The effect of kinetic chain length on the mechanical relaxation of crosslinked photopolymers

This work investigates the effect of kinetic chain length on the network structure of multi-functional (meth)acrylates. Two chain transfer agents, dodecanethiol and m-toluenethiol, were added to a series of crosslinking free radical polymerizations to decrease the kinetic chain length. Using near-IR spectroscopy and dynamic mechanical analysis, the effect of chain transfer on the polymerization rate and network properties (i.e. T_g and modulus) was investigated. The results suggest that macroradical chain length, which has been shown to play a role in the termination kinetics of crosslinking systems, may also impact the network properties of the formed polymer. With the addition of only 0.5-1 wt% chain transfer agent, differences in the polymerization rate and mechanical properties were observed in the crosslinking methacrylate systems. The polymerization rate was significantly suppressed and the T_g of the cured network was found to decrease by up to 10 °C, depending on the monomer formulation. The largest differences in the mechanical properties occurred in the systems with the lowest crosslinking density and diminished as the crosslink density of the cured polymer increased. Furthermore, the differences were less dramatic in the multi-EGDMA systems, that have some inherent ability to chain transfer, and were not discernable in the transfer dominated diacrylate systems.     

Elastomers with two crosslinking systems of different lengths viewed as bimodal networks

Elastomers cured with two crosslinking systems such as sulfur and the polymerization products of p-benzoquinone are shown to have much improved overall mechanical properties. It was thought that this was because of the antioxidizing potency of the quinone polymers that act as radical traps during the oxidative degradation process. However, if the polyquinone crosslinks of the greater length themselves act as elastomeric network chains, then a bimodal network with its, exceptional mechanical properties is produced. Adding commercial antioxidants to the samples will even harvest much tougher samples. The antioxidant added, along with the quinone polymers, will reserve the integrity of the bimodal network produced and lead to better mechanical properties. Tests were also done to examine the effect of the quinone polymers on the hardness and on the onset of the vulcanization process. © 1996 John Wiley & Sons, Inc.     

Equilibrium swelling and mechanical properties of hydrogels of acrylamide and itaconic acid or its esters

Summary The equilibrium swelling and the plateau elastic modulus of a family of hydrogels made by the polymerization of acrylamide with itaconic acid or some of its esters were investigated as a function of composition and crosslinking degree to find materials with satisfactory swelling and elastic properties. We show that an appropriate selection of the comonomers and the concentration of the crosslinking agent is very important to produce hydrogels with large swelling capacity and good mechanical attributes. Tailoring of mechanical properties and swelling can also be achieved by this method. Received: 7 September 1999/Revised version: 3 December 1999/Accepted: 6 December 1999     

Polymerization of dicyclopentadiene: A new reaction injection molding system

Dicyclopentadiene was polymerized by reaction injection molding (RIM) using a catalyst system based on WCl₆ and diethylaluminium chloride. Ring opening polymerization results in formation of a crosslinked polymer with a high crosslink density. The kinetics of the fast exothermic reaction was followed by the adiabatic temperature rise method. In addition to the “adiabatic” polymerization, isothermal reactions were carried out in a thin mold. The properties of the cured samples were determined by dynamic mechanical measurements, solgel analysis, gas chromatography, mass spectrometry, DSC, and IR spectrometry. Gel fraction, glass transition temperature, content of the unreacted monomer, the modulus, and the degree of swelling were used to characterize the cured samples. The system shows very low critical conversion at the gel point (α_c < 0.01) proving a chainwise mechanism of the polymerization. Possible participation of a cationic mechanism is discussed. We found the specific reaction temperature range (T = 100–140°C) for optimum properties of the cured samples. Deterioration of properties (decrease in the crosslinking density, etc.) at a high temperature is a result of a faster deactivation of catalytic centers and a reversibility of the exothermic ring opening polymerization. Reverse cyclodegradation is preferred at a higher temperature.     

Synthesis and characterization of uv-curable acrylated urethane prepolymers,I. Effects of reactive diluents and relative humidity on physical properties

A UV-curable acrylated urethane prepolymer was synthesized from tolylene-2,4-diisocyanate (TDI), a polyether polyol (Arcol 1131) and endcapped with 2-hydroxyethyl methacrylate (HEMA) by addition reaction in the presence of dibutyltin dilaurate as catalyst. UV curing was performed with either diethylene glycol diacrylate or thiodiethylene glycol diacrylate as reactive diluent. The effects of reactive diluent types, their concentrations and the humidity of environment on mechanical properties of cured films were investigated. Changes in the tensile strength, elongation and Young's modulus values of the cured films upon addition of reactive diluents with different concentrations were related to the effect of the diluent on the crosslinking density of cured films. The increase of relative humidity from 50 to 95% caused a decrease of tensile strength and Young's modulus values of cured films. It is proposed that the decrease of these physical properties in high relative humidity is due to the formation of hydrogen bonding in polymer chains caused by water molecules.     

