Characterization of mechanical properties of polymers by nanoindentation tests

A constitutive model for thermoplastic polymeric materials and its finite element implementation are presented. The model was verified by both tensile and indentation experimental results. In addition, a five-step indentation scheme, which is able to separate time-dependent plastic deformation from elastic and visco-elastic–plastic deformations, was formulated to extract the complete list of parameters in the constitutive model. Four types of indentation tests on polymethylmethacrylate (PMMA), following the five-step and other schemes, were performed. Experimental data were used to extract all the parameters in the model and verify the five-step test scheme. Good agreement between the experimental results and model prediction indicate that the five-step indentation scheme is a practical approach to determine the mechanical properties of thermoplastic polymers.     

Determination of solid material elastic modulus and surface energy based on JKR contact model

The JKR contact theory is employed to study the adhesion phenomena between two solid materials in intimate contact. The elastic contact modulus and the work of adhesion of solid materials are obtained during adhesion tests by utilizing a micro force-deflection measuring apparatus. Six of the plastic materials, including polyethylene polyoxymethlene (POM), polyamide (PA), terephthalate (PET), polyvinyl chloride (PVC), polypropylene (PP), and ultra-high molecular weight polyethylene (UHMWPE) are used to evaluate the adhesion effect implied by the JKR theory. Comparison is made between surface energy obtained from the adhesion tests with that by a dynamic contact angle analyser.Results show that the load/deflection data in the loading phase are in good agreement with the predictions of JKR equation, and the experimental data of unloading phase deviate significantly from the JKR theory. The phenomena of adhesion hysteresis in loading tests are responsible for these results due to the effects of molecular reconstruction on solid surfaces in contact. The work of adhesions, and hence surface energies of plastic materials, calculated by the best fitting of JKR equation with the experimental data in the loading phase, agree satisfactorily in a comparable manner with that obtained using the contact angle analyser.     

Radiation effects on poly(methyl methacrylate) induced by pulsed laser irradiations

Poly(methyl methacrylate) (PMMA) was irradiated using a medical UV-ArF excimer laser operating at the fundamental wavelength of 193 nm. Characterized by a beam diameter of 1.8 mm and energy of 180 mJ with a Gaussian energy profile, it operates in a single mode or at 30 Hz repetition rate. Mechanical profilometry was carried out on ablation craters in order to study the rugosity and the ablation yield in the various operative conditions. Optical transmission and reflection measurements at six wavelengths were conducted in order to characterize the optical properties of the irradiated surfaces. Measured crater depths in PMMA were lower with respect to the forecasted ones in corneal tissue, while the lateral crater aperture was maintained. The rugosity produced at the crater bottom after irradiation was about 0.3 μm, and the ablation yield was about 10¹⁵ molecules/laser pulse, while etching depth and diameter show a roughly linear dependence on the number of laser shots. These experiments constitute a base for deeper clinical investigations.     

A comparative study on the effects of Co-60 gamma radiation on polypropylene and polyimide

Polypropylene (PP) and polyimide (PI), which belong to entirely different classes of polymers, are irradiated by Co-60 gamma radiation under similar doses and similar conditions in the dose range varying from 57.6 to 230.4 kGy. The radiation responses of these two polymers are analyzed by various characterizations such as Fourier transform infrared, UV–visible, energy-dispersive spectroscopy, X-ray diffraction, scanning electron microscopy and contact angle. PP shows substantial modifications in its structure and properties while in the same dose range, PI shows remarkable stability. These two different responses are interpreted in terms of physicochemical structure and properties of these polymers.     

Interaction of Domains in Ternary Polymer Melt Blends with Separate Dispersions of the Inner Phases

The morphology evolution in three immiscible ternary polymer systems characterized by separate dispersions of the dispersed phases (i.e., no encapsulation phenomena takes place) was characterized. The components used were three of the following: commercial atactic poly(methyl methacrylate) (PMMA) and polystyrene (PS), crystallizable poly(butylene terephthalate) (PBT) and isotactic polypropylene (PP) and glass microspheres (GMS). In System I PMMA/PS/PP (primary dispersed phase/matrix/secondary, or minor, dispersed phase), all of the components were liquid on blending at 190°C. In System II PP/PS/PBT and System III PP/PSyGMS, at 190°C, the minor PBT and GMS dispersed phases were nondeformable. It was shown that small portions (0.5–1.0 wt%) of the PP minor dispersed phase added to the binary PMMA/PS blend produced a dual action: (a) transition of the PMMA dispersed phase to a cocontinuous one and (b) simultaneous substantial (up to a 6-fold) growth of the degree of dispersion of the blend. Moreover, these effects were accompanied by about a three-fold reduction of the threshold PMMA concentration (C*) at which it formed its own co-continuous phase in the ternary blend compared to that in the PMMA/PS binary mixture. The observed phenomena took place regardless of whether the domains of the minor dispersed phase were liquid (System I) or solid (Systems II and III), and was strongly related to the domain sizes of this phase and blend composition. A mechanism underlying the outlined behavior is proposed.     

