Search

The influence of the crystallization temperature and the pressure on the crystallization kinetics, structure and compression strength of γ-polypropylene was investigated. Samples were produced in a custom-made high-pressure crystallization cell and characterized by differential scanning calorimetry (DSC), wide angle X-ray scattering (WAXS) and transmission electron microscopy (TEM). It could be shown that the strength is mainly affected by the mean lamellar thickness. The strength of γ-PP is at least 40% higher than α-PP at the same mean lamellar thickness. This confirms the hypothesis that the arrangement of non-parallel chains restricts the number of possible slip systems so that almost no mobile dislocations are generated. The mode of nucleation is only slightly affected by the crystallization pressure. The Avrami coefficient is n = 2.5 ± 0.1 at the beginning of crystallization and largely independent of pressure.

Electrically conductive composite polypropylene and poly(ethylene terephtalate) fabrics were obtained by coating with organo-silicon sol containing dispersed graphene particles. The graphene-modified coating was deposited on the fabric using the sol-gel method and the padding technique. To improve bonding of the coating to the fiber surfaces, the latter were activated with atmospheric pressure plasma in the form of corona discharge. The activation resulted in chemical and structural changes of the surfaces, roughening, and the formation of oxygen-containing functional groups, as evidenced by SEM with 3D analysis, EDS and XPS, and also measurements of contact angles with liquids. The padding with graphene-containing organo-silicon sol imparted electrical conductivity to the polymer fabrics. It was found that the pre-coating activation improved durability of the coating and stability of the electrical properties due to better bonding between the conducting coating and the fiber surfaces.

Three star-shaped poly(ʟ-lactide)s (PLLAs) were synthesized, two 6-arm with Mw of 120 and 245 kg/mol and one 4-arm with Mw of 123 kg/mol. Linear PLLAs with Mw of 121 and 339 kg/mol were also prepared. Crystallization, both isothermal and nonisothermal, of all PLLAs was studied by means of DSC. The growth rate of spherulites was measured in a broad range of crystallization temperature using PLM. At Mw close to 120 kg/mol the star architecture slowed down the spherulite growth above 120 °C, up to 145 °C. The spherulite growth in PLLAs with Mw > 200 kg/mol was slower than that in the other polymers and unaffected by the macromolecule architecture. The differences in the spherulite growth rate were reflected in overall crystallization kinetics in isothermal conditions and also in crystallization peak temperature and crystallinity developed during cooling. The highest crystallinity developed during isothermal crystallization in linear and 6-arm PLLAs with Mw close to 120 kg/mol, whereas the lowest in PLLAs with Mw > 200 kg/mol. For Mw > 200 kg/mol the star architecture decreased the melting temperature. The glass transition temperature of all PLLAs was the same, at 61–62 °C, regardless of Mw and macromolecule architecture.

The relations between crystalline structure and micromechanical properties of α-, β- and γ-phase of isotactic polypropylene (iPP) were investigated. The iPP samples with various crystalline phase compositions were prepared by a controlled high-pressure crystallization that enabled us to obtain not only α-iPP and β-iPP modifications, but also a series of almost pure γ-iPP samples with gradually changing morphology, crystallinity and lamellar thickness. The micromechanical properties were assessed by instrumented microindentation hardness testing (MHI), from which we determined indentation hardness (HIT), indentation modulus (EIT), indentation creep (CIT) and elastic part of the indentation work (ηIT). It was demonstrated that γ-iPP does not necessarily exhibit better mechanical performance than α- and β-iPP, as suggested by some previous studies, because the final properties are strongly influenced not only by the phase composition (i.e. by the content of α-, β- and γ-phase), but also by the overall crystallinity.

