A study of the yield and post‐yield behavior of syndiotactic polypropylene

The strain recovery of three syndiotactic polypropylenes (s‐PPs) differing in the percentage of [rrrr] pentad is investigated. A suitable method based on loading–unloading tests at constant displacement rate in tensile loading conditions is adopted to measure the residual and recovered strain components of the applied strain. The method allows to obtain a large amount of data from few tests and to explore a wide strain range. The dependence of the material's strain recovery on the applied strain is analyzed in relation to s‐PP strain‐induced microstructural changes and crystalline form transitions, which are reported in literature. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1276–1282     

Synthesis of networked polymers by crosslinking reactions of polybenzoxazine bearing allyl group in the side chain

A polybenzoxazine bearing allyl group in the side chain was synthesized by the ring‐opening polymerization of N‐allyl‐benzoxazine and was crosslinked by the two different processes, (1) thermally induced oligomerization of the allyl side chains and (2) radical addition of dithiol (thiol‐ene reaction) to the allyl side chains. The former process was promoted by adding 2,5‐dimethyl‐2,5‐di(tert‐butylperoxy)hexane as a radical source, leading to the improved yield of the networked polymer isolated as acetone‐insoluble fraction. The thiol‐ene reaction with using 1,6‐hexanedithiol was also an efficient method for crosslinking the polybenzoxazine. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Photo‐induced thiol‐ene polysulfide‐crosslinked materials with tunable thermal and mechanical properties

Photo‐induced thiol‐ene crosslinked polymeric networks have been extensively explored in constructing a variety of new materials with enhanced mechanical properties for optical, biomedical, and sensing applications. Toward the broad applications, however, tunable mechanical properties are greatly desired. Here, an effective approach utilizing high‐molecular‐weight methacrylate copolymers having pendant thiol and vinyl groups (MCPsh and MCPenes) to modulate thermal and mechanical properties of photo‐induced thiol‐ene crosslinked materials is reported. The MCP copolymers are synthesized by an industrially friendly polymerization method, followed by post‐modification including either a facile coupling reaction or reductive cleavage. Upon UV irradiation, thiol‐ene reactive blends of MCPsh and MCPenes yield highly crosslinked materials through the formation of flexible sulfide linkages. These polysulfide‐crosslinked materials based on rigid MCP backbones exhibit enhanced mechanical properties. Further, their thermal and mechanical properties are tuned by modulating monomer compositions of MCPs as well as varying numbers of pendant SH or vinyl groups (i.e., extent of crosslinking densities). This approach is versatile and effective for development of high performance polymeric materials. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3060–3068     

Crosslinkable polyurethane bearing a methacrylate structure in the side chain

A polyurethane bearing methacrylate groups through urethane linkages was prepared by the addition of 2‐methacryloyloxyethyl isocyanate to the hydroxyl groups in poly(hydroxyurethane) prepared by the polyaddition of a bifunctional cyclic carbonate with 1,12‐diaminododecane. The urethanization proceeded quantitatively in the presence of a catalytic amount of di‐n‐butyltin dilaurate at an ambient temperature, whereas a crosslinked product was obtained from the reaction at 60 °C. The resulting linear polyurethane, bearing a methacrylate structure, was thermally crosslinkable. Its radical copolymerization with vinyl‐type monomers afforded the corresponding crosslinked polymers, whose low glass transition temperatures suggested the flexibility of the polymer chains in the crosslinked product. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3400–3407, 2007     

Healable dual organic–inorganic crosslinked sol–gel based polymers: Crosslinking density and tetrasulfide content effect

In this article, the first generation of healable sol–gel based polymers is reported. A dual organic–inorganic crosslinked network is developed containing non‐reversible crosslinks and reversible (tetrasulfide) groups. The designed polymer architecture allows thermally induced mesoscale flow leading to damage closure followed by interfacial strength restoration due to reformation of the reversible groups. While the reversible bonds are responsible for the flow and the interface restoration, the irreversible crosslinks control the required mechanical integrity during the healing process. The temperature dependent gap closure kinetics is strongly affected by the crosslinking density and tetrasulfide content. Raman spectroscopy is used to explain the gap closure kinetics in air and dry nitrogen. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1953–1961     

