The effect of peroxide crosslinking on thermal,mechanical, and rheological properties of polycaprolactone/epoxidized natural rubber blends

Polycaprolactone (PCL)/epoxidized natural rubber (ENR) blends (PCL/ENR = 70/30, 50/50 wt/wt) were prepared by a melt mixing in an internal mixer in the presence of a small amount (0.5 and 1 phr) of dicumyl peroxide. The effect of peroxide crosslinking on thermal, mechanical, and rheological properties of the blends was investigated by means of DSC, tensile test, and small amplitude oscillating rheometer, respectively. It was revealed that peroxide crosslinking enhanced degree of crystallinity of PCL phase and its non-isothermal melt crystallization temperature. The crosslinked blends behave like a thermoplastic elastomer exhibiting high elongation-at-break and fairly good elastic recovery as well as melt processibility. From melt rheological analysis, the peroxide crosslinked blends showed more pronounced shear thinning effect and higher elasticity compared to simple blends.     

Irradiation effects on polycaprolactone

The structure and some physical properties of γ-irradiated polycaprolactone (PCL), a semi-crystalline linear saturated polyester, were studied as function of the irradiation dose level. The critical dose level for gel formation is 26 Mrad and above this irradiation dose the number of scission events is similar to the number of crosslinking events. G.p.c. results show that the initial rather narrow molecular weight distribution gradually widens with increasing dose in the pre-gelation region. A significant difference between first and second d.s.c. scans of irradiated PCL is shown and explained. Scission and crosslinking reactions associated with the irradiation process occur preferentially in the non-ordered regions. Small irradiation doses, 2–5 Mrad, are shown to have a dramatic effect on the tensile elongation at break by converting ductile PCL samples into brittle materials.     

Thermal and wide angle X‐ray analysis of chemically and radiation‐crosslinked low and high density polyethylenes

Low and high density polyethylenes (PE) were crosslinked by two methods, namely, chemically by use of different amounts of tert‐butyl cumyl peroxide (BCUP) and by irradiation with different doses of electron beam. A comparison between the effects of these two types of crosslinking on crystalline structure, crystallinity, crystallization, and melting behaviors of PE was made by wide angle X‐ray diffraction and DSC techniques. Analysis of the DSC first heating cycle revealed that the chemically induced crosslinking, which took place at melt state, hindered the crystallization process and decreased the degree of crystallinity, as well as the size of crystals. Although the radiation‐induced crosslinking, which took place at solid state, had no significant influence on crystalline region, rather, it only increased the melting temperature to some extent. However, during DSC cooling cycle, the crystallization temperature showed a prominent decrease with increasing irradiation dose. The wide angle X‐ray scattering analysis supported these findings. The crystallinity and crystallite size of chemically crosslinked PE decreased with increasing peroxide content, whereas the irradiation‐crosslinked PE did not show any change in these parameters. As compared with HDPE, LDPE was more prone to crosslinking (more gel content) owing to the presence of tertiary carbon atoms and branching as well as owing to its being more amorphous in nature. HDPE, with its higher crystalline content, showed relatively less tendency toward crosslinking especially by way of irradiation at solid state. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3264–3271, 2006     

Shape‐memory behaviors of sensitizing radiation‐crosslinked polycaprolactone with polyfunctional poly(ester acrylate)

The sensitizing effects of polyfunctional poly(ester acrylate) on the radiation crosslinking of polycaprolactone (PCL) were studied. The influences of the use of the polyfunctional material, the number of functional groups, and the radiation dose on the radiation crosslinking, dynamic mechanical properties, and shape‐memory behaviors of PCL, respectively, were investigated. The radiation crosslinking of PCL, under the conditions in which the polyfunctional material was applied, did not follow the classic Charlesby–Pinner equation but instead followed the Chen–Liu–Tang relation. The efficiency of the radiation crosslinking of PCL was distinctly improved with the polyfunctional material. The greater the usage and functional group number were, the greater gel content and the more distinctive the radiation‐crosslinking effects were. This also indicated that the polyfunctional material directly participated in the crosslinking reaction. Dynamic mechanical analysis indicated that enhanced radiation crosslinking better raised the heat deformation temperature of PCL and presented a higher and wider rubbery‐state plateau; it also produced greater strength at temperatures higher than the melting temperature and provided greater force for recovering the deformation than pure PCL. The shape‐memory results revealed that sensitizing crosslinked PCL presented 100% recoverable deformation and a quicker recovery rate. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 634–639, 2005     