Poly(lactide)/halloysite nanotubes (PLA/HNT) nanocomposites with crystalline matrix were obtained by cold crystallization and examined. Neat HNT and HNT treated with N,N′- ethylenebis(stearamide) (EBS) were used as nanofillers. Reference materials, PLA and PLA/EBS blend, prepared in the same way, were also considered. The influence of HNT and/or EBS content on the crystallinity and morphology of PLA matrix, as well as on the dynamic mechanical and optical properties of the materials, was determined. The nanocomposites contained well-distributed HNT, with only occasional agglomerates. HNT, EBS-treated HNT and EBS influenced the morphology of the crystalline PLA matrix and the amounts of the disorder α’ (termed also δ) and order α crystallographic forms of PLA. Crystallinity increased stiffness of the materials compared to their counterparts with the amorphous matrix. Owing to the crystallinity and the presence of the nanofillers, the storage modulus at 20 °C and 60 °C increased by up to 30 and 60%, respectively, compared to neat amorphous PLA. Interestingly, at lower nanofiller content the crystalline nanocomposites with EBS were more transparent than neat crystalline PLA.

Thermal and mechanical properties of polylactide (PLA) composites with different grades of calcium carbonate, 40 nm and 90 nm nanoparticles, and also with submicron particles, unmodified and modified with calcium stearate or stearic acid, obtained by melt mixing, were compared. Films with amorphous and crystalline matrices were prepared and examined. Tg of PLA in the composites remained unaffected whereas its cold crystallization was enhanced by the fillers and predominantly depended on filler content. Filling decreased thermal stability of the materials but their 5% weight loss temperatures well exceeded 250 °C, evidencing stability in the temperature range of PLA processing. The amorphous nanocomposites with modified nanoparticles exhibited improved drawability and toughness without a significant decrease of tensile strength; nearly two-fold increase of the elongation at break and tensile toughness was achieved at 5 wt% content of the modified nanofiller. Lack of surface modification of the filler, larger grain size with an average of 0.9 μm, and matrix crystallinity had a detrimental effect on the drawability. However, the presence of nanofillers and crystallinity improved tensile modulus of the materials by up to 15% compared to neat amorphous PLA.

Amorphous polylactide/halloysite nanotube (PLA/HNT) nanocomposites were prepared and examined. Neat HNT and HNT treated with N,N' - ethylenebis(stearamide) (EBS) were used as nanofillers. The role of HNT and/or EBS content on the cold crystallization of amorphous PLA matrix, HNT dispersion, as well as on the dynamic mechanical and optical properties of the materials was determined. The PLA/HNT-based nanocomposites contained well-distributed nanotubes and occasionally micron-sized aggregates, especially at high loading. HNT, EBS treated HNT and EBS influenced the cold crystallization of PLA, therefore the formation of the disorder α′ and the order α crystallographic forms of PLA. The nanocomposites exhibited increased stiffness and decreased transparency compared to the neat PLA. Due to the reinforcing effect and additional specific features of HNT, the addition of the nanofiller allows tuning of the properties of the nanocomposites with amorphous PLA matrix.

Shear-induced nonisothermal crystallization of two commercial polylactides (PLAs) differing in optical purity was studied. The molten polymers were sheared at selected temperatures (Ts) and subsequently cooled. The crystallization was followed by a light depolarization method, whereas the specimens were analysed ex-situ by DSC, 2D-WAXS and SEM after etching. It was found that the effect of shear, especially on the crystallinity developed during post-shearing cooling, intensified with a decrease of Ts from 160 to 146 °C, and with increasing shear rate and strain. Moreover, the effect of shear on PLA1.5 with d -lactide content of 1.5% was stronger than PLA2.8 with 2.8% of d -lactide, although maximum crystallinity of both polymers was practically the same. A decrease of cooling rate from 30 to 10 °C/min increased crystallinity of both PLAs, except for those shearing conditions which induced high crystallinity even during faster cooling. Although SEM examination revealed some row-nucleated forms, no significant crystal orientation was detected by 2D-WAXS, indicating that, under the experimental conditions, the shear induced predominantly point-like nuclei.

Nonwovens of commercially available polylactide (PLA) were produced by electrospinning using laboratory scale equipment. The nonwovens were subsequently coated with an aqueous dispersion of multiwall carbon nanotubes (MWCNT), using the padding technique. MWCNT formed 3D networks on fiber surfaces making the nonwovens electrically conducting, with surface electrical resistivity of 77 Ω/sq – 680 Ω/sq. Modification of the nonwovens with methyltrichlorosilane made them superhydrophobic, what was reflected in the water contact angles of 160 − 175°. In turn, electrochemical deposition of silver particles (Ag°) on the conductive nonwovens imparted antibacterial activity against Staphylococcus aureus (gram-positive) and Escherichia coli (gram-negative).