Superhydrophobic elastomer surfaces with nanostructured micronails

New approaches to the fabrication of microstructures of special shape were developed for polymers. Unusual superhydrophobic surface structures were achieved with the use of flexible polymers and hierarchical molds.Flexible polyurethane?acrylate coatings were patterned with microstructures with use of microstructured aluminum mold in a controlled UV-curing process. Electron microscope images of the UV-cured coatings on polymethylmethacrylate (PMMA) substrates revealed micropillars that were significantly higher than the corresponding depressions of the mold (even 47 vs. 35 μm). The elongation was achieved by detaching the mold from the flexible, partially cured acrylate surface and then further curing the separated microstructure. The modified acrylate surface is superhydrophobic with a water contact angle of 156° and sliding angle of < 10°.Acrylic thermoplastic elastomers (TPE) were patterned with micro?nanostructured aluminum oxide molds through injection molding. The hierarchical surface of the elastomer showed elongated micropillars (57 μm) with nail-head tops covered with nanograss. Comparison with a reference microstructure of the same material (35 μm) indicated that the nanopores of the micro?nanomold assisted the formation of the nail-shaped micropillars. The elasticity of the TPE materials evidently plays a role in the elongation because similar elongation has not been found in hierarchically structured thermoplastic surfaces. The hierarchical micronail structure supports a high water contact angle (164°), representing an increase of 88° relative to the smooth TPE surface. The sliding angle was close to zero degrees, indicating the Cassie–Baxter state.     

Surface modification of UHMWPE with γ-ray radiation for improving interfacial bonding strength with bone cement (II)

The surface of ultra high molecular weight polyethylene (UHMWPE) was exposed to γ-ray for improving bonding strength to polymethylmethacrylate (PMMA) bone cement. Two types of irradiation methods, pre-irradiation and syn-irradiation, were engaged in this study. The intensity of irradiation was 5–30 kGy for pre-irradiation, and 1–3 kGy for syn-irradiation. The grafting process was performed in a glass ampule filled with methanol, MMA monomer (60 v%), FeSO₄ · 7H₂O (1.5 × 10⁻⁴ M) and H₂SO₄ (0.1 M). The graft rate of each specimen was measured with time variation. The grafting effect of the acrylates on to the UHMWPE surface was investigated by mechanical test for bonding strength. Pre-irradiation method showed thinner coverage PMMA graft on the surface of the UHMWPE and higher bonding strength than syn-irradiation method. The interfacial bonding strength between UHMWPE and PMMA bone cement was considerably improved by γ-ray irradiation method. For medical application, the pre-irradiation method might be recommended, because the PMMA could be grafted as optimized thickness to the UHMWPE surface.     

Thermal variations of refractive index of PMMA,polystyrene, and poly (4-methyl-1 -pentene)

The refractive indexes of PMMA, polystyrene, and poly-(methyl pentene), three transparent polymers having various morphologies, have been measured between about -100°C and +150°C. The prism method was used in order to make the sample quite free of any contact and mechanical interaction. Thermally stimulated current experiments were concurrently performed to observe and to confirm the existence of the transitions occurring in the various samples. It is verified that the refractive index variations, on the one hand, perfectly follow the Lorentz-Lorenz law and, on the other hand, systematically exhibit anomalies every time a morphological change temperature is overstepped by the material. So, it is shown that measurement of the thermal variations of the refractive index of polymers is another means to follow the behavior of their specific volume while avoiding the application of any stress from the experimental device.     

