Strength and deformation of rigid polymers: structure and topology in amorphous polymers

Glassy polymer is formed because the irregular chain architecture prevents crystallisation. Computer simulations allow Voronoi tessellation of atomic groups (for example monomers) to be carried out and measured along the molecular chain, which reveals significant density fluctuations. A Voronoi polyhedron is constructed for each particle according to a unique mathematical procedure [J Reiner Angew Math 134 (1908) 198]. When measured in terms of Voronoi polyhedra, amorphous structures show wide variations in packing density on the atomic/monomer scale, with a characteristic skewed distribution. The Voronoi method can be applied to all polymers; however, in this paper only uncrosslinked amorphous polymers are considered. Constriction points around a chain segment are defined as a locally specific configuration and arrangement of adjacent chains such that the local density within a sphere of radius approximately equal to two monomer diameters comes close to or below the hypothetical crystalline density. The topological theory of molecular structure developed by Bader defines the concepts of atoms and bonds in terms of the topological properties of the observable charge distribution [Rep Prog Phys 44 (1981) 893]. In polymers the high density regions become an even stronger topological feature, and are referred to as the constriction points.     

Crystallization of nanocomposites of an isotactic random butene-1/ethylene copolymer and layered double hydroxide

The effect of the presence of a layered double hydroxide (LDH) nanofiller on the crystallization behavior of a random isotactic butene-1/ethylene copolymer was investigated. Addition of LDH enhanced heterogeneous nucleation of the ordering process of the polymer matrix leading to an increase of the temperature of formation of the Form II mesophase on cooling the melt. Consequently, the size of spherulites of the polymer matrix was markedly reduced in the nanocomposites. In contrast, the Form II mesophase–Form I crystal phase transition kinetics and the final crystallinity were not affected by the presence of LDH. Addition of the LDH nanofiller led to a beneficial increase of the stiffness which suggests a route for compensating of the lower stiffness of the random copolymer compared to the homopolymer. Random copolymerization accelerates the disadvantageous room-temperature mesophase–crystal transition, but results in a reduction of the crystallinity. The addition of LDH counterbalances the lowering of the crystal fraction.     

Kinetics-controlled compatibilization of immiscible polypropylene/polystyrene blends using nano-SiO2 particles

Rigid inorganic filler has been long time used as a reinforcement agent for polymer materials. Recently, more work is focused on the possibility that using filler as a compatibilizer for immiscible polymer blends. In this article, we reported our efforts on the change of phase morphology and properties of immiscible polypropylene(PP)/polystyrene(PS) blends compatibilized with nano-SiO₂ particles. The effects of filler content and mixing time on the phase morphology, crystallization behavior, rheology, and mechanical properties were investigated by SEM, DSC, ARES and mechanical test. A drastic reduction of PS phase size and a very homogeneous size distribution were observed by introducing nano-SiO₂ particles in the blends at short mixing time. However, at longer mixing time an increase of PS size was seen again, indicating a kinetics-controlled compatibilization. This conclusion was further supported by the unchanged glass transition temperature of PS and by increased viscosity in the blends after adding nano-SiO₂ particles. The compatibilization mechanism of nano-SiO₂ particles in PP/PS blends was proposed based on kinetics consideration.     

Compressive strength of an unsaturated granular material during cementation

The cohesive behavior of unsaturated granular materials is due to the presence of cohesive bonds between grains. These bonds can have various physico-chemical characteristics and may evolve with environmental conditions. We study the case of a granular material partially saturated by an aqueous solution. The bonds are thus initially of capillary type and the mechanical strength is weak. At low relative humidity, the phase change of water involves crystallization of the solute at the contact points between grains, generating thus solid bonds. The mechanical strength of the material is then enhanced. An experimental study of the evolution of the mechanical strength during crystallization of the solute shows clearly the transition from capillary regime to cemented regime. This transition is not correlated with the mass of the crystallized solute, but rather with the residual degree of saturation. This behavior is analyzed here in the light of discrete element simulations. We introduce a local cohesion law that accounts for transition from capillary to cemented bonding. This law is formulated in terms of the degree of crystallization as a result of the evaporation of water at the boundary of the sample. The cohesion of the packing is initially of capillary type. A crystallization front then spreads from the sample boundaries to the center of the sample, and the strength increases as a result. Uniaxial compression allows us to determine the strength at different times. The numerical strength agrees well with the experimental data, and reveals strength enhancement as the solute crystallizes, as well as the transition from capillary to cementation regime.     

