Preparation and characterization of poly(vinyl chloride)/layered double hydroxide nanocomposites with enhanced thermal stability

Poly(vinyl chloride)/layered double hydroxide (PVC/LDH) nanocomposite has been prepared via solution intercalation process using dodecyl sulfate (DS⁻) or stearate anion modified LDH. XRD and TEM results give evidence that the inorganic LDH particles were dramatically exfoliated into nanoscale and homogeneously dispersed in PVC matrix. The enhanced thermal stability was confirmed by conventional Congo Red test. The nanocomposite containing Mg₃Al-LDH-stearate had the maximum of increased dehydrochlorination time, 15 times of that of neat PVC, and more than twice of that of PVC/LDH-NO₃ composite and PVC/LDH-DS nanocomposite at 5 wt% loading. Furthermore, its thermal stability time increased with the LDH content. At 10 wt% loading of Mg₃Al-LDH-stearate, it reached 20 times of that of PVC, and obviously larger than that of the previously reported nanocomposite using alkyl phosphonate (AP) grafted LDH. The reason may lie on the absorption of the released HCl during degradation by both nanoscale dispersed LDH particles and stearate anions.     

Polyhedral oligomeric silsesquioxane as a novel plasticizer for poly(vinyl chloride)

This study focuses on investigating the use of polyhedral oligomeric silsesquioxanes (POSS) to plasticize poly(vinyl chloride) (PVC). Conventional organic plasticizers for PVC, such as dioctyl phthalate (DOP), are somewhat volatile, leading to plasticizer loss and unwanted deterioration of the material properties over time. Previous experimental results indicate that methacryl-POSS, which is much less volatile due to its hybrid organic-inorganic structure, has the ability to plasticize PVC. Methacryl-POSS is miscible in the PVC only up to 15 wt%, thereby limiting its suitability as a plasticizer. However, through the use of ternary compositions it is possible to increase the proportion of methacryl-POSS in PVC substantially. The T_g of appropriately formulated ternary PVC/POSS/DOP compounds can be reduced to near room temperature, and these materials exhibit desirable ductile behavior. Binary (PVC/DOP) and ternary (PVC/POSS/DOP) compounds formulated to the same T_g values showed considerably different mechanical properties. Such findings reveal the possibility of using POSS to engineer the mechanical properties of plasticized PVC.     

Rheological and mechanical properties of PVC/CaCO3 nanocomposites prepared by in situ polymerization

Poly(vinyl chloride) (PVC)/calcium carbonate (CaCO₃) nanocomposites were synthesized by in situ polymerization of vinyl chloride (VC) in the presence of CaCO₃ nanoparticles. Their thermal, rheological and mechanical properties were evaluated by dynamic mechanical analysis (DMA), thermogravimetry analysis (TGA), capillary rheometry, tensile and impact fracture tests. The results showed that CaCO₃ nanoparticles were uniformly distributed in the PVC matrix during in situ polymerization of VC with 5.0 wt% or less nanoparticles. The glass transition and thermal decomposition temperatures of PVC phase in PVC/CaCO₃ nanocomposites are shifted toward higher temperatures by the restriction of CaCO₃ nanoparticles on the segmental and long-range chain mobility of the PVC phase. The nanocomposites showed shear thinning and power law behaviors. The ‘ball bearing’ effect of the spherical nanoparticles decreased the apparent viscosity of the PVC/CaCO₃ nanocomposite melts, and the viscosity sensitivity on shear rate of the PVC/CaCO₃ nanocomposite is higher than that of pristine PVC. Moreover, CaCO₃ nanoparticles stiffen and toughen PVC simultaneously, and optimal properties were achieved at 5 wt% of CaCO₃ nanoparticles in Young's modulus, tensile yield strength, elongation at break and Charpy notched impact energy. Detailed examinations of micro-failure micromechanisms of impact and tensile specimens showed that the CaCO₃ nanoparticles acted as stress raisers leading to debonding/voiding and deformation of the matrix material around the nanoparticles. These mechanisms also lead to impact toughening of the nanocomposites.     

