Molecular conformations in polyethylene after recrystallization or annealing at high pressures

Information on molecular conformations inside polyethylene lamellas following high pressure treatments is obtained by comparing observed and predicted length distributions of nitrated samples. Three types of conformation are inferred. The commonest is that molecules are folded with chain ends included in lamellas, except that ends are not turned in by less than ～600 Å from a surface. This situation accounts in detail for the previously observed agreement between the number fracture-surface distribution of crystal thicknesses and the gel permeation chromatography (GPC) weight distribution following nitration. The second conformation, found especially for samples of low polydispersity, is also of folded molecules but with chain ends excluded from lamellas. This particular case justifies rigorously the initial assumption of folding. Finally there are examples where the molecular length distribution is substantially lower than the crystal thickness distribution, implying that molecular length is not the upper limit of crystal thickness and that molecules can be fully extended inside lamellas.     

Crystallization of short aliphatic polymer chains. I. General chain-folding behavior

Short aliphatic polymer chains of different lengths were prepared by degrading polyethylene samples of appropriately chosen initial fold lengths to the chain lengths which correspond to a single chain traverse through the lamella. The resulting dicarboxylic acids were either used as such for further crystallization experiments or were first converted into diiodides to remove polar endgroups. The resulting short polymers all crystallized by chain folding even if the chains (peak of distribution) were only 1.5–4 times the length of a traverse through the lamella. In the diiodides the fold length varied continuously with crystallization temperature, as is usual in high molecular weight material, but with the dicarboxylic acids such variation, while observable, was only small. The effect of the molecular weight on the fold length due to its influence on supercooling at a given crystallization temperature has become apparent. Renewed degradation with nitric acid and subsequent GPC analysis of the degradation products confirmed the folded nature of the chains in the above crystals. This analysis combined with experiments on the reactivity of chain ends has led to the picture that each chain folds completely, once, twice etc. so that both folds and ends are in the surface zone but are located at varying heights, as appropriate to the overall layer thickness for the molecular weight distribution in question. This picture is consistent with other concurrent work.     

Nascent crystallization of a growing chain on a catalyst surface: a nonequilibrium molecular dynamics simulation study

The growing chain molecular dynamics (GCMD) simulation method, a new nonequilibrium molecular dynamics code, is proposed to simulate the polymer chain aggregation behavior during polymerization on a catalyst surface. We found that the growing chain crystallizes on the surface in two stages: the nucleation stage and the crystal growth stage. In the first part of the nucleation period, the short polymerizing chain first absorbs on the surface and can be in either an ordered or disordered structure. Still in the nucleation period, when the chain reaches a degree of polymerization, about 100 bonds, the chain folds into a stable nucleus on the substrate with 3-5 stems. In the crystal growth stage where the polymerization also proceeds, we observed a stem elongation process in combination with a chain folding process. In the stem elongation step, the number of stems in the nucleus remains constant, and all the stems expand together to a length of ca. 5-25 ns. In the subsequent chain folding step, the stem length decreases about 20 bonds within a period of ca. 0.1-0.5 ns. During chain growth, the elongation process and the folding process occur in an alternating and repeated fashion. The crystallization mechanism of the polymerizing chain was discussed.     

Importance of individual side chains for the stability of a protein fold: computational alanine scanning of the insulin monomer

A new computational approach is proposed to probe the importance of residue side chains for the stability of a protein fold. Computational mutations to estimate protein stability (CMEPS) is based on the notion that the binding free energy corresponding to the complexation of a given side chain, considered as a "pseudo-ligand" of the wild type protein, reflects the importance of this side chain to the thermodynamic stability of the protein. The contribution of a particular side chain to the folding energy is estimated according to the molecular mechanics-generalized born surface area MM-GBSA approach, using a single molecular dynamics simulation trajectory of the wild type protein. CMEPS is a first principles method which does not contain any adjustable parameter that could be fitted to experimental data. The approach is first validated for Barnase and the B1 domain of protein L, for which a correlation coefficient R = 0.73, between experimental and CMEPS calculated DeltaDeltaG values, is found and then applied to the insulin monomer. In the present application, CMEPS replaces each amino acid by an alanine residue. Therefore, most mutations lead to cavities in the protein. From this the change in stability can be correlated with increased cavity volume. For insulin, this correlation is very similar compared with data previously analyzed for T4 lysozyme from an experiment for buried apolar side chains. There, the increased cavity volume has been related to the hydrophobic effect. However, since CMEPS uses the energetics in terms of electrostatic and van der Waals interactions (and not the hydrophobic effect which is difficult to relate to physical interactions), it is possible to study the effect of mutations of polar and solvent accessible side chains. According to CMEPS, residues Leu A16, Tyr A19, Leu B11, Leu B15, and Arg B22 are most important for the stability of the monomeric insulin fold. This is in agreement with experimental data. As a consequence, mutation of these residues may lead to misfolded and inactive insulin analogues.     