Effects of carbon black on tribology of blends of poly(vinylidene fluoride) with irradiated and non-irradiated ultrahigh molecular weight polyethylene

Friction and wear resistance are two vital tribological properties of polymer-based materials but optimization of both is rarely attempted. We have investigated blends of 70 wt% poly(vinylidene fluoride) (PVDF)+30% ultra high molecular weight polyethylene, the latter either un-irradiated or else γ-irradiated. Each sample contained varying amounts of carbon black (CB) and also had a varied degree of crosslinking and irradiation dose. We have determined static and dynamic friction, scratch resistance, and sliding wear in multiple scratching tests. Effects of the irradiation dose and CB concentration have been quantified. The electric conductivity threshold is reflected in a drop of static friction; formation of a continuous phase of the lubricant affects tribology as well as electrical properties—both for irradiated and for un-irradiated samples. The scratch resistance as represented by the residual (healing) depth is affected by crosslinking, by the stage at which irradiation is applied (before or after blending) and by CB addition. Crosslinking by moderate amounts of irradiation provides shallower residual depths while higher doses cause adverse results. Similarly, the CB lubricant can either improve or worsen the scratch resistance. A combination of both approaches produces either better or else worse results than crosslinking alone. Lower friction seems accompanied by higher scratch resistance. A combination of a specific irradiation dose and an optimized CB concentration lowers the sliding wear significantly. Strain hardening in sliding wear determination takes place for all materials studied, irrespective of the extent or radiation-induced crosslinking and of the presence and concentration of carbon black.     

聚己内酯的辐射交联及改性研究

实验研究了多官能团单体(PFM)对聚己内酯(PCL)的辐射交联效应及对PCL的力学性能的影响。结果表明,在相同辐照剂量时,多官能团单体的用量越大,官能团数目越多,所生成凝胶含量越多,辐照交联效应越明显。并研究了无机羟基磷酸钙粉末(CPP)作填料时对PCL的增强作用,发现随着CPP用量的增加,共混物的拉伸强度提高,但对PCL的辐射交联没有影响。     

Improvement of processability of poly(ε‐caprolactone) by radiation techniques

Improvement of processability of Poly(ε‐caprolactone) (PCL) was achieved by introduction of a branch structure using gamma‐irradiation from a ⁶⁰Co source. Irradiated PCL has higher molecular weight by producting a branch structure. Hence, the irradiation at a lower dose, such as 3 Mrad, leads to a higher melt viscosity. The branched structure gave improved properties for dynamic viscoelasticity and elongational viscosity. High elongational viscosity was observed by entanglement due to branch chain formed during irradiation, and the elongational viscosity for 3 Mrad is higher than 1.5 Mrad. Due to a higher elongational viscosity, PCL foam can be produced by a molding process. Foam produced from irradiated PCL pellets at 3 Mrad has honeycomb‐like structure, and the foam showed higher enzymatic degradation compared to film samples. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1815–1820, 1999     