Microhardness Studies of the Interphase Boundary in Rubber‐Softened Glassy Polymer Blends Prepared with/without Compatibilizer

Abstract

The interphase boundary of incompatible polymer blends such as poly(methyl methacrylate) (PMMA)/natural rubber (NR) and polystyrene (PS)/NR, and of compatible blends such as PMMA/NR/epoxidized NR (ENR) and PS/NR/styrene–butadiene–styrene (SBS) block copolymer, where ENR and SBS were used as compatibilizers, was studied by means of microindentation hardness (H) and microscopy. Cast films of neat PMMA and PS, and blended films of PMMA/NR, PS/NR, PMMA/NR/ENR, and PS/NR/SBS were prepared by the solution method using a common solvent (toluene). Hardness values of 178 and 173 MPa were obtained on the surfaces of the neat PMMA and PS, respectively. After the inclusion of soft phases, the binary (incompatible) and the ternary (compatible) blend surfaces show markedly lower H‐values. Scanning electron and optical microscopy reveal a clear difference at the phase boundary of the surface of compatible (smooth boundary) and incompatible (sharp boundary) blends. The compatibilized blends were characterized by using microhardness measurements, as having the thinnest phase boundary (～30 µm), while incompatible blends were shown to present a boundary of about 60 µm. The hardness values indicate that the compatibilizer is smoothly distributed across the interface between the two blend components. Results highlight that the microindentation technique, in combination with microscopic observations, is a sensitive tool for studying the breadth and quality of the interphase boundary in non‐ or compatibilized polymer blends and other inhomogeneous materials.     

Thermal and optical properties of femtosecond-laser-structured PMMA

Thermal diffusivity of laser micro- and nano-structured regions in polymethylmethacrylate (PMMA) was measured by the temperature wave method with a lateral resolution reduced to ～10 μm using an array of micro-sensors. The volume fraction of laser modified phase was maximized by implementing tightly focused femtosecond laser pulses inside PMMA and maintaining distance of few micrometers between the irradiation spots. The absolute value of thermal diffusivity of PMMA 1.066±0.08×10^?7 m²/s was reliably determined with the miniaturized sensors. Regions laser structured by single pulses had no trace of carbonization, almost the same thermal diffusivity as the host PMMA, and a stress-induced birefringence Δn～10^?4 modulated with period ～2 μm.     

Laser cutting of polymeric materials: An experimental investigation

The CO₂ laser cutting of three polymeric materials namely polypropylene (PP), polycarbonate (PC) and polymethyl methacrylate (PMMA) is investigated with the aim of evaluating the effect of the main input laser cutting parameters (laser power, cutting speed and compressed air pressure) on laser cutting quality of the different polymers and developing model equations relating input process parameters with the output. The output quality characteristics examined were heat affected zone (HAZ), surface roughness and dimensional accuracy. Twelve sets of tests were carried out for each of the polymer based on the central composite design. Predictive models have been developed by response surface methodology (RSM). First-order response models for HAZ and surface roughness were presented and their adequacy was tested by analysis of variance (ANOVA). It was found that the response is well modeled by a linear function of the input parameters. Response surface contours of HAZ and surface roughness were generated. Mathematical model equations have been presented that estimate HAZ and surface roughness for various input laser cutting parameters. Dimensional accuracies of laser cutting on polymers were examined by dimensional deviation of the actual value from the nominal value. From the analysis, it has been observed that PMMA has less HAZ, followed by PC and PP. For surface roughness, PMMA has better cut edge surface quality than PP and PC. The response models developed can be used for practical purposes by the manufacturing industry. However, all three polymeric materials showed similar diameter errors tendency in spite of different material properties.     

Morphology of Fractured Polymer‐Polymer Interfaces Bonded at Ambient Temperature as Studied by Atomic Force Microscopy

The amorphous polymer surfaces of polystyrene (PS, M _n=200 kg/mol, M _w/M _n=1.05) and poly(methyl methacrylate) (PMMA, M _n=51.9 kg/mol, M _w/M _n≤1.07) were brought into contact at 21°C to form PS‐PS (for 54 days) and PMMA‐PMMA auto‐adhesive joints (for 11 days). After contact at that temperature corresponding to T _g‐bulk ?81°C for PS and to T _g‐bulk–88°C for PMMA, where T _g‐bulk is the calorimetric glass transition temperature of the bulk sample, the bonded interfaces were fractured and their surfaces were analyzed by atomic force microscopy (AFM). The surface roughness, R _q, of the fractured interfaces was larger by a factor of 3–4 than was that of the free PS and PMMA surfaces aged for the same period of time. A similar increase in R _q was found by comparison of the free PS surface aged at T _g‐bulk+15°C for 1 h and of the surface of the PS‐PS interface fractured after healing at T _g‐bulk+15°C for 1 h. These observations, indicative of the deformation of the fractured interfaces, suggest the occurrence of some mass transfer across the interface even below T _g‐bulk ?80°C.     