GPC analysis of degraded annealed single crystals

Annealed polyethylene single crystals have been degraded with fuming nitric acid, and the molecular weight distribution of the fragments determined by using a gel-permeation chromatograph. Peaks due to chain folding were observed in these distributions as for unannealed single crystals. The peaks moved to lower molecular weight with increasing degradation time. Comparison of the lowest molecular weight peak length after a given degradation time with the low-angle x-ray periodicity before degradation gave information about a disordered surface layer. The thickness of this layer at early states of degradation was dependent solely on annealing temperature, though changes in the layer must have occurred with annealing time, since there was an increase in reaction rate with annealing time. At higher degradation states, the thickness of the layer was dependent solely on the original low-angle periodicity. This has been related to the depth at which some folds are buried beneath the lamellar surfaces. The relevance of these observations to the structure of annealed single crystals is discussed.     

分子极化效应与烷烃、醇的空腔表面积

烷烃以及醇在水中形成的空腔表面积CSA可由下式计算:CSA=122.984+8.65736SBL-103.862ΔMPEI。其中SBL是分子中所有化学键长度的总和, ΔMPEI是含有相同碳原子数目的(烷烃或者醇)支链异构体分子极化效应指数MPEI支与直链异构体分子极化效应指数MPEI直的差值。     

A manifestation of the Ostwald step rule: molecular-dynamics simulations and free-energy landscape of the primary nucleation and melting of single-molecule polyethylene in dilute solution

The paper presents numerical results from extensive molecular-dynamics simulations of the crystallization process of a single polyethylene chain with N=500 monomers. The development of the ordered structure is seen to proceed along different routes involving either the global reorganization of the chain or, alternatively, well-separated connected nuclei. No dependence on the thermal history was observed at the late stages of the crystallization. The folding process involves several intermediate ordered metastable states, in strong analogy with the experiments, and ends up in a well-defined long-lived lamella with ten stems of approximately equal length, arranged into a regular, hexagonal pattern. This behavior may be seen as a microscopic manifestation of the Ostwald step rule. Both the metastable states and the long-lived one are evidenced as the local minima and the global one of the free-energy landscape, respectively. The study of the microscopic organization of the lamella evidenced that the two caps are rather flat, i.e., the loops connecting the stems are short. Interestingly, annealing the chain through the different metastable states leaves the average number of monomers per loop nearly unchanged. It is also seen that the chain ends, the so-called cilia, are localized on the surface of the lamella, in agreement with the experiments, and that structural fluctuations take place on the lamella surface, as noted by recent Monte Carlo simulations. The study of the melting process evidences that the degree of hysteresis is small.     

Failure of one-dimensional Smoluchowski diffusion models to describe the duration of conformational rearrangements in floppy, diffusive molecular systems: a case study of polymer cyclization

Motivated by recent experimental efforts to measure the duration of individual folding∕unfolding transitions in proteins and RNA, here we use simulations to study the duration of a simple transition mimicking an elementary step in biopolymer folding: the closure of a loop in a long polymer chain. While the rate of such a transition is well approximated by a one-dimensional Smoluchowski model that views the end-to-end distance dynamics of a polymer chain as diffusion governed by the one-dimensional potential of mean force, the same model fails rather dramatically to describe the duration of such transitions. Instead, the latter timescale is well described by a model where the chain ends diffuse freely, uninfluenced by the average entropic force imposed by the polymer chain. The effective diffusion coefficient then depends on the length scale of the loop closure transition. Our findings suggest that simple one-dimensional models, when applied to estimate the duration of reactive events in complex molecular systems, should be used with caution.     

Effects of lipid chain length on the surface properties of alkylaminomethyl rutin and of its mixture with model lecithin membrane

Three model flavonoid-based bioactive molecules with different lipid chain lengths (RuCn: n=8, 12, 18) were newly synthesized. The surface properties [surface pressure (π)-area (A), surface potential (ΔV)-surface pressure (π) and dipole moment (u(⊥))-surface pressure (π)] of pure RuCn and the lecithin membrane compounds had been investigated by using the Langmuir monolayer technology. The results suggested that the distinctive monolayer behavior of RuCn is strongly dependent on the lipid chain length. The great differences in the monolayer properties brought by the lipid chain length could be attributed to two major factors: (i) the ionization degree of the bulky hydrophilic head group (including hydroxyl and NH groups) alters its local field solely via the surface potential; (ii) tring molecular (or dipole) packing density within monolayers. The excess Gibbs energy (ΔG((ex))) calculated for the RuCn-lecithin mixed monolayers infers that higher stability of the mixed monolayer can be strengthened as the lipid chain length decreases. And the addition of RuCn into lecithin membrane may increase the total u(⊥) of the binary mixed monolayers, which could inhibit the hydration of the lecithin's hydrophilic head groups. The shorter the lipid chain length of RuCn (e.g., RuC8) is, the higher the surface activity can be. Our findings provide a molecular basis for the application of such class of biomolecules in the functional food, cosmetics and medicine.     

