Drawing and extrusion of semi-crystalline polymers

This review treats mainly the initial transformation of the starting unoriented lamellar material into the final oriented microfibrillar structure and its drawing. If the starting material has partially oriented shish-kebab structure the drawing is a combination of transformation of lamellae into the microfibrils and the drawing of microfibrils. The review article is based on the literature from the last ten years and relys heavily on recent reviewing [1–3]. Older articles are only included if they have contributed to the basic ideas and experiments of drawing.     

Polymorphism of a semi-crystalline diketopyrrolopyrrole-terthiophene polymer

Few semiconducting polymers are known that possess more than one semi-crystalline structure. Guidelines for rationalizing or creating polymorphism in these materials do not exist. Two different semi-crystalline polymorphs, β₁ and β₂, and an amorphous α phase have recently been identified for alternating diketopyrrolopyrrole-quaterthiophene copolymers (PDPP4T). The polymorphs differ structurally by the π–π stacking distance, and electronically by the optical bandgap and charge carrier mobility. Here we investigate the corresponding terthiophene (PDPP3T) derivatives, to study the effect of the relative orientation of adjacent DPP units on the polymorphism. In PDPP3T, the relative orientation of DPP units alternates along the chain, while in PDPP4T it is constant. We show that the two polymorphs, β₁ and β₂, can also be generated for a PDPP3T polymer in solution and thin film. Interestingly, compared to PDPP4T, more solvents can induce the two distinct semi-crystalline polymorphs for PDPP3T via a β₁ → α → β₂ polymorphic transition.     

Infrared spectra of polymer solutions. I. Conformational stability of isotactic polymer chains in solution

The infrared spectra of isotactic polystyrene, polypropylene, and poly-p-chlorostyrene were measured in dilute solutions or gels at various temperatures ranging from room temperature to ?100°C. For isotactic polystyrene and polypropylene, all the absorption bands characteristic of the helical conformation of the molecules increase in intensity with decreasing temperature, and show intensities at low temperature as strong as in the highly crystallized samples. This suggests that the molecules can assume very regular conformations even in solution. Similar temperature dependence of the spectra was observed for a homogeneous gel of isotactic poly-p-chlorostyrene, which is believed to be noncrystalline. These experimental facts lead to the conclusion that the stability of the TG type helical conformation of these isotactic polymers may be ascribed to the intramolecular forces within a molecule. The fact that these spectral changes are reversible has permitted a thermodynamic treatment of the conformational regularity in solution on the basis of the infrared data. The temperature dependence of the absorption intensities has been interpreted quantitatively by a simple statistical mechanical model, and the enthaply and the entropy differences between the helical and the random states of the monomeric residue in the molecular chain have been estimated.     

Simulation study of semi-crystalline polymer interphases

The study of structure and properties of semi-crystalline polymer inter-phases is important to explain and extend polymer applications. In this region, polymer chains exist in three distinct populations: tie chains that bridge the two crystals, chain folds and chain ends. The distribution of these populations influences the properties of the interphase. We have developed off-lattice Monte Carlo simulations of constrained interphases of semi-crystalline polymers which utilize robust off-lattice moves. A united atom model with polyethylene-like interactions and with freely rotating bonds is used to mimic the prototypical flexible chain structure. These simulations capture the limiting distributions of tight and loose chain folds and of tie chains within the metastable phase. The dissipation in order and density between the crystal and amorphous regions has been studied, and results for freely rotating chains indicate that the characteristic decay of anisotropy occurs in a length scale of ca. 10 Å. Simulation results for the effect of system size and molecular weight for freely rotating chains have also been investigated.     

Conformational energies,stereoregularity, and the role of nonstaggered conformations in polymer chains

The role of nonstaggered conformations in polymethylenes and in vinyl polymers is discussed in some detail. In incorporating such conformations into rotational isomeric state treatments, it is essential to so choose rotational states as to assure equitable sampling of configuration space. Tacit identification of rotational states with minima in the conformation energy surface, a common practice at present, may lead to serious errors. The significance and limitations of conformational energy calculations are discussed. It is pointed out that the alleged discrepancy between theory and experiments on the spatial configurations of isotactic polymers of α-olefins in general and of polypropylene in particular is unsupported by valid experiments on polymers of established stereochemical constitution.     