Mechanical Properties and Morphology of Polypropylene/Polypropylene-g-Maleic Anhydride/Long Glass Fiber Composites

Long glass fiber (LGF)-reinforced polypropylene (PP) was prepared using a self-designed impregnation device. The effect of dicumyl peroxide (DCP) and maleic anhydride (MA) content on the compatibilizer, PP grafted with maleic anhydride (PP-g-MA), was investigated by means of scanning electron microscopy (SEM) and mechanical properties. The experimental results demonstrated that the increase of DCP and MA could effectively improve the interfacial interaction between PP and GF. Good interfacial adhesion between PP and GF in PP/ PP-g-MA /LGF composites was observed from SEM studies for the higher contents of MA. The best mechanical properties of PP/ PP-g-MA /LGF(30%) composites were obtained when the content of DCP and MA were 0.4 and 0.8 wt%, respectively. The storage modulus of the PP/PP-g-MA/LGF composites increased and then decreased with the content of MA. When the content of MA was 0.8 wt%, tan δ had the lowest value, indicating that the corresponding composites had the best compatibility.     

Preferential surface segregation of homopolymer and copolymer blend films

Surface film properties of the homopolymers polystyrene (PS), poly(methyl methacrylate) (PMMA), poly(butyl methacrylate) (PBMA) and the copolymer poly(methyl methacrylate)-co-poly(butyl methacrylate) (PMMA-co-PBMA) and their blends with PS have been examined by atomic force microscopy (AFM) and contact angle measurements. The total and the Lifshitz-van der Waals, acid and base components of the surface free energy together with the work of adhesion and its components, the cohesive energy density and the solubility parameters of the homopolymer, copolymer and blend films were determined. Films of about 3 μm were considered. The results are discussed in terms of surface migration mechanisms based on surface free energy and solubilities of the polymers in the solvent, toluene in this paper. AFM imaging and contact angles revealed surface enrichment at the air polymer interface of PBMA for both the PS/PBMA blend and the copolymer PMMA-co-PBMA, whereas the PS/PMMA and PS/PMMA-co-PBMA blend film surfaces show island-like phase-separated structure of typical size 27.4-86.5 nm in diameter and 6.9-15.6 nm in height for PS/PMMA, while for PS/ PMMA-co-PBMA film surface the typical size is 49.6-153.3 nm in diameter and 1.6-14.2 nm in height.     

PMMA and FEP surface modifications induced with EUV pulses in two selected wavelength ranges

Surface modification of PMMA and FEP polymers using extreme ultraviolet (EUV) in two spectral ranges was investigated. A laser-plasma EUV source based on a double stream gas puff target equipped with an Au coated ellipsoidal collector was used for the experiment. The spectrum of the focused radiation from Kr plasma consisted of a narrow feature with a maximum at 10 nm and a long-wavelength tail up to 70 nm. Al and Zr filters were employed for the selection of radiation from these two spectral regions. The radiation fluences in the two cases were comparable. Polymer samples were mounted in the focal plane of the EUV collector and irradiated for 1–2 min with a 10 Hz repetition rate. Weak ablation accompanied by creation of micro- and nanostructures of different kinds was obtained in both cases. Significant differences in the surface structures after irradiation of PMMA and FEP in these two spectral regions were revealed.     

Ionic conductivity and interfacial resistance of electrospun poly(acrylonitrile)/poly(methyl methacrylate) fibrous membrane-based polymer electrolytes for lithium ion batteries

Electrospun poly(acrylonitrile) fibrous membrane (PAN-EFM) is prepared and enhanced by adding poly(methyl methacrylate)(PMMA) and subsequently minimizing the average diameter of the PAN/PMMA blend fibers. Electrospinning of the 50/50 wt% PAN/PMMA solution is carried out with the aim of the simultaneous presence of both polymers on the fiber surface. Their presence in exterior surface is confirmed using the Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) technique next to the leaching of PMMA with acetone. The process parameters are optimized in four stable modes with the average diameter decreasing from 445 to 150 nm. Mechanical strength of the membrane is measured and reported. Comparing the sample electrochemical properties of the EFMs reveals that the addition of PMMA increases ionic conductivity from 1.02 to 3.31 mS cm^?1 and reduces interfacial resistance from ~1000 to ~400?Ω. It is also demonstrated that the ~300-nm reduction in average diameter of the blend fibers increases ionic conductivity from 3.31 to 5.81 mS cm^?1 and reduces interfacial resistance from ~400 to ~200?Ω.     

