Nonradiative energy transfer study on the interface of compatibilized linear low density polyethylene/poly(methyl methacrylate) blends

Blends of chromophore-labeled LLDPE and chromophore-labeled PMMA compatibilized by block copolymer of hydrogenated polybutadiene and methyl methacrylate (PHB-b-PMMA) were studied by nonradiative energy transfer (NRET) technique. The ratio of fluorescence intensity of the donor at 336 nm and the acceptor at 408 nm (I_D/I_A) decreased with an increase in block copolymer content. At about 8 wt.-% block copolymer content I_D/I_A reached a minimum value, indicating the interdiffusion of LLDPE chains and PMMA chains in the interface is strongest. The influence of temperature on the interdiffusion of polymer chains in the interface was also examined. Samples quenched in liquid nitrogen from 140°C showed lower energy transfer efficiencies than those annealed from 150°C to room temperature.     

Calorimetric study of blends of low density polyethylene (LDPE) and linear low density polyethylene (LLDPE) temperature rising elution fractionation (TREF) fractions

Preparative temperature rising elution fractionation (TREF) on commercial linear low density polyethylene (LLDPE) and low density polyethylene (LDPE) samples has been performed. The resulting fractions exhibited a bimodal and a unimodal distribution, respectively. Two LLDPE fractions of low (5 CH₃/1000 C) and high (21 CH₃/1000 C) short-chain branching content were solution-mixed with the LDPE central fraction (16 CH₃/1000 C). Indirect evidence based on differential scanning calorimetry results suggest that the fractions with similar branch contents are more miscible than those with dissimilar ones.     

Synthesis and functionalization of bromomethylated poly(phenylene sulfide)

A soluble poly(phenylene sulfide) (PPS) derivative was synthesized by oxidative polymerization of bis(3,5-dimethylphenyl) disulfide. The obtained poly(thio-2,6-dimethyl-1,4-phenylene) ( I ) (M_w = 22 900, M_n = 8 400) was brominated by the reaction with N-bromosuccinimide (NBS). The bromomethyl substituents on PPS are reactive and were converted quantitatively to hydroxymethyl groups by hydrolysis in NMP at 100°C. The reaction of the bromomethylated PPS with tertiary amines gave the quaternized polymer, which is stable up to 145°C in the solid state and in refluxing methanol.     

Stabilization of polystyrene/poly(methyl methacrylate) blends by in situ compatibilization with graft copolymers

Copolymers of styrene and ortho-vinylbenzaldehyde (o-VBA) are useful precursors to multicomponent polymer systems. Graft copolymers of poly(styrene-stat-o-VBA) can be produced by free-radical chain transfer to methyl methacrylate monomer to yield materials that display some potential as interfacial agents with binary blends of polystyrene and poly(methyl methacrylate). Significant improvements in ultimate tensile strengths and energy to rupture values have been witnessed for polystyrene/poly(methyl methacrylate) mixtures that contain various levels of the graft component. Scanning electron microscopy of representative fracture surfaces demonstrate a decrease in the particle size of the dispersed phase; it is suggested that this morphological factor contributes to the superior mechanical properties of these composites. In this regard, both the isolated and the crude graft copolymers are able to compatibilize, and thereby enhance, the material properties of polystyrene/poly(methyl methacrylate) blends.     

Compatibilizing effect of polystyrene-block-poly(4-vinylpyridine) for poly(2,6-dimethyl-1,4-phenylene oxide)/chlorinated polyethylene blends

The compatibilizing effect and mechanism of compatibilization of the diblock copolymer polystyrene-block-poly(4-vinylpyridine) P(S-b-4VPy) on immiscible blends of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO)/chlorinated polyethylene (CPE) were studied by means of scanning electron microscopy (SEM), differential scanning calorimetry (DSC), mechanical properties and FTIR measurements. The block copolymer was synthesized by sequential anionic polymerization and melt-blended with PPO and CPE. The results show that the P(S-b-4VPy) added acts as an effective compatibilizer, located at the interface between the PPO and the CPE phase, reducing the interfacial tension, and improving the interfacial adhesion. The tensile strength and modulus of all blends increase with P(S-b-4VPy) content, whereas the elongation at break increases for PPO-rich blends, but decreases for CPE-rich blends. The polystyrene block of the diblock copolymer is compatible with PPO, and the poly(4-vinylpyridine) block and CPE are partially miscible.     