Equilibrium statistics of weakly slip‐linked Gaussian polymer chains

We calculate the free energy and the pressure of a weakly slip‐linked Gaussian polymer chains. We show that the equilibrium statistics of a slip‐linked system is different from one of the corresponding ideal chain system without any constraints by slip‐links. It is shown that the pressure of a slip‐linked system decreases compared with the ideal system, which implies that slip‐linked chains spontaneously form aggregated cluster‐like compact structures. These are qualitatively consistent with previous theoretical analyses or multichain simulations. We also show that repulsive potentials between chains, which have been phenomenologically utilized in simulations, can cancel the artificial pressure decrease. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Quantitative molecular characterization of oriented semi-crystalline polymers

The stress-strain birefringence relations obtained from the hot drawing of poly(ethylene terephthalate) films are analysed within the framework of non-Gaussian models of rubber elasticity. It is found that, in spite of the increasing influence of strain-induced crystallization for higher draw ratios, the proposed treatment of experimental results leads to the definition of the number of statistical links per elastic chain in the amorphous part of the sample for any state of elongation. The results suggest a progressive disentanglement of the physical network structure and make possible an evaluation of the limit of extensibility attainable in cold-drawing.     

Conformational statistics of polymer chain terminally attached to wall (II)

The SAW tail chains were studied. The permitted conformational number and the mean square end-to-end distance as a function of the chain length N for such a model tail chain were obtained by computer simulations, including the exact enumeration and Monte Carlo method. These two basic quantities obeyed the relations deduced from the scaling law. The critical exponents and the lattice indexes were given by fitting the data of the computer experiments. It has been shown that there is a certain extension in the size of the SAW tail chains as well as the NRW tail chains in the direction normal to the wall. The normal component of the mean square end-to-end distance is almost twice as large as the parallel component of the short chain SAW. However, as N →∞, the effect of the wall on the chain conformation becomes a little weak because of the self-avoiding behavior for the model. That is quite different from the case of the NRW tail chain. Project supported by the National Natural Science Foundation of China     

The chemical structure dependence of interaction strength between polymers and mobility of polymer chains in the polymer interface

The structural difference of the interface between polypropylene pair (PP/PP) and polypropylene and poly(ethylene-alt-propylene) pair (PP/EAP) was investigated using molecular dynamics and molecular mechanics, under a two-dimensional periodic boundary condition (2DPBC). From the position of the end group of each polymer chain, the influence of the chemical structure and the mobility of the polymer chain were considered. In the PP/PP system, the end group is mainly located in the middle region of each polymer layer. In the PP/EAP system, the PP end group and the ethylene end group of EAP are also located in the middle region of each polymer layer, but the propylene end group of EAP is located very near the interface. These results suggest that the mobility of the middle part of the polymer chain is not so small, and that the similarity in the chemical structure, which is a measure of the interaction strength between polymers, plays an important role in the early stage of adhesion. An easy and efficient estimation method of the interaction strength between polymer pairs is also proposed, and the influence of the component sequence in copolymer on the interaction strength is systematically estimated.     

Conformational and dynamic effects of pendant-PEK chains on rigid-rod polymers by molecular dynamics simulation

Molecular dynamics simulations were performed with models of a single-component ‘hairy-rod’ molecular composite composed of flexible meta poly(aryl ether ketone) (mPEK) chains (the ‘hairs’) grafted to a poly(p-phenylene benzobisthiazole) (PBZT) rigid-rod backbone. The molecular-composite concept, conceived to circumvent solubility problems and improve compressive strength of PBZT, relies on an even distribution of rods in a coil-like matrix. Two-molecule simulations show that the pendant mPEK chains associate with the rods causing large bends in the rod backbone. Simulations of bulk systems imply that the mPEK chains greatly reduce the correlation of rod orientation without altering rod spacing. Radial distribution and orientation correlation functions as well as correlation volumes are computed.     

