Molecular Weight Samples Research Articles

Negatively charged hyperbranched polyether-based polyelectrolytes.

The preparation of carboxylated hyperbranched polyglycerols of narrow polydispersity was achieved by modification (78–90%) of the hydroxyl end groups via Michael addition of acrylonitrile, followed by hydrolysis. High conversion could only be achieved for low molecular weight starting materials (520 and 1,030 g mol−1). The solution properties of the resulting materials were investigated by dynamic light scattering (DLS), showing the formation of large aggregates with size depending on the pH value. After deposition on a negatively charged mica surface, the structures observed by atomic force microscope (AFM) show the coexistence of aggregates and single macromolecules. Most interesting, in the case of the lower molecular weight sample (PG 520 g mol−1), extended and ordered terrace structures were formed, which are unprecedented for hyperbranched polymers and are of interest for surface modification in general.

Multidimensional On-Line SPE for Undisturbed LC-MS-MS Analysis of Basic Drugs in Biofluids

In bioanalytical LC-MS-MS matrix effects influencing the ionization process are a major concern with respect to the quality of the results obtained. In general such matrix effects are directly related to an insufficient sample clean-up of the biofluids. In order to establish a MS-adequate clean-up procedure for basic analytes present in biofluids (e.g. urine, plasma) which is based on solid-phase extraction (SPE) principles a combination of tailor-made SPE column packings and automated column-switching was developed. This novel, multidimensional (MD) SPE platform relies on the combination of a SPE column packed with a restricted access material (RAM) allowing size-exclusion and reversed phase chromatography (SEC-RPC) and a second SPE column packed with a mixed-mode phase (MMP) allowing ion exchange and reversed phase chromatography (IEX-RPC). For the evaluation of this MD-SPE platform 8 tricyclic antidepressants and two metabolites were chosen as model analytes. In order to monitor matrix effects, i.e. ion suppression, postcolumn infusion experiments were performed and compared with a two-dimensional SPE column mode (SEC-RPC). The MD-SPE platform is highly efficient for removal of low and high molecular weight sample components which suppress ionization to varying extend. In addition electrospray ionization of the model analytes is not affected by inter- or intra-individual variations in the composition of the matrix investigated. It is also independent of the species the biofluids originate from. It was demonstrated that the MD-SPE platform has a generic potential with respect to on-line SPE of basic drugs having a pKa > 6.5 and a moderate to low polarity and being present in different biofluids.

Solution thermodynamics of poly(ethylene glycol)/water systems

AbstractThe unperturbed molecular dimensions of poly(ethylene glycol) (PEG) samples (of different molecular weights) have been evaluated in aqueous solutions from viscosity measurements at 25, 30, 35, and 40°C. The unperturbed dimension, Kθ, has been determined from extrapolation methods, i.e., Kurata–Stockmayer–Fixman (KSF), Inagaki–Suzuki–Kurata (ISK), and Berry equations. The hydrodynamic expansion factor, αη, as well as the unperturbed root‐mean‐square end‐to‐end distance, 〈r2〉, found for the system indicated that the polymer coils contract as the temperature is raised from 25 to 40°C. The long‐range interaction (excluded volume) parameter, B, was also evaluated and a significant decrease was found for the PEG/water system between 25 and 40°C. The theta temperatures, θ, were obtained from the temperature dependence of (1/2 − χ) and the second virial coefficient was detected in the temperature interval of 25–40°C for the system and quite a good agreement with the calculated values evaluated via extrapolation and interpolation methods was observed. The thermodynamic interaction parameter χ was evaluated through the sum of the individual values of enthalpy and entropy dilution parameters, χH and χS, for PEG samples. All the unperturbed molecular dimensions of PEG/water system were calculated and compared according to Mw and Mn values of PEG samples. Calculated values were interpreted mainly on the basis of hydrogen‐bond formation between polymer segments and PEG‐water molecules in solution. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 203–216, 2006

Enhanced mixture analysis of poly(ethylene glycol) using high-field asymmetric waveform ion mobility spectrometry combined with fourier transform ion cyclotron resonance mass spectrometry.

