Unperturbed dimensions of stereoregular polymers.- Reinforcement of rubber by carbon black.- Transformations of phenolic antioxidants and the role of their products in the long-term properties of polyolefins.

Properties of polymers

Polyimides containing aliphatic/alicyclic segments in the main chains

Surface lamellar decoration (SLD), surface enhanced Raman scattering (SERS) and optical second harmonic generation (SHG) experiments have been utilized to study the molecular orientation and conformation changes at a rubbed polyimide alignment-layer surface. This aromatic polyimide containing pendent cyanobiphenyl mesogens was synthesized via a polycondensation of 2,2'-bis(3,4-dicarboxy-phenyl)hexafluoropropane dianhydride (6FDA) with bis[omega-[4-(4-cyanophenyl)phenoxy]hexyl] 4,4'-diamino-2,2'-biphenyldicarboxylate (nCBBP, n = 6), abbreviated as 6FDA--6CBBP. Uniform alignment layers, possessing high pretilt angles ranging from 39 degrees to 43 degrees, have been achieved after mechanical rubbing of the polyimide thin film surface at room temperature and subsequent annealing. This is the first time that high pretilt angles have been detected to possess a negative angle (-theta(c)) with respect to the rubbing direction (i.e., opposite to the rubbing direction), considerably different from the conventional pretilt angle (theta(c)) observed along the rubbing direction. This observation is confirmed using magnetic null and SHG methods. Combined polyethylene (PE) SLD and atomic force microscopy experiments reveal that the azimuthal orientation distribution of the long axis of the edge-on PE lamellar crystals is oriented normal to the rubbing direction, indicating that the PE chains are aligned parallel to the rubbing direction. This SLD technique probes the anisotropic surface orientation of the outermost molecules of the rubbed polyimide layer. The SERS results show that prior to rubbing the surface, both the pendent cyanobiphenyls in the side chains and backbones possess nearly planar chain conformations at the polyimide surface. Mechanical rubbing causes not only tilting of the backbone moieties, such as imide-phenylene structure, but also significant conformational rearrangements of the pendent side chains at the surfaces. The molecular mechanism of this unusual alignment is due to the fact that the pendent cyanobiphenyls forms a uniformly tilted conformation on the rubbed surface, and the polar cyano groups point down toward the layer surface deduced from SHG phase measurements. This conformational rearrangement of the side chains results in the formation of fold-like bent structures on the surface, which directly leads to the long axis of cyanobiphenyls having the -theta(c) pretilt angle with respect to the rubbing direction.

Rubbing-Induced Molecular Reorientation on an Alignment Surface of an Aromatic Polyimide Containing Cyanobiphenyl Side Chains

Liquid Crystalline Polyimide Films with High Intrinsic Thermal Conductivities and Robust Toughness

Polyimides (PIs) represent a benchmark for high-performance polymers on the basis of a remarkable collection of valuable traits and accessible production pathways and therefore have incited serious attention from the ever-demanding medical field. Their characteristics make them suitable for service in hostile environments and purification or sterilization by robust methods, as requested by most biomedical applications. Even if PIs are generally regarded as “biocompatible”, proper analysis and understanding of their biocompatibility and safe use in biological systems deeply needed. This mini-review is designed to encompass some of the most robust available research on the biocompatibility of various commercial or noncommercial PIs and to comprehend their potential in the biomedical area. Therefore, it considers (i) the newest concepts in the field, (ii) the chemical, (iii) physical, or (iv) manufacturing elements of PIs that could affect the subsequent biocompatibility, and, last but not least, (v) in vitro and in vivo biocompatibility assessment and (vi) reachable clinical trials involving defined polyimide structures. The main conclusion is that various PIs have the capacity to accommodate in vivo conditions in which they are able to function for a long time and can be judiciously certified as biocompatible.

https://www.mdpi.com/1996-1944/12/19/3166/pdf

Biocompatibility of Polyimides: A Mini-Review

Four poly(oxadiazole-imide)s containing naphthalene rings, with different flexibility and molecular weight, are investigated with respect to their rheological properties to establish the optimal processing conditions from solution phase to film state for liquid crystal orientation purposes. The film uniformity and strength are determined by monitoring the flow behavior and chain entanglements. The solution rheological data are in agreement with film tensile testing, revealing that higher molecular weight favors chain entanglements and implicitly the film mechanical resistance. In order to analyze the suitability of these films as alignment layers their surface is patterned by rubbing with two types of velvet. Liquid crystal alignment of 4′-pentyl-4-biphenylcarbonitrile nematic is tested by polarized light microscopy. The resulting behavior is correlated with the polyimide malleability and characteristics of the textile fibers, namely their polarity, size, and mechanical features. The competitive effects between chain flexibility and entanglements, together with the interactions occurring between the polymer and velvet are analyzed in order to explain the surface regularity, which influences the uniformity of the liquid crystal alignment. The contrast between dark and bright states recorded on the liquid crystal cell indicates that some of these polynaphthalimides are promising candidates for liquid crystal display devices.