Preparation and properties of film materials of poly(aryl ether ketone)–based phthalonitrile resins

A series of poly(aryl ether ketone) polymers (m‐PAEK‐CN) containing phthalonitrile were synthesized by a direct solution polycondensation and characterized by Fourier‐transform infrared spectroscopy and hydrogen nuclear magnetic resonance. Thermal crosslinking of m‐PAEK‐CN, catalyzed by p‐BAPS, was then performed via heating their films up to 350^oC. Dynamic rheology results showed that the rate of diamine‐catalyzed crosslink reaction could be easily controlled by varying the content of cyano groups in the polymer. The uncured synthesized polymers had good solubility, whereas the cured ones became insoluble in common organic solvents. Spectra measurement demonstrated the trimerization reaction of terminal cyano groups to form triazine rings. The resulting cured samples had higher glass transition temperatures, better thermal, and thermo‐oxidative stability with high char yield than the uncured ones. Gel content measurements demonstrated that the cured polymers had high crosslinking density with significantly high gel content over 96.0%. Thermal mechanical analysis indicated that the cured phthalonitrile resins possessed excellent thermal mechanical properties and dimensional stability at increased temperatures. The tensile modulus and strength of the cured m‐PAEK‐CN increased up to 2101–2104 MPa and 96–100 MPa, respectively, which were about 21–23% and 32–56% higher than the uncured ones’. This kind of phthalonitrile resins may be used as a candidate for high‐performance polymeric film materials. POLYM. ENG. SCI., 55:2313–2321, 2015. © 2015 Society of Plastics Engineers     

Preparation and properties of UV-autocurable BTDA-based epoxy-multiacrylate resins. Effects of the degree of polymerization and the epoxy type

A series of UV-autocurable epoxy-multiacrylate resins was synthesized, and the effects of degree of polymerization (DP) and epoxy type on their properties were investigated. These autocurable multiacrylate resins possess good pot life and are cured rapidly when exposed to ultraviolet (UV) without the addition of photoinitiator or photosensitizer. The curing rate of the autocurable resins was probably dependent on the number of abstractable hydrogen in epoxy resins. Stress-strain, differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA) were used to characterize the properties of cured multiacrylate resins. Increased crosslinking density of cured films improved tensile properties. Increasing the molar ratio of epoxy resin in the multiacrylate resins was found to decrease the effective acrylate concentration of resins and to depress crosslinking density of cured resins, which also resulted in an increased elongation at break but a decreased Young's modulus and breaking strength. Furthermore, the different structures of epoxy resins were used to give wide range properties of cured epoxy-multiacrylate resins with a glass transition temperature (T_g range from 74 to 102°C. The film properties of the multiacrylate resins coated on steel plates were also investigated.     

Transesterification and crosslinking of poly(vinyl chloride‐co‐vinyl acetate) copolymers in the melt

A new method to obtain hydroxylated poly(vinyl chloride) (PVC‐OH) and its crosslinking in the melt are studied. Starting from a vinyl chloride‐co‐vinyl acetate copolymer, a transesterification reaction in the presence of an alcohol during the processing of plasticized polymer is investigated as a function of the processing temperature and alcohol nature (1‐butanol or 1‐octanol). Reaction evolution is followed by ¹H‐NMR and IR spectroscopies. The best results are obtained for 1‐octanol, and they show the absence of secondary reactions and the progressive appearance of OH groups in the polymer as acetate groups disappear. On the other hand, crosslinking of the thus‐obtained PVC‐OH with hexamethylene diisocyanate (HMDI) during the processing is also studied. The gel content and the mechanical properties at 140°C are studied as a function of three crosslinking variables: number of OH groups present in the polymer, concentration of HMDI added to the polymer, and time of crosslinking. The results show that by optimizing those parameters it is possible to obtain gel contents up to 100% and an increase of 600% in the Young's modulus and 1300% in the ultimate tensile strength with respect to the plasticized PVC. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 621–630, 1999     

Deformation-morphology correlations in electrically conductive carbon nanotube—thermoplastic polyurethane nanocomposites