The combination of high-field asymmetric waveform ion mobility spectrometry (FAIMS) with Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) makes possible lower detection limits, increased sensitivity, and increased dynamic range in the analysis of poly(ethylene glycol) (PEG) samples of low molecular weight. The signal gain obtained using FAIMS depends on ion identity, with a range between 1.8x and 14x obtained for various molecular ions of PEG 600. A 1.7-fold reduction in noise is obtained using FAIMS due to the elimination of chemical noise. The improved detection performance is predominantly due to a reduction in adverse Coulomb effects as a result of ions being selectively introduced into the mass spectrometer. The high ion transmission obtained using FAIMS combined with the high sensitivity of FTICR-MS detection make possible separation of multiple gas-phase conformers of PEG molecular cations that have low abundance (less than 0.2% relative abundance) and that have not been detected previously. Mixed dications of PEG that have the same nominal mass but differ by the number polymer subunits (m/Delta m up to 25,000) can be separately introduced into the mass spectrometer using FAIMS. Interactions of the carrier gas with the metal ions that are attached to the PEG molecules appear to be the most significant factor in these FAIMS separations.

Hydrolytic degradation of poly( d, l-lactide) as a function of end group: Carboxylic acid vs. hydroxyl

d, l-Lactide was initiated with 1,4-butanediol in the presence of stannous octoate catalyst to provide hydroxyl-terminated poly( d, l-lactide) at 5000 and 20,000 g/mol. Portions of these materials were reacted with succinic anhydride in the presence of 1-methylimidazole to convert the hydroxyl functionality to succinic acid-terminated polymers in relatively high yield. The four materials were placed in a 7.4 pH buffered saline solution at 37 °C and monitored up to 180 days for their relative moisture uptake and weight loss behaviors. Carboxylic acid functionality displayed a dramatic effect on the moisture uptake behaviors for the 5000 and 20,000 g/mol polymers when compared to their respective hydroxyl functional materials. Carboxylic acid functionality significantly increased the hydrolytic degradation rate and mass loss behavior for the 5000 g/mol material, but did not affect the hydrolytic degradation rate for the higher molecular weight sample. These results suggest that moisture uptake is not the rate limiting step for the hydrolytic degradation high molecular weight poly( d, l-lactide).

Effect of MW and concentration of chitosan on antibacterial activity of Escherichia coli

Different molecular weight (MW) chitosans (5.5×10 4 to 15.5×10 4 Da) with the same degree of deacetylation (80%±0.29), were obtained by the method of acetic acid hydrolysis. The effect of antimicrobial activities of chitosan and acetic acid against Escherichia coli were investigated. All of the chitosan samples with MW from 5.5×10 4 to 15.5×10 4 Da had antimicrobial activities at the concentrations higher than 200 ppm. The growth of E. coli was promoted at concentration lower than 200 ppm. The antibacterial activity of chitosan had relationship to the MW at the concentration range from 50 to 100 ppm. The antibacterial activity of low MW chitosan is higher than that of the high MW samples. But the chitosan sample with the middle MW (9.0×10 4 Da) could promote the growth of bacteria. In the different stages of cultivation, the earlier chitosan was added the greater effect it did. And the mechanism of antibacterial activity was that E. coli was flocculated.

Effects of end group polarity and molecular weight on the lower critical solution temperature of poly(N‐isopropylacrylamide)