The effect of low levels of plasticizer on the rheological and mechanical properties of polyvinyl chloride/newsprint-fiber composites

Plasticizers play a key role in the formulation of semirigid and flexible PVC and in determining their physical properties and processability. This study examines the effects of the low phr levels of plasticizer (DOP) on the rheological and mechanical properties of rigid and semi-rigid PVC/newsprint-fiber composites. Mechanical and rheological properties of unfilled PVC compounds and PVC/newsprint-fiber composites at 45 phr were compared over the range of 0 to 15 phr of DOP plasticizer. Analyses of data using SAS procedures of ANOVA were also conducted to discern the effects of concentration of plasticizer on these properties. The following conclusions were reached: (1) DOP was relatively ineffective in improving melt flow index of the composites compared to unfilled PVC. (2) The composites were significantly inferior in tensile strength at yield and stiffer than unfilled PVC. The differences observed were roughly the same over the DOP range studied. (3) Elongation at break and toughness (break energy) were lower for the composites and were essentially independent of DOP level, whereas there was a significant improvement for unfilled PVC over the DOP range studied. (4) Impact strengths of the composites and unfilled PVC were essentially comparable up to about 11.25 phr of DOP. However, at 15 phr DOP, impact strength of the composite was far inferior to that of unfilled PVC. (5) The well-known antiplasticization effect was noted for unfilled PVC, as well as to some extent for the filled PVC.     

Deformation and damage upon stretching of degradable polymers (PLA and PCL)

Microstructure and plastic behavior of poly(lactic acid), PLA, and poly(ε-caprolactone), PCL, are investigated. The injected molded specimens are analyzed as received. Thermomechanical properties are characterized by DSC and DMA and crystalline structure by WAXS. The results show that PLA samples are weakly crystalline (14 wt%) and that amorphous phase is glassy at room temperature. The PCL samples exhibit higher crystallinity (53 wt%) and contain a rubber-like amorphous phase. Mechanical behavior is investigated by means of novel video-controlled materials testing system specially developed to assess true stress vs. true strain curves and to record the volume changes upon stretching. While tested at 50 °C, PLA undergoes extensive plastic deformation with a dramatic yield softening followed by a progressively increasing strain hardening. Volume strain, which characterizes deformation damage, increases steadily over the whole plastic stage until reaching 0.27 for an axial strain of 1, 4. For its part, PCL exhibits at 23 °C a much progressive plastic response with a soft yield point, no softening, and moderate strain hardening at large strain. Volume change is delayed until axial strain reaches 0.4. Subsequent damage grows very quickly, eventually reaching 0.2 for an ultimate strain of 1, 3. Results are discussed on the basis of microscopic damage mechanisms observed in the stretched state.     

Crystallization, structure and properties of plasticized poly(l-lactide)

Poly(l-lactide) (PLA) was plasticized with poly(ethylene glycol)s having M_w of 400 and 600 g/mol. In addition to poly(ethyne glycol)s with hydroxyl end groups, monomethyl ethers of poly(ethylene glycol) having M_w of 550 and 750 g/mol, with chains terminated with hydroxyl groups and methyl groups, were used. The effect of different end groups on the plasticization of both amorphous and semicrystalline PLA was studied. The crystallization, structure, thermal and tensile properties of PLA and PLA with 5 and 10 wt% of plasticizers were explored. No marked effect induced by different end groups of plasticizers was found. All the plasticizers used decreased T_g and increased the ability of PLA to cold crystallization. While an amorphous plasticized PLA could be deformed to about 550%, a semicrystalline PLA with the same total plasticizer content exhibited nonuniform plasticization of the amorphous phase and less ability to the plastic deformation. Nevertheless, a 20% elongation at break was achieved for a semicrystalline PLA with 10 wt% of the plasticizer. The plastic deformation of both neat and plasticized PLA was associated with crazing.     