Morphology,crystallization, and thermal behaviour of isotactic polypropylene/low density polyethylene blends

The morphology, the crystallization and thermal behaviour of isotactic polypropylene (IPP) in its blends with two different samples of low density polyethylene (LDPE) was investigated at temperatures high enough to prevent any solidification of LDPE. It is found that pre-existing liquid LDPE domains are incorporated in intra-spherulitic regions during the isothermal crystallization of iPP. The radial growth rate of spherulites is almost unaffected by the LDPE content. The overall rate of crystallization of iPP, on the contrary, is strongly depressed by the addtion of LDPE. A depression of the equilibrium melting temperature of iPP, due to kinetic and morphological effects, is also observed. The depression of the overal kinetic rate constant is accounted for by the negative effect (decrease in the number of nuclei) that the addition of LDPE has on the primary nucleation process of iPP.     

Poly(phenylene oxide) macromonomers for graft copolymer synthesis via ring-opening olefin metathesis polymerization

Poly(oxy-2,6-dimethyl-1,4-phenylene) samples with bicyclic olefin end groups were obtained by esterification of the hydroxyl end group of the poly(phenylene oxide) with bicyclo[2.2.1]hept-5-ene-2-carbonyl chloride. Mixtures of these macromonomers with bicyclo[2.2.1]hept-5-ene-2-carboxylic acid 2-(2-(2-methoxyethoxy)ethoxy)ethyl ester were copolymerized by ring-opening olefin metathesis polymerization (ROMP) to produce graft copolymers with poly(phenylene oxide) side chains. The catalyst used for the metathesis polymerization was RuCl₃(hydrate). Molecular weights of the graft copolymers were in the range of 130 000 to 250 000. Macromonomers containing exclusively exo-linked bicyclic end groups were found to be more reactive than macromonomers with 55% exo- and 45% endo-substituted end groups.     

Two-component copolymer networks. Synthesis of polystyrene/poly(dimethylsiloxane) copolymer network and its swelling behavior in mixed solvents

A two-component copolymer network comprising polystyrene (PS) and poly(dimethylsiloxane) (PDMS) as copolymer components was prepared by cross-linking PS with telechelic PDMS. In the first stage of the cross-linking reaction, PS having 5–6 mol -% of sodium acrylate units along the main chain was ionically coupled with bifunctional PDMS having 1-methylpyrrolidinium salt groups on both chain ends, and then the ionic coupling was converted to covalent bonds at 100°C by ring-opening of the 1-methylpyrrolidinium salt upon nucleophilic attack of the carboxylate counter anion. The resulting PS/PDMS networks were subjected to investigation of swelling behavior in three kinds of mixed solvents which were chosen so as to contrast with one another in respect of solubility of the component polymers. Three different types of swelling behavior were obtained which regard to the solvent composition dependence.     

Synthesis of a novel structural triblock copolymer of poly(gamma -benzyl-l-glutamic acid)-b-poly(ethylene oxide)-b-poly(epsilon-caprolactone)

A novel structural triblock copolymer of poly(gamma-benzyl-l-glutamic acid)-b-poly(ethylene oxide)-b-poly(epsilon-caprolactone) (PBLG-PEO-PCL) was synthesized by a new approach in the following three steps: (1) sequential anionic ring opening polymerization (ROP) of ethylene oxide and epsilon-caprolactone with an acetonitrile/potassium naphthalene initiator system to obtain a diblock copolymer CN-PEO-PCL with a cyano end-group; (2) conversion of the CN end-group into NH2 end-group by hydrogenation to obtain NH2-PEO-PCL; (3) ROP of gamma-benzyl-l-glutamate-N-carboxyanhydrides (Bz-l-GluNCA) with NH2-PEO-PCL as macroinitiator to obtain the target triblock copolymer. The structures from CN-PEO precursor to the triblock copolymers were confirmed by FT-IR and 1H NMR spectroscopy, and their molecular weights were measured by gel permeation chromatography. The monomer of Bz-l-GluNCA can react almost quantitatively with the amino end-groups of NH2-PEO-PCL macroinitiator by ROP.     