AbstractThe lower critical solution temperatures (LCSTs) for mass fractionated samples of poly(N‐isopropylacrylamide) (PNIPAM) were studied to determine the effect of polymer molecular weight on the LCST using a high throughput temperature gradient apparatus. PNIPAM fractions prepared by a conventional radical polymerization using azoisobutyronitrile (AIBN) as the initiator had LCSTs that were largely invariant with molecular weight or dispersity. Only slight deviations were noted with lower molecular weight samples. An 18‐kDa sample had a 0.6 °C higher LCST. A 56‐kDa sample had a 0.2 °C higher LCST. PNIPAM derivatives prepared with a triphenylmethyl (trityl) functionalized azo initiator were also prepared and mass fractionated. These samples' LCSTs were identical to those of PNIPAM samples prepared using AIBN initiation when higher molecular weight samples were compared. The trityl‐containing PNIPAM fractions' LCSTs varied when the molecular weight decreased below 100 kDa. Acidolysis of the trityl end groups provided a third set of PNIPAM derivatives whose LCST differed only with samples with Mw values < 60 kDa. These results show there is no effect of molecular weight on LCST until the degree of polymerization is such that end group structure becomes significant. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1492–1501, 2006

Antioxidant activity of different molecular weight sulfated polysaccharides from Ulva pertusa Kjellm (Chlorophyta)

Polysaccharides extracted from Ulva pertusa Kjellm (Chlorophyta) are a group of sulfated heteropolysaccharides, the ulvans. In this study, different molecular weight ulvans were prepared by H2O2 degradation and their antioxidant activities investigated including superoxide and hydroxyl radical scavenging activity, reducing power and metal chelating ability. The molecular weights of natural and degraded ulvans were 151.7, 64.5, 58.0, and 28.2 kDa, respectively, as determined by high performance gel permeation chromatography. Among the four samples, U3 (the lowest molecular weight sample) showed significant inhibitory effects on superoxide and hydroxyl radicals with IC50 values of 22.1 μ g mL−1 and 2.8 mg mL−1; its reducing power and metal chelating ability were also the strongest among the four samples. All the other samples also demonstrated strong activity against superoxide radicals. The results indicated that molecular weight had a significant effect on the antioxidant activity of ulvan with low molecular weight ulvan having stronger antioxidant activity.

Thermal stability and degradation behavior of novel wholly aromatic azopolyamide-hydrazides

Thermal stability and degradation behavior of a series of novel wholly aromatic polyamide-hydrazides containing azo groups in their main chains have been investigated in nitrogen and in air atmospheres using differential scanning calorimetry (DSC), thermogravimetry (TG), infrared spectroscopy (IR) and elemental analysis. The influences of controlled structural variations and molecular weight on the thermal stability and degradation behavior of this series of polymers have also been studied. The structural differences were achieved by varying the content of para- and meta-substituted phenylene rings incorporated within this series. Azopolyamide-hydrazides having different molecular weights of all para-substituted phenylene type units were also examined. The polymers were prepared by a low temperature solution polycondensation reaction of p-aminosalicylic acid hydrazide [PASH] and an equimolar amount of 4,4′-azodibenzoyl chloride [4,4′ADBC] or 3,3′-azodibenzoyl chloride [3,3′ADBC] or mixtures of various molar ratios of 4,4′ADBC and 3,3′ADBC in anhydrous N, N-dimethyl acetamide [DMAc] containing lithium chloride as a solvent at −10 °C. All the polymers have the same structural formula except the mode of linking phenylene units in the polymer chain. The results clearly reveal that these polymers are characterized by high thermal stability. Their weight loss occurred in three distinctive steps. The first was small and assigned to the evaporation of absorbed moisture. The second was appreciable and was attributed to the cyclodehydration reaction of the hydrazide groups into 1,3,4-oxadiazole rings by losing water, combined with elimination of azo groups by losing molecular nitrogen. This is not a true degradation but rather a thermo-chemical transformation reaction of the azopolyamide-hydrazides into the corresponding polyamide-1,3,4-oxadiazoles. The third was relatively severe and sharp, particularly in air, and corresponded to the decomposition of the resulting polyamide-1,3,4-oxadizoles. In both degradation atmospheres, the improved resistance to high temperatures was always associated with increased content of para-phenylene moieties of the investigated polymer. The better thermal stability of the wholly para-oriented type of polymer relative to the other polymers is attributed to its greater chain symmetry which is responsible for its greater close packing, rod-like structure and consequently stronger intermolecular bonds which would be more difficult to break and therefore more resistance to high temperatures. Further, with exception of 160–200 °C temperature range, where the lower molecular weight samples showed considerable weight losses which were most probably due to hydrogen bonded DMAc, all the wholly para-oriented phenylene type of polymer samples behaved similarly regardless of their respective molecular weight. This seems to indicate that the structural building units responsible for high thermal stability of the polymers are their characteristic groups, such as aromatic moieties, amide and hydrazide linkages in case of azopolyamide-hydrazides, and 1,3,4-oxadiazole rings, aromatic nuclei and amide linking bonds in case of polyamide-1,3,4-oxadiazoles, rather than the longer chain segments.

