. UHMWPE specimens soaked in 1,7-octadiene, methylacetylene and ethylene, respectively, were irradiated with electron beam to different doses in single or multiple passages. FTIR spectroscopy was used for the chemical characterisation of the crosslinked polymer. Tensile tests were performed with all samples in order to monitor changes in the mechanical properties.Gel fraction measurements proved that crosslinking took place in all the irradiated samples, but 1,7-octadiene turned out to be the most effective additive for the present purpose, exhibiting a good efficiency in enhancing crosslinking. The FTIR analyses showed that consumption of vinyl double bonds is proportional to the irradiation dose, with an efficiency which increases with increasing their initial concentration, confirming the hypothesis of the involvement of these groups in the crosslinking process. In the case of UHMWPE with 1,7-octadiene irradiated to high doses in multiple passages, the results of the tensile tests indicated a significant decrease in both elongation at break (up to 65%) and ultimate stress (up to 25%). A possible explanation of the diminished strength of the crosslinked material has also been proposed.     

Suppression of inhomogeneities in hydrogels formed by free-radical crosslinking copolymerization

Network microstructures of poly(acrylamide) (PAAm) and poly(N,N-dimethylacrylamide) (PDMA) hydrogels were compared by static light scattering and elasticity measurements. The hydrogels were prepared by free-radical crosslinking copolymerization of the monomers acrylamide (AAm) or N,N-dimethylacrylamide (DMA) with N,N′-methylenebis(acrylamide) as a crosslinker. During the formation of PAAm gels, the reaction time dependence of the scattered light intensity exhibits a maximum at a critical reaction time, while in case of PDMA gels, both a maximum and a minimum were observed, corresponding to the chain overlap threshold and the gel point, respectively. This difference in the time-course between the two gelling systems is due to the late onset of gelation in the DMA system with respect to the critical overlap concentration. Compared to the AAm system, no significant scattered light intensity rise was observed during the crosslinking polymerization of DMA. It was shown that, regardless of the crosslinker ratio and of the initial monomer concentration, PDMA gel is much more homogeneous than the corresponding PAAm gel due to the shift of the gelation threshold to the semidilute regime of the reaction system. The results suggest that the spatial gel inhomogeneity can be controlled by varying the gel point with respect to the critical overlap concentration during the preparation of gels by free-radical mechanism.     

Transition into the gel regime for free radical crosslinking polymerisation in a batch reactor

Crosslinking polymerization has been studied by means of a four-dimensional population balance model accounting for chain length, free pending double bonds, crosslinks, and multiradicals as dimensions. The model, for the first time and to a full extent resolves the crosslinking problem as formulated by Zhu et al. [1] and covers both pre-gel and gel regimes, in a straightforward manner. Approximations on radial basis functions have been employed to reduce the size of the system with minimal information loss. The model has been validated with data from an experimental crosslinking polymerization, Methyl Methacrylate with Ethylene Glycol Dimethacrylate. Non-trivial patterns in the time evolution of average quantities like crosslink densities, partly observed in prior studies [2–4], are naturally emerging from the model by computing marginals of the four-dimensional distribution possessing an interesting multimodal structure.     

Chemical crosslinking and biophysical properties of electrospun hyaluronic acid based ultra-thin fibrous membranes

Hyaluronic acid (HA) ultra-thin fibrous membranes by electrospinning technology had been fabricated successfully by our group. However, these HA fibrous membranes dissolve and decompose rapidly in water. In this study, chemical crosslinking of HA and HA based ultra-thin fibrous membranes was studied in detail. It was found that the chemically crosslinked pure HA ultra-thin fibrous membranes with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) as the crosslinking agent did not show an obvious improvement in the water-resistance. However, by simply adding and changing the gelatin content, the degradation time of the HA based fibrous membranes could be effectively adjusted from one week to several months. That is with the increase in gelatin content, the water-resistance of the HA/gelatin bi-component membranes could be increased greatly. The crosslinked HA/gelatin bi-component membranes were evaluated in vitro by seeding with mouse embryonic fibroblasts (3T6-Swiss Albino). The results showed that the HA/gelatin fibrous membranes were cytocompatible and suitable for cell viability. However, cells were hard to attach and grow on the HA based membranes. Thus, the HA/gelatin fibrous membranes with controllable degradation rate could have good potential applications in the anti-adhesion and wound dressing fields.     