Real-time AFM study of lamellar growth of semi-crystalline polymers

Atomic force microscopy (AFM) has been used to study the lamellar development during the crystallization and melting processes of poly(bisphenol A-co-alkyl ether) (BA-Cn) films. High-resolution and real-time AFM phase imaging enables us to observe the detailed growth process of the lamellae. At the early stage of the lamellar growth, embryos appeared firstly and some disappeared on the film surface after a period of time. The stable embryo developed into a single lamella. Then the lamella developed into a lamellar sheaf through branching and splaying. Our results revealed that the branches of the lamellae were formed by induced nucleation and it was also dependent on the crystallization temperature. Real-time AFM study of the melting, recrystallization and remelting processes of lamellae indicated that the thermal stability of different segments of a single lamella is different and that the thermal stability of the different lamellae is also different even if they develop at the same annealing temperature. The orientation and the development of the lamellae at the characteristic eyes and boundaries of the spherulites are observed in details.     

The determination of the lamellar thickness in semi-crystalline polymers by small-angle scattering

Three small-angle scattering methods for determining the thickness of the crystalline lamellae in polymers are described. These are based on 1. observation of the identity period, 2. determination of the specific surface of the phase boundary, and 3. analysis of the shape of the peak at the origin. The conditions for application of these methods and the types of average obtained are discussed, and some results are presented.     

Molecularly imprinted polymers with assistant recognition polymer chains for bovine serum albumin

A new protein molecularly imprinted polymer (MIP) was prepared with grafting polyvinyl alcohol as assistant recognition polymer chains (ARPCs). The ARPCs and acrylamide monomers were interpenetrated and then polymerized on the surface of macroporous acrylate adsorbent spheres. The template BSA was removed by treatment with 2.00 mol L-1 potassium chloride (KCl) solution and the adsorbed proteins were detected with sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). 0.150, 0.500, and 2.00 mo...     

Conformational fluctuations and large fluorescence spectral diffusion in conjugated polymer single chains at low temperatures

The fluorescence of single chains of the conductive polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) was studied by means of single-molecule spectroscopy at 15 K. MEH-PPV was deposited onto a surface from a toluene solution and covered with a polymer cap layer of poly(vinyl alcohol) spin-coated from an aqueous solution for protection against air. Because MEH-PPV is insoluble in water, such sample preparation guarantees that MEH-PPV chains do not mix with the cap polymer. We found that this "host matrix free" environment results in substantially stronger fluorescence spectral diffusion than that observed for conjugated polymer single chains embedded into polymer matrices. The average spectral diffusion range was 500 cm(-1), and the maximum registered value reached 1100 cm(-1), which is approximately 6 times larger than the values reported before. We analyzed spectral diffusion by observation of temporal evolution of the fluorescence intensity, the position of the maximum, and the width of fluorescence spectra. We propose that the transition energy shifts are caused by the differences of the London dispersive forces in slightly different polymer chain conformations. Such conformational changes are possible even at low temperatures because the MEH-PPV single chains in our samples have more freedom for fluctuations than in the usual "in host" arrangement.     

Durable semi-crystalline interphase engineering to stabilize high voltage Ni-rich cathode in dilute ether electrolyte

Ethers are promising electrolyte solvents for secondary Li metal batteries because of their excellent reduction stability. However, their oxidation stability has been mostly relying on the high concentration approach, and limited progress has been made on building effective interphase to protect the cathode from the corrosion of the electrolyte. In this work, we construct a semi-crystalline interfacial layer on the surface of Li(Ni_0.8Co_0.1Mn_0.1)O₂ cath...     