Postgel properties in the statistical crosslinking of heterochains. II. Free‐radical crosslinking copolymerization

The heterochain crosslinking theory is applied to postgel behavior in the free‐radical crosslinking copolymerization of vinyl and divinyl monomers. In this context, the crosslinked polymer formation can be viewed as a system in which the primary chains formed at different times are combined in accordance with the statistical chain‐connection rule governed by the chemical reaction kinetics. Because the primary chains are formed consecutively, the number of chain types N must be extrapolated to infinity, N → ∞. Practically, such extrapolation can be conducted with the calculated values for only three different N values. The analytical expressions for the weight fraction and average molecular weights of the sol fraction are derived for the general primary chain length distribution function in free‐radical polymerization. Illustrative calculations show that the obtained results agree with those from the Monte Carlo method, and that the postgel properties in free‐radical crosslinking copolymerization systems could be significantly different from those in randomly crosslinked systems. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2342–2350, 2000     

Investigation of the boron nitride/polybenzoxazine interphase

The influence of a hexagonal boron nitride powder surface on the polymerization of a benzoxazine monomer is examined by differential scanning calorimetry (DSC). By varying the thickness of the benzoxazine coatings on boron nitride particles, a distinct influence of the surface substrate on the polymerization reaction could be observed. At a coating thickness calculated to be on the order of a monolayer, the heat of reaction is reduced, while the exothermic peak temperature and peak width at half height are increased relative to that of the neat resin values. The reduced heat of reaction and increased exothermic peak temperature indicate that the boron nitride surface is inhibiting the benzoxazine polymerization reaction. In the intermediate coating thickness region, the heat of reaction increases and the exothermic peak temperature and peak width at half height decrease with increasing coating thickness. With even thicker coatings, the heat of reaction and exothermic peak temperature and peak width at half height reach values corresponding to that of the neat resin and no longer change with thickness. In addition, the influence of surface treated boron nitride particles on the thermal polymerization behavior, as well as the effect of the surface treatment on the viscosity, dynamic mechanical properties, and flexural properties of the filled composites are also investigated. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2360–2372, 1999     

Surface and interfacial segregation of polyethersulfone deuterated chain ends determined by neutron reflectivity

The surface localization of polymer chain ends has been shown to be an effective method for surface composition control in amorphous polymer films. This work determines chain end distribution in thin polyethersulfone (PESU) films end‐capped with deuterated compounds of varying size and composition. Neutron reflectivity revealed the preferential localization of chain ends to the PESU‐air interface, independent of chain end identity. The length scale of the chain end concentration gradient was determined to differ from that predicted for flexible chain polymers. Atomic force microscopy and contact angle analysis demonstrated that chain end localization allows for improved control of nanoscale and macroscale surface properties of PESU films. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 293–301     

Molecular segregation and nucleation of poly(ethylene oxide) crystallized from the melt. IV. Computer modeling

A crystal surface is modelled and Monte Carlo simulations are attempted to learn about conformations and motion of a macromolecule that could lead to crystallization. Chains with up to 200 mobile segments have been analyzed. The diffusion distance of a given chain segment increases with positioning closer to the chain end, and completion of crystallization accelerates with decreasing length of remaining amorphous chain ends and increasing temperature.     

Molecular dynamics simulations of surfactant self-organization at a solid-liquid interface

Self-organization of aqueous surfactants at a planar graphite-like surface is studied by means of coarse-grain molecular dynamics simulations. The nonionic surfactant, n-alkyl poly(ethylene oxide), and water are both represented by coarse-grain models while an implicit representation is used for the graphite surface. The observed morphology of the aggregated surfactants depends on the alkyl chain length. Surfactants with a short chain form a monolayer on the graphite surface with a thickness roughly equal to that of the alkane tail. On the other hand, longer-tail surfactants form continuous hemicylinders on the surface with diameter approximately 5.0 +/- 0.5 nm, in good agreement with experimental AFM data.     