Addition of small amounts (0.5-10 vol%) of multiwall carbon nanotubes (CNT) to thermoplastic elastomer Morthane produced polymer nanocomposites with high electrical conductivity (σ∼1-10 S/cm), low electrical percolation (?∼0.005) and enhancement of mechanical properties including increased modulus and yield stress without loss of the ability to stretch the elastomer above 1000% before rupture. In situ X-ray scattering during deformation indicated that these mechanical enhancements arise not only from the CNTs, but also from their impact on soft-segment crystallization. The deformation behavior after yielding of the nanocomposites, irrespective of CNT concentration, is similar to the unfilled elastomer, implying that the mechanistics of large deformation is mainly governed by the matrix. The relative enhancement of the Young's modulus of the nanocomposites is comparable to other elastomeric nanocomposites, implying that to the first order specific chemical details of the elastomeric system is unimportant.     

Preparation of tricalcium phosphate–calcium alginate composite flat sheet membranes and their application for protein release

Tricalcium phosphate–calcium alginate (TCP–CA) composite flat sheet membranes with the thickness of 150–200 μm were prepared by Ca²⁺ crosslinking of TCP and sodium alginate (SA) aqueous solution. The composite flat sheet membranes were characterized by SEM, FTIR, DSC and BET method. The mechanical properties of the membranes were tested with a tensile testing machine in wet form. Hemoglobin (HB) was immobilized in the following four matrices (1) HB was adsorbed onto the TCP powders directly, (2) HB and SA dissolved together, and then crosslinked by CaCl₂, (3) HB was pre‐adsorbed onto TCP powders, and then mixed with SA aqueous solution, finally crosslinked by CaCl₂, (4) HB and TCP powders were individually dispersed in the SA aqueous solution and then crosslinked by CaCl₂. The release behaviors of the four materials were investigated in phosphate buffer (pH = 6.86), Tris‐HCl solution (pH = 7.4) and NaCl solution (0.9 wt%). SEM observation showed that the TCP particles were well embedded and homogeneously distributed in the alginate matrix. DSC and FTIR indicated that there were additional interactions between the TCP and the polymer. TCP can improve the mechanical properties of CA membranes and the swelling ratio of TCP–CA membrane in NaCl solution decreased with the increase of TCP content. The sustained release behaviors of matrix 3 were shown in PBS, Tris‐HCl and NaCl solutions. POLYM. COMPOS., 36:1899–1906, 2015. © 2014 Society of Plastics Engineers     

The effect of synthesis methods on the mechanical properties of self‐crosslinkable poly(n‐butyl methacrylate‐co‐N‐methylolacrylamide) films

The effects of particle size and parent polymer characteristics on the mechanical properties, gel fraction, and swelling index of self‐crosslinkable poly(n‐butyl methacrylate‐co‐N‐methylolacrylamide) films made by two‐stage emulsion or microemulsion polymerization in the presence of variable amounts of the chain transfer agent, n‐butyl mercaptan, are reported here. In films prepared with latexes made by microemulsion polymerization, the crosslinking degree increased greatly on curing; by contrast, in those made by emulsion polymerization, the crosslinking degree practically did not increase after curing. Stress–strain tests of uncured and cured films indicate that microemulsion‐made films are tougher than the emulsion‐made films. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Elastomeric domain-type interpenetrating polymer networks

Elastomeric latex interpenetrating polymer networks (IPNs) can result from a two-stage emulsion polymerization procedure in which styrene is polymerized and cross-linked on a lightly cross-linked polyacrylate (PA) seed latex in a ratio of 75 : 25 PA-PS. The multiphase nature of these IPNs is indicated by two distinct T_gs and is confirmed by cold-stage transmission electron microscopy and by the unique mechanical and rheological properties that are intimately related to the material's structure. PS microdomains reinforce the elastomeric PA, yielding a significant modulus, and interparticle PS physical ties yield a significant ultimate tensile strength. The elastomeric latex IPN's dual thermoset-thermoplastic nature is revealed in a stick, slip, roll flow mechanism of the cross-linked submicrometer particles, which can be injection molded as a thermoplastic. The relationships among the polymerization procedure, the structure, and the physical properties are characterized by the examination of several different materials using a variety of analytic techniques.     