Study of iPP crosslinking by means of dynamic and steady rheology measurements

The crosslinking of isotactic polypropylene (iPP) using crosslinking agents (CAs) based on a peroxide/sulfur/accelerator system is a very attractive new method that has been reported recently. The present work deals with the study of the dynamic rheological behavior of iPP during and after the crosslinking process. The influence of the CA concentration and the processing temperature T on the rheological behavior of the iPP was analyzed. The kinetics of the crosslinking reaction was established using the technique described by G. A. Harpell and D. H. Walrod. This reaction is found to be of order one. At T = 180°C, the crosslinking reaction was faster. By varying the crosslinking agent content, different crosslinking degrees of iPP, expressed by the corresponding gel content, are achieved. On the other hand, the modified polypropylene exhibits an unexpected viscosity‐shear rate pattern, which describes the reverse crosslinking reaction mainly occurring by the opening of the bridges of the new interpenetrating network (IPN) formed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Shape‐memory effects of radiation crosslinked poly(ϵ‐caprolactone)

Poly(?‐caprolactone) (PCL) with different molecular weight were crosslinked by γ‐radiation. The radiation crosslinking features were analyzed by Soxhlet extraction with toluene and the Charlesby–Pinner equation. The crosslinking degree is relative to molecular weight and radiation dose; the relation between sol fraction and dose follows the Charlesby–Pinner equation. All the samples were crystalline at room temperature, and the radiation crosslinking had a little effect on the crystallinity and the melting behavior of PCL. The shape‐memory results indicated that only those specimens that had a sufficiently high crosslinking degree (gel content is higher than about 10%) were able to show the typical shape‐memory effect, a large recoverable strain, and a high final recovery rate. The response temperature of the recovery effect (about 55°C) was related to the melting point of the samples. The PCL shape‐memory polymer was characterized by its low recovery temperature and large recovery deformation that resulted from the aliphatic polyester chain of PCL. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1589–1595, 2003     

Irradiation effects on the relaxation behaviour of EPDM elastomers

γ‐irradiation of ethylene–propylene diene monomer (EPDM) elastomers under oxidant atmosphere was carried out in order to change their mechanical and dielectric behaviour. Three different formulations of EPDM (70 wt% ethylene, 28 wt% propylene; diene monomer: 2 wt% norbornene) were studied: a non‐crosslinked EPDM terpolymer, a crosslinked EPDM and a crosslinked EPDM stabilized with an antioxidant. Dielectric and mechanical relaxation show a β‐sub‐glass relaxation at about ?120 °C (1 Hz) and an α‐relaxation at ?15 °C (1 Hz) associated with the glass transition but influenced by the effects of irradiation. The local mobility associated with the β‐relaxation is only weakly influenced by γ‐irradiation up to 450 kGy. The α‐process is shifted to higher temperatures as a result of crosslinking and changes in the semicrystalline structure. The amplitude of the dielectric α‐process increases as a result of the formation of oxidized species during irradiation under oxygen. In contrast, the mechanical α‐relaxation amplitude decreases as a result of physical and chemical cross‐linking. It was shown that the main factors that determine the crosslinking/chain scission balance are (1) the presence of oxygen together with the irradiation dose, (2) the dose rate and (3) the initial crosslink density of the EPDM material. As a result, the individual contribution of crosslinking and crystallization, and therefore the understanding and prediction of the properties after γ‐irradiation can only be deduced after comparison of the polymer behaviour below and above its melting temperature. Copyright © 2004 Society of Chemical Industry     

Effect of peroxide crosslinking on thermal and mechanical properties of poly(ε‐caprolactone)

Poly(ε‐caprolactone) (PCL), a saturated polyester, derived from ring‐opening polymerization of ε‐caprolactone, was chemically crosslinked with various amounts of benzoyl peroxide (BPO) by a two‐step method by first evenly dispersing the BPO into the PCL matrix and then crosslinking at elevated temperature. The gel fraction increased with an increase in BPO content. The modified Charlesby–Pinner equation was used to calculate the ratio of chain scission and crosslinking. The results showed that both scission and crosslinking occurred, and that crosslinking predominated over scission. The number‐average molecular weight between the crosslinks determined by the rubber elasticity theory using the hot set test showed a decrease with increasing BPO content. The melting temperature and crystallinity decreased with an increase in BPO content, and the crystallization temperature increased after crosslinking. Dynamic mechanical analysis results showed a decrease in the glass transition temperature as a result of chemical crosslinking of PCL. This was explained by the observed reduction in crystallinity and the increase in free volume due to restrictions in chain packing. Moreover, Young's modulus and elongation at break generally decreased with an increase in BPO content, but the tensile strength first increased with BPO content up to 1.0 wt%, reached a maximum, and then decreased. Copyright © 2007 Society of Chemical Industry     