Fragmentation of the Lamellae and Fractionation of Polymer Coils upon Mixing Poly(dimethylacrylamide) with the Lamellar Phase of Aerosol OT in Water

The lamellar mesophase formed by surfactant 1,4-bis(2-ethylhexyl) sodium sulfosuccinate (AOT) in deuterated water is mixed with poly(dimethylacrylamide) (PDMAA) polymers of low molecular weight (Mn= (2-20) x 10(3)). The mixtures separate into microphases (lamellar plus isotropic polymer solution). Their microstructures are studied by microscopy, small-angle X-ray scattering (SAXS), and deuterium NMR (2H NMR). According to SAXS, the lamellar phase fractionates the molecular weight distribution of the polymer, by dissolving only chains with coil sizes smaller than the thickness of the water layers between lamellae, and keeping larger chains segregated from the lamellar phase. The fraction of polymer that is segregated from the lamellar phase grows with Mn of the polymer. In 2H NMR, there are two signals, a quadrupolar doublet (water molecules hydrating the anisotropic lamellar phase contribute to this doublet) and a singlet (water molecules in the isotropic polymer solution contribute to this singlet). These two signals are deconvoluted to analyze the phases. Mixing with the polymer produces the partial dispersion of the lamellar phase into small fragments (microcrystallites). The structure of these microcrystallites is such that they conserve the regular long period spacing of the macrophase, and are thus identified in SAXS, but they are smaller than the minimum size required to produce quadrupolar splitting (about 4 microm), and therefore, in 2H NMR, they contribute to the singlet. 2H NMR can thus not distinguish between small microcrystallites and an isotropic polymer solution segregated from the lamellar phase; instead small microcrystallites are detected as an apparent increase of the isotropic solution. The degree of dispersion produced by the polymer in the lamellar phase is correlated with the degree of segregation that the polymer suffers. Thus, much greater dispersion into microcrystallites is produced by the higher Mn polymers than by the lower Mn polymers (in the range covered by the present samples, although with a much higher molecular weight sample (3 x 10(6)) that is totally segregated no such microcrystallites were detected).

Effect of low molecular λ-carrageenan from Chondrus ocellatus on antitumor H-22 activity of 5-Fu

λ-Carrageenan is a kind of sulfated galactan isolated from some red algae and reported to have more biological activities than other types, such as K-type [Güven KC, özsoy Y, Ulutin ON. Anticoagulant, fibrinolytic and antiaggregant activity of carrageenans and alginic acid. Bot Mar 1991; 34: 429–432]. The molecular weight (MW) of λ-carrageenan reaches up to 600–700 kDa, the high MW decreased their solubility and limited their bioactivities. At the same time, the production and applications of λ-carrageenan are limited for its some characteristics, such as the high viscosity, and the experiments of antitumor and immunomodulation also indicated that MW of polysaccharides had notably effect on the activities [Zhou G, Sun YP, Xin H, Zhang Y, Li Z, Xu Z. In vivo antitumor and immunomodulation activities of different molecular weight lambda-carrageenans from Chondrus ocellatus. Pharmacol Res 2004; 50(1) 47–53, Zhou G, Xin H, Sheng W, Sun Y, Li Z, Xu Z. In vivo growth-inhibition of S180 tumor by mixture of 5-Fu and low molecular λ-carrageenans from C. ocellatus. Pharmacol Res 2005; 51(2) 153–157]. In this research, λ-carrageenan was extracted from C. ocellatus, an important economic alga in China. Then some small MW samples of 650, 240, 140, 15 and 9.3 kDa are obtained by the method of microwave degradation, respectively. The tumor inhibiting activities of the low MW λ-carrageenan and mixture of it and 5-Fu on transplanted H-22 tumor mice were investigated. The weight of immune organ, proliferation ratio of lymphocyte and concentration of Tumor Necrosis Factor-α (TNF-α) and histopathology of tumor from transplanted H-22 tumor mice were also determined. The results indicated that the degraded λ-carrageenan could add the antitumor activities of 5-Fu and improve the immunocompetence damaged by 5-Fu.

