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•Obtain crystal and amorphous layered double oxides (LDOs) using molecular dynamics.•Overall CO2physisorption on amorphous LDO is higher than that on crystalline LDO.•CO2form stable adsorption layer near crystalline LDO, preventing further adsorption.•CO2shows ordered arrangement on crystalline LDO, but random on amorphous LDO.

Chemical functionalization of metal sulfides by adsorption of specially designed ligands is of primary importance in efficiently manipulating the physicochemical properties of the target surface. Despite the widely studied ligand–metal molecular systems and the generally used binding energy concept, reports on the bond character of ligand–metal sulfide and its implication on the design of ligands with enhanced functionalities are rather scarce. Herein, using density functional theory calculations, we studied the intrinsic binding mechanism of the typical S/O-terminated ligands and (100) surface of lead sulfide (PbS), with a particular emphasis on the ligand–PbS(100) bond character. Strikingly, the ligand–PbS(100) bond is found to be predominantly ionic with minor covalency, completely different from the conventional perception of covalent interaction. Further insights into the specific hydrophobization of PbS led us to demonstrate that the ligand electronegativity, rather than the commonly used binding energy, allows more accurate prediction of the relative hydrophobic functionality of the ligand toward PbS. Therefore, this study not only advances our fundamental understanding on the ligand–PbS(100) interaction but also provides new perspectives on the first-principles design of ligands.

Chemical functionalization of metal sulfides plays a critical role in many fields such as materials science and froth flotation. The commonly used thiol-bearing functionalized ligands are generally considered to bind with metal sulfides covalently, and the computational binding energy is widely used to evaluate the functionality of the ligands toward metal sulfides. Herein, we studied the surface chemistry of the model ZnS and its binding with typical S- and O-terminated ligands using density functional theory calculations with an emphasis on the resulting bond character. Surprisingly, it was found that the ligand–ZnS(110) bond is essentially ionic with limited covalency. This very fundamental finding was further extended to the hydrophobization of ZnS in the context of froth flotation and rationalized the previously unresolved phenomenon that the higher the ligand–ZnS(110) binding strength, the lower the hydrophobic functionality of the ligand toward ZnS. Meanwhile, instead of the binding energy, the electronegativity of the ligand was identified as an effective computational descriptor that can accurately predict the relative hydrophobic functionality of the ligand toward ZnS. This work, therefore, further advanced our understanding of the intrinsic ligand–metal sulfide binding mechanism and highlighted the importance of computational parameters, beyond the binding energy, in guiding the first principles design of ligands with enhanced functionalities or optimizing relevant industrial processes.

Owing to the increasing popularity of using waste disposal as a source material, inorganic salts such as CaCl2and MgCl2are frequently present in geopolymerization typically taking place in alkali media. Such contaminants influence the dissolution of the aluminosilicate source materials and consequently the properties of the geopolymers made. This work is particularly aimed at elucidating the dissolution mechanism of a well-known clay, namely, kaolinite, in alkali media with the presence of two aforementioned aqueous medium contaminants using molecular dynamics (MD) simulation. A series of MD simulations was carried out on model kaolinite, with its tetrahedral and deprotonated octahedral surfaces exposed to the alkali solutions containing neat Na+or neat K+cations at two concentrations, 3 and 5 M. Different concentrations of CaCl2and MgCl2contaminants (i.e., 0.1, 0.3, and 0.5 M) were added to such alkali solutions. Atomic density profiles show that all cations, including those from the contaminants adsorbed on the two basal surfaces, intensify the dissociation of the aluminate groups from the deprotonated octahedral surface. The dissociation mechanism is somewhat similar to that of the alkali media without contaminants, in which cations weaken the interaction between the aluminum and bridging oxygen atoms. The number of the aluminate groups dissociated decreased with increasing contaminant concentration. In fact, at the highest contaminant concentration used (i.e., 0.5 M), the number of dissociated aluminate groups was even lower than that of the system without contaminants. This observation was attributed to the fact that most of chloride anions remained in the bulk solution at 0.1 M but an increasing amount of chloride ions started to cluster around the cations at 0.5 M, thereby screening the interaction between cations and the deprotonated octahedral surface. As a result, the screening effect reduced the number of aluminate groups dissociated from the surface. Structural analyses of the deprotonated octahedral surface indicated that the crystallinity of the surface decreased with increasing simulation time and alkali solution concentration. No dissolution of the tetrahedral surface was observed for all systems studied.