Oligomeric ladder silsesquioxanes (LPSQ-R) with side substituents (R = COOH, COOMe, OH) capable of acceptor/donor interactions through hydrogen bonds were used for the preparation of blends with polylactide (PLA). Depending on the strength of their interactions with the PLA, LPSQ-R were differently dispersed in the polyester matrix. The blends exhibited different properties and crystallization behaviour depending on the type and content of LPSQ-R. PLA/LPSQ-COOMe exhibited good transparency. Owing to very good dispersion of the modifier, PLA with 5 wt% of LPSQ-COOMe was ductile, which was reflected in nearly 230% elongation at break, retaining however relatively high yield strength of nearly 40 MPa. Moreover, the spherulite growth rate was accelerated in PLA/LPSQ-COOMe, which caused enhancement of the cold-crystallization. In turn, cold-crystallization of PLA/LPSQ-OH was enhanced by strong nucleation.

The influence of shear on non-isothermal crystallization of commercial poly(butylene adipate-co-terephthalate) (PBAT) was investigated. PBAT melt was sheared at 130 and 150 °C at rates of 10–100/s, and then cooled. The crystallization was followed by a light depolarization technique, whereas the crystallized specimens were analyzed by DSC, 2D-SAXS, 2D-WAXS, PLM and SALS. Shear flow shifted crystallization to higher temperature, and the effect was augmented by lower temperature of shearing as well as by higher shear rate and strain. Crystallization peak rate temperature of PBAT, sheared at 130 °C for 5 min at 100/s, increased by up to 12 °C. However, no evidence of crystal orientation due to shear was found, indicating that the shear induced the point-like nucleation. Only a small increase of melting peak temperatures, by up to 2–5 °C, was observed for the specimens sheared at the highest rates (≥50/s).

Electrical conductivity and hydrophobicity was imparted to a cotton fabric through a sol-gel method by coating with an organosilicon sol containing dispersed graphene oxide (GO). The coating was followed by thermal treatment causing GO reduction and completion of cross-linking reactions. To enhance the reduction, a reductive sol was synthesized from alkoxysilanes with active groups obtained in the hydrolysis and condensation reactions. A siloxane oligomer with covalently bonded –SiH groups was used as a reducing component. The reductive sol was analyzed by means of 1H and 29Si NMR spectroscopy and ATR FTIR. Surface electrical resistivity and hydrophobicity/philicity of the modified fabric were determined, whereas the fiber surfaces were analyzed by SEM. The resistivity of 2.7 MΩ/sq and water contact angle of approx. 150° was reached. Electrical resistance of the coated fabric was sensitive to strain but returned to its initial value after the strain release. Moreover, neither the electrical conductivity nor hydrophobicity worsened after intense sonication, unlike in the case of reference fabric sample prepared by coating with GO alone and GO thermal reduction.

Aromatic component of biodegradable aliphatic–aromatic random copolyester crystallizes. A strong self-nucleation is active with self-nuclei stable below 160 °C. Without self-nuclei the peak crystallization temperature decreases by 25 °C during subsequent cooling. The activity of self-nuclei was incentive to find artificial nucleating agents. Green phthalocyanine, blue phthalocyanine and talc were found to be effective nucleating agents for aliphatic–aromatic copolyester in absence of self-nuclei, increasing both the peak crystallization temperature and the density of nucleation. Crystallization of aliphatic–aromatic copolyester led to the formation of spherulites filling the entire volume of the material although aromatic crystal amounted to 10% of entire volume. SAXS analysis and AFM imaging showed that crystals are neither lamellar nor aligned in stacks. Instead, the crystals are formed as narrow, 3–4 nm thick fibrils radiating from spherulite centers. Crystallization of nucleated copolyester proceeds during cooling in a very peculiar way: first long fibrils are formed that partially immobilize the rest of material. During further cooling partially confined melt crystallizes in the form of short fibrils, however, in significantly smaller amount. The strangest outcome of artificial nucleation is that the crystallinity degree of nucleated copolyester is lower than nonnucleated. By short annealing at 70 °C long fibrils decompose by partial melting and capillary instability freeing the material from constraints.