The in vivo and in vitro degradation behavior of poly(trimethylene carbonate)

The in vivo and in vitro degradation behavior of poly(trimethylene carbonate) (PTMC) polymers with number average molecular weights of 69×10 3, 89×10 3, 291×10 3 and 457×10 3 g/mol (respectively abbreviated as PTMC 69, PTMC 89, PTMC 291 and PTMC 457) was investigated in detail. PTMC rods (3 mm in diameter and 4 mm in length) implanted in the femur and tibia of rabbits degraded by surface erosion. The mass loss of high molecular weight PTMC 457 specimens was 60 wt% in 8 wks, whereas the mass loss of the lower molecular weight PTMC 89 specimens in the same period was 3 times lower. PTMC discs of different molecular weights immersed in lipase solutions (lipase from Thermomyces lanuginosus) degraded by surface erosion as well. The mass and thickness of high molecular weight PTMC 291 discs decreased linearly in time with an erosion rate of 6.7 μm/d. The erosion rate of the lower molecular weight PTMC 69 specimens was only 1.4 μm/d. It is suggested that the more hydrophilic surface of the PTMC 69 specimens prevents the enzyme from acquiring a (hyper)active conformation. When PTMC discs were immersed in media varying in pH from 1 to 13, the non-enzymatic hydrolysis was extremely slow for both the high and low molecular weight samples. It can be concluded that enzymatic degradation plays an important role in the surface erosion of PTMC in vivo.

Study on the drug release property of cholesteryl end‐functionalized poly(ε‐caprolactone) microspheres

End-functionalized poly/oligo(epsilon-caprolactone)s were synthesized through the ring-opening polymerization of epsilon-caprolactone initiated by cholesterol with a hydroxyl group. Using the end-functionalized poly/oligo(epsilon-caprolactone)s with different molecular weights, the microsphere drug delivery systems were fabricated using a convenient melting-emulsion method. The drug release properties of microspheres were investigated with the presence of an enzyme, Pseudomonas cepacia lipase, as well as in the absence of the enzyme. The release profiles can be fitted nicely by the classical empirical exponential expression. Under the hydrolytic condition, the drug release is mainly controlled by Fickian diffusion, and the high molecular weight of the matrix results in a slower drug release rate. Under the enzymatic condition, the drug release is dominated by combined degradation and diffusion mechanism, and the high molecular weight sample exhibits a faster release rate that is mainly caused by the higher degradation rate of the sample with lower cholesteryl moiety content.

Structure determination of the amorphous perfluorinated homopolymer: Poly[perfluoro(4-vinyloxyl-1-butene)

The low molecular weight samples were examined to assign the nuclear magnetic resonance (NMR) signals, and to study the cyclic structure and the end group structure for poly[perfluoro(4-vinyloxyl-1-butene)]. While the penta-cyclic structure was dominant, the hexa-cyclic structure also existed. The ratio was penta/hexa = 0.84/0.16. No preference of cyclic structure (penta- or hexa-cyclic) changed from the initial to the final stage of polymerization. The polymerization temperature did not affect to the ratio of cyclic structures, whereas the modification at side chain part could. The end group was observed, (CH 3) 2CHOCOO for initiator end, CF 2Cl for chain transfer end, then only initiator end group changed to CF 3 with fluorination.