Pushing the boundaries of the investigation of hydrophobic attraction (HA) to the molecular scale readily ensures the collection of experimental results free of secondary effects, thereby facilitating the unraveling of the underlying mechanism by providing clean experimental results that truly reflect the hydrophobic attraction. Regardless of the feasibility of this approach, investigations using this promising method are stagnant due to the difficulties in determining the individual contributions of HA and van der Waals (vdW) interactions at the molecular scale. Here, a novel approach was proposed for the first time to determine the individual contributions of vdW interactions and HA by studying the single-molecule adhesion forces of a neutral oligo ethylene glycol methacrylate copolymer on a MoS2crystal exposed to different water chemistry. The anisotropic surface properties of MoS2enabled the partitioning of vdW interactions and hydrophobic attraction in total single-molecule adhesion forces and also enabled determining the contribution of electrostatic interaction (ESI). When the presence of ESI is excluded, the study of single-molecule adhesion forces using single-molecule force spectroscopy (SMFS) revealed that the contribution of vdW interactions to total molecular interactions was smaller than 9 pN. The strong single-molecule adhesion forces of oligo ethylene glycol copolymer on the hydrophobic basal surface of MoS2demonstrated that HA plays a dominant role with contribution up to 89% to the total single-molecule adhesion force. By utilizing the derived theoretical model, we quantified the individual contribution of each fundamental interaction under a variety of conditions. This study proposed a facile approach to quantitatively clarify the roles of vdW interactions and HA at the molecular scale, which may help assist future experimental and theoretical investigations of hydrophobic (solvophobic) effects and vdW interactions in aqueous solutions.

Using a molecular dynamics simulation, the behavior of water and cyclohexane confined in a model clay (kaolinite) nanopore (∼4–6 nm) was studied at 298.15 K and 1 atm. On a dry clay basis, the water weight concentration ranged from 6 to 30% while that of cyclohexane was kept at 14%. The formation of water bridge(s) connecting the two basal surfaces was observed at all concentrations. Upon analyzing the surface potentials using the Gouy–Chapman theory, the observation was found to be attributed to the overlapping of the positive potential (octahedral) and the negative potential (tetrahedral) of the two surfaces in the interior region of such a confined environment. The wettability of the tetrahedral sheet was found to be dependent on the water content. Larger areas of this surface would become water-wet with the increase in water concentration to minimize the contact area between the oil and water phases. Addition of sodium chloride to the aqueous phase at the concentrations of 0.1, 0.5, and 1 M substantially improved the wetting of basal surfaces. At the highest salt concentration, breakage of the water bridge was observed, and a phase-separated, three-layer structure (water–cyclohexane–water) was formed within the nanopore because of the screening effect of the adsorbed counterions.

Molecular dynamics simulation was used to study the role of water in the intercalation of CO2with a model Mg–Al–Cl-hydrotalcite mineral at ambient pressure and temperature. The ClayFF force field was used along with a model Mg–Al–Cl-hydrotalcite containing different amounts of water (H2O) and carbon dioxide (CO2) molecules in its interlayer spacing. It was observed that high CO2content, say 3.85 mmol g–1, could be achieved at low water concentrations or even without the presence of water. However, high water concentrations (e.g., 2 H2O molecules/hydrotalcite unit cell, the maximum allowed water concentration observed experimentally) could also yield similar CO2content, but in this case, the presence of water led to a significant interlayer spacing expansion (from 23.0 Å (no water) to 28.5 Å). The expansion was likely due to the change in the orientation distribution of the CO2molecules. Analyzing the orientation of CO2molecules revealed that they preferred to orientate parallel to the mineral surface at low water concentrations. However, as water concentration increased, CO2molecules exhibited a wider range of orientations with a significant fraction of them orienting more or less perpendicular to the mineral surface, especially at high CO2contents. The observed change in the orientation of CO2was attributed to the dipole interaction between H2O and CO2molecules and the reduced interaction between CO2and the hydroxyl groups on hydrotalcite. Also, it was observed that water molecules formed extensive hydrogen bond networks. All of the above findings seem to explain the contradicting results reported in the literature that water is needed under certain conditions to increase the amount of CO2captured by hydrotalcites. Here, we showed that high amounts of CO2can be intercalated with the presence of water.

