Effect of entangled state of nascent UHMWPE on structural and mechanical properties of HDPE/UHMWPE blends

In this study, a synthesized ultra‐high molecular weight polyethylene (UHMWPE) with a less entangled state and a commercial UHMWPE with a highly entangled state were blended with high‐density polyethylene (HDPE) by melt blending, respectively. Rheology, 2D small‐angle X‐ray scattering (2D‐SAXS), differential scanning calorimetry (DSC), and tensile test were used to study the relationship between the microstructure and the mechanical properties of blends. It was demonstrated that the UHMWPE with the less entangled state was easy to be oriented at a given flow. More mechanical networks were achieved among the HDPE matrix and the UHMWPE chains due to the fewer entanglements of synthesized UHMWPE, improving the melting recovery of blends. Furthermore, notably oriented structures (shish‐kebabs) with increased long‐periods were made in the blends of weakly entangled UHMWPE and HDPE. The tensile strength of this blend was thus enhanced, i.e., the tensile strength raised for neat HDPE from 45.7 to 83.1 MPa for HDPE/UHMWPE blends containing 10 wt % of less entangled UHMWPE. However, the phase separation of blends was characterized when more weakly entangled UHMWPE was incorporated. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44728.     

Solid-state C NMR analyses of the structures of crystallized and quenched poly(lactide)s: Effects of crystallinity, water absorption, hydrolytic degradation, and tacticity

The effects of crystallinity, water absorption, hydrolytic degradation, and tacticity on the solid structure and chain mobility of poly(lactide)s were investigated by solid-state ¹³C NMR spectroscopy. The following results were obtained from the line shapes of the carbonyl and methine carbons in ¹³C NMR spectra and their spin-lattice relaxation behavior. The crystallized poly(l-lactide) (PLLA) specimens in the dried, hydrated, and hydrolyzed states had two components, rigid and mobile components which can be, respectively, assigned to the crystalline and non-crystalline components. Upon water absorption, the chain mobility in the non-crystalline component of PLLA-C remained unvaried, reflecting a very small effect of the incorporated water molecules at room temperature. In contrast, the elevated chain mobility in the crystalline component and unclear splitting of carbonyl carbon strongly suggest that the water molecules are incorporated in the crystalline lattice. Upon removal of the non-crystalline components by hydrolytic degradation of crystallized PLLA, the chain mobility was slightly elevated in both crystalline and non-crystalline components by the lowered crystalline thickness and shortened non-crystalline chains. The non-crystalline specimens, PLLA (PLLA-Q) and poly(dl-lactide) (PDLLA), could be analyzed to contain two components, rigid and soft components, with the similar conformation but different restricted states of chains which cause high and low chain mobility. The insignificant difference in the spectral shapes and T_1C values between PLLA-Q and PDLLA strongly suggests that the effects of difference in the chain regularity and interaction on the spectral shapes and T_1C values are very low.     

Tailoring molecular structure via nanoparticles for solvent-free processing of ultra-high molecular weight polyethylene composites

Composites of conventional ultra-high molecular weight polyethylene (UHMWPE) and nanoparticles, such as carbon nanotubes or ceramics, require special processing techniques due to the high melt viscosity of the polymer matrix. Recently, we have shown that polymerization with a single-site catalytic system in suitable reaction conditions produces “disentangled” UHMWPE that can be processed in the solid state. In this study, nanoparticles have been used as carriers for the single-site catalytic system in the polymerization of UHMWPE. The high-surface area of the nanoparticles, coupled with controlled reaction conditions, favors the growth of polyethylene chains with a reduced number of entanglements. This novel synthetic route offers several advantages: 1) the catalytic system is more stable and less fouling occurs during the polymerization reaction; 2) nanoparticles are directly embedded in an otherwise intractable polymer matrix; 3) the low amount of entanglements in the UHMWPE matrix allows the resulting composites to be processed in the solid state well below the equilibrium melting temperature in a broad temperature window, to give high strength/high modulus tapes.     