Miscibility Study of Chitosan/Poly(Vinyl Pyrrolidone) Blends in Dilute Solution

Miscibility of chitosan (CHI) with two poly(vinyl pyrrolidone) samples (PVP-1 and PVP-2) of different molecular weights was studied in dilute solution by viscometry. Blends with chitosan weight fraction in the range 0.10–0.90 were studied at 30°C in a buffer solution of 0.1 M acetic acid/0.2 M sodium acetate. Starting with the classical Huggins equation, the results of the viscosity behavior of each parent polymer and their blends were interpreted in terms of miscibility parameters, Δk, α, and Δb. CHI (Mv = 3.3 × 105) and PVP-1 (Mv = 2.5 × 104) were found to be miscible at a blend composition of CHI/PVP 0.90/0.10. PVP-2 (Mv = 1.1 × 106) and CHI were miscible in the full range of blend compositions studied. The existence of favorable secondary interaction between CHI and PVP leading to miscibility was demonstrated by FTIR spectroscopy.

Conductive Fibre Prepared From Ultra-High Molecular Weight Polyaniline for Smart Fabric and Interactive Textile Applications

Panion™ fibre is prepared from a high molecular weight version of chlorine and defect free polyaniline (PANI). It is processed into textile fibre by continuous wet spinning techniques. The room temperature DC conductivity for unstretched and stretched (5x at 373 K) high molecular weight samples was 72 and 725 Ω −1 cm −1, respectively. Resistivity measurements of Panion™, from 4 to 350 K, indicate that the fibre manufacturing process and the polymer synthetic route both serve to minimize structural disorder in the solid-state. The transport resides on the metallic side of the metal-insulator (M-I) phase boundary for unstretched and stretched fibre samples prepared from high M w PANI emeraldine base. The monofilament fibre can be formed into yarns for weaving fabric, braided into 2- and 3- dimensional structures, or used to form knitted fabrics, all with tunable electronic functionality.

Synthesis of Linear α-Olefins via Polyhomologation

Linear α-olefins (LAOs) of controlled molecular weight and low PDI were synthesized in high yield by the polyhomologation reaction of dimethylsulfoxonium methylide (1) with triallylborane (2). Following polymerization, propionic acid or trimethylamine N-oxide dihydrate (TAO) was used to afford α-vinyl-ω-methyl or α-vinyl-ω-hydroxy end-functionalized polymethylene, respectively. Controllable molecular weights up to 13 kDa were obtained by varying the initial monomer/catalyst ratio, and the resultant polymers had very narrow polydispersities (PDI < 1.08). Although polymer yields were generally high, quantities of oxidized side products were observed in α-vinyl-ω-methyl samples; however, these oxidized side products could be removed for samples of low molecular weights (Cn ≤ 36).

Formation of new banded spherulites in polyimides

AbstractThe crystalline morphology and structural development of aromatic polyimides during an optimum continuous thermal imidization procedure were examined by means of polarized optical microscopy and X‐ray diffraction. During thermal imidization, 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride/1,3‐diaminobenzene polyimide samples formed complicated spherulites, which, in addition to zigzag Maltese crosses, also showed concentric extinction rings, which are characteristic of banded spherulites. The factors affecting the formation of banded spherulites were studied. The initial imidization conditions dramatically affected the formation of the banded spherulite morphology: slow heating (0.5 °C/min) or fast heating (20 °C/min) led to relatively small polyimide spherulites and less identifiable extinction rings. The morphological features were also affected by the molecular weight of the polyimide: higher molecular weight samples showed typical banded spherulites, whereas low‐molecular‐weight samples formed degenerated banded spherulites. In all the spherulites formed in 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride/1,3‐diaminobenzene polyimides, special zigzag Maltese crosses, instead of normal Maltese crosses, were observed. The relationship between the imidization procedure and the spherulite morphology formation was also studied. X‐ray and Fourier transform infrared together revealed that after several minutes of thermal treatment, the crystallization was nearly complete, with a 42.5% degree of crystallinity; meanwhile, only some poly(amic acid) converted to the corresponding polyimide, with a 27% degree of imidization. The crystalline morphology and structure formed in the initial stage of the imidization process were maintained during the following imidization processing at an elevated temperature. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1997–2004, 2005