Effect of annealing on the viscoelastic and viscoplastic responses of low‐density polyethylene

Two series of tensile tests with constant crosshead speeds (ranging from 5 to 200 mm/min) and tensile relaxation tests (at strains from 0.03 to 0.09) were performed on low‐density polyethylene in the subyield region of deformations at room temperature. Mechanical tests were carried out on nonannealed specimens and on samples annealed for 24 h at the temperatures T = 50, 60, 70, 80, and 100 °C. Constitutive equations were derived for the time‐dependent response of semicrystalline polymers at isothermal deformations with small strains. A polymer is treated as an equivalent heterogeneous network of chains bridged by temporary junctions (entanglements, physical crosslinks, and lamellar blocks). The network is thought of as an ensemble of mesoregions linked with each other. The viscoelastic behavior of a polymer is modeled as a thermally induced rearrangement of strands (separation of active strands from temporary junctions and merging of dangling strands with the network). The viscoplastic response reflects sliding of junctions in the network with respect to their reference positions driven by macrostrains. Stress‐strain relations involve five material constants that were found by fitting the observations. Fair agreement was demonstrated between the experimental data and the results of numerical simulation. This study focuses on the effects of strain rate and annealing temperature on the adjustable parameters in the constitutive equations. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1638–1655, 2003     

Optical and shape memory properties of semicrystalline poly(cyclooctene) upon cold‐drawing

Semicrystalline thermoplastic poly(cyclooctene) (PCO) shows significant improvement in transparency when cold‐drawn at room temperature, unlike other semicrystalline polymers whose fibrillated chains cause crazing upon cold‐drawing, making the polymers opaque to visible light. Upon heating, transparent cold‐drawn PCO recovers its original opacity as well as its undeformed shape. In situ wide‐ and small‐angle X‐ray diffraction and polarized Fourier transform infrared analyses show that molecular density differences between the PCO crystalline and amorphous phases were reduced due to strain‐induced crystallization and that fibrillated chains and voids, an indication of craze, were not observed due to chain entanglements concentrated in trans double‐bond regions. These two factors explain the unique optical properties of PCO. Finally, it is demonstrated that crosslinked PCO enhanced optical and shape memory recovery without deterioration of the transparency of the polymer. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1595–1607     

Fracture of model polyurethane elastomeric networks

Fracture properties of model elastomeric networks of polyurethane have been investigated with a double‐edge notch geometry. The networks were synthesized from monodisperse end‐functionalized polypropylene glycol precursors and a trifunctional isocyanate. All reagents were carefully purified and nearly defect‐free ideal networks were prepared at a stoichiometry very close to the theoretical one. Three networks were prepared: an unentangled network of short chains (M_n = 4 kg mol^?1), an entangled network of longer chains (M_n = 8 kg mol^?1) and a bimodal network with 8 kg mol^?1 and 1 kg mol^?1 chains. The presence of entanglements was found to increase significantly the toughness of the rubber, in particular at room temperature, relative to the bimodal networks and to the short chains network. Fracture experiments were carried out at different strain rates and temperatures and showed for all three networks a marked decrease in fracture toughness with increasing temperature and decreasing strain rate which mirrored reasonably well the rate and temperature dependence of tan δ, the dissipative factor. However the proportionality factor between tan δ, and G_IC was very material dependent and the shift factors obtained for the master curves of the viscoelastic properties could not be used to build fracture energy master curves. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Fast and excellent healing of hydroxypropyl guar gum/poly(N,N‐dimethyl acrylamide) hydrogels

In this article, a fast and high efficient healing hydroxypropyl guar gum (HPG)/poly(N,N‐dimethyl acrylamide) (PDMA) hydrogel is prepared by a facile synthesis method. HPG networks are formed through hydrogen‐bond interaction between the hydroxyl groups in the HPG chains, and PDMA networks are self‐crosslinked without any chemical crosslinker. The cut hydrogel could heal when nanosilica solution is chosen as the connector that is related to the adsorption of polymer to the surface of nanosilica. The fracture stress of the HPG/PDMA gels presents a fast and almost full recovery within a short time (1 min), while the recovery of fracture strain and elastic modulus is related to time in 2 h. The healing efficiency of HPG/PDMA gel is investigated as a function of healing time, HPG content, and N,N‐dimethyl acrylamide content. The microscopic healing process and healing mechanism are also discussed. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 239–247     