Chemical reactions of polymer crosslinking and post-crosslinking at room and medium temperature

This review focuses on various strategies that enable the crosslinking and post-crosslinking of polymers, excluding crosslinking obtained by radiation (e.g., X-ray, UV, etc.) and that at high temperature. The review is divided into two main parts: systems enabling crosslinking at room temperature and those for which crosslinking occurs at intermediate temperatures (<150 °C). In the first part, various key functional groups can be used, such as (i) carboxylic acid involving reactions with compounds that bear carbodiimide or aziridine functions; (ii) acetoacetyl groups (with isocyanate, activated alkenes, aldehyde, amine functions); (iii) reactions involving activated amines with carbonyl functions (aldehydes, ketones, etc.); (iv) species bearing acetals as pH-sensitive crosslinking agents since they are stable in basic medium but they can self react under acidic conditions; (v) acrylamide functions which are able to self-crosslink; (vi) crosslinking agents able to react with water (such as species that bear a poly(alkoxy)silane for sol-gel process) and derivatives containing isocyanate functions and (vii) systems that require oxygen, for example polymers bearing double bonds, boranes for generating hydroperoxides and acetylenic functions which undergo acetylenic coupling. The second series of systems, used at higher temperatures (yet below 150 °C) involving the following key functions: (i) carboxylic acid that react with oxazoline, or epoxide function where specific catalysts are necessary; (ii) alcohols reacting with protected urethanes, azlactones and methylol amide (for coating applications); (iii) azetidines (obtained from a cyclic amine onto an activated double bond) which self-crosslink; (iv) reversible Diels-Alder reaction (such as furane/bismaleimide reaction), and (v) Huisgen reactions between azido and triple bond.Various examples are presented, along with a discussion of their properties and applications.     

Study on degradation and crosslinking of impact polypropylene copolymer by dynamic rheological measurement

The structural stability of impact polypropylene copolymer (IPC) melt under high temperatures was explored by dynamic rheological measurement. The structure changes including degradation and crosslinking of IPC were discussed through examining the influence of temperature and additive antioxidant on dynamic rheological functions. A plateau of dynamic storage modulus (G′) appeared in low frequency region at high temperatures under air atmosphere. Furthermore, when IPC sample was annealed at 230 °C, its viscoelasticity presented a dramatic change. The time dependences of G′ for pure IPC at different temperatures were investigated. At 190 °C, a slight drop and a succedent rise of G′ for IPC0 appeared. The decrease period of G′ generally shorted and the increase of G′ became more remarkable with the increase of temperature. The decrease of G′ was ascribed to the degradation in IPC and the G′ increase was due to the crosslink reaction. Through incorporation of antioxidant into IPC, the crosslinking in IPC could be effectively inhibited, and the degradation and crosslinking in IPC were believed to result from PP component.     

Preparation conditions and swelling equilibria of dextran hydrogels prepared by some crosslinking agents

Crosslinking reactions of Dextran (Dx) (M_n of 2.0×10⁶ g mol⁻¹) with some selective Cl-, P- and N-containing functional monomers such as epichlorohydrin (ECH), phosphorus oxychloride (POC1₃) and N,N′-methylenebisacrylamide (MBAM) were carried out in the basic aqueous solutions (2.8 N NaOH) at 25-50 °C. The optimum conditions of the effective swelling and crosslinking for the each system studied were found in copper (CuCl₂·2H₂O) solution. The percent swelling, equilibrium swelling, initial rate of swelling, swelling rate of constant, equilibrium water content, and diffusion type and constant values were evaluated for Dx/crosslinker (CL) systems at 1 mg/100 ml copper (CuCl₂·2H₂O) solution. A substantial difference of these parameters observed for the various Dx/CL systems was explained by the effect of nature of crosslinking agents on the mechanism of crosslinking and swelling processes. It was shown that S_eq and M_c values increase depending on the nature of CLs in the following order: ECH>MBAM>POCl₃. General scheme and proposed mechanism of crosslinking reactions in the Dx/CL systems were also described.     