Evolution of lamellar orientation and crystalline texture of various polyethylenes and ethylene-based copolymers in plane-strain compression

Deformation-induced orientation behavior of a series of polyethylenes and ethylene copolymers was studied by WAXS and SAXS. Plane-strain compression was employed as a deformation mode. Compression experiments were performed at room temperature and at 80 °C. Activity of crystallographic slip systems supported by interlamellar shear during deformation process resulted in formation of the (100)[001] primary texture component with chain axis aligned along the flow direction, FD, and (100) planes perpendicular to the loading direction, LD. A second texture component was produced on unloading by {310} twinning of primary component. Intensity of that twinning was controlled primarily by the molecular weight of polymer, through modification of the molecular network and its stress response on applied strain. Molecular weight influenced also the orientation behavior on the lamellar level. In all polymers studied the lamellar shear, intensive on the beginning of deformation and exhausting around the true strain of 0.75, induced cooperative kinks of lamellae. In samples of relative low molecular weight further advancement of deformation led to an additional intense fragmentation of lamellar structure and restructurization into new long period in the direction of flow ( e>1.2). In polymers of higher molecular weight such an intense fragmentation did not occur due to topological constraints. Consequently, the lamellar structure of heavily deformed polyethylenes of low molecular weight consisted of relatively small blocks, oriented with their normals approximately along FD, while high molecular weight species, in absence of intense lamellae fragmentation, developed the structure of relatively long lamellar crystals, oriented in a chevron-like structure. It was found that the orientation produced by deformation process is controlled entirely by the strain applied to the material, while the differences induced by modification of the stress due to the change of the deformation temperature are of minor consequence.

Processing, Morphology, and Mechanical Properties of Liquid Pool Polypropylene with Different Molecular Weights

AbstractSummary: The processability, morphology, and resulting mechanical properties of novel polypropylene (PP) samples of varying molecular weight ($\overline M _{\rm w}$) were studied. A series of homopolymer PP in a wide $\overline M _{\rm w}$ range from 101 000 to 1 600 000 g · mol−1 was polymerised in a liquid pool (LP) under defined conditions. The LP‐PP with a well‐known polymerisation history was manufactured into micro dumbbell specimens by means of a micro injection‐moulding process. The morphology and mechanical properties of the samples were studied by light microscopy, transmission and scanning electron microscopy, and a quasi‐static tensile test. Simulation of the filling behaviour of the molten polymer inside the mould shows that the shear rate increases as the molecular weight increases, up to a maximum shear rate of 750 000 s−1. In addition, the present crystallisation time of the high‐molecular‐weight PP samples is clearly lower than their retardation time; the long macromolecules do not have sufficient time to retard while cooling. As a result of the shear‐induced crystallisation, a highly oriented crystalline structure is formed as a function of the acting shear rate. SEM and TEM investigations show the existence of an oriented shish kebab structure. The density of the shish kebab increases as the molecular weight increases. Evaluations of the shear rate and the morphological structure indicate a critical shear rate of about 300 000 s−1. Above this shear rate level, shish kebab structures are favourably formed. The shear‐induced crystallisation and, therefore, the preferred formation of a highly oriented shish kebab structure lead, obviously, to unusual solid‐state properties of the analysed LP‐PP samples. With a tensile strength up to 100 N · mm−2 and an attainable strain at break of more than 30%, the mechanical performance is much higher than results ever reported in literature.True strain–stress behaviour of moulded the LP‐PP samples of different molecular weight.magnified imageTrue strain–stress behaviour of moulded the LP‐PP samples of different molecular weight.