Transient shear and extensional properties of biodegradable polycaprolactone

Transient shear and extensional properties of two grades of partially crystalline biodegradable aliphatic polyesters, poly ε caprolactone (PCL), were investigated at three different temperatures. Uniaxial extensional viscosity at a constant strain rate was obtained using the Meissner apparatus. The magnitudes of the stress-over-shoot during stress growth experiments were smaller than those typically observed for other polymers. Higher melt temperature and higher strain led to faster relaxation, while the lower molecular weight PCL 767 relaxed faster than the higher molecular weight PCL 787. The relaxation moduli are independent of strain for strain values below 0.1. Transient extensional measurements were conducted at strain rates of 0.01 to 1.0 s⁻¹. At small stresses the extensional viscosity has the threefold value of shear viscosity as predicted by Trouton. There appeared to be no steady state regime for either grade of PCL studied and as a result η_e(ε) could not be determined. The departure from the linear limit is fastest for the highest extensional rate. Extension thickening behavior is observed at Hencky strains ranging from 1.0 to 2.0. PCL 767 displayed greater extension thickening than PCL 787 at the same temperatures. The Wagner integral constitutive equation was found to give an acceptable fit to the stress growth data in shear and extension, with the fit being better for PCL 767 than for PCL 787.     

Measurement of crosslinking degree for electron beam irradiated block copolymers

The molecular weight between crosslinking junctions (Mc) of styrene-butadiene-styrene (SBS) and that of styrene-isoprene-styrene (SIS) irradiated by electron beams (5 Mrad to 100 Mrad) was measured by dynamic modulus, glass transition temperature, static modulus, and swelling. The change in dynamic modulus at low temperature was smaller than that at high temperature in both SBS and SIS. The shift of Tg to higher temperature was observed in both SBS, and SIS systems except for 100 Mrad of SIS. Both dynamic and static moduli increased with irradiation doses, however, the change of static modulus was smaller than that of dynamic one. Those mechanical measurements were compared with the swelling measurements and discussed.     

用动态剪切模量估算NR与SBR硫化胶的化学交联密度

用美国Ｍｏｎｓａｎｔｏ公司的橡胶加工过程分析仪测定ＮＲ和ＳＢＲ硫化胶或混炼胶料的动态剪切模量（Ｇ＇），进而根据Ｇ＿Ｎ￣０＝ｇ×２Ｘ＿０ＲＴ和Ｇｅ＝ｇ＇×２Ｘ＿（ｐｈ）ＲＴ估算其硫化胶的化学交联密度（Ｘ＿ｃｈ），结果发现：ＮＲ／过氧化二异丙苯体系硫化胶和混炼胶料的Ｇ＇是不同的，但用Ｇ＇估算的Ｘ＿ｃｈ与用化学分析法测定的结果基本相近；对ＳＢＲ／促进剂ＴＭＴＤ体系，用这种方法估算的Ｘ＿（ｃｈ）接近用应力－应变法和ＮＭＲ法的估算值；对ＮＲ／促进剂ＴＭＴＭ体系，用Ｇ＇估算的Ｘ＿（ｃｈ）与用溶胀法获得的数据基本接近。用Ｇ＇估算Ｘ＿（ｃｈ）法，十分简单、方便。     

Electron‐beam modification of nitrile rubber in the presence of polyfunctional monomers