Thermal and mechanical properties of microcellular thermoplastic SBS/PS/SBR blend: effect of crosslinking

This study investigates the effect of peroxide crosslinking on the structure and mechanical properties for SBS/PS/SBR foams composed of polystyrene (PS), poly(styrene-b-butadiene) diblock copolymer (SBR-1502), and poly(styrene-b-butadiene-b-styrene) triblock copolymer (SBS). The cell size and its distribution of SBS/PS/SBR foams were investigated by SEM images, showing the smaller and denser of hollow cells for the SBS/PS/SBR foam containing the higher concentration of DCP (dicumyl peroxide). As expected, the density of the SBS/PS/SBR foams increases with increasing the content of DCP. The high density of polymeric foams exhibits the high mechanical properties such as hardness, shrinkage, tensile strength, tear strength, elongation at break, and compression set.     

Imidazole-promoted acceleration of crosslinking in epoxidized natural rubber/dicarboxylic acid blends

The crosslinking of epoxidized natural rubber (ENR) with dodecanedioic acid was tremendously accelerated in presence of 1,2-dimethylimidazole (DMI). The curing reaction of ENR containing 10 or 25 mol % epoxide groups was followed by rheology at 180 °C or 160 °C and the mechanical properties of the cured materials were examined by stress-strain experiments. An equimolar amount of accelerator and carboxylic functions was required to reach optimum tensile properties. Among a variety of usual accelerators, DMI was found the only one efficient, which was associated to the formation of reactive imidazolium carboxylates. The presence of these intermediates was supported by DSC experiments. Instead of a catalytic mechanism, the activation of the crosslinking agent is enabled by the synergetic association of DMI and diacid forming a soluble species in the rubber matrix. This simple system thus enables the efficient crosslinking of functionalized natural rubber without the use of sulphur or peroxides.     

Characterization of the crosslinking reaction in high performance adhesives

In this study, thermal analysis, infrared spectroscopy (near and mid) in conjunction with low field NMR, have been used to characterize the crosslinking reaction involving phenol formaldehyde resin and a crosslinking agent, hexamethylenetetramine (HMTA) used as adhesive in organic-inorganic composites. The strong hydrogen bonds in the resin and the completely crystalline HMTA (T_m = 280 °C) severely hamper the crosslinking process. Yet the addition of a small amount of plasticizer can induce an efficient (> 50% increase) crosslinking reaction as compared to the system without plasticizer. The infrared spectroscopy clarifies the dissolution process of the crystalline crosslinker and the specific interactions needed to achieve miscibility of the reactants. The thermal analysis enabled us to follow the reaction as a function of temperature. The low field NMR with the T₁ inverse recovery technique allowed us to monitor the crosslinking process directly. For the first time, it is now possible to identify the functionality of the plasticizer and correlate the degree of crosslinking achieved in order to assess the macroscopic cohesive strength needed for high performance adhesives.     

DSC and TGA study of the transitions involved in the thermal treatment of binary mixtures of PE and EVA copolymer with a crosslinking agent

The crosslinking process of PE and EVA with different concentrations of a crosslinking agent (CA) has been studied. The extent of the crosslinking has been evaluated by gel fraction and density measurements. The different transitions involved in the thermal processing and decomposition have been studied by DSC and TGA and compared for all the mixtures prepared. Mechanistic pseudo-kinetic models have been suggested and applied involving all the fractions susceptible of undergoing transitions or reactions. The models applied can be of great interest in understanding the phenomena involved, as well as in modeling the heat effects in the whole processing or pyrolysis of this type of products.