In order to modify its physical properties, particularly the drawability and toughness, polylactide (PLA) was melt blended with a set of PEG-b-PPG-b-PEG block copolymers with varying ratio of the hydrophilic (PEG) and hydrophobic (PPG) blocks. Miscibility of the copolymers with PLA depended on their molar mass and PEG content. Interestingly, the best drawability was achieved in the case of partially miscible blends, in which fine liquid inclusions of the modifier were dispersed in PLA rich matrix with glass transition temperature only moderately decreased, to about 50 °C. About 37 fold increase of the elongation at break and about 1.5 fold increase of the tensile impact strength with respect to neat PLA were reached at small content (10 wt.%) of the modifier. Despite the phase separation, the blends remained transparent. In addition, the barrier properties for oxygen were improved.

Polylactide (PLA) was toughened by blending with a novel biodegradable aliphatic–aromatic copolyester - random poly(butylene succinate-co-glutarate-co-adipate-co-terephthalate) (PBSGAT), with low crystallinity, synthesized from respective dimethyl esters of terephthalic acid, succinic acid, glutaric acid, adipic acid, and 1,4-butanediol. PLA blends with PBSGAT were prepared by melt blending and their structure as well as properties were studied. The copolyester formed particles in the blends and exhibited a separate glass transition. 20-fold increase of the ultimate strain and 2.5 fold increase of the tensile impact strength with respect to neat PLA was achieved in the blends with 25–35 wt.% of PBSGAT. TEM and SEM studies of drawn specimens evidenced that the main mechanism of the toughening was cavitation inside the copolyester particles, which promoted shear yielding and further plastic deformation of PLA matrix.

New aliphatic–aromatic copolyester containing three different aliphatic comonomers: butylene adipate, butylene succinate and butylene glutarate, and one aromatic butylene terephthalate comonomer, all of petrochemical origin, has been synthesized (PBASGT) and its molecular characteristic, crystallization aptitude and morphology were examined by various techniques. Using the assignment of 1H NMR resonances due to homogeneous and heterogeneous dyads, the average block lengths were determined and it was found that PBASGT is a nearly random copolymer. During solidification only butylene terephthalate blocks are able to crystallize. However, some of the reflections from crystal planes are shifted towards lower diffraction angles as compared to butylene terephthalate homopolymer. Nonisothermal crystallization revealed the peak of crystallization at 37 °C while melting of those crystals occurred at 108 °C, in contrast to a homopolymer of butylene terephthalate (228 °C). PBASGT crystallizes in the form of thin fibrous crystals, few nanometers thick, which is the main factor influencing the depression of its melting temperature. Although the crystallinity degree is low (around 20%) the crystals form spherulitic aggregates filling the entire volume of the material. The chemical structure, morphology and crystallization behavior of PBASGT were compared with commercial aliphatic–aromatic copolyester of butylene terephthalate and butylene adipate (PBAT), Ecoflex. It appeared that, in contrast to aliphatic part of PBAT, aliphatic part of PBASGT is unable to crystallize, when annealed below room temperature. It appeared also that, similarly as PBAT, PBASGT is biodegradable.

An idea to deform highly crystalline polymer particles with unentangled macromolecules into nanofibers by shearing via viscous media has been exploited in the paper. It has been shown that nanofibers can be generated during uniform shearing of solid polytetrafluoroethylene, PTFE, particles dispersed in a molten polymer. PTFE powder particles should be of high crystallinity with unentangled macromolecules and should consist crystals with melting temperature exceeding that of equilibrium melting temperature (332–336 °C). The larger the deformation ratios and shear rates, and the longer the shearing times the thinner and stronger PTFE nanofibers are obtained. Shearing during compounding results in generation of PTFE nanofibers and thus, forming all-polymer nanocomposite. Isotactic polypropylene, high density polyethylene, low density polyethylene and atactic polystyrene were used as polymer matrices for the generation of all-polymer nanocomposites during compounding with selected PTFE powders. The process is characterized by simplicity just by shearing the dispersed crystalline polymer inclusions in another molten polymer.