Understanding crystal nucleation in solution-segregated polymers

We report dynamic Monte Carlo simulations of crystal nucleation in polymer bulk phase segregated from solutions. We found that poorer solvent enhances crystal nucleation in the concentrated phase of polymers. In addition, when the solvent becomes poor enough, crystal nucleation prefers to occur at the diffuse interfaces. The results are consistent with the predictions from theoretical phase diagrams, but something different from immiscible polymer blends. The surface-enhanced crystallization may explain the bowl-shaped crystal aggregates observed experimentally in poor solvent.     

Porous materials with tunable mechanical properties

We present a study of the mechanical properties of porous polymer materials elaborated from the controlled formulation of emulsions and using a polyHIPE templating approach. This consists in the polymerization of high internal phase water-in-oil emulsions of controlled average size and dispersed phase volume fraction. We investigate the evolution of the linear and non-linear mechanical properties as a function of two sets of parameters, that define the structure (namely pore size and volume fraction) and the composition (chemical composition of the polymer walls) of the material. Pore sizes and volume fraction were varied by tuning the initial emulsion size and volume fraction while the solid wall composition was varied by using a mixture of monomers which yield polymers with very different glass transition temperatures T _g (namely styrene and ethyl hexyl acrylate which yield polymers with a high and low T _g respectively). Unlike the classical models, the two components model that we developed is able to account for the dependence of the measured mechanical properties on the cell size and porosity values. We show that the foam mechanical properties are mainly governed by the T _g of the blends. The foams are shown to vary from an elastic-brittle system to a soft ductile material as the proportion of EHA is increased.     

Effect of phase separation on spherulitic growth rate of PP/EPR in‐reactor alloys

In this work, the effect of phase separation on the spherulitic growth rate of a polypropylene/ethylene–propylene random (PP/EPR) copolymer in‐reactor alloy was investigated. The PP/EPR in‐reactor alloy was either directly quenched from homogeneous melt to crystallization temperature or held at various temperatures for phase separation prior to crystallization. It is found that at lower crystallization temperatures previous phase separation in the melt retards the crystallization rate. The higher the phase separation rate, the smaller the spherulitic growth rate. This can be attributed to faster crystallization rate than the rate of secondary phase separation. The composition of the PP‐rich phase and corresponding depression of the equilibrium melting temperature of PP vary with phase separation temperature. On the other hand, at higher crystallization temperature, previous phase separation in the melt has little effect on the spherulitic growth rate because secondary phase separation can take place prior to crystallization. The transition temperature from regime II to regime III also shifts to lower temperature as the phase separation temperature increases. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Crystallization behavior of nylon-6 confined among ultra-fine full-vulcanized rubber particles

The crystallization behavior of nylon-6 confined in a hybrid of nylon-6 and ultra-fine full-vulcanized powdered rubber was studied. It is found that carboxylic styrene-butadiene ultra-fine full-vulcanized powdered rubber (CSB-UFPR) could behave as a nucleating agent for nylon-6 crystallization, which can increase the crystallization temperature, the onset temperature of crystallization as well as the glass transition temperature of nylon-6 confined in the hybrid of nylon-6 and ultra-fine full-vulcanized powdered rubber. It is also found that nylon-6 molecules confined among CSB-UFPR particles favor the crystalline form of γ than that of α. The crystallization process of γ phase of nylon-6 in nylon-6/CSB-UFPR hybrid is a diffusion-controlled process, and thus it can be accelerated by raising the crystallization temperature or by extending oil into CSB-UFPR. It is expected that these results will benefit the research on crystallization behavior of other constrained geometry polymer, such as ultra-thin film and polymer/clay hybrid, and etc.     