Single molecule force spectroscopy (SMFS) was utilized to study single-molecule interactions between synthesized polymers and MoS2surfaces that provide the guidance to determine candidate polymers for efficient aqueous exfoliation of bulk MoS2into single-layer nanosheets. The technique allowed the direct quantification of the adhesion force of single polymer molecules with both basal and edge surfaces of MoS2. Compared with two studied neutral polymers, highly water-soluble cationic poly(vinylbenzyl trimethyl ammonium chloride) (PVBTA) was shown to be more promising for MoS2exfoliation as a result of strong single-molecule interactions with both basal and edge surfaces of MoS2. In 1 mM NaCl solution of pH around 5.5, the measured single-molecule adhesion forces on basal and edge surfaces were around 59 and 51 pN, respectively. Such strong adhesion force led to high performance of PVBTA in exfoliating MoS2bulk material into single-layer sheets. Compared with reported approaches in the literatures, an order of magnitude higher concentration of single-layer MoS2sheets in water was prepared using an order of magnitude less treatment time in this study. This research demonstrated that SMFS is a powerful tool to select appropriate surface-active polymers for effective exfoliation of MoS2, which could be applied to a variety of practical applications, such as water adhesion  and dispersing.

Exploration aiming to understand how a single polymer chain interacts with interested solid–water interface and its dependence on the environmental stimulus is critical, as it is important for a variety of scientific research and practical applications, such as underwater adhesives or smart systems for controlled attachment/detachment. Here, by single molecule force spectroscopy (SMFS), results on underwater adhesion of a single stimuli-responsive polymer chain on a model hydrophobic surface are reported. Salt concentration was used as an effective stimulus for the transition of the stimuli-responsive polymer from hydrophilic (solvophilic) hydrated state to hydrophobic (solvophobic) state in SMFS experiment. The probed equilibrium single-molecule adhesion force that changed from 45 to 76 pN with increasing salt concentration were free of other interactions that are responsible for cohesions, indicating that solvent quality is critical in determining the strength of single-molecule interfacial adhesion. Moreover, the derived simple quantitative thermodynamic model by combining single-molecule detachment mechanics with hydration free energy illustrated that higher single-molecule adhesion force was resulted from higher solvation/hydration free energy. The experimental study and theoretical analysis presented in this work quantitatively revealed the role of hydrophobic and more general solvophobic interactions in single-molecule level and shed light on the molecular structure optimization for desired applications, such as underwater adhesives or smart interfaces.

BACKGROUND:At present, guidelines are lacking on platelet transfusion in patients with a traumatic intracranial bleed and history of antiplatelet therapy. The aspirin and P2Y12response unit (ARU and PRU, respectively) assays detect the effect of aspirin and P2Y12inhibitors in the cardiac population.OBJECTIVE:To describe the reversal of platelet inhibition after platelet transfusion using the ARU and PRU assays in patients with traumatic brain injury.METHODS:Between 2010 and 2015, we conducted a prospective comparative cohort study of patients presenting with a positive head computed tomography and a history of antiplatelet therapy. ARU and PRU assays were performed on admission and 6 hours after transfusion, with a primary end point of detection of disinhibition after platelet transfusion.RESULTS:One hundred seven patients were available for analysis. Seven percent of patients taking aspirin and 27% of patients taking clopidogrel were not therapeutic on admission per the ARU and PRU, respectively. After platelet transfusion, 51% of patients on any aspirin and 67% of patients on any clopidogrel failed to be reversed. ARU increased by 71 ± 76 per unit of apheresis platelets for patients taking any aspirin, and PRU increased by 48 ± 46 per unit of apheresis platelets for patients taking any clopidogrel.CONCLUSION:A significant percentage of patients taking aspirin or clopidogrel were not therapeutic and thus would be unlikely to benefit from a platelet transfusion. In patients with measured platelet inhibition, a single platelet transfusion was not sufficient to reverse platelet inhibition in almost half.