Chain flexibility versus molecular entanglement response to rubbing deformation in designing poly(oxadiazole-naphthylimide)s as liquid crystal orientation layers

Some novel semi-alicyclic polyimides based on epiclon and three aromatic diamines are investigated. The ellipsometry data reveal that the reduced polarizability provided by the alicyclic dianhydride moieties leads to polyimides with low refractive index, ranging from 1.64 to 1.72. The corresponding dielectric constants decrease from 2.97 to 2.71 as the polarization of the diamine part is lower. The transmittance is of appreciatively 90 % for polyimides containing methylene groups in the diamine residue. The cut-off wavelength decreases, as the backbone conjugation is smaller, being comprised between 305 nm and 317 nm — values comparable to that of a fully aliphatic PI. To obtain the optical energies describing absorption limits, the approach proposed by Tauc and Davis-Mott has been used, due to the similarity of the absorption edges. The values of optical band-gap are found from 3.51 to above 3.78 eV, indicating a low probability of optical absorption transitions and thus a good transparency. The Urbach and Tauc energies changed in the range of 480–806 meV and 1,453–2,786 meV, being smaller as the flexible sequence in the diamine moieties is longer. The novelty of this work consists in relating optical properties with morphological aspects, namely surface defects and entropy of morphology. These new results have a great impact in selecting the proper materials as liquid crystal alignment layers or as cell culture substrates.

Morphological effects on transparency and absorption edges of some semi-alicyclic polyimides

Synthesis of a new partial-alicyclic copolyimide based on bicyclo [2.2.2] oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BOCA) is presented. This polymer was used to obtain porous microspheres by suspension polymerization method. It is copolymerized with N, N’- 4, 4′- diphenylmethanebismaleimide in the presence of two pairs of porogens: 1-methyl-2-pyrrolidone/benzyl alcohol, and 1,4 Dioxane/benzyl alcohol. Influence of different factors, such as diluents composition or crosslinker concentration, on the resulting polymer beads morphology is studied. Various characteristcis including the specific and apparent densities, porosity, pore volume, surface area and mean pore diameter are analyzed. Thermal behavior and tendency to swell in different organic diluents for some chosen samples are also determined. The best polymer beads, obtained in a reaction system with a solubility parameter value of 23.45 MPa ½ and a crosslinker concentration of 40 %, are thermally stable above 400 oC, having a pore volume of 1.28 mL/g, a surface area of 74.20 m2/g and enhanced swelling properties.

New polyimide-based porous crosslinked beads by suspension polymerization: physical and chemical factors affecting their morphology

Novel copolyimides (cPIs) based on 3,3′,4,4′-benzophenonetetracarboxylic dianhydride, 4,4′-diaminodiphenylmethane (DDM) and 1,6-diaminohexane (DAH) were synthesized. Their surface wettability was improved by plasma enhancement of polar component contribution to surface tension from 10 to 50 %. Interfacial energy reduction up to 2.07–2.95 mN/m was depicted in spreading work values that reveal the manner in which cPIs interact with blood constituents. The results showed that by increasing of the aliphatic DAH content in the polymer chain, the rejection of platelets is higher, prohibiting thrombosis and favoring cohesion with plasma proteins. The different water sorptions of the samples were evaluated by considering their morphologies and chemical affinities to water. When DAH prevails to DDM, water sorption capacity is lowered from 6.36 to 4.96 % db, presumably due to a higher chain packing efficiency. Atomic force microscopy data revealed that surface roughness is <8 nm for all polymers, even after plasma treatment, preventing clot formation.

Plasma Modification of Surface Wettability and Morphology for Optimization of the Interactions Involved in Blood Constituents Spreading on Some Novel Copolyimide Films

The impact of rubbing fabric type on surface roughness and tribological properties of some semi-alicyclic polyimides evaluated from atomic force measurements

Andreea Irina Barzic

Papers

Chain flexibility versus molecular entanglement response to rubbing deformation in designing poly(oxadiazole-naphthylimide)s as liquid crystal orientation layers

Morphological effects on transparency and absorption edges of some semi-alicyclic polyimides

New polyimide-based porous crosslinked beads by suspension polymerization: physical and chemical factors affecting their morphology

Plasma Modification of Surface Wettability and Morphology for Optimization of the Interactions Involved in Blood Constituents Spreading on Some Novel Copolyimide Films

The impact of rubbing fabric type on surface roughness and tribological properties of some semi-alicyclic polyimides evaluated from atomic force measurements