Controlled biological response on blends of a phosphorylcholine-based copolymer with poly(butyl methacrylate)

Phosphorylcholine (PC)-based polymers have been used in a variety of medical device applications to improve biocompatibility. Here, solutions containing poly(butyl methacrylate) (PBMA) and 2-methacryloyloxyethyl phosphorylcholine-co-lauryl methacrylate (MPC-co-LMA(2)) copolymer were spin-coated onto glass coverslips at various ratios ranging from 5:1 to 1:0 for each of the two components, respectively. The resulting blend coatings were shown to be phase-separated on the nanometre-scale by atomic force microscopy, the PC copolymer within the blend being preferentially expressed at the surface. The adsorption of two key blood proteins, fibrinogen and albumin were investigated using surface plasmon resonance as an indicator of biocompatibility. The adsorption of protein to a biomaterials' surface can then stimulate further biological responses. This study therefore, also investigates the materials ability to elicit an inflammatory response by studying the adhesion of human mononuclear cells to the material surface. The materials ability to support the adhesion and growth of other tissue cells was also evaluated, looking specifically at the adhesion and proliferation of rabbit corneal epithelial cells. Results suggest that the adsorption of proteins and the adhesion of both corneal epithelial cells and mononuclear cells are dependent on the composition of the PBMA:MPC-co-LMA(2) copolymer.     

Synthesis of poly(vinyl alcohol)/poly(dimethylsiloxane) graft copolymer

Poly(vinyl acetate)/poly(dimethylsiloxane) graft copolymer ( 4a ), with a controlled poly(dimethylsiloxane) graft chain length, was synthesized by radical copolymerization of vinyl acetate with poly(dimethylsiloxane) ( 3 ) having a dimethylvinylsilyl end group. 3 was prepared by living anionic polymerization of hexamethylcyclotrisiloxane ( 1 ) with butyllithium and subsequent termination with chlorodimethylvinylsilane ( 2 ). Poly(vinyl alcohol)/poly(dimethylsiloxane) graft copolymer ( 4b ) was then synthesized by selective saponification of the poly(vinyl acetate) segments in the graft copolymer 4a with K₂CO₃ in methanol.     

Primary thermal degradation processes of poly(ether-sulfone) and poly(phenylene oxide) investigated by direct pyrolysis-mass spectrometry

The thermal degradation processes which occur in poly(ether-sulfone) (PES) and poly(phenylene oxide) (PPO) have been studied by Direct Pyrolysis-Mass Spectrometry (DPMS). The pyrolysis mass spectra of PES and PPO contain molecular ions up to m/z 700, corresponding to pyrolysis compounds of sufficient size to allow the characterization of the primary processes which occur in the thermal cleavage of these polymers. The results obtained indicate that the thermal degradation of PES and PPO proceeds by thermal cleavage of the groups located at the bridges. In the case of PPO the bridges are all homogeneous, and therefore both an intramolecular thermal rearrangement or a radical cleavage process can account for the formation of the primary pyrolysis products. Formation of dibenzofuran units along the PPO chain is a parallel thermal process. In the case PES the simultaneous thermal cleavage of two different bridged units, diphenyl sulfone and diphenyl ether occurs. Extrusion of SO₂, with consequent formation of compounds containing biphenyl units, is observed in the thermal degradation of PES. This process is accompanied by formation of dibenzofuran units along the polymer chain, as expected from the thermal cleavage of diphenyl ether units, and by another rearrangement of the polymer backbone, yielding structures containing more oxygen atoms with respect to that of PES.     