Shear damping function measurements for branched polymers

We present results for step‐strain experiments and the resulting damping functions of polyethylene blends of different structures, including solutions of linear, star and comb polymers. Remarkably, an entangled melt of combs exhibits a damping function close to that for entangled linear chains. Diluting the combs with faster‐relaxing material leads to a more nearly constant damping function. We find similar behavior for blends of commercial low density polyethylene LDPE. Our results suggest a simple picture: on timescales relevant to typical damping‐function experiments, the rheologically active portions of our PE combs as well as commercial LDPE are essentially chain backbones. When strongly entangled, these exhibit the Doi‐Edwards damping function; when diluted, the damping function tends toward the result for unentangled chains described by the Rouse model – namely, no damping. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3117–3136, 2007     

Thermally crosslinkable hole‐transporting poly(fluorene‐co‐triphenylamine) for multilayer polymer light‐emitting diodes

This article reports the synthesis and characterization of a novel thermally crosslinkable hole‐transporting poly (fluorene‐co‐triphenylamine) (PFO‐TPA) by Suzuki coupling reaction, followed with its application in the fabrication of multilayer light‐emitting diodes by wet processes. The thermal, photophysical, and electrochemical properties of PFO‐TPA were investigated by differential scanning calorimeter, thermogravimetric analysis, optical spectroscopy, and cyclic voltammetry, respectively. Thermally crosslinked PFO‐TPA, through pendant styryl groups, demonstrates excellent thermal stability (T_d > 400 °C, T_g = 152 °C), solvent resistance, and film homogeneity. Its highest occupied molecular orbital level (?5.30 eV) lies between those of PEDOT:PSS (?5.0 ～ ?5.2 eV) and poly(9,9‐dioctylfluorene) (PFO: ?5.70 eV), forming a stepwise energy ladder to facilitate hole injection. Multilayer device with crosslinked PFO‐TPA as hole‐injection layer (HIL) (ITO/PEDOT:PSS/HIL/PFO/LiF/Ca/Al) was readily fabricated by successive spin‐coating processes, its maximum luminance efficiency (3.16 cd/A) were about six times higher than those without PFO‐TPA layer (0.50 cd/A). The result of hole‐only device also confirmed hole‐injection and hole‐transport abilities of crosslinked PFO‐TPA layer. Consequently, the device performance enhancement is attributed to more balanced charges injection in the presence of crosslinked PFO‐TPA layer. The thermally crosslinkable PFO‐TPA is a promising material for the fabrication of efficient multilayer polymer light‐emitting diodes because it is not only a hole‐transporting polymer but also thermally crosslinkable. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Strain stiffening and negative normal stress in alginate hydrogels

Alginate hydrogels are polysaccharide biopolymer networks widely useful in biomedical and food applications. Here, we report nonlinear mechanical responses of ionically crosslinked alginate hydrogels captured using large amplitude oscillatory shear experiments. Gelation was performed in situ in a rheometer and the rheological investigations on these samples captured the strain‐stiffening behavior for these gels as a function of oscillatory strain. In addition, negative normal stress was observed, which has not been reported earlier for any polysaccharide networks. The magnitude of negative normal stress increases with the applied strain amplitude and can exceed that of the shear stress at large‐strain. Fitting a constitutive relationship to the stress‐strain curves reveals that the mode of deformation involves stretching of the alginate chains and bending of both the chains and the junction zones. The contribution of bending increases near saturation of G blocks as Ca²⁺ concentration was increased. The results presented here provide an improved understanding of the deformation behavior of alginate hydrogels and such understanding can be extended to other crosslinked polysaccharide networks. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1767–1775     

Relationship of deformation behavior to thermal transitions of ethylene/styrene and ethylene/octene copolymers