Heterogeneous continuous kinetics modeling of PET depolymerization in supercritical methanol

The kinetics of PET depolymerization in supercritical methanol was investigated. A continuous kinetics model was developed to analyze PET decomposition behavior. This model includes molecular weight distribution (MWD) changes in the polymer by random and specific scissions and secondary reactions of monomer components for complex macromolecular reactions. The changes of MWD and monomers as a function of time were simulated by continuous kinetics. Reactions in two phases, polymer melt phase and supercritical phase, were considered. By comparing simulated and experimental results, values of the rate constants were determined. These results indicated that random scission proceeds predominantly in the heterogeneous phase during the initial stage of PET depolymerization in supercritical methanol and specific scission proceeds predominantly in the homogeneous phase during the final stage. It was also shown that mass transfer influences the depolymerization of PET.     

EFFECT OF NONSOLVENT ADDITIVES ON THE PORE SIZE AND THE PORE SIZE DISTRIBUTION OF AROMATIC POLYAMIDE RO MEMBRANES

In this paper discussions are made on the effect of nonsolvent swelling agents on the average pore size and pore size distributions at the surface of polyamide membranes which result from casting solutions involving above nonsolvent swelling agents.

The size of the polymer aggregate in the film casting solution and the size of polymer network pores are correlated to physicochemical data of ions which constitute the electrolytes used as nonsolvent swelling agents. As such ionic properties the charge density and the free energy of transition of ions from polyamide phase to water phase were considered. The validity of the correlation is limited in a range of casting solution composition where the polymer concentration in the casting solution is close to the limiting concentration of polymer at the phase boundary and the molar ratio of the nonsolvent swelling agent to the amide group involved in the polyamide polymer is equal to or slightly more than 0.7.     

Thermo-mechanical characterization of plasticized PLA: Is the miscibility the only significant factor?

PLA is a widely used polymer which has received much attention in the last decade because of its originating from renewable resources and its potential biodegradability. PLA fulfils the packaging industry's requirements for most of the rigid objects but the polymer needs to be plasticized to be used as soft films. In this work, agreed plasticizers for food contact were melt mixed with L-PLA and then, the glass transition, melting, crystallization and mechanical properties of the blends were investigated. The experimental results were compared to the predicted results found through empirical interaction parameters and Fox equations. Molecular scale miscibility is assumed in the amorphous phase whatever the plasticizer. The mobility gained by the PLA chains in the plasticized blends yields crystallization, which is the driving force for various scale phase separations.     

Phase separation,crystallization, and structure formation in immiscible polymer solutions

Morphological and calorimetric studies of phase separation have been carried out in solutions of a crystallizable polymer in poor solvents. Hydrogenated polybutadiene with low branch content was investigated in solutions with diphenyl ether and diphenyl methane, in which the equilibrium phase diagram exhibits both liquid–liquid phase separation and crystallization of the polymer. Emphasis is placed on sample preparation protocols using thermal treatments at low concentrations where it is anticipated that both phase separation mechanisms may influence the resulting morphology. Samples prepared using either ramp cooling or isothermal crystallization exhibit porous structures such as those seen in membrane materials, that predominantly reflect liquid phase separation. However, the interplay between the different kinetics of liquid demixing and crystallization provides a mechanism to control, for instance, pore size. DSC studies during ramp cooling showed evidence of two discrete crystallization processes associated with the two liquid phases expected to be present under these circumstances. Finally, high concentration samples showed morphological evidence of liquid phase separation induced at the growth front of spherulites in otherwise single-phase polymer solutions. © 1995 John Wiley & Sons, Inc.     

Formation of β-modification of isotactic polypropylene in its late stage of crystallization

In the late stage of the crystallization of isotactic polypropylene (IPP), melt inclusions are encapsulated by the crystallized phase. The contraction caused by the proceeding crystallization within the inclusion leads to a reduced pressure accompanied by the appearance of vacuum bubbles. In the surface layer of the melt surrounding the vacuum bubbles, the polymer chains are subjected to extension during the development of bubbles, which leads to the formation of -row nuclei. As has been observed during the shear-induced crystallization of IPP, the row-nuclei formed in situ can also induce the growth of the β-phase in this case. It was demonstrated that a possible reason for the — β transition on the surface of growing -spherulites is the local mechanical stress caused by the contraction. Based on experimental results, suggestions are made for the origin of the strongly birefringent phase observed by Duval et al. during the crystallization of blends of IPP and IPP grafted with maleic anhydride.     