Conformational structures and thermodynamic properties of compact polymer chains confined between two parallel plane boundaries

In this paper, the authors investigated the adsorption phenomenon of compact chains confined between two parallel plane boundaries using a pruned‐enriched Rosenbluth method. The authors considered three cases with different adsorption energies of ε = 0, ?1, and ?3 (in units of k_BT) for the confined compact chains of different chain lengths N, respectively. Several parameters were employed to describe the size and shape of compact chain, and some special behaviors in the conformational structures were investigated for the first time. For example, the size and shape of confined compact chains undergo distinct changes in the adsorption cases of ε = ?1 and ?3, and pass through the maximum values at the characteristic distances D_c. The authors found that this characteristic distance D_c could be scaled as D_c～ (N + 1)^ν (ν = 0.56 ± 0.01) in the case of ε = ?3. In addition, the microstructures of chains were investigated, and several significant results were obtained by analyzing the segment density distribution and the mean fractions of segment in tails, trains, bridges, and loops structures. On the other hand, the thermodynamic properties were also investigated for the confined compact chains, such as average energy per bond, Helmholtz free energy per bond, and elastic force per bond. Results show that elastic forces f have different behaviors in three cases, indicating that it is not necessary to exert an external force on the boundaries in the nonadsorption case. At the same time, the average contact energy of compact chain obviously changes when the distance between the two parallel boundaries D increases, which is similar to those of the size and shape parameters. The authors also conclude that these thermodynamic properties of compact chains depend strongly on not only the adsorption energies but also the chain lengths and the confined condition. In addition, several results of the conformational and thermodynamic parameters, such as the segment density distribution and free energy, were compared with the results from the self‐consistent field theory. These investigations may help us to deepen the knowledge about the adsorption phenomenon of confined compact chains. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2888–2901, 2006     

Research on viscoelastic behavior of semi-crystalline polymers using instrumented indentation

In this study, the viscoelastic behavior of a polyamide 12 (PA12) polymer was evaluated using instrumented indentation technology based on a rheological model. The creep compliance and retardation spectra were obtained to analyze the viscoelastic response during the holding stage according to the rheological model under different preloading conditions. The results showed that the viscoelastic responses were independent of the indentation depth or load under step loading conditions. However, the creep compliance increases, and the peak intensity of the retardation spectrum decreases with a decrease in the preloading rate owing to the structural relaxation observed during the preloading stage under ramp loading conditions. Furthermore, softening dispersion can be completed during the loading stage under continuous stiffness measurement (CSM) conditions. As the preloading strain rate changes, the peak of the retardation spectrum gradually decreases until it disappears completely. Moreover, studies on indentation creep using the CSM method are challenging because of the complicated viscoelastic response observed during the preloading stage.     

Configurational statistics of polyamide chains

The statistical mechanical treatment of polymeric chains in terms of the largest eigenvalue of the product of statistical weight matrices for the rotational interactions of skeletal bonds of the repeat unit becomes excessively complicated if the repeat unit spans more than three or four skeletal bonds. Moreover, such treatment is necessarily limited to chains in which the number of repeat units is indefinitely large. Newer methods are readily applicable to chains of any degree of polymerization comprising repeat units of any realizable length. If interdependence of neighboring bond rotations is confined to bond pairs within a given unit, rotations about a pair of bonds belonging to neighboring units being mutually independent, further simplifications may be introduced without sacrifice of rigor. Polyamides, in which rotation about bonds on opposite sides of the amide group are independent, are polymers of this type. Adherence of the amide group to the planar trans conformation favors a more extended configuration of the chain, but this effect is dominated by the smaller steric repulsions affecting rotations about bonds which are first, second, and third neighbors of the amide group. It is for this reason that the characteristic ratio 〈r²〉₀〉/nl² for poly(hexamethylene adipamide), ca. 6.0 according to experimental results of Saunders, is less than the value, 8.0, for polymethylene at 25°C. The characteristic ratios and molecular dipole moments are computed as functions of the degree of polymerization. The poly(εaminocaproamide) chain also is treated.