The stress‐strain response of low‐crystallinity ethylene‐octene (EO) and ethylene‐styrene (ES) copolymers with 7–20 mol % comonomer was compared over a temperature range that spanned the glass‐transition and crystal melting regions. Above the onset temperature of the glass transition, the copolymers exhibited elastomeric behavior with low initial modulus, uniform deformation to high strains, and high recovery after the stress was released. In the glass‐transition range, an initial low‐stress elastomeric response was followed by a distinct “bump” in the stress‐strain curve. On the basis of the temperature and rate dependence of the stress‐strain curve, local strain‐rate measurements, local temperature changes, and recovery characteristics, the “bump” was identified as high strain yielding. Hence, the stress‐strain curve sequentially exhibited the features of elastomeric and plastic deformation. Following high strain yielding, strain hardening dramatically increased the fracture strength. This behavior was defined as elastomeric‐plastic. Elastomeric‐plastic behavior in the broad glass‐transition range constituted a gradual transition from elastomeric behavior at higher temperatures to low‐temperature plastic behavior with high modulus and macroscopic necking. Because of the lower glass‐transition temperature of EO, ?40 °C as compared with ?10 °C for ES, the onset of elastomeric‐plastic behavior occurred at a significantly lower temperature. The concept of a network of flexible chains with fringed micellar crystals serving as the multifunctional junctions that provides the structural basis for elastomeric behavior of low‐crystallinity ethylene copolymers was extended to elastomeric‐plastic behavior by considering a network with a fraction of rigid, glassy chains. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 142–152, 2002     

Triallyl monomer bearing adamantane‐like core derived from naturally occurring myo‐inositol: Synthesis and polyaddition with dithiols

This paper deals with a triallyl monomer bearing a rigid adamantane‐like core derived from myo‐inositol, a naturally occurring cyclic hexaol. The core structure of the monomer can be readily constructed by orthoesterification of myo‐inositol. The polyaddition of the triallyl monomer with dithiols based on the thermally induced radical thiol‐ene reaction gives the corresponding networked polymers. These networked polymers exhibit much higher thermal stability than the comparative networked polymers obtained from a triallyl monomer bearing less rigid cyclohexyl core. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1193–1199     

Rheological properties of linear and crosslinked polymer blends: Relation between crosslink density and enhancement of elongational viscosity

Strain‐hardening behavior in the elongational viscosity of binary blends composed of a linear polymer and a crosslinked polymer, in which the molecular chains of the linear polymer were incorporated into the network chains of the crosslinked polymer, was studied. Blending the crosslinked polymer characterized as the gel just beyond the sol–gel transition point greatly enhanced the strain‐hardening behavior in the elongational viscosity, even though the amount of the crosslinked polymer was only 0.3 wt %. However, the crosslinked polymer, which was far beyond or below the sol–gel transition point, had little influence on the elongational viscosity as well as the shear viscosity. The stretching of the chain sections between the crosslink points was responsible for the strain‐hardening behavior. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 228–235, 2001     

Mechanical recoverability and damage process of ionic‐covalent PAAm‐alginate hybrid hydrogels

Hydrogels consisting of interpenetrating networks of ionically and covalently crosslinked polymers showed high toughness and mechanical recoverability as a result of the dissociation and re‐formation of ionic crosslinks. The present investigation aimed to provide a quantitative study on the mechanical recoverability and damage process of an example hybrid gel of calcium crosslinked alginate and covalently crosslinked polyacrylamide. Three series of load/unload tests were performed sequentially with the mechanical properties of the gel fully retrieved between the 2nd and 3rd load/unload series while only the partial recovery of mechanical properties was evident from the 1st to 2nd series. The load/unload curves in the three series were modeled by existing mechanical models, and the fitted model parameters clearly demonstrate a damage process for the hybrid gel. When a hybrid gel was deformed above its historic maximum strain, the shortest alginate chains were fully‐stretched, pulling apart the weak ionic crosslinks and dissipating fracture energy. Consequently, the strand density of the intact gel network was reduced and the contour length of the remaining next‐shortest load‐bearing alginate chains became longer. A log‐normal distribution was used to describe the probability distribution for the strand fracture and also to describe the strand length distribution of the ionic network. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 53–63     

Insight into shear‐induced modification for improving processability of polymers: Effect of shear rate on the evolution of entanglement state