Exploring the entangled state and molecular weight of UHMWPE on the microstructure and mechanical properties of HDPE/UHMWPE blends

Three types of ultra-high molecular weight polyethylene (UHMWPE) with different entangled state and molecular weight were blended with high-density polyethylene (HDPE) matrix by melt blending. Rheology, 2D-SAXS, 2D-WAXD, DSC, and mechanical tests were used to study the evolution and difference of microstructure and mechanical properties of the blends. The addition of weakly entangled UHMWPE enhanced the chain diffusion and chain orientation ability under a specific flow field. Thus, the rheological properties and mechanical properties of the blends were improved with the mix of weakly entangled UHMWPE. The mechanical properties enhancement effect of HDPE/UHMWPE blends with weakly entangled UHMWPE was owing to the shish-kebab structure formed in the injection molding process. The molecular chains of UHMWPE with a low degree of entanglement and high molecular weight increased the lamella size and crystallinity of the blends during processing. This leads to the formation of more oriented shish structures and more kebab lamella. Besides, the molecular chains of weakly entangled UHMWPE were better interlocked and intertwined with other polyethylene chains in the amorphous region, acting as the tie molecules, significantly improving the impact resistance.     

Research on the molecular entanglement and disentanglement in the dry spinning process of UHMWPE/decalin solution

The macromolecular entanglement and disentanglement in the dry spinning process of ultrahigh molecular weight polyethylene (UHMWPE)/decalin solution were investigated. By the fitting results of the theoretical model to the experimental data, it is found that the variation tendency of the N_skT value, which reflects the chain slippage at the slip links in the extensional deformation on spinning line of draw‐down process, is in accordance with the fact that each of the relationship curves between the tensile strength, modulus of the UHMWPE fibers through maximized solid state drawing and the draw‐down ratio showed a peak, thus discovering the molecular movement mechanism of “disentanglement on spinning line.” When the entanglements were included in the flow units, their apparent quantity would decrease. Based on this result, the optimum draw‐down ratio can be determined directly by measuring the draw‐down stress at the exit of the spinning duct. The molecular entanglements numbers, which are derived from the theoretical fitting to the experimental data of predrawing and after‐drawing process, abruptly increased in large amounts. It may be concluded that these are mainly not attributed to the topological entanglements, but attributed to the agglomerate entanglements. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 864–875, 2006     

The thickness dependence of elastic modulus of organosilane interphases

Series of organosilanes with varying molecular structure was deposited onto E glass. Solution coating and rf plasma enhanced Chemical Vapor Deposition (PE-CVD) techniques were used. Thickness of the deposited layers varied from 30 nm to 10⁶ nm. Vibrational piezoelectric resonator technique and the speed of Rayleigh wave measurement were used to determine elastic modulus of the layers deposited on the glass substrate as a function of layer thickness. In all cases, increase of the Young modulus, E, of the organosilane layer with decreasing layer thickness was observed. Solution deposited layers exhibited always significantly lower elastic moduli compared to the layers deposited using PE-CVD technique. Elastic moduli of deposited layers increased with increasing hydrocarbon chain rigidity at comparable chain length. Chlorosilane interlayers possessed always significantly greater elastic modulus compared to their ethoxy-analogues. The observed increase of E with the decreasing layer thickness was ascribed to the reduction of the molecular mobility of chains near solid surface compared to their mobility in the bulk. It was shown that increasing the bond strength between the deposited layer and the solid E-glass substrate resulted in immobilized region with constraints imposed by the surface spanning into a distance from the surface. This was in agreement with current models of reinforcement mechanism in nanocomposites based on the idea of immobilized entanglements. It was shown that the deposition technique plays important role in controlling the structure and properties of the deposited interphase. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

A molecular model for structural changes during shear yielding and crazing in amorphous polymers

Summary Based on macroscopic and microscopic observations, a model is proposed concerning molecular conformation and chemical structure changes occurring during mechanical deformation of amorphous polymers below their Tg. It is assumed that a bulk polymer, with high molecular mass, consists of interpenetrating, coiled chain molecules. The intrinsic flexibility and the thickness of chain molecules depend on their chemical nature. Stresses are transferred from one chain to others by physical entanglements.Shear yielding, as a solid state deformation mode, is considered as the result of stretching of the coiled chain segments between the entanglements. Crazing, as a cavitation process, is considered to be the consequence of chain scission after some degree of local shear yielding.     