Synthesis and characterization of novel UV-curable polyurethane–clay nanohybrid: Influence of organically modified layered silicates on the properties of polyurethane

Nanohybrids based on UV-curable polyurethane acrylate (PU) and cloisite 20B (C-20B) have been synthesized by solution blending method using different loading levels of C-20B. The structures of PU/C-20B nanohybrids were confirmed by Fourier transform infrared spectroscopy (FTIR) while X-ray diffraction and transmission electron microscopy (TEM) showed the intercalation of PU into layer silicates. The thermal properties of PU and PU/C-20B nanohybrids were investigated by thermal gravimetric analysis (TGA) and differential scanning calorimetric (DSC). TGA tests revealed that the thermal decomposition temperature (T_d10%) of the nanohybrid containing 5 wt% of C-20B increased significantly, being 61 °C higher than that of pure PU while DSC measurements indicated that the introduction of 5 wt% of clay increased the glass transition temperature from 89.7 to 101 °C. Accordingly, the mechanical and anti-water absorption properties proved also to be enhanced greatly as evidenced by nanoindentation anylsis and water absorptions data in which the nanohybrid containing 5 wt% of clay have highest elastic modulus (4.508 GPa), hardness (0.230 GPa) and lowest water absorption capacity. Thus the formations of nanohybrids manifests through the enhancement of thermal, mechanical and anti-water absorption properties as compared with neat PU due to the nanometer-sized dispersion of layered silicate in polymer matrix.     

Morphology, thermal, and viscoelastic properties of poly(glycidyl methacrylate-co-methyl methacrylate)-based nanocomposites with various organo-modified clays

New nanocomposites of poly(glycidyl methacrylate-co-methyl methacrylate) (PGM) cured with cyclohexanedicarboxylic anhydride (CDCA) and layered silicates of inorganic content 3 and 5 wt% were prepared by casting the solution of the mixture and subsequent cross-linking at finally 200 °C. Non-modified montmorillonite (MMT) and organo-MMTs (ODA-M, ALA-M, LEA-M, and HBP-M) modified with octadecylamine, 12-aminolauric acid, N-lauryldiethanolamine, and hexadecyltributylphosphonium bromide, respectively, were used as layered silicates. X-ray diffraction and morphological studies using transmission electron microscopy revealed that the highly intercalated nanocomposites with the interlayer spacing more than 5.5 nm are formed for the PGM-CDCA/ODA-M, LEA-M, and HBP-M composites with inorganic content 3 wt%. When the inorganic content was increased from 3 to 5 wt%, the degree of intercalation of all the PGM-CDCA/organoclay composites was lowered. Dynamic viscoelastic measurement revealed that the organoclay nanocomposites have significantly higher storage modulus than the PGM-CDCA neat resin. The thermogravimetric analysis revealed that the HBP-M composite with inorganic content 5 wt% has the highest thermal decomposition temperature.     

Ultrafine full-vulcanized powdered rubbers/PVC compounds with higher toughness and higher heat resistance

A new type of rigid PVC compound with higher toughness and higher heat resistance was prepared by using a new type of PVC modifier, ultrafine full-vulcanized powdered rubber (UFPR). The UFPRs used in this paper were butadiene nitrile UFPR-1 (NBR-UFPR-1) with particle size of about 150 nm and butadiene nitrile UFPR-2 (NBR-UFPR-2) with particle size of about 90 nm. Dynamic mechanical thermal analysis (DMTA) showed that glass transition temperature (T_g) of PVC in compounds increased from 77.52 °C of neat PVC to 82.37 and 85.67 °C, while the notched impact strengths increased from 3.1 kJ/m² of neat PVC to 5.2, 5.5 kJ/m², respectively. It can be found that both T_g and toughness of PVC have been improved simultaneously, and the smaller the particle size of NBR-UFPRs, the higher the T_g and the impact strength. The property could be attributed to larger interface and more interfacial interaction between NBR-UFPRs and PVC matrix. Transmission electron microscopy (TEM) showed that NBR-UFPRs could be well dispersed in PVC matrix.     