Polymer-Impregnated mortars I. Effect of polymer state on mechanical behavior

Mortar specimens were impregnated with methyl methacrylate, n-butyl acrylate, styrene, and crosslinking agents in various combinations. After polymerization of the monomers in situ, studies of mechanical properties such as Young's modulus and compressive strength were made. In one experiment, various ccpolymers of methyl methacrylate and n-butyl acrylate were prepared and tested as a function of temperature. Excellent reinforcement was obtained with any combination of monomers as long as the resulting polymer was at a temperature below its glass transition temperature. This suggests that the modulus of the reinforcing polymer is crucial, glassy behavior being required. The addition of crosslinking agents such as TMPTMA increased the high temperature strength, however.     

Hydrophilic elastomer containing poly(tetrahydrofuran) segments and viologen units

Elastomeric polymer consisting of poly(oxytetramethylene) segments and viologen units was prepared by living cationic polymerization of tetrahydrofuran (THF), followed by the end capping of the living polymer by 4,4´-bipyridine. The obtained ionene-type polymer (PTV) exhibited low tensile modulus and high tensile strength with large extensibility, i.e., it is an excellent elastomer from its mechanical properties. The water swelling behavior and effects of water and the sort of counter anions on the tensile properties of PTV were investigated. PTV showed high hydrophilicity; e.g., the water content of PTV with bromide counter anions was ca. 40% after swelling in water at 25°C for 24 h. However, even for the samples with such a high water content, PTVs displayed good elastomeric properties, although the tensile modulus and tensile strength were decreased with increase of water content. The hydrophilicity of PTV was found to depend on the kind of counter anions, which in turn determined the tensile properties of PTVs in a swollen state.     

Influence of compositional variables on the morphological and dynamic mechanical behavior of fatty acid based self-crosslinking poly (urethane urea) anionomers

Fatty acid based self-crosslinking polyurethane urea (PUU) anionomers can find potential applications in coatings field due to enhanced chemical resistance properties. To optimize their performance in coatings, the molecular features that influence the microphase morphology and dynamic mechanical (DM) behavior of polymer films must be understood and exploited. In this work, comprehensive materials characterization of model PUU anionomers films with oxidative-crosslinking microstructure is addressed. For this, linoleic fatty acid based precursor (LPE) was included in polymer backbone which provides reactive sites for autooxidative polymerization. Three series of compositions were prepared with urea content of 8.4%, 13.2% and 18.1% where within each series LPE content has been increasing in same proportion. Different experimental techniques like FTIR, DSC, DMA and mechanical testing were utilized to study the effect of compositional variables on the extent of phase segregation, domain structure and mechanical properties of fully cured polymer samples. The extent soft segment (SS) oxidative crosslinking had marked effect on the microphase morphology and DM properties of materials of lowest urea content. Significant phase mixing was observed with evolvement of single heterogenous phase in the sample with highest LPE content. Samples with 13.2% urea shows less sensitivity toward increased SS crosslinking in their microphase morphology change. Their mechanical and DM properties were observed to be dominated by interlocked hard domains. With higher urea content, such kind of hard segment cohesion results due to greater strength of bidenate H-bonding among urea linkages. While samples with highest % urea, were clearly found to be well microphase separated compared to other two series with highest HS interconnectivity and have marginal effect of extent of SS crosslinking on microphase separation. This study gives an insight about effect of extent of complex oxidative crosslinking on the microphase separation and DM behavior of segmented PUU anionomers based films with different urea content which is useful in designing such materials in coating system with specific surface structure and function.     

Elastomeric latex domain-interpenetrating polymer networks: Physical and rheological properties

The combination of rubbery and rigid polymers in a multiphase structure using staged emulsion polymerization has yielded materials with properties ranging from reinforced elastomers to high impact plastics. The many different particle morphologies that result from a two-stage latex (TSL) polymerization include core/shell, domain, interpenetrating polymer networks (IPN), and various combinations thereof. The sequence of polymerization, crosslinking, grafting, and composition are among the significant parameters that determine the particle morphology. Elastomeric TSL with soft polyacrylates (PA) as the seed particles and polystyrene (PS) as the second stage, with each stage lightly crosslinked, may yield IPN-microdomain particles. The particle morphology has been elucidated through a combination of microscopy and mechanical property analyses. The significant modulus of elastomeric latex interpenetrating polymer networks (LIPN) results from reinforcement by PS intra-particle microdomains and their significant tensile strength from a strength forming mechanism of PS inter-particle microdomains. The increase in the PA seed crosslinking increases the crosslinked PS (xPS) level of molecular mixing with, and grafting via residual unsaturation to, the crosslinked PA (xPA) network and decreases particle deformnability. At higher xPS concentrations the formation of an xPS-rich shell enhances xPS continuity in the molded material through the partial coalescence of the shells, diminishing the PA continuity, and yielding more PS-like properties. The submicron lightly crosslinked latex particles with these different morphologies flow as a pseudoplastie material through a particle slippage flow mechanism exhibiting neither a Newtonian plateau nor a yield stress at low shear rates. The deformable lightly crosslinked particles with interchangeable PS ties which disintegrate at elevated temperatures retain their identity and regain their shape at the cessation of shear. The LIPN can be processed using standard thermoplastic methods and machinery, with power law constants and shear insensitive flow activation energies that are similar to those of thermoplastics at high levels of shear. Uncrosslinked PS shells around crosslinked PA seed particles, on the other hand, completely coalesce upon molding to form a continuous thermoplastic PS matrix that may essentially flow through molecular deformation.     