Compatibilizing effect of a thermotropic liquid-crystalline compatibilizer on liquid-crystalline polycarbonate/poly(p-phenylene oxide) blends

The compatibilizing effect of a thermotropic block-graft copolymer (LCBG) as a compatibilizer on thermotropic liquid-crystalline (LC) polycarbonate (LCPC)/poly(p-phenylene oxide) (PPO) blends was studied. Compatibility, morphology and mechanical properties of the blend systems were investigated by differential scanning calorimetry (DSC), thermal mechanical analysis (TMA) and scanning electron microscopy (SEM). The blend without the compatibilizer formed thermotropic LC phases, based on LCPC domains, and had a phase-separated structure. The LC temperature ranges of the blends with LCBG were wider than that of the blend without the compatibilizer. DSC, SEM and TMA data showed that in the blends containing the compatibilizer the compatibility and the viscoelasticity were improved, because LCBG segments penetrate into both the PPO matrices and the LCPC domains in the blends. These results suggest that LCBG acts as a good compatibilizer for blends.     

Evaluation of cell affinity on poly(L-lactide) and poly(epsilon-caprolactone) blends and on PLLA-b-PCL diblock copolymer surfaces

An evaluation of cell proliferation and adhesion on biocompatible film supports was performed. A series of films were compression molded from commercially available poly (L-lactide), PLLA, and poly(epsilon-caprolactone), PCL, and from their melt mixed blends (PLLA/PCL blends). These were compared with compression molded films of PLLA-b-PCL model diblock copolymers. The samples were analyzed by differential scanning calorimetry (DSC), contact angle measurements, and scanning force microscopy (SFM). Cell adhesion and proliferation were performed with monkey derived fibroblasts (VERO) and with osteoblastic cells obtained either enzymatically or from explants cultures of Sprague-Dawley rat calvaria. Migration studies were performed with bone explants of the same origin. The results obtained indicate that although all materials tested were suitable for the support of cellular growth, a PLLA-b-PCL diblock copolymer sample with 93% PLLA was significantly more efficient. This sample exhibited a unique surface morphology with long range ordered domains (of the order of 2-3 mum) of edge-on PLLA lamellae that can promote "cell contact guidance." The influence of other factors such as chemical composition, degree of crystallinity, and surface roughness did not play a major role in determining cell preference toward a specific surface for the materials employed in this work.     

Synthesis of mainly linear poly(dichlorophenylene oxide)s by thermal polymerization in solution

Thermal decomposition of bis(4-bromo-2,6-dichlorophenoxo)-(N,N,N',N'-tetramethylethy-lenediamiene)copper(II) complex in toluene, bis(4-bromo-2,6-dichlorophenoxo)(N,N-dimethyl-formamide)copper(II) complex in N,N-dimethylformamide and bis(4-bromo-2,6-dichlorophenoxo)(dimethyl sulfoxide)copper(II) complex in dimethyl sulfoxide was achieved at 70°C. 4-Bromo-2,6-dichlorophenol derivatives prefer 1,4-addition to 1,2-addition, leading to highly linear polymers irrespective of the substituted ligands. The complexes were characterized by IR spectroscopy and C, H, N elemental analysis. The characterization of the synthesized polymers were achieved by ¹H NMR, ¹³C NMR and IR spectroscopy. The highest polymer yield was obtained from the decomposition of bis(4-bromo-2,6-dichlorophenoxo)(N,N,N',N'-tetramethyl-ethylenediamine)copper(II) complex.     

Synthesis of star-shaped poly(ethylene oxide)

Three different methods to synthesize star-shaped poly(ethylene oxide) are discussed in the present article. In all three cases, the branches are grown from a plurifunctional initiator. It is established that even though the early stages of the polymerization occur in heterogeneous phase, the consequences on the polymers formed are of minor importance. The most significant method is a core-first process, involving multifunctional poly-DVB cores as the initiating species, made anionically in dilute solution. Although strong association phenomena are occurring during the growth of the branches, star-shaped poly(ethylene oxide)s with a high number of functionalized branches are obtained. The polymers arising from all three methods were characterized accurately.     