The structural changes of nitrile rubber in both the presence and the absence of polyfunctional monomers, such as trimethylolpropane triacrylate (TMPTA), tripropyleneglycol diacrylate (TPGDA), and trimethylolmethane tetraacrylate (TMMT), at different doses of electron beam irradiations, were investigated with the help of FTIR spectroscopy (in the ATR mode), solid‐state NMR, dynamic mechanical thermal analysis, and sol–gel analysis. There was a significant decrease in the concentration of olefinic groups for modified system with 3% TMPTA compared to that of the unmodified nitrile rubber on irradiation. This was also confirmed by the increase in the carbon resonances attributed to C? C linkages from solid‐state NMR for the modified system, indicating more crosslinkages. The increase in crosslinking was also revealed by the increase in % gel content and dynamic storage moduli with radiation dose. The lifetime of spurs formed and the critical dose, an important criterion for overlapping of spurs, were determined for both grafted and ungrafted nitrile rubber using a mathematical model. The ratio of scissioning to crosslinking for nitrile rubber was determined using the Charlesby–Pinner equation. Mechanical properties were studied for the modified and the unmodified systems and the tensile strength was found to increase with grafting of polyfunctional monomers. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 435–447, 2005     

Effect of Thermal History on Structural Changes in Melt‐Intercalated Poly(ε‐caprolactone)/Organoclay Nanocomposites Investigated by Dynamic Viscoelastic Relaxation Measurements

Poly(ε‐caprolactone) (PCL) nanocomposites were prepared using two different types of organically modified nanosilicates by melt intercalation with an internal mixer. Dynamic mechanical analysis revealed possible structural changes in the nanocomposites even during the small deformation occurring during shear oscillatory measurements, as evidenced by a V‐shaped modulus change in the plot of the dynamic storage modulus as a function of stepwise increased temperature. X‐ray diffraction patterns were recorded at different simulated temperatures during the various stages of dynamic measurements. The X‐ray data indicate that the structural changes can be ascribed to a further intercalation of the PCL matrix chains into the silicate layers. This further intercalation is a consequence of the heat treatment during the dynamic mechanical measurements. Furthermore, there is a considerable vertical shift in addition to the horizontal shift in the higher temperature regime, which allows the mapping of a master curve through the application of the time‐temperature superposition principle to the dynamic storage and the loss modulus data obtained at various isothermal temperatures. The present study is also concerned with the relative molecular mobility of both PCL nanocomposites in the given experimental conditions considering the Williams‐Landel‐Ferry (WLF) equation and the Arrhenius relationship between the horizontal shift factor and the activation energy of flow. Moreover, the extent of the vertical shift as a function of temperature made it possible to determine the apparent activation energy of the further intercalation of PCL into the silicate layers. This intercalation is caused by the additional exposure to heat during the dynamic mechanical measurements after mixing, which led to a comparison of the relative diffusivity of the PCL matrix in the two nanocomposites.

Dynamic shear storage moduli G′ of PCLOC25A and PCLOC30B as a function of temperature with increase increments of 20 °C from 60 to 260 °C. The G′ data were obtained from isothermal frequency sweep G′(ω) data at ω = 1 rad · s^?1 at the corresponding temperatures.     

Effects of linear low density polyethylene on physical properties and irradiation effectiveness of polypropylene

Blends of polypropylene (PP)/linear low density polyethylene (LLDPE) were prepared by melt mixing in twin screw extruder at 190 °C. Polyfunctional monomer TMPTMA (trimethylolpropane-trimetacrylate) was added to the mixture as a crosslinking co-agent to improve the crosslinking or branching efficiency of the olefins during irradiation. The effect of LLDPE on the crosslinking or branching effectiveness and physical properties of PP was investigated in conjunction with the monomer content of LLDPE in the blends. Thermal stability, rheological properties and electron beam irradiation effectiveness of PP in presence of LLDPE were analyzed by DSC, TGA and RDS. Solution gel analysis and the presence of ?C=O in FT-IR test supported some crosslinking or branching that occurred after irradiation. Certain decrease in melting temperature (T _m ) that was noticed after irradiation could have been the result of chain scissioning, which decreases the number of tie molecules in the amorphous region and consequently weakens the lamellar connections. Shear thinning effect and zero shear viscosity were improved by irradiation in the PE incorporated samples.     