A novel composite material containing 2 wt% of cellulose nanofibers well dispersed in PLA matrix, both materials being biodegradable, was prepared and studied. Biodegradable composites with 2 and 20 wt% of cellulose fibers with standard diameters were also obtained and examined for comparison. The nanocomposite exhibited markedly higher storage modulus as compared to neat PLA and the composite with the same content of cellulose standard fibers. In addition, yield strength of the nanocomposite was improved in comparison with neat PLA, especially at elevated temperature of 45 °C, at which it was higher by 50%. No negative effect of standard fibers and nanofibers on molar mass of PLA matrix was observed. Moreover, the composite materials, including the nanocomposite, did not show weight loss up to 300 °C.

International audience

Nucleation of crystallization of isotactic polypropylene (iPP) and polyoxymethylene (POM) with dispersed submicron particles of another polymer – poly(tetrafluoroethylene) (PTFE) was studied. The polymers were mixed with various contents of PTFE particles, in the range from 0.005 to 0.5 wt.%. iPP and POM with PTFE particles are all-polymer systems with enhanced nucleation of crystallization. PTFE particles with sizes below 300 nm added to POM and iPP efficiently decreased sizes of polycrystalline aggregates. Moreover, nonisothermal crystallization temperature of iPP by increased by up to 14 °C. iPP and POM with PTFE exhibited the elastic modulus slightly higher, by up to 10–13%, than that of the neat polymers. Other mechanical properties remained unchanged, with the exception of reduced elongation at break of POM with PTFE.

International audience; Shear-induced isothermal crystallization in iPP based nanocomposites with organo-modified montmorillonite was followed by light depolarization technique. Prior to the crystallization, samples were sheared at 1 or 2 s(-1) for 10s in a plate-plate system at crystallization temperature of 136 degrees C. Structure of the solidified specimens was investigated by light microscopy and electron microscopy, X-ray techniques and IR spectroscopy. Strong enhancement of the nucleation and crystallization after shearing was observed in the compatibilized nanocomposites with the clay. Clay exfoliation was found to accelerate strongly the shear-induced nucleation and overall crystallization. However, the sheared samples exhibited only weak orientation of alpha crystals with (040) crystallographic planes parallel to shearing direction that resulted probably from a small population of oriented crystals that formed due to shear-induced orientation of iPP chains and served as nuclei for further nearly isotropic growth.

Plasticization of semicrystalline poly( l -lactide) (PLA) with a new plasticizer – poly(propylene glycol) (PPG) is described. PLA was plasticized with PPG with nominal M w of 425 g/mol (PPG4) and 1000 g/mol (PPG1) and crystallized. The plasticization decreased T g , which was reflected in a lower yield stress and improved elongation at break. The crystallization in the blends was accompanied by a phase separation facilitated by an increase of plasticizer concentration in the amorphous phase and by annealing of blends at crystallization temperature. The ultimate properties of the blends with high plasticizer contents correlated with the acceleration of spherulite growth rate that reflected accumulation of plasticizer in front of growing spherulites causing weakness of interspherulitic boundaries. In PLA/PPG1 blends the phase separation was the most intense leading to the formation of PPG1 droplets, which facilitated plastic deformation of the blends that enabled to achieve the elongation at break of about 90–100% for 10 and 12.5 wt% PPG1 content in spite of relatively high T g of PLA rich phase of the respective blends, 46.1–47.6 °C. Poly(ethylene glycol) (PEG), long known as a plasticizer for PLA, with nominal M w of 600 g/mol, was also used to plasticize PLA for comparison.