Oligoresorcinols fold into double helices in water

We report the first double helices with a controlled helicity in water based on oligoresorcinols as a new, simplest water-soluble structural motif. The molecular strands of the oligoresorcinols self-assemble into double helices with the aid of aromatic interactions in water as characterized by 1H NMR and absorption spectroscopies together with the X-ray crystallographic study of the pentamer. The double helix formation is sensitive to the chain length, solvent composition, and temperature. Moreover, a bias in the screw sense of the double helices was achieved by covalently attaching chiral substituents to both ends of the molecular strands.     

Molecular dynamics simulations of end-to-end contact formation in hydrocarbon chains in water and aqueous urea solution

We probe the urea-denaturation mechanism using molecular dynamics simulations of an elementary "folding" event, namely, the formation of end-to-end contact in the linear hydrocarbon chain (HC) CH(3)(CH(2))(18)CH(3). Electrostatic effects are examined using a model HC in which one end of the chain is positively charged (+0.2e) and the other contains a negative charge (-0.2e). For these systems multiple transitions between "folded" (conformations in which the chain ends are in contact) and "unfolded" (end-to-end contact is broken) can be observed during 4 ns molecular dynamics simulations. In water and 6 M aqueous urea solution HC and the charged HC fluctuate between collapsed globular conformations and a set of expanded structures. The collapsed conformation adopted by the HC in water is slightly destablized in 6 M urea. In contrast, the end-to-end contact is disrupted in the charged HC only in aqueous urea solution. Despite the presence of a large hydrophobic patch, on length scales on the order of approximately 8-10 A "denaturation" (transition to the expanded unfolded state) occurs by a direct interaction of urea with charges on the chain ends. The contiguous patch of hydrophobic moieties leads to "mild dewetting", which becomes more pronounced in the charged HC in 6 M aqueous urea solution. Our simulations establish that the urea denaturation mechanism is most likely electrostatic in origin.     

Radical alternating copolymerization: A strategy for hyperbranched materials

Novel hyperbranched polymers were synthesized in a high yield without gelation through the free‐radical alternating copolymerization of an AB/B′ (allyloxy maleic acid/maleic anhydride) system, in which group B and monomer B′ both could only alternately polymerize with group A. The arm number of the produced highly branched polymers was equal to the product of the linear chain length and the probability of pendent B groups being growing centers. The molecular weight of these novel hyperbranched polymers increased with increasing initiator concentration and prolonged polymerization times. The AB/B′ system, used as described, provides a new general methodology for highly branched and functional polymers. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3074–3085, 2000     

Melting and crystallization behavior of polyhedral oligomeric silsesquioxane‐capped poly(ε‐caprolactone)

In this study, we investigated the melting and crystallization behavior of polyhedral oligomeric silsesquioxane (POSS)‐capped poly(ε‐caprolactone) PCL with various lengths of PCL chains by means of X‐ray diffraction and differential scanning calorimetry. This organic–inorganic macromolecule possesses a tadpole‐like structure in which the bulky POSS cage is the “head” whereas PCL chain the “tail”. The novel organic–inorganic association result in the significant alterations in the melting and crystallization behavior of PCL. The POSS‐terminated PCL displayed the enhanced equilibrium melting points compared to the control PCL. Both the overall crystallization rate and the spherulitic growth rate of the POSS‐terminated PCLs increased with increasing the concentration of POSS (or with decreasing length of PCL chain in the hybrids). The analysis of Avrami equation shows that the crystallization of the POSS‐terminated PCL preferentially followed the mechanism of spherulitic growth with instantaneous nuclei. It is found that the folding free energy of surface for the POSS‐terminated PCLs decreased with increasing the concentration of POSS. It is found that the folding free energy of surface for the POSS‐terminated PCLs decreased with increasing the concentration of POSS. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2201–2214, 2007     

Local segmental relaxation in bidisperse polystyrenes

Dynamic mechanical results are reported for segmental relaxation of monodisperse polystyrenes (PSs) with molecular weights of 0.7, 3, 18, and 104 kg/mol and bidisperse PSs created from blending pairs of these materials. The data for the monodisperse polymers confirm previous findings; namely, there is an increase in the glass‐transition temperature normalized temperature dependence of the segmental relaxation times (fragility) with increasing molecular weight, along with a breakdown of the correlation between the fragility and the breadth of the relaxation function. For both the monodisperse and bidisperse PSs, the glass‐transition temperature is a single function of the average number of chain ends, independent of the nature of the molecular weight distribution. It is also found that these materials exhibit fragilities that uniquely depend on the number‐average molecular weight, that is, on the concentration of chain ends. In blends with linear PS, cyclic PS with a low molecular weight behaves as a high polymer, similar to its neat behavior, reflecting the overriding importance of chain ends. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2604–2611, 2004