Effect of organoclay on non-linear rheological properties of poly(lactic acid)/poly(caprolactone) blends

The nonlinear viscoelastic properties of PLA/PCL blends with and without clay (montmorillonite, MMT) under large amplitude oscillatory shear (LAOS) flow were investigated. The G′ and G″ as a function of strain amplitude, Lissajous plots and FT-rheology methods were used to interpret nonlinear behavior of PLA/PCL blends with and without MMT. Additionally, scanning electron microscopy (SEM) images of PLA/PCL with MMT blends were taken to investigate the effects of clay on the internal structure of the PLA/PCL blends. A relationship between morphological changes and linear and nonlinear rheological properties was observed. SEM image analysis revealed that clay acted as a compatibilizer and then reduced the size of droplets in the PCL domain of the PLA matrix. As a result, nonlinear properties sensitively reflect morphological changes with increasing MMT amount. The nonlinear rheological properties of PLA/PCL/MMT/metallocene-LLDPE (mLLDPE) were also investigated when mLLDPE was used as an impact modifier to improve mechanical properties, and the nonlinear rheological properties of PLA/PCL/MMT and PLA/PCL/MMT/mLLDPE were also compared.     

Fe(acac)3‐bis(imino)pyridine/MAO: A new catalytic system for ethylene polymerization

Formulations of chemically crosslinked and radiation-crosslinked low-density polyethylene (LDPE) containing an intumescent flame retardant such as ammonium polyphosphate were prepared. The influence of blending LDPE with poly(ethylene vinyl acetate) (EVA) as well as the effects of a coadditive such as talc on flammability was investigated. Chemical crosslinking by dicumyl peroxide and crosslinking by ionizing radiation from an electron-beam accelerator were both used and compared. An increase in the limiting oxygen index (LOI) was found by the partial replacement of LDPE with EVA. The effect of talc on the flammability depended on the amount of talc in the formulations. The addition of a small amount of talc increased LOI and reduced smoke during cone calorimeter measurements. A higher amount of talc led to a decrease in the LOI values. Formulations crosslinked by ionizing radiation yielded lower LOI values than chemically crosslinked formulations. This could be attributed to the use of trimethylolpropane triacrylate as a crosslinking coagent in formulations crosslinked by ionizing radiation. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

亲水-疏水PVP-semi-IPN-PCL水凝胶的制备与溶胀性能

N-乙烯基吡咯烷酮（NVP）在聚己内酯（PCL）的乙酸乙酯溶液中进行自由基聚合,制备了亲水-疏水性聚乙烯吡咯烷酮（PVP）/聚己内酯（PCL）半互穿网络水凝胶（PVP-semi-IPN-PCL）。凝胶中PCL的熔融温度T_m无明显变化,而T_m吸热峰形状随PVP含量变化。凝胶平衡溶胀率（ESR）随PVP含量的升高而增大,结合水量的增大尤其显著。由于“笼蔽效应”,低浓度引发剂时,偶氮二异丁睛（AIBN）引发制备的凝胶ESR低于过氧化苯甲酰（BPO）引发剂。交联剂浓度较低时,以戊二醛交联形成凝胶的ESR较N,N-亚甲基双丙烯酰胺（NMBA）交联形成的凝胶大。浓度较高时,戊二醛交联凝胶ESR较NMBA低。PVP含量（质量）分别为20%、40%、60%、80%时,凝胶溶胀动力学Fick模型中的n值分别为0.854、0.471、0.466、0.253,说明在合适的PVP含量时,凝胶的溶胀动力学符合Fick模型。