Mechanical properties of polyketone terpolymer/rubber blends

Blends of aliphatic polyketone terpolymer and a core-shell rubber (CSR) were melt processed with varying CSR concentration of 0-40 wt%. The obtained morphology was of finely dispersed CSR particles in the polyketone matrix. The thermal properties of the matrix polymer remained unaffected by the addition of the CSR phase. The crystallinity remained constant at 35 wt% and the melting temperature was not changed. The tensile modulus and yield stress were decreased by the addition of the rubber phase to the aliphatic polyketone polymer. The deformation was strongly delocalised with increasing CSR content. The temperature development during fracture was also strongly reduced with increasing rubber concentration. The CSR phase was found to toughen the aliphatic polyketone matrix very effectively, the brittle to ductile transition temperature was lowered from 90 to −40 °C with the highest rubber concentration (40 wt%). Cavitation experiments revealed that the macroscopic cavitation strain remained constant with increasing rubber content. A study of the deformation zone below the fracture surface showed that voids were produced by cavitation of the rubber phase. The voids were strongly deformed by the plastic deformation of the matrix polymer. At high strain rates a relaxation layer was found below the fracture surface, where the voids were no longer present. This relaxation zone was found to be due to the adiabatic temperature rise of the material during fracture at high strain rates.     

Effect of α-MSt content in α-MSAN copolymer on the miscibility of PVC/α-MSAN blends

A series of α-methylstyrene, styrene and acrylonitrile(α-MSAN) copolymers with different α-methylstyrene (α-MSt) content were synthesized by altering α-MSt and St ratios with emulsion copolymerization method. By melt blending these copolymers with PVC resin and di-isooctyl phthalate (DOP), PVC/α-MSAN and PVC/α-MSAN/DOP blends were prepared. The miscibility and morphology of the blends were investigated by dynamic mechanical analysis (DMA) and scanning electron microscopy (SEM). The miscibility of PVC/α-MSAN blends is substantially improved with the increasing α-MSt content in α-MSAN copolymer containing identical AN content and the blends show homogeneous morphology as the α-MSt content in α-MSAN copolymer is up to 22.5wt%. α-MSAN copolymer containing 37.5wt% α-MSt is fully miscible with PVC throughout the whole composition range. When DOP was introduced into the PVC/α-MSAN blends, a single Tanδ peak over room temperature in DMA detection is found as α-MSt content in α-MSAN copolymer is from 15 to 75 wt%, and morphology result also shows that the PVC and α-MSAN copolymer is miscible under the influence of DOP.     

Dynamic rheological behavior and microcrystalline structure of dioctyl phthalate plasticized poly(vinyl chloride)

The dynamic rheological behavior of poly(vinyl chloride) (PVC)/dioctyl phthalate (DOP) systems were studied as a function of DOP content and melting temperature. The dynamic rheological behavior of the PVC/DOP systems was found to be remarkably affected by the DOP content. The observed curves of storage modulus (G′) versus frequency were well fitted to an empirical equation (G′ = G′₀ + Kωⁿ, where G′₀ is the low‐frequency yield value of the storage modulus, the exponent n is a dependent index of frequency, K is a constant coefficient, and ω is the angular frequency). The loss tangent and/or phase angle increased remarkably at a higher DOP content. There was an apparent critical DOP content transition where the dynamic rheological behavior of the PVC/DOP systems changed greatly. Scanning electron microscopy observations revealed the existence of a multiscale particle structure in the PVC/DOP systems. For the PVC/DOP (100/70) system, with increasing melting temperature, its dynamic rheological behavior showed an apparent mutation at about 190°C. Differential scanning calorimetry (DSC) analysis confirmed that the high elastic networks in the PVC/DOP systems were closely related to the microcrystalline structure of PVC. The transitions in the curves of the gelation degree and crystallinity versus the DOP content corresponded well to the DOP content transition in the dynamic rheological behavior. DOP could inhibit the secondary crystallite of PVC and almost had no effect on the primary crystallite of PVC. The coexistence of the microcrystalline structure of PVC and the plasticizer (DOP) resulted in high elastic networks in the PVC/DOP systems. The DSC results explained the DOP content transition and the temperature transition in the dynamic rheological behavior of the PVC/DOP systems well. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Synthesis and application of a novel thermostable epoxy plasticizer based on levulinic acid for poly(vinyl chloride)