Many experimental results have revealed that the re‐entanglement kinetics of disentangled polymers is much slower than that predicted by tube theory. This retarded recovery of fully entangled state is of practical significance that shear‐induced modification may offer a way to improve processability for a polymer by reducing viscosity. This work tried to figure out the shear‐rate dependence variation of viscosity in the view of evolution of entanglement state through disentanglement and re‐entanglement, aiming to provide fundamental insights into application prospect of shear‐induced modification in preparing “in‐pellet” disentangled polymers prior to final processing. High‐density polyethylene was sheared on a parallel‐plate rotational rheometer with a linearly increased shear rate. Results showed that higher shear rate could induce further disentanglement, resulting in a lower viscosity with a reduction rate up to 93.7%, larger molecular weight between entanglements M_e , and longer re‐entanglement time. Additionally, less entanglement would give a larger lamellar thickness of sheared samples after nonisothermal crystallization. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 598–606     

On the relevance of the 8‐chain model and the full‐network model for the deformation and failure of networks formed through photopolymerization of multifunctional monomers

Crosslinked networks were synthesized by copolymerization of mono‐functional tert‐butyl acrylate (tBA) with diethyleneglycol dimethacrylate (DEGDMA) or polyethylene glycol dimethacrylates (PEGDMA). By varying the chain length and concentration of the difunctional PEGDMA, we obtained tBA‐PEGDMA copolymer networks while by varying the concentration of difunctional DEGDMA, we obtained tBA‐DEGDMA crosslinked networks. The various materials were submitted to large deformations through uniaxial tension tests. For moderate weight percent of crosslinking agent, up to 20%, the networks showed standard S‐shape stress–strain curves, characteristic of rubber‐like elasticity. Two macromolecular models, the 8‐chain model and the full‐network model, were applied to fit the uniaxial tensile response of the materials. Both models provide good representations of the overall uniaxial stress–strain response of each material. After fitting to stress–strain data, the network models were employed to predict the shear modulus and the elongation at break. Neither the 8‐chain nor the full network model were capable of predicting the failure strain or shear modulus, indicating these models are best used to describe stress–strain relations rather than predict mechanical properties for the network polymers considered here. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1226–1234, 2008     

Multifunctional linear methacrylate copolymer polyenes having pendant vinyl groups: Synthesis and photoinduced thiol–ene crosslinking polyaddition

UV‐induced thiol‐ene crosslinked films composed of linear methacrylate copolymers having pendant enes (MCPenes) are reported. An approach involving a combination of controlled radical polymerization to synthesize well‐controlled pendant hydroxyl containing copolymers (MCP_OHs) with the following facile carbodiimide coupling of the formed MCP_OHs with enes allows for the synthesis of well‐controlled MCPenes with narrow molecular weight distribution. The density of the pendant enes in MCPenes are easily modulated by varying monomer ratios in the feed. Under UV irradiation, the resulting MCPenes undergo thiol‐ene polyaddition reactions with polythiols to form crosslinked films with a uniform network. The results from thermal and mechanical analysis suggest these properties are tuned by adjusting the densities of pendant enes in MCPenes and the amount of thiols in the reactive mixtures. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 572–581     

Crosslinking PMMA: Molecular dynamics investigation of the shear response

Crosslinking can fundamentally change the mechanical properties of a linear glassy polymer. It has been experimentally observed that when lightly crosslinked, poly(methyl‐methacrylate) (PMMA) has a characteristically more ductile response to mechanical loading than does linear PMMA despite having a higher glass transition temperature. Here, molecular dynamics (MD) simulations are used to investigate conformational and energetic differences between linear PMMA and lightly crosslinked PMMA under shear deformation. As consistent with experiments, crosslinked PMMA is found to have a reduced yield stress relative to linear PMMA. Using the probing capabilities of our explicit atom MD approach, it is observed that while the crosslinks have a minimal direct energy contribution to the total system, they can alter how the main chains conform to macroscopic loading. In crosslinked PMMA, the backbone aligns more with the direction of external loading, thereby reducing the force applied to (and associated deformation of) the polymer bonds. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 444–449