Revisiting emulsion polymerization to produce stable,translucent, nanolatex of partially water‐soluble monomers,ethylacrylate–methylmethacrylate

Stable, translucent nanolatex with monomer weight % as high as 25 was obtained through emulsion copolymerization of partially water‐soluble monomers, ethyl acrylate and methylmethacrylate. The kinetics of reaction, studied at monomer/surfactant (M/S) ratio 10 and 50 showed two intervals and higher rate of particle nucleation for KPS initiated systems. However, AIBN initiated system showed phase separation. The copolymer composition was determined through ¹H‐NMR studies and copolymers showed two glass transition temperatures. Dynamic light scattering studies indicated bimodal distribution of polymer particle size. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2593–2603, 2003     

Enthalpic and entropic origins of nucleation barriers during polymer crystallization: the Hoffman-Lauritzen theory and beyond

In the search for a greater understanding of polymer crystallization, numerous experimental observations with regards to microscopic structures and macroscopic properties have been reported in the past half-century. There are generally two types of experimental results to provide information about the mechanisms of polymer crystal growth, i.e. molecular dynamic/scattering and structural/morphological. Since we cannot follow the trajectory of individual chain molecules when they undergo the transition from liquid to solid state during the crystallization process, structural/morphological analysis of polymer crystals reveal information recorded during this process. Namely, the final structure and morphology of polymer crystals have atomic, stem and global chain conformation information embedded in them during crystallization which provides evidence which can be used to deduce molecular aspects of the polymer crystallization process. It is commonly understood that polymer crystallization, from the thermodynamic perspective, is a first-order transition involving the relaxation of a metastable undercooled melt towards the equilibrium state which is rarely reached in polymer crystals. This process is controlled by a free energy barrier. A molecular model is required to describe the landscape of the free energy barriers and to provide an analytical explanation concerning and predictions about polymer crystallization. The Hoffman-Lauritzen (HL) theory, which was put forward more than 40 years ago, was one of the first analytical theories to illustrate how polymers crystallize. Since then, modifications to the HL theory and suggestions for new approaches have been reported, but the core physical picture of the HL theory has largely remained intact. This article consists of four major parts: (1) we will compare the crystallization of small molecules and long chain molecules, and the relationship between crystallization and crystal habits. The diversity of crystalline structures and morphologies of semi-crystalline polymers must be taken into account when studying the crystallization mechanism of polymers (2) this article also serves as a brief review of the HL theory and its importance in our understanding of polymer crystallization (3) we have tried to answer the question: what is the nucleation barrier? Specifically, we will illustrate that the nucleation barrier in polymer crystallization consists of both enthalpic and entropic components as deduced from experimental results. This barrier, from our perspective, consists of selection processes taking place in different length- and time-scales (4) finally, there is a brief discussion on what issues remain, in particular, in the areas of undercooled liquid structures and the initial stages of crystallization.     

Chain packing and phase transition of N-hexacosylated polyethyleneimine comb-like polymer: A combined investigation by synchrotron X-ray scattering and FTIR spectroscopy

In this paper, the chain packing and phase transition of comb-like polymer has been deeply analyzed with N-hexacosylated polyethyleneimine (PEI26C) as a template, fabricated through the reaction between n-hexacosyl bromide and PEI in a homogenous solution. The effect of long alkyl groups on the chain packing and phase transition of PEI26C was systematically investigated by synchrotron X-ray scattering and variable-temperature FTIR spectroscopy. PEI26C comb-like polymer exhibited an interesting structure-evolution process, and phase transformation from orthorhombic (β_O), monoclinic (β_M), to hexagonal (α_H) phase, and finally to amorphous state was demonstrated, indicating that large alkyl domains induced the complicated structures. Size-depended phase transition behavior provides an insight into the formation of metastable structure during the early stage of polymer crystallization.     