BACKGROUND:Premorbid antithrombotic medication may worsen intracranial injury and outcome after traumatic brain injury (TBI). Routine laboratory tests are insufficient to evaluate platelet activity.OBJECTIVE:To profile the spectrum of platelet inhibition, as measured by aspirin and P2Y12 response unit assays, in a TBI population on antiplatelet therapy.METHODS:This single-center, prospective cohort study included patients presenting to our institution between November 2010 and January 2015 with a clinical history of TBI. Serum platelet reactivity levels were determined immediately on admission and analyzed using the aspirin and P2Y12 response unit assays; test results were reported as aspirin response units and P2Y12 response units. We report congruence between assay results and clinical history as well as differences in assay results between types of antiplatelet therapy.RESULTS:A sample of 317 patients was available for analysis, of which 87% had experienced mild TBI, 7% moderate, and 6% severe; the mean age was 71.5 years. The mean aspirin response units in patients with a history of any aspirin use was 456 ± 67 (range, 350-659), with 88% demonstrating therapeutic platelet inhibition. For clopidogrel, the mean P2Y12 response unit was 191 ± 70 (range, 51-351); 77% showed therapeutic response.CONCLUSION:Rapid measurement of antiplatelet function using the aspirin and P2Y12 response assays indicated as many as one fourth of patients on antiplatelet therapy do not have platelet dysfunction. Further research is required to develop guidelines for the use of these assays to guide platelet transfusion in the setting of TBI.

Conservative therapy is often the first-line treatment for many symptoms of various disease processes, including pain. Nevertheless, if pharmacological or medical management fails for those patients with severe and chronic pain, a surgical strategy may be a reasonable option. First performed for psychiatric disorders, cingulotomy now has been recognized as a viable option for the management of pain. The authors review the literature on anterior cingulotomy for intractable pain to provide guidelines for management and expected outcomes.

IMPORTANCE: Salvage surgery for recurrent head and neck squamous cell carcinoma (HNSCC) carries substantial risks of morbidity and mortality. Risk factors for death within 1 year should be better defined. OBJECTIVES: To report preoperative oncologic prognostic factors predictive of short-term (<1 year) survival after salvage surgery in patients with HNSCC, to assess whether preoperative age and comorbidity predicts 1-year mortality, and to report hospital courses after salvage surgery within 1 year. DESIGN, SETTING, AND PARTICIPANTS: A retrospective medical record review of 191 patients with recurrent HNSCC treated with salvage surgery from January 1, 2003, through December 31, 2013, at a tertiary academic center. INTERVENTIONS: Surgical salvage of HNSCC (larynx, oral cavity, oropharynx, or hypopharynx) with curative intent. MAIN OUTCOMES AND MEASURES: Primary outcome was survival 1 year after salvage surgery. Secondary outcomes were length of inpatient hospital stay, days of admissions, and skilled nursing facility disposition within 1 year stratified by survival status. Presalvage Charlson–Age Comorbidity Index (CACI) was calculated. Associations among CACI, oncologic risk factors, and risk of death within 1 year after salvage surgery are investigated using multivariable analysis. RESULTS: Of 191 patients studied, 53 (27.7%) died within 1 year after salvage surgery. Patients who died within 1 year had more total inpatient admissions (P < .001), longer total length of stay (P < .001), and higher risk of discharge to a skilled nursing facility (P < .001) and spent 17.3% (interquartile range, 5.2-36.3) of their remaining days in the hospital. Independent risk factors for death within 1 year are CACI (relative risk [RR], 1.43; 95% CI, 1.16-1.76), primary T3 or T4 stage (RR, 2.34; 95% CI, 1.27-4.31), and disease-free interval of less than 6 months (RR, 5.61; 95% CI, 1.78-16.7). CONCLUSIONS AND RELEVANCE: Medical comorbidity and age as measured by the CACI, primary T3 or T4 stage, and short disease-free interval must be considered in selecting patients ideal for surgical salvage surgery for recurrent HNSCC. Patients with these risk factors should be more strongly considered for palliative measures.