The widely applied approaches to the description of the conversion of melt into spherulites are reviewed and critically assessed. The attention is focused on the polymer crystallization in quiescent conditions. Both isothermal and nonisothermal crystallization processes are considered, including also the crystallization in a temperature gradient. In addition, the approaches to the description of the influence of spatial confinement and additional nucleation processes on surfaces confining the material on the conversion of melt into spherulites are discussed. Since similar phenomena occur also in fiber-reinforced systems, the theoretical predictions of the overall crystallization kinetics in these systems are also included in the present review. The possible reasons of discrepancies between the theoretical predictions and the experimental data are analyzed.

Poly( l -lactide) (PLA) was plasticized with poly(ethylene glycol)s having Mw of 400 and 600 g/mol. In addition to poly(ethyne glycol)s with hydroxyl end groups, monomethyl ethers of poly(ethylene glycol) having Mw 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 Tg 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.

Nanocomposites based on low density polyethylene, containing of 3 or 6 wt.% of organo-modified montmorillonite nanoclay (MMT-ODA) and maleic anhydride grafted low density polyethylene as a compatibilizer were prepared by melt mixing and characterized. Exfoliation of silicate layers was achieved, as confirmed by X-ray diffraction and transmission electron microscopy. The compatibilized nanocomposites exhibit improved thermal stability in air as compared to neat polyethylene and nonexfoliated MMT-ODA composite. The crystallinity and crystallization kinetics of polyethylene matrix is not affected significantly by the presence of MMT-ODA clay. Drawability of the compatibilized nanocomposite with 6 wt.% of MMT-ODA is similar to neat polyethylene, whereas the composition having the same amount of MMT-ODA, without compatibilizer, exhibits poorer drawability. Scanning electron microscopy and density measurements of drawn samples indicate the existence of pores in noncompatibilized composite while no pores and good adhesion to MMT-ODA are found in compatibilized nanocomposites.

The crystallization of nanocomposites of isotactic polypropylenes with organo modified montmorillonite compatibilized by maleic anhydride grafted polypropylene was studied by light microscopy in isothermal conditions in quiescent state and in shear. The isothermal and nonisothermal crystallization of the composites was also investigated by DSC method. Only weak nucleation activity of montmorillonite was observed during crystallization in static conditions. The clay nucleation activity was greatly enhanced in shear-induced crystallization and resulted in a drastic decrease of spherulite sizes. In nanocomposite films crystallized isothermally the intense nucleation of isotactic polypropylene spherulites was observed when the polymer was forced to flow to compensate the volume shrinkage due to crystallization. It was also found that the presence of a glass support enhances spherulite nucleation in nanocomposites which is caused possibly by shear due to a difference in thermal shrinkage of a polymer matrix and a glass support.

Experiments were conducted to determine the influence on impact strength of small amounts of low molecular weight polybutadiene pools dispersed within the glassy matrix of high impact polystyrene (HIPS)/polystyrene blends or acrylonitrile-butadiene-styrene (ABS)/polystyrene blends. The blends of HIPS and polystyrene or ABS and polystyrene were formulated to obtain materials containing small amounts of composite particles to act as craze initiators. Izod impact tests were conducted on standard single notched samples. The fracture surfaces of the Izod specimens and of double-notched tensile specimens were studied by means of scanning electron microscopy, and the internal morphologies of the blends were examined by transmission electron microscopy. Addition of low molecular weight polybutadiene does not increase the impact strength of the blends because, at the high strain rates of the Izod test, the low molecular weight polybutadiene loses its effectiveness as a local plasticizer of craze fibrils. Tensile tests at various strain rates were conducted in order to estimate the strain rate where the mobility of the low molecular weight polybutadiene cannot keep up with the rate of craze propagation through the polystyrene matrix; this critical strain rate is in the region of 10−2s−1 at 20°C for the particular low molecular weight polybutadiene (3400 g mol−1, 74% 1,4 microstructure) employed here.

The solution of the heat conduction equation with appropriate boundary and initial conditions makes it possible to determine the influence of the latent heat of fusion evolving during polymer crystallization on the temperature distribution near the crystallization front. It is shown that the increase in temperature at the crystallization front does not exceed a fraction of a degree.