Investigations of polymer chains in oriented fluid phases with deuterium nuclear magnetic resonance

Deuterium nuclear magnetic resonance (D n.m.r.) is a potentially powerful technique for exploring molecular structural and dynamical properties of polymer chains in bulk fluids and concentrated solutions. A variety of systems can be investigated (the solid state, elastomeric networks, sheared polymer fluids, chain solutes in liquid crystal solvents, and polymeric liquid crystals), over a wide range of dimensions (local chain properties, rotational isomeric state parameters, behaviour between network junctions or entanglements, evolution of tube distributions, and domain sizes of homogeneous chain alignment). A coarse comparison of low molar mass liquid crystals with condensed phases of entangled polymer fluids and elastomeric networks illustrates the key features of the D n.m.r. technique and establishes a common framework for interpreting experiments.     

Effects of molecular weight and molecular weight distribution on creep properties of polypropylene homopolymer

The molecular weights of the industrial-grade isotactic polypropylene (i-PP) homopolymers samples were determined by the melt-state rheological method and effects of molecular weight and molecular weight distribution on solid and melt state creep properties were investigated in detail. The melt-state creep test results showed that the creep resistance of the samples increased by M_w due to the increased chain entanglements, while variations in the polydispersity index (PDI) values did not cause a considerable change in the creep strain values. Moreover, the solid-state creep test results showed that creep strain values increased by M_w and PDI due to the decreasing amount of crystalline structure in the polymer. The results also showed that the amount of crystalline segment was more effective than chain entanglements that were caused by long polymer chains on the creep resistance of the polymers. Modeling the solid-state viscoelastic structure of the samples by the Burger model revealed that the weight of the viscous strain in the total creep strain increased with M_w and PDI, which meant that the differences in the creep strain values of the samples would be more pronounced at extended periods of time.     

Water sorption of poly(methyl methacrylate): 1. Effects of molecular weight

A sample of poly(methyl methacrylate), PMMA, of low molecular weight (10⁴ D) took up less water (1.2%) than samples of high molecular weight (10⁶ D: 2.0%). In contrast, the uptake of water was only slightly dependent on molecular weight for samples made by radiolysis in the glassy state. It is concluded that uptake of water depends on the closer molecular packing possible in polymers of lower molecular weight. However during radiolysis in the glassy state, this potential is not fully realized because of limited mobility. In more detail, the small changes in the diffusion coefficient and uptake of water in irradiated samples were consistent with closer packing in samples with M_n ⩽ 10 000. It is concluded that molecular packing proceeds more readily below the critical molecular weight for formation of an entangled network.     

Role of the Amorphous Morphology in Physical Properties of Ultra High Molecular Weight Polyethylene

The role of amorphous layer in the physical properties of ultra high molecular weight polyethylene (UHMWPE) was investigated. It revealed that the thicker amorphous layer would promote the toughness of polyethylene, but make a negative effect on the rigidity of the polymer. Furthermore, it would be easier for semicrystalline polymers with less entangled chains to bring out the interlamellar slippage that would absorb more energy during the deformation. Finally, promotion of the physical properties of UHMWPE was also achieved with the assistant of nano-modification and an excellent relativity between the physical properties and the amorphous thickness (la) was obtained.     

An Investigation of the Relation Between the Adhesion of Polyurethanes to Solid Surfaces and Network Structure

It is known that polymer interaction with solid surface limits the mobility of the polymer chain and this consequently affects the physico-chemical properties of the polymer.¹ In the course of polymer lattice formation, the presence of a solid surface leads to the formation of more defective spatial network.²     

Heterogeneous structure of poly(glycolic acid) fiber studied with differential scanning calorimeter, X-ray diffraction, solid-state NMR and molecular dynamic simulation

The heterogeneous structures of poly(glycolic acid) (PGA) fibers which have been used as bio-degradable suture were studied by differential scanning calorimeter (DSC),X-ray diffraction and ¹³C solid state NMR. The ¹³C cross polarization NMR spectra without magic angle spinning of the stretched fibers observed by changing the angle between the fiber axis and the magnetic field clearly showed the heterogeneous structures which consist of three components; well-oriented, poorly-oriented and isotropic amorphous components. The local structure, distribution of the fiber axis and fraction of each component were determined quantitatively. Change in the heterogeneous structure by changing the stretching method in the sample preparation and by changing the stretching ratio was also monitored. The X-ray diffraction data of the fibers are in good agreement with the ¹³C CP NMR data. Change in the heterogeneous structures correlate with change in the thermal properties observed by DSC method. The molecular dynamic simulation showed the generation of trans conformation of PGA chain and also change in the fraction of other conformations by stretching, which supports the experimental results obtained above and gives additional structural information.     