Background and purpose Femoral artery injuries are known complications of percutaneous vascular closure devices (VCDs). We studied the incidence of delayed femoral artery angiographic irregularities after neurointerventional procedures in which the EXOSEAL extravascular closure device was used for femoral arterial puncture closure.Methods Adult patients who underwent femoral arterial puncture closures with an EXOSEAL VCD and had a follow-up femoral artery angiogram from June 2012 through August 2013 were reviewed. A blinded radiologist compared pre-deployment and follow-up femoral arteriograms for the presence of femoral artery stenosis, dissection, pseudoaneurysm, or development of an arteriovenous fistula. Hospital records were reviewed for major or minor complications of the groin site or femoral artery.Results The EXOSEAL VCD achieved hemostasis, without evidence of a groin hematoma or requiring subsequent prolonged manual compression, in 400 of 441 closures following transfemoral arterial access, representing a device success rate of 90.7%. A total of 98 patients underwent 102 repeat angiograms following closure with the EXOSEAL VCD. The average time to the repeat angiogram was 73.5 days (range 0–488, median 28). Follow-up femoral arteriography demonstrated an irregularity in seven cases, all of which were vessel stenoses of <50%. There were no dissections, pseudoaneurysms, infections, or ischemic events in the study population.Conclusions Angiographic irregularities were seen in 6.86% of cases after closure with the EXOSEAL VCD. There were no clinically significant vascular complications. Thus, femoral artery closure with EXOSEAL carries a low risk of clinically significant delayed angiographic findings.

Active layer nanomorphology is a major factor that determines the efficiency of bulk heterojunction polymer solar cells (PSCs). Synthesizing diblock copolymers in which acceptor and donor materials are the constituent blocks is the most recent method to control the structure of the active layer. In the current work, a computational method is proposed to predict the nanomorphology of the active layer consisting of a diblock copolymer. Diblock copolymers have a tendency to self-organize and form well-defined nanostructures. The shape of the structure depends on the Flory–Huggins interaction parameter (i.e., χ), the total degree of polymerization (N) and volume fractions of the constituent blocks (φi). In this work, molecular dynamics (MD) simulations were used to calculate χ parameters for two different block copolymers used in PSCs: P3HT-b-poly(S8A2)-C60and P3HT-b-poly(n-butyl acrylate-stat-acrylate perylene) also known as P3HT-b-PPerAcr. Such calculations indicated strong segregation of blocks into cylindrical structure for P3HT-b-poly(S8A2)-C60and intermediate segregation into cylindrical structure for P3HT-b-PPerAcr. Experimental results of P3HT-b-poly(S8A2)-C60and P3HT-b-PTP4AP, a diblock copolymer having very similar structure to P3HT-b-PPerAcr, validate our predictions.

Molecular dynamics (MD) simulation and density functional theory (DFT) were applied to study the wetting behavior of nanoscale thin films of n-heptane, toluene, pyridine, and water at different thicknesses on the two model basal surfaces of kaolinite at room temperature. Despite the hydrophilicity of the basal surfaces, water exhibited the lowest affinity for them, but was statistically comparable to that of n-heptane, as quantified by the corresponding calculated monomolecular layer works of adhesion. The results are consistent with the simulations in which water molecules in monomolecular layer originally sandwiched between a monomolecular layer of aromatic compounds and the two basal surfaces departed from the surfaces, while the aromatic molecules migrated to the surfaces. However, such behavior was not observed in the cases in which water thin films contained multiple molecular layers of water. Interestingly, the corresponding calculated multimolecular layer works of adhesion of water were the highest among the compounds of interest. Analysis of the simulation data on both basal surfaces suggests that such observation is attributed to the water/water long-range charge–charge (69%, averaged over the two basal surfaces) and short-range hydrogen-bond (31%) interactions. Here, interfacial hydrogen bonds play a relatively minor role in the wetting behavior of water.