Epoxy bisphenol acid isooctyl ester (EBAIE) was synthesized and characterized by FTIR and H-NMR. EBAIE was used as poly(vinyl chloride) (PVC) plasticizer for the first time, and its addition amount was 30 wt% of PVC. The mechanical properties, thermal stability, compatibility, and decomposition activation energy of PVC blends were systematically studied. Compared to the commercial plasticizer-dioctyl phthalate (DOP), the tensile strength had been significantly increased from 30.64 to 45.07 MPa, while the elongation at break had little difference. The thermal stability analysis showed that the static stability time at 180°C had been extended from 40 to 300 min. The decomposition activation energy indicated that the EBAIE plasticized PVC was with a higher thermal stability. The extraction and volatility resistance of the novel plasticizer were superior to those of DOP.     

Nanocomposites of poly(vinylidene fluoride) with organically modified silicate

We report a study of the impact of cold crystallization on the structure of nanocomposites comprising poly(vinylidene fluoride) (PVDF) and Lucentite STN™ organically modified silicate (OMS). Nanocomposites were prepared from solution over a very wide composition range, from 0.01 to 20% OMS by weight. Thermal preparation involved cold crystallization at 145 °C of quenched, compression-molded plaques. Static and real-time wide and small angle X-ray scattering (WAXS, SAXS), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC) were used to investigate the crystalline phase of PVDF. For OMS content greater than 0.50 wt%, WAXS studies show that that the silicate gallery spacing increases modestly in the nanocomposites compared to neat OMS film, indicating a level of polymer intercalation.Using Gaussian peak fitting of WAXS profiles, we determine that the composition range can be divided into three parts. First, for OMS greater than 0.5 wt%, alpha phase fraction, ?_alpha, is insignificant (?_alpha∼0-0.01). Second, at the intermediate range, for OMS between 0.5 wt% down to 0.025 wt%, beta phase dominates and the beta fraction, ?_beta, is related to alpha by ?_beta>?_alpha. Third, below 0.025 wt% OMS, alpha dominates and ?_alpha>?_beta. The ability of small amounts of OMS (≥0.025 wt%) to cause beta crystal domination is remarkable. Overall, crystallinity index (from the ratio of WAXS crystal peak area to total area) ranges from about 0.36 to 0.51 after cold crystallization. Real-time WAXS studies during heating of initially cold crystallized nanocomposites show that there is no inter-conversion between the alpha and beta phase PVDF crystals, where these crystals coexist at room temperature. While all samples showed a strong SAXS Bragg peak, indicating existence of two-phase lamellar stacks, the sample containing predominantly beta phase had poorly correlated lamellar stacks, compared to samples containing predominantly alpha phase.     