Mechanical,Thermal, and Surface Properties of Ultrahigh Molecular Weight Polyethylene/Polypropylene Blends

Various compositions of ultrahigh molecular weight polyethylene/polypropylene (UHMWPE/PP) blends were prepared in decalin, with the rheological, mechanical, thermal, and surface properties of the blends being determined using the solution cast film. Viscosity and mechanical properties of the blends decreased below the additivity value with increasing PP content implying that PP molecules disturb the entanglement of UHMWPE. Contact angle of the blend films with a water drop increased with increasing content of PP. The atomic force microscope (AFM) images showed that the surface of cast UHMWPE was very smooth whereas that of cast PP was very uneven. For blends, the surface became rough and uneven with increasing content of PP. The melting temperature of PP (T _mP) decreased in the blends with increasing UHMWPE content while that of UHMWPE (T _mU) remained almost constant in blends.     

Influences of interfacial adhesion on gas barrier property of functionalized graphene oxide/ultra-high-molecular-weight polyethylene composites with segregated structure

The segregated graphene oxide(GO)/ultra-high-molecular-weight polyethylene (UHMWPE) composite films with various interfacial adhesion property were prepared by mechanical blending method from UHMWPE, GO, dodecyl amine (DA) functionalized graphene oxide(DA–GO) or uniform DA–GO/high density polyethylene (DA–GO/HDPE) powder. The results of XRD and XPS indicated that DA chain was successfully grafted onto GO sheets via a chemical method, which enhanced the interfacial adhesion between UHMWPE particles and GO sheets. The characterizations of POM and SEM proved that good segregated structure was only obtained in DA–GO/UHMWPE or DA–GO/HDPE/UHMWPE composite. Strong interfacial adhesion between fillers and matrix exhibits positive effect on gas barrier property. Compared to the GO/UHMWPE composite film, dramatic decrease in O₂ permeability coefficient by 42.2 and 48.1%, from 15.4 × 10^?14 to 8.9 × 10^?14 and 8.0 × 10^?14 cm³ cm cm^?2 s^?1 Pa^?1, is achieved upon the addition of only 0.5 wt% fillers, respectively. The DSC results demonstrated that the enhanced gas barrier performance was ascribed to the strong interfacial adhesion between DA–GO/HDPE and UHWMPE matrix, rather than the crystallinity of UHWMPE matrix. Additionally, the decrease in UHMWPE particle size might be conducive to improving the gas barrier property of composite films due to the formation of more isolation layers perpendicular to the film plane.     

Gamma radiation effects on mechanical properties and morphology of a polyurethane derivate from castor oil

In this study, an adhesive of a polyurethane derivate from castor oil was irradiated with gamma radiation from a ⁶⁰Co source, at doses from 0.2 to 25 kGy. This adhesive polyurethane is considered for use in hospital furniture because it does not liberate dangerous solvents. Hardness and elastic modulus were measured by instrumented indentation with a pyramidal Berkovich indenter, using loads from 0.08–40 mN with a nanoindenter XP. The instrumented indentation hardness was 110 MPa for an untreated sample, increasing to 124 MPa after irradiation with 25 kGy, at penetration depths of about 5 μm. The increases in elastic modulus induced by radiation were less pronounced. This polyurethane is naturally cross-linked and the relative modifications in the hardness are attributed to an additional cross-linking process induced by radiation. X-ray diffraction indicates a slight increase in crystallinity. The roughness measured by atomic force microscopy increases after gamma irradiation.     

压缩CO2中聚合物玻璃化转变温度的实验研究

 玻璃化转变温度（T_g）是聚合物重要的特性参数，压缩CO₂环境中聚合物的玻璃化转变温度的测定，更是超临界流体技术在聚合物科学领域中成功应用的前提条件。根据蠕变柔量实验原理，自建一套测定高压环境下玻璃化转变温度的实验装置。利用该装置对聚对苯二甲酸乙二醇酯（PET）、聚苯乙烯（PS）、聚氯乙烯（PVC）以及聚甲基丙烯酸甲酯（PMMA）在大气中及压缩CO₂环境中的T_g进行了测定。设定实验的平衡吸附温度为室温，平衡吸附压力范围分别为：PET，0~3.5 MPa；PS，0~11.0 MPa；PVC，0~9.0 MPa；PMMA，0~4.5 MPa。在大气中测定的结果与文献中的结果相吻合，表明所设计的实验方法及实验装置是可靠并有效的，可用于高压环境下聚合物的玻璃化转变温度的测定。从压缩CO₂中的聚合物T_g测定结果可以看出，CO₂对聚合物具有较明显的溶胀、增塑作用，可显著降低聚合物的T_g。