Molecular dynamics simulation was applied to study the relative growth rates of seven crystal faces of α-Al 2O 3in the presence of 1,4-butanediol. In particular, the adsorption energy of 1,4-butanediol on various crystal faces was computed using 200 solvent molecules. The following sequence was obtained: ( Eads(012) > Eads(001) > Eads(102) > Eads(113) > Eads(110) > Eads(111) > Eads(010)). The above sequence is in the reverse order of the experimentally observed growth rates of the respective faces. The two faces that exhibit the lowest adsorption energy (i.e., (111) and (010) faces) are the ones that do not emerge in the final morphology of α-Al 2O 3particles in practice. On examination of the data, it was found that 1,4-butanediol molecules preferentially adsorb onto the crystal faces that have larger numbers of octahedral faces per unit area (e.g., the (001) face). In other words, the binding of the growth units of α-Al 2O 3onto such crystal faces is less favorable than the other crystal faces. In addition to the adsorption energy, our results show that the growth habits are also influenced by the mobility of 1,4-butanediol near the crystal growth faces. In fact, the computed diffusion coefficients of 1,4-butanediol near the (012) and (102) faces are the lowest compared to those of the (001), (110), and (113) faces. This may explain why (012) and (102) faces do not emerge unless moderate stirring is applied. Nevertheless, the above conclusions were draw based upon cleaved α-alumina surfaces without relaxation (i.e., surface energy effects of the solid substrate were not included).

We investigate the properties of 1088 Lyman break galaxies (LBGs) at z ~ 3 selected from a ~2.63 deg2 subregion of the First Look Survey field using the ground-based multicolor data and the Spitzer Space Telescope mid-infrared data at 3-8 and 24 mm. With the wide area and the broad wavelength coverage, we sample a large number of "rare" u-band dropouts that are massive (M* > 1011 M), allowing us to perform a statistical analysis of these subsets of LBGs which have not been studied in detail. Optically bright (RAB [?] 24.5 mag) LBGs detected in mid-infrared (S3.6 mm [?] 6 mJy) reside at the most massive and dusty end of the LBG population, with relatively high and tight M/L in rest-frame near-infrared. Most infrared-luminous LBGs (S24 mm [?] 100 mJy) are dusty star-forming galaxies with star formation rates of 100-1000 M yr-1, total infrared luminosity of >1012 L. By constructing the UV luminosity function of massive LBGs, we estimate that the lower limit for the star formation rate density from LBGs more massive than 1011 M at z ~ 3 is [?]3.3 x 10-3 M-1 yr-1 Mpc-3, showing for the first time that the UV-bright population of massive galaxies alone contributes significantly to the global star formation rate density at z ~ 3. When combined with the star formation rate densities at z < 2, our result reveals a steady increase in the contribution of massive galaxies to the global star formation from z = 0 to z ~ 3, providing strong support to the downsizing of galaxy formation.

Based on the expression of the index of coil interpenetration (i.e., the degree of coil overlap, ξb) of amorphous polymers in the bulk state proposed by Qian, the ξb was rewritten as ξb=ρb[η]θ/1.66 via Flory's intrinsic viscosity equation at theta conditions. The ξb of poly (vinyl chloride) (PVC) dense membranes with different molecular weights and cellulose diacetate (CA) dense membranes prepared using different solvents were determined. The permeability, permeation coefficient (P) of nitrogen and permeation flux (J) of liquid through both membranes, respectively, was measured. P and J decreased exponentially with increasing ξb for PVC and CA membranes were revealed.

Adsorption behavior of two normal alkanes (C11and C200) with explicit hydrogens on a relaxed -Al2O3(0001) surface at 150 °C was studied using classical molecular dynamics (MD) simulation along with the use of the COMPASS force field. Prior to the MD simulations, first principle density functional theory (DFT) calculations were carried out to relax the alumina (0001) surface that was created by cleaving the corresponding crystal structure. It was found that orientation of the adsorbed segments and the number of carbons adsorbed seem to be insensitive to the chain length. The computed adsorption energy per mole of adsorbed CH2agrees well with those obtained from inverse gas chromatography measurements. Also, both simulation and experimental results showed that the adsorption energy decreased with increasing chain length. It was observed that molecular planes containing the skeletal carbons of the alkane segments that formed the first adsorption layer tend to orient parallel to the alumina surface, regardless of chain length. However, there existed a small amount of C11segments adopting the perpendicular orientation. For both C11and C200, the average distance between the neighboring adsorbed segments was determined to be 4.6 Å which is slightly lower than the distance between two adjacent normal alkane molecules in their crystalline form but is equal to the inter-atomic distance between the aluminum atoms in the surface layer of the alumina surface. The present work suggests that both the orientation and adsorption energy of normal alkanes on the alumina (0001) surface depends on whether the surface is relaxed or not.