Influence of montmorillonite layered silicate on plasticized poly(l-lactide) blown films

Plasticized poly(l-lactide) (PLA) montmorillonite layered silicate (MLS) nanocomposites were compounded and blown-film processed using a co-rotating twin screw extruder. PLA was mixed with 10 wt% acetyltriethyl citrate ester plasticizer and 5 wt% of an organically modified montmorillonite at various screw speeds. Wide-angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM) determined that the compounded pellets and the blown film PLA/MLS nanocomposites were intercalated. The effect of processing screw speeds on the barrier, thermal, mechanical, and biodegradation properties of the nanocomposites were analyzed and compared to the neat polymer. Nanocomposite films show a 48% improvement in oxygen barrier and a 50% improvement in water vapor barrier in comparison to the neat PLA. The thermogravimetric analysis (TGA) showed an overall 9 °C increase in the decomposition temperature for all of the nanocomposites. Differential scanning calorimetry (DSC) has determined that the glass transition, cold crystallization and melting point temperatures were not significantly influenced by the presence of MLS. Mechanical properties of the nanocomposites showed that the Young's modulus increased by 20% and the ultimate elongation of the nanocomposites were not sacrificed in comparison to the neat samples. Biodegradation rates in soil were slightly greater for the PLA/MLS nanocomposite than the pure PLA. However, none of the PLA pure and nanocomposites achieved significant biodegradation levels after 180 days.     

Hydrothermic aging of plasticized poly(vinyl chloride): Its effect on the dielectric,thermal, and mechanical properties

Accelerated hydrolytic aging (according to the NFT 5166 method) was performed on samples of poly(vinyl chloride) (PVC) plasticized with dioctylphthalate (DOP) and dinonyladipate (DNA) at different concentration ratios. The aging test consisted of immersing the samples in boiling water at 100°C. The samples were removed from water regularly, that is, every 2 h, for mechanical, thermal, and dielectric characterizations. Thermograms of PVC plasticized with DOP revealed no migration of the plasticizer independent of the concentration used. Moreover, the thermal stability of the samples was not affected by the hydrothermal aging. However, for PVC samples plasticized with DNA, a small amount of the plasticizer migrated from the polymer matrix with a considerable effect on the thermal stability. In fact, the data indicated a decrease in the decomposition temperature from 275 to 225°C, particularly for samples containing 50% (w/w) DNA immersed up to 10 h. The mechanical results showed that for a plasticizer content greater than 30% (w/w), the strain at break obtained for samples plasticized with DNA was lower than that for samples plasticized with DOP because the DNA molecules were more likely to be removed by water on account of their polarity and dimension. Finally, the dielectric measurements showed that the permittivity of all the PVC samples plasticized with DOP and immersed in boiling water was higher than that of the virgin samples. On the contrary, the permittivity of the aged unplasticized PVC was less than that of the nonimmersed samples. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3447–3457, 2003     

Plastic membrane electrodes for the determination of flavoxate hydrochloride and cyclopentolate hydrochloride

Four novel ion-exchangers (Fx-Rt (I), Fx-TPB (II), Cp3-PMA (III) and Cp3-PTA (IV)) of antispasmodic and anticholinergic drugs, flavoxate hydrochloride (FxCl), 2-piperidinoethyl-3-methyl-4-oxo-2-phenyl-4h-1-benzopyran-8-carboxylate hydrochloride, cyclopentolate hydrochloride (CpCl) and (2-(dimethylamino)ethyl (RS)-(1-hydroxycyclopentyl)phenylacetate) hydrochloride were synthesized and incorporated into poly(vinyl chloride)-based membrane electrodes for the quantification of FxCl and CpCl in different pharmaceutical preparations. The influence of membrane composition on the potentiometric response of the membrane electrodes was found to substantially improve the performance characteristics. The best performance was reported with membranes having compositions (w/w) of Fx-Rt (2%):PVC (49%):DOP (49%), Fx-TPB (7%):PVC (46.5%):DOP (46.5%), Cp3-PMA (8%):PVC (46%):DOP (46%) and Cp3-PTA (9%):PVC (45.5%):DOP (45.5%). The proposed sensors exhibited Nernstian responses in the concentration ranges of 1.39 × 10⁻⁶-5.00 × 10⁻⁴, 9.90 × 10⁻⁷-3.75 × 10⁻⁵, 1.39 × 10⁻⁵-2.53 × 10⁻³ and 3.21 × 10⁻⁶-8.62 × 10⁻⁴ M, with detection limits of 5.50 × 10⁻⁷, 9.8 × 10⁻⁷, 9.8 × 10⁻⁶ and 2.95 × 10⁻⁶ M for the (I), (II), (III) and (IV) electrodes, respectively. The membrane electrodes performed satisfactorily over pH ranges of 2.0-5.5, 2.0-5.5, 2.0-5.0 and 2.0-7.5, with fast response times of 20, 30, 15 and 20 s for the (I), (II), (III) and (IV) electrodes, respectively. The practical utility of the sensors was demonstrated by the determination of FxCl and CpCl in pure solutions and pharmaceutical preparations using standard additions and potentiometric titration.     