Synthesis of chromophore-labelled polystyrene/poly(ethylene oxide) diblock copolymers

Polystyrene/poly(ethylene oxide) diblock copolymers bearing anthracene or phenanthrene groups at the block junctions were synthesized by sequential anionic polymerization using cumylpotassium as initiator. Labelled copolymers were characterized by means of ultraviolet (UV) spectroscopy, gel-permeation chromatography (GPC) and ¹H NMR spectroscopy. The solutions of the prepared copolymers in water and 1,2-dichloroethane/methanol mixtures were studied by fluorescence spectroscopy. The intermolecular energy transfer process confirms the presence of organized structures (micelles) in solutions.     

Phase structure and compatibility studies in poly(ethylene oxide)/poly(methyl methacrylate) blends

Morphology, crystallinity, and melting behaviour of solution-cast films of poly(ethylene oxide) (PEO)/poly(methyl methacrylate) (PMMA) blends were investigated as a function of composition, using optical and scanning electron microscopy, differential scanning calorimetry, and dilatometry. Up to a content of about 40 wt.-% of PMMA the blend films are completely filled with PEO spherulites, no separated domains of PMMA being observed. This observation suggests, in agreement with small angle X-ray scattering analysis, that for such compositions the PMMA molecules are incorporated in interlamellar regions of PEO spherulites. Blends with higher content of PMMA show islands of crystalline PEO within a matrix of PMMA; large amorphous regions coexist with not well developed PEO spherulites. Addition of PMMA strongly increases the nucleation density for PEO crystals. The solubility parameters δ of PEO and PMMA were calculated from specific volume values as a function of temperature. It was found that at temperatures higher than that of PEO melting the difference δ(PEO) - δ(PMMA) is very low suggesting that the two polymers are compatible in the melt. Compatibility of PEO and PMMA was also predicted by using a theoretical approach which allows to calculate the free energy of mixing as a function of composition and temperature.     

Synthesis and characterization of polycaprolactone / poly(ethylene oxide) / polylactide tri-component copolymers

Polycaprolactonel/poly(ethylene oxide)/polylactide tri-component copolymers (PCEL) with different compositions were synthesized by copolymerization of ε-caprolactone and L-lactide in the presence of poly(ethylene glycol) using stannous octoate as a catalyst. The copolymers were purified and characterized by various analytical techniques such as GPC, FT-IR, H NMR, ¹³C NMR, DSC, and X-ray diffractometry. It was evidenced that these copolymers were pure tri-component compounds which exhibited partially random chain structures, and possessed good mechanical properties and variable biodegradability.     

Morphology,crystallization and thermal behaviour of poly(ethylene oxide)/poly(vinyl acetate) blends

Blends of poly(ethylene oxide) (PEO) and poly(vinyl acetate) (PVAc) show a unique value of the glass transition temperature, intermediate between that of plain polymers. The addition of PVAc to PEO causes a depression in both the spherulite growth rate (G) and the overall kinetic rate constant (K_n). Such depression is larger at higher undercooling and, at a given crystallization temperature, it increases with the content of PVAc. The experimental G and K_n data were analyzed by means of latest kinetic theories in order to determine the influence of composition on the process of surface secondary nucleation. The melt behaviour of PEO/PVAc blends cannot be explained only in terms of diluent effects due to the compatibility of the components in the melt. Especially, at lower undercooling it is likely that annealing and morphological effects must also be taken into account. The morphology of thin films of blends, isothermally crystallized from melt, suggests that an amorphous mixed phase (PEO + PVAc) is formed in interlamellar regions. It was found that plain PEO crystals grow according to a regime I process of surface secondary nucleation while in the case of blends the crystals of PEO grow via regime II mechanism.