Solid-state C NMR investigation of the structure and dynamics of highly drawn polyethylene—detection of the oriented non-crystalline component

The structure and dynamics of highly drawn polyethylene samples were studied by solid-state ¹³C NMR spectroscopy. The analyses of the ¹³C spin-lattice relaxation time (T_1C) and the ¹³C spin-spin relaxation time (T_2C) have revealed that at least three components with different T_1C and T_2C values, which correspond to the crystalline, less mobile non-crystalline, and rubbery amorphous components, exist for these materials, as in the case of isothermally crystallized samples. However, another component with a mass fraction of 0.13-0.18 exists which has a ¹³C chemical shift very close to that of the orthorhombic crystalline phase but has an extremely small T_1C. Since this component is believed to have the all-trans conformation, it is termed fast all-trans. The chemical shift anisotropy (CSA) spectra for various samples that have small T_1C values have been recorded and resolved into those of the non-crystalline and fast all-trans components. As expected, the CSA spectra of the less mobile non-crystalline and rubbery amorphous components that have the smallest T_1C values display only a slight asymmetry. In contrast, the CSA spectrum of the fast all-trans component displays higher asymmetry. However, the spectrum is still much narrower than that of the normal orthorhombic crystalline phase, indicating a high degree of motional averaging. It is proposed that this component should be a highly oriented non-crystalline component, which may exist as taut tie-molecules traversing the non-crystalline region. To account for the narrow CSA, this component must undergo rapid fluctuation with large amplitudes at the torsional potential minimum in each C-C bond and possibly an additional random jump or diffusional rotation around the chain axis. Additional measurements obtained by aligning the draw axis of the sample parallel or perpendicular to the static magnetic field indicate that the fast all-trans component is oriented along the drawing direction and subjected to rapid motion around the chain axis.     

Effect of poly(ethylene oxide) with different molecular weights on the electrospinnability of sodium alginate

Although sodium alginate (SA) could not be electrospun from its aqueous solution, SA-based electrospun nanofibers can be fabricated with the help of polyethylene oxide (PEO). In this study, the influence of PEO on the electrospinnability of SA aqueous solution was investigated and the roles of chain entanglements and conformations of the blend system were emphasized. It was found that a little amount of PEO100 with high molecular weight could improve the electrospinnability of SA aqueous solution. However, a large amount of PEO2 with low molecular weight had no positive effect on the electrospinnability of SA aqueous solution. Dynamic laser light scattering (DLLS) results showed that only when the PEO molecular chains in aqueous solution were in an entangled state, PEO can enhance the electrospinnability of SA aqueous solution. The further study on rheological measurements showed that SA molecular chains could not form significant entanglements for the electrospinning even when the SA solution concentration approached concentrated regime. SA molecular chains are closely “overlapped” due to its rigid and extended conformation and cannot form effective chain entanglement. The main contribution of PEO100 to improve SA electrospinnability is offering entanglement sites and thereby enhancing the applicable entanglement degree of the blend system. Whereas, although the chain interaction between PEO2 and SA may improve slightly the flexibility of SA chains, the significant chain entanglements of the blend solution is not achieved. Three molecular models are proposed to depict visually the effect of PEO with different molecular weights on chain conformations and entanglements of SA.     

Measurement of the C spin-lattice relaxation time of the non-crystalline regions of semicrystalline polymers by a cp MAS-based method

The non-crystalline ¹³C spin-lattice relaxation times of atactic and isotactic polypropylene and those of an ethylene-1-octene copolymer of low crystallinity have been measured by classical inversion and saturation recovery methods as well as by a cp MAS-based pulse sequence. The latter is a saturation recovery-type sequence that involves cross-polarization. It samples preferentially the soft non-crystalline regions of semicrystalline polymers. The method is found to be useful in determining T_1C of the amorphous regions of semicrystalline iPP at room temperature. It is found that the atactic PP molecule and the non-crystalline iPP regions have the same average segmental relaxation rate. The T_1C of some of the carbons investigated was <T_1H and the experimental recovery curves showed complex exponential behavior from the contribution of a transient nuclear Overhauser effect (NOE) to the ¹³C magnetization. Moreover, the experimental data were fitted with a double exponential function obtained from solving the Solomon equations. The fitting leads to T_1C in very good agreement with the values obtained by classical inversion or saturation recovery sequences. The same T_1C value was obtained with the cp-based sequence when transient NOEs were eliminated by saturation of the proton magnetization during the delay period. The hexyl branches of the ethylene copolymer lead to an increased average backbone C-C intermolecular distance in the non-crystalline regions compared to those of the linear polyethylene chain and, thus, to a higher backbone methylene segmental mobility.     