Molecular dynamics (MD) simulations were carried out to study the solid-state structures of single-site (ss) and Ziegler-Natta (ZN) linear low-density polyethylenes (LLDPE) at a temperature slightly below their melting temperatures. The two bulk state models, used to represent the polymers, possessed the same average branch content (10 hexyl branches per 1000 backbone carbons) but with different degrees of interchain branch distribution homogeneity. Both models were first equilibrated at 463 K (i.e., 190 °C) for several nanoseconds, and the resultant structures, which were found to be representative of the corresponding liquid-state structures, were then used as the initial structures for the subsequent quenching process. The quenching temperature was 373 K (i.e., 100 °C), and the structures were then equilibrated at the same temperature for a period of about 10 ns. The structures of the two polymers formed after the low-temperature equilibrations were considerably different. In particular, the ZN-LLDPE model exhibited a higher amount of order, as quantified by a higher trans/gauche ratio, and a longer “stem length” than those of the ss-LLDPE model. The hexyl branches in the ss-LLDPE model distributed more or less evenly in its interphase and amorphous phase while the branches in the ZN-LLDPE model concentrated in the amorphous phase. The concentration of tie molecules in the ss-LLDPE model was significantly higher than that of ZN-LLDPE. We believe that the structures revealed by the MD simulations correspond to those formed in the early stages of the crystallization process since the models and simulation times used precluded us from modeling the complete crystallization process. However, it is also believed that these structures should resemble the chain conformations of the polymers in their solid state because the available thermal energy at 373 K was not sufficient for further significant conformational rearrangements. The results found are consistent with the experimental findings that ZN-LLDPE solids tend to have a higher degree of crystallinity than ss-LLDPE with similar or even lower average branch content and that ss-LLDPE solids possess a higher concentration of tie molecules. Our simulation results also indicated that with the presence of highly branched chains linear chains tended to crystallize faster than the chains with branches, and this is consistent with the experimental observation of Mirabella that thicker lamellae form before the thinner ones J.Polym. Sci., Part B:? Polym. Phys.2001, 39, 2800.

Summary:Effect of density, and hence pressure, on the miscibility of a 50:50 mol/mol PE/PEP blend was studied using a coarse‐grained MC simulation approach on a high‐coordination lattice, with the conformations of the coarse‐grained chains constrained by the RIS model. Interchain pair correlation functions are used to assess the miscibility of the mixtures. Miscibility increases with increasing temperature over the range −50–150 °C. It is rather insensitive to pressure at high temperatures, but at −50 °C, the blend miscibility increases with decreasing pressure. The findings are consistent with the fact that the blend is an UCST blend and that the simulation temperatures used, except −50 °C, were considerably higher than the UCST of the blend. The pressure dependence of the blend miscibility observed near −50 °C is also in agreement with the experimental observation that the blend exhibits a negative volume change of mixing. The present work demonstrates that the coarse‐grained MC approach, when it is used with periodic boundary cells of different sizes filled with the same number of chains, is capable of capturing the pressure dependence of UCST blends. In addition, such a simulation also provides us with insights about the molecular origin of the observed pressure dependence of miscibility. In the present case, the segregation of PE and PEP chains at low temperatures and high pressure simply originates from the fact that fully extended segments of PE chains tend to cluster so that their intermolecular interactions can be maximized. As the temperature increases, there is a decrease in the probability of a transstate at a CC bond in PE, and therefore the attraction between the PE chains is reduced at higher temperatures, promoting miscibility and the UCST behavior.

Inverse gas chromatography was used to measure Flory–Huggins interaction parameters (χ23) for five binary blends consisting of high-density polyethylene (HDPE) and octene-based linear low-density polyethylene (LLDPE) with different compositions at four elevated temperatures. The branch content of the LLDPE used in each pair of the blends ranged from 2 to 87 branches per 1000 backbone carbons. To obtain solvent-independent χ23, the data analysis approach recently proposed by Zhao and Choi (Polymer 2001, 42, 1075) was used. The results indicate that the higher the branch content of LLDPE, the higher the measured χ23, signifying that HDPE/LLDPE blends with low branch content LLDPEs are relatively more miscible than those with high branch contents. In particular, when the branch content of LLDPE is higher than 50 branches per 1000 backbone carbons, phase separation may occur. This result is in good agreement with other researchers' results obtained from different techniques. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1927–1931, 2004