Change of structure and free volume properties of semi-crystalline poly(3-hydroxybutyrate-co-3-hydroxyvalerate) during thermal treatments by positron annihilation lifetime

The structure and free volume properties of semi-crystalline poly(3-hydroxybutyrate-co-3-hydroxy valerate) (PHBV) were investigated in this study. The structure change and conformational motion of PHBV during melting and crystallization process were discussed by in situ FTIR. The free volume within the amorphous phase and its temperature dependence in the PHBV membrane, which was prepared by the compression molding method with isothermal crystallization processes, were characterized by using positron annihilation lifetime (PAL) spectroscopy. From the lifetime parameters, the temperature dependence of free volume size, amount, distribution, and fractional free volume, and the thermal expansion of free volume and/or polymer were discussed. Furthermore, the knee temperature was first observed in the melting process of the crystallized PHBV membranes. It indicated that there was structural transition of polymer chains during melting as the corresponding results observed with in situ FTIR measurement.     

Rigid backbone polymers. XVI. Random copolyamides

Several homologous families of random copolyamides containing aromatic rigid elements and aliphatic or aralkyl flexible elements were prepared and characterized. Lyotropic liquid crystallinity was observed in all such polymers where over 50% of the aromatic residues belonged to rigid elements whose axial ratio surpassed a critical value of 5< x <6. The point where 5< x <6 is reached depends on the nature of the flexible comonomers. The higher their basicity or flexibility, the higher the concentration of rigid monomers in the copolymer at the point of 5< x <6. In concentrated ternary systems of polymer 1/polymer 2/solvent, a single anisotropic phase containing both polymers can be obtained when each polymer can form an anisotropic solution in the solvent. When either polymer is too flexible to form an anisotropic solution by itself, it will then separate from the ternary system into an isotropic phase, leaving the more rigid polymer in the coexisting anisotropic solution.     

Biased diffusion induces coil deformation via a ‘cracking‐the‐whip’ effect of acceleration generated by dynamic heterogeneity along a polymer chain

We performed Brownian dynamics simulations of a single polymer with monomer diffusion biased in a field of constant activation forces. We compared coil shapes among the synchronous and non‐synchronous (random and sequential) updating schemes of monomer positions. The synchronous scheme makes an ideal linear integration of monomer motions, while the random scheme mimics the real nonlinear situation holding dynamic heterogeneity along the polymer chain, and the sequential scheme represents its extremely inhomogeneous situation. We found that, in contrast to the synchronous scheme that raises no deformation, the sequential scheme accumulates the local acceleration generated by dynamic heterogeneity and reveals a ‘cracking‐the‐whip’ effect along the chain from one end to the other to stretch the polymer coil. Meanwhile, the random scheme accumulates the local acceleration towards the middle segment of the chain and thus raises an internal tension for coil deformation as well. Our results demonstrate the dynamic heterogeneity source of coil deformation on biased polymer diffusion, which implies a molecular‐level nonlinear factor in the non‐Newtonian‐fluid behaviors of polymer flows. © 2014 Society of Chemical Industry     

Fast-responsive semi-interpenetrating hydrogel networks imaged with confocal fluorescence microscopy

The composition of semi-interpenetrating polymer networks (semi-IPNs) based on responsive N-isopropylacrylamide (NIPAAm) hydrogels has been shown to affect the kinetics of the volume phase transition. Several N-alkyl-substituted acrylamides were used as the linear polymers in a crosslinked NIPAAm network, and the kinetics was observed as a function of crosslinking density and linear polymer concentration. The time required for collapse of the network could be reduced by as much as 90%, with little change to the corresponding swelling ratio and volume phase transition temperature. However, the underlying changes in network morphology are not known, and here we present kinetics data in combination with imaging of the resulting hydrogel networks. The crosslinked networks and the linear polymers were fluorescently labeled, and the resulting morphology was imaged with confocal fluorescence microscopy and two-photon laser scanning microscopy. The most hydrophilic of the linear polymers was acrylamide, which was shown to phase separate during polymerization. The hydrophilic domains become more interconnected at higher concentrations of the crosslinker and the linear polymers. This correlates well with the kinetics of the volume phase transition for the corresponding networks. The semi-IPNs containing more hydrophobic linear polymers had very similar morphology, but some domains were present, ranging from 500 nm to 2 μm and increasing in size with increased linear polymer concentration. The time scale of collapse was an order of magnitude faster than expected, based on size of the hydrophobic N-alkyl group, when the linear polymer had the same lower critical solution temperature as the hydrogel network. This is an indication that the simultaneous collapse of the linear polymer and the crosslinked network contributes to the fast response of these semi-IPNs.