Entropy reduction phenomenon in the non-equilibrium state of freeze-dried polymethyl methacrylate samples

Polymethyl methacrylate (PMMA) samples with different entanglements are prepared via a freeze-drying processing. Modulated differential scanning calorimetric (MDSC) and Fourier transform infrared spectroscopy (FT-IR) are combined to get insight into the evolution of glass transition behavior dependence of chain entanglements and the kinetics of the entanglements recovery process via annealing experiments. It is interesting to note that glass transition temperature (T_g) did not increase with concentration monotonically after annealing, and tended to be higher than the bulks. MDSC shows that two glass transition regions appeared on the reversing heat flow curves. There seems to be dual amorphous regions with quite different characters in the freeze-dried sample: the free amorphous region and the rigid amorphous region, which are analogous to that in semi-crystalline polymer. Meanwhile, the IR I _1240cm ^-1 /I _1270cm ^-1 ratio determined from the FT-IR curves showed the same progression as T_g, demonstrating that stronger interaction exists in the intermediate concentrated solution. It is proposed that, for intermediate concentration samples, the shielding effect due to the perfect intermolecular proximity and strong interaction between carbonyl group and backbone chain will make the system uneven, which results in entropy reduction and develops into a new quasi-metastable state, thus influencing the glassy-state relaxation behavior.     

Structural characterization of nylon 4 in the solid state by high-resolution solid-state H NMR spectroscopy

High-resolution solid-state ¹H NMR spectra of nylon 4 melt-quenched sample and single crystal sample in the solid state were measured in a wide range of temperatures from room temperature to 505 K by using solid state 300 MHz NMR with the FSLG-2 homo-nuclear dipolar decoupling method combined with high speed magic angle spinning method. From the experimental results, structural characterization on the crystalline and non-crystalline components was carried out. Further, intermolecular interaction between nylon 4 and water contained in the sample was discussed.     

Entanglement Loss in Fast-Flowing Entangled Polymer: Numerical Simulation of The Short-Chain Behavior and Interpretation of Phenomenology

Although some of the important consequences of flow-induced entanglement loss in entangled polymer rheology have recently been recognized, this specific molecular mechanism has rarely been investigated quantitatively based on experiments or molecular theories. For the first time, the amount of entanglement loss of a short entangled linear polymer (i.e., seven entanglements per chain at equilibrium) during fast-flow deformation is directly tracked in the stochastic simulation of an existing reptation model. The primary finding is that significant entanglement loss is observed in both fast elongation and fast shearing, and, contrary to some earlier conjectures, is particularly pronounced in elongational flow when polymer chain stretching formally commences. Furthermore, according to the current simulation in which three different CCR (Convective Constraint Release) schemes are considered, entanglement loss appears to have very prominent effects on the elongational rheology of an entangled linear polymer – an observation that had rarely been recognized or considered before. On the other hand, the currently explored features of flow-induced entanglement loss are tentatively linked to a wide variety of peculiar empirical properties of temporarily entangled polymer liquids. In particular, we are thus able to provide a consistent molecular explanation of the fairly well-known phenomenological effects of polydispersity and long-chain branching leading to a pronounced strain-hardening phenomenon, in view of two newly proposed effects of heterogeneous relaxations in preventing, directly or indirectly, fast entanglement loss during flows.     

The interpretation of orientation — strain relationships in rubbers and thermoplastics

The orientation—strain behaviour typical of non-crystalline polymers is examined with reference to new techniques for determining molecular orientation parameters in a variety of crosslinked and thermoplastic materials, as well as the long-established methods based on the optical anisotropy of natural rubber. It is shown that the affinely-deforming ‘random chain’ assumed by conventional theory is not successful in predicting the relationship between chain orientation and strain unless the network is assumed to change significantly with strain. The implications of this observation are discussed in terms of the behaviour of crosslinks and entanglements: the conventional view of anisotropy needs to be supplemented by an approach in which orientation and strain are seen as distinct aspects of a polymer's response to the stress imposed upon it.