Dual‐phase polymer electrolyte with enhanced mechanical strength

The effect of the heat treatment on the tensile properties and the dynamic moduli of polymer matrix films and polymer electrolytes were studied to further increase the mechanical properties. The crosslinking of latex particles brought about their improvements by heating. We have fabricated the polymer electrolyte with a tensile strength of 3.0 MPa, together with a conductivity above 1 mS/cm for application to a lithium secondary battery. This polymer electrolyte had the highest tensile strength among the known gel polymer electrolytes having conductivity over 1 mS/cm, although mechanical properties of plasticized polymer electrolytes have rarely been reported. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1835–1839, 1999     

A new route for organic-inorganic hybrid material synthesis through reactive processing without solvent

A new route to elaborate organic-inorganic hybrid materials is presented. It is based upon two successive steps, the former is the crosslinking of polymer which contains pendant ester groups such as poly(ethylene-co-vinyl acetate) (EVA) through ester-alkoxysilane interchange reaction in molten state in the presence of dibutyltin oxide as catalyst. The latter is the hydrolysis-condensation reactions of available alkoxysilane groups in the polymer network leading to the silica network co-grafted onto the organic network. More particularly the hydrolysis-condensation reactions in solid state leading to the silica network grafted and confined in the organic network are addressed in the present work. The progress of the hydrolysis-condensation reactions was investigated by gas chromatography, FT-IR spectroscopy, ²⁹Si solid NMR, volume swelling degree at equilibrium and dynamic mechanical analysis. Two side reactions have been evidenced leading to alcohol groups grafted onto EVA. The silanols and these alcohol groups can participate to hydrogen bonds between ester and silica domains. The organic-inorganic hybrids elaborated according to this new chemical route exhibit improved mechanical and thermomechanical properties with respect to the EVA while having an elastomeric behavior with respect to the nanocomposite synthesized by in situ polymerization of tetraethoxysilane.     

Mechanical properties of thermoplastic elastomers based on silicone rubber and an ethylene–octene copolymer by dynamic vulcanization

Thermoplastic vulcanizates (TPVs) are a special class of thermoplastic elastomers that are generally produced by the simultaneous mixing and crosslinking of a rubber with a thermoplastic polymer at an elevated temperature. Novel peroxide‐cured TPVs based on blends of silicone rubber and the thermoplastic Engage (an ethylene–octene copolymer) have been developed. These TPVs exhibit very good overall mechanical and electrical properties. With an increasing concentration of dicumyl peroxide, the tensile strength, modulus, and hardness of the TPVs increase, whereas the elongation at break decreases. Significant correlations have been obtained from oscillating disc rheometer torque values with various physical properties, such as the modulus and tension set of the TPVs. The aging characteristics and recyclability of the silicone‐based TPVs are also excellent. Scanning electron microscopy photomicrographs of the TPVs have confirmed a dispersed phase morphology. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Static and Dynamic Mechanical Behavior of Electron Beam‐Cured Monomer and Monomer/Liquid Crystal Systems

Summary: This paper discusses static and dynamic mechanical properties of electron beam‐cured mixtures made of the nematic liquid crystal (LC) E7 and either tripropyleneglycol diacrylate (TPGDA) or propoxylated glycerol triacrylate (GPTA) as monomers differing essentially by their functionality. Dilution of the initial mixture with LC leads to a significant weakening of the film mechanical strength. Strong effects were found on Young modulus and rubbery state modulus. As the concentration of LC increases, these mechanical parameters drop significantly together with the glass transition temperature of the polymer showing a plasticizing effect. The results for the glass transition temperatures for the polymer and the LC were confirmed by thermograms obtained by differential scanning calorimetry.

Storage tensile modulus versus temperature of EB‐cured TPGDA/E7 films for different LC concentrations.