Improved thermal conductivity of epoxy composites using a hybrid multi-walled carbon nanotube/micro-SiC filler

By adding 6 wt% multi-walled carbon nanotubes (MWCNTs) or 71.7 wt% micro-SiC to an epoxy resin the thermal conductivities of the composites reached maxima that were respectively 2.9 and 20.7 times that of the epoxy alone. To further improve the thermal conductivity a method that partially replaces microfiller with nanofiller was used, and a thermal conductivity, 24.3 times that of the epoxy, was obtained with 5 wt% MWCNTs + 55 wt% micro-SiC.     

Plastic deformation and damage of polyoxymethylene in the large strain range at elevated temperatures

The deformation behaviour of polyoxymethylene has been studied in plane strain compression at temperatures from 120 °C up to 165 °C and in uniaxial tension and simple shear at 160 °C for strain rates from 10⁻⁴ to 1 s⁻¹. In uniaxial tension the stress-strain behaviour was determined by a novel video-controlled testing system. The measurements showed that there was a very significant evolution of volumetric strain, indicating that damage mechanisms play a key role in the plastic deformation behaviour.All tests showed similar deformation stages with a short region of visco-elastic behaviour followed by a rounded yield point. The von Mises equivalent yield stress for these tests showed a linear relationship with logarithmic strain rate, suggestive of an Eyring type thermally activated process. After yielding, all stress-strain curves showed a long plastic deformation regime, which in shear occurred at constant stress. In plane strain compression there was also only a very small increase in stress, in contrast to uniaxial tension where very significant strain hardening was observed at high strains, which is attributed to the onset of structural changes.     

Plastic deformation of polyethylene crystals as a function of crystal thickness and compression rate

Plastic deformation of polyethylene (PE) samples with crystals of various thickness was studied during uniaxial compression with initial compressive strain rates of 5.5×10⁻⁵, 1.1×10⁻³ and 5.5×10⁻³ s⁻¹. Samples with a broad range of crystals thickness, from usual 20 up to 170 nm, were obtained by crystallization under high pressure. The samples underwent recoverable compression below the compression ratio of 1.05-1.07. Following yield, plastic flow sets in above a compression ratio of 1.12. At a compression rate of 5.5×10⁻⁵ s⁻¹ the yield stress increases with the increase of crystal thickness up to 40 nm. For crystals thicker than 40 nm the yield stress levels off and remains constant. This experimental dependence was compared with the model developed on the basis of classical crystal plasticity and the monolithic nucleation of screw dislocations from polymer crystals. In that model contrary to the experimental evidence, the yield stress does not saturate with increase of crystal thickness. The activation volumes determined from strain rate jump experiments and from stress relaxation for crystals thicker than 40 nm are nearly constant at a level of 8.1 nm³. This activation length agrees very well with 40 nm for crystal thickness above which the yield stress levels off. It is proposed, as shown in a companion communication, that for PE crystals thicker than 40 nm two other modes of dislocation emission in the form of half loops of edge and screw dislocations begin to govern the strain rate, which no longer depend on lamella thickness.